Magmatism of the Khambin graben and early history of the Late Mesozoic rift system formation in the Western Transbaikal region

Abstract

The western Transbaikal region was repeatedly subjected to rifting during the Mesozoic. The Early Mesozoic was marked by the formation of a system of grabens filled in with Late Triassic‐Early Jurassic bimodal volcanic sequences of the Tsagan-Khurtei Group and Kunalei Complex, which is traced by massifs of alkalic granites [1‐5]. In the Late Mesozoic, a new rift system, generally conformable with the previous one, appeared [6‐9] and began to develop until the Late Cenozoic. Therefore, the grabens and horsts of the rift system are readily traceable in the present-day topography. The Late Mesozoic epoch commenced with the formation of the bimodal basalt‐trachybasaltic andesite‐trachydacite‐trachyrhyolite‐comendite volcanic association of the Ichetui Formation. Its compositional and structural similarity to the Tsagan-Khurtei Group served as the basis for determining the age of bimodal volcanic associations in some areas of the region, resulting in misleading interpretation of the structure, scale, and geodynamic settings of different-age rifting events. This problem can be exemplified by the Khambin volcanic field (Fig. 1), one of the largest in the region. The volcanic field is outlined as a ridge in geological maps, because it resembles the majority of outcrops of the Tsagan-Khuntei Formation in the topography [10, 11]. In such an interpretation, the field occupied the westernmost part of the Early Mesozoic rift system and, thus, governed its dimensions and structural peculiarities in the pinchout area. At the same time, the Khambin field, located between the western (Dzhida) and central (Khilok‐Tugnui) segments of the Late Mesozoic rift system (Fig. 1, inset), occupies a relatively large fragment of the rift system. In available maps, the fragment is shown as an anomalous zone because of the absence of Late Mesozoic magmatism. In this communication, we present systematic geological and geochronological (Rb‐Sr, K‐Ar) data, which point to the formation of Khambin field lava sequences as a result of Late Mesozoic rifting in several stages of volcanic activity. This conclusion is consistent with the multistage character of magmatic processes in other areas of the rift system. The materials obtained allow us to scrutinize specific features of the manifestation of early stages of Late Mesozoic rifting. The Khambin volcanic field extends in the NNW