Details

Autor(en) / Beteiligte

• WYERS, G. PAUL
• BARTON, MICHAEL

Titel

Polybaric Evolution of Calc-alkaline Magmas from Nisyros, Southeastern Hellenic Arc, Greece

Ist Teil von

• Journal of petrology, 1989-02, Vol.30 (1), p.1-37

Ort / Verlag

Oxford: Oxford University Press

Erscheinungsjahr

1989

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• The lavas of Nisyros were erupted between about 0·2 m.y B.P. and 1422 A.D., and range in composition from basaltic andesite to rhyodacite. Most were erupted prior to caldera collapse (exact date unknown), and the post-caldera lavas are petrographically (presence of strongly resorbed phenocrysts) and chemically (lower TiO2 K2O, P2O5, and LIL elements) distinct from the pre-caldera lavas. The pre-caldera lavas do not form a continuous series since lavas with SiO2 contents between 60 and 66 wt.% are absent. Nevertheless, major element variations demonstrate that fractional crystalliz ation (involving removal of olivine, dinopyroxene, plagioclase, and Fe-Ti oxide from the basaltic andesites and andesites and plagioclase, clinopyroxene, hypersthene, Ti-magnetite, ilmenite, apatite, and zircon from the dacites and rhyodacites) played a major role in the evolution of the pre-caldera lavas. Several lines of evidence indicate that other processes were also important in magma evolution: (1) Quantitative modeling of major element data shows that phenocryst phases of unlikely composi tion or unrealistic assemblages of phenocryst phases are required to relate the dacites and rhyodacites to the basaltic andesites and andesites; (2) The proportions of olivine and clinopyroxene required in quantitative models for the initial stages of evolution differ from those observed petrographically and this is not likely to reflect either differential rates of crystal settling or the curvature of cotectics along which liquids of basaltic andesite to andesite composition lie; (3) The concentrations of Rb, Cs, Ba, La, Sm, Eu, and Th in the rhyod.acites are too high for these lavas to be related to the dacites by fractional crystallization alone; and (4) 87Sr/86Sr ratios for the andesites and rhyodacites are higher than those for the basaltic andesites and dacites, respectively. It is shown that fractional crystallization was accompanied by assimilation, and that magma mixing played a minor role (if any) in the evolution of the pre-caldera lavas. Trace element and isotopic data indicate that the andesites evolved from the basaltic andesites by AFC involving average crust or upper crust, whereas the rhyodacites evolved from the dacites by AFC involving lower crust. Additional evidence for polybaric evolution is provided by the occurrence of distinct Ab-rich cores of plagioclase phenocrysts in the dacites and rhyodacites, which record a period of high pressure crystallization, and by the occurrence of both normal and reverse-zoned phenocrysts in the basaltic andesites and andesites. Furthermore, calculated pressures of crystallization are ˜8 kb for the dacites and rhyodacites and 3·5–4 kb for the basaltic andesites and andesites. It is concluded that the dacites and rhyodacites evolved via AFC from basaltic andesites and andesites largely in chambers sited near the base of the crust whereas the basaltic andesites and andesites mostly evolved in chambers sited at mid-crustal levels. Eruption from different chambers explains the compositional gap in the chemistry of the pre-caldera lavas since eruptive products represent a more or less random sampling of residual liquids which separate (via filter pressing) from bodies of crystallizing magma at various depths. Magma mixing was important in the evolution of the post-caldera lavas, but geochemical data require that these magmas evolved from parental magmas which were derived from a more refractory source than the parental magmas to the pre-caldera lavas.