Folded cauldrons of the Early Proterozoic Labine Group, northwestern Canadian Shield

Abstract

Great Bear Magmatic Zone (1.875–1.84 Ga) is a linear belt, 100 km wide by 800 km long, of little metamorphosed volcanic rocks and allied plutons of calc‐alkaline affinity. The entire zone was folded shortly after magmatism, and sections many kilometers thick through several cauldron complexes are exposed. Eruptions of rhyolite led to collapse of Black Bear Cauldron in which 1.5 km of tuff ponded. The cauldron then became the locus of fluvial and lacustrine sedimentation followed by augite‐plagioclase porphyritic andesite volcanism, both of which also accumulated within the cauldron. Ash flow eruptions of dacite caused collapse of Clut Cauldron which was accompanied by landsliding and avalanching of the cauldron walls. Abundant andesitic and intrusive debris is intercalated with the propylitized intracauldron facies tuff adjacent to the walls and some blocks are as large as 1 km across. Clut Cauldron also became the site for fluvio‐lacustrine sedimentation after collapse and was likely resurgent. Resurgence was probably related to the emplacement of a quartz monzonite pluton which occupies the core of the cauldron. Cornwall Cauldron, which is exposed entirely in cross section, contains 1–2 km of intracauldron facies tuff overlain by a complex of dacite lava flows. The cauldron was intruded by a granodiorite‐monzogranite pluton probably responsible for resurgence. Data from the area suggest the following: (1) the similarity of early Proterozoic cauldrons to Cenozoic examples indicates that cauldron collapse is a process that has occurred since 1.9 Ga; (2) batholiths beneath ash flow tuff fields are probably composite bodies made up of many individual plutons; (3) most of the plutons are sheetlike in cross section; and (4) cauldron collapse begins as soon as ash flow eruptions, and relief on the cauldron margin becomes large early in the subsidence history.