Protolith and ultranhigh-pressure (UHP) metamorphic ages of ultramafic rocks in Weihai area, North Sulu UHP terrane.
北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

Ultramafic rocks as irregular lenses and blocks are commonly identified within gneissic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, and in situ U-Pb LA-ICP-MS dates of zircons provided insight into the protolith and UHP metamorphic ages of these rocks. Zircon grains separated from olivine-bearing pyroxenite (samples W1 and W2) in Weihai area, North Sulu can be subdivided into three types: (1) inherited (magmatic) zircons with clear magmatic CL images and Ol+Cpx+Ap inclusions, which record Paleoproterozoic protolith ages of 1835~1845Ma; (2) some inherited zircons with blurry CL images and Ol+Cpx+Ap inclusions give a younger age group of 250~784Ma related to partial recrystallization and Pb loss during late magmatic and metamorphic events; and (3) metamorphic zircons with homogeneous CL images and Grt+Cpx inclusions that record UHP metamorphism at 230~234Ma, which are consistent with the UHP ages (235~225Ma) recorded by the coesite-bearing zircons from eclogite and its country-rocks in Sulu UHP terrane. Existence of UHP metamorphic zircons in ultramafic rocks indicates that the fluid may play an important role in the UHP metamorphic process of the Sulu terrane. The results from this study not only exactly constrain the protolith and UHP metamorphic ages of ultramafic rocks, but also show important significance for an in-depth insight into of the fluid property and evolution feature, fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane.