辽冀地区条带状铁建造地球化学特征：Ⅱ.稀土元素特征

辽冀地区(鞍山-本溪地区和冀东地区)位于华北克拉通北东部,是我国早前寒武纪条带状铁建造(BIFs)最重要的分布区,主要为新太古代Algoma型。本文系统对比了辽冀地区28个铁矿床200件铁矿石样品的稀土元素特征。结果表明:(1)所有样品的稀土元素特征比较相似:稀土元素总量较低,Y/Ho比值较高;经太古宙后平均澳大利亚页岩(PAAS)标准化后呈现重稀土相对富集、轻稀土相对亏损的配分模式,La异常不明显,强烈的Eu正异常和明显的Y正异常,暗示研究区铁矿石成矿物质主要来源于海底高温热液和海水的混合溶液;与冀东地区BIFs相比,鞍本地区Eu异常更为明显,说明鞍本地区BIFs显示更多的热液特征;(2)铁矿石的Ce/Ce*变化范围为0.77~1.09,缺乏明显的Ce负异常,说明其沉积于还原的海水环境;(3)辽冀地区BIFs的稀土元素总量、Eu异常、Y异常和Y/Ho比值变化范围均比较大,可能与BIFs沉积过程中碎屑物质的加入有关;与鞍本地区相比,冀东地区BIFs的Eu正异常、Y正异常程度均小于鞍本地区,热液和海水特征均不明显,Y/Ho比值更接近球粒陨石(26~28),可能暗示冀东地区有更多的碎屑物质的加入。     

Rare earth element patterns in Archean high-grade metasediments and their tectonic significance

Metasediments from two contrasting types of Archean high-grade terrains are interpreted as being derived from distinct tectonic settings. The Kapuskasing Structural Zone, Canada, represents the deep roots of a typical greenstone belt, whereas the Limpopo Province, southern Africa and Western Gneiss Terrain, Australia, mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism. Upper amphibolite to granulite fades paragneisses from the Kapuskasing Structural Zone have REE patterns similar to those of greenstone belt sediments, except where partial melting has occurred, forming restites with Eu enrichment and melts with Eu depletion. Except in this latter instance, metamorphism has not affected the primary REE patterns. REE patterns in Archean upper amphibolite to granulite facies metasediments from the central Limpopo Province and Western Gneiss Terrain show wide differences, ranging from patterns which resemble those in post-Archean terrigenous sediments, to typical Archean sedimentary rock patterns. The diversity in REE patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances. Samples with “post-Archean” patterns, displaying Eu depletion, are interpreted as being derived from K-rich granitic plutons which were portions of small, stable early Archean terrains, precursors of the widespread late Archean-Proterozoic episode of major cratonic development.     

The Behaviour of Rare—Earth Elements（REE） During Weathering of Granites in Southern Guangxi,China

The behaviour of the rare-earth elements(REE)during the weathering of granites was studied in southern Guangxi,China.Based on the study of the weathering profiles,the soil,weathered and sub-weatereb zones are identified with different REE geochemical behaviours throug the weathering profiles of granite.The Ce anomalies of the weathering profiles cover a large range of values with most falling between 1.02 and 1.43in the soil zone and 0.16and 0.40in the weathered and sub-weathered zones.Light rare-earth elements(LREE) and heavy rare-earth elements(HREE)are enriched to varying degree in the weathering profiles as compared to host granites.In the soil zone,more HREEs are leached than LREEs,and HREEs are more enriched than LREE in the weathered and sub-weathered zones.It is considered that infiltration and adsorption on clays are two processes controlling the enrichment and formation of REE deposits in the weathering profiles of granite.     

REE Geochemistry in the Weathered Crust of Granites, Longnan Area,Jiangxi Province

Warm and humid climate and gentle hilly topography have provided favourable conditions for the devel-opment of the weathering crust of the granite intrusions in the Longnan area, Jiangxi Province. REE is mostlyconcentrated in an adsorption state in clay in the wholly weathered zone. The rare-earth minerals enriched inthe parent rocks provided the source material for the REE enrichment. Exchangeable REE accounts for48-86%. Extraction experiments and stable isotopic study of clay minerals suggest that the downward infiltra-tion of meteoric water and increasing gradient of pH values have played an important role in the enrichment ofREE during the progressive weathering. Slight fractionation of individual REE can not change their distribu-tion patterns in the profiles. which are inherited from the parent rocks.     

Rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at Passa Quatro, Minais Gerais, Brazil

In a lateritic bauxite formed by weathering of nepheline syenite at Passa Quatro, Minas Gerais State, Brazil, bauxites on the hill-tops directly develop from the syenite bed-rock, while downslope, a kaolinitic layer occurs between bauxite and synetie. A petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit. The undisturbed weathered rocks were chemically analysed for major trace elements including REE and Zr. Mass balance calculations were applied, and the behaviour of the REE in the Passa Quatro weathering system was established compared to REE reference chondrite and to REE reference parent rock. In the lateritic bauxite, the results suggest that the first stages of weathering induce a volumetric change of 50%, i.e. collapse, with respect to the parent rock, and remove REE with a slightly larger loss of the LREE, except Ce, compared to the HREE. In the upper layers, where bauxite is more mature, a net mass gain in REE is observed relative to the underlying layers. This gain takes place during the reduction of the upper layer during the downward progression of the weathering front. Very significant REE losses occurs during the bauxitization processes throughout the upslope profile. In addition, the downslope kaolinitic system is demonstrated to be depleted in REE in the same proportions as the upslope bauxite. We proposed that the REE exported in solution from the whole weathering mantle have enriched neighbouring watershed sediments.     

Weathering characterization for landslides modeling in granitoid rock masses of the Capo Vaticano promontory (Calabria,Italy)

This work focuses on developing multidisciplinary researches concerning weathering profiles related to landscape evolution of the Capo Vaticano promontory on the Calabria Tyrrhenian side (southern Italy). In this area, the tectonic uplift, occurred at least since Pleistocene, together with the Mediterranean climatic conditions, is the main cause of deep weathering and denudation processes. The latter occurred on the outcropping rocks of the crystalline-metamorphic basement, made up of weathered granitoids, in turn belonging to the Monte Poro granitoid complex (intermediate to felsic plutonic rocks covered by Cenozoic sedimentary successions). Field observations coupled to borehole explorations, geophysical surveys, and minero-petrographical analyses allowed the characterization of the granitoid outcrops typical of the studied area in terms of kind and degree of slope instability. This characterization was based on suitable correlations verified between several factors as weathering degree, elastic properties of rocks, and discontinuity features. Weathering profiles are mainly composed by rock masses varying from completely weathered rock with corestones of highly weathered rock (classes IV–V) to slightly weathered rocks (class II). The weathered rocks are involved in several landslide typologies such as debris flow (frequency 48.5%), translational slide (frequency 33.3%), and minor rock fall and rotational slide (frequency 9%). The achieved data allowed the establishment of a general correlation between weathering degree and type of slope instability. Debris flow-type instabilities are predominant on the steeper slopes, involving very poor rock masses ascribed to the shallowest portions of the weathering class IV. Translational slides are less widespread than the previous ones and often involve a mixture of soil and highly weathered rocks. Rotational slides are more frequently close to the top of the slopes, where the thicknesses of more weathered rocks increase, and involve mainly rock masses belonging to the weathering classes IV and V. Rock falls mostly occur on the vertical escarpments of the road cuts and are controlled by the characteristics of the main discontinuities. The assessment of rock mass rating and slope mass rating, based on the application of the discontinuity data, allowed respectively an evaluation of the quality of rock masses and of the susceptibility of rock slopes to failure. The comparison between the last one and the real stability conditions along the cut slopes shows a good correspondence. Finally, the geological strength index system was also applied for the estimation of rock mass properties. The achieved results give a worthy support for a better understanding of the relationship between the distribution of landslides and the geological features related to different weathering degrees. Therefore, they can provide a reliable tool to evaluate the potential stability conditions of the rock slopes in the studied area and a general reference framework for the study of weathering processes in other regions with similar geological features.     

贵州平坝县白云岩风化壳中稀土元素分布特征之初步研究

本文报道了平坝白云岩风化剖面风化前缘稀土元素超常规富集(总量达3%以上)现象及其特征,并通过主元素、稀土元素的测定,结合白云岩中“酸不溶物”的提取和代表性样品的化学淋溶实验研究,讨论了白云岩风化壳剖面稀土元素的分异特征,认为风化前缘稀土元素的富集是由3方面的因素共同作用造成的:1)对白云岩风化残积的继承;2)剖面上部一部分轻稀土淋滤聚积;3)碳酸盐岩中非均匀分布的原生含磷矿物风化产生的稀土磷酸盐矿物的聚集,后者是最重要的原因。     

赣南新元古代变质岩稀土矿物及其地球化学特征

近年来赣南地区首次报道了变质岩离子吸附型稀土矿床的发现,为离子吸附型稀土的找矿提供了新思路。赣南地区新元古代变质岩大面积分布,风化壳也广泛发育。文章对30件稀土元素含量(300×10~(-6))高的变质岩矿物样品进行了详细的电子探针分析,旨在查明赣南新元古代不同类型变质岩中的稀土矿物种类及特征,探讨其成因、对全岩稀土元素含量的贡献以及离子吸附型稀土元素的成矿潜力。研究表明,区域上变质岩可大致分为6类,分别是变质凝灰岩类、板岩类、千枚岩类、片岩类、变砂岩类和变粒岩类,不同类型变质岩的稀土矿物组合不同,除了普遍存在的、难风化的独居石、磷钇矿和锆石外,部分岩性中出现易风化的褐帘石、含稀土元素绿泥石和含稀土元素金红石,及表生的水磷酸盐和磷铝酸盐等矿物。这些富稀土矿物贡献了全岩中大部分稀土元素,且部分矿物成因与后期流体作用相关,为成矿提供了良好的条件。文章总结分析认为,赣南地区广泛分布的变质岩中,片岩类、变砂岩类和变质凝灰岩类均具有相对易风化的稀土矿物组合,尤其变质凝灰岩类和变砂岩类,能为离子吸附型稀土成矿提供充足的物质来源,具有可观的离子吸附型稀土成矿潜力。     

特殊地质地貌区填图物化探技术应用

基于物化探技术方法在特殊地质地貌区填图中的应用实验,介绍从区域物化探数据提取地质填图信息的方法,指出重磁与氡-汞气测联合应用是覆盖区探测隐伏岩体、断裂的高效低成本物化探技术组合。运用基于地球化学理论方法的元素地球化学判别技术,在强烈风化区定量划分风化等级为:基岩-弱风化-中度风化-强风化-全风化-残积土等;指出风化壳元素地球化学行为对原生矿物分解、次生矿物形成具有示踪效应;不同风化层稀土元素富集分异显著,强烈富集于全风化层（岩土界面或风化岩石顶界）中的稀土元素分布模式,可作为界定风化壳分层的重要地球化学判别指标。      

不整合面控制内生金属成矿的新实例:山东沂南金铜铁矿床

山东沂南矽卡岩—热液型金铜铁矿床产于燕山期中酸性杂岩体的接触带及其外侧围岩中。近年来,在太古宇与新元古界之间不整合面及其上覆的新元古界地层中新发现了两层矿体,同时在不整合面以下局部地段的太古宇花岗片麻岩中也见有金铜矿化。受控于不整合面的矿体多呈层状、似层状,埋藏深度大(一般介于430~650 m),产状平缓,侧向延伸连续且稳定。作为一种容易失稳的构造薄弱带,不整合面及其上下机械性质不同的岩层之间极易产生层间破碎、滑脱,导致岩浆及其所分泌的含矿气水热液顺层贯入,并通过交代—充填作用成矿。同时,不整合面又是一个地球化学性质的转换带或地球化学屏障,盖层(含砾砂岩、砂岩、页岩夹泥灰岩)与基底岩石(花岗片麻岩、斜长角闪岩)化学成分和性质的显著差异,也是控制不整合面成矿的重要因素之一。不整合面以下太古宇花岗片麻岩中细脉浸染状金铜矿化,则可能与沿不整合面的拆离断裂活动引发的基底张性裂隙有关。该矿床中不整合面附近矿体的发现,对矿山深部、外围乃至整个鲁西地区的成矿规律研究和找矿勘探工作,具有重要的理论和实际意义。     

Major Element, REE, and Other Trace Element Behavior in Amphibolite Weathering under Semiarid Conditions in Southern India

A body of komatiitic amphibolite, an enclave within the Archean high-grade orthogneisses in southern India, shows mild chemical weathering under semiarid conditions. Along fractures, chemical weathering has advanced (Chemical Index of Alteration &sqbl0;CIA&sqbr0;=53; CIA of fresh rock approximately 26) to the extent that secondary Mg-Fe-Al clay minerals have formed and the rock has turned brownish red, soft, and fine grained. The weathering process has resulted in the mobilization and redistribution of the so-called immobile elements Fe, Al, Ti, and REE effected by the nature of secondary mineral formation (talc vs. aluminous clay minerals) and also possibly by soil microbes. In the initial stages of secondary mineral formation, there is a small loss of Fe, Al, and REE (noticeably Eu). However, in the fracture zone as well as in the incipiently altered zone, there is significant REE enrichment, probably affected by a different precipitation mechanism. Mobilized REE may have come from a minor alteration of clinopyroxene.     

Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon

The behaviour of major and trace elements have been studied along two serpentinite weathering profiles located in the Kongo-Nkamouna and Mang North sites of the Lomié ultramafic complex.The serpentinites are characterized by high SiO₂ and MgO contents, very low trace, rare earth and platinum-group element contents. Lanthanide and PGE contents are higher in the Nkamouna sample than in Mang North. Normalized REE patterns according to the CI chondrites reveal that: (i) all REE are below chondrites abundances in the Mang North sample; (ii) the (La/Yb)_N ratio value is higher in the Nkamouna sample (23.72) than in the Mang one (1.78), this confirms the slightly more weathered nature of the Nkamouna sample. Normalized PGE patterns according to the same CI chondrites reveal a negative Pt anomaly in the Mang sample. The Nkamouna sample is characterized by a flat normalized PGE pattern.All element contents increase highly from the parent rock to the coarse saprolite.In the weathering profiles, Fe₂O₃ contents decrease from the bottom to the top contrarily to Al₂O₃, SiO₂ and TiO₂. The contents of alkali and alkaline oxides are under detection limit.Concerning trace elements, Cr, Ni, Co, Cu, Zn and Sc decrease considerably from the bottom to the top while Zr, Th, U, Be, Sb, Sn, W, Ta, Sr, Rb, Hf, Y, Li, Ga, Nb and Pb increase towards the clayey surface soil. Chromium, Ni and Co contents are high in the weathered materials in particular in the saprolite zone and in the nodules.REE contents are high in the weathered materials, particularly in Nkamouna. Their concentrations decrease along both profiles. Light REE are more abundant than heavy REE. Normalized REE patterns according to the parent rock reveal positive Ce anomalies in all the weathered materials and negative Eu anomalies only at the bottom of the coarse saprolite (Nkamouna site). Positive Ce anomalies are higher in the nodular horizon of both profiles. An additional calculation method of lanthanide anomalies, using NASC data, confirms positive Ce anomalies ([Ce/Ce^*]_NASC = 1.15 to 60.68) in several weathered materials except in nodules ([Ce/Ce^*]_NASC = 0.76) of the upper nodular horizon (Nkamouna profile). The (La/Yb)_N ratios values are lower in the Nkamouna profile than in Mang site.PGE are more abundant in the weathered materials than in the parent rock. The highest contents are obtained in the coarse saprolite and in the nodules. The elements with high contents along both profiles are Pt (63–70 ppb), Ru (49–52 ppb) and Ir (41 ppb). Normalized PGE patterns show positive Pt anomalies and negative Ru anomalies.The mass balance evaluation, using thorium as immobile element, reveals that:

– major elements have been depleted along the weathering profile, except for Fe, Mn and Ti that have been enriched even only in the coarse saprolite;

– all the trace elements have been depleted along both profiles, except for Cr, Co, Zn, Sc, Cu, Ba, Y, Ga, U and Nb that have been enriched in the coarse saprolite;

– rare earth elements have been abundantly accumulated in the coarse saprolite, before their depletion towards the top of the profiles;

– platinum-group elements have been abundantly accumulated in the coarse saprolite but have been depleted towards the clayey surface soil.

Moreover, from a pedogenetical point of view, this study shows that the weathering profiles are autochtonous, except in the upper part of the soils where some allochtonous materials are revealed by the presence of zircon grains.     

Robust geochemical indices for redox and weathering on a granitic laterite landscape in Central Uganda

Most geochemical studies of laterite formation have been based on detailed examinations of a small number of hopefully “representative” profiles. Due to spatial variability in bedrock composition and geomorphic processes that sort materials by size, density, and other properties, experimental designs developed for the geochemical analysis of single soil profiles are of limited use in landscape investigations. In this study of a laterite landscape on a granitic rock in Uganda, we used element concentrations from neutron activation analysis (NAA) to develop a suite of robust geochemical indices for landscape-scale analysis of weathering and oxidation-reduction (redox) processes in laterite formation. To capture K-feldspar weathering the K/Hf ratio served as a good proxy for feldspar/zircon mineral ratios while K/Na ratios indexed feldspar concentration to quartz. Using principal components analysis, we derived an index of Rare-Earth Elements (REE) leaching (relative to Th), and an index of Heavy to Light REE (HREE/LREE) fractionation. We used the Fe/Sc ratio to ascertain the absolute (redox) vs. relative (weathering) Fe accumulation in laterite formation—with absolute Fe accumulation also associated with lower Fe/As ratios. Mn-Ce-Ba nodules and concretions were identified by characteristically high Ce values—with individual Ce values as high as 5000 μg g⁻¹. The relatively large amounts of K-feldspar (K_avg is 4000 μg g⁻¹) and low degree of HREE/LREE fractionation suggest that this landscape is weathered less than generally assumed. The Fe- and Mn-derived indices highlight the important role of oxidation-reduction processes in iron accumulation. Overall, our findings suggest that the current ferricrete-mantled hilltops were formerly located on a lower, poorly drained part of the landscape.     

柴达木盆地东坪地区基岩风化壳特征

柴达木盆地阿尔金山前斜坡带发现了规模分布的东坪基岩气田,其基岩风化壳的作用成为关注的问题。依据元素分析、X-射线衍射分析、岩心薄片观察,常规测井响应和成像测井响应特征,识别出东坪地区基岩风化壳发育不同结构层,且不同结构层的储集特征有很大差异。研究表明,基岩风化壳结构可划分为土壤层、完全风化层和半风化层,而半风化层又可进一步分为溶蚀带和崩解带;其中土壤层厚度0~2 m,完全风化层厚度4~15 m,溶蚀带厚度365~164 m,崩解带厚度300~1 000 m。基岩半风化层是储层发育带,其中溶蚀带储集物性好于崩解带,溶蚀带发育较多的溶蚀孔洞和溶蚀加宽的网状裂缝,孔隙度范围2%~16%;而崩解带发育弱溶蚀构造缝和节理缝,孔隙度范围2%~8%。东坪地区大规模发育基岩风化壳为柴达木盆地远离烃源岩灶的斜坡地区寻找油气提供了借鉴依据。     

REE Geochemistry of Loess in Xinjiang ,China

The total rare-earth element values(ΣREE)of loess in the Xinjiang region vary over a range of 128-200 ppm ,with an average of 153ppm .The average REE content of loess lies between the earth‘s crust (155ppm) and sedimentary rocks(151ppm).The Xinjiang loess,with the REE distribu-tion patterns characterized by negative slopes ,is rich in the Ce-family elements, and has a distribu-tion pattern characteristic of sedimentary rocks.The North Xinjiang loess is relatively depleted in Tb,but rich in Yb and Lu.The South Xinjiang loess is relatively rich in light rare-earth elements.This is full proof that the Xinjiang loess comes partly from weathered materials(clay rock,sandstone)in the region studied.The REE distribution patterns in the Xinjiang loess are similar to those in the precipitated dust and Aeolian sand,indicating the same material source.The REE distribution pat-terns in the Xinjiang loess are also similar to those in loess from the middle Yellow River Valley,China and Taskent,the former USSR.This implies that loesses of the three locations(Xinjiang,the mid-dle Yellow River Valley and Taskent) come from a common material source.But the REE patterns in the Xinjiang loess are different from those in wall rocks (volcanic rock,K-bearing volcanic rock).Generally ,LREE/HREE,Eu/Eu* and Ce/Ce* ratios reflect the features of parent materials of loess,indicating that the parent rocks were probably in the early stage of alkaline weathering and the weathered materials existed in an oxidation environment with basic mediums under arid-climatic conditions before transport.As a result,the migration ability of the REE is weak.     

Rare earth element geochemistry of Archean metasedimentary rocks from Kambalda,Western Australia

Archean sedimentary rocks of very limited lateral extent from horizons within basaltic and ultramafic volcanic sequences at Kambalda, Western Australia, are extremely variable in major elements, LIL and ferromagnesian trace element compositions. The REE patterns are uniform and do not have negative Eu anomalies. Two samples have very low total REE abundances and positive Eu anomalies attributed to a very much greater proportion of chemically deposited siliceous material. Apart from these two samples, the Kambalda data are similar to REE abundances and patterns from Archean sedimentary rocks from Kalgoorlie, Western Australia and to average Archean sedimentary rock REE patterns. These show a fundamental distinction from post-Archean sedimentary rock REE patterns which have higher $\text{La}\text{Yb}$ ratios and a distinct negative Eu anomaly.     

The release of Pb and REE from granitoids by the dissolution of accessory phases

The release of Pb and rare earth elements (REE) during granitoid weathering was investigated through dissolution experiments of fresh granite and soil samples. Two aliquots of a granite sample from the El-Capitan Granite, Sierra Nevada, California, were leached several times using a dilute acid at pH = 1. The results of the experiment were compared with Pb and REE data from soils developed on the same rock. During the early stages of granitoid dissolution, Pb and REE were preferentially released from some of the accessory phases (i.e., allanite, sphene, and apatite). This caused higher ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb values and different REE patterns in solution compared with the rock values. Based on Pb isotopes and REE patterns, three stages of rock dissolution can be identified. In the first stage the dissolution of allanite dominates the release of Pb and REE from accessory phases, as ²⁰⁸Pb/²⁰⁷Pb, Ce/Pb, and chondrite-normalized Ce/Yb ratios in solution increase and approach the values of allanite. In the second stage, the dissolution of apatite and sphene become more significant. In the third stage, the isotopic ratios of Pb and the normalized-REE patterns reflect the depletion of accessory phases and the increase in the rate of feldspar dissolution. According to our estimate (based on Si release from the rock) all three stages account for the first 500 kyr of granitoid weathering.Using the isotopic ratios of Pb, major elemental compositions, and REE concentrations both in the experimental solutions and in the soil we were able to establish the following order of the weathering rates of accessory phases: allanite > apatite > sphene. In addition, we have demonstrated that biotite is significantly less resistant to weathering than hornblende under acidic conditions, and is probably dissolved completely after approximately 500 kyr of rock weathering. We also suggest that within 500 kyr of granitoid weathering K-feldspar accounts for 15% of the released K.     

Nature and origins of granitic regolith in bauxite mine floors in the Darling Range,Western Australia

Chemical and physical weathering of primary minerals during the formation of laterite profiles in the Darling Range has formed distinct secondary mineral and morphological zones in the regolith. Erosion and human activity such as mining have exposed large areas of lateritic regolith, and its classification is important for land management, especially for mine rehabilitation. Preserved rock fabrics within regolith may enable the identification of parent rock type and degree of weathering, thus providing explanations for variations in important physical properties such as the strength and water retention of regolith. Feldspar, quartz, biotite and muscovite in porphyritic and fine-grained monzogranite in lateritic profiles have weathered via a series of gradational changes to form saprolite and pedolith consisting of kaolin, quartz, iron oxides, muscovite and gibbsite. Local reorganisation in the upper regolith or pedoplasmation zone has included illuviation of kaolin, which may be iron oxide-stained and which has disrupted the preserved rock fabric of saprock and saprolite. Quartz grain- or matrix-supported fabrics have developed, with greater pedoplasmation resulting in a quartz-grain-supported fabric. The recognition of these processes enables the use of gibbsite grainsize and distribution in regolith to infer original feldspar grainsize. Muscovite-rich or muscovite-deficient kaolin matrix indicates where plagioclase or alkali feldspar, respectively, was present in the parent rock. In some regolith, cementing by iron oxides has faithfully preserved rock fabric. The recognition of these various regolith types provides a basis for identifying the parent materials of lateritic regolith developed from granitic and doleritic rocks. Rock fabric is sometimes preserved in iron oxide-cemented bauxite mine floor regolith (Zh) due to the pseudomorphic gibbsite grains and iron oxide cement which forms a porous, rigid fabric. Plagioclase-rich granitoid is more likely to have weathered to dense clay-rich regolith (Zp), whereas albite and alkali feldspar have weathered to quartz-rich regolith (Zm) with the random orientation of quartz grains indicating that substantial reorganisation of rock fabric has occurred. It is possible to predict the response of regolith materials exposed in mine floors to management practices including ripping and re-vegetation, thus allowing targeted use of deep-ripping and planting density based on regolith type.     

冀东柞栏杖子BIF地球化学特征、形成时代及其地质意义

冀东地区柞栏杖子BIF出露于绿片岩相—低角闪岩相朱杖子岩群变质岩中。铁矿石主要由石英和磁铁矿组成,还含有少量透闪石和黑云母。主量元素主要为Si O2、Fe2O3和Fe O,其次为少量的Ca O和Mg O。较低的Al2O3含量、极低的Ti O2含量和高场强元素(HFSE)暗示,铁矿石中陆源碎屑物质含量很低。铁矿石的稀土元素含量较低,稀土元素配分模式特征为轻稀土元素相对亏损、重稀土元素相对富集。较明显的Eu正异常、轻微的Y正异常及较高的Y/Ho值的稀土元素特征,类似于高温热液和海水的混合热液,暗示成矿物质主要来自于海水和高温热液。对柞栏杖子BIF矿体夹层黑云斜长变粒岩进行SHRIMP锆石U-Pb定年,207Pb/206Pb年龄加权平均值为2572±8Ma(MSWD=5.8),可代表柞栏杖子BIF的形成年龄。综合前人研究,认为冀东地区变质级别不同的BIF物质来源相同、形成年代相近,BIF的变质可能和2500Ma左右华北克拉通东部陆块发生的地幔岩浆底侵事件有关。     

Weathering profiles in granites, Sierra Norte (Córdoba, Argentina)

Two weathering profiles evolved on peneplain-related granites in Sierra Norte, Córdoba province, were examined. Several weathering levels, of no more than 2 m thickness, were studied in these profiles. They had developed from similar parent rock, which had been exposed to hydrothermal processes of varying intensity. Fracturing is the most notable feature produced by weathering; iron oxides and silica subsequently filled these fractures, conferring a breccia-like character to the rock. The clay minerals are predominantly illitic, reflecting the mineral composition of the protolith. Smaller amounts of interstratified I/S RO type are also present, as well as scarce caolinite+chlorite that originated from the weathering of feldspar and biotite, respectively. The geochemical parameters define the weathering as incipient, in contrast to the geomorphological characteristics of Sierra Norte, which point to a long weathering history. This apparent incompatibility could be due to the probable erosion of the more weathered levels of the ancient peneplains, of which only a few relicts remain. Similar processes have been described at different sites in the Sierras Pampeanas. Reconstruction and dating of the paleosurfaces will make it possible to set time boundaries on the weathering processes studied and adjust the paleographic and paleoclimatic interpretations of this great South American region.