- © 2016 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
A suite of samples collected mainly from river cobbles in the Mt. Hibernia Complex in SE Jamaica includes metamorphosed basaltic and clastic sedimentary rocks, which are interpreted here as part of a Cretaceous accretionary prism. The metabasic rocks occur as blueschists and greenschists, whereas the metasedimentary rocks are former greywackes s.l., or psammopelites, and can now be characterized as phyllites. Eight characteristic mineral assemblages can be defined within the metabasic sample suite, involving amphibole (Na amphibole and/or actinolite), pumpellyite, lawsonite, epidote, chlorite, titanite, phengite, albite and quartz. Augite is an important relict phase of the magmatic source rocks. White mica, chlorite and quartz predominate in the phyllites in variable proportions, but Na amphibole, epidote, albite and titanite may also be important. Calculated P–T pseudosections show that the assemblage glaucophane–pumpellyite–chlorite occurs only in a restricted field along the lower P–T limit of the blueschist facies. Pseudosections also indicate that peak P–T conditions for the blueschist samples studied here lie in the range 6.5–7.2 kbar and 310–340°C, that is at the upper stability limit of pumpellyite and lawsonite, and in the range 6.4–7.8 kbar and 340–380°C for greenschist samples. The P–T range for phyllites overlaps with both. Thus, blueschists and greenschists represent comparable pressure but different temperature conditions of formation. Literature data on the mapped distribution of metabasic rock types suggests an isobaric metamorphic field gradient in the Mt. Hibernia Complex, with temperatures increasing slightly but significantly from blueschist to greenschist zones. We interpret this gradient as reflecting either increasing temperatures in the accretionary complex away from the subducting slab or thermal re-equilibration in a cooling subduction zone. Sodium-Ca amphiboles, predominantly winchite, are commonly observed as rims around Na amphibole and formed together with albite porphyroblasts during decompression. Comparison with published information on the miscibility gap between Na and Ca amphiboles shows that many of these compositions fall within this gap for temperatures around 350°C. Thus, these amphiboles either grew metastably below the critical temperature of the gap, or some as yet undefined parameter must be responsible for drastically altering the shape of the gap. Published descriptions of deformation in the Mt. Hibernia Complex show that coaxial pure shear initially predominates, as is common in accretionary complexes; however, a late shearing event, postulated here to be caused by the collision between the Great Arc of the Caribbean and the continental margin of North America at 75–70 Ma, is held responsible for the rapid exhumation of the combined blueschist–greenschist sequence.