Provenance Analysis And Paleogeography Reconstruction Of The Tethys Himalaya:Constraints From The Devonian To Triassic Quartz Sandstone

The most famous collisional orogen,Himalayan Orogen was created by the India-Asia collision.However,the knowledge about the provenance,geometric characteristics and evolution of the northern Indian passive continental margin(the most subducted and frontier of collision)— Tethys Himalaya Sequence(THS)— before collision is rather rare.Quartz sandstone — the most common,typical and significant rock types in passive continental margin sequence — was well-developed in the post-Devonian THS,but the interpretation of the provenances,relationships,flow paths of source-sink and basin distributions towards the quartz sandstones is rather limited.In this thesis,we investigated the Middle-Late Devonian Boqu Formation,Carboniferous Naxing Formation,Early Permian Jilong Formation – Bilong Formation – Zhongba Formation,and Late Triassic Derirong Formation of the THS in the south Tibet.An integrated study of petrography,sedimentology,stratigraphy and detrital zircon U-Pb geochronology was carried out in the representative profiles to provide new constraints to the provenance interpretation,basin distribution and evolutional history of the northern Indian passive continental margin from pre-rift to post-rift.The Boqu Formation comprises quartzose sandstones,siltstones and mudstone-shales,while the Naxing Formation consists of several alternating sequences of quartzose sandstones and siltstone-mudstone-shales.The Jilong Formation and Bilong Formation are similar,mainly composed of quartzose to pure quartzose sandstones and discontinuous diamictites.The Derirong Formation comprises feldspatho-quartzose to pure quartzose sandstones with intercalated bioclastic oosparites in the upper part.The quartz sandstone strata deposited in coastal environments influenced by wave and tidal processes.Given the sandstone petrology and heavy mineral assemblages — indicating a stable provenance of India Craton — and the detrital zircon age spectra characterized by the Pan-African(~520 ma)and Late-Grenvillian(~950 ma)age peaks,after correlation with India and its peripheral tectonic domains,~950 ma zircons in the Devonian to Triassic THS quartz sandstones may have been derived from the Eastern Ghats Mobile Belt in eastern India and the Rayner Orogen in eastern Antarctica,and ~520 ma ones may have been derived from the Eastern African Orogen and Pinjarra Orogen,which enclosed the India subcontinent.Based on sandstone petrography,heavy mineral analysis and detrital zircon U-Pb geochronology,the Zhongba Formation in the Zhongba Microcontinent shared the same provenance with Jilong Formation and Bilong Formation.In addition,previous carbonate microfacies,isotope geochemistry and paleontology study also suggest the affinity of the Zhongba Microcontinent and the THS.A remarkably pattern of bidirectional N-S paleocurrents in the Carboniferous and bidirectional E-W ones in the Permian and Triassic indicates the paleo-shoreline trend and the transgressive direction have been shifted.In addition,progressive eastward thickening of the sandstone layers and eastward younging diachroneity of the Derirong Formation were also ascribed to the Neo-Tethys transgression from W to E.Anyway,the orientation and distribution of the northern Indian passive continental margin are not always consistent with the present E/W-trending THS.Considering the ~55° counterclockwise rotation during northward drifting in the Mesozoic,this thesis suggests that the trend of the northern Indian passive continental margin was N-S before the Neo-Tethys open,turned E-W after the Neo-Tethys rifting and opening and last at least until the end of the Triassic.