Abstract:
The finding of microdiamond and coesite in the high-pressure metamorphic rocks of the Kokchetav massif (Caledonides of Central Asia, Northern Kazakhstan) evidence that the crustal rocks were transformed at great depth (more that 150 km), probably as a result of subduction. Geodynamic interpretations show that the collision of the microcontinent with an island arc in the Cambrian has led to the exhumation of these rocks from great depths. Isotopic and kinetic data (preservation of diamond, coesite, and transformation products) confirm that the rocks exhumed very fast in the period separating their subduction stage (540-520 Ma) and mica cooling stage (515 ± 5 Ma, Ar-Ar). The compilation of geological and metamorphic data of the Kokchetav region permitted recognition of five tectonic domains. Detailed structural information with additional petrostructural data is given for two of them, the Kokchetav microcontinent domain and the metamorphic belt domain, interpreted as a megamelange, with three distinct units of high- and ultrahigh-pressure rocks. Previous systematic mapping and the present work show box folding, preservation of the initial arrangement of metasediments and gneissic basement with eclogites, and other characteristics, suggesting that differential shear in high- and moderate-pressure rocks are reduced as compared wit the nearby low-pressure rocks. It is supposed that strong deformation in low-pressure rocks corresponds to the final stage of exhumation in mid-crustal conditions. This stage is probably related to detachment mechanism during extension and emplacement of granite domes under the obliquely oriented Caledonian collision, but this postulate requires additional structural and geochronological investigations.