Study On The Geological Characteristic And Tectonic Attribute Of The Ophiolite And Island-arc-type Igneous Rocks,Central Belt Of East Kunlun(Eastern Section)

The Central tectonic mélange belt of East Kunlun(CTBEK) is an important suture between the Northern and Southern belt of East Kunlun orogen.In the thesis,we focus on the research of the CTBEK,which is researched systematically intotectonic geology,petrology,geochemistry analysis and isotope chronology,with the plate tectonics theory and continental dynamics research as the guiding ideology.This thesis attempt to restore the Early Paleozoic ocean-land tectonic framework,and analyse the tectonic evolution process of East Kunlun orogenic belt(eastern section),through mainly studying the geological characteristics and tectonic attribute of the ophiolite and island-arc-type igneous rocks in the tectonic belt. Mainly the following new progress and preliminary conclusions:1.CTBEK consists of tectonic mélange rock mass,antique basement and autochthonous sedimentary rocks. The former is composed of Early Paleozoic rock mass,which can be divided into ocean-crust type mixed rock,island-arc-type igneous rocks and Naij Tal Group,and the sedimentary rocks is mainly composed of Maoniushan Formation,Halaguole Formation,Haoteluowa Formation,Hongshuichuan Formation and Yangqu Formation. Each units are intertweaved as a series of tectonic block(fragment),which showed extremely complex tectonic melange visage.2.Tatuo ophiolite is composed of serpentinite,olivine pyroxenites,pyroxenite and gabbro,which contact with thrust fault each other.Ultramafic units is the remain of partial melting original mantle,which was derived from enrich oceanic lithosphere mantle.The gabbro units belong to the island-arc theoilitic series,and show both IAT and MORB geochemical characteristics,indicating the SSZ-type.They were derived from like E-MORB enrichment mantle, which is the result of garnet lherzolite 5%~10% partial melting,then suffering from slab fluids metasomatism and containmination,afterwards,degree fractional crystallization having played a dominant role.The LA-ICP-MS zircon U-Pb age of gabbro is(521.9±3.2)Ma;Qushi’ang ophiolite is composed of serpentinite,serpentinized ultramafic rocks,pyroxenite,gabbro and basalt.The gabbro and basalt from the ophiolite belong to the island-arc-type cal-alkaline series, characterized by IAB.They were derived from like E-MORB enrichment mantle source, which is the result of garnet lherzolite 5%~8% partial melting,then suffering from slab fluids metasomatism,having experienced some degree fractional crystallization. The LA-ICP-MS zircon U-Pb age of gabbro is(504.5±5.5)Ma;Qingshuiquan ophiolite is composed of serpentinite,serpentinized ultramafic rocks,pyroxenite and a few gabbro,basalt.The mantle peridotite were derived from relatively enrich source region,which is the result of garnet lherzolite suffering from 5%~10% partial melting.The gabbro from it can be divided into alkaline and sub-alkaline series,but both show IAB geochemical characteristics.The gabbro were derived from a rather complicated source region as depleted-enrichment mantle source,which is the result of different degree and proportion mixed melting by garnet lherzolite and spinel lherzolite,then suffering from slab fluids metasomatism and containmination to a certain extent.The LA-ICP-MS zircon U-Pb age of gabbro is(481.1±1.3) Ma.The ophiolite from Qushi’ang,Tatuo,Qingshuiquan all belong to SSZ-type ophiolite,which represent the expansion evidence of back-arc basin,formed by subduction northward of“Buqingshan-A’nymaqen ocean”causing arc extension, and in what order record the back-arc limited little basin continued to expand to the Middle Ordovician at least.3.Qushi’ang intermediate-basic complex is composed of massive hornblende gabbro,gabbro diorite and diorite,which show the gradient contact relationship each other.They all show island-arc cal-alkaline geochemical characteristics,which is formed in the mature ancient island arc tectonic environment,and the LA-ICP-MS zircon U-Pb age of hornblende gabbro is(455.6 ± 1.2) Ma.Qingshuiquan hornblende gabbro show island-arc tholeiite geochemical characteristics,which is the product of island arc, and the LA-ICP-MS zircon U-Pb age is(452.1±5.2)Ma.All of these island-arc magmatic rocks represent the product of Central Eastern Kunlun back-arc limited little basin subducting northward in a peak period,represented by Qushi’ang-Tatuo-Qingshuiquan ophiolite.4.The East Kunlun area was in an archipelago ocean framework in Early Palaeozoic,which consists of ocean basin and microplate in different size.The Buqingshan-A’nymaqen tectonic mélange belt and CETMB respectively represent the major ocean basin and back-arc ocean basin of Proto-Tethys,whose outspread degree and scale were different.Considering regional geology and chronology data,The tectonic evolution of East Kunlun Orogenic(east part) in Early Palaeozoic can be devided in following stage:(1) the major oceanic crust formed and expansion stage(Z-∈1);(2) the oceanic crust further expansion and subduction stage(∈2-S1),which contains back-arc small oceanic basin formed and expansion sub-stage(∈2-O2),and primary subduction of oceanic crust sub-stage(O3-S1);(3) demise of the oceanic crust and collision orogenic stage(S2-S3);(4) post-collision extension stage(S32-D1).