The Piedras Grandes Soncor Eruptions, Lascar Volcano, Chile; Evolution of a Zoned Magma Chamber in the Central Andean Upper Crust

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1 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 PAGES The Piedras Grandes Soncor Eruptions, Lascar Volcano, Chile; Evolution of a Zoned Magma Chamber in the Central Andean Upper Crust S. J. MATTHEWS 1, R. S. J. SPARKS 1 AND M. C. GARDEWEG 2 1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, QUEENS ROAD, BRISTOL BS8 1RJ, UK 2 SERVICIO NACIONAL DE GEOLOGIA Y MINERIA, AVENIDA SANTA MARIA 0104, CASILLA 10465, SANTIAGO, CHILE RECEIVED MAY 1, 1998; REVISED TYPESCRIPT ACCEPTED JUNE 7, 1999 Stage II of Lascar Volcano, Chile, involved development of an The Soncor zoned magma chamber developed in the upper crust at andesite to dacite volcanic complex and associated hypabyssal about 6 km depth as a result of repeated influx of hydrous mafic porphyry intrusions above the main magma chamber. The system magmas. The magmas in the chamber evolved by open-system culminated in development of a large zoned magma chamber that fractionation with magma mixing being important in the less evolved erupted in the large-magnitude (8 km 3 ) Soncor explosive eruption magmas. Repeated influxes of hydrous mafic magmas resulted in at 26 5 ka, forming a Plinian pumice deposit and ignimbrite. Ventponents to the chamber. This produced complex histories of individual convective stirring and addition of heat, volatiles and mafic com- derived lithic clasts in the Soncor deposits sample the pre-existing Stage II complex. The Piedras Grandes hornblende andesite unit crystals and heterogeneous character of phenocrysts in individual represents a pre-soncor dome complex. The andesite consists samples. Halogen contents of amphibole, biotite, apatite and glass inclusions, S contents of glass inclusions, stabilization of anhydrite of a heterogeneous phenocryst assemblage of plagioclase in the silicic magmas, and mass balance calculations imply major amphibole orthopyroxene oxides and minor biotite, clinopyroxene, transfer of volatile components from the hydrous mafic magmas into quartz, apatite, anhydrite and olivine together with commingled the interior of the zoned chamber in the form of a co-magmatic basaltic andesite inclusions and streaks. Temperature estimates from fluid phase. zoned orthopyroxenes and Fe Ti oxides and disequilibrium between phenocrysts indicate an origin by remobilization and remelting of an igneous protolith by influx of hydrous mafic magmas so that the andesite is a mixture of partial melt, restite crystals, mafic components and phenocrysts. More silicic Stage II rocks are also interpreted as KEY WORDS: andesite; Andes; magma mixing; compositional zoning partial melts with entrained restite. The zoned Soncor chamber contained dacite (67 wt % SiO 2 ) to silicic andesite (61 wt % SiO 2 ) crystal-rich magmas with an assemblage of plagioclase orthopyroxene clinopyroxene oxides with minor biotite, amphibole, INTRODUCTION apatite, zircon, anhydrite, pyrrhotite and olivine. Hornblende-rich The Soncor ignimbrite and its associated Plinian pumice mafic andesite pumice from late flow units in the ignimbrite provides fall deposit were formed by the largest magnitude exevidence for influxes of hydrous mafic magmas at the base of the plosive eruption of Lascar Volcano, Chile (23 22 S, chamber at sufficient depths to stabilize amphibole. The hydrous W). This eruption took place at 26 5 ka and ejected mafic magmas are interpreted to have evolved in the lower crust by a minimum of 8 km 3 of magma (Gardeweg et al., 1998). high-pressure fractionation with some lower-crustal contamination. The Soncor deposits are heterogeneous, with juvenile Corresponding author. Steve.Sparks@bristol.ac.uk Oxford University Press 1999

2 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 ejecta ranging from dacite (67 6% SiO 2 ) to basaltic ande- four stages in its history. A brief synopsis of this evolution site (56 1% SiO 2 ). The Soncor deposits are complexly is given as a context for the development of the Piedras zoned. The initial Plinian deposit is predominantly com- Grandes and Soncor magma system. posed of silicic andesite and dacite pumice, and there is Stage I began <43 ka ago with the formation of a an upper zone of more diverse pumice including banded stratocone on the eastern end of the present complex, pumice and more mafic compositions. The following and the eruption of two-pyroxene andesite lavas (55 ignimbrite is mostly composed of dacite and silicic ande- 64 7% SiO 2 ) and coarse-grained pyroclastic flow deposits. site pumice, but late flow units contain mafic andesite The Stage I products are preserved as the remnants of scoria and heterogeneous porphyritic amphibole-rich a steep andesite stratocone and as flank lavas and pumices. pyroclastic rocks. Dacite xenoliths in various stages of The evolution of the magmatic system to generate the melting and disaggregation are observed in andesite zoned magma chamber sampled in the Soncor eruption ejecta from younger pyroclastic flow deposits of Stage has been investigated by study of pre-soncor volcanic I. The Stage I products were significantly eroded before rocks, including remnants of the pre-existing volcanic Stage II. complex and lithic clasts within the Soncor deposits that Stage II involved the development of a silicic andesite were vent derived. A sequence of block-and-ash flow dome complex to the west of the Stage I stratocone. deposits and flood deposits, termed the Piedras Grandes Associated block-and-ash flow deposits and deposits from unit, consists of hornblende andesite ( % SiO 2 ) a large flood event are preserved to the west of the with minor basaltic andesite streaks and inclusions (56 volcano. This unit, known as the Piedras Grandes unit, wt % SiO 2 ). This unit represents a dome complex, which preceded the Soncor eruption. Vent-derived lithic clasts developed before the climactic Soncor eruption. Vent- in the Soncor deposits also indicate the existence of derived lithic blocks included in the Soncor deposits domes and shallow-level intrusive rocks of rhyodacite to range in composition from basaltic andesite (56% SiO 2 )to basaltic andesite composition in the Stage II structure. dacite porphyry (68% SiO 2 ). These blocks are commonly The Soncor eruption produced an initial Plinian deposit prismatically jointed, implying high pre-eruption tem- and a coarse-grained pumice- and lithic-rich non-welded peratures. Their textures and compositions indicate that ignimbrite, which extends up to 27 km from Lascar they originate from pre-soncor hypabyssal intrusions and (Calder et al., 1999). Accelerating mass spectrometer dome complexes. (AMS) radiocarbon analysis yields an age of ± This paper documents the petrology and geochemistry 0 5 ka for the eruption. of the Soncor deposits, included lithic blocks from the Stage III built an andesitic to dacitic stratocone over pre-eruption complex, and the Piedras Grandes units, and the source area of the Soncor eruption. Stage IV began demonstrates their petrogenetic coherence. The study with a major basaltic andesite to andesitic pyroclastic provides insights into how large zoned magma bodies flow eruption and formation of a small (1 5 km diameter) are generated within the upper crust of the Central summit caldera at 9 2 ka. Gardeweg et al. (1998) in- Andes. The Piedras Grandes magmas are interpreted in terpreted this pyroclastic flow deposit as the last products terms of remobilization and reheating of a solidified or of Stage III. However, the compositions of this eruption semisolid andesitic source region by influxes of basaltic are much more similar to the Stage IV lavas and ejecta andesite. This solid protolith represents the pre-existing and there is a long period of dormancy of Lascar (at high-level Lascar magma chamber, which cooled and least 6 ka) between Stage III products and this deposit. crystallized following a long hiatus in activity. We present Stage IV included an andesitic lava flow at 7 1 ka and a model for the development of the Soncor magmas from the formation of three deep collapse craters in remnants the Piedras Grandes magma by progressive mobilization of the Stage I edifice. Present eruptive activity is centred of the protolith and interactions between mafic magmas on the westernmost crater of Stage IV. Since 1984, and the products of partial melting of the protolith, activity has consisted of lava dome extrusion, explosive culminating in the development of a compositional strat- eruptions and vigorous degassing, culminating in the ified magma chamber in the upper crust April 1993 explosive eruption, which involved 0 1 km 3 of andesitic magma in the form of pumice flow and tephra fall deposits (Gardeweg & Medina, 1994; Evolution of Lascar Volcano Matthews et al., 1997). The petrology and geochemistry of Lascar volcanic Lascar Volcano, Chile (23 22 S, W, 5580 m) is rocks were described by Matthews et al. (1994b), who the most active volcano of the Andean Central Volcanic attributed the petrological and geochemical features of Zone. Lascar is a stratocone elongated along a WSW Lascar magmas to combined fractional crystallization ENE trending lineament. The evolution of the volcano and magma mixing as a result of periodic replenishment was described by Gardeweg et al. (1998), who recognized and reheating of a steadily evolving magma chamber by 1892

3 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO basaltic andesite influxes. Two types of shallow-level largely composed of white, compositionally heterogeneous, (subvolcanic) magma chambers were distinguished. First, two-pyroxene andesitic to dacitic pumice (62 4 andesitic magma chambers received frequent mafic 67 3% SiO 2 ). The uppermost part of the Plinian deposit magma influxes and underwent efficient magma mixing contains a similar variety of pumice compositions, ranging and vigorous convection. The products of these chambers from dacitic pumice (66 5% SiO 2 ) to darker, more silicapoor are two-pyroxene andesite lavas and pyroclastic flow silicic andesitic pumice (61 1% SiO 2 ), and comare deposits with limited petrological and compositional vari- positionally banded pumice is prominent. The immediately ability but marked disequilibrium textures. Second, more overlying ignimbrite flow units only contain evolved, dacitic, volatile-rich magma chambers were re- white dacitic pumice. However, the ignimbrite, which mobilized by a mafic influx following a long hiatus. This consists of numerous flow units, is compositionally zoned. second group, characterized by the Piedras Grandes and Late flow units contain a diverse assemblage of pumice Soncor magmas, is the subject of this paper. compositions ranging from dacite to mafic andesite (56% SiO 2 ) and with compositionally banded pumice. There are also amphibole-rich, juvenile, less-vesicular crystalrich THE PIEDRAS GRANDES AND pumice clasts. The pattern can be described as double zoning (Smith, 1979; Hildreth, 1981). The Plinian SONCOR DEPOSITS deposit and ignimbrite are each normally zoned. A The Piedras Grandes unit consists of monomict horn- compositional reversal at the boundary between the blende andesite block-and-ash flow deposits related to Plinian deposit and ignimbrite has previously been de- dome-collapse events. A more distal facies, occurring to scribed only in the Cape Riva deposit, Santorini (Druitt, 10 km distance on the western flanks, consists of thin 1985). (<1 m) fluvial sands and gravels with large (up to 15 m) boulders with chilled, glassy selvages and radial cooling joints. This facies was interpreted by Gardeweg et al. (1998) as a jokulhlaup deposit, produced as a result of PETROLOGY flash-melting of a summit glacier by the growing dome The Piedras Grandes unit complex. The Piedras Grandes unit consists of dominantly hornblende The Soncor eruption produced a minimum of 15 km 3 silicic andesite ( % SiO 2 ) with minor of pyroclastic deposits [8 km 3 dense rock equivalent basaltic andesite bands and inclusions (up to 5 cm in (DRE)]. The initial Plinian pumice fall deposit extends diameter). Individual blocks show multiple, parallel, bato the ESE of the volcano, with a thickness of 18 m on saltic andesite bands 5 10 cm across. The andesite is the southern flank and 2 m at a distance of 20 km. poorly vesicular and porphyritic (42 47 vol. % pheno- The ignimbrite is non-welded and coarse grained, with crysts; Table 1; Fig. 1). Phenocrysts are anhedral to abundant lapilli- to block-sized lithic clasts and pumices. euhedral. Plagioclase is the dominant phenocryst phase. There are also pumice-rich facies and proximal coarse Amphibole is the main ferromagnesian phase and ranges lithic-rich lag breccias. The ignimbrite was emplaced from fresh euhedral crystals to those with thin reaction dominantly in quebradas and valleys to the W, NE and rims of pyroxenes, plagioclase and Fe Ti oxides (Fig. 1) SE of the volcano. Vent-derived lithic clast types include to complete pseudomorphs. Minor phenocrysts of or- Stage I lavas, Piedras Grandes hornblende andesite, a thopyroxene, augite, Fe Ti oxides and accessory apatite dacitic porphyry and hypabyssal andesitic intrusive rocks. are present. Rare biotite crystals are commonly pseudo- Prismatic jointing in some of the lithic types indicates morphed by a fine-grained aggregate of pyroxenes, that they were at high temperatures at the time of plagioclase and Fe Ti oxides. Olivine crystals occur the eruption. The high lithic content in the ignimbrite sparsely and are partially or wholly pseudomorphed (average of ~50 wt %) is attributed to the destruction of by a vermicular orthopyroxene magnetite symplectite. the pre-existing Stage II volcanic complex, deep explosive Anhydrite and scarce Fe Cu sulphide and anhydrite cratering into the intrusive interior of the volcano, and inclusions occur in Fe Ti oxide minerals. The ground- erosion during pyroclastic flow emplacement. Criteria mass consists of microphenocrysts of plagioclase, pyroxenes for distinguishing vent-derived lithic clasts from those and Fe Ti oxides set in a matrix of rhyolitic glass entrained in the pyroclastic flows have been given by (Fig. 1). Calder et al. (1999). Vent-derived lithologies in the Plinian The mafic component is represented by a moderately deposit and proximal co-ignimbrite lag breccias are read- phyric (12 vol. % crystals; Table 1) basaltic andesite band ily distinguished from lithic clasts eroded from the ground. (LAS451a). The main phenocryst phases are plagioclase, The Soncor deposits exhibit both large-scale com- olivine with Cr-spinel inclusions and augite with minor positional zoning and strong compositional heterogeneity orthopyroxene, Fe Ti oxides and accessory apatite. Oc- within individual units. The initial Plinian deposit is casional biotite and amphibole crystals are partially to 1893

4 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Table 1: Modal analyses of samples from completely pseudomorphed as in the host andesite. The matrix is a fine-grained groundmass of plagioclase, pyrthe Piedras Grandes flow deposit, quoted on oxenes and Fe Ti oxides. a vesicle-free basis Plagioclase phenocrysts in the andesite are euhedral, up to 2 mm in length (Fig. 1) and range from An 30 to Sample: LAS451a LAS451b LAS61 LAS62 An 71 with both normal and reverse zoned crystals (Fig. 2). Description: Basaltic Hornblende Hornblende Hornblende In the basaltic andesite the compositional range of most Matrix andesite 88 3 andesite 67 4 andesite 64 6 andesite 63 0 phenocrysts is An 38 52, although one crystal with a calcic core (An 85 ) was found. Pyroxene phenocrysts in the andesite are subhedral to euhedral and up to 1 mm in Plagioclase length. Orthopyroxene phenocrysts in four samples of Orthopyroxene andesite have a restricted compositional range (mg-num- Clinopyroxene ber = ; Fig. 3) show no zoning of mg-number, Olivine apart from two crystals with a thin (10 μm) more mag- Hornblende trace nesian overgrowth (mg-number = 0 72) and a magnesian Biotite 0 2 trace inclusion (mg-number = 0 81) in an amphibole pheno- Pseudomorphs cryst. One sample of the andesite contains orthopyroxene with a wider compositional range (mg-number = 0 68 Fe Ti oxides ) with both normal and reverse zonation. Or- Quartz trace thopyroxene in a basaltic andesite band has a Apatite trace trace trace trace compositional range similar to that in the host andesite Total (mg-number = ). Augite phenocrysts in the % Vesicularity andesite (mg-number = ), and a basaltic andesite band (mg-number = ) are normally zoned. Olivine phenocrysts in the basaltic andesite are normally zoned, with core compositions of Fo and rim compositions of Fo Fig. 1. Photomicrograph (crossed polars) of Piedras Grandes andesite, showing subhedral to euhedral, zoned plagioclase phenocrysts, both fresh and slightly reacted hornblende phenocrysts, and small, rounded orthopyroxene phenocrysts, in a glassy matrix containing plagioclase, orthopyroxene and hornblende microphenocrysts. Polars slightly uncrossed to show matrix texture. Field of view on long axis ~2 mm. 1894

5 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Fig. 2. Plagioclase compositions in Piedras Grandes and Soncor rock types and Lascar Stage II biotite porphyries. Xenolith in a porphyry sample denotes coarse groundmass alkali feldspar between quartz xenocrysts. Amphibole phenocrysts (up to 2 mm in length) fall Hornblende andesite pumice clasts ( % SiO 2 ) mainly in the fields of tschermakite and tschermakitic from late ignimbrite flow units contain phenocrysts hornblende (Leake, 1968). The amphibole phenocrysts (32 39 vol. %, calculated vesicle free) of plagioclase, show a wide range of mg-number from 2 9 to 3 5 in amphibole, orthopyroxene, clinopyroxene, Fe Ti oxides individual samples. Amphibole phenocrysts in the basaltic and minor biotite (see Fig. 6b, below) in a low-silica andesite are relatively Al poor and Si rich with a restricted rhyolitic glass matrix (71 74% SiO 2 ). One sample range of mg-number (Fig. 4). Occasional amphibole crys- (LAS191) has a high-silica rhyolitic glass matrix (76 78% tals similar in composition to those in the andesite are SiO 2 ). Crystal clots, involving various combinations of interpreted as xenocrysts from the andesite. Biotite crys- amphibole, plagioclase and orthopyroxene, are abundant. tals in both the andesitic rocks and the basaltic andesite Also present are accessory apatite microphenocrysts, rare band are similar in composition and have a wide comolivine pyrrhotite inclusions in amphibole phenocrysts, and rare positional range (Fig. 4). crystals with orthopyroxene magnetite reaction coronae. Some of these pumice clasts are heterogeneous because they vary locally from hornblende-rich, pyroxene-poor The Soncor ejecta areas to areas with no hornblende and two pyroxenes similar in appearance to the main Soncor two- The main juvenile components are two-pyroxene andesite pyroxene pumice. and dacite pumice ( % SiO 2 ). These pumice Basaltic andesite scoria (56 2% SiO 2 ) contains phenoclasts contain phenocrysts (28 44 vol. %, calculated ves- crysts of olivine, augite, plagioclase, orthopyroxene and icle free; Table 2) of plagioclase, orthopyroxene, augite magnetite in an andesitic groundmass consisting of plaand Fe Ti oxides (see Fig. 6a, below) in a highly vesicular gioclase, pyroxenes and magnetite, and rare ilmenite. (24 68 vol. % vesicles) high-silica rhyolite glass matrix Olivine crystals are rimmed with a fine overgrowth of (76 77% SiO 2 ). Some samples contain minor amphibole augite, orthopyroxene, sub-calcic augite and pigeonite, phenocrysts and amphibole pseudomorphs consisting of and contain inclusions of Cr-spinel and andesitic glass. plagioclase, pyroxene and Fe Ti oxides. Completely fresh Augite crystals contain inclusions of orthopyroxene, euhedral amphibole and coarse-grained pseudomorphs pigeonite, magnetite and dacite glass. can occur in the same thin section. Sparse biotite is Two varieties of glassy lithic clasts are interpreted present in a few samples. Occasional rounded quartz, as juvenile components. Glassy agglutinate clasts are and olivine with reaction coronae of orthopyroxene interpreted as early welded pyroclastic facies of the Soncor magnetite symplectite are present. Apatite is common as eruption disrupted by later pyroclastic flows. They are phenocrysts and as inclusions in phenocrysts. Rare zircon, dense, poorly vesicular, crystal-rich (40 50% crystals) anhydrite and pyrrhotite are present as inclusions in hyalocrystalline silicic andesite. They contain phenocrysts Fe Ti oxides (not in the same individual crystals). of plagioclase, orthopyroxene, augite, amphibole and 1895

6 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Fig. 3. Pyroxene compositions in Piedras Grandes andesite, Soncor ejecta (basaltic andesite, two-pyroxene pumice and hornblende pumice) and included rock types (hypabyssal lithic clasts, agglutinate and vitrophyre clasts). Fe Ti oxides in a brown glassy andesitic matrix. They commonly contain lithic clasts of Stage I andesitic lavas, medium- to coarse-grained granitic rocks, rhyodacite porphyry and skarn xenoliths, as well as rounded and embayed quartz crystals. The matrix is heterogeneous with interbanded streaks of pale and dark brown glass. Dacite vitrophyres ( % SiO 2 ) are pale cream to pink, poorly vesicular welded rocks consisting of glassy fiamme and slightly flattened dacitic pumice. Phenocrysts and microphenocrysts of plagioclase, two-pyroxenes, amphibole and Fe Ti oxides are contained in a cloudy, oxidized rhyolitic glass matrix. Plagioclase phenocrysts in Soncor ejecta have large compositional ranges (An ; Fig. 2) with complex zonation. The most calcic compositions (An ) are found in phenocryst cores, which are commonly rounded and resorbed or sieve-textured. Oscillatory-zoned, more sodic overgrowths are common. A basaltic andesite scoria contains calcic cores (An ) with more sodic (An ) rims and microphenocrysts. Orthopyroxene and augite in all pumice types typically have very large compositional ranges (Fig. 3) with both normal and reverse zonation (mgnumber opx = , mg-number cpx = ). The entire compositional range can sometimes be observed in a single thin section. In contrast, the hornblende andesite pumices mostly lack augite and contain orthopyroxene with more restricted compositional ranges (mg-number = ). Basaltic andesite scoria contains magnesian augite and orthopyroxene (mg-number opx = , mg-number cpx = ) and 1896

7 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO unit, provide information about the early stages of evolution of the magma system that eventually led to the zoned chamber, which was sampled by the Soncor eruption. Prismatic-jointed blocks include both juvenile material and fragments of the pre-existing volcanic edifice. Two main varieties have been recognized. Pale to dark green medium-grained basaltic andesites ( % SiO 2 ) are porphyritic or holocrystalline rocks with hypabyssal textures and containing phenocrysts of plagioclase, pyroxenes and Fe Ti oxides (Fig. 6c). The matrix is an intersertal intergrowth of plagioclase, augite, orthopyroxene, Fe Ti oxides, quartz and minor apatite. Euhedral plagioclase phenocrysts (up to 2 mm across) have large cores and oscillatory-zoned overgrowths. An andesitic prismatic-jointed block contains plagioclase with a restricted compositional range (An ). Some crystals have sieve-textured cores and growth zones. Pyroxene phenocrysts (mg-number opx = , mgnumber cpx = ; Fig. 3) are up to 1 mm in length and euhedral. Orthopyroxene magnetite intergrowths in crystal clots of orthopyroxene and plagioclase are interpreted as pseudomorphed olivine. Pale two-pyroxene dacites ( % SiO 2 ) contain phenocrysts of plagioclase, pyroxenes, amphibole pseudomorphs, Fe Ti oxides and occasional biotite in a matrix of colourless glass with microphenocrysts of plagioclase, pyroxenes and Fe Ti oxides. Plagioclase phenocrysts (up to 4 mm in length) are subhedral to euhedral with large cores and oscillatory-zoned overgrowths. Occasional rounded, embayed and sieve-textured plagioclase crystals Fig. 4. Amphibole compositions from Piedras Grandes and Soncor also occur. Pyroxene phenocrysts (mg-number opx = (primitive and included) samples. Axis labels in formula units, calculated according to Leake (1968). Element covariations are chosen that most , mg-number cpx = ; Fig. 3) are up effectively discriminate between different rock types. to 2 mm in length and subhedral to euhedral. Biotite phenocrysts (up to 2 mm across) are rounded and embayed with a thin rim of Fe Ti oxides. The mineral normally zoned olivine (Fo ). Amphibole phenocrysts assemblage, mineral chemistries and textures are similar in Soncor ejecta have a lower Fe 3+ /Fe 2+ ratio, lower Al to Soncor pumice, and this rock type could represent an and higher Ti and Si than Piedras Grandes amphiboles, early dome of the Soncor eruption. and more restricted range of mg-number (Fig. 4). One The porphyries are dense non-vesicular to poorly vehornblende andesite pumice (LAS191) contains two com- sicular andesitic to dacitic rocks. They are divided into positional groups of amphiboles, one of which has lower three petrographically distinct types. Al and higher Si than the other group. Biotite phenocrysts (1) Type A porphyries are pale grey, fine-grained in two-pyroxene pumice and agglutinates lie within the microporphyritic dacites. Microphenocrysts (48 vol. %) range of Piedras Grandes biotite compositions (Fig. 5). and sparse (<1 mm) phenocrysts of plagioclase, ortho- Biotite phenocrysts in one hornblende-rich pumice clast pyroxene, clinopyroxene, Fe Ti oxides and pseudo- (LAS36-1) have relatively high Ti and low Al. morphed amphibole and olivine are present in a pale brown rhyolitic glass. (2) Type B porphyries commonly occur as inclusions in type A porphyries and in agglutinates. Type B dacite Vent-derived lithic clasts porphyries ( % SiO 2 ) contain phenocrysts of Vent-derived lithic clasts are divided into two main rock plagioclase, biotite, amphibole, pyroxenes, quartz and types: prismatic-jointed blocks and porphyries. These Fe Ti oxides, and microphenocrysts of apatite and zircon lithologies are interpreted to originate from the pre- Soncor Stage II volcanic complex and still hot subvolcanic intrusive bodies. They, together with the Piedras Grandes (Fig. 6d). The matrix consists of a fine-grained rhyolitic (73% SiO 2 ) groundmass of plagioclase, pyroxenes and Fe Ti oxides. Plagioclase phenocrysts are up to 5 mm 1897

8 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Table 2: Modal analyses of samples from the Soncor flow deposit, quoted on a vesicle-free basis φ φ φ φ Sample: LA124 LAS29-2 LAS149 LAS150 LAS151 LAS152 LAS72 Description: Basaltic Andesite 2-pyx 2-pyx 2-pyx 2-pyx 2-pyx andesite scoria pumice pumice pumice pumice pumice Matrix Plagioclase Orthopyroxene Clinopyroxene Hornblende Biotite Acc Olivine Acc. Pseudomorphs 4 0 Fe Ti oxides Quartz Apatite Acc. Acc. Acc. Acc. Acc. Acc. Total % Vesicularity Sample: LAS57 LAS58e SM95/38 SM95/39 LAS47 SM95/41 SM95/26 Description: Hornblende Hornblende Andesitic Andesitic Green Green Pale pumice pumice agglutinate agglutinate PJB PJB andesite Matrix Plagioclase Orthopyroxene Clinopyroxene Hornblende Biotite Acc. 0 1 Olivine 1 7 Pseudomorphs Fe Ti oxides Quartz Apatite Acc. 0 2 Acc. Acc. Acc. Acc. Acc. Total % Vesicularity Sample: SM95/27 SM95/47 SM95/28 SM95/29 Description: Pale Type A Type B Type B dacite Porphyry Porphyry porphyry Matrix Plagioclase Orthopyroxene Clinopyroxene Hornblende Acc. 2 0 Biotite Olivine Pseudomorphs Fe Ti oxides Quartz 0 5 Acc. Apatite Acc. Acc. Acc. Acc. Total % Vesicularity φ, vesicularity (and thus corrected modal composition) from density measurements by water displacement method. 1898

9 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO phenocrysts of amphibole, orthopyroxene, augite, plagioclase and Fe Ti oxides in a medium-grained groundmass of plagioclase, alkali feldspar, quartz, pyroxenes and Fe Ti oxides. Amphibole phenocrysts are euhedral, up to 4 mm long and partially pseudomorphed. The amphibole compositions are compositionally similar to those in the Piedras Grandes magmas. Subhedral plagioclase phenocrysts are up to 3 mm across with antiperthitic texture. Pyroxenes are subhedral to euhedral, up to 1 mm across and commonly inclusion rich. Fig. 5. Biotite compositions from Piedras Grandes and Soncor (primitive and included) samples. Plots of Ti, Fe and Al against Mg (formula units calculated to 22 oxygens assuming all Fe as Fe 2+ ). GEOTHERMOMETRY AND OXYGEN BAROMETRY Methods One-pyroxene temperatures were calculated from individual orthopyroxene and clinopyroxene analyses using the QUILF program ( Frost & Lindsley, 1992; Lindsley & Frost, 1992). This method projects the pyroxene compositions onto the pyroxene quadrilateral and calculates temperatures according to the method of Lindsley (1983). Pressure dependence is slight (+2 C/kbar) and a pressure of 3 kbar was assumed. Aluminous pyroxenes (opx Al 2 O 3 > 2% and cpx Al 2 O 3 > 3 wt %) yielded variable and often very high temperatures, and were assumed to represent disequilibrium compositions produced under rapid-quench conditions (e.g. Schiffman & Lofgren, 1982; Ohnenstetter & Brown, 1992). Errors in the temperature calculations related to uncertainties in the probe analyses are estimated at ±10 C from repeated analyses of the same area of a crystal. The most important source of error is calibration of Si, which affects calculated Fe 3+ / Fe 2+ ratios. Analyses of orthopyroxenes in experimental studies of the hornblende andesite of Montserrat (Barclay et al., 1998) produced calculated temperatures within 5 C of the run temperatures. However, Murphy et al. (2000) estimated an error of ±20 C in temperatures calculated for Soufriere Hills andesite, which is the product of wt % errors in microprobe Ca analysis. We therefore estimate a maximum error of ±20 C in our calculations. Temperature and oxygen fugacity were calculated from average titanomagnetite ilmenite compositional pairs using the QUILF program (Andersen, 1993), which uses the model of Frost & Lindsley (1992) and Lindsley & Frost (1992). This method gave good agreement with glass inclusion melting temperatures in Lascar rocks ( Matthews, 1994). At least 10 titanomagnetite and il- menite crystals were analysed for each sample. Magnetite and ilmenite were checked for equilibrium using Mg Mn partitioning (Bacon & Hirschmann, 1988), and only across. Some crystals have sieve-textured cores. A wide range of plagioclase compositions (An ) is observed with sodic compositions occurring in phenocryst rims and groundmass grains. Amphibole phenocrysts are up to 3 mm in length and are often pseudomorphed. Amphibole phenocrysts have distinctive high Si and low Al and Ti in comparison with other Soncor and Piedras Grandes amphibole phenocrysts (Fig. 5). Anhedral biotite crystals are up to 3 mm in length and are rimmed with Fe Ti oxides. Biotite crystals in these rocks have higher Mg and Ti and lower Fe than in other Soncor and Piedras Grandes samples (Fig. 5). Subhedral quartz crystals are up to 5 mm in length. assemblages with equilibrium compositions, low com- (3) Micro-granodiorite porphyries (one analysis; 62 5% positional variability and absence of exsolution lamellae SiO 2 ) are medium-grained intrusive rocks containing were used. This limited the calculations to rapidly cooled, 1899

10 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Fig. 6. Photomicrographs. (a) Soncor two-pyroxene pumice (crossed polars), showing small plagioclase, orthopyroxene, clinopyroxene and magnetite phenocrysts in a vesicular glassy matrix. The central grey orthopyroxene has a clinopyroxene overgrowth. (b) Soncor hornblende andesite pumice (crossed polars), showing hornblende, orthopyroxene and plagioclase phenocrysts in a vesicular glassy matrix. (c) Basaltic andesite prismatic-jointed block (crossed polars) with flow-aligned plagioclase, orthopyroxene, corroded clinopyroxene and Fe Ti oxides. In (a) and (b) the polars are slightly uncrossed to show matrix glass detail. (d) Type B porphyry (plane-polarized light), with corroded, oxidized biotite, sievetextured plagioclase, and subhedral orthopyroxene phenocrysts in a medium-grained groundmass of plagioclase, pyroxenes and Fe Ti oxides; Field of view on long axis of all photographs ~2 mm. usually dacitic rocks and very slowly cooled, re-equilibrated shallow intrusive rocks. For olivines with Crspinel inclusions in the basaltic andesite scoria, oxygen fugacities were calculated using the method of Ballhaus et al. (1990) and the temperature was calculated from augite one-pyroxene thermometry. Results The Piedras Grandes andesite displays a wide range of calculated temperatures (740 C to over 1060 C) despite the restricted range of pyroxene mg-number (Figs 7 and 8). Clinopyroxenes from a basaltic andesite band give temperatures of C. Many individual ortho- pyroxenes have low-temperature cores and higher-tem- perature rims, with apparent temperature contrasts of up to 150 C. In contrast, the Soncor two-pyroxene pumices have a more restricted range of orthopyroxene temperatures (mostly C) over a wide range of mg-number (Fig. 8). Clinopyroxene temperatures fall in a similar range with a few high- and low-temperature outliers (Fig. 9). Orthopyroxene temperatures increase with increasing mg-number in Soncor pumice, albeit with considerable scatter (Fig. 8). Hornblende pumice samples overlap the Soncor field, but one sample (LAS191) gave lower and more varied temperatures ( C) with a few outliers with lower temperature and lower mgnumber. Core-to-rim zoning trends in Soncor pumice are both up- and down-temperature. The type A and B biotite porphyries gave a wide range of temperatures ( C), up-temperature core-to-rim zoning trends and restricted mg-number, features similar to results from Piedras Grandes samples (Fig. 7). The Piedras Grandes andesite, Soncor juvenile ejecta and vent-derived lithic clasts from the Soncor deposits contain equilibrium Fe Ti oxide assemblages with low compositional variability. Three samples of Piedras Grandes andesite yielded temperatures of C (Table 3). A basaltic andesite band recorded a slightly lower temperature (875 C). Soncor two-pyroxene pum- 1900

11 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Fig. 7. Core-to-rim variations in temperature and mg-number for orthopyroxenes in Piedras Grandes andesite and basaltic andesite, and Soncor Type B porphyry lithic clasts, illustrating the similar compositional ranges and heating trends observed in orthopyroxenes in these rocks (marked by arrows connecting cores to rims). Fig. 8. Temperature mg-number variations of orthopyroxene in Piedras Grandes and Soncor (primitive and included) ejecta. Data (upper diagram) and interpretation (lower diagram). GEOCHEMISTRY Piedras Grandes and Soncor products are medium- to ices, hornblende pumices and a vitrophyre sample yielded high-k basaltic andesites, andesites and dacites. They temperatures in the range C. Two andesitic are compositionally similar to other Lascar and Pleisto- scoria samples recorded a temperature of 900 C. Lower cene Recent Central Andean magmas (e.g. Davidson et temperatures were calculated for an andesitic prismatic- al., 1990; Feeley et al., 1993; Feeley & Davidson, 1994; jointed block (748 C) and two Type B biotite porphyry Matthews et al., 1994b). Major and trace element analyses samples ( C), and are interpreted as (probably are presented in Tables 4 and 5 and illustrated in Figs sub-solidus) closure temperatures for magnetite ilmenite 10 and 11. Analytical methods are listed in the Appendix. equilibrium. Representative electron microprobe analyses of glass mat- Calculated oxygen fugacities of the Piedras Grandes rix and glass inclusions in phenocrysts in Soncor basaltic and Soncor andesites and dacites range from FMQ andesite, hornblende pumice and two-pyroxene pumice (fayalite magnetite quartz) + 2 to (Table 3). samples are presented in Table 6. High oxygen fugacity is supported by the ubiquitous presence of anhydrite in Piedras Grandes andesite, Soncor hornblende and two-pyroxene pumices, and vitrophyres (e.g. Carroll & Rutherford, 1987; Matthews et al., Piedras Grandes and Soncor products show typical Cent- Major element geochemistry 1994a). Basaltic andesite samples range from FMQ + ral Andean variations of decreasing TiO 2,Al 2 O 3,Fe 2 O 3, 1 1 to + 1 2, which is consistent with the interpretation MgO and CaO, and increasing K 2 O with increasing of Matthews et al. (1994a) that oxygen fugacity increases SiO 2 (Fig. 10). Na 2 O contents are highly variable, and with falling temperature in Lascar magmas. significant scatter in TiO 2,Al 2 O 3 and P 2 O 5 contents is 1901

12 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Table 3: Temperature and oxygen fugacity of Lascar ejecta from Fe Ti oxide equilibria Sample Temperature ( C) log f O 2 δfmq Piedras Grandes Hornblende andesite SM93/ SM93/ LAS451B Basaltic andesite LAS451A Soncor Andesite LA LAS Two-pyroxene pumice LA LA LAS Hornblende andesite pumice Fig. 9. Temperature mg-number variations of clinopyroxene in Piedras LAS Grandes and Soncor (primitive and included) ejecta. Data (upper LAS diagram) and interpretation (lower diagram). LAS58e LAS58f observed. Samples of Soncor two-pyroxene pumice with Vitrophyre low Na 2 O (Fig. 10) were hydrothermally altered after SM93/ emplacement, and have cloudy, altered glass and contain large quantities of iron sulphide, silica and various sulph- PJB ate minerals decorating the inner surfaces of vesicles. LAS Inflections in the MgO and CaO trends are observed at Type B porphyry about SiO 2 = 60 62%. The majority of Piedras Grandes SM95/ hornblende andesite analyses are close to this com- SM95/ positional boundary, which separates Lascar basaltic andesite and andesitic compositions from more evolved banded pumice, two-pyroxene pumice and biotite porphyries. than 60 62%. Lascar magmas have light rare earth element (LREE)-enriched patterns when normalized to chondrite, and Eu anomalies are weak or absent Trace elements ( Matthews et al., 1996). Selected trace elements are plotted against SiO 2 in Fig. 11. Rb and Ba increase with increasing SiO 2, although Ba shows increasing scatter in more evolved compositions. Geochemical zonation and heterogeneity V and Co decrease strongly with increasing SiO 2. Basaltic The Soncor Plinian fallout deposit and ignimbrite show andesite magmas have highly variable Cr ( ppm), compositional heterogeneity and a weak but complex Ni ( ppm), V ( ppm), Sr ( ppm) zoning pattern (Fig. 12). The main part of the Plinian and Zr ( ppm), and in the case of V, Zr and Sr deposit, as exposed on the south flank of Lascar, consists this variability extends to more silicic compositions. Cr of white andesitic to dacitic two-pyroxene pumice with and Ni are low for more silicic magmas (>60 61% SiO 2 ). variable SiO 2 ( %). The uppermost 2mofthis Sr and Zr decrease markedly for SiO 2 contents higher deposit in the most proximal exposure also contains 1902

13 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Table 4: Whole-rock major element analyses of Piedras Grandes and Soncor samples Rock type: Piedras Piedras Piedras Piedras Piedras Piedras Piedras Piedras Basaltic Andesite Andesite Andesite Andesite Grandes Grandes Grandes Grandes Grandes Grandes Grandes Grandes andesite Sample: SM93/22 SM93/22 GLA-101 GLA-201 LAS61 LAS62 LAS451a LAS451b LA124 TU-69 LAS29 LAS79 LAS79 SiO TiO Al 2 O Fe 2 O 3 n/a n/a n/a n/a n/a n/a n/a 3 78 n/a FeO n/a n/a n/a n/a n/a n/a n/a 2 89 n/a (Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2 O P 2 O H 2 O + n/a n/a n/a n/a n/a n/a n/a 1 20 n/a LOI Total H 2 O 0 21 n/a n/a n/a n/a n/a n/a n/a n/a n/a Rock type: Andesite Andesite Banded Banded 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice Sample: LAS95a GLA-123 TU-85 TU-86 LA121 LA122 LA147 TU-65 TU-66 TU-87 LAS5 LAS7 LAS35-1 SiO TiO Al 2 O Fe 2 O 3 n/a n/a n/a n/a n/a FeO n/a n/a n/a n/a n/a (Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2 O P 2 O H 2 O + n/a n/a n/a n/a n/a LOI Total H 2 O n/a n/a n/a n/a n/a n/a abundant banded pumice and dark andesitic pumice (61 flanks. The main flow units contain mostly andesitic to wt % SiO 2 ) as well as white pumice, and exhibits a wide dacitic two-pyroxene pumice types ( % SiO 2 ). compositional range ( % SiO 2 ). The ignimbrite The late flow units contain more silica-poor white can be divided into two parts. The main flow units pumice, compositionally banded pumice and andesitic consist of lithic-rich lag breccias and pumice-rich scoria ( % SiO 2 ), hornblende andesite pumice ignimbrite. Late flow units are pumice- and scoria-rich types ( % SiO 2 ) and basaltic andesite scoria distal flow units and the uppermost flow units on the SE (56% SiO 2 ). A crude double zonation is indicated, with 1903

14 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Table 4: continued Rock type: 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice Sample: LAS35-3 LAS50 LAS53 LAS69a LAS69b LAS72 LAS78b LAS145b LAS145c LAS145d LAS145e LAS148 LAS149 SiO TiO Al 2 O Fe 2 O 3 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a FeO n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a (Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2 O P 2 O H 2 O + n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a LOI Total H 2 O n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Rock type: 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx Hornbl Hornbl Hornbl Hornbl Hornbl Hornbl Hornbl pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice Sample: LAS150 LAS151 LAS152 LAS153 LAS190 GLA-208 LAS30 LAS57 LAS58e LAS58f LAS108 LAS109 LAS110 SiO TiO Al 2 O Fe 2 O 3 n/a n/a n/a n/a n/a 1 97 n/a n/a n/a n/a n/a n/a n/a FeO n/a n/a n/a n/a n/a 2 09 n/a n/a n/a n/a n/a n/a n/a (Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2 O P 2 O H 2 O + n/a n/a n/a n/a n/a 0 06 n/a n/a n/a n/a n/a n/a n/a LOI Total H 2 O n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a two cycles in which more mafic magma is erupted at the and inclusions in clinopyroxenes range from 63 to 67% end of a cycle. SiO 2. This trend shows variable TiO 2 and Na 2 O, decreasing Al 2 O 3, MgO and CaO, and increasing K 2 O and Glass compositions P 2 O 5. FeO has a flat but variable trend to 63% SiO 2 Glass inclusions in olivine and clinopyroxene in a basaltic andesite (LA124) define a continuous compositional trend. Inclusions in olivines range from 56 to 62% SiO 2 then decreases sharply. The more silicic glass inclusions coexist with titanomagnetite. Hornblende pumice samples contain low-silica rhyolitic 1904

15 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Rock type: Hornbl Hornbl Vitro- Vitro- Green Green Andesite Dacite Type B Type B Dacite Dacite Green pumice pumice phyre phyre PJB PJB porphyry porphyry PJB Sample: LAS156 LAS191 SM93/44 SM93/44 GLA-125 LAS47 95/026 95/027 95/028 95/029 95/030 95/041 95/042 SiO TiO Al 2 O Fe 2 O 3 n/a n/a n/a n/a 3 08 n/a n/a n/a n/a n/a n/a n/a n/a FeO n/a n/a n/a n/a 4 09 n/a n/a n/a n/a n/a n/a n/a n/a (Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2 O P 2 O H 2 O + n/a n/a n/a n/a 0 18 n/a n/a n/a n/a n/a n/a n/a n/a LOI Total H 2 O n/a n/a 0 28 n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Rock type: Andesite Type B Granoporphyry diorite Sample: 95/043 95/049 LAS101 SiO TiO Al 2 O Fe 2 O 3 n/a n/a n/a FeO n/a n/a n/a (Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2 O P 2 O H 2 O + n/a n/a n/a LOI Total H 2 O n/a n/a n/a n/a, not analysed. glass, both as matrix and as inclusions in amphibole phenocrysts. Glass matrices and inclusions in phenocryst phases in two-pyroxene pumices are more evolved (76 77% SiO 2 ). Hornblende pumice sample LAS191 has matrix glass (and inclusions in minerals) similar in composition to that of the two-pyroxene pumices. There is no continuous compositional trend for all Soncor glasses. Glasses in hornblende and two-pyroxene pumices have separate compositional fields, both from each other and from the compositional trend of glass inclusions from basaltic andesite scoria. These low-silica rhyolitic glasses have relatively low TiO 2 and P 2 O

16 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Table 5: Whole-rock trace element analyses of Piedras Grandes and Soncor samples Rock type: Piedras Piedras Piedras Piedras Piedras Piedras Piedras Basaltic Andesite Andesite Andesite Andesite Andesite Grandes Grandes Grandes Grandes Grandes Grandes Grandes andesite Sample: SM93/22 GLA-101 GLA-201 LAS61 LAS62 LAS 451a LAS 451b LA124 TU-69 LAS29 LAS79 LAS95a GLA-123 Ba Ce Co Cr Cs Cu Ga La Nb Nd Ni Pb Rb Sc Sm Sr Th U V Y Zn Zr Cl F Rock type: Banded Banded 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice Sample: TU-85 TU-86 LA121 LA122 LA147 TU-65 TU-66 TU-87 LAS5 LAS7 LAS35-1 LAS35-3 LAS50 Ba Ce Co Cr Cs Cu Ga La Nb Nd Ni Pb Rb Sc Sm Sr Th U V Y Zn Zr Cl F

17 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Rock type: 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx 2-Pyx pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice Sample: LAS53 LAS69a LAS69b LAS72 LAS78b LAS145b LAS145c LAS145d LAS145e LAS148 LAS149 LAS150 LAS151 Ba Ce Co Cr Cs Cu Ga La Nb Nd Ni Pb Rb Sc Sm Sr Th U V Y Zn Zr Cl F Rock type: 2-Pyx 2-Pyx 2-Pyx 2-Pyx Hornbl Hornbl Hornbl Hornbl Hornbl Hornbl Hornbl Hornbl Hornbl pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice pumice Sample: LAS152 LAS153 LAS190 GLA-208 LAS57 LAS30 LAS58e LAS58f LAS108 LAS109 LAS110 LAS156 LAS191 Ba Ce Co Cr Cs Cu Ga La Nb Nd Ni Pb Rb Sc Sm Sr Th U V Y Zn Zr Cl F

18 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Table 5: continued Rock type: Vitro- Green Green Andesite Dacite Type B Type B Dacite Dacite Green Andesite Type B Granophyre PJB PJB porphyry porphyry PJB porphyry diorite Sample: 93/44 GLA-125 LAS47 95/026 95/027 95/28 95/029 95/030 95/041 95/042 95/043 95/49 LAS101 Ba Ce Co Cr Cs Cu Ga La Nb Nd Ni Pb Rb Sc Sm 2 1 Sr Th U V Y Zn Zr Cl F VOLATILE ELEMENTS basaltic andesite (Fig. 15). Biotite in the Type B porphyries extends from the magnesian end of the Piedras Grandes Chlorine and fluorine range towards high-mg, Cl-rich compositions. Glass in- In Soncor two-pyroxene and hornblende pumice samples clusions in phenocrysts in Soncor hornblende and twoapatites have approximately constant F/Cl ratio in most pyroxene pumices contain wt % chlorine individual samples (Fig. 13a), but ratios differ between (Fig. 16a). Chlorine contents of matrix glasses are lower. samples. Halogen contents increase towards apatite rims. Glass inclusions in olivine and clinopyroxene phenocrysts Apatites from one sample of the Piedras Grandes andesite in the Soncor basaltic andesite scoria LA124 have more (SM93/10) show a trend of variable F at constant Cl variable Cl ( wt %). content whereas another sample (SM93/20) shows no coherence (Fig. 13b). For most Soncor hornblende andesite pumices Cl is low in amphibole, with the exception of Sulphur sample LAS191, which has relatively Cl-rich amphibole Glass inclusions in phenocrysts in Soncor pumices (Fig. 14a). This high-cl trend is continued to higher mg- (Fig. 16b) are relatively sulphur poor (<500 ppm). Hownumber and lower Cl by amphibole in a Type B porphyry. ever, there is a negative correlation with glass SiO 2, For both high- and low-cl groups, Cl increases slightly indicating a melt composition control on sulphur solwith decreasing mg-number. Amphiboles in the Piedras ubility. Glass inclusions in olivine and clinopyroxene Grandes andesite show low Cl in comparison with most phenocrysts in the Soncor basaltic andesite show a strong Soncor ejecta (Fig. 14b). Individual samples of Piedras decrease in S from 5000 ppm at 55% SiO 2 to <1000 Grandes andesite contain hornblende phenocrysts covering ppm at 65% SiO 2. a large range in mg-number, in contrast to Soncor samples, which show more restricted hornblende compositions in individual samples (Fig. 14). Biotite phenocrysts PETROGENESIS in Soncor two-pyroxene pumice have higher Cl In this section we discuss the evolution of the Piedras contents than biotite in Piedras Grandes andesite and Grandes Soncor system based on the assumption that 1908

19 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Table 6: Representative glass analyses from Soncor ejecta Sample: LA124 LA124 LA124 LA124 LA124 LA124 LAS30 LAS30 LAS30 LAS57 LAS57 LAS57 LAS57 Analysis: G2 G3 G6 G14 G15 G20 G52 G51 G50 G13 G2 G8 G7 Host: OLIVINE OLIVINE OLIVINE CPYX CPYX CPYX MATRIX MATRIX MATRIX AMPH AMPH MATRIX MATRIX SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Cl F S Total Normalized to 100% volatile free SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Total Sample: LAS191 LAS191 LAS191 LAS191 LAS72 LAS72 LAS72 Analysis: G12 G3 G7 G8 G5 G6 G2 Host: AMPH AMPH OPYX OPYX OPYX OPYX MATRIX SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Cl F S Total Normalized to 100% volatile free SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Total LA124, basaltic andesite scoria; LAS30, hornblende andesite pumice; LAS57, hornblende andesite pumice; LAS191, hornblende andesite pumice; LAS72, two-pyroxene silicic andesite pumice. 1909

20 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 12 DECEMBER 1999 Fig. 10. Harker diagrams of major element variations for Piedras Grandes and Soncor ejecta. the Piedras Grandes and Soncor vent-derived lithic assemblage preserves information on the development of an evolving intermediate to silicic magma system, which culminated in the formation of the zoned Soncor magma chamber. Origin of mafic magmas The variability of many major and trace elements in the basaltic andesite samples from the Piedras Grandes and Soncor eruptions implies that these magmas were modi- fied before emplacement into the shallow magma system. Variations in Al 2 O 3,Na 2 O, Cr, Ni and Sr with little change in SiO 2 imply fractionation processes involving olivine, clinopyroxene and spinel. The abundance of these early phenocryst phases in the Soncor basaltic andesite sample LA124 is reflected in high Cr, Ni and V compared with other Lascar basaltic andesite samples. Some magmas became depleted in Cr and Ni by frac- tionation of olivine, clinopyroxene and Cr-spinel, whereas others (like LA124) entrained crystals of these phases and were thus enriched in these elements. The geochemical variations do not show any features indicative of pla- gioclase fractionation, which is consistent with evolution of basaltic andesite magmas by fractionation at high pressure. These features suggest that the basaltic andesite magmas were fractionated in the lower crust. There is, however, poor correlation between Cr and Ni and other major and trace elements, indicating that fractionation and accumulation of olivine, clinopyroxene and spinel were not the only important processes. The LREE-enriched and relative heavy rare earth element (HREE)-depleted patterns of Lascar magmas implicate garnet (Kay et al., 1991). A number of models have been proposed for the baseline compositional and isotopic variability of Central Andean magmas, involving MASH (melting, assimilation, storage and homogenization) type processes (e.g. Hildreth & Moorbath, 1988; Rapp & Watson, 1995) in garnet-bearing lower crust. There is also evidence for modification of the mafic magma in the shallow magma chamber. For example, Mg-poor, low- to moderate-temperature ( C) orthopyroxene phenocrysts and hornblende crystals identical to those in the andesite occur in the basaltic andesite band LAS451a of the Piedras Grandes unit. These phe- nocrysts are compositionally and thermally similar to the phenocrysts in the host andesite and are interpreted as crystals that have mixed into the basaltic andesite from the host magma. 1910

21 MATTHEWS et al. PIEDRAS GRANDES SONCOR ERUPTIONS, LASCAR VOLCANO Fig. 11. Trace element variations as function of SiO 2 for Piedras Grandes and Soncor ejecta. supply of heat and to some extent mingled with the basaltic andesite. Orthopyroxene crystals in the andesites of the Piedras Grandes unit and in the dacite porphyries have low- temperature ( C) cores and show rising coreto-rim temperatures. We interpret the low-temperature orthopyroxene cores as restite crystals and the core-to-rim trends of rising crystallization temperature as evidence of reheating and partial melting in their origin. In the case of the Piedras Grandes andesite the presence of basaltic andesite bands and inclusions provides direct evidence that influx of hotter mafic magmas provided the heat input. Several features are consistent with hybridism between the andesite magma and the mafic magmas. Rounded, sieve-textured, anorthite-rich cores are observed in plagioclase crystals, commonly with more sodic, oscillatory zoned overgrowths. These cores have similar compositions to phenocrysts in the basaltic andesite. Rounded and embayed quartz grains with inclusions and fracture-fills of rhyolitic groundmass together with reacted olivine crystals in the andesite are illustrative of the disequilibrium hybrid character of the andesite. The amount of hybridism is, however, limited. The proportion of crystals that might originate from the basaltic andesite end member is low. The geochemical variations in the Pre-Soncor magma evolution The pre-soncor Stage II volcano consisted of porphyritic hornblende andesites of the Piedras Grandes unit and porphyritic biotite dacite porphyries as sampled by the Soncor eruption. We interpret the petrological and geo- chemical data as the consequence of basaltic andesite magmas being emplaced into a high-temperature highly crystalline magma body that was remobilized by the Fig. 12. Whole-rock SiO 2 through the Soncor Plinian and later pyro- clastic flow deposit showing complex compositional variation. The vertical axis represents relative stratigraphic position. 1911