Field and geochemical characteristics of Mesoarchean to Neoarchean volcanic rocks in the Storø greenstone belt,SW Greenland: Evidence for accretion of intra-oceanic volcanic arcs

The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Sm_cn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Yb_cn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Sm_cn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.     

Metallogeny of South Greenland: A review of geological evolution,mineral occurrences and geochemical exploration data

South Greenland has been the site of historic mining of cryolite, copper, graphite and gold, hosts mineral deposits with gold, uranium, zinc, niobium, tantalum, zirconium, hafnium, REE, iron, titanium, vanadium, fluorite and graphite, and has additional potential for lithium, beryllium, phosphorus, gallium and thorium. Data from stream sediment geochemical surveys document that South Greenland is enriched in a range of these elements relative to the rest of Greenland and to estimates of the upper crust composition. Distribution patterns for individual elements within south Greenland exhibit enriched regions that are spatially related to lithological units, crustal structure and known mineralisation.The Northern Domain of South Greenland includes the southernmost part of the orthogneiss-dominated North Atlantic craton. Orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the Mesoarchaean Tartoq Group of mafic metavolcanic rocks. Geochemical exploration indicates that additional potential for gold mineralisation exists within Palaeoproterozoic supracrustal rocks overlying the Archaean basement.Rocks formed during the Palaeoproterozoic Ketilidian orogeny occupy a major part of South Greenland and has been divided into two domains. The Central Domain is underlain by the Julianehåb igneous complex forming a 100 km wide ENE–WSW zone centrally across South Greenland. Intrusive and extrusive, mostly felsic magmatic rocks were emplaced in two main stages (1850–1830 and 1800–1780 Ma) in a continental arc setting. Positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex. Intra-arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone-hosted copper–silver–gold mineralisation at Kobberminebugt and Kangerluluk, whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation. Orthomagmatic iron–titanium–vanadium mineralisation is hosted by a gabbro. A potential for porphyry-type mineralisation related to the late intrusive stages of the Julianehåb igneous complex is suggested by showings with copper, molybdenum and gold together with stream sediment anomalies for these elements. Vein-type uranium mineralisation occurs in fault zones in the Julianehåb igneous complex related to Mesoproterozoic rifting.The Southern Domain contains an assemblage of Palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation, peak HT–LP metamorphism and partial melting with subsequent retrograde exhumation at 1790–1765 Ma. The supracrustal rocks contain syngenetic Au, As, Sb, U, and Zn mineralisation in volcanic or graphite- and sulphide-rich sedimentary environments; graphite was mined historically at two sites. Many stream sediment gold anomalies are located in a NE-trending belt along the boundary between the early Julianehåb complex and the supracrustal rocks to the south. They reflect a number of auriferous quartz vein occurrences, including the Nalunaq gold deposit, hosted in a system of shear zones and probably generated as orogenic gold during Ketilidian accretion. The 1755–1730 Ma, A-type Ilua plutonic suite is the latest magmatic event in the Ketilidian orogen.The 1300–1140 Ma Gardar period involved continental rifting, sedimentation and alkaline magmatism. Numerous dykes and 10 ring-shaped intrusion complexes were formed across South Greenland. An orthomagmatic iron–titanium–vanadium deposit is hosted by troctolitic gabbro. Residual magmas and fluids resulting from extreme magmatic differentiation, possibly combined with assimilation of older crust, created mineral deposits including cryolite that was mined at Ivigtut, large low-grade deposits of uranium–rare earth elements–zinc at Kvanefjeld and tantalum–niobium–rare earth element–zirconium at Kringlerne, in the Ilímaussaq complex, as well as tantalum–niobium–rare earth elements at Motzfeldt Sø in the Igaliko complex.The South Greenland crustal evolution records effects of mantle processes, such as lithospheric extension, subduction and underplating, which resulted in recurrent magma emplacement in tectonically active environments. As such, the geology of South Greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore-forming fluid systems during the Ketilidian orogeny as well as to the development of extreme rock compositions within the Gardar alkaline igneous province.     

Geochronological and geochemical constraints on the petrogenesis of Early Eocene metagabbroic rocks in Nabang (SW Yunnan) and its implications on the Neotethyan slab subduction

Mafic rocks similar to those of the Gangdese belt have been poorly reported in the Nabang area (SW Yunnan Province in SW China) of the Eastern Himalayan Syntaxis. This led to a widely-accepted assumption that Early Eocene mafic rocks are absent in Nabang. This paper reports new zircon U–Pb, Lu–Hf isotopic, whole-rock elemental and Sr–Nd isotopic data for the recently identified Tongbiguan and Jinzhuzhai metagabbroic plutons. Our data show that the two mafic plutons crystallized at 53.2 ± 0.4 Ma and 53.6 ± 0.7 Ma, respectively, with zircon in-situ ε_Hf(t) values ranging from − 3.1 to + 4.9. Our data confirm the presence of Early Eocene mafic rocks in Nabang, contemporaneous with the major magmatic flare-ups of ~ 52 Ma in South Tibet. The rocks show high-K calc-alkaline basalt and basaltic andesite composition. They are characterized by subparallel spiky patterns with enrichment in LILEs, depletion in HFSEs and P–Ti negative anomalies. They show (Nb/La)n = 0.21–0.63, Ce/Pb = 2.99–9.91 and Nb/U = 5.2–14.1, along with high ⁸⁷Sr/⁸⁶Sr(t) ratios of 0.7061–0.7077 and ε_Nd(t) values of − 3.4 to − 5.6. Such geochemical signatures are similar to those of the synchronous Dangxung gabbroic and Yangbajing ultrapotassic rocks. Their least-contaminated samples can petrogenetically be attributed to input of slab-derived fluid into the lithospheric mantle. In conjunction with other available data, the mafic suite can be geochronologically and geochemically correlated to those in South Lhasa and are probably the equivalents of the Gangdese southeastward extension. Their formation might tectonically be related to slab rollback in response to the decreasing convergence rate. The termination of the Neotethyan subduction in SW Yunnan might be later than ~ 52 Ma, identical to that in South Tibet.     

Geochemistry,petrogenesis and tectonic setting of Neoproterozoic mafic–ultramafic rocks from the western Jiangnan orogen,South China

The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834 Ma) and the late Neoproterozoic (770–750 Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive ε_Nd(t) values (+ 2.7 to + 6.6) whereas mafic rocks exhibit negative ε_Nd(t) values (− 5.8 to − 0.9). The basaltic rocks show TiO₂ contents of 0.62–0.69 wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show ε_Nd(t) values (− 1.23 to + 3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO₂ contents of 1.01–1.33 wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750 Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750 Ma.     

Early Mesozoic southward subduction history of the Mongol–Okhotsk oceanic plate: Evidence from geochronology and geochemistry of Early Mesozoic intrusive rocks in the Erguna Massif,NE China

In this paper we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for Early Mesozoic intrusive rocks in the Erguna Massif of NE China, and we use these data to constrain the history of southward subduction of the Mongol–Okhotsk oceanic plate, and its influence on NE China as a whole. The zircon U–Pb dating indicates that Early Mesozoic magmatic activity in the Erguna Massif can be subdivided into four stages at ~ 246 Ma, ~ 225 Ma, ~ 205 Ma, and ~ 185 Ma. The ~ 246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with I-type affinities. The ~ 225 Ma intrusive rocks consist of gabbro–diorites and granitoids, and they constitute a bimodal igneous association. The ~ 205 Ma intrusive rocks are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks. The ~ 185 Ma intrusive rocks are dominated by I-type granitoids, accompanied by minor amounts of A-types. These Early Mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction-related fluids. The rock associations and their geochemical features indicate that the ~ 246 Ma, ~ 205 Ma, and ~ 185 Ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The ~ 225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol–Okhotsk oceanic plate. Every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the Mongol–Okhotsk suture belt. Taking all this into account, we conclude the following: (1) during the Early Mesozoic, the Mongol–Okhotsk oceanic plate was subducted towards the south beneath the Erguna Massif, but with a pause in subduction at ~ 225 Ma; and (2) the southward subduction of the Mongol–Okhotsk oceanic plate not only caused the intense magmatic activity, but was also favorable to the formation of porphyry deposits.     

Sedimentation and magmatism in the Paleoproterozoic Cuddapah Basin,India: Consequences of lithospheric extension

The Cuddapah Basin is one of many Proterozoic, intracontinental sedimentary basins across Peninsular India. The basin comprises several unconformity-bounded successions, the lowermost of which (the Papaghni Group and overlying Chitravati Group) are intruded by dolerite sills that contact metamorphosed their host rocks. A mafic-ultramafic sill from the base of the Tadpatri Formation in the Chitravati Group was previously dated at c. 1885 Ma, and interpreted to be part of a large igneous province (LIP). We have dated two samples of a felsic tuff from the upper part of the Tadpatri Formation at 1864 ± 13 Ma and 1858 ± 16 Ma; combining data from the two samples yields a weighted mean date of 1862 ± 9 Ma. Mafic sills intrude rocks stratigraphically above the tuffaceous beds, indicating that mafic magmatism continued until after c. 1860 Ma. Given that the sills intruded lithified rocks, some of the sills may be considerably younger than 1860 Ma. Mafic volcanic rocks are also known from below the unconformity at the base of the Chitravati Group, within the basal Papaghni Group (> c. 1890 Ma). Collectively, these data indicate that mafic sill emplacement spanned more than 30 myr so that it is likely to have been a protracted event or a series of events, and, therefore unlikely to represent a LIP. The time span for mafic magmatism is more compatible with episodic, lithospheric extension (passive rifting) during basin evolution than it is with a mantle plume (active rifting).     

Neoproterozoic magmatic events in the South Qinling Belt,China: Implications for amalgamation and breakup of the Rodinia supercontinent

Neoproterozoic magmatic rocks in the South Qinling Belt of China provide important clues for understanding the mechanism and timing of the amalgamation and breakup of the Rodinia supercontinent. Here we report new geochemical and high-precision LA-ICP-MS zircon U–Pb–Hf isotopic analyses on magmatic suites from the Liuba and Zhashui areas in the South Qinling Belt. Our data show that the crystallization ages of the granitic intrusions from Tiefodian and Tangjiagou in the Liuba area are 863 ± 22 Ma and 794 ± 11 Ma, respectively, whereas those of the dioritic and gabbroic intrusions at Chishuigou in the Zhashui area are 925 ± 28 Ma and 832.6 ± 4.0 Ma, respectively. The diorites at Chishuigou display arc-related geochemical affinity, characterized by strong depletion in Nb, Ta, P and Ti, and enrichment in large-ion lithophile elements (i.e., Rb, Ba, Th and U), indicating a subduction-related arc setting at ca. 925 Ma. The Tiefodian granitic rocks have high SiO₂ (68.46–70.98 wt.%), Na₂O (3.87–4.51 wt.%), and low K₂O (1.34–2.61 wt.%) contents with TTG affinity. However, their Cr, and Ni contents and Cr/Ni, Nb/Ta ratios are similar to those of continental crust, and together with high negative ε_Hf(t) values (− 4.87 to − 14.84), suggesting a continental margin arc at ca. 863 Ma. The gabbros at Chishuigou have high TiO₂ content (2.74–3.14 wt.%), Zr/Y (3.93–4.24), Ta/Yb (0.19–0.25) ratios and low Zr/Nb ratios (11.37–13.17), similar to the features of within-plate basalts, indicating an intra-continental rift setting at ca. 833 Ma. The granitoids at Tangjiagou exhibit enrichment of LREE, K and Pb, and depletion of Nb, Ta, P and Ti, suggesting an extensional tectonic environment at ca. 794 Ma.The results indicate that Neoproterozoic magmatic rocks in the South Qinling Belt formed before ca. 833 Ma and might represent the amalgamation of the Rodinia supercontinent in an arc-related subduction environment, whereas the magmatic events with the peak ages at ~ 740 Ma during ca. 833–680 Ma represent the breakup of Rodinia. Integrating our new data with those from previous works, we propose a new tectonic model for the evolutionary history of the South Qinling Belt in the Neoproterozoic, including four key stages: 1) an ocean that separated the South Qinling Belt and the Yangtze Block in the Early Neoproterozoic (ca.1000–956 Ma); 2) bidirectional subduction of the oceanic lithosphere during ca. 956–870 Ma; 3) subduction and collision between the South Qinling Belt and the Yangtze Block during ca. 870–833 Ma, thus suggesting that the South Qinling Belt was as a part of the Yangtze Block from this period; and 4) intra-continental rifting during ca. 833–680 Ma, although the blocks were not entirely rifted apart.     

Late Neoarchean thick-skinned thrusting and Paleoproterozoic reworking in the MacQuoid supracrustal belt and Cross Bay plutonic complex,western Churchill Province,Nunavut, Canada

In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500 Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks.The ∼2190 Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9 Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810 Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen.     

Evidence of Mid-ocean ridge and shallow subduction forearc magmatism in the Nagaland-Manipur ophiolites,northeast India: constraints from mineralogy and geochemistry of gabbros and associated mafic dykes

We discuss here the mineralogical and geochemical characteristics of mafic intrusive rocks from the Nagaland-Manipur Ophiolites (NMO) of Indo-Myanmar Orogenic Belt, northeast India to define their mantle source and tectonic environment. Mafic intrusive sequence in the NMO is characterized by hornblende-free (type-I) and hornblende-bearing (type-II) rocks. The type-I is further categorized as mafic dykes (type-Ia) of tholeiitic N-MORB composition, having TiO₂ (0.72–1.93 wt.%) and flat REE patterns (La_N/Yb_N = 0.76–1.51) and as massive gabbros (type-Ib) that show alkaline E-MORB affinity, having moderate to high Ti content (TiO₂ = 1.18 to 1.45 wt.%) with strong LREE-HREE fractionations (La_N/Yb_N = 4.54–7.47). Such geochemical enrichment from N-MORB to E-MORB composition indicates mixing of melts derived from a depleted mantle and a fertile mantle/plume source at the spreading center. On the other hand, type-II mafic intrusives are hornblende bearing gabbros of SSZ-type tholeiitic composition with low Ti content (TiO₂ = 0.54 wt.%–0.86 wt.%) and depleted LREE pattern with respect to HREE (La_N/Yb_N = 0.37–0.49). They also have high Ba/Zr (1.13–2.82), Ba/Nb (45.56–151.66) and Ba/Th (84.58–744.19) and U/Th ratios (0.37–0.67) relative to the primitive mantle, which strongly represents the melt composition generated by partial melting of depleted lithospheric mantle wedge contaminated by hydrous fluids derived from subducting oceanic lithosphere in a forearc setting. Their subduction related origin is also supported by presence of calcium-rich plagioclase (An_16.6–32.3). Geothermometry calculation shows that the hornblende bearing (type-II) mafic rocks crystallized at temperature in range of 565°–625 °C ± 50 (at 10 kbar). Based on these available mineralogical and geochemical evidences, we conclude that mid ocean ridge (MOR) type mafic intrusive rocks from the NMO represent the section of older oceanic crust which was generated during the divergent process of the Indian plate from the Australian plate during Cretaceous period. Conversely, the hornblende-bearing gabbros (type-II) represent the younger oceanic crust which was formed at the forearc region by partial melting of the depleted mantle wedge slightly modified by the hydrous fluids released from the subducting oceanic slab during the initial stage of subduction of Indian plate beneath the Myanmar plate.     

Zircon U–Pb age,Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic–felsic Complex at the southeastern margin of the Yangtze Block

This work presents an integrated study of zircon U–Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic–felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7093–0.7127, low εNd(t) values ranging from −5.6 to −5.4 and corresponding Nd model ages (T_2DM) of 1.6 Ga. Their zircon grains have εHf(t) values ranging from −2.7 to 2.6 and model ages of 951–1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7053–0.7058, εNd(t) values of 0.2–1.6 and corresponding T_2DM of 1.0–1.1 Ga. Their zircon grains have εHf(t) values ranging from 3.2 to 6.1 and model ages of 774–911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7065–0.7117, εNd(t) values from −5.7 to −1.9 and Nd model ages of 1.3–1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled Nd–Hf isotopic systemics, which may be a fingerprint of a previous late Mesoproterozoic to early Neoproterozoic oceanic subduction.     

Crustally-derived granites in the Panzhihua region,SW China: Implications for felsic magmatism in the Emeishan large igneous province

In the Panxi region of the Late Permian (~ 260 Ma) Emeishan large igneous province (ELIP) there is a bimodal assemblage of mafic and felsic plutonic rocks. Most Emeishan granitic rocks were derived by differentiation of basaltic magmas (i.e. mantle-derived) or by mixing between crustal melts and primary basaltic magmas (i.e. hybrid). The Yingpanliangzi granitic pluton within the city of Panzhihua intrudes Sinian (~ 600 Ma) marbles and is unlike the mantle-derived or hybrid granitic rocks. The SHRIMP zircon U–Pb ages of the Yingpanliangzi pluton range from 259 ± 8 Ma to 882 ± 22 Ma. Younger ages are found on the zircon rims whereas older ages are found within the cores. Field relationships and petrography indicate that the Yingpanliangzi pluton must be < 600 Ma, therefore the older zircons are interpreted to represent the protolith age whereas the younger analyses represent zircon re-crystallization during emplacement. The Yingpanliangzi granites are metaluminous and have negative Ta–Nb_PM anomalies, low εNd_{(260 Ma)} values (? 3.9 to ? 4.4), and high I_Sr (0.71074 to 0.71507) consistent with a crustal origin. The recognition of a crustally-derived pluton along with mantle-derived and mantle–crust hybrid plutons within the Panxi region of the ELIP is evidence for a complete spectrum of sources. As a consequence, the types of Panxi granitoids can be distinguished according to their ASI, Eu/Eu*, εNd_(T), εHf_(T), T_Zr(°C) and Nb–Ta_PM values. The diverse granitic magmatism during the evolution of the ELIP from ~ 260 Ma to ~ 252 Ma demonstrates the complexity of crustal growth associated with LIPs.     

Petrology,geochemistry and geochronology of the magmatic suite from the Jianzha Complex,central China: Petrogenesis and geodynamic implications

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO^tot and Cr₂O₃ of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)_N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu¹ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε_Nd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.     

Structure,age, and tectonic development of the Huoshishan–Niujuanzi ophiolitic mélange,Beishan, southernmost Altaids

The Huoshishan–Niujuanzi ophiolitic mélange (HNO) is located near the central part of the Beishan Orogenic Belt in the southernmost Altaids. The HNO consists of ultramafic rocks, cumulate gabbros, gabbros, plagiogranites, diorites, diabases, basalts, andesites, rhyolitic volcaniclastic rocks and siliceous sedimentary rocks, many of which are in a schist matrix (Gongpoquan Group). Geochemical data of the mafic rocks indicate a calc-alkaline or a mixture of calc-alkaline and tholeiitic rocks with negative Nb, Ta and positive Pb, Ba and La anomalies, suggesting formation in an island arc or supra-subduction zone setting. A gabbro from a block in the mélange in the Niujuanzi area has a zircon age of 435.0 ± 1.9 Ma and a plagiogranite with an age of 444.3 ± 1.9 Ma, and another gabbro from the Huoshishan area has an age of 410.5 ± 3.7 Ma. The schist matrix has a zircon age of 512 ± 5.3 Ma and contains Silurian, Devonian and Carboniferous fossils, thus the mélange formed in the late Carboniferous or later. Our structural analysis of fault planes in the HNO, the crenulation cleavages (S₂) of the schist, and fold axial planes of early Permian sandy limestone/quartz veins and late Permian sandstones indicates that the mélange underwent a north-to-south compression, and the orientation of stretching lineations, slickensides and fold hinge lines implies that the HNO experienced top-to-the north (or -northwest) movement. The entire planar and linear structural data set suggests that the subduction polarity was probably to the south in the late Paleozoic. The emplacement age of the HNO was probably near the end-Permian based on the age of the youngest rocks in the ophiolitic mélange, and by the presence of a late Permian unconformity. From our work, integrated with published regional data, we outline a comprehensive geodynamic model for the central BOC.     

Archaean to Palaeoproterozoic high-grade evolution of the Belomorian eclogite province in the Gridino area,Fennoscandian Shield: Geochronological evidence

The Belomorian eclogite province was repeatedly affected by multiple deformation episodes and metamorphism under moderate to high pressure. Within the Gridino area, high pressure processes developed in a continental crust of tonalite–trondhjemite–granodiorite (TTG) affinity that contains mafic pods and dykes, in which products of these processes are most clearly evident. New petrological, geochemical and geochronological data on mafic and felsic rocks, including PT-estimates, mineral chemistry, bulk rock chemistries, REE composition of the rocks and zircons and U–Pb and Lu–Hf geochronology presented in the paper make it possible to reproduce the magmatic and high-grade metamorphic evolution in the study area. In the framework of the extremely long-lasting geologic history recorded in the Belomorian province (3–1.7 Ga), new geochronological data enabled us to define the succession of events that includes mafic dyke emplacement between 2.87 and 2.82 Ga and eclogite facies metamorphism of the mafic dykes between ~ 2.82 and ~ 2.72 Ga (most probably in the time span of 2.79–2.73 Ga). The clockwise PT path of the Gridino association crosses the granulite- and amphibolite-facies PT fields during the time period of 2.72 Ga to 2.64 Ga. A special aspect of this work concerns the superposed subisobaric heating (thermal impact) with an increase in the temperature to granulite facies conditions at 2.4 Ga. Later amphibolite facies metamorphism occurred at 2.0–1.9 Ga. Our detailed geochronological and petrological studies reveal a complicated Mesoarchaean–Palaeoproterozoic history that involved deep subduction of the continental crust and a succession of plume-related events.     

In situ zircon U–Pb dating and O isotopes of the Neoarchean Hongtoushan VMS Cu–Zn deposit in the North China Craton: Implication for the ore genesis

The Hongtoushan volcanogenic massive sulfide (VMS) deposit is the largest Archean Cu–Zn deposit in China, located in the Qingyuan greenstone belt on the northern margin of the North China Craton. The Cu–Zn mineralization was stratigraphically controlled by the interbeds (~ 100 m in thickness) of mafic–felsic volcanic sets and overlain by banded iron layers. However, the relationship between VMS deposits and associated volcanics has not been examined. This study ultimately clarifies the times and sources of the volcanics and mineralization. Based on in situ zircon U–Pb and O isotope on VMS-hosting mafic, felsic volcanic rocks, banded and massive sulfide ores and postmineralization pegmatite vein, we considered that there were two main formation stages for the Qingyuan Cu–Zn deposits; one was exhalative-hydrothermal sedimentation and another was further Cu–Zn enriched by later hydrothermal processes. The timing of the first stage occurred at 2571 ± 6 Ma based on the magmatic zircons in the VMS-hosting mafic volcanic rocks, from which the inherited zircons also indicate the existence of 2.65–3.12 Ga ancient supercrustal rocks in the Qingyuan district. A modern mantle-like δ¹⁸Ozircon value of 5.5 ± 0.1‰ (2SD) for this volcanism was well preserved in the inherited core domains of ore samples. It suggests that the mafic volcanics was most likely sourced from partial melting of juvenile crust, e.g., TTG granites. A large-scale metamorphic or hydrothermal event is documented by the recrystallized zircons in sulfide ores. The timing is tightly constrained by the hydrothermal zircon U–Pb ages. They are 2508 ± 4 Ma for the banded ore, 2507 ± 4 Ma for the massive ore and 2508 ± 2 Ma for the postmineralization pegmatite vein. These indistinguishable ages indicate that the 2507 Ma hydrothermal systems played a significant role in the upgrading of the VMS Cu–Zn orebodies. The weighted δ¹⁸O values of hydrothermal zircons show a successively increasing trend from 6.0 ± 0.1‰ (2σ) for the banded ore, 6.6 ± 0.2‰ (2σ) for the massive ore to 7.3 ± 0.2‰ (2σ) for the later pegmatite vein. This variation might be induced by gradual inputting of the δ¹⁸O-rich oceanic crust and/or oceanic sediment during the hydrothermal cycling system. Considering its modern mantle-like oxygen isotope composition of 2571 Ma volcanism, a submarine volcanic hydrothermal system involving mantle plumes is a preferred setting for the Neoarchean VMS Cu–Zn deposits in the Qingyuan greenstone belt.     

Neoproterozoic granitic activities in the Xingdi plutons at the Kuluketage block,NW China: Evidence from zircon U–Pb dating,geochemical and Sr–Nd–Hf isotopic analyses

Neoproterozoic igneous rocks are widely distributed in the Kuluketage block along the northern margin of the Tarim Craton. However, the published literature mainly focuses on the ca. 800 Ma adakitic granitoids in the area, with the granites that intrude the 735–760 Ma mafic–ultramafic rocks poorly studied. Here we report the ages, petrography and geochemistry of two granites in the Xingdi mafic–ultramafic rocks, in order to construct a new view of the non-adakitic younger granites. LA-ICP-MS zircon U–Pb dating provided weighted mean ²⁰⁶Pb/²³⁸U ages of 743.0 ± 2.5 Ma for the No.I granite (G1) and 739.0 ± 3.5 Ma for the No.II granite (G2). A clear core-rim texture of similar age and a high zircon saturation temperature of ca. 849 ± 14 °C were observed for the No.I granite; in contrast, G2 has no apparent core-rim texture but rather inherited older zircons and a lower zircon saturation temperature of ca. 763 ± 17 °C. Geochemical analysis revealed that G1 is an alkaline A-type granite and G2 is a high-K calc-alkaline I-type granite. Both granites share similar geochemical characteristics of arc-related magmatic rocks and enriched Sr–Nd–Hf isotopes, likely due to their enriched sources or mixing with enriched magma. Whereas G1 and its host mafic rocks form typical bimodal intrusions of the same age and similar Sr–Nd–Hf isotope compositions, G2 is younger than its host mafic rocks and its Sr–Nd–Hf isotope composition indicates a lower crust origin. Although they exhibit arc-related geochemical features, the two granites likely formed in a rift setting, as inferred from thier petrology, Sr–Nd–Hf isotopes and regional tectonic evolution.     

Backarc mafic–ultramafic magmatism in Northeastern Vietnam and its regional tectonic significance

The geology of Northern Vietnam offers critical clues on the convergence history between the South China and Indochina blocks. We constrain the tectonic evolution of the South China and Indochina blocks using geochemical, mineral chemical and geochronological data collected from mafic–ultramafic rocks exposed in the Cao Bang area, Northeastern Vietnam. These rocks show significant enrichment in large ionic lithophile elements (LILEs) such as Cs, Rb, Ba, Th, U, and Pb and depletion in high field strength elements (HFSEs) such as Nb, Ta, Zr, and Ti showing [Nb/La]_N between 0.28–0.41, [La/Yb]_N = 3.94–10.00 and Zr/Y = 2.0–4.4. These geochemical features as well as the petrology and mineral chemistry of the Cao Bang mafic–ultramafic magmas are comparable to those of magmatic complexes formed in a back-arc environment. The basalts yield Rb–Sr whole rock ages of 263 ± 15 Ma, that are consistent with the zircon U–Pb and K–Ar ages reported in previous studies from the same area. The spatial and temporal distribution of the arc magmas within the Indochina block and along the southern margin of the South China block suggest that the Permo-Triassic mafic–ultramafic magmas formed during a tectonic event that is different from the subduction and collision event between the Indochina and South China blocks.     

Petrogenesis of the 2115 Ma Haicheng mafic sills from the Eastern North China Craton: Implications for an intra-continental rifting

The Rhyacian (2300–2050 Ma) is a special era of the Paleoproterozoic represented by large layered intrusions in many cratons. It is well known that there are widespread igneous events at ~ 2100 Ma in the Eastern North China Craton; however, their tectonic environments are under debate: whether they were related to an intra-continental rifting or an arc/back-arc setting along a continental margin. These ~ 2100 Ma igneous events comprise several mafic dykes/sills, with some coeval A-type granites and volcanic events in several rifts; among them, the Haicheng mafic sills in the Liaohe rift are unique as their host rock, the Liaohe Group, bears the world's largest magnesium deposit. Most of the mafic sills are E-W-elongated at present coordinates. Exclusive of superimposition caused by deformation, the widths of the individuals are tens to hundreds of meters and the lengths are hundreds to thousands of meters. They have metamorphosed to an assemblage of plagioclase and hornblende, with minor quartz and accessory chlorite, epidote, apatite, ilmenite, and magnetite. However, relic gabbro and ophitic textures with mainly plagioclase and clinopyroxene are well-preserved. SIMS Pb–Pb dating on baddeleyites from one ~ 1000 m thick sill near Xialiulinzi village yields an average ²⁰⁷Pb/²⁰⁶Pb age of 2115 ± 3 Ma (n = 15, MSWD = 2.3), representing the timing of crystallization. SIMS U–Pb dating on zircon yields a similar forming age. They are tholeiitic in composition (MgO: 4.36–8.88 wt.%; SiO₂: 45.76–53.39 wt.%), enriched in light rare earth elements ((La/Yb)_N = 1.72–4.37) and large ion lithophile elements (i.e., Cs, Rb, Sr, and K) but depleted in high field strength elements (i.e., Nb, Ta, and Ti). These features were unlikely caused by crustal contamination during their emplacement, as there are little variations in Nb/La and Th/Nb. The rocks have experienced significant plagioclase-plus clinopyroxene-dominating fractional crystallization. Their enriched Sr–Nd isotope characteristics (⁸⁷Sr/⁸⁶Sr_t = 0.703 ~ 0.705, εNd_t = − 1.9 ~ 0.6) and trace element patterns indicate that their source(s) could be the ancient subcontinental lithospheric mantle; and this source is similar to those coeval sills from other parts of the craton. Their arc-like trace element features could be inherited from their source regions formed via a subduction process at the late Archean rather than at the middle-late Paleoproterozoic. These sill swarms, throughout the craton, might have developed in an integrated intra-continental rift system at ~ 2100 Ma.     

U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations

As the lowest volcanics-bearing unit of the Neoproterozoic succession, the Beiyixi Formation is the key to understanding the early response to the breakup of the Roninia supercontinent in the Tarim Block. The SHRIMP analyses of zircons from the volcanic rocks at the bottom of the Beiyixi Formation yield a weighted mean ²⁰⁶Pb/²³⁸U age of 755 ± 15 Ma. This is interpreted as the eruption age of the Beiyixi volcanic rocks. The Beiyixi volcanic rocks consist of bimodal basalt and dacite-rhyolite with a SiO₂ gap between 55% and 65%. The mafic rocks display negative ɛ_Nd (755 Ma) values (−9.9 to −10.8), moderate enrichment in LILE and variable depletion in Nb, Ta and P, resembling those of the tholeiitic basalts in continental rift. Geochemical and Nd isotopic characteristics suggest that the mafic rocks were derived from partial melting of an enriched lithospheric mantle reservoir. The felsic rocks show negative ɛ_Nd (755 Ma) values (−7.9 to −9.2), negative Nb, Ta, P and Ti anomalies, very high La_N/Yb_N (62–92) ratios and LILE abundances, and may be generated by melting of eclogites or garnet amphibolites in the lower crust, as a result of basalt emplacement into continental crust during continental rifting. The age of 755 ± 15 Ma indicates that the Beiyixi glaciation took place later than 755 Ma and it could be correlated with the Chang’an glaciation in the Yangtze Block and the Sturtian–Rapitan glaciation in other Rodinia Blocks. The geochemical characteristics of the Beiyixi volcanic rocks resemble those of the rift-related magmatism in other Rodinia Blocks, suggesting that the Beiyixi volcanism was a part of global magmatism during the breakup of Rodinia supercontinent. The age and geochemical features of the Beiyixi volcanic rocks also reveal that the mantle plume activity spread to the northwestern margin of the Rodinia supercontinent and probably resulted in the breakup between Australia and Tarim Blocks.     

Early Permian East-Ujimqin mafic–ultramafic and granitic rocks from the Xing’an–Mongolian Orogenic Belt,North China: Origin,chronology, and tectonic implications

The East-Ujimqin complex, located north of the Erenhot–Hegenshan fault, North China, is composed of mafic–ultramafic and granitic rocks including peridotite, gabbro, alkali granite, and syenite. We investigated the tectonic setting, age, and anorogenic characteristics of the Xing’an–Mongolian Orogenic Belt (XMOB) through field investigation and microscopic and geochemical analyses of samples from the East-Ujimqin complex and LA-MC-ICP-MS zircon U–Pb dating of gabbro and alkali granite. Petrographic and geochemical studies of the complex indicate that this multiphase plutonic suite developed through a combination of fractional crystallization, assimilation processes, and magma mixing. The mafic–ultramafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from a mantle source. The mafic–ultramafic magmas triggered partial melting of the crust and generated the granitic rocks. The granitic rocks are alkali and metaluminous and have high Fe/(Fe + Mg) characteristics, all of which are common features of within-plate plutons. Zircon U–Pb geochronological dating of two samples of gabbro and alkali granite yielded ages of 280.8 ± 1.5 and 276.4 ± 0.7 Ma, placing them within the Early Permian. The zircon Hf isotopic data give inhomogeneous ε_Hf(t) values of 8.2–14.7 for gabbroic zircons and extraordinary high ε_Hf(t) values (8.9–12.5) for the alkali granite in magmatic zircons. Thus, we consider the East-Ujimqin mafic–ultramafic and granitic rocks to have been formed in an extensional tectonic setting caused by asthenospheric upwelling and lithospheric thinning. The sources of mafic–ultramafic and granitic rocks could be depleted garnet lherzolite mantle and juvenile continental lower crust, respectively. All the above indicate that an anorogenic magma event may have occurred in part of the XMOB during 280–276 Ma.