Paleozoic ocean plate stratigraphy unraveled by calcite U-Pb dating of basalt and biostratigraphy

Oceanic mafic volcanic rocks preserve unique information regarding the nature and evolution of tectonic plates. However, constraining their age is commonly challenging because of their lack of datable minerals and high degrees of alteration. We present in situ laser ablation-inductively coupled plasma-mass spectrometry U-Pb dating of calcite phases in altered basalts in a Paleozoic subduction complex (eastern Australia). Calcite enclosed in amygdules and filled in fractures yielded two distinctive ages with contrasting geochemical signatures. These results, combined with new biostratigraphic and whole-rock geochemical data, suggest that oceanic islands formed in the Panthalassa Ocean at about 365 million years ago, accreted to eastern Gondwana at about 330 million years ago, and underwent brittle deformation at about 305 million years ago. Calcite U-Pb geochronology is valuable to help constrain minimum formation ages of volcanic rocks and their deformation history, ultimately improving ability to unravel the geological record of accretionary complexes, and more generally ancient underwater volcanic systems. The formation and tectonic evolution of mafic oceanic crust can be determined through dating of calcite amygdules using U-Pb geochronology as well as geochemistry and biostratigraphic observations, according to a study of the New England Orogen, Australia.