Lu–Hf systematics of magmatic zircons reveal a Proterozoic crustal boundary under the Cretaceous Pioneer batholith, Montana

Abstract

Lu–Hf systematics of magmatic zircons from quartz diorite and granodiorite plutons of the Late Cretaceous Pioneer batholith, Montana, indicate involvement of distinctly different crustal sources in the petrogensis of individual components of the batholith. Plutons of the eastern Pioneer batholith contain magmatic zircons with initial εHf values of −28 to −34 that crystallized in magmas likely derived from dominantly Archean and earliest Paleoproterozoic crust. Contemporaneous granodiorite in the western Pioneer batholith contains magmatic zircons with initial εHf values ranging from −9 to −33, but dominated by values between −18 and −22, which suggest a mixture of Paleoproterozoic and possible Mesoproterozoic sources. These data suggest that distinct segments of crust juxtaposed and produced during formation of the Great Falls tectonic zone (1.78–1.86Ga) and the Belt basin (~1.43–1.47Ga) contributed to magmatic compositions in the batholith and that these contributions are recorded in the magmatic zircons. The contrasting εHf distributions between eastern and western components of the Pioneer batholith suggest that an important crustal and/or lithospheric boundary underlies the Pioneer batholith. The Hf-isotopic results also suggest that the high P-wave velocity lower crust of the northern Rocky Mountains did not form in a single event.