The Forming Environment And Structural Attributes Of The Mesozoic Mugagangri Group In The Gaize Area,Tibet

The Mugagangri Group is a diagnostic tectono-stratigraphic unit of the Bangong-Nujiang suture zone,and its tectonic setting is closely related to the evolution of the Bangong-Nujiang Meso-Tethys Ocean(BNO).However,due to the strong structural shear during the closure of the BNO,there are different understandings of the original sedimentary environment and tectonic attributes.Based on the field geological survey,thin slice analysis,statistics of clastic components and zircon U-Pb chronology,the basic composition and the provenance of clastic materials of the Mugagangri Group in the west section of Bangong-Nujiang suture zone near the Gaize County are analyzed,and its tectonic setting is further discussed.The Mugagangri Group in the study area is mainly composed of two parts: the matrix and the rock blocks.The matrix is a set of deep-sea flysch sediments,and the rock blocks are mainly composed of the intermediate-basic volcanic rock blocks,the marble limestone blocks and the conglomerate blocks comprising gravels from the above two rock units.The matrix and the rock blocks are usually in structural contact,but the sedimentary contact can also be seen locally.The main rock types of its matrix sandstones are the lithic quartz sandstones and the lithic feldspar sandstones.The detrital components of the lithic quartz sandstones are mainly quartz,with medium rounding and poor sorting,and about 2%-15% of the matrix content,indicating that its component maturity is high and its textural maturity is low.The detrital components of the lithic feldspar sandstones are mainly feldspar and magmatic rock debris,with poor rounding and medium sorting,and about 10% of the matrix content,indicating that its component maturity is medium and its textural maturity is low.The Dickinson triangular diagram of the detrital components shows that the lithic quartz sandstones mainly fall into the source area of the recycled orogenic belt in the Qm-F-Lt plot.In the Qp-Lv-Lsm plot,they mainly fall into the source areas of the collision suture belt and fold-thrust belt,with a small amount in the source area of the mixed orogenic belt.The lithic feldspar sandstones mainly fall into the source area of the transition arc in the Qm-F-Lt plot.In the Qp-Lv-Lsm plot,they mainly fall into the source area of the volcanic arc orogenic belt,with a small amount in the mixedorogenic belt.The U-Pb geochronological analysis of zircons shows that the provenance of the Mugagangri Group is closely related to the Qiangtang terrane and independent of the Lhasa terrane.The zircon U-Pb age spectra of the contain six main peak ages: 206 Ma,263 Ma,428 Ma,758 Ma,1902 Ma and 2528 Ma.The zircons with 206 Ma as the characteristic age peak might be derived from the Late Triassic magmatic rocks in the northern Qiangtang terrane.The zircons with 263 Ma as the characteristic age peak might be sourced from the northern Qiangtang terrane.The zircons with 428 Ma as the characteristic age peak might be sourced from the central Qiangtang metamorphic belt.The zircons with 758 Ma as the characteristic age peak might be the product of denudation of the northern Qiangtang terrane,while the zircons 1902 Ma and 2528 Ma as the characteristic age peaks were also related to the Qiangtang terrane.At the same time,the weighted average age of the youngest zircons is 206.7±2.1 Ma,indicating that the Mugagangri Group was deposited later than the Late Triassic.The Mugagangri Group was unconformably overlain by the Qiongmo conglomerates,of which the youngest detrital zircon U-Pb age is 145.8±1.4 Ma.The age of the Mugagangri Group was thus estimated to be the Early-Middle Jurassic time.The sandstone composition and texture,tectonic background of the source rocks,and detrital zircon U-Pb chronological analysis indicate that the detrital materials of the matrix sandstones of the Mugagangri Group in the study area have the characteristics of dual source supply.The lithic quartz sandstones were mainly derived from the Qiangtang terrane,while the lithic feldspar sandstones were mainly sourced from the ocean island volcanic rocks.Based on previous studies,it is concluded that the Mugagangri Group in the study area is a set of sedimentary melanges,which was subjected to strong shear deformation and generally disordered except for locally ordered sequence,and formed in the ocean basin near the trench of the northern Meso-Tethys Ocean.