Chloritie Rocks and Chloritized Basalts as Plausible Precursors of Metamorphic Peridotites and Pyroxenites in the Kokchetav Massif, Northern Kazakhstan

摘要

The Kokchetav Massif in northern Kazakhstan, a unique manifestation of collisional metamorphism of high and ultrahigh pressures, represents a fragment of the Proterozoic crust within the Central Asian Foldbelt . As a result of Proterozoic subduction, the western part of the massif subsided to a depth of at least 140 km. Under conditions of a pressure of >40 kbar and temperature of >900℃, this led to the formation of diamonds in metasedimentary rocks. Judging from the absence of diamonds, the eastern part of the massif subsided to shallower depths. However, the maximal subsidence exceeded 80 km, which is evident from the occurrence of coesite-bearing schists. Exhumation of the subsided crust in the Late Cambrian resulted in its deformation, splitting into tectonic sheets displaced relative to each other, and retrograde metamorphism. The newly formed megamelange complex represents an ensemble of rocks metamorphosed under ultrahigh to low pressures and exhumed from different depths.