A Discussion on the evolution of the Precambrian crust - Evolution of an early Proterozoic continental margin: the Coronation geosyncline and associated aulacogens of the northwestern Canadian shield

Abstract

The Coronation geosyncline developed in the early Proterozoic along the western margin of a continental platform (the Slave Province) of Archaean rocks older than 2300 Ma, and culminated between 1725 and 1855 Ma ago with the emplacement of a pair of batholiths (the Bear Province). The evolution of the geosyncline has a strong family resemblance to Phanerozoic geosynclines believed to delineate ancient continental margins and have been controlled by global plate interactions. Such geosynclines are unknown in Archaean orogenic belts, from which it is inferred that creation of the first large rigid continental platforms marked the end of the Archaean and the beginnings of actualistic plate tectonics. The geosyncline began with deposition of a westward-facing continental shelf, consisting of a lower formation dominated by orthoquartzite, derived from the platform, and an upper cyclic stromatolitic dolomite formation. West of the shelf edge, the dolomite passes abruptly into a much thinner mudstone sequence with dolomite debris-flows, and the orthoquartzite into a thick laminated silt and mudstone sequence with quartzite turbidites. The oldest rocks west of the shelf edge, an area interpreted to have been a continental rise, are pillow basalts and volcanic breccias, extruded above a basement of unknown character. The principal turning point in the evolution of the geosyncline came with the foundering of the continental shelf. It is draped by a thin laminated pyritic black mudstone sequence, overlain by a westward-thickening clastic wedge resulting from intrusion and erosion of the batholiths to the west. The clastic wedge begins with a thick sequence of coarse greywacke turbidites that passes eastward into concretionary mudstone on the platform. The mudstone grades upward into laminated shaly limestone with minor greywacke turbidites, overlain in turn by cross-bedded red lithic sandstone. The supracrustal rocks of the geosyncline have been compressed and tectonically transported toward the platform. Adjacent to the batholithic belt, the continental rise and clastic wedge sequences are penetratively deformed and recrystallized by regional low-pressure metamorphism. To the east, the unmetamorphosed continental shelf and clastic wedge sequences have been flexurally folded and overthrust above a basal detachment surface. East of the thrust zone, relatively thin rocks on the platform are nearly flat-lying except around large anticlinal basement uplifts. Unusual features of the platform are its two aulacogens - long-lived deeply subsiding fault troughs that extend at high angles from the geosyncline far into the interior of the platform. During every phase in the evolution of the geosyncline, the aulacogens received much thicker sedimentary sequences, commonly with the addition of basaltic volcanics, than adjacent parts of the platform. Although equal in thickness to the geosyncline, the aulacogens were never subjected to the batholithic intrusions, regional metamorphism or low-angle overthrusting characteristic of the geosyncline. The Athapuscow aulacogen, in the region of Great Slave Lake, is interpreted as having been an incipient rift, located over a crustal arch, during the continental shelf stage of the geosyncline, but sagged to become a crustal downwarp during the clastic wedge stage, ultimately with sufficient transverse compression to produce broad folds. Finally, the aulacogen became part of a regional transcurrent fault system, along which thick fanglomerates accumulated in local troughs. The batholithic belt consists of two batholiths, eroded to different depths, separated by the northtrending 350 km long Wopmay River fault. The Hepburn batholith, east of the fault, is a composite intrusion of mesozonal granodiorite plutons. The foliated and migmatitic borders of the plutons are normally concordant with wall rock sheaths of sillimanitic paragneiss. Along the eastern margin of the batholith, metamorphosed rocks of the continental rise sequence dip gently to the west beneath the batholithic rocks. Belts of intensely deformed and metamorphosed supracrustal rocks within the batholithic terrain include sequences of pillow basalt, pelites and granite-pebble conglomerate, perhaps the lower part of the continental rise deposited during the initial rifting of the continental margin. The Great Bear batholith, west of the fault, consists of discordant epizonal plutons, mostly adamellite, that intrude broadly folded but regionally unmetamorphosed sequences of welded rhyodacitic ash-flow tuff, trachybasalt and derived sedimentary rocks. The volcanic rocks, intruded by dense dyke swarms radiating from the plutons and by felsite plugs, are interpreted to be comagmatic with the plutons. Mapping is as yet insufficient to establish, speculations aside, the possible relations of the two batholiths to arc-trench systems. Furthermore, the western margin of the batholithic belt, a region of critical importance, is covered by a veneer of younger Proterozoic and Paleozoic sedimentary rocks. Until fossil arc-trench systems are outlined, the contention that the Coronation Geosyncline involved global plate interactions is based on indirect evidence - the analogous evolution of the geosyncline east of the batholithic belt with Phanerozoic geosynclines in which fossil arc-trench systems have been found.