Geochemistry of Paleoproterozoic pseudotachylites of the Anabar Shield and mechanism of its formation

Abstract

484 This study is aimed at solution of the problem of the formation conditions of thick bodies of Paleoprot erozoic pseudotachylite on the Anabar Shield by investigation of their geochemistry, as well as the geochemistry of the parental rocks. Pseudotachylite is a rock formed under the condi tions of amphibolite facies at the expense of rapid fric tion (grinding) and melting of crack walls in upper crustal rocks during earthquakes and large impact events. Pseudotachylites of the Anabar shield on the periphery of the large (250 km in diameter) Kotuikan ring structure (Fig. 1) were studied. They are pre sented by thin veins and veinlets (A type), as well as thick elongated bodies (B type). The formation of these pseudotachylite resulted from an impact event, which occurred 1900 ± 50 Ma and was accompanied by friction melting of crack walls under shock compression and shifts. Their melting occurred in the Archean (model age of pseudotachylite TDM = 3031 and 3099 Ma) crustal (eNd = –9.6...–11.79) protolith [1]. As was demonstrated by H. Melosh [2], the main mystery in the mechanism of the formation of signifi cant pseudotachylite masses related to the B type implies that the initial process of the formation of pseudotachylite composing thin veinlets (A type) must prevent the appearance of much melt because of the decrease in friction between slipping rock blocks. The solution to this mystery is connected to peculiari ties of the formation of a large impact crater, because thick extrusive veins and dykes of pseudotachylite occur in the rings around such basins. This is evident from the example of the Sudbery impact structure [3] and confirmed by the position of thick pseudotachylite bodies on the periphery of the Kotuikan impact struc ture (Fig. 1). It is supposed that, under the influence of a shock wave and strong vibration on the crater mar gins (at the expense of mass deficiency inside it), the initial melt (A type) appears along the shear zones in narrow shift zones. Because of the low viscosity, under the conditions of significant decrease of rock strength, it is moved from gliding plates to adjacent opened cracks and pockets. As a result, vibrating gliding sur faces of crack walls are always touching tightly pro ducing new portions of the melt. Thus, during the for mation of B type pseudotachylite, friction melting occurred not only in “fresh” parental rocks, which did not undergo this process before, but also in the previ ous replaced portions of the melt. Such a mechanism of its formation corresponds to the theoretically and Geochemistry of Paleoproterozoic Pseudotachylites of the Anabar Shield and Mechanism of Its Formation