The Role of Syn-Extensional Lamprophyre Magmatism in Crustal Dynamics-the Case of the Menderes Metamorphic Core Complex, Western Turkey

The Menderes Massif in Turkey represents one of the largest metamorphic core complexes in the world. It is regarded as a section of lower continental crust exhumed along low-angle detachment faults in the Late Miocene during a period of extension that affected the entire Aegean province. Syn-extensional magmatic activity within the Menderes metamorphic core complex is predominantly felsic forming several plutons, whereas mantle-derived magmatism has not been known so far. Here, we present a detailed study of the petrology and geochemistry of previously unreported mafic to intermediate lamprophyres within the Menderes Massif and assess their role in the geodynamic evolution of the core complex. The Menderes lamprophyres are mostly kersantites, with 49-60 wt % SiO2, 3.2-8.4 wt % MgO, 100-360 ppm Cr, 32-132 ppm Ni and Mg# of 37-50. Positive Pb and negative Ti-Nb-Ta anomalies suggest a clear orogenic affinity. Isotopes of Sr and Pb are relatively radiogenic (Sr-87/Sr-86 = 0.70609-0.71076; Pb-206/Pb-204 = 18.88-19.03, Pb-207/Pb-204 > 15.71), while Nd is unradiogenic (epsilon Nd = -1.4 to -3.2). Most phenocrysts are sharply zoned with a primitive core (Mg# 77-85, up to 0.95 wt % Cr2O3 in clinopyroxene; Mg# 72-76 in amphibole) and a more evolved rim (Mg# 68-74, <0.25 wt % Cr2O3 in clinopyroxene; Mg# 69-71 in amphibole). Trace element ratios between different cores may vary significantly (e.g. Dy/Yb 2-5 in amphiboles), whereas rims show less variation but are more enriched than the cores. U-Pb dating of zircons provides an age of 15 Ma for the lamprophyres, coeval with the syn-extensional granite magmatism. The Hf isotopic composition of magmatic zircons is variably unradiogenic (Hf-176/Hf-177(15Ma) = 0.28248-0.28253, epsilon Hf-15Ma = -8.6 to -10.5), while zircon xenocrysts with dominantly Cadomian and older ages show highly variable Hf isotopic signatures at the time of lamprophyre emplacement (epsilon Hf-15Ma = -7.6 to -46.7). The orogenic geochemical signature of the lamprophyres' parental melts is similar to nearby orogenic lavas from the West Anatolian Volcanic Province. Variation in bulk-rock epsilon Nd and in Dy/Yb ratios of phenocryst cores reflect moderate mantle heterogeneity. The chemical heterogeneity of phenocrysts and zircon epsilon Hf values implies intense hybridisation of proto-lamprophyre melts with felsic crustal melts, most probably derived from the melting of augen gneisses of the Menderes basement. We propose that fluid released from the lamprophyre primary melt had a decisive impact on crustal melting and the formation of granitic plutons within the Menderes core complex.