The Great Falls Tectonic Zone after the assembly of Laurentia: evidence for long-term tectonic stability from xenolith apatite

Combined U-Pb geochronology, and trace element analysis (Th, U, Y, Sr, rare earth elements: REE) by LA-Q-ICPMS and halogen chemistry (F, Cl, OH) by SEM-EDX are applied to apatite from a suite of amphibolite- and granulitefacies xenoliths from the Bearpaw Mountains (Montana, USA), to constrain cooling of Paleoproterozoic mid- to lower crust. Xenoliths hosting fluorapatite and hydroxyfluorapatite were emplaced at 54 to 50 Ma within Krich, host lamprophyres (minettes). Granulite xenoliths yield apatite with highly dispersed U-Pb isotopic compositions. But discordia fit through their youngest populations yield Neoproterozoic-Cambrian (c. 650-500 Ma) lower intercepts. A single amphibolite xenolith from a shallower depth contains apatite with older single-grain apatite ages in the range of 1200-890 Ma. These lower intercept ages are much younger than previously reported zircon U-Pb ages from the same samples, which register a thermal maximum in the Palaeoproterozoic (c. 1700-1800 Ma). Trace element analysis of apatite confirms that most grains did not undergo retrogressive recrystallization. Additionally, there is a statistically significant link between apatite size and single grain ages in at least one sample. Our interpretation is that these apatite U-Pb ages represent slow cooling through the apatite Pb partial retention zone (c. 375-450 degrees C). Whether these data represent a distinct cooling event, or simple protracted cooling from a Palaeoproterozoic thermal peak, cannot be strictly constrained by these data. However, previous evidence from rutile U-Pb in xenoliths from nearby locations implies slow cooling from a Palaeoproterozoic peak. If a slow cooling scenario is assumed, by comparing apatite cooling ages to previous results from Ti-in-zircon thermometry on metamorphic zircon from the same xenoliths, a cooling rate of 0.14-0.33 degrees C/Myr can be estimated for these granulite xenoliths. Our apatite data thus contribute to a growing consensus on the geodynamic history of the northern Wyoming Craton and Medicine Hat Block, wherein lithospheric stability was maintained on a giga-year timescale from a granulite-facies metamorphic event associated with the assembly of Laurentia up until Laramide orogenesis. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

Taking samples from the Bearpaw Mountains in Montana, USA, this study conducted U-Pb geochronology, trace element analysis, and halogen chemistry on apatite from amphibolite- and granulite-facies xenoliths. The results suggest a complex cooling history with slow cooling rates represented by apatite U-Pb ages.