Contrasting exhumation P–T paths followed by high-P rocks in the northern Caribbean subduction–accretionary complex: Insights from the structural geology, microtextures and equilibrium assemblage diagrams

The Río San Juan metamorphic complex exposes a segment of a high-pressure subduction–accretionary complex built during convergence between the Caribbean island arc and the North America continental margin. It is composed of accreted arc- and oceanic-derived metaigneous rocks, serpentinized peridotites and minor metasediments forming a structural pile. Combined structural geology, microtextural relations, multi-equilibrium calculations and thermodynamical modelling, together with published isotopic ages, allow reconstructing the metamorphic P–T–t paths of each nappe/unit and their links to the structural evolution. In all units of the complex, three major stages (M1 to M3) in the tectonothermal evolution have been distinguished. The M1 stage corresponds to the prograde evolution towards the pressure-peak of metamorphism under blueschist or eclogitic-facies conditions. The M2 stage is related to the main retrogressive event and is characterized by the S2–L2 fabric development in all lithologies and at all scales. The M3 stage represents continuous exhumation from ductile to ductile–brittle deformation regimes. However, the shape of the retrograde P–T path, the age of the exhumation-related D2 structures and the tectonic significance of D2 deformation are different in each structural unit.In the upper structural levels of the Jagua Clara serpentinitic-matrix mélange, the counter-clockwise P–T path of the eclogite blocks is typical of rocks exhumed in the early stages of intra-oceanic subduction zones. The clockwise P–T path obtained for the lower Cuaba unit is characterized by a strong isothermal decompression from the garnet-epidote amphibolite and eclogite-facies pressure-peak. This P–T evolution can be explained by rapid exhumation caused by extensional tectonics, in relation to a major modification of convergence conditions across the subduction zone. The P–T path also explains local syn-M2 partial melting processes, because it crosses the wet solidus for IAT mafic compositions. The P–T path obtained for the high-P Guineal Schists, with exhumation trajectory following the burial trajectory, can be related to exhumation during active subduction. This exhumation was most likely driven by a combination of underthrusting of tectonic units and erosion processes.Available geochronological data and T–t/P–t estimates reveal a Late Campanian to Maastrichtian retrograde M2 metamorphism in the lower structural units of the complex during a consistent D2 top-to-the-NE/ENE tectonic transport. A similar tectonic transport has also been recognized in the metasedimentary nappes of the Samaná complex during Eocene to earliest Miocene. These relations indicate a northeastward progradation of deformation during the successive tectonic incorporation of arc, oceanic and continental-derived terrains to the developing Caribbean subduction–accretionary complex.

► The Río San Juan metamorphic complex exposes a segment of a high-P accretionary wedge.
► In each structural unit, the shape and age of the retrograde P–T path are different.
► Exhumation mechanisms are mass flow, underthrusting and extensional tectonics.