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Abstract: Two competing hypotheses have been proposed for garnet-bearing ultramafic rocks in the Dominican Republic: (1) The ultrahigh pressure (UHP) – ultrahigh temperature (UHT) hypothesis involves a magmatic protolith of mantle origin, which was then delivered to, and incorporated into deep-subducted oceanic crust (eclogite) at UHP conditions (Abbott et al., 2005, 2006, 2007; Abbott and Draper, 2010; Gazel et al., 2011). (2) The low-pressure (LP) hypothesis involves a plagioclase-bearing, arc-related protolith of crustal origin, which was then subducted to UHP conditions (De Hoog, 2011; Hattori et al., 2010a,b). In both hypotheses, the rocks were uplifted to the surface by an as yet poorly understood mechanism. Here we respond to concerns regarding the integrity of REE analyses, Cpx-Grt REE partitioning, other matters related to the interpretation of the trace element data, and Grt-Spl major-element thermometry. We show that none of the concerns precludes a UHP magmatic origin. [Copyright &y& Elsevier]

Abstract: Ultra high pressure (UHP) garnet-bearing ultramafic rocks from the Dominican Republic may represent the only known example where such rocks were exhumed at an ocean–ocean convergent plate boundary, and where the protolith crystallized from a UHP magma (>3.2GPa, >1500°C). This study focuses on the petrology and geochemistry of one of the ultramafic lithologies, the pegmatitic garnet-clinopyroxenite (garnet+clinopyroxene+spinel+corundum+hornblende). Three distinct types of garnet were recognized: Type-1 garnet (low Ca, high Mg) is interpreted as near magmatic (P>3.2GPa, >1500°C). Type-1′ garnet (high Ca, low Mg) is interpreted as having formed approximately isochemically from magmatic high-Al clinopyroxene. Type-2 garnet (intermediate Ca, high Mg, and low Fe+Mn) formed together with hornblende as a result of late, low-pressure retrograde hydration. Clinopyroxene is close to diopside–hedenbergite (Mg# ~88) and metasomatized by arc-related fluids. Spinel and corundum occur as microinclusions in type-1 and type-1′ garnets in the only reported natural occurrence of coexisting garnet+spinel+corundum, indicative of very high pressure. Chondrite-normalized REEs (rare earth elements) of the garnets show humped or weakly sinusoidal patterns, typically associated with garnet inclusions in diamond and garnet in kimberlite that crystallized at UHP conditions. These humped to weakly sinusoidal REE patterns developed as the result of interaction with a light REE-enriched metasomatic fluid. Partitioning of REEs between type-1′ and type-1 garnets is consistent with the former having inherited its REEs from a high-Al clinopyroxene predecessor. The partitioning preserves a record of near-solidus temperatures (~1475°C). Petrology and phase relationships independently suggest near-solidus conditions >1500°C (the highest temperature conditions reported in a UHP orogenic setting), providing evidence for an origin in a mantle plume. Therefore, the Dominican ultramafic rocks may represent the only example of exhumed “fossil fragments” of mantle plume in an orogenic setting (oceanic or continental). [Copyright &y& Elsevier]

Abstract: The Cuaba Gneiss in northern Dominican Republic hosts an unusual suite of Grt-ultramafites. Two hypotheses for their origin are distinguished by the depth of crystallization of an igneous protolith. In the first hypothesis the protolith crystallized under ultra high pressure (UHP) magmatic conditions (>3.2GPa) in the field of stability for Grt+Spl+Crn. The protolith was then modified by subsolidus processes. In a new hypothesis the protolith crystallized under low pressure (LP) magmatic conditions (<1.1GPa) in the field of stability for plagioclase. Grt+Crn was produced during prograde metamorphism. The LP hypothesis depends on a small Eu anomaly, limited fluid interaction, REE modeling, and a magmatic composition for clinopyroxene. Arguments against the LP hypothesis address, (1) the source of the Eu anomaly, (2) the use of clinopyroxene in assessing provenance and modeling REEs, (3) the proposed prograde mineral reactions, and (4) thermodynamic calculations. [Copyright &y& Elsevier]