Two high-pressure–low-temperature serpentinite-matrix mélange belts, Motagua fault zone, Guatemala: A record of Aptian and Maastrichtian collisions

Abstract

Left-lateral motion along the North American–Caribbean plate boundary has juxtaposed two high-pressure–low-temperature (HP-LT) belts from separate Cretaceous collisions. These two belts have quite different ages and different suites of high-pressure assemblages, yet they both contain jadeitite, a relatively rare rock type. This part of the plate boundary zone follows the Motagua River Valley in Guatemala, where it separates the Maya block (North American plate) from the Chortís block (Caribbean plate). On both sides of the bounding Motagua fault, tectonic slices of serpentinite-matrix mélange host the HP-LT rocks. South of the fault, the mélange slices contain eclogite, lawsonite eclogite, glaucophane eclogite, and blueschist blocks. North of the fault, the mélange slices contain omphacite metabasite, albitite, and garnet amphibolite blocks, but lack intact eclogite. In addition to the dissimilar rock assemblages, ⁴⁰Ar/³⁹Ar geochronology of phengitic micas yields 77–65 Ma for northern and 125–113 Ma for southern blocks. These data suggest that the southern belt formed during Early Cretaceous (Aptian), northeastward-dipping subduction of the Farallon plate and collision of the Chortís block with western Mexico. The block was then displaced southeastward along this suture. In contrast, the northern belt records subduction related to the Maastrichtian collision of an extension of the Chortís block, perhaps the Nicaraguan Rise, with the Maya block.