Evidence for Continuous Mixing of Individually Fractionated, Coeval Felsic and Mafic Magmas Forming Synextensional Plutons, the Menderes Core Complex, Western Turkey

Abstract

The Erigoz and Koyunoba monzogranites in western Turkey contain igneous enclaves that provide important information concerning the magma-forming processes and their petrogenetic origin in a core-complex setting. The Erigoz monzogranite differs from the Koyunoba monzogranite in that it displays a porphyritic texture and contains abundant amphibole and allanite. In addition, two mineralogically contrasting groups of igneous enclaves have also been distinguished in the Erigoz and Koyunoba monzogranites; one group contains both hornblende and biotite, while the only mafic mineral in the other group is biotite. The host rocks and igneous enclaves are compositionally calc-alkaline and metaluminous to slightly peraluminous and belong to the I-type class of granites. The geochemical signatures of the host rocks and igneous enclaves are largely similar, but some enclaves contain high Al2O3, Fe2O3, MgO, CaO, TiO2, MnO, P2O5, Na2O, Mg#, Cu, Zn, Ni, Ga, Nb, V, Ti, and Zr and low SiO2, K2O, and Th contents. The igneous enclaves are strongly depleted in Ba and light rare earth elements (REEs) and moderately depleted in heavy REEs, Nb, P, and Ti with respect to their host rocks. The Koyunoba monzogranite and associated igneous enclaves represent both the most evolved magma and the late stages of crystallization. This study suggests that the progressive fractionation and continuous mixing/mingling of coeval crustal and mantle magmas are important in defining the near-final composition of these granitoid magmas and their igneous enclaves. The host rocks and igneous enclaves have partly overlapping Sr-87/Sr-86(I), epsilon Nd-I, and Pb isotope values that indicate a contribution from crustal assimilation. Development of these processes in synextensional granitoids is consistent with a geodynamic model involving slab rollback-induced asthenospheric upwelling as a heat source, which caused melting and mixing of lower crustal and lithospheric mantle. Rollback-induced extension also played a fundamental role in the generation of conduits for the entry of high-K, mantle-derived mafic melts into the large felsic-magma reservoirs.