Journal of Petrology Volume 43 Number 2 Pages 219-241 2002
© Oxford University Press 2002
Hornblende- and Phlogopite-Bearing Gabbroic Xenoliths from Volcán San Pedro (36°S), Chilean Andes: Evidence for Melt and Fluid Migration and Reactions in Subduction-Related Plutons
1SECTION DES SCIENCES DE LA TERRE, UNIVERSITÉ DE GENÈVE, 13 RUE DES MARAÎCHERS, 1211 GENEVA, SWITZERLAND
2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WISCONSIN–MADISON, 1215 W. DAYTON ST., MADISON, WI 53706, USA
Received June 19, 2000; Revised typescript accepted August 7, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGTEXTURES, AND MINERAL AND...WHOLE-ROCK CHEMICAL COMPOSITIONSDISCUSSIONIMPLICATIONS OF THE PRESENCE...CONCLUSIONSAPPENDIXREFERENCES |
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Two groups of gabbroic xenoliths (I and II) containing large proportions of late-crystallized hornblende (up to 50 vol. %) and Na-rich phlogopite (up to 15 vol. %), were brought to the surface by a late Holocene eruption of Volcán San Pedro, the youngest edifice of the Tatara–San Pedro Volcanic Complex (36°S, Chilean Andes). Group I are inferred to be fragments of partially solidified Holocene plutons because they contain residual interstitial glass, whereas exsolution and deformation textures in Group II indicate that they are fragments of pre-Quaternary plutonic basement. On the basis of textural relations plus the mineral and whole-rock compositions of both groups of xenoliths, we suggest that hornblende and phlogopite with high mg-numbers and Cr contents have formed by reactions between refractory cumulus minerals (olivine, Cr-spinel, pyroxenes or plagioclase) and evolved melts ± aqueous fluids that migrated through partly solidified crystalline frameworks. Thus, the hydrous minerals are not early-crystallized phases in the basaltic magmas from which the cumulus minerals precipitated. The high proportions of hornblende in many subduction-related gabbroic plutons and xenolith suites compared with its paucity in basaltic or basaltic andesitic lavas may be partially explained by multistage plutonic crystallization histories involving reaction and migration of evolved melt ± aqueous fluids that either could have originated within the cumulus pile of the mafic intrusion or were derived externally, from broadly contemporaneous felsic magmas.
KEY WORDS: Tatara–San Pedro; gabbroic xenolith; hornblende; phlogopite; melt migration
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INTRODUCTION |
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Hornblende-bearing gabbroic xenoliths found at subduction-related volcanoes have been inferred to reflect differentiation processes that have operated on magmas of these arcs (Itinome-gata, Japan, Aoki, 1971
; Lesser Antilles, Arculus & Wills, 1980; Adak, Aleutian arc, Conrad et al., 1983; Conrad & Kay, 1984; DeBari et al., 1987; Medicine Lake, California, Grove & Donnelly-Nolan, 1986; Japan, Yagi & Takeshita, 1987; Martinique, Lesser Antilles, Fichaut et al., 1989; Arenal volcano, Costa Rica, Beard & Borgia, 1989; Mt. St. Helens, Cascades, Heliker, 1995; Calbuco volcano, Southern Chile, Hickey-Vargas et al., 1995; compilation from various sites, Beard, 1986). Many such xenoliths have been interpreted as crystal fractionation residues, and their mineral abundances and compositions have been used to constrain fractional crystallization models of arc magmas. The presence of hornblende in these gabbroic xenoliths and certain apparently refractory compositional characteristics (e.g. high Cr contents) have led to inferences that (1) hornblende formed as an early-crystallizing mineral in water-bearing mafic magmas (e.g. Conrad & Kay, 1984; Yagi & Takeshita, 1987; Beard & Borgia, 1989), and (2) fractionation of near-liquidus hornblende contributes to the calc-alkaline differentiation trend (Cawthorn & O’Hara, 1976; Beard, 1986; Grove & Kinzler, 1986; Yagi & Takeshita, 1987). Despite the plausibility of a relation between hornblende stability in water and alkali-rich, high- fO2 mafic magmas (e.g. Sisson & Grove, 1993) and early silica enrichment along the calc-alkaline trend, the high modal abundances of hornblende in arc-related gabbros (xenoliths and plutons) are in marked contrast to the rarity of hornblende-bearing basaltic or basaltic andesitic magmas erupted from subduction-zone volcanoes. This could be explained by the instability of amphibole at low pressure in mafic magma, or by protracted closed-system crystallization reactions in gabbroic plutons. However, the association of hornblende and Na-rich phlogopite in the San Pedro gabbroic xenoliths suggests that reactions in mafic plutons might be triggered also by migrating water-rich melts and aqueous fluids, as has been found in experiments involving interactions between evolved liquids and mafic cumulates (e.g. Prouteau et al., 2001; Costa et al., in preparation).
We propose that two groups of hornblende- and phlogopite-bearing gabbroic crustal xenoliths from the Holocene Volcán San Pedro (Tatara–San Pedro Volcanic Complex, Chilean Andes) are distinct in terms of age and origin, but that both suites are the result of multistage differentiation histories involving migration of evolved melts ± aqueous fluids through cumulate piles. Reactions between early-crystallized refractory minerals (olivine, Cr-spinel, pyroxenes or plagioclase) and percolating melts and fluids have produced substantial proportions of hornblende (up to 50 vol. %) plus Na-rich phlogopite (up to 15 vol. %) with features such as high Cr2O3 contents, which have been interpreted elsewhere as indicating early crystallization from hydrous basaltic magmas. The processes of melt and aqueous fluid migration and reaction-replacement proposed for the San Pedro gabbros could be analogous to those described in tholeiitic intrusions (e.g. Muskox intrusion, Canada, Irvine, 1980; Skaergaard intrusion, Greenland, McBirney, 1995; Bushveld Complex, South Africa, Mathez, 1995; Stillwater Complex, USA, Boudreau, 1999) or those caused by interactions between mafic and felsic magmas (e.g. Sha, 1995).
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GEOLOGICAL SETTING |
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The Quaternary Tatara–San Pedro Complex (TSPC; 36°S, 71°51'W) is a long-lived frontal arc volcanic centre (930 ka; Holocene) of the Southern Volcanic Zone of the Andes (Fig. 1). Exposed basement lithologies are mainly low-grade metavolcanic and metasedimentary rocks intruded by two shallow-level granitoid plutons dated at 6·2–6·4 Ma (Davidson & Nelson, 1994
; Nelson et al., 1999). Gabbroic rocks are not observed at the surface, although gabbro and olivine gabbro (including troctolite and norite) with anhydrous mineralogy are the most common xenolith lithologies and the sources of minor (<10 vol. %) but widespread xenocrystic contamination in mafic to intermediate lavas. The TSPC (
55 km3) consists mainly of basaltic andesitic lavas, although erupted magmas range from primitive basalt to high-SiO2 rhyolite that define calc-alkaline medium- to high-K trends (e.g. Singer et al., 1997; Dungan et al., 2001). Dacitic lavas at the TSPC typically contain minor hornblende phenocrysts that are rarely accompanied by biotite, but no lava with <65 wt % SiO2 contains hornblende phenocrysts.
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, main Quaternary volcanic centres. Pz, Palaeozoic rocks; Mz, Mesozoic rocks. Grey shaded areas indicate Tertiary plutons. Figure modified from Hildreth & Moorbath (1988)
Volcán San Pedro and xenoliths
The principal phases of volcanic construction at Volcán San Pedro (1·5 km3) are divided into a main cone-building stage comprising andesitic and dacitic lavas, and a late event triggered by sector collapse of the eastern flank. The latter comprises an explosive eruption that produced air-fall dacitic tephra (Singer & Dungan, 1992; Singer et al., 1995) followed by extrusion of a succession of lavas from the eastern flank: (1) 0·2 km3 of biotite–hornblende dacite containing up to 10% mafic xenoliths plus minor quenched mafic inclusions (QMI); (2) 0·5 km3 of two-pyroxene dacite with abundant QMI; (3) 0·1 km3 of two-pyroxene andesite with rare QMI. The last volcanic activity at this cone consisted of 0·2 km3 of basaltic andesitic magma erupted from the summit crater. The majority of the xenoliths are gabbroic (22 samples), although scarce granites and metamorphic rocks (hornfels) similar to exposed basement are also present. Small xenoliths are rounded to subrounded, but larger fragments (up to 45 cm in diameter) tend to be angular. The observation that the xenoliths are found exclusively in the first lava flow following structural failure of the east flank of Volcán San Pedro suggests that they are fragments of the conduits or upper parts of the margins of the San Pedro magma chamber that were shattered during the eruption (in a similar fashion to the 18 May 1980 Mount St. Helens eruption; Heliker, 1995).
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TEXTURES, AND MINERAL AND GLASS COMPOSITION OF THE XENOLITHS |
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Major and minor element compositions of minerals and glass were determined by electron microprobe (CAMECA SX-50; see the Appendix). Mineral names, structural formulae and end-members were determined following Morimoto et al. (1988)
for pyroxenes; Leake et al. (1997) for amphiboles; Rieder et al. (1998) and Deer et al. (1962) for micas; Deer et al. (1992) for olivine, plagioclase and apatite; and Stormer (1983) for spinel and ilmenite. Tables with the complete electron microprobe analyses of spinel, ilmenite, pyroxenes, olivine, plagioclase and glass can be downloaded from the Journal of Petrology Web site at http://www.petrology.oupjournals.org.
None of the samples shows evidence of low-temperature hydrothermal alteration. Partial melting along grain boundaries or post-entrainment modification as a result of interaction with the host lava are minimal. On the basis of textures and modal mineralogy (Table 1 and Fig. 2), we have divided the xenoliths into two groups [nomenclature after Streckeisen (1976) and LeMaitre (1989)]:
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- Group I are olivine–hornblende norites and melanorites (three samples) with interstitial glass bounded by euhedral crystal faces. These samples were probably dislodged from partly solidified crystal-rich zones of an active conduit or reservoir system.
- Group II have subsolidus exsolution and deformation textures (19 samples). These observations and 40Ar/39Ar data from two xenoliths (Fig. 3) indicate that they are fragments of the pre-Quaternary, plutonic basement of the volcano. These have been further subdivided into: (a) Group IICL, which are mainly clinopyroxene leuconorites; (b) Group IIHN, which are hornblende norites.
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Group I xenoliths: olivine–hornblende norites and melanorites
These samples consist of olivine, orthopyroxene, hornblende, plagioclase and phlogopite forming a medium-grained (1–5 mm) crystal network with interstitial dacitic to rhyolitic glass (SiO2 67–72 wt %; K2O 3·7–8·6 wt %) bounded by euhedral crystal faces suggesting that the glass is residual from crystallization and not due to partial melting (Fig. 4a). Cr-spinel (Ulv0·01–0·31; Cr2O3 10–18 wt %) occurs as inclusions in olivine, orthopyroxene, hornblende and rarely in plagioclase cores. Olivine (Fo86–76; NiO 
0·05-0·4 wt %) is typically resorbed and surrounded by hornblende, orthopyroxene and phlogopite that formed in reaction relationship with olivine (Fig. 4b). Less commonly, euhedral olivine showing no reaction is in contact with glass (Fig. 4a). Rare clinopyroxene (Wo46–45En46–44Fs9–11; Cr2O3 0·3–0·8 wt %) is also resorbed and has largely been replaced by hornblende. Plagioclase is euhedral, whether it occurs as free crystals or as inclusions within orthopyroxene, hornblende or phlogopite. Most plagioclase crystals consist of a normally zoned core (An86–78) surrounded by a normally zoned rim (An45–26) with an 35 An mol % gap between cores and rims (Fig. 5). The high Fo and NiO contents of olivine, the high Wo and Cr2O3 contents of clinopyroxene and the high An contents of plagioclase cores suggest that these minerals are near-liquidus crystallization products of a water-bearing basaltic magma (e.g. Gaetani et al., 1993; Sisson & Grove, 1993).
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Subhedral to euhedral orthopyroxene, hornblende (magnesiohastingsite) and phlogopite are late-crystallizing minerals, as they are commonly in contact with interstitial glass (Fig. 4a). Despite this, the three minerals are characterized by high mg-numbers [mg-number = 100Mg/(Mg + Fet) in mols, where Fet is total iron] ranging from 77 to 82, and Cr2O3 contents from <0·1 to 1·2 wt % (Tables 2–4). The Cr2O3 concentrations in these three minerals vary irregularly within and between crystals, and they are as high as, or higher than those of clinopyroxene (Fig. 6). This is in accord with the textural relations indicating that orthopyroxene, hornblende and phlogopite are the products of reactions between cumulus minerals (olivine, Cr-spinel and clinopyroxene) and liquid. Most phlogopite is characterized by Na2O contents (2–3·4 wt %; Table 4 and Fig. 7) that are higher than those for most biotite or phlogopite reported in the literature. Such high sodium contents have been interpreted by Costa et al. (2001) as the result of (1) crystallization from a liquid with high Na2O contents, (2) crystal-chemical effects, as incorporation of Na in biotite or phlogopite is enhanced by high Mg/Fe of the mica (Volfinger et al., 1985), and (3) the presence of a solvus between phlogopite and aspidolite (synonymous with the sodium phlogopite end-member; Rieder et al., 1998).
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On the basis of the data discussed above, we propose a two-stage crystallization sequence for these xenoliths: (1) Cr-spinel + olivine + clinopyroxene ± plagioclase crystallized from a water-bearing basaltic magma; (2) reaction occurred between the mafic minerals with an evolved water-rich liquid that produced hornblende + orthopyroxene + phlogopite, accompanied or followed by plagioclase + apatite crystallization. The question of whether these reactions are due to progressive closed-system crystallization or were triggered by ingress of an evolved water-rich liquid is addressed in the Discussion.
Group IICL xenoliths: clinopyroxene leuconorites with subsolidus textures
The majority of these samples are characterized by mosaic, seriate textures, with subsolidus textural re-equilibration along grain boundaries between plagioclase crystals, and between plagioclase and pyroxenes (i.e. constant grain boundary dihedral angles between crystals; Hunter, 1987). Plagioclase is dominantly normally zoned, and there are compositional modes at An85-80 and An60–55 (Fig. 5), which reflect that some samples have mainly anorthite-rich plagioclase (Hx14e, Hx14y) whereas in others, plagioclase is more albitic (Hx14a). Olivine (Fo81–72; NiO 0·25 wt %) is commonly anhedral and surrounded by rims of orthopyroxene, Fe–Ti oxide symplectites and occasionally by small flakes of phlogopite, which we refer to as late phlogopite (Table 1). Rare Cr-spinel inclusions (Ulv0·23; Cr2O3 18 wt %) are present in olivine. Anhedral to subhedral diopsidic to augitic clinopyroxene (Wo40–48En40–44Fs10–17) and orthopyroxene (mg-number 80–65) containing exsolution lamellae commonly occur as clusters interstitial to plagioclase crystals. Cr-poor magnetite (Ulv0·04–0·44) and ilmenite (Ilm0·74–0·96) are typically exsolved, and occur as euhedral inclusions in pyroxenes, or as anhedral oikocrysts surrounding anhedral plagioclase, pyroxenes and occasionally hornblende and phlogopite. Anhedral apatite is the only accessory mineral, and it occurs between grain boundaries of plagioclase crystals or inside phlogopite. Halogen contents and ratios in apatite are variable between samples (Table 5, Fig. 8), which suggests that they crystallized from melts with different halogen compositions (e.g. Boudreau, 1995).
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Subhedral to anhedral hornblende and phlogopite are late-crystallizing minerals, as they typically occur as small (<1 mm) poikilitic crystals surrounding resorbed, anhedral pyroxene, olivine and, in contrast to Group I xenoliths, partly resorbed plagioclase (Fig. 4c). Hornblende is commonly magnesiohastingite, but rare tschermakitic hornblende and magnesiohornblende are also present in sample Hx14y (Table 3). Hornblende and phlogopite have mg-numbers (Table 2) that overlap with those of pyroxene and olivine, whereas their Cr2O3 contents are typically higher (Fig. 6). As previously argued for Group I xenoliths, we interpret the high mg-numbers and Cr2O3 contents of the hydrous minerals as indications of reactions between Mg-rich, Cr-bearing minerals (olivine, pyroxenes and Cr-spinel) and water-rich evolved liquid, and not as evidence for crystallization from mafic magma. However, in contrast to Group I, plagioclase is resorbed in most xenoliths of this group. This could be explained by an increase in the water contents of the interstitial melt (e.g. Sisson & Grove, 1993). The Na2O contents of phlogopite are lower (1–2 wt %; Table 4 and Fig. 7) than those of Group I xenoliths, but are none the less higher than in most biotite or phlogopite analyses reported in the literature (Costa et al., 2001). Low proportions (6·5 vol. %) of rhyolitic glass (SiO2 71–74 wt %; K2O 5·7–6·7 wt %) are present exclusively along resorbed plagioclase–plagioclase, plagioclase–orthopyroxene and plagioclase–hornblende grain boundaries, indicating that glass is a secondary partial melting product that formed after xenolith entrainment rather than as a primary residual melt as is the case for Group I xenoliths.
Deformation and microfracturing
Plagioclase commonly shows bent twins and microcracks (Fig. 4c), whereas hornblende and phlogopite do not display textural evidence of deformation other than kink-bands in phlogopite. In many samples, discontinuous microfractures (0·5 mm in width; Fig. 4d) filled with Fe–Ti oxides, hornblende and phlogopite (and occasionally also orthopyroxene) cut across all minerals except poikilitic hornblende and phlogopite. Where microfractures intersect pyroxene–plagioclase contacts, both minerals are resorbed, and anhedral, and they are mantled by a rim of hornblende or phlogopite (Fig. 4d). Orthopyroxene, hornblende and phlogopite filling microfractures have mg-numbers and Cr2O3 contents that overlap with or are lower than those of poikilitic minerals (Table 2). These compositional and textural observations suggest that the microfractures hosted evolved water-bearing melts or aqueous fluids that reacted with pyroxenes, olivine and plagioclase to produce the poikilitic hydrous minerals with mg-numbers and Cr2O3 contents that are higher than those hosted by microfractures.
Group IIHN xenoliths: hornblende norites with subsolidus textures
Samples from this group are texturally heterogeneous and are characterized by large anhedral hornblende oikocrysts (
1 cm; Fig. 4e) that surround resorbed olivine (Fo79–78; NiO 0·20 wt %), diopsidic to augitic clinopyroxene (Wo42–48En45–47Fs6–11) and orthopyroxene (mg-number 81–77). Plagioclase (An88-80, occasionally An50) is also resorbed when it occurs as inclusions in hornblende. Oxide minerals are Cr-spinel (Ulv0·2; Cr2O3 9 wt %), magnetite (Ulv0·07–0·76) and ilmenite (Ilm0·89). Phlogopite is also present as oikocrysts that include resorbed olivine, orthopyroxene and plagioclase, and occasionally it also occurs inside hornblende oikocrysts (Fig. 4f). Rare anhedral apatite is present along plagioclase grain boundaries.
Late-crystallized hornblende (magnesiohastingite) and phlogopite have mg-numbers and Cr2O3 contents (Table 2) that overlap with, or are higher than those of olivine and pyroxenes (Fig. 6). As in Group IICL xenoliths, resorbed plagioclase inside hornblende could be explained by an increase in the water content of the interstitial melt before or during hornblende (and phlogopite) crystallization-reaction. Phlogopite has extremely high Na2O (1·5–5 wt %), approaching the composition of aspidolite (Table 4 and Fig. 7).
Plagioclase and orthopyroxene inside hornblende oikocrysts are deformed (Fig. 4e), whereas hornblende oikocrysts do not show textural evidence of deformation, and phlogopite occasionally shows kink bands. From these textural relations, we infer that deformation occurred before crystallization of the hydrous minerals.
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WHOLE-ROCK CHEMICAL COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGTEXTURES, AND MINERAL AND...WHOLE-ROCK CHEMICAL COMPOSITIONSDISCUSSIONIMPLICATIONS OF THE PRESENCE...CONCLUSIONSAPPENDIXREFERENCES |
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It is well established that the bulk-rock compositions of many plutonic rocks are not representative of liquids, but are the result of varying degrees of mineral accumulation or/and melt ± fluid migration (e.g. McBirney, 1995
). In the following sections we describe the major, minor and trace element compositions of the xenoliths with respect to the mean composition of 10 basalts that we think are representative of mafic liquids of the TSPC (see the Appendix for the analytical methods and precision of the analyses).
Group I xenoliths: olivine–hornblende norites and melanorites
Compared with the mean basaltic composition of the TSPC (Table 6), the xenoliths have higher MgO (20–21 wt %) and Ni (446–643 ppm), and lower SiO2 (46–47 wt %) and incompatible elements (e.g. K2O, Zr) (Fig. 9). This, together with the high modal proportions of olivine, suggests that the low incompatible element abundances are mainly due to accumulation of olivine. Ratios of elements that are not affected by olivine accumulation, such as P/Zr (9–10), or Rb/Y (1·1–1·5) fall within the range of TSPC basalts (Fig. 10).
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Group IICL xenoliths: clinopyroxene leuconorites with subsolidus textures
The major element compositions of most of these xenoliths are comparable with those of high-alumina basalts typical of subduction zones (e.g. Gust & Perfit, 1987), with 49–52 wt % SiO2, 5·2–8·2 wt % MgO and 17·2–19 wt % Al2O3 (Table 6). Samples Hx14e, Hx14h and Hx14y stand out from the rest by their lower concentrations of SiO2 (46–47 wt %), and generally higher MgO (7·6–10·7 wt %) and Al2O3 (21·8–23 wt %) (Fig. 9). Minor and trace element abundances are highly variable. Most xenoliths have concentrations of incompatible elements (e.g. K2O, Zr, Y) that range from those of the TSPC basalts to much lower values (Fig. 9). Concentrations of compatible elements (e.g. Sr and Ni) of most xenoliths are, however, within the ranges defined by TSPC basalts (Fig. 9), and thus their low incompatible element concentrations cannot be solely explained by plagioclase or olivine accumulation, and suggest that loss of interstitial melt rich in incompatible elements is a more plausible explanation. Some xenoliths show a positive Eu anomaly (Eu/Eu* up to 1·75) when normalized to primitive mantle (McDonough et al., 1992). This could be due to plagioclase accumulation, although it could also occur if the parent liquid initially had a positive Eu anomaly or, as will be discussed later, by loss of interstitial liquid with a negative Eu anomaly. Samples Hx14e, Hx14h and Hx14y also have low concentrations of some incompatible elements (e.g. Y, Zr), although K2O (0·34–2·48 wt %) and Rb (10–80 ppm) are highly variable. Their Ni and Sr concentrations are within the values or higher than those of the TSPC basalts, and thus accumulation of olivine + plagioclase may be a contributing factor for the low abundances of some incompatible elements in these three xenoliths. Ratios of incompatible elements (e.g. K/P, P/Zr, Rb/Y) of all xenoliths of this group are highly variable, ranging from much higher to lower than those of the TSPC basalts (Fig. 10). Such a large range of incompatible element ratios further indicates that processes other than mineral accumulation (e.g. migration of interstitial melt and aqueous fluids) have contributed to bulk compositional variations among the xenoliths.
Group IIHN xenoliths: hornblende norites with subsolidus textures
Compared with the mean basaltic composition of the TSPC these samples have low concentrations of SiO2 (45 wt %) and incompatible elements (e.g. K2O, Zr), and high MgO (13·3–16·5 wt %), Ni (226–335 ppm) and Ca/Na values (5·2–6·3), and thus olivine and plagioclase accumulation could partly explain their low incompatible element abundances (Fig. 9). However, ratios of incompatible elements such as K/P (7–25) and Rb/Y (2–3) range from those of the TSPC basalts to higher (Fig. 10), suggesting that apart from mineral accumulation, migration of interstitial melt and fluids are important processes for understanding the petrogenesis of these xenoliths.
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DISCUSSION |
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Melt migration and reaction in Group I xenoliths
Textural relations and compositions of orthopyroxene, hornblende and phlogopite (e.g. high mg-number and Cr2O3 contents) indicate that they are probably the result of reactions between early-crystallized mafic minerals (Cr-spinel, olivine and clinopyroxene) and an evolved, water-rich liquid. Reactions between clinopyroxene or olivine and liquid to produce hornblende, and between olivine and liquid to produce orthopyroxene have been reported in low-pressure (
0·3 GPa) crystallization experiments using basaltic to andesitic starting compositions (e.g. Holloway & Burnham, 1972; Heltz, 1973; Sisson & Grove, 1993; Grove et al., 1997; Moore & Carmichael, 1998). However, co-crystallization of hornblende, orthopyroxene and phlogopite in reaction relationship with olivine or clinopyroxene has never been reported. In particular, the high Na2O contents of the phlogopite are an uncommon compositional feature, which points to a more complex differentiation history for these xenoliths (Costa et al., 2001). Recent experimental work (Prouteau et al., 2001; Costa et al., in preparation) involving interactions between evolved water-rich liquids and mafic minerals (e.g. forsteritic olivine) have co-crystallized hornblende, orthopyroxene and phlogopite as reaction products, and thus an alternative possibility to closed-system crystallization is that these minerals in the San Pedro xenoliths are due to ingress of a differentiated melt (e.g. dacitic) by displacement of the mafic interstitial liquid in equilibrium with olivine, clinopyroxene and high-anorthite plagioclase. This process could explain several textural and mineralogical features of the xenoliths such as the compositional gap between plagioclase cores (An85–70) and rims (An45–20) [a detailed discussion of trace element composition of plagioclase has been given by Costa (2000)], and the coexistence of forsteritic olivine and rhyolitic glass, both of which are atypical of closed-system crystallization (e.g. Brophy et al., 1996), and are more characteristic of mixing or mingling between felsic and mafic magmas (e.g. Feeley & Dungan, 1996). In the next section we derive some constraints on the amount and composition of the reacting liquid.
Mass-balance constraints on the amount and composition of the reactive melt
To test the hypothesis of melt migration, we have undertaken least-squares mass-balance calculations using the composition of the cumulus minerals (high-anorthite plagioclase, clinopyroxene, Cr-spinel and olivine) and a representative dacitic composition of Volcán San Pedro as an analogue for the replacive interstitial melt. The results show that (Table 7): (1) the residuals (R2) are low (<1) and thus do not preclude the melt migration hypothesis; (2) before reaction the three xenoliths consisted of large amounts of olivine (42–45 wt %), so that for sample Hx14n the calculated amount of olivine before reaction is more than twice the observed amount; (3) the amount of clinopyroxene that was consumed (4·5–6·5 wt %) is also significant; (4) the quantity of reactive liquid is almost the same for all three xenoliths (30–33 wt %). As an alternative means of obtaining a rough estimate of the amount of interstitial melt that could have been displaced, we have used the proportions of post-cumulus minerals (hornblende, orthopyroxene, phlogopite, plus plagioclase rims) and glass present in the xenoliths. However, as hornblende, orthopyroxene and phlogopite are the products of reactions that consumed liquid and minerals, their observed modal abundances do not directly correspond to the porosity at the time of melt migration. As an approximation, the proportions of liquid and minerals that participated in the reactions were taken from the literature: for the hornblende reaction we have used the stoichiometry (wt %) determined by Sisson & Grove (1993): 100 Hbl = 22 Ol + 38 Cpx + 42 liquid. For the orthopyroxene reaction we have estimated the stoichiometry (wt %) suggested by Kelemen (1990): 100 Opx = 60 Ol + 40 liquid. No experimentally determined stoichiometry for the phlogopite reaction was found in the literature, so we assumed that the quantity of liquid consumed by this reaction equals the observed modal proportion of phlogopite. Lastly, we have estimated that plagioclase rims are one-third of the plagioclase in the xenoliths. The calculations (Table 8) give similar results to those obtained from mass-balance constraints, i.e. (1) large amounts of olivine and clinopyroxene were consumed by the reactions and (2) the amount of liquid that was consumed is 32–33 wt %. Although the calculations might not be very accurate because the stoichiometry of the reactions depends on the liquid composition, it illustrates how estimating the amount of ‘trapped melt’ or porosity in cumulate rocks by considering only the amount of post-cumulus minerals could be misleading if there are reactions involved. For example, if we take the amount of post-cumulus minerals and glass as representative of ‘trapped melt’ the values vary between 44 and 58 wt %, up to 1·8 times higher than the calculated amount (32–33 wt %).
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Textural, mineralogical and compositional evidence for melt and fluid migration in Group II xenoliths
The low concentrations of incompatible elements (e.g. Zr) of most xenoliths are not correlated with high concentrations of compatible elements (e.g. Sr, Ni), suggesting that loss of evolved interstitial liquid rather than mineral accumulation is responsible for the low incompatible element abundances. Only six samples have major and trace element abundances that could be partly explained by accumulation of plagioclase (Hx14w) or plagioclase + olivine (Hx14e, Hx14h, Hx14v, Hx14y and Hx14z) (Fig. 9). However, the large range of P/Zr values cannot simply be the result of melt loss from a crystal pile consisting of plagioclase, pyroxenes or olivine, as the partition coefficients of P and Zr in these minerals are <0·1 (e.g. Rollinson, 1993). Thus, we propose that expulsion of interstitial liquid occurred both before and after apatite crystallization. Samples with high P/Zr lost melt after apatite crystallization, whereas xenoliths with P/Zr values within the range of the TSPC basalts (but with low Zr and P2O5 concentrations) lost melt before apatite crystallization (Fig. 10). Xenoliths that lost melt before apatite crystallization are commonly those that show positive Eu anomalies, which suggests that the Eu anomalies could be due to loss of interstitial liquid rich in REE with a negative Eu anomaly.
The large ranges of ratios of incompatible elements involving Rb and K suggest that an aqueous fluid has also modified the bulk-rock composition of some xenoliths. For example, many samples have Rb/Y values similar to or lower than those of the TSPC basalts, whereas others have very high Rb/Y. As Y is highly compatible in apatite (e.g. Pearce & Norry, 1979), the low Rb/Y values of some xenoliths could be explained by loss of interstitial liquid after apatite crystallization. However, the high Rb/Y of other xenoliths suggests that they have gained Rb with respect to Y. Decoupling of K and Rb from the rest of incompatible elements can be produced by the involvement of an aqueous fluid phase, as fluid–melt partition coefficients of K and Rb are much higher than those of Y and Zr (e.g. Keppler, 1996). Accordingly, the high Rb and K2O concentrations and the high Rb/Y (or low P/Rb) of some xenoliths (Fig. 10) could be explained by the arrival of an aqueous fluid phase rich in alkalis (e.g. K, Rb, Na) that dissolved into the remaining melt. Further evidence for the arrival of a fluid is the variable halogen contents of apatite. Fluid addition to a melt can be recorded as high Cl/F in apatite (e.g. Boudreau & McCallum, 1989) because Cl tends to partition into the fluid phase, whereas F remains in the melt (e.g. Candela & Piccoli, 1995; Villemant & Boudon, 1999). Apatite from one xenolith (Hx12a) has low Cl/F (Fig. 8), whereas the rest have much higher Cl/F, suggesting that it has crystallized from melts enriched in Cl by fluid addition. Because it is not apparent from Fig. 9, it is worth noting that samples that lost melt before apatite crystallization also have apatite with high Cl/F (e.g. Hx14y and Hx14e), suggesting that fluid arrival might have post-dated melt migration.
The bent laths and microcracks displayed by the plagioclase and the microfractures filled with hornblende, phlogopite, orthopyroxene and magnetite (Fig. 4c–e) are interpreted as textural evidence of expulsion of interstitial liquid by compaction of a crystal-rich magma. Microfractures commonly cut across bent plagioclase crystals, suggesting that rock deformation changed from ductile to brittle (e.g. Kronenberg & Shelton, 1980), or that the behaviour of plagioclase changed from plastic to cataclastic (e.g. Hacker & Christie, 1990). In many samples poikilitic hornblende contains plagioclase with bent twins (Fig. 4c and e). Thus, deformation of the crystal pile and expulsion of interstitial liquid seem to have occurred before arrival of aqueous fluids. Perhaps during initial ductile deformation of the crystal matrix, melt migrated through the pore spaces, and later, during brittle deformation, melt migration occurred mostly through microfractures. The remaining interstitial melt was enriched by aqueous fluids (and alkalis), which caused dissolution of olivine, pyroxenes and plagioclase and crystallization of poikilitic hornblende and Na-rich phlogopite with high mg-numbers and Cr2O3. The available stable isotope analyses (sample Hx14h, whole rock: 
18O = 5·4; sample Hx14v: hornblende, 18O = 5·3; bulk rock, D = -62, all values relative to SMOW; B. S. Singer, unpublished data, 1993) suggest that the fluids that fluxed the xenoliths were magmatic and not meteoric (e.g. Taylor & Forester, 1979).
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IMPLICATIONS OF THE PRESENCE OF HORNBLENDE AND PHLOGOPITE IN SUBDUCTION-RELATED GABBROIC ROCKS |
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Most of San Pedro gabbroic xenoliths have significant proportions of hornblende and phlogopite, either as small crystals filling microfractures, or as large poikilitic post-cumulus crystals that can make up >50 vol. % of the rock. In this respect they are not unusual, and a survey of the literature shows that the majority of subduction-related gabbroic xenoliths and plutons have important amounts of hornblende, and occasionally phlogopite. In some localities, at least one generation of hornblende is an early-crystallizing mineral (Ulmer et al., 1983
, Adamello batholith, Italy; Sisson et al., 1996, hornblende gabbro sill, California), whereas in others it is a late phase in a reaction relation with other minerals (Smith et al., 1983, Peninsular Ranges batholith, California; Regan, 1985, Coastal batholith of Peru; Whalen, 1985, Uasilau–Yau Yau Intrusive Complex, New Britain; Himmelberg et al., 1987, Yakobi intrusion, Alaska; Beard & Day, 1988, Smartville Complex, California; DeBari & Coleman, 1989, Tonsina Complex, Alaska; Springer, 1989, Pine Hill Complex, California; Kepezhinskas et al., 1993, Kamchatka; DeBari, 1994, Fiambalá intrusion, Argentina; Tepper, 1996, Chilliwack batholith, Washington; Roberts et al., 2000, Quérigut Complex, French Pyrenees). The high proportions of hydrous minerals in gabbroic plutons and xenolith suites is in marked contrast with the rare occurrences of hornblende phenocrysts in basalts or basaltic andesites in arc volcanoes world wide (Sigurdsson & Shepherd, 1974, Kick’em-Jenny Volcano, Lesser Antilles; Arculus, 1976, Grenada, Lesser Antilles; Arculus et al., 1976, Bogoslof Volcano, Alaska; Luhr & Carmichael, 1985, Cerro la Pilita, Mexico; Peterson & Rose, 1985, Ayarza caldera, Guatemala; Rose, 1987, Santa María Volcano, Guatemala). Apart from the possibility that the hydrous minerals are produced by reactions triggered by protracted closed-system crystallization, the petrological, mineralogical and geochemical characteristics of the San Pedro xenoliths suggest that the large proportions of hornblende with high mg-numbers and Cr2O3 in gabbroic rocks can be the result of reactions between early-crystallized refractory minerals (olivine, Cr-spinel, pyroxenes and plagioclase) and evolved melt ± aqueous fluids that percolate through crystal-rich mafic magmas, and do not necessarily imply hornblende crystallization from water-rich mafic magmas. These melts and aqueous fluids could be derived from within the cumulate pile of the intrusion, as has been documented for the Skaergaard intrusion (e.g. McBirney, 1995) or from broadly contemporaneous felsic magmas (e.g. Sha, 1995). Interactions between partially solidified mafic cumulate piles and invading water-rich silicic magmas may provide a means of stabilizing substantially larger quantities of amphibole and mica (e.g. >50 % mode) in subvolcanic reservoirs or plutons than closed-system evolution of either mafic or silicic magmas. Partial melting and recycling of these hydrous and alkali-enriched plutonic facies could have important implications for magma evolution at long-lived subduction-related volcanic centres. Assimilation of partially or completely solidified metasomatically modified cumulates could lead to complex and apparently uncoupled major and trace element trends in the affected lavas (e.g. Upper Placeta San Pedro sequence of the TSPC, Dungan et al., 2001). Partial melting of such gabbroic plutons could also lead to the generation of second-stage silicic magmas (e.g. Feeley et al., 1998). |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGTEXTURES, AND MINERAL AND...WHOLE-ROCK CHEMICAL COMPOSITIONSDISCUSSIONIMPLICATIONS OF THE PRESENCE...CONCLUSIONSAPPENDIXREFERENCES |
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A detailed petrographic and compositional study of two groups of gabbroic xenoliths from the Tatara–San Pedro Complex has shown that these xenoliths record multistage crystallization histories involving migration of melts and aqueous fluids. Reaction of early-crystallized refractory minerals (Cr-spinel, olivine, pyroxenes and plagioclase) with percolating, evolved melts and aqueous fluids triggered crystallization of significant proportions of hornblende and Na-rich phlogopite with compositional signatures (e.g. high mg-numbers and Cr2O3) that might otherwise be attributed to crystallization from primitive basaltic magmas. This implies that the much higher abundance of mafic hydrous minerals in subduction-related gabbros compared with arc basalts or basaltic andesites may be explained by the complex differentiation histories of plutonic crystal-rich magmas.
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APPENDIX |
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Electron microprobe analyses
Analyses were performed with a Cameca SX-50 instrument at the University of Lausanne. Analyses were carried out using a wavelength-dispersive system, with an operating voltage of 15 kV. Beam current was 7 nA for glass, 15 nA for plagioclase, hornblende and phlogopite, 20 nA for olivine and pyroxenes, 25 nA for spinel and ilmenite, and 30 nA for apatite. Beam diameter was
1 µm, except for phlogopite (2 µm) and for glass (5–10 µm).
X-ray fluorescence analyses
Major and trace elements were determined at the University of Massachusetts by standard techniques (see Rhodes, 1988). The 2
relative precisions are as follows: SiO2, 0·6%; TiO2, 0·5%; Al2O3, 1%; Fe2O3, 0·5%; MnO, 6%; MgO, 1·2%; CaO, 0·6%; Na2O, 5%; K2O, 1·5%; P2O5, 4·5%. Rb, 10%; Sr, 1%; Zr, 1%; Nb, 6%; La, 4%; Ce, 12%; Y, 2·5%; V, 2%; Cr, 1·8%; Ni, 2%; Zn, 1%.
Inductively coupled plasma-atomic emission spectrometry analyses
Rare earth element analyses were performed at the University of Geneva. Details of the methods have been given by Voldet (1993). The relative 2 precisions range between 5 and 10% depending on the concentration of the element.
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ACKNOWLEDGEMENTS |
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We would like to thank A. Wulff and M. Rhodes for the X-ray fluorescence analyses, P. Voldet for the inductively coupled plasma atomic emission spectrometry analyses, F. Ramos for the thermal ionization mass spectrometry analyses, Y. Vinzce and T. Thon-That for the 40Ar/39Ar analyses, and F. Parat for the electron microprobe analyses of apatite. We are also grateful to J. Barclay for making available to us her unpublished experimental data and for discussions about occurrences of hornblende-bearing mafic lavas. Reviews of an earlier version of the manuscript by J. Davidson, A. McBirney and P. Ulmer are acknowledged. Comments by Tom Sisson and Richard Arculus helped to clarify our views on hornblende stability in subduction-related mafic magmas. Fieldwork was supported by a grant from the Swiss Academy of Natural Sciences, and by research grants 20-42124-94 and 20-49730-96 of the Swiss National Fonds.
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FOOTNOTES |
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Extended dataset can be found at http://www.petrology.oupjournals.org
*Corresponding author. Present address: ISTO, 1A rue de la Ferollerie, 45071 Orleans, France. Telephone: +33-238255213. Fax: +33-238636488. E-mail: costaf{at}cnrs-orleans.fr
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