AGE AND CHARACTER OF PRECAMBRIAN BASEMENT IN THE LITTLE BELT MOUNTAINS, MONTANA: IMPLICATIONS FOR THE ROLE OF THE GREAT FALLS TECTONIC ZONE IN THE PALEOPROTEROZOIC ASSEMBLY OF NORTH AMERICA

The Great Falls tectonic zone (GFTZ) in western Montana is a NE-trending zone of Proterozoic to Tertiary features and geophysical trends thought to be controlled by inherited basement structures. Paleoproterozoic basement in the GFTZ defines the boundary between the Archean Wyoming province and Medicine Hat block. The Little Belt Mountains (LBM) of west-central Montana provide the only exposures of basement within the GFTZ and therefore the only place to directly assess the role of the GFTZ in Paleoproterozoic assembly of the Laurentia. The northern part of the LBM exposure consists of felsic to mafic migmatites derived from plutonic and meta-volcanic rocks. Preliminary U-Pb zircon data from a leucogranite pod suggests Paleoproterozoic high-grade metamorphism and melting. The central LBM are composed of two homogeneously foliated intermediate plutonic bodies previously dated as 1.8-1.9 Ga. The southern half of the LBM is dominated by compositionally variable weakly foliated leucogranites. Leucogranite contains lenses of older highly foliated/lineated intermediate and mafic rocks. One amphibolite lense has yielded a U-Pb zircon age of ~1.81 Ga, providing a maximum age for the leucogranite. A mappable metadiorite body in the northern part of the leucogranite yielded a U-Pb zircon age of ~2.8 Ga. Leucogranite dikes intrude the intermediate plutonic rocks in the central LBM. Thus, current mapping indicates that the area consists almost entirely of a felsic-to-mafic, igneous/meta-igneous rocks with both Late Archean and Paleoproterozoic ages. The area is dissected by a 1-2 km thick mylonite zone that dips moderately southward, affects a variety of mafic to felsic plutonic rocks, and is locally folded. Mylonitic fabrics are dominantly greenschist-grade, with remnants of amphibolite-facies fabrics suggesting deformation during cooling or during multiple episodes. Lineations plunge gently from WNW to WSW and are roughly parallel to lineations in lower strain rocks away from the mylonite zone. The majority of kinematic indicators from the mylonite and adjacent rocks yield a sinistral shear sense. The lineations and foliations within the area studied thus far are oblique to the NE trend of the GFTZ suggesting that they have either been rotated by later km-scale folding or formed during oblique convergence.