Study On Geological And Geochemical Characteristics Of The Xiongmei And Shesuo Plutons In Xiongmei Area,Tibet

Late Cretaceous magmatism and mineralization in the Xiongmei area are representative in the Tibet Plateau.This paper presents petrology,zircon U-Pb chronology and geochemistry of two typical porphyries?Xiongmei and Shesuo?in Xiongmei area to discusses the origin and evolution of magma and deep geological processes.The zircon U-Pb dating results show that the ages of the three samples from the Xiongmei porphyry are 109.7±1.3 Ma,110.3±1.7 Ma and 105.3±2.2 Ma respectively;the ages of the two samples from the Shesuo porphyry are 110±1.9 Ma and 107.1±2.3Ma.The Xiongmei granodiorite porphyry is strong-peraluminous I-type granite,with arc magmatic characteristics and low magmatic differentiation degree.The Shesuo monzogranite porphyry belongs to weak-peraluminous I-type granite with arc magma characteristics,and plagioclase crystalline differentiation occurred during magma evolution.The Xiongmei granodiorite porphyry has a high Mg#value?39⁵9?and high Cr?10.9³5.5 ppm?and Ni?10²24 ppm?content,suggesting the possible source of mantle-derived melts.In addition,the Xiongmei granodiorite porphyry has a relatively high(⁸⁷Sr/⁸⁶Sr)_i value?0.7093⁰.7103?and a low?Nd?t?value?-6.8 to-8.0?,and also has a high content of incompatible elements?Rb,Ba,Th,U,Pb?and Nb-Ta-Ti negative anomalies.These characteristics indicate that there may be a source region for the Xiongmei porphyry that is metamorphosed by crustal material melting/subduction oceanic crust fluid.Compared to the Xiongmei granodiorite porphyry,the Shesuo monzogranite has lower MgO,Cr,Ni content and negative?Nd?t?values,but has similar negative Nb-Ta-Ti anomalies and positive Pb anomaly.These characteristics indicate that the source areas of the two plutons are similar,while the Shesuo magma source area has more crust-derived materials.Comprehensive analysis suggests that in Early Cretaceous,the Bangong-Nujiang Tethys ocean diving south and then plate retracement?roll-back?took place,which caused small-scale asthenosphere upwelling.This upwelling asthenosphere transmits heat to the subducted oceanic crust and causes slab dehydration,which produces fluid to the contribution of the formation of enriched lithospheric mantle reacting with the overlying ancient lower crust.Underwent long-term crust-mantle interaction,the metal elements?e.g.,copper,gold and molybdenum?were enriched in the contact zone between juvenile lower crust and the overlying ancient lower crust.By the late Early Cretaceous,the subducting slabs were broken off due to deep subduction,which resulted in a large-scale upwelling of asthenosphere materials.Large amounts of melts were generated under regional decompression and heating,which can dilute the metal elements enriched previously.The metal-bearing melt upwelling through structural weak zone and forms the Xiongmei granodiorite porphyry and related copper polymetallic deposits.Metal elements within strata was activated and extracted by the metal-bearing melt to form the Shesuo monzogranite and related copper polymetallic deposits in the shallow crust.