Archean Granite Research Articles

Seismic Imaging of Shallow Depths using Ambient Noise Tomography in the Hyderabad Region, Eastern Dharwar Craton, India

ABSTRACT Hyderabad and its surrounding region are situated within Eastern Dharwar Craton, primarily composed of Archean granite. We use ambient noise tomography to obtain the 3-D shallow subsurface shear seismic velocity structure of 150 x 200 km2 area around Hyderabad. We used continuous waveforms that were recorded by a digital seismic network with 10 three-component broadband seismographs from November 2020 to December 2021. Using the vertical component of waveforms, the Empirical Green’s functions (EGFs) have been estimated, and the group velocity dispersion curves in the time period range of 1.0–10 s have been extracted from these. We inverted these dispersion curves to estimate 3-D variation of shear wave velocity by using direct surface wave tomographic method. The results suggest that the subsurface velocity structure is heterogeneous. The shear wave velocity varies from 3.4 to 3.7 km/s in the shallow subsurface. The limited seismicity in this area is mostly confined to the boundary between high and low-velocity changes.

Regional-Scale Paleoproterozoic Heating Event on Archean Acasta Gneisses in Slave Province, Canada: Insights from K–Ar and 40Ar/39Ar Chronology

Slave Province in Canada is an Archean granite–supracrustal terrane at the northwestern corner of the Canadian Shield. It is bordered by the Thelon–Taltson orogen (2.0 to 1.9 Ga) to the southeast and the Wopmay orogen (1.9 to 1.8 Ga) to the west. Acasta gneisses, exposed in the westernmost Slave Province, and the Wopmay rocks, located close to the gneisses, were systematically collected for K–Ar and laser step-heating 40Ar/39Ar single-crystal analyses of the biotite and amphibole. The K–Ar biotite ages of the four Wopmay samples range from 1816 ± 18 Ma to 1854 ± 26 Ma. The 40Ar/39Ar biotite analyses of the three Wopmay samples yield plateau ages of 1826 ± 21 Ma, 1886 ± 13 Ma, and 1870 ± 18 Ma. These ages fall within the reported U–Pb zircon age range of the Wopmay orogen. The K–Ar biotite ages of the fifteen Acasta gneisses range from 1779 ± 25 Ma to 1877 ± 26 Ma, except for one younger sample (1711 ± 25 Ma). The 40Ar/39Ar analyses of the biotite crystals from three samples give the plateau ages of 1877 ± 8 Ma, 1935 ± 14 Ma, and 1951 ± 11 Ma. The K–Ar amphibole ages from twelve samples range from 1949 ± 19 Ma to 1685 ± 25 Ma. Two samples of them give ages older than the zircon U-Pb age of Hepburn plutons. The 40Ar/39Ar analyses of the amphibole crystals show varied age relations. The two samples give plateau ages of 1814 ± 22 Ma and 1964 ± 12 Ma. Some samples exhibit apparent old ages of ~2000 Ma in the middle temperature fractions. These old fractions result from the amphibole crystals, originally formed in the Archean, being affected by the thermal events during the Wopmay orogeny but not fully resetting. These observations suggest that the K–Ar system ages of the biotite and amphibole in the Archean Acasta gneiss were rejuvenated during the Paleoproterozoic ages. The Discussion explores the possibility that the heat source rejuvenating the K–Ar system ages may have arisen due to asthenospheric extrusion into the wedge mantle, a process likely triggered by subduction rollback.

Geochronology and petrogenesis of granitoids from the Bas Draa inlier (Western Anti-Atlas, Morocco): Revived debate on the tectonic regime operating during early Paleoproterozoic at the NW edge of the West African Craton

Proterozoic terrains of the Anti-Atlas belt (Morocco) represent one of the key areas for studying the Precambrian geodynamics of the northwestern West African craton (WAC). Voluminous Paleoproterozoic granitoids, which outcrop in the basement of the Precambrian inliers of the southern Anti-Atlas, are conventionally regarded as vestiges of a magmatic arc constructed during incipient subduction on Earth. However, except for their calc-alkaline, arc-like signatures, little evidence supports the existence of this arc system and the sources and tectonic context of these granitoids remain elusive. In this study, we present new whole-rock major- and trace-element, and Sr-Nd isotope composition, and in situ zircon U-Pb ages of the composite granitoid intrusions from the Bas Draa inlier. We obtained Rhyacian ages for the quartz-diorite (~2059 ± 6 Ma) and leucogranite (~2051 ± 6 Ma), and Orosirian age for the biotite-bearing granite (~2022 ± 7 Ma). The studied magmatic rocks are classified into three groups according to their petrographic features and chemical composition: a) layered, sanukitoid quartz-diorites, b) biotite-muscovite (i.e., two-mica) leucogranites, and c) biotite-bearing, hybrid granites. All three groups of magmatic rocks share a common geochemical composition with late Archean granites worldwide, including a high Mg# (52.9–64), and Ni (average 40 ppm), Cr (10–250 ppm) and V (139-238 ppm) content. The identification of mantle-derived sanukitoids and hybrid granites, combined with the absence of first-order evidence for subduction, obduction, and collision, as commonly observed in Neoproterozoic and Phanerozoic orogens worldwide (i.e., blueschist facies metamorphics, ophiolites, and accretionary prism units), contests the theory for the onset of plate tectonics during Early Paleoproterozoic in the Anti-Atlas. We instead propose that the available data rather suggests that vertical tectonics, characteristic of the Archean, still prevailed in this domain throughout the Early Proterozoic times.

Rockmass Characteristics and its Influence on TBM Penetration Rate in Archeans and Meta-sedimentary Rocks of India

TBM penetration rate prediction models were developed for the hard Archean granites, granite gneisses and Meta-sedimentary rock groups comprising of quartzites, quartz arenites and sandstone of peninsular India.Construction stage engineering geological mapping comprising of face logging resulted in rockmass characterisation of Archeans and Meta-sedimentary rock groups. Statistical correlation of TBM penetration rate (PR) with Rock quality designation (RQD), unconfined compressive strength (UCS), volumetric joint count (Jv) and rockmass class was carried out. The prediction model provides good correlation only in the Meta sedimentary rock group for RQD, UCS and rockmass class and good correlation only for RQD and rockmass class in the Archean group but poor correlation with UCS. There is an exponential increase of penetration rate with Jv in both the rock groups. Keywords: TBM Penetration Rate, Rockmass Characteristics, Geological Mapping, Rockmass Classification, Rock Tunnel

Chemical Weathering Indices and their Use for Paleoclimatic Reconstructions (on the Example of the Vendian‒Lower Cambrian Section of Podolsk Transnistria)

The article provides an overview of various indices/indicators (Vogt, Parker, CIA, CIW, PIA, MIA, etc.) used in the study of weathering profiles/crusts and reconstruction of paleoclimatic environments of sedimentary sequence accumulation. Their possibilities are demonstrated by the example of Vendian-Lower Cambrian terrigenous deposits of Podolsk Transnistria (southwestern slope of the Ukrainian Shield). The distribution of the ba1 index values in this section indicates the presence of the material most transformed by the processes of chemical weathering in the mudstones of the Nagoryany Formation, the lower part of the Danylivka and the middle part of the Studenytsya formations. For mudstones of the Danylivka–Zbruch interval, the HM values are close to the HMPAAS. For rocks of the Yaryshiv–Nagoryany interval, the HM values are slightly higher, and for mudstones in the lower part of the Yaryshiv Formation, they are comparable to those inherent in continental clays of a hot tropical climate. The average SA index for mudstones of the entire section is 5.6 ± ± 0.7. Mudstones of the Hrushka–Nagoryany interval, where SA SAPAAS, are composed of more weathered material. The WIP values in the mudstones of the Mohyliv and Yaryshiv formations, as well as in the upper part of the Zbruch Formation, correspond to the interval of their values between PAAS and the average Archean granite. Clay rocks of other formations have WIP ≤ WIPPAAS values. The average CIA value for mudstones is 71 ± 4, which practically corresponds to the CIA value (70), which separates the sediments of cold/arid and warm/humid climates. Variations in the CIW index along the section are symbate with variations in the CIA. The vast majority of mudstones are characterized by PIA PIAPAAS values. The average CPA value is 91 ± 4, which is also typical for PAAS. The above and other data show that on the basis of a “direct” interpretation of the values of various indices of chemical weathering inherent in fine-grained clastic rocks, the paleoclimate that existed in the Vendian‒Early Cambrian on the territory of Podolsk Transnistria was rather moderate or warm humid. Comparison of the CIA values of mudstones with the values of this indicator for the particulate suspended matter of modern rivers suggests that the climate in the Vendian‒Early Cambrian resembled dry and humid subtropical or dry tropical with elements of humid.

Dynamics of Early Neoproterozoic accretion, west-central India: I. Geochronology and Geochemistry

The Godhra-Chhota Udepur (GC) sector (west-central India) is the zone of convergence between two crustal-scale accretion zones, i.e. the N/NNE-striking Aravalli Delhi Fold Belt (ADFB) against the E-striking Central Indian Tectonic Zone (CITZ). In this study, we demonstrate the two orogens welded during the Early Neoproterozoic. In the GC sector, recumbently folded basement gneisses, shallow-dipping granitoid mylonites and a suite of allochthonous supracrustal rocks experienced top-to-the south thrusting and nappe formation (D2 deformation). The shallow-dipping crustal domain was modified by the superposition of a network of WNW/W-striking transpressional shear zones and related folds (D3 deformation). The D2-D3 deformations occurred due to oblique crustal convergence marked by the emplacement of pre-D2, post-D2 and syn-D3 granitoids. The post-D2, syn-D3 granite-granodiorites are weakly peraluminous, calc-alkalic, having ferroan to magnesian affinity, and characterised by LREE-enriched moderately fractionated REE patterns with variable negative Eu anomalies. Trace element geochemistry and whole rock Sr-Nd systematics suggest the granitoids were derived from dominantly meta-greywacke precursors. In South-GC, U-Pb zircons in the basement gneisses yield upper intercept/Concordia dates at 1.65–1.60 Ga for high-grade metamorphism. The lower intercept/Concordia dates in the gneisses coincide with the 0.95–0.93 Ga emplacement age of post-D2/syn-D3 granitoids throughout GC. However, the pre-D2 granitoids in South-GC are older, ~1.03 Ga. By contrast, pre-D2 granitoids in North-GC yield Late Neoarchean (2.5 Ga) Concordia/upper intercept emplacement ages, identical to the emplacement age of the Archean granites in the ADFB; the lower intercept/Concordia age is 0.95–0.93 Ga. The 2.5 Ga granites did not experience the 1.65–1.60 Ga high-grade metamorphism, but both the lithodemic units shared the 0.95–0.93 Ga D2-D3 deformation-metamorphism and the emplacement of post-D2 and syn-D3 0.95–0.93 Ga granitoids. The juxtaposition of the ~2.5 Ga granitoids and the 1.65–1.60 Ga gneisses is attributed to the 1.03–0.93 Ga ADFB-CITZ oblique accretion that involved contemporaneous emplacement of syn-collisional S-type granitoids.. • Godhra-Chhota Udepur (GC) is the western end of the Central Indian Tectonic Zone . • At GC, NNE-SSW shortening is contemporaneous with 0.95–0.93 Ga felsic emplacements . • In north GC, 2.5 Ga granite reworked at 0.95 Ga with no Paleoproterozoic overprint . • In south GC, no Archean inheritance noted in 1.6 Ga gneisses reworked at 0.95 Ga . • ~0.95 Ga accretion welded mutually exclusive Archean and Paleoproterozoic terrains .

The diversity of sources of late Archean granites reflects a transition from plume-dominated to plate tectonics in the Superior Province, Canada

The Superior Province in Canada represents an excellent natural laboratory to test various models of crustal growth and evaluate tectonic regimes during the late Archean. Here, we present new geochemical and geochronological data for ca. 2.8–2.7 Ga plutons intruding different lithotectonic units of the north-eastern part of the Superior Province that demonstrate a complex nature and evolution of magma sources in relation to a changing tectonic setting. Four distinct plutonic suites were newly identified: (1) sodic tonalite–diorite (TD) with a composition resembling low-pressure TTG-like melts, (2) sodic tonalite–granodiorite–diorite (TGD) with medium/high-pressure TTG-like signatures, (3) Mg–K-rich monzogranite to monzodiorite (sanukitoids; MMD), and (4) K-rich granodiorite–granite–monzogranite (GGM). These suites are interpreted as recording a temporal evolution from plume-assisted melting of lower mafic continental crust through melting of a subducted oceanic slab at different depths to large-scale re-melting of the previously formed and amalgamated crustal units. In this evolutionary scheme, the ∼2730–2700 Ma subduction-related plutonism is the most voluminous addition of the juvenile arc material and thus represents the most significant crustal growth event. Altogether, the above inferences and published data suggest that the late Archean plutonism in the Superior Province formed during a transitional regime from plume-dominated to plate-tectonic over a short time span between ∼2730 and ∼2700 Ma and that this transition was markedly diachronous across the Province.

Petrography of the Mesozoic Alkaline Rocks of the Taezhniy Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

The article is concerned with the petrographie compositions of the Mesozoic alkaline igneous rocks of the Taezhniy massif and its small bodies (dikes). The Taezhniy massif is located in the central part of the Nimnyr block (Aldan-Stanovoy shield). According to our field observations, it was first determined that the Taezhniy massif has a two-phase structure, its rocks of contain syenite-porphyry and leucocratic syenites. The rocks of the massif intrude the Archean granites, and are themselves intruded by late dikes of bostonites and vogesites. Based on petrographic studies, we also identified two phases of intrusion in the Taezhniy massif – syenite-porphyry and leucocratic syenites. The difference of them is as follows: in the syenite-porphyry rocks, plagioclase predominates over K-feldspar, and the content of dark-colored minerals reaches 10%. The rocks of the second phase are characterized by the predominance of K-feldspar over plagioclase, with the content of dark-colored minerals up to 5%. The order of intrusion of the massif formations is determined by the presence of xenoliths of syenite-porphyry rocks in leucocratic syenites. The rocks of the bostonite dike cutting the massif are characterized by the absence of amphibole and an increased content of pyroxene. This is most likely due to the fact that, the rock contains xenoliths of the host rocks of biotite-pyroxene composition. The dikes also conyain xenoliths of the second phase rocks, which indicates a later age of this dike. The bostonite dike located in the immediate vicinity of the massif is almost identical to the dike found in the massif, except an amphibole in the composition of the rocks. The only dike of vogesites that intersects the body of the massif is characterized by the presence of two varieties of amphibole: common hornblende and a sufficient amount of barkevikite, which phenocrysts stand out clearly against the background of the total microcrystalline mass. Also, in the course of crystal-optical studies, it was noted that in the structure of the massif there is an increase in the leucocratic magmatism from the early phase to the late, but with a decrease in this indicator, taking into account the introduction of vogesite dikes at the late stages of magmatism development. Based on this, the conclusion is made about the antidromic development of the rocks of the Taezhniy massif. When comparing the evolution of magmatism of the Taezhniy massif with the Ryabinoviy massif, a possible gold-ore specialization of the studied object is suggested.

Archean to Paleoproterozoic evolution of the Crixás greenstone belt, Central Brazil: Insights from two contrasting assemblies of metaigneous rocks

The Archean-Paleoproterozoic Crixás greenstone belt is one of the most important components of the Crixás-Goiás Domain, the oldest crustal segment of the Central Brazil Goiás Massif. This gold-bearing greenstone comprises amphibolite facies ultramafic-mafic metavolcanic rocks overlaid by a low-grade metasedimentary succession associated with rocks of intermediate composition. We present new integrated geochemical and isotopic data of the metaigneous rocks to characterize the stages of magmatism and delineate the petrogenetic processes and tectonic environment of the different units. The data give important insights into the geological evolution of the Crixás greenstone belt during the Archean and Paleoproterozoic, including that the Crixás greenstone belt consists of genetically and temporal unrelated rocks that are spatially associated in a structural framework marked by stacking and imbrication. Two lithological assemblies were described: i) a Mesoarchean komatiite-tholeiite association formed by heterogeneous mantle plume melting in an oceanic plateau setting, and ii) a Rhyacian intraoceanic arc association. These units are allochthonous and override an Archean crustal basement dominated by granites and gneisses of mostly TTG composition. Chemical composition of the komatiites is similar to the Barberton-type Al-depleted komatiites, whilst the basalts consist of high-Mg tholeiites with flat chondrite-normalized REE patterns and immobile trace element systematics that resemble Phanerozoic oceanic plateau lavas. Eruption age of the ultramafic and mafic lavas is roughly estimated at ~3.0 Ga, based on previously published SmNd and PbPb isotopic studies, whereas whole-rock ReOs isotopic data of samples of komatiite and basalt presented here yielded a 2609 ± 65 Ma isochron age. This younger age may reflect a Neoarchean overprinting event associated with metasomatic Re-addition recorded in the lower ultramafic-mafic metavolcanic rocks, possibly related to the late stages of the cratonization of the Crixás-Goiás Domain during the emplacement of the youngest Archean granites of the region (~2.70 Ga). Newly described andesites and diorites possess a subduction zone fingerprint. LA-ICP-MS UPb zircon dating of an andesite sample provided an upper intercept Concordia age of 2172.2 ± 12.7 Ma, constraining the age of the intermediate magmatism. Therefore, these rocks are correlated with the Rhyacian syn-orogenic poorly-sorted graywackes and carbonaceous phyllites of the greenstone's upper stratigraphy. Andesites and diorites have homogeneous Paleoproterozoic two-stage Nd model ages of 2.28 to 2.26 Ga and positive initial εNd values of +2.34 to +2.69, indicating a juvenile nature for the arc magmatism. ReOs geochronology of gold-related arsenopyrite from a metasedimentary rock-hosted massive sulfide provided an age of 2137 ± 11 Ma, suggesting that the mineralization is also coeval to the Rhyacian collisional stage.

Hydrochemical evolution and assessment of groundwater quality in fluorosis-affected area, Mandla District, central India

Deterioration of groundwater quality is a matter of serious concern today. In Mandla region of central India, fluorosis is common among the local people due to consumption of fluoride-contaminated groundwater. It is, therefore, important to understand the hydrochemical process of contamination and assess the groundwater quality of this region. A total of 173 samples (pre-monsoon and post-monsoon) were collected from borewells and dug wells of different depths for major ions analysis: K+, Na+, Ca2+, Mg2+, Cl−, CO32−, HCO3−, NO3−, SO42−, and F−; along with pH, temperature, TH (as CaCO3), TDS and EC. Broadly, three hydrogeologic units have been identified, viz. Deccan lava flows, infratrappean Lameta and Archean granite and gneiss. In Deccan, two distinct hydrostratigraphic units are Mandla and Dhuma. Mandla is the most prominent and extensive aquifer having hydraulic conductivity 5–15 mday-1 and yield 10–50 m3day−1. Water quality data vis-à-vis well-depth data indicate that Mandla aquifer, lying at a lower elevation than 500 m amsl, is fluoride-contaminated and harmful for human health. Fluoride (41.0% of total samples) and bicarbonate (54.9% of total samples) concentrations in analyzed water are above the global standard for drinking purpose. However, above 500 m msl groundwater is not contaminated with fluoride and is suitable for drinking and irrigation purpose. Fluoride concentration maps depict spatial and seasonal variation. Spatial variation of fluoride in groundwater is due to change in aquifer lithology, varying degree of rock weathering and orientation of structural lineaments. In fluoride concentration map of post-monsoon season, broadening of contour of uncontaminated zone indicates groundwater recharge, which corroborates with water fluctuation map. The fluoride-contaminated water and low-fluoride water are mainly Na–HCO3 and Ca–HCO3 type, respectively. The hydrochemical processes suggest pre-dominance of rock-water interaction; contribution of Na+ and K+ ions in groundwater is due to silicate weathering, whereas Ca2+ and Mg2+ ions are derived due to both silicate and carbonate weathering. Saturation index calculation point towards calcite-dolomite supersaturation and consequent precipitation, along with fluorite dissolution under alkaline condition.

Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments

This Chapter considers triple oxygen isotope variations and their 4 Gyr temporal evolution in bulk siliciclastic sedimentary rocks and in granites. The d18O and D'17O values provide new insights into weathering in the modern and ancient hydrosphere and coeval crustal petrogenesis. We make use of the known geological events and processes that affect the rock cycle: supercontinent assembly and breakup that influence continent-scale and global climate, the fraction of the exposed crust undergoing weathering, and isotopic values of precipitation. New data from a 5000 m Texas drillhole into the Oligocene Frio Formation demonstrate minimal isotopic shifts from mudrocks to shales during diagenesis, mostly related to expulsion of water from smectite-rich loosely cemented sediment and its conversion to illite-rich shale. Inversion of triple oxygen isotope fractionations return isotopic values and temperatures along the hole depth that are more consistent with weathering conditions in the Oligocene and modern North America (d18O = -7 to -15‰, and T of +15 to +45°C) rather than d18O from 8 to 10‰ diagenetic water in the drill hole at 175-195°C. More precise T and d18Owater are obtained where the chemical index of alteration (CIA) based detrital contribution is subtracted from these sediments. Triple oxygen isotopes from suspended sediments in major world rivers record conditions (T and d18Ow) of their watersheds, and not the composition of bedrock because weathering is water-dominated. In parallel, the Chapter presents new analyses of 100 granites, orthogneisses, migmatites, tonalite-trondhjemite-granodiorite (TTG), and large-volume ignimbrites from around the world that range in age from 4 Ga to modern. Most studied granites are orogenic and anatectic in origin and represent large volume remelting/assimilation of shales and other metasediments; the most crustal and high-d18O of these are thus reflect and record the average composition of evolving continental crust. Granites also develop a significant progressive increase in d18O values from 6-7‰ (4-2.5 Ga) to 10-13‰ (~1.8-1.2 Ga) after which d18O stays constant or even decreases. More importantly, we observe a moderate -0.03‰ step-wise decrease in D'17O between 2.1 and 2.5 Ga, which is about half of the step-wise decrease observed in shales over this time interval. We suggest that granites, as well as shales, record the significant advent and greater volumetric appearance of low-D'17O, high-d18O weathering products (shales) altered by meteoric waters upon rapid emergence of large land masses at ~2.4 Ga, although consider alternative interpretations. These weathering products were incorporated into abundant 2.0-1.8 Ga orogens around the world, where upon remelting, they passed their isotopic signature to the granites. We further observe the dichotomy of high-D'17O Archean shales, and unusually low-D'17O Archean granites. We attribute this to greater contribution from shallow crustal hydrothermal contribution to shales in greenstone belts, while granites in the earliest 3.0-4.0 Ga crust and TTGs require involvement of hydrothermal products with lower-D'17O signatures at moderately high-d18O, which we attribute to secondary silicification of their protoliths before partial melting. The Chapter further discusses evolution of the shale record through geologic history and discusses the step-wise change in d18O and D'17O values at Archean/Proterozoic transition. Denser coverage for shales in the past 1 billion years permits investigation of the rocks and their weathering in the last supercontinent cycle, with observed lighter d18O values, characteristic for the mid-Phanerozoic at the initiation of Gondwana breakup. The continuing increase in d18O values of the shales since 4 Ga is interpreted to reflect accumulation of weathering products via shale accretion to continents, as low-density and buoyant shales tend to not subduct back into the mantle. The rock cycle passes triple oxygen isotopic signatures from precipitation to sedimentary, metasedimentary, and finally to anatectic igneous rocks. Continental crust became progressively heavier in d18O, lighter in D'17O due to incremental accumulation of high-d18O sediments in accretionary wedges. Second-order trends in d18O and D'17O are due to supercontinent cycles and glacial episodes.

Evaluation of springs and waterfalls as Geomorphosites and proposition of strategies to develop Geotourism at Ajodhya hill, Puruliya district, Eastern India

Ajodhya hill is an extended part of Chota Nagpur Granite Gneissic Complex located in Puruliya district, western part of West Bengal, Eastern India. This uplifted plateau is primarily composed of Archean granite and gneissic rock with the sharp break at the plateau fringe. Ground water comes out as spring through the fractures of the granitic rock base situated beneath the permeable red soil. Waterfalls are prominent along the steep slope of the plateau fringe. Thirteen springs and four waterfalls have been identified as important Geomorphosites to analyse their Geotourism potential of Ajodhya hill in response to its geologic and geomorphic background. SWOT analysis has been used for estimating the Geotourism potentialities of Ajodhya hill. To analyse the potentiality, weight and score has been given under each factors and weighted score has been calculated. Few strategies have been suggested for enriching the Geotourism potential of the region with the help of TOWS matrix. Prioritization of the strategies has been ascertained by QSPM (quantitative strategies planning matrix) method through which the Ranking of Total Attractiveness Score (RTAS) has been estimated.

Petrogenesis of Archean TTGs and potassic granites in the southern Yangtze Block: Constraints on the early formation of the Yangtze Block

The southern Yangtze is underlain by Mesoarchean igneous rocks that were emplaced in two pulses, including trondhjemites at ca. 3092–3071 Ma followed by potassic granites at 2920 Ma. These represent the oldest TTG suites identified in the southern Yangtze and the oldest potassic granites in the entire Yangtze Block. Hf isotopic analysis reveals εHf(t) values of −2.49–7.49 and Hf crustal model ages (TDMC) of 3601–3000 Ma for zircons from the trondhjemites, whereas those from the potassic granites show εHf(t) values of −1.79–4.18 and TDMC ages of 3453–3043 Ma. The trondhjemites have high SiO2, Mg# and low Al2O3 and K2O/Na2O, and are highly evolved with low Ni, Cr and Co. The trondhjemites are characterized by low Sr/Y, Eu/Gd and (La/Yb)N, and high Y and HREE, typical of high-HREE and low-pressure TTGs. Their geochemical and isotopic compositions indicate that they are derived from melting of juvenile mafic crust with little contribution from older crustal materials, consistent with a plagioclase-bearing basaltic source. The multielement patterns of potassic granites are similar to that of the trondhjemites, albeit with lower HREE and Y contents. Their high K2O values, strongly fractionated REE patterns and low Y contents indicate the presence of garnet and/or amphibole as residual phases in the source region, which indicates they formed at higher pressure than the trondhjemites. The isotopic and geochemical data imply that the formation of the potassic granites could be related to reworking of older TTGs. The increasing K2O/Na2O of the 3092–3071 Ma trondhjemites to potassic 2920 Ma granites corresponds with stabilization, maturation and thickening of the Archean continental crust in the southern Yangtze. The geochemical data of the TTGs in the Kunming region show important differences from that of contemporaneous TTGs from the Kongling Complex, suggesting that southern Yangtze is composed by an independent old nucleus, which is exotic to the northern Yangtze. A synthesis of petrogenesis, geochronology, geochemistry and isotope data of Archean granites of the Yangtze Block provides important insights into the architecture of its Archean crust. We propose that the Yangtze Block can be subdivided into discrete northwestern, northeastern and southern domains, but which are all affected by late Paleoproterozoic tectonothermal events, suggesting that they had been assembled together by then to form the united Yangtze Block.

Paleoproterozoic granitoids of the Don terrane, East-Sarmatian Orogen: age, magma source and tectonic implications

The Pavlovsk complex granitoid plutons and associated gabbro-diorites are confined to the East Sarmatian Orogen (ESO) that compose the western part of the junction between the Sarmatian and Volgo-Uralia segments of the East-European Craton. The plutons form the largest batholiths with an area of up to 2000 km2 and a total length of more than 330 km. The conditions for their emplacement and crystallization, sources of melts and tectonic position remain debatable. Here we present new results on geochronological, geochemical and isotope- data on the Pavlovsk complex granitoids and associated gabbro-diorites and discuss these in conjunction with previously published data. The geochemical characteristics, such as elevated concentrations of Mg, K, Sr and Ba (usually > 500 ppm), ratios (La/Yb)N (25–73) and Sr/Y (23–223) in the absence of Eu anomalies of the Pavlovsk-type plutons are similar to high-Mg Archean granites. Zircon U-Pb data indicate the formation of the granitoids and associated gabbro-diorites during a short span at 2060–2075 Ma. The magma emplacement occurred in a transitional environment (from late-collisional to post-collisional) after the collision of two crustal segments (Sarmatia and Volgo-Uralia). We identify a significant ancient Archean continental crust in the magma source for the northern plutons and leucogranites including the presence of Archean zircon cores and Archean model TNd (DM) ages. All the plutons are dominated by a mixed magma source generated through the interaction of juvenile lower crustal and enriched mantle components. The Nd isotopic composition of the studied plutons is significantly more radiogenic as compared with the host Archean rocks (εNd (T) from +0.3 to −4.2). Our geochemical data suggest that that the mantle source of the gabbro-diorites and granitoids were modified (metasomatized) by subduction-related processes. The contrasting geochemical and isotopic features of the granitoids in the ESO as compared to the Pavlovsk granitoids suggest different sources for the parent melts.

Geochronological constraints on the formation and evolution of the Huangling basement in the Yangtze craton, South China

The Huangling basement has the largest area and highest metamorphic grade of exposed Archean and Paleoproterozoic rocks in the Yangtze craton basement. Hence, it provides an excellent opportunity for examining the formation and evolution of the Yangtze craton. The Huangling basement consists mainly of the Kongling complex, with a chronotectonic framework established through the geochronological study of amphibolites, khondalites, ultramafic rocks, and diabase dikes. Our results indicate that magmatic events in the Huangling basement are divided into four phases; the first phase is represented by the emplacement of the intermediate–basic volcanic rocks (3.2–3.0 Ga) that formed a typical Archean greenstone belt. The greenstone belt was subsequently intruded by tonalite–trondhjemite–granodiorite rocks (3.0–2.9 Ga) and A-type granite (2.7–2.6 Ga) constituting a typical Archean granite–greenstone terrane. The granite–greenstone terrane was transformed into an Archean continental nucleus during the first metamorphism (2.6–2.5 Ga), resulting in an unconformity between Archean and Proterozoic strata. Stable cratonization and sedimentation are considered to have lasted for 500 Ma (2.5–2.05 Ga), during which sedimentary rocks with a thickness of 2500 m were deposited. Later, the sedimentary rocks were metamorphosed to form the khondalite series during the second metamorphic event at 2.05–1.9 Ga, which caused an angular unconformity between Paleoproterozoic and Mesoproterozoic strata. The Paleoproterozoic high-grade terrane and Archean continental nucleus simultaneously merged to form the Huangling basement. The Kongling complex was intruded by A-type K-feldspar granite and diabase dikes, which were emplaced at 1.87–1.85 Ga and had not undergone deformation and metamorphism, indicating that the formation and evolution of the Kongling complex and the Huangling basement ended before 1.87–1.85 Ga.

Clay minerals and organic matter in shelf sediments off Coromandel Coast of India: Implications for provenance, transportation and depositional processes

Surface sediments collected from 25 stations in the Bay of Bengal were analyzed for grain size, organic matter and clay minerals to understand the spatial distribution, transport pathways, depositional process and provenance of the marine sediments. Granulometric analysis reveals the decreasing trend of grain size with increasing depth from coast to offshore region. The predominance of sandy sediments at shallower region represents their terrigenous origin and prevalence of high energy condition. The sediments from 50-100 m water depth are found to contain higher concentration of organic matter (TOC, TN and Sulfur) suggesting the high productivity zone. The C/N value ranges from 4 to 8 in 52% of sediments which indicates the source of both marine and terrestrial inputs. And 36% of sediment having C/N ratio <4 denotes that they are mainly derived from a marine source of phytoplankton and zooplankton remain and faeces while 12% of sediments C/N ratio ranges from 8 to 14 which suggests terrestrial organic matter input through local riverine systems. The C/S ratio of samples varies from 0.57 to 4.69 with mean value of 2.47 indicates the prevalence of semi-anoxic to anoxic marine conditions. The positive correlation of mud with TOC and sulfur denotes that their distribution is mainly controlled by finer fractions of sediments. Illite, Kaolinite and Smectite are the dominant clay minerals in the order of abundance occurring in the study area, which indicates the terrestrial sources. High amount of illite and kaolinte in the study area is influenced by Pennar River which drains through Archean granites, charnockites, khondalites suites of rock formations and Nellore pegmatites. The abundance of smectite in the study area is mainly from Deccan basalts by Krishna River and also from Cuddapah basin by Pennar River. In the study area, the clay minerals, organic carbon, nitrogen and sulfur distribution in surface sediments are chiefly controlled by the provenance and currents.

Evidence of Archaean metamorphism from the Yerrabali schist belt of Eastern Dharwar craton from EPMA dating of monazite

The Dharwar craton is divided in to the Western and Eastern Dharwar craton (WDC and EDC) by the N–S trending Chitradurga Shear zone. The EDC mainly consists of late Archean granites interspersed with N–S trending linear schist belts. The Yerrabali Schst belt (YSB) is located at the north-eastern corner of the EDC, near the Karimnagar granulite belt. The YSB is metamorphosed in middle-upper amphibolite facies and consists of Banded Magnetite Quartzite (BMQ), ferruginous quartzite, ultramafics, quartz–cordierite–gedrite ± garnet bearing quartzite, dolomite, amphibolite, deformed meta-pyroxenite, tremolite schist, sillimanite–garnet–cordierite schist, and dolerite dykes. We have dated monazites in the sillimanite–garnet–cordierite schist, exposed at the SW part of the YSB by EPMA chemical method. So far no dates are available from the YSB. The monazite chemical ages with three distinct populations, viz., 2658 ± 42, 2817 ± 19 and 3097 ± 34 Ma, probably correspond to three distinct metamorphic phases. The peak metamorphism at around 2800 Ma corresponds with the garnet and cordierite growth. A temperature of 673°C and a pressure of 4.7 kb have been estimated for peak metamorphism.

Characteristics, diagenetic evolution, and controlling factors of the Es1 deep burial high‐quality sandstone reservoirs in the PG2 oilfield, Nanpu Sag, Bohai Bay Basin, China

Significant achievements have been reported for deep oil and gas exploration in the Bohai Bay Basin. Among them, the Nanpu Sag has achieved good deep (greater than 3.5 km) hydrocarbon exploration results. However, the hydrocarbon formation mechanisms and the diagenetic evolution of these high‐quality sandstone reservoirs remain unclear. Mineralogical, petrographic, and fluid inclusion data were selected to investigate the deeply buried sandstone reservoir physical properties, pore systems, and diagenetic features and the main formation mechanisms of the deep high‐quality reservoirs of the Es1 (the top member of the Paleogene Shahejie Formation) sandstones in the Nanpu Sag, eastern China. The results show that the Es1 sandstone reservoirs are dominated by lithic arkoses and have low porosities (average porosity 13.4%) but medium to high permeability (average permeability 188 mD). The pore systems consist of primary intergranular pores, secondary dissolution pores, and microfractures. Loss of porosity is greater due to compaction (average 20.89%) than due to cementation (average 4.82%), and dissolution contributes an improvement in porosity of approximately 5.85%. Two peaks in the inclusion homogenization temperatures were observed: The first fluid inclusion homogenization temperature term ranges from 130°C to 145°C and exhibits yellow fluorescence; the other peak occurs from 155°C to 165°C, showing green fluorescence. Eodiagenesis is due to mechanical compaction, precipitation of early calcite cements and quartz overgrowths, and dissolution of feldspar and carbonate cements. Mesodiagenesis occurs due to mechanical compaction, framework grain (mainly feldspar) dissolution, and precipitation of quartz and late carbonate cements. The main factors controlling the formation of the deep high‐quality reservoirs are the presence of a high hydrodynamic depositional environment (subaqueous distributary channel), the type of provenance (Archean granites), and the creation of secondary porosity by feldspar dissolution.

Early earth geodynamics: cross examining the geological testimony.

Many studies link the presence of continents on Earth to the operation of plate tectonics. Radiogenic isotope data have, however, long consigned the bulk of crust generation and preservation to the murky realm of the Precambrian Earth, where the prevailing geodynamic systems are highly uncertain due to the sparse and complex nature of the geological record of these early eons. The purpose of this paper is to examine the nature of this geological record, considering the biases and artefacts that may undermine its fidelity, and to assess what are the most robust lines of evidence from which meaningful geodynamic inferences can be drawn. This is pursued with reference to Hadean detrital zircons, Archean gneiss complexes and Archean granite-greenstone terranes, and by considering isotopic proxies of crust-mantle interaction. The evidence reinforces long held views that the formation of some of the oldest continental nuclei involved a distinctive mode of planetary geodynamics that rests uneasily within definitions of modern style plate tectonics. A detailed interrogation of the oldest rocks, integrating multi-scale information from the best preserved whole-rock and mineral archives, and emphasizing careful selection at the sampling and analytical stages, will lead to the most robust input data for petrological and thermodynamic models of early Earth processes.This article is part of a discussion meeting issue 'Earth dynamics and the development of plate tectonics'.

Систематика Sc, Cr, Co, Ni, Th и редкоземельных элементов в черных сланцах муравьинской и вёлсовской свит Ляпинско-Кутимского антиклинория (Северный Урал): к реконструкции пород-источников сноса

The features of the Sc, Cr, Co, Ni, Th and rare earth elements distribution in the black shales of the Riphean Murav’inskaya and Velsovskaya formations in southern part of the Lyapin-Kutim anticlinorium (Northern Ural) are analyzed. The contents of the listed elements in 20 samples of black shales are determined by the ICP-MS method at the IGG UB RAS (Ekaterinburg). Samples of the shales of the Murav’insky formation are selected from the outcrops along the river Vels, below the mouth of Pos’mak River, as well as along Pravaya Rassokha River. The selection was made from outcrops more or less evenly distributed over the studied area. The shales of the Velsovskaya formation were studied from the drilling holes in the basin of Sur’ya Vagranskaya River, as well as in outcrops along the Sibiryakovskaya Martayka and Pravaya Rassokha rivers. In the Sc-Th/Sc diagram, the data points of the black shales of the Murav’inskaya and Velsovskaya formations are fairly compact located between the points of the average composition of the Archean and Proterozoic granites, on the one hand, and tonalite-trondhjemite-granite (TTG) associations of the same age. Approximately the same distribution of the data points of the black shales of these formations can be seen in the La/Sc-Th/Co and La/Sm-Sc/Th diagrams. The average value of ΣРЗЭ in black shales of the Murav’inskaya formation is 316 ± 90 ppm, in the shales of the Velsovskaya Formation it is noticeably lower - 200 ± 47 ppm. The black shales of the Murav’inskaya Formation are substantially enriched with light lanthanides (La/Yb)Nmean = 40, and noticeably depleted by heavy lanthanides, (Gd/Yb)Nmean = 5.4. For the black shales of the Velsovsky formation (La/Yb)Nmean = 22, (Gd/Yb)Nmean = 3.1. This ones and a number of the other data make it possible to conclude that the complexes of rock-sources of fine aluminosiliciclastics for black shales of Murav’inskaya and Velsovskaya formations had a rather specific composition. On the basis of a comparison of the geochemical features of the black shales of the southern part of the Lyapin-Kutim anticlinorium and potential sources of fine aluminosiliciclastics, the conclusion is based that such rock complexes could be, from one side, the Archean and/or Proterozoic TTG associations and granites of the same age of the Eastern European platform, and, from another side, certain local rocks with high Cr contents and significant differentiation of REE, similar in composition to the products of kimberlite magmatism.