Abstract
Central area of the Tạ Khoa Anticlinorium is situated in Bắc Yên District, Sơn La Province. This area is characterized by very complex geological structure, which consists of many rock types but it is dominated by high grade metamorphosed sedimentary rocks in the core of the anticlinorium. The core is overlained by younger metamorphic and volcanic formations. These rocks have been strongly penetrated and and metamorphosed by hetrogeneity intrusive rocks. The metasedimentary rocks in the region have experienced at least 5 tectonic deformation phases and 2 metamorphic events. In which, the first deformation phase was the strongest event associated with the first metamorphism (M1) at the amphibolite facies and the formation of fibrolite. The second deformation phase occurred at the same P-T condition as phase 1 to form local crystalization of silimalite. Third deformation phase was related to the second lower grade metamorphic phase causing retrograde replacement of high temperature metamorphic minerals by lower grade metamorphic minerals. The later deformation phases demonstrated weaker indications, which did not significantly affect the regional geological structure. The zircon and monnazite U-Pb dating data of the metasedimentary rocks and pegmatite by SHRIMP technique showed that the latest formation occurred in the Early Devonian (411 Ma.) The Ealiest deformation (B1) may have occurred since the Middle Carboniferous till (At least) the Early Triassic (~250 Ma.), when the peak of 1st metamorphism (M1) took place. The deformation was continued by 2nd phase, which inherited P-T regime from phase 1 and has lasted from ~230 – 240 Ma. The 3rd deformation event occurred coresponding to low temperature metamorphic event. Possibly, the tectonic uplift and erosion of the region has triggered the replacement of high grade metamorphic mineral by lower grade metamorphic minerals. The later deformation phase all have accurred in the P-T decreased condition, corresponding to the uplift and erosion of the Earth crust but they did not affect the regional geological structure.