Tectonic Affinities Of(micro-) Terranes In Southeastern Tibet And Their Constraints On The Tectonic Evolution Of Proto-and Paleo-Tethys Ocean

Southeastern Tibet and western Yunnan are the key areas of the eastern Tethyan orogenic belt,which is composed of different Tethyan ophiolitic melange belts,subduction-related magmatic belts and micro-continental blocks.The multidisciplinary data have indicated that these micro-continental blocks/terranes surrounded by suture belts were involved in the tectonic evolution of not only Tethys oceans but also Rodinia and Gondwana.Magmatic rocks and clastic sedimentary sequences within these micro-continental terranes in response to these tectonic events,bear critical informations on the tectonic processes.However,little attention has paid to the tectonic affinity and evolution of these micro-continental terranes,particularly for the Yidun,Lincang and Bomi-Chayu terranes,and even their correlations with the ambient big blocks.This limits our understanding of the precise spatial-temporal patterns of these terranes and their corresponding Tethyan oceans.Here,we present U-Pb geochronology and Hf isotope,trace elemental geochemistry of detrital zircons from the Neoproterozoic-Paleozoic(meta-)sedimentary and igneous rocks in the Yidun,Bomi-Chayu,Baoshan and Lincang terranes.Our results coupled with literature data,provide insights into the tectonic affinities and paleogeographic position of these terranes during the Late Neoproterozoic to Paleozoic,and decode the spatial and temporal evolution of the Tethyan oceans surrounding them.Detrital zircon provenance analysis in the Yidun terrane indicated that the late Neoproterozoic samples display a similarly unimodal spectra with a major age population at 800-760 Ma,while all the Ordovician samples share similarly multimodal age distribution patterns,with major age populations at 600-500 Ma,860-700 Ma and 1100-900 Ma.Our new results together with published big data from these terranes/blocks reveal that the predominant provenance of the late Neoproterozoic sedimentary sequences in the southeastern Yidun Terrane was the Panxi-Hannan magmatic arc along the northern and western margins of the Yangtze Terrane of South China while the Ordovician successions were sourced primarily from the recycling of the Cambrian strata in the western Yangtze Terrane.The ultimate sources of these Ordovician sediments could be the Eastern Ghats-Rayner Complex and the Kuunga Orogen located in the eastern India region,and Jiangnan Orogen of South China.Our provenance analysis indicates the Yidun terrane kept a close linkage with the Yangtze terrane of South China Block at least from the Middle Neoproterozoic to Early Ordovician.The similarities of facies assemblages and detrital age patterns of time-equivalent strata in India and South China reflect that South China Block was connected with India at least from the late Tonian(ca.830 Ma)to Early Cambrian and formed the South China-India Duo along the western margin of Rodinia.In contrast,the variations in facies assemblages and detrital age patterns of the Ordovician susscesions in these two block hints that South China was finally separated from Indian Gondwana before the Early Ordovician.Based on the study of the detrital zircons from the Demula and Bomi groups in the Bomi-Chayu terrane of southeastern Tibet,they showing two major age groups at 600～500 Ma and 1000～800 Ma are indicative of a tectonic affinity with the Tengchong,Baoshan,South Qiangtang and Himalaya terranes,rather than the Lhasa block during the Cambrian.Such signatures suggest that the Bomi-Chayu terrane is the northern extension of the Tengchong block,but not the eastern extension of the Lhasa block.In addition,the Bomi-Tengchong Duo was located on the active continental edge of the Indian Gondwana in the early Paleozoic.Our new data combined with literature data for the Paleozoic(meta-)sedimentary rocks in the Baoshan,Simao-Indochina and Lincang blocks provide crucial constraints on the paleogeographic position and tectonic affinity of these blocks during Paleozoic times.Collectively,the Cambrian-Carboniferous sequences in the Baoshan Block and the eastern Indian region maintain similar detrital zircon U-Pb age spectrum,with two similarly major age groups at 1100～900 Ma and 600～500 Ma.The Upper Ordovician-Carboniferous sequences of the Simao-Indochina Block have a major age group of 400～500 Ma and a subpopulations of 900～1000 Ma.In contrast,the Lower Paleozoic sequences in the Lincang terrane share similar age spectra with the Baoshan Block but the Upper Paleozoic shows two differently prominent groups at 1300～1100 Ma and 400～300 Ma.Our provenance analysis and other available data indicate that the Simao-Indochina and South China blocks had rifted from the Gondwana before the Late Ordovician,the Simao-Indochina Block was separated from the South China during the Silurian-Devonian,and that the Lincang and Baoshan terranes were linked with each other during Early Paleozoic times.Subsequently,the Lincang Terrane rifted from the Baoshan Terrane due to a back-arc rifting between these two terranes during Late Paleozoic times.Paleogeographically,they were located on the northern margins of Australia and India in Gondwana,respectively.Therefore,the Lincang Block did not represent a continental arc of the Simao Block before the Early Permian,but rather it rifted from the northern margin of the Indian Gondwana in the Early Permian and subsequently became an independent block drifting northward to the west side of the Simao Block.The southern continuation of the Lincang Terrane is the Inthanon zone rather than the Sukhothai zone as proposed previously.Integrating with all published date about southeast Tibetan and ambient blocks,we renew the tectonic evolution model of the Proto-and Paleo-Tethys Ocean and paleogeographic position of those(micro-)terranes in southeastern Tibetan during Neoproterozoic-Paleozoic as follows.(1)During late Neoprotezoic-Early Cambrian times,these microterranes in southeastern Tibetan were located to East Gondwana.(2)During Middle Cambrian time,the Simao-Indochina,Yidun and South China terranes rifted from Gondwana due to the opening of the Proto-Tethys Ocean.The Lincang and Baoshan terranes were still located at the northern margin of Gondwana during the Cambrian-Ordovician.(3)During late Cambrian-Ordovician times,a narrow Changning-Menglian Proto-Tethys Ocean started to subduct westwards and eastwards beneath the Simao-Indochina and Baoshan terranes,respectively.In this case,the Baoshan and western Simao-Indochina terranes developed the arc-related igneous rocks.Meanwhile,a time-equivalent continental arc had also developed along the northern margin of Gondwana and western Simao-Indochina terrane,respectively.(4)During Silurian-Early Devonian times,the Changning-Menglian Proto-Tethys Ocean continued spreading and subducting beneath the Simao-Indochina Terrane,which leds to the formation of the Ailaoshan Tethyan back-arc basin.(5)During Middle Devonian-Early Permian times,a new ocean/rift basin between the Lincang and Baoshan terranes could have developed by the transform faults at least since the Late Devonian,which resulted in the variation in detrital sources in the Lincang terrane during Devonian to Early Permian times.The Simao-Indochina terrane rifted from the South China Block due to the formation of the Ailaoshan Paleo-Tethys Ocean.Our data illustrate that the tectonic evolution of the Proto-Tethys and Paleo-Tethys Oceans within southeastern Tibetan does not means that all the Gondwana-derived terranes of Tethys region was directly rifted from Gondwana,but rather it could rift from the previously separated continent group.