The Sequences And Tectonic Implication Of Late Permain To Middle Triassic Volcanic Rocks, Along The Eastern Margin Of The Lanping Basin

The Southwestern Sanjiang orogenic belt has a long and complex geologic evolution history, and several volcanic belts were found in the belt. Recently a large quantity of geochronological and geochemical data gradually revealed the temporal and spatial relationship of the volcanic rocks. However these belts covered by Mesozoic to Cenozoic sediments of Lanping basin which lies in the middle Sanjiang orogenic belt, lacked the precious geochronological and geochemical datas, which have limited to understand the temporal and spatial relationship of volcanic rocks. Hence, this paper choose the Late Permain to Early Triassic (P3-T1) volcanic rocks which cropped out in Madeng as the major topic, integrating with the volcanic rocks which cropped out in the Misha, along the eastern margin of Lanping basin, comparing with in order to ravel out the stack sequences, volcanism time and tectonic setting by the detailed field observation and measurement. Meanwhile, integrating with the coeval volcanic rocks to discuss the regional tectonic implications, provide the new evidence for the evolution of basin and Paleotethys. By the field geological investigation, observation of thin section, analysis of zircon LA-ICP-MS U-Pb dating and chemistry, this study gets the acknowledgements and achievements as follow:(1) Redefined and established the stratigraphic sequences of eastern and western margin of Lanping basin:Late Permain to Lower-Middle Triassic (P3-T2) stratigraphy, the lower sequence termed formation, formed in Late Permain; the upper sequence termed Pantiange formation, Lower-Middle Triassic. They overlaid by Maichuqing formation (Late Triassic, T3m), Sanhedong formation (Late Triassic, T3s), Nanxin formation (upper Cretaceous, K2n), Yunlong formation (Paleocence-Eocene, E1-2y), or Baoxiangsi group (Oligocene, E3b). In this study, we regarded Shanglan formation as the upper Permain, it is different from previous research which thought the Shanglan group overlaid on Middle Triassic Pantiange formation; Pantiange group was the Lower-Middle Triassic and the basement of Lanping basin, is different from previous study which regarded it as Middle-Upper Triassic; actually, previous defined Jurassic stratum which is almost Shanglan formation (P3s) and the other is Nanxin group (K2n).(2) The Late Permain to Early Triassic (P3-T1) volcanic rocks are made of four stack sequences, has a short time which volcanism started at 250Ma and continued to 244Ma. Bulk geochemistry suggests that the volcanic rocks has high SiO2 and K2O content, low MgO, Mg# and TiO2, and show the highly LREE enriched, and exhibit negative Nb, Ta, Ti and Sr anomalies. These characteristics are similar to arc-like volcanic rocks, suggest it generate in a subduction environment, as the result of Paleotethyan ocean northeastward or eastward subduction.(3)The Late Permain to Early Triassic rhyolitic rocks were found along the Misha, which be consist of three sequences, and volcanism started at 258Ma and continued to 248Ma. Intermidiate-acid lava and volcaniclastic cropped out along Zhongpai-Shideng, a few basalt, western of Lanping basin, four sequences are divided in this succession which the volcanism occurred at Early Triassic to lowermost Middle Triassic(241Ma-229Ma). Comparing the volcanic rocks and volcaniclastic which cropped in the western and eastern margin of Lanping basin, Late Triassic volcanic rocks not be found.(4) Integrating and comparing with previous data, Late Triassic succession cannot find in the Lanping basin. Permain to Middle Triassic volcanic rocks which consist the base of basin exposed along the western and eastern margin of Lanping basin, and stretched northward linking with Jomda-Weixi arc belt, also formed by the Paleotethyan ocean northeastward or northward subduction.