Polygenic nature of granite clastites: Communication 2. Secondary and supergene reworking of granitic clastites

Abstract

Clastogenic rocks spatially associated with granite massifs have been reported in the geological literature from different regions: Caucasus (Leonov, 1974, 1991), Urals (Puchkov, 1968), Kazakhstan (Svarichevskaya and Skublova, 1973), Transbaikal region (Leonov, 2008; Lobanov et al., 1991), Tien Shan (Leonov et al., 2008), North America (Beroush, 1991; Lukin, 1989, 2007; Pippin, 1973). In some places, they represent crushed rocks of indigenous massifs. In other places, they make up accumulations and aprons of clastic products of the granitic composition both on the surface and beneath the sedimentary cover. In the first communication (Leonov et al., 2014) devoted to the origin of granite clastites, we examined specific features of the structure and evolution of granite bodies at the posthumous development stage, i.e., after cooling and introduction into the consolidated layer of the Earth’s crust. It was shown that such rocks are formed at least due to two main processes: supergene1 (chemical and physical weathering) and tectonic (prototectonics and posthumous disintegration). Although the rocks are highly similar in composition, structure, and bedding conditions, they are marked by several specific features described in the first communication that provide insight into their genetic nature. However, the problem of morphostructural characteristics and genetic interpretation of granite clastites cannot be closed here. Reconstruction of the “primary” origin of clastic granitic bodies in some, far from single, cases is complicated by the following fact: the exhumed massifs of tectonically disintegrated granitoids undergo supergene transformations, while sediments in the weathering crust are involved in tectonic reworking. Thus, clastites can be formed in several stages with different successions of events: supergene processes (formation of the weathering crust) can precede the tectonic reworking of rocks or succeed the formation of tectonomixtites. Determination of diagnostic properties of genetically different clastic rocks and stages of their lithostructural alteration is important for solving the issues of regional geology, development of methods for the study of genetically complex sequences, as well as paleogeographic and paleotectonic reconstructions. This problem acquires a specific importance because of two circumstances: first, its solution is at the intersection of two geological disciplines (lithology and tectonics); second, granitic clastite bodies often represent commercial hydrocarbon reservoirs (Areshev et al., 1997; Gavrilov, 2000; Izotov et al., 2003; Lobanov et al., 1991; Lobusev et al., 2002; Lukin, 2007; Martynova, 2002; Pippin, 1973; Sitdikova and Izotov, 2002). Let us discuss two scenarios of the succession of events: scenario 1—“tectonic mixtite” → “supergene reworking”; scenario 2—“weathering crust” → “tectonic reworking”. All other versions are combinations of these two types.