MAGMATIC EVOLUTION OF THE BORZAGO VALLEY CALC-ALKALINE BASIC SATELLITE BODY (SOUTHERN ADAMELLO BATHOLITH-NORTHERN ITALY) AND ROLE OF XENOCRYSTIC PHASES IN ITS GEOCHEMISTRY

The Borzago Valley magmatic rocks constitute a satellite intrusive bod y inside the metamorphic aureole close to the north-eastern edge of th e Re di Castello Superunit (Adamello batholith). The outcropping body mainly consists of gabbro-diorite and diorite with minor hornblende-ga bbro, and sporadic granodiorite and granite. Field relationships betwe en mafic and acidic rock-types are extremely complex. Geological, petr ographic, geochemical and Sr isotope data suggest a separate origin fo r basic and acidic magmas. The former display evolution consistent wit h a two-stage crystallization process (at high and low pressures, resp ectively) in a mafic parental magma. The thermal anomaly due to crysta llizing and rising mafic magma probably induced melting at a deep crus tal level, producing acidic magma which was emplaced subcontemporaneou sly with basic magma. The geochemistry and serial characters shown by basic rocks reflect the presence of xenocrystic phases (An-rich plagio clase and Al-IV-rich amphibole) partially retained during the high-pre ssure crystallization stage. Subsequent re-equilibration processes hin der precise evaluation of the xenocrystic fraction and quantitative pe trogenetic modelling.