Tracing the Boundary between UHP and HP Metamorphic Belts in the Southwestern Sulu Terrane, Eastern China: Evidence from Mineral Inclusions in Zircons from Metamorphic Rocks

Abstract

The southwestern Sulu terrane, eastern China is divided into four fault-bounded lithologic units. From northwest to southeast, they are: (1) orthogneiss unit (Unit I); (2) supracrustal rock unit (Unit II); (3) kyanite-bearing quartzite—marble unit (Unit III); and (4) paragneiss—schist unit (Unit IV). Inclusions of index minerals in zircon separates from more than 90 samples from these units were identified using laser Raman spectroscopy and electron microprobe analysis. Coesite and coesitebearing ultrahigh-pressure (UHP) mineral assemblages occur in zircon separates from various amphibolite-facies metamorphic rocks in Units I and II. In contrast, aragonite- and phengite-bearing high-pressure (HP) mineral assemblages are preserved in zircons from many amphibolite-to greenschist-facies metamorphic rocks in Units III and IV. These together with previous studies yield P-T estimates of peak metamorphic recrystallization at 723°-852°C, and P > 2.8 GPa for UHP rocks, and 500°-600°C, and P = 1.2-2.5 GPa for HP rocks. The spatial distribution of these P-T estimates and recent surface mapping results constrain an exact boundary between UHP and HP belts in the southern Sulu terrane along a ductile shear zone in the Donghai region.