Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

Abstract

Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere-lithosphere history of partial melting and subsequent multi-stage melt-rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5-10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970-1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt-rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt-rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt-rock interaction at decreasing melt mass, and crystallization of small (\3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt-rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt-rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred.

Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins

“…An oceanic depleted mantle is rare in the Alps and only found in the Monte Maggiore in Corsica (Rampone et al , 2008 and probably in harzburgite-dunite associations in southern Lanzo (Piccardo et al 2007). This gabbro-mantle association displays similar isotopic relationships as those described from actual mid-ocean ridges (e.g.…”

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

“…Occurrence of the latter in abyssal and ophiolitic peridotite has often been attributed to reactive porous flow (e.g. Borghini and Rampone 2007;Cannat et al 2006;Dick 1989;Dijkstra et al 2001;Menzies 1973;Müntener and Piccardo 2003;Müntener et al 2004Müntener et al , 2005 Rampone et al 1997;2008;Seyler and Bonatti 1997;Takahashi 2001).…”

Post-deformational impregnation of depleted MORB in Nain lherzolite (Central Iran)

“…The Nain lherzolite possibly provides the best example for the depleted nature of impregnating melts. The Nain melt is more depleted than the melts estimated for other ophiolitic peridotites (e.g., Rampone et al, 1997Rampone et al, , 2008Dijkstra et al, 2001Dijkstra et al, , 2003. The Nain plagioclase lherzolite protomylonites possibly recorded rifting around the margins of the CentralEast Iranian Microcontinent (CEIM) to form the Nain basin as initiation of ocean floor spreading in Mesozoic time.…”

“…4a). Involvement of an ultradepleted melt in the formation of plagioclase peridotites has been reported only from a few ophiolites (e.g., Rampone et al, 1997Rampone et al, , 2008Dijkstra et al, 2001Dijkstra et al, , 2003. The Nain lherzolite possibly provides the best example for the depleted nature of impregnating melts.…”