Trace element distribution within olivine-bearing gabbros from the Northern Apennine ophiolites (Italy): evidence for post-cumulus crystallization inMOR-type gabbroic rocks

The trace element distribution in three selected olivine-bearing gabbros fr om the Northern Apennine ophiolites has been determined. These rocks consis t of euhedral plagioclase and olivine, and subhedral to poikilitic clinopyr oxene. Fe-Ti-oxides, titanian pargasite, orthopyroxene and apatite occur as interstitial accessory minerals. Plagioclase, clinopyroxene and accessory minerals were analysed for rare earth (REE) and selected trace elements by secondary ion mass spectrometry. Both plagioclase and clinopyroxene are com positionally zoned. The plagioclase rims have slightly lower anorthite comp onent and higher light REE (LREE), Ba and K than the cores. Likewise, the c linopyroxene rims show a slight Mg and Cr decrease, and a marked increase i n Zr, REE and Y relative to the core. The rims of plagioclase and clinopyro xene, Fe-Ti-oxides, apatite and titanian pargasite most likely formed throu gh post-cumulus fractional crystallization of interstitial liquid. It is ar gued that such interstitial liquid had an exotic component, probably relate d to the infiltration of highly evolved, slightly LREE enriched liquid in t he cumulate pile. On the basis of mass balance calculations, we show that F e-Ti-oxides play an important role in the Ti budget of the whole rock, as d oes apatite for LREE.