Mafic Source Research Articles

Onset of the supercontinent cycle: Evidence for multiple oceanic arc accretion events in the Paleoproterozoic Sefwi Greenstone Belt of the West African Craton

Greenstone belts preserved in the West African Craton (2300–2070 Ma) provide key information for understanding the petrogenetic processes and geodynamic setting for juvenile crust formation during a nascent Paleoproterozoic plate tectonic regime. New geochemical and isotopic data are presented for the magmatic suites exposed in Sefwi Greenstone Belt, SW Ghana. We define five distinct plutonic suites, with extrusive equivalents, including: 1) two generations of high SiO2, Na2O-rich tonalite-trondhjemite-granodiorite (TTG) granitoids; 2) calc-alkaline, LILE-enriched diorites; 3) tholeiitic pyroxenite, gabbro and layered mafic intrusives; 4) high-K quartz monzonites; and, 5) incompatible element-rich, two-mica granites and muscovite leucogranites. Trace element patterns of most magmatic suites display negative Nb-Ta, P and Ti anomalies and weakly positive Pb anomalies, suggesting magma genesis in a subduction setting with minor crustal contamination/assimilation. Complementary zircon U-Pb and Lu-Hf data indicate contemporaneous emplacement of diorites (εHf(t) = +2.5 to +5.4) and low-HREE TTGs (εHf(t) = +4.3 to +7.6) between ca. 2159 and 2153 Ma, derived from contrasting sources, namely the partial melting of the metasomatised mantle wedge and high-pressure partial melting of low-K mafic sources, respectively. High-HREE TTGs are attributed to the shallow, partial melting of mafic material. The emplacement of high-K quartz monzonites (εHf(t) = +2.5 to +5.2) at ca. 2135 Ma reveals mixing of mantle-derived magmas and remelting of existing TTGs in the crustal pile. Consistently positive εHf(t) values indicate predominantly radiogenic, though heterogeneous, magma sources during crustal growth, derived from precursory material extracted from the depleted mantle and an older crustal component with a minimum age of 2650 Ma. Magmatogenesis of two-mica granites and leucogranites (ca. 2092 and 2081 Ma) is interpreted to be the product of crustal anatexis during an oceanic arc-arc collisional event, with a proposed suture parallel to the NW margin of the Sefwi Belt. We hypothesise that the formation and accretion of the West African Craton reflects the rapid amalgamation of individual oceanic arc terranes, providing potential insight into the transitional plate tectonic regime that may have characterised the Earth between 3000 and 2000 Ma.

Early continental crust generated by reworking of basalts variably silicified by seawater

The Archaean continental crust comprises two major groups of silicon-rich granitoids: the tonalite–trondhjemite–granodiorite and granite–monzonite–syenite suites, which differ in their sodium-to-potassium ratios. How these felsic granitoids evolved from their mafic precursors remains elusive and the subject of great debate. Here, we present silicon isotopic constraints on the formation of representative trondhjemitic and granitic plutons from the Kaapvaal craton that range in age from 3.51–2.69 billion years ago. We identified very consistent silicon isotopic signatures, all uniformly 0.1–0.2‰ heavier than rocks of the modern continental crust. This unusual composition is explained by the melting of a mafic source that included significant proportions (15–35 wt%) of silicified basalts, which were common supracrustal rocks before 3 billion years ago. Before the melting event that formed the granitoid magmas at depth, portions of the mafic source rocks were enriched in silica by interaction with silica-saturated seawater. The addition of silica depresses the stability of amphibole at similar water activity, allowing trondhjemitic and granitic melt production at lower temperatures from protoliths with contrasting silica contents: 52–57 and ≥60 wt%, respectively. This explains why granitoids were able to form very early in Earth history but did not emerge in significant amounts on other rocky planets. Granitic continental crust in the Archaean formed from a basaltic source that was enriched in silica due to interaction with the early oceans before melting, according to silicon isotope analyses on rocks from the Kaapvaal craton.

Pyroxene-bearing low- and high-HREE TTGs from the northeastern margin of the Kaapvaal Craton, southern Africa: Implications for Archean geodynamics

Archean tonalite-trondhjemite-granodiorite (TTG) rocks are generally amphibole- and/or biotite-bearing. Pyroxene-bearing granitoids of TTG affinity are less commonly reported. This study evaluates the range in composition and petrogenesis of pyroxene-bearing TTGs from a high-grade terrane along Archean cratonic margin. The Southern Marginal Zone high-grade terrane along the northeastern margin of the Kaapvaal Craton exposes prominent Mesoarchean pyroxene-bearing granitoids. The present study describes two groups of the granitoids in the Commissiedraai-Hugomond areas. The tonalite-trondhjemites with Fe-rich orthopyroxene (XMg = 0.46–0.49) dominate and occur as massive homogenous and banded varieties, the latter likely due to anatectic overprint. Enclaves of metabasites occur in both varieties. The darker layers of the banded variety are identical in mineralogy, texture and chemistry to the massive dark greenish-grey rocks. A less dominant group of clinopyroxene-rich massive tonalites occur in the Hugomond area. Clinopyroxene exhibit prominent orthopyroxene exsolution, with a pre-exsolution composition of XMg = 0.61–0.63. The spatially associated orthopyroxene-bearing and clinopyroxene-rich rocks are compositionally connected by a two-pyroxene-bearing variety with Mg-rich orthopyroxene (XMg = 0.56–0.59). No textures of pyroxene formation after earlier FeMg minerals occur in the studied rocks, whereas textures of pyroxene destabilization are indicative of late magmatic interaction with a residual melt. Pyroxenes in all rocks are primary magmatic. Biotite-quartz intergrowths and amphibole after pyroxenes are late magmatic in origin. Small garnet crystals occur locally in the orthopyroxene-bearing tonalite-trondhjemites and are related to metamorphic/anatectic overprint. The rocks are affected by late alkali metasomatism close to shear zones. Geochemical characteristics of the orthopyroxene-bearing tonalite-trondhjemites are comparable to low-HREE TTGs, while that of the clinopyroxene-rich tonalites are similar to high-HREE TTGs. The proposed petrogenetic model considers the formation of both groups of pyroxene-bearing granitoids via remelting of mafic sources in a thickened plume-related oceanic plateau. The high temperatures of melting, low water activity and oxidized conditions likely resulted in the formation of charnockitic assemblages, and is supported by phase equilibria modelling. Shallow level melting of a MORB-like source produced clinopyroxene-rich high-HREE TTG. Subsequent melting of a slightly enriched-MORB-like source occurred at relatively greater depths forming orthopyroxene-bearing TTG with low-HREE characteristics. The two-pyroxene-bearing rocks likely resulted from the mingling-mixing of the high- and low-HREE TTG end members. Implications of the results on Archean geodynamics are discussed together with a comparative exercise with similar rocks from other Archean terranes.

Tectonic evolution of north‐eastern Tethyan Himalaya: Evidence from U–Pb geochronology and Hf isotopic geochemistry of detrital zircons

It is well established that the Himalayan Orogen was formed by successive amalgamation of continental slices to the Eurasian continent, with the final collision of the Indian continent. The Upper Triassic Langjiexue Group on the north‐eastern margin of the Tethyan Himalaya has been central to debates on the provenance with diverse models linking it with the northern India, Lhasa terrane, or multiple sources from surrounding terranes including Australia. In order to address this debate, here, we present U–Pb ages, trace element characteristics, and Hf isotope data of detrital zircons. The trace element data suggest that analysed zircons are mostly of igneous origin, with a broad affinity to mafic source rock, and some of the grains showing evidence for hydrothermal alteration. The zircon age spectra show distinct Permian to Triassic age peaks at 200–280 Ma with εHf(t) between −6.1 and 13.4, Neoproterozoic to Cambrian ages at 480–750 Ma with εHf(t) ranging from −24.7 to 8.5, and a broad Meso‐ to Neoproterozoic age range of 850–1,150 Ma with εHf(t) values of −8.1 to 10.1. The age spectra from the Langjiexue Group are in contrast with that of the Lhasa terrane which has pronounced age peaks of 300–325, 550–600, 1,150–1,350, and 1,750–1,900 Ma, suggesting that Lhasa might not be the source of Langjiexue Group detritus. In a similar way, north‐western Australia and the Banda Arc are excluded as sources of the Langjiexue Group. The Hf model age spectra show distinct peaks of 750–800 and 1,200–1,300 Ma, and the second peak agrees with that of the Tethyan Himalaya. We infer that the basin of Langjiexue Group formed within the basement of Tethyan Himalaya in the Triassic and conclude that the sediments were deposited at the passive margin of northern Indian continent. Based on the results, we also propose the palaeogeographic evolution of the Himalayan Orogen.

Provenance and fate of trace and rare earth elements in the sediment-aquifers systems of Majuli River Island, India

In this work we present a completely new dataset of Rare Earth Element (REE) distribution and fractionation in the groundwater and sediments of the world's largest populated river island-Majuli, located in the Brahmaputra River, India. Groundwater (n = 9) and borehole sediments (n = 23) of different depths were collected randomly and analyzed for REEs using inductively coupled plasma mass spectrometry (ICP-MS). Multivariate statistical techniques were applied to determine interrelationships among different REEs. Both water and sediments exhibit a higher content of light rare earth elements (LREEs) than heavy rare earth elements (HREEs), implying that source rock composition primarily controls the REE signatures of these river sediments, largely owing to variations in levels of chlorite and monazite, which are more abundant in bottom river sediments of the Brahmaputra. However, lower LREE/HREE ratios in groundwater samples (8.42–14.48) compared to sediments (8.9–52.6) suggest less dissolved phase mobilization of LREE during weathering and transport. Hierarchical cluster analysis showed more consistent clustering in sediment compared to water and clear demarcations of MREE, HREE and LREE were observed in the sediments. The primitive mantle-normalized REE patterns of borehole sediments were similar to the upper continental crust (UCC) and the compositions of average Himalayan rocks. LREE enrichment compared to HREE suggest dominance of a felsic source. However, a La/V versus Th/Yb plot suggests a small contribution from the mafic source component. The sediments show high Th/Sc (>1 of UCC) and high Zr/Sc. A Eu/Eu* verses GdN/YbN plot suggests that the dominant source with Upper crustal composition has undergone multiple recycling.

Sediment chemistry and detrital zircon record in the Bosque and Paseo del Mar coastal areas from the southwestern Gulf of Mexico

Abstract In this study, we analyzed the mineralogy and bulk chemical composition of 20 sediment samples, as well as trace, rare earth element (REE) concentrations and U–Pb ages of 400 detrital zircons retrieved from the Bosque (PB) and Paseo del Mar (PM) coastal sediments, southwestern Gulf of Mexico, to investigate the provenance. The XRD study revealed that the PB sediments are associated with quartz, muscovite, cordierite, albite, and anorthite. The PM sediments are associated with calcite, aragonite, and biogenic materials, which derived probably within the coastal environment. The weathering indices like Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA) revealed a moderate intensity of weathering for the PB sediments. The chondrite normalized REE patterns of the PB and PM sediments are with negative and positive europium anomalies (Eu/Eu*), suggesting that the sediments were derived by the combination of felsic and mafic source rocks. The Th/U ratios (>0.3) of the PB and PM zircons indicated an igneous origin. The chondrite normalized REE patterns of the PB and PM zircons are depleted in low REE (LREE) and enriched in heavy REE (HREE) with positive cerium and negative europium anomalies. A wide variation in chondrite normalized REE patterns of zircons with respect to ages was observed. The PB is dominated by Proterozoic (~551–2500 Ma) and Paleozoic zircons (~445–480 Ma; Ordovician), whereas PM is dominated by Cenozoic zircons (~5–21 Ma; Miocene), indicating that the zircons were derived from different source terranes. The Chiapas Massif (CMC) and Chuacus complexes are identified as the potential source areas, which contributed Proterozoic zircons to PB. On the other hand, Cuicateco terrane (CUI) and Tuxtla Volcanic Field (TVF) are considered as the potential source areas, which supplied significant number of Cenozoic zircons to the PM coastal area. The zircon U–Pb ages and the bulk sediment geochemistry together indicate that the source terranes, which supplied sediments to the PB and PM coastal areas, consist of mafic and felsic rocks. The results of this study further illustrated the importance of littoral currents in distributing sediments, although rivers are considered as a carrier and agent of distributing sediments along the coastal areas.

Late Mesozoic sediment provenance on Georges Bank: Enlargement of river drainages to the Atlantic Ocean in the Late Jurassic–Early Cretaceous

The detrital mineralogy of Upper Jurassic to Lower Cretaceous sandstones from the Continental Offshore Stratigraphic Test G-2 well in the Georges Bank Basin was studied to better understand the varying supply of clastic sediment to the basin. Heavy minerals separated from cuttings were identified by chemical analyses, zircons were dated, and Raman spectroscopy was applied to identify titania polymorphs (rutile, anatase, and brookite). Upper Jurassic (Oxfordian) sands contain tourmaline, fluorapatite, Mn-almandine and grossular garnets (derived from the central Maine and coastal Maine belts), and pumice and trachytes reworked from volcanics in the Long Island platform. Zircon geochronology (300–450 and 550–700 Ma) supports sources from peri-Gondwanan Appalachian terranes. Lower Cretaceous heavy minerals show that Oxfordian supply was diluted to 30%, mostly by additional ilmenite, zircon, tourmaline, and staurolite, probably by components from central and northern Maine and New Brunswick. Rutile and anatase from metapelites are more common than in the Oxfordian, where mafic igneous sources predominate. Lower Cretaceous zircons include a few Mesoproterozoic grains that might be reworked from middle Paleozoic sedimentary rocks in the Gander terrane of northern Maine and New Brunswick, although subhedral grains together with rare chromite may indicate minor or intermittent supply from the Sable River draining Labrador. Heavy minerals provided important provenance information not available from zircon geochronology alone. Progressive enlargement of the catchment area resulted from an increasingly humid climate in the latest Jurassic and active mostly northeast-trending faults in the Early Cretaceous. This resulted in greater supply of petrographically more-mature sand to the basin.

Petrography, geochemistry, and Nd isotope systematics of metaconglomerates and matrix-rich metasedimentary rocks: implications for the provenance and tectonic setting of the Labrador Trough, Canada

The New Quebec Orogen consists of a supracrustal belt that was reworked when the Superior Craton collided with the Core Zone terrane during the Paleoproterozoic Trans-Hudson Orogeny. Within the New Quebec Orogen, the Kaniapiskau Supergroup can be divided into four terrigenous lithotypes metamorphosed at low-grade: one set with greater compositional and textural sedimentary maturity classified as quartz arenites and subarkoses, and another set with lower textural maturity classified as feldspathic wackes and mudrocks. In contrast, the Laporte Group includes homogeneous lithotypes represented by feldspathic and lithic wackes with a range of matrix contents metamorphosed at low to medium grade. The Kaniapiskau Supergroup rocks have a wide range of SiO2 and Al2O3 contents (SiO2/Al2O3 = 3.7–51) compared to the restricted compositional range of the Laporte Group rocks (SiO2/Al2O3 = 4.4–6.8). In general, the geochemical variations in both formations of the Laporte Group are within the range of the main clast varieties from basal metaconglomerates, although the Deborah Formation (top unit) records higher TiO2, P2O5, MgO, and Ni contents and high Cr/Th, Co/Ba, Th/U, and Rb/Sr ratios indicating additional mafic sources. Our results support the hypothesis that the Kaniapiskau Supergroup was deposited along an intraplate continental margin with predominantly recycled (εNd(1.87Ga) –12) Paleoarchean sources (TDM 3.2 Ga). In contrast, the Laporte Group marks the transition from a continental forearc (Grand Rosoy Fm.) with a typical juvenile source, including granitic clasts (εNd(1.83Ga) –0.1 to +3.1), to a wedge-top depozone (Deborah Fm.) in the context of a collisional proforeland basin. This syncollisional sedimentary environment is characterized by the presence of old crustal components (εNd(1.83Ga) –4.4 to –9.1).

Detrital zircon U-Pb geochronology of stromatolitic carbonates from the greenstone belts of Dharwar Craton and Cuddapah basin of Peninsular India

Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin. Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics. In this study, we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition. The U-Pb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426 ± 26 Ma to 2650 ± 38 Ma whereas the Sandur stromatolites gave an age of 3508 ± 29 Ma to 2926 ± 36 Ma suggesting Paleo- to Neoarchean provenance. The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761 ± 31 Ma to 1672 ± 38 Ma suggesting Neoarchean to Mesoproterozoic provenance. The Rare Earth Element (REE) patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements (LREE) and enrichment in heavy rare earth elements (HREE) with pronounced positive Ce and negative Eu anomalies, typical of magmatic zircons. The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites. The 3508 ± 29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton. The overall detrital zircon ages (3.5–2.7 Ga) obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin (2.7–1.6 Ga) collectively reflect on ∼800–900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.

Geochemistry of the Palaeoproterozoic quartzites of Lower Cuddapah Supergroup, South India: Implications for the palaeoweathering, provenance, and crustal evolution

The lower part of the Proterozoic Cuddapah Basin exposes three quartzite sequences, namely, Gulcheru, Pulivendla, and Gandikota formations. Gulcheru Quartzites show angular to subangular texture, whereas Pulivendla and Gandikota quartzites contain rounded to subrounded detrital grains. Mineralogically, all quartzites are predominantly composed of quartz with subordinate amount of feldspar, mica, and heavy minerals. Clay and silt are present as a matrix while silica and carbonate act as a cementing material. Based on detrital modes and major element geochemistry, these sediments are classified as arkose to subarkose. The petrological and geochemical attributes of the basal formations of the Cuddapah quartzite indicate cyclic weathering under variable humid subtropical climate. The CIA values of these quartzites suggest moderate to intense chemical weathering for the Gulcheru and Gandikota quartzites and low to moderate chemical weathering for Pulivendla Quartzites. The major element ratios (SiO2/Al2O, K2O/Na2O), trace element ratios, chondrite‐normalised REE patterns and UCC normalised multi element diagrams suggest that these quartzites were predominantly derived from the felsic sources, with subordinate contribution from mafic sources. Provenance studies and mixing models, in conjunction with palaeocurrent data suggest that these quartzites of the Lower Cuddapah Supergroup received sediments from various sources including Eastern Dharwar Craton as well as from more juvenile crust.

Geochemistry of Archean–Proterozoic shales, Dharwar Craton, India: Implications on depositional environment

Shales are significant sedimentary units in the Archean greenstone belts and Proterozoic Cuddapah sequence of Dharwar Craton. We present the geochemical studies of the Archean shales from the Chitradurga greenstone belt and the Proterozoic Vempalle Formation of the Cuddapah Basin to evaluate their chemical composition, weathering, provenance, and depositional conditions. The Archean shales are depleted in transition metals (Ni and Co) but enriched in V, Cr, and Sc relative to upper continental crust, while the Proterozoic shales are depleted in Cr, Co, Ni, Sc, and V compared with Post‐Archean Australian shale reflecting on negligible mafic source during their deposition. The REE patterns of these shales are uniform with moderately flat LREE and negative to positive Ce anomalies reflecting on marine conditions of their deposition. The presence of both positive and negative Ce anomalies indicate fluctuating oxic and anoxic conditions. The Archean and Proterozoic shales show negative and positive Eu anomalies, respectively, along with slightly enriched HREE. Compositional variation is observed in the Archean shales, whereas the Proterozoic counterparts were affected by sediment recycling as displayed by the Th/Sc and Zr/Sc relationship. Th/U and Th along with (Gd/Yb)n ratios display significant heavy mineral enrichment in these shales. The chemical index of alteration (CIA) and chemical index of weathering (CIW) of these shales suggest moderate to intense chemical weathering during the Archean and low to moderate weathering during Proterozoic times. The overall geochemical signatures collectively indicate granitic and tonalitic provenance for the Archean and Proterozoic shales, which were deposited in an active and passive continental margins.

Contrasting whole-rock and mineral compositions of ore-bearing (Tongchang) and ore-barren (Shilicun) granitic plutons in SW China: Implications for petrogenesis and ore genesis

A common perception is that oxidized magma is critical for the formation of a porphyry Cu ± Mo ± Au ore deposit. We have used an ore-bearing pluton (Tongchang) and an ore-barren pluton (Shilicun) in the western rim of the South China block to test this idea and to determine other important controls on ore genesis. Zircon UPb ages indicate that the Tongchang and Shilicun plutons were emplaced at 36.3 ± 0.2 Ma and 35.2 ± 0.4 Ma, respectively, broadly coinciding with strike-slip faulting in the region and continental collision that occurred ~300 km to the west. These two plutons are all characterized by significant light REE enrichments and pronounced negative NbTa anomalies, similar to the bulk crust. Apatite separates from these two plutons all have elevated initial 87Sr/86Sr (Tongchang, 0.70690 to 0.70796; Shilicun, 0.70703 to 0.70726) and negative εNd(t) (Tongchang, −6.2 to −7.3; Shilicun, −4.5 to −5.8). The mean εHf(t) and δ18O of zircon from these plutons are −1.4 and 6.8‰ for Tongchang, and −0.3 and 6.5‰ for Shilicun. The Sr-Nd-Hf isotope compositions are all within the ranges of the Neoproterozoic mafic arc lower crust in the region. Whole-rock U-Nb-Ta systematics indicate that the Tongchang and Shilicun plutons originated from an amphibole-bearing source and a garnet-bearing source, respectively, implying a shallower and more H2O-rich mafic source for the former than the latter. Quartz trace element data indicate that the Tongchang pluton was emplaced at a shallower depth than the Shilicun pluton. Zircon Ce4+/Ce3+ and whole-rock V/Sc ratios, and apatite SO3 and MnO contents indicate that the parental magmas of both plutons are as oxidized as those of some porphyry Cu deposits in northern Chile, but the parental magma of the Tongchang pluton is less oxidized than that of the Shilicun pluton. Apatite Cl-F-OH systematics and higher abundant hydrous silicate minerals such as amphibole and biotite in the Tongchang pluton than the Shilicun pluton indicate that the parental magma of the Tongchang pluton has higher Cl and H2O content than that of the Shilicun pluton. Our results show that oxidized magma is important but this alone cannot make a porphyry Cu ± Mo ± Au ore deposit. High contents of H2O-Cl in magma and shallow depth of emplacement are also important.

Dyke intrusion between neighbouring arc volcanoes responsible for 2017 pre-eruptive seismic swarm at Agung

Forecasting explosive eruptions relies on using monitoring data to interpret the patterns and timescales of magma transport and mixing. In September 2017, a distal seismic swarm triggered the evacuation of around 140,000 people from Agung volcano, Bali. From satellite imagery and 3D numerical models, we show that seismicity was associated with a deep, sub-vertical magma intrusion between Agung and its neighbour Batur. This, combined with observations of the 1963 eruption which caused more than thousand fatalities, suggests a vertically and laterally interconnected system experiencing recurring magma mixing. The geometry of the 2017 dyke is consistent with transport from a deep mafic source to a shallow andesitic reservoir controlled by stresses induced by the topographic load, but not the regional tectonics. The ongoing interactions between Agung and Batur have important implications for interpretation of distal seismicity, the links between closely spaced arc volcanoes, and the potential for cascading hazards.

Eoarchaean tectonics: New constraints from high pressure-temperature experiments and mass balance modelling

Four billion years ago the formation and preservation of the Earth’s first continental crust marked a new stage in the long-term geochemical evolution of our planet. This continental material is predominantly composed of plagioclase-rich granitoids that form Eoarchaean Tonalite and Trondjhemite (ETT) rock suites. There are two plausible overarching models for generating ETT: by a variety of intraplate melting mechanisms or through the onset of primitive subduction. Here we use mass balance modelling to show that ETT can only be derived from the hydrous melting of mafic rocks in convergent margins. These mafic source regions could be composed of relatively undepleted (oceanic plateau-like) non-arc metabasalts with or without residual rutile. Melts derived from these metabasalts subsequently mix with a slab-related component to form ETT magmas. Additionally, ETT can be formed from partial melting of metabasalts with geochemical affinities similar to modern island arc basalts. We propose that a primitive form of plate tectonics began on the surface of Hadean magma ocean(s), in a similar way to the plate-like motions observed on modern lava lakes, and subsequently evolved into the plate tectonics operating today. Hydrous melting of subducted basaltic crust and the formation of the proto-continental ETT crust would have followed the onset of accumulation of liquid water on the Earth’s surface.

Tracing source–sink processes and productivity from trace metals (Ba, Zn, Pb, Cd) of the surface sediments off Mahanadi to Pennar, western Bay of Bengal

In this paper, an attempt is made to understand the provenance of sediments and the role of dissolved oxygen in the preservation of metals and the productivity variations. Sixty-one surface sediment samples collected along eight transects from western Bay of Bengal off Mahanadi, Vamsadhara, Godavari, Krishna and Pennar Rivers were analyzed for spatial variations in selected metals (Ba, Zn, Pb, Cd). A change in source of sediment from north to south was very well reflected from the distribution of metals. Ba and Pb were dominant in the northern transects off Mahanadi and Vamsadhara indicating a felsic source for the sediments as Ba and Pb are more associated with rocks rich in feldspar. Conversely, lower Ba and Pb off Godavari, Krishna and Pennar indicated mafic source for the sediments as these metals are less associated with Deccan basalts. Furthermore, the preservation and distribution pattern of the redox-sensitive metals Zn and Ba was largely controlled by the oxygen minimum zone prevalent in the region at intermediate water depths. Negative biogenic Ba values and a poor correlation of Cd with Chl-a and P indicated low Ba and Cd input from plankton productivity, further enhancing the existing knowledge regarding low productivity in western Bay of Bengal.

Variations in trace metal concentrations and Sr, Nd isotopic compositions in sediments from two contrasting settings in the Eastern Arabian Shelf: Implications for provenance and paleoclimate reconstruction

Major and trace elements concentrations as well as Nd and Sr isotopic ratios were measured in sediments from two gravity cores (GC17 and GC19) deposited in the high-sediment flux regime of the Eastern Arabian Shelf (EAS). The core-sampling locations are close to the mouths of the geochemically and isotopically well-characterized Rivers Mandovi and Zuari and are overlain by water columns of contrasting redox conditions. Given the available knowledge of bottom water oxygen concentrations in the studied locations, the compositions of the Mandovi and Zuari sediments as well as the lithology of the drainage basin, the goal of this study was to test the efficacy of different paleo-redox proxies and provenance indicators in a high-sediment flux regime of the ocean.The variations of immobile elements like Al, Ti, Th, and Zr in the studied samples suggest lack of sediment reworking and sorting. Provenance proxies such as Th/Co, La/Sc ratios and variations in La-Th-Sc concentrations suggests a dominantly silicic source rock of granitic to granodioritic composition for these sediments, which is consistent with the dominant lithology of the drainage basins of the Mandovi and Zuari Rivers. Minor contributions from a mafic source (basalt or metabasalt) cannot be ruled out. The chemical index of alteration (CIA) and plagioclase index of alteration (PIA) values of the samples suggest low to moderate weathering of the source rocks. While the Sr isotopic compositions of the GC17 and GC19 sediments overlap with that of the gneissic basement rocks from the Mandovi and Zuari drainage basins, the εNd(0) of the sediments (average −13.72) show significantly more radiogenic values compared to the gneissic basement rocks (average −36). This difference in the Nd isotopic compositions is explained by early selective weathering and removal of rock-forming minerals like plagioclase in the source rocks which results in the supply of radiogenic Nd isotopic composition bearing sediments to the ocean. This argument is consistent with the overlapping εNd(0) of the GC17 and GC19 sediments and suspended particulate matter (SPM) of the Mandovi and Zuari Rivers. Although, the Nd isotopic compositions overlap, the less radiogenic Sr isotopic compositions of the GC17 and GC19 sediments compared to the SPM of the Mandovi and Zuari Rivers suggests the influence of organic matter, formation of authigenic clay and carbonates, as well as the contribution of Sr from submarine groundwater discharge.Trace metal concentrations and ratios were measured in the bulk sediments as well as in the leached authigenic and residual detrital-rich fractions of selected samples. For the GC19 sediments overlain by oxygenated waters, these geochemical proxies correctly infer their deposition under oxic conditions. In contrast, most of these proxies, barring those involving U and Mo, erroneously infer oxic depositional conditions for the GC17 sediments which are overlain by an oxygen minimum zone (OMZ) of the eastern Arabian Sea. We believe that the high sedimentation rate and large terrigenous flux possibly prohibits the equilibration of some of these trace metals in the sediments with the ambient seawater. Our study suggests that U and Mo-based proxies for inferring paleo-redox conditions are robust even under high sedimentation conditions. Overall, this study provides useful insights into the behaviour of redox-sensitive elements and provenance indicators in a continental margin setting impinged by an OMZ.

Chemical weathering of mafic rocks in boreal subarctic environment (northwest Russia) under influence of glacial moraine deposits

Chemical weathering of mafic rocks represents substantial sink of atmospheric CO2 yet the mechanismes of this process in high-latitude regions are poorly understood. This work addresses geochemical migration and partitioning of major and trace elements between rock, soil and surface waters during chemical weathering of mafic rocks and soil formation under the influence of a glacial moraine in the northwestern Russian subarctic. We used multidisciplinary approach which included major and trace element chemical analysis, Sr isotopic measurements, and mineralogical structural and microscopic investigations. Quaternary (Pleistocene) deposits were observed even in the deepest horizons of soil profiles and confirm a strong moraine influence as most soils showed podzolic features. Chemical and Sr isotopic analyses revealed a strong impact of the moraine on soil chemistry and mineralogy. Mineralogical studies showed the presence of non-aeolian quartz and zircon in soils which were not linked to the nature of the parental rocks (i.e. mafic and felsic). At the same time, we observed the presence of etch pit corrosion on the surface of zircons and feldspars, and a newly-formed matter adjacent to the surface of primary phases. The chemical index of alteration (CIA) showed weak or no weathering of rocks within the soil profile and the weathering intensity scale confirmed the “felsic” chemical composition of soils developed over mafic and ultramafic rocks. Analysis of Sr radiogenic isotopes demonstrated preferential removal of easily weatherable and less radiogenic minerals over the full depth of soil profile and a more radiogenic signature of the granitic moraine compared to mafic and felsic bedrocks. The composition of most surface waters reflected the weathering of silicate rocks but did not allow for distinguishing purely mafic source. The chemical weathering rates of ultramafic rocks were calculated to be higher than those for the granitic till, but the moraine depositions which dominate this region hide evidence of the real weathering process. This could be due to retention of Mg and Ca in soil due to precipitation of secondary phases such as Mg-vermiculite or divalent cation adsorption on mineral, organic or organo-mineral phases. As a result, weathering rates estimated for the olivinite rock appear to be lower than the weathering rates of mafic rocks reported in other boreal and subarctic regions. Overall, the felsic moraine deposits are capable to sizably decrease the weathering intensity of mafic rocks which should be taken into account for chemical weathering modelling of high-latitude regions subjected to glaciation in the past.

The Sources of Paleoproterozoic Collisional Granitoids (Sharyzhalgai Uplift, Southwestern Siberian Craton): from Lithospheric Mantle to Upper Crust

Abstract —The paper presents the geochemical and isotope characteristics of rocks and the U–Pb age and Lu–Hf isotope composition of zircons from three plutons of Paleoproterozoic granitoids in the Sharyzhalgai uplift (southwestern Siberian craton). The age of granitoids of the Toisuk (1838 ± 6 and 1827 ± 9 Ma), Nizhnii Kitoi (1846 ± 7 Ma), and Malaya Belaya (1863 ± 16 Ma) plutons corresponds to the Late Paleoproterozoic collision stage and is correlated with the time of mafic magmatism. The studied rocks have a wide range of silica contents. The Toisuk pluton is composed of a range of rocks from monzodiorites to granodiorites (granosyenites) and granites; the Nizhnii Kitoi pluton, of granodiorites and granites; and the Malaya Belaya pluton, of leucogranites. The rocks of the three plutons are highly ferroan, enriched in LREE, Th, and HFSE, and correspond in composition to A-type granitoids. A characteristic feature of melanocratic granitoids of the Toisuk and Nizhnii Kitoi plutons is extremely high contents of Ba: 4080–1500 ppm and 1560–990 ppm, respectively. Based on analysis of experimental data on the melting of various substrates and the results of numerical simulation, it is assumed that monzodiorite–granodiorites of the Toisuk pluton and granodiorites of the Nizhnii Kitoi pluton resulted from the differentiation/melting of a mafic source similar in Ba and Sr contents to intraplate continental basalts. The isotope compositions of zircon and melanocratic granitoids of the Toisuk (εHf from –6.0 to –10.7 and εNd from –5.3 to –10.2) and Nizhnii Kitoi (εHf from –5.0 to –8.1 and εNd = –4.0 and –5.1) plutons argue for the generation of their mafic sources from the enriched lithospheric mantle formed as a result of Neoarchean subduction processes. Vein granites of the Toisuk pluton and leucogranites of the Malaya Belaya pluton formed through the melting of quartz–feldspar (granodiorite) substrate. The contrasting isotope parameters of the Toisuk vein granites (εHf from –6.7 to –10.1, zircons, and εNd = –5.5, rock) and Malaya Belaya leucogranites (εHf from 2.9 to 5.9, zircons, and εNd from +0.7 to –1.9, rocks) indicate melting of the Archean and Paleoproterozoic crust, respectively. The more radiogenic Hf isotope composition of zircons from vein granites as compared with rocks of the Archean crust of the Irkut terrane is evident of the contribution of juvenile material to the granite formation.

Mineralogy and geochemistry of oil shale in Azerbaijan: classification, palaeoweathering and maturity features

Shamakhi-Gobustan and Absheron regions (Azerbaijan) are a part of the South Caspian Basin, which is a subsiding basin located between the colliding of Arabian and Eurasian plates. The intensive rate of sedimentation process creates a favorable condition for the formation of oil shale, hydrocarbon and as well as mud volcanoes in these regions. The purpose of the article. The study of oil shale in Azerbaijan has been mainly devoted to their geological and organic-geochemical characteristics, etc. However, the chemical classifications, provenience, palaeoweathering and maturity characteristics have not been studied. This study is the first attempt to investigate noted issues. The research methodology. 10 samples from the outcrops and eject of mud volcanoes were analyzed. The concentrations of major and trace elements and minerals were measured by “S8 TIGER Series 2 WDXRF”, “Agilent 7700 Series ICP-MS” mass spectrometers and XRD “MiniFlex 600”. The microscopes “Loupe Zoom Paralux XTL 745” and “MC-10” and a digital camera “OptixCam” were used to determine the age of the samples. The major and trace elements in the composition of samples were compared with average shale, NASC, PAAS and average black shale as well as oil shale from the Green River Formation of USA, Kukersit of Estonia, etc. studied in the published literature. A diagram and index were used for the classifications and determination of maturity of rocks. The palaeoweathering characteristic was determined based on CIA versus ICV and some other plots and ratios. Research results. The minerals found in oil shale were classified according to their classes. According to the used classification diagram, it was established that all studied samples correspond to shale. A superiority of clay minerals in the composition of oil shale compared to K-minerals, including K-feldspar was found. The estimates based on geochemistry and some ratios of elements confirm the instability of oxides and minerals, and immaturity of the samples. The values of the CIA, CIA versus ICV plot, etc. confirm moderate to high degree of weathering. The results confirm a conclusion that the original sediments were derived from mafic and intermediate source terrain. The scientific novelty. The scientific analysis presented in the paper is based on several substantial theoretical conclusions, which related to the factual material of research conducted by the co-authors. The mineralogy, classification features, stability characteristics of the major oxides and minerals as well as chemical maturity and palaeoweathering were studied based on the chemical composition of the samples. The practical significance. The results of the current study can be used for the further utilization of oil shale in Azerbaijan and the selection of promising areas in terms of mineral raw materials.

Stratigraphy and Geochemistry of the Goedgenoeg and Makwassie Formations, Ventersdorp Supergroup, in the Bothaville area of South Africa

Abstract The Neoarchaean Ventersdorp Supergroup represents a fully evolved bimodal volcano-sedimentary succession on the Kaapvaal Craton. Stratigraphic information was mainly derived from exploration drilling for gold in the underlying Witwatersrand Supergroup. Following the basal flood basalts of the Klipriviersberg Group, the Platberg Group mostly comprises extensive and voluminous intermediate to felsic volcanics of the Goedgenoeg and Makwassie Formations. Detailed stratigraphic and geochemical reports of these two formations were previously mostly restricted to the so-called Bothaville gap, between the Free State and Klerksdorp Gold Fields. The lower part of the Goedgenoeg Formation succession consists of non-porphyritic andesitic lava flows, followed upwards by dacitic phenocryst-rich feldspar porphyries. The basal lava flows are frequently interbedded with sedimentary rocks of the underlying Kameeldoorns Formation. Andesitic lava-flow units persist upwards, occurring interbedded with the Goedgenoeg Formation feldspar porphyries. The feldspar porphyries originated as ash flows; most are highly welded. The upper feldspar porphyries in the succession are absent in large parts of the Bothaville gap. The overlying Makwassie Formation consists of quartz porphyries, also with intermittent interbedded mafic lava units throughout the succession. The upper part of the succession is composed of phenocryst-rich crystal tuffs, only preserved over limited areas of the Bothaville gap. The quartz porphyries are highly welded, high-temperature ash flows with individual flows of tens of metres in thickness, typically separated by thin tuffs and sediments. The Goedgenoeg and Makwassie interbedded andesitic lavas have similar geochemical composition (the geochemical composition of the overlying Rietgat Formation lavas is also similar to that). The Goedgenoeg dacitic feldspar porphyries are separated into two geochemical groups: a lower high-Zr group and an upper low-Zr group. The Makwassie quartz porphyries are of dacitic compositions, while the upper part is rhyolitic. Two geochemical groups can be distinguished in the rhyolites, based on the Zr/Cr ratio. Although each of the two groups occur as distinct stratigraphic units, in some boreholes the low-Zr/Cr group form the lower part of the rhyolite succession overlain by the high-Zr/Cr group, while in other areas the sequence is reversed. A significant hiatus may have existed between eruption of the Goedgenoeg and Makwassie volcanics. The andesitic mafic lavas persistently co-erupted with the Goedgenoeg dacites and with the Makwassie dacites and rhyolites, indicating a stable mafic magma source that was re-mobilised from time to time to initiate eruption of the more viscous dacite and rhyolite crustal melts. The Goedgenoeg and Makwassie dacites are mixed magmas of the mafic andesitic source with crustal melts, while the Makwassie Formation rhyolites are a distinct crustal melt.