RELIES OF PRE-ALPINE OPHIOLITES IN THE SERIE-DEI-LAGHI (WESTERN SOUTHERN ALPS)

Banded amphibolites with lenses of amphibolite with garnet relies, met agabbro, pyroxenite and spinel peridotite with pyroxenite veins, occur in a continuous horizon in the Serie dei Laghi, a basement unit of th e western Southern Alps, which shows a lower amphibolite facies Varisc an metamorphism. The chemical composition of the amphibolites is homog eneous, that of the associated metagabbros is variable; these rocks ar e quite similar to the MAR tholeiitic basalts and cumulitic gabbros. T he pyroxenites show a wide range of composition, as many ophiolitic py roxenites do. In the peridotites two groups are distinguished: upper m antle peridotites with restitic character (Mg# = 90), peridodites with characteristics of a mantle contaminated by melts or fluids (Mg# less than 88). T and P estimates for the pyroxenites give T > 800 degrees C and P around 12 kb which suggests a lower crustal environment. Amphi bolites with Amph-Pl kelyphite around garnet and Di-Pl symplectite rep resent the products of transformation of an eclogitic assemblage and s uggest even higher pressure conditions. The amphibolite facies overpri nt appears related to the main regional Variscan metamorphism (about 6 00 degrees C and 6-8 kb). The studied mafic and ultramafic rocks likel y represent relies of an ophiolite belt. They were dispersed into turb iditic metasediments within an accretionary prism in the early stages of the Variscan cycle.