首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
古尔班通古特、腾格里和鄂尔多斯三个沙漠不同粒级组分的风成沙具有不同的εNd（0）和87Sr/86Sr 比值：古尔班通古特沙漠中分别为-3~-5.3 和0.712452~0.716895;腾格里沙漠中分别为-9.9~-12.3 和0.716753~0.723033;鄂尔多斯沙漠中分别在-13.1~-18 和0.714028~0.71678。三个沙漠不同的Sr-Nd 同位素特征,表明它们可能有各自的物质源区。Nd 同位素主要受各沙漠所处的地质、地貌单元所控制,Sr 同位素还要受气候变化诱导的矿物差异风化与粒度分选作用的影响。同位素组成在古尔班通古特、腾格里沙漠风成沙不同粒度组分中有相似的变化规律,但在鄂尔多斯沙漠中明显不同。鄂尔多斯沙漠风成沙粗、细颗粒组分有不同的物质来源,细颗粒组分,特别是<2μm 组分可能有沙漠区外远源的贡献,而其他沙漠各自的风成沙不同颗粒组分则来自相同的物源区,沙漠区外远源输入的可能性较小。这一研究对深入认识北方沙漠物质的成因机制有重要的理论意义。  相似文献   

2.
The modal and chemical composition of sands from Cox’s Bazar beach (CBB) and Kuakata beach (KB) areas of Bangladesh has been investigated to infer their maturity, chemical weathering, and provenance signatures. The CBB and KB sands are typically high quartz, low feldspar, and lithic fragments, representing a recycled orogen source. Major element compositions of CBB sands are characterized by high SiO2 (83.52–89.84 wt%) and low Al2O3 (4.39–6.39 wt%), whereas KB sands contained relatively low SiO2 (63.28–79.14 wt%) and high Al2O3 (9.00–11.33 wt%) contents. The major, trace and rare earth element (REE) compositions of beach sands display comparable distribution patterns with enriched Th and SiO2 for both sands relative to upper continental crust (UCC). Pb, Rb, Y, and Fe for KB sands are little higher than UCC and the rest of the elements are marked depleted for both suites reflecting destruction of plagioclase and K-feldspar during fluvial transportation. The CBB and KB sands are compositionally low mature to immature in nature subsequently classified as subarkose and litharenite, respectively. Chondrite-normalized REE patterns for CBB and KB sands show LREE enrichment and nearly flat HREE (LaN/YbN, 7.64–9.38 and 5.48–8.82, respectively) coupled with prominent Eu anomalies (Eu/Eu*, 0.51–0.72 and 0.52–0.76, respectively), suggesting felsic source provenance. The provenance discrimination diagrams, immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr), and REE (∑LREE/HREE, Eu/Eu* and GdN/YbN) parameters indicate that CBB and KB sands were largely derived from felsic source rocks, with compositions close to average rhyolite, granodiorite, granite, and UCC.  相似文献   

3.
A pilot study with Holocene fluvial sands was undertaken in order to evaluate the effects of source rock composition and climate on natural abundances of rare elements (REE) in the first leg of the sedimentary cycle. We have analysed the medium grained sand fraction of samples collected from first order streams exclusively draining granitic plutons in Montana (semi-arid), Georgia (humid), and in South Carolina (humid). Despite compositional differences between parent plutons the REE distribution patterns (but not the total absolute abundances) of the daughter sands are very similar. Averages of the three areas have a La/Lu ratio of ～ 103 showing a depletion of heavy REE with respect to an ‘average granite’ (La/Lu = 79) or the composite of North American Shales (NAS; La/Lu = 55). Also, the Eu/Sm ratio in sands from these areas is ～ 0·22 which is very close to the NAS ratio of 0·21. However, the overall REE distribution of these sands is not similar to that of NAS in any way. We conclude that the major rock type, but neither its minor subdivisions nor the climate (in the source area), controls the REE distribution patterns in first cycle daughter sands. But, the total and the parent rock-normalized abundances of REE in sands from the humid areas are much lower than those in sands from the arid areas.  相似文献   

4.
Robert Cullers 《Lithos》1988,21(4):301-314
A series of soil and stream sediments developed during intense weathering on the metaluminous Danburg granite, northeastern Georgia, U.S.A., have been analyzed mineralogically and chemically. The concentrations of Ba, Na, Rb and Cs in the silt and coarser fractions are controlled mainly by feldspars and biotite. Hf is controlled by zircon, and the REE (rare-earth elements) and Th are largely controlled by sphene. Variations in feldspar, sphene and zircon may produce small variations in Eu/Sm and La/Lu ratios. Ferromagnesian minerals control Ta, Fe, Co, Sc and Cr concentrations.

The mineralogical and chemical composition of the Danburg granite is more closely reflected in the silt than in the sand or gravel fractions of stream sediments. In the silt, the contents of Rb, REE, Th, Ta, Fe, Co and Sc and the ratios of La/Sc, Th/Sc, La/Co, Th/Co, Eu/Sm and La/Lu are similar to those in the unweathered granite. In contrast, these element contents or ratios in the sands and gravels are 0.05−3× the concentration in the unweathered granite. Ta and Ba contents are an exception to the above. The Ta and Ba contents of the sands and gravels are similar to those of the granite.

In the kaolinite-halloysite clays, the content of Na is depleted relative to the source. Rb, Cs, Ba, Hf and Ta are depleted or enriched in the clays relative to the source, while the REE, Th, Fe, Co, Sc and Cr are enriched. The Eu/Sm (Eu anomaly size) and La/Lu ratios, and the REE patterns of the clays are similar to those of the source.

Thus, the mineralogy and element contents of a siltstone developed from metaluminous, granitic sources during intense weathering would be expected to be more similar to the source rock than the sandstones and conglomerates. Claystones should contain similar REE patterns and Eu/Sm ratios as the source rock, but such fine-grained sediments might represent much larger areas of source rocks than the more locally derived sandstones or conglomerates.  相似文献   


5.
The Asian Monsoon forms an important part of the earth’s climate system, yet our understanding of the past interactions between its different sub-systems, the East Asian and Indian monsoons, and between monsoonal winds and other prevailing wind currents such as the Westerly jet, is limited, particularly in central Asia. This in turn affects our ability to develop climate models capable of accurately predicting future changes in atmospheric circulation patterns and monsoon intensities in Asia. Provenance studies of mineral dust deposited in terrestrial settings such as peat bogs can address this problem directly, by offering the possibility to examine past deposition rates and wind direction, and hence reconstruct past atmospheric circulation patterns. However, such studies are challenged by several issues, most importantly the identification of proxies that unambiguously distinguish between the different potential dust sources and that are independent of particle size. In addition, a single analytical method that is suitable for sample preparation of both dust source (i.e. desert sand, soil) and receptor (i.e. dust archive such as peat or soil profiles) material is desirable in order to minimize error propagation derived from the experimental and analytical work. Here, an improved geochemical framework of provenance tracers to study atmospheric circulation patterns and palaeomonsoon variability in central Asia is provided, by combining for the first time mineralogical as well as major and trace elemental (Sc, Y, Th and the rare earth elements) information on Chinese (central Chinese loess plateau, northern Qaidam basin and Taklamakan, Badain Juran and Tengger deserts), Indian (Thar desert) and Tibetan (eastern Qinghai-Tibetan Plateau) dust sources.Quartz, feldspars and clay minerals are the major constituents of all studied sources, with highly variable calcite contents reflected in the CaO concentrations. Chinese and Tibetan dust sources are enriched in middle REE relative to the upper continental crust and average shale but the Thar desert has a REE signature distinctly different from all other dust sources. There are significant differences in major, trace and REE compositions between the coarse and fine fractions of the surface sands, with the finest <4 μm fraction enriched in Al2O3, Fe2O3, MnO, MgO and K2O and the <32 μm fractions in Sc, Y, Th and the REE relative to the coarse fractions. The <4 μm fraction best represents the bulk REE geochemistry of the samples. The provenance tracers Y/∑REE, La/Er, La/Gd, Gd/Er, La/Yb, Y/Tb, Y/La, Y/Nd and to a certain extent the europium anomaly Eu/Eu∗ (all REE normalized to post-Archean Australian shale, PAAS) are particle size-independent tracers, of which combinations of Y/∑REE, La/Yb, Y/Tb, Y/La and Eu/Eu∗ can be used to distinguish the Thar desert, the Chinese deserts, the Chinese loess plateau and the Tibetan soils. Their independence upon grain size means that these tracers can be applied to the long-range provenance tracing of Asian dust even when only bulk samples are available in the source region. Combinations of La/Th, Y/Tb, Y/∑REE, Sc/La and Y/Er distinguish the Tibetan soils from the Chinese loess plateau and the Chinese deserts. La/Th and notably Th/∑REE isolate the signature of the Badain Juran desert and the combination of Sc/La and Y/Er that of the Taklamakan desert. The similarity in all trace and REE-based provenance tracers between the northern Qaidam basin and Tengger desert suggests that these two deposits may have a common aeolian source.  相似文献   

6.
Tracing sources of aeolian sediment is key to reconstructing earth surface processes in arid areas and interpreting the paleoenvironmental significance of aeolian sequences. However, the difference of geochemical characteristics between different fractions of sediments in the Yarlung Zangbo River Basin (YZRB) is still unclear, so we want to identify which fraction of sediments is more suitable for tracing sediment sources. Considering the long distance between different wide valleys in the YZRB, we wonder whether there is any difference between these wide valleys in terms of geochemical characteristics of sediments. Forty-three surface sediment samples in the YZRB are collected, and the grain-size distributions and the major-element composition for 37 samples and the trace and rare earth element composition for the coarse (75–500 μm) and fine (<?75 μm) fractions of all samples are determined. The results reveal the following: (1) the fine fractions of the deposits contain more environmental information, suggesting that the fine fractions cannot be directly compared between different climate zones for provenance identification and that appropriate coarse fractions, based on the grain-size distribution of the targeted sediments, are more ideal for tracing sediment sources; and (2) geochemical characteristics of various sediment types show spatial heterogeneity. The coarse and fine fractions of the loose sediment samples can be divided into two regional groups based on geochemical characteristics: the Maquanhe zone in the upper reach, the Xigaze, Shannan and Mainling zone in the middle reach, which is consistent with the geological background of the YZRB; (3) aeolian deposits in the YZRB are a local origin and predominantly derived from the adjacent loose sediments, and fluvial sediments in the upper reach contribute little to the aeolian sands in the middle reach.  相似文献   

7.
Concentrations of rare-earth elements (REE), Sc, Fe, Co, Cr, Na and Ir in the bulk sample and mineral separates of the isotopically unique Allende inclusion, EK 1-4-1, were determined by instrumental neutron activation analysis. REE concentrations were also determined by mass-spectrometric isotope dilution for two density separates.The inclusion showed enrichment of light REE over heavy REE with a positive Yb anomaly, thus showing a tendency to resemble group II fine-grained inclusions in REE abundances, although EK 1-4-1 is a coarse-grained inclusion. High Cr concentrations also indicate group II affinity. However, high Ir (6 ppm) and Sc (105 ppm), and low FeO (1.80%), and Co (13.8 ppm) concentrations in the bulk sample and low Na concentrations in mineral separates show group I affinity.Between melilite and pyroxene fractions, the same samples in which mass-fractionated O isotope ratios were observed (Clayton and Mayeda, 1977), REE, Sc, Co and Fe showed distributions which are substantially different from those observed for “ordinary” Allende type B inclusions. These two minerals do not appear to be in equilibrium with respect to trace element distributions.The results indicate that the isotopically unique EK 1-4-1 is also unusual in its elemental abundances and distributions.  相似文献   

8.
Nine fine-grained feldspathoid-, grossular-, spinel-, pyroxene-bearing inclusions from the Allende meteorite were analysed by instrumental neutron activation analysis. On the average, these inclusions are enriched in the refractory lithophile elements Ca, Sc, Ta and the rare earths by factors of 5–30 relative to Cl chondrites but are depleted in the refractory and volatile siderophiles, Ir, Co and Au. The volatile elements Fe, Cr and Zn are present at levels of 3.38–8.51%, 326–2516 ppm and 308–1376 ppm, respectively. Textural, mineralogical and chemical data suggest that the fine-grained inclusions formed in the solar nebula by the simultaneous condensation of volatiles and refractory lithophile elements which failed to condense into the coarse-grained, high-temperature condensate inclusions. The marked differences in the enrichment factors for different refractories in the fine-grained inclusions are caused by relatively small differences in their accretion efficiencies into the coarse-grained ones. The trace element data indicate that the refractories in the fine- and coarse-grained inclusions can only be the cosmic complements of one another if the fine-grained ones represent no more than ~ 20% of the most abundant refractory elements.  相似文献   

9.
Geochemical and isotopic studies of aeolian sediments in China   总被引：5，自引：0，他引：5  
The Sr and Nd isotopic, rare earth element (REE) and major element compositions, together with mineral and grain‐size proportions, are reported for aeolian loess deposits and desert sands from several Chinese localities. The study was carried out in order to examine regional variations in the isotopic and geochemical features of these aeolian sediments, and to constrain the provenance of Chinese loess. Samples include loesses from the Tarim and Junggar basins and desert sands from the Taklimakan desert in north‐west China, loess from the Ordos area and desert sands from the Tengger and Mu‐us deserts in north‐central China, as well as loess and desert sands from the Naiman area, north‐east China. REE distributions show minimal variation among the Chinese loess deposits, whereas those for the desert sands show regional variations. New isotopic data document a latitudinal variation in Sr and Nd isotopic features for the loesses and desert sands. The Naiman and Junggar loesses have distinctly lower 87Sr/86Sr ratios and higher εNd(0) values than the loesses from the Tarim Basin, the Ordos area and the Loess Plateau. Among the desert sands, the Naiman samples have higher εNd(0) values than the Taklimakan, Tengger and Mu‐us samples. Isotopic data suggest that loesses of the Loess Plateau were supplied from the Tarim Basin loesses and Taklimakan Desert sand, and that the Naiman loesses were supplied from the Junggar Basin loesses. The latitudinal variation in the loesses and desert sands may be partly explained by isotopic variations reported previously for moraines from the Tianshan and west Kunlun Mountains, which are possible sources for the loesses and desert sands. These inferences on the provenance of the loesses and desert sands are consistent with the dust transport pattern over East Asia.  相似文献   

10.
Oil‐source correlation studies have demonstrated that the crude oils in the Ordos Basin were mainly derived from organic‐rich lacustrine mudstones of the Yanchang Formation. The sedimentology, petrology and organic geochemistry of these mudstones have been studied intensively, but their trace and rare earth element (REE) characteristics have received little attention. In this paper, we present trace and rare earth element data of the Upper Triassic Yanchang Formation mudstones in the southern Ordos Basin to constrain the palaeoenvironment, provenance and depositional setting. Our results show that the REE and trace element concentrations of the Yanchang Formation mudstones are higher than those of the upper continental crust (UCC). The Sr contents and Sr/Ba and Y/Ho ratios of these mudstones indicate the absence of a marine transgression during the sedimentation of the Upper Triassic mudstones. The depositional environment of the Upper Triassic mudstones was slightly oxic as evidenced by the values of Eu/Eu*, Ce/Ce*, Ceanom, δU, U/Th, V/Cr and Ni/Co. The UCC‐normalized distribution pattern of REEs, spider diagrams, the ratios of related elements, the bivariate diagrams of Th/Sc–Zr/Sc and La/Th–Hf and the ternary plots of La–Th–Sc and Th–Sc–Zr/10 signify that the provenances of the Chang9–7 mudstones were mainly derived from a continental island arc, whereas the provenances of the Chang6–3 mudstones were mainly derived from a mixture of continental island arc and active continental margin, and the latter contain less recycled materials. Combined with the previous studies of detrital zircon dating and petrography of the Yanchang Formation sandstones in the southern Ordos basin, we propose that the Qinling orogenic belt served as one of the primary source regions occurring between the Chang7 and Chang6 periods, corresponding to the initial uplift of the west Qinling Mountains due to the collision between the Yangtze and North China blocks. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

11.
Rare earth elements in modern coral sands: an environmental proxy   总被引：1，自引：0，他引：1  
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.  相似文献   

12.
依据Stoke定律将长江小于63 μm的沉积物分成4个粒级.将黄河小于63 μm的沉积物分成6个粒级.采用ICP-MS法分别测试了分粒级沉积物的REE含量,结果显示：相同粒级中长江沉积物的∑REE均高于黄河沉积物.长江沉积物REE的丰度遵循元素的“粒度控制律”,即随粒度变细∑REE含量依次增高;黄河沉积物∑REE呈“高-低-高”的不对称马鞍型分布;北美页岩标准化分布曲线均呈右倾状,轻重稀土分馏明显,相对富集LREE,具弱Ce亏损,明显的Eu正异常.长江与黄河沉积物REE组成特征差异与两条河流流域的风化作用及沉积物的矿物组成密切相关,黄河∑REE的马鞍型分布是细粒级中黏土矿物吸附及粗粒级中相对高含量的重矿物富集作用的结果,而长江沉积物随粒级增大∑REE的衰减趋势主要是随粒级增大逐步增加的石英和长石含量的稀释作用所造成.  相似文献   

13.
中国北方东部沙质荒漠化的地学观   总被引：8，自引：2，他引：8  
中国北方东部的土地荒漠化主要受新构造运动以来的权造－沉积特征、青藏高原隆升所引起的全球气候变化、尤其是亚洲季风等地质背景控制。北方东部沙质荒漠化区域地质分类，按其成因可分为就地起沙型、风沙型和土地粗粒化型等3种类型。其中，就地起沙型沙质荒漠化主要分布在东部毛乌素、科尔沁、浑善达克、呼伦贝尔等沙地内，流经这些沙地的古河道，广泛发育河湖相砂质沉积物以及以砂质沉积物为母质的土壤分布区。风沙型沙质荒漠化主要分布在上述沙地的边缘地带，如毛乌素沙地以南的覆沙黄土区等。土地粗化主要分布在内蒙古锡林郭勒草原－河北坝上草原－河北坝上草原，其土壤类型多为含砾沙土，其土壤母质为基岩风化壳、残坡积或冲洪积物。荒漠化发展趋势上，尽管就地起沙型沙质荒漠化在毛乌素、科尔沁和浑善达达沙地的局部地区均有所逆转，但总体上，在北方广大地区，主要以地表土壤粗化为主的荒漠化和风沙型荒漠化等均有不同程度的扩大。上述不同类型的沙质荒漠化，总体上受地背景控制。其中，第四纪以来的构造－沉积特征控制着不同类型荒漠化的区域分布，气候变化控制着荒漠化的进程，人类活动则是在上述背景的基础上进行。  相似文献   

14.
地表松散沉积物中不同粒级的颗粒蕴含着不同的信息,这种差异与其物质来源、搬运和沉积过程中的分选作用以及沉积后的风化作用等有关。因此,可根据沉积物中不同粒级颗粒地球化学特征与矿物组成的差异,进行沉积物物源追踪或沉积环境重建。选择青藏高原在气候和地貌方面具有典型意义的柴达木盆地、错那湖、雅鲁藏布江流域作为研究区,分别采集了这几个地区的沙丘沙、黄土、湖积物、冲积物、洪积物、残坡积物等地表松散沉积物样品,用干筛法将沉积物样品分为两个部分,粗颗粒部分粒径范围为≥75~500 μm,细颗粒部分粒径<75 μm。对上述粗、细颗粒分别进行稀土与微量元素、Sr-Nd同位素、轻矿物组成的实验室分析测试,并对沉积物中粗、细颗粒的测试结果采用古典多维尺度方法进行相似性分析。结果表明：青藏高原松散沉积物中粗、细颗粒在轻矿物组成、微量和稀土元素含量、稀土特征、元素参数等方面存在差异;细颗粒部分蕴含了更多的环境信息,富含容易受化学风化影响的黏土矿物以及硬度较小、容易被磨蚀的方解石等矿物。在物源示踪中不同气候区的沉积物细颗粒不宜进行直接比较,应根据样品的粒度分布情况优先选择合适的粗颗粒组分进行物源示踪。  相似文献   

15.
Pennsylvanian strata of the Hazard coalfield, Eastern Kentucky, contain fluvial, upper-delta plain facies characterized by thick localized coals, bay-fill shales, levee silt- and sandstones, and channel-fill sands and gravels. Although the deltaic nature of these sediments has long been established, mining and exploration activities in the district require a thorough understanding of small areas within the delta environment. Coal quantity and quality trends in the Hazard #7 seam, the major producer in the area, have been examined in detail.The #7 coal accumulated in a peat swamp restricted laterally by a major fluvial channel. Three types of non-coal parting are recognized. Thin, tabular, fine-grained partings resulted from periods of increased terrigenous influx into the swamp. Lenticular crevasse-splay deposits locally split the coal. Wedge-shaped, fine- to coarse-grained partings, of probable levee origin, are found along the channel margin. Post-swamp deposits consist of thick bay-fill shales, thin shales, silt- and sandstones deposited on floodplains, and channel-fill sandstones and gravels.The modeling technique discussed provides guidance for mine development and regional exploration by prediction of coal seam quantity and quality trends from local geologic features. The thickest #7 coal is split by thin tabular partings and is overlain by thick shale sequences. Coal overlain by silt and sandstone is thinner and unsplit, and typically of higher heat value. Regional seam thinning due to channel scour is recognized. Factors which control the configuration of the coal seam include position within the peat swamp, proximity to the fluvial channel, swamp burial processes, and paleochannel sinuosity.  相似文献   

16.
《International Geology Review》2012,54(16):2016-2029
The Salmas area, in the northernmost part of the Sanandaj–Sirjan zone of Iran, contains a crystalline mafic–intermediate complex that intrudes into the Precambrian metamorphic basement complex and is composed of gabbroic and gabbrodiorite cumulates and fine-grained non-cumulate gabbronorites and diorites. These rocks have fine- to coarse-grained texture and are mainly composed of plagioclase, pyroxenes, and amphibole. Major element geochemistry indicates that the pluton has a low-K with tholeiitic affinity. Variations of major and trace elements on Harker diagrams, including negative correlations MgO, Fe2O3, CaO, and Co and positive correlations Na2O, K2O, Rb, Ba, and La, with increasing SiO2 and chondrite-normalized REE patterns, suggest that fractional crystallization of gabbroic rocks could have played a significant role in the formation of evolved rocks. The chondrite-normalized REE patterns are not fractionated (LaN /LuN = 1.3–5.4) and display strong Eu anomalies (Eu/Eu* = 1.15–1.76) in cumulate rocks, which we attributed to cumulus plagioclase. Sr and Nd isotopic ratios vary from 0.704698 to 0.705866 and from 0.512548 to 0.512703, respectively. Gabbronorites with high 143Nd/144Nd ratios, low 87Sr/86Sr ratios, and high MgO, Ni, and Cr contents indicate that they were generated from relatively primitive magmas. We used petrogenetic modelling to constrain sources. Trace element ratio modelling indicates that the gabbroic rocks were generated from a spinel-peridotite source via 5–20% degrees of fractional melting at a depth of ～52 km. Major and REE modelling shows that the diorites are the products of fractional crystallization of gabbronorites.  相似文献   

17.
The petrology of first cycle (unmodified) and second cycle (reworked) sand at the termini of eleven valley glaciers eroding coarse- to fine-grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands. First cycle sands are coarse- to medium-grained, poorly sorted, fine-skewed, non-modal lithic sands with an average composition of Q21F6L73. Grain-size distributions do not vary with composition or source rock types, although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources. By contrast, second cycle sands are medium-grained, poorly sorted, fine- to coarse-skewed arkosic to lithic sands with an average composition of Q19F40L41, and contain fewer lithic fragments than do first cycle sands. We propose that the origin, mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness, and the particular stress field of the transporting medium. As a result of abrasion, glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain-size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock. Upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point-loading during saltation in addition to shear stress. Point-loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness, so the lithic fragments break into smaller monomineralic grains and/or lithic fragments. Lithic fragments of very coarse sand-size are abundant in first cycle sands, thus refuting, at least for glacial sands, the widely held view that grains of this size are deficient in nature. The presence of fewer grains of this size in second cycle sands indicates that very coarse sand-size lithic fragments can survive recycling, but in greatly reduced proportions due to breakage.  相似文献   

18.
In progressing from a granitoid mylonite to an ultramylonite in the Brevard shear zone in North Carolina, Ca and LOI (H2O) increase, Si, Mg, K, Na, Ba, Sr, Ta, Cs and Th decrease, while changes in Al, Ti, Fe, P, Sc, Rb, REE, Hf, Cr and U are relatively small. A volume loss of 44% is calculated for the Brevard ultramylonite relative to an Al–Ti–Fe isocon. The increase in Ca and LOI is related to a large increase in retrograde epidote and muscovite in the ultramylonite, the decreases in K, Na, Si, Ba and Sr reflect the destruction of feldspars, and the decrease in Mg is related to the destruction of biotite during mylonitization. In an amphibolite facies fault zone separating grey and pink granitic gneisses in the Hope Valley shear zone in New England, compositional similarity suggests the ultramylonite is composed chiefly of the pink gneisses. Utilizing an Al–Ti–Fe isocon for the pink gneisses, Sc, Cr, Hf, Ta, U, Th and M-HREE are relatively unchanged, Si, LOI, K, Mg, Rb, Cs and Ba are enriched, and Ca, Na, P, Sr and LREE are lost during deformation. In contrast to the Brevard mylonite, the Hope Valley mylonite appears to have increased in volume by about 70%, chiefly in response to an introduction of quartz. Chondrite-normalized REE patterns of granitoids from both shear zones are LREE-enriched and have prominent negative Eu anomalies. Although REE increase in abundance in the Brevard ultramylonites (reflecting the volume loss), the shape of the REE pattern remains unchanged. In contrast, REE and especially LREE decrease in abundance with increasing deformation of the Hope Valley gneisses. Mass balance calculations indicate that ≥95% of the REE in the Brevard rocks reside in titanite. In contrast, in the Hope Valley rocks only 15–40% of the REE can be accounted for collectively by titanite, apatite and zircon. Possible sites for the remaining REE are allanite, fluorite or grain boundaries. Loss of LREE from the pink gneisses during deformation may have resulted from decreases in allanite and perhaps apatite or by leaching ofy REE from grain boundaries by fluids moving through the shear zone. Among the element ratios most resistant to change during mylonitization in the Brevard shear zone are La/Yb, Eu/Eu*, Sm/Nd, La/Sc, Th/Sc, Th/Yb, Cr/Th, Th/U and Hf/Ta, whereas the most stable ratios in the Hope Valley shear zone are K/Rb, Rb/Cs, Th/U, Eu/Eu*, Th/Sc, Th/Yb, Sm/Nd, Th/Ta, Hf/Ta and Hf/Yb. However, until more trace element data are available from other shear zones, these ratios should not be used alone to identify protoliths of deformed rocks.  相似文献   

19.
江汉盆地沉积物微量元素特征与长江上游水系拓展   总被引：2，自引：0，他引：2       下载免费PDF全文
江汉盆地是长江出三峡后第一个大型卸载区,近2.77 Ma以来堆积了近300 m的碎屑沉积物,主要由河流相和湖沼相组成,形成了多个沉积旋回。选择江汉盆地中心位置的ZL钻孔,利用ICP-MS方法,展开微量元素组成分析,研究了新近纪以来江汉盆地沉积物物源的变化。结果表明,上新世以来微量元素化学组成的离散程度逐渐减小,且趋近上部陆壳平均值,可能反映了物源供应区范围的扩展过程。1.25 Ma B.P.以来,多种微量元素含量及Th/Sc、Co/Th、La/Sc、Cr/Cu等比值变化很小,显示长江可能形成于1.25 Ma B.P.以后。  相似文献   

20.
Concentrations of the REE, Sc, Co, Fe, Zn, Ir, Na and Cr were determined by instrumental neutron activation and mass spectrometric isotope dilution analysis for mineral separates of the coarseand fine-grained types (group I and II of Martin and Mason's classification) of the Allende inclusions.These data, combined with data on mineral/liquid partition coefficients, oxygen isotope distributions and diffusion calculations, suggest the following: (1) Minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements. On the other hand, differences in oxygen isotope distributions among minerals preclude a totally molten stage in the history of the inclusion. Group I inclusions were formed by rapid condensation (either to liquid or solid) in a supercooled solar nebula; extrasolar pyroxene and spinel dust were included but not melted in the condensing inclusions, thus preserving their extrasolar oxygen isotope composition. REE were distributed by diffusion during the subsequent heating at subsolidus temperatures; because oxygen diffuses much more slowly at these temperatures, the oxygen isotope anomalies were preserved. (2) The fine-grained (group II) inclusions were also formed by condensation from a super-cooled nebular gas; however, REE-rich clinopyroxene and spinel were formed early and REE-poor sodalite and nepheline were formed later and mechanically mixed with clinopyroxene and spinel to form the inclusions. The REE patterns of the bulk inclusions and the mineral separates are fractionated, indicating that REE abundances in the gaseous phase were already fractionated at the time of condensation of the minerals. (3) Pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage thus resetting the 26Al-26Mg chronometer.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号