Sibel Tatar Erkül

Petrogenetic evolution of the Early Miocene Alaçamdağ volcano-plutonic complex, northwestern Turkey: implications for the geodynamic framework of the Aegean region

Extensional-tectonic processes have generated extensive magmatic activity that produced volcanic/plutonic rocks along an E-W-trending belt across north-western Turkey; this belt includes granites and coeval volcanic rocks of the Alaçamdağ volcano-plutonic complex. The petrogenesis of the Early Miocene Alaçamdağ granitic and volcanic rocks are here investigated by means of whole-rock Sr–Nd isotopic data along with field, petrographic and whole-rock geochemical studies. Geological and geochemical data indicate two distinct granite facies having similar mineral assemblages, their major distinguishing characteristic being the presence or absence of porphyritic texture as defined by K-feldspar megacrysts. I-type Alaçamdağ granitic stocks have monzogranitic-granodioritic compositions and contain a number of mafic microgranular enclaves of monzonitic, monzodioritic/monzogabbroic composition. Volcanic rocks occur as intrusions, domes, lava flows, dykes and volcanogenic sedimentary rocks having (first episode) andesitic and dacitic-trachyandesitic, and (second episode) dacitic, rhyolitic and trachytic-trachydacitic compositions. These granitic and volcanic rocks are metaluminous, high-K, and calc-alkaline in character. Chondrite-normalised rare earth element patterns vary only slightly such that all of the igneous rocks of the Alaçamdağ have similar REE patterns. Primitive-mantle-normalised multi-element diagrams show that these granitic and volcanic rocks are strongly enriched in LILE and LREE pattern, high (87Sr/86Sr)i and low εNd(t) ratios suggesting Alaçamdağ volcano-plutonic rocks to have been derived from hybrid magma that originated mixing of co-eval lower crustal-derived more felsic magma and enriched subcontinental lithospheric mantle-derived more mafic magmas during extensional processes, and the crustal material was more dominant than the mantle contribution. The Alaçamdağ volcano-plutonic complex rocks may form by retreat of the Hellenic/Aegean subduction zone, coinciding with the early stages of back-arc extension that led to extensive metamorphic core-complex formation.

An extensional and transtensional origin of elongated magmatic domes and localised transfer faults in the northern Menderes metamorphic core complex, western Turkey

Abstract The northern Menderes metamorphic core complex has complex exhumation history and is one of the key localities to investigate the spatial and temporal relationships of extensional and compressional structures. Detachment faults and syn-extensional plutons are linked to a series of antiforms and synforms and the denudation of the northern Menderes Massif occurred in three stages. The first stage is related to the development of detachment faults under the consistent NE–SW-directed extension. The second stage is represented by a series of elongated magmatic domes that were oriented parallel, oblique and perpendicular to the regional extension direction. Emplacement of these asymmetrical magmatic domes appears to have been controlled by heterogeneous extension and post-dates the extensional Simav detachment fault. On the third stage, progressive heterogeneous extension that led to updoming of plutons has been finally accommodated by a localised and short-lived transfer zone, which was described as the Gerni shear zone for the first time in this study. The transfer zone is formed by a NE-striking, dextral ductile/brittle shear zone that accommodated the propagation of folds, conjugated strike-slip faults and normal- and oblique-slip faults. Mylonites associated with the transfer zone are related to the localisation of strain along the thermally weakened strike-slip fault systems by short-lived intrusions rather than to the development of regional-scale detachment faults. These structures are consistent with a transtensional simple shear model, which properly explains the evolution of extensional and compressional structures exposed in the northern Menderes core complex. Structural setting of the Eğrigöz region is somewhat similar to that of the NE-trending gneiss domes in the northern Menderes Massif and updoming of magma during late stages of detachment faulting appears to have played an important role in the exhumation of lower and upper plate rocks.

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

The Eğrigöz 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 Eğrigöz 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 Eğrigöz 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 87Sr/86SrI, εNdI, 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.