Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

摘要

A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch-like metasediments, meta-ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite-facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al2O3 and CaO and depleted in TiO2, MgO, and SiO2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph-bearing hybridized rocks, an origin by fluid-rock interaction and Ca-metasomatism in the subduction environment is inferred and considered in the Western Alps context.