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1.
Rare-earth element data for 14 gabbro and syenogabbro samples from the Tugtutôq younger giant dyke complex (YGDC) and for three anorthosite inclusions lend support to other geochemical and experimental evidence that the anorthosites represent early fractionates that formed at depth from a magma similar to (but not identical with) the liquid which formed the giant dykes themselves. All samples are light REE enriched and absolute REE concentrations increase with increasing degree of differentiation in both chilled marginal samples and cumulates. Strong positive Eu anomalies in early cumulates decrease with advancing fractionation and there may even be negative Eu anomalies in the most differentiated cumulates. Lack of significant Eu anomalies in the chilled samples confirms that large quantities of anorthosite (large positive Eu anomalies) could not have formed directly from the YGDC magma. A comparison with REE distributions in the nearby, younger Ilimaussaq alkaline complex (from data reported by Gerasimovskiy & Balashov (1968) shows that the late fractionates of the YGDC and the early, augite syenite, phase of Ilimaussaq have very similar REE patterns. The later agpaites of Ilimaussaq, however, are greatly enriched in REEs and show strong negative Eu anomalies; this suggests that substantial feldspar fractionation took place at depth before the emplacement of the agpaites, producing rock suites petrologically similar to those exposed in the YDGC.  相似文献   

2.
Quantitative analysis of REE distribution in differentiated, clearly intrusive charnockites from southwest Norway, reveal extreme variation in both absolute abundances and fractionation patterns. The pyroxene-bearing, charnockitic facies show uniform REE patterns with slight enrichment of the light-REE (about 150x chondrites) and positive, neutral or negative Eu-anomalies. Subsequent amphibole-bearing, adamellitic facies show progressive, preferential enrichment of light-REE, reaching La-values higher than 500x chondrites, and increasingly negative Euanomalies. Finally, highly differentiated biotite-granites show a marked depletion of ight-REE, ending with chondrite-normalized La/Lu ratios about 1 and Eu/ Eu* ratios less than 0.2.Using geochemical model calculation, relating major element variations between three main stages of differentiation in terms of refractory mineral assemblages, stepwise quantitative modelling of the REE distributions reproduces the observed changes, and support an origin of the charnockite series as progressively fractionated residual liquids.Close similarity with the REE patterns of charnockite-rapakivi suites elsewhere implies that these may constitute a series of co-magmatic rocks, derived from related more basic source magmas. The fact that the least differentiated members of the series, the charnockites, generally display remarkably uniform REE-patterns, suggests that they equilibrated with a refractory crystal fraction that produces a uniform, bulk partition coefficient. The present analysis suggests that this would be plagioclase and orthopyroxene in a ratio of about 41 and including minor apatite, which in turn points towards affinity with potentially plagioclase-rich cumulates.  相似文献   

3.
Pre-collisional Eocene–Oligocene arc diorites, quartzdiorites, granodiorites, and volcanic equivalents in the Kerman arc segment in central Iran lack porphyry Cu mineralization and ore deposits, whereas collisional middle-late Miocene adakite-like porphyritic granodiorites without volcanic equivalents host some of the world’s largest Cu ore deposits. Petrological and structural constraints suggest a direct link between orogenic arc crust evolution and the presence of a fertile metallogenic environment. Ore-hosting Kuh Panj porphyry intrusions exhibit high Sr (>400 ppm), low Y (<12 ppm) contents, significant REE fractionation (La/Yb > 20), no negative Eu anomalies (Eu/Eu* ≥ 1), and relatively non-radiogenic Sr isotope signatures (87Sr/86Sr = 0.7042–0.7047), relative to Eocene–Oligocene granitoids (mainly Sr < 400 ppm; Y > 12; La/Yb < 15; Eu/Eu* < 1; 87Sr/86Sr = 0.7053–0.7068). Trace element modeling indicates peridotite melting for the barren Eocene–Oligocene intrusions and a hydrous garnet-bearing amphibolite source for middle-late Miocene ore-hosting intrusions. The presence of garnet implies collisional arc crustal thickening by shortening and basaltic underplating from about 30–35 to 40–45 km or 12 kbar. The changes in residual mineralogy in the source of Eocene to Miocene rocks in the Kerman arc segment reflect probing of a thickening arc crust by recycling melting of the arc crustal keel. Underplating of Cu and sulfur-rich melts from fertile peridotite generated a fertile metallogenic reservoir at or near the crust–mantle boundary, and dehydration melting under oxidizing conditions produced syn- and post-collisional ore-hosting intrusions, while the lack of post-collisional volcanism prevented the venting of volatiles to the atmosphere from sulfur-rich and oxidized adakitic magmas. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

4.
Summary ¶Sm–Nd, Rb–Sr and Pb–Pb isotopic compositions of 34 intrusive AMCG (anorthosite–mangerite–charnockite–granite) suite rocks and spatially related ferrodiorites and gabbros from the Lofoten Islands, northern Norway, suggest that almost all Lofoten intrusive rocks can be modelled as a mixture of mainly two components: Archean lower crustal material and an about 1.8Ga mantle-derived component. Isotopically, the gabbros and anorthosites overlap the mangeritic rocks in all three isotope systems. Hence, the isotopic data are in agreement with a model that relates the mangeritic rocks to the fractionation of crustally contaminated mantle-derived basaltic melts. Overlap in all three isotopic systems indicates that at least part of the assimilation process predates or accompanies fractionation. Whole rock geochemistry supports this model: based on major, minor, and trace element data, primitive 1.79–1.86Ga mangerites could have formed from anorthosite residual liquids – the ferrodiorites – by fractionation processes involving Fe–Ti oxides, Fe–Mg silicates and apatite combined with contamination of the ferrodiorites with about 50wt.% crustal anatectic melt.Received May 7, 2002; revised version accepted November 26, 2002 Published online: April 14, 2003  相似文献   

5.
The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na2O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average LaN/YbN = 8.41) and significant negative Eu anomalies (average Eu/Eu1 = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average LaN/YbN = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu1 = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average LaN/YbN = 15.35), resembling those of Group 1, and an identical average Eu/Eu1 of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu1 = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and GdN/YbN–Eu/Eu1 diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K2O/Na2O, LaN/YbN, LaN/SmN, Eu/Eu1, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.  相似文献   

6.
This paper reports the results of numerical simulation for the behavior of rare earth elements (REE) during decompression degassing of H2O- and Cl-bearing granite melts at pressures decreasing from 3 to 0.5–0.3 kbar under near isothermal conditions (800 ± 25°C). Fluid phase in equilibrium with the melt contains mainly chloride REE complexes, and their behavior during magma degassing is, therefore, intimately related to the behavior of chlorine. It was shown that the contents and distribution patterns of REE in the aqueous chloride fluid phase formed during decompression vary considerably depending on (1) the contents of volatiles (Cl and H2O) in the initial melt, (2) the redox state of the magma, and (3) the dynamics of fluid phase separation from magmas during their ascent toward the Earth’s surface. During decompressiondriven degassing, the contents of both Cl and REE in the fluid decrease, especially dramatically under opensystem conditions. The REE patterns of the fluid phase compared with those of the melt are characterized by a higher degree of light to heavy REE fractionation. A weak negative Eu anomaly may be present in the REE patterns of Cl-rich fluids formed during the early stages of degassing at relatively high pressures. At a further decrease in pressure and Cl content in the fluid, it is transformed into a positive Eu anomaly increasing during decompression degassing. Such an anomalous behavior of Eu during degassing is related to its occurrence in magmatic melts in two valence states, Eu3+ and Eu2+, whereas the other REE occur in melts mainly as (REE)3+. The Eu3+/Eu2+ ratio of melt is controlled by the redox state of the magmatic system. The higher the degree of melt reduction, the more pronounced the anomalous behavior of Eu during decompression degassing. The amount of REE extracted by fluid from melt during various stages of degassing does not significantly influence the content and distribution patterns of REE in the melt.  相似文献   

7.
Low to medium grade crystalline rocks locally known as Bomdila Group extensively covers the Lesser Himalayan region in Western Arunachal Himalaya. This Group consists dominantly of mylonitic gneisses of granitic composition of Palaeoproterozoic age, named as Bomdila mylonitic gneiss (BMG) and a small body of hornblende bearing granite of Mesoproterozoic age known as Salari granite (SG). The BMG is affinity to peraluminous (A/CNK > 1.1) with high content of SiO2, K2O/Na2O ratio, normative corundum, high ratio of FeOt/MgO in biotite (3.21–5.11) that shows characteristics of S-type granite whereas SG has granodiorite composition with high Na2O, low K2O, presence of hornblende, normative diopside, low A/CNK ratio (<1.1) and low FeOt/MgO ratio in biotite (1.58–1.60) indicates metaluminous I-type granite affinity. The SG has more fractionated nature of REE [(Ce/Yb)N = 9.06–18.53] and minor negative Eu anomalies [EuN/Eu* = 0.69–0.94] as compared to BMG which has less fractionation of REE [(Ce/Yb)N = 5.95–9.16] and strong negative Eu anomalies [EuN/Eu* = 0.37–0.43]. Geochemical and petrological studies suggest that the SG and BMG are not genetically related; SG appears to have derived from igneous source whereas the BMG have been derived from sedimentary source, however these granitoids might have produced during the same thermal event.  相似文献   

8.
The Matomb region constitutes an important deposit of detrital rutile. The rutile grains are essentially coarse (> 3 mm), tabular and elongated, due to the short sorting of highly weathered detritus. This study reports the major, trace, and rare-earth element distribution in the bulk and rutile concentrated fractions. The bulk sediments contain minor TiO2 concentrations (1–2 wt%), high SiO2 contents (∼77–95 wt%) and variable contents in Al2O3, Fe2O3, Zr, Y, Ba, Nb, Cr, V, and Zn. The total REE content is low to moderate (86–372 ppm) marked by high LREE-enrichment (LREE/HREE ∼5–25.72) and negative Eu anomalies (Eu/Eu* ∼0.51–0.69). The chemical index of alteration (CIA) shows that the source rocks are highly weathered, characteristic of humid tropical zone with the development of ferrallitic soils. In the concentrated fractions, TiO2 abundances exceed 94 wt%. Trace elements with high contents include V, Nb, Cr, Sn, and W. These data associated with several binary diagrams show that rutile is the main carrier of Ti, V, Nb, Cr, Sn, and W in the alluvia. The REE content is very low (1–9 ppm) in spite of the LREE-abundance (LREE/HREE ∼4–40). The rutile concentrated fractions exhibit anomalies in Ce (Ce/Ce* ∼0.58 to 0.83; ∼1.41–2.50) and Eu (Eu/Eu* ∼0.42; 1.20–1.64). The high (La/Sm)N, (La/Yb)N and (Gd/Yb)N ratios indicate high REE fractionation.  相似文献   

9.
The Archean Shawmere Anorthosite Complex, at the southern end of the Kapuskasing Structural Zone, consists dominantly of anorthosite (An65 –85) with minor gabbroic and ultramafic units, which are completely enclosed and cut by tonalites. Both the anorthosites and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the rocks have undergone high grade metamorphism and are recrystallized so that few igneous textures remain.The anorthosites, gabbros and ultramafic rocks of this complex are cumulates which contain calcic plagioclase (An65–95) and have atomic Mg/(Mg + Fe2+) ratios (Mg#) greater than 0.6; less than 3 ppm Rb; 150–210 ppm Sr; and less than 60 ppm Ba. REE abundanees range from 0.2 to 10 times chondritic and exhibit both light-enriched and light-depleted REE patterns. The lower Mg# for the samples having more enriched light REE indicates substantial fractions of ferromagnesian minerals crystallized in addition to plagioclase during fractional crystallization, suggesting that the parent magma was basaltic, and not anorthositic. The ranges in Sr, Ba and REE abundances required for the magmas are typical of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere Anorthosite Complex may represent cumulates of a crustal-level magma chamber which could have been the immediate source of basic Archean volcanics.One gabbroic anorthositic dike sample has a steeply fractionalted REE pattern with heavy REE abundances less than chondrites and a large positive Eu anomaly. The proposed interpretations is that this rock formed by partial melting of mafic cumulates, perhaps those of the Shawmere Anorthosite Complex itself.  相似文献   

10.
Summary The F-rich Hongshan pluton in the eastern Nanling Range, southern China, is a topaz-bearing albite leucogranite. It is distinctive from other topaz-bearing felsic rocks in South China with respect to age, size, geochemical evolution and topaz mode and morphology. The Hongshan granites are highly peraluminous and characterized by high K2O/Na2O, Si, Rb, Cs, Nb, Ta and F, and low Ca, Ba, Sr, Zr, Hf, P, K/Rb, Zr/Hf and Eu/Eu*. The granites show significant trace-element variations with magma evolution, with increasing Rb, Cs, Nb, Ta, Sn, W and decreasing Sr, Ba, Zr, Hf, Y, REE, Pb, Th, K/Rb, Zr/Hf, Th/U and Eu/Eu*. These changes dominantly reflect fractional crystallization of plagioclase, biotite and accessory minerals such as zircon and monazite. The granites also exhibit a decrease in ɛNd(t = 225 Ma) from −7.9 to −11.7 with magma evolution. Modeling shows that the Nd isotopic variation could result from assimilation of the Taoxi Group wall rocks during fractional crystallization. The Hongshan pluton also shows spatial geochemical variations; the most evolved parts are located in the southeastern part of the pluton, which would be the most likely target area for rare-metal mineralization commonly associated with other topaz-bearing granites. Zircon grains from two rock types in the Hongshan body were analyzed in situ for U–Pb ages and Hf isotopic values. The concordant zircon grains mostly range from 218 to 230 Ma with an average of 224.6 ± 2.3 Ma (Indosinian). Some zircons with different internal structures and Hf isotope compositions, as well as monazite fragments, yield U–Pb ages of ca. 280 to 240 Ma, suggesting older thermal events in the studied area. The ɛHf(t) of these older zircons is strongly negative (−12.3), implying a crustal source with a Paleoproterozoic model age, similar to that for the Proterozoic Zhoutan Group. The main (∼225 Ma) zircon population exhibits less negative ɛHf(t) (−3.0 to −7.6) and Mesoproterozoic model ages, suggesting that the original magma of the Hongshan granite was generated from deeper Mesoproterozoic crust.  相似文献   

11.
新疆赞坎铁矿床位于西昆仑塔什库尔干地块西段,是近年新发现的一个大型沉积变质型磁铁矿床。赋矿岩系布伦阔勒群主要由黑云母石英片岩、斜长角闪片岩、变粒岩、硅质岩及磁铁石英岩等组成。目前探明工业矿体4条,单个矿体长度大于2.5km,矿体厚10~70m;局部见高品位铁矿段(mFe50%),长度达900m,厚度40m左右。矿石类型主要为2种,一种为原生的条纹-条带状磁铁矿(为主);另一种为热液改造形成的块状(高品位铁矿石)及浸染状磁铁矿。矿石稀土元素配分(PAAS)表明,原生条纹-条带状铁矿石Ce和Y元素异常不明显(~1.15、~0.94),Eu具正异常(~1.69),Y/Ho平均值为25,稀土配分模式与沉积变质型铁矿相似。而受改造的矿石中,浸染状矿石具有较高的稀土总量,明显富集轻稀土,La和Ce显示正异常(~1.46、~1.17),Y显示负异常(=0.66~0.72),Eu表现为强烈的正异常(~4.37),稀土配分模式明显不同于原生条纹-条带状铁矿石。矿体围岩斜长角闪片岩(变沉积岩)中的碎屑锆石U-Pb年龄为591±1Ma,结合前人对矿区内侵入体的年代学研究(霏细斑岩,533Ma),大致反映沉积铁矿的形成时代为新元古代至早寒武世。电子探针显示,条带状磁铁矿中的TiO_2、AL_2O_3、MgO、MnO含量较低,标型组分含量与沉积变质型磁铁矿颇为接近,在磁铁矿单矿物成因图解中,条带状磁铁矿整体显示磁铁矿为沉积变质型铁矿;浸染状矿石和块状矿石的组成与典型沉积变质型铁矿的偏离反映了后期岩浆-构造热事件对条带状铁矿石的改造;上述结果显示赞坎铁矿整体属于沉积变质型铁矿(BIF)。调查发现赞坎高品位铁矿体与早寒武世侵入的霏细斑岩联系密切,高品位矿石及其围岩发育一定程度的矽卡岩化,如阳起石化、碳酸盐化和黄铁矿化。本文推测高品位铁矿石的成因可能为霏细斑岩的岩浆热液溶解并运移早期沉积变质铁矿中的含铁物质,在构造发育处充填交代形成块状磁铁富矿石。在早寒武世侵入到矿区中部的霏细斑岩体中,同时发育有角砾状磁铁矿和脉状磁铁矿,因此,岩浆热液改造原生条带状铁矿石形成高品位铁矿石的时代应为早寒武世。  相似文献   

12.
The Maoduan Pb–Zn–Mo deposit is in hydrothermal veins with a pyrrhotite stage followed by a molybdenite and base metal stage. The Re–Os model ages of five molybdenite samples range from 138.6 ± 2.0 to 140.0 ± 1.9 Ma. Their isochron age is 137.7 ± 2.7 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the nearby exposed Linggen granite porphyry gave a 206Pb/238U age of 152.2 ± 2.2 Ma and the hidden Maoduan monzogranite yielded a mean of 140.0 ± 1.6 Ma. These results suggest that the intrusion of the Maoduan monzogranite and Pb–Zn–Mo mineralization are contemporaneous. δ 34S values of sulfide minerals range from 3.4‰ to 4.8‰, similar to magmatic sulfur. Four sulfide samples have 206Pb/204Pb = 18.252–18.432, 207Pb/204Pb = 15.609–15.779, and 208Pb/204Pb = 38.640–39.431, similar to the age-corrected data of the Maoduan monzogranite. These isotope data support a genetic relationship between the Pb–Zn–Mo mineralization and the Maoduan monzogranite and probably indicate a common deep source. The Maoduan monzogranite has geochemical features similar to highly fractionated I-type granites, such as high SiO2 (73.7–75.2 wt.%) and alkalis (K2O + Na2O = 7.8–8.9 wt.%) and low FeOt (0.8–1.3 wt.%), MgO (~0.3 wt.%), P2O5 (~0.03 wt.%), and TiO2 (~0.2 wt.%). The granitic rocks are enriched in Rb, Th, and U but depleted in Ba, Sr, Nb, Ta, P, and Ti. REE patterns are characterized by marked negative Eu anomalies (Eu/Eu* = 0.2–0.4). The Maoduan monzogranite, having (87Sr/86Sr) t  = 0.7169 to 0.7170 and εNd(t) = −13.8 to −13.7, was probably derived from mixing of partial melts from enriched mantle and the Paleoproterozoic Badu group in an extensional tectonic setting.  相似文献   

13.
Four massif-type anorthosite bodies 25–100 km2 in area occur within high-pressure granulite facies supracrustal gneisses in southwestern Madagascar. Two of these bodies (Ankafotia and Saririaky) appear to have been pulled apart by 40 km in a ductile shear zone, but structural features such as sub-vertical stretching lineations indicate an origin by intense west-directed flattening and pure shear. Country rocks (Graphite Series) include abundant graphite schist (some with >60% graphite), marble, quartzite, and minor amphibolite and leucogneiss. Comagmatic granitoids (e.g. charnockites) are conspicuously absent. The anorthosite bodies are dominated by coarse grained anorthosites and leuconorites (feldspars typically 3–5 cm, up to 1 m); minor norites and oxide-rich ferrogabbros occur near the margins, but ultramafic rocks are absent. Typical mineralogy of the anorthositic rocks is: plagioclase (An41–54) + orthopyroxene (En38–66) ± augite (Mg♯ = 32–68) ± ilmenite ± magnetite ± apatite. High-alumina (to 6.1 wt% Al2O3) orthopyroxene megacrysts are widespread; most have exsolutions of calcic plagioclase (An72–85) but some contain garnet lamellae. Metamorphism has produced abundant recrystallization and sporadic coronitic garnet (Mg ♯=12–36) + clinopyroxene assemblages. Rb-Sr isotopic analyses of whole-rocks and minerals reveal no meaningful age relationships. The age of late Neoproterozoic metamorphism is best constrained at 559 ± 50 Ma by a 6-point Sm-Nd mineral isochron (whole rock, plag, pyx, ilm, apat, gar) from a Saririaky oxide-rich gabbro. The igneous crystallization age of the anorthosites is estimated at 660 ± 60 Ma by a 19-point combined whole-rock and mineral Sm-Nd isochron for samples from both the Ankafotia and Saririaky bodies. Initial isotopic ratios calculated at 0.66 Ga among 13 whole rocks are: Nd=+2.6 to +5.2 (mean=+3.7) and ISr=0.70328–0.70407 (mean=0.70347), indicating derivation of the Malagasy anorthosites from a depleted mantle source, and little, if any, contamination with Archean crustal material. One anorthosite sample with Nd=−1.4 and ISr=0.70344 (calculated at 0.66 Ga) probably reflects the effects of assimilation of Early to Middle Proterozoic crustal basement, but typical surrounding graphite schist (Nd=+0.3, ISr=0.70636, both at 0.66 Ga; TDM= 1131 Ma) represents only a minor potential contaminant for the anorthosite bodies. TDM model ages of the Malagasy anorthosites (797–1280 Ma; mean of 14 samples=949 Ma), as those of most other massif-type anorthosites, are older than the true crystallization age, because of crustal contamination effects. Our isotopic data, together with recent U-Pb data from the anorthosites and surrounding country rocks, are consistent with emplacement of the Malagasy anorthosite bodies at or before the start of a protracted, high-grade metamorphic event or series of events between about 630 and 550 Ma. This period coincides with the collision between, and amalgamation of, East and West Gondwana. Received: 19 December 1997 / Accepted: 12 June 1998  相似文献   

14.
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15.
The Turkel anorthosite Complex (TAC) in the Eastern Ghats Belt in India is composed of anorthosites and leuconorites at the centre and ferrodiorites and quartz diorites at the periphery. Here we report whole‐rock geochemistry, and zircon U–Pb data and REE geochemistry from a co‐spatial ferrodiorite and two quartz diorites from the TAC. The diorites have low abundance of High Field Strength Elements (HFSE) and REE, exhibit a flat chondrite‐normalized pattern with slight LREE enrichment and negligible or no Eu anomaly. Our results show weighted mean 207Pb/206Pb ages of 2433 ± 33 Ma for the ferrodiorite. Two quartz diorite samples from Turkel yield mean207Pb/206Pb ages of 2419 ± 32 Ma and 2505 ± 31 Ma. The zircons from all the analysed samples show high REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly, suggesting a common magmatic source. The prominent Neoarchaean to early Palaeoproterozoic magmatic ages from the anorthosite complex deviate from the late Neoproterozoic ages reported from other anorthosite suites in the Eastern Ghats Belt, and suggest an active convergent margin along SE India during Archaean–Proterozoic transition. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

16.
The rare-earth element （REE） concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations （190×10^-6-1191×10^-6; av.=549×10^-6） with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE （151×10^-6-1169×10^-6; av.= 466×10^-6）, while the HREE show low abundance （4×10^-6-107×10^-6; av.=28×10^-6）. These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN （2.30-343.37）, CeN/YbN （5.94-716.87）, LaN/SmN （3.14-11.68） and TbN/YbN （0.58-1.65）. The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu＊ ratios （0.085-2.807; av.=0.9398） indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu＊ ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources.  相似文献   

17.
Rare earth element (REE) distributions and Pb isotope compositions were explored in soils varying in age from ca. 0.4 to ?300 ka, developed on moraines in the Wind River Mountains, Wyoming. Soil extracts (0.6 M HCl) were used to examine the soil labile pool while the major element distribution in soil profiles was used to determine the extent of weathering at different soil depths. The results show that the chondrite-normalized REE patterns of the deepest bulk soil within each profile reflects the composition of the moraine till, except for the oldest soil. Up to ca. 12 ka, the soil extract fraction is enriched in light REE, indicating early release of light REE to the soil labile pool while that of the two oldest soils are relatively enriched in heavy REE. In the soil extracts the La/Sm ratio normalized to the deepest soil (LaD/SmD) decreases systematically with soil age. Similarly, the Eu-anomaly in the deepest soil from each profile (EuD/EuD*) decreases slightly with soil age in the three young soils; however, EuD/EuD* increases with soil age in the older soils. The systematic trends of these two ratios indicate the depletion of light REE in young soils and the enrichment of Eu and heavy REE in the older soils. Based on the Pb isotope ratios, the relative contribution of Pb to the soil labile pool via mineral weathering of U- or Th-rich phases was assessed for the different stages of weathering. The whole-soil profile 208Pb/204Pb ratio was found to decrease with soil age and with LaD/SmD, whereas it increased with the EuD/EuD* ratio. In each horizon, Pb isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) ratio generally decrease with soil age. In order to overcome possible effects from parent material heterogeneity, the amount of radiogenic Pb as compared to the whole-soil composition was calculated and this was found to decrease systematically with soil age.  相似文献   

18.
The Dokhan volcanics are represented by a thick stratified lava flows succession of basalt, andesite, imperial porphyry, dacite, rhyodacite, rhyolite, ignimbrites, and tuffs. These lavas are interbanded with their pyroclastics in some places including banded ash flow tuffs, lithic tuffs, crystal lapilli tuffs, and agglomerates. They are typical calc–alkaline and developed within volcanic arc environment. All rocks show moderate enrichment of most large ion lithophile elements relative to high field strength elements (HFSE). The incompatible trace elements increase from basalt through andesite to rhyolite. The felsic volcanics are characterized by moderate total rare earth elements (REE) contents (162 to 392 ppm), less fractionated patterns {(Ce/Yb)N = (1.24 to 10.93)}, and large negative Eu anomaly {(Eu/Eu*) = (0.15 to 0.92)}. The mafic volcanics have the lowest REE contents (61 to 192 ppm) and are relatively steep {(Ce/Yb)N = (3.2 to 8.5)}, with no negative Eu anomalies {(Eu/Eu*) = (0.88 to 1)}. The rhyolite displays larger negative Eu anomaly (Eu/Eu* = 0.28) than those of other varieties, indicating that the plagioclase was an early major fractionating phase. The mineralogical and chemical variations within volcanics are consistent with their evolution by fractional crystallization of plagioclase and clinopyroxene.  相似文献   

19.
Mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35% olivine, occur at three different levels in Olivine-Bearing zones III and IV of the Middle Banded series of the Stillwater complex. The discordant bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses. Although the host olivine-gabbros are foliated with tabular plagioclase, the discordant bodies lack a discernible fabric and have blocky plagioclase. Average olivine in the host rocks is slightly more magnesian than that of the discordant bodies (Mg#75.8 ± 0.7 versus Mg#74.6 ± 0.3 respectively) but plagioclase compositions are indistinguishable (An77.6 ± 2.0 versus An76.6 ± 4.3– average host and discordant bodies respectively). Whole-rock major- and trace-element compositions of the discordant bodies are generally indistinguishable from cumulates with similar modal abundance. However, bulk compositions of anorthositic cores from the discordant bodies are enriched in K, Na, Ba, Sr and P. We conclude that the discordant bodies formed when cooler volatile fluids or fluid-rich silicate liquids moved upward and encountered a hotter undersaturated solid-plus-liquid assemblage. Continued liquid/fluid fluxing increased the permeability along the flow path and focused the flow, allowing the original bulk compositions to be modified and leaving plagioclase-rich troctolites and anorthosites. The shapes of the discordant bodies suggest that the cumulus pile had anisotropic permeability during late-stage liquid/fluid flow. Chemical and mineralogical evidence from other parts of Olivine-Bearing zones III and IV suggests that the processes that formed the discordant bodies may have influenced other cumulates. In fact, it appears that the same processes that formed the discordant bodies operated within an anorthositic layer, strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence. Received: 10 December 1996 / Accepted: 12 August 1997  相似文献   

20.
Primary massive sulfide gossans (MSG) in the Bathurst Mining Camp (BMC), New Brunswick, Canada, are characterized by relative enrichment of Au, Sb, and As, formation of jarosite group minerals (jarosite, plumbojarosite, and argentojarosite) and little or no fractionation in the rare earth elements (REE), including preservation of large positive Eu anomalies (average [Eu/Eu*]NASC = 4.14 in MSG; 6.61 in massive sulfide mineralization; 0.60 in host rocks). The chemical and mineralogical characteristics of MSG (e.g., Halfmile Lake deposit) imply low pH (<3) and relatively oxidizing conditions during gossan formation; oxidation of a volcanogenic massive sulfide body (comprising pyrite, pyrrhotite, sphalerite, galena, and chalcopyrite) with a falling water table. The lack of light REE or heavy REE fractionation and preservation of positive Eu anomalies characteristic of the original (465 Ma) hydrothermal fluid is consistent with relatively large water-rock ratios during massive sulfide mineralization oxidation, and removal of the REE predominantly as sulfate complexes (LnSO4+, Ln(SO4)2−). Low pH groundwaters recovered from past producing mines in the BMC display REE patterns reflecting those inferred to have occurred during gossan formation. Gossan at the Restigouche deposit, in contrast to the Halfmile Lake deposit, displays mineralogical and chemical evidence for having been chemically reworked since primary gossan formation. Evidence for chemical reworking includes loss of primary massive sulfide mineralization textures, replacement of plumbojarosite with anglesite, almost complete removal of jarosite minerals, loss of Au, Sb, and As and apparent preferential removal of Eu, resulting in loss of positive Eu anomalies for most samples (average [Eu/Eu*]NASC = 1.21 in the gossan, with many displaying strong negative anomalies; 3.65 in massive sulfide mineralization; 0.54 in host rocks). Based on geochemical modeling, conditions inferred for the chemical reworking of the Restigouche deposit include near neutral conditions and either relatively oxidizing conditions with Eu2+ hosted in a preferentially weathered mineral host (possibly through substitution for Pb in plumbojarosite and beudantite) or cycling between reduced and oxidized conditions during gossan reworking.  相似文献   

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