Ga Crustal Growth Research Articles

Late Neoarchean and Paleoproterozoic tectonothermal and metamorphic evolution recorded in high-pressure granulites: Evidence from the Xuanhua Complex, northern North China Craton

Archean cratons usually underwent reworked processes during the late Paleoproterozoic orogeny, which may result in multiple tectonothermal records preserved in some high-grade metamorphic rocks; thus, these rocks provide a key opportunity on understanding the tectonic styles during different periods. In this study, we report new petrological, mineral chemical and geochronological data of a suite of newly discovered mafic and felsic metamorphic rocks in the Xuanhua Complex from the northern North China Craton (NCC), revealing the complex metamorphic thermal history and the transition of tectonic regime from the late Neoarchean to the late Paleoproterozoic. The mafic granulite preserves the prograde quartz and clinopyroxene inclusions within garnets (M1), and the peak stage (M2) is recorded by garnet, clinopyroxene, hornblende, plagioclase, quartz and ilmenite. The first retrograde (M3a) and final cooling stages (M3b) are defined by the appearance of orthopyroxene, and the amphibolite-facies overprinting, respectively. The felsic granulite records the similar peak mineral assemblage (M2), but has higher volumes of feldspar and quartz than the mafic granulite. The retrograde stage (M3) is represented by amphibolite-facies assemblage (hornblende ± biotite ± clinopyroxene) and fine-grained intergrowth of hornblende + plagioclase around garnet. Pseudosection modeling and thermobarometry results suggest that both high-pressure (HP) mafic and felsic granulites record clockwise P–T paths, including isothermal decompression and cooling from the peak stage (M2: 10.4–10.8 kbar/830–860 °C and 9.6–11.2 kbar/850–930 °C, respectively) to the retrograde stages (M3a and M3b: 7.5–8.0 kbar/840–870 °C and 7.8–8.2 kbar/650–700 °C, respectively; M3: 7.2–8.6 kbar/670–800 °C). Zircon U–Pb dating shows that the magmatic precursors of these rocks were emplaced at c. 2.5–2.47 Ga. The metamorphic zircons reveal two groups metamorphic ages of c. 2.51–2.45 Ga and c. 1.90–1.85 Ga. The c. 2.51–2.45 Ga ages indicate a late Neoarchean tectonothermal event, and the c. 1.95–1.85 Ga ages are interpreted to represent the Paleoproterozoic HP granulite-facies metamorphism overprinting. The highest positive εHf(t) value of 7.64 and 2.8–2.6 Ga Hf model ages of c. 2.5 Ga magmatic zircons indicate that the c. 2.5 Ga mantle-derived mafic magma and the c. 2.5 Ga partial melting of 2.8–2.6 Ga basaltic juvenile crust generated the protoliths of mafic and felsic granulites, respectively, revealing 2.8–2.6 Ga and 2.5 Ga crustal growth and c. 2.5 Ga crustal reworking in the northern NCC.To sum up, we consider that the late Neoarchean magmatism and coeval metamorphism recorded in our samples are probably resulted from the amalgamation of micro-blocks. The Paleoproterozoic clockwise P–T–t paths with near-isothermal decompression segments and low geothermal gradients (c. 21–26 °C/km) of HP mafic and felsic granulites indicate a rapid post-collisional exhumation process after the significantly crustal thickening, and this long-term orogeny along the TNCO (c. 1.95–1.85 Ga; > 100 Myr) is related to the incorporation of the NCC into the Columbia supercontinent.

Proterozoic mantle melting recorded by the Re-Os isotopic systematics of ophiolites from the Qilian Orogenic Belt, northwestern China

• The Minhe serpentinites are mantle residues of multi-episode partial melting. • Fluid/melt metasomatism exerted a limited impact on the Re-Os systematics. • Re-Os ages provide temporal constraints on Proterozoic melt extraction. Re-Os isotopes of the mantle-derived ultramafic rocks can provide temporal constraints on the melt extraction of ancient mantle. Here, new geochemistry and Re-Os isotopes of serpentinites from the Minhe ophiolite in the Qilian Orogenic Belt (QOB) are presented. Their geochemical compositions (e.g., abundances of heavy rare earth elements, Al 2 O 3 , Yb, Re, and Th/Yb ratio) suggest that protoliths of the serpentinites originated as mantle residues. They underwent multiple episodes of melt depletion, leading to a highly refractory nature and high degrees of partial melting (> 18%). Moreover, subchondritic 187 Re/ 188 Os (0.0023 – 0.0399) and 187 Os/ 188 Os (0.1135 – 0.1210) ratios of the serpentinites, and decoupling of the Re-Os systematics and metasomatic proxies (e.g., LOI, Sr, La, and Th/Yb), demonstrate that fluid/melt metasomatism exerted a limited impact on their Re-Os isotopic systematics. Their Re-depletion ages (T RD ), varying from 1.22 – 2.21 Ga, are analogues to their Re-Os model ages (T MA , 1.29 – 2.31 Ga). Therefore, Re-Os isotopic ages recorded by ophiolites in QOB indicate that at least two major melt-extraction episodes occurred in the lithospheric mantle during Proterozoic (1.2 – 1.6 Ga and 2.0 – 2.2 Ga). Notably, the 1.2 – 1.6 Ga mantle melt-depletion event may have a genetic affinity with the 1.1 – 1.7 Ga crustal growth. Mantle rocks from an ophiolite have a great potential in tracking ancient geological events, such as Precambrian mantle melting.

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

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

Structural and geochemical evidence for a failed rift crustal evolution model in Western Dharwar Craton, South India

Detailed geological mapping along with structural and geochemical analysis were carried out in the Kibbanahalli Schist Belt (KSB) of the Bababudan Group in the Western Dharwar Craton (WDC). Repeated sequences of sedimentary rocks and volcanic rocks with conformable contact relations are the major lithological association in KSB. Sedimentary structures and field relations indicate that the Bababudan Group begins with basal conglomerate composed of debris (talus) and alluvial fan deposits or fluvial deposits. Volcanic rocks preserve features of explosive volcanism in an aqueous environment.Based on the field and microstructural observations five stages of deformations (D0, D1, D2, D3, and D4) are characterized from the study area. Among these deformation events, D2 is the major regional-scale event. East dipping NW–SE trending thrust faults and F2 parallel folds that are upright and isoclinal mark the D2 event. These F2 folds are sandwiched between D2 thrust faults. D2 structures suggest the temporal evolution of a fold-and-thrust belt by inversion of an intracratonic basin.Trace element geochemical analysis of the mafic to intermediate rocks from KSB and Chitradurga Schist Belt (CSB) shows three types of patterns. Type 1 has Nb–Ta negative anomalies, Type 2 is relatively flat trend and Type 3 has Zr–Hf positive anomalies. Type 1 has slightly enriched LREE trend while relatively flat LREE trend in Type 2 and 3. HREE trend is relatively flat in all samples, except for one sample from Type 3. The geochemical variations are related to the degree of crustal contamination experienced during their eruption in connection with the intracratonic extension.A pair of thrust faults and folded sequences of volcanic, volcanoclastic and sedimentary rocks summarize the major structural association in the study area. This association is identical to an inverted failed rift structure. Geochemical evidence also points to an intracratonic mafic to intermediate volcanism. Our study thus provides a well-preserved example of Archean failed rift. We thus propose the post-3.0 Ga crustal growth throughout the Western Dharwar Craton is therefore initiated by similar failed rifting events.

Diversity of late Neoarchean K-rich granitoid rocks derived from subduction-related crust/mantle interactions in the Jiaobei terrane, North China Craton

The appearance of voluminous K-rich granitoids within nearly all ancient cratons represents one major characteristic of late Archean Earth, which hold the key to understand the transitional geodynamic regimes globally during this period. The genetic regimes and links among different K-rich granitoids and their implications for crustal growth and evolution remain controversial. A series of late Neoarchean K-rich granitoids, including quartz dioritic, granodioritic, and monzogranitic gneisses, occur in the Jiaobei terrane of North China Craton. Zircon U-Pb isotopic data reveal that they emplaced during ~2544–2494 Ma, coeval with regional ~2530–2470 Ma high-grade metamorphism.The quartz dioritic-granodioritic gneisses are magnesian rocks, and they show low Si and Ti, but high K and Mg, similar to Archean low-Ti sanukitoids. The Sr/Y and (La/Yb)N are high (mostly 59.99–119.32 and 8.56–61.42), with moderate to high Nb (up to 11.79 ppm). Geochemical modeling, combined with depleted zircon εHf(t2) (+0.5 − +7.2) and the presence of minor xenocrystic zircons, indicate that these low silica samples were derived from a metasomatized depleted mantle source with inputs of slab-derived fluids and melts, and minor contamination by ancient crustal materials. The monzogranitic rocks are ferroan rocks showing high Si, K, and Fe, but low Mg. They are divided into two subgroups: the first displays low TREE of 44.00–127.00 ppm and positive Eu anomalies (EuN/Eu*N = 1.06–1.60), whereas the second shows high TREE of 85.76–819.02 ppm but negative Eu anomalies (EuN/Eu*N = 0.51–0.62). Geochemical modeling and depleted zircon εHf(t2) of +2.6 − +8.4 suggest their formation by partial melting of juvenile crustal sources involving tonalitic and some metasedimentary rocks at diverse crustal levels.Combined with regional geological data, these late Neoarchean K-rich granitoids were generated by coupled melting of metasomatized depleted mantle and dominantly juvenile crustal materials during crustal stabilization. Furthermore, the Jiaobei terrane experienced ~2.6–2.5 Ga crustal growth under a subduction-accretion setting.

Petrogenesis and tectonic regime of two types of Neoarchaean amphibolites in the northern margin of the North China Craton

ABSTRACT The Neoarchaean crustal evolutionary processes of the North China Craton (NCC) provide a window to understanding the crust–mantle interaction in the Early Earth. The Jiefangyingzi amphibolites are located in the Bainaimiao arc belt, along the northern margin of the NCC. LA–ICP–MS U–Pb isotopic data reveal that the Type 1 amphibolites were formed at ~2562 Ma, and the Type 2 amphibolites were formed at ~2504 Ma, but were subsequently subjected to metamorphism at ~1889 Ma. Lu–Hf isotopic data of the Type 1 amphibolite show positive εHf(t) values (+1.2–+6.0) and TDM from 2612 Ma to 2791 Ma, suggesting a significant episode of 2.6–2.8 Ga crustal growth within the study area. Geochemically, the Type 1 amphibolites show high SiO2 contents (53.37%–55.19%), moderate LREE–enriched patterns (La/Sm)N = 2.42–3.15 and HREE–depleted patterns (Gd/Yb)N = 1.86–2.64, with negative Nb anomalies, which are similar to IAB–like rocks, suggesting that they were derived from partial melting of arc–related depleted mantle with intense subduction–related fluids metasomatism. The Type 2 amphibolites have lower SiO2 contents from 44.28% to 50.77%, and show LREE–depleted REE patterns ((La/Sm)N = 0.67–1.02), nearly unfractionated HREE patterns ((Gd/Yb)N = 0.89–1.77), with negative Nb, Th and positive Sr, Zr anomalies, which are close to N–MORB–like rocks, indicating that they were generated from partial melting of depleted mantle with slight fluid metasomatism. The geochronological, Lu–Hf isotopic and geochemical features show that the IAB–like rocks were formed in an intra–oceanic arc environment, coincide with the significant ~2.5–2.6 Ga episode of crustal growth in the NCC; The slightly younger MORB–like rocks reflect the spreading of the oceanic crust might last until ~2.5 Ga in the central zone.

Neoarchean-Paleoproterozoic magmatic arc evolution in the Wutai-Hengshan-Fuping area, North China Craton: New perspectives from zircon U–Pb ages and Hf isotopic data

The Wutai granite-greenstone belt, which formed in the Neoarchean-Paleoproterozoic, with the adjoining high-grade metamorphic Hengshan-Fuping region, record important transitions in crustal and tectonic evolution, although these evolutionary processes have still been in dispute. In this study, new U–Pb and Lu–Hf LA-ICP-MS zircon analyses were obtained from eight metamorphosed magmatic rocks in the Wutai-Hengshan-Fuping area in North China Craton (NCC). The results reveal that most of Wutai granitoids yield emplacement ages of 2.56–2.52 Ga, with corresponding εHf(t2) of +4.7 to +8.3. The tonalitic gneiss from Dongshandi intrusion, however, gives a new age of 2482 Ma and εHf(t2) of +2.5 to +4.6, which are similar with Yixingzhai Pluton marked by ages of 2501–2499 Ma and εHf(t2) of +3.2 to +4.4. Also, ∼2.7 Ga and ∼2.5 Ga continental crust generation are further revealed. But ∼2.5 Ga crustal growth is more complicated due to the involvement of old crustal components, which can be identified by variable εHf(t2) values ranging from −1.8 to +7.5. Thus, based on the distinctions of Hf isotopes, compositional and spatial features, two consecutive groups of 2.56–2.52 Ga and 2.52–2.48 Ga are recognized and a two-step tectonic evolution model for the Wutai-Hengshan-Fuping area is proposed. During 2.56–2.52 Ga, intra-oceanic subduction led to the formation of Wutai oceanic island arc, accompanied mainly by the development of Wutai granitoids (εHf(t2) of +4.7 to +8.3) and coeval volcanics (predominant arc-tholeiitic chemical affinity), corresponding to the juvenile crustal growth. Between 2.52 Ga and 2.48 Ga, with intervening oceanic crust between intra-oceanic arc and continent consumed, the Wutai intra-oceanic arc then collided and was obducted onto the western edge of the Hengshan-Fuping continental block. Large amounts of TTG gneisses produced in both the Wutai and Hengshan-Fuping (εHf(t) of −1.8 to +7.5), along with calc-alkaline volcanism in Wutai, possibly in response to the thickening through arc accretion and commencing the continental arc. This proposed two-step tectonic evolution model, combined with a late Neoarchean (2.6–2.5 Ga) intra-oceanic arc system along the northwestern of the Eastern Block (EB), probably provides a better explanation for the fundamental geodynamic mechanism for continental arc development at the Proterozoic dawn.

Neoarchean magmatism and implications for crustal growth and evolution of the Kuluketage region, northeastern Tarim Craton

The Tarim Craton is well known as one of the oldest large blocks in China. The Kuluketage region is an important area for studying the Precambrian crustal evolution of the Tarim Craton because rare Precambrian basement rocks are exposed in the craton. In this study, amphibolites and granitic gneisses from the Pargangtage area of eastern Kuluketage have been studied using a combination of ICP-MC-MS zircon U–Pb dating and Lu–Hf isotopic analyses and whole rock major, trace element and Sm–Nd isotopic geochemistry to constrain their magmatic ages and sources. The amphibolites and granitic gneisses yield intrusion 207Pb/206Pb ages of 2524 ± 20 Ma and 2501 ± 17 Ma, respectively. The amphibolites are characterized by enriched LREE, flat HREE patterns, and pronounced negative Nb, Ta, Zr, Hf, Ti, and Y anomalies that indicate an arc affinity. The moderate positive εNd(t) values (+3.43 to +5.52) and zircon εHf(t) values (+1.36 to +6.53) suggest that the parental mafic magma of the amphibolites was mainly generated from a depleted lithospheric mantle source that was metasomatized by subduction-related components. The granitic gneisses show chemical features of high SiO2; low Mg#, Cr and Ni with fractionated REE patterns; and high Sr, (La/Yb)N, and Sr/Y ratios. These values are consistent with the chemical characteristics of Cenozoic adakites, which were derived from the partial melting of mafic rocks in an island arc tectonic setting. The positive εNd(t) values (+3.21 to +4.42) and εHf(t) values (+1.64 to +6.31) of the granitic gneisses indicate they were derived from the partial melting of juvenile crustal materials. This study, combined with previous analytical results, suggests that the northern Tarim Craton mainly experienced continental crustal growth episodes at ∼3.2 Ga, ∼2.9–2.8 Ga and ∼2.65 Ga, and two magmatic events at ∼2.7 Ga and ∼2.5 Ga during the Archean. A continental (oceanic) arc probably played an important role in the ∼2.7 Ga and 2.5 Ga crustal growth and reworking of the northern Tarim Craton.

Hydrothermal modification of zircon geochemistry and Lu–Hf isotopes from the Hongtoushan Cu–Zn deposit, China

The volcanic zircons, inherited pre-magmatic core domains and hydrothermal zircons in felsic meta-volcanic rocks and Cu–Zn orebodies from the Neoarchean Hongtoushan deposit in China were distinguished according to their texture, 207Pb/206Pb ages, Th/U and oxygen isotopic ratios. Their respective element compositional and Lu–Hf isotopic features were studied using LA–(MC)–ICP–MS methods, showing that the compositions of the pre-magmatic core domains are intensely modified by hydrothermal processes, exhibiting no differences with hydrothermal zircons. All zircons show a linearly decreasing trend in the ZrO2 vs SiO2 plot, but similar REE patterns, with steep slopes, Ce and Eu anomalies and enrichments of HREE, indicating their common petrologic origin. Nevertheless, compared with volcanic zircons, pre-magmatic core domains underwent compositional modification in the hydrothermal accretion processes by P and Y substitution mechanisms. Therefore, these core domains are highly enriched in P2O5, Y2O3, REEtotal, and Hf2O as well as some incompatible elements, such as Rb, Sr, Nb, Ta and Pb, similar to the hydrothermal zircons. Moreover, the Lu/Hf ratios are elevated by almost an order of magnitude, which is associated with the addition of Lu rather than changes in Hf, leading to increased 176Lu/177Hf ratios and decoupling of Hf–O isotope systematics. Nevertheless, the hydrothermal influence on the 176Hf/177Hf ratios seems to be undetectable, rising by 0.022% when compared with the ratios of the volcanic zircons. All 176Hf/177Hf ratios plot between depleted mantle and modern known plumes suggest a mantle plume setting corresponding to the ∼2.6Ga crustal growth in the Qingyuan district.

Age and depositional setting of the Paleoproterozoic Gantaohe Group in Zanhuang Complex: Constraints from zircon U–Pb ages and Hf isotopes of sandstones and dacite

The Gantaohe Group, located in the Zanhuang area, contains typical North China Craton Paleoproterozoic strata, and is one of the best-preserved examples of these rocks. It is therefore a significant area for deciphering evolutionary process in the trans-North China Orogen. In this paper, we present petrologic, detrital and volcanic zircon U–Pb ages and Lu–Hf isotopes from three formations of the Gantaohe Group. The zircons from a conglomerate at the base of the Gantaohe Group yielded a young population of ca. 2.1Ga. Dacite from the upper section of the Nansizhang Formation yielded zircon 207Pb/206Pb ages of 2090±14Ma. Thus, the initial deposition of the Gantaohe Group occurred at ∼2.1Ga. The Group underwent greenschist to low amphibolite facies metamorphism, identical to regional 1.87–1.85Ga metamorphism in Zanhuang Complex. Therefore, we propose that the Gantaohe Group was deposited in the period between 2.1Ga and 1.9Ga. Sandstones in the Gantaohe Group are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with only minor lithic fragments, suggesting that detrital components were mainly derived from a cratonic source. Detrital zircons from sandstones exhibit a dominant population of 2.55–2.5Ga grains with minor contributions of ∼2.7Ga and ∼2.1Ga grains, which further indicate they were dominantly sourced from ∼2.5Ga TTG gneisses, late Archean Zanhuang and late Archean to Paleoproterozoic Guandu Groups, with subordinately input from ∼2.7Ga TTG gneisses and the Paleoproterozoic Xuting granite. Given that the sandstone clastic components chiefly comprise quartz and feldspar, there is a lack of volcanic lithics, and the major detrital zircons are 400Ma older than the initial deposition age, we propose that the Gantaohe Group was deposited in a rift-related setting far from any back-arc basin at ∼2.1Ga or 1880–1810Ma syn-collisional processes. A few 2.5–2.7Ga zircons have positive εHf(t) values identical with the contemporaneous depleted mantle and the 207Pb/206Pb ages are similar to their single stage model ages (TDM1), which suggests some degree of 2.5–2.7Ga crustal growth. Most ∼2.5 zircons, however, show 207Pb/206Pb ages that are much younger than the Hf single stage model ages, which indicates intensive crustal reworking in the late Neoarchean. The zircons collectively present two stage model ages ranging from 2.6 to 2.9Ga. Therefore, we suggest that the 2.6–2.9Ga periods represents a time of intensive crustal growth in the Zanhuang Complex.

Neoarchean Andean-type active continental margin in the northeastern North China Craton: Geochemical and geochronological evidence from metavolcanic rocks in the Jiapigou granite-greenstone belt, Southern Jilin Province

The Jiapigou granite-greenstone belt of Southern Jilin Province is located at the northeastern margin of the North China Craton (NCC). The Neoarchean metavolcanic rocks of this region provide strong lines of evidence in petrology and geochemistry for Neoarchean crust–mantle geodynamic evolution and significant crustal growth events. The metavolcanic rock assemblage is dominated by amphibolite, hornblende/garnet biotite plagioclase gneiss, and minor felsic gneiss. Zircon LA–ICP–MS U–Pb isotopic analyses reveal that these supracrustal rocks may be formed by two episodes of magmatism. The first magmatic episode occurred at ∼2688Ma and was recorded by minor felsic gneiss. The second episode occurred at ∼2588–2536Ma and was marked by a major lithological assemblage of metavolcanic rocks in this area. All the supracrustal rocks experienced amphibolite facies metamorphism at ∼2510–2480Ma.Except for the felsic gneiss, seventeen metavolcanic rock samples can be geochemically classified into three groups. Group #1 has tholeiitic basaltic chemical composition, and is characterized by slightly LREE-enriched chondrite-normalized REE pattern with low (La/Yb)N ratio and weakly negative Nb anomalies, similar to those of primitive arc tholeiite. Their magmatic precursor was most likely generated by partial melting of upper sub-arc mantle wedge slightly metasomatized by slab-derived fluids or melts in a subduction initiation environment. Group #2 with basaltic composition is akin to typical calc-alkaline basalt (CAB), showing fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. The magmatic precursor of the Group #2 was derived from partial melting of a sub-arc mantle wedge at relatively deeper levels, significantly metasomatized by slab-derived fluids or melts. Group #3 show chemical composition of andesite to dacite, and is distinguished by wide variations in SiO2, MgO and Mg# values, strongly fractionated chondrite-normalized REE patterns with higher (La/Yb)N ratios, low Yb and Y contents, and negative Nb, Ta, Ti and P anomalies, analogous to the Phanerozoic adakites. They might be derived from magma mixing between the mantle wedge melts metasomatized by slab-derived melts with perhaps an ancient crustal contribution.The felsic gneiss shows positive zircon εHf(t2) values (+2.2 to +4.5), together with ∼2.7Ga metamorphic volcanic rocks in the neighboring area, indicating a ∼2.7Ga crustal growth episode. Most of the metavolcanic rocks display positive εHf(t2) values of +0.4 to +7.4, the highest of which is close to the value of contemporaneous depleted mantle, suggesting a significant crustal growth event at ∼2.59–2.54Ga. Combined with numerous ∼2.83–2.65Ga inherited zircons and ∼2688Ma felsic gneiss, the Jiapigou granite-greenstone belt evolved at an Andean-type active continental margin, recording the transition from primitive arc to mature arc.

Timing of deposition and tectonothermal events of banded iron formations in the Anshan–Benxi area, Liaoning Province, China: Evidence from SHRIMP U-Pb zircon geochronology of the wall rocks

Most of the Algoma-type BIFs and associated volcanic suites in the North China Craton (NCC) have undergone high-grade metamorphism during the ∼2.5Ga tectonothermal event. Isotopic systems in high-grade metamorphic rocks are often reset, thereby losing the earlier records, whereas zircons in low-grade metamorphic rocks are potential candidates to evaluate earlier thermal events. Detailed textural relationship and internal structures of zircons and their age data for eight wall-rock samples with metamorphic grades from greenschist to upper amphibolite facies of the Algoma-type BIFs in the Anshan–Benxi area are used to constrain the age of protolith. Zircons in the greenschist to lower amphibolite facies are generally metamorphism-resistant, whereas those in the upper amphibolite facies rocks show variable recrystallization. SHRIMP zircon U–Pb dating of magmatic zircons in six samples constrains the peak of BIF-deposition at 2.56–2.53Ga, which is consistent with ∼2.5Ga crustal growth in the NCC. In addition, the greenschist-facies metamorphic rocks in the Anshan area also record an event of volcanic activity at 2.72–2.65Ga. We consider that low-grade metamorphic wall rocks were not significantly affected by the ∼2.5Ga tectonothermal event, and the 2.72–2.65Ga age group records another important magmatic event corresponding to ∼2.7Ga crustal growth. The metamorphic zircons from these rocks record a thermal event during 1.88–1.84Ga.

Zircon U–Pb–Hf isotope systematics and geochemistry of Helong granite-greenstone belt in Southern Jilin Province, China: Implications for Neoarchean crustal evolution of the northeastern margin of North China Craton

As one of the major Neoarchean granite-greenstone belts in China, the Helong terrane is located at the northeastern margin (Southern Jilin Province) of the North China Craton (NCC). It is dominated by metamorphic supracrustal rock assemblage and granitoid gneiss. The former contains amphibolite, hornblende plagioclase gneiss, biotite plagioclase gneiss and banded iron formation (BIF), whereas the latter is dominated by the Tonalite–Trondhjemite–Granodiorite (TTG) gneisses. Zircon LA–ICP–MS U–Pb isotopic analyses indicate that the supracrustal rock assemblage was formed at 2683–2654Ma and underwent regional metamorphism (lower amphibolite facies) at 2614–2553Ma before the widespread emplacement of plutonic granitoids at 2551–2532Ma. Both greenstone sequence and plutonic granitoid rocks experienced the subsequent regional metamorphism (greenschist to lower amphibolite facies) at 2508–2400Ma. The supracrustal rocks can be divided into three groups based on their lithological and geochemical characteristics. Detailed petrogenetic and geochemical studies suggest that the Group #1 amphibolite samples were derived from partial melting of depleted upper mantle that had been metasomatized by slab-derived fluids; Group #2 amphibolite samples were derived by partial melting of a depleted mantle wedge that was strongly metasomatized by subduction-derived fluids or melts; and Group #3 hornblende/biotite plagioclase gneiss samples were derived from partial melting of juvenile basalt and minor amounts of older sediments in the lower crust level. The granitoid gneisses in the Helong granite-greenstone belt can be divided into relatively high magnesium group (HMG) and low magnesium group (LMG). The HMG samples were derived from parental magmas generated by partial melting of juvenile basalt and minor amounts of older sediments within the lower crust level, whereas magmatic precursors of the LMG samples were generated by partial melting of a metapelite-dominated source under lower temperature melting conditions.U–Pb dating of inherited or captured zircons, combined with zircon Lu–Hf isotope data, indicates a significant ∼2.9–2.7Ga crustal growth episode, whereas the ∼2.55–2.53Ga magmatism involved crustal recycling in the Helong terrane. Consequently, the Helong terrane preserves a record of Neoarchean tectonic environment of Andean-type active continental margin.

Archean-Paleoproterozoic crustal evolution of the Ordos Block in the North China Craton: Constraints from zircon U–Pb geochronology and Hf isotopes for gneissic granitoids of the basement

The Ordos Block has been considered as an Archean micro-block in North China Craton. However, its formation and evolution are poorly understood for a long time due to lack of available basement rocks. Our LA-ICPMS and SIMS zircon U–Pb dating of gneissic granitoids from the Ordos Block basement identified two distinct periods of granitic magmatism at ∼2.5Ma and 2.2–2.0Ga, and two phases of metamorphisms at ∼1.95Ga and ∼1.85Ga, respectively. Of which most zircons of ∼2.5Ga granitoids show positive ɛHf(t) values ranging from +0.1 to +4.1 with a peak TDM of ∼2.7Ga. Minor zircons have negative ɛHf(t) values from −3.8 to −0.02 and THfC of 2.8–3.1Ga. Moreover, the 3.40Ga inherited zircon and zircons with THfC of 3.1–4.2Ga were found from 2.2Ga to 2.0Ga granitoids. These suggest that there existed a Neoarchean crust in the Ordos Block, which underwent a major crustal growth at ∼2.7Ga and granitic magmatism at ∼2.5Ga, highly consisting with ∼2.7Ga crustal growth and ∼2.5Ga magmatism resulted from juvenile crustal reworking in North China Craton. The zircons from 2.2Ga to 2.0Ga granitoids in Ordos Block basement display variable Hf isotopic composition of positive to negative ɛHf(t) values, much similar to those of 2.2–2.0Ga granitoids related to continental margin arc setting in the Trans-North China Orogen. They were subsequently subjected to ∼1.95 and ∼1.85Ga metamorphisms, which were widespread throughout the Trans-North China Orogen. Based on the highly coherent zircon geochronology and Hf isotopic composition between Ordos Block and other micro-blocks of North China Craton, we propose that there is a Neoarchean crust in Ordos Block, which underwent a tectonic evolution similar to those in East Block in later Neoarchean, and then experienced 2.2–2.0Ga granitic magmatism related to continental margin arc and 1.95–1.85Ga metamorphism resulted from collision between the East and West Blocks along TNCO.

Geochronology and geochemistry of the Nanfen iron deposit in the Anshan-Benxi area, North China Craton: Implications for ∼2.55 Ga crustal growth and the genesis of high-grade iron ores

The Nanfen iron deposit, situated in the Anshan-Benxi area of the North China Craton (NCC), hosts scattered banded iron formations (BIFs) and BIF-hosted high-grade iron orebodies (defined by FeOT>80wt.%). The associated roof and bottom rocks, which occur in parallel to orebodies, are mica quartz schist and chlorite hornblende schist (locally interlayers), respectively. The PAAS-normalized rare earth element (REE) patterns indicate that quartz–magnetite banded iron formations (BIFs) are enriched in Heavy REEs with positive La, Eu and Y anomalies, indicating that the Nanfen BIF precipitated from a mixture of ambient seawater and submarine high-T hydrothermal fluids (∼0.1%). In addition, the BIF exhibits low concentrations of Al2O3 and TiO2 with depletion in HFSEs (high field strength elements, e.g., Nb, Ta, Zr, Hf and Ti), suggesting little contamination with continental detritus. Excepting lower Eu/Eu* values, the high-grade iron ores display similar geochemical features to the BIF, likely implying that they have a same iron source but did not form synchronously. Electron microprobe analyses (EMPA) and in situ LA-ICP-MS analyses reveal that in relative to the magnetite grains in high-grade ores, the magnetite grains in the BIF contain higher MnO, MgO and Zn concentrations, and lower Al2O3 and REE abundances, implying different formation conditions. Thus, we argue that high-grade ores are likely to be alteration products of primary BIFs. Chlorite hornblende schist (meta-basalt) samples exhibit low SiO2 contents (43.78–50.30wt.%) with flat chondrite-normalized REE and N-MORB-normalized trace element patterns, which are similar to those of BABB (back-arc basin basalt). The zircon U–Pb dating results indicate that magmatic and metamorphic zircons from chlorite hornblende schist formed at approximately 2552Ma and 2482Ma, respectively. The former volcanic age could constrain the deposition age of the Nanfen BIF, and the latter reflects a subsequent metamorphic event, which is also reflected by the ∼2480Ma hydrothermal zircons selected from the BIF. Moreover, most zircons from chlorite hornblende schist have positive ɛHf(t) values (0.22–7.96) close to the depleted mantle evolutionary curve, implying that the source magma were derived from a depleted mantle with some contamination from ancient crust. Hence, we propose that the Nanfen BIF is a Neoarchean Algoma-type BIF, and its deposition was most likely benefited due to crustal growth.

Zircon U–Pb–Lu–Hf–O isotopic evidence for ≥3.5 Ga crustal growth, reworking and differentiation in the northern Tarim Craton

Continental crust was largely generated before 2.5Ga through mafic–ultramafic and TTG (tonalite-trondhjemite-granodiorite) magmatism, but it is contentious when did such primitive crust evolve into mature granodioritic to granitic composition similar to modern upper crust. Here we present zircon U–Pb–Lu–Hf–O isotopic data for late Paleoproterozoic metasedimentary rocks (the Xingditag Group) in the Kuruktag area, northern Tarim Craton, NW China. CL-imaging reveals core–rim structures for most zircons from a garnet-bearing paragneiss and a semi-pelitic schist, whereas two quartzites are dominated by metamorphic zircons. SHRIMP and/or LA-ICP-MS U–Pb dating yielded a range of detrital ages from ca. 2.0 to 3.5Ga for the zircon cores and a consistent metamorphic age of ca. 1.93Ga for the rims for the paragneiss and schist. However, zircons from the two quartzites mainly record a ca. 1.85Ga metamorphic event; detrital zircons are rare or absent. These data confirm that the Xingditag Group was deposited after ca. 2.0Ga and was metamorphosed at ca. 1.93 or 1.85Ga. Importantly, the ca. 2.0–3.5Ga concordant detrital zircons exhibit low initial 176Hf/177Hf ratios (as low as 0.28045) and high δ18O values (6.6–11.4‰). These values are interpreted as recording primary magmatic features of the basement rocks in the northern Tarim Craton, because: (1) the dominantly prismatic or fragmentary morphology, oscillatory zoning and moderate Th/U ratios of the detrital zircons indicate a local provenance dominated by igneous rocks; and (2) the within-grain and overall heterogeneities argue against Hf and O isotopic resetting during metamorphism. Linear regressions of the initial 176Hf/177Hf values of these detrital zircons yield a remarkably consistent 176Lu/177Hf ratio of 0.01 for the oldest (TDM2=3.9 and 3.7Ga) and youngest (TDM2=2.8 Ga) crustal components. These observations suggest that significant amounts of felsic continental crust may have been formed, altered and reworked as early as ca. 3.5Ga, marking crustal differentiation and maturation during the Paleoarchean. Hafnium crustal model ages reveal that the oldest crustal component in the northern Tarim Craton may have been generated before ca. 3.9Ga, much earlier than previously thought.

Zircon Hf isotope of Yingfeng Rapakivi granites from the Quanji Massif and ∼2.7 Ga crustal growth

The Quanji (全吉) Massif is located in the Northwest China, which is interpreted as a micro-continent that is composed of metamorphic basement and stable cover strata. There are some controversies of genetic relationship between the Quanji Massif and the major cratons in China. In this study, we obtained in situ zircon U-Pb and Hf isotopic compositions of the Yingfeng (鹰峰) rapakivi granites from the northwest Quanji Massif by application of LA-MC-ICP-MS technique. Twenty U-Pb age measurements points are concordant or near concordant, and their weighted mean 207Pb/206Pb age is 1 793.9±6.4 Ma (MSWD= 1.09), yields an upper intercept age of 1 800±17 Ma (MSWD=0.41); 19 Hf isotope measurements yield a two-stage Hf model ages (T DM2) of 2.63 to 2.81 Ga, with a weighted average age of about 2.70±0.02 Ga and ɛHf(t) values variate between -8.91 to -5.35. This indicates that magma source of the Yingfeng rapakivi granites were produced from partial melting of late stage of Neoarchean juvenile crust, and suggests a significant crustal growth event occurred in the Quanji Massif at that time. The Quanji Massif might be an ancient continental segment detached from the Tarim Craton based on the crustal growth history and other geological records. The Tarim Craton (including the Quanji Massif) and the North China Craton had a similar or homological early crustal evolution around ∼2.7 Ga, which implies that Tarim Craton might be one of the component parts of North China Craton.

Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of metavolcanics from eastern Hebei reveal Neoarchean subduction tectonics in the North China Craton

The Neoarchean metavolcanic rocks of Saheqiao occur within the central domain of the Precambrian terrane in eastern Hebei within the North China Craton. These rocks consist mainly of garnet two-pyroxene granulites, (garnet) clinopyroxene amphibolites, amphibole plagioclase gneisses and biotite plagioclase gneisses. These metavolcanic rocks are exposed as several kilometers scale of massive blocks or several tens of centimeters scale of enclaves within Precambrian granitoid gneisses. Based on whole rock geochemistry, we classify these intermediate-mafic volcanic rocks into four groups. Group #1 exhibits low SiO2, Yb and Nb/Ta ratios, and high Ni and Cr and shows N-MORB-like (Normal Mid-Ocean Ridge Basalts) geochemical features, suggesting that the magmatic precursors of these rocks were derived through partial melting of depleted mantle. Compared to Group #1, Group #2 shows slightly right-declined REE and nearly flat multi-element patterns with variable negative Nb anomalies, matching with primitive arc basaltic rocks formed by partial melting of the mantle wedge. Group #3 exhibits chemical compositions of basaltic and andesitic rocks and is characterized by pronounced LREE enrichment but HREE depletion and shows distinctly negative Nb and Ta anomalies and weakly negative Zr and Hf anomalies. These rocks may compare with island arc volcanic rocks, and were probably generated by partial melting of the mantle wedge modified by dehydration of subducted oceanic slab. The magmatic precursors of the andesites were derived from the crystallization differentiation of the basaltic magma. Group #4 is characterized by clearly right-declined REE patterns and slightly positive or no Nb–Ti anomalies, which are analogous to Nb-enriched basalts (NEB) and originated from melting of HFSE-enriched mantle sources.LA-ICP-MS U–Pb dating of magmatic zircons from five samples from the three groups reveals that Group #1 N-MORB-like rocks formed at 2525±7Ma, Group #2 primitive arc basaltic rocks at 2518±15Ma, and Group #3 typical arc volcanic rocks at 2523±6Ma to 2614±18Ma. Zircons from all the dated samples also preserve signatures of 2450–2370Ma metamorphisms or thermal events. The zircon Lu–Hf data from the three dated samples in Group #3 show εHf(t) values ranging from −1.4 to +9.0 and TDM values of 2627Ma to 2965Ma, suggesting a significant episode of 2.6–2.9Ga crustal growth in this region. The ~2525Ma N-MORB-like basaltic rocks are younger than the arc-related rocks (~2614–2518Ma), suggesting that the MORB-like basaltic rocks were generated in a back arc environment. On the basis of the rock association of back-arc MORB basalts (Group #1), arc-related andesites–basalts (Groups #2 and #3) and NEB-like rocks (Group #4), we suggest that the Neoarchean volcanic rocks of the eastern Hebei region formed in a subduction-related setting within a convergent plate margin.

The latest Neoarchean–Paleoproterozoic evolution of the Dunhuang block, eastern Tarim craton, northwestern China: Evidence from zircon U–Pb dating and Hf isotopic analyses

The Dunhuang block, in the easternmost segment of the Tarim craton, exposes Precambrian mafic granulite and felsic gneiss. To better understand the poorly known formation and evolution of the Tarim craton, we carried out detailed field-based petrological and geochronological investigations on the major lithologies of the Dunhuang block. U–Pb dating and Hf isotopic analyses on the mafic granulite and felsic gneiss (TTG gneiss) of the Dunhuang block reveal two distinct age populations: the Latest Neoarchaean (∼2.5Ga) and the Late Palaeoproterozoic (1.82–1.85Ga). The zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of ∼2.5Ga, supporting the existence of Archean rocks in the Dunhuang block. The short time interval between Latest Neoarchean magmatism and high-grade metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons show Hf-depleted mantle model ages between 2.55Ga and 2.8Ga with a peak at ∼2.7Ga, suggesting that ∼2.7Ga juvenile crust of the Dunhuang block was reworked during a 200–300m.y. period at the end of the Neoarchean. The age of ∼1.85Ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high-pressure granulite facies metamorphism. Combined with the Hf isotopic data, these data suggest that the Dunhuang block experienced ∼2.7Ga crustal growth, a ∼2.5Ga magmatic–metamorphic event and a 1.82–1.85Ga (HP) granulite facies metamorphic event. This sequence of events is very similar to that of the North China Craton, implying that the evolutionary history of the Dunhuang block was consistent with that of the North China Craton from the Neoarchean to Paleoproterozoic.

2.6–2.7 Ga crustal growth in Yangtze craton, South China

A combined study of zircon U–Pb and Lu–Hf isotopes and whole rock major and trace elements and Sr–Nd isotopes has been conducted for 10 tonalitic–trondhjemitic–granodioritic (TTG) and granitic gneisses from the Kongling terrain, the only known Archean microcontinent in the Yangtze craton, South China. The results reveal a significant magmatic event at ∼2.6–2.7Ga, in addition to the previously reported ∼2.9Ga and ∼3.2–3.3Ga magmatism. The ∼2.6–2.7Ga rocks show relatively high REE (530–1074ppm), apparently negative Eu anomaly (Eu/Eu*=0.22–0.35), low #Mg (19.51–22.63) and low LaN/YbN (10.3–24.2). Besides, they have high K-feldspar proportion and relatively evaluated (K2O+Na2O)/CaO, TFeO/MgO, Zr, Nb, Ce and Y contents. Their 10,000×Ga/Al ratios range between 3.00 and 3.54. All these features suggest that the protoliths of these gneisses are A-type granites. Most of the ∼2.6–2.7Ga zircon grains have ɛHf(t) values >0 (up to 7.93, close to the depleted mantle value). This clearly indicates a considerably higher proportion of new crustal components in the ∼2.6–2.7Ga granitoids compared to the ∼3.2–3.3Ga and ∼2.9Ga TTGs. Our results support the conclusion of worldwide studies of igneous and detrital zircons that age peaks at 2.65–2.76Ga represent increases in the volume of juvenile continental crust. The present study also confirms the existence of the two older magmatic events in the Kongling terrain. Both whole rock ɛNd(t) values (−3.74 to 1.59) and the zircon ɛHf(t) values (−11.18 to 3.55) for the ∼2.9Ga TTG and the Hf isotopes of ∼3.2–3.3Ga igneous zircons (−7.37 to 3.12) are chondritic or subchondritic, suggesting that they were mainly generated by reworking of older rocks with a small amount of new crustal additions.