Dyke Rocks Research Articles

Fractionation of the rare earths in a suite of highly evolved metaluminous granitoids and felsic dykes from the Western Cape Province, South Africa

The rare earth element (REE) distribution in a comagmatic, metaluminous, igneous suite, consisting of a monzogranite, differentiating to an even-grained granite and eventually a sequence of dyke rocks, was investigated. During the early stages of the differentiation process, the REE behaved compatibly, the LREE being preferentially removed by fractionation of early allanite and monazite. With increasing fractionation, higher silica content and increasing alkalinity in the dyke rocks, evidence for the incompatible behaviour of the REE were obtained. LREE, Eu and, to a lesser extent, Gd were fractionated from the HREE and concentrated in the dyke centres. Partitioning of F and F-REE complexes towards the highly fractionated melt phase contributed towards this process. An increase in oxidation conditions during the very late evolutionary phases of the dykes again stabilized allanite as an accessory phase. LREE were sufficiently enriched at this stage to be incorporated in the mineral, leading to a subsequent lowering of the La Sm ratio in some highly evolved dyke centres. Late crystallizing fluorite in dyke centres accommodated some of the enriched LREE by a process of coupled substitution. Evidence for these fractionation processes in evolved metaluminous felsic rocks have implications for the use of REE in modelling the generation of such rocks.

Field relationships, geochemistry, and tectonic context of the Tertiary dyke suites in Staffordshire and Shropshire, central England

SUMMARY Field and petrographic data relating to the historically important dyke rocks in Shropshire and Staffordshire are reviewed and trace element geochemistry is used to show that separate dyke suites can be differentiated chemically. The dykes have usually suffered late-stage feldspathization, analcitization, mineralization and weathering, making it difficult to characterize them petrographically. The whole-rock geochemistry described in this paper shows that in Shropshire two tholeiitic suites occur, around Acton Reynald and between Clive and Grinshill, while in Staffordshire alkaline basaltic dykes in the northern Butterton–Swynnerton area are chemically different from those in the southern Yarnfield–Norton Bridge area. These latter suites were derived from different magmatic sources. Chemical comparisons with the nearest suitably aligned Tertiary dyke sets in Anglesey and north Lancashire reveal broad, but not precise, comparability with the Shropshire and Staffordshire dykes respectively. It is argued that dyke emplacements were probably vertical rather than lateral and largely took advantage of pre-existing fracture patterns. The discovery of distinct geochemical suites permits reinterpretation of fission track dating relating to dyke emplacement in the late Cretaceous–early Palaeocene phase of the British Tertiary Volcanic Province. The timing of emplacement of the Shropshire dykes accords well with the existence of a north-east-south-west tensional palaeostress field across the British Tertiary Volcanic Province both before and while the effects ( c. 62 Ma) of the Icelandic mantle plume and dome were first apparent and an abortive Gallic sub-plate was developing. Younger K-Ar and fission track dates in Staffordshire (at 52 and 54 Ma respectively) may relate to resetting of the apatite clock by late-stage hydrothermal alteration. A younger, less reliable K-Ar date in Shropshire (50 Ma) may relate to Cu–Pb–Zn–Co–Ni–V mineralization. The north-north-west–south-south-east Staffordshire dykes may relate generally to emplacement in the same stress field, but their alignment may also have been influenced by inherited upper-crustal Malvernoid fractures in the underlying Midlands microcraton. Post-intrusive normal, oblique and wrench faulting, consistent with later Tertiary and Neogene stress fields, affected dykes in Shropshire, but similar effects in Staffordshire are less well documented.

Geochemistry of the Grenville Dyke Swarm: role of plume-source mantle in magma genesis

Major and trace element data of samples from the approximately 590 Ma Grenville dyke swarm, in the southeastern parts of the Canadian Shield, were used to evaluate the extent of crustal contamination of the dyke magma and to model its probable mantle source. The dyke rocks are mostly saturated, quartz tholeiites. A few samples represent transitional basalts. Their overall chemistries are similar to those of continental flood basalts. Their compositions resemble those of liquids initially produced in the garnet peridotite mantle zone and then subjected to polybaric fractionation. However, fractionation models fail to mass-balance Ti, Fe, K, and P. Geochemical and isotopic criteria indicate that crustal contamination was minimal. The incompatible elements Zr, Y, K, Rb, Nb, V, Ta, Hf, and Th show hyperbolic elemental ratio-ratio distributions indicative of mixing between two end-members. One end-member has elemental ratios similar to those of a “depleted” N-MORB source and the second of a P-MORB i.e. ◂ertile” or “plume” MORB source. The more enriched dyke compositions can be successfully reproduced by mixing models in the ratio of 6:1 between the most depleted “MORB-like” dyke magma and a liquid composition similar to metasomatic glass veinlets coexisting with mica peridotite mantle xenoliths, and subsequent crystal fractionation. The Grenville dykes are probably a synrift swarm that was emplaced along a rift arm related to an Iapetan rrr triple junction that is thought to have formed over a mantle plume. The chemistry of the dyke swarm is consistent with its derivation from a plume head, in that the minor enriched source can be identified with plume-source mantle and the major depleted source with entrained mantle. The magma probably originated in the upper cooler parts of the plume head.

Geochronology and geochemistry of Palaeozoic intrusive rocks in the Rockvale region, southern New England Orogen, New South Wales

Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S‐type granitoids; (ii) a small I‐type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I‐types; and (iii) a suite of dyke rocks ranging in composition from calc‐alkaline lamprophyre through hornblende and biotite porphyrite to aplite. Ion‐microprobe U‐Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 ±3 Ma (weighted mean 206Pb/238U age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S‐type granitoids from a relatively juvenile protolith. Deforma...

北上山地，ＩＶ帯の深成岩類を貫くアルカリに富む岩脈類

Peculiar dyke rocks rich in alkalies and other incompatible elements intrude into the Himekami and Hinomiko plutons of Zone IV in the Kitakami Mountains. Representative rock types are as follows: 1) Shoshonite with the mineral assemblage biotite-orthopyroxene-olivine-clinopyroxene-plagioclase-alkali feldspar; and 2) High-K andesite with biotite-hornblende-plagioclase-alkali feldspar-quartz. The assemblage of the first rock types is similar to those of monzogabbro, which characterizes Zone IV plutons. Phenocrysts of olivine and orthopyroxene are surrounded by small crystals of cummingtonite and pale colored biotite. Composition of olivine is Fo74-70, while phenocryst and groundmass clinopyroxene are Wo40En45Fs15 and Wo43En35 Fs22 respectively, and that of phenocryst orthopyroxene has Wo3En72Fs25 core and Wo1En55Fs44 rim. Compositions of phenocryst plagioclase, groundmass plagioclase and alkali-feldspar are An60-41 An43-21 and Or91-31 respectively. The second rock type is rather coarase and porphyritic having phenocrysts of plagioclase and hornblende, and groundmass biotite, hornblende, plagioclase, alkali feldspar, quartz, apatite and Fe-Ti oxides. Hornblende is distinctly zoned and rich in (Na+K) and Al, different from that of the Cretaceous plutons in the Kitakami Mountains. Major element chemistry of these rocks indicates that both rock types are characteristically rich in K2O and poor in TiO2. They show MORB-normalized patterns poor in Nb and rich in Rb and Ba, which are characteristics of island arc or orogenic shoshonite rock series. The occurrence of these rocks provides important information as to the genesis of plutons and tectonic environments of the Kitakami Mountains during Early Cretaceous age.

Magma Generation in a Deep Seated Magma Chamber: Evidence by Mineral-, Geochemical, and Isotope Data from Pan-African Dyke Rocks (Bir Safsaf/SW Egypt)

Magma Generation in a Deep Seated Magma Chamber: Evidence by Mineral-, Geochemical, and Isotope Data from Pan-African Dyke Rocks (Bir Safsaf/SW Egypt)

Flow direction and groundmass shear zones in dykes, Shimane Peninsula, Japan

AbstractThe groundmass of andesitic dykes at Sezaki, southwest Japan, has trachytic texture and contains microscopic shear zones. The shear zones comprise a conjugate pair formed by flattening of the solidifying dyke rock, probably caused by the magma pressure of the still molten part of the dyke. This pressure shortened the solidifying rock perpendicular to the dyke margins and caused it to extrude parallel to the magma flow direction. The groundmass shears indicate that locally the magma flowed 60° upward in the dykes. It is concluded that while groundmass shears are a useful indicator of flow direction in dykes, phenocryst alignment in dykes is strongly influenced by magma-pressure flattening and thus may be a poor indicator of flow direction

The regional geology of the Poços de Caldas alkaline complex: mineralogy and geochemistry of selected nepheline syenites and phonolites

The Mesozoic Poços de Caldas alkaline complex, the largest known in South America, is circular-shaped with a mean diameter of about 33 km, and developed during continental break-up and drift. It comprises a suite of alkaline volcanic and plutonic rocks (mainly phonolites and nepheline syenites) with average amounts of U, Th and rare-earth elements (REEs). The evolutionary history began with major early volcanism involving ankaratrites, phonolite lavas and volcanoclastics, followed by caldera subsidence and nepheline syenite intrusions forming minor ring dykes, various intrusive bodies and circular structures. Finally, the addition or concentration of strongly incompatible elements led to the formation of eudialyte nepheline syenites and phonolites. Magmatic evolution included deuteric processes indicating a volatile-rich parent magma of upper mantle origin, without appreciable crustal contamination. These processes extended over a large temperature range and resulted in the formation of pegmatitic veins and comprised mineral assemblages including rare metal silicates such as giannettite, incipient alkali exchange reactions of feldspars, various zeolites, fluorite and hematite. Geochemically, the resulting rocks are enriched in potassium when compared to global nepheline syenites and phonolites. Mobilization and concentration of U, Th and REEs did not apparently occur at this stage. At one place (Morro do Ferro) the intermediate nephelinic suite was affected by a possible carbonatite intrusion and the formation of a stockwork of magnetite veins. Very intensive hydrothermal K- and S-rich alteration, associated with contemporaneous volcanic breccias, occurred locally. These processes led to the formation of several important radioactive and REE-rich anomalies. Two of these, the Th-REE occurrence of Morro do Ferro and the U-Zr-REE-Th occurrence of the Osamu Utsumi uranium mine, comprise the study sites of the Poços de Caldas Analogue Project. Later major stages in the evolution of the Poços de Caldas complex involved the emplacement of mafic-ultramafic dyke rocks and the onset of lateritic and allitic weathering, resulting (at the uranium mine) in supergene geochemical redistribution and the formation of redox fronts sometimes related to uranium enrichments. The end of the magmatic and hydrothermal-mineralizing events is likely fixed by the Ar-Ar dating of a lamprophyre dyke at the uranium mine (76 Ma). This study was focused towards the major rock types of the regional nephelinic suite relative to those experiencing more local hydrothermal and final weathering-related alteration. In the studied intrusive, subvolcanic and volcanic nepheline syenites and phonolites, very little variation was observed. This lack of differentiation may be seen as an argument for a short emplacement history of these rock bodies. Present radiometric age measurements suggest a time span of about 10 Ma for igneous activity at the caldera.

Paraná magmatism and the opening of the South Atlantic

Abstract New chemical and isotope results are presented on dyke rocks associated with the Paraná CFB, together with preliminary laser 40 Ar/ 39 Ar analyses on selected Paraná basalts. Dyke rocks from the Ponta Grossa Arch are similar to the Pitanga and Paranapanema magma types in the Paraná lavas, but dykes from the Santos-Rio de Janeiro section include samples with compositions not observed in the overlying lavas. Rather their minor and trace elements are strikingly similar to basalts recently erupted on Tristan de Cunha, and thus these late stage dykes may represent the first direct evidence for the involvement of typical plume-related OIB in the Paraná province. Laser 40 Ar/ 39 Ar analyses of two Gramado low Ti basalts have yielded preferred isochron ages of 132.4 ± 1.4 and 132.9 ± 2.8 Ma. These indicate a short eruption time for at least the Gramado magma type, and that magmatism took place several million years after the species extinction in the Tithonian ( c. 141 Ma). The majority of basalts and basaltic andesites in the Paraná CFB have distinctive trace elements ratios (low Nb/La and Nb/Ba), and relatively enriched Sr, Nd, and Pb isotope compositions. Since such features are not commonly observed in oceanic basalts, and they occur in CFBs which have been screened for the effects of crustal contamination, they are typically attributed to old, incompatible element enriched source regions in the continental mantle lithosphere. In some models the minor and trace element ‘mantle lithosphere’ component was introduced in small degree melts (lamproites) added to asthenosphere derived magmas. However, such models appear to be inconsistent with the data from low Ti CFB, and they also require that the asthenosphere derived magmas have very low incompatible element contents, in marked contrast to the high Nb/La late stage dykes in the Paraná. Alternatively some CFBs may have been generated within the mantle lithosphere in the presence of small amounts of water. The results of preliminary calculations indicate that in the presence of a mantle plume up to 5 km of melt may be generated entirely from within the mechanical boundary layer, for β values of less than 1.2.

Shoshonitic and alkaline lamprophyres with elevated Au and PGE concentrations from the Kreuzeck Mountains, Eastern Alps, Austria

Amphibole and mica lamprophyres and related dykes of Tertiary age from the Kreuzeck Mountains, Central Alps, Austria, have been investigated petrographically and geochemically. They intrude a sequence of early Palaeozoic metapelites, greenstones and amphibolites to the north of the Cretaceous Periadriatic Lineament, a major suture zone of 700 km E-W extent. The dykes are spatially associated with Sb, W, Hg, and Cu-Ag-Au deposits. Most lamprophyres are characterized by primitive chemistry (mg-numbers > 60 and Cr > 200 ppm) and have high contents of LIL elements (K, Rb, Sr and Ba). Geochemically, five different subgroups of calcalkaline/shoshonitic to alkaline affinity can be distinguished. These are: Group 1, amphibole-bearing shoshonitic lamprophyres (0.5–1.0 wt% Ti02, Zr 20); Group 3, alkaline lamprophyres (1.5–2.1 wt% TiO2, Zr > 250 ppm, Nb > 30 ppm, Ba/Rb 10–25); Group 4, low-MgO alkaline lamprophyres (∼ 2.5 wt% TiO2, mg-number < 57, Nb ∼ 20 ppm, Ba/Rb ∼ 20); Group 5, calc-alkaline basaltic dykes (∼ 2.2 wt% TiO2, mg-number <55, Nb < 10 ppm, Ba/Rb < 10). Group 2,3 and 4 dykes have NE-SW orientations and are of Oligocene age (K-Ar age 27–32 Ma); Group 1 and 5 dykes are of Lower Oligocene age (K-Ar age 36 Ma) but have mostly E-W orientations. The Kreuzeck lamprophyres were generated in post-collisional magmatic events, which were probably linked to extensional tectonics following oblique continent-continent collision between the African and Eurasian Plates during the Eocene. Group 1, 2 and 5 dyke rocks have typical calc-alkaline geochemical signatures indicating that they represent partial melting products of subduction-modified lithosphere. Group 3 and 4 alkaline lamprophyres have geochemical features transitional between calc-alkaline and within-plate alkaline igneous rocks (e.g. Ba/Nb ∼ 30–70) indicating that their mantle source-region includes both subduction-modified lithospheric and OIB-type asthenospheric components. There is no apparent relationship between mineralization in the Kreuzeck region, thought to be of Ordovician-Devonian age, and much later lamprophyre intrusion. Alteration of the dykes by late-magmatic fluids has resulted in the formation of secondary minerals, and has occasionally led to increased Au and PGE values in the 10–35 ppb range particularly in close proximity to Cu-Ag-Au deposits.

Geochemistry and petrogenesis of Proterozoic mafic dykes in north Kerala, southwestern Indian Shield — preliminary results

Mafic dyke intrusions occur in three distinct orientations ( NNW-SSE and NW-SE and NE-SW) in north Kerala regions, the southwestern part of the Indian Shield. Dating of two NNW-SSE trending dykes by K-Ar method has yielded Middle Proterozoic ages (ca. 1660 Ma and ca. 1420 Ma respectively). Our initial geochemical results on these dyke rocks (0.65-0.15 wt pct K2O, 0.37-0.38 wt pct P2O5, 3.30-1.00 wt pct TiO2, 11-1 p.p.m. Rb, 250-90 p.p.m. Sr, 230-40 p.p.m. Ba, 160-40 p.p.m. Zr, and 30-10 x chondrite rare earth elemental abundances) indicate a transitional character between abyssal and plateau tholeiites. Petrogenetic modelling suggests that the dyke compositions have been derived by different degrees of partial melting of a heterogenous source mantle. The mantle sources with accessory amphibole and/or garnet, variably enriched in LREE and LIL elements, are compatible with the observed geochemical data.

Faulting of a Middle Jurassic, ultramafic dyke in the Picton Quarry, Picton, southern Ontario

A fault zone coinciding with a Middle Jurassic, ultramafic dyke exposed in the Picton Quarry in Prince Edward County, Ontario, is marked by steeply dipping, generally east–west-striking fractures. The dyke has been affected by faulting, as evidenced by the presence of subhorizontal slickensides on fractures cutting the dyke rock. This discovery constitutes the first known example of Middle Jurassic or younger faulting having affected Paleozoic strata of southern Ontario and indicates that the structural and tectonic history of the region is more complex than commonly believed.

Brioverian volcanism and Cadomian tectonics, Baie de St Brieuc, Brittany: stages in the evolution of a late Precambrian ensialic basin

Abstract During late Precambrian times, the Northern Brittany and Lower Normandy region was the site of extensive submarine basaltic volcanism. On the southern side of the Baie de St Brieuc, two volcanic formations have been identified. In the east the Erquy Volcanic Formation (EVF), over 2 km thick, is a sedimentary-volcanic sequence in which the basalts occur mainly as pillowed, massive or composite massive-pillowed flows, interdigitated with intrusive sheets. Whole- and broken-pillow breccias, hyaloclastites, and peperites are locally common. Further west, the Hillion Volcanic Formation (HVF) has a similar thickness and attitude, and exhibits the same range of lithologies as the EVF, though in a more deformed state and metamorphosed to a higher grade. While assigned by some workers to the Ordovician, several lines of evidence are presented which show that the basic volcanics of the EVF form part of the same late Precambrian volcanic suite as the HVF. The suite was erupted in a marginal basin (the Lanvollon basin) environment founded upon a slightly older crust of calc-alkaline plutonic rocks, now represented in part by the adjacent Penthièvre massif. Basic sheets (the Dahouët dykes), cutting this basement, are considered co-genetic with the volcanics of the EVF and HVF. Chemically, the metabasic lavas, sub-volcanic sills and dyke rocks are subalkaline, tholeiitic basalts and basaltic andesites. The Lanvollon basin was subsequently filled with clastic sediments and then subjected to Cadomian deformation and metamorphism, the grade of which increases westwards, varying from prehnite-pumpellyite to lower amphibolite facies.

Thermal metamorphism and H2O-CO2-NaCl immiscibility at Patapedia, Quebec: evidence from fluid inclusions

The methamorphic history of the Patapedia thermal zone, Gaspe, Quebec, is re-evaluated in the light of results obtained from a study of fluid inclusions contained in quartz phenocrysts of felsic dyke rocks. The thermal zone is characterised by calc-silicate bodies that have outwardly telescoping prograde metamorphic isograds and display extensive retrograde metamorphism with associated copper mineralization. Three distinct fluid inclusion types are recognized: a low to moderate salinity, high density aqueous fluid (Type I); a low density CO2 fluid (Type II); and a high salinity, high density aqueous fluid (Type III). Fluid inclusion Types I and II predominate whereas Type III inclusions form <10% of the fluid inclusion population. All three fluid types are interpreted to have been present during prograde metamorphism. Temperatures and pressures of metamorphism estimated from fluid inclusion microthermometry and isochore calculations are 450°–500° C and 700–1000 bars, respectively. A model is proposed in which the metamorphism at Patapedia was caused by heat transferred from a low to moderate salinity fluid of partly orthomagmatic origin (Type I inclusions). During the early stages, and particularly in the deeper parts of the system, CO2 produced by metamorphism was completely miscible in the aqueous hydrothermal fluid and locally resulted in high XCO2 fluids. On cooling and/or migrating to higher levels these latter fluids exsolved high salinity aqueous fluids represented by the Type III inclusions. Most of the metamorphism, however, took place at temperature-pressure conditions consistent with the immiscibility of CO2 and the hydrothermal fluid and was consequently accompanied by the release of large volumes of CO2 vapour which is represented by Type II inclusions. The final stage of the history of the Patapedia aureole was marked by retrograde metamorphism and copper mineralization of a calcite-free calc-silicate hornfels in the presence of a low XCO2 fluid.

San Jacinto Intrusive Complex: 3. Constraints on crustal magma chamber processes from strontium isotope heterogeneity

Strontium isotope data from three large plutons interpreted as derived from dynamic magma chambers (Hill, this issue; Hill et al., this issue) show the existence of pronounced isotopic heterogeneity within each unit. Ranges of calculated initial 87Sr/86Sr are unit I, 0.7060–0.7076; unit II, 0.7060–0.7074; unit III, 0.7058–0.7073. A limited sample of mafic inclusions and dyke rocks, interpreted as samples of liquid added to the various chambers during inflation, have initial 87Sr/86Sr of 0.7068–0.7084. These variations are regular at the kilometer scale within each pluton and show no identified correlation with any other observed geocheinical or petrological parameter. Slightly older small intrusions that span the compositional range olivine gabbro to granite show a systematic increase in initial 87Sr/86Sr from 0.7057 in the southwest to 0.7077 in the northeast, expressing the pattern previously reported for the northern 600 km of the Peninsular Ranges batholith. The Sr isotope data indicate that melt production, aggregation, transport, and crystallization processes were far from capable of completely homogenizing initial variations in initial 87Sr/86Sr within the liquids from which these rocks crystallized. Whole chamber convection is apparently ruled out by these data; more complex patterns, including double‐diffusively induced horizontal stratification, are permitted.

A comparative analysis of dyke lineaments mapped from Shuttle Imaging Radar and Large Format Camera photography in hyperarid areas of the Eastern Desert, Egypt, and Red Sea Hills, Sudan

Abstract The Red Sea Hills (Sudan) and Eastern Desert (Egypt) have been selected, as areas with hyperarid climate, for a comparative study of SIR-B radar imagery and Large Format Camera (LFC) spaceborne photography. Some areas with extensive sand sheets in which dykes and dyke swarms were partially outcropping were studied in detail and a dyke lineament analysis was made using the different remote sensing data sets. Particularly from the LFC photos much detail could be obtained through stereo analysis. It appeared that the radar images revealed some information on the dyke rocks covered by a shallow sand layer, otherwise invisible on Landsat and LFC images. This suggests some microwave penetration capability, although results were not conclusive. The differences between dyke lineament interpretation from radar images and LFC photos seem not so much influenced by the penetration capability of microwaves, but more by the influence of radar look direction.

Reconsideration of ‘soda-minettes’

AbstractIt is shown here that dyke rocks of type localities for ‘soda-minettes’ are not minettes (= lamprophyres dominated by phlogopite and K-rich feldspar). It has long been suggested that ‘soda-minette’ also be applied to otherwise normal minettes from elsewhere that carry modally variable amounts of groundmass aegirine and/or, more commonly, arfvedsonitic to riebeckitic amphibole. Despite the presence of sodic pyriboles, these rocks are poorer in Na2O then minettes lacking these phases. ‘Soda-minette’ is thus misleading and self-contradictory to the many petrologists to whom ‘soda-’ legitimately connotes that the rock in question either has K/Na &lt; 1 and/or is rich in albitic feldspar. To eliminate this ambiguity it is recommended, with the approval of twenty-six other petrologists, that ‘soda-minette’ no longer be used as a rock name. As minettes with Na-pyribole(s) are markedly K-rich and most are both ultrapotassic and peralkaline, independent of the modal abundance of the Na-pyrobile, it is suggested that one of these three chemical characteristics be utilized adjectivally in naming these minettes.

Constrictional deformation of the Koster dyke swarm in a ductile sinistral shear zone, Koster islands, SW Sweden

The Koster-Kattsund dyke swarm is an important element in the Sveconorwegian province of the Baltic shield. Dyke intrusion took place in the period 1225-1015 Ma. Throughout most of the swarm the dykes are strongly deformed and thoroughly recrystallised into lineated amphibolites as a result of a Sveconor- wegian tectonic event about 1000 Ma ago. However, in the Koster archipelago fresh dolerites can be fol­lowed northwards in to partially recrystallised metadolerites and finally into the totally recrystallised, line­ated amphibolites that characterise the swarm. In the Koster archipelago intense dyking resulted in the formation of a multilayered rock sandwich con­sisting of alternating layers of gneiss and dolerite. The sandwich trends NNE and dips 67°W. The dolerite dykes have a mean thickness of2.2 m and they occupy 15-20% of the total rock mass. To the northeast the sandwich becomes progressively deformed and ultimately shows very high strain of pure constrictional type. The deformation took place in a steep NW-SE-trending ductile shear zone. During the initial shear zone deformation (D4,) the sandwich underwent anticlockwise bending and the large Kyrkosund synform was formed. The fold plunges 303/66 and has a NW-SE-trending axial surface. The bending took place by means of flexural-slip folding in which the layer-parallel shearing was located in incompetent dyke layers. Increasing shearing and recrystallisation in a NW-SE-trending belt crossing the northern limb of the Kyr­kosund synform resulted in a softening of this belt. The succeeding event (D4b) was localised in this initial soft belt, and involved sinistral simple shear combined with pure shear resulting in horizontal widening and vertical shortening of the belt. This composite deformation formed the pure constrictional fabric now seen in the rocks. The strong D4b stretching was followed by the formation of trains of asymmetric folds (D 4c and d4a). It is demonstrated that volume changes in the dyke rock during deformation were negligible, and that no competence contrast between gneiss and dyke rock existed during the D 4b stretching. The finite con­strictional strain ellipsoid has the dimensions X = 7.07, Y = Z = 0.18. The composite simple/pure shear deformation that presumably caused the constriction has a simple shear component y = 10.9, correspond­ing to an angular shear of 84. 7°. The pure shear deformation resulted in a 3.4 times horizontal widening of the initial soft belt. The horizontal sinistral displacement within the shear zone was at least 35 km.

Magnetic susceptibility used in mapping of amphibolite facies recrystallisation in basic dykes

The dense tholeiitic Koster dyke swarm, west Sweden, has undergone progressive deformation under amphibolite facies metamorphic conditions. The varying influence of this event on the dyke rock forms the basis for a regional division of the swarm into three sectors: the sectors of igneous dolerites, partly recrystallised metadolerites, and lineated amphibolites, respectively. Systematic measurements of the magnetic susceptibility in the dykes have demonstrated a correlation between the regional pattern of recrystallisation and the pattern of mean susceptibility given as mean susceptibility per 0.5 km 2. The transformation of dolerites to metadolerites and finally to lineated amphibolites is accompanied by a marked decrease in the susceptibility, which is related essentially to the degree of alteration of titanomagnetite to sphene + Fe-ions incorporated in the silicates.

Trace element geochemistry of high alumina basalt - Andesite - Dacite - Rhyodacite lavas of the Main Volcanic Series of Santorini Volcano, Greece

Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.