Detrital Zircon Geochronology And Hf Isotope Studies Of The Baoshan And Tengchong Terranes,western Yunnan,and Their Geological Significance

The Sanjiang Tethys Tectonic Belt is located in the southeastern part of the Qinghai-Tibet Plateau,including multiple ophiolitic melange belts,(micro)landmasses and magmatic arcs.It completely records the multiple tectonic cycles of the Proto-,Paleo-,Meso-and Neo-Tethys Ocean.The southern section of the Sanjiang Tethys Tectonic Belt is located in western Yunnan Province.Bounding by the Changning-Menglian Tectonic Belt,the Yangtze-derived Simao Terrane is on the east side,while on the west side is the Gondwana-derived Baoshan and Tengchong terranes.The Baoshan Terrane has been regarded as the northern part of the Sibumasu Terrane.However,there is a big controversy about the nature of the Tengchong Terrane.It was considered to be a part of the Sibumasu Terrane,same with the Baoshan Terrane,or the southeastern extension of the Lhasa Terrane in Tibet.In addition,the specific locations of both the Baoshan and Tengchong terranes in the Early Paleozoic Gondwana Super-continent reconstruction(such as the northern margin of Indian or the northern edge of Australian)are still controversial.In the past 20 years,with the advancement and popularization of laser ablation inductively couple plasma mass spectorometry analysis technology,the detrital zircon U-Pb-Hf isotope system has been widely used to constrain the maximum sedimentary age,to study the source of clastic materials and depostion environment for detrital strata and further to trace the affinity between terranes.The Baoshan Terrane contains multiple early Paleozoic metamorphic sedimentary strata,but the sedimentary ages of these strata has not been well defined.Some researchers had carried out detrital zircon U-Pb-Hf isotope study on these strata,but only the eastern part of the Baoshan Terrane was focused mostly and the work in western region is still relatively lacking.Moreover,the Tengchong Terrane has been considered to be lacking in clastic sedimentary strata.In this paper,field research and sampling were carried out for the Gongyanghe Group and the Mengdingjie Group of the Baoshan Terrane.We also have IV found a set of high-grade metamorphic Early Paleozoic-Late Mesozoic detrital sedimentary rocks in the Gaoligonshan Group of the Tengchong Terrane.Detrital zircon U-Pb geochronology and isotope analysis were conducted and the new understanding achieved are as follows:(1)The ages of detrital zircons from the Gongyanghe Group in western Baoshan Terrane ranges from 3862 to 533 Ma with ?Hf(t)value from-37.8 to +11.0.The minimum age peak is at 576 ~ 458 Ma,indicating that the maximum sedimentary age of this group is no early than early Early Cambrian.Considering the Pumanshao Group that covers the upper part of the Gongyanghe Group contains 499 Ma volcanic rock interlayers,we infer that the Gongyanghe Group in the western part of the Baoshan Terrane is of the Cambrian in depositional age.(2)The ages of detrital zircons from the Mengdingjie Group in the eastern part of Baoshan Terrane rang from 3097 to 542 Ma.The minimum age peak period(at 576 ~ 458 Ma)limits the sedimentary age of the Mengdingjie Group to no early than the Late Ordovician.(3)The main age peak of detrital zircons from both the Gongyanghe and Mengdingjie groups of the Baoshan Terrane are of 1000 to 900 Ma and the most likely sediment source area is the Eastern Ghats Orogenic Belt in eastern Indian.In addition,the detrital zircon U-Pb ages of the Mengdingjie Group and the Lancang Group in the east show relatively obvious secondary age peaks ranging from 1200 to 1100 Ma,which are similar to the main age peaks of detrital zircons from the Lhasa terrane and Western Australia.Therefore,the Baoshan Terrane was in the northern margin of India in the reconstruction of the Early Paleozoic Gondwana Supercontinent and its basins mainly accepted the sediments from India while the eastern side of the Terrane was adjacent to Australia and accepted a certain part of sediments from Australia.(4)The youngest age clusters of six samples from the Gaoligongshan Group of the Tengchong Terrane can be divided into three groups with ages of 539 ~ 491 Ma,236 ~ 223 Ma and 115 Ma,that means the maximum sedimentary age of these groups are possible Cambrian,Triassic and Cretaceous,respectively.These results further confirm the Gaoligongshan Group is not a Precambrian crystalline basement but a metamorphic complex.(5)Significantly different from the Baoshan Terrane,the main age clusters of the detrital zircons from the Cambrian and Triassic phases of the Gaoligongshan Group are at ~530 Ma and ~1130 Ma,which are very similar to the detrital zircon age spectrum of the Lhasa terrane in central Tibet and western Australia.It indicates that the Tengchong Terrane was closed to the northern margin of Australia in the reconstruction of the Gondwana Supercontinent.The Early Paleozoic sedimentary basins mainly accepted sediments from Australian.The Cretaceous phase is the deposition record after the collision between the Tengchong and Baoshan terranes.(6)Comprehensive researches of the stratigraphy,magmatism and structural property and the detrital zircon U-Pb-Hf isotope analysis of clastic rocks from the Baoshan,Tengchong,Western Qiangtang and Lhasa terranes,suggest that the Baoshan and Tengchong terranes are the southeast extension of the Western Qiangtang and Lhasa terranes,respectively.During the India-Asia collision,the southeast terranes escaped to the southeast into today's geographical position.