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1.
周义平  任友谅 《沉积学报》1994,12(2):123-132
对滇东?黔西晚二叠世煤系中同沉积火山灰蚀变粘土岩夹矸?正常沉积粘土岩和煤层共采46件样品,用多种仪器分析方法测定微量元素30余种?研究表明,煤系各层段中赋存的夹矸是由性质不同的火山灰沉积蚀变形成的,各层夹矸的微量元素含量及组合关系各有特点并在较大范围内保持良好的稳定性,可用于判定层位;正常沉积粘土岩与夹矸的原始物质性质不同,其微量元素含量和组合面貌各有自己特定的分布范围,两者易于区别?  相似文献   

2.
本文就滇东晚二叠世含煤沉积中同沉积火山灰蚀变粘土岩夹矸和正常沉积粘土岩的锆石进行了统计学和形态学研究.该两种不同成因类型的粘土岩中锆石含量及形态面貌有显著区别.tonsteins中锆石形态参数统计规律因层位而异;同层位tonstein的锆石含量和形态参数在平面上呈有规律地变化.系统研究含煤建造剖面上各类岩石(包括tonsteins)中的锆石特征,能够用以确定物质来源、性质、成因以及以此为基本建立正常地层层序并精确对比层位,因而具有理论和实用意义.  相似文献   

3.
The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO2 contents, variable abundances of K2O and Na2O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.  相似文献   

4.
Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<2 (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<2O/Na2O (<1), K/Rb (60–350), Zr/Hf (30–50), Nb/Ta (2–22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5–22), Th/Yb (0–27), Ba/La (0–80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Ybcn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Ybcn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.  相似文献   

5.
The volcanic origin of a number of major tonstein horizons has been established from mineralogical compositions and from trace elements that are quantitatively retained during diagenesis. Discriminant function analysis using the variables Ti/Al, Cr/Al, Zr/Al and Ni/Al allows the tonsteins to be classified according to original composition. Two main groups are recognized amongst the British tonsteins. Those formed from acid volcanic ash are comparable in composition with French and German tonsteins and a common source is postulated, whereas tonsteins formed from basic volcanic ash contain variable amounts of detrital sediment and are thought to originate from local eruptions. Possible lateral equivalents between tonstein horizons in Britain and elsewhere in Europe are suggested.  相似文献   

6.
The early miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km3 of the Tecuya volcanic rocks. The basalts have Nd(t) values of +2 to +6 and (87Sr/86Sr)i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)N =2.4–5.5; La=28–150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr–Nd isotopic data suggest the involvement of subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. Nd(t) values in rhyolites vary from +1.5 to +3.7 while (87Sr/86Sr)i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)N=10; La=100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu*=0.75) and depletion of Ti and P. Mixing relations in (87/86Sr)i – Nd(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge-induced magmatism along a continental margin.  相似文献   

7.
In progressing from a granitoid mylonite to an ultramylonite in the Brevard shear zone in North Carolina, Ca and LOI (H2O) increase, Si, Mg, K, Na, Ba, Sr, Ta, Cs and Th decrease, while changes in Al, Ti, Fe, P, Sc, Rb, REE, Hf, Cr and U are relatively small. A volume loss of 44% is calculated for the Brevard ultramylonite relative to an Al–Ti–Fe isocon. The increase in Ca and LOI is related to a large increase in retrograde epidote and muscovite in the ultramylonite, the decreases in K, Na, Si, Ba and Sr reflect the destruction of feldspars, and the decrease in Mg is related to the destruction of biotite during mylonitization. In an amphibolite facies fault zone separating grey and pink granitic gneisses in the Hope Valley shear zone in New England, compositional similarity suggests the ultramylonite is composed chiefly of the pink gneisses. Utilizing an Al–Ti–Fe isocon for the pink gneisses, Sc, Cr, Hf, Ta, U, Th and M-HREE are relatively unchanged, Si, LOI, K, Mg, Rb, Cs and Ba are enriched, and Ca, Na, P, Sr and LREE are lost during deformation. In contrast to the Brevard mylonite, the Hope Valley mylonite appears to have increased in volume by about 70%, chiefly in response to an introduction of quartz. Chondrite-normalized REE patterns of granitoids from both shear zones are LREE-enriched and have prominent negative Eu anomalies. Although REE increase in abundance in the Brevard ultramylonites (reflecting the volume loss), the shape of the REE pattern remains unchanged. In contrast, REE and especially LREE decrease in abundance with increasing deformation of the Hope Valley gneisses. Mass balance calculations indicate that ≥95% of the REE in the Brevard rocks reside in titanite. In contrast, in the Hope Valley rocks only 15–40% of the REE can be accounted for collectively by titanite, apatite and zircon. Possible sites for the remaining REE are allanite, fluorite or grain boundaries. Loss of LREE from the pink gneisses during deformation may have resulted from decreases in allanite and perhaps apatite or by leaching ofy REE from grain boundaries by fluids moving through the shear zone. Among the element ratios most resistant to change during mylonitization in the Brevard shear zone are La/Yb, Eu/Eu*, Sm/Nd, La/Sc, Th/Sc, Th/Yb, Cr/Th, Th/U and Hf/Ta, whereas the most stable ratios in the Hope Valley shear zone are K/Rb, Rb/Cs, Th/U, Eu/Eu*, Th/Sc, Th/Yb, Sm/Nd, Th/Ta, Hf/Ta and Hf/Yb. However, until more trace element data are available from other shear zones, these ratios should not be used alone to identify protoliths of deformed rocks.  相似文献   

8.
Geochemical and petrological studies of the well-preserved greywacke horizon of the ‘Middle Aravalli Group’ were carried out to constrain the early evolution of the Aravalli basin. Petrological and geochemical attributes of Middle Aravalli greywackes (MAGs) such as very poor sorting, high angularity of framework grains, presence of fresh plagioclase and K-feldspars, variable Chemical Index of Alteration (CIA) index (46.7–74.5, avg. 61), and high Index of Compositional Variability (ICV) value (~1.05) suggest rapid physical erosion accompanying an active tectonic regime. The sediments record post-depositional K-metasomatism and extraneous addition of 0–25% (avg. ~10%) K is indicated. Assuming close system behaviour of immobile elements during sedimentation, various diagnostic element ratios such as Th/Sc, La/Sc, Zr/Sc, and Co/Th, Eu anomaly and rare earth element patterns of MAG suggest that the Archaean Banded Gneissic Complex (BGC) basement was not the major source of sediments. In conjunction with the dominant 1.8–1.6 Ga detrital zircon age peaks of Middle Aravalli clastic rocks, these data rather indicate that the sediments were derived from a young differentiated continental margin-type arc of andesite–dacite–rhyodacite composition. A highly fractionated mid-oceanic-ridge-basalt-normalized trace element pattern of MAGs, with characteristic enrichment of large-ion lithophile elements (LILEs), depletion of heavy rare earth elements, negative Nb-Ta, Ti and P anomalies, positive Pb anomaly, and distinctive Nb/Ta, Zr/Sm, Th/Yb, and Ta/Yb, Ce/Pb ratios envelop the composition of modern continental arc magmas (andesite–dacite) of the Andes, suggesting a subduction zone tectonic setting for precursor magma. High magnitude of LILE enrichment and high Th/Yb ratios in these sediments indicate that thick continental crust (~70 km) underlay the ‘Middle Aravalli’ continental arc, similar to the Central Volcanic Zone of the modern Andes. We propose that eastward subduction of Delwara oceanic crust beneath the BGC continent led to the formation of a continental volcanic arc, which supplied detritus to the forearc basin situated to the west. This model also explains the opening of linear ensialic basins in the Bhilwara terrain, such as in Rajpura–Dariba and Rampura–Agucha in a classical back-arc extension regime, similar to the Andean continental margin of the Mesozoic. On the basis of the recent 207Pb/206Pb detrital zircon age of Middle Aravalli sediment, a time frame between 1772 and 1586 Ma can be assigned for Middle Aravalli continental arc magmatism.  相似文献   

9.
万斌  关成国  周传明  孟凡巍  庞科  唐卿  饶馨 《岩石学报》2013,29(12):4373-4386
我国华南扬子地台不同地区（湖北宜昌、安徽休宁）埃迪卡拉系底部的盖帽碳酸盐岩中均保存有浅色粘土岩层。岩石薄片和扫描电镜观察,以及X射线衍射分析表明,这些粘土岩层以伊利石和伊/蒙混层矿物为主,同时含有港湾状熔蚀石英、高温透长石及岩浆锆石等斑晶矿物。X荧光光谱仪和ICP-MS等离子质谱仪等地球化学元素分析表明,这些粘土岩层具有相似的元素组成和配分模式,以高K2O、低TiO2,富含Nb、Ta、Zr、Hf、Th、Y等亲石元素为特征,Al2O3/TiO2、Zr/Hf 和Ti/Th 值均指示了酸性火山灰的性质,同时稀土元素配分模式与花岗岩较为相似。岩石学和地球化学特征表明,这些浅色粘土岩层是火山喷发事件形成的钾质斑脱岩。通过两处剖面的对比,可以将安徽休宁地区的钾质斑脱岩年龄限制在大约635Ma左右。岩浆判别图指示了这些钾质斑脱岩的原始岩浆为高钾亚碱性流纹英安岩;依据微量元素特征和构造环境判别图,初步认为原始岩浆形成于大陆弧环境。这些钾质斑脱岩在区域上的分布,表明我国扬子地台埃迪卡拉纪早期发生了广泛的火山活动,“雪球地球”事件的结束可能是导致这期以钾质斑脱岩为代表的火山喷发事件的诱因。  相似文献   

10.
杨多  李萌萌  龚全德  陈天红  毛磊  秦天  赵亮 《地质通报》2019,38(10):1660-1674
对内蒙古阿巴嘎旗甘珠音敖包地区中生代火山岩进行了锆石U-Pb测年和岩石地球化学研究,对其形成时代、岩石成因及构造背景给予制约。研究区主要发育中生代满克头鄂博组酸性火山岩和梅勒图组中性火山岩。LA-ICP-MS锆石U-Pb同位素定年结果显示,2组火山岩形成年龄分别为163.6±0.6Ma和139.6Ma±0.7Ma。岩石地球化学研究表明,满克头鄂博组酸性火山岩为碱性系列,具有高硅、富碱、低TFeO、Al_2O_3、TiO_2、MgO、CaO和Na_2O的特征,轻稀土元素富集、重稀土元素亏损、轻重稀土元素分馏明显,Eu强烈亏损,大离子亲石元素Rb、Th、K明显富集,Ba、Sr明显亏损,高场强元素Nb、Ta、P、Ti强烈亏损,具有A型花岗岩特点,形成于陆壳岩石的部分熔融。梅勒图组中性火山岩亦为碱性系列岩石,富碱、富钠、贫钾,高Al2O3、TiO2、MgO,贫CaO,LREE富集,HREE亏损,轻重稀土元素分馏明显,具微弱的负Eu异常,富集大离子亲石元素(LILEs)Rb、Ba、Th、U、K等,亏损高场强元素Nb、Ta、Ti、Y、Yb、Lu等,来源于下地壳镁铁质岩石部分熔融。结合岩石学及该地区构造背景特征,认为满克头鄂博组和梅勒图组火山岩可能形成于蒙古-鄂霍茨克的俯冲作用导致的大兴安岭西坡—冀北—辽西地区加厚陆壳坍塌或拆沉作用的伸展环境。  相似文献   

11.
南秦岭勉县-略阳结合带中超基性岩类主要为方辉橄榄岩和纯橄榄岩,稀土特征为轻稀土亏损,铕富集型;辉绿岩均为轻稀土富集型。变质火山岩可区分为两种组合：一为轻稀土亏损的洋脊拉斑玄武岩,其Ti／V．Th／Ta、Th／Yb、Ta／Yb表明其为MORB型玄武岩,代表本区消失了的洋壳岩石,指示本区晚古生代一早中生代期间的一个古洋盆。另一类为岛弧火山岩组合,除岛弧拉斑玄武岩外,甚至还有大量中、酸性岛弧火山岩,它们并非洋壳的物质组成,而是大陆弧岩浆活动的产物。这表明勉略洋盆在晚古生代-早中生代期间曾经经历过一个较完整的有限洋盆的发生、发展与消亡过程,这期间秦岭已成为一独立的岩石圈微板块,其南缘具有活动大陆边缘的构造属性,秦岭并非华北与扬子陆块之间的简单碰撞产物。  相似文献   

12.
青海鄂拉山地区陆相火山岩地球化学特征及构造环境   总被引：2，自引：0，他引：2  
鄂拉山岩浆岩带晚三叠世火山岩为中-中酸性火山岩组合,由玄武安山岩、安山岩、英安岩、流纹岩及少量火山碎屑岩等组成,岩石蚀变强烈,成层性差,柱状节理发育,具典型的陆相喷发特点.火山岩属铝饱和类型,里特曼指数(δ)及岩石学等显示具钙碱性特征, (FeO/MgO)、K2 O/Na2O显示可能具有陆缘岛弧环境的特性;轻稀土元素分馏程度高且富集,δEu小于1,为弱负异常,稀土元素配分模式曲线与岛弧型稀土元素配分模式图相似;微量元素Rb、Ba、Th等元素明显富集,而Ti、Y、Yb、Sc、Cr等元素较亏损.Nb/Zr、La/Nb、Th/Ta、Th×Ta/Hf2等特征反映鄂拉山组火山岩产于陆缘火山弧环境.结合区域地质背景、岩石地球化学及构造环境等特征,认为鄂拉山地区晚三叠世火山岩产于大陆碰撞与陆缘弧并存的环境.  相似文献   

13.
Major, trace element, and Sm-Nd isotope data are presented for the garnet-biotite and cordierite-garnet-biotite gneisses from the Early Precambrian granulite complex of the Irkut Block (Sharyzhalgai Uplift, Siberian Craton). The garnet-biotite and cordierite-bearing gneisses of the Irkut Block were formed owing to the granulite metamorphism of metaterrigenous rocks. The chemical index of weathering and the content of clayey (pelitic) components in the normative mineral composition increase from the garnet-biotite gneisses to the cordierite-bearing gneisses, thus reflecting the maturation degree of initial sediments. Protoliths of the studied paragneisses correspond to a rock series ranging from the graywacke siltstones to clayey rocks. The trace and rare-earth element distribution indicates that the terrigenous material of the paragneisses was derived from felsic and mafic provenance. Increase in contents of Fe, Ti, Cr, Ni, and Sc and the Cr/Th ratio and decrease in the La/Sc ratio from the garnet-biotite to the cordierite-bearing gneisses reflect growth of the abundance of mafic rocks in the provenance. Potential sources of the detrital material were intermediate-felsic and mafic volcanic rocks (orthogneisses and basic crystalline schists) of the Irkut Block. The paragneisses show a distinct negative Eu anomaly (Eu/Eu* = 0.38–0.85), which suggests the input of crustal melting products, such as the potassium granites. A wide range of model Nd age (TNd(DM) = 2.4–3.1 Ga) of the paragneisses indicates the Archean to Early Paleoproterozoic age of their protoliths. The complex of isotopic, geochemical, and geochronological data, as well as the character of association of metaterrigenous rocks (mature pelites and carbonate rocks included), implies that sedimentation was separated in time from volcanism. The sedimentation was preceded by metamorphism, granite formation, and tectonic stabilization of the Irkut Block crust.  相似文献   

14.
Seven rare-earth elements (La, Ce, Sm, Eu, Tb, Yb, Lu) and Co, Cr, Sc, Ba, Hf and Th have been determined by non-destructive neutron activation analysis on the Quaternary volcanic rocks in Hokkaido, Japan. The trace-element abundances are discussed in terms of the petrological problems, particularly the origin of calc-alkali magma. On the basis of the La/Sm ratio and the contents of K, Ba, Th and La, lateral variations in the contents of trace elements exist across the Kurile and the northern Honshū arcs. The calc-alkali rocks can be classified into three types which correspond to Kuno's three basalt-magma types. There is no essential difference in the rare-earth patterns between the basaltic rocks and the associated calc-alkali rocks in each petrographic province. This suggests that the calcalkali rocks may be derived from the basaltic magmas by fractional crystallization under certain conditions.  相似文献   

15.
NS935钻孔沉积物不活泼微量元素记录与陆源输入变化探讨   总被引：7，自引：2，他引：7  
探讨了NS93－5钻孔沉积物的部分高场强元素和大离子亲石元素的含量变化，结果表明，Ti、Rb、Ga、Y、Zr、Nb、Cs、Hf、Ta、Th和稀土元素（REE）相互间含量变化具有很好的相关关系，反映这些元素均基本来自碎屑组分。经Ti和Sc标准化后这些元素含量的变化，以及Zr/Y等特征元素比值和Ge异常指数Ce/Ce、Eu异常指数Eu/Eu的变化，均反映出碎屑输入的通量或者组成的变化，例如火山灰的富集。其中一些变化与气候变化有关，可能反映周边陆壳的风化强度的波动。NS93－5钻孔沉积物的不活泼元素记录，可提供反映输入陆壳物源区的环境演变证据。  相似文献   

16.
塔河地区海西晚期火山岩地球化学特征及地质意义   总被引：1，自引：0，他引：1       下载免费PDF全文
崔泽宏  唐跃 《中国地质》2010,37(2):334-346
通过岩心观察,塔河地区海西晚期先后发育基性火山岩和中酸性火山岩两大类。两类火山岩具有不同地球化学组成:基性火山岩K2O+Na2O=3.49%~4.25%,K2O/Na2O=0.18~0.381,为钠质系列玄武岩,富集大离子亲石元素K、Rb、Ba、Th和高场强元素Nb、Ta、Ti、Zr、P,δEu≈1,无明显亏损;中酸性火山岩表现为高场强元素Nb、Ta、Sr、P、Ti亏损,大离子亲石元素K、Rb、Ba、Th较为富集,稀土元素δEu=0.43~0.661,铕具明显负异常特征,K2O+Na2O=6.86%~8.82%,K2O/Na2O=1.25~1.591,为钾玄或高钾钙碱性系列英安岩。强不相容元素Th/Ta、Nb/U比值说明,玄武岩与英安岩成因没有明显亲缘性,玄武岩来源于地幔,英安岩来源于地壳。结合邻区火成岩对比分析,塔河地区发育的玄武岩、英安岩分别与塔中地区玄武岩和南天山分区花岗岩具有相同的地球化学特征,构造判别图解表明,玄武岩形成于板内主裂陷期,英安岩、花岗岩形成于板块边缘岛弧构造环境,作为特殊古构造位置,塔河地区先后发育的玄武岩、英安岩为板内主裂陷盆地边缘和弧火山边缘相互耦合的产物。  相似文献   

17.
Provenance studies on Early to Middle Ordovician clastic formations of the southern Puna basin in north-western Argentina indicate that the sedimentary detritus is generally composed of reworked crustal material. Tremadoc quartz-rich turbidites (Tolar Chico Formation, mean composition Qt89 F7 L4) are followed by volcaniclastic rocks and greywackes (Tolillar Formation, mean Qt33 F42 L25). These are in turn overlain by volcaniclastic deposits (mean Qt24 F30 L46) of the Diablo Formation (late Arenig–early Llanvirn) that are intercalated by lava flows. All units were deformed in the Oclóyic Orogeny during the Middle and Late Ordovician. Sandstones of the Tolar Chico Formation are characterized by Th/Sc ratios > 1, La/Sc ratios ≈ 10, whereas associated fine-grained wackes show slightly lower values for both ratios. LREE (light rare earth elements) enrichment of the arenites is ≈ 50× chondrite, Eu/Eu* values are between 0·72 and 0·92, and flat HREE (heavy rare earth elements) patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition. The εNd(t = sed) values scatter around −11 to −9. The calculated Nd-TDM residence ages vary between 1·8 and 2·0 Ga indicating contribution by a Palaeoproterozoic crustal component. The Th/Sc and La/Sc ratios of the Tolillar Formation are lower than those of the Tolar Chico Formation. Normalized REE (rare earth elements) patterns display a similar shape to PAAS (post-Archaean average Australian shale) but with higher abundances of HREEs. Eu/Eu* values range between 0·44 and 1·17, where the higher values reflect the abundance of plagioclase and feldspar-bearing volcanic lithoclasts. Average εNd(t = sed) values are less negative at −5·1, and Nd-TDM are lower at 1·6 Ga. This is consistent with characteristics of regional rocks of upper continental crust composition, which most probably represent the sources of the studied detritus. The rocks of the Diablo Formation have the lowest Th/Sc and La/Sc ratios, lower LREE abundances than the average continental crust and are slightly enriched in HREEs. Eu/Eu* values are between 0·63 and 1·17. The Nd isotopes (εNd(t = sed) = −3 to −1; TDM = 1·2 Ga) indicate that one source component was less fractionated than both the underlying Early Ordovician and the overlying Middle Ordovician units. Synsedimentary vulcanites in the Diablo Formation show the same isotopic composition. Our data indicate that the sedimentary detritus is generally composed of reworked crustal material, but that the Diablo Formation appears to contain ≈ 80% of a less fractionated component, derived from a contemporaneous continental volcanic arc. There are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the Gondwana margin during the Ordovician.  相似文献   

18.
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19.
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   

20.
A unique type of Nb–Zr–REE–Ga-enriched alkali tonstein of pyroclastic origin occurs exclusively within the late Permian coal measures of southwest China. The alkali tonsteins are located within the lowest Xuanwei or Longtan formations of Wuchiapingian age, indicating that their age is later than the main episode of Emeishan Large Igneous Province (ELIP) magmatism. The alkali tonsteins have intermediate–felsic Al2O3/TiO2 values (12.6–34.2, mean 22.0), light rare earth element-enriched chondrite-normalised patterns, negative δEu and incompatible element ratios similar to those of ELIP alkaline Nb–Ta-enriched syenites. All available evidence shows that the alkali tonsteins from southwest China originated from coeval ELIP alkaline magmatism. The enrichment of Nb–Zr–REE–Ga in alkali tonsteins is derived from the ELIP alkaline Nb–Ta-enriched volcanic ashes and may represent the last stage of mineralisation associated with the Emeishan mantle plume activity.  相似文献   

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