Detrital zircon, detrital titanite and igneous clast U–Pb geochronology and basement–cover relationships of the Colonsay Group, SW Scotland: Laurentian provenance and correlation with the Neoproterozoic Dalradian Supergroup

Abstract

A multidisciplinary provenance study, including in situ U–Pb dating of detrital titanite, was undertaken on the enigmatic low-grade metasediments of the Colonsay Group, SW Scotland to determine their source and assess previous correlations with major Neoproterozoic sedimentary sequences within the Scottish Caledonides. Sm–Nd model ages ( t DM) of the Colonsay Group range from 1664 to 2140 Ma, suggesting a Palaeoproterozoic source. Laser Ablation ICPMS U–Pb detrital zircon ages from six Colonsay Group formations are consistent with sources within Laurentia with predominant input from a ca. 1780 Ma source (Rhinns Complex) and some Grenvillian ( ca. 1.3–0.95 Ga), Pinwarian ( ca. 1.51–1.45 Ga), Labradorian ( ca. 1.71–1.62 Ga) and Ketilidian ( ca. 1.9–1.75 Ga) detritus. Archaean (> ca. 2.5 Ga) input is minimal. Clasts from the basal Octofad Sandstone Formation indicate input from ca. 1795 Ma and ca. 1400 Ma sources (U–Pb SIMS zircon ages). U–Pb (SIMS) analyses of detrital titanite record Grenville metamorphic events in the source terranes and suggest a slightly younger maximum depositional age of ca. 942 Ma compared with the youngest detrital zircon age of ca. 1025 Ma. The data support the interpretation that the Octofad Sandstone Formation is part of the Colonsay Group and that the Colonsay Group rests unconformably on the Rhinns Complex on Islay. Deposition of the Colonsay Group is, therefore, tied to the margins of Laurentia, possibly in the foreland to the Grenville Belt, as the Rhinns Complex does not record any Grenville deformation. The data are consistent with deposition of the Colonsay Group in a foreland basin related to the Rigolet phase ( ca. 1000–980 Ma) of the Grenville Orogeny. On the basis of detrital zircon U–Pb ages the Colonsay Group best correlates with the lowermost Dalradian Grampian Group of Scotland. This correlation has significant implications for Dalradian basin dynamics, timing of Dalradian deposition and timing of Dalradian deformation. In addition, the unconformable contact between the Colonsay Group and the Rhinns Complex on Islay would make it one of the few places where the basement to the Dalradian can be observed.