Geochemistry of high-grade metamorphic rocks of the North Qaidam mountains and their geological significance

The high-grade metamorphic basement in the North Qaidam mountains (NQD) is composed mainly of Jinning age metasedimentary rocks and crustally-derived granites, possibly accompanied by minor Paleoproterozoic and Archaean material. Based on their geochemistry the metasedimentary rocks are divided into two groups. Type I, which makes up the bulk of the high-grade metamorphic basement in the NQD, is characterized by high K2O, low Na2O, relatively high total rare earth elements (TREE=114–357 ppm), and weak fractionation of light (LREE) and heavy (HREE) rare earth elements ((La/Yb)n=8–27). These rocks also have strong negative Eu anomalies (Eu/Eu*=0.3–0.6). Type II rocks crop out mainly in the Wulan area and are characterized by relatively high MgO (2.2–4.1 wt%), lower total REE and strong fractionation of LREE and HREE (La/Yb)n=29–48). Both types of metasedimentary rocks have similar Nd isotope compositions with tDM=1.9–2.2 Ga and εNd=−4.1 to −8.5. The granites are compositionally similar to the Type I metasedimentary rocks. The high-grade metamorphic basement of the NQD is similar to that in the Qilian orogenic belt and the Yangtze Platform. The Central Qilian-NQD terrane was separated from the Dunhuang–Alxa terrane before the Jinning Period, and the two terranes belong to the South China Block and North China Craton, respectively.