Search

A flare‐up of granitoids occurred at 465–445 Ma in the South Altyn Tagh, synchronously with the exhumation of subducted continental crust. Nevertheless, it remains enigmatic whether a petrogenetic connection exists between them. Here, we report a 454–451 Ma monzogranite pluton, which is characterized by abundant inherited zircons, located in the northern margin of the South Altyn Tagh high‐pressure (HP)—ultrahigh‐pressure (UHP) metamorphic terrane. U–Pb ages and Hf–O isotopic compositions of inherited and synmagmatic zircons are investigated to trace the source rocks and petrogenesis of this pluton. The inherited zircons (zircons that predate the magmatism) exhibit a wide range of ages from 2618 to 484 Ma, displaying three major peaks at 1800–1100 Ma, 1000–800 Ma and 500 Ma. By comparing these inheritance age patterns with zircon spectra of main (meta‐)sedimentary sequences and the widespread Early Neoproterozoic granites (presently as granitic gneisses) in South Altyn Tagh, along with zircon εHf(t) and whole‐rock Nd isotopic composition, we argue that the main source rocks of the studied monzogranite are Early Neoproterozoic granitic gneisses and Late Mesoproterozoic paragneisses from the South Altyn Tagh HP–UHP metamorphic terrane. The ca. 500 Ma inherited zircons have a metamorphic origin, which is simultaneous with the peak metamorphic ages of HP–UHP metamorphic rocks in the Altyn Tagh Complex. These observations indicate that the source rocks of the monzogranite pluton are the subducted continental crust, which underwent metamorphism at ca. 500 Ma and followed by partial melting at 454–451 Ma. In addition, synmagmatic zircons exhibit variable δ18O and εHf(t) values ranging from 5.4 to 11.7‰ and from −19 to +10.3, respectively, indicating a minor contribution of mantle‐derived melts in the formation of the monzogranite. Given the studied pluton and contemporaneous extensive granitoids (465–445 Ma), characterized by similar geochemistry and source rocks, are synchronous with the final exhumation of subducted South Altyn Tagh continental crust, we propose that the reworking of exhumed continental crust at middle to lower crustal depths is their main petrogenesis. [ABSTRACT FROM AUTHOR]

A systematic data set of petrography, mineralogy, geochronology, and geochemistry is reported for typical arc granitoids and associated mafic microgranular enclaves (MME) from the Chinese Altai, Central Asian Orogenic Belt. Zircon U‐Pb dating indicates that they crystallized coevally at ∼395 Ma. Magma hybridization modeling suggests that the granitoid parental magma formed by mixing between a mafic and a felsic endmember. The sharp decrease of An values from cores to rims (e.g., from ca. 80 to 40) of plagioclase crystals points to polybaric crystallization accompanied by degassing. Petrographic evidence and plagioclase in situ Sr isotopic compositions ((87Sr/86Sr)i = 0.7053–0.7071) confirm the interaction of at least two isotopically different magmas during the mineral crystallization. A model for the formation of the enclave‐bearing calc‐alkaline plutons in arc settings is presented: At depth, mantle and crustal melting and efficient magma mixing controlled the principal chemical compositions of the granitoid intrusions. Then in the higher level chamber, decompression‐dominated crystallization and the mingling and limited mixing between blobs of mafic magma and the enclosing granitoid magma ultimately determined the texture, mineral composition, and enclave morphology. The MME have (87Sr/86Sr)i of 0.7048–0.7062, εNd(t) of −0.5 to +2.6, and zircon εHf(t) of +2.3 to +5.4, while the host granitoids have similar Sr isotopic compositions ((87Sr/86Sr)i = 0.7054–0.7064), but generally lower whole‐rock εNd(t) and zircon εHf(t) values (−2.2 to +0.4 and +0.6 to +4.6, respectively). The petrogenetic analysis and isotopic characters suggest that granitoid emplacement was a means of crustal growth in the Chinese Altai orogen. Key Points: Efficient magma mixing in the deep source zone controlled the principal chemical compositions of the granitoid intrusionsMafic microgranular enclaves are quenched mafic magma that experienced mingling and limited mixing in the shallow granitic chamberTwo‐stage mafic‐felsic magma interactions operated in the generation of enclave‐bearing calc‐alkaline granitoids [ABSTRACT FROM AUTHOR]

Considerable controversy concerns the consumption and closure processes of the North and South Altyn Oceans resulting in the formation of the North Altyn Tagh subduction-accretion belt and the South Altyn Tagh subduction-collision belt in the Altyn Tagh orogen in southeastern Tarim. This study focused on U–Pb ages and Hf isotopes of detrital zircons from early Paleozoic sedimentary rocks in the Central Altyn Tagh terrane. Our dating results revealed that the studied sedimentary rocks were deposited at ca. 467–429 Ma. The predominant early Paleozoic detrital zircons demonstrate negative εHf(t) values, highlighting their derivation from the coeval intermediate-felsic magmatic rocks sourced from ancient supracrustal materials with enriched isotopes. These inferred, enriched intermediate-felsic magmatic rocks were probably generated in different consumption and closure processes of the North and South Altyn Oceans, based on geochronology and geochemistry of the early-middle Paleozoic intermediate-felsic magmatic rocks in the Altyn Tagh orogen. A number of ca. 520–490 Ma detrital zircons were primarily derived from the coeval, enriched intermediate-felsic magmatic rocks in the local Central Altyn Tagh terrane and the adjacent North Altyn Tagh subduction-accretion belt associated with the advancing subduction of the North Altyn Ocean. The ca. 490–470 Ma detrital zircons are most prominent and suggest a derivation from the coeval, enriched magmatic rocks subject to the rollback of the North Altyn Ocean. These inferred magmatic rocks might have once existed in the region but have been intensely exhumed to be source material for the studied sedimentary rocks. A few ca. 450 Ma detrital zircons indicate a provenance from the North Altyn Tagh subduction-accretion belt during collision after the final closure of the North Altyn Ocean. In contrast, some ca. 470–460 Ma detrital zircons were probably derived from the coeval intermediate- to felsic-magmatic rocks in the Central Altyn Tagh terrane during the exhumation stage of the deep continental collision after the final closure of the South Altyn Ocean. Combination of provenance study and magmatic record provides potential to establish complicated orogenic evolution. [ABSTRACT FROM AUTHOR]

The Jiangnan orogenic belt in South China defines a subduction-collisional zone between the Yangtze and the Cathaysia blocks. The eastern part of this belt comprises the oceanic Huaiyu terrane, the northeast Jiangxi suture zone and the continental Jiuling terrane. We report zircon U–Pb and Hf isotope data and whole rock geochemistry on the arc magmatic rocks of andesite and basalt occurring as olistostromal blocks, as well as conglomerate in the Dengshan Group above the unconformity between cover sequence and the volcanic arc units. The andesites yield weighted mean ages of 864 Ma, 868 and 871 Ma, whereas the detrital zircons from the conglomerate yield a major age population of 863–810 Ma (peak of 833 Ma), with two minor ones of 1120–940 Ma (peak at 960 Ma) and 1780–1560 Ma (1670 Ma). A post-ca. 810 Ma depositional age is thus suggested for the Dengshan Group. The influx of olistostromal deposits into the Dengshan Group must post-date the youngest detrital zircon age of 810 Ma and is possibly related to the widespread Nanhua rifting associated with the breakup of Rodinia. Furthermore, the age distribution of detrital zircons from the phyllite associated with the arc units compared to its depositional age also suggest a convergent setting, and those from the conglomerate are plotted into a similar field. The andesite and basaltic trachyandesite have LREE enriched patterns, with minor negative Eu anomaly, and negative Nb, Ta, Cs and Ti anomalies. However, the basalt displays only minor LREE enrichment, with no Eu anomaly, and almost flat spidergram trace element patterns, except for a minor positive Sr, Ba and Ti anomaly and negative Rb anomaly. The andesite and basaltic trachyandesite are all plotted in the field of volcanic arc, suggesting an arc setting. The basalt is plotted between EMROB and volcanic arc domains, suggesting magma generation from metasomatised sub-arc sources with subducted oceanic components. The Hf isotopic data on zircons from the andesite yield positive ɛ Hf( t ) values of 2.79–15.51 and late Mesoproterozoic model ages, suggesting that the Neoproterozoic arc sources include reworking of subducted late Mesoproterozoic materials. Our results show that the amalgamation of the Yangtze and Cathaysia blocks in the eastern part occurred at some time between ca. 864 Ma and ca. 810 Ma. [ABSTRACT FROM AUTHOR]

Highlights: [•] N-MORB type basalts are identified from the west Jiangnan orogenic belt. [•] Ca. 863Ma Mafic suites in the eastern and western segments of the belt show arc related signatures. [•] Amalgamation of Yangtze and Cathaysia Blocks in the eastern part after ca. 863Ma, followed by breakup at ca. 790Ma. [ABSTRACT FROM AUTHOR]

Highlights: [•] 854Ma mafic–ultramafic rocks from west Jiangnan Orogen were emplaced in arc setting. [•] 830–820Ma S-type Yuanbaoshan granite pluton was emplaced in syn-collisional setting. [•] The assembly age of the Yangtze and Cathaysia Blocks is constrained at ca. 850–830Ma. [Copyright &y& Elsevier]

Abstract: The unified South China Block, comprising the Yangtze Block to the northwest and the Cathaysia Block to the southeast, has been central to recent studies on the Precambrian crustal evolution and position of South China in Proterozoic supercontinents. Here we present results from detailed U–Pb geochronology, Lu–Hf isotopes and rare earth geochemistry on zircons from three Ordovician and Devonian sandstone samples of southeastern Yangtze, together with those from a granodiorite sample and three Triassic sandstone samples in northeast and southeast Cathaysia. The southeastern Yangtze Block is characterized by extensive tectono-thermal events during 850–730Ma and 470–410Ma, which corresponded to the breakup of Rodinia and the Caledonian orogenic event, respectively. Major zircon populations with ages of 2800–2500Ma, 1990–1580Ma and 1260–920Ma are recorded from southeastern Yangtze, which are correlated, respectively, with the Neoarchean crustal growth, and histories of the Paleo-Mesoproterozoic Columbia and Neoproterozoic Rodinia supercontinents. In contrast, zircon U–Pb ages from southeast Cathaysia reveal prominent magmatic events during 2630–2300Ma, 2030–1770Ma and 280–190Ma, correlated with Neoarchean continental growth and the tectonothermal regimes associated with the history of the Columbia supercontinent as well as the Indosinian movement. The sector-zoned zircons from the granodiorite in northeast Cathaysia yielded weighted mean ages of 846.7±9.8Ma and 826.4±7.6Ma and positive ɛHf values, constraining the initial breakup of South China at ca. 848 Ma and extensive rift-related magmatism at ca. 826Ma. The Hf data suggest important episodes of juvenile magmatic addition at 3.1–2.8Ga and 2.4–1.5Ga for Yangtze, and at 3.3–2.3Ga for Cathaysia, indicating the existence of older crustal components in South China. The Hf data suggest that most zircons in both the blocks were derived from crustal magmas. Our results suggest distinct crustal evolution histories for the Yangtze and Cathaysia Blocks albeit with close affinities. [Copyright &y& Elsevier]

Abstract: This paper reports some new results from U–Pb geochronological, Hf isotopic and REE geochemical studies of detrital zircons in the Ordovician sandstones from South Jiangxi within Cathaysia. 426 groups of U–Pb age determinations define five major age populations: 2560–2380Ma (a peak of 2460Ma), 1930–1520Ma (a peak of 1700Ma), 1300–900Ma (a major peak at 970Ma and two subordinate peaks at 1250Ma and 1130Ma), 850–730Ma (a prominent peak of 780Ma) and 670–530Ma (a major peak at 540Ma and a subordinate peak at 650Ma). We also report zircon U–Pb concordia age of 3.96Ga, which is the oldest age so far obtained from Cathaysia. The age peak at 2460Ma correlates with similar ages reported for Neoarchean global continental growth. The 1930–1520Ma population broadly overlaps with the time of amalgamation and disruption of the Columbia supercontinent. The major age peak at 970Ma and two secondary peaks at 1250Ma and 1130Ma reflect multiple tectonothermal events associated with the assembly of Rodinia. Similar ages are widely reported from the South China Craton (SCC). Our study reveals that the 850–730Ma population is consistent with the breakup period of Rodinia, suggesting that the SCC within Rodinia began to break up since 850Ma. Geologically, the evidence for this breakup event is widespread and presented by Neoproterozoic granites, bimodal igneous rocks, basic dyke swarms and formation of continental rift type basins. Our study also reveals a 670–530Ma population that correlates well with the assembly of Gondwana during end Neoproterozoic. However, direct geological evidence for this event has not yet been found within the studied area. Furthermore, the Hf isotopic model age data suggest two major stages of crustal evolution within Cathaysia. The first is the event dated at 1.6–2.8Ga and the second one at 3.5–3.9Ga. The zircons show a large range of εHf(t) values from +8.64 to −30.54, suggesting that they have different origins with a similar age of crystallization. The fact that most detrital zircons show negative εHf(t) values suggests the ancient provenances of Cathaysia were dominated by reworked crustal materials. [Copyright &y& Elsevier]

Highlights • The 848 Ma anorthosite was found in the E-Jiangnan ophiolite and the 860–830 Ma intermediate-felsic rocks display active margin signature. • The 870–860 Ma ophiolite shows N- and E-MORB features and the 817 Ma K-granite marks the collision event. • Identification of western suture zone and the timing of final assembly of South China proposed as 830–800 Ma. Abstract The Jiangnan orogenic belt in South China, formed by the collision of the Yangtze and Cathaysia sub-blocks, represents a key to reconstruct the assembly of the Rodinia supercontinent. Here we investigated the Fuchuan ophiolite mélange and Shexian magmatic units in the eastern Jiangnan belt, and report the first findings of anorthosite and leucogranite that are dated at 848 ± 4 and 840 ± 5 Ma, respectively. The anorthosites yield positive εNd(t) values (2.3–4.2) and lower initial 87Sr/86Sr ratios (0.704616–0.704679), along with zircon εHf(t) of ∼9.2, suggesting a depleted mantle contribution with minor crustal assimilation. The mafic-ultramafic rocks and anorthosite within the Fuchuan ophiolite display N-MORB & E-MORB signatures, likely generated in back-arc marginal sea spreading center and metasomatized by subduction-related fluids. The intermediate-felsic igneous suites from Shexian area in the eastern margin of the Zhanggong Terrane were formed at 850–830 Ma and display continental-arc-related geochemical features. Zircons within these rock suites mostly show εHf(t) values of 3–12, whereas inherited grains yielded negative εHf(t) values with model ages of 2.7–1.9 Ga, implying an active continental marginal environment. Overall relations and data indicate that the disrupted Fuchuan mafic-ultramafic rocks, anorthosite and leucogranite constitute the remnants of an 860–830 Ma ophiolite, whereas the northwestward subduction beneath the Zhanggong terrane occurred at ca. 870–860 Ma to ca. 830 Ma. A ca. 817 Ma muscovite-bearing K-granitic pluton dated at 817 ± 5 Ma emplaced in a post-collisional setting constrains the upper age limit of continental assembly along the Shaoxing- Jiangshan- Pingxiang- Qidong-Quanzhou- Longsheng-Yongfu fault zone. The timing of the final assembly of South China occurred at 830–800 Ma as referred from a collisional-type granitic magmatism ca. 830–815 Ma and an external dynamic process of uplifting-erosion-transpotation-deposition presented by the regional-scale unconformity with ages 820–795 Ma. Age and nature of the Jiangnan orogenic belt, along with sedimentary records and paleomagnetic constraints, indicate that the South China Block was possibly located along the northern Rodinia margin, adjacent to the Tarim and northwestern Australia blocks. [ABSTRACT FROM AUTHOR]

The Jiangnan Orogen and Cathaysia Block constitute integral components in the geological framework of the South China Craton. However, the tectonic attributes of these two regions remain controversial. In this study we present LA–ICP–MS zircon U–Pb ages, Lu–Hf isotopes, and major and trace elements data from a suite of clastic rocks. Two hundred and forty seven zircon U–Pb ages obtained from the Neoproterozoic meta-sandstones in western Jiangnan Orogen define three major age populations: 2564–2395 Ma, 2068–1849 Ma and 963–730 Ma, corresponding to the early Paleoproterozoic crustal growth, amalgamation of the supercontinent Columbia and tectonics associated with Rodinia. Two hundred and forty three zircon U–Pb age data from Cambrian and Ordovician sandstones in the western Cathaysia yield five age populations: 2626–2392 Ma, 2050–1518 Ma, 1173–904 Ma, 857–708 Ma and 630–502 Ma, which can be correlated to the Neoarchean–early Paleoproterozoic continental growth, the tectono-thermal events associated with the Columbia supercontinent, the assembly and breakup of Rodinia, as well as the Pan-African event. The 630–502 Ma population correlates with the assembly of Gondwana supercontinent during late Neoproterozoic to early Paleozoic, although direct geological evidence for this event has not yet been reported from this region. These results suggest that the western Jiangnan belt and western Cathaysia Block have distinct crustal evolution histories albeit with close affinities. The REE geochemisty and Lu–Hf isotope data also indicate that the provenances of the Neoproterozoic and early Paleozoic detritus in western Jiangnan belt and western Cathaysia are mostly composed of reworked crustal materials. The metamorphic rims of zircon grains with core–rim structures yield ages of 912–891 Ma and 853–835 Ma, suggesting two phases of metamorphic events during Neoproterozoic in the source areas. These two events are likely associated with the tectonics related to the collision of Yangtze and Cathaysia blocks in the western and eastern segments, respectively. [ABSTRACT FROM AUTHOR]

Slab-derived components can significantly diversify subduction-related magmatism, for example, fore-arc ophiolites evolving from normal mid-ocean ridge basalts to fore-arc basalts and then to boninites. Here we show a progressive evolution of back-arc ophiolites controlled by slab-derived components but different from fore-arc ophiolites through studying the Kudi back-arc ophiolite in Western Kunlun, NW Tibetan Plateau. Our results reveal that the Kudi ophiolite was formed at 516–512 Ma in a back-arc setting in response to the rollback of the Proto-Tethys oceanic slab. The back-arc setting is supported by geochemistry of basalts (e.g., low Th/Nb and V/Ti ratios), low melting pressure (<10 kbar) revealed by amphibole geobarometer and low δ11B values (−19.1 to −8.1‰) of the mantle units. In such a scenario, the early-stage depleted Kudi basalts are characterized by depletions in light rare earth elements and enrichments in fluid-mobile elements, probably resulting from decompression melting of the primary back-arc mantle slightly modified by slab-derived fluids. Subsequently, partial melting of the back-arc mantle enriched by progressive addition of slab-derived sediments resulted in the formation of residual harzburgites and corresponding primary magmas that evolved into dunites, gabbros, quartz diorites and enriched pillow lavas. Additionally, both the residual harzburgites and cumulated dunites experienced minor impregnation of melts, without significant interstitial melt-crystal reactions. Compared with the early-stage depleted Kudi basalts, the late-stage enriched pillow lavas are characterized by enrichments in light rare earth elements and depletions in high field strength elements, typical arc affinities, confirming the addition of slab-derived sediments. Compared with typical fore-arc ophiolitic series, it is possible that there is a progressive evolution in back-arc basins in which a "new" enriched mantle source (corresponding arc basalts) modified by slab-derived sediments displaces an "old" depleted mantle source (corresponding basalts characterized by enrichments in fluid-mobile elements) modified by slab-derived fluids. • The 516–512 Ma Kudi ophiolite formed in a back-arc setting. • Decompressed melting of the primary mantle formed depleted Kudi volcanism. • Partial melting of an enriched mantle formed volcanics with arc affinities. • Back-arc magmatic stratigraphy exhibits a time-progressive evolution. [ABSTRACT FROM AUTHOR]