Archean Tonalite Trondhjemite Granodiorite Research Articles

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

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

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

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

Late Neoarchean TTG and monzogranite in the northeastern North China Craton: Implications for partial melting of a thickened lower crust

The petrogenesis and dynamic setting of Archean tonalite–trondhjemite–granodiorite (TTG) is widely debated. As a typical Archean terrane in the northeastern North China Craton (NCC), the southern Jilin Province area underwent multi-stage magmatism during the Neoarchean and is therefore a promising region in which to track the Neoarchean evolution of the NCC. This study investigates the petrogenesis of late Neoarchean TTGs and associated monzogranites and their tectonic implications. Zircon U–Pb dating results suggest that the TTGs and monzogranites have emplacement ages of 2.55–2.52 and 2.53–2.51 Ga, respectively. The TTGs have variable SiO2 (61.7–73.3 wt%) and MgO (0.66–2.74 wt%) contents and Mg# values of 33–50. Most have high Sr and low Y and Yb contents, with high Sr/Y and (La/Yb)N ratios. The TTGs have variable εHf(t) (+0.7 to + 8) and εNd(t) (−2.77 to + 2.79) values. These characteristics, together with the results of thermodynamic partial-melting modelling, indicate that the TTGs can be classified as medium- and low- pressure types and were likely generated by amphibole-breakdown partial melting of 2.9–2.7 Ga juvenile basaltic rocks under P–T conditions of 1.0 ± 0.1 GPa and 880 ± 50 °C. The monzogranites have higher SiO2 (66.1–76.0 wt%) and K2O (4.03–5.06 wt%) and lower TFe2O3 (0.89–4.97 wt%) and MgO (0.21–1.27 wt%) contents. They also have high Sr/Y and (La/Yb)N ratios and variable εHf(t) (+0.6 to + 5.4) and εNd(t) (−3.47 to − 0.15) values. The geochemical and Hf–Nd isotopic compositions of the monzogranites indicate that the parental magmas may have been derived through the partial melting of thickened lower crust with heterogeneous sources. The high thermal gradient (>730 °C/GPa−1), anticlockwise metamorphic P–T paths, and 3.4–3.2 Ga remanent zircons in the region suggest that a late Neoarchean thickened lower crust was the likely geodynamic setting for the generation of the TTGs and monzogranites.

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

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

Unsupervised Machine Learning for Lithological Mapping Using Geochemical Data in Covered Areas of Jining, China

Application of (supervised and unsupervised) machine learning algorithms to big geoscience data can facilitate intelligent lithological mapping and interpretation in a data-driven fashion. Among them, unsupervised machine learning could play more important role in lithological classification because lithological labels are often missed in covered places. In this study, principal component analysis (PCA) and self-organizing maps (SOM) were applied to delineate major lithologies in the Jining region, Inner Mongolia, China, by integrating ten major-element stream sediment geochemical (K2O, Na2O, CaO, MgO, Al2O3, ΣFeO, MnO2, SiO2, P2O5 and TiO2) datasets. The results suggest that unsupervised machine learning using major-element stream sediment geochemical data is effective in capturing main lithological variations in a covered area. In detail, the PCA method identified only three interpretable components among nine principal components, whereas the SOM clustering recognized four categories including Archean TTG (tonalite–trondhjemite–granodiorite) gneisses besides felsic series, Cenozoic clastics and Cenozoic basalts. Apart from coinciding well with the pre-existing geological map in outcropping areas, the mapping results added new details regarding spatial distributions of felsic and basaltic rocks, which were previously recognized as Cenozoic clastics. This study highlights the ability of unsupervised learning for intelligent lithological mapping in covered areas using major-element stream sediment geochemical data.

Mineralogical and Geochemical Study on the Yaojiazhuang Ultrapotassic Complex, North China Craton: Constraints on the Magmatic Differentiation Processes and Genesis of Apatite Ores

The differentiation process of ultrapotassic magmas is enigmatic and poorly understood. The Yaojiazhuang ultrapotassic complex is concentrically zoned by late-intruded syenite in the core and early emplaced clinopyroxenite in the periphery, combining a “bi-modal” lithology. Spatially, apatite and iron oxide-apatite (IOA) ores, glimmerite and pseudoleucite occur in the upper part of clinopyroxenite. The syenite and clinopyroxenite are composed of variable amounts of clinopyroxenite, biotite, K-feldspar, magnetite, apatite with minor analcite, titanite and primary calcite. The pseudoleucite clinopyroxenite contains mainly clinopyroxene, biotite and garnet in the matrix, and nepheline–K-feldspar intergrowth with muscovite and minor celestine in the leucite pseudomorph. Geochemically, rocks of the Yaojiazhuang complex are significantly enriched in potassium (K), light rare earth elements (LREE) and large ion lithophile elements (LILE). Crustal contamination by Archean tonalite–trondhjemite–granodiorite (TTG) gneisses basement may play an important role to convert the syenitic melts from silica-undersaturation to saturation. Fractionation crystallization is supported by the mineral crystallization sequence to explain the bimodal lithologies instead of silicate liquid immiscibility. During the magmatic evolution, decompression, fractionation of volatile-poor clinopyroxene and the enhancement by CO2 may result in the exsolution of an aqueous fluid. The late-stage interactions between existing minerals and magmatic fluids in the crystal mush could be a key process in the generation of both leucite pseudomorphs and apatite/IOA ores.

Formation and stabilization of the Dunhuang Block, NW China: Constraints from the Late Paleoproterozoic A-type granites of the Dunhuang Complex

The geochemical composition and temporal evolution of A-type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust. Moreover, these aspects may provide an indication of when orogenic processes ended and craton formation/stabilization began in Early Precambrian blocks. In this study, we report the results of zircon U–Pb ages, Hf isotope data and whole-rock geochemistry for the Proterozoic Huoyanshan potassic granites in the Dunhuang Block, northwest China. Zircon U–Pb dating reveals that the Huoyanshan granites were emplaced at ca. 1791–1786 Ma, representing a Late Paleoproterozoic magmatism event. The granites have high SiO2 (70.71–72.20 wt%), total alkali contents (K2O + Na2O = 8.81–8.96 wt%; K2O/Na2O = 1.57–1.66), TiO2/MgO ratios (2.19–2.57), low CaO (0.77–1.27 wt%), MgO contents (0.12–0.16 wt%), and Mg# values (7.72–8.67), with A/CNK ratios of 0.96–1.04, pointing to alkaline and metaluminous to weakly peraluminous affinities. These granites are characterized by high total rare-earth element concentrations (∑REE = 419–755 ppm) and variable (La/Yb)N values (8–24) with significantly negative Eu anomalies (Eu/Eu* = 0.31–0.42). The primitive mantle-normalized trace element patterns suggest that the Huoyanshan plutons are depleted in Ba, Nb, Ta, Sr, P, and Ti, but enriched in Rb, Th, and Pb. Additionally, the granites have a high zirconium saturation temperature of 855–897 °C, with an average of 878 °C. These geochemical characteristics indicate that Huoyanshan potassic granites are an aluminous A-type granite, and further classified as A2-type. Together with the zircon Hf isotope data (εHf(t) = −11.21 to −2.76), indicates that these granites were generated from a crustal source. The heterogeneous Hf isotope compositions in zircons are principally caused by the disequilibrium melting of Zr-bearing sources. In addition, two-stage Hf model ages of zircons from the granites are ca. 2.6–3.1 Ga and plot on the average crustal evolution trend of Archean tonalite–trondhjemite–granodiorite (TTG) gneiss (~2.5 to 3.0 Ga) in the Dunhuang Block. Thus, we propose that A-type granites in the study area were generated from the disequilibrium partial melting of Archean tonalitic to granodioritic rocks under high-temperature and low-pressure conditions according to their low Al2O3 + CaO + Na2O + K2O (molar) values and high 10000 × Ga/Al values (>2.9). Subsequently, the magma experienced prominent fractional crystallization of K-feldspar, plagioclase, orthopyroxene, and/or magnetite. Our results, withavailabledata, suggest that the Huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting, and serve as a precursor for the formation and stabilization of the Dunhuang Block during the Late Paleoproterozoic. Furthermore, it is probable that the post-collisional magmatism event at ca. 1.79 Ga in the Dunhuang Block was closely related to the assembly of the Columbia supercontinent.

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

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

Determining the Geodynamic Setting of Adakitic Granitoids Using Geochemical Data

Discriminant analysis was performed for representative sets of igneous rocks with adakitic geochemical signatures (granitoids of Archean tonalite–trondhjemite–granodiorite suites, island-arc adakites, and adakites and adakitic granitoids of collisional to postcollisional geodynamic settings). It was shown that the granitoids of Archean tonalite–trondhjemite–granodiorite suites are significantly different from islandarc adakites, as well as from collisional to postcollisional adakites and adakitic granitoids. The following discriminant function was proposed for the geodynamic classification of island arc and collisional-postcollisional adakites and adakitic granitoids on the basis of chemical composition: DF3 =–1.69324TiO2–0.25537Al2O3–0.21269FeO* + 0.06076MgO–0.09796CaO + 0.47377Na2O + 0.29270K2O + 3.57821P2O5 + 0.00431Rb + 0.00036Sr + 0.03119Y + 0.00006Zr + 0.01088Nb–0.00048Ba + 0.01366La + 0.0004Ce + 0.02319Nd–0.18584Sm + 1.29135Eu–0.62229Gd + 0.3819Dy + 2.06583Er–2.62769Yb + 1.6464.

Litogeoquímica e geocronologia dos ortognaisses migmatíticos do Domo de Itabaiana, Sergipe: uma suíte do tipo tonalito, trondhjemito e granodiorito?

O Domo de Itabaiana encontra-se inserido no Domínio Vaza Barris da Faixa de Dobramentos Sergipana, no Estado de Sergipe. Está representado por um complexo gnáissico-migmatítico, constituído por ortognaisses migmatizados, com intercalações de níveis anfibolíticos. O conjunto foi retrabalhado e soerguido durante o processo de deformação Neoproterozoica dessa faixa. Estudos petrográficos revelam que os ortognaisses são de composição tonalítica a granodiorítica, por vezes com feições desde protomiloníticas a miloníticas. A litogeoquímica evidenciou um caráter sódico nessas rochas, as quais foram analisadas em dois grupos distintos, em relação aos seus padrões em ETR e teores de K2O. Os litotipos com teores de K2O < 2,5% compreendem termos da série cálcio-alcalina de baixo K a cálcio-alcalina normal, enquanto aqueles com teores de K2O ≥ 2,5% compreendem termos posicionados na interface da série cálcio-alcalina normal com a série cálcio-alcalina de alto K. Os dois grupos possuem anomalias negativas de Th-U, Ta-Nb e Ti, além de baixos teores de Y, apresentando um padrão fortemente fracionado, com o enriquecimento em terras raras leves e empobrecimento em terras raras pesadas. Apenas um grupo apresenta anomalia positiva de Sr e concavidade nos espectros dos terras raras pesados, feições também típicas dos TTGs arqueanos. Sugere-se que ambos possuem afinidade com TTGs e que o grupo mais enriquecido em K2O sofreu algum tipo de participação crustal em sua gênese. As análises geocronológicas determinaram uma idade de 2729 ± 12 Ma (LA-ICP-MS), interpretada como idade de cristalização dos ortognaisses.

Earth’s evolving subcontinental lithospheric mantle: inferences from LIP continental flood basalt geochemistry

Archean and Proterozoic subcontinental lithospheric mantle (SLM) is compared using 83 similarly incompatible element ratios (SIER; minimally affected by % melting or differentiation, e.g., Rb/Ba, Nb/Pb, Ti/Y) for >3700 basalts from ten continental flood basalt (CFB) provinces representing nine large igneous provinces (LIPs). Nine transition metals (TM; Fe, Mn, Sc, V, Cr, Co, Ni, Cu, Zn) in 102 primitive basalts (Mg# = 0.69–0.72) from nine provinces yield additional SLM information. An iterative evaluation of SIER values indicates that, regardless of age, CFB transecting Archean lithosphere are enriched in Rb, K, Pb, Th and heavy REE(?); whereas P, Ti, Nb, Ta and light REE(?) are higher in Proterozoic-and-younger SLM sources. This suggests efficient transfer of alkali metals and Pb to the continental lithosphere perhaps in association with melting of subducted ocean floor to form Archean tonalite–trondhjemite–granodiorite terranes. Titanium, Nb and Ta were not efficiently transferred, perhaps due to the stabilization of oxide phases (e.g., rutile or ilmenite) in down-going Archean slabs. CFB transecting Archean lithosphere have EM1-like SIER that are more extreme than seen in oceanic island basalts (OIB) suggesting an Archean SLM origin for OIB-enriched mantle 1 (EM1). In contrast, OIB high U/Pb (HIMU) sources have more extreme SIER than seen in CFB provinces. HIMU may represent subduction-processed ocean floor recycled directly to the convecting mantle, but to avoid convective homogenization and produce its unique Pb isotopic signature may require long-term isolation and incubation in SLM. Based on all TM, CFB transecting Proterozoic lithosphere are distinct from those cutting Archean lithosphere. There is a tendency for lower Sc, Cr, Ni and Cu, and higher Zn, in the sources for Archean-cutting CFB and EM1 OIB, than Proterozoic-cutting CFB and HIMU OIB. All CFB have SiO2 (pressure proxy)–Nb/Y (% melting proxy) relationships supporting low pressure, high % melting resembling OIB tholeiites, but TM concentrations do not correlate with % melting. Thus, the association of layered intrusion (plutonic CFB) TM deposits with Archean terranes does not appear related to higher metal concentrations or higher percentages of melting in Archean SLM. Other characteristics of these EM1-like magmas (e.g., S2 or O2 fugacity) may lead to element scavenging and concentration during differentiation to form ore deposits.

Lithological, structural, and geochemical characteristics of the Mesoarchean Târtoq greenstone belt, southern West Greenland, and the Chugach – Prince William accretionary complex, southern Alaska: evidence for uniformitarian plate-tectonic processes

The Mesoarchean Târtoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) with well-preserved duplex structures, representing a relict accretionary prism. The Târtoq greenstone belt is a remnant of a supra-subduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite–trondhjemite–granodiorite suites. Field observations, along with geochemical and zircon U–Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012 ± 4 and 2993 ± 6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach – Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and (or) accreted oceanic crust during slab window magmatism. Archean granitoid–greenstone terrains share many geological characteristics of Phanerozoic subduction–accretion complexes such as the Alaskan and Altaid subduction–accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of tonalite–trondhjemite–granodiorite generation in subduction–accretion complexes.

Magmatic oxygen fugacity estimated using zircon-melt partitioning of cerium

Using a newly-calibrated relation for cerium redox equilibria in silicate melts (Smythe and Brenan, 2015), and an internally-consistent model for zircon-melt partitioning of Ce, we provide a method to estimate the prevailing redox conditions during crystallization of zircon-saturated magmas. With this approach, oxygen fugacities were calculated for samples from the Bishop tuff (USA), Toba tuff (Indonesia) and the Nain plutonic suite (Canada), which typically agree with independent estimates within one log unit or better. With the success of reproducing the fO2 of well-constrained igneous systems, we have applied our Ce-in-zircon oxygen barometer to estimating the redox state of Earth's earliest magmas. Using the composition of the Jack Hills Hadean zircons, combined with estimates of their parental magma composition, we determined the fO2 during zircon crystallization to be between FMQ −1.0 to +2.5 (where FMQ is the fayalite–magnetite–quartz buffer). Of the parental magmas considered, Archean tonalite–trondhjemite–granodiorite (TTG) compositions yield zircon-melt partitioning most similar to well-constrained modern suites (e.g., Sano et al., 2002). Although broadly consistent with previous redox estimates from the Jack Hills zircons, our results provide a more precise determination of fO2, narrowing the range for Hadean parental magmas by more than 8 orders of magnitude. Results suggest that relatively oxidized magmatic source regions, similar in oxidation state to that of 3.5 Ga komatiite suites, existed by ∼4.4 Ga.

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognised that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

Evidence for Archean inheritance in the pre-Panafrican crust of Central Cameroon: Insight from zircon internal structure and LA-MC-ICP-MS U[sbnd]Pb ages

The main geological feature of Central Cameroon is the wide spread occurrence of granitoids emplaced in close association with transcurrent regional shear zones. The basement of this vast domain is a Paleoproterozoic ortho-and para-derivative formation, which has been intensely reworked, together with subsequent intrusions and sediments, during the Panafrican orogenesis in the Neoproterozoic. As consequence, the area underwent pervasive metamorphism and intense deformation. This makes it difficult to distinguish between Panafrican metasediments or syntectonic plutonites and their respective basement. Our study presents zircon features (CL-BSE-SE) and in-situ U–Th–Pb LA-MC-ICP-MS geochronology of a meta-sedimentary pyroxene-amphibole-bearing gneiss of the Méiganga area in Central Cameroon. Based on the Internal structures of the zircon four characteristic zonation patterns can be deciphered: 1) cores with magmatic oscillatory zonation 2) zircons with oscillatory or sector zonation, 3) zircons with sector zoning or blurred zoning, and 4) narrow bright un-zoned rims. These groups suggest that the rock experienced a number of geological events. Considering this zircon characteristic, the U–Th–Pb data allow to distinguish four ages: 2116 ± 57 Ma, consistent with ages from the Paleoproterozoic West Central African Belt; 2551 ± 33 Ma which marks a late Neoarchean magmatic event; 2721 ± 27 Ma related to a Neoarchean magmatic even in Central Cameroon, similar to one found in the Congo Craton. A zircon core gives ages around 2925 Ma which provides some evidence of the presence of the Mesoarchean basement prior to the Neoarchean magmatism. A weighted average of lower intercepts ages gives a value of 821 ± 50 Ma, representing the age of later metamorphism event. The various characteristic group and related ages reflect not only the complexity of the history of the pyroxene amphibole gneiss, but also show that the meta-sediment has at least three zircon contributing sources. It is likely that erosion, transport and deposition took place between 2116 and 821 Ma. Geochemical data show that the REE, Y, Yb, Sr/Y of some samples are similar to the known Archean craton formations (depletion in REE, Y ≤ 10 ppm, Yb ≤ 1 ppm, Sr/Y ≥ 30). These characteristics are known as specific for the Archean TTG (Tonalite—Trondhjemite—Granodiorite). It means that: i) Archean TTG contribute significantly to the detritus of the sedimentary basin, ii) The depositional basin and the source rock were close and the detritus was immature.Our results show that the Pre-Panafrican history of central Cameroon includes Meso- to Neo-Archean crustal accretion and associated magmatism prior to the Paleoproterozoic event of the West Central African Belt. In respect to this new insight, any evolutionary reconstruction of the area should integrate the presence of Archean crust.

Petrogenesis of the Archean tonalite–trondhjemite–granodiorite (TTG) and granites in the Lushan area, southern margin of the North China Craton: Implications for crustal accretion and transformation

Tonalite–trondhjemite–granodiorite (TTG) assemblages and abundant high-K calc-alkaline magmatism are considered to be closely related to craton stabilization, and usually record a tectonic transition. Three episodes of granitic magmatism are identified in the Lushan area, at the southern margin of the North China Craton (NCC) on the basis of new LA-ICP-MS zircon U–Pb ages and whole-rock geochemical data. They are TTG gneisses, sodic–potassic granite (SPG) suite and potassic granite dated at ∼2840Ma, ∼2760Ma and ∼2570Ma, respectively. The TTG gneisses have high SiO2, variable Mg#, Cr and Ni contents with high (La/Yb)N and Sr/Y ratios, suggesting that the parental magmas were derived by partial melting of subducted oceanic crust. They show no obvious correlations between Sr/Y and Dy/Yb or (La/Yb)N and Dy/Yb, and all have negative Nb and Ta but positive Zr and Hf anomalies and variable Dy/Yb, Nb/Ta, Zr/Hf and Zr/Sm ratios, suggesting melting of a deep source (>45km) with rutile eclogite and garnet amphibolite in the residue. The TTG samples have positive ɛHf(t) values (+2.6 to +7.0) with the youngest TDMC (2896Ma) close to the crystallization age, indicating that they were produced by partial melting of juvenile crustal materials in an arc setting. Rocks of the SPG suite have high SiO2 contents, variable Na2O/K2O ratios (0.41–10.91), and low Y and Yb contents with high Sr/Y ratios, indicating an affinity to I-type granites. They display depleted and concave-upward REE patterns between middle and heavy REEs and higher Zr/Sm than TTG gneisses, possibly resulting from amphibole removal by fractional crystallization or as a residual phase during melting. Meanwhile, the rocks have a large variation of Dy/Yb without obvious correlations between Sr/Y and Dy/Yb, and (La/Yb)N and Dy/Yb, in combination with variable but uniformly low Y contents. Therefore, we suggest that the SPG samples were derived from partial melting with garnet in the residue. Magmatic zircons have ɛHf(t) values from −4.3 to +4.2 with TDMC ages from 3200Ma to 2877Ma, indicative of older mafic lower crustal sources. The rocks formed within a short time interval with the TTG, and their emplacement was accompanied by regional granulite and amphibolite metamorphism, implying that they may also have formed in an arc setting. The potassic granite has high SiO2 (74.79wt.%) and K2O+Na2O (9.22wt.%) contents with a low 10,000Ga/Al (1.89) ratio and TZr (670°C). Magmatic zircons have a narrow range of ɛHf(t) values (−2.0 to +4.1) with TDMC ages (2942–2692Ma) older than the formation age, but consistent with the formation age (ca. 2760Ma) of the Lushan SPG suite. The potassic granite has a Hf isotope composition identical to that of the SPG suite and is enriched in LREE and depleted in Y with a positive Eu anomaly, suggesting that the SPG suite is the main source. One zircon yielded an inherited age of 2837±36Ma consistent with the age of the TTG gneisses, indicating that the gneisses could also be the source of the potassic granite. The two types of source rocks indicate a shallow crust depth. Hence the heat required for partial melting of granodioritic rocks to form the potassic granite could have been provided by the upwelling of mantle or mantle-derived melts in an extensional setting. Therefore, the TTG gneisses, SPG suite and potassic granite in the Lushan area record a tectonic transition from a compressive to an extensional setting in the southern margin of the NCC between 2.85 and 2.50Ga.

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.

Constraints from experimental melting of amphibolite on the depth of formation of garnet-rich restites, and implications for models of Early Archean crustal growth

The felsic continental crust formed in the early Earth most likely resulted from melting of basaltic protoliths, but the geodynamic processes leading to partial melting are still debated. Attempts to reconcile geochronological data, thermal modeling and experimental results have led to two major alternative scenarios: (1) partial melting of subducted oceanic slabs and (2) dehydration melting at the base of thick (or thickened) oceanic/simatic protocrusts. Existing experimental data on melting of metabasalt suggest that garnet only becomes an important residual phase (>10wt%) at depths >50–60km, which has been seen as difficulty for model 2. We present results of amphibolite dehydration-melting experiments at pressures of 5–15kbar and provide constraints on melting reactions of a hydrated metabasalt with SiO2 of 47.5wt% and Al2O3 of 16.4wt%. Our experiments demonstrate that felsic melts and complementary restites with ∼20wt% garnet can form at ca. 900°C and 12kbar, conditions corresponding to the base of a 40-km thick basaltic protocrust that might be prevalent in the Early Archean. Based on phase proportions determined experimentally and trace element partitioning data, our modeling suggests that such partial melts resemble the Early Archean tonalite–trondhjemite–granodiorite (TTG) suites, including high Al2O3 and low MgO contents, and modestly high La/Yb and Sr/Y ratios. The garnet-rich restite is calculated to be denser than the underlying Early Archean lherzolitic upper mantle and would have the potential to delaminate. Our experimental results and combined geochemical modeling are consistent with models where the initial growth of continental crust on the Archean Earth occurred in non-subduction settings by anatexis of the base of basaltic plateaus.

Geochemical and Hf–Nd isotopic constraints on the crustal evolution of Archean rocks from the Minnesota River Valley, USA

The 3.5–2.6Ga rocks of the Minnesota River Valley (MRV) provide a unique opportunity to examine rocks that span the now recognized crucial time (3.2–3.0Ga) on the Earth when Wilson style plate tectonic processes likely started. To elucidate the crustal evolutionary history of these ancient rocks we obtained Hf isotopic compositions by LA-MC-ICPMS from zircon, previously dated by SHRIMP U–Pb methods; whole-rock Nd isotopic compositions by TIMS; and whole-rock major and trace element concentrations. The oldest, ca. 3500Ma rocks have major and trace element compositions similar to other Archean tonalite–trondhjemite–granodiorite (TTG) suites, which are consistent with derivation from a mafic source. The ca. 3500Ma Morton and Montevideo gneisses have similar initial ɛHf values that range from 1.9 to −2.6 at 3500Ma, with model ages ranging from 3.83 to 3.64Ga. The model ages are interpreted as resulting from melting of a source that is >100Ma older than the ca. 3500Ma gneisses. Elevated Nb/Ta ratios and large negative Nb, Ta and Ti anomalies suggest that the source may have been eclogite, consistent with melt formation in an overthickened basaltic lithosphere.Younger (3380Ma), more chemically evolved, intrusions into both the Morton and Montevideo gneisses plot on a closed-system isotopic evolution path (Nd, Hf), indicating that they were in situ melts of the older ca. 3500Ma rocks. These intrusions contrast with a more mafic 3140Ma intrusion, which has zircon with more positive ɛHf than the closed-system isotopic evolution defined by the older gneisses. The last major event was the emplacement of the 2604Ma Sacred Heart granite and formation of a garnet-biotite paragneiss. Isotopic values for the Sacred Heart granite show a large juvenile component relative to all other rocks, suggesting it is a mix of juvenile material and recycled Morton or Montevideo gneiss. The transition from closed to open system evolution of rocks in the MRV at 3140Ma, and continuing at 2600Ma, is consistent with a proposed global change in continental crust formation and evolution that began at 3.2–3.0Ga.

Origin of Archean tonalite–trondhjemite–granodiorite (TTG) suites and granites in the Fiskenæsset region, southern West Greenland: Implications for continental growth

Mesoarchean to Neoarchean orthogneisses (2.95–2.79Ga) in the Fiskenæsset region, southern West Greenland, are composed of an older suite of metamorphosed tonalites, trondhjemites, and granodiorites (TTGs), and a younger suite of high-K granites. The TTGs are characterized by high Al2O3 (14.2–18.6wt.%), Na2O (3.4–5.13wt.%), and Sr (205–777ppm), and low Y (0.7–17.4ppm) contents. On chondrite- and N-MORB-normalized trace element diagrams, the TTGs have the following geochemical characteristics: (1) highly fractionated REE patterns (La/Ybcn=14–664; La/Smcn=4.3–11.0; Gd/Ybcn=1.5–19.7); (2) strong positive anomalies of Sr (Sr/Sr*=1.0–15.9) and Pb (Pb/Pb*=1.4–34.9); and (3) large negative anomalies of Nb (Nb/Nb*=0.01–0.34) and Ti (Ti/Ti*=0.1–0.6). The geochemical characteristics of the TTGs and trace element modeling suggest that they were generated by partial melting of hydrous basalts (amphibolites) at the base of a thickened magmatic arc, leaving a rutile-bearing eclogite residue. Field observations suggest that spatially and temporarily associated tholeiitic basalts (now amphibolites) in the Fiskenæsset region might have been the sources of TTG melts. The high-K granites have steep REE patterns (La/Ybcn=3.8–506; La/Smcn=2.7–18.9; Gd/Ybcn=0.92–12.1) and display variably negative Eu anomalies (Eu/Eu*=0.37–0.96) and moderate Sr (84–539ppm) contents. Four outlier granite samples have variably positive Eu (Eu/Eu*=1.0–12) anomalies. Given that the granodiorites have higher K2O/Na2O than the tonalites and trondhjemites, it is suggested that the granites were derived from partial melting of the granodiorites. It is speculated that the dense eclogitic residues, left after TTG melt extraction, were foundered into the sub-arc mantle, leading to basaltic underplating beneath the lower rust. Melting of the granodiorites in response to the basaltic underplating resulted in the production of high-K granitic melts. Formation of the Fiskenæsset TTGs, the foundering of the eclogitic residues into the mantle, and the emplacement of the high-K granites led to the growth of Archean continental crust in the Fiskenæsset region.