The first five papers of this issue of the Mineralogical Magazine have as a common theme Tertiary Igneous Geology. In May 1983 the Society held a thematic meeting on the igneous rocks of the Tertiary Province and from this has followed a collection of papers, grouped here together for convenience. The range of topics covered is broadly based and the papers demonstrate the extent of current research activity in the area.

All readers of the Magazine will be aware of the extent of the contribution made to igneous petrology and geochemistry by work done on the Tertiary Igneous Province. The Tertiary age of the igneous rocks had been recognized from work during the last century by Sir Archibald Geike and Professor J. W. Judd.

The Dippin sill, which is emplaced into the Triassic sediments of SE Arran, is an alkaline basic sheet which displays pronounced hydrothermal alteration. The 40-m-thick sill has suffered pervasive contamination with radiogenic Sr, introduced from the Triassic sediments by hydrothermal fluids. Stable isotope measurements suggest that fluids were of meteoric origin, but were restricted to a small closed-system circulation. Initial 87Sr/86Sr ratios in the sill were raised from an original value of 0.7032 to a maximum of 0.7091, contamination being especially pronounced near the contacts at Dippin Head itself (localities 12 and 14) and in a drill core section through the sill above Dippin. Hydrothermal Sr was incorporated into an early-formed high-CaO, high-Sr analcime, which replaced unstable high-silica nepheline in interstitial patches. However, this high-CaO analcime, along with plagioclase, was later replaced by a low-CaO, low-Sr analcime, allowing Sr leaching from the margins of the sill. Hydrothermal fluids are thought to have migrated up to 1 km laterally, up the dip of the sill, mainly via tension joints forming in the cooling intrusion. Pooling of hot fluids at the upper end of the sill probably raised water/rock ratios in this region and allowed greater Sr contamination during mineralogical alteration. The undersaturated mineralogy of the sill accounts for its pervasive hydrothermal Sr contamination, which contrasts markedly with the relatively undisturbed Sr isotope compositions of Hebridean granites involved in hydrothermal systems.

The Myggbukta Complex is a shallowly dissected central volcano superimposed on the early Tertiary Plateau Basalts of NE Greenland. This, and the Kap Broer Ruys centre, 30 km to the east, appear to be the most northerly central complexes of the North Atlantic Tertiary Province. The Myggbukta Complex comprises a suite of extrusions and minor intrusions ranging from picritic basalt to potassic rhyolite: most of the suite appears related by relatively low pressure (< 10 kbar) crystal fractionation. A small rise in initial 87Sr/86Sr (0.70593) with silica content is attributed to minor contamination through crustal anatexis. A basic dyke-swarm associated with the complex, precedes it and shows more limited differentiation. The basalts of the dyke-swarm and the Myggbukta Complex are genetically intimately related to the lavas forming the upper part of the earlier plateau basalt succession (UPLS). It is proposed that a large shield volcano developed some 100 km west of the developing spreading centre (Mohns/Aegir ridge), of which the UPLS, the dyke-swarm and the Myggbukta Complex represent three successive evolutionary stages. A generalized increase in differentiation through time can be recognized from one stage to the next. The acid intrusions of the Kap Broer Ruys area are probably also largely residues of basalt fractionation like their Myggbukta counterparts. However, higher initial 87Sr/86Sr ratios (0.70625–0.71034) imply a greater degree of crustal contamination.

An alternative explanation for the occurrence of chrome-spinel layers in the Eastern Layered Series of the Rhum intrusion is suggested by extreme concentrations of chrome-spinel in small-scale structures in the layer at the unit 7–8 boundary (Brown, 1956). These take the form of downward pointing cones several centimetres across and deep, and lined or wholly filled with chrome-spinel; lamination in the underlying allivalite exhibits quaquaversal dips around these cones. By comparing these structures to fluid escape structures in clastic sediments, it is proposed that spinel is the product of mixing and reaction of upward moving interstitial liquid and more primitive liquid newly emplaced in the chamber. Further evidence for the presence of a second liquid during spinel crystallization is provided by spherical silicate inclusions within spinel grains. Complex zoning in feldspars in the underlying allivalite suggests that the newly emplaced primitive liquid was able to penetrate the crystal mush on the intrusion floor.

Geochemical data (including REE determinations) are presented for all five Mourne Mountains granites and three Northern Ireland rhyolites. These confirm (1) the extremely fractionated nature of some of the rocks (Sr and Ba < 10 ppm, Rb > 400 ppm, Eu/Eu* < 0.1, and K/Rb < 100), and (2) a major revision to the outcrops of the E. Mourne granites G1 and G2 in which much of the former is reclassified as G2. Combined petrographic and geochemical studies have also indicated that magmatic pulses were involved in the emplacement of Mourne intrusions G2 (Revised)-G5 inclusive.

The N. Ireland Tertiary acid rocks exhibit general geochemical similarities to their analogues elsewhere in the British Tertiary Igneous Province (in which Sr is generally < 100 ppm and CeN/YbN generally < 8 with Eu/Eu* often < 0.6), but as a suite the Mourne granites are enriched in Rb and some other LIL elements relative to their N. Arran counterparts.

The more fractionated acid magmas of NE Ireland are believed to have evolved from primitive granitic parent liquids by crystal fractionation at depth which involved major and accessory phases (including zircon and allanite). In the Mourne (and County Antrim) areas the primitive acid compositions lie at the ends of basaltic (tholeiitic) differentiation series, and in the Mourne central complex there is a complete geochemical sequence from basic rocks through intermediate members to primitive and ultimately highly evolved, subalkaline, granitic intrusions. It is concluded that the data are consistent with the Mourne granites and Northern Ireland rhyolites being essentially basaltic differentiates, although Sr isotope evidence indicates some (probably minor) crustal involvement.

Major- and trace-element data are presented for basic dykes intruding the Eastern Red Hills district of the Tertiary igneous centre of Skye. In addition to a group of transitional basic intrusions, members of two other distinctive suites have been identified: (1) a series of basic alkaline intrusions (akin to the ‘Beinn Dearg Type’ of Harker, 1904), with characteristic high alkali contents, distinctive values of K2O/(K2O + Na2O) and rare-earth-element (REE) patterns similar to those of many of the Tertiary lavas of north Skye, and, (2) tholeiitic dykes, with low concentrations ofK2O, Ti/Zr values of c. 100, and flat REE patterns similar to those of the so-called Fairy Bridge magma type (Mattey et al., 1977). From these data and a consideration of the time relationships of the various components of the Skye Centre, agreement is found with Thompson et al.'s (1972, 1980) model of a mantle thermal anomaly which generates the magmatic sequence: transitional basic magmas, giving way to olivine-bearing tholeiitic magmas (at the culmination of the anomaly), and, finally, late-stage basic alkaline magmas at the end of the activity.

The powder diffractograms of twenty wet chemically analysed samples in the isomorphous triphylite-lithiophilite series and five synthesized members with Fe/(Fe + Mn) = 1, 0.75, 0.50, 0.25, and 0.0, were recorded. Their unit cell dimensions were accurately refined in order to find a reliable method for semi-quantitative determination of the divalent cation content of these minerals. A multivariate best fit analysis based on Kummell’s procedure shows the marked influence of Fe2+ and Mg2+ on the cell dimensions, as well as that of small amounts of Fe3+ substituting for Mn2+, following LiMn2+ → □ Fe3+. The best representation of the correlation between chemical composition and cell parameters is given by the equations:

a = 6.1041 − 0.0245 Fetot − 0.049 Mg2+

b = 10.4511 − Fetot − 0.082 Mg2+

c = 4.7400 − 0.0130(Fe2+ + Mg2+) − 0.025 Fe3+.

No evidence of non-linearity has been found for the variation of the three cell dimensions with the chemical composition. Assuming the absence of appreciable amounts of Mg2+, the following set of equations is proposed: Fetot = 41(6.104–a); Fetot = 35(10.451–b); Fetot = 77(4.740–c) in which the c dimension gives a relatively poor estimate.

Two sets of determinative graphs were constructed, one based on the cell parameter variation, and the other on the 311, 222, and 142 reflection angular positions, v. the total iron content of these minerals. These two methods, whose reliability is examined, can be used for determination of the divalent cation content, provided the samples contain less than about 0.5 wt.% Na2O, 0.5 wt.% CaO, and 3 wt. % Fe2O3, and are homogeneous.

Electron probe microanalyses carried out on thirty areas of lithiophorite (Al,Li)MnO2(OH)2 from five Australian localities indicate wide variations in its Ni, Co, Cu, and Zn content. The concentration of these elements varies inversely with the alumina content, though there is a marked change in their concentration at 18% alumina, suggesting the existence of two varieties of lithiophorite with differing concentration of transition elements. EPMA line scans also indicate definite changes in Co content in lithiophorite grains less than 0.2 mm in dimension. The variations in alumina and transition elements may be explained if lithiophorite is considered to be an irregular mixed-layer-lattice intergrowth of pure lithiophorite with members of the recently discovered asbolane-type minerals.

The result of electron probe microanalyses, along with X-ray and optical data obtained for tin sulphides from two different localities in south-west England indicate that the mineral hitherto described from St Michael’s Mount as stannite is in fact kesterite, Cu2(Fe0.23Zn0.77)SnS4. The yellow ‘stannite’ reported from this locality proved to be stannoidite, Cu8(Fe2.39 Zn0.61)SnS2S12. Kesterite, Cu2(Fe0.09Zn0.90SnS4, and zincian stannite, Cu2(Fe0.63Zn0.38)SnS4, are also reported from Cligga Head. It is suggested that a more cautious approach should be taken in identifying these tin sulphides solely on their optical properties.

The first occurrence of mpororoite and anthoinite outside the African Continent has been found in an altered skarn from the Kara mine, Tasmania, forming a white powdery mixture of the two minerals pseudomorphing scheelite. The wet chemical analysis of the mixture shows WO3, Al2O3, and H2O to be the essential constituents, proving the non-essential nature of Fe2O3 in mpororoite; the original mpororoite contained high Fe2O3 substituting for Al2O3. The thermal treatment of the mixture at 100°C for 6 hours in air brings about the conversion of mpororoite into anthoinite, which re-hydrates into mpororoite, although the associated anthoinite does not. The source of Al is ascribed to the decomposition of aluminian andradite in the skarn.

Grandidierite occurs in the outer margin of a biotite-rich pelitic xenolith in norite close to the contact of a tourmaline-bearing granite in the Haddo House norite-gabbro complex. The boron for the grandidierite was derived by metasomatism from the granite. The assemblage is (decreasing order): An55–51 plagioclase (65%), biotite (10%), spinel (10%), cordierite (5%), corundum (5%), ilmenite, grandidierite (< 1%), rutile, sphene, zircon. XMg range is grandidierite (0.80–0.78) > cordierite (0.78) > biotite (c. 0.59) > spinel (0.35–0.27). The grandidierite has little chemical variation: SiO2, 19.9–20.5%; Al2O3, 51.3–51.9%. A compilation of all grandidierite analyses in the literature shows that the only substitutions are Fe2+ ⇌ Mg and Al ⇌ Fe3+, the XMg range being 0.98 to 0.19.

The Carnmenellis granite and its aureole contain the only recorded thermal groundwaters (up to 52 °C) in British granites. They occur as springs in tin mines at depths between 200 and 700 m and most are saline (maximum mineralization 19 310 mg 1−1). Mining activity has disturbed the groundwater circulation pattern developed over a geological time-scale and levels of bomb-produced tritium (> 4 TU) indicate that a significant component (up to 65 %) of the most saline waters are of recent origin. All components of all the mine waters are of meteoric origin. Radiogenic 4He contents, 40Ar/36Ar ratios, and uranium series geochemistry suggest that the thermal component has a likely residence time of at least 5 × 104 years and probably of order 106 years.

The thermal waters have molar Na+/Cl− ratios considerably less than 1 but they are enriched relative to sea water in all major cations except Mg. The groundwater is also particularly enriched in Li with contents ranging up to 125 mg 1−1. The groundwater salinity, which may reach a maximum of 30 000 mg 1−1, is shown to result from weathering reactions of biotite (probably through a chloritization step) and plagioclase feldspar, to kaolinite. On volumetric considerations, fluid inclusions cannot contribute significantly to the groundwater salinity, and stable isotope ratios rule out any contribution from sea water.

Groundwater silica contents and molar Na+/K+ ratios suggest that the likely equilibration temperature is 54°C, which would imply a depth of circulation of about 1.2 km.

Blue-coloured gem-quality spinel from Nigeria was analysed by wet chemical methods (using atomic absorption spectrophotometry) and investigated by X-ray diffraction. The results showed it to be gahnite (unit cell dimension a=8.091±0.003 Å) containing 36.7% ZnO, 3.58% FeO, and 0.12% MgO. The spinel has an RI of 1.79 and density between 4.4 and 4.59. Broadening of the n[111] XRD reflections indicates a measure of compositional heterogeneity. The gahnite analyses were compared with compositions of zinc spinels from other parts of the world. The analyses cluster into two distinct groups, Mg-rich spinels of metamorphic origin and Mg-poor spinels (including the Nigerian gahnite) with igneous affinities. Diadochy seems to operate within the zinc spinel structure between (Zn + Mn) and (Fe + Mg).

The thermal expansions of trimerite and beryllonite have been determined up to 800°C. No anomalies were observed in the thermal expansion curves but the expansion coefficients for beryllonite are approximately twice those of trimerite. The thermal expansion behaviour of the minerals is interpreted in terms of a tetrahedral tilting model with the main change in apparent tilting being about the monoclinic 010 plane, i.e. the 0001 pseudo-hexagonal plane.

Dypingite, Mg5(CO3)4(OH)2·5H2O, was found in materials obtained during the production of high purity magnesia from crude magnesite by the calcination/CO2-leaching process. The snow-white, finely crystalline dypingite contained 41.0% MgO; the theoretical value is 41.5%. The X-ray powder data revealed a long period structure with a strong peak at 31.0 Å. The five strongest reflections (Å) and their estimated intensities (I/I0) were 10.4 (100), 31.0 (40), 15.62 (40), 5.86 (30), and 6.34 (20). The DTA curve showed ten endothermic reactions with maxima at 43, 50, 77, 86, 105, 127, 180, 260, 420, and 514°C and an exothermic reaction at 495°C.

A number of rare-element minerals from the Meldon aplite are described, e.g. native arsenic and antimony, arsenolite, stibiconite, and various niobium-tantalum minerals. Some of the minerals are new to the occurrence and some may be new to the British Isles. Particular attention has been paid to the Nb-Ta minerals and a large number of electron-microprobe analyses for manganocolumbite, manganotantalite, and microlite are presented. Examination of numerous, minute crystals and fragments of columbite-tantalite revealed a complex oscillatory zoning, indicating a slow rate of cooling and perhaps incomplete miscibility of the isomorphous columbite-tantalite series.

Ilvaite is an alteration product of carbonate, which itself replaced clinopyroxene, in a Precambrian tholeiitic dyke. Additional secondary minerals are ferroactinolite, cummingtonite, chlorite, biotite, prehnite, and epidote. Microprobe analyses demonstrate that the ilvaite is close to the ideal end-member composition [CaFe22+Fe3+Si2O8 (OH)] and that only cummingtonites, Mg/(Mg+Fe2+) 0.77–0.39, and carbonates (solid solutions between calcite and dolomite, dolomite and ankerite, magnesite and siderite) show significant compositional variations. It is estimated that ilvaite formed at temperatures < 470 °C and at pressures < 2 kbar, probably during a low-grade metamorphic event (or events) associated with the Caledonian orogeny. The fluids involved in the alteration process contained both CO2 and H2O, initial fluids probably being richer in CO2 than later ones. Ilvaite may be more common in hydrothermally altered igneous rocks than previously supposed.