Pargasite and ilmenite exsolution texture in clinopyroxenes from Hujialing garnet-pyroxenite, Su-Lu UHP terrane, Central China: a geodynamic implication

Abstract

We report here the discovery of amphibole exsolution texture in the Hujialing garnet-pyroxenite, Su-Lu ultrahigh pressure (UHP) metamorphic terrane. These rocks together with coesite-bearing eclogites were exhumed from mantle depths. The garnet-pyroxenite consists mainly of garnet and diopside with minor pargasitic amphibole, ilmenite, Mg-Al spinel, magnetite, tremolite, orthopyroxene, and olivine. Diopside contains abundant garnet inclusions as well as pargasite and ilmenite lamellae. The present mineral assemblages observed in the Hujialing garnet-pyroxenite indicate that the parental mineral of the reported exsolution texture should have a composition of mixture of pargasite, ilmenite and diopside. Primary clinopyroxene in the Hujialing garnet-pyroxenite contains ∼2000 ppm H 2 O, suggesting a peak metamorphic pressure probably at 8.0 GPa. Prevailing exsolution of pargasite (stage I) and ilmenite (stage II) within clinopyroxenes suggests an at least two-stage exsolution process, which implies a complex exhumation history for the Hujialing garnet-pyroxenite. Texture relationships apparently show that the pargasite lamellae formed earlier than the ilmenite lamellae in diopsides. These observational data implies that the exhumation of these UHP metamorphic rocks were accomplished in two steps, one at depths ∼100 km, another at much shallower levels.