Potassic Granites Research Articles

Mineral thermobarometry and its implications for petrological constraints on Mesoarchean granitoids from the Carajás Province, Amazonian Craton (Brazil)

This paper describes the first investigation of the crystallization parameters of the Mesoarchean granitoids from Ourilândia do Norte, which is located in the midwestern part of the Carajás Province. These rocks are divided into three major groups: (i) sanukitoids composed of amphibole-bearing granitoids and associated intermediate to mafic rocks; (ii) potassic granites composed of biotite monzogranites and associated high-Ti granodiorite; and (iii) subordinated TTG-affinity granitoids (amphibole-bearing trondhjemites and biotite-bearing tonalitic xenoliths). These rocks were formed from different sources at different depths, including enriched mantle (sanukitoids), continental crust (monzogranite), mantle-crust mixture (high-Ti granodiorite) and oceanic crust (TTG-affinity granitoids). Potassic granites and porphyritic sanukitoid facies have high magnetite contents, and consequently high magnetic susceptibility (MS), compared to the equigranular sanukitoid facies and trondhjemite. Thermometers based on apatite saturation in whole-rock indicate liquidus temperatures above 800 °C for all granitoids, reaching 959 °C in the high-Ti granodiorite. Amphibole-based and Si-sensitive geothermometers indicate solidus temperatures from 785 to 738 °C for sanukitoids and 892 to 772 °C for trondhjemite, with estimated emplacement pressures varying from 297 to 80 MPa and 580 to 263 MPa, respectively. Because the potassic granites display an amphibole-free assemblage, no geothermobarometric estimate based on amphibole composition was possible. However, potassic granites are associated with coeval sanukitoids (2.88 Ga), which indicates similar pressure conditions and emplacement depths for these rocks. Crystallization constraints from the Ourilândia sanukitoids are comparable to those of the geochemically similar Rio Maria sanukitoids of the Carajás Province, although the pressure estimates and water content show differences. The Fe/(Fe + Mg) ratios of both biotite and amphibole indicate high to intermediate redox conditions for the sanukitoids and potassic granites, which are mostly above that of nickel-nickel oxide (+1.0 < NNO < + 1.9), although less oxidized conditions are estimated for the trondhjemite (– 0.5 < NNO < + 1.0). High water content was estimated for the sanukitoid-related granitoids (> 5%) relative to the TTG-affinity granitoids (< 4%) using amphibole-based water calibrations. Compared with sanukitoids from other Archean cratons, the Ourilândia sanukitoids crystallization constraints show similarities with the more oxidized members from the Baltic Shield, the Closepet-type granites and the Matok-type sanukitoids but differ from the reduced redox conditions of the Elan-type granitoids from the East Sarmatian orogen.

Multi-scale spatial distribution of K, Th and U in an Archaean potassic granite: a case study from the Heerenveen batholith, Barberton Granite-Greenstone Terrain, South Africa

AbstractWe describe the multi-scale distribution of K, Th and U in the ca. 3.1 Ga Heerenveen batholith of the Barberton Granite-Greenstone Terrain. Data were obtained with a combination of tools, including a portable gamma-ray spectrometer from the scale of the whole batholith to the scale of outcrops, and autoradiography for the thin section scale. U is concentrated preferentially in minor phases in the border shear zones of the batholith and, within these shear zones, in late pegmatites as well as fractures. The processes responsible for the concentration of U in the Heerenveen batholith is discussed in terms of magmatism, hydrothermalism (redistribution of U in fissures associated with magmato-hydrothermal fluids), and supergene alteration. The statistical properties of K, Th and U concentrations are different. K shows spatial correlation over large distance, largely mirroring mappable rock types, with increased variability at larger scales. In contrast, U is dominated by small-scale variations (“nugget effect”) and its variability is, averaged and smoothed by large-scale integration. Spatial and statistical features thus offer useful and complementary insights on petrogenetic and metallogenic processes in granitoids in addition to standard approaches (petrography, geochemistry).

On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt

The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield in southeast Egypt. It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite, granodiorite and monzogranite. The host tonalite, in turn, encloses several, fine-grained amphibolite enclaves. U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ± 18 Ma for tonalites; 754 ± 3.9 Ma for trondhjemite; 738 ± 3.8 Ma for granodiorite; and 717 ± 3.2 Ma for monzogranite), suggesting crystallization of independent magma pulses. The high positive εNdi (+6–+8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust. Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745 ± 31 °C for tonalite to 810 ± 25 °C for trondhjemite; 840 ± 10 °C for granodiorite; and 868 ± 10 °C for monzogranite. The pressure of partial melting is loosely constrained to be below the stability of residual garnet (<10 kbar) as inferred from the almost flat HREE pattern ((Gd/Lu)N = 0.9–1.1), but >3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases. The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization. The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting. It lacks distinctive features for melt derived from a subducted slab (e.g. high Sr/Y and high (La/Yb)N ratios), and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge. Comparison with melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite, trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith. Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples, it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting of tonalite and the emplacement of monzogranite melt within the SGC. The emplacement of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalite-trondhjemite-granodiorite) most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield. Eventually, after complete consolidation, the whole SGC was subjected to regional deformation, most probably during accretion to the Saharan Metacraton (arc–continent collisions) in the late Cryogenian -Ediacaran times (650–542 Ma).

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

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

Delineation of the early Neoarchean (2.75 to 2.70 Ga) crustal growth and reworking processes in the southeast base of Taihang Mountain, North China Craton

Numerous studies suggest that various secular geologic and geochemical transitions occurred between ~3.2 Ga and 2.5 Ga. During this age-window, the ~2.70 Ga tectono-thermal event of the Neoarchean is by far the most influential, and is unusual in terms of coeval mafic-felsic magmatic rocks which were interpreted to reflect widespread crustal accretion. Here we report the early Neoarchean TTG and potassic granite association preserved in the Yunmengshan area, Taihang Mountain, North China Craton (NCC). TTG and potassic granites emplaced at 2712 ± 65 to 2644 ± 25 Ma, followed by ~2.57 to 2.50 Ga metamorphism and partial anatexis. TTG has geochemical features corresponding to high-Al medium-pressure (MP) TTG. Their high CaO, low to moderate Al2O3/(FeOT + MgO), and positive εHf (t) values (+3.8 to +7.2) with TDM2 of 2.96 to 2.75 Ga, indicate that they formed by partial melting of juvenile low-K mafic rocks, representative of crustal accretion. Besides, TTG can be subdivided into two groups based on Eu anomalies. Those with Eu/Eu* < 1.0 underwent advanced amphibole and plagioclase fractionation. The possible reason for their weakly negative Eu anomalies is that plagioclase has negated the effects of amphibole fractionation which is theoretically accompanied by plagioclase removal. Those with positive Eu anomalies accord with “slab-melt” identification criterion (Sr > 300 ppm plus elevated Sr/Y > 40, (La/Yb)N > 12 and Eu/Eu* > 1.0). The high Eu/Eu*, Sr/Y and low Yb probably represent the contribution of amphibole fractionation and plagioclase accumulation during the magma evolution. The evidence favors an arc-related setting for the TTG. The early Neoarchean potassic granites are monzogranite to syenogranite, and formed shortly after TTG. They belong to high-K calc-alkaline to shoshonitic I-type granite, and display depleted and concave-upward REE patterns between middle and heavy REEs and higher Zr/Sm ratios (69.6 on average) than TTG (45.4). The calculated εHf (t) values are mainly positive (+1.4 to +7.0) with TDM2 from 3.03 Ga to 2.72 Ga, together with low Al2O3/(FeOT + MgO), low to moderate CaO and high K2O/Na2O ratios, indicating a high-K mafic crustal source with metapelite involvement. Meanwhile, we reviewed geochemical data of the early Neoarchean TTGs published in the NCC and other cratons abroad. The results show that samples from Trans-North China Orogen in NCC have lower Y contents, higher Sr/Y and (La/Yb)N ratios, and more depleted HREE than those from Eastern Block, which could be interpreted to reflect an increase in the depth of melting. Alternatively, according to previous studies and comparison globally, elevated Sr/Y values could also reflect increased consumption of plagioclase in melting reactions due to higher temperatures, or magma fractionation. Therefore, it mostly lacks a clear distinction between the “arc” and “non-arc” settings based on geochemistry. However, many researchers put forward evidence that subduction was well attested in the late Archean (3.0 to 2.5 Ga), but it might be unstable.

LA–ICP–MS zircon U[sbnd]Pb dating, Lu[sbnd]Hf, Sm[sbnd]Nd geochronology and tectonic setting of the Mesoarchean mafic and felsic magmatic rocks in the Sangmelima granite-greenstone terrane, Ntem Complex (South Cameroon)

The Ntem Complex represents the northwestern margin of Congo Craton in southern Cameroon. It consists mainly of charnockites, TTG suites, potassic grantitoids, minor supracrustal and mafic rocks. The latter include gabbronorite enclave or sill, gabbro and dolerite dykes. The metagabbro are basaltic to andesitic in composition and show geochemical features of low-La Archean tholeiitic basalts, and LREE-depleted (LaN/SmN < 1) akin to modern N-MORB. The doleritic dykes show high TiO2/P2O5 (7.50–11.47) and (La/Yb)N ratios, and low NbTa values similar to modern intraplate continental basalts and intermediate-La Archean tholeiitic basalt, least affected by contamination from granitic continental crust. The partial melting of low-to high-K mafic rocks of oceanic and continental arc affinity generated the charnockites and low−/medium-pressure TTGs. The potassic granites were formed by partial melting of charnockite or TTG, and inherited many geochemical features of their sources. New LA–ICP–MS UPb zircon dates suggest that the charnockites and TTG suites formed at ~3155–2850 Ma. The gabbro intrusion was dated at 2866 ± 6 Ma. Migmatisation of TTG-gneiss was coeval whit the regional high–grade metamorphism event at 2843 ± 7 Ma. The potassic granites were probably crystallized at ~2758 Ma. The garnet-whole rock SmNd age of 2744 ± 31 Ma may represent the age of the second anatexis event coeval with D2 deformation. Literature data indicate the disruption or open isotopic systems in the Ntem Complex around 2500–2600 Ma.

Petrogenesis of Archean TTGs and potassic granites in the southern Yangtze Block: Constraints on the early formation of the Yangtze Block

The southern Yangtze is underlain by Mesoarchean igneous rocks that were emplaced in two pulses, including trondhjemites at ca. 3092–3071 Ma followed by potassic granites at 2920 Ma. These represent the oldest TTG suites identified in the southern Yangtze and the oldest potassic granites in the entire Yangtze Block. Hf isotopic analysis reveals εHf(t) values of −2.49–7.49 and Hf crustal model ages (TDMC) of 3601–3000 Ma for zircons from the trondhjemites, whereas those from the potassic granites show εHf(t) values of −1.79–4.18 and TDMC ages of 3453–3043 Ma. The trondhjemites have high SiO2, Mg# and low Al2O3 and K2O/Na2O, and are highly evolved with low Ni, Cr and Co. The trondhjemites are characterized by low Sr/Y, Eu/Gd and (La/Yb)N, and high Y and HREE, typical of high-HREE and low-pressure TTGs. Their geochemical and isotopic compositions indicate that they are derived from melting of juvenile mafic crust with little contribution from older crustal materials, consistent with a plagioclase-bearing basaltic source. The multielement patterns of potassic granites are similar to that of the trondhjemites, albeit with lower HREE and Y contents. Their high K2O values, strongly fractionated REE patterns and low Y contents indicate the presence of garnet and/or amphibole as residual phases in the source region, which indicates they formed at higher pressure than the trondhjemites. The isotopic and geochemical data imply that the formation of the potassic granites could be related to reworking of older TTGs. The increasing K2O/Na2O of the 3092–3071 Ma trondhjemites to potassic 2920 Ma granites corresponds with stabilization, maturation and thickening of the Archean continental crust in the southern Yangtze. The geochemical data of the TTGs in the Kunming region show important differences from that of contemporaneous TTGs from the Kongling Complex, suggesting that southern Yangtze is composed by an independent old nucleus, which is exotic to the northern Yangtze. A synthesis of petrogenesis, geochronology, geochemistry and isotope data of Archean granites of the Yangtze Block provides important insights into the architecture of its Archean crust. We propose that the Yangtze Block can be subdivided into discrete northwestern, northeastern and southern domains, but which are all affected by late Paleoproterozoic tectonothermal events, suggesting that they had been assembled together by then to form the united Yangtze Block.

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

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

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.

Origins of two types of Archean potassic granite constrained by Mg isotopes and statistical geochemistry: Implications for continental crustal evolution

The chemical composition of the upper continental crust (UCC) dramatically changed from predominantly sodic to more potassic during the late Archean (3000–2500 Ma), although how this change occurred remains poorly constrained. The origins of Neoarchean potassic granites may provide clues for this crucial change, as their formation dominated the compositional transition of the UCC. In this study, we conducted high-precision Mg isotopic analyses of Neoarchean potassic granites (2558–2520 Ma) and tonalite–trondhjemite–granodiorites (TTGs; 2595–2574 Ma) from the North China Craton and statistical geochemistry in K2O/Na2O ratios of global Archean granitoids. Results show that these TTGs, with K2O/Na2O ratios of ~0.61 and δ26Mg values of −0.39‰ to −0.22‰, were generated from heterogeneous basaltic sources. In contrast, potassic granites yield two distinctive groups of δ26Mg values and can be subdivided into I- and S-types. The I-type granite has mantle-like δ26Mg values of −0.28‰ to −0.22‰ and weak enrichment in K2O (K2O/Na2O = 1.0–1.5), and was likely derived by partial melting of igneous sources. The S-type is characterized by much higher δ26Mg values of +0.60‰ to +0.91‰ and strong K2O enrichment (K2O/Na2O = 1.57–23.26), with geochemical characteristics suggesting derivation by partial melting of sediments. Based on the geochemical characteristics of the S-type potassic granites in this study, we sort the S-type granites out of 4066 Archean felsic igneous rocks worldwide. Their temporal distribution suggests that S-type potassic granites have appeared in the middle Archean (3500–3000 Ma) and became widespread in the late Archean (3000–2500 Ma), but were scarce in the early Archean (4000–3500 Ma). The onset of S-type granites indicates reworking of supracrustal rocks may have started since 3500 Ma. And widespread S-type potassic granites during 3000–2500 Ma play an important role for the high K2O/Na2O ratio of UCC at the end of Archean. A mass-balance calculation shows that 25.88 wt% I-type, 4.12 wt% S-type potassic granite and 70 wt% TTGs could increase the K2O/Na2O ratio of the UCC to ~0.83 which is the global average value for felsic rocks at the end-Archean. To increase K2O/Na2O of the Archean UCC, contributions made by I- and S-type potassic granites could probably reach 3:2, respectively. Consequently, the evolved crustal composition from predominantly sodic to potassic results from the synergy of re-working of igneous (unaltered) and sedimentary (weathered) rocks, by which the former contributed more quantitatively, while the latter contributed more efficiently.

Petrogenesis of the Bakoudou TTGs and high-K biotite granites and associated gold mineralization, southern Gabon: An Archean example of a porphyry–epithermal transitional gold system

The Bakoudou gold deposit, southern Gabon, is hosted in the Archean basement granitoids of the Chaillu Massif. These granitoids of the Bakoudou deposit are subdivided into two main groups: TTGs and porphyritic biotite granites. The TTGs show geochemical characteristics similar to those of other TTGs worldwide, such as high Al2O3 (13.49–18.93 wt%) and Na2O (3.27–4.66 wt%) and low K2O (1.01–1.98 wt%), with low K/Na (0.15–0.32) ratios. Their Mg# (Mg/(Mg + Fe)*100) values vary from 34.81 to 50.76, with low to moderate Nb/Ta and Zr/Sm ratios (7.03–23.43 and 19.13–75.25, respectively). They are characterized by moderate (La/Yb)N (6.70–41.71) and Sr/Y (20.80–72.13) ratios similar to those of adakites, negative Nb, Ta and Ti anomalies and LREE-enriched and HREE-depleted patterns, without significant Eu anomalies (Eu/Eu* = 0.93–1.11, average = 1.00). These geochemical characteristics suggest that the TTG rocks were formed by partial melting of hydrous basaltic crust, such as garnet-rich amphibolite. The porphyritic biotite granites show high-K calc-alkaline affinity. They are characterized by high SiO2 (71.01–74.86 wt%) and K2O (3.29–4.92 wt%) but relatively low MgO (0.38–0.92 wt%), with Mg# values varying between 31.78 and 40.27. The rocks show low to moderate Sr/Y (13.75–26.66) ratios and LREE-enriched and HREE-depleted patterns, with negative Eu anomalies (Eu/Eu* = 0.36–0.58, average = 0.47). These geochemical features indicate that the porphyritic biotite granites originated from partial melting of TTG lithologies.The orebody of the Bakoudou gold deposit occurs as quartz veins hosted in the TTG shear zones. Quartz is the main gangue mineral, with minor sulfides such as pyrite and chalcopyrite. Gold is present in solid solution (Au+1) and as nanoparticles of native gold (Au0) in pyrite and mostly as free gold. Pyrite samples analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) show high Ni (3.24–3600 ppm) and Co (4.68–4140 ppm) and low As (<34.50 ppm) and Au concentrations (<2.90 ppm) compared to Cu, which has rather high concentration of <3570 ppm. Moreover, the Au–As relationships of pyrites show a porphyry–epithermal transitional signature. Therefore, the Bakoudou gold deposit can be considered an example of Archean porphyry–epithermal transitional gold system, formed from magmatic fluids originating from a hidden porphyry system at depth during cooling and emplacement of the potassic granites. The disseminated free gold grains in the quartz veins may have originated from boiling of the mineralizing fluid (phase separation) and/or from remobilization of invisible gold in pyrite by later tectonic events and surficial weathering.

Peel-back controlled lithospheric convergence explains the secular transitions in Archean metamorphism and magmatism

The preserved archive of continental crust suggests that various secular geologic and geochemical transitions took place during the Mesoarchean and Neoarchean (∼3.2–2.5 Ga). These transitions, which are imprinted in the metamorphic and magmatic rock-record, include the emergence of paired metamorphism, an increasing depth of tonalite-trondhjemite-granodiorite (TTG) formation, the widespread appearance of granites and changes in the composition of mafic rock. Previous studies have argued that these secular transitions reflect secular cooling of the mantle and herald a gradual transition from pre-plate tectonic to plate tectonic regimes. However, their tectonic driver remains elusive, in part due to a lack of detailed understanding of lithospheric dynamics prevalent during this transitional period when the mantle was warmer compared to the present-day. Here, we demonstrate that lithospheric convergence driven by peel-back (/peeling) process under warmer mantle conditions – termed as peel-back convergence – may explain the late Archean secular transitions. This tectonic phenomena features large-scale peeling (a form of delamination, but not dripping) of the mantle lithosphere with or without lower crustal rocks during convergence. We simulated numerical models of peel-back convergence under Archean crust-mantle conditions and predicted the pressure-temperature (P-T) conditions of crustal metamorphism and melting. The evolution of peel-back convergent setting features juxtaposition of a colder, compressional regime with thickened crust that forms at the site of peeling, and a warmer, extensional regime with thinned crust forming behind it. The metamorphic and magmatic P-T conditions prevalent in these two tectonothermal sites replicate the features of the late Archean rock record, including the: appearance of coeval high-T/P and intermediate-T/P metamorphic rocks; greater abundance of high-T/P relative to intermediate-T/P metamorphic rocks; dominant formation of TTGs at higher pressures where garnet ± rutile is stable in the residue; reworking of pre-existing felsic rocks/sediments to produce potassic granites; and interaction of crustal melts and mantle to form hybrid granitoids. Thus, peel-back convergence can explain the thermobaric bimodality of late Archean rocks within an asymmetric tectono-thermal framework. However, unlike modern convergent plate boundaries, the asymmetry is controlled by lithospheric peeling.

A Neoarchean arc-backarc pair in the Linshan Massif, southern North China Craton

The Linshan Massif is located in the southern North China Craton (NCC) and consists mainly of tonalitic (TTG) and dioritic gneisses, and a volcano-sedimentary-dominated assemblage, all of which were intruded by post-collisional potassic granites. The volcano-sedimentary-dominated assemblage is composed mostly of deformed and foliated basaltic amphibolite and meta-sedimentary rocks, with minor interlaying meta-gabbros, forming a stack of tectonically imbricated sedimentary and volcanic rocks. The TTG gneisses formed at 2.6–2.52 Ga, and the meta-gabbros, basaltic amphibolites and diorites formed at 2.51–2.50 Ga. They underwent subsequent metamorphism at 2.49–2.48 Ga. Furthermore, all their igneous zircons possess large positive εHf (t) values (+2.9 to +7.4). The basaltic amphibolites can be divided into two groups: depleted basalts and enriched basalts. Depleted basalts are characterized by flat LREE patterns and low Th/Nb ratios with positive Nb anomalies, similar to N-MORB-type patterns. Enriched basalts display LREE-enriched patterns with high Th/Nb ratios and pronounced negative Nb anomalies, showing an arc-related affinity. The meta-gabbros have LREE-enriched patterns with negative Nb and Zr anomalies, consistent with subduction-related signatures. Considering mixed arc- and MORB- signatures with positive zircon εHf(t) values, we propose that the meta-gabbros and basaltic amphibolites may have formed in a back-arc setting. The tonalites and the diorites showing LREE-enriched patterns and depleted in HFSEs are interpreted to represent arc-related magmatism. Combined with the documented 2.55 Ga Dengfeng fore-arc accretionary complex, we propose the following geodynamic evolution model for the Southern NCC: (1) at 2.6–2.54 Ga, a TTG-dominated oceanic arc developed above a west-dipping subduction zone, forming the 2.55 Ga Dengfeng accretionary complex and related tectonic units; (2) at 2.52–2.50 Ga, slab rollback subsequently drove back-arc rifting of the mature TTG-dominated intra-oceanic arc, resulting in the formation of two arc segments with the intervening Linshan back-arc basin; and (3) at 2.50–2.48 Ga, the TTG-dominated arc collided with the Eastern Block resulting in the closure of the back-arc basin and formation of the accreted arc-back-arc package in the Linshan Massif. We suggest that the Neoarchean Central Orogenic Belt in the southern NCC contains at least two amalgamated oceanic arc terranes, separated by a belt of back-arc basin magmatic rocks caught within an intra-oceanic mélange/suture zone.

Origin of late Neoarchean granitoid diversity in the Western Shandong province, North China Craton

Diverse late Neoarchean granitoids are widely exposed in the Western Shandong province (WSP). According to geological relationships, lithological features, magmatic zircon U–Pb–Lu–Hf isotopes and whole-rock chemical characteristics, eight main lithological assemblages are identified, and their petrogenesis is determined. (1) The ~2.54 Ga and ~2.51 Ga potassic granites show the highest SiO2 contents (67.06–75.84 wt%) and were sourced from partial melting of metamorphic greywackes. (2) The ~2.53 Ga quartz syenites exhibit the highest alkali contents (10.32–11.23 wt%) and were sourced from mantle-derived basaltic rocks and recycled sediments. (3) The ~2.53–2.52 Ga quartz diorites (less-differentiated sanukitoids) formed from partial melting of metasomatized mantle. (4) The ~2.53 Ga hornblendites have crystallization ages and Lu–Hf isotopes similar to those of the quartz diorites, implying that they are cogenetic cumulates from the quartz dioritic magmas. (5) The ~2.56–2.52 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses were formed by melting of thickened crust. (6) The ~2.53 Ga high-Mg granodiorites (differentiated sanukitoids) were formed by mixing of crust-mantle melts. (7) The ~2.51–2.49 Ga gabbros originated from partial melting of fluid-metasomatized mantle. (8) The ~2.54–2.53 Ga high-Mg andesites (boninites) originated from partial melting of refractory mantle that experienced the extraction of basaltic magma at earlier stage and was then enriched in LILEs and LREEs by slab-derived melts.Integrating the petrogenesis of these late Neoarchean diversified granitoids and associated volcanic rocks, we consider that the late Neoarchean was a crucial period of crust-mantle interaction in the WSP, and partial melting of the metasomatized mantle produced intermediate-mafic melts, then underplating of the mantle-derived magmas caused the partial melting of crustal materials, further led to the mixing of crust-mantle magmas. The above complex crust-mantle interactions triggered the transfer of energy and materials from the Earth’s interior to the surface, forming modern-like continental crust thickness and geothermal gradient. Moreover, combining late Neoarchean intense granitic magmatism and ~2.49 Ga mafic dike swarms implies that the North China Craton experienced cratonization processes at the end of the Neoarchean.

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.

Neoarchean and Paleoproterozoic K-rich granites in the Phan Si Pan Complex, north Vietnam: Constraints on the early crustal evolution of the Yangtze Block

Precambrian igneous and metamorphic rocks of the Phan Si Pan Complex, North Vietnam, constitute the southern extension of the Yangtze Block, and provide a valuable record of the early evolution of the continental crust. We present results of U-Pb zircon geochronology and geochemistry for Precambrian granites in this complex to constrain their emplacement age and genesis. Granites from three plutonic bodies yielded ages of 2848 ± 15 Ma, 2768 ± 19 Ma and 1869 ± 30 Ma, which represent newly-recognized late Archean to Paleoproterozoic potassic granite plutonism in the southern Yangtze Block. The average εHf(t) values range from −6.2 to 0.1 for the 2.85 to 2.77 Ga granitic rocks and −13.1 to −9.2 for the ca. 1.86 Ga granitic rocks, with two-stage model ages of 3.64 to 3.20 Ga and 3.31 to 3.07 Ga, respectively, suggesting derivation from partial melting of Paleoarchean and Mesoarchean crust. The late Archean potassic granites exhibit high K2O, and high Sr/Y and (La/Yb)N ratios with negligible Eu anomalies, indicating derivation from melting of the thickened lower crust, which is inferred to have occurred in an active margin setting. The late Paleoproterozoic alkali feldspar granites are characterized by high FeOT/(FeOT + MgO)(0.96–0.99) and 10000 * Ga/Al (2.75–2.94) ratios, showing an affinity of A-type granite. These A-type granites exhibit flat chondrite-normalized HREE patterns and strong negative Eu anomalies, and low Sr/Y and (La/Yb)N ratios, corresponding to melting at a shallow depth, probably in a post-collisional extension setting.Comparison of the rock units and events recorded by the Phan Si Pan Complex with other Archean to Paleoproterozoic complexes (Houhe, Dongchuan, Yudongzi, Douling, Zhongxiang and Kongling complexes) in the Yangtze Block indicate spatially distinct histories of crustal growth, and thus may reflect independent terranes. The ca. 1.86 Ga post-collisional magmatism, which succeeds a 2.0–1.9 Ga metamorphic event, is distributed throughout the Yangtze Block, including the Phan Si Pan Complex, suggesting assembly of the disparate terranes and final cratonization of the Yangtze Block overlaps with, and may be related to, assembly of the Nuna supercontinent.

Formation of Paleoarchean-Mesoarchean Na-rich (TTG) and K-rich granitoid crust of the Singhbhum craton, eastern India: Constraints from major and trace element geochemistry and Sr-Nd-Hf isotope composition

The Singhbhum craton in eastern India is underlain by Paleo- to Mesoarchean Tonalites-Trondhjemites-Granodiorites (TTGs) and granites that formed in two pulses at 3.45–3.44 Ga and 3.35–3.32 Ga, followed by potassic granite activity at 3.1 Ga. The TTGs are metaluminous to slightly peraluminous and highly evolved with low Ni, Cr, and MgO. Low heavy-REE concentrations, Sr/Y and Eu/Gd, and high Nb, Ta, and Y concentrations indicate they belong to the low HREE, medium- to low-pressure TTG type. Their compositions are similar to TTGs derived from chondritic source with amphibolitic residue, consistent with rutile-free and garnet-bearing metabasic source. The granites have similar REE and multielement patterns as the TTGs, albeit with pronounced negative Eu, Sr, and Ti anomalies. High K2O/Na2O, higher heavy-REE concentration, low Sr/Y and Eu/Gd, and negative Eu anomalies of the 3.05 Ga Mayurbhanj potassic granites are consistent with shallow plagioclase-bearing crustal sources. The increasing K2O/Na2O of the granitoids from 3.45 Ga to 3.05 Ga is related to gradual thickening of the continental crust and increasing contributions of intracrustal melts in younger granitoids. Neodymium and Hf isotope data indicate the 3.45–3.44 Ga granitoids have juvenile as well as Eoarchean TTG crust-like signature. The 3.35–3.32 Ga granitoids were produced by partial melting of the same mafic crust that gave rise to the 3.45–3.44 Ga TTGs with some components requiring up to 50% crustal contribution. The Mayurbhanj granites represent intracrustal melts of more potassic members of the Paleoarchean TTGs. The trace-element characteristics of the granitoids reflect melt generation at different depths in a tectonic setting producing both TTGs as well as granites contemporaneously. Plausible geodynamic settings could be very hot subduction or “dripduction” regime involving shallow melting of delaminated mafic crust producing the 3.45 Ga juvenile TTGs and minor granites. Large scale delamination and melting of mafic lower crust and felsic proto-crust in a thickened crustal pile in a subduction-like regime could have produced granitoids with juvenile as well as recycled isotopic signatures. The Nd-Hf isotope data indicate that the Paleoarchean upper mantle was moderately depleted in response to a spurt in Paleoarchean continental growth.

Genesis of the Singhbhum Craton, eastern India; implications for Archean crust-mantle evolution of the Earth

The Singhbhum Craton in eastern India, one of the four major Archean cratons of the Indian Shield, is underlain by Paleoarchean granitoids (trondhjemite-tonalite-granodiorite i.e. TTGs, and potassic granites) that are encircled by greenstone belts rich in banded iron formation known as Iron Ore Group. This study presents whole rock major-trace element geochemistry, Sr and Nd isotopes, K-feldspar common Pb isotopes, along with in-situ zircon U–Pb ages and Hf isotope data from the granitoids and the greenstone volcanic rocks to characterize their source(s) and to constrain the evolution of the Paleoarchean continental crust. Zircon U–Pb ages show that these granitoids were emplaced during two magmatic episodes at ca. 3.47–3.44 Ga and 3.36–3.28 Ga. The greenstone volcanic rocks in the eastern part of the craton are basalts and basaltic andesites with rare komatiites, whereas those in the western part are basaltic-andesites and andesites. Whole rock Sm–Nd isotope regression lines for volcanic rocks from the eastern (including komatiites) and western regions yield ages of 3746 ± 340 Ma (MSWD = 169) and 2961 ± 420 Ma (MSWD = 3.3), respectively. The major and trace element data show that the volcanic rocks of the eastern and western greenstone belts had a different petrogenesis. The granitoids are highly evolved with SiO2 contents up to 76 wt%. The major and trace element data indicate that the granitoids of the Singhbhum Craton were derived by partial melting of a mafic crust at variable depths (ca. 10–15 kbar). The isotopic compositions of Pb measured on leached K-feldspars from the granitoids show undepleted mantle-like characteristics and negate the possible involvement of much older evolved crust in the formation of these rocks. The whole rock initial Nd isotopic compositions (εNdi = −0.3 to +2.2) and zircon in-situ initial Hf isotopic compositions (εHfi = −0.3 to +2.0) of the granitoids calculated using zircon crystallization ages indicate that they were derived from a near-chondritic reservoir. These data suggest that only a limited volume of continental crust formed prior to ca. 3.5 Ga, and major crust extraction from the mantle started at ca. 3.5 Ga. Using this time as the start of major crustal growth and concomitant mantle depletion, Nd and Hf mantle depletion histories are proposed for the Singhbhum Craton that can be described by the equations εNdt = −2.85 × (tGa) + 10, and εHft = −4.68 × (tGa) + 16.4. These curves do not change the model ages for young rocks, but they make a significant difference for the Archean rocks.

Geochronology and geochemistry of Meso- to Neoarchean magmatic epidote-bearing potassic granites, western Dharwar Craton (Bellur–Nagamangala–Pandavpura corridor), southern India: implications for the successive stages of crustal reworking and cratonization

Abstract We present field and petrographical characteristics, zircon U–Pb ages, Nd isotopes, and major and trace element data for the magmatic epidote-bearing granitic plutons in the Bellur–Nagamangala–Pandavpura corridor, and address successive reworking and cratonization events in the western Dharwar Craton (WDC). U–Pb zircon ages reveal three stages of plutonism including: (i) sparse 3.2 Ga granodiorite plutons intruding the TTG (tonalite–trondhjemite–granodiorite) basement away from the western boundary of the Nagamangala greenstone belt; (ii) 3.0 Ga monzogranite to quartz monzonite plutons adjoining the Nagamangala greenstone belt; and (iii) 2.6 Ga monzogranite plutons in the Pandavpura region. Elemental data of the 3.2 Ga granodiorite indicate their origin through the melting of mafic protoliths without any significant residual garnet. Moderate to poorly fractionated REE patterns of 3.0 Ga plutons with negative Eu anomalies and Nd isotope data with ε Nd (T) = 3.0 Ga ranging from −1.7 to +0.5 indicate the involvement of a major crustal source with minor mantle input. Melts derived from those two components interacted through mixing and mingling processes. Poorly fractionated REE patterns with negative Eu anomalies of 2.6 Ga plutons suggest plagioclase in residue. The presence of magmatic epidote in all of the plutons points to their rapid emplacement and crystallization at about 5 kbars. The 3.2 Ga intrusions could correspond to reworking associated with a major juvenile crust-forming episode, whilst 3.0 Ga potassic granites correspond to cratonization linked to melting of the deep crust. The 2.6 Ga Pandavpura granite could represent lower-crustal melting and final cratonization, as 2.5 Ga plutons are absent in the WDC.