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

Autor: Robert Bolhar, Brendan C. Hoare, Gary O'Sullivan, Kenneth D. Collerson, Yashirvad Thakurdin, Péter Horváth
Rok vydání: 2021
Předmět:
Zdroj: Lithos. :105977
ISSN: 0024-4937
DOI: 10.1016/j.lithos.2021.105977
Popis: 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 granulite-facies 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 K-rich, 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 °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 °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.
Databáze: OpenAIRE
Externí odkaz: