Pb- and Sr-isotope, major element and trace element analyses are presented for 38 Tertiary igneous rocks and older country rocks from Arran. Tertiary igneous rocks define silica-undersaturated and -oversaturated differentiation trends, but acidic and basic examples of the latter trend, exemplified in composite dykes, are separated by a wide Daly gap. In contrast, Pb-, and to some extent Sr-isotope compositions of these rocks display continuous variation, almost independent of major element composition.

Isotopic variations are interpreted as resulting principally from crustal contamination of the differentiation products of mantle-derived basic magma. A dominant rôle by mantle heterogeneity or crustal anatexis in producing the isotopic variation is ruled out. Contamination is believed to have occurred primarily after differentiation, but some rocks, such as the fine unit of the Northern Granite, display evidence of post-contamination differentiation.

Tertiary igneous rocks define an array on a Pb/Pb isochron diagram whose slope corresponds to an apparent age of 770±180 Ma (2σ). This points to the existence of a late Proterozoic basement complex under Arran.

Acidic and basic members of composite dykes are interpreted as basaltic differentiates which have been variably affected by contamination, yielding wide ranges of isotopic composition. Composite intrusion is ascribed to the simultaneous presence of acidic and basic magma in the crust.

Orthopyroxene- and kyanite-lineage eclogites are distinguished on the basis of silicate and carbonate petrography and chemistry. Different textural and chemical types of magnesite, dolomite and calcite can be satisfactorily attributed to one of the three sequential metamorphic stages of eclogite history at anhydrous eclogite facies, hydrous eclogite facies and amphibolite facies. The chronological sequence of development of magnesite, dolomite and calcite matches the carbonate sequence found experimentally in synthetic peridotite-CO2-H2O systems during decompression. Magnesite and dolomite show reasonably regular Fe/Mg distribution relationships with silicates.

A new geothermobarometer based upon is provisionally calibrated. It yields T estimates for given P very similar to those given by the geothermobarometer, e.g. 759–829°C at 20 kb and 803–894°C at 40 kb. P is tentatively estimated at 30±10, 23±6 and 10 ± 3 kb respectively for the three metamorphic stages.

Fluid conditions after equilibration during each stage were either low or fluid-absent, but not low . A unique quartz-magnesite-dolomite vein represents an influx of CO2 in the P-T regime of instability of CO2 + pyroxene. Tectonic emplacement by deep-level obduction of diverse foreign source materials remains the petrogenetic model which best fits the data for these eclogites.

Accurate identification of iron and manganese oxides in soils presents great difficulty because, not only are they always in admixture, but the poor crystallinity of the species present leads to difficulties in interpretation of diffraction patterns. A method involving the combined use of electron microscopy, selected area electron diffraction and electron probe microanalysis on the same crystals gives unambiguous identification. Its application is described in relation to the various iron and manganese oxides found in a set of soils from all parts of the world. The occurrence of hausmannite in soils has been definitely established.

Analysed compositions of coexisting phases from cordierite-amphibole-bearing rocks (cordierite-orthoamphibole-cummingtonite-garnet-staurolite-spinel-opaque ores) and mica schists (garnet-staurolite-biotite-muscovite-plagioclase-andalusite-quartz) have been used in conjunction with experimentally determined equilibria to estimate pressures and temperatures of metamorphism for the rocks of the Outokumpu district. The data do not determine a single point in P-T space, but bracket an area such that T ≈ 600 ± 50°C and P ≈ 3·5 ± 1 kb. These conditions represent the maximum temperature attained by the rocks, and the associated sulphide ore bodies, during the Svecokarelian orogeny and are consistent with a regional palaeogeotherm of c. 50°C/km.

Assessment of the composition of the coexisting phases in the cordierite-amphibole-bearing rocks, particularly the Fe-Mg-bearing phases, indicates that it is unlikely that either iron or magnesium was mobile during metamorphism and that the equilibrium assemblage represents essentially isochemical metamorphism of a rock composition that predates the metamorphism. A similar genesis is considered likely for the cordierite-anthophyllite rocks of Orijärvi in southern Finland which forms the basis of the classic statement of the magnesium-metasomatism hypothesis.