Metamorphic core complexes of the Transbaikalia: review

Abstract

Metamorphic core complexes (hereafter MCC) revealed in the Transbaikalia have similar features of their patterns. Three levels can be distinguished by structural­material indicators: core, zone of mylonites (dynamically metamorphosed rocks) and overlying formations. The cores are composed of the Paleozoic granites and granitogneisses. Zones of mylonites skirt the cores and are characterized by various tectonites which are formed at the expense of the core rocks. The overlying formations include volcanogenic­sedimentary series of the Mesozoic and the Upper Palaeozoic. The rocks are not metamorphosed, yet subject to brittle deformations. Structurally, they are detached and deposited above the zone of mylonites. In Transbaikalia, MCC are characterized by synmetamorphic structural paragenesises of one type: low­angle schistosity, micro­ and macro­structures (folds, mineral streaking, boudinage, pressure shadows, C–S structure, kick­bends). According to the kinematic analyses, they were formed by the simple shear mechanism along the zones of deeply penetrating regional dislocations which plunged in the south­eastward direction. Tectonic transportation of the materials developed in the same direction, i.e. the top parts of tectono­stratigraphic sections were displaced against the lower parts in the south­eastward direction. Extension deformations tended in the north­west – south­east direction. Such movements facilitated formation of synthetic listric normal faults and rift basins. The most intensive tectonic exposure period is determined as 112–123 mln years, while the period of metamorphism is assessed as 140–130 mln years. The rocks in depth of the deep dislocation were transformed in conditions of amphibole facies of metamorphism (Т=590–640 °С; Р=3.2–4.6 kbar). According to our structural-­geological, petrological and isotopic data, the age of the majority of the metamorphic formations of the Transbaikalia is determined as the Late Mesozoic. They were formed in the extension regime due collapse of the Late Mesozoic orogeny, that was caused by accretion­collision events during the Early Mesozoic. Thickening of the continental crust contributed to increase of heat flow and higher plasticity at the crustal bottom. The orogen was thus unstable and flowing and caused regional extension and dislocations at the middle­crust level. Thinning of the crust was accompanied by isostatic uplifting which facilitated emergence of the structural metamorphic complexes of the middle­crust levels on the surface and formation of the metamorphic core complexes.