Journal of Petrology

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Journal of Petrology | Volume 44 | Number 2 | Pages 355-385 | 2003
© Oxford University Press 2003

Glass-rich, Cordierite–Biotite Rhyodacite, Valle Ninahuisa, Puno, SE Peru: Petrological Evidence for Hybridization of ‘Lachlan S-type’ and Potassic Mafic Magmas

HAMISH A. SANDEMAN^* and ALAN H. CLARK

DEPARTMENT OF GEOLOGICAL SCIENCES AND GEOLOGICAL ENGINEERING, QUEEN'S UNIVERSITY, KINGSTON, ON., CANADA, K7L 3N6

Corresponding author. Present address: Canada-Nunavut Geoscience Office, PO Box 2319, Iqaluit, Nunavut, Canada X0A 0H0. E-mail: hsandema{at}nrcan.gc.ca

RECEIVED March 29, 2002; ACCEPTED August 29, 2002

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
The Revancha dyke, a 24·1 Ma, 5 m wide body of cordierite–biotite rhyodacite exposed in the Central Andean Inner Arc of southeastern Peru, is interpreted as a pristine analogue of a Lachlan-type, strongly peraluminous, S-type monzogranite. The preservation of both glass and microphenocrysts, as well as the hypabyssal setting, provide evidence of crystallization processes that are largely disguised in the more widespread plutonic or the largely pyroclastic extrusive examples of such rocks. The dyke contains up to 74% undevitrified glass, phenocrysts of plagioclase, biotite, Fe-cordierite and rare quartz and sanidine, and accessory sillimanite, apatite, zircon, ilmenite and monazite. Both plagioclase and biotite phenocrysts exhibit complex zonation and convincing evidence of magmatic dissolution, whereas groundmass microphenocrysts of these minerals commonly exhibit, respectively, more calcic and magnesian compositions than the phenocrysts. The magma was highly reduced, the sparse ilmenite approaching stoichiometric FeTiO₃. Although comparable in most salient aspects with S-type granitoids of the Lachlan Fold Belt of SE Australia, the rhyodacite is enriched in K₂O, Na₂O, Al₂O₃, P₂O₅, U and Rb, but depleted in MgO, CaO, FeO^T, TiO₂, V and Co. The Revancha rhyodacite, like many peraluminous S-type suites, exhibits mineralogical, textural and chemical features strongly supporting shallow-crustal hybridization with a mafic melt, or melts, of mantle derivation, but the geochemical relationships suggest that the latter were probably markedly potassic rather than basaltic. The Revancha rhyodacite is a unit of the Picotani Group, a 22–26 Ma suite of diverse mafic and felsic rocks emplaced along the foreland boundary of the Central Andean orogen during the Aymará tectonic event. High heat-flow and relatively thin orogenic crust at this time promoted anatexis of clastic rocks at shallow depths. This geotectonic setting contrasts markedly with that of the subsequent, 7–17 Ma, Macusani Formation ash-flows, and other strongly peraluminous, ‘Himalayan-type’, two-mica rhyolites and granites of the Quenamari Group, emplaced during the major crustal thickening of the Quechuan orogeny.

KEY WORDS: glassy rhyodacite; Lachlan S-type; sieve textures; K-rich mafic melt hybridization; petrogenesis

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
Chappell & White (1974) first proposed, on the basis of field studies of two restricted areas of the Palaeozoic Lachlan Fold Belt (LFB) of eastern Australia, that granitoid rocks globally fall into two broad categories, namely I-type, i.e. derived from (meta-)igneous protoliths, and S-type, ultimately of (meta-)sedimentary derivation. The former are generally hornblende bearing and hence metaluminous or, in more silicic members, weakly peraluminous [aluminium saturation indices (Zen, 1986) 1·1], whereas the latter were described as lacking hornblende but containing the Al-rich minerals muscovite (‘common’) and, less consistently, garnet and/or cordierite, in addition to abundant biotite. In a recent reappraisal of this critical concept, drawing on a quarter-century of continued research on the granitoid rocks of the Lachlan Fold Belt, Chappell & White (2001) maintained the fundamental validity of the two-fold classification, although noting that the more felsic members of the two suites converge towards similar minimum melt compositions. Although questioning the general significance of enclaves, they stated that hornblende-bearing and metasedimentary enclaves occur, respectively, only in I- and S-type suites, a distinction ‘supported by all subsequent studies’ (p. 495), a relationship considered to reflect the postulated ubiquitous retention of restite material (White & Chappell, 1977). They moreover argue for a close inherent relationship between muscovite-rich and cordierite-bearing peraluminous granites, interpreting the former as more strongly fractionated, and hence more hydrous equivalents of the latter. Finally, they propose that all markedly peraluminous granites are products of ‘low-temperature’ melts, emphasizing that the vast majority of such rocks in the Lachlan Fold Belt contain over 65 wt % SiO₂.

Significantly different petrogenetic models have, none the less, been proposed by other workers. Thus, Barbarín (1996), in a review of strongly peraluminous granitoid suites, argued that ‘two-mica’, i.e. muscovite-bearing granites (MPG) and rhyolites differ critically from cordierite–biotite-bearing rocks (CPG) in exhibiting positive rather than negative correlations of aluminium saturation index with SiO₂ content and other differentiation indices. Sylvester (1998) similarly concluded that these two clans of peraluminous rocks, distinguished as Himalayan- and Lachlan-type, respectively, record fundamentally differing petrogenetic and geotectonic environments, the two-mica suites forming under higher pressures and lower temperatures in response to greater degrees of orogenic, post-collisional, crustal thickening. Patiño-Douce (1999), integrating the results of melting experiments with the major element compositions of diverse granitic rocks, has reiterated the case for distinguishing peraluminous, muscovite-rich leucogranites and rhyolites (‘PLGS’) from peraluminous S-type granites (‘PSGS’), which characteristically exhibit low-P, high-T mafic aluminous minerals, i.e. cordierite, spinel and/or Al-rich orthopyroxene. He emphasizes that among common granite types only the former are likely to represent pure anatectic melts, interpreting the generally less siliceous magmas, including PSGS, as incorporating residual (restite) and/or peritectic phases generated through the reaction of basaltic magmas with quartzo-feldspathic crustal protoliths. PSGS are thus considered as products of ‘hybrid magmas’ produced through shallow-crustal dehydration-melting of biotite-rich metapelites.

Persisting controversy regarding the genesis of true S-type granitic rocks, and particularly the extent to which their mineralogy and chemistry are influenced by restite retention (see Wyborn et al., 1981; Clemens & Wall, 1984), in part reflects the predominance of coarse-grained phaneritic intrusive facies, in which melt compositions are not preserved and crystallization history is disguised. Moreover, most volcanic representatives are either altered or pyroclastic, and the compositions of microphenocrysts, which in many volcanic rocks record the conditions of late-stage magmatic consolidation, have not been recorded in S-type suites. The salient objective of the present contribution is the documentation of a young Lachlan-type, or PSGS, rock emplaced in a transitional, hypabyssal setting, thereby preserving both unaltered phenocrysts and microphenocrysts, as well as undevitrified glass. The subject of discussion is a member of a well-documented suite in a clearly defined Central Andean geodynamic environment which was also the site of emplacement of muscovite-bearing, Himalayan-type, granites and rhyolites, including the well-known Macusani rhyolites. Moreover, a nearby, closely allied and coeval cordierite–biotite granite pluton hosts the richest giant hard-rock Sn deposit, San Rafael, and clarification of the petrogenesis and crystallization history of the S-type magmas of the region has clear implications for metallogenesis and ore-genesis. Comparisons are made herein with the classic S-type associations of eastern Australia, and with the few similarly fresh cordierite-phyric suites that have been described, namely, the largely ignimbritic Canberra (Wyborn et al., 1981) and Violet Town (Clemens & Wall, 1984) volcanics of the LFB, a dacite from the Upper Miocene Cerro del Hoyazo volcanic centre, SE Spain (Zeck, 1970, 1992), and the Upper Miocene cordierite rhyolites of the Morococala Meseta, Bolivia (Morgan et al., 1998).

	GEOLOGICAL SETTING

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
The uppermost Oligocene, rhyodacitic Revancha dyke crops out in the upper Ninahuisa Valley (Fig. 1), at 14° 4'4''S latitude, 70° 35'3''W longitude in Puno Department of southeastern Peru. The valley is eroded into the SE quadrant of the Meseta de Quenamari, a high tableland underlain largely by the flat-lying Middle to Upper Miocene Macusani Formation (Fig. 1). The meseta and a chain of 4500–5200 m a.s.l. (above sea level) glaciated peaks rimming its southern margin were assigned by Laubacher (1978) to the Precordillera de Carabaya, a southwestern outlier of the main Cordillera de Carabaya, itself a segment of the Cordillera Oriental. The area lies within the Inner Arc magmatic domain of Clark et al. (1983, 1984, 1990), a petrographic subprovince characterized since the Permian by compositionally diverse magmatism with both crustal and mantle sources, focused along the craton–orogen interface (Kontak et al., 1984; Kontak, 1985). The Inner Arc is contrasted to the more homogeneous Main Arc of the Central Andes, which underlies the entire Cordillera Occidental and much of the Altiplano at this latitude.

View larger version (46K): [in this window] [in a new window]   Fig. 1. Location map (inset) and simplified geological map showing the location of the Revancha dyke in southern Peru with respect to the major Tertiary rock units of the Cordillera de Carabaya region.

 
The Cenozoic volcanic and sedimentary rocks of the Cordillera de Carabaya region were originally grouped together by Laubacher et al. (1988) as the Cayconi Formation, but were assigned to the Crucero Supergroup by Sandeman (1995) and Sandeman et al. (1995, 1997a) to express their petrological and petrogenetic diversity. On the basis of stratigraphic and geochronological relationships, this subaerial succession was subdivided into an older Picotani Group, 22–26 Ma in age and which includes the Revancha dyke, and a younger, 6·5–17 Ma, Quenamari Group, each associated with a cogenetic intrusive suite.

The study area is underlain by a thick sequence of clastic sedimentary strata of the Ordovician Sandia Group and Carboniferous Ambo Group, as well as basalts and trachyandesites of, respectively, the Permian Mitu Group and Jurassic AllincÄ&!aacute;pac Group (Kontak et al., 1990a). These are unconformably overlain by silicic pyroclastic rocks and high-K calc-alkaline basaltic flows of the Picotani Group (Sandeman, 1995; Sandeman et al., 1997a), which record the mid-Cenozoic revival of Andean magmatism along the foreland margin of the orogen, coincident with the Aymará (Pehuenchean) tectonic event (Sandeman et al., 1995). Tectonic and geomorphological relationships suggest that the Picotani Group was emplaced into continental crust of normal, or even below-normal thickness, whereas the ensuing Quenamari Group magmatic activity clearly accompanied major crustal thickening throughout the Middle and early Late Miocene (Kontak et al., 1990b; Sandeman et al., 1995).

Commingling and mixing of Lachlan S-type granitic magmas with absarokitic basaltic melts have been documented for Picotani Group suites elsewhere in the Precordillera de Carabaya by Kontak et al. (1986) and Kontak & Clark (1997, 2000), and Carlier et al. (1992, 1993, 1997), Sandeman & Clark (1993) and Sandeman et al. (1997a; unpublished data, 1995) have described similar relationships between S-type and minette and/or lamproite magmas in the Picotani Group and Intrusive Suite. Commingling between mantle-derived potassic and Lachlan-type, PSGS crustal melts therefore appears to have been wide-spread in the SE Peruvian Inner Arc domain in the late Oligocene–Early Miocene.

The Revancha dyke, a minor unit of the Picotani Intrusive Suite (Sandeman et al., 1997a), attains a maximum exposed width of 5 m. It cuts rhyodacitic ash-flows and monzogranitic intrusive rocks, with which it shows close mineralogical and chemical similarities, on the southern slope of the valley of the Ninahuisa River, 1.6 km WNW of Hacienda Ninahuisa. The dyke strikes 120°, dips 67° SW, and has been traced for 400 m. To the NW, it is covered by fluvio-glacial sediments. Chilled margins, commonly flow-banded (Fig. 2a), enclose a porphyritic interior (Fig. 2b), but centimetre-scale variations in grain-size and crystallinity are locally observed. The rock has a striking, but misleading, mafic appearance in outcrop, with dark mauve cordierite phenocrysts scattered through a matrix of blackish glass. The dyke is unweathered and unaffected by hydrothermal activity.

View larger version (96K): [in this window] [in a new window]   Fig. 2. Photographs of two representative facies of the Revancha dyke. (a) Glass-rich, flow-banded variety with phenocrysts of biotite (Bt), plagioclase (Pl) and quartz (Q). (b) Crystal-rich, porphyritic facies with phenocrysts of biotite (Bt), plagioclase (Pl), quartz (Q), sanidine (S) and cordierite (Cd).

 

	PETROGRAPHY AND MINERAL CHEMISTRY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
Both microscopic and outcrop-scale textural relationships indicate that, although the flow-banded, crystal-poor facies is characteristic of the margins of the dyke, in general this intruded more crystal-rich, coarser-grained rhyodacite; thus, flow-bands in the latter are truncated by the boundaries of the former. The two facies exhibit significantly different textural features and mineral compositions, and are hence distinguished in the ensuing discussion as, respectively, the glass-rich (GR) and glass-poor (GP) facies. Sanidine and quartz phenocrysts occur only in the GP facies, in which cordierite and biotite are also coarser, attaining diameters of 10 and 5 mm, respectively. Modal compositions of the two facies, based on at least 2000 points/section for 22 sections, are given in Table 1.

View this table: [in this window] [in a new window]   Table 1: Modal proportions (%) of mineral phases and groundmass glass in eight samples of the Revancha dyke

 
Plagioclase
Plagioclase is the dominant phenocryst phase in both GP and GR facies of the dyke, comprising 13–18 modal % of the rock. It exhibits a wide variety of grain-sizes, morphologies and textures. Phenocrysts, generally 0·5–4 mm in diameter, occur as resorbed, subhedral grains or grain fragments. The majority incorporate zones with sieve textures (Tsuchiyama & Takahashi, 1983), which may be classified as either coarse, interconnected, ‘honeycomb’ networks of glass tubules or ‘dusty’ associations of very small, typically unconnected glass inclusions. Elongate, prismatic microphenocrysts (microlites), generally 0.5 mm in length, are common throughout both facies of the dyke.

Approximately 80% of the plagioclase phenocrysts in the GR facies are rounded to subhedral and exhibit extensive honeycomb sieve textures. Clear, but locally mineral inclusion-rich, cores (Fig. 3a) are enclosed by honeycombed mantles, which are in turn overgrown by clear, strongly zoned rims. The core-zones commonly display albite or carlsbad twinning that persists through the surrounding sieved zones (Fig. 3a). The honeycomb texture characteristically comprises concentric arrays of glass tubules, and is confined to specific internal surfaces such as twin planes, oscillatory zones whose composition appears to have been more sodic than that in equilibrium with the melt, and zones overgrowing truncated zoning (evidence of earlier thermal resorption). The outer zones of the grains exhibit marked gradational extinction (Fig. 3b), but Nomarski differential interference contrast (NDIC) images of fluoroboric acid-etched polished surfaces reveal very fine oscillatory laminations, which are typically truncated by irregular resorption surfaces at the grain boundaries. A few honeycomb-textured crystals show a complete spectrum of these features. Plagioclase phenocrysts with zones of fine ‘dusty’ sieve texture are rare in the GR facies of the dyke. Microlitic plagioclase grains in this facies are, like the phenocrysts, rounded, and similarly widely exhibit honeycomb textures, evidence of temperature rise at a very late, near-surface stage of emplacement. It should be emphasized that, like most plagioclase microphenocrysts in a wide range of volcanic rocks (e.g. Pearce & Clark, 1989), those in the dyke exhibit fine-scale oscillatory zoning in NDIC images. Such zoning is truncated by the honeycomb tubules, and these textures do not represent skeletal products of rapid growth.

View larger version (79K): [in this window] [in a new window]   Fig. 3. Photomicrographs and corresponding An values obtained by electron microprobe traverses across a variety of plagioclase phenocryst morphotypes. (a) A representative plagioclase phenocryst from the glass-rich facies of the dyke showing a honeycomb sieve-textured margin (hct), a normally zoned central region (nz) and dusty core zone (dcz). Gl, glass; Bt, biotite. (b) A strongly sieved plagioclase phenocryst from the glass-rich facies of the dyke having a strongly zoned mantle. The compositional gaps evident in the traverse represent glass inclusion-bearing zones for which it was difficult to obtain a reasonable analysis because of the large diameter of the electron beam relative to the sieve-free zones within the plagioclase. Note the 10 An unit jump across the sieved zone between the core and the fresh, strongly zoned rim. The marked, progressive decrease in An values in the rim implies that the melt was not significantly enriched in calcium overall. (c) A large rounded plagioclase phenocryst from the glass-poor facies of the dyke exhibiting patchy extinction. Note the highly variable An values for patchy extinction zones. (d) A large rounded plagioclase phenocryst, from the glass-poor facies of the dyke, characterized by fine, dusty sieve texture. Gl, glass; Bt, biotite. (e) A subhedral, weakly rounded plagioclase phenocryst from the glass-poor facies of the dyke characterized by a dusty core zone having a distinct optical boundary with a clear plagioclase overgrowth. The higher An values in the dusty core vs the clear overgrowth should be noted.

 
Plagioclase phenocrysts in the GP facies of the dyke conform to three distinct morphotypes. Most abundant are subhedral grains that incorporate core zones with patchy extinction (Fig. 3c), and resorbed and weakly sieved margins. Carlsbad and albite twinning is largely obscured in the patchy textured cores, but well developed elsewhere. Glomerocrysts are abundant and comprise 3–5 grains, each with a patchy textured core zone. The second morphotype comprises subhedral and resorbed grains exhibiting extensive areas of fine, ‘dusty’ sieve texture (Fig. 3d), and the third, which may be xenocrystal, comprises subhedral, weakly resorbed grains that exhibit mineral inclusion-rich cores and oscillatorily zoned overgrowths (Fig. 3e). The matrix of the GP facies is characterized by numerous flow-aligned, albite-twinned, subhedral to euhedral plagioclase microphenocrysts that lack sieve texture but commonly exhibit minor resorption.

Plagioclase compositions (Table 2), including all phenocrysts (Figs 4a and b, 5a and b) and microphenocryst textural variants (Figs 4c and 5c) in both facies of the dyke, vary widely. Phenocrysts from the GP facies (An_20–45: Figs 4a and 5a) are, however, overall less variable in composition than those of the GR facies (An_20–67: Figs 4b and 5b). The most calcic plagioclase in the GR facies occurs at the inner boundaries of the oscillatorily zoned mantles surrounding areas of intense honeycomb sieve texture. These mantles commonly display, near the outer boundaries of the sieved zones, a strong increase in An (up to 12 mol %) relative to the cores (Fig. 3b). Such ‘calcic spikes’ are, however, succeeded outwards by abrupt and continuous decreases in An towards the rims of the grains. The most calcic analyses overlap in composition with a field defined by 60 phenocryst and microlitic plagioclases from uppermost Oligocene basaltic rocks of the region (MVP, Fig. 4, H. A. Sandeman, unpublished data, 1995). Patchy textured (Figs 3c and 5a) and fine dusty sieve-textured grains (Figs 3d and 5a) are typically more sodic, ranging from An₂₂ to An₄₄. Analysis of dusty sieve-textured grains also reveals high apparent K concentrations, attaining Or₃₁, but this probably reflects incorporation of potassic glass (Fig. 3d). Phenocrysts with apparently undisturbed normal oscillatory zoning range overall from An₂₀ to An₅₉ (Fig. 5a and b), but compositions more calcic than An₂₆ typically predominate. Many of these oscillatorily zoned grains (see Fig. 3e) exhibit a progressive rimward decrease in An content. Similarly simple normal zonation has been reported for plagioclase in comparable volcanic rocks, the Hawkins, Goobarragundra and Laidlaw Suites of the LFB, by Wyborn et al. (1981), who indeed interpreted the plagioclase, biotite and quartz phenocrysts of the Hawkins Suite as restitic phases. Clemens & Wall (1984), however, recorded plagioclase grains in the Violet Town volcanics, also in the LFB, with up to two internal sieved and Ca-rich zones indicative of partial resorption.

View this table: [in this window] [in a new window]   Table 2: Representative electron microprobe analyses of plagioclase phenocrysts and microphenocrysts from specimens of the Revancha dyke

 

View larger version (23K): [in this window] [in a new window]   Fig. 4. (a) Or–Ab–An ternary diagram showing the compositions of all analysed plagioclase phenocrysts from the glass-poor facies of the Revancha dyke. (b) Or–Ab–An ternary diagram showing the compositions of all analysed plagioclase phenocrysts from the glass-rich facies of the dyke. (c) An Or–Ab–An ternary diagram showing the compositions of all analysed plagioclase microphenocrysts from the dyke. The compositions of representative grains are given in Tables 2 and 3. (d) Or–Ab–An ternary diagram showing the compositions of all analysed sanidines from the dyke. , Ma8915b; •, MAC175; , MAC137.

 

View larger version (26K): [in this window] [in a new window]   Fig. 5. Histograms summarizing the compositional data (in mol % An) for different textural varieties of plagioclase grains from both facies of the dyke. (a) Plagioclase phenocrysts from the glass-poor facies of the dyke. (b) Plagioclase phenocrysts from the glass-rich facies of the dyke. (c) Microphenocrysts from both facies of the dyke.

 
Plagioclase microphenocrysts in the dyke exhibit wide compositional ranges (Figs 4c and 5c) and, remarkably, many are as calcic as the most Ca-rich zones in the phenocrysts. This is particularly evident in the GP facies, in which microphenocrysts have compositions of An_33–61, most being more calcic than the associated phenocrysts. Rounded and sieved, albite-twinned microphenocrysts in the GR facies exhibit a similar overall range from An₂₅ to An₆₁ (Figs 4c and 5c), but on average are less calcic than those in the GP facies. It is therefore apparent that both textural facies of the dyke record complex thermal histories, the late-crystallizing microphenocrysts revealing apparent temperature ranges similar to those over which the phenocrysts had formed. Moreover, the GP facies may have experienced an overall prograde evolution, its slightly more calcic microphenocrysts suffering less dissolution than those in the GR facies. The extreme variability in the composition of ‘contiguous’ microphenocrysts in both facies, however, is evidence for marked spatial variations in T.

View this table: [in this window] [in a new window]   Table 3: Representative electron microprobe analyses of sanidine from three specimens of the Revancha dyke

 
Cathodoluminescence (CL) images (not shown) of plagioclase phenocrysts reveal that the oscillatory zoning is associated with variations in the intensity of a greenish yellow CL response at a comparable scale. CL emission spectra for several grains (A. N. Mariano, personal communication, 1997) are in conformity with excitation by Fe²⁺ and/or Mn²⁺. No crystals exhibit the reddish brown response characteristic of Fe³⁺ excitation, a feature shown by the corroded outer zones of phenocrysts in several volcanic units of the Picotani Group that record the mixing of reduced granitic and more oxidized minette magmas (Sandeman & Clark, 1993; Sandeman et al., 1997a). Indeed, the majority of the Revancha plagioclase phenocrysts are cut by networks of thin veinlets exhibiting intense yellow CL response; microphenocrysts are similarly affected. Electron microprobe X-ray mapping of plagioclase phenocrysts reveals that the selvages of the microfractures are enriched in Fe rather than Mn. The crosscutting features, most of which are not visible in transmitted light, may contain films of glass. We propose that the feldspar grains experienced late-stage thermal shock with the concentration of Fe²⁺-enriched plagioclase within the resultant fractures, i.e. local Fe metasomatism, or redistribution, occurred under reduced conditions.

Sanidine
In the GP facies of the dyke, large (2·5 cm), euhedral phenocrysts of translucent sanidine comprise up to 20 modal % of the rock, resulting in a porphyritic texture that closely resembles that in the adjacent phaneritic monzogranite. These crystals enclose numerous subhedral inclusions of oscillatorily zoned plagioclase, biotite and quartz, none exhibiting features suggestive of a restitic origin. This, along with its restriction to the coarsely crystalline facies, suggests that sanidine was paragenetically late.

Representative sanidine analyses are given in Table 3 and the entire dataset is plotted in Fig. 4d. The composition ranges from Or₇₀Ab₃₀ to Or₇₅Ab₂₅, with 1.0 wt % An and up to 0·39 wt % BaO. Sanidine in the glass-poor facies of the Revancha dyke is very similar in composition to alkali feldspars in unmetasomatized, but holocrystalline, Lachlan-type PSGS granites in SE Peru (Kontak, 1985; H. A. Sandeman, unpublished data, 1995), and generally comparable with those in the S-type granites and volcanic rocks of the Kosciusko and Melbourne Basement terranes of the Lachlan Fold Belt (Wyborn et al., 1981; Elburg, 1996a).

Quartz
Quartz occurs only in the GP facies of the dyke, in which it attains 5 modal %. It forms large (7 mm), rounded and embayed bipyramidal phenocrysts that commonly contain inclusions of apatite, biotite and plagioclase. It displays arcuate, anastamosing fractures, typically decorated by small melt inclusions, implying that fracturing occurred before solidification of the melt. Such features could result from heating during adiabatic magma ascent, but their close association with mafic enclaves elsewhere in the Picotani Group supports an origin through thermal shock during commingling with mafic magmas (Kontak et al., 1986; Kontak & Clark, 1997). Because quartz occurs only in the GP, it is inferred to have crystallized at a relatively late stage.

Biotite
Biotite forms 8–14 modal % of the rock, occurring both as ragged, stubby grains up to 5 mm in size and as elongate prismatic microphenocrysts (0·5 mm). The former exhibit pale to dark brown pleochroism and contain abundant inclusions of apatite, monazite and zircon, and sparse inclusions of ilmenite. Many have rounded, ragged and/or embayed forms (Fig. 6a), and enclose numerous blebs of glass and quartz. Those occurring in GR domains are typically more strongly resorbed than those in the GP facies, implying that the former may represent a hotter melt that intruded the latter. Biotite microphenocrysts in both facies, however, exhibit similar forms and, unlike the large phenocrysts, lack sieve textures.

View larger version (160K): [in this window] [in a new window]   Fig. 6. Photomicrographs of various textural and mineralogical features of the Revancha dyke. (a) A phenocryst of biotite (Bt) enclosed in glass (Gl) showing rounded, ragged and resorbed grain margins. (b) A subhedral apatite microphenocryst (Ap) enclosed in glass (Gl), from a glass-rich facies of the dyke. The elongate biotite microphenocrysts (Bt) should be noted. (c) The only observed grain of orthopyroxene (Opx) surrounded by glass (Gl) with an adjacent phenocryst of cordierite (Cd). The elongate biotite microphenocrysts (Bt) should be noted. (d) Photomicrograph of a micro-inclusion comprising the assemblage garnet + plagioclase + biotite + cordierite + orthopyroxene + quartz. Bt, biotite; Pl, plagioclase; Gt, garnet. (e) A micro-inclusion comprising the assemblage plagioclase + biotite + spinel + fibrolitic sillimanite. Bt, biotite; Pl, plagioclase; Sp, spinel; Si, fibrolitic sillimanite.

 
Representative biotite analyses are reported in Table 4 and a more comprehensive dataset is plotted in Fig. 7, including both large, stubby sieve-textured grains and elongate prismatic microphenocrysts. Electron microprobe analysis was supplemented by titrametric determination of FeO on high-purity separates (250–354 µm) of micas from four specimens, permitting estimation of FeO:Fe₂O₃ ratios, at least on the multi-grain scale. The rocks were gently ground under inert liquids, and we consider it improbable that the micas were oxidized during either crushing or titration. The FeO content determined for a separate was assumed to apply to all microprobe analyses of phenocryst grains (>250 µm) in that specimen. Biotite phenocryst compositions are characterized overall by elevated TiO₂ (3·2–4·5 wt %) and high MgO (10·4–13·7 wt %). There is extensive compositional overlap for phenocrysts in the GP and GR facies, but the former exhibit overall higher Fe(Fe + Mg) ratios (Fig. 7) paralleling the contrast in plagioclase phenocryst compositions. Biotite microphenocrysts from the dyke are characterized, on average, by more variable, and typically higher, TiO₂ (3·17–5·79 wt %) and higher MgO (11·4–15·7 wt %) than the associated phenocrysts. Microphenocrysts in the GP facies exhibit a wider range of Fe(Fe + Mg) ratio than those in GR (Fig. 7), but there is probably no significant difference in the two populations. All microphenocrysts show wide variations in Al^IV, but this feature is particularly evident in the GR facies.

View this table: [in this window] [in a new window]   Table 4: Representative electron microprobe analyses of biotite phenocrysts and microphenocrysts from specimens of the Revancha dyke

 

View larger version (36K): [in this window] [in a new window]   Fig. 7. A plot after Clarke (1981) of AlIV vs Fe/(Fe + Mg) for biotites from the Revancha dyke. It should be noted that the biotites from the dyke are distinct from those of two-mica, Himalayan-type rocks from SE Peru (shaded field: Kontak, 1985; Yamamura, 1991), from biotite + cordierite + garnet-bearing S-type granitoids from the LFB of SE Australia (dotted field: Clemens & Wall, 1984) and from hornblende + biotite-bearing, I-type calc-alkaline granitoids (diagonally shaded field: Dodge & Moore, 1968; de Albuquerque, 1973). Also plotted are biotites from the commingled Minastira pluton of SE Peru (filled triangles: Kontak & Clark, 1997), the cordierite rhyolites of the Morococala Field of Bolivia (five-pointed stars: Morgan et al., 1998), biotite from micro-inclusion 1 () and biotite from micro-inclusion 3 (eight-pointed stars). The wide range in AlIV values for the microphenocrysts, in particular those from the glass-rich facies, and also for the commingled Minastira pluton (Kontak & Clark, 1997) should be noted.

 
Figure 7 highlights the elevated Mg contents of the Revancha biotites in comparison with those from a range of peraluminous (Lachlan- and Himalayan-type), and even I-type rocks. There is, in contrast, considerable overlap in the Al^IV contents. The Revancha phenocryst biotites overlap with the Al^IV and Mg-rich sector of the field delimited for micas from other cordierite–biotite monzogranitic or rhyodacitic rocks of the Picotani Group and Intrusive Suite (Kontak, 1985; Kontak et al., 1986; Kontak & Clark, 1997), particularly those from units known to have undergone commingling with mafic potassic magmas (Sandeman & Clark, 1993). Close similarities are apparent with the biotite from the commingled Minastira pluton (Kontak & Clark, 1997). The biotites predictably exhibit higher mg-numbers than those in the Himalayan-type, two-mica suites of the Cordillera de Carabaya and are significantly more magnesian than those in the cordierite rhyolites of the Morococala Field, Bolivia (Morgan et al., 1998). They are, moreover, more magnesian than biotites in the S-type plutonic and volcanic rocks of the LFB. Microphenocryst biotites exhibit highly variable Al^IV and even more magnesian compositions than the stubby, sieved phenocrysts, and represent exceptional compositions for biotite in a silicic rock.

Analysis of biotite phenocryst separates indicates that these are much richer in Fe³⁺ than would be expected from the reduced nature of the bulk-rock, as indicated by the absence of magnetite and the scarcity of ilmenite. The unweathered and unaltered condition of the dyke implies that little post-crystallization alteration has occurred, and the rocks do not contain xenocrysts, such as of Cr-rich phlogopite, derived from mixing with minette magmas, a common feature in other units of the Picotani Group (Sandeman & Clark, 1993). The analysed biotites are therefore interpreted as true phenocrysts. The bulk compositions plot adjacent to the hematite–magnetite buffer in the Fe²⁺–Fe³⁺–Mg²⁺ diagram of Wones & Eugster (1965). Apparent intra-sample variation is evident, but this is ascribed to the assignment of a single FeO content to the several microprobe analyses determined for each hand-specimen: thus, the variation in Fe₂O₃ is estimated by difference, and is probably an exaggeration of the real range. However, an inter-sample variation in the total iron (Fe_T) and Mg contents is evident, with a decrease in Mg from sample MAC-2 to sample MAC-175, respectively GR (74% glass) and GP (45%). That the biotites crystallized, or recrystallized, under variable oxidation conditions, ranging from the Ni–NiO to the Hem.–Mag. buffers, is shown by the variable Fe²⁺:Fe³⁺ ratios (Table 4), and it is inferred that those in the GR facies record the lower f_O2 conditions. The apparent trend in biotite composition with crystallization is comparable with that documented for the Ploumanac'h granitoids by Barrière & Cotton (1979), who attributed it to early fractionation of ilmenite with subsequent late- or post-magmatic oxidation of Fe²⁺. We therefore suggest that the magmatic f_O2 at the time of initial biotite crystallization is revealed by the least oxidized grains, i.e. it slightly exceeded the Ni–NiO buffer.

It should be noted that comparable, or even higher, Fe³⁺:Fe²⁺ ratios were recorded by Kontak (1985) for biotites from the highly reduced Macusani Formation rhyolites: the Fe³⁺ enrichment, equivalent to f_O2 values well above the Ni–NiO buffer, was tentatively ascribed to late-stage oxidation associated with volatile concentration before eruption. These chemical data were reported, but not discussed, by Pichavant et al. (1988b, table 6). It should also be noted that Kontak (1985) recorded a wide range of Fe²⁺:Fe³⁺ ratios in biotite in several holocrystalline cordierite–biotite monzogranites of the Picotani Intrusive Suite, ranging from the QFM (quartz–fayalite–magnetite) buffer to well above the Ni–NiO buffer, and inferred that the silicic magmas in several of the hypabyssal centres experienced significant late-magmatic oxidation.

View this table: [in this window] [in a new window]   Table 6: Representative electron microprobe analyses of apatite microphenocrysts in two specimens of the Revancha dyke (1–4) and analyses of ilmenite from five specimens of the Revancha dyke (5–10)

 
Cordierite
Cordierite, with an intense mauve colour, forms up to 4 modal % of the rock as euhedral, pseudo-hexagonal prisms, typically 4 mm in diameter but attaining 10 mm in GP specimens (Fig. 6c). The phenocrysts commonly exhibit sector-twinning and incipient alteration to pinite assemblages along grain boundaries and fractures, and contain rare sillimanite inclusions. Because of its abundance and large grain-size in the GP domains, and the absence of feldspar inclusions, cordierite is inferred to be paragenetically early (see Morgan et al., 1998). Cordierite phenocrysts are normally zoned, the cores and rims exhibiting mg-numbers [100 x molecular Mg/(Mg + Fe^T)] of 71 and 55, respectively, with a mean value of 60. Representative compositions are presented in Table 5. The cordierite compositions are comparable with those from other plutonic and volcanic settings (Deer et al., 1986), including the broadly similar PSGS granites of the Cornubian batholith (Stimac et al., 1995), the Violet Town, Hawkins and Goobarragandra Volcanics of the LFB (Wyborn et al., 1981; Clemens & Wall, 1984) and the PSGS volcanic rocks of the Morococala Field, Bolivia (Morgan et al., 1998). Cordierite from both SE Australian and SE Peruvian PSGS monzogranites contains slightly less SiO₂ than that reported by Montel et al. (1986), but is significantly lower in Al₂O₃ and higher in MgO and FeO than cordierite-like phases from the Macusani Formation (Pichavant et al., 1988b).

View this table: [in this window] [in a new window]   Table 5: Representative energy-dispersive, electron microprobe analyses of cordierite from three samples of the Revancha dyke

 
Accessory phases
Sillimanite occurs as sparse, isolated, prismatic microphenocrysts in the glass matrix, but more commonly forms acicular crystals included in sanidine, plagioclase and, rarely, biotite and cordierite. Ilmenite, the only Fe–Ti mineral, is rare, occurring both as small subhedral microphenocrysts in the glass matrix and, more widely, as inclusions in other phases, in particular biotite. Monazite and zircon occur as tiny dispersed prismatic grains but more commonly form inclusions in other phases. Apatite occurs as subhedral microphenocrysts (Fig. 6b) and as inclusions in biotite, and commonly hosts numerous micro-inclusions of sillimanite, monazite, zircon and ilmenite.

Representative electron microprobe analyses of selected accessory phases are presented in Table 6. Apatites in the Revancha dyke are fluorapatites, with compositions similar to those from the Cornubian batholith (Stimac et al., 1995); it should be noted that F is mobile under the analytical conditions employed, and the data should be considered semi-quantitative. Cathodoluminescence emission spectra of several Revancha apatites (A. N. Mariano, personal communication, 1997) record enrichment in heavy rare earth elements (HREE).

The composition of the ilmenite has been examined in detail. No evidence was found for intragrain zonation. The Fe³⁺/(Fe²⁺ + Fe³⁺) ratios range narrowly from 0·000 to 0·094 for all analysed grains (Table 6), and the data form a tight cluster near stoichiometric FeTiO₃, possibly with a restricted extension towards TiO₂. The elevated MgO contents (1·38–2·35 wt %), confirmed by both WDS and EDS techniques, are interpreted to record crystallization from a relatively primitive, high-T magma. Similar MgO contents, i.e. 3·13 wt %, have been determined by WDS analysis for ilmenites from the coeval San Rafael cordierite–biotite monzogranite (A. H. Clark, unpublished data, 1997). The close approach to stoichiometry implies strongly reducing conditions, a feature in apparent conflict with the variable compositions of the biotite phenocrysts. However, it should be noted that the comparably reduced Macusani Formation rhyolitic ash-flows contain ilmenite with a markedly more variable composition, embracing not only a wide range of Mn:Fe ratios, but also much more extensive solid solution of Fe₂O₃ and apparent alteration to ‘pseudorutile’ (Pichavant et al., 1988b). Both the Macusani and Morococala ilmenites are significantly less magnesian, with 0·35 and 0·41 ± 0·16 wt % MgO, respectively, than those from Revancha, a contrast paralleling that shown by the associated biotites. Moreover, the ilmenite from the Morococala quartz latite, a rock similar to the Revancha rhyodacite in SiO₂ content, is markedly less magnesian (0·67 ± 0·25% MgO: Morgan et al., 1998), although the Cr contents of these rocks are comparable.

Possible xenocryst material
A painstaking search of the 22 sections cut from the dyke revealed only four polycrystalline micro-inclusions that may represent restite, xenocryst bodies or, less probably, material incorporated from a cogenetic mafic melt. In addition, a single grain of orthopyroxene (Fig. 6c) was observed. The micro-inclusions comprise: (1) aggregates of 2 mm diameter garnet + plagioclase + biotite + cordierite + orthopyroxene + quartz (Fig. 6d); (2) 1 mm aggregates of plagioclase + biotite + spinel + fibrolitic sillimanite (Fig. 6e); (3) 1·5 mm plagioclase + biotite aggregates; (4) 2·5 mm aggregates of cordierite + fibrolitic sillimanite + spinel. Orthopyroxene, spinel and garnet have been observed only in the above settings, but spinel and garnet do not occur together. Plagioclase grains in three of the micro-inclusions are mainly intergrown in a granoblastic texture, show albite twinning and, except in the first-mentioned inclusion, do not exhibit sieve textures (see below). They are clearly distinct from both phenocryst and microphenocryst plagioclase grains in the rocks. Spinel grains are colourless to pale green, anhedral, and generally intimately intergrown with sheaves of fibrolitic sillimanite. Garnet is pale red to pink, and anhedral, with embayed outlines apparently resulting from corrosion. In the inclusions, cordierite forms large, anhedral to subhedral grains, intergrown with the replacing anhedral orthopyroxene; it is in part sector-zoned and commonly is weakly pinitized along grain margins and fractures.

The compositions of silicate minerals in these sparse aggregates, including the single orthopyroxene grain from the glass-rich facies of the dyke and the four micro-inclusions, are presented in Table 7. The orthopyroxene yielded compositions of En_59·8Fs_39·6Wo_0·6 to En_58·2Fs_41·2Wo_0·6, with no discernible zonation in BSE images. Plagioclase from inclusion 3 ranges from An₅₅ to An₇₁ (2% Or), and is thus significantly more calcic than the majority of the plagioclase phenocrysts and microphenocrysts, but comparable in composition with plagioclase from high-K mafic volcanic rocks of the region (H. A. Sandeman, unpublished data, 1995). However, plagioclase intergrown with garnet in inclusion 1 displays a composition, An_24–42, similar to that of the average plagioclase in the rock (An_20–45). Garnet from micro-inclusion 1 is almandine–pyrope (Alm_76–77Py_17–20) with minor grossular (Grs_2–4) and spessartine (Sps_0·5–4·3), and exhibits a slight enrichment in Mn from core to rim. Fibrolitic sillimanites from inclusions 2 and 4 approach Al₂SiO₅, with only 0·15–0·16 wt % FeO and 0·05 wt % of other cations. Spinels from inclusions 2 and 4 are hercynitic with up to 10·5 wt % ZnO (Table 7). Biotite intergrown with spinel in inclusion 2 is more Fe-rich (17·2–17·4 wt % FeO_T) than the microphenocrysts of the host dyke, but similar in composition to the phenocrysts therein. Biotite grains occurring adjacent to garnet in inclusion 1 have compositions comparable with those of the phenocryst and microphenocryst grains, whereas those coexisting with plagioclase in inclusion 3 exhibit Fe/(Fe + Mg) values similar to the biotite microphenocrysts, but with lower calculated Al^IV (Fig. 7). Cordierite in inclusions 1 and 4 is comparable in composition with the phenocrysts in the dyke.

View this table: [in this window] [in a new window]   Table 7: Representative electron microprobe analyses of silicate phases in micro-inclusions occurring in the Revancha dyke and for the isolated orthopyroxene grain

 
Groundmass glass
Averaged, replicate analyses of groundmass glass from several samples of the dyke are presented in Table 8. The electron microprobe analyses were obtained using ‘macusanite’ glass sample JV-1 (Pichavant et al., 1987) as a secondary standard, thereby obviating problems of Na migration (see Appendix), and permitting direct comparison with strongly peraluminous obsidians from the Macusani Formation. Macusanite has a composition comparable with rare-element pegmatites and has been interpreted as a product of extreme fractionation of a two-mica peraluminous magma (Pichavant et al., 1987, 1988a, 1988b). Glass from Revancha has much lower Al₂O₃ contents (12·5–13·2 vs 15·8–16·4 wt %), but significantly higher K₂O:Na₂O ratios (1·74–2·65 vs 0·75–0·86) than macusanite, whereas the remainder of the major element concentrations are comparable (Table 8). High K₂O:Na₂O ratios similar to those of the Revancha glass have been reported for finely crystalline matrix assemblages, inferred to represent devitrified glass, in the Ministira granite, another unit of the Picotani Intrusive Suite (Kontak & Clark, 1997). Groundmass glasses from the cordierite rhyolites of the Morococala Field are higher in Al₂O₃ and SiO₂, but are characterized by lower abundances of CaO, MgO and, in particular, lower K₂O/Na₂O ratios than the Revancha dyke.

View this table: [in this window] [in a new window]   Table 8: Electron microprobe analyses of groundmass glass for specimens of the Revancha dyke

 
Although the Revancha glasses probably represent residual rather than original melt compositions, they provide, to our knowledge, the first direct evidence for melt compositions in a relatively unfractionated S-type system and, as such, directly complement the data for the Morococala cordierite rhyolites (Morgan et al., 1998).

	WHOLE-ROCK GEOCHEMISTRY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
Major elements
The Revancha dyke is strongly peraluminous (sensu Sylvester, 1998), with whole-rock A/CNK ratios of 1·14–1·22 (Table 9) and normative corundum contents of 2·71–3·55 wt %. It is dacitic in terms of total alkalis vs SiO₂ (LeBas et al., 1986) or monzogranitic in the QAP (quartz–alkali feldspar–plagioclase) granitoid classification plot (LeBas & Streckeisen, 1991), and falls within the field defined by other Lachlan-type PSGS granitoids of SE Peru (Kontak, 1985; Kontak et al., 1986; Sandeman, 1995; Kontak & Clark, 1997). Herein, we retain the term rhyodacite, which appropriately distinguishes these intermediate rocks from the locally abundant rhyolites and less common dacites of the Crucero Supergroup (Sandeman et al., 1997a).

View this table: [in this window] [in a new window]   Table 9: Whole-rock geochemistry for seven samples of the Revancha dyke

 
Figure 8, a plot of normative corundum vs wt % SiO₂, compares the Al enrichment of the Revancha rhyodacite with that of other peraluminous rock suites. The dyke samples occupy a tight data cluster, lying within the field for other Peruvian cordierite + biotite monzogranites but having low normative corundum values in comparison with the S-type granitoids and volcanic rocks of the Kosciusko and Melbourne Basement terranes of the Lachlan Fold Belt with moderate SiO₂ contents (Hine et al., 1978; Wyborn et al., 1981; Clemens & Wall, 1984; Elburg, 1996a, 1996b; OZCHEM National Whole-rock Geochemical Database). Cordierite rhyolites of the Morococala Field of Bolivia have higher SiO₂ but comparable normative corundum contents. Also shown are fields for the Himalayan-type, muscovite + biotite ± andalusite ash-flow tuffs and obsidian (‘macusanite’) of the Macusani Formation, SE Peru (Noble et al., 1984; Pichavant et al., 1988b; Sandeman, 1995). It is apparent that Lachlan-type PSGS rocks are comparable in normative corundum content with the more silicic, Himalayan-type, two-mica rhyolitic or leucogranitic suites. Matrix and quartz-enclosed glasses from the Morococala cordierite rhyolites are somewhat higher (2·7–3·9 and 2·8–3·6 wt %, respectively) in normative corundum than those from the dyke (2·4–3·1 wt %) and similarly overlap in composition with Himalayan-type PLGS rocks. Although glasses from the Revancha dyke exhibit higher SiO₂ contents than the whole-rocks, their normative corundum contents and A/CNK ratios are, on average, slightly lower. If the whole-rock analyses are close to liquid compositions, this implies that melt peraluminosity decreased with fractionation, a feature considered by Barbarín (1996) to be characteristic of Lachlan-type PSGS suites. A similar relationship is exhibited by the Morococala cordierite rhyolites, whereas in the ‘Himalayan’ Macusani rhyolites, glass is markedly more aluminous than the corresponding whole-rocks.

View larger version (34K): [in this window] [in a new window]   Fig. 8. A plot of wt % SiO2 vs normative corundum content for whole-rock and groundmass glass from seven samples of the Revancha dyke. The whole-rock and corresponding glass analyses are joined by tie-lines demonstrating that the glasses are, in general, less peraluminous than the bulk rocks. Shown for comparison are fields for PSGS, Lachlan-type monzogranites of SE Peru (Kontak, 1985; Sandeman, 1995); S-type rocks of the Kosciusko and Melbourne Basement terranes of the LFB of SE Australia (Hine et al., 1978; Elburg, 1996a, 1996b; OZCHEM National whole-rock geochemical database); the two-mica ash-flow tuffs of the Macusani Formation; and macusanite obsidian glass (Noble et al., 1984; Pichavant et al., 1988b; Sandeman, 1995). Also shown are whole-rock analyses (five-pointed stars) and corresponding groundmass glasses (+) compositions for the cordierite rhyolites of the Morococala Field (Morgan et al., 1998) and for the Cerro del Hoyazo dacite (eight-pointed star: Zeck, 1970).

 
Figure 9a and b compare major element data for the dyke and its residual glass with various experimental peraluminous glasses as well as with selected rock suites. The glasses synthesized by Patiño-Douce & Johnston (1991) are generally depleted in CaO relative to the Revancha rhyodacite, presumably reflecting the pelitic source composition used in their experiments, whereas those generated in the experiments of Green (1976) and Vielzeuf & Holloway (1988), using metagreywackes as starting materials, are more comparable with those of the Revancha dyke. The cordierite rhyolites of the Morococala Field and their glasses have somewhat more aluminous compositions (Morgan et al., 1998), whereas the Cerro del Hoyazo dacitic lava is strongly enriched in Fe and Mg relative to the Revancha dyke (Zeck, 1992).

View larger version (19K): [in this window] [in a new window]   Fig. 9. (a) A' (Al2O3 – Na2O – K2O – CaO) – K (K2O) – F (FeOT + MgO) plot (molar) for whole-rock and groundmass glass analyses for the Revancha dyke. Also shown are the fields for the experimental products of Patiño-Douce & Johnston (1991: PD & J), Vielzeuf & Holloway (1988: V & H) Green (1976), as well as fields for Lachlan S-type rocks of the Kosciusko and Melbourne Basement terranes (Hine et al., 1978; Elburg, 1996a, 1996b; OZCHEM National Whole-rock Geochemical Database), SE Peruvian PSGS granitoids (Kontak, 1985; Sandeman, 1995), two-mica ash-flow tuffs of the Macusani Formation and Macusanite obsidian glass (Noble et al., 1984; Pichavant et al., 1988b; Sandeman, 1995). Data for cordierite rhyolites of the Morococala Field are from Morgan et al. (1998) and the single analysis of the Cerro del Hoyazo cordierite dacitic lava (C.H.) is from Zeck (1970). (b) A (Al2O3-Na2O-K2O)-C (CaO) F (FeOT+ MgO) plot (molar) for whole-rock and groundmass glass analyses for glassy rhyodacite. [See (a) for further details.] Symbols as in Fig. 8. Opx, orthopyroxene; Bt, biotite; Pl, plagioclase; Cd, cordierite; K-spar, potassium feldspar; Si, sillimanite and andalusite; Ga, garnet; Mu, muscovite; Osm, osumilite.

 
The dyke is similar in major element composition to other, largely holocrystalline, cordierite–biotite monzogranites of SE Peru (Kontak, 1985; Kontak et al., 1986, 1987; Sandeman, 1995), overlapping with the more SiO₂-poor examples of the suite. Like the other monzogranites of SE Peru, the dyke differs in composition from the S-type rocks of the Kosciusko and Melbourne Basement terranes of the Lachlan Fold Belt (Hine et al., 1978; Wyborn et al., 1981; Clemens & Wall, 1984; Elburg, 1996a, 1996b; OZCHEM National Whole-rock Geochemical Database), being generally enriched in K₂O, Na₂O, Al₂O₃ and P₂O₅, but depleted in MgO, CaO, Fe₂O₃^T and TiO₂ [Fig. 10; also shown for comparison are the average compositions of the S- and I-type granitoids of the LFB White & Chappell, (1983)]. Cordierite rhyolites from the Morococala Field of Bolivia have higher SiO₂, significantly lower MgO, slightly lower CaO, Fe₂O₃^T and TiO₂, and comparable contents of Al₂O₃, Na₂O, K₂O and P₂O₅.

View larger version (55K): [in this window] [in a new window]   Fig. 10. Selected major and trace element Harker variation diagrams for seven specimens of the Revancha dyke () and their groundmass glass (•) compared with analyses of PSGS monzogranites from southeastern Peru ( Kontak, 1985; Sandeman, 1995). Shown for comparison is a field for Lachlan S-type rocks of the Kosciusko and Melbourne Basement terranes (cross-hatched field), as well as the average compositions of S-type () and I-type () granites from that area (Hine et al., 1978; Elburg, 1996a, 1996b; White & Chappell, 1983; OZCHEM National Whole-rock Geochemical Database). Also shown are whole-rock (+) and groundmass glass (five-pointed stars) analyses for the cordierite rhyolites of the Morococala Field (Morgan et al., 1998) and for the single analysis of the Cerro del Hoyazo dacite, Spain (12 pointed star: Zeck, 1970).

 
Trace elements
The Revancha rhyodacite is similar in its trace element contents to coeval cordierite–biotite monzogranites of SE Peru (Kontak, 1985; Kontak et al., 1986; Kontak & Clark, 1997; Sandeman, 1995), all being enriched in U, Rb, F and Pb, but depleted in V, Co, Zr, Y and Yb relative to the Australian rocks of similar mineralogy (Hine et al., 1978; Wyborn et al., 1981; Clemens & Wall, 1984; Elburg, 1996a, 1996b; OZCHEM National Whole-rock Geochemical Database: Fig. 10). In comparison with the cordierite rhyolites of Bolivia, the Revancha dyke has much higher Cr and Th, mildly higher Ba and Zr, comparable Rb and U, and lower Sr abundances. However, it is comparable in composition with the quartz latites documented by Morgan et al. (1998).

Rare earth elements
Chondrite-normalized (Sun & McDonough, 1989) rare-earth element (REE) contents of seven specimens of the dyke are shown in Fig. 11. All are enriched in the light-REE (LREE; La_N/Lu_N = 19·4–27·7) and exhibit minor negative Eu anomalies (Eu/Eu^* 0·52–0·62: Eu^* after Taylor & McLennan, 1985). The compositions fall within a field defined by 14 samples of monzogranite from the region (Kontak, 1985; Sandeman, 1995), although the dyke corresponds to the most REE-enriched holocrystalline granites and also exhibits a generally less pronounced negative Eu anomaly (Fig. 11). The cordierite rhyolites of Bolivia have REE abundances and patterns very similar to those of the Revancha rocks. In comparison, selected S-type rocks of the Kosciusko and Melbourne Basement terranes of the LFB (Hine et al., 1978; Elburg, 1996a; OZCHEM National Whole-rock Geochemical Database), with LREE abundances very similar to the Revancha dyke, exhibit higher HREE abundances, possibly recording the presence of an HREE-bearing phase, probably garnet, in the source of the Revancha magmas.

View larger version (39K): [in this window] [in a new window]   Fig. 11. Chondrite normalized (Sun & McDonough, 1989) rare earth element patterns for the Revancha dyke compared with a field for 14 PSGS monzogranites of SE Peru (diagonally shaded field: Kontak, 1985; Sandeman, 1995) and a field defined by three specimens of the Violet Town Volcanics, a series of S-type ash-flow tuffs exposed in the Melbourne Basement terrane of SE Australia (dashed field: Elburg, 1996a). Also shown is a field for five granitoids of the PSGS, Young Batholith (Kosciusko Terrane), SE Australia (dash–dot field: OZCHEM National Whole-rock Geochemical Database).

 

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
Implications of field and geochronological data
The Revancha Dyke is a 24·1 ± 0·2 Ma body of glass-rich rhyodacite that intruded pyroclastic and porphyritic monzogranitic rocks of similar age and mineralogy, together making up part of the 20–26 Ma Picotani Group of the Crucero Supergroup in the Cordillera Oriental of SE Peru (Sandeman, 1995; Sandeman et al., 1997a). The dyke exhibits two distinct facies, glass rich (74 vol. % glass) and glass poor (51%), and textural evidence implies that the former intruded the latter. The occurrence of high-K mafic rocks of the same age as the Revancha dyke and the other S-type rocks of the region, in conjunction with widespread evidence for felsic–mafic magma commingling and mixing, implies that mantle-derived magmas were both heat sources for the production of peraluminous magmas in southeastern Peru, and mafic end-members involved in magmatic commingling. However, unambiguous macroscopic evidence of commingling with mafic magmas is not observed in the Revancha dyke itself.

Implications of mineral textures and composition
The preserved phenocryst mineral assemblage, plagioclase + biotite + cordierite ± quartz ± sanidine, and the corresponding microphenocryst assemblage, ilmenite (minor) + monazite + apatite + zircon ± sillimanite, are compatible with an origin through dehydration melting of a semipelitic, reduced crustal protolith (Clarke, 1981; Vielzeuf & Montel, 1994; Patiño-Douce & Beard, 1995). The compositions of sanidine, cordierite and ilmenite are comparable with those documented in Lachlan-type PSGS suites worldwide. However, the textural features and compositional variations of plagioclase and biotite in both facies of the dyke are complex, and provide evidence for magmatic resorption of these phenocrysts, which is inferred to have accompanied major temperature rises in the melt, probably during interaction of the PSGS magma with a mafic melt.

Abundant sieve textures in phenocryst plagioclase in both glass-rich and glass-poor facies of the dyke, and in microphenocryst plagioclase in the glass-rich facies, as well as the resorption of large, stubby biotite phenocrysts in both facies, are evidence for thermal disturbance of the magma. The predominance of honeycomb-textured plagioclase in the glass-rich facies, characterized by strongly zoned, unsieved overgrowths that, in turn, exhibit marginal resorption, indicates that the magma experienced at least two distinct thermal perturbations during crystallization. Honeycomb-textured plagioclase grains incorporate calcic spikes (12 An %) on the inner margins of the sieved zones, but the mantles of these grains exhibit a strong outward depletion in calcium, a relationship implying that the enclosing melt as a whole did not experience enrichment in CaO. Comparable honeycomb textures were not observed in the glass-poor facies, but a prograde early thermal history may be recorded by the mottled, patchy extinction in the cores of many of the large plagioclase grains (Fig. 3c). Such zones have commonly been interpreted as representing residual, restitic plagioclase, but these mottled zones are characterized by highly variable compositions, with calcic zones immediately adjacent to sodic ‘patches’, a feature not readily attributable to crystallization during either magmatic cooling, or residuum development at the site of anatexis. The Ca-rich patches may instead represent regions of calcic plagioclase originally surrounded by sodic glass resulting from partial melting of plagioclase crystals in a magma. This is in accord with the observation of Tsuchiyama & Takahashi (1983) that residual, resorbed plagioclase becomes increasingly more calcic with rising temperature. We therefore tentatively interpret the unusual extinction patterns as recording early honeycomb sieve-textured domains that have partially annealed, and infer that, like the glass-rich, the glass-poor facies experienced an early thermal perturbation.

Plagioclase microphenocrysts in both facies of the dyke not only exhibit large ranges in composition but are on average more calcic than most phenocrysts. The microphenocrysts of the glass-poor facies, in particular, are significantly more calcic than the associated phenocrysts, mimicking the compositional variations of the calcic mantles on honeycomb sieve-textured phenocrysts in the glass-rich facies. These relationships imply that the melt from which the microphenocrysts crystallized was hotter and possibly more calcium-rich than that from which the bulk of the phenocrysts precipitated at an earlier stage. Indeed, the latest textural feature preserved in the plagioclase phenocrysts in all specimens is a corrosion of their outer surfaces, implying that the melt was exposed to thermal perturbation before quenching.

The textural–compositional relationships of the plagioclase are parallelled by those of biotite. Thus, biotite phenocrysts in the glass-rich, and to a lesser extent the glass-poor facies of the dyke, exhibit ragged margins and commonly host numerous glass inclusions. Similar textural features have been described by Brearley (1987) for biotite undergoing dehydration breakdown in experimental charges. Thus, following phenocryst growth, the magma is inferred to have experienced a thermal pulse, probably at 850–900°C. Biotite microphenocrysts in both facies, however, exhibit generally euhedral, elongate prismatic forms and are enriched in MgO and TiO₂ relative to the phenocrysts. These microphenocrysts exhibit large ranges in calculated Al^IV contents, a feature not readily explained through crystallization from a homogeneous silicic PSGS melt. We suggest instead that the microphenocrysts crystallized from a heterogeneous, hybrid melt fraction, or fractions, characterized by variable Al₂O₃. In conjunction with the documented low Al₂O₃ and high K₂O contents of the Revancha groundmass glass (Table 7), this model offers an explanation for the wide range in plagioclase and biotite microphenocryst compositions and the variable Al^IV contents of the latter. The combined implication of these observations is that the PSGS magma represented by both facies of the dyke underwent late-stage mixing with hotter, more mafic, potassic to ultrapotassic magmas.

Physical evolution of the Revancha magmas
The evolution of the Revancha dyke may be deduced from the textures and corresponding compositional variations in the plagioclase and biotite, supplemented by the textural and compositional relationships preserved in the sparse polycrystalline grain aggregates (Table 10). In particular, the plagioclase and biotite microphenocrysts and in situ matrix glass provide an unusually comprehensive record of crystallization history, evidence lacking in pyroclastic suites such as the Morococala cordierite rhyolites and the Macusani Formation. We propose that the preserved textures reflect processes in a large-scale magma chamber prior to hypabyssal intrusion and quenching of the magmas. The rare, calcic, mineral inclusion-rich, plagioclase core zones (e.g., Fig. 3e) plausibly represent either residual plagioclase or grains incorporated from a more mafic melt. Similarly, the rare micro-inclusions comprising distinctive mineral assemblages probably incorporate either accidental xenoliths, restitic material entrained from the source (inclusion 1: garnet + plagioclase + biotite + cordierite + orthopyroxene; and inclusion 2: plagioclase + biotite + spinel + fibrolite; and inclusion 4: cordierite + fibrolitic sillimanite + spinel), or admixed magmatic assemblages (inclusion 3: biotite + calcic plagioclase).

View this table: [in this window] [in a new window]   Table 10: Magma generation and crystallization history for the Revancha dyke based on mineral textural and compositional variations and on whole-rock geochemistry

 
The earliest-formed features suggestive of thermal disequilibrium are the zones of ‘annealed’ mottled extinction in the cores of some large plagioclase grains in the glass-poor facies of the dyke, and the coarse honeycomb sieve-textured plagioclase grains in the glass-rich facies. Although it is possible that the glass-poor facies experienced an early thermal perturbation that did not affect the glass-rich facies, this model does not account for the thermal and compositional structure of the magma chamber or the specific locations therein of the magmas represented by the distinct dyke facies. We infer, therefore, that the glass-rich facies represents a volume of hotter, less crystallized melt that was situated adjacent to an underplating heat source, whereas the glass-poor facies formed from a somewhat cooler, more fractionated melt, at a greater distance from the latter. In this model, both magmas experienced the same early thermal event, but the glass-rich facies remained longer at high temperature, so that its biotite phenocrysts underwent more pronounced resorption, producing ragged, sieved grain morphologies.

This thermal and, probably, chemical perturbation was followed by an interval of crystallization in the two associated melts, characterized in the glass-rich facies by a calcic spike at the outset of the growth of mantles on the plagioclase phenocrysts, followed by pronounced oscillatory zoning and outward depletion of calcium in the new plagioclase. No such compositional gradients are preserved, however, in the glass-poor facies. This crystallization interval was, we infer, accompanied by a more pronounced cooling in the more distal glass-poor assemblage, resulting in more extensive crystallization of the latter, entering the liquidus field for the assemblage cordierite + biotite + plagioclase + sanidine + quartz. This caused the progressive annealing of plagioclase sieve-textures generated during the earlier underplating event. The lack of overgrowths on the biotite phenocrysts, however, suggests that this mineral did not regain thermal and compositional equilibrium with the surrounding melt, possibly because of the slow rates of diffusion of the ferromagnesian elements relative to the alkalis in granitic magmas (Watson & Jurewicz, 1984). However, new, microphenocryst biotite began to crystallize during this interval, and its composition suggests that, overall, the melt probably experienced a bulk increase in MgO and TiO₂. The large variation in Al^IV is difficult to interpret, but it may reflect biotite microphenocryst crystallization from a hybrid melt, or melts, having variable Al₂O₃ contents (see above). This interval of crystallization and annealing was followed by a second, less intense thermal event prior to magma emplacement and quenching, evidenced by rounding and sieving of the margins of both plagioclase phenocrysts and microphenocrysts in the glass-rich facies and of phenocrysts in the glass-poor facies of the dyke, probably through further underplating by hot mafic(?) melts. These repeated thermal perturbations suggest that the magmas were stored in an episodically replenished magma chamber.

The PSGS magmas represented by the two facies of the dyke are inferred to have formed the thermally layered carapace of a large, presumably bimodal magma chamber. The base of this chamber was periodically invaded by significantly hotter, mantle-derived mafic magmas. The most likely candidates for the latter would be the contemporaneous absarokitic lavas and/or the ultrapotassic dykes and flows exposed locally in the region (Kontak et al., 1986; Sandeman & Clark, 1993; Carlier et al., 1997). The crystal-laden, glass-poor melt is inferred to have overlain the crystal-poor, glass-rich melt, and therefore to have been partially insulated from the thermal effects of the underplating mafic melt. The glass-rich melt intruded and commingled with the glass-poor and both were together rapidly injected into the overlying country rocks. Mechanical introduction of the mafic magma into the silicic roof of the magma chamber is inferred to have occurred through syn-plutonic dyking. This readily explains the documented complex textural and compositional features of the magmas represented by the dyke. Significantly, Sandeman & Clark (1993) reported minette enclaves with scalloped boundaries in a cordierite + biotite monzogranite stock exposed 5 km west of the Revancha dyke, and similar features are shown by the San Rafael stock (A. H. Clark, unpublished data, 1997), compelling evidence in favour of the syn-plutonic dyking hypothesis.

Physico-chemical conditions of magma genesis and fractionation
We have attempted to determine the P–T conditions in the anatectic source region of the Revancha magmas from the mineral phases preserved in the micro-inclusions. Unfortunately, equilibrium relationships are disguised because of the extensive resorption of the restitic minerals, a feature that is interpreted to have resulted from rapid anatexis in the source. Estimates of P–T conditions calculated with the TWQ computer program (Berman, 1991, 1992), using the version 2.01 database (Berman & Aranovich, 1996; Aranovich & Berman, 1997) yielded P–T conditions of 755°C and 440 MPa. Because the inclusion is resorbed, we interpret these P–T conditions as minima for the site of generation of the PSGS magma, and they probably represent low-P, low-T re-equilibration of the magma at a late stage. Zircon saturation temperatures determined from the calibration of Watson & Harrison (1983) and P–T estimates from the two-feldspar thermometric calibration of Fuhrman & Lindsley (1988) yielded temperatures of 755°C and 694°C (P = 400 MPa), respectively. These temperatures and pressure determinations are in general agreement with 755°C minimum temperature and 440 MPa pressure obtained for the garnet-bearing inclusion, implying complete re-equilibration of the inclusion mineral phases during late cooling and crystallization of the magma.

Implications of whole-rock geochemistry
The major and trace element compositions of the Revancha dyke are broadly in accord with partial melting of a reduced, immature, metasedimentary source. Thick sequences of Lower Palaeozoic slate and greywacke underlie much of the axis of the Cordillera de Carabaya (Fig. 1) and the upper slopes of the Sub-Andean ranges, and structural and geophysical data imply that these rocks extend at depth beneath the study area. Although geochemical and isotopic data for these rocks are lacking, a source of this nature is supported by the general similarity of the major element composition of the Revancha dyke to that of other S-type suites and, moreover, to glasses generated through experimental melting of feldspathic, metasedimentary and meta-igneous assemblages (Le Breton & Thompson, 1988; Vielzeuf & Holloway, 1988; Holtz & Johannes, 1991; Patiño-Douce & Johnston, 1991). These conclusions are supported by the REE abundances and chondrite-normalized REE patterns, which are comparable with those of other S-type monzogranites from SE Peru and suggest a role for an HREE-bearing phase in their source (Hanson, 1978; Morgan et al., 1998). Differences in the HREE contents of the Revancha rocks and those from the Kosciusko and Melbourne Basement terranes of the Lachlan Fold Belt of SE Australia are probably a function of the relative roles of a HREE-bearing phase during melt generation. The occurrence of garnet in one micro-inclusion (see above), suggests that it was present in the source and responsible for generating the strongly fractionated REE patterns of the Revancha dyke. The variable, but generally minor, negative Eu anomalies imply the presence of plagioclase and/or K-feldspar in the source, or, alternatively, their fractional crystallization from the melt.

The whole-rock data, reflecting a bulk composition generally compatible with an origin of the Revancha magmas as anatectic PSGS melts, imply that if magma mixing occurred, then it must have involved a large volume of PSGS silicic crustal melt and only minor mafic melt at the present exposure interval. This model similarly supports the important role of mechanical interaction via syn-plutonic dyking of mafic magma.

Fingerprinting the cogenetic mafic end-member
Textural and compositional data for both plagioclase and biotite and compositional data for groundmass glass indicate that the Revancha magma underwent a temperature rise on at least two occasions, and that these events were accompanied by probably minor increases in bulk-magmatic CaO, MgO, TiO₂ and, possibly, K₂O. This suggests that the thermal perturbation generating the sieve textures did not involve mixing with a significantly more calcic mafic melt. The coeval ultrapotassic dykes and flows exposed in the region indeed have CaO (3·75 wt %), MgO (3·15–9.26 wt %), TiO₂ (0·54–1·56 wt %) and K₂O (4·1–8·0 wt %) contents (Sandeman & Clark, 1993; Carlier et al., 1997) overlapping with or somewhat exceeding those of the granitic S-type rocks of the region (typically 2·6 wt % CaO, 2·32 wt % MgO, 0·63 wt % TiO₂ and 7·96 wt % K₂O). Hence, although a melt generated through mixing/hybridization of rhyodacitic and mafic-to-ultrapotassic mafic liquids might not be significantly more calcic than an S-type felsic melt, its solidus would exceed that of the latter and, moreover, the ultrapotassic mafic melt would contain significantly higher MgO, FeO^T, TiO₂ and possibly K₂O than the silicic melt (H. A. Sandeman, unpublished data; Carlier et al., 1997). On the basis of a comparison of the whole-rock compositions of the Picotani Group ultrapotassic dykes and flows with those of the calc-alkaline to shoshonitic basaltic suites of the region, we suggest that the Revancha magma experienced temperature rise resulting from magmatic underplating and mixing with coeval ultrapotassic mafic magma.

Comparable glassy cordierite–biotite-phyric rocks
The cordierite–biotite dacite from Cerro del Hoyazo, SE Spain, described by Zeck (1968, 1970, 1992) displays many mineralogical and compositional similarities to the Revancha rhyodacite. This Upper Miocene lava comprises 50% glass, with 10% ‘simply zoned’ plagioclase, 9% cordierite and 8% biotite as phenocrysts. Major differences between the occurrences, however, are the abundant inclusions in the Spanish dacite of mafic igneous rocks, including both ‘cognate’ and commingled bodies, and particularly, aluminous restite associations, the latter amounting to 10–15% of the rock. ‘Basaltoid’ enclaves host numerous restitic grains, a feature interpreted as evidence for basalt–dacite melt mixing before commingling. Zeck (1992) proposed that several episodes of dacitic–basaltic commingling were probably involved. Abundant, dispersed plagioclase crystals exhibit honeycomb-textured, patchy core zones with residual compositions as calcic as An₉₀. These are texturally akin to some of the phenocrysts in the glass-rich facies of the Revancha rhyodacite, but the calcic compositions and abundance of (meta-) gabbroic enclaves favour (Zeck, 1992) an origin through disaggregation of such bodies. This is a possible model for the Revancha rocks, but it should be noted that even strictly cognate plagioclase phenocrysts at Cerro del Hoyazo have core compositions of An₈₅ (compare with An₆₅ at Revancha). The ‘hotter’ mineralogy and markedly greater abundance of both restitic and basic-magmatic enclaves exhibited by the Spanish rocks imply a magma much closer to the site of anatexis than at Revancha, as is exemplified by the earlier description by Zeck (1970) of the Cerro del Hoyazo dacite as an ‘erupted migmatite’. However, it is possible that underplating and commingling with large volumes of basaltic melt both heated and reduced the effective viscosity of a restite-charged dacitic melt, permitting it to rise to the surface.

The Upper Miocene Morococala volcanic field (Ericksen et al., 1990; Luedke et al., 1990; Morgan et al., 1998) constitutes a more southerly segment of the Central Andean Inner Arc and ‘Tin Belt’, which host the Revancha rhyodacite (Clark et al., 1984, 1990), and overall exhibits a range of peraluminous rock-types comparable with those of the Crucero Supergroup (Sandeman et al., 1997a). The closest analogue of the Revancha rocks is a cordierite- and biotite-phyric tuffaceous rhyolite (whole-rock: 70·0–71·2 wt % SiO₂). This is very similar in phenocryst/microphenocryst mineralogy to the Revancha rhyodacite, although cordierite was considered by Morgan et al. (1998) to have crystallized later than sanidine and quartz on the basis of sparse mutual inclusions. The Morococala and Revancha plagioclase phenocrysts exhibit comparable ranges in An %, but the former do not attain the most calcic compositions displayed by the latter, as would be expected from the higher SiO₂ content of the Bolivian rocks. More critically, the complex compositional patterns shown by the Revancha plagioclases are not matched at Morococala, where zoning is normal overall, and no ‘calcic spikes’ are described, although resorption surfaces are locally exhibited by plagioclase in the vitric quartz latite flows (Morgan et al., 1998). Biotite phenocrysts are markedly less magnesian in the Morococala rhyolites than at Revancha, not exceeding 7·43 wt % MgO, whereas biotite in the younger quartz latite (68·7–69·5 wt % MgO) overlaps in composition with the latter. The absence of microphenocrysts of plagioclase and biotite in the pyroclastic Bolivian rhyolites precludes comparison with the late-stage Ca- and Mg-rich mineral assemblages at Revancha. Similarly, the lack of Fe²⁺ analyses for the Morococala biotites prevents recognition of any late-magmatic oxidation. Ilmenite in the Morococala rhyolites is, like that at Revancha, close to stoichiometric MTiO₃, implying reduced conditions, but is markedly depleted in MgO (0·41 ± 0·16 vs 2·35 wt %) in comparison.

The Morococala rocks have mineralogical and compositional features closely akin to those of the Revancha rhyodacite, and in permissive agreement with an origin through anatexis of a similar reduced aluminous protolith, probably at slightly lower temperatures. Morgan et al. (1998) estimated magmatic conditions of 730–750°C and 350–450 MPa for the cordierite rhyolite, but noted the weak convergence of two-feldspar and zircon saturation temperatures. However, and notwithstanding the overall temporal trend in the Morococala Meseta volcanic succession to more primitive compositions, there is apparently no evidence for either commingling or mixing of the cordierite rhyolite magma with hotter melts. Morgan et al. considered the possibility that the andalusite rhyolite, cordierite rhyolite and quartz latite were successively derived from a single, zoned, magma chamber, but argued that the widely separated eruption centres and protracted (2 Myr) history of volcanism make this unlikely.

Geodynamic setting of Lachlan-type PSGS magmatism, SE Peru
The geological relationships of the Revancha dyke and other PSGS members of the Picotani Intrusive Suite are established by integrated lithostratigraphic and geochronological studies (Kontak, 1985; Sandeman, 1995; Sandeman et al., 1996, 1997a) in the Cordillera and Precordillera de Carabaya. Eruption of high-K calc-alkaline basalts and basaltic andesites, absarokites and shoshonites, and minettes extended from 26 Ma to at least 22·3 ± 1·4 Ma. This mafic volcanism was strictly contemporaneous with the eruption of cordierite- and biotite-phyric dacites, rhyodacites and rhyolites, including lava flows, agglutinized flows and ash-flows. Several units unambiguously record both commingling and mixing of felsic and mafic magmas, the latter including absarokites (Kontak et al., 1986; Kontak & Clark, 2000) and minettes (Sandeman & Clark, 1993; Sandeman, 1995; Sandeman et al., 1997a). Hypabyssal cordierite–biotite monzogranitic/rhyodacitic stocks, including the Revancha dyke, clearly cogenetic with the Picotani Group volcanic rocks and similarly displaying intimate relationships with a varied mafic magma suite, were emplaced over the interval 24·57 ± 0·21 to 23·15 ± 0·22 Ma (⁴⁰Ar–³⁹Ar incremental-heating age plateaux: Sandeman et al., 1997a).

In this transect of the Inner Arc, muscovite-phyric volcanic and intrusive rocks, constituting the Quenamari Group, were not emplaced until 17·04 ± 0·20 Ma, thereafter continuing to 7 ± 1 Ma (Cheilletz et al., 1992; Sandeman et al., 1997a) and culminating with eruption of the Macusani Formation and several mineralogically and compositionally correlative hypabyssal units (Farrar et al., 1990; Yamamura, 1991).

The uppermost Oligocene to Upper Miocene Crucero Supergroup thus exhibits the supercedence of Lachlan-type PSGS magmatism, constituting the Picotani Group, by Himalayan-type PLGS magmatism, with a clear temporal separation. The intervening 6–7 Myr apparent hiatus in volcanism and intrusion coincided with a period of uplift of the Cordillera de Carabaya, recorded by sparse apatite fission-track ages for Upper Triassic granitoid rocks of 17·5 ± 5·9 to 19·9 ± 6·7 Ma (Kontak et al., 1990b), and corresponding broadly to the Quechua I ‘orogenic pulse’ of Benavides-Cáceres [1999; see Sandeman et al. (1995) for a review of the Neogene tectonic history of this Andean transect]. The uplift, which affected the entire Cordillera Oriental, Altiplano and Cordillera Occidental at this latitude (e.g. Tosdal et al., 1984), was a response to crustal thickening. Although mantle-derived melts were probably involved in the generation of the two-mica, Himalayan-type PLGS rocks such as the Macusani Formation (Kontak, 1985; Pichavant et al., 1988a), the observed volumes of mafic rocks of this age are minimal, and evidence of commingling and mixing with mantle-derived magmas is vestigial.

The geotectonic context of the Picotani Group is problematic. Thus, Sandeman et al. (1995) ascribed the late Oligocene Early Miocene mafic magmatism in the Cordillera Oriental to the foundering of a slab segment detached from a flat slab which is inferred to have underlain southern Peru since 52 Ma (Clark et al., 1990; Clark, 1993). This triggered potassic basaltic and lamprophyric melt generation in the overlying asthenosphere and lithosphere, respectively, with resulting extreme heat flow and high-T–low-P anatexis in the crust. In contrast, James & Sacks (1999) proposed that the Picotani Group magmatism was caused by asthenospheric influx above a retreating slab in the course of a transition from flat to ‘normal’ subduction, a model which, in our opinion, does not account for the documented spatial and temporal distribution of igneous rocks in the transect. Although physiographic uplift in this transect was initiated in the late Oligocene, during the Aymará event (Tosdal et al., 1984; Sandeman et al., 1995), or Incaic IV orogenic pulse (Benavides-Cáceres, 1999), it is unlikely that the Inner Arc was underlain by crust exceeding 35–40 km at this time. Such tectonomagmatic conditions would be ideal for the generation of Lachlan-type, true S-type, PSGS magmas in the models of Sylvester (1998), although the ‘post-collisional’ setting envisaged by that author would be inappropriate in this Andean context. Similarly, the supplanting of Lachlan- by Himalayan peraluminous magmatism as the Inner Arc crust thickened during mid-Miocene orogenesis would be a predictable outcome.

The tectonic environment of the Morococala meseta and its broadly Lachlan-type cordierite rhyolite differs significantly from that which we document in SE Peru. By the Late Miocene, the crust underlying the Bolivian Cordillera Oriental probably already exceeded 60 km in thickness (Beck et al., 1996). Moreover, the cordierite rhyolite (6·8 Ma) overlies a succession of andalusite rhyolite pyroclastics (8·4 Ma) of clear Himalayan type, corresponding in all salient aspects to the andalusite- and sillimanite-bearing two-mica rhyolites of the Middle to Upper Miocene Quenamari Group.

	CONCLUSIONS: IMPLICATIONS FOR THE ORIGIN OF LACHLAN-TYPE PSGS MAGMAS

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Like S-type granitoids from the Lachlan Fold Belt type-locality, the Revancha rhyodacite is enriched in MgO, FeO^T, Cr and Ni (see Fig. 10), the concentrations of the transition metals exceeding those of most I-type calc-alkaline granites of similar SiO₂ content (Chappell & White, 1974; White & Chappell, 1983). Hine et al. (1978) and White & Chappell (1983) argued that the elevated Cr and Ni in S-type granites of the LFB reflect adsorption of these metals onto clay particles during deposition of the marine sediments inferred to constitute the source rocks. Although this is a viable hypothesis, we emphasize that many, and perhaps most, Lachlan-type, PSGS silicic suites worldwide exhibit evidence of commingling and mixing with mafic, mantle-derived melts (Kontak et al., 1986; Sandeman & Clark, 1993; Sandeman, 1995; Stimac et al., 1995; Elburg, 1996b; Carlier et al., 1997; Sandeman et al., 1997a). Thus, the LFB and analogous, SE Peruvian PSGS suites may derive their high abundances of the cafemic metals through mixing processes involving mantle-derived magmas.

The essentially pristine nature of the Revancha rhyodacite provides information on magmatic conditions that may be obliterated or disguised in phaneritic S-type granites. The glass-rich character of the rocks, the occurrence of euhedral, sector-zoned cordierite lacking abundant mineral inclusions, the abundant oscillatorily zoned plagioclase phenocrysts, and the rarity of inherited restitic phases indicate that the magmas probably comprised a high proportion of magmatic crystals and coexisting melt rather than a low proportion of melt with abundant entrained crystals. Magma consisting of 74% melt and only minor restitic material (2%) would be highly fluid in comparison with the crystal mushes widely advocated for Lachlan-type PSGS systems (White & Chappell, 1977; Patiño-Douce & Johnston, 1991). Moreover, the repeated underplating of the silicic magma by a hotter mafic melt would also have heated and hence lowered the viscosity of the Revancha magmas, and fluxing of volatiles such as H₂O, F and Cl from underplated potassic mafic melts would similarly lower the viscosity of the overlying silicic PSGS melt. We propose that such peraluminous silicic magmas are likely to evolve through fractional crystallization processes rather than through restite separation (see Holtz & Johannes, 1991; Patiño-Douce & Johnston, 1991).

The Revancha dyke exhibits whole-rock compositional characteristics that approach the average for all documented S-type monzogranites exposed in the Cordillera de Carabaya. If a suite of comagmatic granitoids is likely to span a range of compositions representing, at the silica-poor extremity, ‘cumulates’ and, at the silica-rich, ‘differentiates’, it may be inferred that the Revancha rocks probably represent a ‘best-estimate’ of a least-fractionated, possibly primary, cordierite + biotite, PSGS monzogranitic magma in the SE Peru context. We emphasize, however, the significant differences in the abundances of some elements in the Peruvian rocks and those of the Kosciusko and Melbourne Basement terranes of the LFB of SE Australia: the former are enriched in K₂O, Rb, U and Al₂O₃ but depleted in Fe₂O₃*, Na₂O, CaO, MgO, FeO, TiO₂ and Co. The Ba, Sr, Cr, Ni, La and Th contents are, however, comparable with those of the Australian S-type. The differences, largely in the major element compositions of the suites, are either a function of the composition and mode of the metasedimentary source (i.e. the source of the Lachlan Fold Belt S-type granites contained more calcic plagioclase and probably more magnesian biotite than that of the Revancha dyke) or result from the variable interaction and mixing of the primary Lachlan-type, PSGS magmas with mantle-derived melts. The latter hypothesis, involving mixing with diverse basaltic magmas, has been demonstrated on large and small scales for Lachlan Fold Belt PSGS granites by McCulloch & Chappell (1982) and Elburg (1996a, 1996b), respectively. Significantly, the ultrapotassic rocks of the Peruvian Cordillera Oriental are enriched in K₂O, Rb, Ba and U relative to the associated PSGS silicic rocks. This may, in part, explain why the PSGS suites of SE Peru are enriched in these elements in comparison with their SE Australian counterparts.

The unambiguous record in this region of the coexistence of mafic and silicic Lachlan-type PSGS magmas implies that potassic to ultrapotassic mantle-derived melts acted as heat sources and commonly mixed with silicic PSGS melts. They therefore played a critical role in crustal anatexis and petrogenesis of Neogene suites in this Andean domain. Furthermore, we emphasize that this process may be a prevalent, rather than rare phenomenon in the production of Lachlan-type, PSGS suites in general.

	APPENDIX: ANALYTICAL METHODS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING PETROGRAPHY AND MINERAL... WHOLE-ROCK GEOCHEMISTRY DISCUSSION CONCLUSIONS: IMPLICATIONS FOR... APPENDIX: ANALYTICAL METHODS REFERENCES

 
Mineral analyses
The majority of the mineral compositions were obtained by energy-dispersive electron microprobe analysis using an ARL-SEQM probe, on-line with a Tracor Northern computer system. Operating parameters included an accelerating potential of 15 keV, a beam current of 75 nA and count times of 200 s with a beam diameter of 1 µm. Fluorine was determined by wavelength-dispersive analysis using an Ovonyx Company, multilayered synthetic crystal, a 15 keV accelerating voltage and a beam current of 300 nA. Owing to the presence of an Fe L line in the lower-background measurement, multiple analyses of the same grain were performed in order to obtain reasonable statistical values for the F peak/background ratios, a procedure carried out at least 4 times per grain. In order to limit volatilization of light elements, a 15 µm wide beam was used during both EDS and WDS analysis of hydrous and alkali-metal-bearing mineral phases and glasses. We recognize, however, that these light elements may be mobile under high current electron beams and therefore these data can be treated as qualitative only. Data reduction for the EDS analyses was performed using the matrix corrections of Bence & Albee (1968) and applying the alpha-factors of Albee & Ray (1970). ZAF corrections were applied to the F peak/background ratios and these were then integrated with the EDS data and the 12 element analyses were recalculated using ZAF.

A standard reference glass (NBS-470) was used to establish operating parameters for all analyses. For the analysis of all phases, comparable mineral standards (e.g. Dorfgastein albite; Springwater olivine; and a range of internal reference materials) were used as secondary checks and analytical results were within 10% of the accepted values. For the analyses of groundmass glass, the peraluminous glass standard JV-1 (Pichavant et al., 1987) was used as a secondary check. Fluorine and C1 secondary standards included Durango apatite, a synthetic scapolite and a natural biotite. Errors for fluorine analyses were ±15% and ±10% for chlorine.

Oxides, garnet, orthopyroxene, biotite and plagioclase microphenocryst compositions were obtained by wavelength-dispersive analysis using a Cameca SX-50 electron microprobe equipped with 4 wavelength-dispersive spectrometers at the Geological Survey of Canada. The raw counts were corrected to elemental concentrations using the Cameca PAP program (Pouchou & Pichoir, 1985). Representative mineral compositions used in geothermobarometry calculations are given in Table 8. Standards used were a mixture of natural and synthetic metals, oxides and simple silicates. Counting time on both the peaks and background was 10 s.

Whole-rock analysis
Seven specimens of the Revancha dyke were crushed, pulverized in both tungsten carbide and chrome steel swing mills and analysed for major, trace elements and REE. Samples with the prefix MAC- were analysed twice. For the first sequence of analyses the major elements and Cr were determined by X-ray fluorescence spectrophotometric analysis (XRF) of fused glass discs at Queen's University. The elements F, B and Li were determined at XRAL Laboratories, Don Mills, Ontario, Canada, by specific-ion electrode analysis, direct-coupled plasma emission spectrometry and atomic absorption spectrophotometry, respectively. Most trace elements were determined by XRF analysis of pressed rock-powder discs at Queen's University, but the REE, Co, Sc, W, Ta, Hf, U, Th, As, and Cs were determined by instrumental neutron activation analysis (INAA) at the Royal Military College, Kingston, Ontario; elemental concentrations measured by INAA were calculated as outlined in Sandeman et al. (1997b). The second set of analyses were obtained at the Geological Survey of Canada following the procedures of Sandeman et al. (1999).

All major and trace element and REE analyses for samples Ma-89-15a and -15b were obtained by ICP-optical emission spectroscopy at Centre de Recherches Pétrographiques et Géochimiques et École Nationale Supérieure de Géologie de Nancy, France.

FeO determinations
FeO contents of four biotite grain separates (Table 4) and five whole-rock specimens (Table 9) were obtained through titrametric techniques at Queen's University following the methods of Wilson (1960). The accuracy of the FeO analyses is calculated to be better than2% based on 16 analyses of international reference materials MRG-1, SY-2 and NBS-278.

	ACKNOWLEDGEMENTS

 
This is a contribution to the Queen's University Central Andean Metallogenetic Project (CAMP), and is an outcome of doctoral studies by H.A.S. at Queen's University. The project was supported through NSERC grants to A.H.C. and Edward Farrar, and scholarships awarded to H.A.S. by the Queen's University School of Graduate Studies. The Instituto Peruano de Energía Nuclear is warmly thanked for logistical support in the field. Guido Arroyo provided co-operative and stimulating discussion in the field. Alain Cheilletz took part in the initial stages of the fieldwork and kindly arranged for the analysis of two samples at Nancy, France. Tony Mariano generously provided cathodoluminescence data. Katharine Venance assisted with electron microprobe analysis at the Geological Survey of Canada. We wish to acknowledge the invaluable logistic help of Señor Willy Delgado Aragón of Macusani, and the indefatigable assistance in manuscript preparation provided by Joan Charbonneau. The text of this paper benefited greatly from reviews by A. Patiño-Douce, G. Morgan (IV) and M. Elburg. M. Wilson provided much help.

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