Metamorphic Conditions in the Westphalia Schists of the Blue Mountain Inlier, Jamaica: Tectonic Implications

Abstract

Two groups of metamorphic rocks—the Westphalia Schists and Mt. Hibernia Schists—occur as fault-bounded blocks on the southwestern side of the Blue Mountain Inlier in eastern Jamaica. The metamorphosed sediments and volcanic rocks are part of an Early Cretaceous accretionary complex. The Mt. Hibernia Schists reflect Sanbagawa facies-series metamorphism. Metamorphic conditions of the Westphalia Schists are less well known. In Westphalia metabasites, two metamorphic zones are recognizable—a low-T zone (I) is related to a high-T zone (II) by the reaction bio + epi = gar + hnb. The diagnostic assemblage in pelitic rocks is the same, gar + bio + chl, throughout the Westphalia Schists. These observations indicate high pressure within the amphibolite facies.

In the Westphalia Schists, temperature estimates based on Fe-Mg exchange in gar-hnb are 580 to 650° C; estimates based on Fe-Mg exchange in garbio are lower—460 to 540° C. Pressure estimates for gar-hnb-plg equilibria are 6.2 to 8.6 kbar. Although temperature estimates for the Mt. Hibernia Schists (270 to 450° C) are lower than those for the Westphalia Schists, pressure estimates (5.5 to 9.0 kbar) are comparable. This indicates that the Westphalia Schists may be a high-T facies within the same Sanbagawa facies series as the Mt. Hibernia Schists. The estimated conditions, K-Ar radiometric ages on hornblende and biotite (76.5 Ma and 48.8 to 52.9 Ma, respectively) (Lewis et al., 1973), and textural relationships indicate a retrograde P-T-t path. Projected timing for the highest estimated P and T (for the hnb-gar-plg equilibrium) is approximately 85 Ma. Thus uplift began prior to this time—i.e., prior to the Campanian. Retrograde metamorphism may have commenced with a proposed Aptian-Albian orogenic event (Pindell, 1994). This supports and clarifies the supposition that the Westphalia Schists and Mt. Hibernia Schists are among the oldest rocks in Jamaica (Robinson, 1994).