Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet

Abstract

This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 “carbonatite-like” kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼ 0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼ 415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼ 292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.