GEOCHEMISTRY AND GENESIS OF MONZONITOIDS OF THE DZHALTUL TRAP INTRUSION, NORTHWEST SIBERIAN PLATFORM

Abstract

It has been established that the acid rocks in the northwest of the Siberian Platform, earlier recognized as an independent granitoid formation, are related to formation of the unique Dzhaltul trap intrusive (DI) famous for macroquantities of native iron. This intrusion, located in the Kureika - Gorbiachin interfluve, formed by supply of magma from an extended magma column into the chamber. The northern part of the intrusion, characterized by wells and exposures along the Gorbiachin River, formed from melts of the of the frontal zone of the magma colomn. Three zones are observed in this section. The lower and middle zones are made up of basic rocks, and the upper zone is composed of magnetite gabbros, gabbro-pegmatites, gabbro-diorites and monzodiorites. Sills consisting of acid rocks, earlier considered to be undependent bodies, are actually tongues of the DI roof. Geological features, chemistry, fluid composition, and behavior of trace elements in all the differentiates of the intrusion have been studied. Mineralogical and chemical evidence interaction between reducing methane-hydrogen fluid and toleiite-basaltic magma is described. This fluid has acidic properties and provides favorable conditions for migration of many elements. As a result, a significant redistribution of alkalies in the DI matters is observed. Such elements as U, La, Ba, Sr, Rb, Th, Nb, Y, and Yb were transported, together with K, from rear to front zone, enriching the DI monzonitoids up to 5-18 times. The uncommon distribution of REE is explained well enough by the effect of acid fluid. Analysis of Rb-Sr isotopic data shows initial Sr-isitopic heterogeneity of the magmatogenic source and magma evolution under open-system conditions. On the basis of all the obtained information, it is concluded that the monzonitoids formed from a silica- and alcali-enriched melt, which was generated in the extended magma column as a result of interaction of the evolved tholeiite-basaltic magma with reducing acid fluid according to the principle of acid-base interaction. The proposed genetic model shows an alternative way of formation of associations of basic and acid rocks in continental zones of the Earth's crust.