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| Titel |
Bulk rock and melt inclusion analyses indicate bimodal distribution in Calbuco volcano (Chile) |
| VerfasserIn |
Salvatrice Montalbano, Olivier Bolle, Pierre Schiano, Nicolas Cluzel, Jacqueline Vander Auwera |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090643
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| Publikation (Nr.) |
 EGU/EGU2014-4898.pdf |
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| Zusammenfassung |
| Calbuco is an active stratovolcano situated in the central SVZ (Southern Volcanic Zone) of the Andes at 41.2°S. The dominant rock-type is basaltic andesite containing macrocrysts of plagioclase (An57-91), olivine (Fo60-81), clinopyroxene (Mg# 74-85), orthopyroxene (Mg# 66-75) and rare amphibole (mostly pargasitic) in a micro-crystalline matrix. Orthopyroxene frequently occurs as a reaction rim surrounding olivine suggestive of a peritectic reaction. The oldest lava unit (Calbuco 1) contains basaltic andesites that are notably lower in MgO and higher in Al2O3 than the other samples. Some dacitic compositions have also been identified. Bulk rock analyses define a low-K calc-alkaline trend with however two basalts plotting in the tholeiite field in the AFM diagram. Bulk rocks display a differentiation trend of decreasing CaO, FeOt and MgO and increasing K2O and P2O5 with increasing SiO2. Typical negative anomalies in Nb, Ta and Th are shown in spiderdiagrams whereas there is no Eu anomaly in REE patterns. In variation diagrams, a clear compositional gap occurs between 61 and 65 wt. % SiO2. Investigation of melt inclusions was performed on homogenized and naturally quenched inclusions hosted in olivine and clinopyroxene crystals. Their composition mimics the differentiation trend observed in the bulk samples, including a bimodal distribution. The melt inclusions analyzed in olivine range in composition from 45 to 58 wt. % SiO2 whereas those occurring in clinopyroxene range from 70 and 76 wt. % SiO2. The compositional gap of the melt inclusions thus overlaps that of the whole rocks. The observed differentiation trend from basalt to basaltic andesite (49 to 58 wt. % SiO2) perfectly fits published experimental trends acquired on hydrous basalts at different crustal pressures, water concentrations and oxygen fugacities at subduction zones and can be accounted for by a fractional crystallization process where a bulk cumulate made of plagioclase, olivine, clinopyroxene and magnetite of intermediate compositions is subtracted from the least differentiated basaltic andesite to drive the melt composition up to 58 wt. % SiO2. The origin of the compositional gap (immiscibility, density filter, rapid evolution of the liquid composition at intermediate SiO2 contents, critical crystallinity) at Calbuco has still to be resolved. |
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