The Geochemical Characteristics, Petrogenesis And Tectonic Significance Of The Late Cretaceous-Early Paleocene Basic Intrusive Rocks In The Western Lhasa Block

The Gangdese granite batholith and corresponding volcanic rocks widely distributed in the Lhasa terrane in the southern Tibetan Plateau recorded the deep process of subduction and closure from the Neo-Tethys Ocean to the Indian-Asian continental collision.Therefore,it is of great significance to study the origin,evolution and spatial and temporal distribution of their magma.However,compared with the eastern section of the Lhasa block,there are relatively few studies on magmatic rocks in the western section,especially mantle-derived magmatic rocks,which greatly restricts people's understanding of the Tethys ocean subduction and continental collision process.Based on this,this paper reports geochemical data for the Late Cretaceous?⁸7Ma?Gabbro-norite and the Early Paleocene?⁶3 Ma?hornblende gabbros in the western part of Lhasa terrane to carry out detailed chronological,mineralogical and geochemical studies.Identify their source area composition and magma evolution process,and try to further explore the deep structure and magmatism during the subduction of the New Tethys Ocean.We mainly made the following progress and understanding:The zircon U-Pb dating results indicate that The Dajiacuo Gabbro-norite was formed in the Late Cretaceous?⁸7Ma?.In terms of geochemical composition,it belongs to the High-K calc-alkaline rock series,with high K₂O?1.54-2.79wt.%??Na2O?2.44-2.89 wt.%?;medium SiO2?53.5-59.7wt.%??Al₂O₃?15.4-17.0wt.%?and low Fe₂O₃T?6.94-8.99wt.%??MgO?4.18-5.25wt.%??Mg#?56.8-59.9?.and have arc-type trace element features such as enrichment in light rare earth elements and large ion lithophile elements?e.g.,Rb,Sr and Ba?,and depletion in high field-strength elements?HFSE e.g.,Nb and Ta?.The combination of high zircon Ti temperature,two-pyroxene temperature and previous research data indicates that the Dajiacuo Gabbro-norite was formed in the tectonic setting of the Neo-Tethyan slab roll-back.We suggest that it formed by were formed by partial melting of lithosphere mantle metasomatized by slab-released fluid,during the ascending and emplacement process,it was subjected to assimilation and contamination by ancient substrates,which enriched Sr-Nd-Hf isotopes((⁸⁷Sr/⁸⁶Sr)_i=0.7077-0.7089,?_Nd?t?=-4.8^-7.0,?_Hf?t?=-1.7^-9.2))and subsequently undergone crystal fractionation of clinopyroxene,orthopyroxene and plagioclase.The zircon U-Pb dating results indicate that The Naburu hornblende gabbros was formed in the Early Cretaceous?⁶3Ma?.In terms of geochemical composition,it belongs to the calc-alkaline rock series,with low SiO₂?46.7-52.2 wt.%?,MgO?3.0-6.8wt.%?and high Al₂O₃?16.4-19.0 wt.%?and have arc-type trace element features such as enrichment in light rare earth elements and large ion lithophile elements?e.g.,Rb,Sr and Ba?,and depletion in high field-strength elements?HFSE e.g.,Nb and Ta?.Combining with their uniform Sr-Nd isotopes((⁸⁷Sr/⁸⁶Sr)_i=0.7046-0.7050,?_Nd?t?=0.64-1.32),we suggest that the Naburu hornblende gabbros formed by were formed by partial melting of lithosphere mantle metasomatized by slab-released fluid and subsequently undergone crystal fractionation of olivine,clinopyroxene and Fe-Ti oxides.The Naburu hornblende gabbros were formed at the end of the Neo-Tethys oceanic subduction,but present indistinguishable Sr-Nd isotopes from the prior-collision mafic rocks of the eastern Lhasa Terrane,indicating that the compositional difference of post-collisional lithospheric mantle between the eastern and western Lhasa terrane was not resulted from subduction of the Neo-Tethys Oceanic slab.