Detrital Sedimentary Rocks Research Articles

RKV01 Rutile – A New Potential Archaean Reference Material for Microbeam U‐Pb Dating

Rutile stands as a classical mineral within U‐Pb geochronology, offering insights into both magmatic and metamorphic events, and contributing to the understanding of provenance of detrital sedimentary rocks. Addressing the paucity of high‐quality Archaean rutile reference materials suitable for microbeam U‐Pb dating, we present a natural rutile (RKV01) sourced from the Kaap Valley pluton, South Africa, positing it as a potential Archaean reference material. Major element determination was executed via electron probe microanalysis, while trace element determination was conducted using laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS). The determination of U‐Pb ages involved the application of isotope dilution‐thermal ionisation mass spectrometry (ID‐TIMS), secondary ion mass spectrometry (SIMS) and LA‐ICP‐MS. The ID‐TIMS mean 207Pb/206Pb age is 3225.61 ± 0.64 Ma (2s, MSWD = 0.83), interpreted as the best estimate for the crystallisation age. The relatively high U mass fraction (~ 83 μg g‐1), extremely low Th mass fraction (~ 0.003 μg g‐1), negligible common Pb and isotopically homogeneous compositions collectively facilitated precise 207Pb/206Pb age determination via microbeam methods. Both LA‐ICP‐MS and SIMS 207Pb/206Pb ages are characterised by reproducibility and were consistent with the ID‐TIMS age within 2s analytical uncertainty. The results underscore the potential of RKV01 rutile as an Archaean reference material for microbeam U‐Pb dating.

Improving the chronostratigraphic framework of the Transvaal Supergroup (South Africa) through in-situ and high-precision U-Pb geochronology

The Transvaal Supergroup, on the Kaapvaal Craton in South Africa, is widely accepted as one of the best-preserved sedimentary archives to constrain planetary-scale environmental changes during the late Archean and early Proterozoic, yet the sedimentation age for certain stratigraphic intervals remains poorly constrained. To improve the temporal control on some of the first-order global changes recorded in these rocks, we carried out U-Pb analyses of detrital zircon populations from several clastic and volcano-clastic sedimentary units of the Transvaal Supergroup. We applied the Chemical Abrasion-Isotope Dilution-Thermal Ionization Mass Spectrometry (CA-ID-TIMS) technique on detrital and volcanic zircon populations that had been previously screened using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) technique. We report new maximum depositional age estimates for the Pannetjie (2456.6 ± 7.0 Ma), the Heynskop (2451.5 ± 2.5 Ma), the Makganyene (2423.1 ± 1.0 Ma) and the Hekpoort formations (2248.0 ± 1.1 Ma).A ca. 2.25 Ga-old cluster of LA-ICP-MS analyses in the Makganyene Formation was identified to be spurious, since it was completely removed during the chemical abrasion. Thus, we speculate that circulation of hydrothermal fluids and associated Pb-loss from a radiation-damaged lattice during the emplacement of the much younger Hekpoort Formation or possibly the Ophthalmia Orogeny, recorded in Western Australia, may have reset the U-Pb system of this zircon population. This implies that the accurate maximum depositional age of the Makganyene Formation is ca. 2.42 Ga instead, which denotes the age of the oldest glacial event of global extent during the Paleoproterozoic. Therefore, we suggest that the combination of both dating techniques is essential to ensure accurate maximum depositional age constraints for ancient detrital sedimentary rocks. Additionally, our data provides temporal constraints on a period characterized by major fluctuations in atmospheric oxygen. Finally, and supporting the complex nature of the Paleoproterozoic, linkages between widespread glaciations and atmospheric oxygen fluctuations remain to be explored.

Erosional reservoir for the northern segment of the Arabian-Nubian shield: Constrains from U-Pb geochronology of the lower palaeozoic succession, North Eastern Desert, Egypt

Zircon U-Pb geochronology of detrital sedimentary rocks is a precious and unique tool to explore older eroded crust which, frequently, no longer exists. This is because the clastic sedimentary rocks represent a detritus reservoir for the surrounding eroded region. A total of 207 (196 concordant ages) zircon grains from the Lower Palaeozoic Araba and Naqus formations were analysed to investigate the possible rock provenances and examine the different components of the Arabian-Nubian Shield in the region before being intensively eroded between the Cambrian and the Carboniferous. Somr El-Qaa area in the North Eastern Desert (NED) contains one of the rare remnant exposures of the Lower Palaeozoic sediments where the catchment area is expected to be extended through the NED and Southern Sinai. Three main chronological provenances sources were represented: (1) Pre-Pan-African (Pre-Cratonization) source, represented by ca. 35% of Araba Formation and ca. 10% of Naqus Formation total populations (44 grains) with ages varies between 2289 ± 113 Ma and 907 ± 45 Ma, indicates a probable pre-Pan-African crust palaeo-existence in the region. (2) Pan-African (Cratonization) source, represented by ca. 58% of Araba and ca. 76% of Naqus dated zircons (137 grains) with ages varying between 887 ± 44 Ma and 501 ± 25 Ma. Surprisingly, nearly half of the dated grains belong to the age zones of the island arc and syn-orogenic suites which suggests their palaeo-dominance in the NED and being intensively eroded. (3) Post-Pan-African (Post-Cratonization) source, represented by ca. 3% of Araba and ca. 12% of Naqus total analysed grains (15 grains) with ages ranging from 489 ± 24 Ma to 312 ± 16 Ma which refers to probable unidentified magmatic events during the Cambrian-Silurian along with the Hercynian tectonic event. Furthermore, it documents being the so-called Lower Palaeozoic system open in the studied region until Pennsylvanian time.

Quality Assessment of Groundwater Based on Geochemical Modelling and Water Quality Index (WQI)

Potable groundwater contamination through arsenic (As) is a common environmental problem in many developing countries, including Pakistan, with significant human health risk reports. The current research was conducted in District Nankana Sahib, which is a major industrial site in Punjab, Pakistan. According to the Punjab Directorate of Industries in Pakistan, there are more than a thousand industries in this area. These industries produce a lot of waste and effluent, which contaminate the environment with harmful and toxic materials. Continuous irrigation with industrial effluent and sewage sludge may make groundwater sources vulnerable. Therefore, this research was aimed as assessing the hydrochemical profile of groundwater concerning As contamination in the study area using sixty-seven groundwater samples. Multivariate statistical analysis, graphical plots, geochemical modeling, and the water quality index (WQI) were applied to investigate the hydrochemistry of the research area. The outcomes of the WQI revealed that 43% of the samples were of poor quality and not fit for human consumption. About 28% of the groundwater samples showed high arsenic contamination beyond the permitted limit of the World Health Organization (WHO). The piper plot identified three distinct types of water in the research area: calcium, chloride, and calcium-chloride types. The Gibbs diagram illustrated that rock–water interaction influenced the hydrochemistry. Terrigenous sedimentary rocks, such as stream deposits, flood plain deposits, and detrital sedimentary rocks, among other types of sedimentary rocks covered most part the study area. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) indicated that the arsenic in groundwater exhibited a significant positive correlation for pH, Fe and As. Health risk assessments indicated a hazard quotient (HQ) greater than 1, indicating a 28% contribution showing that groundwater ingestion is highly toxic to the local habitats. The results of this study further help in managing future sustainable groundwater management approaches in the Nankana District, Punjab, Pakistan.

The Cu-Ag-Pb Polymetallic Mineralization of Agdim-Ait Elfersi Sector, North-Eastern Part of the Saghro Massif, Morocco: Geological Setting, Ore Petrography and Geochemistry

The Cu-Ag-Pb polymetallic mineralization of the Agdim-Ait Elfersi sector is situated in the eastern Anti Atlas Moroccan belt, the northeastern part of the Saghro inlier. The mineralization is hosted in the volcanic rocks of late Neoproterozoic and the detrital sedimentary rocks of the Lower Cambrian. A network of faults has affected this area; it is a brutalizing deformation with NW-SE, NE-SW, E-W, and NNE-SSW directions. NW-SE and NE-SW structures have a metallogenic significance. This tectonic canvas was used as a target of a mineralization vein with Cu, Ag, and Pb in this area. By combining geological, structural, and metallogenic characteristics, it can be assumed that the Cu-Ag-Pb mineralization of the Agdim-Ait Elfersi sector is linked to two major events. A late Pan-African event linked to the extensional tectonics of the late Neoproterozoic, responsible for the establishment of mineralization at the level of the basement, and then a late Hercynian-Atlasic event, which manifests by the remobilization of metals in NE-SW oriented structures. Geochemical analysis shows the polymetallic character of this mineralization. The high contents of silver and mercury observed indicate the presence of similarities of an Ag-Hg epithermal type of mineralization in this area. Geological context, as well as mineralogical and textural characters of the Agdim-Ait Elfersi sector are reminiscent of epithermal-type mineralization as in the Imiter and Zgounder deposit.

Episodic growth of felsic continents in the past 3.7 Ga.

Continents form the most accessible parts of Earth, but their complex compositions make their origin difficult to investigate. A novel approach based on a comprehensive compilation of samarium-neodymium isotopic compositions of detrital sedimentary rocks is here used to unravel continental growth through time. This record reveals that continents were as felsic as today in the past 3.7 Ga (billion years) and that their growth was not continuous but episodic. Reworking of preexisting crust was a ubiquitous process during most of Earth history, but at least six periods of continental growth can be identified every 500 to 700 Ma (million years) in the past 3.7 Ga. This recurrence could be accounted for by changes in tectonic plate velocities favoring periods of rapid subduction and enhanced production of juvenile felsic crust.

Significance and Petrochemistry of Upper Mantle–mantle Transition Zone Metaperidotites and Oceanic Crust Metagabbros within the Kazdağı Metaophiolite: Implications for the Geological Evolution of the Izmir–Ankara–Erzincan Ocean, Northwestern Turkey

AbstractThe Kazdağı metaophiolite crops out in the Kazdağı (Ida) Mountains in the Biga Peninsula in northwestern Turkey. It is in stratigraphic contact with the high–grade metamorphic rocks of the Kazdağı Massif. Metaophiolitic and high–grade metamorphic rocks are tectonically overlain by low–grade metamorphic units of the Permo‐Triassic Karakaya Complex of the Sakarya Zone. Late Oligocene‐Early Miocene granites intruded these tectonic units (Okay and Satır, 2000; Duru et al. 2012).In the Kazdağı metaophiolitic sequence, upper mantle peridotites are represented by metaharzburgite and metadunite, whereas the mantle transition zone metaperidotites are composed of metadunite, metapyroxenite and minor plagioclase‐bearing metalherzolite. The upper part of the metadunites in the mantle transition zone show intercalation with metagabbros. Gabbros of oceanic crust experienced amphibolite facies metamorphism and are transformed into amphibolite, garnet amphibolite and migmatitic gabbros. The metagabbros and amphibolites display MORB‐ and IAT‐like geochemical features.The Kazdağı metaophiolite is conformably overline by basal conglomerates and hemi‐pelagic carbonate rocks continuing upward into forearc‐type flysch–like detrital sedimentary rocks interspersed with mafic volcanic intervals. These cover units underwent high–grade metamorphism into gneisses, migmatites, amphibolites and marbles in a compressional regime during the Alpine orogeny. New U–Pb zircon data from the metagabbros show two crystallization peaks at ∼52 Ma and ∼73 Ma. This has implications for the age of subduction of the Izmir–Ankara–Erzincan Ocean, generally assumed to be northward under the Sakarya Zone. During the Triassic to Middle Eocene, progressive overthrusting of the Sakarya Zone via a N–S compresional regime created by the Alpine orogeny onto subduction–accretion‐ and forearc‐units resulted in high–grade metamorphic conditions in the Biga Peninsula.

Areally Extensive Surface Bedrock Exposures on Mars: Many Are Clastic Rocks, Not Lavas.

Areally extensive exposures of intact olivine/pyroxene-enriched rock, as well as feldspar-enriched rock, are found in isolated locations throughout the Martian highlands. The petrogenetic origin(s) of these rock units are not well understood, but some previous studies favored an effusive volcanic origin partly on the basis of distinctive composition and relatively high thermal inertia. Here we show that the regolith development, crater retention, and morphological characteristics for many of these "bedrock plains" are not consistent with competent lavas and reinterpret the high thermal inertia orbital signatures to represent friable materials that are more easily kept free of comminution products through eolian activity. Candidate origins include pyroclastic rocks, impact-generated materials, or detrital sedimentary rocks. Olivine/pyroxene enrichments in bedrock plains relative to surrounding materials could have potentially formed through deflation and preferential removal of plagioclase.

Sadiola Hill: A World-Class Carbonate-Hosted Gold Deposit in Mali, West Africa

The ~8-Moz Sadiola Hill gold deposit is located in the Kedougou-Kenieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone.

The Tethyan Himalayan detrital record shows that India–Asia terminal collision occurred by 54 Ma in the Western Himalaya

The Himalayan orogen is a type example of continent–continent collision. Knowledge of the timing of India–Asia collision is critical to the calculation of the amount of convergence that must have been accommodated and thus to models of crustal deformation. Sedimentary rocks on the Indian plate near the suture zone can be used to constrain the time of collision by determining first evidence of Asian-derived material deposited on the Indian plate. However, in the Himalaya, for this approach to be applied successfully, it is necessary to be able to distinguish between Asian detritus and detritus from oceanic island arcs that may have collided with India prior to India–Asia collision. Zircons from the Indian plate, Asian plate and Kohistan–Ladakh Island arc can be distinguished based on their U–Pb ages combined with Hf signatures. We undertook a provenance study of the youngest detrital sedimentary rocks of the Tethyan Himalaya of the Indian plate, in the Western Himalaya. We show that zircons of Asian affinity were deposited on the Indian plate at 54 Ma. We thus constrain terminal India–Asia collision, when both sutures north and south of the Kohistan–Ladakh Island arc were closed, to have occurred in the Western Himalaya by 54 Ma.

Genesis of the boron potential of the Taukha metallogenic zone, Sikhote Alin, and boron sources during formation of the Dal’negorsk boron deposit

It is shown that the formation of borosilicate skarn in the Taukha metallogenic zone completes a series of successive stages of the formation and transformation of the folded sequences of the Taukha accretionary wedge. The Early Cretaceous sedimentary stage, including accumulation of detrital tourmaline-rich sedimentary rocks, was implemented in the marginal sea of the Paleopacific Sino-Korean segment. In the Turonian–Campanian, the boron-bearing folded sequences of the accretionary wedge were involved in anatexis to generate siliceous S-type boron-bearing melts. The thus-formed magmatic chambers were emptied during catastrophic volcanic eruptions. At the final Middle Campanian volcanic stage, fluid-magmatic differentiation of the melt in the residual chambers generated fluid flow. The infiltration interaction of the fluids, which inherited enrichment in boron, with limestones of the olistostrome sequence resulted in the formation of a giant zone of grossular–wollastonite skarns and danburite lodes. The boron potential of the Taukha boron–lead–zinc metallogenic zone may be considered as a reproduction of the Precambrian boron metallogeny of the eastern Eurasian margin, which was implemented in the Late Mesozoic during recycling of the continental crust.

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures.

Biological activity is a major factor in Earth's chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth's evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth's sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life's origin in the Hadean eon (>4,000 Ma).

Composition of conglomerates analyzed by the Curiosity rover: Implications for Gale Crater crust and sediment sources

AbstractThe Curiosity rover has analyzed various detrital sedimentary rocks at Gale Crater, among which fluvial and lacustrine rocks are predominant. Conglomerates correspond both to the coarsest sediments analyzed and the least modified by chemical alteration, enabling us to link their chemistry to that of source rocks on the Gale Crater rims. In this study, we report the results of six conglomerate targets analyzed by Alpha‐Particle X‐ray Spectrometer and 40 analyzed by ChemCam. The bulk chemistry derived by both instruments suggests two distinct end‐members for the conglomerate compositions. The first group (Darwin type) is typical of conglomerates analyzed before sol 540; it has a felsic alkali‐rich composition, with a Na2O/K2O > 5. The second group (Kimberley type) is typical of conglomerates analyzed between sols 540 and 670 in the vicinity of the Kimberley waypoint; it has an alkali‐rich potassic composition with Na2O/K2O < 2. The variety of chemistry and igneous textures (when identifiable) of individual clasts suggest that each conglomerate type is a mixture of multiple source rocks. Conglomerate compositions are in agreement with most of the felsic alkali‐rich float rock compositions analyzed in the hummocky plains. The average composition of conglomerates can be taken as a proxy of the average igneous crust composition at Gale Crater. Differences between the composition of conglomerates and that of finer‐grained detrital sediments analyzed by the rover suggest modifications by diagenetic processes (especially for Mg enrichments in fine‐grained rocks), physical sorting, and mixing with finer‐grained material of different composition.

Basin evolution and stratigraphic correlation of sedimentary-exhalative Zn–Pb deposits of the Early Cambrian Zarigan–Chahmir Basin, Central Iran

The Zarigan–Chahmir basin is placed in the southern part of a crustal domain known as the Central Iranian microcontinent, at the northwestern margin of Gondwana. This basin hosts abundant mineral deposits, particularly of the iron oxide–apatite (IOA), Fe–Mn exhalative, and Zn–Pb sedimentary-exhalative (SEDEX) types. The evolution of this basin is governed by the Proto-Tethys oceanic crust subduction beneath the Central Iranian microcontinent and by the resulting continental arc and back-arc. This evolution followed two major stages of rifting: (I) Stage I or syn-rift phase, related to intra-basin extension, is indicated by coarse-grained detrital sedimentary rocks and bimodal volcanism (basis of the Early Cambrian Volcano-Sedimentary Sequence; ECVSS), which filled half-graben systems. During this stage, tuff-hosted stratiform, exhalative Fe–Mn deposits along with Kiruna-type IOA deposits formed. The former deposits (e.g., Narigan) are related to early submarine rhyolithic volcanism of the rift-phase sequence, whereas the latter (e.g., Esfordi, Choghart, Chadormalu, Chahgaz) are connected to hydrothermal activity directly linked to the arc calk-alkaline magmatism. (II) Stage II or sag-phase involved the deposition of calcareous shales, siltstones and carbonates (upper part of the ECVSS). Sedimentation during this phase was controlled by basin subsidence and by the reactivation of the half-graben faults. SEDEX deposits are hosted within a carbonaceous, black siltstone unit of the sag-phase sequence, which was deposited during a period of rapid basin subsidence and under anoxic conditions. The location of synsedimentary faults and the prevalence of poorly-oxygenated bottom water conditions were key factors controlling SEDEX mineralization processes in the basin. A high geothermal gradient caused by maximum syn-rift magmatism and sub-crustal lithospheric thinning, affecting primarily the center, western and northwestern basin, may have driven a basin-scale hydrothermal fluid circulation. This finding explains the occurrence of larger, well correlable SEDEX deposits in the northwestern and central parts of the Zarigan–Chahmir basin.

Growth of continental crust: a balance between preservation and recycling

AbstractOne of the major obstacles to our understanding of the growth of continental crust is that of estimating the balance between extraction rate of continental crust from the mantle and its recycling rate back into the mantle. As a first step it is important to learn more about how and when juvenile crust is preserved in orogens. The most abundant petrotectonic assemblage preserved in orogens (both collisional and accretionary) is the continental arc, whereas oceanic terranes (arcs, crust, mélange, Large Igneous Provinces, etc.) comprise <10%; the remainder comprises older, reworked crust. Most of the juvenile crust in orogens is found in continental arc assemblages. Our studies indicate that most juvenile crust preserved in orogens was produced during the ocean-basin closing stage and not during the collision. However, the duration of ocean-basin closing is not a major control on the fraction of juvenile crust preserved in orogens; regardless of the duration of subduction, the fraction of juvenile crust preserved reaches a maximum of ∼50%. Hafnium and Nd isotopic data indicate that reworking dominates in external orogens during supercontinent breakup, whereas during supercontinent assembly, external orogens change to retreating modes where greater amounts of juvenile crust are produced. The most remarkable feature of εNd (sedimentary rocks and granitoids) and εHf (detrital zircons) distributions through time is how well they agree with each other. The ratio of positive to negative εNd and eHf does not increase during supercontinent assembly (coincident with zircon age peaks), which suggests that supercontinent assembly is not accompanied by enhanced crustal production. Rather, the zircon age peaks probably result from enhanced preservation of juvenile crust. Valleys between zircon age peaks probably reflect recycling of continental crust into the mantle during supercontinent breakup. Hafnium isotopic data from zircons that have mantle sources, Nd isotopic data from detrital sedimentary rocks and granitoids and whole-rock Re depletion ages of mantle xenoliths collectively suggest that ≥70% of the continental crust was extracted from the mantle between 3500 and 2500 Ma.

Protoliths of enigmatic Archaean gneisses established from zircon inclusion studies: Case study of the Caozhuang quartzite, E. Hebei, China

A diverse suite of Archaean gneisses at Huangbaiyu village in the North China Craton, includes rare fuchsite-bearing (Cr-muscovite) siliceous rocks – known as the Caozhuang quartzite. The Caozhuang quartzite is strongly deformed and locally mylonitic, with silica penetration and pegmatite veining common. It contains abundant 3880–3600 Ma and some Palaeoarchaean zircons. Because of its siliceous nature, the presence of fuchsite and its complex zircon age distribution, it has until now been accepted as a (mature) quartzite. However, the Caozhuang quartzite sample studied here is feldspathic. The shape and cathodoluminescence petrography of the Caozhuang quartzite zircons show they resemble those found in immature detrital sedimentary rocks of local provenance or in Eoarchaean polyphase orthogneisses, and not those in mature quartzites. The Caozhuang quartzite intra-zircon mineral inclusions are dominated by quartz, with lesser biotite, apatite (7%) and alkali-feldspar, and most inclusions are morphologically simple. A Neoarchaean orthogneiss from near Huangbaiyu displays morphologically simple inclusions with much more apatite (73%), as is typical for fresh calc-alkaline granitoids elsewhere. Zircons were also examined from a mature conglomerate quartzite clast and an immature feldspathic sandstone of the overlying weakly metamorphosed Mesoproterozoic Changcheng System. These zircons have oscillatory zoning, showing they were sourced from igneous rocks. The quartzite clast zircons contain only rare apatite inclusions (<1%), with domains with apatite habit now occupied by intergrowths of muscovite + quartz ± Fe-oxides ± baddeleyite. We interpret that these were once voids after apatite inclusions that had dissolved during Mesoproterozoic weathering, which were then filled with clays ± silica and then weakly metamorphosed. Zircons in the immature feldspathic sandstone show a greater amount of preserved apatite (11%), but with petrographic evidence of replacement of other apatites by quartz and mica. From the zircon morphology and inclusions studies, our studied Caozhuang quartzite is most likely <3500 Ma immature detrital sedimentary rock of local provenance. This strengthens the case for Eoarchaean rocks with a substantial 3880–3800 Ma component occur close to Huangbaiyu.

SHRIMP zircon U-Pb dating of late Paleoproterozoic kondalites in the Daqing Mountains area on the North China Craton

The Daqing Mountains area comprises a typical occurrence of the Khondalite Belt in the Western Block of the North China Craton (NCC). In this area, both early and late Paleoproterozoic metasedimentary rocks have been identified in what was originally called the Upper Wula Mountains “Subgroup”. Six metasedimentary rock samples yielded SHRIMP U-Pb zircon ages of 2.56–2.04 Ga for detrital and 1.96–1.83 Ga for metamorphic zircons. Based on these data and previously published results, the following conclusions can be drawn: 1) The source region for the late Paleoproterozoic detrital sedimentary rocks is mainly 2.55–2.4 and 2.2–2.04 Ga in age, consistent with the early Precambrian geological history identified widely in the basement of the NCC. 2) The majority of sedimentary rocks of the khondalite series were deposited between 2.04 and 1.95 Ga, and then in a protracted period (1.96 and 1.83 Ga) underwent a complex history of amphibolite to granulite-facies metamorphism.

Structural Evolution of a Precambrian Segment: Example of the Paleoproterozoic Formations of the Mako Belt (Eastern Senegal, West Africa)

The western part of the Kedougou Kenieba Inlier is located in the West African Craton. It consists of paleoproterozoic NE-trending elongate belts (subprovinces) of metavolcanic and granitic rocks that alternate with metasedimentary belts. Major linear fault such as the MTZ which also approximate a north-easterly trend form the eastern boundaries. The field observations and geophysics analyses were completed by a microscopic study. Based on these data we define across this region four lithostructural domains from east to west. The western domain is structurally complex. The rocks of this domain have been subjected to a complex history of polyphase deformation and metamorphism. The structural analyse allow us to distinguished three deformation events. The deformation results in the formation of D1 thrust tectonic and D2 and D3 transcurrent tectonic. The structural evolution of the Mako Belt is characterized by deformation dominated by the intrusion of large TTG batholiths (D1) followed by basins formation and transpression accommodating oblique convergence and collision (D2 and D3). The change from thrusting (D1 deformation to transcurrent motion (D2 and D3) is recorded in the marginal basin of the central domain and in Tinkoto pull apart basin. The timing of these basins indicates a diachronous evolution. Deformation styles within the basin are compatible with a dextral transpression which terminated at ca 2090 Ma. Small extensional basins formed over the rocks of the Mako Belt are filled with continental detrital sedimentary rocks that show weak foliation and active felsic volcanism. We suggest that the sinistral transpressive tectonic associated with oblique subduction may have generated the pull-apart basin and subaqueous volcanism. In part these features are now related to terrain accretion, thrusting and strike slip movement during oblique convergence. The inversion of the large scale structural evolution from thrusting to strike slip is common to modern orogenies.

Elemental and Sm-Nd isotopic geochemistry on detrital sedimentary rocks in the Ganzi-Songpan block and Longmen Mountains

Systematic results of major and trace element geochemistry and Sm-Nd isotopic geochemistry on detrital sedimentary rocks of Precambrian to Triassic in the Ganzi-Songpan block and Longmen Mountains are presented. The rocks are classified into greywackes or feldspar sandstones, grains of which are the mixtures of mafic rocks, felsic rocks, and quartz+calcite. Total rare earth elements (REE) contents of the rocks increase gradually and negative Eu anomalies become more obvious from Precambrian to Triassic, which may indicate intensifying crustal anatexis. Tectonic setting was stable during the Late Paleozoic, therefore there are obvious negative Ce anomalies. Nd model ages are between 1.6 Ga and 2.4 Ga, which are very similar to those of the Yangtze craton, South Qinling and North Qinling belts and quite different from those of the North China craton. Therefore, provenance of the sedimentary rocks in the Ganzi-Songpan block and Longmen Mountains was the Yangtze craton and/or the Qinling orogen, which evolved on the basis of the Yangtze craton. The correlation between provenances and tectonostratigraphic strata of the western Yangtze craton shows that the source materials should be primarily from Neoproterozoic. Secondary sources were Archean and Paleoproterozoic strata. Triassic clastic sedimentary rocks contain Late Paleozoic mantle-derived materials, represented by the Emeishan Permian flood basalts. Spatial distribution of initial Nd isotopic compositions indicates that denudating areas were in the east and the north and depositing areas of deep water were in the west and the south for the Ganzi-Songpan basin during Triassic.

Neoproterozoic crustal evolution in Southern Chad: Pan-African ocean basin closing, arc accretion and late- to post-orogenic granitic intrusion

In the Lake Léré region, southern Chad, Neoproterozoic terrains are distributed in four lithostructural groups that reveal the geotectonic evolution of a part of the Pan-African orogenic domain. The first group includes basaltic volcanic rocks and fine-grained detrital sedimentary rocks of pre-tectonic basins that were emplaced in an extensional regime, close to a volcanic arc. The second and third groups include calc-alkaline gabbroic intrusions emplaced at an upper crustal level and a midcrustal tonalite, respectively, that are interpreted to be the roots of an active margin volcanic arc. These first three groups experienced WNW to ESE compression, and may belong to a fore-arc basic—volcanic arc—back-arc basin system that was accreted eastward to the Palaeoproterozoic Adamaoua-Yadé Block. The fourth group includes post-tectonic granite plutons invading the older groups. This paper documents the accretion processes in the southern margin of the Saharan Metacraton.