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1.
Loess geochemistry generally reflects paleo-weathering conditions and it can be used to determine the average composition of the upper continental crust (UCC). In this study, major and trace element concentrations were analyzed on loess samples from southwestern Hungary to determine the factors influencing their chemical compositions and to propose new average loess compositions. All studied loess samples had nearly uniform chemical composition, suggesting similar alteration history of these deposits. Chemical Index of Alteration values (58–69) suggested a weak to moderate degree of weathering in a felsic source area. Typical non-steady state weathering conditions were shown on the Al2O3–CaO + Na2O–K2O patterns, indicating active tectonism of the Alpine–Carpathian system during the Pleistocene. Whole-rock element budgets were controlled by heavy minerals derived from a felsic magmatic or reworked sedimentary provenance. Geochemical parameters indicated that dust particles must have been recycled and well homogenized during fluvial and eolian transport processes.  相似文献   

2.
最近130ka来黄土物质化学风化的空间观察   总被引：1，自引：1，他引：0       下载免费PDF全文
高玲  丁仲礼 《第四纪研究》2008,28(1):162-168
选取黄土高原48个末次间冰期以来沉积剖面中的248个黄土、古土壤和弱发育土壤层的代表性样品,对它们的酸不溶物进行了主量元素的分析。结果表明,在已有的几个不受粒度影响的化学风化指标中,(CaO MgO Na2O)/TiO2比值对气候条件变化具有相对较高的敏感性,且受沉积速率的影响较小。在源区风化状况相对稳定的前提下,它是一个反映夏季风变化较为有效的指标。根据该指标在黄土高原上的空间等值线图,发现其从北到南间冰期的变幅大于冰期,这可能表明间冰期时夏季风降水在黄土高原的变化梯度要大于冰期。  相似文献   

3.
Surface water resources play a crucial role in the domestic water delivery system in Ghana. In addition, sustainable food production is based on the quality and quantity of water resources available for irrigation purposes to supplement rain-fed agricultural activities in the country. The objective of this research was to determine the main controls on the hydrochemistry of surface water resources in the southern part of Ghana and assess the quality of water from these basins for irrigation activities in the area. R-mode factor and cluster analyses were applied to 625 data points from 6 river basins in southern Ghana after the data had been log transformed and standardized for homogeneity. This study finds that surface water chemistry in the south is controlled by the chemistry of silicate mineral weathering, chemistry of rainfall, fertilizers from agricultural activities in the area, as well as the weathering of carbonate minerals. A Gibb’s diagram plotted with total dissolved solids (TDS) on the vertical axis against (Na+ + K+)/(Ca2+ + K+ + Na+) on the horizontal axis indicates that rock weathering plays a significant role in the hydrochemistry. Activity diagrams for the CaO–Na2O–Al2O–SiO2–H2O and CaO–MgO–Al2O3–SiO2–H2O systems suggest that kaolinite is the most stable clay mineral phase in the system. In addition, an assessment of the irrigation quality of water from these basins suggests that the basins are largely low sodium—low to medium salinity basins, delivering water of acceptable quality for irrigation purposes.  相似文献   

4.
Geochemical variations in stream sediments (n = 54) from the Mahaweli River of Sri Lanka have been evaluated from the viewpoints of lithological control, sorting, heavy mineral concentration, influence of climatic zonation (wet, intermediate, and dry zones), weathering, and downstream transport. Compositions of soils (n = 22) and basement rocks (n = 38) of the catchment and those of <180 μm and 180–2000 μm fractions of the stream sediments were also examined. The sediments, fractions, soils and basement rocks were analyzed by X-ray fluorescence to determine their As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, Fe2O3, TiO2, MnO, CaO, P2O5 and total sulfur contents. Abundances of high field strength and ferromagnesian elements in the sediments indicate concentration of durable heavy minerals including zircon, tourmaline, rutile, monazite, garnet, pyriboles, and titanite, especially in <180 μm fractions. The sediments show strong correlation between Ti and Fe, further suggesting presence of heavy mineral phases containing both elements, such as ilmenite and magnetite. The basement rocks range from mafic through to felsic compositions, as do the soils. The river sediments lack ultrabasic components, and overall have intermediate to felsic compositions. Elemental spikes in the confluences of tributary rivers and high values in the <180 μm fractions indicate sporadic inputs of mafic detritus and/or heavy minerals to the main channel. Al2O3/(K2O + Na2O) and K2O/Na2O ratios of the sediments and LOI values of the soils correlate well with the climatic zones, suggesting intense weathering in the wet zone, lesser weathering in the intermediate zone, and least weathering in the dry zone. Low Sr and CaO contents and Cr/V ratios in stream sediments in the wet zone also suggest climatic influence. Fe-normalized enrichment factors (EFs) for As, Pb, Zn, Cu, Ni and Cr in stream sediments in the main channel are nearly all <1.5, indicating there is no significant environmental contamination. The chemistry of the sediments, rocks and the soils in the Mahaweli River are thus mainly controlled by source lithotype, weathering, sorting, and heavy mineral accumulation.  相似文献   

5.
The distribution of chemical elements in the Weinan loess section shows that: (1) carbonate is the critical constituent affecting loess chemistry. The leaching of carbonates may cause “apparent” enrichment of some other constituents. (2) CaCO3, SiO2, FeO, MgO, K2O and Na2O are mobile while Al2O3, Fe2O3 and TiO2 are inert components. The mobility sequence may follow the order of CaCO3>FeO>MgO>Na2O>K2O>SiO2>Al2O3>TiO2>Fe2O3. (3) No obvious migration is noticed of iron, except changes in valence and forms, during the pedogenic process. Little iron has been supplied to paleosol from precipitation. (4) Carbonates and, to a lesser extent, iron are most sensitive to, and can be therefore used as good indicators of, environmental changes. This research project was financially supported by the National Natural Science Foundation of China (Grant No. 49672137).  相似文献   

6.
Precise Fe/Mn ratios and MnO contents have been determined for basalts from the Hawaiian shields of Ko’olau and Kilauea by inductively coupled plasma mass spectrometry. It is well known that the youngest Ko’olau (Makapu’u-stage) shield lavas define a geochemical endmember for Hawaiian lavas in terms of CaO and SiO2 contents and isotopic ratios of O, Sr, Nd, Hf, Pb, and Os. We find that their MnO content is also distinct. Despite the small range in MnO, 0.146 to 0.176 wt%, the precision of our data is sufficient to show that among unaltered Ko’olau lavas MnO content is correlated with Nd-Hf-Pb isotopic ratios, La/Nb and Al2O3/CaO elemental ratios, and contents of SiO2, MgO and Na2O + K2O adjusted for olivine fractionation. These trends are consistent with two-component mixing; one endmember is a SiO2-rich, MnO-, and MgO-poor dacite or andesite melt, generated by low degree (10-20%) partial melting of eclogite. Since this low-MgO endmember (dacite or andesite melt) has very low FeO and MnO contents, mixing of high Fe/Mn dacite or andesite melt with a MgO-rich picritic melt, the other endmember, does not significantly increase the Fe/Mn in mixed magmas; consequently, Ko’olau and Kilauea lavas have similar Fe/Mn. We conclude that the high Fe/Mn in Hawaiian lavas relative to mid-ocean ridge basalt originates from the high MgO endmember in Hawaiian lavas.  相似文献   

7.
The ancient weathered crust studied occurs on the razed surfaces of various physiographic periods at different altitudes in the Sichuan red basin. A typical profile of the ancient weathered crust can be divided into three weathered belts from the surface to the basement: the clastic clay belt (Belt I), the clastic breccia-clastic clay belt (Belt II), and the clastic breccia belt (Belt III). These belts are different in clay mineral composition, reflecting varying degrees of chemical weathering. Wide variations are recognized in the abundance of chemical elements in the profile of th ancient weathered crust. In comparison with the underlying purple parent rocks, Fe2O3 > A12O3 > TiO2 > SiO2 are relatively concentrated in the ancient weathered crust, while CaO>P2O5>MnO>Na2O>MgO>K2O are relatively depleted due to leaching in order of Belt I > II > III. The extent of chemical weathering in the ancient weathered crust in the Sichuan red basin is closely related to the time of formation of the razed surface. For this reason, the contents of A12O3 and Fe2O3 or their concentration values, and the leaching amounts of CaO, MgO, K2O, Na2O, and P2O5 are higher in the ancient weathered crust at high altitudes than those at low altitudes.  相似文献   

8.
The effect of fluorine and fluorine + chlorine on melt viscosities in the system Na2O-Fe2O3-Al2O3-SiO2 has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na2O + FeO) < (Al2O3 + Fe2O3)] to peralkaline [(Na2O + FeO) > (Al2O3 + Fe2O3)] were determined over a temperature range 560-890 °C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 108.8 to 1012.0 Pa s. The compositions are based on addition of FeF3 and FeCl3 to aluminosilicate melts with a fixed amount of SiO2 (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na2O-Fe2O3-Al2O3-SiO2 melts. Am. Mineral.91, 344-352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed.  相似文献   

9.
The Yarlung Tsangpo-Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo-Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo-Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km−2 yr−1 and the silicate cation flux more than 34 tons km−2 yr−1. In total, chemical weathering in this area consumes 15.2 × 105 mol CO2 km−2 yr−1, which is twice the Brahmaputra average. These data show that 15-20% of the total CO2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na+ and Cl− fluxes throughout southern Tibet, comprising more than 50% of all Na+ in some stream systems. Carbonate weathering generates 80-90% of all dissolved Ca2+ and Mg2+ cations in much of the Yarlung Tsangpo catchment.  相似文献   

10.
The structure of silicate melts in the system Na2O·4SiO2 saturated with reduced C-O-H volatile components and of coexisting silicate-saturated C-O-H solutions has been determined in a hydrothermal diamond anvil cell (HDAC) by using confocal microRaman and FTIR spectroscopy as structural probes. The experiments were conducted in-situ with the melt and fluid at high temperature (up to 800 °C) and pressure (up to 1435 MPa). Redox conditions in the HDAC were controlled with the reaction, Mo + H2O = MoO2 + H2, which is slightly more reducing than the Fe + H2O = FeO + H2 buffer at 800 °C and less.The dominant species in the fluid are CH4 + H2O together with minor amounts of molecular H2 and an undersaturated hydrocarbon species. In coexisting melt, CH3 - groups linked to the silicate melt structure via Si-O-CH3 bonding may dominate and possibly coexists with molecular CH4. The abundance ratio of CH3 - groups in melts relative to CH4 in fluids increases from 0.01 to 0.07 between 500 and 800 °C. Carbon-bearing species in melts were not detected at temperatures and pressures below 400 °C and 730 MPa, respectively. A schematic solution mechanism is, Si-O-Si + CH4?Si-O-CH3+H-O-Si. This mechanism causes depolymerization of silicate melts. Solution of reduced (C-O-H) components will, therefore, affect melt properties in a manner resembling dissolved H2O.  相似文献   

11.
The Haast and Clutha rivers drain opposing flanks of New Zealand’s Southern Alps. Major element analyses of grain size fractions (2–1 mm, 1 mm–355 μm, 355–63 μm, and <63 μm) from bedload sediments collected throughout the reach of each river suggest that weathering is strongly partitioned between the chemical weathering of carbonates and the physical weathering of silicates. Sand size fractions from both rivers are depleted in CaO (∼0.2–2.1 wt%) relative to source schists (∼3 wt% CaO), while silt fraction CaO concentrations range from 2–5 wt%. The depletion of CaO in the sediments is interpreted to be due at least in part to removal of carbonate during chemical weathering of the schist protolith in the soil zone. The observed covariance of CaO and P2O5 concentrations in all river sediment suggests that most CaO is bound in a combination of phosphate-bearing minerals such as apatite along with other heavy mineral phases with similar hydrodynamic properties (e.g. epidote). Chemical index of alteration (CIA) values for grain size fractions from both rivers are similar (Haast: 54–63, Clutha: 49–61) and do not systematically vary with grain size or sample location. Al2O3–CaO∗ + Na2O–K2O (A–CN–K) relationships suggest that CIA values are controlled by albite–muscovite mixing rather than feldspar weathering. Both A–CN–K relationships and modal mineralogical calculations from Clutha river samples indicate progressive downstream attrition of muscovite from coarser to finer grain size fractions. In contrast, Haast river sediments display less variable normative muscovite concentrations and no downstream enrichment/depletion trends.  相似文献   

12.
In this study, an assessment of the lithogenic concentrations of trace metals in soils and saprolite over basement rock units in Ibadan, SW-Nigeria is presented in respect of bedrock types and geochemical controls on the weathering-associated release of trace metals. Consequently, soil, weathered and fresh rock samples from the Precambrian Basement of SW Nigeria were collected from three different bedrock units within Ibadan metropolis and subjected to mineralogical and geochemical analyses. The analytical results revealed major proportions of oxides in the range of 18–20% Al2O3, 2–6% Na2O and 1–6% K2O for weathered profiles over granite-gneiss and pegmatite units, compared to 2–3% Al2O3, <0.5% Na2O and <1.0% K2O over schist-quartzite. For the trace elements, weathered profiles on granite-gneiss and schist-quartzite settings exhibit similar enrichment trends (enrichment factor, EF l) for most of the trace elements, unlike the pegmatite bedrock. However, enrichments are relatively greater in the top soil unit compared to the intermediate saprolite unit, especially for Pb, Ni, Zn, Cr, Co, Rb, Sr and Ba, a situation attributed to leaching and redistribution within the weathered profiles through pedogenetic process and percolating groundwater.Furthermore, the estimated weathering indices using Ruxton Ratio (RR = {SiO2/Al2O3}) and Chemical Index of Alteration (CIA = 100{Al2O3/[Al2O3 + CaO + Na2O + K2O]}) revealed RR of 2.9–3.7 and CIA of 54–73% for granite-gneiss and pegmatite units, implying medium levels of weathering, compared to RR of 30.8–35.5 and CIA of >60% for schist-quartzite units, which suggest weak chemical weathering. Also, the estimated high percentage loss, especially for Pb, Rb, Sr, Ba relative to the bedrocks, shows that the trace elements can be mobilized within the weathering profiles even at a low degree of chemical weathering. Such weathering-induced release of trace metals is of environmental significance as natural lithogenic input sources and as background reference for future monitoring of possible human/anthropogenic impacts.  相似文献   

13.
Conventional graphical and statistical methods were used with water quality indices to characterize the hydrochemistry of groundwater from the northern part of the Volta region of Ghana. The objective was to determine the processes that affect the hydrochemistry and the variation of these processes in space among the three main geological terrains: the Buem formation, Voltaian System and the Togo series that underlie the area, and to determine the suitability of groundwater from the area for drinking purposes. The Q-mode cluster analysis reveals three main water groups. The groups established from the Q-mode HCA appear to indicate different degrees of weathering which could further indicate varying levels of fracturing aquifer hydraulic properties. R-mode HCA and factor analysis (using varimax rotation and Kaiser Criterion) were then applied to determine the significant sources of variation in the hydrochemistry. This study finds that groundwater hydrochemistry in the area is controlled by the weathering of silicate and carbonate minerals, as well as the chemistry of infiltrating precipitation. Mineral activity diagrams for the CaO–Na2O–Al2O3–SiO2–H2O and CaO–MgO–Al2O3–SiO2–H2O systems plotted for the area indicate stability in the smectite field and attribute hydrochemistry to the weathering of silicate minerals. Silicate mineral weathering and the effects of precipitation appear to be pervasive among all the three main geological terrains, whereas carbonate weathering is localized among the Voltaian aquifers. Cation exchange does not appear to play a significant role in the hydrochemistry but mild Water quality indices (WQI) were calculated for the samples using the concentrations of Na+, Ca2+, Mg2+, Cl−, NO3 −, F−, and EC at the various sample locations. The WQI values indicate that groundwater from the study area is of excellent quality for drinking purposes. WQI values from groundwater samples are averagely higher than samples taken from surface water sources in the area. This implies that geology has had an impact on the WQI of groundwater in the area.  相似文献   

14.
The 2.7 Ga Kambalda Sequence comprises a mafic to ultramafic dominated volcanic rock sequence of the Kalgoorlie Terrane, Yilgarn Craton, Western Australia. The Sequence is divided into Lower and Upper Units separated by the Kambalda Komatiite Formation. Five basalt suites of the Lower Unit are tholeiitic where MgO spans 5-10 wt.% MgO, with minor assimilation-fractional crystallization (AFC), whereas six volcanic suites identified in the Upper Unit are tholeiitic to komatiitic-basalts with MgO 24-5 wt.% having generally greater degrees of AFC. Upper suites plot at Al2O3/TiO2 (17-26) close to the primitive mantle ratio of 21, and Pt + Pd (19-31 ppb), whereas the PGE-depleted Lower basalts plot at generally lower Al2O3/TiO2 (<16) and Pt + Pd (<10 ppb). Most suites have an average Pt/Pd ratio of 1.11, despite large variations in MgO contents, broadly consistent with the Pt/Pd ratio in the primitive mantle. On primitive mantle-normalised PGE plots, Upper suites generally display less fractionated patterns of the IPGE (Os, Ir, Ru and Rh) from the PPGE (Pt and Pd) relative to the Lower basalts. Most suites exhibit patterns with positive slopes reflecting relative enrichment of Pd, Pt, Au and Cu relative to Ni and IPGE. In suites of both Units, the concentrations of Ir and Ru fall with decreasing MgO contents, indicating their broadly compatible behaviour during magmatic evolution that involved AFC. Platinum and Pd behave as incompatible elements in the high-MgO suites, whereas Pt and Pd behave compatibly during crystallisation of the Lower basalt magmas, an interpretation consistent with progressively higher Cu/Pt and Cu/Pd ratios at decreasing MgO contents, and with falling Pt/Ti, collectively due to sulphur saturation induced by AFC as recorded in an antivariance of Pd/Ir with Nb/Th, a monitor of AFC.Collectively, the data suggest that several of the Lower Basalt suites crystallised under sulphide-saturated conditions, whereas most of the Upper Basalt Sequences remained sulphur undersaturated during magmatic evolution. Alteration, and fractional crystallisation of silicate and oxide phases, can be ruled out as factors governing PGE distribution in these mafic-ultramafic suites. Instead, the data suggest that discrete PGE-bearing phase (s) fractionated from the magmas. Such phases could be platinum group minerals (PGM; e.g., laurite) and/or alloys, or discrete PGE-rich nuggets or sulphides.  相似文献   

15.
The Hong (Red) River drains the prominent Red River Fault Zone that has experienced various tectonic activities—intrusion of magma, exhumation of basement rocks, and influx of thermal waters—associated with the Cenozoic collision of India and Eurasia. We report dissolved major element and Sr isotope compositions of 43 samples from its three tributary systems (Da, Thao/Hong main channel, and Lo) encompassing summer and winter seasons. Carbonic acid ultimately derived from the atmosphere is the main weathering agent, and sulfuric acid from pyrite oxidation plays a minor role. Seasonality is manifested in higher calcite saturation index and Mg/TZ+ and lower Ca/Mg in summer, suggesting calcite precipitation, and in higher Si/(Na∗ + K) ratios in summer suggesting more intensive silicate weathering. We quantified the input from rain, evaporite, carbonate, and silicate reservoirs using forward and inverse models and examined the robustness of the results. Carbonate dissolution accounts for a significant fraction of total dissolved cations (55-97%), and weathering of silicates makes a minor contribution (1-40%). Our best estimate of the spatially averaged silicate weathering rate in the Hong basin is 170 × 103 mol/km2/yr in summer and 51 × 103 mol/km2/yr in winter. We tested for correlations between the rate of CO2 consumption by silicate weathering and various climatic (air temperature, precipitation, runoff, and potential evapotranspiration) and geologic (relief, elevation, slope, and lithology) parameters calculated using GIS. Clear correlations do not emerge (except for ?CO2 and runoff in winter) which we attribute to the complex geologic setting of the area, the seasonal regime change from physical-dominant in summer to chemical-dominant in winter, and the incoherent timescales involved for the different parameters tested.  相似文献   

16.
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17.
Various iron-bearing primary phases and rocks have been weathered experimentally to simulate possible present and past weathering processes occurring on Mars. We used magnetite, monoclinic and hexagonal pyrrhotites, and metallic iron as it is suggested that meteoritic input to the martian surface may account for an important source of reduced iron. The phases were weathered in two different atmospheres: one composed of CO2 + H2O, to model the present and primary martian atmosphere, and a CO2 + H2O + H2O2 atmosphere to simulate the effect of strong oxidizing agents. Experiments were conducted at room temperature and a pressure of 0.75 atm. Magnetite is the only stable phase in the experiments and is thus likely to be released on the surface of Mars from primary rocks during weathering processes. Siderite, elemental sulfur, ferrous sulfates and ferric (oxy)hydroxides (goethite and lepidocrocite) are the main products in a water-bearing atmosphere, depending on the substrate. In the peroxide atmosphere, weathering products are dominated by ferric sulfates and goethite. A kinetic model was then developed for iron weathering in a water atmosphere, using the shrinking core model (SCM). This model includes competition between chemical reaction and diffusion of reactants through porous layers of secondary products. The results indicate that for short time scales, the mechanism is dominated by a chemical reaction with second order kinetics (k = 7.75 × 10−5 g−1/h), whereas for longer time scales, the mechanism is diffusion-controlled (DeA = 2.71 × 10−10 m2/h). The results indicate that a primary CO2- and H2O-rich atmosphere should favour sulfur, ferrous phases such as siderite or Fe2+-sulfates, associated with ferric (oxy)hydroxides (goethite and lepidocrocite). Further evolution to more oxidizing conditions may have forced these precursors to evolve into ferric sulfates and goethite/hematite.  相似文献   

18.
合肥下蜀土地球化学特征及其古气候意义   总被引：1，自引：0，他引：1  
合肥地处长江中下游北岸,下蜀黄土广泛分布,通过对合肥地区BK2钻孔剖面岩性、氧化物含量及其地球化学风化参数的分析,研究合肥地区风成沉积物地球化学元素特征及其气候变化规律。结果表明:野外钻孔岩芯剖面显示地层较好连续性,结合年龄数据划分了该孔的第四纪沉积框架,Qh底界为1.20 m,Qp3底界为5 m,5~35.10 m属Qp2中晚期地层;合肥下蜀土主要化学成分（SiO2+Al2O3+Fe2O3）的平均含量之和达88.99%,这种显著的富硅铝铁现象表明了该区气候较为湿润;化学风化程度较强,下蜀土的CIA平均值及其脱Ca、Na、K的程度均大于洛川黄土,说明其堆积期古气候比同期堆积的洛川气候温湿,较宣城干凉,与南京、镇江气候较为接近;近0.5 Ma以来总体经历由湿热-干冷的变化,大致可分为35~14.50 m、14.50~4.50 m、4.50~1.20 m和1.20~0 m四个阶段,气候变化由温暖湿润-温暖偏干-冷凉偏干-温暖湿润,显示了区内更新世中晚期以来的气候变迁具有全球一致性特征。   相似文献   

19.
The origin of chemical and mineralogical heterogeneity in tens-of-microns wide layers and domains in enderbite-hosted couple-of-centimeters wide pseudotachylite vein is examined based on the results of BSE and X-ray element imaging, and electron probe microanalyses of major elements of host-rock minerals, clasts, microphenocrysts, and pseudotachylite matrix. The pseudotachylite layers and domains containing variable proportions of orthopyroxene and magnetite microphenocrysts continue as mantles around quartz, K-feldspar, plagioclase and garnet clasts. The clasts are chemically modified along margins and intra-clast pseudotachylite injections. The chemical modifications are extensive in smaller clasts <5 μm diameter. At least three chemically distinct layers and domains in the pseudotachylite, and their fine-grained matrices, plot in sharply defined, well-segregated and non-overlapping fields in FeO + MgO−Al2O3–SiO2, FeO–CaO–MgO and CaO–Na2O–K2O and FeO vs. FeO/FeO + MgO diagrams. The compositions of the layers and domains—smeared between a feldspar + quartz component and a ferromagnesian component of garnet + Fe–Ti oxides (±orthopyroxene)—possibly correspond to fractionated quenched melts, or admixtures of microphenocrysts that cannot be resolved by the microbeam techniques employed. The compositional variations are incompatible with deformation-driven crystal fractionation in melt. Instead the layers and domains possibly are crystal-melt mushes produced by syn-deformation ultra-high temperature (1,250–1,375°C) melting reactions involving variable proportions of host-rock minerals determined by time-transient local phase aggregates experiencing strain. The similar element variation trends in pseudotachylite examined here and those reported from anorthosite, metapelite and charnockite elsewhere suggests local phase aggregate controlled multi-reaction melting is a phenomenon commoner than hitherto realized in pseudotachylites.  相似文献   

20.
The major, trace and rare earth element (REE) contents of metapelite (MPL), metapsammite (MPS) and metamarl (MM) samples from the Cambro-Ordovician Seydi?ehir Formation were analyzed to investigate their provenance and tectonic setting. The MPS, MPL, and MM samples have variable SiO2 concentrations, with average values of 72.36, 55.54, and 20.95 wt%, moderate SiO2/Al2O3 ratios (means of 6.88, 3.23, and 3.80), moderate to high Fe2O3 + MgO contents (means of 5.14, 9.55, 3.56 wt%), and high K2O/Na2O ratios (means of 3.26, 3.64, 2.90), respectively. On average, the chemical index of alteration (CIA) values of the MPS and the MPL are 65.87 and 71.96, respectively, while the chemical index of weathering (CIW) values are 74.54 and 85.09, respectively. These data record an intermediate to high degree of alteration (weathering) of plagioclase to illite/kaolinite in the samples’ provenance. The chondrite-normalized REE patterns of all the sample groups are similar and are characterized by subparallel light rare earth elements (LREE)-enriched, relatively flat heavy rare earth elements (HREE) patterns with pronounced Eu anomalies (mean of 0.69) and moderate fractionation [average (La/Yb)N = 8.7]. Plots of sediments in ternary diagrams of La, Th, Sc and elemental ratios (La/Sc, Th/Sc, Cr/Th, Eu/Eu*, La/Lu, Co/Th, La/Sc and Sc/Th), which are critical for determining provenance, and REE patterns indicate that the metaclastic units of the Seydi?ehir Formation were derived dominantly from felsic to intermediate magmatic rocks and not from a mafic source. The La–Sc–Th and Th–Sc–Zr/10 ternary diagrams of the Seydi?ehir Formation are typical of continental island arc/active continental margin tectonic settings. The geologic location and geochemistry of the Seydi?ehir Formation suggest that it was deposited in an Andean-type retroarc foreland basin during the Late Cambrian–Early Ordovician period. The Neoproterozoic intermediate to felsic magmatic rocks and metaclastic sediments with felsic origins of the Sand?kl?–Afyon Basement Complex (SBC) and their equivalent units, which are thought to be overlain by the younger units in the study area, may be the dominant source rocks for the Seydi?ehir Formation.  相似文献   

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