Geochemistry and petrogenesis of the Jitang granitoids in Tibet, SW China

The Lancang River tectonic zone in southwestern Yunnan and eastern Tibet is important for the study of the evolution of the Paleotethys. The most important component of the Lancang River tectonic zone is the Triassic granitoids which occur over 1000km from Leiwuqi, Tibet in the north to Yunnan in the south. The Jitang granitoid intrusion is a representative body in the northern part of this granite belt. It consists of granodiorite and monzonitic biotite granite. The outcrop of this intrusion is 70km long and 2 similar to 10km wide. The overall straking of the intrusion is NNW. It occur in the west side of Lancang River, similar to 3km west of Jitang Town, Changdu City. Whole-rock Rb-Sr isochron age is similar to 220Ma according to a previous study. This paper reports whole-rock major and trace element compositions, and Sr, Nd isotopes for the Jitang granitoid intrusion. These data are used to evaluate source characteristics and tectonic setting for the intrusion. The Jitang granitoid samples contain 63.5% similar to 70.9% SiO2, 2. 8% similar to 4. 2% Na2O and 1.7% similar to 3.4% K2O. The alumina-saturation index (A/CNK molar) of the samples vary from 1. 1 to 1. 5. These characteristics, together with the presence of CIPW-norm corundum in the samples (1. 5% similar to 5. 6%) indicate that these samples belong to the peraluminous and calc-alkaline series. According to the classification of Chappel and White (1992), the Jitang granitoids belong to S-type granite. The granodiorite samples from the Jitang intruson have initial Sr-87/Sr-86 ratios of 0. 7280 similar to 0. 7395 and epsilon(Nd) (i) values of -14. 1 similar to -16. 2. These data suggest that the parental magma was not produced by subduction zone magmatism, but by anatexis of ancient crustal materials. The depleted mantle Nd model ages (t(DM)) of the Jitang granitoids are close to 2. 0Ga, similar to the age of gneisses of the Jitang Group which formed by metamorphism of dacite or dacitic pyroclastic protoliths. The Sr-Nd isotopic compositions of the Jitang granitoids are similar to that of the Jitang gneisses. Strong depletion of Eu and Sr in whole rocks suggests residual plagioclase in the source region; depletion of Ba in whole rocks suggests that K-fieldspar was also a residual phase. These features, together with high whole-rock CaO/Na2O ratios of similar to 0. 8 support an interpretation that the parental magma was derived by biotite-dehydration melting from a source with composition similar to greywacke, not pelite. We proposed that the melting took place at about 25km in the middle crust due to extension of the thickened crust after the closure of the paleo-Lancang ocean. Our results support the hypothesis that the Lancang River tectonic zone represents a remnant Paleo-Tethyan main ocean preserved in the collision suture between a Gondwana-derived microcontinent and the continental margin of the Yangtze craton. The Paleo-Tethyan ocean in the Lancang River area should have been closed before the formation of the Jitang granitoid pluton at similar to 220Ma. The closure is more likely to have taken place at 280Ma (Early Permian) based on other geological data.