扬子地台北缘白勉峡组和三湾组火山岩形成构造环境及岩石成因的地球化学约束

依据中基性火山岩主量和微量元素地球化学特征的差异,白勉峡组可分两部分,一部分火山岩TiO_2大于1%,变质程度较高,主要分布在下段;另一部分火山岩TiO_2小于1%,变质程度较浅,主要分布在上段.下段火山岩属拉斑玄武岩系列,上段主体属钙碱系列,稀土总量高(∑REE=83.4～180.8μg/g),轻重稀土分异较低(LREE/HREE=2.17～5.85),有弱的Eu负异常(δEu=0.79～1.01),微量元素原始地幔蛛网图显示有弱的Nb、Ta亏损,具有板内火山岩的地球化学特点,形成于板内裂谷环境.上段火山岩稀土总量低(∑REE=40.3～82.4μg/g),轻重稀土分异较高(LREE/HREE=2.3～7.6),无Eu负异常(δEu=0.90～1.11),微量元素原始地幔蛛网图发育明显的Nb-Ta槽和Zr-Hf槽,Ti、Sr发育较强的低谷,具有典型岛弧玄武岩的地球化学特点,形成于岛弧或大陆边缘弧环境.三湾组玄武岩和安山岩稀土元素分配型式呈LREE亏损的左倾型或呈近平坦型,类似于N-MORB,明显不同于白勉峡组,岩石组合和地球化学特点类似于弧后盆地火山岩.火山岩及相关侵入岩LA-ICPMS锆石U-Pb定年及元素及Sr-Nd同位素地球化学研究揭示,白勉峡组下段火山岩形成时代可能为1144Ma,其源区为与洋岛玄武岩类似的软流圈地幔源区,部分熔融发生在石榴子石二辉橄榄岩稳定区,岩浆在演化过程中经历了一定分离结晶作用(分离结晶矿物为斜长石+单斜辉石)和地壳混染作用.白勉峡组上段火山岩形成时代可能为437Ma,有可能跨到晚古生代,其源区为受俯冲作用改造的富集地幔区,部分熔融亦发生于石榴子石二辉橄榄岩稳定区.三湾组中基性火山岩源于N-MORB近似的亏损地幔源区.白勉峡组下段代表中元古代末板内拉张事件的地质记录,白勉峡组上段和三湾组目前的火山岩样品可能代表了古生代同一洋陆转化的地质记录.     

Geochemistry of an ophiolitic complex from New Caledonia

The ophiolites of New Caledonia are composed of ultramafics overlain by mafic rocks, all of which were affected by low P metamorphism. The mafic rocks studied (gabbroic cumulates, and basaltic flows and dikes) from Montagne des Sources are similar to recent mid-ocean ridge rocks. They are olivine-normative with Mg/Mg+Fe²⁺ ratios ranging from 0.69 in lavas to 0.90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of Ti and V with the Mg/Fe ratio. The lavas have a flat REE pattern with a slight depletion of light REE and a La/Yb ratio <2. The dikes have three different types of REE patterns. The first type is nearly parallel to that of lavas, the second one is enriched in LREE (La/Yb4) and the third type with the lowest REE contents and a distinct LREE depletion is similar to that of cumulitic pyroxene gabbro. The variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of Langmuir et al. (1977). In agreement with structural and tectonic observations, the geochemical data suggests that the ophiolites were formed during the spreading of a mid-ocean ridge with a spreading half-rate of about 1 cm/ year.     

Geochemical Constrains on Petrogenesis and Tectonic Setting of Volcanic Rocks from the Baimianxia and Sanwan Formations in the Northern Margin of the Yangtze Plate

On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units: high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high-grade metamorphism, but the latter is less than 1% and crops out in the upper part with low-grade metamorphism. The high-TiO2 unit is dominated by tholeiitic lavas showing high rare earth element (REE) contents (ΣREE?=?83.4–180.8?μg/g), high light/heavy REE (LREE/HREE) ratios (LREE/HREE=2.17–5.85) and weak negative Eu anomaly (Eu=0.79–1.01). Its trace element patterns display weak Nb-Ta anomalies with respect to Th, K, La, Ce, showing within-plate basalt affinities. In contrast, the low-TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb-Ta anomalies, indicating typical arc or continental arc signature. Chondrite-normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid-oceanic basalt. Therefore, we suggest that these lavas should be formed in a back-arc basin setting. Sr-Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ~1144?Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic-island basalt-like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid-oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift-related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.     

Rare-Earth Abundances in Basalts and Metabasalts from the Troodos Massif,Cyprus

Abundances of some rare-earth elements (REE) in twelve pillow lavas from the Upper Pillow Lavas and Axis Sequence of the Troodos Massif are reported. The samples consist of three fresh basalts and nine zeolite facies metabasalts, metamorphosed at temperatures between 0 and 200° C. All give similar light rare-earth element (LREE) depleted patterns, indicating that hydrothermal metamorphism within this temperature range does not appreciably affect these REE patterns. The LREE depletion is consistent with a petrogenetic model in which Troodos formed at a spreading axis. Variations in profile shape indicate that mantle melting beneath the axis may have taken place during a series of discrete episodes.     

Metavolcanic host rocks,mineralization, and gossans of the Shaib al Tair and Rabathan volcanogenic massive sulphide deposits of the Wadi Bidah Mineral District,Saudi Arabia

The Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation.     

Rare earth element mobility in vesicular lava during low-grade metamorphism

A geochemical comparison of basaltic relicts and spilitic domains from two burial metamorphosed flows in central Chile, of similar original composition and rich and poor in amygdules, respectively, demonstrates a relationship between initial vesicularity and rare earth element (REE) mobility. During spilitization the REE were partly leached from permeable parts of the flows and precipitated in voids, now amygdules and veinlets. The REE (excluding Eu) moved coherently in the highly amygdaloidal flow: spilitic domains and amygdules inherited the basaltic REE pattern. Besides being characterized by a positive Eu anomaly, epidotes separated from amygdules have a REE distribution which mimics that of the basalt; the absolute contents range widely, suggesting local and/or temporal REE variations in the metamorphic fluids. Pumpellyite differs by being strongly enriched in heavy REE. Similar ratios of Th, Hf and Ta in samples as contrasting as relict basalt and a geode are consistent with coherent leaching. Coherent mobility, when established for a rock system, can be used to elucidate, for example, whether minerals in cross-cutting veins were formed by local redistribution or from introduced material.     

Rare earth element investigation of the Cliefden Outcrop,N.S.W., Australia

The effect of low grade hydrous burial metamorphism (prehnite-pumpellyite facies) upon the rare earth elements (REE) has been studied by using samples from the Cliefden Outcrop, New South Wales. The REE, together with other reputedly immobile elements, have been mobilised during the metamorphism. Although mobile, the REE have behaved remarkably coherently with little light rare earth (LREE) fractionation. This is reflected in the chondrite normalised patterns which are sub-parallel to parallel in shape. High correlations of REE with other elements can be used to predict the maximum likely variation of these elements in the studied outcrop. The high correlations do not necessarily mean that, for similarly metamorphosed terrains, crystallisation-differentiation processes have operated but may rather have resulted from strong geochemical coherence during post-crystallisation elemental redistribution. The REE do not appear to be strongly domain controlled within the Cliefden Outcrop.     

Occurrence and paragenesis of diagenetic monazite in the upper Triassic black shales of the Marvast region, South Yazd, Iran

Upper Triassic-Jurassic black shale at Marvast, Iran, contains grey to green-grey ellipsoidal nodules of monazite ranging from 0.1 to 2?mm across. The presence of host-rock mineral inclusions within the monazite grains, low Th content in the monazite, lack of relict yellow cores (characteristic of igneous monazite), and the absence of monazite in the other sedimentary sequences in the Marvast area rule out a detrital origin for the monazite nodules and suggest authigenic crystallization during sediment compaction. Enrichment of the cores of the monazite grains in mid-range to heavy rare-earth elements (REE) and their rims in La + Ce point to variations in the degree of REE mobility and/or evolving composition of the diagenetic mineralizing fluid during nodule growth. The phosphorus and REE required for monazite crystallization were probably derived from seawater and adsorbed on clays and Fe-Mn hydroxides. The interstitial fluids expelled from the sediments during burial compaction and diagenesis became enriched in P and REE through complexing. The association of the Marvast monazite nodules with the black shale may indicate that organic complexes aided in the mobilization and transport of the REE into the pore fluids. Detailed field investigations in the study area and vicinity show that authigenic monazite in the upper Triassic-Jurassic shale sections is spatially associated with quartz lenses. It is likely that these lenses are surface expressions of shallow intrusive magmas, which provided the heat that promoted the mobilization and redistribution of the REE and P, and initiated precipitation of monazite in the overlying sediments.     

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano,Central Italy: bearing on evolutionary processes in potassic magma chambers

Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO₂, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. ⁸⁷Sr/⁸⁶Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills.     

Rare earth element mobility during prograde granulite facies metamorphism: significance of fluorine

 Mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the Quetico subprovince of the Superior Province, Ontario, Canada, and exhibit concentric zoning with a biotite-rich margin, orthopyroxene-rich outer zone, clinopyroxene-rich central zone, and, occasionally, patches of relict amphibolites within the clinopyroxene-rich zone. The granulites (biotite-, orthopyroxene- and clinopyroxene-rich zones) in the mafic gneisses are characterized by significant amounts of rare earth element (REE)-bearing fluorapatite (1–10 vol.%) and other REE-rich minerals (allanite, monazite and zircon). Fluorapatite shows an increase in modal abundance from the biotite- and orthopyroxene-rich zones to the clinopyroxene-rich zone, but is rare in the relict amphibolites. Textural evidence and element partitioning indicate that fluorapatite (and other REE-rich minerals) was part of the peak metamorphic assemblages. Whole-rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of REE and high field strength elements (HFSE), whereas the relict amphibolites are geochemically typical of tholeiitic basalts. Mass-balance calculations reveal that REE and HFSE were introduced into the mafic gneisses during the prograde granulite facies metamorphism, pointing to REE mobility under granulite facies metamorphic conditions. The presence of high F contents in the REE-rich minerals and their associated minerals (e.g. biotite and hornblende) suggests that REE and HFSE may have been transported as fluoride complexes during the granulite facies metamorphism. This conclusion is supported by previously published results of hydrothermal experiments on the partitioning of REE between fluorapatite and F-rich fluids at 700°C and 2 kbar. Received: 2 May 1995 / Accepted: 28 September 1995     

Rare earth and trace element geochemistry of a fragment of jurassic seafloor,Point Sal,California

A suite of rocks from the Point Sal ophiolite, California, were analyzed for rare earth elements (REE), Sc, Co, Na₂O, Cr, Zn and FeO. The lavas all have either flat or slightly light REE (LREE) depleted profiles relative to chondrites. The lavas contain smectite or greenschist facies mineralogy and some have radiogenically enriched ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. This is interpreted as evidence of basaltseawater interaction (Hopsonet al., 1975; Davis and Lass, 1975). The smectite and zeolite bearing lavas that have been exposed to seawater for prolonged periods have anomalous Ce abundances. At higher grades of metamorphism, the lavas show no marked changes in light REE. The plutonic igneous rocks vary from early cumulus dunite to late stage, noncumulus diorite. All the plutonic rocks are light REE depleted with total REE abundance varying by a factor of 100 × between the dunites and diorites. Analyses of clinopyroxene and hornblende separates indicate that these two minerals strongly influence the REE characteristics of the early cumulates and late stage fractionates, respectively.In general, REE contents are: hornblende > clinopyroxene > plagioclase > orthopyroxene > olivine. Estimates of the REE compositions of parental lavas were obtained by calculating the REE contents of liquids in equilibrium with early cumulate clinopyroxenes. This reveals that the parent to the stratiform sequence was more depleted in light REE than the parent to the lava pile.     

Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland

Major and trace element analyses are reported for 70 lavas from the Tertiary succession of eastern Iceland. The low grade regional metamorphism responsible for the zeolite zones appears to have caused significant mobilization of some elements, particularly Si, Mg, K, Rb, Sr and light rare earth elements (LREE). In contrast, values for Ti, P, Zr, Y, Nb, Ta, Hf and some of the rare earth elements show a high degree of correlation and this is taken to imply that these elements have been relatively unaffected by metasomatic transport. The demonstrated mobility of Sr and LREE suggest that Sr isotopic and rare earth data obtained from the eastern Iceland lavas must be interpreted with caution.     

Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset,S. W. Greenland

The meta-volcanic amphibolites closely associated with the Fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry; by far the most abundant type has light rare-earth element (LREE) depleted REE patterns and displays a wide range in trace element abundances. Chemically comparable amphibolites can be recognised throughout the ca. 2800 M.yr. West Greenland terrain.The geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes, although variable degrees of partial melting may also be important. The required crystal extract (plagioclase dominated) in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the Fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites.The application of trace element discrimination to assign tectonic environment in the Archaean is arguable. However, details of the trace element chemistry (especially chondritic La/Ta ratios) are taken to suggest, out of a range of likely tectonic environments, an ocean floor, rather than island arc, affinity for the Fiskenaesset amphibolites. The large ion lithophile (LIL) elements display erratic distribution patterns, but are generally enriched relative to the REE. This appears not to be related to high-grade metamorphism but may be a relict feature of seafloor alteration. The association of the cumulate sequence with meta-volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion.     

珠江三角洲晚第四纪风化层稀土元素地球化学特征*

珠江三角洲地区上更新统与全新统之间广泛发育1层杂色黏土,其成因多认为主要是由上更新统沉积物在末次冰盛期暴露于地表风化而成。对取自珠江三角洲3条钻孔（PRD09、PRD16和PRD17）的岩心样品分析表明,受风化作用的影响,其稀土元素含量和分异特征发生了较明显的变化。杂色黏土层的稀土总量大大低于下伏沉积物,而在邻近风化层的下伏沉积物中稀土元素却表现为明显富集,尤其是重稀土元素的富集。风化作用强度较大的PRD09孔和PRD17孔下伏沉积物中的稀土富集程度高于风化作用强度相对较小的PRD16孔。珠江三角洲在末次冰盛期时普遍发育的酸性介质条件,促进了风化层的稀土元素发生溶解和迁移。在风化过程中,由于轻、重稀土元素具有不同的溶解迁移能力和吸附能力,导致杂色黏土层的REE指标值（LREE/HREE、(La/Gd)_N和(La/Yb)_N）高于下伏沉积物。风化过程对Ce、Eu异常有一定的影响,但不十分明显,杂色黏土层的Ce、Eu异常值仅略低于下伏沉积物。     

REE chemistry and Sm-Nd systematics of late Archean weathering profiles in the Fortescue Group, Western Australia

Two weathering profiles, each consisting of an upper, sericite-rich zone and a lower, chlorite-rich zone, are preserved between flows of the Mt. Roe Basalt in the Fortescue Group, Hamersley Basin, Western Australia. REE concentrations in samples from these two profiles, which originally developed ca 2,760 Ma, show large variations depending on stratigraphic position. LREE abundances and (La/Yb)N are greatest at depths of 3-6 m below the paleosurface of the Mt. Roe #1 profile and are somewhat lower in samples above this level. The LREEs reach concentrations 6-9 times greater than in the underlying basalt, and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part. LREE concentrations in the #2 profile show a similar distribution but with a sharp increase in all REE concentrations within 50 cm of the paleosurface. The distinction between the REE profiles in the two paleosols may be related to the difference in the overlying material. The #1 paleosol is overlain by a few meters of sediments and then by basalt, whereas the #2 paleosol is directly overlain by basalt. The LREEs appear to have been mobilized both during chemical weathering of the parental basalt and during later lower-greenschist-facies metamorphism and metasomatism of the paleosols. Remobilization of the REEs during the regional metamorphism of the Fortescue Group is confirmed by a whole-rock Sm-Nd reference isochron of Mt. Roe #1 samples with an age of 2,151 +/- 360 Ma. Variable initial 143Nd/144Nd values of unweathered basalt samples which may represent the paleosol protolith prevents a confident determination of the magnitude of LREE mobility. Both the initial mobilization of the REEs during weathering and the metasomatic remobilization appear to have taken place under redox conditions where Ce was present dominantly as Ce3+, because Ce anomalies are not developed within the sericite zone samples regardless of concentration. Europium anomalies in the paleoweathering profile are somewhat variable and were probably modified by mobilization of Eu2+ at metamorphic conditions. In all samples, the HREEs appear to have been relatively immobile and correlate with Al, Ti, Cr, V, Zr, and Nb. Sm-Nd systematics and REE patterns of four unweathered basalt samples indicate derivation of the Mt. Roe Basalts from a heterogeneous and enriched source having epsilon Nd between -4.0 and -7.4. Initial 143Nd/144Nd values of these basalts are even lower than those reported by NELSON et al. (1992) for Fortescue Group basalts and indicate a substantial crustal component in the generation of Mt. Roe Basalts.     

Rare earth elements in modern coral sands: an environmental proxy

The concentration rare earth elements and Yttrium (REE + Y) were determined in coral sands from Kavaratti Island, Arabian Sea, India. Chondrite-normalized REE + Y patterns show: (1) high REE concentration particularly light REE (LREE) enrichment; (2) consistent negative Ce anomaly; (3) nearly chondritic Y/Ho ratios. All these features are consistent with the geochemistry of well oxygenated seawater with significant terrestrial contribution. The seawater composition of Nd/Yb ratio inferred from the coral record point to the dominance of LREE more than the heavy REE (HREE). The high terrestrial input rich in LREE and property of adsorption/scavenging processes of LREE than that of HREE may be the cause. Terrigenous contributions were detected on the basis of co-occurring trace element concentrations (Sc, Hf and Th) and Y/Ho ratio. Except for La, the REE distribution coefficients, KD(REE)s, are between 100 and 300. KDs are high comparing to the other elements in biogenic calcite which is attributed to detrital contamination during elemental incorporation. This study may not reflect original seawater chemistry but it can be a good proxy to indicate proximity of corals to terrigenous input sources.     

山东沂水雪山岩体和林家官庄岩体的同位素年代学

研究了山东沂水地区的两个太古宙花岗岩类岩体,雪山岩体是花岗闪长岩-花岗岩体。林家官庄岩体属奥长花岗岩-英云闪长岩系列。稀土元素均具轻稀土富集型的分配型式。分别具轻微到明显,和轻微负Eu异常。用质谱计双带源逐层蒸发法进行了单颗粒锆石的详细定年。确定了雪山岩体侵入时代为2530Ma(2531,2532).林家官庄岩体的侵入时代为2507Ma,两个岩体都形成于新太古代。雪山岩体中还发现了年龄为2910Ma的继承性锆石。推测这种错石来自于表壳岩包体。     

贵州瓮安陡山沱组磷块岩稀土元素地球化学特征与沉积期后变化

贵州瓮安陡山沱组磷块岩保存了可能是全球最早的后生动物化石 (瓮安动物群 ),对于瓮安动物群出现过程中的古海洋环境重建具有重要科学意义。但必须对成岩后生作用对磷块岩中的稀土元素改造进行评估。通过对贵州瓮安陡山沱组磷块岩的磷质碎屑、磷质和白云质胶结物、磷条带和泥条带等的稀土元素地球化学特征研究,确定沉积期后变化对稀土元素的改造影响不大。上矿层磷块岩沉积期形成的磷质碎屑、成岩期形成的白云质和磷质胶结物具相同的稀土元素配分模式,暗示了沉积期后的改造作用对瓮安陡山沱组磷块岩保存的原生沉积信息影响不大。瓮安陡山沱组磷块岩具有显著的重稀土亏损特征。磷块岩的磷质和白云质胶结物、伴生磷质碎屑、强风化磷块岩相近的ErN/LuN 比值,表明沉积期后的改造作用不是重稀土元素亏损的主要原因。磷块岩的ErN/LuN、LaN/NdN 与Ce/Ce 间的相关性,表明越氧化的沉积环境中,轻和重稀土元素亏损越强。     

REE concentrations of garnet and omphacite in eclogites from the Dabie Mountain, central China

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients (Kd=CiOmp/CiGrt) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks.     

云南兰坪盆地古近系细碎屑岩地球化学特征及其地质意义

采用电感耦合等离子体质谱(ICP-MS)方法对兰坪盆地古近系104件细碎屑岩样品进行了稀土元素及微量元素分析,结果显示∑LEE含量较高,轻稀土含量较富集、重稀土含量较亏损,显示出明显的"右倾"型配分模式。根据稀土元素和微量元素特征、w(Zr)-w(Th)、La-Th-Sc、Th-Sc-Zr/10等多种沉积构造背景判别图解及多种交叉分析方法,对兰坪盆地古近系细碎屑源岩构造背景进行了详细研究。利用La/Th-Hf和La/Yb-∑REE判别图解对兰坪盆地古近系源岩属性进行了分析,结果表明:兰坪盆地古近系碎屑源岩主要以上地壳长英质岩石为主,并混有少量基性岩,反映其物源区构造背景为早期为大陆岛弧构造背景,至晚期逐渐过渡为被动大陆边缘构造背景。