Genetic relationship between 1780 Ma dykes and coeval volcanics in the Lvliang area, North China.

• 1780 Ma bimodal dykes and volcanics are identified in the Lvliang area. • The low-Si dykes are conduits for basalt-andesite while high-Si ones for rhyolite-dacite. • Both bimodal dykes and volcanics were originated from subcontinental lithospheric mantle. • The lithosphere was probably metasomatized by a paleo-plume at 1780–1730 Ma. The 1780 Ma dykes and coeval volcanics constitute a large igneous province in the North China Craton; however, whether these dykes and volcanic rocks of varying compositions originated from one or multiple sources remains controversial. The ca. 1780 Ma Lvliang dykes are characterized by widespread E-W-oriented dykes and minor N-W-oriented diabase dykes. The E-W-oriented dykes are bimodal and comprise two subgroups: one is dominated by acidic high-silica dykes (SiO 2 >63 wt%), while the other is dominated by mafic to intermediate relatively low-silica dykes (SiO 2 <63 wt%). The ca. 1780 Ma volcanics (in the Xiaoliangling Group) in the Lvliang area are also bimodal and dominated by rhyolite to dacite volcanics and basalt to andesite volcanics with few clastic interlayers. SIMS U-Pb dating on zircons from one high-silica dyke and a rhyolite volcanic layer yields 207Pb/206Pb ages of 1783 ± 7 Ma and 1776 ± 6 Ma, respectively, and SIMS U-Pb dating on baddeleyites from a low-silica dyke yields a 207Pb/206Pb age of 1789 ± 5 Ma. They are all tholeiitic in composition (MgO: 0.3–6.1 wt%; SiO 2 : 51–74 wt%), enriched in light rare earth elements ((La/Yb) N = 7.9–20.2) and show negative anomalies in Nb, Ta, Sr and Eu (Eu/Eu* = 0.3–0.9). They also have similar in-situ Hf isotopes of baddeleyite/zircon (εHf values of −14.5 to −1.8) and bulk rock εNd (−6.7 to −3.0) isotopes. Furthermore, immiscible textures are identified from the Xiaoliangling Group volcanics, which are characterized by some silica-rich melts globules in the Fe-rich magma. These spatiotemporal affinities and geochemical similarities, as well as the identified immiscible textures, reveal that the low-silicate dykes are conduits for the basalt-andesite with significant fractionation of clinopyroxene and plagioclase; while the high-silica dykes are conduits for the rhyolite-dacite with distinct immiscible segregation prior to eruption. In addition, there is a sharp change of εHf values at 1780–1730 Ma, and their Hf T DM ages are ∼2750–1950 Ma. As these rocks originated from the subcontinental lithospheric mantle, this may indicate that the subcontinental lithospheric mantle was metasomatized during this time period, possibly by a paleo-plume event. [ABSTRACT FROM AUTHOR]