On Some Accessory Minerals in Upper Paleozoic Granites of the Khrami Crystalline Massif
Keywords:Georgia, the Khrami massif, accessory mineral, garnet, allanite
The Khrami crystalline massif is exposed within the Black Sea – Central Transcaucasian terrane (Georgia). The massif is built up of Preсambrian gneiss-migmatite complex, the packet of Middle Paleozoic (?) metasandstones and protrusions of mantle serpentinites; Paleozoic gabbroids; Middle-Upper Paleozoic granitoids and rocks of volcanogenic-sedimentary complexes and Upper Paleozoic quartz-porphyry - granite-porphyry formations. A number of scientists have studied the granitoids of the Khrami crystalline massif, but detailed investigations on some minerals using modern analytical facilities have not been conducted yet. The Late Variscan granitoids of the Khrami massif are represented by biotite-hornblende-allanite-, biotite-microcline- and biotite-garnet-bearing granites and aliaskites. We have studied in detail the accessory minerals - garnet and allanite in the Late Variscan granites of the massif. Their microprobe analysis has been performed. According to the microprobe analysis of the studied garnets, the centre of the crystal is homogeneous, while the peripheral part is zonal. The amount of FeO decreases slightly from the centre to the periphery, while the amount of CaO and MgO significantly decreases; a well-expressed increase in MnO content is recorded. The variation in almandine content is insignificant. The amount of pyrope and grossular drops sharply from the centre to the periphery while the number of spessartine increases. The amount of Al2O3 in all varieties of granitoids is similar. In our opinion, the garnet formation occurred in two stages: during the crystallization process of granitic magma and its subsequent post-magmatic stage. The core of the garnet was formed in the magmatic stage, but its periphery - was in the cooling stage of granitic magma. According to the results of the microprobe study of allanite, the same - two stages of their formation have been conducted: during the crystallization process of granitic magma and in its next post-magmatic stage. The genesis of allanite in the biotite-hornblende-allanite-bearing granites of the massif is due to the concentration of REE in the granitic magma and the introduction of calcium by hydrothermal solutions. These allanites belong to the cerium variety with high content of Ce2O3, La2O3 and Nd2O3.
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