National Research Center for Geoanalysis, Beijing 100037, China

Systematic petrological and geochemical analysis were conducted Late Silurian Bitu complex in northern West Junggar, with the attempt to reveal its petrogenesis and tectonic setting. The Bitu complex mainly consists of syenogranite and subordinate alkaline granite, syenite and diorite. The syenogranite and syenite are highly-fractionated I-type granitoids, characterized by low contents of MgO (0.3%~0.5%), CaO (0.8%~1.1%), high REE (105~204 ��g/g) and strong Eu anomalies. The alkaline granite shows low Al₂O₃ (9.9%), high total alkalis (6.8%) and REE (271 ��g/g), strong depletion in Sr, Ba, Eu, with the affinity of A-type granite. The diorite exhibits high MgO (4.1%) and CaO (7.5%),and relatively high REE abundance (93 ��g/g). It belongs to high-K calc-alkaline series and was formed by partial melting of H₂O-rich mantle. Geochemical data suggest that granitic rocks of Bitu complex were originated from crystallization differentiation of partial melting juvenile crust. The Bitu complex is enriched in alkali, light rare earth elements and large ion igneous elements (such as Rb and K) and slightly depleted in Nb and Ta, indicating that it was generated in a post-collisional extensional setting.