Petrogenesis And Tectonic Implications Of The Early Paleozoic Granitoids From South Altun

The orogenic belts are important features of the convergent plate margins,and the research on their formation and evolution has always been focused topic in geosciences.The Altun orogenic belt(AOB)marks the northern margin of the Qinghai–Tibet Plateau and is an important part of the central orogenic belt(COB)of China.The south Altun orogenic belt(SAOB)is a major secondary tectonic unit of the AOB,which makes the region a hotspot for the Altun geoscience research.Granitoids in collisional orogenic belt can place precise constraints on the collision mechanism and crustal differentiation process.Similarly,the related mafic rocks can also provide us a significant access to reveal the process of magmatism and regional tectonic–magmatic evolution of the orogenic belt.However,the early Paleozoic tectonic evolution of the SAOB is still controversial.The spatial–temporal distribution characteristics of the granitoids within the area and detailed magmatic responses to the corresponding tectonic evolution remain unclear.In addition,the research of some granitic plutons and related mafic rocks still needs to be performed.The above situation restricts our understanding of the early Paleozoic tectonic–magmatic evolution of the SAOB and COB.In this thesis,based on the previous studies,we conducted systematic research on petrological,petrography,zircon U–Pb geochronology and Hf isotopic composition of 6granitic plutons and related mafic rocks within the region,as well as the related mafic rocks to determine the tectonic–magmatic evolution of the SAOB and COB.The major insights obtained from this thesis are shown as follows.(1)The early Paleozoic granitoids within the SAOB can be divided into five episodes.Episode 1(517～494 Ma)granitoids are represented by the granodiorite(505～504 Ma)from north Yumuquan pluton.Episode 2(483～462 Ma)granitoids are represented by the north quartz diorite(480～478),porphyritic monzogranites(476 Ma)from the Yusupualeke tagh pluton,quartz monzonite(483 Ma)and monzogranite(478Ma)from the Mengguaigele pluton.Episode 3(456～442 Ma)granitoids are represented by the granodiorites(453 Ma)in the Yusupualeke tagh pluton and middle–fine–grained granites(456～455 Ma)from the Qimubulake pluton.Episode 4(436～419 Ma)granitoids,represented by the quartz diorites(434 Ma)of the Jianshishan pluton and monzonites(419 Ma)of the Huangtuquan pluton.Episode 5 granitoids were formed in411～403 Ma,represented by the Mangya alkali–feldspar granites.(2)The early Paleozoic granitoids within the SAOB are mainly belong to I–type granites.The primary magma originated from the partial melting of Mesoproterozoic juvenile crust and formed in an active continental margin setting.Besides,the granodiorites from the north Yumuquan pluton show geochemical characteristics of adakite,and belongs to the partial melting of the thickened mafic lower crust.The quartz diorites from the north Yumuquan pluton and fine–grained granites in the Qimubulake pluton are derived from the partial melting of the late Paleoproterozoic to early Mesoproterozoic ancient crust.The studied mafic rocks are originated from the partial melting of the lithospheric mantle that was subjected by the fluid metasomatism.(3)In this thesis,we put forward a new model of the early Paleozoic tectonic–magmatic evolution.Stage 1,the central Altun ocean was northward subducted beneath the central Altun continental block,forming the early oceanic adakitic granite(517 Ma).The northward subduction of the south Altun ocean lasted until 500 Ma.The south Altun continental block was deeply subducted under the dragging of the subducting oceanic crust,forming high–pressure/ultrahigh–pressure(HP/UHP)metamorphic rocks and adakitic granites(granodiorite from north Yumuquan pluton of this study).At the same time,the south Altun ocean was northward subducted beneath the south Altun continental block,forming the oceanic adakitic granites(511 Ma)and I–type granites(501～494 Ma).Stage 2(483～462 Ma),the deep subducted slab broke off while the south Altun ocean was continued subducted beneath the south Altun continental block,forming a series of I–type granitoids including the Yusupualeketage monzogranites.Mafic magmas in the root of the orogenic belt were emplaced along the granitoids and formed hornblende gabbro(465 Ma).Stage 3(456～442 Ma),with the rollback of the deep subducted slab,the HP/UHP metamorphic rocks underwent retrograde metamorphism.The south Altun ocean was closed.The tectonic regime changed from collision to post–collision extension.The mantle–derived mafic magmas emplaced and forming the granodiorite in the Yusupualeketage pluton.Stage 4(436～419 Ma),the tectonic regime of the SAOB was further transformed to the extension,forming numerous I–type granites(Jianshishan quartz diorites and Huangtuquan monzogranites)and a little A–type granites.Stage 5(411～403 Ma),the whole SAOB entered into a post–orogenic extensional environment,forming numerous A–type granites and a small number of I–type granites.(4)By comparing the early Paleozoic tectonic–magmatic evolution of the SAOB,the north Altyn orogenic belt,the western Kunlun orogenic belt,the eastern Kunlun orogenic belt,the northern Qilian orogenic belt and the northern Qinling orogenic belt,it is concluded that except for the eastern Kunlun orogenic belt,the early Paleozoic tectonic–magmatic evolution of the COB is: 520～460 Ma,the subduction and collision of oceanic crust and deep subduction of continental crust;460～440 Ma,with broke off of the subducted slab,the tectonic regime changed from collision to post–collision extension;～420 Ma,the whole COB entered into a post–collision extensional setting.