The compositions of rock-forming and accessory minerals from the Gemeric granites (Hnilec area, Gemeric Superunit, Western Carpathians)


Authors: Broska I, Kubiš M, Williams CT, Konečný P

Published in: Bulletin of Geosciences, volume 77, issue 2; pages: 147 - 155; Received 19 March 2002; Accepted in revised form 3 May 2002;

Keywords: granite, two-mica granite, rock-forming minerals, accessory minerals, chemical composition, zircon, boron, fluorine, niobium, phosphorus, tantalum, tin, greisenization, albitization, Western Carpathians,

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The Hnilec granites represent a suite of specialized S-type granite massifs in the Western Carpathians, with a primary enrichment in elements B, F, Sn, Nb, Ta and W being reflected in a suite of characteristic minerals (e.g. tourmaline, cassiterite, and Nb-Ta phases). The highly evolved nature of these granites results from a lowering of the viscosity of the primary melt, which is due to the high content of the volatiles (B, F), and an increased activity of P. Phosphorus is incorporated into primary apatite as well as in the K-feldspar, whereas the Na-feldspar (albite) is P-bearing only in the exsolved perthitic part of the K-feldspar. The majority of the perthitic albite crystals contain abundant exsolved apatite grains that in general, do not exceed 3-5 µm in size and due to the apatite exsolution, albite is low in P. Compositionally, secondary apatite in albite differs from the primary one in having low concentrations of Mn, Fe and REE. The LREE are predominantly hosted in apatite, and to a lesser extent monazite, whereas the HREE are mainly distributed in zircon, xenotime and apatite. The morphology of zircon crystals is similar in all the granitic rock types investigated, i.e. in the deep-seated coarse-grained granites, upper fine-grained granites and the greisenized cupola. Cathodoluminesce images of zircon crystals show a strong primary magmatic zonation and some secondary alteration features. These aspects of zircon morphology and composition suggest that zircon crystallized mainly in the deep magma chamber and was incorporated within the ascending differentiated melt. Some of the zircons can be classified as G1 and L subtypes which represents a late magmatic population of zircon. The contribution of the early magmatic zircon to the bulk rock. HREE distribution is considered to be significant. The co-magmatic features of the granites are evident from extreme differentiation leading to the zonality of the plutons and alteration of the granite, i.e. albitization and greisenization, by fluids in the cupola.