EMPLACEMENT OF THE LATE ARCHEAN LOUIS LAKE BATHOLITH DURING REGIONAL CONTRACTION, SOUTHERN WIND RIVER MOUNTAINS, WYOMING

The 2.63-Ga, calc-alkaline Louis Lake batholith (LLb) has previously been described as a post-tectonic, Late Archean intrusion. However, new geologic mapping and structural studies along the southern margin of the LLb in the southern Wind River Mountains, Wyo., indicate syntectonic emplacement of several early batholithic phases of this composite intrusion. Evidence for syntectonic emplacement includes: (1) an abrupt transition from magmatic/hypersolidus flow fabrics in the interior parts of the LLb to penetrative, solid-state fabrics near the external, intrusive contact; (2) early batholithic phases, quartz diorite and granodiorite porphyry dikes, have brittle and mylonitic fabrics, respectively; and (3) inclusions of mylonitic leucogranite gneiss (part of the host-rock assemblage) and mylonitic granodiorite porphyry are contained within deformed mafic dikes interpreted as early elements of the composite LLb. These observations, coupled with shear-sense criteria consistent with north-directed thrusting of the Miners Delight allochthon on the Roundtop Mountain deformation zone (a nearby plastic-to-brittle terrane boundary [i.e., Hull, 1988]), suggest that emplacement of the LLb was synchronous with terrane accretion. Such synchroneity of granitic magmatism and regional deformation, involving the accretion of a tectonostratigraphic terrane of oceanic affinity, is characteristic of continental margin crustal growth in many Phanerozoic orogens such as the Klamath Mountains of NW California and SW Oregon. Our field data support the concept that a major, composite crustal growth event occurred along the southern margin of the Wyoming province in the Late Archean (~2.64-2.62 Ga) (e.g., see Frost, C.D., et al., 1998, Precambrian Res., 89, 145-173; Chamberlain et al., 2003, CJES, 40, 1357-1374). This event involved both the accretion of an oceanic terrane as well as the emplacement of a calc-alkaline, composite batholith.