PETROLOGICAL AND RAMAN SPECTROSCOPICAL STUDY OF PROGRADE METAMORPHIC EVOLUTION IN SULU UHP ROCKS

Research on the Sulu terrane of eastern China has provided abundant geological data on ultrahigh-pressure (UHP) rocks, which facilitates understanding of the geodynamic processes occurring in continental collision zones. Although the peak UHP conditions and subsequent decompression path are well constrained in the Sulu terrane, information on the pre-UHP metamorphic stage is scarce. A new approach for verifying the prograde evolution of UHP rocks is required.

Residual pressure is detected as a frequency shift in the Raman spectrum of quartz inclusions in garnet. This technique, known as quartz-Raman barometry, can be a useful tool for determining the prograde pressure conditions of alpha-quartz stability. However, the ability of quartz-Raman barometry to estimate the pressure conditions during the early stages of UHP metamorphism, at the time of alpha-quartz stability, remains inconclusive. In this study, we use combined Raman spectroscopical and petrological approaches to examine UHP eclogites from Yangzhuang region, and document the prograde evolution of the Sulu rocks.

Garnet porphyroblasts have inner and outer segments with the boundary being marked by discontinuous changes in the grossular content. In the inner segment, the SiO₂ phases are alpha-quartz that have retained high residual pressure with no coesite or radial cracks. The metamorphic conditions estimated by calculation from the residual pressure and thermodynamic calculation range from 500 to 630 °C and from 1.3 to 2.3 GPa for the inner segment. Coesite and its pseudomorph occur as inclusions in the outer segment. The occurrence of coesite in the outer segment is consistent with the P–T conditions of 660–725 °C and 3.1 GPa estimated for the peak metamorphic stage by geothermobarometry. Our results suggest that the quartz inclusions in the inner segment were trapped by garnet under alpha-quartz stable conditions and survived phase transition to coesite at the UHP stage. The SiO₂ phases and lower-pressure minerals in the inner segment of garnet indicate that the garnet has sufficiently acted as a pressure vessel, even for the case of silica polymorph in the UHP metamorphism. Therefore, the combined Raman spectroscopical and petrological studies are effective in recognizing the signatures of the lower-pressure metamorphic conditions in UHP rocks.