SHRIMP U-PB DATING OF RECURRENT CRYOGENIAN AND LATE CAMBRIAN-EARLY ORDOVICIAN ALKALIC MAGMATISM IN CENTRAL IDAHO: IMPLICATIONS FOR RODINIAN RIFT TECTONICS

Seven composite alkalic plutonic suites and a tuffaceous diamictite, which are discontinuously exposed in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the 200 km-long northwest-aligned Big Creek-Beaverhead belt across central Idaho. SHRIMP U-Pb dating documents two discrete magmatic pulses that occurred at 665-650 Ma along the entire extent of the Big Creek-Beaverhead belt and at 500-485 Ma along the southeastern half of it. Together with the nearby parallel exposures of the 685-Ma Edwardsburg Formation volcanic rocks (lower Windermere Supergroup), the present study indicates recurrent extensional magmatic pulses that spanned 200 Ma. Additionally, recurrent Cryogenian-early Paleozoic uplift along the regionally coincident Lemhi arch is reflected in the stratigraphic record of same-age cratonal-platform and miogeoclinal basins to the northeast and southwest. Preserved miogeoclinal rocks originated on a narrow continental shelf, overlying a narrow, northwest-striking zone of thinned continental crust.

The Big Creek-Beaverhead belt is interpreted to have originated as a northeast-extending upper-plate extensional zone that formed an eastern Washington to southeastern Idaho segment of the Cryogenian-early Paleozoic Rodinian rift margin of western Laurentia. It was flanked on the north by the St. Mary-Moyie transform zone (south of a narrow southern Canadian upper-plate segment) and on the south by the Snake River transfer zone (north of a broad Great Basin lower-plate segment). These are central segments of a zigzag-shaped Cordilleran rift system of alternating northwest-striking extensional zones offset by northeast-striking transfers and transforms. In combination with similar-aged rift-related igneous rocks in the Canadian Cordillera, the recurrent igneous activity along the Big Creek-Beaverhead belt reflects polyphase rift and continental separation events, including (1) Cryogenian-Ediacaran pre- and syn-Windermere rifting, (2) Windermere margin subsidence, (3) late Ediacaran-Cambrian rifting, and (4) well-developed Cordilleran miogeocline passive-margin subsidence. Timing and geometries support synchronous but opposing divergence along Cordilleran and Atlantic rifts with a junction in southern California-Sonora.