PRECAMBRIAN TECTONIC EVOLUTION OF KANGDING-LUDING AREA, WEST SICHUAN—AN EXAMPLE FOR CRUSTAL POLY-ACTIVATION AND ALTER-NATE PROGRESS OF "MOBILIZATION" AND "STABILIZATION"
WU Genyao(Institute of Geology, Academia Sinica)
A new stratigraphic sequence of early Precambrian in Kangding-Luding area, West Sichuan, is put forward. The oldest basement rock is named Kangding Gneiss, followed by, in ascending order, Luding's metavolcano-edimentary rocks, Yaside Formation and Yanjing Group. They could be correlative, respectively, with Yakou Gneiss, Miyi Group, Lixi Group and Hekou Formation in Southwest Sichuan Area. The tectonic evolution of the area could be separated into nine main stages: Late Archaean gneissic basment, greenstone belt basin, Early Proterozoic Protoplatform, Frotogeosyncline, Mid Proterozoic uplifting and erosion, Jinningian magmatic activition, Early Sinian graben, Chengjinangian magmatic activation, and Late Sinian platform. It might reveal a crust rising development of poly-activation and alternate progress of "mobilization" and "stabilization".Three main kinds of activation are explored. (1) Basement activation. The gneissic basment melted on a large scale dur- ing the end of Archaean when the greenstone belt basin closed and sag-subducted. It remelted violently in Jinning movement accompanying the underthrusting and collision, and remelted again (only in a small region) in Chengjiang movement; during that time the graben closed and fracture transformed. (2) Protoactivization. The protoplatform broken and disintegrated by fracturing and rifting during the end of Early Proterozoico The volcanism was characterized by bimodal one, accompanied by mafic-ultramafic intrusion. The ancient platform was reformed and distorted beyond recognition. It should belong to activation of diwa (geodepression)-type. (3) Magmatic activation. Owing to the subduction of oceanic crust and collision, between continental block and folded belt, mid-acidic magmatic intrusion occurred in a wide area during Jinning movement. Its contents and sequence seem some what similar to the tectonic-magmatic process taking place in Baltic Shield 1800-1600 Ma ago. The Chengjiangian magmatic was characterized by potash feldspar-granite, which might be a featured rock in Southwest Baltic dated 1000-900Ma.