Geochemical and isotopic evidence for crystal fractionation, AFC and crustal anatexis in the genesis of the Rensen Plutonic Complex (Eastern Alps, Italy)

Petrological, geochemical and isotopic data are reported for the Rensen Massif, a Late Alpine (ca. 30 Ma) plutonic complex sited in the Eastern Alps. It consists of tonalites and granodiorites with minor quartz diorites and granites. The latter are represented by small masses with gneissic textures which outcrop on the northern edge of the complex. Many of the major and trace elements display relatively smooth variations with the differentiation index (DI): other trace elements, and Sr isotope ratios show much more complex patterns. Sr, Th, Ta, LREE and other “incompatible” trace elements display a wide scattering of values in rocks of similar major-element composition. Initial isotope ratios range from 0.70752 to 0.70877 in quartz diorites and granodiorites, whereas values of 0.70996 to 0.71071 are attained in the granite gneisses. Field, geochemical and isotopic data suggest different origins for the granite gneisses and the quartz diorite-tonalite-granodiorite suite. The latter appears to have evolved by multistage, polybaric evolutionary processes which involved several steps of fractional crystallization and interaction with metamorphic host rocks. A high-pressure crystallization process, starting with a dioritic liquid from which a garnet-bearing solid phase separated, is believed to have generated some tonalitic magmas. These, in turn, evolved to a granodioritic composition by assimilation-fractional crystallization (AFC) or bulk wall-rock assimilation. Other batches of dioritic magma, in contrast, appear to have undergone AFC processes at lower pressures with the generation of Sr-poor, unfractionated-HREE tonalitic and granodioritic magmas. The granitic rocks have geochemical and isotopic signatures which are consistent with an origin by melting of a pelitic source, possibly with a significant contribution from a component less rich in radiogenic Sr. The occurrence of gneissic textures in these rocks, which is not apparent in the quartz diorite-tonalite-granodiorite suite, points to an early emplacement and crystallization of acidic crustal-derived melts, and for a deformation event which predates the crystallization of the main plutonic mass.