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1.
Robert Cullers 《Lithos》1988,21(4):301-314
A series of soil and stream sediments developed during intense weathering on the metaluminous Danburg granite, northeastern Georgia, U.S.A., have been analyzed mineralogically and chemically. The concentrations of Ba, Na, Rb and Cs in the silt and coarser fractions are controlled mainly by feldspars and biotite. Hf is controlled by zircon, and the REE (rare-earth elements) and Th are largely controlled by sphene. Variations in feldspar, sphene and zircon may produce small variations in Eu/Sm and La/Lu ratios. Ferromagnesian minerals control Ta, Fe, Co, Sc and Cr concentrations.

The mineralogical and chemical composition of the Danburg granite is more closely reflected in the silt than in the sand or gravel fractions of stream sediments. In the silt, the contents of Rb, REE, Th, Ta, Fe, Co and Sc and the ratios of La/Sc, Th/Sc, La/Co, Th/Co, Eu/Sm and La/Lu are similar to those in the unweathered granite. In contrast, these element contents or ratios in the sands and gravels are 0.05−3× the concentration in the unweathered granite. Ta and Ba contents are an exception to the above. The Ta and Ba contents of the sands and gravels are similar to those of the granite.

In the kaolinite-halloysite clays, the content of Na is depleted relative to the source. Rb, Cs, Ba, Hf and Ta are depleted or enriched in the clays relative to the source, while the REE, Th, Fe, Co, Sc and Cr are enriched. The Eu/Sm (Eu anomaly size) and La/Lu ratios, and the REE patterns of the clays are similar to those of the source.

Thus, the mineralogy and element contents of a siltstone developed from metaluminous, granitic sources during intense weathering would be expected to be more similar to the source rock than the sandstones and conglomerates. Claystones should contain similar REE patterns and Eu/Sm ratios as the source rock, but such fine-grained sediments might represent much larger areas of source rocks than the more locally derived sandstones or conglomerates.  相似文献   


2.
武当地块耀岭河群中两类不同性质的酸性火山岩研究   总被引：1，自引：0，他引：1  
耀岭河群火山岩中可划分出两类不同性质的酸性火山岩:酸性岩A主要属碱流岩-白碱流岩类,酸性岩B主要属流纹英安岩/英安岩类。总体上看二者主量元素组成差异不大:SiO2含量为65.00%～79.85%,(Na2O+K2O)为5.26%～9.68%。强烈富集Th、Nb、Ta、La、Ce、Nd、Zr、Sm和Hf等微量元素,强烈亏损Ba、K、Sr、P、Ti等元素,稀土元素特征表现为∑REE很高(339.94～1152.85μg/g),具有强烈的铕负异常(δEu=0.18～0.32),与典型的大陆裂谷流纹岩的稀土曲线一致。酸性火山岩B显示K、Rb、Ba、Th和LREE的正异常和Sr、P、Ti和Nb的负异常,Eu具弱负异常(δEu=0.64～0.88),表现出与酸性火山岩A迥然不同的微量元素及稀土元素特征,而与耀岭河群基性火山岩相似,显示其成因可能与之有相同源区。  相似文献   

3.
骆文娟  张招崇  侯通  王萌 《岩石学报》2011,27(10):2947-2962
茨达复式岩体位于中国西南扬子地台西缘的攀西裂谷内,其岩性从基性到酸性连续变化,SiO2含量为40.06％ ～68.54％,但以基性和酸性岩石为主,中性岩石较少,而且非常不均匀,通常具有斑杂构造特征.从基性岩到酸性岩,各岩石样品由轻稀土弱富集型变为较强富集型.微量元素表现为酸性岩中Rb、Th、K、La、Ce、Pb、Nd、Zr、Hf、Sm呈正异常和Ba、Nb、Ta、Sr、P、Ti的负异常;基性岩除Ti负异常和Pb正异常外,其它异常不明显;中性岩具有Ti、Sr负异常和Pb正异常,其它特征介于基性岩和酸性岩石之间.野外和岩相学特征明显指示出中性岩石具有混合特征.酸性端元岩浆准铝质的特征以及相对低的SiO2含量指示其起源于玄武质下地壳的部分熔融,而基性端元岩浆的地球化学特征以及高温特征暗示着其起源于地幔柱源区.锆石U-Pb年龄数据表明,该复式岩体中基性端元LA-MC-ICP-MS U-Pb锆石年龄为243.76±0.77Ma,酸性端元年龄为240.5±0.76Ma,可能代表了峨眉山大火成岩省岩浆活动的尾声阶段.  相似文献   

4.
本文对满洲里地区灵泉盆地中生代火山岩进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,以便对其岩石成因和区域构造演化给予制约。研究区内9个代表性火山岩中的锆石均呈自形-半自形晶,显示出典型的岩浆生长环带或条痕状吸收,结合其高的Th/U比值(0.25~7.78),暗示其岩浆成因。测年结果表明,研究区中生代火山岩可分为3期,分别以中侏罗世(约166Ma)塔木兰沟组玄武岩类、早白垩世早期(约142Ma)吉祥峰组流纹岩类和早白垩世晚期(约125Ma)上库力组流纹岩类和伊列克得组玄武岩类为代表。中侏罗世(塔木兰沟组)火山岩具有富碱、高钾和高的La/Yb、La/Nb、Hf/Sm、Nb/U和Ce/Pb比值,以及富集大离子亲石元素(LILEs)、轻稀土元素(LREEs)和亏损Nb等特征,暗示其形成应与陆壳加厚之后的岩石圈伸展作用有关;早白垩世早期(吉祥峰组)火山岩具有高硅、富碱、富K、Rb、Th及低Al、Mg、Ca、Ni、Cr、Ti等特点,类似于A型流纹岩,揭示其形成于陆内伸展环境;早白垩世晚期火山岩具双峰式组合特点,基性端元具有富碱、高钾、高度富集LILEs和LREEs,类似于钾玄质玄武岩,而酸性端元显示A型流纹岩特点,揭示该期火山岩应形成于一种与裂谷发育过程相似的强烈引张环境。结合区域中生代火山岩的空间展布特征,认为中-晚侏罗世和早白垩世早期火山岩的形成应与蒙古-鄂霍茨克缝合带的演化有关,而早白垩世晚期双峰式火山岩的形成应与太平洋板块向欧亚大陆的俯冲作用相联系。  相似文献   

5.
New major and trace element data on the Proterozoic Chimalpahad layered anorthositic Complex and associated basaltic amphibolites of the Nellore Schist Belt of South India provide new constraints on their petrogenesis and geodynamic setting. The Complex consists of layered anorthosites, leucogabbros, gabbros, ultramafic rocks and is spatially associated with basaltic amphibolites. Despite deformation and metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the Complex. Whereas the amphibolites display diverse REE systematics, the Chimalpahad anorthositic–gabbroic rocks are characterized by moderately depleted to strongly enriched LREE patterns and by flat to depleted HREE patterns. The field relations, major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic–gabbroic rocks by fractional crystallization. The anorthositic rocks and the basaltic amphibolites share the depletion of Nb relative to Th and La on primitive mantle-normalized diagrams. They exhibit signatures of arc magmatic rocks, such as high LILE and LREE relative to the HFSE and HREE, as well as high Ba/Nb, Ba/Zr, Sr/Y, La/Yb ratios that mimic chondrite-normalized REE and primitive mantle-normalized trace element patterns of arc magmas. Similarly, on log-transformed tectonic discrimination diagrams, the Chimalpahad rocks plot within the field of Phanerozoic magmatic arcs, consistent with a subduction zone origin. On the basis of field relations and geochemical characteristics, the Chimalpahad Complex is interpreted as a fragment of a magma chamber of an island arc, which is tectonically juxtaposed against its original volcanic cover. A new preliminary Sm–Nd date of anorthosite from the Chimalpahad Complex indicates a model age of 1170 Ma.  相似文献   

6.
Ten rare-earth elements (La, Ce, Nd, Sm, Eu, Gd, Tb, Tm, Yb and Lu) and Ta, Th and Hf contents in eight kimberlites and inclusions from Greenland and Zambia have been determined by instrumental neutron activation. All the samples have highly fractionated rare-earth (REE) distribution patterns. La/Yb ratios in the Greenland kimberlites (hypabyssal facies) vary from 111.8 to 188.4, and the total rare-earth contents range from 204.8 to 380.3 ppm. No europium anomaly is present. The Zambian kimberlites (diatreme facies) are altered and carbonated. Rare-earth patterns in these are also light REE-enriched. A significant difference is shown to exist between the diatreme and hypabyssal facies of kimberlites.  相似文献   

7.
Analytical data for Sr, Rb, Cs, Ba, Pb, rare earth elements, Y, Th, U, Zr, Hf, Sn, Nb, Mo, Ni, Co, V, Cr, Sc, Cu and major elements are reported for eocene volcanic rocks cropping out in the Kastamonu area, Pontic chain of Northern Turkey. SiO2% versus K2O% relationship shows that the analyzed samples belong to two major groups: the basaltic andesitic and the andesitic ones. High-K basaltic andesites and low-K andesites occur too. Although emplaced on continental type basement (the North Anatolian Crystalline Swell), the Pontic eocene volcanics show elemental abundances closely comparable with typical island arc calc-alkaline suites, e.g. low SiO2% range, low to moderate K2O% and large cations (Cs, Rb, Sr, Ba, Pb) contents and REE patterns with fractionated light and almost flat heavy REE patterns. REE and highly charged cations (Th, U, Hf, Sn, Zr) are slightly higher than typical calc-alkaline values. Ferromagnesian elements show variable values. Within the basaltic andesite group the increase of K%, large cations, REE, La/Yb ratio and high valency cations and the decrease of ferromagnesian element abundances with increasing SiO2% content indicate that the rock types making up this group developed by crystalliquid fractionation of olivine and clinopyroxene from a basic parent magma. Trace element concentration suggest that the andesite group was not derived by crystal-liquid fractionation processes from the basaltic andesites, but could represent a distinct group of rocks derived from a different parent magma.  相似文献   

8.
鄂拉山岩浆岩地处东昆仑、西秦岭、南祁连造山带交汇部位,内部结构复杂,是中国西部重要的构造结和矿集区之一.海相火山岩出露广泛,岩石类型主要由杏仁状玄武安山岩、杏仁状安山岩、安山质火山角砾岩、安山质晶屑岩屑凝灰岩、安山质角砾凝灰熔岩及英安岩、流纹岩等组成,以火山地层、夹层状、透镜状等形式赋存于洪水川组索拉曲砂岩、幸福村细浊积岩、河卡山粗浊积岩中,属海相喷发环境的产物.火山岩岩石化学成分与岛弧钙碱性安山岩平均化学成分相似,δEu平均为0.75,显示为弱负异常,(La/Sm)N、(Gd/Yb)N值平均分别为5.09、1.76,轻重稀土元素之间分馏程度显著,轻稀土元素分馏程度较高且富集.Ba、Rb、Th元素强烈富集,Ti、Cr等元素较亏损.Th×Ta/Hf2值在0.029～0.162,平均为0.079,具有陆缘火山弧的特性.由上所述,洪水川组火山岩产于碰撞后的拉伸环境.  相似文献   

9.
李平  王洪亮  徐学义  陈隽璐  过磊  奚仁刚 《岩石学报》2014,30(12):3553-3568
新疆西准噶尔北部地区的早泥盆世马拉苏组出露有少量富钠低钾的拉斑质中基性熔岩,这些分布于谢米斯台断裂北侧的玄武安山岩和玄武岩多呈夹层状断续产出于火山碎屑岩之中。马拉苏中基性熔岩的Mg#与主、微量元素协变关系及Th-Th/Nd图反映了其并非同源岩浆演化的结果。马拉苏火山岩中的玄武安山岩富集LILE、亏损HFSE,具有较高的Th含量及较低的Hf/Th和(Nb/Th)PM比值,显示出弧火山岩的地球化学特征。其中的玄武岩则具有略为平坦的稀土元素分配样式,较低的Th含量及较高的Hf/Th和(Nb/Th)PM比值,此同MORB地球化学特征极为相似;虽然其也显示有轻微的LILE富集、HFSE亏损,但是较高的La/Nb比值则暗示这同地壳或俯冲物质组分的卷入有关,且一系列构造环境判别图解也进一步印证了马拉苏组内的玄武岩应属似MORB基性熔岩。此外,两类岩石的高场强元素比值Zr/Nb、Hf/Ta同全球平均大洋中脊玄武岩的相应比值极为接近,反映了马拉苏组中基性火山岩的物质源区主体均为MORB地幔物质源区。La/Yb-Gd/Yb原始地幔标准化比值的模拟计算进一步显示了马拉苏组玄武安山岩与受改造(俯冲沉积物或地壳物质的混染)的石榴子石或尖晶石-石榴子石地幔橄榄岩物质源区的部分熔融作用有关,而似MORB型玄武岩则源自尖晶石地幔橄榄岩源区的部分熔融。结合区内同期的蛇绿岩、火山岩和碱性花岗岩的地球化学研究,我们可以进一步推断此类兼具有似MORB和弧火山岩地球化学特征的早泥盆世马拉苏火山岩应当是西准噶尔地块北部在早古生代受后期俯冲作用影响下经历弧后扩张形成的火山-岩浆地质记录。  相似文献   

10.
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.  相似文献   

11.
雀莫错一带那益雄组火山岩是由玄武岩组成的基性熔岩,岩石化学表现为低TiO2(<1％)和Al2O3,Na2O>K2O,属钙碱性系列。地球化学特征表现为轻稀土富集,Eu平坦或轻微负异常,富集大离子亲石元素(Rb,Sr,Ba,K)和高强场元素Th,亏损高强场元素Nb和Ta,反映了岛弧构造环境。上二叠统那益雄组岛弧型火山岩的发现,说明长江源地区晚古生代有岛弧存在,且该岛弧应为晚古生代特提斯洋向东俯冲在中北段形成的多岛-弧盆系统的一部分。  相似文献   

12.
The paper presents data on the leaching efficiency of rare-earth (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) and radioactive (Th and U) elements by various reagents from alkaline rocks of the Lovozero Massif. Element concentrations were analyzed by ICP-MS and instrumental neutral activation (INAA). A new complex technique is suggested for analyzing elements on the solid phase of polymer hydrogels. This technique makes it possible to enhance the sensitivity and selectivity of INAA when these elements are simultaneously contained in rocks. Data are presented on the selective leaching of trace elements and the application of environmentally safe reagents.  相似文献   

13.
Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu)cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO2, Al2O3, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.  相似文献   

14.
辽东半岛东南部地区出露大面积南辽河群变质表壳岩系,其下部尤以广泛发育变质火山岩为特征。本文通过对黑云斜长片麻岩、斜长角闪岩以及黑云母变粒岩等岩石系统的岩石学、全岩地球化学研究,并结合锆石U-Pb年代学以及Lu-Hf的研究结果来制约其物质组成、原岩形成和变质时代、岩石成因及形成环境,进而探讨胶—辽—吉活动带的大地构造属性。锆石显微结构、微区成分及LA-ICP-MS U-Pb年代学分析结果表明,研究区南辽河群变质火山岩原岩可能形成于2.19Ga左右,并记录了1.90Ga左右的变质事件。元素和同位素地球化学分析结果表明,其原岩以玄武安山岩-安山岩和英安岩及少量的流纹岩为主,主体属于亚碱性系列,具有钙碱性演化趋势。其中,基性端元以相对较低的SiO_2质量分数,富集MgO、TFe_2O_3和轻稀土元素(LREE)及Cr、Co和Ni为特征,强烈亏损重稀土元素(HREE)和高场强元素(HFSE,如Nb、Ta和Ti),岩浆应起源于受俯冲流体或熔体交代的亏损岩石圈地幔楔;而中—酸性端元则具有高的SiO_2质量分数和较低的MgO、TFe_2O_3质量分数,富碱,富集Ba、Th、U和K,以及亏损Nb、Ta、P和Ti等特征,应来自于新生地壳物质的部分熔融。结合区域地质资料表明,南辽河群变质火山岩应形成于一个典型的活动大陆边缘的构造环境,暗示胶—辽—吉活动带北段古元古代中期以前的形成演化机制应与弧-陆碰撞作用有关。  相似文献   

15.
《International Geology Review》2012,54(13):1668-1690
The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high εNd(t) values (5.3–5.6) and initial 87Sr/86Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted εNd(t) (5.6–6.4) and εHf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.  相似文献   

16.
龙脖河铜矿床位于滇中地区红河成矿带南段,矿区火山岩发育、与铜矿化时空密切共生。区内火山岩遭受了不同程度的蚀变或区域变质作用,根据矿物组合可分为变安山岩、变次火山岩和变基性火山岩。三类变火山岩均相对富Na,在地球化学分类图解上样品主要位于碱性玄武岩区域。除个别微量元素外,三类岩石的微量元素含量相近,与各种构造环境玄武岩相比,本区变火山岩富集HFSE(Th、Nb等)、亏损LILE(Sr、Ba等),微量元素配分模式出现明显Th、Nb正异常和Sr、Ba负异常。三类岩石的REE含量范围相互重叠,除Eu异常外,其REE配分模式均为相似的LREE富集型。各种地球化学证据表明,本区三类变火山岩为同源岩浆不同演化阶段的产物,其原始岩浆为大陆板内拉张构造环境交代富集地幔部分熔融的产物,结晶分异作用在岩浆演化过程中具有重要地位。  相似文献   

17.
徐雯雯  刘栋  赵志丹  车悦  齐宁远  雷杭山  朱锐 《岩石学报》2021,37(12):3735-3758
希腊Lesvos岛广泛出露的中新世钾质岩浆岩为深入探讨地中海东部地区新生代构造演化和深部过程提供了宝贵机会。本文对Lesvos岛碰撞后钾质岩石开展了系统的矿物电子探针分析、锆石U-Pb定年和Hf同位素分析及全岩地球化学分析。研究结果表明：Lesvos岛出露的碰撞后火山岩主要由玄武粗安岩、玄武安山岩和粗面岩组成,锆石U-Pb年龄为17.2Ma。三类岩石均表现出相对富集大离子亲石元素以及相对亏损高场强元素的地球化学特征。玄武粗安岩和玄武安山岩样品具有相似且富集的Sr-Nd-Pb同位素组成,其较低的Dy/Yb和整体较高且变化范围较大的Ba/La、Zr/Hf比值表明这两类钾质基性岩石起源于受到俯冲陆源沉积物起源流体/熔体交代的尖晶石相岩石圈地幔的低度部分熔融。相比之下,同时代粗面岩具有更亏损的Sr-Nd同位素组成,其均匀的同岩浆锆石Hf同位素组成排除了在岩浆演化过程中发生显著地壳混染或者岩浆混合作用的可能。而较高的Y和Yb含量、相对较低的Sr/Y和（La/Yb）N比值则表明这种粗面质岩浆起源于中-上地壳的部分熔融。考虑到地震层析结果和Pelagonia陆块与Apulian陆块初始碰撞的时间,研究区各类碰撞后钾质岩浆活动可能反映了深俯冲的Pindos洋壳在中新世发生了板片断离过程。  相似文献   

18.
扬子岩浆岩带东段基性岩地球化学   总被引：20，自引：7，他引：13  
邢凤鸣 《地球化学》1998,27(3):258-268
长江中下游中生代岩浆岩带称为扬子岩浆岩带。该带岩浆岩属高钾钙碱性岩系和橄榄安粗岩系,共基性端员玄武岩和辉长岩高钾富碱,硅弱不饱和,富集Ｒｂ、Ｂａ、Ｔｈ、Ｋ、ＬＲＥＥ等强不相容元素,强烈亏损Ｃｒ、Ｎｉ等强相容元素;ＲＥＥ球粒陨石标准化曲线为右倾型,在Ｌａ／Ｓｍ－Ｌａ图上排列成一斜线,是地幔不同程度部分熔融为主的产物。基性岩εＮｄ较高,Ｉｓｒ较低,在εＮｄ－ＩＳｒ图上沿地幔排列及其延长线分布,略向右漂  相似文献   

19.
Geochemical mapping of Shikoku in the SW Japan Arc was conducted using stream sediments (<0.18 mm fraction) with sample densities of 1 site per 106 km2. Concentrations of 51 elements in 173 stream sediments were determined. Areas with especially high Cr, Ni, Mg and Co concentrations run intermittently and linearly through Shikoku, and coincide with exposures of the Mikabu greenstone complex composed of oceanic basaltic and gabbroic rocks with minor ultramafic bodies. Areas with higher concentrations of Fe, V, Sc, Ti, Mn, Cr, Ni, Mg, Co and Eu are distributed widely along the Shikoku Island axis; they mainly overlap with the zones of the high-pressure type metamorphic rocks (Sanbagawa Belt) and of the Jurassic accretionary complexes with sedimentary rocks (Chichibu Belt): in contrast, areas with lower concentrations spread over the northern coastal zone of Cretaceous granitoids (Ryoke Belt) and in the southern coastal zone of Cretaceous-Paleogene accretionary complexes (Shimanto Belt). Distribution patterns of these elements reflect that various amounts of mafic and ultramafic materials, possibly of oceanic origin, are associated with the rocks in the Sanbagawa and Chichibu Belts, although such components occur sparsely in the Shimanto and Ryoke Belts. Calcium, Sr, Na, Nb, Sm and Gd contents are lower along the southern coastal zone. High values of Th, U, La, Ce and Ba are associated with granitic rocks. The positive anomalies of Cu and Cd coincide with the distribution of stratabound Cu deposits in the Sanbagawa Belt. Positive anomalies of Sb in the northern area are associated with Sb deposits near the large Median Tectonic Line fault zone. A high-Hg zone is present in the southern fore-arc area. The median concentrations for stream sediments in Shikoku are higher in Hg, Cu, Ni, Cr, Rb, Li, Cs, K, slightly higher in Nb, Ta, La, Ce, Sn and lower in Ca than those for average Japanese stream sediments. The median concentrations for the four geologic zones show systematic wide variations of mafic associated elements, and narrow variations of lithophile elements generally more abundant in felsic rocks. The chemical compositions of the stream sediments in Shikoku largely reflect the concentrations in the pre-Neogene accretionary complexes and in the high-P metamorphic rocks mainly formed from clastic materials derived from continental margins, and ratios of mafic and ultramafic materials within surface rocks; they are partly influenced by sulfide mineralization and fluid migration.  相似文献   

20.
Abundant cone sheets form one of the last magmatic stages in the Tertiary central complex on the Scottish peninsula of Ardnamurchan and can be grouped into a younger inner and an older outer suite relative to a gabbro intrusion. Most of the cone-sheets consist of tholeiitic to transitional basalt with MgO contents between 7.5% and 4%, although more evolved rocks also occur (to 0.5% MgO). The mafic samples are slightly enriched in the light rare earth elements (Chondrite-normalized La/Sm ∼1.1), the enrichment increases in the more evolved rocks. The compositional variation of the basaltic rocks is mainly due to crystal fractionation of olivine and clinopyroxene at depths of ∼10 km but trace elements show simultaneous assimilation of Archean Lewisian granulite crust. The andesitic to rhyolitic lavas formed by fractional crystallization from the contaminated basaltic magma coupled with assimilation of Proterozoic Moine metasediments at uppermost crustal levels. The occurrence of composite cone-sheets with basaltic and rhyolitic parts and mixtures between these magmas implies that the melts ascended successively but within a short period of time. The parental magmas of the Ardnamurchan cone-sheets must have formed at relatively shallow depths in the mantle and are comparable to the youngest tholeiitic lavas from the neighbouring island of Mull. Received: 5 June 1997 / Accepted: 12 November 1997  相似文献   

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