Geochemical evidence for exhumation of eclogite via serpentinite channels in ocean-continent subduction zones

Abstract

Retrograde eclogites (ranging from un altered eclogite to retrograde blueschist and greenschist mantling the eclogite boulders) from Ring Mountain on the Tiburon Peninsula, near San Francisco, California, were examined for whole-rock major and trace elements to assess protolith compositions and the geochemical signature of fl associated with retrogression. High fi eld strength elements are highly correlated, indicating relatively immobile and conservative behavior during retrogression. These immobile elements were used to assess the relative losses or gains of other elements during retrogression. Rare earth elements and FeO content show only minimal opensystem behavior. The rare earth abundance patterns, FeO contents, and Nb/Ta and Nb/La ratios show that the proto liths of these rocks were most likely normal to enriched mid-oceanic ridge basalts. Massbalance considerations reveal two independent styles of metasomatic enrichment during retrogression. One style involves coupled enrichment in large ion lithophile elements (Cs, Rb, Ba, K, and Tl), likely caused by fl uids from sediments or reaction with sediments. Another style involves coupled enrichment in Cr, Mg, Ni, and Pb, which may refl ect overprinting by reaction of eclogite boulders with serpentinite, the latter of which are highly enriched in these elements. Pb is shown here and elsewhere to be nearly universally enriched in serpentinites and is likely to be selectively mobilized into eclogite-serpentinite reaction zones. Because all retrograde lithologies show reaction with serpentinites and sediments, exhumation of the eclogite must have been accompanied by chemical interaction with serpentinites along the entire retrograde path. The simplest interpretation is that the eclogites were transported within a deeply rooted serpentinite channel, presumably formed along the slab-mantle interface by infi ltration of slab-derived fl uids into the overlying mantle wedge. Physical models of channel fl ow show that the rapid exhumation rates required to preserve eclogites in a hydrous carrier matrix, such as serpentinite, are possible due to the buoyant and low-viscosity nature of serpentinite. However, the most rapid ascent rates occur during oblique subduction, suggesting that eclogite exhumation could be favored by, but not confi ned to, oblique subduction zones.