Geochemistry and petrogenesis of the Serifos Granite, in relation to other Aegean granitoids, Greece.

The young 9Ma Serifos pluton is the latest of a group of Miocene age medium- to high-K calc-alkaline granites, granodiorites and tonalites that were emplaced into the upper crust of the southern Aegean during back-arc extension. Serifos granitoid consists of a granodiorite, minor tonalite and abundant quartz-diorite enclaves. Most of these S. Aegean plutons have characteristics which vary between I-type and S-type, but their petrogenesis has been controversial. Here, different petrogenetic models are assessed using field criteria, major and trace element geochemistry, and Sr-Nd and O isotope systematics. The major element compositions of the Serifos granodiorie-tonalite and other I-type Cyclades granitoids are metaluminous to subaluminous, and have MgO, CaO, and TiO2 contents consistent with liquid compositions produced during amphibolite dehydration- melting experiments. Modelling of crystal fractionation processes using major elements would imply that removal of large amounts of plagioclase+hornblende +/- biotite is necessary to reproduce the Serifos evolved biotite-rich granodiorite from the quartz diorite magma; however, this is not consistent with the trace element variations. Conversely, mixing of the latter type of magma with a partial melt derived from the basement metasedimentary gneiss is consistent with both major- and trace-element trends, and new experimental data on melting of mixed amphibolite-sediment sources. The LILE elements particularly are controlled by the metasedimentary component. The Sr, Nd, and O isotope ratios of the Serifos and other Cyclades I-granitoids strongly indicate a crustal origin. Relationships between Rb/Sr, Sr, Nd, initial 87Sr/86Sr ratios, Nd/O isotope ratios are also consistent with binary mixing: magmas being derived from a mixed amphibolite/greywacke gneiss source available in the Cyclades basement, and with a significant metasedimentary contribution. Mixing of these two components in the source is a very important mechanism for explaining the distinct individual isotopic signatures of Serifos and the other Cyclades granitoids, but at the same time accounts for the continuous mixing trend (in terms of Sr, Nd, initial 87Sr/86Sr, Nd) seen for all I-type and S-type granites in the Cyclades, and indeed for most of the Eastern Mediterranean. The model could therefore be widely applicable. The geochemistry of the quartz diorite enclaves indicates some involvement of primary mantle-derived magma. Granite generation in the Aegean requires augmented thermal gradients, probably associated with lithospheric decompression from the early-Miocene to late-Miocene/Recent; this thermal perturbation may be a result of slab-breakoff. The increasing mantle heat input then accounts for the greater proportions of hornblende- dehydration melts incorporated in the progressively younger I-type granites (Naxos 12.5Ma, Mykonos 11.5Ma, Serifos 9Ma).