Petrogenesis of the Cenozoic volcanic rocks from the northern part of Qinhai-Xizang (Tibet) plateau

Based on electron probe analyses of the minerals and bulk composition of the Cenozoic volcanic rocks from Yumen and Hoh Xil lithodistricts, Qinghai-Xizang plateau, the forming conditions including the temperature and pressure of those rocks are studied in this paper. According to the thermodynamic calculation results of mineral-melt equilibrium, the depth of the asthenosphere superface (about 75 – 130 km) for the northern part of the Qinghai-Xizang plateau during the Cenozoic is suggested. Finally, this paper indicates that the Cenozoic volcanic rocks in the northern part of the Qinghai-Xizang plateau mainly consist of shoshonite series. Their forming temperature is 630 – 1039°C and forming pressure is between 2.3 – 4.0 GPa. The rocks were formed in the intracontinental orogenic belt, of which the primary magma was originated from a particular enrichment upper mantle and accreted crust-mantle belt or directly from asthenospheric superface as a result of partial melting of pyrolite. The project was supported by the National Natural Science Foundation of China (No. 49234080), the Geological Sectional Foundation of MGMR (No. 8506201, 8506207) and the Science Foundation of Northwest University (97NW23).     

Trace Element Geochemistry of Cenozoic Volcanic Rocks in Shandong Province

The Cenozoic volcanic rock of Shandong Province are mainly alkalic and strongly alkalic basaltic rocks.The Contents of major and trace elements including transitional,incompatible and rare-earth elements were determined.The chemical characterisitics of major and trace elements indicate that these basaltic rocks were derived from a mantle source and probably represent a primary magma,I,e.,unmodifiecd partical melts of mantle peridotite in terms of Mg values,correlatione between P2O5 and Ce,Sr,Ni and Rb concentrations,mantle xenoliths,etc.The abundances of trace elements vary systematically from west to east.The compatible transition elements such as Co,Ni,and Cr show a remarkable depletion,whereas the incompatible and rare-earth elements are abundant as viewed from the chondrite-nor-malized patterns.The chemical composition and correlation are consistent with the tectonic setting.According to the batch and fractional partial melting theory,the trace element contents of Shandong volcanic rocks can be calculated from the two-component mixing model.     

藏南定日白垩纪火山岩岩石学与地球化学初步研究

本文描述了西藏定日白垩纪火山岩的地质、岩石学、岩石地球化学特征和构造造背景。该火山岩岩石类型有蚀变英安岩和蚀变安山岩,其岩石化学成分特征表明岩石为二氧化硅过饱、铝过饱和类型,属亚碱性拉斑玄武岩系列。岩石稀土元素配分曲线呈强烈右倾的轻稀土富集型;微量元素配分模式图呈锯齿状"多隆起"型,类似于岛弧火山岩的微量元素配分模式。综合判定该期火山岩形成于大陆边缘造山带岛弧构造环境。     

Age,main geochemical characteristics,and sources of late Cenozoic volcanic rocks in the Udurchukan volcanic area (Amur Region)

This paper presents new mineralogical, petrographic, geochemical, and isotope–geochronological data on the Cenozoic basaltic trachyandesites from the Udurchukan volcanic area (Amur region), which occupies the watersheds of the Uril, Mutnaya, and Khingan rivers. Based on the available geochrolonological data and new K–Ar dating, the basaltic trachyandesites are middle Miocene in age (18.9–17.1 Ma). Petrogeochemically, they are divided into two groups. These groups differ in the contents of MgO, TiO₂, P₂O₅, as well as Sr, Ba, Nb, Ta, and LREE, which is presumably related to the different degrees of metasomatic reworking of the mantle sources and their melting. In terms of the trace-element distribution and ratios, the basaltic trachyandesites from the Udurchukan area are close to the within-plate rocks and were contributed by enriched lithospheric mantle previously subjected to fluid metasomatism.     

K—Ar Geochronology and Evolution of Cenozoic Volcanic Rocks in Eastrn China

Cenozoic volcanic rocks widespread in eastern China constitute an important part of the circum-Pacific volcanic belt.This paper presents more than 150K-Ar dates and a great deal of petrochemical analysis data from the Cenozoic volcanic rocks distributed in Tengchong,China‘s southeast coast,Shandong,Hebei,Nei Monggol and Northeast China.An integrated study shows that ubiquitous but uneven volcanic activities prevailed from the Eogene to the Holocene,characterized as being multi-eqisodic and multicycled.For example,in the Paleocene(67-58Ma),Eocene(57-37.5Ma),Miocene(22-18,16-19Ma),Pliocene(8-3Ma),and Early Pleistocene-Middle Pleistocene(1.2-0.5Ma) there were upsurges of volcanism,while in the Oligocene there was a repose period.In space,the older Eogene volcanic rocks are distributed within the region or in the central part of the NE-NNE-striking fault depression,while the younger Neogene and Quaternary volcanic rocks are distributed in the eastern and western parts.Petrologically,they belong essentially to tholeiite-series and alkali-series basalts,with alkalinity in the rocks increasing from old to youg.The above regularities are controlled by both global plate movement and regional inherent tectonic pattern.     

Strontium isotopes in Cenozoic volcanic rocks from southeastern Papua New Guinea

Strontium isotope data are presented from 28 rock samples representing four distinct episodes of late Cenozoic volcanism in southeastern Papua. Eocene tholeiitic basalts have initial ⁸⁷Sr/⁸⁶Sr ratios (0.7037) which are higher than those in chemically comparable basalts and are thought to have been enhanced by rock-sea water interaction. Late Cenozoic are trench type volcanoes in the Papuan islands have initial ⁸⁷Sr/⁸⁶Sr ratios which show little variation (0.7041±2) in contrast to those from the contiguous Papua New Guinea mainland (0.7036-0.7054). This isotopic discontinuity does not appear to be due to contamination by immediately underlying sialic metamorphics. High-K trachytes in the Lusancay Islands north of the late Cenozoic are have comparable initial ⁸⁷Sr/⁸⁶Sr ratios. Wide variation (0.703-0.710) in the initial ratios measured in a group of apparently closely related rocks ranging in composition from transitional basalt to peralkaline rhyolite cannot be explained by differences in age or by late magmatic fractionation and continue to pose an enigma.     

Cenozoic volcanic rocks of North Kamchatka: In search of subduction zones

Two belts of subaerial volcanic rocks—the Eocene Kinkil belt and the Neogene belt of the Sredinny Range—extend along the Kamchatka Isthmus. It is suggested that their formation is related to subduction of the oceanic lithosphere beneath the continental margin of North Kamchatka. The oceanic lithosphere consumed in the subduction zones could have been formed as a result of active spreading in the Komandorsky Basin. In the simplest case, both spreading and subduction reflect the northwestward motion of the lithosphere of the Komandorsky Plate relative to Kamchatka, the Shirshov Ridge, and the Aleutian Basin combined into one relatively immobile plate conventionally called the North American Plate. The authors perform a simulation of conjugate spreading and subduction. The most important parameter determining the regional geodynamics—the velocity of the Komandorsky Plate moving relative to the North American Plate—is taken as 2.5, 5.0, and 7.5 cm/yr. The calculated ages of the onset and end of volcanic activity in the aforementioned belts are compared with the dates obtained with the isotopic and paleontological methods. For the Eocene Kinkil belt, where volcanism started 44 Ma ago, the model age of the onset of subduction depends on the accepted velocity of the motion of the Komandorsky Plate and varies from 54 Ma at the velocity of 2.5 cm/yr to 47.5 Ma at the velocity of 7.5 cm/yr. It can be assumed that the model of fast subduction in this age interval is most consistent with the geological data. For the Miocene-Pliocene belt of the Sredinny Range, assuming the velocity of the motion of the Komandorsky Plate at 5.0 and 7.5 cm/yr, multiple rifting at the boundary with the Shirshov Ridge should be assumed. Therefore, for the end of the Neogene, a model with low velocity (2.5–5.0 cm/yr, i.e., about 4.0 cm/yr) is preferable.     

Isotopic evidence for the origin of Cenozoic volcanic rocks in the Pinacate volcanic field, northwestern Mexico

Six volcanic rocks, reconnaissance samples representing most of the temporal and compositional variation in the Pinacate volcanic field of Sonora and Arizona, are characterized for major element and Nd---Sr isotopic compositions. The samples consist of basanite through trachyte of an early shield volcano, and alkali basalts and a tholeiite from later craters and cinder cones. With the exception of the trachyte sample, which has increased ⁸⁷Sr/⁸⁶Sr due to crustal effects, all ⁸⁷Sr/⁸⁶Sr values fall between 0.70312 and 0.70342, while ε_Nd values are all between + 5.0 and + 5.7. Clinopyroxene in a rare spinel-lherzolite nodule derived from the uppermost mantle beneath the field has ⁸⁷Sr/⁸⁶Sr of 0.70320 but ε_Nd of + 8.8, three ε_Nd units higher than the volcanic rocks. Both the volcanic rocks and the nodule record the presence of asthenospheric, rather than enriched lithospheric mantle beneath Pinacate. This is consistent with one or both of (a) proximity of Pinacate to the Gulf of California spreading center and (b) presence of similar asthenospheric mantle signatures in volcanic rocks over a wide contiguous area of the southwestern USA. We consider the comparison to other southwestern USA magma sources as the more relevant alternative, although a definite conclusion is not possible at this stage.     

长江源区新生代火山岩的年代学研究

长江源区的火山岩包含了青藏高原北部新生代火山岩形成及演化的全部信息，其年代学研究对查明青藏高原北部新生代火山活动时空演化具有重要意义。通过K—Ar同位素年代学研究，将长江源区新生代的火山活动划分为3期：第Ⅰ期(40．1-35．1Ma)火山喷发较弱，形成小范围分布的细粒安粗岩和斑状粗面岩；第Ⅱ期(26．5～17．6Ma)火山喷溢最为强烈，形成大面积厚层黑色气孔状安粗岩；第Ⅲ期(5．5-4．61Ma)由潜火山作用形成球粒状、斑状粗面岩岩丘。长江源区3个期次的火山喷发与青藏高原北部火山活动的阶段性相对应。长江源区新生代火山岩的演化特征为：从早到晚，岩石系列由高钾钙碱性系列→钾玄岩系列演化；岩性由安粗岩→粗面岩演化。     

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge,Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ?_Nd (+8.0–+8.4) and low ⁸⁷Sr/⁸⁶Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (?_Nd=+7.5?+8.4; ⁸⁷Sr/⁸⁶Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ?_Nd (+4.6–+4.9) and (⁸⁷Sr/⁸⁶Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.     

青藏高原北部黑石北湖新生代钾质火山岩的成因

昆仑岩带新生代钾质火山活动是青藏高原北缘深部动力学过程的重要记录。有关岩浆源区的性质和岩浆产生的机制一直存在较大的认识分歧。阐述了昆仑岩带黑石北湖钾质火山岩的岩石学和地球化学特征,并与西昆仑康西瓦和泉水沟新生代火山岩进行了成因对比分析。研究表明,昆仑火山岩带玄武质岩石的K2O和Na2O含量变化较大,从钠质过渡到钾质,硅碱图显示玄武质初始岩浆向钾质安粗岩方向演化。黑石北湖火山岩的高场强元素和Sr、Nd同位素组成显示岩浆源区具有EMⅡ的性质。岩石LREE的强烈富集和HREE的强烈分馏指示岩浆来自富集型含石榴子石地幔源区。不相容元素K/Nb、Ba/La等比值具有岛弧火山岩与OIB型玄武岩的过渡特征,指示源区为曾遭受软流圈流（熔）体交代的古俯冲地幔楔。     

吉林省敦化地区新生代火山岩地质特征

吉林省敦化地区新生代火山岩主要分布在敦化—密山断裂带及其两侧。本文以其分布状况、岩石类型、岩石化学与地球化学特征、火山岩相以及构造环境等方面论述该区新生代火山活动规律、岩浆演化等,并与区域资料进行分析对比,从而为该区新生代火山的深入研究提供实际的基础地质资料。     

青藏高原北部黑石北湖新生代钾质火山岩的成因

昆仑岩带新生代钾质火山活动是青藏高原北缘深部动力学过程的重要记录。有关岩浆源区的性质和岩浆产生的机制一直存在较大的认识分歧。阐述了昆仑岩带黑石北湖钾质火山岩的岩石学和地球化学特征,并与西昆仑康西瓦和泉水沟新生代火山岩进行了成因对比分析。研究表明,昆仑火山岩带玄武质岩石的K2O和Na2O含量变化较大,从钠质过渡到钾质,硅碱图显示玄武质初始岩浆向钾质安粗岩方向演化。黑石北湖火山岩的高场强元素和Sr、Nd同位素组成显示岩浆源区具有EMⅡ的性质。岩石LREE的强烈富集和HREE的强烈分馏指示岩浆来自富集型含石榴子石地幔源区。不相容元素K/Nb、Ba/La等比值具有岛弧火山岩与OIB型玄武岩的过渡特征,指示源区为曾遭受软流圈流（熔）体交代的古俯冲地幔楔。     

坦桑尼亚伦圭新生代火山岩的地球化学特征及岩石成因

伦圭新生代火山岩位于东非裂谷西支最南端,已有的大量地质与地球化学证据表明伦圭火山岩是典型的碱性双峰式火山岩,其SiO2含量在46％～56％之间出现明显的间断.基性火山岩和酸性火山岩均表现出富集轻稀土元素,轻、重稀土元素强烈分馏,无明显Eu异常的特征.基性岩大离子亲石元素Rb、Ba、Th、U、Sr富集,高场强元素Zr、H...     

可可西里湖地区新生代火山岩同位素地球化学特征及火山成因、源区性质讨论

在青藏高原腹地可可西里地区发育一套新生代碱性、高钾钙碱性——钾玄岩火山岩类,由于该时期正是亚洲大陆内部岩石圈发生剧烈变形,青藏高原强烈隆升的重大变革时期,火山岩的形成在这些重大事件中扮演什么角色?很值得探讨。通过对区内火山岩Rb、Sr、Sm、Nd、Pb同位素的研究,在火山岩的岩浆来源、成因等方面获得了大量信息。研究表明:新生代火山岩主要来源于富集 型地幔区,部分为具极高εSr和极低εNd的酸性火山岩,它是与区内钾质熔岩同源的晚期分异产物;碱性火山岩在成因上与板内初始裂谷有关。研究成果为青藏高原特有的"壳-幔混合层"的形成提供了证据。     

羌塘赤布张错地区新生代火山岩研究

根据K-Ar同位素年代学研究成果,将赤布张错新生代火山活动划分为3期:第Ⅰ期(40.1~35.1Ma)火山喷发较弱,形成小范围分布的细粒安粗岩和斑状粗面岩;第Ⅱ期(26.5~17.6Ma)火山喷溢最为强烈,形成大面积厚层黑色气孔状安粗岩;第Ⅲ期(5.5~4.61Ma)由潜火山作用形成球粒状、斑状粗面岩岩丘。新生代火山岩系包括高钾钙碱性系列和钾玄岩系列,高钾钙碱性火山岩形成于始新世,属于壳源岩浆系列;钾玄岩系火山岩形成于中新世-上新世,属于幔源岩浆系列。由老到新,岩石系列由高钾钙碱性系列→钾玄岩系列演化,岩性由安粗岩→粗面岩演化。     

青藏高原北部新生代火山岩的成因机制

Based on the electron probeanalyses result of the minerals and the bulk rock composition of the Cenozoic volcanicrocks from Yumen and Hoh Xil lithodistricts on the Qinghai-Tibet Plateau, the formingconditions including temperature and pressure of those rocks are studied.According to thethermodynamics calculation result of the mineral-melt equilibrium,the depth of theasthenosphere superface (about 75～130km) for the northern part of the Qinghai-TibetPlateau during Cenozoic era is suggested. Finally, this paper indicates that the Cenozoicvolcanic rocks in the northern part of the Qinghai-Tibet Plateau mainly consist ofshoshonite series.Their forming temperature is 630～1039℃ and forming pressure isbetween 2.3～4.0 GPa and formed in the intracontinental orogenic belt,where the primarymagma was originated from a particular enriched upmantle and accreted crust-mantle belt ordirectly originated from asthenosphere superface by partial melting of pyrolite.     

Geochemical Characteristics of Cenozoic Basaltic High-K Volcanic Rocks from Maguan Area, Eastern Tibet

The major element, trace element and Nd-Sr isotopic composition of Cenozoic basaltic volcanic rocks from the Maguan area, eastern Tibet, indicates that the volcanic rocks are enriched in alkalis, especially K (K2O up to 3.81%) and depleted in Ti (TiO2 = 1.27%-2.00%). These rocks may be classified as two groups, based on their Mg# numbers: one may represent primary magma (Mg# numbers from 68 to 69), and the other, the evolved magma(Mg# numbers from 49 to 57). Their REE contents are very high (∑REE = 155.06-239.04μg/g). Their REE distribution patterns are of the right-inclined type, characterized by LREE enrichment [(La/Yb)N =12.0-19.2], no Ce anomaly (Ce/Ce*=1.0), and weak negative Eu anomaly (Eu/Eu*=0.9). The rocks are highly enriched in Rb, Sr and Ba (59.5-93.8μg/g, 732-999 μg/g, and 450-632 g/g, respectively), high in U and Th (1.59-2.31μg/g and 4.73-8.16 μg/g, respectively), and high in Nb, Ta, Zr and Hf (70-118 μg/g,3.72-5.93 μg/g, 215-381 μg/g, and 5.47-9.03 μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, Nb, Ta, Zr, Hf and P show positive anomalies, whereas Ba, Ti and Y show negative anomalies. The 87Sr/86Sr ratios range from 0. 704029 to 0.704761; 143Nd/144Nd from 0. 512769 to 0. 512949; and εNd from 2.6 to 6.1. These geochemical features might suggest that the potential source of the basaltic high-K volcanic rocks in the Maguan area is similar to the OIB-source mantle of Hawaii and Kergeulen volcanic rocks.     

长白山区图们江流域新生代火山岩的岩石化学研究

图们江流域新生代火山岩以溢流式喷发为特征，其主要岩石类型以拉斑玄武岩为主，含少量玄武质粗安岩、碱性玄武岩、玄武质安山岩和粗面玄武岩。根据图们江流域火山岩的Mg#（Mg#=100Mg^2＋／（Mg^2＋＋Fe^2＋））值多数≤60，Ni含量〈100μg／g和主、微量元素地球化学特征综合表明，形成火山岩的岩浆不是原始岩浆，它们是原始岩浆在上地幔经历了橄榄石、辉石分离结晶作用后形成的以拉斑玄武岩为主，包括碱性玄武岩和粗面玄武岩等演化岩浆，结晶分异作用是岩浆演化过程的主要控制因素。拉斑玄武质岩浆在上升过程中发生了不同程度的壳源物质同化混染作用，发生在上地壳遭受较大程度混染的岩浆K2O含量明显偏高（〉2．6％），形成玄武质粗安岩。     

羌塘西北部松西地区新生代火山岩岩石地球化学特征及成因讨论

羌塘西北部松西地区新生代火山岩由安山岩、英安岩和晚期火山颈相流纹斑岩3种岩石类型组成,属于钙碱性-高钾钙碱性岩石系列.岩石富集大离子亲石元素和LREE,相对亏损高场强元素,Nb、Ta、Ti负异常,反映源岩具有壳源特征,基性端员的SiO2含量＜53%,表明松西地区玄武安山岩不可能完全由陆壳直接局部熔融产生,应该有少量基性的地幔物质加入.岩浆Eu负异常不明显,说明岩浆来源于加厚陆壳中下部,是印度板块与欧亚板块发生长期碰撞挤压导致青藏高原北部包括羌塘地区的陆壳缩短和加厚、拉萨地块大陆岩石圈的北向俯冲作用以及羌塘陆块之下上涌的软流层物质的底侵作用,引发增厚下地壳发生部分熔融形成的.