Early Crustal Evolution Research Articles

Evolution of seismic layer 2B across the Juan de Fuca Ridge from hydrophone streamer 2‐D traveltime tomography

How oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the Earth's interior and the oceans. Although considerable work has been done characterizing the evolution of seismic layer 2A, there has been little analysis of the processes that affect layer 2B after formation. Here we present high‐resolution 2‐D tomographic models of seismic layer 2B along ∼300 km long multichannel seismic transects crossing the Endeavour, Northern Symmetric, and Cleft segments of the Juan de Fuca Ridge. These models show that seismic layer 2B evolves rapidly following a different course than layer 2A. The upper layer 2B velocities increase on average by 0.8 km/s and reach a generally constant velocity of 5.2 ± 0.3 km/s within the first 0.5 Myr after crustal formation. This suggests that the strongest impact on layer 2B evolution may be that of mineral precipitation due to “active” hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber. Variations in upper layer 2B velocity with age at time scales ≥0.5 Ma show correlation about the ridge axis indicating that in the long term, crustal accretion processes affect both sides of the ridge axis in a similar way. Below the 0.5 Ma threshold, differences in 2B velocity are likely imprinted during crustal formation or early crustal evolution. Layer 2B velocities at propagator wakes (5.0 ± 0.2 km/s), where enhanced faulting and cracking are expected, and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2B. Analysis of the layer 2B velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2B velocities, respectively. Additionally, the distance between present up‐flow and down‐flow zones is related to the amount of sediment cover because, as sediment cover increases and basement outcrops become covered, direct pathways from the igneous basement through the seafloor are cut off, forcing convective cells to find alternate paths.

Precise and accurate determination of 147Sm/ 144Nd and 143Nd/ 144Nd in monazite using laser ablation-MC-ICPMS

The 147Sm– 143Nd radioactive decay system has been widely used in the Earth sciences as a geochronometer and isotopic tracer. Recent studies have shown that laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is capable of precise and rapid 143Nd/ 144Nd measurements with high spatial resolution, but its ability to measure 147Sm/ 144Nd accurately is still poorly constrained. A critical analytical issue is the fractionation effects between 147Sm and 144Nd in LA-MC-ICPMS, which must be carefully corrected to determine robust initial 143Nd/ 144Nd particularly for ancient geological samples. Here we show that LA-MC-ICPMS can produce substantial (> 5%) 147Sm/ 144Nd fractionation during analysis of monazite and that the fractionation can be corrected with uncertainty of ~ 1% by using an independently defined 147Sm– 143Nd isochron for an Archean monazite standard. In addition, we describe a method for improving the precision and accuracy of LA-MC-ICPMS isotopic measurements by calibration of Faraday amplifier response and isotopic ratio normalization using a synthetic standard. This method has been applied to one Neogene and two Archean monazites, which were also analyzed by isotope dilution-thermal ionization mass spectrometry (ID-TIMS). The LA-MC-ICPMS data were acquired from 16–37 μm single spots. The LA-MC-ICPMS determinations of the 145Nd/ 144Nd for all samples and 143Nd/ 144Nd for the Neogene monazite compare well with the ID-TIMS results, indicating the robustness of our method to measure Nd isotopic ratios. For the Archean monazites, the method yields initial 143Nd/ 144Nd values identical to the ID-TIMS values with reproducibilities (2 s.d.) of 39 and 58 ppm. Analytical precisions of individual spot analyses for initial 143Nd/ 144Nd were calculated by taking into account the uncertainty in (i) the Nd isotopic normalization to the synthetic monazite, (ii) the correction factor for 147Sm/ 144Nd fractionation and (iii) the age, which results in 2σ of 45–87 ppm. These results clearly demonstrate the capabilities of the LA-MC-ICPMS technique applied to monazite Sm–Nd isotope systematics for early crustal evolution studies.

Geochronology, geochemistry and Hf–Sr–Nd isotopic compositions of Huziyan mafic xenoliths, southern Hunan Province, South China: Petrogenesis and implications for lower crust evolution

The Huziyan basalts, occurring in the southern Hunan province, contain abundant mafic xenoliths. These xenolith and the basalt samples offer a unique opportunity to investigate the origin and nature of the lower crust beneath the inland area of the South China Block and to shed new light on the study of the early crustal evolution in the region. We carried out combined SHRIMP U–Pb geochronology and Hf isotope analyses on zircons from both the Huziyan mafic xenoliths and the host basalts. We have also carried out mineral chemistry, whole-rock geochemical, and Sr–Nd isotopic analyses on these mafic xenoliths. Our data show that these xenoliths were once seated in the lower crust according to petrographic examination and P– T estimation. We discovered one of the oldest zircon ( 207Pb/ 206Pb age = 3.73 Ga, and T DM = 4.08 Ga) in the Cathaysia Block in the Huziyan basalts, which implies the existence of a crustal remnant as old as 3.7 Ga. Three representative xenoliths yield consistent U–Pb ages of about 220 Ma (220 ± 9 Ma for H4-2 determined through two baddeleyites, 230 ± 8 Ma for H4-3 and 233 ± 5 Ma for H4-5-2). Some of the zircons with ages of about 220 Ma display depleted mantle-like Hf isotopic compositions. It is thus deduced that some juvenile crust formed at 220 Ma when the mafic melt, derived from depleted mantle, underplated the lower crust. Such a thermal event in the deep crust might have induced intensive crust–mantle interaction and contributed to the formation of contemporaneous Indosinian granitoids in peripheral areas. Another thermal event leading to crustal growth at about 800 Ma can also be identified both from U–Pb ages (770–850 Ma) of xenocrystic zircons in the basalts and xenoliths, and from upper intercept ages of discordia in two xenoliths (933 ± 65 Ma and 828 ± 80 Ma). Episodic crustal growths might also have taken place at 2.7 Ga and 1.9 Ga. This is implied by Hf isotope model ages of some of the zircons. Large variation in Hf isotopic compositions among zircons of the same age ( ɛ Hf( t) = − 5.4 to 20.8 for 220 Ma zircons, and − 8.3 to 10.9 for 800 Ma zircons) suggests mixing and interaction between DM-derived melt and old crustal components at both of these times. Furthermore, petrographic and geochemical characteristics suggest that these xenoliths were formed through accumulation of pyroxene and plagioclase, and were then metamorphosed in the lower crust. Multiple episodes of juvenile growth, crustal reworking, and mafic underplating resulted in complexity and heterogeneity in the lower crust, as demonstrated by diversities in bulk geochemistry, Sr–Nd isotopic compositions and zircon Hf isotopic composition of these xenoliths.

Complex 3670–3500 Ma Orogenic Episodes Superimposed on Juvenile Crust Accreted between 3850 and 3690 Ma, Itsaq Gneiss Complex, Southern West Greenland

The Itsaq Gneiss Complex of the Nuuk region, southern West Greenland, is dominated by 3850–3690 Ma tonalites intruded into and intercalated with lesser amounts of different ≥3850–3700 Ma supracrustal units. Published whole‐rock Sr and Nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source. From our field studies and SHRIMP U/Pb zircon dating, we argue that this juvenile crustal accretion was spread over ca. 170 m.yr. (≥3850–3690 Ma). Following 3850–3690 Ma juvenile crustal accretion events, the evolution of the Itsaq Gneiss Complex continued with numerous crustal‐reworking events between 3670 and 3500 Ma. Examples of these reworking events are as follows: (1) there is intrusion of several generations of geochemically diverse granites (sensu stricto) with subordinate gabbros and diorites; (2) there are superimposed, multiple episodes of amphibolite to granulite facies metamorphism (illustrated with Akilia Island samples by detailed dating of metamorphic zircon overgrowths in several rocks); (3) part of the complex around Amiitsoq and the hills to the north contains the youngest tonalitic gneisses at ca. 3660 Ma, indistinguishable in age from the oldest recognized granites (sensu stricto) elsewhere; and (4) the hills north of Amiitsoq also contain tectonized remnants of volcanosedimentary basins that were still being filled between 3650 and 3600 Ma (from dating youngest detrital zircons) and that had been inverted and metamorphosed by ca. 3570 Ma (dating of oldest in situ metamorphic overgrowths). These 3670–3500 Ma events are interpreted as reflecting a complex orogen (tentatively collisional and/or strike slip) superimposed on the products of the earlier (3850–3690 Ma) juvenile crustal accretion regimes. Implications for early crustal evolution studies are discussed.

Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China: Implications for early crustal evolution of the North China Craton

Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6–3.8 Ga granitic rocks.In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust. The ∼3.8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks (granitic rocks) were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before ∼3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Craton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.

Crustal structure of the Kaapvaal craton and its significance for early crustal evolution

High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is ∼15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km 2 footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.

The episodic development of intermediate to silicic volcano–plutonic suites in the Archaean West Pilbara, Australia

Four felsic igneous rock suites in the Archaean West Pilbara have been identified based on geochemistry and geochronology. A voluminous TTG suite formed at ca. 3260 Ma, which appears to be from melting of a mafic-subducted oceanic slab and thus represents generation of new continental crust. A tholeiitic to calc-alkaline volcanic assemblage and coeval granitoids formed at ca. 3120 Ma in an extensional environment. Further TTG magmatism occurred at ca. 3000 Ma, generating both large granitoid complexes and small plutons, again adding new continental crust to the West Pilbara. At 2930-Ma crustal reworking, most likely of the 3000-Ma rocks, generated small plutons that are coeval with layered ultramafic–mafic intrusions in the region. The changes from new crustal material to crustal reworking infer changing tectonic regimes, which is important for models of Archaean continental crust generation. The data presented here indicate that crustal generation mechanisms varied and were episodic in the West Pilbara, implying that early crustal evolution was a result of periodic changes in tectonic regime, which is reflected in the geochemistry of the rocks.

Seismic imaging and crustal architecture across the Lachlan Transverse Zone, a possible early cross‐cutting feature of eastern Australia

A 2‐D crustal velocity model has been derived from a 1997 364 km north‐south wide‐angle seismic profile that passed from Ordovician volcanic and volcaniclastic rocks (Molong Volcanic Belt of the Macquarie Arc) in the north, across the Lachlan Transverse Zone into Ordovician turbidites and Early Devonian intrusive granitoids in the south. The Lachlan Transverse Zone is a proposed west‐northwest to east‐southeast structural feature in the Eastern Lachlan Orogen and is considered to be a possible early lithospheric feature controlling structural evolution in eastern Australia; its true nature, however, is still contentious. The velocity model highlights significant north to south lateral variations in subsurface crustal architecture in the upper and middle crust. In particular, a higher P‐wave velocity (6.24–6.32 km/s) layer identified as metamorphosed arc rocks (sensu lato) in the upper crust under the arc at 5–15 km depth is juxtaposed against Ordovician craton‐derived turbidites by an inferred south‐dipping fault that marks the southern boundary of the Lachlan Transverse Zone. Near‐surface P‐wave velocities in the Lachlan Transverse Zone are markedly less than those along other parts of the profile and some of these may be attributed to mid‐Miocene volcanic centres. In the middle and lower crust there are poorly defined velocity features that we infer to be related to the Lachlan Transverse Zone. The Moho depth increases from 37 km in the north to 47 km in the south, above an underlying upper mantle with a P‐wave velocity of 8.19 km/s. Comparison with velocity layers in the Proterozoic Broken Hill Block supports the inferred presence of Cambrian oceanic mafic volcanics (or an accreted mafic volcanic terrane) as substrate to this part of the Eastern Lachlan Orogen. Overall, the seismic data indicate significant differences in crustal architecture between the northern and southern parts of the profile. The crustal‐scale P‐wave velocity differences are attributed to the different early crustal evolution processes north and south of the Lachlan Transverse Zone.

Evolution of the Continental Crust in the Proterozoic Eastern Ghats Belt, India and new constraints for Rodinia reconstruction: implications from Sm–Nd, Rb–Sr and Pb–Pb isotopes

For this study Nd, Sr and Pb isotope compositions were analyzed for ortho- and paragneisses from the Eastern Ghats Belt of India in order to determine its crust formation and crustal evolution. This belt represents a Proterozoic orogen that extends along the east coast of Peninsular India and forms part of the mobile belts in East Gondwana and Rodinia. The Eastern Ghats Belt was affected by Mesoproterozoic granulite facies metamorphism in the western segment (Western Charnockite Zone) and a Grenvillian regional-scale high-grade event in the central and eastern segments (Western Khondalite Zone, Charnockite Migmatite Zone and Eastern Khondalite Zone) as well as a local Pan-African overprint. The results of the isotope studies are used for the large-scale reconstruction of the indo-antarctic part of the Rodinia supercontinent. Based on Nd model ages and Pb isotope ratios from leached feldspars four crustal domains can be distinguished in the Eastern Ghats Belt. These domains can in part be correlated with the lithological division of the belt: (1) The Western Charnockite Zone south of the Godavari Graben is characterized by Nd model ages between 2.3 and 2.5 Ga for orthogneisses and 2.6 and 2.8 Ga for metasediments (Domain 1). The Pb isotopes are primitive indicating reworking of dominantly Archean and mixing with minor Proterozoic material; (2) North of Godavari Graben Nd model ages for orthogneisses are significantly higher with values ranging from 3.2 to 3.9 Ga. The Pb isotopes are strongly retarded; (3) The north-eastern parts of the Charnockite Migmatite Zone and Western Khondalite Zone form a distinct and almost homogeneous crustal domain (Domain 3) with Nd model ages between 1.8 and 2.2 Ga; (4) Between the isotopically homogeneous terranes stretches a broad transition zone (Domain 2) enclosing parts of the Western Khondalite Zone, Charnockite Migmatite Zone and Eastern Khondalite Zone. The Nd model ages for metasediments (2.1–2.5 Ga) are younger than paragneiss ages of the adjoining Western Charnockite Zone. The Nd model ages for orthogneisses (1.8–3.2 Ga) display a large spread, which is consistent with the Pb isotope signatures that indicate mixing of Archean with Proterozoic material. The border zone between Domain 3 and the Archean Eastern Indian Craton forms a second transition zone (Domain 4) characterized by metasediments with Nd model ages between 2.2 and 2.8 Ga and orthogneisses with model ages around 3.2 Ga. Reworking of Archean crustal material is most intense along the border zones of the belt and ‘juvenile’ material is more dominant away from the orogenic front. This scenario is indicative of an active continental margin setting for the two Proterozoic episodes of orogenesis in the Eastern Ghats Belt. A correlation of Domain 3 with the Rayner Complex and the Prydz Bay region, Antarctica, for the early crustal evolution is supported by the similarity of the isotope signatures. The Napier Complex is very different to the Eastern Ghats Belt and an early joint evolution of these terranes is ruled out on the basis of the Pb–Pb and Sm–Nd systematics.

Initial Pb of the Amı̂tsoq gneiss revisited: implications for the timing of early Archaean crustal evolution in West Greenland — Reply

Abstract The rapid change in 207 Pb / 206 Pb ratio during early Earth history provides a powerful constraint for temporal resolution of Archaean crustal evolution. New and published data for Amitsoq gneiss feldspars and whole rocks define the Earth's least radiogenic silicate Pb known until now. The most important feature of the data is that independent of the exact slope of the regression in a common Pb diagram, a very well-defined intercept with plausible terrestrial Pb evolution lines is obtained between 3.63 and 3.66 Ga, identical to a combined Pb/Pb regression age of 3654±73 Ma (MSWD=17.6). We conclude that the magmatic precursors of the Amitsoq orthogneisses separated from a depleted mantle-like source at ca. 3.65 Ga. This contrasts sharply with claims based on ion-probe U–Pb zircon dating that the Amitsoq gneisses had a prolonged, episodic, magmatic evolution extending from ca. 3.87 to 3.60 Ga. Pb isotopes offer no evidence that any Amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3.85 Ga. We propose that zircons significantly older than 3.65 Ga were inherited from chemically evolved rocks as old as ca. 3.85 Ga. Geochemical arguments are presented to support this proposal. New feldspar and whole rock Pb isotope data are also reported for sheets of quartz-dioritic gneiss on Akilia Island that yield the oldest zircon U–Pb age clusters in the range 3.84–3.87 Ga. These sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose C-isotope ratios are regarded as biogenic in origin. As above, Pb-isotopes in the sheets are consistent with a magmatic age of 3.65 Ga, which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers. Additional Pb-isotope data, as well as limited published Sm–Nd and U–Pb zircon data on the Akilia enclaves themselves, suggest an age between ca. 3.65 and 3.70 Ga for their original deposition, some 150–200 Ma younger than proposed previously. Thus the question of overlap of earliest life with a bolide impact scenario terminating ≥3.80 Ga (as on the moon) becomes irrelevant.

Geochemical Characteristics and Crustal Evolution of the Archaean High-Grade Kongling Terrain in the Yangtze Craton, Southern China

The North China and Yangtze cratons are two most important continental blocks in eastern China, and collided in Late Palaeozoic-Early Mesozoic with the formation of the Qingling-Dabie-Sulu orogenic belt. The early geological history of both cratons is essential for understanding the evolution of the belt. However, geological and geochemical data on Archaean rocks of the Yangtze have not been available due to lack of known exposures. The recently confirmed Archaean Kongling terrain in the north of the Yangtze craton provide an opportunity to study the nature of the Yangzte crust in the Archaean (Gao and Zhang, 1990; Ling et al., 1998). This paper reports results of a systematic study on Sm-Nd isotopes of associated felsic gneisses amphibolites and metapelites from the Kongling terrain. Based on these data, tectonic environments and early crustal evolution of the Yangtze craton are discussed.

The Itsaq Gneiss Complex of southern West Greenland; the world's most extensive record of early crustal evolution (3900-3600 Ma)

The Itsaq Gneiss Complex of southern West Greenland contains the best-preserved occurrences of ⩾ 3600 Ma crust. Its known area is ∼ 3000 km 2 with almost continuous exposure in some places. SHRIMP UPb zircon geochronology shows that the gneiss complex had a complicated early history, having been added to, and modified in, several events starting at ∼ 3900 Ma. Supracrustal, mafic and ultramafic rocks comprise approximately 10% of the complex and range in age from ⩾ 3870 to ∼ 3600 Ma. A large portion of the Isua supracrustal belt and some other bodies contain sequences of LREE-enriched, mafic (locally pillow-structured) to felsic volcanic and volcaniclastic rocks (some deposited from turbidity currents) and abundant diverse chemical sediments. These sequences might have formed in an environment analogous to present-day volcanic arcs. Other (mostly ⩾ 3800 Ma?) units dominated by LREE-depleted, high Ti/Zr mafic rocks (as found in komatiites and komatiitic basalts free of crustal contamination) with layers of banded iron formation might have been derived from volcanic edifices formed as a result of plume activity. Only in the youngest supracrustal sequences (∼ 3600 Ma) are detrital sediments derived from mixed-provenance clastic sources an important component. Layered anorthosite, gabbro and peridotite units (some ⩾ 3800 Ma) derived from large deep crustal intrusions are also widespread. In addition there are massive dunites and harzburgites. In the rare cases where their original mineralogy and texture are preserved, these contain aluminous spinel, indicating equilibration in the lowermost crust or upper mantle. Approximately 90% of the complex consists of grey gneisses, the dominant protoliths having been tonalites and less abundant trondhjemites, quartz-diorites, diorites and granodiorites. The protoliths were intruded in several events starting at ∼ 3870 Ma. Like other suites of Archaean grey gneisses, they were formed by partial melting, probably in an arc environment, of buried (subducted?) amphibolite, leaving residual hornblende ± garnet. Granites form approximately 10% of the complex. The oldest are 3650 Ma leucogranites, which probably formed by deep crustal anatexis of predominantly tonalitic gneisses. There are also ∼ 3625 Ma augen granites and ferrogabbros, whose chemistry resembles that of some A-type, within-plate granite suites. The evolution of the Itsaq gneiss complex is marked by increasing compositional diversity with time. Pre-3650 Ma lithologies are predominantly mafic and ultramafic rocks, tonalites and diorites. The first recorded regional metamorphic event occurred at 3650 Ma, and was marked by localised partial melting and intrusion of leucogranites. This might record crustal thickening, brought about by collision of different blocks of tonalite-dominated crust. A further thermal event occurred just after 3600 Ma, as shown by PbPb titanite and feldspar ages and local intrusion of granites. Deposition of sediments at ∼ 3600 Ma which were derived from mixed-age sources suggests their derivation from an extensive block of “continental” crust.

Two Types of Archaean Grey Gneiss in North China and Their Geological Significance1,2

Abstract Grey gneisses are tonalitic intrusive rocks that have Archaean characteristics and tectonic significance. Archaean grey gneisses in North China may be classified into two types: the Zunhua type and the Hengshan type. Grey gneisses of the Zunhua type alternate with basic granulites, forming a bimodal migmatitic suite. The rock is characterized by complex mineral composition, highly varied REE patterns and positive Eu anomaly and originated from autochthonous or partly autochthonous deep ‐ level migmatization with a high degree (30% ±) of fusion. As the sub ‐ stratum in the root of the early ‐ stage high ‐ grade greenstone belt, they prevented basic rocks from sinking. Grey gneisses of the Hengshan type are characterized by a relatively distinct intrusion form, large ‐ amplitude upward emplacement, rather simple mineral composition, only slight changes of REE patterns and absence of Eu anomaly. The rock is a product of subduction and underthrusting of the high ‐ grade greenstone belt and then fusion and emplacement with a low degree (15% ±) of fusion. In this paper an attempt is made to furnish important information about palaeoplate tectonics and early crustal evolution by means of a study of grey gneisses.

Gneiss-greenstone relationships in the Ancient Gneiss Complex of southwestern Swaziland, southern Africa, and implications for early crustal evolution

The Ancient Gneiss Complex (AGC) of Swaziland is made up of a variety of Archaean gneisses derived from granitoid intrusions of TTG composition, emplaced between 3.6 and 3.2 Ga ago, and infolded amphibolite-grade greenstone remnants locally referred to as Dwalile Supracrustal Suite (DSS). The greenstones were considered to postdate the oldest phases of the AGC, known as Ngwane gneiss, and this is now confirmed by conventional UPb zircon geochronology and SHRIMP single zircon ages on detrital grains from Dwalile clastic metasediments, suggesting that these rocks were derived from a granitoid source some 3.42 to 3.57 Ga in age. Trondhjemitic Ngwane gneiss structurally underlying the greenstones has a zircon age of 3521±23 Ma and contains a 3683±10 Ma old xenocryst. This together with Nd T DM model ages between 3.7 and 3.9 Ga and a pronounced negative Euanomaly for the metasediments strongly argues for the presence of very old crustal components in the AGC. Nd isotopic systematics of Dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of Ngwane type, and resulting Nd model ages (3.60–3.99 Ga) are likely to be too old. The major and trace element chemistry of the Dwalile metavolcanics is remarkably similar to that of Onverwacht Group rocks in the nearby Barberton Greenstone Belt (BGB) and this, together with a similar age for these two suites, points to a genetic link and a similar mechanism of formation. Our data as well as field relationships do not favour an intra-oceanic origin for the DSS, as has been inferred for the Onverwacht Group, nor do they support a juvenile origin for the exposed Ngane gneisses of the AGC. The Ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification, particularly through remelting and deformation, gave rise to the various compositional varieties now seen in the Ngwane suite.

Experimental melting of hydrous low-K tholeiite: evidence on. the origin of Archaean era tons

Experimental melting studies were performed on a natural high-Al basalt and a synthetic average Archaean tholeiite (AAT) composition (0.3 wt.% K20) with variable amounts of H20. Microprobe analyses of quenched melts (glass) from runs at 5-30 kbar and 750°-l100°C showed that typical Archaean tonalitic and trondhjemitic "grey gneiss" compositions were produced from the average Archaean tholei-ite over the entire experimental range, with 15% to less than 1 % H20. The high-Al basalt produced liquids too high in Al203 (18--23%) for realistic grey gneiss compositions. The persistent generation in our experiments of low-K calc-alkaline magmas directly by vapor-undersaturated partial melting of low-K Archaean tholeiite strongly suggests this mechanism for the origin of early continents. Temperatures of 850°-l000°C and pressures around 15 kbar are appropriate melting conditions. Ton­alitic magmas are favored by higher temperatures, lower pressures, and higher H20 contents in the source. Trondhjemitic magmas are favored by lower temperatures, higher pressures, and lower H20 contents. Heavy REE depletion of magmas would be possible for partial melting above 15 kbar because of the stability at higher pressures of residual garnet. Unfractionated REE patterns of magmas could result from melting at lower pressures, where garnet does not coexist with liquid. The low-K trends of melts are maintained by very refractory amphibole (up to 0.7 wt. % KzO) which coexists with liquid for bulk H20 contents of 2 wt. % or more. Shallow subduction-zone melting of amphibolite with magma extraction, and partial melting of amphibolite under deep-crustal metamorphic conditions are models for early crustal evolution which appear to satisfy the experimental constraints.

Early crustal evolution as viewed from archean basic rocks of China

Early crustal evolution as viewed from archean basic rocks of China

Precise, small sample U-Pb geochonology: A critical tool for the study of early crustal evolution

Precise, small sample U-Pb geochonology: A critical tool for the study of early crustal evolution

Early Proterozoic geology of Arizona

The Early Proterozoic geology of Arizona and adjoining regions was the topic of a workshop convened by Clay M. Conway (U.S. Geological Survey (USGS), Flagstaff, Ariz.), Karl E. Karlstrom (Northern Arizona University (NAU), Flagstaff), and Leon T. Silver (California Institute of Technology (Caltech), Pasadena) in Flagstaff, October 3–5, 1985. The meeting, sponsored by USGS, NAU, Caltech, and the Arizona Geological Survey, was attended by 73 geologists from industry, academia, and governmental agencies. The workshop brought together for the first time workers in a variety of disciplines who have been studying facets of Early Proterozoic crustal evolution in the southwest. From responses during and following the workshop, we judge that the meeting successfully accomplished its objective of furthering communication, cooperation, and collaboration. The meeting encouraged contributions, including progress reports, from all participants and concentrated on specific problems of stratigraphy, structure, petrology, geochemistry, and ore formation, with a view toward understanding overall orogenic evolution and continental accretion.

Precambrian Sulphide Deposits

This book is dedicated to Howard S. Robinson, who was born and educated in the United States, but who spent his professional career in Canada with McIntyre Porcupine Mines, concentrating on Precambrian mineral deposits. Although his career in mineral exploration was distinguished, his major contribution to earth science was probably as one of the founders of the Geological Association of Canada, an institution to which he made a bequest in his will. With this background, the strong emphasis on Canadian Precambrian mineral deposits should come as no surprise; of the 23 papers in this book, 21 are solely or primarily devoted to Canadian deposits. The two exceptions—those describing the Balmat, N.Y., zinc mines (at times the largest zinc producer in the United States) and the Crandon, Wisconsin, volcanogenic zinc‐copper massive‐sulfide deposit (the largest deposit of its kind found in the 1970s)—are each within a couple of hundred kilometers of the Canadian border. Although the title of the book is more expansive than the actual topics discussed, Canada is rich in Precambrian rocks and ore bodies, and Canadian scientists have been especially alert to tectonic influences in the formation of mineral deposits. These features, plus the fact that the country contains a very well exposed expanse of Archean rocks which is the largest in the world, facilitate the study of early crustal evolution and make the book of particular interest to geophysicists.

Mantle convection and early crustal evolution

The existence of peridotitic komatiites in the Archaean suggests that the Archaean mantle was significantly hotter than the modern mantle. This evidence is contradicted by estimates of Archaean continental geothermal gradients, based on the pressure and temperature recorded in metamorphic rocks, which suggest that there is no marked difference between Archaean and modern continental geothermal gradients. Numerical modelling shows that small changes in the mantle temperature can have an important influence on convection. If the average temperature of the upper mantle is increased by 200°C, convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe. The crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction. Later, when the mantle temperature falls to 100°C above its present level, the upper mantle partial melt zone contracts away from subduction areas. It is suggested that the first primitive felsic magmas were generated at subduction zones. The appearance of these magmas at ∼3.8 Ga permitted the formation of buoyant continents and eventually led to crustal thickening. As a consequence of this thickening the proto-continents, consisting of a bimodal suite of basalts and sodic granodiorites, contained two types of latent energy: (1) radioactive energy held in elements such as Th, K and U; and (2) potential energy resulting from the elevation of the continents above sea level. The potential energy of the continents led to sedimentation. The increase in the rate of sedimentation during the Archaean resulted from increased crustal buoyancy. At the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust. This caused a major metamorphic, metasomatic and crustal melting event which produced the potassic granites of the late Archaean. Once the radioactive elements had been removed from the lower crust, that region of the continent become tectonically stable. The Proterozoic shelf sediments were deposited at the margins of these stable cratons. Convection models of the Archaean mantle show hot diapirs rising from the boundary layer above the core—mantle interface. We suggest that these diapirs began to melt at a depth of ∼ 450 km, giving rise to komatiitic magmas. This model requires the average temperature of the Archaean upper mantle to be ∼ 100°C above that of the modern mantle. The similarity between Archaean and modern continental geothermal gradients can be explained if Archaean continents formed above subduction zones. Raising the temperature of the Archaean mantle by 100°C (1) halves the thickness of the oceanic lithosphere, (2) increases the oceanic geothermal gradient at the mid-point of a convection cell, (3) decreases the viscosity of the mantle by at least an order of magnitude. The combination of these effects produces a marked decrease in the strength of the Archaean lithosphere and mantle. Thus the form of Archaean tectonics can be expected to have been very different from modern tectonics.