Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.

No abstract

Bimodal associations of basalt and rhyolite of Upper Ordovician age which were erupted in a submarine environment occur within the Caledonian orogenic belt of South Britain at Parys Mountain (Anglesey), in Snowdonia (North Wales) and at Avoca (SE Ireland). The volcanic rocks have experienced hydrothermal alteration and low-grade metamorphism, and therefore immobile elements (e.g. Ti, Zr, Nb, Y) have been used to identify the original geochemical characteristics. The basalts have characters transitional between volcanic ‘arc’ and ‘within plate’ types consistent with eruption on an extensional part of an active continental margin. Two groups of rhyolites have been identified. A low-Zr group (Zr<500ppm), represented at all three locations, is interpreted as originally of high-K, subalkaline type. A high-Zr group (Zr>500ppm), represented at Snowdonia and Avoca, is interpreted as originally being peralkaline in composition; their high Zr/Nb ratios (>10) are typical of peralkaline rhyolites erupted above subduction zones. The bimodal nature of the associations and the peralkaline character of some rhyolites indicates magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably syn-sedimentary. High-level, rhyolitic magma chambers are thought to have driven convection of the hydrothermal fluids from which the sulphides precipitated.

Stonehenge on Salisbury Plain is one of the most impressive British prehistoric(c.3000–1500 BC) monuments. It is dominated by large upright sarsen stones, some of which are joined by lintels. While these stones are of relatively local derivation, some of the stone settings, termed bluestones, are composed of igneous and minor sedimentary rocks which are foreign to the solid geology of Salisbury Plain and must have been transported to their present location. Following the proposal of an origin in south-west Wales, debate has focused on hypotheses of natural transport by glacial processes, or transport by human agency. This paper reports the results of a programme of sampling and chemical analysis of Stonehenge bluestones and proposed source outcrops in Wales.Analysis by X-ray-fluorescence of fifteen monolith samples and twenty-two excavated fragments from Stonehenge indicate that the dolerites originated at three sources in a small area in the eastern Preseli Hills, and that the rhyolite monoliths derive from four sources including northern Preseli and other (unidentified) locations in Pembrokeshire, perhaps on the north Pembrokeshire coast. Rhyolite fragments derive from four outcrops (including only one of the monolith sources) over a distance of at least 10 km within Preseli. The Altar Stone and a sandstone fragment (excavated at Stonehenge) are from two sources within the Palaeozoic of south-west Wales. This variety of source suggests that the monoliths were taken from a glacially-mixed deposit, not carefully selected from anin situsource. We then consider whether prehistoric man collected the bluestones from such a deposit in south Wales or whether glacial action could have transported bluestone boulders onto Salisbury Plain. Glacial erratics deposited in south Dyfed (dolerites chemically identical to Stonehenge dolerite monoliths), near Cardiff, on Flatholm and near Bristol indicate glacial action at least as far as the Avon area. There is an apparent absence of erratics east of here, with the possible exception of the Boles Barrow boulder, which may predate the Stonehenge bluestones by as much as 1000 years, and which derived from the same Preseli source as two of the Stonehenge monoliths. However, 18th-century geological accounts describe intensive agricultural clearance of glacial boulders, including igneous rocks, on Salisbury Plain, and contemporary practice was of burial of such boulders in pits. Such erratics could have been transported as ‘free boulders’ from ‘nunataks’ on the top of an extensive, perhaps Anglian or earlier, glacier some 400,000 years ago or more, leaving no trace of fine glacial material in present river gravels. Erratics may be deposited at the margins of ice-sheets in small groups at irregular intervals and with gaps of several kilometres between individual boulders.‘Bluestone’ fragments are frequently reported on and near Salisbury Plain in archaeological literature, and include a wide range of rock types from monuments of widely differing types and dates, and pieces not directly associated with archaeological structures. Examination of prehistoric stone monuments in south Wales shows no preference for bluestones in this area. The monoliths at Stonehenge include some structurally poor rock types, now completely eroded above ground. We conclude that the builders of the bluestone structures at Stonehenge utilized a heterogeneous deposit of glacial boulders readily available on Salisbury Plain. Remaining erratics are now seen as small fragments sometimes incorporated in a variety of archaeological sites, while others were destroyed and removed in the 18th century. The bluestones were transported to Salisbury Plain from varied sources in south Wales by a glacier rather than human activity.

Quaternary-Recent volcanism in the Andes has occurred in three regions: 45–33°S, 28–16°S and 2°S–5°N, each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics. Isotope ratios of O and Sr are lowest and those of Pb least variable in the southern volcanic zone (SVZ) where medium-K lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent. The SVZ lavas were probably derived from an asthenospheric mantle source above a shallow Benioff zone. Parental basaltic magmas rose largely unmodified through thin continental crust, where differentiation occurred by low-pressure crystal fractionation without concurrent modification of isotopic composition. The slight enrichment of 206 Pb in the northern volcanic zone (NVZ), where the crust is thin and the Benioff zone deep, suggests a greater subduction-zone component for parental medium-K magmas. The slight 18 O enrichment suggests a small amount of lower crustal interaction. Isotope ratios of O and Sr are highest and those of Pb most variable in the central volcanic zone (CVZ). Parental magmas in the CVZ were probably generated within the same mantle source region as those in the SVZ and NVZ. Subsequently, during transit through the exceptionally thick continental crust of the central Andes, high-K magmas were produced by a combination of bulk contamination in the lower crust and later low-pressure fractional crystallization-assimilation processes in the upper crust which altered both chemical and isotopic compositions.