Preliminary investigations of 18O/16O and D/H compositions in rhyo-ignimbrites in the In Hihaou (In Zize) Magmatic Center,central Ahaggar,Algeria

The 620 M.y.-old in Hihaou (In Zize) magmatic complex located at the north-western boundary of the Archaean In Ouzzal block (western Ahaggar), is composed of massive alkaline rhyo-ignimbrites and rhyolitic domes, which are intruded by a granophyric and granitic body. The whole is preserved in a cauldron structure. Extrusive rocks are strongly ¹⁸O-depleted, with -values as low as –1.5/SMOW, while granophyres are less depleted (minimum -¹⁸O value=+2.0/SMOW. The granite has values around + 6/SMOW. D/H compositions are rather low, with D–90 to –110/SMOW. Isotopic zoning of quartz phenocrysts, ¹⁸O/¹⁶O fractionation among coexisting phases, and heterogeneity of the whole-rock -¹⁸O values, suggest that the volcanic rocks have interacted with meteoric water after the eruption. Several mechanisms of isotopic alteration are discussed. The hydrothermal alteration does not seem to have been controlled by the granitic intrusion, but rather seems to have followed the deposition of thick pyroclastic deposits on permeable arkosic sandstones and fluvio-glacial conglomerates. Pervasive circulation of water through the cooling volcanic deposits could have produced the observed ¹⁸O depletion.     

Oxygen-isotope systematics of a strongly recrystallized granitic rock complex,Grenvillian Belt,SW Sweden

In the eastern, external part of the Grenvillian Belt in SW Sweden, five formations of granitic rocks were found in the basement of the Dalslandian Supracrustal Group. The granitic rocks have been strongly recrystallized but have preserved most of their granitic texture in the process. Most magmatic crystals have been pseudomorphed by metamorphic minerals: quartz, albite, chlorite, biotite, white mica, epidote, titanite, hematite, pyrite and carbonate. Two of the formations have subsequently been affected by a cataclastic deformation and at present consist of mylogneisses. ¹⁸O whole-rock values for the granitic rocks vary from +3.0 to +11.1. Quartz-apatite, quartz-zircon and quartz-titanite pairs show ¹⁸O/¹⁶O fractionations corresponding to equilibrium temperatures of 550–700° C, which are believed to reflect in the main continued closed-system isotopic exchange at high temperatures following solidification. In contrast highly positive ¹⁸O/¹⁶O fractionations for quartz-K-feldspar, quartz-biotite, quartz-chlorite and quartz-sericite pairs in some granitic samples indicate that these rocks have exchanged oxygen with heated, meteoric, H₂O dominated fluids. Other granitic samples, however, show virtually undisturbed magmatic ¹⁸O/¹⁶O fractionation values for the same mineral pairs, even though these rocks are equally altered.It is concluded that all granitic rock units recrystallized under greenschist facies conditions during the infiltration of fluids under the influence of hydrothermal convection systems set up by the intrusion of the granitic plutons. The fluids probably had a range of ¹⁸O values from ca. -14 to ca. +10, indicating the mixing of distinct fluid reservoirs, one of meteoric origin and the others of magmatic and/or metamorphic origin. The temperature of alteration is estimated at 450–500° C.Estimation of pre-alteration ¹⁸O whole rock values for the five granitic units suggests that three units should be assigned a dominantly S-type origin, where as the other two units may partly or wholly have an I-type origin.     

Contrasting fluid/rock interaction between the Notch Peak granitic intrusion and argillites and limestones in western Utah: evidence from stable isotopes and phase assemblages

The Jurassic Notch Peak granitic stock, western Utah, discordantly intrudes Cambrian interbedded pure limestones and calcareous argillites. Contact metamorphosed argillite and limestone samples, collected along traverses away from the intrusion, were analyzed for ¹⁸O, ¹³C, and D. The ¹³C and ¹⁸O values for the limestones remain constant at about 0.5 (PDB) and 20 (SMOW), respectively, with increasing metamorphic grade. The whole rock ¹⁸O values of the argillites systematically decrease from 19 to as low as 8.1, and the ¹³C values of the carbonate fraction from 0.5 to –11.8. The change in ¹³C values can be explained by Rayleigh decarbonation during calcsilicate reactions, where calculated is about 4.5 permil for the high-grade samples and less for medium and low-grade samples suggesting a range in temperatures at which most decarbonation occurred. However, the amount of CO₂ released was not anough to decrease the whole rock ¹⁸O to the values observed in the argillites. The low ¹⁸O values close to the intrusion suggest interaction with magmatic water that had a ¹⁸O value of 8.5. The extreme lowering of ¹³C by fractional devolatilization and the lowering of ¹⁸O in argillites close to the intrusion indicates oxgen-equivalent fluid/rock ratios in excess of 1.0 and X(CO₂)_F of the fluid less than 0.2. Mineral assemblages in conjunction with the isotopic data indicate a strong influence of water infiltration on the reaction relations in the argillites and separate fluid and thermal fronts moving thru the argillites. The different stable isotope relations in limestones and argillites attest to the importance of decarbonation in the enhancement of permeability. The flow of fluids was confined to the argillite beds (argillite aquifers) whereas the limestones prevented vertical fluid flow and convective cooling of the stock.     

Oxygen isotope geochemistry of hydrothermally-altered synmetamorphic granitic rocks from South-Central Maine,USA

Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock ¹⁸O (SMOW) values 10.7 to 13.8. Constituent quartz, feldspar, and muscovite have ¹⁸O in the range 12.4 to 15.2, 10.0 to 13.2, and 11.1 to 12.0, respectively. Mean values of _Q–F ( ¹⁸O_quartz– ¹⁸O_feldspar)=2.4 and _Q–M ( ¹⁸O_quartz– ¹⁸O_muscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured _Q–F and _Q–M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic ¹⁸O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have D (SMOW) values in the range –65 to –82.Feldspars in the granitic rocks are mineralogically altered to either (a) muscovite+calcite, (b) muscovite+calcite+epidote, (c) muscovite+epidote, or (d) muscovite only. A consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks, quartz, and feldspar. Rocks with assemblage (a) have whole-rock ¹⁸O>12.1 and contain quartz and feldspar with ¹⁸O>13.8 and >11.4, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock ¹⁸O<11.4 and contain quartz and feldspar with ¹⁸O< 13.1 and <11.0, respectively. The correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration.Three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400°–150° C, 100°–150° C below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons. Calculations of mineral-fluid equilibria indicate that samples with assemblage (a) coexisted during the event with CO₂-H₂O fluids of and ¹⁸O=10.8 to 12.2 while samples with assemblages (b), (c), or (d) coexisted with fluids of and ¹⁸O=9.4 to 10.1. Compositional variations of the hydrothermal fluids were highly correlated: fluids enriched in CO₂ were also enriched in ¹⁸O. Because CO₂ was added to the granites during hydrothermal alteration and because fluids enriched in CO₂ were enriched in ¹⁸O, some or all of the variation in ¹⁸O of altered granites may have been caused by addition of ¹⁸O to the rocks during the hydrothermal event. The source of both the CO₂ and ¹⁸O could have been high-¹⁸O metasedimentary country rocks. The inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in ¹⁸O close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism.A close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from Maine is in marked contrast to hydrothermally-altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium. The difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics. Both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration. Deeper hydrothermal events occur at higher temperature and are longer-lived. Minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium. Shallower hydrothermal events occur at lower temperatures and are shorter-lived. Generally there is insufficient time for fluid to closely approach equilibrium with all minerals.     

Petrogenesis of the magmatic complex at Mount Ascutney,Vermont, USA

The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had ⁸⁷Sr/⁸⁶Sr=0.7039, ¹⁴³Nd/¹⁴⁴Nd=0.512678 ( _Nd=+3.8) and ¹⁸O=6.1, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in ¹⁸O, to about 7.8, is the result of fractional crystallization. Mafic rocks show a large range in initial ¹⁴³Nd/¹⁴⁴Nd ratio, from about 0.51267 to 0.51236 ( _Nd= +3.7 to –2.5), which is correlated with elevated ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.     

An oxygen isotope study of the Loon Lake pluton and the Apsley gneiss,Ontario

The Loon Lake pluton in the Grenville province of Southeastern Ontario consists of a quartz monzonite rim surrounding a monzonite core containing inclusions of gneiss, gabbro and diorite. The pluton was emplaced in amphibolite facies Apsley gneiss, amphibolite and marble. Abnormally high ¹⁸O values are observed in all igneous rock types: quartz monzonite (8.9–13.9), monzonite (8.9–9.7), diorite-gabbro (8.0–9.3). High ¹⁸O contents are attributable to interaction between pluton and country rocks, through either isotopic exchange or direct mixing of mobilizate anatectically produced in the contact aureole of the pluton.The Apsley gneiss displays a ¹⁸O range from 8.3 to 16.9. There is no difference in ¹⁸O distribution between rocks inside and outside the contact aureole, although intermineral isotopic fractionations in the aureole are smaller than those outside. A chemical composition discriminant function that distinguishes rocks of igneous origin (DF>0) from sedimentary (DF<0) is inversely correlated with ¹⁸O of the gneisses, indicating that low ¹⁸O values are inherited from a silicic volcanic protolith. Al₂O₃/Na₂O, an index of maturity of sediments, increases with ¹⁸O for the DF<0 group but is almost constant for the DF>0 group over a ¹⁸O range from 8.3 to 13.4. The DF<0 group is inferred to have formed from a series of clastic sediments of varying degree of weathering or maturity; the DF>0 group formed either from tuffs partially altered to zeolites, or from hydrated volcanic flows or ignimbrites.     

Oxygen isotopic study of the Cretaceous granitic rocks in Japan

The ( ¹⁸O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2 relative to SMOW, while ten Cretaceous granitic rocks from the non-metamorphic zone are +7.9 to +9.8. The ¹⁸O-enrichment in the former rocks is mainly attributed to oxygen isotopic exchange between the granitic magma and the surrounding metamorphic rocks during regional metamorphism. The assimilation of ¹⁸O-rich country rocks is also possible in the cases such as gneissose granite and migmatite.The oxygen isotopic ratios of quartz-biotite pairs in the granitic rocks indicate that they are isotopically in near-equilibrium with each other. The quartz-biotite isotopic equilibrium temperatures estimated for these rocks range from 550° to 670° C. Feldspar is occasionally isotopically in disequilibrium with other minerals. This may be caused by exchange of oxygen isotopes between feldspar and hydrothermal or meteoric water after crystallization.     

Altersbestimmungen an Gesteinen des Ostbayerischen Grundgebirges und Ihre Geologische Deutung

Zusammenfassung Rb/Sr-Altersbestimmungen an Biotiten aus Gneisen und Graniten des Moldanubikums lieferten übereinstimmende Alterswerte von 330–345 Millionen Jahren. Das Biotitalter eines vormoldanubischen Granat-Disthen-Gneises ergab sich zu 440 Millionen Jahren, das eines Metagranodiorits der Münchberger Gneismasse zu 385 Millionen Jahren. Zirkone aus einem moldamibischen Gneis zeigen nahezu konkordante Alter um 450 Millionen Jahre.Diese Ergebnisse deuten darauf hin, daß die Münchberger Masse sowie weite Teile des Moldanubikums eine frühvaristische Aufheizung, wahrscheinlich im Zuge regionaler Metamorphose, erlebt haben. Die Beziehungen dieser beiden stark metamorphen Komplexe zu den benachbarten weniger beanspruchten Sedimenten des Saxothuringikums (im Falle der Münchberger Masse) und des Barrandiums (im Falle des Moldanubikums) werden im Sinne einer varistischen Stockwerkstektonik erklärt. Die höheren Alterswerte scheinen wenigstens teilweise auf kaledonische Intrusionen hinzuweisen.

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Rb/Sr biotite ages of Moldanubian gneisses and granites fall in the range of 330–345 million years (m.y.). The biotite age of a pre-Moldanubian garnetkyanite gneiss was found to be 440 m.y., that of a metagranodiorite of the Münchberg Mass 385 m.y. Zircons from a Moldanubian gneiss yielded nearly concordant ages of approximately 450 m.y.These results indicate that both the Münchberg Mass and large parts of the Moldanubicum have undergone considerable heating during early Variscan (=early Hercynian) time, probably in the course of regional metamorphism. The relations of these strongly metamorphosed complexes with the neighboring less affected sediments of the Saxothuringicum (in the case of the Münchberg Mass) and the Barrandium (in the case of the Moldanubicum) are presumed to be caused by different depths of metamorphism in Variscan time. Higher ages seem to be related at least partly to Caledonian intrusions.

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Résumé Des déterminations de la teneur Rb/Sr dans les biotites des gneiss et des granites du Moldanubique ont fourni les indications d'un âge de 330 à 345 millions années. La biotite d'un gneiss à grenat et disthène prémoldanubique a donné l'âge de 440 m.a.; celle d'une métagranodiorite du massif gneissique de Münchberg a fourni l'âge de 450 m.a. Les zircons d'un gneiss moldanubique montrent l'âge approximatif de 450 m.a.Ces résultats indiquent que le gneiss de Münchberg ainsi qu'une grande partie du Moldanubique ont subi une métamorphose varisque ancienne. Les relations entre ces deux ensembles très métamorphiques et les sédiments moins affectés du Saxothuringien et du Barrandien s'expliquent par une tectonique varisque à plusieurs étages structuraux de nature différente. On suppose que les valeurs donnant un âge plus ancien indiquent des intrusions calédoniennes.

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/, 330–345 . . -- 440, 385 . . 450 . . , . . .

     

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.     

Application of Sr and O isotope relations to the petrogenesis of the alkaline rocks of the Red Hill complex,New Hampshire,USA

The Red Hill ring complex in central New Hampshire is composed of apparently cogenetic syenites, nepheline-sodalite syenite, and granite. The ages and petrogenetic relations among five of the six recognized units have been investigated by rubidiumstrontium and oxygen isotope analysis of whole rocks and separated minerals. Whole-rock samples from three syenite units are consistent with a single Rb-Sr isochron which gives an age of 198±3 m.y. and an initial (⁸⁷Sr/⁸⁶Sr)_o ratio of 0.70330±0.00016 (±2 sigma; =1.42× 10^–11y^–1). However, Sr isotope data for two other units, nepheline syenite and granite, are not consistent with this isochron but rather indicate higher initial ratios which range from 0.7033 to about 0.707. Whole-rock O isotope analyses give ¹⁸O values which range from+6.2 to+9.3 Sr and O isotope analyses on mineral separates indicate that observed whole-rock variations in (⁸⁷Sr/⁸⁶Sr)_o are primary and are not due to any secondary process. The fact that the isotope systematics correlate with rock type, suggests that crustal interaction is likely to have played a significant role in the development of this over-and undersaturated association. Such process(es), while still not fully delineated, could be of fundamental importance to the genesis of associations of critically undersaturated and oversaturated intrusives. The data support the idea that interaction between magmas and crustal materials strongly influenced the compositional relations of similar complexes elsewhere including those of the White Mountain magma series.     

Hydrothermal alteration of Variscan granites,southern Schwarzwald,Federal Republic of Germany

Hercynian S-type granites from the southeastern Schwarzwald granite series represent cogenetic biotite-and two-mica granites. Oxygen- and hydrogen-isotope data show that hydrothermal alteration invoking isotopically light surface waters resulted in a drastic reduction in ¹⁸O and D and pronounced disequilibrium between the minerals. Effective water-rock ratios are calculated to be high, about 0.8 vol units. A shift in the¹⁸O/¹⁶O and the chemical composition of the fluid due to water-rock interaction is continuously traced from pure H₂O with meteoric isotopic character in the deep-seated biotite granites to slightly saline water with rock-equilibrated isotopic composition in the two-mica granites at a shallower level. Substantial retrograde hydrometamorphism in the temperature range 500° to 200° C resulted mineralogically in high-temperature chloritization of biotite, and low-temperature muscovitization as well as feldspar alteration, respectively. Another result of the re-equilibration of cations is strong disturbance of the Rb–Sr system which affects measured ages and initial⁸⁷Sr/⁸⁶Sr values. Hydrothermal differentiation and alteration probably overlap to a very large extent magmatic differentiation processes.     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

The oxygen isotope composition of granitoid and sedimentary rocks of the southern Snake Range,Nevada

Six diverse intrusive igneous types are exposed as discrete outcrops within an area of 900 km² in the southern Snake Range, White Pine County, Nevada. The previously recognized variety among these igneous types is reflected in the wide range of ¹⁸O values (–1.1 to 13.4 permil) found in these rocks. This range of ¹⁸O values probably results from differences in source material and post-crystallization history of the different intrusive types.The Jurassic intrusive of the Snake Creek-Williams Canyon area represents the chemical equivalent of a large part of a differentiation sequence, with the entire range of composition (63–76 percent SiO₂) exposed over a horizontal distance of about five km. The rather regular increase of ¹⁸O values from the most mafic to the most felsic parts of this pluton, together with ¹⁸O values determined for constituent minerals recovered from five of the samples, supports a fractional crystallization model. The high ¹⁸O values found (10.2–12.2 permil) indicate that the magma likely was derived from or assimilated sedimentary materials.Nine samples of the Cretaceous two-mica granite of the Pole Canyon-Can Young Canyon area have ¹⁸O values in the range 10.6–12.1 permil. These high ¹⁸O values, an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7165, and the presence of muscovite along with an accessory mineral suite limited to monazite, apatite, zircon, and an allanite-like mineral, characterize this intrusive mass as an S-type granite. It probably formed through anatexis of late Precambrian pelitic rocks.The granitoid rock exposed in the Young Canyon-Kious Basin area is Tertiary (32 m.y.). Most of this intrusive has been cataclastically deformed as a result of late (18 m.y.) movement on the overlying Snake Range decollement. The undeformed portion of this intrusive has ¹⁸O values of 8.7–10.0 permil. However, the deformed portion of this intrusive has ¹⁸O values as low as –1.1 permil, apparently resulting from isotopic exchange between this rock and ground water at the time of cataclasis.Although the igneous types exposed in the southern Snake Range differ petrologically and range in age from Jurassic to Tertiary, most have relatively high ¹⁸O values compared with other granitoid rocks of the Basin-Range Province.     

Patterns and processes of oxygen isotope exchange in a fossil meteoric hydrothermal system,Cuillins Gabbro Complex,Isle of Skye,Scotland

Oxygen isotope compositions of whole rock specimens and mineral separates from the Cuillins Gabbro Complex, Isle of Skye, Scotland, are employed to determine the patterns and processes of¹⁸O depletion in the Outer Unlayered Gabbro (OUG) and associated dikes. Whole rock ¹⁸O values range from +4.8 to –1.1 (SMOW) and dike ¹⁸O values range from +4.7 to –2.8 Mineral separates from three OUG samples yield ¹⁸O values from +5.3 to +4.8 for augite and +4.1 to +0.8 for plagioclase. An early, small-scale hydrothermal circulation system was initiated by the OUG prior to the large-scale hydrothermal convection established by the later Layered Cuillins Complex (LCC). Dikes were emplaced in the OUG after intrusion of the LCC and had only a minor effect on hydrothermal circulation in the OUG. There is evidence of enhanced fluid flow along dike/gabbro contacts. Isotopic compositions of augite separates demonstrate a normal ¹⁸O value for the OUG magma with all¹⁸O depletion in the OUG due to subsolidus exchange processes including diffusion and surface reaction. The mineral separates yield a pattern of¹⁸O depletion consistent with a diffusion mechanism, the bulk of the exchange having occurred in the plagioclase. Secondary mineral formation played a subordinate role in the¹⁸O depletion of the OUG. The calculated water to rock mass ratio necessary to effect the observed¹⁸O depletion in the OUG is on the order of 0.2, although a much greater amount of water circulation probably occurred. The cooling duration required to explain the measured¹⁸O depletion in the OUG by diffusion is very short (140 years at 750° C, 2400 years at 550° C) compared to the duration necessary for pure conductive cooling (10⁵ to 10⁶ years). Rapid local cooling rates in the OUG due to meteoric water convection are consistent with the observed¹⁸O depletion in OUG samples.     

Oxygen,sulfur, and strontium isotope and fluid inclusion studies of barite deposits from the Iglesiente-Sulcis mining district,Southwestern Sardinia,Italy

This paper deals with barite from stratiform, karst, and vein deposits hosted within Lower Paleozoic rocks of the Iglesiente-Sulcis mining district in southwestern Sardinia. For comparison sulfates from mine waters are studied. Stratiform barite displays ³⁴S=28.8–32.1, ¹⁸O=12.7–15.6, and ⁸⁷Sr/⁸⁶Sr=0.7087, in keeping with an essentially Cambrian marine origin of both sulfate and strontium. Epigenetic barite from post-Hercynian karst and vein deposits is indistinguishable for both sulfur and oxygen isotopes with ³⁴S=15.3–26.4 and ¹⁸O=6.6–12.5; ⁸⁷Sr/⁸⁶Sr ratios vary 0.7094–0.7140. These results and the microthermometric and salinity data from fluid inclusions concur in suggesting that barite formed at the site of mineralization by oxidation of reduced sulfur from Cambrian-Ordovician sulfide ores in warm, sometimes hot solutions consisting of dilute water and saline brine with different ¹⁸O values. The relative proportion of the two types of water may have largely varied within a given deposit during the mineralization. In the karst barite Sr was essentially provided by carbonate host rocks, whereas both carbonate and Lower Paleozoic shale host rocks should have been important sources for Sr of the vein barite. Finally, ³⁴S data of dissolved sulfate provide further support for the mixed seawater-meteoric water composition of mine waters from the Iglesiente area.     

Oxygen and hydrogen isotope compositions as indicators of granite genesis in the New England Batholith,Australia

Oxygen and hydrogen isotope studies of a number of granite suites and mineral separates from the New England Batholith indicate that O¹⁸ can be used to discriminate the major granite protoliths. The granite suites previously subdivided on the basis of mineralogical and geochemical criteria into S-type (sedimentary) and I-type (igneous) have O¹⁸ values consistently higher in the S-type granites (10.4–12.5) than in the spatially related I-type plutons (7.7–9.9). There appears to be a systematic variation in O¹⁸ from the most S-type to the most I-type granites, the dividing point between the two occuring at O¹⁸ equal to 10. A group of leucocratic granites that form about half of the batholith and difficult to classify mineralogically and geochemically is found to have low O¹⁸ values (6.4–8.1), suggesting an affinity to the most I-type granites. A single leucogranite pluton with minor muscovite has a O¹⁸ of 9.6 which is significantly higher than other leucogranites indicating a different origin perhaps involving amphibole fractionation.The behavior of D in the plutonic rocks is much less systematic than O¹⁸. Excluding samples collected adjacent to major faults, the D values show a rough positive correlation with water content similar to, but less pronounced than, the trend previously observed in the Berridale Batholith, southeastern Australia. This relation is considered to reflect an interaction between meteoric water and the granites, the largest effect being observed in samples with the least amount of water. Of note is the generally lower D values of the upper Paleozoic New England Batholith compared with the Silurian Berridale Batholith. This difference may be related to a near equatorial paleolatitude of 22 °S in the Silurian and near polar paleolatitudes in the late Carboniferous that have been inferred for these regions. Granite samples collected from near major faults, and one ignimbrite sample of rhyodacite composition, have very low D values (less than –120) suggesting a much greater degree of interaction with meteoric water.     

Non-equilibrium,metasomatic18O/16O effects in upper mantle mineral assemblages

Kyser, O'Neil, and Carmichael (1981, 1982) measured the ¹⁸O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Based mainly on the systematics revealed in ¹⁸O-olivinevs. ¹⁸O-pyroxene diagrams, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena. On such- diagrams, the mantle nodules exhibit data arrays that cut across the ¹⁸O=zero line; these arrays strongly resemble the non-equilibrium quartz-feldspar and feldspar-pyroxene ¹⁸O arrays that we now know arediagnostic of hydrothermally altered plutonic igneous rocks. Thus, the peridotites appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO₂ magma, H₂O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels. This accounts for the correlation between ¹⁸O pyroxene-olivine and the whole-rock ¹⁸O of the peridotites, which is a major difficulty with the equilibrium interpretation. The metasomatic¹⁸O-enrichments of the peridotites can be related to metasomatic enrichments in LIL elements and the development of amphibole and phlogopite. This type of precursor metasomatic activity can explain the development of alkali basalt magmas, as well as leucitites and nephelinites (all of which tend to be slightly¹⁸O-rich relative to MORB, with ¹⁸O=+6 to +7.5). Fluids with appropriate ¹⁸O values to explain the open-system metasomatic effects can be produced by exchange with ancient subducted oceanic crust (eclogite). However, fluid/rock ratios of about 0.4 to 2.5 are required, indicating that this cannot be a mantle-wide phenomenon. Also, these non-equilibrium effects are apparently transient phenomena, probably associated with the eruptive events that brought the nodules to the surface; at characteristic mantle temperatures, the effects would likely disappear in a few tens of millions of years, or less, implying that the ultramafic nodules are not typical samples of the upper mantle.Contribution No. 4156, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

Oxygen isotope compositions of selected laramide-tertiary granitoid stocks in the Colorado Mineral Belt and their bearing on the origin of climax-type granite-molybdenum systems

Quartz phenocrysts from 31 granitoid stocks in the Colorado Mineral Belt yield ¹⁸O values less than 10.4, with most values between 9.3 and 10.4. An average magmatic value of about 8.5 is suggested. The stocks resemble A-type granites; these data support magma genesis by partial melting of previously depleted, fluorine-enriched, lower crustal granulites, followed by extreme differentiation and volatile evolution in the upper crust.Subsolidus interaction of isotopically light water with stocks has reduced most feldspar and whole rock ¹⁸O values. Unaltered samples from Climax-type molybdenumbearing granites, however, show no greater isotopic disturbance than samples from unmineralized stocks. Although meteoric water certainly played a role in post-mineralization alteration, particularly in feldspars, it is not required during high-temperature mineralization processes. We suggest that slightly low ¹⁸O values in some vein and replacement minerals associated with molybdenum mineralization may have resulted from equilibration with isotopically light magmatic water and/or heavy isotope depletion of the ore fluid by precipitation of earlier phases.Accumulation of sufficient quantities of isotopically light magmatic water to produce measured depletions of ¹⁸O requires extreme chemical stratification in a large magma reservoir. Upward migration of a highly fractionated, volatile-rich magma into a small apical Climax-type diapir, including large scale transport of silica, alkalis, molybdenum, and other vapor soluble elements, may occur with depression of the solidus temperature and reduction of magma viscosity by fluorine. Climax-type granites may provide examples of ¹⁸O depletion in magmatic systems without meteoric water influx.     

Deep fluid circulation in alpine shear zones,Pyrenees, France: field and oxygen isotope studies

A combined field, stable isotope, and whole-rock chemical study was made on late Cretaceous to Tertiary metasomatic shear zones cutting Hercynian gneisses in the Aston Massif, Pyrenees, France. Mylonitisation occurred during the early stages of Alpine compression under retrograde conditions at 400–450°C and about 10 km depth. Whole-rock ¹⁸O values of (+11 to +12 in the gneisses) was lowered to +5 to +9 in the shear zones, with the quartz-muscovite ¹⁸O/¹⁶O fractionations of about 2 to 4 essentially unchanged. These ¹⁸O/¹⁶O systematics, together with D muscovite=-40 to-50 indicate that large volumes of formation waters or D-rich meteoric waters passed through the shear zones during deformation. The same fluids also redistributed major elements, as shown by the correlation of ¹⁸O shift with muscovitisation and albitisation reactions in granitic wall rocks. However, even though ¹⁸O was universally lowered within the shear zones, the ¹⁸O/¹⁶O ratios were not homogenised, nor do they correlate in detail with the presence or absence of muscovitisation, suggesting that fluid flow was probably fracture-controlled and episodic. Field mapping shows that, along the length of a particular shear zone, muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss. Thus, muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re-equilibration of acidic, chloride-rich, aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss. Extremely high material-balance fluid-rock ratios (10³) are required to explain the extent of muscovitisation along this shear zone, implying integrated fluid mass fluxes of about 10⁸ kg/m²; this is probably close to the maximum value for other shear zones in the network. Similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites, showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux. For reasonable pressure gradients and time scales of fluid movement, effective permeabilities of 10^-15 to 10^-17 m² are required. Such values can be accounted for by short-lived, widely-spaced cracks produced during seismic activity. A model is presented in which formation waters were seismically pumped down an underlying, shallow, southward-dipping decollement and then upward through the steeply-dipping shear zone network.Contribution No 4711: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA     

U-Pb and Pb-Pb zircon dating of the older orthogneiss suite in the Silvretta nappe,eastern Alps: Cadomian magmatism in the upper Austro-Alpine realm

Two orthogneiss suites dominate the Silvretta nappe. Primary crystallization of the larger suite (younger orthogneisses) is assumed to be Ordovician in age. The second, adjacent magmatic suite consists of older, alkaline to calc-alkaline, ultrabasic, basic to intermediate and granitic rocks known as older orthogneisses. U-Pb data of multigrain zircon fractions, as well as single zircon stepwise evaporation ²⁰⁷Pb/²⁰⁶Pb results suggest a latest Proterozoic to early Cambrian intrusion age for the protoliths of the older orthogneisses as both dating methods yield early Cambrian crystallization ages of 526±7 and 519±7 Ma for an alkaline granite gneiss; similar results were obtained for two neighbouring calc-alkaline orthogneisses (²⁰⁷Pb/²⁰⁶Pb ages of 533 ± 4 and 568 ± 6 Ma, respectively). The crystal habitus corresponds to P5, S19 and S9 zircons of magmatic origin. Whole-rock initial Sr isotope ratios indicate a primitive source. The igneous protoliths of these older orthogneisses represent a fragment of a Cadomian (Pan-African) crust found in places within the basement of the European Hercynides.