Archaean Cratons Research Articles

Archaean deformation patterns in Zimbabwe: true indicators of Tibetan-style crustal extrusion or not?

Abstract Recent models of the structural evolution of the Zimbabwe Archaean craton have stressed the role of WSW-trending sinistral strike-slip shears in accommodating the WSW-directed extrusion of crust thickened in response to NNW-SSE compression and shortening. This relationship between crustal thickening and crustal extrusion is analogous to the structural response of Tibetan crust to the Tertiary collision of India and Asia. We have re-examined many of the structures of the Zimbabwe craton and find the extrusion model largely untenable. Instead, a conjugate set of ESE-striking dextral shears and NNE-striking sinistral shears have been identified, on both regional and outcrop scale, which apparently reflect part of the structural response of the Zimbabwe crust to NNW-SSE shortening resultant from collision of the craton with the Central zone of the Limpopo belt to the south. This collision, at about 2.58 Ga, post-dated the 2.68 Ga aged collision between the Central zone and the Kaapvaal craton which represents the Limpopo orogeny sensu stricto. The suture between the Zimbabwe craton and the Central Zone was re-activated at c. 2.0 a as a major intracontinental dextral strike-slip shear zone. Partial melting of the lower crust at c. 2.58 Ga accompanied thickening of the Zimbabwe Archaean crust following collision with the Central zone. The conjugate shears deform fabrics that date from that collision, are synchronous with, or post-date, granite emplacement and may have acted as conduits for transport of the Chilimanzi monzogranitic magmas from the lower to the upper crust. Contacts between greenstone belts and granites are commonly sheared. These marginal shear zones are often parallel to the regional conjugate shears, although local variations in stress trajectories imposed by pluton emplacement may result in them being non-parallel to the major shears.

Early Precambrian crustal development: changing styles of mafic magmatism

Abstract The work of Sutton & Watson (1951) was one of the cornerstones in the interpretation of the evolution of early Precambrian (Archaean and early Proterozoic) gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted Archaean cratons. Since then, the concept of geological terranes and terrane accretion has been extended back to the Archaean, and different events can be distinguished by increasingly precise isotopic dating techniques. There is growing evidence that, with certain clear provisos, the principle of uniformitarianism can be applied as equally to the early Precambrian as it can through the rest of the geological record. In terms of mafic magmatism, there are similarities between some ancient and modern suites, both intrusive and extrusive, but there are also some major differences, such as the presence of komatiites in the Archaean and the formation of large-scale layered intrusions and extensive dyke swarms at the end of the late Archaean and in the early Proterozoic. The mantle has evolved continuously with the progressive extraction of mafic magmas but, allowing for this, continental tholeiites appear to have changed little with time. Ancient mid-ocean ridge basalts are elusive because of their poor preservation record. The geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present-day arc systems. The relative abundance of early Precambrian intrusive mafic suites parallels the crustal growth curve, reflecting the global crustal accretion-differentiation superevent towards the close of the Archaean. Bimodal tholeiitic-noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle-mafic crust-continental crust system. This, in turn, gives a possible link between the respective evolutions of Archaean to Recent komatiitic, noritic and boninitic suites.

Origin of felsic sheets in the Scourian granulites: new evidence from rare earth elements

Synopsis Felsic sheets within the Scourian granulites of NW Scotland are shown to be granites, trondhjemites and tonalites. Their origin has been extensively debated. Previously they have been considered to be either a product of anatexis of Scourian gneisses during granulite facies metamorphism, or part of a Scourian igneous complex which predated the granulite facies metamorphism. The trace element geochemistry of the felsic sheets is, however, incompatible with an anatectic origin. It is proposed that the felsic sheets were formed by the partial melting of a mafic source at > 15 kbar and that their origin is linked to that of tonalite–trondhjemite–granodiorite suites common in Archaean cratons.

Neoproterozoic sinistral displacement along the Darling Mobile Belt, Western Australia, during Gondwanaland assembly

Evidence is provided for Neoproterozoic sinistral displacement along the Darling Mobile Belt between Greater India and Precambrian terranes of Western Australia. A dilatational bend in the shear zone is thought to have controlled emplacement of alkali granites, anorthosite and mafic intrusives of the Leeuwin Complex which was deformed during progressive deformation. Faults formed in this event extend across the Albany Mobile Belt, and control the formation of the Stirling Basin on the southern margin of the Yilgarn Craton. The regional extent of this event and the rotation of Mesoproterozoic trends in the Albany Mobile Belt through 90° suggests that the Darling Mobile Belt may be part of a shear system of regional importance during assembly of proto-East Gondwanaland. Much attention is being focused on the assembly of Gondwanaland and the relative displacements between Archaean cratons along Proterozoic mobile belts (e.g. Powell 1993 ; Unrug 1994 ; Yoshida 1994 ). Southwestern Australia is a key area within proto-East Gondwanaland as, in reconstructions such as de Witt et al. 1988 and Powell et al. 1988, it constitutes the junction between the Archaean Yilgarn Craton of Western Australia, India/Greater India, and the East Antarctic Shield (Fig. 1). This paper examines the timing and nature of displacements along the Darling Mobile Belt (Glikson & Lambert 1973; Mathur & Shaw 1982) on the western margin of the Yilgarn Craton and contemporaneous structures within the Albany Mobile Belt and southern Yilgarn Craton (Fig. 2). The Darling Mobile Belt. The Darling Mobile Belt, referred to by Myers (1990) as the

Diversity of 1.8 Ga potassic granitoids along the edge of the Archaean craton in northern Scandinavia: a result of melt formation at various depths and from various sources

The edge of the Archaean craton in northern Scandinavia had been intensively reworked during the Svecofennian orogeny 1.93-1.86 Ga ago and was subsequently intruded by potassic granitoids of 1.79–1.80 Ga age. Despite similar or even identical ages and overlapping areas of occurrence, these rocks belong to two different groups, the Edefors and Lina granitoids, which have contrasting geochemistries and SmNd isotopic characteristics. The Edefors granitoids range from syenites to granites, and are alkali-rich and distinctly metaluminous. They crystallized from dry magmas. This is indicated by the scarcity of pegmatites and aplites. The contacts to older rocks are often distinct, but gradual transitions to Lina-type granitoids are common. The Edefors granitoids have high contents of Zr but not of elements such as Y, REE, Ta and Nb, and have low Mg Mg+Fe ratios. They also frequently have positive Eu anomalies, even in the quartz rich varieties. Initial ϵ Nd values range from −2.1 to +1.4, indicating that the Edefors granitoids were formed by the mixing of mantle-derived magmas and continental crustal materials. The amount of crustal component was probably less than 35% in most cases. The Lina granitoids are accompanied by abundant pegmatites and aplites. Ghost structures and remnants of country rock are common. True granites predominate, but also quartz monzonites occur. The content of HFS elements is low and the Mg Mg+Fe ratios are higher than in the Edefors granitoids. Initial ϵ Nd values range from −9.3 to −3.7, reflecting a significant portion of Archaean Nd in the source materials. The Lina granitoids are largely the result of remobilisation of continental crust with a small input of juvenile material. However, the dominant source for these crustally derived granitoids are c. 1.9 Ga old granitoids. These carry a large proportion of Archaean Nd. The most probable environment of the formation of potassic migmatite granitoids, such as the Lina type, is a collision zone between two masses of felsic crust (e.g. arc-continent or continent-continent), but the details of such a collision in the Baltic Shield remain to evaluated. The formation of the Edefors granitoids could have been associated with an extensional zone developed due to delamination caused by separation of the down-dip oceanic lithosphere from the continental lithosphere.

The Archaean/Proterozoic contact zone in West Africa: a mountain belt of décollement thrusting and folding on a continental margin related to 2.1 Ga convergence of Archaean cratons?

Lithotectonic and metamorphic studies in various parts of the Man Shield, and in particular at the periphery of the Archaean nucleus, have shown that the contact zone between the Archaean and the Palaeoproterozoic Birimian terranes is a major thrust zone. The thrusting took place during the earliest stage (D1) of the Palaeoproterozoic Birimian tectonic and metamorphic evolution. Lithologic, sedimentologic, tectonic and geochronologic studies together provide a model of progressive D1 deformation and thickening reflecting progressive tectonic accretion of Proterozoic rocks onto the Archaean nucleus. In particular, the sediments of part of the lower Birimian (B1) were being deposited while the earlier lithotectonic pile was being progressively built up through thrusting (early D1). These Birimian B1 flysch-type sediments, derived from erosion of the domain undergoing thickening, were themselves later accreted through thrusting and folding (late D1), which implies progressive burial of the earlier terrane by continued tectonic stacking. This is confirmed by prograde early metamorphic evolution (under increasing pressure) with the transition andalusite-kyanite and the late-tectonic development of staurolite-kyanite assemblages. The tectonic and metamorphic evolution of the thrust belt is characteristic of a Precambrian mountain belt that underwent: (1) a collisional stage, with important crustal thickening and prograde metamorphic evolution, syntectonic granitization, and late-kinematic migmatization when sillimanite developed at the expense of kyanite; (2) a late-orogenic stage characterized by (a) strong granitization and locally intensified migmatization with doming, (b) retrograde metamorphism, with the development of andalusite or cordierite at the expense of sillimanite marking a lowering of pressure, related to isostatic and tectonic uplift, the latter in turn related to transcurrent tectonism, and (c) numerous post-D1 intracontinental volcanic areas (tholeiitic, locally komatiitic, magmatism), developed in a distensive or transtensive tectonic setting (of Basin and Range style?) during the uplift and before the paroxysmal transcurrent activity, and associated molasse basins locally coeval with the latter calc-alkaline rocks. The interpretation proposed for this belt is “a belt of decollement thrusting and folding on a continental margin” that was built ∼ 2.15-2.10 Ga during a period of convergence. This implies subduction for which geochemical evidence exists in Palaeoproterozoic mafic rocks of the northern Man Shield. This D1 mountain belt appears to form part of a broad orogenic domain characterized by thrusting related to a global eastward displacement of the Archaean Guapore, Man and Reguibat terranes. The cause of the displacement could have been subduction along a suture, a segment of which has been indicated in the northeast of the orogenic domain.

The Kibaran of southern Africa: Tectonic evolution and metallogeny

The tectono-magmatic and metallogenic evolution of the ∼ 1100 Ma (Late Kibaran) high-grade Namaqua-Natal metamorphic belt is reviewed and compared with that of the Grenville Province and the Kibaran Belts of central and eastern Africa. In Namaqualand, 1300–1600 Ma supracrustal rocks are recognised, including arc-related sequences in the east and extensional foreland basin sequences in the west. These were laid down either on a heterogeneous basement of Eburnian (∼ 2000 Ma) age or on juvenile oceanic crust, prior to Late Kibaran deformation, metamorphism and multiple plutonic intrusion. The pre-Namaqua basement infrastructure has been recognised largely in the eastern and NW parts of the orogen, adjacent to areas of exposed Eburnian crust and probably only the Areachap Terrane in the east comprises juvenile Kibaran crust. In Natal, ∼1400 Ma, predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the ∼ 1050 Ma tectonothermal event. The northern terrane is interpreted as an ophiolitic assemblage, obducted to the NE onto the Kaapvaal Craton, whereas the southern domains are predominantly arc-related. No older basement is recognised and the entire belt comprises juvenile crustal material, none of which is apparently older than ∼ 1400 Ma. The kinematic evolution of the entire Namaqua-Natal Belt is comparable. In both areas, early SW- and NE- d directed thrusting and nappe emplacement during collision, were followed by pervasive steep transcurrent ductile shearing. In Natal. the later shearing event gave rise to major WNW-trending oblique sinistral transcurrent “steep belts”, whereas the same event in Namaqualand is manifested as major, subvertical, NW-trending dextral transcurrent shear zones. The difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent Archaean craton margin, within a progressive, consistently oriented, NE-SW directed zone of plate convergence. In eastern Namaqualand and Namibia, the transcurrent shearing gave rise to a north-propagating stricke-slip rift system in which the volcano-sedimentary Koras-Sinclair-Ghansi sequences were deposited. Mineral cooling ages of ∼ 950 Ma, reported from throughout the belt, are considered to signal the end of the Kibaran cycle in southern Africa. The metallogenic evolution is discussed in the light of the tectono-magmatic model presented. Economically important stratiform volcanic and sedimentary exhalative base-metal occurrences are extensively associated with early supracrustal sequences throughout the belt. Other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites, which characterise the younger rocks in both Natal and Namaqualand. The metallogenesis of the dominantly convergent-collisional Namaqua-Natal Belt is similar to that seen in the Grenville Province, but distinct from, and younger than, the lower grade, strike-slip basin-related Kibaran Belt of central and eastern Africa.

Implications of UPb ages of columbite-tantalites from granitic pegmatites for the Palaeoproterozoic accretion of 1.90–1.85 Ga magmatic arcs to the Baltic Shield

The Palaeoproterozoic growth of the Baltic Shield involved the accretion of several ∼1.90–1.85 Ga old magmatic arcs to the southwest of the Archaean craton and the deformation and migmatization of sedimentary basins located between the arcs. During crustal thickening after the collision of the arcs, the sedimentary basin fill became intruded by peraluminous two-mica granites. Locally, columbite-bearing pegmatites are genetically associated with these granites. Columbites from lithium-cesium-tantalum-type (LCT-type) pegmatites from the Stockholm area (Sörmland gneisses) yield UPb ages at 1815–1820 Ma, while niobium-yttrium-fluorine-type (NYF-type) pegmatites from the same area are younger (1795±2 Ma, 2σ). Farther to the north, LCT-type pegmatites from the central Bothnian Basin, that correspond geochemically and mineralogically to those of the Stockholm area, yield UPb columbite ages at 1795–1800 Ma, while LCT-type pegmatites in the sedimentary basin between Skellefteå and Luleå yield less well constrained UPb columbite ages at 1765–1775 Ma. LCT-type pegmatites are mainly associated with crustal melts that form during postcollisional thickening of continental crust. They represent markers for the time when the Palaeoproterozoic Baltic Shield suffered sufficient thickening to yield voluminous anatectic melts. The UPb columbite ages from the LCT-type pegmatites indicate that comparable phases of post-collisional crustal thickening of the Svecofennian area of the Baltic Shield occurred later to the north.

The characterisation and origin of graphite in cratonic lithospheric mantle: a petrological carbon isotope and Raman spectroscopic study

Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13 CPDB = − 12.3 to − −3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.

Sinistral transpression and hydrothermal activity during emplacement of the Early Proterozoic Julianehåb batholith, Ketilidian orogenic belt, South Greenland

The first systematic investigations of the central part of the Early Proterozoic Ketilidian orogen in the vicinity of Søndre Sermilik in the early 1960s suggested that this part of the orogen comprised a mixture of the Julianehåb granite, altered supracrustal rocks and older orthogneisses. Recent field work has shown that the area consists only of a variably deformed suite of granitic to dioritic plutonic rocks and a range of hornblende-bearing dykes of the appinite suite which all belong to the Julianehåb batholith. Steep to vertical shear zones with widths from a few centimetres to more than one kilometre are a significant element of the structure. The principal shear zones trend north-east and they are parallel to the schistosity and subhorizontal linear structures in the granitoid rocks. Kinematic indicators in many of the shear zones indicate sinistral transcurrent displacements. The relationships between granite fabrics, shear zones and mafic dykes suggest that the Julianehåb batholith was emplaced during subduction from the south towards the Archaean craton in the north-west in a sinistral transpressional system. Effects of hydrothermal alteration, mainly in the form of quartz veining, silicification, chloritisation, epidotisation and pyritisation, are common within and adjacent to the largest shear zones. These effects are believed to be related to late stages of the evolution of the batholith. Gold anomalies appear to be closely tied to the hydrothermal phenomena.

Reconstruction of Crustal Genesis in a High-Grade Gneiss Terrane Using Sm/Nd-Whole Rock Systematics and Pb-Isotopes of Leached Feldspar

Sm/Nd whole rock and Pb-feldspar data each provide important information on the history of old continental crust. However, the combination of both data sets allows new insights that prove most valuable in high-grade and polymetamorphic terranes, where a lot of information is lost during metamorphism. The data set from Tanzania comprises granitoid rocks from the Archaean craton and granulite and eclogite facies rocks from the polymetamorphic Mozambique Belt (MB) in Eastern Tanzania. The evolution of the Mozambique Belt in eastern Africa (Holmes 1951) has been a matter of discussion for a long time, with essentially two contrasting models: the older one proposing an emialic orogeny with reworking of older crustal material (Watson 1976; Krrner 1980) while the second, new model is based on plate tectonics involving continent-continent collision (Shackleton 1976; Muhongo 1989) with generation of new ernst. However, the amount of new crust added during metamorphic events to pre-existing Archaean material and its distribution within the metamorphic belt was hitherto unknown. Some U-Pb zircon data on the granulites showed either evidence for Proterozoic (Maboko et al., 1985) or Archaean (Coolen 1982) precursors. Several zircon samples just yielded upper intercepts around 700Ma that were interpreted as the age of Panafrican metamorphism. Although Nd model ages are not unequivocal in their interpretation due to mixing effects in the ernst and possible fractionation of the Sm/Nd ratio during high-grade metamorphism, they provide first order constraints on age provinces within metamorphic belts. Whereas the Sm/Nd ratio suffers most severe fractionation during melt extraction from the mantle i.e. the forming of new crust, changes in Th/Pb and U/Pb ratios occur afterwards during crustal processes such as metamorphism, sedimentation and weathering. Pb isotope systematies of leached feldspars from the same samples, therefore, give complementary results to Nd systematics because they reveal the time integrated effects of the crustal history and preserve information on the time of present parent-daughter fractionation.

Delineation and character of the Archaean-Proterozoic boundary in northern Sweden

Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used SmNd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative ϵ Nd( t ) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ∼ 100 km farther to the north. The ∼ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ∼ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ∼ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.

Electrical conductivity distribution of the lithosphere in the central Fennoscandian Shield

Abstract Geoelectric information on the crustal structures of the central Fennoscandian Shield consists of magnetovariational (MV) and magnetotelluric (MT) data. MV-data are obtained from magnetometer arrays devised to map lateral variations of subsurface resistivity; MT-data are obtained from soundings yielding knowledge on vertical resistivity distribution in the form of cross-sections. The electromagnetic data base for Fennoscandia includes data from about 350 magnetometer sites and about 600 magnetotelluric soundings. The results show that the crustal resistivity of the Archaean and Palaeoproterozoic crust varies greatly both laterally and vertically. The model resistivities range from several tens of thousands of Ωm to below 1 Ωm, which are extreme values amongst those normally observed in crustal rocks. Several large elongated and inclined crustal conductors, which mark fossil tectonic boundaries, have been discovered in the Shield. These include: (1) the Lake Ladoga, Outokumpu, Kainuu Schist Belt, and Oulu conductors that form a discontinuous conductive boundary between the domains of Archaean and Palaeoproterozoic crust; (2) the Southern-Finland conductor caused partly by nearly vertical, several kilometres deep graphite-bearing metasedimentary layers that were tilted and deeply buried in a Svecofennian collision; (3) the Kokkola, Bothnian, Skelleftea, Storavan and Oulu conductors representing a collisional belt in a northwards accreted Palaeoproterozoic terrain; and (4) the Lapland Granulite Belt conductors imaging a sequence of sheared thrust sheets that developed as the granulites were upthrusted southwestwards onto the Archaean craton. Zones of enhanced electrical conductivity delimit more resistive and homogeneous crustal blocks including the Archaean Kuhmo block in eastern Finland and the Svecofennian Central Finland Granitoid Complex (CFGC). Highly resistive upper crust ( > 30,000 Ωm ) in the CFGC coincides with the granitoid intrusions. Groundwater, which fills the fractures and pores of the granitoids, is non-conductive (100 Ωm) and is thus not able to enhance the bulk conductivity. Therefore, the resistivity of the upper crust approaches that of dry rocks. At depths of 10 to 14 km, a decrease in resistivity by one order of magnitude coincides with a seismic low velocity (vP) zone that appears related to a porous fractured zone filled with saline water. A lower crustal conductor beneath the CFGC at depths of 30 to 40 km has a thickness of 10 to 15 km and a resistivity of 20 to 100 Ωm. It is most probably caused by graphite that can have been precipitated from CO2-rich fluids during the peak metamorphism of the Svecofennian orogeny and the subsequent cooling period. Free fluids trapped in the lower crust appear less plausible. Magnetotelluric soundings in the resistive CFGC in the central Fennoscandian Shield have not provided indications of a well-developed electrical asthenosphere within the uppermost 300 km.

Svecofennian detrital zircon ages—implications for the Precambrian evolution of the Baltic Shield

Metasediments intruded by 1.90-1.87 Ga old plutonic rocks form the oldest major Proterozoic crustal component in the Svecofennian Domain of the Baltic (Fennoscandian) Shield. Their NdTDM model ages and conventional multigrain zircon UPb ages between 2.4 and 2.1 Ga have previously been interpreted either as mixing ages between ∼ 1.9 Ga old juvenile materials and a minor Archaean component, or as actual rock and protolith ages. To resolve the ensuing controversy, 120 individual detrital zircons from Svecofennian metasediments in Sweden and Finland were analysed using the SHRIMP ion microprobe. The oldest materials in this array are a 3.44 Ga old zircon from the Tampere Schist Belt in Finland and a 3.32 Ga old crystal from southeastern Sweden. About 30% of the analysed crystals are 2.97-2.60 Ga old, while ∼ 65% have ages between 2.12 and 1.88 Ga. Thus there is no evidence of 2.6-2.1 Ga old protoliths, but the age range of the Proterozoic zircons indicates that a major area of 2.1-1.9 Ga old crust was in erosional position 1.9 Ga ago. This implies that the formation of Palaeoproterozoic crust in the Baltic Shield or its one-time close neighbourhood must have commenced 100–200 Ma earlier than hitherto assumed. In conjunction with previously obtained isotopic data, the youngest detritus ages of the present study constrain the age of Svecofennian sedimentation. It can also be concluded that the Archaean zircons found in quartzites from southern Sweden may have been derived from source areas to the southwest of the central-Svecofennian marine depositional basin, the so-called Bothnian Basin, separating southern Sweden from the Archaean craton in the northeastern part of the Shield.

Petrochemical and petrological characteristics of 1.9 Ga old volcanics in northern Sweden

Abstract Palaeoproterozoic, ∼ 1.9 Ga old, subaerial felsic to mafic volcanics of the Kiruna-Arvidsjaur Porphyry Group (KAPG) are abundant in northernmost Sweden. The felsic-dominated KAPG is situated north of a proposed north-dipping subduction zone along the boundary between the Archaean craton in the northeastern part of the Baltic Shield and juvenile Proterozoic crust in the south. The geochemistry of the volcanics indicates that plate-tectonic and volcanic-arc processes similar to recent ones occurred in northern Sweden ∼ 1.9 Ga ago. The volcanics were formed in extensional as well compressional environments. Calc-alkaline volcanics mark compression in the south and in the east. Mildly alkaline volcanics occurring in the west were formed in an extensional setting. The same rift zone gave rise to a basin filled with a thick sequence of younger sediments. On the basis of regional differences in geochemistry and petrology, the KAPG has been divided into the Kiruna, Arjeplog. Arvidsjaur and Lulea subprovinces. The volcanics in the Skellefte ore district south of the KAPG occurrence area have a volcanic-arc character and can be related to the volcanics of the Arvidsjaur subprovince immediately to the north; however, the latter are subaerial whereas the Skellefte volcanics represent a marine facies. The northerly Kiruna subprovince represents a different tectonic environment with a mafic to intermediate, trachytic volcanism that indicates extensional environments. The pattern of the different geotectonic settings indicates that the development of the ∼ 1.9 Ga old volcanics cannot be assigned solely to the action of a subduction zone dipping northwards beneath the Skellefte district.

Chronology of Proterozoic orogenic processes at the Archaean continental margin in northern Sweden

Abstract The Proterozoic Svecofennian orogeny was characterized by the rapid formation of great amounts of juvenile continental crust and extensive remobilization of older crust. This paper considers UPb zircon ages and some geochemical and lithostratigraphic features of the Svecofennian in the continental-margin area between the Skellefte ore district and the boundary of the Archaean craton farther north in Sweden. New UPb zircon ages constrain the igneous activity of Svecofennian crust formation in northern Sweden to between 1930 and 1870 Ma ago. The same time constraints apply also to crust formation farther east in Finland. Orogeny along the entire Archaean-Proterozoic boundary zone was thus simultaneous and does not represent an east-to-west event succession. It is argued that rocks with similar major element compositions but distinctly different trace element characteristics were formed simultaneously but in different plate-tectonic environments. By ∼ 1875 Ma ago, the Svecofennian volcanic arc had matured and a variety of syn- to late-orogenic igneous rocks appeared in both tensional and compressional settings. Shortly thereafter, the Svecofennian magmatic activity ceased altogether, probably as a result of collision between the arc and the Archaean continent in the north. It is also suggested that pre-Svecofennian rifting of the Archaean craton had created a passive continental margin and that the transition to an active margin with subsequent island-arc magmatism and subduction beneath the Archaean craton commenced prior to 1930 Ma ago. It may well have been initiated by a relative drift of the Archaean craton towards the present southwest as a consequence of the formation of the collisional Proterozoic Lapland Granulite Belt in the northernmost Baltic Shield.

Early Precambrian crustal development: changing styles of mafic magmatism

The work of Sutton & Watson (1951) was one of the cornerstones in the interpretation of the evolution of early Precambrian (Archaean and early Proterozoic) gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted Archaean cratons. Since then, the concept of geological terranes and terrane accretion has been extended back to the Archaean, and different events can be distinguished by increasingly precise isotopic dating techniques. There is growing evidence that, with certain clear provisos, the principle of uniformitarianism can be applied as equally to the early Precambrian as it can through the rest of the geological record. In terms of mafic magmatism, there are similarities between some ancient and modern suites, both intrusive and extrusive, but there are also some major differences, such as the presence of komatiites in the Archaean and the formation of large-scale layered intrusions and extensive dyke swarms at the end of the late Archaean and in the early Proterozoic. The mantle has evolved continuously with the progressive extraction of mafic magmas but, allowing for this, continental tholeiites appear to have changed little with time. Ancient mid-ocean ridge basalts are elusive because of their poor preservation record. The geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present-day arc systems. The relative abundance of early Precambrian intrusive mafic suites parallels the crustal growth curve, reflecting the global crustal accretion–differentiation superevent towards the close of the Archaean. Bimodal tholeiitic–noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle–mafic crust–continental crust system. This, in turn, gives a possible link between the respective evolutions of Archaean to Recent komatiitic, noritic and boninitic suites.

Deep seismic profile across a Proterozoic collision zone: surprises at depth

COLLISIONS between continents generate the world's most striking mountains; the deep structure of these collision zones, inferred from seismic reflection profiles1–3, can be equally dramatic, if less well understood. Improved knowledge of the structure of ancient collisional belts, now known to have developed through plate tectonic processes4,5, can provide important new insights into the mechanisms of crustal growth. Although many such belts have been partly destroyed by severe post-collisional deformation6,7 or magmatism8, some have been preserved in the stable interiors of continents9: one example is the Early Proterozoic Trans-Hudson orogen9,10, in western Canada. Here we present seismic reflection images across the entire orogen, which reveal a broadly symmetric structure, with reflections linked to island-arc rocks dipping beneath both bounding Archaean cratons. The observed reflection geometries imply a doubling of crustal thickness during the collision, along crust-penetrating faults. The presence of a significant amount of Archaean crust at depth, the extent of which could not have been determined from surface mapping, suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11,12.

Seismic and geoelectric evidence for collisional and extensional events in the Fennoscandian Shield implications for Precambrian crustal evolution

Extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles, normal-incidence reflection profiles, several hundred magnetotelluric soundings, and several magnetovariational array observations. Results are compiled into four maps: (1) Moho depth; (2) thickness of the lower crustal high-velocity layer ( V p > 7 km/ s); (3) depth to the upper interface of the high-velocity layer; and (4) distribution of upper and middle crustal electrical conductors. The Moho depth is approximately 40 km in the Archaean Domain and varies between 40 and 65 km in the Svecofennian Domain. Most of the thickness variation (0–24 km) is concentrated in the lower crustal high-velocity layer. Electrically, the crust is formed of several rather homogeneous, either resistive or conductive, blocks that are bounded by upper and middle crustal conductive zones. The cratonized Archaean crust experienced an extensional phase during which a thin, mafic, lower crustal high-velocity layer was produced in association with mafic dykes (2.2–2.1 Ga) and well-conducting volcanogenic and metasedimentary belts (2.5–2.0 Ga). The Moho of the Archaean Domain was, consequently, reformed in the Early Proterozoic. Major thickening of the crust (50–65 km) occurred when a Svecofennian island arc and the Archaean craton collided. The Svecofennian orogeny (1.76–1.9 Ga) resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc. The internal terrane boundaries are seen as good reflectors and/or inclined or vertical conductors that extend at least to the middle crust. The thick high-velocity (> 7 km/s) lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of Late Svecofennian subduction. The collisional boundary is also preserved as a discontinuous conductive zone between the Archaean and Svecofennian Domains. The thickened crust was subsequently thinned to 42 km along E—W-striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths, coeval mafic dykes, and gabbro-anorthosite bodies (1.6–1.5 Ga). The crust of the central part of the shield has since remained virtually intact.

Contrasting Pb isotopic compositions in two intrusive complexes of the Gardar Magmatic Province of South Greenland

Pb isotopic compositions were determined in rocks from two Proterozoic alkaline complexes in the Gardar Province of South Greenland. Both complexes, Kûngnât Fjeld and Tugtutôq Younger Giant Dyke, embrace a comparable range of compositions (alkali gabbro-ferrosyenogabbro-syenite-granite) but the two lie in contrasting crustal environments. Kûngnât lies within the southern marginal part of the Archaean craton; Tugtutôq lies off the craton in early Proterozoic (Ketilidian) granites. There are significant initial isotopic Pb differences between the two as well as marked Pb isotopic variations within each complex. These latter are attributed to increasing degrees of crustal contamination accompanying crystal fractionation processes whereas the overall differences between the two complexes reflect the differing isotopic signatures of the country rocks, viz. the relatively unradiogenic Archaean host rocks for Kûngnât and the more radiogenic Proterozoic granitic hosts for Tugtutôq.