Determination of Elemental Concentrations in Nine CMEA Rock and Ore Standards,in NBS 98 Plastic Clay and in Fired Clay Gent (FCG) by Instrumental Neutron Activation Analysis*

Data for as many as 31 elements were determined by instrumental thermal neutron activation analysis for nine European Council for Mutual Economic Assistance (CMEA) rock and ore standards. The National Bureau of Standards plastic clay 98 and the University of Gent fired clay FCG were also analyzed. Synthetic, multielement standards were used and USGS rock standards provided reference samples. Correction factors for uranium fission products on cerium and molybdenum, and also for less commonly encountered spectral interferences, such as those due to the 213 ppm tantalum in granitoid 2B, were evaluated.     

Contact Metamorphism of Serpentinite, Chloritic Blackwall and Rodingite at Paddy-Go-Easy Pass, Central Cascades, Washington

The 2 km wide contact aureole produced from serpentinite bythe intrusion of the Mount Stuart Batholith into the IngallsComplex at Paddy-Go-Easy Pass contains the following ultramaficassemblages, in order of increasing grade: serpentine-forsterite-diopside,serpentine-forsterite-tremolite, forsterite-talc, forsterite-anthophyllite,forsterite-enstatite-anthophyllite, forsterite-enstatite-chlorite,forsterite-enstatite-spinel. Associated metarodingites displayfive metamorphic zones, the diagnostic assemblages of whichare, in increasing grade: grossular-idocrase-chlorite, grossular-diopside-chlorite,epidote-diopside-chlorite, epidote-diopside-spinel, plagioclase-grossular-diopside.Mafic hornfels in the aureole contains no orthopyroxene, indicatingthat the conditions of pyroxene hornfels facies were not reached. The breakdown of chlorite is best displayed in aluminous blackwallreaction zones around mafic inclusions in the peridotite. Attemperatures above those of the anthophyllite-out isograd, butwithin the field of forsterite+tremolite, these chlorite-richrocks react to form the assemblage: forsterite-enstatite-spinel.Calculations show that cordierite did not form as a result ofchlorite breakdown in the natural system because impurities,such as iron and chromium, displaced the equilibrium: forsterite+cordierite=enstatite+spinelto much lower pressures than the three kilobars found in thepure system. The primary chromite of the peridotite has been altered to chrome-magnetitein the serpentinite. This alteration seems to be isochemicalover the whole rock, as true chromite, formed by metamorphism,occurs at grades above that of the forsterite-enstatite-anthophylliteassemblage. Calcic amphibole in high-grade metaperidotite is tremolite,even in the presence of aluminous chromite, whereas that inmetamorphosed blackwall rock grades from tremolite into hornblende.The pattern of substitution appears to be: Mg₂Si₃(Na,K)(Al^VI)₂(Al^IV)₃.     

Low-grade Metamorphism of the Karmutsen Volcanics, Vancouver Island, British Columbia

Low-variance assemblages occurring within amygdules of Karmutsenlavas from the Elk Creek and Upper Campbell Lake areas, VancouverIsland, British Columbia, provide important constraints on thepressure and temperature of metamorphism as well as on the compositionof the attendant fluid. The P-T stability of the assemblagesepidote-muscovite-K-feldspar-prehnite and epidote-prehnite-quartz-wairakitecoupled with mean isochores derived from homogenization temperaturesof H₂O inclusions within amygdaloidal quartz indicate that theUpper Campbell Lake area was subjected to metamorphism at 1?5kb (?0?5 kb), 260 ?C (? 15?C) and the Elk Creek area at somewhatlower P or higher T. Isobaric T-a(CO₂) diagrams show that the occurrence of epidote-oligoclase,prehnite-orthoclase-albite, and prehnite-andesine assemblagescollected from the Elk Creek area is consistent with the P-Tconstraints and that these assemblages formed in water-richfluids containing very low concentrations of CO₂. The presenceof Ca-zeolite-epidote assemblages in the Upper Campbell Lakearea is also compatible with P-T estimates. The consistencyof epidote and prehnite rim compositions in low-variance assemblagesand the lack of incompatible phases in these assemblages demonstratethat equilibrium was obtained on limited domains within amygdules. Because epidote and prehnite compositions in low-variance assemblagesare very sensitive to changes in concentration of CO2, low-varianceassemblages involving these phases can serve as monitors offluid composition. It is postulated that low-variance assemblagesin Karmutsen flows originated by reaction of previously formed,high-variance assemblages with infiltrating CO₂-bearing aqueousfluids during a subsequent hydrothermal event. These fluidspreferentially exploited more permeable amygdaloidal portionsof the Karmutsen flows. The low-variance assemblages not onlyrecord the extremely H₂O-rich composition of the permeatingfluid, but also outline the paths the fluid took. ^* Offprint requests to B. R. Frost     

Fluid Transfer in High-grade Metamorphic Terrains Intruded by Anorogenic Granites: The Thor Range, Antarctica

A composite intrusive igneous complex in the central mountainrange of Queen Maud Land (Thor Range), Antarctica, displayscharacteristic features of anorogenic granites. A suite of massiveintrusives and various sets of dykes and satellite intrusionsare ferroan, alkalic to alkali–calcic, and weakly peraluminous.An early set of plutons consists of charnockitic alkali-granites;a later group of plutons comprises fayalite Qtz-syenites. Coarsemesoperthite is the dominant mineral in all rocks, quartz isabundant and plagioclase is a minor mineral. Olivine (fayalite)is the characteristic mafic mineral, but subcalcic augite andoccasionally pigeonite or orthopyroxene are present. In mostsamples, amphibole is the dominant mafic mineral and its compositionis close to end-member hastingsite. It contains high concentrationsof F and Cl. Some samples contain igneous fluorite. Thermobarometrysuggests a temperature of 900 ± 25°C and a pressureof 0·4 ± 0·1 GPa for the crystallizationconditions of the pyroxene–olivine assemblages. The solidustemperature of 800–850°C for both suites of plutonicrocks is typical of water-deficient granitic melts. The estimatedlow water activity of 0·3–0·5 at solidusconditions is consistent with the high halogen content of thebulk-rocks and their constituent minerals. In the absence ofan aqueous fluid, the halogens remained in the minerals at thesolidus. Oxygen fugacity stayed below QFM in all igneous rocksabove solidus. This is typical of melts derived from partialmelting of mafic source rocks. The igneous rocks were locallyaffected by at least three distinct episodes of hydration. Asthe melt approached solidus conditions, fayalite and pyroxenewere locally transformed into hastingsite as a result of increasingfugacity of volatile components. Fayalite-free and fayalite-bearingigneous rocks are arranged in banded structures. Subsolidushydration locally modified the igneous rocks and transformedpyroxene- and fayalite-bearing granites into biotite-granitesand hornblende-granites in which all evidence of former high-Thistory was erased. This local hydration of igneous rocks occurredin response to uptake of H₂O that had been given off by gneissicxenoliths as a result of progressing, continuous, dehydrationreactions. The reactions in the gneiss xenoliths were drivenby contact metamorphism. This exchange of H₂O between igneousand metamorphic rocks occurred in a fluid-absent regime at temperaturesof about 750°C. Late reaction veins formed by hydraulicfracturing of the plutonic rocks and indicate the presence ofa low-density fluid phase at amphibolite facies conditions. KEY WORDS: anorogenic granite; fayalite; hastingsite; fluid recycling; Antarctica     

Caledonian Magma Genesis and Crustal Recycling

The isotopic compositions of Sr, Nd, Pb and O together withabundance data for Rb, Sr, Sm, Nd, U and Pb are reported forsamples from the component parts of the c. 400 Ma old EtiveComplex, temporally and spatially related Lorne and Glencoelavas, and the Dalradian country rocks into which the Complexhas been emplaced. These and published data available for otherCaledonian granites are used to evaluate the petrogenesis ofthe Etive Complex in particular, and the role of crustal recyclingin the generation and evolution of the Caledonian granites ingeneral. Nd-isotope compositions of Etive samples at 400 Ma range from – 9.9 to – 4.7 compared with–8.4 < – 3.2 for the associated volcanics investigated here, and an estimatedvalue for depleted mantle 400 Ma ago at approximately ? 7. Dalradiancountry rocks have – 23.4 < – 7.5 and two partially digested metasedimentaryxenoliths within the granite have values of –9. 3 and -4.0. Initial ⁸⁷Sr/ ⁸⁶Sr ratios forthe Etive Complex range from 0–7043 to 0–7079, whereasDalradian metasediment in the immediate vicinity of the granitehas an initial ratio of 0–726. Oxygen isotopes in theComplex have 7. 6 per mil <¹⁸O < 10.0 per mil, all inexcess of typical values of mantle oxygen and reflect a crustalcomponent. An upper limit of 25 per cent Dalradian assimilationis set by the Nd-Sr isotopic variations with the granites andxenoliths. The Etive complex parent magma prior to Dalradian xenolith assimilationis estimated to have values between – 10 and – 5. In order to satisfy the Srand Pb isotope composition, additional components from a deepersource within the lithosphere (lower crust or continental lithosphericmantle) with relatively unradiogenic Sr, Nd and Pb are required. The crustal residence ages of the Etive Complex average about1.5 Ga, similar to those of many other late and post-tectonicCaledonian granites. The generation of the Etive Complex andCaledonian granites in general has been dominated by recyclingof the continental lithosphere, rather than the addition ofnew material from asthenospheric sources.     

Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U–Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U–Pb apatite ages are 2.4–2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz–biotite–plagioclase–garnet–staurolite–kyanite in the pelitic schists; (b) quartz–biotite–plagioclase–low-Ca amphiboles–kyanite in Mg–Al-rich schists, and locally (c) hornblende–plagioclase–garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg–Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from >7 kbar to <3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78–1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.     

The Geochemical Evolution of Anorthosite Residual Magmas in the Laramie Anorthosite Complex, Wyoming

Olivine- and pyroxene-bearing Fe-enriched dioritic rocks inthe 1434 Ma Laramie anorthosite complex are interpreted to representvariably fractionated and contaminated magmas residual afterthe crystallization of anorthosite. Geochemical characteristicsof this suite include the following: high contents of TiO₂,, and P₂O₅; high incompatibletrace element contents; rare earth element patterns with a largerange of Eu anomalies; and isotopic compositions that reflectthe geographic location of individual samples, with I_Sr increasingand _Nd decreasing from south to north. After extraction fromanorthosite, fractionation of ferrodioritic residual magmasresulted in secondary residual monzodioritic melts and complementaryoxide-rich ferrodiorite cumulates. Geographic trends in isotopiccomposition reflect an increasing Archean crustal componentfrom south to north. Dioritic dikes and cumulates with isotopiccompositions similar to associated anorthosites were derivedlocally. Large isotopic discrepancies between some dioritesand their hosting anorthosites reflect preferential contaminationof residual magma during ascent and emplacement of mantle-derivedplagioclase-rich diapirs, followed by subsequent extractionand isolation of Fe-enriched interstitial melt. Strong isotopiccontrasts between anorthosite and associated Fe-enriched rocksin anorthosite complexes do not preclude a direct relationshipbetween them and reflect the diversity and complexity of processesduring their petrogenesis. KEY WORDS: anorthosite; ferrodiorite; geochemistry; Laramie anorthosite complex; residual magma ^*Corresponding author: Present address: Ceramic Science and Technology, Mail Stop K762, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Phone: (505) 665-3934. Fax: (505) 665-3935. E-mail: jeremy{at}lanl.gov.     

南阿尔金茫崖地区花岗岩类锆石SHRIMPU-Pb定年、Lu-Hf同位素特征及岩石成因

南阿尔金茫崖地区早古生代花岗岩锆石SHRIMP U-Pb定年结果表明,阿克提山花岗岩为264±1Ma,柴水沟花岗岩分别为404±5Ma、406±4Ma,其中的辉绿岩为454±4Ma,常春沟花岗岩分别为411±5Ma、406±3Ma,茫崖镇北石英闪长岩为466±5Ma,阿卡龙山花岗岩为469±6Ma。锆石Lu-Hf同位素分析表明,εHf(t)值大多数为正值,少数继承性锆石为负值,反映了它们的源岩以新生地壳为主,同时,也混有少量的古大陆壳的成分。结合区域地质特征和各岩体的岩石地球化学特征,将南阿尔金茫崖地区早古生代花岗质岩浆活动划分为3期,第一期(465~469Ma)岩石组合为石英闪长岩+花岗闪长岩+花岗岩,具有岛弧火成岩的地球化学属性,其形成可能与洋壳的俯冲作用有关;第二期(404~411Ma)岩石组合为花岗闪长岩+二长花岗岩+正长花岗岩,具有A型花岗岩的地球化学特征,可能与板块碰撞后造山带块体均衡调整有关,第三期(264Ma)岩石组合为石英闪长岩+二长花岗岩+正长花岗岩,也具有I型花岗岩的特征,可能与阿尔金断裂的活动有关。     

Petrogenesis of Mesozoic, Peraluminous Granites in the Lamoille Canyon Area, Ruby Mountains, Nevada, USA

Two groups of closely associated, peraluminous, two-mica graniticgneiss were identified in the area. The older, sparsely distributedunit is equigranular (EG) with initial