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 Furassic 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 lherzolites partly recrystallized in the plagioclase lherzolite stability field. Clinopyroxenes stable in the spinel-facies assemblage have nearly flat REE patterns (Ce-N/Sm-N = 0.6-0.8) at (10-16) x C1 and high Na, Sr, Ti and Zr contents. Kaersutitic-Ti-pargasitic amphiboles also occur in the spinel-facies assemblage. Their LREE-depleted REE spectra and very low Sr, Zr and Ba contents indicate that they crystallized front hydrous fluids with low concentrations of incompatible elements. Thermometric estimates on the spinel-facies parageneses yield lithospheric equilibrium temperatures in the range 1000-1100 degrees C, in agreement with the stability of amphibole, which implies T < 1100 degrees C. Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, plot within the depleted end of the MORB field (Sr-87/Sr-86 = 0.70222-0.70263; Nd-143/Nd-144 = 0.513047-0.513205) similar to many subcontinental orogenic spinel lher- zolites from the western Mediterranean area (e.g. Ivrea Sone and Lanzo N). The interpretation of the EL lherzolites as subcontinental lithospheric mantle is reinforced by tire occurrence of one extremely depleted isotopic composition (Sr-87/Sr-86 = 0.701736; Nd-143/Nd-144 = 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 spinel- to plagioclase-facies assemblage, accompanied by progressive deformation (front granular to tectonite-mylanite textures), indicate that the EL lherzolites experienced a later, subsolidus decompressional evolution, starting from subcontinental lithospheric levels. Sm/Nd isochrons an plagioclase-clinopyroxene pairs furnish ages of similar to 165 Ma. This early Furassic subsolidus decompressional history is consistent with uplift 63, 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 sub-continental lithospheric mantle during early stages of opening of an oceanic basin.
PETROLOGY, MINERAL AND ISOTOPE GEOCHEMISTRY OF THE EXTERNAL LIGURIDE PERIDOTITES (NORTHERN APENNINES, ITALY)
OTTOLINI L
1995
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 Furassic 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 lherzolites partly recrystallized in the plagioclase lherzolite stability field. Clinopyroxenes stable in the spinel-facies assemblage have nearly flat REE patterns (Ce-N/Sm-N = 0.6-0.8) at (10-16) x C1 and high Na, Sr, Ti and Zr contents. Kaersutitic-Ti-pargasitic amphiboles also occur in the spinel-facies assemblage. Their LREE-depleted REE spectra and very low Sr, Zr and Ba contents indicate that they crystallized front hydrous fluids with low concentrations of incompatible elements. Thermometric estimates on the spinel-facies parageneses yield lithospheric equilibrium temperatures in the range 1000-1100 degrees C, in agreement with the stability of amphibole, which implies T < 1100 degrees C. Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, plot within the depleted end of the MORB field (Sr-87/Sr-86 = 0.70222-0.70263; Nd-143/Nd-144 = 0.513047-0.513205) similar to many subcontinental orogenic spinel lher- zolites from the western Mediterranean area (e.g. Ivrea Sone and Lanzo N). The interpretation of the EL lherzolites as subcontinental lithospheric mantle is reinforced by tire occurrence of one extremely depleted isotopic composition (Sr-87/Sr-86 = 0.701736; Nd-143/Nd-144 = 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 spinel- to plagioclase-facies assemblage, accompanied by progressive deformation (front granular to tectonite-mylanite textures), indicate that the EL lherzolites experienced a later, subsolidus decompressional evolution, starting from subcontinental lithospheric levels. Sm/Nd isochrons an plagioclase-clinopyroxene pairs furnish ages of similar to 165 Ma. This early Furassic subsolidus decompressional history is consistent with uplift 63, 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 sub-continental lithospheric mantle during early stages of opening of an oceanic basin.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
