The coastal region of Guangdong Province, China, is characterized by well-developed dyke swarms. The dykes with widths of decimetres to metres and lengths of tens of metres occur along straight and planar fractures cutting granites or volcanic rocks with Jurassic to Early Cretaceous ages. They show steep attitudes with strikes varying from NNW to NNE and from NE to SE, consistent with a stress regime transition from E–W to N–S extension. Major-element analysis on representative dyke samples reveals a composition range from basaltic to andesitic with a few dacitic outliers. Trace elements of most samples show notable Nb–Ta negative anomalies and Pb positive anomalies on the primitive mantle normalized spidergrams, characteristic of arc-related rocks. Among these, several dacite samples show notable fractionated medium rare earth elements and heavy rare earth elements and high Sr/Y (47–74) and La/Yb (15–21) ratios indicative of adakitic affinity. A few samples lack anomalies in Nb and Ta and have primitive trace-element ratios (e.g. Nb/La and Sr/Nd) or initial ϵNd values resembling ocean island basalt-like rocks. Rare earth element distribution patterns of all samples are right inclined and generally absent of Eu anomalies, which together with other trace-element indexes indicate an origin from heterogeneous mantle sources with depths below the stability field of plagioclase. Radiogenic isotopes, especially those of Nd and Sr (or Nd–Pb), define an array extended from the depleted to enriched mantle (EMII) provenance. LA-ICP-MS dating of zircon obtains ages between 110 and 70 Ma indicating emplacement of these dykes in latest Early to Late Cretaceous time, a time just before initiation of the South China Sea basin. In the context of regional geodynamics, it is proposed that these dykes were formed in a tectonic setting transiting from a Pacific-related back-arc to a passive continental margin pertaining to the development of the South China Sea basin.

The Cumbrae islands are unmatched for study of minor intrusions associated with the Early Carboniferous Clyde Lava Plateau. The transitional to subalkaline rock suite is present in a linear swarm of c. 250 ENE to NE dykes – 82 ‘basaltic’ (basalt to hawaiite) and 18 ‘trachytic’ (benmoreite to felsite), with a crustal dilation of 13·3. This swarm provides a unique window on the shallow crustal feeder system of the lava plateau. A ‘trachytic’ secondary swarm, focused on the plateau volcano of Misty Law in the Renfrewshire Hills, is analogous to a similar swarm in the Campsie Fells linked to the Meikle Bin volcano, and a ‘basaltic’ secondary swarm, unusually dense, with crustal dilation of 24·7, may record projection of the Dumbarton–Fintry Volcano–Tectonic lineament from the Campsie and Kilpatrick hills into the Firth of Clyde. Accompanied by bosses, plugs, vents and sheeted structures, the swarms are inferred to overlie fracture systems in the Caledonian basement of the Midland Valley. Internal relationships among extrusive and intrusive rocks of the Early Carboniferous suite and external relationships to dyke suites recording regional tectono-magmatic events of Late Carboniferous and Palaeogene age allow formulation of a new relative chronology for magmatic activity in the western off-shore Midland Valley.