Journal of Petrology | Volume 40 | Number 12 | Pages 1891-1919 | 1999
© Oxford University Press 1999
The Piedras Grandes–Soncor Eruptions, Lascar Volcano, Chile; Evolution of a Zoned Magma Chamber in the Central Andean Upper Crust
1 Department of Earth Sciences, University of Bristol Wills Memorial Building, Queens Road, Bristol BS8 1RR, 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
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Abstract |
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Stage II of Lascar Volcano, Chile, involved development of an andesite to dacite volcanic complex and associated hypabyssal porphyry intrusions above the main magma chamber. The system culminated in development of a large zoned magma chamber that erupted in the large-magnitude (8 km3) Soncor explosive eruption at 26.5 ka, forming a Plinian pumice deposit and ignimbrite. Vent-derived lithic clasts in the Soncor deposits sample the pre-existing Stage II complex. The Piedras Grandes hornblende andesite unit represents a pre-Soncor dome complex. The andesite consistsof a heterogeneous phenocryst assemblage of plagioclase–amphibole–orthopyroxene–oxides and minor biotite, clinopyroxene, quartz, apatite, anhydrite and olivine together with commingled basaltic andesite inclusions and streaks. Temperature estimates from 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 partial melts with entrained restite. The zoned Soncor chamber contained dacite (67 wt % SiO2) to silicic andesite (61 wt % SiO2) crystal-rich magmas with an assemblage of plagioclase–orthopyroxene–clinopyroxene–oxides with minor biotite, amphibole, apatite, zircon, anhydrite, pyrrhotite and olivine. Hornblende-rich mafic andesite pumice from late flow units in the ignimbrite provides evidence for influxes of hydrous mafic magmas at the base of the chamber at sufficient depths to stabilize amphibole. The hydrous mafic magmas are interpreted to have evolved in the lower crust by high-pressure fractionation with some lower-crustal contamination. The Soncor zoned magma chamber developed in the upper crust at about 6 km depth as a result of repeated influx of hydrous mafic magmas. The magmas in the chamber evolved by open-system fractionation with magma mixing being important in the less evolved magmas. Repeated influxes of hydrous mafic magmas resulted in convective stirring and addition of heat, volatiles and mafic components to the chamber. This produced complex histories of individual crystals and heterogeneous character of phenocrysts in individual samples. Halogen contents of amphibole, biotite, apatite and glass inclusions, S contents of glass inclusions, stabilization of anhydrite in the silicic magmas, and mass balance calculations imply major transfer of volatile components from the hydrous mafic magmas into the interior of the zoned chamber in the form of a co-magmatic fluid phase.
KEY WORDS: andesite; Andes; magma mixing; compositional zoning
* Corresponding author. e-mail: Steve.Sparks{at}bristoLac.uk
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