Laser-ICP-MS U-Pb zircon ages and geochemical and Sr-Nd-Pb isotopic compositions of the Myasar plutonic complex, Iran: constraints on petrogenesis and tectonic evolution

摘要

We conducted geochemical and isotopic studies on the Oligocene-Miocene Niyasar plutonic suite in the central Urumieh-Dokhtar magmatic belt, in order better to understand the magma sources and tectonic implications. The Niyasar plutonic suite comprises early Eocene microdiorite, early Oligocene dioritic sills, and middle Miocene tonalite + quartzdiorite and minor diorite assemblages. All samples show a medium-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of large-ion lithophile elements (LILEs, e.g. Rb, Ba, Sr), enrichment of light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, e.g. Nb, Ta, Ti, P). The chondrite-normalized rare earth element (REE) patterns of microdiorite and dioritic sills are slightly fractionated [(La/Yb)_n = 1.1-4] and display weak Eu anomalies (Eu/Eu* = 0.72-1.1). Isotopic data for these mafic mantle-derived rocks display I_(Sr) = 0.70604-0.70813, ε_(nd) (microdiorite: 50 Ma and dioritic sills: 35 Ma, respectively) = +1.6 and -0.4, TDM = 1.3 Ga, and lead isotopic ratios are (~(206)Pb/~(204)Pb) = 18.62-18.57, (~(207)Pb/~(204)Pb) = 15.61-15.66, and (~(208)Pb/~(204)Pb) = 38.65-38.69. The middle Miocene granitoids (18 Ma) are also characterized by relatively high REE and minor Eu anomalies (Eu/Eu* = 0.77-0.98) and have uniform initial ~(87)Sr/~(86)Sr (0.7065-0.7082), a range of initial Nd isotopic ratios [sNd(T)] varying from -2.3 to -3.7, and Pb isotopic composition (~(206)Pb/~(204)Pb) _ 18.67-18.94, (~(207)Pb/~(204)Pb) = 15.63-15.71, and (~(208)Pb/~(204)Pb) = 38.73-39.01. Geochemical and isotopic evidence for these Eocene-Ologocene mafic rocks suggests that the magmas originated from lithospheric mantle with a large involvement of EMH component during subduction of the Neotethyan ocean slab beneath the Central Iranian plate, and were significantly affected by crustal contamination. Geochemical and isotopic data of the middle Miocene granitoids rule out a purely crustal-derived magma genesis, and suggest a mixed mantle-crustal [MASH (melting, assimilation, storage, and homogenization)] origin in a post-collision extensional setting. Sr-Nd isotope modelling shows that the generation of these magmas involved ~60% to 70% of a lower crustal-derived melt and ~30% to 40% of subcontinental lithospheric mantle. All Niyasar plutons exhibit transitional geochemical features, indicating that involvement of an EMII component in the subcontinental mantle and also continental crust beneath the Urumieh-Dokhtar magmatic belt increased from early Eocene to middle Miocene time.