Observations on the effect of proximity to rhyolitic intrusions on Kübler index

Abstract

The Upper Ordovician sedimentary succession of southeast Ireland is riddled with almost synsedimentary igneous intrusions. A study was required to determine any effects of these intrusions on Kübler Index (KI) so that further sampling for a regional study could be undertaken and the results would be unbiased. Sampling was carried out on the low-grade metapelites (Southeast Ireland) around the rhyolitic and doleritic intrusions, which were intruded into the still soft sediment. Clay mineralogy was determined using X-ray diffractometry and KI values were measured; SEM and whole rock elemental analyses were applied to complement the results of the XRD work. KIs immediately around rhyolitic intrusions were observed in all cases to be lower than the regional KI, however this was not seen around doleritic intrusions. SEM work shows illite grain sizes following the KI trend and K2O were noted to be highest where KI was lowest. Rare earth elements (REEs) show some mobilization which also follows the KI trend. This is considered to be a residual effect of hydrothermal fluids / volatiles hosted by the rhyolites but not encountered in the dolerites. Chlorite crystallinity is not affected as it most likely crystallized after the hydrothermal event. The KI is often used as a proxy for illite crystallinity in studies of low grade metamorphism but is influenced by many parameters and care should be taken when applying this technique to studies of regional metamorphism.