Early Paleozoic Proto-tethys Tectonic Evolution In SW Yunnan: Constraints From Detrital Zircon U-Pb Geochronology And Granite Associations

The Sanjiang Paleo-Tethys Orogen in SW China constitutes an essential part of the eastern Tethyan tectonic domain,it is mainly composed of microcontinents,ophiolitebearing sutures,magmatic arcs,seamounts and flysch deposits,and preserves abundant rock records of evolution of the Proto-Tethys,Paleo-Tethys Ocean,Paleo-Tethys and branch oceans.The Sanjiang Orogen constitutes an important part of the Tethyan tectonic domain.The Neoproterozoic to early Paleozoic magmatism,coeval metamorphism,regional sedimentary unconformity produced in the margin of East Gondwana(e.g.central Iran,Turkey,Himalaya,Lhasa,Qiangtang,Tengchong,Baoshan).Two possible tectonic regimes have been proposed as the context for this late Cambrian to Early Ordovician magmatism:(1)a late extensional stage of the longlasting Pan-African orogenic cycle,which ended with the formation of the Gondwana supercontinent;and(2)an Andean-type active continental margin related to the subduction of Proto-Tethyan oceanic lithosphere and the assembly of outboard microcontinents.The Changning-Menglian Belt is a N-S trending structural zone.It is separated from the the Yangtze-derived Simao Block to the east,and from the Gondwana-derived Baoshan Block to the west.Many authors have suggested that the early paleozoic margin of Gondwana consisted of some or all of several major terranes namely South China,North China,Tarim,Indochina,Sibumasu(or Shan–Thai Terrane,Baoshan Block in part),West Burma,Lhasa and Qiangtang,thus,the Sanjiang Orogen in the Southeastern Qinghai–Tibet Plateau is an ideal region to investigate the geodynamic processes related to the evolution of the Proto-Tethys Ocean.In this contribution,we report zircon U–Pb ages and Hf isotope compositions,and whole-rock element and data of early Paleozoic granite from gneiss domes of Qianmai,Bulangshan and Mayidui and early Paleozoic sedimentary of Mengtong,Mengdingjie and Lancang Group.These new data provide robust constraints on the age and petrogenesis of the granites and sedimentary.In combination with published data in SW Yunnan and East Gondwanaderived microcontinent,we intend to draw implications for the nature of Paleozoic events in these blocks and further test tectonic models for the evolution of the eastern Gondwana supercontinent.(1)Age spectra for the two samples from the Lancang Group exhibit a similar range of ages,which fall into four groups: 562 Ma,970 Ma,1122 Ma and 2382 Ma,and with ?Hf(t)values of-22.9 to +16.1.The youngest age from the two samples is 455 Ma.Age spectra from the Mengtong Group yielded 562 Ma,970 Ma,1122 Ma and 2382 Ma,and with ?Hf(t)values of-25.8 to +28.3.The youngest age from the two samples is 558±15 Ma(n=5).Age spectra from the Mengdingjie Group yielded 552 Ma,962 Ma,1170 Ma and 1606 Ma.The youngest age from the two samples is 552±5 Ma(n=12).Detrital zircon ages of the Lancang and Mengtong Groups in SW Yunnan show that the deposition of the groups began in the Ordovician rather than previously inferred Neoproterozoic.(2)The distribution pattern of the U-Pb ages and Hf isotopic data of zircons suggests that the Lancang Group(Lancang Block)was paleogeographically close to the Baoshan Block on the northern margin of East Gondwana during the Early Paleozoic.Detritus from the Wilkes-Albany-Fraser belt in southwest Australia and from the Rayner-Eastern Ghats belt in India were the main sources for Paleozoic sediments of the Baoshan Block and the Lancang Block.(3)The sedimentary from Lancang Group,Mengtong Group and Mengdingjie Group have high Si O2 and Al2O3,but low K and Na,the provenance was dominated by quartzose sediments and felsic igneous rocks.The chondrite–normalized REE patterns of samples generally present enrichment in light rare earth elements(LREEs)and display relatively flat heavy rare earth element(HREE)pattern,forming an active continental margin.(4)Early Paleozoic magmatic events are identified in western margin of Simao Block,including Qianmai(484-496 Ma),Bulangshan(478-481 Ma)and Mayidui Pluton(459-463 Ma).The Qianmaitrondhjemites have high Na and low K.The trondhjemites are characterized by weakly fractionated REE with moderate to negligible negative Eu anomalies,similar to Na-rich intermediate-to-felsic rocks in subduction zones.The formation of Na-rich granitic rocks is correlated to the subduction of Proto-Tethys ocean.The Bulangshan A-type granite have high K and low Na,are characterized by weakly fractionated REE with moderate to negligible with strong negative Eu anomalies.The geochemistry of these A-type granites indicates they were likely generated by the reworking of crustal material with little juvenile mantle material.,They formed in an extensional setting associated with roll-back of the subducted Proto-Tethys oceanic slab.The geochemistry of Mayisui S-type granite are Phanerozoic circum-Pacific orogenic belts S-type granites.S-type plutonism is triggered by renewal of arc magmatism following thickening,when hot basaltic magmas are intruded into the thickened backarc crust once slab retreat is reestablished.(5)Our new data with published data constrains the subduction time span of the Proto-Tethys Ocean and the Early Paleozoic tectonic history of the Sanjiang orogen related to the Proto-Tethys Ocean.The age and geochemical data indicate that both the western Simao Block and eastern Baoshan Block were active continental margins during the Cambrian to Silurian,favoring a bi-directional subduction model for the evolution of the Proto-Tethys during that period.During the early Devonian,the Changning-Menglian opened as a back-arc basin in response to the Proto-Tethys subduction.(6)In this contribution we present a model of the Early Paleozoic orogeny along the northern Gondwana particularly focused on the sedimentary,metamorphic and magmatic evolution.1)In Turkey,Iran and Arghanistan,southward subduction of Proto-Tethys begin at 570 Ma or more early and cease at 530 Ma;2)In Himalya-Tibet,southward subduction of Proto-Tethys begin at 510 Ma or more early,the South Qiangtang and North Qiangtang collided at Silurian,leading to closure of Proto-Tethys;3)In Sanjiang area,a bi-directional subduction model for the evolution of the ProtoTethys during Cambrian to Silurian.