Pre-Rodinian (Mesoproterozoic) supercontinental rifting along the western margin of Laurentia: geochemical evidence from the Belt-Purcell Supergroup

Recent efforts at supercontinental reconstruction for the late Precambrian (e.g., the SWEAT hypothesis Moores, 1991) have commonly invoked an uncertai n lacustrine interpretation for deposition of the Mesoproterozoic Belt-Purc ell Supergroup, with the Belt basin placed centrally within the assembled l andmass. The utility of the Belt-Purcell system in addressing Proterozoic c ontinental configurations has improved with refined constraints on the timi ng and setting of deposition. Contrary to a lacustrine interpretation, elem ental carbon-sulfur (C-S) relationships and stratigraphic S isotope trends for black shales of the Helena Embayment of western Montana, the easternmos t extension of Belt deposition, reveal that the early Belt basin received a t least episodic fluxes of open marine water into a variably restricted set ting. In the present study, this model of restricted marine deposition was tested and corroborated by generation of an analogous S isotope stratigraph y for the time equivalent Prichard Formation, located to the west of the em bayment within the main Belt basin. Data for both pyrite and coexisting pyr rhotite show comparatively high delta S-34 values and systematic S isotope shifts in excess of 20 parts per thousand over stratigraphic intervals span ning several hundred meters, suggesting limited and temporally varying sulf ate availability in a bacterially driven marine system. Additional geochemi cal and sedimentological proxies (e.g. delta C-13 of carbonate lithologies, diagnostic abiotic carbonate precipitates, and the presence of glauconite) further support a repeated or continued marine linkage throughout Belt dep osition. A model for a restricted marine setting, when viewed in light of i ndependent tectonic evidence (i.e. syndepositional mafic magmatism with pea ks in activity at approximately 1470 and 1370 Ma; hydrothermal activity,/mi neralization along basin-bounding, high-angle normal faults; and high rates of sedimentation that resulted in sediment thicknesses of up to similar to 16 km) defines a history of active and likely episodic rifting for the Bel t basin. Collectively, these observations are temporally consistent with ne w models for Precambrian supercontinent cyclicity and suggest a complex his tory for western Laurentia that includes both episodes of convergence and a t least incipient rifting prior to the final assembly of Rodinia. This rela tionship reflects a stepwise continental assembly perhaps analogous to the formation of Gondwana and the subsequent assembly of Pangea. (C) 2001 Elsev ier Science B.V. All rights reserved.