Early Paleozoic High- and Ultrahigh-Pressure Complexes in the Western Part of the Central Asian Orogenic Belt: Ages, Compositions, and Geodynamic Models of Formation.

The western part of the Central Asian Orogenic Belt, which comprises folded areas in Kazakhstan, Kyrgyzstan, and northwestern China, includes a number of large Precambrian sialic massifs that are framed by deformed and dismembered Paleozoic ophiolites and by island arc and flysch formations. The basements of the massifs are commonly made up of diverse metamorphic complexes, some of which were metamorphosed under high and ultrahigh pressures in the Early Paleozoic at ~480–530 Ma. These metamorphic formations are the Zerendy Group of the Kokchetav massif in northern Kazakhstan; Akdzhon Group of the Issyk-Kul massif in the northern Tien Shan); Aktyuz, Kemin, and Koyandy complexes of the Chu-Kendyktas and Zheltau massifs in southern Kazakhstan and the northern Tien Shan; and the Kassan Group of the Ishim–Naryn massif in the central Tien Shan. The paper reviews data on the structures, compositions, and metamorphic evolutions of the high- and ultrahigh-, and medium-pressure metamorphic rocks of these massifs. Numerous P–T assessments have been made for the near-peak and/or post-peak retrograde metamorphism, and some prograde P–T paths have been calculated for the key rock types over the past three decades of the studies. Near-peak and/or post-peak metamorphic ages and some ages of retrograde metamorphism are estimated for most of the high- and ultrahigh-pressure rocks. The paper discusses problems faced by the researcher when building geodynamic models for the high- and ultrahigh-pressure complexes in various massifs of the western part of the Central Asian Orogenic Belt. It is shown that any reliable model shall be underlain by detailed information on the compositions, ages, and formation environments of the protoliths for the ultrahigh-, high-, and medium-pressure rocks and complexes. Moreover, the structures and compositions of Paleozoic complexes surrounding the Precambrian massifs shall also be taken into consideration. [ABSTRACT FROM AUTHOR]