C.R. Anhaeusser

A Reappraisal of Some Aspects of Precambrian Shield Geology

Attention is drawn to some of the misconceptions regarding the ancient crystalline shields, and an attempt is made to clarify some of the ideas held on their geology. In Southern Africa, the Precambrian shield is well represented and exposed, and recent studies of it reveal a clear and well-defined pattern of events in its evolution. The very ancient, stable, cratonic nuclei incorporating the greenstone belts are termed “The Earliest Precambrian.” Traversing the shield areas and surrounding the cratonic nuclei are large, elongated, highly metamorphosed and granitized “Precambrian Mobile Belts.” Although younger than, and totally different in character to, the cratons which they tend to encircle, they nevertheless form an integral part of the crystalline shields. The fundamental elements of the geology of the greenstone belts within the cratonic nuclei, together with a distinctive pattern of relationships between the greenstone belts and their surrounding granitic terrain are repeated with remarkable consistency in other shield areas of the world, especially in Canada and Western Australia. The geological features which typify and contribute to the establishment of this highly distinctive pattern are outlined in the text and demonstrated with the aid of tables and diagrams. The world-wide uniformity of the stratigraphy, structure, metamorphism, mineralization, associated granites, and geotectonic setting of the greenstone belts is stressed. An attempt is made to reconstruct an evolutionary model of the development of the early Precambrian granite-greenstone belt terrain. This avoids direct comparisons with younger geological features and events, particularly the younger, Alpine-type orogenic belts with which early Precambrian geology has frequently been compared and equated. The mobile belts are briefly discussed, and again it is suggested that their evolution was not necessarily along the lines suggested for Alpine orogenesis.

The origin of fluids and the effects of metamorphism on the primary chemical compositions of Barberton komatiites: New evidence from geochemical (REE) and isotopic (Nd, O, H, 39Ar40Ar) data

Numerous greenstone relics, all containing the two lowermost formations of the Onverwacht Group, occur in the Archean trondhjemitic/tonalitic gneiss terrains south of the Barberton Greenstone Belt. In this study, we report detailed petrological, geochemical and isotopic (Nd, O, H, 40Ar39Ar) data obtained on komatiites from the Schapenburg Greenstone Remnant (SGR), the largest and best-preserved greenstone relic. The main goals are 1. (1) to date the metamorphism affecting the SGR using the 40Ar39Ar dating method on amphiboles, 2. (2) to evaluate the effect of metamorphism on the preservation of primary isotopic and chemical signatures, 3. (3) to estimate the temperature and water/rock ratios that prevailed during metamorphic recrystallization in order to constrain the composition and origin of the reacting fluid phase. 40Ar39Ar ages of 2.9 Ga obtained on two amphibole separates from the Schapenburg metavolcanics reveal the existence of a metamorphic event younger than the emplacement age (3.5 Ga). This metamorphic event belongs to a series of discrete periods of thermal activity from 3.4 to 2 Ga, each of which coincides with a major episode of magmatic activity. The ultrabasic lava flows acquired their δ18O values (from +3.2 to +5%.) at high temperature (≈450°C) under high water/rock ratios. The reacting water had initial isotopic values typical of metamorphic fluids (δ18O = +5 to +7%.; δD = −65 to −50%.). REE patterns were not disturbed by metamorphic recrystallization. Despite the long time interval between emplacement and metamorphism (≈600 Ma), ϵNd(T) values are uniform throughout the whole magmatic suite, indicating that the Sm-Nd system was closed on the sample scale during metamorphism. The mantle source of these greenstones was depleted in LREE as evidence by ϵNd(T) ≈ +2.5. Chemical fluxes during metamorphism were calculated for elements unfractionated by olivine removal (e.g. Na, Ca, Ti, Al, and Sr), by normalizing to Nd. They suggest a significant mobility of most major elements in the cumulate zones of the lava flows. By contrast, spinifex zones appear to have preserved most of their primary chemical signatures during metamorphic recrystallization. Their CaOAl2O3 and Al2O3TiO2 ratios can be used with confidence to determine the PT conditions of melting in the mantle source. A general model of water-rock interactions applied to the sedimentary and magmatic rocks of the Onverwacht Group is also presented. The model involves conditions of metamorphism deduced from this study and available data from the literature. In addition to metavolcanic rocks, most of the oxygen isotope compositions of carbonates and cherts (except the highest values) can be explained by reequilibration with metamorphic fluids under greenschist-amphibolite facies conditions.

The Johannesburg Dome, South Africa: new single zircon U–Pb isotopic evidence for early Archaean granite–greenstone development within the central Kaapvaal Craton

Abstract The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas to better understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety of Archaean granitic rocks intruded into mafic–ultramafic greenstone remnants. This study presents new precise U–Pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the Johannesburg Dome. A trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 3340±3 Ma and represents the oldest granitoid phase recognized so far. This result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c.3.34 Ga. This initial magmatic episode, involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome, was followed by the emplacement of a 3201±5 Ma hornblende–biotite–tonalite gneiss in the south. Following the trondhjemite–tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses (3340–3200 Ma) and later, crosscutting, potassic granitoids. These rocks, consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at 3121±5 Ma. Their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of 3114±2.3 Ma. An age of 3117±12 Ma, from zircons extracted from one of the mafic dykes possessing granitic microveins, provided confirmation of the timing of this third magmatic event. Lastly, pegmatites that crosscut all these earlier granitoid events are younger than 3114 Ma and might be at least 3.0 billion-years old. These new data provide confirmation of the conclusion that the Witwatersrand Basin was deposited after c.3074 Ma on an Archaean basement as young as c.3120 Ma. The data, combined with that from other parts of the Kaapvaal Craton, further supports the view that the evolution of the Craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the c.3.5 Ga Barberton-Swaziland granite–greenstone terrane situated in the southeastern part of the Craton.

EPISODIC GRANITOID EMPLACEMENT IN THE WESTERN KAAPVAAL CRATON : EVIDENCE FROM THE ARCHAEAN KRAAIPAN GRANITE-GREENSTONE TERRANE, SOUTH AFRICA

Abstract Field, petrological, geochemical, isotopic and geophysical data have been assembled to determine the nature and extent of Archaean Kraaipan granite-greenstone rocks on the western edge of the Kaapvaal Craton, southern Africa. The Kraaipan greenstone belts, consisting of metamorphosed mafic volcanic rocks and interlayered metasediments (mainly banded iron formations, jaspilites and ferruginous cherts), occur poorly exposed beneath cover sequences comprising mainly Neoarchaean Ventersdorp Supergroup volcanic rocks and a blanket of Tertiary-Recent Kalahari sediments. A variety of granitoid rocks intruded the Kraaipan greenstones, which, on the basis of whole rock PbPb dating of banded iron formations, have yielded an age of 3410+61/-64 Ma. The earliest granitic rocks, which comprise tonalites and trondhjemitic gneisses, were dated using the single grain Pb evaporation technique on zircons, and yielded minimum ages ranging from 3162±8 to 3070±7 Ma in the study area. This, coupled with 3250-3030 Ma ages reported for gneisses in the Kimberley and other areas on the western edge of the Kaapvaal Craton, suggests a prolonged evolution for the basement gneisses which were also disturbed between 2940 and 2816 Ma ago, probably during episodes of migmatisation. Potassium-rich granitoids, also dated using the single grain Pb evaporation method, range in age from 2880±2 to 2846±22 Ma and extend from the Schweizer-Reneke area in the south to the Botswana border and beyond in the north. Geophysical evidence (aeromagnetic and Bouguer gravity data) suggest that the intrusions may be interconnected and might have been emplaced episodically across the study area. A close spatial relationship exists between these granodiorites and adamellites, and known Au mineralisation present in the Kraaipan-Madibe areas in the north and the Amalia area in the south. This suggests a possible genetic link which could be of significance in mineral exploration. Lastly, a late granitoid pluton, the Mosita Adamellite, yielded a Pb evaporation age of 2749±3 Ma and is the youngest intrusive body recorded in the Kraaipan granite-greenstone terrane. Its presence beneath Kalahari sand cover is defined by Bouguer gravity data. The Kraaipan granite-greenstone terrane, with a prominent north-south trend, appears to represent an Archaean crustal segment that may have accreted episodically on to the western edge of the Kaapvaal Craton. In a manner similar to the Murchison granite-greenstone terrane in the northeastern part of the craton, the region may also have constituted an important potential source of placer Au mineralisation found in the Witwatersrand Basin.

Episodic granitoid emplacement in the Archaean Amalia–Kraaipan terrane, South Africa: confirmation from single zircon U–Pb geochronology

Abstract The Amalia–Kraaipan granite–greenstone terrane, located in the western part of the Kaapvaal Craton, South Africa consists of metamorphosed mafic volcanic rocks and interlayered ferruginous and siliceous metasediments (mainly banded iron formations), intruded by a variety of granitoid rocks comprising tonalitic and trondhjemitic gneisses, granodiorites and adamellites. This study presents new single zircon dating for several granitoid rocks in order to define the chronology of magmatic events influencing this terrane. New TIMS and SHRIMP U–Pb data demonstrate episodic granitoid emplacement events, which occurred over a time-span of approximately 250 Ma. The oldest granitoid rocks so far recognized are dated at ca. 3008 Ma and are represented by biotite–trondhjemite gneisses located southwest of the town of Amalia. Homogeneous leuco-trondhjemite dykes intruding these gneisses yield an age of ca. 2940 Ma. Two granodiorite samples from the central part of the Amalia–Kraaipan terrane were dated at 2913±17 and 2915±12 Ma, respectively, while a further two granodiorite samples from the central and northern parts of the study area yielded ages of 2879±9 and 2879±11 Ma. The youngest granitoid body, the Mosita adamellite, which is exposed in the northwest sector of the Amalia–Kraaipan terrane, was dated at 2791±8 Ma. Since the 3008 Ma biotite–trondhjemite gneisses contain conformably interlayered amphibolite xenoliths, at least some of the Kraaipan Group volcano-sedimentary rocks are probably older than 3008 Ma. Based on available age data the 3010–2920 Ma Amalia–Kraaipan granitoids described in this paper could represent a possible source for some of the Witwatersrand sediments 100 km to the east and their contained gold. The ca. 2791 Ma age obtained for the Mosita adamellite permits a possible genetic link between this granitoid body and the ca. 2781 Ma Gaborone Granite Complex exposed approximately 120 km north in neighbouring Botswana.

The tectonostratigraphy, granitoid geochronology and geological evolution of the Precambrian of southern Ethiopia

Abstract Two distinct tectonostratigraphic terranes, separated by repeatedly reactivated deformation zones, are recognised in the Precambrian of southern Ethiopia: (1) granite-gneiss terrane, which is classified into sub-terranes and complexes, and (2) ophiolitic fold and thrust belts. The granite-gneiss terrane consists of para- and orthoquartzofeldspathic gneisses and granitoids, intercalated with amphibolites and sillimanite–kyanite-bearing schists. The paragneisses resemble gneisses from northern Kenya that were derived from sediments that filled the Kenyan sector of the “Mozambique Belt basin” between 1200 and 820 Ma. The volume of sediments formed during this period is comparatively small in southern Ethiopia, implying that the “Mozambique Belt basin” became progressively narrower northwards. The granitoid rocks in the study area vary from granitic gneisses to undeformed granites and range compositionally from diorites to granites. The granitoid gneisses form an integral part of the granite-gneiss terrane, but are rare in the ophiolitic fold and thrust belts. The ophiolitic fold and thrust belts are composed of mafic, ultramafic and metasedimentary rocks in various proportions. Undeformed granitoids are also developed in these belts. Eight granitoids from southern Ethiopia have been dated by U–Pb single zircon SHRIMP and laser probe 40Ar–39Ar dating. The SHRIMP ages range from ∼880 to 526 Ma, and are interpreted as close approximations of the respective magmatic emplacement ages. The 40Ar–39Ar data range from 550 to 500 Ma. The available geochronological data and field studies allowed classification of the granitoids of the Precambrian of southern Ethiopia into seven generations: Gt1 (>880 Ma); Gt2 (800–770 Ma); Gt3 (770–720 Ma); Gt4 (720–700 Ma); Gt5 (700–600 Ma); Gt6 (580–550 Ma); and Gt7 (550–500 Ma). The period 550–500 Ma (Gt7) is marked by emplacement of late- to post-tectonic and post-orogenic granitoids and presumably represents the latest tectonothermal event marking the end of the East African Orogen. Five tectonothermal events belonging to the East African Orogen are recognised in the Precambrian of southern Ethiopia: (1) Adola (1157±2 to 1030±40 Ma); (2) Bulbul–Awata (∼876±5 Ma); (3) Megado (800–750 Ma); (4) Moyale (700–550 Ma); and (5) Berguda (550–500 Ma).

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

Abstract 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.

Regional and detailed field and geochemical studies of Archean trondhjemitic gneisses, migmatites and greenstone xenoliths in the southern part of the Barberton Mountain Land, South Africa

Abstract A wide variety of Archean granites and greenstones occupy the extensive tract of country southwest of the Barberton Greenstone Belt. The greenstones, which occur as xenoliths wedged between numerous trondhjemitic gneiss diapirs, contain lithologies that are indisputably equatable with assemblages found in the lower formations of the Onverwacht Group. Intrusive into this environment are a complex suite of potash-rich granites of batholitic dimensions, as well as several smaller granitic and syenitic bodies. Mafic dykes of various ages also transect the region. Field investigations show that complex areas of migmatite in the area are not randomly distributed but are intimately linked with the greenstone remnants and are probably the result of granite—greenstone interaction. It can furthermore be demonstrated on a variety of scales that, in certain places, lit-par-lit intrusion of the greenstones by the granites has taken place. The mapping, in detail, of selected platform exposures has led to the recognition of a variety of granitic phases hitherto unrecognized in the district. At one locality two phases of trondhjemitic gneiss are recorded and the initial observations suggest that the earlier trondhjemite gneiss may predate the greenstone xenolith with which it is juxtaposed. A second exposure shows trondhjemite intrusive into an older felsic migmatite—gneiss unit which, it is suggested, may represent the migmatized equivalent of an acid tuffaceous rock unit from a typical greenstone assemblage. This paper reports some of the findings to date and draws attention to some of the anomalies that exist in the region. Work still in progress should provide a greater understanding of the interactive processes between granites and greenstone and will result in a much refined model for the nature and development of the Archean crust in the Barberton region.

Amphibolite facies metamorphism in the Schapenburg schist belt: A record of the mid-crustal response to ~3.23 Ga terrane accretion in the Barberton greenstone belt

The Schapenburg Schist Belt is one of several large greenstone remnants exposed in the granitoid-dominated terrane to the south of the Barberton greenstone belt and is unique in that it contains a well-developed metasedimetary sequence in addition to the typical mafic-ultramafic volcanic rocks. Detrital zircons within the metasediments have ages as young as ~3.24 Ga and consequently, these rocks correlate with Fig Tree Group sediments exposed in the central portions of the Barberton greenstone belt some 60km to the north, where they are metamorphosed to greenschist facies conditions. The Schapenburg metasediments are relatively K2O-poor, and are commonly characterised by the peak metamorphic assemblage garnet + cordierite + gedrite + biotite + quartz ± plagioclase. Other assemblages recorded are garnet + cummingtonite + biotite + quartz, cordierite + biotite + sillimanite + quartz and cordierite + biotite + anthophyllite. In all cases the peak assemblages are texturally very well equilibrated and the predominantly almandine garnets from all rock types show almost flat zonation patterns for Fe, Mg, Mn and Ca. Analysis using FMASH reaction relations, as well as a variety of geothermometers and barometers, has constrained the peak metamorphic pressure-temperature conditions to 640 ± 40°C and 4.8 ± 1.0 kbar. The maximum age of metamorphism is defined by the ~3.23 Ga age of a syntectonic tonalite intrusion into the central portion of the schist belt. Thus, sedimentation, burial to mid-crustal depths, and amphibolite facies equilibration were achieved in a time span similar to ~15 Ma. The strong bedding-parallel foliation in the metasediments dips to the east at an angle of 75 to 85° In this foliation plane elongated cordierite rods produced during prograde metamorphism define a close to vertical mineral lineation. The metasedimentary succession youngs to the east and in this direction is overlain by older Onverwacht Group rocks. Thus, the cordierite lineation might represent a transposed lineation developed during thrust stacking. Post-kinematic static recrystallization has largely obscured the prograde history of the rocks. However, the pressure-temperature conditions of peak metamorphism, the age of metamorphism, the rapidity of metamorphism following sedimentation, the presence of cordierite on the prograde path, as well as the timing of metamorphism relative to deformation strongly suggest that metamorphism occurred in an arc-style collisional setting. Thus, the rocks of this study most likely represent an exhumed mid-crustal terrane equilibrated during the proposed ~3230 Ma terrane accretion event in the Barberton Greenstone Belt, and record an apparent geothermal gradient typical for portions of modern orogenic belts of this type.

Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons

Abstract How and when continents grew and plate tectonics started on Earth remain poorly constrained. Most researchers apply the modern plate tectonic paradigm to problems of ancient crustal formation, but these are unsatisfactory because diagnostic criteria and actualistic plate configurations are lacking. Here, we show that 3.5–3.2 Ga continental nuclei in the Pilbara Craton, Australia, and the eastern Kaapvaal Craton, southern Africa, formed as thick volcanic plateaux built on a substrate of older continental lithosphere and did not accrete through horizontal tectonic processes. These nuclei survived because of the contemporaneous development of buoyant, non-subductable mantle roots. This plateau-type of Archean continental crust is distinct from, but complementary to, Archean gneiss terranes formed over shallowly dipping zones of intraoceanic underplating (proto-subduction) on a vigorously convecting early Earth with smaller plates and primitive plate tectonics.