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Petrology, Mineral and Isotope Geochemistry of the External Liguride Peridotites (Northern Apennines, Italy) |
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| Authors: | RAMPONE E; HOFMANN A W; PICCARDO G B; VANNUCCI R; BOTTAZZI P; OTTOLINI L |
| Institution: | 1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSIT DI GENOVA CORSO EUROPA 26, 16132 GENOVA, ITALY 2MAX-PLANCK-INSTITUT FOR CHEMIE POSTFACH 3060, 55020 MAINZ, GERMANY 3CNR-CENTRO DI STUDIO PER LA CRISTALLOCHIMICA E LA CRISTALLOGRAFIA VIA ABBIATEGRASSO 209, 27100 PAVIA, ITALY 4DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSIT DI PAVIA VIA ABBIATEGRASSO 209, 27100 PAVIA, ITALY |
| Abstract: | Mantle peridotites of the External Liguride (EL) units (NorthernApennines) represent slices of subcontinental lithospheric mantleemplaced at the surface during early stages of rifting of theJurassic Ligurian Piemontese basin. Petrological, ion probeand isotopic investigations have been used to unravel the natureof their mantle protolith and to constrain the timing and mechanismsof their evolution. EL peridotites are dominantly fertile spinelIherzolites partly recrystallizfd in the plagiodase Iherzplitestability field Clinopyroxenes stable in thespinel-facies assemblagehave nearly fiat REE patterns (CeN/SmN=06–08) at (10–16)C1and high Na, Sr, Ti and Zr contents. Kaersutitic-Ti-pargasiticamphiboles also occur in the spinel-facies assemblage. TheirLREE-depleted REE spectra and very low Sr, Zr and Ba contentsindicate that they crystallized from hydrous fluids with lowconcentrations of incompatible elements. Thermometric estimateson the spinelfacies parageneses yield lithospheric equilibriumtemperatures in the range 1000–1100C, in agreement withthe stability of amphibole, which implies T<1100C. Sr andNd isotopic compositions, determined on carefully handpickedclinopyroxene separates, plot within the depleted end of theMORB field (87Sr/86Sr=070222–070263; 143Nd/144Nd=0513047–0513205)similar to many subcontinental orogenic spinel Iherzolites fromthe western Mediterranean area (e.g. Ivrea Zpne and Lanzfl N).The interpretation of the EL Iherzolites as subcontinental lithosphericmantle is reinforced by the occurrence of one extremely depletedisotopic composition (87Sr/86Sr=0701736; 143Nd/144Nd=0513543).Sr and Nd model ages, calculated assuming both CHUR and DM mantlesources, range between 24 Ga and 780 Ma. In particular, the12-Ga Sr age and the 780-Ma Nd age can be regarded as minimumages of differentiation. The transition from spinel-to plagioclase-faciesassemblage, accompanied by progressive deformation (from granularto tectonite-mylonite textures), indicate that the EL Iherzolitesexperienced a later, subsolidus decompressional evolution, startingfrom subcontinental lithospheric levels. Sm/Nd isochrons onplagioclase-clinopyroxene pairs furnish ages of  165 Ma. Thisearly Jurassic subsolidus decompressional history is consistentwith uplift by means of denudation in response to passive andasymmetric lithospheric extension. This is considered to bethe most suitable geodynamic mechanism to account for the exposureof huge bodies of subcontinental lithospheric mantle duringearly stages of opening of an oceanic basin.
*Corresponding author. Present address: Dipartimento di Stienze della Terra, Univenit di Geneva, Corso Europa 26,16132 Genova, Italy |
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