Archaean Granitoids Research Articles

Importance of water for Archaean granitoid petrology: a comparative study of TTG and potassic granitoids from Barberton Mountain Land, South Africa

A new model for Archaean granitoid magmatism is presented which reconciles the most important geochemical similarities and differences between tonalite-trondhjemite-granodiorite (TTG) and potassic granitoids. Trace element abundances reveal a strong arc magmatism signature in all studied granitoids from Barberton Mountain Land. Characteristic features include HFSE depletion as well as distinct enrichment peaks of fluid-sensitive trace elements such as Pb in N-MORB normalisation, clearly indicating that all studied granitoids are derived from refertilised mantle above subduction zones. We envisage hydrous basaltic (s.l.) melts as parental liquids, which underwent extensive fractional crystallisation. Distinctive residual cumulates evolved depending on initial water content. High-H2O melts crystallised garnet/amphibole together with pyroxenes and minor plagioclase, but no olivine. This fractionation path ultimately led to TTG-like melts. Less hydrous basaltic melts also crystallised garnet/amphibole, but the lower compatible element content indicates that olivine was also a liquidus phase. Pronounced negative Eu-anomalies of the granitic melts, correlating with Na, Ca and Al, indicate plagioclase to be of major importance. In the context of our model, the post-Archaean disappearance of TTG and concomitant preponderance of granites (s.l.), therefore, is explained with secular decrease of aqueous fluid transport into subduction zones and/or efficiency of deep fluid release from slabs.

Correlations between the Eastern Block of the North China Craton and the South Indian Block of the Indian Shield: an Archaean to Palaeoproterozoic link

Abstract Recent tectonometamorphic data reveal that the North China Craton formed by amalgamation of an Eastern Block (EB) and a Western Block (WB) along the Trans-North China Orogen (TNCO), and the Indian Shield formed by amalgamation of the South Indian Block (SIB) and the North Indian Block (NIB) along the Central Indian Tectonic Zone (CITZ). The EB of the North China Craton and the SIB of the Indian Shield share many similar magmatic and sedimentary features. The early Archaean supracrustal sequences in both the EB and SIB consist of 3.6–3.4 Ga (fuchsite) quartzites, pelitic gneisses, calc-silicates, marbles, banded magnetic quartzites and amphibolites, called the Caozhuang Group in the EB and the Older Metamorphic Group (OMG) in the SIB. The middle Archaean assemblages in both the blocks can be divided into an early (3.4–3.3 Ga) phase, represented by the Chentaigou supracrustal rocks and orthogneisses in the EB and the Holenarsipur supracrustal rocks and the Gorur orthogneisses in the SIB, and a late (3.1–2.9 Ga) phase, represented by the Qianan supracrustal rocks and the Yangyashan orthogneiss in the EB and the Sargur Group and the Peninsular gneiss in SIB. The late Archaean Taishan Group and its equivalents in the EB are stratigraphically similar to the Dharwar Supergroup in the SIB. Moreover, both the EB and SIB are characterised by near-contemporaneity of late Archaean granitoid intrusive and metamorphic events. In addition, dome-and-basin structures are dominant in both the EB and SIB, and anticlockwise P – T paths charaterise the metamorphic evolution of the late Archaean granulites in the two blocks. The Palaeoproterozoic sedimentary–volcanic rocks in both blocks comprise lower clastic-rich, middle volcanic-rich and upper clastic+carbonate sequences. The strong resemblance of the Archaean to Palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the EB and SIB are dispersed remnants of what was once a single continent. A possible fit is proposed for the reconstruction of the EB and SIB, where the northern margin of the EB is placed adjacent to the western margin of the SIB, with the TNCO and WB representing the continuations, respectively, of the CITZ and NIB. This fit suggests that the EB+SIB and the WB+NIB represent two discrete continental blocks that joined along the TNCO+CITZ orogen during a Palaeoproterozoic collisional event. Fragmentation of the EB and SIB may have occurred at the beginning of the Mesoproterozoic and, by 1.4 Ga, the EB and SIB may have separated into two different continental blocks, as evidenced by striking differences in the Mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks.

Granitoid Series in Terms of Magnetic Susceptibility: A Case Study from the Barberton Region, South Africa

Magnetic susceptibilities were measured on a representative collection of Archaean granitoids of the Barberton region using a portable KT5 magnetic susceptibility meter. The studied granitoids comprise, (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) granitoids (132 samples), (2) late-tectonic calc-alkaline granitoids (402 samples) and (3) post-tectonic low-Ca and high-Ca granitoids (12 samples). Most of the early-stage syn-tectonic granitoids (∼3450 Ma) have low magnetic susceptibilities, less than 3 × 10 −3 SI units, and correspond to ilmenite-series granitoids. The late-stage Kaap Valley tonalite pluton (∼3230 Ma) contains sporadically distributed higher magnetic susceptibility values (greater than 3 × 10 −3 SI units), which are less than one-third in magnetic susceptibility of typical magnetite-series TTG of the Japanese Island Arc and thus strictly belong to an intermediate series. The Barberton TTG suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the Earth surface during the Archaean Eon. The more oxidized nature of the Kaap Valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate. Late-tectonic granodiorite - adamellite batholithic complexes (∼3105 Ma) belong mostly to the magnetite series, and seem to suggest that relatively oxidized continental crust, reflecting oxic atmosphere and subduction mechanism operating, had evolved it by this time. Post-tectonic granitic plutons formed largely between circa 2900 Ma and 2700 Ma can be subdivided into low-Ca ilmenite series and high-Ca magnetite series.

Late Palaeoproterozoic monazite and titanite U–Pb ages in the Archaean Suomujärvi Complex, N-Finland

The paper documents a late Palaeoproterozoic episode of metamorphism and anatexis in the Suomujärvi Complex, an Archaean enclave in the Palaeoproterozoic Central Lapland Granitoid Complex of northern Finland. U–Pb ages were obtained from monazite and titanite in four Archaean granitoid rocks, one quartzite and a massive granite. The results show that growth of monazite and titanite in these rocks spanned a period of about 15 million years between 1780 and 1765 Ma, presumably reflecting complex processes of metamorphic (re-) crystallisation, invasion by granitic dykes, veins and fluids and post-peak-metamorphic retrogression. The timing of this activity corresponds to post-orogenic granitoid magmatism throughout the Baltic Shield. In this region, between the Lapland-Kola and that of Svecofennian orogens, it may represent a retarded response to their coeval, ca. 1.9 Ga convergent collision. The results support a proposed affinity of the Suomujärvi Complex with the Belomorian rather than the Archaean Karelian Province.

Timing of Palaeoproterozoic crustal shearing in the central Fennoscandian Shield according to U-Pb data from associated granitoids, Finland

VAASJOKI, MATTI, KARKI, AULIS and LAAJOKI, KAUKO 2001. Timing of Palaeoproterozoic crustal shearing in the central Fennoscandian Shield according to U-Pb data from associated granitoids, Finland. Bulletin of the Geological Society of Finland 73, Parts 1–2, 87–101. Granitoids in the Finnish part of the Fennoscandian Shield were created by numerous Archaean and Palaeoproterozoic tectonic events. Late phases of Palaeoproterozoic, Svecokarelian deformation created ductile, crustal-scale shear zones, of which the N-S striking Hirvaskoski Shear Zone and the NE-SW striking Oulujarvi Shear Zone were strongly affected by the latest deformation stages, D3 and D4. These shear tectonic events deformed ductilely prekinematic granitoids and caused intrusion of synkinematic igneous rocks, but the D3 and D4 fault rocks are also penetrated by postkinematic intrusives. Archaean granitoids behaved mostly as rigid blocks unaffected by Palaeoproterozoic deformation. However, inherited Archaean zircons occur in some obviously Proterozoic granitoids, and others contain heterogeneous zircon populations. Granitoids lacking structures of the earliest deformations, D1 and D2, contain inherited zircon populations partially reset during the Svecokarelian orogeny and have been sheared ductilely during stages D3 and D4. Granitic rocks yielding zircon ages of 1.82-1.80 Ga show no features associated with these shearing events and penetrate the D4 fault rocks setting a minimum age for these deformation events. This age marks also the end of convergent tectonics in the central Fennoscandian Shield. Monazite data from a variety of rocks demonstrates a regional cooling below the 600°C isotherm at 1.8 Ga.

Combined chemical separation of Lu, Hf, Sm, Nd, and Rees from a single rock digest: precise and accurate isotope determinations of Lu-Hf and Sm-Nd using multicollector-ICPMS.

A combined procedure for separating Lu, Hf, Sm, Nd, and rare earth elements (REEs) from a single sample digest is presented. The procedure consists of the following five steps: (1) sample dissolution via sodium peroxide sintering; (2) separation of the high field strength elements from the REEs and other matrix elements by a HF-free anion-exchange column procedure; (3) purification of Hf on a cation-exchange resin; (4) separation of REEs from other matrix elements by cation exchange; (5) Lu, Sm, and Nd separation from the other REEs by reversed-phase ion chromatography. Analytical reproducibilities of Sm-Nd and Lu-Hf isotope systematics are demonstrated for standard solutions and international rock reference materials. Results show overall good reproducibilities for Sm-Nd systematics independent of the rock type analyzed. For the Lu-Hf systematics, the reproducibility of the parent/daughter ratio is much better for JB-1 (basalt) than for two analyzed felsic crustal rocks (DR-N and an Archaean granitoid). It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.

Evidence of crustal contamination of mafic rocks associated with rapakivi rocks: an example from the Nordingrå complex, Central Sweden

Gabbro and leucogabbro are volumetrically important rocks in the Nordingrå rapakivi complex, East Central Sweden. Plagioclase, ortho- and clinopyroxenes, and olivine dominate the gabbro. Perthitic orthoclase and quartz are interstitial in relation to the major minerals. The present work is based on 232 major-element and a large number of trace element analyses together with 15 whole rock Sm–Nd isotope analyses of the Nordingrå gabbroic rocks. εNd(T) values are negative, −1.1 to −3.2; the most negative values come from the gabbro. Most rocks are enriched in iron, some extremely enriched; none represent primitive mantle melts. The range of Mg-numbers is the same in the gabbro and the leucogabbro. Plots of the Ni-content vs. the Mg-number are scattered, but there is a positive correlation between these two parameters. The primary mantle-normalized ratios between similar trace elements are normally strongly different from one. Values larger as well as smaller than one are found for the same ratio in different rocks. The rare earth elements are only weakly fractionated with small Eu anomalies, negative for the gabbros and positive for the leucogabbros. The primary magma of the Nordingrå gabbro-anorthosite is thought to have been derived from a mildly depleted mantle source. Variations in the degree of partial melting of a reasonably homogeneous enriched mantle do not explain the observed chemical evolution. Crystal differentiation can account for some geochemical features, especially the Fe-enrichment. Crustal contamination is required by other characteristics as, for example, the negative εNd(T) values and the irregular and sometimes high primary-mantle normalized incompatible trace-element ratios. Al-rich relic material from the formation of the rapakivi granite melt is another source of assimilation. Most probably contaminants are heterogeneous, including undepleted crust (represented, for example, by early Svecofennian and Archaean granitoids), depleted crust (restitic after rapakivi magma extraction), and to some degree the associated rapakivi magma itself. Significant parts of this crust should be Archaean in age.

On the scarcity of >3900 Ma detrital zircons in ≥3500 Ma metasediments

Constant volume models for the continental crust require a flux of crustal material back into the mantle (recycling), equal in volume to that of the juvenile igneous suites added to the continental crust throughout time. In growth of crustal volume models, there is not equilibrium between the volume of juvenile crustal additions and any recycling (destruction) of crust. By establishing the proportion of >3900 Ma detrital zircons in early Archaean sediments it might be possible to constrain the relative importance of crustal growth and recycling. Gneiss complexes in western Greenland, northern Labrador and northeastern China contain rare ≥3500 Ma detrital metasediments. In sediments deposited between 3500 and 3600 Ma, ≥3900 Ma zircons have not been detected in a suite of 117 detrital grains. Based on statistical considerations, at the 95% confidence level any ‘missed’ ≥3900 Ma component forms <3% of this suite. Likewise, >3900 Ma detrital grains do not occur amongst 54 detrital grains from (even rarer) 3700–3800 Ma sediments, arguing with 95% certainty that any ‘missed’ ≥3900 Ma component forms <5% of this most ancient suite. If the age spectra of these detrital zircon suites are representative of the complexes in which they reside, then constant volume (recycling=new additions) models require that by 3500 Ma, >97% of >3900 Ma crust was destroyed by recycling. Such an extremely high recycling rate (≈25% of the crust 100 Ma −1) is hard to reconcile with the diversity of initial Nd and Sr isotopic ratios of well preserved early Archaean granitoid suites in the same complexes, because significant average crustal residence times are required to permit the radiogenic isotopic systems to evolve. The most likely interpretation of the detrital zircon record in the Greenland, Labrador and China sediments is that in their provenance areas the volume of continental crust was small at 3900 Ma, and that it grew significantly during the early Archaean. If the measured ≥3500 Ma detrital sediment suites are globally representative, they support growth models for the continental crust in the early Archaean, rather than models involving recycling of a voluminous >3900 Ma sialic crust. Because of its global coverage and the dating of thousands of grains, the age spectra for detrital zircons from 3000–3200 Ma sediments provides a more reliable impression of crustal ages. However, as they were deposited 700–900 Ma after 3900 Ma, the globally small proportion of ≥3900 Ma detrital grains in them (from Jack Hills, Mt. Narryer and Wyoming) can be accommodated in both crustal growth and moderate recycling models.

2.8–3.0 Ga plutonism and deformation in the SE Amazonian craton: the Archaean granitoids of Marajoara (Carajás Mineral Province, Brazil)

The granitoids of Marajoara in the Rio Maria terrain (Carajás Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guarantã); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrão) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

Archaean–Proterozoic geochronology of the Vallen-Alhamn area, northern Sweden

A minor massif of approximately 2700 Ma Archaean granodiorite and tonalite is exposed in a Svecofennian environment at Vallen-Alhamn, near to the Archaean–Proterozoic Palaeoboundary in northern Sweden. Close to this massif occur conglomerate-like rocks (in part interpreted as a magmatic, hydraulic breccia) with fragments of Palaeoproterozoic metavolcanites and granitoids as well as of the Archaean granitoids. The present work comprises dating of the granitoids of the massif, c. 2700 and 1900 Ma granitoid fragments in two occurrences of conglomerate-like rocks, and the volcanogenic matrix of one of the conglomerate-like rocks. The age of igneous-looking zircons in the matrix (1871±2 Ma) is interpreted to be close to the time of formation of the conglomerate-like rock, and points to an Upper Svecofennian stratigraphic position. The bedrock of the Vallen-Alhamn area is thus a unique example of Archaean rocks in close geographic association with Upper Svecofennian supracrustal or shallow-level intrusive rocks.

RbSr isotope constraints on the timing of late to post-Archa↭ tectonometamorphism affecting the southeastaean Kaapvaal Craton

Biotite separates from Archaean granitoid lithologies on the Kaapvaal Craton north of the roterozoic Namaqua-Natal Belt in south eastern South Africa exhibit RbSr model dates of 967 ± 24 Ma for samples from within 25 km of the present northern limit of the Proterozoic thrust front. Samples from further north (>50 km to 170 km) have model RbSr dates of 2614 ± 74 Ma. The younger dates are interpreted as dating cooling after northwards emplacement of Proterozoic crust onto the Kaapvaal Craton, whereas the older dates are presumed to relate to an Archaæan metamorphic episode, possibly associated with intrusion of the post-Pongola granites.

Evolution of the Western Margin of Australia during the Rodinian and Gondwanan Supercontinent Cycles

The proto-Darling Fault zone and its successor, the Darling Fault, extend for 1, 000 km along the western continental margin of Australia and appear to have been active at several periods during the geological past. Deformation commenced at ∼2,570 Ma and affected Late Archaean granitoids along the western margin of the Yilgarn Craton. Much of the later activity reflects events related to the accretion and breakup associated with the Rodinia and Gondwanaland supercontinent cycles. In the north, rocks of the Northampton and Mullingarra Complexes form part of a high-grade Grenvillian orogenic belt lying to the west of the Darling Fault, referred to as the Pinjarra Orogen. They underwent granulite facies metamorphism ∼1080 Ma ago and form part of the global collisional event that resulted in the amalgamation of Rodinia. These rocks extend southward beneath Phanerozoic sedimentary cover (the Perth Basin), where they are constrained to the east by the Darling Fault and to the west by the Dunsborough Fault, the latter marking the eastern boundary of the Leeuwin Complex. The Leeuwin Complex is a fragment of Pan-African crust that has traditionally been considered part of the Pinjarra Orogen. It is composed predominantly of upper amphibolite to granulite facies felsic orthogneisses derived from A-type, anorogenic granitoids. Conventional and SHRIMP U-Pb zircon geochronology has established that the granitoids evolved between ∼780 Ma and ∼520 Ma and were metamorphosed at ∼615 Ma. These events are equated with rifting associated with the breakup of Rodinia. Sm-Nd whole rock data support the juvenile nature of the crust and provide no evidence for the involvement of pre-existing Archaean continental material. During the Phanerozoic, the Dunsborough and Darling Faults were reactivated, as normal faults defining the inner arm of a major rift system within Eastern Gondwanaland and controlling sedimentation in the Perth Basin that now overlies the Grenvillian terrane. Major normal movement on the Darling Fault ceased by the Late Jurassic and it appears that continental breakup in the Early Cretaceous occurred along fractures closely related to the western boundary of the Leeuwin Complex that defined the eastern margin of the outer arm of the rift system. Breakup between Australia and Greater India commenced at ∼132 Ma and was followed by eruption of the Bunbury Basalt at 130 Ma and 123 Ma. This possibly resulted from hot spot activity beneath Eastern Gondwanaland and may have been a reflection of the Kerguelen plume, though the evidence is equivocal. It is argued from the petrographic, geochemical and isotopic characteristics, together with the likely contiguity of the Eastern Gondwanaland continents since the assembly of Rodinia, that the Leeuwin Complex evolved within an intracrustal rift and is not an exotic terrane. It is distinct from adjacent portions of the Pinjarra Orogen and should be considered a separate terrane. It is recommended that use of the term ‘Pinjarra Orogen’ be confined to rocks recording the Grenvillian events, thereby excluding those rocks (the Leeuwin Complex) that evolved during the later Pan-African orogeny.

A comparison of fluids causing post-depositional hydrothermal alteration in Archaean basement granitoids and the Witwatersrand Basin

Some Archaean granitoids occurring along the north-western and western edge of what is classically known as the Witwatersrand Basin have been hydrothermally altered in two different styles. Both styles - pervasive and vein-controlled alteration - are characterised by sulphide mineralisation as well as by thorium-, uranium-, gold- and REE-rich nodules of carbonaceous material (fly-speck carbon). All of the granitoids displaying these characteristic alteration styles are associated with secondary moderate-to high-salinity, Ca-rich fluid inclusions with low homogenisation temperatures in magmatic quartz and quartz veins. C-isotopes of the fly-speck carbon and the fluid composition of the associated fluid inclusions are typical of modified basin brines. Similar types of Ca-rich fluid inclusions were found in authigenic quartz and hydrothermal quartz veins from gold-bearing, hydrothermally altered sediments of the Witwatersrand Basin, although the salinities of the fluids in the basement granitiods are somewhat higher. The moderate- to high salinity of this inclusion fluid and its present composition is considered to be the result of modifications by fluid-rock interaction during transport and subsequent metamorphism within the Witwatersrand Basin. Available age data on the hydrothermally derived minerals in these granitoids range between 2.7 and 2.0 Ga indicating several fluid pulses, with fluids repeatedly expelled during the dewatering of the Witwatersrand Basin. These data are consistent with the conclusion that major parts of the alteration are the result of sediment dewatering which affected both the granitoids and the gold-bearing strata within the Witwatersrand Basin.

The interpretation of complex zircon U–Pb systems in Archaean granitoids and gneisses from the Jack Hills, Narryer Gneiss Terrane, Western Australia

SHRIMP U–Pb ages of zircons from Archaean TTG granitoids and gneisses from the Jack Hills region of Western Australia show a range of complexity which reflects the origin and geological history of the rocks and the stability of the zircon U–Pb systems under metamorphic and magmatic conditions. Strong light rare earth element (LREE) enrichment and similarities in the major and trace element chemistry suggest that TTG gneisses formed by fractional melting of a garnet-bearing amphibolitic source. Maximum ages, of 3.75 to 3.6 Ga, obtained from low U zircon cores are interpreted as the period of formation of the TTG parent rocks. However, most zircons in these populations are complex, consisting of zoned and unzoned parts, and have disturbed U–Pb systems which form a concordant to discordant array between 3.7 and 3.3 Ga on a concordia plot, suggesting these grains experienced one or more disturbance events within this time span. The range of complexity of U–Pb systems observed in a number of the zircon populations is also present within individual zircon crystals indicating that secondary movement and loss of Pb and possibly U has occurred within and from zircons, causing variable isotopic discordance and yielding a range of apparent ages within a single grain. Disturbance of the zircon isotopic systems appears to be due to a combination of early Archaean events together with recent Pb loss. The presence of 3.3 Ga zircon rims around older cores in samples of tonalite and porphyritic granodiorite, together with the exclusive presence of 3.3 Ga zircons in one sample of porphyritic granite, is interpreted as evidence of a major magmatic and possibly metamorphic event at this time. The presence of ca 3.5 Ga ages in a number of unzoned zircons in the tonalite and two samples of granodiorite suggests that this could also have been a time of magmatism and zircon crystallization. Evidence for 3.3 Ga magmatism appears to be restricted to zircons from TTG rocks south of the Jack Hills Metasedimentary Belt, raising the possibility that the present position of the belt defines a major suture within the Narryer Gneiss Terrain. The post-tectonic monzogranites have LREE enriched patterns with strong negative Eu anomalies, similar to the older gneisses, but in contrast to these rocks, zircons from the monzogranites have relatively simple U–Pb systems, which define an age of 2654±7 Ma, and only rarely do zircons have inherited cores with nearly concordant ages of ca 3.6 Ga. The very low content of inherited zircon raises doubts about the formation of the granites by reworking of older TTG gneisses and an alternative explanation that the monzogranites formed from new 2.65 Ga crust, with minor 3.6 Ga crustal contamination, is seen as more likely.

Low-temperature deformation mechanisms at a lithotectonic interface near the Sudbury Basin, Eastern Penokean Orogen, Canada

Southwest of the Sudbury Basin, variations in macroscopic strain, microstructure and deformation mechanisms within Archaean granitoid basement and overlying Proterozoic rocks suggest that rock strength was strongly influenced by large amounts of fluids. Ductilely deformed granitoids with strong shape fabrics are concentrated in a 150–200-m-wide zone parallel to the subvertical basement-cover interface. High fluid activity in this zone is indicated by: (1) thickness and close spacing of foliation-parallel quartz veins, (2) abundant fluid-inclusion trails in quartz; (3) quartz c-axis fabrics: and (4) transformation of feldspar into phyllosilicates under middle greenschist-facies metamorphic conditions. Away from this zone, brittle-ductile deformation in Archaean granitoids caused grain size reduction of feldspar by fracturing and crystal-plastic deformation of quartz indicating largely ‘dry’ conditions. Strain was concentrated at the interface, in a region of bulk horizontal shortening and linked kinematically to large-scale shortening of the Sudbury Igneous Complex and thrusting in the South Range Shear Zone. After early open buckling of the Sudbury Igneous Complex, reverse shearing in this zone may have been controlled by ductile deformation at the basement-cover interface. Physical continuity of, and age constraints on, strain fabrics suggest that the South Range Shear Zone represents a segment of a large ductile deformation zone which is most likely linked to the Murray fault and on which higher-grade rocks ascended between 1.85 and 1.23 Ga ago.

Nd Isotopic Constraints on the Genesis of Archaean Tonalitic Gneisses from Southern Zimbabwe

We here report Nd isotopic data on selected undeformed archaean granitoids from three locations in the South of Zimbabwe craton. These Masahaba, Chingezi and Shabani areas belong to the Belingwe belt, where the oldest recognised basement of 3.5-2.9 Ga age is exposed. Using elements such as Y and heavy REE (Yb) as tracers of garnet in the source because of their high K~ arnet partition coefficient, previous studies have shown that these tonalite-trondhjemite-granodiorite samples have been generated by melting of metamorphosed basaltic source with variable amount of garnet in the residue (Luais and Hawkesworth, 1994). Because of high I~ m/Ndgarnet (I~ m/Nd = 4), we will evaluate on Nd isotopic ratios the inferences of garnet in the source, as well as the consequences on crustal Nd isotopic evolution.

Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust1,2,3. Recent studies4,5,6, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8 Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

Emplacement features of Archaean TTG plutons along the southern margin of the Barberton greenstone belt, South Africa

The Barberton greenstone belt in South Africa and its surrounding granitoid terrane represents one of the type localities for Archaean granite-greenstone tectonics. Contact relationships between early Archaean granitoids and greenstone sequences along the southern margin of the greenstone belt illustrate multiple, simultaneously operating emplacement mechanisms for the plutons. These emplacement mechanism, which can be identified in individual plutons comprise ductile wall-rock deformation, stoping, assimilation, and the intrusion of ring dykes. The interaction between these various mechanisms is interpreted to be responsible for the complex and highly variable discordant and concordant contact relationships observed in the granite-greenstone terrane as well as the deformation observed within wall rocks and country-rock xenoliths. The absence of regionally consistent fabrics within the plutons suggests considerable competence contrasts between the tonalitic-trondhjemitic plutons and, in many cases, the hydrated greenstone cover-sequence during later subhorizontal crustal shortening which occurred parallel to the greenstone-granitoid interface. This competence contrast is interpreted to have resulted in the accentuation of cuspate-lobate folds along the granitoid-greenstone interface, being partly responsible for the typical arcuate, dome-and-keel geometry of the Barberton greenstone belt.

SmNd evidence for late Archaean crust formation in the Lapland-Kola Mobile Belt, Kola Peninsula, Russia and Norway

A SmNd model age traverse across the Kola Peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early Archaean “Saamian” crust in the region. Granitoid gneisses and subordinate metasediments have been sampled from the main Archaean geological units, i.e. from northeast to southwest: the Murmansk domain (model ages of 2.68–2.96 Ga); Central Kola domain including samples from the Superdeep Borehole (2.74–2.92 Ga); Belomorian domain, including the eastern part of the Tersk area (2.70–2.93 Ga). The narrow range of depleted mantle model ages from 2.68 to 2.96 Ga for Archaean granitoid orthogneisses, close to the oldest crystallization ages from the region, indicate a major episode of late Archaean crustal growth and reveal no indication of ancient “Saamian” material in keeping with other geochronological evidence. Similar results (2.86 to 2.93 Ga) for Archaean metasediments support this conclusion. Proterozoic model ages for supracrustals from the western part of the Tersk area (2.16 to 2.23 Ga) indicate that the crust in this area is much younger than previously recognized. Initial ϵ Nd values for late Archaean samples with reasonable age constraints fall within a range of +2.3 to −2.5. Two well-dated samples from the Belomorian domain have the highest initial ϵ Nd values [ ϵ Nd(2.81 Ga) = + 2.0 and ϵ Nd(2.78 Ga) = + 2.3] and suggest that a depleted mantle source was present in the Kola region in late Archaean times. Dated samples from the Central Kola domain have lower initial ϵ Nd (between − 2.5 and + 1.2) suggesting a possible regional difference in mantle character. In contrast to the depleted signature of the Archaean orthogneisses between 2.7 Ga and ∼ 2.8 Ga, previous studies of earliest Palaeoproterozoic rift-related majic intrusives in the Kola and Karelia regions report negative ϵ Nd values at 2.4–2.5 Ga. Possibly these data reflect a change to more enriched-mantle values.

Melting of Archaean sialic crust under high aH 2O conditions: genesis of 3300 Ma Na-rich granitoids in the Mount Edgar Batholith, Pilbara Block, Western Australia

The voluminous 3300 Ma old granitoids of the Mount Edgar Batholith, Pilbara Block, Western Australia, are typically trondhjemites and silicic tonalites with SiO 2 ranging from 65–75 wt%. They have high Sr (average: 425 ppm at 70.6% SiO 2) and Na 2O (3.5–5.0 wt%) contents, with K 2O rapidly increasing (1.0–4.5 wt%) with increasing silica, so that the most fractionated are granodioritic. A lack of restite minerals, calcic plagioclase cores, and mafic microgranitoid enclaves suggests that the granitoids were produced from chemically evolved sources and that they crystallised from melts approximating the bulk composition of the batholith (70.6 wt% SiO 2). Based on experimental data, mass balance and trace element modelling, these silicic melts are considered to have been produced by high degrees of partial melting, involving complete plagioclase dissolution, of tonalitic/dacitic precursors similar in composition to the dacites of the ∼ 3450 Ma greenstone succession. The data are consistent with field and isotopic evidence from the gneiss complex of the batholith, where orthogneisses have formed virtually in situ from older (∼3450 Ma) banded tonalitic gneisses that resemble coeval metamorphosed felsic volcanic rocks of the Duffer Formation, or deeper-level equivalents. A number of granitoids from other batholiths of the Pilbara Block are geochemically similar to the Mount Edgar Batholith, and probably were derived from similar sources. The following three-stage model of crustal growth is envisaged: (1) production of a thick, primitive mafic crust at (∼360 Ma, remnants of which exist as gabbroic anorthosite xenoliths in some granitoids (McNaughton et al., 1988); (2) partial melting of the mafic crust at 3500-3450 Ma to produce tonalitic/ dacitic rocks at shallow mantle depths ( < 60 km), associated with formation of the major greenstone belts of the Pilbara Block; (3) infracrustal melting of the tonalitic/dacitic rocks at 3300 Ma to produce the abundant silicic tonalitic/trondhjemitic (sodic) granitoids. Uniformitarian and non-uniformitarian tectonic models involving arc magmatism, rifting, mantle plumes and delamination are discussed for the three-stage crustal growth process, but the available geological evidence does not favour any particular model. The data indicate that Archaean granitoids are similar to those produced in modern subduction-related environments, but the process of Archaean magmatism might have been quite different.