Petrology of contact metamorphic metapelites from the southern rim of the Permian Brixen Granodiorite (South Tyrol, italy)

The Brixen Granodiorite is part of the Permian calc-alkaline plutonic association (Brixen Granodiorite, Ifinger Granite, Kreuzberg Granite, Cima d’Asta Granitoid) that intruded the Variscan Southalpine metamorphic basement. The Brixen Granodiorite is located to the south of the Periatriatic Lineament in the eastern part of the Southalpine basement complex and comprises a series of tonalitic, granitic and granodioritc intrusions, which were emplaced during the Permian (280 Ma) into the country rocks of the Brixen Quarzphyllites. The depth of these Southalpine granodioritic intrusions was less than 10 km (P ≤ 0.3 GPa) and solidus temperatures were 670-720 °C (Visona, Mem Sci Geol 47:111-124, 1995; Acquafredda et al., Miner Petrogr Acta XL:45-53, 1997; Wyhlidal et al., Austr J Earth Sci 102:181-192, 2009). Only a small, about 200 m wide, contact aureole formed at the southern rim of the Brixen Granodiorite near the village Franzensfeste/Fortezza (South-Tyrol, Italy). Within the contact aureole four different zones can be distinguished based upon mineralogical, mineral chemical and textural features. Approximately 200 m from the granite contact zone I occurs. The rocks from this zone are macroscopically still quartzphyllites and are characterized by two texturally and chemically different generations of micas (muscovite, biotite) and the appearance of cordierite. Zone II is characterized by quartzphyllites containing pseudomorphs of cordierite + biotite after garnet. The inner contact aureole (zone III) starts approximately 50 m from the granite contact and shows already typical hornfels textures. This zone is characterized by the first occurrence of andalusite. In the innermost area (zone IV), ca 10 m from the granite contact, spinel and corundum occur. Geothermometry (two-feldspar-, Ti-in-biotite) yielded an increase in temperature from 540 °C in the outermost aureole (zone I) to <740 °C in the innermost aureole (zone IV). Pseudosection modelling of hornfelses from zones III and IV also resulted in similar P-T conditions of <0.28 GPa and <620 °C. This contact aureole represents one of the few well-developed remaining areas of Permian contact metamorphism in the Southalpine domain, which are otherwise mostly obliterated by late-stage hydrothermal alteration in the course of the Alpine tectonic overprint.