The petrogenetic significance of the garnets in the intrusive massifs of Bressanone and Vedrette di Ries (Eastern Alps - Italy)

Petrographic and microchemical studies on the garnets occurring in some rocks supplied new elements for the genetic and evolutionary interpretation of the intrusive massifs of Bressanone and Vedrette di Ries, respectively of Hercynian and Alpine age. The garnet cores of the tonalites and granodiorites of the Vedrette di Ries massif, independent of their Xenolithic or magmatic nature, indicate crystallization conditions of P≥ 10 Kb (≥ 36 km), T ≤950 °C, and H2O~ 5%. The later, relatively rapid ascent of the magma and its crystallization at shallower crustal levels (~ 2 Kb corresp. to ~ 7 km) are suggested by the presence of reaction borders (plag + qz + bio i amph) round the garnet and by marked zoning of the plagioclase. The Bressanone massif is heterogeneous, not only because of the possible hetero- geneity of its original materials, but also because of the pressure conditions under which melting took place. In effect, as well as the garnet-bearing granitoids in which garnet indicates melting at P7-10 Kb (25-36 km), there are cordierite- bearing granitoids for which the anatexis of the original materials took place at P < 7 Kb (< 25 km). The granodioritic-granitic complex intruded at shallow crustal levels (~ 10 km), while the tonalites of the northern border formed and crystallized at depth (P≥ 10 Kb): granodiorites, granites and tonalites reached the same crustal level only after their complete crystallization. Lastly, the composition and the phase relationships of the garnets in the aplitic (Vedrette di Ries) and granitic “Group A” rocks (Bressanone massif) indicate their genesis from residual melts at somewhat low pressure (2-3.5 Kb). The possible genetic link between the Bressanone tonalitic northern belt and the Vedrette di Ries tonalites is to be stressed, together with the possible petro- logical alien character existing between the Bressanone granodioritic massif and the Bressanone tonalitic belt.