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GEOCHEMICAL SPECIFICITY OF THE BASITES OF ANCIENT PALEORIFTS OF THE SIBERIAN PLATFORM
Abstract
The diversity of the material composition of the middle Paleozoic basites is determined by the geodynamic regime. Paleorift dike belt record several groups of basites, differing in their material composition. The most common basites were formed in the regime of extension - magma has not experienced significant changes after leaving magma formation zone. These are typical basites of the Middle Paleozoic paleorifts of the eastern part of the Siberian platform. Group of leucocratic gabbro-dolerites and plagiodolerites, which were formed when extension was changed to compression at the peripheral zones of dike belts. Such magmatites reflect manifestation of lateral zonation within the limits of thick dike belts. A separate group consists of the bodies of magmatites, which material composition records long-term fractionation of the melt in pre-chamber deep-seated setting. As a result of long-term closing of the system, anorthosite or monzonite type of magma differentiation are formed in deep-seated magmatic chambers. The unique nature of the Vilyui and Olenek paleorifts is in combination, within their limits, of basic and kimberlitic magmatism. Dikes of high-Ti basites localized nearby the kimberlite pipes form a separate group. As regards the time of emplacement, the dikes within the limits of the Nakyn kimberlite field are post-kimberlitic. Chemical composition of the basites is specifically characterized by high TiO2 content (up to 5%). Besides, they are slightly undersaturated in silicic acid and contain higher contents of K2O and volatile constituents. Figurative compositional points of the basites plot mainly into the field of the subbalkaline series. Alkali basites within the Nakyn kimberlite field are shown to have metamagmatic origin and, along with the accompanying explosive breccias, are good indicators of fluid permeable zone and kimberlite-controlling structures. Locality of their distribution, closeness in time to kimberlites make it possible to use intrusives of alkaline basites as prospecting indicators of primary diamond deposits
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