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.     

Sr–Nd isotope data of basement rocks from the northernmost argentine Precordillera and its implications for the early Paleozoic evolution of SW Gondwana margin

The Precordillera terrane (Cuyania) in western Argentina is commonly accepted as an exotic fragment derived from Laurentia in the Early Paleozoic. Evidence supporting such an interpretation is manly based on similarities in the sedimentary cover successions and their paleontological content. Little is known about the basement of the Precordillera terrane. Its isotopic characterization is essential to better constrain the present areal distribution of the terrane and it may provide more insight into the pre-rifting evolution of the Precordillera terrane along the Iapetan margin of Laurentia. We present new Sr and Nd isotope data of pre-Late Ordovician meta-igneous rocks from the Río Bonete region in NW Argentina, interpreted as the northernmost extent of the Precordillera. The Nd systematics of the Río Bonete basement rocks including greenschists and metagabbros (εNd₍₄₇₀₎ = +2.14–−0.19; T_DM = 0.99–1.2 Ga), a garnet-amphibolite (εNd₍₄₇₀₎ = −0.53; T_DM = 1.32 Ga) and a quartz-phyllite (εNd₍₄₇₀₎ = −3.83; T_DM = 1.55 Ga), are similar to other pre-Ordovician meta-igneous rocks from Sierra de Umango, Pie de Palo and the Ullum xenoliths, usually interpreted as the basement of the Precordillera terrane. Nd model ages around 1.2 Ga are also typical from the Mesoproterozoic Grenvillian basement of southern North America, currently exposed in the Llano region. In addition, the greenschists and metagabbros show a robust correlation with the Late Neoproterozoic Catoctin volcanics in the central Appalachians. The Sr isotope data (when not disturbed) also supports this novel interpretation and suggests the presence of the Blue Ridge rifting event in Precordillera. According to our interpretation, some lithotypes included within the basement complex of the Río Bonete area belonged to the basement of the Precordillera terrane supporting previous correlation between both regions.     

Geochemistry of the volcanic rocks from Bioko Island(“Cameroon Hot Line”): Evidence for plume-lithosphere interaction

Bioko Island(3008 m a.s.l) is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine(83% Fo 87%) and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase(An62-67Ab35-32Or3-1). Major element variation diagrams show an increase in Si O2, Al2O3, Na2 O and K2 O with increasing Mg O for the studied rock groups. The rocks are characterized by low(86Sr/87Sr)i ratios(0.70320e0.70406), high 3Nd(t) values(2.56e4.33) and high(206Pb/204Pb)i ratios(20.032e20.035) values.Basalts are enriched in LILE and LREE, and have(Hf/Sm)N? 0.57e1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.     

K-Ar dating,whole-rock and Sr-Nd isotope geochemistry of calc-alkaline volcanic rocks around the Gümüşhane area: implications for post-collisional volcanism in the Eastern Pontides,Northeast Turkey

Volcanic rocks from the Gümü?hane area in the southern part of the Eastern Pontides (NE Turkey) consist mainly of andesitic lava flows associated with tuffs, and rare basaltic dykes. The K-Ar whole-rock dating of these rocks range from 37.62?±?3.33 Ma (Middle Eocene) to 30.02?±?2.84 Ma (Early Oligocene) for the andesitic lava flows, but are 15.80?±?1.71 Ma (Middle Miocene) for the basaltic dykes. Petrochemically, the volcanic rocks are dominantly medium-K calc-alkaline in composition and show enrichment of large ion lithophile elements, as well as depletion of high field strength elements, thus revealing that volcanic rocks evolved from a parental magmas derived from an enriched mantle source. Chondrite-normalized rare-earth element patterns of the volcanic rocks are concave upwards with low- to-medium enrichment (La_CN/Lu_CN?=?3.39 to 12.56), thereby revealing clinopyroxene- and hornblende-dominated fractionations for andesitic-basaltic rocks and tuffs, respectively. The volcanic rocks have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70464 to 0.70494) and εNd_(i) values (+1.11 to +3.08), with Nd-model ages (T_DM) of 0.68 to 1.02 Ga, suggesting an enriched lithospheric mantle source of Proterozoic age. Trace element and isotopic data, as well as the modelling results, show that fractional crystallization and minor assimilation played an important role in the evolution of the volcanic rocks studied. The Eocene to Miocene volcanism in the region has resulted from lithospheric delamination and the associated convective thinning of the mantle, which led to the partial melting of the subduction-metasomatized lithospheric mantle.     

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H₂O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).     

Geochemistry and geochronology of Upper Permian–Upper Triassic volcanic rocks in eastern Jilin Province,NE China: implications for the tectonic evolution of the Palaeo-Asian Ocean

We present zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotope results for the Upper Permian–Upper Triassic volcanic rocks to constrain the timing of the final closure of the eastern segment of the Palaeo-Asian Ocean. The volcanic rocks were mainly collected from the Yanbian area in eastern Jilin Province, northeastern China. The zircon U–Pb dating results indicate that the samples can be classified as Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) and Upper Triassic dacites (ca. 216 Ma). The whole-rock geochemical results indicate that the rocks predominately belong to the medium-K and high-K calc-alkaline series. The basalts are enriched in large ion lithophile elements (LILEs, e.g. Ba and K) and depleted in high field strength elements (HFSEs, e.g. Nb and Ta), with weak positive Eu anomalies. The dacites are enriched in LILEs (e.g. Rb, Ba, Th, and K) and light rare earth elements (LREEs) and marked depletion in some HFSEs (e.g. Nb, Ta, and Ti), with significant negative Sr, P, and Eu anomalies. Moreover, the Upper Permian–Lower Triassic basalts have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7037–0.7048) and high ε_Nd values (4.4–5.4). In contrast, the Upper Triassic dacites possess relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7052) compared with their low ε_Nd values (1.4). The basaltic magma likely originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids, and the felsic magmas likely originated from the partial melting of a dominantly juvenile source with a minor component of ancient crust. Taken together, the Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) are arc basalts that formed in an active continental margin setting, and the Upper Triassic dacites (ca. 216 Ma) are A-type granitic rocks that formed in an extensional setting. Therefore, the final closure of the Palaeo-Asian Ocean occurred during the Middle–Late Triassic.     

Geochronological and geochemical investigation of the late Mesozoic volcanic rocks from the Northern Great Xing’an Range and their tectonic implications

Precise age dating and systematic geochemical investigation were performed on the widely distributed late Mesozoic volcanic rocks in the North Great Xing’an Range (NGXR). In situ zircon U–Pb age measurements indicate that the volcanic eruption commenced from 163 Ma ago and lasted to 113 Ma ago. These volcanic rocks show a wide range in compositions from basaltic andesite, trachyandesite and trachydacite to rhyolite. The majority of volcanic rocks exhibit high-K calc-alkaline affinity with the subordinate showing shoshonitic features. The volcanic rocks are characterized with low MgO contents, LILE, LREE enrichment and HFSE depletion. Elemental and isotopic variations suggest that fractional crystallization with the predominant removal of olivine and orthopyroxene play an important role in the evolution of magma. Most of the basic and intermediate volcanic rocks are generated from an enriched lithospheric mantle which was metasomatised by fluids released from subducted slabs during the closure of the Paleo-Asian and Mongol-Okhotsk oceans. The generation of such widely distributed volcanic rocks was caused by the decompressional partial melting of enriched lithospheric mantle in an extensional regime, which resulted from the gravitational collapse and upwelling of asthenosphere after the final closure of the Mongol-Okhotsk oceans in late Jurassic and from then the Mongol-Okhotsk orogen turned into the post-orogenic stage.     

Origin of megacrysts in volcanic rocks of the Cameroon volcanic chain – constraints on magma genesis and crustal contamination

Lavas of the Biu and Jos Plateaus, Northern Cameroon Volcanic Line (CVL), contain abundant genetically related megacrysts of clinopyroxene, garnet and subordinately plagioclase, ilmenite and amphibole. P, T-estimates of crystallization for the primitive group of cpx and gnt megacrysts are 1.7–2.3 GPa and ~1,400 °C. Because crustal thickness in these areas is only ~30 km (~0.9 GPa), megacrysts must have formed within the lithospheric mantle. Primitive Biu and Jos lavas are isotopically heterogeneous in Sr-Nd isotope space (⁸⁷Sr/⁸⁶Sr=0.70285–0.70360 and _Nd=7.5–4.6). Biu Plateau megacrysts overlap the range of Biu lavas in Sr-Nd isotope composition, indicating that crustal contamination of Biu lavas was minor. Jos Plateau lavas are isotopically more enriched than their associated megacrysts. Therefore an additional contamination of Jos lavas due to assimilation of continental crust (~5%) or enriched shallow lithospheric mantle is indicated. Lavas of Biu and Jos Plateau do not reflect simple fractionation or equilibrium crystallization products, but instead reflect mixing between primary melts and their fractionated derivatives.Editorial Responsibility: I. Carmichael     

Sources of primitive alkaline volcanic rocks from the Central European Volcanic Province (Rhön,Germany) inferred from Hf,Os and Pb isotopes

Basanites and nephelinites from the Tertiary Rhön area (Germany), which are part of the Central European Volcanic Province (CEVP), have high MgO, Ni and Cr contents and prominent garnet signatures indicating that they represent near-primary magmas formed by melting of a CO₂-bearing peridotitic mantle source at high pressure. The Pb and Hf isotope (and previously published Nd and Sr isotope) ratios of the Rhön lavas are rather uniform, whereas the Os isotope composition is highly variable. For the most primitive basanites, Pb, Os and Hf isotope compositions fall within the range of enriched MORB and some OIB. Other basanites and nephelinites with low Os concentrations have distinctly more radiogenic Os (¹⁸⁷Os/¹⁸⁸Os: 0.160–0.469) isotope compositions, which are inferred to originate from crustal contamination. The samples with the highest Os concentrations have the lowest Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os_(23 Ma): 0.132–0.135), and likely remain unaffected by crustal contamination. Together with their fairly depleted Sr, Nd and Hf isotope ratios, the isotopic composition of the Rhön lavas suggests derivation from an asthenospheric mantle source. Prominent negative K and Rb anomalies, however, argue for melting amphibole or phlogopite-bearing sources, which can only be stable in the cold lithosphere. We therefore propose that asthenospheric melts precipitated at the asthenosphere-lithosphere thermal boundary as veins in the lithospheric mantle and were remelted or incorporated after only short storage times (about 10–100 million years) by ascending asthenospheric melts. Due to the short residence time incorporation of the vein material imposes the prominent phlogopite/amphibole signature of the Rhön alkaline basalts but does not lead to a shift in the isotopic signatures. Melting of the lithospheric mantle cannot strictly be excluded, but has to be subordinate due to the lack of the respective isotope signatures, in good agreement with the fairly thin lithosphere observed in the Rhön area. The fairly radiogenic Pb isotope signatures are expected to originate from melting of enriched, low melting temperature portions incorporated in the depleted upper (asthenospheric) mantle and therefore do not require upwelling of deep-seated mantle sources for the Rhön or many other continental alkaline lavas with similar Pb isotope signatures.     

Petrology of volcanic and plutonic rocks from the Uimen-Lebed’ terrain,Gorny Altai

The Uimen-Lebed’ volcanoplutonic terrane is located at the junction of the Gorny Altai, Gornaya Shoriya, and western Sayan structures and is part of the Devonian-Early Carbonaceous Salair-Altai volcanoplutonic belt. The volcanic facies of the terrane composes the contrasting Nyrnin-Sagan Group, which includes basalt-basaltic andesite and basalt-rhyolite associations. The plutonic facies makes up the multiplet Elekmonar Group, which includes two independent complexes: monzogabbro-monzodiorite-granodiorite-granite and granodiorite-granite-leucogranite. The volcanic and plutonic rocks are asymmetrically distributed: volcanic sequences fill inherited depressions in the eastern part of the terrane, whereas plutonic complexes are located in its western part at the fault system branching from the transregional Kuznetsk-Teletsk-Kurai fault zone. The basalts of the Nyrnin-Sagan Group show geochemical signatures of both suprasubduction and rift-related rocks. The evolution of basaltoid magmatism reflects the formation and development of a suprasubduction mantle wedge in the inner part of an active continental margin accompanied by the influence of an intraplate mantle source. The silicic volcanism was generated under lower crustal conditions (P > 10 kbar) at the expense of metabasic materials and was accompanied by the influx of potassium into the anatectic zones. The gabbroids of the Elekmonar Group show suprasubduction geochemical features and no signatures of rift-related structures. The composition of the Elekmonar granitoids indicates significantly shallower (compared with the silicic volcanics) depths of their generation. The Uimen-Lebed’ volcanoplutonic terrane in the northeastern part of Gorny Altai was formed in the inner part of an active continental margin of the Andean type. Its magmatic complexes were formed over a considerable time range, from the early Emsian, when the formation of the active continental margin began, to the end of the Eifelian or, more likely, the beginning of the Givetian stage.     

Zircon U-Pb geochronology,Lu-Hf isotope systematics,and geochemistry of bimodal volcanic rocks and associated granitoids from Kotri Belt,Central India: Implications for Neoarchean–Paleoproterozoic crustal growth

The Bastar Craton of Central India has a thick sequence of volcano–sedimentary rocks preserved in Kotri–Dongargarh belt that developed on a tonalite-trondhjemite-granodiorite (TTG) basement followed upwards by the Amgaon, Bengpal, Bailadila, and Nandgaon Groups of rocks. Here, we report the U-Pb geochronology and Lu-Hf isotope systematics and whole rock geochemistry of volcanic rocks and associated granitoids belonging to the Pitepani basalts, Bijli rhyolites, and Dongargarh granite in the Nandgaon Group of the Kotri belt. The volcanic rocks of the Nandgaon Group are bimodal in nature in which the basalts exhibit intergranular, porphyritic to spherulitic texture composed of pyroxenes, plagioclase, tremolite, actinolite, and chlorite ± Fe oxides. The rhyolites display porphyritic texture consisting of K-feldspar, quartz, and plagioclase as phenocrysts. The associated porphyritic granitoids have K-feldspar, microcline, plagioclase, and biotite phenocrysts within a groundmass of similar composition. The bimodal suite displays LILE, LREE enrichment, and HFSE depletion with significant negative Nb-Ta anomalies combined with slightly fractionated REE patterns in the basalts and highly fractionated patterns and prominent negative Eu anomalies in the rhyolites endorsing their generation in an island-arc/back-arc tectonic setting. The geochemical features of the associated granitoids indicate that these are potassic and classify as within-plate A-type granites. Zircons from the basalts show clear oscillatory zoning in their CL images. They cluster as a coherent group with ²⁰⁷Pb/²⁰⁶Pb spot ages ranging from 2446 to 2522 Ma and weighted mean age of 2471 ± 7 Ma. Zircons from the rhyolite samples are subhedral to euhedral and show simple oscillatory zoning with some heterogeneous fractured domains. The data from two samples define upper intercept ages of 2479 ± 13 Ma and 2463 ± 14 Ma. Zircon grains in the granite show clear oscillatory zoning and their U-Pb data define an upper intercept age of 2506 ± 50 Ma. The Lu-Hf isotopic data on the zircons from the basalts show initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.280925 to 0.281018. Their ε_Hf(t) values are in the range of − 10.0 to − 6.7. The Hf-depleted model ages (T_DM) are between 3038 Ma and 3171 Ma, and Hf crustal model ages (T_DM^C) vary from 3387–3589 Ma. The zircons from the rhyolites show initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.280919 to 0.281020 and from 0.281000 to 0.281103, respectively, with ε_Hf(t) values varying from − 10 to − 6.4 and from − 7.5 to − 3.9. Among these, one sample shows T_DM between 3038 Ma and 3182 Ma, and T_DM^C varies from 3377 to 3596 Ma, whereas the other sample shows ages of 2925 Ma and 3072 Ma with T_DM^C varying from 3208 to 3432 Ma. The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the granites range from 0.280937 to 0.281062 with ε_Hf(t) values of − 8.8 to − 4.3. The T_DM shows a range of 2979 Ma and 3170 Ma, and T_DM^C varies from 3269 to 3541 Ma. The predominant negative ε_Hf(t) values of zircons from these rocks suggest that the source material was evolved from the Paleoarchean crust. The geological, geochemical, and geochronological evidence suggests coeval tectonic and magmatic episodes of volcanic and plutonic activity in an island-arc setting where the arc migrated toward the continental margin and played a significant role in the Neoarchean–Paleoproterozoic crustal growth of the Kotri belt of Central India.     

Recycling of crustal materials and implications for lithospheric thinning:Evidence from Mesozoic volcanic rocks in the Hailar–Tamtsag Basin,NE China

Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar-Tamtsag Basin,northeast China,provide important insights into the recycling processes of crustal materials and their role in late Mesozoic lithospheric thinning.The Late Jurassic Nb-rich basaltic andesites(154 ± 4 Ma) are enriched in large-ion lithophile and light rare earth elements,slightly depleted in high-field-strength elements,and have high TiO_2,P_2 O_5,and Nb contents,and(Nb/Th)PM and Nb/U ratios,which together with the relatively depleted Sr-Nd-Hf isotopic compositions indicate a derivation from a mantle wedge metasomatized by hydrous melts from subducted oceanic crust.The Early Cretaceous high-Mg adakitic volcanic rocks(129-117 Ma) are characterized by low Y and heavy rare earth element contents,and high Sr contents and Sr/Y ratios,similar to those of rocks derived from partial melting of an eclogitic source.They also have high Rb/Sr, K_2 O/Na_2 O,and Mg#values,and high MgO, Cr, and Ni contents.These geochemical features sugge st that the adakitic lavas were derived from partial melting of delaminated lower continental crust,followed by interaction of the resulting melts with mantle material during their ascent Our data,along with available geological,paleomagnetic,and geophysical evidence,lead us to propose that recycling of Paleo-Pacific oceanic crustal materials into the upper mantle due to flat-slab subduction and rollback of the Paleo-Pacific Plate during the late Mesozoic likely provided the precondition for lithospheric thinning in northeast China,with consequent lithospheric delamination causing recycling of continental crustal materials and further lithospheric thinning.     

Geochemistry of mafic rocks and melt inclusions and their implications for the heat source of the 14.0°S hydrothermal field,South Mid-Atlantic Ridge

Fresh rocks sampled from the 14.0°S hydrothermal field of the South Atlantic Ridge can be divided into two categories： olivine-gabbro and basalt. The olivine-gabbro is composed mainly of three types of minerals： olivine, clinopyroxene and plagioclase, while a multitude of melt inclusions occur in the plagioclase phenocrysts of the basalts. We analyzed the whole-rock, major and trace elements contents of the basaks, the mineral chemistry of phenocrysts and melt inclusions in the basalts, and the mineral chemistry of olivine-clinopyroxene-plagioclase in the olivine-gabbro, then simulated magma evolution within the crust using the COMAGMAT program. The whole-rock geochemistry shows that all the basalts exhibit typical N-MORB characteristics. In addition, the mineral chemistry characteristics of the olivine-gabbro （low-Fo olivine, low-Mg# clinopyroxene, high-TiO2 clinopyroxene, low-An plagioclase）, show that strong magma differentiation occurred within the crust. Nevertheless, significant discrepancies between those minerals and phenocrysts in the basalts （high-Fo olivine, high-An plagioclase） reflect the heterogeneity of magma differentiation. High Mg# （-~0.72） melt inclusions isobaric partial crystallization simulations suggest that the magma differentiation occurred at the depth shallower than 13.03 km below the seafloor, and both the vertical differentiation column shows distinct discrepancies from that of a steady-state magma chamber. Instead, a series of independent magma intrusions probably occurred within the crust, and their corresponding crystallized bodies, as the primary high-temperature thermal anomalies within the off-axis crust, probably act as the heat source for the development of the 14.0°S hydrothermal system.     

Geochemistry of sedimentary rocks from Permian–Triassic boundary sections of Tethys Himalaya: implications for paleo-weathering,provenance, and tectonic setting

The geochemical characteristics of two sections—the Permian–Triassic boundary (PTB) Guryul Ravine section, Kashmir Valley, Jammu and Kashmir, India; and the Attargoo section, Spiti Valley, Himachal Pradesh, India—have been studied in the context of provenance, paleo-weathering, and plate tectonic setting. These sections represent the siliciclastic sedimentary sequence from the Tethys Himalaya. The PTB siliciclastic sedimentary sequence in these regions primarily consists of sandstones and shales with variable thickness. Present studied sandstones and shales of both sections had chemical index of alteration values between 65 and 74; such values reveal low-to-moderate degree of chemical weathering. The chemical index of weathering in studied samples ranged from 71 to 94, suggesting a minor K-metasomatism effect on these samples. Plagioclase index of alteration in studied sections ranged from 68 to 92, indicating a moderate degree of weathering of plagioclase feldspars. The provenance discriminant function diagram suggests that the detritus involved in the formation of present studied siliciclastic sedimentary rocks fall in quartzose sedimentary and felsic igneous provenances. These sediments were deposited in a passive continental margin plate tectonic setting according to their location on a Si₂O versus K₂O/Na₂O tectonic setting diagram.     

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.     

Geochemistry,geochronology, and Sr–Nd isotopic compositions of Permian volcanic rocks in the northern margin of the North China Block: implications for the tectonic setting of the southeastern Central Asian Orogenic Belt

International Journal of Earth Sciences - The southeastern part of the Central Asian Orogenic Belt (CAOB), which records the collision of the North China Block (NCB) with the South Mongolian...     

Continental rift and oceanic protoliths of mafic–ultramafic rocks from the Kechros Complex,NE Rhodope (Greece): implications from petrography,major and trace-element systematics,and MELTS modeling

The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO₂ content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO₂ loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.     

Geochemistry of Early Cretaceous volcanic rocks in the Northeastern Great Xing’an Range,northeast China and implication for geodynamic setting

ABSTRACT

The mechanism that triggered large-scale Late Mesozoic magmatism in the northeastern Great Xing’an Range (NE GXAR) is strongly controversial. In this paper, we present whole rock geochemistry and zircon trace element, U-Pb and Hf isotopic data on the volcanic rocks in the Longjiang and Guanghua formations in the northeastern Xing’an Block. Zircons with ages of 120–119 Ma indicate that these volcanic rocks were formed in the Early Cretaceous. Combined with previous data, it is clear that volcanic rocks in the NE GXAR erupted between 128 and 108 Ma. The andesite samples of the Longjiang Formation show high contents of Al₂O₃, CaO, and MgO, significant negative Nb, Ta, and Ti anomalies; ε_Hf (t) values of zircons from the andesite sample vary from +4.13 to +7.67, indicating an enriched mantle source. The rhyolites of the Guanghua Formation show high SiO₂ and K₂O concentrations, low P₂O₅, MgO, Cr, and Ni contents and Mg# values. The positive ε_Hf (t) values (+5.72 to +10.58) with two-stage Hf model ages ranging from 939 to 701 Ma indicate that the rhyolites are derived from the partial melting of basaltic lower crust. Combined with the regional geological evolution, we conclude that the generation of the Early Cretaceous volcanic rocks in the NE GXAR might be triggered by the dehydration, disintegration, and foundering of the Mongol-Okhotsk Oceanic flat-slab and the subsequent upwelling of the asthenosphere.     

Chronostratigraphic basin framework for Palaeoproterozoic rocks (1730–1575 Ma) in northern Australia and implications for base‐metal mineralisation

Sequence‐stratigraphic interpretations of outcrop, drillcore, wireline and seismic datasets are integrated with SHRIMP zircon and palaeomagnetic determinations to provide a detailed chrono‐stratigraphic basin framework for the base‐metal‐rich Palaeoproterozoic rocks of the southern McArthur, Lawn Hill and Mt Isa regions. The analysis forms a basis for future correlations across northern Australia. Nine second‐order unconformity‐bounded supersequences are identified. Supersequences have a duration of 10–20 million years; some hitherto‐unrecognised unconformity surfaces record up to 25 million years of missing rock record. The second‐order supersequences contain a series of nested third‐, fourth‐ and fifth‐order sequences many of which can be correlated across the Mt Isa, Lawn Hill and southern McArthur regions. The analysis relates accommodation history to major intraplate tectonic events evident on the apparent polar wander path for northern Australia. Major tectonic events at approximately 1735 Ma, 1700 Ma, 1670 Ma, 1650 Ma, 1640 Ma, 1615 Ma, 1600 Ma and 1575 Ma impacted on accommodation rates and basin shape in northern Australia. Sub‐basin depocentres, the hosts for major sulfide mineralisation, are attributed to reactivated faults that controlled local subsidence. Pb/Pb model ages of 1653 Ma, 1640 Ma and 1575 Ma for the Mt Isa, McArthur River and Century Pb–Zn–Ag deposits, suggest that changes to intraplate stresses at tectonic events of like age resulted in the migration of metal‐bearing fluids into the sub‐basins. A Pb/Pb model age of 1675 for the Broken Hill deposit suggests that intraplate stresses manifest in northern Australia also affected rocks of similar age further south. Magmatic events close to 1700 Ma (Weberra Granite) and 1675 Ma (Sybella Granite) coincide with times of regional incision and the formation of supersequence‐bounding unconformity surfaces.