Granitoid Samples Research Articles

Lithium and trace-element concentrations in trioctahedral micas from granites of different geochemical types measured via laser ablation ICP-MS

AbstractThe compositions of trioctahedral micas from 51 samples of granitoids with different geochemical affiliations and grades of differentiation from the Bohemian Massif, Central Europe, were analysed using electron microprobe (major elements) and laser ablation inductively coupled plasma mass spectrometry (Li, Sc, Ga, Ge, Nb, In, Sn, Cs, Ta, W, Tl). The micas form a continuous evolutionary series from phlogopite to zinnwaldite. The phlogopites and biotites from the I-type rocks are characterized by 5.5–5.7 Si, 2.4–2.6 Al, <0.1 Li atoms per formula unit [apfu] and Mg/(Mg + Fe) = 0.4–0.8. The biotites from the S-type granites usually contain 5.3–5.7 Si, 3.2–3.6 Al, 0.1–0.3 Li apfu and Mg/(Mg + Fe) = 0.15–0.4. The annites and zinnwaldites from the rare-metal granites contain 5.7–6.8 Si, 3.2–3.8 Al, 0.6–2.6 Li apfu and Mg/(Mg + Fe) < 0.1. The concentrations of F, Rb, Cs and Tl increase from the phlogopites and biotites to zinnwaldites: F 0.1 → 8 wt.%, Rb2O 0.05 → 1.7 wt.%, Tl 2 → 50 ppm and Cs 40 → 2000 ppm. The concentrations of Sn, Nb, Ta and W in phlogopites and biotites from the I- and S-type granitoids generally correlate with those of the parent rocks and reach values of (in ppm) 20–100 Sn, 20–250 Nb, 1–20 Ta and <5 W. The highest concentrations were found in the Li-annites in the relatively early facies of rare-metal granites (in ppm): 250–600 Sn, 400–600 Nb, 60–120 Ta and 50– 120 W. The zinnwaldites in the late rare-metal granites facies are impoverished in these elements, which is explained by contemporaneous crystallization of cassiterite and columbite. Lithium enters the crystal lattice of trioctahedral micas via the exchange vector Li3□Si3Fe–6Al–1up to concentrations of ∼2.5 wt.% Li2O (1.5 apfu Li). At higher Li concentrations, Li is incorporated through the exchange vector Li3Si1□–1Fe–2Al–1.

Palaeoproterozoic continental arc magmatism, and Neoproterozoic metamorphism in the Aravalli-Delhi orogenic belt, NW India: New constraints from in situ zircon U-Pb-Hf isotope systematics, monazite dating and whole-rock geochemistry

Presently, the extent, origin and petrogenesis of late Palaeoproterozoic (ca. 1.85Ga) magmatism in the north-central Aravalli-Delhi orogenic belt, NW India and subsequent metamorphic overprints are poorly constrained. Results of new in situ zircon U-Pb-Hf isotope analyses in combination with whole-rock elemental and isotopic data provide the first hard evidence that granitoid magmatism occurred in a continental magmatic arc setting between 1.86 and 1.81Ga. The Hf-Nd model ages of 3.0–2.6Ga and inherited zircon grains of 3.3–2.5Ga indicate abundant reworking of Archaean crust. Flat HREE patterns with negative Eu anomalies furthermore reveal that the granitoids were generated from garnet-free and plagioclase-rich sources at shallow depths. Significant isotope variation among granitoid samples (εHft=−3.7 to −9.0; εNdt=−4.8 to −7.9) indicate that the reworked Archaean crust was not completely homogenised during the Palaeoproterozoic. This is best reflected by zircon Hf-isotope variation of ca. 9.5 epsilon units within the oldest granitoid sample. Zircon grains from this sample define three discrete Hf-isotope groups at εHf1.86Ga=−8.9, −4.8 and −1.6. These are interpreted to result from mixing of zircon-saturated magmas derived from three distinct sources within the crust prior to solidification. A monazite U-Pb isochron age of 868±4Ma from one of the granitoid samples furthermore indicates that the Aravalli fold belt was affected by an important post-magmatic overprint, perhaps related to the widespread metasomatic, granulite metamorphic and/or magmatic events during the same time span.

Iron isotopic compositions of adakitic and non-adakitic granitic magmas: Magma compositional control and subtle residual garnet effect

Here we present iron (Fe) isotopic compositions of 51 well-characterized adakitic and non-adakitic igneous rocks from the Dabie orogen, Central China and Panama/Costa Rica, Central America. Twelve I-type non-adakitic granitoid samples from the Dabie orogen yield δ56Fe ranging from −0.015‰ to 0.184‰. The good correlations between δ56Fe and indices of magma differentiation (e.g., SiO2, FeOt, Mg#, and Fe3+/ΣFe) suggest Fe2+-rich silicate and oxide minerals dominated fractional crystallization with Δ56Femelt-crystal∼0.06‰ may account for the δ56Fe variation in these samples. One A-type granite sample from the Dabie orogen has δ56Fe as high as 0.447‰, likely indicating less magnetite crystallization and an increase in 103lnβmelt with magma (Na+K)/(Ca+Mg). Combined with the literature data, most high silica (SiO2⩾71wt.%) granitic rocks define a good positive linear correlation between δ56Fe and (Na+K)/(Ca+Mg): δ56Fe=0.0062‰×(Na+K)/(Ca+Mg)+0.130‰ (R2=0.66). Given that fractional crystallization also tends to increase δ56Fe with (Na+K)/(Ca+Mg), this correlation can serve as the maximum estimate of the magma compositional control on Fe isotope fractionation. Low-Mg adakitic samples (LMA) have δ56Fe ranging from 0.114‰ to 0.253‰. The melt compositional control on LMA δ56Fe could be insignificant due to their limited (Na+K)/(Ca+Mg) variation. Except for one sample that may be affected by late differentiation, 14 out of 15LMA have δ56Fe increasing with (Dy/Yb)N, reflecting a subtle but significant effect of residual garnet proportion. This serves as evidence for that source mineralogy may play an important role in fractionating Fe isotopes during partial melting. Dabie and Central America high-Mg adakitic samples have homogeneous Fe isotopic compositions with mean δ56Fe of 0.098±0.038‰ (2SD, N=11) and 0.085±0.045‰ (2SD, N=11), respectively. These samples have undergone melt-mantle interaction, which may neutralize the possible δ56Fe variation of their pristine magmas derived by partial melting of eclogitic rocks.

Uplift history of the Jiaodong Peninsula, eastern North China Craton: implications for lithosphere thinning and gold mineralization

AbstractThe link between lithosphere thinning and formation of world-class gold deposits is well established in the Jiaodong Peninsula within the eastern North China Craton (NCC). However, the timing of initiation and duration of the lithospheric thinning process as well as the depth of formation of the mineralization remain uncertain. Since these parameters are fundamental to formulate exploration strategies, in this study we perform fission track (FT) analysis on zircon and apatite grains in Late Mesozoic granitoid samples from the Jiaodong Peninsula and provide new constraints for the mode and duration of lithospheric evolution and mineralization depth. The zircon FT ages range from 64.3 to 90.9 Ma and those of apatite show a range of 32.8–50.9 Ma. The data collectively display age peaks at ~60–80 and ~30–50 Ma. Reverse modelling of the apatite FT results indicates rapid crustal uplift during ~30–80 Ma in the Jiaodong Peninsula. This period coincides with the timing of maximal sedimentation in the neighboring basins and voluminous basaltic eruptions in the eastern NCC. We suggest that the Jiaodong Peninsula has experienced two stages of crust uplift in the Late Cretaceous and Paleogene as a consequence of the continuing lithosphere thinning, together with the surrounding basins, forming the horst–graben system in the eastern NCC. The Late Mesozoic granitoids are the main wall rocks for gold deposits in Jiaodong, and thus the crust denudation history gathered from the FT data suggest that the gold mineralization formed at depths ofc.6–11 km.

Geochemistry of metavolcanics and granitic intrusive from western margin of northern afar depression, Dallol, Northern Ethiopia

The western margin of northern Afar Depression constitutes various rock units of Neoproterozoic basement complex. Major oxide data of metavolcanic and granitoid rock samples from basement complex indicate significant variation in the concentration of major oxides (e.g. Fe 2 O 3 , TiO 2 , CaO, MgO). Metavolcanic rocks of Dallol demonstrate heterogeneous nature of magma source. The mafic metavolcanics indicate two distinct groups, (i) CaO-enriched and (ii) tholeiitic composition. Another group of magmatic series, the felsic metavolcanic group, shows a co-genetic relationship with the tholeiitic metavolcanics. Granitoids on the other hand have a wider range of SiO 2 (61.5 – 76.65 wt %) and are peraluminous to metaluminous in nature. Keywords : Geochemistry, Metavolcanics, Granitioids, Dallol, Northern Afar Depression, Ethiopia.

Geochronology and geochemistry of the TTG and potassic granite of the Taihua complex, Mts. Huashan: Implications for crustal evolution of the southern North China Craton

Tonalite-trondhjemite-granodiorite (TTG) suite and potassic granite volumetrically expose in the Neoarchean to Paleoproterozoic Taihua complex, Mts. Huashan region, southernmost segment of the Trans-North China Orogen (TNCO). Zircon U–Pb dating on the trondhjemite, granitic gneiss, K-feldspar granite and coarse-grained granite samples show three episodes of magmatism (2.55–2.49Ga, 2.33–2.25Ga and 1.87–1.80Ga) with distinct geochemical features in this area during the Paleoproterozoic. The trondhjemite samples have high SiO2 (67.46–71.73wt%), Na2O (4.83–5.95wt%), low Mg# (31–40) and HREE contents, with moderate (La/Yb)N values (16.8 to 35.3) and Sr/Y ratios (15.8 to 34). The potassic granite samples show high SiO2 (65.2–75.43wt%), K2O (3.9–8.6), low Na2O (1.74–4.17wt%), Mg# (2–46) and HREE contents, with moderate to high (La/Yb)N values (10.3 to 226) and Sr/Y ratios (12.8 to 208). All these granitoid samples are characterized by high SiO2 and low Mg#, Cr, Ni with moderate to high (La/Yb)N values and Sr/Y ratios. The absence of evidence of any pre-existing high Sr/Y and La/Yb sources in this region led to that the granitoids probably generated from partial melting of hydrous mafic rocks with garnet and amphibole in the residue. Therefore, partial melting of thickened lower curst is the most likely origin. Combined with previous studies, the first episodic magmatism (2.55–2.49Ga) represented by TTG and granitic gneisses with positive whole rock ƐNd(t) and zircon ƐHf(t) values is interpreted as melts from partial melting of juvenile thickened lower crust. The second episodic magmatism (2.33–2.25Ga) represented by TTG, granitic gneisses and K-feldspar granite with positive to negative whole rock ƐNd(t) and zircon ƐHf(t) values is suggested as the product of partial melting of both juvenile and pre-existing crustal material. The third episodic magmatism (1.87–1.80Ga) is represented by these potassic granites, which is synchronism with the metamorphism recorded in this region and probably resulted from partial melting of pre-existing crustal materials in a syn-orogenic or post-orogenic setting during the collision between the Eastern and Western Blocks of the North China Craton. Multistage continental accretion (at 2.84–2.72Ga, 2.57–2.43Ga and 2.36–2.25Ga) and reworking (at 2.36–2.25Ga, 2.19–2.07Ga and 1.87–1.80Ga) occurred in the southern segment of the TNCO from Neoarchean to Paleoproterozoic. Transformation from an Andean-type continental margin arc setting on the western margin of the Eastern Block to continent-arc-continent collision in the southern TNCO during Late Neoarchean to Late Paleoproterozoic is suggested in this paper.

Early Paleozoic intracontinental orogeny in the Yunkai domain, South China Block: New insights from field observations, zircon U–Pb geochronological and geochemical investigations

Debate on whether the Early Paleozoic tectono-magmatic event in South China is related to a subduction–collision or an intracontinental orogen has been lasted for decades within the geoscience community. This study deals with LA-ICP-MS zircon U–Pb ages, whole-rock chemistry, rare earth elements, trace elements and Hf isotopes from granitoid samples collected in the Yunkai domain in order to better constrain the Early Paleozoic tectonic evolution of the South China Block. The weighted mean 206Pb/238U ages for eight samples range from 426Ma to 443Ma, representing the crystallization ages of the magma. Fourteen samples were analyzed for geochemistry, all of which are characterized by a peraluminous signature with A/CNK values greater than 1.0. The REE geochemistry reveals enrichment in light rare earth element. LREE/HREE values range from 2.81 to 30.36 and (La/Yb)N vary from 1.23 to 55.14 (mean of 14 analyses is 14.69). All the samples exhibit distinct negative Ba, Sr and Nb anomalies and enrichment in Rb, Th, U and Pb. Hf isotopic analyses indicate negative εHf (t) values mainly ranging from −3 to −12, corresponding to two model age distributing from 1637Ma to 2208Ma. The geochemical analyses indicate that the Silurian granitic magmas in the Yunkai domain were derived from partial melting of crustal materials with little or no input of mantle source. These new data support the intracontinental subduction model already proposed to account for the Early Paleozoic tectonic, metamorphic and magmatic event of South China.

Geology, mineralogy and evolution of iron skarn deposits in the Zanjan district, NW Iran: Constraints from U-Pb dating, Hf and O isotope analyses of zircons and stable isotope geochemistry

The Zanjan iron skarn deposits are located in the northwest Central Iranian terrane, along the Soltaniyeh mountains. The deposits are hosted by late Proterozoic to late Cambrian carbonate rocks (Soltaniyeh, and Mila Formations) and by Permian (Ruteh Formations), intruded by the Arjin granite, the Khakriz granodiorite and the Inche-Rahbari quartzmonzonite (53±1Ma to 55±0.4Ma, early Eocene). These granitoids show high-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of LILEs and LREEs, and depletion in high field strength elements (HFSEs) and HREE. The Arjin, Khakriz and Inche-Rahbari granitoid samples show zircon εHf(t) values of −0.7 to +5.8 (TDM2=1052–1640Ma) and δ18O values of +5.55‰ to +7.42‰ suggesting partial melting of older crustal material then mixing with juvenile mantle-derived melts. The thermal anomaly induced by underplating mafic melts resulted in partial melting of continental crust and formation of Arjin, Khakriz and Inche-Rahbari magmas emplaced during several magmatic pulses at 53–55Ma.The Zanjan iron skarn deposits are magnesian (Arjin and Inche-Rahbari) and calcic (Khakriz) with endoskarn and exoskarn facies. In these skarn facies forsterite, garnet and pyroxene comprise the main prograde skarn minerals, whereas magnetite, serpentine, amphiboles of the tremolite-ferroactinolite series (sometimes asbestiform), epidote, talc and calcite are the principal retrograde skarn minerals. Iron exoskarns formed at the contacts of the intrusions parallel to the fabric of the host rocks and in networks of crosscutting veins and veinlets. The assemblage of ore minerals consists of magnetite, hematite (specularite), pyrite, chalcopyrite and bornite; in addition, minor galena and sphalerite occur in the Khakriz deposit. The sulfur isotopic compositions in the Arjin deposit range from +15.99 to +18.35‰ and in the Inche-Rahbari and Khakriz deposits from +2.96 to 5.30‰. The oxygen and carbon isotope ratios of the fresh or partly altered limestones from Soltaniyeh and Mila Formations reveal δ13C values of −0.64 to +0.03‰ and δ18O composition of +13.12 to +24.41‰ typical of marine carbonate. The isotopic compositions of the hydrothermal calcite (skarn calcite) range from −5.32 to −2.01‰ for carbon isotope and 3.29 to 20.67‰ for oxygen isotope. Complex water-rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulfur, oxygen and carbon mixing. Field observations, mineral paragenesis, and stable isotopic signatures in these deposits are consistent with ore mineralization formed by the interaction of magmatic fluids with both cooler meteoric water and carbonate host rocks.

Geochronology and geochemistry of Late Cretaceous–Paleocene granitoids in the Sikhote-Alin Orogenic Belt: Petrogenesis and implications for the oblique subduction of the paleo-Pacific plate

We present zircon U–Pb ages, major and trace element analyses, and zircon Hf isotope data on the Late Cretaceous–Paleocene granitoids at the southern end of the Sikhote-Alin Orogenic Belt of the Russian Far East. These data are used to discuss the petrogenesis of the granitoids in the context of the paleo-Pacific subduction beneath the eastern Eurasia. Zircons from four granitoid samples give emplacement ages of ~56, ~83, ~91, and ~92Ma. These granitoids with high SiO2 (73.43–76.76wt%) are metaluminous to weakly peraluminous (A/CNK=0.97–1.03) and belong to the high-K calc-alkaline series (K2O=3.75–4.95wt%). They are all enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and relatively depleted in high field strength elements (HFSEs) with striking depletion also in Ba, Sr, P and Eu. They are petrographically and geochemically consistent with being of I-type granitoids. The zircons have εHf (t) values (−0.8 to +7.6) higher than whole-rock εHf (t) values predicted from whole-rock εNd (t) (−4.1 to +0.5) in the literature. All these observations suggest that primary magmas parental to these granitoids were likely to have derived from partial melting of a juvenile lower crust accompanied by assimilation with ancient mature crust during magma ascent and evolution. A recent study illustrates that the collision of an exotic terrane carried by the paleo-Pacific plate with the continental China at ~100Ma accreted the basement of the Chinese continental shelf (beneath East and South China Seas), and resulted in a new plate boundary of transform nature between the NNW moving paleo-Pacific plate and the eastern margin of the shelf. Our new data and analysis of existing data support this hypothesis, but we hypothesize that this transform becomes transpressional in its northern segment with oblique subduction of the paleo-Pacific plate beneath northeastern Asia as manifested by the Late Cretaceous-Paleocene granitoids in the Russian Far East-NE China (97–56Ma) and South Korea-SW Japan (96–71Ma). These tectonic and magmatic events took place prior to the opening of the Huanghai, Bohai and Japan seas, which are of rift-origin from continental China much later. Our study emphasizes the necessity and significance of plate tectonics reconstruction in the greater northwestern Pacific region since the Cretaceous.

Age and Origin of Early Paleozoic and Mesozoic Granitoids in Western Yunnan Province, China: Geochemistry, SHRIMP Zircon Ages, and Hf-in-Zircon Isotopic Compositions

AbstractThis work presents evidence for early Paleozoic magmatism in the Baoshan block and Mesozoic magmatism in the Tengchong block and along the Gaoligong fault system in the Tethyan belt, western Yunnan Province, southwestern China, on the basis of SHRIMP dating of zircons from 12 granitoid samples. Deformed granitoids with gneissic textures contain zircons with magmatic oscillatory zoning that define weighted mean 206Pb/238U ages ranging from 481 ± 3 to 493 ± 3 Ma and are interpreted as the crystallization time of the host rocks. The granitoids are characterized by enriched light rare earth elements, high Th and U contents, and negative Nb anomalies on a primitive mantle–normalized spider diagram, which is indicative of crustal melting. The peraluminous, silica-rich compositions, high initial 87Sr/86Sr ratios, as well as negative eNd(t) and eHf(t) values also suggest that the granitoid rocks originated through crustal reworking. Zircons of 216–226 Ma, Late Triassic S-type granites of the Tengchong blo...

Petrology and geochemistry of Early Cretaceous A-type granitoids and late Mesozoic mafic dikes and their relationship to adakitic intrusions in the lower Yangtze River belt, Southeast China

ABSTRACTThe relationship among magmatism, large-scale metallogenesis of Southeast China, and subduction of the Pacific plate has long been debated. The lower Yangtze River belt (LYRB) in the northeastern edge of Southeast China is characterized by intense late Mesozoic magmatism and associated polymetallic mineralization such as copper, gold, iron, tungsten, molybdenum, etc. The copper-related adakitic rocks (148–130 Ma) in this belt are the oldest episode of magmatism and intruded as small intermediate-acid intrusive bodies. The Huayuangong granitoids (HYG), located in the southern part of this belt, however, are copper-barren. Three granitoid samples from this pluton give zircon U–Pb ages of 126.4 ± 1.6 Ma, 125.9 ± 1.9 Ma, and 126.2 ± 1.2 Ma, respectively. The HYG has A-type affinity with metaluminous to weakly peraluminous, high FeOT/(FeOT+MgO) ratios, and high Zr+Nb+Ce+Yb contents. Meanwhile, 10 late Mesozoic mafic samples from the LYRB exhibit similar trace element characteristics to those of ‘continental arc andesite’ (CAA) and suggest an enriched lithospheric mantle source with depletion in high field strength elements (e.g. Nb, Ta, Zr, Hf, and Ti) and enrichment of large ion lithophile elements (e.g. Rb, Th, U, and Pb). Although the HYG exhibits similar Sr–Nd isotope composition with the mafic dikes, distinct whole-rock Pb isotope ratios imply that the granitoids and mafic magmas originated from heterogeneous mantle sources. Compared with coeval Baijuhuajian A-type rocks that are exposed along the Jiang–Shao fault of Southeast China, the HYG shows enriched Hf isotope ratios of zircon with εHf(t) values ranging from −4.8 to −11.1. In the Yb/Ta versus Y/Nb diagram, being different from the major asthenospheric mantle-origin Baijuhuajian pluton, a large range of and high Y/Nb ratios as well as high Zr contents of the HYG pluton suggest a magmatic source of mixing between the asthenospheric and enriched crustal component in the LYRB. Compared with early-stage copper-related adakitic rocks (148–130 Ma) with subduction-related affinities and high oxygen fugacity, the copper-barren HYG has with-plate A-type affinities and lower oxygen fugacity. Summarizing, the production of early-stage (i.e. subduction related) adakitic rocks followed by late-stage A-type granitoids in the LYRB is ascribed to the rollback of the Palaeo-Pacific plate beneath Southeast China and associated with asthenospheric upwelling and lithospheric thinning during the late Mesozoic era.

Late Palaeozoic mineralization and tectonic evolution of the West Junggar metallogenic belt, Central Asia: constraints from Re–Os and 40Ar/39Ar geochronology

ABSTRACTThe West Junggar Metallogenic Belt (WJMB) is located between the Tianshan fault system and the Ertix fault system in the western part of the Central Asian Metallogenic Domain (CAMD). The belt features widespread late Palaeozoic granitic plutons, strike-slip faults, and porphyry copper and orogenic gold deposits. We collected nine molybdenite samples from the Baogutu III–IV Cu–Mo deposit and the Suyunhe Mo–W deposit, and 12 granitoid samples from the Jiaman, Kangde, Kulumusu, Bieluagaxi, Hatu, Akbastau, Miaoergou, Baogutu, Karamay, and Hongshan plutons in the WJMB. Molybdenite Re–Os dating gives metallogenesis ages of 312.7 and 299.7 Ma for the Baogutu III–IV and Suyunhe deposits, respectively. 40Ar/39Ar thermochronology yields biotite ages ranging from 326 to 302 Ma and K-feldspar ages from 297 to 264 Ma, indicating a regional medium-temperature cooling history in the WJMB during the late Carboniferous to middle Permian. By integrating these data with previous zircon U–Pb, amphibole 40Ar/39Ar, and zircon and apatite fission-track ages, we reconstruct the whole thermal history of the WJMB, which includes late Palaeozoic intrusive magmatism, porphyry Cu and W–Mo mineralization, and late Mesozoic tectonic uplift and exhumation of the WJMB. The regional 40Ar/39Ar cooling ages are consistent with the timing of regional sinistral strike-slip faulting, thereby indicating the tectonic significance of the cooling ages. We suggest that the biotite 40Ar/39Ar ages represent the static cooling of the granitic plutons after emplacement, since the ages are consistent with the U–Pb ages of the plutons. Thereafter, the oldest K-feldspar 40Ar/39Ar age may record the initiation of sinistral strike-slip movement on the Darabut, Mayile, and Baerluke faults. The regional faulting resulted in significant uplift of the WJMB during the early and middle Permian.

Unravelling the record of Archaean crustal evolution of the Bundelkhand Craton, northern India using U–Pb zircon–monazite ages, Lu–Hf isotope systematics, and whole-rock geochemistry of granitoids

Combined whole-rock, U–Pb zircon and monazite dating, in situ zircon (MC)-SF-ICP-MS Lu–Hf-isotope data are reported for more than 20 Archaean granitoid samples to constrain magmatic events and their sources in the Bundelkhand Craton, northern India. The results are crucial for understanding the extent of reworking vs juvenile crustal growth during Palaeoarchaean–Neoarchaean and also provide new information on the temporal gap between Palaeoarchaean TTG and Neoarchaean granodiorite–granite magmatism. These data reveal that the Bundelkhand Craton was affected by at least four discrete episodes of Palaeoarchaean TTG magmatism between 3.55Ga and 3.20Ga at regular intervals of 100–150 Myr. After a period of about 500 Myr of near magmatic shutdown, it was followed by a relatively short-lived, but more voluminous granitoid emplacement event during the Neoarchaean at 2.58–2.50Ga, during which minor granodioritic gneisses and voluminous granites intruded. The new data also provide evidence for a metamorphic overprint at ca. 3.20Ga, and a Pan-African tectonothermal overprint at 0.50–0.60Ga.Hf-isotope and whole-rock geochemical data of the TTGs suggest a complex process of crust formation and stabilisation during the Palaeoarchaean, mainly by the reworking of older mafic and felsic crust between 3.55 and 3.20Ga. The CHUR parallel array for the TTGs may indicate that these rocks were formed by partial melting of an oceanic crust that was derived from chondritic mantle sources, shortly before formation of TTG. This interpretation is inconsistent with Hf-isotope data from worldwide sources, which provide evidence for a successive mantle depletion since at least 4.0Ga, resulting in εHft of about +5.0 at 3.0Ga. Intense melting of the heterogeneous TTG crust during the Neoarchaean resulted in the coeval formation of granodioritic gneisses and voluminous granites, and subordinate hybrid granites with a minor contribution of melts from an enriched mantle source, affected by subduction-zone processes prior to 2.55Ga.

Zircon U–Pb geochronology, Sr–Nd–Hf isotopic composition and geological significance of the Late Triassic Baijiazhuang and Lvjing granitic plutons in West Qinling Orogen

The Qinling Orogen was a consequence of continental collision of the South China Craton with the North China Craton in the Triassic and caused widespread granitoid magmatism. However, the petrogenesis of these granitoids remains controversial. In this paper, we choose the Baijiazhuang (BJZ) and Lvjing (LJ) plutons in the West Qinling Orogen for a combined study of the zircon U–Pb geochronology, whole-rock major and trace element compositions and Sr–Nd–Hf isotopic characteristics. We obtained zircon crystallization ages of ~216Ma and ~212Ma for the BJZ and the LJ plutons, respectively. The granitoid samples from both plutons have high K2O metaluminous to peraluminous compositions. They are enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field-strength elements (HFSEs) with significant negative Eu anomalies. The BJZ samples have initial Sr isotopic ratios of 0.7032 to 0.7078, εNd(t) of −10.99 to −8.54 and εHf (t) of −10.22 to −6.41. The LJ granitoids have initial Sr isotopic ratios of 0.7070 to 0.7080, εNd(t) of −5.37 to −4.58 and εHf(t) of −3.64 to −1.78. The enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust. However, two BJZ samples show depleted Sr isotope compositions, which may infer possible involvement of mantle materials. Mantle-derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. That is, the two granitic plutons are magmatic responses to the closure of the Mianlue ocean basin and the continental collision between the Yangtze and South Qinling crustal terranes.

Effect of sample size on the fluid flow through a single fractured granitoid

Most of deep geological engineered structures, such as rock caverns, nuclear waste disposal repositories, metro rail tunnels, multi-layer underground parking, are constructed within hard crystalline rocks because of their high quality and low matrix permeability. In such rocks, fluid flows mainly through fractures. Quantification of fractures along with the behavior of the fluid flow through them, at different scales, becomes quite important. Earlier studies have revealed the influence of sample size on the confining stress–permeability relationship and it has been demonstrated that permeability of the fractured rock mass decreases with an increase in sample size. However, most of the researchers have employed numerical simulations to model fluid flow through the fracture/fracture network, or laboratory investigations on intact rock samples with diameter ranging between 38 mm and 45 cm and the diameter-to-length ratio of 1:2 using different experimental methods. Also, the confining stress, σ3, has been considered to be less than 30 MPa and the effect of fracture roughness has been ignored. In the present study, an extension of the previous studies on “laboratory simulation of flow through single fractured granite” was conducted, in which consistent fluid flow experiments were performed on cylindrical samples of granitoids of two different sizes (38 mm and 54 mm in diameters), containing a “rough walled single fracture”. These experiments were performed under varied confining pressure (σ3 = 5–40 MPa), fluid pressure (fp ≤ 25 MPa), and fracture roughness. The results indicate that a nonlinear relationship exists between the discharge, Q, and the effective confining pressure, σeff., and Q decreases with an increase in σeff.. Also, the effects of sample size and fracture roughness do not persist when σeff. ≥ 20 MPa. It is expected that such a study will be quite useful in correlating and extrapolating the laboratory scale investigations to in-situ scale and further improving theoretical/numerical models associated with fluid flow through rock masses.

Geochronology of the central Tanzania Craton and its southern and eastern orogenic margins

Geological mapping and zircon U–Pb/Hf isotope data from 35 samples from the central Tanzania Craton and surrounding orogenic belts to the south and east allow a revised model of Precambrian crustal evolution of this part of East Africa. The geochronology of two studied segments of the craton shows them to be essentially the same, suggesting that they form a contiguous crustal section dominated by granitoid plutons. The oldest orthogneisses are dated at ca. 2820Ma (Dodoma Suite) and the youngest alkaline syenite plutons at ca. 2610Ma (Singida Suite). Plutonism was interrupted by a period of deposition of volcano-sedimentary rocks metamorphosed to greenschist facies, directly dated by a pyroclastic metavolcanic rock which gave an age of ca. 2725Ma. This is supported by detrital zircons from psammitic metasedimentary rocks, which indicate a maximum depositional age of ca. 2740Ma, with additional detrital sources 2820 and 2940Ma. Thus, 200Ma of episodic magmatism in this part of the Tanzania Craton was punctuated by a period of uplift, exhumation, erosion and clastic sedimentation/volcanism, followed by burial and renewed granitic to syenitic magmatism.In eastern Tanzania (Handeni block), in the heart of the East African Orogen, all the dated orthogneisses and charnockites (apart from those of the overthrust Neoproterozoic granulite nappes), have Neoarchaean protolith ages within a narrow range between 2710 and 2630Ma, identical to (but more restricted than) the ages of the Singida Suite. They show evidence of Ediacaran “Pan-African” isotopic disturbance, but this is poorly defined. In contrast, granulite samples from the Wami Complex nappe were dated at ca. 605 and ca. 675Ma, coeval with previous dates of the “Eastern Granulites” of eastern Tanzania and granulite nappes of adjacent NE Mozambique. To the south of the Tanzania Craton, samples of orthogneiss from the northern part of the Lupa area were dated at ca. 2730Ma and clearly belong to the Tanzania Craton. However, granitoid samples from the southern part of the Lupa “block” have Palaeoproterozoic (Ubendian) intrusive ages of ca. 1920Ma. Outcrops further south, at the northern tip of Lake Malawi, mark the SE continuation of the Ubendian belt, albeit with slightly younger ages of igneous rocks (ca. 1870–1900Ma) which provide a link with the Ponte Messuli Complex, along strike to the SE in northern Mozambique.In SW Tanzania, rocks from the Mgazini area gave Ubendian protolith ages of ca. 1980–1800Ma, but these rocks underwent Late Mesoproterozoic high-grade metamorphism between 1015 and 1040Ma. One granitoid gave a crystallisation age of ca. 1080Ma correlating with known Mesoproterozoic crust to the east in SE Tanzania and NE Mozambique. However, while the crust in the Mgazini area was clearly one of original Ubendian age, reworked and intruded by granitoids at ca. 1Ga, the crust of SE Tanzania is a mixed Mesoproterozoic terrane and a continuation from NE Mozambique. Hence the Mgazini area lies at the edge of the Ubendian belt which was re-worked during the Mesoproterozoic orogen (South Irumide belt), providing a further constraint on the distribution of ca. 1Ga crust in SE Africa.Hf data from near-concordant analyses of detrital zircons from a sample from the Tanzania Craton lie along a Pb-loss trajectory (Lu/Hf=0), extending back to ∼3.9Ga. This probably represents the initial depleted mantle extraction event of the cratonic core. Furthermore, the Hf data from all igneous samples, regardless of age, from the entire study area (including the Neoproterozoic granulite nappes) show a shallow evolution trend (Lu/Hf=0.028) extending back to the same mantle extraction age. This implies the entire Tanzanian crust sampled in this study represents over 3.5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the Tanzanian Craton that was subsequently reworked was composed of a very depleted, mafic source with a very high Lu/Hf ratio. Our study helps to define the architecture of the Tanzanian Craton and its evolution from a single age-source in the early Eoarchaean.

Syn-collisional granitoids in the Qilian Block on the Northern Tibetan Plateau: A long-lasting magmatism since continental collision through slab steepening

In this paper we present a new model that can explain the large zircon age spectrum of ~510 – 420Ma within a single sample from the Gangcha (Gcha) biotite granodiorite and the Huangyuan (HY) two-mica monzogranite on the northern Tibetan Plateau. The large age spread recorded in zircons is characteristic of granitoid samples from the studied region, which is best explained by the long-lasting magmatism since the onset of continental collision at ~500Ma, followed by slab steepening and the ultimate slab break-off at ~450Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (ISr[450]=0.709 to 0.715), Nd (ԐNd[450]=−6.5 to −3.7), Hf (ԐHf[450]=−4.3 to 1.5) and Pb (206Pb/204Pb[450]=17.70 to 17.17; 207Pb/204Pb[450]=15.60 to 15.69; 208Pb/204Pb[450]=38.04 to 38.73) isotopic variation. The small negative whole rock ԐNd[450] and ԐHf[450] values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative ԐHf[450] values (e.g. −50) are indicative of input from the lower continental crust and subducted sediments. The correlated variations among major elements, trace elements and radiogenic isotopes are best interpreted as reflecting melting-induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation. The inherited zircons of Palaeo-Proterozoic age and the Archean crustal model ages signify the involvement of ancient basement rocks.

Comparison between natural radioactivity levels andgeochemistry of some granitoids in western Turkey

Granitoids commonly include K-feldspar, biotite and zircon, apatite, titanite, allanite, and xenotime crystals, which are known to contain common radioactive elements. Radioactive isotopes of 40K, 238U, and 232Th can be harmful to human health with increasing dosage and their quantification should be well defined to assess the suitability of wall-rock granitoids for indoor and outdoor purposes. In this study, it is aimed to correlate the possible relationship between concentrations of natural radionuclides and SiO2, Na2O, K2O, and CaO together with elements U, Th, Zr, Y, Ba, Rb, and Sr to provide a basic approach to the compatibility of geochemical data with natural radioactivity levels of granitic to dioritic rocks in western Turkey. 226Ra, 228Ac, and 40K radioactivity concentrations of the granitoid samples from seven locations range between 15.6 ± 1.5 and 139.7 ± 11.2, 12.0 ± 1.1 and 93.4 ± 9.0, and 297.5 ± 15.5 and 880.2 ± 47.5 Bq/kg, respectively. The lowest 226Ra, 228Ac, and 40K values occur in the Karaburun granitoids while the Buldan granitoids have the highest values. Our data confirm that the silica-rich acidic granitoids have higher natural radioactivity levels than silica-poor basic granitoids and high natural radioactivity levels have been closely associated with high SiO2, Na2O, K2O, Rb, and Ba contents, which may be explained by postmagmatic events of metasomatism and alteration. CaO, Sr, Y, and Zr do not show any correlation with natural radioactivity levels. Natural radioactivity parameters of the studied granitoids are within the safe dosage limits specified in international standards and are safe for use as construction materials. However, metasomatized or strongly altered granitoids may have elevated natural radioactivity levels and hence careful attention is needed for such granitoids.

Geochronological, geochemical and mineralogical constraints of emplacement depth of TTG suite from the Sinassi Batholith in the Central African Fold Belt (CAFB) of northern Cameroon: Implications for tectonomagmatic evolution

The Sinassi Batholith in the Central African Fold Belt (CAFB) of northern Cameroon represents the largest volume of plutonic rocks or granitoids massif of the Western Cameroonian Domain. It is made up dominantly of tonalite-trondhjemite-granodiorite (TTG) suite and lesser granite which are locally more or less deformed, and composed of varying proportions of quartz, plagioclase, K-feldspar, biotite, hornblende, sphene, magnetite, apatite and zircon.Major and trace element compositions of fifteen rock samples of granitoids (Djourdé granodiorite, Sinassi quartz diorite and orthogneisses groups) indicate that investigated rocks from the Sinassi Batholith are characterized by medium- to high-K calc-alkaline affinity and metaluminous I-type signature. In addition, their chondrite- and primitive mantle-normalized trace element patterns are strongly fractionated ((La/Yb)N = 2.96–61.40) and show respectively enrichment in LREE relative to HREE and enrichment in LILE compared to HFSE with moderate to slight negative Nb–Ta, Ti and Eu anomalies consistent with a continental magmatic arc setting related to a subduction zone.Geothermobarometric calculations using hornblende-plagioclase thermometry and aluminum-in-hornblende barometry on eleven rock samples indicate that plutons from Sinassi Batholith were emplaced at average temperatures and pressures ranging between 698 and 720 °C and 4.06–5.82 kbar (Djourdé granitoids), 698–728 °C and 4.04–5.34 kbar (Sinassi granitoids) and 667–670 and 4.23–4.76 kbar (orthogneisses group) respectively. The average emplacement depths estimates for the investigated granitoids is constrained at ca 16–18 km, indicating that at least 16 km of crustal rocks of the Sinassi Batholith must have been eroded or uplifted at approximately exhumation rates of 0.08–0.10 mm/year.Regardless of their Th/U ratios, geochronological results highlight three main events characterizing the Neoproterozoic tectonomagmatic evolution within the Sinassi Batholith at the northern margin of CAFB of northern Cameroon: (1) inherited magmatic sources ranging from one group to another at ca 692, 713, 722, 746 and 772 Ma; (2) successive emplacement ages at ca 670–686 Ma (orthogneisses group), ca 661–666 Ma (synchronous emplacement of the Djourdé and Sinassi groups) and ca 644 Ma (emplacement age of eyespots quartz diorite in the Sinassi group); and (3) post-magmatic alteration or metamorphism at ca 600 Ma.Correlations with the neighboring Mayo Kebbi Batholith in southwestern Chad show geochemical and geochronological similarities, demonstrating that the Sinassi and Mayo Kebbi granitoids beyond political boundaries are parts of a unique Sinassi-Mayo Kebbi Batholith witnessing a Neoproterozoic subduction-related continental magmatic arc setting of Andean-type.

Geochronology, geochemistry and tectonic implications of Late Triassic granites in the Mongolian Altai Mountains

Although the closure of the Paleo-Asian Ocean in western China and western Mongolia occurred in the Late Carboniferous and Early Permian, widespread intra-continental magmatism continued to occur across this region from the Late Permian to the end of the Triassic. In this study we document field relationships and geochemical characterization of a Late Triassic felsic intrusive complex in the western Mongolian Altai. The plutonic complex occurs as sills, dikes, and small stocks and its composition varies from biotite granite, two-mica granite, to leucogranite. Structurally, the plutonic complex occurs in the hanging wall of a segment of the regionally extensively (>1500km long) Irtysh–Ertix–Bulgan thrust zone. As the plutonic bodies both cut and are deformed by the shear fabrics in this regional thrust shear zone, the duration of felsic magmatism and regional thrusting was temporally overlapping. This suggests that magmatism was coeval with crustal thickening. Major- and trace-element data and isotopic analysis of granitoid samples from our study area indicate that the felsic intrusions were derived from partial melting of meta-sediments, with the biotite and two-mica granite generated through vapor-absent melting and the leucogranite from flux melting. Although the Mongolian Altai intrusions were clearly originated from anatexis, coeval granite in the Chinese Altai directly west of our study area in the hanging wall of the Irtysh–Ertix–Bulgan thrust was derived in part from mantle melting. To reconcile these observations, we propose a Himalayan-style intracontinental-subduction model that predicts two geologic settings for the occurrence of felsic magmatism: (1) along the intracontinental thrust zone where granite was entirely generated by anatexis and (2) in the hanging wall of the intracontinental thrust where convective removal and/or continental subduction induced mantle melting.