The rocks of the Jasra intrusive complex (Shillong Plateau, northeastern India) include phlogopite clinopyroxenites (with olivine or perovskite relics), alkali gabbros/monzodiorites, syenites and nepheline syenites. They have a potassic affinity (Na2O/K2O ∼1), and their mineralogy is dominated by clinopyroxene with which phlogopite, olivine, amphibole, feldspars, feldspathoids, oxides, orthopyroxenes, perovskite, titanite and other accessory phases are variably associated. The Jasra intrusive rocks are cumulates derived from at least two distinct magmatic liquids. The potassic affinity of the Jasra rocks differs from the nearby Sung Valley ijolitic-carbonatitic complex and from the ultrapotassic lamproitic rocks of the Damodar Valley, which are of approximately the same age. This suggests major variability in the mantle sources of small-volume alkaline volcanism in the Early Cretaceous of northeastern India.

Detailed remapping of the Palaeogene Ardnamurchan Centre 3 gabbros, NW Scotland, suggests that this classic sequence of ring-intrusions forms a composite layered lopolith. The area mapped by previous studies as the Great Eucrite gabbro intrusion comprises 70% by area of Centre 3. Field observations suggest that most of the other smaller ring-intrusions of Centre 3 (interior to the Great Eucrite) constitute either distinct petrological facies of the same intrusion, or included country-rock or peridotite blocks. These observations, together with syn-magmatically deformed inward-dipping modal layering, are used here to support the interpretation that significant central sagging occurred in the intrusion at a late stage in its crystallization history.

The paradox of Lachlan Fold Belt (LFB) granitoids is that although contrasted chemical types (S- and I-types) imply melting of distinct crustal sources, the simple Nd–Sr–Pb–O isotopic arrays indicate a continuum, suggesting mixing of magmatic components. The paradox is resolved by the recognition that the previously inferred, isotopically primitive end-member is itself a crust-mantle mix, so that three general source components, mantle, lower crust and middle crust, comprise the granitoids. Based on Nd isotopic evidence, mantle-derived basaltic magmas melted and mixed with Neoproterozoic-Cambrian, arc-backarc-type material to produce primitive I-type, parental granitoid magmas in the lower–middle crust. Ordovician metasediment, locally underthrust to mid-crustal levels, was remobilised under the elevated geotherms and is most clearly recognised as diatexite in the Cooma complex, but it also exists as gneissic enclaves in S-type granites. The diatexite mixed with the hybrid I-type magmas to produce the parental S-type magmas. Unique parent magma compositions of individual granite suites reflect variations within any or all of the three major source components, or between the mixing proportions. For example, chemical tie-lines between Cooma diatexite and mafic I-type Jindabyne suite magma encompass almost all mafic S-type granites of the vast Bullenbalong supersuite, consistently in the proportion Jindabyne: Cooma, 30:70. The modelling shows that LFB S-type magmas are heavily contaminated I-type magmas, produced by large-scale mixing of hot I-type material with lower temperature diatexite in the middle crust. The model implies a genetic link between migmatite and pluton-scale, crustally derived (S-type) granites.

Given the chemical and isotopic contrasts of the crustally derived source components, and their typically unequal proportions in the magmas, it is not surprising that the LFB granitoids are so distinctive and have been categorised as S- and I-type. The sublinear chemical trends of the granitoid suites are considered to be secondary effects associated with crystal fractionation of unique parental magmas that were formed by three-component mixing. The model obviates the necessity for multiple underplating events and Proterozoic continental basement, in accordance with the observed tectonostratigraphy of the Lachlan Fold Belt.

Metamorphosed abyssal ocean-floor tholeiitiic rocks, little affected by crustal contamination and probably formed at a spreading ridge, occur as a dyke complex in banded gabbros with oceanic affinities and as small tectonic lenses in serpentinised peridotite. Igneous textures and mineral assemblages have been partly or completely replaced in both dykes and gabbros. Metamorphic development of clinopyroxene and very Ti-rich hornblende at 900-1000°C was followed by a low temperature actinolite-chlorite assemblage and then by pectolite-bearing veins. Geochemical variations resulting from magmatic and metasomatic processes have been distinguished and plots of major and trace elements used to establish tectonic environment. The very high temperature metamorphic mineral growth associated with beerbachite formation indicates that the dyke rocks were in a high heat-flow environment for a considerable time, presumably in the vicinity of a spreading ridge. Obduction of a very hot slab of oceanic lithosphere accounts for the superimposed mineral assemblages.

Spatially associated pillow lavas have been affected by crustal contamination and are not comagmatic with the dykes. There are petrochemical characters indicating that some of the lavas are ocean-floor basalts. For other lavas, features suggesting emplacement in a continental or an island-arc environment could also be consistent with development in a marginal basin. However, there is so much variation in composition of pillows from margin to centre, from small to large and from metasomatic activity, that their compositional fields cannot be used for discrimination of basaltic type with the same confidence as for the dykes. Accordingly, it is suggested that the petrochemistry of sheeted dyke complexes, rather than of pillow lavas, be used for this purpose in the study of ophiolites.