福建东山县澳角村澳角群花岗质石榴黑云片麻岩锆石SHRIMP U-Pb定年及其地质意义

本文对福建东山县澳角村澳角群花岗质石榴黑云片麻岩中17颗锆石进行了SHRIMP U-Pb同位素年龄测定。结果显示锆石Th/U比值变化于0.31~1.01,平均值为0.48,为典型的岩浆锆石;其中16个分析点在谐和图上分布较为集中,206Pb/238U年龄加权平均值为(152.8±1.6)Ma(n=17,MSWD=1.12);结合锆石自形、发育岩浆环带等特点,该年龄被解释为澳角群亲营山组花岗质石榴黑云片麻岩的形成年龄。其所指示的地质意义:(1)福建东山县澳角村澳角群亲营山组花岗质石榴黑云片麻岩形成于晚侏罗纪,而非前泥盆纪;(2)福建东山县澳角村澳角群亲营山组应解体为表壳岩与变质侵入体。     

Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100°C to 550°C: Case study in the Liassic black shale formation of Central Swiss Alps

Hydrocarbon distributions and stable isotope ratios of carbonates (δ¹³C_car, δ¹⁸O_car), kerogen (δ¹³C_ker), extractable organic matter (δ¹³C_EOM) and individual hydrocarbons of Liassic black shale samples from a prograde metamorphic sequence in the Swiss Alps were used to identify the major organic reactions with increasing metamorphic grade. The studied samples range from the diagenetic zone (<100°C) to amphibolite facies (∼550°C). The samples within the diagenetic zones (<100 and 150°C) are characterized by the dominance of C_<20n-alkanes, suggesting an origin related with marine and/or bacterial inputs. The metamorphic samples (200 to 550°C) have distributions significantly dominated by C₁₂ and C₁₃n-alkanes, C₁₄, C₁₆ and C₁₈n-alkylcyclopentanes and to a lesser extend C₁₅, C₁₇ and C₂₁n-alkylcyclohexanes. The progressive ¹³C-enrichment (up to 3.9‰) with metamorphism of the C_>17n-alkanes suggests the occurrence of cracking reactions of high molecular weight compounds. The isotopically heavier (up to 5.6‰) C_<17n-alkanes in metamorphic samples are likely originated by thermal degradation of long-chain homologous with preferential release of isotopically light C₁ and C₂ radicals. The dominance of specific even C-number n-alkylcyclopentanes suggests an origin related to direct cyclization mechanism (without decarboxylation step) of algal or bacterial fatty acids occurring in reducing aqueous metamorphic fluid conditions. The regular increase of the concentrations of n-alkylcycloalkanes vs. C_>13n-alkanes with metamorphism suggests progressive thermal release of kerogen-linked fatty acid precursors and degradation of n-alkanes. Changes of the steroid and terpenoid distributions are clearly related to increasing metamorphic temperatures. The absence of 18α(H)-22,29,30-trisnorneohopane (Ts), the occurrence of 17β(H)-trisnorhopane, 17β(H), 21α(H)-hopanes in the C₂₉ to C₃₁ range and 5α(H),14α(H),17α(H)-20R C₂₇, C₂₉ steranes in the low diagenetic samples (<100°C) are characteristic of immature bitumens. The higher thermal stress within the upper diagenetic zone (150°C) is marked by the presence of Ts, the disappearance of 17β(H)-trisnorhopane and thermodynamic equilibrium of the 22S/(22S + 22R) homohopane ratios. The increase of the ααα-sterane 20S/(20S + 20R) and 20R ββ/(ββ + αα) ratios (from 0.0 to 0.55 and from 0.0 to 0.40, respectively) in the upper diagenetic zone indicates the occurrence of isomerization reactions already at <150°C. However, the isomerization at C-20 (R → S) reaches thermodynamic equilibrium values already at the upper diagenesis (∼150°C) whereas the epimerisation at C-14 and C-17 (αα → ββ) arrives to constant values in the lower anchizone (∼200°C). The ratios Ts vs. 17α(H)-22,29,30-trisnorneohopane [(Ts/(Ts + Tm)] and 18α(H)-30-norneohopane (C₂₉Ts) vs. 17α(H),21β(H)-30-norhopane [C₂₉Ts/(C₂₉Ts + C₂₉)] increase until the medium anchizone (200 to 250°C) from 0.0 to 0.96 and from 0.0 to 0.44, respectively. An opposite trend towards lower values is observed in the higher metamorphic samples.The occurrence of specific hydrocarbons (e.g., n-alkylcyclopentanes, cadalene, hydrogenated aromatic compounds) in metamorphic samples points to kerogen degradation reactions most probably occurring in the presence of water and under reducing conditions. The changes of hydrocarbon distributions and carbon isotopic compositions of n-alkanes related to metamorphism suggest that the organic geochemistry may help to evaluate the lowest grades of prograde metamorphism.     

Hydrogen isotopic fractionation in lipid biosynthesis by H2-consuming Desulfobacterium autotrophicum

We report hydrogen isotopic fractionations between water and fatty acids of the sulfate-reducing bacterium Desulfobacterium autotrophicum. Pure cultures were grown in waters with deuterium (D) contents that were systematically varied near the level of natural abundance (−37‰ ? δD ? 993‰). H₂ of constant hydrogen isotope (D/H) ratio was supplied to the cultures. The D/H ratios of water, H₂, and specific fatty acids were measured by isotope-ratio mass spectrometry. The results demonstrate that D. autotrophicum catalyzes hydrogen isotopic exchange between water and H₂, and this reaction is conclusively shown to approach isotopic equilibrium. In addition, variation in the D/H ratio of growth water accounts for all variation in the hydrogen isotopic composition of fatty acids. The D/H ratios of fatty acids from cultures grown on H₂/CO₂ are compared with those from a separate set of cultures grown on D-enriched formate, an alternative electron donor. This comparison rules out H₂ as a significant source of fatty acid hydrogen. Grown on either H₂/CO₂ or formate, D. autotrophicum produces fatty acids in which all hydrogen originates from water. For specific fatty acids, biosynthetic fractionation factors are mostly in the range 0.60 ? α_FA-water ? 0.70; the 18:0 fatty acid exhibits a lower fractionation factor of 0.52. The data show that α_FA-water generally increases with length of the carbon chain from C₁₄ to C₁₇ among both saturated and unsaturated fatty acids. These results indicate a net fractionation associated with fatty acid biosynthesis in D. autotrophicum that is slightly smaller than in another H₂-consuming bacterium (Sporomusa sp.), but much greater than in most photoautotrophs.     

变质流体研究新进展

变质流体是变质过程的主要动力学因素之一。目前变质流体研究主要集中在下部地壳麻粒岩相变质流体，俯冲带高压－超高压变质流体和接触变质流体等方面。研究的主要问题是流体流动机制和元素迁移，流体－岩石相互作用和流体来源。下部地壳麻粒岩相变质流体以ＣＯ２为主，具有较低的ａＨ２Ｏ。δ１３Ｃ研究表明大约２／３ＣＯ２是深成的。富ＣＯ２流体流动是紫苏花岗岩形成和热扰动的原因之一，也是麻粒岩形成和大离子亲石元素亏损的主要因素。俯冲带是高压、超高压变质作用发生和流体活动最活跃的场所。流体富含Ｈ２Ｏ、ＣＨ４和ＣＯ２，可以诱导部分熔融反应和岛弧岩浆作用。高压变质条件下的矿物稳定性也与流体有关。同位素研究表明，在超高压变质期间没有化学上完全相同的流体大规模循环。流体－熔体系统模式能更有效地解释下插板片的元素再循环。接触变质流体研究主要集中在含有易于发生流体－岩石反应的不纯碳酸盐岩地区。硅灰石带中流体／岩石比率高达４０∶１，表明接触变质岩石中有大量流体存在。接触变质过程流体成分有较大差异，主要取决于流体来源、原岩性质和侵入体特征。流体流动和循环模式受控于构造变形，岩浆作用和变质过程的动力学条件及流体成分。     

阿尔金淡水泉早古生代泥质高压麻粒岩及其P-T演化轨迹

南阿尔金构造带淡水泉一带出露的含石榴石蓝晶石黑云母片麻岩是一套典型的泥质高压麻粒岩,其峰期特征矿物组合为石榴子石+蓝晶石+钾长石+金红石+石英.根据矿物内部一致性热力学数据和Thermocalc 3.23程序计算,确定其峰期变质温压条件为T>850℃和P>11kbar.结合岩相学研究和P-T视剖面图计算,可识别出该岩石经历了4个阶段的变质演化,构成了一个早期快速等温降压,后期近等压降温的顺时针型的退变质P-T演化轨迹,为与陆壳俯冲碰撞有关的高压变质事件的产物.该岩石锆石阴极发光图像显示其内部具有明显的核.边结构,核部为残留的原岩碎屑锆石,边部则表现为面状或扇状生长的变质锆石的特征.微区原位LA-ICP-MS微量元素分析表明,核部测点的重稀土含量较高,对应Th/U接近于0.4,具有岩浆锆石的特征;边部测点的重稀土相对亏损,重稀土配分曲线平坦,对应Th/U比值均小于0.1,显示与石榴子石平衡共生的变质锆石特征.LA-ICP-MS微区定年获得其变质年龄为486±5Ma,该年龄值与阿尔金江尕勒萨依和英格利萨依两地超高压变质岩石的变质年龄相近,进一步证明沿阿尔金构造带南缘断续存在一条早古生代的高压-超高压变质岩带.另外,本次研究在获得该泥质高压麻粒岩峰期变质时代的同时,还获得该岩石原岩的形成时代上限值约为719Ma,从而限定阿尔金构造带南缘阿尔金群的形成时代可能不属古元古代,而应属新元古代.     

Seasonal variation of fatty acids from drip water in Heshang Cave, central China

In order to investigate how lipids in cave water respond to seasonal climate change, drip water samples were collected from 2006 to 2008 in Heshang Cave, central China for fatty acid analysis. These lipids are abundant in the drip water. Their compositions are dominated by lower-molecular-weight nC16:0, nC18:0 and nC14:0 acids, together with mono-unsaturated nC18:1, nC16:1 and nC14:1. Analysis of one water sample revealed marked differences between the dissolved and particulate fractions. The dissolved fraction contains total fatty acids one order of magnitude higher than that of the particulate fraction. The distributional patterns of the fatty acids suggest that microbes living in the overlying soils and/or groundwater system contribute most fatty acids to the drip water. This 2-a monitoring experiment reveals that the abundance of mono-unsaturated fatty acids relative to the saturated homologues (nC16:1/nC16:0 and nC18:1/nC18:0) relate inversely to the changes of synchronous external air temperature. Higher values occur under cold conditions (winter/spring), while lower values appear in warm intervals (summer). Further studies are needed to elucidate the dynamic processes by which the external temperature affects fatty acids in drip water and to confirm the potential application of fatty acid ratios such as nC16:1/nC16:0 and nC18:1/nC18:0 in paleotemperature reconstructions.     

西昆仑赞坎铁矿地质和地球化学特征及矿床类型探讨

赞坎铁矿位于西昆仑造山带塔什库尔干地块西段,是近年新发现的一个大型沉积变质型磁铁矿床。铁矿体的顶一底板围岩分别为古元古代布伦阔勒群斜长角闪片岩和黑云母石英片岩;矿区南部出露早古生代花岗岩岩体;矿石类型以条带状及浸染状磁铁矿石为主,兼具少量块状磁铁矿石。在详细矿区地质观察的基础上,本文报道了矿石的地球化学特征与锆石U-Pb年代学分析结果。元素地球化学分析表明,条带状铁矿石具有较高的稀土总量,明显富集轻稀土,未呈现明显Ce和Y元素的异常(Ce/Ce~*=0.93~1.12、Y/Y~*=0.74~1.30);部分矿石显示Eu正异常(Eu/Eu~*=1.66~4.46),稀土配分形式与沉积变质型铁矿相似。矿体围岩变粒岩锆石U-Pb年龄为2 416±54 Ma,大致反映沉积铁矿的形成时代,而807±51 Ma的年龄反映变质作用时代。矿区一有铁矿捕虏体的花岗岩锆石U-Pb年龄为504±26 Ma。该期花岗岩的侵入对铁矿有明显的改造和叠加作用。     

淄博市瓦泉寨矿区铁矿床地质特征及其成因

沉积变质型铁矿是山东省鲁西地区重要成因类型之一,含矿层位为新太古代泰山岩群。经过地表工程和钻探工程验证,瓦泉寨矿区C1、C2、C3、C5磁异常是由铁矿体引起。矿体总体走向NW,倾向NE,倾角55°~88°,厚度1.44~6.29m;主要矿物为磁铁矿;矿石主要呈粒状变晶结构、条带状构造。同时,本文对沉积变质型铁矿床的成因进行了分析,铁质来源与基性火山活动关系密切。     

滇西哀牢山变质岩系锆石U-Pb定年及其地质意义

哀牢山-红河构造带是滇西地区最著名的带状变质带之一,其主体由哀牢山深变质岩系(哀牢山岩群)组成,一直被认为是扬子陆块古元古代结晶基底.本文选取哀牢山深变质岩系内的花岗片麻岩(11 ALl7-1和11AL09-1)和石英岩(11AL08-1),以及邻区的花岗岩(11ALl2-1)进行LA-ICP-MS锆石U-Pb定年.结果显示,花岗片麻岩11 ALl7-1有岩浆和变质两类锆石,两者的206Pb/238U年龄加权平均值分别为700±6Ma(MSWD=1.4,n=14)和27.4±1.2Ma(MSWD=1.9,n=3),代表原岩形成时代和变质年龄.花岗片麻岩llAL09-1岩浆锆石206 pb/238U年龄为220±3Ma(MSWD=3.1,n=14),变质锆石年龄为31.2±2.3Ma(MSWD =6,n=5),分别代表原岩结晶时代和后期变质年龄.石英岩11AL08-1中所有锆石具有核-边结构,92颗锆石核部年龄集中分布在6组,分别为493～528Ma(n=42)、635 ～ 640Ma(n=2)、701～784Ma(n=44)、976 ～980Ma(n=2)、1839Ma(n=1)和2487Ma(n=1).92个核部分析点具有高的Th/U比值(＞0.23),指示岩浆来源.最年轻一组的42个核部年龄加权平均值为509Ma,代表石英岩原岩的最大沉积时代.7颗锆石变质边年龄为26～ 75 Ma内,代表变质年龄.花岗岩11 ALl2-1锆石206pb/238U年龄加权平均值为750±4Ma(MSWD =0.6),代表岩石形成时代.这些年龄表明哀牢山变质岩系是一个原岩复杂的变质杂岩带,它的原始物质至少包含新元古代～ 700Ma岩浆岩、～509 Ma沉积地层及220 ～ 240Ma的岩浆岩和地层,而不是以往认为的古元古代结晶基底.现今所见的哀牢山岩群“古老”岩石面貌主要是由地质历史上的浅变质或未变质的地层和岩浆岩在新生代26～31Ma发生变质变形作用改造的结果.哀牢山变质带的源区物质特征和主要岩浆事件与扬子陆块西缘十分相似,具有亲扬子的构造属性.     

Did prolonged two-stage fragmentation of the supercontinent Kenorland lead to arrested orogenesis on the southern margin of the Superior province?

Recent geochronological investigations reinforce the early suggestion that the upper part of the Paleoproterozoic Huronian Supergroup of Ontario,Canada is present in the Animikie Basin on the south shore of Lake Superior.These rocks,beginning with the glaciogenic Gowganda Formation,are interpreted as passive margin deposits.The absence of the lower Huronian(rift succession) from the Animikie Basin may be explained by attributing the oldest Paleoroterozoic rocks in the Animikie Basin(Chocolay Group)to deposition on the upper plate of a north-dipping detachment fault,which lacks sediments of the rift phase.Following thermal uplift that led to opening of the Huronian Ocean on the south side of what is now the Superior province,renewed uplift(plume activity) caused large-scale gravitational folding of the Huronian Supergroup accompanied by intrusion of the Nipissing diabase suite and Senneterre dikes at about 2.2 Ga.Termination of passive margin sedimentation is normally followed by ocean closure but in the Huronian and Animikie basins there was a long hiatus- the Great Stratigraphic Gap- which lasted for about 350 Ma.This hiatus is attributed to a second prolonged thermal uplift of part of Kenorland that culminated in complete dismemberment of the supercontinent shortly before 2.0 Ga by opening of the Circum-Superior Ocean.These events caused regional uplift(the Great Stratigraphic Gap) and delayed completion of the Huronian Wilson Cycle until a regional compressional tectonic episode,including the Penokean orogeny,belatedly flooded the southern margin of the Superior province with foreland basin deposits,established the limits of the Superior structural province and played an important role in constructing Laurentia.     

Trace elements in various genetic types of zircon from syenite of the Sakharjok massif,Kola Peninsula

Zircon is an accessory mineral in alkali and nepheline syenites of the Neoarchean Sakharjok intrusion. Zircon in association with britholite and pyrochlore forms orebodies in nepheline syenite of this massif. Zircon crystals reveal an inhomogeneous zonal, occasionally mosaic structure comprising fragments and zones related to magmatic, hydrothermal, and metamorphic stages of mineral formation. Magmatic zircon differs by a high REE concentration (1769 ppm, on average), distinct Ce maximum (Ce/Ce* = 105, on average), and Eu minimum (Eu/Eu* = 0.19) as compared with other genetic types. No correlation between these parameters has been established. Hydrothermal zircon is characterized by a low Ce/Ce* ratio (0.7–3.9 and 2.0, on average), elevated LREE contents, and lowered ratios of MREE and HREE to La. Metamorphic zircon differs from magmatic by a sharply lower REE concentration (385 ppm, on average), lowered Th/U (0.32) and Ce/Ce* (31.9, on average) ratios. In the Ce/Ce* versus MREE/La plot, the lowest values of these ratios are typical of hydrothermal zircon, while the intermediate and maximum values are inherent to metamorphic and magmatic zircons, respectively. These variations make it possible to delineate reliable fields of their compositions. The distribution of data points in the above-mentioned plots shows that REE chemical activity depends on the redox conditions of zircon crystallization.     

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。     

Geochronology and geochemistry of leucosomes in the North Dabie Terrane, East China: implication for post-UHPM crustal melting during exhumation

Migmatites are widespread in the North Dabie ultrahigh-pressure metamorphic terrane (NDT) of Dabie orogen, East China. Idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase, quartz and biotite, suggesting formation of leucosome by fluid-present melting of biotite + plagioclase + quartz-bearing protoliths at P = 5–7 kbar, T = 700–800 °C. Precise SIMS zircon U–Pb dating indicates that migmatization of Dabie orogen initiated at ~140 Ma and lasted for ~10 Ma, coeval with the formation of low-Mg^# adakitic intrusions in Dabie orogen. Based on mineralogical, petrographic and geochemical data, leucosomes in NDT can be subdivided into three groups. (1) High La/Yb_(N)–Medium Sr/Y group (Group I), whose high Dy/Yb_(N) but medium Sr/Y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses. (2) Low La/Yb_(N)–Low Sr/Y group (Group II), whose flat HREE patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss. (3) High La/Yb_(N)–High Sr/Y and Eu^# group (Group III), whose extremely high Sr and Eu but low other REE concentrations are caused by accumulation of plagioclase and quartz. Although Group I and III fall in the adakitic fields on La/Yb_(N)–Yb_(N) and Sr/Y–Y diagrams, they are chemically distinct from contemporary high-pressure adakitic intrusions in Dabie orogen in a series of geochemical indexes, for example, lower Dy/Yb_(N) and/or Sr/Y ratios at given La/Yb_(N) ratio, lower Sr/CaO ratios, lower Rb concentration but higher K/Rb ratios. Therefore, leucosomes are produced by anatexis of the exhumed ultrahigh-pressure metamorphic rocks at middle crustal level, instead of partial melting of thickened lower crust with garnet-rich and plagioclase-poor residual. The coeval occurrence of migmatites and high-pressure adakitic intrusions in Dabie orogen indicates large-scale partial melting of middle to thickened lower crustal column in the early Cretaceous. The required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed.     

贺兰山——阿拉善地区下、中前寒武系的划分对比及其变质、成矿作用特征

据近期成果,贺兰山—阿拉善地区出露的巨厚变质杂岩可划分为中太古界贺兰山群和叠布斯格群(其全岩Rb—Sr等时年龄为3108.3和3218.8Ma),上太古界阿拉善群和下元古界的赵池沟群、阿拉坦敖包群;它们具不同的变质矿物共生组合,太古界变质岩属低压高温变质的麻粒岩相;下元古界为低—低中压区域动力(热流)变质的绿片岩相岩石。太古界有较强的混合岩化、花岗岩化作用,并蕴藏有铁、石墨、矽线石、刚玉等多种矿产。     

Genesis of proterozoic iron-formation: sulphur isotope evidence

Seven units of carbonaceous shale or sulphide-facies iron-formation have been sampled. They are associated with Proterozoic iron-formations that range in age from ~ 1.9 to ~2.5 Ga: Sokoman and Gunflint (Canada), Riverton (United States), Penge (South Africa) and Brockman (Australia). Sulphur isotope ratios have been determined on the sulphides removed from these shales by both physical and chemical means.The mean δ³⁴S composition of the seven units varies between ?4.9%. and +6.6%. and the sample variance is low within each unit. These distributions are more characteristic of hydrothermal sulphide than sulphide produced by biogenic reduction. This hydrothermal sulphide is believed to have originated from high temperature reduction of seawater sulphate and from magmatic sulphide. A model is suggested whereby this sulphide was exhaled into stratified anoxic/oxic basins. The sulphide and associated base metals were deposited in the reduced sediments beneath the anoxic waters, while some iron and manganese was deposited on oxygenated shelves.The data support, but do not prove, a hydrothermal exhalative origin for lower Proterozoic iron-formation.     

康滇地轴中段前震旦系马鞍山组的发现及特征

四川会东县“马鞍山组”的发现，是近年来康滇地轴中段前震旦系地质研究的重要进展，对提高基础地质研究，进一步寻找会理拉拉式铜矿具有重要意义。“马鞍山组”的剖面特征和岩石学研究表明，该组相于古元古界河口群，岩性组合主要为海相喷发的钠质火山岩－细碧岩组合，蚀变强烈；火山岩岩石化学特征表现为富钠、贫钾，高钙、低铁，岩性偏中酸性，火山岩属碱性玄武岩系列。微量元素以Cu-Au-U-Zn-As-Sb-Fe-Cr-Co组合为特征。岩石的稀土总量低，轻重稀土比值大，为轻稀土富集型，具有等铕亏损和微弱铈亏损；其配分模式图呈向右倾斜的“V”字型。     

Archean chemical weathering at three localities on the Canadian Shield

Sapropelite developed on a granodiorite basement below the Archean Steeprock Group in the western Superior Structural Province of Canada, weathered materials from a 3.15 Ga granitic basement to the Archean Yellowknife Supergroup in the Slave Structural Province, and weathered detritus from contemporaneous komatiites and gneissic basement of the Archean Prince Albert Group in northern Churchill Structural Province, have been analysed. The deviation of the weakly metamorphosed weathered material from the chemical composition of their precursors is comparable to that seen in more recent weathered profiles from similar source materials. This includes the enhancement of ferric to ferrous iron ratios, and potash to soda, lime and magnesia ratios. There is also an increase in lighter REE and a depletion of heavier REE. Quartz rich sediments immediately above the unconformity, derived from the basement granitoids, may be enriched in aluminum and iron, and yield chloritoid under proper metamorphic conditions. Sediments formed from mixed sources, such as weathered gneissic basement and contemporaneous komatiitic volcanics, yield anomalous resistate sedimentary mixtures such as chromiferous quartzites. Based on the data given here, there is no need to postulate that different weathering and/or ground water systems were operative during the Archean, compared to more recent times. On the other hand the complicated weathering and diagenetic history that altered material undergoes below a subsiding unconformity, relies as much or more on the buffering capacity of the enclosed rock mass as on the atmospheric conditions prevalent at the time of weathering. Hence weathering profiles, old and new, are more a response to local environmental conditions within the rock than the composition of the atmosphere.     

Adaminaby Group west of Batemans Bay: Deformation and metamorphism of the Narooma accretionary complex,NSW

This study provides new structural data that show that the Adaminaby Group is part of the Narooma accretionary complex and has been overprinted by HT/LP metamorphism associated with Middle Devonian Moruya Suite intrusions. The grade of metamorphism based on Kübler Indices is the same in the Wagonga and Adaminaby Groups at Batemans Bay inferring that these rocks were involved in the same accretionary event. White micas in slates of the Adaminaby Group record apparent K–Ar ages of 384.6 ± 7.9 Ma and 395.8 ± 8.1 Ma. These ages are believed to represent the age of Middle to Upper Devonian Buckenbowra Granodiorite. Kübler Index values indicate lower epizonal (greenschist facies) metamorphic conditions and are not influenced by heating in metamorphic aureoles of the plutons. All b cell lattice parameter values are characteristic of intermediate pressure facies conditions although they are lower in the metamorphic aureole of the Buckenbowra Granodiorite than in the country rock, defining two areas with dissimilar baric conditions. East of the Buckenbowra Granodiorite, b cell lattice parameter values outside the contact aureole (x = 9.033 Å; n = 8) indicate P = 4 kb, and assuming a temperature of 300°C, infer a depth of burial of approximately 15 km for these rocks with a geothermal gradient of 20°C/km. In the metamorphic aureole of the Buckenbowra Granodiorite, b cell lattice parameter values (x = 9.021 Å; n = 41) indicate P = 3.1 kb inferring exhumation of the Adaminaby Group rocks to a depth of approximately 11 km prior to intrusion. A geothermal gradient of 36°C/km operated in the aureole during intrusion. An extensional back-arc environment prevailed in the Adaminaby Group during the Middle to Upper Devonian.     

北苏鲁仰口地区变辉长岩中锆石U-Pb定年、微量元素和Hf同位素特征及其地质意义

北苏鲁仰口地区出露超高压的变辉长岩.锆石阴极发光图像和其内部矿物包体激光拉曼测试的联合研究结果表明,变辉长岩锆石具有弱发光效应的岩浆韵律环带的核和被改造的强发光效应的边.岩浆韵律环带的核部保存大量而复杂的矿物包体,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(P1)+石英(Qtz)+黑云母(Bt)+钛铁矿(Ilm)+磷灰石(Ap);边部保存的矿物包体则相对较少,包括普通辉石(Cpx)+斜方辉石(Opx)+斜长石(Pl)+磷灰石(Ap).尽管岩浆韵律环带核部的稀土元素总合量比被改造的锆石边部明显偏高,但二者稀土元素配分模式具有明显的相似性,主要表现为轻稀土相对亏损,而重稀土明显富集,相应的(La/Yb)N=0.00015～0.00039,并具有明显的负Eu异常(Eu/Eu*=0.20～0.26)、相对明显的正Ce异常(Ce/Ce*=71.5～147.4)和较高的Th/U比值(1.97～2.69).上述特征表明,仰口地区变辉长岩中的锆石均为继承性的岩浆锆石,而没有新生的变质锆石.LA-(MC)-ICP-MS锆石原位U-Pb定年和Lu-Hf同位素分析结果表明,两件锆石样品Y1和Y2的年龄数据所构成的不一致线显示了十分接近的上交点和下交点年龄.其上交点年龄分别为785±15Ma(2σ)和784±12Ma(2σ),应代表原岩的形成时代,表明变辉长岩的原岩与Rodinia超大陆裂解的岩浆事件存在密切的成因关系;而下交点年龄分别为226±24Ma(2σ)和228±26Ma(2σ),与苏鲁其它类型超高压岩石中含柯石英锆石微区记录的变质年龄十分吻合,应代表变辉长岩的超高压变质时代.岩浆结晶锆石的核部具有明显偏高的176Lu/177Hf(0.00044～0.00291)和176Yh/177Hf(0.0165～0.1168)比值,而176Hf/177Hf比值变化于0.281956～0.282048之间,相应的εHf(t)=-8.5～-14.0,tDM2=2.03～2.32Ga,表明仰口地区变辉长岩的原岩起源于古元古代时期的富集地幔或发生部分熔融的下地壳残留体.被改造的岩浆结晶锆石的边部则具有明显偏低的176Lu/177Hf(0.00029～0.00060)和176Yh/177Hf(0.0112～0.0200)比值,而176Hf/177Hf(t)比值变化于0.281953～0.282002之间,相应的εHf(t)=-10.2～-11.9,tDM2=2.12～2.21Ga.与岩浆结晶锆石核部相比,被改造的岩浆锆石边部的176Lu/177Hf、176Yb/177Hf、176Hf/177Hf(t)比值和εHf(t)和tDM2值的变化范围更小,表明中-新三叠纪的超高压变质作用使岩浆结晶锆石边部的Lu-Hf同位素体系发生调整,更趋向于均一化.     

Deformation‐driven,regional‐scale metasomatism in the Hamersley Basin,Western Australia

Mafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km² Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc , in subsets of the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–Fe₂O₃, successfully reproduces observed metamorphic mineral assemblages, giving conditions of ~210 °C, 2 kbar in the north and 335 °C, 3.2 kbar in the south. Superimposed on this metamorphic gradient, regional‐scale metasomatism in the Fortescue Group progressively produces a suite of prehnite‐bearing and pumpellyite–quartz/epidote–quartz‐dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260–280 °C, 2.5–3 kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional‐scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.