Origin and significance of the ophiolitic high-P mélanges in the northern Caribbean convergent margin: Insights from the geochemistry and large-scale structure of the Río San Juan metamorphic complex

Abstract

The Río San Juan metamorphic complex exposes a segment of a high-pressure accretionary wedge, built during Late Cretaceous intra-oceanic subduction below the Caribbean island-arc. Detailed mapping, large-scale structural analysis, U–Pb/Ar–Ar geochronology, in situ trace element analyses of clinopyroxene, and bulk-rock geochemical data show that the northern half of the complex is composed of arc- and oceanic-derived units, whose large-scale internal structure consists of an imbricate stack of mainly high-P rocks. In the lower structural levels, the Gaspar Hernández serpentinized peridotite–tectonite is composed of massive, serpentinized harzburgite that grades into surrounding sheared serpentinite or gouge. Gabbroic rocks and dolerite sills with N-MORB geochemical signatures are sparse and altered to greenschists facies. U–Pb data indicate a Lower Cretaceous magmatic age (136.40±0.32Ma). These rocks are interpreted as ophiolitic fragments of the proto-Caribbean Ocean. In the intermediate Jagua Clara serpentinite-matrix mélange, the massive serpentinite with relict peridotite textures was overprinted by schistose and sheared serpentinite, and serpentinite gouge, recording various degrees and conditions of internal deformation. Serpentinite schistosity surfaces warp around mélange hard blocks (<1m to 2.5km). Blocks are massive serpentinized peridotite and exotic high-P crustal rocks. The protoliths of the mafic metaigneous rocks originated from arc-like and non-arc-like magmas. The Jagua Clara serpentinite-matrix mélange represents the deep subduction channel, formed during intra-oceanic subduction. In the upper structural levels, the Morrito unit is composed of high-P Puerca Gorda and Guineal Schists. Protoliths are basic-intermediate volcanic rocks with IAT, low-Ti IAT and boninitic geochemical signatures, similar in composition to the Lower Cretaceous Caribbean island-arc. The Morrito basal fault-zone thrusted the Puerca Gorda Schists onto the Jagua Clara mélange. The geochemical characteristics of many mafic metaigneous rocks found within the Jagua Clara mélange, and particularly in the Morrito unit, indicate that they occupied a supra-subduction zone position at some time in their history and, therefore, the mélange reflect the position of the Late Cretaceous intra-oceanic subduction zone. The presence in the mélange of exotic blocks plucked from both the upper plate (Caribbean island-arc) and the lower plate (proto-Caribbean Ocean) suggests that the arc crust was eroded at its base by tectonic processes in the Late Cretaceous. Regional structural data indicate that the Morrito basal fault-zone now forms the suture between the Caribbean island-arc terranes and the accreted units of the paleo-continental margin of North America.