Petrological and geochronological constraints on the origin of HP and UHP kyanite-quartzites from the Sulu orogen, Eastern China

Abstract

Kyanite (Ky)-quartzites occur in both the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belts in the southern Sulu orogen. The HP Ky-quartzites consist of quartz, kyanite and minor rutile with or without topaz and phengite, whereas those from the UHP unit consist of quartz, kyanite, phengite and rutile. The HP Ky-quartzites are characterized by high Al 2O 3 (up to 32.9 wt.%) and low SiO 2 (down to 60.4 wt.%) with very low other oxides contents (<1 wt.%); whereas the UHP Ky-quartzites are characterized by high SiO 2 (77.7–78.4 wt.%) and relatively low Al 2O 3 (19.1–19.5 wt.%). Both HP and UHP quartzites have variable trace element concentrations, but show distinctly negative Nb and Ta anomalies, and positive Zr anomalies. Rare earth element (REE) patterns for both types of quartzites show LREE enrichment and HREE depletion without Eu anomalies. Zircons from HP quartzites have inherited magmatic cores and metamorphic rims; most zircon cores show oscillatory zoning, contain low-pressure (LP) mineral inclusions of Kfs, Pl and Qz, and have relatively high REE contents (av. 1236.6 ppm) and high Th/U ratios (av. 1.57). The metamorphic rims show patchy zoning, contain HP inclusions of Ky and Tpz and have relatively low REE contents (av. 199.1 ppm) and Th/U ratios (av. 0.015). Most zircons from the UHP quartzites show irregular or patchy zoning, have mineral inclusions of Coe, Ph, and Rt, low REE contents (av. 332.8 ppm) and low Th/U values (av. 0.027). Only a few zircons from the UHP quartzite have inherited magmatic cores. LA-ICP-MS zircon U–Pb dating yields Neoproterozoic protolithic ages (790–764 Ma) for magmatic zircon cores of the HP quartzites, whereas the rims give Triassic (222–218 Ma) ages for HP metamorphism. Similarly, zircons from the UHP quartzites yield a Neoproterozoic protolithic age (819 Ma) and Triassic UHP metamorphic ages (223 and 221 Ma). Based on the comparable petrochemical compositions, and consistent protolithic and metamorphic ages between the quartzites and their country rock gneisses, we conclude that these quartzites were metasomatic products of granitic gneisses. Thus, they were interpreted as a part of the coherent Sulu terrane that was subjected to the coeval Triassic HP and UHP metamorphism at different subduction depths of the Yangtze plate.