Plutonic Imaging: Evidence for Two Proterozoic Mafic Underplating Events in the NW Wyoming Province from Hf-Isotopes of Zircon from Cretaceous Granitoids

(Photo of the roof of the Pioneer batholith, SW Montana, where it
intrudes Paleozoic carbonates near Browns Lake. The Pioneer batholith
was mainly derived from partial melting of Paleoproterozoic mafic lower crust.

Plutonic imaging refers to the extraction of discrete elemental and isotopic (U-Pb, Hf, O, etc.) information from in-situ measurements of individual zircons and other accessory minerals as a complement to secondary isotopic systematics of plutonic rocks. It can provide critical constraints on crust-mantle intereactions and an image of the distribution of temporally and compositionally distinct lithospheric components. Plutonic imaging provides a critical complement to geophysical studies that cannot provide estimates of crustal age diversity and true composition.

The Late Cretaceous (ca. 70 Ma) Pioneer, Philipsburg, and Mt. Powell batholiths were emplaced along the boundary between the Archean Wyoming craton and Proterozoic-Archean Great Falls tectonic zone. Hf-isotopes of magmatic zircons from granodiorite and tonalite from the Pioneer batholith give eHf values for most grains of -28 to -32 with depleted mantle model ages of 1.8-2.0 Ga. The dominant population of magmatic zircons from the Mt. Powell and Philipsburg plutons gives eHf of -16 to -20 along with a few grains that extend the overall range from -12 to -30. Single stage model ages for the -16 to -20 group  are 1.4-1.6 Ga, other grains range from 1.1 to 1.9 Ga. These data are consistent with derivation of these metaluminous batholiths from partial melting of two discrete, but likely mafic, lower crustal sources, one that is mainly Paleoproterozoic (ca. 1.9 Ga) and the other Mesoproterozoic (ca. 1.5 Ga). The Pioneer batholith plutons were largely derived from partial melting of Paleoproterozoic crust similar in age to the Little Belt magmatic arc of the Great Falls tectonic zone, whereas the Philipsburg and Mt Powell batholiths were derived mainly from mafic rocks emplaced during the formation of the ca. 1.47 Ga Belt Basin.

These data suggest that the thick (~20 km) mafic (P-wave velocities >7 ms-1) lower crust of the Wyoming province and Great Falls tectonic zone is diachronous and significantly younger than the Archean-Paleoproterozoic upper crust. This implies that the lower crust along the northwestern margin of the Wyoming craton was underplated (either tectonically or magmatically) subsequent to the formation of the Archean upper crust of the Wyoming craton.