Conditions of magma genesis and syn-tectonic emplacement of the granitic rocks of Tinos Island (Cyclades, Greece)

<p>During the Miocene, the Aegean region experienced rapid extension, accommodated by a series of crustal-scale detachments, several of which were intruded by syn-kinematic granitoids. This extensional event led to the exhumation of the Cycladic Blueschist Unit (CBU) and the formation of the Cycladic metamorphic core complexes (MCCs). On Tinos Island, the NE part of the metamorphic dome is intruded by a weakly metaluminous I-type, Bio-Hbl granodiorite, surrounded by smaller bodies of peraluminous Kfs- Grn-bearing leucogranite (S-type). Zircon U-Pb dating shows that the two different types of granitoids were synchronous, i.e., 14.73 &#177; 0.22 Ma and 14.4 &#177; 0.2 Ma for I-type and S-type, respectively. The applied geothermometers, geobarometers and hygrometers indicate that the I-type granites crystallized at higher temperatures, whereas crystallization pressures are within the same range for both types. Although the S-type melt appears more hydrous than the I-type, both compositions indicate H₂O-undersaturated conditions. These empirical observations are consistent with the geochemistry of these granites, suggesting a derivation through dehydration melting of Bio-Ms schist for the S-type and Bio-gneiss/amphibolite for the I-type granites. I-type granites appear enriched in Zr, possibly associated with higher magmatic temperatures and their metaluminous affinity that enhances the solubility of Zr, in contrast to peraluminous melts.</p> <p>Upper-crust normalized compositions of the I-type granites point to a dominant metasedimentary source, with mild negative Ba anomalies and mildly positive- to no-anomaly in Sr, suggesting the presence of plagioclase in the source rock. On the other hand, the S-type granites have pronounced negative Ba and Sr anomalies and high Rb/Sr. The Rb-Sr concentrations in Tinos granites appear to have been controlled by the dehydration melting of muscovite and biotite. For the S-type granites, this is consistent with the dehydration melting at ~ 5.5 kbar, as indicated by the Grn-Bio-Plg barometer. Peritectic garnet has been found in the Tinos S-type granites showing resorption features and containing inclusions of quartz, K-feldspar and melt. Biotite-dehydration melting triggers a substantial increase in the Rb/Sr ratio with little effect on either Ba or Sr since these are hosted mainly in plagioclase.</p> <p>The similarity in crystallization pressures of the S- and I-type granites could imply that crystallization occurred at the structural level of emplacement along with the S-type magma. This could be due to a lower viscosity of the I-type granitoid resulting from its drier and high-T nature. Thus, it is postulated that elevated temperatures at the lower crust during crustal extension and probably asthenospheric upwelling, resulted in the dehydration melting of the Cycladic basement, producing the I-type melt. The ongoing crustal thinning and the lower melt viscosity enhanced magma ascent and resulted in a rapid emplacement at the middle crust, within the CBU unit.&#160;</p>