Provenance constraints on the Late Triassic ejecta layer from Churchwood Quarry, SW England: An impactite suite from Manicouagan

Abstract

In situ LA-ICP-MS/MS U-Pb and Rb-Sr geochronology combined with geochemical analysis and electron microscopy have been performed on ejecta components sampled from the Mid-to-Late Triassic Mercia Mudstone Group at Churchwood Quarry, SW England. The layer comprises altered impact spherules, melt-rich and fragment-rich accreted grain clusters (AGCs), along with shocked mineral phases. Late Triassic ages are obtained: a U-Pb age of 219 ± 72 Ma from variably shock-metamorphosed apatite and a Rb-Sr age of 213 ± 31 Ma from melt-rich AGCs. A post-depositional U-Pb age of 200 ± 7.5 Ma obtained from the carbonate host matrix correlates with an early Jurassic dolomitization event associated with regional marine transgression. Several links to the Manicouagan impact structure, Canada, are identified that complement previous provenance studies: (1) rare earth element compositions of impact spherules and melt-rich AGCs match those of the Manicouagan impact melt sheet; (2) the preservation of Archean and Paleo- to Neoproterozoic target rock U-Pb ages in zircon and apatite match those recorded within Manicouagan basement lithologies; and (3) impact spherules and melt-rich AGCs record initial ⁸⁷Sr/⁸⁶Sr compositions that overlap with those of the Manicouagan impact melt sheet and the target rocks involved in their generation.