Juvenile Crustal Growth Research Articles

Changing provenance of late Neoarchean metasedimentary rocks in the Anshan-Benxi area, North China Craton: Implications for the tectonic setting of the world-class Dataigou banded iron formation

The Anshan-Benxi (AnBen) area is situated in the northeastern part of the North China Craton and is considered to be the most important iron metallogenic province in China. The giant Dataigou banded iron formation (BIF) has the potential to be the largest iron deposit in this area, even in Asia. Here, we present in situ zircon U–Pb-Hf-O isotopic and whole-rock geochemical data for metasedimentary rocks (metapelites and metaarenites) and trondhjemitic gneiss associated with this BIF, in order to provide constraints on the setting of the deposit, with implications for geological and exploration models. SIMS zircon U–Pb dating results indicate that the trondhjemitic gneiss crystallized at 3450±19Ma, and thus it is the oldest rock so far indentified in the Benxi area. The youngest group of detrital zircons from the metaarenite samples constrains their maximum depositional age at ~2.54Ga. In combination with the earlier metamorphic age of ~2.51Ga for the metaarenites, the deposition age of the Dataigou BIF can be constrained between 2.54 and 2.51Ga. The metasediments have undergone varying degrees of source weathering. Source rocks of the metapelites have undergone moderate to severe chemical weathering, whereas those of the metaarenites have subjected to relatively weak chemical weathering. Diagnostic geochemical features such as the Al2O3/TiO2 values, trace element ratios, and REE patterns suggest that the metaarenites were predominantly derived from felsic igneous sources, whereas the metapelites were sourced mainly from mafic tholeiitic rocks. The dominant late Neoarchean detrital zircons further indicate that they were most likely sourced from almost synchronous volcanic rocks in the AnBen area. Taking into account the lithostratigraphic, geochemical, and geochronological data, the Dataigou BIF is interpreted as having been deposited in a back-arc basin, most probably on the margin of an ancient continental crust. Continuous provenance variations of these metasediments during highstand times and transgression can be ascribed to changes in natures of nearby volcanisms. Integrated with previous zircon Hf isotopic data from other supracrustal rocks in the AnBen area, our Hf-O data reveal major juvenile crustal growth stages at 3.0Ga and during 2.8–2.7Ga, and a crustal reworking event with minor juvenile addition at ~2.5Ga in the studied area.

Geochemistry and Sr–Nd–Hf isotopes of Middle Devonian igneous rocks of the Sarsuk polymetallic Au deposit: implications for understanding the tectonic evolution of the south Altay Orogenic Belt, Northwest China

ABSTRACTThe medium-tonnage Sarsuk polymetallic Au deposit is located in the Devonian volcanic–sedimentary Ashele Basin of the south Altay Orogenic Belt (AOB), Northwest China. Within the deposit, the rhyolite porphyries and diabases are widespread, emplaced into strata. The orebodies are hosted by the rhyolite porphyries. We studied the petrography, geochemistry, and Sr–Nd–Hf isotopes of the rhyolite porphyries and diabases, in order to understand the petrogenesis of these rocks and their tectonic significance. They display typical bimodality in geochemistry compositions. The diabases are characterized by SiO2 contents of 44.84–59.77 wt.%, high Mg# values (43–69), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb and Ta, low (87Sr/86Sr)i (0.706687–0.707613) values, positive εNd(t) (4.8–6.8) values, and positive and high εHf(t) (7.15–15.19) values, suggesting a depleted lithosphere mantle source that might have been metasomatized by subduction-related components. The rhyolite porphyries show affinity to sanukitoid magmas contents [high SiO2 (78.6–81.82 wt.%) and MgO (3.38–5.94 wt.%, one sample at 0.61 wt.%), and enrichments in LILE and LREE], they were derived from the equilibrium reactions between a mantle source and subducted oceanic crust materials. Those characteristics together with the positive εNd(t) (4.1–8.4) and εHf(t) (2.88–15.17) values indicate that the diabases and rhyolite porphyries were generated from the same mantle peridotite source. But the rhyolite porphyries underwent fractional crystallization of Fe–Ti oxides, plagioclase, and apatite due to their negative Eu (δEu = 0.21–0.28) and P anomalies. According to the geochemical and isotopic data, the Sarsuk Middle Devonian igneous rocks are considered to be the products of the juvenile crustal growth in an island arc setting. The Sarsuk polymetallic Au deposit formed slightly later than the Ashele Cu–Zn deposit in the Ashele Basin, but they have the same tectonic setting, belonging to the trench–arc–basin system during extensional process in the south AOB.

Pb isotope composition of granitoids from the Hercynides of the Central Asian orogenic Belt: Evidence for growth of the juvenile crust

Based on study of the Middle Paleozoic (Hercynian) structures of the Central Asian Fold Belt located in Southwest Mongolia, the problem of the sources of juvenile crust growth registered in the Pb isotope composition of postaccretional granitoids is considered. Our study shows that the Pb isotope system of Late Paleozoic granitoids from the Hercynides of the Central Asian Fold Belt provides evidence for the juvenile nature of the continental crust in this region. The evolution of the Pb isotope composition in the Hercynides of Mongolia corresponds to the model parameter μ = 9.25, according to the Stacey–Kramers twostage model. The juvenile source of the Hercynian crust should be characterized by a lower μ value. In addition, according to the Stacey–Kramers two-stage model, the Th/U value in this crust is 3.58.

Petrogenesis of taxitic dioritic–tonalitic gneisses and Neoarchean crustal growth in Eastern Hebei, North China Craton

The taxitic dioritic–tonalitic gneisses in the Zunhua–Qinglong microblock of Eastern Hebei, North China Craton, are composed of dioritic, quartz dioritic, tonalitic and trondhjemitic gneisses. They show taxitic or banded structures and were subjected to amphibolite- to local granulite-facies metamorphism. Based on their whole rock geochemistry, the dioritic and quartz dioritic gneisses (DQ Group) can be classified into a high Ni Subgroup (HNDQ Subgroup) and a low Ni Subgroup (LNDQ Subgroup), and the tonalitic and trondhjemitic gneisses (TT Group) into a low-fractionated Subgroup (LFTT Subgroup) and a high-fractionated Subgroup (HFTT Subgroup). LA-ICP-MS zircon U–Pb isotopic dating reveals that the magmatic precursors of the HNDQ, LNDQ and LFTT subgroups were emplaced from 2522±13 to 2533±14Ma, whereas those of the HFTT Subgroup emplaced at 2499±4Ma. Studies in petrogenesis revealed that the magmatic precursors of the HNDQ Subgroup were derived from partial melting of mantle peridotites metasomatized chiefly by slab melts, while those of the LFTT Subgroup were formed by fractional crystallization of the magmas of the HNDQ Subgroup, with clinopyroxene, magnetite, plagioclase and apatite as the major fractionated phases. Magmatic precursors of the LNDQ Subgroup were derived from partial melting of mantle peridotites metasomatized in a dominant manner by slab-derived fluids, with melts experiencing fractionation of hornblende, magnetite, plagioclase and apatite during the magmatic evolution. Further, magmatic precursors of the HFTT Subgroup were derived from partial melting of volcanic sedimentary rocks formed in the fore arc to trench belt, which were subducted along with oceanic slabs, with the melts contaminated by mantle peridotites during their ascent through the mantle wedge.These new data, combined with previous investigations on metavolcanic rocks and granitoid orthogneisses, suggest that the magmatic precursors of these taxitic dioritic–tonalitic gneisses were formed in an arc to back-arc tectonic setting from north to south. Furthermore, Lu–Hf isotopic data from the late Neoarchean metavolcanic rocks and granitoid gneisses in the Zunhua–Qinglong microblock reveal that the Zunhua–Qinglong microblock experienced three major periods of juvenile crustal growth during Neoarchean at 2.72–2.70Ga, 2.64–2.56Ga and 2.53–2.50Ga, respectively, with the latter two periods coupled with reworking of old crustal materials.

Unravelling the record of Archaean crustal evolution of the Bundelkhand Craton, northern India using U–Pb zircon–monazite ages, Lu–Hf isotope systematics, and whole-rock geochemistry of granitoids

Combined whole-rock, U–Pb zircon and monazite dating, in situ zircon (MC)-SF-ICP-MS Lu–Hf-isotope data are reported for more than 20 Archaean granitoid samples to constrain magmatic events and their sources in the Bundelkhand Craton, northern India. The results are crucial for understanding the extent of reworking vs juvenile crustal growth during Palaeoarchaean–Neoarchaean and also provide new information on the temporal gap between Palaeoarchaean TTG and Neoarchaean granodiorite–granite magmatism. These data reveal that the Bundelkhand Craton was affected by at least four discrete episodes of Palaeoarchaean TTG magmatism between 3.55Ga and 3.20Ga at regular intervals of 100–150 Myr. After a period of about 500 Myr of near magmatic shutdown, it was followed by a relatively short-lived, but more voluminous granitoid emplacement event during the Neoarchaean at 2.58–2.50Ga, during which minor granodioritic gneisses and voluminous granites intruded. The new data also provide evidence for a metamorphic overprint at ca. 3.20Ga, and a Pan-African tectonothermal overprint at 0.50–0.60Ga.Hf-isotope and whole-rock geochemical data of the TTGs suggest a complex process of crust formation and stabilisation during the Palaeoarchaean, mainly by the reworking of older mafic and felsic crust between 3.55 and 3.20Ga. The CHUR parallel array for the TTGs may indicate that these rocks were formed by partial melting of an oceanic crust that was derived from chondritic mantle sources, shortly before formation of TTG. This interpretation is inconsistent with Hf-isotope data from worldwide sources, which provide evidence for a successive mantle depletion since at least 4.0Ga, resulting in εHft of about +5.0 at 3.0Ga. Intense melting of the heterogeneous TTG crust during the Neoarchaean resulted in the coeval formation of granodioritic gneisses and voluminous granites, and subordinate hybrid granites with a minor contribution of melts from an enriched mantle source, affected by subduction-zone processes prior to 2.55Ga.

A review of the Paleozoic tectonics in the eastern part of Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogen in the world, which is responsible for considerable Phanerozoic juvenile crustal growth. The NE China and its adjacent areas compose the eastern segment of the CAOB, which is a key area for providing important evidence of the CAOB evolution and understanding the NE Asian tectonics. The eastern segment of the CAOB is composed tectonically of four micro-blocks and four sutures, i.e. Erguna block (EB), Xing'an block (XB), Songliao-Xilinhot block (SXB), Jiamusi block (JB), Xinlin-Xiguitu suture (XXS), Heihe-Hegenshan suture (HHS), Mudanjiang-Yilan suture (MYS) and Solonker-Xar Moron-Changchun-Yanji suture (SXCYS). The EB and XB were amalgamated by westward subduction, oceanic island accretions and final collision in ca. 500Ma. The XB and SXB were amalgamated by subduction-related Early Paleozoic marginal arc, Late Paleozoic marginal arc and final collision in the late Early Carboniferous to early Late Carboniferous. The JB probably had been attached to the SXB in the Early Paleozoic, but broken apart from the SXB in the Triassic and collided back in the Jurassic. The closure of Paleo-Asian Ocean had experienced a long continue/episodic subduction–accretion processes on margins of the NCC to the south and the SXB to the north from the Early to Late Paleozoic. The final closure happened along the SXCYS, from west Solonker, Sonid Youqi, Kedanshan (Keshenketengqi), Xar Moron River through Songliao Basin via Kailu, Tongliao, Horqin Zuoyizhongqi, Changchun, to the east Panshi, Huadian, Dunhua, Yanji, with a scissors style closure in time from the Late Permian-Early Triassic in the west to the Late Permian-Middle Triassic in the east. The amalgamated blocks should compose a united micro-continent, named as Jiamusi-Mongolia Block (JMB) after Early Carboniferous, which bounded by Mongo-Okhotsk suture to the northwest, Solonker-Xar Moron-Changchun suture to the south and the eastern margin of JB to the east.

Crustal evolution of the Eastern Block in the North China Craton: Constraints from zircon U–Pb geochronology and Lu–Hf isotopes of the Northern Liaoning Complex

The Northern Liaoning Complex in northeastern China constitutes an important component of the Eastern Block in the North China Craton. The major lithologies consist of Archean granitoid gneisses with minor supracrustal rocks occurring as tectonic lenses. This study presents zircon Lu–Hf isotopic data for the first time as well as new SHRIMP zircon U–Pb data of the major lithologies from the Northern Liaoning Complex, in order to elucidate the crustal evolution of the complex and provide new constraints on the Neoarchean crustal evolution of the Eastern Block in the North China Craton. Magmatic zircon U–Pb data from this study show that the protolith magmas of the supracrustal rocks and granitoid gneisses were generated during ∼2.55–2.50Ga. Metamorphic zircons document consistent metamorphic ages at 2.49–2.48Ga, suggesting a regional metamorphic event immediately after the magmatism at the end of the Neoarchean in the Northern Liaoning Complex. Inherited/detrital zircons of 2.79–2.60Ga suggest possible existence of ancient crust in this region. Zircon Lu–Hf isotopic compositions show that the magmatic zircons have variable ɛHf(t) values from −4.0 to +9.0 with depleted mantle model ages of 3.6–2.5Ga, of which most ɛHf(t) values are positive with a model age peak at 2.9–2.7Ga. These zircon Hf signatures reveal major juvenile crustal growth with additions of older crustal materials during 2.9–2.7Ga, and a crustal reworking event with minor juvenile additions at 2.6–2.5Ga in the studied area. Integrated with previous data from other Neoarchean complexes in the Eastern Block, both the major juvenile crustal growth during 2.9–2.7Ga and the strong crustal reworking at 2.6–2.5Ga contribute to the extensive Neoarchean crust formation of the Eastern Block in the North China Craton. The North China Craton share similar Neoarchean continental crustal evolution to other cratons in the world, though it is distinctively featured by intensive tectonothermal overprinting at the end of the Neoarchean.

Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

Continental flood basalts (CFBs) represent large-scale melting events in the Earth’s upper mantle and show considerable geochemical heterogeneity that is typically linked to substantial contribution from underlying continental lithosphere. Large-scale partial melting of the cold subcontinental lithospheric mantle and the large amounts of crustal contamination suggested by traditional binary mixing or assimilation-fractional crystallization models are difficult to reconcile with the thermal and compositional characteristics of continental lithosphere, however. The well-exposed CFBs of Vestfjella, western Dronning Maud Land, Antarctica, belong to the Jurassic Karoo large igneous province and provide a prime locality to quantify mass contributions of lithospheric and sublithospheric sources for two reasons: (1) recently discovered CFB dikes show isotopic characteristics akin to mid-ocean ridge basalts, and thus help to constrain asthenospheric parental melt compositions and (2) the well-exposed basaltic lavas have been divided into four different geochemical magma types that exhibit considerable trace element and radiogenic isotope heterogeneity (e.g., initial e Nd from −16 to +2 at 180 Ma). We simulate the geochemical evolution of Vestfjella CFBs using (1) energy-constrained assimilation-fractional crystallization equations that account for heating and partial melting of crustal wall rock and (2) assimilation-fractional crystallization equations for lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle lithosphere) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1–15 wt%) contamination of asthenospheric parental magmas by melts from variable lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal growth events rather than just recycling of old lithospheric materials.

Geochemistry and petrogenesis of Mesoproterozoic A-type granitoids from the Danish island of Bornholm, southern Fennoscandia

Granitoids and gneisses from the Danish island of Bornholm have been investigated using whole rock geochemistry, Sr and Nd isotope geochemistry and Hf isotopes in zircon. Recent U–Pb dating shows that the rocks were formed during a short time interval at 1.45 to 1.46Ga, penecontemporaneous with ongoing deformation. The strong similarity in geochemical signatures indicate that they all belong to a single igneous suite composed of alkali-calcic biotite–hornblende quartz monzonites to more evolved biotite granites, albeit with an apparent gap in SiO2 content at around 70wt%, dividing the suite into an intermediate and a felsic part. These dominantly metaluminous rocks are strongly ferroan and potassic, and with highly elevated concentrations of many trace elements, traits that are typical for A-type granitoids. The presence of magnetite and titanite indicates relatively oxidized compositions, and Nb/Y ratios designate them to the A2 subtype. Initial whole rock εNd values range between +1 and -2 (with one outlier at +4), and initial zircon εHf values between +3 and -4. These values may be explained by melting of relatively juvenile crust similar to that forming the Transscandinavian Igneous Belt alone, but the spread in Hf and Nd isotope compositions to values overlapping with the Svecofennian mantle at 1.45Ga suggests involvement of a mantle-derived component. This indicates the magmatism was associated with juvenile crustal growth. There are no systematic differences in isotope or trace element characteristics between the orthogneisses and the less deformed granitoids, suggesting similar origins for both rock types, and no systematic changes in isotopic composition with SiO2 concentration.Trace element compositions indicate a within-plate setting, similar to other 1.45Ga granites in southwest Fennoscandia, in spite of the close relation between magmatism and deformation on Bornholm. We therefore suggest intracratonic A-type magmatism within an active continental shear zone as a tentative model for the Mesoproterozoic magmatism on Bornholm. However, a close relationship to the nearby Hallandian tectono-magmatic activity in southern Sweden, attributed to continental margin orogenic processes, is also evident. Intermediate magmas presumably formed by ponding of enriched mantle-derived basic magma at the crust-mantle boundary or within the lower crust, causing extensive partial melting and assimilation of mafic to intermediate crustal rocks related to the Transcandinavian Igneous Belt. The magmas then evolved to granitic compositions by removal of an assemblage similar to that seen petrographically, i.e. plagioclase, amphibole, magnetite, titanite, and late-stage K-feldspar.

Zircon U–Pb ages and geochemical characteristics of granitoids in Nagqu area, Tibet

This paper reports zircon LA-ICP-MS U–Pb ages and Hf–O isotopic ratios, and whole-rock major and trace element data of Early Cretaceous felsic intrusive rocks from Nagqu area, the northern Lhasa subterrane, southern Tibet. LA-ICP-MS zircon U–Pb dating of biotite granites and biotite monzogranites in the area yields magmatic crystallization ages of ca. 112Ma, which suggests that they were emplaced in the late Early Cretaceous. Both rocks show high-K calc-alkaline to shoshonitic composition and slightly–moderately peraluminous signature. They are enriched in the alkalis, Rb, Th, K, U and light rare earth elements, depleted in Nb, Ta, Ti and P, and characterized by high Al2O3 contents (12–16wt.%), high Rb/Sr ratios (1.3–33) and low Mg# values (15–39). Their magmatic zircons have negative εHf(t) values (from −25.9 to 0.5) and high positive δ18O values (from 7.9‰ to 11.5‰). All the above characteristics indicate that Nagqu biotite monzogranites and biotite granites were likely derived from hybrid melts of sediments from the continent crust with minor mantle-derived input, then experienced varied degrees of fractional crystallization. The Nagqu intrusion is a component of the late Early Cretaceous magmatic flare-up event that occurred during ~120–100Ma in the northern and partly central Lhasa subterranes. This magmatic flare-up is marked with a great compositional diversity (basalt, rhyolite, adakitic rocks, dioritic enclave, biotite monzogranite and granite) that might be caused by the slab breakoff of the southward subducting Bangong–Nujiang oceanic lithosphere, or more likely by slab window opening, which may have significantly contributed to juvenile crustal growth of the northern Lhasa subterrane.

Zircon U-Pb and Hf isotopic study of gneiss and granodiorite from the southern Beishan orogenic collage: Mesoproterozoic magmatism and crustal growth

The distribution of Precambrian basement rocks and their tectonic attributes are of key importance for understanding the tectonic architecture and the crustal growth model of the Central Asian Orogenic Belt (CAOB). We present LA-(MC)-ICP-MS zircon U-Pb and Hf isotopic data for the biotite-plagioclase gneiss and granodiorite from the southern Beishan orogenic collage, on the southern margin of the CAOB. The magmatic zircon grains from the gneiss yielded a protolith age of 1408±4 Ma, indicating the occurrence of Mesoproterozoic basement rocks in the southern Beishan. Furthermore, these zircon grains have high positive e Hf( t ) values (4.1-9.9), and young Hf model ages (1.50-1.72 Ga) close to the crystallization age, suggesting that the protolith of this gneiss was derived from the depleted mantle or the juvenile crust. This indicates the involvement of an important Mesoproterozoic (~1.4 Ga) event in juvenile crust growth in the southern Beishan. Conversely, the granodiorite intruding the biotite-plagioclase gneiss yielded a crystallization age of 257±3 Ma, and numerous ~1.4 Ga inherited zircon grains were found in this rock. These inherited zircon grains exhibit crystallization ages and Hf isotope compositions consistent with those of the biotite-plagioclase gneiss, further confirming the occurrence of Mesoproterozoic crustal basement rocks beneath the southern Beishan. By comparing our new data with previous results from the Precambrian basement in the Dunhuang Block (part of the Tarim Craton) and the Central Tianshan Block (microcontinent in the Chinese Tianshan), we postulate that the Precambrian microcontinents in the southern Beishan have similar crustal evolutionary histories to the Central Tianshan Block, and they may not have been derived from the Tarim Craton. The Mesoproterozoic basement rocks investigated here provide new clues for examining the accretionary orogenic processes of the southern CAOB.

Detrital provenance evolution of the Ediacaran–Silurian Nanhua foreland basin, South China

We report here in-situ U–Pb and Hf isotopic results of detrital zircons from sixteen Cambrian–Silurian siliciclastic samples across the Nanhua foreland basin, South China. Together with published data from Ediacaran–Silurian sandstones in the region, we establish the temporal and spatial provenance evolution across the basin. Except for samples from northeast Yangtze, all other Ediacaran–Silurian samples exhibit a prominent population of 1100–900Ma, moderate populations of 850–700Ma and 650–490Ma, and minor populations of 2500Ma and 2000–1300Ma, grossly matching that of crystalline and sedimentary rocks in northern India. Zircon Hf isotopes further reveal four episodes of juvenile crustal growth at 2.5Ga, 1.8Ga, 1.4Ga and 1.0Ga in the source regions. Utilizing the basin history and late Neoproterozoic to early Paleozoic paleogeography of South China, we conclude that the Ediacaran–Cambrian sediments in the Nanhua foreland basin were mainly sourced from northern India and adjacent orogens, and the Ordovician–Silurian sediments were derived from both locally recycled Ediacaran–Cambrian rocks and eroded Cathaysian basement. The Wuyi–Yunkai late-orogenic magmatic rocks also contributed to the Silurian sediments in the basin. The upper-Ordovician to Silurian samples in northeast Yangtze received higher proportions of local Cryogenian (850–700Ma) magmatic rocks which were uplifted during late-Ordovician to Silurian time. We speculate that there was an Ediacaran–Cambrian collisional orogen between South China and northern India, shedding sediments to the early Nanhua foreland basin. Far-field stress during the late stage of this collisional orogeny triggered the Ordovician–Silurian intraplate Wuyi–Yunkai orogeny in South China, and erosion of the local Wuyi–Yunkai orogen further provided detritus to the late Nanhua foreland basin.

Reworking of the Gangdese magmatic arc, southeastern Tibet: post‐collisional metamorphism and anatexis

AbstractThe Gangdese magmatic arc, southeastern Tibet, was built by mantle‐derived magma accretion and juvenile crustal growth during the Mesozoic to Early Cenozoic northward subduction of the Neo‐Tethyan oceanic slab beneath the Eurasian continent. The petrological and geochronological data reveal that the lower crust of the southeastern Gangdese arc experienced Oligocene reworking by metamorphism, anatexis and magmatism after the India and Asia collision. The post‐collisional metamorphic and migmatitic rocks formed at 34–26 Ma and 28–26 Ma respectively. Meta‐granitoids have protolith ages of 65–38 Ma. Inherited detrital zircon from metasedimentary rocks has highly variable ages ranging from 2708 to 37 Ma. These rocks underwent post‐collisional amphibolite facies metamorphism and coeval anatexis under P–T conditions of ~710–760 °C and ~12 kbar with geothermal gradients of 18–20 °C km−1, indicating a distinct crustal thickening process. Crustal shortening, thickening and possible subduction erosion due to the continental collision and ongoing convergence resulted in high‐P metamorphic and anatectic reworking of the magmatic and sedimentary rocks of the deep Gangdese arc. This study provides a typical example of the reworking of juvenile and ancient continental crust during active collisional orogeny.

Zircon ages and Hf isotopic compositions of plutonic rocks from the Central Tianshan (Xinjiang, northwest China) and their significance for early to mid-Palaeozoic crustal evolution

We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475–473 Ma) granitoid rocks have zircon εHf(t) values either positive (+0.3 to +9.5) or negative (−6.0 to −12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician–Silurian (ca. 458–425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458–442 Ma) has negative zircon εHf(t) values (−6.3 to −13.1), indicating a predominantly crustal source; zircon from later events (ca. 434–425 Ma) has positive zircon εHf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410–404 Ma) rocks have near-zero zircon εHf(t) values, either slightly negative or positive (−1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (−11.2 to −14.8). The Late Devonian (ca. 368–361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock εNd(t)values (−2.4 to −5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle–crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust.

Provenance and Crustal Evolution of the Northern Yangtze Block Revealed by Detrital Zircons from Neoproterozoic–Early Paleozoic Sedimentary Rocks in the Yangtze Gorges Area, South China

AbstractDetrital zircon U-Pb ages of six sedimentary rocks in the northern Yangtze Block reveal three major age groups of 740–890 Ma, 1.9–2.1 Ga, and 2.4–2.55 Ga and two minor groups of 2.6–2.7 and 2.9–3.1 Ga. Although Lu-Hf isotope analysis suggest both juvenile crustal growth and reworking of old crust for all the age populations, juvenile input occurred mainly in the Archean and Neoproterozoic time, whereas Paleoproterozoic is a period dominated by recycling of preexisting crust. The predominant Neoproterozoic detritus is mainly derived from coeval igneous rocks distributed along the western and northwestern margin of the Yangtze Block. However, most of the pre-Neoproterozoic sediments cannot be matched with exposed basement rocks within the Yangtze Block. The similar age patterns of the sedimentary rocks in this study imply only a slight variation in the source regions from the Neoproterozoic to Silurian.

From Rodinia to Gondwanaland: A tale of detrital zircon provenance analyses from the southern Nanhua Basin, South China

The paleogeographic positions of the South China Block (SCB) during the Neoproterozoic and early Paleozoic are important for understanding the transition from the break-up of the supercontinent Rodinia to the formation of Gondwanaland. Integrated in situ U-Pb ages and Hf-O isotope analyses of detrital zircons from Cambrian sedimentary rocks in the southwestern SCB reveal major age populations at 2500 Ma, 1100 to 900 Ma, 850 to 750 Ma and 650 to 500 Ma, with a predominant group at ∼980 Ma that counts for ∼50 percent of all analyses. Zircon Hf-O isotopic results suggest three Precambrian episodes of juvenile crustal growth for the source area(s) (3.0 Ga, 2.5 Ga and 1.0 Ga), with major crustal reworking at 580 to 500 Ma. The source provenance as defined by the U-Pb and Hf analyses is distinctly different from the known tectonomagmatic record of the SCB, or that of western Australia or western Laurentia, but matches well with that of the Ediacaran (latest Neoproterozoic)–Cambrian clastic sedimentary rocks and granitic intrusions in the NW Indian Himalaya. The SCB–NW India provenance linkage appears to have started from the Ediacaran. We propose that after breaking away from central Rodinia, the SCB collided with NW India during the Ediacaran–Ordovician time, causing the “Pan-African” Kurgiakh/Bhimphedian orogeny at the northern margin of India as well as the intraplate Wuyi-Yunkai orogeny (>460 Ma − 415 Ma) in South China. The Ediacaran–lower Paleozoic clastic sedimentary rocks in the Nanhua Basin are therefore interpreted to be foreland deposits formed during the collision of the SCB with Gondwanaland.

Mesozoic intraplate granitic magmatism in the Altai accretionary orogen, NW China: Implications for the orogenic architecture and crustal growth

The Central Asian Orogenic Belt (CAOB) is the world9s largest Phanerozoic accretionary orogen and is the most important site for juvenile crustal growth in the Phanerozoic. In this work, we employed U-Pb zircon geochronology to identify the early and middle Mesozoic intraplate granitic intrusive events in the Chinese Altai segment of the southern CAOB in order to better understand the crustal architecture of the CAOB. We also used whole-rock geochemical, Sr-Nd isotopic and zircon Hf isotopic data to constrain the generation for these granitic rocks and to evaluate the implications for vertical crustal growth in this region. The Early Mesozoic granitic intrusions were emplaced between 220 and 200 Ma in the central Altai “microcontinental terrane” (also widely referred to as Units 2 and 3). The granites have shoshonitic and high-K calc-alkaline affinities and show the characteristics of differentiated I-type granite. The whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios (0.7058-0.7128) and ε_Nd(210) values (−0.6 to −4.3), as well as the zircon ε_Hf(t) values (−4.0 to +5.0) and two-stage Hf model ages (0.94-1.52 Ga), suggest that the granitic magmas were produced from a mixed source with both mantle-derived and recycled crustal components. The middle Mesozoic granites were emplaced at ∼150 Ma in the southern Altai “accretionary terrane” (Units 4 and 5). They show A-type characteristics with the REE tetrad effect and have positive ε_Nd(151) whole-rock values of +1.0 to +5.2 and two-stage Nd model ages (T_DM2) of 0.6 to 1.0 Ga. Zircon Hf data show positive zircon ε_Hf(151) values of +1 to +8 and two-stage Hf model ages of 0.6 to 1.2 Ga. The Nd-Hf isotopic data suggest that the granitic magmas were derived from short-lived juvenile mantle-derived materials. Thus, the isotopic signatures of all the Mesozoic granites from the central (old terrane) and southern (young accretional terrane) Altai suggest that the basement of both terranes has retained its original nature. The data further imply that the Altai orogen has kept its original architecture of Paleozoic horizontal accretion during Mesozoic time, as commonly observed in accretionary orogens where horizontal tectonics are dominant. All the early Mesozoic intrusions in the Altai were emplaced in an intraplate anorogenic setting; hence are distinguished from the contemporaneous syn- or post-orogenic magmatism in the eastern CAOB. We conclude that the early Mesozoic granites in the CAOB were emplaced in a variety of tectonic settings.

Zircon U–Pb geochronology and Hf isotopes of major lithologies from the Jiaodong Terrane: Implications for the crustal evolution of the Eastern Block of the North China Craton

The Jiaodong Terrane of the Eastern Shandong Complex in the Eastern Block of the North China Craton (NCC) consists predominantly of Archean granitoid gneisses with minor supracrustal rock enclaves or lenses. This study presents new zircon LA-ICP-MS U–Pb and Lu–Hf isotopic data for these lithologies, which help to better understand the Archean crustal evolution of the Eastern Block of the NCC. Magmatic zircon U–Pb data reveal that zircons in the supracrustal rocks and granitoid gneisses were generated by multi-stage events at ~2.9Ga, ~2.7Ga and ~2.5Ga. Metamorphic zircon U–Pb data obtained for these rocks show distinct metamorphic ages at ~2.50 and ~1.9–1.8Ga, suggesting that the Jiaodong Terrane experienced a regional metamorphic event at the end of the Neoarchean and encountered reworking by a tectonothermal event that was associated with the formation of the Paleoproterozoic Jiao–Liao–Ji Belt. Magmatic zircons have variable εHf(t) values from −5.5 to +7.7 with model ages of 3.92–2.57Ga, of which most εHf(t) values are positive with a predominant peak of model ages at 3.4–3.1Ga and a subordinate peak at 2.8–2.7Ga. These Hf features reveal major juvenile crustal growth stages with significant additions of older crustal materials at 3.4–3.1Ga and 2.8–2.7Ga, and a crustal reworking event with minor juvenile additions at ~2.5Ga in the Jiaodong Terrane.

Crustal growth stages in the Songino block of the early caledonian superterrane in Central Asia: II. Geochemical and Nd-isotope data

Geochemical and Nd isotope data are reported for Late Riphean metamorphic complexes and granitoids of the Bayannur zone of the Songino block in the Early Caledonian superterrane of Central Asia. Geological, geochronolgical, geochemical, and isotope data were integrated to discuss rock sources and main mechanisms responsible for the formation and evolution of the Late Riphean continental crust. It was estab� lished that lithotectonic complexes of the Bayannur zone were formed on a convergent plate during Late Pro� terozoic tectonogenesis (around 1.3-0.78 Ga). This period primarily produced a juvenile crust represented by paleooceanic (Nand EMORB types) and island arc basalts. An interval of 800-880 Ma was marked by the formation of rocks of the Bayannur complex and metaterrigenous sequence (accretionary wedge) of the Kholbonur complex, and the emplacement of quartz diorites and granodiorites of the Gashunnur pluton due to erosion and melting of both Late Riphean juvenile sources and ancient possibly Early Precambrian crustal material in a setting of ensialic island arc. At the final stage of the Late Riphean evolution of the Bayannur zone, postkinematic granitoids of the Bayannur pluton, and gabbrodiorites and anorthosites of the Ontsula pluton were derived from mantle juvenile and crustal sources in a withinplate setting. In terms of isotope characteristics, the crystalline complexes of the Bayannur zone are comparable with the Japantype modern island arc systems. A synthesis of geological, geochronological, and isotopegeochemical data indicates a much wider distribution of the Late Riphean juvenile crustforming processes than considered previously and remobilization of continental crust in the eastern segment of the Central Asian Fold Belt. The Vendian-Pale� ozoic stage in the evolution of this segment was characterized by an intense growth of juvenile crust, while magmatism during Late Riphean stage was determined by mixing of Late Riphean juvenile and ancient Early Precambrian sources.

Tectonic evolution of the southeastern margin of the Yangtze Block: Constraints from SHRIMP U-Pb and LA-ICP-MS Hf isotopic studies of zircon from the eastern Jiangnan Orogenic Belt and implications for the tectonic interpretation of South China

The Jiangnan Orogenic Belt is a NE-SW trending fold belt in the southeastern Yangtze Block of South China. The eastern segment of the belt, located in northeastern Jiangxi and southern Anhui provinces, is predominated by greenschist-facies metamorphosed volcanic-sedimentary strata with minor ophiolite, and their protoliths are considered to have formed in a back-arc basin. Zircon SHRIMP U-Pb dating reveals that these rocks were deposited/emplaced in the period 842–825Ma. Zircon LA-ICP-MS Hf analyses show that most igneous zircons from the volcanic-sedimentary rocks possess positive ɛHf(t) values of 0.09–12.74, corresponding to depleted-source model ages of 0.9–1.4Ga. This suggests a significant Neoproterozoic juvenile crustal growth event in the southeastern part of the Yangtze Block, which is also documented by gabbro (833Ma) from ophiolite with positive ɛHf(t) values of 5.24–13.25 and hafnium model ages of 0.87–1.20Ga. These new data, combined with previous results, indicate the presence of an arc and back-arc basin system along the southeastern margin of the Yangtze Block at 842–825Ma. Collision of the arc with the proposed Shuangxiwu Composite Terrane, which had previously formed away from the southeastern margin of the Yangtze Block through possible subduction and collision, led to the closure of the back-arc basin and the formation of the Jiangnan Belt in the period 825–815Ma. We suggest that South China is an accretionary orogen and that orogen-parallel motion along terrane-bounding shear zones may have played a significant role in its amalgamation.