The Magmatic Origin Of Banggunjianshan And Polunshan Granitoids In Tengchong Block, Western Yunan

The Ruili region is located in the Tenghcong block, know as a part of the East Tethyan belt. In this region there are widespread Mesozoic to Cenozoic granitic intrusions. In this study, we carry out geochronology and geochemistry study on the Banggunjianshan(BGJS) and Polunshan(PLS) granitoids. Polunshan pluton is composed of granodiorite, dated at 62.8-67.5Ma with zircon U-Pb method. North BGJS pluton is composed of granodiorite and some diorite enclaves, both dated at 73. l-74.7Ma. South BGJS pluton is mainly composed of granite, dated at 49.0-50.8Ma. Geochronology data suggests that there are three magmatic activities during late Cretaceous, early Paleocene and early Eocene in the Ruili Region.BGJS and PLS plutons are featured by high alkaline contents. PLS pluton is highl-K calc-alkaline and BGJ pluton is apt to shoshonitic. These rocks are enriched in large-ion lithophile elements (LILEs) such as potassium and rubidium, and depleted in high field-strenth elements (HFSEs), with ACN/K=0.95-1.03, considered as metaluminous-weak peraluminous. BGJS pluton is comparatively enriched in rare earth elements (REEs), especially light REEs, and more distinct depleted in Eu, Sr, Ba, P and Ti elements, suggesting a more significant fractional crystallization process. Whole rock Sr-Nd isotope data of PLS pluton shows an enrich signature (average 87Sr/86Sr(t)≈ 0.716028, ε Nd(t)≈-15.0). Further evidence is given by Paleoproterozoic inherited zircon, suggesting a magma source mainly composed of Paleoproterozoic crust, with negligible mantle contribution. This Paleoproterozoic crust can be regarded as a widespread basement, which also appears in Sibumasu and Lhasa terrain. The whole rock Sr-Nd signature of South BGJS pluton is slightly depleted (average 87Sr/86Sr(t) ≈0.712385, ε Nd(t)≈-9.5). Paleoproterozoic inherited zircons are also found in the pluton, suggests the magma source of South BGJS pluton is the mixing product of Paleoproterozoic crust and mantle-derived magma. The two-component mixing equations constrain the proportion of asthenosphere mantle and an enrich mantle newly modified by slab-derived fluids. The North BGJS pluton is slightly complicated. A granite and a diorite enclave sample is featured by a εNd(t) around -12, while the other granite and diorite enclave is featured by a εNd(t) around -7. They might be two different melting product of different components during the Ceno-Tethys subduction.The geochronology and geochemistry signatures of PLS and BGJS plutons suggest the following scenario:during early Paleocene and early Eocene, Ceno-Tethys is experiencing the transformation from subduction to closure. In the meantime, the ancient crust of Tengchong block is reworked. The interaction between mantle and crust is significantly enforced.