Two fault-bounded sequences of metamorphic rocks are exposed in the Blue Mountains of eastern Jamaica. Westphalia Schist is dominated by amphibolite facies hornblende schist and mica schist. Mt. Hibernia Schist is dominated by blueschist-greenschist facies metabasalts. New whole-rock geochemistry and 40Ar/39Ar ages clarify the tectonic setting of the protoliths, timing of post-metamorphic cooling, and evolution of the northern margin of the Caribbean plate.
Westphalia Schist is geochemically variable, with mafic igneous protoliths or volcaniclastic sedimentary protoliths. Regardless of the protolith, the trace-element geochemistry is consistent with an island-arc tectonic environment. These rocks most likely represent metamorphosed equivalents of the regionally extensive Early Cretaceous Greater Antilles arc that is preserved discontinuously along the present-day northern margin of the Caribbean plate. Mt. Hibernia Schist shows little geochemical variability, with an igneous protolith of subalkaline basaltic composition. Flat rare-earth-element patterns and flat extended trace-element patterns are consistent with an enriched mid-ocean ridge basalt or oceanic plateau environment. However, in terms of immobile elements, Mt. Hibernia Schist is geochemically indistinguishable from nearby ca. 90 Ma basalt of the Bath-Dunrobin Formation, which is a product of Caribbean plate–forming ocean plateau magmatism; i.e., Caribbean large igneous province. Hence, an ocean plateau environment is inferred for the Mt. Hibernia protolith.
The Westphalia and Mt. Hibernia Schists are currently juxtaposed along the Blue Mountain fault, yet were subjected to very different subduction-related metamorphic histories. Stratigraphic relationships require that the metamorphic rocks were uplifted, and exposed at the surface by the Early Paleocene. 40Ar/39Ar ages indicate that the two units were affected differently by burial metamorphism related to Paleocene–Early Eocene transtensional tectonics. Final juxtaposition of Westphalia Schist and Mt. Hibernia Schist was accomplished through a combination of vertical and horizontal displacements during Neogene transpression along the Plantain Garden fault.