Main stages in the evolution and geodynamic setting of the South Hangay metamorphic belt, Central Asia

Abstract

This paper reports new geological, geochronological (U-Pb method) and Sm-Nd isotope-geochemical data on the metamorphic and magmatic complexes of the South Hangay metamorphic belt located in the southern part of the Hangay group of the Precambrian blocks, along the junction zone between the Baidarik block and the Vendian-Late Riphean ophiolite complex of the Bayanhongor zone. A lower age limit of the early metamorphic episode of the disthene-sillimanite facies series (M2) is provided by the age of 603 ± 3 Ma obtained for gabbroids of the island arc association distinguished in the Tatsaingol complex. An upper age limit is determined by granites with ages of 562 ± 2, 564 ± 5, and 571 ± 9 Ma. Later superimposed metamorphism of the andalusite-sillimanite facies series (M2) was established in the southeastern part of the belt. The lower boundary of this metamorphism is constrained by the emplacement of quartz diorites at 552 ± 2 Ma, while the upper boundary is marked by postkinematic granites of the Tatsaingol pluton with an age of 545 ± 5 Ma. The protoliths of high grade metamorphic rocks contained Early and Late Precambrian rocks. The former could belong to the basement of the Baidarik block and/or its shelf, while the latter, to the rocks of the oceanic and island-arc complexes of the Bayanhongor paleobasin. Virtually, the continental crust, fragments of which are represented in the Tatsaingol complex, was formed within an interval around 610-560 Ma. The final stages in the formation of the South Hangay metamorphic belt coincided with the initiation of the Vendian-Early Paleozoic paleoocenic basins, volcanic islands, and island arcs in the Early Caledonian framing of the Precambrian Hangay block structure in the Late Vendian. By the time of amalgamation of structures of the Early Caledonian superterrane of Central Asia in the Late Cambrian-Early Ordovician (500-470 Ma), an “agglomerate” of the Precambrian Hangay blocks has been already formed.