Proterozoic Dyke Swarms Research Articles

Paleomagnetism and U–Pb geochronology of easterly trending dykes in the Dharwar craton, India: feldspar clouding, radiating dyke swarms and the position of India at 2.37 Ga

A U–Pb baddeleyite age of 2367 ± 1 Ma from a diabase dyke together with previously published age data, suggests that a major early Proterozoic dyke swarm cuts across the structural grain of the Archean Dharwar craton in India. Paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30° with convergence to the west. It was originally emplaced at high latitudes, and together with the Widgiemooltha dykes of the Yilgarn block of Australia, may have been a segment of a larger radiating swarm related to a long-lived plume event that was active for about 50 My from 2418 to 2367 Ma. A regional change in the intensity of brown feldspar clouding in the dykes suggests that the Dharwar craton was tilted northwards, in harmony with previous observations on the structure and metamorphism of the Archean rocks. Towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker, more “sooty” appearance. The black clouding, whose precise origins remain unknown, is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at ∼800 Ma, and with a shear zone and change in structural trend related to Pan-African deformation at ∼550 Ma. Paleomagnetic studies suggest that (i) the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and (ii) as a more general observation, diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black.

Petrology and petrogenesis of a Late Proterozoic dyke swarm in South-Jordan

Petrology and petrogenesis of a Late Proterozoic dyke swarm in South-Jordan

Multivariate statistical analysis of geochemical data exemplified by Proterozoic dyke swarms in Sweden

The multivariate statistical analysis of chemical data in petrology and geochemistry entails principles that are little known among geological practitioners. Such data are compositional, and hence, lie in a sub-space of full space, the simplex. The concept of product-moment correlation of full space cannot be validly applied to them, because of the subcompositional incoherency of that statistic in the simplex. A second type of problem concerns the stability of multivariate estimates. Appropriate techniques, such as principal component cross-validation, are discussed exemplified and contrasted with inappropriate procedures using geochemical data on Swedish Proterozoic dyke swarms. Essential features of compositional covariance structure are exemplified in an Appendix.

Geochemistry and petrogenesis of the Proterozoic dykes in Tamil nadu, southern India: another example of the Archaean lithospheric mantle source

Approximately 1650-Ma-old NW/SE and NE/SW-trending dolerite dykes in the Tiruvannamalai (TNM) area and approximately 1800-Ma-old NW/SE-trending dolerite dykes in the Dharmapuri (DP) area constitute major Proterozoic dyke swarms in the high-grade granulite region of Tamil nadu, southern India. The NW- and NE-trending TNM dykes are compositionally very similar and can be regarded as having been formed during a single magmatic episode. The DP dykes may relate to an earlier similar magmatic episode. The dolerites are Fe-rich tholeiites and most of the elemental variations can be explained in terms of fractional crystallisation. Clinopyroxene and olivine are the inferred ferromagnesian fractionation phases followed by plagioclase during the late fractionation stages. All the studied dykes have, similar to many continental flood basalts (CFB), large-ion lithophile element (LILE) and light rare-earth element (LREE) enrichment and Nb and Ta depletion. The incompatible element abundance patterns are comparable to the patterns of many other Proterozoic dykes in India and Antarctica, to the late Archaean (∼2.72 Ga) Dominion volcanics in South Africa and to the early Proterozoic (∼2.0 Ga) Scourie dykes of Scotland. The geochemical characteristics of the TNM and DP dykes cannot be explained by crustal contamination alone. Instead, they are consistent with derivation from an enriched lithospheric mantle source which appears to have been developed much earlier than the dyke intrusions during a major crustal building event in the Archaean. The dyke magmas may have been formed by dehydration melting induced by decompression and lithospheric attenuation or plume impingement at the base of the lithosphere. These magmas, compared with CFB, appear to be the minor partial melts from plume heads of smaller diameter and of shallow origin (650 km). Therefore, the Proterozoic thermal events could induce crustal attenuation and dyke intrusions in contrast to the extensive CFB volcanism and continental rifting generally associated with the Phanerozoic plumes of larger head diameter (>1000 km) and of deeper origin (at crust mantle boundary).

SHRIMP U-Pb single zircon geochronology of a Proterozoic mafic dyke, Isukasia, southern West Greenland

Noritic dykes which are orientated north-south are early members of the Proterozoic dyke swarms which cut the Archaean craton of West Greenland. A diffuse-margined felsic patch within a >100 m wide dyke at Isukasia is interpreted to have crystallised at the same time as the rest of the dyke. Three zircons were separated from a 500 g sample of the felsic patch. Two of the grains are prismatic, euhedral, metamict with very high U contents, and are interpreted to have crystallised at the same time as the felsic patch. The most concordant analyses from the first two grains yield a weighted mean ^"Pb/^Pb age of 2214 ± 10 Ma (2F). The third grain is somewhat rounded, non-metamict with a low U content, and four spots yield ^"Pb/^Pb ages between 3390 and 3310 Ma. This is within error of whole-rock Rb-Sr and Sm-Nd isochron ages (with large uncertainties) for pegmatites forming part of the country rocks to the dyke. 2214 ± 10 Ma is interpreted as the time of emplacement and crystallisation of the dyke, during which it assimilated blocks of country rocks, giving rise to the felsic patches and inherited zircons such as the >3300 Ma grain.

The palaeomagnetism of middle Proterozoic dyke swarms of the Gardar Province and Mesozoic dykes in SW Greenland

SUMMARY The Precambrian metamorphic basement in SW Greenland was subjected to widespread emplacement of dolerite dykes between circa 1290 and 1160 Ma; during this period the regional stress field rotated in a counter-clockwise sense. A regional palaeomagnetic study of 59 of these dykes and contacts is reported. Whilst contact tests generally indicate primary magnetizations, two- and three-component structures in some dykes record partial overprinting at later stages of the igneous cycle. Component directions are distributed from shallow negative westerly to steep NW positive. Over most of the outcrop the former magnetizations are found in the earlist (E-W trending) dykes. Steepening and rotation of magnetization directions correlates with rotation of dyke trends towards later (NE-SW) trends. The exception occurs where dykes are deflected in a sigmoidal way through an older alkaline complex. The quasi-continuous nature of dyke emplacement records migration of the palaeofield direction between circa 1290 and 1160 Ma and representative sequential pole positions are (A1) 215.3dE, 3.1dN (21 dykes, dp/dm= 4.8/9.6d), (A2) 220.4dE, 7.4dN (5 dykes, dp/dm= 4.3/8.5d) and (A3) 222.3dE, 33.5dN (30 dykes, dp/dm= 4.5/7.0d). Palaeomagnetic poles from the Gardar Igneous Province define the major part of a double APW loop anchored by dated poles from igneous complexes and executed between 1300 and 1140 Ma. This interval appears to have been dominated by one field polarity. Palaeomagnetic results are also reported for five dykes belonging to the Mesozoic coast parallel swarm, and palaeomagnetic results from these dykes in south and south-west Greenland are reassessed. They define a dual-polarity axis of intermediate inclination (D/I = 336/51d, 13 sites, palaeomagnetic pole (TD1) at 168.3dE, 55.6dN. dp/dm= 4.7/6.9d) plus a steeper single-polarity group (mean direction D/I = 329/69d, 14 sites, palaeomagnetic pole (TD2) at 207.7dE, 68.6dN, dp/dm= 6.6/7.8d). The TD1 pole corresponds to North American apparent polar wander at circa 165 Ma compatible with the 168-138 Ma age constraint, whilst the TD2 pole correlates with near-static APW between 129 and 88 Ma. It appears to record an episode of dyke emplacement across south Greenland during the Cretaceous Normal Superchron shortly before commencement of sea-floor spreading between Greenland and Labrador.

Constraints on the nature of the Kapuskasing structural zone from the study of Proterozoic dyke swarms

Petrochemical, paleomagnetic, and structural studies on Early Proterozoic dyke swarms show that the rocks of the Kapuskasing structural zone (KSZ), central Superior Province, were upthrust about 10–15 km along the Ivanhoe Lake fault after intrusion of 2.04 Ga Kapuskasing dykes. This uplift was part of a more widespread deformation in flanking terranes that involved sinistral motion along north–north-northwest-trending faults, dextral displacement along east-northeast–northeast-trending faults, and a distortion of the 2.45 Ga Matachewan swarm to the west and northwest of the Ivanhoe Lake fault. The most spectacular demonstration that the KSZ is largely a product of Proterozoic deformation is that Matachewan dykes change their magnetic polarity on crossing the zone, a consequence of remanence acquisition at deep crustal levels being delayed until after a reversal of the earth's magnetic field and prior to uplift of the dykes.Matachewan and Kapuskasing dykes within the amphibolite- to granulite-grade rocks of the KSZ are relatively fresh and contain a high-alumina green amphibole and feldspar laths that exhibit tea-coloured clouding due to the presence of submicroscopic inclusions of magnetite. These features may be diagnostic of crystallization at deeper crustal levels because they are found in regions where Rb–Sr biotite ages from tonalites suggest broad-scale uplift of the crust after about 2 Ga. About 50 km west of the KSZ, dykes exhibiting clouded feldspar and high-alumina amphiboles occur on the upthrown side of the Budd Lake fault zone, which has a trend similar to that of the Ivanhoe Lake fault. Therefore a second thrust block may exist, raising the possibility that the KSZ represents the basal member of a series of imbricate thrust slices that extends westwards from the Ivanhoe Lake fault. Radiometric age data suggest that the region affected by this Early Proterozoic deformation continues to the northwest for a further 300 km.

SmNd mineral isochron ages of Late Proterozoic dyke swarms in Australia: evidence for two distinctive events of mafic magmatism and crustal extension

Two distinctive events of widespread dyke intrusion have been identified in central-southern Australia. Crystallization ages of the dykes were determined using SmNd mineral isochrons obtained by splitting clinopyroxene and plagioclase into different magnetic and density fractions, respectively. The Stuart Dyke Swarm in the southern Arunta Inlier and the Kulgera Dyke Swarm in the eastern Musgrave Inlier define essentially identical crystallization ages of 1076 ± 33 and 1090 ± 32 Ma, respectively, marking the first episode of post-orogenic mafic magmatism in the region. Two samples from the Gairdner Dyke Swarm in the Stuart Shelf yield SmNd mineral isochron ages of 867 ± 47 and 802 ± 35 Ma, respectively, whilst two samples from the Amata suite in the central Musgrave Inlier, 790 ± 40 and 797 ± 49 Ma. These four ages represent the second episode of mafic magmatism in central-southern Australia. These ages allow new constraints to be placed on the timing of large-scale crustal extension and formation of intracratic depositional basins in the region. Initial ϵ Nd-values of − 7.8 to − 6.6 for the Stuart dykes and + 0.7 for the Kulgera dykes suggest they were derived from different mantle sources with the Stuart dykes being produced from strongly enriched sources. In contrast, positive and relatively uniform initial ϵ Nd-values of + 2.5 to + 4.1 for the younger Amata Suite and Gairdner dykes indicate that they were derived from relatively depleted and homogeneous mantle sources. The 1076 ± 33-Ma SmNd age of the Stuart Dyke Swarm is significantly older than its Rb-Sr mineral isochron age of 897 ± 9 Ma previously reported, suggesting mobility of Rb and/or Sr and partial resetting of Rb-Sr isotopic systematics. This study demonstrates that reliable and relatively precise SmNd ages can be obtained for Precambrian dykes, despite the limited number of mineral phases available in the dykes.

Amphibole and plagioclase chemistry in two Proterozoic dyke swarms and its relation to the uplift history of the Kapuskasing Structural Zone

The gneisses of the Chapleau portion of the Kapuskasing Structural Zone (KSZ) and of the Wawa Gneiss Terrane (WGT) can be partitioned into subregions on the basis of magnetic polarity of 2.45 Ga Matachewan diabase dykes emplaced within them. West of the Ivanhoe Lake fault zone (ILFZ) the polarity sequence across the trend of the dyke swarm is normal–reverse–normal (N–R–N) over a traverse distance >100 km. Highly correlated with dyke N polarity is the presence of tea-coloured groundmass plagioclase and hornblende with high Al, Na, and Ti content. Reversely magnetized dykes within the WGT and in dykes of either polarity in regions far removed from the KSZ have groundmass plagioclase with hydrous alteration and relatively Al-poor amphiboles. Although N and R magnetized dykes have groundmass plagioclase with comparable Fe contents, the plagioclase of N dykes and Kapuskasing dykes in the highest grade country rock contain discrete, micrometre-sized, Ti-poor magnetite particles. The mineralogical variations are independent of whole-rock bulk chemistry and are ascribed to greater crystallization depth for N magnetized dykes. The magnitude of the amphibole compositional change in dykes at the western N–R boundary within the WGT is comparable to that across the ILFZ. The western N–R boundary is wholly within the WGT, whereas the ILFZ juxtaposes upper and lower crustal levels. As a consequence, only some of the differential uplift between the KSZ and the Abitibi belt can be accounted for in post-Matachewan dyke time. Limited data from the amphiboles in the 2.04 Ga east-northeast-trending Kapuskasing dykes suggest that this phase of faulting along these block boundaries postdates Kapuskasing dyke emplacement.

Geochemistry of Precambrian basaltic rocks from the Central African Republic (Equatorial Africa)

The two Archaean greenstone belts (Bandas and Bogoin) in the Central African Republic (Equatorial Africa) are 250 and 150 km long. The metavolcanic rocks in the belts are predominantly komatiitic and tholeiitic basalts. Komatiites include both Al-depleted and Al-undepleted types. The komatiites and light-REE-depleted tholeiites were probably derived from a similar upper mantle source. However, the tholeiitic basalts enriched in light REE from the upper volcanic strata of the Bandas belt were generated from a different source. The dolerites from Proterozoic dyke swarms and sills differ from the basalts mainly in their abundances and ratios of several incompatible elements such as K, Rb, Th, and light REE. They were derived from a distinct, incompatible-element-enriched upper mantle source.The average background gold levels in the Bandas belt and dolerite dyke swarms are comparable to those in equivalent rocks from North America. The exception is the Bogoin greenstone belt, which has anomalously high gold abundances.

Trace element variations in Atlantic Ocean basalts and Proterozoic dykes from northwest Scotland: Their bearing upon the nature and geochemical evolution of the upper mantle

Trace element data on ocean floor and ocean island basalts from the Atlantic and from early Proterozoic dyke swarms are considered in relation to the nature and causes of upper mantle heterogeneity. Studies of dredged and drilled basalts from the North Atlantic have enabled the compositions of erupted ridge basalts to be monitored in space and time and have revealed the following features: (a) that while melting processes may give rise to appreciable REE variation at a particular site, ratios of the more incompatible elements remain remarkably consistent in basalts erupted at a particular ridge segment for tens of millions of years and (b) that there are major differences in incompatible element ratios in basalts erupted at different ridge segments. The relationships suggest that the volumes of mantle with these characteristic incompatible element ratios must be large, but that the processes associated with basalt generation are not capable of significantly changing incompatible element ratios. Nevertheless, the processes which have produced these incompatible element variations must be incorporated into any model for mantle evolution. Our preferred model invokes both crustal extraction and incipient mantle melting as the major mechanisms for changing incompatible element mantle source regions: both represent very small degrees of mantle melting. A depleted upper mantle variably veined with incompatible element enriched liquids or fluids (derived through incipient melting at greater depths) is most consistent with the trace element and isotopic compositions of North Atlantic basalts, many of which are not normal mid-ocean ridge basalts (N-type MORB). Ocean island basalts may represent more intensely veined and, in some cases at least, more refractory sources sampled at lower degrees of partial melting. Mantle from which incipient melts have been removed probably represents the N-type MORB source. Extraction of crustal material, having high contents of the large ion lithophile elements but low contents of the high field strength ions (ie., Ta and Nb), from a veined/metasomatized mantle source, leaves the upper mantle residue enriched in Ta and Nb relative to the large ion lithophile elements. This might explain the enrichment of Ta and Nb in most mantle sources of continental and ocean island basic lavas. Mantle sources enriched by incipient melting processes are continually dissipated by mantle convection, but may be preserved for a significant time beneath stable continents. Trace element data on early Proterozoic tholeiitic to picritic dyke swarms from Scotland suggest that short-term mantle heterogeneities may have existed beneath the region at that time and could have been related to the processes of crustal generation 600 m.y. earlier.