Evolution of the lithospheric mantle in an extensional setting: Insights from ophiolitic peridotites

Abstract

We present a model of mantle lithosphere evolution in an extensional setting based on the structure and petrology of the Alpine-Apennine ophiolitic peridotites from the Jurassic Ligurian Tethys Ocean. Continental extension and rifting in the Ligurian domain were induced by far-fi eld tectonic forces and were characterized by the interplay of tectonic and magmatic processes. Extension and thinning of the lithosphere induced adiabatic upwelling and decompression melting of the asthenosphere. Mid-ocean-ridge basalt (MORB) melts percolated by porous fl ow through the extending lithospheric mantle and were trapped therein by interstitial crystallization. Therefore, melt emplacement at the seafl oor during continental rifting and breakup was prevented, and the resulting rifted margins of the basin were nonvolcanic. The asthenosphere-lithosphere interaction induced signifi cant rheological modifi cations of the percolated mantle. Weakening and softening of the mantle lithosphere signifi cantly lowered the total lithospheric strength. Accordingly, thermo-mechanical erosion of the mantle lithosphere promoted lithosphere stretching and thinning and enhanced the transition from distributed continental deformation to localized oceanic spreading.

Study of Serpentinized Ultramafic Rocks of Bela Ophiolite, Balochistan, Pakistan

“…The low content of Al 2 O 3 (av. 0.96%) in the study area indicates depleted peridotites which generally formed at more distal (internal, oceanic) settings of the basin, represent refractory residua after asthenosphere partial melting and MORB extraction and to be similar to modern abyssal peridotites (Piccardo et al, 2009). The occurrence of depleted mantle is associated with fast spreading ridges, which favour the formation of most refractory peridotites (Godard et al, 2008).…”

“…29.62%) showing relevance with Alpine peridotites. The Alpine peridotites are characterized by strong compositional heterogeneity, because they face multiple processes including partial melting and melt extraction (Piccardo et al, 2009).…”

Orogenic, Ophiolitic, and Abyssal Peridotites

“…In their scheme, the HT LP plagioclase peridotites have undergone a complex history of regional-scale melt infiltration and melt-rock reaction that has erased most of the ancient (lithospheric) history that is otherwise preserved in LT LP spinel peridotites. This process, coupled with thermomechanical erosion by upwelling asthenosphere, would result in the rejuvenation ('asthenospherization') of subcontinental lithosphere beneath rifted margins (see also, for the Alps, Mü ntener and Piccardo, 2003;Mü ntener et al, 2004Mü ntener et al, , 2005Piccardo and Guarnieri, 2010;Piccardo et al, 2004bPiccardo et al, , 2009Ranalli et al, 2007;. Several studies have reported geochemical analyses of LP orogenic peridotites from the Western Alps.…”

“…In contrast with mantle xenoliths, the massif and abyssal peridotites do not generally represent lithospheric mantle that has been stabilized for a sufficient period of time to be traversed by intraplate magmatism. Some orogenic peridotites probably include relatively old lithospheric mantle components (see Section 3.4.5.2.1), but they were generally strongly 'asthenospherized' -both thermally and chemically -before their exhumation (Le Roux et al, 2007;Lenoir et al, 2001;Mü ntener et al, 2005Mü ntener et al, , 2010Piccardo et al, 2007aPiccardo et al, , 2009.…”

“…This process would account for the depletion of the most incompatible trace elements (including LREEs) while leaving almost unchanged the rest of the peridotite composition. An alternative to this scenario is to envisage that some fertile orogenic lherzolites have acquired their geochemical signature as a result of melt-rock interaction processes associated with the thermomechanical erosion of lithospheric mantle by upwelling asthenosphere (e.g., Lenoir et al, 2001;Mü ntener et al, 2005Mü ntener et al, , 2010Piccardo et al, 2007aPiccardo et al, , 2009. In this scheme, refertilization of lithospheric peridotites by (and reequilibration with) mildly LREE-enriched or LREE-depleted melts is an alternative to the small degrees of melt extraction to account for LREE depletion in otherwise fertile lherzolites (Le Roux et al, 2007).…”