Age and evolution of the Precambrian crust of the Tobacco Root Mountains

Mueller, P., Wooden, J., Heatherington, A., Burger, H, Mogk, D. and D’Arcy, K., 2004, in Brady, John B., Burger, H. Robert, Cheney, John T., and Harms, Tekla A., eds., Precambrian Geology of the Tobacco Root Mountains, Montana: Geological Society of America Special Paper 377, p. 181-202.

Abstract

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90 Ga to 1.77 Ga.  The oldest U-Pb age reported (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak metamorphic suite, although younger ages of detrital grains suggest the protoliths of the metasedimentary rocks were deposited subsequent to 3.2 Ga.  Alternatively, a Pb-Pb age of ~2.45 Ga from a single zircon extracted from a quartzite within the SPMS suggests the possibility that this quartzite and other SPMS lithologies may have formed in the Proterozoic.  Although a Proterozoic depositional age is possible, Sm-Nd model ages of mafic and metasedimentary SPMS rocks are Archean and the age-distribution of zircons from the quartzite are very similar to the age-distribution present in Archean quartzites in the region.  In either case, the protoliths of the volumetrically dominant quartzofeldspathic gneisses were apparently emplaced 3.3 – 3.5 Ga, and are interpreted to be the basement upon which the younger (meta) sedimentary rocks were deposited.  The Archean gneisses and metasupracrustal rocks were also affected by Paleoproterozoic metamorphism and deformation.  U-Pb analyses of zircons from an anatectic luecosome yield ages ranging from 1.77 to 3.48 Ga.  The 1.77 Ga age is interpreted to represent the time of formation of the leucosome.  In addition, U-Pb analyses of zircons extracted from a mafic dike that both cuts across Archean gneissic banding and exhibits a granulite facies paragenesis indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga.  The well defined field relations between the mafic dike, melt pod, and the rocks of the Spuhler Peak metamorphic suite clearly demonstrate that the Spuhler Peak association was tectonically emplaced between 2.06 Ga and 1.77 Ga.  In addition, the protoliths of the Spuhler Peak rocks apparently reached granulite facies conditions at 1.77 Ga, most likely in response to burial as a result of terrane collision (e.g., Wyoming and Hearne provinces) or to post-collisional mafic underplating.

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