PETROGENESIS OF SOME LIGURIAN PERIDOTITES .1. MINERAL AND BULK-ROCK CHEMISTRY

Bulk-rock and mineral compositions of eleven lherzolitic specimens have been determined. Eight samples are from the Erro-Tobbio thrust sheet of the Gruppo di Voltri, western Liguria; three samples are from allochthonous eastern Ligurian masses within the External Ligurides. All specimens consist of spinel (±plagioclase) in addition to ol + opx + cpx, and have been derived from mantle portions of the South Alpine-Apennine lithospheric plate. Electron microprobe analyses of 11 olivines, 17 orthopyroxenes, 13 clinopyroxenes, eight spinels and one plagioclase have been performed. Except for Ca-rich and Ca-poor lamellae in some pyroxenes, and iron-enriched rims in a few spinels, the analyzed phases are remarkably homogeneous. Mineral compositions vary insignificantly from rock to rock. Consideration of major element partitioning in coexisting opx + cpx pairs, combined with the petrogenetic grid for aluminous lherzolite bulk compositions, yield the following provisional P-Tequilibration conditions: Erro-Tobbio complex, 1150 ± 50°C, 16 ± 6 kbar; External Ligurides, 1000 ± 50°C, 14 ± 6 kbar. Evidently the western Ligurian peridotites equilibrated at higher temperatures and possibly at slightly greater depths with the upper mantle than those from eastern Liguria. The lherzolite samples have nearly identical bulk-rock major element compositions. Comparisons with estimated compositions for the upper mantle, and for model pyrolite indicate that these Ligurian peridotites are primitive or only very slightly depleted in basaltic constituents. Thus, based on major element chemistry, the Ligurian peridotites cannot be regarded as residua left after extensive partial fusion, hence evidently cannot have produced large proportions of oceanic tholeiite liquid; on the other hand, they seem to represent a fertile mantle source for such melts. Relics of garnet have not been found, and apparent temperatures for both Erro-Tobbio and eastern Ligurian ultramafic suites exceed the classical intraplate oceanic geotherm; accordingly, these bodies may represent rising mantle material that recrystallized during ascent in the general vicinity of a spreading center, without having undergone extensive partial melting. © 1979.