Trace element distribution within olivine-bearing gabbros from the Northern Apennine ophiolites (Italy): evidence for post-cumulus crystallization in MOR-type gabbroic rocks |
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Authors: | Riccardo Tribuzio Massimo Tiepolo Riccardo Vannucci Piero Bottazzi |
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Institution: | (1) Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, I-27100 Pavia, Italy Fax: 382 505890; E-mail: tribuzio@crystal.unipv.it, IT;(2) C.N.R – Centro di Studio per la Cristallochimica e la Cristallografia (CSCC), Via Ferrata 1, I-27100 Pavia, Italy, IT |
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Abstract: | The trace element distribution in three selected olivine-bearing gabbros from the Northern Apennine ophiolites has been determined.
These rocks consist of euhedral plagioclase and olivine, and subhedral to poikilitic clinopyroxene. 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 compositionally zoned. The plagioclase rims have slightly lower anorthite component and higher light REE (LREE), Ba and
K than the cores. Likewise, the clinopyroxene rims show a slight Mg and Cr decrease, and a marked increase in Zr, REE and
Y relative to the core. The rims of plagioclase and clinopyroxene, Fe-Ti-oxides, apatite and titanian pargasite most likely
formed through post-cumulus fractional crystallization of interstitial liquid. It is argued that such interstitial liquid
had an exotic component, probably related to the infiltration of highly evolved, slightly LREE enriched liquid in the cumulate
pile. On the basis of mass balance calculations, we show that Fe-Ti-oxides play an important role in the Ti budget of the
whole rock, as does apatite for LREE.
Received: 15 January 1998 / Accepted: 22 September 1998 |
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