A high resolution record of multiple diagenetic events: Ultraviolet laser microprobe Ar/Ar analysis of zoned K-feldspar overgrowths

Abstract

High spatial resolution UV laser Ar/Ar analyses were performed in situ on K-feldspar overgrowths from sandstone hosted Pb–Zn bodies from central Sweden. Two different populations of ages were detected—Precambrian–Cambrian (528 ± 10 to 567 ± 8 Ma) and late Silurian (403 ± 6 to 453 ± 8 Ma). These were both detected in single pore spaces, with the older population adjacent to the detrital feldspar whilst the younger population was always distal to the detrital grain. This younging of authigenic K-feldspar ages with distance from the detrital grain margin is discussed in terms of disturbance to the argon system within an authigenic overgrowth that is of a single age: (a) the overgrowths are Precambrian–Cambrian in age and the late Silurian population might be an artefact of argon-loss due to partial resetting, (b) the overgrowths are late Silurian and the Precambrian–Cambrian population might have resulted from excess argon uptake, and (c) the overgrowths are late Silurian but the Precambrian–Cambrian population is an experimental artefact arising from the analysis of overlapping detrital/authigenic feldspars. Diffusion modelling shows that the partial resetting scenario is a very unlikely cause for the observed age variation because it requires a very particular arrangement of sub-grain sizes. Excess argon was not detected in either the K-feldspar or quartz. The data are best interpreted as the ages of two discrete diagenetic events—an early burial diagenesis and a later tectonically induced fluid flow event relating to the collapse of the Caledonian orogen. Further, the use of 38Ar Cl/ 39Ar K as a proxy for Cl/K ratio highlights the physical boundaries between feldspar populations, where all other petrographic, SEM and CL analyses have failed to do so. This study shows that high spatial resolution UV laserprobe Ar/Ar analysis is a powerful tool for extracting age information from complex hydrothermal minerals as a means to investigating fluid flow events.