Tracking the evolution of the Grenvillian foreland basin: Constraints from sedimentology and detrital zircon and rutile in the Sleat and Torridon groups, Scotland

Abstract

The Grenville Orogen, although occupying a key position in the Rodinia supercontinent, lacks a clear foreland basin in its type area in eastern Canada. Early Neoproterozoic siliciclastic rocks in northern Scotland, however, are now interpreted as remnants of a proximal Grenvillian foreland basin. Analysis of the sedimentology and detrital zircon and rutile of the Torridon and underlying Sleat groups provide new constraints on the evolution of this basin. Youngest U-Pb detrital zircon grains yield ages of 1070–990Ma in both groups, consistent with a Grenvillian source. The proportions of older age components vary throughout the stratigraphy. The lower Sleat Group shows a dominant ca. 1750Ma peak, likely derived from local Rhinnian rocks in Scotland and Ireland uplifted within the Grenville Orogen. In the upper Sleat Group and Torridon Group, detrital zircon peaks at ca. 1650Ma and ca. 1500–1100Ma become increasingly important. These latter peaks correspond with Labradorian Pinwarian, Elzevirian and early Grenvillian protolith ages within the eastern Grenville Province in Canada, and reflect exhumation and erosion of different mid-crustal complexes within that sector of the orogen. There is no difference in detrital zircon ages across the low-angle Sleat/Torridon unconformity. Detrital rutile in the Torridon Group yields a significant ca. 1070Ma, Grenville-age peak, but older grains (1700–1200Ma) also occur, suggesting derivation from the cool (T<600°C) upper orogenic crust. The detrital mineral data and sedimentology suggest the following evolution of the Grenvillian foreland basin in Scotland: i) early deposition in a narrow marine foreland basin (lower Sleat Group), sourced from the Irish-Scottish sector of the Grenville Orogen, with orogen-normal fill; ii) within the narrow Sleat Group basin a gradual switch to more distal sources in the Canadian sector of the Grenville Orogen, via axial transport; iii) an abrupt switch in basin dynamics (but not in source) across the Sleat-Torridon boundary to fluvial braidplain deposition in a much wider, Torridon-Morar basin; iv) followed by a gradual retrogradation of that basin. The Torridon-Morar groups represent a major denudational event of the Grenville Orogen that we infer was linked to more distal deposits in East Greenland, Svalbard and Norway.