MELT DIFFUSE REFERTILIZATION AND FOCUSED MIGRATION IN LITHOSPHERIC MANTLE OF A FOSSILE RIFTED MARGIN: THE MT. NERO PERIDOTITE (EXTERNAL LIGURIDES, ITALY)

Abstract

Present knowledge on the External Ligurides (EL) ophiolitic peridotites indicate that they derived from the subcontinental mantle lithosphere, to be successively exposed at the sea-floor of along the ocean-continent transition zone (OCT) of the slow spreading Jurassic Ligurian Tethys. During exhumation, they underwent significant melt percolation and impregnation forming large areas of plagioclase peridotites, which replace preexisting granular and tectonite spinel peridotites (Piccardo et al., 2004). Plagioclase peridotites from Mt. Nero (EL) show relict tectonite foliation and deformed spinel-facies assemblage. Sporadically they preseve relict textures (i.e. olivine borders surrounding and patially preserving exsolved and diformed mantle pyroxenes) suggesting previous pyroxenedissolving, olivine-precipitating events, suggesting that these peridotites, prior to melt impregnation, underwent melt-peridotite interaction with pyroxene-undersaturated melts. Under spinel-facies conditions, these peridotites experienced: i) the interstitial cristallization of unstrained plagioclase and pyroxenes, sometimes concentrated in mmsize gabbroic pods, ii) the magmatic overgrowth of mantle pyroxenes and iii) the widespread replacement of deformed mantle olivine by undeformed magmatic orthopyroxene. These microstructures document pervasive porous flow percolation, interstitial crystallization and impregnation of melts. Locally, spinel-facies tectonite fabrics were almost completely recovered during melt impregnation and isotropic, coarse granular textures were acquired by the impregnated peridotites. Plagioclase peridotites have commonly high plagioclase modal contents (up to 15-20%): spinels are remarkably Aldepleted, Cr-Ti-enriched, and clinopyroxenes are Al-Na-depleted with respect to minerals of the pre-impregnation lithospheric spinel lherzolites of the same peridotite mass. Clinopyroxene shows significant enrichment in transition trace elements (i.e. REE, Ti, Sc, V, Zr, Y), having convexupward REE patterns (MREE up to 30xC1). Accordingly, calculated liquids in equilibrium with these cpx, using solidliquid partition coefficient determined for SiO2-undersaturated anhydrous systems at relatively high T and P (e.g. Hart and Dunn, 1993), have higher REE contents and patterns than liquids in equilibrium with a DMM asthenospheric mantle source. Taking into account the most probable silicasaturation of the impregnating melts, i.e. higher polymerisation than silica-undersaturated melts, the higher solid-liquid partition coefficients of Vannucci et al., (1998) have been applied, and a more realistic composition of liquids have been obtained, corresponding to very low degrees of fractional melting. Accordingly, it can be deduced that melt impregnation was operated by slightly enriched melts, showing a transitional MORB affinity. In places, plagioclase peridotites are strongly deformed along decameter-scale bands, forming plagioclase tectonitemylonite shear zones: these deformed zones have, frequently, decimeter- to meter-wide bands of spinel harzburgites, running parallel to the main foliation. These harzburgites are characterized by coarse granular textures, with rounded orthopyroxene aggregates, rare clinopyroxene relics and are completely lacking of plagioclase. Microstructural evidence, i.e. reaction microstructures (namely pyroxene dissolution / olivine precipitation), and coarsening of the olivine crystals, indicate that these spinel harzburgites are reactive in origin. They, most probably, were formed by complete consumption of plagioclase and most of clinopyroxene by the focused and reactive migration of melt along these shear zones, producing the complete recovering of the previous deformed structures. Geochemical data on clinopyroxene relics evidence two different types of these reactive spinel harzburgites. The first group (RH1) have clinopyroxenes showing convex- upward REE patterns, with a maximum at EuN (up to 10 xC1), and rather low REE absolute contents (LaN = 2.5; YbN ~ 4.2); they show, accordingly, HREE and LREE negative fractionations (GdN/YbN = 1.56, CeN/SmN = 0.59, respectively). These cpx show, moreover, evident Eu, Sr, Zr and Ta positive anomalies. Their trace element concentrations and patterns cannot be derived by any realistic partial melting process but, most probably, these cpx record transient geochemical gradients induced in the migrating melts by the melt-peridotite interaction. Particularly, the relatively high Na contents and Eu e Sr positive anomalies suggest that they were determined by the progressive demolition of the plagioclase-bearing assemblage of the percolated plagioclase peridotite tettonite. The second group (RH2) have clinopyroxenes showing convex-upward REE patterns, with maximum in the region Sm-Eu, at more than 10 xC1, negative HREE (SmN/YbN ~ 2; YbN ~ 5.4 ) and LREE fractionations (LaN/SmN in the range 0.33-0.40; LaN = 3.5-4.4). In addition, cpx of RH1 are characterized by absence of Sr, Zr and Hf anomalies and significant Nb, Ta, Th and U contents. LREE contents of cpx are significantly high, being practically coincident with estimates for cpx in the asthenospheric mantle source DMM. Information on the composition of the percolationg melts within the harzburgite channels is better approached by calculating the composition of liquids in equilibrium with RH2 cpx, utilizing distribution coefficient reliable for silica undersaturated system at high T conditions (i.e. Ionov et al., 2002). Calculated liquids show significant LREE/HREE fractionation (LaN/YbN = 5-6.5), and high LREE concentrations (LaN = 65-80). They, moreover, have relatively high HFSE(4+,5+), except for Ti. Similar concentrations and fractionations are more consistent with liquids with alkaline affinity rather than tholeiitic. This work describes some melt-related steps of the composite evolution of a sector of subcontinental mantle lithosphere, during exhumation at the ocean-continent transition zone of the slow spreading Jurassic Ligurian Tethys ocean. Pristine spinel lherzolites, deriving from the subcontinental lithosphere mantle underwent diffuse porous flow percolation and refertilization by upwelling pyroxene-saturated asthenospheric melts. Melt impregnation caused glogging of the migration pathways and collapse of the diffuse porous flow percolation, forming impregnated plagioclase peridotites. Impregnated plagioclase peridotites, later on, were strongly deformed along extensional shear zones with formation of plagioclase-bearing peridotite tectonites and mylonites. Subsequent melt migration was forced within these plagioclase-bearing peridotite shear zones: the reactive percolation of pyroxene-undersaturated melts dissolved plagioclase and most of the clinopyroxenes forming coarse granular spinel harzburgite channels.