Geochemical and Isotopic Constraints on the Genesis of Granitoids in the Aegean Subduction Zone

Granitoid magmas are abundant in subduction zones and form large portions of the upper continental crust. However, the formation of granitoid magmas remains debated, with models proposing (1) evolution from mantle-derived mafic melts by assimilation-fractional crystallization (AFC), (2) partial melting of lower to middle continental crust induced by mafic underplating, and (3) partial melting of metasomatized pyroxenite in the mantle. Since the Oligocene, slab rollback and trench retreat have caused the Aegean subduction zone to migrate approximately 350 km southwestward, resulting in extensive mafic to felsic magmatism with numerous granitoid intrusions in the region. We present new whole-rock major and trace element data together with Sr-Nd-Pb isotope compositions for the 15 to 8 Ma granitoids from Tinos, Mykonos, Naxos, Paros, Lavrion, and Serifos, as well as metasedimentary rocks of the Cycladic Blueschist Unit (CBU) from Tinos, Syros, Andros, and Sifnos. The CBU metamorphic rocks comprise low-grade metamorphic schists, marbles, and high-pressure mélanges and were subducted at the Aegean subduction zone. The metasediments received a high-pressure metamorphic imprint between 55 and 30 Ma. They exhibit element compositions similar to modern Eastern Mediterranean sediments, but many have higher initial ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb than the sediments from the Hellenic Trench. These differences indicate that the composition of subducted sediments changed over time at the Aegean subduction zone. Most granitoids display geochemical signatures characteristic of arc magmas and represent an isotopic end-member of Aegean magmatism in Sr-Nd-Pb isotope space. The isotopic compositions of many granitoids overlap with those of sediments and CBU metasediments, whereas others display distinctly more radiogenic (Pb) signatures. The Cyclades Continental Basement has much higher Sr isotope ratios than the granitoids. Consequently, the isotope composition of the granitoids does not support partial melting of lower continental crustal rocks. Partial melting of metasomatized pyroxenite is unlikely, as most granitoids lie on fractional crystallization trends. The high Th/Nd and low Ce/Pb of the granitoids indicate a fractionation of these elements by accessory minerals during partial melting of the upper crustal rocks. We propose that most granitoid magmas in the Aegean form by fractional crystallization of mafic magmas derived from mantle sources modified by subducted upper continental crustal components. The granitoids require a more radiogenic (Sr and Pb) subducted component than observed in the CBU metasediments or modern sediments, possibly related to the subduction of microcontinental fragments.