Magmatism of the British Tertiary Volcanic Province

Abstract

Synopsis This review concentrates on those aspects of research into the magmatism of the British Tertiary Volcanic Province (BTVP) which have seen rapid accumulation of data and changes of views during the last few years. There seems to be general agreement at present amongst those who have made detailed geochemical studies of the problem that the pervasive hydrothermal metamorphism which affects most BTVP igneous rocks has not caused significant changes in the compositions (other than hydrogen and oxygen isotopes) of samples collected away from the immediate vicinities of the major passageways for hydrothermal fluids. Wide ranges of silica saturation characterize the basic members of both the major traditional BTVP magma types—Plateau and Non-Porphyritic Central (NPC). It is only the evolved members of these suites, the ne-normative Plateau-type hawaiites and the Q-normative NPC tholeiitic andesites, which show clearly the long-emphasized contrast between alkalic and tholeiitic compositions. Prior to their final uprise and eruption or emplacement, the Plateau-type magmas appear to have equilibrated near the base of the continental crust, whereas the NPC magmas evolved at upper-crustal depths. The NPC basalts (as traditionally defined) seem to be polygenetic in terms of mantle-derived magma types. Most of them are related to the MORB(Mid Ocean Ridge Basalt)-like Preshal Mhor basalts of Skye and Mull but some appear to be batches of Plateau-type magma which equilibrated in the upper crust. The Rio Grande Rift, southwestern U.S.A., provides a present-day analogue to the Palaeocene magmatic plumbing envisaged beneath the BTVP. The mantle-derived basic magmas upwelling beneath the BTVP were affected by fractional crystallisation and sialic contamination to varying extents as they rose through the continental crust. The earlier magma batches mostly interacted only with granulite-facies Lewisian gneisses, from which they appear to have melted selectively the most-fusible minor acid members. In addition, there is little doubt that Pb equilibrated selectively, in a vapour or non-silicate liquid phase, between the magmas and Archean sial, whilst Sr- and Nd-isotope ratios in the BTVP igneous rocks may also be in part the products of selective magma-crust interactions. If those BTVP basic rocks which contain negligible components of sialic contaminant are considered, it is apparent that there are consistent differences in various incompatible-element ratios between the Plateau-type basalts of Skye and Mull, the two areas studied in most detail at present. All of the BTVP Plateau-type basalts are exceptionally depleted in the strongly-incompatible elements, relative to worldwide examples of basalts with similar major-element compositions. This feature is consistent with genesis from a volume of upper mantle which had previously lost a very small fraction of strongly-alkalic melt. The Permian lamprophyres of the region are tentatively identified as samples of this postulated pre-Tertiary alkalic magma. A case is also made for considering the Caledonian Iapetus Suture as the tectonic feature along which most of the BTVP igneous centres are distributed. Seen from this point of view, the origins of BTVP magmatism are to be found in upper-mantle convection during and after the subduction which closed Iapetus—subsequently re-activated by the regional tension and Palaeocene mantle convection associated with the opening of the North Atlantic. At the climax of BTVP magmatism, when the bulk of the dyke swarms and the plutons of the central complexes were emplaced, a final increment of the progressive partial fusion of the upper mantle beneath the region gave rise to the MORB-like Preshal Mhor-type magmas. Although it is apparent from their Sr-, Nd- and Pb-isotope ratios that most of the BTVP acid magmas contained appreciable fractions from sialic sources, there are few instances where mantle-derived magma does not appear to have been the dominant ingredient in the hybrid liquids which evolved—by fractional crystallisation—to acid residua. Nevertheless, when the sequence of granite emplacement within individual BTVP intrusive centres is considered, it is apparent that the evolution of each acid magma batch was a complex and variable process, involving combinations of such mechanisms as fractional crystallisation, fusion of both sialic crust and earlier BTVP acid rocks, and mixing of magmas at various states of evolution.