Abstract
The crystalline basement of the Western Tatra Mts. comprises of metamorphic rocks, intruded by the post-metamorphic Tatra Granite. The age of the granitoid magmatism was established at 340-370 Ma (Gaweda 1995; Janak et al., 1998), while the cooling age was dated by (super 40) Ar/ (super 39) Ar and Rb-Sr at 300-330 Ma (Maluski et al., 1993; Janak, 1994). The metamorphic complex of the Western Tatra Mts. is comprised of two structural units: Upper Structural Unit (USU) and Lower Structural Unit (LSU), metamorphosed respectively under upper amphibolite facies (T = 700-780 degrees C, P = 7,5-11 kbars) and upper greenschist to lower amphibolite facies conditions (T = 540-580 degrees C, P = 5-8 kbars). Both units are divided by a shear zone system (Gaweda et al., 1998). The mylonites contact with both granitic intrusions. They are composed of fine-grained matrix, and porphyroclast of quartz, plagioclase, K-feldspar, garnet and apatite. The mylonitic matrix is composed of sericite, quartz, goethite, chlorite and not transparent minerals. Syndeformational minerals: sillimanite and muscovite could be often found in mylonites. Sillimanite is represented by two generations: a) prismatic sillimanite and b) fibrolitic sillimanite, growing at the expense of muscovite. Mylonites from Siwa Przelecz contain also kyanite. The penetrative ESE-SE dipping foliation is almost parallel to the lithologic boundaries. This suggest that the shear zones had been developed on older, anisotropic planes. In mylonites with S-C structures the penetrative foliation S (sub 2) corresponds to C plane, however a foliation plane S (sub 1) is concordant with S plane. The foliation planes are uderlined by sheet minerals orientation and an alternating micas and quartz-feldspars laminae. Mineral lineation occurs on the C-type planes of the S-C structures and it is recognize as a stretching lineation. The S-C structures, extension crenulation cleavage, porphyroclast's tails, pressure shadows and dominoes structures describe the non-coaxial deformation. These structures define the sinistral sense of shear during mylonitization and later weak dextral deformation. The mineral assemblages Bt+Sil+Pl(An 23%)+Qtz reflects the metamorphic conditions of the USU. The mylonites which contain kyanite, probably reflect the metamorphic conditions of the LSU. There are no mineral equilibrium stated in mylonites, so the calculation of the metamorphic conditions is impossible. The chloritisation of biotite marks the retrogression part of the metamorphic history. The occurrence of mylonitic gneisses, fyllonites, granitic mylonites and porphyroclasts assemblage point out the different types of the parent rocks and heterogeneity shearing processes: from protomylonites to mylonites. Two chemically different groups of muscovite could be distinguished. The first group has a phengite chemical formulae. The phengites crystallized in the preferred sides of deformational structures. The second group shows chemical formulae close to ideal muscovite and its origin can be bound to granitic fluids flux. The K-Ar ages of muscovites from shear zones are in the range of 318.6+ or -12.1-298+ or -11.3 Ma. Similar data were established by Maluski (1993), Janak & Onstott (1993) and Janak (1994). The muscovites from granitoids and migmatites have given range of ages 300-330 Ma ( (super 40) Ar/ (super 39) Ar method). These ages are interpreted as cooling of granitic pluton and its metamorphic envelope. The age of the muscovite from the alaskites pegmatite (343+ or -11 Ma) is compatible with the Rb-Sr age--about 345 Ma. The pegmatite cuts the shear zone and is an evidence of the early (pre-intrusive) stage of mylonitization.