Characteristics and identification of weathering crust of Pennsylvanian volcanic rocks in Shixi area,Junggar Basin

Based on drill core and thin section observation,major geochemical element analysis and con-ventional well log analysis,this study summarizes the characteristics and thickness of weathering crust of Pennsylvanian volcanic rocks(Carboniferous)in Shixi area,Junggar Basin.The weathering crust is identified and divided into three types according to the petrological characteristics and well log interpretations in Shixi area,and the isopach of weathering crust is mapped.The results show that:(1)With the increase of depth,the weathering weakens,and the rocks become less fractured with decreased porosity;(2)the weathering crust of the Upper Carboniferous volcanic rocks can be divided into strongly weathered and mildly weathered layers in Shixi area;(3)the weathering crust is relatively thicker in Dinan uplift and Shixi uplift.This study provides research basis for further evaluation of Upper Carboniferous volcanic reservoir,and will benefit for well location deployment and potential oilfield development in the Shixi area.

Zircon U-Pb age of the volcanic rock from the Huoshiling Formation in the Yingtai Fault Depression, Songliao Basin, China

1. Objective The Songliao Basin is a superposed basin filled with two layers. The lower layer is characterized by a fault-bounded volcanogenic succession comprising of intercalated volcanic,pyroclastic and epiclastic rocks. The upper layer is composed of normal sedimentary rocks. The Huoshiling Formation is the earliest volcanic stratum in the Songliao Basin. The report of geological age of it was rare, because of the lack of credible dating samples.

Paleoproterozoic volcanic rocks in the southern margin of the North China Craton, central China:Implications for the Columbia supercontinent

The volcanic rocks of the Xiong’er Group are situated in the southern margin of the North China Craton(NCC).Research on the Xiong er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic.In this study,to constrain the age of the Xiong’er volcanic rocks and identify its tectonic environment,we report zircon LA-ICP-MS data with Hf isotope,whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopes of the volcanic rocks of the Xiong’er Group.The Xiong’er volcanic rocks mainly consist of basaltic andesite,andesite.dacite and rhyolite,with minor basalt.Our new sets of data combined with those from previous studies indicate that Xiong’er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism.These volcanics have extremely low MgO.Cr and Ni contents,are enriched in LREEs and LILEs but depleted in HFSEs(Nb,Ta,and Ti),similar to arc-related volcanic rocks.They are characterized by negative zirconεHft values of-17.4 to 8.8,whole-rock initial 87Sr/86Sr values of 0.7023 to 0.7177 andεNd(t)values of-10.9 to 6.4.and Pb isotopes(206Pb/204Pb=14.366-16.431,207Pb/204Pb=15.106-15.371,208Pb/204Pb=32.455-37.422).The available elemental and Sr-Nd-Pb-Hf isotope data suggest that the Xiong’er volcanic rocks were sourced from a mantle contaminated by continental crust.The volcanic rocks of the Xiong’er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean.Thus,we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic,rather than in an arc setting.

Geochemistry of Mesoproterozoic Volcanic Rocks in the Western Kunlun Mountains: Evidence for Plate Tectonic Evolution

Mesoproterozoic volcanic rocks occurring in the north of the western Kunlun Mountains can be divided into two groups. The first group (north belt) is an reversely-evolved bimodal series. Petrochemistry shows that the alkalinity of the rocks decreases from early to late: alkaline→calc-alkaline→tholeiite, and geochemistry proves that the volcanic rocks were formed in rifting tectonic systems. The sedimentary facies shows characteristics of back-arc basins. The second (south belt) group, which occurs to the south of Yutian-Minfeng-Cele, is composed of calc-alkaline island arc (basaltic) andesite and minor rhyolite. The space distribution, age and geochemistry of the two volcanite groups indicate that they were formed in a back-arc basin (the first group) and an island arc (the second group) respectively and indicate the plate evolution during the Mesoproterozoic. The orogeny took place at -1.05 Ga, which was coeval with the Grenville orogeny. This study has provided important geological data for exploring the position of the Paleo-Tarim plate in the Rodinia super-continent.

Chronology and Geochemistry of Mesozoic Volcanic Rocks in the Linjiang Area,Jilin Province and their Tectonic Implications

Zircon U-Pb ages and geochemical analytical results are presented for the volcanic rocks of the Naozhigou, Ergulazi, and Sidaogou Formations in the Linjiang area, southeastern Jilin Province to constrain the nature of magma source and their tectonic settings. The Naozhigou Formation is composed mainly of andesite and rhyolite and its weighted mean ^206pb/^238U age for 13 zircon grains is 222±1 Ma. The Ergulazi Formation consists of basaltic andesite, basaltic trachyandesite, and andesite, and six grains give a weighted mean ^206pb/^238U age of 131±4 Ma. The Sidaogou Formation consists mainly of trachyandesite and rhyolite, and six zircon grains yield a weighted mean ^206pb/^238U age of 113±4 Ma. The volcanic rocks have SIO2=60.24%-77.46%, MGO=0.36%-1.29% （Mg#=0.32-0.40） for the Naozhigou Formation, SIO2=51.60%-59.32%, MGO=3.70%-5.54% （Mg#=0.50-0.60） for the Ergulazi Formation, and SIO2=58.28%-76.32%, MGO=0.07%-1.20% （Mg#=0.14-0.46） for the Sidaogou Formation. The trace element analytical results indicate that these volcanic rocks are characterized by enrichment in light rare earth elements （LREEs） and large ion lithophile elements （LILEs）, relative depletion in heavy rare earth elements （HREEs） and high field strength elements （HFSEs, Nb, Ta, and Ti）, and negative Eu anomalies. Compared with the primitive mantle, the Mesozoic volcanic rocks in the Linjiang area have relatively high initial ^87Sr/^86Sr ratios （0.7053-0.7083） and low εNd（t） values （-8.38 to -2.43）, and display an EMⅡ trend. The late Triassic magma for the Naozhigou Formation could be derived from partial melting of a newly accretional crust with the minor involvement of the North China Craton basement and formed under an extensional environment after the collision of the Yangtze Craton and the North China Craton. The Early Cretaceous volcanic rocks for the Ergulazi and Sidaogou Formations could be formed under the tectonic setting of an active continental margin related to the westward subduction of the Izanagi plate.

Chronology and Geochemistry of Volcanic Rocks in the Cretaceous Suifenhe Formation in Eastern Heilongjiang,China

Zircon U-Pb ages and geochemical data of volcanic rocks in the Suifenhe Formation in eastern Heilongjiang Province are reported, and their petrogenesis is discussed in this paper. The Suifenhe Formation mainly consists of basalt, andesite, and dacite. Zircon from andesite and dacite are euhedral in shape and show typical oscillatory zoning with high Th/U ratios （0.18-0.57）, implying its magmatic origin. Zircon U-Pb dating results by laser ablation inductively coupled plasma mass spectrometer （LA-ICP-MS） indicate that the ^206Pb/^238U ages of zircons from andesite range within 105- 106 Ma, yielding a weighted mean age of 105.5±0.8 Ma （n=14）, and that ^206pb/^238U ages of zircons from dacite are between 90-96 Ma, yielding a weighted mean age of 93.2±1.3 Ma （n =13）. The volcanic rocks from the Snifenhe Formation are subalkaline series and show a calc-alkaline evolutionary trend with SiO2 content of 47.69%-65.47%, MgO contents of 1.42%-6.80% （Mg^#= 45-53）, and Na2O/K2O ratios of 1.83-3.63. They are characterized by enrichment in large ion lithophile elements （LILE） and lightrare-earth elements （LREE）, depletion in heavy rare earth elements （HREE） and high field strength elements （HFSE） （e.g., Nb, Ta, Ti）, and low initial ^87Sr/^86Sr ratios （0.7041-0.7057） and positive εNd（t） ValUes （0.39-4.08）, implying that they could be derived from a depleted magma source. Taken together, these results suggest that the primary magma of the volcanic rocks might originate from partial melting of the mantle wedge metasomatized by fluids derived from subducted slab under a tectonic setting of active continental margin.

Neoproterozoic Volcanic Rocks in the Southern Quruqtagh of Northwest China:Geochemistry,Zircon Geochronology and Tectonic Implications

The Early Neoproterozoic Beiyixi Formation volcanic rocks of the southern Quruqtagh comprise mainly of a suite of tholeiitic basalts, alkaline andesites, and calc-alkaline rhyolites. The rhyolites are characterized by variably fractionated enrichment in light rare earth elements （LREE） and fiat in heavy rare earth elements （HREE）, and strongly negative Eu anomalies. Compared to the rhyolites, the andesites also exhibit enrichment in LREE and flat HREE （chondrite-normalized values of La/Yb,and La/Sm are 13.30-41.09, 3.18-6.89 respectively）. Their rare earth element patterns display minor negative Eu anomalies. Both of them exhibit coherent patterns with strongly to moderately negative anomalies of Nb, Zr, Ti, and Hf on spider diagrams. Two rhyolite and one andesite magmatic zircons with defined oscillatory zoning yielded weighted mean 206pb/23Su ages of 743 ± 7 Ma, 741±2 Ma, and 7274 Ma. These ages are interpreted to represent the timing of volcanic eruptions. According to geochemistry and rock type, these volcanic rocks formed within a continental island-arc environment following subduction of the oceanic crust during the Early Neoproterozoic period.

U-Pb Dating of Volcanic Rocks and Granites along the Wuyishan Belt:Constraints on Timing of Late Mesozoic Tectonic Events in Southeast China

Five volcanic rock samples and two granite samples taken from the volcanic basins in western Fujian and southern Jiangxi were dated by using the zircon laser albation-inductively coupled plasma mass spectrometry U-Pb method. Together with previously dated ages, the dates obtained provide important constraints on the timing of late Mesozoic tectonic events in SE China. The volcanic rock samples yield ages of 183.1±3.5 Ma, ca. 141 Ma to 135.8±1.1 Ma, 100.4±1.5 to 97.6±1.1 Ma, confirming three episodes of late Mesozoic volcanic activities, which peaked at 180±5 Ma, 140±5 Ma and 100±5 Ma, respectively, along the Wuyishan belt. Moreover, based on field investigations of these volcano-sedimentary basins, we have recognized two compressional tectonic events along this belt. The early one was characterized by Upper Triassic to Middle Jurassic NNE-trending folds that were intruded by late Jurassic granites; and the late one caused the Lower Cretaceous volcano-sedimentary layer to be tilted. The dated age 152.9±1.4 Ma of the granitic samples from the Hetian granitic pluton in the Changting Basin and that from the Baishiding granitic pluton, 100.2±1.8 Ma, in the Jianning Basin, give the upper boundaries of these two tectonic events respectively. Hence, the late Mesozoic tectonic evolution of SE China was alternated between extension and compression.

Geodynamic Background of Intracontinental Cenozoic Alkaline Volcanic Rocks in Laojiezi, Western Yangtze Craton: Constraints from Sr-Nd-Hf-O Isotopes

The Laojiezi alkaline volcanic rocks, which are located in the intraplate region of the Yangtze craton, coincide with the formation of the Jinshajiang-Ailaoshan-Red River alkaline rock belt. Although this belt has been widely studied by geologists because of its porphyry-related Pb-Ag-Au polymetallic deposit and geotectonic location, the material sources of this belt are still debate. Whole-rock analyses show that these rocks have high total alkali contents(3.73–11.08 wt%), and their aluminum saturation index(ASI) values widely vary from 0.82 to 3.07, which comprise a metaluminous-peraluminous magma series. These rocks are characterized by high K(K2 O/Na2 O>1) and low Ti and Mg contents; enrichment in large-ion lithophile elements, such as Rb, Ba, K and light rare earth elements; and depletion in high field strength elements, such as Ta, Nb, P, and Ti. These rocks exhibit moderate Eu(Eu/Eu*=0.86–1.04) and Ce(Ce/Ce*=0.63–0.96) anomalies. Their(87 Sr/86 Sr)i, εNd(t), zircon εHf(t) and δ18 O values range from 0.70839 to 0.71013, from-10.16 to-12.45, from-19.6 to-5.8, and from 5.69‰ to 8.54‰, respectively, and their Nd and Hf two-stage model ages(TDM2) are 1.67–1.86 Ga and 1.27–2.02 Ga, respectively. These data reflect the primary partial melting of Paleoproterozoic to Mesoproterozoic lower crust with minor residual continental lithospheric mantle and supracrustal metasediments. The lithosphere was likely thickened along the southeastern margin of the Tibetan Plateau following the Indian-Asian continentcontinent collision(65–41 Ma). During the post-collision phase(36–16 Ma), the transition from a compressional to extensional setting triggered the convective removal of the over-thickened CLM beneath the Yangtze craton, which led to the upwelling of asthenospheric materials. This process created alkali-rich and high-K magma through the partial melting of the thickened lower crust. Magma that carried Cu-Au-Pb-Ag minerals was emplaced by strike-slip motion along the E-to W-or ENE-to WSWtrending tectonically weak zone, finally forming an alkaline porphyry Cu-Au-Pb-Ag polymetallic deposit.

Cenozoic Adakite-type Volcanic Rocks in Qiangtang,Tibet and Its Significance

Volcanic rocks in the study area, including dacite, trachyandesite and mugearite, belong to the intermediate-acid, high-K calc-alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that the rocks are characterized by SiO2>59%, enrichment in A12O3(15.09-15.64%) and Na2O (>3.6%), high Sr (649-885 μg/g) and Sc, low Y contents (<17 μg/g), depletion in HREE (Yb<1.22 μg/g), (La/Yb)N>25, Sr/Y>40, MgO<3% (Mg<0.35), weak Eu anomaly (Eu/Eu=0.84-0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd and Sr isotope data (87Sr/86Sr=0.7062-0.7079, 143Nd/144Nd=0.51166-0.51253, εNd= -18.61-0.02), show that the magma resulted from partial melting (10%-40%) of newly underplated basaltic lower crust under high pressure (1-4 GPa), and the petrogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will give an insight into the uplift mechanism of the Tibetan plateau.

Discovery and Significance of High CH_4 Primary Fluid Inclusions in Reservoir Volcanic Rocks of the Songliao Basin,NE China

Comparing compositions of the fluid inclusions in volcanic rocks to the contents and isotopes of the gases in corresponding volcanic reservoirs using microthermometry, Raman microspectroscopy and mass spectrum analysis, we found that： （1） up to 82 mole% methane exists in the primary inclusions hosted in the reservoir volcanic rocks; （2） high CH4 inclusions recognized in the volcanic rocks correspond to CH4-bcaring CO2 reservoirs that are rich in helium and with a high ^3He/^4He ratio and which show reversed order of 813C in alkane; （3） in gas reservoirs of such abiotic methane （〉80%） and a mix of CH4 and CO2, the enclosed content of CH4 in the volcanic inclusions is usually below 42 mole%, and the reversed order of δ^13C in alkane is sometimes irregular in the corresponding gas pools; （4） a glassy inclusion with a homogeneous temperature over 900℃ also contains a small portion of CH4 although predominantly CO2. This affinity between gas pool and content of inclusion in the same volcanic reservoirs demonstrates that magma-originated gases, both CH4 and CO2, have contributed significantly to the corresponding gas pools and that the assumed hydrocarbon budget of the bulk earth might be much larger than conventionally supposed.

Zircon SHRIMP U–Pb age of Late Jurassic OIB-type volcanic rocks from the Tethyan Himalaya: constraints on the initial activity time of the Kerguelen mantle plume

This work presents zircon U–Pb age and wholerock geochemical data for the volcanic rocks from the Lakang Formation in the southeastern Tethyan Himalaya and represents the initial activity of the Kerguelen mantle plume. SHRIMP U–Pb dating of zircons from the volcanic rocks yielded a ^(206) Pb/^(238) U age of 147 ± 2 Ma that reflects the time of Late Jurassic magmatism. Whole rock analyses of major and trace elements show that the volcanic rocks are characterized by high content of Ti O_2(2.62 wt%–4.25 wt%) and P_2O_5(0.38 wt%–0.68 wt%), highly fractionated in LREE/HREE [(La/Yb)N= 5.35–8.31] with no obvious anomaly of Eu, and HFSE enrichment with no obvious anomaly of Nb and Ta, which are similar to those of ocean island basalts and tholeiitic basaltic andesites indicating a mantle plume origin. The Kerguelen mantle plume produced a massive amount of magmatic rocks from Early Cretaceous to the present, which widely dispersed from their original localities of emplacement due to the changing motions of the Antarctic, Australian, and Indian plates. However, our new geochronological and geochemical results indicate that the Kerguelen mantle plume started from the Late Jurassic. Furthermore, we suggest that the Kerguelen mantle plume may played a significant role in the breakup of eastern Gondwanaland according to the available geochronological, geochemical and paleomagnetic data.

Development features of volcanic rocks of the Yingcheng Formation and their relationship with fault structure in the Xujiaweizi Fault Depression,Songliao Basin,China

The Xujiaweizi Fault Depression is located in the northern Songliao Basin,Northeast China.The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formation（K 1 yc）.Based on seismic interpretation,drill cores and the results of previous research,we analyzed the distribution of faults and the thickness of volcanic rocks in different periods of K 1 yc,and studied the relationship of volcanic activities and main faults.Volcanic rocks were formed in the Yingcheng period when the magma erupted along pre-existing fault zones.The volcanic activities strongly eroded the faults during the eruption process,which resulted in the structural traces in the seismic section being diffuse and unclear.The tectonic activities weakened in the study area in the depression stage.The analysis of seismic interpretation,thin section microscopy and drill cores revealed that a large number of fractures generated in the volcanic rocks were affected by later continued weak tectonic activities,which greatly improved the physical properties of volcanic reservoirs,and made the volcanic rocks of K 1 yc be favorable natural gas reservoirs.The above conclusions provide the basis to better understand the relationship of the volcanic rock distribution and faults,the mechanism of volcanic eruption and the formation of natural gas reservoirs in volcanic rocks.

Permian Tectonic Evolution in the Middle Part of Central Asian Orogenic Belt: Evidence from Newly Identified Volcanic Rocks in the Bilutu Area, Inner Mongolia

In this study, zircon U-Pb ages, geochemical and Lu-Hf isotopic data are presented for the newly identified volcanic rocks which were considered as Bainaimiao group in Bainaimiao Arc Belt(BAB), Inner Mongolia, which could provide important constraints on the evolution of the northern part of North China Block(NCB) and BAB. Basalt to basaltic andesite and andesite to dacite were collected from two sections, which showed eruption ages of 278.2±4.1 Ma and 258.3±3.0 Ma respectively. All samples are characterized by high abundances in Al2O3, LREEs, and LILEs, but depleted in HFSEs. Together with high Mg# ratios and low K/tholeiite to calc-alkaline series, these features indicated that basalt to andesite was likely derived from relatively low degree partial melting of the subduction-fluid related mantle in the spinel phase. And dacite was mainly from the partial melting of crust, then affected by mantle. All samples barely went through fractional crystallization process with the slight Eu anomaly. Compared with the contemporary basalt in NCB, rocks in BAB have a complex composition of zircon and a more positive εHf(t) value(-6.6-6.4), indicating that they had different magma sources of rocks. Though with different basements, NCB and BAB have become an integrated whole before 278 Ma. Therefore, it could be concluded that NCB and BAB belonged to the active continental margin and the PAO had not closed yet until late Permian and then it disappeared gradually and the CAOB developed into a condition of syn-post collision.

Zircon U-Pb Geochronology and Geochemical Characteristics of the Volcanic Host Rocks from the Tongyu VHMS Copper Deposit in the Western North Qinling Orogen and Their Geological Significance

Precise in situ zircon U-Pb dating and Lu-Hf isotopic measurement using an LA-ICP-MS system, whole-rock major and trace element geochemistry and Sr-Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo-Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle-normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE（e.g. Th, U, Pb and La） and depletion of HFSE（e.g. Nb, Ta, Ti, Zr and Hf）. Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr（0.703457 to 0.708218） and εNd（t）（-2 to 5.8） indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr-εNd（t） diagram. The involvement of crustal-derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon εHf（t） values, which range from-3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores（2616±39 Ma and 1297±22 Ma）. The mineralization of the Tongyu copper deposit shows syn-volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U-Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo-Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore-forming elements. The magma and ore-bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS-type copper deposit.

Rare Earths, Trace Element Characteristics of High-Mg Volcanic Rocks of Yixian Formation in Sihetun West Liaoning Province and Its Apocalypse

The high-Mg volcanic rocks of the Yixian Formation in the Sihetun area have the obvious characteristics of mantie-derived lava in rare earth, trace element characteristics with high Mg # （62 - 70） and high content of compatible elements. In the meantime, the volcanic rocks also have the obvious characteristics of Crust-source material in rare earth, trace element characteristics with high ∑ REE （ 158.78 × 10^-6 - 359.66 × 10^-6 ）, high （La/Yb） N （ 14.61 - 29.60）, high La/Nb（2.37 - 7.52） and high Ba/Nb （67.58- 205.96）, obvious positive anomaly of Pb and negative anomaly of Nb, Ta in trace element spider-gram. In Sr-Nd-Pb isotope the （^87Sr/^86Sr）i ratio is higher than 0.706 and the εNd（ t ） ratio is from - 3.4 to - 13, both reflect enriched Mantle characteristics. The characteristics above of the volcanic rocks combined with the content of Sr, Ba, Y, Yb and the ratio of Sr/Y show that the volcanic rocks have the property of the Sanukite rocks in Setouchi Japan beside subduction zone, and illuminate that the Sanukite rocks can be formed not only in island-arc near subduction zone but also in intro-plate. The analysis indicates that the high-Mg volcanic rocks in the Sihetun area result from the collective function of mantle-derived lava and crust-source materials. The result illuminates that the West Liaoning region is very special in tectonic geochemical background in Cretaceous in East China, and is an ideal region for us to further study the characteristics of magmatic activity as well as the process of Crust-Mantle interaction in Eastern China.

Geochemistry and Petrogenesis of Volcanic Rocks in the Yeba Formation on the Gangdise Magmatic Arc, Tibet

The Early Jurassic bimodal volcanic rocks in the Yeba Formation, situated between Lhasa, Dagze and Maizhokunggar, composed of metabasalt, basaltic ignimbrite, dacite, silicic tuff and volcanic breccia, are an important volcanic suite for the study of the tectonic evolution of the Gangdise magmatic arc and the Mesozoic Tethys. Based on systematic field investigations, we carried out geochemical studies on representative rock samples. Major and trace element compositions were analyzed for these rock samples by XRF and ICP-MS respectively, and an isotope analysis of Rb-Sr and Sm-Nd was carried out by a MAT 262 mass spectrograph. The results show that the SiO2 contents in lava rocks are 41 %-50.4% and 64 % -69 %, belonging to calc-alkaline basalt and dacite. One notable feature of the basalt is its low TiO2 content, 0.66%-1.01%, much lower than those of continental tholeiite. The ∑REE contents of basalt and dacite are 60.3-135 μg/g and 126, 4--167.9μg/ g respectively. Both rocks have similar REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts have depleted Ti, Ta and Nb and slightly negative Nb and Ta anomalies, with Nb = 0.54--1.17 averaging 0. 84. The dacites have depleted P and Ti and also slightly negative Nb and Ta anomalies, with Nb= 0. 74 -1. 06 averaging 0. 86. Major and trace elemental and isotopic studies suggest that both basalt and dacite originated from the partial melting of the mantle wedge at different degrees above the subduction zone. The spinal Iherzolite in the upper mantle is likely to be their source rocks, which might have been affected by the selective metasomatism of fluids with crustal geochemistry. The LILE contents of both rocks were affected by metamorphism at later stages. The Yeba bimodal volcanic rocks formed in a temporal extensional situation in a mature island arc resulting from the Indosinian Gangdise magmatic arc.

Geochemical and Geochronological Constraints on the Origin of the Meta-basic Volcanic Rocks in the Tengtiaohe Zone, Southeast Yunnan

The meta-basic volcanic rocks in the Tengtiaohe Zone yield zircon U–Pb ages of 258.8±2.5 Ma and 259.2±1.8 Ma, respectively which agree with the ages of flood basalts of ELIP and are similar to the basaltic rocks and komatiites from the Song Da Zone in northern Vietnam. The results suggest that the age of meta-basic volcanic rocks is Late Permian, rather than the Early Permian or Early Carboniferous ages as previously inferred. Most meta-basic volcanic rocks are strongly enriched in LREEs relative to HREEs and display trace element patterns similar to the ELIP high-Ti basalts, and are enriched in LILEs with negative Sr anomalies. Their initial 87^Sr/86^Sr ratios range from 0.705974 to 0.706188 and εNd（t） from-0.82 to-2.11. Their magmas were derived from an enriched and deep mantle source without significant crustal contamination. These meta-basic volcanic rocks formed in ELIP. Therefore, the Tengtiaohe Zone is not an ophiolite zone and can link to the Song Da Zone in northern Vietnam.

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.

Sodium-rich volcanic rocks and their relationships with iron deposits in the Aqishan-Yamansu belt of Eastern Tianshan,NW China

The volcanic rocks hosting the iron deposits in the Aqishan–Yamansu metallogenic belt are sodium-rich.The geochronology,petrography,and geochemistry of minerals and sodium-rich rocks as well as the relationship between these rocks and the iron deposits are studied.Geochemically,the ore-hosting volcanic rocks are sodiumrich(the averages of Na2O and Na2O/K2O are 4.31 wt.%and 8.56,respectively)and belong to the calc-alkaline series.They are enriched in LREEs and LILEs(Ba,U,K,and Sr),but depleted in HFSEs(Nb,Ta,and Ti).SHRIMP zircon U–Pb dating of the crystal tuff in the Aqishan Formation and the dacite in the Tugutu Bulak Formation yields ages of 337.5
2.3 Ma(n?15,MSWD?0.85)and 313.0
3.3 Ma(n?13,MSWD?0.74),respectively,indicating that the sodium-rich volcanic rocks formed from the early–late Carboniferous.Electron microprobe data from plagioclases demonstrate that albites and/or oligoclases were formed in the basic–intermediate–acid volcanic rocks.Two stages of albitization are identified,and the latter is likely attributed to the dissolution of iron in the Aqishan–Yamansu belt.The sodium-rich volcanic rocks probably formed by the interaction between volcanic lava and seawater after volcanoes erupted on the seafloor;meanwhile,the albites formed by element substitution in a low-metamorphic environment.The spatiotemporal coupling relationship between sodium-rich volcanic rocks and iron deposits in the Aqishan–Yamansu belt is favorable.Iron dissolved from the dark minerals of basic–intermediate volcanic rocks through sodium metasomatism is one of the material sources for the iron deposits.