新疆沙尔布尔山卡拉岗组火山岩岩石化学及其构造环境分析

对卡拉岗组火山岩岩石化学特征分析和研究结果表明，火山岩碱性程度为碱性－钙碱性，岩石系列为高铝玄武岩系列、拉斑玄武系列碱性玄武岩系列，求得钙碱指数为52，属张性环境，在全碱－二氧化硅图上样品落入玄武岩区及流纹岩区，分异指数频率曲线表明具有大陆拉张的双峰式特点，认为该区下二叠统计卡拉岗组火山岩产于双峰式大陆拉张构造环境。     

西天山塔尔得套地区乌郎组地球化学特征和构造环境

新疆尼勒克县塔尔得套地区二叠纪火山岩产于伊犁微板块中,为陆相火山岩,主要岩性为玄武岩和流纹岩,具双峰式火山岩系组合特征。基性火山岩属高钾钙碱系列,轻微富集LREE,HREE分馏较明显((Gd/Yb)N=1.52~2.68),富集大离子亲石元素(LILE),亏损高场强元素(HFSE)。流纹岩的Eu负异常明显(δEu=0.33),在微量元素蛛网图上,流纹岩除了具LILE富集和HFSE亏损特征外,Sr,P,Ti等元素的强烈亏损。稀土元素和微量元素特征表明火山岩形成于大陆拉张环境,处于裂谷演化的早期阶段。     

内蒙古东北部额尔古纳地区上护林-向阳盆地中生代 火山岩LA-ICP-MS锆石U-Pb年龄和地球化学特征

LA-ICP-MS锆石U-Pb测年结果表明,研究区中生代火山岩可分为2期,分别是早白垩世早期(约144Ma)吉祥峰组流纹岩类和早白垩世晚期(约125Ma)上库力组流纹岩类与梅勒图组玄武岩类。早白垩世早期吉祥峰组流纹岩富硅、富碱、富钾(K2O/Na2O>1),富集轻稀土元素和Rb、Th、U等元素,亏损重稀土元素、高场强元素(Nb、Ta、Ti)及Sr、P,具有A型流纹岩的特征,暗示其形成于伸展环境。早白垩世晚期火山岩显示双峰式岩石组合特征,基性端元富碱、高钾,富含轻稀土元素和Rb、Ba,亏损重稀土元素和高场强元素(Nb、Ta、Ti、Y),类似于钾玄质玄武岩,酸性端元显示A型流纹岩的特征。双峰式火山岩组合的存在暗示其形成于陆内拉张的构造环境。结合区域上中生代火山岩的空间分布规律,认为早白垩世早期火山岩的形成与蒙古-鄂霍次克缝合带的演化有关,早白垩世晚期双峰式火山岩的形成与古太平洋板块向欧亚大陆下的俯冲作用相联系。     

内蒙古东北部额尔古纳地区上护林-向阳盆地中生代火山岩LA-ICP-MS锆石U-Pb年龄和地球化学特征

LA-ICP-MS锆石U-Pb测年结果表明,研究区中生代火山岩可分为2期,分别是早白垩世早期（约144Ma）吉祥峰组流纹岩类和早白垩世晚期（约125Ma）上库力组流纹岩类与梅勒图组玄武岩类。早白垩世早期吉祥峰组流纹岩富硅、富碱、富钾（K2O/Na2O〉1）,富集轻稀土元素和Rb、Th、U等元素,亏损重稀土元素、高场强元素（Nb、Ta、Ti）及Sr、P,具有A型流纹岩的特征,暗示其形成于伸展环境。早白垩世晚期火山岩显示双峰式岩石组合特征,基性端元富碱、高钾,富含轻稀土元素和Rb、Ba,亏损重稀土元素和高场强元素（Nb、Ta、Ti、Y）,类似于钾玄质玄武岩,酸性端元显示A型流纹岩的特征。双峰式火山岩组合的存在暗示其形成于陆内拉张的构造环境。结合区域上中生代火山岩的空间分布规律,认为早白垩世早期火山岩的形成与蒙古-鄂霍次克缝合带的演化有关,早白垩世晚期双峰式火山岩的形成与古太平洋板块向欧亚大陆下的俯冲作用相联系。     

勉略结合带五里坝火山岩的地球化学研究及其构造意义

五里坝火山岩位于扬子与秦岭微板块的分界断裂:巴山弧两河—饶峰—石泉—高川蛇绿构造混杂带内,岩石为一套双峰式火山岩组合,由亚碱性玄武岩和亚碱性英安流纹岩组成。基性岩和酸性岩均具有显著的Nb、Ta亏损,玄武岩的Th/Yb-Ta/Yb和Ti/Zr-Ti/Y不活动痕量元素组合特征指示这套双峰式火山岩具有岛弧火山岩的地球化学特征,形成于特殊的洋内岛弧弧内裂陷的大地构造环境。五里坝火山岩应是勉略洋盆在古生代晚期—中生代早期发育期间洋壳俯冲及裂陷的岩浆作用产物。它可能是勉略缝合带在巴山弧地区出露的重要岩石学证据。     

西藏冈底斯带东段石炭纪构造环境讨论

冈底斯带东段石炭纪构造环境存有争议。本文通过岩石地球化学分析,发现石炭纪诺错组和来姑组火山岩具有碱性系列向拉斑系列过渡的特征,具有双峰火山岩特点;石炭纪玄武岩样品的平均化学成分与大陆拉斑玄武岩的平均值较为接近;微量元素除Ti、Yb、Y之外,其他元素都富集,其中Rb、Th、Ce富集程度相对较高;稀土元素特征表现为轻稀土富集,铕、铈异常不明显。显示出石炭纪玄武岩的地球化学特征均与大陆拉斑玄武岩相似。构造环境判别图解显示,石炭纪玄武岩样品主要落入板内环境的大陆拉张带(或初始裂谷)玄武岩区。因而认为冈底斯东段在石炭纪属于陆内裂谷或被动陆缘裂谷环境。     

大兴安岭嫩江地区中生代双峰式火山岩锆石U-Pb定年、地球化学特征及其地质意义

本文对嫩江地区中生代双峰式火山岩进行了锆石LA-ICP-MSU-Pb年代学和岩石地球化学研究。测年结果显示嫩江地区中生代双峰式火山岩形成于127.5Ma的早白垩世晚期。岩石地球化学研究表明,早白垩世晚期火山岩具有双峰式组合特点,基性端员富碱,富含轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素（Nb、Ta、Ti、Y）,形成于富集的岩石圈地幔的部分熔融和分离结晶作用,形成的过程可能含有少量的陆壳混染。酸性端员显示A型流纹岩的特征,为幔源岩浆底侵,使中下地壳岩石发生部分熔融的成因。双峰式火山岩组合的存在暗示其形成于陆内拉张的构造环境。结合区域上中生代火山岩的空间展布特征,嫩江地区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有关。     

新疆柯坪库木如吾祖克地区二叠纪火山岩

新疆柯坪库木如吾祖地区二叠纪火山岩产于柯坪微地块中。为陆相火山岩，主要由基性熔岩（辉石玄武岩）和酸性凝灰岩组成，具双峰式火山岩系组合特征，火山岩为3个火山喷发旋回的产物，从下至上基性熔岩主元素成分成规律性变化，表明基性熔岩为同源岩浆分异演化的产物。基性熔岩具有大陆火山岩性质，形成于大陆拉张环境。该二叠纪火山岩为天山造山带石炭－二叠纪大规模裂谷作用在柯坪古老微地块上的反映。     

不同构造环境中双峰式火山岩的主要特征

总结了双峰式火山岩研究的最新进展。研究表明,双峰式火山岩可以出现在多种大地构造背景中,如大陆裂谷、洋岛、大陆拉张减薄、弧后扩张、造山后、洋内岛弧和成熟岛弧／活动陆缘等。流纹岩主要有两种成因,一种流纹岩与玄武岩来自同一源区,流纹岩通常是玄武岩浆演化的产物,出露面积相对很少,并与玄武岩具有的地球化学特征;另一种流纹岩与玄武岩不同源,流纹岩是地壳深熔作用形成的,流纹岩相比玄武岩常常是占优势的中双峰式并且     

新疆昆仑式火山岩型块状硫化物铜矿床及成矿地质环境

新疆西昆仑奥依塔克-恰尔隆拗陷带内阿克塔什-萨落依成矿带发现多处火山岩型块状硫化物铜矿床。这些矿床产在石炭系双峰式火山岩系之内，沿着下石炭统基性火山岩和上石炭统酸性火山岩两个层位产出，分别以玄武岩和流纹岩为容矿主岩，可以明显地分成基性火山岩型和酸性火山岩型两种类型。矿石主成矿元素均以铜为主，含少量的锌，几乎不含铅，矿床类型属于铜型。这些基性火山岩型和酸性火山岩型矿床被统称为昆仑式火山岩型块状硫化物铜矿床。根据矿床产出地质环境、双峰式火山岩系、沉积建造以及火山岩地球化学特征，推断昆仑式火山岩型块状硫化物铜矿床最可能形成于泥盆纪-石炭纪弧后拉张构造环境。     

The geochemistry of Archaean shales derived from a Mafic volcanic sequence, Belingwe greenstone belt, Zimbabwe: provenance, source area unroofing and submarine versus subaerial weathering

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 (La_N/Yb_N = 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 (La_N/Yb_N = 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.     

A geochemical study of magmatism associated with the initial stages of back-arc spreading

Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading.Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with Ce_N/Yb_N = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although Ce_N/Yb_N ratios and ⁸⁷Sr/⁸⁶Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher ⁸⁷Sr/⁸⁶Sr (0.7039) and Ce_N/ Yb_N (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present ⁸⁷Sr/⁸⁶Sr ratios.Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.     

Rare-earth elements as source indicators of Pan-African granites from Obudu Plateau,Southeastern Nigeria

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.     

Major Element Compositions and Rare—earth Element Abundaces of Cenozoic Basalts in Eastern China：Implications for a Pressure Control over LREE／HREE Fractionation in Continental Basalt

Major element compositions and rare-earth element (REE) and transition element(Ni,Cr and V) abundances have been determined on 44 basalt samples from eastern China.These basalts have SiO2 contents ranging from 38.63 to 55.24(wt.%),and Na2O K2O from 3.1 to 9.4(wt.%).Ni and Cr abundances are largely variable,respectively falling in ranges 60-605 and 78-1150 ppm.REE abundances,especially light rare-earth elements(LREE), are highly variable.La/Sm and La/Yb ratios vary 2.8 to 7.6 and 1.8 to 8.1. Although the segregation mainly of olivine and clinopyroxene is requested to account for the vari-able and low MgO,CaO/Al2O3,Cr and Ni characteristic of these basalts studied here,the differ-ences in REE composition of the basalts are still related mainly to the partial melting process.Obvious varations in REE abundances could be principally attributed to the partial melting process.Obvious variations in REE abundances could be principally attributed to the partial melting processes that took place at different depths,in spite of some variations caused by the fractional crystallization processes.REE abundances and La/Sm and La/Yb ratios systematically decrease with increasing SiO2,which probably indicated that the basaltic magma derived from a deeper level has higher LREE and LREE/HREE ratios than that from a shallower level.As viewed from the fact that the D^Yb/D^La ratios of clinopyroxenes in the basaltic system increase with increasing pressure,the increase of LREE/HUEE ratios with increasing melting depth can be interpreted as the pressure dependence of bulk D^HREE/D^LREE ratios of silicate minerals,in addition to the pressure control over the melting degree.     

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/La_pm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Yb_n=1–8), high Th/Nb_pm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO₂, Ta, P₂O₅, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Yb_n tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Yb_n of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs. Received: 2 June 1999 / Accepted: 29 December 1999     

Petrology,geochemistry and tectonic setting of the Khoy ophiolite,northwest Iran: implications for Tethyan tectonics

The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest–southeast trending ophiolite complex consists from bottom to top (east to west) of a well-defined basal metamorphic zone, peridotites (dunite, harzburgite) and serpentinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately adjacent to the peridotite and the gabbros and the low-grade rocks are in contact with the Precambrian Kahar Formation. Based on mantle-normalized incompatible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virtually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N-MORB. The chondrite-normalized REE patterns for the pillow basalts are LREE-depleted [(La_N/Sm_N)_ave=0.70], similar to patterns for the mean diabase composition for the Oman ophiolite and LREE-depleted basalts of the Band-e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basalts are similar in general REE abundances to the pillow basalt patterns, but they are slightly LREE-enriched [(La_N/Sm_N)_ave=1.09] and their patterns cross those of the pillow basalts. The REE patterns for the gabbros and diorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This basaltic melt was most likely generated by approx. 20–25% partial melting of a simple lherzolite source and had REE concentrations of roughly 10× chondrite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a tectonic link between the Khoy ophiolite and the rest of the Iranian ophiolites. Our results suggest that Khoy ophiolite is equivalent to the inner group of Iranian ophiolites (e.g. Nain, Shahr-Babak, Sabzevar, Tchehel Kureh and Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Iranian microcontinent.     

NdSr isotope and REE geochemistry of alkali basalts from the Massif Central,France

Nd and Sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the Massif Central, in France. Samples show a typical enrichment in incompatible elements. In particular, the REE patterns exhibit a strong fractionation characterized by a ($\text{La}\text{Yb}\text{)}{}_{\mathrm{<mtext>N</mtext>}}$ ratio of about 20. The Yb_N content is about 10 times chondrite. The $\text{143}\text{Nd}\text{144}\text{Nd}$ ratios exhibit a range from 0.512775 to 0.512989, values quite comparable to those from oceanic island basalts. The $\text{87}\text{Sr}\text{86}\text{Sr}$ ratios vary between 0.70338 and 0.70458 and are anti-correlated with the Nd isotopic ratio.The isotopic and the trace element (in particular REE) data have been used in order to quantitatively model the genesis of the alkali basalts. Among the several types of models tested here, the most likely one appears to be the model of mantle metasomatism. A semi-quantitative approach shows that the source of alkali basalts from the Massif Central was metasomatized prior to melting. In such a model, the basalts could be produced by rather high degrees of partial melting (such as 10 or 15%) of the metasomatically enriched mantle.     

Geochemistry of black shales from the Lower Cretaceous Paja Formation,Eastern Cordillera,Colombia: Source weathering,provenance, and tectonic setting

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 La_N/Yb_N = 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 La_N/Yb_N = 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 La_N/Yb_N = 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 Gd_N/Yb_N–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, La_N/Yb_N, La_N/Sm_N, 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.     

Depositional conditions,characteristics and source of rare earth elements in carbonate strata of the Albian Asu River Group,Middle Benue Trough,north central Nigeria

Major, trace and rare earth elements (REE) concentrations in limestone beds of the Asu River Group within the Middle Benue Trough were measured to understand the depositional conditions, characteristics and source of REE. The limestone has high content of CaO (Average of 46.55%), followed by SiO₂ (Average of 7.90 %), Fe₂O₃(t), MgO and Al₂O₃. The limestones are depleted in most of the trace elements (Co, V, Rb, Ba, Zr, Y, Nb, Hf and Th) when compared with the Post-Archean Australian Shale (PAAS). The observed large variations in ΣREE contents among various limestones of the present study (12.22 to 142.53ppm) are mainly due to the amount of terrigenous matter present in them. The characteristics of non-seawater-like REE patterns, elevated REE concentrations, high La_N/Yb_N ratios and low Y/Ho ratios, suggest that the observed variations in ΣREE contents are mainly controlled by the amount of detrital sediments in the limestones of the Asu River Group in the middle Benue trough. The observed variations in Ce contents and Ce anomalies in the studied samples resulted from detrital input. The limestones show positive Mn* values (0.30 to 0.78) and low contents of U (～0.60–3.20 ppm) suggesting that they were deposited under oxygen-rich environment.     

Permo-Triassic plume magmatism of the Kuznetsk Basin,Central Asia: geology,geochronology, and geochemistry

The Kuznetsk Basin is located in the northern part of the Altai–Sayan Folded Area (ASFA), southwestern Siberia. Its Late Permian–Middle Triassic section includes basaltic stratum-like bodies, sills, formed at 250–248 Ma. The basalts are medium-high-Ti tholeiites enriched in La. Compositionally they are close to the Early Triassic basalts of the Syverma Formation in the Siberian Flood basalt large igneous province, basalts of the Urengoi Rift in the West Siberian Basin and to the Triassic basalts of the North-Mongolian rift system. The basalts probably formed in relation to mantle plume activity: they are enriched in light rare-earth elements (LREE; La_n = 90–115, La/Sm_n = 2.4–2.6) but relatively depleted in Nb (Nb/La_PM = 0.34–0.48). Low to medium differentiation of heavy rare-earth elements (HREE; Gd/Yb_n = 1.4–1.7) suggests a spinel facies mantle source for basaltic melts. Our obtained data on the composition and age of the Kuznetsk basalts support the previous idea about their genetic and structural links with the Permian–Triassic continental flood basalts of the Siberian Platform (Siberian Traps) possibly related to the activity of the Siberian superplume which peaked at 252–248 Ma. The abruptly changing thickness of the Kuznetsk Late Permian–Middle Triassic units suggests their formation within an extensional regime similar to the exposed rifts of Southern Urals and northern Mongolia and buried rifts of the West Siberian Basin.