O-Sr isotopic variations in Miocene granitoids from the Aegean: evidence for an origin by combined assimilation and fractional crystallization |
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| Authors: | Rainer Altherr Friedhelm Henjes-Kunst Alan Matthews Hans Friedrichsen Bent Tauber Hansen |
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| Institution: | (1) Institut für Petrographie und Geochemie, Universität Karlsruhe, Kaiserstrasse 12, D-7500 Karlsruhe 1, Federal Republic of Germany;(2) Institute of Earth Sciences, Department of Geology, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel;(3) Institut für Geologie, FU Berlin, Altensteinstrasse 34A, D-1000 Berlin 22;(4) Zentrallabor für Geochronologie (ZLG), Institut für Mineralogie, Universität Münster, Corrensstrasse 24, D-4400 Münster, Federal Republic of Germany |
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| Abstract: | Sr, O, and D/H isotopic compositions have been analyzed in Miocene metaluminous to slightly peraluminous (I-type) granitoids of the central Aegean. Individual plutonic complexes display significant variations in their 
18O and initial87Sr/86Sr compositions.D and
18O compositions of minerals and whole-rocks are mostly in the magmatic range. Some samples from Naxos and Mykonos/Delos show low D and
18O values characteristic of meteoric-water-hydrothermal interaction, but as a whole the changes in
18O and Sr isotopic compositions as a result of hydrothermal alteration were slight, even in instances where marked alteration is petrographically observable. Consequently, the bulk-rock variations of
18O from 8.1 to 12.0 and of87Sr/86Sr from 0.70438 to 0.71450 may be regarded as primary and indicative of the conditions of their evolution. Heterogeneous isotopic compositions observed in the individual plutons of Serifos, Ikaria, Samos and Kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses, with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization. The granitoids of Serifos, Ikaria and Kos display weak correlations between the initial87Sr/86Sr and
18O and 1/Sr. The granitoid province shows a positive correlation between87Sr/86Sr and
18O and a non-linear relationship between87Sr/86Sr and 1/Sr, whereby 1/Sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases. It is argued that assimilation of older continental material by mantle-derived arc magmas with combined fractionation (AFC) is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock-types trend from granodiorites in the (south)west, near the inferred Oligocene-Miocene suture, to granites in the center and monzonites in the (north)east of the province. |
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