Pressure-temperature-time path of ultrahigh-pressure metamorphic terranes constrained by zircon petrochronology: A case study of orthogneisses from the northern Sulu belt, China

Reconstruction of pressure-temperature-time (P-T-t) path of volumetrically-dominant orthogneisses in deeply subducted continental crusts is challenging due to their simple amphibolite-facies mineral assemblages. To provide new insights into this issue, a detailed study of mineral inclusions, trace elements and U-Pb ages within robust zircon containers, together with titanite U-Pb dating and petrology were conducted on ultrahigh-pressure (UHP) orthogneisses from the northern Sulu belt. The following P-T-t conditions of three metamorphic stages were constrained: (i) cathodoluminescence (CL)-grey zircon cores (ca. 238-237 Ma) with flat heavy rare earth element (HREE) patterns include garnet, phengite and omphacite that yield estimated P-T conditions of 2.3-2.8 GPa/730-800 degrees C, corresponding to prograde high-pressure (HP) metamorphism before the UHP peak. (ii) CLdark zircon mantles (ca. 224 Ma) contain multiphase solid inclusions of plagioclase +K-feldspar + quartz and show extremely low Th/U ratios (<= 0.08) and negative Eu anomalies, demonstrating their crystallization from hydrous melt at 2.0-1.8 GPa/890-820 degrees C during HP eclogite-facies exhumation. In addition, titanite grains are characterized by high LREE contents (1939-5239 ppm), Th/U ratios (0.37-0.81) and crystallization temperatures (830-801 degrees C), suggesting their peritectic origin. The lower intercept age (220 +/- 4 Ma) of titanite is broadly consistent with the ages of the zircon mantles, indicating that they both crystallized from the melt at the same stage. (iii) CL-bright zircon rims (ca. 212-211 Ma) preserve low-pressure mineral inclusions of plagioclase, Kfeldspar and quartz, and they show low Th/U ratios (0.03-0.12) and steep HREE patterns with negative Eu anomalies, witnessing amphibolite-facies overprinting at 1.0-0.8 GPa/610-550 degrees C. Combined with the metamorphic peak conditions of 3.2 GPa/840 degrees C at ca. 232 Ma constrained by previous studies from the same area, the P-T-t path of the northern Sulu UHP belt is reconstructed. The data reflects two-stage exhumation of this terrane, including an early-stage (5.8 km/m.y.) and a later-stage slower (3.1 km/m.y.) exhumation processes. The faster exhumation during early decompression was probably enhanced by extensive melting activity recorded by the CL-dark zircon mantles from the orthogneisses.