The Palaeogene Bracken Bay–Straiton dyke: composition and controls on intrusion

Abstract

Synopsis The Bracken Bay–Straiton dyke, a member of the Palaeogene Mull dyke swarm, is one of the Solitary dykes of southern Scotland and northern England recognized by Geikie (1897). It shows a remarkable (unique?) change of strike direction, deviating from the regional NW–SE trend to propagate along the Southern Upland Fault for 16 km before apparently resuming its southeasterly course in the Southern Uplands. The dyke is a clinopyroxene–plagioclase–phyric quartz tholeiite, showing little petrographic variation along strike. However, geochemical data indicate that the dyke segment south of the Southern Upland Fault is not comagmatic with the main dyke; it is here distinguished as the Kello Water dyke. The reason for the dyke’s deviation at the fault is uncertain but may be related to a significant role for crustal heterogeneities, particularly Caledonian fractures which had earlier been utilized by the Late Carboniferous quartz dolerite dyke swarm. Supplementary material: Electron microprobe analysis data are available at http://www.geolsoc.org.uk/SUP18737 .

Large‐volume lateral magma transport from the Mull volcano: An insight to magma chamber processes

“…This range is represented by the shaded region in Figure c, and dike samples with MgO wt % higher than 7.5% are labeled as “high MgO” and those with lower than 2.5 wt % are labeled as “low MgO” (Figure b and Table ). Type 1–4 dikes include previously reported long‐distance dikes [ MacDonald et al ., ]. Type 1 dikes includes part of Eskdalemuir and Stevenston‐Coylton dikes, Type 2 includes Moneyacres and Barrmill‐Hartfell, Type 3 includes Cleveland, Dalraith, and Hawick‐Acklington, and Type 4 includes Bracken Bay‐Straiton (Figures and ).…”

“…At distances of >50 km from Mull, individual dikes can be traced across southern Scotland and northern England for >400 km [ Dagley et al ., ; MacDonald et al ., ]. Figure c combines the data from this study with existing data for these distal individual dikes, which tend to fall naturally into three/four groupings [ MacDonald et al ., ; Dagley et al ., ]. Each group represents a magma fractionation pathway or potentially a mixing trajectory between mafic and felsic magmas.…”

“…Our new data include dikes with high Fe 2 O 3 values relative to the published data (Figure b). This is because previously published dike data are mostly for long‐distance dikes which show decreasing Fe 2 O 3 with decreasing MgO, while our data include a number of proximal dikes which show first increasing, then decreasing Fe 2 O 3 with decreasing MgO [ Dagley et al ., ; MacDonald et al ., ].…”

“…Boundaries between Types 1–4 are shown in Figure 6c and are defined using a fractionation lines with slope = 0.786 as Type 1 = Fe 8.0 < 12.0, Type 2 = Fe 8.0 12.0–13.4, Type 3 = Fe 8.0 13.4–16.1, and Type 4 = Fe 8.0 > 16.1. Data include previously reported long‐distance dikes [ MacDonald et al ., ; Dagley et al ., ].…”

The Mull Paleocene dykes: some insights into the nature of major dyke swarms

“…No analysis of the stress fields associated with these swings in direction has been made postdating that of Anderson (1951) and we are unaware of occurrences of similar magnitude in the literature on which to base comparisons. Macdonald et al (2014) called on a significant role for crustal heterogeneities, specifically citing Caledonian fractures which had earlier been utilized by the late Carboniferous quartz dolerite swarm. Lineament L4, for example, runs parallel to the generally E-W strike of that swarm (Fig.…”