Trondhjemitic Gneisses Research Articles

New discovery of 3.84−3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance

Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838−3754 Ma trondhjemitic gneisses (4 samples), 3786−3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786−3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4−3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4−56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86−1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2−11.3) and lower Eu/Eu* values (0.62−0.79). The potassic granites have (La/Yb)N ratios of 20.7−55.2 and Eu/Eu* values of 0.75−1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3−3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1−3.5 Ga (mainly 4.1−3.8 Ga) and δ18O of 4.6‰−7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.

Building and reworking of Archean continental crust in the nucleus of the Yangtze Craton: Constraint from 3.4 Ga and 2.94 Ga granitoid gneisses in the Southern Kongling Complex

The TTG gneisses, a dominant lithological assemblage within Archean cratonic provinces, provide a window into the early evolution of the continental crust. The Kongling complex in South China preserves voluminous Archean granitoids, recording the early formation and evolution history of the Yangtze Craton. We present results of a comprehensive study involving field investigation, petrographic characterization, zircon U–Pb geochronology and Lu-Hf isotope tracing, and whole-rock geochemistry for the Archean granitoid gneisses from the Southern Kongling Complex. Igneous zircons from two granitic gneisses yielded weighted mean 207Pb/206Pb ages of 3399 ± 15 Ma (MSWD = 0.99, n = 24) and 3389 ± 12 Ma (MSWD = 1.02, n = 20). Zircons from the 3.4 Ga granitic gneiss also record a ca.3.0 Ga anatexis event which is contemporaneous with the widespread granitoid gneisses in the Kongling Complex. Geochemical and zircon Hf isotope data (εHf(t) values of − 3.41 to + 0.77 and two-stage Hf model ages of 3.65–3.80 Ga) for the granitic gneisses support a derivation from pre-existing Eoarchean (TDM2 = 3.73 Ga) crust. LA-ICP-MS zircon U-Pb dating yielded emplacement ages of 2948 ± 17 Ma for a migmatized trondhjemitic gneiss, and 2928 ± 14 Ma for a quartz monzonite gneiss. These two gneisses have similar zircon Hf model ages (TDM2 = 3.45 Ga) and both were emplaced during a major phase of mafic magma diapiric process that significantly reworked the old crust. The occurrences of 2.94 Ga trondhjemite and associated mafic blocks suggest intra-crustal differentiation by partial melting of low-K mafic rocks. We suggest that intra-crustal remelting and differentiation played a key role in the formation, evolution, and maturation of the micro-continents in Kongling during the Paleo- to Meso-Archean. The whole-rock geochemical and zircon Hf isotopic compositions of the different generations of granitoid gneisses from the Southern Kongling Complex are similar to those of the coeval gneisses (3.45 Ga and 2.95 Ga) from the northern part of the Kongling Complex, indicating a common tectono-magmatic history shared by both segments tracing back to as early as 3.4 Ga. The Archean granitoid gneisses from the Kongling Complex record remelting and differentiation of Eo- to Paleo-Archean crusts during the Paleo- to Meso-Archean, implying that Eoarchean crust is likely to have been preserved within the Kongling complex.

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.

Neoarchean magmatism in the southern Scott and Raggatt Mountains, Napier Complex, east Antarctica

The Napier Complex of Enderby Land and Kemp Land is a unique part of the Eastern Antarctic Shield, as it preserves history of crustal growth extending from the Eo- to Neoarchean. It is composed mainly of enderbitic and charnockitic gneisses and granulites metamorphosed at ca. 2.5 Ga, and locally at ca. 2.8 Ga, under high- to ultra-high temperature conditions. These events have been well-characterized in the central and western parts of the complex; however, the south-southwestern region has been little studied. Samples from the southern Scott and Raggatt Mountains were selected for zircon U-Pb dating by Secondary Ion Mass Spectrometry and whole-rock geochemical analysis. Tonalitic gneiss from the Raggatt Mountains and trondhjemitic gneiss from the southern Scott Mountains have protolith ages of 2711 ± 5 Ma and 2726 ± 6 Ma, respectively. In contrast, tonalitic samples from the southern Scott Mountains have protolith ages of ca. 2539 ± 5, 2533 ± 5 and 2522 ± 9 Ma. The younger generation of magmatism overlaps the timing of metamorphic zircon growth between 2540 and 2440 Ma observed in this study. The geochemistry of ca. 2720 Ma gneisses indicates that magmatism was generated by medium-pressure melting (>1.5 GPa) of basaltic crust, whereas ca. 2530 Ma magmas were generated at lower pressures (<1.2 GPa). The ca. 2.7 Ga magmatism may be associated with continental crust formed at 2.8 Ga, or be newly formed crust that was assembled with older prior to or during a tectonometamorphic event at ca. 2.5 Ga. Tonalitic magmatism at 2.53 Ga during the first half of the 2585–2420 Ma metamorphic event may be a product of partial melting during high-grade metamorphism. Although protolith ages across the complex are still scarce, differences between regions raise the possibility that the complex contains crustal domains of different ages and origins that may not have been assembled until the ca. 2.5 Ga tectonothermal event.

Neoarchean reworking of Mesoarchean and Paleoarchean crust (3.4 ∼ 3.0 Ga) within the North China Craton: Constraints from zircon U-Pb geochronology and Lu-Hf isotopes from the basement of the Bohai Bay Basin

The Bohai Bay Basin basement is mainly composed of Archean granitoid gneisses with minor supracrustal rocks and is the largest basin in the Eastern Block of the North China Craton. Due to a cover of Mesozoic and Cenozoic strata, little is known about the age and crustal evolution of this basement. In this study we report new zircon SHRIMP and LA-ICP-MS U-Pb and Lu-Hf isotope data for drill core samples, including TTGs (granodiorite, tonalite gneiss, trondhjemite gneiss), granites (monzogranite, syenogranite) and a leptite, with the aim of revealing the Archean crustal evolutionary history of the Bohai Bay Basin basement. The U-Pb age of magmatic zircons from these granitoids reveals that basement rocks were mainly generated by two-stage events at ∼3.1 Ga and ∼2.5 Ga. The εHf(t) values of ∼3.1 Ga magmatic zircons vary from +0.56 to +8.27, and their corresponding single-stage model ages range from 3.3 Ga to 3.0 Ga. The εHf(t) values of ∼2.5 Ga magmatic zircons range from −12.87 to −0.07, their corresponding two-stage model ages range from 3.8 Ga to 2.9 Ga with most ages from 3.4 Ga to 3.0 Ga. The Hf isotopic characteristics show that the crustal growth of basement beneath Bohai Bay Basin occurred mainly between 3.4 Ga and 3.0 Ga, different from crustal accretion ages of 2.9–2.7 Ga on the periphery of the Bohai Bay Basin. However, both areas were reworked by the ∼2.5 Ga tectono-thermal event. Integration of this new data from the basin basement with previous data, indicates that the Eastern Block of the North China Craton may be controlled by a mantle plume during the ∼2.5 Ga period. The results from this study are significant in assessing the tectonic environment of the eastern basement in the North China Craton.

Paleo-Mesoarchean crustal growth and reworking in the western Dharwar Craton, southwestern India: Evidence from trondhjemitic gneiss and granitic gneiss

Paleo-Mesoarchean felsic orthogneisses are widely exposed in the southern part of the western Dharwar Craton (WDC), southern India shield. The felsic orthogneisses studied in this paper from the WDC can be divided into two groups based on their geological, geochronological, and geochemical data: the 3.4–3.3 Ga trondhjemitic gneiss (including trondhjemitic veins) and the ∼3.1–3.0 Ga granitic gneiss. The 3.4–3.3 Ga trondhjemitic gneiss is characterized by high contents of SiO2 (70.4–73.1 wt%), Na2O (5.49–5.91 wt%) and Sr (647–740 ppm), high ratios of Na2O/K2O (4.7–7.7) and Sr/Y (53–106), but relatively lower Mg# values (32–39), and Cr (9.5–28.7 ppm) and Ni (1.34–5.33 ppm) concentrations, as well as insignificant Eu anomalies, fractionated REE patterns and mostly positive zircon εHf(t) values (+0.8-+5.9). These data indicate that the trondhjemitic gneiss was derived from partial melting of juvenile mafic crust in a relatively moderate-shallow depth, with residual mineral assemblage of garnet, amphibole and clinopyroxene, implying crustal growth event at 3.4–3.3 Ga. By contrast, the ∼3.1–3.0 Ga granitic gneiss is characterized by relatively lower contents of Na2O (3.82–4.63 wt%) and Sr (96.9–291 ppm), low ratios of Na2O/K2O (1.4–1.7) and Sr/Y (3.02–8.65), as well as significantly negative Eu anomalies and negative to positive zircon εHf(t) values (-5.6-+4.5). Therefore, the granitic gneiss was probably derived from partial melting of pre-existing crustal rocks, suggesting a prominent crustal reworking event at ∼3.1–3.0 Ga. This study emphasizes that the western Dharwar Craton is dominated by Paleoarchean (3.4–3.2 Ga) crustal growth event and a strong crustal reworking event in early Mesoarchean (∼3.1–3.0 Ga), though the Mesoarchean crustal growth event is discernible, evidenced by regional ∼3.0 Ga TTG rocks. The early Mesoarchean (∼3.1–3.0 Ga) crustal reworking event, represented by series of potassium-rich granites, is also reported in other Archean cratons, and thus it may have global significances.

Petrogenetic links between rare metal-bearing pegmatites and TTG gneisses in the West African Craton: The Mangodara district of SW Burkina Faso

We describe the geological context of rare metal-bearing pegmatites from the Mangodara district (South-West Burkina Faso, Paleoproterozoic West African Craton) and discuss their petrogenesis and links with the host rocks. The Mangodara district exposes a gneiss-granitoid complex structured in a regional-scale dome, mantled by granodioritic gneiss enclosing rafts of amphibolite, micaschist and paragneiss, and cored by tonalitic to trondhjemitic gneisses. These gneisses enclose granitoid plutons, and four populations of rare metal-bearing pegmatites: titanite-allanite-, apatite-zircon-, garnet-columbite (Li, Nb) and garnet-REE (Ti, Y, HREE)-bearing varieties.Rafts of migmatitic amphibolite, micaschist and paragneiss are chemically equivalent to nearby Birimian greenstone belts. An origin of the gneiss-granitoid complex by partial melting is suggested by diffuse contacts of rafts with the gneisses as well as the presence of garnet-bearing or hornblende-bearing leucosome in paragneiss and in amphibolites, respectively. Textural continuity between pegmatites and granitic veins concordant to the foliation of the gneisses point to syntectonic segregation of the pegmatite-forming magmas.The compositions of gneisses and plutonic rocks spread between a Na-rich pole and a K-rich pole. Granodioritic gneiss, hornblende-biotite granodiorite and potassic biotite granitoids define a K-rich series attributed to relatively low-pressure high-degree partial melting of dominantly paragneiss, which accounts for low to absent LILE and HFSE fractionation compared to the paragneiss. Tonalitic-trondhjemitic gneiss, a biotite trondhjemite and a peraluminous two-mica trondhjemite belong to a Na-rich series characterized by low K content and strong depletion in REE and HFSE, which might reflect plagioclase clustering after partial melting at a relatively high pressure of amphibolite, and fractionation of HFSE-REE-bearing minerals. Thermodynamic and mass balance modeling further corroborate these propositions implying that partial melting of paragneiss and amphibolite at about 5 to 10 kbars for a temperature of 780 to 850 °C followed by melt/solid segregation and fractional crystallization are susceptible to generate the Na- and K-rich series.U-Pb dating of zircon from a titanite-allanite-bearing pegmatite at 2094.3 ± 8.8 Ma and of apatite from tonalitic-trondhjemitic gneiss, apatite-zircon-bearing pegmatite and granodioritic gneiss at 2094 ± 21 Ma, 2055 ± 20 Ma, and 2041 ± 33 Ma respectively, confirm that partial melting, melt segregation and crystallization-cooling occurred during the Eburnean orogeny.Based on these data, we propose that (i) titanite-allanite- and apatite-zircon-bearing pegmatites result from syntectonic segregation of residual melt during crystallization of the tonalitic-trondhjemitic gneiss, (ii) garnet-columbite-bearing pegmatites originate from syntectonic melt segregation from the migmatitic paragneiss, and (iii) garnet-REE-bearing pegmatites derive from segregation of a residual melt within the granodioritic gneiss.

Geochronology, geochemistry and petrogenesis of the Neoarchean magmatism in the Jiefangyingzi area, northern North China Craton: Implications for crustal growth and tectonic affinity

The Bayan Obo-Duolun-Chifeng-Kaiyuan fault is considered as the craton-orogen boundary between the northern North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB). However, locations of this boundary from Duolun, Chifeng to Kaiyuan have not been well constrained. The Jiefangyingzi area northeast to Chifeng is located at the junction of the Bainaimiao arc belt and northern NCC, and has long been regarded as part of the Bainaimiao arc belt of the CAOB. This paper presents new LA-ICP-MS zircon U-Pb geochronological, whole-rocks geochemical and Nd-Hf isotopic data of some recently identified Neoarchean magmatic rocks from the Jiefangyingzi area. These rocks consist mainly of amphibolites, dioritic gneisses, quartz dioritic gneisses, tonalitic gneisses and trondhjemitic gneisses formed at 2569–2536 Ma and minor amphibole plagioclase gneisses formed at ca. 2640 Ma, and are distributed in an east–west belt that is about 13 km long and 1.5 km wide. Geochemical and Nd-Hf isotopic compositions suggest that the amphibolites were derived from partial melting of a depleted mantle that was metasomatized by subduction-derived fluids with significant crustal contamination, and the dioritic gneisses were generated by partial melting of mantle peridotite metasomatized by slab-melts. The quartz dioritic and tonalitic gneisses originated from high degree partial melting of the subducted oceanic crust with garnet-bearing amphibolite facies in the residue, while the trondhjemitic gneisses originated from low degree partial melting of the thickened mafic lower crust with eclogite facies in the residue due to underplating of mafic magmas. The occurrences of Neoarchean basement rocks, combined with previous Nd-Hf isotopic data of the late Paleozoic magmatic rocks, suggest that the Jiefangyingzi area has a tectonic affinity with the Eastern Block of the NCC rather than the CAOB, and the craton-orogen boundary between the northern NCC and the CAOB should be situated in places north to the Jiefangyingzi area. Our results support that the late Neoarchean (2.6–2.5 Ga) is also a period of crustal growth as well as extensive reworking of the 2.9–2.7 Ga ancient crust in the northwestern margin of the Eastern Block of the NCC. In addition, the transition from tonalite-trondhjemite-granodiorite (TTG) and other potassium-poor granitoid gneisses to crustal-derived potassium-rich granitoid gneisses at the end of Neoarchean (ca. 2.53 Ga) marked the initial cratonization of the NCC and ca. 2.53 Ga is a critical transformation period of maturation of the continental crust in the NCC.

Early crustal evolution of the Yangtze Block: Constraints from zircon U-Pb-Hf isotope systematics of 3.1–1.9 Ga granitoids in the Cuoke Complex, SW China

The newly identified Cuoke Complex preserves voluminous early Precambrian granitoids, bearing key information for the early evolution of the Yangtze Block, South China. Here, we report new field observations, petrology, zircon U-Pb-Hf isotopes and trace elements for eight granitoids from this complex. The analyzed zircon domains are of magmatic origin, as evidenced by their oscillatory zoning, high Th/U ratios and magmatic rare earth element patterns. Three trondhjemitic gneisses yield crystallization ages of 3.11–3.06 Ga, documenting the oldest known rocks in the southern Yangtze Block. Their εHf(t) values vary from −3.2 to + 2.6 with depleted mantle two-stage (TDM2) model ages of 3.6–3.3 Ga, indicating reworking of Paleoarchean crustal materials. Three gneissic granitoids crystallized at 2.86–2.85 Ga and reveal a significant episode of granitoid magmatism in the late Mesoarchean. They possess positive εHf(t) values (+0.2 to +5.0) and slightly older TDM2 model ages (3.3–3.0 Ga), suggesting that these granitoids were most likely derived from remelting of juvenile continental crust. Two monzogranites with crystallization ages of 1.94 and 1.89 Ga display negative εHf(t) values (−13.0 to −8.1) and older TDM2 model ages (3.3–3.1 Ga), indicating substantial involvement of ancient crustal materials. Available zircon U-Pb-Hf isotopic data from the Cuoke Complex have revealed that the earliest crustal growth probably occurred in the early Paleoarchean, with significant crustal growth and reworking during the early Mesoarchean, followed by multistage crustal reworking during the late Mesoarchean to Paleoproterozoic. Integrating the results of this study with records from other Archean basement complexes in the Yangtze Block (i.e., Kongling, Zhongxiang, Douling, Yudongzi and Phan Si Pan), we propose that these complexes have diverse formation histories and generally experienced tectonothermal events at ca. 2.0–1.9 Ga, probably responding to the global-scale assembly of the Nuna supercontinent.

The complexities of Mesoarchean to late Paleoproterozoic magmatism and metamorphism in the Qixia area, eastern North China Craton: Geology, geochemistry and SHRIMP U-Pb zircon dating

Qixia is a typical area of early Precambrian basement in eastern Shandong Province, eastern North China Craton. Many TTG (tonalite-trondhjemite-granodiorite) assemblages were once considered to be supracrustal rocks (the Jiaodong Group), and the formation ages of the rocks have only been determined in a few outcrops as shown on the early geological map of the area. We carried out geological mapping, geochemical study and SHRIMP U-Pb zircon dating in order to determine the temporal and spatial distribution and origins of the TTG rocks. In the newly compiled geological map (1:50,000), the main rock types of the Archean basement are ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga tonalitic gneisses with local occurrences of trondhjemitic gneisses, granodioritic gneisses, (quartz) dioritic gneisses and meta-gabbro showing the same age range. Supracrustal rocks with ages of ∼2.9 Ga and ∼2.5 Ga are locally identified. All rocks broadly extend in a NW-SE direction as a result of strong tectonothermal events of the late Neoarchean and late Paleoproterozoic. Although the late Paleoproterozoic tectonothermal event strongly influenced all 2.7 to 2.9 Ga rocks in the area, metamorphic zircon ages are not widely recorded in these rocks because the high-grade metamorphism at ca. 2.5 Ga caused the older 2.7 to 2.9 Ga rocks to become relatively dry systems. The three generations of TTG rocks are similar in major element composition, characterized by high Na2O and low K2O, except for the late Neoarchean granodioritic gneisses, which locally occur and are relatively high in K2O. All the TTG rocks of different ages commonly have zircon O isotopic compositions within the range determined by Valley and others (2005) for Archean magmatic zircon. The ∼2.9 Ga TTG rocks show large Sr/Y and La/Yb variations and depletion in whole-rock Nd and zircon Hf isotopic compositions. The ∼2.7 Ga TTG rocks are similar in Nd-Hf isotopic compositions to the ∼2.9 Ga TTG rocks but have low Sr/Y and La/Yb ratios. The ∼2.5 Ga TTG rocks are similar in trace element composition to the ∼2.9 Ga TTG rocks, showing large variations in Sr/Y and La/Yb ratios. They can be further subdivided into two types in terms of Nd-Hf isotopic compositions with the depleted type mainly including tonalitic gneisses [eNd(t) = +1.86 to +4.59, eHf(t) = +1.0 to +8.7] and the enriched type including trondhjemitic and granodioritic gneisses [eNd(t) = −2.38 to −0.06, eHf(t) = −1.6 to −2.9]. It is concluded that the ∼2.9 Ga TTG rocks were formed in an oceanic environment (oceanic plateau or intra-ocean subduction), and the 2.7 Ga TTG rocks were formed by mantle underplating that resulted in partial melting of lower crustal mafic rocks under relatively low pressure conditions. More ancient continental materials played a role in the ∼2.5 Ga magmatic process, but more work is required to determine the tectonic environment (underplating or arc magmatism).

Synchronous late Neoarchean Na- and K-rich granitoid magmatism at an active continental margin in the Eastern Liaoning Province of North China Craton

The appearance of voluminous K-rich granitoid rocks during Late Archean marks gradual maturation and stabilization of the continental crust. While most K-rich granitoid magmatism followed TTG magmatism, they occur synchronously sometimes, and this relationship is crucial for our understanding of the evolution of late Archean continental crust and its geodynamic setting.In this study, a series of ~2.57–2.52 Ga coeval and diverse granitoid rocks, including quartz dioritic-trondhjemitic and granodioritic to monzo-/syenogranitic gneisses, were identified in the southern Fushun area of Eastern Liaoning Province, North China Craton. The ~2.57 Ga quartz dioritic gneisses show moderate MgO (≤ 3.96 wt.%) and moderate to high Mg# (37.3–75.5). Geochemical modeling, together with mildly fractionated REE patterns and negative Eu anomalies and depleted zircon ƐHf(t2) (+2.1 − +7.4), suggest that they were differentiated from a depleted mantle source that was metasomatized by slab-derived fluids. The younger trondhjemitic gneisses (~2.55–2.52 Ga) are divided into two subgroups, i.e., an older subgroup (~2547–2540 Ma) characterized by mildly fractionated REE patterns and negative Eu anomalies, and a younger subgroup (~2533–2517 Ma) with strongly fractionated REE patterns and positive Eu anomalies. The trondhjemitic gneisses lack evidence for magma differentiation, and they are inferred to have been formed by reworking of amphibolites/greywackes at diverse crustal levels, with some inputs of mantle materials in the earlier subgroup. The coeval ~2550–2529 Ma granodioritic and monzo-/syenogranitic gneisses are characterized by high K2O/Na2O (0.67–2.45) but low MgO and Mg# (mostly <2 wt.% and < 50, respectively). Geochemical modeling data indicate that the granodioritic gneisses are less differentiated, which could have been derived from the partial melting of high-K mafic rocks (e.g., regional late Neoarchean calc-alkaline meta-basaltic rocks), as further supported by the constant A/CNK (1.00–1.14) and zircon ƐHf(t2) (+2.0 − +9.3) values. In comparison, the monzo-/syenogranitic gneisses show variable A/CNK (0.78–1.32) and zircon ƐHf(t2) (−2.4 − +7.8) values. They are explained to be formed by the partial melting of mixed sources of high-K mafic and sedimentary rocks, with the primary magmas showing plagioclase and apatite fractionation.Taken together, the above granitoid rocks record gradually decreasing zircon ƐHf(t2) values and increasing crustal thickness. Considering the petrogenetic information, regional geological data and the presence of some ~3.45–2.70 Ga crustal materials, the late Neoarchean granitoid magmatism of southern Fushun were likely generated via complex crust-mantle interaction processes at an active continental margin. It is further emphasized that Archean active continental margins are key sites for the initial maturation of the crust, and this tectonic scenario acted as a trigger for granitoid diversification during both the subduction and subsequent collisional stages.

Lu-Hf analyses of zircon from the Makoppa Dome and Amalia-Kraaipan area: implications for evolution of the Kimberley and Pietersburg blocks of the Kaapvaal Craton

Abstract Previously dated zircon crystals from the Amalia-Kraaipan granite-greenstone belts and Makoppa Dome were analysed for their Lu-Hf isotopic characteristics to refine the geological evolution of these areas. Samples from the Makoppa Dome, belonging to the Pietersburg Block, largely fall within the epsilon Hf-age range for granitoids from the eastern part of the block. However, the oldest 3.01 to 3.03 Ga trondhjemitic gneisses show that reworking of juvenile mafic crust started earlier in the western than the eastern part of the block, suggesting a diachronous tectonic evolution. The three granitoids from the Amalia-Kraaipan area fall within the field for Pietersburg and Kimberley block granitoids. Contribution from older crustal material is seen in a 3.08 Ga schist, likely derived from a volcanic protolith, from the Madibe Belt, in the far east of the Kimberley Block, with a mantle extraction age of 3.25 to 3.45 Ga. The data suggest that the Kimberley Block, like the Pietersburg Block, also contains (minor) ancient crustal components, derived from a depleted mantle source prior to 3.1 Ga. The new data suggest that the Kimberley and Pietersburg blocks underwent a very similar Paleo- to Mesoarchean crustal evolution, with a major crust formation event at 3.1 to 3.0 Ga followed by successive crust reworking until 2.77 Ga. Lavas of the Ventersdorp Supergroup, for which zircon grains from a ca. 2.75 lapilli tuff give εHfi of +2, are the first evidence of a juvenile source, after 300 Myr of crustal reworking.

The nature of subduction system in the Neoarchean: Magmatic records from the northern Yangtze Craton, South China

The Neoarchean is an important era during which plate tectonics began to operate widely on Earth. The style of plate tectonics in the Neoarchean should be different but poorly constrained. Here we report geochemical studies on felsic to mafic igneous rocks from the northern Yangtze Craton, which record a subduction at ca. 2.65–2.70 Ga with distinctive magmatic and geodynamic features from modern subduction system. The metaigneous rocks of felsic composition, trondhjemite gneisses (TTG), from the northern margin show high sodic contents, positive Eu anomalies, high (La/Yb)cn and Sr/Y ratios, low Yb and Y contents. The metaigneous rocks of mafic-intermediate composition from the northern margin are mainly calc-alkaline and show less fractionated REE patterns. They have similar zircon U-Pb ages of 2651 ± 30 Ma to 2703 ± 9 Ma, similar chondritic zircon εHf(t) values and whole-rock εNd(t) values of −1.6 to 3.4. Their zircon δ18O values are mostly 4.6–6.6‰, similar to or slightly higher than normal mantle values. Additionally, they all record a metamorphic event at ca. 2.47–2.50 Ga. The potassic granites from the interior are A-type in geochemistry and show similar zircon U-Pb ages and εHf(t) values to the metaigneous rocks in the northern margin but obviously lower zircon δ18O values from 3.6‰ to 5.4‰. The two types of rocks exhibit quartz δ18O values of 12.0–15.2‰ and 3.5–8.2‰, respectively. The trondhjemite gneisses and calc-alkaline metaigneous rocks were derived from partial melting of the subducted crust and the metasomatized mantle wedge, respectively. The A-type granites are the products of high temperature magmatism in an extensional setting in response to the contemporaneous subduction. This demonstrates the development of a subduction system in the northern Yangtze Craton during 2.65–2.70 Ga. The contemporaneous development of both slab-derived and mantle wedge-derived magmatism is suggested as a feature of the Neoarchean subduction system that is distinguished from the modern ones. The lack of TTG magmatism at 2.50–2.65 Ga but the reburst of TTG and metamorphism at 2.47–2.50 Ga imply that the subduction in the Neoarchean is intermittent.

Crust-mantle geodynamic origin of ~2.7 Ga granitoid diversification in the Jiaobei terrane, North China Craton

The crust-mantle geodynamic origin of global late Archean granitoid diversification remains enigmatic. A series of early Neoarchean diversified granitoids were newly identified in the Jiaobei terrane of the North China Craton, including ~2749–2739 Ma Na-rich quartz dioritic, tonalitic, and trondhjemitic (DTTG) gneisses, and ~2718–2701 Ma K-rich quartz monzodioritic, granodioritic, and monzogranitic gneisses.The quartz dioritic and TTG gneisses commonly show transitional field relationships. They have low K2O/Na2O (0.19–0.59), and are characterized by decreasing MgO, FeOT, TiO2, P2O5, and CaO, changing Eu anomalies from negative to positive, and more fractionated REEs, with increasing SiO2. Both the quartz dioritic and TTG gneisses possess depleted zircon εHf(t2) values (+1.4 to + 7.0). Through quantitative petrogenetic modeling, a fractional crystallization model is invoked, i.e., the quartz dioritic gneisses were derived from partial melting of a depleted mantle source metasomatized by slab-derived melts, whereas TTG gneisses (a low silica and a high silica group) formed by different degrees of fractionation from the quartz dioritic magmas, with inferred liquidus minerals of hornblende, clinopyroxene, plagioclase, and apatite. In comparison, the K-rich granitoids show higher K2O/Na2O (0.69–2.87), with some containing euhedral muscovites. The quartz monzodioritic and granodioritic gneisses have higher MgO and Mg#, but lower Fe* (FeOT/(FeOT + MgO)) than the monzogranitic gneisses. The zircon εHf(t2) values (-0.5 to + 5.6) are lower than those of DTTG gneisses. It is suggested that these K-rich granitoids formed by partial melting of either a metasomatized mantle source (with recycled sediments) or metasedimentary rocks at diverse crustal levels.Early Neoarchean crust-mantle interactions of the Jiaobei terrane are reconstructed, including (1) earlier slab-mantle wedge interactions under a continental marginal arc, forming quartz dioritic and the derivative tonalitic and trondhjemitic rocks; (2) ongoing convergence led to high relief of the arc mountain, facilitating erosion and reworking/recycling of sediments; and (3) partial melting of coupled crust-mantle system following accretion between the above arc system and regional ~2.9 Ga or older continental nucleus generated different K-rich granitoids. It is emphasized that the construction and erosion of thickened arcs are prone to the formation of both high pressure TTGs and K-rich granitoids, leading to the late Archean granitoid diversification globally.

Mesoarchean geodynamic regime evidenced from diverse granitoid rocks in the Anshan-Benxi area of the North China Craton

The petrogenesis and inner genetic relations of Mesoarchean granitoid rocks have been poorly researched in the Anshan-Benxi (AB) area of the North China Craton. These Mesoarchean granitoids are mainly composed of ~3.14 Ga Lishan trondhjemitic gneisses (LSTG), and 2.95–3.01 Ga Donganshan – Jinjialing trondhjemitic to monzogranitic gneisses (DTMG) and Tiejiashan – Gongchangling high-K granitoid gneisses (TGKG), which show essentially consistent ages of magmatic crystallization.The LSTG are characterized by SiO2 (70.71–73.72 wt%) and Al2O3 (14.25–15.99 wt%) contents, and low MgO (0.46–1.12 wt%) contents with low Mg# values (32–49) and K2O/Na2O values of 0.43 to 0.66, and total rare earth elements (TREE) of 97 to 151 ppm with (La/Yb)N ratios of 14.19 to 83.15. Integrated with Hf–O isotope characteristics from previous research and partial melting simulation, the geochemical characteristics of LSTG indicate that the trondhjemitic magma was generated by the partial melting of the source consisting of dominant ~3.3 Ga juvenile crustal material and some Paleo- to Eoarchean sediments at the lower crust level. In contrast with the LSTG, the DTMG shows higher SiO2 (73.10–75.99 wt%) contents, slightly lower Al2O3 (13.41–15.00 wt%) and MgO (0.21–0.61 wt%) contents with Mg# values (18–48), higher K2O/Na2O values (0.53–1.26) and similar TREE contents of 85 to 162 ppm, (La/Yb)N ratios (30.78–125.02), and εHf(t2) values from −5.38 to 3.16. Detailed petrogenetic investigations suggest that the magma of the DTMG was derived from the partial melting of the source comprising main ~3.3 Ga basaltic material and more Mesoarchean sediments at the lower crust level, then experienced slight plagioclase fractionation crystallization. The TGKG is characterized by the highest SiO2 (72.86–77.57 wt%) concentrations, but the lowest Al2O3 (11.96–14.47 wt%) and MgO (0.01–0.54 wt%) contents with K2O/Na2O values of 0.86 to 2.53 and Mg# values of 2 to 41. The wide TREE contents (142–766 ppm) and (La/Yb)N ratios (6.41–86.87), εHf(t2) value range of −9.71 to −2.14, suggesting that the magma of TGKG formed from the partial melting of abundant Eo- and Paleoarchean recycled sediments and igneous rocks. This petrogenesis of TGKG magma indicates that Eo- to Paleoarchean supracrustal material may exist under the Tiejiashan – Gongchangling area.Integrating previous studies on the AB area, these diverse granitoids record two episodes of major crust growths at ~3.65–3.8 Ga and ~3.3 Ga. Subsequently, the geological processes were dominated by the recycling of ~3.3 Ga basaltic material and Eo- to Paleoarchean crustal materials during ~3.14–2.95 Ga. The petrogenesis of Mesoarchean diverse granitoids indicates that voluminous supracrustal materials with different ages were transferred into the lower crust level during 2.95–3.14 Ga, revealing that pre-subduction buckling convergence may be the most probable Mesoarchean geodynamic regime in the AB area of the North China Craton.

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.

The late Neoarchean transition from low- to high-K granitoids in western Liaoning Province, NE China: Clues from geochemistry and zircon U–Pb–Hf–O isotopes

High-K granitoids are among the most abundant rock types in many Archean cratons. Late Neoarchean monzogranitic to syenogranitic gneisses with high-K affinities are widely distributed in the Anshan, Suizhong, Qinhuangdao, and Aolaishan areas on the northeastern margin of the North China Craton (NCC). In this contribution, we present an integrated study of zircon U–Pb–Hf–O isotopic compositions and whole-rock elemental compositions of amphibolites and trondhjemitic and monzo–syenogranitic gneisses of the Jinzhou area in the metamorphic basement of eastern Hebei–western Liaoning, with the aim of constraining their petrogenesis and geodynamic setting. Emplacement ages of the amphibolites and trondhjemitic–monzogranitic gneisses are 2543 ± 27, 2532 ± 19, and 2513 ± 7 Ma, respectively. The amphibolites are tholeiitic in composition with SiO2 contents of 49.7–50.8 wt%, variable degrees of light rare-earth-element (LREE) enrichment and high-field-strength element (HFSE) depletion, and high zircon εHf(t) values of +2.6 to +6.3, suggesting a depleted lithospheric mantle origin. The major- and trace-element compositions of the trondhjemitic gneisses are similar to those of the low-pressure tonalite–trondhjemite–granodiorite (TTG) suite. The zircon Hf (εHf(t) = +1.6 to +3.9) and O (δ18O = +3.76‰ to +5.73‰) isotopic compositions of the trondhjemitic gneisses indicate a juvenile basaltic source at the base of a thickened magmatic arc. The monzogranitic gneisses differ from their TTG counterparts in that they have lower SiO2 and higher MgO, K2O, and incompatible-element (especially Ba, Th, Sr, P, and LREE) contents. They also have slightly evolved zircon εHf(t) values (+0.6 to +3.8) and higher δ18O values (+4.69‰ to +6.13‰). These features suggest that the monzogranitic gneisses represent sanukitoid-type rocks, with a mantle source modified by crust-derived melts. The weakly deformed syenogranitic gneisses are characterized by high SiO2 and K2O, and very low MgO, Cr, and Ni contents, suggesting that they were formed by partial melting of local TTG rocks. Our results, together with those of previous investigations, suggest that the 2554–2513 Ma low- to high-K magmatism in the Jinzhou area most likely originated in an arc–back-arc tectonic setting on the northern margin of the NCC. The large volumes of high-K granitoids in eastern Hebei–western Liaoning are related to extensive mantle–crust interactions and crustal reworking in such a setting.

Evidence for a late Cambrian juvenile arc and a buried suture within the Laurentian Caledonides of Scotland: Comparisons with hyperextended Iapetan margins in the Appalachian Mountains (North America) and Norway

Abstract Uranium-lead (U-Pb) zircon dating establishes a late Cambrian (Drumian) protolith age of 503 ± 2 Ma for a trondhjemitic gneiss of the calc-alkaline Strathy Complex, northern Scottish Caledonides. Positive εHf and εNd values from trondhjemitic gneisses and co-magmatic amphibolites, respectively, and an absence of any inheritance in zircon populations support published geochemistry that indicates a juvenile origin distal from Laurentia. In order to account for its present location within a stack of Laurentia-derived thrust sheets, we interpret the complex as allochthonous and located along a buried suture. We propose that a microcontinental ribbon was detached from Laurentia during late Neoproterozoic to Cambrian rifting; the intervening oceanic tract closed by subduction during the late Cambrian and formed a juvenile arc, the protolith of the Strathy Complex. The microcontinental ribbon was reattached to Laurentia during the Grampian orogeny, which transported the Strathy Complex as a tectonic slice within a nappe stack. Peak metamorphic conditions for the Strathy Complex arc (650–700 °C, 0.6–0.75 GPa) are intermediate in pressure between those published previously for Grampian mineral assemblages in structurally overlying low-pressure migmatites (670–750 °C, &lt;0.4 GPa) that we deduce to have been derived from an adjacent backarc basin, and structurally underlying upper amphibolite rocks (650–700 °C, 1.1–1.2 GPa) that we interpret to represent the partially subducted Laurentian margin. This scenario compares with that of the northern Appalachian Mountains and Norway where microcontinental blocks are interpreted to have their origins in detachment from passive margins of the Iapetus Ocean during Cambrian rifting and to have been re-amalgamated during Caledonian orogenesis.

A Neoarchean K-rich granitoid belt in the northern North China Craton

The Archean metamorphic basement in the southeastern part of Eastern Hebei to Western Liaoning provinces, northern North China Craton, is dominated by Neoarchean K-rich granitoids, with a small amount of Neoarchean supracrustal rocks and Na-rich dioritic-tonalitic-trondhjemitic-granodioritic gneisses, and minor Paleo- to Mesoarchean rocks. The Paleo- to Mesoarchean lithological signatures include TTG gneisses and supracrustal rocks at the Caozhuang and Caochang areas. The Neoarchean K-rich granitoids form a ca. 350 km long belt extending in a NE direction, and primarily consist of porphyritic monzodioritic-quartz monzodioritic-granodioritic-monzogranitic (MQGM) gneisses and secondary monzogranite-syenogranites and charnockites, with minor Neoarchean TTG gneiss enclaves. Based on their major mafic mineral phases, the MQGM gneisses can be divided into an amphibole-dominated group and a biotite-dominated group. Zircon U-Pb isotopic dating reveals that the Caozhuang trondhjemitic gneisses, amphibole-dominated MQGM gneisses, and biotite-dominated MQGM gneisses were emplaced at ∼2.90 Ga, 2.55–2.53 Ga, and 2.55–2.53 Ga, respectively. The amphibole-dominated porphyritic MQGM gneisses are geochemically analogous to sanukitoids, and the sanukitoid magma was derived from the partial melting of lithospheric mantle metasomatized by dehydration fluids and melts from subducted slabs and related sediments, following the fractionation of amphibole and clinopyroxene during their magmatic evolution. The magmatic precursors of the biotite-dominated MQGM gneisses were most likely generated by the partial melting of medium- to high-K mafic-intermediate rocks and locally generated by the partial melting of metagreywackes at medium pressures. Within the K-rich granitoid belt, the 2.55–2.53 Ga porphyritic MQGM gneisses experienced 2.53–2.51 Ga E-W extension and strike-slip shearing deformation in the Jinzhou-Xingcheng structural domain, and diapiric upwelling by the formation of dome structures in the Jielingkou-Anziling-Suizhong structural domain. All these structures then suffered a top-to-the-SE-NEE thrusting in the northern margin during 2.51–2.50 Ga. The 2.53–2.51 Ga dioritic-tonalitic-trondhjemitic-granodioritic gneisses and charnockites in the Taipingzhai-Santunying-Zunhua structural domain firstly underwent diapiric upwelling during 2.51–2.50 Ga, then together with the 2.52–2.50 Ga volcanic-sedimentary rocks in the Shuangshanzi-Qian’an-Lulong structural domain overprinted by nearly N-S-striking dextral strike-slip shearing deformation during 2.50–1.80 Ga. The temporal-spatial relationships, petrogenesis, and structural patterns of these various Neoarchean lithological assemblages confirm that the Neoarchean K-rich granitoid belt in the northern North China Craton was most likely developed in an active continental margin setting, and our studies suggest this involved progressively southeastward multi-stage retreating oceanic slab subduction and collisions between an oceanic arc and an ancient continental nucleus.

Sm-Nd and Rb-Sr Datings, Petrogenesis and Thermometry of the Ngovayang Area (South-West Cameroon): Isotopic Data Insight of Recycling Crust and Convergence Orogen

The Ngovayang trondhjemitic gneisses belong to the Nyong Complex at the Northwest boundary of the Ntem Complex. A combined study of whole-rock major, trace elements and isotopic data, Sm-Nd and Rb-Sr indicate that trondhjemitic gneisses are peraluminous and range from weakly I-type to S-type with nearly constant A/CNK values between 1.04 and 1.24. Their I- and S- type characters suggest that juvenile materials have been partially remobilized or recycled. These trondhjemitic gneisses have slightly to moderate MgO, Cr and Ni contents, ISr (0.703677 - 0.741911) and low eNd(t) (from -16.48 to -10.6) values. Such geochemical features suggest a small mantle-source contribution, coupled with assimilation of some upper and lower crustal materials and indicate the implication of old crust probably with both Archean and Early eburnean origin (2597 - 2318 Ma). They also exhibit geochemical features typical of calc-alkaline, crustal contaminated calcalkaline rocks, volcanic arc and trondhjemitic nature. These results show that old crust has recycled and trondhjemitic gneisses are linked to convergent geodynamic system. The petrogenetic diagrams of AFM vs CFM, MgO vs SiO2 and Rb/Ba vs Rb/Sr and lower K2O/Na2O ratio values (0.38-0.51), indicate that the magmatic source materials are mainly composed of plagioclase-rich sources such as metagreywackes - metabasic mixtures, metagreywackes and metabasics and basalt or igneous rocks. These sources materials occurring in the melt originated from both slab melting and assimilation of upper and lower crust at ca. 732.4 - 928.7°C and at 2597 - 2318 Ma. This study shows that Archean crust may have a North-West extension beyond the actual boundary with Nyong Complex.