Bimodal volcanic rocks in the northeastern margin of the Yangtze Block: Response to breakup of Rodinia supercontinent

Mid-Neoproterozoic protoliths of high-ultrahigh-pressure metamorphic rocks in the northeasternmargin of the Yangtze Block preserve one of the best examples of O-18-depleted magmatic provinces worldwide, which is the significant record for lithospheric extension associated with continental rifting. In this contribution, we report the depleted O-18 signature of felsic meta-volcanic rocks from the Zhangbaling Uplift (ZBLU), a segment of the Tan-Lu fault zone, which is located between the Dabie and Sulu orogenic belts. The low-delta O-18 signature of these rocks provides crucial information for understanding their origin and tectonic setting. Zircons yield U-Pb protolith ages of 771 +/- 5 Ma to 761 +/- 5 Ma and show a narrow range in delta O-18 values of 1.97% to 3.11%. The typical syn-magmatic characteristics of these zircons indicate that they record inheritance from low-delta O-18 magmas during the mid-Neoproterozoic, which suggests the occurrence of high-temperature surface-water-rock interaction in a continental rift setting. Quartz keratophyres in the ZBLU exhibit A-type granite geochemical compositionswith high contents of high-field-strength elements (Zr+ Nb+ Ce+ Y> 350 ppm) and a zircon saturation temperature of 829 degrees C. The zircon grains have slightly depleted eHf(t) values of -3.4 to +1.9, which suggestMesoproterozoic juvenile crustal formation in response to the amalgamation of the Yangtze Block with the Rodinia supercontinent. A few coeval spilites (758 Ma) crop out sporadically in the ZBLU. Their geochemical characteristics include high SiO2 contents (51.06-57.03 wt%), low Mg# values (42-53), and arclike trace-element distribution patterns. The variable zircon epsilon(Hf)(t) values for the spilite samples can be divided into two groups: negative epsilon(Hf)(761 Ma) values of -16.4 to -11.1, and positive epsilon(Hf)(744 Ma) values of +5.9 to +8.5, which supports source mixing between enriched mantle and residual oceanic crustal material. In conclusion, the ZBLU spilite-keratophyre sequence is composed of a typical bimodal rock association, providing new petrological evidence for the origin of low-delta O-18 magmas and for a continental rift setting related to the separation of the Yangtze Block from the Rodinia supercontinent at ca. 771 Ma. (c) 2021 Elsevier B.V. All rights reserved.