Search

The Yangtze Craton is one of the crucial cratons constitued the eastern Asia continent. It is characterized by Archaean to Paleoproterozoic basement rocks, making it an excellent area to study the Precambrian geologic and tectonic regime. Recently, abundant Archaean rocks have been reported from the Dabie Orogen, which providing important information to understanding the early evolution of the Northern Yangtze Craton. In this contribution, we present petrological, whole-rock geochemical, and zircon U – Pb – Hf isotopic analyses on newly discovered Neoarchean granite in the Dabie Orogen. We identified 2744 ± 10 Ma and 2715 ± 11 Ma (~2.74 Ga) Na-rich granitic gneisses, 2691 ± 6 Ma and 2696 ± 8 Ma (~2.69 Ga) A2-type granitic gneisses, and 2664 ± 8 Ma, 2664 ± 6 Ma, 2672 ± 7 Ma, 2642 ± 10 Ma (~2.66 Ga) A1-type granitic/syenite gneisses from the Daqishan area of the Dabie Orogen. Geochemical and zircon Hf isotope data indicate that the ~ 2.74 Ga Na-rich granitic gneisses were partial melting products of thickened mafic crust with limited depleted mantle input, while the 2.69 Ga and 2.66 Ga A-type granitic gneisses represent felsic magmatism in a post-collisional to an extensional tectonic setting. Based on our research, we categorized the Northern Yangtze Craton into the Kongling domain and Dabie domain by comparing with the distribution of the zircon U-Pb magmatic ages and metamorphic events. We suggest that the initial unified basement of the northern proto-Yangtze Craton formed during the Nuna supercontinent cycle at ~ 2.0 Ga. [ABSTRACT FROM AUTHOR]

The Neoarchean Qingyuan greenstone belt (QGB) is located at the northeastern margin of the North China Craton (NCC) and has experienced amphibolite- to granulite-facies metamorphism. The supracrustal rock assemblage is composed of biotite-hornblende (Bt-Hb) monzonitic gneiss, garnet-biotite-hornblende (Grt-Bt-Hb) monzonitic gneiss, Grt-Bt-Hb plagioclase gneiss, amphibolite and orthopyroxene-biotite-hornblende (Opx-Bt-Hb) plagioclase gneiss. This study involves thorough field and petrographic observations, whole-rock geochemistry and LA-ICP-MS zircon U‒Pb ages of these rocks to constrain the Neoarchean geodynamic setting of the QGB. Zircon U‒Pb dating indicates that these metavolcanic rocks formed during 2.57‒2.52 Ga and experienced subsequent regional metamorphism at 2.52‒2.47 Ga. Lithological and geochemical characteristics show that the QGB protoliths were tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites. The andesite-dacite-rhyolite assemblage exhibits high SiO2 and Mg# values, low Yb and Y contents, strongly fractionated chondrite-normalized REE patterns and depletion in Nb, Ta, Ti and P, resembling Phanerozoic adakites. These geochemical characteristics suggest an origin from the partial melting of a subducted slab interacting with mantle wedge material. The tholeiitic to calc-alkaline basalts exhibit slightly LREE-enriched chondrite-normalized REE patterns and negative Nb and Ti anomalies, like island arc basalts. These basalts might have originated by partial melting of a mantle source affected by metasomatism from subduction-derived fluids and melts. The lithological associations and geochemical characteristics imply that the Qingyuan greenstone belt developed in a continental margin arc setting. [ABSTRACT FROM AUTHOR]

We present new results of geochronological, rock magnetic, paleomagnetic and paleointensity studies of the olivine gabbro dyke located at the northern part of the Murmansk craton, NE Fennoscandia (the Kola Peninsula). According to its geochemistry, petrographic and geochronology features, the dyke belongs to the 2.68 Ga dyke swarm, as confirmed by Sm-Nd mineral isochron. We find a significant difference in the rock magnetic and paleomagnetic characteristics of the central and marginal parts of the dyke, which is independently supported by petrography and geochemistry. It is shown that the rocks of the central part of the dyke retained not only their primary mineral composition, but also the primary component of the natural remanent magnetization. We use its direction to determine the 2.68 Ga virtual geomagnetic pole for the Murmansk craton: Slat = 68.64292° N, Slong = 37.7945° E, N = 41 specimens, Plat = –73.5°, Plong = 138.9°, dp/dm = 3.2°/3.4°, paleolat = –65.9°. We also obtain reliable estimates (17 samples) of the Earth's magnetic field intensity at ca. 2.68 Ga: VDM value is found to be 1.85 × 1022 A m2 corresponding to the geomagnetic field several times weaker than the present-day field. [ABSTRACT FROM AUTHOR]

Archean porphyry-style mineralization is one of the sources of Au and Cu in greenstone belts. Archean porphyries have been modified by regional deformation and late fluid circulation, and questions remain on the timing of Au mineralization. Indeed, Au may have been introduced at the magmatic-hydrothermal (porphyry) stage, or by a post-magmatic hydrothermal fluid (overprinting orogenic gold system), or a combination of these two processes. Using the Cu–Au Corner Bay deposit as a case study and high-quality chemical mapping performed using laser ablation coupled to a time-of-flight mass spectrometer (LA-FF-ICP-ToF-MS), this study demonstrates that pyrite chemistry has the potential to reconstitute the metallogenic model for Archean porphyry systems. The Corner Bay deposit is associated with the ~2718–2715 Ma Chibougamau pluton, located in the northeastern corner of the gold-endowed Abitibi greenstone belt. Pyrite chemistry points to the following succession of events at Corner Bay: (1) an early porphyry stage that produced pyrite-mineralized quartz-anhydrite veins associated to chloritization and sericitization; (2) followed by a porphyry stage that includes brecciation in response to fluid overpressure and deposition of Cu-Au mineralization by magmatic-hydrothermal fluids with a multi-metal signature (Cu, Ag, Bi, Te, etc.); and (3) a post-porphyry deformation event(s) that induced parallel fracturing and minor carbonatization. This study proposes that, at Corner Bay, Au was introduced during the porphyry-stage of mineralization and was neither remobilized, nor further introduced, during the ~2700 Ma regional deformation stage that is coeval with most orogenic gold-style of mineralization in the southern part of the Abitibi greenstone belt. [ABSTRACT FROM AUTHOR]

The Zanhuang Complex is situated on the eastern margin of the Trans-North China Orogen, with the Huangcha Pluton being a constituent of this complex. To ascertain the nature of the approximately 2.5-billion-year-old Huangcha Pluton, crucial evidence for understanding its extensional setting was sought through petrogenesis and dating investigations. LA-ICP-MS dating of zircon from the granite yielded an age of (2488 ± 6) Ma. Primarily composed of porphyritic monzonite with sporadic melanocratic enclaves, the Pluton's phenocrysts are predominantly feldspar with minor quartz. The granite exhibits high SiO2 content (72.64%–74.16%) and alkali levels, with Na2O + K2O ranging from 7.59% to 9.07%, classifying it as a shoshonitic series with a slightly peraluminous feature. Enrichment in large-ion lithophile (LIL) elements (Rb, Th, and U) and depletion in Sr, V, Cr, Co, and Ni were observed, with high Rb/Sr and Ga/Al ratios ranging from 0.73 to 2.72 and 2.75 × 10−4 to 3.11 × 10−4, respectively. The rock exhibits high εNd(t) values, ranging from −0.06 to 0.88, with TDM2 ages falling between 2.79 and 2.87 billion years. Zircon grains display 176Hf/177Hf ratios ranging from 0.281266 to 0.281412 and εHf(t) values spanning from 0.96 to 6.18, calculated using the 207Pb/206Pb age. It is suggested that the Huangcha Pluton represents A-type granite formed via anatexis of the Neoarchean TTG in an extensional setting following orogenic processes. The formation of the Huangcha Pluton further corroborates the stabilization of the North China Craton towards the end of the Neoarchean. This finding supports the hypothesis that the North China Craton may belong to the Rae-family cratons, sharing similar magmatic and tectono-metamorphic records around ~2.5 billion years ago. [ABSTRACT FROM AUTHOR]

Volcaniclastic deposits are cynosures for reconstructing volcanic archives and evaluating the tectonic imprints of the study area. This paper presents an elaborate geological field litholog, petrography and geochemistry of ~1.2 km thick sequence of the quartz-feldspar porphyry (QFP) and rhyolites in the Kadiri–Hindupur section of the Kadiri Greenstone belt from Eastern Dharwar Craton (EDC). The QFP exhibits primary stratification; the thickness of the beds varies from very thin to thick (3–8 cm) and shows large-scale tabular cross-bedding in the lower half of the section. The bedding surfaces of QFPs are marked by current crescent-shaped marks (long axis: 5–25 cm, short axis: 2–8 cm), speculated as possible remnants of volcanic bombs and pyroclastic debris during the explosive volcanism. Petrographically, these QFPs are characterised by spectacular blue opalescent quartz grains with embayed grain boundaries, anhedral feldspar with sieved texture, fractured plagioclase and polycrystalline groundmass suggesting a volcaniclastic origin. They are also endowed with significant post-depositional textures such as domino-listric faulting and sigma-shaped porphyroblasts. Geochemically, QFPs are unaltered with trachyte-trachy dacite compositions showing high-potash, calc-alkaline nature displaying shoschonitic affinity. The primitive mantle normalised trace and rare earth elements are marked by an enriched LREE-LILE pattern coupled by negative zircon–hafnium, niobium–tantalum and titanium anomalies affirming their common genetic link to a prominent island arc–back-arc system where explosive volcanism produced the felsic crystal tuffs that were deposited and preserved as volcaniclastic QFP. The associated blocks and bombs produced the current crescent-shaped marks preserved on the bedding surfaces of QFPs. Research highlights: Research Highlights Quartz-feldspar porphyry (~1.2 km thick) occurs in the Kadiri Greenstone Belt, Eastern Dharwar Craton. Blue opalescent quartz and deformed plagioclase porphyry indicate volcaniclastic origin. Enigmatic current crescent-shaped marks represent remnants of volcanic bombs and pyroclastic debris. The primitive mantle normalised REE diagram is characterised by LREE-LILE enrichment. They represent Neoarchean pyroclastic volcanism and sedimentation in an island arc–back-arc setup. [ABSTRACT FROM AUTHOR]

Wulanwusu phosphate deposit, one of the important phosphate deposits in western Liaoning Province, has a large-scale resource reserve and associates with beneficial component (magnetite). The phosphate orebodies are occurred in the Neoarchean Xiaotazigou complex, mainly controlled by greenstone-type phosphorus magnetite formation. The ore mineral assemblage includes apatite, magnetite, titanomagnetite, hornblende, biotite and plagioclase. The apatite content is positively correlated with the content of dark components such as iron, titanium and manganese, indicating the siderophile affinity of phosphorus. The deposit is of metamorphic apatite type genetically. [ABSTRACT FROM AUTHOR]

The Anshan–Benxi area, situated in the northeast of the North China Craton (NCC), is home to not only the oldest rocks in China (~3.8 Ga) but also a diverse range of granitoids dated between 3.8 and 2.5 Ga. The Lianshanguan batholith, covering an area of approximately 250 km2 with an east–west trend, predominantly consists of syenogranites (K2O > 4 wt. % and K2O/Na2O ratios > 1.3). Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb analyses of the two syenogranites yielded concordant ages of 2541 ± 22 and 2512 ± 13 Ma, respectively. These syenogranites had zircon εHf(t) values ranging from −20 to +4.9 with two-stage Hf model ages (TDM2(Hf)) spanning 3.9–2.7 Ga. Based on petrological, geochemical, and isotopic characteristics, we conclude that the Lianshanguan syenogranites are mainly resulted from the reworking of complicated Eoarchean–Mesoarchean crustal materials, possibly with a small proportion of ~2.7 Ga juvenile crustal materials. When compared with coeval syenogranites from the Northern Liaoning and Western Liaoning–Eastern Hebei areas, ~2.5 Ga syenogranites from the Anshan–Benxi area displayed more complicated TDM2(Hf) ages, hinting at a pronounced late Neoarchean reworking of the Eoarchean to Mesoarchean continental crust (including metasedimentary sources) primarily in the Anshan–Benxi region of the North China Craton. This scenario significantly bolsters the arc–continent collision model. [ABSTRACT FROM AUTHOR]

Multiple interpretations exist regarding the tectonic evolution model of the Neoarchean North China Craton, requiring a more in-depth study of metamorphic processes. Systematic petrographic observations, mineral chemical analysis, phase equilibrium modeling, and zircon dating were conducted on the basic granulites from Qingyuan in northern Liaoning to elucidate their metamorphic evolution processes and geological significance. The selected samples of mafic granulites were divided into the garnet-bearing domain (19DJ07-GD) and garnet-free domain (19DJ07-NGD), with the garnet-bearing region displaying a banded and inhomogeneous distribution. Both domains exhibit two generations of granulite facies assemblages. In the garnet-bearing domain, the first-generation metamorphic mineral assemblage includes garnet + clinopyroxene + orthopyroxene + hornblende + biotite + plagioclase + quartz. Notably, the first-generation plagioclase (Pl1) exhibits a complex compositional zoning, with anorthite content (xAn) increasing from the core to the mantle and then decreasing towards the rim. Similarly, the titanium component zoning in the first-generation amphibole (Amp1) follows a pattern of increasing from the core to the mantle and then decreasing towards the rim. Based on mineral assemblages and corresponding component zoning, it is inferred that the first-generation granulite facies metamorphic process followed a counterclockwise P-Tpath, involving a pre-peak P-T rise stage and a post-peak P-T drop stage. Phase equilibrium modeling constrains the peak conditions at 0.8~0.9 GPa/900~950 ℃, indicative of high-ultrahigh-temperature (HT-UHT) metamorphism conditions. Zircon dating results yielded a post-peak cooling age of 2498±6.9 Ma (MSWD=0.39). Considering the regional "dome-and-keel" tectonics, the counterclockwise P-T path, and the metamorphic timing of supracrustal rock nearly synchronous with late-stage TTG magmatic pulses, the UHT metamorphism of the supracrustal rocks is believed to be controlled by the unique Archean vertical tectonics/sagduction system. The second-generation metamorphic assemblage is characterized by locally formed coronas or symplectites of garnet + quartz ± clinopyroxene, representing high-pressure (HP) granulite facies metamorphism associated with a Paleoproterozoic orogenic event.

Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supra-crustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic biomass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity. [ABSTRACT FROM AUTHOR]

In order to enhance the understanding of Archean continental crust formation and evolution history of Liaonan block in North China Craton, the Neoarchean TTG(tonalite-trondhjemite-granodiorite)gneisses in Shijianfang area of Liaonan block were focused on, and the detailed field investigation, zircon U-Pb dating and Lu-Hf isotope analysis, and whole-rock geochemical analysis were intergrated. The results show that TTG gneiss in Shijianfang area crystallizes at(2 499.7±5.4)Ma. The TTG gneiss samples have high contents of SiO2(66.1%-69.1%)and Al2O3(14.6%-15.6%), are enriched in Na and depleted in K(K2O/Na2O is 0.41-0.51)with weakly positive Eu anomalies(1.06-1.59); the samples are relatively enriched in large ion lithophile elements(Ba, Rb, K, etc.)and high field strength elements(Zr, Hf, etc.), featured by high Sr/Y(44.3-69.5)and(La/Yb)N(11.8-24.0), and depleted zircon Hf isotopic composition with two-stage model ages of 2 903-2 687 Ma. Compared with TTG rocks derived from partial melting of the thickened lower crust, TTG gneisses in Shijianfang area have relatively higher contents of MgO, Cr and Ni. The above geochemical characteristics reveal TTG gneisses in Shijianfang area are typical medium-pressure TTG rocks, which is the product of partial melting of Meso-Neoarchean basaltic crust. The source area is rich in garnet and amphibolite, but short of rutile and plagioclase. Combined with previous studies, TTG gneisses in Shijianfang area are likely formed by the partial melting of subduction slabs, indicating the warm subduction tectonic system of the ancient continental margin. The North China Craton might be controlled by both plate tectonics and mantle plume tectonics during Late Neoarchean. [ABSTRACT FROM AUTHOR]

Archean gneiss domes are a very common structure in old cratons, these are different from the structural patterns in the post‐Archean linear orogenic belts related to subduction and collision. There are two tectonic models in Archean tectonics: horizontal tectonism and vertical tectonism. The formation mechanisms of Archean gneiss domes are still debated. Here we present data from the Anziling gneiss dome, a typical gneiss dome structure from the eastern North China Craton, which was formed dominantly by vertical tectonism during Neoarchean times (2554–2452 Ma). The Anziling gneiss dome is located in the east of Shuangshanzi, East Heibei Province and consists of the Anziling gneiss (a diopside‐monzonitic gneiss) in the center outwards surrounded by the Jiguanshan gneiss (hornblende‐plagioclase gneiss) and the Niuxinshan gneiss (trondhjemitic gneiss), together showing an irregular elliptical shape with a NE‐ward extending tail on the surface. But in the cross‐section, it represents a deflective dome, both north‐western and south‐eastern limbs dip approximately to the west. The dome suffered inhomogeneous ductile shear deformation, much stronger with more displacement in the north‐western half (especially close to the margin) than that in the eastern half. The core area shows only a relatively weak deformation. Therefore, we suggest that the Anziling gneiss dome was formed by the TTG (trondhjemite, tonalite, granodiorite) pluton rising obliquely up towards the east by diapirism in the first stage, followed by sub‐horizontal emplacement towards the north and then to the north‐east associated with clockwise rotation in the second stage. The Anziling gneiss dome and its neighbouring Shuangshanzi shear zone were formed simultaneously, indicating that the Anziling gneiss dome was driven by both gravitational forces and magma diapirism. [ABSTRACT FROM AUTHOR]

U–Th–Pb LA-ICP-MS was used for dating detrital zircon extracted from the cement of Neoarchean and Paleoproterozoic polymict conglomerates of the Karelian Province, Fennoscandian Shield. The chemical composition of terrigenous and volcanogenic rocks of the Gimolskaya Formation is presented for the first time. The data obtained showed that the detrital zircon from the cement of conglomerates of different ages yields similar results of about 2750 Ma. The clasts of the gneiss tonalite and plagioporphyry dominate in the Neoarchean conglomerates of the Sukkosero area. They are identical in chemical composition with the Neoarchean (2.78–2.73 Ga) TTG, granodiorite, and metadacite of the Central Karelian domain. These rocks are distinguished by an increased content of Ba (900–1200 ppm) and Sr (600–700 ppm), and the ratios Sr/Y = 34–90 and (La/Yb)n = 17–30. The chemical composition of the cement (values of CIA < 55 and negative values of the function DF(x)) from –1.0 to –0.4 is similar to the composition of clasts, which means the cement is composed of finer clasts of the same rocks. The Paleoproterozoic conglomerates of the Vottomuks area contain zircon from felsic rocks of the Neoarchean basement. The chemical composition of the cement of the Paleoproterozoic conglomerates and the predominance of basaltic andesites of Sumian are evidence of the post-Sumian(<2.45 Ga) age of these rocks. The absence of the Paleoproterozoic zircon in cement could have resulted from its absence in the Paleoproterozoic source rocks. The age of Neoarchean conglomerates from the Gimolskaya Formation is less than 2.75 Ga. [ABSTRACT FROM AUTHOR]

A whole-rock geochemical and zircon U–Pb and Lu–Hf isotope study of the Dengfeng complex in the Jishan area was performed to constrain the Neoarchean continental crust evolution in the southern margin of the North China Craton (NCC). The Dengfeng complex can be divided into supracrustal assemblage and plutonic intrusion. In the supracrustal assemblage, the detrital zircon of quartz schist from the Guojiayao Formation display oscillatory growth zones and streaky absorption with relatively high Th/U ratios (0.36–2.10), suggesting a magmatic origin. The 207Pb/206Pb ages range from 2963 to 2531 Ma, with a peak age of 2555 Ma, and the εHf(t) values are between + 2.3 and + 12.1. Combined with previous studies, the depositional age of the Guojiayao Formation can be constrained between 2542 and 2522 Ma, indicating that it mainly originated from the Neoarchean accretive basement material of the southern NCC. The Ruzhou gneissic syenogranites yield U–Pb ages from 2515 ± 13 Ma to 2504 ± 20 Ma, show that the plutonic intrusions also formed in the Late Neoarchean. They have high SiO2 (64.03–73.99 wt%) and Na2O + K2O (8.79–14.58 wt%) contents but low MgO (0.19–0.36 wt%) and CaO (0.02–0.27 wt%) contents, and belong to slightly peraluminous I-type granites with shoshonitic affinity. Magmatic zircon grains have positive εHf(t) values (+ 0.2 to + 6.7) with average crustal Hf model ages ranging from 2911 to 2671 Ma (average of 2765 Ma), indicating that the Ruzhou gneissic syenogranites were likely produced by partial melting of the accretive lower crust. The positive εHf(t) values and similarly ancient model ages suggest that a large-scale continental crust growth occurred in the southern margin of NCC at ca. 2.7 Ga. The Dengfeng complex was mainly derived from an arc-related setting associated with subduction in the Late Neoarchean. [ABSTRACT FROM AUTHOR]

The Bayan Obo–Duolun–Chifeng–Kaiyuan (BDCK) Fault is considered the boundary between the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB). However, the site of this boundary from Duolun to Chifeng and Kaiyuan remains highly controversial. Here, we present an integrated study of petrology, whole‐rock geochemistry, and zircon U–Pb and Sr–Nd–Hf isotope compositions for tonalites and amphibolites in the Bainaimiao arc belt (BAB) to better constrain the northern NCC margin. The tonalites are similar to NCC tonalite–trondhjemite–granodiorites (TTGs), characterized by relatively high Na2O/K2O and Sr/Y ratios and fractionated rare earth element (REE) patterns, with high light REE (LREE) but low heavy REE (HREE) concentrations. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yields formation ages of 2599–2674 Ma, with a weighted average of 2649.6 ± 3.7 Ma. The εHf(t) values (3.5–7.5), two‐stage model ages (2673–2793 Ma), whole‐rock positive εNd(t) values (1.3–1.9), and two‐stage model ages (2841–2880 Ma) suggest that the Neoarchean tonalites likely originated from remelting of thickened Meso‐Neoarchean mafic crust. LA–ICP–MS U–Pb isotope data reveal that the amphibolites formed at ~2610 Ma. They have high‐Fe tholeiitic compositions and are close to normal mid‐ocean ridge basalt (N‐MORB)‐like rocks, indicating that they were generated from partial melting of the depleted mantle. The existence of Neoarchean basement rocks, combined with seismic and isotope data, suggests that the Jiefangyingzi area is part of the Trans‐North China Orogen (TNCO) in the NCC rather than part of the CAOB and that the northern boundary of the NCC in Chifeng is located north to the Jiefangyingzi area. [ABSTRACT FROM AUTHOR]

Copyright of Industrial Minerals & Processing / Huagong Kuangwu yu Jiagong is the property of Industrial Minerals & Processing Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Deposition of Neoarchaean carbonate platform sediments of the Campbellrand Subgroup (Transvaal Supergroup) are cyclic and exhibits a full range of facies, which extend from subtidal to supratidal deposition. The strata are host to Mississippi Valley-type lead-zinc deposits including the Pering mine and Bushy Park prospect, the latter of which is the focus of this study. Core spanning the top of the Klipfontein Heuwel Formation to the top of the Kogelbeen Formation where described in terms of the sedimentary environments that likely prevailed at the time. Key facies were identified that were used to identify parasequences and depositional sequences, which were based on stacking patterns of shoaling upward sequences and flooding surfaces. Progradational facies indicate sea level rise and retrogradational facies mark sea level fall of different magnitudes. Multiple breccia types were observed including early diagenetic, karst related breccias, slump breccias, as well as structurally controlled collapse breccias. Although an autocyclic origin for the 4th and 5th order parasequences cannot be ruled out, sedimentary parasequences in the Neoarchaean strata at Bushy Park are very similar to those recognised in Phanerozoic carbonate sections where deposition and accommodation space is believed to be controlled by orbitally forced eustatic sea level rise and fall (Milankovitch cycles). This work identifies a need to test the possible effects of Milankovitch cycles not only on other tectonic basins of similar Archean age but also other rocks through time to account for the possibility that Archean cycles are longer in duration than those of the Phanerozoic Eon. • Documentation of possible orbital forcing affecting cyclicity in Neoarchean platform carbonates. [ABSTRACT FROM AUTHOR]

The results of geochronological and petrological studies of the largest mafic dyke in the northern part of the Fennoscandian Shield, called the Great Dyke of the Kola Peninsula (GDK), are presented. According to U-Pb D-TIMS baddeleyite dating, the GDK crystallization age is 2680 ± 6 Ma. The age of host granites is 2.75–2.72 Ga (U-Pb, zircon, SHRIMP-II). The dyke has a simple internal structure with no signs of multistage melt injection. It comprises equigranular and plagioclase-porphyritic dolerites and gabbro that are amphibolitized to varying degrees. All rocks are low-Mg (Mg# less than 0.37) with low concentrations of Cr and Ni, and were derived through differentiation of more primitive melts. The analysis of geochemical and Sr-Nd isotopic data suggests that GDK melts could be formed by mixing of two types of mantle melts: depleted asthenospheric melt and enriched melt formed via melting of a lithospheric mantle. The weakly fractionated HREE patterns indicate that primary GDK melts originated at shallow (<60 km) depths outside the garnet stability field. The generation and injection of melts of the Neoarchean GDK occurred immediately after large-scale granitic magmatism and main crustal growth event in the Murmansk Craton and marked the cratonization of the continental lithosphere in the northeastern part of the Fennoscandian Shield. [ABSTRACT FROM AUTHOR]

The Alxa Block is the westernmost part of the North China Craton (NCC), and is regarded as one of the basement components of the NCC. Its geological evolution is of great significance for the understanding of the NCC. However, the Precambrian basement of the Alxa Block is still poorly studied. In this study, we present new in situ LA‐ICP‐MS zircon U‐Pb and Lu‐Hf isotope data from the Diebusige Metamorphic Complex (DMC) which located in the eastern Alxa Block. Field and petrological studies show that the DMC consists mainly of metamorphic supracrustal rocks and minor metamorphic plutonic rocks and has experienced amphibolite‐granulite facies metamorphism. Zircon U‐Pb dating results suggested that the amphibolite sample yields a crystallization age of 2636 ± 14 Ma and metamorphic ages of 2517–2454 Ma and 1988–1952 Ma, proving the existence of exposed Archean rocks in the Langshan area and indicating that late Neoarchean to Paleoproterozoic metamorphic events existed in the Alxa Block. Two paragneiss samples show that the magmatic detrital zircons from the DMC yield 207Pb/206Pb ages ranging from 2.48 Ga to 2.10 Ga with two youngest peaks at 2.13 Ga and 2.16 Ga, respectively, and they were also overprinted by metamorphic events at 1.97–1.90 Ga and 1.89–1.79 Ga. Compilation of U‐Pb ages of magmatic detrital and metamorphic zircons suggested that the main part of the DMC may have been formed at 2.1–2.0 Ga. Zircon Lu‐Hf isotope data show that the source materials of the main part of the DMC were originated from the reworking of ancient Archean crust (3.45–2.78 Ga). The Hf isotope characteristics and the tectonothermal event records exhibit different evolution history with the Khondalite Belt and the Yinshan Block and the other basements of the Alxa Block, indicating that the Langshan was likely an independent terrain before the middle Paleoproterozoic and was subjected to the middle to late Paleoproterozoic tectonothermal events with the Khondalite Belt as a whole. [ABSTRACT FROM AUTHOR]

The Mewengo area belongs to the northwestern edge of Nyong Group in South Cameroon. Whole-rock geochemistry and U–Pb zircon dating were performed on granitic gneisses, the main country rock of Mewengo iron deposit. Petrographically, Mewengo granitic gneisses are high-grade metamorphic rocks made up of quartz, K-feldspar, biotite, plagioclase, garnet, and accessory minerals, displaying heterogranular granoblastic to mylonitic microstructures. Chemical composition of the rocks shows high SiO2 (71.78 to 76.03 wt%) and total alkali (6.44 wt% ≤ Na2O + K2O ≤ 7.14 wt%), low CaO (≤ 2.76%wt), and Al2O3 (≤ 14.69%wt) contents. The rocks yield metaluminous, I-type granite protoliths and belong to high-K Calc-alkaline to Calc-alkaline series, which crystallized in a continental crust-arc-related setting. The lack of Ce anomalies (Ce/Ce* = 0.88–1.10) indicates weak overprints of late alteration processes. Trace element contents in zircon indicate their igneous origin and confirm the continental arc setting for the magmatism in Mewengo area. U–Pb LA-ICP-MS dating on zircons show that the crystallization of the protolith occurred at the age of 2837 ± 100 Ma. The 2.8 Ga magmatic event represents the Archean magmatic inputs within the Paleoproterozoic Nyong Group, which were later subjected to high-grade metamorphism during the Paleoproterozoic orogeny (2.3–2.05 Ga). This finding is consistent with the previous works in the Nyong Group. [ABSTRACT FROM AUTHOR]

Archean rock exposures in the Yangtze Craton of South China are scarce and have been only studied in small-scale outcrops for understanding its early crustal evolution. Geological investigations have been carried out for three Neoarchean gneissic granitic plutons from the Dabie orogen in the northeastern margin of the Yangtze Craton. Zircon U-Pb geochronology constrains the emplacement of high-K monzogranite at 2645±30 Ma, high-K syenogranite at 2630±37 Ma and Na-rich granodiorite at 2497±29 Ma. These findings provide direct evidence for the existence of ∼2.7–2.5 Ga rocks in the Dabie orogen, which significantly contribute to the distribution and evolution of Neoarchean basement rocks in the Yangtze Craton. Zircon crystals from the three granite samples give εHf(t) values of −0.9 to 3.7, −4.2 to −0.4 and −4.7 to 0.1, respectively, corresponding to TDM2 ages of 3186–2909, 3372–3141 and 3297–3005 Ma. The magmatic zircons have δ18O values of 5.06±0.27‰ to 5.79 ±0.30‰ (average 5.59‰), 6.14±0.24 to 7.35±0.26‰ (average 6.87‰) and 5.95±0.17‰ to 7.09±0.17‰ (average 6.55‰), respectively. Considering the possible decrease of δ18O value due to the lead loss during post-crystallization alteration, the primary δ18O values might be substantially higher than those of the normal mantle zircon value (δ18O=5.3±0.6‰). These isotopic data indicate that the Jiamiao Neoarchean granites were largely generated through reworking of Paleo-Mesoarchean basement rocks with a minor contribution of supracrustal material. Integrating our findings with previous results, we infer that the Archean-Paleoproterozoic complexes/terranes of the Yangtze Craton have distinct petrogenesis and the timings of the transition from Na-rich to high-K granites, crustal growth, and tectonothermal evolution before ∼2.0 Ga. However, all of them underwent metamorphism at about 2.0 Ga. These observations also reveal that the Yangtze Craton might comprise several microcontinents that evolved individually and collided to form a unified craton at ∼2.0 Ga, synchronous with the assembly of the Columbia supercontinent. However, the lack of relevant data from the southeastern Yangtze Craton may have a critical influence on this conclusion and should be solved in the future. [ABSTRACT FROM AUTHOR]

The Vila Jussara Suite (VJS) is formed by Neoarchean (~ 2.75 Ga) granites that are located in the Sapucaia Domain of the Carajás Province (CP), Amazonian Craton. Four petrographic varieties were identified in the VJS: biotite-hornblende monzogranite (BHMzG); biotite-hornblende tonalite (BHTnl); biotite monzogranite (BMzG); and hornblende-biotite granodiorite (HBGd). In terms of magnetic signature, BHMzG has two subgroups: the first subgroup has low magnetic susceptibility (MS) values (0.16 × 10-3 to 0.81 × 10-3) and more commonly contains ilmenite with titanite rims; the second subgroup shows moderate to high MS (1.91 × 10-3 to 6.02 × 10-3) and magnetite dominant over ilmenite. BHTnl has moderate MS (0.85 × 10-3 to 1.36 × 10-3) and dominance of pyrite followed by magnetite. BMzG and HBGd have comparatively high MS (3.35 × 10-3 to 19.3 × 10-3 and 2.14 × 10-3 to 25.0 × 10-3, respectively), with magnetite dominant over pyrite. The granite varieties of the VJS were formed under different oxygen fugacity (fO2) conditions, varying from reducing (< fayalite-magnetite-quartz (FMQ)) to oxidizing conditions (nickel-nickel oxide (NNO) to NNO+1). In addition, biotite-hornblende syenogranite occurs subordinately and shows high MS values and extremely high FeOt/(FeOt + MgO) ratios, both in whole-rock and amphibole and biotite. The granites of the VJS are similar to other Neoarchean granites of the Carajás province. The reduced VJS granites are akin to the ferroan granites of Planalto suite, Estrela Complex and Vila União area and the magnesian granites of VJS approach the magnesian granites of Vila União area. [ABSTRACT FROM AUTHOR]

Fluids play a major role in facilitating igneous/metamorphic processes in the Earth’s crust and mantle. In this study, we investigate the nature and composition of fluids in Earth’s interior by studying the lower crustal rocks. We compare accessory minerals (e.g., apatite, monazite, allanite, and titanite), their texture, mineral reactions and composition among regionally distributed metamorphosed mafic and felsic rocks representing the roots of Neoarchean arc magmatism from the Nilgiri Block of the Southern Granulite Terrane in India. Regional trends in accessory minerals show the formation of monazite, allanite, and titanite in the felsic rocks. Apatite is depleted in REEs in all the rock types, irrespective of the difference in their whole-rock chemistry. Textural features and mineral reactions show that these accessory minerals were affected by fluids present in the lower crustal conditions. By comparing our results with those from previous experimental results, we further show that acidic CO2-H2O-HCl-HF fluids stable in lower crustal conditions could have resulted in these chemical and textural features. Dielectric constant of water is high (10–35 compared to lower crustal conditions) in high-pressure and low-temperature conditions of subduction zones and the upper mantle. Such conditions would enhance dissociation of HCl (compared to lower crust) and result in acidic fluids during dehydration reactions in subduction zones and in the upper mantle. Our results have important implications in understanding the nature and composition of fluids in Earth’s interior and would be helpful to model the tectonic and deep geochemical processes in both early and modern conditions in planetary interiors.

Abstract The Vila Jussara Suite (VJS) is formed by Neoarchean (~ 2.75 Ga) granites that are located in the Sapucaia Domain of the Carajás Province (CP), Amazonian Craton. Four petrographic varieties were identified in the VJS: biotite-hornblende monzogranite (BHMzG); biotite-hornblende tonalite (BHTnl); biotite monzogranite (BMzG); and hornblende-biotite granodiorite (HBGd). In terms of magnetic signature, BHMzG has two subgroups: the first subgroup has low magnetic susceptibility (MS) values (0.16 × 10-3 to 0.81 × 10-3) and more commonly contains ilmenite with titanite rims; the second subgroup shows moderate to high MS (1.91 × 10-3 to 6.02 × 10-3) and magnetite dominant over ilmenite. BHTnl has moderate MS (0.85 × 10-3 to 1.36 × 10-3) and dominance of pyrite followed by magnetite. BMzG and HBGd have comparatively high MS (3.35 × 10-3 to 19.3 × 10-3 and 2.14 × 10-3 to 25.0 × 10-3, respectively), with magnetite dominant over pyrite. The granite varieties of the VJS were formed under different oxygen fugacity (fO2) conditions, varying from reducing (< fayalite-magnetite-quartz (FMQ)) to oxidizing conditions (nickel-nickel oxide (NNO) to NNO+1). In addition, biotite-hornblende syenogranite occurs subordinately and shows high MS values and extremely high FeOt/(FeOt + MgO) ratios, both in whole-rock and amphibole and biotite. The granites of the VJS are similar to other Neoarchean granites of the Carajás province. The reduced VJS granites are akin to the ferroan granites of Planalto suite, Estrela Complex and Vila União area and the magnesian granites of VJS approach the magnesian granites of Vila União area.

• The metabasites and TTG gneiss in the Linshan complex mainly formed at 2.6–2.5 Ga. • The metabasites in the Linshan complex formed in a Neoarchean back-arc basin. • There may have been several Neoarchean "forearc-arc-backarc" systems in the NCC. The onset of plate tectonics and crustal growth processes in the early Earth have been controversial scientific issues in the geoscience. The North China Craton (NCC) preserves widespread 3.8–2.5 Ga rocks, providing an ideal place to understand early continent formation and evolution. The Linshan complex located in the southern segment of the Central Orogenic Belt (COB) of the NCC, is mainly composed of TTG (tonalite-trondhjemite-granodiorite)-diorite gneisses and metamorphic volcanic-sedimentary units dominated by gabbro, basalt, basaltic andesite and biotite-plagioclase gneiss. Detailed mapping on the scale of 1:100 of a structural transect shows that the Linshan complex has mainly experienced two major deformation events including top-to-the-SE thrust faults and late NE-trending high-angle normal faults. Detailed zircon U-Pb dating shows that gabbro, basaltic andesite, and TTG gneiss mainly formed at ca. 2.52–2.50 Ga. Gabbros and basalts display enrichment of LREE and negative Nb and Ta anomalies, and basaltic andesites display mixed MORB-IAT geochemical affinities. Basalts and basaltic andesites are members of the Nb-enriched basalt series with high absolute Nb contents (>6 ppm). TTG gneisses are geochemically divided into high-pressure and low-pressure TTGs. High-pressure TTGs are characterized by high ratios of La/Yb cn (26.29–45.73) and fall into the adakitic region in the La/Yb cn -Yb cn diagram. Considering the close contact with Nb-enriched basaltic series, it is proposed that high-pressure TTGs may have formed by partial melting of a subducting oceanic slab with garnet and amphibole and/or rutile as residues. Low-pressure TTGs are characterized by low ratios of La/Yb cn and Sr/Y with marked negative Eu anomalies, indicating partial melting at shallow crustal levels. Regional tectonic relations have defined the Neoarchean Dengfeng island arc-forearc accretionary complex to the east of the Linshan complex. Thus, we propose that the gabbros-basalts-basaltic andesites in the Linshan complex mostly formed in a Neoarchean suprasubduction back-arc basin by rifting of a TTG-dominated island arc terrane. The final closure of the back-arc basin resulted in their tectonic juxtaposition forming thrust-imbricated structures. There may have been several Neoarchean "forearc-island-arc-backarc" systems in the NCC that are similar to modern accretionary tectonic orogens, indicating that plate tectonics has been in operation since at least 2.55–2.50 Ga. [ABSTRACT FROM AUTHOR]

Studies of lithotectonic formations within the Keivy domain of the NE Baltic Shield have shown that the domain was tectonically overlapped by adjacent microcontinents during regional collision processes in the Late Archean. As a consequence, the continental crust of the Keivy domain was submerged, relative to other blocks of the continental crust, and the described domain acquired the features of a classical median massif. Surrounded on all sides by collision systems, the Keivy median massif entered the cratonization regime. This led to intensive processes of denudation of the surrounding domains of the crust and the accumulation of a thick sedimentary cover on the surface. The described processes occurred during the formation of the first supercontinent (Monogea) in the history of the Earth and the manifestation of the Early Precambrian Huronian glaciation, which left its traces on most domains of the Earth’s continental crust. Thus, the processes of peneplain formation within the Keivy massif occurred under the cold weather conditions, high volcanic activity in the peripheral zones, and sedimentary cover saturation with the products of the physical and chemical mineral transformation of tonalite–trondhjemite and greenstone rock assemblages. The unique combination of certain geodynamic and climatic cycles on the Baltic Shield in the Late Archean led to the accumulation of extensive stratiform deposits of alumina raw materials within the Keivy median massif.

• A calc-alkaline bimodal volcanic complex in the Yinshan Block was formed at ∼ 2570 Ma. • The high-Mg basalt/andesite were derived from a metasomatised depleted mantle. • The adakitic rhyolites were derived from a thickened juvenile mafic lower crust. • This bimodal complex was formed in a modern-style arc setting. Plate tectonics plays an important role in Earth's evolution, but it is a difficult and controversial issue to trace the operation of plate tectonics in the Archean. Calc-alkaline magmas are a diagnostic feature of modern subduction-related magmatism and thus identification of Archean subduction-related calc-alkaline magmas is crucial for understanding when Archean plate tectonics operated. Here we present zircon U-Pb, whole-rock geochemical and Sr-Nd isotopic data for a metamorphosed Neoarchean calc-alkaline bimodal volcanic complex from the Yinshan Block, the North China Craton. Zircon U-Pb dating results indicate that these meta -volcanic rocks were erupted in the late Neoarchean (∼2570 Ma). Its mafic unit is characterized by high contents of K 2 O (0.68–4.20 wt%), MgO (Mg# = 62.0-73.4), Cr (332-904 ppm), Ni (105-269 ppm), and is enriched in light rare earth elements (REEs), but depleted in heavy REEs and Nb, Ta and Ti. These geochemical features are similar to those of high-Mg basalts in the Phanerozoic arc setting. They have positive ε Nd (t) values (+1.3 to + 2.4), indicating that they were derived from the depleted mantle wedge. The felsic unit has variable K 2 O content (1.42–4.95 wt%) and K 2 O/Na 2 O ratios of 0.23–1.36 with positive ε Nd (t) values (+0.7 to + 3.1). They show characteristics of modern adakites with high Sr/Y and La N /Yb N ratios, suggesting that they were generated by partial melting of the thickened juvenile mafic lower crust. We conclude that the studied Neeoarchean bimodal meta -volcanic complex was formed during subduction-zone retreating and root delamination in a Neoarchean arc. Our results further indicate that plate tectonics had already operated in the Neoarchean in the Yinshan Block. [ABSTRACT FROM AUTHOR]

This study reports electron probe micro-analysis of relict clinopyroxenes in Neoarchean Kotima basalt of the Dongargarh Supergroup, Central India and their petrogenetic significance. Kotima clinopyroxenes plot in the field of augite and show an overall limited sub-alkaline fractionation trend. Inter-elemental variation diagrams reveal an overall magmatic trend and preservation of magmatic chemistry, whereas their euhedral to subhedral form indicates near equilibrium conditions of crystallisation. Besides wollastonite, enstatite and ferrosilite major components, 'other components' comprises Ca–Al tschermak, esseneite, CaTiAl2O6 (Tp), kosmochlor, aegirine, jadeite and johannsenite. Single clinopyroxene geothermobarometers indicate magmatic temperatures (1180–1050°C) and shallow depths (2–3 kb pressure) of crystallisation of Kotima clinopyroxenes. The clinopyroxene chemistry and chondrite-normalised whole-rock rare-earth element plots imply the derivation of Kotima basalt from a relatively primitive parent magma, in which pH2O and fO2 decreased with fractionation, a hallmark of arc-related volcanics. The overall low-Ti (and Al) contents of the Kotima clinopyroxenes imply their formation from a subduction-related sub-alkaline magma, whereas discriminant plots of Si vs. Al, Na vs. Ti, Al vs. Ti and Ti vs. Alz reveal their formation in a mixed rifting (non-orogenic) continental arc tectonic setting in response to ridge subduction. [ABSTRACT FROM AUTHOR]

Clast morphometry and sedimentary facies analysis of polymictic conglomerate of the Hiriyur Formation in the Gadag greenstone Belt, Western Dharwar Craton, India, was conducted to constrain the palaeohydraulic condition, depositional environment, and type of sedimentary basin. The conglomerate grades from clast‐supported to matrix‐supported with siltstone‐sandstone intercalations. Eight lithofacies along with five major lithofacies associations are recognized in the Majjur and Attikatti Domains of the Gadag greenstone Belt. The lithofacies architecture suggests sedimentation commenced with rapid deposition of slope‐derived debris in a proximal high‐energy alluvial fan setting followed by a braided river environment. Clast morphometry and palaeohydraulic data indicate a low palaeoslope (0.000024 m/m), and stream discharge value (1.913 m3/s) calculated from the channel fill conglomerates. Greywacke overlying the conglomerate has been interpreted as turbidites from a continental slope. The accompanying 40‐m‐thick banded iron formation and carbonate rocks also support a marine context. The lack of transitional deposits between river and deep marine settings suggests that the greywacke turbidites sequence is separated from the conglomerate deposits by substantial unconformities. This change in depositional setting could be due to a change in basin subsidence rate, tectonic rejuvenation, or major sea‐level changes. [ABSTRACT FROM AUTHOR]

New data have been obtained on composition and age of the Archean supracrustal complexes of the Yalonvara–Pertinjarvi, Korpijarvi, and Pastajarvi structures in the marginal part of the Karelian Craton of the Fennoscandian Shield in the conjunction area with the Svecofennian Orogen. The structures were typically attributed to the single Yalonvara–Ilomantsi greenstone belt. On the basis of geochronological, petrographic, and geochemical data, only the Korpijarvi and Pastajarvi structures with an age of 2.73 Ga can be considered as a continuation of the Hattu greenstone belt in the Ilomantsi province (Finland). More ancient rocks of the Yalonvara–Pertinjarvi structure, estimated at 2.95–2.93 Ga, can be compared with the Mesoarchean basalt–andesite–dacite–rhyolite stratotectonic association of the Hautavaara greenstone structure. The younger Neoarchean rocks metamorphosed under the high-temperature amphibolite facies conditions are composed of gneisses and amphibolites. The ancient greenstone rocks metamorphosed under the greenschist and epidote–amphibolite facies conditions are of tuff nature. The rocks of both complexes are similar in terms of geochemistry to volcanic rocks of the suprasubduction settings, but they are different in proportions of the material of the mantle wedge and subduction plate. [ABSTRACT FROM AUTHOR]

Geodynamic and petrogeochemical study of the Keivy domain in the Baltic Shield has shown that it was overlapped by surrounding microcontinents in the Archean. As a result, the Baltic Shield was subject to cratonization. Thus, the Keivy microcontinent can be considered as the most ancient known median massif of the continental crust. Specific features of its geodynamic evolution predetermined conditions for formation of unique in scale stratiform deposits of alumina raw materials on the surface of the microcontinent. The paper describes and justifies the mechanism of alumina formation by physical-chemical decomposition of Archean and early Proterozoic metamorphosed sedimentary complexes in the Kola region of the Baltic Shield. Besides, the study of metallogenic features of transformation processes in the Archean continental crust of the Keivy microcontinent and its rimming suggests that carbonaceous schists of the Keivy domain that formed in the sedimentary cover can be significantly rich in nanogold. It will allow considering the central part of the Kola region as a major gold-bearing province. The authors believe that the metallogenic forecast based on geodynamic reconstructions is promising for further research of early Precambrian complexes. This work continues a series of publications on structural-material study of the Keivy domain.

Pyrite corona nodules from the ∼2.7 Ga Kapai Slate, a thin, sulfidic carbonaceous shale horizon interbedded with basaltic lava flows in the Yilgarn Craton, Western Australia, have concentric compositional and isotopic zoning with distinctive textural differences between cores and mantles. The sieved-textured cores are enriched in highly compatible trace elements, incl. Ni, As, Ag, Te, Sb, Bi and Pb, and depleted in incompatible Mo and Tl, whereas the radiating-textured mantles are strongly depleted in compatible elements and enriched in incompatible elements, relative to the cores. A striking feature of the data is that both the compatible and incompatible elements are linearly correlated, with correlation coefficients as high as 0.99. A marked drop in the concentration of compatible elements and an increase in incompatible elements at the core-mantle boundary is attributed to a sudden change in the rate of growth of the nodules produced by eruption of the voluminous overlying Paringa Basalt. The weight of the basalt produced sudden compaction of the unconsolidated clays below resulting in upward advection of pore fluid, which thinned the boundary layer around the growing nodules, leading to a marked increase in the rate of pyrite growth. Rapid pyrite growth led to a dramatic depletion in highly compatible elements, and to a build-up in incompatible elements, in the boundary layers around the growing nodule mantles, which resulted in extreme depletion of compatible elements, and enrichment in incompatible elements in the nodule mantles, relative to the cores. The corona nodules are also isotopically zoned with cores that have higher δ34S, with small positive Δ33S values, and mantles that have lower δ34S and higher Δ33S. The increase in Δ33S towards the rims is attributed to S 8 being advected to the growing mantles by upward fluid movement during sudden compaction, and the decrease in δ34S to the lighter S isotope, with its higher reactivity and diffusivity, being preferentially incorporated into the fast growing pyrite mantle. The extreme changes in the growth rates of the Kapai Slate corona pyrite nodules provide a new constraint on the partition coefficients of the trace elements between Archean ocean water and sedimentary pyrite. The compatibility of the analysed trace elements decreases in the order Bi > Te > Sb > Ag > Cu > Pb > Ni ≈ As > (Co, Zn, Se, Cd, Mn, W) > Tl > Mo, which is consistent with the order obtained from modern sedimentary pyrites by Large et al. (2014), except for the redox-sensitive elements Mn, Tl and Mo. These differences are attributed to the lower oxygen content of the Archean atmosphere and oceans. [ABSTRACT FROM AUTHOR]

The Songling and Majiayu peridotite blocks occur in the Eastern Block, North China Craton (NCC). Geothermobarometry data show that the Songling peridotites began exhumation from a depth of c. 70 km (c. 23 kbar). During exhumation, the Songling peridotites were intruded by an upper-crust-derived, high-δ18O (up to +7.28‰ in zircon) trondhjemitic dyke at 2.47 Ga and experienced granulite-facies metamorphism. The Songling peridotites have hybrid mantle wedge (HMW) -like high SiO2 (> 45 wt%), high FeOt (c. 10 wt%) content, high modal orthopyroxene abundance (> 35%) and high ϵNd(t) (+18.4 to +21.4), which were generated by the reaction between peridotite and eclogite-derived melts. The clinopyroxenes from the Songling peridotites were in equilibrium with a Nb-, Zr- and Ti-depleted arc-like magma. The Majiayu peridotites are characterized by depletion of Nb, Zr and Hf and are highly enriched in FeOt, Th and light rare earth elements (LREEs), which can be interpreted as an open system reaction between hydrous melts and fore-arc mantle peridotites. These two peridotite blocks are considered to be arc-related mantle peridotites that experienced melt extraction and metasomatism in different styles. They were exhumed to the north margin of the North China Craton during the c. 2.47 Ga arc–continent collision along the Zunhua structural belt. [ABSTRACT FROM AUTHOR]

Crystalline basement and Precambrian crustal growth of the continental massifs within the Central Asian Orogenic Belt (CAOB) are still pending problems. Our geological and geochemical investigations identified an Archean (2606 Ma) granitic pluton in the Biliya area of the Erguna Massif. The Neoarchean granitoids show high and positive in‐situ zircon ∊Hf(t) (+0.3 to +10.0), whole‐rock ∊Nd(t) (+4.8) and whole‐rock ∊Hf(t) values (+2.1). They are characterized by high Y + Ce + Zr + Nb (> 220 ppm) and Zr contents and could be classified as A‐type granites. These granitoids are characterized by high Zr saturation temperatures (TZr) (796–836°C). They were derived from partial re‐melting of juvenile mafic lower crust in an intracontinental back‐arc extensional environment. This newly identified Neoarchean granitic pluton may represent the crystalline basement of the several continent massifs within the CAOB, and their high ∊Hf(t)–∊Nd(t) values may also indicate the occurrence of lateral crustal growth events in these massifs during the Neoarchean. [ABSTRACT FROM AUTHOR]

Banded iron formations (BIFs) were deposited before and concurrent with the Great Oxidation Event at ∼2.33 Ga. They provide useful archives that document the transformation of the Precambrian hydrosphere from anoxic to progressively oxygenated conditions. Their formation involves removal of oceanic Fe by either inorganic or biologically promoted Fe2+ oxidation, or both. To evaluate depositional settings, elemental sources that affect seawater chemistry, and oxidation pathways, we present the first combined highly siderophile element (HSE) and Fe-Os isotope study for the ∼2.7 Ga Temagami BIF, Abitibi Greenstone Belt, Ontario (Canada). HSE abundances and 187Os/188Os ratios show no systematic variation between alternating magnetite and (meta)chert bands of the Temagami BIF. Whereas HSE concentrations mostly resemble modern crustal values, present-day 187Os/188Os ratios range from ∼0.17 to ∼10.8. Magnetite samples define a regression line corresponding to an age of 2661 ± 126 Ma. A chondrite-like 187Os/188Os initial value is in agreement with earlier studies on Neoarchean marine sediments and is thought to reflect seawater composition, which, unlike modern oceans, is dominated by mantle-like 187Os inventory most likely derived from deep-sea hydrothermal vents. Our δ56Fe data vary from about +0.6‰ to +0.9‰ and define a sawtooth-like pattern between alternating magnetite and (meta)chert layers. Partial oxidation of hydrothermally sourced Fe(II) and a lack of microbially mediated dissimilatory iron reduction provide the most plausible explanation for the positive δ56Fe values. Notably, our δ56Fe data for Temagami are in accord with trends defined by literature results for other Algoma-type BIFs that were deposited throughout the Archean. [ABSTRACT FROM AUTHOR]

Strongly peraluminous granitic rocks (SPG), defined by an aluminum saturation index =1.1, become abundant in the rock record in the Neoarchean. This study identifies three different varieties of Neoarchean SPG in the Archean Wyoming Province, U.S.A. These include calcic SPG, represented by the Webb Canyon Gneiss and Bitch Creek Gneiss of the Teton Range; calc-alkalic to alkali-calcic suites composed entirely of SPG, including the Rocky Ridge garnet granite gneiss of the northern Laramie Mountains and the Bear Mountain granite in the Black Hills; and calc-alkalic to alkali-calcic suites that include both weakly and strongly peraluminous granitic rocks, such as the Mount Owen batholith, Wyoming batholith, and Bears Ears granite. Although the petrogenesis of all the SPG suites involves partial melting of crustal sources, the composition of those sources, the melting conditions, and the tectonic settings vary. The calcic suites originate by dehydration melting or water excess melting of hornblende-plagioclase rocks at relatively high temperature. The suites composed entirely of SPG formed by partial melting of metasedimentary rocks by reactions involving muscovite at lower temperatures. Suites with both weakly and strongly peraluminous granite may form by partial melting of metasedimentary rocks by reactions involving biotite or by assimilation of aluminous melts of felsic crust by differentiated calc-alkalic magma. Most of the Wyoming SPG appear to have formed in collisional orogens, but SPG of the Wyoming batholith and Bears Ears granite are associated with continental arc magmatism. The appearance of SPG in the Neoarchean rock record marks the time when subduction enabled the formation of strong, thick, increasingly felsic continental crust, which in turn allowed the development of a mature, clastic sedimentary cover. Lateral movement of crustal blocks led to collisional orogeny, SPG magma genesis, and the formation of the first supercontinents. [ABSTRACT FROM AUTHOR]

Microbial activity is often invoked as a direct or indirect contributor to the precipitation of ancient chemical sedimentary rocks such as Precambrian iron formations (IFs). Determining a specific metabolic pathway from the geological record remains a challenge, however, due to a lack of constraints on the initial conditions and microbially induced redox reactions involved in the formation of iron oxides. Thus, there is ongoing debate concerning the role of photoferrotrophy, that is the process by which inorganic carbon is fixed into organic matter using light as an energy source and Fe(II) as an electron donor, in the deposition of IFs. Here, we examine ~2.74‐Ga‐old Neoarchean IFs and associated carbonates from the Carajás Mineral Province, Brazil, to reconstruct redox conditions and to infer the oxidizing mechanism that allowed one of the world's largest iron deposits to form. The absence of cerium (Ce) anomalies reveals that conditions were pervasively anoxic during IF deposition, while unprecedented europium (Eu) anomalies imply that Fe was supplied by intense hydrothermal activity. A positive and homogeneous Fe isotopic signal in space and time in these IFs indicates a low degree of partial oxidation of Fe(II), which, combined with the presence of 13C‐depleted organic matter, points to a photoautotrophic metabolic driver. Collectively, our results argue in favor of reducing conditions during IF deposition and suggest anoxygenic photosynthesis as the most plausible mechanism responsible for Fe oxidation in the Carajás Basin. [ABSTRACT FROM AUTHOR]

• The study reveled the formation of the Neoarchean conglomeratic Ament Bay basin. • The basin is similar to the Timiskaming assemblage in sedimentation and setting. • The basin is dissimilar to the latter in crustal architecture and gold endowment. • The lack of gold in the basin is due to existence of underlying gneissic basement. • Timiskaming-like basins similar in geology may not contain similar gold endowment. The Neoarchean conglomeratic Timiskaming assemblage is considered to be one of the most gold-endowed rock associations in the Abitibi greenstone belt of the southern Superior craton. Identifying Timiskaming-like basins, therefore, may facilitate new discoveries of gold deposits in other Archean greenstone belts. This study focuses on the conglomeratic Ament Bay assemblage (ABA) in the Neoarchean Sturgeon Lake greenstone belt of the western Wabigoon terrane of the western Superior craton. Geologic mapping and petrographic analysis show that the ABA consists of a ≥1000 m thick conglomerate-arenite lithofacies and a conformably overlying ≥200 m thick greywacke-mudstone lithofacies, which are interpreted to indicate a sub-aerial fan delta that transitioned into subaqueous turbidites. The depositional age is constrained to be between 2697.5 ± 1.2 and 2693.2 ± 0.9 Ma by the statistically defined minimum age of the ABA detrital zircons and igneous zircons of an alkalic intrusion within the ABA, respectively. Detrital zircons from seven ABA samples are characterized by ca. 2930, 2780–2770, and 2750–2690 Ma age populations along with minor zircons of ca. 3300–3000 Ma, which are consistent with proximal sources mostly in the western Wabigoon terrane and secondarily in the adjacent Winnipeg River terrane. Petrography of conglomerate clasts and sandstone supports these provenances and reveals additional sources from proximal mafic volcanic rocks. The ABA is interpreted to result from uplift and erosion associated with the late-stage convergence between the western Wabigoon and Winnipeg River terranes and to be partially controlled by a regional fault zone and accompanied by broadly coeval alkalic magmatism. Overall, the ABA is similar to the typical Timiskaming assemblage in lithofacies, depositional environments, structural setting, and syn -depositional alkalic magmatism, but dissimilar to the latter in gold endowment. The apparent lack of gold in the ABA is likely due to assimilation of crustal components into the coeval alkalic melts and a shallow-penetrating fault zone, in contrast to the juvenile nature of alkalic magmatism and deep-penetrating fault zones associated with the Timiskaming assemblage. This study suggests that Timiskaming-like basins similar in geology are not necessarily equivalent to the typical Timiskaming assemblage in gold endowment. [ABSTRACT FROM AUTHOR]

• Craton-wide structural and geochemical data from main Yilgarn Craton shear zones. • Syn-orogenic shear zones reflect pulses of Neoarchean synmagmatic transpression. • Syntectonic granitic magmatism from upper-mantle and lower-crustal sources. • Synorogenic magmatism played a major role in shaping Yilgarn lithosphere. • Syn-orogenic shear zones played critical role in development of mineral systems. Between the Neoarchean and the early Proterozoic, plate tectonics was gradually established on our planet. How this transition took place, and what kind of geodynamic paradigm dominated before, is a matter of debate, and systematic multidisciplinary studies help improving our understanding of such a transitional period of Earth's evolution. The Yilgarn Craton of Western Australia exposes upper-crustal granite–greenstone terranes that are juxtaposed along large-scale, Neoarchean shear zones, which mainly developed during the 2730 – 2660 Ma Yilgarn Orogeny. By combining structural, microstructural and geochemical data, we present here a systematic study of the main shear zones occurring in the best-exposed, northwestern half of the craton. Our structural data demonstrate that large granitic magma sheets were emplaced during shearing. Although these shear zones exhibit a variety of fabrics that developed over a wide range of temperatures, all preserve memory of at least one stage of shearing at near-solidus temperatures. Our results allow tracing a c. 100 Myr-long tectonomagmatic evolution of this portion of the Archean crust, with implications for lithosphere rheology, crustal evolution, and mineral systems. The major synorogenic structures can be interpreted as inclined transpressional shear zones, along which the bulk of the 3D deformation was efficiently partitioned between greenstones and crystallizing granitic sheets within the shear zones, with the latter accommodating large amounts of orogen‐parallel strain through suprasolidus viscous flow. The positive feedback between protracted magmatism and transpression promoted the extraction and upward transfer of syntectonic magma originated between the uppermost mantle and the mid-crust, as demonstrated by our geochemical dataset. This synkinematic mode of granitic magma transfer contrasts fundamentally to the diapiric mode that was dominant during the lithospheric extension phase that pre-dated the Yilgarn orogeny. Our study suggests that the shear zone system studied here likely played a major role in controlling magma/fluid pathways throughout the late Archean lithosphere, therefore playing a critical role in controlling the development of near-surface mineral systems. [ABSTRACT FROM AUTHOR]

The cratonization history of the North China Craton (NCC) and the nature of tectonothermal events are still highly controversial. Tonalite-trondhjemite-granodiorite (TTG) gneisses, as the dominant lithological assemblages in Archean metamorphic terranes, can provide significant clues to the magmatic and metamorphic evolution of Precambrian crust. This study presents zircon laser-ablation inductively-coupled-plasma mass spectrometry U–Pb ages, trace-element, and in-situ LA-MC-ICP-MS zircon Hf isotope data for the TTG gneisses from the Bengbu-Wuhe area on the southeastern margin of the NCC. Cathodoluminescence images and trace elements indicated that magmatic zircons display the characteristics of euhedral-subhedral crystals with oscillatory growth zoning structures, high ΣREE contents, marked Ce positive anomalies, and Pr–Eu negative anomalies. The metamorphic zircons display the spherical-oval crystals with distinct core-rim structures, high and homogeneous luminescent intensity, lower ΣREE, Nb, Ta, Hf contents, relative flat REE patterns, weak Ce positive anomalies, and Pr-Eu negative anomalies. The Ti–in–zircon geothermometer data indicate that the crystallization temperature of the TTG gneiss ranged from 754 to 868 °C. Zircon U–Pb ages indicate that the TTG gneisses formed at 2.79–2.77 Ga and 2.50 Ga and underwent metamorphism at 2.57–2.52 Ga. The Hf isotopic data indicate that the magmatic zircons exhibit high, positive εHf(t) values close to those of the coeval depleted mantle, whereas the metamorphic zircons exhibit negative or nil εHf(t) values. This implies that the TTG gneisses were derived from the partial melting of the ~ 2.9–2.6 Ga juvenile crustal sources mixed with ~ 3.0–2.8 Ga ancient crustal materials. Combined with the regional tectonic evolution, we propose that the metamorphic basement at the southeastern margin of the NCC underwent episodic crustal growth at ~ 2.7 and ~ 2.5 Ga and subsequently underwent crustal reworking or re-melting of the ancient crust during the Neoarchean. The Neoarchean TTG gneisses might have been derived from the partial melting of lower crustal materials related to plate subduction. [ABSTRACT FROM AUTHOR]

Based on field relationships, microstructure, phase petrology, and whole‐rock elemental geochemistry, we have unravelled the redox conditions and petrogenetic evolution of Neoarchean granites (sensu lato) from the central part of the Bundelkhand Craton, Central India. The studied Neoarchean granites are sanukitoids (sensu lato), high‐K, low‐magnesian granites (HKLMG), and oxidized A‐type granites (OAG). Chemistry of biotite in conjunction with amphibole compositions from sanukitoids indicates their crystallization in calc‐alkaline felsic host magma formed in a subduction setting at elevated fugacity of oxygen (fO2), buffered at +2 fayalite‐magnetite‐quartz (FMQ), corresponding to magnetite series granite. Chemistry of biotite and amphiboles from HKLMG suggests that host melts of these phases buffered above +1 FMQ, that is, moderate fO2 condition corresponding to magnetite series granites. The nature of the HKLMG magma is peraluminous to alkaline which originated at a relatively high‐P condition. The biotites from OAG exhibit pronounced Al‐Fe substitution buffered above +0.5 FMQ to represent oxidized A‐type host granite magma, belonging to magnetite series granite. The sanukitoids bear high ferromagnesian minerals (FeOt + MgO + MnO + TiO2 (FMMT) >4.5; Mg# = 28–45), and enriched incompatible elements without significant Eu anomalies, probably originated from melt‐enriched mantle source in a subduction setting. The HKLMG are characterized by relatively low Mg#, TiO2, high K‐LILEs‐REEs with negative Eu anomalies, and high Th/Nb, Ce/Pb, and La/Nb ratios, which suggest mixed source signatures, derived from the enriched mantle and continental crust. A‐type granites (OAG) bear high NK/A (molar Na2O + K2O/Al2O3 ~ 1.17), FeOt/MgO, K2O/Na2O, Ga/Al, and Ce–Y, which indicate the emplacement of OAG magmas in syn‐ to post‐collisional tectonic environments. Thus, the Neoarchean granites from the Bundelkhand Craton represent a collage of subduction related sanukitoids to high‐K magnesian granites, and syn‐ to post‐collisional oxidized A‐type granites. [ABSTRACT FROM AUTHOR]

In this article, we present the textural, mineralogical, and geochemical studies of a suite of volcanic rocks and their economic mineral potential from the Kadiri Volcanic Province (KVP) – a greenstone belt in the Eastern Dharwar Craton (EDC) of southern peninsular India. The volcanic rocks are represented by primary pyroclastic deposits dominated by tuff breccia, minor lapilli tuff, and pyroclastic flows together with lava flows, namely, metabasalts, andesites, dacites, and rhyolites. Metabasalts and high‐Mg andesites in the KVP are associated with sporadic occurrences of disseminated‐type gold. The ancient works on gold are confined to the ductile‐brittle shear zones in en‐echelon pattern. The source of gold is likely to be from deep fractures (>200 km) at mantle depths that paved the magma to move upper crustal levels due to partial melting of peridotites which tapped the siderophile elements (Au–Ag) from deeper source. Some andesites and dacites contain atomic minerals [uranium, thorium, and rare earth element (REE)], which requires attention in future studies. Monazite occurring in such dacites is a major source of REE such as cerium (Ce), lanthanum (La), neodymium (Nd), and Yttrium (Y), contributing 50–60% of REEs. Presence of thorite (ThSiO4) in dacite, a metamict mineral, strongly radioactive containing upto 10% uranium (U). This study confirms multi‐metal mineralization in KVP, for example, gold, silver, atomic (U and Th), rare‐earths along with base metals. Emplacements of rhyolite and alkali granite appears to be synchronous with the major thermal event at 2.51 Ga of the Dharwar Craton. The diverse volcanic associations are formed in a subduction–accretion orogeny at ~2.7 Ga, coeval with the global accretion event and gold mineralization. Au–U–Th–REE abundances in the composite arc‐back arc system of Kolar‐Hutti‐Kadiri‐Jonnagiri greenstone belts of EDC are primarily attributed to devolatilization and melting of subducted oceanic slab, fluid‐fluxed metasomatism of mantle wedge and elemental cycling associated with different stages of subduction. [ABSTRACT FROM AUTHOR]

TIMS Pb-Pb geochronological data allow determination of the timing and genesis of the Fazenda Coqueiro volcanic‑hosted massive sulfide (VHMS) Zn-Pb deposit hosted in the Neoarchean Mundo Novo greenstone belt (MNGB), NE São Francisco Craton. The deposit is inserted in the Rhyacian-Orosirian Contendas-Jacobina lineament between Archean cratonic blocks. The basement of the deposit is composed of Paleoarchean metagranite and metarhyolite nuclei tectonically emplaced within the supracrustal rocks. The volcanic-sedimentary rocks comprise the ocean floor western metabasalt, calcsilicate rock, aluminous schist, metachert, banded iron formation, and tremolitite of the middle sequence and metasedimentary siliciclastic rocks of the uppermost sequence of the MNGB. The western metabasalt is hydrothermally altered, which resulted in the formation of two alteration zones. They consist of carbonate zone, proximal, hosting massive sulfides composed mainly of sphalerite and galena with minor chalcopyrite; and sericite-chlorite zone, distal, hosting mainly disseminated chalcopyrite. Pb-Pb galena, chalcopyrite and sphalerite data from the massive and disseminated zones in the Fazenda Coqueiro deposit yielded model ages of 2,804 ± 11.15, 2,794 ± 11.2, and 2,767 ± 11.1 Ma, respectively, with the Pb sourced from the upper crust, based on the uranogenic and thorogenic diagrams. The Pb-Pb isochron mineralization age of 2,747 ± 16 Ma obtained from chalcopyrite and sphalerite samples from the massive and disseminated zones suggests that the sulfides were coeval and do not record later metamorphic-hydrothermal events. Therefore, the Fazenda Coqueiro deposit would have formed from Neoarchean ocean floor volcanic-exhalative processes. The Rhyacian-Orosirian tectonic event compressed the deposit between Archean blocks along the Contendas-Jacobina lineament, preserving the sulfides from remobilization processes.

Abstract The Pedra Branca suite (2.75 Ga) is located in the Canãa dos Carajás domain in the southeastern Amazonia Craton. It diverges from typical TTG in mineralogical and geochemical terms, by the presence of hornblende and clinopyroxene and because it has a high content of HFSE (Zr, Y, Ti, and Nb). It belongs to the low-K subalkaline series, which varies from metaluminous to peraluminous, and are mostly calc and ferroan granitoids. Amphibole is calcic and classified as ferroan-edenite, and hastingsite. Plagioclase is mainly oligoclase. The Fe/(Fe + Mg) ratios found in the amphiboles indicate that these granitoids were formed under high to moderate fO2 conditions. Geobarometric calculations suggest pressures between 9.3 and 71 Kbar for the origin and 4.8-53.4 Kbar for the emplacement. Geothermometric calculations suggest initial crystallization temperatures between 945 and 862°C, and the water content in the magma is estimated to be higher than 4 wt%. The magma source was defined as tholeiitic continental gabbro melted in an extensional setting (Carajás Rift) with geochemical features similar to diabase from Nova Canadá (PA). The Pedra Branca magma was originated by partial melting (~28%), leaving a residue with plagioclase (An40), hornblende, clinopyroxene, and may or may not have magnetite.

Copyright of Anuario do Instituto de Geociencias is the property of Universidade Federal do Rio de Janeiro, Instituto de Geociencias and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Zircon crystals from the alkaline and nepheline syenites of the Saharjok massif, which were formed during the magmatic (2645 ± 7 Ma), hydrothermal (1832 ± 7 Ma), and metamorphic (1784 ± 22 Ma) crystallization stages, were studied. Zircon from alkaline syenites displays lower εHf and εNd values than zircon from ocean island basalts (OIB); these values indicate their affinity to mantle-derived protoliths, which are similar to CHUR in isotope composition. It is assumed that the Neoarchean Keivy alkaline province formed as a result of the plume–lithosphere interaction, induced by the upwelling of a Neoarchean plume, which facilitated metasomatic alteration and the subsequent partial melting of rocks of the subcratonic lithospheric mantle and the crust. The parental melts of the Saharjok massif could have been formed during the resumption of plume activity 20–30 million years after alkaline granite emplacement in the province by the selective melting of the metasomatized lithospheric mantle rocks. [ABSTRACT FROM AUTHOR]