A-type granites and related rocks: Evolution of a concept, problems and prospects

Clinopyroxene composition as a method of identification of the magmatic affinities of paleo-volcanic series

Some remarks on the movement of small melt fractions in the mantle

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.

Magmatism of the British Tertiary Volcanic Province

The partitioning of Fe and Mg between garnet and aluminous orthopyroxene has been experimentally investigated in the pressure-temperature range 5–30 kbar and 800–1,200° C in the FeO-MgO-Al2O3-SiO2 (FMAS) and CaO-FeO-MgO-Al2O3-SiO2 (CFMAS) systems. Within the errors of the experimental data, orthopyroxene can be regarded as macroscopically ideal. The effects of Calcium on Fe-Mg partitioning between garnet and orthopyroxene can be attributed to non-ideal Ca-Mg interactions in the garnet, described by the interaction term:W

 CaMg
 ga
 -W

 CaFe
 ga
 =1,400±500 cal/mol site. Reduction of the experimental data, combined with molar volume data for the end-member phases, permits the calibration of a geothermometer which is applicable to garnet peridotites and granulites:
 
$$T(^\circ C) = \left\{ {\frac{{3,740 + 1,400X_{gr}^{ga} + 22.86P(kb)}}{{R\ln K_D + 1.96}}} \right\} - 273$$

 with
 
$$K_D = {{\left\{ {\frac{{Fe}}{{Mg}}} \right\}^{ga} } \mathord{\left/ {\vphantom {{\left\{ {\frac{{Fe}}{{Mg}}} \right\}^{ga} } {\left\{ {\frac{{Fe}}{{Mg}}} \right\}}}} \right. \kern-\nulldelimiterspace} {\left\{ {\frac{{Fe}}{{Mg}}} \right\}}}$$

 and
 
$$X_{gr}^{ga} = (Ca/Ca + Mg + Fe)^{ga} .$$

 The accuracy and precision of this geothermometer are limited by largerelative errors in the experimental and natural-rock data and by the modest absolute variation inK
D with temperature. Nevertheless, the geothermometer is shown to yield reasonable temperature estimates for a variety of natural samples.

An experimental study of the partitioning of Fe and Mg between garnet and orthopyroxene

The liquidus temperature (1198 °C) and equilibrium phase relations of a sample of Columbia River basalt from the Picture Gorge section have been determined at I atmosphere by heating in a controlled atmosphere When this basalt is cooled from above its liquidus temperature the liquidus phase (plagioclase) may fail to crystallize depending on the degree of undercooling and the duration of the experiment A field in temperature-time space in which plagioclase fails to crystallize on cooling is separated from another in which plagioclase always crystallizes by a third in which the nucleation of plagioclase is unpredictable in terms of temperature and time The extent to which this basaltic liquid can be supercooled without the crystallization of plagioclase is independent of the time it is held above the liquidus or the temperature in excess of the liquidus to which it is heated The exceptionally long times required to ensure the nucleation of plagioclase at or near the liquidus temperature suggest that many so-called ‘equilibrium’ phase relations determined from experiments of a few hours' duration could be in serious error if the ‘equilibration’ involves a nucleation process It is demonstrated that, over a range of cooling rates, the temperature at which plagioclase begins to crystallize on cooling varies markedly and the temperature and times required for both possible and certain nucleation of plagioclase are calculated for a range of constant cooling rates The range of cooling rates over which the nucleation temperature of plagioclase varies is likely to occur in nature only in certain lava flows and small minor intrusions In such cases this could lead to changes in the order in which the minerals appear on cooling and other petrologically significant effects

Supercooling and the crystallization of plagioclase from a basaltic magma

The Zoned Clinopyroxenes of the Shiant Isles Sill, Scotland

High-temperature, very deep, geological disposal: a safer alternative for high-level radioactive waste?

We have tested the ability of some 12 different barometer and 20 different thermometer formulations to reproduce the experimental P-T equilibration conditions of ‘natural’ multi-component garnet lherzolite assemblages. For natural rock compositions it is essential to take account of the influence of both Cr and Fe on the garnet-orthopyroxene Al exchange reaction customarily used as a barometer for such assemblages and accordingly our results demonstrate that the formulation of Nickel and Green (1985) is the most satisfactory. No single thermometer formulation was judged to be reliable throughout the P-T range of interest. In our view equilibration temperatures are best assessed by consideration of a combination of the most satisfactory thermometer formulations, based on the two-pyroxene solvus and the Fe2+-Mg2+ exchange reactions between mineral pairs. Our results further indicate that use of the barometer (MacGregor 1974) and thermometer formulations recommended by Finnerty and Boyd (1984 and 1986) will lead to inaccurate assessment of the temperatures and pressures of equilibration for most garnet lherzolite xenolith assemblages and hence to incorrect interpretation of their depth of derivation within the mantle.

Evaluation of mineral thermometers and barometers applicable to garnet lherzolite assemblages

It has been advocated that convective and crystal settling processes play significant, and perhaps crucial, roles in magmatic differentiation. The fluid dynamics of magma chambers have been extensively studied in recent years, both theoretically and experimentally, but there is disagreement over the nature and scale of the convection, over its bearing on fractionation and possibly over whether it occurs at all. The differential distribution of modal olivine with height in differentiated alkaline basic sills provides critical evidence to resolve this controversy, at least for small to medium-large magma chambers. Our own and others' published data for such sills show that, irrespective of overall olivine content, modal olivine contents tend to increase in a roughly symmetrical manner inwards from the upper and lower margins of the sill, i.e. the distribution patterns are more often approximately D-shaped rather than the classic S-shape generally ascribed to gravity settling. We concur with the majority of other authors that this is an original feature of the filling process which has survived more or less unchanged since emplacement. We therefore conclude that the magmas have not undergone turbulent convection and that gravity settling has usually played only a minor modifying role since the intrusion of these sills. We offer a possible explanation for the apparent contradiction between fluid dynamical theory and the petrological evidence by suggesting that such sills rarely fill by the rapid injection of a single pulse of magma. Rather, they form from a series of pulses or a continuous pulsed influx over a protracted interval during which marginal cooling severely limits the potential for thermal convection.

Fergus G. F. Gibb

Papers

Supercooling and the crystallization of plagioclase from a basaltic magma

The Zoned Clinopyroxenes of the Shiant Isles Sill, Scotland

High-temperature, very deep, geological disposal: a safer alternative for high-level radioactive waste?

Evaluation of mineral thermometers and barometers applicable to garnet lherzolite assemblages

Convection and crystal settling in sills