Petrographic, geochemical and geochronological investigation on granitic pebbles from Permotriassic metasediments of the Tisia terrain (eastern Papuk, Croatia)

Abstract

Granitic pebbles occurring in the Permotriassic metasedimentary sequence of eastern Papuk, Slavonian Mountains, Croatia, were recognized to represent a coherent group of felsic, muscovite-albite metagranites. Fabrics, modal compositions and geochemical data imply that the rocks are derivatives of S-type granites formed through a combination of igneous and subsequent metasomatic processes. A Variscan formation age is demonstrated by K-Ar dating on coarse muscovite (range of 329–317 Ma) as well as by electron microprobe based Th-U-Pb monazite dating (338 ± 15 Ma). Additionally to the Variscan metasomatic processes of albitization and greisenisation, which led to an almost complete replacement of K-feldspar and biotite by albite and coarse muscovite, pebbles were affected by a younger phase of alteration resulting in the formation of a fine-grained sericitic matrix. The fine sericite yields K-Ar ages of 91–83 Ma. A substantial reheating of the rocks during the Cretaceous is also indicated by the growth of new monazite dated at 106 ± 10 Ma. Yttrium-contents of the Cretaceous monazite from the granite pebbles (0.3–0.9 wt% Y2O3) are compatible with metamorphic temperatures of ~350–400°C. These data confirm recent concepts according to which large parts of the Slavonian Mountains received a pervasive Cretaceous low-T regional metamorphic overprint. Furthermore, the pebbles provide useful information on the nature of the eroded Variscan crust of the Tisia Terrain, which has obviously contained considerable amounts of evolved high-level S-type granites modified through albitization and greisenization.