Mafic dykes are typically emplaced through primary hydraulic fracturing of undeformed crust or may make use of pre-existing crustal inhomogeneities, representing the plumbing systems of a large igneous province. The Eastern Dharwar Craton has dense exposures of several generations of Paleoproterozoic mafic dyke swarms ranging from ca. 2.37 Ga to ca. 1.79 Ga. Herein, using anisotropy of magnetic susceptibility fabric data of mafic dykes and associated host granites, the emplacement systematics of the NW- to W-trending ca. 2.21 Ga Anantapur–Kunigal dyke swarm, displaying a radiating geometry, have been studied to understand magma flow dynamics. A low-angle relationship between the silicate and opaque fabrics and good correlation with magnetic lineation, identified via petrographic studies and shape preferred orientation analyses of multiple oriented thin sections, suggest a primary flow-related magnetic anisotropy for the studied dyke samples. The classic subparallel relationship between the trend of the dyke planes and magnetic fabric of the associated host granites suggests that the radiating geometry of the ca. 2.21 Ga dyke swarm was supported by a favourable pre-existing structural grain of the country rock. We interpret the magma for the studied dyke swarm was fed laterally from a distant plume. It was emplaced as laterally propagating primary dyke fractures as well as injected into the pre-existing subparallel crustal inhomogeneities. Corroborating all these inferences, a detailed emplacement model for ca. 2.21 Ga Anantapur–Kunigal dyke swarm is also proposed.

An ultramafic lamprophyre dyke is described from the otherwise tholeiitic Ferrar magmatic province of Antarctica. We report an Ar-Ar age of 183 ± 2.2 Ma for the dyke, indistinguishable from those of the Ferrar tholeiites. However, the dyke has mineralogical and major and trace element compositions, and radiogenic isotopes ratios, very different from the Ferrar tholeiites. The sample consists of olivine and rare clinopyroxene phenocrysts with perovskite and spinel microphenocrysts in a groundmass of amphibole, nepheline and biotite. Carbonatitic globules contain calcite, dolomite, Fe-rich carbonate, nepheline, biotite, orthoclase, pyrite, clinopyroxene, apatite and silicate glass, and were formed by liquid immiscibility. The rock is mildly potassic and classifies as an ouachitite. It is strongly enriched in both moderately and highly incompatible trace elements and is the first high-Ti rock to be described from the Ferrar magmatic province. The rock has similar initial 143Nd/144Nd to OIB, notably Bouvet, Crozet and Réunion, but significantly higher initial 87Sr/86Sr. The lamprophyre magma is interpreted as having been generated by low-degree partial fusion of metasomatized lithospheric mantle as a result of heat conducted from an underlying Jurassic mantle plume. The same mantle plume was probably also responsible for generating one of the world’s largest layered gabbro bodies, the Dufek-Forrestal intrusions.

Zircon U–Pb ages, major and trace element geochemistry and Sr, Nd and Pb isotope compositions of diorite and diorite porphyry dykes from the Jinchanggouliang (JCGL) gold ore field on the northern margin of the North China craton (NCC) were studied to investigate their sources, petrogenesis and geodynamic significance. LA-ICP-MS zircon U–Pb dating reveals three major age groups of 2500 Ma (n = 2), 253 ± 7 Ma (n = 5) and 227 ± 1 Ma (n = 9). The inherited ages of 2500 Ma, contemporary with the Archaean NCC continental growth, imply that crustal material was involved in the magma source. The igneous zircons with a concordia age of 227 ± 1 Ma may record the emplacement age of the JCGL dykes. Both diorite and diorite porphyry exhibit a wide range of SiO2 and MgO contents and are characterized by high concentrations of Na2O+K2O and Al2O3, and low abundances of P2O5 and TiO2. They are enriched in large ion lithophile elements and light rare earth elements without significant Eu anomalies, and depleted in high-field-strength elements; all are categorized as shoshonitic rocks. All samples show a narrow range of Sr isotope compositions with initial 87Sr/86Sr ratios from 0.70394 to 0.70592, variable εNd(t) values (1.1 to −12.0) and TDM2 ages of 913–1972 Ma. Their Pb isotope compositions form continuous variation trends and plot in the fields between enriched mantle 1 (EM1) and lower continental crust (LCC). The above results suggest that the JCGL dykes studied could have been derived from mixing of lower crust, lithospheric mantle of the NCC and ascending asthenospheric melt in a post-orogenic extensional geodynamic setting. These shoshonitic dykes, together with the geochronological data of regional ENE-trending retrograded eclogites, ophiolites, continental arc magmatic belt, A-type granite, alkaline intrusions and metamorphic core complex from the northern NCC and Central Asian Orogenic Belt (CAOB) suggest that closure of the Palaeo-Asian Ocean (i.e. stage of pre-collision to collision) had completed during latest Permian to earliest Triassic time, and that the CAOB was subsequently tectonically dominated by post-orogenic extensional regimes. The involvement of asthenospheric melt in the magma source implies that the sub-continental lithospheric mantle (SCLM) of the NCC had been modified, and the onset of lithospheric destruction and thinning beneath the northern NCC may have occurred in Middle–Late Triassic time as a result of post-orogenic subducting slab detachment and lithospheric delamination.

The hot emplacement of the Ronda peridotites (Betic Cordilleras) developed a dynamothermal aureole and partial melts that led to the intrusion of granite dykes in the peridotites. Previous geochronological data place rather broad limits for this event between 22 and 19 Ma. Analyses of neocrystalline zircon rims from large zircon populations yield a U–Pb SHRIMP age of 22.3±0.7 Ma for the dynamothermal aureole formation, and intrusion ages of granite dykes between 22.6±1.8 and 21.5±3.8 support that conclusion. Therefore, these new ages provide a more robust constraint on the hot emplacement of the Ronda peridotites at middle crustal levels.

Thurston Island, and the adjacent Eights Coast and Jones Mountains, record Pacific margin magmatism from Carboniferous to Late Cretaceous times. The igneous rocks form a uniformly calc-alkaline, high-alumina, dominantly metaluminous suite; some relatively fractionated granitoids are mildly peraluminous. The magmas were hydrous, a result of subduction. Gabbros have compositions outside the range of mafic volcanic and hypabyssal rocks, as a result of cumulate processes. Trace element compositions of the mafic magmas range from a low La/Yb, Th/Ta end-member close to E-MORB in composition, perhaps contaminated by crust, to a high La/Yb, Th/Ta end-member, close to shoshonite, with strong magmatic arc trace element character. This variation may be a result of mixing of tholeiitic and shoshonitic end-members. Most silicic rocks could have been generated batch-wise from mafic magmas by fractional crystallization of a phenocryst assemblage dominated by plagioclase, pyroxene ± amphibole, as seen in the cumulates. Cessation of magmatism at about 90 Ma approximately coincided with collison of a spreading centre between the Phoenix and Pacific oceanic plates with the continent margin subduction zone. The rifting of New Zealand from West Antarctica and associated extension probably was responsible for emplacement of a coast-parallel Cretaceous dyke swarm.

The management of earthwork sites and their protection from erosion is a constant problem. Here Kevin Jones assesses methods and materials used on archaeological sites in Britain. It is essential to find compatible, economic land uses for most earthworks but this use should not lead to diminished expectations of good protection and historic interpretation.