| The Sanjiang Tethys zone was known as three rivers,Nu River,Lancang River and Jinshan River in Yunnan,in SW China.The Sanjiang Tethys zone is mainly composed of aboundant microcontinents,ophiolite mélange sutures,magmatic arc zones,and preserves abundant rock records of evolution of the Proto-Tethys,Paleo-Tethys Ocean,Paleo-Tethys and branch oceans.Many geologists recognized that there was a Proto-Tethys Ocean in SW Yunnan,but it is blank report about the early Paleozoic ophiolite Mélange which is the effective evidence for a real ocean.As the results,those geological events such as magmatism,orogen and metamorphism were proposed in different tectonic regimes in Himalaya,Lhasa,South Qiangtang,Tengchong,Baoshan,Indochina and South China along the north margin of East Gondwana.In recently years,more and more ophiolitic records were found in East Asian,and it is possible that the Proto-Tethys Ocean was location at the northern margin of East Gondwana in Early Paleozoic.And a branching ocean of Proto-Tethys have been inferred with the early Paleozoic ophiolite mélange in the center Tibet,as well as that several early Paleozoic ophiolite mélanges was found from Yunxian to Menghai in SW Yunnan.The studied area is belonging to SW Yunnan,and including Baoshan block,Changning-Menglian suture zone and Simao block from west to east,and the two blocks are separated by the Changning-Menglian zone which a N-S trending structural Belt.The Baoshan Block is a northern part of sibumasu block,and affinity of Gondwana.The Simao Block is Yangtze-derived,located at northern Indochina.In early Paleozoic,Baoshanblock?Subimasu block?and Simao block?Indochina block?were regarded as a part of the north margin of Gondwana and a part of Asian Hun superterane,respectively,and the Proto-Tethys Ocean have been suggested between Gondwana and Asian Hun superterane.So,it is possible that some early Paleozoic ophiolitic rocks are found to study tectonic events of Proto-Tethys Ocean in SW Yunnan.Using the results of geological survey,we found several ophiolite mélanges along Yunxian to Menghai in SW Yunnan,domes of Mayidui ophiolite mélange in Yunxian,Wanhe ophiolite mélange and Yinchanghe ophiolite mélange in Shuangjiang,Qianmai ophiolite mélange in Lancang and Damengling ophiolite mélange in Jinghong.In this contribution,we report zircon U–Pb ages and Hf isotope compositions,and whole-rock element and Sr–Nd compositions of early Paleozoic ophiolite.These new data provide robust constraints on the age and petrogenesis of the ophiolite.In combination with field results of geological survey in SW Yunnan,we intend to draw implications for the nature of early Paleozoic events and further test tectonic models of Proto-Tethys evolution.?1?The Proto-Tethys ophiolite mélanges were N-S trending along Yunxian–Shuangjiang–Lancang–Menghai from north to south,and extending to northern Burma,herein,named as Yunxian-Menghai ophiolite mélange belt.Yunxian-Menghai ophiolite mélange belt is depended at eastern part of Changning-Menglian suture zone,and parallel to Tongchangjie-Niujingshan ophiolite mélange belt which was regarded as the oceanic ridge residues of Paleo-Tethys.Yunxian-Menghai ophiolite mélange belt was consisted of Mayidui,Wanhe,Yichanhe,Qianmai and Damenglong ophiolite mélanges.The Mayidui and Damenglong ophiolite mélanges were the oceanic ridge residues of Proto-Tethys,and the Yichanhe ophiolite mélange was the typical oceanic island series of deposit and OIBs,Wanhe ophiolite mélange was formed in the oceanic intra-arc,the Damenglong ophiolite mélange shows island arc magmatic rocks and sedimentary.?2?Early Paleozoic magmatic events are identified in Yunxian-Menghai Ophiolite mélange belt,including Mayidui?458-447 Ma?,Wanhe?461-458 Ma?,Yichanghe?462-455 Ma?,Qianmai?507-492 Ma?and Damenglong?481 Ma?.The mafic rocks from Mayidui and Damenglong Ophiolite mélanges have low K,high Ti,and tholeiite feature,and show uniform N-MORB-like REE pattern and spider diagram,greatly positive?Hf?t?values and positive?Nd?t?values,suggesting that these mafic rocks were derived from depleted mantle in oceanic ridge.The mafic rocks from Yinchanghe ophiolite mélanges are characterized by low-middle K,high Ti,and show alkaline nature,have OIB-type REE and trace element patterns with significantly positive?Nd?t?values and slightly negative?Hf?t?values.The Yinchanghe ophiolitic rocks likely represent fragments of oceanic island or seamount,and originated from incremental partial melting of enrichment mantle in an intra-oceanic setting.The Wanhe ophiolitic rocks show similar E-MORB-like geochemical characters of affinities to tholeiitic series,weakly negative Nb and Ti anomalies,and have positive?Nd?t?and variable?Hf?t?values,suggesting that they were originated from enriched mantle and formed in an intra-oceanic setting related to subduction of the Proto-Tethys ocean.The magmatic rocks of Qianmai ophiolite mélange were composed of mafic rocks,intermediate rocks and granitoids.The basaltic rocsks are characterized by high Mg O,Nb,Sr and low Y concentrations,and intermediate and granitoids have high Mg#values and Sr elements and low Al2O3concentration,Y elements.All of the magmatic rocks show enrichment LREEs and LILEs,and no fractionated LREEs with moderate negative Nb,Ta,Sr,Eu anomalies,and they are similar to those of basaltic-to-felsic rocks in subduction zones,and are correlated to the subduction of Proto-Tethys ocean.The basaltic rocks were derived from enrichment mantle,and the geochemistry of these granitoids indicate that they were likely generated by the reworking of crustal material with little juvenile mantle material.The Qianmai ophilite were formed in an extensional setting associated with roll-back of the subducted Proto-Tethys oceanic slab in Cambrian.?3?The Early Paleozoic tectonic history of Proto-Tethys will be discussed by our new data and published data in SW Yunnan:firstly,the Proto-Tethys ocean,named as Yunxian-Menghai ocean,had been opening,then turning into mature ocean in pre-Cambrian;the magmatic data in SW Yunnan should constrain the evident of subduction time span of the Yunxian-Menghai ocean during 518-481 Ma.The age and geochemical data indicate that both the western Simao Block and eastern Baoshan Block were active continental margins during Ordovician to early Silurian?480-440 Ma?;Later than 440Ma,an atrophic ocean of Proto-Tethys could still subducted forward to Simao block,a back-arc basin opened in the west margin of the Simao block in response to the Proto-Tethys final subduction.?4?The Sanjiang Tethys realm in the eastern margin of the Tibetan Plateau,and there were abundant early Paleozoic magmatic records in the Longmu Co-Shuanghu suture in the center Tibetan,which could be the branch to the north of Yunxian-Menghai suture belt,and the both were the oceanic residues of the SE branch ocean of Proto-Tethys.The Proto-Tethys should be suggested diachronic tectonic history along the northern Gondwana particularly focused on the early Paleozoic orogeny and magmatic records.In Turkey and Iran,southward subduction of Proto-Tethys begin in late Neoproterozoic and cease in early Cambrian;while in center Tibet-SW Yunnan,strongly southward subduction of Proto-Tethys began in middle Cambrian,and the bi-directional subduction been occurred during Ordovician to early Silurian,and a rudimental ocean could be survived during middle Silurian. |
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