Kaapvaal Research Articles

Metamorphic history of amphibolite facies metapelites from the Southern margin of the Limpopo Belt: In situ U-Pb dating of Zircon, monazite and rutile

The Southern Marginal Zone (SMZ) comprises, in part, Archean granulite facies metapelitic rocks deposited at ∼ 2733 Ma and metamorphosed to granulite facies at ∼ 2713 Ma of the Limpopo Belt in direct contact with the Kaapvaal craton. The SMZ also contains amphibolite facies metapelites within a fragment that is in direct contact with the thrust-faulted tectonic boundary with the Kaapvaal craton. These metapelites are proposed to represent former granulites that were comprehensively rehydrated under amphibolite facies. However, no previous study has directly investigated the conditions of metamorphism or timing of the proposed higher-grade events in these retrograde rocks. Neither has the timing of retrogression in the retrogressed metapelites been well constrained. Here, detailed petrographic analysis, mineral composition and in situ U-Pb dating of zircon, monazite and rutile are presented for the amphibolite facies metapelites. These rocks are characterized by assemblages consisting of garnet, orthoamphibole, biotite, quartz, plagioclase, rutile, kyanite and graphite. Although there is pervasive retrogression to orthoamphiboles (anthophyllite to gedrite series), rare relics of orthopyroxene are preserved. The compositions of the first garnet (Grt 1) generation suggests granulite facies peak metamorphic conditions of 860 ± 10 °C and 11 ± 0.4 kbar. Metamorphic zircon grains and overgrowth rims and monazite, included in Grt 1, indicate that peak metamorphic conditions occurred between 2714 ± 7 and 2713 ± 4 Ma. Dating of rutile inclusions in Grt 1 from granulite yield a Concordia age of 2677 ± 6 Ma. This indicates post-granulite facies cooling and shows that later retrograde metamorphism did not reach temperatures high enough to reset rutile inclusions in Grt 1 garnet. Orthoamphibole and second garnet with kyanite inclusions (Grt 2) compositions suggest that retrogression occurred under amphibolite facies conditions of 600 ± 10 °C and 6.0 ± 0.5 kbar. Rutile included in Grt 2 and as discrete grains equilibrated with the retrograde textures record a range of Paleoproterozoic ages (2212 ± 19, 2148 ± 12 and 2012 ± 10 Ma). Thus, the metamorphic history of the amphibolite facies metapelites of the SMZ is hotter than thought and similar to their Archean granulite facies counterparts, except for younger Paleoproterozoic ages, likely indicating a pervasive retrogression to amphibolite facies conditions.

A 187Re-187Os and highly siderophile element study of diamondiferous kimberlite melt-mantle interactions and the inferred age of continental lithosphere

The ∼ 1.15-billion-year-old (Ga) Premier kimberlite pipe (Cullinan diamond mine), South Africa, is composed of several distinct kimberlite facies (Grey, Brown, Pale Piebald, Dark Piebald, Black Coherent [Type 3C], Blue/Brown Transitional, and Fawn). We report bulk rock Re-Os isotope data for Premier kimberlite facies, as well as for a suite of entrained peridotite and mafic xenoliths. These data are complemented by bulk rock highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os), major- and trace-element abundances. Measured 187Os/188Os for the kimberlite facies range from 0.1223 to 0.1672 (ΥOsi of −2.5 to + 17.4), peridotite xenoliths range from 0.1096 to 0.1244 (ΥOsi of −13.3 to −1.1), and pyroxenite xenoliths range from 0.1796 to 0.938 (ΥOsi of + 27 to + 419). A single measured amphibolite xenolith has the most radiogenic measured 187Os/188Os of 2.86 (ΥOsi of + 43). Harzburgite xenoliths yield time of rhenium depletion model ages (TRD) of ∼ 1.5 to 2.8 Ga, consistent with average TRD ages for Premier peridotites (2.4 ± 0.4 Ga). With these and published data, we considered the relationships between kimberlite and mantle xenoliths, compare estimates of relative peridotite incorporation to sampled diamond grade, and explore recratonization versus refertilization arguments with regards to TRD model ages. Kimberlite melt infiltration into Premier peridotite xenoliths is evident from melt veins accounting for ∼ 2 and ∼ 14 modal % of samples, and has led to incompatible element enrichment, including elevated Re. In turn, kimberlites show geochemical evidence for addition of peridotite xenolith fragments, with Type 3C having > 30 % more peridotite contribution than the Brown volcaniclastic facies. Kimberlites and peridotites plot on a 187Re/188Os versus 187Os/188Os mixing line (R2 = 0.92), with kimberlites having older apparent ages than the true age of crystallization. This mixing line provides estimates of lithospheric incorporation into the kimberlites, where the units with higher peridotite incorporation do not correlate with diamond grade. This is likely due to lithological and post-emplacement alteration heterogeneity within the kimberlite units, perhaps also reflecting the eclogitic paragenesis of many Premier diamonds. The peridotites provide evidence for the nature of the lithosphere beneath Premier prior to ∼ 1.15 Ga. Metasomatism of the peridotites is possibly linked to the Bushveld Igneous Event at ∼ 2 Ga, as well as to other magmatic events that affected the Kaapvaal craton from the Archean to the Mesoproterozoic. Premier peridotites do not suggest that the cratonic lithosphere beneath the region was completely replaced. Samples with Proterozoic TRD eruption model ages may represent Archean lithosphere that experienced alteration by metasomatism and modification, such as during the Bushveld Igneous Event.

A billion-year shift in the formation of Earth’s largest ore deposits

Banded iron formations (BIFs) archive the relationship between Earth's lithosphere, hydrosphere, and atmosphere through time. However, constraints on the origin of Earth's largest ore deposits, hosted by BIFs, are limited by the absence of direct geochronology. Without this temporal context, genetic models cannot be correlated with tectono-thermal and atmospheric drivers responsible for BIF upgrading through time. Utilizing in situ iron oxide U-Pb geochronology, we provide a direct timeline of events tracing development of all the giant BIF-hosted hematite deposits of the Hamersley Province (Pilbara Craton, Western Australia). Direct dating demonstrates that the major iron ore deposits in the region formed during 1.4 to 1.1 Ga. This is one billion to hundreds of millions of years later than previous age constraints based upon 1) the presence of hematite ore clasts in conglomerate beds deposited before ~1.84 Ga, and 2) phosphate mineral dating, which placed the onset of iron mineralization in the Province at ~2.2 to 2.0 Ga during the great oxidation event. Dating of the hematite clasts verified the occurrence of a ~2.2 to 2.0 Ga event, reflecting widespread, but now largely eroded iron mineralization occurring when the Pilbara and Kaapvaal cratons were proximal. No existing phosphate mineral dates overlap with obtained hematite dates and therefore cannot be related to hematite crystallization and ore formation. New geochronology conclusively links all major preserved hematite deposits to a far younger (1.4 to 1.1 Ga) formation period, correlated with the amalgamation of Australia following breakup of the Columbia supercontinent.

Confocal Raman spectroscopic study of melt inclusions from peridotite xenoliths in economic and barren kimberlites from Kaapvaal Craton

AbstractMelt inclusions trapped in minerals inside xenoliths from kimberlites can help to examine the composition of kimberlite melt and/or metasomatic processes in the subcratonic lithospheric mantle as well as shed more light on the role of these melts in diamond destruction. In this study, confocal Raman spectroscopy of secondary melt inclusions in olivine from xenoliths in five different Kaapvaal Craton kimberlites was used for testing any compositional differences between melt inclusions from economically productive (Bultfontein and Frank Smit) and uneconomic diamond barren (Matsoku, Thaba Putsoa, and Pipe 200) kimberlite pipes. The xenoliths represent a range of pressures (37–45 kbar) and temperatures (1000–1300°C). The 26 daughter minerals identified within melt inclusions include Ca–Mg (±Na, K, P, Cl)‐bearing carbonates, alkali (±Ca, Ba, Cl, F, H2O, CO2)‐bearing sulfates, phosphates, oxides, silicates, and a rare nitrate. The mineral assemblages in melt inclusions are similar in both economic and barren kimberlite pipes from the interior and the edge of the craton, indicating the similar composition of the entrapped melts in all studied samples. However, the petrographic study revealed different metasomatic processes recorded by xenoliths from barren and economic kimberlites. Metasomatism by a melt enriched in K, Ca, and H2O could be instrumental in diamond destruction and the low diamond grade of the three barren kimberlites from Lesotho. Our study revealed no effect of kimberlite melt composition on diamond preservation in the studied kimberlites: instead, diamond grade is most likely affected by diamond destruction in the mantle source prior to kimberlite emplacement with kimberlite ascent.

Factors responsible for Witwatersrand gold mineralisation

Abstract The exceptional gold mineralisation in quartz pebble conglomerates of the Witwatersrand Basin is attributed to a combination of factors. These factors are linked to the co-evolution of the atmosphere, hydrosphere, lithosphere and biosphere, at a very specific time in Archaean geological history and the evolution of the Kaapvaal Craton. Following craton stabilisation and its subaerial emergence, intense chemical weathering and erosion of large volumes of granitoid-greenstone basement released detrital and dissolved gold. Shallow-marine reworking in a long-lived and slowly subsiding basin subjected to episodic compressional deformation and relative sea-level oscillations led to sedimentary concentration of detrital gold. The interaction between acidic, anoxic, and sulfurous surface runoff and more oxidizing marine waters in a near-coastal oxygen oasis supported microbially mediated diagenetic pyrite formation and incorporation of dissolved gold in the pyrite crystal lattice. Erosion and reworking of diagenetic pyrite gave rise to detrital pyrite that characterise most reefs. Abundance of detrital uraninite in conglomerates, derived from erosion of Mesoarchaean granites, and episodes of hydrocarbon migration through sedimentary strata during deep burial set the scene for further enhancement of gold grades in the reefs. Granular and seam pyro-bitumen formed by radiation-induced polymerisation of hydrocarbons around detrital uraninite. Gold dissolved in migrating hydrothermal fluids was then reduced and precipitated upon interaction with the reef pyro-bitumen during hydrothermal placer modification.

Sedimentary and stratigraphic architecture of the Duitschland and Rooihoogte formations (Palaeoproterozoic, South Africa): implications for tempo of the Great Oxidation Event

Abstract Determining the tempo and causality of key palaeoclimate events recorded in sedimentary strata depends on high-resolution numerical ages and well-constrained stratigraphic correlations at regional and global scale. This requirement is not necessarily met in Precambrian strata due to poorer age resolution, limited preservation, and secondary overprints. A good example includes the Palaeoproterozoic Rooihoogte and Duitschland formations in South Africa, which document the disappearance of mass-independent fractionation of sulphur isotopes (MIF-S) and contain glacial diamictites at their base. They are thus key records of Earth’s surface oxygenation during the Great Oxidation Event (GOE). However, previous studies have either correlated these units, resulting in a unidirectional oxidation trend; or have regarded them as successive strata, causing an interpretation of oscillating oxidation. This study uses extensive outcrop and new core material to investigate correlation between these two units, and to establish depositional models. Results show that key stratigraphic markers can be traced around the entire Kaapvaal craton both in outcrop and the subsurface. In particular, the basal Bevets breccia and the top Duitschland breccia are here re-interpreted as two separate units that are present at the base and top of both formations, supporting correlation of the formations. Consequently, the base of both formations records a major craton-wide event of uplift and karstification, leading to carbonate dissolution and chert brecciation. Erosion of older rocks from across the craton also delivered material for the basal glacial diamictite. The majority of mixed siliciclastic-carbonate sediments were deposited on a storm- and/or delta-influenced shelf. Depositional packages in both formations reflect post-glacial relative sea level rise, followed by progradation of a deltaic, storm or shoreline depositional system. There is a relatively short-lived depositional hiatus to overlying shales of the Timeball Hill Formation. Both Rooihoogte and Duitschland formations thus record only a single glacial event at their base, and a unidirectional trend of oxidation.

Kaapvaal lamproite melts (aka orangeites): A new type of alkali-carbonate liquid? insights from olivine-hosted multiphase inclusions (Silvery Home, South Africa)

Kaapvaal lamproites (aka orangeites) are a group of volatile-rich (H2O, CO2), micaceous, ultrapotassic igneous rocks that are unique to the Kaapvaal craton in southern Africa. However, the composition of the melts that give rise to these rocks remains poorly understood due to overprinting effects of contamination by mantle and crustal material, volatile exsolution, fractional crystallisation and post-magmatic alteration. Consequently, this lack of reliable data on the initial composition of the Kaapvaal lamproite melts hampers our understanding of their source, petrogenesis and ascent mechanisms.Olivine is a common mineral comprising the Kaapvaal lamproites that has both xenocrystic (i.e., mantle) and magmatic origins. Multiphase inclusions (melt/fluid) entrapped within olivine have been consistently demonstrated as an effective tool for gaining fundamental insights into the composition and evolution of melts that produce both kimberlites and lamproites, prior to processes, such as eruption, devolatization and syn-/post-magmatic alteration.In this study, multiphase inclusions of both secondary and pseudosecondary origin hosted in olivine from a Kaapvaal lamproite (Silvery Home, South Africa) provide novel insights into the composition of the melt(s) that initially transported olivine to the surface and then crystallised after emplacement to form the lamproite groundmass. The inclusions in our study contain daughter mineral assemblages consisting of diverse Ca-Mg carbonates, including K-, Na-, Ba-, and Sr-bearing varieties, moderate K-rich silicates (phlogopite, tetraferriphlogopite), and subordinate oxides, phosphates, sulphides, sulphates, and halides. Based on these daughter mineral assemblages, we suggest that the composition of the melt entrapped by olivine was SiO2-poor, Ca-Mg carbonate-rich and contained elevated concentrations of K, Na, Ba, Sr, P and Cl. The mineral and reconstructed melt compositions are in stark contrast to the mineral association of the groundmass and the bulk-rock composition of the Silvery Home lamproite, respectively. We suggest that alkali-/alkali-earth carbonates, phosphates, sulphides, sulphates, and halides represented a potentially significant, or even dominant, component of the melt that crystallised the Silvery Home lamproite but were likely removed by degassing and/or interaction with syn-/post-magmatic fluids. We show that olivine-hosted multiphase inclusions from the Silvery Home lamproite share many compositional similarities to melt inclusions hosted in olivine from kimberlites but are distinct from ‘classic’ cratonic olivine lamproites worldwide.

Metasomatism of mafic-intermediate rocks in the Motloutse Complex, Botswana: Effect on ore mineralization and implication for the Paleoproterozoic Large Alkali Metasomatic Province (LAMP) in southern Africa

Alkali metasomatism is a common overprint in Archean medium- to high-grade metamorphic terranes. Since mafic-intermediate rocks are able to host ore mineralization, it is important to understand the effect of metasomatism on its distribution. This study presents field, petrographic, mineralogical, geochemical, and U-Pb/40Ar-39Ar geochronologic characteristics of metasomatized amphibolite, amphibole gneiss and gabbro along a regional E-W section of the southeastern Motloutse Complex in eastern Botswana. The progressive metasomatism accompanies shear deformation and is visually manifested by varying degrees of pinkish discoloration of the host rocks. The metasomatic reactions in amphibolite, amphibole gneiss and gabbro are manifested in replacement of amphibole, clinopyroxene, ilmenite and plagioclase by the assemblages of chlorite, epidote, titanite, magnetite, sericite, albite and K-feldspar, up to near complete albitization and K-feldspathization with few relics of earlier minerals. Mineral compositions reflect the effects of a progressive metasomatic overprint. The metasomatic overprint resulted in the break down and dispersal of primary Cu-Ni sulphides. The pseudosection modelling (PERPLE_X) in terms of lg(aK2O) and lg(aH2O) parameters for representative compositions of leucocratic and melanocratic amphibolites, amphibole gneiss and gabbro indicates that the metasomatic process involved an increase of potassium activity followed by hydration. The modelling explains variable extent of metasomatism in different rock types. Whole-rock compositional variations indicate a general enrichment in Na2O, K2O and SiO2, and depletion in FeO and MgO in rocks during the metasomatism. 40Ar/39Ar amphibole and U-Pb titanite geochronology from metasomatized amphibolite and gabbro gave comparable ages of c.2.01–1.96 Ga. Alkaline fluids producing metasomatism were likely of crustal origin and moved along the reactivated regional c.2.01–1.95 Ga shear zones at the boundaries of the Motloutse-Limpopo complexes with the adjacent Zimbabwe and Kaapvaal cratons. Implications for a Paleoproterozoic Large Alkali Metasomatic Province (LAMP) in southern Africa, and its relation to the Bushveld large igneous province (LIP) is explored.

Multi-stage ultrahigh temperature metamorphism in the lower crust of the Kaapvaal craton recorded by U–Pb ages of garnet

U–Pb ages were determined by split-stream LA-SF/MC-ICPMS in garnets from UHT granulite xenoliths (Star mine, South Africa; 124 Ma). They give a considerable age range of 400 million years with well-defined maximas at 3.09, 3.01 and 2.75 Ga. The oldest peak overlaps with the changeover from tonalites to K-granites at 3.14–3.04 Ga and with zircon ages of the mid-crustal granulites of the Vredefort dome (3.1 Ga) in the wake of the 3.2 Ga collision of three terrains that compose the Witwatersrand block. Subduction (or sagduction) of the uppermost crust in an ultrahot orogen setting brought shales and greenstones to the lower crust. Ultrahigh temperature (UHT) conditions are the result of high mantle potential temperatures and self- heating by the radioactive inventory of the subducted lithologies. Metamorphism, anatexis to very high degrees and melt extraction left UHT granulites as residue. Rejuvenation of UHT conditions was brought about by Dominion Group magmatism between 3.0 and 2.95 Ga. Magmatic uprise caused intense shearing in the lower crust followed by recrystallisation of the shear zones to generate the younger garnet age group. Ventersdorp flood basalt volcanism caused similar processes at around 2.72 Ga and generated the third garnet age group. Zircon gives U–Pb ages mainly around 2.72 Ga (both literature and our own data) i.e. zircon adjusted or newly crystallized at the youngest UHT event. Only few zircon grains retained older ages up to 2.94 Ga. Still unconstrained, but very high closure temperatures (≥ 1100 °C) for the U–Pb system in garnet keep the memory of the oldest ages in UHT granulites. Such ages can only be reset by recrystallization. This way, garnet records a prolonged high-temperature history of the lower crust of the Kaapvaal craton.

The legacy of African kimberlite and diamond research and exploration for the earth sciences

Kimberlite is a rare and volumetrically insignificant magmatic rock, which nevertheless has an outsized economic importance as the world's principal source for natural diamonds. Discovered in 1870, in South Africa, it was named after Kimberley, the town that grew around the first mines developed in the pipe-shaped bodies of the newly discovered, ultramafic diamond source rock. Apart from their economic significance, kimberlites are of enormous scientific interest. Originating at great depth, they have entrained a variety of ultramafic and ultrabasic rock fragments (xenoliths) and their disaggregated mineral grains (xenocrysts) that provide a direct window into the upper mantle, making it possible to identify regional variations in the nature of the upper mantle and to understand mantle processes. After a flurry of studies on the heels of the initial kimberlite discoveries, there was a long hiatus in publications between the 1930's and mid-1960's. Kimberlite, diamond, and upper mantle studies received a lasting stimulus in the early 1970's, largely as the result of the seminal First International Kimberlite Conference (1 IKC), held at the University of Cape Town in 1973, and 10 follow-up conferences held since on every continent except Antarctica. This review explores examples of advances and controversies resulting from African-centered research on the subject. The definition and nomenclature of “kimberlite” progressed from applying the term to all igneous rocks with diamond, to distinguishing two types of diamond-bearing kimberlite (Groups I and II), and eventually recognizing that Group I (“archetypal” kimberlite) and Group II (formerly orangeite, now assigned to the olivine lamproite clan) are entirely different rock types, originating below and within the lower part of the lithosphere, respectively. As to the shape of kimberlite pipes, the classic southern African model of a carrot-shaped magmatic system, consisting of a root zone, a steep-sided diatreme, and a crater zone, was based on a composite of the highly eroded Kimberley and Jagersfontein pipes, for the lower part, and the significantly less eroded Orapa pipe in Botswana, for the upper part. Restriction of diamond-bearing igneous rocks to the ancient cratons of Africa led to the formulation of “Clifford's Rule”, an important tool in diamond exploration worldwide. Studies of peridotite xenoliths and Mesoarchean ages determined on peridotitic garnet inclusions in diamond xenocrysts from the Kaapvaal craton revealed that the craton is underlain by a deep diamondiferous lithospheric root which is older, cooler, and more refractory (harzburgitic) than the shallower lithosphere under adjacent Proterozoic orogenic belts, in fact providing a petrologic explanation for Clifford's rule. The correlation between harzburgitic garnet and diamond was expanded into a powerful exploration tool, developed, and tested in Africa, that was successfully applied in many other parts of the world. Interpretations of eclogite xenoliths changed from being cognate nodules in kimberlite, to pieces of an eclogite layer, to high-pressure cumulates from eclogitic melts in a peridotitic mantle, to fragments of subducted oceanic crust. Combining the latter interpretation with Archean ages for eclogitic mineral inclusions in diamond xenocrysts, it was recognized that plate tectonic processes began as early as ca. 2.9 Ga and were involved in the formation of the Kaapvaal craton, the oldest major Archean craton in the world. Based on the model developed for typical kimberlite pipes in southern Africa, it has been possible to quantify erosion processes across the southern part of the continent, particularly since the disruption of Gondwana. Dispersion of “pathfinder” kimberlitic indicator minerals (KIMs), including diamond, into drainage networks provide tracers to reconstruct past river systems. Being well endowed with kimberlite pipes, Africa has also been central to the important debate about the relevance of plumes as triggers for kimberlite and other alkaline magmatism. Nevertheless, despite the importance of resolving this geological conundrum, it remains highly controversial. Many kimberlites are capped by well-preserved crater-lake facies sediments, most of which are dated as Cretaceous to early Cenozoic, within the time of the proposed Angiosperm Terrestrial Revolution. As the associated plant fossil records are the only ones for the interior of southern Africa during this time interval, they may contribute to solving what has been coined as “Darwin's Abominable Mystery”, the poorly understood origin of angiosperms (flowering plants).

Petrogenesis and tectonic implications of the Bloubergstrand basaltic andesites, Tygerberg Formation, Malmesbury Group, Pan-African Saldania Belt, southwestern Africa

Abstract The Bloubergstrand Member, consisting of mafic to intermediate tuffs and flows within the Tygerberg Formation of the Malmesbury Group, is a crucial but understudied component of the western Saldania Belt in southwestern Africa. With a U-Pb zircon age of 555 ± 5 Ma, these volcanic rocks provide valuable insights into the origins and, more broadly, the tectonic setting of the Saldania Belt during the final stages of Malmesbury Group sedimentation in the context of the construction of southwestern Gondwana at approximately 560 to 540 Ma. The Bloubergstrand Member amygdaloidal volcanic rocks vary from basaltic andesitic to andesitic and have experienced varying degrees of alteration. The volcanic rocks are enriched in the large ion lithophile elements (LILE) relative to the high field strength elements (HFSE). When normalised to primitive mantle values, they show enrichment in Cs, Rb, K and Pb and mild enrichment in Th, U, Zr and Hf, with the light rare earth elements (LREE) enriched relative to the heavy rare earth elements (HREE). They are depleted in Ba, Nb, Ta, Sr, Eu, P and Ti. εNd(t) values are mildly negative ranging from -3.60 to -2.39, and Nd TDM model ages range from 1.4 to 1.7 Ga. Initial 87Sr/86Sr ratios vary from 0.70478 to 0.70620, with one higher value of 0.72270, the latter likely due to extensive alteration. The Bloubergstrand Member volcanic rocks exhibit characteristics suggesting their origin from partial melting of lithospheric to transitional asthenospheric upper mantle, influenced by sediment-derived melts and variable degrees of crustal contamination. With continental arc basalt compositions and similarities to shoshonites, the samples reflect variable degrees of partial melting and source heterogeneities. Fractional crystallisation of pyroxenes, hornblende, and plagioclase contributes to compositional variability. Erupting in a continental arc margin, likely part of the Arachania block of the Kalahari craton, separated during the Tonian break-up of Rodinia, the volcanics were associated with the Marmora basin formed by eastward-directed subduction below the western Kaapvaal Craton margin. Extruded in a distal position relative to the Cuchilla Dionisio Arc in present-day southern Brazil and Uruguay, these volcanic rocks preceded the closure of the southern Marmora basin. The mantle melting, possibly a result of slab roll-back, break-off, ridge subduction, or a combination, served as a precursor to the lower crustal heating that generated the granitic magmas of the Cape Granite Suite.

Mélange Signatures and Low Oxygen Fugacity in Eclogite Xenoliths From the Crust‐Mantle Transition Below a Mesoproterozoic Collision Belt

AbstractMass transfer across the crust‐mantle boundary is a fundamental process governing planetary differentiation, the evolution of geochemical reservoirs and ore formation, controlled by physicochemical conditions at the crust‐mantle interface. In situ trace‐element, clinopyroxene 87Sr/86Sr and garnet Fe3+/ΣFe of kimberlite‐borne eclogite xenoliths from the deep (∼50 km) crust‐mantle transition below the ca. 1.2–1.0 Ga Namaqua‐Natal Fold Belt (southwestern Kaapvaal craton margin) were determined to elucidate their origin and evolution, and to constrain the oxygen fugacity of this pivotal but largely inaccessible environment. Based on a garnet source signature (NMORB‐normalized Er/Lu > 1) in pristine “gabbroic” eclogites with pronounced positive Eu, Sr, and Pb anomalies, the suite is interpreted as originating as plagioclase‐rich cumulates in oceanic crust from melts generated beneath mature oceanic lithosphere, subsequently subducted during the Namaqua‐Natal orogeny. Enriched eclogites have higher measured 87Sr/86Sr in clinopyroxene (up to 0.7054) than gabbroic ones (up to 0.7036), and show increasing bulk‐rock Li, Be and Pb abundances with increasing δ18O in clinopyroxene, and muted Eu‐Sr‐Pb anomalies. These systematics suggest interaction with a siliceous fluid sourced from seawater‐altered oceanic sediment in a subduction mélange setting. Garnet Fe3+/ΣFe in deep crustal eclogites is extremely low (0.01–0.04, ±0.01 1σ), as inherited from the plagioclase‐rich cumulate protolith, and owing to preferred partitioning into clinopyroxene at low temperatures (∼815–1000°C). Average maximum oxygen fugacities (∆logƒO2(FMQ) = −3.1 ± 1.0 to −0.5 ± 0.7 relative to the Fayalite‐Magnetite‐Quartz buffer) are higher than in deeper‐seated on‐craton eclogite xenoliths, but mostly below sulfate stability, limiting the role of S6+ species in oxidizing the mantle wedge.

Thermo-tectonic evolution of the Neoarchaean Southern Marginal Zone of the Limpopo granulite Complex (South Africa)

Abstract Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event.

Growth faults and avalanches: Reconstructing Paleoarchean basins in the Pilbara and Kaapvaal cratons

Four new volcano-sedimentary complexes (VSC's), respectively c. 3510, 3460, 3430, and 3320 Ma old, are identified in the East Strelley, Coongan, and Kelly belts of the East Pilbara craton and compared with the extraordinarily complete c. 3450 Ma Buck Reef-VSC in the southern African Kaapvaal craton. The VSC’s reveal an intricate relationship between volcanic deposition, sedimentation, development of syndepositional fault arrays, both extensional and contractional, and magmatism. The geometry and kinematics of these fault systems were analyzed after restoring the tilt of the stratigraphic sequences back to the depositional horizontal. The growth fault arrays are interpreted to have been generated by lack of lateral support of depositional basin margin prisms as known from present-day deltas and passive margins. This ‘basin margin collapse’ scenario for deformation and kinematics of the topmost section of the crust relies on topographic relief combined with vertical crustal oscillation. The relationship between supracrustal collapse and coeval deformation on deeper-seated detachments within the basement of the Pilbara remains as yet unsolved.The restoration corroborates the (semi-)circular basin architecture we proposed in 2017, which preceded, and is unrelated to, the present-day configuration of granitoid complexes and greenstone belts. The new findings also assess an early, syndepositional presence of the east–west Warrawoona Lineament as a major dividing line of as yet unknown structural character between two areas of basin superposition.For the Coonterunah Subgroup a multistorey architecture is established: the Table Top/Coucal-VSC with its near-water level chert top is overlain by regularly bedded Double Bar Basalt. The ensemble is truncated by the newly introduced tectono-stratigraphic Bergamina Unit, interpreted as a mega-avalanche emplaced somewhere between 3496 and 3466 Ma, possibly time-equivalent to the Duffer Fm.The crustal depths of the detachments below the (water-level) tops of all VSC’s identified, in other words the thicknesses of the VSC’s, are used as proxy for determining minimum basin centre depths ranging from 1000 to 3800 m. The greatest depth was reached in the North and South Coongan Basins, where voluminous bimodal volcanism of the Duffer Fm, maximum subsidence and deposition rates resulted in maximum basin margin instability.

Zircon U–Pb–Hf isotope and geochemical constraints on the petrogenesis and tectonic setting of Mesoarchean granitoids from the Carajás province, Amazonian craton, Brazil

Based on U–Pb–Hf isotope and whole-rock geochemical data, this paper investigates the origin and tectonic setting of Mesoarchean granitoids from the Ourilândia do Norte area, Carajás province (northern Brazil) and compares them with other Archean units. The following units were selected: (i) tonalite-trondhjemite-granodiorite (TTG) from the Mogno and Rio Verde suites, represented by a 2.92 Ga tonalitic xenolith [εHf(2.92 Ga) = +2.0 to –0.2] and a 2.92 Ga porphyritic trondhjemite [εHf(2.92 Ga) = +2.3 to –3.5], respectively; (ii) the Ourilândia sanukitoid suite, composed of the 2.92 Ga Arraias granodiorite [εHf(2.92 Ga) = +1.9 to –4.4] and 2.88 Ga Ourilândia tonalite-granodiorite complex [εHf(2.88 Ga) = +3.2 to –1.0]; and (iii) the Boa Sorte granite (Canaã dos Carajás granitic suite), which provides four zircon populations dated at 3.04 Ga, 2.98 Ga, 2.93 Ga and 2.88 Ga. The 2.88 Ga population represents the granitic crystallization age, with εHf(2.88 Ga) values ranging from –0.8 to –4.1. The Paleoarchean geological record from the Carajás province is scarce and restricted to detrital zircons (<3.7 Ga), rare xenocrystals (<3.7 Ga) and crustal model ages (Hf-TDMC = 3.6–3.1 Ga), which indicate Paleoarchean crust extraction from the mantle. The zircon U–Pb–Hf isotope and whole-rock geochemistry data from the Mesoarchean granitoids support the hypothesis that the Carajás province experienced mantle enrichment and progressive crustal reworking during 3.1–2.8 Ga, similar to the Mesoarchean record from the Pilbara and Kaapvaal cratons. A proposed three-stage tectonic model explains the petrogenesis and Hf isotope signatures of the studied granitoids. Regardless of the extent of the Paleoarchean geological record, the available data allow us to postulate that the formation of Paleoarchean crust in the Carajás province involved a long-lived dome-and-keel setting (3.6–3.1 Ga), and that this crust was later recycled, allowing mantle enrichment during Mesoarchean low-angle subduction (3.07–2.92 Ga). Then, a short-lived collision defined by peak regional metamorphism (2.89–2.84 Ga) and associated with crustal thickening and slab breakoff gave rise to mantle- and crust-derived magmas at 2.88 Ga.

Sheared Peridotites from Northern Lesotho: Metasomatism-Induced Deformation and Craton Destabilization

Abstract Sheared peridotite xenoliths are snapshots of deformation processes that occur in the cratonic mantle shortly before their entrainment by kimberlites. The process of deformation that caused the shearing has, however, been highly debated since the 1970s and remains uncertain. To investigate the processes involved in the deformation, we have studied 12 sheared peridotites from Late Cretaceous (90 Ma) kimberlites in northern Lesotho, on the southeast margin of the Kaapvaal craton. Various deformation textures are represented, ranging from porphyroclastic to fluidal mosaic. Our sample suite consists of eleven garnet peridotites, with various amounts of clinopyroxene, and one garnet-free spinel peridotite with a small amount of clinopyroxene. All of the peridotites are depleted in Fe, and the Mg# of olivine and orthopyroxene range from 91 to 94. Three groups of sheared peridotites are present and have been identified primarily on the basis of Ca contents of olivine and orthopyroxene. The porphyroclasts preserve pre-deformation P–T conditions of 3.5 to 4.5 GPa and 900°C to 1100°C (Group I), 5 to 5.5 GPa and 1200°C to 1250°C (Group II) and 6 ± 0.5 GPa and 1400 ± 50°C (Group III). Group III samples lie above the 40 mW/m2 conductive geothermal gradient, indicating thermal perturbation prior to deformation. The sheared peridotites from Lesotho were affected by various metasomatic events. Pre-deformation metasomatism, involving melts and fluids, is recorded in the porphyroclasts. In Group II and III samples, the clinopyroxene porphyroclasts have similar compositions to Cr-rich and Cr-poor clinopyroxene megacrysts, respectively, that have previously described from southern African kimberlites. This suggests a relationship between them. Younger pre-deformation metasomatism is preserved in a zoned garnet from Group II (enrichment in Ti, Zr, Y + HREE) and orthopyroxene in a Group I sample. The latter exhibits a complex zonation, with a highly enriched (Fe, Ti) inner rim and a less-enriched outer rim. These enrichments must have occurred shortly before deformation. Metasomatism during deformation is revealed by the complex chemical changes recorded in olivine neoblasts with, depending on the sample, increasing or decreasing contents of Ti, Ca, Al, Cr, Mn and Na. Crystallographic preferred orientations of olivine neoblasts are consistent with bimodal, B, C, E, AG-type fabrics and indicate the presence of a hydrous metasomatic agent. We suggest that, akin to the shallower sheared peridotites (Group I), Groups II and III were influenced by early (proto-)kimberlite melt pulses and propose the following model: (proto-)kimberlitic melts invaded the lower lithosphere. These melts followed narrow shear zone networks, produced by deformation at the lithosphere–asthenosphere boundary, heated and metasomatized the surrounding peridotites and were responsible for megacryst crystallization. Sheared peridotites from close to the melt conduits (Group III) have compositions comparable to Cr-poor megacrysts, while those located at a greater distance (Group II) resemble Cr-rich megacrysts. Reactive infiltration of volatile-rich proto-kimberlite melts caused rheologically weakening of olivine in the lithospheric mantle. The consequence of this positive feedback mechanism of metasomatism, weakening and deformation—due to the high magmatic and metasomatic activity in the Late Cretaceous—is the progressive perforation of the lower Kaapvaal lithosphere by rheologically weak zones and the destruction of the protecting dry and depleted layer at its base. This could have caused the observed thinning and destabilization of the lower lithosphere below the southern margin of the Kaapvaal craton.

Petrology of an apatite-calcite segregationary kimberlite from the Andriesfontein pipe, Kaapvaal craton

Here, we present a case study of an apatite-calcite segregationary kimberlite from the Andriesfontein pipe (Kaapvaal craton) using EPMA and LA-ICP-MS data of olivine macrocrysts and groundmass phases to constrain magma evolution. The primary objectives are to first constrain magma evolution in the SCLM using olivine followed by late-stage evolution and the timing of fluid exsolution using groundmass phases and importantly, apatite. Olivine cores display a range in Mg# (87.2–93.6) with no distinguishable gap in compositions. Aluminium, however, clearly divides the cores into two populations: 1) high-Al cores (>100 ppm Al; >400 ppm Ca; Mg# 87.2–92.7; 52% of cores) interpreted to predominantly reflect the disaggregation of megacrysts, and 2) low-Al cores (<100 ppm Al; <200 ppm Ca; Mg# >92; 48% of cores) interpreted to reflect sampling of garnet peridotite at depths of 84.5–138 km estimated using the Al-in-olivine thermometer. Groundmass phlogopite and spinel are well-zoned recording evolution of the kimberlite melt with increasing oxidation state indicated by the presence of MUM spinel and tetraferriphlogopite rims. Perovskite trace elements concentrations are typical of kimberlite perovskite worldwide. Apatites are observed in late-stage segregations as both equant grains and radiating clusters of acicular laths, however, their compositions are identical (SrO ∼2.2 wt%, SiO2 < 1 wt%, and low REE e.g., La2O3 + Ce2O3 - <0.01 wt%). The relatively high SrO and low SiO2 suggests formation from a fluid phase rather than directly from the melt, consistent with the presence of apatite in segregations and predominantly acicular habit. Chondrite normalised REE profiles for apatite display sinusoidal patterns, which can be modelled by melt evolution through small degrees of predominantly perovskite fractionation. Ultimately, we present a five-stage model for melt evolution from melt-SCLM interactions to magma evolution through groundmass crystallisation and late-stage fluid exsolution, highlighting the complexity of kimberlite magma evolution from source to surface.

Titanium and iron isotopic records of granitoid crust production in diverse Archean cratons

Archean granitoid provides critical clues on how Earth's felsic crust was established and its geodynamics evolved. In this study, we present Fe-Ti isotope and trace-element data for granitoids (mostly TTGs) from the ∼3.8-3.6 Ga Itsaq Gneiss Complex and ∼3.3 Ga East Pilbara Terrane, Pilbara Craton. TTGs from those localities and several other cratons follow the same Ti isotopic fractionation trend as modern calc-alkaline rocks. This similarity hides important petrogenetic differences as partial melting could have played a much more important role in establishing the felsic character of TTGs compared to modern granites of calc-alkaline affinity. Our thermodynamics-based modeling of isotopic fractionation shows that partial melting of hydrated metabasite can explain the Fe and Ti isotopic compositions of many TTGs but reworking of tonalite crust is needed to explain the most extreme compositions. The same Ti isotopic fractionation trends have been found now in several cratons (West Greenland, Slave, Pilbara, and Kaapvaal cratons) in granitoids formed in different geothermal gradients, meaning that it was likely a global signature of the Archean crust. Sediment provenance studies that use those TTG compositions and point to a primarily felsic crust in the Archean are therefore valid.

New insights into the tectonics and geodynamics of Botswana from intracrustal structures revealed by receiver function analysis

SUMMARY Recent improvements in seismic coverage allow for an increasingly detailed knowledge of the crustal structure beneath Botswana to be obtained. For the first time, shear wave velocity–depth models throughout Botswana are obtained from genetic algorithm inversion of receiver functions. In this approach, averaging of important structural variations is minimized by separating receiver function waveforms into backazimuth and slowness bins. Division of crustal layers into sediment, upper, middle and lower crust reveals important features that enhance our knowledge of tectonics and geodynamics of Botswana. In the west, beneath the Ghanzi-Chobe belt, as well as at the border region of the Zimbabwe craton with Magondi belt, the crust has been thickened through a thickening of the lower crust, most likely by collisional tectonics during the Damara Orogeny. A significant difference in structure is observed between the western and central-to-eastern Kaapvaal craton. Western Kaapvaal craton has a thin mafic lower crust (&amp;lt;10 km) that has survived the extensive melting of the Ventersdorp tectonomagmatic event at 2.7 Ga, contrary to previous reports of a lack of a mafic lower crust. Meanwhile in central-to-eastern Kaapvaal craton, a lower average ${{{}{V}}}_{{}{s}}$ (∼3.3 km s−1) in the upper crust suggests the presence of a suture zone that separates the two blocks of the Kaapvaal craton of different ages and crustal thicknesses. A relatively low ${{{}{V}}}_{{}{s}}$ in the middle crust from the Okavango rift zone to central and SE Botswana suggests reactivation of Palaeoproterozoic shear zones along a thin and weak crust by intraplate relative motion has enabled fluid infiltration from the mantle which causes seismicity in the region. Furthermore, a NW–SE trending region of high Vs in crustal layers is likely related to Karoo flood magmatism that formed the NW–SE trending Okavango dyke swarm. There is also an indication that a dedicated study is required to further investigate shallow converted phases from mobile belts and sedimentary basins in Botswana, however, the results from this study can provide a good starting point for such a study.

Th e age of detrital zircon from metasedimentary rocks of the Ternuvate strata (West Azov block of the Ukrainian Shield)

The Ternuvate strata comprise metamorphic rocks that make up the Haichur arcuate structure, which is about 72 km long. Its western part lies within the Andriivka fault zone, which separates the Vovcha and Huliaipole blocks, while the eastern part is located within the Ternuvate fault zone, traced on the Remivka block. The rocks composing the Haichur structure have irregular and laterally variable composition, changeable thickness, and exhibit dynamometamorphic structures of boudinage and schistosity. The upper part of the Ternuvate strata mainly consists of metasedimentary rocks, including gneisses, biotite schists, garnet-biotite, magnetite-amphibole, and feldspar quartzites. The lower part comprises volcanogenic rocks such as amphibolites, metaultrabasites, and biotite-amphibole gneisses. Using the LA-ICP-MS method, 38 zircon crystals from muscovite-biotite gneisses in the upper part of the Ternuvate strata were analyzed. Based on the geochemical data, these zircons are metamorphosed graywackes. The zircon crystals belong to several age populations (3.65-3.45 and 3.3-2.95 Ga), corresponding to the major stages of Archean crust formation in the West Azov domain, including the formation of the oldest basement and granite-greenstone complexes of the Paleoarchean and Mesoarchean ages. Identical populations of detrital zircon were found in the early Precambrian metaterrigenous rocks of the Krutobalka Formation in the Sorokyne greenstone structure. The correspondence between the Paleoarchean crust (3.45-3.65 Ga) of the West Azov block of the Ukrainian Shield and the Kursk-Besedine granulite-gneiss area of the Kursk Magnetic Anomaly block is evident, while the Paleoarchean and Mesoarchean complexes (2.9-3.3 Ga) correspond to the Mykhailivka and Orel-Tim granite-greenstone area of the Kursk Magnetic Anomaly block. The Archean complexes of Sarmatia are of the same age as similar formations in the Kaapvaal craton in South Africa, Bastar craton in India, North China Craton, Slave craton in Canada, and others, which formed during the Eoarchaean.