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1.
以山东西部西董地区中生代青山群火山岩为研究对象,在进行系统的元素-同位素分析的基础上,探讨其岩浆源区和岩石成因,同时丰富该区域岩石地层的地球化学特征。西董青山群火山岩Si O2含量变化于52.8%~59.8%之间,主要为钙碱性系列岩石,岩石组合为玄武质粗面安山岩-玄武质安山岩-粗面安山岩,Mg O、TFe2O3、Mn O、Ti O2、Ca O与Si O2具有明显的负相关关系,Al2O3和Na2O与Si O2具有正相关关系;微量元素富集Rb、Ba、Th、U和K等大离子亲石元素,亏损Nb、Ta和Ti等高场强元素,稀土元素总量∑REE变化范围为100×10-6~187×10-6,(La/Yb)N比值范围为8.2~11.1,无明显的Eu异常(δEu=0.9~1.02);显示富集且均一的Sr-Nd同位素组成((87Sr/86Sr)t=0.7047~0.7050,εNd(t)=-16.5~-14.2)。西董青山群火山岩的Nd、Sr同位素组成与Si O2缺乏明显负相关关系,Nb/Ta与La/Nb之间不具有负相关关系以及Nb与Th之间具有正相关关系,指示岩浆在上升和演化过程中地壳混染作用对青山群火山岩岩浆化学组成的影响并不显著;火山岩Mg O含量明显高于玄武岩部分熔融形成的熔体,说明青山群中基性火山岩岩浆的形成过程可能与大陆地壳和地幔岩系之间的相互作用有关。综合分析表明,西董中生代青山群火山岩是大陆下地壳物质经拆沉作用于地幔环境中发生部分熔融,与上地幔岩石发生交代作用后进入地表形成。 相似文献
2.
青藏高原北部黑石北湖新生代钾质火山岩的成因 总被引：3，自引：2，他引：1
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。 相似文献
3.
长白山区图们江流域新生代火山岩的岩石化学研究 总被引：5，自引：3，他引：5
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。 相似文献
4.
近年来,东北地区地幔热演化过程的相关研究相对较少,而揭示东北地区地幔热演化过程的有效手段就是研究东北地区玄武岩的成分变异特征.系统总结并对比了大兴安岭北段早白垩世玄武质岩石和新生代玄武质岩石的化学成分变异,以便揭示研究区中生代晚期-新生代的地幔热演化过程.大兴安岭北段早白垩世玄武岩在化学上属于拉斑玄武岩系列,以亏损Nb、Ta、Ti等高场强元素为特征,它们的La/Nb和La/Ta比值分别介于1.8~5.6和30~87,暗示岩浆起源于岩石圈地幔;它们的初始87Sr/86Sr值、εNd（t）和εHf（t）值分别介于0.704 5~0.706 9、-1.52~+3.60和+1.74~+7.77,表明岩浆源区属于弱亏损-弱富集的岩石圈地幔;早白垩世玄武质岩石的Sr-Nd-Pb同位素成分指示岩浆源区是由DM和EMⅡ型地幔端元混合而成,并经历了俯冲流体的交代.表明大兴安岭北段早白垩世玄武质岩浆源区为受早期俯冲流体交代的岩石圈地幔.新生代超钾质和钾质玄武岩具有Nb-Ta的弱负异常,87Sr/86Sr值为0.704 7~0.705 7、εNd（t）值为-6.3~-0.8,而地幔捕掳体具有Sr-Nd同位素亏损特征;钠质玄武岩具有Nb-Ta的正异常,较超钾质和钾质玄武岩具有低的87Sr/86Sr（0.703 5~0.704 2）以及高的εNd（t）值（+3.4~+6.6）,类似MORB的同位素组成,这些特征说明大兴安岭北段新生代玄武质岩石起源于软流圈地幔.综上所述,大兴安岭北段早白垩世和新生代玄武质岩石成分的差异不仅指示其岩浆源区从岩石圈地幔转变为软流圈地幔,更为重要的是它揭示了研究区地幔的热演化过程——从早白垩世高的地温梯度到新生代低的地温梯度的转变.这一过程也是岩石圈从中生代晚期到新生代逐渐增厚的过程.结合区域构造演化,可以得出大兴安岭北段早白垩世的玄武质岩浆作用与岩石圈伸展、减薄形成的裂陷作用相关,而新生代玄武质岩浆作用则与陆内裂谷作用相关. 相似文献
5.
藏北蚕眉山地区中新世火山岩地球化学特征及成岩过程研究 总被引：1，自引：0，他引：1
青藏高原北部蚕眉山地区中新世火山熔岩属于可可西里新生代火山岩带中段的羌巴欠岩省,从早至晚由玄武粗安岩、粗安岩、粗面英安岩等3类岩石组成,全岩K-Ar法年龄为12.81~14.51Ma。玄武粗安岩、粗安岩和粗面英安岩的Si O2含量分别为53.85%~56.02%、59.22%~61.84%、64.81%~67.70%。火山岩K2O+Na2O含量为6.21%~8.20%,K2O含量为3.17%~4.94%,K2O/Na2O=1.01~1.52,Ti O2含量为0.77%~1.88%,属钾玄岩系列,ACNK=0.54~0.90,KNA=0.57~0.72,属偏铝质岩石。LREE强烈富集,ΣREE=297×10-6~939×10-6,(La/Yb)N=45.1~127.4,δEu=0.73~0.95,高度富集LILE(K、Rb、Cs、Sr、Ba、U、Th),相对贫HFSE(Nb、Ta、Zr、Hf)。根据地球化学特征并结合区域对比,认为岩浆应来源于EMⅡ型地幔的部分熔融。氧化物Harker图解、Mg#值对微量元素协变图解、La/Sm La图解以及Mg#值和稀土元素含量变化特征等表明玄武粗安岩和粗安岩属同源岩浆演化产物,并经历了较高程度的结晶分异演化;而粗面英安岩则属于另一岩浆系列,由先期经一定程度演化的岩浆在地壳深部冷却固结的岩石后期经再次部分熔融所形成。据此认为,青藏北部新生代深部地幔熔融等岩浆作用过程应有比地表火山岩更宽的时、空尺度,区域挤压构造体制使部分幔源岩浆被封闭在深部而冷却成岩,因此,新生代火山岩的时空分布规律,一定程度上与区域构造体制的发展和迁移有关。 相似文献
6.
长白山火山活动历史、岩浆演化与喷发机制探讨 总被引：18，自引：0，他引：18
广义的长白山火山在我国境内包括著名的天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。图们江火山和望天鹅火山活动都始于上新世,喷发活动分别介于上新世—中更新世（5.5~0.19 Ma）和上新世—早更新世（4.77 ~2.12 Ma）。天池火山和龙岗火山属于第四纪火山,喷发活动从早更新世（~2 Ma）持续到全新世。图们江火山岩为溢流式喷发的拉斑玄武岩,望天鹅火山、天池火山和龙岗火山母岩浆都是钾质粗面玄武岩,但经历了不同的演化过程。天池火山和望天鹅火山都经历了钾质粗面玄武岩造盾、粗面岩造锥和晚期碱性酸性岩浆（碱流岩和碱性流纹岩）的喷发;龙岗火山来自地幔的钾质粗面玄武岩浆则未经演化和混染直接喷出地表。图们江火山岩以溢流式喷发的拉斑玄武岩为主,少量玄武质粗安岩等。天池火山造盾之后,地壳岩浆房和地幔岩浆房具互动式喷发特点,来自地幔的钾质粗面玄武岩浆一方面在天池火山锥体内外形成诸多小火山渣锥,另一方面持续补给地壳岩浆房发生岩浆分离结晶作用和混合作用,分别导致双峰式火山岩分布特征和触发千年大喷发。火山岩微量元素和Sr-Nd-Pb同位素示踪揭示,长白山东（图们江火山、望天鹅火山和天池火山）、西（龙岗火山）两区显示地幔非均一性,东区岩浆源区具有软流圈地幔与富集岩石圈地幔混合特征,西区岩浆源区具有相对亏损的较原始地幔特征。西太平洋板块俯冲—东北亚大陆弧后引张是长白山火山活动的动力学机制。 相似文献
7.
《吉林大学学报(地球科学版)》2015,(Z1)
安山岩广泛出露于汇聚板块边界,表明其成因与板块俯冲有关。另一方面,大陆地壳整体上被认为是地幔部分熔融的产物,平均成分为安山质,是与亏损地幔(DM)互补的地球化学储库。然而,地幔部分熔融的产物通常具有玄武质成分,这与大陆地壳的安山岩平均成分不符。这个问题一直困扰着地球化学家们。因此,揭示安山岩的成因对于理解大陆地壳的形成和壳幔分异历史具有十分重要的意义。目前普遍接受的汇聚板块边界安山岩的成因模型包括:1)玄武岩输入模型;2)安山岩模型。这两者的主要区别在于地幔来源的初始岩浆是玄武质还是安山质。玄武岩输入模型认为初始岩浆为玄武质,安山岩是由初始玄武质岩浆在壳内的分异如分离结晶、地壳混染以及岩浆混合等地质过程形成的。安山岩模型则认为初始岩浆为安山质,是富水地幔橄榄岩部分熔融或俯冲板片部分熔融产生的埃达克质熔体与地幔橄榄岩反应的产物。无论哪种模型,俯冲隧道内的板片-地幔相互作用是形成安山岩地幔源区的关键过程,俯冲地壳物质是形成汇聚板块边界安山岩重要的组分来源。板片俯冲进入地幔是引起地幔化学不均一性的重要地质过程。在大洋俯冲带,俯冲洋壳释放流体/熔体交代上覆地幔楔并引发地幔楔发生部分熔融,从而形成同俯冲弧岩浆岩。大陆俯冲带通常缺乏对应的同俯冲弧岩浆岩,但在造山带内部和仰冲板块边缘通常发育大量同折返和碰撞后岩浆岩,这些岩浆岩为研究俯冲陆壳物质再造和再循环及其壳幔相互作用提供了重要的研究载体。大别造山带是三叠纪华南板块向华北板块俯冲碰撞形成的碰撞造山带,发育有大量的碰撞后火成岩,其中含有安山质火山岩。我们对大别造山带北淮阳碰撞后安山质火山岩进行了详细的地球化学研究,包括全岩主微量、Sr-Nd-Pb-Hf同位素分析,锆石SIMS U-Pb定年、O同位素和Lu-Hf同位素分析。这些火山岩具有变化的Si O2质量分数(50.3%~63.9%),Mg O质量分数(1.2%~4.7%),Mg#(32.4~63.6),Na2O+K2O质量分数(5.0%~8.5%),其岩性主体为粗面安山岩,还有少量玄武粗面安山岩、安山岩、英安岩和粗面岩。锆石SIMS U-Pb定年结果显示,北淮阳安山质火山岩的形成年龄为早白垩世(123~130 Ma)。另外,它们含有丰富的残留锆石核,其U-Pb年龄为新元古代和三叠纪,分别与大别造山带超高压变质岩的原岩和变质年龄一致。它们具有弧型的微量元素分布特征,即富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损高场强元素(HFSE);具有富集的Sr-Nd-Hf同位素组成,即高的初始87Sr/86Sr比值(0.707 5~0.711 0),低的εNd(t)值(-23.1~-15.0)和εHf(t)(-29.8~-18.3)。锆石具有低的εHf(t)值(-31.0~17.8)和变化的δ18O值(4.4‰~6.8‰)。这些元素和同位素特征表明,北淮阳碰撞后安山质火山岩来源于富集的造山带岩石圈地幔。三叠纪华南陆块与华北陆块碰撞过程中,俯冲的华南陆壳在地幔深度发生部分熔融产生的长英质熔体在大陆俯冲隧道内交代上覆华北岩石圈地幔楔橄榄岩,熔体-橄榄岩反应形成了富沃富集的地幔交代体。该地幔交代体在早白垩世发生部分熔融就形成了北淮阳碰撞后安山质火山岩。因此,大陆俯冲隧道内的板片-地幔相互作用是形成碰撞造山带安山质岩石的关键过程。 相似文献
8.
长白山天池火山粗面玄武岩的喷发历史与演化 总被引：18，自引：11，他引：18
本文新提出的年代学和岩石化学结果，进一步从天池火山与区域火山活动的关系，论述了天池火山造盾、造锥历史和岩浆结晶分异转型的时间约束，早更新世早期（2Ma前）开始粗面玄武岩的造盾，早更新世晚期（约1Ma）粗面玄武岩向粗安岩、粗面岩演化，中更新世是粗面岩造锥的主阶段，到了晚更新世（约0．1Ma）粗面岩向碱流岩演化。在中-晚更新世来自地壳岩浆房的粗面岩、碱流岩造锥过程中，来自地幔的粗面玄武岩浆喷发活动始终没有间断过。由于来自地幔粗面玄武质岩浆持续向地壳岩浆房补给，所以天池火山是一座长寿命的火山。岩浆的结晶分异作用和混合作用是天池火山岩浆演化的两个最重要过程，前者形成天池火山双峰式火山岩分布特征，后者成为天池火山喷发的触发机制。天池火山在晚更新世-全新世碱流质岩浆主喷发期兼有少量玄武质粗安岩、粗安岩或粗面质岩浆的交替喷出，揭示了天池火山的地壳岩浆房熔体的分层结构特点，由于来自地幔粗面玄武质岩浆注入地壳岩浆房，导致不同层位岩浆的扰动和混合作用，触发天池火山的喷发。 相似文献
9.
再循环洋壳和沉积物在壳.幔循环过程中扮演着重要的角色。前人研究指出板片俯冲与海南地幔柱形成关系密切,然而地幔柱源区中是否存在再循环沉积物仍然不得而知。本文将首次报导海南岛新生代玄武质熔岩的Hf同位素,并将新的Sr-Nd-Pb同位素与前人数据相结合,限定再循环物质的组成及其年龄。与同位素地幔趋势相比,给定εNd时,海南岛玄武质熔岩的εHf值更低;而且Eu/Eu*>1、Sr/Sr*>1,与Al2O3、MgO都不存在相关性,说明Eu和Sr的正异常是继承了源区的特征,指示源区中存在再循环洋壳物质的参与。海南岛玄武质熔岩的Ce/Pb与εNd、εHf呈正相关关系,Nd/Pb与87Sr/86Sr呈负相关关系,它们都指向了沉积物端元。根据εHf-εNd模型和Pb同位素演化模型,推断源区中再循环洋壳和沉积物相对年轻,其年龄应小于1.0Ga。这些再循环洋壳和沉积物将进入深部地幔,并与地幔中的橄榄岩反应形成二阶段辉石岩,随着海南地幔柱不断上升而发生熔融,最终熔体喷发至地表形成海南岛玄武质熔岩。 相似文献
10.
西藏高原西北缘钾质火山岩地球化学特征及其地质涵义 总被引：1，自引：0，他引：1
西藏高原西北缘钾质火山岩属橄榄玄粗岩(shoshonite)系列。其K-Ar同位素年龄变化于0.28~7.79Ma,属中新世末—更新世;其K2O/Na2O几乎均>1,Mg#值有较宽的范围(从0.51~0.64),且具有较低的Cr,Ni含量(均≤326×10-6)。它们代表着经历了不同程度分异演化的岩浆结晶产物。研究区钾质火山岩(shoshon-ites)以高度富集LREE和LILE(K,Rb,Sr,Ba,Th)以及高度亏损HREE和HFSE(Nb,Ta,Zr,Hf,Ti)等元素为特征。(La/Yb)n高达29.82~84.94。钾质火山岩对球粒陨石标准化的REE配分型式均为向右陡倾的LREE富集型。对原始地幔标准化的痕量元素蛛网图均具有明显的Nb,Ta,Ti负异常。钾质火山岩具有较高的87Sr/86Sr(0.707755~0.710426)和较低的143Nd/144Nd(0.51196~0.512439)比值。上述特征表明这些火山岩来源于富集的地幔源区。钾质火山岩较高的Ba/Nb比值(40.84~97.28)、较高的Th/Ta及Ce/Yb比值(131~366)、和较低的Nb/Y比值(0.9~3.3),结合Th/Yb-Ta/Yb及(Th×100)/Zr-(Nb×100)/Zr判别图,提示了它们也形成于活动大陆边缘,但与其伴生的钙碱性火山岩为早期与俯冲作用有关的火山岩,而钾质火山岩则为晚期岩石圈拆沉作用的产物。 相似文献
11.
火山渣锥是白头山(或长白山)火山喷发的重要产物,主要沿熔岩台地周边呈圆锥状寄生小火山锥体分布。野外特征显示,火山渣是火山渣锥的重要组成;岩石手标本显示,火山渣具有气孔构造,样品自顶部至底部,颜色从赤色、赤褐色,向褐色、灰色发生转变。地球化学特征表明,火山渣岩性包括玄武岩、粗面玄武岩、玄武岩质粗面安山岩、粗面安山岩岩,具有较高SiO_2(46.22%~55.38%),Al_2O3含量(15.28%~22.11%),低MgO(2.05%~4.94%),FeOT(6.79%~14.76%)的特征;同时具有较高的碱Na_2O/K_2O(Na_2O/K_2O1)比值,为钠质火山岩。其轻稀土(LREE)和重稀土(HREE)分异明显,具有弱的δEu正异常,并且具有富集K、Rb、Ba、Sr等大离子亲石元素(LILE)和相对亏损Nb、Ti等高场强元素(HFSE)的特征。此外,火山渣的分异指数(DI)范围为36.93~64.48,高于造盾阶段的幔源玄武岩的分异指数;其固结指数(SI)为10.73~24.09,低于早期幔源玄武岩的固结指数(SI=25~45),这些特征说明火山渣成分发生了较高程度的岩浆分异作用。同时,火山渣的Nb/La、Sm/Nd、La/Nb和Ba/Nb比值几乎全部介于幔源玄武岩和大陆地壳之间,说明具有明显的地壳混染的特征。因此,我们认为研究区火山渣的岩浆可能是由幔源基性玄武岩上升过程中发生分离结晶和地壳混染作用形成的。 相似文献
12.
Yung-Tan LEE Ju-Chin CHEN Jyh-Yi SHIH Kung-Suan HO Huai-Jen YANG Meng-Lung LIN Yen-Tsui HU Ching-Hsing CHIU 《《地质学报》英文版》2013,87(1):102-117
Cenozoic（Miocene to Pleistocene） basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt.We present major,trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area.On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province,we suggest that these basalts may belong to the same magmatic system.The alkali basalts found in Jiangsu province have higherΣFeO,MgO,CaO,Na2O, TiO2 and P2O5 and incompatible elements,but lower Al2O3 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine,pyroxene and minor plagioclase.In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/ΣFeO ratios.The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB.Partial loss of the mantle lithosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma.The 143Nd/144Nd vs.87Sr/86Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EMI component in the study area.According to Shaw’s equation,the basalts from Jiangsu province may be formed by l%-5%partial melting of a depleted asthenospheric mantle source.On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen（1982）,we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts. 相似文献
13.
A geochemical study of magmatism associated with the initial stages of back-arc spreading 总被引：4，自引：0，他引：4
Stephen D. Weaver Andrew D. Saunders Robert J. Pankhurst John Tarney 《Contributions to Mineralogy and Petrology》1979,68(2):151-169
Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading.Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with CeN/YbN = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although CeN/YbN ratios and 87Sr/86Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher 87Sr/86Sr (0.7039) and CeN/ YbN (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present 87Sr/86Sr ratios.Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas. 相似文献
14.
Heterogeneous mantle source and magma differentiation of quaternary arc-like volcanic rocks from Tengchong,SE margin of the Tibetan Plateau 总被引：7，自引：0，他引：7
The Tengchong volcanic field north of the Burma arc comprises numerous Quaternary volcanoes in the southeastern margin of
the Tibetan Plateau. The volcanic rocks are grouped into four units (1–4) from the oldest to youngest. Units 1, 3 and 4 are
composed of olivine trachybasalt, basaltic trachyandesite and trachyandesite, and Unit 2 consists of hornblende dacite. The
rocks of Units 1, 3, and 4 form a generally alkaline suite in which the rocks plot along generally linear trends on Harker
diagrams with only slight offset from unit to unit. They contain olivine phenocrysts with Fo values ranging from 65 to 85 mol%
and have Cr-spinel with Cr# ranging from 23 to 35. All the rocks have chondrite-normalized REE patterns enriched in LREE and
primitive mantle-normalized trace element patterns depleted in Ti, Nb and Ta, but they are rich in Th, Ti and P relative to
typical arc volcanics. Despite minor crustal contamination, 87Sr/86Sr ratios (0.706–0.709), εNd values (−3.2 to −8.7), and εHf values (+4.8 to −6.4) indicate a highly heterogeneous mantle source.
The Pb isotopic ratios of the lavas (206Pb/204Pb = 18.02–18.30) clearly show an EMI-type mantle source. The underlying mantle source was previously modified by subduction
of the Neo-Tethyan oceanic and Indian continental lithosphere. The present heterogeneous mantle source is interpreted to have
formed by variable additions of fluids and sediments derived from the subducted Indian Oceanic lithosphere, probably the Ninety
East Ridge. Magma generation and emplacement was facilitated by transtensional NS-trending strike-slip faulting. 相似文献
15.
We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr–143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNdi (??5.2 to ??9.4) and radiogenic εSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events. 相似文献
16.
The postshield and posterosional stages of Haleakala Volcano contain intercalated alkalic basalt and evolved alkalic lavas. Isotopic and incompatible element abundance ratios in the Haleakala postshield basalts changed systematically with time, providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources. Specifically, a depleted, i.e. low87Sr/86Sr and high143Nd/144Nd, mantle component is more abundant in younger lavas. However, as magma-production rates decreased during the postshield and posterosional stages, basaltic melts in magma reservoirs cooled and fractionated, leading to evolved residual melts such as hawaiite. Because primary basalt compositions changed with time, the evolved Haleakala lavas formed from a range of parental compositions. However, basalts and evolved lavas of similar age and isotopic ratios (Sr and Nd) have major and trace element contents that are consistent with a crystal-fractionation model. Although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both Haleakala and Mauna Kea volcanoes, there are important differences between their lavas. For example, compositional differences between the hawaiite suites at Haleakala and Mauna Kea indicate that, on average, the evolved lavas at Haleakala formed at lower pressures. Also, at Haleakala basalts are intercalated with hawaiites, whereas at Mauna Kea basalts and hawaiites are separated by a sharp boundary. These differences probably reflect a higher magma supply rate to the Haleakala volcano. 相似文献
17.
《International Geology Review》2012,54(2):206-223
We report elemental and Nd–Sr isotopic data for three types of Ordovician volcanic and gabbroic rocks from the Sharburti Mountains in the West Junggar (Xinjiang), Northwest China. Gabbros and Type I lavas occur in the Early Ordovician Hongguleleng ophiolite whereas Type II and III lavas are parts of the Middle Ordovician Bulukeqi Group. Gabbros and Type I lavas are tholeiites with a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a crystallization sequence of plagioclase–clinopyroxene, suggesting formation at a mid-oceanic ridge. Type II lavas are Nb-enriched basalts (NEBs, Nb = 14–15 ppm), which have E-MORB-like REE patterns and Nb/Yb and Th/Yb ratios. They come from mantle metasomatized by slab melts. Type III lavas are further divided into two sub-types: (1) Type IIIa is tholeiitic to calc-alkaline basalts and andesites, with REE patterns that are flat or slightly LREE enriched, and with a negative Nb anomaly and Th/Yb enrichment, indicating that they were generated above a subduction zone; (2) Type IIIb is calc-alkaline basalts and andesites, which are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb enrichment, suggesting generation in a normal island-arc setting. The initial 87Sr/86Sr ratios of Type III lavas range from 0.70443 to 0.70532 and ?Ndt ranges from +1.5 to +4.5, suggesting that these melts were derived from mantle wedge significantly modified by subducted material (enriched mantle I (EMI)) above a subduction zone. Contemporary tholeiitic to calc-alkaline basalt–andesite and NEB association suggest that the NEBs erupted during development of the tholeiitic to calc-alkaline arc. We propose a model of intra-oceanic subduction influenced by ridge subduction for the Ordovician tectono-magmatic evolution of the northern West Junggar. 相似文献
18.
D. T. Vaniman B. M. Crowe E. S. Gladney 《Contributions to Mineralogy and Petrology》1982,80(4):341-357
Hawaiite-type lavas were erupted in three cycles (3.7, 1.2, and 0.3 M.y.) at Crater Flat, Nevada. The compositions of all three cycles, considered together, form a straddling alkalic series as defined by Miyashiro, in which the less evolved basalts plot near the normative olivine-diopside divide and the more evolved basalts project into hypersthene or nepheline fields. Fractionation modeling based on the oldest cycle allows the removal of olivine, clinopyroxene, and amphibole to arrive at the more evolved hawaiite compositions. In general, fractionation of phlogo-pite or feldspar is limited by the fractionation modeling and by Eu/REE relations. In detail, all hawaiites within one cycle (3.7 M.y.) need not be derived from a single parent magma. Varied parentage is more evident between cycles although all cycles are consistently of hawaiite composition. Basalts of the youngest two cycles are generally enriched in trace elements. Superimposed on this enrichment is a lack of Rb variation, leading to Rb/Sr ratios far lower than required to generate the high 87Sr/86Sr ratio (0.707) typical of basalts in this region. The very low Rb/Sr ratios limit processes that may lead to trace-element enrichment during magma evolution (cyclic recharge of a fractionating magma chamber). Decreased fractions of mantle melting leaving phlogopite in the residuum or an earlier event of metasomatic transport from phlogopite-bearing mantle rocks into a phlogopite-absent mantle assemblage might explain the observed trace-element enrichment with low Rb/Sr. 相似文献
19.
分布于云南宾川地区、厚逾5000m的晚二叠世峨眉山玄武岩以拉斑玄武岩为主,少量碱性玄武岩。根据它们的岩相学和主量元素、微量元素特征,将其划分为两个地球化学类型:低钛玄武岩(LT)和高钛玄武岩(HT)。低钛玄武岩主要分布于岩石剖面的中下部,其主要地球化学标志为低Ti/Y(<500),高Mg#(048～067),低的∑REE(<120μg/g)、(La/Yb)N(<6)和(Sm/Yb)N(<25)。高钛玄武岩分布于剖面的顶部,以高Ti/Y(>500),低Mg#(039～053)、高的∑REE(>150)、(La/Yb)N(>9)和(Sm/Yb)N(>3)为特征。根据LT的分异指数和Th、U异常特征等,将其分为LT1和LT2两个亚类。其中LT1位于岩石剖面下部,表现为明显的Th和U正异常,高Mg#(061～067)和低Nb/U比值等。LT2位于岩石剖面中部,具Rb、Ba正异常,无Th和U异常和较低的分异指数(Mg#=048～054)。LT玄武岩可能是峨眉地幔热柱主活动期地幔柱头部熔融的产物,后经较弱的橄榄石+单斜辉石±斜长石结晶分异形成。剖面最底部的LT1玄武岩显示较强的“壳源”印记,可能与岩石圈地幔中富集组份的活化有关。这些壳源物质的参与程度自底部向上有降低的趋势。晚期HT玄武岩为地幔热柱消亡期的产物,其在地壳浅部经历强烈的以斜长石为主的结晶分异,壳源物质的混染不明显。 相似文献
20.
L. López-Escobar M. A. Parada R. Hickey-Vargas F. A. Frey P. D. Kempton H. Moreno 《Contributions to Mineralogy and Petrology》1995,119(4):345-361
Calbuco volcano is a Late Pleistocene-Holocene composite stratovolcano located at 41°20 S, in the southern region of the Southern Volcanic Zone of the Andes (SSVZ; 37°–46° S). In contrast to basalt and basaltic andesite, which are the dominant lava types on the volcanic front from 37° to 42° S, Calbuco lavas are porphyritic andesites which contain a wide variety of crustal xenoliths. They have SiO2 contents in the 55–60% range, and have comparatively low K2O, Rb, Ba, Th and LREF abundances relative to other SSVZ centers. Incompatible element abundance ratios are similar to those of most SSVZ volcanics, but 87Sr/86Sr and 143Nd/144Nd are respectively higher and lower than those of adjacent volcanic centers. Basalts from nearby Osorno stratovolcano, 25 km to the northeast, are similar to other basaltic SSVZ volcanoes. However, basalts from several minor eruptive centers (MEC), located east of Calbuco and Osorno volcano along the Liquiñe-Ofqui fault zone (LOFZ), are enriched in Ba, Nb, Th and LREE, and have higher La/Yb and lower Ba/La, K/La and Rb/La. 87Sr/86Sr and 143Nd/144Nd in MEC basalts are respectively lower and higher than those of Osorno and Calbuco lavas. We suggest that MEC basalts were produced by lower extents of mantle melting than basalts from Osorno and other SSVZ stratovolcanoes, probably as a result of lower water content in the source of MEC basalts. Calbuco andesites formed from basaltic parents similar to Osorno basalts, by moderate pressure crystallization of a hornblende-bearing assemblage accompanied by crustal assimilation. Hornblende stability in the Calbuco andesites was promoted by the assimilation of hydrous metasedimentary crustal rocks, which are also an appropriate endmember for isotopic trends, together with magma storage at mid-crustal depths. The unique characteristics of Calbuco volcano, i.e. the stability of hornblende at andesitic SiO2 contents, low 143Nd/144Nd and high 87Sr/86Sr, and abundant crustal xenoliths, provide evidence for crustal assimilation that is not apparent at more northerly volcanoes in the SSVZ. 相似文献