Geochemical Characteristics, UHP Metamorphic Age, and Genesis of the Hujialing Garnet Clinopyroxenite, Snlu Terrane, China

摘要

The bulk composition of the Hujialing garnet clinopyroxenite from Rizhao, Sulu ultrahigh-pressure (UHP) metamorphic belt, is rich in fertile and low in refractory elements compared with primary mantle. This composition is characterized by high MgO (12.77-18.73 wt percent) and CaO/Al_2O_3 (1.6-8.0), and low alkali and Mg~# (74.8-86.1), suggesting that the rock has an affinity to alumina-depleted komatiite. The REE pattern resembles that of clinopyroxene with relative HREE depletion of (Gd/Yb)_N = 3.51-5.87. The PGE concentrations lie in the range of 19.18-59.44 ppb, higher than that of mantle xenolith and Alpine type peridotites, with (Pd/Ir)_N of 8.58-11.07. These chemical data indicate that the magma that recrystallized as garnet clinopyroxenite was formed mainly by melting of clinopyroxene from a mantle source containing residual garnet. The high concentration and positive correlation between PGE abundances and atomic number show that their distribution was controlled by a high degree of partial melting in the absence of sulfide segregation. A zircon U-Pb age of 226.3 + - 1.8 Ma represents the time of UHP metamorphism of the garnet clinopyroxenite, which is coincident with 220-240 Ma regional UHP events for the Dabie-Sulu terrane.