Cathaysia Block In South China Research Articles

Ediacaran diamictite deposition in South China: Detrital zircon u-pb age evidence and macro sedimentary texture of the Aiqiling formation in southeastern Hunan Province

A better understanding of Ediacaran (Gaskiers) glaciations, including their ages, global distribution and types, is critical to our understanding of the Earth’s evolution following the Neoproterozoic “Snowball Earth”. The present study focuses on a diamictite that forms part of a continuous succession of Neoproterozoic clastic sedimentary rocks in the northwestern margin of the Cathaysia block in South China. Detailed sedimentary sequence established here shows that the diamictite, previously interpreted to be Cryogenian in age, is a part of the Ediacaran Aiqiling Formation. Detrital zircon U-Pb ages constrain its maximum deposition age to ca. 571 Ma, supporting its Ediacaran age. The macro sedimentary textures of the diamictite unit, including matrix-supported sediments with angular and randomly distributed clasts, combined with regional geochemical results, suggest a glaciogenic origin. The U-Pb detrital zircon age data also indicate an uplifting source area to the southeast that migrated northwestward since the Ediacaran.

Early Precambrian basement components of the Cathaysia Block in South China exhumed by Phanerozoic orogenic events

Ancient rock assemblages formed during the early history of the Earth are normally regarded as continental nuclei and they occur in old cratons. In recent years, they have been reported from different orogenic belts, but the reasons why they occur in orogenic belts are yet to be constained. The Cathaysia Block in South China is a continental block that witnessed multiple episodes of orogenic events and ancient basement rocks sporadically occur in the Wuyi area, where intense metamorphic alteration can be seen. This area is therefore ideal for addressing the above issue. New data, in combination with a review of previous studies, show that the ancient basement components of the Cathaysia Block formed during > 1.93 – 1.75 Ga and are mostly reworked ancient crustal materials (∼3.8 – 2.5 Ga). The first episode of metamorphism recorded in this area can be traced back to the Paleoproterozoic, but the features of this metamorphism are unclear. The early Paleozoic metamorphic rocks occur across the Wuyi area, and their protolith ages are dominantly Neoproterozoic, with a small amount of Paleoproterozoic rocks. This episode of metamorphism is characterized by high geothermal gradients, possibly corresponding to an arc setting. The early Mesozoic high-grade metamorphism of the Wuyi area is characterized by low geothermal gradients, indicating significant crustal thickening during the orogenic event. The spatial distribution of the early Mesozoic high-grade metamorphic rocks and the early Precambrian basement rocks of the Cathaysia Block are almost the same. Considering that the ancient basement rocks of the Cathaysia Block have been found in the Wuyi area, it is inferred that both the early Paleozoic and the early Mesozoic orogenic events should have facilitated the exhumation of the early Precambrian basement components. It is further suggested that the final exhumation of these ancient basement rocks is dominated by the early Mesozoic orogenic event because compared with other orogenic events of this continental block, this orogeny influenced rocks from deeper crustal levels (with the highest metamorphic pressures). Such a mechanism of ancient basement rocks exhumed by overprinted orogenic events might apply to many other high-grade terranes.

The Joining of North and South China During the Permian: Coherent Metamorphic Evidence From East Asia Orogenesis

AbstractThe joining of the North and South China blocks marks the formation of the united East Asian continent, which is an integral part of Pangea. The amalgamation of these two continental blocks also resulted in the formation of the world‐class ultrahigh pressure orogenic belt, namely the Dabie–Sulu orogenic belt. Figuring out the time of the initial collision between the two continents is critical for resolving questions such as the duration and the geodynamic processes of the orogeny. A Triassic joining time has been suggested by the geochronology of the eclogite facies rocks in this orogenic belt. However, paleomagnetic and paleontological studies suggested a Permian docking time for these continental blocks. In this paper, we present new metamorphic ages of rocks from northern Dabie and the Permo–Triassic intracontinental orogen of South China, which are all closely associated with this continental collisional event. New age dating results, as well as a synthesis of recent studies on metamorphic rocks occurring in southeastern North China and the Cathaysia Block of South China, show that the onset of the collisional orogenesis along the eastern part of the orogenic belt dates back to the Middle Permian (270–252 Ma). Considering these data and other geological records, we provide a new tectonic model for the major continents of East Asia, in which we show that the initial collision between North and South China occurred in the east during the Middle Permian, and then propagated westward. Continental collisions forming mountains and closing seaways in East Asia thus apparently occurred before the end‐Permian mass extinction, suggesting these paleogeographic changes might have also preconditioned and facilitated the end‐Permian biospheric crisis in the region.

Complex Crustal Deformation Beneath the Cathaysia Block in South China: Evidence From Receiver Function Analysis

AbstractBy harmonically fitting the arrival times of the P‐to‐S converted phases from the Moho, we systematically investigate the crustal anisotropy beneath the Cathaysia Block (CaB) and the surrounding areas in South China for the first time. The measured anisotropy parameters show strong lateral variations with an average splitting time of 0.35 ± 0.12 s. The resulting fast directions exhibit clear geographic variations with two possible boundaries that separate the CaB into three parts. The inconsistent fast direction of crustal anisotropy among each part, and the comparisons with teleseismic XKS splitting measurements indicate that the mechanisms of crustal deformation vary between distinct regions. The crust deforms coherently with the upper mantle in the western CaB, which is possibly driven by mantle flow. The fast directions of crustal anisotropy in the central CaB exhibit a radial pattern and show large discrepancies with XKS splitting measurements. We suggest that the crust has a complex deformation mechanism in this region. We propose a new scenario that emphasizes the interaction of the crust and upper mantle for interpreting the radial‐patterned crustal anisotropy, which involves the processes of subduction plate rollback, asthenospheric upwelling, crustal melting and lithospheric mantle intrusion. The fast directions of crustal anisotropy in the eastern CaB are dominantly parallel to the continental margin, suggesting subduction‐controlled crustal deformation in this region.

Evidence for crustal magma chamber associated with metallogeny based on P-wave velocities, South China

Recent seismic studies presented evidence for mantle upwelling and magmatic intrusions in the Cathaysia Block in South China. However, the key evidence or crustal velocity structure to understand crustal magma chambers that contributed to ore mineralization remain unclear. In this study. We collected a large number of Pg phase arrivals from 2009 to 2017 and carried out a P-wave tomography analysis in South China. Our results reveal a low-velocity anomaly adjacent to the Shaoguan region, which may be associated with concealed magmatic intrusions in this area. These intrusions may be linked with the widespread Yanshanian metallogeny in this region.

Petrology, phase equilibria modeling and zircon U-Pb geochronology of charnockites from the Badu Complex in southwestern Zhejiang: Implications for Indosinian orogenic event of the Cathaysia Block, South China

As the oldest basement rocks in the Cathaysia Block of South China, the Paleoproterozoic Badu Complex underwent high-grade metamorphism and preserved a suit of charnockites that their petrogenesis is still controversial. Comprehensive studies of petrology, mineral chemistry, metamorphic P-T calculation, and zircon U-Pb geochronology trend to discuss the petrogenesis of garnet-bearing charnockites from the Badu Complex in the Suichang-Dazhe area of southwestern Zhejiang Province in detail. The charnockites contain an assemblage of garnet, clinopyroxene, orthopyroxene, biotite, plagioclase, anti-perthite, K-feldspar, quartz, and ilmenite. Three metamorphic stages can be recognized: The prograde (M1) mineral assemblage is mainly composed of garnet (core) + biotite + clinopyroxene + plagioclase + quartz occurring as inclusions in garnet porphyroblast or pyroxene; The peak metamorphic (M2) mineral assemblage is identified as garnet (mantle) + clinopyroxene + orthopyroxene + biotite + plagioclase + K-feldspar + anti-perthite + ilmenite + quartz + melt, typical of granulite-facies mineral assemblage; minerals in plagioclase symplectic corona around garnet porphyroblast and replacing orthopyroxene constitute the retrograde assemblage (M3), including garnet (rim) + clinopyroxene + biotite + plagioclase + quartz ± Ilm. Conventional geothermobarometry in combination with phase equilibria modeling employing the NCKFMASHTO system yield metamorphic P-T conditions for each metamorphic stage as 810–825 °C, 7.5–8.3kbar (M2), 570–640 °C, 5.9–6.4 kbar (M3). It suggests that the charnockites experienced granulite-facies metamorphism and overprinted decompression and cooling along a clockwise P-T path. LA-ICP-MS zircon U-Pb data show a protolith age of 1860 ± 40 Ma and metamorphic age of 229.1 ± 5.7 Ma from the charnockites respectively. Combined with previous studies, we infer that the protolith of charnockites could be the Paleoproterozoic A-type granites and underwent granulite-facies metamorphism during the Indosinian orogenic event. Considering high-grade metamorphism and clockwise P-T path in charnockites with other metamorphic rocks in the Badu Complex, we suggest that the Indosinian orogenic event in the Cathaysia block could be attributed to continental collisions of South China with Indochina and North China. Significant underthrusting of Badu Complex along plate margins with subsequent rapid exhumation might be involved in this process, leading to granulite-facies metamorphism and later decompression and cooling in Paleoproterozoic basement rocks such as charnockites in the Suichang-Dazhe area.

Lithospheric electrical structure beneath the Cathaysia Block in South China and its tectonic implications

To better constrain the heat source of the geothermal system and lithospheric thinning mechanism of the Paleo-Pacific subduction zone, data from 35 broadband magnetotelluric (MT) stations were collected in 2020 along a 180-km-long NW-trending profile in the Cathaysia Block. Based on the measured data, we obtained the lithospheric electrical structure of the Cathaysia Block in South China using the 3D MT inversion code of ModEM. The final resistivity structure along the profile shows a series of highly conductive anomalies and highly resistive anomalies. The highly resistive anomalies in the upper crust correspond well with the distribution of magmatic rocks, which extend downwards to depths of 20 km. The highly resistive anomalies with a depth of 20–40 km are interpreted as crustal mafic sills derived from the mantle. The crustal conductors at depths of 5–15 km are typically interpreted as accumulations of aqueous fluids, which originate from the dehydration of the slab during the flat-slab subduction of the Paleo-Pacific Plate and are confined in the crust by impermeable layers. Fluid fractions of 0.8–9% would be required to explain the observed conductance values. The MT results indicate that there is no magma chamber or partial melt beneath the geothermal system. Therefore, the heat source of the geothermal system in the Cathaysia Block is radiogenic heat produced by the decay of radioactive elements in the crust. Thermal erosion has played a central role in lithospheric thinning and crust–mantle interaction of the Cathaysia Block.

Mantle Upwelling Beneath the Cathaysia Block, South China

AbstractA large low‐velocity anomaly (LLVA) beneath the Cathaysia Block in South China that may be associated with mantle upwelling was revealed by seismic tomography. However, seismological and geological studies indicate that ascending mantle must have a higher temperature than the surrounding mantle, and a low‐velocity anomaly alone is not indicative of mantle upwelling. The receiver function technique has been used as a robust tool to characterize the topographic relief of the upper mantle discontinuities, which are closely related to temperature variations and mantle upwelling. In this study, we extracted 25,187 receiver functions and performed common‐conversion‐point stacking of those receiver functions in South China and across the location of the LLVA beneath the Cathaysia Block. The results show that the 410 km discontinuity is depressed and that the 660 km discontinuity is elevated at the location of the LLVA. The structure of the mantle transition zone seems to be the passage of an upwelling mantle plume, which implies that the LLVA is a signature of mantle upwelling.

First report of phengites in the Longyou paragneiss in the northern Early Paleozoic Wuyi-Yunkai orogen, South China: P-T conditions, zircon U-Pb ages and tectonic implications

Our recent investigations into the Longyou paragneiss (country gneiss of the Longyou garnet amphibolite) in the northern Wuyi-Yunkai orogen of South China lead to a recognition of high-pressure metamorphic phengites in the paragneiss. The paragneiss consists of phengite, muscovite, biotite, K-feldspar, plagioclase, quartz, and minor graphite and ilmenite or magnetite. Mineral compositions show that Si contents in phengites are between 3.40 and 3.25 (p.f.u., per formula unit), belonging to high-pressure metamorphic phengites. P-T estimates suggest that peak high-pressure metamorphic stage (M2) has P-T conditions of ~ 13.0 kbar and ~ 690 °C, whereas post-peak retrograde stage (M3) has lower P-T conditions of ~ 6.0 kbar and ~ 695 °C. The two-stage P-T conditions define a clockwise P-T path of post-peak isothermal decompression, reflecting a typical collisional orogenic process. SIMS zircon U-Pb age data show a weighted mean 206Pb/238U age of 445 ± 3 Ma, consistent with the metamorphic age of the Longyou garnet amphibolite, suggesting that the peak high-pressure metamorphic event occurred in the Early Palaeozoic orogeny in the Cathaysia block in South China. Thus, we consider that both the Longyou paragneiss and enclosed garnet amphibolite together experienced a phase of high-pressure metamorphism during the Early Palaeozoic continent–continent collisional orogeny likely along the Zhenghe-Dapu fault between the South China block and an unknown block on the northern margin of eastern Gondwana continent.

Early Paleozoic high-temperature metamorphism of garnet amphibolite in the Longyou area, Cathaysia Block of South China: P–T path and tectonic implications

Garnet amphibolite can provide crucial information on tectonic processes. Recent progresses on garnet amphibolite in the Longyou area of the Cathaysia Block, South China have attracted geologists’ wide concerns and discussions about the regional metamorphic history and geodynamic setting. The Longyou garnet amphibolite shows a prominent porphyroblastic texture involving garnet (~40 vol%), hornblende, clinopyroxene, quartz and ilmenite, with minor plagioclase (<5 vol%), sphene, rutile, pyrite and apatite. Garnet presents as aggregations with double-/multi-nuclear structure and has embayed rims surrounded by quartz and plagioclase. Clinopyroxene is partially replaced by late hornblende and plagioclase. A clockwise high-temperature P–T evolution involving four stages has been determined. The generalized prograde stage shows increases of P–T conditions. The Pmax and Tmax stages lie in P–T conditions of 13 ± 0.6 kbar/808 ± 13 °C and 10 ± 0.9 kbar/840 ± 10 °C, reflecting apparent thermal gradients of ~17 °C/km and 22–23 °C/km, respectively. Both stages match medium-pressure facies series metamorphism. The late retrograde stage shows decompression and cooling. A P–XMg [=Mg/(Mg + Fe) in mole] pseudosection drawn at T = 840 °C suggests that low bulk-rock XMg contents (=0.29–0.33) in the Longyou garnet amphibolite are responsible for the high content of garnet in the rock. A P–M(Na2O) pseudosection at T = 840 °C reveals that the low content of plagioclase in the rock is caused by low bulk-rock Na2O contents (=0.44–1.18 mol.%). The integrated P–T–t data from the Cathaysia Block indicate that the early Paleozoic metamorphism can be connected to an intraplate orogenic setting.

India in the Nuna to Gondwana supercontinent cycles: Clues from the north Indian and Marwar Blocks

Evolution of the Indian Block can be traced through Earth9s Phanerozoic and Precambrian supercontinent cycles. The Paleoproterozoic tectonostratigraphic record of the North Indian Block and the Aravalli Delhi Fold Belt in the Nuna supercontinent assembly shows a close link with the events in the Cathaysia Block of South China. Accretion of the two terranes is documented by 1.97 to 1.92 Ga continental arc igneous rocks and 1.91 to 1.81 Ga syn- and post-collisional magmatism in the Lesser Himalaya, along with 1.88 to 1.86 Ga granulite metamorphism in both continental blocks. The connection between the North Indian Block and the Cathaysia Block continued through Nuna dispersal and was followed by the accretion of a series of terranes/microcontinents along the western margin of this united North India-Cathaysia Block during Rodinia assembly (ca. 1.0 Ga). This is recorded by accretion of the Marwar Block to the North Indian Block and Yangtze Block to the Cathaysia Block. Long-lived active continental margins continued along Marwar (NW India), Yangtze, Madagascar and the Seychelles until ca. 720 Ma that jointly occupied a peripheral or even independent paleoposition in the Rodinia reconstructions. The eastern margin of India sutured with the Western Australia-Mawson blocks along the Kunnga Orogen during the final assembly of Gondwana in the early Paleozoic, whereas microcontinental blocks including south Qiangtang and north Lhasa, were accreted to the northern margin of Gondwana in the vicinity of India. The collision of this ensemble of blocks with Africa (western Gondwana) is marked by the East African Orogen/Mozambique Belt, extending through central east Africa, Madagascar, South India and Antarctica. However, further north, India was separated from the Arabian-Nubian Shield by an embayment of the proto-Tethys that remained integral until the breakup of Gondwana. The accretion of Laurussia to Gondwana in the mid-Paleozoic during the assembly of Pangea corresponds with lithospheric extension along the northern margin of India (Gondwana) and separation of several continental blocks including South China, south Qiangtang, and north Lhasa, which then drifted northward across the Paleo-Tethys to collide with the Asian segment of Pangea in the Permo-Triassic.

Cogenetic Dykes the Key to Identifying Diverse Magma Batches in the Assembly of Granitic Plutons

AbstractThis study demonstrates that dykes that are coeval and cogenetic with plutons can provide an important tool for recognizing discrete batches of magma with similar overall chemical compositions and physical attributes, but different isotopic characteristics, and which contributed to pluton formation. The Qianlishan granitic pluton, located in the Qin–Hang fault zone separating the Yangtze block from the Cathaysia block in South China, was emplaced at 155–152 Ma in the Late Jurassic. It consists of a central zone of strongly differentiated zinnwaldite-bearing equigranular granite surrounded by a less differentiated porphyritic granite. The pluton is spatially associated with an extensive granitic dyke swarm dated here at 153–152 Ma, demonstrating a coeval relationship. Amongst the dykes, two discrete end-member sources can be identified from the bimodal nature of their zircon hafnium and oxygen systematics, with one group showing a range in εHf(t) of -11·9 to -8·0 and in δ18O of 9·0–10·4‰, whereas in the other group the ranges are from -7·3 to -4·1 and 8·4–9·4‰, respectively. This contrasts with the two phases of the Qianlishan pluton, which record wide ranges in εHf(t) of -11·1 to -5·1 and in δ18O of 8·3–10.4‰, but without bimodality. Hence, the overlapping Hf–O isotopic profiling shows the dykes and pluton to be cogenetic. Small-volume magma batches, with their rapid transport through the crust and quick cooling, are all typical features of dyke generation, thus preserving the original heterogeneous Hf–O isotopic signatures that are characteristic of two distinct crustal sources. However, although the pluton was formed from similar sources to the dykes, the bimodal source identity was lost during its assembly through mixing of the magma batches. These findings also provide a potential explanation for the wide range of zircon hafnium isotopic systematics typical of granitic plutons, as shown by sampling at all scales.

Texture and geochemistry of pyrite from the Gaoche gold deposit in South China: Implications of ore-forming processes

The Gaoche lode-type gold deposit is hosted in the Neoproterozoic metasedimentary rocks at the northeastern Cathaysia Block in South China. The deposit comprises three ore-bodies that are controlled by the NE-trending faults. Four generations of pyrite and associated paragenesis minerals have been identified. The Py0, formed in the early sericite-quartz alteration period unrelated to the gold mineralization, contains abundant silicate inclusions and has low Au (0–0.03 ppm), Ag (0.79–6.4 ppm) and As (0.93–402 ppm). The Py1 shows oscillatory zonings and has high invisible Au (0.01–1.8 ppm), Ag (0.01–106 ppm) and As (0.46–1163 ppm), representing the first stage mineralization. The Py2 also has high invisible Au (0.08–8.9 ppm), Ag (0.06–37 ppm) and As (5.5–10275 ppm), reflecting pseudomorphic dissolution of the Py1 and reprecipitation at the second stage mineralization. The Py3 lacks mineral inclusions but is chemically similar to the Py2. Available δ34S values for the mineralization-related pyrites range from −11.4‰ to +7.2‰, and their variation suggests the fluids were originally released from magmas followed by fluid-rock interaction under redox states at shallow depth. In addition, the Py1 and Py2 show high lead isotope compositions which lie above the orogen curve in the Pb-Pb isotope plots, indicating that the ore-forming materials were probably sourced from the metamorphic basement rocks. In combination with the regional geology and mineralization, the Gaoche gold deposit is suggested to have been genetically associated with regional metamorphism and deformation in relation to the western subduction of the paleo-Pacific oceanic slab during the Mesozoic.

Linking South China to North India from the late Tonian to Ediacaran: Constraints from the Cathaysia Block

Neoproterozoic paleogeography of the Cathaysia Block of South China is directly linked to the formation of Rodinia, its break-up, and the subsequent amalgamation of Gondwana. Two diamictite units occur in the Neoproterozoic sedimentary successions on the western margin of Cathaysia Block. In combination with our detrital zircon U-Pb dating results and corrected chemical weathering index data (CIAcorr), these units coincide with the global Sturtian and Marinoan glaciations. Late Tonian to early Ediacaran sedimentary rocks from western Cathaysia contain abundant late Mesoproterozoic to early Neoproterozoic (1150–870 Ma) and some mid-Neoproterozoic (800–720) detrital zircons. Paleocurrent data indicate sediment transport to the northwest. The timing and character of the ages of detrital zircons match the records of North India and East Antarctica, indicating that the Cathaysia Block received detritus from Gondwana. Thus, South China lay adjacent to northern Gondwana since at least 720 Ma, and possibly considerably earlier.

Exotic origin of Pingtan Island in the Pingtan-Dongshan Metamorphic Belt (SE China): Zircon U-Pb age and Hf isotope evidences

Cathaysia Block in South China had accreted tremendous amounts of Mesozoic magmatic rocks. Recent zircon UPb age results revealed existence of some Early Jurassic gneissic granites (200–190 Ma) in the Pingtan-Dongshan Metamorphic Belt (PDMB), coastal SE China which corresponds one stage of magmatic quiescence in the rest part of East Cathaysia. To better understand Mesozoic thermal evolution of the PDMB, UPb geochronology and Hf isotope composition of zircon separates (LA-ICPMS) and inclusions (NanoSIMS, age only) are examined for two granitic rocks and one meta-sandstone in Pingtan area (northern PDMB) and three granites in Matsu Islands further north. For Pingtan rocks, ages of discrete zircons give two Early Jurassic ages (196 Ma and 173 Ma) and prossess in dominance positive ɛHf(t) values (−0.2 to 7.5). On the other hand, zircon inclusions within rock-forming minerals yield a wide age span from Permo-Triassic to Early Cretaceous in six clusters, namely 270–206 Ma, 191–170 Ma, 168–151 Ma, 146–132 Ma, 126–119 Ma and 109–100 Ma that basically coincide the major Mesozoic thermal episodes in Cathaysia except the second cluster. For Matsu granites, discrete zircons exhibit Late Jurassic (162–158 Ma) and Early Cretaceous (137 Ma) ages with ɛHf(t) values of −9.1 to −1.6 and −4.9 to −0.8, respectively. Nevertheless, zircon inclusions show ages corresponding to Pingtan samples in the last four clusters. With reference to Mesozoic magmatic rock units in the neighboring Southeast China Magmatic Belt (SCMB), our data suggest irrelevance of Pingtan samples to the Cathaysia and linkage of off-shore Matsu granites to the PDMB. Noting that there are similarities of zircon ages and ɛHf(t) values between the granite-schist associations in Tananao Metamorphic Belt (TMB) of eastern Taiwan and Pingtan island of northern PDMB, these Early Jurassic granites were probably originated from a same source. Three consistent Early Cretaceous age clusters of zircon inclusions over the studied samples match two well-known magma prevailing episodes in the coastal SE China, with one tectonic episode (128–119 Ma) in the PDMB that is often invisible from the zircon separates remains unexplainable. In response to a plausible tectonic evolution, we support the earlier proposition for a now-concealed microcontinent that had impinged on Cathaysia around 125 Ma, but further suggest that it left Pingtan Island behind as a remnant fragment when drifted away at later time.

Phanerozoic magma underplating associated with remelting of the lower crust beneath the Cathaysia Block: Evidence from zircon U[sbnd]Pb ages and Hf[sbnd]O isotopes of granulite xenoliths from Daoxian, South China

Widespread Mesozoic granitoids in the Cathaysia Block of South China are associated with intensive reworking of the lower crust as a result of magma underplating. This inference is based mainly on studies of mafic igneous rocks, whereas there is little evidence from lower-crustal rocks. Lower-crustal xenoliths in Mesozoic basalts in the Daoxian region within the Cathaysia Block might record information on the relationship between magma underplating and remelting of the pre-existing crust beneath the block. The xenoliths are mainly mafic granulites, with minor felsic granulites. The mafic granulites have low SiO2 contents (47.22–49.46 wt%) and high Mg# values (77.8–82.4), suggesting that their protoliths were derived from a mantle source. The felsic granulite xenoliths have high SiO2 (69.56–70.27 wt%) and low MgO (1.63–2.50 wt%) contents, and zircons in these xenoliths yield negative εHf(t) values (−6.1 to −12.6) and high δ18O values (6.8–7.6‰), consistent with a crustal source. Both mafic and felsic granulite xenoliths record magmatic (226–218 Ma) and metamorphic (218–193 Ma) events, suggesting a genetic link between mafic and felsic rocks in the lower crust. We propose that the magma underplating was responsible for the origin of the mafic granulites and partial melting of pre-existing lower crust, as recorded by the felsic xenoliths. The granulite xenoliths also contain abundant inherited zircons that formed during the Archean–Neoproterozoic (2584–659 Ma), early Paleozoic (peaking at ca. 425 Ma), and late Paleozoic (peaking at ca. 261 Ma). Zircons from the Daoxian granulite xenoliths have distinct HfO isotopic compositions that record the multistage evolution of the lower crust beneath the Cathaysia Block. This evolution involved the accretion of juvenile crust during the late Archean (εHf(t) = +4.2 to +4.6) and late Paleozoic (εHf(t) = +1.3 to +5.3; δ18O = 5.8–6.2‰), crustal reworking during the Neoproterozoic (εHf(t) = −7.5 to −11.8; δ18O = 5.1–9.5‰) and early Paleozoic (εHf(t) = −0.5 to −2.2; δ18O = 7.3–7.5‰), and crustal accretion with significant reworking during the early Mesozoic (εHf(t) = −19.2 to +5.9; δ18O = 6.8–7.6‰). The UPb ages and HfO isotopic data of zircons from the Daoxian granulite xenoliths are consistent with the distribution of Phanerozoic igneous rocks in the Cathaysia Block. These data support the inference that Mesozoic magma underplating triggered the remelting of pre-existing crustal materials and produced extensive granitoid magmatism.

Unexposed Archean components and complex evolution beneath the Cathaysia Block: Evidence from zircon xenocrysts in the Cenozoic basalts in Leizhou Peninsula, South China

Magmatic systems exposed at the surface can sample the deep crust during the ascent of magmas from deep lithosphere. Zircons captured by such rocks, especially those with pre-eruption ages (xenocrysts), can be used to extract information on the formation and evolution of the deep continental crust. Integrated U–Pb dating and Hf-isotope analysis of zircon xenocrysts from the Cenozoic (<30 Ma) basalts on the Leizhou Peninsula, in the southwestern part of the Cathaysia Block of South China, offer a unique glimpse at the evolution of the deep crust beneath the block. 90 zircons from the basalts yield concordant or nearly concordant U–Pb ages varying from 2726 Ma to 61 Ma. There are a few Archean ages (2.7–2.5 Ga), but the majority of zircons fall into Proterozoic, Early Paleozoic (most strongly represented) and Mesozoic age populations. The oldest grain (2.7 Ga) has εHf(t) of −9.5, corresponding to a TDM model age of 3.4 Ga and a Tcrust model age of 3.8 Ga, respectively, while 30% of the analyzed grains have Tcrust model ages ≥2.5 Ga. The zircons in the basalts also record multiple post-Archean tectono-thermal events, including 1.9–1.7 Ga, 1.4–1.2 Ga, 1.0–0.8 Ga, 600–450 Ma, 250–150 Ma. These thermal events, and particularly the strong Caledonian and obvious Indosinian events are similar to those recorded in the Yunkai, Naling and Wuyi regions. The similarity between the studied area and these other regions indicates that the Leizhou Peninsula, covered by Quaternary volcanic rocks and sedimentary rocks, represents the southwestern part of the Cathaysia Block. The zircons with ages of Paleoproterozoic (1.9–1.7 Ga), late Mesoproterozoic (1.0–0.8 Ga) events have positive εHf(t), up to +10.2, showing the addition of juvenile mantle-derived materials in these three periods. Meanwhile, those with ages of the early Mesoproterozoic (1.4–1.2 Ga), late Neoproterozoic to early Paleozoic (650–550 Ma), Phanerozoic (400–450 Ma, 250–150 Ma) have negative εHf(t), down to −35, suggesting the reworking of old crustal components at these times. These data imply that there are significant volumes of Paleoarchean to Mesoarchean (>3.4 Ga) components (or remnants) as old as 3.8 Ga beneath the studied area. Combining with the scattered Archean ages in other parts of the Cathaysia Block (such as Guangxi, Yunkai, Naling and Wuyi), they suggest that ancient deep crustal components may be more widely distributed than would be indicated by the exposed rocks in the block.

Ancient crustal recycling in modern island arcs: A tale of the world's youngest charnockite from SW Japan

Modern accretionary orogens are considered as zones of extensive juvenile crustal growth on our planet with the Central Asian Orogenic Belt (CAOB) and the western Pacific domain as typical examples. Here we report magmatic charnockites (orthopyroxene-bearing anhydrous monzogranite) from the Shikoku Island in SW Japan intruding the Shimanto accretionary belt. Petrological and geochemical features of the intrusion are identical to Archean, Paleoproterozoic and Cretaceous charnockites from various parts of the world and show typical arc magmatic affinity and formation in a convergent margin setting. Zircon UPb data indicate that the pluton was emplaced at 13.75 Ma, revealing one of the youngest magmatic charnockite occurrences in the world. Zircon LuHf data yield epsilon Hf(t) values in the range of −3.2 to 2.8 suggesting that the magma source involved a substantial amount of reworked material, in addition to juvenile input. The Hf modal age (TDM) shows a major peak at 701 Ma, suggesting reworking of Neoproterozoic basement rocks beneath the young island arc. The data confirm the model of the birth of Proto-Japan along the periphery of the Cathaysia block in South China. The modal age peak corresponds to the timing of magmatism associated with Rodinia rifting. Our study provides evidence for the involvement of both ancient recycled components and juvenile material associated with crust building in a young island arc within an ongoing subduction system.

Geochronology, geochemistry and Hf isotopic composition of Late Cretaceous Laojunshan granites in the western Cathaysia block of South China and their metallogenic and tectonic implications

Sn-, In- and W-rich granites are identified in Laojunshan area in the western Cathaysia block, South China, including coarse-grained two-mica monzogranite (CMG) and fine-grained two-mica monzogranite (FMG). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating of CMG and FMG are from 85.6 ± 0.8 Ma to 90.37 ± 0.7 Ma and 89.9 ± 1.4 Ma, respectively. All samples have relatively high total alkalis contents (5.74 wt% to 9.73 wt%) and Al saturation indices (0.95 to 1.79), indicating that most of granites belong to the strongly peraluminous granites. The granites are enriched in P, Rb, Cs, Sn, Li, Be, and U, and are depleted in Ti, Mg, Co, Ni, Sr, Ba, Zr, and Hf. The fractional crystallization of plagioclase and K-feldspar was the principal process of magmatic differentiation that controlled Rb, Sr, Ba and Eu concentrations. While rare earth elements were fractionated by accessory minerals such as apatite, monazite or ilmenite. These geochemical features suggest that most of rocks are highly fractionated S-type granites. The εHf(t) values of the zircon samples from CMG and FMG range from −12.88 to −1.19, with the two-stage Hf model ages ranging from 1.2 Ga to 1.9 Ga. The geochemical and isotopic data show that the Laojunshan granites are derived from a crystalline basement that has metamorphosed pelitic rocks from the Mes-paleoproterozoic continental crust. The upwelling of the asthenosphere caused by a strong lithospheric extension led to the partial melting of the metamorphosed pelitic rocks to form widespread Late Cretaceous S-type granites in the Laojunshan area. Lithospheric extension and Cretaceous magma in SE Yunnan are probably related to the Palaeo-tethys ocean crustal delamination, which subducted beneath the Indochina block.

The Mesoproterozoic Baoban Complex, South China: A missing fragment of western Laurentian lithosphere

Abstract New age data for the Baoban Complex, South China establishes that it lay outboard of western Laurentia in the early Mesoproterozoic but was not part of the Cathaysia Block, with which it is traditional linked, until the mid-Paleozoic. Our geochronology data for detrital zircon and authigenic monazite grains from metasedimentary rocks indicate accumulation between ca. 1.55 Ga and 1.45 Ga for the Gezhencun succession of the Baoban Complex and ca. 1.45 Ga and 1.30 Ga for the Ewenling succession. The former unit is dominated by detrital zircon populations between 1900 Ma and 1500 Ma with two peaks at 1780 Ma and 1580 Ma. The Ewenling succession has detrital zircon peaks at 1720 Ma and 1450 Ma. Newly discovered gneissic granites were emplaced at 1550 Ma and intruded by 1450 Ma leucogranite dykes that are coeval with 1460–1430 Ma bimodal magmatism. The whole Baoban Complex was metamorphosed over the range of 1.3–0.9 Ga based on ages of authigenic zircon and monazite. Depositional ages of metasedimentary rocks are coeval with successions of the Belt-Purcell Basin, western Laurentia. Detrital zircon from the two regions have similar age populations and Lu-Hf compositions, and display a synchronous provenance shift at ca. 1.45 Ga. Basement lithologies on Hainan Island range in age from ca. 1.55–1.43 Ga and underwent metamorphism during 1.3–0.9 Ga. This is younger than basement rocks on the mainland of the Cathaysia Block in South China, suggesting the two regions are spatially unrelated at this time and hence the Mesoproterozoic record of the island cannot constrain the location of the Cathaysia Block in the Nuna and Rodinia supercontinents.