THE EXHUMATION OF THE ERRO-TOBBIO MANTLE PERIDOTITES (LIGURIAN ALPS, ITALY) AS RECORDED BY MULTIPLE MELT INTRUSION EVENTS

Abstract

The Erro-Tobbio (ET) peridotites (Voltri Massif, Ligurian Alps) represent subcontinental lithospheric mantle tectonically exhumed during Permo-Mesozoic extension of the Europe-Adria lithosphere. Previous studies have shown that exhumation started during Permian times, and occurred along km-scale lithospheric shear zones which enhanced progressive deformation and recrystallization from spinelto plagioclase-facies conditions (Hoogerduijn Strating et al., 1993; Rampone et al., 2005a). Ongoing field and petrologic investigations have revealed that the peridotites experienced, during uplift, a composite history of diffuse melt migration and multiple episodes of ultramafic-mafic intrusions (Piccardo et al., 2004; Borghini et al., 2005), which record their progressive exhumation from deep lithospheric depths to the sea floor. In this paper we present the results of field, structural and petrologic-geochemical investigations into a sector of the Erro-Tobbio peridotite unit which preserves this multiple intrusion history. In the investigated area, peridotites are mostly constituted by low-strain spinel tectonites. The oldest intrusion event recorded is represented by the diffuse occurrence of centimeter- scale pyroxenite bands. They often display tight isoclinal folds which are crosscut at medium-high angle by the mantle tectonite foliation. This indicates that their primary intrusion relationships have been almost completely transposed by old deformation events. As already documented by Hoogerduijn Strating et al. (1993), these features suggest that pyroxenite intrusion represents an old deep-seated magmatic event, which preceded the lithospheric exhumationrelated evolution of the ET mantle. Pyroxenites consist of a coarse-grained primary mineral assemblage, made by spinel and variable amounts of clinoand ortho-pyroxene, their modal compositions thus ranging from clinopyroxenites to spinel-websterites. The spinelbearing mineral association is severely recrystallized to a plagioclase-bearing assemblage. This is recorded by the crystallization of plagioclase + olivine aggregates around relict Cr-rich spinels, and by the development of large opx+plag exsolutions in spinel-facies pyroxenes. Preliminar geochemical investigations on two pyroxenite samples have revealed that clinopyroxenes hold an unusual trace element signature, being characterized by very high Sc (120-155 ppm) and V (765-913 ppm) contents, and strongly fractionated REE spectra, with very low MREE-HREE ratios (GdN/YbN = 0.39-0.52) and HREE concentrations up to 20xC1. Similar compositional features were documented by Vannucci et al. (1993) in clinopyroxenes from plagioclasereequilibrated spinel pyroxenites of Zabargad (Red Sea), and they were interpreted to be inherited from a precursor garnet-bearing primary magmatic assemblage. The investigated ET pyroxenites also show significantly fractionated bulk-rock REE spectra, with marked LREE depletion (CeN/YbN = 0.049), absent EuN anomaly, and HREE abundances at 7xC1. All these features point that pyroxenites originated as high-pressure cumulates. Microstructural and geochemical characteristics suggest that plagioclase formation in the studied pyroxenites was mostly driven by subsolidus reaction. The likely occurrence of garnet in the primary magmatic assemblage constrains the depth of intrusion and crystallization to P > 15-20 kbar (Hirschmann and Stolper, 1996; and quoted references). Subsequent decompression of the ET pyroxenite-peridotite association is documented by the recrystallization of the pyroxenite bands to spinel- and plagioclase-bearing assemblages. At shallower lithospheric levels, the ET peridotites were diffusely migrated and impregnated by melts. Melt impregnation is documented by significant enrichment of interstitial plagioclase between mantle minerals, and by the crystallization of unstrained poikilitic orthopyroxene replacing deformed tectonitic mantle olivine and exsolved clinopyroxene: this indicates that the impregnating melts were opx-saturated. Melt-rock interaction caused chemical changes in mantle minerals (e.g. Al decrease and REE increase in cpx; Ti and Cr# enrichment in spinel). Reacted clinopyroxenes, in spite of the overall increase in the REE contents, still exhibit strong LREE depletion (CeN/SmN =0.006-0.011), indicating a depleted signature for the percolating melts. Melt impregnation was thus related to diffuse porous flow migration of opx-saturated depleted MORB-type melt fractions, as inferred by Piccardo et al. (2004). The impregnated peridotites are intruded by a hectometre- scale stratified cumulate ultramafic body, mostly consisting of troctolites and wehrlites, showing gradational, interfingered contacts with the host mantle rocks. Subsequent intrusion events are revealed by the occurrence of olivine gabbros as decameter-wide lenses, variably thick (cm- to mscale) dykes and thin dykelets, which crosscut both the peridotite foliation and the magmatic layering in the cumulates. Overall, major and trace element compositions of minerals in the intrusives indicate that they represent variably differentiated cumulus products crystallized from rather primitive N-MORB-type aggregated melts. Slightly more evolved compositions are shown by olivine gabbros, relative to the troctolites and wehrlites of the ultramafic body. Peculiar mineral chemistry features (e.g. the Fo-An correlation and high Na, Ti, Mg# in cpx) indicate that the studied intrusive rocks crystallized at moderate pressure conditions (3-5 kbar, i.e. 9-15 km depth). Sm-Nd isotope data on two olivine gabbros have yielded a magmatic crystallization age of 180 + 14 Ma (Rampone et al., 2005b). Chemical and petrologic characteristics of ultramafic and gabbroic rocks point to compositional differences between their parental melts (olivine-saturated, N-MORB-type aggregated melts) and melts which impregnated the host peridotites (orthopyroxene- saturated single depleted melt increments). Field and geochemical evidence thus indicate that peridotite impregnation and cumulate intrusion represent unrelated events. Peridotite impregnation was caused by diffuse migration of single depleted melt fractions and occurred before the intrusion of MORB-type aggregated melts. The transition from porous flow melt migration to emplacement of magmas in fractures, most likely reflects progressive change of the lithospheric mantle rheology during extension-related uplift and cooling of the ET mantle.