Petrogenesis And Geological Significance Of Cenozoic K-rich Intermediate-acid Plutons Along Ailao Shan-Red River Fault Zone

The Ailao Shan-Red River(ASRR) fault zone, in the southeastern margin of Qinghai-Tibet Plateau, is the tectonic boundary between the Three River region and the west margin of Yangtze craton. A Cenozoic K-rich igneous rocks zone, especially its K-rich intermediate-acid plutons, along the ASRR fault zone is about a thousand kilometers in length,50-80Km in width, which is belonged the southern part of the Jinshajiang-Red River potassic alkali plutons zone. This thesis focus on the Cenozoic potassium-rich intermediate-acid plutons from NE area (3plutons) and SW area (4plutons) of the ASRR fault zone, and the data of petrology, major-trace element geochemistry, Sr-Nd-Pb isotopic composition, as well as the LA-ICP-MS U-Pb geochronology of the typical plutons are presented in the thesis. In combination with the geochemical data and regional tectonic setting, we trace the source region and discuss the petrogenesis, geodynamic implication of the potassium-rich intermediate-acid plutons. This thesis yielding the the following results and understanding:1. Zircon LA-ICP-MS U-Pb dating revealed a crystallization age of35-37Ma for the formation of the Cenozoic K-rich intermediate-acid plutons along the ASRR fault zone, in the period of late-collisional orogeny (40-26Ma) between Indian and Euroasian plates. The K-rich igneous activity was derived in the intercontinental transpression of the Oblique collision zone between Indian and Euroasian plates:Three River region, southeast Qinghai-Tibet.2. Geochemical characteristics and Source region:The formation of the Cenozoic K-rich intermediate-acid magma was mainly controlled by partial melting, and generally not affected by fractional crystallization or magmatic contamination. The K-rich plutons are a suit of shoshonitic-high potassium calc-alkaline series, characterized by high in SiO2, K2O, K2O/Na2O, and low in MgO(Mg#), TiO2, Cr, Ni. Trace element geochemistry shows that the suit of rocks have relatively high contents of Sr, low Yb, relatively enrich in LREE&LILE, depleted in HFSE(especially Nb, Ta), and lack of apparent Eu anomalies. Combined with the geochemical composition of the regional lower crustal rock, and the data of High P&T experimental petrology, the Cenozoic K-rich intermediate-acid plutons belong to high-pressure syenites or C-type adakites in geochemistry, and are derived from partial melting of the base of thickened continental crust (≥55Km). Previous systemtical studies on Sr-Nd-Pb isotopic characteristics, indicate that the source region has similar geochemical characteristics of EM Ⅱ enriched mantle, which is likely related to the mixing of the lower crust and the mantle with fingerprints of the subducted Paleo-Tethys ocean crust.3. Geodynamic implication:The continental crust of southeast Qinghai-Tibet has been thickened in the late period of main-collisional orogeny (60-41Ma) between Indian and Euroasian plates, which resulted in a large-scale lithospheric accretion and convergence under compressive stresses. In the early period (35-37Ma) of late-collisional orogeny (40-26Ma) between Indian and Euroasian plates, the K-rich intermediate-acid plutons along the ASRR fault zone are very likely to be originated from partial melting of the base of thickened crust, which is induced by the tectonic transform from compresso-shear to tenso-shear, and related to upwelling, thermal erosion of asthenospheric melts initiated by the oblique collision and face to face intra-continental subduction between the Indiachina plate and the Yangtze plate. In this process, the source region has been extensively metasomatized by the fluid released from old subducted slab. Then, these K-rich intermediate-acid plutons are divided into two different areas, about600kilometers from NE area to SW area of the ASRR fault zone, because of the large-scale left-strike slip of ASRR fault zone after35Ma.