Late proterozoic A-type granites of the chernorechenskii massif in the Yenisei ridge: New geochemical and geochronological data

Abstract

The problems in estimation of the geochronologi� cal boundaries in the history of the breakup of the Rodinia Supercontinent are mostly far from their final solution. This is explained by the lack of precise dat� ings, which limits significantly the possibilities of age correlations for global geological processes. Because of this, the problem of geochemical specifics and dat� ing of Neoproterozoic granitoid complexes, i.e., the main components of the continental crust and indica� tors of geodynamic environments of key significance for reconstruction of events in the lithosphere evolu� tion, is still important. Recently new results on the geochemistry and geo� chronology of granitoids from the Yenisei Ridge formed under contrasting geotectonic environments were obtained (1). These crystalline massifs are located in the axial part of the Central cratonic block, mostly in the Tatarsko-Ishimbinskaya suture zone. In this relation, the western part of the Central block composed of rocks of the Garevka metamorphic com� plex (GMC), the oldest in Transangaria, is the least studied. It consists of a number of intrusive massifs located within the Yenisei regional suture zone (YRSZ), the large deformational metamorphic struc� ture of the lineament type separating the Central cra� tonic block and the Isaakovskii islandarc terrane (2). This zone is a continuation of the Baikal-Yenisei fault and located on the left side of the Yenisei River along the western margin of the Siberian Craton for more than 200 km at a width from 50 to 70 km. Because of the absence of reliable geochemical and geochrono� logical data, the problem of the age and tectonic set� ting of these intrusions is debatable. This is especially important for rocks of the Chernorechenskii massif occurring in the composition of the Posol'nenskii and Ayakhtinskii magmatic complexes of various age and origin (3). To resolve these contradictions, we obtained new data on the geochemistry, age, and nature of these granitoids formed under the environment of extension along the western margin of the Siberian Craton. The Chernorechenskii massif (CM) is located in the GMC, on the western flange of the YRSZ charac� terized by contrast metamorphism, intense foliation, and dynamic recrystallization of rocks (4). This massif occurs on the right side of the Yenisei River, in the Raz� gromnaya River basin, and is located in the same tec� tonic zone as small massifs of rapakivitype granites (5) and bimodal complex of granite and basic dykes (2). The intrusive body of a lenslike shape and an area of ≈140 km 2 occurs at large submeridional faults. With account for the geophysical data (3), it is suggested that the massif has subvertical dipping, a "platelike" shape, and tectonic contacts with host rocks. Three main stages with different thermodynamic regimes and values of geothermal gradients are distin� guished in the evolution of western areas of the YRSZ (6). At the first stage related to the Grenville orogeny (1050-850 Ma), zoned lowpressure metamorphic complexes with a gradient of dT/dH = 20-30°C/km typical of orogenesis were formed. At the second stage in the nappe zone, these rocks underwent Late Riph� ean (801-793 Ma) collisional metamorphism of mod� erate pressures with low dT/dH ≤ 10°С/km. The last stage was accompanied by synexhumation metamor� phism (785-776 Ma) with dT/dH ≤ 12°С/km reflect� ing the tectonic environments of rapid uplift in the dis� location-thrust zones of the crust. The final stages of the orogen evolution in the region include the forma� tion of the bimodal dyke belt with an age of 797- 785 Ma intruding gneiss and crystalline schist of the GMC (2). These events are controlled by the processes of extension along the western margin of the Siberian Craton and consistent with the beginning of breakup of the Rodinia Supercontinent. Later a number of rif� togenic troughs and grabens filled by Neoproterozoic