Evidence for 930 Ma metamorphism in the Shetland Islands, Scottish Caledonides: implications for Neoproterozoic tectonics in the Laurentia-Baltica sector of Rodinia

摘要

Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U–Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 ± 8 and 925 ± 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migmatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c. 650–700 °C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c. 650 °C and 8–9 kbar. In situ electron probe microanalysis (EPMA) U–Th–Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician–Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia–Baltica provenance. Deposition of the Westing Group is constrained to between c. 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway.