Petrogenesis of South Armorican serpentinized peridotites

Twenty three serpentinite samples collected from five outcrops in south Brittany (Audierne near Quimper, Champtoceaux near Nantes), western France, were studied using optical microscopy, electron microprobe (EMP), inductively-coupled optical emission spectroscopy (ICP-OES), inductively-coupled plasma mass spectrometry (ICP-MS), and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Their bulk-rock major and trace element contents recomputed on an anhydrous basis are broadly characteristic of mantle-derived peridotites, as are the covariation trends of inert elements despite evidence of serpentinization-related remobilization of some fluid-mobile elements (FME; e.g., Ca, La, Ce, Sr, U). One outcrop near Champtoceaux shows fertile lherzolite compositions and chondrite-normalized rare earth element patterns consistent with low (5-7%) degree of mantle partial melting. The other four occurrences are harzburgites displaying higher partial melting degrees (15-25%). Regardless of their degree of fertility, our South Armorican peridotites bear evidence of high-temperature melt/fluid - rock metasomatic interaction yielding an overall enrichment in highly incompatible elements (HIE; Cs, Rb, Ba, Th, U, Pb, La). Hydrous modal metasomatism has been identified in both lherzolites and harzburgites. The lherzolites reacted with HIE-enriched small-volume fluids at P = 1.5-2 Gpa for T > 900 °C that produced a Ti-poor pargasite. The Audierne harzburgites were pervasively refertilized by alkali-rich hydrous melts that precipitated K- and Cr-rich pargasite. Taken as a whole, South Armorican peridotites record a great diversity of protoliths, from supra-subduction zone ophiolites (Audierne) to arc-fore arc provenance.