Shear-influenced partial melting in the Western Tatra metamorphic complex: Geochemistry and geochronology

The geochemical and isotopic characteristics as well as the age relationship between the migmatites and leucogranites, present in the metamorphic basement of the Western Tatra Mts. (Tatric Superunit - Central Western Carpathians), were studied. The suggested process for their formation was the progressive melting of the same metapelitic source under differing P-T-aH2O conditions during shearing. The migmatitic leucosomes started to form by vapour-present partial melting. As the melting developed, with increasing temperature and stress, muscovite dehydration-melting under both vapour-present and vapour-absent conditions was continued, giving rise to the formation of leucogranite melt pockets of different size. Coupled feldspar-governed fractionation and restite removal influenced the chemical and mineralogical zonation in leucogranite bodies, accumulated in the pressure shadows. No fractionation was observed in migmatitic leucosomes. U-Pb TIMS and LA-MC-ICP-MS zircon and monazite ages obtained from anatectic leucosome and leucogranite indicate that the partial melts crystallized at ca. 360-365 Ma. The zircon U-Pb ages reflect the span of time encompassing partial melting, melt separation and crystallization. The effect of the incipient crystallization of the partial melt is recorded by the inner part of the isometric zircons (Th/U < 0.1) which yielded an age of 365.1 ± 2.4 Ma (2σ). Their growth continued until the metamorphic climax at ca. 750 °C and 8 kbar. Elongate zircons (Th/U > 0.1) yielding an age of 360.5 ± 1.9 Ma (2σ) crystallized during initial cooling accompanied by melt crystallization. The two zircon populations bracket the time interval during which the analyzed monazite-(Ce) crystallized (ca. 365-360 Ma). The U-Pb age of the leucogranite zircons (359.1 ± 1.2 Ma) supports the thesis that the leucogranites crystallized from melt expelled from the migmatised metapelites.