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 (Sr-87/Sr-86(i) = 0.7070 - 0.7075; epsilon(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 Os-187/Os-188(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.