Cretaceous (possibly older) metamorphic rock occurs mainly in the Blue Mountain inlier in eastern Jamaica. Fault-bounded blocks reveal two styles of metamorphism, Westphalia Schist (upper amphibolite facies) and Mt. Hibernia Schist (blueschist (BS)-greenschist (GS) facies). Both Westphalia Schist and Mt. Hibernia Schist preserve detailed records of retrograde P-T paths. The paths are independent, but consistent with different parts of the type-Sanbagawa metamorphic facies series in Japan. For each path, phase relationships and estimated P-T conditions support a two-stage P-T history involving residence at depth, followed by rapid uplift and cooling. Conditions of residence vary depending on the level in a tectonic block. For the critical mineral reaction (isograd) in Westphalia Schist, conditions were P $7.5 kbars, T $6008C (upper amphibolite facies). Retrograde conditions in Hibernia Schist were P ¼ 2.6-3.0 kbars, T ¼ 219-2378C for a(H 2 O) ¼ 0.8-1.0 (GS facies). Mt. Hibernia Schist may represent a volume of rock that was separated and uplifted at an early time from an otherwise protracted P-T path of the sort that produced the Westphalia Schist. Reset K-Ar ages for hornblende and biotite indicate only that retrograde metamorphism of Westphalia Schist took place prior to 76.5 Ma (pre-Campanian). Uplift may have commenced with an Albian-Aptian ($112 Ma) orogenic event.
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 40 Ar/ 39 Ar 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 mafi c igneous protoliths or vol caniclastic 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 Carib bean plate. Mt. Hibernia Schist shows little geochemical variability, with an igneous protolith of subalkaline basaltic composition. Flat rare-earth-element patterns and fl at 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 Paleo cene. 40 Ar/ 39 Ar 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.
The Mt. Hibernia Schist occurs in 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 metamorphism reflects a Sanbagawa facies series, wherein two facies, blueschist and greenschist, and transitional schist are recognized. The typical assemblages are: blueschist: Gln + Pmp + [Epi] + Act + Chl + Qtz + Alb + Cpx; transitional: Gln + Pmp + Epi + Act + Chl + Qtz + Alb + Cpx; greenschist: Epi + Act + Chl + Qtz + Alb + Cpx, where only traces of epidote (square brackets) occur in the blueschist assemblage. The clinopyroxene is relict in all assemblages. P-T conditions for the blueschist and transitional schist were estimated by multiple equilibria analysis involving mineral components in NCMASH. Estimated P-T conditions for the blueschist are 5.1-6.2 kbars and 290-366°C. Estimated P-T conditions for the transitional schist are 2.6-3.5 kbar and 235-240°C.Textural relationships indicate a retrograde paragenesis (blueschist to greenschist), which can be summarized by a reaction of the general form: Gln + Pmp + Cpx + Qtz = Act + Epi + Chl + Alb (1). Details of the transition are explained by three NCMASH+Fe 2 O 3 equilibria, here generalized: Gln + Epi + Qtz + H 2 O = Act + Chl + Alb (2); Pmp = Epi + H 2 O (3); and Gln + Pmp + Qtz = Act + Epi + Chl + Alb + H 2 O (4). The highest P-T conditions are represented by the blueschist assemblage, Gln + Pmp + Act + Chl + Qtz + Alb + (relict Cpx). Upon decompression and cooling, conditions meet reaction (4). The equilibrium accounts for the transitional assemblage, Gln + Pmp + Epi + Act + Chl + Qtz + Alb + (Cpx), marked by the presence of epidote. Viewed as a reaction, equilibrium (4) ceases when either the glaucophane or the pumpellyite is used up, resulting in two possible assemblages-Act + Epi + Pmp + Chl + Alb + Qtz or Gln + Act + Epi + Chl + Alb + Qtz-neither of which is the greenschist assemblage. The final greenschist assemblage comes about as the result of the elimination of any remaining glaucophane or pumpelleyite by reactions (2) and (3), respectively. We propose a two-stage retrograde P-T-time path, involving (1) prolonged residence at a particular depth, consistent with a pressure of about 5 kbar for the transitional schist, followed by (2) rapid uplift and exhumation.
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