Magmatic Evolution of the Ordovician Snowdon Volcanic Centre, North Wales (UK)

Geochemical variation in the Penmaenmawr intrusion (North Wales)

“…Often (Figure 6a). The changes in the graphs at this SiO 2 value can be compared with the two-stage fractionation for the igneous rocks of this area advocated by Ball and Merriman (1989), Howells et al (1991) and Thorpe et al (1993). Despite this possibility of sudden changes at 62 .…”

“…For reasons which are considered later, it cannot be the sub-LRT type of basalt advocated by Howells et al (1991) or the Rhobell fawr type advocated by Thorpe et al (1993). It is, however, possible to calculate a probable mineral extraction to ®t the observed changes of composition found between 58% and 62 .…”

“…Howells et al (1991) accepted that the low-Zr group of rocks in the ®rst magmatic cycle (including the Penmaenmawr intrusion) was similar to the low-Zr group of the second cycle (the Snowdon Volcanic Group) and advocated that the latter were derived from basalts of the sub-Lower Rhyolitic Tu Formation type. Later, Thorpe et al (1993) concluded that these rocks came instead from a basic parent of the type found in the Rhobell fawr Volcanic Complex, some 30 km SE of Snowdon (Kokelaar 1986). But on a logarithmic TiO 2 vs. Zr graph ( Figure 10) the path of the Penmaenmawr rocks shows major dierences from the line of descent advocated by Thorpe et al (and from that previously suggested by Howells et al).…”

“…Similar dierentiation trends were recognized by Howells et al (1991) in the second magmatic cycle coming from two basic parents, both present as extrusive phases (as well as a third trend from a postulated high-thorium basalt, not exposed). Thorpe et al (1993) later reconsidered the nature of the basic parent of the low-Zr rock group of the second cycle, but their conclusions will be discussed more fully later.…”

A striated, far travelled clast of rhyolitic tuff from Thames river deposits at Ardleigh, Essex, England: evidence for early Middle Pleistocene glaciation in the Thames catchment

“…On this diagram, the Snowdonia and Llŷn rhyolites plot as a series of ellipsoidal clusters, one of which includes the Ardleigh clast (Nb/Zr = 0.064); the Nb/Y ratio of 0.36 for the Ardleigh clast also compares with certain of the Llŷn and Snowdonia rhyolite groupings. When plotted on diagrams showing ratios and fractionation trends between Nb/Th, Ta/Zr and TiO 2 /Zr, the Ardleigh clast lies within the fields of Snowdonian rocks, with particular affinities to the subalkaline rhyolite groupings 'A2' and 'B2' of Thorpe et al (1993).…”