山东东部中生代中基性火山岩的地球化学特征及成因探讨

本文对山东东部中生代中一基性火山岩的岩石化学、微量元素、稀土元素及铅同位素特征作了系统研究．该区中生代火山岩主要为玄武岩、粗面玄武岩、玄武质粗面安山岩、安山岩和粗面安山岩．玄武岩属钙碱性系列的拉斑玄武岩,微量元素特征显示大离子亲石元素和高场强元素均稍富集;稀土配分模式略呈右倾型,轻、重稀土之间的分馏不明显,基本无Eu异常;铅同位素组成显示玄武岩形成于原始地幔．粗面玄武岩和安山岩类属钙碱一弱碱性系列,微量元素特征显示大离子亲石元素富集,高场强元素含量低;稀土配分模式呈右倾型,轻、重稀土之间的分馏明显,基本无Eu异常;铅同位素组成显示粗面玄武岩形成于亏损地幔,安山岩类形成于富集地幔,具EMI属性．这些特征表明,山东东部中生代火山岩的岩浆来源于不同的源区．形成于早白垩世的安山岩类,产于活动大陆边缘拉张动力背景下,是由富集型地幔部分熔融形成的岩浆直接喷发形成;形成于晚白垩世的粗面玄武岩,也产于活动大陆边缘拉张动力背景下,随着弧后扩张引起拉伸作用,岩石圈伸展,从地幔来源的玄武质岩浆在上升过程中受到了陆壳物质的混染,之后沿着大断裂喷出地表形成本区显示亏损地幔来源特征的粗面玄武岩;形成于晚白垩世的玄武岩产于大陆板内拉张环境,是由原始地幔部分熔融而成的岩浆直接喷发形成．     

琼中古-中元古代变质基性火山岩地球化学特征

琼中斜长角闪(片麻)岩为变质的古-中元古代基性火山岩,属于高铝和低钛的高钾钙碱性玄武岩系.其LILE和LREE明显富集,HFSE相对亏损,具有典型岛弧玄武岩特征.微量元素、Sm-Nd同位素组成还揭示其岩浆源区偏离原始地幔,但未受成熟地壳物质的明显污染,是消减带上部地幔楔与俯冲洋壳析出的流体二元混合物部分熔融的产物.古-中元古宙时期,海南地块可能经历了由拉张到挤压的地球动力学转变,早期拉张阶段形成琼西洋脊型和过渡型拉斑玄武岩,中晚期强烈俯冲作用形成琼中岛弧高钾钙碱性玄武岩,该俯冲期与华南统一地块古-中元古宙板块俯冲时期相对应.     

浙闽沿海晚中生代壳幔作用研究

总结了浙闽沿海早白垩世基性．酸性火成岩所构成的复合岩流、岩墙和岩体 的岩石地球化学特征．通过Nd,sr,Pb同位素及大离子亲石元素(LIIJE)、轻稀土 (眦E)、高场强元素(HFSE)等的地球化学研究,认为浙闽沿海晚中生代玄武岩中大 离子亲石元素的高丰度是太平洋板块俯冲导致的“源区富集”和玄武岩浆上侵过程中 “地壳混染”两方面地质过程所致,这些晚中生代复合基性一酸性火成岩是活动大陆边 缘“壳幔作用”的产物．     

甘肃野马南山地区北大河岩群一岩组变质岩原岩特征及构造意义

野马南山地区位于中祁连西段,区内最古老的地层为古元古代北大河岩群,由于变质程度较深,原岩特征难以识别。通过岩石地球化学方法,对北大河岩群一岩组变质岩系及原岩进行研究,其中斜长角闪岩原岩为玄武岩,花岗片麻岩原岩为酸性火山岩。该套火山岩整体缺少Si O253%~66%之间的中性火山岩部分,基性火山岩来源为幔源部分融入的玄武岩,上升过程中混染了部分壳源物质,酸性火山岩来自壳源熔融,整体具双峰式火山岩特征,类似于大陆裂谷火山岩组合,可能为原始壳幔形成初期陆核—陆块转变过程的产物。     

南秦岭岚皋基性火山岩的地质学 、地球化学及其构造意义

为了研究南秦岭岚皋地区基性火山岩的岩石成因和区域构造背景, 选择岚皋县境内的玄武岩和火山角砾岩进行岩相学、常量元素、微量元素和稀土元素的分析。通过分析发现, 它们同属于碱性玄武岩系列, 轻重稀土分异明显, Rb, K 相对Th, Hf 亏损, 不具 Nb, Ta, Zr, Ti 负异常, 整体表现出与 OIB( 洋岛玄武岩) 高度一致的稀土图谱和微量元素特征。岩石成因分析表明, 它们为同源岩浆演化序列, 其原始岩浆起源于尖晶石-石榴石二辉橄榄岩和石榴石二辉橄榄岩地幔源区的低度部分熔融, 在上升过程中基本未受地壳混染的影响。岩浆演化过程中主要经历了单斜辉石、少量斜长石的分离结晶作用。综合区域地质学和微量元素特征, 该组火山岩最有可能形成于洋盆台地或大洋岛构造环境。     

俯冲带环境下的火成岩岩石组合及其意义

大洋板块俯冲是板块构造中威尔逊旋回的重要环节。俯冲带环境下具有特殊的火成岩岩石组合,并产出有大量金属矿产。大洋岛弧与陆缘弧的火成岩组合以拉斑玄武岩、钙碱性玄武岩、安山岩和英安岩组合为主,其中陆缘弧的中酸性组分相对偏多。埃达克岩产出在特殊的俯冲带环境下,例如热的年轻洋壳、低角度缓慢俯冲及特殊的地热等环境。埃达克质岩浆上升侵位过程中与地幔楔发生交代作用,形成富Nb玄武岩。埃达克岩通常伴生有大量的金、铜等矿产资源。     

新疆东天山金滩金矿区安山岩岩石学、地球化学及地质意义

新疆金滩金矿发育一套与成矿密切相关的安山岩,对该安山岩元素质量分数和相关性进行分析。研究结果表明:安山岩中锆石结构以及Th和U的质量分数之比ω(Th)/ω(U)(0.30~1.02)均显示其为岩浆成因,锆石LA-ICP-MS U-Pb年龄为(303.65±4.8)Ma,代表安山岩形成时代;安山岩ω(Si O_2)为54.29%~69.05%,ω(Al_2O_3)为11.27%~19.46%,全碱ω(K_2O+Na_2O)为2.46%~6.81%,ω(Mg O)为3.60%~10.45%,Mg指数为46~66。安山岩轻稀土元素(LREE)富集、轻重稀土元素分异作用明显,绝大多数样品具中等Eu负异常(δω(Eu)=0.44~0.77)和轻微Ce负异常(δω(Ce)=0.93~0.98),富集K,Rb,Ba,Th和U等大离子亲石元素,相对亏损Nb和Ti等高场强元素;岩浆经历了分离结晶作用,且发生部分熔融;安山岩可能是康古尔塔格洋向南俯冲,洋壳脱水进入地幔楔,被俯冲流体交代的地幔楔部分熔融,同时产生结晶分异作用而形成,而且遭受明显的陆壳物质混染;东天山地区在晚石炭世依旧存在康古尔洋的俯冲消减作用。     

青藏高原改则火山岩地球化学特征

对分布于西藏改则地区班公湖-怒江缝合带的早白垩世去申拉组火山岩进行了岩石学、地球化学及大地构造研究。该区火山岩为一套低-中钾的中-基性钙碱性岩石,其中玄武岩原始岩浆来源于不均一地幔,安山岩岩浆由玄武岩岩浆结晶分异形成,岩浆在上升过程中未发生地壳混染作用;该区火山岩形成于早白垩世大陆板内裂谷构造背景下。     

房山岩体的岩浆来源、形成机制及动力学意义

房山岩体是华北东部典型的燕山期中酸性侵入杂岩体,其地球化学特征为富Si、Na、Al、Sr,亏损高场强元素(Nb、Ta),轻重稀土分异强烈,Sr-Nd同位素具有类似EMI型富集地幔的特点,表明其岩浆来自于加厚下地壳的部分熔融,并且中生代华北东部下地壳已经被早先的富集岩石圈地幔置换.不同基性程度岩浆的不完全混合是形成微粒闪长质包体的原因.侵入杂岩的角闪石压力计显示下地壳部分熔融形成的中酸性岩浆先后在19~8 km里的深度范围内结晶就位.古太平洋板块在地幔过渡带深度于早白垩世水平俯冲至太行山一线,洋壳脱水导致上覆地幔部分熔融,后者成为加热下地壳部分熔融的热源.     

辽西中侏罗统海房沟组埃达克质岩的确认及地质意义

海房沟组火山岩是辽西中生代火山岩的重要成员，其岩石共生组合为粗安岩—安山岩—英安岩—粗面英安岩，属于高钾钙碱性—钙碱性火山岩系。火山岩主量元素、稀土元素和微量元素研究表明：海房沟组火山岩属埃达克质岩；这些火山岩的源岩较深，可能为古老的下地壳玄武质岩石部分熔融产物。海房沟组埃达克质岩的确认对探讨辽西地区火山岩浆起源、壳幔相互作用、中生代构造应力场的重大转换及大规模断陷盆地群的形成具有重要意义。     

Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua

Resolving flow geometry in the mantle wedge is central to understanding the thermal and chemical structure of subduction zones, subducting plate dehydration, and melting that leads to arc volcanism, which can threaten large populations and alter climate through gas and particle emission. Here we show that isotope geochemistry and seismic velocity anisotropy provide strong evidence for trench-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua. This finding contradicts classical models, which predict trench-normal flow owing to the overlying wedge mantle being dragged downwards by the subducting plate. The isotopic signature of central Costa Rican volcanic rocks is not consistent with its derivation from the mantle wedge or eroded fore-arc complexes but instead from seamounts of the Galapagos hotspot track on the subducting Cocos plate. This isotopic signature decreases continuously from central Costa Rica to northwestern Nicaragua. As the age of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, minimum northwestward flow rates can be estimated (63-190 mm yr(-1)) and are comparable to the magnitude of subducting Cocos plate motion (approximately 85 mm yr(-1)). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones.     

Continental subduction channel processes: Plate interface interaction during continental collision

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth＇s surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of perido- tile, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust- and man- lie-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism.     

Carboniferous adakites and Nb-enriched arc basaltic rocks association in the Alataw Mountains, north Xinjiang： interactions between slab melt and mantle peridotite and implications for crustal growth

Adakites and Nb-enriched arc basaltic rocks (NEABs) in arc setting, which are closely correlated in petrogenesis, have recently been widely followed with interest[1—9]. In general, adakite is derived from partial melting of subducting oceanic crust[1]. When adakitic magma (slab melt) passes through the mantle wedge, the interactions between slab melt and mantle peridotite will occur: slab melt is contaminated by peridotite, meanwhile peridotite is metasomated by slab melt. The NEABs are d…     

Mantle wedge control on back-arc crustal accretion

At mid-ocean ridges, plate separation leads to upward advection and pressure-release partial melting of fertile mantle material; the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away. In back-arc basins, the subducting slab is an important control on the pattern of mantle advection and melt extraction, as well as on compositional and fluid gradients. Modelling studies predict significant mantle wedge effects on back-arc spreading processes. Here we show that various spreading centres in the Lau back-arc basin exhibit enhanced, diminished or normal magma supply, which correlates with distance from the arc volcanic front but not with spreading rate. To explain this correlation we propose that depleted upper-mantle material, generated by melt extraction in the mantle wedge, is overturned and re-introduced beneath the back-arc basin by subduction-induced corner flow. The spreading centres experience enhanced melt delivery near the volcanic front, diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away. Our model explains fundamental differences in crustal accretion variables between back-arc and mid-ocean settings.     

Holocene volcanic rocks in Jingbo Lake region ? Diversity of magmatism

During the time from 5500 a to 5200 a BP more than 10 Holocene volcanoes in Jingbo Lake region erupted and the volcanic rocks covered an area of about 500 km2. Holocene volcanic rocks in Jingbo Lake region belong to the potassium?rich rocks and contain three rock types: trachybasalts, basanites and phonotephrites. Various types of magmatism formed in a small area and in a short period of time came from partial melting of potassically?metasomatised lithospheric mantle. The diversity of magmatism can be explained by that Jingbo Lake is situated in the back?arc extensional region of East Asian continent subducted by the Pacific Ocean, and potassic fluid derived from mantle wedge or dehydration of subducted slab can result in a high heterogeneity of the mantle beneath this region. Based on the pressure estimation of clinopyroxene megacrysts, we estimate that phonotephrite magma fractionally crystallize at ca. 52?54 km down the earth.     

Contrasting crustal production and rapid mantle transitions beneath back-arc ridges

The opening of back-arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading. During this process, the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced (with high volatile contents derived from the subducting plate). Current models predict gradational, rather than abrupt, changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction-influenced mantle. In contrast, here we show that across-strike and along-strike changes in crustal properties at the Eastern Lau spreading centre are large and abrupt, implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes. With incremental separation of the ridge axis from the volcanic front of as little as 5?km, seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills, upper crustal seismic velocities abruptly increase by over 20%, and gravity anomalies and isostasy indicate crustal thinning of more than 1.9?km. We infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge, such that stable, broad triangular upwelling regions, as inferred for mid-ocean ridges, cannot form near the mantle wedge corner. Instead, the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water-rich low-viscosity mantle when it is near the arc. As the ridge moves away from the arc, a tipping point is reached at which that material is rapidly released from the upwelling zone, resulting in rapid changes in the character of the crust formed at the ridge.     

华北板块北缘中——-西段早白垩世钾玄岩地球化学特征及对比研究

对内蒙古阿拉善盟苏宏图地区一套早白垩世富钾碱性玄武岩中主量元素和微量元素进行测试, 结果表明: 岩石以富碱(K₂O+Na₂O含量为5.20%~8.92%)、高K₂O/Na₂O比值(0.59~1.03)和高TiO₂ (1.59%~2.05%, 平均1.82%)为主要特征, 为钾玄岩系列。该火山岩富集大离子亲石元素、轻稀土元素及高场强元素, 有微弱的Eu异常(δEu=0.88~1.11)。岩石地球化学特征表明, 华北板块北缘西段的苏宏图组火山岩与中段四子王旗地区及白云鄂博南部的早白垩世火山岩有较大的相似性。苏宏图组火山岩、四子王旗钾玄岩以及白云鄂博南部玄武岩均为钾玄岩系列, 三地火山岩在喷出过程中都较少经历地壳混染作用, Nb和Ta的负异常更可能是古亚洲洋俯冲物质影响的结果。同位素和微量元素分析结果表明, 苏宏图组火山岩与四子王旗钾玄岩源区为被古亚洲洋俯冲板块交代的EMⅠ型富集软流圈地幔, 白云鄂博南部玄武岩富集程度稍弱于其他两地, 但三地源区有相似的演化历史, 都经历了亏损的软流圈地幔交代富集过程。三地火山岩相似的源区演化历史表明, 华北板块北缘中?西段在早白垩世可能有相似的构造运动, 在古亚洲洋闭合后, 晚三叠世南北向挤压缩短, 地壳强烈加厚, 而后下部岩石圈拆沉, 已俯冲的古亚洲洋板块的物质重熔, 对亏损的软流圈地幔进行交代富集, 最终富集的软流圈地幔上涌, 形成沿华北板块北缘中?西段东西向呈线状分布的早白垩世钾玄岩。     

Identification of Hercynian shoshonitic intrusive rocks in central Hainan Island and its geotectonic implications

Hercynian (Variscan) orogenic belts, including the European-NW African orogen, the Appalachian orogen of North America and the central Asia-Mongolia- Hinggan orogen, etc., are widely distributed in the world. Their extensions are often several thousand ki…     

粤东中生代花岗质火山-侵入杂岩成因的Nd同位素制约

运用Ｎｄ同位素及稀土元素Ｎｄ的含量关系对岩浆成因的制约,结合区域地质地球化学特征,确定粤东地区中生代花岗质火山－侵入杂岩的成因机制,不是幔源岩浆同化地壳物质并发生分离结晶作用以及壳幔两种来源岩浆的混合作用,而是地壳深熔作用,即古老地壳基底岩石的部分熔融作用。