Complementary <i>in situ</i> garnet Lu-Hf geochronology and monazite U(-Th)-Pb petrochronology: an example from the Archean Slave craton, Northwest Territories, Canada

Abstract Advances in analytical instrumentation over the past decade have facilitated the development of new geochronological methods. In particular, the triple quadrupole inductively coupled plasma mass spectrometers with in-line reaction cells have made it possible to investigate beta decay geochronological systems, including Lu-Hf in garnet, in situ via laser ablation. While these new methods allow different kinds of studies to be carried out, their utility relative to well-established methods has not been investigated in detail. Herein we present the results of in situ garnet Lu-Hf and monazite U(-Th)-Pb geochronological analyses of two adjacent Archean meta-pelitic rocks from the Northwest Territories, Canada. Whereas the garnet Lu-Hf analyses define single population isochrons of ca. 2570 Ma, monazite U(-Th)-Pb analyses define two chemically distinct populations. An older ca. 2570 Ma monazite population has low Y concentrations, while a younger, ca. 2530 Ma monazite population has high Y concentrations. The change in Y concentrations in the monazite is consistent with coeveal growth with garnet during prograde metamorphism whereas the younger high Y monazite is interpreted to reflect growth during garnet breakdown during decompression. The results presented herein demonstrate that incorporating both in situ garnet and monazite geochronological data takes advantage of each method (i.e. the spatial precision, variable (re)crystalization under different conditions, and the ability to determine a date without relying on an isochron for monazite U-Pb, and the ability to date a primary metamorphic phase for garnet Lu-Hf) to build a more robust geochronological history.