Petrogenesis Implications Of The Late Triassic Ludian Granite Batholith In Northwestern Yunnan

The Jinshajiang suture zone,located in the Paleo-Tethys tectonic regime,is an important place to study the processes of oceanic spreading,slab subduction beneath continent,continental collision as well as post-collisional magmatism.Several magmatic arcs are distributed along the Jinshajiang suture zone,among which the Jomda-Weixi magmatic arc is an ideal place to study post-collisional magmatism.Previous studies mainly focused on regional geochronological framework and tectonic setting of magmatic rocks,while the magma sources and magmatic processes are still not well constrained.This study focuses on the Ludian granite batholith,Jomda-Weixi magmatic arc in order to reveal the origin of magmas and magmatic processes of the batholith.Twenty one samples were collected across the central of the Ludian granite batholith in this study.These samples are divided into two groups based on petrographic and geochemical characteristics.Group 1 consists of 11 granodiorite and5 monzogranite,while Group 2 includes 5 syenogranite.LA-ICP-MS zircon U-Pb dating results show that:1)Two monzogranite and four granodiorite from Group 1have similar age of²35 Ma.One granodiorite in this group has slightly younger age of 229.7±2.7 Ma?2??;2)One dated syenogranite vein of Group 2 has an age of228.9±1.8 Ma?2??.The samples of Group 1 have SiO₂ contents of 65.27–71.41wt.%with Mg^#of 39–51?They belong to high-K calc-alkaline series and are weakly peraluminous with A/CNK varying from 0.99 to 1.14?mostly<1.1?and therefore belonging to I-type granite.The samples of Group 1 have low contents of Y?<18 ppm?and Yb?<1.9 ppm?,variable Sr/Y ratios?9.07–34.88?and?La/Yb?_N ratios?13.85–33.58?,and moderately negative Eu anomaly?Eu/Eu*=0.45–0.70?.The samples of Group 2 have high SiO₂ contents?74.86–78.83 wt.%?belonging to high silica granite and have low Mg^#of 12–27.They vary from low-K calc-alkaline series to shoshonitic series and are weakly peraluminous with A/CNK varying from 0.98 to1.07 and therefore belonging to fractionated I-type granite.The samples of Group 2have low contents of Sr?47–193 ppm?and low Sr/Y ratios with majority less than 5.They have flat chondrite normalized REE patterns with variable REE contents.They have variable?La/Yb?_N ratios of 2.12–6.40 and strongly negative Eu anomaly?Eu/Eu*=0.09–0.57?.Zircon Hf isotopes indicate that the two groups have similar Hf isotopic compositions and two stage Hf model ages?T_D^C _M?.One monzogranite from Group 1 has?_Hf?t?of-9.3 to-7.5,with T_D^C _M varying from 1719 Ma to 1828 Ma.One granodiorite from Group 1 has?_Hf?t?of-9.6 to-6.2,with T_D^C _M varying from 1634 Ma to 1848 Ma.One syenogranite from Group 2 has?_Hf?t?of-9.7 to-7.9,with T_D^C _M varying from 1740 Ma to 1848 Ma.Based on our new data and previously reported data,this study suggests that the Group 1 samples of Ludian granite batholith were derived from partial melting of crustal amphibolite and their geochemical variation resulted from fractional crystallization of plagioclase,Fe-Ti oxide,biotite,and apatite with negligible magma mixing or contamination.The Group 2 samples are genetically related with Group 1and are fractionated products of Group 1 samples.