Active Continental Margin Setting Research Articles

Crust–mantle interaction during Early Jurassic subduction of Neo-Tethyan oceanic slab: Evidence from the Dongga gabbro–granite complex in the southern Lhasa subterrane, Tibet

Gabbro–granite complexes that form in active continental margins provide an excellent opportunity to investigate the genetic relationships between acid and basic magmas and offer important insights into crust–mantle interaction. In this paper, we present detailed petrographic, mineralogical, whole-rock geochemical, Sr–Nd isotopic, zircon U–Pb, and Hf isotopic data for the Dongga gabbro–granite complex of the southern Lhasa subterrane, with the aim of elucidating the petrogenesis of these rocks and gaining insights into the processes of crust–mantle interaction associated with the northward subduction of the Neo-Tethyan oceanic slab. The Dongga complex consists predominantly of tonalite and granodiorite with abundant mafic microgranular enclaves (MMEs) and lesser amounts of hornblende gabbro. LA–ICP–MS zircon U–Pb dating yields crystallization ages of 178.0±1.5Ma and 192.3±1.5Ma for the hornblende gabbros, and 178.1±1.7Ma and 178.8±1.8Ma for the tonalites and MMEs, respectively. Geochemically, the gabbroic rocks have low SiO2 and Na2O+K2O concentrations and high Mg# values. These gabbroic rocks are enriched in large ion lithophile elements (LILEs; e.g., Ba, K, Pb, and Sr) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, P, Zr, and Hf). The tonalites are subalkaline and metaluminous and are also characterized by an enrichment of LILEs and LREEs and a depletion of HFSEs. The MMEs are silica poor and alkali rich and have broadly similar trace element signatures to those of the host tonalites. The gabbroic rocks, tonalites, and MMEs have similar initial 87Sr/86Sr ratios (ISr) of 0.7035–0.7036 and positive εNd(t) values of +5.3 to +6.8. However, zircon εHf(t) values from the hornblende gabbros (+14.0 to +18.2) are somewhat higher than those of the tonalites (+11.4 to +15.0) but are similar to the MMEs (+14.4 to +18.2). The combined petrological, geochemical, and isotopic features of the Dongga gabbro–granite complex suggest that the southern Lhasa subterrane was located in an active continental margin setting during the Early Jurassic. The gabbroic rocks were generated by the hydrous partial melting of a depleted mantle wedge metasomatized by aqueous fluids released from the subducted Neo-Tethyan oceanic slab. The tonalites and MMEs were produced by the mixing of mafic magmas related to the gabbroic rocks and felsic magmas derived from the remelting of the newly formed juvenile lower crust. The underplating of mantle-derived magmas and the resulting crustal melting and magma mixing occurred during the Early Jurassic subduction of the Neo-Tethyan oceanic slab. The mantle-derived magmas not only provided substantial heat to melt the lower crust but also mixed with the crust-derived magmas to form granitoids and MMEs.

Geochronology and geochemistry of the Heilongjiang Complex and the granitoids from the Lesser Xing'an-Zhangguangcai Range: Implications for the late Paleozoic-Mesozoic tectonics of eastern NE China

An integrated study on two subduction-related units (the Heilongjiang Complex and magmatic rocks in the Lesser Xing'an-Zhangguangcai Range) was carried out to explore the Paleozoic to early Mesozoic tectonic framework of the eastern Northeast China, with a focus on the Jiamusi block. The Heilongjiang Complex, which is considered as the suture between the Jiamusi and Songliao blocks, is composed of blueschist, amphibolite and ultramafic rocks. The blueschist from the Yilan area shows geochemical features similar to OIB and has a 206Pb/238U age of 288±2Ma, which is interpreted as the crystallization age of protolith formed in an ocean island setting. In contrast, amphibolite from the Xiachengzi area shows geochemical features similar to N-MORB. Zircons in this amphibolite show magmatic character and yield a 206Pb/238U age of 248±4Ma, which is interpreted as the protolith age. Calcic amphiboles in the amphibolite give a 40Ar/39Ar plateau age of 195±3Ma, which is interpreted as the metamorphic age of the Heilongjiang Complex. Magmatic rocks in the Lesser Xing'an-Zhangguangcai Range mostly belong to I-type granites that were formed in an active continental margin setting. Zircon U-Pb ages of these granites range from ~200Ma to ~174Ma with a westward younging trend shown as two groups of emplacing ages of ~200Ma in the east and ~180Ma in the west. Our new data support an early occurrence (likely since the early Permian) of a large ocean (namely the Mudanjiang Ocean) in the area, whose subduction resulted in the metamorphic unit (the Heilongjiang Complex) and the magmatic unit (the Lesser Xing'an-Zhangguangcai Range) in the Jurassic.

Geochemistry and Genesis of Craton-derived Sediments from Active Continental Margins: Insights from the Mizoram Foreland Basin, NE India

This study investigates relative controls of source compositions and sedimentary processes (weathering+mineral sorting) on the bulk-rock geochemistry of clastic rocks deposited along an active continental margin setting by considering the Miocene Mizoram basin of NE India. It is a classic foreland accretionary sedimentary basin related to Himalaya orogeny and is ideally located to understand tectonic evolution of the Eastern Himalayas, Indo-Burmese arc, and Indian cratonic margin [the Meghalaya Plateau (also known as Shillong Plateau)]. These three tectonic domains represent potential suppliers of clastic sediments to the basin. The Mizoram basin comprises sandstone, siltstone and shale deposited under shallow marine conditions. Depletions in Ca and Sr indicate plagioclase-dominated weathering of the sources. The Mizoram strata are first-cycle sediments; however, quartz dilution has resulted in the decrease of both major (except SiO2) and trace elements, carbonate dilution has increased Ca and Sr, and heavy minerals have influenced Zr, HREE, Hf, U and Y abundances in a few of the studied Mizoram sediments. Newly formulated Rb/V-Zr/Zn-Sc/Nb and Rb/V-Zr/Zn-Eu/Eu* ternary plots distinguish sediments derived from mafic versus felsic sources, and allow the estimation of mafic:felsic proportions in the sedimentary provenances. We thus suggest an average tonalite-granodiorite source terrain for the Mizoram sediments. The geochemical evidences, including TiO2 (>0.7wt%), Cr/Cr* (>1), V/V* (>1), Th/U (10.23), Nb/U (9.01) and Th/Rb (0.124) values, indicate that the Mizoram sediments were derived chiefly from old cratonic crust of the Meghalaya Plateau. The source for the Miocene Mizoram sediments is slightly mafic than that supplying the modern Brahmaputra and Ganga sediments (dominated by the juvenile Himalayas). The Mizoram clastics provide indirect evidence for rise of the Meghalaya Plateau and a possible change in the Brahmaputra drainage as controlled by the uplift. We suggest that the Mizoram sediments are archives of the cratonic provenance in a foreland basin dominated by orogeny-derived detritus.

Palaeoproterozoic metasedimentary rocks of the Ji'an Group and their significance for the tectonic evolution of the northern segment of the Jiao–Liao–Ji Belt, North China Craton

AbstractIn this paper we present new petrological and whole-rock geochemical data for the Palaeoproterozoic metasedimentary rocks in the upper part of the Ji'an Group within the Jiao–Liao–Ji Belt, China, as well as zircon U–Pb age dates andin situLu–Hf isotope data. The new data improve our understanding of the original nature of the metasedimentary rocks, further providing insights into their tectonic setting and the evolutionary history of the northern segment of the Jiao–Liao–Ji Belt. The zircons can be divided into two groups, viz., one of magmatic origin and the other of metamorphic origin. Zircon U–Pb dating gave mean or statistical peak ages for the magmatic zircons at 2035, 2082, 2178, 2343–2421, 2451–2545, 2643–2814 and 2923–3446 Ma, and mean peak ages for the metamorphic zircons at 1855 and 1912 Ma, which indicate a maximum depositional age of 2.03 Ga and two-stage metamorphic events atc. 1.91 and 1.85 Ga for the metasedimentary rocks. Geochemical data show that (1) the protoliths of these rocks were mainly sandstones, greywackes and claystones, together with some shales; (2) the main sources of the sedimentary material were Palaeoproterozoic granites and acid volcanic rocks, with minor contributions from Archaean granitic rocks; and (3) the sediments were deposited in an active continental margin setting. Moreover, along the northeastern margin of the Eastern Block of the North China Craton there is evidence of ancient crustal materials as old as 3.76 Ga, and multiple crustal growth events at 3.23–3.05, 2.80–2.65, 2.54–2.45 and 2.28–2.08 Ga.

Sedimentary microfacies and organic geochemical characterization of mudstones in the Keluke Formation in northeastern Qaidam, China

During the Early Carboniferous, the northeastern Qaidam Basin probably formed in an active continental margin setting. Due to marine regression after the late Early Carboniferous, the marginal shelf began to receive more terrestrial clastic sediments, and alternating occurrence of carbonates and siliciclastics characterized this region. Consequently, shales or mudstones with considerable thickness were deposited in the Upper Carboniferous, especially in the Keluke Formation. In order to illuminate a more accurate sedimentary environment of shales and further evaluate the shale gas potential, multidisciplinary traditional methods were conducted including rock core examinations and laboratory analysis. Results reveal that the sedimentary microfacies of shales in the Keluke Formation can be divided into the upper shoreface, the lower shoreface, and the upper to transition offshore. These microfacies developed on a homoclinal carbonate ramp with a gentle slope. The thickest succession of mudstone develops in the lower shoreface, which is a counterpart with the lagoonal setting of a stationary hydraulic system. The quartz content of shale samples ranges from 30% to 66% and clay minerals from 24% to 43% by bulk compositional analysis, which indicates the content of brittle minerals and shales fissility to some extent. The total organic carbon content ranges from 0.5% to 7.6% by weight, with an average value of 2.69% in ZK1‐1 and 1.11% in ZK3‐2, indicating abundant organic matter richness. The kerogen is mainly of type II, which indicates that source for the organic matter is a mixture of higher terrestrial plants and acquatic organisms. The vitrinite reflectance ranges from 1.22% to 1.55%, indicating a moderate to high thermal maturity. Three types of pores are recognized in the shales, including organic pores, intrapartical pores, and interparticle pores. The significance of this work emphasizes two aspects. First, it gives a more detailed classification of sedimentary environment of the mudstone succession; second, it acquires the preliminary subsurface data regarding to the shale gas potential evaluation.

Metamorphic rock-hosted orogenic gold deposit style at Bombana (Southeast Sulawesi) and Buru Island (Maluku): Their key features and significances for gold exploration in Eastern Indonesia

In Indonesia, gold is commonly mined from epithermal-, porphyry-, and skarn-type deposits that are commonly found in volcanic belts along island arcs or active continental margin settings. Numerous gold prospects, however, were recently discovered in association with metamorphic rocks. This paper focuses on metamorphic rock-hosted gold mineralization in Eastern Indonesia, in particular the Bombana (SE Sulawesi) and Buru Island (Maluku) prospects. At Bombana, gold-bearing quartz-veins are hosted by the Pompangeo metamorphic complex. Sheared, segmented veins vary in thickness from 2 cm to 2 m. Gold is mainly present in the form of ‘free gold’ among silicate minerals and closely related to cinnabar, stibnite, tripuhyite, and in places, minor arsenopyrite. The gold distribution is erratic, however, ranging from below detection limit up to 134 g/t. At least three generations of veins are identified. The first is parallel to the foliation, the second crosscuts the first generation of veins as well as the foliation, and the late-stage laminated deformed quartz-calcite vein represents the third mineralization stage. The early veins are mostly massive to crystalline, occasionally brecciated, and sigmoidal, whereas the second-stage veins are narrower than the first ones and less subjected to brecciation. Gold grades in the second- and third-stage veins are on average higher than that in the earlier veins. Microthermometric and Raman spectrometric studies of fluid inclusions indicate abundant H2O-NaCl and minor H2O-NaCl-CO2 fluids. Homogenization temperatures and salinities vary from 114 to 283 ºC and 0.35 to 9.08 wt.% NaCl eq., respectively. Crush-leach analysis of fluid inclusions suggests that the halogen fluid chemistry is not identical to sea water, magmatic or epithermal related fluids, but tends to be similar to fluids in mesothermal-type gold deposits. In Buru Island (Gunung Botak and Gogorea prospects), two distinct generations of quartz veins are identified. Early quartz veins are segmented, sigmoidal discontinuous and parallel to the foliation of the host rock. This generation of quartz veins is characterized by crystalline relatively clear quartz, and weakly mineralized with low sulfide and gold contents. The second type of quartz veins occurs within the ‘mineralized zone’ of about 100 m in width and ~1,000 m in length. Gold mineralization is intensely overprinted by argillic alteration. The mineralization-alteration zone is probably parallel to the mica schist foliation and strongly controlled by N-S or NE-SW-trending structures. Gold-bearing quartz veins are characterized by banded texture particularly following host rock foliation and sulphide banding, brecciated and rare bladed-like texture. Alteration types consist of propylitic (chlorite, calcite, sericite), argillic and carbonation represented by graphite banding and carbon flakes. Ore mineral comprises pyrite, native gold, pyrrhotite, and arsenopyrite. Cinnabar and stibnite are present in association with gold. Ore chemistry indicates that 11 out of 15 samples yielded more than 1 g/t Au, in which 6 of them graded in excess of 3 g/t Au. All high-grade samples are composed of limonite or partly contain limonitic material. This suggests the process of supergene enrichment. Interestingly, most of the high-grade samples contain also high concentrations of As (up to 991ppm), Sb (up to 885ppm), and Hg (up to 75ppm). Fluid inclusions in both quartz vein types consist of 4 phases including L-rich, V-rich, L-V-rich and L1-L2-V (CO2)-rich phases. The mineralizing hydrothermal fluid typically is CO2-rich, of moderate temperature (300-400 ºC), and low salinity (0.36 to 0.54 wt.% NaCl eq). Based on those key features, gold mineralization in Bombana and Buru Island tends to meet the characteristics of orogenic, mesothermal types of gold deposit. Metamorphic rock-hosted gold deposits could represent the new targets for gold exploration particularly in Eastern Indonesia.

Triassic volcanism along the eastern margin of the Xing'an Massif, NE China: Constraints on the spatial–temporal extent of the Mongol–Okhotsk tectonic regime

We present new zircon U–Pb–Hf and whole-rock geochemical data for volcanic rocks along the eastern margin of the Xing'an Massif of NE China in order to further our understanding of the history of subduction towards the SE and the spatial extent of the Mongol–Okhotsk tectonic regime. Zircon U–Pb dating indicates that the Triassic volcanism in the Xing'an Massif occurred in two stages during the Middle (ca. 242Ma) and Late (ca. 223–228Ma) Triassic. Middle Triassic basaltic andesites in the Heihe area have an affinity to arc-type volcanic rocks. The zircon εHf(t) values (+8.5 to +12.7) suggest that the primary magma was generated by the partial melting of a relatively depleted mantle wedge that had been metasomatized by subduction-related fluids. The Late Triassic andesites in the Handaqi area exhibit geochemical affinities to high-Mg adakitic andesites. Their zircon εHf(t) values (+11.5 to +14.5) and TDM2 ages (313–484Ma) indicate that their primary magma was derived from the partial melting of a young subducted oceanic crust, followed by interaction with melts derived from mantle peridotite. The Late Triassic basaltic andesites, andesites, and dacites in the Zhalantun–Moguqi area have features similar to those of igneous rocks formed in subduction zones. Their zircon εHf(t) values (+8.4 to +15.4) and TDM1 ages (260–542Ma) indicate that their primary magma was derived from the partial melting of a depleted mantle wedge that had been metasomatized by subduction-related fluids. These data suggest that the Triassic volcanic rocks of the Xing'an Massif formed in an active continental margin setting associated with the southward subduction of the Mongol–Okhotsk oceanic plate towards the SE. We conclude that the Mongol–Okhotsk tectonic regime extended at least as far as the eastern margin of the Xing'an Massif, and that the tectonism spanned the period from the late Permian to early Early-Cretaceous.

Geochronology and Petrogenesis of Mesozoic Granitoids in the Geological Corridor of Western Liaoning Province, China

Geochronology and Petrogenesis of Mesozoic Granitoids in the Geological Corridor of Western Liaoning Province, China

Composition and conditions of formation of the parental melts of Jurassic dolerites of southwestern Crimea: Evidence from melt inclusions in olivine phenocrysts

This study focuses on Jurassic shallow intrusions and subvolcanic bodies from around Trudolyubovka village in the southwestern Crimea. All the rocks are similar in mineral composition and have similar geochemical features and occur in close spatial and geological association. This allows us to assign the intrusions to a single magmatic series and interpret them as differentiation products of a single parental melt. The investigation of melt inclusions in olivine from the most magnesian sample showed that the composition of igneous melts ranged from basalt to basaltic andesite of a moderately potassic subalkaline affinity. Compared with N-MORB, they are enriched in LILE, but have similar HFSE and REE contents. The early magmatic melts crystallized at temperatures ranging from 1240 to 1125°C, pressures of 6–8 kbar, and an oxygen fugacity of ΔQFM = +0.6; and later melts crystallized at 1090–940°C, ~1.5 kbar, and oxygen fugacity increasing from ΔQFM + 0.9 to ΔQFM + 2.3. The minimum pressure of groundmass crystallization was estimated as 40–60 bar. The primitive melts were formed in a mature island arc or an active continental margin setting by ~13% melting of a DMM-like source. The melting occurred at spinel-facies depths under the influence of a slab-derived fluid at a temperature 25°C below the dry peridotite solidus.

Geochronology and geochemistry of early Paleozoic intrusive rocks from the Khanka Massif of the Russian Far East

The Russian Far East and Northeast (NE) China are located in the eastern part of the Central Asian Orogenic Belt (CAOB), which consists of a series of micro‐continental massifs including the Erguna, Xing'an, Songnen–Zhangguangcai Range, Bureya, Jiamusi, and Khanka massifs. The Khanka Massif is located in the easternmost part of the CAOB, mainly cropping out in the territory of Russia, with a small segment in NE China. To the north and west of the Khanka Massif are the Jiamusi and Songnen–Zhangguangcai Range massifs, respectively. The boundary between these massifs is marked by the Dunhua–Mishan Fault. To the south lies the North China Craton, and to the east is the Sikhote–Alin Orogenic Belt separated by the Arsenyev Fault.However, the early Paleozoic evolution and tectonic attributes of the Khanka Massif are debated. These conflicting ideas result from the lack of systematic research on early Paleozoic igneous rocks from the Russian part of the Khanka Massif.It is generally accepted that the CAOB represents the largest known Phanerozoic accretionary orogenic belt. However, questions remain concerning the nature of the deep crust beneath the Khanka Massif, and whether Precambrian crust exists within the massif itself.In this paper, we report new zircon U–Pb ages, Hf isotopic data, and major‐ and trace‐element compositions of the early Paleozoic intrusive rocks from the Khanka Massif of the Russian Far East, with the aim of elucidating the early Paleozoic evolution and the tectonic attributes of the Khanka Massif, as well as the nature of the underlying deep crust.New U–Pb zircon data indicate that early Paleozoic magmatism within the Khanka Massif can be subdivided into at least four stages: ∼502 Ma, ∼492 Ma, 462–445 Ma, and ∼430 Ma.The ∼502 Ma pyroxene diorites show negative Eu anomalies, and the ∼492 Ma syenogranites, intruding the ∼502 Ma diorites, show positive Eu anomalies. These observations indicate that the primary parental magmas of these rocks were derived from different origins.The 462–445 Ma magmatism is made up of syenogranites and tonalites. The ∼445 Ma Na‐rich tonalites contain low REE concentrations, and are enriched in Eu and Sr. These observations, together with the positive εHf(t) values, indicate that they were derived from magmas generated by partial melting of cumulate gabbros.The ∼430 Ma I‐type granodiorites and monzogranites from the northern Khanka Massif, and the A‐type monzogranites from the central Khanka Massif display zircon εHf(t) values ranging from −5.4 to +5.8. This suggests that they formed from magmas generated by partial melting of heterogeneous lower crustal material.Zircon Hf isotopic data reveal the existence of Precambrian crustal material within the Khanka Massif. The geochemistry of the Middle Cambrian intrusive rocks is indicative of formation in an extensional setting, while Late Cambrian–middle Silurian magmatism was generated in an active continental margin setting associated with the subduction of a paleo‐oceanic plate beneath the Khanka Massif. Regional comparisons of the magmatic events indicate that the Khanka Massif has a tectonic affinity to the Songnen–Zhangguangcai Range Massif rather than the Jiamusi Massif.

Reply to comment on “Geochronology and geochemistry of rhyolites from Hormuz Island, southern Iran: A new Cadomian arc magmatism in the Hormuz Formation” by Atapour, H. and Aftabi, A

We appreciate the discussion by Atapour and Aftabi (2016) and would like to reply to their comments in detail. Atapour and Aftabi (2016) do not dispute our key findings concerning zircon geochronology, geochemistry, and thermometry. In fact, their criticism is focused on two topics: 1) our geological mapping, which they claim is not robust; 2) that the Hormuz rhyolites are not of the I-type, formed from subduction-related magmatism in an active continental margin setting but rather claim that these rocks are A-type rhyolites formed in a rift-related or within plate setting.

Genesis of the Tianbaoshan Polymetallic Ore District, Yanbian, NE China: Constraints from Geochronology and Isotopic Analysis

AbstractThe large‐scale Tianbaoshan polymetallic (Cu, Pb, Zn, and Mo) ore district in the Yanbian area of China hosts the Xinxing Pb–Zn, Lishan Pb–Zn, Dongfeng Zn–Cu (Pb), and Dongfengbeishan Mo deposits. We report zircon U–Pb and Re–Os age data, geochemical data, and zircon Hf isotope compositions of granitic rocks associated with the Tianbaoshan ore district. Zircon U–Pb dating of the Xinxing, Lishan, Dongfeng, and Dongfengbeishan granitic rocks, all of which are associated with polymetallic mineralization, yielded weighted mean 206Pb/238U ages of 272.1 ± 2.1, 272.7 ± 1.8, 272.5 ± 2.7, and 199.6 ± 1.4 Ma, respectively. The Dongfengbeishan Mo deposit yields molybdenite Re–Os model ages of 200.3 and 190.3 Ma, an isochron age of 196.5 ± 3.1 Ma, and a weighted mean model age of 194.6 ± 2.6 Ma. This suggests that mineralization took place in the ore district in two stages during the middle Permian and Early Jurassic. The intrusive rocks of these two stages are identified as medium‐ to high‐K calc‐alkaline I‐type granites. They are enriched in large ion lithophile elements, depleted in high field‐strength elements, and contain positive zircon εHf(t) values (+4.6 to +13.9), indicating the primary magma originated from the partial melting of juvenile lower crust. The granitic rocks show geochemical characteristics similar to those of coeval rocks in NE China. Based on the new geochemical data and other regional geological studies, we propose that the mineralization of the two stages of granitic rocks in the Tianbaoshan polymetallic ore district occurred in active continental margin settings, but was controlled by different geotectonic mechanisms: the subduction of the Paleo‐Asian Oceanic Plate in the middle Permian and the subduction of the Paleo‐Pacific Plate in the Early Jurassic.

Geochemistry and zircon U-Pb-Hf isotopes of the late Neoarchean granodiorite-monzogranite-quartz syenite intrusions in the Northern Liaoning Block, North China Craton: Petrogenesis and implications for geodynamic processes

Late Neoarchean weakly foliated to massive granodiorite-monzogranite-quartz syenite intrusions are widely distributed in the Northern Liaoning Block, North China Craton. LA-ICP-MS zircon dating reveals that these rocks were emplaced during 2.56–2.53Ga and underwent regional metamorphism at ∼2.51Ga. These granitoid rocks that contain abundant K-feldspar and high K2O contents with high K2O/Na2O ratios can be subdivided into four lithological types based on their lithological and geochemical characteristics, including porphyritic granodiorites, medium-grained granodiorites, medium-grained monzogranites and porphyritic quartz syenitic gneisses. The porphyritic granodiorites contain biotite as the main mafic mineral, and are characterized by high (La/Yb)N (16–60) and Sr/Y (46–93) ratios, low Yb and Y contents, and weakly positive Eu anomalies for most samples. They also show depletion of Nb, Ta and Ti in the primitive mantle-normalized spidergrams, with zircon εHf(t2) values of +3.8 to +6.3, suggesting that they were derived from partial melting of juvenile high-K mafic rocks at the lower crust. The medium-grained granodiorites have major mafic minerals of hornblende and biotite. They display zircon εHf(t2) values (+4.6 to +7.5) similar to the porphyritic granodiorites, and have higher CaO contents, molar CaO/(MgO+FeOtot) ratios and Yb, Y contents, with relatively lower (La/Yb)N (9–34) and Sr/Y (29–43) ratios for most samples. They show weakly negative Eu and Sr anomalies for most samples, with significant Nb, Ta and Ti depletion, indicating that they might be produced by partial melting of calc-alkaline volcanic sources with higher CaO contents at relatively higher levels of the lower crust, compared with the porphyritic granodiorites. The medium-grained monzogranites show the highest SiO2 contents (69.71–77.64wt%), strongly to moderately fractionated REE patterns ((La/Yb)N=25–69 for most samples), and weakly positive to significantly negative Eu and Sr anomalies, with zircon εHf(t2) values of +2.4 to +4.7, indicating that they may be produced by partial melting of metagreywackes at middle to lower crustal levels. The porphyritic quartz syenitic gneisses exhibit the highest K2O concentrations of 5.11–7.05wt% and are attributed to the shoshonitic series, and they are featured by high (La/Yb) ratios (34–45), and positive Eu and Sr anomalies, with zero-straddling zircon εHf(t2) values (−2.4 to +4.1). We consider that they might be derived from partial melting of a mixed source of juvenile mafic rocks and ancient continental materials in the deep lower crust.Integrating the available data from the previous studies, we conclude that a late Neoarchean active continental margin setting can account for the generation of these granodiorite-monzogranite-quartz syenite intrusions.

Geochronology and geochemistry of the Paleoproterozoic Yinyugou Group in the southern North China Craton: Implications for provenance and tectonic evolution

The Paleoproterozoic Yinyugou Group in the southern North China Craton is a littoral facies sedimentary sequence which is mainly composed of feldspar quartzites and mica schists with minor carbonate rocks. U-Pb isotopic dating of detrital zircons bracket the depositional age of the Yinyugou Group at ∼2.43–2.30Ga. Chemical Index of Alteration (CIA) values of most feldspar quartzite and schist samples are lower than 75, indicating generally weak to moderate source weathering conditions for the Yinyugou rocks. The majority of the studied Yinyugou samples give a large range of Index of Compositional Variability (ICV) values of 0.93–2.59, revealing a chemically immature source which deposited in an active continental margin setting. Chondrite-normalized REE patterns and discrimination diagrams involving La, Th, Hf, Co and Sc reveal that the Yinyugou Group dominantly received detritus from felsic sources with minor contribution from mafic igneous rocks. U-Pb ages of detrital zircons from the Yinyugou Group can be divided into two groups: 2.90–2.40Ga and 3.50–3.20Ga, with a predominant age peak at ∼2.50Ga. The 3.50–3.20Ga detrital zircons have negative εHf(t) values ranging from −11.2 to −3.8 and old crustal model ages (TDMC) of 4246 to 3909Ma, indicating a mature crustal source. Most ∼2.50Ga zircons show sub-depleted mantle positive εHf(t) values and TDMC ages mainly ranging from 3.10 to 2.70Ga, representing the time when juvenile materials were extracted from the depleted mantle. Taking all these data into account, the Yinyugou Group was most likely deposited in a back-arc setting. In combination with previous studies on other Paleoproterozoic volcanic-sedimentary sequences and magmatic rocks in the NCC, a cycle of subduction (collision)-extension (rift)-subduction processes is proposed to explain the tectonic evolution of the NCC during Paleoproterozoic.

Contrasting mantle-crust melting processes within orogenic belts: Implications from two episodes of mafic magmatism in the western segment of the Neoproterozoic Jiangnan Orogen in South China

Partial melting of crust and mantle is the dominant mechanism controlling Earth’s differentiation. The two melting processes are commonly associated because mantle-derived magmas could simultaneously provide heat and volatiles to promote partial melting of crust. To explore how the two melting regimes interact during the evolution of orogenic belts, we evaluated the petrogenesis and possible relationships with the Rodinia supercontinent of the two episodes of Neoproterozoic mafic magmatism in the western segment of the Jiangnan Orogen in South China. The first episode of mafic magmatism took place at ca. 830Ma in the Baotan area and was associated with widespread peraluminous granitoids. In contrast, the second formed at ca. 770Ma in the Longsheng area without significant granitic magmatism. The ca. 770Ma Longsheng mafic-ultramafic rocks show radiogenic and coupled Nd [εNd(t) values of −1.3 to +3.1)] and Hf isotopes [εHf(t) values of +2.6 to +6.7], whereas the ca. 830Ma Baotan mafic rocks show unradiogenic and decoupled Nd-Hf compositions [εNd(t)=−8.3 to −0.9, εHf(t)=−6.3 to −2.0]. Crustal contamination cannot result in the enriched isotopic features and the arc-like (such as strong deletions of Nb and Ta) geochemistry for the Baotan rocks because of the absence of correlations of Nd isotopes with incompatible elements and SiO2 contents, but crust-derived components may have been incorporated into the mantle source (<60km) by oceanic subduction. Geochemical modeling suggests that the Longsheng mafic-ultramafic rocks with steep rare earth element patterns formed from a deeper (deep to 120km) mantle source with little crustal contamination. We suggest that the Baotan mafic rocks formed in an active continental margin setting where a lot of volatiles can be released from the underplating mafic magmas to facilitate further crustal melting to generate the associated granitoid rocks. In contrast, the Longsheng rocks formed in a post-orogenic extensional setting where mafic magmas were apt to ascend so that extensive crustal melting was prohibited. The transition between the two episodes of mantle-derived magmas shed light on the secular evolution of lithospheric mantle in the western segment of the Jiangnan Orogen. The diverse mantle-crust melting processes in orogenic belts can serve as an indicator of tectonic transition of Precambrian orogenic belts in the history of the earth.

The origin and tectonic significance of the volcanic rocks of the Yeba Formation in the Gangdese magmatic belt, South Tibet

Zircon U-Pb geochronology, Hf isotope and whole-rock geochemistry were performed on the tuffs of the Yeba Formation in the Gangdese magmatic belt, South Tibet. The results are used to detail the age, source nature and tectonic processes that led to the formation of the Gangdese belt. Dating results indicate that the rhyolitic-andesitic tuffs were formed at 174-170 Ma. Positive and variable zircon e Hf(t) values of the rhyolitic tuffs reveal that the source was dominated by juvenile material, however, experienced crustal contamination. The basaltic tuffs have low HREEs, high contents of compatible elements (V and Cr) and no Eu anomaly. In contrast, the rhyolitic-andesitic tuffs show low compatible trace elements, depletion in Eu but enrichment in incompatible elements (Rb, Zr and Hf). According to the discrimination diagrams of P2O5-SiO2 and Th-Rb, the rhyolitic-andesitic tuffs show a close affinity to I-type granitoids. Moreover, these tuffs are marked by significant depletion in Nb, Ta and Ti, plotted in calc-alkaline field, and with the andesitic-rhyolitic tuffs falling into an active continental margin setting. We suggest that these tuffs of the Yeba Formation were probably generated in an active continental margin above the northward subduction of the Neo-Tethyan oceanic lithosphere.

Permian copper–polymetallic mineralization in the southern Great Xing’an Range, Northeast China: the Daolundaba copper–polymetallic deposit example

ABSTRACTThe Daolundaba Cu–polymetallic deposit is a newly discovered Cu–W–Sn deposit on the western slopes of the southern Great Xing’an Range, and its mineralization was related to an early Permian coarse-grained biotite granite. However, there is little information on the age of formation of the deposit. In this article, we present the results of our investigation into the age of the Daolundaba Cu–polymetallic deposit, which involved the selection of chalcopyrite and pyrrhotite samples for Rb–Sr isochron dating. A Rb–Sr isochron defined by the chalcopyrite samples yielded a Rb–Sr isochron age of 290.0 ± 11 Ma (MSWD = 1.2) with an initial Sr isotopic composition (ISr) of 0.71446. The pyrrhotite samples yielded a Rb–Sr isochron age of 283.0 ± 2.6 Ma (MSWD = 1.16) with an initial Sr isotopic composition (ISr) of 0.71447. The Rb–Sr isochron age determined from the chalcopyrite and pyrrhotite is 282.7 ± 1.7 Ma (MSWD = 1.13). These results indicate that the Daolundaba Cu–polymetallic deposit formed during the early Permian (282.7–290.0 Ma). The Rb and Sr contents of the chalcopyrite and pyrrhotite range from ~0.1325 to ~3.6810 ppm and from ~0.1219 to ~9.5740 ppm, respectively, and the initial Sr isotope ratios (ISr) range from 0.71047 to 0.71869, with an average of 0.714723. These isotopic characteristics indicate the ore-forming minerals of the Daolundaba Cu–polymetallic deposit originated mainly from the crust, but with small amounts of mantle material involved. The copper was derived from the associated magma whereas the W and Sn was derived from the surrounding strata. The Permian mineralization of the Xing’an–Mongolia region occurred in an active continental margin setting during subduction of the Palaeo-Asian oceanic plate beneath the Siberian Plate.

Age, tectonic setting, and metallogenic implication of Phanerozoic granitic magmatism at the eastern margin of the Xing’an–Mongolian Orogenic Belt, NE China

The eastern margin of the Xing’an–Mongolian Orogenic Belt is characterised by widespread Phanerozoic granitic magmatism, some of which is closely related to significant ore mineralisation. This paper presents new geochronological, petrogenetic, and tectonic data for selected intrusions. Zircon U–Pb geochronology for five granitoid plutons indicates they were emplaced during the middle–late Permian (264–255Ma) and Cretaceous (106–94Ma), and thus granitic magmatism occurred throughout the Phanerozoic, Permian (268–252Ma), Early–Middle Triassic (248–240Ma), Early Jurassic (183Ma), and Cretaceous (112–94Ma). The Permian granitoids consist of monzogranite, granodiorite, tonalite, and quartz diorite, characterised by enrichment in Na2O (3.60–4.72wt.%), depletion in K2O (0.97–2.66wt.%), and a negative correlation between P2O5 and SiO2. Together with the presence of hornblende, these geochemical features are indicative of an I-type affinity. The Permian granitic magmatism is associated with quartz-vein-type tungsten deposits (252Ma; unpublished Sm–Nd isochron age), which formed in an active continental margin setting related to subduction of the Palaeo-Asian Ocean. The Cretaceous quartz diorites have an adakitic affinity, having relatively high Sr (374–502ppm), low Yb (0.51–0.67ppm) and Y (8.7–10.7ppm), and high Sr/Y (39.4–46.8) and (La/Yb)N values (16.2–34.7), suggesting that they were related to the partial melting of subducted oceanic crust. In addition, they are associated with porphyry Au–Cu deposits. We conclude that the Cretaceous granitic rocks and associated porphyry Au–Cu mineralisation occurred in an extensional tectonic setting related to the subduction of the Palaeo-Pacific Plate beneath the Eurasian Plate. In addition, the large-scale Early–Middle Triassic syn-collisional granite belt at the eastern margin of the Xing’an–Mongolian Orogenic Belt extends from the middle of Jilin Province to the Wangqing–Hunchun region, constraining the timing of the final collision between the North China Craton and the Jiamusi–Khanka Massif, and suggesting that the Xra Moron River–Changchun Suture likely extends eastward into the eastern Hunchun region. This collision caused the Middle Triassic mesothermal lode gold mineralisation.

Plume magmatism in the northeastern part of the Altai–Sayan region: Stages, source compositions, and geodynamics (exemplified by the Minusinsk Depression)

The results of geochronological (U–Pb, Ar–Ar), geochemical, and isotopic (Sr, Nd) studies of the Ordovician and Devonian mafic volcanic–subvolcanic rock associations of the Minusinsk Depression are presented. The obtained ages of magmatic associations and the basite composition, considering previous studies, witness to the impact of two mantle plumes different in age (Late Cambrian–Ordovician and Devonian) on suprasubduction rock complexes in active continental margin settings.

Geochemistry of Cretaceous Mudrocks and Sandstones from Douala Sub-Basin, Kumba Area, South West Cameroon: Constraints on Provenance, Source Rock Weathering, Paleo-Oxidation Conditions and Tectonic Environment

Geochemical data of fifteen Cretaceous sediment samples from Kumba area in the Douala sub-basin are presented to determine the provenance, source rock weathering, tectonic setting and paleo-oxidation conditions of the depositional setting of these rocks. For this purpose, the whole-rocks were analyzed for their major and trace element, including rare earth elements (REEs), contents by ICP-AES and ICP-MS methods respectively. On the basis of their major element composition, the rocks have been classified mainly as Fe-shale, shale, arkose and Fe-sandstone. For the provenance, the plot of Zr vs. TiO2, Y/Ni vs. Cr/V, TiO2 vs. Al2O3 diagrams, high LREE/HREE ratios (5.84 to 20.91) and negative and positive Eu anomalies (Eu/Eu*= 0.87 to 1.62) suggest that the studied rocks were mainly derived from felsic igneous rocks with lesser contribution of mafic components. The higher values of paleo-weathering indices such as Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW), ranging from 79.63 to 99.90 and 87.57 to 99.92 respectively, suggest that the sediments and their potential source rocks experienced intense weathering. Whereas the variable Chemical Index of Alteration (CIA) values (63.01 to 99.50) coupled with high content of K2O (up to 7.5) in most samples could be indicative of K-addition diagenetic processes. The A-CN-K plot also suggests a possible K-addition. The redox conditions during the sedimentation were suboxic to oxic as evidenced by the Ce anomalies of 0.88 to 1.03. This is also confirmed by the values of Ni/Co (less than 7), U/Th (less than 1.25) and Cu/Zn (mostly less than 1). Tectonic discrimination diagrams (e.g., SiO2-K2O/Na2O and Th-Sc-Zr/10) show that these rocks were deposited mainly in an active continental margin setting, and in various tectonic environments. This reflects probably the recycling effect experienced by the samples studied.