Tectonic Setting Discrimination Research Articles

Unraveling the early Paleozoic tectonic history of the South Qinling Belt: Evidence from geochronology, geochemistry, and Sm-Nd isotopes of meta-sedimentary rocks

The determination of the petrogenesis and age of meta-sedimentary rocks plays a crucial role in enhancing our understanding of regional tectonic evolution. In this study, we conducted comprehensive geochemical, zircon U-Pb, and Sm-Nd isotope dating analyses of the biotite gneiss of the Changjiaoba group to investigate the tectonic evolution of the South Qinling belt. The age range of detrital zircons from the biotite gneiss was estimated to be 446–2798 Ma, with prominent peaks observed at approximately 454, 660, 790, 1998, and 2770 Ma. The earliest data indicate that the final sedimentary age of the Changjiaoba group corresponds to the late Ordovician. The protoliths of the biotite gneiss were argillaceous rocks, classified as parametamorphic rock. The chondrite-normalized rare earth element (REE) diagram exhibited right-inclined patterns with negative Eu anomalies. The upper continental crust (UCC)-normalized trace element patterns exhibited insignificant enrichment of Ba, Sr, and K; negative anomalies of Hf, Th, U, and Ti; and depletion of Cr and Ni. Geochemical indicators suggest that the precursor sediments are felsic rocks that have undergone moderate weathering. The εNd (t) values of the biotite gneiss (−10.2 to −3.4) suggest a mixture of a few juvenile crust components and abundant ancient material, indicating an arc-realted setting. Collectively, our findings suggest that the primary material sources of the Changjiaoba group were predominantly the North Qinling belt during the Paleozoic and the Yangtze block during the Neoproterozoic. The tectonic setting discrimination diagrams imply that the biotite gneiss were formed in an active continental margin environment, supporting the notion of the northward oceanic subduction during the early Paleozoic. In conclusion, our study provides valuable insights into the composition and deposition history of the Changjiaoba group in the South Qinling belt, contributing to our understanding of the regional geology.

The discrimination of tectonic settings using trace elements in magmatic zircons: A machine learning approach

The discrimination of tectonic settings using trace elements in magmatic zircons: A machine learning approach

Geochronology and geochemistry of zircon from Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA

New zircon U–Pb, trace element, and Lu–Hf laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data are presented for five Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA. These samples were previously analyzed by U–Pb LA-ICP-MS and whole-rock geochemical methods and the new data generally corroborate the earlier results. Coarse-grained alkali-feldspar granite from northwest of the Fundy magnetic anomaly, the interpreted fault boundary in the offshore between Gondwanan microcontinents Ganderia to the northwest and Avalonia to the southeast, yielded a crystallization age of 414 ± 2 Ma. Southeast of the inferred fault, crystallization ages are 385 ± 3 Ma and 386 ± 3 Ma for two crystal tuff samples near the fault, 403 ± 3 Ma for an alkali-feldspar granite ~50 km southeast of the fault, and 399 ± 5 Ma for syenogranite ~25 km southeast of the fault, which also yielded inherited grains at ~1.3 Ga and between 613 ± 15 Ma and 558 ± 9 Ma. Lu–Hf LA-ICP-MS data for zircon retaining igneous crystallization ages have εHf(t) between 2.9 and 13.1 and model ages based on felsic sources between 0.52 and 1.04 Ga, reflecting a mix of late Mesoproterozoic (Avalonian?) basement and primitive melt, possibly in an extensional setting. Zircon Nb/Hf ratios generally greater than 0.001 indicate a predominately within-plate/anorogenic/rift setting, consistent with their whole-rock chemistry. U/Yb-Nb/Yb and U/Yb-Hf tectonic setting discrimination diagrams show ocean island to continental arc signatures, with a stronger continental arc signature for the syenogranite. Most zircon grains have Eu/Eu* values less than 0.1, indicating a crustal thickness of ~30 km or less at the time of their crystallization.

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Geochemistry of Triassic metasediments from easternmost part of Sakar Unit, Sakar-Strandzha Zone, SE Bulgaria

Triassic metasediments from the easternmost part of Sakar Unit, Sakar-Strandzha Zone, SE Bulgaria, show a high variety in their mineral and chemical composition from shales, wakes, litharenites to arkoses. Geochemical signatures suggest low to moderate weathering degree at their source and upper continental crust composition. The Triassic metasedimentary rocks were derived from felsic to intermediate igneous rocks with some recycled sediment input. On tectonic setting discrimination diagrams, based on immobile trace elements, the studied metasediments fall in the fields of continental island arc and active continental margin.

Petrogenesis and tectonic significance of Late Paleozoic magmatism in the Xilinhot micro-continent, central Xingmeng orogenic belt

ABSTRACT The evolution history of the Hegenshan Ocean is an important part of studying the evolution of the Xingmeng orogenic belt, but it is still controversial. Here we present an integrated study of petrology, zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd isotopes on a series of intrusive rocks from the Baiyinguole area, Xiwuqi, central Inner Mongolia, NE China. Chronological data indicate that the intrusive rocks can be divided into three stages: 326–317, 297–295, and 259–254 Ma. The 326–317-Ma gabbros and diorites belong to the calc-alkaline series, and are characterized by positive whole-rock εNd(t) values (+1.1–+6.7), relatively low La/Ba ratios (0.04–0.10), and high La/Nb ratios (1.82–3.18), indicating that the magma source is from a subduction-modified lithospheric mantle. These samples have arc features owing to the southward subduction of the Hegenshan oceanic crust. The 297–295-Ma granodiorites and monzogranites show positive εNd(t) values (+1.9–+2.4), low 87Sr/86Sri ratios (0.704185–0.705234), and young Nd two-stage model ages (TDM2), which is consistent with typical I-type granites. On various tectonic setting discrimination diagrams, the granodiorites and monzogranites are plotted in the syn-collisional field. The 259–254-Ma syenogranites have low abundances of mafic minerals, low contents of Zr/Hf (18.31–31.35) and Nb/Ta (6.80–11.80) values, high SiO2 (76.36–78.54 wt.%) content, A/CNK (1.08–1.76), and D.I. of 96.8–97.4 (average 97.1), and notable negative Eu anomalies (δEu* = 0.07–0.17) and weakly tetrad-effects in REE pattern, which is affinity to the characteristics of highly differentiated granites. Moreover, they have positive εNd(t) values (+2.0–+2.1) with scattered TDM2 of 730–1217 Ma and low Nb/U ratios. This study suggests that the syenogranites are formed in a post-collisional tectonic setting, which is related to the closure of the Hegenshan Ocean. In addition, based on a review of the pre-existing literature data, a model of southward ridge subduction is proposed to explain the tectonic evolution of the Hegenshan Ocean.

The depositional environment of the Koeris Formation in the Aggeneys-Gamsberg ore district, South Africa

Abstract The Koeris Formation in the Mesoproterozoic Aggeneys-Gamsberg ore district (South Africa) is located in the hanging wall of an unconformity, only metres above the giant Gamsberg and Big Syncline Pb-Zn ore deposits and potentially contains critical information on the metallogenesis and its geodynamic setting. We, therefore, conducted geochemical analyses of amphibolite and metasedimentary rocks of this formation in the core of the Gamsberg fold, the results of which shed new light on the plate tectonic processes after deposition of the sedimentary exhalative ore. Major elements previously used for the characterisation of the amphibolite and the discrimination of the plate tectonic setting of its protolith had been mobilised during retrograde metamorphic overprint and are thus of little tectonic significance. More informative are certain trace elements: The amphibolite shows a strong depletion in Nb, Ta and P, typical of subduction-related magmatism, whereas exceptionally strong enrichment in mobile elements like Rb and Ba indicate the participation of continental material (crust/sediment) in the enrichment of the mantle wedge. This is in perfect agreement with existing U-Pb age data, which attest formation of the mafic metavolcanic rocks of the Koeris Formation at the very end of the Okiepian orogenesis (1 210 to 1 180 Ma), when subduction was terminated. Generally, very high Zn and Pb contents speak for metasomatic interaction of the Koeris Formation with the Zn-and Pb-rich Gamsberg deposit prior to peak metamorphism during the Klondikean orogeny (1 040 to 1 020 Ma). The fact that especially the immobile trace elements (e.g. Nb, Ta) in all metasedimentary rocks mirror the chemical characteristics of the metavolcanic rocks indicates local amphibolite detritus as a main source of the sediments. We conclude that the metamorphic volcano-sedimentary sequence of the Koeris Formation is not, as previously assumed, the product of a large back-arc basin but was deposited in response to tectonic uplift in a small-scale intra- or inter-montane basin at the very end of the Okiepian orogeny. The thick amphibolite layers concentrated within this topographic depression may have played a crucial role in the conservation of the underlying ore. Therefore, the occurrence of the Koeris Formation may indicate favorable locations for further exploration.

Provenance, tectonic setting, paleoweathering and paleoclimatic conditions of early to mid-Eocene sandstones of the Dalbuing Formation, Arunachal Pradesh, NE India: Inferences from petrography and geochemistry

Petrography and geochemistry of sandstones of the Dalbuing Formation of Eastern Himalaya have been investigated to unveil their provenance, tectonic setting, paleoclimate and source rock weathering. This study suggests that the sandstones are wacke type and their provenance is quartzose recycled to transitional recycled orogenic provenance. The chondrite-normalized Rare Earth Element (REE) patterns of the sandstones show an enrichment of LREE, flat HREE patterns and negative Eu anomalies similar to those exhibited by Upper Continental Crust (UCC) and Post Archean Australian Shale (PAAS). This is an indicative of felsic igneous source for the sandstones and A–CN–K plot suggest their intermediate source. The study of paleoweathering conditions based on as Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), Chemical Index of Weathering (CIW) and A–CN–K (A = Al2O3, CNCaO + Na2O and KK2O) plot indicate moderate to intense chemical weathering in the source area. Tectonic setting discrimination study based on geochemical data of these sandstones suggests both the passive margin (PM) and the active continental margin (ACM) tectonic settings of depositional condition. Paleoclimatic condition of the basin as suggested by the quartz-feldspar-lithic fragment (QFL) plot of petrographic study was arid to humid and by the bivariant log/log plot of (Qp/F + RF) versus (Qt/F + RF) was semi-arid to semi-humid. However, the bivariant SiO2 versus Al2O3 + K2O + Na2O plot suggest arid paleoclimatic condition with increasing trend of chemical maturity during the entire process of sedimentation. The C-value plots as well as Sr/Cu ratio suggests humid climatic condition specifically during the time of deposition of the sediments.

Geochemical Characteristics and Constraints on Provenance, Tectonic Setting, and Paleoweathering of Middle Jurassic Zhiluo Formation Sandstones in the Northwest Ordos Basin, North-Central China

To further explore the uranium-bearing prospects of the Zhiluo Formation, the petrography, major elements, trace elements and rare earth elements of Zhiluo sandstone samples collected from four boreholes were analyzed in this research to determine the provenance, tectonic setting and paleoweathering of the formation. The results of the analysis reveal that the Zhiluo Formation sandstone comprises primarily feldspar sandstone, with quartz, feldspar, and mica as the main mineral components. The rare earth elements are mainly characterized by enrichment in light rare earth elements and loss of heavy rare earth elements. The ratio of light to heavy rare earth elements (LREEs/HREEs) is 5.55–7.79, with an average of 6.33. The value of (La/Yb)CN is 12.96–22.33, with an average value of 17.41, indicating obvious fractionation of LREEs and HREEs. The chemical index of alteration (CIA) value of this sandstone is 56.30–63.04, with an average of 59.75, which indicates that the parent rock experienced weak chemical weathering in a dry climate. The discrimination diagrams of the source area and parent rock show that the source area of the Zhiluo sandstones had a mixed intermediate-felsic composition, and that the main parent rocks were andesite and granite. The tectonic setting discrimination diagram indicates that the tectonic setting of the source area was the passive margin. Thus, the provenance of the sandstone of the Zhiluo Formation is proposed to be the volcanic-sedimentary rock series developed on the northern margin of the Ordos Basin.

Chromite Composition and Platinum-Group Element Distribution in the Proterozoic Chimalpahad Anorthosite Complex, South India: Implications for Magmatic Processes and Discrimination of Tectonic Setting

Abstract The Proterozoic Chimalpahad Anorthosite Complex (CAC) is a deformed and dismembered complex in South India comprising of anorthosite-leucogabbro-gabbro-ultramafic rocks. It is located in the Nellore-Khammam Schist Belt (NKSB) within the contact zone between the Eastern Dharwar Craton (EDC) and the Eastern Ghats Belt (EGB). The chromitites occur as small dismembered lensoidal bodies within the ultramafic rocks. Presence of micrometric inclusions of iridium-like platinum-group minerals (IPGM), such as laurite (RuS2), erlichmanite (OsS2), irarsite (IrAsS), and Ir-Os-Ru alloys, is reported for the first time in the CAC chromites. The chromite cumulates preserve magmatic character (Cr#: 0.61 to 0.77; Mg#: 0.29 to 0.55), while disseminated chromites and rims of some grains (Cr#: 0.59 to 0.89; Mg#: 0.10 to 0.25) show postmagmatic modifications. The chromites are characterised by higher Cr and Al contents and lower Fe and Ti contents. The mineral chemistry of chromites such as Cr# and Mg# contents of Al2O3 (10.05–20.07 wt.%) and TiO2 (0.1 to 0.25 wt.%) typically suggests that the CAC chromitites and their ultramafic hosts were generated from the S-undersaturated boninite melt in a magmatic arc in suprasubduction tectonic setting. A general negative slope of bulk-rock PGE distribution with upward convex pattern is observed for the CAC chromitites in the chondrite-normalized plot. These geochemical and mineral chemical characters in addition to location of the chromitites and the CAC in the tectonic zone between Eastern Dharwar Craton and Eastern Ghats Mobile Belt point towards ophiolitic affinity of the CAC chromitites. Laurite (RuS2) grains are trapped close to the cores of chromites, whereas erlichmanites (OsS2) are trapped away from the cores. Textural relations indicate primary orthomagmatic nature of laurite and Or-Ir-Ru alloys and their precipitation prior to or concomitant with chromite from an S-undersaturated melt at ~1200–1300°C and log fS2 varying between −2 and −1.3. Ru-Os disulfide and erlichmanite crystallized subsequently with a gradual decrease in temperature in the differentiating ultramafic magma. Precipitation of irarsite and base-metal sulfides can be attributed to local and rapid fluctuations of temperature, fS2, and fAs during the formation of the chromitites due to influx of fresh batch of melts or crystallization in an open magmatic system. The compositional variations of laurite-erlichmanite record heterogeneous physicochemical environment during their crystallization. In addition to zoning, preservation of IPGMs in the CAC chromites points towards fast cooling of the magma possibly associated with rapid exhumation of the chromitites and their ultramafic hosts from the roots of the subcontinental lithospheric mantle (SCLM) towards their final emplacement into the crust. Occurrence of the CAC along a linear extension with the Kondapalli Ultramafic Complex and deformed remnants of ophiolites (Kandra and Kanigiri complexes) along the eastern margin of the Eastern Dharwar Craton (EDC) bordering the Eastern Ghats Belt (EGB) indicates this zone to be a Proterozoic convergent margin.

Characteristics, Source Area-weathering, Sedimentary Processes, Tectonic Setting and Taxonomy of Vertisols Developed on Alluvial Sediments in the Benue Trough of North Cameroon

Although Vertisols have been highly documented owing to technological breakthroughs, numerous aspects are still not fully understood such as implication of geochemistry on source area-weathering, provenance, tectonic setting and sedimentary processes as well as problems of classification and management. This work aims to highlight the geochemical characteristics of Vertisols formed on alluvial deposits in the Benue Basin of North Cameroon and to highlight their source area-weathering, sedimentary processes, tectonic setting, taxonomic level and possible management strategies. The work was done in the field and in the Laboratory. The main results showed that smectite is the predominant clay mineral. Chemical composition revealed high Si, Al and Fe contents. Heavy minerals contents are of plutonic (augite, aegerine and aegerinic augerinic augite), metamorphic (kyanite, sillimanite and andalusite) and volcanic (tourmaline) origin. This agrees with the Al2O3/TiO2 ratio of 8.72 to 30.40 reflecting sediments from felsic rocks and minor mafic rocks. The CIA (chemical index of alteration: 70 to 85), PIA (plagioclase index of alteration: 66 to 82) and Ruxton’s index (SiO2/Al2O3: 2.27 to 3.55) suggest a warm and humid climate during moderate to intense chemical weathering probably prevailing during a more humid pre-depositional period. The K2O/Na2O ratio <1 suggests high sediments chemical maturity. The predominance of angular quartz grains suggests short fluvial transport distance and low sorting. The tectonic setting discrimination ternary diagram indicates that alluvial sediments, parent material of Vertisols, originate from an Active Continental Margin while the discriminant function-based multidimensional tectonic diagram indicates an arc-collisional setting suggesting that parent materials are from the Pan-African basement of the Central African Fold Belt. The Vertisols are classified as Ustic Haplusterts Clayey Isohyperthermic (United States Department of Agriculture) and as Gleyic Stagnic Vertisols (Pellic, Hypereutric, Clayic) (World Reference Base for Soil Classification).

Petrography and geochemistry of the Middle Jurassic Fort Member Sandstone, Jaisalmer Formation, Western India: Implications for weathering, provenance, and tectonic setting

Detrital mineralogy, major oxides, trace, and rare earth elements (REE) were examined in the Middle Jurassic Fort Member Sandstone (FMS) of the Jaisalmer Formation in Western India to understand palaeoweathering, provenance of sediments, and to decipher the tectonic setting. The sandstones are classified as arkose to sub‐litharenite types according to their relative percentages of quartz, feldspar, and lithic fragments, and major elemental ratios. The values of Chemical Index of Alteration (67–85), Plagioclase Index of Alteration (78–97) and Chemical Index of Weathering (83–97) along with A‐CN‐K diagram suggest moderate to intense chemical weathering in the source region. Presence of fresh feldspars and dominance of monocrystalline quartz, showing both parallel and undulose extinction, indicate plutonic and volcanic felsic igneous provenance. Contribution from metamorphic orogen sources is indicated by the lithic fragment population. Various geochemical parameters such as Th/Sc, Cr/Th, Ti/Zr, and Eu/Eu* support a dominant felsic provenance. Higher Zr/Sc ratios relative to the magmatic Zr/Sc‐Th/Sc trend, higher Zr and Hf concentrations in the FMS indicate some extent of sedimentary recycling and zircon sorting. LREE enrichment, flat HREE pattern, negative Eu anomaly and trace element ratios, in conjunction with palaeocurrent data, indicate a likely sediment contribution from basement source terrains including Erinpura‐Mount Abu‐Sirohi granites, Malani Igneous Suite, and Aravalli‐Delhi orogens. The major and trace element tectonic setting discrimination diagrams for the FMS indicate a passive margin setting.

Geochemistry of Upper Eocene-Oligocene sandstones from Tuzgölü Basin (Central Anatolia)

In this study, mineralogical and geochemical features of Upper Eocene-Oligocene sandstones exposed by the Tuzgölü Fault Zone (TFZ) at the eastern border of the Tuzgölü Basin were investigated. The absence of zircon enrichment in the Zr/Sc-Th/Sc diagram indicated no sedimentary recycling. This shows that the sandstones are first cycle sediments, that is, the material transported directly from the source. Critical element ratios for provenance such as La/Sc, La/Co, Th/Sc, Th/Co, Th/Cr, Zr/Sc, Zr/Co, Ba/Sc, and Ba/Co, Th/Sc-Eu/Eu* diagram and average Rare Earth Element (REE) pattern suggest a provenance in “intermediate magmatic” composition. The variation in the negative Ce anomaly effect observed between the lower and upper parts of the sequence indicates variation in the oxygen level of the water. In each of the La-Th-Sc, Th-Co-Zr/10, and Th-Sc-Zr/10 tectonic setting discrimination diagrams, the sandstone average fell onto the “Continental Island Arc” position. This tectonic setting defines the arc that developed along the continental margin of the subduction zone. The tectonic setting found for the basin, supported the evolution model that the Tuzgölü Basin developed as a fore-arc basin.

New geochemical constraints on the development of active continental margin in Southeast China during the Middle Permian and its tectonic implications

Two competing hypotheses that passive or active continental margins developed in South China during the Late Paleozoic are still under debate. Previous studies were mainly focused on the remnant magmatic rocks, but little attention has been paid to the contemporary sediments deposited in the back-arc basin. Therefore, a deeper knowledge of the tectonic link between the continental arc and regional basins is not only vital for understanding their tectono-sedimentary evolution, but also provide the potential to evaluate these hypotheses. Here two Middle Permian sections of bedded chert-mudstone sequence (Gufeng Formation) and an overlying siliciclastic succession (Yinping Formation) from the Lower Yangtze region, were analyzed to understand their provenance, tectonic and depositional setting, and to provide new constraints on the Late Paleozoic tectonics of the South China. Discrimination of tectonic setting indicates that the Middle Permian sedimental source rocks of the Lower Yangtze basin probably originated in a transitional setting from continental island arc to active continental margin, consistent with the development of an active margin magmatic arc along Southeast China during that time. Provenance analysis also showed that, the Gufeng Formation was mainly derived from an Andesitic arc source with mixed mafic and felsic source, whereas the Yinping Formation received detritus from a more felsic source. This provenance switching could reflect a possible tectonic transition related to the continental arcs’ evolution that from Andean-type normal/high-angle subduction to flat-slab subduction, which weakened the arc volcanism and intensified the bedrock uplift in South China. The depositional environments of the Lower Yangtze basin also evolved from an open deep ocean environment to a continental margin setting during the deposition of Gufeng Formation, and then transformed into a coastal/neritic shallow water setting during the deposition of Yinping Formation. This sedimentary evolution can be attributed to the tectonic transition during the Middle Permian.

Geochemistry of the mudrocks and sandstones from the Bredasdorp Basin, offshore South Africa: Implications for tectonic provenance and paleoweathering

Abstract An inorganic geochemical investigation of mudrocks and sandstone from the southern Bredasdorp Basin, off the south coast of South Africa was carried out to unravel the provenance, paleoweathering, and tectonic setting of the basin. Seventy-seven representative samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 underwent geochemical analysis involving major and trace elements. The major oxide compositions show that the sandstones could be classified as sub-arkose and sub-lithic arenite. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the Bredasdorp sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr, and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th∗10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Also, the closely similar compositions of the analysed samples and recent sedimentary rocks of the East African Rift System perhaps suggest a rifted basin tectonic setting for the Bredasdorp Basin. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations.

Th, Nb and Zr characteristics and plume causes identification of Emeishan basalts

Having compared with the characteristics of world typical mantle plume basalts and analyzed Th, Nb and Zr characteristics of Emeishan basalt, this paper points out that although there were some differences about the formation environment of the three rock areas (WEST, CENTRE and EAST rock area), main rocks of them shared common geochemistry characteristics of ocean island basalts, implying that rocks came from metasomatic enriched mantle. Further studies showed that Emeishan basalt is generated from the mantle plume, but the crustal hybridism, the fractional crystallization of basaltic magma and different magma source regions lead to the fact that rock is generally rick in Th, Th/Nb ratio. In the tectonic setting discrimination diagram, their samples mostly fall into the tholeiite area of intracontinental rift and epicontinental rift, as opposed to the mantle plume area the samples of typical basalt of the world would fall into. The paper also indicates that the data collection must have certain criteria, this means that the samples must have low crustal contamination, little magma fractional crystallization, namely as representative of the rock of forming original magma as possible when using Th/Hf - Ta/Hf double logarithmic diagram (proposed by Wang et al. (2001)) and discrimination diagrams of basalt tectonic settings (proposed by Sun (2003)) to discriminate the tectonic setting of magmatic rock formation.

Geochemical (REE and Trace elements) characteristics and tectonic setting discrimination of Permo-Carboniferous sandstone from Sikkim Lesser Himalaya, NE India

Geochemical (REE and Trace elements) characteristics and tectonic setting discrimination of Permo-Carboniferous sandstone from Sikkim Lesser Himalaya, NE India

Petrology and zircon U-Pb geochronology of mafic - intermediate dykes in the West-Central Taurides: Implications for magma source during the Late Precambrian – Early Palaeozoic

ABSTRACT Cadomian magmatic activity is represented by a mafic – intermediate dyke suits in the Karacahisar dome (West – Central Taurides) formed by the southward subduction of the Proto-Tethyan ocean beneath North Gondwana. These dyke suites are classified as diabase, andesite and dacite. They display calc-alkali, high-K calc-alkali and shoshonitic character, and intruded into the siliciclastic rocks deposited in a back-arc rift setting. In this study, we present new zircon U-Pb ages, Sr-Nd isotopic and geochemical data to better understand the petrological evolution of the Cadomian magmatism. Zircon U-Pb concordia ages of 551.3 ± 3.5 Ma, 557.9 ± 3.7 Ma and 569 ± 7.2 Ma were obtained for these dykes. On the N-MORB normalized spider diagrams, mafic – intermediate dykes exhibit enrichment in large ion lithophile elements (LILE: Sr, K, Rb, Ba, Th), relatively less enrichment in light rare earth elements (LREE: La, Ce and Nd) and depletion in high field strength (HFS: Tb, Ti, Y and Yb) elements. Dyke samples have significant depletions of Ta and Nb relative to neighbouring LILE and LREE and display a significant subduction component. On the chondrite normalized REE diagrams, a weak Eu anomalies are observed, and this indicates some plagioclase accumulation. Dyke samples fall into the volcanic arc (island arc and/or continental arc), and back-arc fields on tectonic setting discrimination diagrams. The εNd(i) values of dykes range from −1.47 to 1.76, and Nd depleted-mantle model (TDM) ages vary between 1.32 and 1.81 Ga, and show enriched mantle or metasomatized subcontinental lithospheric mantle source signatures. The primary magmas of the dykes were derived from amphibole- and garnet-bearing mantle, and were affected by fractional crystallization, mixing and assimilation processes during ascent through the upper crust. All available data show that this magmatic activity developed in a back-arc rift setting subsequent to Cadomian arc magmatism in the West-Central Taurides.

Geochemical and Petrographical Characteristics of the Madzaringwe Formation Coal, Mudrocks and Sandstones in the Vele Colliery, Limpopo Province, South Africa: Implications for Tectonic Setting, Provenance and Paleoweathering

The sedimentary rocks of the Madzaringwe Formation in the Tuli Basin have been investigated using geochemical and petrographic methods to reveal their source area composition, tectonic setting, provenance and paleoweathering conditions. The petrographic studies show that the rocks consist mostly of clay minerals and quartz. The major elements geochemistry indicates that the rocks of the Madzaringwe Formation have the same source area. Based on the discriminant function plots, it can be inferred that the rocks are of quartzose sedimentary provenance, suggesting that they were derived from a cratonic interior or recycled orogen. The binary plots of TiO2 versus Zr and La/Sr against Th/Co shows that the rocks were derived from silicic or felsic igneous rocks. The tectonic setting discrimination diagrams of SiO2 against Log (K2O/Na2O), Th–Sc–Zr/10, and TiO2 versus (Fe2O3 + MgO) support passive-active continental margin settings of the provenance. The A–CN–K (Al2O3–CaO + Na2O–K2O) ternary diagram and binary plot of the index of compositional variability (ICV) against chemical index of alteration (CIA) shows that the rocks have been subjected to moderate to intensive weathering. Geochemical and petrographic characteristics of the rocks point to uplifted basement source areas predominantly composed of sedimentary rocks and/or granite-gneiss rocks. These source areas might have been from adjacent areas near the Tuli coalfield which include the Limpopo Belt (igneous and sedimentary rocks), and basement uplifted rocks of the Beit-Bridge Complex, consisting of the granite, granite-gneisses and schists.

Geochemistry and provenance of the metasedimentary rocks surrounding the Santa Quitéria magmatic arc, NE Brazil: Tectonic and paleogeographic implications for the assembly of West Gondwana

Western Gondwana resulted from the closure of the Pharusian-Goianides Ocean, a process that culminated in the formation of magmatic arcs, high and ultra-high-pressure (HP-UHP) belts, and sedimentary basins. These tectonic units occur in the NW part of the Borborema Province (Northeastern Brazil), which encloses the Neoproterozoic Santa Quitéria magmatic arc (SQMA). Metasedimentary rocks (sillimanite-, garnet- and biotite-bearing gneiss, quartzite, and schist) that constitute the Canindé Complex and the Independência Unit surround the western and eastern margins of this arc. According to A/CNK and N/K ratios and the Chemical Index of Alteration calculated for each rock type, the protolith of these rocks correspond to moderately weathered pelites, greywacke, and lithic-arenite. Trace element distributions (LILE, HFSE and REE) indicate that the Canindé Complex rocks are depleted in Sc, V, Cr, Co, Ni and LILE and are formed by mature and stable minerals such as quartz, muscovite and K-feldspar, similarly to passive margin sediments. In contrast, the Independência rocks present higher concentrations of transition trace elements, constituted by unstable-system minerals (iron-magnesium minerals) and immature sediments, typical of active margin settings. In general, these rocks present felsic-source components, with ample fractionation between LREE over HREE (LaN/YbN = 26.2 to 4.16) and with a slight to strong negative Eu anomaly (Eu/Eu* = 1.26 to 0.33). Isotopic Hf data identified a mixture of crustal- and mantle-derived sources. According to tectonic setting discrimination diagrams (Co-Th-Zr/10 and Sc-Th-Zr/10) and the concentrations of characteristic trace elements and ratios (e.g., Th, V, Co, La, Ce, LaN/YbN), these rocks were interpreted as being deposited in basins formed in active and passive margin settings associated with continental and intraoceanic arcs. U-Pb zircon data indicates source rocks from the Archean (3.0 to 2.6 Ga) to the Neoproterozoic (820 to 541 Ma). According to our results, the Canindé and Independência supracrustal rocks were deposited in basins affected by different tectonic regimes, controlled by the installation of the Santa Quitéria Magmatic Arc (SQMA), starting with a passive margin or intracratonic basins that evolved into an active margin setting on the western SQMA margin (Canindé Complex) and a predominantly passive margin setting on the eastern SQMA margin (Independência Unit), leading to the formation of forearc and backarc basins.