Proterozoic Dykes Research Articles

Ti-rich plagioclase-phyric dykes of southern West Greenland

The investigation of Proterozoic basic dykes in southern West Greenland stemmed from the programme of systematic mapping of the Archaean craton in that region by the Geological Survey of Greenland (GGU). This work began in the southern Frederikshåb region in the early 1960s (Jensen, 1968, 1969) and progressed northwards, from bases in the Fiskenæsset (Kalsbeek & Myers, 1973; GGU, 1976), Godthåb (Allaart et al., 1977) and Sukkertoppen areas (Allaart et al., 1978). The results of most of this mapping work were summarized by Bridgwater et al. (1976) and compiled onto a 1:500 000 scale geological map sheet by Allaart (1982). The distribution of the major Proterozoic dykes which cut the entire region is shown on this map. While the basic dykes are individually minor intrusions, many are up to 50 metres wide and continuous for several tens of kilometres, and collectively they represent a major magmatic event. As many of the Archaean terrains of the world possess Proterozoic basic dyke swarms, their compositions are crucial to a correlation of events from one craton to another and to an understanding of crustal and mantle evolution after the world-wide late Archaean sialic crust-forming event.

Palaeomagnetism of middle Proterozoic (c. 1.25 Ga) dykes from central North Greenland

From a nunatak in central North Greenland (81.5°N, 44.7°W) nine sites of Middle Proterozoic basic dykes, cutting Archaean basement, were palaeomagnetically investigated. After AF and thermal cleaning the nine dyke sites and three adjacently baked gneiss sites give a stable characteristic remanent mean direction of D = 265°, I = 21.5° (N= 12, α95= 5.6°), the direction being confirmed by a detailed and positive baked contact test. The polarity of the dykes in the nunatak area is opposite to that of the Zig-Zag Dal Basalts and the Midsommersø Dolerites in eastern North Greenland some 200–300 km away, the volcanics of which are assumed to be of similar age (about 1.25 Ga). The remanent directions of the two sets of data are antiparallel within the 95 per cent significance level of confidence. When rotating Greenland 18° clockwise back to North America by the ‘Bullard fit’, the pole of the central North Greenland dolerites (NDL) falls at (14.3°N, 144.3°W). The reversed pole (14.3°S, 35.7°E) fits well on to the loop between 1.2 and 1.4 Ma on the apparent polar wander swath of Berger & York for cratonic North America. The palaeomagnetic results from the Middle Proterozoic basic dykes from central North Greenland thus strengthen previous palaeomagnetic results from the Midsommersø Dolerites and Zig-Zag Dal Basalts from the Peary Land Region in eastern North Greenland, suggesting that Greenland was part of the North American craton at least for the period between c. 1.3 and 1 Ma (and probably up to the end of Cretaceous time). The major geographical meridian of Greenland was orientated approximately E–W, and the palaeo-latitude of Greenland was about 10°–15°.

Lamproites and other potassium-rich igneous rocks: a review of their occurrence, mineralogy and geochemistry

Summary In this paper the geological occurrence, geochemistry and mineralogy of ultrapotassic (K 2 O/Na 2 O > 3 (molar ratio)) and perpotassic (K 2 O/Al 2 O 3 > 1 (molar ratio)) igneous rocks, especially lamproites, are reviewed and discussed in the context of compositionally-similar mantle-derived melts. Lamproites are K- and Mg-rich igneous rocks (typically K 2 O > 5 wt.%, MgO > 5 wt.%) which possess an exotic and diagnostic mineralogy and geochemistry. Known lamproites occur in 21 major suites or localities in continental regions with a variety of geological and tectonic environments; they range in age from the early Proterozoic dykes at Holsteinsborg, W Greenland, and Chelima, India, and Precambrian pipe at Argyle, W Australia, to the Middle Pleistocene flows and the Recent volcanics of the Leucite Hills, Wyoming, and Gaussberg, Antarctica, respectively. Intrusive and extrusive forms of lamproites include flows, a variety of pyroclastics (welded tuffs, piperno, air-fall tuffs, volcanic breccias etc.), cinder cones, dykes, sills and diatremes. Whereas kimberlite diatremes tend to be carrot shaped, the shape of olivine lamproite diatremes approximates a sherbet-glass. The recent discovery of diamondiferous lamproties of large volumetric proportion in the E and W Kimberleys, NW Australia, and the reclassification of the diamondiferous micaceous peridotite at Prairie Creek, Arkansas, as a lamproite substantiate their economic importance. The 21 lamproite suites considered here tend to be localized marginal to continental craton cores in areas that overlie fossil Benioff zones, in contrast with the general occurrence of kimberlites more interior to continental cratons. The petrographic diversity of lamproites has historically hindered the development of a concise and universal classification and nomenclature. Lamproites are distinguished from kimberlites and alkali basalts (and lamprophyres) in terms of mineralogy, mineral chemistry, geochemistry and volcanic extrusive character. Relative to kimberlites, lamproites are enriched in K, Si, Ti, Al, Rb, Sr, Zr and Ba and depleted in CO 2 , Ca, Mg, Fe, Ni, Co and Cr. Lamproites are characterized by the general presence of phlogopite, diopside, leucite and K-richterite, occasional glass, olivine, sanidine, priderite, perovskite, wadeite, apatite and chrome spinel, and very rare ilmenite. Lamproite amphiboles, diopsides and phlogopites are distinctly depleted in Al 2 O 3 relative to those of nearly all other igneous rocks. Lamproite magmas are produced by the partial melting of old refractory mantle peridotite (approaching a dunite or harzburgite in mineralogy) that was enriched in K-bearing and other incompatible-element-enriched phases, such as phlogopite and apatite, most probably as a result of some metasomatic event which occurred prior to melting. In contrast with alkali basalt and kimberlite melts which are apparently produced from the partial melting of a CO 2 -enriched mantle periodtite (i.e. a source with a relatively high CO 2 /H 2 O ratio), water is the key volatile species involved with lamproite petrogenesis (source with a low CO 2 /H 2 O ratio).

The boundary between Proterozoic and Archaean crustal blocks in central West Greenland and northern Labrador

Summary The contact between the Early Proterozoic Nagssugtoqidian Mobile Belt and the Archaean craton in West Greenland, and that between the Churchill Province and the Archaean (Nain) craton in Northern Labrador represent two cross-sections of an Early Proterozoic orogenic boundary. The Archaean cratons in both regions display the peripheral effects of Proterozoic orogenic events. The best indicators of Proterozoic effects on the Archaean cratons are the Late Archaean-Early Proterozoic dykes and the Early Proterozoic supracrustal rocks which unconformably overlie the gneisses and dykes. In Greenland the boundary zone evolution is characterized by Late Archaean (Nag. 1) transcurrent shearing, Early Proterozoic Kangâmiut dyke intrusion and subsequent (Nag. 2) ductile overthrusting. Ductile overthrusts are characteristic of the Nagssugtoqidian-Archaean boundary and attest to substantial shortening of the crust in this region. In Labrador the boundary region has been divided into several distinct zones. Although facets of its evolution are analogous to the boundary in Greenland there are several fundamental differences, the most important of which is that transcurrent movement appears to have been dominant over ductile overthrusting. These differences are provisionally related to contrasting tectonic settings of the two areas during Early Proterozoic continental collision, the Greenland segment being the frontal collision zone whilst the Labrador segment documents slightly oblique transcurrent motion.

A boninitic dyke in the eastern Sukkertoppen region: geochemistry of the boninitic-norite dyke swarm of southern West Greenland

Large early Proterozoic (c. 2100 Ma) basic dykes occur throughout the Archaean craton of southern West Greenland (Allaart, 1975, 1982; Bridgwater et al., 1985; Kalsbeek & Taylor, 1985) and have become known as the MD (meta-dolerite) dykes (Bridgwater et al., 1976). Four generations (MD1, MD2, MD3a and MD3b) can be distinghushed by their mutuaIly discordant relationships and their progressively evolved geochemistry along an apparently common tholeiitic trend (Rivalenti, 1975; Hall et al., 1985). However, the earliest generation comprises two distinct lithological types, one noritic and the other doleritic. Noritic dykes predominate in the eastern Sukkertoppen region (cf. Garde et al., 1983), while dolerite dykes become increasingly abundant towards the south. A few N-S trending noritic dykes extend southwards through the eastern Godthåbsfjord and Fiskenæsset regions and similar dykes occur rarely in the Frederikshåb region (Andrews, 1973, personal communication, 1985). Textural and geochemical evidence presented here from a quenched variety of the norite dykes demonstrates that these dykes crystallized from magmas with strong boninitic affinities, as has also been suggested on geochemical grounds for other dykes from the same swarm by Bridgwater et al. (1985), and that they are not genetically related to the dolerites of the tholeiitic MD swarm. In view of this evidence, and since these noritic dykes are neither metamorphosed nor dolerites, it is suggested that they should no longer be considered to be part of the MD dyke swarm, but that they warrant a new stratigraphic name.

Silver deposits associated with the Proterozoic rocks of the Thunder Bay District, Ontario

The silver deposits to the immediate north and west of Lake Superior are divided into three groups. The Mainland veins, the largest group, occur along a zone of normal faults near the western margin of the Proterozoic rocks of the Southern Province. The most economically productive deposits, the Island group, arc in or very near a northeast-trending swarm of gabbro dykes lying immediately offshore the northwestern shore of Lake Superior. The third group occurs near the western margin of the Port Coldwell alkalic complex; these veins are in a shear zone that cuts both Archean metasedimentary rocks and a Proterozoic diabase dyke. The Mainland deposits occur in the Rove shale, immediately below the contact with Logan diabase sills. The veins locally extend upwards into the sills, but the silver-bearing portions, consisting of acanthite and native silver associated with base-metal sulphides, fluorite, barite, quartz, and calcite, are largely bounded by locally silicified shale. The Island veins, typified by the Silver Islet mine, are in fractures perpendicular to the gabbro-dyke host rocks. These veins contain both native silver and acanthite, associated with a Ni–Co sulpharsenide suite and the same mineral assemblage as the Mainland deposits. The veins near Coldwell are rich in sphalerite and galena.Lead-isotope data indicate that the Mainland and Island veins are genetically related and that the Mainland veins formed from an inhomogeneous fluid. Two-stage calculations indicate an early Proterozoic source rock, possibly the Rove shale. The Island veins are more isotopically homogeneous, and their metals may have been derived partially from the gabbro. The Coldwell veins contain lead that is less radiogenic than that of the other two groups and is possibly derived from the adjacent Archean rocks. All three groups of deposits have isotopic compositions that are much less uranogenic and more thorogenic than the nearby Pb–Zn–Ba veins of the Dorion area. Preliminary fluid-inclusion data from the Mainland veins indicate that deposition occurred from a fluid whose temperature varied from approximately 200 °C to more than 400 °C; deposition occurred during boiling induced by adiabatic expansion of the fluid at relatively shallow crustal depths. The Mainland veins developed in the shale (rather than the diabase), as its high fissility, and hence permeability, made it susceptible to intense fracturing by the expanding fluid. Both the Mainland and Island groups were deposited in structures formed dominantly by listric normal faulting during late stages of intracontinental rifting. Heat was supplied by abundant mafic intrusions that formed coincident with rifting. The ore fluid was probably formed as a result of metamorphic dewatering, with metals released to the fluid because of silicate and sulphide recrystallization.

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.