Compositions,Structural Deformation And Geodynamics Of The Early Paleozoic Melange Belt In North Altyn Tagh

The Altyn Tagh,situated in the northern margin of the Qinghai-Tibet Plateau,is one of the key areas to understand the geodynamic evolution and geomorphic formation of the Tibetan Plateau,even the western China.The Hongliugou-Laipeiquan Ophiolitic Melange Belt(HLOM),be consider as an ancient plate suture in the Early Paleozoic,had undergone complicated evolution process of breakup,spread,subduction,accretion and closure for North Altyn Ocean and recorded material compositions,structure and tectonic deformation during the processes of formation and evolution in the North Altyn.Hence,the HLOM is a critical site for studying the Early Paleozoic geological evolution,reconstruct plaeostructure environment and geodynamic architecture of North Altyn.In this paper,we present compositions and structure deformation on the HLOM in North Altyn,integrated with published regional data,in order to reconstruct geodynamic evolution and mechanisms of the North Altyn in the Early Paleozoic.Hongliugou-Lapeiquan Ophiolitic Melange Belt(HLOM)consists of various tectonic blocks and matrixes,including ultramafic rocks,basaltic lava,turbidite,cherts,limestone,HP/LT metamorphic rocks and exotic blocks.Detailed field mapping and analyses of the rock associations and structure deformation features,HLOM can be subdivided into south incoherent series,middle coherent series and north incoherent series.In the south incoherent series and north incoherent series,amalgamations of various kinds and sources of tectonic blocks as a result of the subduction and accretion of the North Altyn Ocean,forming accretionary wedge/accretionary complex,suggesting the North Altyn has basic characteristics of an accretionary orogen.Moreover,detailed observation and analysis of rock association in the south incoherent series suggest that the paleo-seamounts existed in North Altyn Ocean.integrated with exotic blocks and more terrigenous detrital material have been identified in the HLOM and the geochemistry data of the cherts exhibit that they are formed in the continental margin basin,implying that North Altyn Ocean was not a widely ocean,but rather a little-limited ocean basin,which was close to continental margin,with seamounts and ocean islands.Field observations and structural analysis displaying that E-W strike imbricate thrusts and folds were well exposed in the HLOM.The fossils of the strata involved in the deformation and the age of the undeformed dikes which cut out folds and thrust faults constrain these folds and thrusts formed between middle-late Ordovician and early Devonian,implying those structures were related to the subduction and collision processes of North Altyn Ocean.In addition,upright,inclined and overturned folds were products of progressive deformational in the south incoherent series,and the thrust faults are characterized by the northward thrusting in the south incoherent series and the southward thrusting in the north incoherent series,suggesting that North Altyn Ocean had undergone divergent double-sided subduction in the Early Paleozoic period.Furthermore,detailed field investigation,micro-structural analyses,electron backscattered diffraction(EBSD)fabric analyses and ziron U-Pb dating indicate that the southern ductile shear zone is characterized by top-to-the-NW with dextral thrust ductile shearing,while the North Altun brittle-ductile shear zone is featured by dextral strike-slip ductile shearing.The timing of the two ductile shear events have been constrained to be 499.0±2.2Ma-511.0±2.4Ma and 427.3±5.7Ma?478.1±2.1Ma,respectively.It is proposed that the southern ductile shear zone is associated with the process of southward subduction of North Altyn Ocean and the North Altun brittle-ductile shear zone can be interpreted as a tectonic response caused by the oblique collision between Tarim plate and Azhong Terrane.SHRIMP zircon U-Pb dating for the Kaladawan monzogranites which occur an the southern margin of the HLOM yields an age of 484.2±4.9 Ma.The geochemical feature suggests that the monzogranites were formed in an island-arc setting.Zircon Hf isotopic data(?Hf(t)=+4.08?+8.26)and summarize previous research results of Early Paleozoic magmatism in the North Altyn and North Qilian,suggesting the arc-related magma which in the southern margin of the HLOM had been formed through a magmatic mixing between juvenile crustal materials and old crustal materials.In addition,numerous intermediate-acidic magmatic rocks also were found in the northern margin of the HLOM.Zircon U-Pb analyses,using SHRIMP and LA-ICP-MS U-Pb,indicate that the intermediate-acidic magmatic rocks were formed in the Late Cambrian to Middle Ordovician.Elemental geochemistry indicates typical island arc affinities for those acidic magmatic rocks,which is comparative with the Early Paleozoic magmatic rocks in the southern margin of the HLOM.These ages,combined with the geochemistry and locations of the plutons,indicate the existence of Early Paleozoic volcanic arc assemblages on both sides of the North Altyn,supporting that the divergent double-sided subduction model in the North Altyn Ocean.From our work,combining with previous studied,we outline a comprehensive geodynamic model for North Altyn during Mid-Neoproterozoic to Early Paleozoic:1)initial breakup and spread stage of North Altyn Ocean from Mid-Neoproterozic to Eearly Cambrian,(2)divergent double subduction-accretion stage of North Altyn Ocean in Middle Cambrian to Late Ordovician,(3)collision orogenic stage of North Altyn in Early Silurian to Early Devonian.