THE CASE FOR EPISODIC RIFTING, SUBSIDENCE AND DEPOSITION IN THE BELT-PURCELL BASIN, 1470-1370 MA: GEOLOGIC PIERCING POINTS FOR RODINIA RECONSTRUCTIONS

Through the efforts of many research groups, there exist enough precise U-Pb dates on mafic magmatism associated with deposition in the Belt-Purcell basin to start to evaluate whether rifting and subsidence was a slow, steady progression or occurred in episodic pulses. Available dates suggest several distinct pulses of mafic magmatism throughout the Belt-Purcell basin ca. 1470, 1455, 1440, and 1380 Ma. Each of these rifting events can be correlated to major shifts in depositional sites and environments in the Belt-Purcell Supergroup further strengthening an interpretation of episodic rather than continuous deposition.

Mafic magmatism at 1470 Ma is well-documented from western Montana to British Columbia and correlates closely with collapse of the Belt-Purcell basin and a northward shift in the depo-center for the Lower Belt-Purcell turbidites (Cressman, 1989). The ca. 1455 Ma event is documented from the Paradise sill of western Montana and newly dated meta-diorite sills from the Hauser Lake Gneiss in the Priest River Complex of northern Idaho. This mafic event may correlate with tectonic subsidence of the Belt-Purcell basin recorded by the transition from the terrestrial Ravalli Group to the subaqueous Middle Belt Carbonate, U-Pb dated to 1454 ± 9 Ma (Evans et al., 2000). Eruption of the 1443 ± 7 Ma Purcell lavas (Evans et al., 2000) was associated with graben formation in Canada (Höy, 1993) and tectonic re-configuration of the basin recorded in the upper Missoula Group (cf. Link et al., 1993). The ca. 1440 Ma event also appears in the Hauser Lake sills. The 1380 Ma event is well documented in the Shoup area of central Idaho, where mafic sills, rapakivi granite and burial metamorphism have all been directly dated. Similar aged A-type granites to the west document a basin-wide rift with rapid sediment accumulation at least locally, such as the Garnet Range Formation in the upper Missoula Group of central western Montana.

The new dates of mafic sills reported here from the Hauser Lake gneiss of the Priest River complex establish the Belt-Purcell history at the westernmost margin of Laurentia. Similar mafic magmatism and episodic rifting can be expected for the conjugate continent in Rodinia reconstructions. Potential matches include the Kuonamka and 1380 Ma Cheiress dike swarms (Ernst et al., 2000) of Anabar massif of Siberia.