Zircon LA-ICPMS geochronology of the Cornubian Batholith, SW England

Abstract

Available UPb age data for the Cornubian Batholith of SW England is based almost entirely on monazite and xenotime, and very little zircon UPb age data has been published. As a result, no zircon inheritance data is available for the batholith, by which the nature of the unexposed basement of the Rhenohercynian Zone in SW England might be constrained.Zircon LA-ICPMS data for the Cornubian Batholith provides Concordia ages (Bodmin Moor granite: 316±4Ma, Carnmenellis granite: 313±3Ma, Dartmoor granite: ~310Ma, St. Austell granite: 305±5Ma, and Land's End granite: 300±5Ma) that are consistently 20–30Ma older than previously published emplacement ages for the batholith and unrealistic in terms of geologic relative age relationships with respect to the country rock. This discrepancy is likely as a consequence of minor pre-granitic Pb inheritance. Several of the batholith's granite plutons contain a component of late-Devonian inheritance that may record rift-related, lower crustal melting or arc-related magmatism associated with subduction of the Rheic Ocean. In addition, the older granites likely contain Mesoproterozoic inheritance, although the highly discordant nature of the Mesoproterozoic ages precludes their use in assigning an affinity to the Rhenohercynian basement in SW England.