Mass flow in serpentinite-hosted subduction channels: P–T–t path patterns of metamorphic blocks in the Rio San Juan mélange (Dominican Republic)

Serpentinite mélanges formed in the subduction zone channel of an intra-oceanic arc system are exposed in the Rio San Juan complex of northern Hispaniola. This study presents an array of pressure–temperature paths for 14 metamorphic blocks. The P–T paths were constructed from isochemical P–T phase diagrams (“pseudosections”) as well as multi-equilibrium calculations on mineral assemblages, providing for the first time the means to compare P–T paths in both time and space. The study involves metabasic, meta-andesitic and metagranitic lithologies. Subduction was slow, so that the uninterrupted thermal development of the subduction zone can be followed over a time span of more than 60 Myr. Early eclogites mirror “warm” subduction zone gradients (∼18°/km); both clockwise and anticlockwise paths are found. The maximum P–T conditions reached are ∼800 °C and 26 kbar, thus indicating sufficient buoyancy for return flow from depths exceeding the stability of serpentine. As the subduction zone matures, P/T gradients gradually steepen to ∼6°/km, and the maximum depth from which blocks are exhumed decreases in a regular way from 26 to 17 kbar. The exhumed metamorphic products are initially omphacite-bearing blueschists and evolve to jadeite- and lawsonite-bearing blueschists with veins of jadeitite jade. Lawsonite eclogites are not found, because in the latest history of the subduction zone the predominantly basic geochemistry of the blocks evolved to more intermediate compositions inappropriate for this rock type. There are no indications of systematic differences or episodic variations in subduction/exhumation velocities, so that a continuous cycling of material in the subduction zone channel is indicated. In two blocks of eclogite and garnet blueschist an earlier amphibolite-facies history at 575–700 °C and 7–8 kbar can be documented, suggesting erosion of the base of the overlying intra-oceanic island arc. The fossil subduction zone studied in detail here is part of a much larger feature exposed at intervals around the perimeter of the Caribbean plate where the intra-oceanic arc has collided with North and South America. The continuous history documented here from before 120 Ma to after 60 Ma constrains regional plate tectonic models to incorporate a similarly uninterrupted continuous history of eastward movement of the Caribbean plate with respect to North and South America.

Research highlights
► Reconstructing mass flow in a serpentinite subduction channel using P–T–time paths.
► >60 Myr of continuous cycling of material in an intra-oceanic subduction channel.
► Dynamic thermal structure in a self-organizing intra-oceanic subduction zone.
► Return flow from depths exceeding serpentinite stability in a subduction channel.
► Uninterrupted westward subduction below the Caribbean Plate from >120 to <60 Ma.