CRYSTAL RETENTION, FRACTIONATION AND CRUSTAL ASSIMILATION IN A CONVECTING MAGMA CHAMBER, NISYROS VOLCANO, GREECE

Nisyros island is a calc-alkaline volcano, built up during the last 10 0 ka. The first cycle of its subaerial history includes the cone-build ing activity with three phases, each characterized by a similar sequen ce: (1) effusive and explosive activity fed by basaltic andesitic and andesitic magmas; and (2) effusive and extrusive activity fed by dacit ic and rhyolitic magmas. The second eruptive cycle includes the calder a-forming explosive activity with two phases, each consisting of the s equence: (1) rhyolitic phreatomagmatic eruptions triggering a central caldera collapse; and (2) extrusion of dacitic-rhyolitic domes and lav a flows. The rocks of this cycle are characterized by the presence of mafic enclaves with different petrographic and chemical features, whic h testify to mixing-mingling processes between variously evolved magma s. Jumps in the degree of evolution are present in the stratigraphic s eries, accompanied by changes in the porphyritic index. This index ran ges from 60% to about 5% and correlates with several geochemical param eters, including a negative correlation with Br isotope ratios (0.7033 84-0.705120). The latter increase from basaltic andesites to intermedi ate rocks, but then slightly decrease in the most evolved volcanic/roc ks. The petrographic, geochemical and isotopic characteristics can be largely explained by processes occurring in a convecting, crystallizin g and assimilating magma chamber, where crystal sorting, retention, re sorption and accumulation take place. A group of crystal-rich basaltic andesites with high Sr and compatible element contents and low incomp atible elements and Sr isotope ratios probably resulted from the accum ulation of plagioclase and pyroxene in an andesitic liquid. Re-entrain ment of plagioclase crystals in the crystallizing magma may have been responsible for the lower Sr-87/Sr-86 in the most evolved rocks. The g aps in the degree of evolution with time are interpreted as due to liq uid segregation from a crystal mush once critical crystallinity was re ached. At that stage convection halted, and a less dense, less porphyr itic, more evolved magma separated from a denser crystal-rich magma po rtion. The differences in incompatible element enrichment of pre- and post-caldera dacites and the chemical variation in the post-caldera do me sequence are the result of hybridization of post-caldera dome magma s with more mafic magmas, as represented by the enclave compositions. The occurrence of the quenched, more mafic magmas in the two post-cald era units suggests that renewed intrusion of mafic magma took place af ter each collapse event.