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1.
MORB 是玄武岩中研究得最详细的玄武岩类,可分为N-MORB 和E-MORB 两类。通常认为,N-MORB 和OIB 都是独立的端元,分别来自亏损和富集的地幔源岩,而E-MORB 则是N-MORB 与OIB 混合的结果。本文研究表明,E-MORB 具复杂的成因,洋脊深度、洋脊扩张速率及源区部分熔融程度及压力不是造成E-MORB 富集的主要原因。压力及部分熔融程度对玄武岩成分的影响远小于地幔不均一性的影响。推测E-MORB 可能有两个主要的形成方式：1） 由较深处略富集的地幔发生部分熔融而成;2） 由N-MORB 与OIB 混合形成。玄武岩微量元素频率直方图表明,N-MORB 基本上保持了来自亏损地幔源区的特征;OIB 则多多少少受到外来物质加入或与N-MORB 混合的影响; E-MORB 则是N-MORB 受OIB 影响的产物。OIB 与E-MORB 似乎没有本质上的区别, 仅仅是受影响和混合程度的不同而已。OIB 富集LILE,可能既有继承了来自源区的特征（深部富集地幔、循环的古洋壳、循环的陆壳、大陆岩石圈地幔、LVZ 熔体层或早期交代岩脉等）,也可能有外来物质加入的影响（与N-MORB 发生不同程度的混合作用）。3 类玄武岩的87Sr/86Sr 和143Nd/144Nd 同位素频率分布与早先的结论一致,但206Pb/204Pb、207Pb/204Pb和208Pb/204Pb同位素频率分布显示OIB 具有更加复杂的特征。 相似文献
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新疆甜水海地块种羊场石炭纪火山岩年代学和地球化学:岩石成因和地质意义 总被引：1，自引：1，他引：0
甜水海地块西段的种羊场地区发育一套互层状产出的玄武岩-玄武安山岩-流纹岩,本文对其进行了岩石学、同位素年代学和地球化学研究。结果表明,流纹岩LA-ICP-MS锆石U-Pb定年获得三组年龄:343.5±4.1Ma表明火山岩的形成时代为早石炭纪,2439±26Ma和1988±36Ma说明甜水海地块存在前寒武纪结晶基底。其中玄武质岩石岩性从拉斑系列、钙碱性系列向碱性系列过渡,呈现出E-MORB(OIB)、大陆板内拉张和岛弧的混合特征,与典型弧后盆地Okinawa玄武岩有一定的差异,表明其可能是异常陆缘弧后盆地拉张裂解的产物。玄武质岩石和流纹岩的主量元素、稀土元素和微量元素比值对的差异表明它们不是同源岩浆演化的产物,玄武质岩石的源区为类似E-MORB(OIB)的岩石圈地幔,且发生了部分熔融,原始岩浆上升过程中经历了矿物分离结晶和地壳混染作用。流纹岩属于高硅高碱的钙碱性火山岩,是上地壳部分熔融的产物。种羊场早石炭纪火山岩可能代表了古特提洋西端早期扩张的记录,为西昆仑-喀喇昆仑地区晚古生代多岛洋格局提供了新的证据。 相似文献
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《Journal of Asian Earth Sciences》2000,18(1):109-121
The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(LaN/SmN)ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(LaN/SmN)ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent. 相似文献
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云南香格里拉县洛吉地区出露由洛吉组玄武岩和基性岩墙群组成的基性杂岩,其母岩浆起源于亚碱性-碱性系列的拉斑玄武系列,地球化学特征表明该基性杂岩体形成于构造变动的复杂环境,并与不同源区岩浆混合及一定程度的壳幔混染有关,其中的碱性玄武岩与基性岩墙属具有相似性OIB玄武岩,是深部地幔石榴石橄榄岩与尖晶石橄榄石小部分熔融的产物,形成于大陆内或陆缘裂谷转变为盆缘造山带的过渡环境,亚碱性玄武岩属于E-MORB并有向OIB过渡趋势,由源区相对浅的尖晶石橄榄石熔融形成,为洋中脊向大陆过渡环境的蛇绿岩质分子,属甘孜-理塘结合带残余产物。 相似文献
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N-MORB和E-MORB数据挖掘——玄武岩判别图及洋中脊源区地幔性质的讨论 总被引：11，自引：7，他引：4
MORB分为N-MORB和E-MORB,二者的区分通常是以LREE亏损和富集为标志。玄武岩的基本理论主要是在MORB研究成果的基础上建立起来的。通常认为,N-MORB和OIB是两个独立的端元,E-MORB是N-MORB与OIB不同程度混合形成的。在20世纪70~80年代,玄武岩构造环境判别图的建立极大促进了玄武岩在地球动力学及大地构造背景研究的进展,其方法应用持续盛行了几十年。然而,国外学者现在已逐渐淡化玄武岩判别图解的应用,但在中国仍有方兴未艾之势。本文利用Pet DB数据库5万多个MORB数据的投图表明,全体样品投图的效果显然比抽样和典型样品得出的结果更扎实可靠,许多原先被广泛应用的主量、微量元素判别图无论从实践上还是理论上均存在一些问题。本文的研究结果进一步表明,MORB样品既有LREE亏损的,又有富集的,但亏损和富集程度与早先定义的差异较大,揭示了洋脊下地幔不均一性要比早先认识的更为复杂,成分范围变化更大,仅少数是强烈亏损的,部分可能是相当富集,甚至可以与OIB源区相类比;玄武岩构造环境判别图需要重新审视,基于岩浆成因机理的多维度、高置信度的判别体系有待于建立。 相似文献
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Prasanta K. Patro Ujjal K. Borah G. Ashok Babu B. Veeraiah S. V. S. Sarma 《Journal of the Geological Society of India》2017,89(6):711-718
The geochemistry of the mafic xenoliths from Baspa valley of Himachal Pradesh, India has been investigated to characterize their protoliths on the basis of immobile elements, especially trace elements including REE. The mafic xenoliths occur within the Kinnaur Kailash granite (KKG) and their geochemistry show that they have tholeiitic nature with basaltic composition. Compositionally, they range from ‘depleted’ to ‘enriched’ MORB as observed on the binary diagrams of Ti vs V and Zr vs Ti and on ternary diagrams of Zr-Ti-Y and Th-Zr-N. Likewise, they match with various enriched or ‘transitional’ MORB types as evident from their Zr vs Nb binary plot. Their enriched character when compared with N-MORB, E-MORB and OIB rocks on chondrite and primordial mantle normalized plots reveals that it is intermediate to that of E-MORB and OIB. The geochemistry of the rocks suggest that the enriched components are probably derived by melting of a mantle source with E-MORB or OIB rather than due to the crustal contamination. The study carried out emphasize that the mafic xenoliths have developed in rift environment, and that they are not volcanic rocks of island arc related to subduction tectonics. It is visualized that the mafic xenoliths were formed as cumulate rocks from the tholeiitic magmas that were rising to lower crust levels in a rift environment, which at a later stage got entrapped as restitic material in the host Kinnaur Kailash granite formed in a collision environment, and propose a change of regime from rift related to collision environment prior to Palaeozoic period. 相似文献
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S. Harangi 《Lithos》1994,33(4):303-321
Early Cretaceous volcanic rocks (basanite to phonolite) from the Mecsek Mountains (South Hungary) represent the products of Late Mesozoic extension-related alkaline magmatism at the southern margin of the European plate. Two mafic groups have been distinguished: ankaramite-alkali basalt and Na-basanite-phonotephrite. Phonolites could have been formed from the Na-basanitic magma by low-pressure fractionation. The major and trace element characteristics of the Mecsek basalts are similar to those of alkaline basalts of other intraplate areas and have a St. Helena-type OIB affinity. The mantle source of the Mecsek volcanics could be similar to that proposed by Wilson and Downes (1991) as one of the mantle endmembers for extension-related Tertiary-Quaternary alkaline basalts in Europe. Geochemical modelling indicates that the primary magmas of the Na-basanite series were formed by about 4% partial melting, whereas ankaramites and alkali basalts originated by about 6% partial melting of a garnet-peridotite source. 相似文献
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《Journal of Asian Earth Sciences》1999,17(3):319-332
The Kermanshah ophiolite is a highly dismembered ophiolite complex that is located in western Iran and belongs to the Zagros orogenic system. The igneous rocks of this complex consist of both mantle and crustal suites and include peridotites (dunite and harzburgite), cumulate gabbros, diorites, and a volcanic sequence that exhibits a wide range in composition from subalkaline basalts to alkaline basalts to trachytes. The associated sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-water sedimentary rocks (e.g., dolomite, limestone, and pelagic sediments, including umber). Also present are extensive units of radiolarian chert. The geochemical data clearly identifies some of the volcanic rocks to have formed from two distinct types of basaltic melts: (i) those of the subalkaline suite, which formed from an initial melt with a light rare earth elements (LREE) enriched signature and incompatible trace element patterns that suggest an island arc affinity; and (ii) those of the alkaline suite with LREE-enriched signature and incompatible trace element patterns that are virtually identical to typical oceanic island basalt (OIB) pattern. The data also suggests that the trachytes were derived from the alkaline source, with fractionation controlled by extensive removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse volcanics together with the occurrence of a variety of Triassic–Cretaceous sedimentary rocks and radiolarian chert indicate that the studied volcanic rocks from the Kermanshah ophiolite represent off-axis volcanic units that were formed in intraplate oceanic island and island arc environments in an oceanic basin. They were located on the eastern and northern flanks of one of the spreading centers of a ridge-transform fault system that connected Troodos to Oman prior to its subduction under the Eurasian plate. 相似文献
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哀牢山-大象山变质杂岩带位于青藏高原东南缘，是西南三江地区重要的北西向造山带。杂岩带主要由各种类型的副片麻岩、片岩、石英岩、大理岩和斜长角闪岩构成，岩石发生强烈的糜棱岩化。本文在哀牢山变质杂岩带东南段元江和金平地区以及大象变质杂岩带北段老街和Pho Rang地区发现了中二叠-早三叠世的斜长角闪岩。岩石地球化学和锆石U-Pb年龄研究表明，这些斜长角闪岩可分为4组：第1组为元江斜长角闪岩，具有类似于E-MORB的稀土和微量元素配分曲线特征，锆石U-Pb定年显示其原岩形成年龄为272.5±1.7Ma；第2组为勐桥-马鞍底斜长角闪岩，相比于元江斜长角闪岩（第1组）具有更高的轻/重稀土分馏程度，其稀土和微量元素配分曲线类似于E-MORB，微量元素比值（Nb/Yb、Th/Yb等）显示具有沿MORB-OIB序列演化的趋势，勐桥和马鞍底斜长角闪岩中的Ti含量和Ti/Y比值等特征分别类似于峨眉山低Ti玄武岩和高Ti玄武岩。锆石U-Pb定年结果表明，勐桥-马鞍底斜长角闪岩形成于265.2±1.0Ma~266.2±1.0Ma和250.4±1.5Ma~248.7±1.6Ma，其中晚期岩石中含有261.2±1.5Ma~257.9±1.6Ma的继承锆石，该年龄与峨眉山玄武岩（约260Ma）近于同期。上述证据表明勐桥-马鞍底斜长角闪岩岩浆演化过程中可能混染了部分具有OIB属性的峨眉山地幔柱物质成分；第3组为大象山老街-Pho Rang斜长角闪岩，具有类似于OIB的稀土和微量元素配分曲线特征，微量元素含量和比值（TiO2=3.28%~4.31%，Nb/La=0.84~1.01，Ti/Y>500等）特征显示与峨眉山高Ti玄武岩相似的地球化学属性，表明峨眉山玄武岩在哀牢山-大象山变质杂岩带内广泛分布，该岩石成分可能为勐桥-马鞍底斜长角闪岩（第2组）的端元组分之一；第4组为大象山Pho Rang斜长角闪岩，该组岩石具有与元江斜长角闪岩（第1组）相似的E-MORB属性特征。进一步的研究表明，哀牢山-大象山变质杂岩带中具有类似于E-MORB属性的斜长角闪岩均表现出不同程度的Nb、Ta元素亏损和Rb、Ba等大离子元素富集，微量元素比值（Nb/Yb（0.72~5.29），Th/Yb（0.11~0.87），La/Nb（0.91~8.83））等特征类似于岛弧玄武岩，这些特征指示其原岩岩浆可能是俯冲环境下地幔楔岩石部分熔融的产物。结合哀牢山-大象山变质杂岩带、哀牢山-马江缝合带以及扬子地块之间的时-空关系，本文推测哀牢山-大象山变质杂岩带内的E-MORB类型岩浆岩形成于东古特提斯支洋（即哀牢山-马江洋）向东的俯冲过程，其俯冲持续时间为中二叠-早三叠世（272~248Ma）。 相似文献
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华南板块西南缘、越北地块以北桂西那坡县城以西及西南一带发育一套晚二叠世基性岩,由层状、似层状次火山岩相辉绿岩、辉绿玢岩及球状岩组成。根据岩石地球化学特征,那坡基性岩可划分为高Ti(TiO_22.8%和Ti/Y500)和低Ti两部分。高Ti基性岩为碱性玄武岩,而低Ti基性岩为拉斑玄武岩。与低Ti基性岩相比,高Ti基性岩整体具有相对较低的SiO_2、MgO和较高的FeO_t、P_2O_5,轻、重稀土分馏明显,富集大离子亲石元素(LILE)和高场强元素(HFSE),显示出似OIB地球化学特征,与峨眉山高Ti玄武岩具高度亲缘性;低Ti基性岩具有相对较高的SiO_2、MgO和较低的FeO_t、P_2O_5,稀土配分曲线较平坦,富集LILE,严重亏损HFSE(Nb、Ta),与岛弧玄武岩地球化学特征类似。从微量元素比值及相关图解对岩浆源区和构造环境判别,那坡高Ti基性岩来自富集OIB地幔源区,而低Ti基性岩兼具OIB和岛弧岩浆源区的过渡特征。结合岩石地球化学特征及区域地质背景,认为那坡高Ti基性岩可能为峨眉山地幔柱岩浆作用的产物,低Ti基性岩为古特提斯俯冲与峨眉山地幔柱共同作用的产物,揭示了那坡地区晚二叠世同时受到峨眉山地幔柱和古特提斯俯冲相互作用的影响。 相似文献
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辽河盆地沙三期火山-侵入岩地球化学与岩石成因 总被引：8，自引：0，他引：8
辽河盆地沙三期火山-侵入岩为一套偏碱性的双峰式岩系，其基性端元为碱性玄武岩，中偏碱性端元为粗面质熔岩和侵入岩。碱性玄武岩富集高场强元素(如Nb、Th、Zr、Hf、V等)和轻稀土、Sr、Ba等大离子亲石元素，而亏损Rb和K，具有与板内碱性玄武岩和洋岛玄武岩类似的特征。粗面质岩石显示与基性端元相似的地球化学特征，其不相容元素含量总体上高于碱性玄武岩，但具强烈的Sr和Eu亏损。矿物学、岩石学及地球化学证据表明，玄武质岩石是软流圈地幔低程度部分熔融的产物，并经历了橄榄石和辉石的分离结晶作用，其源区可能有金云母和石榴石残留。玄武质岩浆上升到较浅部位后进一步发生橄榄石、辉石、斜长石和磁铁矿等的分离结晶作用而形成粗面质岩浆，地壳物质混染作用不显著。 相似文献
13.
仲岗洋岛位于班公湖-怒江板块缝合带中西段,前人对仲岗洋岛的研究主要集中在东段,中段洞错一带尚未有人研究。对仲岗洋岛的岩石组合及岩石地球化学进行研究,并对洋岛火山岩的源区及构造环境进行分析。在仲岗洋岛中段洞错北部的一条剖面上采集了8个玄武岩和8个玄武安山岩地球化学样品,主量元素特征表明,二者皆为具有富Ti特征的碱性玄武岩,微量元素特征显示二者富集Nb、Ta元素。稀土元素配分曲线和微量元素蛛网图与典型洋岛玄武岩曲线相似。化学成分指示,仲岗洋岛玄武岩与玄武安山岩可能来自于同一岩浆源区,且二者来源于具有洋岛玄武岩特征的地幔,相容元素Cr、Ni的亏损表明,成岩过程中发生了橄榄石和辉石的分离结晶作用。洋岛形成于大洋板块内以洋壳为基底的洋岛环境。 相似文献
14.
随着研究的不断深入,在中亚造山带(CAOB)不断有不同时代的洋岛玄武岩(OIB)被识别出来。在中亚造山带西南缘的西准噶尔地区的多条蛇绿混杂岩带中,也存在具有OIB特征的玄武岩。这些玄武岩呈枕状,与超基性岩、辉长岩、块状玄武岩、灰岩及紫红色硅质岩等紧密共生。地球化学研究表明枕状玄武岩均为碱性系列,具有较高的TiO2含量(大多>2.5%)、强烈富集轻稀土元素、无明显Nb、Ta负异常,与典型的OIB极为相似,认为其可能形成于大洋板内与地幔柱有关的海山环境。通过对海山的发展阶段分析认为,西准噶尔地区海山应该发展到爆炸海山阶段,因为其中发育大量的枕状熔岩。海山中火山岩或火山碎屑沉积物富集大离子亲石元素和高场强元素,海山的俯冲将对弧及弧后地区火山岩地球化学产生明显影响,而西准噶尔地区泥盆纪-石炭纪火山岩中恰恰存在海山的信号。因此,海山俯冲模式可能能更好地解释西准噶尔地区火山岩中存在OIB特征火山岩的成因。另外,海山俯冲还存在潜在的资源效应,因此应该寻找和研究古海山及火山岛链俯冲的迹象,将对进一步找金铜等矿提供可靠依据。 相似文献
15.
《Geodinamica Acta》2001,14(1-3):159-167
Pliocene–Pleistocene volcanism accompanied strike-slip-related transtensional deformation along the Kızılırmak fault segment of the Central Anatolian fault zone (CAFZ) in the west of Şarkışla (Sivas–central Turkey). These volcanic rocks are represented by alkali olivine basalts. They can be divided into four different sub-groups on the basis of their Zr, Nb, TiO2 contents. A primitive mantle-normalized incompatible trace element diagram for four subgroups shows close similarity to typical OIB pattern. Some of the incompatible trace element ratios (Ce/Y, Zr/Nb, La/Ba, La/Nb) are also akin to OIB values. Highly fractionated REE patterns (La/YbN=24.7–9.2) with no Eu anomaly are the main features of the alkali basalts and are comparable to alkaline volcanism in continental rift zones. On the basis of Al2O3/TiO2, Nb/Y, Zr/Y Zr/Nb ratios, the geochemical differences among four sub-groups can be explained by variable degrees of partial melting of compositionally similar mantle source. Th/Nb, Th/Y, Nb/Y ratios and the primitive mantle-normalized trace element diagram suggests significant amount of crustal involvement for most of the alkali olivine basalts erupted along the CAFZ. Rupture of the continental lithosphere by strike-slip-related transtensional deformation might have caused decompressional partial melting of the asthenospheric mantle and generating alkali olivine basalts in this region. 相似文献
16.
N. A. Krivolutskaya D. V. Kuzmin B. I. Gongalsky I. A. Roshchina N. N. Kononkova N. M. Svirskaya T. V. Romashova 《Geochemistry International》2018,56(5):419-437
A periodic character of the evolution of trap magmatism was inferred by many researchers from the fact that sequences of volcanic rocks consist of alternating units of lava flows and tuff. A new phase of studying magmatic rocks in the Siberian Platform was related to the possibility of apply high-precision geochemical techniques in studying trace elements and Sr, Nd, and Pb isotopic compositions. The use of these techniques made it possible not only to identify small individual cycles in the vertical sections of volcanic rocks but also to distinguish larger stages. The currently most widely acknowledged scenario of the origin of volcanic rocks involves three stages, during which oceanic-island basalts (OIB), transitional series (intermediate between OIB and WPB), and within-plate basalts (WPB) were formed. This scenario was inferred mostly from data on rocks in the western part of the Norilsk area (Kharaelakh Trough). This publication presents recently obtained data on the inner structure of the sequences of volcanic rocks and the geochemistry of basalts in the eastern part of the territory, where no rocks show transitional characteristics have ever been found. They can be classified into two types that have clearly different composition and occur in different areas. These types characterize two major stages of the origin of volcanic rocks: rift-related and trap magmatism itself. The rocks produced during these stages occur at neighboring territories. 相似文献
17.
Formation Environment of the High-Pressure Metamorphic Rocks, Western Tianshan, Xinjiang, China 总被引：1，自引：0，他引：1
1.IntroductionFromexperimefltalphaseequilibrium,stableisotOPe,andthermo-barometricstudies,ProgradebineschistdineralparageneseshavebeenproducedexclusivelyatrelativelyhighPadratios(DeRoever,1956;Miyashiro,1961;DobretsovandSobolev,1984;Emst,1973,1988;Maruyamaetal.,1996).InthelastthreedeCades,withtheadvanceofplatetectonics,manygeologistssuggestedthatblueschists,representinghigh-Pressurelow-tCmperamre~rphism,areformedbysubductionofoceanicplate(Emst,1973).Blueschistshavealsobeenregardedasoneof… 相似文献
18.
D. V. Kovalenko V. I. Lebedev A. A. Mongush Kh. N. Sat O. A. Ageeva E. V. Koval’chuk 《Petrology》2016,24(2):178-195
The paper reports original isotopic and geochemical data on Early Precambrian lavas in the Ozernaya Zone in Mongolia. According to their normalized trace-element patterns, the rocks are classified into the following groups: (1) rocks similar to N-MORB; (2) rocks similar to E-MORB; (3) basalts enriched in trace elements, with HFSE minima; and (4) basalts depleted in trace elements, with HFSE minima. All of the lava types could be produced in an island arc—backarc basin system. The magmatic rocks of group (1) were likely formed in a spreading backarc basin, and those of group (2) were likely generated within the influ- ence zone of a hotspot or were derived from heterogeneous upper mantle domains. The lavas of group (3) seem to be fragments of an ensimatic, relatively primitive island arc. The basalts and basaltic andesites of group (4) were likely produced by mixing melts of groups (1) and (3). The fact that lavas of groups (1) and (4) sometimes intercalate within a single stratigraphic section suggests that the extension and subduction zones were closely spaced and operated simultaneously. The magmas of groups (1), (2), and (3) were derived from different mantle sources, which possessed different ratios of trace elements and were different in isotopic composition. 相似文献
19.
Mobility of trace elements during subduction metamorphism as exemplified by the blueschists of the Kurtushibinsky Range,Western Sayan 总被引：1，自引：0，他引：1
N. I. Volkova S. I. Stupakov G. A. Babin S. N. Rudnev A. A. Mongush 《Geochemistry International》2009,47(4):380-392
In order to evaluate the mobility of trace elements during subduction metamorphism, the geochemistry of blueschists of the Dzhebash Group from the Kurtushibinsky Range of the Western Sayan (basins of the Koyard and Oresh rivers) was studied, and the chemical compositions of high-pressure rocks were compared with weakly altered basalts from the same region. The protoliths of the blueschists were probably metabasalts of similar age from the ophiolitic dike complex, the pillow lavas of the Verkhnekoyardsky Formation crowning them, and the pillow basalts of the Kurtushibinsky Formation. The spatial association of the blueschists with the Kurtushibinsky Formation basalts and identical trace element patterns in these rocks allow us to suppose the cogenetic character of their protoliths. Geological and geochemical data suggest their formation in an oceanic plateau setting, whereas the mafic rocks of the dike complex and the Verkhnekoyardsky Formation show island-arc affinity. A comparison of the Dzhebash Group blueschists with the chemically equivalent Kurtushibinsky basalts showed that high-pressure metamorphism caused only minor changes in their compositions. These rocks are almost indistinguishable with respect to such fluid-immobile components as Ti, P, Zr, Hf, Y, and middle and heavy rare earth elements. On the other hand, the blueschists are strongly depleted in potassium. The selective removal of Rb and Ba during blueschist metamorphism was observed only in those samples that showed the most extensive removal of potassium. 相似文献
20.
对青藏高原羌塘地块中部双湖地区发育的二叠系碱性系列和拉斑系列玄武岩进行了详细的地球化学研究。碱性系列玄武岩富集LILE和LREE,其La/Nb比值和OIB相近,Ti/V比值明显高于典型MORB,在微量元素构造环境判别图解上位于OIB区域。拉斑系列玄武岩具有相对平坦的稀土元素配分模式,和典型MORB相比,其Ti/V比值也明显偏高,在微量元素构造环境判别图解上位于MORB和OIB重合的区域,表明岩石起源于一个低度富集的地幔源区。结合区域地质背景,认为双湖二叠系拉斑系列-碱性系列玄武岩组合可能形成于陆间裂谷到小洋盆环境,这套玄武岩的产出可能代表古特提斯洋沿龙木错—双湖构造带在不同地区的发育程度不同,双湖地区在二叠纪应为一个陆间裂谷到小洋盆环境。 相似文献