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.     

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     

Composition and geodynamic setting of the volcanic rocks from ophiolites of the Ust’-Belaya Mountains,Chukchi Peninsula

In addition to ophiolites in the structure of the Otrozhnaya sheet, the igneous rocks were established within the Middle Devonian-Lower Carboniferous tuffaceous-terrigenous complex earlier considered to be the cover of the ophiolite association. In order to establish their geodynamic formation setting, the geochemical study of igneous rocks was conducted. The volcanic rocks from the ophiolite complex are similar to MORB; subvolcanic rocks of tuffaceous-terrigenous complex have a suprasubduction origin. An abundance of pyroclastic rocks and the type of sediments allow us to conclude about their formation in an island arc setting. The existence of the Middle Devonian-Lower Carboniferous island arc complex within the Ust’-Belaya Mountains gives rise to continue the Koni-Taigonos arc inside the region and testifies to its subsistence in the Devonian.     

New isotope–geochemical and mineralogical data on the ultramafic xenoliths in the volcanic rocks of the Kamchatka–Koryak region: Two types of mantle protolith in the modern island-arc system

The paper presents new isotope–geochemical and mineralogical data on mantle xenoliths of the “island-arc” (Avacha, Shiveluch, and Kharchinsky volcanoes) and “within-plate” (Valovayam River, Cape Navarin, and Bakening Volcano areas) types. In terms of paragenesis and mineral composition, the “islandarc” xenoliths correspond to the olivine–plagioclase depth facies, while the “within-plate” xenoliths came from spinel lherzolite to wehrlite facies, which is transitional to the olivine–plagioclase equilibrium. The majority of the “within-plate” xenoliths are enriched in high-field-strength elements (Ti, Nb, Hf, Zr, Yb). The “island-arc” xenoliths in general are depleted in REE, while the “within-plate” xenoliths are enriched in all REE. The former have low Pb isotope ratios, being in isotope equilibrium with lower crustal basites, while most of the latter group are enriched in radiogenic Pb. The island-arc xenoliths are of magmatic origin and were derived from the sublithospheric crust–mantle mixture, while the “within-plate” xenoliths reflect the composition of the asthenospheric mantle source. The primary appearance of the xenoliths is obliterated by secondary recrystallization and metasomatic reworking.     

Mineralogy and petrology of alkaline granophyric xenoliths from the Thorsmörk ignimbrite,southern Iceland

The Thorsmörk ignimbrite, southern Iceland, contains a suite of granophyre xenoliths displaying magmatic or high-temperature sub-solidus mineral assemblages. These granophyres are consanguineous with the erupting comenditic magma. Four types of mineral assemblages are distinguished:

• 1.(A) oligoclase, edenitic hornblende, salitic pyroxene, magnesian biotite, magnetite and sphene;
• 2.(B) oligoclase, manganoan to sodic ferro-augite, fayalite, richterite, ilmenite and magnetite;
• 3.(C) anorthoclase, ferrohedenbergite to aegirine hedenbergite, ilmenite, magnetite and (riebeckite);
• 4.(D) cryptoperthite, aegirine hedenbergite to (aegirine), aenigmatite, arfvedsonite, ilmenite and magnetite.

Geothermometry shows that the xenoliths have crystallized between 900°C and 500°C at moderate oxygen fugacities, just above the FMQ buffer. It is further demonstrated that a hot vapour phase heavily charged with sodium and halogens, played a major role in the late sub-solidus crystallization of the different types.     

Underplating and assimilation–fractional crystallization of Mesozoic intrusions in the Tongling area,Anhui Province,East China: evidence from xenoliths and host plutons

This paper presents new petrographic observations and geochemical and microprobe analyses for the Laomiaojishan, Xiaotongguanshan, and Tianebaodanshan intrusions in the Tongguanshan mineral district, East China. The plutons vary in composition from quartz monzonitic diorite to pyroxene monzonitic diorite, and contain gabbroic to dioritic xenoliths. The Xiaotongguanshan intrusion yields a SHRIMP zircon U–Pb age of 139.5±2.9 Ma, indicating Late Jurassic to Early Cretaceous magmatism in the Lower Yangtze River Valley. Relative to host rocks, the gabbro and diorite xenoliths are low in SiO₂ (52.03–54.61 wt‐%), Al₂O₃ (12.87–14.43 wt‐%), and total alkalis (Na₂O+K₂O; 5.26–6.30 wt‐%), but high in MgO (5.41–11.66 wt‐%); the host rocks have high SiO₂ (59.97–64.44 wt‐%), Al₂O₃ (16.43–17.59 wt‐%), and total alkalis (6.67–8.25 wt‐%), but are low in MgO (1.52–2.50 wt‐%). Concentrations of rare earth elements (REEs) in the xenoliths (165.70–190.40 ppm) are similar to those in the host rocks (166.12–185.95 ppm), although the ratio of light REEs to heavy REEs in the xenoliths (3.39–4.27) is lower than that in the host plutons (4.86–5.94). All of the analysed rocks show similar REE patterns, although the xenoliths display marked positive Eu anomalies and the host rocks show slightly negative Eu anomalies. Values of epsilon Nd (t) ranges from ?4.9 to ?9.9 in the gabbro xenoliths and from ?11.4 to ?11.9 in the host intrusives. Initial ⁸⁷Sr/⁸⁶Sr ratios are 0.7064–0.7073 in the xenoliths and 0.7072–0.7084 in the quartz monzonitic diorite host rocks. Crystallization temperatures of hornblende and plagioclase in the gabbro xenoliths, diorite xenoliths, and host rocks are 816, 773–790, and 664–725°C, respectively, based on an amphibole–plagioclase geothermometer. The pressures recorded by these phases indicate that they formed at depths of 26, 12–15, and 3–4 km, respectively, based on an aluminum‐in‐hornblende geobarometer. The petrological and geochemical features of the analysed intrusions and xenoliths are consistent with their derivation from basic to intermediate‐acidic magmas that possibly formed via a series of complex interactions between underplated, mantle‐derived basaltic magma and varying amounts of middle‐ to lower‐crustal material, followed by assimilation–fractional crystallization.     

Mineral chemical constraints on the petrogenesis of mafic and intermediate volcanic rocks from the Erciyes and Hasandağ volcanoes,Central Turkey

Hasandağ and Erciyes stratovolcanoes, which produced both calc-alkaline and alkaline eruptive products, are the two important volcanic complexes in Central Anatolia. There are three geochemical evolution stages in the history of the Hasandağ strato volcanic complex: (1) Keçikalesi tholeiitic, (2) Hasandağ calc-alkaline and (3) Hasandağ alkaline. Volcanologic and petrologic characteristics of the Hasandağ and Erciyes calc-alkaline series show that water played an important role on the genesis of these rocks. These rocks are phenocryst-rich with vesicular texture, and contain hydrous mineral phases. The approximate pressure and temperature estimates obtained from the mineral chemistry studies of the Hasandağ strato volcanic complex indicate crystallization temperature of 1100 °C with 2.5–3.4 kbar pressure interval for the first stage of Keçikalesi tholeiitic volcanism, and about 850 °C temperatures with 4.3–9.6 kbar pressure intervals for the second stage of Hasandağ calc-alkaline volcanism.The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline, (2) Koçdağ calc-alkaline and (3) Erciyes calc-alkaline. The temperature of Koçdağ alkaline volcanism is 1097–1181 °C and in a range of 5.1–6.7 kbar pressure, for Koçdağ calc-alkaline volcanism 850–1050 °C temperature to 2.0–6.6 kbar pressure interval, and for Erciyes calc-alkaline volcanism about 950 °C temperature, to 3.2–7.9 kbar pressure intervals were calculated. Polybaric origin of magma chambers for calc-alkaline and alkaline rocks and disequilibrium parameters observed in phenocrysts indicate that the rocks were affected by magma mixing processes in crustal magma chambers. The disequilibrium features of amphibole and plagioclase phenocrysts in these rocks point the latent heat in magma chambers and periodic recharging with mafic magma chambers and also show that magmas reequilibrate before the eruption.     

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.     

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.).     

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.     

Olivine and Cr-spinel as indicators of the petrogenesis and partial melting conditions of the high-MgO ultramafic volcanic rocks from NW Ad Dhala Province—Yemen

Acta Geochimica - The high-MgO ultramafic volcanic rocks in the NW Ad Dhala province are classified as meimechite according to the IUGS classification scheme. This province represents the...     

Character and significance of spectacular layering features developed in the thin, alkali-basaltic sills of the Ulvö Gabbro Complex, Sweden

Summary The Ulv? Gabbro Complex consists of alkali-olivine basaltic circular bodies ∼30–80 km in diameter. These intrusions were emplaced at shallow depths (∼3 km) as thin sheets (∼300 m). Among other things, the gabbroic cumulates of the complex display: modal layering, grain-size variations, trough structures, and slump structures. The crystallization sequence is olivine+plagioclase, ulv?spinel, clinopyroxene, and apatite. A nearly continuous exposure across one of these intrusions, the Norra Ulv?n gabbro, is subdivided into: a Lower Zone (LZ), a Rhythmically Layered Zone (RZ) and an Upper Zone (UZ). LZ and RZ were formed at the floor, while UZ grew from the roof downward. Major-element variations in the cores of the cumulus minerals define fractionation trends from the base of the intrusion to the RZ-UZ boundary interpreted as a “sandwich horizon”. Modeling suggests that a significant amount of crystallized interstitial liquid is required to produce the observed stratigraphic relations. Our results suggest that the small size and shallow emplacement depth of the intrusions of the Ulv? Gabbro Complex helped to preserve evidence of primary accumulation processes. However, it is also clear that despite the limited time available postcumulus processes such as diffusional homogenization and compaction of some grains were important. Correspondence: S. ?. Larson, Earth Sciences Centre, Department of Geology, G?teborg University, POB 460, SE 40530 G?teborg, Sweden     

Petrogenesis and tectonic setting of volcanic rocks in the Xiaoshan and Waifangshan areas along the southern margin of the North China Craton: Constraints from bulk-rock geochemistry and Sr–Nd isotopic composition

As part of the Xiong'er volcanic belt along the southern margin of the North China Craton, volcanic rocks in the Xiaoshan and Waifangshan areas have a compositional range from the basaltic andesite, andesite, dacite to rhyolite, which display consistent variation trends in terms of their major and trace elements and Sr–Nd isotopic compositions. The variable Yb contents with nearly constant La/Yb and Tb/Yb ratios of volcanic rocks in two areas suggest that the fractional crystallization may have played an important role in the differentiation from the basaltic andesite, through andesite and dacite, to rhyolite. The volcanic rocks in these two areas are characterized by the LILE and LREE enrichments and negative HFSE anomalies, implying hydrous melting of a mantle wedge in a subduction zone. Variable Sr/Nd ratios of the basaltic andesite and andesite are interpreted as a result of the fluid addition from a subducting slab. Non-radiogenic Nd isotopic compositions as well as high Zr/Y and Nb/Y ratios suggest that the volcanic rocks in these areas were derived from an enriched mantle source. On the other hand, the volcanic rocks of the basaltic andesite and andesite possess markedly higher Fe–Ti and HFSE concentrations than those of typical intra-oceanic arcs, implying that the mantle source from which the volcanic rocks were derived was metasomatised by siliceous melts during the Archean to Paleoproterozoic subduction/collision in the Trans-North China Orogen. These data suggest that in the Paleo-Mesoproterozoic, the southern margin of the North China Craton was most likely an Andean-type continental arc in which slab dehydration not only induced the melting of a pre-existing metasomatised mantle source, but also released LILE-enriched fluids into the mantle source, masking the inherent HFSE-enriched characteristics of the volcanic rocks along the southern margin of the craton. The results of this study indicate that the North China Craton, like many other continental components (e.g. North America, Greenland, Baltica, Amazonia, Australia, etc.) of the supercontinent Columbia (Nuna), also underwent a subduction-related outgrowth along its southern margin during the Paleo-Mesoproterozoic time.     

Recognition of the early permian volcanic rocks and Its geological significance in chifeng region of inner Mongolia北大核心CSCD

In order to investigate the geological significance of the large area of volcanics in the Uplift bell of Mingshan in western Chifeng,When carrying out the I :5 million regional mineral geological survey in the area,Through the research work of field geology investigation, petrology, LA-1CP-MS zircon U-Pb isotopic dating and regional stratigraphic correlation and so on.Found that the volcano rocks in the area is different from the Late Jurassic-Early Cretaceous volcanic rocks. And the obvious green alteration phenomenon was developed : The dating results of the Hornblende andesite in the lower parts is 273. 6±2. 2 Ma,it should belong to the Permian Elitu formation^ The Volcanics in the area belong to Calc-Alkaline-high-k calc-Alkaline series volcanic rocks. And volcanic magma experienced the separation and crystallization of minerals, it was the product of the continuous evolution of cognate magma, having the Characteristics that from bottom to top Intermediate-Mafic to Acidic magmatic evolution and the REE content increased gradually 0 Studies suggest that the Formation of volcanic rocks in the area is related to the ancient Asian Ocean to the North China block subduction in the Late Paleozoic period . It should be formed in the Archean to Proterozoic crystalline basement and late devonian magma Kemelting eruption, the tectonic environment formation of the volcanics in this area was andreessen arcc.     

Mid-Late Neoproterozoic China： Geological records Rodinia rift-related volcanic rocks in of rifting and break-up of

Early Cambrian and Mid-Late Neoproterozoic volcanic rocks in China are widespread on several Precambrian continental blocks,which had aggregated to form part of the Rodinia supercontinent by ca.900 Ma.On the basis of petrogeochemical data,the basic lavas can be classified into two major magma types:HT(Ti/Y>500) and LT(Ti/Y<500) that can be further divided into HT1 (Nb/La>0.85) and HT2(Nb/La≤0.85),and LT1(Nb/La>0.85) and LT2(Nb/La≤0.85) subtypes, respectively.The geochemical variation of the HT2 and LT2 lavas can be accounted for by lithospheric contamination of asthenosphere-(or plume-) derived magmas,whereas the parental magmas of the HT1 and LT1 lavas did not undergo,during their ascent,pronounced lithospheric contamination.These volcanics exhibit at least three characteristics:(1) most have a compositional bimodality;(2) they were formed in an intracontinental rift setting;and(3) they are genetically linked with mantle plumes or a mantle surperplume.This rift-related volcanism at end of the MidNeoproterozoic and Early Cambrian coincided temporally with the separation between AustraliaEast Antarctica,South China and Laurentia and between Australia and Tarim,respectively. The Mid—Late Neoproterozoic volcanism in China is the geologic record of the rifting and break-up of the supercontinent Rodinia.     

Chemistry of chromium spinel in high-Mg rocks from the Morungava Intrusion,Cretaceous Paraná Igneous Province,southernmost Brazil

Mineralogy and Petrology - Mafic-ultramafic intrusions within continental flood basalt terrains are frequently associated with Cu-Ni-PGE mineralization. This study aims to constrain the...     

Zones of impact metamorphism in the crystalline rocks of the Nördlinger Ries crater

Fragments of crystalline basement rocks in suevite, a tuff-like breccia found in the area of the Nördlinger Ries crater, display a continuous gradation of metamorphism. This is interpreted as an effect of the shock waves caused by meteoritic impact, the degree of metamorphism reflecting a radial gradient of pressure and temperature diminishing outward from the point of impact. This phenomenon gives further support to the theory that Ries crater is of meteoritic origin and that suevite is not volcanic in nature. A classification of varying impact facies in crystalline rocks is proposed. It is hoped that the system proposed may prove valid for impact structures in other areas.