19 new K–Ar mineral ages of 78-201 Ma and 3 Rb–Sr whole rock isochron ages of 81 ± 10, 127±4 and 181±30 Ma are presented from units of continental crust, mafic complex and island arc assemblage on South Georgia. The Drygalski Fjord Complex, part of the possible floor of the marginal basin in the southern part of the island, includes granodiorite and gabbro plutons of minimum age 180–200 Ma. Together with older metasediments they have been affected by a major thermal event at about 140 Ma, thought to have resulted from the emplacement of a mafic complex (Larsen Harbour Formation) during the initial opening of the marginal basin. Rocks of the Larsen Harbour Formation are cut by the Smaaland Cove intrusion dated by Rb–Sr whole rock isochron at 127±4 Ma. An island arc assemblage exposed to the SW of South Georgia consists of pyroclastic rocks cut by monzodiorite and andesite intrusions, which give radiometric ages of 81–103 Ma. These data suggest that the marginal basin opened during the late Jurassic (pre-140 Ma); that part of an earlier (early Mesozoic) magmatic arc is preserved in continental crust making up part of the floor of the basin; and that subduction continued beneath the island arc until at least the Senonian time. The younger plutons in the arc were emplaced at roughly the same time as turbidite facies rocks at deep levels in the marginal basin were being affected by penetrative deformation and metamorphism. The timing of events on South Georgia agrees closely with that deduced for the continuation of the same island arc–marginal basin system in South America. The 180–200 Ma plutons correlate with an older suite of plutonic rocks reported from the Antarctic Peninsula and southern Andes; they are part of a once-continuous magmatic arc related to subduction of the Pacific plate beneath Gondwanaland during the early Mesozoic.

The Hardy Formation, a sequence of Upper Mesozoic volcanic rocks exposed in Peninsula Hardy (Isla Hoste) in the southernmost archipelago of Chile represents, at least in part, the island-arc assemblage of an island-arc-marginal-basin system related to an eastward dipping subduction zone. This island arc was founded on South American continental crust and is also represented in the island of South Georgia 2000 km to the E. The island-arc assemblage includes pyroclastic rocks, characterized by a high proportion of vitric material, and lava intercalations ranging in composition from rhyolite to basalt. These rocks underwent zeolite and prehnite-pumpellyite facies metamorphism and are gently folded, in contrast with the intense folding exhibited by the rocks exposed to the north of Peninsula Hardy. Silicic volcanics assigned to this assemblage underlie pillow lavas, and are intruded by dolerites and gabbros probably related to a Late Jurassic-Early Cretaceous ophiolite magmatism associated with the generation of a quasioceanic marginal basin. Volcanic turbidites (Yahgan Formation) were deposited into the marginal basin.

It is suggested that in pre-marginal basin times the Hardy Formation interfingered towards the Atlantic with the silicic volcanics of the Tobifera Formation. However, recent geochemical work on the Tobifera Formation suggest an origin by continental crust anatexis in a volcano-tectonic rift zone related to upper mantle diapirism, whereas an island arc origin is favoured for at least the andesitic and basaltic components of the Hardy Formation. Therefore, the geology of Peninsula Hardy as presented here, confirms early assumptions of the splitting apart of a Middle–Upper Jurassic volcanic terrain along the Pacific margin of South America during the generation of a marginal basin. The spreading axis of the latter seems to have been located at the boundary of two somewhat overlapping petrotectonic assemblages: and island arc on the Pacific side and a silicic volcano-tectonic rift zone towards the Atlantic. A probably Cenozoic volcanic complex discordantly overlies the Yahgan and Hardy formations.

Within blueschist facies rocks it is common to find that mineral assemblages represent a series of attempts to arrive at changing states of equilibrium. To interpret such rocks it is necessary to study how the mineral constrains and determines chemical interaction. The gradual process of chemical exchange between minerals, during changes in conditions, is interrupted by the nucleation of a new mineral species. Mineral reactions are instigated by the nucleation of one new mineral species, though when a reaction is seen at an equilibrated completion the reaction path cannot be traced. In low temperature metamorphism the preservation of partial mineral reactions is very widespread. The growth, adaptation and resorption of a mineral are considered in order to demonstrate: (1) how and why a mineral should zone during growth: (2) the competitive reaction paths that may allow a mineral to change composition, even across and internal solvus; and, (3) through the consideration of a few simple parameters, the prediction of a variety of resorption textures. It is proposed that through an understanding of the way in which the mineral can store information on the chemical history of the rock it is possible to obtain a metamorphic analysis without recourse to the conventional phase equilibrium metamorphic model - a frequent misapplication in the blueschist facies.

On Fuerteventura, inner Canary Islands, a phase of early Cretaceous deposition on a passive continental margin was terminated in Albian time by strong localized uplift associated with onset of calcareous pelagic sedimentation. Later, during early Tertiary times, spasmodic episodes of submarine alkali volcanism led to the construction of a thick lava pile. After folding of the lower submarine volcanic rocks, and the underlying pre-volcanic sedimentary rocks, parts of the Basal Complex were emergent by Oligocene to Miocene times, fringed by coral and algal reefs. With renewed early Miocene uplift, the reefs disintegrated, followed by block faulting, intrusion of plutons, and a final phase of submarine dyke intrusion. There was then strong uplift coupled with deep erosion, extrusion of sub-horizontal Neogene plateau lavas and further intrusion of plutons. Regional comparisons suggest that tectonic events on Fuerteventura may have a bearing on sedimentary hiatuses and gravity sliding events which affected the adjacent West African continental margin.

Distances measured between homologous reference points on fragmentary fossil material can be used to recover a three-dimensional framework by operations of standard matrix algebra. A procedure that combines both linear and non-linear algorithms is described and illustrated in a simple reconstruction of Waptia fieldensis. The approach attempts to provide a modelling process in which a palaeontologist can explore the geometrical consequences of fossil measurements through continuous interaction with programmed reconstructions.