New palaeomagnetic results from the Whin Sill complex: evidence for a multiple intrusion event and revised virtual geomagnetic poles for the late Carboniferous for the British Isles

Journal of the Geological Society, Nov 2004 by Liss, D, Owens, W H, Hutton, D H W

Abstract:

A large-scale palaeomagnetic study (125 specimens from 88 sampling localities) has been carried out on the Whin Sill complex and its associated dykes in northern England. The dominant carrier of remanent magnetization of the doleritic rocks is relatively pure magnetite of pseudo-single domain size, and the characteristic remanent magnetizations are typically well defined and unidirectional. Regionally consistent palaeomagnetic directions are obtained from three geographically distinct parts of the Whin Sill complex, here named as the Holy Island Sill (Declination/Inclination (D/I)= 189.5/ -2.8, α^sub 95^ = 6.3), the Alnwick Sill (D/I = 194.6/ - 25.6, α^sub 95^ = 8.1) and the Hadrian's Wall-Pennines Sill (D/I = 189.2/3.3, α^sub 95^ = 3.5). Although the Holy Island Sill and the Hadrian's Wall-Pennines Sill are of similar age (c. 296 Ma) and indistinguishable on palaeomagnetic grounds, the difference in palaeomagnetic directions between them and the Alnwick Sill demonstrates the multi-component nature of the complex; magma-flow indicators suggest that the Holy Island Sill and the Hadrian's Wall-Pennines Sill may also represent separate intrusion events. Dykes local to the three sills have remanence directions corresponding to their respective sills and can tentatively be identified as their feeder dykes. Virtual geomagnetic poles for the three palaeomagnetic sites are: Holy Island Sill and Holy Island Dyke, 346.8E, 35.4S; Alnwick Sill, High Green Dyke and St. Oswald's Chapel Dyke, 337.1E, 47.1S; Hadrian's Wall-Pennines Sill and Hett Dyke, 347.1E, 32.9S.

Keywords: sills, dykes, palaeomagnetism, emplacement, dolerite.

The late Carboniferous doleritic Whin Sill complex of northern England represents the archetype for sheet-like concordant intrusions; from it, indeed, geology acquired the term 'sill' (e.g. Dunham & Strasser-King 1982; Johnson & Dunham 2001). There is widespread evidence of igneous activity in NW Europe in the late Carboniferous and early Permian; for example, in the Midland Valley of Scotland (Dunham & Strasser-King 1982), the Oslo Rift (Neumann 1994; Sundvoll & Larsen 1994) and southern Sweden (Obst 1999). The suggestion of Ernst & Buchan (1997, 2001) that all these occurrences are related and are manifestations of a plume-related ' jutland event', has refocused interest on igneous activity in the region. Although the Whin Sill complex has been studied for more than a century (references given by Dunham & Strasser-King 1982; Randall 1995; Johnson & Dunham 2001; and a Presidential Address by Francis (1982) to the Society), problems regarding its mode of emplacement and the location of feeder dykes still remain.

As the dolerites of the Whin Sill complex carry a stable characteristic remanent magnetization, palaeomagnetic results from the complex have provided an important datum. A study by Storetvedt & Gidskehaug (1969), following pioneering work by Creer et al. (1959), has provided a magnetization direction that is accepted as defining a key virtual geomagnetic pole (VGP) for the late Carboniferous, contributing to the apparent polar wander path of the British Isles (Torsvik et al. 1989; Piper et al. 1991).

This paper reports on an extensive palaeomagnetic study carried out on the Whin Sill complex and its associated dykes. The results of the study provide new information on both the intrusion history of the Whin Sill complex and its accepted VGP, revealing previously unresolved detail in both.

Geological setting and sampling

The Whin Sill complex is a large tholeiitic quartz dolerite intrusion in northern England (Fig. 1). On land its outcrop extends for c. 120 km in a north-south direction and c. 80 km in an east-west direction but, as the sill complex generally dips underneath the North Sea, its true extent is unknown. The complex has a maximum observed thickness of c. 80 m and its average thickness is of the order of 30 m. The stratigraphical horizons into which it is emplaced range from Dinantian to Westphalian and the overburden thickness at the time of emplacement has been estimated as between 0.5 and 1 km (Francis 1982).

Four major ENE-WSW-trending doleritic dykes are associated with the Whin Sill complex (Fig. 1); from north to south these are the Holy Island Dyke, the High Green Dyke, the St. Oswald's Chapel Dyke and the Hett Dyke. The thickest of these dykes is the High Green Dyke, which reaches a width of up to c. 65 m. The close geographical relationship and the petrological affinity between the dykes and the sill complex have led several workers to propose that the four dykes formed the feeders to the complex (Holmes & Harwood 1928; Andcrson 1951; Francis 1982; Goulty et al. 2000). Additional geochemical data seem to confirm this finding (Thorpe & MacDonald 1985). Nevertheless, no direct relationship between the dykes and the Whin Sill complex can be demonstrated anywhere in the field, and there is some dispute in the literature as to whether the dykes acted as feeders (Smythe 1930; Johnson & Dunham 2001).