Ga TTGs Research Articles

Neoarchean granitoid magmatism and geodynamic process in the northeastern North China craton

Abstract The composition of Archean granitoid rocks changed from predominantly tonalite-trondhjemite-granodiorite (TTG) gneisses in the early Archean (4–3 Ga) to diversified granitoid rock assemblages in the late Archean (3.0–2.5 Ga), marking a crucial transformation in the geodynamic processes of early Earth. However, the reason for this major transition remains enigmatic because the petrogenetic features of different granitoid assemblages and their crust-mantle interactions during different periods are poorly understood. We use variations in the spatial-temporal distribution, lithological association, chemical composition, and petrogenesis of Neoarchean (2.7–2.5 Ga) granitoids and inferred correlative crust-mantle interactions in the Eastern Liaoning Range (ELR) of the northeastern North China craton to explore this geodynamic transition. The early Neoarchean (ca. 2.7 Ga) ELR granitoids were dominated by TTG gneisses, and the late Neoarchean (2.6–2.5 Ga) ELR granitoid typology and compositions became more complex, changing into TTGs and more K2O-rich granitoid rocks. The TTGs can be subdivided into a high-Ca group and a low-Ca group: The 2.71–2.68 Ga high-Ca group TTG magma originated from partial melting of subducted juvenile oceanic crust, and the low-Ca group TTG magma was derived from fractionation crystallization of the high-Ca group TTG magma. The chemical composition of the magmatic sources played a dominant role on the 2.60–2.50 Ga TTG magmatism: the high-Ca and low-Ca group TTG magmas came from low-K mafic rocks and tonalites, respectively. The 2.58–2.49 Ga K2O-rich granitoids can be divided into three petrogenetic series: (1) The high-Ca-Mg group K2O-rich granitoid magma originated from partial melting of high-K mafic rocks, (2) the low-Ca-Mg group K2O-rich granitoid magma was derived from partial melting of sedimentary rocks, and (3) the transition group K2O-rich granitoid magma was sourced from metagreywackes. The 2.71–2.68 Ga TTGs were generated in an island arc belt, and subducted slab melting and subsequent magmatic differentiation were the dominant mechanisms of the TTG magmatism. The 2.60–2.50 Ga diversified granitoids were formed in the oceanic-continental subduction process under the active continental margin; the complicated oceanic slab subduction and arc-arc and arc-continent collisions contributed to the diversity of late Neoarchean granitoid magmatism.

Petrogenesis of 2.7–2.65 Ga TTGs in the Wutai Complex: Constraints on the Neoarchean crustal evolution of the North China Craton

The Wutai complex, located on the boundary between the Eastern and Western Blocks of the North China Craton (NCC), offers valuable insight into early crustal growth and evolution of the NCC. In this study, a suite of 2.7–2.65 Ga TTG gneisses has been identified in the Wutai complex. Zircon grains from eight samples indicate that their protoliths were emplaced between 2706 Ma and 2640 Ma. The trondhjemites have low K2O/Na2O, high La/Yb and Sr/Y ratios, and positive Eu anomalies. The observed trend of decreasing Dy/Yb and Dy/Dy* ratios with increasing La/Yb ratios support for an amphibole-dominated fractional crystallization process during the TTG formation. Zircon Hf and oxygen isotope analyses (εHf(t) = 3.5–7.0, TDM = 2.86–2.71 Ga; δ18O = 5.5–6.3 ‰) indicate that the parental magma may originate from juvenile crustal sources altered by surface water at low temperature. When combined with the previous studies of volcanic rocks and TTGs formed around 2.5 Ga, it becomes evident that the Wutai complex experienced significant crustal growth during the period from 2.7 to 2.5 Ga. The identification of 2.7–2.65 Ga magmatic activity as well as the older detrital/xenocryst zircons up to 3.6 Ga, provide increasing evidence that the Wutai complex has a much longer and nearly continuous geological history. Combined with geological evidence, zircon data compilations and the Re-depletion model age distribution of the mantle xenoliths of the NCC, it can be inferred that the period from 2.6 to 2.5 Ga was likely the most significant for the crustal generation within the NCC's geological history.

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.

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

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

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

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

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

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

Petrogenesis of newly identified Neoarchean granitoids in the Qingyuan of NE China: Implications on crustal growth and reworking of the North China Craton

• A new ca. 2.7–2.5 Ga granitoid suite was identified in the Qingyuan, NE China. • The 2.68 Ga TTG was derived from ancient lower crust as well as juvenile crust. • Qingyuan granitoids record two periods of crustal growth and reworking events. Discovery of ca. 2.7 Ga tonalite-trondhjemite-granodiorite (TTG) in the North China Craton (NCC) can provide the key clue to understanding the Neoarchean crustal evolution. Most recently, we mapped out ca. 2.7 Ga TTGs from the northern Qingyuan at the northeastern margin of NCC. Together with the associated ca. 2.6–2.5 Ga high-K granites and high-Mg diorites, this study provides new constraints on crustal growth and reworking during the Neoarchean. Zircon U–Pb dating indicates that the TTGs, high-K granites and high-Mg diorites were formed at ca. 2.68 Ga, ca. 2.56 Ga and ca. 2.54 Ga, respectively. All of these lithologies exhibit enrichments in large ion lithophile elements (e.g., Rb and Ba) but depletions in high field strength elements (e.g., Nb and Ta), with positive εHf(t) and εNd(t) values. The TTGs have moderate SiO 2 , low Al 2 O 3 and variable MgO contents with high Sr/Y and (La/Yb) N ratios. They were likely derived from partial melting of mafic lower crust. High SiO 2 and K 2 O but low MgO and Cr contents as well as high Sr/Y and (La/Yb) N ratios reflect an origin of recycling of ancient crustal rocks (e.g., pre-existing TTGs) for the high-K granites. The high-Mg diorites have high MgO and Cr contents but low Sr/Y and (La/Yb) N ratios, which indicate that they could be sourced from partial melting of metasomatized mantle peridotite. Combined with previous studies, the northeastern NCC experienced two major periods of ca. 2.7 Ga and ca. 2.6–2.5 Ga crustal growth and reworking events.

Nickel isotopic composition of the upper continental crust

Establishing the nickel (Ni) isotopic composition of the upper continental crust (UCC) is crucial for using the Ni isotope system to trace biogeochemical processes and understand crust-mantle interactions. This study reports the Ni isotopic composition of eighty-four well-characterized upper crustal samples, including granites, granodiorites, tonalite-trondhjemite-granodiorite (TTG), loess, river sediments and glacial diamictites, to constrain the Ni isotopic composition of the UCC. Significant variations in δ60Ni are revealed for I-type (0.02–0.26‰), A-type (−0.05–0.08‰) and S-type (0.08–0.36‰) granites for the first time. These Ni isotopic variations are attributed to magmatic differentiation for I- and A- type granites and source heterogeneity for S-type granites. The δ60Ni values of fine-grained clastic sediments (including loess, river sediments and glacial diamictites) range from −0.01‰ to 0.23‰. Such δ60Ni variations cannot be explained by Ni isotopic fractionation during chemical weathering because there are no clear correlations between δ60Ni and Ni/Al2O3, or the chemical index of alteration (CIA). Instead, the δ60Ni variations in fine-grained clastic sediments are likely inherited from source rocks. The δ60Ni values of our samples for 3.2–3.5 Ga TTGs (0.00–0.13‰), 2.4–2.5 Ga TTGs (0.04–0.13‰) and < 0.4 Ga granites (excluding S-type granites) are statistically indistinguishable (P < 0.05, student's t-test), implying limited variation of δ60Ni in the felsic igneous UCC since 3.5 Ga. Similarly, the δ60Ni values of glacial diamictites suggest insignificant temporal variation in the weathered UCC since 2.4 Ga. The data gathered in this study combined with literature data yields an arithmetic mean δ60Ni value of 0.12 ± 0.15‰ (2SD) for the UCC (ranging from −0.07‰ to 0.36‰). And the weighted average δ60Ni is estimated to be 0.07 ± 0.10‰ (2SD) or 0.11 ± 0.09‰ (2SD) depending on the assumed δ60Ni of the metamorphic rocks. Thus, a lithology-weighted average δ60Ni needs to be further determined by future studies when the δ60Ni values of metamorphosed sedimentary rocks in the UCC are constrained.

The origin of early continental crust: New clues from coupling Ge/Si ratios with silicon isotopes

The recent discovery of heavy Si isotopic compositions in both high-Na Tonalite-Trondhjemite-Granodiorite (TTG) and High-K Granite-Monzonite-Syenite (GMS) suites of early continental crust requires that a notable seawater-derived silica-rich component had been added to their respective protoliths prior to melting. Here we use the Ge/Si ratio as a complementary tracer to δ30Si in order to delineate the exact role of modal quartz and silicified basalts from the Archean seafloor among the primary controls of the early appearance of felsic melts on Earth. We have approached the question by (1) specifying the Ge/Si signatures of various Archean and post-Archean rock types by compiling the Ge-SiO2 data stored within the GEOROC database; (2) coupling Ge/Si investigation to silicon isotopes on a large selection of silicified and unsilicified altered mafic and ultramafic greenstones, felsic volcanics, TTG and GMS granitoids and mineral separates from TTGs of the Barberton Greenstone Belt (BGB) of South Africa. The GEOROC compilation demonstrates that Archean TTGs and granites display much lower Ge/Si (1.15±0.10 and 1.13±0.11 μmol/mol, respectively) than post-Archean adakites, granites, tonalites and granodiorites (with average Ge/Si in the range of 1.64 to 1.85 μmol/mol). This result is corroborated by the Ge/Si ratios we report from BGB rocks that formed prior to Kaapvaal craton stabilisation, including 3.5 Ga Theespruit Formation felsic volcanic rocks, 3.5-3.2 Ga TTGs and 3.2-3.1 Ga GMSs, all of which exhibit low ratios (0.68±0.23; 0.92±0.17; 1.05±0.19 μmol/mol, respectively). Based on their low TTG-like Ge/Si, pre-cratonic GMSs are dismissed as being derived from melting of both TTG and metasedimentary sources. Instead, TTGs and GMSs originated from similar Ge-depleted sources. Low Ge/Si ratios, coupled to heavy Si isotopic signatures (−0.14‰<δ30Si<+0.27‰) are also a characteristic feature of mafic and ultramafic BGB greenstones subjected to low-temperature hydrothermal seafloor alteration, especially at the interface between silicified (0.2<Ge/Si<1.2 μmol/mol) and unsilicified (1.8<Ge/Si<3.1 μmol/mol) portions of the altered Archean seafloor. We infer that both Na-rich and K-rich Archean felsic melts are derived from a unique class of protoliths: Ge-depleted metabasalts containing a significant modal proportion of supracrustal quartz generated by the silicification of the Eo-Paleoarchean basaltic seafloors. The transition from Na-rich to K-rich felsic melts in the BGB is assumed to be connected to a gradual increase of potassium as a key element associated with seafloor silicification. In contrast, younger (3.07-2.69 Ga), post-cratonic BGB granites have higher Ge/Si (1.93±0.23 μmol/mol) and were generated through the reworking of a TTG-like basement, by incongruent melting of biotite and hornblende (1.79<Ge/Si<2.97 μmol/mol) leaving an oligoclase-rich (0.64<Ge/Si<0.72) residue. The Earth changed from a prevalent Ge-depleted felsic crust in early-middle Archean times to a widespread Ge-enriched post-Archean crust, which emphasizes the importance of late Archean changes in the Earth dynamics. In particular, our data suggest the likelihood of generating primitive felsic continents by rather shallow melting processes, without the need for inducing high pressures by subduction, as long as Archean ocean floors were silicified.

Petrogenesis and implications of ∼2.1 Ga Jingqishan granites in the Jiaobei Terrane, North China Craton

Widely distributed 2.2–2.1 Ga A-type granites in the North China Craton (NCC) have not only a significant role in unveiling the regional Palaeoproterozoic tectonic evolution but also crucial implications for the spatial and temporal evolution of A-type granite. This paper presents zircon U-Pb dating, geochemistry and Nd and Hf isotopes of Jingqishan Palaeoproterozoic gneissic granites from the Jiao-Liao-Ji Belt (JLJB) of the eastern NCC. The Jingqishan granites are mainly monzogranites. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb analyses of three samples yield concordant ages of 2124 ± 7 Ma, 2089 ± 11 Ma and 2088 ± 10 Ma. The Jingqishan granites are ferroan, calc-alkalic to alkali-calcic, and metaluminous-peraluminous. The high Ga/Al ratios, Zr + Nb + Y + Ce contents and Zr saturation temperatures (861 °C on average) of these granites are consistent with the features of A-type granite. The granites have εNd(t) values of −5.7 to −0.8 with two-stage Nd model ages of 2.99–2.63 Ga and zircon εHf(t) values of −7.3 to −2.9 with two-stage Hf model ages (TDM2(Hf)) of 2.94–2.61 Ga. These TDM2(Hf) ages are similar to those of ∼ 2.5 Ga Jiaobei Tonalite-Trondhjemite-Granodiorite (TTG) gneisses. Based on petrology, geochemistry and isotopic characteristics, we infer that the Jingqishan A-type granites are mainly partial melts of the Jiaobei 2.5 Ga TTGs. Combining our data with those published for 2.2–2.1 Ga JLJB granites and extensive data, we discover that both 2.1 Ga and 2.18 Ga A-type granites of the JLJB have A2-type granite characteristics and formed in a back-arc basin but were generated from the partial melting of different sources at two magmatic stages. Most 2.2–2.1 Ga A-type granites of the NCC belong to A2-type that sourced from late Achaean crustal rocks in an extensional setting.

Crustal reworking and Archean TTG generation in the south Gavião Block, São Francisco Craton, Brazil

Origin of early Earth’s felsic crust is a matter of growing debate in which the rock record from old cratons plays a key role. In northeast Brazil, the São Francisco Craton preserves the oldest felsic rocks of South America and understanding its Archean evolution is an important piece of this debate. Here we present new SHRIMP U-Pb zircon ages and whole-rock geochemistry, as well as a large, compiled dataset to discuss the evolution of the south Gavião Block (GB), a major component of this craton. The GB contains Paleo- to Mesoarchean (3.36–3.18 Ga) TTGs, high-K granites, and minor diorites of the Gavião Complex followed by renewed magmatism, reworking and anatexis between 3.27 and 3.22 Ga. The TTG rocks of Gavião Complex can be divided into high-Sr (high pressure) and low-Sr (low- to medium pressure), evidencing that the sodic magmatism likely occurred at different depths diachronically between 3.36 and 3.20 Ga. We report a crystallization age of a ca. 3.35 Ga for an Archean unusual high-K calc-alkaline meta-granite which marks the first potassic magmatism of the Gavião Complex, suggesting reworking of preexisting TTGs. Zircon U-Pb dates from a TTG migmatite constrained an age of ca. 3.27 Ga, interpreted as a metamorphic event within the basement of the GB, stretching back in ca. 120 My the oldest metamorphic age known so far for the whole São Francisco Craton. Additionally, at ca. 2.7 Ga, the GB was intruded by the intraplate, alkaline magmatism of the Caraguataí Suite, marking thus the stability of this Block during Neoarchean. We argue that crustal reworking was the main mechanism responsible for the formation of the GB during the Paleo- to Mesoarchean. The data suggest the GB rocks share an old source possibly including Hadean components. We propose that the formation of the TTGs of GB is a result of the melting of low-K mafic crust at the base of a progressively thicker oceanic plateau.

Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO2 concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t2) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)N values (13.76–98.19) and positive ɛHf(t2) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO2 (69.09–73.46 wt%) and K2O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.

Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments

This Chapter considers triple oxygen isotope variations and their 4 Gyr temporal evolution in bulk siliciclastic sedimentary rocks and in granites. The d18O and D'17O values provide new insights into weathering in the modern and ancient hydrosphere and coeval crustal petrogenesis. We make use of the known geological events and processes that affect the rock cycle: supercontinent assembly and breakup that influence continent-scale and global climate, the fraction of the exposed crust undergoing weathering, and isotopic values of precipitation. New data from a 5000 m Texas drillhole into the Oligocene Frio Formation demonstrate minimal isotopic shifts from mudrocks to shales during diagenesis, mostly related to expulsion of water from smectite-rich loosely cemented sediment and its conversion to illite-rich shale. Inversion of triple oxygen isotope fractionations return isotopic values and temperatures along the hole depth that are more consistent with weathering conditions in the Oligocene and modern North America (d18O = -7 to -15‰, and T of +15 to +45°C) rather than d18O from 8 to 10‰ diagenetic water in the drill hole at 175-195°C. More precise T and d18Owater are obtained where the chemical index of alteration (CIA) based detrital contribution is subtracted from these sediments. Triple oxygen isotopes from suspended sediments in major world rivers record conditions (T and d18Ow) of their watersheds, and not the composition of bedrock because weathering is water-dominated. In parallel, the Chapter presents new analyses of 100 granites, orthogneisses, migmatites, tonalite-trondhjemite-granodiorite (TTG), and large-volume ignimbrites from around the world that range in age from 4 Ga to modern. Most studied granites are orogenic and anatectic in origin and represent large volume remelting/assimilation of shales and other metasediments; the most crustal and high-d18O of these are thus reflect and record the average composition of evolving continental crust. Granites also develop a significant progressive increase in d18O values from 6-7‰ (4-2.5 Ga) to 10-13‰ (~1.8-1.2 Ga) after which d18O stays constant or even decreases. More importantly, we observe a moderate -0.03‰ step-wise decrease in D'17O between 2.1 and 2.5 Ga, which is about half of the step-wise decrease observed in shales over this time interval. We suggest that granites, as well as shales, record the significant advent and greater volumetric appearance of low-D'17O, high-d18O weathering products (shales) altered by meteoric waters upon rapid emergence of large land masses at ~2.4 Ga, although consider alternative interpretations. These weathering products were incorporated into abundant 2.0-1.8 Ga orogens around the world, where upon remelting, they passed their isotopic signature to the granites. We further observe the dichotomy of high-D'17O Archean shales, and unusually low-D'17O Archean granites. We attribute this to greater contribution from shallow crustal hydrothermal contribution to shales in greenstone belts, while granites in the earliest 3.0-4.0 Ga crust and TTGs require involvement of hydrothermal products with lower-D'17O signatures at moderately high-d18O, which we attribute to secondary silicification of their protoliths before partial melting. The Chapter further discusses evolution of the shale record through geologic history and discusses the step-wise change in d18O and D'17O values at Archean/Proterozoic transition. Denser coverage for shales in the past 1 billion years permits investigation of the rocks and their weathering in the last supercontinent cycle, with observed lighter d18O values, characteristic for the mid-Phanerozoic at the initiation of Gondwana breakup. The continuing increase in d18O values of the shales since 4 Ga is interpreted to reflect accumulation of weathering products via shale accretion to continents, as low-density and buoyant shales tend to not subduct back into the mantle. The rock cycle passes triple oxygen isotopic signatures from precipitation to sedimentary, metasedimentary, and finally to anatectic igneous rocks. Continental crust became progressively heavier in d18O, lighter in D'17O due to incremental accumulation of high-d18O sediments in accretionary wedges. Second-order trends in d18O and D'17O are due to supercontinent cycles and glacial episodes.

Constraining provenance for the uraniferous Paleoproterozoic Francevillian Group sediments (Gabon) with detrital zircon geochronology and geochemistry

The Paleoproterozoic sandstones of the ca. 2.1 Ga Francevillian Group, southeast Gabon, host the oldest known redox-controlled high-grade uranium (U) deposits. Uranium-bearing detrital minerals (e.g., monazite) in the Francevillian Group sediments were derived by erosion of Archean basement rocks from the East Gabonian block of the northwestern Congo craton surrounding the Francevillian Basin. They are viewed as primary sources for U, but the nature of these sources remains poorly constrained. Here, U-Pb ages and Hf isotope data for detrital zircon grains are combined with literature geochemical data for igneous rocks from the Archean basement, to characterize the nature, age, and origin of the provenance for the Francevillian Basin and the ultimate source of U for the high-grade uranium deposits.Detrital zircon ages range from ca. 3.1 to 2.6 Ga with a dominant population between ca. 2.90 and 2.80 Ga, suggesting a major contribution from ca. 2.88 to 2.81 Ga high-K granitoids with a limited contribution from ca. 3.1 to 2.84 Ga TTGs and 2.75 to 2.7 Ga granitoids of the East Gabonian block. Most detrital zircon grains share similar depleted-mantle Hf model ages between ca. 3.08 and 3.30 Ga, indicating significant recycling of a Paleoarchean to Mesoarchean crust. We suggest that ca. 2.9 to 2.8 Ga high K, U, and Th granitoids, which reflect hybrid melts of both mantle and crustal parentage were the major detrital sources of U to the Francevillian Basin and, ultimately, for the U deposits. We propose that ca. 2.9 to 2.8 Ga detrital zircons with Hf TDM ages between ca. 3.1 and 3.3 Ga may thus allow for vectoring U deposits in sedimentary basins across the Congo craton.

Building the core of a Paleoarchean continent: Evidence from granitoids of Singhbhum Craton, eastern India

A significant part of many Archean cratons formed during Paleoarchean. Yet, the mechanism and tectonic setting of formation of Paleoarchean continental crust remain highly debated. In this contribution, we present field, petrographic, geochemical, zircon U-Pb age and Hf isotope data on Paleoarchean granitoids from west-central part of the Singhbhum Craton (Champua-Hat Gamharia corridor), India. The whole process starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The core of the craton nucleated with formation of 3.45–3.40 Ga TTGs showing juvenile character (zircon ɛHft = +0.6 to +7.1). These rocks show slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu* = 0.90 to 1.00) and moderate Sr/Y (25–64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. A second generation of TTG formation took place at 3.32 Ga in the area by deeper melting of a juvenile mafic crust (zircon ɛHft = +1.3 to +5.7) as suggested by strongly depleted HREE and Y, and high Sr/Y (52–155) implying significant amount of garnet retention in the residue. Subsequently, the area witnessed intracrustal melting at 3.28 and 3.25 Ga which tapped moderately old to juvenile (zircon ɛHft = −1.9 to +4.5), mostly TTG sources at variable depths generating potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites marks the final cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated crustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. A synthesis of rock assemblage, regional geological setting and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean. Further, a comparison of juvenile crustal growth and crustal reworking events of the Singhbhum and other Indian cratons show that these cratons record distinct evolutionary histories and were probably nucleated at different sites.

Tungsten isotopic constraints on homogenization of the Archean silicate Earth: Implications for the transition of tectonic regimes

The short-lived 182Hf-182W system has been used to constrain the early Earth’s accretion and differentiation. This paper reports high precision W isotopic compositions and trace element concentrations for Archean (4.0–3.0 Ga) intermediate to felsic rocks from the Slave, North China, and Kaapvaal cratons to discuss early Earth differentiation and mantle mixing processes. The 4.0–3.8 Ga diorite and trondhjemite samples from the Acasta Gneiss Complex (AGC) in the Slave Craton and Anshan Complex in the North China Craton have positive μ182W values ranging from 8.3 ± 3.6 to 14.5 ± 4.0, whereas the 3.36–2.95 Ga TTG rocks from the North China Craton have a uniform modern mantle-like μ182W value of 0.7 ± 3.9 (2 SD, n = 11); with one exception, the 3.32 Ga old sample F28-2, which has a positive μ182W anomaly (7.3 ± 3.9 and 13.0 ± 3.2 for two individual analyses). The Earth’s oldest potassic granites, occurring as conglomerates in the Moodies Group, an intrusive trondhjemite, and an amphibolite with ages of ~3.55 Ga in the Kaapvaal Craton exhibit W isotopic compositions indistinguishable from what is proposed for the modern mantle. The positive 182W anomalies in the 4.0–3.8 Ga rocks could possibly be the result of either early mantle differentiation that occurred within the lifetime of 182Hf or a partial lack of late accreted material. The signature of 182W excess in the 3.32 Ga sample (F28-2) is inherited from earlier crust, presumably similar to the 3.8 Ga TTGs, either by melting or crustal contamination. The transition in μ182W values from positive to near-zero would indicates that a significant event occurred at ∼3.6 Ga which caused efficient mixing of early differentiated mantle and late accreted material, possibly indicating the transition of tectonic regimes from plume to plate tectonics on the Earth.

Identification of ca. 2.65 Ga TTGs in the Yudongzi complex and its implications for the early evolution of the Yangtze Block

In this contribution, we carry out an integrated study of petrology, whole-rock geochemistry, as well as zircon morphology, trace element, U–Pb and Lu–Hf isotopes for granitic gneisses and leucogranites from the Yudongzi complex, a poorly studied Archean outcrop in the northwestern Yangtze Block. The granitic gneisses have the TTG affinity characterized by relatively high Na2O/K2O and Sr/Y ratios, and fractionated REE pattern with relatively high LREE but low HREE concentrations. Zircon grains in the granitic gneisses show magmatic cores and metamorphic rims. U–Pb dating on zircon core of four granitic gneiss samples yielded their formation ages as 2660–2632 Ma with a weighted average of 2650 ± 21 Ma (MSWD = 1.6; n = 4). Their average εHf (t) values range from −0.6 to 0.5 and two-stage model ages from 3.20 to 3.10 Ga, suggesting that the granitic gneisses were likely generated by remelting of thickened Mesoarchean proto-mafic crust. The metamorphic zircon rims have relatively high Th/U ratios and identical Hf isotope compositions to their corresponding cores, indicating that the rims were formed via solid-state recrystallization process. The metamorphic zircons recorded a metamorphic event at ca. 2.5 Ga. U-Pb dating and Lu-Hf isotopes on magmatic zircons from a leucogranite sample revealed a magmatic event at 2477 ± 18 Ma. The recognization of the 2.65 Ga TTG magmatism and the 2.48 Ga intercrustal remelting event in the Yudongzi complex demonstrates the widespread Neoarchean magmatism in the Yangtze Block. Combined with the age spectra of previous detrital zircons, we show that the Yangtze Block has experienced a major tectonothermal event in the late Archean. The Yudongzi complex has a distinct crustal evolution history compare with the Paleoarchean continent nucleus in the eastern part of the Yangtze Block, suggesting that the continent nucleus might have subjected to lateral growth by accretion processes.

An Archaean Tonalite–Trondhjemite–Granodiorite Association of the Kursk Block (Voronezh Massif): Composition, Age, and Correlation with the Ukrainian Shield

Framing of the Archaean greenstone belts of the Kursk Block (KB) of the East Sarmatia preserves rocks of the TTG association: those do not form massifs with distinct boundaries, but occur as fields gradually transiting into gneisses and migmatites. According to Sm–Nd isotope–geochemical data, the TTG are characterized by positive values of eNd(2960) = +0.3…+1.6 and protolith model ages of ТNd(DM) = 3100–3200 Ma. Magmatic protoliths of the Kursk Block TTG were formed about 2960 Ma by melting from a juvenile basite source. These age estimates are significantly younger than heterochronous (3.19, 3.13 and 3.07 Ga) TTGs of the Middle Dnieper granite–greenstone terrane. On the other hand, the similarity of eNd(T) implies a single source of their protoliths. Consequently, the KB TTGs, apparently, are a result of transformation of an older sial crust preserved within the Middle Dnieper Block.

Petrogenesis of the Neoarchean diorite-granite association in the Wangwushan area, southern North China Craton: Implications for continental crust evolution

The crustal evolutionary scenarios and geodynamic driving mechanisms of the North China Craton (NCC) during the late Neoarchean (∼2.5 Ga) are still lacking comprehensive understanding due to subsequent strong deformation and metamorphic overprinting events. The widespread ∼2.5 Ga tectono-thermal activities throughout the NCC can be studied to constrain the tectonic evolution at this period. The Wangwushan Neoarchean diorites and high-K granites in the southern NCC were formed at ∼2.52–2.51 Ga. The diorites have high Mg#, Th, Y, low Sr and Sr/Y ratios and variably positive zircon εHf(t) (+2.4 – +8.4) and whole rock εNd(t) values (−0.26 – +3.24), indicating a depleted mantle wedge source which was metasomatized by the melted subducted silicic sediments. The depleted Nb, Ti and enriched LILE and Pb features indicate that the diorites were derived from slightly metasomatized mantle wedge in a subduction-related setting. The high-K granites show a shoshonite affinity and peraluminous features. The rocks have low 10,000 Ga/Al ratios, Zr and Zr + Nb + Ce + Y concentrations, and calculated zircon saturation temperature (TZr = 686–837 °C, 774 °C on average), belong to fractionated I-type granite. Low Sr, Y, flat HREE, and negative Eu anomalies indicate the presence of Ca-rich plagioclase and absence of garnet in the residue during partial melting of the sources, thus further indicating a shallower source with a pressure of <10 kbar and a depth less than 35 km. Zircon Hf and whole-rock Nd isotopic compositions (εHf(t) = −2.7 to +7.4, εNd(t) = +1.25 to +3.09, TDM2 ages are 3.20–2.56 Ga and 3.20–2.65 Ga, respectively) are identical to those of the Neoarchean TTGs, amphibolites and diorites in the Wangwushan area, which likely represent plausible sources for the high-K granites. The partial melting was probably triggered by underplating of mantle-derived mafic magmas in a post-collision setting. Together with 2.57–2.52 Ga TTGs and amphibolites in this area, the magmatic spectrum of these igneous rocks are typical of subduction-related magmatism, involving multi-stage processes in a convergent plate margin, and likely record a transitional regime from early oceanic plate subduction to late post-collisional extension.

Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane

The Pilbara Craton, Western Australia, is one of the best preserved Palaeo- to Mesoarchaean terrains on Earth. The East Pilbara Terrane is the archetypical granite-greenstone belt, the dome-like complexes of which were formed through three major magmatic events. These granite domes are comprised of metamorphosed granitic igneous rocks that exhibit a magmatic evolution from early tonalite-trondhjemite-granodiorite (TTG) rocks towards K-rich granites over the period 3.53–2.83Ga. Accordingly, East Pilbara has been a focus for workers seeking to constrain early Archaean geodynamic processes. One way to inform on this debate is to interrogate successive igneous supersuites using tools sensitive to magmatic source. We present new zircon Hf and whole-rock Nd isotope data from four major supersuites of the Mount Edgar Dome, of the East Pilbara Terrane. Early ca. 3.45Ga TTGs exhibit isotopic signatures that imply their partial derivation from existing crust, with addition of some juvenile material. Subsequent Palaeoarchaean magmatic events show a secular trend towards more evolved isotopic signatures, interpreted as a dominance of increasing reworking of existing crust, with only minor addition of new juvenile crust. The implication of this is that these later Palaeoarchaean supersuites were largely derived from the melting of older granitic crust, with mass balance modelling suggesting an input of ca. 20% juvenile (depleted mantle) material. The limited addition of juvenile material and increased reworking of existing crust with time, does not support a model of modern-style subduction, but is consistent with vertical tectonic processes in a volcanic plateau-type setting for the East Pilbara Terrane >3.2Ga. All Palaeoarchaean Mount Edgar samples resolve to two-stage Hf model ages of ca. 3.7Ga and Nd model ages of ca. 3.6Ga. These Eoarchaean model ages support the existence of a cryptic pre-3.5Ga protocrust, albeit of unknown extent. Analysis of late Mesoarchaean granites yields highly evolved Hf isotope signatures of K-rich monzogranites. This is consistent with a trend in the East Pilbara Terrane from sodic TTG type magmatism in the early Palaeoarchaean to K-rich magmas in the late Mesoarchaean.