Trace-element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills,Australia

Hydrothermal zircon can be used to date fluid-infiltration events and water/rock interaction. At the Boggy Plain zoned pluton (BPZP), eastern Australia, hydrothermal zircon occurs with hydrothermal scheelite, molybdenite, thorite and rutile in incipiently altered aplite and monzogranite. The hydrothermal zircon is texturally distinct from magmatic zircon in the same rocks, occurring as murky-brown translucent 20–50 μm-thick mantles on magmatic cores and less commonly as individual crystals. The hydrothermal mantles are internally textureless in back-scatter electron and cathodoluminescence images whereas magmatic zircon is oscillatory zoned. The age of the hydrothermal zircon is indistinguishable from magmatic zircon, indicating precipitation from a fluid evolved from the magma during the final stages of crystallization. Despite indistinguishable U-Pb isotopic compositions, the trace-element compositions of the hydrothermal and magmatic zircon are distinct. Hydrothermal zircon is enriched in all measured trace-elements relative to magmatic zircon in the same rock, including V, Ti, Nb, Hf, Sc, Mn, U, Y, Th and the rare-earth elements (REE). Chondrite-normalized REE abundances form two distinct pattern groupings: type-1 (magmatic) patterns increase steeply from La to Lu and have Ce and Eu anomalies—these are patterns typical for unaltered magmatic zircon in continental crust rock types; type-2 (hydrothermal) patterns generally have higher abundances of the REE, flatter light-REE patterns [(Sm/La)_N = 1.5–4.4 vs. 22–110 for magmatic zircon] and smaller Ce anomalies (Ce/Ce* = 1.8–3.5 vs. 32–49 for magmatic zircon). Type-2 patterns have also been described for hydrothermally-altered zircon from the Gabel Hamradom granite, Egypt, and a granitic dyke from the Acasta Gneiss Complex, Canada.Hadean (∼4.5–4.0 Ga) zircon from the Jack Hills, Western Australia, have variable normalized REE patterns. In particular, the oldest piece of Earth—zircon crystal W74/2-36 (dated at 4.4 Ga)—contains both type-1 and type-2 patterns on a 50 μm scale, a phenomenon not yet reported for unaltered magmatic zircon. In the context of documented magmatic and hydrothermal zircon compositions from constrained samples from the BPZP and the literature, the type-2 patterns in crystal W74/2-36 and other Jack Hills Hadean (JHH) zircon are interpreted as hydrothermally-altered magmatic compositions. An alteration scenario, constrained by isotope and trace-element data, as well as α-decay event calculations, involving fluid/zircon cation and oxygen isotope exchange within partially metamict zones and minor dissolution/reprecipitation, may have occurred episodically for some JHH zircon and at ∼4.27 Ga for zircon W74/2-36. Type-2 compositions in JHH zircon are interpreted to represent localized exchange with a light-REE-bearing, high δ¹⁸O (∼6–10‰ or higher) fluid. Thus, a complex explanation involving “permanent” liquid water oceans, large-scale water/rock interaction and plate tectonics in the very early Archean is not necessary as the zircon textures and compositions are simply explained by exchange between partially metamict zircon and a low volume ephemeral fluid.     

Proterozoic volcanism in the Jack Hills Belt,Western Australia: Some implications and consequences for the World's oldest zircon population

Rare felsic volcanic rocks of dacitic to rhyolitic composition occur in the central part of the Jack Hills metasedimentary belt in the Narryer Terrane of Western Australia, interleaved with clastic sedimentary rocks and amphibolite. Representative samples of the four identified felsic volcanic units reveal a similar complex pattern of zircon age distribution, with all samples containing zircon populations at ∼3.3–3.4, ∼3.0–3.1, ∼2.6 and ∼1.8–1.9 Ga. The ∼3.3–3.4 Ga zircons show well-developed oscillatory zoning in cathodoluminescence (CL) images and are interpreted as inherited igneous zircon derived from granitic precursors, similar to the ∼3.3 Ga trondhjemitic granitoids currently exposed along the northern and southern margins of the belt. The ∼3.0–3.1 Ga zircons also reveal well-developed oscillatory zoning in CL and are most likely derived from granitoid and/or volcanic rocks of this age, as recorded in the Murchison domain to the south and possibly also present in the Narryer Terrane. The ∼2.6 Ga population matches the age of nearby late Archean granitoids intruding the Jack Hills belt and their oscillatory zoning and U–Th chemistry is consistent with their origin from such a source. The youngest discrete group of zircon grains, with ages ranging from ∼1970 to ∼1775 Ma, show strong oscillatory zoning and average Th/U ratios of 0.76, features consistent with an igneous origin. These younger zircons are therefore interpreted as defining the age of crystallisation of the volcanic rocks. These results establish that the Jack Hills metasedimentary belt contains significant post-Archean components. Taken together with similar results obtained from zircon occurring as detrital grains in clastic sedimentary rocks at Jack Hills, these results overturn the generally-accepted view that the belt is entirely Archean in age and that sedimentation was completed around 3.0 Ga ago. Instead, there is a distinct possibility that much of the material currently exposed in the Jack Hills belt formed in the Proterozoic. A further implication of this study is that the metamorphism affecting these rocks also occurred in the Proterozoic and consequently the rocks should not be considered as forming an Archean greenstone or metasedimentary belt. The paucity of zircons >4 Ga in the known Proterozoic sedimentary rocks and their total absence in the felsic volcanic rocks suggests that such ancient source rocks were no longer present in the area.     

A Precambrian fayalite granite from the south coast of Western Australia

A Precambrian fayalite granite outcropping at Lower King, near Albany, Western Australia, is interpreted as a high-Fe²⁺/(Fe²⁺ + Mg) analogue of charnockite. Calculation of the original titanomagnetite composition from analytical data on ilmenomagnetite ‘exsolution’ intergrowths suggests initial crystallisation of opaque oxides at about 940°C and 10^?12 bars f_o2. This result indicates a magmatic origin for the rock. Other determinable points on the T-f_o2 cooling curve of the fayalite granite pluton include crystallisation of biotite at roughly 800–820°C and 10^?14.5 bars f_o2, and final equilibration of opaque oxides below 550°C and 10^?23 bars f_o2. Mineralogical data on nearby granulite facies country rocks suggest a regional total pressure of roughly 5 kb, and hence the depth of pluton emplacement was probably around 18–19 km. Thus the Lower King fayalite granite is believed to have crystallised from water-deficient, high-T melt or partial melt generated, possibly from metasedimentary rocks, deep in the crust under granulite facies conditions.     

青海祁漫塔格山乌兰乌珠尔斑状正长花岗岩 LA-MC-ICPMS锆石U-Pb定年及地质意义

青海祁漫塔格地区乌兰乌珠尔正长花岗岩具有高硅、高碱(AR为3.02~5.88,NK/A为1.04~1.17,A/CNK为0.96~1.07)、高TFeO/MgO和贫CaO、MgO的特征;∑REE较高,LREE略富集,轻重稀土元素分馏不十分明显,Eu负异常明显,稀土元素配分模式呈稍右倾的V字形;明显富集Rb、K、Th等大离子亲石元素,相对富集Zr、Hf等高场强元素,相对亏损Ba、Sr、P、Ti、Nb等。这些资料显示了A2型铝质花岗岩的特征,其来源可能与洋壳、岛弧建造组成的年轻地壳有关。采用LA-MC-ICPMS锆石U-Pb定年方法,获得乌兰乌珠尔正长花岗岩体的206Pb/238U年龄为388.9Ma±3.7Ma(MSWD=3.1),代表该岩体的形成年龄。结合岩石地球化学、构造特征和区域地质背景,认为该岩体形成于后造山或造山作用演化晚期相对伸展的环境。     

浙江天台地区赖家组的划分与分布

柴达木盆地是中国目前保存较为完整的、连续的、巨厚的河－湖相沉积记录的盆地，通过对ZK701＋801，水6孔，涩中6和涩深1井等典型剖面孢子花粉组合特征和分布规律的研究，详细地论述了第四纪孢粉组合带及其所代表的植被，气候演变进程，自1．87MaB.P.以来共识出了59个孢粉组合带，划分出29个完整的冷暖气候旋回，并且同中国西北第四纪黄土－古土壤系列的37个气候旋回以及深海钻孔氧同位素气候旋回进行了较好的对比。     

Significance of ~ 1.5 Ga zircon and monazite ages from charnockites in southern Lithuania and NE Poland

The presence of 1.52–1.50 Ga charnockites from the anorthosite–mangerite–charnockite–granite (AMCG) Mazury complex in southern Lithuania and NE Poland, in the western East European Craton (EEC) is revealed by secondary ion mass-spectrometry (SIMS) and EPMA geochronology. Early 1.85–1.82 Ga charnockites are related to major orogeny in the region whereas the newly studied charnockites intrude the already consolidated crust. The 1.52–1.50 Ga charnockite magmatism (SIMS data on zircon) was followed by high-grade metamorphism (EPMA data on monazite), which strongly affected the surrounding rocks. The 1.85–1.81 Ga zircon cores in Lazdijai and 1.81 Ga monazite domains in the Lanowicze charnockites represent the protolith age of a volcanic island arc. The 1.52–1.50 Ga charnockite magmatism and metamorphism are likely related to the distal, Danopolonian, orogeny further to the west, at the margin of Baltica. The c.1.52–1.50 Ga AMCG magmatism and metamorphism in the western EEC as well as the paired accretionary-rapakivi suites in Amazonia, may be the inboard manifestations of the same early Mesoproterozoic orogeny associated with the juxtaposition of Amazonia and Baltica during the amalgamation of the supercontinent Columbia.     

青海祁漫塔格山乌兰乌珠尔斑状正长花岗岩LA-MC-ICPMS锆石U-Pb定年及地质意义

青海祁漫塔格地区乌兰乌珠尔正长花岗岩具有高硅、高碱(AR为3.02~5.88,NK/A为1.04~1.17,A/CNK为0.96~1.07)、高TFeO/MgO和贫CaO、MgO的特征;∑REE较高,LREE略富集,轻重稀土元素分馏不十分明显,Eu负异常明显,稀土元素配分模式呈稍右倾的V字形;明显富集Rb、K、Th等大离子亲石元素,相对富集Zr、Hf等高场强元素,相对亏损Ba、Sr、P、Ti、Nb等。这些资料显示了A2型铝质花岗岩的特征,其来源可能与洋壳、岛弧建造组成的年轻地壳有关。采用LA-MC-ICPMS锆石U-Pb定年方法,获得乌兰乌珠尔正长花岗岩体的206Pb/238U年龄为388.9Ma±3.7Ma(MSWD=3.1),代表该岩体的形成年龄。结合岩石地球化学、构造特征和区域地质背景,认为该岩体形成于后造山或造山作用演化晚期相对伸展的环境。     

Palynology of Archean microfossils (c. 3.0 Ga) from the Mount Grant area, Pilbara Craton, Western Australia: Further evidence of biogenicity

Palynological techniques have only rarely been applied to Archean rocks, mainly because of concerns about contamination. However, where microstructures are abundant and well documented, palynology can add considerably to interpretations based on thin-section examination. A modified palynological preparation method, which avoids vigorous physical or chemical methods that might cause the fragmentation of fragile specimens, was applied to black chert from the c. 3.0 Ga Farrel Quartzite of the Mount Grant area, Pilbara Craton, Western Australia, known to contain abundant microstructures of putative biogenic origin.Four main morphological types have been reported from chert thin sections: threads, films, hollow spheres and spindles. These microstructures exhibit morphological and chemical signatures consistent with a biological origin as well as showing features indicative of taphonomic degradation. Nevertheless, the possibility remains that they are artefacts. Similar structures have been attributed to physical processes, such as the accretion of fine particulate matter that has subsequently been redistributed as a result of crystal growth.By extracting structures palynologically, we have increased the probability of a biogenic origin. Firstly, examination of the morphology free of the encompassing matrix demonstrates that they are indeed three-dimensional structures with complex morphology; secondly, the extraction of entire specimens militates against them being particulate matter aggregations, which would simply disintegrate during preparation and thirdly, the chemicals used remove silicates and most other minerals, confirming the organic composition of the structures. To date, we have extracted films, hollow spheres and a few poorly preserved spindles, but preparation continues. The fact that the specimens retain their morphological integrity during palynological extraction argues strongly in favour of a biogenic origin for the microstructures.     

北大别白垩纪花岗岩中发现2.65 Ga新太古代继承锆石

<正>1研究目的（Objective）太古宙岩石是研究大陆地壳早期形成与演化最重要的载体,在探讨早期地壳物质成分、地壳增生与再造以及早期地球构造体制和板块构造启动时限等方面尤为重要,因此长期以来备受地学界关注。然而,华南板块的太古宙岩石出露较为稀少,目前仅在扬子北缘的崆岭、钟祥、鱼洞子、大别山黄土岭和扬子西南缘地区零星出露。位于扬子陆块北缘的大别造山带经历了三叠纪华南板块与华北板块陆-陆碰撞及白垩纪巨量岩浆活动的叠加改造,但仍有少量太古宙年龄信息的报道,     

Evidence for SEDEX-style mineralization in the 1.7 Ga Tawallah Group,McArthur Basin,Australia

The Paleoproterozoic McArthur Basin (McArthur Group) of northern Australia hosts world-class sedimentary ‘exhalative’ (SEDEX) McArthur type Zn–Pb deposits, which are largely hosted within a sequence of 1.64 Ga pyritic carbonaceous shales deposited in an extensional rift setting. A well-known example of these is McArthur River (or Here's Your Chance [HYC] Zn–Pb–Ag deposit). The ~ 1.78 Ga McDermott and ~ 1.73 Ga Wollogorang formations (Tawallah Group) both contain carbonaceous shales deposited in similar environments. Our observations suggest the carbonaceous facies of the Wollogorang Formation were deposited under mostly euxinic conditions, with periodically-high concentrations of sedimentary pyrite deposition. The carbonaceous shales in the older McDermott Formation contain considerably less early pyrite, reflecting a mostly sulfide-poor, anoxic depositional environment. Localized fault-bound sub-basins likely facilitated lateral facies variations, which is evident from synsedimentary breccias.The presence of evaporitic oxidized facies within the McDermott and Wollogorang formations, alongside evidence for synsedimentary brecciation in reduced shales are favourable criteria for SEDEX-style base metal deposition. Both formations overlie volcanic units, which could have been sources of base metals. Detailed X-ray petrography, new geochemical data and sulfur isotope data from historical drill cores indicate multiple horizons of stratiform and sediment breccia-hosted base metal sulfide within carbonaceous shale units, with high-grade Zn concentrations. A close association between sphalerite and ferromanganean dolomite alteration draws comparisons with younger SEDEX mineralization at HYC. Additionally, SEDEX alteration indices, used demonstrably as a vector to the younger orebodies, indicate the sedimentary rocks analyzed in this study are marginally below the ore window when compared to the overlying mineralized stratigraphy.Our data imply that localized active circulation of metalliferous brines occurred in the Tawallah Group basin. High-grade sulfide deposition in reduced facies alteration may represent distal expressions of larger SEDEX-style deposits. Furthermore, abundant pyrite and high molybdenum in the Wollogorang Formation suggest the global oceanic sulfate concentration was sufficient by ~ 1.73 Ga to engender intermittent but strong bottom-water euxinia during shale deposition, thus providing a robust chemical trap for base metal sulfide mineralization.     

辽宁鞍山地区东山杂岩带3.3~3.1 Ga期间的岩浆作用——锆石SHRIMP U-Pb定年

报道了鞍山地区东山杂岩带奥长花岗岩和二长花岗岩的锆石SHRIMP U-Pb年龄。中粗粒奥长花岗岩中岩浆锆石的年龄为3329 Ma ± 22 Ma (MSWD=9.6),存在年龄为3687~3784 Ma的残余锆石。细粒奥长花岗岩和二长花岗岩中岩浆锆石的年龄分别为3141 Ma ± 8 Ma (MSWD=1.5)和3142 Ma ± 5 Ma (MSWD=0.35)。研究表明,约~3.3 Ga和3.1 Ga是鞍山地区2个重要的地壳演化阶段。     

Taxonomy and biogenicity of Archaean spheroidal microfossils (ca. 3.0 Ga) from the Mount Goldsworthy–Mount Grant area in the northeastern Pilbara Craton, Western Australia

Microstructures recently reported from an Archaean sedimentary succession (ca. 3.0 Ga) in the Mount Goldsworthy–Mount Grant area in the northeastern Pilbara Craton meet the criteria for compelling evidence of biogenicity [Sugitani, K., Grey, K., Allwood, A., Nagaoka, T., Mimura, K., Minami, M., Marshall, C.P., Van Kranendonk, M.J., Walter, M.R., 2007. Diverse microstructures from Archaean chert from the Mount Goldsworthy–Mount Grant area, Pilbara Craton, Western Australia: microfossils, dubiofossil, or pseudofossils. Precambrian Res. 158, 228–262]. The structures are morphologically diverse. Although they were tentatively classified into five major morphological types (thread-like, film-like, small (<15 μm) and large (>15 μm) spheroidal, and spindle-like), the possible taxonomic significance of these groups was not discussed. Building on our earlier analysis, we focus on the morphology of the larger spheroids, as well as presenting further evidence relating spindles and several bizarre forms, and attempt to group them taxonomically and adduce additional evidence for their biogenicity.Taphonomic features were identified in each of the various morphological groups, but the range of morphological diversity of the spheroids cannot be attributed solely to taphonomic alteration. Four subdivisions of spheroids are proposed: (1) simple single-walled spheroids, (2) thin-walled spheroids having a diffuse envelope, (3) thick-walled spheroids, and (4) spheroids having an extensively folded wall. Simple single-walled spheroids, 15–60 μm in diameter, with various wall textures but commonly lacking envelopes or appendages form the dominant subgroup. Other complex morphologies are present and include aligned or associated bodies of thin-walled spheroids with diffuse envelopes, and spindle-like structures containing inner spheroidal bodies. The degree of morphological complexity and associations between structures suggest the presence of reproductive phases. If correct, this implies that the early Earth (ca. 3.0 Ga) showed a higher level of biodiversity than is currently postulated.     

A Rb-Sr isotopic study of weathering in the Mertondale granite,Western Australia

Fresh samples of the Mertondale granite give a Rb-Sr age of 2580 ± 16 m.y. (λRb⁸⁷ = 1.39 × 10^?11 yr^?1), representing the time of emplacement. Very little change in age is detected with progressive alteration of the granite, apparently associated with weathering, even when it is converted to a kaolinite-quartz assemblage. Preservation of the whole-rock age was unexpected, as the region has been subjected to Tertiary lateritisation to which the alteration of the granite would normally be attributed. We conclude that either the apparent weathering of the granite must be re-interpreted as a late-stage hydrothermal kaolinisation, or that the weathering occurred in the Archaean within about 15 m.y. of the granite's crystallisation.     

Pb/Pb and U/Pb zircon dating of subvolcanic rhyolite as a time marker for Hercynian granite magmatism and Sn mineralisation in the Eibenstock granite, Erzgebirge, Germany: Considering effects of zircon alteration

Tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo- and exocontact of the Eibenstock granite, Erzgebirge, Germany. The same structures which cut the granite control the rhyolite emplacement and the location of ore-bearing greisen bodies. Albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock. Comprehensive zircon studies by scanning electron microscopy (secondary and backscattered electron as well as cathodoluminescence imaging; EDX measurements) reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity, including distortion of the U-Pb isotope system when affected by greisenisation. Single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a ²⁰⁷Pb/²⁰⁶Pb age of 290±5 Ma, whereas zircon from little altered granite gave a ²⁰⁷Pb/²⁰⁶Pb age of 320±8 Ma. The single evaporation age of zircon from rhyolite is confirmed by ²³⁸U/²⁰⁶Pb SHRIMP ages of 297±8 Ma. The significant time gap of at least 20 Ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the Eibenstock granite cannot be regarded as a source for tin-ore forming fluids as previously assumed.     

New evidence for ~ 4.45 Ga terrestrial crust from zircon xenocrysts in Ordovician ignimbrite in the North Qinling Orogenic Belt,China

Evidence for the earliest known terrestrial crust comes predominantly from Jack Hills in Western Australia, where hafnium isotopic results from > 3.8 Ga detrital zircons indicate crustal precursors as old as ~ 4.4–4.5 Ga. We present evidence from magmatic cores in > 3.9 Ga xenocrystic zircons from a felsic volcanic rock in the North Qinling Orogenic Belt, China, of similar Hf crustal model ages up to 4.45 Ga. These lie on the same Lu/Hf trajectory as the least disturbed Jack Hills and Apollo 14 zircons, therefore providing only the second example of the earliest known generation of continental crust on Earth. In addition, the rims of two zircon grains record later growth at 3.7 Ga and, when combined with the fact that the grains are incorporated in Paleozoic volcanic rocks, imply long-lived crustal residence within the basement of the North China Craton. These results therefore establish the wider distribution and survival of the most ancient crustal material on the Earth and highlight the possibility for the further discovery of ancient crustal remnants.     

The behaviour of uranium, thorium and tin during leaching from a coarse-grained porphyritic granite in an arid environment

During the routine analysis of chip samples from water boreholes it was noted that the DeBakken granite south of Kenhardt has high uranium and tin contents. Rock samples collected on surface, however, revealed low concentrations of both these elements indicating that they have been leached from the surface samples. The distribution of thorium in the borehole profiles showed that while both tin and uranium have been removed above the water table, other “immobile” elements and thorium remained unaffected.The geological data showed that the leaching took place since the start of the Tertiary. Initially, the movement of uranium was vertical and accumulated in pedogenic calcrete above the granite. This vertical leaching was controlled by fluctuations in the water table and took place from the Pliocene to Pleistocene. Later, in the Holocene, the pedogenic calcrete was removed and deposited as non-pedogenic calcrete along the rivers. These nodular calcrete deposits, which are abundant in Namaqualand, are mineralized only along those rivers which drain the DeBakken granite, indicating that this granite was the source of the uranium.When the water table reached depths of ten metres or more the vertical migration of uranium ceased and horizontal leaching caused by the movement of ground water became active. Where the water table cuts the surface, such as in the pans, uranium deposition due to evaporation of groundwater is still active.Leaching of uranium from this granite, together with the formation of the secondary deposits is mineralogically controlled, and is ascribed to the fact that uranium is not hosted in zircon. Granites such as the DeBakken granite, in which the uranium is hosted in biotite, monazite and apatite, cannot be recognized by surface sampling and hence a lithogeochemical approach, using sub-surface samples has to be adopted. Calculations showed that approximately 1400 tons of uranium metal has been leached from this granite since the start of the Tertiary and that the maximum reserve of the province as a whole is relatively small.The distribution of tin is erratic, and although it has been leached from the surface samples, it has not been transported for any significant distance. Thus the tin distribution in the profiles does not show the same degree of leaching as does uranium. This is ascribed to the fact that samples from a percussion drill would include both the rock fragments from which tin has been leached and the clay-rich alteration products in which it has been trapped. Tin is generally immobile when present as cassiterite, but when enclosed in biotite, and when the groundwater is enriched in chlorine and fluorine, it leaches readily from the biotite.     

The rare earth element geochemistry of granite,gneiss, and migmatite from the Western Metamorphic Belt of South-eastern Australia

Four muscovite-biotite granites from the Western Metamorphic Belt of South-eastern Australia have rare earth element patterns characterized by: (i) light rare earth element enrichment; (ii) slight Eu depletion; (iii) varying degrees of heavy rare earth element depletion. The rare earth element and major element chemistry of three of these muscovite-biotite granites (the Koetong, Lockharts and Yabba Granites) can be approximated very closely by a model involving 20% partial melting of an ultrametamorphosed pelitic sediment and contamination of this minimum melt by the residual material left after melting, in the ratio 60% melt: 40% residue. Granitoids can be very largely solid material at the time of emplacement.The other muscovite-biotite granite studied (the Hawksview Granite) has major and trace element characteristics which distinguish if from the other three granitoids and these differences are attributed to variations in source material at the site of melt generation.The rare earth element and major element chemistry of a garnet-cordierite gneiss from the Western Metamorphic Belt can be modelled assuming 5% partial melting of a pelitic metamorphic rock and contamination of the minimum melt by the residue in the ratio 30% melt: 70% residue.Separated granitic and biotitic portions of a migmatite from the Western Metamorphic Belt have rare earth element characteristics which are inconsistent with a simple partial-melting model, but it is suggested that re-equilibration following, or during, separation of the vein material could obscure the process by which the vein of the migmatite developed. It is however certain that the vein developed in situ from a pelitic meta-sediment leaving the biotite rich selvage, without the introduction of material from an external source.Leucogranites which crop out to the east of the Western Metamorphic Belt are high level intrusions of highly fractionated granitic melt. Their Sr isotopic characteristics and features of their major and trace element chemistry suggest that they derive from an igneous source and are not directly related to the granites and gneisses to the west.     

Supergene alteration of the Mt Windarra nickel sulphide ore deposit,Western Australia

Three main zones of progressive oxidation, termed the transition, violaritepyrite and oxide zones, can be delineated in the supergene profile of the Mt Windarra massive/matrix ore deposit. In the broad transition zone from pure primary ore, pentlandite is progressively oxidised to an iron rich violarite of composition Co_0.02Fe_1.38Ni_1.60 S₄, releasing Fe²⁺ and Ni²⁺ ions into solution. Up to 43% of this Ni²⁺ moves to nearby pyrrhotite margins which are replaced firstly by nickeliferous smythite and then by a second lamellar-textured violarite with an even higher iron content but lacking in cobalt (approximately Fe_1.6Ni_1.4S₄). On completion of violaritisation of the pentlandite, violaritisation of the pyrrhotite also ceases and the remainder of the pyrrhotite is rapidly replaced by secondary pyrite/marcasite, siderite and void space, this reaction defining the top of transition zone. Both sulphur and nickel are extracted from solution and further Fe²⁺ ions are released into solution. The violarite-pyrite zone is characterised by the absence of pentlandite and pyrrhotite and continued stability of violarite and secondary iron disulphides. Most, if not all, of the iron generated by these oxidation reactions precipitates as magnesian siderite at the expense of magnesite, giving rise to solutions containing mainly Mg²⁺ and Ni²⁺ ions. At and just above the water table atmospheric oxygen is reduced while the sulphides are oxidised to sulphate and hydroxides. Much of the iron remains in situ as characteristic goethite relicts while nickel and copper are leached, producing the enrichment below the water table. The overall genetic model proposed is electrochemical and is analogous to the corrosion of a piece of metallic iron partially immersed in differentially aerated water.     

Determination of magnetic anisotropy and a ca 1.2 Ga palaeomagnetic pole from the Bremer Bay area,Albany Mobile Belt,Western Australia

Granulite facies tonalitic gneiss, mafic granulite and late metadolerite dykes from Bremer Bay in the Mesoproterozoic Albany Mobile Belt yield palaeomagnetic remanence that were acquired between ca 1.2 Ga and 1.1 Ga. A well‐constrained pole (66.6°N, 303.7°E) fits the ca 1.2 Ga part of the Precambrian Australian apparent polar wander path. This pole is in agreement with the high‐latitude position of Australia at ca 1.2–1.1 Ga shown on some Rodinia reconstructions. More data are required before any significance can be attributed to a second, poorly defined pole (41.8°S, 243.7°E) that falls at some distance from the ca 0.8 Ga part of the Australian apparent polar wander path. Magnetic anisotropy measurements from all samples except late granite dykes indicate northeast‐southwest elongation (i.e. parallel to the local trend of the orogenic belt) and northwest‐southeast contraction. This is in agreement with the orientation of principal strain axes deduced from structures formed during late stages of ductile deformation. The mean magnetic fabric lineation (long axis of the strain ellipsoid) is subparallel to a mineral elongation lineation and the axes of late upright to inclined folds. Short axes of the strain ellipsoid determined from magnetic fabric measurements are in a similar orientation to poles to the axial surfaces of these folds and to the associated cleavage. This mean shortening axis bisects late conjugate ductile shear zones that overprint the folds. This study has shown that structurally complex high‐grade gneisses and intrusive rocks with variable timing relationships may yield meaningful palaeomagnetic results for late stages of metamorphism. Magnetic anisotropy analysis is also seen to be a valuable tool in providing principal strain directions for late ductile deformation.     

Pb / Pb-ages of zircons from syn-collisional I-type porphyritic granites of the central Ribeira belt, SE Brazil

In the central segment of the Ribeira belt, southeast Brazil, several foliated porphyritic granitic bodies intrude high-grade migmatitic gneisses of the Andrelândia and Juiz de Fora domains and Embú Complex. Results of geological, geochemical and geochronological investigations of the Maromba, Pedra Selada, Serra do Lagarto and Funil porphyritic I-type granites provide profound similarities, except for the distinct geochemical behavior of the Funil Granite, perhaps related to a different crustal source. These granitoids show similar structural, textural and mineralogical features. Pb-evaporation of single zircons provided ages of 586 ± 6, 579.6 ± 6.3, 586.3 ± 4.8 and 584 ± 5 Ma for the granites, respectively, coincident with the syn-collision I episode of the central Ribeira belt. The intrusion of I-type porphyritic granitoids coeval with the main collisional event has not often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature or even a slightly peraluminous I-type character. However, the occurrence of coeval I- and S-type syn-tectonic granites along the central Ribeira belt, as observed in the investigated area and discussed in this paper is noteworthy.