Trondhjemite Granodiorite Research Articles

Archean granitoids of the Aravalli Craton, northwest India

Abstract The Archean granitoids of the Aravalli Craton (NW India) are represented by orthogneisses (3.3–2.6 Ga) and undeformed granitoids ( c. 2.5 Ga). Here we present whole-rock geochemical (elemental and Nd-isotope) data of the granitoids from the Aravalli Craton with an aim of understanding the evolution of the continental crust during the Archean. These Archean granitoids have been classified into three compositional groups: (1) TTG – tonalite–trondhjemite–granodiorite; (2) t-TTG – transitional TTG; and (3) sanukitoids. Based on the geochemical characteristics, it is proposed that the TTGs have formed from the partial melting of subducting oceanic plateau. The t-TTG formed owing to reworking of an older continental crust (approximately heterogeneous) in response to tectonothermal events in the craton. For the formation of the sanukitoids, a two-stage petrogenetic model is invoked which involves metasomatization of the mantle wedge, followed by slab breakoff and asthenospheric upwelling, which leads to the melting of asthenosphere and the metasomatized mantle wedge. It is also proposed that subducted sediments contributed to the genesis of sanukitoid magma.

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.

Geochemical Features and Geological Processes Timescale of the Achaean TTG Complexes of the Ingozero Massif and the Pechenga Frame (NE Baltic Shield)

This article provides a geological review and results of the structural, metamorphic, and geochronological studies of the Pechenga frame outcrops located in the NW part of the Central-Kola terrain and the Ingozero massif outcrops situated in the northeastern part of the Belomorian mobile belt of the Kola Region (NW Baltic Shield). As a result of the work, the deformation scales and ages of the geological processes at the Neo-Archaean–Paleoproterozoic stage of the area’s development were compiled, and the reference rocks were dated. The petrochemical and geochemical characteristics of the Ingozero rocks are similar to those of tonalite–trondhjemite–granodiorite (TTG) complexes established on other Archaean shields. The isotope U–Pb dating of individual zircon grains from the biotite gneisses provided the oldest age for magmatic protolith of the Ingozero gneisses, which is 3149 ± 46 Ma. Sm–Nd model ages showed that the gneisses protolite initial melt formed at 3.1–2.8 Ga. Ages of metamorphic processes were determined by using isotope U–Pb dating ID TIMS (isotope dilution thermal ionization mass spectrometry): Biotite gneisses—2697 ± 9 Ma; amphibole–biotite gneisses—2725 ± 2 Ma and 2667 ± 7 Ma; and biotite–amphibole gneisses 2727 ± 5 Ma. Ages of granitoids, which cut the deformed gneisses, are 2615 ± 8 Ma and 2549 ± 31 Ma for plagiogranites and pegmatoid veins in gneisses, respectively. The following age sequence of geological processes was established by using U–Pb zircon dating: 2.8 Ga—The time of the garnet–biotite gneiss metamorphism; 2722 ± 9 Ma—The granodiorite crystallization time; 2636 ± 41 Ma—The aplite emplacement age and 2620 ± 16 Ma—The age of pegmatites origin, which marked final stages of the Archaean evolution; 2587 ± 5 Ma—The age of gabbros emplacement and 2507 ± 7 Ma—The age of gabbros metamorphism; 2522–2503 Ma—The origin time of the iron quartzite interpreted as the age of gabbros and biotite gneiss metamorphism.

S-type granites in the western Superior Province: a marker of Archean collision zones

Detailed field observations indicate that Neoarchean S-type granites were emplaced along and (or) proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator mineral, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K calc-alkaline, moderately to strongly peraluminous (ANKC >1.1), and contain >1% CIPW normative corundum. Compared with more voluminous, older I-type granitoids in tonalite–trondhjemite–granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (SiO2 >72 wt.%) contents with a small silica range (SiO2 = 72.2–81.2 wt.%), but a large range of Zr/Hf (17.1–43.8) and Nb/Ta (0.3–16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr (<400 ppm) and Yb (<2 ppm) contents and low Sr/Y (<40) and La/Yb (<20) ratios are consistent with a partial melting process that left a granulite-facies residue consisting of plagioclase, pyroxene, and ± garnet. The S-type granites display low zircon saturation temperatures (mostly <800 °C) and low emplacement pressures (<300 MPa), similar to strongly peraluminous leucogranites formed in the Himalayas. Therefore, we propose that the Neoarchean S-type granites in the western Superior Province, whose source rocks were deposited between colliding tectonic blocks between 2720 and 2680 Ma, may serve as a geological marker of some Archean terrane boundary zones.

Geochronology and Geochemistry of Archean TTG and Tremolite Schist Xenoliths in Yemadong Complex: Evidence for ≥3.0 Ga Archean Continental Crust in Kongling High-Grade Metamorphic Terrane, Yangtze Craton, China

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units and the familiar metabasite xenoliths they host in the Yangtze Craton, China, remain controversial, and resolving these issues is important if we are to understand the evolution of the early Yangtze Craton. We focused on biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).

Geochemistry, Zircon U-Pb Geochronology and Hf-O Isotopes of the Banzhusi Granite Porphyry from the Xiong’ershan Area, East Qinling Orogen, China: Implications for Petrogenesis and Geodynamics

The Banzhusi granite porphyry is located in the Xiong’ershan area, East Qinling orogenic belt (EQOB). This study presents an integrated whole-rock geochemistry and zircon U-Pb-Hf-O isotope analysis of the Banzhusi granite porphyry. These rocks have metaluminous, high-K alkali-calcic and shoshonitic features and show significant enrichment in light rare earth elements (LREEs) over heavy rare earth elements (HREEs) with negative Eu anomalies. These samples are also greatly enriched in Rb, Ba, K, Pb, Th and U and depleted in Nb, Ta, P and Ti, and they mostly overlap the ranges of the Taihua Group tonalite–trondhjemite–granodiorite (TTG) gneiss. Magmatic zircons from three samples of the Banzhusi granite porphyry yield U-Pb ages of 125.1 ± 0.97 Ma, 128.1 ± 1.2 Ma and 128.2 ± 1.3 Ma. The Hf-O isotope features of zircons from the three samples are very similar (δ18Ozircon = 4.84‰ to 6.51‰, εHf(t) = −26.9 to −14.4). The co-variations of geochemical and isotopic data in these granite porphyries imply that the Banzhusi granite porphyry resulted from the mixing of the partially melted Taihua Group and mantle-derived material in a post-collisional setting from 128–125 Ma.

Petrogenesis of the Neoarchean granitoids and crustal oxidation states in the Western Shandong Province, North China Craton

Na-rich and K-rich granitoid rocks contain geological signatures that are crucial for understanding the global evolution and oxidation states of the Archean crust. Here we present an integrated investigation of the lithology, geochronology, and geochemistry of Neoarchean plutonic granitoid gneisses and supracrustal rocks from Xintai to Feixian within the Western Shandong Province (WSP) granite-greenstone belt of the North China Craton (NCC). Six main lithological assemblages are identified within the WSP granite-greenstone belt: (1) The ∼2.69–2.68 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses, which were derived from the partial melting of low-K mafic rocks at lower crust, have low MgO contents (0.68–2.11 wt%) and high (La/Yb)N ratios (16.7–92). (2) The ∼2.53 Ga granodioritic gneisses exhibit a low MgO content (0.87–1.91 wt%) but high K2O content (3.06–4.66 wt%), their magma originated from the partial melting of high-K mafic rocks. (3) The ∼2.54–2.49 Ga monzogranitic gneisses show the highest SiO2 content (71.67–75.26 wt%), and their magma was formed from the partial melting of metagreywackes. (4) The ∼2.56–2.52 Ga TTG gneisses also have low MgO content (1.42–1.69 wt%) and relatively low (La/Yb)N ratios (17.1–23); their magma was produced by the partial melting of low-K mafic rocks at lower crustal levels. (5) The ∼2.53–2.52 Ga quartz syenites have the highest alkali content (9.66–11.23 wt%) but relatively low MgO content (0.71–1.50 wt%), and their magma was generated by the partial melting of juvenile basaltic rocks associated with recycled sediments. (6) An ∼2.73 Ga xenolith of plagioclase amphibolite is characterized by high positive εHf(t2) values from +5.2 to +8.8, and fractionated chondrite-normalized REE patterns, indicating it may have been derived from a depleted mantle source enriched either by the addition of melt or by fluid metasomatism.The oxygen fugacity values of the crust-derived Neoarchean granitoid magmas are calculated based on a zircon trace element oxybarometer, the results show that the log (f O2) values for the ∼2.7 Ga TTG gneisses that range from FMQ-7.4 to FMQ+5.5 with an average value of FMQ+0.5, whereas the various ∼2.5 Ga granitoid gneisses have log (f O2) values ranging from FMQ-4.3 to FMQ+8.9 with an average value of FMQ+3.3. In conjunction with mineralogical results, we consider that the ∼2.7 Ga continental crust was in a relatively more reduced state than the ∼2.5 Ga continental crust in the WSP, which may be associated with differences in their source regions and compositions.

The Mesoarchean Tonalite-Trondhjemite-Granodiorite Associations of Eastern Sarmatia: Age and Geological Setting

Two episodes of Mesoarchean magmatism (ca. 2.95 and 3.03 Ga) have been recognized within the Kursk domain of Eastern Sarmatia. They led to the formation of the Saltyki granitoid complex, which is similar in composition to the granitoids of the typical tonalite–trondhjemite–granodiorite (TTG) associations of Archean cratons. These episodes correlate well with the previously determined emplacement start and completion dates of the Mesoarchean TTG associations in the Middle Dnieper and Azov domains of the Ukrainian shield. This confirms the assumption that the Kursk domain is a part of the joint Mesoarchean nucleus of Eastern Sarmatia. The geochemistry and Sm–Nd isotope characteristics of the Mesoarchean Saltyki granitoids suggest that their protoliths were the more ancient TTG-gneisses.

Early continental crust generated by reworking of basalts variably silicified by seawater

The Archaean continental crust comprises two major groups of silicon-rich granitoids: the tonalite–trondhjemite–granodiorite and granite–monzonite–syenite suites, which differ in their sodium-to-potassium ratios. How these felsic granitoids evolved from their mafic precursors remains elusive and the subject of great debate. Here, we present silicon isotopic constraints on the formation of representative trondhjemitic and granitic plutons from the Kaapvaal craton that range in age from 3.51–2.69 billion years ago. We identified very consistent silicon isotopic signatures, all uniformly 0.1–0.2‰ heavier than rocks of the modern continental crust. This unusual composition is explained by the melting of a mafic source that included significant proportions (15–35 wt%) of silicified basalts, which were common supracrustal rocks before 3 billion years ago. Before the melting event that formed the granitoid magmas at depth, portions of the mafic source rocks were enriched in silica by interaction with silica-saturated seawater. The addition of silica depresses the stability of amphibole at similar water activity, allowing trondhjemitic and granitic melt production at lower temperatures from protoliths with contrasting silica contents: 52–57 and ≥60 wt%, respectively. This explains why granitoids were able to form very early in Earth history but did not emerge in significant amounts on other rocky planets. Granitic continental crust in the Archaean formed from a basaltic source that was enriched in silica due to interaction with the early oceans before melting, according to silicon isotope analyses on rocks from the Kaapvaal craton.

An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes

Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved silicon at low concentration. The unique heavy silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands. Archaean granitic rocks formed by melting of silica-enriched subducted basaltic crust through interaction with seawater, according to heavy silicon isotopes measured in Archaean samples.

Diverse middle Neoarchean granitoids and the delamination of thickened crust in the Western Shandong Terrane, North China Craton

Middle Neoarchean granitoids contain crucial clues for understanding crust–mantle interactions and further investigating the mechanism of the compositional transformation of the crust in the Neoarchean. In the Western Shandong Terrane (WST), diverse Neoarchean magmatism is recognized, including ∼2.62 Ga tonalitic gneisses, ∼2.59 Ga granodioritic gneisses and ∼2.61–2.58 Ga monzogranitic gneisses, which generally intruded into ∼2.70 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses. The ∼2.70 Ga TTG gneisses show high SiO2 contents (70.17–72.89 wt%), but low MgO contents (0.58–0.69 wt%) and high (La/Yb)N ratios (30–48) and originated from partial melting of low-K mafic rocks at lower crustal levels. The ∼2.62 Ga tonalitic gneisses exhibit low K2O/Na2O ratios (0.10–0.39), but high (La/Yb)N (7.60–12.7) and Sr/Y (18.8–48) ratios and Mg# (47–57). With high εHf(t2) values ranging from +2.7 to +7.4, these rocks were derived from dehydration melting of subducted oceanic slabs. The ∼2.61–2.58 Ga monzogranitic gneisses show high SiO2 (70.35–75.12 wt%) and K2O (4.35–5.81 wt%) contents but low MgO (0.20–0.77 wt%) contents with positive εHf(t2) values (+2.0 − +6.6), suggesting they were derived from partial melting of juvenile greywackes. The ∼2.59 Ga granodioritic gneisses exhibit high SiO2 (66.34–73.04 wt%), K2O (2.23–4.32 wt%) and MgO (1.39–3.51 wt%) contents with dispersed εHf(t2) values (−0.2 − +5.3), indicating a mixing process between crust- and mantle-sourced magmas. The ∼2.59 Ga granodioritic gneisses show consistent major and trace element compositions (i.e., high Mg#, lack of Eu anomalies and uniform chondrite-normalized REE patterns), and accordingly, their magmatic precursors were derived from partial melting of delaminated lower crustal materials.The lithological assemblages in the WST became increasingly diverse between ∼2.71–2.58 Ga, changing from TTG gneisses to arc-related rock series, and then to delamination-related granodioritic gneisses, crust-derived monzogranitic and trondhjemitic gneisses and mantle-derived tholeiitic basalts and diorites. By integrating these data with a reduction in the continental crustal thickness from ∼2.70 Ga to 2.55 Ga, a subduction–delamination model is proposed in which a long-term lateral accretion beginning at ∼2.7 Ga and persisting through ∼2.68–2.62 Ga led to the delamination of the lower crust at ∼2.61–2.58 Ga, which may have further triggered the transition of the continental crust composition from basaltic to andesitic.

Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: Age, origin, and tectonic implications

The Permian magmatism in the Alxa region provides critical geological evidence to constrain the geodynamic processes in the southern Central Asian Orogenic Belt. The Shouji batholith is located in the southern Langshan, eastern Alxa region, and contains monzogranites, K-feldspar granites, and gabbros. New zircon UPb age data show that the Shouji batholith formed at 282–268 Ma. The Shouji granites have high (Na2O + K2O), Nb, and Zr contents, and high 10000∗Ga/Al, FeOT/(FeOT + MgO), Yb/Nb, and Yb/Ta ratios, similar to the values of A2-type granites. The Shouij granites exhibit enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE); depletion in Nb, Ti, and Sr; and enriched isotopic compositions. These Shouji granites were generated by partial melting of dehydrated tonalite–trondhjemite–granodiorite rocks of the Diebusige Complex in the lower crust. The Shouji gabbros are sodium-rich and low-K calc-alkaline rocks. They are enriched in LREE and LILE, depleted in high field strength elements, and have enriched isotopic compositions. These Shouji gabbros were generated by partial melting of a lithospheric mantle source metasomatized by melts derived from an ancient continental crust in an active continental margin. Roll-back of the Enger Us subducting slab accounts for generations of the Shouji A2-type granites and gabbros. The zircon UPb ages of Carboniferous–Permian magmatism show a younging trend toward the northwest in the southern Alxa region. Evidence from late Paleozoic magmatism and arc-derived sediments indicates that the southern Alxa switched from an active continental margin to an intra-oceanic arc during the early Carboniferous–Late Permian as a result of retreat of the subducting Enger Us slab.

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

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

The geochemistry of Au–Ag minerals and base‐metal sulphides as indicators for gold precipitation: Case study of the Shihu gold deposit, central North China Craton

The Shihu quartz vein‐type gold deposit, the largest gold deposit in the central North China Craton, occurs within wall rocks of Neoarchean TTG (tonalite–trondhjemite–granodiorite) gneiss and shows a close relationship with the early Cretaceous Mapeng granitoid pluton and various intermediate–mafic dykes. The occurrence, morphology, composition, and precipitation mechanism of the Au–Ag minerals are short of research, which is the purpose of this study. The primary mineral assemblage of orebody contains quartz, calcite, and base‐metal sulphides including pyrite, galena, sphalerite, chalcopyrite, and minor pyrrhotite and enargite. The Au–Ag minerals can be classified into three distinct types, namely, (a) quartz‐hosted, (b) pyrite‐hosted, and (c) base‐metal sulphidehosted, corresponding to formation sequence. Electron microprobe analysis indicates that these Au–Ag minerals show fineness (=Au/[Au + Ag] × 1,000) ranging from 976 to 366 with an average of 667 and are rich in Bi, Te, and Hg while deficient of Cu, As, and Se. Bismuth and tellurium in the Au–Ag minerals show a linear relationship with Au, whereas in the coexisted sulphides are almost below detection limits. We suggest that Bi and Te fundamentally influenced the transportation, enrichment, and precipitation of gold as scavengers to concentrate and deposit gold at the location with an “appropriate distance” from magma. Phase analyses of native bismuth and Bi–Te minerals included in pyrite, reported for the first time in Shihu Gold deposit, imply that the gold precipitation witnessed the ore‐forming fluid evolution with log fTe2 decreasing from approximately −11 to approximately −15 and log fS2 increasing from approximately −13 to more than −11. Mineralogical mapping with the fineness of Au–Ag minerals (fineness contours) suggest that the ore‐forming fluids came from the deep earth beneath the north of the mining area with excellent gold potential, which has been verified by later drilling.

Late Neoarchean slab rollback in the Jiaoliao microblock, North China Craton: Constraints from zircon U-Pb geochronology and geochemistry of the Yishui Complex, Western Shandong Province

Earth's plate tectonic system was initiated between 3.0 and 2.5 Ga and became the dominant regime for crustal growth and differentiation. However, it is unclear whether subduction–accretion processes, including slab rollback, occurred regionally in the area of the western Shandong Province Granite–Greenstone Terrane of the North China Craton (NCC) during the early development of plate tectonics. We quantified petrology, whole-rock geochemistry, and zircon U–Pb–Hf isotopes for felsic, mafic, and ultramafic rocks from the Yishui Complex (YC) in the NCC to constrain the origin of these rocks and the associated tectonic setting(s). The granites yield 207Pb/206Pb ages of 2509 ± 20 to 2484 ± 26 Ma, and magmatic zircon grains from these rocks have εHf(t) values of 0.6–6.4. The coeval mafic–dioritic rocks represent the fractional crystallization products of a primary magma that was derived from low-degree (<7%) partial melting of a metasomatized mantle source. The granodiorites were derived from the partial melting of oceanic slab material, with slab melts being contaminated by mantle wedge material during their ascent. The high-K granites represent the crustal reworking products of regional tonalite–trondhjemite–granodiorite and sedimentary rocks. Late Neoarchean magmatism in the YC was related to slab subduction, and slab rollback at ~2.50 Ga led to the generation of potassic rocks, coeval mafic–ultramafic xenoliths, and granulite metamorphism with a counterclockwise P–T–t path in the Yishui area.

Hotter mantle but colder subduction in the Precambrian: What are the implications?

On contemporary Earth, subduction recycles mafic oceanic crust and associated volatile elements, creating new silicic continental crust in volcanic arcs. However, if the mantle was hotter in the Precambrian, the style of subduction, the depth of devolatilization and the formation of silicic continental crust may have been different. Consequently, the generation of the tonalite–trondhjemite–granodiorite (TTG) suite, which is characteristic of Archean crust, may not have been related to subduction. Here, we use a two-dimensional numerical magmatic-thermomechanical model to investigate intraoceanic subduction for contemporary mantle conditions and at higher mantle temperatures, as appropriate to the Precambrian. In each case, we characterize the thermal structure of the subducting plate and investigate magma compositions and production rates. We use these results to assess the potential growth of silicic continental crust associated with intraoceanic subduction at different mantle temperatures. For the Precambrian, in a set of experiments with ΔT = 150 K and decreasing subducting plate velocity, we find that the contemporary style of subduction was preceded by an arc-free regime dominated by rapid trench rollback and vigorous upwelling of asthenospheric mantle into the space created above the retreating slab. In this regime, formation of magmas by fluid-fluxed melting of the mantle is suppressed. Instead, decompression melting of upwelling asthenospheric mantle results in the widespread development of voluminous plateau-like basaltic magmas. In addition, retreating subduction at higher mantle temperature causes faster descent of the downgoing slab, leads to colder thermal gradients, similar to those associated with active subduction in the western Pacific today, and suppresses melting of the basaltic crust, limiting production of silicic (adakite-like) magmas. With increasing maturity of the subduction system, retreat of the subducting plate ceases, the role of decompression melting strongly decreases and fluid-fluxed melting of the mantle coupled with melting of the hydrated slab begins to produce basaltic and felsic arc volcanic rocks similar to those formed during contemporary subduction. In an additional series of individual experiments at various ΔT, an increase of the mantle temperature above ΔT = 150 K leads to episodic and short-lived subduction accompanied by limited production of silicic continental crust. The results of our experimental study demonstrate that a hotter mantle in the Precambrian changes dramatically both the slab dynamics and the processes of magma generation and crustal growth associated with intraoceanic subduction zones. These changes may preclude growth of the early Precambrian silicic continental crust by processes that were dominantly similar to those associated with contemporary subduction.

Neoarchaean magmatism through arc and lithosphere melting: Evidence from eastern Dharwar Craton

The Neoarchaean era is characterized by rapid crustal growth corresponding to some fundamental global changes in geodynamic processes. However, the nature of crustal growth including the mechanism and tectonic setting of the Neoarchaean are controversial issues. The eastern Dharwar Craton (EDC) exposes widespread Neoarchaean granite‐greenstone belts, which provide an opportunity to evaluate the various models proposed for Neoarchaean crustal growth. In this study, we present field, petrographic, and geochemical data and discuss the petrogenesis and significance for crustal evolution for a suite of previously undescribed banded gneisses, TTG (tonalite–trondhjemite–granodiorite), biotite granites, alkali feldspar granite and gabbro. These rocks are associated with Neoarchaean metavolcanic and metapelites rocks of the Tsundupalle greenstone belt, in the eastern fringe of the EDC. Whole‐rock major and trace element geochemical data are consistent with diverse sources, including both crust and enriched mantle in an evolving subduction zone. A convergent orogenic setting is proposed for interpreting the association of various granitoids in the Tsundupalle area. Finally, intrusion of crustally derived, highly silicic, alkali‐rich granite, and mantle‐derived gabbro emplaced in a post‐subduction regime is proposed. Lithospheric delamination and attendant mantle melting are suggested as possible mechanisms for generation of these rocks. The understanding of generation of the different granitoid types along with gabbro provides significant insights into the mechanism of Neoarchaean crustal growth.

New insights into Neoarchean–Paleoproterozoic crustal evolution in the North China Craton: Evidence from zircon U–Pb geochronology, Lu–Hf isotopes and geochemistry of TTGs and greenstones from the Luxi Terrane

Tonalite–trondhjemite–granodiorite (TTG) suites constitute dominant components of Precambrian continental crust and provide important insights into the crustal evolution history of the early Earth. Here we investigate a suite of TTGs and associated greenstones from the Luxi Terrane in the south–eastern part of the North China Craton (NCC). We present data from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes to characterize these rocks and understand their genesis. Mineral assemblage in the greenstones and TTGs suggests that the rocks have undergone amphibolite–facies metamorphism with regression under greenschist–facies. Zircon U–Pb data from the magmatic grains yield upper intercept ages of 2753 ± 37 and 2582 ± 40 Ma for greenstone, 2552 ± 13 Ma for TTG gneiss and 2476 ± 18 Ma for tonalite, and weighted mean 207Pb/206Pb age of 2507 ± 22 Ma for granodiorite. Lu–Hf isotope systematics show εHf(t) values ranging from −2.13 to 7.29 and crustal model ages (TCDM) in the range of 2603–3125 Ma for the TTGs, whereas the εHf(t) values are up to 8.53 with TCDM ages of 2644–3006 Ma for the greenstone. The results suggest that the protolith for the greenstone was derived from a Meso- to Neoarchean mantle contaminated by the crust source, whereas the TTGs were mostly sourced from juvenile crustal sources with reworking of older basement. The geochemical features of greenstone are similar to island arc basalt, with low Na2O, K2O and TiO2, low total rare earth elements (REEs) values, relative enrichment in light REEs and depletion in heavy REEs absence of obvious Ce anomaly, and negative Nb, Ta and Ti anomalies. The TTGs show similar features, except for the high K2O content and moderate total REEs values. We propose that the greenstone likely represent a part of the early and initial crustal growth at ca. 2750–2600 Ma, and the TTGs were generated subsequently at ca. 2550–2450 Ma. Our results are consistent with the previous models on the crustal growth models in the NCC and provide further insights into the Neoarchean to Paleoproterozoic tectonic history of the craton.

Deciphering the Mesoarchean to Neoarchean history of crustal growth and recycling in the Caochang region of the Eastern Hebei Province, North China Craton using combined zircon U–Pb and Lu-Hf isotope analysis

In Archean cratons, the end of the Archean aeon (3.0–2.5 Ga) is marked by a considerable diversification in both the nature and petrogenesis of granitoid rocks. We reported the studies of a series of Mesoarchean-Neoarchean diorite and tonalite–trondhjemite–granodiorite (TTG) and granitic gneisses in the Caochang area, Eastern Hebei region of the North China Craton. Combined zircon U-Pb and Lu-Hf isotope analyses and CL imaging analysis reveal three major magmatic events (3.2-3.0 Ga, 2.9-2.8 Ga, ~2.5 Ga) and one major metamorphic overprint (~2.5 Ga). The close temporal relationship for different zircon domains reveals that the late-Neoarchean crustal materials were recycled or reworked immediately after their formation, immediately followed by ~2.5 Ga recrystallized metamorphism.The voluminous mantle-derived mafic rocks indicate that the 2.5-2.6 Ga is an important period of crustal addition in the Eastern Hebei region. The peak of Hf model age and the widespread Mesoarchean or Paleoarchean crustal materials in Eastern Hebei may point to another important period of crustal addition before the Neoarchean.

Statistical analysis on secular records of igneous geochemistry: Implication for the early Archean plate tectonics

In recent years, application of statistical analysis to a sufficiently large geochemical database has become more and more popular to reveal the onset and evolution of plate tectonics as a consequence of the development of computer science and storage technology. Here, we introduce a robust statistical method to process a filtered geochemical dataset including predominantly continental basaltic rocks through Earth's history. The results show that the average Sr concentrations gradually increased from 3.8 to 0 Ga, indicating a progressively growing depth of magma source as a result of secular cooling of the mantle. However, the average La/Yb and Sm/Yb ratios only started to increase at ca. 3.0 Ga. Considering the La–Yb and Sm–Yb fractionations are commonly attributed to the residual garnet crystallization, we interpreted that the geotherm of the magma source of continental basaltic rocks before 3.0 Ga might be higher than the stability limit of garnet. As such, there were little or no garnets in the residue during partial melting of the upwelling asthenospheric mantle or mantle plumes before 3.0 Ga, which also implied a higher continental geotherm during that time. Such a high geotherm could not support the formation of contemporaneous high‐pressure or medium‐pressure TTGs (tonalite–trondhjemite–granodiorite) by vertical tectonic models associated with upwelling mantle heating or delaminated lower crust melting in the early Archean. We therefore conclude that the plate tectonics had occurred by ca. 3.8 Ga in a hot subduction style and contributed to the formations of high‐ and medium‐pressure TTGs subsequently.