CONSTRAINING RODINIAN RIFT-RELATED MAGMATISM FROM ZIRCON U-PB AGES AND TRACE ELEMENT COMPOSITIONS

There is a limited record of Cambrian and Neoproterozoic magmatism associated with the break-up of Rodinia along the western margin of Laurentia. Previous work identified two main periods of magmatism. Magmatism between 690 and 660 Ma with the deposition of the Windermere Supergroup, while latest Neoproterozoic magmatism (~575 Ma) relates to the final rifting away of the western landmass and initiation of the passive margin. Sauk sequence sandstones, at the base of the passive margin, often yield Neoproterozoic and Cambrian detrital zircons and may provide a more complete record of magmatism at this critical time in the margin.

The timing of magmatism along the margin, as inferred from the detrital zircon record, shows spatial trends that we relate to the timing of crustal thinning along the margin. Magmatism in western Laurentia at the latitude of Alberta and British Columbia occurred in four separate phases with peaks at 752 Ma, 683 Ma, 646 Ma, and 535 Ma. At the latitude of Utah magmatism was somewhat younger with detrital zircons yielding an age of 522 Ma. In the southwest US magmatism occurred at 556 Ma, 523 Ma, 518 Ma and 508 Ma. The proportion of Proterozoic zircons decreases to the south with many samples yielding no Cryogenian dates. Trace element compositions for Neoproterozoic and Cambrian detrital zircons suggest derivation from magmas of mafic to intermediate compositions consistent with a rift setting.

Our results support previous models that invoke a southward propagation of Rodinian rifting during the late Neoproterozoic and Cambrian. While, northern Cambrian samples are dominated by dispersed Cryogenian and Cambrian ages, southern Cambrian samples lack latest Neoproterzoic grains suggesting a later onset of rifting there.