Dating mafic magmatism by integrating baddeleyite, zircon and apatite U–Pb geochronology: A case study of Proterozoic mafic dykes/sills in the North China Craton

Abstract

Direct dating of mafic dykes/sills is crucial for understanding mantle geodynamic evolution and crust-mantle interaction, but is also challenging, especially in ancient samples. Here, we provide a reliable approach for dating Proterozoic mafic dykes/sills through an integrated investigation of U–Pb isotopes and elemental distribution maps of the coexisting baddeleyite, zircon and apatite. This study focuses on Paleo- to Neo-Proterozoic mafic dykes/sills in the North China Craton.In the studied samples, baddeleyite is undoubtedly magmatic, with sensitive high resolution ion microprobe (SHRIMP) 207Pb/206Pb ages that may record crystallization ages of the dykes/sills. Common lead incursion along rims and fractures of baddeleyite is identified via laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) elemental mapping as the likely origin of age uncertainties. In contrast, magmatic zircons from the Proterozoic mafic intrusions are somewhat affected by metamictization as roughly indicated by high alpha dose (Dα), and thus show intricate U–Pb isotopic features. Intensely metamict zircons are susceptible to Pb loss due to fluid infiltration, as clearly illustrated by opposing enrichment trends of Pb and U in zircon maps. Where metamict zircons are unaffected by fluid alteration, they are able to preserve the original 207Pb/206Pb ratios and thus give high precision age constraints on emplacement of the ancient dykes/sills.As for apatites, abundant euhedral coarse-grained crystals were separated from the mafic rocks, rendering them suitable for LA-Q-ICP-MS measurement. U–Pb data define Tera-Wasserburg isochrons with lower intercept ages that match crystallization of the host rocks. Age uncertainties of this method lie in instrumental analysis errors caused by low concentrations of U, Pb and a low proportion of radiogenic Pb in the total Pb, with these parameters commonly showing consistent enrichment trends from crystal cores to rims. By combining U–Pb dating and elemental mapping, this study provides a novel approach to the geochronology of mafic rocks, by highlighting the potential of ubiquitous apatite, offering additional means for error analysis and age interpretation of baddeleyite and zircon.