Many recent studies have investigated the replacive formation of troctolites from mantle protoliths and the compositional evolution of the percolating melt during melt–rock interaction processes. However, strong structural and geochemical constraints for a replacive origin have not yet been established. The Erro–Tobbio impregnated mantle peridotites are primarily associated with a hectometre-size troctolitic body and crosscutting gabbroic dykes, providing a good field control on melt–rock interaction processes and subsequent magmatic intrusions. The troctolitic body exhibits high inner complexity, with a host troctolite (Troctolite A) crosscut by a second generation of troctolitic metre-size pseudo-tabular bodies (Troctolite B). The host Troctolite A is characterized by two different textural types of olivine, corroded deformed millimetre- to centimetre-size olivine and fine-grained rounded undeformed olivine, both embedded in interstitial to poikilitic plagioclase and clinopyroxene. Troctolite A shows melt–rock reaction microstructures indicative of replacive formation after percolation and impregnation of mantle dunites by a reactive melt. The evolution of the texture and crystallographic preferred orientation (CPO) of olivine are correlated and depend on the melt/rock ratio involved in the impregnation process. A low melt/rock ratio allows the preservation of the protolith structure, whereas a high melt/rock ratio leads to the disaggregation of the pre-existing matrix. The mineral compositions in Troctolite A define reactive trends, indicative of the buffering of the melt composition by assimilation of olivine during impregnation. The magmatic Troctolite B bodies are intruded within the pre-existing Troctolite A and are characterized by extreme textural variations of olivine, from decimetre-size dendritic to fine-grained euhedral crystals embedded in poikilitic plagioclase. This textural variability is the result of olivine assimilation during melt–rock reaction and the correlated increase in the degree of undercooling of the percolating melt. In the late gabbroic intrusions, mineral compositions are consistent with the fractional crystallization of melts modified after the reactive crystallization of Troctolites A and B. The Erro–Tobbio troctolitic body has a multi-stage origin, marked by the transition from reactive to fractional crystallization and diffuse to focused melt percolation and intrusion, related to progressive exhumation. During the formation of the troctolitic body, the melt composition was modified and controlled by assimilation and concomitant crystallization reactions occurring at low melt supply. Similar processes have been described in ultraslow-spreading oceanic settings characterized by scarce magmatic activity.

Oceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono-magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back-arc basin. Bathymetry of the Mado Megamullion reveals spreading-parallel corrugations extending 25 km from the breakaway to the termination. Samples from several locations include peridotites, gabbros, dolerite, and rare pillow basalts. Gabbros range from granular to varitextured olivine gabbros and oxide gabbros. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono-metamorphic evolution including (i) dynamic recrystallization within shear zones, developed under granulite-to upper-amphibolite-facies conditions, and (ii) intrusion of highly evolved melts forming felsic segregations. This tectono-metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep-seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid-ocean ridge lower crustal rocks. We infer that the water-bearing character of the primary melts in this back-arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe-Ti oxides. Such modifications in phase saturation are likely to be directly related to the back-arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.

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