MORPHOLOGY AND INTERNAL STRUCTURE OF THE KAMENSK AND GUSEV ASTROBLEMES

The Kokchetav and Dabie Shan complexes are typical examples of ultrahigh-pressure metamorphic complexes (UHPM) and are important units of the largest suture zones within the Eurasian continent. The Dabie Shan complex is located in the center of a long Permian-Triassic high-pressure (HP) belt between the Sino-Korean and Yangtze cratons. Other members of this belt are the Sulu region of of NE China, the Imjingang belt in Korea, the Sangun and Marginal Hida belts in Kyushu, the Spassk zone in the Sikhote-Alin of the Russian Far East, and the Bikou, Animaqing, Ailaoshan, and Lancang belts in China bounding the western margin of the Yangtze craton. The Kokchetav complex is located in the center of the largest Early Paleozoic HP belt in Asia, which includes the North Qilian complex, the Kekesu and Atbashi zones of the Tien Shan, and the Aktyuz and Makbal areas in the North Kyrgyz Range.

The structure of the Kokchetav complex is interpreted as a mega-melange zone that consists of seven tectonic units separated by tectonic thrusts or faults. There are many similarities between the Kokchetav and Dabie Shan tectonic units. Principal differences relate to the rocks of coeval island-arc series abundantly exposed in the Kokchetav area, but absent in the Dabie Shan, and to the ongoing subduction and island-arc magmatism in Kokchetav after the collision and UHP metamorphism compared to the final collision after UHP metamorphism in the Dabie Shan.

The Caledonian Kokchetav complex formed in the Early Paleozoic, whereas the Indosinian Dabie Shan complex formed in the Early Mesozoic; however, both complexes are characterized by a close succession of events and the occurrence of a Late Proterozoic protolith. In both cases magmatic events occurred in 150-m.y. intervals. Retrograde stages, cooling histories, and exhumation processes are similar for both complexes.

Comparison of mineral assemblages in those complexes indicates higher temperature and pressure in the Kokchetav peak assemblages. The best containers for preserved UHP mineral assemblages are metacarbonate rocks and zircon and garnet from metapelites and felsic rocks in both regions. The Dabie Shan UHP assemblages are better preserved than the Kokchetav ones, which has to do either with their higher temperature or with specific kinetics. Oxidation conditions deduced from mineral distributions, mineral chemistry, and composition of fluid inclusions indicate the higher oxygen potential in the Dabie Shan than in the Kokchetav rocks.

The comparison allows us to conclude the following:

1. The small size of sheets and blocks of UHPM rocks supports a model for reverse flows within a subduction-accretionary wedge or tectonic exhumation of thin sheets, but not uplifting of large blocks.

2. The preservation of coesite and diamond, and the presence of thin reactionary rims (primarily in the Dabie Shan), provides evidence for a very short time of retrograde reactions and high velocity of block uplifting. Thus, three exhumation stages are accepted: (1) superfast uplifting; (2) rapid uplifting up to the sole of the continental crust; and (3) slow uplifting within the continental crust. In the Kokchetav complex, the first stage is absent.

3. For the Dabie Shan we suggest a complex scenario implying two-stage subduction and subsequent collision. Comparison with the Kokchetav complex shows that UHP metamorphism is not likely to have resulted from a collision, but the latter was responsible for the superfast exhumation of thin sheets of UHPM rocks from depths of over 100 km.