Permian contact metamorphism in the Southalpine (S-Tyrol)

During the Permian the Austroalpine and Southalpine units were affected by widespread magmatism, high-T/low-P metamorphism and extensional tectonics. Metamorphism was characterized by a high geothermal field gradient and reached amphibolite- to granulite-facies conditions at about 270 Ma. Subsequently the rocks cooled until ca. 190 Ma, when the steady state geotherm was reached again. Geodynamically, this Permo-Triasssic extensional event is related to extension of the lithosphere where early Permian decompression melts from the lithospheric mantle intruded near to the crustal/mantle boundary, causing widespread high-T/low-P metamorphism and anatectic melting in the crust. The accompanying magmatism lead to intrusions of gabbros, granites, pegmatites and extensive volcanism in the Austroalpine-Southalpine realm. The Permian Brixen- (Bressanone), Iffinger- (Ivigna) and Kreuzberg- (Monte Croce) granitic- to granodioritic plutons are a major geomorphological feature of South Tyrol, and cover an area of ca. 200 km2. Although the chemical evolution of these intrusive bodies is well established, and contact aureoles were identified and partly mapped at the turn of the last century, there are absolutely no quantitative mineral chemical and petrological data available from any of the hornfelses of the contact aureoles. The main goal of this project thus is fill this gap by performing detailed mapping and sampling of the contact aureoles, the three large intrusive bodies mentioned above and the adjacent Southalpine basement rocks, to obtain mineral chemical data, and to compare petrological (thermobarometric) data with results obtained from theoretical models (calculations of petrogenetic grids and pseudosections) with experimental investigations on natural rocks from this area. In addition geochronological constraints on the timing of contact metamorphism as well as the cooling path (T-t path) will be placed by dating the plutons and the basement host rocks with U-Pb, Ar- Ar, Rb-Sr and fission track methods. This will lead to a comprehensive understanding of the Permian metamorphic event which is the prerequesite for geodynamic models of the Southalpine metamorphic basement.

Many granitic plutons form by partial melting of the continental crust and, therefore, are important markers of geodynamic processes such as orogenies. These magmas cystallize in the shallow continental crust causing thermal metamorphism of the country rocks. In the Eastern Alps, large Permian (ca. 270 - 280 Ma) granite/granodiorite massifs such as the Brixen/Bressanone, Iffinger/Ivigna and Kreuzberg/Monte Croce plutons occur along the tectonic border between the Southalpine and Austroalpine units, the Periadriatic Lineament. From the phase relations of the plutons and the country rocks (Brixen Quartzphyllites), the geodynamic setting like depth of magma formation and emplacement, geothermal regime of the crustal rocks, chemical environment, and age relations can then be derived. These informations are indipensable for a better knowledge of the evolution of this part of the lithosphere prior to the eo-Alpine orogeny. Contact metamorphosed pelitic mineral assemblages vary systematically with pressure and temperature and, therefore provide valuable quantitative information about the tectonothermal evolution of metamorphic belts. Unfortunately, very little attention has been devoted to examining the phase relations of low-pressure contact metamorphic pelites compared to regionally metamorphosed pelites, perhaps because spotted slates and hornfelses are spatially very restricted, extremely fine-grained, and commonly contain altered minerals such as pinitized cordierite. This is especially true when it comes to the study of Permian contact metamorphic rocks from the Eastern Alps. The Permian extensional event has recently been recognized in many units of the Eastern Alps, but due to the subsequent pervasive Alpine metamorphic overprint, pre-Alpine features have mostly been erased and hence very little geological evidence remains of the Permian contact metamorphic rocks in most units of the Eastern Alps. The Southalpine basement with its large Permian intrusive complexes (Brixen-, Iffinger- and Kreuzberg pluton) lacks the Alpine overprint and thus provides a perfect opportunity to study Permian contact metamorphism.