Evidence of two-phases of Rodinian rift-related magmatism in Sauk sequence sandstones of western North America

An understanding of the breakup of Rodinia along the western margin of Laurentia is hindered by a limited record of rift-related magmatic rocks. Previous work identified two main periods of magmatism associated with episodes of crustal thinning: (1) magmatism synchronous with initial rifting of western Laurentia and deposition of the Windermere Supergroup, and: (2) late Ediacaran and Cambrian magmatism associated with initiation of the Paleozoic Laurentian passive margin. Here we use detrital zircon from basal Sauk sequence sandstones along 2300 km of the western Laurentian margin from New Mexico, USA to central British Columbia, Canada as an additional record of rift-related magmatism. Over 600 zircon grains from each of twenty samples of basal Sauk sequence sandstones were measured to identify Neoproterozoic and Cambrian grains. These grains were then re-ablated to improve the uncertainty of the dates and to measure trace-element compositions. Samples from strata in Canada contain grains yielding dates of 780 Ma, 750 to 660 Ma and 580 to 535 Ma. Trace-element compositions of the 780 to 660 Ma populations are consistent with mafic source rocks while the 580 to 535 Ma populations derive from sources with mixed compositions. In Cambrian samples from the southwestern USA, Neoproterozoic zircon populations are rare. Late Ediacaran to Cambrian, 550 to 502 Ma, grains are common and derive from sources with mixed compositions. Our new U-Pb detrital zircon dates and trace element data show spatial and temporal trends that improve our understanding of the timing and type of magmatism along the margin. A gap in magmatism between 640 and 600 Ma is present along the length of the study area. The gap separates mafic magmatism, associated with crustal thinning that led to Windermere deposition, from magmatism of mixed chemistry that led to the formation of the Paleozoic passive margin. The temporal separation of the two magmatic events, and their different chemistries, suggest that different lithospheric processes were operating in western Laurentia during these events.