A-type granitoids from the Eastern Pontides, NE Turkey: Records for generation of hybrid A-type rocks in a subduction-related environment

Abstract

The Eastern Pontides is characterized by the many of intrusive bodies formed throughout the late Mesozoic–early Cenozoic. Most of these are I-type granitoids, but here, we present for the first time an A-type pluton from the region to assess source characteristics and geodynamic implications. The A-type Pirnalli pluton has a SHRIMP zircon U–Pb age of 81.2±1Ma. It is composed of granite, syenite and quartz monzonite, and its enclaves are monzonitic in composition with elevated Ga/Al ratios and low Mg# (<42). The pluton is dominantly metaluminous (A/CNK=0.88 to 1.00) and belong to shoshonitic and ultra-potassic series. The samples are highly enriched in LREE and show significant negative Eu (Eu/Eu*=0.33 to 0.92), Ba, Nb, Sr and Ti anomalies in the spidergrams. The host rocks and their enclaves posses quite similar Sr–Nd isotopic compositions (ISr=0.70693 to 0.70736, εNd (81Ma)=−2.6 to −2.0, TDM=0.94 to 1.12Ga), pointing to a lower crustal parental magma with a minor contribution of lithospheric mantle. We hypothesize that upwelling of asthenosphere triggered melting of chemically enriched upper mantle and basic magma formed. Melting of lower crust was provided by the underplating of this basic magma. Mixing between lower crust- and mantle-derived melts at depths of lower crust appears to be most reasonable petrogenetic process responsible for generation of the pluton. Sr–Nd isotope modeling suggests mixing of 82–90% of the lower crustal-derived melt with ~10–18% of the mantle-derived melt. Then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types. Our data also suggest that the A-type Pirnalli pluton likely formed at an extensional environment of active continental margin throughout the late Cretaceous. Ongoing extension then led to opening of Black Sea as a back-arc basin further north of the Eastern Pontides.