Archean Crust Formation Research Articles

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.

Evolution of the sources of TTG and associated rocks during the Archean from in-situ 87Sr/86Sr isotope analysis of apatite by LA-MC-ICPMS

Radiogenic isotopes provide an important means towards elucidating Archean crustal evolution. The global Hf and Nd isotope record of Archean crustal fragments has been instrumental to unveiling the history of ancient crustal growth and differentiation. The Rb-Sr system could provide valuable complementary constraints in this regard, as this system is particularly sensitive to magmatic fractionation processes, and the chemical and isotopic evolution of magma sources. Application of this system has so far been complicated, however, by its susceptibility to isotope re-equilibration or alteration of the Rb/Sr parent-daughter ratio. In-situ Sr isotope analysis of primary igneous minerals with very low Rb/Sr, such as apatite, provides a new means to determine the initial 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr i ) values for igneous rocks directly. In this study, we apply in-situ Sr isotope analysis of apatite by LA-MC-ICPMS to tonalite-trondhjemite-granodiorite (TTG) rocks and end-member sanukitoids from Archean cratons worldwide. The 87 Sr/ 86 Sr i values of sanukitoids are relatively radiogenic, supporting the model in which such rocks are formed by flux melting of a mantle strongly enriched by metasomatism, possibly by slab-derived fluids. The 87 Sr/ 86 Sr i values for TTGs formed between 3.72 and 3.45 Ga are generally radiogenic, indicating aged amphibolite sources. The 87 Sr/ 86 Sr i values of younger TTGs are systematically lower and were derived from mafic sources that had an average age of ≤0.2 Gyr. This evolution matches with observations from Hf isotopes for TTGs of similar age and indicates a systematic change in the nature or efficiency of TTG crust formation during the Paleoarchean. In-situ Sr isotope analysis of apatite provides a useful method to uncover the Sr record of the early continental crust, and enables constraints on local source evolution and the general two-step evolutionary process of Archean crust formation. • New Sr isotope record of Archean TTGs from LA-MC-ICPMS analysis of apatite • TTG Sr isotope compositions change from radiogenic to unradiogenic at c. 3.5 Ga. • Neoarchean sanukitoids uniformly provide unradiogenic Sr isotope signatures. • The change represents a systematic shift in the average age of mafic TTG sources. • This shift occurred within a small time window (3.6–3.4 Ga) in different cratons.

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

Constraints on Archean crust formation from open system models of Earth evolution

Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge–Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ∼30% and ∼75% of the present-day mass of the CC, respectively. Models proposing formation of most (∼90%) of the present-day CC during the initial 1 Gyr or nearly 50–60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (∼ 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).

Rapid mantle convection drove massive crustal thickening in the late Archean

The lithospheric mantle beneath Archean cratons is conspicuously refractory and thick compared to younger continental lithosphere (Jordan, 1988; Boyd, 1989; Lee and Chin, 2014), but how such thick lithospheres formed is unclear. Using a large global geochemical database of Archean igneous crustal rocks overlying these thick cratonic roots, we show from Gd/Yb– and MnO/FeOT–SiO2 trends that crustal differentiation required continuous garnet fractionation. Today, these signatures are only found where crust is anomalously thick (60–70 km), as in the Northern and Central Andes and Southern Tibet. The widespread garnet signature in Archean igneous suites suggests that thickening occurred not only in the lithospheric mantle but also in the crust during continent formation in the late Archean. Building thick crust requires tectonic thickening or magmatic inflation rates that can compete against gravitational collapse through lower crustal flow, which would have been enhanced in the Archean when geotherms were hotter and crustal rocks weaker. We propose that Archean crust and mantle lithosphere formed by thickening over mantle downwelling sites with minimum strain rates on the order of 10−13–10−12 s−1, requiring mantle flow rates associated with late Archean crust formation to be 10–100 times faster than today.

Generation of felsic crust in the Archean: A geodynamic modeling perspective

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. Higher mantle temperatures and higher radiogenic heat production in the Archean Earth would have impacted the thickness and composition of the crust. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean gray gneiss complexes is the sodic tonalite–trondhjemite–granodiorite (TTG) suite. Melting of hydrated basalt at garnet amphibolite, granulite or eclogite facies conditions is considered to be the dominant process that generated the Archean TTGs. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical tectono-magmatic numerical model with initial conditions appropriate to the Eoarchean–Mesoarchean. Based on the result of our experiments, we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust: (1) delamination and dripping of the lower mafic crust into the mantle; (2) local thickening of the crust; and (3) small-scale crustal overturns. In the context of a stagnant-deformable lid tectono-magmatic geodynamic regime that is terminated by short-lived subduction, we identify two distinct types of continental crust. The first type is a pristine granite–greenstone-like crust with dome-and-keel geometry formed over delaminating–upwelling mantle which is mostly subjected to vertical tectonics processes. By contrast, the second type is a reworked (accreted) crust comprising strongly deformed granite–greenstone and subduction-related sequences and subjected to both strong horizontal shortening and vertical tectonics processes. Thus, our study has identified a possible spatial and temporal transition from pristine granite–greenstone-like crust with dome-and-keel geometry to reworked (accreted) crust forming more felsic gneiss terranes in the Archean. We suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the Archean geological record.

Timing of Archean crust formation and cratonization in the Awsard-Tichla zone of the NW Reguibat Rise, West African Craton: A SHRIMP, Nd–Sr isotopes, and geochemical reconnaissance study

The Awsard-Tichla zone, in the Moroccan sector of the Reguibat Rise, comprises three lithodemic units, the TTG Aghaylas Suite, the Tichla greenstone belt, and the intrusive feldspathoidal syenites of the Awsard-Lechuaf group. The Aghaylas suite consists of Mesoarchean tonalites and trondhjemites with minor granodiorites and granites that are locally migmatised. These rocks are notably depleted in HREE, Y, Nb, Ta, and U, with elevated Th/U and K/Rb, 87Sr/86Sr(t) from 0.7003 to 0.7030, ɛNd(t) from +2 to +5, and Nd model ages in the range TCR=3.03–2.92Ga. Eight TTG gneisses yielded SHRIMP U–Pb zircon crystallisation ages between 3.04Ga and 2.92Ga. One TTG leucocratic gneiss from the neighbouring Oulad Dlim sector has a TCR=3.09 and a zircon crystallisation age of 2.94Ga but also contains a 3.11Ga population of pre-magmatic zircons. The Tichla greenstone belt, on the other hand, was formed between 3.03Ga and 3.01Ga, though Nd model ages suggest the contribution of an older crust. Field evidence and the chemical and isotopic compositions of the TTGs are consistent with crustal recycling of slightly younger juvenile TTG. After 2.92Ga there is no trace of magmatic activity until the intrusion of the Awsard feldspathoidal (kalsilite and nepheline) syenites at 2.46Ga. This reveals that, for reasons that are not yet well understood, the Archean crust in this sector of the Reguibat Rise stabilised long before than any other surrounding cratonic area. Furthermore, the crust stayed stable until the Archean-Proterozoic transition that, here, was marked by a peculiar high-K silica-subsaturated magmatism more extreme in composition than in any other known Archean terrane.

Isotopic geochronological evidence for the Paleoproterozoic age of gold mineralization in Archean greenstone belts of Karelia, the Baltic Shield

The Rb-Sr age of metasomatic rocks from four gold deposits and occurrences localized in Archean granite-greenstone belts of the western, central, and southern Karelian Craton of the Baltic Shield has been determined. At the Pedrolampi deposit in central Karelia, the dated Au-bearing beresite and quartz-carbonate veins are located in the shear zone and replace Mesoarchean (∼2.9 Ga) mafic and felsic metavolcanic rocks of the Koikar-Kobozero greenstone belt. At the Taloveis ore occurrence in the Kostomuksha greenstone belt of western Karelia, the dated beresite replaces Neoarchean (∼2.7 Ga) granitoids and is conjugated with quartz veins in the shear zone. At the Faddeinkelja occurrence of southern Karelia, Aubearing beresite in the large tectonic zone, which transects Archean granite and Paleoproterozoic mafic dikes, has been studied. At the Hatunoja occurrence in the Jalonvaara greenstone belt of southwestern Karelia, the studied quartz veins and related gold mineralization are localized in Archean granitoids. The Rb-Sr isochrons based on whole-rock samples and minerals from ore-bearing and metasomatic wall rocks and veins yielded ∼1.7 Ga for all studied objects. This age is interpreted as the time of development of ore-bearing tectonic zones and ore-forming hydrothermal metasomatic alteration. New isotopic data in combination with the results obtained by our precursors allow us to recognize the Paleoproterozoic stage of gold mineralization in the Karelian Craton. This stage was unrelated to the Archean crust formation in the Karelian Block and is a repercussion of the Paleoproterozoic (2.0–1.7 Ga) crust-forming tectonic cycle, which gave rise to the formation of the Svecofennian and Lapland-Kola foldbelts in the framework of the Karelain Craton. The oreforming capability of Paleoproterozoic tectonics in the Archean complexes of the Karelian Craton was probably not great, and its main role consisted in reworking of the Archean gold mineralization of various genetic types, including the inferred orogenic mesothermal gold concentrations.

Mechanisms of Archean crust formation inferred from high-precision HFSE systematics in TTGs

It has been proposed that Archean tonalitic–trondhjemitic–granodioritic magmas (TTGs) formed by melting of mafic crust at high pressures. The residual mineralogy of the TTGs (either (garnet)-amphibolite or rutile-bearing eclogite) is believed to control the trace element budget of TTGs. In particular, ratios of high-field-strength elements (HFSE) can help to discriminate between the different residual lithologies. In order to place constraints on the source mineralogy of TTGs, we performed high-precision HFSE measurements by isotope dilution (Nb, Ta, Zr, Hf) together with Lu–Hf and Sm–Nd measurements on representative, ca. 3.85–2.8 Ga TTGs and related rock types from southern West Greenland, W-India and from the Superior Province. These measurements are complemented by major and trace element data for the TTGs. Texturally homogeneous early Archean (3.85–3.60 Ga old) and Mesoarchean (ca. 3.1–2.8 Ga old) TTGs have both low Ni (<11 ppm) and Cr contents (<20 ppm), indicating that there was little or no interaction with mantle peridotite during ascent. Ratios of Nb/Ta in juvenile Eoarchean TTGs range from ca. 7 to ca. 24, and in juvenile Mesoarchean TTGs from ca. 14 to ca. 27. Even higher Nb/Ta (14–42) were obtained for migmatitic TTGs and intra-crustal differentiates, most likely mirroring further fractionation of Nb from Ta as a consequence of partial melting, fluid infiltration and migmatisation. In the juvenile TTGs, positive correlations between Nb/Ta and Gd/Yb, La/Yb, Sr/Y, Zr/Sm and Zr/Nb are observed. These compositional arrays are best explained by melting of typical Isua tholeiites in both, the rutile-bearing eclogite stability field (>15 kbar, high Nb/Ta) and the garnet-amphibolite stability field (10–15 kbar, low Nb/Ta). With respect to the low end of Nb/Ta found for TTGs, there is currently some uncertainty between the available experimental datasets for amphibole. Independent of these uncertainties, the TTG compositions found here still require the presence of both endmember residues. A successful geological model for the TTGs therefore has to account for the co-occurrence of both low- and high-Nb/Ta TTGs within the same geologic terrane. An additional feature observed in the Eoarchean samples from Greenland is a systematic co-variation between Nb/Ta and initial εHf(t), which is best explained by a model where TTG-melting occured at progressively increasing pressures in a pile of tectonically thickened mafic crust. The elevated Nb/Ta in migmatitic TTGs and intra-crustal differentiates can shed further light on the role of intra-crustal differentiation processes in the global Nb/Ta cycle. Lower crustal melting processes at granulite facies conditions may generate high-Nb/Ta domains in the middle crust, whereas mid-crustal melting at amphibolite facies conditions may account for the low Nb/Ta generally observed in upper crustal rocks.

ReOs and UPb geochronological constraints on the eclogite–tonalite connection in the Archean Man Shield, West Africa

Mantle-derived eclogite xenoliths and tonalite–trondhjemite–granodiorites (TTG) that occur in the Man Shield, West Africa, sample different levels of Archean lithosphere. Chemical and oxygen isotope systematics indicate that low MgO eclogites from the Koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional Archean crust formation. ReOs whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an Archean age of 3.44±0.76 Ga (2 σ), with ReOs model ages ranging from 1.4 to 3.9 Ga. This wide range of model ages overlaps with the age range for crust formation and metamorphism in the Man Shield. In situ UPb ages of zircons from crustal rocks have been measured by laser ablation ICP-MS. A tonalitic gneiss has discordant zircons with rare old cores (∼3.6 Ga) and an upper concordia intercept at 2890±9 Ma (2 σ). Zircons from a mafic lower crustal granulite xenolith are concordant at 2686±32 Ma. Our results, together with previously published ages for Man Shield rocks, indicate an early Archean crust formation event followed by major crustal growth at 2.9–3.0 Ga and a last major metamorphic event at 2.7 Ga. These data show that the eclogites and the continental crust of the West African Craton overlap in time of formation (but only at the very broad age uncertainty provided by the eclogite ReOs results). They are permissive of Archean crustal growth by melting of the protoliths of the materials now sampled as the Koidu eclogite xenoliths. If so, this suggests that Archean crustal growth in the Man Shield occurred in a convergent margin setting.

Assembly of the Armorica Microplate: A Strike‐Slip Terrane Delivery? Evidence from U‐Pb Ages of Detrital Zircons

Abstract Detrital zircons separated from the Ordovician Armorican quartzite in the foreland of the Variscan belt of Iberia were dated by laser ablation inductively coupled plasma mass spectrometry. The age groups found correspond to Proterozoic and Archean crust formation events. The Proterozoic events reflected by zircon ages are Avalonian‐Cadomian‐Pan‐African (550–800 Ma), Grenvillean‐Sunsas (900–1300 Ma), and Eburnian‐Transamazonian‐Icartian (1800–2200 Ma). Archean zircon ages are comprised between 2500 and 2800 Ma (Central‐Amazonian‐Liberian events). The presence of Mesoproterozoic (Grenvillean) zircons in the Armorican quartzite, which was deposited in a shallow platform bordering the NW African craton (north Gondwana margin), suggests that the NW Iberian terrane drifted from the South American margin, where it was located during the Neoproterozoic, toward Africa. The drift of the Iberian block, likely achieved via transform faults, was arrested by its docking and amalgamation with the peri–NW Africa...

Assembly of Archean cratonic mantle lithosphere and crust: plume–arc interaction in the Abitibi–Wawa subduction–accretion complex

Recent thermodynamic models suggest that direct interaction between mantle plumes and island arcs will enhance long-term arc buoyancy and contribute disproportionately to the crustal record. However, crustal growth models have also proposed a range of differing mechanisms for Archean crust formation that emphasize specific rock types, such as tonalite–trondhjemite–granodiorite (TTG) plutons or high magnesian andesites. Studies in the Abitibi–Wawa subprovince, allow these proposals to be evaluated in the context of the world's largest greenstone belt. Crustal growth in the southern Superior Province was the product of subduction–accretion tectonics enhanced and modified by mantle plume processes, particularly mantle plume–island arc interaction. High Archean geothermal gradients promoted volumetrically minor slab melts from the earliest phases of the Abitibi–Wawa arc, resulting in the adakite–high magnesium andesite–Niobium–enriched basalt association. However, recent flat subduction models for the formation of adakites also provide important insights into the generation of syn-tectonic Archean TTG batholiths that were probably derived from subducted, rather than accreted, oceanic crust. The distribution of Niobium enriched basalts (NEB) in the southern Superior Province may reflect plume controlled shallow subduction beneath the Abitibi belt that limited melting depths within a restricted asthensopheric mantle tongue. A well-constrained tectonic history, the mantle source requirements of successively-formed components of the Abitibi–Wawa crust, and detailed seismic interpretations of crustal architecture all preclude the presence of an autochthonous mantle lithospheric root beneath the Abitibi–Wawa arc. Instead, the late diapiric ascent of buoyant refractory plume residue and subducted oceanic crust resulted in the coupling of the mantle root and arc crust 10's of million years following batholith emplacement. High-velocity material identified at the base of the crust and centered beneath the Abitibi–Pontiac suture zone is probably Archean aged rather than Proterozoic. It corresponds to minor melts generated during ascent of the plume residue diapir and underplated prior to and during formation of the >200 km thick Abitibi–Wawa continental mantle lithosphere root.

Geochemistry of xenolithic eclogites from West Africa, part I: A link between low MgO eclogites and archean crust formation

—Oxygen isotope, mineral trace element, and measured and reconstructed whole rock compositions are reported for low MgO (6–13 wt.% MgO in the whole rock) eclogite xenoliths from the Koidu kimberlite complex, Sierra Leone. The δ 18O values of garnet (4.7–6.8‰), determined by laser fluorination on clean mineral separates, extend beyond the range for mantle peridotites. All low MgO eclogites have reconstructed trace element patterns that are depleted in Ba, Th, U, and light rare earth element (LREE), with jadeite-rich samples having more variable trace element patterns than jadeite-poor samples. These observations, coupled with low SiO 2 contents, and Nb-rich but LREE-depleted reconstructed whole rock compositions, suggest the low MgO eclogites are remnants of altered oceanic crust that was partially melted during subduction. Partial melting of a mafic protolith at high pressure (leaving a garnet-bearing residue) is the preferred model to explain the origin of Archean tonalite-trondhjemite-granodiorite (TTG) suites, which make up large portions of the crust in Archean cratons. We therefore suggest that the Koidu low MgO eclogites may be residues from Archean continental crust formation.

Migmatization by metamorphic segregation at subsolidus conditions: implications for Nd–Pb isotope exchange

We have carried out major and trace element analyses as well as Nd and Pb isotope measurements on a stromatic migmatite in the Phikwe Complex (Botswana) in order to assess the effect of high-grade metamorphic event leading to migmatization on apparent Nd model ages. The Phikwe Complex, considered as an entity of the Central Zone of the Limpopo Belt, underwent a high-grade metamorphic event at 2.0 Ga leading to migmatite formation. Migmatites in this area present a very good layering of migmatitic components. Leucocratic layers were segregated from mafic layers, dominated either by hornblende or biotite, on a centimeter to millimeter scale. Accessory minerals such as monazite, apatite and zircon were not identified in hornblende-rich and quartzo-feldspathic layers. The combination of petrography and trace element concentrations in all layers indicates that the migmatite may have been formed either by partial melting according to biotite and/or hornblende dehydration reactions, or by metamorphic segregation at subsolidus conditions. REE modeling, applying batch, fractional and disequilibrium melting equations, allows to discriminate between these two processes. The modeled REE results of the quartzo-feldspathic layers match those observed via metamorphic segregation at subsolidus conditions. The ε Nd values at 2.0 Ga range from 0.3 to −8.9 with lower values in the REE-rich migmatitic layers. Thus, the distribution of major and trace elements as well as the scatter Nd isotope data are interpreted as reflecting the proportional amount of each mineral in the different migmatitic components. The positive ε Nd value of hornblende-rich layers suggests hornblende crystallization from a mantle-derived source and Sm/Nd fractionation at protolith formation age. Apparent Nd model ages ranging from 2.3 to 3.1 Ga, do not reflect the crustal evolution in Southern Africa in Archean to Proterozoic time. Based on the geochemical features and the Pb–Pb WR isochron at 2.0 Ga, mass conservation during migmatization can be assumed. Therefore, chemical and Nd isotopic compositions of the protolith prior to migmatization can be calculated. The results indicate a Si and K poor diorite as protolith. Moreover, calculated protolith compositions yield a two-point Sm–Nd `isochron' indicating an age of 2.7 Ga, and very similar Nd model ages of 2.73–2.75 Ga. Those ages constrain the timing of an Archean crust formation episode in the Phikwe Complex.

Reworking of sialic crust as represented in late Archean–age gneisses, northern Labrador

Regionally reworked sialic crust has been recognized in the form of ∼ 2,800-m.y.-old nebulitic rocks (Kiyuktok gneisses) with high initial 87Sr/86Sr ratios (∼ 0.7081) and Pb isotope ratios that suggest a history of the source back to at least 3,500 m.y. B.P. The field, geochemical, and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the ∼ 3,600-m.y.-old Uivak gneisses by the introduction of aqueous fluids some 2,800 m.y. ago. Contemporary but subordinate tonalitic-granodioritic orthogneisses derived from discrete intrusive bodies have contrasting Sr isotope compositions. Some display low initial 87Sr/86Sr ratios (∼ 0.7030) that suggest derivation from a source with mantlelike Sr-isotope composition. However, most of the late Archean–age intrusive rocks display higher initial 87Sr/86Sr ratios (∼ 0.7056, 0.7059). Recognition of these contrasting isotopic patterns suggests that processes of late Archean crust formation and stabilization in the North Atlantic craton were more complex than previously realized. Crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material.