普朗斑岩铜矿岩浆混合作用：岩石学及元素地球化学证据

普朗斑岩铜矿花岗闪长斑岩中存在大量的随机分布的镁铁质微粒包体,包体与寄主岩存在渐变接触关系。包体成分为闪长质,具有岩浆结构,存在针状磷灰石,显示了快速冷却结晶的特征。包体内可见具有暗色矿物镶边的眼球状石英,表明存在岩浆混合作用。寄主岩岩体规模较小,呈岩枝状产出,包体和寄主岩均为似斑状结构,说明两种岩浆侵位和发生岩浆混合作用的深度较浅。元素地球化学特征显示包体与寄主岩的之间有成分交换。包体和寄主岩强不相容元素均富集,高场强元素Nb、Ta、Ti均表现出显著的负异常,具有典型的岛弧岩浆岩的微量元素特征,包体和寄主岩Mg＃较高,在同等Si条件下比玄武岩部分熔融体富K2O和MgO,意味着源区必须有幔源岩浆的贡献。普朗斑岩铜矿蚀变矿化模式反映了其成矿环境偏基性,暗示镁铁质岩浆的加入对成矿具有贡献。     

华南与花岗岩有关的一种新类型的锡成矿作用：矿物化学、元素和同位素地球化学证据

华南是我国最重要的锡成矿省,产有大量的与花岗岩有关的大型-超大型锡多金属矿床。近年来,在湘南新探明一个超大型锡矿床-芙蓉锡矿床,其中,最重要的锡矿化产在骑田岭花岗岩体西南部的破碎蚀变带内,与绿泥石化密切相关。骑田岭花岗岩富含角闪石,具有较高的氧逸度,显示出准铝的地球化学特征,在花岗岩形成过程中发生过壳-幔岩浆混舍作用。这些特点都表明骑田岭花岗岩并不同于一般的S型含锡花岗岩,而显示出A型花岗岩的地球化学特征。同位素定年分析表明,芙蓉锡矿床主成矿阶段的形成时代要晚于骑田岭花岗岩侵位年龄近20Ma。氢、氧同位素分析表明,发生过水-岩反应的大气降水在成矿流体中占有很重要的地位。硫同位素分析表明花岗岩和地层都提供了成矿所需的硫。因此,用花岗岩浆结晶分异过程中分离出富锡的岩浆流体来形成锡矿的传统模式并不适合于解释芙蓉锡矿的形成。我们认为芙蓉锡矿的形成主要与骑田岭花岗岩的绿泥石化蚀变有关,循环的大气降水与花岗岩发生水-岩反应,富锡的铁镁矿物在蚀变成绿泥石的同时释放出Sn和Ti等金属到流体中,当物理化学条件改变时,沉淀形成锡矿体。这是一种比较独特的锡矿化模式,丰富了华南与花岗岩有关的锡矿化类型。     

Origin and geochemical evolution from ferrallitized clays to karst bauxite: An example from the Lower Cretaceous of NE Spain

Four outcrops of Lower Cretaceous (Barremian) karst bauxites located in Teruel (NE Spain) were analysed. The deposits show a heterogeneous-chaotic lithostructure consisting of pisolitic bauxite blocks embedded in lateritic red clays filling karst cavities. The research has focused on the geochemical study of major, minor, and trace elements (including some critical to industry) of both the bauxites and clays. The objective was to investigate the bauxite precursor material and to characterize the system’s geochemical evolution. Geochemical analyses were carried out by inductively-coupled plasma optical emission and mass spectroscopy. An absolute weathering index has been calculated to estimate element mobility, assuming Ti as an immobile element and the Upper Continental Crust (UCC) as parent material. Further, selected samples were observed by field emission scanning electron microscopy. The data indicate that both the bauxites and red clays originated by intense chemical weathering from more mafic argillaceous sediments than the UCC. Ongoing weathering caused the bauxitization of the upper parts of the original profile, preventing the lower parts from being bauxitized, thus producing the ferrallitized clays underlying the pisolitic bauxites. Subsequent karst reactivation gave rise to the current lithostructure. Ferrallitization is related to Fe, Sc, and V enrichment. On the other hand, although bauxites are relatively enriched in some elements compared to clays, the more intense chemical weathering associated with bauxitization led to chemical homogenization and widespread element depletion. During the bauxitization, Al, Ti, Zr, Cr, and probably Hf and the critical element Nb behaved as more immobile elements in the system. Bauxitization also enhanced homogenization and depletion of the REE, which is more pronounced for the LREE. HREE trends seem to be partly related to the concentration of Ti oxides in the bauxites, whereas P-bearing phases, more frequent in the clays, control the LREE. Subsequent to bauxitization, partial kaolinization of the bauxite took place related to the circulation of acid solutions that also caused the karst reactivation. These late processes caused some Al depletion in the bauxites and enhanced Fe loss together with V and, to a lesser extent, Ge.     

Trace element and sulfur isotopic evidence for redox changes during formation of the Wallaby Gold Deposit,Western Australia

Wallaby is a major gold deposit of the St Ives Gold field of Western Australia, with an estimated resource of 8 million ounces of gold. It has a well-established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time; from hematite to magnetite. Micro-analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions. The sulfur isotope composition has a δ³⁴S range of −7.7 to +9.8‰ using 3 μm spots on the Sensitive High Resolution Iron Micro Probe-Stable Isotope (SHRIMP-SI). Negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite (which coexists with hematite) that also contains high concentrations of As, Ni, Zn, Ag, Sb, Cu and Pb. Conversely, positive values representative of reduced sulfur are found in later generations of pyrite, with lower concentrations of As, Ni, Sb, Cu, Zn and Pb. These later pyrite crystals display higher As/Ni, As/Sb, and As/Bi, and lower Cu/Te. These geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure. Previous studies suggested a single orogenic event formed the Wallaby Deposit. This is not supported by the present study. Trace element ratios such as As/Ni clearly delineate the high Au generations and could be used as an exploration tool. It is suggested that pyrite from the Wallaby Gold Deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated. Utilizing pyrite to link the entire fluid history of Wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold-bearing fluid systems.     

Metasomatism in the subcontinental mantle beneath the Eastern Carpathians (Romania): new evidence from trace element geochemistry

A wide range of trace elements have been analysed in mantle xenoliths (whole rocks, clinopyroxene and amphibole separates) from alkaline lavas in the Eastern Carpathians (Romania), in order to understand the process of metasomatism in the subcontinental mantle of the Carpatho-Pannonian region. The xenoliths include spinel lherzolites, harzburgites and websterites, clinopyroxenites, amphibole veins and amphibole clinopyroxenites. Textures vary from porphyroclastic to granoblastic, or equigranular. Grain size increases with increasing equilibrium temperature of mineralogical assemblages and results from grain boundary migration. In peridotites, interstitial clinopyroxenes (cpx) and amphiboles resulted from impregnation and metasomatism of harzburgites or cpx-poor lherzolites by small quantities of a melt I with a melilitite composition. Clinopyroxenites, amphibole veins and amphibole clinopyroxenites are also formed by metasomatism as a result of percolation through fracture systems of large quantities of a melt II with a melanephelinite composition. These metasomatic events are marked by whole-rock enrichments, relative to the primitive mantle (PM), in Rb, Th and U associated in some granoblastic lherzolites and in clinopyroxene and amphibole veins with enrichments in LREE, Ta and Nb. Correlations between major element whole-rock contents in peridotites demonstrate that the formation of interstitial amphibole and clinopyroxene induced only a slight but variable increase of the Ca/Al ratio without apparent modifications of the initial mantle composition. Metasomatism is also traced by enrichments in the most incompatible elements and the LREE. The Ta, Nb, MREE and HREE contents remained unchanged and confirm the depleted state of the initial but heterogeneous mantle. Major and trace element signature of clinopyroxene suggests that amphibole clinopyroxenites and some granoblastic lherzolites have been metasomatized successively by melts I and II. Both melts I and II were Ca-rich and Si-poor, somewhat alkaline (Na > K). Melt I differed from melt II in having higher Mg and Cr contents offset by lower Ti, Al, Fe and K contents. Both were highly enriched in all incompatible trace elements relative to primitive mantle, showing positive anomalies in Rb, Ba, Th, Sr and Zr. They contrasted by their Ta, Nb and LREE contents, lower in melt I than in melt II. Melts I and II originate during a two-stage melting event from the same source at high pressure and under increasing temperature. The source assemblage could be that of a metasomatized carbonated mantle but was more likely that of an eclogite of crustal affinity. Genetic relationships between calc-alkaline and alkaline lavas from Eastern Carpathians and these melts are thought to be only indirect, the former originating from partial melting of mantle sources respectively metasomatized by the melts I and II. Received: 17 March 1997 / Accepted: 14 July 1997     

贵州务川瓦厂坪铝土矿床稀土元素地球化学特征及其对成矿过程的指示

瓦厂坪铝土矿床是黔北务川-正安-道真铝土矿成矿区内的大型古风化壳沉积型铝土矿床,其含矿岩系位于下二叠统栖霞组(P2q)灰岩和志留系中下统韩家店组(S1-2hj)泥页岩之间的下二叠统梁山组(P2l)地层中。铝土矿石的矿物组成主要以硬水铝石、勃姆石、高岭石、蒙脱石以及赤铁矿为主,其次还含有少量的三水铝石、黄铁矿、针铁矿、锐钛矿、锆石、长石和石英等。矿物学特征表明,铝土矿物质经历了搬运和重新沉积。研究表明铝土矿石及下伏志留系韩家店组泥页岩的稀土元素地球化学特征较相似,暗示韩家店组泥页岩可能为铝土矿主要的成矿原岩。铝土矿床的形成是下伏韩家店组泥页岩通过长期原位风化作用逐步转变而来的。铝土矿剖面上稀土元素的地球化学演化是多方面因素综合作用的结果,这些因素包括:原岩类型、p H值、矿物颗粒的大小、地下水的化学性质、剖面上Fe的浓度变化、所含矿物的种类以及稀土元素的地球化学性质等因素。根据地质学、矿物学和地球化学数据,对务川瓦厂坪铝土矿床提出了4阶段的成矿模式。     

新疆喇嘛苏铜矿床成因：来自石榴子石元素地球化学的证据

喇嘛苏矿床是中国西天山地区首例大型铜矿床,矿体主要产于花岗闪长斑岩与碳酸盐岩的接触带矽卡岩中。石榴子石在矽卡岩中大量发育,是矽卡岩的主要造岩矿物,分为外矽卡岩带红棕色粗粒石榴子石(Grt-Ⅰ)、外矽卡岩带浅棕色细粒石榴子石(Grt-Ⅱ)、外矽卡岩带黄绿色不等粒石榴子石(Grt-Ⅲ)3类。文章对不同产状石榴子石开展了电子探针分析和LA-ICP-MS原位微区分析,探讨了石榴子石成因及其对成矿过程的指示。喇嘛苏矿床内3类石榴子石均以钙铁榴石为主,其次为钙铝榴石,具有相似的微量元素组成,亏损Rb、Ba、Sr等大离子亲石元素,富集Th、U、Nb、Ce高场强元素。3类石榴子石的稀土元素特征差别较大,其中,Grt-Ⅰ类石榴子石的∑REE为101.47×10^-6～262.87×10^-6,具有较陡峭的REE配分模式,LREE/HREE为3.81～68.50,具有正Eu异常;Grt-Ⅱ类石榴子石核部的∑REE为163.49×10^-6～249.52×10^-6,显示负Eu异常,LREE/HREE为2.00～4.71,而外环...     

Metabasalts from the Troodos Massif,Cyprus: Genetic implication deduced from petrography and trace element geochemistry

A wide variety of data support the contention that the Troodos massif, Cyprus is a fragment of oceanic lithosphere formed at a constructive margin some 85 m.y. ago. However, Troodos rocks differ significantly in major and trace element content and mineral parageneses from those formed at present day active constructive margins. It is here argued that the geochemical and petrographic characteristics of the massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.     

Geochemical behaviors of different element groups during shale weathering at the Susquehanna/Shale Hills Critical Zone Observatory

Major and trace element concentrations were measured in bedrock, regolith and stream sediments from a first-order catchment developed entirely on grey shale in central Pennsylvania, USA. These elements can be classified into five major groups based on statistical data analysis. The presence of different elemental groups is due to the mineralogical origin, cycling processes, and geochemical properties of these elements during soil formation. A better understanding of the behaviors of these elements during chemical weathering would allow for their possible use as natural tracers in Critical Zone processes.     

Petrology and geochemistry of prograde deserpentinized peridotites from Happo-O’ne, Japan: Evidence of element mobility during deserpentinization

The prograde deserpentinized peridotites from the talc zone in the Happo-O’ne complex, central Japan, show differences in their field relation and mineral assemblage with the high-P retrograde peridotites of the other part of the complex. They show a mineral assemblage, olivine + talc + antigorite ± prograde tremolite ± chlorite, formed by thermal metamorphism around the granitic intrusion at T, 500-650 °C and P < 7 kbar. The olivine has numerous opaque inclusions and high Fo (91.5-96.5) relative to the retrograde olivine, reflecting its formation by deserpentinization. The prograde tremolite, which is low in Al₂O₃ (<1.0 wt.%), Cr₂O₃ (<0.35 wt.%), and Na₂O (<0.6 wt.%) but high in Mg# (up to 0.98) and SiO₂ (up to 59.9 wt.%), is different in size, shape and chemistry from the retrograde tremolite. The prograde peridotites display a U-shaped REE pattern (0.02-0.5 times PM), similar to diopside-zone retrograde metaperidotites, possible protoliths. They are enriched in LILE (e.g., Cs, Pb, Sr, Rb) relative to HFSE (e.g., Ta, Hf, Zr, Nb), like their protoliths, because of their local re-equilibration with the fluid released during dehydration of the protoliths. They have high contents of REE and some trace elements (e.g., Cs, Th, U, Ta) relative to their protoliths because of an external-element addition from the granitic magma. In-situ analyses of peridotitic silicates confirmed that the prograde tremolite and talc display a spoon-shaped primitive mantle (PM)-normalized REE pattern (0.1-3 times PM) in which LREE are higher than HREE contents. The prograde tremolite is depleted in Al, Na, Cr, Sc, V, Ti, B, HREE and Li, but is enriched in Si, Cs, U, Th, HFSE (Hf, Zr, Nb, Ta), Rb and Ba relative to the retrograde tremolite; the immobile-element depletion in this tremolite is inherited from its source (antigorite + secondary diopside), whereas the depletion of mobile elements (e.g., Li, B, Na, Al) is ascribed to their mobility during the deserpentinization and/or the depleted character of the source of tremolite. The enrichment of HFSE and LILE in the prograde tremolite is related to an external addition of these elements from fluid/melt of the surrounding granitic magma and/or in situ equilibrium with LILE-bearing fluid released during dehydration of serpentinized retrograde metaperidotites and olivine-bearing serpentinites (protoliths). The prograde olivine is higher in REE and most trace-element contents than the retrograde one due to the external addition of these elements; it is enriched in B, Co and Ni, but depleted in Li that was liberated during deserpentinization by prograde metamorphism.     

Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra–Jamuna River, Bangladesh: Inference from geochemistry

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj–Tangail (downstream) section of the Brahmaputra–Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO₂, Fe₂O₃, MgO, K₂O, P₂O₅, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO₂/Al₂O₃ and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na₂O, Sr and Pb increase with increasing SiO₂/Al₂O₃ and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra–Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra–Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra–Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A–CN–K ternary diagram, most of the samples close to the plagioclase–K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra–Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz.     

Major and trace element geochemistry of plutonic rocks from Francistown, NE Botswana: evidence for a Neoarchaean continental active margin in the Zimbabwe craton

The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.

The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al₂O₃ (>>16%), low TiO₂ (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite.     

Petrography and geochemistry of the Paleogene sandstones from the Ningnan Basin,NW China: implications for provenance,weathering and tectonic setting

Petrographical and geochemical characteristics of the Paleogene sandstones in the Ningnan Basin, NW China, were studied by analyzing thin sections and major, trace, and rare earth element (REE) compositions. The sandstones of the Sikouzi and Qingshuiying formations are sublitharenite to feldspathic litharenite and characterized by low-medium compositional and textural maturity. Petrographic provenance study indicates a quartzose-recycled provenance with higher concentration of Zr in the Qingshuiying Formation. Depositional environment of the Sikouzi and Qingshuiying sandstones was mainly oxic as evidenced by lower values of δEu, δCe, V/Cr, Ni/Co, U/Th, and δU. The Chemical Index of Weathering (CIW’) and the Th/U ratios (av. 3.3 and 3.54) reveal weak and moderate weathering history in the source area. The Zr, Nb, Y, Th, and U concentrations in the Qingshuiying Formation are comparable to upper continental crust (UCC) values and the negative Eu anomalies support contribution from a felsic source. However, a significantly larger range of Ce anomalies in the Sikouzi Formation also reveals mixing of felsic and mafic/ultramafic source terrains. The petrology and geochemistry analyses of the Sikouzi sandstones imply that the provenance was mainly derived from diabase in Xiang Shan and lower Cretaceous strata in the study area whereas the Qingshuiying sandstones were derived from the granites in the southern part of Liupan Shan, which means that the northeastward expansion of the Tibetan Plateau did not make the Liupan Shan a high relief during Paleogene. The present study has let to document the origin of the Ningnan Basin that was formed during the late phase of Ordos Basin reformation, along with the other synchronous peripheral fault-bounded basins of the Ordos Basin, and was not related to the northeastern extrusion of Tibetan Plateau.     

新疆克拉玛依岩体的岩浆混合作用成因：岩石地球化学证据

新疆西准噶尔地区克拉玛依花岗质岩体中发育大量闪长质微粒包体,并形成有岩浆混合成因的岩浆混合岩——石英闪长岩。包体成分主要为闪长质,显微镜下具有岩浆岩结构,岩浆混合特征十分明显,如:针状磷灰石,角闪石包裹辉石残晶,长石斑晶的溶蚀环带等特征。岩体中寄主岩石、岩浆混合岩、闪长质微粒包体、闪长玢岩脉分别代表岩浆混合演化过程中两端元岩浆按不同比例混合的产物。在岩石地球化学方面,包体与寄主岩石的主要氧化物之间具有良好的线性关系,寄主岩石和包体的稀土元素配分曲线和微量元素蛛网图形态相似;各种地球化学元素参数特征显示,寄主岩石与包体在岩石形成过程中发生过成分交换及均一化。特征元素比值及同位素等特征表明,闪长质包体的端元岩浆可能为幔源基性岩浆,寄主岩石的端元岩浆可能是以壳源为主的酸性岩浆。岩石地球化学特征进一步佐证了该区岩浆混合作用的存在,同时也暗示岩浆混合作用可能是新疆北部后碰撞过程中重要的岩浆活动形式。     

Terrestrial environments during MIS 11: evidence from the Palaeolithic site at West Stow,Suffolk, UK

Multidisciplinary investigations of the sequence at Beeches Pit, West Stow (Suffolk, UK), have a direct bearing the age of the Hoxnian Interglacial and its correlation with the continental Holsteinian and with the global marine record. At this site, glacial deposits (till and outwash gravels) referable to the Anglian Lowestoft Formation fill a subglacial channel cut in Chalk bedrock. Above these glacial deposits a series of interglacial sediments occurs, consisting of limnic, tufaceous and colluvial silts, lacking pollen but rich in shells, ostracods and vertebrates. Lower Palaeolithic flint artefacts of Acheulian character have also been recovered, including refitting examples. Charred material is abundant at certain horizons and many of the bones have been burned. Several discrete areas of burnt sediment are interpreted as hearths. The molluscan fauna comprises some 78 taxa and includes species of considerable zoogeographical and biostratigraphical importance. The land snail assemblage from the tufa consists of woodland taxa with no modern analogue, including species that are either extinct (e.g. Zonitoides sepultus) or which no longer live in Britain (e.g. Platyla polita, P. similis, Neniatlanta pauli). This is also the type locality of Retinella (Lyrodiscus) skertchlyi, which belongs to a subgenus of zonitid land snail now living only on the Canary Islands. There are indications from this fauna (‘the Lyrodiscus biome’) that the climate was wetter and perhaps warmer than the present day. The vertebrate fauna is also noteworthy with species of open habitats, such as rabbit (Oryctolagus cf. cuniculus), and of closed forest, such as squirrel (Sciurus sp.) and garden dormouse (Eliomys quercinus) present at different times. The occurrence of southern thermophiles, such as Aesculapian snake (Zamenis longissimus), indicates temperatures warmer than those of eastern England today. The upper levels include much material reworked from the interglacial sediments, although there is clear faunal evidence for climatic deterioration. Both the molluscan and vertebrate faunas suggest correlation of the interglacial sediments with the Hoxnian. Uranium series dates from the tufa (∼455 ka BP), TL dates from burnt flints (414±30 ka BP) and a range of amino acid racemization data all support correlation of this interglacial with MIS 11. However, four OSL dates from sand beneath the interglacial sequence yield a mean age of 261±31 ka BP, far younger than all other age determinations and far younger than implied by the biostratigraphy. Archaeologically the site is unusual in showing prolonged human occupation within closed deciduous forest and evidence for controlled use of fire in a Lower Palaeolithic context. Biostratigraphical correlations with other Lower Palaeolithic sites support the suggestion that Acheulian and Clactonian industries both occurred in southern Britain during the same substage of the Hoxnian, although not necessarily at precisely the same time. The characteristics of the MIS 11 interglacial in Britain are discussed in the light of evidence from Beeches Pit and elsewhere.     

葡萄牙NeVes COrVO块状硫化物矿床锡石的微量元素地球化学

对葡萄牙Neves Corvo Cu.Sn多金属矿床锡矿石，富铜硫化物矿石和贫铜硫化物矿石中锡石的微量元素电子探针分析结果的综合研究表明，该矿床存在高温（＞300度），富W的早期锡矿化阶段，钞晚的贫W，富Cu，温度略低（245－295度）的Cu,Sn主矿化阶段，以旬晚的贫W，贫Cu温度更低（约200度）的Cu,Sn矿化阶段，结合Pb同位素研究可以推测矿床中Sn可能来源于矿体下部深处未出露的，与板块俯冲有关的富Sn古老地壳岩石，与其他类型锡矿床的微量元素对比表明，Neves Corvo矿床属于处于火山岩容矿（VHMS）型和沉积岩容矿（SHMS）型块状硫化物矿床之间的一种过渡类型－－伊比利亚型。     

Degassing, crystallization and eruption dynamics at Stromboli: trace element and lithium isotopic evidence from 2003 ashes

During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity, Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards. Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses, along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower δ⁷Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ⁷Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined with a degassing model based on open-system Rayleigh distillation and on the assumption that ^melt/fluidD_Li > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre- and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system.     

湖南香花铺钨矿床含钙矿物的稀土元素地球化学

香花铺矿床是湘南地区唯一的萤石-白钨矿型矿床。本文利用高精度LA-ICP-MS对该矿白钨矿和方解石中的稀土元素进行了原位分析。结果表明,该矿的白钨矿表现为LREE富集和正Eu异常特征,方解石则具有LREE富集型和相对平坦型两种稀土配分模式,且均呈现负Eu异常特征。该矿属于与岩浆活动有关的热液矿床,成矿流体在还原环境下的运移过程中,首先结晶出白钨矿和LREE富集型方解石,而后沉淀形成了具有相对平坦型REE模式的方解石。此外,稀土元素在该区矿物中的分布存在明显的不均一现象,在不同微区REE的含量、配分模式以及分异程度等均可能存在差异,其原因除了结晶过程中水动力学条件的变化,还可能与矿物结晶后颗粒外表层受流体作用的改造有关。     

Evolution of the Archean-Palaeoproterozoic Bundelkhand Massif, central India—evidence from granitoid geochemistry

The Bundelkhand massif of Archean-Palaeoproterozoic age is primarily a granite-gneiss complex. Three distinct granitoid suites have been identified within the massif hornblende granitoids, biotite granitoids and leucogranitoids, in order of decreasing age. These granitoids were emplaced in previously deformed basement consisting of gneisses, banded iron formations and other metasediments, mafic to felsic volcanics.
The granitoids exhibit a large compositional range from quartz diorite to syenogranite and show a calc-alkaline trend. They are metaluminous to peraluminous and have I-type characteristics. The SiO₂ content ranges from 49 to 77 wt%. Low K₂O/Na₂O characterizes the granitoids. The oldest hornblende granitoids have low Rb and Yb contents compared to the younger biotite granitoids and leucogranitoids. Rb/Sr values for most of the granitoids are low (< 1). K/Rb ratios range from 95 to 373 which is, in general, comparable with other calc-alkaline suites. Y/Nb ratios of the granitoids are > 1.2 which is a characteristic feature of magmas derived from sources chemically similar to island arc or continental margin basalts.
The features mentioned above coupled with concentrations of Rb, Y, Nb, Yb, Ta and Th indicate a volcanic-arc tectonic setting for the granitoids. It is proposed that the massif represents subduction-related magmatism of an ocean in the southern part of the massif (an Andean plate margin).     

Late Cretaceous redbeds from the Gan-Hang Belt in Southeast China: petrography and geochemistry implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in Southeast China were generated by crustal extension associated with subduction of the Palaeo-Pacific plate during the late Mesozoic. Compared with adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstones from the Late Cretaceous Guifeng Group in the Yongchong Basin, Southeast China. Detrital grains are subangular to subrounded, poorly sorted, and rich in lithic fragments. Variable Chemical Index of Alternation values (59.55–79.82, avg. 66.79) and high Index of Compositional Variability (ICV) values (0.67–3.08, avg. 1.40) indicate an overall low degree of chemical weathering and rapid physical erosion of source rocks. Such features are consistent with an active extension tectonic setting. Other chemical indices (e.g. Al₂O₃/TiO₂, Th/U, Cr/Th, Th/Sc, Zr/Sc) also suggest significant first-cycle sediment input to the basin and a dominant felsic source nature. Thus, the Guifeng Group possibly underwent moderate to low degrees of weathering upwards. Sandstone framework models and geochemical characteristics suggest the provenance was likely a combination of passive margin (PM) and active continental margin (ACM) with minor continental island arc (CIA) tectonic settings. Sediment derivation from Neoproterozoic metamorphic rocks and Cambrian to Triassic granitoids indicates PM provenance, whereas sediments derived from Jurassic to Cretaceous granitoids suggest ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben basin under the back-arc extension regime when Southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.