Tholeiitic Dykes Research Articles

Geochemistry and tectonic significance of late Gothian mafic dykes in the Østfold-Marstrand belt of S.E. Norway and W. Sweden

Studies of the poly-orogenic Proterozoic crust in W. Sweden and S.E. Norway suggest a crustal evolution which essentially took place during the Gothian period (ca 1.76-1.5 Ga) and under the control of east dipping subduction. Later, the crust underwent tectono-thermal reworking during the Sveconorwegian orogeny (1.1-0.9 Ga). In the intervening period, granites and tholeiitic dykes intruded an unroofed Gothian crust possibly under conditions of crustal thinning and high heat flow. This paper investigates late Gothian (ca 1.57-1.5 Ga) net-veined and mafic dykes from an area untouched by Sveconorwegian strain which provides information about late Gothian crustal conditions. The net-veined dykes consist of a quartz tholeiitic component, geochemically similar to the mafic dykes, and veins of a calcalkaline granite including hybrids. Based on field evidence and geochemistry, the net-veining granite is thought to be derived from crustal material from which it inherited a typical subduction-related signature. The moderately evolved mafic rocks are enriched in large ion lithophile elements and light rare earth elements presumably resulting from contamination by the granites, and show an affinity to present-day ocean island tholeiite and to mid-ocean ridge basalt of transitional type. The mafic rocks are thought to be extracted from an asthenospheric, possibly normal mid-oceanic ridge basalt mantle source enriched by material from a rising plume in a continental back-arc position. The back-arc model suggests that the eastwards dipping Gothian subduction under western Sweden and south-eastern Norway still continued in late Gothian time, but in a position considerably displaced to the west, and that the Gothian orogenic section drifted away from the subduction modified mantle wedge prior to dyke emplacement.

Petrogenetic relationship between Palaeoproterozoic tholeiitic dykes and associated high-Mg noritic dykes, Labrador, Canada

Trace-element and SrNd isotopic studies of the Palaeoproterozoic (2235 Ma) Kikkertavak dyke swarm in Labrador provide information on early sub-continental lithosphere development. There are three distinct chemical dyke types: two are tholeiitic, the third of high-Mg norite composition. Petrogenetically, it is not possible to relate any of the chemical groups one to another by simple fractional crystallisation processes; nor can they be derived by contaminating asthenospheric magmas with local crustal components. From their SrNd isotopic characteristics in relation to their rare-earth and multielement patterns, it is clear that significant heterogeneity existed in the source regions of the dyke magmas at the time they were generated, and that the mantle processes responsible can be linked with the phase of crustal growth in the region at ∼3.0 Ga. Modelling suggests that the observed compositions can be accounted for by lower lithosphere-asthenosphere mantle sources which have been variably depleted by previous episodes of basalt magma extraction, and then re-enriched by a lamproitic melt component and possibly by progressive infiltration of melts from a subducted sediment or subduction component. High-Mg noritic dykes are relatively common worldwide in the Palaeoproterozoic (and are often broadly coeval with tholeiitic dykes), but are much less common in younger dyke suites. Most have compositions which require a refractory major-element composition coupled with significant re-enrichment in incompatible lithophile elements. Recent models for cyclic mantle overturns with related deep mantle plumes begin to provide the right thermal and compositional environment beneath immature lithosphere which can generate the dyke-types observed.

Continental break-up and collision in the Neoproterozoic and Palaeozoic — A tale of Baltica and Laurentia

During the Neoproterozoic and Palaeozoic the two continents of Baltica and Laurentia witnessed the break-up of one supercontinent, Rodinia, and the formation of another, but less long-lived, Pangea. Baltica and Laurentia played central roles in a tectonic menage a trois that included major orogenic events, a redistribution of palaeogeography and a brief involvement of both with Gondwana. Many of these plate re-organisations took place over a short time interval and invite a re-evaluation of earlier geodynamic models which limited the speeds at which large continental plates could move to an arbitrarily low value. Baltica and Laurentia probably shared a common drift history for the time interval 750 – 600 Ma as they rotated clockwise and drifted southward from an equatorial position during the opening of the Proto-Pacific between Laurentia and East Gondwana (initial break-up of Rodinia). On their combined approach toward the south pole, Baltica and Laurentia were glaciated during the Varanger glaciations. Although the two continents drifted toward the south pole during the Late Proterozoic, they began to separate at around 600 Ma (rift to drift) to form the Iapetus Ocean through asymmetric rifting and relative rotations of up to 180°. Initiation of rifting on the Baltic margin is marked by the 650 Ma Egersund tholeiitic dykes (SW Norway) which contain abundant lower crustal zenoliths, and the tholeiitic magma was probably derived from a mantle plume. In latest Precambrian time, the final redistribution of Rodinia is characterised by high plate velocities. In particular, Laurentia began a rapid, up to 20 cm/yr, ascent to equatorial latitudes and essentially stayed in low latitudes throughout most of the Palaeozoic. The high velocities suggest either that Laurentia was pushed off a lower mantle heat anomaly originating from supercontinental mantle insulation or that Laurentia was pulled toward a subduction-generated cold spot in the proto-Pacific. Baltica, except for a short and rapid excursion to lower latitudes in the Late Vendian, remained mostly in intermediate to high southerly latitudes and closer to the Gondwana margin until Early Ordovician times. In Early Ordovician times, Arenig-Llanvirn platform trilobites show a broad distinction between the continents of Laurentia/Siberia/North China Block (Bathyurid), Baltica (Ptychopygine/ Megalaspid) and the areas of NW Gondwana/Avalonia/Armorica (Calymenacean-Dalmanitacean). During the Ordovician, Baltica rotated and moved northward, approaching close enough to Laurentia by the late Caradoc for trilobite and brachiopod spat to cross the intervening Iapetus Ocean. Docking appears to have been irregular both in time and manner: the collision between Scotland/Greenland and western Norway resulted in the early Scandian Orogeny in the Silurian (c. 425 Ma), but further south, there is evidence of late Silurian impingement with subduction of Avalonian continental crust (in England and Ireland) below the eastern edge of Laurentia until the Emsian. In the northern Appalachians the main time of collision appears to have been during the Emsian/Eifellian Acadian Orogeny. Recent analyses invalidates the traditional concept of a sustained orthogonal relationship between Baltica and Laurentia across a single Iapetus Ocean throughout the Caledonide evolution. The active margin of Baltica (Scandinavian Caledonides) faced Siberia during the Late Cambrian and Early Ordovician with oceanic separation between these landmasses in the order of 1200–1500 km. This may explain the local occureences of Siberia-Laurentian type Bathyarid tribobite faunas in Central Norwegian Caledonian nappes, earlier interpreted as Laurentia-Baltica trilobite mixing. Subsequent counterclockwise rotation of Baltica transferred the Caledonian margin in the direction of Laurentia by Silurian times, when the two continents once again started to collide to form Euramerica. This rotation, along with the strongly asymmetric opening of the Iapetus at around 600 Ma, demonstrates a complexity in Precambrian-Palaeozoic plate tectonics, i.e. a collage of metastable plate boundaries which have perhaps too often been simplified to an orthagonal Wilson cycle tectonic scenario.

Nature of mantle source contributions and the role of contamination and in situ crystallisation in the petrogenesis of Proterozoic mafic dykes and flood basalts Labrador

Proterozoic tholeiitic dyke swarms share many compositional features with, and pose similar petrogenetic problems to, Phanerozoic continental flood basalts, but there are few extrusive equivalents of such swarms. The Mesoproterozoic (1.27 Ga) Harp dyke swarm in Labrador is one where possible extrusive equivalents exist in the Seal Lake group, but are slightly displaced in space and time, and can probably be related by models of progressive crustal extension. Here we try to evaluate the roles of crystal differentiation, in situ crystallisation, crustal assimilation and the relative contributions of asthenosphere- and lithosphere-derived melts in the petrogenesis of the mafic magmas. Modelling of the major and trace element variations both within individual dykes and between dykes, and within the lava sequence, does not suggest an important role for continental crust involvement. While in situ crystallisation processes could account for some of the compositional variations, the most successful models invoke mixing or contamination of asthenospheric magmas with/by veined material in the lower lithosphere / upper asthenosphere which carries the ‘continental’ characteristics. The results imply an important role for hydrous phases such as phlogopite and hornblende in the sub-lithosphere mantle. Much of the low-MgO character of mafic dykes may result from significant removal of mafic phases during in situ crystallisation within the lithosphere.

Boninitic and tholeiitic dykes in the Lewis Hills mantle section of the Bay of Islands ophiolite: implications for magmatism adjacent to a fracture zone in a back-arc spreading environment

A detailed, integrated field, petrographic, and geochemical study of the Springers Hill area of the Bay of Islands ophiolite exposed in the Lewis Hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite, orthopyroxenite, and clinopyroxenite, and their associated dunites, hosted by a refractory harzburgite–dunite mixture. A geodynamic situation is presented, which is constrained by previous studies requiring formation of the Springers Hill mantle section at a ridge–fracture zone intersection, and the whole of the Bay of Islands ophiolite within a back-arc spreading environment. The veins and dykes formed during magmatism at the ridge–fracture zone intersection and along the fracture zone, as progressively hotter, more fertile (richer in clinopyroxene) asthenosphere ascended and was channelled up and along the fracture zone wall. Shallow melting of refractory harzburgite in the presence of subduction-derived hydrous fluids produced light rare earth element (LREE)-enriched boninitic magma from which crystallized chromitites, some of their associated dunites, and orthopyroxenites. This melting event dehydrated much of the mantle in and around the zone of partial melting. Continued rise and shallow partial melting of hotter, more fertile mantle under conditions of variable hydration generated LREE-depleted, low-Ti tholeiitic magma. This magma crystallized olivine clinopyroxenite, some associated dunite, and clinopyroxenite. The final magmatic event may have involved partial melting of mid-ocean-ridge basalt-bearing mantle at depth, ascent of the magma, and formation of massive wehrlite–lherzolite bodies at the ridge–fracture zone intersection and along the fracture zone. Ridge–fracture zone intersections in suprasubduction-zone environments are sites of boninitic and tholeiitic magmatism because refractory asthenospheric mantle may melt as it is channelled with subduction-derived fluids to shallow depths by the old, cold lithospheric wall of the fracture zone. Heat for melting is provided by the ascent of hotter, more fertile mantle. Extremely refractory magmas do not occur along "normal" oceanic fracture zones because volumes of highly refractory mantle are much less, subduction-derived hydrous fluids are not present, and fracture zone walls extend to shallower depths.

Palaeostress analysis, a contribution to the understanding of basin tectonics and geodynamic evolution. Example of the Permian/Cenozoic tectonics of Great Britain and geodynamic implications in western Europe

Microtectonic analysis in association with tectonic-sedimentological observations enables us not only to define palaeostress tensors but also to date each of them. Such a study, carried out in central and northeastern England and in southernmost Wales has permitted us to point out several tectonic stages during the Permian-Cenozoic period. (1) Inferred from syn-sedimentary faulting in the Late Permian series, the Permian/Early Triassic tectonic regime is characterised by a NNW-SSE-oriented extension. (2) During the Late Triassic to the early Late Jurassic, an E-W- to ENE-WSW-oriented extension was acting as suggested by syn-sedimentary normal faulting in Keuper, Lias and Middle Dogger sediments and post-sedimentary normal faulting in Oxfordian sediments. (3) The following tectonic event probably started during the Malm and was particularly active during the Early Cretaceous. It was characterised by a N-S- to NNE-SSW-oriented extension and was associated with an E-W- to WNW-ESE-oriented strike-slip tectonic regime (transtension). (4) A locally observed NW-SE-oriented transpressional strike-slip tectonic regime is thought to correspond to the ‘Laramide’ inversion phase of the Middle Paleocene. (5) The later tectonic event is characterised by a N-S-oriented transpressional strike-slip regime and affects a Paleocene tholeiitic dyke (‘Cleveland dyke’). It is supposed to be synchronous with the so-called Eocene ‘Pyrenean’ to the Early Miocene ‘Helvetic’ compressional stages. These paleostress tensors are compared with others defined in France, the Benelux countries and Germany and are also collated to the tectonic evolution of oil basins assessed from geophysical analyses. These comparisons raise discussions about the paleostress distribution within the geodynamic evolution of the West European shelf.

Magmatisme fissural triasico-liasique dans l'ouest du Massif armoricain (France): pétrologie, géochimie, âge, et modalités de la mise en place

The dolerite dykes at the western end of Britanny (France), which is located on the North Atlantic margin, were emplaced in a NW–SE field of faults at the Trias–Lias time limit, between 210 and 195 Ma. These rocks are quartz tholeiites enriched in TiO2, with a trace element composition close to that of the Deccan basalts. The initial concentrations of Ba, K, and Rb, though slightly modified by the alteration, and of Nb seem to have been affected by crustal contamination during the magma transfer. These dykes are comparable, in age and composition, to the tholeiitic dykes exposed in the Iberian Peninsula, in the Moroccan Atlas and in the Northern Appalachians; they however have a different orientation. Furthermore, these various domains differ in their postintrusion history: the rifting leading to the opening of the Central Atlantic occurred soon after the intrusions of the dykes, whereas the opening of the North Atlantic began several tens of millions of years later, following the direction of the Armorican array of dykes. The study of the mode of emplacement of the intrusions shows evidence of two sets of dykes: "simple dikes", 5 to 10 m thick, resulting from a single magmatic injection and "multiple dykes", about 30 m thick, composed of five magmatic units that are individually 5 to 10 m wide. The western Armorican dyke field is the result of the intrusion of successive sets of dolerite having a uniform thickness and composition, suggesting a cyclic magmatic supply from homogeneous source. [Journal Translation]

Adinoles revisited: hydrothermal Na(Ca)-metasomatism of pelite screens adjacent to tholeiitic dykes in the Dublin terrane, Ireland

AbstractThe albitisation of pelitic metasediments to form adinoles adjacent to minor mafic intrusions has in the past been attributed to Na-bearing metasomatizing fluids emanating from the intrusions themselves. The chemistry and mineralogy of adinoles associated with dykes forming a high-intensity swarm in the Tallaght area, County Dublin, Ireland, confirms a metasomatic origin for the adinoles described, with Na and to a lesser extent Ca introduced into the country rocks at the expense of K. We suggest, however, that the source of the metasomatizing fluids was external to the dykes — possibly involving a hydrothermal system driven by an underlying magma reservoir that was parental to the dykes and also contributed volatiles to the hydrothermal system by degassing. Heat flow from the dykes was instrumental in promoting the reaction between muscovite in the country rock pelites and the Na(Ca)-bearing fluids to form albite, temperatures of ∼350°C in the country rocks within a metre of the contact being indicated. The mechanism of adinolization proposed is shown to be compatible with recent experimental work on the hydrothermal alteration of greywackes and basalts and also with the likely temperature gradients adjacent to dykes.

Early Proterozoic mafic dykes in the North Atlantic and Baltic cratons: field setting and chemistry of distinctive dyke swarms

Abstract Widespread swarms of basic dykes intruded the Archaean North Atlantic craton (NAC) and the NW Baltic shield between 2.5 and 1.9 Ga. The oldest Mg-rich dykes and sills are 2.4–2.5 Ga and are associated with rift controlled acid and basic magmatism. In SW Greenland the main high-Mg swarms are c. 2.2 Ga. The high-Mg dykes have high Si, LILE and LREE and are low in Ca, Al, Ti and Nb. Isotopic compositions (Sr, Nd and Pb) demonstrate a high degree of crustal contamination. Individual dykes are remarkably constant in composition but there are large variations between adjacent dykes. Tholeiitic swarms were emplaced at c. 2.2 Ga in the southern part of the craton in SW and SE Greenland and Labrador. The majority are normal moderately evolved tholeiites with no marked LILE or LREE enrichment, a few c. 2.2 Ga comparatively mafic tholeiitic dykes show a LILE and LREE enrichment. Dense swarms of distinctive Fe-enriched hornblende-bearing tholeiitic dykes (the c. 2.0 Ga Kangâmiut dykes) intruded the northern part of the NAC in Greenland. Many are composite with intermediate centres. The Kangâmiut dykes were emplaced at depth during shearing in a regional N-S compressional regime and developed metamorphic mineral assemblages at the time of dyke injection. The 1.83 Ga Avayalik dykes from the eastern foreland of the Torngat orogen, northern Labrador show comparable syn-tectonic emplacement and autometamorphic assemblages. Emplacement was controlled by regional sinistral shearing after the first calc-alkaline igneous activity had occurred within the orogen.

Basaltic magmatism related to the early stages of rifting along the Benue Trough: The Obudu dolerites of south‐east Nigeria

AbstractDolerites of the Obudu Plateau of south‐eastern Nigeria occur predominantly as dykes cutting Precambrian basement gneisses. Minor dolerite sills have also been mapped. The dolerites are dominantly olivine tholeiites and are geochemically of the oceanic basalt type. They were emplaced in a within‐plate tectonic setting and yield a 40Ar/39Ar plateau age of 140.5 ± 0.7 Ma, which is interpreted as the age of their emplacement. This age indicates that the Obudu dolerites are the products of basic magmatism related to the early stages of rifting along the Benue Trough of Nigeria. The Obudu dolerites are comparable with Mesozoic tholeiitic dykes of the northern Amazon craton, whose intrusion preceded the break‐up of Africa and South America.

Geological affinities of the Glenelg river complex, western Victoria

A detailed comparison between the Glenelg River Complex in the Wando Vale district of western Victoria and the South Australian Kanmantoo Group of the Mt Lofty Ranges reveals many common elements. As the basis of comparison the geology of Wando Vale is described concisely with emphasis on its magmatic and structural history. Both have similar metasedimentary rocks, which are siliciclastically dominated with a substantial calcareous component. Geochemically, these rocks are relatively rich in CaO and Na2O, and compositionally distinct from the mature turbidites of the Lachlan Orogenic Belt. Four deformational episodes are identified in the Glenelg River Complex and are comparable to the history of the Kanmantoo Group in the Tungkillo‐Palmer region. The regional metamorphic zonation of both units extends into the amphibolite facies and is of the andalusite‐sillimanite type with similar mineral crystallization sequences. Tholeiitic dykes with comparable timing, the geochemical affinities of mid‐ocean ridge b...

U-Pb zircon dating of mafic dykes and its application to the Proterozoic geological history of the Vestfold Hills, East Antarctica

The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic Shield, comprises a high-grade metamorphic basement complex intruded by at least nine generations of Early to Middle Proterozoic mafic dykes. Extensive U-Pb ion microprobe (SHRIMP) analyses of zircons, derived predominantly from late-stage felsic differentiates of the mafic dykes, provide precise crystallisation ages for several dyke generations. These new ages enable constraints to be placed on both the history of mafic magmatism in the Vestfold Hills and the timing of the various interspersed Proterozoic deformation events. In addition to demonstrating the utility of zircons derived from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study also places doubt on previous wholerock Rb-Sr dating of mafic dyke suites in this and other areas of East Antarctica. The 207Pb/206Pb zircon ages of 2241±4 Ma and 2238±7 Ma for the Homogeneous and Mottled Norites, respectively, provide a younger emplacement age for associated group 2 High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424±72 Ma) originally interpreted as the age of all high-Mg intrusives in the Vestfold Hills. Zircon ages of 1754±16 Ma and 1832±72 Ma confirm the previously defined Rb-Sr age of the group 2 Fe-rich tholeiites. Two later dyke generations, the group 3 and 4 Fe-rich tholeiites, are distinguished on the basis of field orientations and cross-cutting relationships, and yield zircon emplacement ages of 1380±7 Ma and 1241±5 Ma which also define minimum ages for two suites of lamprophyre dykes. Xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478±5 Ma to ∼2740 Ma, indicating the presence of Archaean crustal source rocks of this antiquity beneath the Vestfold Hills.

Northeast-trending Early Proterozoic dykes of southern Superior Province: multiple episodes of emplacement recognized from integrated paleomagnetism and U–Pb geochronology

Integrated paleomagnetic and U–Pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of Early Proterozoic tholeiitic dykes of northeast trend in the southern Superior Province, previously referred to collectively as Preissac dykes. It is demonstrated that they are readily separated on the basis of paleomagnetism into subsets, referred to as the Biscotasing and Senneterre swarms. In addition a pair of unnamed dykes may be associated with the north-and northwest-trending Matachewan swarm farther west.Biscotasing dykes have a down-west magnetization of single polarity with a corresponding paleopole at 27.8°N, 136.7°W (dm = 12.3° and dp = 9.4°). Senneterre dykes carry an up-north (or occasionally down-south) direction with corresponding paleopole at 15.3°S, 75.7°W (dm = 7.0°, dp = 4.4°). The Senneterre direction is indistinguishable from the primary N1 remanence direction that dominates the magnetization of Nipissing sills of the Southern Province. Paleomagnetic field tests described herein or in earlier studies indicate that Biscotasing and Senneterre directions are primary and, hence, that two ages of intrusion are involved, with the age of Senneterre dykes coinciding with the intrusion of most Nipissing sills. U–Pb dating of baddeleyite conducted at a paleomagnetic sampling site yields an age of 2214.3 ± 12.4 Ma for the Senneterre swarm, indistinguishable from the age of 2217.2 ± 4 Ma reported from an N1 Nipissing sill site in another study. A U–Pb age on baddeleyite and zircon of 2166.7 ± 1.4 Ma was obtained from a paleomagnetic site in the Biscotasing swarm. The primary paleopoles for the Senneterre, Nipissing, and Biscotasing rocks define a direction of polar wander opposite to that of the most widely used polar wander paths for North America for this period, suggesting that these paths should no longer be used.

Evidence of magmatic activity related to Middle Jurassic and Lower Cretaceous rifting from northeastern Brazil (Ceará-Mirim): K/Ar age, palaeomagnetism, petrology and Sr/1bNd isotope characteristics

The Mesozoic magmatic activity in the easternmost part of NE Brazil (Ceará-Mirim) is mainly represented by two-pyroxene tholeiitic dykes; only few dykes have alkaline character. K/Ar ages and palaeomagnetism data indicate that Ceará-Mirim dykes are of Middle Jurassic (175-160 Ma) and Early Cretaceous (140-130 Ma) age. Both Middle Jurassic and Lower Cretaceous dykes have high incompatible-element concentrations and TiO 2 with the lower contents generally confined to the Middle Jurassic dykes; rare tholeiitic dykes low in TiO 2 and incompatible elements are present. Sr/1bNd isotopic and other chemical data do not support appreciable crustal contamination and in a 87 Sr 86 Sr vs. 143 Nd 144 Nd diagram almost all the dykes plot in a narrow area of the enriched quadrant of the “mantle array”. Tholeiitic and alkaline Ceará-Mirim dykes are related to different parent magmas probably originating by different degrees of melting of garnet-peridotite mantle sources characterized by distinct Sr/1bNd isotope compositions and small-scale chemical heterogeneities. Ceará-Mirim tholeiitic dykes show close mineralogical, chemical and isotopic similarities with the Lower Cretaceous high-TiO 2 tholeiites from Maranhão (Sardinha intrusives) and Paraná (Northern Province) basins. In general, chemical and Sr/1bNd isotope data for high- and low-TiO 2 Brazilian Mesozoic tholeiites appear to be related to time-integrated enriched and depleted mantle components, respectively. Subcontinental mantle heterogeneity is believed to be due to variable “enriched” components related to “metasomatic” processes. The Ceará-Mirim Jurassic dykes correlate well in terms of composition and tectonic setting with the analogue tholeiitic dyke swarms from the Benue trough, and can be related to the opening of the Central Atlantic Ocean. The Lower Cretaceous Ceará-Mirim magmatic activity corresponds to the coeval one of the Benue trough and can be related to the early rifting events responsible for the opening of the Equatorial Domain of the South Atlantic whose oceanic crust formed between 120 and 100 Ma.

Deccan-related magmatism west of the Seychelles-India rift

Abstract The rocks of the Seychelles can be divided into two age groups, namely Precambrian granites and younger (Cretaceous/Tertiary) intrusive rocks. The latter can be further subdivided into alkaline ring complexes (as found on the islands of Silhouette and North Island) and basic dykes (on Praslin, Felicité and Mahé islands). Evidence from offshore seismic work and drill holes suggests that Cretaceous/Tertiary magmatism occurred over the whole Seychelles Bank, producing both flood basalts and central volcanic complexes. The flood basalts extend at least as far south as 10°S/60°E. The younger igneous rocks of the Seychelles show close similarities to the Deccan igneous rocks of India. Tholeiitic dykes from Praslin have previously been shown to resemble Bushe Formation tholeiites from the Deccan, and here we show that the Felicité Island dykes also resemble Bushe. We show also that the alkaline dykes of Mahé and North Island are chemically similar to the dykes at Murud on the west coast of India. Isotopically the Seychelles undersaturated rocks fall within the fields of the Deccan tholeiites. In India, alkaline magmatism post-dates the tholeiitic magmatism; the age difference is of the order of 3 Ma. This is similar to the age difference between shield-building and rejuvenated-stage magmatism on Hawaiian volcanoes, which has been related to reactivation of the volcanoes by the passage of the Hawaiian Arch. We propose that the Deccan alkaline magmatism is a continental equivalent of oceanic rejuvenated-stage volcanism.

The difficulties of dating mafic dykes: an Antarctic example

Archaen gneisses of the Vestfold Hills of East Antarctica are transected by several compositionally discrete suites of tholeiitic dykes. A representative of one of those suites, which has been dated in the present study, shows that not only Rb−Sr whole-rock isochrons, but also U−Pb zircon techniques (if not properly applied) can produce erroneous crystallisation ages. Two zircon populations were recovered from the mafic dyke itself, one of which is 2,483±9 Ma in age and clearly of xenocrystic origin. The other yields an age of 1,025±56 Ma, which is not ascribed to the magmatic crystallisation of the dyke, but rather to the time that it underwent metamorphic/metasomatic alteration as a response to high-grade tectonism in the adjacent mobile belt. It is presumed that the zircon in question formed by the breakdown of another mineral or minerals (possibly magmatic baddeleyite), due either to ingress of a siliceous fluid, or more probably by the release of silica from the breakdown of pyroxene to amphibole. A cogenetic 1–2 cm wide felsic vein, of late magmatic/early hydrothermal origin, also contains two zircon populations. Again, most of the grains therein, which are interpreted as of xenocrystic origin, grew at 2,483±9 Ma. However, a few euhedral zircons with very high U and Th contents grew at 1,248±4 Ma, which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke.

Tholeiitic dykes in the Seychelles and the original spatial extent of the Deccan

We report geochemical data from c . Deccan age tholeiitic dykes from Praslin Island in the Seychelles, a continental fragment which lay adjacent to the coast of India before sea-floor spreading started in the Arabian Sea at 64 Ma. The dykes have compositions very similar to the distinctive Bushe magma-type from the Deccan, and hence constitute the first traces of Deccan activity recognized on the Seychelles. Fitting the Seychelles and India back together to reconstruct the original extent of the Deccan, we find that the present estimate of 1 million km 2 needs to be revised upwards by c . 25% to account for the Deccan magmatism in the Seychelles. If dipping reflectors found on seismic lines to the northwest of the Seychelles are also Deccan age flows, this would increase still further the area and volume estimates.

Lower Paleozoic tholeiitic dykes from central New Brunswick: possible evidence for the early opening of an ensialic Taconian back-arc basin

Cambrian and Early Ordovician sedimentary rocks of the Lower Tetagouche Group from the southern Miramichi Terrane in New Brunswick were intruded by a series of mafic dykes. The age of dykes is unknown, but their stratigraphic context and state of deformation indicate that they were intruded during the Taconian orogeny. The geochemistry of the dykes shows that they are a cogenetic magmatic suite of basaltic composition (48–55 wt.% SiO2) and tholeiitic affinity. The dykes are low in silica and high in TiO2 (1.5–3.5 wt.%) and FeO* (8–16 wt.%). Magmatic evolution of the suite resulted from processes involving both crystal fractionation and crustal contamination, as indicated by depletion of compatible elements, important enrichment in high-field-strength elements, and the presence of inherited zircons of Precambrian age. The most primitive samples resemble mid-ocean-ridge basalts, except for higher alkali contents, or continental tholeiites depleted of light rare-earth elements. In comparison, the most evolved dykes have a transitional character similar to within-plate basalts.The general evolution of the Lower Tetagouche Group dykes is similar to that of nascent basins formed within volcanic-arc complexes. Dyke intrusion resulted from the early development of an ensialic back-arc basin during the Taconian orogeny. The paroxysmal stage of this tectonic event is represented by the ocean-floor fragment of the Elmtree Terrane, north of the Miramichi Terrane.

New occurrences of undersaturated syenites from the southern Blosseville Coast, East Greenland

This note reports hitherto undescribed nepheline-sodalite syenites from I. C. Jacobsen Fjord and Ryberg Fjord. Numerous syenite erratics have been encountered at I. C. Jacobsen Fjord. They are believed to come from a single intrusion in the immediate neighbourhood which has not yet been located. They consist Iargely of perthite with nepheline, sodalite, salitic pyroxene, hastingsitic amphibole, sphene and Fe-Ti oxide. They resemble the pulaskites of the Kangerdlugssuaq intrusion. On the side of the Sorgenfri Gletscher at the head of Ryberg Fjord an extensive area of peralkaline syenite has been found. It consists of perthite, nepheline, sodalite, aegirine, arfvedsonite and very Fe-rich biotite. It is cut by tholeiitic dykes of a late generation of the coastal dyke swarm. These syenites bridge a gap between previously known examples and suggest that the magma type was much commoner than thought originally.

Lower Cretaceous tholeiitic dyke swarms from the Ponta Grossa Arch (southeast Brazil): Petrology, Sr-Nd isotopes and genetic relationships with the Paraná flood volcanics

The Lower Cretaceous dykes of the Ponta Grossa Arch, the most important dyke swarms in Brazil, are associated with the flood basalts and rare acid flows of the northern Paraná basin. The Ponta Grossa (PG) dykes are formed by two-pyroxene tholeiites and rare acid rocks. The basaltic dykes may be distinguished into two main groups: a dominant, high-TiO 2 (> 2 wt.%; HTi) group and a subordinate, low-TiO 2 (< 2 wt.%; LTi) group, characterized, for similar MgO content, by high and low incompatible-element contents, respectively. Most PG dykes do not show chemical and isotope evidence supporting important crustal contamination. PG dykes with ( 87 Sr 86 Sr ) 0 < 0.7060 plot in the mantle array ( ϵ Sr ≈ + 17 and ϵ Nd ≈ −3) and can be considered virtually uncontaminated. Intra-dyke chemical variations suggest that the normal or reverse differentiation trends may be related to pre-emplacement low-pressure differentiation processes. The important chemical differences between incompatible-element-poor and incompatible-element-rich basaltic dykes cannot be accounted for in terms of fractional crystallization, but are compatible with different melting degrees of a garnet peridotite mantle source, e.g. 9% and 20% melting for the HTi and LTi tholeiites, respectively. Field distribution of the magmatism, chemistry and Sr-Nd isotope compositions support that the PG dykes may be related to the flood volcanics of northern Paraná, and not to those of the central and southern Paraná. Paleomagnetic data indicate that PG dykes are younger than the Paraná volcanics. All the results suggest that PG dykes were probably feeders of the stratoid volcanics erupted in northern Paraná towards the continental margin and later eroded. Chemistry and isotopic data reveal that asthenospheric mantle components were not significant factors in the genesis of PG dykes. Dyke emplacement occurred during early phases of rifting and/or flexuring cutting flood volcanic suites.