REE Geochemistry of Loess in Xinjiang ,China

The total rare-earth element values(ΣREE)of loess in the Xinjiang region vary over a range of 128-200 ppm ,with an average of 153ppm .The average REE content of loess lies between the earth‘s crust (155ppm) and sedimentary rocks(151ppm).The Xinjiang loess,with the REE distribu-tion patterns characterized by negative slopes ,is rich in the Ce-family elements, and has a distribu-tion pattern characteristic of sedimentary rocks.The North Xinjiang loess is relatively depleted in Tb,but rich in Yb and Lu.The South Xinjiang loess is relatively rich in light rare-earth elements.This is full proof that the Xinjiang loess comes partly from weathered materials(clay rock,sandstone)in the region studied.The REE distribution patterns in the Xinjiang loess are similar to those in the precipitated dust and Aeolian sand,indicating the same material source.The REE distribution pat-terns in the Xinjiang loess are also similar to those in loess from the middle Yellow River Valley,China and Taskent,the former USSR.This implies that loesses of the three locations(Xinjiang,the mid-dle Yellow River Valley and Taskent) come from a common material source.But the REE patterns in the Xinjiang loess are different from those in wall rocks (volcanic rock,K-bearing volcanic rock).Generally ,LREE/HREE,Eu/Eu* and Ce/Ce* ratios reflect the features of parent materials of loess,indicating that the parent rocks were probably in the early stage of alkaline weathering and the weathered materials existed in an oxidation environment with basic mediums under arid-climatic conditions before transport.As a result,the migration ability of the REE is weak.     

REE Geochemical Characteristics of Volcanic Rocks in Zhejiang and Jiangxi Provinces and Their Gological Significance

Based on the data of 64 samples ,the REE geochemical characteristics of volcanic rocks in northern Zhejiang and eastern Jiangxi provinces are discussed in this paper.The REE distribution patterns in acid and intermediate-acid volcanic rocks in these areas display some similarities,as indicated by rightward-inclined V-shaped curves with negative Eu anomalies,which are parallel to earch other.In addi-tion,their REE parameters(ΣREE,ΣLREE/ΣHREE,δEu,Ce/Yb,La/Sm,La/Yb,etc)also va-ry over a narrow range with small deviations.HREE are particularly concentrated in the volcanic rocks as-sociated with uranium mineralization.The initial ^87Sr/^86Sr ratio in the volcanic rocks is about 0.7056-0.7139.All these features in conjunction with strontium isotopic data indicate that the rock-forming materials come from the sialic crust.The REE distribution patterns and REE geochemical parameters of the volcanic rocks ,as well as La/Sm-La and Ce/Yb-Eu/Yb diagrams may be applied to the sources of rock-forming and ore-forming materials.     

Geochemistry and petrogenesis of granitoids of Lesser Himalayan crystallines,Western Arunachal Himalaya

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 FeO^t/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 FeO^t/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 [Eu_N/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 [Eu_N/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.     

Geochemical Characteristics of Cenozoic Basaltic High-K Volcanic Rocks from Maguan Area, Eastern Tibet

The major element, trace element and Nd-Sr isotopic composition of Cenozoic basaltic volcanic rocks from the Maguan area, eastern Tibet, indicates that the volcanic rocks are enriched in alkalis, especially K (K2O up to 3.81%) and depleted in Ti (TiO2 = 1.27%-2.00%). These rocks may be classified as two groups, based on their Mg# numbers: one may represent primary magma (Mg# numbers from 68 to 69), and the other, the evolved magma(Mg# numbers from 49 to 57). Their REE contents are very high (∑REE = 155.06-239.04μg/g). Their REE distribution patterns are of the right-inclined type, characterized by LREE enrichment [(La/Yb)N =12.0-19.2], no Ce anomaly (Ce/Ce*=1.0), and weak negative Eu anomaly (Eu/Eu*=0.9). The rocks are highly enriched in Rb, Sr and Ba (59.5-93.8μg/g, 732-999 μg/g, and 450-632 g/g, respectively), high in U and Th (1.59-2.31μg/g and 4.73-8.16 μg/g, respectively), and high in Nb, Ta, Zr and Hf (70-118 μg/g,3.72-5.93 μg/g, 215-381 μg/g, and 5.47-9.03 μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, Nb, Ta, Zr, Hf and P show positive anomalies, whereas Ba, Ti and Y show negative anomalies. The 87Sr/86Sr ratios range from 0. 704029 to 0.704761; 143Nd/144Nd from 0. 512769 to 0. 512949; and εNd from 2.6 to 6.1. These geochemical features might suggest that the potential source of the basaltic high-K volcanic rocks in the Maguan area is similar to the OIB-source mantle of Hawaii and Kergeulen volcanic rocks.     

河南舞阳地区赵案庄铁矿床成矿时代及地质意义——中国最古老的岩浆型铁矿床

河南舞阳地区赵案庄铁矿床是产于超基性岩中的隐伏矿床。超基性岩侵位于新太古界太华群赵案庄组透辉斜长角闪片麻岩中。矿体呈似层状,具有多层。矿石成分较复杂,主要矿物为磁铁矿、蛇纹石、氟磷灰石等。矿床规模为中等,其成因是与超基性岩有关的岩浆晚期磷灰石-磁铁矿矿床。在磷灰蛇纹磁铁矿石中选出的锆石样品为变质锆石,利用激光烧蚀多接收器电感耦合等离子体质谱仪(LA-MC-ICPMS)进行微区原位U_Pb同位素测年,获得不一致线的上交点年龄为(1943±5)Ma(MSWD=2.3)。同时对锆石阴极发光(CL)图像研究,表明该年龄代表锆石遭受强烈变质作用的年龄,可以限定矿体形成时代的上限,即矿体形成时代不晚于1943 Ma。超基性岩浆演化晚期,岩浆充填构造裂隙呈透辉石岩脉产出。透辉石脉与矿体同期并且穿切铁山庙组BIF型铁山铁矿。铁山庙组形成时代在2300~2500 Ma之间。透辉石岩脉形成时代不早于该组年龄,可作为赵案庄铁矿床形成时代的下限。赵案庄铁矿床的形成时代为1943~2300 Ma,为古元古代,这是中国目前发现的最古老的岩浆型铁矿床。该成矿地质时代反映了在华北陆块东南缘曾发生过一次地壳扩张和裂陷活动及华北克拉通裂解事件。     

Elemental geochemistry and Nd isotopic characteristics of the metasedimentary rocks from the metamorphic belt in central Jiangxi： Provenance and tectonically environmental constraints

The metamorphic belt in central Jiangxi, located in the compound terrain within the Cathaysia, Yangtze Block and Caledonian fold zone of South China, is composed dominantly of meta-argillo-arenaceous rocks, with minor amphibolite. These rocks underwent amphibolite-facies metamorphism. The meta-argillo-arenaceous rocks show large variations in major element composition, but have similar REE patterns and trace element composition, incompatible element and LIE enrichments [ high Th/Sc (0.57-3.59) , La/Sc ( 1.46 - 12.4), La/Yb (5.84 - 19.0) ] and variable Th/U ratios, with ∑REE = 129-296μg/g, δEu =0.51 -0.86, and (La/Yb)N = 3.95 -12.9. The Nd isotopic model ages tDM of these rocks vary from 1597 to 2124 Ma. Their 143 Nd/144 Nd values are low [εNd (0) = - 11.4 to -- 15.8]. Some conclusions have been drawn as follows: (1) The metamorphic rocks in central Jiangxi Province are likely formed in a tectonic environment at the passive continental margin of the Cathaysia massif. (2) The metamorphosed argillo-arenaceous rocks are composed dominantly of upper crustal-source rocks (Al- and Krich granitic or/and sedimentary rocks of Early Proterozoic), which experienced good sorting, slow deposition and more intense chemical weathering. (3) According to the whole-rock Sm-Nd isochron ages (1113±49 to 1199 ± 26 Ma) of plagioclase-amphibole (schist) and Nd isotopic model age tDM ( 1597 - 2124Ma) of meta-argillo-arenaceous rocks, the metamorphic belt in central Jiangxi Province was formed during the Middle Proterozoic ( 1100 - 1600 Ma).     

Dokhan volcanics of Gabal Monqul area,North Eastern Desert,Egypt: geochemistry and petrogenesis

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.     

Geological framework of the Archean and Paleoproterozoic Tanami Region, Northern Territory

The Tanami Region, a poorly exposed, mostly Paleoproterozoic province within the North Australian Craton, hosts a number of significant gold deposits in diverse settings. Rare exposures of 2,520–2,500 Ma amphibolite facies Archean gneiss and metasedimentary rocks form basement to the thick overlying metasedimentary succession of the 1,880–1,830 Ma Tanami Group. The basal unit of the Tanami Group is the Dead Bullock Formation, a fining-upward deep-water succession dominated by siltstone, carbonaceous siltstone, iron-rich siltstone and mafic sills. Carbonaceous- and iron-rich lithologies in the upper Dead Bullock Formation represent important hosts for gold mineralization. The conformably overlying Killi Killi Formation represents turbiditic sedimentary rocks that are correlated with the widespread Lander Rock beds of the Arunta Region. Sedimentation of the Tanami Group was terminated by regional deformation and greenschist to amphibolite facies metamorphism during the Tanami Event (D₁/M₁), at around 1,830 Ma. The Tanami Group is unconformably overlain by rhyolite, siliciclastic sedimentary rocks, and felsic ignimbrite of the Ware Group that were deposited at about 1,825–1,810 Ma. Subsequent ESE–WNW to SE–NW directed shortening (D₂), followed by NE–SW to E–W directed shortening (D₃), has resulted in open NE F₂- and NW F₃-trending folds in both the Tanami and Ware Groups. Voluminous granitoids, dominated by I-type, biotite granodiorite, and monzogranite were intruded in the interval 1,825–1,790 Ma and have been subdivided using geochemical criteria into the Birthday, Frederick, and Grimwade Suites. Basalt and immature sedimentary rocks of the Mount Charles Formation are restricted in extent to the Tanami mine corridor, and are interpreted to reflect a continental rift succession that was deposited around 1,800 Ma, with an early Archean sedimentary provenance. Steep S to SE dipping F₄-fold structures of Tanami and Ware Group metasedimentary rocks, many spatially associated with 1,825–1,790 Ma granitoid intrusions, indicate a period of SSE-directed regional shortening (D₄) syn-to-post the regional granitoid intrusive phase. A network of N to NW striking faults, several of which are interpreted as oblique thrusts with a component of left lateral movement, indicates a period of D₅ convergence during WSW–ENE to E–W directed shortening. The Tanami mine corridor fault system comprises a network of N, NE to ENE striking D₅ faults that merge with N to NW striking faults and probably accommodated movement between granite core domains. D₅ faulting is associated with the main phase of gold mineralization in suitable structural–lithological traps. The Paleoproterozoic basement of the Tanami Region is unconformably overlain by quartz sandstone, lithic arenite, and conglomerate of the Pargee Sandstone. Pargee Sandstone may represent syn-tectonic sedimentation related to the 1,730 Ma Strangways Orogeny, and is unconformably overlain by the late Paleoproterozoic platform cover succession of the Birrindudu Group. The Paleoproterozoic basement and cover sequences have subsequently undergone several episodes of faulting, collectively termed D₆₊. The Paleoproterozoic evolution of the Tanami Region is interpreted to have occurred in an intracratonic setting, but was fundamentally influenced by tectonic events in the adjacent Halls Creek Orogen (1,835–1,805 Ma Halls Creek Orogeny) and Arunta Region (1,815–1,800 Ma Stafford Event). The boundaries between the Tanami Region and Kimberley Region to the northwest and the Arunta Region to the southeast are transitional, and are largely defined by the presence or absence of identifiable Dead Bullock Formation.     

滇西勐满金矿床成矿物质来源分析及矿床成因探讨——来自年代学、矿物学及地球化学的证据

勐满金矿床位于云南省勐海县境内,地处临沧-景洪褶皱束之临沧-澜沧复背斜南部,截至2005年探明金矿资源量17t,主矿体属于地表红土型氧化矿,平均品位为0.60×10-6。文章基于对勐满金矿床矿区内新元古界曼来组片岩和中侏罗统花开左组碎屑岩及矿区外围东部的临沧花岗岩的LA-ICP-MS锆石U-Pb年龄测定,探讨了曼来组片岩和花开左组碎屑岩地层的物质来源,追溯母岩的物质源区,并结合矿区普遍发育的硅化岩和硅质脉体的地球化学特征,分析了金成矿的物质来源。勐满金矿床曼来组中主要碎屑锆石年龄为(957.0±9.8)Ma,属新元古代早青白口纪;中侏罗系花开左组主要碎屑锆石年龄为(224.2±2.8)Ma,与临沧花岗岩基中2件黑云母二长花岗岩的年龄(分别为(228.7±2.8)Ma和(229.4±2.0)Ma)一致,表明花开左组地层的物质来源可能与临沧花岗岩有关。微量元素测试结果显示,矿区硅化岩与硅质脉体具有相似的稀土元素配分模式与微量元素组成,均表现为富集U、Th、Pb等大离子亲石元素,亏损Zr、Hf、Nb、Ta等高场强元素,轻稀土元素明显富集,重稀土元素亏损,中-弱的Eu负异常,Ce异常不明显,与矿区围岩及临沧花岗岩体南部的黑云母花岗岩具有相似的变化趋势。硅化岩稀土元素总量为42.3×10-6~311.0×10-6,平均为131.8×10-6;硅质脉体稀土元素总量为5.0×10-6~280×10-6,平均为56.8×10-6,显示硅化岩相对于硅质脉体具有更高的稀土元素总量。硅质脉体((La/Yb)N为4.3~133.4,平均36.7)相对于硅化岩((La/Yb)N为2.1~26,平均为8.5)具有更高的轻、重稀土元素分馏特征。硅化岩的地球化学特征变化介于围岩与硅质脉之间,表明硅化岩既继承了围岩特征,又有经热泉活动改造的痕迹。综合锆石年代学结果与沉积物源分析,认为矿区花开左组砂岩主要为临沧岩体经受风化搬运沉积的产物,曼来组向花开左组提供金的量不足以形成花开左组中的碎屑岩型矿体,热泉活动产生的流体是金成矿的另一重要物质来源,同时是导致金后期富集的重要因素。据此推断,花开左组是赋矿层位,而并非矿源层。结合低硫型矿床标志性矿物冰长石的出现,提出勐满金矿属于典型的与热泉活动有关的低硫型浅成低温热液金矿。     

REE Geochemical Characteristics of Apatite,Sphene and Zircon from Alkaline Rocks

The accessory minerals apatite and sphene are the main carriers of REE in alkaline rocks.Their chondrite-normalized REE patterns decline sharply to the right as those of the host rocks,In the patterns an obvious negative Eu anomaly and a positive Ce anomaly can be seen in apatite and sphene,respectively.Zircon from alkaline rocks is different in REE pattern,I,e,. a nearly symmetric“V“-shaped pattern with a maximum negative Eu anomaly.Compared with the equivalents from granites,apatite,sphene and zircon from alkaline rocks are all characterized by higher (La/Yb)N ratio and less Eu depletion,As to the relative contents of REE in minerals,apatite,sphene and zircon are enriched in LREE,MREE and HREE respectively,depending on their crystallochemical properties.     

First results of U/Pb dating of detrital zircons from Early Paleozoic and Devonian sandstones of the Baltic-Ladoga region (south Ladoga Area)

The first results of U/Pb isotopic dating (LA ICP MS) of detrital zircons from sands from the Middle Cambrian Sablinka Formation, Upper Cambrian Ladoga Formation, Low Ordovician Tosna Formation, and calcareous sands from Syas’ Formation (Sargaevskii horizon of the Upper Frasnian) from Baltica-Ladoga Glint (BLG) of the Southern Ladoga area are presented. The obtained ages of detrital zircons span the intervals 492.7 ± 5.1-3196.4 ± 5.1 Ma (Sablino Formation); 577.9 ± 7–2972.6 ± 13.4 Ma (Ladoga Formation); 509.4 ± 8.5–3247.6 ± 10.1 Ma (Tosna Formation); 451.1 ± 14.7–2442.2 ± 6.9 Ma (Syas’ Formation). A comparison of the obtained isotopic ages of detrital zircons to ages of crystalline complexes composing the Kola-Karelian, Svecofennian, and Sveconorwegian domains of Baltic Shield and Pre-Uralian-Timanian structures of Subpolar and Polar Urals and basement of Pechora Basin was carried out. It is proposed that the Middle Paleozoic sedimentary basin accumulated Upper Frasnian rocks of Syas’ Formation. The basin ranged northward from the present-day BLG and occupied the eastern part of the Baltic Shield.     

Petrogenesis and source characteristics of metatholeiites from the Archean Ramagiri schist belt, eastern part of Dharwar craton, India

The N–S trending, 2–4 km wide Ramagiri schist belt is made up of three blocks dominated by metavolcanic rocks, separated and surrounded by granitic rocks of distinct characteristics. The metavolcanic rocks are tholeiitic in composition and are very similar in their major element composition as well as in their abundances of some trace elements. However, the rare earth elements (REE) require distinct sources. The rocks of the amphibolite facies eastern block have LREE depleted REE patterns ([Ce/Yb] = 0.7–0.9), requiring derivation from depleted mantle-like sources. The greenschist facies metatholeiitic rocks of the central block have LREE enriched REE patterns ([Ce/Yb] = 3–6), reflecting the nature of their source(s). The Nd isotopic data require that the LREE enriched nature could not have been attained significantly prior to its melting. The fine-grained, upper greenschist facies metatholeiites of the western block have flat to slightly LREE depleted patterns ([Ce/Yb] = 0.8–0.95). Minor fractional crystallization of rock forming minerals may relate a few samples to each other among samples from each of the three blocks. Different extents of partial melting of distinct mantle sources have played a dominant role in the generation of the parent magmas to the central versus eastern and western block metatholeiites. The geochemical data suggest that the mantle sources were non-lherzolitic, and that these sources may have seen previous episodes of melt addition and extraction prior to melting that gave rise to the parent melts to the rocks ∼2750 Ma ago. The REE data indicate that while the sources of the eastern and western block rocks were similar to depleted mantle (ɛ_{Nd( i )} about +2), the source of the central block rocks (ɛ_{Nd( i )} about +3.5) were enriched in large ion lithophile element (LILE)-rich fluids/melts probably derived from subducting oceanic crust. This and other trace element signatures point to magma extraction in tectonic settings similar to modern island arcs. Subsequent to magma emplacement and crystallization, all the three suites of rocks were affected by interaction with low-temperature, crustal derived fluids (ɛ_{Nd 2750Ma} of about −8 to −12), probably during the accretion of the three blocks of the belt in the present form. The inferred source characteristics, tectonic setting of magma generation and the crustal fluid processes seem to suggest that Phanerozoic-style tectonic processes may have been important in the generation of Archean crust in the Dharwar craton. Received: 31 July 1995 / Accepted: 12 May 1997     

Geochemical characteristics of pyrite in Duolanasayi gold deposit, Xinjiang

The Duolanasayi gold deposit, 60 km NW of Habahe County, Xinjiang Uygur Autonomous Region, is a mid-large-scale gold deposit controlled by brittle-ductile shearing, and superimposed by albitite veins and late-stage magma hydrothermal solutions. There are four types of pyrite, which are contained in the light metamorphosed rocks (limestone, siltstone), altered-mineralized rocks (chlorite-schist, altered albite-granite, mineralized phyllite), quartz veins and carbonatite veinlets. The pyrite is the most common ore mineral. The Au-barren pyrite is present mainly in a simple form and gold-bearing pyrite is present mainly in a composite form. From the top downwards, the pyrite varies in crystal form from {100} and {210} {100} to {210} {100} {111} to {100} {111}. Geochemical studies indicate that the molecular contents of pyrite range from Fe1.057S2 to Fe0.941S2. Gold positively correlates with Mn, Sr, Zn, Te, Pb, Ba and Ag. There are four groups of trace elements: Fe-Cu-Sr-Ag, Au-Te-Co, As-Pb-Zn and Mn-V-Ti-Ba-Ni-Cr in pyrite. The REE characteristics show that the total amount of REE (ΣREE) ranges from 32.35×10 -6 to 132.18×10 -6; LREE/HREE, 4.466-9.142; (La/Yb)N, 3.719-11.133; (Eu/Sm)N, 0.553-1.656; (Sm/Nd)N, 0.602-0.717; La/Yb, 6.26-18.75; δEu, 0.628-2.309; δCe, 0.308-0.816. Sulfur isotopic compositions (δ 34S=-2.46‰--7.02‰) suggest that the sulfur associated with gold mineralization was derived from the upper mantle or lower crust.     

内蒙古锡林浩特跃进公社地区温都尔庙群年代学、地球化学特征及构造环境

温都尔庙群为一套绢云绿泥石英片岩、绢云石英片岩、二云石英片岩、黑云斜长片麻岩、变粒岩、石英岩等的变质岩组合,形成于新元古代。LA-ICP-MS锆石U-Pb同位素定年结果显示,温都尔庙群中的岩浆热事件年龄为459.0±1.8Ma,变质年龄为898.9±5.2Ma,即锡林浩特地区存在900Ma左右的变质基底岩系。温都尔庙群片岩样品显示轻稀土元素富集的中等分异特征,并有一定的负Eu异常,其稀土元素分布模式接近于安第斯型;在微量元素原始地幔标准化图解中,出现明显的高场强元素Nb负异常,表明原岩受地壳混染作用影响;Sr/Ba平均值为0.30,小于1,基本反映了副变质岩的特点。通过岩石地球化学图解分析,认为本区温都尔庙群哈尔哈达组的原岩为沉积岩,岩性主要为砂岩和泥岩。地球化学特征及形成构造环境判别结果表明,温都尔庙群哈尔哈达组片岩原岩沉积环境为活动大陆边缘体系。     

西藏神公地区典中组火成岩锆石U-Pb年龄及地球化学特征

新特提斯洋俯冲消减、印度-亚洲板块碰撞导致西藏冈底斯构造带分布大量火成岩,其形成年龄尚存在一定争议,形成时代及构造意义对揭示板块运动信息具有重要意义。对神公地区典中组火成岩样品分别进行了锆石U-Pb同位素测年和元素地球化学分析,结果表明,典中组底顶部火成岩锆石U-Pb年龄值分别为65.37±0.58Ma和57.42±0.20Ma;该套火成岩主要为钙碱性系列岩石,稀土元素配分曲线表现为轻稀土元素富集、重稀土元素亏损的右倾形态,且负Eu异常明显,K_2O/Na_2O值平均为1.57,微量元素Rb/Sr值平均为1.78,Nb/Ta值平均为13.28,Rb、Th、U、Pb等大离子亲石元素富集,Nb、Ta、Ti亏损。由此认为,典中组火成岩是在俯冲碰撞背景下,来自地幔和地壳的岩浆以不同比例混合形成的,可能指示了新特提斯洋关闭引起的洋壳俯冲作用。     

Geochemical Characteristics and Sedimentary Environment of the Salawusu Section

The chemical compositions of and the contents of CaCO3, free Fe2O3 and REE in the sediments of the Salawusu section have been analyzed to investigate the geochemical features of different sedimentary facies and explore their sedimentary enironmetnts during the Late Pleistocene.The con-tents of CaCO3, free Fe2O3 and ∑REE in lacustrine deposits of the middle part of the section are higher than those in the other parts ,except SiO2 which shows an opposite trend.According to the distribution characteristics of the components mentioned above, the section may be divided into four parts which are equivalent to those divided in terms of lithologic characters and sedimentary facies.More remarkable characteristics were observed with respect to the distribution of CaCO3, free Fe2O3,∑REE and SiO2/Al2O3 and FeO/Fe2O3 ratios, which can be adopted as the geochemical indices for paleoclimate division.From the above, four paleoclimate stages have been distinguished for the deposition process.The first stage ,dated at 0.20-0.15Ma, is characterized by a dry and cold climate,the second stage, ranging from 0.15 to 0.07 Ma ,by a warm and slightly dry climate,the third stage ,about 0.07-0.01Ma ,by a cold and humid climate;and prevailing in the last ten thorsand years is the fourth stage,with the climate changing graduall from warm-humid to warm-dry.     

内蒙古科右前旗大石寨组火山岩锆石LA-ICP-MSU-Pb年龄及其形成背景

对大石寨组火山岩的形成时代和形成背景的认识存在较大的分歧。对采自建组剖面底部英安岩样品中的锆石进行阴极发光照相、微区原位LA-ICP-MS微量元素分析和U-Pb测年。锆石U-Pb同位素分析结果显示,锆石发育典型的岩浆震荡环带, 高Th/U(0.51~1.57),轻稀土元素亏损,重稀土元素富集,具有强烈的正Ce异常和强烈的负Eu异常等特征,表明锆石均属于岩浆成因。22个分析点获得的谐和年龄为(314±1)Ma(MSWD=0.88),代表了大石寨组火山岩的形成时代。结合前人的资料,认为大石寨组火山岩为裂陷早期的产物,而晚古生代的沉积岩石组合为在这一基础上发展起来的具有裂陷盆地性质的沉积组合。     

抚顺南部早前寒武纪变质杂岩的地质事件序列

抚顺南部早前寒武纪变质杂岩是华北克拉通北缘辽北-吉南早前寒武纪变质地块的一个重要组成部分,主要由浑南群石棚子组角闪岩相变质火山岩、火山碎屑岩及相伴生的沉积岩等表壳岩系和侵位于其中的石英闪长质片麻岩、英云闪长质-奥长花岗质-花岗闪长质(TTG)片麻岩和花岗闪长岩-二长花岗岩-钾长花岗岩岩石组合组成。LA-ICP-MS锆石U-Pb同位素分析结果显示,侵位于表壳岩中的石英闪长质片麻岩样品12LN39-3的岩浆结晶年龄为2571±7Ma,指示存在老于该年龄的表壳岩系。英云闪长质片麻岩样品12LN04-1和奥长花岗质片麻岩样品13LB49-3的岩浆结晶年龄分别为2544±4Ma和2550±10Ma,记录了一期重要的英云闪长质-奥长花岗质片麻岩侵位事件。斜长角闪岩(样品12LN25-2)的岩浆结晶的最小年龄为2530±5Ma,指示另一火山喷发阶段。晚期钾长花岗岩样品12LN01-1和奥长花岗质片麻岩样品12LN27-1分别侵位于2522±4Ma和2518±23Ma,说明它们的岩浆作用发生于同一时期。而采自于晚期未变形侵入体的石英闪长岩样品12LN30-2的岩浆结晶年龄为2496±18Ma,与上述表壳岩和深成侵入体的主要变质作用(2510~2470Ma)同期发生。这些年代学结果表明,抚顺南部地区新太古代大规模的铁镁质火山喷发作用在大于2571±7Ma已经发生,紧接着2571±7Ma发生石英闪长质岩浆侵位,在2550±10Ma~2544±4Ma之间发生英云闪长质-奥长花岗质岩浆侵位。接下来铁镁质火山再度喷发(~2530±5Ma),随后为钾长花岗岩和奥长花岗质岩浆的侵位(2522±4Ma~2518±23Ma)。晚期为角闪岩相变质作用时期(2510~2470Ma),伴随一定规模的石英闪长岩侵位。     

REE Geochemistry of Precambrian Metamorphic Rocks in Wutaishan Region

The metasedimentary-volcanic series of the Wutai and Hutuo groups experienced regional metamorphism and thus turned into moderate-to low-grade metamorphic rocks.REE abundances and REE distribution patterns in the Shizui and Taihuai Subgroup metasedimentary-volcanic rocks are typical of the Archean,whereas the Gaofan Subgroup and the Hutuo Group show post-Archean REE geochemical char-acteristics.Five types of REE distribution pattern are distinguished:(1)rightward inclined smooth curves with little REE anomaly(Eu/Eu*=0.73-0.95) and heavy REE depletion (e.g.the Late Archean metasedimentary rocks);(2)rightward inclined V-shaped curves with sharp Eu anoma-ly (Eu/Eu*=0.48-0.76) and slightly higher ∑REE (e.g.the post-Archean metasedimentary rocks);(3) rightward inclined steep curves with negative Eu anomaly(Eu/Eu*=0.73-0.76) and the lowest ∑REE (e.g.the post-Archean dolomites);(4)rightward inclined,nearly smooth curves with both positive Eu anomaly and unremarkable positive Eu anomaly(Eu/Eu*=0.95-1.25)(e.g.the meta-basic volcanic rocks);and (5) rightward inclined curves with Eu anomaly(Eu/Eu*=1.09-1.19)and heavy REE depletion(e.g.the meta-acid volcanic rocks).Strata of the two groups are considered to have been formed in an island-arc belt-an instable continental petrogenetic environment.     

大兴安岭中段扎赉特旗地区满克头鄂博组火山岩锆石U-Pb年龄及地球化学特征

对大兴安岭中段扎赉特旗地区满克头鄂博组火山岩进行了锆石U-Pb年龄和地球化学研究。用LA-ICP-MS测得流纹质隐爆角砾岩(TW6)和流纹岩(TW14)中锆石的~(206)Pb/~(238)U年龄分别为157.2±1.2Ma和154.8±1.1Ma,表明火山岩形成于晚侏罗世。岩石地球化学特征表明,火山岩富硅碱、贫钙镁,为过铝质钙碱性系列;(La/Yb)_N=20.24~22.68,轻、重稀土元素分馏明显;球粒陨石标准化稀土元素配分模式图呈右倾形式,Eu为正异常;原始地幔标准化蛛网图显示,大离子亲石元素Rb、K相对富集,高场强元素Nb、Ti、P强烈亏损,显示壳源岩浆的特征。综合本区火山岩的特征并结合他人研究成果,认为扎赉特旗地区满克头鄂博组火山岩的形成与蒙古-鄂霍次克洋闭合后伸展作用关系密切。