Alaskity Tatr Zachodnich : zapis wczesnowaryscyjskiej kolizji w prakontynencie Karpat

Abstract

The crystalline basement of the Western Tatra Mountains consists of metamorphic cover and granodiorite intrusion, called Rohacze Granite. The metamorphic cover is composed of two units: Upper Structural Unit (GJS), metamorphosed in upper amphibolite facies conditions (P = 7.5-9 kbar; T = 670-780°C), and Lower Structural Unit (DJS), metamorphosed in the epidote-amphibolite to lower amphibolite facies conditions (P = 5-8 kbar; T = 540-580°C), forming together the inverted metamorphic zonation (Janak, 1994; Gawęda, Kozłowski, 1998; Gawęda et al., 1998; Kozłowski, Gawęda, 1999). Both units are divided by the ductile shear zone. Small lens-like or tongue-shaped leucogranite intrusions, called traditionally alaskites (Jaroszewski, 1965; Burchart, 1970) are located in the fold cores, shear zones and in tectonic discontinuities at the base of the Upper Structural Unit (GJS). The modal analyses of the alaskites show the variation in proportions of plagioclases to K-feldspars and in the amount of quartz. For each studied locality a shift in mineral compositions from the margin to the centre of each intrusion was observed (from high Kfs/Pl and Qtz content about 20% vol to low Kfs/Pl ratio and Qtz content more than 30%). Such features mirror the changes in magmatic fluid character and the predominance of CO2/CH4 over H2O (Ebadi, Johannes, 1991). This fact is in agreement with the character of the host rocks (rich in graphite - Gawęda, Cebulak, 1999) with low f02, and supported by the lack or scarcity of magmatic muscovite and the local presence of positive Eu anomalies in REE chondrite-normalized diagram. The magmatic fabric (phot. 6, 10) and porphyroclasts rotation (phot. 3), point out the syntectonic character of the alaskite intrusions, connected with the overthrusting of GJS over DJS (Kozłowski, Gawęda, 1999). The resister and restite minerals (Pl, Grt, Bt, Zr, Tnt, and Ap) are present in different amount in the alaskites: from 0-10% vol. in the centres of alaskite bodies, up to 50-60% vol. in the border zones, which can be classified, in fact, as migmatites. Restite and resister minerals subjected the partial recrystallization and/or chemical homogenisation inside the alaskite magma. Alaskites are peraluminous in composition, with A/CNK > 1 and A/NK >1.1, poor in CaO (tab. 13, fig. 28). They plot on or slightly below the cotectic plane in the quaternary Ab-Or-Q-An diagram (James, Hamilton, 1969; fig. 30). As the products of partial melting, alaskites are assumed to be formed according to the reaction of the muscovite dehydration-melting: Ms + Qtz = Kfs + Sil + L, modified by the presence of graphite or tourmaline. The range of partial melting and melt composition depended on the petrographical characteristics of the source material and the composition and accessibility of volatile components. The melting ratio varied in the range of 9-36% vol. (tab. 18 a, b), but did not exceed the Rheologically Critical Melt Fraction (defined after Wickham, 1987 a, b). According to mass-balance calculations, carried out for the main elements, the metamorphic rocks of GJS could be a source for the partial leucocratic melts. The syntectonic emplacement of alaskite magma and lost of the fluid phase caused the enhanced crystallisation, and - as a further consequence - an increase of magma viscosity. The increase of magma viscosity inhibited removal of the resister/restite minerals, which underline the primary magmatic fabric (phot. 15). The conditions of magma generation were consistent with the GJS peak metamorphic conditions: P = 7-10 kbar, T = 700-800°C. The rapid loss of volatiles as well as the peritectic reactions, related to this process (for example tourmaline crystallisation), caused the local perturbations in crystallisation temperatures. As an effect, the local increase of temperature, up to the level of “dry” granite magma - over 800°C up to 900°C - took place (fig. 40). As a result of the mentioned perturbations, the decomposition of Ti-rich biotite to sillimanite “matts” (Gawęda et al., 1999 a, b) and sapphirine occurred. The alaskites of the Western Tatra Mountains have the features of syntectonic granitoids, originated due to collision of microplates (fig. 47 a, b, c, d). The collision and leucogranite magma generation took place at 340-370 Ma. Alaskites in the Western Tatra metamorphic basement are the remnants of the Early-Variscan collisional stage, which formed the pre-continent of the Carpathians. The presence of leucogranitic syntectonic bodies could help in reconstruction of the broken up and separated crystalline cores, present in the Inner Western Carpathians.