End Of Archean Research Articles

Reconstructing the evolution of an ancient continental arc: Implications from the temporally-spatially related shoshonitic to calc-alkaline intrusive magmatism in the North China Craton

Coeval shoshonitic and calc-alkaline to high-K calc-alkaline rock series were discovered in the eastern North China Craton (NCC). Here, we report zircon and titanite UPb dating, zircon oxygen isotopic, whole-rock major and trace element and SmNd isotopic data for these rocks. Zircon and titanite UPb geochronology suggests that the coeval shoshonitic to calc-alkaline mantle-derived magmas from the Jinan terrane were emplaced at ∼2.5 Ga and recorded multi-phases of metamorphic imprints. The shoshonitic series in Group I show higher K2O, but in contrast, the calc-alkaline to high-K calc-alkaline series (Group II) are characterized by higher Na2O. The two groups have enriched light rare earth elements (LREE) and depleted/nearly flat heavy rare earth elements (HREE). They also show negative anomalies in some HFSE and positive anomalies in LILE. Therefore, the geochemical data, SmNd and O isotopes included, show that the Jinan shoshonitic to calc-alkaline magmas were generated by melting of the metasomatized mantle at an ancient continental arc. Distinct geochemical features possibly suggest different mantle source features (metasomatic agents). The elevated zircon δ18O values imply that the weathering was significant during the end of Archean. In addition, the crustal reworking or intra-crustal recycling process played an important role in the continental crust evolution. The low-δ18O magmas documented here possibly indicate high-temperature water-rock interaction that occurs between continental crust and meteoric water. This study represents an example of co-existed shoshonitic and calc-alkaline mantle-derived magma derived from the melting of metasomatized mantle above an ancient subduction zone. Similar coeval associations of shoshonitic to calc-alkaline mantle-derived rocks, in combination with high-P granulites and crust-derived granites, may be used to constrain the ancient subduction-collisional belts.

Exploring the link between spatiotemporal patterns of plutonism and geodynamic regimes at the end of Archean: an example from the northeastern Superior Province, Canada

This paper presents new U–Pb zircon ages obtained using laser ablation ICP-MS from various plutonic units that intruded along the southern margin of the Bienville domain, a presumed Neoarchean magmatic arc in the northeastern Superior Province, Québec, Canada. The U–Pb ages indicate that the arc plutons were emplaced episodically between ca. 2745 Ma and ca. 2697 Ma, synchronous with regional transpressional deformation. Furthermore, the observed intra-pluton age variations show that the largest examined intrusion, the Radisson pluton, grew incrementally over a period of at least 15 My, perhaps in a series of elongated, sheet-like magma batches straddling the presumed arc margin and emplaced successively, younging inwards. This emplacement style is comparable (though not unique) to modern arc plutons emplaced along or within regional transpressional shear zones. The new ages are then complemented with a statistical analysis of previously published U–Pb pluton zircon ages from the adjacent units (La Grande, Opinaca, Opatica), which revealed a subtle southward-younging trend (i.e., towards the presumed trench) in otherwise broadly coeval and areally extensive plutonism characterized by diverse compositions. The observed spatiotemporal pattern of plutonism is transitional between unfocused, plume-related magmatic centers and focused, unidirectionally migrating magmatism of modern arcs, perhaps as a result of hotter environment and complex interplay between the waning plume-related regime and increasingly dominating modern-style plate tectonics at the end of Archean.

Paleoproterozoic (1.96–1.86 Ga) granites in Xinyang record zoned deep crustal structure and multi-stage reworking beneath the southern North China Craton

As one of the oldest cratons on Earth, the North China Craton (NCC) is a frequent area for geological research due to its complex crustal evolution. Regarding the Paleoproterozoic crustal evolution of the craton, most studies have focused on continental amalgamation (i.e. horizontal crustal evolution), but few on the vertical crustal evolution. Here we report whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotope geochemistry for the Xinyang granites to constraint the vertical crustal structure and evolution of the southern NCC. The studied granites comprise two-mica K-feldspar granite and garnet-bearing K-feldspar granite. All the rocks have high SiO2 and total alkali, but low CaO, MgO and Fe2O3T contents. Their A/CNK values (1.05–1.28) suggest that they are peraluminous rocks. These granites are enriched in LILE (Rb, Th and K), depleted in Sr, P and HFSE (Nb and Ti). The two-mica K-feldspar granites show obviously fractionated REE patterns ((La/Yb)N = 18.3–387) with negative or positive Eu anomalies (δEu = 0.47–1.03, 4.85–6.72), while the garnet-bearing K-feldspar granites have flat REE patterns ((La/Yb)N = 1.69–4.89) with extraordinary negative Eu anomalies (δEu = 0.22–0.42). The magmatic zircons of the two-mica K-feldspar granites yield crystallization ages ranging from ~ 1.86 Ga to ~ 1.90 Ga, and those of the garnet-bearing K-feldspar granites give crystallization ages of ~ 1.90 Ga. Previous work also reported a biotite K-feldspar granite pluton formed at ~ 1.96 Ga in the Xinyang area. Taken together, three major episodes of Paleoproterozoic granitoid events (~1.86 Ga, ~1.90 Ga and ~ 1.96 Ga) are recognized in the area. Combined with regional geochronological investigations, the peraluminous granitic magmatic activities occurring during 1.96–1.86 Ga in the Xinyang area could be linked to the collision between the Eastern and Western Blocks of the NCC. Alternatively, zircon Hf isotopic compositions suggest that these rocks derived from partial melting of pre-existing crust with heterogeneous compositions. Zircons from the ~ 1.86 Ga two-mica K-feldspar granites yield εHf(t) values of −5.2–1.7 with Tcrust of 2.40–2.86 Ga, compared to zircons from the ~ 1.90 Ga two-mica K-feldspar granites which yield wide range of εHf(t) values of −13.5–1.0 with Tcrust of 2.50–3.42 Ga. In contrast, the ~ 1.90 Ga garnet-bearing K-feldspar granites have homogeneous negative zircon εHf(t) values ranging from −11.8 to −10.4, with Tcrust ranging from 3.23 to 3.30 Ga. Moreover, zircons from the biotite K-feldspar granites have εHf(t) values of −0.6–2.6, corresponding to Tcrust of 2.44–2.77 Ga. Based on abovementioned geochemical signals, we propose that the Paleoproterozoic crust in the southern NCC likely consisted of two parts in vertical, the upper Middle-Late Archean crust and the lower early Paleoproterozoic crust with Eo-Paleoarchean crustal remnants. During the Paleoproterozoic, the southern NCC experienced multi-stage crustal reworking, indicating that a stable continental crust likely had been formed by the end of Archean.

Origins of two types of Archean potassic granite constrained by Mg isotopes and statistical geochemistry: Implications for continental crustal evolution

The chemical composition of the upper continental crust (UCC) dramatically changed from predominantly sodic to more potassic during the late Archean (3000–2500 Ma), although how this change occurred remains poorly constrained. The origins of Neoarchean potassic granites may provide clues for this crucial change, as their formation dominated the compositional transition of the UCC. In this study, we conducted high-precision Mg isotopic analyses of Neoarchean potassic granites (2558–2520 Ma) and tonalite–trondhjemite–granodiorites (TTGs; 2595–2574 Ma) from the North China Craton and statistical geochemistry in K2O/Na2O ratios of global Archean granitoids. Results show that these TTGs, with K2O/Na2O ratios of ~0.61 and δ26Mg values of −0.39‰ to −0.22‰, were generated from heterogeneous basaltic sources. In contrast, potassic granites yield two distinctive groups of δ26Mg values and can be subdivided into I- and S-types. The I-type granite has mantle-like δ26Mg values of −0.28‰ to −0.22‰ and weak enrichment in K2O (K2O/Na2O = 1.0–1.5), and was likely derived by partial melting of igneous sources. The S-type is characterized by much higher δ26Mg values of +0.60‰ to +0.91‰ and strong K2O enrichment (K2O/Na2O = 1.57–23.26), with geochemical characteristics suggesting derivation by partial melting of sediments. Based on the geochemical characteristics of the S-type potassic granites in this study, we sort the S-type granites out of 4066 Archean felsic igneous rocks worldwide. Their temporal distribution suggests that S-type potassic granites have appeared in the middle Archean (3500–3000 Ma) and became widespread in the late Archean (3000–2500 Ma), but were scarce in the early Archean (4000–3500 Ma). The onset of S-type granites indicates reworking of supracrustal rocks may have started since 3500 Ma. And widespread S-type potassic granites during 3000–2500 Ma play an important role for the high K2O/Na2O ratio of UCC at the end of Archean. A mass-balance calculation shows that 25.88 wt% I-type, 4.12 wt% S-type potassic granite and 70 wt% TTGs could increase the K2O/Na2O ratio of the UCC to ~0.83 which is the global average value for felsic rocks at the end-Archean. To increase K2O/Na2O of the Archean UCC, contributions made by I- and S-type potassic granites could probably reach 3:2, respectively. Consequently, the evolved crustal composition from predominantly sodic to potassic results from the synergy of re-working of igneous (unaltered) and sedimentary (weathered) rocks, by which the former contributed more quantitatively, while the latter contributed more efficiently.

Thermal evolution of the Upper Yangtze Craton: Secular cooling and short-lived thermal perturbations

The Upper Yangtze Craton (UYC) has stayed in a long-term stabilization state. Its thermal history can be portioned into two parts: secular evolutionary trend and short-lived thermal perturbations. The former is simulated by a two-dimensional forward transient thermal model, and the latter is discussed by reviewing previous studies. The numerical modeling indicates the UYC tended to be cooling as it adjusted to the asynchronous rates of change in the internal radiogenic heat production and in the convecting mantle temperature below the cratonic root. The average cooling rate of the UYC has been ~58 °C/Ga since the end of Archean, faster than that of the convecting mantle. The surface heat flow presents a decreasing trend from ~82 mW/m2 at 2.5 Ga to ~53 mW/m2 at present-day, which constructs the long-term evolutionary path. The thermal perturbations derived from the occasional tectono-thermal events, such as mantle plume activity and regional lithosphere extension, serves as paroxysmal factors. The thermal effects of the Emeishan mantle plume were very strong, yielding to the maximum heat flow anomaly of ~100 mW/m2 at the Late Permian. In contrast, the influences of regional lithosphere extension during both the early Paleozoic period and the Early Permian-Middle Triassic period were rather weak, resulting in a surface heat flow increase of <10 mW/m2. These thermal anomalies disappeared after a time period depending on the thermal relaxation without altering the stability of the UYC, and then the thermal evolution of craton returned to its original cooling route. The thermal history of the UYC, the overprinting of the secular cooling and the short-lived perturbations, helps better understand the craton evolution and provide the fundamental thermal data for petroliferous-basin analysis.

Constraints on Archean crust formation from open system models of Earth evolution

Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge–Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ∼30% and ∼75% of the present-day mass of the CC, respectively. Models proposing formation of most (∼90%) of the present-day CC during the initial 1 Gyr or nearly 50–60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (∼ 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).

Eoarchean to Paleoproterozoic crustal evolution in the North China Craton: Evidence from U-Pb and Hf-O isotopes of zircons from deep-crustal xenoliths

Granulite-facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age, origin and history of the lower continental crust. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from the North China craton. The geochronological data demonstrates that the oldest lower crustal remnants formed at ∼3.82 Ga and related magmatism continued until ∼3.55 Ga. The Eo-Paleoarchean zircons, with exception of one analysis, have subchondritic initial Hf isotopes with negative εHf(t) and are characterized by normal mantle to elevated δ18O values (5.37–6.90‰), indicating a development of a low Lu/Hf reservoir and hydrosphere-crust interactions in the early Earth. Magmatic zircons from lower crustal xenoliths of North China define a strongly episodic distribution of ages at 3.82–3.55 Ga, ∼2.7 Ga, ∼2.5 Ga and 1.95–1.85 Ga and a non-linear Hf isotope-age array for nearly 2 Gyr, indicating episodic crustal generation and reworking through time. The lower crustal xenoliths record a change in zircon O isotope compositions from a restricted range at 3.8–2.6 Ga to more variable after ∼2.5 Ga. Both heavier (δ18O up to 10.25‰) and lighter (δ18O low to 2.16‰) O isotope compositions than the normal mantle are observed in ∼2.5 Ga magmatic zircons. The secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials (including high-δ18O sediments, weathered and altered rocks and low-δ18O altered oceanic crust) into magma sources at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time.

Effects of atmospheric composition on apparent activation energy of silicate weathering: II. Implications for evolution of atmospheric CO2 in the Precambrian

The apparent activation energy of silicate weathering is a key parameter for understanding the regulation of atmospheric CO2 and surface-temperature of the Earth. Combining the atmospheric composition effects on the apparent activation energy with the compensation law for silicate-weathering flux, the relationship between the temperature dependence of atmospheric CO2 (ΔHCO2′), temperature (T) and silicate-weathering flux (FCO2) has been recently established (Kanzaki and Murakami, 2018). The present study examined the effects of atmospheric CO2 and CH4 on silicate weathering in the Precambrian based on the above T-ΔHCO2′-FCO2 relationship and the greenhouse effects of CO2, which represent ΔHCO2′ on the global scale, with and without the presence of CH4. Calculation of the ratio of the change in FCO2 to the corresponding change in the partial pressure of atmospheric CO2 (PCO2) as an indicator of the silicate-weathering feedback on CO2 revealed hitherto unknown weathering-climate interplays. The states where PCO2 < 10−0.5 atm and T > ∼30 °C are unstable due to the positive feedback, and immediately change with slight CO2 changes to either the states of PCO2 > 10−0.5 atm or those of PCO2 < 10−0.5 atm and T < ∼30 °C, both of which are stable due to the negative feedback. The latter states are especially stable against glaciations, because the feedback becomes more negative as temperature decreases, possibly explaining the stable climates in the Mesoproterozoic. When CH4 is present in atmosphere with CH4/CO2 ratio within a limited range (∼0.03–0.15), a positive feedback operates at low temperatures (<∼5 °C) and thus global glaciations can occur, which may explain the glaciation in the late Archean.The temperature and PCO2 transitions in the Precambrian were finally calculated based on the relationship between ΔHCO2′, T and FCO2 and the greenhouse effects of CO2. The calculated CO2 levels are high enough that the temperature could have been maintained at >0 °C only by CO2 through the Precambrian. The consistent PCO2 estimates from paleosols (fossil weathering profiles) in the literature support the argument. The calculated temperatures suggest that the Earth could have been cool to hot until around the end of Archean and cool to moderate afterwards.

Anorthosites from an Archean continental arc in the Dharwar Craton, southern India: Implications for terrane assembly and cratonization

Anorthositic rocks have attracted attention in terms of their possible role as the primordial crust of our planet, and also as markers of Archean-Paleoproterozoic plate tectonic regimes. Here we investigate the Konkanhundi gabbro-anorthosite suite from Peninsular India, adjacent to the major collisional suture between the Western and Eastern Dharwar cratonic blocks. The anorthosites display high Al2O3 contents with negative correlation against MgO, high CaO and Sr contents, and positive Eu anomalies indicating plagioclase flotation in the magma chamber. The pyroxenite and gabbroic rocks of the suite show high Ni, Cr, Co contents and geochemical features typical of pyroxene cumulation. LA-ICPMS U-Pb analyses of magmatic zircon grains yield weighted mean 207Pb/206Pb ages of 2601 ± 12 Ma for pyroxenite, 2616 ± 12 Ma for gabbro, 2615 ± 13 Ma and 2627 ± 14 Ma for anorthositic gabbro, 2594 ± 16 Ma for anorthosite, and 2605 ± 27 Ma for microgabbro. The age data from the different rock types in the anorthosite suite are broadly consistent, marking the emplacement time as ca. 2.6 Ga, and providing insights on one of the oldest anorthosite complexes in Peninsular India. Zircon grains in the surrounding TTG gneisses into which the gabbro-anorthosite suite was emplaced show weighted mean 207Pb/206Pb age of 3321 ± 11 Ma suggesting Mesoarchean basement. The zircon grains display high Th/U values and REE patterns typical of magmatic crystallization with enriched HREE, positive Ce and Sm anomalies, and negative Pr, Nd, Eu anomalies. Zircon Lu-Hf analysis yield negative εHf(t) values of −4.9–−0.7 with TDMC (two stage model ages) of 3123–3376 Ma for pyroxenite, −4.5–−1.7 and 3192–3366 Ma for gabbro, −5.3–0.9 and 3034–3410 Ma for anorthositic gabbro, and −3.9–-2.4 and 3221–3311 Ma respectively, suggesting the incorporation of Mesoarchean components within the Neoarchean magmatic suite. However, zircon grains in the TTG gneiss possess more ‘juvenile’ εHf(t) values in the range of −0.2–1.3 with TDMC in the range of 3554–3646 Ma. The zircon Hf isotopes and trace element data, together with the whole rock geochemical features suggest that the parent magma of the Konkanhundi gabbro-anorthosite suite was derived from subduction-related depleted mantle source that also incorporated continental crustal components. The time of emplacement of the gabbro-anorthosite complex broadly correlates with widespread arc magmatism and crust production as well recycling in other domains of the Dharwar Craton. We envisage multiple convergence of microblocks during craton assembly at the end of Archean in Dharwar, with the greenstone belts marking zones of paleo-ocean closure.

Possible magmatic underplating beneath the west coast of India and adjoining Dharwar craton: Imprint from Archean crustal evolution to breakup of India and Madagascar

The shear wave velocity of the crust along a ∼660 km profile from the west to the east coast of South India is mapped through the joint inversion of receiver functions and Rayleigh wave group velocity. The profile, consisting of 38 broadband seismic stations, covers the Archean Dharwar craton, Proterozoic Cuddapah basin, and rifted margin and escarpment. The Moho is mapped at a depth of ∼40 km beneath the mid-Archean Western Dharwar Craton (WDC), Cuddapah Basin (CB), and the west and east coasts formed through the rifting process. This is in contrast with a thin (∼35 km) crust beneath the late-Archean Eastern Dharwar Craton (EDC). Along the profile, the average thickness of the upper, middle and lower crust is ∼4 km, 12±4 km and 24±4 km respectively. Above the Moho, we observe a high-velocity layer (HVL, Vs > 4 km/s) of variable thickness increasing from 3±1 km beneath the EDC to 11±3 km beneath the WDC and the CB, and 18±2 km beneath the west coast of India. The seismic wave velocity in this layer is greater than typical oceanic lower crust. We interpret the high-velocity layer as a signature of magmatic underplating due to past tectonic processes. Its significant thinning beneath the EDC may be attributed to crustal delamination or relamination at 2.5 Ga. These results demonstrate the dual signature of the Archean Dharwar crust. The change in the geochemical character of the crust possibly occurred at the end of Archean when Komatiite volcanism ceased. The unusually thick HVL beneath the west coast of India and the adjoining region may represent underplated material formed due to India–Madagascar rifting, which is supported by the presence of seaward dipping reflectors and a 85–90 Ma mafic dyke in the adjoining island.

Peri-Amazonian provenance of the Euxinic Craton components in Dobrogea and of the North Dobrogean Orogen components (Romania): A detrital zircon study

The Romanian South Carpathians foreland is located southwards of the East European Craton (EEC) and consists of three major tectonic units: the Scythian Platform, the North Dobrogean Orogen and the Euxinic Craton. The Euxinic Craton is represented in the Dobrogea area by two components: (i) Central Dobrogean Shield and (ii) East Moesia. In central-northern Dobrogea, the Trans European Suture Zone (TESZ), including the Teisseyre–Tornquist Zone, presumably strikes below the surface. Because the paleogeographic provenance of the above tectonic units was poorly documented until present, a systematic sampling for detrital zircon has been conducted in all tectonic units except for the Scythian Platform basement which is entirely covered by the Danube Delta. In all, this totals 26 samples and around 1833 analyzed zircon grains. The distribution of zircon grains dates along the time scale for all the sampled terranes suggests that the sampled components of the Euxinic Craton in Dobrogea and the constitutive terranes of the North Dobrogean Orogen have a peri-Gondwanan, peri-Amazonian provenance. Their ages are Neoproterozoic or younger. In addition, the Euxinic Craton can be depicted as a paleo-Amazonian crustal fragment that was marginally affected by the Avalonian–Cadomian orogenic events. Along with its margin, the Avalonian type terranes accreted or Ganderian type terranes have evolved. Primary relationships between the Amazonian derived terranes and EEC are obscured due to an intricate geologic history exhibited by the North Dobrogean Orogen involving Caledonian, Variscan and Cimmerian orogenic events, while the geologic history of the Scythian Platform is also poorly established. The general distribution diagram of U–Pb detrital zircon ages shows some major zircon sources, shared with those detected at a global scale. They are episodic and coincident along with the time scale, such as those at the end of Archean or those from the Pan-African orogens. Finally, among these discussed tectonic components, only the Scythian Platform can possibly have an EEC provenance.

High-grade metamorphism during Archean–Paleoproterozoic transition associated with microblock amalgamation in the North China Craton: Mineral phase equilibria and zircon geochronology

Metamorphic regimes in Archean terranes provide important keys to the plate tectonic processes in early Earth. The North China Craton (NCC) is one of the ancient continental nuclei in Asia and recent models propose that the cratonic architecture was built through the assembly of several Archean microcontinental blocks into larger crustal blocks. Here we investigate garnet- and pyroxene-bearing granulite facies rocks along the periphery of the Jiaoliao microcontinental block in the NCC. The garnet-bearing granulites contain peak mineral assemblage of garnet+clinopyroxene+orthopyroxene+magnetite+plagioclase+quartz±biotite±ilmenite. Mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak P–T condition of 800–830°C and 7–8kbar for metamorphism. Isopleths using XMg of orthopyroxene and XCa of garnet in another sample containing the peak mineral assemblage of garnet+orthopyroxene+quartz+magnetite±fluid yield peak P–T conditions of 860–920°C and 11–14kbar. Geochemical data show tonalitic to granodioritic composition and arc-related tectonic setting for the magmatic protoliths of these rocks. Zircon LA-ICP-MS analyses yield well-defined discordia with upper intercept ages of 2562±20Ma (MSWD=0.94) and 2539±21Ma (MSWD=0.59) which is correlated with the timing of emplacement of the magmatic protolith. A younger group of zircons with upper intercept ages of 2449±41Ma (MSWD=0.83); N=6 as 2449±41Ma (MSWD=0.83; N=6) and 2480±44Ma (MSWD=1.2; N=9) constrains the timing of metamorphism. Zircon Lu–Hf data show dominantly positive εHf(t) values (up to 8.5), and yield crustal residence ages (TDMC) in the range of 2529 to 2884Ma, suggesting magma sources from Meso-Neoarchean juvenile components. The high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction–collision tectonics associated with microblock amalgamation in the NCC at the end of Archean. Together with the evidence for high pressure metamorphism from an adjacent locality, our results correlate with models that predict paired metamorphism at the Archean–Proterozoic transition with the onset of modern style plate tectonics.

Ferropicrite-driven reworking of the Ungava craton and the genesis of Neoarchean pyroxene-granitoids

Voluminous, pyroxene-bearing, intermediate to felsic plutons were emplaced during a 20–50 million year long, spatially extensive Neoarchean igneous event that culminated in the cratonization of North Americaʼs ∼500 km-wide Ungava craton. The crystallization ages of pyroxene-bearing plutons coincide with the emplacement of numerous ca. 2.72–2.70 Ga, Fe-rich, ultramafic/mafic intrusions of the Qullinaaraaluk suite (Q-suite) that are scattered across the disparate domains of the Ungava craton. A high proportion of relatively sodic pyroxene-bearing granitoids with intermediate silica contents fall in a compositional gap between the Q-suite and pyroxene-free granitoids, suggesting that the pyroxene-granitoids may be formed by the simultaneous fractional crystallization and assimilation of older tonalitic and trondhjemitic (TT) crust by the Q-suite magmas. We estimate that pyroxene-granitoids containing ∼65 wt.% SiO2 may reflect ∼40–50 wt.% contamination of mantle-derived picritic magma by trondhjemitic melts of the pre-2.74 Ga TTG crust. The craton-wide occurrence of the Q-suite intrusions and pyroxene-granitoids suggests that underplating by ferropicritic magmas played a key role in the cratonization of the Ungava craton at the end of Archean. A major contribution of mantle-derived magmas to the petrogenesis of the ca. 2.74–2.70 Ga pyroxene-granitoids is consistent with the proposed global generation of voluminous juvenile continental crust ca. 2.7 Ga.

Ca. 2.5 billion year old coeval ultramafic–mafic and syenitic dykes in Eastern Hebei: Implications for cratonization of the North China Craton

A group of extremely rare coeval ultramafic–mafic and syenitic dykes was discovered in the Eastern Hebei region of the North China Craton. These dykes intrude the 3.8–2.55 Ga old Caozhuang complex. An olivine gabbro dyke and syenite dyke yield, respectively, SHRIMP zircon U–Pb ages of 2516 ± 26 Ma and 2504 ± 11 Ma, interpreted as the magmatic crystallization age. Their zircons have single-stage Hf model ages 2677 Ma and 2705 Ma. The olivine gabbros have Mg # values of 59–63, similar to high-magnesian tholeiitic basalt. They show relatively LREE-enriched patterns without Eu anomalies (La/Yb N = 9.28–9.78, Gd/Yb N = 2.8, δEu = 0.96–1.00), with enrichment in LILEs and depletion in HFSEs. They also contain high Cr and Ni, and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle. The syenites are alkaline in composition, with 8.67–8.88 wt.% Na 2O + K 2O, and show high total REE contents (543–854 ppm) and strong LREE-enriched patterns with minor Eu negative anomalies (La/Yb N = 50–101, Gd/Yb N = 5.2–5.3, δEu = 0.80–0.82). Trace element diagrams show strong LILE-enrichment relative to HFSE. Zr/Hf and Nb/Ta ratios are 47–49 and 26–76, respectively, which are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archean continental crust. The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source, which implies that the NCC probably had continental crust of considerable thickness at this time. Combined with evidence for ∼2.5 Ga granite intrusion and metamorphism in the Eastern Hebei region and adjacent areas, we propose that the NCC has been a present scale craton at the end of Archean.

Biogenic nitrogen and carbon in Fe‐Mn‐oxyhydroxides from an Archean chert, Marble Bar, Western Australia

To quantify and localize nitrogen (N) and carbon (C) in Archean rocks from the Marble Bar formation, Western Australia, and to gain insights on their origin and potential biogenicity, we conducted nuclear reaction analyses (NRA) and carbon and nitrogen isotope ratio measurements on various samples from the 3460‐Myr‐old Fe‐rich Marble Bar chert. The Marble Bar chert formed during the alteration of basaltic volcanoclastic rocks with Fe‐ and Si‐rich hydrothermal fluids, and the subsequent precipitation of magnetite, carbonates, massive silica, and, locally, sulfides. At a later stage, the magnetite, sulfides, and carbonates were replaced by Fe‐Mn‐oxyhydroxides. Nuclear reaction analyses indicate that most of the N and C resides within these Fe‐Mn‐oxyhydroxides, but a minor fraction is found in K‐feldspars and Ba‐mica dispersed in the silica matrix. The N and C isotopic composition of Fe‐oxides suggests the presence of a unique biogenic source with δ15NAIR values from +6.0 ± 0.5‰ to 7.3 ± 1.1‰ and a δ13CPDB value of −19.9 ± 0.1‰. The C and N isotope ratios are similar to those observed in Proterozoic and Phanerozoic organic matter. Diffusion‐controlled fractionation of N and C released during high combustion temperatures indicates that these two elements are firmly embedded within the iron oxides, with activation energies of 18.7 ± 3.7 kJ/mol for N and 13.0 ± 3.8 kJ/mol for C. We propose that N and C were chemisorbed on iron and were subsequently embedded in the crystals during iron oxidation and crystal growth. The Fe‐isotopic composition of the Marble Bar chert (δ56Fe = −0.38 ± 0.02‰) is similar to that measured in iron oxides formed by direct precipitation of iron from hydrothermal plumes in contact with oxygenated waters. To explain the N and C isotopic composition of Marble Bar chert, we propose either (1) a later addition of N and C at the end of Archean when oxygen started to rise or (2) an earlier development of localized oxygenated environments, where biogeochemical cycles similar to modern ones could have developed.

The lower precambrian of china

The Lower Precambrian of China consists of Archean and Lower Proterozoic formations formed probably prior to ca. 1,800-1,900 Ma. They are exposed chiefly in the North China Platform. Archean rocks are composed mainly of gneisses, granulitite and plagloclase-amphibolite of the amphibolite facies, with the lower part containing pyroxene-gneiss and granulite of the granulite fades. The parent rocks were not well differentiated sedimentaries and volcanics, forming two volcano-sedimentary cycles. During the Archean (before 2,500-2,600 Ma), the tectonic environment over an extensive area, was quite uniform yet fairly active. Towards the end of Archean there prevailed median- to high-grade metamorphism often accompanied by rather intensive migmatization. In the first Early Proterozoic epoch, a thick sequence of volcano-sedimentaries were accumulated in some marine troughs regarded as eugeosynclinal and developed on the Archean sialic basement, such as the Wutai Group. The protoliths were lhe rather widespread volcanics, the semipelitic and pelitic types and turbidites, mainly of greenschist fades and partly amphibolite facies, occasionally accompanied by migmatization probably not later than 2,300 Ma. After that, a stratigraphic pile accumulated in the miogeosynclinal basins or troughs as represented by lhe HutuoGroup, and was composed of coarser clastics, pelitics and stromatolite-bearing Mg-rich carbonates which show rhythmic deposition. Their greenschist metamorphism probably occurred during ca. 1,800-1,900 Ma and this marked the end of the Early Precambrian history.