The Evolution Of Metamorphism And Geochemistry For The Cangshan And Julin Groups In Central Yunnan,China

Systematic studies on metamorphism and geochemistry of the Proterozoicbasements in Central Yunnan in the conjunct area among the Yangtze, Cathaysia andIndochina Blocks are very important not only for interpreting the mechanism of theProterozoic mineralization and using mineral resources but also for understandingthe formation of geochemical steep zones and affirming the location of the SouthChina Block within the Neoproterozoic Rodinia supercontinent.Based on the comprehensive studies of the mineral growth sequence, P-Tconditions of metamorphism, garnet growth zoning and ~39Ar-~40Ar dating, a cunter-clockwise P-T-t path has been reconstructed for a staurolite-kyanite isograde belt inthe Julin Group. The prograde path was related to the magmatic accretion in arcsettings, indicating the existence of an arc setting during the Mesoproterozoic-Neoproterozoic period. The retrograde near-isobaric cooling path after the peakmetamorphism reveals that rapid erosion did not occur and sustained heat suppliesfrom the upper-mantle were maintained after magmatic accretion in the Yuanmouarea. The metamorphic conditions estimated by the author, together with thepublished P-T data for the Ailao Shan-Cang Shan Groups show that al least twoepisodes of metamorphism occurred in this area, i.e., the early episode is themedium-pressure type, and the late episode is low-pressure type.The elemental geochemistry shows that two types of metabasites occurred in theCangshan and Julin Groups. One is the meta-tholeiite formed in a transitional P-MORB and arc tectonic environment, and the other is the meta-alkali basaltic dykeformed in a within-plate extension environment. The characteristics of the Nd-Sr-Pbisotope system further indicate that the meta-alkali basaltic dykes were originat6dfrom a mantle source mixed between two end-members of DMM and EM II sources,indicating that their source was reworked by subduction in the Mesoproterozoic-Neoproterozoic time; and the meta-tholeiites resulted from a mantle source, fOrmedby mixing between the DMM and EM lI end-members for the Cangshan GrouP but theDMM and EM l for the Julin Group in the P a l eo protero zo ic -MesoprotCrozo ic period,indicating that the two Gruops respectively belonged to the different blocks beforethe Mesoproterozoic time.The comparative studies of the Proterozoic basements in the westem margin ofthe YangtZe block show that the metamorphic basements were mainly formed duringthe Mesoproterozoic-Neoproterozoic period (2.0 ~ l.6 Ga) but might belong todiffereni blocks; the tectonic activities during the Mesoproterozoic-Neoproterozicinterval (l.0 ~ 0.8 Ga) may be related to the assembly and breakup of the RodiniasuPercoatinent.