Differentiation of mafic magma in a continental crust-to-mantle transitionzone

The Braccia gabbro complex (Eastern Central Alps, Northern Italy) intruded the boundary between the Adriatic lowermost continental crust and the subco ntinental upper mantle in Permian times. The gabbro complex consists mainly of gabbro norites with minor dykes of quartz diorite and Fe-Ti-P-rich dior ite. The gabbro-norites contain abundant cumulus clino- and orthopyroxene a nd only small amounts of olivine, indicating crystallization at high pressu re (similar to 1.0 GPa). Slow, near-isobaric cooling of the gabbro erased c ompositional zoning of major and trace elements in magmatic minerals. Bulk- rock composition and the combination of mg-number in pyroxenes and modellin g of mineral trace element data permit us to track the trace element enrich ment in minerals and whole rocks. Some trace element rich gabbros represent frozen liquids, whereas others are cumulates formed from highly differenti ated residual liquids. Differentiation of the gabbro complex is mainly driv en by fractional crystallization of pyroxenes and plagioclase, resulting in a tholeiitic differentiation trend. The trace element composition of the p arental melt was calculated from the most primitive cumulate found in the B raccia gabbro complex. This parental melt is similar to transitional mid-oc ean ridge basalts, although the Braccia gabbro crystallized at the base of the continental crust. It is suggested that the parental melt originated fr om decompression melting of upwelling mantle within the spinel peridotite f ield. Thus, a thinned lithosphere with a high geothermal gradient existed a t the northern part of the African plate in Permian times.