International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Variscan thrusting in I- and S-type granitic rocks of the Tribeč Mountains, Western Carpathians (Slovakia): evidence from mineral compositions and monazite dating

Pages: 455 - 471

Authors: IGOR BROSKA, IGOR PETRÍK

Abstract: The Tribeč granitic core (Tatric Superunit, Western Carpathians, Slovakia) is formed by Devonian/Lower Carboniferous, calc-alkaline I- and S-type granitic rocks and their altered equivalents, which provide a rare opportunity to study the Variscan magmatic, post-magmatic and tectonic evolution. The calculated P-T-X path of I-type granitic rocks, based on Fe-Ti oxides, hornblende, titanite and mica-bearing equilibria, illustrates changes in redox evolution. There is a transition from magmatic stage at T ca. 800—850 °C and moderate oxygen fugacity (FMQ buffer) to an oxida- tion event at 600 °C between HM and NNO up to the oxidation peak at 480 °C and HM buffer, to the final reduction at ca. 470 °C at ∆NN = 3.3. Thus, the post-magmatic Variscan history recorded in I-type tonalites shows at early stage pronounced oxidation and low temperature shift back to reduction. The S-type granites originated at temperature 700—750 °C at lower water activity and temperature. The P-T conditions of mineral reactions in altered granitoids at Variscan time (both I and S-types) correspond to greenschist facies involving formation of secondary biotite. The Tribeč granite pluton recently shows horizontal and vertical zoning: from the west side toward the east S-type grano- diorites replace I-type tonalites and these medium/coarse-grained granitoids are vertically overlain by their altered equivalents in greenschist facies. Along the Tribeč mountain ridge, younger undeformed leucocratic granite dykes in age 342 ± 4.4 Ma cut these metasomatically altered granitic rocks and thus post-date the alteration process. The overlaying sheet of the altered granites is in a low-angle superposition on undeformed granitoids and forms “a granite duplex” within Alpine Tatric Superunit, which resulted from a syn-collisional Variscan thrusting event and melt formation ~ 340 Ma. The process of alteration may have been responsible for shifting the oxidation trend to the observed partial reduction.

Keywords: I- and S-type granitic rocks, granite duplex, Tribeč Mts, Western Carpathians, monazite, xenotime, titanite, dating, oxygen fugacity

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