Abstract:

In order to research the Indosinian tectonic evolution features of the Zongwulong tectonic belt, the studies of petrology, geochronology and geochemistry were conducted for the Shailekeguolai granodiorite samples and Chahannuo granodiorite samples from that area. The results of zircon SHRIMP UPb dating of three samples for the Shailekeguolai granodiorite and Chahannuo granodiorite were 249.2±2.6 Ma, 242.7±1.9 Ma and 243.5±2.4 Ma respectively, which confirmed that the intermediateacidic intrusive rocks of the Zongwulong tectonic belt were formed in the Early and Middle Triassic, and the existence of Indosinian tectonicmagmatic activity in North Qaidam has been proved. The Shailekeguolai granodiorite and Chahannuo granodiorite are weak peraluminous Itype granites of middlehigh potassium calcalkaline series, characterized by high SiO2, high Na2O and high Al2O3, with A/CNK ratio ranging between 1.021.06 and 1.011.05, Rittmann index ranging between 1.541.73 and 1.071.61 respectively. The two granodiorites experienced a relatively insufficient crystallizationdifferentiation process, which is reflected by K/Rb(234 and 279 on average respectively) and DI(72.9576.06 and 64.4976.42 respectively). The rocks show LREE enrichment(LREE/HREE=6.839.70 and 3.297.40 respectively) and weak negative and positive Eu anomalies. The primitive mantle normalized trace element spider diagrams are characterized evidently by positive anomalies of Rb, U, La, Pb, Sr and negative of Nb, P, Ti. Microgranular mafic enclaves(MMEs) are developed in the Chahannuo granodiorite. Compositional and textural disequilibrium between plagioclase and hornblende in the MMEs and host rocks had been found, suggesting a magma mixing origin for the granodiorites. The granodiorites are products of lower crustmantle mixed magma due to the crustal thickening caused by active continental margin subduction and arccontinent collision during the Indosinian period in the North Qaidam. Based on the comparison of geologic features of the magmatites between the Zongwulong tectonic belt in North Qaidam and the Ela mountain tectonic belt in East Qaidam, the formation of the Zongwulong tectonic belt may have a close genetic relationship with the intense oblique collision of West Qinling along the Gonhe aulacogen towards Qaidam blocks.

Key words: Zongwulong tectonic belt, Shailekeguolai, Chahannuo, granodiorite, Indosinian, magmamixing origin, subduction and collision