Geochemistry of Upper Cambrian-Lower Ordovician black shale along a northeastern Appalachian transect

Abstract

Samples of Upper Cambrian to Lower Ordovician black shale, collected in a transect extending across the Canadian Appalachians from the Humber Zone in Quebec, the Gander and Avalon Zones in New Brunswick, to the Meguma Zone in Nova Scotia, were analyzed for 40 elements, including rare earths. These geochemical data lend support to previously established plate-tectonic models of the region. They also provide information on provenance and depositional environments of sediments that are too fine grained for more traditional mineralogical and sedimentological studies. Generally high Al 2 O 3 contents, high Al/Ti values, and steep rare-earth-element (REE) distribution patterns are consistent with a continental-margin depositional setting for shale from the Humber and Meguma Zones on opposite sides of the Iapetus ocean. Generally higher K 2 O contents, lower La/Th values, and lower absolute REE abundances distinguish shale of the Humber Zone deposited on the Laurentian margin from that deposited in the Meguma Zone on the Gondwanan margin. High La/Th values indicate a similar Gondwanan source for shale from both the Meguma and Avalon Zones. Shale from the Gander Zone contains less Al 2 O 3 at increasing distances from its boundary with the Avalon Zone. Low Al/Ti and less fractionated REE distribution patterns suggest a greater component of volcanic detritus in shale of the Avalon and Gander Zones. The presence of distinctive Balto-Scandian signatures (high U, V, and Mo) in shale of the Avalon and northwestern Gander Zones is possibly related to deposition in isolated peri-Gondwanan back-arc basins during a highstand of sea level.