Felsic magmatism has occurred over a large region of East Asia since Jurassic times and has provided important mineral resources such as tin, tungsten, base metals and gold. The circum-Japan Sea region preserves various geological records of active continental margins, including Jurassic to Early Tertiary magmatic arcs and subduction zones and pre-Jurassic continental basements, which were separated by the opening of the Japan Sea during the Miocene. The felsic magmatism in this region shows a wide variation in terms of redox state and related mineralisation, encompassing east-west contrasts around the Pacific Ocean. A review of granitoids and associated ore deposits in this region indicates that the character of the crust, sedimentary versus igneous, is an essential factor to control the redox state, and a tectonic setting may be an additional factor in some cases. The reduced-type granitoids, characterised by tin mineralisation, were generated in carbon-bearing sedimentary crust which was composed mainly of accretionary complex material and not influenced by previous magmatism. Involvement of sedimentary materials is corroborated by oxygen, sulphur and strontium isotope data. The oxidised-type granitoids, characterised by gold or molybdenum mineralisation, were generated in igneous crust which was depleted in reducing agents as a result of previous magmatism. Granitoid magmatism in a given area tends to become more oxidised with time. Jurassic accretionary complexes in East Asia are thought to have been largely displaced from the original place of accretion and stacked up against the northeastern margin in the Khingan and Sikhote-Alin Mountains. This region, dominated by sedimentary crust, was subsequently subjected ot Cretaceous felsic magmatism and converted to a large province of reduced-type granitoids and tin-tungsten mineralisation. Diverse geodynamic processes, including the change of the arc-trench system, the creation and collapse of the back-arc basin and the collision of continents, may have prepared many favourable sites for the generation of reduced-type granitoids in northeast Asia. These processes may have resulted in a remarkable contrast with the Pacific margin of North America, where repeated arc magmatism during the Mesozoic formed granitoid batholiths of the oxidised-type. The granitoid types may also be controlled by the tectonic setting and mode of magma emplacement. In the northern Kitakami area of Northeast Japan, Early Cretaceous episodic magmatism occurred in a Jurassic accretionary complex, and formed the oxidised-type granitoids accompanied by submarine bimodal volcanism associated with kuroko mineralisation. Granitoids of fissure-filling type emplaced under extensional environments may be oxidised, irrespective of basement geology, because of insignificant crustal input.

Microcontinents are common in the accreted continental geological record, but relatively rare in modern settings. Many of today's microcontinents are found in the Tasman Sea and Indian Ocean. These include the East Tasman Rise, the Gilbert Seamount Complex, the Seychelles, Elan Bank (Kerguelen Plateau), and possibly fragments of the Lord Howe Rise and Norfolk Ridge, and the Wallaby Plateau. We review their history of formation, and propose that the mechanisms that led to their isolation were mostly plume-related. Tasman Sea continental fragments formed by ridge jumps onto adjacent continental margins after sea-floor spreading in the southern Tasman Sea commenced. The East Tasman Plateau was separated from the Lord Howe Rise at about chron 34 (83 Ma) and the Gilbert Seamount Complex rifted off the South Tasman Rise at roughly 77 Ma, by ridge jumps in opposing directions. Evidence for thermal anomalies under the central Lord Howe Rise, Ross Sea and possible eastern Australian margin explain ridge jumps that led to the isolation of the East Tasman Plateau, Gilbert Seamount and possibly the northern Lord Howe Rise and Dampier Ridge. In the central Indian Ocean, spectacular exposures of granite make the Seychelles a type example of a microcontinent. As in the Tasman Sea, ridge-plume interactions have been responsible for separating a thinned continental sliver from a large continent (India). Elan Bank, aspart of the Kerguelen Plateau, represents another example of a continental fragment in the Indian Ocean. Newly identified M-sequence anomalies in the Ender by Basin, off Antarctica, suggest that this microcontinent was detached from India no earlier than 124 Ma when a northward ridge jump towards the Kerguelen plume may have isolated Elan Bank. This Interpretation implies that a mantle thermal anomaly due to the incipient Kerguelen plume pre-dated the Rajmahal Traps, emplaced at about 118 Ma, by ∼6 million years. An early Kerguelen hot-spot position north of Elan Bank, with subsequent southward migration, is also supported by palaeomagnetic data and mantle convection models. The Wallaby Plateau off Western Australia, located between the Perth and Cuvier Abyssal Plains, may also include slivers of continental crust, but unequivocal evidence is not available, as it has never been drilled. Here it is suggested that most of these microcontinents formed by re-rifting of a young continental margin in the vicinity of a mantle plume stem. The weak inner flank of a rifted margin weakens further when passing over a mantle plume, causing a nearby spreading ridge to jump onto this zone of weakness. This process isolates a passive margin segment, and leaves a narrow passive margin behind. After the onset of subduction and ocean basin destruction, such micro-continents may be accreted again to an active plate margin.