UHP MAGMATIC PARAGENESIS, GARNET PERIDOTITE, CUABA UNIT, RIO SAN JUAN COMPLEX, DOMINICAN REPUBLIC

Alpine-type spinel-bearing garnet peridotite and corundum-bearing variants in the Cuaba unit of the Cretaceous Rio San Juan complex show evidence for ultra high pressure partial melting and magmatic fractionation. The paragenesis involves the following sequence of assemblages (plus inferred melt) with declining T (mineral abbreviations, at end of abstract): (1) Grt + Oli + Dio + Spi + Liq (partial melt assemblage), (2) Grt + Dio + Spi + Liq (cumulate Dio with interstitial Grt), and (3) Grt + Dio + Spi + Cor + Liq (pegmatite, Dio with interstitial Grt and late Cor). Comparison with 3-Gpa liquidus relationships (Milholland and Presnall, 1998) and extrapolation to pressures above which sapphirine is not possible (>3.4 Gpa at ~1570 ^oC) show that assemblage 1 is consistent with the equilibrium, Spi + Dio=Oli + Grt + Liq. Liquid fractionated from this assemblage crystallized in equilibrium with Grt + Dio + Spi (2). Further fractionation resulted in crystallization of Cor according to the equilibrium, Grt + Dio + Cor=Spi + Liq (3). This last equilibrium is only possible at P > 3.4 Gpa and T > 1570 ^oC (no H₂O) or T > 1350 ^oC (H₂O present). The assemblages constrain a short but well-defined liquid line of descent, which terminated in the first reported natural occurrence of the assemblage, Grt + Dio + Spi + Cor. (NOTE: Neither sapphirine nor plagioclase is involved in any assemblage). The inferred conditions of T (with or without H₂O) are much higher than previous estimates that did not take into account a melt. Previously estimated conditions (P=2.8-3.4 Gpa, T=740-810 ^oC) are presumed to reflect subsolidus re-equilibration. 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. (Grt=garnet, Oli=olivine, Dio=diopside, Spi=spinel, Cor=corundum, Liq=melt).