Constraining The Age,nature And Metamorphic-hydrothermal History Of The Lancang Group,SW Yunnan,with Implications To The Proto- And Paleo-Tethys Evolution

West Yunnan in SW China preserves abundant rock records of Tethys orogenic evolution from the early Paleozoic to the Cenozoic,and hence is an ideal natural laboratory for studying the multi-stage tectonic evolution of the Tethys orogenic belts.In particular,the Lancang Group that crops out between the early Paleozoic YunxianMenghai suture zone and the late Paleozoic Changning-Menglian suture zone,has witnessed the long-term tectono-thermal evolution of the Tethys orogenic belt.However,although numerous studies have been carried out in this area,previous work has been mainly focused on geochemistry and geochronology of the granitoids(Lincang Batholith),detrital zircon and provenance studies of sedimentary rocks,P-T-t patterns of the limited high-grade metamorphic rocks.Many important scientific questions remain debated,these include the spatiotemporal relationship between the Proto-and Paleo-Tethys,the age and property of the Lancang Group,the nature of the basement to the Lancang Group,and the metamorphic-hydrothermal history of the group.The Lancang Group is composed of metasedimentary and volcanic rocks,and consists of five formations,which are in ascending order the Nanmuling,Mengjingshan,Manlai,Huimin and Nankenghe formations,two of which(Manlai and Huimin)contain abundant metavolcanic rocks.This project aims to investigate the Lancang Group through a comprehensive study of zircon U-Pb geochronology and bulk-rock geochemistry on the volcanic and sedimentary rocks,providing new data to refine the age,provenance characteristics and nature of the group.The new results are compiled with published data to reconstruct the Lancang Terrane.Using the low-grade metasedimentary rocks of the Lancang Group,monazite and xenotime SHRIMP U-Th-Pb geochronology was carried out to reveal the metamorphic and hydrothermal history of the group.Major findings and conclusions of this project are summarized below:1.New age constraints on volcanic history of the Lancang Group.Five representative volcanic rock samples from the Huimin Formation yielded zircon U-Pb ages of 478 ± 5 Ma,470 ± 6 Ma,459 ± 4 Ma,449 ± 8 Ma,and 442 ± 4 Ma.These results reveal a significantly wider age range,suggesting a more protracted duration of volcanism spanning > 35 m.y.throughout much of the Ordovician.Together with data from the Mailai Formation mafic volcanic rocks,which have been recently dated at 495–490 Ma,these new geochronology data provide robust age constraints on the volcanic-bearing formations of the Lancang Group.2.Refining the depositional time frame of the Lancang Group.The volcanic rocks of the Lancang Group are mainly confined within the Huimin Formation and,to a lesser extent,the Manlai Formation,both occupying the upper-middle part of the group,whose depositional ages have been well bracketed by the dated volcanic layers.In the absence of volcanic components,the youngest graphical age peak(YPP)is used to define maximum depositional ages of sedimentary strata.The youngest graphical age peak of detrital zircon for the Nanmuling,Mengjingshan,Manlai,Huimin and Nankenghe formations are 576 Ma(n = 2),554 Ma(n = 20),491 Ma(n = 3),463 Ma(n = 5)and 436Ma(n = 5),respectively,constraining the maximum depositional age of the respective formations.The YPP detrital zircon ages of 463 Ma and 491 Ma for the Huimin and Manlai formations,respectively,are indistinguishable within uncertainty from the ages of volcanic rocks they host.Together,U-Pb age data of detrital and volcanic zircons from the Huimin and Manlai formations bracket their deposition to 495–442 Ma.However,the grouped ages of 576 Ma and 554 Ma for the youngest detrital zircon clusters from the Nanmuling and Mengjingshan formations,respectively,permit their deposition to have commenced in the latest Neoproterozoic.3.Geochemical characteristics of volcanic rocks of the Lancang Group and their tectonic implications.In terms of rock type and geochemical characteristics,the volcanic samples from the Huimin Formation show systematic differences,but also share some similarities.The lower part of the formation is dominated by basic rocks,whereas the upper part by intermediate to felsic rocks.On the chondrite-normalized REE diagram and primitive mantle-normalized incompatible trace element spidergram,the Huimin volcanics display enriched light REEs relative to heavy REEs with enhanced negative anomalies of Nb,Ta and Ti,yet the basaltic samples from the lower formation have significantly higher REEs and strongest light-to heavy-REE fractionation.The Huimin Formation volcanics are characterized by uniformly negative εNd(t)value values(–7.61 to –5.57).The volcanic rocks of the Manlai(495–490 Ma)and Huimin(478–442 Ma)formations are the products of arc magmatism induced by the Proto-Tethys Ocean subduction.From the Manlai Formation to the Huimin Formation,the volcanic rocks of the Lancang Group demonstrate systematic changes in geochemistry,such as more abundant REE(96–231 ppm to 82–529 ppm),and the increasingly unradiogenic εNd(t)values(Manlai: –1.32 to 3.42;Huimin: –7.60 to –5.57),reflecting the secular evolution of subduction-zone magmatism during that period.4.Reconstructing the Lancang Terrane and its geological significance.Because no robustly-dated Precambrian rocks have been documented in the study area,xenocrystic zircons identified from the volcanic rocks of the Lancang Group and granitoids of the Lincang Batholith,both of which are believed to be produced by autochthonous magmatism,are used to derive indirect constraints on the age and nature of the underlying basement.This is aided by available whole-rock Nd and zircon Hf model ages from the two types of magmatic rocks.Xenocrystic zircons from the Lincang Batholith granitoids and the Lancang Group volcanic rocks record U-Pb ages ranging from 2.84 to 1.01 Ga,with a major peak at ca.1.74 Ga,indicating the existence of late ArcheanMesoproterozoic basement beneath the Lancang Group.This is supported by Hf model ages of 2.98–1.01 Ga for igneous zircons and whole-rock Nd model ages of 2.82–1.05 Ga for magmatic rocks of the group.Based on these evidences,we reconstruct the Lancang terrane as a continental sliver largely bounded by the Changning-Menglian suture to the west and the Yunxian-Menghai suture(or the Lancangjiang tectonic zone)to the east.Detrital zircon data from the Lancang Group define two major age peaks at ca.1000–900 Ma and 600–500 Ma,with minor age peaks at ca.2500–2400 Ma,1200–1100 Ma and 500–400 Ma.The predominant provenance sources are orogenic belt in Gondwana and magmatic activities in Baoshan-Lancang blocks,indicating an affinity of the Lancang Terrane to Gondwana.During the early Paleozoic,the Lancang Terrane,together with the Baoshan-Sibumasu blocks,formed part of a passive continental margin of the Proto-Tethys along the northern margin of Eastern Gondwana.Along with westdipping subduction of the Proto-Tethys ocean from the Cambrian to the Ordovician,arclike volcanic and granite rocks were formed in the Lancang and Baoshan blocks.Sometime during the Silurian-Early Devonian,back-arc extension in response to subduction of the Proto-Tethys led to the formation of a rift basin west of the Lancang Terrane,which gradually developed into a major ocean known as the Paleo-Tethys represented by the Changning-Menglian suture.As a consequence of the opening and spreading of the Paleo-Tethys ocean,the Lancang Terrane split off northern Gondwana and drifted northeastwards until eventually amalgamated with the Simao Block of South China marking the demise of the Proto-Tethys.5.Low-temperature metamorphic-hydrothermal history of the Lancang Group.Phosphate minerals monazite and xenotime are identified in the Lancang group metasedimentary rocks,in which they are observed to intergrow with metamorphic assemblage biotite,muscovite and chlorite,or overgrow on detrital monazite/xenotime grains,indicative of an authigenic origin.The authigenic monazite and xenotime were analyzed in situ(in polished thin sections)by the SHRIMP icon microprobe,to reveal the metamorphic-hydrothermal history of the host rocks.Monazite and xenotime from the Nanmuling Formation yielded U-Th-Pb ages of 228 ± 2 Ma,225 ± 4 Ma,224 ± 3 Ma and91–73 Ma,monazite from the Manlai Formation and xenotime from the Huimin Formation yielded U-Pb ages of 81 ± 2 Ma and 231 ± 4 Ma,respectively,and monazite and xenotime from the Nankenghe Formation yielded U-Th-Pb age of 406 Ma,377–367Ma,255–241,223–214 Ma and 119 Ma.Put together,authigenic monazite and xenotime age data define 4 peaks at 240 ± 2 Ma(n = 19),226 ± 2 Ma(n = 45),215 ± 3 Ma(n = 13)and 80 ± 2 Ma(n = 14),spanning the Indosinian and late Yanshanian orogenies.The major Indosinian peak at 226 Ma is interpreted to record metamorphic-hydrothermal activities related to the Paleo-Tethys closure and the associated continental collision between the Baoshan,Lancang and Simao blocks,whereas the late Yanshanian age(80 ±2 Ma)records monazite growth during the closure of the Neo-Tethys Ocean and the collision of the Tengchong and Baoshan blocks.