GEOCHEMISTRY AND TECTONIC SETTING OF CRETACEOUS (?) METAMORPHIC ROCKS IN THE BLUE MOUNTAIN INLIER, EASTERN JAMAICA

Two separate, fault-bounded sequences of metamorphic rocks (both of presumed Cretaceous age) are exposed in the Blue Mountain inlier of eastern Jamaica. Westphalia Schist is dominated by upper amphibolite facies mica schist and hornblende schist, with presumably sedimentary and volcanic protoliths. Mt. Hibernia Schist consists of fine-grained blueschist and greenshist facies metamorphic rocks, with predominately basaltic protoliths (tuff, ash and flows). While the metamorphic histories of these rocks have been well established, little detail is known about the ages and tectonic settings of their protoliths.

Major and trace element whole-rock geochemical data have been obtained from representative mafic rock samples from each of the sequences. This study focuses on elements considered to be relatively immobile during alteration and metamorphism. The Westphalia samples are geochemically variable with basaltic to andesitic compositions, enrichment in LREEs relative to chondrites (up to 100x), and have pronounced negative Nb, Ta, and Ti anomalies. The Mt. Hibernia samples show little variability, with subalkaline basaltic compositions, essentially flat REE patterns (10-20x chondrites), and no Nb, Ta, or Ti anomalies. Various tectonic discrimination diagrams indicate a strong calc-alkaline subduction-related, island arc environment for the Westphalia protoliths, and indicate a strong MORB environment for the Mt. Hibernia protoliths. Details in the trace element geochemistry of the Mt. Hibernia samples are consistent with eruption (MORB) in an ocean-plateau province (i.e., Caribbean oceanic plateau).

It is clear from the mafic rock geochemistry that the Westphalia schist and Mt. Hibernia schist do not share the same protolith. The Mt. Hibernia metabasalts are however geochemically indistinguishable from nearby ~90 Ma basaltic rocks of the Bath-Dunrobin Formation (also in the Blue Mountain inlier). We suggest that Mt. Hibernia metabasalts may be the metamorphosed equivalent of the Bath-Dunrobin basalts. The Westphalia rocks, with their island arc signatures, may be related to minor island arc tuffs in the Bath-Dunrobin Formation.