Mineralogy and chemistry of biotites from Eastern Pontide granitoid rocks, NE-Turkey: Some petrological implications for granitoid magmas

The Eocene Zigana Granitoid in the northern zone and the Middle-Upper Eocene Dolek and Saricicek Plutons in the southern zone of the Eastern Pontides (NE-Turkey) consist of different rock series ranging in composition from intermediate to felsic compositions containing from <1% to 9% of biotite. The biotites are Mg-rich (XFe = Fe2+/Fe2+ + Mg) = 0.28 - 0.49) in the southern zone plutons and Mg- to Fe-rich (XFe = Fe2+/Fe2+ + Mg) = 0.33 - 0.78) in the northern zone pluton. They coexist with K-feldspar, plagioclase, quartz, amphibole, pyroxene, low titanium magnetite, minor ilmenite titanite. Biotites have been buffered by the Ni-NiO. The plutons equilibrated under low oxygen fugacities (LogfO2) ranging from -16.01 to -20.23 bars for Dolek and Saricicek, and -16.70 to -17.90 bars for Zigana. EPMA indicate that the composition of biotite is related to the mineral assemblages of Dolek, Saricicek and Zigana biotites as a consequence of chemistry of the magma and the physical conditions during crystallization. The biotite compositions were used to discriminate the tectonic setting of the plutons. The intrusions appear to be related to calc-alkaline to high-K calc-alkaline orogenic suites. Furthermore, Mg(6)-Fetot substitution has played a significant role during the crystallization of the biotites.