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Journal of Petrology | Volume 45 | Number 4 | Pages 739-758 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.
Mineralogy, Textures and P–T Relationships of a Suite of Xenoliths from the Monaro Volcanic Province, New South Wales, Australia
CRC LEME, DEPARTMENT OF EARTH AND MARINE SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, A.C.T. 0200, AUSTRALIA
RECEIVED SEPTEMBER 1, 2002; ACCEPTED SEPTEMBER 9, 2003
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONIntraplate basalts of the Eocene–Oligocene Monaro Volcanic Province (MVP), in southeastern New South Wales, include lower-crustal and refractory to weakly metasomatized upper-mantle xenoliths. Lower-crustal-derived xenoliths appear to be all two-pyroxene plagioclase granulites (CpxFe:Mg:Ca 0·17–0·56:0·63–0·77:0·28–0·89 OpxFe:Mg:Ca 0·39–0·52:1·37–1·47:0·02 An72–86 and An48–50) but may also include garnet pyroxenites at depth. Mantle-derived xenoliths are principally spinel-bearing lherzolites (Fo89·8–90·6 CpxFe:Mg:Ca 0·07–0·45:0·70–1·70:0·01–0·94 OpxFe:Mg:Ca 0·16–0·19:1·62–1·75:0·01–0·10) but also include amphibole ± spinel-bearing lherzolite (Fo88·7–89·1 CpxFe:Mg:Ca 0·09–0·21:0·61–0·91:0·73–0·93 OpxFe:Mg:Ca 0·09–0·31:0·70–1·54:0·03–0·91), spinel-bearing harzburgite (Fo90·5–90·7 CpxFe:Mg:Ca 0·08:0·91–0·93:0·74–0·84 OpxFe:Mg:Ca 0·16–0·18:1·73–1·79:0·00–0·02), wehrlite, pyroxenite (CpxFe:Mg:Ca 0·08–0·10:0·84–0·90:0·80–0·85 OpxFe:Mg:Ca 0·16–0·33:1·51–1·73:0·02–0·03) and rare garnet pyroxenite (GtFe:Mg:Ca 0·83–0·95:1·60–1·70:0·45–0·48 CpxFe:Mg:Ca 0·14–0·21:0·69–0·77:0·78–0·86 Opx Fe:Mg:Ca 0·31–0·42:1·43–1·56:0·02–0·03) and amphibole–apatite composites. Xenolith textures are generally weakly to moderately foliated, a few are mosaic-porphyroblastic and rare samples are veined or highly strained. MVP xenoliths appear to have equilibrated under similar pressure–temperature (P–T) conditions to other southeastern Australian xenoliths equivalent to the South Eastern Australia (SEA) palaeogeotherm. P–T estimates for the MVP suite of xenoliths reveal a heterogeneous lower crust and upper mantle that is thickly underplated to c. 1·8 GPa or c. 50 km depth. MVP xenolith P–T data are compared with those used to derive the SEA palaeogeotherm, which is shown to be in need of revision using more modern geothermometers and geobarometers and new xenolith coexisting mineral data.
KEY WORDS: xenolith; petrography; texture; geotherm; Monaro; eastern Australia
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INTRODUCTION |
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This paper describes the mineralogy, mineral chemistry and textures of a suite of crustal and mantle xenoliths and megacrysts from the Monaro Volcanic Province (MVP) of southeastern New South Wales. The T and P–T relationships of MVP xenoliths are examined through application of some widely used mineral geobarometers and geothermometers. These results are compared with similar calculations used to derive the South Eastern Australia (SEA) palaeogeotherm (O'Reilly & Griffin, 1985
) and to develop a lithospheric model beneath the MVP. |
OVERVIEW OF THE MONARO VOLCANIC PROVINCE |
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The MVP is an Eocene–Oligocene (56–34 Ma; Taylor et al., 1990
) intraplate basaltic lava field located in the Southern Highlands of New South Wales (Fig. 1). Weakly sub-alkaline tholeiitic and transitional lavas are overlain in turn by alkaline ankaramite, alkali olivine basalt, nepheline basanite and olivine nephelinite lavas (Roach, 1996, 1999). The lava pile and surrounding Palaeozoic basement is intruded by over 65 separate eruption sites (Fig. 1). These comprise volcanic plugs, dykes and rare maars composed of primary to moderately evolved rocks including olivine nephelinite and melanephelinite, nepheline basanite, alkali olivine basalt, tephrite and K-trachybasalt. Numerous mantle- and crustal-derived xenoliths and megacrysts are present within the more silica-undersaturated primary to near-primary (in the sense of the magma being in equilibrium with Fo90–En90 mantle assemblages) volcanic plugs and dykes (Table 1). More detail regarding the volcanology, mineralogy, petrology and geochemistry of the MVP has been provided by Brown et al. (1992, 1993), Pratt et al. (1993), Lewis et al. (1994), Roach et al. (1994) and Roach (1996, 1999).
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The MVP is one of a number of large (>1000 km2) intraplate basaltic lava fields situated along the eastern Australian margin. These lava fields are enigmatic in that they do not follow the well-known latitude–age relationships of the hotspot-related ‘central’ volcanoes of eastern Australia, which become younger with increasing latitude [e.g. Wellman (1983)
and references given by Johnson (1989)]. Instead, the lava fields are scattered in time and space throughout eastern Australia and no clear-cut explanation exists for their genesis, although several increasingly sophisticated petrogenetic models in the literature include hotspot- or hotline-related (e.g. Wellman, 1983; McDonough et al., 1985; Sutherland, 1991), lithosphere-extensional (e.g. Lister et al., 1986; Lister & Etheridge, 1989), diapiric related to lithosphere extension (e.g. O'Reilly & Zhang, 1995) and lithosphere motion (Sutherland & Fanning, 2001). Mineralogical, whole-rock geochemical and P–T information from xenolith suites can help constrain petrogenetic models through their use in developing lithospheric (and possibly upper asthenospheric) models for the vicinity of the lava fields. |
PREVIOUS STUDIES |
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Monaro Volcanic Province and its xenoliths and megacrysts
Xenoliths and megacrysts in the MVP basalts have been recognized since Browne (1914)
noted olivine occurring ‘in nodules or segregations, in much greater proportion than in the normal basalt’ at North Brother, south of Cooma, now recognized as one of three large mantle xenolith-bearing nepheline basanite volcanic plugs (Roach, 1991; Brown et al., 1993; Roach et al., 1994). Kesson (1973; included in the study by Wass & Irving, 1976) noted three types of inclusions—‘gabbroic’, quartz-rich and spinel harzburgite—from a small volcanic plug near Beloka, SW of Dalgety. Wass & Irving (1976) also described Cr-diopside-bearing xenoliths and clinopyroxene, mica and olivine megacrysts in olivine nephelinite float at Russell Creek near Cathcart. Knutson & Brown (1989) recorded kaersutite and rare biotite megacrysts in many of the alkaline plugs and dykes of the MVP. Roach (1991) sampled several volcanic plugs in the central MVP for whole-rock geochemical and petrographical analysis and recognized a wide range of mantle and crustal inclusions.
Edgecombe (1992) analysed several spinel-bearing mantle xenoliths from the Rosemount cone sheet (Brown et al., 1993) and from plugs at Amphibole Hill, Wangellic Hill, Telegraph Hill and Inverlochie (Roach, 1991). Equilibration temperatures for spinel-bearing peridotites were calculated using the Wells (1977) and Sachtleben & Seck (1981) geothermometers for comparison against the SEA palaeogeotherm of Griffin et al. (1984) and O'Reilly & Griffin (1985). Edgecombe encountered problems equating equivalent pressure derived from MVP spinel lherzolites with the garnet lherzolite- or pyroxenite-derived SEA palaeogeotherm, which could not be tested because no garnet-bearing xenoliths were then known from the MVP.
Kesson (1973), O'Reilly & Griffin (1984) and O'Reilly & Zhang (1995) discussed the nature of the mantle beneath the MVP based on the geochemistry and Sr- and Nd-isotopic characteristics of volcanic rocks from eastern Australia. Kesson (1973) proposed a weakly metasomatized mantle based on whole-rock geochemical analyses. O'Reilly & Griffin (1984) reanalysed Kesson's samples, amongst others, and correlated high values of 87Sr/86Sr (
0·705) in the basalts with the presence of modal metasomatic minerals (amphibole, mica, apatite) in the local upper mantle. 87Sr/86Sr values of 0·7036–0·7054 for the MVP are consistent with a pervasively, but modally unevenly, metasomatized mantle that provided variable K and Ti to basalts derived from this source (see also Roach, 1999). O'Reilly & Zhang (1995) added Nd isotopes (143Nd/144Nd) to the Sr-isotope database and correlated whole-rock trace element and isotopic signatures with a mixed ocean island basalt (OIB) plume signature and sub-continental lithospheric mantle (SCLM) signature in eastern Australian basalts.
Eastern Australia
Crustal and mantle xenoliths and megacrysts are known from a wide range of tectonic settings and basaltic host rocks from eastern Australia. This summary is limited to observations in a few areas important to the development of the SEA palaeogeotherm. Additional details have been given by Wass (1979), O'Reilly & Griffin (1987), O'Reilly (1989a) and O'Reilly et al. (1989), following work elsewhere by Wilshire & Shervais (1975), Harte (1976) and Frey & Prinz (1978). Mantle xenoliths described here are classified after Le Maitre (1989).
The Newer Volcanics Province of western Victoria and diatremes at Delegate and Jugiong in southern New South Wales host a large variety of spinel- and garnet-bearing mantle and crustal xenoliths and megacrysts. Other Mesozoic–Cenozoic suites that include garnet-bearing mantle xenoliths occur at Bow Hill and Table Cape, Tasmania (Sutherland et al., 1984, 1989, 1994), the Walcha Province (Stolz, 1984; Sutherland et al., 1994), the McBride Volcanic Province, northern Queensland (Rudnick & Taylor, 1987, 1991) and central Queensland provinces (Griffin et al., 1987).
Xenoliths and megacrysts of the Newer Volcanics Province, one of the youngest eastern Australian lava fields, are among the most easily accessible and well-preserved in eastern Australia. Those from the twin maars of Bullenmerri–Gnotuk have received particular attention (e.g. Ellis, 1976; Andersen et al., 1984; Griffin et al., 1984; Nickel & Green, 1984; Hollis, 1985; O'Reilly & Griffin, 1985; Arculus et al., 1988; Griffin et al., 1988; Stolz & Davies, 1988; Sutherland et al., 1998). It is from this area that Griffin et al. (1984) calculated a mantle xenolith-derived palaeogeotherm. The Bullenmerri–Gnotuk samples include lherzolites of the ‘Cr-diopside suite’, interpreted to be magma conduit wall-rocks (Griffin et al., 1984), and pyroxenites and metapyroxenites (± garnet and spinel) of the ‘Al-augite suite’, interpreted to be the products of mantle magmatism, metamorphism and metasomatism (Griffin et al., 1984; O'Reilly, 1989a).
The Griffin et al. (1984) palaeogeotherm was augmented by O'Reilly & Griffin (1985) using additional xenolith data from eastern Australian suites (Irving, 1974; Wilkinson, 1974; Ferguson et al., 1977; Sutherland & Hollis, 1982; Wass & Hollis, 1983; Nickel & Green, 1984; O'Reilly & Griffin, 1985, R. J. Arculus, personal communication, 1985). These data constrain what O'Reilly & Griffin (1985) and O'Reilly (1989b) regarded as an average palaeogeotherm for all of southeastern Australia, the ‘SEA’ palaeogeotherm. Further discussion by Adam et al. (1992), Sutherland et al. (1994, 1998) and O'Reilly et al. (1997, 1998) has argued either for the reinforcement of the SEA palaeogeotherm or against over-simplification of numerous ‘fossil’ perturbed palaeogeotherms over the length of eastern Australia. Sutherland et al. (1998) maintained that eastern Australian palaeogeotherms should be widened to include multiple palaeogeotherms based on new xenolith thermobarometry data.
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XENOLITH- AND MEGACRYST-BEARING ROCK TYPES OF THE MVP |
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Xenoliths and megacrysts are mostly concentrated in the more silica-undersaturated volcanic plugs and dykes (Table 1, Fig. 1). Plugs and dykes range from aphanitic to coarse-grained in texture but only the former contain xenoliths and megacrysts.
The types of xenoliths and megacrysts found at each location broadly reflect the geochemistry of the intrusive rock types, which in turn controls their rheology. Olivine nephelinites and melanephelinites contain either a wide variety of xenoliths and megacrysts or only large amounts of amphibole megacrysts including kaersutite, titanian pargasite, pargasitic hornblende and lesser titanian phlogopite (Roach, 1999). Nepheline basanite and alkali olivine basalts contain less variety. Fractionated feldspar-rich rock types (tephrite, K-trachybasalt) carry only small plagioclase (labradorite–andesine) feldspar megacrysts if any. Ankaramitic rocks contain numerous clinopyroxene megacrysts; these will be described in a separate study.
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PHYSICAL CHARACTERISTICS OF XENOLITHS AND MEGACRYSTS |
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Xenoliths within the MVP are commonly sub-angular to rounded in cross-section and are often highly weathered. Many xenoliths are <2 cm diameter but
5 cm in diameter at Rosemount and Jinny Brother Peak, and weathered rounded xenoliths 10 cm diameter are present at The Peak (Loc. 34, Fig. 1) and Gourock East volcanic plug (Fig. 1). Nepheline basanite boulders containing relatively fresh angular to sub-angular spinel peridotite and two-pyroxene granulite xenoliths 15 cm length occur at Amphibole Hill, a zoned dyke with an amphibole-rich margin and an amphibole + mantle xenolith-rich core. Generally, small xenoliths enclosed within solid basalt are well preserved but larger xenoliths are weathered because joints and fractures penetrate from the host basalt. Only one garnet pyroxenite and one clinopyroxene–plagioclase–spinel–garnet rock (a partially decompression-reacted garnet pyroxenite) were collected from float beside the Gourock 4 volcanic plug. Only one amphibole–apatite xenolith was collected from Nimmitabel Hill but such xenoliths are probably more numerous at this site.
Mantle xenolith and megacryst textures
Most mantle xenoliths from the MVP are Cr-spinel lherzolites, the remainder (in order of decreasing abundance) include: wehrlite and harzburgite (± Cr-spinel); meta-pyroxenite; amphibole lherzolite (± Cr-spinel); dunite; garnet pyroxenite; amphibole–apatite; and olivine–plagioclase. No Al-augite suite xenoliths are known (Roach, 1991, 1999; Edgecombe, 1992 and personal communication, 1998). Most mantle xenoliths have relatively simple mosaic-porphyroclastic textures featuring, in thin section, trails of large, unstrained to moderately strained porphyrocrysts of olivine and orthopyroxene, and unstrained clinopyroxene separated by bands of smaller neoblasts (Fig. 2a). Strained olivine and orthopyroxene porphyrocrysts display sharp kink banding in thin section. Other textures include: non-foliated to moderately foliated coarse-equant to tabular texture (Fig. 2b); rare spinel lherzolite with an olivine–pyroxene–spinel mylonite zone c. 5 mm wide consisting of microscopic olivine and pyroxene neoblasts and stretched Cr-spinel, all enclosed by highly strained olivine and pyroxene porphyrocrysts (Fig. 2c); and porphyroclastic xenoliths with olivine and pyroxene megacrysts surrounded by randomly distributed neoblasts. Pyroxene-rich xenoliths (olivine websterite, olivine clinopyroxenite, pyroxenite) range in recrystallization textures. Clinopyroxene and orthopyroxene megacrysts in pyroxenites show prominent exsolution lamellae of pyroxene and Fe- or Cr-spinel and rare vein-like patterns of neoblastic pyroxene rimming and intruding megacrysts. This texture appears to result from a fluid or melt moving along grain boundaries and fractures, catalysing recrystallization, although volatile-bearing phases have not been detected.
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Evidence of modal mantle metasomatism in the MVP comes from: rare titanian pargasite-, pargasitic hornblende- and kaersutite-bearing lherzolite xenoliths (± Cr-spinel) (Fig. 2d) from Glen Lee and Amphibole Hill (Roach, 1991
; Roach et al., 1994; Roach & Edgecombe, 1996); phlogopite-bearing foliated wehrlite veins in spinel-free lherzolite (Fig. 2e) from the Rosemount cone sheet (Edgecombe, 1992); a 1 cm diameter amphibole–apatite xenolith (Fig. 2f) from Nimmitabel Hill (Roach, 1999); scattered titanian phlogopite megacrysts, particularly at Glen Lee (Roach, 1991); and abundant kaersutite, titanian pargasite and pargasitic hornblende megacrysts in plug rocks. Amphibole in the Glen Lee amphibole lherzolite xenoliths features spongy rims as a result of decompression reaction (Fig. 2d) but was formerly in textural equilibrium in the coarse-equant texture xenoliths, as shown by relict linear to curvilinear grain boundaries and some triple points. Pargasite on the external rims of these xenoliths is commonly replaced by reaction products including orthopyroxene, rhönite, nepheline or analcime, and olivine. One pargasite lherzolite xenolith from Amphibole Hill has amphibole surrounding Cr-spinel and is surrounded by an outer rim of rhönite, nepheline or analcime, and orthopyroxene in a similar fashion to examples from the Newer Volcanics Province (O'Reilly et al., 1989). The reaction rims around amphibole within these xenoliths are interpreted as a decompression feature, occurring during transport or after the xenolith was cooling within the volcanic plug. Additionally, olivine in mantle xenoliths commonly features large numbers of single-phase fluid inclusions in healed cracks.
Edgecombe (1992) described phlogopite phenocrysts in an extremely rare foliated wehrlite band in one recrystallized Rosemount xenolith (Fig. 2e). Larger titanian phlogopite megacrysts (1·5 cm diameter), presumably derived from metasomatic veins in the upper mantle, occur in the Glen Lee volcanic plug. An amphibole–apatite xenolith from Nimmitabel Hill has small (0·2–0·3 mm diameter) rounded to euhedral apatite crystals enclosed within an unzoned kaersutite megacryst (Fig. 2f). The apatite crystals commonly display hexagonally arranged sets of prominent dark inclusions. Apatite is cloudy towards the edges of the kaersutite megacryst and is replaced by microcrystalline opaque minerals where enclosed by the host basalt. One olivine–plagioclase xenolith was collected from Brown Mountain.
Spinel-bearing xenoliths typically contain <5% modal Cr-spinel, which is commonly distributed in stringers or symplectitic intergrowths with orthopyroxene ± clinopyroxene ± olivine. In thin section, spinel is commonly brown Cr-spinel, but rare, opaque, black Al–Fe spinel megacrysts were collected from Mount Emerald.
Garnet-bearing xenoliths include sample R152, a moderately foliated porphyroblastic garnet pyroxenite with clinopyroxene porphyroblasts showing prominent exsolution lamellae (visible in hand specimen) of orthopyroxene and microscopic (<0·1 mm diameter) rounded garnet blebs (Fig. 2g). Garnet also occurs at the junctions of clinopyroxene porphyroblasts as small (0·1 mm), clear rounded blebs. Sample R152C is a partially decompression-reacted garnet pyroxenite. It comprises clinopyroxene megacrysts bordered by a symplectitic intergrowth of clinopyroxene + orthopyroxene + green spinel + microscopic (<0·1 mm diameter) garnet with prominent kelyphytic rims, all surrounded by plagioclase enclosing green spinel blebs. This xenolith also contains a small cross-cutting vein of spinel lherzolite.
Crustal xenoliths and megacrysts
Crustal xenoliths include local Palaeozoic basement and deeper, mafic plutonic rocks. The most common upper-crustal xenoliths are quartzites consisting of sub-rounded to angular quartz grains. Some of these quartz grains have undulose extinction and poorly defined triple-point junctions or seriate grain boundaries, and all xenoliths are surrounded by a rim of brown glass and radiating titanian augite microlites where they contact the host basalt. The quartzites contain variable amounts of feldspar and muscovite/sericite, and are similar in mineralogy and texture to local Ordovician–Silurian flysch rocks. There are rare granitoid and vein quartz xenoliths at Brown Mountain. Both exhibit glassy rims similar to the quartzite xenoliths and the vein quartz xenoliths consist solely of clear quartz featuring prominent single-phase fluid inclusion trails in healed cracks. Rare quartz megacrysts within thin sections from plug rocks feature prominent brown glass rims lined with radially arranged titanian augite microlites and probably represent disaggregated upper-crustal xenoliths. The Brown Mountain basalt plug contains the most abundant upper-crustal inclusions of any site in the MVP.
Lower-crustal two-pyroxene plagioclase granulites are very common within the xenolith-rich plugs and dykes. They commonly have coarse equant to tabular textures or rarely a weakly foliated appearance with grain sizes of <3 mm diameter and well-developed curvilinear to triple-point grain boundaries (Fig. 2h). They contain a monotonous mineral assemblage of pyroxene(s) + plagioclase and rarely include microscopic rounded yellowish crystals, possibly scapolite, and blebs or rims of kelyphytic pigeonite surrounding pyroxene.
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EXPERIMENTAL METHODS |
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Xenoliths were collected in bulk basalt samples from plugs and dykes around the MVP. Basalt blocks were sawn to reveal the freshest xenoliths, which were then impregnated, thin-sectioned and polished in preparation for analysis using the Cameca wavelength dispersive (WDS) electron microprobe (EMP) at the Research School of Earth Sciences, Australian National University. Some analyses (other than the garnet pyroxenites) were obtained with the EMP's energy dispersive (EDS) system. Analytical errors with the WDS or EDS EMP analyses are about 1% for major oxides of >10 wt % abundance and <10% for minor oxides <10 wt %, similar to those of Griffin et al. (1984)
and O'Reilly & Griffin (1985). The EMP was calibrated using internal standards, particularly San Carlos Olivine. Where necessary, FeOtotal was recalculated to FeO and Fe2O3 using the method of Cawthorn & Collerson (1974) for pyroxenes and by charge balance for spinel and garnet, depending on the requirements of the geobarometer or geothermometer applied. |
MINERAL COMPOSITIONS OF MANTLE AND CRUSTAL XENOLITHS AND MEGACRYSTS |
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Olivine
Olivine within mantle xenoliths of the MVP is compositionally unzoned and is Fo89·8–90·6 in spinel lherzolites and Fo88·7–89·1 in amphibole lherzolites. In the eastern MVP, olivine in lherzolites is Fo89·9–90·3 and in harzburgites is Fo90·5–90·7 (Edgecombe, 1992
). Olivine xenocrysts from disaggregated mantle xenoliths, which show ragged edges and kink bands, are Fo85·2–89·4 and are chemically zoned, reflecting Mg–Fe exchange with the surrounding melt. Olivine in modally metasomatized rocks, e.g. Amphibole Hill, where it is in equilibrium with titanian pargasite, contains 0·29 wt % CaO compared with olivine in unmetasomatized lherzolites with <0·20 wt % CaO. Olivine in titanian pargasite-bearing lherzolite xenoliths contains 0·10 wt % TiO2. Representative olivine analyses from a range of MVP xenoliths are presented in Table 2.
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Clinopyroxene
Clinopyroxene in MVP mantle xenoliths consists predominantly of diopside (Table 3, Fig. 3) based on the classification of Morimoto et al. (1988)
. Some clinopyroxene megacrysts and clinopyroxene in the garnet pyroxenites and two-pyroxene plagioclase granulites are low-Cr diopsides. Clinopyroxene megacrysts are distinguished from phenocrysts by their clear to green-coloured cores, which are commonly surrounded by a rim of purplish titanian augite crystallized in optical continuity with the core. Edgecombe (1992) noted lower Al2O3 contents in diopsides in harzburgites (2·0–2·75 wt % Al2O3) compared with diopsides in lherzolites (3·9–4·25 wt % Al2O3) in a suite of xenoliths from the eastern MVP. Clinopyroxene within mantle xenoliths is generally chemically unzoned. Megacrysts, however, are zoned in Fe–Mg–Ca as a result of interaction with the surrounding melt. Clinopyroxene compositions are bimodal: those in two-pyroxene granulites, garnet pyroxenites and xenocrysts are more Fe-rich; and those from spinel lherzolites, harzburgites, amphibole lherzolites and pyroxenites are more Mg-rich. Some MVP mantle xenoliths, particularly pyroxenites, contain two clinopyroxene types, probably arising from the influence of exsolution or decompression reactions on mineralogy.
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Orthopyroxene
All orthopyroxene within MVP xenoliths is broadly classified as enstatite (Morimoto et al., 1988
). However, there is some variation in composition between rock types (Table 4, Fig. 3). Mg, Fe and Si contents vary most, whereas Ca and Al contents are generally low. Orthopyroxene in garnet pyroxenite, two-pyroxene granulite and some pyroxenite is the most Fe-rich and Mg-poor, suggesting a more Fe-rich source than the spinel lherzolites. In detail, two types of orthopyroxene occur in garnet pyroxenite and two-pyroxene granulite xenoliths: a relict primary orthopyroxene (slightly more Mg-rich, Mg 
1·5 atoms on the basis of six oxygens); and a secondary orthopyroxene (slightly more Fe-rich, Mg 1·4 atoms on the basis of six oxygens), probably crystallized from the re-equilibration of other phases. Overall Al and Ca levels in orthopyroxene from harzburgites are lower than those of lherzolites, reflecting Al depletion by earlier partial melting (Jaques & Green, 1980).
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Spinel and other oxides
Several types of spinel occur within MVP xenoliths: in thin section, most are brown Cr-rich Al-spinels (Mg0·69–0·83Fe0·24–0·52Cr0·25–0·74Al1·15–1·68O4) varying in Mg–Fe and Al–Cr between xenolith rock types (Table 5). The oxide in garnet pyroxenite rocks (sample R152) is ilmenite (Fe0·77–0·88Mg0·17–0·25Ti0·96–0·98O3). Green Al-rich spinel (Mg0·71Fe0·34Al1·95O4) is a reaction product in garnet–clinopyroxene–orthopyroxene–spinel symplectites, and within the plagioclase of the decompression-reacted garnet pyroxenite (sample R152C).
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Garnet
Garnet in the two garnet-bearing xenoliths is pyrope-rich (Table 6). In garnet pyroxenite sample R152, garnet averages Py57·34Alm25·14Gr11·27And4·4Maj2·79Sp1·01, corresponding to Mg3·22Fe1·86Mn0·08Ca0·91Al3·92Si5·98O24. In the decompression-reacted pyroxenite sample (R152C), garnet averages Py54·75Alm28·12Gr11·95And2·98 Maj1·65Sp1·28, corresponding to Mg3·39Fe1·67Mn0·06 Ca0·96Al4·02Si5·94O24. Garnet in sample R152 is slightly more Fe-rich than garnet in sample R152C, but both contain minor Ti, Mn, Na and K.
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Figure 4 compares MVP garnet with other eastern Australian xenolith garnet using the scheme of Coleman et al. (1965)
. MVP garnet falls between Coleman et al.'s Field 5 (Group A eclogite), which includes garnet derived from eclogites associated with ultramafic rocks such as peridotite and dunite, and Field 3 (Group B eclogite), which includes garnet derived from gneisses and migmatites. Garnet from other eastern Australian suites falls within those fields but ranges into Field 4 (Group A eclogite), which includes garnet derived from eclogites in kimberlite pipes. Garnet from the nearby Delegate nephelinite breccia pipes (Lovering & White, 1969; Irving, 1974) shows the greatest compositional range.
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Amphibole
Amphibole within MVP lherzolite (± Cr-spinel) xenoliths is either titanian pargasite with >c. 0·15 atoms of Ti (>c. 1·4 wt % TiO2) or kaersutite with >0·5 atoms of Ti (>c. 5·0 wt % TiO2), on the basis of 23 oxygens and the classification of Leake et al. (1997)
. Amphibole is rare within MVP xenoliths, found only within two volcanic plugs, but is very common as megacrysts. Amphibole is interpreted to be derived from modally metasomatized peridotite, metasomatic stockworks or magma conduit walls in the upper mantle and lower crust. A continuum of Mg/Fe in amphibole in mantle xenoliths and megacrysts from 0·87 (in amphibole lherzolite) to 0·57 (in amphibole megacrysts) reflects crystallization in varying proximity to mantle wall rocks equilibrated at Mg/Fe = 0·89–0·91 (O'Reilly & Griffin, 1989).
Feldspar
Feldspar is present in most upper-crustal xenoliths and two-pyroxene granulite xenoliths from the MVP. In two-pyroxene granulites, it is either calcic (bytownite An72–86) or intermediate (labradorite/andesine An48–50), and in the decompression-reacted garnet pyroxenite (sample R152C), it is moderately calcic (labradorite An58–65).
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GEOTHERMOBAROMETRY OF XENOLITHS FROM THE MVP |
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Background
Most mantle- and lower-crustal-derived xenoliths in basaltic eruptives probably ascend relatively quickly, perhaps in less than 60 h (Kushiro et al., 1976
; Ozawa, 1983; Bezant, 1985) or at speeds of up to 70 km/h for a kimberlitic magma (Mercier, 1979). Furthermore, primary metamorphic and igneous textures within the xenoliths indicate that original P–T conditions are retained within the mineral chemistry (Harte & Hawkesworth, 1996). Therefore these xenoliths are useful for determining P–T conditions using some form of calibrated geobarometer and/or geothermometer. Considerable experimental petrological research has investigated the levels and rates of element exchange between minerals under different P–T conditions with varying degrees of success. Geobarometers and geothermometers now provide relatively well-calibrated, robust mathematical relationships for coexisting minerals in equilibrium assemblages (e.g. Finnerty & Boyd, 1984).
The SEA palaeogeotherm for eastern Australian basaltic xenolith suites (Griffin et al., 1984; O'Reilly & Griffin, 1985) was developed using the garnet–orthopyroxene geobarometer of either Wood (1974) or Harley & Green (1982), coupled iteratively with the Ellis & Green (1979) garnet–clinopyroxene geothermometer, to estimate P–T. Griffin et al. (1984) also found that the Wood & Banno (1973) two-pyroxene geothermometer gave results close to the Ellis & Green (1979) garnet–clinopyroxene geothermometer. The Sachtleben & Seck (1981) (SS81) olivine–spinel–orthopyroxene geothermometer for spinel-bearing xenoliths was also found to give similar results to the Ellis & Green (1979) garnet–clinopyroxene geothermometer (Griffin et al., 1984) in composite xenoliths.
Later research showed that the Ellis & Green (1979) garnet–clinopyroxene geothermometer produces scattered results (Finnerty & Boyd, 1984), and consistently overestimates T at low equilibration temperatures by 70–100°C (Brey & Kohler, 1990; Taylor, 1998) or by up to 300°C (Alaoui et al., 1997). Explanations for the overestimation of T include the inability of EMPs to analyse for Fe2+/Fe3+ (Finnerty & Boyd, 1984, 1987) or the failure to compensate for Ca concentration in garnet (Ai, 1994; Alaoui et al., 1997). Additionally, Taylor (1998) has shown that even minor amounts of Na and Ti in spinel, garnet and pyroxene solid solutions can affect P–T estimations, causing calculated equilibrium P–T between fertile and infertile peridotites to vary dramatically. Most recently, Taylor (1998) developed a robust new geobarometer–geothermometer combination for fertile and infertile lherzolite, websterite and pyroxenite based on the Nickel & Green (1985) garnet–orthopyroxene geobarometer and the Brey & Kohler (1990) two-pyroxene geothermometer. Taylor (1998) introduced a correction for Al/Ti in orthopyroxene in the Nickel & Green (1985) geobarometer, producing the Nickel & Green (1985) ‘mod’ geobarometer, and the Taylor (1998) geothermometer, based on the Brey & Kohler (1990) two-pyroxene geothermometer, introducing corrections for the enstatite activity, tschermakitic components, and Fe and Ti. Taylor (1998) also showed that the Wells (1977) two-pyroxene geothermometer gave results closer to his own than any other geothermometer and recommended its use for spinel-bearing and non-spinel-bearing mantle xenoliths.
This study
P–T relationships of coexisting minerals from the MVP, including those of Edgecombe (1992), were initially determined using the same geobarometer–geothermometer combinations as applied by Griffin et al. (1984) and O'Reilly & Griffin (1985) (Table 7) to retain consistency. MVP data are then compared against those used by O'Reilly & Griffin (1985) from which the SEA palaeogeotherm was developed, but using more modern geobarometer–geothermometer combinations. The T relationships of garnet-bearing samples from the MVP are then compared against the rest of the spinel-bearing MVP suite to test the implications of using different geothermometers including: (1) the Harley (1984) garnet–orthopyroxene geothermometer and the Wells (1977) two-pyroxene geothermometer; (2) the Taylor (1998) modification of the Brey & Kohler (1990) two-pyroxene geothermometer and the Wells (1977) two-pyroxene geothermometer.
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P–T estimations for the MVP
Calculated equilibration T for all MVP xenoliths is presented in Table 8 (except for the two garnet granulite xenoliths, which are included below) and Fig. 5, following the methods of Griffin et al. (1984)
and O'Reilly & Griffin (1985). P–T estimation results for the two garnet pyroxenite xenoliths are: R152, 1·43/1060–0·78/895–1·32/970 GPa/°C; R152C, 1·62/1130–0·71/890–0·45/745 GPa/°C. Each P–T pair is estimated using the Wood (1974) ‘c’ modification geobarometer and Ellis & Green (1979) geothermometer; the Wood (1974) ‘c’ modification geobarometer and Harley (1984) geothermometer; and the Nickel & Green (1985) ‘mod’ geobarometer and Taylor (1998) combinations, respectively.
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Equilibration T values for MVP xenoliths, according to the Sachtleben & Seck (1981)
(FeO only) and Ellis & Green (1979) (FeOtotal recalculated as FeO and Fe2O3) geothermometer combination (Fig. 5a), are distributed around a slightly right-skewed mode at c. 1000°C. Garnet-bearing xenoliths have higher T than spinel-bearing xenoliths, and spinel pyroxenite xenoliths (wehrlite, websterite) fall into the higher range for the spinel-bearing xenoliths.
Figure 5b illustrates T calculated using the Wells (1977) and Harley (1984) geothermometers (FeOtotal recalculated as FeO and Fe2O3 for both). In this histogram, garnet-bearing xenoliths no longer occupy the higher end but rather the centre of the data, and spinel-bearing xenolith T data are spread over a wide range because of the recalculation of FeOtotal to FeO and Fe2O3 for the Wells (1977) and Harley (1984) geothermometers. This histogram differs from that in Fig. 5a in that it has a much higher range of equilibrium T (c. 750°C compared with c. 375°C in Fig. 5a), significant numbers of xenoliths equilibrating at c. 1000°C, and garnet pyroxenite xenoliths plot within the spinel lherzolite field, rather than a higher T field.
Figure 5c illustrates the MVP xenolith suite equilibrium T derived using the Wells (1977) and Taylor (1998) geothermometers (FeOtotal recalculated as FeO and Fe2O3 for both). Spinel-bearing xenolith T remains unchanged from Fig. 5b, but the partially recrystallized garnet pyroxenite (sample R152C) now plots separately at c. 745°C, signifying significant re-equilibration near-surface. The other garnet pyroxenite plots at a higher T (c. 950°C). Sample R152C contains a vein consisting of an olivine–orthopyroxene–clinopyroxene–spinel assemblage, which is calculated, using the Wells (1977) geothermometer, as equilibrating at c. 1245°C (annotated as spinel lherzolite in Fig. 5b and c).
Figure 6 compares the P–T estimates for the two MVP garnet-bearing xenoliths and T of the spinel-bearing xenoliths with the P–T results of garnet-bearing xenoliths from Griffin et al. (1984) and O'Reilly & Griffin (1985). Overall, MVP garnet-bearing xenoliths plot slightly above the SEA palaeogeotherm. One of the MVP suite xenoliths (R152) falls within the error limits of the SEA palaeogeotherm but R152C plots c. 70°C above the SEA palaeogeotherm (beyond analytical error). O'Reilly & Griffin (1985) considered that SEA was well constrained to c. ±50°C between 1·0 and 1·8 GPa. Equivalent pressures of equilibration for spinel-bearing xenoliths may be estimated from the SEA palaeogeotherm line.
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DISCUSSION |
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Inconsistencies between geobarometers–geothermometers
Figure 6 illustrates that calculated equilibrium P–T values for MVP suite xenoliths are generally in accordance with those calculated for the suite of xenoliths used by O'Reilly & Griffin (1985)
to calculate the SEA palaeogeotherm. However, the formulation of newer thermobarometric algorithms permits a re-examination of the P–T relationships of mantle and lower-crustal xenoliths in basaltic volcanics, particularly for garnet-bearing xenoliths on which geotherms and palaeogeotherms are primarily based. The differences between older and more modern formulations is highlighted in Fig. 5. In Fig. 5a, calculated using the same geothermometers as applied by O'Reilly & Griffin (1985), garnet-bearing xenoliths plot on the high-T side of the histogram at c. 1060°C (sample R152) and c. 1130°C (sample R152C). However, in Fig. 5c, calculated using the Wells (1977) and newer Taylor (1998) geothermometers, garnet-bearing xenoliths plot at c. 970°C (sample R152) and c. 745°C (sample R152C). The calculated equilibrium T difference between two geothermometers for sample R152C is c. 385°C. This sample is a decompression-reacted garnet pyroxenite that is now a garnet–pyroxene–plagioclase–spinel rock intruded by a vein of spinel lherzolite that is calculated to have equilibrated at 1245°C using the Wells (1977) geothermometer. Coexisting minerals are in disequilibrium, perhaps as a result of decompression and/or fluid invasion, but recrystallization was probably catalysed by magmatic heating during ascent. Sample R152 shows an equilibrium assemblage with well-developed triple-point boundaries between adjoining minerals and a 195°C disparity between calculated equilibrium T using the older and newer geothermometers. Similarly, the inability of EMPs to analyse for Fe3+ and the subsequent need to recalculate FeOtotal to FeO and Fe2O3 by stoichiometry or charge balance also results in a wide difference between calculated equilibrium T between successive generations of geothermometer algorithms, highlighted in Fig. 5a–c. The compositional data used to calculate the SEA palaeogeotherm should be re-examined using newer geobarometer–geothermometer algorithms that do not rely so heavily on Fe–Mg exchange and that also include adjustments for small amounts of other elements (e.g. Na and Ti) that may affect the apportionment of FeOtotal to FeO and Fe2O3 during mineral formulae recalculation.
A lithospheric model for the MVP
Xenoliths from the MVP provide data on the region's uppermost lithosphere. The P–T results (Fig. 5c) show a heterogeneous, stratified upper mantle and lower crust, based on the Wells (1977) and Taylor (1998) geothermometers. Shallow, cooler two-pyroxene granulites pass through garnet pyroxenites into deeper, hotter spinel lherzolites and pyroxenites. Instead of existing as lenses and veins in the spinel lherzolite field, the garnet pyroxenites of the MVP are more likely to be underplated rocks crystallized closer to the base of the continental crust than was previously realized.
MVP data, combined with those used for the SEA palaeogeotherm, are used to develop a lithospheric model below the MVP (Fig. 7). This model is simplified from that of Griffin & O'Reilly (1987) and O'Reilly (1989b) in that it contains less information about underplated magmas, which those researchers described as forming ‘mafic lenses’ between 20 and 70 km depth. Unfortunately, no reliable xenolith information for the MVP, based on Wells (1977) and Sachtleben & Seck (1981) temperatures in crustal xenoliths, is available in the mid-crust interval between c. 1·0 GPa (c. 28·5 km depth) and the base of the upper crust, located at c. 15–20 km depth (O'Reilly, 1989b; Gray et al., 1998). Magmatic underplating played a major role in the formation of the MVP, as indicated by the prominent ankaramitic lavas [discussed in more detail by Roach (1999)]. Pyroxene crystallization pressures derived from porphyritic pyroxene cores in the ankaramites using the Mercier (1976, 1980) single-pyroxene geobarometer show a large peak at around 1·0–2·0 GPa (c. 30–60 km). Recalculation of these data using the Taylor & Nimis (1998) single-pyroxene geobarometer shows that clinopyroxene porphyrocryst cores most frequently crystallized at c. 1·8 ± 0·3 GPa (c. 54 ± 9 km), slightly below the Mohorovi
i
discontinuity in this area (see below), indicating considerable addition of magma near this level. These magmas would crystallize into mafic lenses, now including recrystallized garnet and spinel granulites, two-pyroxene granulite and garnet–spinel pyroxenites, sampled as lower-crustal–upper-mantle xenoliths by ascending basalts. Cenozoic underplating related to the MVP would have added to the crust, which was already underplated to some extent in the Mesozoic, as evidenced by xenoliths in other New South Wales breccia pipes at Delegate (e.g. Irving, 1974), Jugiong (e.g. Fergusson et al., 1977) and Gloucester (e.g. Wilkinson, 1974).
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The base of the underplated zone of the MVP crust appears to coincide with the crust–mantle boundary. Collins (1991)
identified the depth of the Mohorovii discontinuity (Moho—here regarded as the crust–mantle boundary) as coinciding with the transition from seismic P-wave velocities of <7·8 km/s to >7·8 km/s and the depth to the Moho under the MVP as c. 50 km, based on refracted upper-mantle P-wave arrival times. Collins (1991) also commented that the Moho is a transition zone (of variable thickness but at least of several kilometres), consisting of laminated, laterally discontinuous, higher- and lower-velocity materials corresponding to underplated rocks of alternating more mafic and more felsic compositions, each between 50 and 200 m thick. This interpretation is also supported by S-wave seismic tomography data (the Australian National University Research School of Earth Science's SKIPPY project), which estimate the depth to the Moho at between 48 and 50 km beneath the MVP (Zielhuis & van der Hilst, 1996), close to the estimated peak of pyroxene crystallization within ankaramites at c. 1·8 ± 0·3 GPa or c. 54 ± 9 km depth. Thus, although the model (Fig. 7) depicts spinel lherzolites under mafic granulites at the basal lower crust (from the geothermometry of MVP suite xenoliths), this region may also include mafic lenses of garnet and spinel pyroxenites. |
CONCLUSIONS |
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The MVP xenolith suite reveals a heterogeneous lithosphere under this region. Thermobarometry suggests a complex succession of rocks from two-pyroxene granulite and garnet pyroxenite in the lower crust to relatively refractory spinel lherzolite, harzburgite and then garnet lherzolite [from the Griffin & O'Reilly (1987)
and O'Reilly (1989b) model] in the upper mantle (Fig. 7). Additional garnet-bearing xenoliths are required to more precisely establish the palaeogeothermal conditions of the MVP xenolith suite.
It is important to recognize that geotherms are transitory (Irving, 1976) and probably wax and wane over periods of several tens of millions of years (Lister & Etheridge, 1989). The SEA palaeogeotherm, based primarily on recently erupted Bullenmerri–Gnotuk suite xenoliths, may reflect present-day P–T conditions in the central Victorian lithosphere but is not necessarily applicable to the whole of eastern Australia. New xenolith data and more precise geobarometer–geothermometer combinations may discriminate different palaeogeotherms throughout its Mesozoic and Cenozoic volcanic history.
The mineral chemistry of MVP mantle xenoliths reveals a relatively Fe- and Al-rich group of garnet pyroxenites, spinel pyroxenites and two-pyroxene granulites, and a relatively Fe- and Al-poor group including the more refractory spinel-bearing peridotite xenoliths. This implies that MVP lithospheric xenoliths were derived from two sources: relatively Fe- and Al-rich magmas underplated at the base of the crust; and relatively Fe- and Al-depleted upper mantle.
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ACKNOWLEDGEMENTS |
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This research was conducted largely during postgraduate studies at the Centre for Australian Regolith Studies, University of Canberra. The author acknowledges the invaluable financial assistance of the University of Canberra Higher Degrees and Scholarships Committee towards part of this research. The author also wishes to thank: supervisors Dr Max Brown, Associate Professor Ken McQueen and Professor Graham Taylor; Dr Lin Sutherland of the Australian Museum (Sydney), who supplied extra analyses and references to expand the data compilation; and Dr Wayne Taylor and Mr Nick Ware of the Research School of Earth Sciences, ANU, who helped with applying geobarometer–geothermometer algorithms and EMP sessions, respectively. The author gratefully acknowledges Sue Edgecombe for data supplied from her Department of Geology, ANU, 1992 Honours thesis. The author gratefully acknowledges the constructive criticism of Lin Sutherland and Richard Arculus (Department of Geology, ANU) on drafts of this paper.
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FOOTNOTES |
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* Telephone: +61 2 6125 0030. Fax: +61 2 6125 5544. E-mail: Ian.Roach{at}anu.edu.au
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