Alpine stacking of two Variscan granite blocks recognised from mineral stabilities, age and structural data (Western Carpathians)

This study presents petrological, geochemical, geochronological and structural data that serve to identify two different Alpine tectonic units formed from the Variscan-aged Tribec-Zobor granitic rocks. The proposed model for juxtaposition of two granite blocks in the Tribec-Zobor basement during the Alpine orogeny is based on the degree of granite alteration, particularly related to differences in monazite stabilities of the S-type granitic rocks along the studied NW- SE profile through the Tribec-Zobor mountain range. Along the cross-section, the Variscan crystalline basement in the mountain crest is composed of hydrothermally altered and metamorphosed S-type monzogranite (zircon age: 355.2 +/- 1.2 Ma). Below this monzogranite there are hypidiomorphic non-altered granitic rocks with ages spanning from 357.8 +/- 0.66 Ma to 351.2 +/- 0.7 Ma for northeast S-type and 358.2 +/- 0.85 Ma to 347.9 +/- 0.94 Ma for I-type on southwest part of profile. U-Th-Pb dating revealed both low- and high-Y types of monazite in the S-type granites. Low-Y monazite from both the structurally lower unaltered, and the structurally upper, metamorphosed S-type granitic rocks provides Tournaisian Variscan ages in the range of 354-349 Ma. The high-Y monazite from the same rocks records Visean ages in the range of 348-335 Ma, likely indicating later-staged thermal rock overprinting. Monazites from non-altered S-type granodiorites are stable due to rapid emplacement in the upper crustal position. In contrast, the monazites in the overlying metamorphosed Stype granites are retrogressed to allanite as a result of slower, nearly isobaric cooling of the granites emplaced at deeper crustal levels. Thus, monazite behaviour indicate two different Variscan granite blocks, similar in age but with different emplacement depths. The conditions for metamorphism (450-500 degrees C, 750-800 MPa) of the structurally upper granites are inferred from phase equilibrium thermodynamic modelling, occurring during the Alpine orogeny. These metamorphic conditions are similar to those calculated for granites of the Western Carpathians that were pervasively reworked during Alpine orogenesis, indicating that structurally upper granites have an affinity to the Alpine Fatric Unit, whereas the lower granite is rather related to the Tatric Unit. An Alpine date of c. 78 Ma yielded by single grain fusion 40Ar/39Ar geochronology of muscovite is interpreted to post-date Alpine thrusting. The present position of the metamorphosed Variscan Fatric granitic rock in the Tribec-Zobor crystalline basement, overlying both S- and I- non-altered Tatric granites, resulted from large-scale, northdirected Alpine thrusting. The geometry of the thrust, elucidated by tectonic elements from mylonites and hydrothermally altered Triassic quartzites deposited on Fatric granites, is inclined 20-40. to the northwest.