Differentiation of Mafic Magma in a Continental Crust-to-Mantle Transition Zone

摘要

The Braccia gabbro complex (Eastern Central Alps, Northern Italy) intruded the boundary between the Adriatic lowermost continental crust and the subcontinental upper mantle in Permian times. The gabbro complex consists mainly of gabbro-norites with minor dykes of quartz diorite and Fe-Ti-P-rich diorite. The gabbro-norites contain abundant cumulus clino- and orthopyroxene and only small amounts of olivine, indicating crystallization at high pressure (∼1·0 GPa). Slow, near-isobaric cooling of the gabbro erased compositional zoning of major and trace elements in magmatic minerals. Bulk-rock composition and the combination of mg-number in pyroxenes and modelling of mineral trace element data permit us to track the trace element enrichment in minerals and whole rocks. Some trace element rich gabbros represent frozen liquids, whereas others are cumulates formed from highly differentiated residual liquids. Differentiation of the gabbro complex is mainly driven by fractional crystallization of pyroxenes and plagioclase, resulting in a tholeiitic differentiation trend. The trace element composition of the parental melt was calculated from the most primitive pure cumulate found in the Braccia gabbro complex. This parental melt is similar to transitional mid-ocean ridge basalts, although the Braccia gabbro crystallized at the base of the continental crust. It is suggested that the parental melt originated from decompression melting of upwelling mantle within the spinel peridotite field. Thus, a thinned lithosphere with a high geothermal gradient existed at the northern part of the African plate in Permian times