Stenian Granitoids of the West Kyrgyz Ridge (North Tien Shan):Position, Structure, and Age Determination

摘要

In the structure of west Kyrgyz Ridge (North Tien Shan), a great role is played by complexly dislocated Upper Precambrian—Cambrian terrigenous-carbonate and shale strata, as well as by granitoids that comprise several coupled WNW-striking synforms and antiforms, the largest of which is the Makbal antiform.Southeast of the core of this antiform, granitoids comprise the large Kara Dzhilga massif and several massifs that are of lesser size and have tectonic correlations with the hosting terrigenous-carbonate strata. In the Kara Dzhilga massif, the rocks of three penetration phases are distinguished; contacts between rocks are often of tectonic character. The early phase is presented by monzonite and monzodiorite; the main one, by large-porphyric biotitic granites; and the additional one, by aplitic granites and pegmatites. By the chemical composition, granites of Kara Dzhilga massif of the main phase correspond to subalkaline granites of high-potassium calc-alkali series. The age of their crystallization (zircon-based U—Pb method) is 1131 ±4 Ma (Stenian). The formation of Stenian granitoids in the North Tien Shan may be related to development of Grenville fold belts, whose fragments were identified in the units of the Central Asian Belt. Tectonic correlations between these granitoids and hosting terrigenous-carbonate strata appeared as a result of immersion to significant depths and subsequent exhumation into the upper crustal horizons in the Early Ordovician.