Distribution and tectonic setting of Ordovician K-bentonites in the United Kingdom

Abstract

Twenty-five Ordovician K-bentonite samples ranging in age from Llanvirn to Ashgill and representing both sides of the Iapetus suture were analysed by INAA and XRF and compared on magmatic and tectonic discrimination diagrams. The proportional distribution of Nb, Y, Ti, Zr, Hf, Ta, Th and REE indicates that source volcanics were erupted in a setting that changed from subduction-related in the Llanvirn/Llandeilo to marginal basin extensional volcanism in the Caradoc/Ashgill. Welsh K-bentonites appear to be derived from volcanoes within the Welsh Basin, but K-bentonites from the Southern Uplands to not have characteristics consistent with a fore-arc setting, further suggesting a source related to ensialic marginal basin extensional volcanism.

The Lower Silurian Osmundsberg K-bentonite. Part II: mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance

“…Mean Eu/Eu* values for 16 samples of the Osmundsberg K-bentonite are 0.83 with a standard deviation of 0.15. These features are characteristic of more highly evolved peralkaline magmas (Jakes & White, 1972) and resemble previously reported Cambrian, Ordovician and Silurian K-bentonite compositions Huff et al 1993). The lack of correspondingly depleted HREE indicates the fractionation of phases such as garnet and clinopyroxene did not…”

Chemical signature of two Permian volcanic ash deposits within a bentonite bed from Melo, Uruguay

“…Using ratios rather than absolute values, the projection from Winchester and Floyd (1977) can be applied to altered volcanic ma-depth (m) depth (m) Fig. 5a -GROUP 1 terials such as bentonites (Huff et al 1993). In the present case, most of the studied samples are in the rhyolite composition field or near its boundary with comendite/pantellerite (Fig.…”

Geochemistry and origin of Carboniferous (Mississippian; Viséan) bentonites in the Namur-Dinant Basin, Belgium: evidence for a Variscan volcanic source

“…The whole-rock geochemistry of altered volcanic rocks such as bentonites and tonsteins has been employed in several other studies to gain information about the composition of the original (unaltered) ash layers and ultimately the magma from which they were derived (e.g. Spears & Kanaris-Sotiriou, 1979;Merriman & Roberts, 1990;Huff et al, 1993;Christidis et al, 1995). The major elements, which are routinely used to classify fresh or slightly altered volcanic rocks, are of limited use when classifying extensively altered volcanic rocks because several elements, including K and Na, are known to be mobile during weathering and diagenesis (Winchester & Floyd, 1977;Floyd & Winchester, 1978;Zielinski, 1982;Christidis, 1998).…”

“…The major elements, which are routinely used to classify fresh or slightly altered volcanic rocks, are of limited use when classifying extensively altered volcanic rocks because several elements, including K and Na, are known to be mobile during weathering and diagenesis (Winchester & Floyd, 1977;Floyd & Winchester, 1978;Zielinski, 1982;Christidis, 1998). Instead, the classification of altered volcanic rocks relies on trace elements including Ti, the high-field-strength elements (HFSE) Hf, Nb, Ta, Zr and the rare-earth elements, which are generally considered to be immobile during most upper crustal processes and are also indicators of petrogenetic processes (Floyd & Winchester, 1978;Huff et al, 1993). There are potential pitfalls associated with using whole-rock geochemistry; for example, aeolian fractionation of volcanic particles during transportation can modify the bulk composition of a distal ash layer (e.g.…”

“…There are potential pitfalls associated with using whole-rock geochemistry; for example, aeolian fractionation of volcanic particles during transportation can modify the bulk composition of a distal ash layer (e.g. Huff et al, 1993). Likewise, post-depositional processes such as sedimentary reworking can modify wholerock geochemistry (e.g.…”