Whiteschist genesis through metasomatism and metamorphism in the Monte Rosa nappe (Western Alps)

Abstract Whiteschists from the Monte Rosa Nappe were examined in field as well with petrographic, geochemical, and isotopic methods to constrain controversial origin of these rocks their Alpine metamorphic context. occur ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, consist mainly chloritoid, talc, phengite, quartz. The transition whiteschist metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated deformation. development reaction zones, geometry suggest pervasive fluid infiltration, facilitated canalized fingering. Whole-rock compositions whiteschists metagranites indicate an enrichment MgO H 2 O depletion Na O, CaO, Ba, Sr, Pb, Zn relative metagranite. Trace- rare-earth elements, together all other major notably K SiO , uncertainty not mobile. Such Si saturated, undersaturated compatible previous interpretations fluids derived ultramafic rocks, evaporites, or Mg-enriched seawater due mantle interactions. Together large variations δD δ 18 values, this indicates fluxes during metasomatism. Calculated values equilibrium support magmatic–hydrothermal source, does chemical alteration pattern. Bulk rock 87 Sr/ 86 Sr ratios confirm pre-Alpine age infiltration. subsequently partially homogenized closed system metamorphism. In conclusion, granite was locally affected late alteration, which resulted sericite–chlorite granite. entire nappe underwent high-pressure metamorphism orogeny mineralogy produced dehydration metasomatic assemblage under otherwise closed-system While each locality needs be studied detail, in-depth study suggests that many found granitic bodies Alps might similar origin.