Richard N. Abbott

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.

Internal structures in part of the South Mountain batholith, Nova Scotia, Canada

The Devonian South Mountain batholith (SMB) in southern Nova Scotia provides an opportunity to examine the deformation produced in a magma during emplacement. The SMB was emplaced in three principal stages, from oldest to youngest, (1) granodiorite, (2) monzogranite, and (3) late granite porphyry, aplite, and pegmatite. This investigation focuses on the monzogranite that underlies the eastern end of the batholith, approximately 15 km south of the city of Halifax. The deformation consists of various types of mesoscopic and macroscopic flow folds, defined by mapping the preferred orientation of alkali feldspar megacrysts. The fold geometry is similar to that produced during melt-dominated (>30% liquid) flow in migmatites. The deformation did not affect the envelope of Cambrian-Ordovician metasedimentary rocks of the Meguma Group; hence, the stresses that caused the folding were internal. The response to the stresses was by ductile deformation (folding) and only locally by fracturing (schlieren, megacrystic dikes) during the late stages of consolidation. The magma behaved as a Bingham substance during emplacement, alternately plastic or brittle, depending on the stress.

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.

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.

Metamorphic conditions in the Ashe Metamorphic Suite, North Carolina Blue Ridge

Taconian metamorphism of mafic rocks in the Ashe Metamorphic Suite can be characterized by reference to an isograd corresponding to the reaction bio + epi = hbl + gar, which separates rocks into two zones of low-variance assemblages. Temperatures and pressures estimated from mineral exchange geothermometers and a barometer suggest that this reaction occurred at approximately 600-650 °C and 7.5 kbar. Phase equilibria between biotite and hornblende, as well as the sharpness of the mapped isograd, indicate that the reaction is discontinuous. Inferred differences in metamorphic grade between Ashe amphibolites and mafic dikes in the underlying basement suggest that these units are in faulted contact. Isograd patterns in pelitic rocks suggest an elongated domal uplift that developed after metamorphism and thrusting, the core of which is exposed in the adjacent Grandfather Mountain window.

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.

Protolith provenance and thermotectonic history of metamorphic rocks in eastern Jamaica: Evolution of a transform plate boundary

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 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...

The Case for UHP Conditions in the Cuaba Terrane, Río San Juan Metamorphic Complex, Dominican Republic

From the Cuaba terrane in northern Dominican Republic. Ultrahigh pressure (UHP) conditions are indicated for the Cuaba terrane on the basis of phase relationships in garnet-bearing ultramafic rock. Dikes and orthocumulate textures indicate a magmatic origin. Mineral assemblages define a line of descent controlled by fractional crystallization. The original estimate of the magmatic conditions (P>3.4GPa, T>1550°C) was inferred previously from available high-P melting experiments in the CMAS system and high-P experimental determination of the sapphirine-out reaction in the MAS system. Revised estimates of magmatic conditions (P>3.2GPa, T>1500°C) take into account the influence of other components, especially Fe. We propose an origin in the mantle-wedge above a subduction zone. The rock was delivered to the subduction zone by forced convection in the mantle wedge (corner-flow), coupled with erosion of the hanging wall. Thermobarometry indicates >850°C and >3.4GPa when the ultramafic rock was incorporated into eclogite (deep-subducted oceanic crust). Evidence for UHP conditions in the retrograded eclogite is not obvious. Two types of symplectic intergrowths, plagioclase + clinopyroxene (Sym-I) and plagioclase + epidote (Sym-II), are interpreted as the products of the decomposition of two types of omphacite, Omp-I and Omp-II. Theoretically, Omp-II formed as the result of a retrograde reaction of the form, Omp-II + coesite = Omp-I + kyanite + /- garnet, according to which the maximum pressure for Omp-II is between ~2.8GPa (~850°C) and ~4.2GPa (~950°C), consistent with subsolidus conditions for the garnet-bearing ultramafic rocks. For eclogite, the highest-pressure mineral assemblage would have been Omp-I + kyanite + garnet + coesite.