Trevor A. Jackson

The Miocene palaeobathymetry and palaeoenvironments of Carriacou, the Grenadines, Lesser Antilles

Carriacou, a small island in the Grenadines, Lesser Antilles, has a Cenozoic rock record that has been important in interpreting the geologic history of the Southern Lesser Antilles Arc Platform. The Lower-Middle Miocene sedimentary succession of the southeast and east coasts, consisting of the Belmont, Kendeace, Carriacou and Grand Bay formations, has been interpreted as a shallowing-upward sequence from turbidite basin to nearshore?/beach? palaeoenvironments. An earlier interpretation of the Belmont Formation as having been deposited in shallow water is at variance with the turbiditic nature of the succession; the included fossils are considered allochthonous. However, an interpretation of the Grand Bay Formation as deep water is supported by multiple lines of evidence, including sedimentology (turbidites), ichnology (autochthonous association of burrows typical of deep-water environments) and palaeontology (terrestrial, planktic, and shallow and deep water benthic species mixed together). The minimum depth of deposition of the Grand Bay Formation was 150–200 m. This suggests that the (unseen) contact between the Carriacou and Grand Bay formations is either an unconformity, formed following rapid deepening of the basin, or a fault, the Grand Bay Formation being deposited in a separate basin from the shallowing-upwards Belmont-Kendeace-Carriacou formations, against which it is now juxtaposed

Blueschist-Greenschist Transition in the Mt. Hibernia Schist, Union Hill, Parish of St. Thomas, Jamaica

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+Fe2O3 equilibria, here generalized: Gln + Epi + Qtz + H2O = Act + Chl + Alb (2); Pmp = Epi + H2O (3); and Gln + Pmp + Qtz = Act + Epi + Chl + Alb + H2O (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.

The tectonic significance of basalts and dacites in the Wagwater Belt, Jamaica

The Wagwater Belt is a fault-bounded structural unit in which more than 3000 m of early Tertiary sedimentary and volcanic rocks are exposed. Geochemical analyses of the volcanic rocks show that they comprise a bimodal suite of plateau-type tholeiitic basalts and calcalkaline dacites. The extrusion of these volcanics is associated with the development of an interarc basin, the Wagwater Basin, at the beginning of the Cenozoic Era. The Wagwater Basin formed as a result of the splitting of a mature late Cretaceous volcanic arc into a frontal and third arc represented by the Blue Mountain Massif and the Clarendon Block respectively. This model for Jamaica can be correlated with tectonic events occurring in the NW Caribbean during the early Tertiary. The creation of the Wagwater Basin and the eruption of the basalts is related to the initial opening of the Cayman Trough. The cessation of dacite volcanic activity in Jamaica signified the separation of the Caribbean Plate from the East Pacific Farallon Plate.

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

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...

Petrology and Inferred Tectonic Setting of the Mountain Pine Ridge Granitoids, Maya Mountains, Belize

The Mountain Pine Ridge batholith is the largest of three major granitoid intrusions that crop out in the Maya Mountains of Belize. These plutons intrude a thick sequence of Upper Paleozoic metasedimentary and metavolcanic rocks known as the Santa Rosa Group. The mineralogy of the granitic rocks at Mountain Pine Ridge varies and the rocks are classified into four petrographic types—biotite leucogranite, muscovite leucogranite, granodiorite, and tonalite. The essential felsic minerals in these rocks include quartz, microcline, plagioclase feldspar ± perthite, and muscovite, whereas the only mafic mineral is biotite. Microprobe analysis shows that significant differences exist between the biotites of the different petrographic types, especially in the levels of Al(vi), Mn, Mg, total Fe, and the Fe/Fe+Mg ratio. Major, minor and trace-element chemistry of the four petrographic types indicates that the Mountain Pine Ridge batholith is composed of three different suites of granitic rocks. Although all three sui...

The volcaniclastic turbidites of the Grand Bay Formation, Carriacou, Grenadines, Lesser Antilles

Abstract. The Middle Miocene Grand Bay Formation is exposed on the eastern half of the island of Carriacou, Grenada Grenadines. The formation was deposited in water depths of greater than 150 m and is mainly composed of a sequence of bioclastic and volcanogenic turbidites. The dominant rock type in the Grand Bay Formation is fine- to coarse-grained sandstones. The volcaniclastic sandstones are poorly sorted and immature, and contain volcanic clasts, clinopyroxene, amphibole, plagioclase feldspar and opaque crystals in an argillaceous matrix. Within the Grand Bay Formation are beds of subaequeous lapilli and ash tuff representing pyroclastic fall deposits that have a mineralogy that is similar to the heavy minerals in the turbidites. The mineral assemblage of the lapilli tuffs suggests that these eruptions were basaltic andesite or andesite in composition. It is postulated that the volcanogenic turbidites in the Grand Bay Formation formed as a consequence of volcanic eruptions along the southern Lesser Antilles arc during the Early to Middle Miocene. The lithic volcanic fragments and mineral composition of the volcaniclastic turbidites indicate a monomagmatic source in which reworked pyroclastic deposits were temporarily stored on a shallow shelf prior to deposition downslope, in a deep water basin.

THE SERPENTINIZATION OF PERIDOTITE FROM CEDAR VALLEY, JAMAICA

Previously undescribed, microscopic textures in partially serpentinized peridotite shed light on the process of serpentinization. Fracturing and fragmentation of the peridotite protolith produced a modest increase in volume and opened channels to reactive aqueous fluids. Prior to, or in conjunction with, the first appearance of serpentine, micrometer-scale grains of magnetite crystallized on the surfaces of fractures. This decoration of fracture surfaces faithfully preserved the original, angular outlines of fragments of forsterite and pyroxene through all stages of serpentinization. Simple geometric considerations indicate that the volume of a given fragment did not change during replacement. Partially serpentinized forsterite shows a penetrative, micrometer-scale, sieve or sponge texture. Fifty percent or more of the volume of optically continuous forsterite can consist of micrometer-scale perforations, filled with serpentine. This sponge texture has not been described elsewhere. The delicate connection...

A starfish bed in the Middle Miocene Grand Bay Formation of Carriacou, The Grenadines (West Indies)

A starfish bed in the Middle Miocene Grand Bay Formation of Carriacou, The Grenadines (West Indies)

A review of volcanic island evolution and magma production rate: an example from a Cenozoic island arc in the Caribbean

Different stages of volcanic island evolution are evident in the Cenozoic Lesser Antilles arc developed near the eastern edge of the Caribbean Plate. The early stage of seamount evolution is represented by the Kick-em-Jenny submarine volcano near the southern end of the arc. The volcanically active islands known as the Volcanic Caribbées are composed of a series of composite volcanoes and are examples of an emergent volcanic island stage. Those islands that were formerly active, known as the Limestone Caribbées, provide evidence that an erosional to submergent stage occurred in the northern part of the island arc during the Oligocene and Miocene epochs. Growth of the islands and underlying crust is dependent upon the magma production rate along the arc, which has for the last 100 ka ranged from less than 1 km3 to 40 km3. These differences in magma production and volcanic rock composition along the arc are attributed to the obliquity of plate convergence and the plate convergence rates.