Apatite in situ Lu-Hf and U-Pb geochronology in layered mafic intrusions

Abstract

Layered Mafic Intrusions (LMI) represent the solidified remnants of basaltic magma chambers and provide important insights for understanding subsurface igneous processes, including those linked to the formation of economic deposits of Fe-Ni-Cu-Co sulphides, platinum group elements (PGE), Cr-Fe-Ti-V-oxides and apatite. However, constraining the timing of emplacement of LMI can be challenging, as datable minerals like zircon have a relatively low abundance in mafic rocks. In this study, we use apatite, a common accessory mineral in mafic rocks that can reach high modal abundances in the most evolved parts of LMI to evaluate the robustness of the in-situ U-Pb and Lu-Hf dating methods by laser ablation – inductively coupled plasma – reaction cell mass spectrometry (LA-ICP-MS/MS) for constraining the timing of LMI petrogenesis. We present in-situ apatite U-Pb and Lu-Hf dates from apatite-rich lithologies from four major LMI localities: the Fe-Ti Boulder Lake North deposit (Duluth Complex, USA), the Upper Zone (Bushveld Complex, South Africa), the Nebo-Babel Intrusion (Giles Complex, Musgrave Province, Australia), and the Proterozoic anorthosite massifs of the Grader Intrusion and Lac Perron deposit (Grenville Province, Canada). Trace element discrimination plots, coupled with petrological data, indicate that apatite from all four localities exhibit typical mafic compositions and are classified as cumulus apatite. The U-Pb dates in apatite either represent primary magmatic ages or exhibit varying degrees of Pb-loss, as seen in the Nebo-Babel intrusion, and in the Bushveld Complex, where the degree of Pb loss is dependent on apatite crystal size. In contrast, the apatite Lu-Hf system is undisturbed and records primary LMI crystallisation ages as demonstrated for the Duluth and Bushveld Complexes, and Nebo-Babel intrusion. In the Grenville Province, the long-lived magmatic systems likely record younger apatite growth ages compared to zircon ages. Hence, this study demonstrates in-situ LA-ICP-MS/MS Lu-Hf of apatite as a robust tool for obtaining primary magmatic ages of LMI.