Richard St J. Lambert

Petrochemistry and Sr, Pb, and Nd isotopic geochemistry of early precambrian rocks, Wutaishan and Taihangshan areas, China

Abstract In the Wutaishan and Taihangshan region, China, the lowermost high-grade metamorphic complex of grey-gneiss and amphibolite, the Fuping Complex, gives a 2.2 ± 0.2 Ga Pb-Pb isochron age with a first stage growth μ = 7.73, and a 2.37 ± 0.07 Ga SmNd isochron age with (143Nd/144Nd)0 = 0.50963 ± 0.00005 or ϵNd(T) = 1.50. Nd-depleted mantle model dates (TDM) of amhibolites 2.48 to 2.60 Ga, those of gneisses are 2.43 to 2.46 Ga. The unconformably overlying medium- to low-grade metavolcanic complex, the Wutai Complex, gives a 2.0 ± 0.1 Ga RbSr isochron age with (87/86Sr) 0 = 0.7025 ± 2, a 2.27 ± 0.02 Ga PbPb isochron date with afirst stage growth μ = 7.73, and a 2.26 ± 0.06 Ga SmNd isochron first stage (143Nd/144Nd)0 = 0.5097±2 orϵNd(T)=1.08. Individual sample Nd depleted mantle model date are ∼ 2.5 Ga. Metavolcanic samples from low-grade metasedimentary rocks of the Hutuo Group all lie close to a 2.3 Ga reference line on a RbSr isochron plot, have Nd TDM = 2.32, 2.34, and 2.62 Ga. From previous geological work, published UPb zircon dating, and ourisotopic and geochemical results, we infer that the Fuping rocks formed ∼ 2.6 Ga ago in an environment like a modern island arc. The Wutai Complex formed by 2.5 Ga ago, mostly formed in a tectonic setting similar to that of the Fuping Complex, with the exception that one volcanic cycle formed in an environment like a modern MOR and one unit formed in an environment transitional between modern within-plate and plate margin settings. The Hutuo Group formed 2.4 Ga ago in a within-plate tectonic environment. There is no evidence for continental crust before 2.6 Ga in the Wutaishan and Taihangshan region. This contradicts previous assignment of the Fuping Complex to 2.8 Ga ol older. The Fuping Complex is not an early nucleus of the Sino-Korean Craton. The new data also confirm that the Wutai Complex is older than its original assignment as Early Proterozoic. The SmNd systems for metabasaltic rocks in the region, either in amphibolite facies or in greenschist facies, are all significantly disturbed, in contrast with the undisturbed SmNd reported in previous studies of the Sino-Korea Craton. This study provides further evidence that the Archean igneous rocks in China formed at different times, from heterogeneous and depleted mantle sources.

The chemistry and origin of the Lewisian gneisses of the Scottish mainland: The Scourie and Inver Assemblages and sub-crustal accretion

Abstract New chemical data for 21 elements with appropriate factor analyses and new Sr isotope data, together with previously published chemical, geochronological and Pb isotope data are used to re-examine the origin and history of the garnet-pyroxene granulites of the Scottish Mainland Lewisian and their amphibolised equivalents. The quartz-dioritic average composition is allied to chemical variations which bear resemblances to a calc-alkalic differentiation trend, but the behaviour of Na, K, Sr, Zr, Zn and Cr differ from a standard volcanic pattern. The arguments used hitherto in favour of a volcanic or sedimentary origin or both, are rejected in favour of a modified plutonic igneous hypothesis in which magma was accreted to a primitive crust from below in an underplating process, with crystallisation occurring under granulite-facies conditions accompanied by synchronous deformation. As a result, the primitive andesitic magma crystallised directly to rock singularly devoid of some incompatible elements, especially K, Rb, Nb, Y, Th and U. The garnet-pyroxene granulites of the Scourie assemblage have exceptionally high K/Rb and Ca/Y ratios, and low K/Ba and K/Sr ratios to a degree unusual even in the granulite facies elsewhere. The extreme deficiency in Rb and U has led to a 87 Sr/ 86 Sr ratio today of only 0.7021 ± 0.0004 and to equally primitive Pb isotope ratios. These latter may indicate an age-stratified crust with older isotopic ages at the top, compatible with an underplating process occurring over a considerable period, which we name slow sub-crustal accretion . Subsequent to the development of the Scourie assemblage, release of stored stresses led to near-isochemical metamorphism in a succession of shear zones with the formation of the amphibolite-facies Inver assemblage. On its northern margin, adjacent to the more alkaline Laxford assemblage, enrichment in Li, K and Rb occurred in Inver assemblage rocks, a phenomenon not seen elsewhere.

Petrochemistry and Sr, Pb and Nd isotopic geochemistry of the paleoproterozoic Kuandian complex, the eastern Liaoning Province, China

Abstract A geochemical and isotopic study has been carried out on the Paleoproterozoic Kuandian Complex and some associated granitoids in the eastern Liaoning Province, China. The Kuandian amphibolite and granite give an imprecise 1.9±0.4 Ga (all errors are 2σ) Rb-Sr “errorchron” with ( 87 Sr / 86 Sr ) o = 0.711 ± 0.006 , a 2.11 ± 0.06 Ga PbPb isochron with a first-stage growth μ= 8.43, and a 2.31 ± 0.10 Ga SmNd isochron with ( 143 Nd / 144 Nd ) o =0.50970±0.00009 or ϵnd (T) =1 ± 1. Nd-depleted mantle model dates (TDM) of amphibolite are 2.75 to 2.46 Ga, those of granite are 2.53 to 2.36 Ga. Zircons from the Kuandian granite give a minimum 2.14±0.05 Ga upper intercept date. The Kuandian amphibolite is chemically similar to the high-Mg member of Columbia River basalts, and the Kuandian granite is an anorogenic granite. They are genetically related and formed 2.4 to 2.3 Ga ago. Fractional crystallization of olivine, pyroxene and plagioclase from precursor magma of the Kuandian amphibolite can produce magma with composition of the Kuandian granite. The Paleoproterozoic mantle source in the eastern Liaoning Province is less depleted than that inferred from the average mantle evolution curve, in contrast with the Archean mantle in the Sino-Korean craton, which is more depleted than the average. This study also reveals that the Lishan granite of the eastern Liaoning Province is older than 3 Ga and that some granites and dykes previously mapped as Proterozoic are of Mesozoic age.

Yttrium geochemistry applied to petrogenesis utilizing calcium-yttrium relationships in minerals and rocks

Abstract A survey of Y data from all sources shows that Y behaves systematically in igneous, metamorphic and sedimentary rock series, due to its incorporation in a predictable and uniform manner in Ca minerals. Compared with average calc-alkali basalt melts, plagioclase, kaersutite, augitic clinopyroxene and calcite have low Y for their CaO contents; whereas hornblende, garnet, orthopyroxene, apatite, sphene, zircon and most K, Na rich minerals other than plagioclase have high Y for their Ca contents. In sedimentary processes, Ca/Y becomes lower in shales and sandstones, but higher in limestones than their source. In metamorphic processes Y appears to be inert. In all igneous series for which Y data has been assembled Ca/Y falls as Ca falls. These series can be classified into three categories: (i) a standard calc-alkali trend, used as a reference (ii) J-type trends which become progressively impoverished in Y as Ca falls when compared with the standard trend, and (iii) L-trends which become progressively enriched in Y as Ca falls when compared with the standard trend. Despite little knowledge of partition coefficients, the J-type trends appear to have a significant component of hornblende control in their fractionation processes, whereas plagioclase and augitic clinopyroxene dominate the L-type trend. Alternative names for these series might therefore be the standard series, the hornblendic and the pyroxenic series respectively. Modern ocean floor basalts appear to be significantly richer in Y than modern calc-alkali basalts, and these in turn may be richer in Y than their Archaean counterparts.

Reflectance Spectra of Mafic Silicate-Opaque Assemblages with Applications to Meteorite Spectra

Abstract The reflectance spectra of wustite and mixtures of mafic silicates plus carbon or magnetite can be used to interpret meteorite and asteroid spectra. Mafic silicate + magnetite spectra show many features characteristics of magnetite-bearing meteorites— an overall decline or constant reflectance and lown overall reflectance. Mafic silicate + amorphous carbon spectra show low overall reflectance and a red slope unlike that seen in CV and CO carbonaceous chondrite spectra, probably because the meteoritic carbon is in a more ordered form. The reflectance spectra of ureilites are largely consistent with an assemblage of mafic silicates and abundant carbon. Ordinary chondrite reflectance spectra cannot be reproduced by any of the laboratory mixture spectra. The reflectance spectrum of wustite is a reasonable match to the spectrum of ordinary chondrite metal, suggesting that most ordinary chondrite metal grains are probably coated with an optically thick layer of an oxide. Ordinary chondrite and mafic silicate reflectance spectra are consistently less red-sloped than S-class asteroid spectra. The various spectral criteria use to deco0nvolve mafic silicate spectra are also applicable to CV and CO carbonaceous chondrites, ureilites, and ordinary chondrites, because the opaque phases present in these meteorites are spectrally neutral.

Evaluating crustal contamination in continental basalts: the isotopic composition of the Picture Gorge Basalt of the Columbia River Basalt Group

Crustal contamination of basalts located in the western United States has been generally under-emphasized, and much of their isotopic variation has been ascribed to multiple and heterogeneous mantle sources. Basalts of the Miocene Columbia River Basalt Group in the Pacific Northwest have passed through crust ranging from Precambrian to Tertiary in age. These flows are voluminous, homogenous, and underwent rapid effusion, all of which are disadvantages for crustal contamination while en route to the surface. The Picture Gorge Basalt of the Columbia River Basalt Group erupted through Paleozoic and Mesozoic oceanic accreted terranes in central Oregon, and earlier studies on these basalts provided no isotopic evidence for crustal contamination. New Sr, Nd, Pb, and O isotopic data presented here indicate that the isotopic variation of the Picture Gorge Basalt is very small, 87Sr/86Sr=0.70307–0.70371, ɛNd=+7.7-+4.8, δ18O=+5.6±6.1, and 206Pb/204Pb=18.80–18.91. Evaluation of the Picture Gorge compositional variation supports a model where two isotopic components contributed to Picture Gorge Basalt genesis. The first component (C1) is reflected by low 87Sr/86Sr, high ɛNd, and nonradiogenic Pb isotopic compositions. Basalts with C1 isotopic compositions have large MgO, Ni, and Cr contents and mantle-like δ18O=+5.6. C1 basalts have enrichments in Ba coupled with depletions in Nb and Ta. These characteristics are best explained by derivation from a depleted mantle source which has undergone a recent enrichment by fluids coming from a subducted slab. This C1 mantle component is prevalent throughout the Pacific Northwest. The second isotopic component has higher 87Sr/ 86Sr and δ18O, lower ɛNd, and more radiogenic Pb isotopic compositions than C1. There is a correlation in the Picture Gorge data of Sr, Nd, and Pb isotopes with differentiation indicators such as decreasing Mg#, and increasing K2O/TiO2, Ba, Ba/Zr, Rb/Sr, La/Sm, and La/Yb. Phase equilibrium and mineralogical constraints indicate that these compositional characteristics were inherited in the Picture Gorge magmas at crustal pressures, and thus the second isotopic component is most likely crustal in origin. Mixing and open-system calculations can produce the isotopic composition of the most evolved Picture Gorge flows from the most primitive compositions by 8 to 21% contamination of isotopic compositions similar to accreted terrane crust found in the Pacific Northwest. Therefore, in spite of the disadvantages for crustal contamination and their narrow range in isotopic compositions, the process controlling isotopic variation within the Picture Gorge Basalt is primarily crustal contamination. We suggest that comprehensive analyses for basaltic suites and careful consideration of these data must be made to test for crustal contamination, before variation resulting from mantle heterogeneity can be assessed.

The Lewisian of Lochinver, Sutherland; the type area for the Inverian metamorphism

The Lochinver district contains gneisses, pegmatites and dykes generated in five main episodes: (1) pre-2600 m.y, garnet pyroxene granulite facies gneisses with low angle dips, formed in their final state by the Badcallian metamorphism; (2) potash-rich pegmatites of age range 2540 to 2310 m.y.; (3) 2310–2200 m.y. amphibolite facies gneisses occurring in vertical WNW zones, produced by a broadly isochemical metamorphic event; (4) vertical NW-trending dykes emplaced at 2200 m.y. and perhaps down to 1900 m.y. and (5) epidoteamphibolite facies gneisses produced in narrow, discontinuous NW shear zones during the succeeding Laxfordian events. The petrographic and structural features of each sector of the complex are described and new whole rock Rb–Sr ages given for the pegmatite suite, suggesting a maximum age of 2540 m.y. for the suite (plus intrusion of a further suite at 2310 m.y. or recrystallisation of the first suite at 2310 m.y.). Other pegmatite ages confirm the presence of pre-2000 m.y. activity NE of Scourie and at Gruinard. Field and petrographic evidence at Lochinver demonstrates the existence of amphibolite facies metamorphism in post pegmatite pre-dyke times, which produced well-defned rock types and WNW vertical structures, and is defined as the Inverian metamorphic event.

A geochronological study of the Lewisian from Loch Laxford to Durness, Sutherland, N. W. Scotland

Potassium-argon biotite and hornblende ages from near Laxford Bridge, Sutherland, range up to 1770 m.y.: higher apparent ages of 2060 and 1840 m.y. occur in the biotite pods along the southern margin of the Laxford assemblage. A rubidium-strontium isochron from whole-rock specimens from the gneisses of the Laxford assemblage gives 1850 ± 50 m.y., initial 87Sr/86Sr = 0.7061 ± 0.0005 (T 1 2 = 4.85. 1010y). It is suggested that 1850 ± 50 m.y. is a minimum age for the climax of the Laxfordian metamorphic episode, and that this metamorphism had effectively terminated by 1750 m.y. the highest K-Ar age of a mineral from the type area. Younger apparent ages of minerals from and close to the type area spread down to 1575 ± 50 m.y., associated with late effects of the metamorphism, continued pegmatite-formation, and a general slow-cooling. Closure of radiogenic systems occurred later at Durness than at Laxford Bridge. Comparison with the sparse data from nearby shield areas suggests a closer correlation of the Lewisian with comparable rocks in Greenland than with Fennoscandia.

NdSr isotopic study of Proterozoic to Triassic sediments from southeastern British Columbia

Abstract Sedimentary rocks from the southeastern Canadian Cordillera varying in age from Mid-Proterozoic to Triassic have been analysed for their Nd isotopic composition. e Nd (T) values range from +1.5 to −17.2 and Nd model ages T DM (Nd) vary from 1.00 to 2.32 Ga, much in excess over their stratigraphic ages. The difference between the two ages ranges from 0.43 to 1.93 Ga, decreasing as the sediments become stratigraphically older. Nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the Cordilleran miogeocline: (a) an older source ( e Nd (0) from −20.3 to −23.8) with apparent crustal residence ages of 2.26–2.32 Ga, and (b) a younger source (average e Nd (0) ⋍ −14.0 ) with the crustal residence age of about 1.8 Ga. The older residence ages are interpreted to be due to mixtures of materials from the Archean and Proterozoic crusts. Sediments belonging to the Laib Formation (Lower Cambrian) and lower clastic part of the Milford Group (Namurian) appear to have derived from an Archean continental segment, with some admixture of sediments from the Early Proterozoic source. The younger crustal source, which seems to be the principal supplier of sediments for most of the stratigraphic units (Aldridge Formation (Middle Proterozoic), Horsethief Creek Group (Upper Proterozoic), Hamill and Lardeau Groups (Lower Cambrian), Nelway Formation (Middle Cambrian), Active Formation (Ordovician) and part of Milford Group) is not the Archean Canadian shield immediately to the east. Recent subsurface mapping of the Alberta basement beneath the Phanerozoic sedimentary cover using U/Pb zircon dating of igneous lithologies [1] has established the extensive Archean and Early Proterozoic crustal provinces. It is proposed here that the Early Proterozoic province of west-central Alberta is the most likely source of the easterly derived sedimentary rocks of the Cordilleran miogeocline with T CR ages from 1.94 to 1.50 Ga. On the basis of sedimentologic, tectonic and isotopic evidence, we suggest that the younger sediments could be to some extent recycled older sediments. For the Triassic Slocan and Ymir Groups (eastern part of Quesnel terrane), sedimentologic, lithologic, tectonic and isotopic evidence suggest the source was a western one with about 1.8 Ga crustal residence age. Comparison of the crustal residence ages with stratigraphic ages shows the best fit Nd evolution curve for the Cordilleran miogeocline approximately coincides with the continental growth curve deduced for the North Atlantic Province and similar to the growth curve for Gondwana during the Phanerozoic. This curve also passes through the North American Composite. The levelling-off of crustal residence ages for the Mid-Proterozoic to Triassic sediments at about 1.8 Ga supports the idea of negligible crustal growth since at least 1.7 Ga, the average crustal residence age of the modern miogeoclinal and cratonic sediments.

A strontium isotopic study of Newberry volcano, central Oregon: Structural and thermal implications

Abstract Ratios of 87 Sr/ 86 Sr were determined in twenty-seven basaltic rocks from Newberry. These ratios range from 0.70330 to 0.70414; most fall between 0.7035 and 0.7038. Covariance of Al 2 O 3 /FeO ∗ ratio, TiO 2 content, and Th content with 87 Sr/ 86 Sr suggests that some isotopic ratios reflect shallow crustal contamination, but the data also suggest that two mantle sources, a prominent one at 0.7036 and a less-prominent one at 0.7033 or less, are represented. The hypothesis of crustal contamination is supported by variations in 87 Sr/ 86 Sr ratios with locations of the vents from which these rocks erupted. Sr tends to be more radiogenic in rocks from vents either close to a silicic complex such as the caldera of the volcano or the Amota Butte-Sugar Pine Ridge complex, or close to axes of the fault zones that underlie the edifice. Presumably, local heating of the upper crust facilitates contamination of basaltic magma rising through it.