Grenville Draper

Thrust emplacement of the Hispaniola peridotite belt: Orogenic expression of the mid-Cretaceous Caribbean arc polarity reversal?

Articulo sobre el emplazamiento fallero en la zona de peridotitas de la Hispaniola (Republica Dominicana), estudiando si es la expresion orogenica de la polaridad reversa en la zona del Caribe en el cretacico medio.

Denudation Rates, Thermal Evolution, and Preservation of Blueschist Terrains

The erosion of low temperature, high pressure blueschist facies metamorphic rocks presents a number of problems. If their rate of erosion is not sufficiently rapid, then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages. The problem has been investigated using a one dimensional, two layer model to mimic an accretionary prism overlying oceanic lithosphere. The results suggest that, if no erosion takes place, the maximum time that blueschist conditions may exist is about 26 m.y. if levels of radioactive self heating are high, and approximately 100 m.y. if levels are very low. If erosion takes place, the maximum temperature reached by an element of rock is at a higher level in the earths crust than its initial depth of burial. Rocks may re-equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved. This could result in the geothermal profile of metamorphic rocks being concave to the temperature axis, in contrast to the convex shape of the original profile. The minimum erosion rate which would produce no overprint on original assemblages is estimated by us to be about 0.04 cm/year when radioactive self heating is negligible and rises to 0.14 cm/year for higher levels of self heating. Variations of erosion rates during unloading result in different thermal histories. An initial rapid rate of erosion which then slows is less likely to result in an overprint than the converse. Examination of estimates of erosion rates for the present suggests that normal fluvio-glacial erosion rates may be inadequate to successfully exhume blueschist assemblages without overprinting. Unloading may thus be tectonic or quasi-tectonic. This may explain the common association of blueschist blocks within olistostrome and melange like bodies.

Blueschists and associated rocks in eastern Jamaica and their significance for Cretaceous plate-margin development in the northern Caribbean

Blueschists and associated low-grade greenschists and amphibolites occur in the Blue Mountains of eastern Jamaica and are found in proximity to volcanic-arc–generated rocks. In the blueschist and greenschist rocks, known as the “Mount Hibernia Schists,” metamorphosed basalts, gabbro, and peridotites are found in a structurally disrupted region along with meta-cherts, marbles, meta-conglomerates, and meta-epiclastic rocks. The amphibolite-facies rocks, known as the “Westphalia Schists,” occur in a belt adjacent to the Mount Hibernia Schists and consist of banded mafic to quartzofeldspathic schists. A small outcrop of similar amphibolites also occurs in Green Bay, several kilometres southwest of the Blue Mountains. The metaigneous rocks of the Mount Hibernia Schists have tholeiitic affinities, but the Westphalia Schists seem to be more calc-alkaline. The Mount Hibernia blueschists contain minerals typical of high-pressure–low-temperature terranes, including crossite, riebeckite, pumpellyite, and stilpnomelane. The Mount Hibernia greenschists are low-grade, chlorite-zone rocks lacking biotite. In contrast, the Westphalia amphibolites typically contain the higher-grade minerals hornblende, biotite, and plagioclase. Both groups of schists have a low-grade greenschist overprint characterized by epidote, chlorite, and chloritoid. The lithological associations, chemical composition, and metamorphism of these rocks strongly suggest that they were formed in the accretionary prism or subduction complex of the Jamaica–Nicaragua Rise island-arc system. The present geological setting and consideration of their thermal evolution indicate that these rocks probably recrystallized in Early Cretaceous time. The metamorphic rocks are now juxtaposed against a Campanian ophiolite body and are found in close association with Upper Cretaceous andesites and granodiorites. A model for the genesis of the Jamaican Schist terrane describes the formation and subsequent erosion of these rocks in a pre-Campanian, accretionary complex. The tectonic regime changed in Campanian time and involved the obduction of the ophiolite body and a 150-km outward stepping of the site of subduction, and hence of volcanism. This latter change resulted in the later Cretaceous arc complex being superimposed on the Lower Cretaceous accretionary fore-arc complex. Tertiary, left-lateral transcurrent faulting and consequent uplift and erosion exposed the present blueschist-ophiolite-magmatic arc complex.

P-T Path for Ultrahigh-Pressure Garnet Ultramafic Rocks of the Cuaba Gneiss, Rio San Juan Complex, Dominican Republic

Ultrahigh-pressure (UHP) rocks in the Cuaba Gneiss include Grt ultramafic rocks, mafic eclogite, and partially retrograded equivalents. The Grt ultramafic rocks (Spl-bearing Grt peridotite, Splbearing Grt clinopyroxenite, Crn-Spl-bearing Grt clinopyroxenite) are of igneous origin, with magmatic conditions of P > 3.5 GPa, T > 1550°C. The magmatic history took place in the asthenosphere. New chemical analyses of minerals give the following subsolidus conditions: Grt peridotite, 3.0-4.2 GPa, 838-867°C; Grt clinopyroxenite (Grt + Cpx + Spl + Crn), 2.75 GPa, 807°C. The ultramafic rocks are associated with partially retrograded eclogite, interpreted as deep-subducted oceanic crust. New chemical analyses of minerals in the eclogite give conditions that relate to the retrograde decomposition of Grt + Omp + Qtz to Grt + Pl + Di + Qtz, 1.8 GPa, 730°C. The P-T path for the Grt ultramafic rocks is modeled in three parts: (1) slow, isobaric (> 4 GPa) cooling in the mantle, >1550°C down to ∼850°C; (2) relatively rapid, nearly adiabatic (?) decompression, ∼4 GPa (∼850°C) down to ∼1 GPa (∼700°C); and (3) relatively rapid, non-adiabatic decompression and cooling. The first part of the path (1) took place in the mantle above the subduction zone and relates to delivery of the Grt ultramafic rocks to the subduction zone. Incorporation of Grt ultramafic rocks in the deep-subducted oceanic crust (eclogite) marked the end of this part of the path. The second part of the path (2) was associated with transport up the subduction zone. Retrograde P-T conditions for the eclogite fall on this part of the path, supporting the idea that the Grt ultramafic rocks were transported as blocks in the eclogite. The third part of the path (3) relates to final uplift to the surface.

UHP Magma Paragenesis, Garnet Peridotite, and Garnet Clinopyroxenite: An Example from the Dominican Republic

Spinel-bearing garnet peridotite and corundum-bearing variants in the Cuaba Gneiss of the Cretaceous Rio San Juan Complex show evidence for ultrahigh pressure (UHP) partial melting and magmatic fractionation (orthocumulate textures). The paragenesis involves the following sequence of assemblages (plus inferred melt) with declining T: (1) Grt + Ol + Spl + Cpx + Liq (partial melt assemblage); (2) Grt + Spl + Cpx + Liq (cumulate Cpx with interstitial Grt); (3) Grt + Spl + Crn + Cpx + Liq (pegmatite, Cpx with interstitial Grt and late Crn). Comparison with 3 GPa liquidus relationships in CMAS (Milholland and Presnall, 1998) and extrapolation to pressures above which sapphirine is not possible (> 3.4 GPa at ~1570°C) show that assemblage (1) is consistent with the equilibrium Spl + Cpx = Ol + Grt + Liq. Liquid fractionated from this assemblage crystallized in equilibrium with Grt + Cpx + Spl (2). Further fractionation resulted in the crystallization of Crn according to the equilibrium, Cpx + Grt + Crn = Spl + Liq (3). This last reaction is only possible at P > 3.4 GPa and T > 1550°C. The assemblages constrain a short but well-defined liquid line of descent. The inferred conditions of T are much higher than previous estimates that did not take melt into account. Previously estimated conditions (P = 2.8-3.4 GPa, T = 740-810°C) are presumed to reflect subsolidus reequilibration. Evidently, the rocks originated in the deepest part of the lithosphere, or shallowest part of the asthenosphere, and cooled more or less isobarically as they were delivered to the subduction zone, prior to ascent.

UHP Magma Paragenesis Revisited, Olivine Clinopyroxenite and Garnet-Bearing Ultramafic Rocks from the Cuaba Gneiss, Rio San Juan Complex, Dominican Republic

Narrow (1-4 cm) dikes of spinel-bearing clinopyroxene garnetite intruded olivine clinopyroxenite. The latter represents the earliest magmatic rocks yet discovered on a previously described liquid line of descent for a suite of magmatic rocks from the UHP terrane of northern Hispaniola. The extended liquid line of descent produced the following mineral assemblages, from high to low temperature: (I) Ol + Cpx + Opx + Mag; (II) Oli + Cpx + Grt (inferred); (III) Oli + Cpx + Grt + Spl; (IV) Cpx + Grt + Spl; (V) Cpx + Grt + Spl + Crn. Phase relationships in the CMAS system constrain the magmatic conditions to P > 3.4 GPa and T >1540°C, but thermobarometry shows that the mineral assemblages generally re-equilibrated at subsolidus conditions (~850°C, ~4.2 GPa). The subsolidus temperatures reflect Mg-Fe adjustments affecting to a greater extent olivine, clinopyroxene, and orthopyroxene, and to a lesser extent garnet and spinel. Mg-Fe partitioning between Grt and Spl in olivine-bearing assemblages (i.e., assemblage III) is consistent with near-magmatic temperatures (~1150°C to ~1500°C). Mg-Fe partitioning between Liq and Spl for the same assemblage (III) is consistent with Mg#(Liq) 31-36, a result of protracted fractional crystallization at depth. The extended paragenesis and newly determined conditions support a magmatic origin in the asthenosphere, and better constrain the P-T path from depth to the surface.

Optical anisotropy of the human cornea determined with a polarizing microscope

We have investigated the optical anisotropy of the human cornea using a polarizing microscope normally used for optical mineralogy studies. The central part of the cornea was removed from 14 eyes (seven donors). With the sample placed on the microscope stage, we consistently observed hyperbolic isogyres characteristic of a negative biaxial material. The angle between the optic axes, generally similar in both eyes, ranged from 12 degrees to 40 degrees (mean+/-SD=31 degrees +/-8 degrees ). The optic axial plane always inclined downward in the nasal direction at 1 degrees -45 degrees below the horizontal (mean+/-SD=22+/-13 degrees ). The retardance produced by the corneas was estimated to be less than 200 nm. In conclusion, the human cornea possesses the anisotropy of a negative biaxial material. Both the angle between the optic axes and the retardance were fairly constant among the majority of samples, suggestive of uniformity in corneal structure.

Late Jurassic Oceanic Crust and Upper Cretaceous Caribbean Plateau Picritic Basalts Exposed in the Duarte Igneous Complex, Hispaniola: A Discussion

Lapierre et al. (1999) have reported new chemical data on the composition of the Duarte complex and two Ar/Ar ages on amphiboles, from which they conclude that at least part of the Duarte complex is part of the Caribbean-Columbian-Cretaceous Igneous Province (CCCIP) of Kerr et al. (1997). They also conclude that emplacement of the central Hispaniolan ophiolite (CHO) took place in the Late Cretaceous. We would like to highlight some aspects of the regional geology at odds with Lapierre et al.’s conclusions and offer an alternative interpretation of their results. We argue that the age of the Duarte complex is Late Jurassic—or, at youngest, Early Cretaceous—and that Lapierre et al.’s 86–87-Ma ages are the result of the thermal effects associated with granitoid intrusion and do not represent primary igneous crystallization of the Duarte complex.

Petrogenesis of UHP Eclogite from the Cuaba Gneiss, Rio San Juan Complex, Dominican Republic

Ultrahigh-pressure (UHP) conditions were established for the Cuaba Gneiss terrane, northern Dominican Republic, on the basis of phase relationships in garnet-bearing ultramafic rock, a relatively minor constituent of the terrane. Evidence for much more abundant eclogite comes in the form of two types of symplectic intergrowths involving, respectively, Pl + Cpx (Sym-I) and Pl + Ep (Sym-II), interpreted as the products of the decomposition of two types of omphacite, respectively, Omp-I and Omp-II. Sym-II (hence Omp-II) forms mantles on garnet and aggregates of epidote. Otherwise, estimated P-T conditions for the mineral assemblage Grt + Pl + Cpx + Ep + Hbl + Qtz are consistent with upper amphibolite-facies (P ~1.2 GPa, T ~750°C). Thus far, the retrograded eclogite has revealed nothing diagnostic of UHP conditions—e.g., coesite, evidence for coesite (radiating micro cracks in garnet, palisade quartz, etc.), or micro inclusions of diamond. However, by inference from the P-T history of the garnet ultramafic rock, the eclogite must have encountered conditions on the order of P ~4.2 GPa and T ~850°C (within the fields of stability for coesite and diamond), in order for the latter to have incorporated blocks of the former. Compositions for the original omphacite, Omp-I and Omp-II, were reintegrated from Sym-I and Sym-II respectively, using linear algebraic methods. Stoichiometric arguments show that much of the retrograde epidote was derived from kyanite. Omp-II formed as the result of a reaction of the form Omp-II + Coe = Omp-I + Grt + Ky, according to which the maximum estimated pressure for Omp-II is between ~2.8 GPa (~850°C) and ~4.2 GPa (~950°C), depending on the compositions of Omp-I and Omp-II. Therefore, the highest pressure mineral assemblage was Omp-I + Ky + Grt ± Coe. The conditions are consistent with previously estimated conditions for the decompression part of the P-T path for garnet-bearing ultramafic rock. Evidently, deep-subducted ocean-floor basalt (eclogite) was delivered to the surface from depths exceeding 85-125 km.

Garnet peridotite found in the Greater Antilles

Although Alpine peridotites are relatively common in collisional orogenic zones, garnet-bearing peridotites are rare and only associated with high pressure/ultra-high pressure or temperature (HP/UHP or T) terranes [Brueckner and Medaris, 2000; Medaris, 1999]. Until recently all reported occurrences of Alpine-type garnet peridotites and HP/UHP terranes were in Eurasia and Africa, with one occurrence in the Seward Peninsula, Alaska [Till, 1981;Lieberman and Till, 1987]. Now a new Alpine-type garnet peridotite locality has been discovered in the Caribbean island of Hispaniola. This discovery is the second of its kind in the Americas.