Chapter 1 The igneous rocks of south-west England

Introduction and site synthesis

The magmatic rocks of south-west England fall within the northern European Variscan fold belt; they are dominated by pre-orogenic basic–acid volcanics and post-orogenic granites, together with minor volcanics, that span the Devonian and Carboniferous systems. These major magmatic groups have played their part in the evolution of petrogenetic theory, but on a more limited scale than, say, the igneous rocks of the British Tertiary province, and generally relative to the develop­ment of the Variscan fold belt. For example, the small-volume, effusive volcanic rocks of Devonian–early Carboniferous age in south-west England were identified in the early European literature as representative of the so-called 'spilite–keratophyre geosynclinal association'; that is, the association of basic and acidic volcanics in a deep basinal setting. They have their temporal counterparts throughout the Variscan Orogenic Belt of North­ern Europe and provided the scientific battle­ground for argument over the primary versus secondary origin of spilitic rocks (for example, Vallance, 1960; Amstutz, 1974), rocks which we now recognize as metamorphosed basalts.

Volcanic activity in Britain during the Devonian–Carboniferous can be broadly divided into two geographically separate areas that show contrast­ing eruptive and tectonic settings. The volcanic rocks of south-west England are dominated by medium- to deep-water submarine extrusives, shallow intrusives and volcaniclastics generated within rifted ensialic troughs and narrow ocean basins which appear to characterize the Variscides as a whole. Subsequently, they were extensively tectonized and metamorphosed dur­ing the different stages of the Variscan Orogeny and are thus characteristic of pre-orogenic vol­canism. On the other hand, the foreland contin­ental environment to the north in central-northern England and southern Scotland was outside the active orogenic belt and, as a consequence, deformation of volcanic rocks was relatively limited. The eruptive setting was also different. The calc-alkaline Old Red Sandstone volcanics of southern Scotland are dominated by subaerial lavas and volcaniclastics interbedded with thick sequences of intermontane sedimentary debris. Similarly, the extensive basaltic volcanics of Carboniferous age in northern England and the Midland Valley of Scotland are characterized by subaerial lavas and shallow, but often thick, intrusive complexes. Another significant differ­ence is that volcanism continued throughout the Carboniferous in the northern area, whereas in south-west England it terminated in the Viséan in response to thrust-generated crustal shortening (Floyd, 1982a, fig. 15.2).

The post-orogenic granite batholith volumetri­cally dominates the magmatic rocks found in south-west England. The granite batholith and its associated metalliferous ore bodies have provided the type area for fractionated, high-level, high heat-flow granitic terranes and models for hydrothermally induced zonal mineralization. From the economic viewpoint, the special character of the granite batholith has been used as a model for tin mineralization and late-stage alteration associated with acidic magmatism throughout the world. Direct comparisons can be made with the chemically distinct Caledonian granites, some of which also feature high heat flow, but lack the extensive mineralization and late-stage effects exhibited in south-west England. Recently the radioactive-element-enriched Cornubian granite batholith has also attracted national attention as a hot, dry rock energy source and as a potential environmental hazard due to the emission of seeping radiogenic radon gas.

Of no less importance is the Lizard Complex in south Cornwall, and the local problem of its age and tectonic significance. Recent work has firmly placed it in the Variscan tectonic regime, as a fragment of obducted ophiolite with an attendant sedimentary melange. If the early Devonian age for this dismembered ophiolite is correct, then it has European significance as being one of the few remnants of ocean crust exposed in the external zone of the Variscides.

The above brief introduction places the south­west England Devonian–Carboniferous igneous rocks in their regional and tectonic context relative to contemporaneous, often more exten­sive, magmatic episodes in Britain. However, compared with the magmatic character of the northern 'stable' foreland, the rocks of south-west England are characteristic of the various stages of evolution of an orogen and, in particular, illus­trate the spectrum of magmatic events relative to tectonic features within the external zone of the Variscan fold belt (Figure 2.1).

The general localities of the sites that have been selected are shown in (Figure 1.1) (numbered as listed in (Table 1.1)) and an outline of the significant features exhibited by each group of sites is given below. Details of the spatial and temporal location, emplacement environment and origin of the volcanic and plutonic rocks within the tectonic zones of the Variscan Orogen and the local geological framework of south-west England are given in Chapter 2.

Two interrelated criteria were used for the selection of sites in south-west England:

1. to provide a full stratigraphical coverage of different magmatic activity throughout the Hercynian fold belt;
2. to illustrate the special or unique petrological and chemical characteristics of different mag­matic units and their petrogenesis.

This allowed the continuum of magmatic activity within a fold belt to be documented, as well as highlighting special features best displayed in this region relative to elsewhere in the United Kingdom. The justification for choosing these specific sites, rather than others showing similar features, often rested on a combination of adequate geological exposure, lithological fresh­ness and accessibility.

The sites can be conveniently grouped into four main units (A–D) that roughly relate to stratigraphical age and major magmatic events within the Variscan. Some significant geological features exhibited by these units are summarized below:

Lizard ophiolite, mélange and Start Complex

The plutonic complex of the Lizard ophiolite includes the serpentinized peridotite, gabbro and basaltic dykes, together with heterogeneous acid–basic gneisses. As representatives of oceanic crust, these units play an important role in the interpretation of early Variscan basins in south Cornwall; they also provide evidence for subse­quent northward obduction. Although a volcanic carapace to the ophiolite is not present in sequence, tectonically associated, metamorphosed lavas (now hornblende schists) chemically simi­lar to mid-ocean ridge basalts are consistent with a Lizard ocean-crust model. Metamorphism and tectonism possibly occurred both in a suboceanic setting and during obduction. The metavolcanic greenschists of the Start Complex also exhibit mid-ocean ridge chemical features and may represent another tectonized segment of the early Variscan ocean floor in this region.

Pre-orogenic volcanics

This unit comprises various stratigraphically localized volcanic rocks which were erupted contemporaneously with basinal sedimentation. They range in age from Devonian to early Carboniferous, but culminated in late Devonian–Viséan times. Although they represent a bimodal basic–acid suite (the old 'spilite-keratophyre association'), the volcanics are dominated by basaltic pillow lavas and high-level intrusives of both tholeiitic and alkaline character. Basic and acidic tuffaceous volcaniclastics are also common throughout the Upper Palaeozoic. The volcanics invariably have been altered subsequent to con­solidation and deposition, exhibiting secondary assemblages indicative of the prehnite–pumpellyite and lower greenschist facies of meta­morphism.

Cornubian granite batholith

The culmination or late stages of the Variscan Orogeny were marked by the emplacement of the Cornubian batholith at the end of the Carboniferous. This body is often interpreted as the product of the melting of sialic crust induced by continent–continent collision. Although, pre­dominantly a two-mica calc-alkaline granite, the batholith is composed of multiple intrusions, ranging in age from about 300 to 270 Ma and encompasses a number of highly fractionated acidic members including Li- and F-rich variants. Late-stage alteration effects are well displayed and include extensive tourmalinization, greisenization and kaolinization. The granite was also the source of the hydrothermal Sn–W mineralization as well as the heat engine for associated Cu–Pb–Zn–Fe–As deposits within the margins of plutons and their aureoles.

Group A sites: Lizard ophiolite and mélange

Group B sites: Pre-orogenic volcanics

Group C sites: Cornubian granite batholith

Group D sites: Post-orogenic volcanics

(Table 1.1) List of GCR igneous rock sites in south-west England (see (Figure 1.1) for locations.

Post-orogenic volcanics

Shortly after consolidation of the batholith and regional uplift, a post-orogenic volcanic episode began in the late Carboniferous–early Permian. This comprised both suprabatholithic acid volcanism fed by late granite-porphyry dykes, and mafic extrusives and intrusives related to fault-bounded troughs. The latter group include various lamprophyres which often characterize the last stages of magmatic activity, apparently associated with granites of continental origin.

References