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

摘要

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) O1 + Cpx + Opx + Mag; (II) Oli + Cpx + Grt (inferred); (III) Oli + Cpx + Grt + Spl; (IV) Cpx + Grt + Spl; (V) Cpx + Grt + Spl + Cm. Phase relationships in the CMAS system constrain the magmatic conditions to P > 3.4 GPa and T >1540 deg C, but thermobarometry shows that the mineral assemblages generally re-equilibrated at subsolidus conditions (approx 850 deg C, approx 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 (approx 1150 deg C to approx 1500 deg 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 astheno-sphere, and better constrain the P-T path from depth to the surface.