The role of accreted continental crust in the formation of granites within the Alpine style continental collision zone: Geochemical and geochronological constrains from leucogranites in the cataldag Metamorphic Core Complex (NW Turkey)

摘要

The recently discovered cataldag Metamorphic Core Complex (cMCC) of Eo-Oligocene age is situated in NW Turkey, which is a part of the Alpine collisional belt. The footwall of the cMCC consists of a granite-gneissmigmatite complex (GGMC) with a domal structure. Here, we document new major-trace element geochemistry, Sr-Nd-Pb isotope characteristics and Ti-in-zircon thermometry data from granites and syn-plutonic mafic dykes that are connected to the gneiss-migmatite dome of cMCC, and discuss granitic melt evolution within the context of the syn-to post-collisional evolution of the western Anatolian orogenic crust. Granites of the GGMC include both peraluminous garnet-bearing leucogranite and two-mica granite that emplaced from 32 to 35 Ma. Both types of leucogranite are enriched in light rare earth elements (Rb, U, K, Pb) and depleted in HFSE (Nb, Ta, Zr, Ti). Their (87)sr/(86)sr, Pb-206/Pb-204 and Pb-207/Pb-204 initial isotope values range from 0.7094 to 0.7113, 18.79 to 18.91, and 15.71 to 15.73, respectively, and ENd(33) values vary between -5.13 and -7.79. On the other hand, mafic dykes show similar isotopic characteristics (Sr-82/Sr-86((33)) = 0.7055, epsilon Nd-(33) = -1.8 and Pb-206/Pb-204 = 18.8) to enriched mantle melts. Geochemical modelling shows that the granites have dominant crustal melt component (85-70%) and a minor mantle component (<30%). According to partial melting modelling and Ti-in-zircon thermometer calculations, granitic melt was formed by water-absent muscovite dehydration melting of a micaschist source (a melt fraction of max. 35%) at >= 7-10 kb and 739-840 degrees C. Whole rock geochemistry, isotopic characteristics and inherited zircon chronology, combined with the geology of the region, indicate that granitic melts were derived by partial melting of Anatolide-Tauride crustal units that were underthrusted below the Sakarya Continent along the Izmir-Ankara Suture Zone. On the other hand, the source of the syn plutonic mafic dykes emplaced in the core of the CMCC is inferred to derived from enriched mantle (EM II) that lay beneath the western Anatolian orogenic crust. We infer that partial melting and melt migration to produce leucogranites are most probably related to thermal weakening and partial removal of western Anatolian young orogenic lithosphere during the transitional phase between the latest phase of collision and the earliest phase of extension, in the Eo-Oligocene. (C) 2019 Elsevier B.V. All rights reserved.