A geochemical investigation into the provenance of the Neoproterozoic Port Askaig Tillite, Dalradian Supergroup, western Scotland

Abstract

The major and trace element geochemistry of matrix materials from glaciogenic diamictites (unsorted polymictic conglomerates consisting of clasts scattered throughout a finer grained matrix) of the Neoproterozoic Port Askaig Tillite of western Scotland shows evidence of an upward change in provenance. Values for a carbonate-corrected Chemical Index of Alteration (CIA) are fairly high (∼70–80) in the lower diamictites, suggesting that they were derived mainly from an older sedimentary cover (older Dalradian Formations) that had undergone a previous weathering cycle. Low CIA values, more typical of unweathered material, were obtained from diamictites in the upper part of the formation. Thus, deposition of the Port Askaig diamictites is thought to involve a change in provenance, with unroofing of a cover of older sedimentary rocks and exposure of basement rocks. In spite of these inferred differences in provenance, diamictite materials throughout the formation have very uniform chondrite-normalized rare earth element (REE) distributions. REE distribution patterns from the diamictites closely resemble that of average post-Archean shales (PAAS), with a significant negative Eu anomaly and typical enrichment in LREE. These results, together with the presence of uniform (LaYb)N ratios throughout the formation, suggest that the sedimentary cover rocks from which the lower diamictites were derived, were themselves derived from a similar post-Archean basement. A ThSc versus Sc plot suggests that siliciclastic matrix material in the lowest diamictite was derived essentially from provenances similar to PAAS. The higher diamictites appear to have been derived from a mixture of granitic material and PAAS, in a ratio of up to ca 4: 1. Sparse sedimentary structures suggest derivation of the Port Askaig Tillite from the southeast but the exact source remains unknown.