Zircon U-Pb, geochemical and isotopic constraints on the age and origin of A- and I-type granites and gabbro-diorites from NW Iran

摘要

The continental crust of NW Iran is intruded by Late Cretaceous I-type granites and gabbro-diorites as well as Paleocene A-type granites. SIMS and LA-ICPMS U-Pb analyses of zircons yield ages of 100-92 Ma (Late Cretaceous) for I-type granites and gabbro-diorites and 61-63 Ma (Paleocene) for A-type granites. Late Cretaceous gabbro-diorites (including mafic microgranular enclaves; MMEs) from NW Iran show variably evolved signatures. They show depletion in Nb and Ta on N-MORB-normalized trace-element spider-diagrams and have high Th/Yb ratios, suggesting their precursor magmas were generated in a subduction-related environment. Gabbro-diorites have variable zircon epsilon Hf(t) values of +1.2 to +8, delta O-18 of 6.4 to 7.4 parts per thousand and bulk rock epsilon Nd(t) of -1.4 to similar to +4.9. The geochemical and isotopic data attest to melting of subcontinental lithospheric mantle (SCLM) to generate near-primitive gabbros with radiogenic Nd isotopes (epsilon Nd(t) = similar to +4.9) and high Nb/Ta and Zr/Hf ratios, similar to mantle melts (Nb/Ta similar to 17 and Zr/Hf similar to 38). These mafic melts underwent further fractionation and mixing with crustal melts to generate Late Cretaceous evolved gabbro-diorites.