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

Abstract

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.