Tonalite-trondhjemite-granodiorite Research Articles

Geology, geochemistry and zircon SHRIMP U–Pb geochronology of Mesoarchean high-Mg granitoids: constraints on petrogenesis, emplacement timing and deformation of the Água Limpa suite in the Carajás Province, SE Amazonian Craton

Determining the age, geochemical characteristics and conditions of deformation of Mesoarchean sanukitoids is crucial in unravelling the petrogenesis and tectonic significance of mantle-derived rocks. We report the whole-rock major and trace element contents, zircon U–Pb geochronology and geochemical modelling of samples from the Água Limpa sanukitoid suite in the Sapucaia subdomain (the Carajás Province, Amazonian Craton). This suite includes two plutons, named Água Limpa and Água Azul, which are exposed across east–west transpressive sinistral shear zones that act as boundaries between the Rio Maria and Carajás domains. These plutons are composed of porphyritic granodiorites, tonalites and subordinate monzogranites, with amphibole, biotite and epidote as the main mafic mineral components. They are characterized by high contents of Cr (102–383 ppm), Ni (32–117 ppm), MgO (2.50–4.78 wt%), Sr (477–912 ppm) and Ba (519–1966 ppm), with high Mg# values of 49–64. The rare earth element patterns show significant fractionation of the heavy rare earth elements, with high La N /Yb N (18–170) ratios and no or weak Eu anomalies (Eu/Eu* 0.66–1.21). The studied rocks are predominantly medium-K, calc-alkaline and metaluminous, and are geochemically distinct from tonalite–trondhjemite–granodiorites (TTGs) and Archean leucogranodiorites. The Água Limpa and Água Azul plutons yield coeval crystallization ages of 2870 ± 4 and 2872 ± 5 Ma, respectively, with other zircon population ages of 3063 ± 6, 2926 ± 12 and 2849 ± 12 Ma. The first of these other zircon populations is interpreted as xenocrysts, the second as crystals inherited from the modelled TTG-type source and the third as crystals reflecting the age of metamorphism. Most of the suite fractionated from a common mafic parent through 20% partial melting of a mantle previously enriched by 32% TTG-type melt in the garnet stability zone and under oxidized conditions. The combined dataset indicates that the studied sanukitoids are syntectonic intrusions, with distinct pulses of magma transfer facilitated by shear zone reactivation and subordinate small-scale dyking.

High-pressure mafic granulite and supracrustal rocks in the southern Hengshan area, North China Craton: Metamorphic P-T-t evolution and geotectonic significance

Amphibolite- to granulite-facies rocks are widely distributed in the Hengshan Complex, middle Trans-North China Orogen, and the high-pressure (HP) mafic granulite has been recently identified in the southern Hengshan area. The HP mafic granulite and amphibolite occur as rootless tectonic boudins/lenses within the TTG (tonalite–trondhjemite–granodiorite) gneiss/metapelite, indicative of typical “block-in-matrix” texture of metamorphic-tectonic mélange. Three to four generations of metamorphic mineral assemblages that correspond to the prograde (M1), peak (M2), and retrograde (M3-M4) stages, are recognized in these rocks. Conventional geothermobarometry and phase equilibrium modeling yield peak P–T conditions of 10.8–13.8 kbar/754–799 °C for the HP mafic granulite and 7.3–9.0 kbar/690–725 °C for the supracrustal rocks, respectively. A clockwise P–T path with near-isothermal decompression (ITD) and subsequent near-isobaric cooling (IBC) segments is reconstructed for the HP mafic granulite, indicating a dynamic subduction–collision–exhumation process that unfolded during an orogenic event. Secondary ion mass spectrometry (SIMS) U-Pb dating of zircon yields metamorphic ages of ca. 1.91–1.83 Ga, representing the long-lived tectono-metamorphic event caused by collision between the Eastern and Western Blocks along the Trans-North China Orogen. It is hypothesized that the tectono-metamorphic mélange in the area originated from the tectonically juxtaposition of metamorphic rocks that had diverse protoliths, different peak P–T conditions and discrepant metamorphic ages. This complexity may be a hallmark of Paleoproterozoic orogens, drawing intriguing parallels with the intricate characteristics observed in Phanerozoic orogenic belts.

Crustal structure beneath the Western Dharwar Craton segment of Western Ghats: Insights from ambient noise correlation technique

The Western Ghats (WG) comprise three major tectonic blocks: the Deccan Volcanic Province (DVP), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT) from north to south. Among these, the WDC region encompasses the oldest basement rocks, including greenstone belts and calc-alkaline plutons dating back to ∼3.0 to 3.4 Ga. While the crustal structure of the DVP and SGT segments of the WG has been extensively studied, knowledge regarding the crustal structure beneath the WDC segment of the WG is limited due to the scarcity of seismic stations. To address this gap, we utilized continuous waveform data from a newly established network of seismic stations by the National Centre for Earth Science Studies (NCESS), in addition to other available stations and deciphered the shallow crustal shear-wave velocity structure along a profile beneath the WDC segment of the WG using ambient noise cross-correlation and surface wave dispersion. The results reveal a continuous upper layer characterized by a high shear wave velocity of ∼3.6 km/s with notable variation in thickness along the profile. Furthermore, a thick middle layer with velocities reaching up to ∼3.9 km/s and a lower layer with velocities of ≥4.2 km/s are observed. Intriguingly, variations in the thickness of the upper layer, ranging from 5 km to 20 km, are noted beneath the study region. The thickness of this layer exhibits changes in relation to topographical variations, with this dependency diminishing for deeper layers. Notably, high shear wave velocity perturbations (∼10 %) are observed in the upper layer beneath low-elevation regions, possibly indicating the presence of Tonalite–Trondhjemite–Granodiorite (TTG)-type gneiss − a significant rock type in the WDC region, alongside greenstone belts and late calc-alkaline to potassic plutons. These findings suggest that the uneven topography of the WG may result from long-term erosion, exposing deeper and higher-density rocks and leading to higher shear wave velocities at shallow depths. This study serves as a foundation for further exploration into the influence of the upper crustal layer and its correlation with the evolution of the WG.

A gradual Proterozoic transition from an unstable stagnant lid to the modern plate tectonic system

When did plate tectonics begin on Earth, and what preceded it? Published thermo-mechanical mantle evolution models imply that the early history of planets with a composition and size similar to Earth and Venus should be characterized by periodic mantle overturns of 30–100 Myr duration, separated by stable lid phases of 100–300 Myr. This is best described as an unstable stagnant lid because this term captures the Jekyll and Hyde duality of such worlds, which alternate between a stagnant lid sensu stricto phase between mantle overturns, and a mobile lid phase during overturns. Mantle overturn upwelling zones would rework and resurface large tracts of pre-existing Hadean crust and basalt-dominated Archean-style oceanic lithosphere (ASOL). Basal anatexis of ASOL c. 40–50 km thick (or melting within down-drips) could generate tonalite–trondhjemite–granodiorite (TTG) melts and create proto-continental nuclei, while garnet pyroxenite restites delaminate into the mantle. With further reworking, low-K tonalitic rocks would remelt to produce granodiorite and granite, completing the transfer of radioactive elements out of the lower crust. Mantle overturns would generate large-scale lateral currents in the upper mantle that would push against Archean-aged sub-continental lithospheric mantle keels, causing continental drift and orogenesis despite the absence of plate boundary forces such as slab-pull. This is corroborated by the observed displacement of Lakshmi Planum (>1000 km) on Venus, a planet with no arcs nor ridges. Recent models suggest that the Abitibi Greenstone Belt formed as an oceanic tract behind a detached ribbon continent during the partial break-up of the Southern Superior Craton and may be a sample of Archean oceanic lithosphere. The Abitibi Greenstone Belt has c. 50 km of apparent stratigraphy composed of 2–10 Myr mafic–felsic bimodal volcanic cycles that follow assimilation–fractionation trends indicating the contamination of mantle-derived basalts with TTG-like anatectites derived from older basalts. ASOL of this type would be difficult to subduct due to its weakness and buoyancy, but would be fertile and could generate large amounts of second-stage melts. There are no sheeted dykes, precluding a seafloor spreading model, while the absence of basal cumulates or attached mantle means that this type of ASOL should not be called an ophiolite. Archean–Proterozoic unconformities are followed by the deposition of iron formations, clastic and volcanic rocks, which are only rarely affected by sagduction. The increase in siliciclastic input and decreasing sagduction reflect near-global Late Archean emergence of stiffening granitic continents from the oceans due to secular cooling and intra-continental differentiation. Albeit associated with continent-derived siliciclastic debris, many Paleoproterozoic volcanic (and plutonic) rocks resemble Archean rocks geochemically. The similarity of magmatic rocks and hot orogenic styles in the Archean and Paleoproterozoic could imply that the overall geodynamic regime was similar in both. The Siderian–Rhyacian quiet period could therefore represent a stagnant lid phase that followed the 2.5 Ga Archean overturn. When the next mantle overturn ruptured the lid at c. 2.2–2.0 Ga (and again at c. 1.9–1.8 Ga), continents would have been set into motion, forming arcs and ridges. Once initiated, arc and ridge segments needed to multiply and propagate to create a world-girdling system. Mesoproterozoic rocks preserve clear evidence of plate mobility, subduction and orogenesis, but, ophiolites, the geological record of seafloor spreading, are extremely rare prior to 1 Ga. The Earth at 2.0 Ga was probably still largely covered by ASOL, possibly similar to the Abitibi Greenstone Belt, but how and where it was all destroyed and replaced by modern oceanic lithosphere are mysteries. Given the volume of ASOL involved, recognizable by-products of this global-scale reworking process should exist. Voluminous anorthosite–mangerite–charnockite–granite/gabbro suite rocks are mostly of Proterozoic age, requiring either an ephemeral source or a unique process. Trace element inversion models applied to massif anorthosites imply they crystallized from high-La/Yb melts that do not resemble tholeiitic basalts, invalidating the notion that they are flotation cumulates from basaltic underplates. Model anorthosite-forming melts can, however, be explained by high-pressure melting of an ASOL-like basalt source with garnet-bearing residues. I posit that massif anorthosites record the destruction of an ephemeral source (ASOL) at Proterozoic convergent margins. When the last ASOL was crushed between converging continents or consumed by an overprinting arc ( c. 0.8–1 Ga), anorthosite–mangerite–charnockite–granite/gabbro suite rocks ceased to form and the Earth became a modern plate tectonic planet.

Archean UHT metamorphism with counterclockwise P–T path: Insights from pelitic granulites from East Hebei, North China Craton

Archean supracrustal rocks that occur as rafts within tonalite–trondhjemite–granodiorite (TTG) gneisses can reach ultra-high temperature (UHT) conditions. Their metamorphic P–T paths and geodynamic regimes are debated. Herein, we present detailed analyses of UHT pelitic migmatites (JD1730 and JD1733) from the East Hebei, North China Craton, using petrology, phase equilibria modelling, and zircon dating, to recover the metamorphic history. Sample JD1730 is a sillimanite–orthopyroxene–cordierite migmatite that contains the sillimanite + orthopyroxene (+cordierite) symplectites, which are interpreted to be the pseudomorphs after sapphirine. JD1733 is a corundum-bearing garnet–cordierite–sillimanite migmatite that contains the corundum + spinel ± ilmenite/rutile aggregates, representing a stable mineral association of low pressures in local Si-unsaturated domains. The two rocks record consistent counterclockwise P–T paths involving pre-peak up-P processes to the UHT peak conditions followed by cooling. The pre-peak up-P processes occurred at 5–7 kbar and > 970 °C constrained from the textural relations involving (i) spinel enveloped by sillimanite and cordierite and mantled by the Sil + Opx symplectites in JD1730, and (ii) the inclusion variation in garnet of JD1733 with the Spl + Crn (±Ilm) aggregates embayed by sillimanite in the core and mantle, and worm-like quartz in the rim. The UHT peak conditions can be constrained to be 7–8 kbar, 980–1060 °C based on the stability of the inferred peak mineral assemblages in P–T pseudosections added with the plots of the re-integrated anorthite contents from perthite. The post-peak cooling processes to 5–8 kbar / 820–850 °C are marked by the growth of biotite in leucosomes of both samples. Zircon dating and geothermometry results suggest that the UHT metamorphism might have persisted for more than 20 Myr from ∼ 2.52 Ga to ∼ 2.50 Ga and the post-UHT cooling continued another 20 Myr or even 40 Myr. Considering the geological features in the terrane, a vertical sagduction regime was preferred for the Neoarchean tectonic evolution of the East Hebei terrane.

Testing the TTG–Metabasite Connection in the Southern Superior Province: an Integrated Geochemical, Isotopic, and Petrogenetic Modelling Approach

Abstract Archean cratons are dominated by tonalite–trondhjemite–granodiorite (TTG) suites, the products of crustal differentiation that formed early continental crust. These rocks may have been primarily generated by partial melting of hydrated basaltic crust in a variety of settings including subduction zones or the deep lithosphere. Sources are mainly inferred from examination of TTGs alone, as potential source rocks are rarely exposed. In the southern Superior Province, Canada, the Kapuskasing Uplift exposes an important crustal cross-section with upper- to middle-crustal TTGs and lower-crustal metabasites, which show evidence of having produced trondhjemitic anatectic melt. Here, we test the hypothesis that these metabasites were the source of the Mesoarchean to Neoarchean TTGs in the Kapuskasing Uplift by combining phase equilibrium and melt trace element modelling with whole-rock and zircon Lu–Hf isotope analysis and geochronology (garnet Lu–Hf and zircon U–Pb) of metabasic samples. By comparison of the results with existing data from TTGs in the Kapuskasing Uplift, we determined that the metabasites are plausible source rocks for the TTGs. The Lu–Hf systematics of the metabasites and TTGs are the most robust evidence of a genetic connection. Modelling results support an increase in TTG source depth over time. New geochronological data constrain partial melting of metabasite and crystallization of anatectic melt between ca. 2685 Ma and ca. 2600 Ma, coeval with crystallization of only the youngest TTGs. Overall, these results indicate a complex history of intracrustal differentiation in the Kapuskasing Uplift, with partial melting of two isotopically distinct lower-crustal metabasic sources at different times and depths.

Layered Archean lower continental crust: Constraints from granulite terrains and xenoliths

Our understanding of crust–mantle differentiation is hampered by the large uncertainty in the composition of the inaccessible lower continental crust. The lower-crustal estimates derived from granulite terrains are compositionally much more evolved than those based on granulite xenoliths and there is no consensus on the origin of the compositional discrepancy. Here an integrated study of granulite terrains, granulite xenoliths and lower crustal–derived magmatic rocks from the Zhangjiakou region of the northern North China craton and a combination of geochronological, mineralogical, geochemical and geophysical data enable us to establish a complete Archean lower crustal cross-section for the first time. It shows that the late Archean lower crust beneath the North China craton is layered, with an intermediate upper lower crust and a mafic lowermost crust. The tonalite–trondhjemite–granodiorite (TTG)-dominated granulite terrains may merely represent the upper lower crust. Our results successfully solve the long-standing question regarding the compositional difference between granulite terrains and xenoliths. It is inferred that a layered Archean lower crust persists in many other cratons (e.g., the Siberian, Slave, Superior, and Tanzanian cratons) and it has an overall composition different from current lower crustal estimates. We further show that the Archean mafic lower crust is temporally coupled with the upper crust. This finding is significant as it raises an important question whether the mafic lower crust and exposed TTGs are genetically linked, which may be a key to understanding crust generation and differentiation processes.

Geochemical fingerprinting of continental crust trapped in Cadomian volcanic arcs along northern Gondwana

During the Ediacaran to Cambrian periods, the extensive Avalonian–Cadomian accretionary orogen formed along the northern margin of the Gondwana (Pannotia) supercontinent. One of the best-preserved magmatic arcs within this orogenic system is the Davle volcanic complex (DVC) in the Teplá–Barrandian unit of the Bohemian Massif. Using detailed field observations, petrography, major/trace element concentrations, Nd–Hf isotopic compositions as well as U–Pb zircon dating, we interpret a complex evolutionary history of the DVC as comprising three main stages. The first stage (∼610–570 Ma) is represented by a volcano-plutonic association of andesite, dacite, rhyolite and tonalite–trondhjemite–granodiorite (metamorphosed to orthogneiss). A wide range of major/trace element contents together with highly variable εNd values (+3.6 to –5.1) suggest derivation from a juvenile (mantle-derived) source with subsequent assimilation of cratonic crust during magma emplacement, also supported by the modelling of the assimilation–fractional crystallization (AFC) process. The second stage (∼580–560 Ma) is characterized by a gradual termination of the volcanic arc activity, followed by a deposition of marine siliciclastic succession comprising, from the bottom to the top, laminated tuffs, silicified black shales (Lečice Member), and a thick turbidite greywacke–shale–conglomerate sequence (Štěchovice Group). The third stage of poorly constrained age (∼570–500 Ma) is represented by an intra-arc extension and intrusion of juvenile trondhjemite and related rhyolite dykes (εNd +4.8 to +8.4). Finally, we compare these findings with zircon U–Pb ages and whole-rock Nd isotopic data from other Cadomian terranes and propose that the cratonic crust played an important role in recycling along the whole orogenic belt. Thus, we conclude that the Cadomian Orogen formed as a collage of fringing (such as modern Japan, Taiwan, or Philippines) and continental (as modern Andes) arcs of variable evolutionary stages rather than as a belt dominated by intra-oceanic island arcs.

Mesoarchean–Neoarchean coupled crust–mantle differentiation followed by gravity-driven lithospheric delamination and subduction initiation in the North China Craton

The geodynamic regime that formed Archean silicic continental crust is unclear. The episodic Archean TTGs (tonalite–trondhjemite–granodiorite rocks) and associated granitoids, with emplacement ages of ca. 2.9, 2.7, and 2.5 Ga, occurred in the Jiaobei Terrane, North China Craton (NCC), provide a continuous record of the formation of the Mesoarchean–Neoarchean silicic continental crust. To investigate the crust–mantle differentiation, crustal reworking and recycling, and geodynamic regime associated with formation of the Mesoarchean–Neoarchean silicic continental crust, we undertook zircon U–Pb dating and Hf–O isotope, and whole-rock major and trace element analysis of Archean TTGs and associated granitoids. The episodic Archean TTGs and juvenile crust coexisted spatially–temporally in the Jiaobei Terrane, indicating Mesoarchean-Neoarchean synchronous differentiation of crust and mantle. In addition, the ca. 2.9 and 2.7 Ga TTGs have an identical Mesoarchean juvenile crustal source and similar mantle-like zircon δ18O values, whereas the ca. 2.5 Ga TTGs were derived mainly from younger Neoarchean juvenile crust, indicating removal and replacement of the Mesoarchean juvenile lower crust. Some ca. 2.5 Ga TTGs, granitic gneisses, and sanukitoids have markedly higher zircon δ18O values compared with mantle δ18O values, which are indicative of Neoarchean supracrustal recycling. Consequently, we can constrain the geodynamic processes responsible for the formation of the Mesoarchean–Neoarchean silicic continental crust in the Jiaobei Terrane, NCC. Episodic hot upwelling mantle (or plume)–lithosphere interactions at ca. 2.9, 2.7, and 2.5 Ga resulted in coupled crust–mantle differentiation, which produced the TTGs, juvenile crust, and dense lower crustal restites. Subsequently, lithospheric delamination triggered by gravitational instability occurred followed by continental uplift, subduction of altered oceanic crust, and upwelling of asthenosphere and mantle-derived mafic melts. This resulted in extensive anatexis and metamorphism with anticlockwise P–T paths in the middle to lower crust at ca. 2.5 Ga. Furthermore, following the large-scale melting and cooling of the mantle, a thick, stable, cratonic lithosphere had likely developed by the end of the Neoarchean. The geodynamic processes during the Mesoarchean–Neoarchean were diverse, and episodic hot upwelling mantle (or plume)–lithosphere interactions could have been responsible for the formation of Archean silicic continental crust in the Jiaobei Terrane, NCC.

Low δ18O and δ30Si TTG at ca. 2.3 Ga Hints at an Intraplate Rifting Onset of the Paleoproterozoic Supercontinent Cycle

AbstractThe start of the Paleoproterozoic supercontinent cycle is typically taken as the initiation of orogenesis at ca. 2.1 Ga leading to the assembly of Earth's first supercontinent, Columbia. However, the dearth of ca. 2.5–2.2 Ga geological records makes it difficult to deduce tectonic factors during the onset of the Paleoproterozoic supercontinent cycle. The petrogenesis of tonalite–trondhjemite–granodiorite (TTG) provides useful proxies for tracing prevailing geodynamic regimes of early continental evolution. However, marked decreases of TTG and other magmatism occurred across the Archean–Paleoproterozoic transition and have previously precluded forming testable hypotheses. Early Paleoproterozoic TTGs have been identified in the North China Craton (NCC) and other cratons, which may represent the last major pulse of TTGs globally. Here we present low δ18O and δ30Si ca. 2.3 Ga TTGs from the NCC, together with thermodynamic modeling and compilation of stable O and Si isotopes for TTGs globally through time. The ca. 2.3 Ga TTGs were derived from the partial melting of Archean basaltic crust and give lighter average zircon δ18O (3.15 ± 0.35‰) and whole‐rock δ30Si values (−0.17 ± 0.08‰) than most Archean TTGs. Considering coeval mafic‐felsic igneous rocks, and lithospheric thinning since ca. 2.5 Ga based on estimated crustal thickness through the Neoarchean–Paleoproterozoic, we posit the onset of an intraplate rifting consistent with the anomalous low‐δ18O magmatism. Continental rifting of Archean cratons/supercratons plausibly hints at the formation of rifts driving subduction initiation as the veritable onset of the Paleoproterozoic supercontinent cycle.

Genesis of the Early Indosinian Darongshan Granitoid in South China: Response to the Subduction of the Eastern Paleo-Tethys Ocean

The Bangxi–Chenxing suture zone is an essential area from which information about the closure history of the eastern Paleo-Tethys Ocean can be obtained. The Darongshan granitoid, which is adjacent to this suture, lies among the widely distributed granitic rocks and few basic rocks in the southern Guangxi Province. Herein, we report the petrogeochemistry, zircon U–Pb ages, and zircon Hf isotopic data of the Darongshan pluton in this region. The LA-ICP-MS U–Pb zircon analysis indicates that the Darongshan pluton had formed at 249.9±2.6 Ma. The Darongshan granites are silica-rich (SiO2=65.68–72.91 wt%, mean=69.89 wt%) with high Na2O contents (Na2O=0.46–6.58 wt%, mean=3.49), relatively high Mg (Mg#=35.12–73.31, mean=57.73), and an average Fe2O3T+TiO2+MnO+MgO of 4.96. These features are similar to those of the Mg-andesitic/dioritic rock- (MA-) like tonalite–trondhjemite–granodiorites (TTGs). Chemical analyses show that all rocks are enriched in large-ion lithophile elements (Rb, Th, and U) and light rare earth elements, with weak negative Eu anomalies (Eu/Eu∗=0.27–0.67), and Ta, Nb, and Ti depletion, with typical arc-like affinity. The zircon Hf isotopic results show zircon ƐHft values ranging from -18.2 to -7.4 and the TDM2 model ages 1.74–2.41 Ga. The petrogeochemistry and zircon Hf isotopic signatures indicate the magma generation of the Darongshan granitoid with fluid/melt released from the subducted slab and the fluid/melt assimilated and mixed with the mantle peridotite during ascent. Combining previous extant information on Permo–Triassic subduction/collision-related magmatism in the Bangxi–Chenxing with that of the Jinshajiang–Ailaoshan–Song Ma suture zones, the Darongshan granitoid is interpreted as a magmatic formation that was generated in an active continental margin arc environment during the subduction of the Early Indosinian eastern Paleo-Tethys Ocean and the South China Block, further supporting the idea that closure occurred during the Middle–Late Triassic.

Redistribution of heat‐producing elements during melting of Archean crust

AbstractHeat generated from the decay of K, Th, and U plays a fundamental role in the differentiation and stabilization of Earth's continental crust. This is particularly important in the construction of Archean cratons that form the nuclei of Earth's continents. The Kapuskasing uplift is a rare exposure of an Archean‐age crustal cross‐section that provides a snapshot of crustal melting, differentiation, and compositional stratification. We integrate field observations, whole‐rock compositions, thermodynamic equilibrium and accessory mineral modelling with heat production and latency time modelling to provide insights into the partitioning of heat‐producing elements between residue and melt during anatexis of metabasites as well as the resulting effects on metamorphic timescales and the production of tonalite–trondhjemite–granodiorite (TTG) suites. We model six metabasite compositions ranging from relatively fertile greenschist facies metabasites to melt‐depleted residual mafic (upper‐)amphibolites to granulites. Heat‐producing elements are modelled to be partitioned between melt and residue; the dominant minerals in the residue that host these elements are apatite, hornblende, K‐feldspar, and epidote. At 800–850°C epidote is no longer stable, and the melt fraction is predicted to contain roughly half of the heat production capacity for the system. Apatite and melt are expected to be the dominant repositories for Th and U during anatexis; zircon is predicted to be completely consumed by 850°C, whereas apatite persists to higher temperatures and allanite is expected only in minor modal abundances at high‐P, low‐T conditions. The partitioning of heat‐producing elements into relatively low‐density melt decreases the heat production of the residual system during anatexis. Due to their high density and affinity for U and Th, epidote and apatite retain heat production capacity in the residue during metabasite melting. Thermal latency modelling of metamorphism suggests that enriched metabasite compositions require 38–46 My to increase the temperature from ~650 to 850°C (solidus temperature to peak metamorphic temperature of the Kapuskasing uplift), whereas estimates are considerably shorter for depleted compositions (7–25 My). Four of the six samples modelled require 60–70 My to reach 1000°C from the solidus. Our modelling of heat‐producing element partitioning and predicted proportions of melt suggest that enriched basaltic compositions are the most reasonable source of TTG magmas and our heating time modelling indicates the mantle as an equal to dominant source of heat for metabasite anatexis compared with radiogenic heat production.

Zircons reveal the history of fluctuations in oxidation state of crustal magmatism and supercontinent cycle

We apply a zircon redox index to a global compilation of detrital zircons to track the variation of oxidation state, expressed as ΔFMQ, through Earth’s history. Those from I-type rocks, which comprise mantle and crustal igneous protoliths, including tonalite–trondhjemite–granodiorites (TTGs), generally have a high oxidation state (ΔFMQ > 0). In contrast, zircons from igneous rocks derived from supracrustal source rocks (S-type) are commonly reduced (ΔFMQ < 0). With the probability density function derived from the Gaussian-Kernel-Density-Estimation, we use the maximum likelihood estimation (MLE) to distinguish S-type from I-type zircons through Earth’s history using zircon redox. Voluminous S-type magma production shows a ca. 600 Ma cyclicity that is closely related to the supercontinent cycle. We link a cyclic drop in redox values after 2.6 Ga to periodic S-type magma generation associated with burial and melting of metasedimentary rocks during supercontinent assembly and amalgamation. The ΔFMQ of the detrital zircons rise at ∼3.5 Ga followed by a consistent average ΔFMQ > 0 over the last 3 Ga. Given that the redox state of magmas is independent of crustal thickness and silica variation, and elevated values are likely more closely related to tectonic setting, we suggest that the consistent average ΔFMQ > 0 from ca. 3.5 Ga onwards relates to recycling of oceanic lithosphere back into the mantle in what eventually became established as subduction zones. The more reduced magmas associated with sedimentary sources, became established at 2.6 Ga, presumably in response to continental rocks rising above sea-level, and follow peaks of productivity associated with the supercontinent cycle.

Heavy silicon and oxygen isotope signatures of TTGs formed in distinct tectonic settings

Tonalite–trondhjemite–granodiorite suites (TTGs) are key components of Archean continental crust and are therefore crucial to understanding the evolution of Archean Earth. However, the tectonic settings of their formation remain unresolved. Here we present new in situ zircon Si–O–Hf isotopes and quartz Si–O isotopes from 2.95 to 2.54 Ga TTGs of the Jiaodong terrane in the North China Craton. Through first-principles calculations, the corresponding TTG melt δ30Simelt and δ18Omelt values are determined as –0.03 ‰ to 0.06 ‰ and 7.12 ‰ to 7.60 ‰, respectively, indicating heavier isotope signatures than those of mantle-derived juvenile rocks. The 2.95–2.93 Ga and 2.56–2.54 Ga TTGs have depleted Hf isotopes [εHf(t) = +3.7 to +4.9, and +2.5 to +5.5, respectively] and higher (La/Yb)N ratios compared with the 2.75–2.71 Ga TTGs, which have relatively less depleted Hf isotopes [εHf(t) = +0.9 to +2.0] and lower (La/Yb)N ratios. Although the heavy Si–O isotope compositions indicate the involvement of supracrustal materials in the melting sources of all studied TTGs, the variable zircon Hf isotopes and (La/Yb)N ratios suggest that these rocks formed in distinct tectonic settings. The 2.95–2.93 Ga and 2.56–2.54 Ga TTGs are inferred to have been generated in a subduction setting involving the melting of juvenile oceanic crust under high-pressure conditions, whereas the 2.75–2.71 Ga TTGs are inferred to have been derived from the melting of buried ancient oceanic crust under a mantle-plume-induced lower-pressure environment. We conclude that the heavy Si–O isotope signatures of Archean TTGs may not be directly related to plate-tectonic processes.

Neoarchaean DTTGs from the Dunhuang Block, Tarim Craton: insights into petrogenesis and crust–mantle interactions

ABSTRACT Earth’s first continental crust is formed by Archaean and mainly consisted of tonalite–trondhjemite–granodiorite with a small amount of diorites (DTTGs), which has an essential role in probing early crust–mantle dynamic regime and in understanding the formation mechanism of continental crust. Here, we present zircon U‒Pb dating and Lu‒Hf isotopes, whole-rock geochemistry, and petrography on DTTGs rocks in the Dunhuang Block. Three episodes of DTTGs were emplaced circa 2.67 Ga, 2.60 Ga, and 2.50 Ga. The circa 2.67 Ga TTGs exhibit high SiO2 contents (68.14–71.70 wt%), low MgO contents (0.65–1.31 wt%), and high ratios of (La/Yb)N (146 on average), with their enriched Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t) = -5.77–0.53], indicating origination from partial melting of amphibolites at thickened lower crust. In contrast, the circa 2.60 Ga transitional TTGs exhibit relatively high MgO contents (2.80–3.39 wt%), flat REE (Rare earth element) patterns with moderate ratios of (La/Yb)N (20.49 on average), and dispersed Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t)= −3.99–3.08]. Accordingly, circa 2.60 Ga transitional TTGs melts were produced by partial melting of the shallower crust induced by mantle-derived magma upwelling. The circa 2.50 Ga diorites exhibit low SiO2 (55.72–59.11 wt%) but high MgO (3.51–4.52 wt%) contents with positive Nd-Hf isotopes [ƐHf (t) = -0.16–4.17 and ƐNd (t) = 2.00–4.45], suggesting that they originated from partial melting of mantle wedges metasomatized by fluid from subduction slabs. Combined with the detailed petrogenetic studies and crustal thickness variation, we conclude that the complex crust–mantle interaction may be an essential reason for the Neoarchaean diversity of DTTGs from the Dunhuang Block, which experienced prolonged arc accretion before Neoarchaean, followed by delamination between 2.67 and 2.60 Ga and subsequently transitioned to subduction.

Decoupled Zircon Si–O Isotopes Tracing the Supracrustal Silicification and Komatiitic‐Derived Fluids in the Source of TTGs

AbstractThe tonalite–trondhjemite–granodiorite suites (TTGs) are key components of the Archean continental crust and therefore crucial to the understanding of the evolution of the early Earth. Here, we present in situ zircon Si–O isotope data of TTGs from Barberton. Results show that the 3.45–3.42 Ga (Group 1) and 3.24–3.23 Ga TTGs (Group 2) have elevated δ30Simelt values but mantle‐like δ18Ozrc values, whereas the 3.23–3.22 Ga TTGs (Group 3) have coupled elevated δ30Simelt and δ18Ozrc values relative to mantle‐derived rocks. We suggest that the Group 1 and 2 TTGs had a silicified source that was affected by low‐δ18O fluid released from the komatiitic rocks. The low‐δ18O fluid decreased the δ18Ozrc values of Group 1 and 2 TTGs but had negligible influence on their δ30Simelt values. The Group 3 TTGs were generated solely from the silicified source, as the low‐δ18O fluid had become exhausted at that time.

Geochemistry and mineralogy of the Upper Ordovician Wufeng formation, Southwestern China: Implications for Paleoclimate construction

Wufeng Formation shale is an important source rock of unconventional hydrocarbons in the Lower Paleozoic shales of Sichuan Basin. However, the study on its provenance and paleoclimate is still relatively limited. In this study, mineralogical and geochemical data of the shales from the Upper Ordovician Wufeng Formation in southwestern China has been used to interpret the provenance and conditions of weathering and paleoclimate. The Wufeng shales have intermediate to high SiO2 (57.72–82.38 wt. %, av. = 68.84 wt. %) and Al2O3 (5.26–16.17 wt. %, av. = 10.62 wt. %), are rich in transition metal elements (i.e. V, Ni, Cu, Co and Cr) and Y as well as moderate depletion in Na2O and Sr, relative to the concentrations of the upper continental crust (UCC). In the chondrite-normalized (CN) rare earth elements (REE) distributions, these rocks display light REE (LREE) enrichment (La/YbCN = 6.69–12.63, av. = 9.28), flat heavy REE (HREE) (Gd/YbCN = 1.35–2.41, av. = 1.70), and clearly negative Eu anomalies (Euan = 0.50–0.66, av. = 0.58), showing similar characteristics with the CN post-Archean Australian Average Shales (PAAS). Wufeng Formation shales are immature composition without evident recycling sediments, and they are originated from an intermediate-felsic igneous source composed of tonalite–trondhjemite–granodiorite (TTG), granitic and andesitic igneous rocks. The chemical weathering conditions of studied shales decreased from moderate to low in the provenance region, suggesting a gradual cooling trend of the climate at Late Ordovician Thus, this article will be helpful to discern the provenance and variations of chemical weathering conditions and paleoclimate of Wufeng Formation shales.

Archean Crustal Evolution of the Alxa Block, Western North China Craton: Constraints from Zircon U-Pb Ages and the Hf Isotopic Composition

The Alxa Block is an important component of the North China Craton, but its metamorphic basement has been poorly studied, which hampers the understanding of the Alxa Block and the North China Craton. In this study, we conducted geochronological and geochemical studies on three TTG (tonalite–trondhjemite–granodiorite) gneisses and one granitic gneiss exposed in the Langshan area of the eastern Alxa Block to investigate their crustal evolution. The zircon U-Pb dating results revealed that the protoliths of the TTG and granitic gneisses were formed at 2836 ± 20 Ma, 2491 ± 18 Ma, 2540 ± 38 Ma, and 2763 ± 42 Ma, respectively, and were overprinted by middle–late Paleoproterozoic metamorphism (1962–1721 Ma). All gneiss samples had high Sr/Y ratios (41–274) and intermediate Mg# values (44.97–55.78), with negative Nb, Ta, and Ti anomalies and moderately to strongly fractionated REE patterns ((La/Yb)N = 10.6–107.1), slight Sr enrichment, and positive Eu anomalies, displaying features of typical high-SiO2 adakites and Archean TTGs. The magmatic zircons from the 2.84 Ga and 2.49 Ga TTG rocks had low εHf(t) values of −1.9–1.7, and −3.83–2.12 with two-stage model ages (TDMC) of 3.24–3.11 Ga and 3.10–3.01 Ga, respectively, whereas those from the 2.54 Ga TTG rock exhibited εHf(t) values ranging from −1.1 to 3.46 and TDMC from 3.0 Ga to 2.83 Ga, suggesting that the crustal materials of the basement rocks in the eastern Alxa Block were initially extracted from the depleted mantle during the late Paleoarchean to Mesoarchean era and were reworked in the late Mesoarchean and late Neoarchean era. By contrast, the Alxa Block probably had a relative younger crustal evolutionary history (&lt;3.24 Ga) than the main North China (&lt;3.88 Ga), Tarim (&lt;3.9 Ga), and Yangtze (&lt;3.8 Ga) Cratons and likely had a unique crustal evolutionary history before the early Paleoproterozoic era.

Vertical tectonics of a Neoarchean gneiss dome in the eastern North China Craton

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.

Modern-style subduction during the late Neoarchean to early Paleoproterozoic? Geochemical evidence from ca. 2.45 Ga arc-type magmatism in the Feidong Complex, northeastern Yangtze Craton, South China

The late Neoarchean to early Paleoproterozoic tectonic evolution of the Yangtze Craton remains poorly constrained as a result of the poor exposure of contemporaneous basement rocks. This study presents new zircon U–Pb–Hf isotopic data and whole–rock geochemical and Sr–Nd isotopic data for orthogneisses from the Feidong Complex of the northeastern Yangtze Craton. Two orthogneiss samples yielded weighted mean ages of 2416 ± 21 and 2436 ± 21 Ma, respectively, indicating the timing of protolith formation. These samples contain relatively high concentrations of SiO2 (54.04–63.25 wt%), but low concentrations of Na2O (3.29–4.12 wt%) and K2O (1.87–2.84 wt%), indicating they are calc–alkaline intermediate rocks. They are enriched in the light ion lithophile elements (LILE) and the light rare earth elements (LREE) but are depleted in Nb–Ta, Ti, the heavy REE (HREE), and Y. They also plot within the arc affinity fields of (La/Yb)N vs. (Yb)N and Sr/Y vs. Y diagrams, consistent with their relatively high Th/Yb and Ta/Yb and low Ce/Pb and Nb/U values, all of which are indicative of rocks formed by arc–type magmatism. The two orthogneiss samples have zircon εHf(t) values that vary between –6.83 and +2.54 with two–stage model ages (TDM2) of 3.35–2.79 (peak at 3.2 Ga) and with similar Nd isotopic compositions. The high total REE contents and Nb–Ta depletions within the orthogneiss samples may reflect derivation from magmas sourced from regions containing fluids released from a subducting slab. These data indicate that the calc–alkaline protoliths of these orthogneisses formed from magmas generated by the partial melting of mantle wedge material driven by the addition of fluid released from a subducted slab. This process induced the partial melting of ca. 3.2 Ga lower crustal material in a manner similar to that associated with modern arc–type magmatism. Taken collectively, the regional magmatism recorded by the ca. 2.50–2.47 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses of the Douling Complex, the ca. 2.45 Ga calc–alkaline arc-type magmatic rocks in the Feidong Complex, and contemporaneous metamorphism within the north margin of Yangtze Craton all provide evidence that modern–style subduction occurred within the northeastern Yangtze Craton between the late Neoarchean and the early Paleoproterozoic.