Mg-metasomatism of oceanic gabbros and its control on Ti-clinohumite formation during eclogitization |
| |
| Authors: | Marco Scambelluri Elisabetta Rampone |
| |
| Institution: | (1) Dipartimento di Scienze della Terra, Università di Genova, Corso Europa 26, I-16132 Genova, Italy Tel.: 0039 10 3538315; Fax: 0039 10 352169; e-mail: msca@dister.unige.it, IT;(2) Dipartimento di Scienze della Terra, Università di Milano, Via Botticelli 23, I-20133 Milano, Italy, IT |
| |
| Abstract: | In the high-pressure meta-ophiolites of Western Liguria (Italy), serpentinized ultramafites host bodies of eclogite, metarodingite
and Ti-clinohumite ± Ti-chondrodite-bearing rocks. The latter contain relics of augite, ilmenite and apatite, which suggest
derivation from pristine Fe-Ti-rich gabbros. The composition of relict mantle clinopyroxene in the host serpentinites indicates
primary depleted peridotite compositions. Compared with their inferred protoliths, the Ti-clinohumite dikelets and the host
serpentinites display significant changes in their major and trace element concentrations, indicating element exchange between
the two rock systems. In particular, the Fe-Ti-rich gabbros were depleted in CaO and FeO and were strongly enriched in MgO.
Analogous compositional variations are shown by altered gabbros enclosed in serpentinized peridotites from the obducted ophiolite
sequences of the Northern Apennine. This evidence suggests that the observed Mg-enrichment recorded by the Ti-clinohumite
metagabbros occurred in oceanic environments as the result of diffusive exchange between ultramafites and gabbros in presence
of fluids related to serpentinization of the ultramafic country rocks. Alteration of the gabbro and concomitant Mg-uptake
mostly caused extensive chloritization of the igneous plagioclase. Survival of igneous ilmenite and augite and their reaction
with the hydrothermal chlorite during high-pressure metamorphism produced the observed Ti-clinohumite and Ti-chondrodite assemblages.
The data presented thus indicate that crystallization of Ti-clinohumite assemblages was facilitated by a stage of oceanic
alteration leading to Mg-enrichment of original Fe-Ti-rich gabbros. We suggest that during alteration, Mg-metasomatism occurred
prior to rodingitization and was related to the earlier stages of peridotite serpentinization. Survival of oceanic chemical
heterogeneities in the Ti-clinohumite rocks, indicates that element mobility during high-pressure recrystallization of these
rocks was on a limited scale. This allowed preservation of their pre-subduction alteration features.
Received: 13 July 1998 / Accepted: 3 November 1998 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
