Geochronology,Geochemistry Of Early Paleozoic Igneous Rocks In Tashisayi Area,South Altyn Tagh And Its Geological Signification

On the basis of geology,petrology,geochronology,geochemistry,whole rock Sr-Nd isotope composition and zircon Hf values of the magmatic rock in the Tashisayi area of South Altyn Tagh,the thesis provides the following conclusions.?1?A variety of Early Paleozoic mafic-ultramafic and intermediate-acid magmatic complexes have been identified in the Altyn complex in the Tashisayi area,which is divided into three stages of magmatism through zircon U-Pb chronology:stage 1??500Ma?,which is featured by OIB-type mafic-ultramafic rocks,E-MORB and shoshonitic monzonite?garnet-bearing monzonite,amphibole monzonite and mylonitize monzonite?;Stage 2?⁴67Ma?is adakitic granite;Stage 3?⁴00Ma?,which consists of gabbro,diorite and granitic veins in diorite.?2?OIB-type mafic-ultramafic rocks have characteristics of low SiO₂?50.4%⁵3.5%?,high TFe₂O₃,TiO₂,MgO?11.12%¹9.40%,1.78%⁷.72%,5.02%¹6.10%,respectively?.They show OIB-like trace element patterns with enrichments in Nb,Ta,Ti.They may have been generated by partial melting of garnet-bearing asthenospheric mantle source in an ocean island/seamount environment.E-MORB type basalts belong to low-K tholeiitic basalt.They have E-MORB type trace element patterns with depletion in Nb,Ta,which may have originated from an enriched mantle source or a mixture source between depleted asthenospheric mantle source and mantle plume in a mid-ocean ridge of an initial ocean expansion settting.⁵00Ma Shoshonitic monzonites?garnet-bearing monzonite,amphibole monzoniteandmylonitizemonzonite?havefeaturesofhighAlkaline?K₂O+Na₂O=6.43%¹0.66%?,K₂O/Na₂O ratio?1.40⁴.44?and low TiO₂?0.43%¹.41%?with enrichment of LREE and LILE,similar to featrues of shoshonitic rock.?_Hf?t?values?-6.4^-0.1,-3.1^-0.4,-2.5².0?and enrichment of Nb and Ta of these monzonites suggest that they may derived from decompression melting of lithospheric mantle source,triggered by contemporaneous continent subduction.The low Ba/Rb ratios?0.19⁰.68?and high Rb/Sr?1.15⁴.34?of garnet-bearing monzonite and amphibole monzonite suggest they originated from partial melting of phlogopite-bearing lithospheric mantle source.The high Ba/Rb ratios?26.45³1.94?and low Rb/Sr?0.08⁰.10?of mylonitize monzonite suggests they were derived from partial melting of amphibole-bearing lithospheric mantle source.?3?The adakite granites?⁴67Ma?have high contents of SiO₂?66.44%⁶7.04%?and Al₂O₃?16.08%¹6.79%?and ratios of Na₂O/K₂O?2.81%⁵.50%?ratios.They also have high concentration of Sr(394×10^-6552×10^-6),low concentration of Y(3.40×10^-64.05×10^-6)and Yb(0.29×10^-60.35×10^-6).All of the geochemical characteristics indicate that the granite belongs to adakite.The high?_Hf?t??10.83¹5.52?and?_Nd?t??4.0⁴.3?values and similarity between model age and formation age?⁴67Ma?suggest that they were partial melting of newly basaltic lower crust heated by upwelling mantle in a post-collisional extensional setting.?4?The gabbro and diorite?⁴00Ma?are cal-alkaline series rock.They enriched in LREE and LILE?Cs?Rb?Sr?Ba?and depleted in HFSE?Nb?Ta?Ti?,similar to island arc basalt,but the features of major element,trace elements?high Nb,Zr,and low Zr/Y ratio?,?_Hf?t?values and Sr-Nd isotope compositions indicate that they derived from partial melting of the lithospheric mantle modified by asthenospheric mantle,undergoing crust contamination and fractional crystallization in a continental extensional setting.Compared to diorite,the granitic veins in diorite show similar spider diagram of trace element with diorite,but more negative Eu?0.55⁰.61?,higher LREE/HREE ratio.The diorite and granitic veins have the same age?⁴00Ma?,?_Hf?t?values and Sr-Nd isotope composition.These geochemical features and field occurrences reveal that the granitic veins were derived from crystallization differentiation of the diorite with low contribution from crust.?5?The Tashisayi OIB-type mafic-ultramafic rocks and E-MORB type basalts can be compared with ophiolite assemblage in South Altyn Tagh subduction-collision belt.Shoshonitic monzonite suggests ocean crust subducted before⁵00Ma.The adakite granites?⁴67Ma?together with Changshagou post-collisional mafic-ultramafic rocks indicate that the South Altyn Tagh were in the continental extending setting in middle Ordovician.The gabbro and diorite?⁴00Ma?further demonstrate the mantle source magmatic activity and the interaction of the mantle source and crust in a post-orogenic extensional settting.