Kongling Complex Research Articles

Paleoproterozoic porphyries and coarse-grained granites manifesting a vertical hierarchical structure of Archean continental crust beneath the Yangtze Craton

Granites generated by anatexis are ideal crustal probes to decipher the differentiation processes within deep continental crust. In this study, the successive emplacement of granites, including the porphyries and the coarse-grained granites, was found in the nucleus (the Kongling Complex) of the Yangtze Craton. The porphyries, intruding at 1840 ± 17 Ma (SHRIMP ages), occur as veins and contain phenocrysts of K-feldspar (∼15%), quartz (∼5%), plagioclase (∼4%) and garnet (∼1%) setting in a fine-grained groundmass (∼75%) of K-feldspar, plagioclase, quartz and biotite. The coarse-grained granites, emplacing at 1821 ± 8 Ma (SHRIMP ages), are outcropped as pluton and comprise K-feldspar (∼60%), quartz (∼30%), biotite (∼5%), plagioclase (∼5%). They are similar to A-type granites with relatively high SiO2 (69.2–71.5 wt%, 72.6–74.5 wt%), high Fe2O3T/MgO (21.5–27.7, 5.9–19.5), 104*Ga/Al ratios (3.6–4.3, 2.7–3.5) respectively and enriched HFSEs (Th, U, Pb, Ga). The porphyries contain higher total trace element contents and zirconium saturation temperature (mean 892 °C) than those of the coarse-grained granites (mean 803 °C). Magmatic zircons from the porphyries and coarse-grained granites have εHf(t) values of −17.6 to −11.7 and −21.0 to −14.8, which yield two-stage Hf model ages of 3.6–3.3 Ga and 3.8–3.4 Ga respectively, both falling into the evolutionary trend of the 2.9–2.8 Ga Kongling TTGs. It implies both granites are anatectic (crustal reworking) derivatives from partial melting of the Archean TTGs under sustaining extension in Paleoproterozoic. Applying trace element modelling, we further identify a unique tonalitic source for both granites which is analogous to the exposed natural Kongling tonalite, and find the porphyries slightly affected by the juvenile crust. Based on the intrusion of porphyry veins into granitic gneisses, we envisage the crustal structure beneath the craton is layered from older (∼2.94 Ga) tonalites upward to younger (∼2.87 Ga) granitic gneisses. Such hierarchical structure could shed new light on the imperceptible vertical differentiation process as cratonization.

Early crustal evolution of the Yangtze Craton, South China: New constraints from zircon U-Pb-Hf isotopes and geochemistry of ca. 2.9–2.6 Ga granitic rocks in the Zhongxiang Complex

We report here new zircon U-Pb age and Hf-isotope as well as geochemical analyses of the recently discovered Archean-Paleoproterozoic Zhongxiang Complex in the northern-central Yangtze Craton, South China, and interpret the early crustal evolution of the Yangtze Craton. Zircon LA-ICP-MS dating yielded magmatic crystallization ages of ca. 2.90–2.87 Ga for two monzogranites, ca. 2.77 Ga for a trondhjemitic gneiss, and ca. 2.67–2.62 Ga for three potassic granites. The trondhjemitic gneiss also contains zircon rims that record a metamorphic event at ca. 2.08 Ga. These results demonstrate that the Zhongxiang Complex is significantly older than previously thought, and it contains the only potential TTG with ages of 2.8–2.7 Ga for the Yangtze Craton. These granitoids vary considerably in zircon Hf isotopic and geochemical characteristics. The ca. 2.90–2.87 Ga monzogranites correspond in composition to alkaline and calc-alkalic peraluminous granites originated from mixing of ancient and juvenile crustal components at relatively shallow depths. In contrast, the ca. 2.77 Ga trondhjemitic gneisses are calc-alkalic and peraluminous with relatively high Sr/Y and (La/Yb)N ratios and slightly evolved zircon Hf signatures, but without significant Eu anomaly that is, typical of a medium-pressure TTG component. Consequently, they are interpreted to be the products of partial melting of a basaltic source with appreciable involvement of pre-existing crustal components (i.e., tonalites), with garnet and amphibole in the residue. The ca. 2.67–2.62 Ga potassic granites are alkaline, faintly peraluminous and ferroan, exhibiting a high-silica A-type granite affinity. They are characterized by low Al2O3/TiO2 ratios with dominantly undepleted zircons, suggesting their derivation from melting of meta-sedimentary rocks under high-temperature conditions with plagioclase as the main residual phases. The Zhongxiang Complex thereby differs from the Kongling Complex and other Archean provinces in the Yangtze Craton by preserving distinct magmatic pulses (ca. 2.90–2.87 Ga; ca. 2.77 Ga; ca. 2.67–2.62 Ga) with different nature and sources. A comparative analysis of the crustal evolution of the Zhongxiang Complex and other Archean provinces of the Yangtze Craton indicates a compositionally heterogeneous Archean Yangtze Craton, which likely comprises several Archean terranes that accreted together to form a uniform craton during the late Archean to Paleoproterozoic time.

Complex growth and reworking processes in the Yangtze cratonic nucleus

The Kongling Complex (KC), including the northern (NKC) and southern parts (SKC), represents a rare relic of Archean crust in the Yangtze Craton, and an ideal target to investigate the Precambrian evolution of the continental crust. In the SKC, history of Archean-Paleoproterozoic growth and reworking has not yet been clearly understood. New geochemical and geochronological data on the basement rocks (i.e., gneiss, migmatite, amphibolites) and their overlying metasediments in the SKC, combined with published results from the NKC, allow a further subdivision into western, middle and eastern zones, which has proven useful in understanding the Precambrian history of the Yangtze cratonic nucleus.U-Pb dating and Lu-Hf isotopes of zircons in biotite gneisses (with or without amplibole) and amphibolites suggest that the SKC experienced a crustal re-melting (reworking) event at ∼2.9 Ga, an accretion/reworking event at ∼2.7 Ga, and later reworking at ∼1.73 Ga and ∼0.8 Ga. These records are partly consistent with what is known about the NKC. The main differences between them are that rocks with ages > 3.0 Ga and 2.1–2.0 Ga are only found in the NKC but not in the SKC. Hence, the SKC is not a copy of the NKC. According to the consistent rock assemblages and ages, the SKC may have affinity to the western zone of the NKC. That affinity is also supported by geophysical results that suggest the presence of a fossil Huangling magma chamber beneath the SKC and the western NKC at 10–20 km depth (Mei et al., 2013). Therefore, our data provide a new understanding of the Kongling Complex: the SKC stayed together with the western part of the NKC from Mesoarchean to Neoproterozoic (2.9–0.8 Ga) time, and they were separated by the intrusion of the Huangling magmas at ∼0.8 Ga.

A ca. 2.2Ga Acidic Magmatic Event at the Northern Margin of the Yangtze Craton: Evidence from U‐Pb Dating and Hf Isotope Analysis of Zircons from the Kongling Complex

A ca. 2.2Ga Acidic Magmatic Event at the Northern Margin of the Yangtze Craton: Evidence from U‐Pb Dating and Hf Isotope Analysis of Zircons from the Kongling Complex

2.2Ga Subduction‐Related Mafic Magmatic Rocks in the Kongling Complex: Evidence for the Assembly of the Columbia Supercontinent

2.2Ga Subduction‐Related Mafic Magmatic Rocks in the Kongling Complex: Evidence for the Assembly of the Columbia Supercontinent

Geochemical and Nd isotopic compositions of the Palaeoproterozoic metasedimentary rocks in the Kongling complex, nucleus of Yangtze craton, South China block: implications for provenance and tectonic evolution

AbstractPalaeoproterozic metasedimentary rocks, also referred to as khondalites, characterized by Al-rich minerals, are extensively exposed in the nucleus of the Yangtze craton, South China block. Samples of garnet–sillimanite gneiss in the khondalite suite were collected from the Kongling complex for Nd isotopic and elemental geochemical study. These rocks are characterized by variable SiO2 contents ranging from 35.71 to 58.07 wt%, and have low CaO (0.45–0.84 wt%) but high Al2O3 (18.56–29.04 wt%), Cr (174–334 ppm) and Ni (42.5–153 ppm) contents. They have high CIW (Chemical Index of Weathering) values (90.4–94.7), indicating intense chemical weathering of the source material. The samples display light rare earth elements (LREE) enrichment with negative Eu anomalies (Eu/Eu*=0.40–0.68), and have flat heavy rare earth elements (HREE) patterns. The high contents of transition elements (e.g. Cr, Ni, Sc, V) and moderately radiogenic Nd isotopic compositions suggest that the paragneisses might be those of first-cycle erosion products of predominantly mafic rocks mixing with small amounts of felsic moderately evolved Archaean crustal source. Geochemical and Nd isotopic compositions reveal that at least some of the protoliths of Kongling khondalite were sourced from local pre-existing mafic igneous rocks in a continental arc tectonic setting. Combined with documented zircon U–Pb geochronological data, we propose that the Palaeoproterozoic high-pressure granulite-facies metamorphism, rapid weathering, erosion and deposition of the khondalites in the interior of the Yangtze craton might be related to a Palaeoproterozoic collisional orogenic event during 2.1–1.9 Ga, consistent with the worldwide contemporary orogeny, implying that the Yangtze craton may have been an important component of the Palaeoprotorozoic Columbia supercontinent.

A Paleoproterozoic ophiolitic mélange, Yangtze craton, South China: Evidence for Paleoproterozoic suturing and microcontinent amalgamation

We document for the first time the presence of a Paleoproterozoic ophiolitic mélange in the Archean–Paleoproterozoic Kongling Complex in the northern Huangling Dome, Yangtze craton, South China. Detailed field mapping, petrographic, geochronological and geochemical studies reveal that the mélange consists of a suite of amphibolite facies mafic and ultramafic tectonic blocks including serpentinized harzburgite, olivine pyroxenite, pyroxenite, diabase, gabbro and basalt dispersed in a strongly sheared metasedimentary matrix. The mélange displays polyphase deformation and northwest-verging nappe structures, and underwent amphibolite to granulite facies metamorphism in the Paleoproterozoic (2.0–1.95Ga). The metasedimentary rocks are composed mainly of garnet-biotite-plagioclase gneiss, mica schist, mica-graphite schist, marble, quartzite, and banded iron formation (BIF). The mafic–ultramafic rock association shares the lithological and geochemical characteristics of Phanerozoic suprasubduction zone ophiolites.LA-ICP-MS U–Pb dating of zircons from the diabase yield ages between 2142 and 2148Ma for the magmatic cores and between 2042–2048Ma for the metamorphic rims. Zircon cores have εHf (t) values ranging from +5.4 to +10.3 (average=+7.2) with corresponding TDM1 ages of 2.24Ga, indicating that these rocks are remnants of juvenile crust derived from the depleted mantle at ca. 2.2Ga. All rock types in the mélange underwent Paleoproterozoic (2.0–1.95Ga) amphibolite–granulite facies metamorphism and were intruded by syn-metamorphic granite (∼2.0Ga), post-collisional (ca.1.85Ga) granite and mafic dykes. Intrusion age of one quartz monzonitic dyke crosscutting the serpentinized harzburgite is 1999Ma, constraining the minimum emplacement age of the mafic–ultramafic complex. The mafic–ultramafic rocks within the metasedimentary matrix in the northern Huangling Dome are fragments of a ca. 2.15Ga suprasubduction zone ophiolite that was incorporated into the suture zone during the accretionary-collisional process. The mélange recognized in the northern Yangtze craton provides important evidence for the Paleoproterozoic subduction and accretion processes possibly associated with the amalgamation of the proposed Columbia supercontinent.

Sedimentary Environment of Ediacaran Sequences of South China: Trace Element and Sr-Nd Isotope Constraints

Post-Marinoan sedimentary rocks from the Yangtze block in South China were investigated to seek constraints on their provenance, genetic relationship with paleoclimate, depositional environment, and changes. Sedimentary rocks from the Doushantuo Formation in the Jiulongwan section of the Yangtze block were collected for whole-rock trace elements and Sr-Nd isotopic analyses. Flat-to-slightly middle rare earth element (REE)–enriched REE-plus-yttrium patterns, negative-to-positive Ce anomalies (Ce/Ce* of 0.86–1.40), positive La anomalies (La/La* of 1.02–1.42), and suprachondritic Y/Ho ratios (36.8–49.5) obtained for the Doushantuo cap carbonates indicate their deposition in a fluctuating inner-shelf setting where freshwater and seawater mixed together and the influence of hydrothermal fluid from the seafloor is absent. All Doushantuo carbonates and shales have variable Sr isotopic compositions, with initial Sr isotopic ratios (0.704030–0.712720) that support significant river-water contribution during the deposition of the Doushantuo Formation. Samples of the Doushantuo Formation collected across sedimentary profiles show distinct variations in Nd isotopic compositions and diagnostic trace elemental ratios (Th/Sc). The cap carbonates received significant input from a broad continental area undergoing extensive weathering immediately after the cease of the glaciation era. In contrast, the upper Doushantuo Formation sediments received more mature detritus from the nearby Archean Kongling complex, reflecting receding sea levels. In contrast to the negative Ce anomalies observed in lower Doushantuo rocks, negligible Ce anomalies in upper Doushantuo rocks suggest an anoxic setting, in agreement with the growth of phosphatic concretions and the presence of framboids of pyrite and a higher concentration of organic matter.

Petrogenesis and tectonic implications of Paleoproterozoic metapelitic rocks in the Archean Kongling Complex from the northern Yangtze Craton, South China

The Archean Kongling Complex in the northern Yangtze Craton is an ideal target to investigate the Precambrian accretion and evolution of continental crust in South China. This study aims to unravel the crustal evolution and tectonic setting of the Yangtze Craton during the Paleoproterozoic time, using integrated studies of petrography, zircon U–Pb and Hf isotopes and whole-rock geochemistry of Paleoproterozoic metapelitic rocks in the Kongling Complex. Four representative metapelitic rocks contain garnet, and three of them include high-temperature metamorphic minerals as sillimanite or staurolite. Zircons from the metapelitic rocks show nebulous-sector zoning and rim-core microstructures, suggesting a metamorphic origin or a detrital origin with metamorphic overprints. The metamorphic zircon grains and metamorphic overgrowths have concordant 207Pb/206Pb ages at ∼2.0Ga, while detrital grains yield three distinct age populations of >2.5Ga, 2.4–2.2Ga and 2.2–2.1Ga. The age patterns indicate that the depositional age of the metasedimentary rocks was 2.1–2.0Ga. Arc-related magma production represented by the Houhe gneiss in the northern part of the craton could be the source of the 2.2–2.1Ga inherited zircons, because of the similarities of whole-rock geochemical compositions and zircon U–Pb–Hf isotopic signatures between the investigated metapelitic rocks and the gneiss. The 2.4–2.2Ga zircons have Hf model ages (TDM2) of ∼3.5–2.6Ga, and the >2.5Ga zircons have TDM2 ages varying from 3.3Ga to 2.9Ga. Our data support previous knowledge that there were three episodes of growth and reworking events of the Archean Yangtze continental crust, ca. 3.3–3.2Ga, 2.9Ga, 2.7–2.6Ga, and show that the Yangtze Craton has experienced Paleoproterozoic reworking during 2.4–2.2Ga and the growth and reworking during 2.2–2.1Ga.Combined with available data, the new results in this study suggest a continent–arc–continent evolution model to explain the tectonic history of the Yangtze Craton during the Paleoproterozoic time. The western and eastern parts of Yangtze Craton were originally two individual continents with Archean basements. The western part of Yangtze Craton had undergone the crustal re-melting event during 2.4–2.2Ga and growth and reworking event caused by arc generation during 2.2–2.1Ga. It subsequently collided with the eastern part of Yangtze Craton at ∼2.0Ga and experienced later ∼1.85Ga extension.

2.1–1.85 Ga tectonic events in the Yangtze Block, South China: Petrological and geochronological evidence from the Kongling Complex and implications for the reconstruction of supercontinent Columbia

This paper presents petrography, zircon U–Pb ages and Hf isotopic data as well as whole-rock Sm–Nd isotopic data for mafic granulites, metapelitic rocks and high-grade marble from the Kongling Complex in the Yangtze Block, South China. Petrographic observations indicate that these three types of rocks experienced high-pressure metamorphism. Their mineral assemblages and P–T conditions define a clockwise P–T path involving isothermal decompression following the peak high-pressure metamorphism, which is considered to record a continent–continent collisional event. This is systematic documentation of the tectonic evolution of the Kongling Complex from 2.1-2.0Ga deposition (constrained by youngest detrital zircon and metamorphic zircon) through ~2.0Ga collision (high-pressure metamorphism) and syn-collisional partial melting (S-type granite and migmatization of TTG gneiss) to ~1.85Ga post-collisional extension (A-type high-K granite and mafic dyke). These ages are broadly coincident with global collisional events (2.1–1.8Ga) that led to the assembly of the Palaeo-Mesoproterozoic Columbia (or Nuna) supercontinent. Therefore, this study provides strong evidence that the Yangtze Block in South China was a component of the Columbia supercontinent.

Formation and evolution of Precambrian continental lithosphere in South China

An overview is presented for the formation and evolution of Precambrian continental lithosphere in South China. This is primarily based on an integrated study of zircon U–Pb ages and Lu–Hf isotopes in crustal rocks, with additional constraints from Re–Os isotopes in mantle-derived rocks. Available Re–Os isotope data on xenolith peridotites suggest that the oldest subcontinental lithospheric mantle beneath South China is primarily of Paleoproterozoic age. The zircon U–Pb ages and Lu–Hf isotope studies reveal growth and reworking of the juvenile crust at different ages. Both the Yangtze and Cathaysia terranes contain crustal materials of Archean U–Pb ages. Nevertheless, zircon U–Pb ages exhibit two peaks at 2.9–3.0Ga and ~2.5Ga in Yangtze but only one peak at ~2.5Ga in Cathaysia. Both massive rocks and crustal remnants (i.e., zircon) of Archean U–Pb ages occur in Yangtze, but only crustal remnants of Archean U–Pb ages occur in Cathaysia. Zircon U–Pb and Lu–Hf isotopes in the Kongling complex of Yangtze suggest the earliest episode of crustal growth in the Paleoarchean and two episodes of crustal reworking at 3.1–3.3Ga and 2.8–3.0Ga. Both negative and positive εHf(t) values are associated with Archean U–Pb ages of zircon in South China, indicating both the growth of juvenile crust and the reworking of ancient crust in the Archean. Paleoproterozoic rocks in Yangtze exhibit four groups of U–Pb ages at 2.1Ga, 1.9–2.0Ga, ~1.85Ga and ~1.7Ga, respectively. They are associated not only with reworking of the ancient Archean crust in the interior of Yangtze, but also with the growth of the contemporaneous juvenile crust in the periphery of Yangtze. In contrast, Paleoproterozoic rocks in Cathaysia were primarily derived from reworking of Archean crust at 1.8–1.9Ga. The exposure of Mesoproterozoic rocks are very limited in South China, but zircon Hf model ages suggest the growth of juvenile crust in this period due to island arc magmatism of the Grenvillian oceanic subduction. Magmatic rocks of middle Neoproterozoic U–Pb ages are widespread in South China, exhibiting two peaks at about 830–800Ma and 780–740Ma, respectively. Both negative and positive εHf(t) values are associated with the middle Neoproterozoic U–Pb ages of zircon, suggesting not only growth and reworking of the juvenile Mesoproterozoic crust but also reworking of the ancient Archean and Paleoproterozoic crust in the middle Neoproterozoic. The tectonic setting for this period of magmatism would be transformed from arc–continent collision to continental rifting with reference to the plate tectonic regime in South China.

LA-ICPMS U–Pb zircon age constraints on the provenance of Cretaceous sediments in the Yichang area of the Jianghan Basin, central China

Detrital zircon U–Pb ages have been shown to be a powerful tool for provenance analysis and determining the exhumation of sediment source areas. This paper presents the results of detrital zircon LA-ICPMS U–Pb ages for Cretaceous sediments from the Yichang area of the Jianghan Basin, central China. The results provide new information on the provenance of these sediments and the detailed exhumation process of the Huangling Dome. Zircons with different age populations have been derived from the strata of the Huangling Dome. The Liantuo, Gucheng and Nantuo formations and the Kongling complex were exhumed, leading to deposition of the early Cretaceous Wulong Formation, which provides the sources of zircons with age peaks at 3.1–3.0, 2.5 and 1.8 Ga. Exhumation of the Huangling granitoid and contemporary volcanics provided the source of the late Cretaceous Luojingtan Formation, which contains zircons with age peaks at 1.1–0.95 and 0.83–0.74 Ga. The Qinling-Dabie orogeny supplied zircons with an age cluster of 0.27 to 0.18 Ga. These results indicate the timing of initial exhumation for the Huangling granite. They also show how overlying strata was first uplifted and eroded, followed by exposure of underlying strata at the surface during continued exhumation.

Zircon U-Pb age and Hf isotope of Quanyishang A-type granite in Yichang: signification for the Yangtze continental cratonization in Paleoproterozoic

Zircon U-Pb age and Hf isotope, and major and trace element compositions were reported for granite at Quanyishang, which intruded into the Kongling complex in Yichang, Hubei Province. The results show that the Quanyishang granite is rich in silicon and alkalis but poor in calcium and magnesium, and displays enrichment in Ga, Y, Zr, Nb but depletion in Sr and Ba, exhibiting the post-orogenic A-type affinity. 90% zircons from the granite are concordant, and give a middle Paleoproterozoic magmatic crystallization age (mean 1854 Ma). Initial Hf isotope ratios (176Hf/177Hf)i of the middle Paleoproterozoic zircons range from 0.280863 to 0.281134 and they have negative εHf(t) values with a minimum of −26.3. These zircons give the depleted mantle model ages (T DM) of 2.9–3.3 Ga (mean 3.0 Ga), and the average crustal model ages (T crust) of 3.6–4.2 Ga (mean 3.8 Ga). A Mesoarchean grain with 207Pb/206Pb age of 2859 Ma has a slightly high T DM (3.4 Ga) but similar T crust (3.8 Ga) to the Paleoproterozoic zircons. All these data suggest that the source materials of the Quanyishang A-type granite are unusually old, at least ⩾2.9 Ga (even Eoarchean). The event of crustal remelting, which resulted in the formation of the Quanyishang granite in the middle Paleoproterozoic, recorded the cratonization of the Yangtze continent. The process may have relation to the extension and collapse of the deep crust with Archean ages, in response to the transition stage of the assembly and breakup of the Columbia supercontinent.

U-Pb Zircon and Sm-Nd isotopic study of the huangtuling granulite, dabie-sulu belt, China: Implication for the paleoproterozoic tectonic history of the yangtze craton

SIMS data demonstrate Archean (2.9 to 2.7 Ga) and Paleoproterozoic (1991 ± 43 Ma) ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from Huangtuling, Dabie-Sulu orogenic belt, China. The granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the Yangtze Craton. The rounded zircon cores show oscillatory zoning and have Th/U ratios (0.17 − 0.78) systematically higher than those of the unzoned luminescent rims (0.005 − 0.03). The age of the detrital zircon cores and whole rock Nd isotope model age suggest that the Huangtuling granulite was possibly derived from a provenance dominated by Archean rocks similar to the Kongling Complex exposed 200 km in the southwest. Multiple phases of metamorphism have been identified for the granulites, with the dominant granulite-facies mineral assemblage, defining an early clock-wise metamorphic P-T path. Orthopyroxene, plagioclase and whole-rock material define a linear trend in a ^147^Sm/^144^Nd − ^143^Nd/^144^Nd diagram, corresponding to an age of 1926 ± 146 Ma. This result and the U-Pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the Yangtze Craton at ∼2.0 Ga, inviting further study on whether the Yangtze Craton was involved in the assembly of the recently-proposed Paleo-Mesoproterozoic supercontinent Columbia.

Neoproterozoic anatexis of Archean lithosphere: Geochemical evidence from felsic to mafic intrusions at Xiaofeng in the Yangtze Gorge, South China

An integrated study of geochronology and geochemistry was carried out for Neoproterozoic intrusions at Xiaofeng in the Yangtze Gorge. The results are used to provide constraints on their origin, with potential resolution to hotly debated models concerning the source and process of contemporaneous igneous rocks in South China. The Xiaofeng intrusions are composed of felsic–mafic dykes and their wall-rock granitoids in an area of <10 km 2. They show intrusion relationship to TTG rocks of the Archean Kongling Complex. Zircon U–Pb dating indicates that the dykes and granitoids were crystallized almost contemporaneously at 800 ± 3 Ma. They share similar distribution patterns of REE and trace elements, i.e. enrichment in LILE and LREE but depletion in HFSE. The dykes have whole-rock ɛ Nd( t) values of −9.9 to −6.4 and zircon ɛ Hf( t) values of about −12.9 to −9.0, and the granitoids have whole-rock ɛ Nd( t) values of −10.6 to −9.2 and zircon ɛ Hf( t) values of −10.5 to −8.3. Furthermore, their zircon Hf model ages are similar to each other, from 2.8 to 3.2 Ga. These results indicate that they both are derived from reworking of Mesoarchean lithosphere, with the difference only in major element composition. Zircon has δ 18O values of 5.4–6.8‰ for them, typical for I-type granite. Mineral O isotopes indicate small-scale high-T water–rock interaction during the intrusion of the dykes into the granitoids. No growth of juvenile crust during the middle Neoproterozoic magmatism is identified to occur in the Xiaofeng intrusions. Thus, neither mantle plume nor oceanic arc is responsible for their origin, ruling out the plume-rift and slab-arc models for their petrogenesis. Instead, the Neoproterozoic anatexis of the Archean lithosphere can be explained by the tectonic collapse of a thickened intracontinental orogen that formed by Paleoproterozoic arc-continent collision to the Archean Kongling Complex, but it became molten due to lithospheric extension in response to plate reorganization prior to breakup of the supercontinent Rodinia. This provides a positive test to the plate-rift model for their petrogenesis.

Contents of Volume

The Dabieshan area between the Sino–Korean and Yangtze cratons in east-central China has become the focus of much recent attention because of the discovery of abundant coesite and rare micro-diamond inclusions in both eclogites and their enclosing country rocks. The Dabieshan metamorphic complex, previously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmatitic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd–Sr isotope data show that most Dabieshan orthogneisses are distinctly different from the nearby Kongling gneisses of the Yangtze basement, which are Archean high-Al TTG rocks with an average Nd model age of 3.3±0.2 Ga with volcanic arc granitic affinity. The protoliths of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite units display geochemical signatures of post-Archean granites and may have resulted from Neoproterozoic magmatism in a rift environment; (2) some of the felsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusion during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic rocks; and (3) the felsic gneisses of the dome region show geochemical signatures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga, attributed to mixing between Neoproterozoic mantle-derived material and the Archean Kongling gneisses. Numerical modeling shows that mixing between mantle-derived melts and Kongling gneiss can account for the Nd–Sr isotopic variation of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly during early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neoproterozoic rifting. The orogen was later significantly modified, especially by Jurassic–Cretaceous migmatization and magmatism.

The roots of the Dabieshan ultrahigh-pressure metamorphic terrane: constraints from geochemistry and Nd–Sr isotope systematics

The Dabieshan area between the Sino–Korean and Yangtze cratons in east-central China has become the focus of much recent attention because of the discovery of abundant coesite and rare micro-diamond inclusions in both eclogites and their enclosing country rocks. The Dabieshan metamorphic complex, previously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmatitic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd–Sr isotope data show that most Dabieshan orthogneisses are distinctly different from the nearby Kongling gneisses of the Yangtze basement, which are Archean high-Al TTG rocks with an average Nd model age of 3.3±0.2 Ga with volcanic arc granitic affinity. The protoliths of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite units display geochemical signatures of post-Archean granites and may have resulted from Neoproterozoic magmatism in a rift environment; (2) some of the felsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusion during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic rocks; and (3) the felsic gneisses of the dome region show geochemical signatures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga, attributed to mixing between Neoproterozoic mantle-derived material and the Archean Kongling gneisses. Numerical modeling shows that mixing between mantle-derived melts and Kongling gneiss can account for the Nd–Sr isotopic variation of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly during early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neoproterozoic rifting. The orogen was later significantly modified, especially by Jurassic–Cretaceous migmatization and magmatism.

An Sm-Nd isotopic dating study of the Archean Kongling Complex in the Huangling area of the Yangtze Craton

Sm-Nd isotopic compositions of associated amphibolites, TTG gneisses and granitic gneisses from the Archean Kongling Complex of the Yangtze Craton, South China are systematically analyzed. The results show that the amphibolites and TTG gneisses have a similar age of ca. 2 700 Ma, which well agrees with zircon U-Pb dating of previous studies. However, their eND(t) are different, suggestive of possible dissimilarity in source composition.