Tonalitic Gneisses Research Articles

Zircon U-Pb Age and Hf-O Isotope Evidence of the Taihua Complex on the Southern Margin of the North China Craton: Implications for Its Magmatic Process During the Early Paleoproterozoic

The Xiaoqinling is the largest terrane of the Taihua Complex and records Paleoproterozoic silicic magmatism, which is one of the most potential areas for understanding secular changes in tectonic processes during the Tectono-Magmatic Lull (TML). In this study, we use zircon U–Pb dating, Lu–Hf and O isotopic analysis to constrain the age, provenance, and magmatic processes of the protoliths for the Paleoproterozoic granitic gneiss, tonalitic gneisses, and migmatitized tonalitic gneiss from the Xiaoqinling area. The isotopic composition of zircon provides a key proxy for querying the crustal record. Integration of our new U–Pb and Hf–O isotope data with available published data from other regions of the North China Craton (NCC), allows us to better assess the secular changes of the North China Craton during the magmatic lull at ∼2.3 Ga. This study shows that the protolith of the tonalitic gneisses in the study area have crust-like Hf–O isotopic compositions with slightly negative to positive εHf(t) (−3.0− +2.1) and low to moderately elevated δ18O values (2.71–7.87‰), indicating a mixed origin of continental crust assimilated by mantle-derived magmas.

Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology

AbstractThe oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar).

Review of temporal trend in Archean granitoid chemistry- implications for Eoarchean (3.7–3.9 Ga) transition in geodynamic regime of crust formation

In this study, we review changes in Archean granitoid chemistry using stacked probability density estimation and statistical change-point analyses. A significant change in Archean sodic granitoid composition occurs in the 3.9–3.75 Ga period, marked by a shift of SiO2, Al2O3, Na2O, V, Cr, Sr, Sr/Y to higher, and TiO2, FeOT, MnO, P2O5, A/NK, A/CNK, Sc, Ba, Ta, Nb, Hf, Y, and rare earth elements to lower values, which reflects different formation P-T regime and petrogenetic processes. Using phase equilibria and trace element modelling, we demonstrate that the pre-3.9 Ga Acasta Tonalite Gneiss (ATG)-like granitoids formed at shallow depths (∼3 km) but are chemically distinct from impactites of all known large impact craters with basaltic/mixed target rocks. Impact melting produces MgO-rich melts at relatively greater “apparent” depths, reflecting target rock chemistry and contribution from the upwelling mantle, and is unlikely to have produced the shallow-seated ATG rocks. The ATG trace element chemistry is similar to many modern-day non-arc settings felsic rocks, which suggests that the melting regime in these extensional settings also existed before 3.9 Ga and ATG formed in a mantle plume-driven extensional tectonic regime involving melting of long-lived mafic proto-crust at shallow crustal depths above mantle upwelling. In contrast, the 3.9–3.75 Ga transitional and post-3.75 Ga TTGs formed by deep-seated (25–50 km) melting in equilibrium with garnet-bearing amphibolite. When considered together with Ti isotopic evidence of shift from tholeiitic to calc-alkaline magmatism in the 4.0–3.75 Ga period, the evidence for a transition from a regime dominated by crustal reworking to one of significant juvenile input at 3.60–3.85 Ga, noted in trace element/Hf-isotope composition of detrital zircon from several cratons and the reported ultra-high pressure metamorphism/interleaving arc-plume sequence, require a greater depth of melting. We interpret this transition to mark a gradual shift from stagnant-lid to intermittent mobile-lid-type plate tectonics involving deeper subduction of oceanic lithosphere. However, from the single occurrence of pre-3.9 Ga ATG rocks and their similarity with some Archean Fe-rich silica-saturated rocks chemistry, it is unknown whether plume magmatism was the only active mechanism or operated in conjunction with mobile-lid tectonics on early Earth, like in Archean.

Late Neoarchean plume-induced crust-mantle interaction and crustal reworking: Evidence from the Jianping Complex, North China Craton

The Archean basement of the North China Craton (NCC) is predominantly composed of 2.6–2.5 Ga lithological assemblages, while the tectonic regime that governed the formation and evolution of these Neoarchean basement rocks has been a controversial issue. Based on field relationships, petrographical features, whole-rock chemistry and zircon Hf isotope data, diverse late Neoarchean granitoids exposed in the Jianping Complex of the NCC are categorized into TTGs, sanukitoids and potassic granites. Their petrogenesis are associated with partial melting of metabasalts, melting of metasomatized mantle, and melting of crustal lithologies including tonalite and metasediments, respectively. Zircon U-Pb results suggest that different types of Neoarchean granitoids in the Jianping Complex are nearly coeval with crystallization ages of ∼ 2.54–2.52 Ga and metamorphic ages of ∼ 2.49–2.48 Ga. New Hf isotopic data reveal that these granitoids show positive εHf(t) values, with the tonalitic gneisses of εHf(t) values closer to the depleted mantle than those of granodioritic gneiss, sanukitoids and potassic granite. This result reflects potentially more enriched source for other granitoids than the tonalitic gneisses, which is linked with increasing level of crustal-mantle interaction and crustal reworking the late Neoarchean. Integrating other evidence from geochemical, structural and metamorphic studies, it is inferred that the plume activities play an important role in the emplacement of late Neoarchean granitoids in the Jianping Complex, with TTGs formed through the partial melting of juvenile crust induced by plume activity, sanukitoids produced from melting of metasomatized mantle resulted from sagduction process, and potassic granites formed by reworking of earlier crustal lithology caused by sustained underplating of mantle material. Overall, we interpreted that the late Neoarchean crustal growth and crustal-mantle interaction in the Jianping Complex can be attributed to upwelling of mantle plume and potential sagduction process in a vertical tectonic regime.

Exotic vs local Caribbean origin for the arc-related Mabujina Amphibolite Complex (Cuba): implications for segmentation of the Caribbean arc during the Cretaceous

ABSTRACT The Mabujina Complex exposed in south-central Cuba consists of an elongated body of metamorphosed volcanic and subvolcanic mafic rocks and tonalitic gneisses. The complex is largely composed of greenschist facies rocks (Porvenir Formation) directly below the Cretaceous volcanic arc section and of a deeper sequence of deformed amphibolite facies rocks (Mabujina Amphibolite Complex). The origin of the protolith of the Mabujina Complex has been the subject of intense debate in the past. While some authors consider the Mabujina Complex to represent the deepest and oldest (Early Cretaceous) exposed section (root) of the Cretaceous Caribbean volcanic arc others suggested an exotic (non-Caribbean related) Early Cretaceous volcanic arc section. The later proposal implies the tectonic emplacement underneath the Caribbean arc of a segment of the Pacific-related Early Cretaceous Mexican Guerrero volcanic arc terrane. A new set of whole-rock geochemical analyses presented in this study shows that the protoliths of the Mabujina Amphibolite Complex constitute an island-arc sequence with a) less abundant island-arc tholeiitic mafic rocks similar to the Early Cretaceous tholeiitic island arc of the Caribbean and b) dominant transitional to calc-alkaline mafic and felsic igneous rocks that resemble the Early to Late Cretaceous magmatism of the Caribbean arc. The high initial Nd ratios of the rocks from the Mabujina Amphibolite Complex are indistinguishable from the tholeiitic to calc-alkaline sequences from the Caribbean arc, reflecting a depleted MORB mantle source with limited subduction-related component input. The present data, together with published geochemical and geochronological data of the protoliths, allow identifying a Caribbean origin for the Mabujina Complex instead of an exotic provenance in the Guerrero Arc of western Mexico. However, even if the interpretation of the geochemical data favours a Caribbean origin, the Mabujina Amphibolite Complex cannot represent the root of the volcanic arc. Here, we propose that the Mabujina Amphibolite Complex is a fragment of the Early to Mid-Cretaceous Caribbean arc that was tectonically emplaced below this same arc as a likely consequence of arc segmentation and strike-slip trans-compression triggered by oblique subduction during the mid-Cretaceous.

U–Pb AGE OF DETRITAL ZIRCON IN THE CEMENT OF EARLY PRECAMBRIAN POLYMICTIC CONGLOMERATES OF CENTRAL KARELIAN DOMEN, KARELIAN PROVINCE, FENNOSCANDIAN SHIELD

U–Th–Pb LA-ICP-MS was used for dating detrital zircons extracted from the cement of the Neoarchean and Palaeoproterozoic polymictic conglomerates of the Karelian province, Fennoscandian shield. For the first time obtained a chemical composition of terrigenic and volcanogenic rocks of the gimolskaya formation. Obtained data showed that the detrital zircons from the cement of conglomerates of different ages yield similar results of about 2750 Ma. In the Neoarchean conglomerates of the Sukkosero area dominate clasts of the gneissic tonalite and plagioporphyry. They are identical in chemical composition with the Neoarchean (2.78–2.73 Ga) TTG, granodiorite, and metadacite of the Central-Karelian domain. The feature of these rocks is increased content of Ba (900–1200 ppm), Sr (600–700 ppm), and ratios Sr/Y = 34–90, (La/Yb)n = = 17–30. The chemical composition of the cement (the values of CIA 55 and the negative values of the function DF(x)) is similar to the composition of clasts, which means cement is composed of finer clasts of the same rocks. The Paleoproterozoic conglomerates of the Vottomuks area contain zircon from felsic rocks of the Neoarchean basement/ The chemical composition of the cement of the Paleoproterozoic conglomerates and the predominance of basaltic andesites of sumii is evidence of the post-sumii (2.45 Ga) age of these rocks. The absence of the Paleoproterozoic zircon in cement can be the result of the absence of it in the Paleoproterozoic source rocks. The age of Neoarchean conglomerates from the gimolskaya formation is less than 2.75 Ga.

Nature and extent of shear deformation along the eastern and southern terrane boundaries of the Motloutse Complex, eastern Botswana

The Motloutse Complex at the southwestern margin of the Zimbabwe Craton is an Archean medium-grade allochthonous terrane in eastern Botswana. The high-grade gneisses of the Limpopo Complex lie east of the Motloutse Complex and those of the Mahalapye Complex to the south. A number of shear zones are postulated to delineate the aerial extent of the Motloutse Complex, and separate it from the cratonic domain and high-grade terranes. Except the northern margin, these terrane boundaries are poorly understood. This study aims to characterize the nature and extent of shear deformation along the eastern (Dikalate Shear Zone) and southern (Sunnyside Shear Zone) boundaries of the Motloutse Complex. Representative areas from the eastern (Masikate Hills), and southern (Radisele-Mogome) regions were selected for geologic mapping. In view of the poor exposures outside these areas, aeromagnetic data is used to delineate the regional extent of the shear zones. Shear deformation is prominent along the N–S elongate Masikate Hills made up of tonalite gneiss, amphibolite, anorthosite and ultramafic rocks. A dominant right lateral sense of movement, with minor left lateral movement is inferred for the shear deformation. The extent of the N–S trending Dikalate Shear Zone, and its branches, is delineated in the aeromagnetic imagery.In comparison, the Radisele-Mogome area, exposes two set of rocks – tonalite gneiss, amphibolite, ultramafic rocks (part of the Motloutse Complex) to the NE, and granodiorite, diorite gneiss, leucogranite (part of the Mahalapye Complex) to the SW. This is well exemplified in the aeromagnetic image, with change in magnetic signature between rocks in the NE and SW parts. Shear deformation variably overprints rocks along the contact zone. Mylonitic deformation is prominent in the SW part. As along the eastern terrane boundary, a dominant right lateral sense of shear movement is inferred. The extent of the NW-SE trending Sunnyside Shear Zone and its branch is delineated in the aeromagnetic imagery. The results highlight an example of using a combination of geological and geophysical methods to delineate the nature and extent of shear zones that constitute terrane boundaries.

Geochemistry, geochronology and metamorphism of high-pressure mafic granulites in the Huai'an Complex, North China Craton: Implications for the tectonic evolution of the Paleoproterozoic orogeny

The Paleoproterozoic was a critical tectonic transition stage for the North China Craton (NCC), with several collisional orogenic belts identified. However, the tectonic boundaries, temporal-spatial evolution, and geodynamic mechanisms of these orogens remain controversial. In this contribution, we document a newly identified largest high-pressure (HP) mafic granulite outcrop at Xizhaojiayao in the Huai'an Complex, which is composed of metamorphosed supracrustal (volcano-sedimentary) sequences, including HP mafic granulites (metabasalts), marbles (metalimestones), quartzites (metasilicalites), and pelitic granulites (metapelites). They occur as tectonic slices within Neoarchean migmatized tonalitic gneisses. The Xizhaojiayao HP mafic granulites show different field occurrences and rock assemblages from the dike-type HP mafic granulites that are widely documented in the Huai'an and Hengshan Complexes. Microstructural studies, geothermobarometer calculations, and phase equilibrium modeling of the HP mafic granulites defined a clockwise pressure–temperature (P–T) path with prograde evolution from amphibolite-facies (M1, 5.2–5.5 kbar/538–668 °C) to HP granulite-facies (M2, 11.2–12.2 kbar/850–880 °C), followed by a significant near-isothermal decompression to medium-low-pressure granulite-facies (M3-1, 8.4–9.1 kbar/800–900 °C; M3-2, 4.9–5.6 kbar/779–805 °C). Zircon/titanite U–Pb dating constrains peak HP metamorphism, granulite-facies decompression, and amphibolite-facies cooling retrograde at 1902–1909 Ma, ∼1830 Ma, and < 1818 Ma, respectively. Whole-rock major and trace-element compositions, as well as positive εNd(t) (+2.0 to + 4.9) and zircon εHf(t) values (+2.9 to + 8.8) of these HP mafic granulites, indicate a mid-ocean ridge basalt geochemical affinity, whereas an island-arc signature is common for the dike-type HP mafic granulites. The Xizhaojiayao supracrustal sequences, which represent dismembered and metamorphosed upper oceanic crustal fragments, were subducted from the upper crust to depths of 35–40 km, followed by rapid exhumation to the middle-shallow crust and were tectonically involved in the melt-weakened tonalitic crust. These tectono-metamorphic scenarios illustrate the Paleoproterozoic orogenic evolution of a consecutive subduction-collision-exhumation process from 1.95 to 1.80 Ga, which led to the final assembly of the NCC.

Tensile strength of anisotropic rocks from enhanced Brazilian laboratory testing and data analysis protocols

The tensile strength of rock and rock-like materials is a critical material property in rock engineering design and the prediction of rockmass behaviour to mitigate any potential failure that may affect personnel safety or damage property. This study presents new instrumentation guidelines to the Brazilian Tensile Strength (splitting tensile strength) test to enhance the data processing techniques to determine true tensile strength. For this study a total of eighteen (18) specimens of two lithologies (Pointe Du Bois tonalite gneiss and Key Anacon metabasalt) were instrumented, tested, and reviewed to determine the true tensile strength of the material. This paper also identifies three potential failure modes and presents a failure envelope based on the identified potential failure modes.

A newly identified cryogenian (ca. 806 ma) basement tonalite gneiss from the Eastern Karakoram, NW India: Constraints from geochemistry and zircon U-Pb geochronology

The Karakoram Terrane (KT) represents the southern margin of the Eurasian Plate, mainly consisting of Late Jurassic-Early Cretaceous subduction-related granites and post-collisional Miocene leucogranites, which intrude the Late Neo-Proterozoic basement. We report for the first time the existence of the Cryogenian KT basement as recorded from the geochemistry and geochronology of tonalite gneiss (ca. 806 Ma) in the southeastern Karakoram terrane, NW India. Geochemically, the studied tonalite gneiss is slightly peraluminous (Molar Al2O3/CaO+Na2O+K2O=1.1), calc-alkaline volcanic-arc granitoid, strongly fractionated REE (LaN/YbN=33.99), and high Sr/Y =19.75, more akin to its affinity with Tonalite–trondhjemite–granodiorite (TTG)/adakite. The whole-rock elemental data suggest that tonalite gneiss is more likely sourced from ancient mafic lower crust where garnet remained in the residue. The petrogenetic modeling of REE suggests that the melt similar to the observed tonalite gneiss can be generated through ∼50% partial melting of a mafic lower crust with garnet, clinopyroxene, and amphibole assemblage. The synthesis and comparison of present and published Proterozoic magmatic records on the rocks from KT strongly dictate that the produced partial melt similar to observed tonalite gneiss most likely served as the parental melt for the development of TTGs in the Southern Pamir and more evolved granitoid in the Central Tibetan terrane. We propose that the studied tonalite gneiss from the southeast Karakoram is a product of Neoproterozoic Andean-type orogeny formed on the northwestern margin of the Rodinia supercontinent. Thus, our study favors the first time, the position of KT within the Cimmerian belt along with other East Asian continental blocks.

Integration of Geological, Petrographic and Abrasion Resistance Data of the Gneisses from Santa Catarina Granulithic Complex: An Application in the Road Paving Sector

Among the factors that contribute to the adequate performance of the aggregates, the abrasion resistance of these materials stands out, a parameter strongly related to the mineral composition of the rock. Therefore, this work aims to analyze and integrate data from Los Angeles abrasion tests to the petrographic analysis of gneisses of the Santa Catarina Granulithic Complex in order to understand the petrographic characteristics that can interfere or control the resistance results, such as: composition, texture, mineralogical alterations, granulometry and microfractures. The work was developed in four stages: (1) field activities, where descriptions of the outcrops of the four quarries studied were performed, focusing on the characterization of the compositional, textural and structural aspects of the rocks, in addition to sample collection and photographic survey; (2) petrography via conventional optical microscope and BSE images and semi quantitative analysis by electron microscope; (3) analysis of Los Angeles abrasion test data samples from the studied quarries and (4) their integration with petrographic data. Two main lithologies, enderbitic and tonalitic gneiss were identified. The results of the Los Angeles abrasion tests showed that the crushed aggregates (gneiss) of Gaspar Quarry presented the lowest abrasion values, followed by the Vale Selke, Rio Branco and 1001 quarries. Regarding composition, the enderbitic gneiss presented an average loss of 17.2%, while in the tonalitic gneiss the loss was 17.7%. The composition (high content of mafic minerals), texture and the presence of localized microfractures were the main factors that contributed to the higher values found in quarry 1001. However, both lithologies present acceptable abrasion results, in view of the specifications of the regulatory services, DNIT (031/2006) and DNIT (141/2010). The present study shows a good correlation between the result of the Los Angeles abrasion and the petrographic data, showing that composition is the main determining factor of Los Angeles abrasion results.

Delineating the dismembered remnants of Archean layered complexes in medium-high-grade terranes: Scenario from the Motloutse Complex, eastern Botswana

Ultramafic-mafic rocks in medium-to high-grade metamorphic terranes commonly occur as fragmented units of once continuous layered intrusions. A number of factors can complicate the delineation of the dismembered remnants and reconstruction of the original lithostratigraphy. This study focuses on remnant ultramafic rocks of the Mesoarchean Lechana layered complex in the southern Motloutse Complex, eastern Botswana. In view of the limited exposures, including the prominent cover rocks, geologic characterization was supplemented with geophysical techniques. The NE-SW traverse covered in the study includes the Tamasane and Sapolamorori hills, on either side of the cover rocks. Both hills are comprised of layered ultramafic rocks, with tonalite gneiss and amphibolite forming the surrounding rocks. The inferred layered lithostratigraphy includes serpentinized dunite, peridotite, peridotite-pyroxenite, pyroxenite-peridotite, and pyroxenite. Magnetitite seams constitute the main mineralization, and are associated with the serpentine-rich rocks. Aeromagnetic data provide insights into the bulk magnetic susceptibility of the ultramafic lithologies along the transect. The highest magnetic susceptibilities occur as semi-circular closed patches. The two anomalies at either end of the transect correlate with the layered ultramafic bodies at the Tamasane and Sapolamorori hills. The rest approximately correlates with high Ni ± Cr ± Cu soil anomalies reported from the region. This redefines the known extent of the Lechana layered complex beneath the cover rocks. The study highlights the potential of integrating geological and geophysical data to delineate the dismembered remnants of Archean layered complexes.

Defining the contact zone between Archean-Proterozoic terranes with distinct geologic histories: The scenario from eastern Botswana

Understanding when Archean and Proterozoic terranes of distinct geologic histories became proximal to each other is an important topic to investigate. This study aims to define the contact zone between the southern Motloutse Complex, an Archean medium-grade terrane with prominent Neoarchean-Mesoarchean rock units, and the northern Mahalapye Complex, a Paleoproterozoic high-grade granitoid-gneiss terrane, in eastern Botswana. The contact zone between the two terranes is obscured by cover rocks. Field reconnaissance aided with remote sensing and aeromagnetic imaging delineated the NW-SE trending Radisele-Mogome corridor beneath the cover rocks. Tonalite gneiss with amphibolite and layered ultramafic rock xenoliths dominate the NE part, and are common rock types in the southern Motloutse Complex. Granodiorite with diorite gneiss xenoliths dominate the SW part, and are extensions of the NW-SE elongate c.2.04 Ga Mokgware pluton in northern Mahalapye Complex. The c.2.02 Ga Mogome granite intrudes across the contact zone, and contains xenoliths of rocks from both complexes. Shear deformation and metasomatism variably overprint the rocks along the contact zone. U-Pb zircon and 40Ar/39Ar amphibole–biotite geochronology were carried out on representative rock types to understand the chronology of events along the contact zone. The 2056 ± 40 Ma (MSWD = 5.2; n = 56) diorite gneiss from the NE margin of the contact zone contains c.2.6 Ga and c.2.9 Ga xenocrystic zircons. No Archean zircons were found in the 2041 ± 88 Ma (MSWD = 8.4; n = 13) diorite gneiss from the SW margin of the contact zone. The 2036 ± 12 Ma (40Ar/39Ar amphibole) and 2039 ± 11 Ma (40Ar/39Ar biotite) ages, respectively from the NE and SW margins, argue for the c.2.04 Ga age to represent the first common event along the contact zone. Shear deformation and metasomatism are respectively dated at 1987 ± 7 Ma and 1951 ± 11 Ma (40Ar/39Ar biotite), and account for the Pb-loss in dated zircons. It is argued that the Archean Motloutse Complex and Paleoproterozoic Mahalapye Complex terranes came together at c.2.04 Ga, with the emplacement of syntectonic Mokgware granodiorite. The results highlight the factors that are important to define the contact zone between Archean-Proterozoic terranes with distinct geologic histories.

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 to Rhyacian crustal records along the Middle Xingu River area, Amazonian craton

The Carajás (3.0–2.5 Ga) and Xingu-Iricoumé (1.99–1.86 Ga) blocks comprise the Central Amazonia Province (CAP) that is in contact with the Maroni-Itacaiúnas Province (MIP) within the Amazonian craton. The CAP is the oldest portion (Nd-TDM) of the craton and corresponds to an Archean nucleus bordered by younger Paleo-Mesoproterozoic mobile belts, including the MIP. Because the location and tectonic boundaries between these provinces are insufficiently known, we carried out a geological survey along the Middle Xingu River, cutting the WNW-ESE regional trend, to further understand cratonic evolution of the MIP and its southeastern boundary in this key area. Geochronologic results (Pb-evaporation and U-Pb SHRIMP in zircon and monazite), supported by petrographic and field observations, allowed identification of the following lithotypes and their ages: migmatitic gneisses (2859–2080 Ma), tonalitic gneisses (2554 ± 3 Ma, 2480 ± 9 Ma), enderbites (2114 ± 3 Ma), charnockites (2094 ± 4 Ma, 2084 ± 2 Ma), granodiorites (2079 ± 3 Ma), leucogranitic vein (2075 ± 2 Ma), and pelitic paragneisses (2062 ± 8 Ma). These ages are related to the reworking of Archean crust during Rhyacian magmatic arc amalgamation (2.22–2.13 Ga) and collision in the Transamazonian cycle (ca. 2.1 Ga).

Source characteristics of Late Neoarchean diatexite in the Yishan area, western Shandong

鲁西沂山地区高级深熔混合岩中存在丰富的不同时代、不同类型的岩石包体。在详细野外地质工作基础上，本文对高级深熔混合岩中的包体进行了锆石SHRIMP U-Pb定年和Hf同位素分析及全岩地球化学研究，以期探讨沂山地区高级深熔混合岩的源区性质。英云闪长质片麻岩形成时代为~2.7Ga，存在2.69Ga和2.53~2.51Ga两期基性岩浆作用，并记录了2.52~2.50Ga变质作用。角闪斜长片麻岩与黑云角闪变粒岩互层产出，它们可能为变质火山岩，其岩浆锆石年龄为2.51Ga。变质基性岩的岩浆锆石ε_Hf（t）为0~+8.1，一阶段Hf模式年龄为2.83~2.54Ga；变质火山岩的岩浆锆石ε_Hf（t）为-1.2~+5.8，一阶段Hf模式年龄为2.89~2.71Ga。结合前人研究，这套高级深熔混合岩的源区组成包括了新太古代早期的英云闪长质片麻岩和变质辉长岩，新太古代晚期的石英闪长质片麻岩、变质火山岩及斜长角闪岩等岩石类型。~2.5Ga基性岩浆作用在鲁西地区发育广泛，为新太古代晚期深熔作用及高级深熔混合岩和壳源花岗岩的形成提供了热源。;There are abundant enclaves of different ages and types within diatexite in the Yishan area, western Shandong. Based on field work, this paper carried out zircon SHRIMP U-Pb dating and Hf isotope analysis and whole-rock geochemical studies on enclaves in the diatexite. A tonalitic gneiss enclave has a formation age of ~2.7Ga. Two episodes of Neoarchean mafic magmatism are recognized at 2.69Ga and 2.53~2.51Ga, respectively, and they have metamorphic ages of 2.52~2.50Ga. Hornblende-plagioclase gneiss was interbedded with fine-grained biotite-hornblende gneiss, and these two types of gneisses are considered to be meta-volcanic rocks. The hornblende-plagioclase gneiss has a magmatic zircon age of 2.51Ga. The magmatic zircon from the meta-gabbro has ε_Hf(t) values of 0~+8.1 with t_DM1 of 2.83~2.54Ga, whereas the magmatic zircon from the hornblende-plagioclase gneiss has ε_Hf(t) values of -1.2~+5.8 with t_DM1 of 2.89~2.71Ga. Combined with previous work, it is speculated that the source of the diatexite includes Early Neoarchean tonalitic gneiss and meta-gabbro, and Late Neoarchean quartz diorite gneiss, meta-volcanic rock and amphibolite.~2.5Ga mafic magmatism is widely developed in western Shandong, which provides the heat source for the Late Neoarchean anataxis, resulting in the formation of diatexite and crustally-derived granite.

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

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.

Paleoarchean evolution of the Singhbhum Craton, eastern India: New constraints from geochemistry and geochronology of granitoids of Bonai and Champua area

The Singhbhum Craton, one of five major Archean cratons in the Indian subcontinent, contains abundant well-preserved Paleoarchean supracrustals and granitoids. This study presents zircon U-Pb ages and whole rock geochemistry of tonalite-trondhjemite-granodiorites (TTGs) and granites from the Bonai Granite Complex (BGC) and Older Metamorphic Tonalite Gneiss (OMTG), which are separated from each other by the Jamda-Koira-Noamundi Iron Ore Group (IOG) supracrustals. Emplacement ages obtained in this study indicate that a major episode of TTG magmatism took place in BGC around 3368 ± 8 Ma (1σ), followed by granitic magmatism around 3331 ± 33 Ma (1σ). In contrast, a TTG from the Deo Nala area representing OMTG yielded crystallization age of 3312 ± 8 Ma (1σ). The emplacement and evolution of the BGC were coeval with granitoid magmatism from the central part of the Singhbhum Craton. Whole rock geochemical data identify both high- and low-HREE TTGs in both the BGC and OMTG to the west and east of the IOG basin, respectively. The trace element systematics of high-HREE Bonai TTG are similar to those of Icelandic dacites, suggestive of their derivation from a garnet free, plagioclase rich amphibolite. The low-HREE TTGs of the BGC and OMTG were derived from an amphibolite source with varying amounts of garnet. The potassic granites of the BGC were sourced from the older TTGs which had undergone partial melting at a shallow depth. The evolution of the BGC and OMTG can be attributed to the partial melting under a thickened mafic crust. Dome and keel structures and emplacement ages of granitoids from the west and east of the Jamda-Koira-Noamundi IOG basin, support the origin of these Paleoarchean granitoids in a stagnant lid regime. High geothermal gradients induced by heat supplied by mantle upwelling appear to have induced the melting of the thickened crust, to form the TTG. Delamination induced mafic–ultramafic underplating resulted in melting of early formed TTGs, to form the younger potassic granites of the BGC at ∼ 3.33 Ga.

Zircon U–Pb geochronology and Hf isotopic compositions of Palaeoproterozoic meta‐granitoids in the Lesser Himalaya, Nepal: Tectonostratigraphic implications

The Lesser Himalayan Belt in the northern margin of Indian Shield preserves abundant sedimentary and magmatic records related to the assembly and breakup of the Columbia supercontinent. However, the Palaeoproterozoic tectonic history of the northern margin of the Indian Shield and its position in the Columbia supercontinent are still controversial. Based on detailed geological investigations in the Dailekh area of western Nepal, we conducted zircon U–Pb dating and Lu–Hf isotopic analysis for meta‐granitoids that intruded into the Lesser Himalayan Crystalline and Metasediments. The Dungeshworl granitic augen gneiss, meta‐monzogranite, and tonalitic gneiss yielded crystallization ages of 1,829 ± 12 Ma, 1,856 ± 11 Ma, and 1,852 ± 11 Ma respectively, which demonstrate that the deposition limits of both the Lesser Himalayan Crystalline and Metasediments are the Palaeoproterozoic. Zircon εHf(t) values of these meta‐granitoids range from −3.99 to +0.18, with corresponding depleted mantle two‐stage model (TDM2) ages of 2.41–2.62 Ga, indicating that they formed by reworking of Neoarchean to Palaeoproterozoic crust. Synthesizing existing geochronological, petrochemical, and zircon Lu–Hf isotopic data for metamorphic sedimentary and igneous rocks from the entire Lesser Himalayan Belt, we suggest that the Palaeoproterozoic magmatism in the northern margin of Indian Shield may be formed within subduction‐related tectonics responded to the assembly of Columbia supercontinent.