Geochemical and Sr-Nd isotopic constraints on the genesis of the Soheyle-PaKuh granitoid rocks (central Urumieh-Dokhtar magmatic belt, Iran)

摘要

Soheyle-Pakuh granitoid rocks, with a variety of quartz diorite, quartz monzodiorite, granodiorite, tonalite, and granite, have been emplaced into the Tertiary volcanic rocks in the Urumieh-Dokhtar magmatic arc in central Iran. Zircon U-Pb dating yields an age of 39.63 +/- 0.93 Ma for the crystallization of this body. Whole-rock compositions show that SiO(2)changes from 52.31 to 65.78 wt.% and Al(2)O(3)varies from 15.54 to 18.24 wt.%, as well as high concentrations of large-ion lithophile elements (LILE, e.g. Cs, Rb, Ba, and K) and quite low contents of high field strength elements (HFSE, e.g. Nb, Ti, P), as expected in I-type arc granitoids formed in an active continental margin setting. Initial ratios of(87)Sr/Sr-86 and(143)Nd/Nd-144 exhibit ranges 0.7043-0.7047 and 0.51284 to 0.51287, respectively, with positive epsilon Nd((t))from +4.9 to +5.5 with a young TDM1 age (483-674 Ma); this tracer isotopic data suggesting that the SPG originated from juvenile basaltic crust derived from depleted mantle (~90%) with variable contributions from undepleted mantle and approximately 10% old lower crust, despite diverse processes (e.g. magma mixing and fractional crystallization) during their evolution and emplacement into a local extensional setting within the continental margin arc. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly mantle-derived juvenile crustal protoliths and indicating extensive addition of new continental crust, during Cambrian-Neoproterozoic time, in the suprasubduction zone beneath the central Urumieh-Dokhtar magmatic arc. Generation of these types of granitoids favours a model whereby rollback and (or) break-off of a subducted slab with subsequent lithospheric extension triggered by mantle upwelling, heat advection, and underplating resulting in melting of the central UDMA mantle-derived juvenile lower continental crust in the Late Eocene.