Archean Terranes Research Articles

Quantitative modelling of the morphology of age plateaux in muscovite argon spectra and the implications for thermochronology

For muscovite in ancient Proterozoic and Archean terranes, quantitative modelling of age spectrum morphology shows that a significant plateau segment does not form in an age spectrum unless there was rapid cooling that either began before and continued through, or which initiated at, the time given by the plateau age. Further, the modelling shows that a well-developed plateau is not preserved unless such rapid cooling continues until ambient temperatures are reached that are <∼220–200 °C. It thus follows that the age of a well-developed plateau segment in a muscovite age spectrum is an accurate constraint on when the rock was first exhumed to (and/or last cooled at) shallow crustal levels. In a sample held at a higher ambient temperature, argon continues to be lost from the lattice at a significant rate, well past the time of so-called ‘isotopic closure’ predicted by application of Dodson’s formula. Parametric modelling shows that monotonic cooling paths do have a well-defined point, at which isotopic closure takes place, but to form an age plateau: (1) the rate of cooling must be rapid; and (2) cooling must continue at the same or similar rates, until temperatures are reached that are low enough to inhibit significant further loss of argon to the environment. Quantitative modelling shows how Dodson’s formula begins to fail once ambient temperatures rise above these numerically computed values for isotopic closure. Yet these numerically computed closure temperatures are far below the nominal closure temperature for muscovite, typically quoted as ∼ 350–400 °C. Therefore, these results have significant implications for models that utilise closure temperatures based on estimates using Dodson’s formula, when attempting to invert thermochronological datasets to obtain constraints on the temperature–time path.

The tectonic development of the Central African Plateau: evidence from shear-wave splitting

SUMMARY The Central African Plateau comprises a mosaic of numerous Archean terranes—the Congo, Bangweulu and Kalahari Cratons—sutured in a series of Proterozoic to early Cambrian orogenic events. Major upper-crustal deformation and complex craton margin fault zones reflect the region’s diverse tectonic history: rifting during the Neoproterozoic, collision during the Pan-African orogeny, and more recently, Permo-Triassic Karoo rifting and the Pliocene development of the Southwestern branch of the East African Rift. The tectonic evolution and extent to which the lithospheric mantle has been re-worked by each tectonic event is poorly understood. New seismograph networks across the Plateau provide fresh opportunity to place constraints on the plate-scale Precambrian-to-Phanerozoic processes that have acted across the region. Utilizing data from seismograph deployments across the Central African Plateau, including the new Copper Basin Exploration Science network—a NW–SE-trending, 750-km-long profile of 35 broad-band stations—we explore lithospheric deformation fabrics associated with past and present tectonic events via a shear-wave splitting study of mantle seismic anisotropy. Results reveal short length-scale variations in splitting parameters (fast direction: $\phi$, delay time: $\delta$t), suggestive of a fossil lithospheric fabric cause for the observed anisotropy. A lack of fault-parallel $\phi$ across the Mwembeshi shear zone, suggests it may be too narrow at mantle depths, a thin-skinned, crustal-scale feature, and/or did not experience sufficient fault parallel shear-strain during its last active phase to form a lithospheric deformation fabric discernible via teleseismic shear-wave splitting. In the heart of the Lufilian Arc, we observe abrupt changes in splitting parameters with NE–SW, N–S and NW–SE $\phi$ and 0.5 s $&amp;lt; $$\delta$t$&amp;lt; $ 1.2 s evident at short length-scales: no single, uniform, anisotropic lattice preferred orientation (LPO) fabric defines the entire region. This is consistent with the view that multiple episodes of deformation shaped the Lufilian Arc, or perhaps that pre-existing fabrics, relating to Neoproterozoic Katangan Basin development, have failed to be completely overprinted by the Pan-African orogeny. Near the Domes, where most intense crustal re-working is thought to have taken place during the Pan-African orogeny, there is a cluster of null and low $\delta$t splits which likely reflects the lack of organized LPO fabrics, perhaps due to the presence of depth-dependent anisotropy. The neighbouring Congo Craton margin is marked by consistently weak anisotropy ($\delta$t$\lt $ 0.7 s) indicating a weak horizontal alignment of olivine at mantle lithospheric depths, typical of several Archean terranes worldwide.

Polymetamorphic evolution of the Vestfold Block in East Antarctica and implications for the amalgamation of terranes

The Vestfold Block, a typical polymetamorphic Archean terrane in East Antarctica, is a key area to understand amalgamations of Rodinia and East Gondwana continents. However, multiphase overprinting makes it difficult to determine the timing and nature of each tectonothermal event. In this study, we present P–T estimates, zircon, monazite U(–Th)–Pb and biotite/K–feldspar Rb–Sr isochron ages of paragneisses from the SE Vestfold Block. One paragneiss sample, which is assigned to the Chelnok Paragneiss, has experienced a protracted metamorphism from the Neoarchean to the early Paleoproterozoic. Phase equilibria modeling constrained the peak P–T conditions to 7.2–9.6 kbar and 850–880 ℃, and the post–peak metamorphism to 4.2–5.6 kbar and 720–790 ℃, respectively. On the other hand, a paragneiss sample close to the ice sheet documented a high–grade metamorphic event at 918 ± 23 Ma, with peak P–T conditions of 6.0–8.0 kbar and 860–880 ℃. Biotite/K–feldspar Rb–Sr dating for these two samples yields isochron ages of 474 ± 12 and 442 ± 7 Ma, respectively, representing the cooling ages of the Pan–African reworking. Collectively, an integrated application of diverse chronometers, combined with published data, indicates that the Vestfold Block may have experienced at least three major thermal events with variable intensities and extents. Initially, the supracrustal rocks in this region pervasively underwent a protracted high–grade thermal event from the Neoarchean to the early Paleoproterozoic, which formed the backbone of the block. Thereafter, the southern Vestfold Block experienced a Grenvillian granulite facies metamorphism, indicating that the Vestfold Block has been locally involved in the Rayner orogeny (i.e. the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and East Antarctica). Ultimately, the whole Vestfold Block may have been reworked under relatively low temperatures during the Pan–African Prydz tectonic event.

Geochemical and geochronologic evidence for a contiguous northeastern Wyoming Province

The extent and nature of the Wyoming Province, an Archean craton in southwestern Laurentia, are poorly understood due to limited exposure between spatially isolated basement-cored uplifts. This lack of exposure has led to debate about whether the northeastern Wyoming Province is underlain by contiguous Archean crust or Proterozoic rocks and suture zone associated with the Trans-Hudson orogeny. To assess these models we analyzed samples recovered from drill cores in the buried northeastern Wyoming Province that straddle the proposed Proterozoic suture. Whole-rock geochemical and Nd-Pb-O isotopic data suggest the rocks formed by partial melting of > 3.5 Ga hydrated mafic to tonalitic sources similar to elsewhere in the northern Wyoming Province. Zircon U-Pb dates record Mesoarchean magmatism and metamorphism (3.0–2.8 Ga). Published whole-rock Rb-Sr, hornblende and biotite K-Ar, and apatite fission track dates suggest these rocks have not been heated above 300 °C since 2.5–2.1 Ga. Geophysical potential field data are consistent across northeastern Wyoming contrasting with major discontinuities associated with documented Proterozoic orogens on all other margins of the Wyoming Province. Hence, geochemical, isotopic, and geochronologic data along with geophysical imaging can be most simply interpreted in terms of continuous Archean crust in the northeastern Wyoming Province. A geophysically-imaged reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and sources, similar to the boundary between the Montana metasedimentary terrane and Beartooth-Bighorn magmatic zone in the northwestern Wyoming Province. This work emphasizes the value of integrating geologic and geophysical constraints to constrain Archean provinces and their tectonic evolution.

Tectono-metamorphic evolution of the Central Bundelkhand Craton, India from geochemistry and bulk composition modelling of amphibolite enclaves

The Bundelkhand Craton (BuC) represents a significant Archean terrane within the northern Indian Craton and yet its tectono-metamorphic evolutionary history remains relatively understudied. Our investigation involves detailed petrography, geochemistry, and bulk composition modeling of both garnet-bearing and garnet-absent amphibolites with a two-fold objective: (i) to constrain the protolithic nature and tectonic settings involved in the genesis of these rocks, and (ii) to propose a tectono-metamorphic evolutionary history for the BuC. The BuC amphibolites originate from basalt and andesitic-basalt protoliths. Their trace element compositions reveal negative anomalies in Nb, Ta, and Ti, while their rare earth element (REE) normalized patterns indicate enrichment in light REEs over heavy REEs. The basaltic protolith is interpreted to have formed during orogeny in a compressional tectonic regime at the active margins of island arcs in a subduction-related setting. This interpretation is supported by various discrimination plots for amphibolites, such as Nb/Th vs Zr/Nb, Zr vs Zr/Y, and Th/Nb vs Ce/Nb, as well as high Th/Yb and low Nb/Yb contents— all of which suggest an island arc setting influenced by subduction. These amphibolites have undergone three distinct phases of metamorphism, as evidenced by petrography, mineral chemistry, and bulk composition modeling. This interpretation is further supported by geochemical discrimination diagrams which indicate that a subduction tectonic setting was active during peak metamorphism. During the pre-peak phase, the garnet-bearing amphibolites experienced pressure and temperature conditions ranging from 6.25 to 6.5 kbar and 580 to 590ºC, while the garnet-absent amphibolites underwent conditions from 5.0 to 5.8 kbar and 400 to 450ºC. Peak metamorphism was observed at pressures ranging from 6.8 to 7.4 kbar and temperatures from 760 to 805ºC for the garnet-bearing amphibolites, and at pressures from 7.0 to 7.4 kbar and temperatures from 785 to 810ºC for the garnet-absent amphibolites. The metamorphic retrograde conditions for the garnet-bearing amphibolites are defined by P-T conditions ranging from 4.45 to 4.75 kbar and 585 to 615ºC, while for the garnet-absent amphibolites, it ranges from 3.1 to 4.0 kbar and 620 to 710ºC. The mineral assemblages and P-T conditions delineate a clockwise P-T path for both, garnet-bearing and garnet-absent amphibolites from the Babina and Mauranipur regions. This suggests that the rocks underwent a burial process amid subduction tectonic settings in an arc-related environment, followed by a decompression stage that brought them to the surface.

Late Neoarchean-early Paleoproterozoic K-rich granitoid rocks and mafic dykes from the Liaodong Bay Depression, Bohai Sea Basin: Implications for cratonization of the North China Craton

Late Neoarchean to early Paleoproterozoic potassic granites and mafic dykes are widely distributed across the Eastern Block (EB) of the North China Craton (NCC), which provide important information about the geodynamic origin and tectonic evolution of Archean terranes. Here, we are first to report the zircon U–Pb–Hf isotopic and whole-rock compositions of potassic granites and mafic dykes from the Liaoxi Rise of the Liaodong Bay Depression (LBD) in the offshore Bohai Sea Basin (OBSB) of the northeastern NCC. The potassic granites have crystallization ages of 2499–2460 Ma and are characterized by variable SiO2 content (63.2 to 72.3 wt%) and high K2O (3.27–6.88 wt%). In addition, they show geochemical features akin to post-collisional related granites and possess positive zircon εHf(t) values of 0.9 to 4.95, suggesting that they mainly originated from the partial melting of high-K mafic rocks and formed under the post-collisional setting. The mafic dykes have a crystallization age of 2485 Ma. They show high Nb (8.64–9.39 ppm) and high Nb/U ratio (10.3–14.8), as well as radiogenic initial zircon Hf isotopes (﹢1.26–﹢4.39). These values indicate affinities with Nb-enriched basalts and that they originated from mantle-derived magma under a subduction-related back-arc extensional setting. Combined with previous research results, we believed that magmatic rocks of the LBD area may have formed under remote effects of oceanic slabs being subducted from the northwest to southeast of the eastern Hebei–western Liaoning Province back-arc basin system during the late Neoarchean to early Paleoproterozoic. Meanwhile, the K-rich granitoid rocks and coeval mafic dyke sequences imply the increased maturity of the continental crust and late Neoarchean to early Paleoproterozoic basement initiation of cratonization in the NCC.

Geochemical mapping of lithospheric architecture disproves Archean terrane accretion in the Yilgarn craton

Abstract The basement of Mesoarchean to Neoarchean greenstone basins in the Yilgarn craton is composed of fragments of evolved crust up to 3.7 Ga old. New cratonwide geochemical and isotopic data with unparalleled spatial resolution image a NE- to ENE-trending architecture in pre–2.73 Ga crust. These trends cannot be reconciled with plate-tectonic models, as they persist across younger NNW-striking structural fabrics, including a proposed suture previously interpreted to result from exotic terrane accretion. Our results suggest that, in spite of their substantial strike length, the NNW-trending structures have limited horizontal displacement and, although important for understanding regional geology, may be a geodynamically insignificant overprint of the primary ENE-trending architecture. We propose that these greenstone provinces or belts include individual basins formed in rifts with location, size, and orientation influenced by the interaction between basement fragments and regional crustal extension.

Copper Isotope Fractionation in Archean Hydrothermal Systems: Evidence From the Mesoarchean Carlow Castle Cu‐Co‐Au Deposit

AbstractCopper isotope analysis has emerged as a promising tool for understanding genetic processes in Cu ore deposits. However, applications of this analytical technique to Archean Cu deposits have been extremely limited, even though Archean terranes are among the most economically endowed on Earth. As such, this study presents the first Cu isotope analysis of an Archean Cu deposit, the Mesoarchean Carlow Castle hydrothermal Cu‐Co‐Au deposit. Archean primary Cu sulfide ore samples and Cenozoic supergene Cu ore samples were analyzed. Primary ore samples are isotopically light, with δ65Cu values ranging between −0.80 ± 0.02‰ and 0.00 ± 0.007‰, whilst supergene samples are isotopically heavier and range between −0.50 ± 0.01‰ and 0.62 ± 0.005‰. In primary ore samples, a relationship is observed between the Cu isotope signature, ore grade, and alteration assemblage that records the isotopic and physicochemical evolution of the Carlow Castle deposit's hydrothermal ore‐forming system. A mafic igneous source is suggested as a metal source in the Carlow Castle Cu‐Co‐Au deposit. The limited heavy isotopic fractionation of supergene Cu ore samples in this study is interpreted to reflect limited redox cycling of Cu due to in situ oxidative weathering of vein‐hosted Cu sulfides in the overlying Cenozoic supergene system. This differs from previously studied deposits where significant Cu transport and multiple stages of isotopic enrichment are often evident in supergene Cu enrichment layers. The results of this study suggest that Cu isotope analysis could be valuable in understanding genetic processes in hydrothermal Cu deposits, including Archean ore deposits and terranes.

Geochemistry as a Clue for Paleoweathering and Provenance of Southern Apennines Shales (Italy): A Review

The southern Apennines (Italy) chain is a fold-and-thrust belt mainly derived from the deformation of the African–Apulian passive margin where shallow-water, basinal, and shelf-margin facies successions, including fine-grained sediments, occur. Here, we provide a review of the geochemistry of Meso–Cenozoic shales from the Lagonegro basin to elucidate provenance and paleoweathering. The different suites of these shales are dominated by 2:1 clay minerals and are Fe shales and shales. An R-mode factor analysis suggests Ti, Al, and LREE (F1) and K2O-MgO (F2) covariance, likely related to the illite → smectite → kaolinite evolution during weathering. HREE and Y are distributed by phosphate minerals, suggesting LREE/HREE fractionation. The CIA paleoweathering proxy rules out non-steady-state weathering conditions and indicates that the source area was affected by moderate to intense weathering. The paleoprecipitation values derived from the CIA-K and CALMAG indices show median values in the 1214–1610 mm/y range. The Eu/Eu*, Sm/Nd, and Ti/Al provenance ratios point toward a UCC-like source excluding any mafic supply and suggest that the Lagonegro basin was connected, through a southern area, with the African cratonic area. However, the Eu/Eu* median value of the southern Apennine shales is quite similar to the value of the Archean shales, possibly indicating a less differentiated component. This is consistent, in many samples, with the value of the (Gd/Yb)ch ratio, suggesting that the shales likely incorporated ancient sediments derived from African Archean terranes through a cannibalistic process.

Exhumation of an Archean Granulite Terrane by Paleoproterozoic Orogenesis: Evidence from the North China Craton

Abstract Granulites represent high-grade metamorphic rocks of the deep continental crust. The metamorphism and exhumation of granulites from Archean terranes provide insights into the crustal evolution of Archean cratons and shed light on the formation and reactivation of cratons. We present petrology, U–Pb geochronology (zircon, rutile, and titanite), and pressure–temperature (P–T) paths for metadiabase dikes in an Archean granulite terrane of the North China Craton. Garnet (Grt) coronae in the metadiabase dikes are developed between plagioclase (Pl) and clinopyroxene (Cpx) via the reaction Pligneous + Cpx → Pl1 + Grt ± quartz. The reaction proceeds inward within the plagioclase, progressively consuming Pligneous (XAn = 0.65–0.69) and leaving Ca-poor Pl1 (XAn = 0.46–0.53) as the residue. Geothermobarometry and P–T pseudosections suggest peak conditions for garnet formation at ~800°C and 10–13 kbar. During retrograde metamorphism, Grt broke down to Ca-rich Pl2 (XAn = 0.73–0.74), and ilmenite replaced rutile. Geothermobarometry and Zr-in-titanite temperatures constrain the P–T conditions of retrograde metamorphism at 700–750°C and 4.5–7.5 kbar. Zircon and titanite U–Pb geochronology shows that the protolith of the metadiabase dike was formed at 2.4 Ga and underwent granulite-facies metamorphism at 1.86 Ga. The intrusion of mafic dikes into the Archean granulite terrane indicates that the Archean basement was also heated and buried in the Paleoproterozoic. The metadiabase dikes and the hosting Archean basement underwent Paleoproterozoic granulite-facies metamorphism at a depth of ~40 km, followed by near-isothermal decompression and subsequent near-isobaric cooling (cooling rate of 1–3°C Myr−1) at depths of 15–25 km. Crustal shortening and thickening may have been caused by the underplating of the Khondalite series beneath the Archean basement during the amalgamation of supercontinent Columbia. The Paleoproterozoic orogeny induced a second generation of metamorphism of the Archean basement along the margin of the craton and drove the exhumation of the Archean granulite terranes to the middle crust (~15 km).

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Did the Neoproterozoic Revolution Extend to the Deep Mantle?

Late Neoarchean TTG and monzogranite in the northeastern North China Craton: Implications for partial melting of a thickened lower crust

The petrogenesis and dynamic setting of Archean tonalite–trondhjemite–granodiorite (TTG) is widely debated. As a typical Archean terrane in the northeastern North China Craton (NCC), the southern Jilin Province area underwent multi-stage magmatism during the Neoarchean and is therefore a promising region in which to track the Neoarchean evolution of the NCC. This study investigates the petrogenesis of late Neoarchean TTGs and associated monzogranites and their tectonic implications. Zircon U–Pb dating results suggest that the TTGs and monzogranites have emplacement ages of 2.55–2.52 and 2.53–2.51 Ga, respectively. The TTGs have variable SiO2 (61.7–73.3 wt%) and MgO (0.66–2.74 wt%) contents and Mg# values of 33–50. Most have high Sr and low Y and Yb contents, with high Sr/Y and (La/Yb)N ratios. The TTGs have variable εHf(t) (+0.7 to + 8) and εNd(t) (−2.77 to + 2.79) values. These characteristics, together with the results of thermodynamic partial-melting modelling, indicate that the TTGs can be classified as medium- and low- pressure types and were likely generated by amphibole-breakdown partial melting of 2.9–2.7 Ga juvenile basaltic rocks under P–T conditions of 1.0 ± 0.1 GPa and 880 ± 50 °C. The monzogranites have higher SiO2 (66.1–76.0 wt%) and K2O (4.03–5.06 wt%) and lower TFe2O3 (0.89–4.97 wt%) and MgO (0.21–1.27 wt%) contents. They also have high Sr/Y and (La/Yb)N ratios and variable εHf(t) (+0.6 to + 5.4) and εNd(t) (−3.47 to − 0.15) values. The geochemical and Hf–Nd isotopic compositions of the monzogranites indicate that the parental magmas may have been derived through the partial melting of thickened lower crust with heterogeneous sources. The high thermal gradient (>730 °C/GPa−1), anticlockwise metamorphic P–T paths, and 3.4–3.2 Ga remanent zircons in the region suggest that a late Neoarchean thickened lower crust was the likely geodynamic setting for the generation of the TTGs and monzogranites.

The Relationship Between Kimberlitic Magmatism and Electrical Conductivity Anomalies in the Mantle

AbstractKimberlites are igneous rocks whose formation remains enigmatic due to their severely altered nature, highly variable compositions and rapid ascent through the lithosphere. The spatiotemporal distribution of kimberlites suggests that mantle metasomatism may play an essential role in their emplacement. Since the magnetotelluric (MT) method is sensitive to metasomatic mantle modification in the forms of interconnected minerals and water, we compiled MT models from Australia, Brazil, Botswana, Canada, Namibia, South Africa and the USA, calculated water content variations and compared them to the distribution of kimberlites. Results indicate that kimberlites mostly ascend through hydrated/metasomatized lithosphere and avoid both dry and heavily metasomatized lithosphere. Diamondiferous kimberlites preferentially occur on moderately metasomatized lithosphere, with the diamondiferous rate increasing with the degree of metasomatism in Archean terranes and decreasing in Proterozoic terranes. These results link geophysical observations of mantle metasomatism to kimberlite magmatism and will improve mineral systems models for diamond exploration.

Testing the importance of sagduction: insights from the Lewisian Gneiss Complex of northwest Scotland

Archean cratons often contain rock units that are interpreted as having been juxtaposed from different structural levels. A range of uniformitarian and non-uniformitarian processes have been invoked to explain these occurrences, prominent amongst which is the density-driven ‘sagduction’ of high-density upper-crustal lithologies into the underlying dominantly felsic mid-crust. In this paper we test the geological importance of sagduction, using a combination of petrology, phase equilibria modelling, and mechanical modelling. We use the Lewisian Gneiss Complex of northwest Scotland as a test case, analysing the range of observed subordinate felsic–ultramafic bodies within the dominantly felsic crust, but our approach is applicable to Archean terranes globally owing to their analogous lithological ranges. We find that for our thermodynamically-estimated densities of the lithologies present in the Lewisian Gneiss Complex, unrealistically hot temperatures are required for sagduction to be important, given the observed body sizes and available constraints on event durations, geotherms, crustal rheology and emplacement depths. These results suggest sagduction is not responsible for emplacement of the observed subordinate lithologies within the Lewisian felsic mid-crust, and instead support uniformitarian tectonic interpretations. Additionally, our results cast doubt on the importance of sagduction in the structural evolution of granite-greenstone belts. Overall, our study indicates that sagduction was not an important Archean tectonic process.

Recent pegmatite-hosted spodumene discoveries in Western Australia: insights for lithium exploration in Australia and globally

ABSTRACT The discovery and development of world-class Lithium–Caesium–Tantalum (LCT) spodumene-bearing pegmatites in Western Australia underpins growth of a significant new sector of its mining industry. Recently, several new spodumene discoveries have been delineated in the Yilgarn and Pilbara Cratons. Contrary to exploration narratives that new economic mineral discoveries will generally be made at increasingly greater depths, beneath barren cover rocks, or in remote geological environments, all new lithium discoveries have clear surface expressions in relatively ‘mature’ greenstone belts. The exploration implication is that the search space for pegmatite-hosted spodumene deposits in Western Australia remains immature. These recently discovered LCT pegmatites have geological features relevant to exploration including their age, amphibolite-facies metamorphic setting and syn-metamorphic timing, and 3D geometry, particularly their typically gentle dips, that match other such world-class pegmatites globally. Further spodumene discoveries within pegmatites at or near surface are likely in the Archean terranes of Western Australia based on these consistent exploration criteria and supportive capital market conditions.

Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

AbstractThe nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

Regional gravity field distribution over cratonic domains of the Indian shield: Implications for lithospheric evolution and destruction

• Detailed analysis of regional gravity field over Indian cratons. • Extremely low Bouguer gravity of −120 mGal associated with Dharwar Craton. • Gravity field is largely positive over the north Indian cratonic segments. • Cratons with positive gravity have thinner lithosphere and higher heat flow. The nature of crust and lithospheric mantle evolution of the Archean shields and their subsequent destruction through intraplate tectonic processes are important in understanding continental dynamics and resources. The Indian shield, which has remained dynamic throughout the past for more than three billion years of geological history, is unique in terms of its lithospheric architecture compared to other Archean terranes on the globe. It consists of five major cratons, the Dharwar, Singhbhum, Bundelkhand, Bastar and Aravalli, which are separated by active rift valleys, mega lineaments and sutures zones. Here, we present a detailed analysis of gravity field over these areas, and their possible relationship with lithospheric thickness variations. Our results show a large variation in residual gravity field among the different cratons. A highly negative anomaly of around -70 mGal occurs over the western part of the Dharwar Craton, beneath which the lithosphere is about 160–200 km thick. In comparison, the anomalies are conspicuously positive, reaching around (i) +10 mGal over Central Dharwar Craton, Eastern Ghats Belt and the eastern part of Singhbhum Craton, (ii) +35 mGal over the Nellore Schist and Aravalli-Delhi Fold Belts, and (iii) +60 to +70 mGal over eastern part of SGT. In these regions, the lithosphere is markedly thinner between 70 and 135 km. Such areas have been associated with episodic magmatism, moderately high heat flow and elevated Moho temperatures, indicating a large-scale upwarping of isotherms leading to lithospheric destruction to the tune of 100–150 km, caused by mechanical, thermal and chemical erosion. Our study confirms that many of the Archean cratons on the globe, like the Indian shield, have undergone substantial erosion of their roots through subsequent geotectonic processes.

Origin of ultramafic–mafic bodies on the Isles of Lewis and Harris (Scotland, UK): Constraints on the Archean–Paleoproterozoic evolution of the Lewisian Gneiss Complex, North Atlantic Craton

The Lewisian Gneiss Complex (LGC) is a tonalite-trondhjemite-granodiorite (TTG)-dominated fragment of the North Atlantic Craton (NAC) in northwest Scotland. End-member models describe the LGC as representing either a continuous piece of Archean crust or up to 12 geologically distinct Archean terranes, with interpretations sitting on a spectrum between these end-members. There is particular uncertainty over the correlations between the Archean–Paleoproterozoic magmatic and metamorphic events recorded by mainland part of the LGC and the part exposed on the Outer Hebridean islands of Lewis and Harris. In this paper, we present the results of field mapping, petrography, and major, trace and platinum-group element (PGE) bulk-rock geochemistry for four ultramafic–mafic bodies in Lewis and Harris, namely: Maaruig, Loch Mhorsgail, Coltraiseal Mor and Beinn a’ Chuailean. We consider the effects of metamorphism and element mobility, their petrogenesis, and potential correlations with ultramafic–mafic rocks elsewhere in the LGC. Our data indicate that the studied ultramafic–mafic rocks can be subdivided into two petrologically distinct groups. Metaperidotites and metapyroxenites from Maaruig and Loch Mhorsgail are interpreted as Archean (>2.8 Ga) cumulates distinct from anything currently identified in the mainland LGC, with this interpretation based on distinctive modal layering, a discordance with surrounding TTG gneiss, fractionated PGE patterns ([Pd/Ir]N = 1.3–6.6) and negative HFSE anomalies ([Nb/La]N = 0.2–0.8). Metagabbronorites from Coltraiseal Mor and Beinn a’ Chuailean, which also exhibit negative high field strength-element (HFSE) anomalies ([Nb/La]N = 0.2–0.7) and show mildly fractionated ([Pd/Ir]N = 1.2–2.8) PGE patterns, most likely represent deformed Paleoproterozoic dykes. These occurrences could be correlatives of a suite of ca. 2.4 Ga mafic dykes exposed throughout the mainland LGC (the Scourie Dykes), with the Outer Hebridean occurrences having experienced more intense Paleoproterozoic (Laxfordian) deformation/reworking. These interpretations suggest that the LGC lithologies of Lewis and Harris were proximal to the mainland LGC’s Central Region by the early Paleoproterozoic but raises the possibility that they were distinct crustal blocks in the Mesoarchean.

Crustal structure around the margins of the eastern Superior craton, Canada, from receiver function analysis

The Precambrian domains of eastern Canada preserve a remarkable record of continental crust formation and evolution spanning at least 3 billion years, and thus present an excellent opportunity to investigate the imprints of early tectonic processes on the present-day continental crust. Archean domains in the region include the eastern Superior craton, part of one of the largest cratons on Earth, the western portion of the North Atlantic craton, and a collage of continental fragments. The Paleoproterozoic Trans-Hudson Orogen and its eastern branches played a fundamental role in the assembly of Laurentia. In the southeast, a succession of orogenic events culminated in the Mesoproterozoic Grenville Orogen and assembly of supercontinent Rodinia. To better understand crustal formation and evolution in this complex region, we use teleseismic P-wave receiver functions recorded by a network of broadband seismographs distributed across Precambrian eastern Canada. At each station, back-azimuthal variations in receiver function waveforms indicate significant lateral crustal heterogeneity, leading us to model different representative directions separately. The stations situated on Archean terranes present a relatively simple crustal structure with a well-defined Moho at ∼33–46 km depth. In contrast, those on Proterozoic domains show a more complex structure, higher velocities in the lower crust and a deeper, more diffuse Moho at ∼46–55 km depth. Across the entire region, bulk crustal composition is largely felsic to intermediate (Vp/Vs ∼1.66–1.76), except for a station situated in an area dominated by anorthosite massifs whose composition is significantly more mafic (Vp/Vs ∼1.85). We combine our new models with those from previous natural- and active-source seismic studies of eastern Canada, and compare our results to those from other Precambrian regions. We suggest that the present-day crustal structure of Precambrian eastern Canada is more strongly influenced by the tectonic processes that assembled Laurentia than secular variations in initial crustal formation.

Zircon U-Pb isotopic and geochemical study of metanorites from the chromite-mineralised Bacuri Mafic-Ultramafic Complex: Insights of a Paleoarchean crust in the Amapá Block, Guyana Shield, Brazil

The age and tectonic history of the chromite-mineralised Bacuri Complex, a layered mafic–ultramafic intrusion emplaced into Archean terranes of the Guyana Shield in the Amazonian Craton, are here investigated. The stratigraphy of the Bacuri Complex consists of an Ultramafic Zone composed of interlayered metaperidotites and chromitites and two mafic zones composed of metamafic rocks (amphibolites). In this study we present whole-rock lithogeochemical data integrated with zircon U-Pb isotopic and trace-element data from amphibolites from the Mafic Zone. The investigated amphibolites consist of hornblende + plagioclase ± diopside ± biotite with whole-rock compositions like noritic rocks of Archean layered intrusions. These samples are representative of amphibolites with variable proportions of hornblende and plagioclase, interpreted as resulting from amphibolite facies metamorphism of interlayered norite and leuconorite. We suggest that progressive fractionation of a primitive parental magma led to the formation of plagioclase and orthopyroxene cumulates in the mafic zones. We found that zircons from three metanorite samples comprise euhedral crystals with oscillatory zoning (interpreted to preserve a magmatic crystallisation age), as well as variably recrystallised metamorphic zircons. Their U-Pb 207Pb/206Pb ages span over 682 million years, from 3628 Ma to 2942 Ma. Trace element data suggest that magmatic zircons precipitated from late-stage magmatic fluids at ∼ 3.34 Ga (3343 ± 3.5 Ma weighted average age, n = 48, MSWD = 1.3), which defines the Bacuri Complex as one the oldest and best-preserved layered intrusions known. We show that fluid-mediated coupled dissolution-precipitation recrystallisation during metamorphism altered the composition and ages of magmatic zircons. We could not accurately define the number and age of metamorphic events in the 2942–3379 Ma age interval of metamorphic or ‘disturbed’ zircon domains. Our results indicate that continental crust was already established in the Amazonian craton in the Paleoarchean and possibly in the Eoarchean and that it has been reworked since at least the Mesoarchean.