Discussion
The wealth of structural and petrologic studies on the ophiolite sequences from the Alpine and Apennine terrains allow to reconstruct the main characteristics of formation, composition and evolution of the oceanic lithosphere of the basin of provenance, the Jurassic Ligure-Piemontese basin.
The palaeogeographic distribution of the different peridotite massifs indicate that continental break-up exposed at the sea-floor the sub-continental mantle, variably modified by melt-peridotite interaction. During progressive opening of the basin, shallower lithospheric mantle levels that had largely escaped melt percolation were expposed at the ocean floor earlier along the proximal margins, whereas deeper lithospheric mantle levels profoundly modified by melt percolation and melt-rock interaction were exposed in the more distal parts of the basin later on.
The heterogeneity of the Alpine-Apennine peridotites
Alpine-Apennine ophiolitic peridotites show strong compositional heterogeneity and exhumed sub-continental peridotites and melt-modified peridotites characterize, respectively, the ophiolite sequences deriving from marginal and distal settings of the basin.
The marginal Cpx-rich fertile lherzolites preserve structural and paragenetic features that indicate that these peridotites: i) were uplifted from garnet-facies conditions (P >= 2.5 GPa) and ii) were equilibrated at pressures compatible with spinel-facies conditions and mean temperatures of 1000°C, to an average continental geotherm. They are in places strongly deformed in up to km-scale shear zones, showing spinel- to plagioclase- to amphibole-(chlorite)-peridotite facies syn-tectonic metamorphic assemblages, followed by shallow serpentinization. The marginal peridotites maintain, accordingly, rather fertile compositions and spinel-facies assemblages that characterized the sub-continental lithospheric mantle protoliths and record the composite structural-compositional evolution that took place under progressive exhumation during pre-oceanic lithosphere extension and rifting.
The distal peridotites show extreme compositional heterogeneities, varying from pyroxene-depleted spinel harzburgites to plagioclase-enriched peridotites to spinel dunites. Their structural and compositional features indicate the effects of significant melt-rock interaction processes. Geochemical evidence indicates that impregnating melts were fractional melt increments showing MORB affinity, formed under Sp-facies conditions by fractional melting of a DM asthenospheric mantle source, suggesting that melting conditions were attained in the asthenosphere after significant adiabatic upwelling. Reactive and impregnated peridotites were strongly deformed in extensional shear zones that were exploited for the upward focused and reactive migration of MORB-type melts.
Compositional evidence (i.e. transition from early silica-undersaturated to late silica-saturated melts) suggests that the asthenospheric melts which percolated and interacted at spinel-facies conditions were progressively saturated by the reactive interaction (pyroxene dissolution/olivine precipitation) with the host peridotite, and were subsequently entrapped by interstitial crystallization at plagioclase-facies conditions within the lithospheric mantle.
A fundamental role in producing the extreme heterogeneity of the Alpine-Apennine ophiolitic peridotites was played by the upward migration by diffuse porous flow through the lithospheric mantle of MORB melts formed in the underlying melting asthenosphere during the rifting stage in the Ligure-Piemontese system. Depending on the melting process and melt dynamics in the melting source (fractional vs aggregate melts), the melt composition (silica-undersaturation vs saturation) and the depth and mode of percolation (spinel- or plagioclase-facies conditions, diffuse vs focused percolation, open system migration, high melt-rock ratios, low time-integrated melt-rock ratios), strongly different rock types are formed, both depleted and enriched in basaltic components, that cannot be formed by simple partial melting and melt extraction processes on a DM asthenospheric mantle.
The oceanic lithospheric mantle of the Ligure-Piemontese basin
The marginal peridotites consist of sub-continental fertile spinel lherzolites and remnants of these protoliths are preserved within the distal peridotites, nothwithstanding their profound structural and compositional modification by melt-peridotite interaction.
The marginal peridotites, that were exposed at the sea-floor at ocean-continent transition zones, derived from shallower lithospheric levels where they were only sporadically reached by the percolation fronts. The distal peridotites, that were exposed at the sea-floor at more intra-oceanic settings, were exhumed from deeper lithospheric levels where they were more profoundly modified by melt percolation and interaction. Present knowledge indicate that the whole oceanic lithospheric mantle was derived by the sub-continental mantle and was exhumed in different positions in the developing oceanic basin depending on its original location in the extending lithosphere.
Since the early eighties, the distal depleted spinel peridotites of the Internal Liguride ophiolites were considered refractory residua after extraction of the associated N-MORB (i.e. the Internal Liguride ophiolitic basalts) (e.g. Beccaluva et al., 1984). The distal depleted spinel peridotites from the Internal Liguride Units and Corsica were considered similar to modern abyssal peridotites, having experienced an “oceanic-type” evolution, namely asthenospheric upwelling and MORB-type melting (e.g. Rampone et al., 1996, 1997).
Recent studies (e.g. Piccardo, 2003; Rampone et al., 2004; Piccardo et al., 2007a; Piccardo and Vissers, 2007; Piccardo and Guarnieri, 2010a and 2010b) revealed that the depleted distal peridotites record contrasting bulk and mineral chemistry characteristics that cannot be simply induced by oceanic partial melting and melt extraction. They moreover show diffuse olivine-forming/pyroxene-dissolving melt-peridotite reaction micro-textures. The bulk of compositional and structural features of these depleted peridotites support their origin as reactive spinel harzburgites formed by melt-peridotite interaction at the expenses of sub-continental lithospheric protoliths.
The Triassic-Jurassic evolution of the Ligure-Piemontese realm
Passive continental extension, already active during Triassic times, caused the formation of extensional shear zones in the sub-continental lithosphere that was progressive exhumed to shallow lithospheric levels. Extension caused lithosphere necking and asthenosphere upwelling. After significant adiabatic upwelling, asthenosphere underwent decompression partial melting along the axial zone of the extensional system, most probably during Early Jurassic times (Fig. 14) (see discussion in Piccardo et al., 2009, and references therein).
Asthenospheric melts percolated by porous flow through the lower lithospheric mantle, under spinel-facies conditions. Shear zones could have acted as preferential ways for initial focused percolation. Percolating melts were strongly depleted single melt fractions that interacted with the lithospheric mantle starting under spinel-facies conditions and formed pyroxene-depleted/olivine-enriched reactive spinel peridotites. They maintained their geochemical signature during reactive percolation but they were modified from olivine-saturated to orthopyroxene-saturated (Piccardo and Guarnieri, 2010d).
Figure 14. Schematic scenario of the geodynamic evolution of the Ligure-Piemontese basin (from top to bottom) (redrawn and modified after Brunn and Beslier, 1996, and Piccardo et al., 2009).
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Late Triassic times - Extension of the continental lithosphere by far field tectonic forces, tectonic exhumation of lithospheric mantle and adiabatic upwelling of the asthenosphere. Lithosphere extension was already active during Triassic (220-225 Ma), and was accomodated by km-scale extensional shear zones, as recorded in the subcontinental lithospheric peridotites exposed at the marginal settings of the basin during Late Jurassic continental break-up; Early Jurassic times - The adiabatically upwelling asthenosphere underwent decompressional partial melting (1). The asthenospheric melts infiltrated by porous flow mechanisms through the extending lithospheric mantle (2), most probably facilitated by the presence of extensional shear zones, and formed sporadic gabbroic intrusions (black bodies); Jurassic times - Ongoing extension caused further lithosphere exhumation and significant asthenosphere upwelling. Refractory residua after Jurassic partial melting were accreted to the base of the lithosphere (3); Late Jurassic times - The continental crust underwent complete break-up and failure, the non volcanic passive margins were formed and the sub-continental lithospheric mantle was exhumed and exposed at the sea-floor. The sub-continental lithospheric mantle peridotites (4), deriving from shallower mantle levels, were exposed at marginal OCT settings of the basin, close to the continental margins, whereas the deeper sub-continental lithospheric mantle, profounfly modified by melt-peridotite interaction processes, was exhumed and exposed at more distal intra-oceanic settings of the basin.
The strongly depleted, silica saturated melt fractions reached shallower, plagioclase facies conditions. They underwent interstitial crystallization in the percolated peridotites, forming impregnated plagioclase peridotites. The percolation pathways in peridotites were clogged by interstitial crystallization and pods and dykelets of gabbro-norite intrusives were formed (Piccardo and Guarnieri, 2010c). These strongly depleted, variably silica-saturated melts were, accordingly, entrapped and stored into the shallow lithospheric mantle. This early fractional melts stagnated and refertilized the shallow lithospheric mantle forming widespread impregnated plagioclase peridotite bodies. These melts never reached the surface since lava flows with similar compositional characteristics did never erupt at the sea-floor of the extending basin (Piccardo et al., 2009).
All the pre-existing rock types and, particularly, the impregated plagioclase peridotites were deformed along km-scale shear zones that acted as structural discontinuities for further upward migration of MORB melts (e.g. Piccardo and Vissers, 2007), that were in many cases aggregated MORB magmas. These aggregated MORB melts were delivered to shallow lithospheric depths, where intruded as ephemeral gabbroic intrusions, or extruded at the sea-floor as pillowed basaltic lava flows. These intrusive and extrusive products of the upwelling aggregated MORBs constituted the oceanic crustal rocks of the Ligurian lithosphere.
From diffuse continental extension to focused oceanic spreading
The Ligure-Piemontese oceanic basin was formed by passive stretching by far field tectonic forces of the pertinent Europe-Adria lithosphere (see discussion in Piccardo and Vissers, 2007). The extension of the lithospheric mantle was, most probably, an ultra-slow process and was accommodated by a network of shear zones. As discussed by Piccardo et al. (2009), continental extension in the Europe-Adria system was already active, most probably, during Triassic times (around 220-225 Ma, e.g. Montanini et al., 2006; Muentener and Hermann, 2001) and the onset of major rifting occurred in Liassic times (around 190 Ma, e.g. Capitanio and Goess, 2006), whereas only minor amount of extension was contributed by earlier very slow continental extension and rifting (e.g. Dercourt et al., 1986; Froitzheim and Manatschal, 1996).
Continental extention and stretching facilitated the progressive adiabatic upwelling of the asthenosphere which underwent partial melting under decompression and MORB melt extraction after more than 40 Ma of passive lithosphere extension. Information on the inception of asthenosphere melting under decompression are furnished by the first appearance of asthenospheric MORB melts intrusions into the extending lithospheric mantle. The oldest gabbroic bodies yielded Early Jurassic intrusion ages (180-179 Ma) (Tribuzio et al., 2004; Borghini et al., 2007).
Asthenospheric melts infiltrated through, and were entrapped in the mantle lithosphere. The spinel-facies lithospheric mantle protoliths record temperatures in the range 900-1100°C that are related to their residence in the sub-continental lithosphere, whereas the melt-modified peridotites of the distal ophiolites record peak temperatures of 1250-1300°C (Piccardo et al., 2009, and references therein). This indicates that significant heating by asthenosphere upwelling and melt percolation induced asthenospheric thermal conditions in the percolated lithospheric mantle. This implies that significant rheological modifications were induced in the extending “cold” mantle lithosphere along the axial zone of the extending system in connection to asthenosphere upwelling and melt percolation. Accordingly, abundance of “hot” melt-modified peridotites in the distal ophiolites indicates that: 1) substantial volumes of melts from the asthenosphere were entrapped in the lithospheric mantle, and 2) significant portions of lithospheric mantle underwent thermo-chemical erosion.
The thermal erosion of the lithosphere along the axial zone of the extending system induced the rapid decrease in the total strength of the lithosphere. The significant rheological modification of the mantle lithosphere should have caused a significantly faster extension and have played an important role in the geodynamic evolution of the system, enhancing transition from ultra-slow diffuse continental extension to localized oceanic spreading (Ranalli et al., 2007). Accordingly, the entrapment of asthenospheric melts in the shallow mantle lithosphere during the rifting stages of the system played a crucial role in the geodynamic evolution of the extensional system, enhancing the transition from ultra-slow diffuse lithosphere extension to focused oceanic spreading.
The oceanic stage in the Ligurian Tethys
It is widely accepted that the “oceanic stage” in the Jurassic Ligure-Piemontese basin was characterized by the complete removal of the continental crust, the formation of non-volcanic passive continental margins, and the exposure at the sea floor mantle peridotites bearing gabbroic intrusions. The new oceanic crust was characterized by a discontinuous basaltic cover on top of the exposed mantle peridotites, and by the deposition of oceanic sediments. Present knowledge on depleted spinel peridotites from the distal peridotite massifs evidence that they represent reactively modified, former sub-continental lithospheric mantle, and not refractory residua after MORB-forming asthenosphere partial melting. Shallow sub-continental lithospheric mantle was exhumed and exposed at marginal settings of the basin, whereas deeper lithospheric mantle, strongly modified by interaction with percolation asthenospheric MORB-type melts, was exhumed and exposed at more distal intra-oceanic settings during more advanced oceanization stages. Accordingly, the oceanic basin was floored by mantle peridotites deriving from the sub-continental mantle.
The lack of Jurassic oceanic refractory residua within the peridotite massifs exposed in distal settings poses a basic question about the meaning of the “oceanic stage” in the Ligurian Tethys, since the oceanic lithosphere of the basin, as represented by the distal ophiolites, was not formed by the products of asthenosphere partial melting, i.e. basalts and gabbros derived from the melt fraction, and abyssal peridotites representing mantle refractory residua. The transition from marginal peridotites to distal peridotites does not represent the transition from exposed sub-continental mantle to oceanic mantle (i.e. Jurassic refractory residua).
Accordingly, on the basis of present knowledge, it can be envisaged that the “oceanic stage”, frequently related in the past to the distal ophiolites of the Ligure-Piemontese basin, was characterized mainly by the failure of the continental crust. Although the lithosphere was drastically softened and thinned, complete break-up of the sub-continental mantle lithosphere did not occur in the Ligurian Tethys basin, and a complete oceanic lithosphere, consisting of magmatic rocks and peridotite refractory residua, deriving from Jurassic asthenosphere partial melting, was not formed.
The lack of refractory residua after oceanic partial melting within the known distal Alpine-Apennine ophiolitic peridotites and, accordingly, the lack of associated refractory residua and produced melts, evidence that the basin did not reach a “mature” stage in the classic sense.