Origin of lamprophyres from the northern margin of the North China Craton: implications for mantle metasomatism

Abundant lamprophyre dykes occur near the northern margin of the North China Craton and offer a unique opportunity to study the nature of the mantle source. The dykes are minettes composed of phlogopite, sanidine and calcite. ⁴⁰Ar/³⁹Ar dating yields ages of 234 ± 2 and 222 ± 6 Ma. The lamprophyres are near-primary, mantle-derived ultrapotassic melts, having low SiO₂ (31.0 – 41.5 wt%) and high K₂O (4.40 – 7.12 wt%) contents, high Mg# (62 – 84) and high contents of compatible elements. They are characterized by fractionated rare earth element patterns, radiogenic Sr and unradiogenic Nd isotope compositions (⁸⁷Sr/⁸⁶Sr_i= 0.7070 – 0.7075; ε_Nd(t) = –12.8 to –9.2). A small amount of mafic crustal rocks (<4.4%) may have been assimilated during magma ascent, as revealed by ¹⁸⁷Os/¹⁸⁸Os_i ratios of 0.4548 – 0.8068. These data suggest that the lamprophyres originated from a low degree of partial melting of an enriched subcontinental lithospheric mantle source with abundant phlogopite, clinopyroxene and carbonate. The source has been metasomatized by carbonate- and potassium-rich fluids derived from carbonated sediments recycled via subduction of Palaeo-Mongolian oceanic slab beneath the North China Craton.