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1.
Abstract

The geochemistry of Early Proterozoic sedimentary rocks from the low‐grade metamorphic terrain W of the South Alligator Hinge Zone in the Pine Creek Geosyncline is reported and discussed in terms of the stratigraphy and differences in rock type. Major element trends are dominated by lithological and mineralogical variations; in particular, the dolomitic nature of pelites in the Mount Partridge and Namoona Groups, the presence of chlorite in Crater Formation pelites, and the volcanic affinity of pelites within the Gerowie Tuff. Concentration of Th, U, Zr, Y, Nb, Ce and La in the clastic sediments of the Finniss River and South Alligator Groups is probably related to the presence of felsic volcanics in these groups; high levels of Sn in these groups may be similarly related, and highlight the apparent volcanogenic source for this metal. The South Alligator Group is a preferred host for base‐metal mineralization, and the regional abundance of several metals in this group enhance its economic potential and support a syngenetic, possibly exhalative, origin for these deposits. Anomalous Co, Ni, and V values in Masson Formation pelites are probably related to the nearby mafic Stag Creek Volcanics. Detrital minerals derived from reworked Archaean basement account for high Zr and Th in the Crater Formation. The geochemical data support the shift in REE patterns related to changes in crustal composition about the Archaean‐Proterozoic boundary, though patterns within the Early Proterozoic have also been influenced by felsic volcanism in the South Alligator Group. The chemical index of alteration (which reflects the degree of weathering of sedimentary rocks) reflects the existence of a major unconformity between the Mount Partridge and Namoona Groups; however, other hiatuses observed in or inferred from the rock record are not evident. The dominant felsic volcanic component of the Gerowie Tuff pelites is also indicated by this technique.  相似文献   

2.
Preliminary isotopic data for Late Proterozoic (～ 1100 Ma) granulite-facies metamorphics of the Prydz Bay coast indicate only very minor reworking (i.e., remetamorphism) of Archaean continental crustal rocks. Only two orthopyroxene—quartz—feldspar gneisses from the Rauer Group of islands, immediately adjacent to the Archaean Vestfold Block, show evidence for an Early Archaean origin (～ 3700—3800 Ma), whereas the vast majority of samples have Middle Proterozoic crustal formation ages (～ 1600–1800 Ma). The Prydz Bay rocks consist largely of garnet-bearing felsic gneisses and interlayered aluminous metasediments, although orthopyroxene-bearing gneisses are common in the Rauer Group; in contrast, Vestfold Block gneisses are predominantly orthopyroxene-bearing orthogneisses. The extensive Prydz Bay metasediments may have been derived by erosion of Middle Proterozoic rocks, such as the predominantly orthogneiss terrain of the Rauer Group, and deposited not long before the Late Proterozoic metamorphism. Data from nearby parts of the East Antarctic shield also suggest only limited Proterozoic reworking of the margins of the Archaean cratons.As in the Prydz Bay area, high-grade metamorphies in nearby parts of the East Antarctic shield show a secular increase in the sedimentary component. Archaean terrains like the Vestfold Block consist mainly of granitic orthogneisses derived by partial melting of igneous protoliths (I-type), whereas Late Proterozoic terrains (such as the Prydz Bay coast) include a much higher proportion of rocks derived either directly or by partial melting (S-type granitic orthogneisses) from sedimentary protoliths. Related chemical trends include increases in K2O2, Rb, Pb, and Th, and decreases in CaO, Na2O2 and Sr with decreasing age, essentially reflecting changes in the proportions of plagioclase and K-feldspar.  相似文献   

3.
The geochemistry of sediments is primarily controlled by their provenances, and different tectonic settings have distinctive provenance characteristics and sedimentary processes. So, it is possible to discriminate provenances, depositional environments and tectonic settings in the development of a sedimentary basin with the geochemistry of the clastic rocks. The analytical results of the present paper demonstrate that sediments in the Songliao prototype basin are enriched in silica (SiO2=66.48-80.51 %), and their ΣREE are 30-130 dmes of that of chondrite with remarkable Eu anomalies. In discriminating diagrams of Eu/Eu vs eeeeeREE and (La/Yb)N vs ΣREE, most samples locate above the line Eu/ Eu=l, on the right of the line Eu/Eu/ΣREE=1 and under the line La/Yb)N/eeeeeREE=1/8, which indicates that the depositional environment of sediments in the basin was oxidizing. In addition, variations of MgO, TiO2, A12O3, FeO+Fe2O3, Na2O and CaO vs SiO2 reflect a tendency of increasing mineral maturity of sediments  相似文献   

4.
In the year 1787, while visiting the Isle of Arran in southwestern Scotland, James Hutton made the initial discovery of what is now called an angular unconformity. In his example, Dalradian (Neoproterozoic - Cambrian) metasedimentary rocks are overlain by Early Carboniferous (Dinantian) sedimentary rocks of fluvial origin. The exact position of the plane of unconformity has been uncertain because of the presence of a resistant carbonate-rich zone (cornstone) on the Dalradian paleosurface. Whereas the unconformity was previously placed in an erosional re-entrant beneath the prominent carbonate-rich layer, it should be located at the contact between the Dalradian metasedimentary rocks, including those affected by cornstone formation, and overlying Dinantian pebbly sandstones. A prominent set of joints in the Dalradian rocks is sub-parallel to bedding in the overlying Carboniferous rocks and has contributed to ambiguity concerning placement of the plane of unconformity. A second exposure of Hutton's unconformity, at the SW end of the same small Carboniferous outlier on the Arran coast, displays the contact relationships unambiguously. Geochemical investigation of samples from the new locality shows that the cornstones are enriched in CaO, volatiles (LOI), MgO, and MnO. Formation of the cornstone involved replacement of siliciclastic Dalradian metasediments by dolomite. Relative to the fresh Dalradian rocks, the cornstones are depleted in most trace elements, but display enrichment in Sr, Y, Cu, and REE. The cornstone is enriched in REE in a systematic fashion, with a particularly strong concentration of MREE. A mirror-image pattern in the associated Dalradian rocks suggests that the REE enrichment observed in the cornstone is a result of leaching from the country rocks by groundwater, followed by evaporative concentration.  相似文献   

5.
Major, trace and rare earth element (REE) compositions of upper Proterozoic metavolcanic and metasedimentary rocks from the Tsaliet and Tembien Groups in the Werri district of northern Ethiopia were determined to examine their tectonic setting of eruption, provenance and source area weathering conditions. Tsaliet Group metavolcanic rocks in the Werri area have sub-alkaline chemistry characterized by low to intermediate SiO2 contents, high Al2O3, low MgO and very low Cr and Ni. High field strength element (HFSE) abundances are highly variable. ∑REE abundances vary from 66.7 to 161.3 ppm, and chondrite-normalized REE patterns are moderately fractionated, with LaN/YbN values of between 3.1 and 9.0. Europium anomalies are variable (Eu/Eu* 0.80–1.21) but are generally positive (average Eu/Eu* 1.06). On tectonic discrimination diagrams, most samples have either volcanic-arc chemistry or fall in the overlap field with mid-oceanic ridge basalt (MORB). However, primitive mantle-normalized trace element abundances are comparable with sub-alkaline basalts from developed island arcs. 147Sm/144Nd ratios range from 0.1167 to 0.1269 (n = 3), yielding initial εNd(800 Ma) of +3.8 to +4.9 and mean TDM model age of 0.96 Ga, indicative of derivation from juvenile Neoproterozoic mantle. Metasediments from three locations (Werri1, Werri2 and Tsedia) in the Werri and Tsedia Slates have similar Al2O3, TiO2 and HFSE contents but variable and low Na2O, CaO and K2O. Cr and Ni are slightly enriched in the Werri2 and Tsedia suites. SiO2 is very variable, with average values of 70.75, 72.2 and 66.4 wt.% in the Werri1, Werri2 and Tsedia suites, respectively. ∑REE abundances in the metasediments (14.74–108.1) are lower than in the metavolcanics, and are slightly less fractionated, with LaN/YbN ratios of 0.8–5.9. Europium anomalies vary (Eu/Eu* 0.80–1.21) but are insignificant on average (Eu/Eu* 0.96). High values for the Chemical Index of Alteration (generally 70–90), and Plagioclase Index of Alteration (>75) in the Werri metasediments indicate moderate to severe chemical weathering in their source. Average major and trace element compositions of the metasediments and their REE patterns are comparable with the metavolcanics. 147Sm/144Nd ratios of the metasediments range from 0.1056 to 0.1398 (n = 4), with initial εNd(800 Ma) of +3.4 to +5.0 and mean TDM model age of 0.97 Ga, indicating derivation from juvenile Neoproterozoic crust similar to the underlying metavolcanics, with minimal (4–10%) contribution from older crust. The most sensitive tectonic setting discriminators indicate the Werri metasediments represent developed oceanic island arc sediments. The chemical similarity of the Werri metavolcanics to the nearby Adwa metavolcanics, Nakfa terrane in Eritrea, and volcanic units in central Saudi Arabia imply that juvenile Neoproterozoic Arabian Nubian Shield crust extended south at least as far as the Werri area of northern Ethiopia. The comparable geochemistry of the metasediments and their underlying lithologies attests to their derivation from this juvenile crustal material.  相似文献   

6.
We report new data on the major, minor and trace element compositions of metasedimentary quartzcordierite gneisses (QCG), an important member of the Archaean Malene supracrustal suite found throughout the Godthåbsfjord region of West Greenland. The analyzed QCG contain assemblages of quartz+cordierite+biotite±garnet±anthophyllite/gedrite±staurolite±sillimanite±plagioclase (with abundant accessory zircon, and minor rutile, monazite and allanite), and broadly resemble cordierite-orthoamphibole rocks found in a great number of other metamorphic terrains. Chemically, the QCG are characterized by: (1) high but variable SiO2 (59–87 wt%), relative enrichments in MgO, FeO, and Al2O3 (mg～0.35–0.85), and depletions in Na2O, K2O and especially CaO; (2) low concentrations of Sc, Cr, Co, Ni, and Sr; (3) high concentrations of Y, Nb, Zr, Hf, Ta, Th, U, and REE (rare earth elements)-with prominent negative Eu-anomalies in each case; (5) high concentrations of Ga (18–55 ppm), with variable Ga/Al ratios that are significantly higher than average crustal material. Low Cr and Ni, together with enriched and fractionated REE (displaying negative Eu-anomalies), distinguish the Malene QCG from published accounts of most other Archaean sedimentary rocks. Furthermore, all of the above-mentioned trace element characteristics distinguish the QCG from “ordinary” Malene clastic metasediments (quartzites, psammites, and pelites), suggesting a separate origin for the QCG. These data point towards chemically evolved felsic igneous rocks being the source of the QCG. Consequently, we propose that the Malene QCG represent metamorphosed felsic volcaniclastic sediments that underwent hydrothermal alteration by heated seawater prior to metamorphism, which resulted in gain of Mg (and Fe?), loss of alkalis and lime, and possibly Eu and Sr. The overall trace-element characteristics of the QCG (elevated Ga, Zr, Nb, REE, etc.) are features shared by A-type granites and their volcanic equivalents. Such igneous rocks may represent the ultimate source material for the QCG protolith.  相似文献   

7.
鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义   总被引：2，自引：1，他引：1  
对鄂西黄陵背斜南部宜昌太平溪、邓村一带崆岭岩群中的元古宙庙湾岩组强烈变形变质超镁铁—镁铁质岩的研究表明,镁铁质岩主要为似层状细粒斜长角闪岩,变辉长岩岩体、岩脉及辉绿岩岩脉,超镁铁质岩则主要为蛇纹石化纯橄榄岩、方辉橄榄岩,呈构造岩片、岩块分布于斜长角闪岩之中。细粒斜长角闪岩TiO2=1.14%～1.48%,稀土元素配分型式为略亏损—平坦型,无明显的Eu异常,(La/Yb)N=0.87～1.12,La/Nb、Ce/Zr、Zr/Nb、Zr/Y、Ti/Y平均值分别为1.04、0.15、18.78、2.53、290.51,Nb/Th平均为9.88,显示为大洋中脊构造环境形成的N-MORB型拉斑玄武岩;变辉长岩具典型的堆晶结构特征,稀土元素配分型式为平坦型,具明显的Eu正异常;蛇纹石化纯橄榄岩的稀土元素配分型式具中稀土元素略亏损的U形特征,显示为LREE略富集的地幔岩。上述特征表明,黄陵背斜南部崆岭岩群中的元古宙庙湾岩组实际上是一套混杂堆积的古大洋蛇绿岩残片。元古宙庙湾蛇绿岩的发现为华南扬子克拉通存在中元古代洋盆和哥伦比亚超大陆聚合、裂解构造事件提供了重要的证据。  相似文献   

8.
The Eastern Ghats Granulite Belt (EGGB) forms part of a continuous Precambrian metamorphic terrain in Gondwana. It is characterised by widespread development of an Archaean khondalite suite of metasedimentary rocks, Archaean to Late-Proterozoic charnockites and Late Proterozoic anorthositic, granitic and syenitic emplacements. A 1900 Ma megacrystic granitoid suite, containing varying proportions of charnockites and granites, forms an important and widely distributed litho-unit in the central khondalite and eastern migmatite zones of the EGGB. It contains metasedimentary enclaves, megacrystic K-feldspar, quartz, plagioclase ovoids, biotite, garnet (porphyroblasts and coronas), apatite, zircon, ilmenite, magnetite, etc. Hypersthene is present in the charnockite phase. Monazite is present in some garnet-free granites. It is characterised by low Na2O/K2O ratios, high alumina saturation index, CaO, MgO, and ÝREE, negative correlation of TiO2, Al2O3, Fe2O3t, MgO, MnO, CaO, P2O5, Ba, Sr, Zr and V with SiO2, positive correlation of K2O, REE, Th and Rb with SiO2, fractionated LREE, relatively flat HREE and negative Eu anomalies.The data suggest S-type nature of the suite. Fractionation of the granitic magma and local variations in pH2O and fCO2 caused the formation of megacrystic charnockites. Formation of the corona garnet is related to the reworking of the suite during late Proterozoic (ca. 1250 Ma) isothermal decompression associated with channelised CO2-rich fluid flux along narrow shear zones.  相似文献   

9.
云南鲁春晚三叠世硅质岩的地球化学特征及成因   总被引：1，自引：0，他引：1       下载免费PDF全文
云南鲁春晚三叠世火山沉积盆地是在江达－维西火山弧的基础上，通过拉张作用形成的碰撞后拉张盆地，盆地内广泛发育一套由玄武岩和流纹岩组成的“双峰式”火山岩及与其相伴的硅质岩。硅质岩可分为灰质硅质岩和纹层状硅质岩两类。通过对两类硅质岩所进行的地球化学研究和分析，表明云南鲁春晚三叠世火山沉积盆地中的凝灰质硅质岩属火山沉积成因，在而纹层状硅质岩为海底喷流热液／热泉（热水）沉积成因的热水硅质岩。  相似文献   

10.
对采自羌塘盆地那底岗日地区布曲组碳酸盐岩烃源岩进行了稀土元素地球化学研究,分析结果表明:海相碳酸盐岩烃源岩稀土总量(∑REE)最大值75.21μg/g,最小值20.58μg/g,平均值为36.67μg/g.稀土元素北美页岩标准化后具有相对富集轻稀土,亏损重稀土的特点.布曲组碳酸盐岩烃源岩Ce/Ce*值为0.83～0.9...  相似文献   

11.
首次利用稀土元素和微量化学元素,分析鄂尔多斯盆地中部上三叠统延长组母岩类型和物源方向。研究表明盆地中部延长组砂岩和泥岩的稀土元素地球化学特征、REE分配模式及微量化学元素蜘蛛图解相同,表现为轻稀土富集,重稀土亏损,铕、铈元素亏损;REE分配模式均呈“右倾斜、LREE富集、HREE平坦”型;微量化学元素蜘蛛图解具“四峰三谷一平台”型,曲线呈阶梯状分布;砂岩和泥岩的地球化学特征、REE分配模式及微量化学元素蜘蛛图解与盆地东北缘的太古代、元古代变质岩相一致,而与火成岩的重稀土富集、轻稀土亏损、“V”字型REE分配模式和“三峰两谷”型、曲线呈“W”字型分布的蜘蛛图解的特征有显著的差别。说明延长组母岩是盆地东北缘的太古代、元古代变质岩,主要物源方向为北东向。依据物源方向、沉积环境、砂岩岩石学特征、砂岩稀土元素和微量化学元素的地球化学特征,将鄂尔多斯盆地中部上三叠统延长组沉积划分为两大三角洲沉积体系,即北东向的安塞三角洲沉积体系、志靖三角洲沉积体系、安边三角洲沉积体系和北西向的盐定三角洲沉积体系。在三角洲沉积体系划分的基础上,详细阐述了不同三角洲沉积体系的岩石学特征、稀土元素和微量化学元素的地球化学特征及延长组长6-长2期三角洲沉积体系的演化过程。  相似文献   

12.
The supracrustal rocks of the Wuyang metamorphic terrain are divided into the Zhao anzhuang,Tieshanmiao and Yangshuwan Formations.These three Formations were dated at 3000-2550Ma,2550-2300Ma and 2300-2200Ma,respectively.∑REE and La/Yb)n of the Zhao anzhuang Formation volcanic rocks are obviously higher than those of the Tiesanmiao Formation equivalents,suggesting a sedimentary gap(2550 Ma boundary)between these two formations,The Zhao‘anzhuang Formation is older than the Tieshanmiao Formation.The sediments of these two Formations show no obvious differences in REE and are generally characterized by low ∑REE and positive Eu anomalies.On the contrary,the sediments of the Yangshuwan Formation are characterized by high ∑REE and negative Eu anomalies.Detailed discussions demonstrate that the Yangshuwan Formation was deposited in an oxidizing environment whereas the other two formations were formed in a reducing environment.At the end of the evolution of the Tieshanmiao Formation about 2300 Ma ago,the sedimentary environment was transformed from reducing to oxidizing .On the basis of the SHAB (soft/hard acid and base)theory,an oxidation-reduction model for sedimentary REE evolution has been established .It is proposed that the mantle tends to become gradually depleted in REE.especially in LREE,and the indices ∑REE and La/Yb) n of mantle-dervived volcanic rocks also tend to become lower and lower.  相似文献   

13.
Magmatic Cu-Ni sulfide deposits are generally associated with mafic-ultramafic rocks and it has not been reported that lamprophyre is one of the surrounding rocks of Cu-Ni sulfide deposits. The Dhi Samir deposit in Yemen, however, is a rare example of Cu-Ni deposits which are hosted in lamprophyre dikes. In this paper, comprehensive research is made on petrology, petrochemistry and isotope geochemistry for Cu-Ni-bearing rocks in the Dhi Samir area and the results show that dark rocks related to Cu-Ni orebodies are sodium-weak potassium and belong to calc-alkaline series lamprophyre, especially camptonite, characterized by enriched alkali, iron and titanium. In these rocks large-ion-lithophile elements are obviously concentrated, while high field strength elements slightly depleted, showing clear negative anomalies of Ta and Nb, and weak deficiency of Ti. The SREE is very high (225.67-290.05 ppm) and the REE partition curves are flat and right-inclined, featuring a LREE-enriched pattern with low negative Eu anomalies. Study of magmatic source areas indicates that the rocks have low (87Sr/86Sr) and high εNd(t), and the magmas were probably derived from the enriched mantle I (EM-I) end-member. Based on the LA-ICPMS on zircon U-Pb isotope dating, the lamprophyre in the Dhi Samir mining area has an age of 602±2.6 Ma, indicating that the rock was formed in the late Proterozoic and in an intraplate setting due to magmatism of an extensional environment in the post-Pan-Africa orogeny.  相似文献   

14.
Shales of the ∼2.7 Ga Zeederbergs Formation, Belingwe greenstone belt, Zimbabwe, form thin (0.2-2 m) horizons intercalated with submarine lava plain basalts. Shales of the overlying Cheshire Formation, a foreland basin sedimentary sequence, form 1-100 m thick units intercalated with shallow-water carbonates and deep-water, resedimented basalt pebble conglomerates. Zeederbergs shale is characterized by high contents of MgO and transition metals and low concentrations of K2O and LILE as compared to average Phanerozoic shale, indicative of an ultramafic to mafic source terrain. Cheshire shales have similar major and trace element contents, but MgO and transition metals are less enriched and the LILE are less depleted. Zeederbergs shales have smoothly fractionated REE patterns (LaN/YbN = 2.84-4.45) and no significant Eu anomaly (Eu/Eu* = 0.93-0.96). REE patterns are identical to those of the surrounding basaltic rocks, indicating local derivation from submarine reworking. Cheshire shales have rather flat REE patterns (LaN/YbN = 0.69-2.19) and a small, negative Eu anomaly (average Eu/Eu* = 0.85), indicative of a mafic provenance with minor contributions of felsic detritus. A systematic change in REE patterns and concentrations of transition metals and HFSE upwards in the sedimentary succession indicates erosion of progressively more LREE-depleted basalts and ultramafic volcanic rocks, followed by unroofing of granitoid crust. Weathering indices confirm the submarine nature of Zeederbergs shale, whereas Cheshire shale was derived from a source terrain subjected to intense chemical weathering.  相似文献   

15.
寻乌岩组变沉积岩发育在江西南部新元古代—早古生代南城—寻乌盆地内,时代为震旦—寒武纪。寻乌岩组的Si O2含量变化较大,除1个含量较低的样品外,平均为63.01%,具有相对较高的K2O、Al2O3及(TFe O+Mg O)值,较低的Na2O、Ca O。相容元素含量与后太古代澳大利亚页岩(PAAS)十分接近,高于中国东部上地壳,与其较高的(TFe O+Mg O)特征相吻合,说明源区具有一定数量的中基性组分。稀土元素总量高于上地壳及PAAS,但其球粒陨石标准化配分模式与典型的上地壳及PAAS相似,表现为明显向右倾斜,轻稀土富集,重稀土平坦,铕负异常显著,铈负异常不明显。岩石地球化学特征显示其原岩属于一种中等成熟度的陆源碎屑岩,物源主要为被改造的上地壳长英质和花岗质物源区,少部分物源为中基性火山岩。高场强元素及稀土元素等不活动元素地球化学特征表明其沉积环境为浅海—半深海,其构造环境属于发育有裂谷系的被动大陆边缘,该认识从沉积岩地球化学方面为华南早古生代构造属性提供了新资料。  相似文献   

16.
The Bandas belt, one of two prominent Archaean greenstone belts in the Central African Republic (Equatorial Africa), is ca. 250 km long. At the southernmost part of the belt, a metasedimentary—metavolcanic rock suite is preserved only in brachysynclines. The suite can be divided into two lithostratigraphic units. The lower unit is composed predominantly of volcanic rocks, while the upper one contains mainly metasedimentary rocks. The volcanic rocks, which are part of a sequence ca. 3600 m thick, can be sub-divided according to stratigraphic position, lithology and geochemistry into three groups. The lowermost group includes low-K tholeiitic basalts depleted in light REE. The second group consists of tholeiitic basalts with light REE-enriched patterns and the third, uppermost, group includes andesites, which are similar in several respects to Recent calc-alkaline andesites.The tholeiitic basalts of the first two groups are probably related to different upper mantle sources. The andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light REE or equilibrium melting of eclogite or garnet amphibolite.  相似文献   

17.
The Mesoproterozoic metasedimentary units of Chandil Formation belong to the northern part of Singhbhum crustal province, eastern India. The Formation lies north of Dalma volcano-sedimentary belt and is separated from the Meso- Neoproterozoic Chhotanagpur granite gneissic complex (CGGC) by Tamar-Porapahar shear zone (TPSZ), i.e. South Purulia shear zone, (SPSZ) at its northern boundary. The metasedimentary unit comprises of quartz-mica-sericite-schist, quartzite, amphibolites and carbonaceous phyllites, which have not been studied well, so far in terms of their geochemistry, source rock and provenance characterization. In this work, the existing gaps in all those aspects are studied to infer the provenance of these metasedimentary sequences. The high silica contents of the Chandil Formation (~95% for the quartzites and ~70% for the metapelites) and low trace element concentrations indicate silica-rich source. The REE patterns of the quartzites and the metapelites exhibit LREE enrichment and HREE depletion with moderate negative Eu-anomalies which strongly favor felsic nature of the provenance. The significant enrichment of LREE, flat HREE patterns and negative Eu-anomalies of the metapelites suggest their derivation from an old upper continental crust, which is composed of felsic components and the negative Eu-anomalies indicate intra-crustal differentiation. The higher concentrations of the HFSE i.e. Zr, Hf and Ta, within the Chandil metasediments relative to those of EPC (Early Proterozoic Continental Crust), also suggests a felsic provenance. The geochemical signatures of the Chandil metasediments indicate that the sediments were derived during a prolonged period of weathering due to the slow upliftment and unroofing of the southern evolving Singhbhum granitic complex (SGC).  相似文献   

18.
《Precambrian Research》2001,105(2-4):165-181
In the Palaeoproterozoic Nagssugtoqidian orogen of West Greenland reworked Archaean and juvenile Proterozoic orthogneisses occur side by side and are difficult to differentiate in the field. Archaean gneisses have tonalitic to trondhjemitic compositions with relatively low Al2O3 and Sr, and may have been derived from magmas formed by melting of basaltic or amphibolitic rocks at moderate pressures. The Proterozoic rocks are on average more mafic, and it is likely that they crystallised from mantle-derived magmas. Felsic varieties of the Proterozoic igneous suite probably formed from the original magma by fractional crystallisation, in which hornblende played an important role, and at SiO2 > 65% Archaean and Proterozoic rocks have very similar major and trace element compositions (including REE), illustrating that different modes of origin may lead to very similar results.  相似文献   

19.
Siliciclastic metasediments of the Ladoga Group that is the Kalevian stratotype in Karelia correlative with the Kalevian siliciclastic succession in Finland are studied in terms of geochemistry and Sm-Nd isotopic systematics. The results obtained show that rocks in the Ladoga Group lower part are enriched, as compared to rocks of the upper part, in TiO2, Fe2O3, MgO, Cr, Co, Ni, and Sc, being comparatively depleted in Al2O3 and Th that is a result of compositional changes in provenances. The Sm-Nd isotopic data evidence that siliciclastic sediments of the Ladoga Group have accumulated during the erosion of rocks, which originated at the time of the Archean and Early Proterozoic crust-forming processes. Siliciclastic material with the Archean and Early Proterozoic TNd(DM) values, which are characteristic of metasediments in the group lower part, was derived respectively from granite gneisses of the Archean basement in the Karelian megablock of the Baltic Shield and from volcanic rocks of the Sortavala Group. Volcanic rocks of island-arc terranes of the Svecofennian foldbelt represented main source of siliciclastic material that accumulated in upper part of the succession.  相似文献   

20.
The Eu/Eu*, Eu/Sm, and Ce/La ratios can serve as indicators of zones permeable to ascending fluid fluxes that mark time spans when the kinematics of faults in the basement changed (extension and compression phases). The Eu/Eu* ratio is more informative, the variations in the Eu/Sm ratio are correlated with transgressions in the territory of the anteclise, and the variations in the Ce/La ratio could be caused by both metasomatic processes and low-temperature hydrothermal solutions. In the Voronezh anteclise, whose sedimentary cover is relatively thin and provides no evidence of catagenetic processes, high As concentrations and Eu/Eu* ratios of sedimentary rocks can be utilized as indicators of deep zones of fluid generation centres. Comparable average Eu/Eu*, Eu/Sm, and Ce/La ratios in Proterozoic sedimentary rocks in the platform cover and metasediments of Proterozoic banded iron formations (BIF) confirm that the tectonic environments in which sedimentary units of various age were accumulated were similar.  相似文献   

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