The Moel‐y‐Golfa Andesite: An Ordovician (Caradoc) intrusion into unconsolidated conglomeratic sediments, Breidden Hills Inlier, Welsh borderland

Abstract

The Moel‐y‐Golfa Andesite is a discordant intrusive body of porphyritic pyroxene andesite, 1 km across and at least 200 m thick. Textural variation defines a central facies and a marginal facies, the latter displaying well‐formed lobes, tubes, and pillows. The vertically stacked lobes, brecciated tubes, and elongate pillows of andesite intrude and locally enclose enclaves of altered host sediment and are cut by steeply dipping slide scars. The host sediments are locally strongly silicified and cut by numerous, anastomosed vertical and subvertical fissures. Fissures are filled by disrupted host sediment and fragments of brecciated andesite. The formation and filling of each fissure system was probably concomitant with the brecciation of an intrusive magma lobe. The complex relationship of igneous and sedimentary material indicates that the Moel‐y‐Golfa Andesite was emplaced into the host sediments whilst they were unconsolidated or only partly lithified.

Palaeomagnetism of Late Ordovician igneous intrusions from the northern Welsh Borderlands: implications to motion of Eastern Avalonia and regional rotations

“…Major features of this inlier were recognized by Watts (1885Watts ( ,1925 including the general anticlinal form, but the complex structural history and the relationship to other nearby Ordovician inliers have only become apparent from recent mapping (Dixon, 1990;Cave & Dixon, 1993). The Breidden Anticline is a complex faulted structure with an arcuate N-S to ENE-WSW trend and easterly plunge (Fig.…”

“…Although exposure is generally poor, the igneous rocks are well exposed in quarries. Sites 1 and 2 come from a large abandoned quarry in the intrusive Moel-y-Golfa Andesite mapped in detail by Dixon (1990). At this location a massive andesite body (incorporating site 1) and a brecciated zone of stacked lobes and pillows is separated by a transition zone from hosting volcanic conglomerates and altered sediments.…”

“…Ph.D. thesis, 1990). Evidence of magma/wet sediment interaction suggests that intrusion took place soon after deposition (Dixon, 1990). A similar marine succession which includes volcanoclastic rocks (the Forden Mudstone Formation) is exposed in the Montgomery area 21 km to the southwest (Fig.…”

“…Discordant intrusive contacts occur with host volcanoclastic sediments of Caradocian age (Wedd et al 1929;Brenchley, 1978). Although considered to be Devonian in age by Wedd et al (1929), Dixon (1990) has drawn attention to the regional tectonic setting and the strong similarities with the Moel-y-Golfa Andesite. Both units are emplaced into shallow marine rocks containing the zonal trilobite Broeggerolithus broeggeri (Brenchley & Pickerill, 1980;Dixon, 1990) and are characterized by a massive central zone; this grades outwards into flow-folded and brecciated marginal zones apparently formed by emplacement into unlithified host sediment.…”

“…Ph.D. thesis, Univ. Cardiff, 1990) and showing the distribution of palaeomagnetic sampling sites of this study, (b) Detailed geological map of the Moel-y-Golfa intrusive andesite after Dixon (1990) with palaeomagnetic sampling sites, (c) Regional location of the study area.…”