Shallow deformation kinematic history, a new insight from carbonates U-Pb direct dating by LA-ICP-MS imaging technique.

This project aims to refine direct dating of carbonates by the U-Pb system, using a new LA-ICP-MS imaging technique that incorporates complementary element and textural analysis information. The direct dating of carbonates in deep time has been considered desirable for decades (e.g. Jahn and Cuvellier, 1994) given their ubiquity in the Earth system, and carbonates are a key phase for dating geological processes such as brittle-ductile deformation in carbonate successions. This new method facilitates detailed (on the scale of tens of microns) mapping of U-Pb isotope and element distributions (cf Drost et al., 2018), and is here applied to carbonate vein dating to constrain local and regional histories of deformation or fluid activity.

In this presentation we focus on a sample from the Carboniferous North Dublin Basin, Ireland. The basin has been affected by deformation that led to tight chevron folds and kinematically-linked dextral en-echelon vein sets. Additionally bedding-parallel veins and  slickenfibres are common. The deformation has been conventionally assumed to be of Variscan age, and some Variscan U-Pb ages are recorded in this study. However many calcites analysed yield late Eocene ages, a deformation phase that is hitherto undetected on the Irish mainland. Our data indicate repeated fault slip over a peroid of at least c. 4 my during late Eocene times and, thus, demonstrate the ability of the LA-ICP-MS imaging approach to not only unravel complex polyphase deformation histories in carbonates but also to resolve processes on fine temporal and spatial scales.

 

 

DROST, K., CHEW, D., PETRUS, J. A., SCHOLZE, F., WOODHEAD, J. D., SCHNEIDER, J. W. & HARPER, D. A. T. 2018. An Image Mapping Approach to U-Pb LA-ICP-MS Carbonate Dating and Applications to Direct Dating of Carbonate Sedimentation. Geochemistry, Geophysics, Geosystems, 19, 4631-4648.

JAHN, B.-M. & CUVELLIER, H. 1994. Pb-Pb and U-Pb geochronology of carbonate rocks: an assessment. Chemical Geology, 115, 125-151.