Crystal retention,fractionation and crustal assimilation in a convecting magma chamber,Nisyros Volcano,Greece |
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| Authors: | L Francalanci J C Varekamp G Vougioukalakis M J Delant F Innocenti P Manetti |
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| Institution: | (1) Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira 4, I-50121 Florence, Italy;(2) Department of Earth & Environmental Sciences, Wesleyan University, 06459-0139 Middletown, CT, USA;(3) IGME, Messogion 70, H-11527 Athens, Greece;(4) Department of Geology, University of Southern Florida, 33620 Tampa, FL, USA;(5) Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via S. Maria 53, I-56100 Pisa, Italy |
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| Abstract: | Nisyros island is a calc-alkaline volcano, built up during the last 100 ka. The first cycle of its subaerial history includes the cone-building activity with three phases, each characterized by a similar sequence: (1) effusive and explosive activity fed by basaltic andesitic and andesitic magmas; and (2) effusive andextrusive activity fed by dacitic and rhyolitic magmas. The second eruptive cycle includes the caldera-forming explosive activity with two phases, each consisting of the sequence: (1) rhyolitic phreatomagmatic eruptions triggering a central caldera collapse; and (2) extrusion of dacitic-rhyolitic domes and lava flows. The rocks of this cycle are characteized by the presence of mafic enclaves with different petrographic and chemical features which testify to mixing-mingling processes between variously evolved magmas. Jumps in the degree of evolution are present in the stratigraphic series, accompanied by changes in the porphyritic index. This index ranges from 60% to about 5% and correlates with several teochemical parameters, including a negative correlation with Sr isotope ratios (0.703384–0.705120). The latter increase from basaltic andesites to intermediate rocks, but then slightly decrease in the most evolved volcanic rocks. The petrographic, geochemical and isotopic characteristics can be largely explained by processes occurring in a convecting, crystallizing and assimilating magma chamber, where crystal sorting, retention, resorption and accumulation take place. A group of crystal-rich basaltic andesites with high Sr and compatible element contents and low incompatible elements and Sr isotope ratios probably resulted from the accumulation of plagioclase and pyroxene in an andesitic liquid. Re-entrainment of plagioclase crystals in the crystallizing magma may have been responsible for the lower 87Sr/86Sr in the most evolved rocks. The gaps in the degree of evolution with time are interpreted as due to liquid segregation from a crystal mush once critical crystallinity was reached. At that stage convection halted, and a less dense, less porphyritic, more evolved magma separated from a denser crystal-rich magma portion. The differences in incompatible element enrichment of pre-and post-caldera dacites and the chemical variation in the post-caldera dome sequence are the result of hybridization of post-caldera dome magmas with more mafic magmas, as represented by the enclave compositions. The occurrence of the quenched, more mafic magmas in the two post-caldera units suggests that renewed intrusion of mafic magma took place after each collapse event. |
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| Keywords: | Calc-alkaline volcanism Geochemistry Isotopic study Petrology Magmatic evolution Aegean Arc Nisyros volcano |
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