Compositions Of The Neoproterozoic To Early Paleozoic Basement Rocks And Its Tectonic Significance

South China Block(SCB)and Indochina Block(ICB),as the most important Precambrian continental blcks in southern Asia,experienced distinctive crustal evolution histories.The SCB consists of the Yangtze and Cathaysia Blocks,which have different compositions and evolution of the Precambrian basements.Both blocks collided and assembled a united continent,becoming one of the most complicated Precambrian blocks in southern Asia.However,the boundary between the ICB and SCB,and the southwest extension of the boundary between the the Yangtze and Cathaysia Blocks has always been controversial.In this study,early Paleozoic Nanwenhe-Song Chay granitic Complx and its peripheral basement metamorphic rocks in the southeastern Yunnan(China)and northern Vietnam have been selected for further study,to provide valuable information on constraing the above-mentioned block boundaries.The Nanwenhe-Song Chay area straddling the border between SE Yunnan and and NE Vietnam is located at the tectonic junction of the Yangtze Block,Cathaysia Block and ICB.Due to the lack of a systematic understanding of the Precambrian basement rocks in this area,the tectonic attribute of this area is still enigmatic.The Nanwenhe-Song Chay complex(NSCC),as the only one huge early Paleozoic pluton exposed in study area,is a high-K calc-alkaline and peraluminous S-type granitic complex.LA-ICP-MS U-Pb dating of zircons from twelve granitic samples in the complex gives its crystallization ages in the range of 436-423 Ma.Zircon U-Pb-Hf isotopes and geochemical data show that the Nanwenhe granites in the northern part of the NSCC probably were mainly derived from the Neoproterozoic metamorphic pelites analogous to those in the Xiajiang and Danzhou Groups in the southwestern Jiangnan Orogen,Yangtze Block.In contrast,the Song Chay granites in the southern part of the NSCC probably originated from the Neoproterozoic metamorphic psammites analogous to those in the Fanjingshan and Sibao groups in the southern Yangtze Block.These granites from the complex contain abundant inherited zircons with a main age peak at 836-808 Ma and subordinate peaks at?734 Ma and 528-665 Ma.The existence of a few 730-500 Ma inherited zircons indicates that the magma sources probably contain a minor younger metasedimentary rocks,e.g Cambrian sediments.The inherited zircons have age distribution patterns and Hf-isotopic compositions similar to those for the detrital zircons from Neoproterozoic sedimentary rocks in the southern part of the Yangtze Block,but different from those of the Cathaysia Block and Indochina Block.Further study has been carried on the basement meta-sedimentary rocks surrounding the NSCC.Zircon U-Pb dating results suggest that the Mengdong Group in SW China and the Thac Ba Formation in NE Vietnam were deposited in the late Neoproterozoic(<619-592 Ma),while the Tianpeng Formation in SW China and the Ha Giang Formation in NE Vietnam were deposited in the Cambrian(<527-507 Ma).Bulk geochemical data and zircon Hf-isotope compositions indicate that the sources of these sedimentary rocks were dominated by felsic rocks.The sources of the late Neoproterozoic sedimentary rocks probably underwent weaker weathering under drier and colder conditions than those Cambrian ones.The late Neoproterozoic Mengdong Group and the Thac Ba Formation contain abundant 802-747 Ma detrital zircons.Their age distributions and Hf-isotope compositions are similar to those of Neoproterozoic sedimentary rocks in the southern Yangtze Block(Fanjingshan-Sibao area).In contrast,the Cambrian Tianpeng and Ha Giang formations are characterized by abundant Grenvillian detrital zircons with an age peak at?980 Ma.The age spectra and Hf-isotope compositions of the detrital zircons from these rocks are similar to those in the Cathaysia Block(Nanling-Yunkai area)and/or Indochina Block.Integration of our data with published geological data suggests that the Precambrian basement under the Nanwenhe-Song Chay area is the part of the Yangtze Block,and the NSCC may represent the westernmost tip of the early Paleozoic orogenic belt in the SCB.The western boundary between Yangtze and Cathaysia blocks must be located to the south or southeast,rather than along the Shizong-Mile fault in the north,and the boundary between the SCB and ICB should be to the southwest of the NSC area,rather than along the Dian-Qiong suture.Numerous Pan-African(600-500 Ma)detrital zircons and the diagnostic age spectra of detrital zircons in the early Paleozoic sedimentary rocks indicate that the SCB was located at the northern margin of East Gondwana during the early Paleozoic.The dramatic change in the clastic components from the late Neoproterozoic to Cambrian sedimentary rocks suggests that the NSC area was probably affected by the Pan-African orogeny.The Nam Co belt is located to the northeast of Song Ma Fault in NW Vietnam.The tectonic attribute of this bellt is also hotly debated.Zircon U-Pb dating results suggest that the Nam Co Formation in the Nam Co belt is most likely a middle Neoproterozoic stratum.The lower Nam Co Formation probably formed in 821-726 Ma,and the upper Nam Co Formation deposited not earlier than 726-721 Ma.The sedimentary rocks from both parts of Nam Co Formation are dominated by Neoproterozoic(900-800 Ma)detritus,but the upper Nam Co Formation contain more Meso-Paleoproterozoic materials than the lower Nam Co Formation.Their age distribution and Hf-isotope compositions are all similar to those of Neoproterozoic sedimentary rocks from western Jiangnan belt in the southern Yangtze Block and Panxi belt in western Yangtze Block.Zircon U-Pb-Hf-isotope compositions and bulk geochemical data indicate that the sources of the sedimentary rocks from the Nam Co Formation were dominated by felsic rocks with minor mefic components,and clastic materials were mainly derived from Yangtze Block.Moreover,the lower Nam Co Formation probably received more middle Neoproterozoic materials derived from the Panxi belt in the western Yangtze Block than the upper part.Many lines of evidence suggest that the Nam Co belt is also a part of Yangtze Block.This further comfirms that the boundary between the SCB and ICB would most likely be the Song Ma fault,which is located to southwest of the study area.The Nam Co Formation was deposited in a Post-orogenic Intracontinental rift basin at early stage(821-726 Ma),and the depositinal environment became more stable at late stage(<726-721 Ma).After then,the Nam Co belt and SW SCB probably turn into crustal extension setting and became a stable intraplate depositional basin.