~2.6 GA TECTONISM IN THE SOUTHERN WYOMING PROVINCE: RECORD OF ACCRETIONARY OROGENESIS OR SUPERCONTINENT BREAKUP? (Invited Presentation)

The last major Archean event in the Wyoming Province was intrusion of voluminous 2.62-2.63 Ga granitic batholiths. This magmatism coincided with proposed supercontinent assembly. Following supercontinent breakup, Laurentia formed during the Paleoproterozoic, incorporating the Wyoming craton. A rifted margin in southern Wyoming suggests breakup occurred ~2.1 Ga but alternatively 2.4-2.5 Ga tectonism and magmatism could represent such a record (Mueller et al., 2016). Here we present evidence of previously unknown 2.60 Ga tectonism in the southern Wyoming province and evaluate its significance in the context of supercontinent formation and breakup.

The dominant structural grain of the southern Wyoming province is marked by NE-trending foliations and shear zones in Archean gneiss. This fabric is present in granitic orthogneiss as young as 2635 Ma but is absent in the 2625 Ma Wyoming batholith. Some shear zones were reactivated and were conduits for metasomatizing fluids that altered the composition of the rock. Reactivated shear zones within Paleoarchean Sacawee orthogneiss are occupied by qtz-bt-crd gneiss. Shear zones within supracrustal rocks have assemblages qtz-bt-cord-ms-sil, and-bt-ms-qtz, and st-grt-bt-ms-qtz. Biotite defines the foliation and retrogressive chlorite is always present.

Monazite grains in these rocks exhibit a variety of shapes, sizes, and compositional zoning patterns. Only one age population of monazite was identified from shear zones in supracrustal rocks: 2602±6 Ma (15SR21a), 2598±11 Ma (15SR7b), and 2600 ± 9 Ma (SQ2). Monazite from a shear zone in Paleoproterozoic orthogneiss also yielded analyses at ~2.6 Ma, along with older dates (15BG6).

This ~2.6 Ga event is previously unknown in the Wyoming province. It is distinctly older than 2.4-2.5 Ga events elsewhere along the margins of the Wyoming craton. By analogy with the timescales and patterns of accretion of Phanerozoic terranes along western North America, we suggest that the ~2.6 Ga tectonism is more likely to be related to final stages of terrane accretion along the southern margin of the Wyoming province rather than to rifting associated with supercontinent break-up.