Yongzhu Mesozoic Magmatic-tectonic Events In The Middle Section Of Bagnongco-Nujiang Suture Zone,and Its Geological Significance

The Yongzhu area is located in the middle section of the Shiquan river-Yongzhu-Namuco ophiolite melange zone(Slainajap Ocean),central Tibet.There are extensive ophiolite and intermediate-acidic magmatic rocks in our research region,which is a favorable area for geological study.Research on the petrogenesis of these rocks is important for understanding the formation and evolution of the middle section of the Bangongco-Nujiang suture zone.On the basis of a detailed field geology survey and laboratory study,in this paper we describe the spatial and temporal distribution of ophiolite and intermediate-acidic magmatic rocks in the Yongzhu area,the geochemical characteristics of the ophiolite and intermediate-acidic magmatic rocks,and the zircon U-Pb ages and zircon Hf isotopic compositions of the intermediate-acidic magmatic rocks,and comparing with understanding of the Bangongco-Dingqing ophiolite belt.The petrogenesis,diagenetic age and tectonic background of Yongzhu ophiolite and intermediate-acidic magmatic rocks have been revealed,and the tectonic evolution model of the middle part of the Banggongco-Nujiang suture zone has been built.The main research results are as follows.(1)The ophiolite in Yongzhu district,composed of serpentinized pyroxene peridotite and dunite,pyroxenite,gabbro,pillow basalt,gaberite wall and radiolarian-bearing siliceous rocks,is a relatively integrated ophiolite suite.The ultramafic rocks are mainly distributed in the Yongzhu-Guomangco area.The mafic rocks are primarily located in Shesuo,Baerguo and Qvcun district while the pillow basalt and radiolarian-bearing siliceous rocks outcrop in the Shesuo are principally.The Yongzhu ophiolite has a fault contact relation with the Neoproterozoic and Paleozoic-Mesozoic country rocks and is emplaced by tectonism.(2)Geochemistry indicates that the Yongzhu ophiolite was formed in the mid-ocean ridge.Chondrite-normalized REE patterns of serpentinized pyroxene peridotite,basalt,gabbro and diabase are similar to the N-MORB pattern.In primitive mantle-normalized spider diagrams,the pyroxene peridotite is enriched in high strong field element such as Ta and Ti et al.,depleted in large ion lithophile like Ba,Th and Sr;the mafic rocks are characterized by the enrichment Ta,depleted of Th,showing the features of ocean ridge basalt.The flat REE pattern with low fractionation and the N-MORB-like Zr/Nb ratio of mafic rock show that the magmatic source is is ashenospheric mantle in the mid-ocean.The La/Sm ratio,Sm/Yb ratio and Ce/Y ratio reflect the magma is mainly from spinel peridotite partial melt.(3)Ophiolitic rocks in the Shiquan river-Yongzhu-Namuco belt and the Bangongco-Nujiang belt have silimar petrography,geochemistry and geochronology.Bangongco-Nujiang Tethys ocean opend during late Permian to early Triassic and Shiquan river-Yongzhu-Namuco Tethys ocean(Slainajap ocean)opened at late Triassic.Bothe of them evolved during early-middle Jussic and closed in early Cretaceous gradually.Apart from the semblable evolution,similarities in chondrite-nomalized REE pattern,primitive mantle-normalized trace elements spider diagram and tectonic background of the mafic rocks suggest that the Shiquan river-Yongzhu-Namuco ophiolite was formed with the Bangong-Nujiang ophiolite,and both of them have experienced approximate evolution process.(4)The early Cretaceous intermediate-acidic magmatic rocks in the Yongzhu district,including rhyolite,andesite,minor dacite,biotite monzonite granite,granite porphyry and granodiorite,are belong to continental margin arc magmas.Zircon U-Pb dating of the volcanics and plutons are 115^-124Ma and 118?121Ma respectively and the REE chondrite-nomalized patterns show "V" type,enriching the LREE.In primitive mantle-normalized spider diagrams,the early Cretaceous magmatic rocks are characterized by the enrichment of large ion lithophile element Rb,K and radioactive element U,Th,and the depletion of Nb,P,Ti,Ba,and Sr,which represents that they are island-arc igneous rocks,formed in a continental-marginal arc setting.The Hf isotopic research of the early Cretaceous magmatic rocks produces a range from-1.21?+5.35 which correlates to two stage model ages of 0.84Ga?1.26Ga,representing a mixing source of mantle magma and crust magma.The value of Mg#of rhyolite and biotite monzonite granite exceeds 40 and of andesite exceeds 50 which also indicate the mixing source of mantle and crust.Combined with geological background,we recognize that the early Cretaceous intermediate-acidic magmatic rocks in Yongzhu were formed in the continental margin arc of Shiquan river-Yongzhu-Namuco ocean.(5)Given all the petrology,geochemistry,geochromology and stratigraphic distribution of the ophiolite and intermediate-acidic magmatic rocks,we have revealed the evolution process of Shiquan river-Yongzhu-Namuco Tethys ocean(Slainajap ocean):The Slainajap ocean opened following the Bangongco-Nujiang Tethys during Late Triassic to early Jurassic(218?180Ma).The northward subduction of the Slainajap oceanic crust began during middle Jurassic to early Cretaceous(180?140Ma).Caused by heterogeneity of subduction,island-arc magmatic rock fromed in the Shiquan river district while plagiogranite and metamorphic rocks in Laguoco area.But the district from Aso to Namuco,the subduction was more gentle and slowe without triggering magmation.From the middle of early Cretaceous(l30?120Ma),the Slainajap oceanic subduction became more steep and rapid,which triggered magmatism at about 120Ma in continental arc margin of the Yongzhu area.After 110Ma the Slainajap ocean closed gradually,followed by the collision between the southern Qiangtang terrane and northern Lhasa Terrane.