Evidence For The Existence Of The Proto-tethys Ocean In SW Yunnan

The southwestern Yunnan of China is located in the eastern segment of the Tethys orogenic belt,preserving and recording the geological evolution process of the Tethys tectonic domain at different stages from the Paleozoic to the Cenozoic,and is a hot area for Tethys research.In recent years,many Early Paleozoic magmatic-metamorphic events and regional stratigraphic unconformities have been discovered in southwestern Yunnan.In response to these Early Paleozoic tectonic events,some scholars have identified them as the product of the Pan-African orogeny,representing a response to the final convergence of Gondwana.The northern margin of Gondwana should have been a passive continental margin during the Early Paleozoic.However,the other scholars have proposed that these tectonic events are in response to the Anden-type orogeny caused by the subduction of the Proto-Tethys Ocean,and the northern margin of Gondwana should be an active continental margin during the Early Paleozoic.With the continuous research in recent years,geological evidence of ophiolite,volcanic-sedimentary sequences and intrusive rocks,which representing the remnants of paleo-oceanic basins and oceanic subduction in the Early Paleozoic,have been found in the Baoshan Block and its surrounding areas,which mark the existence of the Proto-Tethys Ocean in southwestern Yunnan.However,there are still many unresolved questions about the tectonic evolution of the Proto-Tethys Ocean in southwestern Yunnan,such as the subduction polarity,the tectonic evolution pattern and the relationship between the Proto-Tethys and Paleo-Tethys Ocean.Therefore,in this paper,we focus on the Early Paleozoic sedimentary and magmatic rocks of the Baoshan Block and adjacent Lancang Block,conduct detailed field geological surveys,petrography,geochronology,whole-rock major and trace geochemistry,and isotope geochemistry studies.The youngest detrital zircon of the sedimentary rock samples from the Gongyanghe and Mengtong Groups of the Baoshan Block and the Lancang Group of the Lancang Block are 526 Ma,501 Ma,and 422 Ma,respectively,defining the lower limit of the sedimentary age of the stratum.Combined with volcanic rock intercalations aged 499 Ma and 460 Ma in the upper part of the Gongyanghe Group and Mengtong Group,and chlorite and crossite aged 410 Ma±15 Ma and 409.8±23.6 Ma in the upper part of the Lancang Group,the sedimentary ages of these three strata are respectively defined as Cambrian,Ordovician,and Silurian.The source analysis study shows that the Early Paleozoic strata in the study area have similar age distribution patterns of detrital zircons,which have primary age peaks of～0.95 Ga and 0.6-0.5 Ga and secondary age peaks of～1.2 Ga,1.8-1.7 Ga and 2.5-2.4 Ga,along with a small number of detrital zircons with ages ranging from 0.8-0.6 Ga.The Pan-African detrital zircons were derived from the Gondwana orogenic belt associated with the convergence of Gondwana,while the Grenville detrital zircons were derived from the Eastern Ghats-Rayner orogenic belt in India and East Antarctica,and the Albany-Fraser orogenic belt in Southwest Australia.Therefore,the sediment sources of the Early Paleozoic strata of the Baoshan Block and Lancang Block mainly come from Indian,with a small portion coming from Western Australia.During the Early Paleozoic,the paleogeographic location of the Baoshan Block and Lancang Block on the northern margin of Eastern Gondwana should be located between the Indian and Western Australia,closer to the Indian.Geochemical analysis of the sedimentary rocks shows that the CIA values of the Early Paleozoic sedimentary rock samples range from 63.1 to 85.8 and the Th/U ratios range from 1.55 to 3.06,indicating that the source area suffered from weak-moderate weathering.The ICV values of the samples of the Mengtong Group and Lancang Group range from 0.72 to 1.38,with most of them close to 1,indicating that the material maturity of the source area is relatively poor.The ICV values of the samples from the Gongyanghe Group,Dakuangshan Formation,Laojianshan Formation and Renheqiao Formation,which are located in the interior of the Baoshan Block,range from 0.41 to 1.00,and most of the samples have ICV values much less than 1,with an average value of 0.68,indicating a relatively high maturity of the rock components in the source area.The Ti O₂/Zr ratio of the samples in the study area is 6-38,the average La/Sc value is 7.08,and the average Sc/Th value is 0.50,indicating that the sediment source is mainly from felsic igneous rocks in the upper crust.These samples have relatively high Ti O₂,Al₂O₃,（Fe₂O₃^T+Mg O）contents and Eu/Eu^*ratios,while the Si O₂,K₂O/Na₂O and（La/Yb）_N ratios are relatively low and have roughly the same composition as the gray wacke from the island arc area of the active continental margin,which indicate that they might have been formed in an active continental margin.The Early Paleozoic magmatism is widely distributed on both sides of the Yunxian-Menghai ophiolite belt,and its formation age has lasted from the Early Cambrian to the Late Silurian.In this paper,we selected Caojian biotite monzogranites and Longling biotite monzogranites from the Baoshan Block for geochronological and geochemical analyses.Their weighted average ages are 454.9±5 Ma and 467.9±5.1 Ma,respectively,which are similar to the ages of the meta-volcanic rocks in the Lancang Group.Among them,Caojian S-type biotite monzogranites is characterized by high silica,high-K calc-alkaline and strongly peraluminous,and the fractionation of light and heavy REE is not obvious,with obvious negative Eu anomalies,which is the product of partial melting of ancient lower crustal materials.The Longling biotite monzogranites is characterized by weak peraluminous and calc-alkaline,and belongs to highly fractionated I-type granite.The primitive magma originated from partial melting of lower crustal material and has similar characteristics to calc-alkaline granites formed in island arc or active continental margin environments.The Early Paleozoic strata of the Baoshan and Lancang Block have similar age distribution patterns of detrital zircons,indicating that the Lancang Block was very close to the Baoshan block during the Early Paleozoic.Based on the evidences such as the ⁴⁰Ar/³⁹Ar ages of metamorphic minerals in the rocks of the upper part of the Lancang Group,ophiolites with geochemical characteristics of basalts in a back-arc extensional environment in the Nantinghe area and eclogites discovered in the Mengku area,the Lancang Block should be a part of the Baoshan Block in the Early Paleozoic.And then,the intense subduction of the Proto-Tethys Ocean led to the extension of the back-arc basin in the Changning-Menglian area,and the Lancang Block separated from the Baoshan Block into a continental margin arc.By synthesizing the results of this paper and previous research in the region,we redefine the time frame of the Early Paleozoic magmatism in southwest Yunnan and reshape the tectonic evolution history of the region as follows:In the Early Cambrian,the Proto-Tethys Yunxian-Menghai Ocean began to subduct southward（now westward）under the Baoshan Block,and the northern margin of East Gondwana changed from a passive continental margin to an active continental margin.In the Late Cambrian,the Proto-Tethys Yunxian-Menghai Ocean began to subduct northward（now eastward）under the Simao-Indochina Block,forming a bisubduction model.The intense subduction resulted in the extension of the back-arc basin in the Changning-Menglian area on the eastern margin of the Baoshan block and the Lincang area on the western margin of the Simao block,and the Lancang Block separated from the Baoshan block into a continental margin arc.During the Late Ordovician to the Early Silurian,the scale of the Changning-Menglian back-arc basin gradually decreased under the influence of continuous southward subduction of the Proto-Tethys Ocean,and finally closed during the Late Silurian to the Early Devonian.The Lancang continental margin arc collided with the Baoshan Block again.In the Late Silurian,the southward subduction of the Proto-Tethys Ocean tended to stop,while the northward subduction continued.In the Devonian,the Paleo-Tethys inherited and developed on the basis of the Proto-Tethys,and the expansion coexisted with eastward subduction.The northern margin of Eastern Gondwana changed from an active continental margin to a passive continental margin.During the Late Permian to the Early Triassic,the Lincang back-arc basin closed and arc continental collision occurred.In the Late Triassic,the Paleo-Tethys Ocean finallly closed and the Baoshan Block collided with the Simao Block.During the Early Paleozoic to the Early Mesozoic,the southwest Yunnan region started from the Changning-Menglian suture zone in the west,through the Lancang Block and the Yunxian-Menghai ophiolite belt,to the Lincang granite batholith in the east,which has undergone a multi-phase,multi-stage and long time span orogenic superposition,constituting a regional composite orogen,including the expansion of the Proto-Tethys Ocean,subduction and ablation,arc continental collision,the expansion and extinction of the Paleo-Tethys Ocean and the final continental-continental collision.