Archaean Granitoids Research Articles

Melt breccias in parautochthonous basement rocks of the Morokweng impact structure, South Africa – Evidence for a friction origin

Archaean granitoid gneisses and younger metadolerite and dolerite intrusions in a drill core from the late-Jurassic Morokweng impact structure, host mm- to m-wide aphanitic dykes that are spatially and geometrically associated with cataclasite-bearing reverse-slip faults. The dykes contain lithic and mineral clasts derived from their wallrocks. Petrographic and BSEM analysis indicates that the original dyke matrices are completely altered to a smectite-zeolite-magnetite paragenesis. However, a melt precursor is argued based on evidence for low-viscosity flow, high dyke aspect ratios and intrusive relationships into both the wallrocks and cataclasite, and the sharply contrasting matrix hydrothermal mineral grain size and textures relative to those found in the cataclasites. A subset of cm-wide pink-orange monomictic dykes have a felsic composition closely resembling their granitic wallrocks. More voluminous, polymictic, red dykes show compositions intermediate between the mafic and felsic wallrock endmembers in the drill core, which both occur in close proximity to the dykes. These hybrid compositions are interpreted to have developed either through mixing of mafic and felsic melts along faulted contacts between the doleritic and gneissic rocks, or by mechanical mixing, with or without assimilation, of a dolerite-derived melt with incohesive granitoid-derived cataclasite within the fault zones. The structural, petrographic and geochemical evidence support the dykes originating by friction melting (pseudotachylite, s.s.) caused by high-strain-rate faulting of the crater basement during the excavation and/or modification stages of the Morokweng impact, rather than by contamination of impact melt intrusions. Based on their spatial context relative to the impact melt sheet and the moderately high shock levels of their host rocks, these pseudotachylite dykes are interpreted as part of the parautochthonous peak ring of the Morokweng impact structure.

Earth's early continental crust formed from wet and oxidizing arc magmas.

Formation of continental crust has shaped the surface and interior of our planet and generated the land and mineral resources on which we rely. However, how the early continental crust of Earth formed is still debated1-7. Modern continental crust is largely formed from wet and oxidizing arc magmas at subduction zones, in which oceanic lithosphere and water recycle into the mantle8-10. The magmatic H2O content and redox state of ancient rocks that constitute the early continental crust, however, are difficult to quantify owing to ubiquitous metamorphism. Here we combine two zircon oxybarometers11,12 to simultaneously determine magmatic oxygen fugacity (fO2) and H2O content of Archaean (4.0-2.5 billion years ago) granitoids that dominate the early continental crust. We show that most Archaean granitoid magmas were ≥1 log unit more oxidizing than Archaean ambient mantle-derived magmas13,14 and had high magmatic H2O contents (6-10 wt%) and high H2O/Ce ratios (>1,000), similar to modern arc magmas. We find that magmatic fO2, H2O contents and H2O/Ce ratios of Archaean granitoids positively correlate with depth of magma formation, requiring transport of large amounts of H2O to the lower crust and mantle. These observations can be readily explained by subduction but are difficult to reconcile with non-subduction models of crustal formation3-7. We note an increase in magmatic fO2 and H2O content between 4.0 and 3.6 billion years ago, probably indicating the onset of subduction during this period.

AGE OF THE HANNIVKA GRANITE (MIDDLE-DNIEPER MEGABLOCK OF THE UKRAINIAN SHIELD)

The Middle-Dnieper megablock, which is a fragment of the craton, differs from other cratons found on Earth. This is because of the large variety of granitoids (Tokiv, Mokro-Moskowka, and Demuryne complexes) in the former that were formed after the Mesoarchean TTG. Thus, the Middle-Dnieper megablock is important for studying the genesis and geodynamic formation conditions of Late Archaean granitoids. The granitoids in the Middle-Dnieper megablock are not well understood. They include the Hannivka granites of the East Hannivka monocline of the Kryvyi Rih-Kremenchuk structure, whose age and stratigraphic position has been a matter of a long-standing debate. The purpose of the work is to study the geochemistry, genesis and U-Pb age of the Hannivka granites. Based on our results, the Hannivka granites possibly formed in the crust resulting from the melting of older rocks. They differ from other Late Archean granitoids of the Middle-Dnieper megablock by their high U (56.4 ppm) content and the presence of Mo (4.3 ppm). The Hannivka granites underwent tectonic reworking during a collisional event about 2.0 billion years ago, which is probably associated with the kalishpatization of these rocks. The U-Pb age of the cores of zircons sampled from the Hannivka granites, determined by LA-ICP-MS method, are about 2827±16 million years in age. Younger rims probably formed during kalishpatization. The Hannivka granites are the same age as the granitoids of the Mokro-Moskowka and Tokiv complexes. Late Archean granitoids were formed between 2.99-2.7 Ga in the Middle-Dnieper granite-greenstone block and in the geologically similar granite-greenstone block KMA are 2.6 Ga in age. In the Pilbara craton, which is a Paleoarchean granite-greenstone complex, the age of biotite and feldspar granites is similar to the age of the rocks on the Middle-Dnieper megablock (2.94-2.93 Ga). The difference in magmatism ages may be due to the drift of the different cratons above mantle plumes of different ages.

Age disequilibrium between zircon and their granitoid hosts caused by intracrustal reworking: Nd-Hf-Ar isotope evidence of Archaean Granitoids from Barberton Mountain Land (Kaapvaal craton, South Africa)

The degree of early Archaean terrestrial differentiation into mantle and crust is long debated. A number of studies rely on bulk rock Hf and Nd isotope data to gain further insight. Here we present Hf-Nd data from two igneous basements suites of Barberton Mountain Land, namely a tonalite-trondhjemite-granodiorite (TTG) and granodiorite-granite-monzogranite (GGM) suite to further decipher crust formation and its timing in the Archaean. TTG show high initial ε(Hf) and ε(Nd) if corrected using their respective U/Pb zircon ages that can be interpreted in two ways: (i) early differentiation of bulk silicate Earth or (ii) overcorrection of these data caused by decoupling of zircon from the ‘zircon-free’ bulk rock due to metamorphic overprint. Indeed, regional metamorphism affected the early TTG ca. 250 Ma after their emplacement and disturbed their bulk rock Hf and Nd systematics. Correction of bulk rock Hf and Nd data to the age of metamorphism yields values between +0.9 and − 6.2 and from −1.3 to −4.3, respectively, thus indicating open-system behaviour. Lu-Hf and Sm-Nd mineral isochron ages of two TTG plutons are significantly younger than their corresponding U/Pb zircon age, but agree within uncertainty perfectly with external age constraints for the age of the metamorphic overprint. The same accounts for the 40Ar-39Ar systematics of the investigated plutons. Amphibole samples consist of a mixture of heterochemical generations that yield ages that are both older and younger than the metamorphic overprint at 3.2 Ga. The metamorphic minerals in each plutons provide a highly divergent age information from the coexisting zircon. Hence, the positive ε(Nd) and ε(Hf)-values that resulted from age-correction to the respective U/Pb zircon age are most probably over-corrected. Finally, if all data is age-corrected to the metamorphic overprint all samples agree with models for the Nd-Hf isotopic evolution in the mantle that do not require a large-scale early differentiation of Earth into enriched and depleted reservoirs. Mantle extraction ages (TDM) of TTG are interpreted as minimum ages for the generation of felsic Archaean crust whereas TDM of GGM are interpreted as maximum ages for the last major crust growth episode in the Barberton Mountain Land.

Editorial: Biographical Memoirs, Volume 69

Editorial: Biographical Memoirs, Volume 69

Stephen Moorbath. 9 May 1929 — 16 October 2016

Stephen Moorbath was an intellectual with eclectic interests across the sciences and humanities. In 1939, as a ten-year-old, he fled from Germany to England with his father. Stephen spent almost the whole of the rest of his life—from schoolboy to university professor—in Oxford, where he became one of the world's leading isotope geochemists. His academic career began with setting up Europe's first radiometric rock and mineral dating laboratory. In this laboratory, together with numerous colleagues and students, he applied the lead–lead, rubidium–strontium, potassium–argon and, later, samarium–neodymium isotopic dating methods to the solution of many geological problems. He made major contributions in establishing the chronology for the geological and tectonic evolution of Precambrian crust in the Scottish Highlands and Islands, in West Greenland, Zimbabwe, South India, and Ghana. He developed isotopic criteria for assessing the relative contributions of mantle and crustal sources to Tertiary igneous rocks in Scotland, Andean volcanics and the late Archaean granitoids of West Greenland. He established dating methods for sedimentary rocks: Rb–Sr for shales and Pb/Pb for Archaean limestone. Stephen's abiding geological passion was the study of the early Archaean, especially the Godthåbsfjord and Isukasia areas of West Greenland.

South African Hydrostratigraphy: A conceptual framework

Abstract South African geology, geomorphology and climate are distinctly variable, resulting in a complex hydrological cycle superimposed on equally complex ground conditions. With fractured and karstic systems dominating the hydrogeology, thick vadose zones comprising soil and rock and at highly variable moisture conditions contribute to complex hydrostratigraphic systems comprising various confining and hydraulically connected units. This paper proposed standard terminology for basic concepts pertaining to the description of ground and water in the subsurface to eventually propose a hydrostratigraphic classification based on abiotic factors fairly constant over short periods of time (geology, geomorphology and climate), as well as those temporally highly variable (climate) and those introduced by human involvement (society). Ten major hydrostratigraphic units are eventually described, namely the Cape Fold Belt, Kalahari Desert, Witwatersrand Supergroup, Malmani Subgroup, Cenozoic Coastal Deposits, Saldanian Basement, Karoo Main Basin, Namaqua-Natal Metamorphics, Waterberg Group, and Archaean Granitoids.

An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes

Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved silicon at low concentration. The unique heavy silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands. Archaean granitic rocks formed by melting of silica-enriched subducted basaltic crust through interaction with seawater, according to heavy silicon isotopes measured in Archaean samples.

Zircon U–Pb ages and geochemistry of the late Archaean granitoids in the Zanhuang Complex: Records of an arc–continent collision event at the end of Archaean

The late Archaean witnessed a critical transition of the composition of the continental crust from predominantly sodic tonalite‐trondhjemite‐granodiorites (TTGs) to a combination of TTGs and increasing proportion of potassic calc‐alkaline granitoids. This shift is also well recorded in the Zanhuang Complex located in the central part of the North China Craton (NCC). In this work, we present a detailed geochemical study of these granitoids and subdivide them into four groups, that is, TTGs, sanukitoids, crustal‐sourced granitoids, and hybrid granitoids. The TTGs in the Zanhuang Complex may have formed at a longer and earlier period during ~2.7 to ~2.5 Ga as the first stage. They show typical TTG characteristics, such as high SiO2, Na2O, and Sr/Y and depletion of Nb, Ta, Ti, and HREE. The TTGs are interpreted to have formed in a subduction‐related environment. The other three groups of granitoids formed at a shorter and later stage. The sanukitoid sample shows high MgO and Mg# and other geochemical signatures of typical Archaean sanukitoid affinity, yielding a weighted mean zircon 207Pb/206Pb age of 2,517 ± 5 Ma as the crystallization age. The sanukitoid magma derived from partial melting of a hydrous mantle peridotite source metasomatized by slab‐derived melts or fluids. The crustal‐sourced granitoids, including a new sample (~2,510 Ma) in this work, the Jiandeng granite (~2,490 Ma), and the Huangcha/Wangjiazhuang granite (2,488 to 2,517 Ma) show signatures of a mainly crustal source such as high SiO2 and K2O, low MgO and Mg# values, and weakly peraluminous. The hybrid granitoids represented by the Haozhuang granitoid (2,511 to 2,528 Ma) are characterized by enrichment in both incompatible (LILE and LREE) and compatible (Mg, Ni, and Cr) elements and may have formed by mixing/contamination of multiple magmas or sources. The temporal and spatial distributions of these groups of granitoids indicate that the Zanhuang Complex experienced a subduction–collision event between the Fuping arc and the Eastern Block of the NCC at the end of Archaean.

Archaean granitoids: classification, petrology, geochemistry and origin

Archaean granitoids: classification, petrology, geochemistry and origin

Uranium and Thorium Anomalies in the ~2.5 Ga Vendodu Leucogranite, Nellore Schist Belt, SE India and its Potential to Generate Uranium Deposits

Abstract The Vendodu intrusive, emplaced at 2483±3 Ma within the Nellore schist belt, SE India, is a K-rich per-aluminous A-type leucogranite composed of quartz and alkali feldspar (essential minerals), biotite, zircon, allanite, titanite, magnetite and apatite (magmatic accessory minerals) and muscovite, haematite, fluorite and uraninite (hydrothermal accessory minerals); zircon and uraninite could be both magmatic and hydrothermal. The Vendodu leucogranite is characterized by high Rb, Zr, Nb, Th, U and REE, low Ca, Al, Ba and Sr abundances, and large negative Eu anomalies. U content in the Vendodu leucogranite averages 17.63 ppm and is 6 to 11 times higher than average concentration of U in Upper Continental Crust (UCC) and Archaean granitoids. Similarly, Th content averages 61.38 ppm and is 3 to 20 times higher than average concentration of Th in Archaean granitoids and UCC. The U distribution in the Vendodu leucogranite is influenced by both magmatic and high-temperature hydrothermal processes. Processes that have enriched U have also increased LREE, Nb and Cr contents in the leucogranites. Geochemical proxies including Th/U, Zr/U and V/Yb suggest both magmatic and high-T hydrothermal (deuteric) enrichment of U over a wide range of temperatures and oxygen fugacities. High Th/U ratios in the Vendodu leucogranite (1.62–9.76) hint that the hydrothermal (deuteric) fluids were possibly magmatic. Petrographic and geochemical evidences suggest that the Vendodu leucogranite experienced magmatic and deuteric U enrichment that has potential to form mineralized zones either in situ or in pegmatitic veins.

Highlights on Geochemical Changes in Archaean Granitoids and Their Implications for Early Earth Geodynamics

The Archaean (4.0–2.5 Ga) continental crust is mainly composed of granitoids, whose geochemical characteristics are a function of their formation mechanisms and components, as well as physical conditions of their source. Therefore, revealing changes in Archaean geodynamic processes requires understanding of geochemical changes in Archaean granitoids. This paper compares key geochemical signatures in granitoid occurrences from the Eoarchaean to Neoarchaean Eras and aims to highlight changes or variations in their geochemical signatures. The study is performed by exploring and comparing geochemical and geochronological datasets of Archaean granitoids compiled from literature. The results show that two end-members of sodic TTGs (tonalite–trondhjemite–granodiorite) occur throughout the Archaean: low- and high-HREE (heavy rare earth elements) types. A profound change in granitoid geochemistry occurred between 3.0 and 2.5 Ga when multi-source high-K calc-alkaline granitoid batholiths emerged, possibly indicating the onset of modern-type plate tectonics.

Precise microbeam dating defines three Archaean granitoid suites at the southwestern margin of the Kaapvaal Craton

Precise microbeam U-Pb zircon dates have been determined for 17 granitoid samples which crop out along the southwestern margin of the Kaapvaal Craton. In the Marydale High two main types of granitoid are distinguished mainly by their normative Quartz – Alkali Feldspar – Plagioclase mineral proportions. The Draghoender Granite type is generally tonalitic to trondhjemitic and some samples have very low heavy Rare Earth Elements, probably originating as melts of eclogitic protoliths. Their ages vary by ∼50 Ma from 2946 ± 9 Ma to 2892 ± 6 Ma (2σ), reflecting a long period of magmatism possibly due to subduction. The granodioritic to monzogranitic Skalkseput Granite type intrudes the Draghoender granite in places and has a coherent age of 2901 ± 14 Ma. These two granite types thus overlap in time and space. The more evolved Skalkseputs type could have been derived by melting of the Draghoender type during assembly of the Kaapvaal Craton by collision of the Kimberley and Witwatersrand Terranes.A third granite type is exemplified by the 2721 ± 6 Ma monzogranitic Steenkop Granite Gneiss which occurs south of Prieskapoort, and corresponds in age to a 2718 ± 8 Ma monzogranite dyke which intrudes a 2871 Ma Skalkseput monzogranite. The previous age of 2718 ± 8 Ma for the Skalkseput Granite probably also dated a Steenkop-type dyke. A basal conglomerate to the Zeekoebaard Formation of the Ventersdorp Supergroup has a single 2720 ± 4 Ma detrital zircon population.South of the Marydale High a narrow strip of Draghoender-type Granite extends to Franzenhof where it is exposed in a Dwyka glacial outwash surface and is dated at 2931 ± 9 Ma. The southernmost exposures of granite are windows in the Dwyka group about 130 km southeast of Marydale on both sides of the Doornberg Fault. The 2905 ± 7 Ma Maritzdam Granite which is basement to Ventersdorp Supergroup volcanic rocks at Kuip is monzogranitic and corresponds to the Skalkseput type. On the southwest side of the Doornberg Fault the 2907 ± 4 Ma Welgevonden Granite underlies Marydale Group volcanic rocks, showing that the Kaapvaal basement extends across the fault.

A hitherto unknown river type from the Archaean at Bhurkuli (Jharkhand, E India)

AbstractThe Archaean granitoid pluton of the Singhbhum craton in E India is overlain by Archaean to Palaeoproterozoic metasediments. These sediments are still poorly known and their stratigraphy is under debate. Several scattered, most probably Meso- to Neoarchaean, conglomerates are present in the state of Jharkhand that differ so much in characteristics that they are probably not related to each other. The sedimentology of a series of conglomerate patches and layers near Bhurkuli has been investigated, including the characteristics of the clasts. It is deduced on the basis of these characteristics and the sedimentological context that the Bhurkuli conglomerates represent the channel facies of a river system that differed from the types of fluvial systems that exist nowadays.

Rock strength measurements on Archaean basement granitoids recovered from scientific drilling in the active Koyna seismogenic zone, western India

Reservoir triggered earthquakes have been occurring in the Koyna area, western India for the past five decades. Triaxial tests carried out on 181 core samples of Archaean granitoids underlying the Deccan Traps provide valuable constraints on rock strength properties in the Koyna seismogenic zone for the first time. The data include measurements on granite gneiss, granite, migmatitic gneiss and mylonitised granite gneiss obtained from boreholes KBH-3, KBH-4A, KBH-5 and KBH-7 located in the western and eastern margins of the seismic zone. Salient results are as follows. (i) Increase of rock strength with increasing confining pressure allow determination of the linearized failure envelopes from which the cohesive strength and angle of internal friction are calculated. (ii) Variable differential stresses at different depths are the manifestations of deformation partitioning in close association of fault zone(s) or localized fracture zones. (iii) Fractures controlled by naturally developed weak planes such as cleavage and fabric directly affect the rock strength properties, but the majority of failure planes developed during triaxial tests is not consistent with the orientations of pre-existing weak planes. The failure planes may, therefore, represent other planes of weakness induced by ongoing seismic activity. (iv) Stress-strain curves confirm that axial deformation is controlled by the varying intensity of pre-existing shear in the granitoids, viz., mylonite, granite gneiss and migmatitic gneiss. (v) Frequent occurrences of low magnitude earthquakes may be attributed to low and variable rock strength of the granitoids, which, in turn, is modified by successive seismic events.

About this title ‐ Crust–Mantle Interactions and Granitoid Diversification: Insights from Archaean Cratons

This Special Publication sheds light on crust formation and tectonic processes in early Earth by focusing on Archaean granitoids and related rocks from West Greenland in the North Atlantic Craton, Karelia Province of the Fennoscandian Shield, Eastern Dharwar and Bundelkhand cratons in the Indian Shield and Bug Complex of the Ukrainian Shield. Resulting from the IGCP-SIDA 599 project ‘The Changing Early Earth’, this compilation of papers provides explanations on the nomenclature of Archaean granitoids and explores the petrology, element and isotope geochemistry, geochronology and metamorphism of granitoids and supracrustal rocks of variable metamorphic grade. This volume provides information on the increase and timing of crust-mantle interactions and granitoid diversification from early Archaean protoliths of island arc origin to the emergence of multi-source high-K calc-alkaline granitoid batholiths at convergent continental margins. The formation of abundant granitoid batholiths suggests a significant change in mantle dynamics and plate tectonics towards the end of the Archaean.

Archaean granitoids: an overview and significance from a tectonic perspective

Abstract This special publication presents a collection of papers on Archaean igneous rocks which aim to provide evidence of a tectonic scenario that is increasingly accepted by scientists studying the evolution of the early Earth. Papers on diverse igneous rocks from the North Atlantic Craton, as well as Archaean terrains of the Fennoscandian, Indian and Ukrainian shields, have a common focus on crust–mantle interactions and granitoid diversification, especially at the end of the Archaean, accompanied by insights into metamorphic rocks. This volume, together with present research, provides evidence for a change in global tectonic regime close to the Archaean–Proterozoic boundary. After the long-term episodic formation of tonalite–trondhjemite–granodiorite (TTG) suites of oceanic origin, convergent continental margins with abundant batholiths of potassic granitoids appeared for the first time at 3.0–2.5 Ga. The batholiths involve both mantle-derived and recycled crustal material. It seems that the diversification of granitoids was caused by increased crust–mantle interactions, reflecting a significant change in mantle dynamics and plate tectonics during the Neoarchaean.

The diversification of granitoids and plate tectonic implications at the Archaean–Proterozoic boundary in the Bundelkhand Craton, Central India

Abstract The Bundelkhand Craton in Central India holds a large Archaean granitoid complex consisting of cores of TTG (tonalite–trondhjemite–granodiorite) gneisses of island arc or oceanic origin surrounded by abundant younger high-K calc-alkaline granitoids. Major and trace element groupings and ion probe U–Pb zircon datings of the groups show a time gap of 130 Ma between the main formation episodes of the TTGs (3.5/3.3–2.7 Ga) and the emplacement of the first high-K granitoids (2.57–2.54 Ga). Based on their geochemical diversity, the high-K calc-alkaline granitoids can be divided into low-silica high-magnesium (LSHM) granitoids such as sanukitoids and Closepet-type granitoids, and high-silica low-magnesium (HSLM) monzogranites with low-HREE and low-Eu subgroups. The former group points to mantle or mixed mantle and crustal sources, and the latter to pure crustal sources. All the varieties of the high-K granitoids formed within a narrow time span, which indicates large-scale partial melting and fluid activity in the mantle and crust, possibly resulting from a slab breakoff or delamination at the margin of an Archaean TTG continent. Supplementary material: Major and trace element concentrations and U-Pb results of granitoids from the Bundelkhand Craton are available at https://doi.org/10.6084/m9.figshare.c.3576377

Unravelling the record of Archaean crustal evolution of the Bundelkhand Craton, northern India using U–Pb zircon–monazite ages, Lu–Hf isotope systematics, and whole-rock geochemistry of granitoids

Combined whole-rock, U–Pb zircon and monazite dating, in situ zircon (MC)-SF-ICP-MS Lu–Hf-isotope data are reported for more than 20 Archaean granitoid samples to constrain magmatic events and their sources in the Bundelkhand Craton, northern India. The results are crucial for understanding the extent of reworking vs juvenile crustal growth during Palaeoarchaean–Neoarchaean and also provide new information on the temporal gap between Palaeoarchaean TTG and Neoarchaean granodiorite–granite magmatism. These data reveal that the Bundelkhand Craton was affected by at least four discrete episodes of Palaeoarchaean TTG magmatism between 3.55Ga and 3.20Ga at regular intervals of 100–150 Myr. After a period of about 500 Myr of near magmatic shutdown, it was followed by a relatively short-lived, but more voluminous granitoid emplacement event during the Neoarchaean at 2.58–2.50Ga, during which minor granodioritic gneisses and voluminous granites intruded. The new data also provide evidence for a metamorphic overprint at ca. 3.20Ga, and a Pan-African tectonothermal overprint at 0.50–0.60Ga.Hf-isotope and whole-rock geochemical data of the TTGs suggest a complex process of crust formation and stabilisation during the Palaeoarchaean, mainly by the reworking of older mafic and felsic crust between 3.55 and 3.20Ga. The CHUR parallel array for the TTGs may indicate that these rocks were formed by partial melting of an oceanic crust that was derived from chondritic mantle sources, shortly before formation of TTG. This interpretation is inconsistent with Hf-isotope data from worldwide sources, which provide evidence for a successive mantle depletion since at least 4.0Ga, resulting in εHft of about +5.0 at 3.0Ga. Intense melting of the heterogeneous TTG crust during the Neoarchaean resulted in the coeval formation of granodioritic gneisses and voluminous granites, and subordinate hybrid granites with a minor contribution of melts from an enriched mantle source, affected by subduction-zone processes prior to 2.55Ga.

Chapter 8 Archaean sedimentation on the Singhbhum Craton: depositional environments of conglomerates in Jharkhand (east India)

Abstract Archaean to Palaeoproterozoic metasediments overlie the Archaean granitoid pluton of the Singhbhum Craton in east India. Relatively little is known about the Archaean metasediments, but they comprise several conglomerates in the state of Jharkhand. Their age is most probably Mesoarchaean, possibly earliest Neoarchaean. They occur at separate localities and their exposures commonly consist of scattered patches with a limited extent. Their interrelationships are consequently poorly known. Sedimentological analysis of their structure, architecture and clast composition suggests that most of them represent individual occurrences that partly have been largely eroded away. Other occurrences must have had a limited extent from the very beginning. The various conglomerates were deposited in different environments. Some represent braided or meandering fluvial environments, whereas others seem to have been deposited by mass flows with different degrees of viscosity, possibly mainly on alluvial fans.