WEIYA QUARTZ SYENITE IN EARLY INDOSININA FROM EASTERN TIANSHAN MOUNTAINS: PETROGENESIS AND TECTONIC IMPLICATIONS

Abstract

Voluminous literature has resulted from study of the geology and tectonic environments of the eastern Tianshan Mountains in Paleozoic, whereas tectonic evolution of the eastern Tianshan Mountains in Indosinian has not been well understood. In this contribution, we report the petrography and geochemical data for quartz syenite within the Weiya complex from eastern segment of the Middle Tianshan Mountains to constrain petrogenesis and discuss tectonic implications. Petrographically, the rock is composed of quartz (5% ~ 10%), plagioclase (15% ~ 20%, An16 ~ 18) and K-feldspar (60% ~ 65%) with minor Fe-rich biotite (5%) and calcic-hornblende (≈5%). No pyroxene or olivine has been found. Geochemically, SiO2 content of the syenite varies between 62.67% and 66.22 %. It has high content of Na2O + K2O (10.25% ~ 11.00%) and TiO2 (0.45% ~ 0.70%), high ratios of K2O/Na2O (0.85 ~ 1.60) FeOT/MgO (5.7 ~ 15.86) and CaO/Na2O (0.37 ~ 0.68), whereas the content of MgO (0.34% ~ 0.43%) and ratio of Al2O3/TiO2 (23.77 ~ 38.27) are low. The rock is metaluminous with aluminium saturation indices (ASI) of 0.89 ~ 0.98. The Weiya quartz syenite is characterized by enrichment in LILE such as Rb, Ba and Pb and in HFSE such as U, Th, Zr, Hf, but depletion in LILE such as Sr and in HFSE such as Nb and Ti. Contents of ∑REE in the rock are low and vary from 96.57 to 153.08 (× 10-6). Contents of ∑LREE and ∑HREE in this rock are 76.00 × 10-6 ~ 139.54 × 10-6 and 13.54 × 10-6 ~ 22.67 × 10-6, respectively; the ratios of (La/Sm)N, (Gd/Yb)N, (La/Yb)N are 1.86 ~ 4.78, 0.73 ~ 1.64 and 2.10 ~ 9.64, respectively, indicating that fractionation between LREE is larger than that between HREE while there is no significant fractionation between LREE and HREE. The δEu values varying from 0.94 to 2.83 indicate that Weiya quartz syenite is characteristics of positive Eu anomalies. Studies in petrography and geochemistry of the Weiya quartz syenite indicate that the rock was derived from crust-rather than mantle-origin. Based on the results of experimental petrology and geochemical comparison with other syenite bodies, it has been concluded that the Weiya quartz syenite, formed at high temperature (950°C ~ 1100°C) and pressure (15 ~ 20 kbar), was derived from dehydration melting of felsic granulites at the bottom of a thickened continental crust as a consequence of temperature rising and decompression. Magma generation could have been induced by northward subduction of the Tethys Ocean from late Hercynian to early Indosinian which triggered intense intra-continental A-type subduction followed by extension in the Tianshan Mountains. It is supposed that a geodynamic conversion from the Paleo-Asian ocean subduction-collision system to the Paleo-Thethys ocean regime took place during the late Permian to early Triassic time in the eastern Tianshan Moutains.