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 (similar to 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 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.