Age Of Dykes Research Articles

Defining the Cimmerian orogeny in the West Pontides: geochronological and geochemical data from Permo-Triassic dykes in the İstanbul-Zonguldak Tectonic Unit

ABSTRACT Permo-Triassic intrusive rocks and back-arc basins are exposed in the İstanbul-Zonguldak Tectonic Unit in the West Pontides. Their tectono-magmatic history, thus to the evolution of the back-arc basins, is poorly understood. Here, I present on olivine-bearing basalt and andesite dykes and granite dykes and pyroxene-bearing andesite dykes of Permo-Triassic crystallization and their Pb-loss ages from the basement rocks and sedimentary rocks in the İstanbul-Zonguldak Tectonic Unit (NW Turkey). They have holocrystalline and porphyritic to vitrophyric textures. U-Pb dating of igneous zircon grains from olivine-bearing basalt and andesite dykes and granite dykes yielded Middle Permian ages of ca. 270.1 ± 1.1 to 261.4 ± 1.7 Ma (2σ), and pyroxene-bearing andesite dykes obtained Early Triassic ages of ca. 252.1 ± 2.4 to 250.1 ± 2.0 Ma (2σ). Their young ages of the dykes yielded Late Triassic ages of ca. 228 to 220 Ma (2σ). Geochemically, the Middle Permian dykes have a calc-alkaline affinity and are notable for subduction components, as indicated by the presence of distinct negative Nb anomalies. Early Triassic dykes show alkaline signatures and show within-plate character, corresponding to prominent positive Nb anomalies. Combined with data from the literature, the Middle Permian dykes are magmatic products associated with the Tethys Ocean subducting under the Pontides and the Early Triassic dykes are magmatic pulses related to the rifting of the Permo-Triassic back-arc basins. The Late Triassic young ages of the Permo-Triassic dykes in the İstanbul-Zonguldak Tectonic Unit represent a deformation event corresponding to the Cimmerian Orogeny, and this implies that the Permo-Triassic back-arc basins closed during the late Triassic.

Age and provenance of the Chaung Magyi Group, Yeywa Dome, Myanmar, based on U-Pb dating of detrital zircons

The unfossiliferous Chaung Magyi Group of the Northern Shan Plateau of Myanmar, comprises metasedimentary rocks, in particular greywacke, slates, phyllites and schists, of inferred Proterozoic age that unconformably underlie the late Cambrian Pangyun Formation. To address the problem of the youngest stratigraphic extent of the group, four samples of the Chaung Magyi Group from the Yeywa Dome, East of Kyaukse, Mandalay Division, were collected for U-Pb dating of detrital zircons. Based on the youngest detrital zircons present in sandstone samples from the western Yeywa Dome area, at least the upper part of the Chaung Magyi Group is of latest Proterozoic age and possibly extends to the late Cambrian (~500 Ma). Comparison of detrital zircon age-frequency histogram distributions show only slight differences in distributions of detrital zircons in the Cambrian of NW Thailand and the Chaung Magyi Group samples (Sibumasu Terrane), with a concentration of ages between 1300 Ma and 820 Ma, with two major peaks at 1086 Ma and 1022 Ma. A younger Cryogenian age peak at 647 Ma is also observed. A spread of older concordant ages ranges from 3093 Ma to 1406 Ma. The minimum depositional age for the Chaung Magyi Group of the west of the Bawdwin Mine (982 Ma) based on ages of cross-cutting dykes (reported in Mitchell, 2018), and the maximum depositional age for Yeywa Dome (~600–500 Ma) based on detrital zircons suggest the age difference (~>400 my) is too great for the Chaung Magyi Group to be considered a single group, the dyke ages require re-investigation.

Temporal correlation between dyke swarms and crustal extension in the middle Palaeozoic Vilyui rift basin, Siberian platform

This paper presents results from new 40Ar/39Ar isotope dating of nine mafic dykes from three large dyke swarms (Vilyui–Markha, Kontai–Dzherba, and Chara–Sinsk) of the Yakutsk–Vilyui large igneous province (LIP), in addition to a reconstruction of the subsidence history of the middle Palaeozoic Vilyui paleorift basin (eastern Siberian platform). All previously published 40Ar/39Ar and U–Pb dates are summarized. Statistical analysis of the dyke ages reveals repeated magmatic events in the study area. Two major pulses of mafic magmatism are identified: one at the Frasnian–Famennian boundary, with a main peak at ca. 374.1Ma, and another in the latest Devonian with a peak at ca. 363.4Ma. The time of maximum intensity of dyke intrusion coincides (within uncertainty) with rapid subsidence in the Vilyui basin. The minimum total volume of middle Palaeozoic magmatism produced in the Yakutsk–Vilyui LIP is 100–215Kkm3, which is much less than earlier estimates. Most of the mafic material within the Yakutsk–Vilyui LIP is related to the Vilyui basin and associated dyke swarms. Backstripping analysis of sedimentation in depressions of the Vilyui basin was carried out. Estimates were obtained for the spatial distribution of the stretching factor of the crust and mantle lithosphere, averaging 1.17 and 1.44, respectively. The amount of extension due to dyke intrusion is estimated to be ~6%. Backstripping analysis of sedimentation in the Vilyui basin was used to assess the effect of both intraplate far-field forces and upwelling magma flows initiated by a mantle plume. A numerical thermomechanical model was developed to investigate the relations between two possible mechanisms by which the Vilyui rift was initiated: intraplate extension (passive rifting) and the ascent of a mantle magmatic diapir (active rifting). A model considering both of these mechanisms shows the contribution of the far-field extension forces and the effect of convective flows around the mantle plume, assuming a temperature of 1500°C for a heat source operating for ≥10Myr. The best-fit model predicts the formation of the Vilyui basin under extensional conditions with a depth-integrated horizontal driving force of 2.0×1013N/m, which corresponds to a lithostatic pressure of 0.73.

Evidence for a Neoarchean LIP in the Singhbhum craton, eastern India: Implications to Vaalbara supercontinent

We report eight new Pb-Pb baddeleyite ages and paleomagnetic results on a series of hitherto unknown Neoarchean NNE-SSW trending mafic dyke swarms intruding the Paleoarchean basement rocks in the Singhbhum craton, eastern India. Being the most dominant dyke swarms in the region, they occur over the entire Singhbhum craton with an areal distribution extending to about 30,000km2. Of the eight dyke samples analysed across the entire dyke swarm, six were emplaced at 2762.4±2.0Ma (weighted mean of 6 dyke ages), two with similar strike directions, yielded older and younger ages of 2800.2±0.7Ma and 2752.0±0.9Ma respectively. The older 2800.2±0.7Ma dyke event appears to be coeval with several other magmatic events reported earlier from the Singhbhum craton. Emplacement of dacite tuffs within the Malaigiri basin, on the southern margin of the craton, the Budhapal and Rengali granite from the south-western and southern parts of the craton, along with the Temperkola granite activity and associated acid volcanics in the western part of the craton, and perhaps the Mayurbhanj granite (fine-grained phase) are also contemporaneous, suggesting a fairly wide spread thermal event in the region at this time.Well defined craton-wide magmatic events contemporaneous to the 2762.4±2.0Ma Singhbhum dyke activity have been reported from the Pilbara. The younger 2752.0±0.9Ma age dyke event occurring in the Singhbhum is also know from the eastern Pilbara. An older event at 2783±1.2Ma (Gaborone-Kanye-Plantation Porphyry-Derdepoort-Modipe episode) was identified in the Kaapvaal craton. Comparison of paleomagnetic data obtained on the Singhbhum Neoarchean dyke swarms with the Pilbara and Kaapvaal data, show all these continents were located at steep latitudes and could have been proximal to each other during this time. If Pilbara and Kaapvaal formed supercraton Vaalbara during the Neoarchean time, we propose Singhbhum with similar Archean geology could also have had its ancestry in the same supercraton.

Aeromagnetic interpretation in the south-central Zimbabwe Craton: (reappraisal of) crustal structure and tectonic implications

Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological–tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8–2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.

Baddeleyite U–Pb ages and gechemistry of the 1875–1835 Ma Black Hills Dyke Swarm across north-eastern South Africa: part of a trans-Kalahari Craton back-arc setting?

Eleven new baddeleyite U–Pb crystallisation ages and associated whole-rock geochemistry on NE–NNE-trending tholeiitic dykes cutting across the north-eastern corner of the Archaean Kaapvaal Craton, the overlying Transvaal basin and the Bushveld and Phalaborwa igneous complexes collectively define a 1875–1835 Ma Black Hills Dyke Swarm (BHDS). Dyke ages do not discriminate between dyke trends or geographic location, but subdivide the BHDS into an older set of four more primitive dykes (MgO = 9.4–6.8 wt.%) and a younger set of seven dykes with more differentiated compositions (MgO = 5.6–4.2 wt.%). Despite being emplaced over a c. 40 Myr period, major element compositions are remarkably consistent with a single inversely modelled bulk fractionating assemblage of 57.5% plagioclase, 29.5% augite and 13.0% olivine. This fractionating assemblage requires an additional assimilation of bulk continental crust (at a low r-value of 0.2) for reversed modelling of parental rare earth elements. Even though this crustal assimilation indicates that primary magmas could potentially have been derived from a spinel-bearing ambient primordial and asthenospheric mantle source, anomalously low Nb and high Pb values for the more primitive older dykes may also have been inherited from a sub-continental lithospheric mantle source. The ages for the BHDS bridge a gap between c. 1889 and 1867 Ma mafic sills and c. 1830 Ma rhyodacitic pyroclasts, interbedded in the top of a ~3 km-thick Sibasa basalt sequence, which combine into a continuous c. 1.89–1.83 Ga igneous province. Similar geochemical signatures are consistent with all sills, volcanic rocks and BHDS feeders collectively belonging to a very voluminous and coherent igneous province, which arguably formed behind active Magondi and Okwa-Kheis arcs, along the western margin of the proto-Kalahari Craton.

Paleomagnetism of ca. 2.3 Ga mafic dyke swarms in the northeastern Southern Granulite Terrain, India: Constraints on the position and extent of Dharwar craton in the Paleoproterozoic

The pole positions reported for the mafic dykes occurring in the northeastern Southern Granulite Terrain and easterly trending dykes of Dharwar craton are alike while their ages differ by ca. 700Ma confounding tectonic reconstruction. To resolve the discordance and to constrain the position of the Indian continent in the Proterozoic, paleomagnetic and geochronological studies have been carried out on the mafic dyke swam in the northeastern Southern Granulite Terrain. Two distinct pole positions are obtained, one from the group of Tiruvannamalai dykes as 27.7°S, 231.5°E (dp, dm=12°, 14°) with corresponding mean declination (Dm) and inclination (Im) as 125° and −73.8° (κ, α95=22, 7.6°) respectively, and second from the East Coast Dykes as 2.32°S, 188.2°E (dp, dm=5°, 8°) with corresponding mean Dm and Im as 88.9° and −33.8° (κ, α95=48, 7.1°) respectively. The Remanent Efficiency of Magnetization values between 0.01 and 0.08 suggest that the dykes have been cooled through Curie temperature of magnetite which is the carrier of magnetization. The baked contact test confirms that the Characteristic Remanent Magnetization is the primary magnetization. The Sm–Nd mineral-whole-rock isochron, from one of the Tiruvannamalai dykes, yielded an age of 2318±60Ma (MSWD=1.9). This age is the time of intrusion of the Tiruvannamalai dykes and is similar, within errors, to the age of dykes in the Dharwar craton. Thus, the contiguity of Dharwar craton with the Northern Block of Southern Granulite Terrain, as early as 2.3Ga ago, is established. The pole position thus obtained corresponds to a high-latitudinal position of Dharwar craton during early Paleoproterozoic.

A POSSIBLE MESOARCHAEAN IMPACT STRUCTURE AT SETLAGOLE, NORTH WEST PROVINCE, SOUTH AFRICA: AEROMAGNETIC AND FIELD EVIDENCE

A 25 to 30 km wide magnetic anomaly within the >2.79 Ga granite-greenstone rocks of the northwestern Kaapvaal Craton is spatially associated with megabreccia outcrops near the village of Setlagole in the North West Province, South Africa. The breccia comprises angular to rounded clasts of TTG gneisses, granites and granodiorites, with lesser amounts of amphibolite, calc-silicate rock and banded iron-formation as well as unusual dark grey to black, irregular, centimetre- to decimetre-sized clasts that show evidence of fluidal behaviour and plastic deformation during incorporation into the breccia. The largest clasts reach up to several metres in size. Evidence of fluvial transport is found in rare thin sandy to gritty layers that show crude bedding and upward-fining with layers dipping gently to the northeast. The breccia matrix is highly variable but is dominated by angular mineral clasts (mainly quartz and feldspar, with subsidiary biotite, amphibole and epidote) with interstitial chlorite. The clasts show variable amounts of alteration (saussuritization, sericitization, chloritization of biotite and amphibole). The dark clasts contain angular quartz and feldspar and small biotite fragments in a cryptocrystalline chlorite-dominant matrix. Textures indicate a lower greenschist facies metamorphic overprint. The absence of lava, dolomite or quartzite clasts suggests that the breccia formed prior to the deposition of the Neoarchaean Ventersdorp and Eoproterozoic Transvaal Supergroups, whereas the metamorphic grade indicates that it postdates the ca. 2.79 Ga amphibolite-facies metamorphic peak in the region. This suggests a late Mesoarchaean or early Neoarchaean (ca. 2.79 to 2.71 Ga) age for the breccia. A similar age is inferred for the magnetic anomaly based on postulated cross-cutting dyke ages. Despite a comprehensive search, unequivocal shock-diagnostic microdeformation features have not yet been found in either the breccia or the highly-weathered granitic gneiss outcrops in the central parts of the anomaly. The unusual plastically-deformed dark clasts may represent chloritized mud clasts or impact melt clasts. Geochemical data on these clasts and other components of the megabreccia provide no conclusive support for a meteoritic origin, but the unparalleled composition of the clasts and their high trace element abundances of Ni, Cr, V, Zn and Co relative to rocks of the Kraaipan granite-greenstone basement, suggests an unusual origin for this matrix material. Given the distinctive nature of the breccia and its proximity to a large circular magnetic anomaly, it is postulated that the megabreccia could represent a mass or debris flow in a marine setting triggered by an impact tsunami or resurge. Subsequent faulting may have led to the preferential preservation of these deposits. This interpretation of the Setlagole megabreccia and geophysical anomaly is evaluated in terms of other possible modes of origin and it is concluded that a meteoritic source best fits the available data.

Precise U–Pb baddeleyite ages of mafic dykes and intrusions in southern West Greenland and implications for a possible reconstruction with the Superior craton

The Archaean block of southern Greenland constitutes the core of the North Atlantic craton (NAC) and is host to a large number of Precambrian mafic intrusions and dyke swarms, many of which are regionally extensive but poorly dated. For southern West Greenland, we present a U–Pb zircon age of 2990 ± 13 Ma for the Amikoq mafic–ultramafic layered intrusion (Fiskefjord area) and four baddeleyite U–Pb ages of Precambrian dolerite dykes. Specifically, a dyke located SE of Ameralik Fjord is dated at 2499 ± 2 Ma, similar to a previously reported 40Ar/ 39Ar age of a dyke in the Kangâmiut area. For these and related intrusions of ca. 2.5 Ga age in southern West Greenland, we propose the name Kilaarsarfik dykes. Three WNW-trending dykes of the MD3 swarm yield ages of 2050 ± 2 Ma, 2041 ± 3 Ma and 2029 ± 3 Ma. A similar U–Pb baddeleyite age of 2045 ± 2 Ma is also presented for a SE-trending dolerite (Iglusuataliksuak dyke) in the Nain Province, the rifted western block of the NAC in Labrador. We speculate that the MD3 dykes and age-equivalent NNE-trending Kangâmiut dykes of southern West Greenland, together with the Iglusuataliksuak dyke (after closure of the Labrador Sea) represent components of a single, areally extensive, radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the North Atlantic craton. Comparison of the magmatic ‘barcodes’ from the Nain and Greenland portions of the North Atlantic craton with the established record from the north-eastern Superior craton shows matches at 2500 Ma, 2214 Ma, 2050–2030 Ma and 1960–1950 Ma. We use these new age constraints, together with orientations of the dyke swarms, to offer a preliminary reconstruction of the North Atlantic craton near the north-eastern margin of the Superior craton during the latest Archaean and early Palaeoproterozoic, possibly with the Core Zone craton of eastern Canada intervening.

The British Tertiary Igneous Province: Palaeomagnetism and Ages of Dykes, Lundy Island, Bristol Channel

Summary Of 66 dykes collected 52 gave stable directions: 44 reversed, 4 normal and 4 intermediate. The mean direction of magnetization was N3 - 6 E, + 62 7 (down,) equivalent to a pole position of 82.6 ON, 155.0 E. This is significantly different from the present pole and displays right-handed farsidedness. The bulk, at least, of the dykes fall within the apparent span 45-54 My. Yet they display approximately the same small proportion of normal dykes as do the older dyke swarms of the province which were formed during a different polarity period or periods.

1214 ± 5 Ma dyke from the Darling Range, southwestern Yilgarn Craton, Western Australia

High thorium euhedral, twinned and elongate zircons from the felsic part of a mafic dyke located in the Archaean Yilgarn Craton approximately 30 km northeast of Perth and approximately 2 km east of the Darling Fault, have consistent 207 Pb/ 206 Pb ages of 1214 ± 5 Ma. This age is interpreted as the age of dyke emplacement and is identical, within the uncertainties, with other U–Pb dyke ages reported for the southwest Yilgarn Craton. The present result extends the known occurrence of ca 1210 Ma dykes to the western margin of the Yilgarn Craton and confirms earlier conclusions that a major mafic dyke emplacement occurred throughout the southern Yilgarn Craton during a short‐lived magmatic pulse at ca 1210 Ma.

High-rate flexure of the East Greenland volcanic margin: constraints from 40Ar/ 39Ar dating of basaltic dykes

In the North Atlantic domain, the SE Greenland volcanic margin developed in response to continental break-up at 57–54 Ma. Progressive tilting and intrusion of dykes reflect a major tectonic seaward-dipping flexing of the continental crust. We report eight new ages of tilted (pre-flexure) and vertical (post-flexure) dykes, determined by 40Ar/ 39Ar incremental heating experiments. Despite strong excess argon on plagioclase, measured 40Ar/ 39Ar plateau and mini-plateau ages of dykes from detailed step-heating experiments on mineral separates appear reliable and a systematic difference of ages is found between tilted and vertical dykes. In both studied areas – Kap Wandel (66°20′N) and Kap Gustav Holm (66°40′N) – tilted dykes show ages of ∼54–55 Ma whereas vertical dykes are younger with ages of ∼51 Ma. Therefore, it appears that the crustal flexing of the volcanic passive margin is a short-duration event (<2.9 Ma). These new dates and estimates of finite extensional strain in the studied areas suggest a lower boundary for extensional strain rates as high as 7±2×10 −15 s −1. This value is, however, a clear underestimation as it is difficult to estimate the full finite extension in the studied area. It suggests that the continental lithosphere at volcanic passive margins is softened by high thermal gradients.

Age constraints and characteristics of subaqueous volcanic construction, the Archean Hunter Mine Group, Abitibi greenstone belt

The Archean Hunter Mine Group (HMG) is a 6–7 km-thick south-facing volcanic sequence at the southern limit of the Northern Volcanic Zone, Abitibi greenstone belt. The HMG is composed of (1) felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits; (2) pyroclastic fire-fountain products and high- to low-concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron-formation; (3) mafic pillowed and brecciated flows; (4) a prominent felsic dyke swarm; and (5) abundant mafic intrusions. The stratigraphy is complex, as inferred by detailed volcano-sedimentary facies mapping. A 5–7 km-wide, N-trending felsic dyke swarm, coupled with abundant synvolcanic and synsedimentary faults, chaotic breccias and mineralization suggest a central volcanic depression, which is consistent with a caldera structure. Voluminous effusive activity and explosive fire-fountaining eruptions contributed to caldera development. Rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills. The lithofacies and overlying komatiites flows are consistent with a deep-water marine setting. U-Pb age determinations help constrain the HMG caldera formation to ≈6 million years. The sampled quartz–feldspar–phyric dykes yielded U-Pb zircon ages of 2728.3+4/−3.4 and 2728.9±0.8 Ma and together with a previously published dyke age of 2729.6±1.4 Ma from the same dyke swarm, a case is made that the swarm evolved over 1 million years (2728.6–2729.6 Ma). A lobate flow and flow breccia revealed similar ages of 2724.6+4.6/−1.7 and 2727.6+4.2/−2.0 Ma, whereas the up to 1 km-thick, gabbro-diorite Roquemaure sill precedes both felsic dykes and flows, as indicated by the 2731.8+2.2/−2.0 Ma age. The 2706.4+5.7/−4.3 Ma feldspar–quartz–phyric dykes occurred late in the history of the HMG and were not associated with subaqueous volcanic construction. The 207Pb/ 206Pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the HMG complex. Collectively, these age determinations and inheritance, as well as stratigraphy, support a tripartite division into a 2734–2730 Ma lower formational stage, a 2732 Ma middle formational stage and a 2730–2728 Ma upper formational stage. The study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice.

Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on late Archean events in the Zimbabwe craton and Limpopo belt

U–Pb dating of zircon and rutile from bronzitites of the P1 pyroxenite layer of the Great Dyke precisely constrains the crystallization age of this part of the intrusion to 2575.4±0.7 Ma. Whole rock Rb–Sr and Sm–Nd data of various rock types sampled along the entire length of the Great Dyke record inhomogeneous initial isotope ratios, and also later (<1.5 Ga) disturbances of the Rb–Sr and Sm–Nd isotope systems. The 2575.4±0.7 Ma emplacement age of the Great Dyke is ≈120 Ma older than assumed until recently, and calls for new interpretations of the crustal development of the Zimbabwe craton. Close temporal links between the intrusion of the Great Dyke and the emplacement of late Archean granitoids (Chilimanzi and Razi suites) of the Zimbabwe craton are indicated by the new precise age data. The Chilimanzi and Razi suites of granitoids form at least two sub-suites, which can be described as pre- and post-Great Dyke in age. The Great Dyke age now falls within the range of ages for tectonic events in the Limpopo belt including granitoid magmatism, metamorphism, and thrusting of the Northern Marginal Zone over the Zimbabwe craton. A west-to-east diachroneity in both thrusting and crustal stabilization is suggested by the observations that the Great Dyke cuts across the thrust in the west, but syn-tectonic granitoids that are younger than the Great Dyke are deformed by the thrusting in the east. Intrusion of the Great Dyke cannot be linked to collision of the Zimbabwe and Kaapvaal cratons. The overlap in ages of intrusion of the Great Dyke and late Archean events in the Zimbabwe craton shows that Archean crust was cratonized shortly after large-scale melting and granite intrusion.

The Silurian(?) Passamaquoddy Bay mafic dyke swarm, New Brunswick: petrogenesis and tectonic implications

The volcanic and sedimentary rocks of the Passamaquoddy Bay (PB) area of southeastern New Brunswick are part of the Silurian–Devonian Coastal Volcanic Belt (CVB), an extensive belt of bimodal volcanic rocks. The PB sequence is 4 km thick, has four cycles of mafic and felsic volcanism, and is intruded by mafic dykes at all levels. There are two ages of dykes, those related to the Late Silurian PB volcanism (PB dykes) and Mesozoic dykes (the Minister Island Dyke) related to the opening of the North Atlantic. The PB mafic dykes are subalkalic basalt to basaltic andesite, within-plate tholeiites. The dykes are moderately to highly evolved (Mg# = 66.6 to 26.6), with trends of major and trace elements typical of the fractionation of olivine, pyroxene, plagioclase, and ilmenite. The PB mafic dyke swarm comprises over 155 dykes which represent a greater range of compositions than the associated flows, suggesting that they give a more complete representation of the Late Silurian PB mafic magmas. They exhibit incompatible element characteristics best accounted for by crustal contamination. The dykes plot on a linear array away from mantle mixing lines between depleted and enriched mantle sources and toward the composition of the PB felsic units, suggesting that these felsic units are representative of partial melts and fractionates of the source contaminate. The variable TiO2 contents (1.2–4.3 wt.%) and incompatible element ratio trends plotted against a fractionation index suggest that mantle metasomatism, either fluid or melt derived, may also have influenced the mantle source of the dykes. The dykes dip steeply and have a relatively consistent strike to the north. Most dykes range in thickness from 0.5 to 2 m, but range up to 9 m. The single orientation of the dykes, along with their chemical characteristics and volume, and association with a bimodal intraplate volcanic sequence, are consistent with an extensional tectonic setting. Constraints of the regional geology suggest that this extension was associated with convergence, perhaps in a back-arc setting.

Early Cretaceous Basalt and Picrite Dykes of the Southern Etendeka Region, NW Namibia: Windows into the Role of the Tristan Mantle Plume in Paraná–Etendeka Magmatism

Abundant dykes in the southern Etendeka region, NW Namibia, mostly contain 8–20% MgO. Almost all can be allocated to previously described Early Cretaceous magma types. Horingbaai-type basalts–picrites occur up to 120 km inland. Some have superficially mid-ocean ridge basalt (MORB)-like compositions: (La/Nb)n ∼1·0; (Sm/Lu)n ∼1·5; εNd >8; initial 87Sr/86Sr ∼0·7032. Others show major- and trace-element, and Sr–Nd–Pb isotopic evidence of contamination during upwelling by up to a few per cent of K-feldspar-rich upper crust. Extremely magnesian olivine macrocrysts (Fo91–93·3) in some Horingbaai picrites indicate that komatiitic (MgO ∼24%) liquids were associated with this suite, although they were too dense to reach the crustal levels currently exposed. Ferropicrite dykes resemble closely the nearby Etendeka ferropicrite basal lavas, with total iron (as Fe2O3) >MgO; (La/Nb)n ∼1·0–1·5; (Sm/Lu)n ∼2·5–7; εNd ∼2–3, except in samples with geochemical evidence of contamination by Archaean–Proterozoic lower crust. A few low-Ti dykes resemble geochemically the lavas of the main southern Etendeka succession, with (La/Nb)n ∼2·0–2·5; εNd = 0 to −8; initial 87Sr/86Sr >0·707. Dykes north of the Huab river define a fourth magma type, Nil Desperandum (ND), that may have fed part of the Huab sill complex. The dyke ages are constrained by their field relationships. Ferropicrite and low-Ti dykes are consanguineous with lavas erupted at 133 and 132 Ma, respectively. Both Horingbaai and ND dykes cut 132 Ma Etendeka lavas, and the main swarm of Horingbaai picrites with forsteritic macrocrysts is cut by the 131 Ma Brandberg plutonic complex. Forward and inverse modelling of the genesis of the ferropicrites and the Horingbaai picrite–komatiites gives two temporal ‘windows’ into physicochemical conditions within the head of the impacting Tristan mantle plume. At ∼133 Ma the southern Etendeka lithosphere was >100 km thick and only incipient melting of predominantly Fe-rich peridotite streaks occurred in the rising plume head (Tp = 1500°C). By ∼2 my later, pre-Atlantic extension had reduced the lithosphere thickness beneath and adjacent to NW Namibia, and there was intense melting (Tp = ∼1500–1700°C) of even depleted peridotite in the plume head.

1.2 Ga Mafic dyke near York, southwestern Yilgarn Craton, Western Australia

A SHRIMP 207Pb/206Pb zircon age of 1204 ± 10 Ma is reported for an east west trending dolerite dyke from near York in the southwestern Yilgarn Craton. This age is identical within analytical uncertainty to previously reported ages of ca 1210 Ma for dykes from the central Wheatbelt and the Western and Eastern Goldfields. The consistency of the dyke ages and the wide areal extent of the dykes suggests that emplacement occurred as a single magmatic pulse at ca 1210 Ma throughout the southwestern Yilgarn Craton. The similarities between the age of the dykes and the ages of late events in the Albany Fraser Orogeny, and the approximate parallelism of the east west trending dykes to the margin of the orogen, raises the possibility that these events are related.

Neoproterozoic geochronology and palaeogeography of the Seychelles microcontinent: the India link

The geology of the Seychelles Islands in the Indian Ocean is dominated by granitoid rocks and to a lesser extent by basaltic dykes. A U–Pb zircon age from the Takamaka dolerite dyke (Mahé Island) gives an intrusion age of 750.2±2.5 Ma. The dyke age is considerably older than previous age estimates and suggests that some of the Mahé dolerite dykes are almost coeval with the granitoid rocks. The Mahé dykes show variable degrees of magnetic overprinting, but the proposed oldest magnetization, component A (Decl.=001.4°, Incl.=+49.7° and α 95=11.2; palaeomagnetic pole: Lat.=54.8°N and Long.=057.6°E), is identified as a high unblocking component in most dykes, and compares favorably with palaeomagnetic data from the Mahé granitoids. A new Seychelles–India fit (Euler pole: Lat.=25.8°, Long.=330° and rotation angle=28°) produces a good match of palaeomagnetic poles from ca. 750 Ma magmatic rocks in the Seychelles and NW India (Malani), and places these regions only 600 km apart. Together with Madagascar, this tectonic trio formed an outboard continental terrane of the Rodinia supercontinent during the Neoproterozoic (ca. 750 Ma). The position of the Seychelles at this time marks the incipient formation of a microcontinent because there is no evidence for older continental crust than the 750–755 Ma granitoid rocks. The Seychelles formed at 30°N and most likely as part of an Andean-type arc along the western margin of the former Rodinia supercontinent.

Discussion on Palaeogene dyke swarm, NW Wales: evidence for Cenozoic sinistral fault movement

R. H. Maddock , W. P. Aspinall , E. A. Hailwood , Ting Fung &amp; E. H. Rutter write: In a paper concerning the ages of fault movements in NW Wales, Bevins et al. 1996 cite palaeomagnetic dating by Hailwood et al. 1992 in support of their argument for large sinistral fault displacements of post-Tertiary dyke age. Bevins et al. misinterpret the observations and results published by Hailwood et al. We wish to correct this misinterpretation and to make further comments. Bevins et al. state To date the only evidence of Cenozoic faulting in North Wales is provided by Hailwood et al. 1992 who, from an investigation of palaeomagnetism and fabric analysis of a clay-rich fault gouge in northern Anglesey demonstrated 180 m of sinistral strike-slip or 215 m dip-slip movement during the last 20 million year period (with the minimum age for last movement as 0.73 Ma)'. In fact, the observations and results reported by Hailwood et al. support the contrary view that the greater part of the movement on the fault in question, the E-W-trending Porth-y-pistyll fault, took place well before the intrusion of a Tertiary dyke which cuts and intrudes the fault. Hailwood et al. noted the following: (1) The offset of the boundary between the (early Palaeozoic) Gwna and New Harbour Groups across the fault was consistent with end-member (total) displacements of 215 m normal dip-slip or 180 m sinistral strike-slip movement. (2) A sub-vertical Palaeozoic dyke showed a 10 m dextral strike-slip offset across the fault. Over the

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.