Petrogenesis of Early Cretaceous high-Mg# granodiorites in the northeastern Lhasa terrane, SE Tibet: Evidence for mantle-deep crustal interaction

Abstract

High-Mg# (molar Mg/[Mg + Fe]) intermediate to felsic rocks, such as high-Mg andesitic rocks (HMA) and Mg andesites (MA), are usually produced by melting of mantle-derived peridotites and/or interaction with mantle-derived peridotites. Both are crucial in tracking mantle-crust interaction processes in the deep crust. We present LA-ICP-MS in-situ zircon U-Pb isotopic data, whole-rock geochemistry and whole-rock Sr-Nd-Pb-Hf isotopic data for the Basu granodiorites which are exposed between the Jiali fault and the Bangong-Nujiang suture, SE Tibet. The LA-ICP-MS zircon U-Pb data show that the granodiorites were formed at ca. 122.1 ± 1.0 Ma, coeval with other granitoids in this region. The granodiorites belong to the high-K calc-alkaline and high-Mg# (49.7–53.7) series, similar to typical MA. The decreasing A/CNK from 0.97 to 0.87 and increasing FM (molar Fe + Mg) from 0.08 to 0.16 indicate a clinopyroxene entrainment and I-type trend. The rocks also display enriched Rb, Th, U and Pb, but depleted P, Ti and obvious negative Eu anomalies. Evolved whole-rock isotopic compositions of Basu granodiorites include high initial 87Sr/86Sr (0.7097–0.7114), negative εNd(t) (−12.0 to −7.2) and εHf(t) values (−9.6 to −7.9), and lower to upper crustal affinity of Pb isotopes which imply that they were derived from an ancient and evolved lower crust source. Therefore, we suggest that these granodiorites were products of interaction between partial melting of ancient basaltic lower crust and mantle-derived peridotites. Together with a geochronology dataset from the literature, we suggest that there was intensive granitic magmatism occurred in the Early Cretaceous range from 133 to 113 Ma in the northeastern Lhasa terrane, similar to those in the central and northern Lhasa terrane. Furthermore, we have compared with the dataset of in-situ zircon Lu-Hf isotopes, whole-rock Sr-Nd from the literatures and the relationship with zircon U-Pb ages in the northeastern Lhasa terrane, all of which indicate the importance of evolved and ancient continental crust source in the Early Cretaceous magmatism in the northeastern Lhasa terrane, but also that the mantle-derived materials play a necessary role in the generation. In addition, the amount of enriched and evolved isotopic component gradually decreased from ca.140 to 100 Ma.