Precambrian Metasedimentary Rocks Research Articles

Geology and ore fluids geochemistry of the Hamzeh–Gharanin orogenic gold deposit, NW Iran

The Hamzeh–Gharanin gold deposit (HGD) is located in the Qolqoleh–Kasnazan shear zone in the northwest of Sanandaj–Sirjan Zone (SSZ) and 40 km SW of Saqez. The main rock units are an assemblage of the Precambrian metasedimentary (pelitic) rocks and the Cretaceous phyllite associated with marble interlayers. The post–Cretaceous intrusive granitoids with NW–SE trends intruded within these two units. Mineralization occurs mainly within the NE–SW extensional veines and veinlets in proto–mylonitic, mylonitic and ultra–mylonitic rocks. Hydrothermal alterations consist of sericitization, carbonatization, silicification and chloritization. The main ore minerals include pyrite, chalcopyrite, arsenopyrite, pyrrhotite, sphalerite, galena, gold, magnetite and hematite. The fluid inclusions study from quartz minerals associated with the mineralization shows four different phases: single vapor phase (V; type I), single liquid phase (L; type II), liquid–vapor biphasic (LV; type III) and three phases (LLV; type IV). Microthermometric studies were performed on types III and IV of fluid inclusions. Homogenization temperatures (Th) for Types III and IV inclusions vary between 138 and 310 °C (mean 230 °C) and 250–335 °C (mean 300 °C) and their salinity varies between 0.52 and 13.87 and 4.79 to 9.58 (wt% NaCl), respectively. Moreover, the density of Types III and IV inclusions vary from 0.71 to 0.927 g/cm3 and 0.700–0.870 g/cm3, respectively. The oxygen (in quartz: 4.58–7.80 ‰; mean 5.82 ‰) and δ34S (in pyrite: 5.54–6.28 ‰; mean 5.91‰) isotope compositions of the mineralizing fluids indicate an insignificant meteoric water input during hydrothermal evolution and a magmatic origin of sulfur in the deposit which is broadly similar to those of orogenic gold deposits worldwide.

Gold endowment and unloading along pathway for giant gold mineralization: Insights from spatiotemporal variations of in-situ pyrite geochemistry and gold fineness from the Jiaodong gold deposits, north China Craton

Over 5000 tonnes of gold were accumulated in the Jiaodong Peninsula at the early Cretaceous, the sources and critical ore-forming processes of which are still controversial. Here, we conducted comprehensive in-situ textural, elemental and isotopic analyses on pyrite and gold grains from the different mineralization styles, hydrothermal stages and depths for three representative gold deposits (Linglong, Xiadian and Jiangjiayao) from the northwestern Jiaodong Peninsula, with the aim to exactly constrain the sources and ore-forming processes. Three groups of δ34Spyrite were identified in the studied deposits, of which the Group 1 shows stable and low δ34S (4.8–7.6‰) throughout the hydrothermal evolution while the Group 2 has elevated δ34S (6.0–12.8‰) coupled with increasing As and Au concentrations. The Group 3 shows moderate δ34S (5.2–9.8‰) accompanied by high Al and Si concentrations. The Group 1 suggests a low-δ34S primary auriferous fluid derived from a mantle-related source, whereas the Group 2 indicates the primary auriferous fluid leaching additional S, As and Au from the Precambrian metasedimentary rocks along pathway. The Group 3 is petrographically coupled with numerous silicate inclusions, indicating enhanced fluid-rock interaction between the auriferous fluids and the granitic wallrcoks. Gold fineness decreases from the early to the late hydrothermal stages, combined with increasing deposition of polymetallic sulfides, suggesting an increase of pH resulting in efficient Au deposition. Gold fineness and Bi concentrations in pyrite decrease with shallowing, suggesting that cooling affected Au deposition as well. Bismuth-Te-S minerals were identified in the deep depths, combined with the well correlated Bi and Ag, implying that Bi-Te complexes might play a role in transporting Au and Ag at high-temperature conditions. The above results corroborate that multiple sources and ore-forming processes were responsible for the giant gold mineralization, of which the gold endowment from Precambrian wallrocks needs to be further concerned.

Formation of an Intracontinental Orogen Above the Permo-Triassic Mantle Convection Cell in the Paleo-Tethys Tectonic Realm due to Far-Field Stress Derived From Continental Margins

The identification of intraplate orogens seemingly poses challenges to the plate tectonic theory. Delineating the formation processes of intraplate orogens can provide clues for the better understandings of the above issue. Although still controversial, the Indosinian (Permo-Triassic) orogeny in the South China Block (SCB) is potentially a good example of intracontinental orogen. In this paper, we carry out studies on the Indosinian high-grade rocks in the northeastern Cathaysia Block of the SCB, hoping to cast light on the features and formation processes of intraplate orogenic belts. These rocks exhibit HP/HT granulite facies mineral assemblages and reaction textures imply that they witnessed eclogite-facies metamorphism. Their clockwise P-T trajectories with isothermal decompression stages suggest significant crustal thickening followed by quick orogenic collapse. Immobile whole-rock trace elements indicate basaltic protoliths features, resembling E-MORB and OIB, respectively. SIMS zircon U-Pb age dating confirms Indosinian metamorphic ages of ∼248 Ma and a protolith age of ∼953 Ma. The mantle-like O isotopic compositions of the Neoproterozoic magmatic zircon cores further attest that they were primarily mantle derived rocks. The whole-rock Sm-Nd isotopic compositions show more enriched features because of metamorphic alteration, while zircon Lu-Hf isotopic results show primitive characteristics with Neoproterozoic model ages. These features suggest that the high-grade mafic rocks, as well as the metamorphosed early Precambrian metasedimentary rocks hosting them, are all continental crust components and juvenile oceanic crust components featuring plate margins are absent during the SCB Indosinian orogeny. Characteristics of these high-grade rocks and their spatial occurrences are both consistent with the proposal of an intracontinental orogen. After summarizations and comparisons of the Indosinian plate margin activities around the SCB, we suggest that this northeast-southwest trending orogenic belt is geometrically consistent with two mantle convection cells, with one conveying the SCB northward to collide with the North China Craton, and the other conveying the Paleo-Pacific plate northwestwards to form an active continental margin along the southeast SCB. The driving mechanism of the formation of the SCB Indosinian intracontinental orogenic belt could have broad implications for other intraplate orogens around the world.

Precambrian tectonic affinity of the Beishan Orogenic Belt: Constraints from Proterozoic metasedimentary rocks

The Precambrian metasedimentary rocks that are extensively distributed in the Beishan Orogenic Belt (BOB) can provide crucial geological information to constrain its tectonic affinity and geological evolution. Field investigation, zircon U–Pb dating and Lu–Hf isotopic analyses were conducted in combination for investigating them. Detrital zircon U–Pb dating shows that a quartzite sample of the Beishan Complex yields the youngest age at ca. 1710 Ma, with a major age peak at 1828–1778 Ma. Five quartzite samples from the Gudongjing Group and Pingtoushan Formation exhibit similar detrital zircon age distributions mainly from the late Paleoproterozoic to Mesoproterozoic, with the youngest ages at ca. 953 Ma, ca. 1000 Ma, ca. 978 Ma, ca. 960 Ma and ca. 985 Ma. Considering the 1.4 Ga granitoids and age peak of the early Neoproterozoic metasedimentary rocks, the supracrustal rocks within the Beishan Complex were probably deposited during the late Paleoproterozoic to early Mesoproterozoic (ca. 1.7–1.4 Ga). Moreover, owing to the presence of ca. 933–871 Ma granitoids, the deposition ages of the Gudongjing Group and Pingtoushan Formation can be constrained during the early Neoproterozoic (ca. 953–933 Ma). The metasedimentary rocks, the same as the Meso-Neoproterozoic granitic magmatism, support the existence of a Precambrian microcontinent in the BOB. The early Neoproterozoic metasedimentary rocks have continuous detrital zircon ages during the Mesoproterozoic, mainly with positive εHf(t) values. Integrated with the magmatic records of ca. 0.9 Ga and ca. 1.4 Ga, we believe that the Precambrian basement of the BOB could be a fragment drifted from the Fennoscandia of the Baltica Craton. Field investigation revealed that carbonate rocks, mature sandstones and siltstones primarily comprise the protoliths of the early Neoproterozoic metasedimentary rocks, implying the sedimentation in a passive continental margin. However, an almost contemporaneous granitic magmatism was formed in a continental arc setting associated with the assembly of Rodinia. Therefore, it can be inferred that the BOB could be located in the periphery of the Rodinia supercontinent, where the sedimentation of the passive continental margin occurred at its outboard position.

Anniversary of the commencement of the sg-3 Kola superdeep borehole drilling operations

The presented article reviews the history of drilling the deepest well in the world, located in the northeastern part of the Early Proterozoic Pechenga structure (Kola Peninsula). In 1990, the Kola Superdeep Borehole (SG-3) reached a record depth of 12,262 m. SG-3 fully completed all the assigned tasks, both in technical and scientific terms. The 4024 linear meters of core were extracted, including drill samples from a record depth. Overall, a whole complex of logging geophysical works with a total length of 400 thousand km was performed through the use of 25 methods. This allowed obtaining the latest information on the structure and composition of the deep horizons of the earth's crust, which qualitatively changed the data of previous deep seismic studies. Also, the emerging data on the chemical and mineral composition, as well as the research of physical properties of deep rocks, proved the presence of ore mineralization and metal-bearing hydrothermal systems. SG-3 at its depth uncovered new ore bodies of copper-nickel ores; six types of ore mineralization have been identified throughout the section of the borehole. High permeability for fluids and hydrothermal solutions of deep zones of the earth's crust was established, which is associated with increased porosity and rocks fracturing. It was also proved that the earliest Precambrian metasedimentary rocks were formed with the active participation of biological processes. The presented analysis of the deep-hole drilling experience allows the author to confirm the status of SG-3 as the deepest artificial point on Earth.

Comment on “How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology’’ by Owona et al. [Precambrian Res. 359 (2021) 106220

On the basis of published data, interpretations for the sources, deposition ages, and timing of Eburnean and Pan-African orogeneses in the Nyong Complex (NC) and the Yaoundé Group (YG) are questioned. We argue that the supracrustal rocks in the NC and YD derived from various Meso- to Paleoproterozoic and Archean to Neoproterozoic sources, respectively. The minimum time interval for sediment deposition in the NC is ca. 2500–2005 Ma, compared to ca. 900–580 Ma in the YG. Apart from 2092 ± 1 Ma and ca. 1800 Ma overprinting events recorded in the NC only, tectono-metamorphic/magmatic events dated at ca. 900 Ma, ca. 700 Ma, ca. 650 Ma, ca. 600 Ma, and ca. 550–500 Ma have affected both the NC and the YG. It follows that Pan-African orogeny was a long-lasting event of more than 300 Ma.

Reply to comment on “How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology’’ by Hubert Mvondo and Thierry Bineli Betsi

The Congo craton hosts banded iron formations (BIFs) in southern Cameroon. The Gouap iron deposit is in the central part of the Nyong Complex, next to the Ntem Complex in the northwestern part of the East Gabonian Block of the Congo craton. The Gouap BIFs are interbedded with metasiliciclastic rocks consisting of gneisses, schists, and quartzites. We present the first integrated study combining whole-rock and Sm-Nd isotope geochemistry with SIMS and LA-ICP-MS U-Pb-Hf-O isotope analyses of detrital zircons from metasiliciclastic rocks from the Gouap iron deposit to assess the provenance and tectonic setting during deposition of BIFs as well as the crustal evolution at the northwestern margin of the East Gabonian Block. Geochemical data for the metasiliciclastic rocks indicate that their protolith comprised a sequence of greywacke. Along with regional geology, our U-Pb-Hf-O detrital zircon and Sm-Nd whole-rock isotope data suggest that these rocks were predominantly derived from Neoarchean charnockite and TTG suites of the Ntem Complex with a variable and episodic contribution from ca. 2.2–2.0 Ga charnockites of the Nyong and Ogooué complexes. Metasiliciclastic rocks and BIFs were deposited between ca. 2.1 and 2.0 Ga in a foreland basin developed along the northwestern continental margin of the East Gabonian Block. So far unrecognized ca. 2.50–2.45 and 2.45–2.22 Ga magmatic rocks were exposed in their provenance. The Gouap succession experienced amphibolite-facies metamorphism when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny to form the core of the Nuna/Columbia supercontinent. With the subduction dipping to the west, under the West Gabonian Block, slices of Paleoproterozoic oceanic crust and sediments scraped off the margin of the East Gabonian Block formed an accretionary prism. BIFs of the Nyong Complex likely represent a deep-water temporal equivalent of shallow-marine Mn-deposits of the Francevillian basin, Gabon, and together they belong to the same redox-fractionated deep-water upwelling system that was charged with iron and manganese via hydrothermal alteration of oceanic crust.

How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology

The Nyong Complex and the Yaoundé Group represent two crustal sequences of West Gondwana in Cameroon. We use U-Pb zircon geochronology on meta-sedimentary rocks to analyze their depositional age, provenance, and age of the high-grade metamorphic overprint. Due to the undoubtedly Precambrian age of these rocks, we also pursue questions regarding the veiling of the provenance signal by subsequent overprints, the diversity of the sources, and the role of crustal reworking in such an old setting. Although both the Nyong and the Yaoundé meta-sedimentary rocks were strongly affected by the Pan-African orogeny, Discordia analysis and the evaluation of remnant concordant zircons allow addressing these classic provenance questions with the LA-ICP-MS U-Pb zircon technique. We show that the Nyong Complex meta-sedimentary rocks likely have a <2.4 and> 2.2 Ga depositional age (∼Rhyacian, early-middle Paleoproterozoic) and were locally sourced from the Meso-Neoarchean Congo Shield (∼2.8 Ga); they were overprinted during the Paleoproterozoic Eburnean orogeny (∼2.2–2.0 Ga). The Yaoundé Group meta-sedimentary rocks likely have a <0.7 Ga depositional age (Cryogenian, middle Neoproterozoic); they lack a major Archean source, and have distinct, likely locally-sourced Eburnean (∼2.0 Ga), and Tonian-Cryogenian (∼1.0 and 0.7 Ga) sources. The meta-sedimentary rocks of both units share the Pan-African (∼0.6 Ga) orogenic overprint. Our study highlights the limited source diversity of these old basin deposits with a provenance from the few existing crustal masses.

Regional variations of sulfur isotope compositions for metallic deposits in South Korea

AbstractSouthern Korean peninsula comprises five major geotectonic provinces, throughout which various metallic deposits are distributed. We reviewed sulfur isotope data (n = 1,574) of sulfide minerals collected from previous works for 177 metallic deposits in the provinces to interpret the sulfur isotope characteristics of each province, comprising different wall rocks and geologic settings. The averaged δ34S values of each metallic deposit associated with Precambrian metamorphic rocks and Jurassic granitoids in the Gyeonggi massif and Yeongnam massif range from −7.1 to +10.3‰ (av. +4.5‰) and from −3.6 to +7.8‰ (av. +3.5‰), respectively. The Taebaeksan basin produced the highest δ34S value among the five, −0.4 to +13.2‰ (av. +6.1‰). This was influenced by sulfate sulfur derived from marine carbonate host rock. The Okcheon metamorphic belt, comprising metasedimentary and metavolcanics rocks, shows an isotope range from +1.9 to +8.3‰ (av. +5.7‰). The sulfur isotope distribution of the Gyeongsang basin with a range from −1.2 to +11.7‰ (av. +5.2‰) can be divided into two zones: higher δ34S values from the inner zone related to the volcanic rocks and magnetite‐series Cretaceous granitoids, and lower δ34S values from the outer zone related to the organic‐rich sedimentary rocks. Sulfur isotope variations of metallic deposits in each geotectonic province were mainly influenced by igneous sulfur and inherent wall rock sulfur sources, 32S‐enriched sedimentary sulfur (e.g., Precambrian metasedimentary rocks and biogenic sulfur‐rich sedimentary rocks), and 34S‐enriched seawater sulfur (e.g., carbonates and acid to intermediate volcanic rocks). These wall rocks also contributed to the changes in δ34S values for granitoid rocks and metallic deposits by time, ore genetic type, and ore species in South Korea.

Genesis of the Hebaoshan gold deposit in Fujian Province of Southeast China: constraints from a combined fluid inclusion, H-O-C-S-Pb-He-Ar isotope and geochronological study

The large Hebaoshan gold deposit (41.5 t Au, average grade: 3.5 g/t) is located in a hitherto poorly documented gold province in the northeastern part of the South China Block. It is hosted by Precambrian metasedimentary rocks that experienced Triassic greenschist- to amphibolite-facies metamorphism. Three hydrothermal stages can be distinguished: quartz + sericite + pyrite + pyrrhotite (stage I), quartz + magnetite + sulfide (stage II), and quartz + carbonate + pyrite + hematite (stage III). Auriferous pyrite samples yielded a Rb-Sr isochron age of 234 ± 3 Ma (2σ, MSWD = 0.24), and ore-related hydrothermal sericite yielded a 40Ar-39Ar plateau age of 226.4 ± 2.0 Ma (2σ, MSWD = 1.01), which are distinctly younger than ca. 430 Ma granitic rocks in the mining district and older than 170–130 Ma granites in the region, ruling out any genetic link between granite emplacement and gold mineralization. The mineralizing fluid system can be described as carbonic-aqueous with low to moderate salinity (2.2–10.1 wt% NaCl equiv.) and medium temperature of 290 to 350 °C (stage I) and 230 to 280 °C (stage II). The δ18O quartz ranges from 13.3 to 15.4 ‰, and the δD values for fluid inclusions in quartz range from − 97 to − 60 ‰. The calcite has C-isotopes ranging from − 6.1 to − 3.8 ‰ and O-isotopes from 2.8 to 13.6 ‰. H-O-C isotope data are consistent with a metamorphic fluid derived from devolatilization of Neoproterozoic basement rocks during regional metamorphism in the Late Triassic Indosinian period. The δ34S values of sulfides for stages I, II, and III are 0.1 to 7.8 ‰, − 10.6 to 5.1 ‰, and − 18.9 to − 14.1 ‰, respectively, indicating an increase in oxygen fugacity during fluid ascent which resulted in more negative sulfur isotope values of sulfides and precipitation of gold. Pyrite separates have 206Pb/204Pb ratios of 17.340 to 17.687, 207Pb/204Pb ratios of 15.539 to 15.604, and 208Pb/204Pb ratios of 37.749 to 38.094. The S-Pb isotope data suggest derivation of S and Pb from the Precambrian metasedimentary country rocks. Auriferous pyrite yielded elevated 3He/4He ratios (0.78–1.46 Ra), which suggest a mantle component as can be expected from fluids derived from a subduction setting. It is concluded that Hebaoshan is an orogenic gold deposit that formed during Triassic flat-slab subduction of the paleo-Pacific plate beneath the South China Block and thus constitutes the first Triassic deposit of this type recognized in the coastal area of Southeast China.

Recognition of a Permian to Triassic foreland basin in the central Beishan Orogen, NW China: Provenance variations and their constraints on latest Palaeozoic orogeny

The Beishan Orogen, located in the southernmost Central Asian Orogenic Belt, is a key area for research on accretionary orogens. The formation of the Beishan Orogen is closely related to the evolution of the Paleo-Asian Ocean, but the closure time of a significant branch of the Paleo-Asian Ocean in the Beishan area, i.e., the Hongliuhe–Niujuanzi–Xichangjing (HNX) Ocean, remains controversial. This study investigates the sedimentary sequences of Baihu Basin adjacent to the HNX suture zone and presents new detrital zircon UPb geochronology and whole-rock geochemistry data to reveal depositional ages, provenance variations, paleogeography changes and attributes of Baihu Basin. Detrital zircon ages indicate that deposition began in the Permian and lasted until the Triassic. Two coarse sandstones in Member 1 and one siltstone in Member 2 contain abundant detrital zircons older than 700 Ma, in addition to the typical age peak that characterises the Gongpoquan Arc at ca. 450 Ma, while two sandstones in Member 3 contain rare detrital zircons older than 700 Ma. The whole-rock geochemistry shows that the lower part of the Hongyanjing Formation possesses numerous old components and higher chemical index of alteration values. The sediment in the lower part of the Hongyanjing Formation was derived from both the Gongpoquan Arc, mainly comprising Palaeozoic arc-related magmatic rocks, and the Shuangyingshan Arc, including large quantities of Precambrian meta-sedimentary rocks. The upper part originated solely from the juvenile Gongpoquan Arc. During the collision, the Shuangyingshan Arc played a role similar to a craton. Craton-derived source rocks and fining-upward sequences in the lower part indicate uplift of the Shuangyingshan Arc, which may represent the development of a forebulge and foredeep. In the upper part, the source rocks were derived from the Gongpoquan Arc and accretionary wedges, with coarsening-upward sequences reflecting the springback of the Shuangyingshan Arc and disappearance of the forebulge. Based on the sedimentary sequences, provenance variations, and paleogeographic changes, we suggest that Baihu Basin is a foreland basin associated with the collision between the Gongpoquan Arc and Shuangyingshan Arc, marking the final closure of the HNX Ocean from the Late Permian to Early Triassic. We investigate the closure of the HNX Ocean and reconstruct the evolution of the central Beishan Orogen in the youngest Palaeozoic.

Hydrothermal evolution and ore genesis of the Laozuoshan Au skarn deposit, northeast China: Constrains from mineralogy, fluid inclusion, and O–C–S–Pb isotope geochemistry

The Laozuoshan mine, located in the central Jiamusi block, is the largest gold skarn deposit in Northeast China. It comprises three ore zones (East, Middle, West) with complex stratigraphic and structural controls. The skarn-type orebodies mainly occur in the East and Middle ore zones along the contact zones between Hercynian granite and Precambrian metasedimentary rocks. The earliest stage of alteration is characterized by potassium hornfels. Subsequently, prograde skarn formation occurred, characterized by garnet in the grossular-andradite series and pyroxene in the hedenbergite-diopside series. This was followed by a retrograde skarn overprint of hydrous silicates and ore minerals. Three types of garnet with distinct textures and elemental compositions are recognized, which indicate fluctuating physicochemical conditions in the hydrothermal fluids.The fluids associated to the prograde skarn are characterized by high temperature and high salinity, which underwent fluid boiling that led to precipitation of minor pyrite and arsenopyrite. The main gold and sulfide mineralization is related to the retrograde skarn overprint, where cooling and strong fluid-rock interactions occurred. Gold precipitation is estimated to have occurred at temperatures of 380–470 °C, based on the arsenopyrite geothermometer and fluid inclusion microthermometry. The fluid inclusion and C-O isotope data indicate that the ore-forming fluids were dominated by magmatic water during the prograde skarn stage and the main ore-forming stages. Mixing between meteoric and magmatic fluids occurred during the latest stages of the retrograde skarn formation. Both sulfur and lead isotope compositions of sulfides support a magmatic origin for the ore-forming materials.On the basis of the available geological and geochemical evidence, we propose that the Laozuoshan deposit is a reduced gold skarn deposit, genetically related to the granitic pluton.

Os processos geológicos por trás dos sítios arqueológicos da Serra do Espinhaço Meridional

The southern Espinhaco range, an important physiographic landmark in Minas Gerais territory, hosts a vast network of archaeological sites, whose material and cultural collection have been crucial for investigating the early americans. Most of these sites occur in natural rock shelters such as ridges and caves, developed in precambrian metasedimentary rocks. This paper aims to present, in a didactic way, the main geological processes involved in the genesis of these shelters. The sedimentation, diagenesis, metamorphism and deformation processes influence the composition and structure of the rocks, and these characteristics, in turn, exert strong control over the weathering and erosion processes that, ultimately, give rise to the relief forms that favor human occupation. This demonstrates an intimate relationship between the inorganic and the organic, between the geological and the anthropological, in time and space, in an excellent example of interaction between the various and complex elements of the Earth System.

Crustal contamination and genesis of transitional alkaline-tholeiitic intrusions: Insights from the José Fernandes Suite, Paraná Magmatic Province, Brazil

The José Fernandes suite, located in the Ponta Grossa Arch (PGA) region of southern Brazil, is an approximately 3 km2 composite intrusion associated with the Early Cretaceous (134 Ma) intrusive magmatism that formed the Paraná-Etendeka Magmatic Province. The intrusion is emplaced in Precambrian metasedimentary rocks and composed of gabbroic rocks (including a marginal quartz-bearing variety) intruded by synplutonic alkaline dikes. Petrographic and geochemical characteristics indicate the important influence of crustal contamination on the evolution of the suite. Disequilibrium textures and mineral chemical zoning are widespread in the rocks composing the pluton and are evidence of open-system processes. Orthopyroxene (Fe-enstatite) (Mg# 49-68) occurs in reaction rims of iron-rich olivine (Mg# 42-61) and symplectic intergrowths, together with magnetite and ilmenite. The presence of Fe-enstatite can be related to partial melting of host rocks resulting in non isochemical reactions of olivine with fluids under oxidizing conditions. The positive correlation of SiO2 and Sr—Pb isotope initial ratios and negative correlation with εNd indicate different levels of crustal contamination. The alkaline dikes that cut the pluton have basanite to basaltic trachyandesite compositions and vary from primitive to contaminated (87Sr/86Sri = 0.7049 to 0.7070) dikes, confirming that the intrusion formed from several batches of alkaline magma with distinct crustal contributions. Given the enrichment in K2O over Na2O in samples with high initial 87Sr/86Sr (>0.707) and evidence of a low time-integrated Th/U component typical of the upper crust, metapelitic country rocks are considered potential contaminants in the upper crustal chamber environment. A pole of contamination with low 143Nd/144Nd signatures indicates contamination by gneissic basement with Archean crustal residence at lower crustal levels. Isotopic modeling is consistent with an evolution in which several batches of basanite magma evolved to more radiogenic compositions by assimilation of different proportions (up to ~35%) of partial melts from different crustal contaminants. These contaminants have older crustal residence times than those involved in the generation of silicic magmas associated with the tholeiitic basalts of the Paraná-Etendeka Province.

Geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of granitoids in eastern Tibet and implications for tectonic correlation with southeastern Asia

Understanding the configuration of continental blocks within the Tibetan orogen prior to Indo-Asian collision is vital to determining their accretionary history. However, complex tectonic overprinting, including the development of the eastern and western Himalayan syntaxes, has hampered the investigation of correlations between tectonic blocks in the Tibetan Plateau with those in southeastern Asia. The North Lancangjiang tectonic belt is known to connect with the Longmu Co–Shuanghu suture to the west and the Changning-Menglian suture to the south, but limited work has been undertaken in eastern Tibet, and no ophiolite suites or high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic belts have been identified within this belt. This paper presents new in situ zircon U-Pb, whole-rock major- and trace-element, and whole-rock Sr-Nd-Hf isotopic data for three granitic plutons (Chaya, Jinba, and Batang) within the North Qiangtang block. These plutons yield ages from ca. 265 Ma (Chaya) to ca. 230 Ma (Batang). The Chaya and Batang granites are chemically similar to I-type granites, whereas the Jinba granite is an S-type granite. Sr-Nd-Hf isotopic data suggest that the Chaya granite formed from magmas generated by the partial melting of subduction-modified mantle wedge material and that the Jinba granite formed from magmas generated by the partial melting of Precambrian metasedimentary rocks mixed with a small amount of juvenile mantle–derived crustal material. The Batang granite formed from magmas generated by the partial melting of mafic lower-crustal material. The convergence of the North Qiangtang block with neighboring blocks was a consequence of the closure of the Paleo-Tethys Ocean, and the plutons in this area record subduction-related or syncollisional magmatism. New data from granites within the North Lancangjiang magmatic belt and the Jinshajiang suture indicate that the North Lancangjiang Paleo-Tethyan slab subducted eastward, whereas the Jinshajiang Paleo-Tethyan slab subducted westward under the North Qiangtang block at ca. 267 Ma. The timing and style of block convergence between North Qiangtang–Simao–Indochina and neighboring blocks are similar to those recorded within central Tibet and southeastern Asia. This, combined with the presence of similar Middle Mesoproterozoic–Lower Paleozoic basement materials and Paleozoic sediments hosting Cathaysian warm-climate fossils, indicates that North Qiangtang, Simao, and Indochina represent a single block.

Zircon U-Pb-Hf isotope studies of the early Precambrian metasedimentary rocks in the Kongling terrane of the Yangtze Block, South China

Some metamorphosed pelites (commonly called as kondalites) expose over the Archean crystal basement in the Kongling terrane of the Yangtze Block. These sedimentary rocks mainly contain biotite – plagioclase gneisses, garnet bearing biotite – plagioclase – silimanite gneisses, garnet bearing biotite – plagioclase – silimanite – cordierite gneisses, graphite schists, and some marbles and quartzites. Amphibolites and mafic granulites always occur as lenses or layers in these metasedimentary rocks. In this study, three kinds of metasedimentary rocks, which are Grt-Bi-Pl gneiss, Grt-Sil-Crd-Bi-Pl gneiss and Grt-Sil-Bi-Pl gneiss, are chosen for petrographic, zircon U-Pb, and Lu-Hf isotopic analysis. The mineral assemblages of these three rocks are Grt + Bi + Pl + Q, Grt + Sil + Crd + Bi + Pl + Q + Spl + Kf and Grt + Sil + Bi + Pl + Q respectively, which imply granulite facies metamorphism. In addition, garnet porphyroblasts in both of these metasedimentary rocks and the mafic granulites which are enclaves in these meta-sedimentary rocks develop decompressional coronas, indicating isothermal decompression (ITD) processes during retrograde metamorphism. Core-Rim structures in zircons can be observed in all these three samples based on cathodoluminescence images. Morphological characteristics, Th/U ratios and trace element compositions indicate that zircon cores are of magmatic origin while zircon rims and some single grain zircons are of metamorphic origin. Ages of magmatic zircon cores in Grt-Bi-Pl gneiss are between 3532 ± 20 Ma to 2286 ± 47 Ma while those of metamorphic zircons are 2022 ± 22 Ma. Most zircons in the Grt-Sil-Crd-Bi-Pl gneiss are of metamorphic origin and they have a weighted mean 207Pb/206Pb age of 2017 ± 15 Ma. Magmatic zircon cores in Grt-Sil-Bi-Pl gneiss have a weighted mean 207Pb/206Pb age of 2160 ± 31 Ma and metamorphic zircons have a weighted mean 207Pb/206Pb age of 2028 ± 43 Ma. One zircon core (3199 ± 25 Ma) in Grt-Bi-Pl gneiss possesses the oldest two-stage Hf model age of 3676 ± 85 Ma. Two-stage Hf model (TDM2) ages of all the other analyzed zircon cores in this sample range from 3452 ± 40 to 3277 ± 43 Ma, and the metamorphic zircons give a weighted mean TDM2 age of 3471 ± 14 Ma. Metamorphic zircons in the Grt-Sil-Crd-Bi-Pl gneiss have a weighted mean two-stage Hf model (TDM2) age of 3468 ± 16 Ma. These TDM2 ages indicate that the sources of these metasedimentary rocks are obviously older than those documented by previous studies and they also suggest that protolith of these metasedimentary rocks belong to Archean crustal materials of the Kongling terrane, but with obviously different sedimentary sources. The widespread decompressional textures around garnet in various samples in the study area suggest that the Kongling area underwent a high-pressure granulite facies metamorphism and an isothermal decompression process at ∼2.0 Ga, corresponding to the assemblage of the Paleoproterozoic Nuna Supercontinent.

Petrogenesis of the Late Jurassic peraluminous biotite granites and muscovite-bearing granites in SE China: geochronological, elemental and Sr–Nd–O–Hf isotopic constraints

Biotite granites and muscovite-bearing granites are dominant rock types of the widespread granites in SE China. However, their petrogenesis has been enigmatic. A combined study of zircon U–Pb dating and Lu–Hf isotopes, whole-rock element geochemistry and Sr–Nd–O isotopes was performed for three late Mesozoic granitic plutons (Xinfengjie, Jiangbei and Dabu) in central Jiangxi province, SE China. All the plutons are composed of biotite granites and muscovite-bearing granites that have been poorly investigated previously. The new data not only allow us to assess their sources and magma evolution processes, but also helps us to better understand the genetic link to the large-scale polymetallic mineralization in SE China. LA-ICP-MS zircon U–Pb dating shows that three plutons were emplaced in the Late Jurassic (159–148 Ma) and that the muscovite-bearing granites are almost contemporaneous with the biotite granites. The biotite granites have SiO2 contents of 70.3–74.4 wt% and are weakly to strongly peraluminous with ASI from 1.00 to 1.26, and show a general decrease in ASI with increasing SiO2. They have relatively high zircon saturation temperatures (T Zr = 707–817 °C, most > 745 °C) and show a general decrease in T Zr with increasing SiO2. They have high initial 87Sr/86Sr ratios (0.7136 to 0.7166) and high δ18O values (9.1–12.8‰, most > 9.5‰) and clearly negative e Nd (T) (− 9.5 to − 11.8) and e Hf (T) (in situ zircon) (− 13.1 to − 13.5). The muscovite-bearing granites have high SiO2 contents (74.7–78.2 wt%). They are also weakly to strongly peraluminous with ASI of 1.04–1.18 but show a general increase in ASI with increasing SiO2. They have relatively low T Zr (671–764 °C, most < 745 °C) and also show a general decrease in T Zr with increasing SiO2. The muscovite-bearing granites have high Rb (up to 810 ppm) and high (K2O + Na2O)/CaO (up to 270), Rb/Sr (up to 42) and Rb/Ba (up to 30) as well as low K/Rb (< 150, down to 50), Zr/Hf (< 24, down to 11) and Nb/Ta (< 6, down to 2). They show similar Nd–O–Hf isotopic compositions to the biotite granites with e Nd (T) of − 8.7 to − 12.0, δ18O of 8.7–13.0‰ (most > 9.5‰) and e Hf (T) (in situ zircon) of − 11.3 to − 13.1. Geochemical data suggest the origin of the biotite granites and muscovite-bearing granites as follows: Partial melting of Precambrian metasedimentary rocks (mainly two-mica schist) in the lower crust at temperatures of ca. 820 °C generated the melts of the less felsic biotite granites. Such primary crustal melts underwent biotite-dominant fractionation crystallization, forming the felsic biotite granites. Progressive plagioclase-dominant fractionation crystallization from the evolved biotite granites produced the more felsic muscovite-bearing granites. Thus, the biotite granites belong to the S-type whereas the muscovite-bearing granites are highly fractionated S-type granites. We further suggest that during the formation of the muscovite-bearing granites the fractional crystallization was accompanied by fluid fractionation and most likely the addition of internally derived mineralizing fluids. That is why the large-scale polymetallic mineralization is closely related to the muscovite-bearing granites rather than biotite granites in SE China. This is important to further understand the source and origin of biotite granites and muscovite-bearing granites in SE China even worldwide.

Origin of the granites and related Sn and Pb-Zn polymetallic ore deposits in the Pengshan district, Jiangxi Province, South China: constraints from geochronology, geochemistry, mineral chemistry, and Sr-Nd-Hf-Pb-S isotopes

The Pengshan Sn and Pb-Zn polymetallic deposits are located in the south margin of the Jiujiang-Ruichang (Jiurui) district of the Middle-Lower Yangtze River Metallogenic Belt in South China. Four large deposits include Huangjinwa, Zengjialong, Jianfengpo, and Zhangshiba, the former three are Sn-dominant deposits which occur as stratiform orebodies in the contact zones of the Pengshan granites and within the country rock strata, whereas Zhangshiba consists of stratiform Pb-Zn orebodies within the Precambrian metasedimentary strata. In this study, we present results on zircon U-Pb ages, major and trace elements, and mineral chemistry as well as Sr-Nd-Hf isotope data of the granites, Pb and S isotopes of both the Sn-dominant and Pb-Zn dominant deposits, and U-Pb dating of cassiterite from the Pengshan district. SHRIMP and LA-ICP-MS zircon U-Pb dating shows that the Pengshan granites were emplaced in the Early Cretaceous (129–128 Ma), which is in good agreement with the U-Pb dating (130–128 Ma) of cassiterite from the Jianfengpo Sn deposit. The Pengshan granites consist mainly of weakly peraluminous highly fractionated I-type affinity granitic rocks. Detailed elemental and isotopic data suggest that the granites formed by partial melting of Mesoproterozoic metamorphic basement materials with minor input of mantle-derived melts. The mineral chemistry of biotite demonstrates that the Pengshan granitic magma had a low oxygen fugacity, thereby precluding the tin dominantly partitioning into the rock-forming silicate minerals and favoring accumulation in the exsolved residual liquid during magma crystallization stages. Sulfur isotopes show a relatively heavy sulfur isotopic composition from 5.8 to 17.6 ‰, and no difference for sulfur isotopes between the Sn deposits (5.8–13.4 ‰, Huangjinwa, Zengjialong, Jianfengpo) and the Pb-Zn deposit (mostly 7.1–13.0 ‰, except for one 17.6 ‰, Zhangshiba). The sulfur isotope data of pyrite from the host sedimentary rocks show relatively higher δ34S values of 22.7–28.7 ‰. Lead isotopes indicate that the granites have higher 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios than the host Precambrian metasedimentary rocks, and the sulfide minerals from the orebodies fall in between them. Again, there is no difference for Pb isotopes between the Sn deposits (Huangjinwa, Zengjialong, Jianfengpo) and the Zhangshiba Pb-Zn deposit. The coincidence in S and Pb isotope compositions for Sn and Pb-Zn deposits suggests that similar sources for these mineralization, possibly derived from a mixed source of the granitic magmas and the Precambrian metasedimentary rocks. These data therefore favor of a magmatic-hydrothermal origin for both the Sn and Pb-Zn polymetallic mineralization in the Pengshan district.

Conservation implications of the provenance of modern sediment on a shell-rich platform of Lake Tanganyika (Kigoma, TZ)

Shell-rich substrate (shell beds) in the littoral reaches of Lake Tanganyika, Africa, form a unique substrate that host a number of endemic organisms, including sponges and shell-dwelling cichlid fish. The predominant gastropod, Neothauma tanganyensis, whose shells comprise the majority of the shell beds, are themselves endemic to the lake. Sedimentologic and observational data suggest that one of these shell beds, located between the towns of Kigoma and Ujiji, Tanzania, on the western side of central Lake Tanganyika, is impacted by modern sedimentation. In the northern portion of the study discrete patches of the shell beds between ~6–30 m water depth are blanketed by 20 to >50 wt% mud; in these areas there are few to no sponges nor shell-dwelling cichlid fish (as evidenced by lack of occurrences of fish nests with shells). However, in the southern part of the study area, within the same depth range, the shell beds contain less mud, and sponges and fish nests with shells are more common. Mineralogic and geochemical data suggest the patterns and composition of the sediment are consistent with two sources. The sediment accumulating on the shell beds in the northern portion of the study area matches the chemistry of the weathered Precambrian metasedimentary rocks in hillslopes adjacent to the lake, within a region undergoing severe land-use changes. The sediment accumulating along the southern portion of the study site, particularly in deeper water, matches the chemistry of sediment from the Luiche River, which forms a delta at the southern end of the study site. This delta has prograded up to 1 km within the last 40 years; however, in this setting the bulk of deltaic sediment is shunted into deeper water and so appears to represent a less severe threat to the shallow-water shell-bed habitat than the local hillslopes. This is one of a few detailed studies of the sedimentology of a shell-bed habitat in Lake Tanganyika and the first to tie the sediment to its source. This has important implications for conservation of these habitats as methods and practices of conservations vary based on the types of disturbance. The study also has implications for biologists interested in community structure as certain species of cichlids appear to be efficient engineers of the substrate, forming shelly nests, but in areas of high mud content, are absent.

Detrital zircon geochronology and geochemistry of metasediments from the Vorontsovka terrane: implications for microcontinent tectonics

ABSTRACTThe Vorontsovka terrane (VT) is an important component of the East Sarmatian Orogen (ESO) which divides the Precambrian cores of the Sarmatian and Volgo-Uralia segments of the East European Craton (EEC). The tectonic framework of the VT remains controversial due to poor constraints from geochemical and geochronological studies. In this article we present detrital zircon U–Pb ages and geochemical features of the Precambrian meta-sedimentary rocks from the VT, which occur interlayered with calc-silicate rocks and metabasites. Most of the zircons from metasediments possess oscillatory zoning and high Th/U ratios (>0.2), indicating magmatic provenance. Their 207Pb/206Pb ages cluster around 2093 ± 7, 2126 ± 7, 2158 ± 12, 2189 ± 16, and 2210 ± 31 Ma, correlating with the ages of magmatic zircon cores from the surrounding igneous suites, and reflecting a single tectono-magmatic cycle (~2200–2100 Ma) in the source area. Age of the youngest detrital zircon grain from the metasedimentary rocks and the cores of zircon grains from igneous suites show 207Pb/206Pb ages at 2094 and 2106 Ma, respectively. Together with the largest age clusters of 2126 ± 7 and 2158 ± 12 Ma of the magmatic cores of the detrital zircons, the timing of sedimentation is inferred as ~ 2100–2170 Ma.The metapelites display strong rare earth element fractionation with variable Eu anomalies ((La/Yb)N = 7.0–14.5, Eu/Eu* = 0.49–1.23). In contrast, the calc-silicate rocks and metabasites lack Eu anomalies ((La/Yb)N = 5.2–11.5, Eu/Eu* = 0.87–1.00). The large-ion lithophile (LILE) and high field strength element (HFSE) concentrations of most samples are comparable with those of the upper continental crust (UCC). The rocks possess negative anomalies of Th, Nb, Sr, and Zr relative to UCC. Their high Index of Compositional Variability (0.85–1.32, up to 1.8 in metabasites) and relatively low Chemical Index of Alteration (46.1–70.4) indicate that the metapelitic sediments were immature to weakly immature and probably underwent minor chemical weathering. The protoliths of the metabasites are interpreted as interlayered volcano-sedimentary and pyroclastic material. Relict clastic textures of the VT rocks, their geochemical features, and the grain morphology of detrital zircons suggest that the sediments were derived from intermediate and felsic provenances, which were most likely deposited in an environment with active volcanism. We envisage an active continental margin setting in the southwestern part of the Volgo-Uralia segment of the EEC related to the assembly of the Palaeoproterozoic Columbia supercontinent. Combined with recent data from surrounding terranes of the ESO, our results suggest that the VT represents an accretionary prism along a continental arc within the Sarmatia and Volgo-Uralia oceanic realm in the Palaeoproterozoic.