Study On The Geochronology And Geochemical Characteristics Of The Jurassic Ruocuo Intrusions, Lazi County, Tibet

The Gangdese magmatic belt is located at the southern margin of the Lhasa terrane.The magmatism within the belt recorded the northward subduction of the Neo-Tethys and the India–Asia collisional orogenic processes.Previous investigations have focused on the geodynamic processes and petrogenesis of the Cretaceous and Cenozoic igneous rocks in the Gangdese magmatic belt.However,Early Mesozoic?Middle Triassic to Middle Jurassic?magmatic rocks distributed sporadically in the belt and the study on these rocks is relatively poorly,so that both the geodynamic background of the Early Mesozoic magmatism and its relationship with the early-stage evolution of the Neo-Tethys remain debated so far.Therefore,the Ruocuo Jurassic intrusions in the southern margin of the Lhasa terrane are studied.On the basis of detailed field geological survey,we further carried out detailed petrography,geochronology,elemental geochemistry,Sr-Nd-Pb isotope geochemistry and zircon Lu-Hf isotope analysis.Then,we ascertained their formation age and petrogenesis.By combining our results with the previously reported results,we discuss the geodynamic background of the Early Mesozoic magmatism in the southern margin of the Lhasa terrane and regional exploring direction.Through the study of the thesis,the main achievements and research progress are summarized as follows.?1?Field geological surveys and petrographic studies have shown that the lithology of the Jurassic Ruocuo intrusions is hornblende quartz diorite porphyry and quartz diorite porphyry,in addition,the mineralization characteristics of porphyry deposits are found in the hornblende diorite porphyrite.It contains metal minerals such as malachite,chalcopyrite and limonite and propylitic alteration.?2?The zircon LA-ICP-MS U-Pb dating results show that the crystallization age of the hornblende quartz diorite porphyry and quartz diorite porphyry are 181.7±2.9Ma and 170±2.5 Ma,respectively,which are formed in the Early-Middle Jurassic,in response to an Early Mesozoic magmatic event in the Gangdese magmatic belt of the southern margin of the Lhasa terrane.?3?The geochemical characteristics of the Ruocuo hornblende quartz diorite porphyry and quartz diorite porphyry are similar.They have SiO₂ contents of60.62%–65.63%and 57.46%–60.04%,respectively,Al₂O₃ contents of 14.23%–18.23%and 18.32%–20.14%,respectively,Sr contents of 344×10^-6–571×10^-6 and 514×10^-6–614×10^-6,respectively,Y contents of 9.12×10^-6–13.0×10^-6 and 11.0×10^-6–14.9×10^-6,respectively,and Yb contents of 0.93×10^-6–1.31×10^-6 and 1.08×10^-6–1.52×10^-6,respectively.They are both show geochemical characteristics of adakites.?4?The Sr–Nd–Pb–Hf isotopes of the Ruocuo hornblende quartz diorite porphyry and quartz diorite porphyry are relatively depleted.They show relatively low ratios of(⁸⁷Sr/⁸⁶Sr)_i?0.7038–0.7045?,(²⁰⁶Pb/²⁰⁴Pb)_i?18.31–18.42?,(²⁰⁷Pb/²⁰⁴Pb)_i?15.56–15.66?,and(²⁰⁸Pb/²⁰⁴Pb)_i?38.36–38.77?and relatively high values of?_Nd?t??+5.54 to+6.33?and?_Hf?t??+11.8 to+17.3?.The Sr–Nd–Pb–Hf isotopes of hornblende quartz diorite porphyry and quartz diorite porphyry are similar to those of the Yarlung Tsangpo ophiolites and Indian Ocean MORB.Combined with the geochemical characteristics of major and trace element,indicating that they were predominantly generated by the partial melting of the subducted Neo-Tethys oceanic slab,and that the upwelling asthenosphere,triggered by the roll-back of the subducted Neo-Tethys oceanic slab,provided the heat that was required for slab melting.?5?The subducted Neo-Tethys oceanic slab-derived adakitic rocks that are present in the southern margin of the Lhasa terrane provide strong evidence for proving the presence of oceanic subduction during the Early–Middle Jurassic.By combining our results with the previously reported results,indicating that the Ruocuo adakitic rocks and Early Mesozoic?Late Triassic to Middle Jurassic?magmatic rocks in the southern Lhasa subterrane formed in a magmatic arc setting related to the northward subduction of the Neo-Tethys oceanic slab rather than in a back?arc setting related to the southward subduction of the Bangong?Nujiang oceanic lithosphere,that the initial subduction of the Neo-Tethys oceanic slab occurred prior to the Early Jurassic.?6?By combining our results with the previously reported results,we consider that the Early–Middle Jurassic subducted oceanic slab-derived adakitic intermediate-acid intrusions contain a greater potential for porphyry Cu–Au mineralization than the Early Mesozoic normal magmatic rocks in the southern margin of the Lhasa terrane.In addition,when looking for the Early Mesozoic porphyry deposit in the future,we should pay attention to the northern margin of the Xigaze forearc basin on the southern side of the Yarlung Tsangpo River.