The island of Montserrat is made of the products of five major and three parasitic volcanoes of Pliocene to Recent age. Apart from South Soufrière Hill where some basalt lava flows and pyroclast falls occur, the volcanoes have erupted only andesites, mostly dome lavas and pyroclast flows. The Montserrat lavas belong to the calc-alkaline series. Major and trace element analyses of rocks and minerals are presented and used in a computer test which shows that fractionation of a basaltic parent could account for the composition of all the Montserrat lavas. Other considerations, notably of field relationships and relative volumes of rock types, suggest that andesites are produced in this way only at South Soufrière Hill. Alternative origins for the Montserrat lavas are considered briefly.

From the Middle Jurassic onwards persistent igneous activity in the southern Andes around 46 °S was controlled by easterly dipping subduction along the Pacific margin. Cogenetic plutonic rocks belonging to the Patagonian batholith, and calc-alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products. Erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences, for example in the Cretaceous-Lower Tertiary Divisadero formation. The Coyhaique region marks the northerly extension of a narrow back-arc basin in which the marine Neocomian successions accumulated. Volcaniclastics from the island arc, which presumably lay to the west, are intercalated with the sediments. Although the marine basin was short-lived a mildly extensional back-arc regime may have existed through much of Mesozoic-Recent times. Widespread basalt-rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment. Remnants of the Patagonian basalt plateau at latitude 45-47 °S extend from the Argentine-Chile frontier to Lago Colhue Huapi. Four principal age and compositional groups have been distinguished in the lavas, (i) The oldest, which are about 80 Ma, occur in sections at Senguerr and Morro Negro. They are almost exclusively tholeiitic, but show some calc-alkaline affinities and resemble in other respects basalts from marginal basins, (ii) The second group (57-43 Ma) occur in the lower part of the Chile Chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite. (iii) The upper part of the main plateau sequence, where the flows are in the range 25-9 Ma, are dominantly of alkali basalt composition, (iv) Post-plateau flows from small cinder cones on the surface of the plateau range in age from ca. 4 Ma to 0.2 Ma or less. They are mostly highly undersaturated basanites, with occasional leucite basanites, enriched in incompatible elements. A few of the earlier tholeiites with calc-alkali traits may have been closely associated with subduction or marginal basin processes. The younger lavas are more alkalic intraplate types generated in the remote back-arc extensional zone.

Over 100 samples from the submarine Black–stones Bank Igneous Centre have been collected by scuba divers. The samples include gabbros, dolerites and metamorphosed sediments but ultramafic rocks, expected in view of the high positive gravity anomaly on the bank, have not so far been sampled. The mode of occurrence, petrography and chemical composition of the samples show many similarities between the Blackstones Bank Centre and the Tertiary igneous centres of NW. Scotland. However, K-Ar age determinations on a basaltic dyke which cuts a gabbro suggest a minimum age for the Blackstones Centre of 70 Ma, while most plutonic rocks in the British Tertiary Province have ages of c . 59 and 53 Ma.

No abstract

TRACE ELEMENT STUDIES are increasingly important in igneous petrogenetic investigations in as much as they place constraints on possible modes of origin of igneous rocks and also give evidence of the nature and composition of the mantle/crust systems within which magmas are generated and modified. The conference saw trace element studies applied to a wide range of igneous rock compositions, occurring within a wide range of environments. Four papers were concerned with areas in which basalt lavas predominate, i.e. Mull (Beckinsale et al.), the islands associated with the axial trough of the Red Sea (Henderson & Parry), the East Scotia Sea (Saunders & Tarney) where basalts are generated within an intra-oceanic marginal basin and the South Shetland Islands and Antarctic Peninsula (Weaver et al.) where different periods of magma genesis have been related to subduction and extension. Other papers dealt with metabasalts from ophiolite complexes (Pearce), K-rich volcanics (Parker), diorites and granodiorites (Brown et al), granites and charnockites (Petersen) and the ultramafic to granitic rocks of the Fongen-Hyllingen Gabbro Complex (Esbensen). An important recent development in geochemistry has been to extend the range of material which can be usefully analysed. For example, important petrogenetic information is now being obtained from altered and metamorphosed igneous rocks using ratios based on trace elements such as Ti, Zr, Y, Nb, P, Ce, Sm, Yb, abundances of which appear to be unchanged by secondary processes (Pearce). Volcanic rock geochemistry, which until recently was virtually synonymous with lava geochemistry, has now expanded its scope to