Mesozoic titanite U–Pb age from mafic granulites of the Wuhe Complex, southeastern North China Craton (NCC)

As an important component of the lower crust, mafic granulites can provide a great deal of information about orogens’ metamorphic and tectonic evolution, and thus are studied extensively. According to the previous studies, the Wuhe Complex experienced the Late Paleoproterozoic metamorphic event. Here, we report the newly discovered Mesozoic metamorphic titanite age from the mafic granulites of Wuhe Complex and provide some clues to the Mesozoic metamorphic event of the southeastern NCC. Mafic granulite (sample 20BB44) is composed of garnet (12–15 vol.%), clinopyroxene (30–35 vol.%), hornblende (3–6 vol.%), plagioclase (40–50 vol.%), and quartz (1–2 vol.%) with minor ilmenite, pyrite, apatite, zircon, and titanite. Titanite grains are subhedral, euhedral, or homogeneous with grain sizes of 50–300 μm, and have inclusion minerals of hornblende, plagioclase, quartz, and ilmenite. Titanites have variable contents of U (1.0–17.2 ppm), Th (0.2–29.6 ppm), Pb (2.2–6.5 ppm), and Zr (20–259 ppm) with Th/U ratios of 0.07–4.53. According to the Zr-in-Titanite thermometer (Hayden et al., 2008: the estimated pressure was assumed as 0.5 GPa, and the activity of SiO2 (αSiO2) and TiO2 (αTiO2) were assumed as 1 and 0.8, respectively), the titanites may form at the temperature of 607–725 ℃ (689 ℃ on average). Thirty analysis spots on 29 titanite grains yield a lower intercept U–Pb age of 163 ± 28 Ma (MSWD = 1.17). Titanite U–Pb age of 163 Ma may represent the Mesozoic metamorphic event of southeastern NCC and may relate to the subduction of the Paleo-Pacific plate.