JD-QTZ-BEARING METAPLAGIOGRANITE FROM THE MONVISO META-OPHIOLITE (WESTERN ALPS)

Abstract

In the Basal Serpentinite Unit of the Monviso meta-ophiolite (Val Varaita, Western Alps), a large jadeite-quartz-bearing metaplagiogranite body has been found, which shows magmatic contacts with eclogitic Pl-Cpx-FeTi oxide metagabbro. Primary texture and magmatic minerals in the metaplagiogranite are mostly obliterated by multistage Alpine re-equilibrations. The highest-pressure assemblage consisted of jadeite, quartz, phengite and rutile. Garnet and zoned ferroglaucophane developed immediately after the peak assemblage, and were followed by aegirine-rich clinopyroxene. Calculated phase compatibilities and phengite geobarometry indicate T = 575°C and P = 1.95 GPa as minimal temperature and pressure of formation for the peak assemblage in the metaplagiogranite, in agreement with garnet-clinopyroxene geothermometry from the associated FeTi-oxide metagabbro which yields T = 545±35°C at P = 2.0 GPa. Major, trace, and REE data are typical of oceanic plagiogranite, and similar to those of metaplagiogranite from other meta-ophiolite units of the Western Alps, Northern Apennines and Corsica. Petrological and preliminary geochemical data suggest that the Monviso plagiogranite likely represents late stages of the plutonic activity in this sector of the Western Tethys, and could have formed by fractional crystallization of ocean-floor tholeiite magmas in Late Jurassic times.