THE ZABAIKALIAN PARAMETRIC BOREHOLE: A FIRST EVIDENCE ABOUT DEEP STRUCTURE OF THE BORSHCHOVOCHNY METAMORPHIC CORE COMPLEX

The Zabaikalian parametric borehole drilled out a metamorphic core complex on 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 identified three main structural elements of metamorphic core complexes: the upper plate, detachment, and the lower plate. The upper plate (weakly metamorphosed rocks) is represented by serpentinite melange, which is separated from the underlying rocks of the lower plate by a zone of chlorite breccias (detachment). The lower plate, which is part of the crystalline basement complex, includes 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; it forms layered veins with signs of synkinematic formations. According to preliminary estimates, the age of biotite granite is about 130 Ma. The schist is characterized by mylonite structure at intervals of interbedding with biotite granite veins. In accordance with data of geophysical surveys in the borehole and the petrophysical characteristics of cored rocks, the mylonite zone is the provenance of a series of inclined reflectors on the seismic reflection-CDP section, which can be traced to a depth of at least 10 km. The granite-plagiogneiss subcomplex is represented by plagiogneiss, leucogranites, and gneissic granites. In the Zabaikalian parametric borehole, a detailed section of the orshchovochny metamorphic core complex, fully characterized by core samples, of the was obtained, which should be considered as a reference for studying these structures. The investigation of this section will answer later many questions about the formation and evolution of the Central Asian fold belt.