Petrology, Mineral and Isotope Geochemistry of the External Liguride Peridotites (Northern Apennines, Italy)

Abstract

Mantle peridotites of the External Liguride (EL) units (Northern Apennines) represent slices of subcontinental lithospheric mantle emplaced at the surface during early stages of rifting of the Jurassic Ligurian Piemontese basin. Petrological, ion probe and isotopic investigations have been used to unravel the nature of their mantle protolith and to constrain the timing and mechanisms of their evolution. EL peridotites are dominantly fertile spinel Iherzolites partly recrystallized in the plagioclase Iherzolite stability field. Clinopyroxenes stable in the spinelfacies assemblage have nearly flat REE patterns (Ce-s/ SmN = 0-6-0-8) at (10-16) xCl and high Na, Sr, Ti and %r contents. Kaersutitic—Ti-pargasitic amphiboles also occur in the spinelfacies assemblage. Their LREE-depleted REE spectra and very low Sr, %r and Ba contents indicate that they crystallized from hydrous fluids with low concentrations of incompatible elements. Thermometric estimates on the spinelfacies parageneses yield lithospheric equilibrium temperatures in the range 1000-1100°C, in agreement with the stability of amphibole, which implies T<1100°C. Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, plot within the depleted end of the MORB field CSr/Sr = 0 • 70222-0 • 70263; Nd/Nd = 0 -5130470 -513205) similar to many subcontinental orogenic spinel Iherzolites from the western Mediterranean area (e.g. Ivrea Z and Lanzo N). The interpretation of the EL Iherzolites as subcontinental lithospheric mantle is reinforced by the occurrence of one extremeh depleted isotopic composition (Sr/Sr-0-701736; Nd/Nd = 0-513543). Sr and Nd model ages, calculated assuming both CHUR and DM mantle sources, range between 2-4 Ga and 780 Ma. In particular, the 1-2-Ga Sr age and the 780-Ma Nd age can be regarded as minimum ages of differentiation. The transition from spinelto plagioclase-fades assemblage, accompanied by progressive deformation (from granular to tectonite—mylonite textures), indicate that the EL Iherzolites experienced a later, subsolidus decompressional evolution, starting from subcontinental lithospheric levels. Sm/Nd isochrons on plagioclase—clinopyroxene pairs furnish ages of ~165 Ma. This early Jurassic subsolidus decompressional history is consistent with uplift by means of denudation in response to passive and asymmetric lithospheric extension. This is considered to be the most suitable geodynamic mechanism to account for the exposure of huge bodies of subcontinental lithospheric mantle during early stages of opening of an oceanic basin.