REGIONAL PATTERN OF CORDILLERAN MIOGEOCLINE LOW-GRADE METAMORPHISM FROM ILLITE TO MUSCOVITE TRANSITION

The thermal history of the Cordilleran miogeocline has implications for Paleozoic paleogeography, timing of Paleozoic-Mesozoic thrusting, and patterns of Cenozoic extension. An important step in understanding this history is quantifying regional variations in low-grade metamorphism of Paleozoic strata that arise from burial beneath the westward-thickening passive margin sequence and structural position within thrust sheets. Here we utilize the illite to muscovite transformation to quantify low-grade metamorphism of the oldest pelitic rocks that are widespread in both cratonic and miogeoclinal sections: the Middle Cambrian Bright Angel Shale on the Colorado Plateau and the correlative Carrara Fm. in the southern Great Basin. With increasing metamorphic grade, illite crystallinity decreases (illite crystallites thicken) and there is an increase in the proportion of muscovite (the 2M1 polytype of illite). Illite data were measured on Middle Cambrian shales from eight locations along a 200 km cross-margin transect extending from the Grapevine Mtns. in the NW to Frenchman Mtn. in the SE. In terms of both illite crystallinity and polytypes, there is an overall increase in metamorphic grade to the NW. Illite crystallinity is 0.3 to 0.4 °2θ in the Grapevine Mtns. and Bare Mtn. ("low anchizone" or prehnite-pumpellyite facies) and about 1 at Frenchman Mtn. ("deep diagenetic zone" or zeolite facies). Illite polytypes range from essentially 100% 2M1 in the Grapevine Mtns. to ~10% at Frenchman Mtn. Most of this regional variation occurs between Frenchman Mtn. and the Resting Spring Range, corresponding with the transition from cratonal to miogeoclinal facies and also with the leading edge of the Sevier thrust belt. At Frenchman Mtn., illite crystallinity and polytypes are fairly constant in several Cambrian shales, but both parameters are apparently higher-grade in shale at the base of the Pennsylvanian Callville Limestone. We suggest this up-section change in illite composition reflects input of detrital muscovite shed from basement-cored uplifts of the Ancestral Rocky Mtns. ⁴⁰Ar-³⁹Ar data from the regional transect may empirically constrain Ar diffusion in illite and help to refine estimates for the timing of metamorphism and exhumation of the Cordilleran miogeocline.