The Trans-Baikal Parametric Borehole: First Evidence of the Deep Structure of the Borshchovochnyi Metamorphic Core Complex

Abstract

The Trans-Baikal parametric borehole drilled out a metamorphic core complex in the reference geological and geophysical profile within the Central Asian fold belt, south of the Mongol–Okhotsk suture zone. In the section of the borehole drilled to a depth of 2600 m, a set of geophysical, geological, and laboratory–analytical studies made it possible to identify three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (slightly metamorphosed rocks) is represented by a serpentinite mélange separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate as a part of the crystalline basement complex comprises rocks of granite–schist and granite–plagiogneiss subcomplexes. The granite–schist subcomplex is composed of mafic and intermediate schist with amphibolites and quartzites. Biotite granite is widespread in the section as layered veins with synkinematic signs. According to the preliminary estimates, the age of biotite granite is about 130 Ma. Schist is characterized by a mylonite structure at intervals of interbedding with biotite granite veins. In accordance with the geophysical survey data on the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors in the seismic RSS–CDP section, which can be traced to a depth of at least 10 km. The granite–plagiogneiss subcomplex is represented by plagiogneiss, leucogranite, and gneissic granite. In the Trans-Baikal parametric borehole, a detailed section of the Borshchovochnyi metamorphic core complex, fully characterized by core samples, was obtained. It should be considered as a reference for studying these structures. The investigation of this section will make it possible later to answer many questions about the formation and evolution of the Central Asian fold belt.