Retrograde Reactions of an Ultrahigh-Pressure Metamorphic Spinel Pyroxenite Lens, Northeast Sulu UHP Terrane, Eastern China

摘要

A approx 5 cm thick garnet-rich layer, with relict spinel grains in its center, is intercalated within garnet pyroxenite at Rongcheng. Both the garnetite layer and the host garnet pyroxenite underwent the same P-T evolution, but exhibit quite different mineral assemblages, microtextures, and associated mineral reactions. Microtextures and mineral compositions confirm our previous suggestion that the Rongcheng garnet pyroxenite recrystallized from a spinel pyroxenite cumulate derived from a gabbroic magma. The present study concludes that: (1) The garnet-rich layer resulted from a net transfer reaction between primary spinel and clinopyroxene, Ca_(1.5)(FeMg)_(0.5) Si_2O_6 + (FeMg)Al_2O_4 + TiO_2 = Ca_(1.5)(FeMg)_(1.5)Al_2(Si_2Ti)O_(12), and mass-balance reactions between clinopyroxene end-mem-bers, 2[Ca_(0.75)(FeMg)_(0.25)][(FeMg)_(0.5)Al_(0.5)](Si_(1.5)Al_(0.5))O_6= Ca_(1.5)(FeMg)_(1.5)Al_2Si_3O_(12), under UHP condi-tions that produced garnet-I. (2) Garnet-I decomposed during exhumation of the UHP terrane, leading to the assemblage of Grt-II + Ilm + Cpx-II + Amp; ilmenites were partially released to the relict spinel along micro-fractures to form intergrowths of Spl-I + Ilm. A corona assemblage of Cpx-II + Spl-II + minor Amp developed along contacts between garnet and relict Spl-I at approx 890 deg C and approx 9 kbar. And (3), during a later stage of retrogression, aqueous fluid infiltrated heterogeneously through the garnet-rich layer, leading to the formation of hydrous minerals. Parageneses and mineral compositional maps suggest that Ti may have substituted for Si and entered the T site in the garnet structure, represented by a garnet component such as M_3Al_2[Si_(3-x))Ti_x]O_(12), in addition to the commonly suggested M~(2+)Ti~(4+) - > 2Al~(3+) substitution (forming the garnet component M_(3-0.5x)[Al_(2-x)Ti_x]Si_3O_(12)). Heterogeneous microtextures and mineral compositions indicate that the domain equilibrium was controlled by mass transfer at least at the mineral-grain scale due to various degrees of fluid infiltration.