The early Paleozoic granitoids of the North Qinling Terrane have been widely viewed as the products of an Andean-type margin related to the long-lasting subduction of the Proto-Tethys (Shangdan) ...ocean. However, the spatial and temporal distributions of the arc granitoids coupled with subduction-accretion processes are still a subject of debate. Here, we report LA-ICP-MS UPb monazite and titanite ages and trace element concentrations for granites and a paragneiss from the North Qinling Terrane. The results, together with data in the literatures, are used to reconstruct the magmatic evolution related to the subduction of the Shangdan ocean during the early Paleozoic. As revealed by UPb geochronology, subduction-related magmatism in the southern belt of the North Qinling magmatic arc (NQMA) was most active between 423 and 404 Ma, and therefore younger and shorter than previously speculated. The ages are in agreement with a metamorphic monazite UPb age (415 Ma) obtained from the paragneiss, suggesting that crustal anatexis of the NQMA was contemporaneous with the magmatism. Overall, magmatism in the NQMA moved progressively from north to south. This southward propagation is consistent with roll-back of the Shangdan oceanic slab and transition from an oceanic arc into a back-arc basin setting. The timing of magmatism and migmatization as well as high-temperature granulite metamorphism in the NQMA signifies that the Shangdan ocean retreated to the North Qinling Unit before c. 423 Ma. Monazites from NQMA generally show HREE depletion and negative Eu anomalies, and some display the REE tetrad effect. These geochemical features indicated that the NQMA granites were highly fractionated. In addition, the abrupt changes in REE compositions displayed by titanites probably record a mixing event with a mafic magma along the Shangdan suture zone.
•U-Pb ages of monazite and titanite suggest younger and shorter arc magmatism at c. 423–404 Ma.•The new age data reveals a southwestward migration of the arc magmatic activity from c. 440 Ma.•A revised tectonic model involving slab roll-back is proposed for the North Qinling orogen during the early Paleozoic.•Monazite is well suited for dating highly evolved granites, whereas titanite could record detailed petrogenetic information.
The Pamir orogen originated through the amalgamation of various arc terranes. Despite undergoing numerous studies, the subduction polarity and time regarding the accretion processes remain ...inadequately constrained. To address these issues, we carried out a detailed study of zircon and apatite U-Pb ages, zircon Hf isotopes, and whole-rock geochemistry for the Permian to Cretaceous gabbro, volcanic rocks and granitoids from the Southern Pamir and the Rushan-Pshart suture. Three stages of magmatism are identified, including 279–266 Ma gabbro and andesite to dacite, 209–208 Ma granitoids, and 108–103 Ma gabbro and granitoids, respectively. The Permian gabbro and basaltic andesite to dacite in Pshart Range are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but are depleted in Ta, Nb, and Ti, suggesting formation from an enriched mantle source in a subduction zone. The Late Triassic I-type granitoids are typical arc calc-alkaline magmatic rocks with εHf(t) values varying from −8.7 to −3.5, which mainly sourced from the partial melting of mixed crustal sources predominately consisting mafic rocks with minor metasedimentary rocks. The Early Cretaceous Pshart granites are I-type granites with relatively low εHf(t) values ranging from −14.6 to −2.9, suggesting that they were crust-derived rocks. The Early Cretaceous Bazardara granites are identified as A2-type granites with the εHf(t) values ranging from −18.3 to +5.3, and were derived from mixing of mantle-derived and crust-derived magmas, or interaction between mantle melts and old crustal rocks. The early Cretaceous gabbro exhibits -MORB-like geochemical characteristics with flat REE pattern (La/YbN = 4.96), moderate enrichment of LILEs and weak depletion of Nb, Ta and Ti (Nb/LaPM = 0.94). Thus, the early Cretaceous A2-type granite, E-MORB-like gabbro and volcanic rocks were formed in an extensional setting. Our new data indicate that the southward subduction of the Rushan-Pshart oceanic slab started in the Early Permian and continued to the Late Triassic. Moreover, combining the previous data, we conclude that the Early Cretaceous granites and gabbro in the Southern Pamir formed in an extensional environment due to the southward rollback after northward near-flat subduction of the Neo-Tethyan oceanic lithosphere.
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•Permian, Triassic, and Cretaceous arc rocks are identified in the South Pamir.•Southward subduction of the Rushan-Pshart Ocean began before the Permian.•The southward subduction formed the late Triassic I-type granites.•Slab roll-back of the Shyok Ocean caused within-plate magmatism in the South Pamir.
The architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components, but the fundamental ...paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined, because of unavailable data. The Central Asian Orogenic Belt (CAOB) provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage. In this paper we review several lines of available evidence, which enable us to propose a new tectonic model of huge roll-back in the formation of the accretionary tectonics of the Mongolian collage in Central Asia. In the early Paleozoic the Mongolia collage comprised the southern Siberian and the Tuva-Mongol Oroclines. The Siberia Craton and the Mongolia collage jointly formed a giant “tadpole-shape” within the Paleo-Asian and Panthanlassic oceans; its head (Siberia) was to the south, and the tail (Tuva-Mongol) to the northwest. The structures and tectonic zonation of the Mongolia collage are characteristic of multiple arcs, which have been separately described in detail in different segments southwards from the Southern Siberia-East Sayan, West Sayan-Gorny Altai-Chara, via the Lake Zone-Junggar-Tianshan, Gobi Altai-Beishan-Alxa, to the Manlay-Hegenshan-Baolidao-Solonker segments. Almost all segments underwent Early Paleozoic to Permian, or even Triassic, frontal subduction and accretion, while rifting in the Late Carboniferous to Permian or Triassic occurred in the outward/oceanward (westward) advancing Mongolian collage. Therefore, we suggest that a huge complex roll-back, active from the Carboniferous to Permian or even to late Triassic, facilitated the formation of the Mongolian collage. The outward multiple roll-back process was compatible and almost coeval with the start of the Tuva-Mongol Orocline and rotation of the Siberian Craton, as confirmed by paleomagnetic and structural data. During the roll-back processes an archipelago paleogeography was formed behind the frontal subduction and accretion, in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times. The roll-back process was affected by the collision of the Kazakhstan collage along the Chara and Karamay zones in the Early Permian, the collision of the Tarim Craton along the South Tianshan zone in the Early Permian, the collision of the Dunhuang Block along the Liuyuan zone in the Early Permian-Triassic, the collision of the Alxa block along the Qugan Qulu zone in the Permian, and the collision of the North China Craton along the Solonker zone in the Middle-Late Triassic. The tectonic styles and architecture of accretionary orogenic belts like the CAOB are characterized both by the amalgamation of multiple terranes and by oroclinal bending. The systematic anatomy of the multiple roll-back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens.
The latest Devonian–early Carboniferous granitic intrusions in the Central Tianshan block are composed mainly of monzogranites and granodiorites. Here we present the petrology, geochemistry, and in ...situ zircon U–Pb ages and Hf isotopes of these intrusions. Bulk geochemistry suggests that the monzogranites and granodiorites are high-K, calc-alkaline, I-type granites. LA-ICP-MS zircon dating shows that the monzogranites and granodiorites formed at ca. 362Ma and ca. 354Ma, respectively. They are characterized by relatively high initial 176Hf/177Hf ratios (0.282571–0.282764) and positive εHf(t) values (+2.1 to +7.2). We interpret them to have been derived from partial melting of the Mesoproterozoic metamorphic basement of the Tianshan block and a significant addition of juvenile material. Compared to the monzogranites, the granodiorites are characterized by higher Sr (373–599ppm), low Y (12.5–20.5ppm), and Yb (1.21–2.04ppm) contents, with relatively higher Sr/Y (26–32) ratios, analogous to those of modern adakitic rocks. The differences in geochemical characteristics between the monzogranites and granodiorites may reflect differences in the P–T conditions experienced by the two lithologies during partial melting. We propose that the latest Devonian monzogranites were possibly generated by partial melting of the Tianshan Mesoproterozoic basement rock with an influx of juvenile material in an arc setting. However, the granodiorites were likely related to the slab roll-back of subducted north Tianshan Ocean during the early Carboniferous.
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•The monzogranites and granodiorites in Central Tianshan emplaced 362Ma and 354Ma, respectively.•Their parental magmas include both the Mesoproterozoic basement and juvenile material.•A transition from normal to steeper angle subduction induced upwelling of hot asthenosphere.
Mafic dike–granitoid associations are common in extensional tectonic settings and provide important opportunities for understanding mantle and crust melting during the tectonic evolution of host ...orogenic belts. We report results of petrologic, whole rock geochemical, Sr–Nd isotopic data and in situ zircon U–Pb and Hf isotopes for a mafic dike–granitoid association from the Zhongyangchang pluton in the western Tianshan, in order to constrain their petrogenesis and tectonic significance. The intrusive rocks are mainly composed of granodiorite, monzogranite, and minor granitic dikes, with mafic dikes intruded into the pluton. Zircon LA-ICP-MS U–Pb ages indicate that the Zhongyangchang intrusive rocks were all emplaced during a short interval in the Late Carboniferous (317–310Ma), establishing that the mafic and felsic magmas were coeval. The mafic rocks have low SiO2 and high MgO concentrations, with low 87Sr/86Sr ratios from 0.7048 to 0.7053 and positive εNd(t) and zircon εHf(t) values from +2.9 to +3.8 and +12.2 to +13.6, respectively. They are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), which can be explained by an origin from melting of a depleted lithospheric mantle source and source fluxing by fluids derived from the down-going slab. Granitoids from the pluton have high SiO2 contents and low MgO concentrations, suggesting that they were mainly derived from crustal sources. They also have positive whole rock εNd(t) and zircon εHf(t) values ranging from +0.2 to +2.8 and +6.6 to +15.3, respectively, similar to those of the mafic dikes. They were generated by partial melting of juvenile basaltic lower crust as a result of magma underplating. The Late Carboniferous mafic dike–granitoid association was not related to a post-collisional setting, but rather formed in an arc environment related to oceanic subduction. The most likely tectonic model accounting for the genesis of these rocks involves an extensional environment in the western Tianshan during the Late Carboniferous as a tectonic response to the roll-back of subducted Junggar oceanic lithosphere, which could also account for the “flare-up” of Late Carboniferous magmatism.
•A mafic dike–granitoid association, emplaced in the Late Carboniferous (317–310Ma)•The mafic dike originated from melting of a depleted metasomatized lithosphere•The granitoid was generated by melting of juvenile basaltic lower crust.•Tectonic response to the roll-back of subducted oceanic lithosphere
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•The Paleocene high-Mg diorite and gabbro-diorite plutons (HMDPs) identified in India–Asia collision zone.•The HMDPs belong to the metaluminous calc-alkaline series with a sanukitic ...affinity.•The HMDPs likely originated from subduction-modified lithospheric mantle and mixed with sediment melt.•The HMDPs were generated during the roll-back of the Neo‐Tethys slab.
Zircon U–Pb ages, geochemical data, and Sr–Nd–Hf isotopic data are reported for the Paleocene Qulong high-Mg dioritic plutons (HMDPs) in southern Tibet of the India–Asia collision zone to investigate their source, petrogenesis, and tectonic setting. The Qulong HMDPs consist of diorite with a 206Pb/238U age of 63.45 ± 0.87 Ma and gabbro-diorite with a 206Pb/238U age of 63.8 ± 0.57 Ma. They are metaluminous calc-alkaline rocks and exhibit sanukitic HMD affinity with moderate SiO2 and TiO2 and high MgO (5.24–8.64 wt%) and Mg# (55.78–65.89). They exhibit large ion lithophile element and light rare earth element enrichment and Nb–Ta depletion, relatively low (Th/Nb)N ratios, and high (La/Sm)N, La/Ta, and La/Nb ratios, which are similar to oceanic island basalts derived from the subduction-related mantle source. In addition, relatively low initial 87Sr/86Sr ratios (0.7056–0.7062), positive εNd(t) (+2.23 to + 3.34), and εHf(t) (+4.55 to + 12.35) indicate the origin of the Qulong HMDPs to be the subduction-related lithospheric mantle which had been modified by the asthenosphere, and mixed with the sediment melt. This study suggests that the Qulong HMDPs are a product of the upwelling of the asthenosphere triggered by the slab roll-back of the subducting NeoTethys slab of the India–Asia collision at ca. 64 Ma.
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•The Paleocene high-silica granites originated from the juvenile lower crust.•Fractional crystallization significantly affects the formation of high-silica granite.•The crust-mantle ...magma mixing plays an important role in the crustal reworking.
The history of crustal growth in the western Central Lhasa (CL) subterranean during India-Eurasian collision remains unclear. The crucial problem is lack of suitable and ideal magmatism recorded information of the early stages of continental collision. Here, we report the newly discovered high-silica granites (K-feldspar granite with aplite granite) hosting diorite enclaves from the Geji area in western CL subterrane. This suite of magmatic rocks is of great significance to reveal the crustal growth of CL during the India-Eurasia collision. Zircon U-Pb dating results show that the K-feldspar granite, aplite granite, and diorite enclaves were formed at 57.2–57.9 Ma, 58.7 Ma and 58.2 Ma, respectively. Petrology, mineralogy, and mineral chemistry of the plagioclase from the diorite enclave indicates that it was generated by magma mixing between mafic and felsic magmas. The mafic melt was sourced from partial melting of mantle wedge and metasomatized by subduction fluid and sediment melt. This has been supported by the positive zircon εHf(t) values (+0.6 to + 4.9), enrichment of large ion lithophile elements (Rb, K, Th, and U) and depletion of high field strength elements (Nb and Ta), and the high Th/Ce ratios of diorite enclaves. The K-feldspar granite also has positive zircon εHf(t) values (+1.3 to +6.2), arc magma geochemical features, and a lower crust Nb/Ta ratio (8.3). These indicate that the K-feldspar granite was originated from the juvenile lower crust. In conclusion, we think that the crust-mantle magma mixing has a substantial impact on the crustal growth of the western CL subterrane during the subduction slab roll-back of the Paleocene Neo-Tethys Ocean when paired with other research and the context of regional tectonic development.
New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure ...rocks derived from oceanic crust protoliths. The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions. Eclogite facies conditions, better recorded in mafic protoliths, are revealed by relic lawsonite and phengite, bleb- to worm-like diopside-albite symplectites, as well as garnet core composition. Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole, augite, and oligoclase recrystallization, as well as the record of partial melting leucosomes.
Phase equilibria and pressure-temperature (P-T) path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2–24.5 kbar and 465–580 °C, followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5–14.2 kbar and 580–720 °C. Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286–400 °C at pressures below 6.5–11 kbar. The obtained clockwise P-T path, the garnet zonation pattern revealing a decrease in Xgrs/Xprp related to Mg# increment from core to rim, the presence of partial melting veins, as well as regional constraints, document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous. Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered. This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period. The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms, representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.
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•New field, petrographic and mineral chemistry data on Pijao Metamorphic Complex.•Phase equilibria and P-T path modeling suggest thermally overprinted high-pressure rocks.•Subduction thermal structure modification during ongoing metamorphism due to hot mantle inflow.•First report of slab roll-back related metamorphism in the Northern Andes and Caribbean region.
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•Zircon U–Pb dating indicates the emplacement of Ruocuo adakitic porphyries during the Early–Middle Jurassic (182–170 Ma).•Ruocuo adakitic porphyries were generated by the partial ...melting of the Neo-Tethys oceanic slab.•Formation of Early Mesozoic magmatic rocks in the southern Lhasa subterrane in a magmatic arc setting.•The northern Xigaze forearc basin has a potential for subduction-related porphyry Cu–Au mineralization.
Jurassic adakitic rocks in the southern Lhasa subterrane are relatively rare, poorly documented, and their petrogenesis and geodynamic setting are unclear. To explore these issues, we present zircon U–Pb ages along with Hf isotopic, whole-rock geochemical, and Sr–Nd–Pb isotopic data for hornblende quartz diorite porphyry (HQDP) and quartz diorite porphyry (QDP) in the Ruocuo area, southern margin of the Lhasa terrane, Tibet. Zircon U–Pb dating for these rocks indicates that they were emplaced in the Early–Middle Jurassic (182–170 Ma). Geochemically, HQDP and QDP both show characteristics of adakites, with intermediate SiO2 (60.62–65.63 wt% and 57.46–60.04 wt%, respectively), relatively high Al2O3 (14.23–18.23 wt% and 18.32–20.14 wt%, respectively) and Sr (344–571 ppm and 514–614 ppm, respectively), and low Y (9.12–13.0 ppm and 11.0–14.9 ppm, respectively) and Yb (0.93–1.31 ppm and 1.08–1.52 ppm, respectively). In addition, they show relatively low ratios of (87Sr/86Sr)i (0.7038–0.7045), (206Pb/204Pb)i (18.31–18.42), (207Pb/204Pb)i (15.56–15.66), and (208Pb/204Pb)i (38.36–38.77) and relatively high values of εNd(t) (+5.54 to +6.33) and εHf(t) (+11.8 to +17.3). The Sr–Nd–Pb–Hf isotopes of HQDP and QDP are similar to those of the Yarlung Tsangpo ophiolites, indicating that they were predominantly generated by the partial melting of the subducted Neo-Tethys oceanic slab. The subducted Neo-Tethys oceanic slab-derived adakitic rocks that are present in the southern Lhasa subterrane provide strong evidence for proving the presence of oceanic subduction during the Early–Middle Jurassic. By combining our results with the previously reported results, we suggest that the Ruocuo adakitic rocks and Early Mesozoic (Late Triassic to Middle Jurassic) magmatic rocks in the southern Lhasa subterrane formed in a magmatic arc setting related to the northward subduction of the Neo-Tethys oceanic slab, that the initial subduction of the Neo-Tethys oceanic slab occurred prior to the Early Jurassic, and that the upwelling asthenosphere, triggered by the roll-back of the subducted Neo-Tethys oceanic slab, provided the heat that was required for slab melting. However, contrasting these results with the geochemical characteristics of the No. 2 deposit in the Xiongcun Cu–Au district leads us to consider that the Early–Middle Jurassic subducted oceanic slab-derived adakitic porphyries contain a greater potential for porphyry Cu–Au mineralization than the Early Mesozoic normal magmatic rocks in the southern Lhasa subterrane. Furthermore, mineralized Jurassic porphyry has been reported in the Ruocuo area, adjacent to the northern Xigaze forearc basin, indicating that the northern Xigaze forearc basin also has the potential for subduction-related porphyry Cu–Au mineralization.
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