HF-ISOTOPES OF IGNEOUS ZIRCON FROM CRETACEOUS BATHOLITHS REVEAL CA. 1.9 AND 1.5 GA MAFIC LOWER CRUST IN THE NORTHERN ROCKY MOUNTAINS

The Late Cretaceous Pioneer, Philipsburg, and Mt. Powell batholiths were emplaced along the boundary between the Archean Wyoming craton and the mainly Proterozoic Great Falls tectonic zone. Initial epsilon Hf values of magmatic zircons, with crystallization ages of ~75-69 Ma, from granodiorite and tonalite plutons of the Cretaceous Pioneer batholith are between -28 and -34. One pluton yielded zircon with a value of +11. Magmatic zircons from the nearby Philipsburg batholith give initial epsilon Hf values from -16 to -21. Single-stage depleted mantle model ages for most of the Pioneer batholith zircons are 1.8-2.1 Ga, while those of the Philipsburg batholith zircons are 1.4-1.6 Ga. Single-stage model ages produce a minimum estimate of the time the source separated from the depleted mantle, because of the very low Lu/Hf ratio in zircon. In this case the single-stage model age is consistent with the age and isotopic composition of the Little Belt magmatic arc in the Great Falls tectonic zone, where the Pioneer batholith is located, and the age of mafic rocks associated with the Mesoproterozoic Belt Basin, which hosts the Philipsburg batholith. This suggests that the crustal source of the Cretaceous magmas was the deeper parts of the Little Belt arc and instigated by an influx of mantle-derived melt. A population of 1.8-1.9 Ga inherited zircons in the Pioneer batholith plutons is consistent with a Little Belt arc source as are 1.4-1.5 Ga inherited zircons in the Philipsburg batholith. Two-stage or crustal residence Hf model ages for the zircons calculated using an estimated Lu/Hf ratio equivalent to average crust range from 2.3 to 3.1 Ga. These values are more difficult to reconcile with the rocks in the Great Falls tectonic zone and Belt Basin. This approach is, however, appropriate if the source rocks were not zircon bearing, which was probably not the case for the Little Belt arc and Belt basin crust. These results indicate that the thick, mafic (P-wave velocity >7 ms^-1), lower crust of the northern Rocky Mountains is diachronous and not a continuous underplated layer. Moreover, a significant lower crustal age-boundary related to the Belt rift appears to lie between the Pioneer and Phillipsburg batholiths.