TYING TOGETHER TEXTURES, TEMPERATURES, AND TIMING IN THE WESTERN TATRA MOUNTAINS, SLOVAKIA

We are applying microstructural and microanalytical techniques (Ti-Q thermometry, EBSD fabric analysis, monazite EMPA) to assess temperatures, textures, and timing of the deformed metamorphic and igneous rocks exposed in the Western Tatra Mountains of Slovakia. Medium grade rocks exposed in the lower reaches of the Western Tatras form a tectonic window beneath Variscan high-grade metamorphic rocks and granites. Much geologic research in the Western Tatras has focused on the high grade and better exposed upper unit; less attention has been given to the seemingly monotonous kilometer thick package of mica schists making up the lower unit. However, detailed study of the mica schists show that they are heterogeneously deformed, polymetamorphosed, and preserve a variety of microtextures. The dominant metamorphic imprint of the lower unit occurred at c. 370 Ma. This coincides with peak metamorphic conditions and the onset of plutonism in the upper unit from 370-350 Ma (Moussallam, 2010), likely associated with initial Variscan collision. The similar peak metamorphic ages from the upper and lower units may suggest the formation of an initial mid to deep ‘normal’ metamorphic gradient at c. 370 Ma. Continued Variscan magmatism (at 340 Ma) and exhumation of the upper unit rocks are intimately linked as suggested by the occurrence of granites solely within the overthrust unit (Janak et al., 2001). Rapid deep-crustal overthrusting contributed to continued magmatism (via decompression melting?) and to post-peak metamorphism of the upper unit, but had little effect on the lower unit. Initial metamorphic studies of the Western Tatras (Janak, 1994) interpreted the resulting inverted metamorphic sequence as forming syn-tectonically via the ‘hot iron’ model. The absence of 340 Ma metamorphic ages from the lower unit seemingly argues against this. Instead metamorphic inversion at 340 Ma occurred during overthrusting of a deep-seated thrust nappe above mid-crustal footwall rocks without downward transfer of heat into the footwall. The lack of footwall thermal overprinting suggests rapid hanging wall exhumation via erosional or extensional unroofing of the upper unit occurred during thrusting.