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1.
An enrichment of light rare earth elements (LREE) is characteristic for most of the acidic, Fe- and SO4-rich pit lakes and groundwaters in the lignite mining area of Lower Lusatia (Germany). One of these acidic lakes – the pit lake “RL 1223” – has a strong thermal and chemical stratification. The upper water layer (0–9 m) shows pH values of about 3 during all seasons. The monimolimnion (10–17 m) of the lake is anoxic and has pH values of about 7. The rare earth element (REE) patterns of the upper lake water show enriched LREE (LaN/YbN = 1.6) whereas the opposite patterns (depletion of LREE, LaN/YbN = 0.4) are found in the anoxic water of the monimolimnion. Experiments were conducted to observe the behaviour of REE during Fe oxidation in water from the monimolimnion (depth 14 m). The sampled monimolimnion water was placed in plastic bottles, and the changing water chemistry was observed for 40 weeks after sampling. Due to the initial anoxic conditions almost all Fe precipitated in the investigated water, and the pH value decreased from about 7 to 3 during the oxidation. The Fe precipitates are identified as ferrihydrite which is transformed into goethite within the oxidation process. Stable pH conditions (pH 3.0) were reached after about 10 weeks of oxidation.The original REE patterns of the investigated water are generally reflected in the Fe precipitates collected at the beginning of the experiment as well as after up to 40 weeks of oxidation. However, in the corresponding water LREE were temporally enriched with a maximum LaN/YbN ratio of 1.0 and a maximum LaN/SmN ratio of 2.3 after 6 weeks of oxidation time (pH 3.8–4.9). Although complex geochemical changes took place between the start and the end of the experiment REE patterns observed at these points in time are nearly identical. These differences of the REE pattern can be explained by the sampling procedure. The experimental findings can be transmitted to the mining dump aquifers of the study area where geochemical conditions comparable to the experimental oxidation time from 3 to 6 weeks are found and hydrous ferric oxides are precipitating. Groundwater passing through the mining dumps can preferentially desorb LREE from the Fe precipitates and display the typical LREE enrichment and carry it to the epilimnion of the acidic pit lakes in Lower Lusatia.  相似文献   

2.
对采自羌塘盆地那底岗日地区布曲组碳酸盐岩烃源岩进行了稀土元素地球化学研究,分析结果表明:海相碳酸盐岩烃源岩稀土总量(∑REE)最大值75.21μg/g,最小值20.58μg/g,平均值为36.67μg/g.稀土元素北美页岩标准化后具有相对富集轻稀土,亏损重稀土的特点.布曲组碳酸盐岩烃源岩Ce/Ce*值为0.83~0.9...  相似文献   

3.
The Lesser Qinling carbonatite dykes are mainly composed of calcites. They are characterized by unusually high heavy rare earth element concentrations (HREE; e.g. Yb > 30 ppm) and flat to weakly light rare earth element (LREE) enriched chondrite-normalized patterns (La/Ybn = 1.0–5.5), which is in marked contrast with all other published carbonatite data. The trace element contents of calcite crystals were measured in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Some crystals show reduced LREE from core to rim, whereas their HREE compositions are relatively constant. The total REE contents and chondrite-normalized REE patterns from the cores of carbonate crystals are similar to those of the whole rock. The carbon and oxygen isotopic compositions of calcites fall within the range of primary, mantle-derived carbonatites. The initial Sr isotopic compositions (0.70480–0.70557) of calcites are consistent with an EM1 source or mixing between HIMU and EM1 mantle sources. However these sources cannot produce carbonatite parental magmas with a flat or slightly LREE enrichment pattern by low degrees of partial melting. Analyses of carbonates from other carbonatites show that carbonates have nearly flat REE pattern if they crystallize from a LREE enriched carbonatite melt. This implies that when carbonates crystallize from a carbonatite melt the calcite/melt partition coefficients (D) for HREE are much greater than the D for the LREE. The nearly flat REE patterns of the Lesser Qinling carbonatites can be explained if they are carbonate cumulates that contain little trapped carbonatite melt. Strong enrichment of HREE in the carbonatites may require their derivation by small degrees of melting from a garnet-poor source.  相似文献   

4.
The mineralogy, geochemistry, and radiocarbon ages of two sediment cores (GMX1 and GMX2) collected from the deep sea area of the Southwestern Gulf of Mexico (∼876–1752 m water depth) were studied to infer the sedimentation rate, provenance, heavy metal contamination, and depositional environment. The sediments are dominated by silt and clay fractions. The mineralogy determined by X-Ray diffractometry for the sediment cores reveals that montmorillonite and muscovite are the dominant clay minerals. The sections between 100 and 210 cm of the sediment cores GMX1 and GMX2, respectively, are characterized by the G. menardii group and G. Inflata planktonic foraminiferal species, which represent the Holocene and Pleistocene, respectively. The radiocarbon-age measurements of mixed planktonic foraminifera varied from ∼268 to 45,738 cal. years B.P and ∼104 to 25,705 cal. years B.P, for the sediment cores GMX1 and GMX2, respectively. The variation in age between the two sediment cores is due to a change in sediment accumulation rate, which was lowest at the location GMX1 (0.006 cm/yr) and highest at the location GMX2 (0.017 cm/yr).The chemical index of alteration (CIA), chemical index of weathering (CIW), and index of chemical maturity (ICV) values indicated a moderate intensity of weathering in the source area. The total rare earth element concentrations (∑REE) in the cores GMX1 and GMX2 vary from ∼94 to 171 and ∼78 to 151, respectively. The North American Shale Composite (NASC) normalized REE patterns showed flat low REE (LREE), heavy REE (HREE) depletion with low negative to positive Eu anomalies, which suggested that the sediments were likely derived from intermediate source rocks.The enrichment factor of heavy metals indicated that the Cd and Zn concentrations in the sediment cores were impacted by an anthropogenic source. The redox-proxy trace element ratios such as V/Cr, Ni/Co, Cu/Zn, (Cu + Mo)/Zn, and Ce/Ce* indicated that the sediments were deposited under an oxic depositional environment. The similarity in major element concentrations, REE content, and the NASC normalised REE patterns between the cores GMX1 and GMX2 revealed that the provenance of sediments remained relatively uniform or constant during deposition for ∼4.5 Ma. The major and trace element based multidimensional discrimination diagrams showed a rift setting for the core sediments, which is consistent with the geology of the Gulf of Mexico.  相似文献   

5.
吉林省桦甸油页岩中稀土元素和微量元素的研究   总被引:5,自引:0,他引:5  
对桦甸油页岩及其灰渣的矿物成分、主量元素、稀土元素和微量元素含量进行测定。结果表明:油页岩中稀土元素含量低于北美页岩(NASC)中的平均含量,REE球粒陨石标准化的分布模式曲线表现为负斜率,(La/Yb)N的平均值大于1,属于轻稀土富集型;REE北美页岩标准化的分布模式曲线较平缓,(La/Yb)S的平均值接近于1,轻重稀土分馏不明显。与球粒陨石和北美页岩相比,Eu有较严重的正异常。油页岩中的微量元素与北美页岩和地壳的平均值相比较,Sb、Nb、Cs、Zn、Bi、W等元素具有较高的富集度。油页岩灰渣中稀土元素和微量元素富集度均高于油页岩。  相似文献   

6.
The distribution of REE in the bottom sediments of Amur Bay accumulated over the last 100 years was studied, and the REE contents were evaluated. The REE contents normalized to North American Shale Composite (NASC) show a negative Ce anomaly and a predominance of LREE and MREE. The inflow and accumulation of REE in the bottom sediments are influenced mainly by natural sources, whereas their dependence on anthropogenic factors is minimal.  相似文献   

7.
Over one hundred samples, representing mainly clayey-organic- and carbonate-rich shales (Kupferschiefer) but also other members of different ore sections, including hangingwall dolomites (Z1 Werra) and footwall Weissliegend sandstone (Lower Permian), have been collected in different mines of the Lubin–Głogów mining district, mainly near the contact (transitional zone) between the copper-mineralized zone and secondarily oxidized (Rote Fäule = RF) zone. In general, the Polish Kupferschiefer shales are enriched in MREE in comparison to NASC. In a typical copper-rich ore section the REE amounts and patterns depend on lithologies, being generally similar in shales and dolomite. ∑REE vary among sandstones, shales and dolomites (average 73, 143 and 85 ppm, respectively), probably reflecting mainly their clay contents. Sandstones have strongly convex REE patterns with positive Eu and negative Gd anomalies and depletion in LREE and enrichment in MREE relative to HREE. The REE patterns of shale and dolomite are similar to one another and rather flat, with strong negative Eu anomalies, and a small positive Gd anomaly in the case of shales.The REE patterns of shales from the mineralized Cu-zone are generally convex (MREE enriched) and have negative Eu anomalies. However, in a section with Cu-, Zn- and Pb-shales the REE pattern of Pb-bearing shales shows a positive Eu anomaly, in contrast to other shales and overlying dolomite. More oxidizing conditions of deposition can be assumed for Pb-shales.No significant differences between REE distributions in transitional and oxidized zones have been observed. Their REE patterns are more convex and are much higher (av. 247 ppm) than those in the mineralized zone and they do not show Eu anomalies. The strongly convex pattern may suggest either enrichment in MREE or relative depletion in LREE due to localized precipitation of light REE minerals, both in shales and in the uppermost part of the sandstones.Two unique sections, one Cu-rich and one Cu-lean (partly oxidized), comprising three shale beds interbedded with dolomites have been compared. Generally ∑REE contents are similar in these two sections. Also similar are changes in contents of REE between beds in both sections, which decrease significantly upwards (from 157–171 ppm to 54–60 ppm). The REE patterns of the lowermost beds (directly overlying sandstones) are ramp-like, with LREE enrichments. The upper beds have concave REE patterns. Comparison between sections shows generally stronger negative Eu and positive Gd anomalies in the highly-mineralized section.There is a highly significant positive relationship between Cu and ∑ REE contents in Cu-rich shales and slightly less significant negative relationship for their concentration in oxidized and transitional shales. There is a moderate significant positive correlation between P2O5 and ∑ REE contents in Cu-rich shales.The observed differences in REE contents cannot be provenance dependent but have been caused by diagenetic processes, possibly related to mineralization and oxidation processes. Europium anomalies, generally reflecting different Eh conditions in the deposit, can be eliminated by the prolonged oxidation. Strong enrichment of the RF zones in REE may result from their desorption from large volumes of oxidizing, including mineralizing, solutions which probably emerged from the underlying molasse lithologies into the Rote Fäule areas. Higher contents of REE in the lowermost shales suggest upward movement of solutions from the underlying sandstones also far away from the RF areas.  相似文献   

8.
采用中子活化法对采自内蒙古大青山煤田石炭-二叠纪煤系的12个高岭岩样品进行了稀土元素(REE)含量测定。结果表明,样品的稀土元素含量变化很大,ΣREE最低31.98×10-6,最高616.3×10-6,平均178.6×10-6,与NASC的173.21×10-6相近;ΣLREE/ΣHREE介于6.32~46.14之间,平均17.5,明显高于NASC;中—弱Eu亏损,弱Ce正异常。通过不同对比样品(北美页岩、超基性岩、基性岩、中性岩、酸性岩以及偏碱性岩等)所作的标准化分析结果表明,研究样品不具有典型的沉积型泥岩特征;结合薄片鉴定及前人分析结果推论,高岭岩可能由中—基性或偏碱性火山碎屑物质原地蚀变而成,泥炭化所提供的酸性介质条件对火山灰向高岭岩转化以及稀土元素的分异提供了必要的条件。   相似文献   

9.
长江与黄河沉积物REE地球化学及示踪作用   总被引:66,自引:4,他引:62  
杨守业  李从先 《地球化学》1999,28(4):374-380
长江与黄河沉积物的稀土元素(REE)组成特征不同。长江沉积物REE含量较高,元素含量变化也大于黄河样品;球粒陨石标准化模式表明长江沉积物的(La/Lu)N、(La/Yb)N、(Gd/Yb)N的值也相应地比黄河沉积物中的高10%左右,分布曲线均呈明显的石倾状,轻重稀土分馏明显,相对富集LREE。且长江样品比黄河样品更富集LREE,但Eu亏损不及黄河样品;两者的北美页岩标准化曲线均呈平坦稍右倾状,具有  相似文献   

10.
The rare earth element (REE) contents of sixteen surficial calcareous sediments from the southwestern Carlsberg Ridge, Indian Ocean, have been determined. The total REE vary from 35 ppm to 126 ppm and are inversely related to the calcium carbonate content. REEs show a strong positive correlation with Al + Fe + K + Mg + Na (r 2= 0.98) and Mn + Fe + Cu + Ni (r 2= 0.86) suggesting that the REE is associated with a combined phase of clays (mainly illite) and Mn-Fe oxyhydroxides. The aeolian input into these sediments is suggested from the weak positive Eu/Eu* anomaly. Shale-normalized (NASC) pattern along with La(n)/Yb(n) ratio suggest enrichment of heavy REE (HREE) relative to the light REE (LREE) with a negative Ce/Ce* anomaly implying retention of a bottom water REE pattern. An erratum to this article is available at .  相似文献   

11.
大同盆地是典型的干旱-半干旱内陆盆地,盆地中部地下水碘含量异常,对当地饮用水安全造成了严重威胁.对盆地高碘地下水分布区沉积物组成及稀土元素(REE) 进行了地球化学研究,结果表明,地下水系统呈弱碱性(pH值为7.18~9.64) 的偏还原环境,沉积物多为Ce正常或轻微负异常及Eu负异常;沉积物中碘含量为0~1.78×10-6;ΣREE含量较高,ΣLREE/ΣHREE比值为2.79~4.14,即富集轻稀土元素(LREE) 而亏损重稀土元素(HREE).ΣREE与碘含量呈负相关关系,虽然铁氧化物/氢氧化物矿物的还原性溶解可导致二者的释放,但由于沉积物有机质产生的低结晶矿物对碘的强吸附性,使沉积物中碘含量较高;弱碱性环境中REE的再吸附过程会导致沉积物中富集LREE;沉积物中碘含量与氧化还原敏感组分TOC、U、V及[Eu]N的关系也表明,地下水系统的氧化还原条件及有机质含量是影响碘富集的重要因素.   相似文献   

12.
“寨背式”离子吸附型稀土矿床多类型稀土矿化及其成因   总被引:1,自引:0,他引:1  
赵芝  王登红  邹新勇 《岩石学报》2022,38(2):356-370
赣南寨背离子吸附型稀土矿床产于寨背复式花岗岩体的风化壳中,自20世纪80年代发现以来一直以轻稀土型开采,近年在轻稀土型花岗岩风化壳中发现了重稀土矿。为了探讨轻稀土型花岗岩风化过程中重稀土元素的迁移、分馏和富集机制,本文选择了区内三个具有代表性的风化壳钻孔(ZK1、ZK2和ZK4)对其进行了全相和离子交换相稀土元素地球化学研究。结果显示:钻孔ZK4中离子交换相稀土含量介于14.90×10-6~835.8×10-6之间,并富集轻稀土(LREE/HREE=2.28~10.78);钻孔ZK1中离子交换相稀土含量达1470×10-6(9件样品均值),具有从轻稀土型向重稀土型过渡的配分特征(LREE/HREE=1.30~1.65),并且剖面自上而下显示轻、重稀土逐渐富集的趋势;钻孔ZK2中离子交换相稀土含量为492.4×10-6(8件样品均值),自上而下稀土配分类型从轻稀土型过渡至重稀土型(LREE/HREE=0.43~2.25),且轻稀土富集在全风化层上部而重稀土则富集在下部。三个钻孔的Nb/Ta和Zr/Hf...  相似文献   

13.
江汉平原被确认为我国南方新的饮水型砷中毒病区,目前对于江汉平原高砷地下水的成因机理研究还有待完善.综合运用水文地球化学分析与PHREEQC地球化学模拟计算,分析了地下水和沉积物中REE分异特征及其沿地下水流向形态变化规律.江汉平原地下水REE含量为0.032~0.843 μg/L,富集LREE,具显著Eu正异常,且地下水中Eu异常与As含量呈正相关关系.地下水中REE形态主要以LnCO3+及Ln(CO3)2-为主,沿地下水流向LnCO3+降低、Ln(CO3)22-升高.地下水REE浓度分布受到HCO3-的络合作用及Fe氧化物矿物的还原性解吸附过程控制,径流途径中继承沉积物矿物的REE配分模式及Fe氧化物矿物对LREE的优先解吸附可能是地下水富集LREE的原因,并且沿流向上REE形态分布受到pH控制.研究区中Eu含量及Eu正异常对地下水As富集程度具有一定的指示意义.   相似文献   

14.
《Applied Geochemistry》2000,15(9):1369-1381
Thirty-eight samples of stream sediments draining high-grade metamorphic rocks in the Walawe Ganga (river) Basin, Sri Lanka, were analysed for their REE contents, together with samples of metamorphic suites from the source region. The metamorphic rocks are enriched in light REE (LREE) compared to heavy REE (HREE) and are characterised by high La/Lu ratios and negative Eu anomalies. The chondrite-normalised patterns for these granulite-grade rocks are similar to that of the average post-Archaean upper crust, but they are slightly enriched with La and Ce. The REE contents of the <63-μm fraction of the stream sediments are similar to the probable source rocks, but the other grain size fractions show more enriched patterns. The <63-μm stream sediments fraction contains lower total REE, more pronouncd negative Eu anomalies, higher EuN/SmN and lower La N/LuN ratios relative to other fractions. The lower La N/LuN ratio is related to the depletion of heavy minerals in the <63-μm fraction. The 63–125-μm and 125–177-μm grain size fractions of sediments are particularly enriched in LREE (average ΣLREE=2990 μg/g and 3410 μg/g, respectively). The total HREE contents are surprisingly uniform in all size fractions. However, the REE contents in the Walawe Ganga sediments are not comparable with those of the granulite-grade rocks from the source region of the sediments. The enrichment of REE is accounted for by the presence of REE containing accessory mineral phases such as zircon, monazite, apatite and garnet. These minerals are derived from an unknown source, presumably from scattered bodies of granitic pegmatites.  相似文献   

15.
煤系高岭岩的地球化学判别标志   总被引:7,自引:0,他引:7  
朱如凯 《地质论评》1997,43(2):121-130
本文主要研究了煤系高岭岩的微量、稀土、氧同位素地球化学特征。根据高岭岩产出层序、岩石学、矿物学、地球化学特征的研究,将煤系高岭岩分为两类:(1)铝土质高岭岩,微量元素含量、稀土总量,氧同位素值高,稀土配分模式与典型北美页岩相似,Eu负异常,反映其源岩为风化壳化学风化作用产物。(2)夹矸高岭岩,微量元素含量,稀土总量,氧同位素值低,稀土配分模式部分与典型北美页岩相似,反映其源岩与铝土质高岭岩类似;部  相似文献   

16.
以新疆西准噶尔玛依拉山-萨雷诺海蛇绿岩为重点进行讨论.该区出露的火成岩种类较多,表现出构造混杂特点.变质橄榄岩是具有一定亏损程度的地幔残体,LREE的富集主要是后期蚀变的结果.玄武岩的稀土分布型式是平坦型的或LREE略亏损型的.与MORB相比,REE的总量较高,高Fe和Ti,Mg值低,是一种演化的岩石,与东太平洋中隆地高Fe和Ti的铁质拉斑玄武岩成分相当,地球化学特征类似于MORB,可能形成于小洋盆环境.  相似文献   

17.
With the aim of contributing to the knowledge of the geochemical behaviour and mobility of the rare earth element (REE) in the natural water systems, the ground and surface waters of the Ottana-Orani area (Central Sardinia, Italy) were sampled. The study area consists of albititic bodies included in Hercynian granodiorites. The waters have pH in the range of 6.0-8.6, total dissolved solid (TDS) of between 0.1 and 0.6 g/l, and major cation composition dominated by Ca and Na, whereas predominant anions are Cl and/or HCO3.The pH and the major-element composition of the waters are the factors affecting the concentration of REE in solution. The concentrations of ∑REE+Y in the samples filtered at 0.4 μm vary between 140 and 1600 ng/l, with La of between 14 and 314 ng/l, and Yb of between <6 and 12 ng/l. A negative Ce anomaly, especially marked at high pH, is observed in the groundwaters. The surface waters show lower REE concentrations, which are independent of pH, and negligible Ce anomaly.Speciation calculations, carried out with the EQ3NR computer program, showed that the complexes with the CO32− ligand are the dominant REE species at pH in the range of 6.7-8.6. The REE3+ ions dominate the speciation at pH <6.7 and only in the light REE (LREE).The relative concentrations of REE in water roughly reflect those in the aquifer host rocks. However, when concentrations of REE in water are normalised relative to the parent rocks, a preferential fractionation of heavy REE (HREE) into the water phase can be observed, suggesting the greater mobility and stability of HREE in aqueous solution.  相似文献   

18.
华北北部山西组煤系粘土岩稀土元素地球化学特征   总被引:1,自引:1,他引:0  
分别采用电感耦合等离子发射光谱(ICP-AES)和中子活化法(INAA)对采自河北兴隆、天津蓟玉、山西大同和阳泉、内蒙老窝铺以及宁夏石嘴山和石炭井二叠系山西组的11个粘土岩样品进行了稀土元素含量测定。结果表明:研究样品的稀土元素含量变化很大,ΣREE最低49.43 mg/kg,最高478.66 mg/kg,平均165.04 mg/kg,稍低于北美页岩(NASC)的173.21 mg/kg;ΣLREE/ΣHREE介于3.44~21.23之间,平均9.74,与NASC的7.68相近;多为中—重度Eu亏损以及弱—中度Ce负异常。研究样品不具有典型的沉积型泥岩特征,结合薄片鉴定及前人分析结果推论,粘土岩可能分别由中、基性及酸性(偏碱)火山碎屑物质原地蚀变形成,并不同程度地混有陆源沉积物。   相似文献   

19.
Geochemical and mineralogical studies were conducted on the 12-m-thick weathering profile of the Kata Beach granite in Phuket, Thailand, in order to reveal the transport and adsorption of rare earth elements (REE) related to the ion-adsorption type mineralization. The parent rock is ilmenite-series biotite granite with transitional characteristics from I type to S type, abundant in REE (592 ppm). REE are contained dominantly in fluorocarbonate as well as in allanite, titanite, apatite, and zircon. The chondrite-normalized REE pattern of the parent granite indicates enrichment of LREE relative to HREE and no significant Ce anomaly. The upper part of the weathering profile from the surface to 4.5 m depth is mostly characterized by positive Ce anomaly, showing lower REE contents ranging from 174 to 548 ppm and lower percentages of adsorbed REE from 34% to 68% compared with the parent granite. In contrast, the lower part of the profile from 4.5 to 12 m depth is characterized by negative Ce anomaly, showing higher REE contents ranging from 578 to 1,084 ppm and higher percentages from 53% to 85%. The negative Ce anomaly and enrichment of REE in the lower part of the profile suggest that acidic soil water in an oxidizing condition in the upper part mostly immobilized Ce4+ as CeO2 and transported REE3+ downward to the lower part of the profile. The transported REE3+ were adsorbed onto weathering products or distributed to secondary minerals such as rhabdophane. The immobilization of REE results from the increase of pH due to the contact with higher pH groundwater. Since the majority of REE in the weathered granite are present in the ion-adsorption fraction with negative Ce anomaly, the percentages of adsorbed REE are positively correlated with the whole-rock negative Ce anomaly. The result of this study suggests that the ion-adsorption type REE mineralization is identified by the occurrence of easily soluble REE fluorocarbonate and whole-rock negative Ce anomaly of weathered granite. Although fractionation of REE in weathered granite is controlled by the occurrence of REE-bearing minerals and adsorption by weathering products, the ion-adsorption fraction tends to be enriched in LREE relative to weathered granite.  相似文献   

20.
苏普特片麻岩为含堇青石、矽线石及红柱石等变质矿物的眼球状、条纹条带状黑云斜长片麻岩、黑云二长片麻岩、二云母片麻岩等。其SiO2含量为63.12%~68.34%,Al2O3为13.64%~15.91%,TiO2为0.538%~0.772%,TFe2O3为6%左右,MgO含量较高,多数样品为3%左右。根据其岩石化学特征判断,原岩为泥质岩、砂岩等沉积岩。苏普特片麻岩稀土元素含量较高,ΣREE为155.21×10-6~271.93×10-6,轻稀土元素富集,具中等程度的负Eu异常;在以北美页岩(NASC)标准化的稀土配分型式图上具有较平缓的配分曲线,无明显的Ce、Eu异常。依据地球化学特征,推断其原岩的形成环境为活动大陆边缘。苏普特片麻岩中锆石具有岩浆锆石特征,其U-Pb SHRIMP年龄为282 Ma,可能代表了早二叠世的一次构造-岩浆-混合岩化事件。  相似文献   

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