Significance of serpentine mud volcanism in convergent margins

Abstract

Serpentinites are distinctive components of on-land exposures of former convergent-margin terranes throughout the world, commonly mingled with high-pressure, low-temperature blueschist-facies metamorphic rocks in complexmelange assemblages that have long perplexed geologists. Serpentinite blocks within these melanges commonly retain vestiges of original igneous textures and were once widely considered to be igneous intrusive rocks, altered in situ. These chaotic assemblages never show contact-metamorphic aureoles, but usually do show extensive internal shearing and are commonly found along fault zones; thus an alternative explanation was that they formed as tectonically intrusive (i.e., protrusive) bodies. Early workers suggested that these serpentinites were emplaced during or after deformation and metamorphism. Particularly puzzling were occurrences of sedimentary serpentinite, masses of originally unconsolidated serpentine enclosing clasts of serpentinized peridotite and metabasites (some reaching blueschist-grade metamorphism), showing evidence of sedimentary structures and occasionally incorporating cherts and marine fossils. Recent discoveries of large submarine serpentine mud volcanoes containing blueschist materials in the forearc of the active Mariana convergent plate margin show that serpentine mud, crystals, and clasts can be emplaced directly on the seafloor by protrusive processes. Mudflows from these serpentine seamounts are incorporated in the forearc strata before subsequent emplacement on land and before the deformation and associated metamorphism of the materials enclosing them. The Mariana forearc mud volcanism is episodic and brings up materials derived both from the descending plate and from the mantle and crustal sections of the overriding plate. These materials rise along deep-seated faults above the subduction zone and probably represent fault gouge mobilized by seismicity and rising fluids released during dehydration of materials on the downgoing plate. The serpentine mud volcanoes of the Mariana forearc are the only ones known to be currently active and to be venting slab-derived fluids. These mud volcanoes contain a wide variety of metamorphosed rocks either as clasts or as disaggregated crystals disseminated throughout the muds. Blueschist materials discovered in these seamounts indicate depths of origin of ≤30 km. Probably these materials derive from the top of the descending Pacific plate. Other, unmetamorphosed rocks of plate origin have also been recovered from several of these seamounts. Many examples of sedimentary serpentinite exist throughout the world in ancient convergent-margin settings; thus the phenomenon of serpentine mud volcanism is likely a more important agent of recycling of subducted constituents than previously thought.