CLINOPYROXENITE FROM THE SULU ULTRAHIGH-PRESSURE TERRANE, EASTERN CHINA: ORIGIN AND EVOLUTION OF GARNET EXSOLUTION IN CLINOPYROXENE

Garnet clinopyroxenite occurs as small mantle slices faulted into subducted continental crust in the Sulu ultrahigh-pressure (UHP) terrane, China. Two representative garnet clinopyroxenites (garnet + clinopyroxene + ilmenite) with exsolution microstructures were studied. (1) A massive ilmenite-rich specimen is composed of coarse-grained Cpx containing abundant exsolution rods of grossular-rich garnet (>20 vol%) + ilmenite (~3-4 vol%) and fine-grained matrix of Cpx + Grt + Ilm. (2) A megacrystic garnet-bearing sample is characterized by garnet containing inclusions of Cpx and Ilm; the Cpx inclusions show exsolution rods or blebs with variable amounts of Grt (>1-15 vol%) and minor Ilm. Both exsolved and matrix garnets have similar Grs-rich compositions. The aggregate composition of Cpx + Grt + Ilm intergrowths is similar to the whole-rock composition. We propose that the parental phase of Grt + Ilm exsolution + Cpx host was either a homogeneous clinopyroxene (hypothesis A) or a majoritic garnet (hypothesis B) that experienced three discrete evolution stages. If the parent phase was clinopyroxene (ABO3) with a Si deficiency in the B site, the Rizhao garnet clinopyroxenite could have an initial assemblage Cpx ± Grt ± Ilm formed at near-solidus conditions (1400 oC, about 4-5 GPa) in the upper mantle. The second stage is defined by garnet exsolution from the primary clinopyroxene involving decreasing temperature and/or increasing pressure, related to continental subduction. Coexisting clinopyroxene host and garnet exsolution recrystallized at temperatures of ~900 oC at assumed minimum P of 5 GPa. The third stage is represented by recrystallization of exsolution phases to form the fine-grained matrix of Cpx + Grt + Ilm at 700 oC and >=3 GPa during the early exhumation of the UHP terrane. The hypothesis B is an open question at the present time, but if correct would involve an extremely deep origin of the protolith.