Cenozoic granitoid in Tengchong, western Yunnan: Genesis type and implication for tectonics

Cenozoic granitoid developed in Tengchong, an area located in the SE are structural belt of east structural knot of Himalayan orogen, recorded lots of information about the time of India-Asia continental collision, structural evolution and continent dynamics in the area. In term of the lithofacies and intrusion age, the granitoid can be subdivided into monzonite granite, syenogranite, muscovite granite and muscovite-albite granite. The 40Ar/39Ar data indicate that ages of the firstly intrusived monzonite granite and muscovite granite range from 66Ma to 58Ma, and ages of the syenogranite and monzonitic granite, which intrusived with a large scale, range between 54 - 52 Ma and 43 - 41 Ma. According to Aluminium Saturation Index (ASI) of the granite, we can conclude that monzonitic granite and syenogranite, which ASI value approaches to 1, belong to metaluminous-peralummous high K calc-alkaline series, and muscovite(-albite) granite (ASI: 1.02 - 2.63) are peraluminous-strongly peraluminous granite. K2O/Na2O value of all the granitoid exceed 1, but from syenogranite, monzonitic granite to muscovite (-albite) granite, K2O/Na2O value and SiO2 content increase gradually. The syenogranite and monzonitic granite are rich in Sr, Ba and poor in Rb, with light-REE enriched and strongly negative Eu anomalies. In constrast, muscovite (-albite) granite are characterized by obviously higher Rb, unusually lower Sr and Ba, and swallow-shaped REE pattern, suggesting that they either originated from different magma reservoir or experienced different melt process. According to magma sequence and rock assemblage, we can conclude: (1) during 66 - 59Ma, contact and strong collision between India and Asia continent happened, resulting in crust thickening greatly and melting in deep part; (2) after 5Ma stress adjusting, short-lived relaxation and hypergene emplacement of syenogranite occurred between 54Ma and 52Ma; (3) during 43-41Ma, crust extension appeared, accompanying with development of monzonite granite and syenogranite.