Proterozoic Mafic Dykes Research Articles

First evidence of Archean mafic dykes at 2.62 Ga in the Yilgarn Craton, Western Australia: Links to cratonisation and the Zimbabwe Craton

The Archean Yilgarn Craton in Western Australia hosts at least five generations of Proterozoic mafic dykes, the oldest previously identified dykes belonging to the ca. 2408–2401 Ma Widgiemooltha Supersuite. We report here the first known Archean mafic dyke dated at 2615 ± 6 Ma by the ID-TIMS U-Pb method on baddeleyite and at 2610 ± 25 Ma using in situ SHRIMP U-Pb dating of baddeleyite. Aeromagnetic data suggest that the dyke is part of a series of NE-trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton, here named the Yandinilling dyke swarm. Mafic magmatism at 2615 Ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation, and was coeval with the formation of late-stage gold deposit at Boddington. Paleogeographic reconstructions suggest that the Yilgarn and Zimbabwe cratons may have been neighbours from ca. 2690 Ma to 2401 Ma and if the Zimbabwe and Kaapvaal cratons amalgamated at 2660–2610 Ma, the 2615 Ma mafic magmatism in the southwestern Yilgarn Craton may be associated with the same tectonic event that produced the ca. 2607–2604 Ma Stockford dykes in the Central Zone of the Limpopo Belt. Paleomagnetic evidence and a similar tectonothermal evolution, including coeval low-pressure high-temperature metamorphism, voluminous magmatism, and emplacement of mafic dykes, support a configuration where the northern part of the Zimbabwe Craton was adjacent to the western margin of the Yilgarn Craton during the Neoarchean. Worldwide, reliably dated mafic dykes of this age have so far been reported from the Yilgarn Craton, the Limpopo Belt and the São Francisco Craton.

Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, USA

AbstractFluids can play an important role in the localization of deformation in the deep crust, yet the specific mechanisms active during the complex interactions between metasomatism, metamorphism and deformation remain elusive. Precambrian metagabbronorite dykes in southwest Montana contain fractures filled with Hbl±Grt and discrete cm‐scale shear zones with well‐preserved strain gradients. This system offers an ideal opportunity to constrain the chemical and mechanical processes that facilitated strain localization. An early M1 assemblage of Grt1+Cpx1+Pl1+Qz developed at conditions of 0.51–0.85 GPa and 500–700°C and is preserved largely as a static replacement of relict igneous phases (Opx, Pgt, Pl) in coronitic textures. An M2 assemblage characterized by Grt2+Pl2±Cpx2+Hbl+Scp+Qz developed at 0.86–1.00 GPa and 660–730°C coincided with fluid flow and deformation associated with shear zone development. Microstructural observations in marginal protomylonite/mylonite and laminated ultramylonite suggest a shear zone evolution that involved (1) nucleation from pre‐existing fractures that were sites for major fluid infiltration, (2) initial widening coincident with grain‐size reduction by microfracturing, dislocation creep, and synkinematic metamorphic reaction by solution transfer, and (3) a switch in the dominant deformation mechanisms active in the ultramylonite from grain‐size insensitive mechanisms to grain‐size sensitive granular flow accommodated by fluid‐assisted diffusion. Throughout this evolution, the effective bulk compositions of the rock volumes responding to metamorphism changed through a combination of mechanical and metasomatic processes.

Geodynamic Significance of the Updated Statherian-Calymmian (at C. 1.65 and 1.46 Ga) Palaeomagnetic Results from Mafic Dykes of the Indian Shield

A reassessment of the recent palaeomagnetic data on Proterozoic mafic dykes in the Bundelkhand and Bastar cratons permits a robust estimate of 1.466 Ga (Calymmian) pole (λ = 49.4°N; Φ; = 132.9°E; A 95 = 6.6°; N = 11) for the Indian shield. The pole corresponds to a mean direction of D = 40.5°; I = 56.4° (α 95 = 5.5°; K = 70). The Indian pole at c. 1.65 Ga (Statherian) is suggested to have been situated at λ = 59.6°N and Φ = 47.9°E (A 95 = 8.1°; N = 6); it is estimated from a mean direction of D = 336.4°; I = 66.0°N (α 95 = 5.3°; K = 159). The 1.466-Ga-old dykes are confined to the Eastern Ghats orogenic front in the easternmost part of the Bastar craton. Geochemically, the shoshonitic/high-K calc-alkaline affinity of these dykes is uniquely distinct from the tholeiitic composition found in Mesoor Palaeoproterozoic dykes in other parts of the Indian shield. Testing the existing pre-Rodinia Mesoproterozoic tectonic reconstructions negates the Columbia reconstructions in which the Indian shield is shown in juxtaposition with North China/Laurentia. On the other hand, palaeomagnetic and geological data suggest that the linkages between the Indian shield and Western Australia proposed earlier for the Palaeoproterozoic appear to persist during the Mesoproterozoic as well. The linkages may be further extended into Baltica.

Geochemistry and petrogenesis of Paleo–Mesoproterozoic mafic dyke swarms from northern Bastar craton, central India: Geodynamic implications in reference to Columbia supercontinent

Field setting, petrography, geochemistry and available radiometric ages of Proterozoic mafic dykes from the northern Bastar craton have helped to identify four sets of mafic dykes; two Paleoproterozoic [viz. NW–SE North Bastar dykes (NBD) and ENE–WSW Dongargarh-Chhura dykes (DCD)] and two Mesoproterozoic [viz. 1.42Ga ENE–WSW Bandalimal dykes (BDD) and 1.44Ga N–S Lakhna dykes (LKD)]. Their petrographic and geochemical characteristics are very distinct and suggest their derivation from different mantle melts. Chemistry of all the four sets suggests different petrogenetic histories and samples of each distinct set are co-genetic nature. The NBD, the DCD and the BDD samples are sub-alkaline tholeiitic in nature, whereas the LKD samples show alkaline nature. Very distinct REE patterns are observed for all the four sets again suggesting their different petrogenetic histories. Geochemical comparison between the studied samples and mafic dyke samples of southern and central parts of the Bastar craton suggests very different picture for the northern Bastar craton. Only one set of northern Bastar dykes, i.e. the NBD, matches with BD1 dykes; no other dyke sets match with any of the dyke swarms identified in southern and central Bastar craton. Geochemically it is not straightforward to confirm crustal contamination, however, on the other hand, possibility of crustal contamination cannot be ruled out completely. A petrogenetic model based on trace element data suggests that all the four sets are derived from different mantle melts. The NBD and the DCD are probably generated within spinel stability field, whereas the BDD and the LKD may be derived from melts generated within garnet stability field. Available geological and geochemical data support the emplacement of studied dykes in a stable continental rift tectonic setting, however earlier intrusions have chemistry similar to N-MORB. The available geological, geochemical and geochronological data on the four indentified sets of mafic dykes from the northern Bastar craton indicate their relation to the assembly and break-up of Columbia supercontinent.

A palaeomagnetic and 40Ar/39Ar study of mafic dykes in southern Sweden: A new Early Neoproterozoic key-pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops

We present the results of palaeomagnetic and 40Ar/39Ar studies of the Proterozoic mafic dykes in the Norrköping and Falun areas of the southern Sweden. The primary remanence of two 939±3Ma dykes is supported by the rigorous baked contact test. The remanence direction of two other dykes, one of which was previously U–Pb dated at 946±1Ma is close to the reverse direction of 939Ma dykes. Using these results together with previously published 935±5Ma palaeomagnetic data from the Göteborg-Slussen mafic dykes and some dykes from the Falun area we calculated the mean 946–935Ma palaeopole for Baltica (0.9°S, 240.7°E, A95=6.7), which can be qualified as the key pole. Using this pole together with other date we conclude that the Grenville and Sveconorwegian loops of Laurentian and Baltican Apparent Polar Wander Paths are temporary displaced by 100–150m.y. We propose new palaeogeographic reconstructions of Baltica and Laurentia at ca. 940Ma and ca. 850Ma. We also present two new Mesoproterozoic non-key poles from 1410Ma and 1595Ma dykes.

Proterozoic mafic dykes as monitors of HP granulite facies metamorphism in the Grenville Front Tectonic Zone (western Quebec)

Metamorphosed mafic dykes and pods in the Grenville Front Tectonic Zone (GFTZ) are correlated, on the bases of map patterns and geochemical trends, with Paleo- and Mesoproterozoic dykes that cross-cut Archean rocks of the foreland, NW of the Grenville Front. The GFTZ consists of a SE-dipping 30 km thick slab of quartzo-feldspathic granulites and biotite–garnet (±opx) gneisses overriding, along the McLaurin thrust, a 4 km thick zone of muscovite–sillimanite micaschists that abuts on the Grenville Front. The granulite slab in turn dips under amphibolite-grade migmatites of the Réservoir Dozois terrane along the Dorval detachment. According to published U–Pb and new U–Th–Pb ages on monazite, the granulites, the gneisses and the schists of the GFTZ were metamorphosed at ca. 2.7–2.8 Ga and reworked at ca. 1.0 Ga (Grenvillian). Proterozoic mafic dykes and pods cross-cutting the GFTZ are thus monocyclic, i.e. they went through the Grenvillian metamorphic event only. Several of these metamorphosed mafic dykes and pods have preserved evidence of emplacement in a brittle regime such as intersection of the gneissic foliation, chilled margins, magmatic layering, this in spite of a strong metamorphic overprint attested by coronitic fabrics. In particular, metamorphosed mafic dykes of the granulite slab, contain large pyroxenes rimmed by garnet coronas with minute quartz inclusions, suggesting the reaction pl + opx = grt + qtz. These garnet coronas, in turn, are locally replaced by symplectites of opx, hbl, pl and qtz, probably formed through back-reactions grt + cpx + qtz = pl + opx and grt + cpx = hbl + qtz. Thermobarometric calculations using the above assemblages indicate peak equilibration at about 1.2–1.5 GPa at temperatures of about 800 °C followed by a quasi-isothermal decompression down to 0.9 GPa at temperatures around 700 °C. Proterozoic mafic dykes thus record a Grenvillian HP granulite metamorphism resulting from burial of upper crustal levels down to sub-Moho depths with concomitant heating. This event was followed by rapid exhumation of the GFTZ zone probably controlled by normal displacement along the Dorval detachment.

Magnetic and compositional variations in Proterozoic mafic dykes in Finland, northern Fennoscandian shield

Proterozoic mafic dykes of both tholeiitic basaltic and more magnesian compositions from four areas in Finland have been studied for their magnetic and compositional variations to explain the variation in their aeromagnetic patterns. The highly magnetic more magnesian dykes are characterized with distinctive aeromagnetic anomalies, whereas the tholeiitic dykes are weakly magnetic and associated with faint magnetic signatures. To evaluate the factors that control the primary magnetization and the effects of later geological processes on their ferrimagnetic mineralogy, petrophysical properties of the dykes were correlated with their bulk mineral compositions. The role of ferric iron was of particular interest, as those iron-bearing silicates which participate in the reactions where titanomagnetites are formed or destroyed contain ferric iron in their crystal structures. The early crystallization of hydrous mafic silicates, biotite and amphibole, versus the anhydrous mafic silicate, clinopyroxene (augite), seems to be of key importance to formation of titanomagnetites. As clinopyroxenes can accommodate only minor amounts of ferric iron compared to hydrous mafic silicates, the excess ferric iron can be consumed to form primary titanomagnetites. Therefore it is concluded that the water vapour pressure at the emplacement depth of the dykes is most relevant in terms of the primary ferrimagnetic mineralogy. The originally high total magnetization of rocks that already contain abundant ferrimagnetic minerals will not be greatly changed in secondary processes, as the volume percentage of the secondary fine-grained magnetite is generally small. In addition, the total magnetization of the rocks that are originally deficient in titanomagnetites does not increase significantly.

Magnetic and compositional variations in Proterozoic mafic dykes in Finland, northern Fennoscandian shield

Proterozoic mafic dykes of both tholeiitic basaltic and more magnesian compositions from four areas in Finland have been studied for their magnetic and compositional variations to explain the variation in their aeromagnetic patterns. The highly magnetic more magnesian dykes are characterized with distinctive aeromagnetic anomalies, whereas the tholeiitic dykes are weakly magnetic and associated with faint magnetic signatures. To evaluate the factors that control the primary magnetization and the effects of later geological processes on their ferrimagnetic mineralogy, petrophysical properties of the dykes were correlated with their bulk mineral compositions. The role of ferric iron was of particular interest, as those iron-bearing silicates which participate in the reactions where titanomagnetites are formed or destroyed contain ferric iron in their crystal structures. The early crystallization of hydrous mafic silicates, biotite and amphibole, versus the anhydrous mafic silicate, clinopyroxene (augite), seems to be of key importance to formation of titanomagnetites. As clinopyroxenes can accommodate only minor amounts of ferric iron compared to hydrous mafic silicates, the excess ferric iron can be consumed to form primary titanomagnetites. Therefore it is concluded that the water vapour pressure at the emplacement depth of the dykes is most relevant in terms of the primary ferrimagnetic mineralogy. The originally high total magnetization of rocks that already contain abundant ferrimagnetic minerals will not be greatly changed in secondary processes, as the volume percentage of the secondary fine-grained magnetite is generally small. In addition, the total magnetization of the rocks that are originally deficient in titanomagnetites does not increase significantly.

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.

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.

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.

Retrogression, geochemical alteration and deformation in Proterozoic mafic dykes, Hopedale block, Labrador

Studies of Precambrian high-grade terranes have shown low-grade retrogressive events to be common-place and to be most intense where pre-existing steep surfaces such as dyke margins can be exploited by circulating fluids. Petrographic and geochemical alteration has been studied in two mafic dyke swarms which intrude the Archaean Hopedale block, Labrador, Canada. The older swarm, the Kikkertavak dykes, was intruded in the early Proterozoic and pre-dates the Makkovikian orogeny, which juxtaposed the south east of the Hopedale block and the Makkovik province. Degree of tectonism, secondary mineral growth and element mobility in the Kikkertavak dykes increases approaching the Hopedale block — Makkovik boundary, which is marked by the Kanairiktok shear zone, suggesting that tectonic deformation increased the flux of metasomatic fluids by increasing rock permeability. Within the interior of the Hopedale block, many moderately hydrated Kikkertavak dykes are similarly altered and depleted in mobile elements whereas others show a positive relationship between combined water content and large ion lithophile element concentration. This second pattern is interpreted as being the result of deuteric alteration during petrogenesis of the dykes. The younger Harp dyke swarm intruded in the mid-Proterozoic post-dates the Makkovikian orogeny and is therefore not severely retrogressed. However, the presence of coarse dioritic lenses and a similar positive correlation of LIL element enrichment with combined water content suggests that minor alteration in the Harp dykes is deuteric in nature.

Petrogenesis of middle proterozoic mafic dykes in the São Francisco Craton, Brazil: implications for a fossil mantle plume beneath the Northern Espinhaço Range

The geochemistry of mafic rocks in continental areas may provide valuable information regarding regional geologic evolution if the nature of the processes involved in their genesis is reasonably well constrained. It is of general acceptance that intracontinental mafic dykes and flood basalts usually carry the geochemical fingerprints of ocean flood basalts (MORS) and the continental crust (UGHTFOOT & HAWKESWORTH, 1988; DUPUV et ai., 1988; HERGT et aI., 1989), possiblyas the result of the growth and development of the sub-continentallithospheric rnantle, from which some of them derive, or through MORS contamination and mixing with continental material. Sometimes asthenospheric MORS-Jike magrnas can reach the Earth's surface without being too much contamlnated by the continental lithosphere as is possibly the situation of most of the Deccan basalts (UGHTFOOT & HAWKESWORTH, 1988). In other situations lithospheric rnantle sources must be considered, such as in the case of the Parana high-phosphorus, high-titanium basalts (MANTOVANI et ai., 1985; HAWKESWORTH et ai. , 1988). The heat sources required for rnantle partlal melting is a matter of dispute, but decompresslon beneath stretched continental or oceanic crust anel the rise of hot mantle plumes are usually invoked. Studies of basaltic rocks from the ocean basins have shown that magmatism assoclated with mantle plume or hotspots is dominantly alkaline anel enriched in highly Incompatible trace elements (e.g. LeROEX et ai., 1983, 1985; HAWDAV et aI., 1990). Alkaline compositions can also result through low degree of melting of depleted asthenospheric mantle sources (FITTON & DUNLOP, 1985).

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.

Paleomagnetism of Middle Proterozoic (1.01 to 1.08 Ga) mafic dykes in southeastern Bahia State—São Francisco Craton, Brazil

Paleomagnetic results from profuse Middle-Late Proterozoic mafic dyke swarms in southeastern Bahia State (Sa˜o Francisco Craton) yield either an easterly direction with high upward inclination or a westerly direction with high downward inclination isolated during AF and/or thermal treatments. Thermal demagnetization behavior and thermochronologic and petrologic considerations indicate that these remanent components originated as primary TRM's. Four groups of directions were distinguished from dykes in spatially distinct areas: Ilheus normal polarity ( D = 60.0°; I = −68.8°; α 95 = 2.6° ; N = 17) , Olivença normal polarity ( D = 82.4°; I = −71.0°; α 95 = 5.1 ; N = 31), Itajúdo Coloˆnia ( D = 99.0; I = −71.9 ; α 95 = 5.9° ; N = 23) and Olivença reversed polarity ( D = 298.8°; I = 60.7°; α 95 = 6.4° ; N = 18), which yield paleomagnetic poles located at 100.4°E; 30.3°N ( I N ), 107.0°E; 16.1°N ( O N ), 111.0°E; 7.7°N ( IC) and 280.2°E; 17.0°N ( O R ), respectively. These poles define an APW path for the Sa˜o Francisco Craton between the time interval 1.01–1.08 Ga which is characterized by at least two polarity intervals.

Preservation of primary geochemical signatures in polymetamorphosed tholeiite: The Long Range dyke swarm, Newfoundland, Canada

Latest Proterozoic (0.6 Ga) mafic dykes cutting the Grenvillian Long Range Inlier of western Newfoundland show a polymetamorphic imprint attributed to Appalachian-Caledonian orogenesis. Primary assemblages including augite (Wo 35En 48Fs 17), labradorite and ilmenite are replaced by calcic amphiboles, relatively sodic plagioclase, titanite, epidote, biotite and quartz in dykes recrystallized during crustal reactivation within a 15 km wide zone along the southeastern margin of the inlier. Samples containing relict igneous assemblages have a more evolved tholeiitic composition than the metamorphosed dykes. Mass-balance and Rayleigh-fractionation calculations indicate that these are primary geochemical features, resulting largely from the fractionation of plagioclase, augite and olivine from the Long Range dyke magma, rather than from metasomatism. Metamorphism of the Long Range dykes is therefore interpreted to have proceeded nearly isochemically, and principally involved the hydration and oxidation of the igneous assemblages.

Petrography and geochemistry of Proterozoic mafic dykes from the Guiana Shield, northern Amazon Craton

Petrography and geochemistry of Proterozoic mafic dykes from the Guiana Shield, northern Amazon Craton

Proterozoic mafic dykes near Port Lincoln, South Australia: Composition, age and origin

Igneous‐textured and recrystallized amphibolite or pyroxene granulite dykes cut Proterozoic granitoid gneisses in the Port Lincoln area of South Australia. The igneous‐textured dykes are largely Ol‐normative, and the amphibolites Qz‐normative, tholeiites. The igneous‐textured dykes comprise norites, gabbronorites and plagioclase‐phyric dolerites which may be readily distinguished by their different Hy/Di ratios. All three igneous‐textured groups are LREE enriched, but they are further distinguished by their different degrees of HREE fractionation, distinct Zr/Ti and Zr/Y ratios and different combinations of normalized negative Ti, Nb, P, Sr and V anomalies. The compositional variation within the three groups may be modelled by crystal fractionation of opx + cpx + plag and cpx + plag dominated assemblages. Rb‐Sr isotopic data from the norites and previously published Rb‐Sr and K‐Ar isotopic data from gabbros northwest of the Port Lincoln area collectively suggest mafic magmatism was widespread about 1600 Ma. The norites and gabbros define three apparently distinct isochrons with different initial 87Sr/86Sr ratios which show a crude positive correlation with the average Rb/Sr ratio. The Sr isotope and major element data suggest sial assimilation during fractional crystallization was not important in the norites. Some of the norites are highly magnesian and together with the plagioclase‐phyric dolerites share a boninite‐like geochemical signature. The gabbronorites resemble more closely within‐plate basalts. The association of the dykes with shelf‐like supracrustals overlying Archaean continental crust suggests a possible origin for the dykes at a rifted continental margin.