A stable isotope study of the migmatitic rocks in the Ballachulish contact aureole, Scotland

The highest grade pelitic and semipelitic rocks of the Ballachulish aureole are dominantly potash feldspar + cordierite + biotite hornfelses with widely variable amounts of quartz, plagioclase, andalusite, sillimanite and corundum (together with accessory phases). On a microscopic scale these hornfelses show textural evidence of the presence of melt, whilst on a mesoscopic scale they contain a variety of leucosomes. Oxygen isotope studies have been carried out on both whole rocks and mineral separates in order to: (1) assess the sources of molten and volatile constituents and (2) determine the extents of isotopic homogenization and equilibration. Data from localities with both restricted and extensive evidence of leucosomes and melt development are compared, as well as one locality with petrographic evidence of melt incursion from the igneous complex. The whole-rock δ¹⁸O values of the leucosomes (10.5–14.9%.) are in general similar to the immediately adjacent mesosomes (9.9–14.5%.) which are typically cordierite- and feldspar-rich hornfelses. Isotopic evidence is thus consistent with an in-situ partial melt origin for the leucosomes, without the substantial addition of externally derived components. In the area of extensive melt development, the ‘chaotic zone’, it is possible there was addition of an H₂O-rich fluid phase (6-13 wt%) from the igneous complex which resulted in a slight lowering of δ¹⁸O values by 0.5–1.0%. Quartz mineral separates were used to assess the degree of local isotopic homogenization. In the extensively molten area (chaotic zone) there is extensive homogenization between rock layers (quartz δ¹⁸O usually within 1.0%), whilst in less molten areas δ¹⁸O quartz has a range of c. 3.0%. The greater homogenization in the chaotic zone is attributed to the increased degree of melting and infiltration of H₂O-rich fluid from the igneous complex.     

Fluid heterogeneity during granulite facies metamorphism in the Adirondacks: stable isotope evidence

The preservation of premetamorphic, whole-rock oxygen isotope ratios in Adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived, hot CO₂-H₂O fluids during granulite facies metamorphism. This conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations. The data suggest that these rocks were not an open system during metamorphism; that fluid/rock ratios were in many instances between 0.0 and 0.1; that externally derived fluids, as well as fluids derived by metamorphic volatilization, rose along localized channels and were not pervasive; and thus that no single generalization can be applied to metamorphic fluid conditions in the Adirondacks.Analyses of 3 to 4 coexisting minerals from Adirondack marbles show that isotopic equilibrium was attained at the peak of granulite and upper amphibolite facies metamorphism. Thus the isotopic compositions of metamorphic fluids can be inferred from analyses of carbonates and fluid budgets can be constructed.Carbonates from the granulite facies are on average, isotopically similar to those from lower grade or unmetamorphosed limestones of the same age showing that no large isotopic shifts accompanied high grade metamorphism. Equilibrium calculations indicate that small decreases in ¹⁸O, averaging 1 permil, result from volatilization reactions for Adirondack rock compositions. Additional small differences between amphibolite and granulite facies marbles are due to systematic lithologie differences.The range of Adirondack carbonate ¹⁸O values (12.3 to 27.2) can be explained by the highly variable isotopic compositions of unmetamorphosed limestones in conjunction with minor ¹⁸O and ¹³C depletions caused by metamorphic volatilization suggesting that many (and possibly most) marbles have closely preserved their premetamorphic isotopic compositions. Such preservation is particularly evident in instances of high ¹⁸O calcites (25.0 to 27.2), low ¹⁸O wollastonites (–1.3 to 3.5), and sharp gradients in ¹⁸O (18 permil/15m between marble and anorthosite, 8 permil/25 m in metasediments, and 6 permil/1 m in skarn).Isotopic exchange is seen across marble-anorthosite and marble-granite contacts only at the scale of a few meters. Small (<5 m) marble xenoliths are in approximate exchange equilibrium with their hosts, but for larger xenoliths and layers of marble there is no evidence of exchange at distances greater than 10 m from meta-igneous contacts.     

Post-emplacement serpentinization and related hydrothermal metamorphism in a kimberlite from Venetia, South Africa

The common serpentine–diopside matrix assemblage in volcaniclastic kimberlite (VK) at the Venetia Mine, South Africa is ascribed to a secondary origin, because of post‐emplacement serpentinization and associated hydrothermal metamorphism. Volcaniclastic deposits with 20–30% porosity infill kimberlite pipes in the waning stages of kimberlite eruptions. Olivine macrocrysts are typically rimmed by talc and are pseudomorphed by lizardite, with minor magnetite. The fine matrix consists of mixtures of lizardite, chlorite, smectite, brucite, calcite, titanite and andradite, an assemblage which either pseudomorphed microcrysts or in‐filled voids. Locally we recognize microcryst pseudomorphs rich in sub‐microscopic mixtures of lizardite with smectite, and other microcryst pseudomorphs and void‐filling matrix rich in chlorite and lizardite. Interstitial lizardite and associated phyllosilicates (brucite, smectite and chlorite) crystallized progressively from meteoric or hydrothermally derived pore waters, and Si⁴⁺ and Mg²⁺ released into the fluid phase during serpentinization of olivine macrocrysts. Radial‐fibrous fringes of diopside microlites around crystals display void‐filling textures because of unrestricted growth into pore spaces. Secondary diopside is attributed to Si⁴⁺, Mg²⁺ and Ca²⁺ cations released into the fluid phase by interaction with olivine, calcite and plagioclase in siliceous xenoliths. The paucity of primary, fine‐grained groundmass phases resistant to alteration, for example, perovskite and spinel, precludes an origin for the intergrain matrix as altered interstitial ash, glass or a late‐stage kimberlite melt. Isovolumetric replacement of olivine results in a volume increase of 60% so that pore spaces in the original deposit can be easily filled up with serpentine. The source of Al³⁺ to form chlorite and smectite is attributed to alteration of plagioclase in xenoliths which comprise 20–30 vol.% of the deposit. Titanite, hydro‐andradite and second‐generation diopside precipitate as hydrothermal minerals from calcium‐bearing serpentinizing fluids in replacement reactions and as void‐filling minerals. Consideration of mineral equilibria in the CaO‐MgO‐SiO₂‐H₂O‐CO₂ system constrains the common matrix assemblage of lizardite and diopside in X_CO2)–T space. At 300 bar, the assemblage is stable only at temperatures below 370 °C and X_CO2 < 0.01. This upper limit on temperature is well below the plausible solidus of ultrabasic magmas. Furthermore, the requirement of trace CO₂ in the fluid phase implies a post‐emplacement external source rather than ‘autometamorphism’ from kimberlite‐derived fluids, because of high P_CO2 commonly inferred for kimberlite magmas.     

Particulate metal speciation in surficial sediments from Loch Dee, southwest Scotland, U.K.

The geochemical partitioning of ten elements in stratified Holocene sediments from Loch Dee, southwest Scotland, has been established by use of a five-stage sequential extraction procedure. Samples from below 15 cm sediment depth show minimal evidence of modification by anthropogenic contamination or active diagenesis and hold Fe, Mg, Cu, Cd, Co, Pb, and Ni primarily in detrital silicates or organic complexes, while Mn, Ca, and Zn reside largely in adsorbed and reducible oxide phases. In the uppermost ca 15 cm of sediment, enhanced total concentrations of Zn, Cu, and Pb reflect increased atmospheric deposition during the postindustrial period. Of these metals, only Pb displays any notable adjustment of partitioning in the enriched zone, showing disproportionate accumulation in labile oxides and organic-Pb phases. The lack of Pb and Zn carbonates in the contaminated horizon may reflect inherent thermodynamic instability under the acid surface and pore-water conditions of Loch Dee. Increments to total Mn and Co in the surficial ca 5 cm of sediment are attributable to the accumulation of secondary oxides and adsorbed species, consistent with precipitation from the interstitial pore-waters across a sedimentary redox front. The presence of metals such as Zn and Cd in soluble or acid-volatile phases in the interfacial sediment has implications for the future management of the Loch Dee basin, with leaching into the overlying waters likely, given the continuation of current trends of lake acidification.     

A preliminary stable isotope study on Mogok Ruby, Myanmar

The primary occurrence of ruby in the Mogok area, northern Myanmar is exclusively found in marble along with spinel–forsterite-bearing marble and phlogopite–graphite marble. These marble units are enclosed within banded biotite–garnet–sillimanite–oligoclase gneisses. Samples of these marbles collected for C–O stable isotope analysis show two trends of δ¹³C–δ¹⁸O variation resulting most likely from fluid–rock interactions. Ruby-bearing marble and phlogopite–graphite marble follow a trend with coupled C–O depletion, whereas spinel–forsterite-bearing marble follows a δ¹⁸O depletion trend with relatively constant δ¹³C values. Ruby formation might have resulted from CO₂-rich fluid–rock interaction, while spinel–forsterite-bearing marble was genetically related to CO₂-poor fluid–rock interaction. Both fluids may have arisen from external sources. Based on graphite Raman spectral thermometry, the estimated temperature for phlogopite–graphite marble, and probably ruby-bearing marble, was lower than 607 °C, and for spinel–forsterite-bearing marble, lower than 710 °C. Contrasting C/O diffusion between graphite/ruby/spinel/forsterite and calcite, local variations of isotopic compositions of newly formed minerals as a result of non-pervasive fluid infiltration, and open-system isotopic disturbance during cooling may have affected C-/O-isotopic fractionations between minerals. The estimated high formation temperatures for ruby and spinel/forsterite imply that the parental fluids may have been related to nearby igneous intrusions and/or metamorphic processes. Whether these two types of fluid were genetically related is unclear based on the present data.     

A stable isotope study of retrograde alteration in SW Connemara,Ireland

Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures 275±15°C and in the MGS P_fluid is estimated to be 1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29 (fluid inclusions),-47 to-61 (chlorites) and-11 to-31 (epidotes). Chlorite ¹⁸O values are +0.2 to +4.3, while ¹⁸O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a - plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had D -20 to-30 in all lithologies, but the fluid ¹⁸O varied both spatially and temporally within the range-4 to +7. The fO₂ of the fluid that deposited the epidotes in the MGS varied with its ¹⁸O value, with the most ¹⁸O-depleted fluid being the most oxidizing. The D values, together with low (<0) ¹⁸O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid ¹⁸O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at 400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.     

A stable isotope study of cave seepage waters

An isotope study (δ¹⁸O, δ²-H) of 62 samples of waters from both the interior of the Nerja cave (Málaga, Southern Spain) and the exterior sinkholes and springs at the site was carried out. Rainfall water in the study area was also studied. The mean value of seepage waters can be considered as the mean isotopic value of the precipitation waters of the area. The origin and aquifers of these different types of waters has also been studied, obtaining a clear meteoric origin both for the seepage waters and the waters from the wells and springs, but the aquifer of the latter is different, showing more negative isotopic values. This suggests that they could have mixed with waters from other aquifers which are either deeper or originate at other altitudes.     

A stable isotope study of the Palaeoproterozoic Tallberg porphyry-type deposit,northern Sweden

The Tallberg deposit is situated in the Skellefte District in northern Sweden. It is a Palaeoproterozoic equivalent of Phanerozoic poryphyry-type deposits. The mineralization is situated within the Jörn granitoid complex and is associated with intrusive quartz-feldspar porphyries. The granitoids are coeval with mainly felsic volcanic rocks hosting several massive sulphide deposits. The alteration is generally of a mixed phyllic-propylitic type, but areas or zones associated with high gold grades exhibit phyllic alteration. Ore minerals are pyrite, chalcopyrite, sphalerite, magnetite, and trace amounts of molybdenite. In this stable isotope study, quartz, sericite, and chlorite from the alteration zones were sampled. The magmatic quartz has a ¹⁸O composition of + 6.2 to +6.7 whereas the quartz in the hydrothermal alteration zones have values ranging from +7.5 to +10.6. The calculated temperatures for this fractionation range from 430° to 520°C. The sericites have ¹⁸O ranging from +4.6 to +8.2 (average +6.6) and D -31 to -54 (average -41). Chlorites range from ¹⁸O +4.2 to +7.7 and D from –34 to –44. The range of ³⁴S of 11 pyrite samples is +3.8 to +5.5 with an average of +4.6 ± 0.5, suggesting a relatively homogeneous sulphur source, probably of magmatic origin. Modelling waters in equilibrium with the minerals indicates early magmatic fluids with ¹⁸O of 6.5. This fluid mixed with a low ¹⁸O and high D fluid, which is tentatively identified as seawater. The ¹⁸O signature of sericite and chlorite also indicates significant water-rock exchange, explaining the positive ¹⁸O values for the waters in equilibrium with the hydrated minerals.     

Garnet zoning and metamorphism of the Barrovian type area,Scotland

A microprobe investigation of the high grade metamorphic zones from the Barrovian type area in Angus, Scotland, shows the importance of local zones of retrograde cation exchange between garnet, staurolite and biotite. The interpretation of this zoning, established during a slow cooling history, is critical to any study of metamorphic reactions or conditions. The extent and intensity of these diffusion effects are dependent on a number of parameters including grainsize, fabric orientation, heating and cooling history, and the modal abundance of the phases. Increasing diffusion within garnets with metamorphic grade, and the subsequent retrograde effects are modelled using Temperature-Time-Transformation diagrams and provide information on the activation energy for Fe-Mg diffusion in garnet.     

Regional metamorphism of some Dalradian pelites in the Buchan area,N.E. Scotland

Regional metamorphic zones, based on mineral assemblages in pelites, are presented for the Dalradian rocks of Aberdeenshire and Banffshire, in the type area of Buchan metamorphism; electron microprobe analyses of cordierite (C), staurolite (S), chlorite (Chl), biotite (B) and white mica (Ms) are reported for rocks from the classic sections of the Banffshire coast and the valley of the Ythan.A low grade biotite zone, separates two NE-SW trending sets of higher grade zones, in which the sequence s defined by the entry of cordierite, andalusite (A) and (in the west only) staurolite. The zones are characterised by the assemblages (with quartz and muscovite) B-Chl, C-B-Chl, A-C-B and S-A-B.The western sequence contains a transition towards higher pressure, Barrovian type, metamorphism. The isograds arise from continuous reactions affecting rocks of restricted bulk composition (M/FM). With increasing grade, there is a regular trend towards more magnesian ranges of composition for the assemblages C-B-Chl, A-C-B and finally (as P increases in the west), S-A-B. The isograds form when these assemblages intersect the most Fe⁺⁺-rich rock compositions present which occurs in each case when the biotite M/FM=40. A complex of divariant equilibria, derived for the system KFMASH, is used to model the natural reactions.     

A strontium,neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif,Pyrenees, France

Nd, Sr, and O isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310–340 Ma Hercynian-age metamorphic terrane in the Pyrenees, France. Lower Paleozoic shales and phyllites have ⁸⁷Sr/⁸⁶Sr values of 0.707–0.717 at 310 Ma, but model values at 310 Ma of 0.709–0.736 (based on assumed depositional age of 450 Ma and an initial ⁸⁷Sr/⁸⁶Sr=0.707). On a regional scale, ⁸⁷Sr/⁸⁶Sr was homogenized to about 0.713 to 0.717 in the higher-grade pelitic schists during metamorphism. Much of this ⁸⁷Sr/⁸⁶Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompanied the δ ¹⁸O lowering of the phyllites (+13 to +16) during their transformation to andalusite- and sillimanite-grade schists (δ ¹⁸O=+11 to +12); all of these effects are attributed to pervasive interactions with hydrothermal fluids (Wickham and Taylor 1985). The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing of a relatively mafic magmatic end-member (⁸⁷Sr/⁸⁶Sr～ 0.7025–0.7050 and δ ¹⁸O～ +7.5 to +8.0) with two metasedimentary sources, both having ⁸⁷Sr/⁸⁶Sr～0.715 and δ ¹⁸O～ +10.0 to +12.0, but with one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels within the complex. The less homogeneous end-member that dominates at high levels is clearly derived from the local Paleozoic pelitic schists. A Rb-Sr age of 330±20 Ma was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism, as well. Although a post-metamorphic granodiorite magma body at Trois Seigneurs also displays heterogeneities in δ ¹⁸O and ⁸⁷Sr/⁸⁶Sr (and thus does not give a clear-cut Rb-Sr isochron), the data are consistent with an emplacement age between 260 and 310 Ma, similar to ages of other late granodiorites in the Pyrenees. ¹⁴³Nd/ ¹⁴⁴Nd is very uniform within the Hercynian crust, both at Trois Seigneurs (? _Nd=?3 to ?7) and elsewhere in the Pyrenees; almost all igneous lithologies have depleted-mantle, mid-Proterozoic model ages, consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 Ma or earlier. Rb-Sr mineral ages exhibit a complex cooling history reflecting late Hercynian and Mesozoic thermal events. Our results show that profound homogenization of the ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas; this may explain why many peraluminous Hercynian granitoids of Western Europe have anomalously low (⁸⁷Sr/⁸⁶Sr) initial values compared to their probable sedimentary parent rocks.     

Mo稳定同位素研究进展

随着表面热离子质谱(TIMS)和多接收器电感耦合等离子体质谱(MC-ICP-MS)的广泛应用以及同位素分析方法的改进,近10年来非传统稳定同位素(Cu、Zn、Fe、Se、Mo、Cr、Hg等)的研究得到迅速发展.其中,由于Mo同位素的分馏明显受氧化还原条件的控制,使其在指示古环境及古气候的变化方面有独特的地球化学指示意义.同时,Mo同位素在指示成矿物质来源和海洋Mo循环等方面也取得较大成果.因此,Mo同位素地球化学研究已成为国际地学领域的一个前沿和热点.本文综合前人的研究成果,结合近期自己的工作,论述了Mo同位素地球化学研究领域的一些重要进展,详细介绍了Mo同位素的化学分离、提纯和质谱分析技术,并对其应用前景进行了展望.     

Isotopic contrasts,chronology of elemental transfers and high-grade metamorphism: the Sri Lanka Highland granulites,and the Lewisian (Scotland) and Nuk (SW Greenland) gneisses

Crust formation ages inferred on the basis of Nd model age, U-Pb zircon and Pb-Pb data for granulites from central Sri Lanka, and the late Archean Lewisian (Scotland) and Nuk (south-western Greenland) high-grade gneisses are comparable, and in the range 2.2–3.0 Ga and 2.8–3.0 Ga respectively. The time interval between crust formation and high-grade metamorphism (with its attendant depletions of incompatible and heat-producing elements) is in the order of 2 Ga for the 0.6 Ga Highland granulites of Sri Lanka, but are less than 0.1–0.2 Ga for the Lewisian and Nuk gneisses. Severe depletions of U and Th produced the distinctive unradiogenic present-day Pb ratios of the Lewisian and Nuk gneisses. The Sri Lanka granulites, in contrast, preserve a record of an extended residence in a high U/Pb, high Th/Pb environment and display radiogenic »upper crustal« — like present-day isotope ratios. Relative to primitive mantle compositions, the Pb isotopes identify the Lewisian and Nuk terrains as crustal reservoirs depleted in Th and U (relative to Pb), whereas the Highland granulites are enriched. The global Pb isotope mass balance requires the existence of low U/Pb, low Th/Pb and high Th/U reservoir(s) for successful solutions, and the lower crust is a likely candidate for this reservoir. High-grade terrains like Sri Lanka show depletions of measured U and Th abundances relative to Pb, but do not show Pb isotope signatures of »model bulk lower crust«. Archean high-grade gneisses display the appropriate combination of retarded Th/Pb and U/Pb although their absolute compositions are much less radiogenic than those of »model bulk lower crust« predicted from simple mass balance calculations.

---

Zusammenfassung Nd-Modellalter, U-Pb-Zirkondaten sowie Pb-Pb-Alter weisen auf vergleichbare Krustenbildungsalter von 2.2–3.0 Ga für den zentralen Granulitgürtel Sri Lankas bzw. 2.8–3.0 Ga für die spätarchaischen Granulitgebiete der Lewisian-(Schottland) und Nuk-Komplexe (südwestliches Grönland) hin. Die extrem lange Zeitspanne von ca. 2 Ga zwischen Krustenbildung und hochgradiger Metamorphose und die daraus resultierende Verarmung an hochinkompatiblen Elementen unterscheidet jedoch die ca. 0.6 Ga alten Granulite der Highland- und Südwest-Gruppe Sri Lankas von den Lewisian- und Nuk-Gneisen. Hier erfolgte die Granulitmetamorphose verhältnismäßig bald nach der Krustenbildung ( 0.1–0.2 Ga), und die frühe signifikante Verarmung an U und Th ist verantwortlich für die heutige, wenig radiogene Pb-Isotopie in diesen Gesteinen. Im Gegensatz dazu zeigen Konzentrationsmessungen an den Granuliten Sri Lankas zwar eine Verarmung von U und Th relativ zu Pb, was aber aus der hochradiogenen Pb-Isotopie nicht ersichtlich ist. Der Grund hierfür ist, daß dieser Krustenkomplex eine sehr lange Zeit zur kontinentalen Oberkruste gehörte, mit entsprechend hohen U/Pb- und Th/Pb-Verhältnissen und dem daraus entstandenen Zuwachs an radiogenem Blei. Die terrestrische Massenbilanz des Bleis erfordert, daß die zeitintegrierten erhöhten U/Pb-, Th/Pb- und erniedrigten Th/U-Werte sowohl der Oberkruste als auch fast aller Mantel-derivate durch ein Reservoir, wie z.B. die Unter-kruste, mit langfristig erniedrigtem U/Pb und Th/ Pb und erhöhtem Th/U kompensiert werden muß. Krustenkomplexe wie Sri Lanka, die erst in jüngster Zeit in die Unterkruste versenkt wurden, kommen dafür nicht in Frage. Archaische Granulitgebiete, jedoch, zeigen das entsprechende Th/Pb und U/Pb, wenn auch ihre absolute Zusammensetzung deutlich weniger radiogen ist als es die Modellunterkruste verlangt.

---

Résumé Les âges modèles de formation de la croûte, estimés à partir des méthodes Nd, U-Pb (zircon) et Pb-Pb, pour les granulites du Sri Lanka central d'une part et les gneiss de degré élevé lewisiens (Archéen sup.) d'Ecosse et de Nuk (sud-ouest du Groenland) d'autre part, sont comparables et se situent respectivement dans les intervalles de 2,2–3,0 Ga et 2,8–3,0 Ga. L'intervalle de temps qui sépare la formation de la croûte et le pic du métamorphisme (accompagné de pertes d'éléments incompatibles et producteurs de chaleur) est de l'ordre de ± 2 Ga pour les granulites du Sri Lanka, âgées de ± 0,6 Ga, mais n'excède pas 0,1 à 0,2 Ga pour le Lewisien et les gneiss de Nuk. Des pertes sévères en U et en Th sont responsables des rapports Pb actuels peu radiogéniques dans le Lewisien et les gneiss de Nuk. Par contre, les granulites du Sri Lanka se sont maintenues dans des conditions de rapports U/Pb et Th/Pb élevés et fournissent des rapports isotopiques analogues à ceux de la croûte supérieure actuelle. Par rapport aux compositions mantéliques primitives, les isotopes du Pb identifient le Lewisien et les formations de Nuk comme des réservoirs crustaux appauvris en Th et en U (par rapport au Pb), tandis que les granulites du Sri Lanka ont été enrichies. Le bilan de masse des isotopes du Pb à l'échelle globale requiert l'existence d'un ou de plusieurs réservoirs à faible U/Pb, faible Th/Pb et haut Th/U; la croûte inférieure pourrait constituer ce réservoir. Des formations très métamorphiques comme celles du Sri Lanka présentent un appauvrissement des teneurs en U et en Th par rapport au Pb, mais ne montrent pas les signatures isotopiques du Pb correspondant à un modèle de croûte inférieure globale. Les gneiss archéens de degré élevé possèdent la combinaison adéquate des rapports Th/Pb et U/Pb déprimés, bien que leurs compositions absolues soient moins radiogéniques que celle de la »croûte inférieure type« prévue par les bilans de masse.

---

, - Nd U-Pb , Lewisian () Nuk (- ); : 2,2-3,0 2,8-3,0 . ( , , ) — 2 0,6 -, -0,1 -0,2 . , . - U/Pb Th/Pb, . , , , . U/Pb Th/Pb Th/U; , . , -, U Th , « ». Th/Pb U/Pb, , , .

     

Regional and contact metamorphism within the Moy Intrusive Complex,Grampian Highlands,Scotland

In central Scotland, the Moy Intrusive Complex consists of (1) the Main Phase — syntectonic peraluminous granodiorite to granite emplaced at c. 455 Ma, intruded by (2) the Finglack Alaskite — post-tectonic leucocratic granite emplaced at 407+/-5 Ma. The Main Phase was emplaced into country rocks at amphibolite facies temperatures. Rb-Sr dates and a compositional spectrum of decreasing celadonite content in Main Phase muscovite suggest the persistence of c. 550° C temperatures for c. 30 Ma but with a declining pressure regime, i.e. isothermal uplift. The Finglack Alaskite was intruded at high structural level, leading to the development of a contact metamorphic aureole in the Main Phase. The thermal effects of contact metamorphism include intergrowths of andalusite, biotite and feldspar in pseudomorphs after muscovite. This is associated with recrystallized granoblastic quartz. Muscovite breakdown and reaction with adjacent biotite, quartz and feldspar, i.e. a function of local mineral assemblage rather than bulk rock composition, is postulated to explain the occurrence of metamorphic andalusite in a granitoid rock.The Main Phase pluton of the Moy Intrusive Complex lies within a NNE trending belt of c. 450 Ma Caledonian tectonic and magmatic activity paralleling the Moine Thrust, and extending from northern Scotland to the Highland Boundary Fault. Syntectonic S-type magmatism with upper crustal source areas implies crustal thickening and suggests an intracratonic orogeny.     

A reconnaissance study of stable isotope ratios in archaean rocks from the yilgarn block,Western Australia

Isotopic compositions of sulphur, carbon, and oxygen have been determined for constituents from a total of 103 samples of sedimentary rocks, mafic and ultramafic igneous rocks, nickel ores, and gold ores from the Archaean Yilgarn Block.

Sulphides in the bulk of the sedimentary rocks have δ³⁴S values close to 0‰ and appear to have precipitated from solutions which incorporated magmatic sulphur (either juvenile or derived from older rocks). There is no evidence for widespread sulphate reduction.

δ³⁴S values of sulphides in the nickel deposits and associated mafic/ultramafic igneous rocks are within the magmatic range. The small, high‐grade deposits of the Kambalda‐Nepean‐Scotia type have small positive δ³⁴S values, and the large, low‐to‐medium grade dunite‐associated deposits of the Mount Keith‐Perseverance type have small negative δ³⁴S values.

Sulphides in the Kalgoorlie gold ores are enriched in ³²S relative to those in their host dolerite, supporting an epigenetic origin for the gold, under moderately high fO₂ conditions.

The δ¹³C values do not provide unequivocal evidence for the source(s) of the reduced carbon (kerogen) in the sedimentary rocks. Whilst they are compatible with biogenic derivation, it is not possible to rule out contributions from pre‐biotic organic ‘soup’ or from hydrothermal solutions of deep‐seated origin.

Carbonate in the sedimentary rocks are predominantly in epigenetic, sulphide‐bearing veinlets. In many cases, their δ¹³C values suggest precipitation from hydro‐thermal solutions containing magmatically derived CO₂. In only two samples are the petrographic features and δ¹³C values compatible with marine carbonates. Talc‐carbonate altered ultramafic igneous rocks have δ¹³C values consistent with their incorporation of magmatically derived CO₂.

The ?δ¹³C (carbonate‐kerogen) values for most of the sedimentary rock studied fall in a narrow range around +10‰, suggesting isotopic exchange between oxidized and reduced carbon species at moderately high temperatures (>250°C).

δ¹⁸O values of carbonate from both sedimentary rocks and igneous rocks are mainly within the range +7.2‰ to +18.0‰. If the values are primary they are consistent with the formation of carbonate from hydrothermal solutions of magmatic and/or metamorphic origin. However, it is also possible the δ¹⁸O values are the result of post‐depositional equilibration with meteoric waters.     

内蒙古甲乌拉大型Pb-Zn-Ag矿床稳定同位素地球化学研究

内蒙古甲乌拉银多金属矿床位于大兴安岭成矿带北段,为近年来发现的大型银铅锌多金属矿床。矿床矿体分布完全受到断裂构造的控制,金属矿物组成主要为方铅矿、闪锌矿、黄铜矿、黄铁矿、磁黄铁矿、毒砂、辉钼矿及磁铁矿等。文中重点分析了矿床的硫、氢、氧、碳和铅稳定同位素地球化学特征。研究结果表明：金属硫化物δ34S集中为1.37‰~4.10‰,平均为3.10‰(n=13),极差为2.73‰;石英和方解石δ18Owater的变化范围较大(-18.96‰~+1.08‰) (n=9),均值为-11.36‰;δDV SMOW的变化范围比较集中(-133.6‰~-103.4‰) (n=9);27件样品的铅同位素组成为：206Pb/204Pb=18.228 3~18.758 7、207Pb/204Pb=15.457~15.880和208Pb/204Pb=37.841~39.049,矿床的铅组成基本为正常的放射性成因铅;方解石δ13CV PDB变化范围为-5.2‰~-8.4‰,平均为-6.8‰(n=2)。矿石硫化物的硫同位素及方解石的碳同位素均指示成矿物质可能来源于深部的岩浆活动;石英和方解石的氢氧同位素组成表明成矿流体早期以岩浆流体为主,成矿晚期加入了大量加热补给的大气降水;铅同位素组成表明成矿流体中铅的来源主要为幔源,矿床形成过程中混入少量的壳源铅。矿床稳定同位素组成显示成矿流体主要来源于深部的岩浆热液,特别与燕山晚期的火山次火山热液有较为密切的联系,在流体演化过程中大气降水的加入对矿床成矿元素的聚集和沉淀也起到有利作用。成矿作用的发生是在一种总硫浓度比较低、中等氧化环境、相对开放的非平衡体系中进行的。矿床形成的地球动力学背景为一种岩石圈大规模快速减薄的过程。甲乌拉大型Pb Zn Ag矿床的成因类型属于火山次火山热液脉状银多金属矿床。     

Mineralogy, Petrology, and Geochemistry of an Ultrapotassic Basaltic Suite, Central Sierra Nevada, California, U.S.A.

Ultrapotassic basaltic lavas erupted 3.4–3.6 m.y. ago(K/Ar) in the central Sierra Nevada and originated by partialmelting of a phlogopite-enriched, garnet-bearing upper mantlesource. Ultrapotassic basanites (K₂O: 5–9 per cent), whichare spatially related to contemporaneous potassic olivine basalts(K₂O: 3–5 per cent) and alkali olivine basalts (K₂O: 1–3per cent), contain the K₂O-bearing minerals phlogopite, sanidine,and leucite as well as olivine, diopside, apatite, magnetite,and pseudobrookite. The presence and modal abundance of theK₂O-bearing minerals closely reflects the east to west increasein K₂O throughout the basaltic suite. Many lines of evidence support the derivation of the ultrapotassicbasanites and the related basalts from an upper mantle source:TiO₂ in phlogopite phenocrysts and groundmass crystals, 2–3and 7–9 per cent respectively, support phlogopite phenocrystcrystallization at high pressure, whole rock Mg values (100Mg/Mg + 0.85 Fe) range from 66–78, phlogopite-rich pyroxeniticand periodotitic nodules are included in some flows, and geobarometriccalculations indicate depths of generation at 100–125km. Also, model calculations show that the major, rare earth,and trace elements, except for Ba, Rb, and Sr, can be accuratelygenerated by 1.0–2.5 per cent melting of a phiogopite-and garnet-bearing clinopyroxene-rich upper mantle source. Partialmelting occurred after a general upper mantle enrichment beneaththe Sierra Nevada, the phlogopite- and clinopyroxene-rich sourceof the ultrapotassic lavas being the extreme result of the enrichmentprocess. Clinopyroxene enrichment of the upper mantle probablyoccurred by introduction of a partial melting fraction intothe upper mantle source areas. Enrichment of the upper mantlein the alkali and alkali-earth elements was not accomplishedby a partial melt, but resulted from influx of a fluid phaserich in Ba, K, Rb, Sr, and, probably, H₂O The continuous rangein K₂O of the erupted lavas implies that the upper mantle enrichmentis a cumulative process. The inverse relationship in the SierraNevada between uplift and the K₂O content of the erupted basaltsimplies that a critical relationship may exist between upliftand upper mantle enrichment.     

Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia

The well-studied Paleozoic Cooma metamorphic complex in southeastern Australia is characterized by a uniform siliciclastic protolith, of uniform age, with a continuous range of metamorphic grade from subgreenschist- to upper amphibolite-facies, and migmatite-grade in an annular pattern around the Cooma granodiorite. Those conditions are optimal for investigating variations of N concentrations and δ¹⁵N values during progressive metamorphism. Nitrogen concentrations decrease and δ¹⁵N increases with increasing metamorphic grade (sub-chlorite zone: 120 ppm N, δ¹⁵N = 2.3‰; chlorite zone: 110 ppm N, δ¹⁵N = 3.0‰; biotite and andalusite zone: 85 ppm N, δ¹⁵N = 3.8 ‰; sillimanite and migmatite zones: 40 ppm N, δ¹⁵N = 10.7‰). Covariation of K and N contents is consistent with N substituting for K as NH₄⁺ in micas. Observed trends of increasing δ¹⁵N values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in ¹⁵N with progressive metamorphism, which causes an enrichment of ¹⁵N in the residual nitrogen of the rock. Equilibrium models for Rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by N₂-NH₄⁺ exchange at temperatures of 300-600 °C, whereas observed large fractionations for the upper amphibolite-facies and melt products in the migmatite-grade samples may be interpreted as NH₃-NH₄⁺ exchanges at temperature of 650-730 °C. Lower values in the highest grade zones may also stem in part from input of ¹⁵N-depleted fluids from the granodiorite.The magnitude of isotope fractionation of nitrogen is about 1-2‰ during progressive metamorphism of metasedimentary rocks from sub-chlorite zone to biotite-andalusite zone, which is consistent with previous studies. Consequently, the large spread of δ¹⁵N values in Archean greenschist-facies metasedimentary rocks of −6‰ to 30‰ can be accounted for by variable mixtures of mantle plume-dominated volatiles with a δ¹⁵N of −5‰, and a ¹⁵N-enriched marine sedimentary kerogen component inherited from a CI chondrite veneer having δ¹⁵N of 30‰ to 42‰.     

A fluid inclusion and light element stable isotope study of the gold-bearing quartz vein system,Falun, Sweden

The Falun gold quartz vein mineralization is located ca 230 km NW of Stockholm, Sweden, within the Early Proterozoic volcano-sedimentary sequence of Bergslagen. The mineralization consists of a system with subparallel quartz veins that crosscut the alteration zone to the Falun massive sulphide deposit. Early barren and late gold-bearing quartz veins follow tectonic structures postdating the formation of the massive sulphide ore. Both generations of veins are epigenetic to the massive sulphide ore and were formed by hydrothermal processes. Fluid inclusion study of the gold-bearing quartz veins indicates a low-moderately saline fluid (0.3 to 17.4 equiv wt% NaCl). Heterogeneous trapping is indicated by coexisting inclusions showing a variable CO₂ content from 100% CO₂ ± CH₄ to 100% aqueous fluid. Temperatures of total homogenization also show a wide spread from 116–350°C with a slightly bimodal distribution with peaks at ca 180°C and 280°C. MeasuredδD values — 69 to — 63%0 (SMOW), of inclusion fluid and calculatedδ ¹⁸O values of hydrothermal fluids — 7.5 to — 1.4%0 (SMOW), strongly suggest a meteoric origin for the fluids. The quite consistentδD values and the range inδ ¹⁸O values indicate that major water-rock interaction led to the evolution inδ18O of the hydrothermal fluids.     

A fluid inclusion and stable isotope study of 200 Ma of fluid evolution in the Galway Granite, Connemara, Ireland

Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H₂O-CO₂-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V₁) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ¹⁸O across the batholith (9.5 ± 0.4‰n = 12) suggests V₁ precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V₁ indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H₂O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ¹⁸O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V₂) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H₂O-NaCl-CaCl₂-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ¹⁸O (−3 to + 1.2‰), δ¹³C_CO2 (−19 to 0‰) and δ³⁴S_sulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V₃). Correlations of salinity, temperature, δD and δ¹⁸O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ¹⁸O (1.2–2.5‰), high-δ¹³C_CO2 (> −4‰), low-δ³⁴S_sulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ¹⁸O (−5.4 to −3‰), low-δ¹³C (<−10‰), high-δ³⁴S_sulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V₃ veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997