Ancient ultra-slow spreading peridotites in the Alpine-Apennine ophiolites

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Piccardo, Giovanni Battista۱; Nakamura, Eizo۲; Shimizu, Nobumichi۳; Vannucci, Riccardo۴; Guarnieri, Luisa۱; Padovano, Matteo۱
Holding Date 29 September 2008

A major characteristic feature of ultra-slow spreading ridges is the large compositional variability of the abyssal peridotites. This heterogeneity is interpreted as formed by: i) regional partial melting, ii) large-scale reaction with percolating melts, and iii) impregnation of transient melts.
Recently Shimizu et al. (2006) stressed out that the large local-scale variability of clinopyroxenes from abyssal peridotites in terms of modal abundance, trace element and isotopic composition is produced by melt-rock reaction during melt migration. These features are recognized as distinct characteristics of ultra-slow spreading ridge peridotites and are a potential tool in identifying the ancient ultra-slow spreading ridge peridotites among the "orogenic lherzolite massifs" of the collisional belts.
The majority of ophiolitic peridotites cropping out in the Alpine-Apennine (AA) system derives from the oceanic lithosphere of the Jurassic Alpine (or Ligurian) Tethys basin. They show extreme compositional heterogeneity (i.e. modal variation and trace element contents of clinopyroxene) on m-scale distance. The strong lithological variability has been induced by interaction (both reactive depletion and refertilization) of pristine sub-continental lithospheric peridotites with MORB-type asthenospheric melts that percolated via diffuse and focused porous flow.
Available isotope data concur to indicate DMM to MORB signatures for most AA peridotites. Noticeably, clinopyroxenes from the different rock types of the Lanzo Massif peridotites (Bodinier et al., 1991, and unpublished data) record a even more variable isotopic composition ranging from DMM to OIB-type signatures. Lithospheric lherzolite remnants display DMM signatures (143Nd/144Nd = 0.513479-0.513347), impregnated plagioclase peridotites show MORB signatures (143Nd/144Nd = 0.513399-0.513108), whereas an OIB-like fingerprinting (143Nd/144Nd = 0.512962-0.512706) characterizes clinopyroxene in replacive harzburgites.
Model ages and geochronologic data suggest that the AA mantle protoliths, which escaped at some extent melt percolation, still record early lithosphere accretion and long time depletion when residing in the subcontinental mantle. On the contrary, the whole AA ophiolitic peridotites that underwent MORB melt interaction align conformably to a 164 Ma Sm-Nd isochron, suggesting that asthenosphere partial melting, lithosphere melt percolation and impregnation processes occurred during the Jurassic rifting of the Europe-Adria system.
As a whole, field, petrologic and geochemical evidence documents the extreme local-scale compositional heterogeneity of most AA ophiolitic peridotites that are thus regarded as fossil analogues of modern ultra-slow spreading ridge peridotites. Available lines of evidence are used to draw a composite scenario for the geodynamic evolution of the lithosphere/asthenosphere system during the Jurassic rifting and opening of the ultra-slow spreading Ligurian Tethys basin.