Lu-Hf Analysis of Zircons in the Little Belt Mountains Suggest Paleoproterozoic Subduction in the Great Falls Tectonic Zone

In the assembly of Southwestern Laurentia two competing regions have been proposed as the location of a Paleoproterozoic suture between the Archean Wyoming and Hearne provinces; either the Vulcan Structure, located between the Hearne Province and the Medicine Hat Block, or the Great Falls tectonic zone (GFTZ) between the Medicine Hat Block and the Wyoming Province. Due to a general lack of Precambrian exposure in these areas, discerning the true location of the collision has proven challenging, though mounting evidence indicates the GFTZ as the site for this suture. Among this evidence is a Sm-Nd model age of 1.9 Ga for the Little Belt Mountains of the Eastern GFTZ. Samples collected from across multiple units in the Little Belt Mountains have undergone Lu-Hf analysis on individual zircons using laser ablation, MC-ICP-MS. Among these units the Helispot Granite is a recrystallized granite to granodiorite that locally contains strongly foliated, recrystallized porphyroclastic mylonitic textures and has a mean Lu-Hf model age of 1.99 Ga (TDM) with a range of 1.92-2.17 Ga as well as a U-Pb age of 1842 ± 42 Ma. The Cemetery Migmatite is locally banded and derived from igneous components of varied composition with a mean Lu-Hf age of 2.02 Ga (TDM) and a range of 1.85-2.14 Ga as well as a U-Pb age of 1817 ± 16 Ma. The Hoover Ridge Unit is a diverse collection of felsic to mafic intrusives with a mean Lu-Hf age of 2.63 Ga and a range of 2.54-2.88 Ga (TDM) in addition to a U-Pb age of 1846 ± 9 Ma. The Aspen Paragneiss, a migmatitic pelitic paragneiss with local melt layers and injections, has a Lu-Hf age of 2.77 Ga with a range of 2.53-3.19 Ga (TDM). The O’Brien Creek Unit contains fine grained granitic rocks with a mean Lu-Hf model age of 2.21 Ga ranged between 1.59-2.5 Ga (TDM) and a U-Pb age of 1800-1820 Ma. The Sheep Creek Complex contains a collection of granitoids including a sheeted, garnet bearing leucogranite as well as abundant amphibolites. This unit has a mean Lu-Hf model age of 2.14 Ga ranged from 1.32-2.97 Ga (TDM). Also dated was a megacrystic granitoid with a mean Lu-Hf age of 2.02 Ga and a range of 1.97-2.09 Ga (TDM). These data, taken together with an arc signature in the trace element composition and Sm-Nd model ages of these rocks indicates a Paleoproterozoic suture between the Wyoming Province and the combined Hearne-Medicine Hat Block along the GFTZ, which resulted in the formation of the Little Belt Arc and reworked some archean material. This paleoproterozoic island arc and its associated reworked archean crustal material is preserved today in the calc-alkaline rocks of the Little Belt Mountains.