Plutonic Imaging: A View into the Lower Crust and Upper Mantle of the Great Falls Tectonic Zone

Secondary isotopic systematics of Sr, Nd, and Pb in young, metaluminous rocks have been used to define isotopic/age provinces in many regions around the world. Secondary isotopic systematics of whole rocks, however, typically reflect a variety of mixing processes and are, therefore, strongly influenced by both age and the elemental abundances of mantle-derived magmas and older lithosphere. The ability to extract discrete elemental and isotopic information from individual zircons (zirconology) using in situ techniques (e.g., laser ablation, ion microprobe) provides critical insight into the age and composition of individual sources and the nature of magmatic mixing processes. Plutonic imaging, therefore, refers to the extraction of diverse geochemical information from igneous rocks using both secondary systematics and zirconology and is critical to discerning the age and composition of the lower crust and sub-continental lithospheric mantle in complex orogenic settings. We are applying the plutonic imaging approach to the complex and enigmatic crust of the Great Falls tectonic zone (GFTZ) of the northern Rocky Mountains, which marks the boundary between the Archean Wyoming craton and accreted Proterozoic terranes. Our research has focused on the widespread Late Cretaceous plutonic rocks emplaced within the western GFTZ and the Eocene alkalic rocks of the Montana alkalic province (central GFTZ). Although the secondary isotopic systematics of Pb suggest a homogeneous source approximately 2.1 Ga in age for both the central and western GFTZ, data from individual zircons strongly suggest a more diverse crust ranging in age from 1.1 to 3.3 Ga in the western GFTZ. These data also suggest an inverted crustal section with Proterozoic crust and/or lithosphere underlying and/or interleaved with Archean crust. The central GFTZ, however, lacks any evidence of an Archean component, which supports proposals that the GFTZ was characterized by the production of relatively juvenile Paleoproterozoic crust.