Geology of the don series from the early precambrian Voronezh crystalline massif

Abstract

The Don series from the early Precambrian Oboyan pluton-metamorphic complex of the Voronezh Crystalline Massif occurred in the Don belt among batholiths and extensive migmatite fields of Pavlovsk-type granitoids is considered. Previously, the researchers have been reported the Don series/ sequence/suite as two contrasting rock associations. The first, gneiss-granitoid association (GGA) is represented by medium-coarse-grained "gneisses" of quartz-feldspar composition. The second, stratified association (STA) consists of alternating mostly fine-grained biotite (~10%), biotite-amphibole gneisses and amphibolites (> 50%), marbles and calc-silicate rocks (15–25%). The features of thermal effect on xenoliths, microtextural attributes typical of igneous rocks, zoning and the presence of trace elements in rock-forming minerals, idiomorphism and oscillatory zoning of zircon, contradict the probability of nonplutonic origin of the GGA. The regular spatial relation of GGA and Pavlovsk complex rocks, their similarity at macro and microscopic scales, as well as the rock-forming mineral zoning and compositions, and the whole rock chemistry attest to their co-genetic properties. The GGA is characterized by low liquidus crystallization temperatures (840–900 °C) relative to the Pavlovsky complex batholiths (up to 1050 °C), though coincident emplacement and crystallization pressures of 2,1–3,8 kbar and 2,3–3,4 kbar are observed, respectively. The GGA is not related to the Don series/association, but is banded and contaminated counterparts (injected migmatites) of the Pavlovsk complex rocks from the vast endocontact zone from abyssal part of the batholiths. The STA is deposited at a distance from the batholith influence zone or occur as large xenoliths/interlayers among the GGA and represents the bulk of Don series. All STA litho-types are characterized by close grain sizes, similar blastic microtextures, various structures from massive to banded, frequent alternation of rock types that supports their belonging to stratified objects. The temperatures and pressures of metamorphism of STA were defining 784±1 3 °C and 3,8±0,7 kbar. The STA protoliths have been proposed: (1) calc-alkaline high-K ferroam and high-magnesian volcanogenic rocks, (2) terrigenous and carbonate (limestones and marls) sediments. The Don series sedimentation occurred in the marine basin.