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1.
A suite of Australian shales, greywackes and subgreywackes ranging in age from Proterozoic to Triassic were analyzed for the rare earth elements (REE) in order to detect any secular changes in rare earth distribution. These post-Archean sediments show remarkably similar relative rare earth patterns. They are characterized by negative Eu anomalies of almost constant magnitude (average Eu/Eu* = 0.67 ± 0.05) relative to chondrites and nearly constant ratios of light to heavy rare earths (average ∑LREE/∑HREE = 9.7 ± 1.8). The REE abundances are generally higher in the younger sediments which may suggest that the absolute abundances of the rare earths in clastic sediments have gradually increased with time. Since no secular change in relative rare earth distribution was detected in the post-Archean sediments, a uniform process of crustal growth and evolution seems to have operated over the past 1500 million years at least. Australites show rare earth distributions very similar to that of the average clastic sediment. This suggests that the tektite parent material originated in the upper crust. 相似文献
2.
东太结核主要为半埋藏和埋藏型,发育于以黏土和硅质组分为主的沉积环境。东太结核的锰相矿物主要有水羟锰矿和钡镁锰矿,具有较高的REY、Cu、Ni含量和Mn/Fe比值,显示遭受间隙水的影响,落入水成成因和成岩成因两个区间范围。西太结核主体暴露在海水中,周围沉积物主要由深海黏土组成。西太结核的锰相矿物几乎只有水羟锰矿,具有较高的REY、Co含量和低Mn/Fe比值,属于典型的水成成因型。两个区域的多金属结核的稀土北美页岩标准化模式均显示Ce正异常、Y负异常和无或弱Eu异常,与海水稀土特征构成良好的耦合关系,是多金属结核对海水稀土选择性富集的结果。西太结核相对东太结核具有更高的Ce含量和 δCe,Co、Ti与Ce具有良好的正相关关系。研究认为海水中溶解氧并不一定是控制结核Ce正异常程度的关键因素,Co、Ti等元素及其相关组分能够引起Ce与其他稀土元素的强烈分馏,也可能是影响多金属结核Ce正异常程度的控制因素。研究区多金属结核和富钴结壳表层样的 εNd范围为-6.6~-2.5,是全球最富放射性成因Nd的海洋铁锰壳层。结合稀土模式以及Eu异常特征,本研究认为多金属结核的稀土主要来自 εNd相对较高的周围陆壳,可以通过河流或者大气沉降等方式输送到大洋,而研究区广泛分布的海山玄武岩释放的放射性成因Nd同位素对海水的影响微弱。 相似文献
3.
洒西白钨矿床位于滇东南老君山矿集区内,呈层状、似层状产出。矿区围岩为一套中深变质岩,其稀土元素组成特征为:矿区围岩LREE/HREE比值为4.37~6.87,δEu为0.67~1.49,δCe值为0.62~0.98;外围混合片麻岩LREE/HREE比值为4.51~5.58,δEu平均为0.14。矿石Eu正异常与Ce负异常同时存在说明矿石沉淀时相对高温的热水流体和较低温的海水在海底发生了对流混合,部分样品中Eu负异常的形成可能与后期岩浆热液叠加改造作用有关。结合矿床地质特征,洒西白钨矿床成因为喷流沉积-后期岩浆热液改造型。 相似文献
4.
通过对西太平洋34件海水样品的稀土元素（REY:REE+Y）测试及其与研究区富钴结壳稀土耦合特征分析,揭示了海水稀土特征及其成因.海水的稀土含量随水深呈现逐渐增加的趋势,∑ REY范围为14.0×10-12~65.5×10-12,平均值为31.9×10-12,其中Y的绝对值（均值为6.0×10-12~24.1×10-12）和相对值（（Y/Ho） N均值为1.98）均较高,La含量次之（均值为1.8×10-12~11.6×10-12）,Ce含量相对较低（均值为2.4×10-12~8.8×10-12）,δCe范围为0.33~1.03（均值为0.66）,（La/Yb） N平均值为0.71.海水稀土元素北美页岩标准化后显示左倾模式,具有显著的Ce负异常、Y正异常和无明显的Eu异常特征.研究区普遍发育水成成因的富钴结壳,即其稀土元素和其他组分均源自海水.富钴结壳的稀土含量相对海水富集6~7个数量级,其Ce正异常和Y负异常的稀土模式与海水构成良好的耦合关系,指示富钴结壳类组分对海水稀土清扫具有选择性,是造成海水稀土模式的重要因素.海山上发育的磷块岩以及周围盆地深海泥中的磷酸盐组分,它们具有较高的稀土含量和类似于海水的稀土模式,指示海洋磷酸盐消耗稀土时并未分馏而是继承海水模式.海水独特的稀土模式特征是补给与消耗平衡作用的结果,铁锰氧化物和海洋磷酸盐是两种典型的海洋自生组分,它们对海水稀土特征的形成至关重要. 相似文献
5.
川北元坝气田二叠系长兴组49个碳酸盐岩样品的稀土元素(REE)和钇(Y)含量的测试结果表明:稀土元素总量(ΣREE)普遍较低(平均1.50×10 -6),说明岩石样品几乎无陆源碎屑的混入,且未受到深部热液的影响。页岩标准化后的白云岩REE特征表现为轻稀土(LREE)亏损、正La异常、负Ce异常(0.49～1.08,平均0.75)、普遍的正Eu异常(0.89～46.00,平均4.68)及较高的Y/Ho值(28.82～63.47,平均43.04),总体表现出与海水的REE配分特征类似,表明白云化流体为还原性的海源流体,白云岩形成环境为埋藏环境。元坝气田长兴组构造平缓,白云岩横向上连续分布,碳酸盐岩 δ18O(-7.07‰～-2.92‰,平均-4.95‰)相对较重,岩心与岩石薄片未观察到角砾状或条带状的热液白云岩典型构造,均说明元坝气田长兴组未受到深部热液流体的影响,白云岩较高的正Eu异常与深部热液流体无关;电子扫描显微镜下观察到大量的热化学硫酸盐还原反应(TSR)的产物单质硫和富硫沥青,表明元坝气田长兴组基质白云岩形成之后的TSR作用导致了地层水极度还原的环境,从而造成了白云岩较高程度的正Eu异常,反过来说明了TSR作用对白云岩储集层具有一定的改造作用。 相似文献
6.
广西那坡裂陷盆地位于右江盆地南缘,晚古生代该盆地广泛分布包括硅质岩、泥岩和海相玄武岩在内的深水相沉积.对盆地内上泥盆统榴江组和中下二叠统四大寨组硅质岩地球化学特征研究表明,硅质岩SiO 2含量为88.55%~99.03%,PAAS组成含量小于20%,指示其含有较低的陆源碎屑组成.硅质岩的Al/(Al+Fe+Mn)值为0.45~0.94,Eu/Eu*值为0.51~0.95,为非热液成因硅质岩.除去SiO 2稀释作用的影响后,硅质岩具有较高的稀土元素含量(∑REE+Y含量相当于PAAS组成的2~5倍),指示其形成于相对远离陆源供应的环境.岩信和鱼塘上泥盆统榴江组硅质岩具有中等的Ce负异常(Ce/Ce*值分别为0.37~0.72和0.58~0.89)以及较明显的Y正异常(Y/Ho值分别为39.05~83.74和34.33~36.70),形成于远离陆源的开阔裂谷盆地环境.鱼塘中下二叠统四大寨组硅质岩具有明显的Ce负异常(Ce/Ce*值为0.12~0.33),显示成熟洋盆的地球化学特征.结合右江其他地区硅质岩的地球化学特征认为,晚古生代硅质岩的地球化学特征记录了右江盆地从晚泥盆世裂谷盆地到早中二叠世扩张为开阔洋盆的过程. 相似文献
7.
The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities. The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al2O3 (>>16%), low TiO2 (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite. 相似文献
8.
The Uintjiesberg kimberlite diatreme occurs within the Proterozoic Namaqua–Natal Belt, South Africa, approximately 60 km to the southwest of the Kaapvaal craton boundary. It is a group I, calcite kimberlite that has an emplacement age of 100 Ma. Major and trace element data, in combination with petrography, are used to evaluate its petrogenesis and the nature of its source region. Macrocryst phases are predominantly olivine with lesser phlogopite, with very rare garnet and Cr-rich clinopyroxene. Geochemical variation amongst the macrocrystic samples (Mg# 0.85–0.87, SiO 2=27.0–29.3%, MgO=26.1–30.5%, CaO=10.9–13.5%) is shown to result from 10% to 40% entrainment and partial assimilation of peridotite xenoliths, whereas that shown by the aphanitic samples (Mg# 0.80–0.83, SiO 2=19.1–23.0%, MgO=17.9–23.9%, CaO=16.5–23.7%) is consistent with 7–25% crystal fractionation of olivine and minor phlogopite. Changing trajectories on chemical variation diagrams allow postulation of a primary magma composition with 25% SiO 2, 26% MgO, 2.3% Al 2O 3, 5%H 2O, 8.6% CO 2 and Mg#=0.85. Forward melting models, assuming 0.5% melting, indicate derivation of the primary Uintjiesberg kimberlite magma from a source enriched in light rare earth elements (LREE) by 10× chondrite and heavy REE (HREE) by 0.8–2× chondrite, the latter being dependent on the proportion of residual garnet. Significant negative Rb, K, Sr, Hf and Ti anomalies present in the inferred primary magma composition are superimposed on otherwise generally smooth primitive mantle-normalized trace element patterns, and are inferred to be a characteristic of the primary magma composition. The further requirement for a source with chondritic or lower HREE abundances, residual olivine with high Fo content (Fo94) suggests derivation from a mantle previously depleted in mafic melt but subsequently enriched in highly incompatible elements prior to kimberlite genesis. These requirements are interpreted in the context of melting of continental lithospheric mantle previously enriched by metasomatic fluids derived from a sublithospheric (plume?) source. 相似文献
9.
The migmatites from Punta Sirenella (NE Sardinia) are layered rocks containing 3–5 vol.% of centimeter-sized stromatic leucosomes which are mainly trondhjemitic and only rarely granitic in composition. They underwent three deformation phases, from D 1 to D 3. The D 1 deformation shows a top to the NW shear component followed by a top to the NE/SE component along the XZ plane of the S 2 schistosity. Migmatization started early, during the compressional and crustal thickening stage of Variscan orogeny and was still in progress during the following extensional stage of unroofing and exhumation. The trondhjemitic leucosomes, mainly consisting of quartz, plagioclase, biotite ± garnet ± kyanite ± fibrolite, retrograde muscovite and rare K-feldspar, are locally bordered by millimeter-sized biotite-rich melanosomes. The rare granitic leucosomes differ from trondhjemitic ones only in the increase in modal content of K-feldspar, up to 25%. Partial melting started in the kyanite field at about 700–720 °C and 0.8–0.9 GPa, and was followed by re-equilibration at 650–670 °C and 0.4–0.6 GPa, producing fibrolite–biotite intergrowth and coarse-grained muscovite. The leucosomes have higher SiO2, CaO, Na2O, Sr and lower Al2O3, Fe2O3, MgO, TiO2, K2O, P2O5, Rb, Ba, Cr, V, Zr, Nb, Zn and REE content with respect to proximal hosts and pelitic metagreywackes. Sporadic anomalous high content of calcium and ferromagnesian elements in some leucosomes is due to entrainment of significant amounts of restitic plagioclase, biotite and accessory phases. The rare granitic leucosomes reveal peritectic K-feldspar produced by muscovite-dehydration melting. Most leucosomes show low REE content, moderately fractionated REE patterns and marked positive Eu anomaly. Proximal hosts and pelitic metagraywackes are characterized by higher REE content, more fractionated REE patterns and slightly negative Eu anomaly. The trondhjemitic leucosomes were generated by H2O-fluxed melting at 700 °C of a greywacke to pelitic–greywacke metasedimentary source-rock. The disequilibrium melting process is the most reliable melting model for Punta Sirenella leucosomes. 相似文献
10.
Abstract: The black shales of the Lower Cambrian Niutitang Formation in Weng’an, on the Yangtze platform of south China, contain voluminous polymetallic sulfide deposits. A comprehensive geochemical investigation of trace, rare earth, and platinum group elements (PGE) has been undertaken in order to discuss its ore genesis and correlation with the tectono-depositional setting. The ore-bearing layers enrich molybdenum (Mo), nickel (Ni), vanadium (V), lead (Pb), strontium (Sr), barium (Ba) , uranium (U) , arsenic (As), and rare earth elements (REE) in abundance. High uranium/thorium (U/Th) ratios (U/Th>1) indicated that mineralization was mainly influenced by the hydrothermal process. The dU value was above 1.9, showing a reducing sedimentary condition. The REE patterns showed high enrichment in light rare earth elements (LREE) (heavy rare earth elements (HREE) (LREE/HREE=5–17), slightly negative europium (Eu) and cerium (Ce) anomalies (dEu=0.81–0.93), and positive Ce anomalies (dCe=0.76–1.12). PGE abundance was characterized by the PGE-type distribution patterns, enriching platinum (Pt), palladium (Pd), ruthenium (Ru) and osmium (Os). The Pt/Pd ratio was 0.8, which is close to the ratios of seawater and ultramafic rocks. All of these geochemical features suggest that the mineralization was triggered by hydrothermal activity in an extensional setting in the context of break-up of the Rodinian supercontinent. 相似文献
11.
鄂尔多斯盆地西缘北部的内蒙古桌子山地区奥陶纪地层出露完整,沉积类型丰富,是研究鄂尔多斯盆地西缘奥陶纪沉积的关键地区。该区在中奥陶世时位于从浅海向深海的转换部位,达瑞威尔期沉积的克里摩里组由深灰色薄层状石灰岩和灰黑色泥岩组成,是研究鄂尔多斯盆地西缘奥陶纪深水氧化-还原条件的一个较为理想的层位。本文在详细的野外观察基础上,测试克里摩里组薄层细粉晶-泥晶石灰岩和灰黑色泥岩的稀土元素（REY）和其他与氧化-还原条件相关的微量元素,主要包括V、Cr、Ni、Co等,着重分析探讨其形成时的氧化-还原条件。结果表明：1）石灰岩和泥岩稀土总量（REE）分别为28.18×10 -6和100.37×10 -6,轻、重稀土比值分别为9.60和9.69;2）石灰岩和泥岩稀土元素Y/Ho、（La/Yb） N、（La/Sm） N、（La/Nd） N值分别为29.01、1.35、0.92、0.95和25.72、1.21、0.96、1.02;3）石灰岩无Ce负异常或具有微弱的正异常,同时具有明显的Eu正异常,泥岩具有明显的Ce负异常和较弱的Eu正异常;4）石灰岩和泥岩的其他微量元素比值V/Cr、V/（V+Ni）、Ni/Co分别为1.10、0.62、7.75和3.54、0.89、5.15。依据本文研究结果,结合沉积特征和前人研究成果可以推测,克里摩里组沉积于风暴浪基面之下,远离海岸,但同时又受淡水影响,其中石灰岩形成于氧化环境,泥岩形成于还原环境,同一沉积背景下这种氧化与还原环境的交替是由等深暖流造成的,可称为水动力氧化环境。 相似文献
12.
A decrease in temperature (Δ T up to 45.5 °C) and chloride concentration (ΔCl up to 4.65 mol/l) characterises the brine–seawater boundary in the Atlantis-II, Discovery, and Kebrit Deeps of the Red Sea, where redox conditions change from anoxic to oxic over a boundary layer several meters thick. High-resolution (100 cm) profiles of the methane concentration, stable carbon isotope ratio of methane, and redox-sensitive tracers (O 2, Mn 4+/Mn 2+, Fe 3+/Fe 2+, and SO 42−) were measured across the brine–seawater boundary layer to investigate methane fluxes and secondary methane oxidation processes. Substantial amounts of thermogenic hydrocarbons are found in the deep brines (mostly methane, with a maximum concentration up to 4.8×105 nmol/l), and steep methane concentration gradients mainly controlled by diffusive flow characterize the brine–seawater boundary (maximum of 2×105 nmol/l/m in Kebrit Deep). However, locally the actual methane concentration profiles deviate from theoretical diffusion-controlled concentration profiles and extremely positive δ13C–CH4 values can be found (up to +49‰ PDB in the Discovery Deep). Both, the actual CH4 concentration profiles and the carbon-13 enrichment in the residual CH4 of the Atlantis-II and Discovery Deeps indicate consumption (oxidation) of 12C-rich CH4 under suboxic conditions (probably utilizing readily available—up to 2000 μmol/l—Mn(IV)-oxihydroxides as electron acceptor). Thus, a combined diffusion–oxidation model was used to calculate methane fluxes of 0.3–393 kg/year across the brine–seawater boundary layer. Assuming steady-state conditions, this slow loss of methane from the brines into the Red Sea bottom water reflects a low thermogenic hydrocarbon input into the deep brines. 相似文献
13.
运用岩石学和地球化学的方法研究滇东南丘北地区晚二叠世吴家坪期硅质岩,取得以下主要认识:（1）滇东南丘北地区硅质岩SiO 2/Al 2O 3值除样品P29外均大于80,Al/（Al+Fe+Mn）为0.46~0.76,表明硅质岩主要为生物成因,受陆源碎屑影响较小;（2）硅质岩Ce/Ce*为0.517~0.858,La N/Ce N为1.12～1.80,经球粒陨石标准化的稀土模式则表现为轻稀土元素富集,且具有明显负Eu异常的右倾型,表明沉积环境为大陆边缘;（3）样品P29硅质岩Eu/Eu*为1.129,反映其受到一定程度热液的影响,可能受峨眉山地幔柱或（和）断裂的共同影响。 相似文献
14.
广西南宁附近从早泥盆世埃姆斯期到晚泥盆世弗拉期发育了一套黑色层状硅质岩,常夹凝灰岩(或凝灰质硅质岩)或与之互层,在弗拉期晚期硅质岩变为浅黄色。本文对该区埃姆斯期、艾菲尔期、吉维特期到弗拉期硅质岩的稀土元素地球化学特征进行了系统研究,其∑REE平均值分别为:18.59×10 -6,12.28×10 -6,27.44×10 -6和43.07×10 -6;页岩标准化后Ce含量在这4个阶段均表现为弱-中等的负异常,Ce/Ce *平均值分别为:0.85,0.72,0.85和0.67;Eu/Eu *为弱的正异常,平均值分别为:1.08,1.21,1.46和1.04;(La/Ce) N值中等,平均值分别为:1.42,1.50,1.35和1.73;(La/Yb) N值变化较大,平均值分别为:1.19,1.06,1.24和0.45。这些结果表明该区硅质岩开始形成于陆内裂谷盆地,受陆源影响较大,在晚泥盆世迅速扩张为比较开阔的海盆环境,陆源影响明显减弱。硅质岩的形成虽受到一定热液活动的影响,但热液活动的影响普遍较弱。 相似文献
15.
Abstract: Major and rare earth element contents are reported for Late Archean banded iron formations (BIFs) in the Bababudan Group of the Dharwar Craton, South India. The BIFs are mostly composed of SiO 2 (average1ρ = 54.88.1 wt%) and Fe 2O 3* (44.38.2 wt%). The Al 2O 3 and TiO 2 contents are remarkably low, suggesting that detrital components were starved during the BIF deposition. The BIFs have a LREE-enriched pattern with a relatively high (La/Yb) N (6.644.07). Total REE concentrations (RE) vary from 5.2 to 65.3 ppm. The REE patterns are characterized by the presence of a very large negative Ce anomaly (Ce/Ce*: 0.13-0.83) and a positive Eu anomaly (Eu/Eu*: 0.96-2.45). The Eu/Eu* decreases and (La/Yb) N increases with a increase of RE. These correlations of REE indices are similar to those of modern hydrothermal iron-rich sediments near a mid-ocean ridge (MOR). Greenstones associated with the BIFs have MORB-like geochemical features. These geochemical and geological lines of evidence indicate that the depositional site of the BIFs was remote from a continent and/or island arc and that the BIFs were in situ hydrothermal sediments near a MOR. A striking negative Ce anomaly in the BIFs indicates that oxygenated deep-sea environments emerged at 2.9-2.7 Ga. The existence of contemporaneous Mn deposits in the Dharwar Craton supports this assertion. Our scenario of oxygen in the Earth's surface of the Late Archean is different from long-held notion that the atmosphere and ocean were persistently anoxic throughout the Archean. 相似文献
16.
High-pressure, low-temperature metamorphic Mn-rich quartzites from Andros and Evvia (Euboea) islands, Greece, situated in the Eocene blueschist belt of the Hellenides, reveal different Mn-Al-Ca-Mg-silicate assemblages in response to variable metamorphic grade. On Evvia, piemontite- and/or braunite-rich quartzites which are associated with low-grade blueschists ( T<400° C, P> 8 kbar) show the principle mineral assemblage quartz + montite + sursassite + braunite + Mg-chlorite + hematite + rutile + titanite. The Mn-Al-silicate sursassite, basically (Mn 2+, Ca) 4 Al 2(Al, Fe 3+, Mn 3+, Mg) 4Si 6O 21(OH) 7, thus far reported as a rare mineral, locally occurs as a rockforming mineral in cm- to m-thick layers. On Andros, higher-grade quartzites ( T450–500° C, P>10 kbar) of similar composition contain the assemblage quartz + piemontite + spessartine + braunite + Mg-chlorite+hematite + phengite+ phlogopite + rutile. Rare sursassite is present only as a relict phase. Additional, mostly accessory minerals in quartzites from Evvia and Andros are ardennite, Na-amphibole, acmitic clinopyroxene, albite, apatite, and tourmaline. The chemical composition of the main phases is characterized in detail.Disequilibrium textures and mineral compositions in some samples from Andros and Evvia imply the reactions sursassite + braunite + quartz = spessartine+clinochlore±hematite + H 2O + O 2 (1) sursassite + braunite + phengite + quartz = spessartine + phlogopite±hematite + H 2O + O 2 (2) and in braunite-free assemblages sursassite + Mn 3+Fe
–1
3+
[hematite, piemontite] + hematite + quartz = spessartine + clinochlore + H 2O+O 2 (3) Reactions (1) to (3) have positive P-T slopes. They are considered to account for the breakdown of sursassite and the formation of spessartine during prograde metamorphism of the piemontite quartzites and related rocks.
P-T data from Andros and Evvia and geological data from few other occurrences reported suggest sursassite+ quartz±braunite to be stable at T<400–450° C over a considerable pressure interval at least up to 10 kbar. Theoretical phase relations among Mn 3+-Mn 2+-silicates in the pseudoquaternary system Al-Mn-Ca-Mg with excess quartz, H 2O, and O 2 indicate that low-grade assemblages containing sursassite (±braunite±pumpellyite±viridine±piemontite + quartz) are likely precursors of higher-grade assemblages including spessartine, Mg-chlorite, braunite, viridine, and piemontite reported from greenschist-, amphibolite-, and high-grade blueschist-facies rocks of appropriate composition. 相似文献
17.
通过剖面实测、薄片鉴定、阴极发光分析和地球化学分析,对川东北地区下寒武统龙王庙组白云岩成因进行了研究。研究结果表明:研究区龙王庙组自下而上发育灰岩—过渡岩性—白云岩,以颗粒结构为主,白云石主要为泥—粉晶,呈它形—半自形晶,阴极发光呈昏暗光—暗红光。样品的MgO与CaO呈负相关,白云岩Mg2+/Ca2+较低,高Na含量,低Sr、Fe、Mn含量,白云石有序度较低,各岩性稀土元素配分曲线平行一致分布,δCe无异常,δEu负异常,灰岩δ13C偏正、δ18O处于同期海水内,白云岩δ13C、δ18O偏正,87Sr/86Sr高于同期海水值。结合区域地质背景、岩石学特征和地球化学分析结果,研究区白云石化流体为高盐度蒸发海水,研究区白云岩为渗透回流白云石化作用形成。 相似文献
18.
晚石炭纪-早二叠纪,在准噶尔盆地西北缘,发育区域性的火山岩建造,岩性以酸性英安岩和流纹岩为主,同时发育少量中性玄武安山岩和安山岩,多属于中-低钾的钙碱性系列。样品全碱(Na 2O+K 2O)含量为0.99％ ～8.12％,K 2O的含量较低,为3.01％ ～0.05％,稀土元素总量(∑ REE)较低。稀土元素轻重分异,轻稀土元素(LREE)相对于重稀土元素(HREE)富集:(La/Yb) N=0.76～5.18,且部分样品显示轻微的Eu负异常和Ce正异常:δEu=0.533～1.148,δCe=0.979～1.224。微量元素中大离子亲石元素(LILE)富集,高场强元素Nb、Ta亏损。分析认为石炭系末期-二叠系早期,研究区所处构造环境为洋内弧附近的前弧盆地,发育的火山岩是母源岩浆经历了一定程度分异作用后的产物,以岛弧环境为主,兼具有部分板内大陆环境特征。 相似文献
19.
详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明,研究区BIF具有非常相似的特征：稀土总量均较低;经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征;Y/Ho比值较高;具有明显的Eu、Y、La的正异常,且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常,但不同类型、不同沉积年龄BIF的铕异常程度不同：与吕梁地区Superior型铁矿相比,冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常;并且自中太古代-新太古代-古元古代,BIF的铕正异常逐渐减小,这可能反映了随着BIF沉积年龄的减小,进入到该地区海水中的高温热液流体逐渐减少;同时,研究区BIF缺乏明显的Ce负异常,可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。 相似文献
20.
滇东南麻栗坡位于右江盆地西南缘,该处蛇绿岩被认为是古特提斯洋壳的遗迹,对蛇绿岩所代表八布洋的研究是全面了解右江盆地与华南西南缘古特提斯构造演化的重要环节。研究区广泛发育一套夹有硅质岩的深水相沉积,对麻栗坡八布和西牛堡中—下二叠统他披组硅质岩的地球化学特征研究表明:硅质岩Al/(Al+Fe+Mn)值为0.43~0.70,Al 2O 3/TiO 2值为25.5~44.8,Eu/Eu*值为0.81~1.02,为非热液成因硅质岩。硅质岩Ce/Ce*值分别为0.67~1.29和0.77~1.18,Y/Ho值分别为28.84~31.23和26.41~28.91,形成于大陆边缘环境。结合其他地区硅质岩地球化学特征,认为早—中二叠世八布洋盆的西南缘可能为活动大陆边缘,其洋壳的俯冲方向为南西向。 相似文献
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