Geochemistry and geochronology of Carboniferous volcanic rocks in the eastern Junggar terrane,NW China: Implication for a tectonic transition

The Carboniferous tectonic setting of the Junggar terrane, northern Xinjiang, NW China, has long been a matter of debate. Voluminous Carboniferous volcanic rocks are widely distributed in the Karamaili area, the southern part of the eastern Junggar terrane. Early Carboniferous rocks comprise basalts and basaltic andesites, with enrichment of LREE and LILE and depletion of HFSE, and uniformly high ε_Nd(t) (+ 3.7 to + 4.0). Late Carboniferous rocks consist of basalts, basaltic andesites, rhyolites and minor dacites, and can be subdivided into basic and felsic groups. The basic rocks are depleted in HFSE, and show variable high ε_Nd(t) (+ 4.8 to + 6.9). They have higher Cr and Ni and lower Na₂O, U and Th contents than early Carboniferous basic rocks. The felsic rocks show A-type affinity, with typical enrichment of alkalis, LREE and HFSE and strong depletion in Ba, Sr, Eu and Ti. They have high values of ε_Nd(t) and zircon ε_Hf(t) (+ 11.6 to + 17.9). New LA-ICPMS zircon U–Pb analyses constrain their emplacement to late Carboniferous time (306.5–314.3 Ma).The Carboniferous basic rocks show negative Zr-Hf anomalies and low Th/Ce (< 0.07) and Th/La (0.06–0.16), excluding significant crustal contamination during magma evolution. They have low La/Ba (0.03–0.12), Ce/Y (< 3) and (Tb/Yb)_N (< 2) and variable Ba/Th (28–318) and Ba/La (3.1–34), suggesting that they were derived from a main spinel with minor garnet lherzolite mantle source metasomatized by slab-derived fluids. The late Carboniferous felsic rocks were produced when upwelling asthenosphere triggered partial melting of juvenile lower crust. The early Carboniferous volcanism occurred in an island-arc setting related to the southward subduction of the Paleo-Junggar Ocean plate, whereas the late Carboniferous rocks erupted in a post-collisional extensional setting. Thus, a rapid tectonic transition from arc to post-collisional extension may have occurred between early and late Carboniferous, and probably resulted from slab break-off or lithospheric delamination.     

Late Carboniferous seismic and volcanic record in the northwestern margin of the Junggar Basin: Implication for the tectonic setting of the West Junggar

The northwestern margin of the Junggar Basin is a transition zone between the Zaire-Hala'alate mountains and the Junggar Basin in West Junggar, which developed a large amount of volcanic rocks in the late Carboniferous. An investigation of the tectonic evolution of this area will be helpful for understanding the accretionary process of West Junggar. Here, we selected well-preserved drill core samples of andesite and andesitic tuff for detailed petrogenesis and geochemical studies, while high-quality seismic and resistivity prospecting cross-sections were also used to reveal the tectonic setting. Zircon U-Pb dating results of three andesite and andesitic tuff samples show that these rocks were erupted at ca. 312 Ma. The results of the geochemical and isotopic analyses of seven samples are characterized by relatively high MgO (1.84–5.52 wt%), Cr (26.19–246.61 ppm), Ni (16.53–82.85 ppm) contents, Ba/La (14.19–218.48) ratios, and high positive ɛHf_(t) (+4.8 to +14.2) values, but low TiO₂ (0.68–1.25 wt%) contents, FeO^T/MgO (1.18–2.81), Sr/Y (5.63–27.40), and Th/Yb (0.19–2.18) ratios, which are similar to the Bieluagaxi sanukitoids of West Junggar. LREEs are enriched in all samples, while Nb and Ta are significantly depleted. All the evidence suggests the volcanics were most likely derived from partial melting of oceanic slab that was contaminated by the overlying mantle wedge during the magma ascent. The seismic and resistivity cross-sections show apparent southeastward-vergent imbricate thrust fault systems, implying northwestward subduction in the late Carboniferous. Together with previous studies and our geochemical and geophysical data, we suggest that the formation of these late Carboniferous volcanic rocks is probably related to a northwestward ridge subduction process.     

Carboniferous porphyry Cu (-Au) mineralization of the West Junggar region,NW China: the Shiwu example

ABSTRACT

The West Junggar region, located in the Central Asian Orogenic Belt (CAOB), is characterized by extensive Carboniferous magmatism and porphyry Cu (-Au) deposits. The Shiwu porphyry Cu-Au deposit, located in the east of the Barluk Mountains, the West Junggar region, is not only a newly discovered deposit but also a representative porphyry Cu-Au deposit in this area. The volcanic rocks (including andesite and tuff) and intrusive rocks (including diorite, quartz diorite, quartz diorite porphyry, and tonalite porphyry) occurred in the Shiwu area and the mineralization associated with the quartz diorite porphyry. The secondary ion mass spectrometry (SIMS) zircon U–Pb ages of quartz diorite porphyry and tonalite porphyry are 310.4 ± 2.3 Ma and 310.1 ± 2.4 Ma, respectively, indicating that the Shiwu deposit is related to the Late Carboniferous magmatism. Intrusive rocks, which were characterized by the enrichment of large ion lithophile elements (LILEs) and pronounced negative high field strength elements (HFSEs), belong to the calc-alkaline or tholeiitic series. Their (⁸⁷Sr/⁸⁶Sr)_i, (¹⁴³Nd/¹⁴⁴Nd)_I, and εNd(t) values range from 0.703569 to 0.704311, 0.512488 to 0.512512, and 4.9 to 5.3, respectively. Volcanic rocks, which belong to the calc-alkaline series, have similar geochemical features as the intrusive rocks, and their (⁸⁷Sr/⁸⁶Sr)_i, (¹⁴³Nd/¹⁴⁴Nd)_i, and εNd(t) values, respectively, are 0.703704–0.704071, 0.512520–0.512542, and 5.49–5.92. These characters indicate that the igneous rocks in the Shiwu area derived dominantly from the mantle and formed in an island arc setting. These characters also further confirmed that the Barluk Mountains is still in an island arc setting in the Late Carboniferous and the accretionary orogenesis can exist until 310 Ma at least.     

Origin of two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton: implications for early Mesozoic lithosphere thinning

New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian–Early Triassic at ca. 255–245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO₂ and K₂O, moderate initial ⁸⁷Sr/⁸⁶Sr, low negative ε_Nd(t) and ε_Hf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust–mantle boundary in syncollisional to post-collisional/post-orogenic tectonic settings. The late rock suite, erupted during the Middle–Late Triassic at ca. 238–228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high Al₂O₃, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial ⁸⁷Sr/⁸⁶Sr; high negative ε_Nd(t) and ε_Hf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle–Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.     

Early Cretaceous adakitic rocks in the Anqing region,southeastern China: constraints on petrogenesis and metallogenic significance

ABSTRACT

The Anqing region in Lower Yangtze River metallogenic belt is one of the important Cu polymetal producers in China. The origin of Cu polymetallic deposits in the region is closely related to Early Cretaceous adakitic intrusions. To constrain the petrogenetic and metallogenic significance of the adakitic rocks, a detailed geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic study was performed. The Anqing adakitic rocks (SiO₂ = 57.4–64.2 wt.%) consist mainly of quartz monzodiorite, formed at 138.2 ± 1.7 Ma (Mean Standard Weighted Deviation (MSWD) = 0.61). They have high MgO, Al₂O₃, Sr, and low Rb, Y, Yb contents, together with high Sr/Y (50.5–222) and La/Yb (31.9–46.9) ratios. They also show negative whole-rock ε_Nd(t) (?9.8 to ?8.5) and zircon ε_Hf(t) (?10.0 to ?5.4), and high oxygen fugacity (mainly ?17.0 to ?8.01) values and radiogenic Pb isotopic compositions with (²⁰⁶Pb/²⁰⁴Pb)_i = 17.692–17.884, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.413–15.511, and (²⁰⁸Pb/²⁰⁴Pb)_i = 37.611–37.943. Coupled with negative Nb–Ta anomalies, low K₂O/Na₂O ratios (0.39–0.62), and high Mg# values (0.44–0.71), these data suggest the adakitic rocks and associated large-scale Cu–Au mineralization of the Anqing region resulted from partial melting of the high oxidized subducted oceanic crust. Addition of mantle-derived magmas and assimilation of crustal materials during emplacement are also possible.     

Petrogenesis and tectonic implications of early Carboniferous alkaline volcanic rocks in Karamay region of West Junggar,Northwest China

ABSTRACT

Zircon U–Pb geochronological and geochemical analyses are reported for a suite of the early Carboniferous volcanic rocks from West Junggar (Northwest China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded concordant weighted mean ²⁰⁶Pb/²³⁸U age of 345 ± 3 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The early Carboniferous volcanic rocks consist of basalt, basaltic andesite, and andesite. Geochemically, all the samples bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with minor variations in SiO₂ compositions (45.13–53.05 wt.%), high concentrations of Na₂O + K₂O (5.08–8.89 wt.%) and TiO₂ (1.71–3.35 wt.%), and LREE enrichment and HREE depletion ((La/Yb)_N = 7.1–12.4), with weak Eu anomalies (Eu/Eu* = 0.9–1.1) and no obvious Nb, Ta, and Ti negative anomalies. These features suggest that the early Carboniferous volcanic rocks were derived from an OIB-related source that consists of oceanic lithosphere with ~1–3% degree partial melting of garnet lherzolite. From these observations, in combination with previous work, we conclude that the early Carboniferous alkaline volcanic rocks in Karamay region formed by upwelling of asthenospheric mantle through a slab window in a forearc setting during consumption of the West Junggar Ocean. Meanwhile, seamounts, which formed in the Late Devonian and were accreted and subducted in Karamay arc, also brought geological effects in the subduction zone.     

Elemental and Sr–Nd–Pb isotopic geochemistry of Late Paleozoic volcanic rocks beneath the Junggar basin, NW China: Implications for the formation and evolution of the basin basement

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (<1.4), typical for oceanic crust derived from asthenospheric melts. Zircon Pb–Pb ages indicate that the Kexia andesite, with a volcanic arc affinity, formed in the early Carboniferous (345 Ma), whereas the Luliang rhyolite and the Wucaiwan dacite, with syn-collisional to within-plate affinities, formed in the early Devonian (395 and 405 Ma, respectively). Positive ε_Nd(t) values (up to +7.4) and low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of the intermediate-silicic rocks suggest that the entire Junggar terrain may be underlain by oceanic crust, an interpretation consistent with the juvenile isotopic signatures of many granitoid plutons in other parts of the Central Asia Orogenic Belt. Variation in zircon ages for the silicic rocks, different Ba, P, Ti, Nb or Th anomalies in the mafic rocks, and variable Nb/Y and La/Nb ratios across the basin, suggest that the basement is compositionally heterogeneous. The heterogeneity is believed to reflect amalgamation of different oceanic blocks representing either different evolution stages within a single terrane or possibly derivation from different terranes.     

Petrogenesis of Late Carboniferous granitoids in the Chihu area of Eastern Tianshan,Northwest China,and tectonic implications: geochronological,geochemical, and zircon Hf–O isotopic constraints

ABSTRACT

This contribution presents new SIMS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf–O isotope systematic on an example of Late Carboniferous granodiorite and porphyritic granodiorite intrusions from the Chihu area of Eastern Tianshan, Xinjiang. SIMS zircon U–Pb dating indicates that the Chihu granodiorite and porphyritic granodiorite formed at 320.2 ± 2.4 Ma and 314.5 ± 2.5 Ma, respectively. These rocks are metaluminous to weakly peraluminous with an A/CNK value of 0.92–1.58, as well as low 10000 Ga/Al, Zr + Nb + Y + Ce, and Fe₂O₃^T/MgO values, which suggest an I-type normal island arc magmatic suite. The porphyritic granodiorite has a slightly higher Sr/Y ratio (28–37) and lower Y (6.9–11.7 ppm) and Yb (0.98–1.49 ppm) contents, suggesting mild adakite affinities. In situ Hf–O isotopic analyses using LA-ICP-MS-MC and SIMS indicate that the ε_Hf(t) and δ¹⁸O values of granodiorite zircons vary from +11.5 to +14.9 and 4.80 to 5.85 ‰, respectively, similar to values for porphyritic granodiorite zircons, which vary from +11.9 to +17.2 and 3.78 to 4.71 ‰, respectively. The geochemical and isotopic data imply that the Chihu granodiorite and porphyritic granodiorite share a common origin, most likely derived from partial melts of the subduction-modified mantle. Based on the regional geological history, geochemistry of the Chihu intrusions, and new isotopic studies, we suggest that the Late Carboniferous magma was generated during the period of the northward subduction of the Palaeo-Tianshan ocean plate beneath the Dananhu–Tousuquan island arc.     

U–Pb (zircon) and geochemical constraints on the age,origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu–Au district,southern Mongolia

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (ε_Nd(t) range from + 3.1 to + 7.5 and ²⁰⁶Pb/²⁰⁴Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO₂) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.     

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

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

Palaeoproterozoic A-type magmatism in northern Wuyishan terrane,Southeast China: petrogenesis and tectonic implications

ABSTRACT

In this article we present zircon U–Pb ages, Hf isotopes, and whole-rock geochemistry of the Longzhu rhyolite porphyry from the Cathaysia Block, Southeast China to constrain its petrogenesis and provide insights into the early Precambrian tectonic evolution of the Cathaysia Block. LA-ICP-MS zircon U–Pb dating of a representative sample yields a weighted mean ²⁰⁶Pb/²⁰⁷Pb age of 1819 ± 16 Ma, interpreted as the crystallization age of the Longzhu rhyolite porphyry. Zircons from this sample have ε_Hf(t) values ranging from – 8.4 to – 2.2 and T^Hf_DM2 model ages from 2.76 to 2.46 Ga. The whole-rock Nd isotopic data from the Longzhu rhyolite porphyries yield ε_Nd(t) values spanning – 6.3 to – 4.7 and T^Nd_DM2 model ages from 2.81 to 2.69 Ga. The rhyolite porphyries have geochemical features similar to those of the typical A-type granites (rhyolites), with high SiO₂, total alkali contents and FeO^t/MgO ratios, and low CaO and MgO contents. Additionally, the rhyolite porphyries have high total rare earth element concentrations (627 ~ 760 ppm), high (La/Yb)_N values (14.5 ~ 26.9), strongly negative Eu anomalies (δEu = 0.28 ~ 0.41), and display enrichments of Rb, Ga, Th, and U and depletions of Sr, Nb, Ta, Eu, and Ti. The geochemical and Nd-Hf isotopic features suggest that the Palaeoproterozoic Longzhu rhyolite porphyries were generated by partial melting of source rocks similar to those of the Badu Complex in an intra-plate extensional setting. The results from this study, when combined with existing geochronological data, further demonstrate that the Palaeoproterozoic rocks of Wuyishan terrane probably represent a remnant of the Columbia supercontinent.     

Zircon U–Pb geochronology and geochemistry of the post-collisional volcanic rocks in eastern Xinjiang Province,NW China: implications for the tectonic evolution of the Junggar terrane

We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial ⁸⁷Sr/⁸⁶Sr values (0.7034–0.7042) and positive ε_Nd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive ε_Nd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).     

Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of Early Triassic granites in the Wulonggou area of the Eastern Kunlun Orogen,Northwest China

Widespread granitic intrusions in the northeast part of the Wulonggou area were previously thought to be emplaced into the Palaeoproterozoic Jinshuikou Group during the Neoproterozoic. This contribution presents detailed LA-ICP-MS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf isotope systematic on the Wulonggou Granodiorite and Xiaoyakou Granite from the Wulonggou area. Three granodiorite samples yielded U–Pb zircon ages of 247 ± 2, 248 ± 1, and 249 ± 1 Ma, and one granite sample yielded U–Pb zircon age of 246 ± 3 Ma. The granodiorite samples are metaluminous with an alumina saturation index of 0.90–0.96, as well as intermediate- to high-alkali contents of 5.49–6.14 wt.%, and low Zr+Nb+Ce+Y contents, and low Fe₂O_3T/MgO ratios, which suggest an I-type classical island arc magmatic source. The granite samples are peraluminous with an alumina saturation index of 1.02–1.03, Sr content of 305.00–374.00 ppm, Sr/Y ratios of between 17.68 and 28.77, (La/Yb)_N values of 16.98–25.07, low HREEs (Yb = 1.10–2.00 ppm), and low Y (13.00–21.10 ppm), which suggest adakite-like rocks. All granodiorite samples have zircons ε_Hf(t) values ranging from ?2.9 to +3.9, and granite samples have zircon ε_Hf(t) values ranging from ?7.8 to +3.2. These Hf isotopic data suggest that the Early Triassic granites were derived from the partial melting of a mafic Mesoproterozoic lower crust, although the degree of ancient crustal assimilation may be higher for the Xiaoyakou Granite. It is suggested here that the ca. 246–248 Ma magma was generated during the northward subduction of the Palaeo-Tethys oceanic plate.     

Carboniferous ridge subduction in the Xingmeng Orogenic Belt: Constraints from geochronological,geochemical, and Sr-Nd-Hf isotopic analysis of strongly peraluminous granites and gabbro-diorites in the Xilinhot micro-continent

The Xingmeng Orogenic Belt evolved through a long-lived orogeny involving multiple episodes of subduction and accretion. However, there is a debate on its tectonic evolution during the Late Paleozoic. Here, we report geochemical, geochronological, and isotopic data from strongly peraluminous granites and gabbro-diorites from the Sunidzuoqi–Xilinhot region. Zircon U–Pb ages suggest that the intrusive rocks were emplaced during the Early Carboniferous (333–322 Ma). The granites exhibit geochemical characteristics similar to S-type granites, with high SiO₂ (72.34–76.53 wt.%), Al₂O₃ (12.45–14.65 wt.%), and A/CNK (1.07–1.16), but depleted Sr, Nb, and Ta contents. They exhibit positive ε_Nd(t) and ε_Hf(t) values (?0.3 to 2.8 and 2.7–5.7, respectively) and young Nd and Hf model ages (T_DM2(Nd)=853–1110 Ma and T_DM2(Hf)=975–1184 Ma), suggesting that they may be the partial melting products of heterogeneous sources with variable proportions of pelite, psammite, and metabasaltic rocks. The meta-gabbro-diorites from the Maihantaolegai pluton have low SiO₂ (47.06–53.49 wt.%) and K₂O (0.04–0.99 wt.%) contents, and demonstrate slight light rare earth element (REE) depletion in the chondrite-normalized REE diagrams. They have high zircon ε_Hf(t) values (14.41–17.34) and young Hf model ages (T_DM2(Hf)= 230–418 Ma), indicating a more depleted mantle source. The variations of the Sm/Yb and La/Sm ratios can thus be used to assess the melting degree of the mantle source from 5% to 20%, suggesting a quite shallow mantle melting zone. We propose that the petrogenesis and distribution of the strongly peraluminous granites and gabbro-diorites, as well as the tectonic architecture of the region, can be explained by a ridge subduction model. Based on these results, and previous studies, we suggest a southward ridge subduction model for the Sunidzuoqi–Xilinhot region.     

Early Carboniferous intra-oceanic arc and back-arc basin system in the West Junggar,NW China

A series of Lower Carboniferous volcanic rocks occur in the Hatu, Darbut, and Baogutu areas of Xinjiang Province. Secondary ion mass spectrometry (SIMS) zircon U–Pb isotopic data indicate that two samples of these rocks coevally erupted at 324.0 ± 2.8 Ma and 324.9 ± 3.4 Ma. Three detailed profile measurements show that the volcanics include the Hatu basalt, the Baogutu andesite and dacite, and the Darbut andesite. Whole-rock compositions suggest that the Hatu volcanics are tholeiites and have a mid-ocean ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Their isotopic compositions (?Nd(t) = +2.2 to +4.0, (⁸⁷Sr/⁸⁶Sr)i = 0.70414 to 0.70517) suggest a mixing origin from depleted to enriched mantle sources. In contrast, the Baogutu and Darbut rocks are andesite and dacite possessing a transitional tholeiite to calc–alkaline character and have E-MORB-like and OIB signatures, with a marked negative Nb anomaly and Th/Yb-enrichment, indicating that they were generated in a subduction zone setting. Isotopically, they display consistently depleted Sr–Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)i = 0.70377–0.70469, ?Nd(t) = 1.0–5.2], suggesting that they were derived from a depleted mantle, and that fluid and sediments were involved in their petrogenesis. These features suggest that an early Carboniferous intra-oceanic arc and back-arc basin system generated the studied volcanic units in the West Junggar.     

Sm–Nd geochronology and geochemistry of a Neoproterozoic gabbro in the Kuluketage block,north-western China

In this article, we report whole-rock and mineral Sm–Nd isotopic and whole-rock elemental and Sr–Nd isotopic data of Xingdi No. 1 mafic–ultramafic intrusion in the western Kuluketage block, north-eastern Tarim. Xingdi No. 1 mafic–ultramafic intrusion is the largest in the Xingdi mafic–ultramafic belt, with an exposed area of ca. 20 km². It intruded into the Palaeoproterozoic basement. Gabbro is the major rock type and there is minor olivine pyroxenite. Sm–Nd geochronometry of the gabbro gives an isochron age of 761.2 ± 31.2 million years, identical to the intrusive age of Xingdi No. 2 pluton (760 ± 6 million years). The gabbro is systematically enriched in large ion lithosphile elements and light rare earth elements and depleted in high field strength elements and heavy rare earth elements. The studied rocks are characterized by low whole-rock and mineral ?Nd_(t) values (?7.8 to??7.1) and elevated (⁸⁷Sr/⁸⁶Sr) _i values (0.7066–0.7073). These geochemical characteristics, together with the presence of abundant hornblende, biotite, bladed biotite enclosed in amphibole, and crescent-shaped Palaeoproterozoic wall-rock xenoliths in the intrusion, are key features of magma mixing in the source or assimilation during its emplacement. The rocks have a Zr/Y ratio of 3.81–13, which falls in the within-plate basalt area. As Xingdi No. 1 and No. 2 plutons formed at the same period and display similar geochemical characteristics, we propose that they formed within the same tectonic setting and were derived from the same source, but No. 1 pluton experienced a higher extent of evolution and contamination. Previous studies have shown that the Neoproterozoic tectonic and magmatic events in Kuluketage comprise syn-collisional granite around TC (ca. 1.0–0.9 Ga), post-collisional K-rich granite and alkaline mafic–ultramafic intrusions (ca. 830–800 Ma), and rifting-related mafic–ultramafic plutons, dikes, and bimodal volcanic rocks (ca. 774–744 Ma).     

A geotraverse across two paleo-subduction zones in Tien Shan,Tajikistan

We present first LA-ICP-MS U–Pb zircon ages as well as geochemical and Sr–Nd–Pb isotope data for 14 magmatic rocks collected along ca. 400 km profile across the Chatkal-Kurama terrane in the Mogol-Tau and Kurama ranges and the Gissar Segment of the Tien Shan orogen in Tajikistan. These new data from supra-subduction and post-collisional magmatic rocks of two Late Paleozoic active margins constrain a tectonic model for terrane motions across two paleo-subduction zones: (1) The 425 Ma old Muzbulak granite of the Mogol-Tau range formed in a supra-subduction setting at the northern margin of the Turkestan Ocean. The north-dipping plate was subducted from the Early Silurian to the earliest Middle Devonian. Thereafter the northern side of the Turkestan Ocean remained a passive margin until the Early Carboniferous. (2) In the Early Carboniferous, subduction under the northern margin of the Turkestan Ocean resumed and the 315 to 305 Ma old Kara-Kiya, Muzbek, and Karamazar intrusions formed in a supra-subduction setting in the Mogol-Tau and Kurama ranges. (3) At the same time, in the Early Carboniferous, rifting of the southern passive margin of the Turkestan Ocean formed the short-lived Gissar Basin, separated from the Turkestan Ocean by the Gissar micro-continent. North-dipping subduction in the Gissar Basin is documented by the 315 Ma Kharangon plagiogranite and the voluminous ca. 321–312 Ma Andean-type supra-subduction Gissar batholith. The Kharangon and Khanaka gabbro-plagiogranite intrusions of the southern Gissar range have geochemical and Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) 0.7047–0.7056, εNd of + 1.5 to + 2.3) compatible with mantle-derived origin typical for plagiogranites associated with ophiolites. The supra-subduction rocks from the Gissar batholith and from the Mogol-Tau Kurama ranges have variably mixed Sr–Nd–Pb isotopic signatures (⁸⁷Sr/⁸⁶Sr(t) 0.7057–0.7064, εNd of − 2.1 to − 5.0) typical for continental arcs where mantle-derived magmas interact with continental crust. (4) In the latest Carboniferous, the Turkestan Ocean and the Gissar Basin were closed. The Early Permian Chinorsay (288 Ma) and Dara-i-pioz (267 Ma) post-collisional intrusions, emplaced in the northern part of the Gissar micro-continent after a long period of amagmatic evolution, have intraplate geochemical affinities and isotopic Sr–Nd–Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr(t) 0.7074–0.7086, εNd of − 5.5 to − 7.4) indicating derivation from Precambrian continental crust which is supported by old Nd model ages (1.5 and 1.7 Ga), and by the presence of inherited zircon grains with ages 850–500 Ma in the Chinorsay granodiorite. The post-collisional intrusions in the southern Gissar and in the Mogol-Tau and Kurama ranges (297–286 Ma), emplaced directly after supra-subduction magmatic series, have geochemical and isotopic signatures of arc-related magmas. The distinct shoshonitic affinities of post-collisional intrusions in the Mogol-Tau and Kurama ranges are explained by the interaction of hot asthenospheric material with subduction-enriched wedge of lithospheric mantle due to slab break-off at post-collisional stage. Despite origination from different tectonic environments, all magmatic rocks have relatively old Nd model ages (1.7–1.0 Ga) indicating a significant proportion of Paleoproterozoic or older crustal material in their sources and their model ages are similar to those of post-collisional intrusions from the Alai and Kokshaal Segments of the South Tien Shan.     

Early Cretaceous Adakitic Rocks in the Northern Great Xing’an Range, NE China: Implications for the Final Closure of Mongol-Okhotsk Ocean and Regional Extensional Setting

A large amount of igneous rocks in NE China formed in an extensional setting during Late Mesozoic. However, there is still controversy about how the Mongol-Okhotsk Ocean and the Paleo-Pacific Ocean effected the lithosphere in NE China. In this paper, we carried out a comprehensive study for andesites from the Keyihe area using LA-ICP-MS zircon UPb dating and geochemical and Hf isotopic analysis to investigate the petrogenesis and tectonic setting of these andesites. The U-Pb dating yields an Early Cretaceous crystallization age of 128.3±0.4 Ma. Geochemically, the andesites contain high Sr(686–930 ppm) and HREE contents, low Y(11.9–19.8 ppm) and Yb(1.08–1.52 ppm) contents, and they therefore have high Sr/Y(42–63) and La/Yb(24–36) ratios, showing the characteristics of adakitic rocks. Moreover, they exhibit high K_2O/Na_2O ratios(0.57–0.81), low Mg O contents(0.77–3.06 wt%), low Mg# value(17–49) and negative εHf(t) values(-1.7 to-8.5) with no negative Eu anomalies, indicating that they are not related to the oceanic plate subduction. Based on the geochemical and isotopic data provided in this paper and regional geological data, it can be concluded that the Keyihe adakitic rocks were affected by the Mongol-Okhotsk tectonic regime, forming in a transition setting from crustal thickening to regional extension thinning. They were derived from the partial melting of the thickened lower crust. The closure of the Mongol-Okhotsk Ocean may finish in early Early Cretaceous, followed by the collisional orogenic process. The southern part region of its suture belt was in a post-orogenic extensional setting in the late Early Cretaceous.     

Geochronology and geochemistry of the Nantianwan mafic–ultramafic complex,Emeishan large igneous province: metallogenesis of magmatic Ni–Cu sulphide deposits and geodynamic setting

The Nantianwan mafic–ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA–ICP–MS U–Pb zircon dating of the gabbronorites yields an age of 259.7 ± 0.6 million years, consistent with the ages of other mafic–ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu–Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO₂ contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22–30.16 ppm for lherzolites and 25.21–61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (⁸⁷Sr/⁸⁶Sr) _t (t?=?260 Ma) ratios and ?_Nd(t) values of the mafic–ultramafic suite vary from 0.70542 to 0.70763, and??0.4 to 1.7, respectively. Compared to the Cu–Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (⁸⁷Sr/⁸⁶Sr) _t . Their ?_Nd(t) versus (Th/Nb)_PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)_PM versus (Yb/Sm)_PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04–102.67) and gabbronorites (0.88–5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.     

Geochemistry and Sr–Nd isotope composition of Carboniferous volcanic rocks of the Jueluotage Orogenic Belt: implications for the tectonic evolution of Eastern Tianshan,China

The major and trace element, and Sr–Nd isotopic compositions of the Carboniferous Qi’eshan, Wutongwozi, and Yamansu volcanic rocks from the northern and southern parts of the Jueluotage Orogenic Belt in East Tianshan, China, were analysed to understand their genesis and geodynamic implications. The early Carboniferous Qi’eshan basalts are characterized by high Al₂O₃, with La/Sm (1.38–1.79) and Ba/La (27.06–58.76) values higher than those of typical normal mid-ocean ridge basalt. They are relatively enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements. Overall, their initial Nd–Sr isotopic compositions are ε_Nd(t) = (5.6–7.0) and Isr = 0.70397–0.70429, implying the magma originated from a mantle wedge source that was metasomatized by subduction-related fluids. In contrast, the late Carboniferous Wutongwozi basalts have lower Ba/La (4.86–12.82), La/Nb (0.87–2.45), and LILE concentrations. They have the isotopic characteristics of depleted asthenosphere, relatively high and heterogeneous ε_Nd(t) (9.3–9.4), and high Isr (0.70471–0.70533). Thus, the late Carboniferous Wutongwozi basalts may have been derived from the partial melting of mantle sources during asthenospheric upwelling. The early Carboniferous Yamansu acid volcanic rocks are characterized by high Mg# (46–48) and Lu/Y (~0.15), and low K₂O/Na₂O (0.01–0.20), similar to M-type granites. However, their ε_Nd(t) (5.0–5.5) and Isr (0.70642–0.70768) values are lower than those of depleted mantle, indicating they were contaminated by lower crustal material. The magma source originated from a mantle-derived magma that was contaminated by middle Tianshan massif in a continental margin arc setting. Based on the results and previous field-based studies, we conclude that the Carboniferous volcanics in the Jueluotage Orogenic Belt formed in a complex trench–arc–basin setting in the Kuguertage–Aqikuduke Suture Zone.