Typical Archean Research Articles

Successful subduction of oceanic plate after failed attempts in the Late Archean: Petrological and geochemical constraints

When and how plate tectonics started and evolved to the style as we know it today is a fundamental yet highly controversial question. Numerical geodynamic modelling predicts that the transition into plate tectonics in the Archean was episodic with possible alternation between success and failure of subduction. However, direct geological evidence for failed subduction is scarce. While this possibility has been suggested by numerical modelling, it is still lack of geological evidence. Here we present a combined study of zircon U-Pb ages and Hf-O isotopes, as well as whole-rock major and trace elements, for meta-igneous rocks from the Alxa Block in the westmost North China Craton (NCC). Two periods of Late Archean magmatism ca. 2.75 Ga and 2.5 Ga are identified to occur surrounding a pre-3.0 Ga continental nucleus. Geochemical analyses of the ca. 2.75 Ga meta-mafic and meta-felsic igneous rocks show two different modes of tectonic regime. The felsic rocks resemble typical Archean TTG (tonalite-trondhjemite-granodiorite) in composition, with variable εHf(t) values from -2.8 to 11.0 and δ18O values from 4.3 ‰ to 7.9 ‰. Petrogenetic modelling suggests that their magmatic source was the ca. 3.1–2.75 Ga oceanic crust that was hydrothermally altered at different temperatures and then mixed with the older continental crust and partially melted in lower crust in the garnet stability field. This requires tectonic accretion of the oceanic crust to the continental nucleus, signifying an attempted or failed subduction. On the contrary, the meta-mafic rocks exhibit arc-like trace element patterns and calc-alkaline evolution trend, indicating a metasomatic mantle source due to successful subduction of the oceanic slab to subarc depths. Taken together, the present results provide robust constraints on the behavior of oceanic slab at the Archean convergent margin, where successful oceanic subduction would be achieved after several failed attempts. Such failure-success processes of oceanic subduction may be applicable to the whole NCC and other cratons elsewhere in the world, reflecting the gradual maturation of plate tectonics in the Archean, consistent with predictions of numerical modelling.

Mesoarchean synchronous emplacement of TTG gneisses and potassic granitoids in the Nyabessane granite-greenstone terranes, NW Congo Craton (southern Cameroon): Zircon U[sbnd]Pb geochronology, petrogenesis and tectonic implications

Archean granitoids played key roles in the generation and differentiation of the Archean continental crust and provide clues to understand crustal processes in the early Earth. Abundant Mesoarchean granitoids were emplaced in the Nyabessane granite-greenstone terrane (NGB), part of the Ntem Complex of the Northwest Congo Craton. They include charnockites, tonalite-trondhjemite-granodiorite (TTG), granitic and monzogranitic gneisses. Here, we present a geochemical and geochronological (zircon LA-ICP-SF-MS UPb) study of these granitoids to determine their petrogenesis and to better constrain the crustal evolution of the Ntem Complex. Field and petrographic observations indicate that most of these granitoids underwent extensive metamorphism and deformations, associated with anatexis. Zircon UPb dating results suggest that the charnockite, TTGs and granitic gneisses, and monzogranites have emplacement ages of 2910 ± 11 Ma, 2870–2865 Ma and 2852 ± 31 Ma, respectively. The charnockites have low SiO2 (55–58 wt%) and high Al2O3 (16–18 wt%), CaO (7–8 wt%) and MgO (∼ 4.5 wt%) content with Mg# ∼ 54, and exhibit magnesian, metaluminous characteristics of the Cordilleran granitoid-type formed in magmatic arc. The 2.87–2.86 Ga TTG gneisses are silica-rich (55–58 wt% SiO2), sodic (3–5 wt% Na2O, Na2O/K2O = 1–3), with HREE-depleted, and display the typical Archean medium- to low pressure TTG geochemical features groups. Their chemical compositions are characteristics of TTG-like melts derived from the partial melting of hydrated low- to high-K metabasic/thickened lower crust at various depths followed by magmatic differentiation during ascent. When compared to the TTG gneisses in the NGB, the ∼2.87 Ga granitic gneisses are K-rich, and high Gd/Yb, Th/Yb, Th/Nb, Sr/Y and La/Yb ratios but lower MgO, Yb, V, Y, Cr, Ni and Sr contents, matching typical Archean hybrid and potassic granitic rocks. We propose that the granitic gneisses were derived from the intra-crustal melting of a pre-existing felsic crust. The granitic and the TTG gneisses were generated contemporaneously through the same magmatic event at ∼2.87–2.86 Ga. The ∼2852 Ma monzogranitic gneisses are ferroan and metaluminous rocks, and show LREE enrichment with strongly fractionated REE patterns and positive Eu anomalies. Geochemical features, together with the presence of ∼2.9 Ga-old inherited zircon grains are consistent with the remelting of Mesoarchean granitoids. Considering the petrogenetic, regional geological and geochronological data, the Mesoarchean granitoid magmatism of the Ntem Complex was likely generated via complex transitional geodynamic regimes involving subduction and accretion processes.

Rare earth element and yttrium (REY) geochemistry of 3.46–2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification.

Paleoproterozoic high-pressure granulite facies metamorphism in the Yinshan Block, North China craton

The Yinshan Block located in the northern margin of the North China Craton (NCC) has been regarded as a typical Archean cratonic block, but it is argued whether the block had been affected by Paleoproterozoic tectonic and metamorphic events. This paper reports the late Paleoproterozoic high-pressure (HP) granulites for the first time from the Xiwulanbulang area in the Yinshan Block, and presents a systematic study on their petrography, mineral chemistry, phase equilibria modelling and zircon dating to confirm their metamorphic evolution and tectonic implications. Two representative samples contain the typical HP basic granulite assemblages involving garnet, clinopyroxene, plagioclase, amphibole and quartz, with garnet commonly occurring as coronae around clinopyroxene and plagioclase. Both samples show clockwise P–T paths with their peak conditions of ∼930 °C/∼15kbar (20NM01) and 970–980 °C/∼13.5 kbar (21NM05) respectively, constrained using the maximum Ti contents in amphibole and minimum anorthite (XAn) in the mantle of plagioclase on P–T pseudosections. Zircon dating yields a magmatic age of ∼ 2.3 Ga and metamorphic ages of ∼2.5 Ga and ∼1.8 Ga. The ∼2.3 Ga magmatic age represents the time of protolith gabbro crystallization, which can be comparative to the extension-related bimodal magmatism in the Khondalite Belt. The ∼ 1.8 Ga age was determined from metamorphic zircons that have flat HREE patterns with the presence of garnet, interpreted as the time of the post-peak cooling and uplifting of the HP granulite facies metamorphism. The ∼2.5 Ga is defined from the captured zircon grains by gabbroic intrusions from the country Neoarchean rocks. The HP granulite facies metamorphism registers a geothermal gradient of 19–20 °C/km, indicating a collision-related orogenic event that may have occurred at ∼1.85 Ga along the northern margin of the NCC. Therefore, the Yinshan Block may represent a weak deformation domain in the orogenic belt on the northern margin of the NCC.

Spatial heterogeneity of the lithospheric destruction of the North China Craton: Evidence from an extended magnetotelluric sounding profile

To study the spatial heterogeneity of the North China Craton (NCC) destruction, this paper used a magnetotelluric sounding (MT) profile that passes through almost the entire NCC from west to east. Three-dimensional inversion is used to obtain a lithospheric resistivity model of the NCC. The results show that the upper crust of the Ordos Block is characterized by high resistivity. The lower crust to the upper mantle is characterized by low resistivity. The resistivity structure below the Trans-North China Orogen (TNCO) has stratification features; The Shanxi Graben shows high-low-high-low resistivity features from the upper crust to the asthenosphere; The lithosphere of the Lüliang and Taihang uplifts show high-resistivity features, and only some local relatively low-resistivity areas appear at the crust-mantle boundary. The upper crust on both sides of the Tan-Lu Fault Zone is characterized by high resistivity, but the resistivity structures of the lower crust and the lithospheric mantle are significantly different; The lower crust and the lithospheric mantle of the Sulu Orogenic Belt on the east are characterized by high resistivity; The Luxi Uplift on the west is represented by low resistivity. We propose that the mantle low-resistivity bodies (C1 and C4) of the Western and Eastern blocks may be related to the upwelling of partial melting materials along the ancient structurally weak zones in the lithosphere. The TNCO still has a typical Archean cratonic lithosphere, and the low-resistivity body C2 may be the remnant of the subducted oceanic crust. The Tan-Lu Fault Zone is structurally weak in the Eastern Block, while its western branch is a channel for the asthenospheric upwelling. We propose that the lithosphere of the northwestern Ordos Block and the Yinchuan-Hetao area is being destructed, and the TNCO is in the initial stage of being destructed. In contrast, the lithosphere of the Eastern Block has been severely destructed. In conclusion, affected by the subduction of the paleo-Pacific plate and the collision of the Indian and Eurasian plates, the ancient structures in the NCC were reactivated in the Mesozoic and Cenozoic, resulting in the spatial heterogeneity of the NCC destruction.

Comparative orotomy of the Archean Superior and Phanerozoic Altaid orogenic systems.

We compare and contrast the materials and mechanisms of continental crustal growth in the largest preserved regions of Phanerozoic and Archean juvenile additions to the crust, to test for similarities or differences in the formation of continents through time. We accomplish this through a comparison of map patterns, lithological contents, and structural and metamorphic evolution of the Phanerozoic Altaid orogenic system of Asia, with the Archean Superior Province of the North American Craton, using a method termed comparative orotomy. Both orogenic systems consist of collages of curvilinear belts of eroded arcs, some older continental slivers, and vast tracts of former subduction/accretionary complexes. These contain numerous shreds of portions of the ophiolite suite, slivers of island and continental arcs, and accreted oceanic plateau, all intruded by multiple magmatic suites during or between multiple deformation events, then sliced by large transcurrent fault systems and bent into large oroclinal structures. We make this comparison because the Superior Province is a typical Archean craton that was later, in the Paleoproterozoic, incorporated into the larger North American Craton, and has occupied a central position in several supercontinents (e.g. Kenorland and Nuna, which then formed the core of Columbia, Rodinia, Laurentia and Pangea) during its longevity. Since it is the largest single fragment of Archean continental cratonic lithosphere preserved on Earth, the Superior Province is widely regarded as a testing ground for how Earth's continental crust was formed. Likewise, the Altaids encompass the largest region of crustal growth for the Phanerozoic. Our comparison with the Altaids is needed, as in recent years many myths about how the planet may have responded to higher heat production and flow in the Archean have emerged, because of trends in the science where regional geology is ignored in favor of numerical models, isotopic proxies for assumed models of chemical behavior for crust-forming or tectonic processes, or comparisons with other-worldly bodies that bear little resemblance to our hydrous Earth. Thus, we return to the geological record, and here describe the map patterns, lithological associations, structural patterns and evolution of both the Altaids and Superior Province, showing how comparative tectonics, orotomy, is useful in the absence of meaningful paleomagnetic or biostratigraphic data. We pay particular attention to the style of preservation of disaggregated members of the ophiolite suite (ophirags) and their relationships with other tectonic units, and to the widespread but largely overlooked role of late-stage major transcurrent motions and structural slicing of both Archean and Phanerozoic orogenic systems in defining the present-day architecture of both orogenic systems.

Late Neoarchean TTG and monzogranite in the northeastern North China Craton: Implications for partial melting of a thickened lower crust

The petrogenesis and dynamic setting of Archean tonalite–trondhjemite–granodiorite (TTG) is widely debated. As a typical Archean terrane in the northeastern North China Craton (NCC), the southern Jilin Province area underwent multi-stage magmatism during the Neoarchean and is therefore a promising region in which to track the Neoarchean evolution of the NCC. This study investigates the petrogenesis of late Neoarchean TTGs and associated monzogranites and their tectonic implications. Zircon U–Pb dating results suggest that the TTGs and monzogranites have emplacement ages of 2.55–2.52 and 2.53–2.51 Ga, respectively. The TTGs have variable SiO2 (61.7–73.3 wt%) and MgO (0.66–2.74 wt%) contents and Mg# values of 33–50. Most have high Sr and low Y and Yb contents, with high Sr/Y and (La/Yb)N ratios. The TTGs have variable εHf(t) (+0.7 to + 8) and εNd(t) (−2.77 to + 2.79) values. These characteristics, together with the results of thermodynamic partial-melting modelling, indicate that the TTGs can be classified as medium- and low- pressure types and were likely generated by amphibole-breakdown partial melting of 2.9–2.7 Ga juvenile basaltic rocks under P–T conditions of 1.0 ± 0.1 GPa and 880 ± 50 °C. The monzogranites have higher SiO2 (66.1–76.0 wt%) and K2O (4.03–5.06 wt%) and lower TFe2O3 (0.89–4.97 wt%) and MgO (0.21–1.27 wt%) contents. They also have high Sr/Y and (La/Yb)N ratios and variable εHf(t) (+0.6 to + 5.4) and εNd(t) (−3.47 to − 0.15) values. The geochemical and Hf–Nd isotopic compositions of the monzogranites indicate that the parental magmas may have been derived through the partial melting of thickened lower crust with heterogeneous sources. The high thermal gradient (>730 °C/GPa−1), anticlockwise metamorphic P–T paths, and 3.4–3.2 Ga remanent zircons in the region suggest that a late Neoarchean thickened lower crust was the likely geodynamic setting for the generation of the TTGs and monzogranites.

Trondhjemites from the Qianxi Complex, North China Craton: Implications for Neoarchean crustal growth

The TTG (tonalite‐trondhjemite‐granodiorite) rock suites are major building blocks of the Archean continental crust and their timing of formation and petrogenetic evolution are important keys to understanding the growth of continents in the Early Earth. Here we investigate trondhjemites from the Qiangxi Complex in the North China Craton and present petrological, geochemical and zircon U–Pb, REE and Lu–Hf data to understand the timing of their formation and petrogenetic history. The trondhjemites in our study belong to typical Archean Na‐rich TTG rocks and their geochemical features are similar to high‐Al TTG rocks. The geochemical characteristics of these rocks and the associated amphibolite indicate derivation from a subduction‐related hydrous basalt. The magmatic zircon grains from the trondhjemite samples show a peak 207Pb/206Pb age around 2,515 Ma. Magmatic zircon in the associated amphibolite also shows a similar age representing the timing of the protolith formation. Both the trondhjemites and amphibolite also contain metamorphic zircon grains which constrain the timing of metamorphism as 2,451–2,490 Ma, suggesting high‐grade metamorphism closely following the magmatism, associated with the assembly of the microblocks in the NCC. The geochemical characteristics and zircon age data presented in this study, in conjunction with those in previous studies from the Qiangxi Complex, suggest subduction‐related arc building occurred dominantly during 2.6–2.5 Ga. The Lu‐Hf isotope data from zircon grains in the trondhjemite and amphibolite samples of this study show dominantly positive εHf(t) values, indicating that the protolith magma was derived from juvenile components or depleted mantle. Our new results in conjunction with the data reported in earlier studies indicate a major crust‐building event through multiple magmatism during Neoarchean. The depleted mantle ages (TDM) of zircon in our samples range from 2,857 Ma to 2,605 Ma indicating that active subduction might have initiated in the late Mesoarchean, with the peak of magmatism during 2.6–2.5 Ga, followed by metamorphism at ca. 2.45–2.49 Ga.

Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO2 concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t2) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)N values (13.76–98.19) and positive ɛHf(t2) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO2 (69.09–73.46 wt%) and K2O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.

The Yunmengshan iron formation at the end of the Paleoproterozoic era

Precambrian iron formations (IF) carry rich information on the interplay between the geosphere and the early biospheres. Here, we report mineralogical and petrologic characterizations of the Yunmengshan IF deposited at ~1.7 Ga, the transition between Paleoproterozoic and Mesoproterozoic eras. The IF contains granular hematite with evident oncoidal structures and lacks laminated structures, which are distinct to the typical banded iron formation (BIF) or granular iron formation (GIF) deposited early in the marine environments with high silica content. Based on observations of clastic sedimentary structures around the Yunmengshan IF, including parallel bedding, cross-bedding, asymmetric ripple mark, and mud crack, we deduce that the Yunmengshan IF was mainly deposited in a supratidal to intertidal zone. Electron microscopic observations show two types of hematite euhedral crystals: laminar hematite of 200–500 nm thick and 3-5 μm in diameter and granular hematite of 200–300 nm in diameter randomly distributed in the matrix. Stilpnomelane and minnesotaite, as Fe2+-silicates in the typical Archean- and the early Paleoproterozoic BIF, are identified but with extremely low abundance. The morphology and paragenetic association of these minerals imply that the Yunmengshan IF was probably deposited in an aquatic environment with low Si concentration. These results indicate that the Yunmengshan IF represents a transitional type of iron deposition between BIF/GIF and ironstone in the geological history of the iron cycle on the surface of Earth.

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

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

Complex Neoarchean mantle metasomatism: Evidence from sanukitoid diorites-monzodiorites-granodiorites in the northeastern North China Craton

Mantle metasomatism is a major factor leading to diverse arc magmatism. Late Neoarchean sanukitoid diorites-quartz diorites-monzodiorites-granodiorites (DQMGs) are widespread in the North Liaoning Block, northeastern North China Craton, and were evidently emplaced into Neoarchean tonalite-trondhjemite-granodiorite (TTG) gneisses and metamorphic calc-alkaline volcanic rocks. Integrated with these slightly earlier arc-related TTGs and volcanic rocks, DQMG magmatism may preserve crucial signatures of complicated Neoarchean mantle metasomatism.These DQMGs show emplacement ages of ~2.56 to 2.53 Ga and wide ranges of SiO2 (51.34–68.74 wt%), K2O (1.67–4.56 wt%), MgO (1.48–6.68 wt%), Cr (29–334 ppm) and V (46–329 ppm) concentrations and high Mg# values of 45 to 65, with zircon ɛHf(t2) values from +0.0 to +6.9, suggesting that the DQMGs are typical Archean sanukitoids. According to geochemical characteristics and petrogenesis, these sanukitoid DQMGs are subdivided into Group #1 and Group #2. The Group #1 magma was derived from partial melting of a fluid-metasomatized mantle source altered further by adakitic melts, and the Group #2 magma from the mantle source was further metasomatized by subducted sediment melts. The relatively high K2O contents, K2O/Na2O, Nb/Zr ratios and various TDM(Hf) values of the sanukitoid DQMGs were derived from a lithospheric mantle source that was metasomatized by subducted fluids, slab- and sediment-derived melts. Therefore, the complex mantle metasomatism may be an essential reason for the appearance of a mass of K-rich granitoids with sanukitoid composition and the Neoarchean diversity of granitoid rocks.

Geochemistry of molybdenum in the continental crust

The use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 µg/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion (i.e., UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates (Greaney et al., 2017; Jenner and O’Neill, 2012). On average, granites are missing ∼60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings.

Archean TTGs and sanukitoids from the Jiaobei terrain, North China craton: Insights into crustal growth and mantle metasomatism

U–Pb zircon dating for 10 rock samples collected from the Archean Jiaobei terrain of the eastern block of the North China craton (NCC) yield three groups of ages: ∼2.9Ga, ∼2.7Ga and ∼2.5Ga. Thirteen rock samples studied can be classified into a low Mg# (molecular Mg/(Mg+Fe)) (Mg#=34–52) and a high Mg# group (Mg#=52–61), respectively. The low Mg# rocks are typical Archean TTGs with emplacement ages of ∼2.9Ga, ∼2.7Ga and ∼2.5Ga whereas the high Mg# rocks are Archean sanukitoids with age of ∼2.5Ga. Despite having different ages, all the Jiaobei TTGs have zircon δ18O values between 5.6‰ and 6.3‰, essentially identical to those of the ∼2.7Ga TTGs from adjourning Taishan area and Archean TTGs from other parts of the world. Magmatic zircons from the Jiaobei sanukitoids have δ18O values between 6.4‰ and 7.5‰, significantly higher than the Archean TTGs. The whole-rock Nd and the zircon Hf isotopic compositions indicate that the ∼2.9Ga and ∼2.5Ga TTGs and ∼2.5Ga sanukitoids represent crustal growth at ∼2.9Ga and ∼2.5Ga in the Jiaobei terrain. Unlike the ∼2.7Ga TTGs in the adjourning Taishan area that are considered to reflect juvenile crust, the Jiaobei ∼2.7Ga TTGs, however, are most likely resulted from the reworking of the pre-existing ∼3.0Ga crust. This is in contrast to the previous suggestion that ∼2.7Ga is the most significant crust-forming episode and ∼2.5Ga a period of crustal reworking in the Jiaobei terrain. The high Mg# and high contents of Cr, Ni, LREE and LILE of the ∼2.5Ga Jiaobei sanukitoids, require a metasomatized peridotitic mantle source for their origin. The significant negative Zr and Hf anomalies and the elevated δ18O values of the ∼2.5Ga sanukitoids, are inconsistent with a mantle source metasomatized solely by the TTG melts. Instead, we suggest that the Jiaobei sanukitoids could have originated from a mantle wedge metasomatized by a high δ18O component, most likely the slab-derived fluids. The similarity of multi-element patterns and the zircon δ18O values between the Jiaobei and Superior Province sanukitoids may imply similar processes involved to form sanukitoids in both Jiaobei and Superior Province terrains.

Geochronology, redox-state and origin of the ore-hosting porphyry in the Tongkuangyu Cu deposit, North China Craton: Implications for metallogenesis and tectonic evolution

Tongkuangyu Cu deposit, located at the southern edge of the North China Craton, is hosted by a suite of sedimentary-magmatic rocks. Genesis of this giant ore deposit has been debated for over half a century. New data from geochronological, geochemical and isotopic analyses on the ore-hosting monzogranitic porphyries are presented. Seventy four zircon U-Pb dates by laser ablation ICPMS suggest that the porphyries were emplaced at ca. 2180Ma. Geochemically, they show transitional features between typical Archean TTG and sanukitoid, e.g. displaying sodium enrichment with elevated K contents, and exhibiting strong REE fractionation with elevated HREE, Mg, Cr contents. Ti-in-zircon thermometer reveals a crystallization temperature of 675±16°C. Cerium anomalies in zircons indicated that the porphyry magmas have low oxygen fugacity of 0.5 log unit below the fayalite-magnetite-quartz buffer (ΔFMQ −0.5). Zircons from the porphyries have ɛHf(t) values of −4.9 to 2.5 with two-stage depleted mantle model ages of ∼2.8Ga, which point to a magma source resembles the newly-discovered 2.7Ga TTGs and diorite in this region. Zircons from the porphyries are slightly more enriched in 18O than that of mantle zircons, with δ18O values of 5.2–5.9‰ (averaging 5.6±0.1‰). Geochemical modeling suggests that the porphyry magmas were likely sourced from melting of the 2.7Ga diorite leaving behind hornblende and biotite as residues, with minor contribution (<20%) from juvenile, mantle-derived materials. The reducing nature of the porphyry magmas makes them incapable of sequestering sufficient Cu and S, thus rendering them unsuitable for porphyry-style copper mineralization. By tentative inference, we suggest that magmatic sulfide in the porphyries may have acted as a reducing agent to precipitate copper from the ore-forming fluids. Combining the above evidence and previous interpretations, we propose a tectonic evolution model envisaging an oceanic subduction at ∼2.7Ga, followed by a continental collision at ∼2.5Ga, and ended up with an extension at ∼2.2Ga.

A 2.5 Ga fore-arc subduction-accretion complex in the Dengfeng Granite-Greenstone Belt, Southern North China Craton

The Dengfeng granite-greenstone belt (DGGB), located in the southern segment of the Central Orogenic Belt (COB) of the North China Craton (NCC), consists of a volcano-sedimentary assemblage, intruded by tonalite, trondhjemite, granodiorite (TTG suite), diorite, granites and late mafic dikes. The volcano-sedimentary assemblage in the DGGB mainly consists of tectonically imbricated basaltic amphibolites, meta-gabbroic rocks with minor ultramafic rocks, and metagreywacke, marble and quartzite, consistent with characterisitics of typical Phanerozoic subduction-accretion complexes. The basaltic amphibolites yield a metamorphic zircon 207Pb/206Pb age of 2507±26 Ma, interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction/accretion of the basaltic protolith. The basaltic amphibolites are characterized by a tholeiitic affinity, and flat LREE patterns with minor negative Nb and Zr anomalies. Based on mixed MORB- and arc-affinities, the basaltic amphibolites in the DGGB are interpreted to have formed in a fore-arc tectonic setting. One late potassic granite dike cutting across the fabrics of the volcano-sedimentary assemblage yields an intrusion age of 2492±35 Ma, constraining the minimum deformation age for tectonic assembly of the package. The TTG gneisses and diorites intruded the western margin and the center of the subduction-accretion complex, respectively. One TTG sample and one diorite sample yield igneous zircon 207Pb/206Pb ages of 2514±26 Ma and 2518±36 Ma, respectively, constraining their intrusion ages. The TTG gneisses display high ratios of (La/Yb)cn and Sr/Y, and depletion in HFSE with negative Nb, Ta and Ti anomalies, consistent with those of typical Archean TTGs. The TTG gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and/or accreted arc amphibolites. The diorites have high concentrations of MgO (2.89–6.05wt.%), Ni (148–178ppm) and Cr (85.7–120.6ppm), and highly fractionated REE patterns and are depleted in HFSEs with negative Nb, Ta, Zr, Hf and Ti anomalies. We suggest that the high-Mg diorites in the DGGB may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction-derived melts and/or fluids. Collectively, a Neoarchean subduction-accretion-collision event is therefore proposed to have generated the DGGB. The volcano-sedimentary assemblage in the DGGB represents a fore-arc subduction-accretion complex, which is interpreted to be related to the suture zone of ca. 2.5 Ga arc-continent collision between a TTG-dominated arc terrane in the COB and the Eastern Block of the NCC. We further propose a long N-S striking Neoarchean suture zone occurring in the eastern margin of the COB mainly consists of the ca. 2.5 Ga subduction-accretion complex in the DGGB to the south, the ca. 2.5 Ga mélange belt in the Zanhuang Complex in the central, and the ca. 2.5 Ga ophiolitic mélange belt in the Zunhua-Dongwanzi structural belt to the north, which separate an arc terrane in the COB from the Eastern Block of the NCC.

Petrology and geochemistry of the Guyang hornblendite complex in the Yinshan block, North China Craton: Implications for the melting of subduction-modified mantle

The hornblendite complexes hosted in the Guyang granite-greenstone terrane form part of the Neoarchean basement in the Western Block of the North China Craton. In this study, we focus on the largest one from this block, previously named the Guyang komatiite, and present results from lithological, geochronological and geochemical studies. The dominant lithology in the Guyang hornblendite complex is greenschist facies hornblendite, and can be divided into clinopyroxene hornblendite (∼75%) and olivine-orthopyroxene hornblendite (∼25%). The oldest calculated Re depletion model ages (TRD) of these hornblendites is of 2454Ma, and the zircon U-Pb age of the wallrock is 2480Ma, with single-stage depleted mantle Nd model ages TDM1(Nd) varying from 2.61 to 2.88Ga. These data suggest that the Guyang hornblendite complexes formed during Neoarchean-Paleoproterozoic time. The hornblendites show low SiO2, high MgO contents and are enriched in Cr, Ni, and LREE with negative Ce anomalies and depleted in Ti, Nb and Ta. Their 187Os/188Os ratios range from 0.11145 to 0.11279, with γOs(2.5Ga) varying from −3.9 to +1.4. Geochemically, the olivine-orthopyroxene hornblendite shows little variation with typical cumulate feature. In contrast, the clinopyroxene hornblendite shows a large range of chemical variation. In the CaO vs. MgO and CaO/Al2O3 vs. MgO diagrams, the clinopyroxene hornblendite shows Opx fractionation trend. Combined with lithological information, we infer that the hornblendite magma was emplaced as a crystal mush, with orthopyroxene as cumulus phase. The Os and Nd isotope composition, negative Nb, Ta, Ce anomalies, and the relationships in Th/Yb–Nb/Yb diagram suggest that the Guyang hornblendites formed from a source mantle that was modified by subduction-related melts/fluids derived from a seawater-altered basaltic slab. Compared with typical Archean komatiites, the rocks of present study show significant differences in lithological and geochemical characteristics, and thus it cannot be named as komatiite. To explain these characteristics, we propose a geodynamic model involving ridge subduction and slab window mechanism to account for the formation of the Guyang hornblendite complex.

Episodic Paleoarchean-Paleoproterozoic (3.3–2.0 Ga) granitoid magmatism in Yangtze Craton, South China: Implications for late Archean tectonics

The Archean Kongling Terrane preserves voluminous Paleoarchean-Paleoproterozoic granitoids, recording the early formation and evolution history of the Yangtze Craton, South China. We present petrography, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of twelve gneissic granitoids from this terrane, including trondhjemites, biotite-granites, and two-mica granites. Magmatic zircons in these granitoids yield emplacement ages of 3.31–3.29Ga for trondhjemites, 2.81–2.78 and 2.66Ga for biotite-granites, as well as 2.70–2.64, 2.42, and 2.00Ga for two-mica granites. The 2.8–2.6Ga granites contain minor 3.4–2.9Ga inherited zircons and 2.0Ga metamorphic zircons that formed mainly by recrystallization. Major and trace element compositions of the trondhjemites suggest a garnet amphibolite-facies low-K mafic source, similar to the typical Archean medium-pressure trondhjemite-tonalite-granodiorites (TTGs), whereas those of the biotite- and two-mica granites indicate tonalitic and tonalitic-sedimentary sources, respectively. The Lu–Hf data (chondritic ɛHf(t) values −0.8 to 0 and Eoarchean two-stage Hf model ages 3.75–3.70Ga) for the trondhjemites support partial melting of a long-lived ancient depleted-mantle-derived or a juvenile lower-mantle-derived mafic crust. The Lu–Hf data and inherited zircon ages also suggest that the 2.8Ga granites were derived from the local 3.4–2.9Ga TTGs. Together with previously addressed 3.2 and 3.0–2.9Ga TTG magmatism, a remarkable change is observed at 2.8Ga in the petrology, geochemistry, and petrogenesis of the episodic 3.3–2.0Ga granitoid magmatism from TTG- to granite-dominated in the Kongling Terrane, which may reflect a transition from subduction- to collision-related tectonics. In addition, we note that the 2.70–2.64Ga biotite- and two-mica granites (this study) as well as 2.67–2.62Ga A-type granites (published data) were emplaced shortly after a 2.75–2.72Ga high-grade metamorphic event (published data) in the Kongling Terrane. These granites could be generated as a result of orogenic root collapse and subsequent mantle upwelling, implying a 2.7Ga orogenic event in the Yangtze Craton.

The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening

AbstractGeologically rapid (tens of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes, such as North China Craton, southern Africa, and the North Atlantic Craton, and has been linked with thermomechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermomechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long‐term erosion from normal plate‐mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid‐melt metasomatism, and our models show that this is essential for the thinning of originally stable continental roots. The long‐term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within tens of Myr if affected by metasomatic weakening.

A synthesis of geochemistry and Sm–Nd isotopes of Archean granitoid gneisses in the Jiaodong Terrane: Constraints on petrogenesis and tectonic evolution of the Eastern Block, North China Craton

The Jiaodong Terrane is situated in the northeastern part of the Shandong Province in eastern China, experiencing multi-stage magmatism during the Mesoarchean (∼2.9Ga) and Neoarchean (∼2.7Ga and ∼2.5Ga). It consists dominantly of granitoid gneisses with minor amphibolite enclaves or lenses. This study presents new whole-rock geochemical and Sm–Nd isotopic data for the Archean granitoid gneisses from the Jiaodong Terrane in order to constrain their petrogenesis and tectonic setting. Geochemical studies and petrological features suggest that the protoliths of most granitoid gneisses are typical Archean tonalitic–trondhjemitic–granodioritic (TTG) suites, which are high in SiO2 (60.9–76.8wt.%), Al2O3 (12.6–16.8wt.%), Na2O (3.3–5.3wt.%), Sr (135–604ppm) and Sr/Y ratios (13.9–84), but low in MgO (0.4–4.5wt.%), K2O (0.7–3.4wt.%), TiO2 (0.2–0.9wt.%), Cr (<0.5–37.9ppm), Ni (<0.5–24ppm), Y (2.8–19ppm) and Mg number (Mg#=30–45). They are generally enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), with slight Eu anomalies. These geochemical features suggest that they were derived from partial melting of metabasaltic rocks leaving eclogite or garnet–amphibolite dominant in the residue. Whole-rock Nd isotopes reveal that the protoliths of Mesoarchean (∼2.9Ga) granitoid gneisses were derived mainly from juvenile sources, whereas the early Neoarchean (∼2.7Ga) granitoid gneisses were derived from juvenile sources with significant additions of crustal material and the late Neoarchean (∼2.5Ga) granitoid gneisses were mainly derived from continental crustal sources. Combined with other geological considerations, a mantle plume model is favored to account for the generation of both early and late Neoarchean granitoid gneisses, whereas the tectonic setting for the Mesoarchean granitoid gneisses remains uncertain due to lack of geological data.