R. S. Thorpe

Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the Southern British Caledonides

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.

Petrology and geochemistry of volcanic rocks of the Cerro Galan Caldera, Northwest Argentina

At least 2000 km 3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the > 1000km 3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO 2 ) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87 Sr/ 86 Sr = 0.7108–0.7181; 143 Nd/ 144 Nd = 0.51215–0.51225, and δ 18 O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87 Sr/ 86 Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87 Sr/ 86 Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges ( 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material(> 1.5 Ga). Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO 2 = 51–57%; K 2 O = 2–3% and normative plagioclase compositions of An 37–48 , and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87 Sr/ 86 Sr ranges from 0.7055 to 0.7094 and 143 Nd/ 144 Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO 2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87 Sr/ 86 Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites. The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.

The Geological Sources and Transport of the Bluestones of Stonehenge, Wiltshire, UK.

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 an in situ source. 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.

The distribution and sources of archaeological pitchstone in Britain

Abstract Pitchstone is a glassy volcanic rock, distinguishable from obsidian by having a higher water content. It forms lava flows and minor intrusions at several centres within the British Tertiary Volcanic Province (BTVP) in the west of Scotland and Ireland. Pitchstone artifacts and waste pieces have been recorded from 101 archaeological localities in Scotland and northern England, ranging in time from possibly Mesolithic to Bronze Age. To characterize the sources, 17 pitchstone samples from 11 sources within the BTVP were analysed for major elements by energy-dispersive X-ray fluorescence (EDXRF), and for trace elements by instrumental neutron activation analysis (INAA). 28 pitchstone pieces from 22 archaeological sites were analysed, mainly non-destructively, by INAA and interpreted as having originated from Arran sources. The Arran sources are divided into four petrographic types; the Corrygills, Tormore, Glen Shurig and Glen Cloy Types. Petrographic study of 15 archaeological samples indicated that three were of Corrygills type and two were of Tormore type. The remainder were less confidently attributed to the petrographic types distinguished. Arran pitchstone is found as far as 300 km from the Arran sources, at the chambered tomb of Ord North in Highland.

Late-orogenic alkaline/subalkaline Silurian volcanism of the Skomer Volcanic Group in the Caledonides of south Wales

Basaltic, andesitic and rhyolitic volcanism was widespread during Ordovician time in the Welsh, Basin. Minor Silurian volcanic rocks occur in the Welsh Borderland and in Pembrokeshire (Dyfed), where they represent the youngest volcanic activity associated with the Caledonian orogeny in Wales. The Skomer Volcanic Group comprises c. 760 m of lavas and pyroclastic rocks with minor sedimentary rocks which are best exposed on Skomer Island but crop out over 43 km from the Smalls and Grassholm (in the Irish Sea) to mainland Pembrokeshire. New chemical analyses have been used to characterize these rocks and to compare them with earlier Ordovician volcanic rocks. The Skomer Volcanic Group comprises hawaiite/mugearite lavas and rhyolitic flows and pyroclastic rocks. The rhyolitic rocks are subdivided into a high-Zr group, (originally of peralkaline composition) and an unrelated low-Zr group. The hawaiite/mugearite lavas and high-Zr rhyolites are considered to form a single group, related by fractional crystallization, analogous to basalt–hawaiite–mugearite–comendite associations erupted in within-plate oceanic and continental settings. However concentrations of high field strength elements (Ti, Zr, Nb, Ta, Hf) suggest that the parent magmas may have been derived from a within-plate (ocean island basalt) source modified by the effects of earlier or contemporaneous Caledonian subduction.

Mons Claudianus and the problem of the granito del foro': a geological and geochemical approach

Granito del foro is a distinctive igneous rock, in fact a granodiorite rather than a granite, long known and named for its use in buildings of the Roman Forum. Exactly what is it? Where does it come from? Where else was it used and not used? What does the granito del foro say about ownership and empire?

Snowdon basalts and the cessation of Caledonian subduction by the Longvillian

Basalts of the Bedded Pyroclastic Formation, Snowdon Volcanic Group, North Wales of Longvillian (Caradoc) age, are heterogeneous in trace element composition, forming a mixing trend between ocean island basalt (OIB)-type and magmatic arc basalt. The two end-member compositions were partial melts of two distinct upper mantle sources. The OIB end-member is similar to Icelandic OIB, and is the only known example of basalt of this type in the Ordovician-Silurian Welsh Basin. Since ocean island basalt magma type is thought never to erupt above well-established subduction zones, the eruption of the Longvillian OIB provides a minimum age for the end of subduction of Iapetus oceanic lithosphere below the Welsh Basin. A model for the evolution of the mantle below the Welsh Basin is proposed which accounts for the geochemistry of magmas erupted from the Tremadoc to the early Silurian.

Ordovician volcanism in the Welsh Borderland

Basaltic, andesitic and rhyodacitic/rhyolitic volcanism was widespread during Ordovician time in the Welsh Basin. New chemical data are presented for Llanvirn to Caradoc lavas and tuffs from the Welsh Borderland which, during Ordovician time, formed the southeastern margin of the Welsh Basin. In view of the observed chemical alteration, immobile elements are used in the interpretation of the original geochemical character. The data indicate that the Llanvirn Stapeley volcanic group of the Shelve inlier was a bimodal basalt/basaltic andesite – rhyodacite/rhyolite association. The basalts have trace element contents of tholeiitic associations with a subduction-related character. The Caradoc Whittery and Hagley volcanic groups of the Shelve inlier comprise lavas and tuffs of calc-alkaline andesite. Blocks sampled from the Breidden Hills show that these were also derived from a calc-alkaline volcano. Associated Caradoc pumice- and ash-flow deposits from the Breidden Hills are probably of altered calc-alkaline rhyodacite/rhyolite composition. The Sibdon Carwood basalt flow, the only known example of Ordovician volcanism east of the Pontesford–Linley and Church Stretton lineaments, has transitional tholeiitic to alkaline character, with trace element contents influenced by subduction-related processes. The overall tholeiitic to calc-alkaline nature of the magmatism is consistent with the view that, during Llanvirn to Caradoc time, the Welsh Basin was an ensialic marginal basin.

Geochemical evidence for the emplacement of the Whin Sill complex of northern England

The Whin Sill comprises a major quartz tholeiite sill of late Carboniferous age underlying an area of c. 5000 km2 and with a volume of c. 200 km3, associated with contemporaneous dykes emplaced within Carboniferous sedimentary rocks in northeast England. New trace element analyses of chilled margins, sill interiors and dykes indicate that the Whin Sill complex magmas show significant chemical variations in terms of the relatively stable trace elements Th, Ce, Y, Zr, Nb and Ni. These data indicate that the complex was fed by a large number of compositionally distinct magma pulses, and that certain of the dykes may have formed feeder channels for the sill. The chemical characteristics of the sill and dyke samples are consistent with derivation by extensive polybaric fractional crystallization of olivine tholeiite magma derived by partial melting of compositionally heterogeneous mantle peridotite and/or crustal contamination of mantle-derived magmas.

Composite Ordovician lamprophyre (spessartite) intrusions around the Midlands Microcraton in central Britain

Lamprophyre sills and dykes of Ordovician age were emplaced within Cambrian–Lower Ordovician sedimentary rocks around the northern margins of the Midlands Microcraton. The intrusions show internal mineralogical and chemical variations indicating emplacement as multiple intrusions of co-magmatic pulses. The chemical characteristics of the lamprophyre magmas indicate formation by small-degree volatile-rich partial melting of lithospheric mantle enriched and modified by Lower Palaeozoic subduction (Th/Ta 5.3–11.6, La/Ta 29–82.3), together with a contribution from within-plate mantle source (Zr/Yc. 6) and/or mineralogically heterogeneous lithosphere, followed by varying degrees of fractional crystallization during uprise.