Petrogenesis Of Hornblende Gabbro Plutons From The Huangshan Area In The Jueluotage Belt,Eastern Part Of Tianshan

This thesis takes Early Permian hornblende gabbros in the Huangshan area as the object of study.Based on the detailed geological investigation,here we report the LA-ICP-MS U-Pb zircon ages,whole-rock geochemistry,and Sr-Nd isotope data,taking into account previous magmatism,tectonism and sedimentation,our aims are to decipher their formation age,petrogenesis,source nature and tectonic setting.The hornblende-gabbro plutons at Huangshan area in eastern Hami city of Xinjiang Uygur Autonomous Region,which located in the eastern Jueluotage belt in tectonic background,as irregular rock strain along the Kangguer-Huangshan fault rim,as a intrusion in Carboniferous Wutongwozi and Gandun formations.Detailed petrology revels that these hornblende grbbros have massive structure and variable textures,including gabbro,poikilitic and cumulate tectures.These mafic rocks are mainly composed of plagioclase and hornblede,with accessory minerals of magnetite and apatite.Two representative hornblede gabbro samples in the Huangshan area were selected for LA-ICP-MS zircon U-Pb analyses.Zircon grains from these samples are subhedral to euhedral,and display internal growth zoning in CL images.Generally,they have high Th/U ratios,suggesting a magmatic origin.Two samples yielded similar weighted mean²⁰⁶Pb/²³⁸U ages of 281±1.2 Ma and 282±1.3 Ma?within error?interpreted as magmatic crystallization ages.Whole-rock geochemistry shows that the Huangshan hornblede gabbros have features from calc-alkaline to low-K tholeiitic,with low SiO₂?46.5%⁵2.8%?and K₂O?0.34%⁰.51%?concentrations,and high FeO?5.94%¹0.3%?,MgO?5.15%⁸.42%?and CaO?7.80%⁹.12%?contents similar to island-arc basalt.These hornblede gabbros are characterized by enrichment in LREE and slight positive Eu anomalies in the REE patterns.They are obviously enriched in LILEs?e.g.,Rb,Ba,K,and Sr?and are depleted in HFSEs?e.g.,Nb,Ta,Zr,P,and Hf?,suggesting the input of crustal material.In addition,the Huangshan hornblede gabbros have extremely positive?Nd?t?from+10.66 to+12.54?t=282 Ma?,indicating a depleted mantle origin metasomasised by subduction fluid,which also contaminated with asthenosphere material.Taking into account previous igneous rock association,sedimentation and tectonic evolutionary history in this region,we suggest that the studied hornblede gabbros were formed in an extensional tectonic setting after the collision of Tuha Block and Middle Tianshan Block.