Trondhjemite–granodiorite Gneisses Research Articles

Petrogenesis of late Paleoproterozoic post-collisional magmatism in southern north China Craton: Insights from geochemistry and Nd–Hf isotopic compositions of A-type granites

A-type granites are produced commonly in extensional settings and they are important proxies to track breakup of continent. Here we report systematic studies on geochronology and geochemical features of A-type granites in Songxian–Ruyang region, southern margin of the North China Craton (NCC), in order to constrain their petrogenesis and tectonic implications. Zircons from these granites show oscillatory zoning and high Th/U ratios (0.12 to 1.26), indicating a magmatic origin. LA–ICP–MS zircon U–Pb dating shows that they were emplaced at 1813 to 1789 Ma, indicating that they were formed in late Paleoproterozoic. Geochemically, these rocks are high in SiO2 (67.51–74.42 wt%) and total alkalis (Na2O + K2O = 7.76–10.19 wt%), and low in MgO (0.09–1.31 wt%), CaO (0.09–1.11 wt%) and P2O5 (0.12–0.26 wt%) contents, with negative Nb, Ta, Sr, P and Ti anomalies. They are characterized by high FeOT/(MgO + FeOT), 10000 Ga/Al ratios (1.57–3.15) and Zr + Nb + Ce + Y contents (566–835 ppm). They are also characterized by high calculated Zr saturation temperatures (892–918 °C), high Ti-in-zircon temperatures (693–912 °C) and metaluminous to weakly peraluminous natures, which are typical features of aluminous A-type granites, further belonging to the A2 granites. The low zircon εHf(t) values (–13.8 to –8.6), along with the low whole rock εNd(t) values (–8.62 to –6.10) and low apatite εNd(t) values (–8.31 to –6.56), indicate that the Songxian–Ruyang A-type granites were derived from partial melting of the calc-alkaline tonalitic–trondhjemitic–granodioritic gneisses in Taihua Complex, and they were formed in post-collisional extensional setting. Combined with coeval regional granitoids, the south margin of the NCC is in a continuous extensional setting during 1.8–1.6 Ga, and the Songxian–Ruyang A-type granites can represent a response of the breakup of Columbia supercontinent.

Clockwise P−T−t paths of late Neoarchean high-pressure pelitic granulites from the Qingyuan terrane, eastern North China Craton

Precambrian high-pressure (HP) granulites provide record for the geodynamic process of ancient continental nuclei. Here we report the newly discovered late Neoarchean HP pelitic granulites in the Qingyuan terrane, eastern North China Craton (NCC). The Neoarchean basement of the Qingyuan terrane comprises meta-supracrustal rocks, tonalite–trondhjemite–granodiorite (TTG) gneisses, and a small amount of charnockites. The HP pelitic granulites were recognized from the meta-supracrustal rocks as fragments within the TTG gneisses. Detailed petrographic observations, mineral chemistry and phase equilibrium modelling results show that the prograde stage (M1) of HP pelitic granulites can be presented by the inclusions of garnet porphyroblasts including kyanite, plagioclase, quartz, biotite and ilmenite. The prograde P−T conditions were constructed via melt-reintegration approach and Ti-in-quartz thermometer (710–745 °C at 8–9.3 kbar). The peak stage (M2) can be represented by the core of garnet porphyroblasts and matrix minerals containing kyanite, plagioclase, quartz, biotite, K-feldspar and rutile with the P−T conditions of 12.2–13.6 kbar and 800–825 °C retrieved by Ti-in-quartz thermometer, garnet-Al2SiO5-plagioclase-quartz (GASP) barometer, garnet barometer and phase equilibria modelling. The peak metamorphic P−T conditions suggest that surficial sediments have been brought to a depth over 40 km. The post-peak stage (M3) is presented by the inner rim of garnet porphyroblasts (recorded in QY01) or the core of garnet (recorded in QY02) with some matrix minerals containing sillimanite, plagioclase, quartz, biotite, K-feldspar and ilmenite, indicating the P−T conditions of 7.8–9.0 kbar and 810 °C combined with Ti-in-quartz thermometer, GASP barometer, garnet barometer and phase equilibria modelling. The final retrograde stage (M4) is presented by the minor matrix sillimanite, plagioclase, quartz, K-feldspar, ilmenite and biotite with the absence of melt and the P−T conditions of 668–733 °C at 5–8 kbar using Ti-in-biotite thermometer. Monazite LA-ICP-MS U-Pb dating reveals the peak or near peak metamorphic ages ∼2.49 Ga of the HP pelitic granulites. Therefore, the HP pelitic granulites of the Qingyuan terrane record the clockwise P−T−t paths with sequential isothermal decompression (ITD) and isobaric cooling (IBC) segments, suggesting a subduction−collision environment at the late Neoarchean (∼2.49 Ga) in the eastern NCC.

Zircon U-Pb Dating and Metamorphism of Granitoid Gneisses and Supracrustal Rocks in Eastern Hebei, North China Craton

Granitoid gneisses dominated by tonalitic–trondhjemitic–granodioritic (TTG) compositions, with metamorphic supracrustal rocks consisting of sedimentary and volcanic rocks, are widely exposed in the Eastern Hebei terrane, North China Craton (NCC). This study presents systematic zircon U–Pb geochronological and whole-rock geochemical data of the Neoarchean granitoid gneisses and supracrustal rocks in Eastern Hebei. Zircon U–Pb isotopic dating for the representative samples reveals that magmatic precursors of granitoid gneisses were emplaced between 2524 ± 7 and 2503 ± 12 Ma, and the protoliths of the pelitic granulites were deposited in the Late Neoarchean era. Both of them have been subjected to granulite facies metamorphism during 2508 ± 10 to 2468 ± 33 Ma, coeval with the intrusion of syenogranitic pegmatite (2488 ± 5 Ma). Zircon ages of 2.45–2.01 Ga obtained from the analyzed samples were considered mixed data from 2.53–2.48 Ga and 1.9–1.8 Ga and were chronologically meaningless. Paleoproterozoic metamorphic zircon ages of 1.9–1.8 Ga were usually neglected because of hardly being obtained from TTG gneisses and supracrustal rocks. The tectonic regime during the Neoarchean era was considered to be dominated by vertical tectonism in the Eastern Hebei terrane.

Origin of the ca. 3.1 Ga Luanjiajie rock assemblage in the northeastern margin of the North China Craton: New constraints on the Mesoarchean geodynamic regime

The North China Craton (NCC) is one of the oldest cratons on Earth and preserves abundant Mesoarchean igneous rocks. However, the petrogenesis of the Mesoarchean magmatism and the associated geodynamic regime in the NCC remain controversial. In this paper, we conducted a systematic study to constrain the petrogenesis of the recently discovered Mesoarchean Luanjiajie rock assemblage (LRA) and infer the regional Mesoarchean tectonic setting. The LRA is composed mainly of tonalite–trondhjemite–granodiorite (TTG) gneiss with minor dioritic gneiss and amphibolite. The amphibolite shows intrusive contact with the surrounding TTG gneiss, whereas the dioritic gneiss displays transitional boundary with the TTG gneiss. Zircon U-Pb dating revealed that these three lithologies were roughly contemporaneous, with crystallization and metamorphic ages of ca. 3.1 and ca. 2.5 Ga, respectively. Geochemical characteristics (e.g., high Sr/Y and LaN/YbN ratios and weak positive Eu anomalies), zircon Hf isotopic features (TDM2 ages of 3822 to 3531 Ma with εHf(t) values of −2.44 to +0.97), and slightly elevated δ18O values (+5.48‰ to +7.24‰) indicate that the ca. 3.1 Ga TTG was derived from the partial melting of ancient hydrated low-K mafic rocks. The Luanjiajie TTG gneiss and other reported Mesoarchean TTGs in the NCC are classified mainly as medium- to low-pressure types. The linear trends between SiO2 content and Fe2O3T, MgO, Y, Yb, Sm, and Gd contents of the ca. 3.1 Ga dioritic gneiss reflect that the dioritic gneiss might be a product related to amphibole fractionation of the coeval TTG magma. The ca. 3.1 Ga amphibolite displays OIB-affinity geochemistry and positive εHf(t) (up to +4.65) and εNd(t) (+0.2 to +7.7) values. The amphibolite was most likely formed through the partial melting of a depleted garnet-peridotite mantle and underwent minor crustal contamination during magma ascent. Hence, the LRA was most likely formed under a plume-related tectonic setting involving basaltic magma underplating and recycling of ancient crust. Combining our new results with existing zircon Hf data, we suggest that the NCC experienced initial continental nuclei formation during the Eo- to Paleoarchean, and followed by episodic crustal growth and recycling during the Meso- to Neoarchean.

Neoarchean granitoids and tectonic regime of lateral growth in northeastern North China Craton

The Archean Eastern Liaoning metamorphic basement block is located in the northeastern North China Craton (NCC), in which ∼2.9 to 3.8 Ga lithological assemblages are well developed. The region which exposed these ancient lithological assemblages is usually called as an ancient continental nucleus. Toward north from the northeastern margin of the ancient continental nucleus, ∼2.5 to 2.7 Ga lithological assemblages continuously developed, forming abundant ∼2.5 Ga tonalite- trondhjemite- granodiorite (TTG) and high-K granitoids. Some ∼2.7 Ga TTG gneisses and metamorphic calk-alkaline volcanic rocks have also been documented recently in the Eastern Liaoning to the Southern Jilin of northeastern NCC. Our geological investigations found that many big ∼2.7 Ga TTG gneiss enclaves are preserved in the ∼2.5 Ga granitoids in the northeastern NCC. The early Neoarchean (∼2.7 Ga) TTG gneisses are exposed in the Waitoushan-Baishan areas, together with many contemporaneous subduction-related metamorphic calk-alkaline volcanic rocks in the Helong area of Southern Jilin province, constituting a nearly E-W-trending ∼2.7 Ga island arc belt in the northeastern NCC. However, the ∼2.7 Ga geological record is absent within the Anshan-Benxi-Waitoushan ancient microblock (ABWN) domain, suggesting that the ∼2.7 Ga arc belt was formed far away from the ABWN domain. The petrogenesis of ∼2.52–2.59 Ga TTG and sanukitoid gneisses, integrating with contemporaneous top-to-the-NE thrusting structural features, indicate that the new oceanic crust in the Waitoushan-Weiziyu-Jiubing (WWJ) area subducted to the southwest from ∼2.53–2.60 Ga. At the Archean end, the northern Liaoning-eastern Hebei intraoceanic arc system subducted to the southeast, which led to the final amalgamation among the ABWN, WWJ and northern Liaoning areas. Subsequently (∼2.50–2.53 Ga), the subducted slab broke off and the collisions between the arc and ancient continental nucleus, as well as the arc and arc, resulted in the generation of extensive crust-derived high-K granitoids.

Thermal state and evolving geodynamic regimes of the Meso- to Neoarchean North China Craton

Constraining thickness and geothermal gradient of Archean continental crust are crucial to understanding geodynamic regimes of the early Earth. Archean crust-sourced tonalitic–trondhjemitic–granodioritic gneisses are ideal lithologies for reconstructing the thermal state of early continental crust. Integrating experimental results with petrochemical data from the Eastern Block of the North China Craton allows us to establish temporal–spatial variations in thickness, geothermal gradient and basal heat flow across the block, which we relate to cooling mantle potential temperature and resultant changing geodynamic regimes from vertical tectonics in the late Mesoarchean (~2.9 Ga) to plate tectonics with hot subduction in the early to late Neoarchean (~2.7–2.5 Ga). Here, we show the transition to a plate tectonic regime plays an important role in the rapid cooling of the mantle, and thickening and strengthening of the lithosphere, which in turn prompted stabilization of the cratonic lithosphere at the end of the Archean.

Mesoarchean to Paleoproterozoic Crustal Evolution of the Belomorian Province, Fennoscandian Shield, and the Tectonic Setting of Eclogites

Abstract—The Belomorian Province (BP) of the Fennoscandian Shield is a high-grade belt composed of Meso- to Neoarchean tonalite– trondhjemite–granodiorite (TTG) gneisses with subordinate supracrustal complexes. The Belomorian crust is underlined by a thick mantle keel, a structural element typical of Archean cratons. Belomorian rocks were metamorphosed under conditions of mainly high-pressure amphibolite to granulite facies in both Archean and Paleoproterozoic times. The TTG gneisses contain numerous blocks of almost completely retrogressed eclogite (eclogite-1). This paragenetic association of eclogite-1 and gneisses can be classified as an Archean eclogite–TTG gneiss mélange, a component of the Belomorian continental crust produced by subductional, accretionary, and collisional processes of the Belomorian collisional orogeny 2.9–2.66 Ga. The Paleoproterozoic history of the BP comprises of two prominent tectonic periods: (i) early Paleoproterozoic (~2.5–2.4 Ga), related to a superplume, and (ii) late Paleoproterozoic (2.0–1.85 Ga), resulted from crustal reworking during the Lapland–Kola collisional orogeny that produced strong penetrative metamorphic and local deformational overprint. The Paleoproterozoic highest-grade metamorphic overprint is represented by patches of eclogites (eclogite-2) in Paleoproterozoic mafic dikes and eclogite-1. Field relations between eclogite-1 and eclogite-2 are described in the Gridino area of the western coast of the White Sea. So, the BP is a high-grade polymetamorphic belt formed by a superposition of the Neoarchean Belomorian and Paleoproterozoic Lapland–Kola orogenies, whose characteristic features are eclogites produced by subduction and collision.

Metamorphic history and Neoarchean–Paleoproterozoic crustal growth of the central Trans-North China Orogen: Evidence from granulite- to amphibolite-facies rocks of the Hengshan complex

The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional orogen exposed within the North China Craton (NCC). The orogen has undergone a complex tectonic evolution, but its role in the growth of continental crust is contentious. High-grade metamorphic rocks may carry key information required to clarify these issues. Here, using petrography, mineral chemistry, and zircon U–Pb–Hf isotopes, we investigate a suite of Neoarchean–Paleoproterozoic high-grade metamorphic rocks in the Hengshan Complex within the central TNCO. Garnet-bearing mafic granulites and amphibolites occur as lenses or dikes within tonalite–trondhjemite–granodiorite (TTG) gneisses. Three stages of metamorphic mineral assemblages are identified in a mafic granulite from the northern Hengshan Complex: (1) the prograde assemblage is preserved as small plagioclase and apatite inclusions within garnet porphyroblasts; (2) the peak mineral assemblage is recorded by a high-Ca garnet core coexisting with clinopyroxene, plagioclase, hornblende, biotite, and quartz; and (3) the retrograde assemblage is represented mainly by vermicular symplectites of hornblende + plagioclase and plagioclase coronae surrounding low-Ca garnet rims. Phase equilibria modeling and conventional geothermobarometry suggest that the mafic granulite records a clockwise P–T path, including decompression and cooling from the granulite-facies peak (12.5–14 kbar and >750 °C) to amphibolite-facies conditions during retrogression (7.3–8.3 kbar and 615–721 °C). In contrast, the amphibolite in the southern Hengshan Complex preserves only amphibolite-facies assemblages (hornblende + plagioclase + quartz), consistent with the metamorphic pressures of <7.1 kbar and temperatures below 720 °C.Zircon U--Pb dating of magmatic zircons shows that the protoliths of the TTG gneisses and amphibolite were emplaced at ca. 2.5 and 2.1 Ga, respectively. The metamorphic zircons reveal ages of 1.92–1.80 Ga for the mafic granulite (weighted mean of ca. 1.87 Ga) and 1.92–1.83 Ga for the amphibolites (weighted mean of ca. 1.88 Ga). These 1.88–1.87 Ga metamorphic ages are interpreted to represent the time when the rocks cooled to the solidus. By comparing Paleoproterozoic metamorphic ages across the TNCO, we infer that the subduction–collision–amalgamation between the Western and Eastern Blocks of the NCC lasted from 1.95 to 1.80 Ga at least, marking the final cratonization of the NCC. The ca. 2.5 Ga magmatic zircons within the TTG gneisses show positive εHf(t) values (+0.49 to +7.12) with model ages of 2.8–2.7 Ga, suggesting that the protoliths were derived from the partial melting of Neoarchean juvenile mafic crust. Zircon Hf model ages (TDM = 2.1–2.0 Ga) of the mafic granulite may represent the approximate time at which the mafic magma was generated in the mantle. Thus, the Hengshan Complex within the central TNCO records two episodes of crustal growth at 2.8–2.7 and 2.1–2.0 Ga, respectively. To further understand the processes involved in crustal evolution, we reconstructed a layered crust model for the Hengshan Complex, and the layered crust was strongly influenced by Paleoproterozoic thermal events.

Geochemical and isotopic studies of potassic granite from the western Dharwar Craton, southern India: Implications for crustal reworking in the Neoarchean

The Neoarchean potassic granite from the Hosadurga region adjacent to the Chitradurga greenstone belt (CGB), part of the western Dharwar Craton (WDC), is studied for its petrogenesis. The combined field, petrographic, whole‐rock elemental, and Nd–Sr isotope data of granite and associated Tonalite–Trondhjemite–Granodiorite (TTG) gneiss from this region are presented here. The granites are alkali‐calcic to calc‐alkalic in terms of modified alkali lime index (MALI) and are weakly peraluminous (molar Al2O3/(CaO + Na2O + K2O) = 1.0–1.1). The geochemical data indicate that these are formed by the melting of TTG gneiss at depths above the garnet stability field. Isotopic data (εNd = −2.9 to −7.6 at T = 2,600 Ma, ISr = 0.715–0.729, TDM &gt; 3.0 Ga) also show the involvement of Archean crustal sources. Trace element modelling was carried out using non‐modal equilibrium melting equation, which constraints partial melting of TTG source to a low degree (11%) for the formation of this granite. This model could also produce a major element composition consistent with this granite. The upper bounds on the magmatic temperatures (724–785°C) assessed from zircon saturation thermometry suggest that fluid influx is required. TTG samples show variable and very high positive epsilon Nd values (1.5–8.9), higher than the contemporaneous mantle, suggesting re‐melting of mafic crust. The observed high Nd isotope ratios in TTG indicate involvement of a mafic source that formed as early as ~3.8 Ga, probably as a result of global differentiation of the early Earth.

Episodic crustal growth and reworking at the southeastern margin of the North China Craton: evidence from zircon U–Pb and Lu–Hf isotopes of Archean tonalite–trondhjemite–granodiorite gneisses in the Bengbu-Wuhe area

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

U–Pb–Hf–O–Nd isotopic and geochemical constraints on the origin of Archean TTG gneisses from the North China Craton: Implications for crustal growth

Tonalite–trondhjemite–granodiorite (TTG), an Archean dominant lithological assemblage within the cratonic provinces, is believed to have been formed by partial melting of the hydrous basalts in the garnet stability field. The origin of TTG is a window to decipher the evolution of Archean continental crust, whereas the petrogenesis is not well constrained. To solve this issue, we have carried out field observations, zircon U–Pb–Hf–O isotope, as well as whole-rock Nd–O isotope and geochemical analyses on the TTG and associated dioritic gneisses from the North China Craton. U–Pb isotopic dating using the LA–ICP–MS on zircons reveals that the granitoid gneisses in northern Liaoning were contemporaneously generated during ca. 2.56–2.51 Ga and immediately experienced a regional metamorphic event at 2.48–2.45 Ga. The dioritic gneisses occur as quenched inclusions with rounded to ellipsoidal globules in shape rather than lithospheric relics. Such occurrence, together with isotopic and geochemical data, suggests that the mafic enclaves and host TTG represent two co-existing, compositionally distinct magmas. The TTG gneisses are featured by elevated SiO2 (64.72–86.46 wt%) contents, (La/Yb)N (68 on average) and Sr/Y (119 on average) ratios, and low MgO (0.35–2.58 wt%) contents and Mg# values, with radiogenic Hf and Nd isotopes (εHf(t) = +3.5 to +9.0 and εNd(t) = +0.8 to +5.4), indicating the derivation from partial melting of the juvenile basaltic sources at lower crustal levels. Compared with the TTG gneisses, the dioritic gneisses have lower SiO2 contents, higher MgO contents and Mg# values, with similar Hf–Nd isotopes (εHf(t) = +3.6 to +9.5 and εNd(t) = +3.3 to +5.1), implying the depleted mantle source. A model is proposed whereby partial melts from juvenile basaltic sources evolved via mixing with mafic enclaves (viz. dioritic magma) and fractional crystallization, and finally were contaminated by the sedimentary rocks, forming the TTG magma. The granitoid gneisses in northern Liaoning witnessed the major vertical growth of continental crust at the end of the Neoarchean, which is supported by Hf–Nd–O isotopic data and zircon Hf–O isotopic modeling. Our results show that the geochemistry of TTG (such as elevated La/Yb and Sr/Y) might be affected by complex petrogenetic processes, i.e., partial melting of individual sources, magma mixing, fractional crystallization, and crustal contamination, rather than solely controlled by the P–T conditions of crustal anataxis as previously thought, which might have been also overlooked in other Archean cratons.

Open-system fractional melting of Archean basalts: implications for tonalite–trondhjemite–granodiorite (TTG) magma genesis

Tonalite–trondhjemite–granodiorite (TTG) gneiss forms a major component of Archean continental crust. Resolving the origin of TTG gneisses and the secular change in their compositions is critical for better understanding how the continental crust has evolved and when a global plate tectonic regime began on Earth. Archean TTGs were generated by partial melting of hydrous basalts, although the geodynamic setting in which this occurred is still debated. In this study, an integrated modeling including thermodynamic modeling, accessory mineral solubility modeling, and trace element modeling is conducted on Coucal basalts from Pilbara craton and averaged Archean arc-like basalts along various thermal gradients for both closed and melt-drained systems. The results show the amount and composition of melts would be primarily controlled by source compositions, although geothermal gradient and the ability for melt to leave the system also contribute. Of both bulk compositions, averaged Archean arc-like basalts generate more melts with a medium- to low-pressure signature by fluid-absent melting during intracrustal loading. High-pressure TTGs may be generated by mixing residua and melts derived by fluid-absent melting or just fluid-present melting in a hot subduction zone. Most TTGs produced before the Neoarchean were likely generated at the lower levels of thickened basaltic crust; however, rare Eoarchean and Mesoarchean TTGs with high-pressure signatures suggest subduction occurred on the early Earth in isolated localities. We interpret a secular increase in the proportion of high-pressure TTGs at c. 3.0–2.5 Ga as recording a transition from localized subduction to a global plate tectonic regime.

Persistence of melt-bearing Archean lower crust for &amp;gt;200 m.y.—An example from the Lewisian Complex, northwest Scotland

Abstract Geochronological data from zircon in Archean tonalite–trondhjemite–granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean.

Petrogenesis of the Neoarchean granitoids and crustal oxidation states in the Western Shandong Province, North China Craton

Na-rich and K-rich granitoid rocks contain geological signatures that are crucial for understanding the global evolution and oxidation states of the Archean crust. Here we present an integrated investigation of the lithology, geochronology, and geochemistry of Neoarchean plutonic granitoid gneisses and supracrustal rocks from Xintai to Feixian within the Western Shandong Province (WSP) granite-greenstone belt of the North China Craton (NCC). Six main lithological assemblages are identified within the WSP granite-greenstone belt: (1) The ∼2.69–2.68 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses, which were derived from the partial melting of low-K mafic rocks at lower crust, have low MgO contents (0.68–2.11 wt%) and high (La/Yb)N ratios (16.7–92). (2) The ∼2.53 Ga granodioritic gneisses exhibit a low MgO content (0.87–1.91 wt%) but high K2O content (3.06–4.66 wt%), their magma originated from the partial melting of high-K mafic rocks. (3) The ∼2.54–2.49 Ga monzogranitic gneisses show the highest SiO2 content (71.67–75.26 wt%), and their magma was formed from the partial melting of metagreywackes. (4) The ∼2.56–2.52 Ga TTG gneisses also have low MgO content (1.42–1.69 wt%) and relatively low (La/Yb)N ratios (17.1–23); their magma was produced by the partial melting of low-K mafic rocks at lower crustal levels. (5) The ∼2.53–2.52 Ga quartz syenites have the highest alkali content (9.66–11.23 wt%) but relatively low MgO content (0.71–1.50 wt%), and their magma was generated by the partial melting of juvenile basaltic rocks associated with recycled sediments. (6) An ∼2.73 Ga xenolith of plagioclase amphibolite is characterized by high positive εHf(t2) values from +5.2 to +8.8, and fractionated chondrite-normalized REE patterns, indicating it may have been derived from a depleted mantle source enriched either by the addition of melt or by fluid metasomatism.The oxygen fugacity values of the crust-derived Neoarchean granitoid magmas are calculated based on a zircon trace element oxybarometer, the results show that the log (f O2) values for the ∼2.7 Ga TTG gneisses that range from FMQ-7.4 to FMQ+5.5 with an average value of FMQ+0.5, whereas the various ∼2.5 Ga granitoid gneisses have log (f O2) values ranging from FMQ-4.3 to FMQ+8.9 with an average value of FMQ+3.3. In conjunction with mineralogical results, we consider that the ∼2.7 Ga continental crust was in a relatively more reduced state than the ∼2.5 Ga continental crust in the WSP, which may be associated with differences in their source regions and compositions.

Diverse middle Neoarchean granitoids and the delamination of thickened crust in the Western Shandong Terrane, North China Craton

Middle Neoarchean granitoids contain crucial clues for understanding crust–mantle interactions and further investigating the mechanism of the compositional transformation of the crust in the Neoarchean. In the Western Shandong Terrane (WST), diverse Neoarchean magmatism is recognized, including ∼2.62 Ga tonalitic gneisses, ∼2.59 Ga granodioritic gneisses and ∼2.61–2.58 Ga monzogranitic gneisses, which generally intruded into ∼2.70 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses. The ∼2.70 Ga TTG gneisses show high SiO2 contents (70.17–72.89 wt%), but low MgO contents (0.58–0.69 wt%) and high (La/Yb)N ratios (30–48) and originated from partial melting of low-K mafic rocks at lower crustal levels. The ∼2.62 Ga tonalitic gneisses exhibit low K2O/Na2O ratios (0.10–0.39), but high (La/Yb)N (7.60–12.7) and Sr/Y (18.8–48) ratios and Mg# (47–57). With high εHf(t2) values ranging from +2.7 to +7.4, these rocks were derived from dehydration melting of subducted oceanic slabs. The ∼2.61–2.58 Ga monzogranitic gneisses show high SiO2 (70.35–75.12 wt%) and K2O (4.35–5.81 wt%) contents but low MgO (0.20–0.77 wt%) contents with positive εHf(t2) values (+2.0 − +6.6), suggesting they were derived from partial melting of juvenile greywackes. The ∼2.59 Ga granodioritic gneisses exhibit high SiO2 (66.34–73.04 wt%), K2O (2.23–4.32 wt%) and MgO (1.39–3.51 wt%) contents with dispersed εHf(t2) values (−0.2 − +5.3), indicating a mixing process between crust- and mantle-sourced magmas. The ∼2.59 Ga granodioritic gneisses show consistent major and trace element compositions (i.e., high Mg#, lack of Eu anomalies and uniform chondrite-normalized REE patterns), and accordingly, their magmatic precursors were derived from partial melting of delaminated lower crustal materials.The lithological assemblages in the WST became increasingly diverse between ∼2.71–2.58 Ga, changing from TTG gneisses to arc-related rock series, and then to delamination-related granodioritic gneisses, crust-derived monzogranitic and trondhjemitic gneisses and mantle-derived tholeiitic basalts and diorites. By integrating these data with a reduction in the continental crustal thickness from ∼2.70 Ga to 2.55 Ga, a subduction–delamination model is proposed in which a long-term lateral accretion beginning at ∼2.7 Ga and persisting through ∼2.68–2.62 Ga led to the delamination of the lower crust at ∼2.61–2.58 Ga, which may have further triggered the transition of the continental crust composition from basaltic to andesitic.

New insights into Neoarchean–Paleoproterozoic crustal evolution in the North China Craton: Evidence from zircon U–Pb geochronology, Lu–Hf isotopes and geochemistry of TTGs and greenstones from the Luxi Terrane

Tonalite–trondhjemite–granodiorite (TTG) suites constitute dominant components of Precambrian continental crust and provide important insights into the crustal evolution history of the early Earth. Here we investigate a suite of TTGs and associated greenstones from the Luxi Terrane in the south–eastern part of the North China Craton (NCC). We present data from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes to characterize these rocks and understand their genesis. Mineral assemblage in the greenstones and TTGs suggests that the rocks have undergone amphibolite–facies metamorphism with regression under greenschist–facies. Zircon U–Pb data from the magmatic grains yield upper intercept ages of 2753 ± 37 and 2582 ± 40 Ma for greenstone, 2552 ± 13 Ma for TTG gneiss and 2476 ± 18 Ma for tonalite, and weighted mean 207Pb/206Pb age of 2507 ± 22 Ma for granodiorite. Lu–Hf isotope systematics show εHf(t) values ranging from −2.13 to 7.29 and crustal model ages (TCDM) in the range of 2603–3125 Ma for the TTGs, whereas the εHf(t) values are up to 8.53 with TCDM ages of 2644–3006 Ma for the greenstone. The results suggest that the protolith for the greenstone was derived from a Meso- to Neoarchean mantle contaminated by the crust source, whereas the TTGs were mostly sourced from juvenile crustal sources with reworking of older basement. The geochemical features of greenstone are similar to island arc basalt, with low Na2O, K2O and TiO2, low total rare earth elements (REEs) values, relative enrichment in light REEs and depletion in heavy REEs absence of obvious Ce anomaly, and negative Nb, Ta and Ti anomalies. The TTGs show similar features, except for the high K2O content and moderate total REEs values. We propose that the greenstone likely represent a part of the early and initial crustal growth at ca. 2750–2600 Ma, and the TTGs were generated subsequently at ca. 2550–2450 Ma. Our results are consistent with the previous models on the crustal growth models in the NCC and provide further insights into the Neoarchean to Paleoproterozoic tectonic history of the craton.

Neoarchean granitoid gneisses in Eastern Hebei, North China Craton: Revisited

Neoarchean granitoid gneisses are widely distributed throughout Eastern Hebei, eastern North China Craton, which are dominated by deformed and metamorphosed tonalite–trondhjemite–granodiorite (TTG), diorite, and granite. This study presents the results of systematic zircon U–Pb geochronological, whole-rock geochemical and Sm–Nd isotopic analyses of the Neoarchean granitoid gneisses in Eastern Hebei. These data provide insights into the Archean–Paleoproterozoic multiple tectonothermal events as well as the petrogenesis of these gneisses. Zircon U–Pb isotopic dating indicate that the granitoid gneisses were contemporaneously emplaced between 2546 ± 10 and 2510 ± 10 Ma, reflecting a giant Neoarchean igneous event throughout Eastern Hebei. Subsequently these rocks were subjected to regional amphibolite facies metamorphism at 2.48–2.45 Ga. The close spatial and temporal relationships in terms of magmatism and metamorphism at ca. 2.5 Ga suggest a uniform tectonothermal evolution of Eastern Hebei. The granitic gneisses have mainly originated from the partial melting of juvenile metamorphosed greywackes, with minor involvement of basalts. The large geochemical and isotopic variations within the dioritic and TTG gneisses both provide evidence for the mixing of mafic and felsic magmas, coupled with fractional crystallization. However, the chemical differences between the dioritic and TTG gneisses might result from individual mafic magma sources, viz. basaltic and high-Mg melts. The mafic magma may also form the basalt or komatiite within the greenstone belt or evolve via fractional crystallization prior to the magma mixing. Large-scale granitoid activities were possibly related to mafic magma underplating. The combined geochronological, geochemical, and geological data support an Archean proto-mantle plume model for a geodynamic interpretation of the eastern North China Craton during the Neoarchean.

Early Precambrian tectono-thermal events in Southern Jilin Province, China: implications for the evolution of Neoarchean to Paleoproterozoic crust in the northeastern North China Craton

The early Precambrian basement of the North China Craton, China, records a complex history during the Neoarchean and Paleoproterozoic. Southern Jilin Province is one of the best regions to evaluate the early Precambrian crustal growth of the Eastern Block of the North China Craton. We herein present new zircon cathodoluminescence images, U–Pb dates, trace element and Lu–Hf isotope data for three late Neoarchean tonalitic–trondhjemitic–granodioritic (TTG) gneisses and one Paleoproterozoic mafic dyke from Tonghua region, southern Jilin Province. These results lent convincing support to the occurrence of multiple tectono-thermal events in southern Jilin Province during the Archean and Paleoproterozoic, and shed light on the formation and evolution of continental crust in the northeastern Eastern Block. Zircon LA–ICP–MS U–Pb isotopic analyses indicated three episodes of magmatism. Inherited (or captured) zircons with ages of ca. 2.6 Ga provided strong evidence for a pre-2.5 Ga magmatic event in this region. The second magmatic episode occurred at 2556–2522 Ma, as evidenced by extensive exposures of TTG gneisses. The third episode occurred at ca. 2200 Ma and is recorded by several mafic dykes that intruded the Archean TTG gneisses. Metamorphic zircons yielded consistent ages of 2493–2465 Ma, indicating a regional metamorphic event immediately after the late Neoarchean magmatism in southern Jilin Province. Zircon Hf isotope data of the TTG gneisses indicated that the main phase of crustal growth occurred in the late Neoarchean, with a minor input of Mesoarchean continental crustal materials. The combined geochronological, geochemical, and geological data suggested that the three episodes of crustal growth in the northeastern Eastern Block occurred at 2.8–2.7, 2.6–2.5, and 2.2 Ga.

Pressure–Temperature History of the &gt;3 Ga Tartoq Greenstone Belt in Southwest Greenland and Its Implications for Archaean Tectonics

The Tartoq greenstone belt of southwest Greenland represents a well-preserved section through &gt;3 Ga old oceanic crust and has the potential to provide important constraints on the composition and geodynamics of the Archaean crust. Based on a detailed structural examination, it has been proposed that the belt records an early style of horizontal convergent plate tectonics where elevated temperatures, compared to the modern-day, led to repeated aborted subduction and tonalite–trondhjemite–granodiorite (TTG) type melt formation. This interpretation hinges on pressure–temperature (P–T) constraints for the belt, for which only preliminary estimates are currently available. Here, we present a detailed study of the pressure–temperature conditions and metamorphic histories for rocks from all fragments of the Tartoq belt using pseudosection modelling and geothermobarometry. We show that peak conditions are predominantly amphibolite facies, but range from 450 to 800 °C at up to 7.5 kbar; reaching anatexis with formation of TTG-type partial melts in the Bikuben segment. Emplacement of the Tartoq segments into the host TTG gneisses took place at approximately 3 Ga at 450–500 °C and 4 kbar as constrained from actinolite–chlorite–epidote–titanite–quartz parageneses, and was followed by extensive hydrothermal retrogression related to formation of shear zone-hosted gold mineralisation. Tourmaline thermometry and retrograde assemblages in mafic and ultramafic lithologies constrain this event to 380 ± 50 °C at a pressure below 1 kbar. Our results show that the convergent tectonics recorded by the Tartoq belt took place at a P–T gradient markedly shallower than that of modern-day subduction, resulting in a hot, weak and buoyant slab unable to generate and transfer ‘slab pull’, nor sustain a single continuous downgoing slab. The Tartoq belt suggests that convergence was instead accomplished by under-stacking of slabs from repeated aborted subduction. The shallow P–T path combined with thermal relaxation following subduction stalling subsequently resulted in partial melting and formation of TTG melts.

Challenges in constraining theP–Tconditions of mafic granulites: An example from the northern Trans‐North China Orogen

AbstractSome mafic granulites in the Sanggan area of the northern Trans‐North China Orogen (TNCO) have a relatively simple mineralogy with low energy grain shapes that are compatible with an assumption of equilibrium, but the rock‐forming minerals show variations in composition that create challenges for thermobarometry. The mafic granulites, which occur as apparently disrupted dyke‐like bodies in tonalite–trondhjemite–granodiorite gneisses, are divided into two types based on petrography and chemical composition. Type 1 mafic granulites are fine‐ to medium‐grained with an equilibrated texture and an assemblage of plagioclase+clinopyroxene+garnet+magnetite+ilmenite and sometimes minor hornblende±orthopyroxene. Type 2 mafic granulites are coarse‐grained and hornblende bearing with a peak assemblage of garnet+clinopyroxene+plagioclase+hornblende and variably developed coronae and symplectites of plagioclase+hornblende+orthopyroxene partially replacing porphyroblastic garnet±clinopyroxene.SIMSU–Pb dating of metamorphic zircon from two type 1 mafic granulites yields metamorphic ages ofc. 1.84 and 1.83 Ga, consistent with published ages of the type 2 mafic granulites. Based on phase equilibrium modelling, we use the common overlap ofP–Tfields defined by the mineral assemblage limits, and the mole proportion and composition isopleths of different minerals in each sample to quantify the metamorphic conditions. For type 1 granulites, overlap of the mineral proportion and composition fields for each of three samples yields similarP–Tconditions of 710–880°C at 0.57–0.79 GPa, 820–850°C at 0.59–0.63 GPa and 800–860°C at 0.59–0.68 GPa. For the type 2 granulites, overlaying the peak assemblage fields for three samples yields commonP–Tconditions of 870–890°C at 1.1–1.2 GPa. For the retrograde assemblage, overlap of the mineral proportion and composition fields for each sample yields similarP–Tconditions of 820–840°C at 0.85–0.88 GPa, 860–880°C at 0.83–0.86 GPa and 880–930°C at 0.89–0.95 GPa. TheP–Tconditions appear distinct between the two types of mafic granulite, with the mineralogically simple type 1 mafic granulites recording the lowest pressures. However, there are significant uncertainties associated with these results. For the granulites, there are uncertainties related to the determination of modes and composition of the equilibration volume, particularly estimation of O and H2O contents, and in the phase equilibrium modelling there are uncertainties that propagate through the calculation of mole proportions and mineral compositions. The compound uncertainties on pressure and temperature for high‐Tgranulites are large and the results of our study show that it may be unwise to rely onP–Tconditions determined from the simple intersection of calculated mineral composition isopleths alone. Since the samples in this study are from a limited area—a few hundred square metres—we infer that they record a singleP–Tpath involving both decompression and cooling. However, there is no evidence of the high‐Pgranulite facies event at 1.93–1.90 Ga that is recorded elsewhere in theTNCO, which suggests that the precursor basic dykes were emplaced late during the assembly of the North China Craton.