Sedimentary Fill And Tectonic Evolution Of The Upper Triassic Sequence Within Eastern Himalayan Orogen

The Upper Triassic Langjiexue Group and Nieru Formation,which lie immediately south of the Yarlung-Tsangpo Suture Zone in the Shannan area of southeastern Tibet,are an important part of the Tethyan Himalayan Sequence?THS?.The Upper Triassic sequence not only preserves tectonic memory of the Tethyan Ocean prior to the India-Asia collision,but also carries information on post-collisional deformation of the Himalayan Orogen,representing an ideal geologic entity to investigate the pre-collisional to post-collisional evolution of the eastern Tethyan Himalayan realm.Based on structural analysis,this study has constructed one tectono-stratigraphic unit termed as the Luolin Group.This study presents new data on sandstone petrographic composion,heavy mineral assemblage,geochemical indicator and detrital zircon U-Pb geochronology to constrain the provenance and paleogeographic reconstruction.Combining with X-ray diffraction,fluid inclusion microthermometry,Raman spectroscopy,EMPA and ⁴⁰Ar-³⁹Ar geochronology,this study attempts to discuss tectono-metamorphic evolution of the Upper Triassic sequence.The Luolin Group consisting of meta-sandstones and sericite phyllites is subdivided into six units,of which from north to south are the Jiacha Unit,Sangdong Unit,Pumu Unit,Jiangre Unit,Sela Unit and Banglang Unit.The Pumu Unit is characteristic of extensive syn-deformational quartz veins.Garnet amphiboles and garnet mica schists can be recognized in the Sela Unit.Two stages of foliations are detectable in the Luolin Group.D2 north-verging folds and thrusts superimposed earlier deformational event,highlighted by predomination of the south-dipping S₂ foliation.D1 south-verging folds and thrusts are the main deformational style in the Langjiexue Group and Nieru Formation.Primary sedimentary beddings?S₀?were folded with development of axial cleavages.The deformational style similar to positive flower structure was established in the Shannan area.The dominance of quartz grains and felsic volcanic lithic fragments suggests that the sandstones are litho-quartzose.The heavy mineral assemblages consisted mainly of zircon and rutile indicate felsic provenance contributions.The geochemical signatures of sandstones and detrital zircons suggest derivation from felsic igneous sources.The detrital zircon age spectra display three major peaks:1200-900 Ma,750-500 Ma and300-200 Ma.The youngest age cluster is incompatible with sources from neigghbouring terranes,including the South Qiangtang terrane,Lhasa terrane,THS and the Higher Himalayan Crystalline.Correlations of the Permian-Triassic zircons with those of time-equivalent strata in northwest Australia,west Burma,and the Banda Arc unveil a potential connection to the Tasmanides along the convergent margin of eastern Australia.The New England Orogen?300-230 Ma?could have supplied the Upper Triassic sequence with magmatic materials via continent-scale drainage systems or a submarine fan complex.Results from illite crystallinity?IC?indicate that metamorphic grade decreases from epizonal in the Luolin Group?303??to anchizonal in the Langjiexue Group?297??and Nieru Formation?292??.The fluids from two stages of syn-deformational quartz veins are characterized by moderate homogenization temperatures?ca.260??and low salinity.EMPA analyses of garnet amphiboles and garnet mica schists show that the Sela Unit experienced metamorphism of⁴80-500?and³-4 Kbar.Age of the D1 event is determined by ⁴⁰Ar-³⁹Ar dating at ca.47 Ma.Correlations with regional deformational events suggest the D2 event occurred between ca.30 Ma and 22 Ma.