Proterozoic Dykes Research Articles

Redbed diagenesis links district-scale mineralising fluids to Cu source rock in the Polaris Zn-Pb(-Cu) district, Arctic Canada

Red sandstone of the Proterozoic Aston Formation (Nunavut, Canada) is exposed in the same geographic area as the Storm Cu showing in the Paleozoic-carbonate-hosted Polaris Zn-Pb district (Arctic Canada). The ∼800-m-thick Aston Formation experienced successive burial and exhumation episodes, with maximum burial (>3 km) at the time of the Devonian Ellesmerian orogeny. A petrographic and in situ SIMS oxygen isotope study identified redbed diagenetic events, associated fluid types and temperatures, and their relative timing. The first hematite coat (Hem1) developed in the Proterozoic. Intrusion of Proterozoic dykes and sills heated local formation water to high temperatures (130–223 °C), resulting in precipitation of patchy Qz1 cement in a shallow-burial environment. The Fe and Si were probably derived through alteration of non-quartz silicate detritus. In the Paleozoic, a bleaching (reducing) event removed most Hem 1; the reduced fluid may have been related to regional Zn mineralisation throughout the Polaris district during the mid-Paleozoic Ellesmerian orogeny. Ensuing pressure-solution during maximum burial was probably also Ellesmerian. A second hematisation episode (Hem 2) encloses the pressure-solved quartz-grain contacts. Coeval with Hem 2 was pervasive Qz2 cementation from a low- to mid-latitude, meteoric-derived, oxidised hydrothermal (140–180 °C) fluid that supplied externally derived Si and Fe, probably in the Early Carboniferous. The nature and timing of the fluid events indicated by the Aston Formation’s diagenetic history closely match the fluid history recorded in both Cu and Zn mineralisation in the Polaris district. This relationship strongly supports the concept that Cu in sedimentary-rock-hosted ore deposits can be sourced from redbeds, even those whose depositional age and environment is much older than, and geodynamically unrelated to, the fluid event(s) that caused diagenetic reddening and Cu transfer from source rock to mineralising site.

A review of anisotropy of magnetic susceptibility analysis of Indian dykes: Implications for magma emplacement

The analysis of Anisotropy of Magnetic Susceptibility (AMS) is a powerful and rapid technique to examine the preferred orientations of mineral (magnetic) fabrics and can indicate the nature of a magma transport (vertical or lateral). The relationship between magnetic fabric and geometry of a dyke swarm enables us to understand magma emplacement processes. Depending on the mutual relationship of magnetic fabric and individual dyke geometry, mode of magma transport is interpreted. The knowledge on the nature of magma transport combined with information on geometry, magmatic overpressure and geochemistry enable us to comment on dyke emplacement processes, the location of possible feeders, syn-emplacement and post-emplacement deformations and prevailing stress regime during emplacement. A number of dykes and dyke swarms have been emplaced into the Indian shield at different points in time. Their ages vary from the Mesoarchean to Tertiary. We present here a review of three case studies where AMS technique was applied to the samples collected from Indian dykes. Two case studies are on the Proterozoic dykes that intruded into the Dharwar craton and the third case study is on Mesozoic dykes that punctured the South Indian Granulite Terrain (SIGT). The dykes generally show “normal” anisotropy fabric to indicate vertical magma emplacement with few exceptions where lateral/inclined magma flow was suggested or the results were inconclusive. We present here a critical review on the interpretation of such “anomalous” fabrics and comment on further studies that can be carried out to extract more information from such results.

Break-up related 2170–2120 Ma mafic dykes across the North Atlantic craton: Final dismembering of a North Atlantic-Dharwar craton connection?

Progress with supercontinental reconstructions relies on accurate age determinations for continental geological units such as mafic dyke swarms. Here we present zircon and baddeleyite U-Pb ID-TIMS isotope data of six mafic dykes from the Archean North Atlantic craton in present-day southern West and South-East Greenland. Two dykes in southern West Greenland yield crystallization ages of 2021 ± 4 Ma, for the NE-trending Hamborgersund dyke, and 2125 ± 9 Ma, for the E-trending Angissunguaq dyke. Additional age determinations of mafic dykes from South-East Greenland are 2166 ± 8 Ma and 2158 ± 8 Ma for two E–W trending dykes, herein named the Skjoldungen dykes, and 2137 ± 11 and 2124 ± 11 for two ENE and NE trending dykes, respectively. The name Ruinnæsset dykes is proposed for this slightly younger, ca. 2137–2124 Ma, generation of dykes in SE Greenland, and Nuuk dykes for coeval ca. 2125 Ma dykes in SW Greenland. The Skjoldungen and the Ruinnæsset dykes have primordial mantle geochemical signatures, with only minor LILE-enrichments. These signatures differ from other, more ‘lithospheric’, Proterozoic dykes within the region, and may reflect highly attenuated lithospheric extension during their emplacement. Coeval mafic magmatism in North Atlantic and Dharwar cratons suggests that these two fragments coexisted within a common Paleoproterozoic supercraton. A lack of younger age-matches further argue for ca. 2170–2140 Ma rifting and subsequent break-up of this Dharwar-North Atlantic connection, prior to the 2137–2124 Ma emplacement of the Ruinnæsset dykes. Other global age correlations are discussed and a likely paleogeographic reconstruction of North Atlantic craton together with the Dharwar and Superior cratons within a Paleoproterozoic supercraton is presented.

Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.

Paleomagnetism of Devonian dykes in the northern Kola Peninsula and its bearing on the apparent polar wander path of Baltica in the Precambrian

Mafic dykes and large alkaline and carbonatite intrusions of Middle-Late Devonian age are widespread on the Kola Peninsula in NE Fennoscandia. These magmatic rocks are well characterized with petrographic, geochemical and geochronological data but no paleomagnetic results have been reported yet. We studied dolerite dykes from the northern part of the Peninsula and isolated three paleomagnetic components in these rocks. A low-temperature component is aligned along the present-day field, while a major constituent of natural remanent magnetization is an intermediate-temperature component (Decl.=79.6°, Inc.=78.5°, α95=5,9°, N=17 sites) that is present in most Devonian dykes but is found in some baked metamorphic rocks and Proterozoic dykes too. Finally, a primary Devonian component could be reliably isolated from two dykes only. Rock-magnetic studies point to presumably primary low-Ti titanomagnetite and/or pure magnetite as the main remanence carriers but also reveal alteration of the primary minerals and the formation of new magnetic phases.The directions of a major component differ from the Middle Paleozoic reference data for Baltica but closely match those for the 190±10Ma interval recalculated from the apparent polar wander path of the craton. We assume that this Early Jurassic component is a low-temperature overprint of chemical origin. The main impact of the new results is not to mid-Paleozoic or Early Mesozoic times but to much older epochs. Analysis of paleomagnetic data shows that the directionally similar remanences are present in objects with the ages ranging from 500Ma to ~2Ga over entire Fennoscandia. Hence we argue that an Early Jurassic remagnetization is of regional extent but cannot link it to a certain process and a certain tectonic event. If true, this hypothesis necessitates a major revision of the APWP for Baltica over a wide time interval.

Petrological discrimination among Precambrian dyke swarms: Eastern Kaapvaal craton (South Africa)

Age-determinations on a complex array of mafic dykes across the eastern Kaapvaal craton ( Olsson et al., 2010) are complemented herein by field evidence, dyke trend analysis, and petrological characterization of 58 dyke samples. ∼2.95 Ga SE-trending, a ∼2.65 Ga E- to SE-radiating, and ∼1.90 Ga NE-trending swarms can be distinguished. Prominent Archean (∼2.95 and ∼2.65 Ga) dyke ridges contain basement xenoliths, and have a more quartz-oversaturated, andesitic and calc-alkaline character. Proterozoic (∼1.90 Ga) dykes are, on the other hand, more aeromagnetically prominent tholeiitic basalts with higher modal (Fe, Ti)-oxide contents. Multi-elemental statistics indicate that the best geochemical discrimination is found between Archean high-Sr/V and La/Yb and Proterozoic low-Sr/V and La/Yb dyke swarms. The calc-alkaline character of Archean dyke swarms is augmented by LILE-enriched spider-element patterns with steeper REE-slopes as compared to the Proterozoic swarm. Geochemical similarities are roughly consistent with the ∼2.95 and ∼2.65 Ga dykes having fed coeval lavas within the Nsuze Group and Allanridge Formation (upper Ventersdorp Supergroup), respectively. ∼1.90 Ga dykes match coeval sills on both the Zimbabwe and Kaapvaal cratons, and are presumed feeders to Soutpansberg Group lavas. This new information provided by dated feeder dyke swarms leads to a re-evaluation of petrotectonic settings, ranging from continental back-arc to radiating swarms emanating from igneous (plume?) centers.

Early to middle Proterozoic dykes in the Mt. Riiser-Larsen area of the Napier Complex, East Antarctica: tectonic implications as deduced from geochemical studies

Abstract NE–SW- and north–south-striking dykes were emplaced into ultrahigh-temperature (UHT) granulites apparently after UHT metamorphism in the Mt. Riiser-Larsen area of the Archaean Napier Complex, East Antarctica, of which the north–south-striking dykes interrupt the NE–SW-striking ones. The NE–SW-striking dykes are tholeiite basalt (THB) and high-magnesian andesite (HMA) in composition. The THB dykes display relict doleritic textures, whereas the HMA dykes shows blastoporphyritic textures characterized by phenocrysts of clinopyroxene and plagioclase. Both sets of dykes exhibit large ion lithophile element and light rare earth element enrichment and negative anomalies of Nb, Ti and/or P in a spider diagram normalized to primitive mantle, which is reminiscent of modern subduction-related arc volcanism or continental flood volcanism. The isotope ratios of the THB dykes define isochron ages of 2.0–1.9 Ga: 1979±80 Ma in the Rb–Sr system (initial ratio ( I 0 ): 0.70239±0.00035) and 2078±104 Ma in the Sm–Nd system ( I 0 : 0.50964±0.00012). Such moderate 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd initial ratios may represent source materials closely related to the mantle wedge of a subduction zone. The north–south-striking dykes are compositionally divided into two basalt types. One is an alkaline basalt (AL) showing intergranular texture and characterized by high concentrations of incompatible elements, similar to those of ocean island basalt. They yield an isochron age of c. 1.2 Ga: 1161±238 Ma in the Rb–Sr system ( I 0 : 0.7047±0.0012). The other type (THB-m) is doleritic (ophitic) in texture, and has a tholeiitic affinity with a flat chondrite-normalized REE pattern, which is comparable with that of enriched mid-ocean ridge basalt. A comparison with dykes reported from other areas of the Napier Complex suggests that the north–south-striking dykes occur in restricted areas, whereas the NE–SW-striking dykes are more regional in occurrence. The 2.0–1.9 Ga magmatism of the NE–SW-striking dykes may have been related to the formation of continental crust of the Rayner Complex.

Microwave paleointensities from dyke chilled margins: a way to obtain long-term variations in geodynamo intensity for the last three billion years

Absolute paleointensity values of the geomagnetic field, measured using microwaves, are obtained from chilled margin samples of proterozoic dykes from the southern superior province of the Canadian Shield. Two age groups of dykes are studied; the 2446–2473 Ma Matachewan swarm, and 2076–2170 Ma dykes from the Biscotasing, Marathon and Fort Frances swarms. All samples selected for paleointensity study carry a dominant, primary magnetization component as determined from detailed paleomagnetic studies. Paleointensity results quoted below are based on individual sample determinations having a quality factor greater than 10. For the Matachewan dykes, the mean virtual dipole moment (VDM) is 2.53 ± 0.93 A m 2 × 10 22, based on 12 samples from 12 dykes. These results compare favourably with Thellier data from the same swarm by who obtained a mean VDM value, based on 25 samples from five dykes, of 2.80 ± 0.87 A m 2 × 10 22. For the Biscotasing, Marathon and Fort Frances swarms, the mean VDM, based on 12 samples from 12 dykes is 4.13 ± 1.82 A m 2 × 10 22, a value significantly greater than that obtained by Macouin et al. for the same swarms of 0.98 ± 0.10 A m 2 × 10 22, based on 25 samples from nine dykes. Possible reasons for this difference are discussed. The microwave method is shown to be an effective means for measuring paleointensity on dyke chilled margins and offers the opportunity that dyke swarms, which occur frequently throughout the last three billion years of geological history, can be used to track any long period variations in the geomagnetic field intensity that reflect stages in the evolution of the earth's core.

The Karoo triple junction questioned: evidence from Jurassic and Proterozoic 40Ar/ 39Ar ages and geochemistry of the giant Okavango dyke swarm (Botswana)

The lower Jurassic Karoo–Ferrar magmatism represents one of the most important Phanerozoic continental flood basalt (CFB) provinces. The Karoo CFB province is dominated by tholeiitic traps and apparently radiating giant dyke swarms covering altogether ca. 3×10 6 km 2. This study focuses on the giant N110°-trending Okavango dyke swarm (ODS) stretching over 1500 km across Botswana. This dyke swarm represents the main (failed) arm of the so-called Karoo triple junction that is generally considered as a key marker of the impingement of the Karoo starting mantle plume head. ODS dolerites yield six new plagioclase 40Ar/ 39Ar plateau (and miniplateau) ages ranging from 178.7±0.7 and 180.9±1.3 Ma. The distribution of the ages along a narrow Gaussian curve suggests a short period of magmatic activity centered around 179 Ma, i.e., ∼5 Ma younger than the emplacement age of Karoo mafic magmas in the southern part of the Karoo CFB province (∼184). This age difference indicates that Karoo magmatism does not represent a short-lived event as is generally the case for most CFB but lasted at least 5 Ma over the whole province. In addition, small clusters of plagioclase separated from 28 other dykes and measured by “speedy” step-heating experiments (with mostly two to three steps), gave either “Karoo” or Proterozoic ages. Integrated ages of the Proterozoic rocks range from 851±6 to 1672±7 Ma, and one plateau age (959.1±4.6 Ma) and one possibly geologically significant weighted-mean age (982.7±4.0 Ma) were obtained. Proterozoic and Karoo mafic rocks are petrographically similar, but Proterozoic dykes display clear geochemical differences (e.g., TiO 2<2.1%) with the Karoo high-Ti ODS (TiO 2>2.1%). Geochemical data combined with available Ar/Ar dates allow the identification of the two groups within a total set of 77 dykes investigated: ∼10% of the bulk ODS dykes are Proterozoic. Thus, the Jurassic Karoo ODS dykes were emplaced along reactivated Proterozoic structures and there is no pristine Jurassic Nuanetsi triple junction as commonly proposed. This throws into doubt the validity of the “active plume head” Karoo CFB rift models as being responsible for the observed “triple junction” dyke geometry.

The Karoo triple junction questioned: evidence from Jurassic and Proterozoic 40Ar/39Ar ages and geochemistry of the giant Okavango dyke swarm (Botswana)*1

The lower Jurassic Karoo–Ferrar magmatism represents one of the most important Phanerozoic continental flood basalt (CFB) provinces. The Karoo CFB province is dominated by tholeiitic traps and apparently radiating giant dyke swarms covering altogether ca. 3×10 6 km 2 . This study focuses on the giant N110°-trending Okavango dyke swarm (ODS) stretching over 1500 km across Botswana. This dyke swarm represents the main (failed) arm of the so-called Karoo triple junction that is generally considered as a key marker of the impingement of the Karoo starting mantle plume head. ODS dolerites yield six new plagioclase 40 Ar/ 39 Ar plateau (and miniplateau) ages ranging from 178.7±0.7 and 180.9±1.3 Ma. The distribution of the ages along a narrow Gaussian curve suggests a short period of magmatic activity centered around 179 Ma, i.e., ∼5 Ma younger than the emplacement age of Karoo mafic magmas in the southern part of the Karoo CFB province (∼184). This age difference indicates that Karoo magmatism does not represent a short-lived event as is generally the case for most CFB but lasted at least 5 Ma over the whole province. In addition, small clusters of plagioclase separated from 28 other dykes and measured by “speedy” step-heating experiments (with mostly two to three steps), gave either “Karoo” or Proterozoic ages. Integrated ages of the Proterozoic rocks range from 851±6 to 1672±7 Ma, and one plateau age (959.1±4.6 Ma) and one possibly geologically significant weighted-mean age (982.7±4.0 Ma) were obtained. Proterozoic and Karoo mafic rocks are petrographically similar, but Proterozoic dykes display clear geochemical differences (e.g., TiO 2 <2.1%) with the Karoo high-Ti ODS (TiO 2 >2.1%). Geochemical data combined with available Ar/Ar dates allow the identification of the two groups within a total set of 77 dykes investigated: ∼10% of the bulk ODS dykes are Proterozoic. Thus, the Jurassic Karoo ODS dykes were emplaced along reactivated Proterozoic structures and there is no pristine Jurassic Nuanetsi triple junction as commonly proposed. This throws into doubt the validity of the “active plume head” Karoo CFB rift models as being responsible for the observed “triple junction” dyke geometry.

Low paleointensities recorded in 1 to 2.4 Ga Proterozoic dykes, Superior Province, Canada

Paleointensity studies have been conducted on six mafic dyke swarms from the Superior Province in Canada with U–Pb ages between 1141 Ma and 2473 Ma (Buchan et al., Tectonophysics 319 (2000) 167–198). The mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests. This primary component was isolated over a very narrow range of high unblocking temperatures (usually 550–580°C), indicating that it is carried by magnetite or low titanium titanomagnetite. Magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo-single domain or single domain grains. Taken together, the characteristics of this component suggest that it is thermal in origin. Paleointensity experiments were conducted with a specially designed oven, using a revised approach of the Thellier–Coe method. Eighty-five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms, doubling the number of existing well-constrained Precambrian data. The paleofield estimates for the dyke swarms vary between 5 and 11.2 μT, yielding virtual dipole moments between 0.85±0.1 and 2.8±0.87×1022 Am2. The overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000–2400 Ma period than has been recorded between 0.3 and 400 Ma. However, additional determinations are required, particularly for the 400–2000 Ma interval, in order to establish whether a long-term evolution in the time-averaged field intensity can be linked to the onset of the growth of the Earth’s inner core.

The French Guyana doleritic dykes: geochemical evidence of three populations and new data for the Jurassic Central Atlantic Magmatic Province

A petrographic and geochemical study of 15 Early Jurassic and 7 Proterozoic dolerites of French Guyana, and of one Jurassic dolerite from Ivory-Coast were carried out. The Early Jurassic SSW-NNE trending dykes have doleritic aphyric or gabbroic phyric texture. Their chemical compositions, slightly under-saturated to over-saturated, show moderate to low Mg-ratios (63–36), high TiO 2 contents (1.85–3.56 wt.%), weak rare earth element fractionation [1.8<(La/Yb) n <4.6], negative Sr-anomalies (0.41<Sr* <0.91) and positive TiO 2-anomalies (1.2<TiO 2*<2.15). They correspond to sub-alkaline high-Ti basalts (HTiB). The La/Nb ratio (1.18±0.23) is close to that of the OIB-type Deccan basalts and incompatible element ratios: Rb/Nb n (1.07±0.3), Zr/Y (4.6±0.8) are intermediate between those of HTiB in Deccan and those of HTiB in Paraná. These geochemical signatures and recent isotopic data of Bertrand et al. [Bertrand, H., Liegeois, J.P., Deckart, K., Féraud, G., 1999. High-Ti tholeiites in Guinea and Their Connection with the Central Atlantic CFB Province: Elemental and Nd-Sr-Pb Isotopic Evidence for Preferential Zone of Mantle Upwelling in Cause of Rifting. AGU spring meeting (Abst. p 317)] suggest that their magmatic source is different from that of the other basalts of the Central Atlantic Magmatic Province (CAMP). Such signatures are restricted to a central zone coinciding with the Panafrican Rokelides suture. We propose a model of sub-lithospheric preferential channelling of an asthenospheric ascent in this zone. Two other groups of dykes were identified in French Guyana. Compared to the Jurassic ones the Proterozoic dykes have NNW-SSE and E-W trending direction, more important LILE enrichment, low TiO 2 contents (<2 wt%) and Nb–Ta negative anomalies. Their calc-alkaline signature could be the result of a previous subduction and may be related to the 1800 Ma Venturi-Tapajós event, which contaminated the mantle source.

Petrogenesis of the Early Proterozoic Matachewan dyke swarm, Canada, and implications for magma emplacement and subsequent deformation

Major, minor, and rare-earth element analyses of fine-grained marginal material from 249 dykes of the Matachewan giant radiating swarm show that the range in compositions cannot be produced by any simple fractionation model. Instead a two-stage process is required, whereby melts were modified by both fractionation and assimilation of lower crustal material, before rising and ponding at shallower crustal levels (15–20 km maximum), where a combination of fractional crystallization, plagioclase phenocryst formation, and periodic melt replenishment took place. These shallow crustal chambers acted as feeders for the numerous dykes. One hypothesis compatible with these observations involves an extensive rising plume that would provide heterogeneous melts over a large area accompanied by stresses that could produce a radial pattern of dykes. In a search for indications of lateral magma flow, a comparison of longitudinal variations in composition along four sample traverses across the western subswarm showed no truly consistent trends. However, Hf/Zr values are highest in the traverse nearest the focal region and lowest in the traverse 250 km away, suggesting a change in magma composition with distance from the focus of the swarm, the assumed plume centre. Given the presently available data, the variations cannot be attributed to progressive fractionation during lateral magma flow in the crust nor to a northward increase in amounts of contamination by known lower crustal assimilants, all of which have Hf/Zr ratios higher than the distal values recorded by the dykes. Instead the variations are thought to reflect a possible compositional zonation of the plume head. The distortion of the western subswarm from a simple radiating pattern may have been facilitated by a low viscosity lower crust, softened by Matachewan igneous activity, and is consistent with deformation by the Blezardian orogen at about 2.2–2.4 Ga.

Exploring Through Cover - The Integrated Interpretation of High Resolution Aeromagnetic, Airborne Electromagnetic and Ground Gravity Data from the Grant’s Patch Area, Eastern Goldfields Province, Archaean Yilgarn Craton Part C: Combining Geophysical Methods for a Holistic Exploration Model.

Grant’s Patch is a semi-mature lode gold corridor located 45 km NW of Kalgoorlie in Western Australia. Gold mineralisation at Grant’s Patch sits within geological structures less than 20 m wide (lithological contacts, ductile shears and brittle cross-faults). These structures are masked by 10 to 100 m of regolith cover and are often impossible to correlate between exploration drillholes.The southern part of this corridor is covered by high-resolution aeromagnetic, airborne electromagnetic and ground gravity data. This provides dense, regularly spaced data coverage to image patterns reflecting the sub-surface geometry of the regolith and bedrock geology. There is also uniform exploration drillhole coverage to non-weathered bedrock and very little surface disturbance from mining infrastructure.Aeromagnetics identifies magnetic greenstone units, such as mafic to ultramafic sills and dolerite horizons, faults as disruptions crossing proterozoic dykes, and paleochannels containing ferruginous gravel. Patterns in the filtered gravity data reflect subtle density differences between rock units, regolith features, and weathered fault zones that cannot be detected in the aeromagnetic data. The airborne electromagnetic (AEM) response is dominated by conductive clays, predominately in situ as saprolite, and saline groundwaters in the regolith that follow lithodependant weathering of bedrock units. Cross-faults are mapped as disruptions of the conductive units and may be either conductive or resistive.Geophysical ‘pattern mapping’ from such surveying is relatively inexpensive when compared to drilling. It increases success rates by identifying mineralised structures and fluid pathways at the early stages of an exploration program and should also be routinely applied in ‘brownfields’ project areas.

Diachronous deformation and a strain gradient beneath the Selkirk allochthon, northern Monashee complex, southeastern Canadian Cordillera

Structural relationships of granitoid rocks dated by the U–Pb method indicate that deformation was diachronous and a strain gradient exists in a 6-km-thick section beneath the Selkirk allochthon, in the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera. At high structural levels, immediately beneath a crustal-scale thrust zone that transported the allochthon eastward, a metasedimentary-dominated cover sequence was strongly affected by kilometre-scale east-verging isoclinal folds (F 1) and outcrop-scale folds (F 2) that are associated with the dominant foliation and lineation. The F 2 folding occurred, at least in part, after 58 Ma and ceased by 55 Ma. In deeper levels of the cover sequence and the underlying orthogneiss-dominated basement, F 2 folding occurred, at least in part, after 52 Ma and ceased by 49 Ma. Proterozoic dykes in the basement were locally weakly affected by D 2. These new findings require that: (i) D 2 compression youngs structurally downward, synchronous with the thermal peak of metamorphism; (ii) D 2 in deeper levels is synchronous with extension above the complex that was partly responsible for its exhumation; and (iii) a D 2 strain gradient lies between strongly deformed cover rocks and weakly D 2-deformed basement rocks. We propose a model in which rocks that were tectonised at different places and times within the orogen were juxtaposed, likely during east-verging kilometre-scale F 1 folding and shearing along the isocline limbs (a similar model was previously proposed to explain a pattern of downward younging thermal peak ages and an inverted metamorphic sequence in higher rocks). The rapid downward decrease in deformation intensity suggests that the lower limit of significant Cordilleran strain lies in the exposed basement. Cessation of deformation at this level is attributed to the fact that the basement attained elevated temperatures and began straining when the Cordilleran tectonic regime changed from compressional to extensional.

Petrogenesis of the Early Proterozoic Matachewan dyke swarm, Canada, and implications for magma emplacement and subsequent deformation

Petrogenesis of the Early Proterozoic Matachewan dyke swarm, Canada, and implications for magma emplacement and subsequent deformation

Remagnetisation and magnetic refraction in proterozoic dykes from Central Scandinavia during Caledonian deformation

Mafic dykes, related to the Central Scandinavian Dolerite Group, changed their geometry during the Caledonian orogeny from flat-lying bodies in the Autochthon to steeply inclined in the thrusted and stacked Lower Allochthon units. During tectonic transport a remagnetisation occured and the post-Jotnian paleomagnetic signature was completely obliterated. In the Bångonåive window this remagnetisation occurred at a deeper crustal level probably during metamorphism and deformation under amphibolite-facies conditions, while in the Børgefjell window it was caused by alteration in a more shallow crustal level. In both tectonic windows the remanence direction is influenced by the structure, and in the strongly anisotropic samples of the Bångonåinve window a distinct refraction of the remanence direction (up to 15°) towards the magnetic lineation is observed.

Palaeomagnetic and Low-Field AMS Studies of Proterozoic Dykes and their Basement Rocks Around Harohalli, South India

Abstract Palaeomagnetic and Low-field Anisotropy of Magnetic Susceptibility (AMS) results are reported from 8 unmetamorphosed dolerite dykes, and from one site each of the Closepet Granite (CG), the Peninsular Gneiss (PG) and the Charnockite (Ch) in the Harohalli area of the Dharwar craton. The AMS results exhibit "normal" magnetic fabrics for three dykes. Rest of the dykes exhibit either "inverse" or "anomalous" magnetic fabrics, indicating the dominance of single domain (SD) uniaxial magnetite grains and a complex mixture of SD and multi-domain (MD) grains respectively. Almost similar AMS fabrics are exhibited by the Peninsular Gneiss, Closepet Granite and Charnockite. This magnetic fabric is in close conformity with the structural features of the terrain and indicates E-W compressional stress. Two dykes have yielded a mean characteristic remanent magnetization (ChRM) direction (D = 130°, I = -88°, a95 =33°; N=2) which is similar to a previous result from the same dyke swarm and the Tiruvannamalai dykes in the adjacent South Indian Granulite Terrain (SIGT). Thus we support the earlier suggestion (Radhakrishna and Joseph, 1993) that no relative motion took place between the granite-greenstone terrain (GGT) and the SIGT, at least since the emplacement of Tiruvannamalai dykes ca. 1600 Ma ago. The palaeomagnetic directions from the PG (D = 285°, I = 43°), CG (D = 300°, I = 43°), and Ch (D = 300°, I = 43°) are statistically indistinguishable and the corresponding pole positions fall close to those reported for 1100-1000 Ma old rocks. Thus partial remagnetization/streaking in these sites occurred probably during the Eastern Ghats orogeny. It appears that the Eastern Ghats orogeny has penetrated selectively into the adjacent Dharwar craton.

40Ar/ 39Ar and K/Ar geochronology of the dykes from the south Indian granulite terrain

40Ar/ 39Ar and conventional K/Ar data on mafic dykes and a felsic dyke, together with previous data have been used to constrain the ages of the dyke magmatism in the south Indian granulite terrain. The age of the mainly ENE–WSW-trending Agali–Anaikatti swarm is estimated to be 1980±25 Ma, with a very limited occurrence of upper Cretaceous (80–90 Ma) dykes in the area. The NW–SE Dharmapuri swarm is dated as at least 1800 Ma (possibly 1855±9 Ma). Both the NW–SE and the NE–SW dyke swarms of Tiruvannamalai are dated at 1650±10 Ma. Only a few dykes in north Kerala are of middle Proterozoic (ca. 1700–1650 Ma) age. Furthermore, the 40Ar/ 39Ar results of Dharmapuri dykes exhibit age spectrum characteristic of partial degassing; they provide strong evidence of latest Proterozoic overprinting by a thermal event probably related to the Pan-African orogeny. Most of the Phanerozoic dykes are confined to the coastal region in Kerala province. The dolerites of central Kerala and the majority of north Kerala dolerites intruded during the 70–65 Ma period; whereas a large gabbro and a felsic dyke in central and north Kerala, respectively, are of upper Cretaceous age (ca. 85 Ma) and the mean 81±1.5 Ma age from these dykes may represent cooling age. The oldest Phanerozoic (Upper Jurassic; 144±6 Ma) dyke intrusions have a restricted occurrence in south Kerala, very close to the southern tip of India. The Proterozoic dykes are interpreted as being lateral intrusions from under-plating igneous bodies beneath the eastern shield region. The geological record of the terrain does not show indications of significant pre-magmatic extension and rifting, but dyke magmatism may have led to extension, rifting and the consequent development of the India–Antarctica–Sri Lanka Proterozoic high-grade terrain. The data are also interpreted to suggest that the south Indian granulite terrain is an early Proterozoic tectono-metamorphic terrane with strong thermal imprints of Neoproterozoic/early Palaeozoic age. It is difficult to reconcile its extensions into Proterozoic Sri Lanka or Antarctica where tectono-metamorphism is dated to be of Neoproterozoic age. The late Phanerozoic dykes are predominantly related to the Deccan trap event, and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the Seychelles and the Indian continent. The few older (ca. 145 Ma) dykes in south Kerala may be related to the rift tectonics that preceded the separation of Australia–Antarctica and India.

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.