Archaean Times Research Articles

Deep, hot,ancient melting recorded by ultralow oxygen fugacity in peridotites.

The oxygen fugacity (fO2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude1-3. Although much attention has been given to mechanisms that drive variations in mantle fO2 between tectonic settings1,3,4 and to comparisons of fO2 between modern rocksand ancient-mantle-derived rocks5-10, comparatively little has been done to understand the origins of the high variability in fO2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of fO2. Refractory peridotites from the Gakkel Ridge record fO2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow fO2 can be produced by large degrees of melting at high potential temperature (Tp), beginning in the garnet field and continuing into the spinel field-conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.

Mineral chemistry of high-Al chromian spinel from ultramafic rocks of the Babina–Prithvipur transect, Bundelkhand Craton, central India: Implication for petrogenesis and tectonic setting

Bundelkhand Craton is an important Archaean cratonic nuclei of the Central Indian Shield and comprises two greenstone complexes, i.e., the Babina–Mauranipur Greenstone Belt and the Girar–Madawara Greenstone Belt. The E–W trending Babina–Mauranipur Greenstone Belt in the central part of the craton, encloses several isolated lensoid shaped ultramafic bodies which has suffered various degrees of alteration and metamorphism. As per modal mineral analysis, the ultramafic rocks of the Babina–Prithvipur section belong to harzburgite and also contain high-Al chromian spinel along with olivine, ortho-pyroxene and amphibole. Mineral chemistry reveals that the spinels are of high Al and Cr poor variety, where Al2O3 and Cr2O3 contents range from 40.06 to 54.34 wt.% and 9.05 to 14.89 wt.%, respectively. The TiO2 content is extremely low (average ≈0.07 wt.%). The Cr# value of the spinel is <0.2, whereas Mg# ranges from 0.495 to 0.633. The forsterite contents of the olivine ranges from 86.088 to 88.105 wt.%. Average CaO and NiO contents of the olivine stand 0.03 and 0.24 wt.%, respectively. Composition of the analyzed ortho-pyroxene belongs to En84.20–87.75Wo0.15–0.39 with low CaO content of 0.080 to 0.207 wt.%. As per mineral chemistry, these harzburgite rocks of the Babina–Prithvipur section belong to mantle peridotite. Melt calculation for the spinel also suggests a least differentiated magmatic product, which is also supported by the olivine spinel mantle array diagram as all the samples are plotted within the mantle array field very close to the fertile mantle source. Low TiO2 and high Al contents of spinel also reflect the MORB type peridotite characteristics for these ultramafic rocks which probably originated from least differentiated plagioclase free mantle derived harzburgite/lherzolitic magma in a rift related spreading centre. As a whole these ultramafic rocks appear to be the remnant of the early crust that existed during the Archaean time.

Incompatible element-enriched mantle lithosphere beneath kimberlitic pipes in Priazovie, Ukrainian shield: volatile-enriched focused melt flow and connection to mature crust?

ABSTRACT Major, minor and trace element compositions of mantle xenocrysts from Devonian kimberlite pipes in the Priazovie give an insight into the mantle structure beneath the SE Ukranian Shield and its evolution. Garnets yield low temperature conditions as determined by monomineral thermobarometry. The mantle lithosphere is sharply divided at 4.2 GPa, marked by a high temperature Cpx-Ilm-Phl trend, eclogites and changes in pyrope geochemistry. Seven layers are detected: Ist layer at 2.5–1 GPa is enriched mantle (Fe#Ol ~ 0.11 − 0.14) with Gar- pyroxenites and Sp peridotites; IInd at 2.5–3.2 GPa – Gar-Sp (Fe#Ol 0.08 − 0.10) peridotite. IIId at 4.3–3.2 GPa is formed of Archaean- Proterozoic peridotites with Fe#Ol ~0.07 − 0.095. IVth at 3.2–5 GPa- contains pyroxenitic Gar with higher Ca, eclogites, Chr and Cpx (Fe#Ol ~0.10 − 0.125); Vth at 5.8 − 5 GPa is marked by sub-Ca garnets, Cr-rich chromites and Mg-Cr ilmenites; VIth layer at 5.8–6.8 GPa contains Fe-enriched pyropes, almandines and Cr-Mg ilmenites near the lithosphere base; VIIth layer > 6.8 GPa consists of ‘hot’ Fe-rich garnets. Garnets show increasing enrichment in LREE, LILE, Hf, Zr with decreasing pressure. Primitive garnets have round REE patterns; depleted ones have S-type patterns inflected at Nd. Garnets from 6.5 to 3 GPa show increasing La/Ybn, Zr-Hf, LILE. Peridotitic clinopyroxenes have inclined linear trace element patterns rounded from La to Pr with high LILE and HFSE levels. The Fe-rich group (reacted with eclogites) shows bell-shaped irregular patterns with LILE close to the LREE levels. A possible reason for LILE (HFSE and) enrichment of the upper part of the mantle is subduction metasomatsm in Archaean times (with participation of mature continental sediments) activated by plumes at 1.8 Ga and earlier which produced pervasive focused melt flow with remelting of mica-amphibole metasomatites giving continuous REE and LILE enrichment in mantle lithologies from 5.8 to 2.5 GPa.

An Overview of the Carbonatites from the Indian Subcontinent

AbstractCarbonatites are carbonate-rich rocks of igneous origin. They form the magmas of their own that are generated in the deep mantle by low degrees of partial melting of carbonated peridotite or eclogite source rocks. They are known to occur since the Archaean times till recent, the activity showing gradual increase from older to younger times. They are commonly associated with alkaline rocks and be genetically related with them. They often induce metasomatic alteration in the country rocks forming an aureole of fenitization around them. They are host for economically important mineral deposits including rare metals and REE. They are commonly associated with the continental rifts, but are also common in the orogenic belts; but not known to occur in the intra-plate regions. The carbonatites are known to occur all over the globe, majority of the occurrences located in Africa, Fenno-Scandinavia, Karelian-Kola, Mongolia, China, Australia, South America and India. In the Indian Subcontinent carbonatites occur in India, Pakistan, Afghanistan and Sri Lanka; but so far not known to occur in Nepal, Bhutan, Bangladesh and Myanmar. This paper takes an overview of the carbonatite occurrences in the Indian Subcontinent in the light of recent data. The localities being discussed in detail cover a considerable time range (&gt;2400 Ma to &lt;0.6 Ma) from India (Hogenakal, Newania, Sevathur, Sung Valley, Sarnu-Dandali and Mundwara, and Amba Dongar), Pakistan (Permian Koga and Tertiary Pehsawar Plain Alkaline Complex which includes Loe Shilman, Sillai Patti, Jambil and Jawar), Afghanistan (Khanneshin) and Sri Lanka (Eppawala). This review provide the comprehensive information about geochemical characteristics and evolution of carbonatites in Indian Subcontinent with respect to space and time.

Initial 87Sr/86Sr as a sensitive tracer of Archaean crust-mantle evolution: Constraints from igneous and sedimentary rocks in the western Dharwar Craton, India

Among the rocky planets of the solar system only the Earth has “granitic” continental crust. The timing and processes involved in the formation of Earth’s first extensive crust is still enigmatic. The chemical and isotope compositions of ancient crustal rocks preserve a record of their genesis. The Rb-Sr system proves to be an efficient proxy for the reconstruction of crust-mantle evolution since it can bring together information from seawater as preserved in chemical sedimentary rocks and information from magmatic rocks that can trace the time and extent of crust formation and concomitant mantle depletion during the Archaean eon. The Dharwar Craton in India preserves a suite of metamorphosed igneous and sedimentary rocks that record its early crustal evolution. To overcome the susceptibility for resetting and the difficulty in determining initial 87Sr/86Sr, the minerals barite and apatite are used to obtain precise and accurate 87Sr/86Sr, because these minerals preferentially incorporate Sr and exclude Rb and preserve the initial Sr isotope compositions at the time of their formation. Initial 87Sr/86Sr of apatite were obtained in situ using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry. The robustness and only minor dispersion of the isotope ratios demonstrate the capability of matrix apatites in preserving initial Sr ratios. The least radiogenic value is used as the best estimate for the initial 87Sr/86Sr. The 87Sr/86Sr ratios of apatite from igneous rocks that formed from 3.5 Ga to 2.6 Ga constrain the Rb/Sr of the source over the whole time-span. A comparison of the Sr isotopes between seawater-derived barite and initial Sr isotope ratios in apatite from igneous rocks reveals that significant mafic to intermediate crust had formed by 3.2 Ga. Studying the entire Archaean time window, a dominantly mafic crust was the main source for the granitoid rocks in the Dharwar Craton from 3.5 to 3.1 Ga, whereas the rocks from 2.9 to 2.7 Ga were extracted dominantly from the depleted mantle.

Age and provenance of detrital zircons from the Oligocene formations of the Marseille–Aubagne basins (SE France): consequences on the geodynamic and palaeogeographic evolution of the northern Gondwana margin

Eight samples from Oligocene sedimentary rocks of the Marseille–Aubagne basins have been analysed for their detrital zircon age spectra. These age spectra provide information about the regional evolution, from Oligocene to Archaean times. The Carboniferous Variscan and the Late Cretaceous to Eocene Pyreneo-Provencal belts represent the latest main tectonic, magmatic, and volcanic events that formed the major zircon age populations found in studied sediments. The obtained detrital zircon age record of the Marseille–Aubagne basins comprises eleven detrital zircon age clusters. They reflect the long and complex geologic history of the sediments source areas and can be ascribed to the opening of the western Mediterranean, the Variscan, Cadomian and Pan-African belts, to an unknown Mesoproterozoic event, to the Eburnean orogeny of West Africa and to the different tectono-metamorphic events that took place in Archaean times. In general, the Palaeo- and Mesozoic events are ascribed to the dispersal of Western and Eastern Gondwana and the Pangaean supercontinent cycle. Thus, the successive recycling of zircon grains from older and the incorporation of them to younger belts lead to new geodynamical models for the northern Gondwana margin evolution. Significant amounts of Mesoproterozoic detrital zircon are at odds with previous hypotheses and re-open the question of the provenance of these zircon age populations. Therefore, this tiny tertiary basin is a natural archive which records the main geological events in SE France and its vicinity.

Possible biogenic structures from the Lower Cambrian strata in Yunnan Province, South China

AbstractMicrobially induced sedimentary structures (MISS) are microbial traces in sandy deposits. They have been formed by various modes of microbial behaviour in response to the prevailing physical dynamics in shallow-marine environments since early Archaean time. An association of fossils, phosphatic structures and grains, stromatolites and microbial laminated levelling structures is documented from the Zhongyicun Member of Lower Cambrian strata in the Baideng section dated close to 535 Ma. SEM examination demonstrates that microbial laminated levelling structures are the result of the development of microbial mats composed of filamentous structures. We propose that there was a short hiatus after the dolomite layer deposited when the dolomite layer was weathered to form centimetre-scale valleys firstly, and then microbes accumulated in these valleys and formed the microbial laminated levelling structures.

Th-U-Pb zircon geochronology of the Palaeoproterozoic Hartley Formation porphyry by six methods, with age uncertainty approaching 1 Ma.

The Palaeoproterozoic Hartley Formation in the Olifantshoek Group was deposited in one of the rift-related Waterberg (sensu lato) red bed basins which formed on the Kaapvaal Craton after the 2.05 Ga Bushveld intrusions and coeval thermal event. The age of these basins is not well constrained due to the shortage of directly dateable rock types. The Hartley Formation contains rare quartz-porphyry lavas interbedded with the dominant basalts and these provide the means to date the formation by analyses of zircon. In this work zircon from one sample has been dated by six Th-U-Pb methods, namely Laser Ablation ICP Quadrupole Mass Spectrometry, Laser Ablation ICP High-resolution Mass Spectrometry, Laser Ablation ICP Multicollector Mass Spectrometry U-Pb (also Lu-Hf), Nordsim Ion probe U-Pb and Th-Pb; and Krogh method ID-TIMS. Our precise ages give a combined age of 1915.2 ± 1.1 Ma. Including one published ion probe date from the only other known occurrence of quartz porphyry, the results only agree if the quoted analytical errors are increased by 20%, which gives a combined result of 1915.6 ± 1.4 Ma. This is considered a reliable, precise and accurate age for the Hartley Formation and supersedes the published Kober method 207Pb/206Pb age of 1928 ± 4 Ma. The new Lu-Hf zircon data, supported by published whole rock Sm-Nd and Rb-Sr data, suggests that both the dominant basalts and the rare quartz porphyries of the Hartley Formation were derived from mafic source rocks which had been in the crustal domain from Archaean times. By contrast with the intracratonic rifts of the other Waterberg Basins, the Olifantshoek Supergroup reflects the development of a western passive margin as the Archaean Kaapvaal Craton rifted and drifted. This was followed by accretion of the Rehoboth Province along the Kalahari Line, accompanied by the development of the east-vergent Kheis Province thrust complex. This created a larger cratonic block against which the 1.2 Ga collisions of Namaqua-Natal terranes impacted. The Kheis Province now yields ~1.17 Ma cooling ages, reflecting the Namaqua collisions, but the true age of the Kheis event is still enigmatic.

Lewisian Complex of Strath Dionard–Rhiconich and its significance in the early history of the NW Highlands of Scotland

Synopsis Remapping of a quartzofeldspathic gneiss-dominated part of the Lewisian Complex in the NW Highlands of Scotland, originally mapped by the Geological Survey in the latter part of the nineteenth century, has added new information about the lithologies and their distribution, and established a tectonothermal–igneous history. Linking new data for the gneisses of Strath Dionard–Rhiconich to the results of isotopic studies points to the development of their protolith in late Archaean times as the result of arc magmatism similar to that at a modern ocean–continent margin. Compositionally, most of the gneisses, and those southwards towards Laxford, correspond to granodiorite with adakitic affinities. Using a comparison with a well-substantiated model for coeval late Archaean crust formation of part of the Wyoming Craton, they are interpreted as representing a stage in a tectonomagmatic cycle when the products of metasomatism and melting in the mantle were emplaced as sheet-like banded plutons at middle crustal levels. Less common granitic gneisses represent products of further melting in the thickened middle crust. Xenolithic blocks of subducted older sedimentary rocks, carried upwards by the magmas, are now seen as elongate lensoid masses of calc-silicate rock and quartzite that are grossly concordant with the banding in the gneisses. Also generally concordant with this banding are elongate lensoid masses of amphibolite, the protolith of which is considered to have been the product of mafic magmatism during the convergent margin arc activity. In early Proterozoic times the late Archaean assemblage was extensively reworked. The development of penetrative foliation in both gneisses and amphibolites was associated with isotopic homogenization that, in the southerly continuation of the domain towards the type locality of the Laxfordian orogenic belt at Laxford, has been dated at c . 1860 Ma. This preceded the imposition of the current major structural pattern that included the formation of the Strath Dionard antiform and the parasitic Gharbh Mhòr folds. Abundant granites and pegmatites, subsequently emplaced in an injection complex at Strath Dionard, are linked with the c . 1855 Ma suite of acidic intrusions that are prominent at Laxford. The rock assemblage and sequence of development are like those of other early Proterozoic mobile belts formed as the result of extensive reworking of late Archaean cratons. Limited mineralogical and structural modification of the ≥1850 Ma gneiss–amphibolite–granite assemblage, and pegmatite emplacement associated with (micro)-plate collision, took place during the development of the c . 1750 Ma Calvian orogenic belt. Further limited modification and pegmatite emplacement was at c. 1670 Ma. The structural–metamorphic–igneous histories described, integrated with isotopic data, do not support interpretations proposing a transition between the Strath Dionard–Rhiconich (–Laxford) gneiss–amphibolite domain and the adjacent granulite–dolerite dyke domain; or that the quartzofeldspathic gneiss–amphibolite domains in some other parts of the NW Highlands can be correlated with the type area of the Laxfordian orogenic belt.

Superposed deformation and inherited structures in an ancient dilational step-over zone: Post-mortem of the Rengali Province, India

In the eastern Indian shield, a dextral strike-slip system juxtaposed the Archaean Singhbhum Province against the Proterozoic Eastern Ghats Belt at ∼490–470 Ma. Two WNW–ESE trending strands of the strike-slip system enclose a multiply deformed (D1 to D3) intervening domain called the Rengali Province, with D3 representing dextral shearing. In a granulite lens within the province, an early fabric (Sgr) was deformed by an amphibolite facies D1–D2 deformation continuum in the late Archaean time, forming cylindrical folds. In the surrounding quartzofeldspathic gneisses, quartzites and mica schists of the province, superimposition of syn-D3 shortening on D1-D2 folds generated complex non-cylindrical geometries; the granulites escaped D3 strain. Microstructures in the province-bounding shear zones confirm that D3 deformation was associated with mylonitization, dynamic recrystallization and greenschist facies metamorphism. In the quartzites, syn-D3 folds can be correlated with rotation of D1–D2 structures through the shortening zone of bounding dextral shears. Since the province-bounding shears form a step-over zone, the structural complexity within the Rengali Province arises from superposition of syn-D3 shortening structures on initially asympathetically oriented inherited cylindrical D1-D2 folds. Hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied D3 as the step-over zone was dilational in nature.

Native gold and ore mineral inclusions in detrital zircon and quartz from the early precambrian quartzites and quartz gravelstones of the Karelian Craton

The study of solid mineral inclusions in detrital hydrothermal and hydrothermally altered zircons and quartz from Archaean and Palaeoproterozoic (Sumian and Jatulian) terrigenous quartz-rich clastic rocks of the Karelian Craton on a VEGA II LSH-TESKAN microanalyzer has revealed, for the first time, micron-sized native gold inclusions and associated syngenetic inclusions of ore minerals, such as galena, sphalerite, pyrite, chalcopyrite, molybdenite, molybdoscheelite, native bismuth and cerussite, as well as barite and complex hydroaluminophosphates of the goyacite-gorceixite-florencite-plumbogummite group. This mineral association in detrital material could indicate the ore-forming nature of its quartz vein source. These data, together with the earlier results of the local isotopic (SHRIMP-II and LA-ICP-MS) dating of these and similar heterogeneous detrital zircons, are indicative of several hydrothermal events, dated at 3.3, 3.15–2.9 and 2.76–2.65 Ga, which occurred in the region in Archaean time. The two latter ages are consistent with the time of formation of quartz vein ore systems with some of which gold ore-forming processes were associated.

Palaeomagnetism of Precambrian dyke swarms in the North China Shield: The ∼1.8 Ga LIP event and crustal consolidation in late Palaeoproterozoic times

The North China Shield (NCS) is cut by a laterally-extensive dyke swarm emplaced at 1.78–1.76 Ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by ∼1.85 Ga. Palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with NE to N declinations and rare opposite polarity. Dykes with NE shallow magnetic declination (A1, D/ I = 36/−1°) recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations (A2, D/ I = 8/2°) in lower grade metamorphic terranes to the south. Contact tests indicate a primary cooling-related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable. Petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at ∼1.85 Ga. A temporal migration A2 → A1 is implied by the higher crustal level and earlier acquisition of the former, and the deeper source and later acquisition of the latter. A third population of dyke magnetisations (A3, D/ I = 18/43°) is distributed towards steeper inclinations and close to the Mesozoic-Recent palaeofield. These are either partial or complete overprints of A1–A2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra, or are much younger dykes of Mesozoic-Tertiary age. A minority fourth (later Precambrian but presently undated) dual polarity population has a magnetisation (11 dykes, D/ I = 108/7°) with contact tests indicating a primary cooling-related origin. The ∼1.78–1.76 Ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major Large Igneous Province (LIP)-related events. The new definition of a ∼1.83–1.76 Ga APW swathe from the North China Shield permits a comparison with other shields and yields a constraint to continental configurations during the late Palaeoproterozoic. A quasi-integral reconstruction of Palaeopangaea is tested here and supported by conformity of predominantly of uplift-related palaeopoles from the ∼1.90–1.70 Ga tectono-thermal belts and from SW → NE trending APW implied by the distribution of poles from the ∼1.80 Ga igneous suites including the LIP events. This trend incorporates the A2 → A1 migration and the granulite terrane cooling polar swathe from North China. The reconstruction indicates that continental crust consolidated in Palaeoproterozoic times by accretion of ∼2.3–1.7 Ga orogenic belts around a hemispheric and crescent-shape core already established by Late Archaean times. The North China Shield is interpreted to have bordered the western cratonic margin of the Indian Shield in a proximity supported by correlation of geological features and suggested by a number of previous workers. The Central Orogenic Zone of the North China shield characterised by tectono-thermal activity prior to ∼1.85 Ga was then contiguous with a comparable zone running through the centre of the Indian Shield and continuing into the Capricorn Belt of Western Australia. The ∼1.78–1.76 Ga dykes in North China continue into dyke swarms in the South India Shield and may have been sourced in a plume-related LIP focussed near the continental margin in the Xiong’er Aulacogen.

Archaean fluid-assisted crustal cannibalism recorded by low δ18O and negative εHf(T) isotopic signatures of West Greenland granite zircon

The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U–Pb, δ18O and LA-MC-ICPMS 176Hf isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two ~3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qôrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial εHf zircon values (n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one 207Pb/206Pb age, and correlated with 176Hf/177Hf compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite–trondhjemite–granodiorite (TTG) gneisses. Zircon from the ~3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic 176Hf/177Hf and mantle-like δ18O compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions (n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean δ18O values ranging from 5.8 ± 0.6 to 3.7 ± 0.5‰. The 3.64 Ga granitic augen gneiss sample displays the highest δ18O with a mildly supra-mantle composition of 5.8 ± 0.6‰. Inherited Eoarchaean TTG-derived zircon shows mantle-like values. Igneous zircon from all other samples, spanning more than a billion years of Archaean time, record low δ18O sub-mantle compositions. These are the first low δ18O signatures reported from Archaean zircon and represent low δ18O magmas formed by the remelting and metamorphism of older crustal rocks following high-temperature hydrothermal alteration by meteoric water. Meteoric fluid ingress coupled with crustal extension, associated high heat flow and intra-crustal melting are a viable mechanism for the production of the low δ18O granites, granodiorites and trondhjemites reported here. Both high and low δ18O magmas may have been generated in extensional environments and are distinct in composition from Phanerozoic I-type granitic plutonic systems, which are typified by increasing δ18O during intra-crustal reworking. This suggests that Archaean magmatic processes studied here were subtly different from those operating on the modern Earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement.

Precambrian mafic magmatism in south Indian granulite terrain

South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.

Geochemistry and petrogenesis of Neoarchaean high-Mg low-Ti mafic igneous rocks in an intracratonic setting, Central India craton: Evidence for boninite magmatism

The southern Central India craton (also known as Bastar craton) has experienced several episodes of mafic magmatism during the early Precambrian time. These mafic igneous rocks are emplaced as dyke and volcanic rocks. A set of mafic dyke swarm and few exposures of mafic volcanics, both emplaced in the Neoarchaean time, contain high-SiO2 (>52 wt%), high-MgO (>8 wt%), and low-TiO2 (mostly <0.5 wt%) and classified as boninites on IUGS classification scheme. Al2O3/CaO ratio classifies these high-Mg mafic igneous rocks into high-Ca and low-Ca (Type 3) boninites. High-Ca boninites show typical boninitic geochemical characteristics, whereas low-Ca variety shows geochemical characteristics similar to the high-Mg norites. Siliceous high-magnesium basalts (SHMB) show similar geochemical properties as observed for the high-Mg norites. Early Precambrian high-Mg norites are reported from the rift setting environment but, on the other hand, boninitic magmatism represents an enigmatic style of magmatism and commonly formed at convergent margin settings. Most boninites are Phanerozoic in age and rarely reported from the Archaean terrains. The established geological and tectonic setting of the Archaean Bastar craton clearly suggests existence of a stable continental rift environment in the region since the Archaean time. Thus, report of Neoarchaean boninite-like rocks in an intracratonic setting is an important feature noticed in the Archaean terrain of Indian peninsula. A highly refractory mantle source region is required to form boninitic magma. Refractory lithosphere may be result of extensive extraction of mafic magma during Mesoarchaean. The preservation of refractory reservoir beneath the Bastar craton for long period of time corroborates a high degree of crustal stability during the Meso-Neoarchaean time. Trace element modelling suggests that high-Mg mafic igneous rocks under study are product of different pulses of boninitic magma produced by ∼20-25% melting of a refractory mantle source.

Source mantle heterogeneity and its role in the genesis of Late Archaean–Proterozoic (2.7–1.0 Ga) and Mesozoic (200 and 130 Ma) tholeiitic magmatism in the South American Platform

A comparative study of Precambrian and Mesozoic tholeiitic basalt magmatism from the South American Platform reveals that both Late Archaean–Proterozoic and Mesozoic tholeiites are characterized by high TiO 2 (>2 wt.%; HTi) and low TiO 2 (<2 wt.%; LTi), and high and low incompatible elements, respectively. The Early Jurassic (ca. 200 Ma) Central Atlantic Magmatic Province (CAMP) and the Early Cretaceous Paraná (ca. 130 Ma) HTi and LTi tholeiites have major and trace element contents and Sr–Nd isotopic compositions similar to their Precambrian analogues, particularly for the tholeiites belonging to the same craton. Both the Precambrian and Mesozoic LTi and HTi tholeiites are characterized by “depleted” (low LREE/HREE) to “enriched” (high LREE/HREE) features. The “enriched” tholeiitic basalts have negative Nb anomaly (relative to K and La), while the “depleted” ones may show a slight positive Nb spike. Sr and Nd isotopes of both the HTi and LTi Precambrian tholeiites, as well as most of HTi Mesozoic ones, fit the mantle array trend, while the LTi tholeiites of Mesozoic age trend towards EM II. The Precambrian and Mesozoic tholeiites from the South American Platform (SAP) with high Sr and low Nd isotopic compositions cannot be easily explained by AFC and mixing processes between crustal and mantle components if a basaltic major element composition should be preserved. This suggests that the Precambrian and Mesozoic SAP tholeiitic melts reflect a heterogeneous source mantle, including EM I (e.g. fluids, small volume melts) and EM II (e.g. ancient subduction-related metasomatism and/or silicic melt veining) components. All the data indicate that mantle source heterogeneity was well established at least since Late Archaean times, as documented by the Precambrian and Mesozoic SAP tholeiites which have similar compositional features, particularly the tholeiites cropping out on the same craton. The concentration of the Precambrian and Mesozoic SAP tholeiitic magmatism towards craton/mobile belt boundaries suggests that an important role in its genesis was played by upper mantle “edge drive convection” geodynamics.

SHRIMP U–Pb zircon geochronology of granitoids from Dongping area, Hebei Province, China: constraints on tectonic evolution and geodynamic setting for gold metallogeny

The Dongping area in the central part of the northern marginal zone of the North China Craton (NCC) is one of the most important gold-producing areas in China, and has been intruded by a number of alkaline and granitic batholiths or plutons spatially associated with gold deposits. Previous dates on these intrusions yield contradictory data, depending on the isotopic methodology used, and have led to ambiguity in understanding magmatic evolution and gold mineralization of the region. Detailed SHRIMP U–Pb zircon geochronology of the alkaline and granitic rocks in the study area now reveals details of its tectono-magmatic history. Zircons from a granitic gneiss hosting the Xiaoyingpan gold deposit yielded concordant ages of ca. 2.5 Ga. This constrains regional metamorphism of the Archaean basement to late Archaean time. A second magmatic event at ca. 1.8 Ga is revealed by xenocrystic/inherited zircons with this age. These two Precambrian events are coeval with those in other parts of the NCC. The largest igneous intrusion in the region, the Shuiquangou syenite–monzonite complex, was emplaced at ca. 390 Ma, as indicated by the magmatic zircon populations of the two samples from the central and eastern parts of the complex. The accretion of a volcanic arc onto the northern margin of the NCC is considered as a plausible cause of this alkaline magmatism. The emplacement of the Guzuizi porphyritic granite pluton at 236±2 Ma represents another major magmatic event in the region, probably a response to postcollisional orogeny following amalgamation of the North China and Angara cratons. Latest magmatism is marked by the emplacement of the Shangshuiquan monzogranite at 142.2±1.3 Ma, which is most likely related to an underplating event that took place in the Dongping and its adjacent areas at ca. 140 Ma. More importantly, these results suggest that gold mineralization in the area, essentially a single event as indicated by similar deposit characteristics, is broadly coeval with this latest granite magmatism. Hence, this mineralization is about 250 Ma later than the emplacement of the Shuiquangou alkaline complex and, thus, is interpreted to preclude the widely accepted model that suggests gold deposits have genetic relationships with alkaline magmatism. The late timing of gold mineralization with respect to felsic magmatism in the area is similar to those observed in other major gold-producing provinces in the NCC. In all these major gold provinces, we suggest that gold mineralization is coeval with the latest granite magmatic event at about 140–120 Ma. This farther suggests that these mesothermal gold deposits were formed from similar tectono-magmatic environments.

Global modelling of continent formation and destruction through geological time and implications for CO 2 drawdown in the Archaean Eon

Abstract The possible drawdown of a massive CO 2 atmosphere in early Earth history is discussed using two working hypotheses: first, that this removal of CO 2 from the atmosphere occurred mainly via silicate weathering; second, that crust-to-mantle recycling rates found from forward modelling of crust-mantle history can be used to estimate rates of this ancient silicate weathering. Previous U-Th-Pb and Sm-Nd forward modelling efforts are reviewed, from which it was concluded that an insignificant amount of continental crust existed at 4.4 Ga, i.e. so-called ‘no-growth models’ for the continental crust appear untenable. New modelling carried out is based on a crustal growth curve starting with zero mass at 4.2 Ga and reaching 75% of the present crust mass by 2 Ga. It concerns variations in crust-to-mantle recycling rates through geological time. Best fits to isotope data are obtained if it is assumed that erosion rates (mass removal per unit surface) were approximately constant from early Archaean time to the present. From the results it can be estimated that drawdown of a massive CO 2 atmosphere by silicate weathering could have been completed by the end of Archaean time at the earliest, and about 1.5 Ga ago at the latest.

Carbon dioxide cycling through the mantle and implications for the climate of ancient Earth

Abstract The continental cycle of silicate weathering and metamorphism dynamically buffers atmospheric CO 2 and climate. Feedback is provided by the temperature dependence of silicate weathering. Here we argue that hydrothermal alteration of oceanic basalts also dynamically buffers CO 2 . The oceanic cycle is linked to the mantle via subduction of carbonatized basalts and degassing of CO 2 at the mid-ocean ridges. Feedback is provided by the dependence of carbonatization on the amount of dissolved carbonate in sea water. Unlike the continental cycle, the oceanic cycle has no thermostat. Hence surface temperatures can become very low if CO 2 is the only greenhouse gas apart from water. Currently the continental cycle is more important, but early in Earth’s history the oceanic cycle was probably dominant. We argue that CO 2 greenhouses thick enough to defeat the faint early Sun are implausible and that, if no other greenhouse gases are invoked, very cold climates are expected for much of Proterozoic and Archaean time. We echo current fashion and favour biogenic methane as the chief supplement to CO 2 . Fast weathering and probable subduction of abundant impact ejecta would have reduced CO 2 levels still further in Hadean time. Despite its name, the Hadean Eon might have been the coldest era in the history of the Earth.

Pb isotope variations in Archaean time and possible links to the sources of certain Mesozoic-Recent basalts

Abstract The present-day Pb isotope ratios of mafic and felsic rocks, feldspars and ore samples (galenas) from Archaean cratons ranging in age from 3.8–3.7 Ga to 2.6–2.7 Ga have been used to investigate their initial 207 Pb/ 204 Pb ratios, and μ 1 ratios (source U/Pb ratios). Two broad trends of initial 207 Pb/ 204 Pb evolution are observed, with samples from Greenland, Abitibi and SW India tending to have lower initial 207 Pb/ 204 Pb ratios through time than those from eastern India, Australia and southern Africa (Kaapvaal and Zimbabwe). The calculated μ 1 values are in the range of 7–9 and they tend to be constant for a given craton through time, with Zimbabwe as a notable exception, for both mafic and felsic rock types. It follows that each craton is characterized by specific μ 1 values, or initial 207 Pb/ 204 Pb ratios; μ 1 for Greenland is 7.37 ± 0.27, and they are higher for Abitibi at 7.57 ± 0.13, which are similar to SW India, where μ 1 = 7.57 ± 0.13. In Scotland μ 1 = 7.94 ± 0.152, in Finland μ 1 is 8.03 ± 0.08, in eastern India it is 8.03 ± 0.1, and for Australia μ 1 = 8.07 ± 0.14, and there tend to be higher values of 8 ± 0.2 and 8.32 ± 0.29 in the Kaapvaal and Zimbabwe, respectively. Overall, there is therefore a strong provinciality of μ 1 values in the Archaean areas, and unexpectedly the high μ 1 Archaean terranes are close to the areas of high μ 1 Mesozoic continental flood basalts (CFB), and the DUPAL anomaly in recent oceanic basalts. The cratons with high μ 1 values are in the southern hemisphere near the 130–190 Ma CFB from the Gondwana supercontinent, and the maximum DUPAL anomalies at latitudes 30–45°S south of Africa and 0–15°S south of India. The variations in μ 1 are based on 207 Pb/ 204 Pb ratios, and so they primarily reflect events that took place early in Earth history. In one model, the high μ 1 values were initially generated at the core-mantle boundary, through processes linked to core formation and late accretion events, and they were mobilized in plumes responsible for at least some of the early Archaean mafic crust. Preservation of domains characterized by different U/Pb ratios is inferred to have been at relatively shallow levels, as they are preserved in both felsic and mafic rocks, and they appear to have survived for long periods of time, despite the effects of mantle convection and plate motion. Recycling of early Archaean lithosphere in the mantle by delamination might perpetuate these anomalies through time in a given area, perhaps even contributing to the modern DUPAL Pb anomalies. Delamination rather than subduction as the process of lithosphere recycling might explain why such regional domains are not observed in the kappa (κ 1 : source Th/U) ratios of these Archaean samples.