Mineralogical and Petrological Peculiarities of Grorudites of Eastern Azov Area (Ukraine)
Authors: O.V. Dubyna, S.G. Kryvdik, O.A. Vyshnevskyi
Number of views: 44
Alkaline granites and their hypabyssal and effusive varieties have a limited distribution on the Ukrainian Shield, unlike other Precambrian platforms and shields. Grorudite dikes in the Eastern Azov region have discrete chemical and mineral composition and are considered as analogues of alkaline granites — pantellerites (high-Ti or aegirine type) or comendites (low-Ti or amphibole type). High-Ti grorudites are highly enriched in incompatible elements (REE, Zr and Nb) in comparison to low-Ti
type. As a result of high-Ti grorudite investigation, new for these rocks titanium minerals (titaniferous hematite with high MnO content, minerals of the FeTiO3–MnTiO3–ZnTiO3 series), previously unknown in these rocks Ca-Na-amphiboles, as well as
structural relationships between aegirine and alkaline amphibole were revealed. Despite of high or increased content of HFSE
in grorudites, information about their accessory minerals is quite restricted. In the aegirine-bearing varieties of grorudites, such rare elements minerals: monazite-(Ce), bastnäsite-(Ce), britholite-like mineral and REE-enriched apatite, zircon, a Na-rich mineral similar to calciocatapleiite, as well as unidentified zirconium minerals and minerals of the aeschynite group were recognized.
In amphibole-bearing dikes zircon and Nb-bearing rutile were only found. These minerals are often very small in size,
the largest of them can reach 15—20 μm in size; mostly do not exceed 10 μm. It greatly complicates their identification. An interesting and unusual fact is that many of rare minerals, especially bastnäsite-(Ce), aeschynite group minerals and zircon are present as inclusions in titaniferous hematite. It is assumed that a significant part of rare metals isomorphically incorporate to rock-forming minerals (alkaline pyroxenes and amphiboles). Some of rare minerals (especially zirconium ones) are secondary mineral and were formed as a result of replacement of primary Na (eudialyte, catapleiite, hilairite) or Ca (gittinsite, calciocatapleiite) Zr-silicates, which are more typical in peralkaline (agpaitic) rocks. Taking into account the petrological features and the nature of the rare-earth mineralization of grorudites of the Eastern Azov region, there is a reason to believe that such type of mineralization in these rocks occurs in course of differentiation of the initial magmatic melt(s). Such melts are similar to high-Ti basalts (characteristic rocks of this region) of Devonian age. Secondary hydrothermal processes are weakly manifested in the studied rocks and probably led to substitution of primary accessory minerals. Considering the increased or high concentrations of Nb in high-Ti grorudites and an insignificant amount of Nb-minerals, it is likely that differentiation conditions (low F, high f O2 and hypabyssal conditions of crystallization) did not promote prolonged crystallization with significant accumulation of rare-metal minerals. However, the presence of grorudites with high content of rare metals found in this region increases the prospects for discovery of small alkaline granite intrusions (holocrystalline analogues of grorudites) which may host deposits and/or occurrences of Nb, REE, Zr, Sn, Be.