Melts and fluids evolution in the process of crust and mantle formation in Paleo-Proterozoic (2,2—1,75 Ga ago). Stratigraphy and magnetic activity

Abstract

Stratigraphy of metamorphic Precambrian complexes related to Paleo-Proterozoic is controversial. An additional sign permitting to correlate strata may be their material composition because in the Precambrian composition of deep fluids changed regularly. Then some regularity in changes of composition of magmatic and metamorphic rocks has to exist. The rocks produced during this time are dolomite and calcite. Quartzite, carbonaceous shist and graphite gneiss are also abundant. Only during this activation of Precambrian carbonatite volcanic activity became apparent. The author’s method has been used generalizing the special features of magmatic and metamorphic rocks of the Ukrainian shield, which allowed reconstructing the course of the deep process and composition of melts and fluids in the layer of melting. Subdivision of the shield into blocks of two types has been proposed: inherent blocks and the areas spatially attached by suture zones. The blocks differ by decisive role of the fault zones formed before granitization; the presence of gabbro-montsonite-trachite granites massifs formed before granitization (2,1—1,99 Ga ago); mutual transitions between intusive and palingenous granites; formation of anorthosite-granite massifs after granitization. Activation consists of two tectono-magmatic cycles separated by granitization. Composition of rocks attached to suture zones is effected by oxidized deep melts and fluids. We can make estimation on geodynamic process and composition of deep fluids in blocks by composition of magmatic rocks. In blocks fault zones are generated with multiple intrusion of melts along them, differentiated under the crust. Neutral-alkalescent aquatic-chloride-potassium fluids take part in differentiation of melts. Composition of fluids changes after granitization under the whole shield. Immediately after it the massifs of carbonatites and alkaline rocks are formed where alkaline ultrabasites are also present. In the territories attached to suture zones carbonate ferruginous layers appear, which include xenocrysts. In blocks in places of intersection of sub-longitudinal and sub-latitudinal zones, in relict foci mixing of melts takes place diluted by aquatic-chloride-potassium fluids and added deep carbonate-fluoride-sodium ones. Massifs of anorthosites-rapakivi-granites with titanium are formed. They are accompanied by pegmatite fields, albite-uranium metasomatites when joint participation of carbonate-fluoride-sodium and aquatic fluids is required.