Large Negative Eu Anomalies Research Articles

Geochemical and Nd isotopic compositions of the metasedimentary rocks in the La Ronge Domain, Trans-Hudson Orogen, Canada: implications for evolution of the domain

Metasedimentary rocks of greenschist to amphibolite facies occur in various belts in the Paleoproterozoic La Ronge Domain of the Trans-Hudson Orogen. Metasedimentary rocks in the Central Metavolcanic greenstone belt are low to intermediate in SiO 2 contents, have ϵ Nd values of +2.3 to +4.5, with two rare earth element (REE) patterns of (La/Yb) n <4 and >8.5, respectively. The predominant component, meta-arkoses, of the overlying McLennan Lake belt, have distinctly high K 2O and SiO 2 contents, high (La/Yb) n ratios (11–46), large negative Eu anomalies (Eu/Eu*=0.1–0.58) and uniformly low ϵ Nd values (−0.8 to +0.2). The MacLean Lake belt is comprised of a variety of metasedimentary gneisses. The volumetrically minor amphibole- and diopside-rich gneisses have high ferromagnesian element (Sc and Co) concentrations, high ϵ Nd values of +4.1 to +5.7, and flat REE patterns with no Eu anomalies. The predominant lithological unit in the MacLean Lake belt, the psammitic gneisses, is geochemically more variable. The mafic psammitic gneiss samples in the MacLean Lake belt and the metasedimentary rocks in the Central Metavolcanic belt are similar in trace and major element geochemistry and Nd isotope composition, whereas those more felsic psammitic gneisses are similar to the McLennan Lake meta-arkoses. Geochemical data, combined with lithological and geochronological evidence, indicate that sediments in the Central Metavolcanic belt were derived mainly from contiguous volcanic debris, with minor addition of felsic components from exotic sources. In contrast, the McLennan Lake meta-arkoses were derived from a distinct felsic source, whereas the psammitic gneisses in the MacLean Lake belt are mixtures of volcanic materials in the Central Metavolcanic belt and meta-arkoses-like felsic components. Nd isotopic data and ages of detrital zircon suggest that Proterozoic juvenile materials were dominant sources for metasedimentary rocks in the La Ronge Domain. Comparison of geochemical signatures of the metasedimentary rocks with those of the felsic plutonic rocks in the Central Metavolcanic belt suggests that the felsic materials in the McLennan Lake and MacLean Lake belts could not have been derived predominantly from the plutonic rocks, implying more distal sources. Topographic changes during collision may have resulted in a shift from mixing of both local and distal sources for the rocks in the MacLean Lake belt, to the dominance of a more distal source for the McLennan Lake meta-arkoses.

Geochemistry and petrogenesis of El Gezira ring complex, Egypt: a monzosyenite cumulate derived from fractional crystallization of trachyandesitic magma

El Gezira ring complex (GRC) is a high-level Permo-Triassic intrusion related to the alkaline province of southern Egypt. It comprises an older, outer ring of alkali basalt flows and agglomerates and an inner ring of monzosyenite. Crescent-shaped bodies of alkali gabbros separate the two rings. The core of the complex is dominated by monzosyenite and volcanic flows which range in composition from trachyandesite to trachydacite. The monzosyenite constitutes a major part of the complex and rarely grades to mafic-rich segregations. Mg-rich augites are the dominant mafic minerals in most rock types of GRC. Olivine is common in basic rocks, whereas secondary amphiboles and annitic biotite are more abundant in intermediate-felsic rocks. The rocks of GRC are metaluminous and exhibit geochemical features of A-type anorogenic magmatism such as high Fe/Mg ratio, total alkalis, Nb, Zr and Y contents with concomitant low values of Ca, Mg and Sr. The basic rocks have flat primitive REE patterns with no Eu anomalies suggesting that they probably represent the parental basaltic melt from which the complex was evolved. The trachyandesite, trachydacite and the monzosyenite all have steep LREE-enriched patterns with no enrichment in HREE. Pronounced positive Eu anomalies (Eu/Eu *=1.83–3.50) are characteristic for the monzosyenites on which, together with remarkably high Ba contents indicate the role of feldspar accumulation. The trachyandesite and trachydacite exhibit large negative Eu anomalies (Eu/Eu *=0.45–0.75; 0.72) indicating the dominant separation of feldspars. Field and geochemical features of the complex supported by mass-balance calculations of major elements and REE modelling suggest two distinct stages of fractional crystallization in the evolution of the GRC. The first stage includes crystal-melt fractionation dominated by clinopyroxene, calcic plagioclase and olivine from the basaltic source to yield the trachyandesitic magma. Further, fractional crystallization of the trachyandesitic melt with the alkali feldspar as dominant separating phase and minor contribution of clinopyroxene, biotite, Fe–Ti oxides and apatite yielded the more evolved felsic trachydacite (stage II). The cumulus mineralogy and its modelled REE abundances of stage II closely match the modal and REE compositions of the monzosyenite, which reinforces the cumulate origin of the rock.

Evolution of the lunar crust: SIMS study of plagioclase from ferroan anorthosites

The lunar crust, down to a depth of ∼65 km, is composed of older (>4.5 Ga) ferroan anorthosites and younger (4.43-4.17 Ga) Mg-suite lithologies which include dunites, troctolites, and norites. The anorthosites are generally inferred to represent floating cumulates in a lunar magma ocean (possible depth 800 km, moon's radius ∼ 1,738 km). The cumulates that are inferred to be located near the base of the magma ocean are dominantly olivine and pyroxene. The last dregs of the magma ocean are enriched in incompatible elements and have been named KREEP (K, rare earth elements, P). KREEP, formed in this manner, is probably concentrated near the crust/mantle boundary at ∼70 km depth. We are attempting to characterize melts parental to ferroan anorthosites and Mg-suite norites by analyzing REEs (La, Ce, Nd, Sm, Eu, Dy, Er, Yb) and Ba, Sr, and Y in their cumulus plagioclase. If the cumulus grains have not been compromised by postcrystallization effects and if we know the relevant mineral/melt partition coefficients (Ds) we can invert the trace element data for plagioclase to parental melt compositions. Melts parental to ferroan anorthosites are estimated to contain REE at concentrations ten to fifty times chondrites. Melts parental to the earlier crystallizing anorthosites (lower REE) have virtually no Eu anomaly, while melts parental to later crystallizing anorthosites (higher REE) have small negative Eu anomalies. This is qualitatively consistent with the fractionation of Eu relative to other REE by crystallization of plagioclase with large positive Eu anomalies. Melts parental to the Mg-suite have much higher total REE and very large negative Eu anomalies. The characteristics of Mg-suite norite parental magmas may have been derived by the assimilation of KREEP (near the crust/mantle boundary) by Mg-rich basaltic melts formed deeper in the cumulate pile or near the contact between the lower cumulate horizons of the magma ocean and primitive, unprocessed lunar mantle (∼800 km).

Post-collision, A-type granites of Homrit Waggat Complex, Egypt: petrological and geochemical constraints on its origin

The Homrit Waggat Complex (HWC) is a ring complex which belongs to the younger granite province of the Nubian Shield. It comprises a central biotite granite encircled by an intermediate ring of mylonitized pink granite. both of which have been emplaced by cauldron subsidence in a granodiorite body. The mylonitized pink granite is locally cut across by hypersolvus coarse-grained red granite. The mylonitized pink granite is characterized by fully-developed mylonitic fabrics and suffered inflation and flattening due to a ‘ballooning effect’ which suggests that it has experienced stronger ductile deformation than the granodiorite and biotite granite. The HW granites are mildly peraluminous, highly fractionated and enriched in Rb, Nb. Y, REE (except Eu) and F with corresponding depletion in Mg, Fe, Ti, Ca, Sr and Ba. REE fractionation patterns and Eu Eu∗ values decrease from granodiorite ( La Yb n = 6.24−6.69 ; Eu Eu∗ = 0.95−1.56 ) to biotite granite ( La Yb n = 2.48−4.68 ; Eu Eu∗ = 0.25−0.75 ). The mylonitized pink and red granites display flat patterns and large negative Eu anomalies ( La Yb n = 0.33−1.09 ; Eu Eu∗ = 0.01−0.07 , respectively). Although accessory minerals (e.g. apatite, allanite and fluorite) exerted some control on REE concentrations, volatile transfer largely controls REE abundances, particularly HREE (heavy rare-earth element). Geochemical data indicate that the granites of HWC are post-collision, A-types. The less fractionated granodiorite has an I-type and volcanic arc geochemical signature. Least-squares analyses of major elements and modelling of REEs indicate that the HW A-type granites were formed through two stages of differentiation. A first stage, crystalmelt fractionation, was dominated by the removal of plagioclase, biotite, quartz and apatite from the granodiorite magma to yield the biotite granite. Further crystal fractionation of the biotite granite liquid with K-feldspar as a dominant separating phase in addition to plagioclase, biotite and quartz gave rise to the mylonitized pink granite (second stage). Volatile phase transfer occurred during the second stage of fractionation in the magma chamber. The red granite represents the last crystallizing granite type from a residual, volatile-rich fluids which accumulated and emplaced as ring dykes in the solidified part of the mylonitized pink granite.

Conflicting mineral and whole-rock isochron ages from the Late-Archaean Lewisian complex of northwestern Scotland: Implications for geochronology in polymetamorphic high-grade terrains

We report major and trace element (including rare-earth element, REE) geochemistry and whole-rock Sm Nd geochronology on two suites of mafic/ultramafic inclusions in an agmatite complex within a trondhjemitic host at Gruinard Bay, in the Late Archaean Lewisian Complex of northwest Scotland. A suite of medium-grained equigranular amphibolites (main amphibolite suite, MAS) has broadly chondritic high field strength element (HFSE) ratios (Ti/Zr ca. 115, Ti/Y ca. 318, Y/Cr ca. 0.39), flat heavy REE patterns at five to fifteen times chondrites, usually with mild light REE enrichment (La N < 50) and small Eu anomalies. This is consistent with an origin as basaltic liquids which may represent suitable mafic precursors for tonalite-trondhjemite-granodiorite (TTG) suite magmas, as argued previously by Rollinson and Fowler (1987). A Sm Nd regression line (MSWD = 3.4) obtained from the MAS yields an age of 2.943 ± 0.091 Ga with an initial ε Nd value of +2.69 ± 0.62. A suite of coarser grained hornblendites and metagabbros (hornblendite-metagabbro suite, HMS) displays significant geochemical differences from the MAS, exemplified by significantly light REE-enriched patterns in which the hornblendites have the highest total REE with La N/Nd N< 1 and large negative Eu anomalies, and the most felsic metagabbros have low total REE and large positive Eu anomalies. Samples from the HMS yield a Sm Nd isochron (MSWD = 0.52) age of 2.846 ± 0.073 Ga with an initial ε Nd value of +1.10 ± 0.72. The HMS geochemistry is consistent with an origin as amphibole and plagioclase cumulates to the local TTG magmas. Combining the new data with previously published data from the host trondhjemites at Gruinard Bay (Whitehouse, 1989a) yields a Sm Nd isochron (MSWD = 1.09) age of 2.795 ± 0.028 Ga with an initial ε Nd value of +0.66 ± 0.38. The relatively low initial ε Nd values for the HMS and combined HMS-trondhjemite isochrons compared to contemporaneous depleted mantle (ε Nd(2.8) ca. + 2.1 to + 3.3) is consistent with derivation of a parental magma from light-REE-enriched components of a mafic (basaltic) precursor which was itself derived from a depleted mantle source approximately 100–200 Ma previously. The MAS suite at Gruinard Bay may be representative of such a mafic precursor. Our new geochronological data differ significantly from the ca. 3.3 Ga mineral Sm Nd and Pb Pb regression ages presented recently for two members of the HMS by Burton et al. (1994) which, if correct, require radical revision of the early geological history of the Lewisian Complex. Comparison of the relatively old mineral ages with our new whole-rock age contradicts the conventional assumption that mineral isotope systematics are less robust to later disturbance than whole-rock systematics. However, we argue that extensive retrogression, evident in all lithologies at Gruinard Bay, makes them less than ideal samples for a mineral isotopic study and it is likely that the ca. 3.3 Ga ages may be spurious, the product of disturbance of isotope systematics during the Laxfordian. On the basis of the geochronology presented here, evolution of the Lewisian closely follows generally accepted models for late-Archaean crust formation.

Geochemistry and geochronology of the Mkhondo suite, Swaziland: evidence for passive-margin deposition and granulite facies metamorphism in the Late Archean of Southern Africa

The Archean Mkhondo suite in southern Swaziland is a multiply deformed succession of metasediments intruded with amphibolite dykes and sills and granitoid gneisses. Mineral and textural relationships indicate an early period of granulite facies metamorphism, followed later by amphibolite facies metamorphism. Geothermobarometry indicates maximum temperatures of 700–900°C and burial depths of 25–3 km. Paragneisses and biotite quartzites have LREE enriched patterns with small negative Eu anomalies, whereas white quartzites show variable REE patterns and low REE concentrations. BIF has slight LREE enrichment and Eu anomalies. Amphibolites have moderate LREE enrichment and depletions in TaNb and P. Unlike many Archean granitoids, the Mkhondo granitoid gneisses are high in K and other LILE, have large negative Eu anomalies and are not depleted in HREE. SHRIMP U Pb isotopic analyses of detrital zircons from a biotite quartzite define a source age of ∼3600–3460 Ma. A deformed granitoid in tectonic contact with the Mkhondo suite yields a zircon evaporation 207 Pb 206 Pb mean age of 3192±5 Ma, which is interpreted as the age of emplacement. A zircon evaporation age of a granitic melt patch in paragneiss, as well as whole-rock and garnet SmNd isotopic ages, suggest that the peak of high-grade metamorphism in the Mkhondo suite occurred at about 2750 Ma. This is the first evidence for Late Archean high-grade metamorphism in the southeastern Kaapvaal craton. The age data of this study restrict deposition of the Mkhondo suite to between ∼3.2 and ∼2.75 Ga. Mkhondo paragneisses are interpreted as shales with biotite quartzites as iron- and quartz-rich detrital sediments. Geochemical mixing calculations indicate that the sediment sources were composed of basalt (±komatiite), TTG and Eu-depleted granitoids. The Mkhondo assemblage may have been deposited along a passive continental margin or in a continental interior basin. The presence of minor BIF with positive Eu anomalies suggests minor hydrothermal input into the sedimentary basin. Intense chemical weathering was probably most important in production of the relatively pure quartz sands.

Geochemical characteristics of granitoid rocks from within the Archean Michipicoten Greenstone Belt, Wawa Subprovince, Superior Province, Canada: implications for source regions and tectonic evolution

Three distinct tonalite-trondhjemite-granodiorite (TTG) magma series occur syngenetically with bimodal mafic-felsic cyclical volcanics in the middle to late Archean Michipicoten Greenstone Belt (MGB), Canada. Between 2.72 and 2.60 Ga, two additional series are recognised as pre-tectonic, late tectonic, and post-tectonic syenite-quartz monzonite granitoid stocks (SMG series) and an alkali feldspar syenite-alkali feldspar quartz syenite stock (SS series). The synvolcanic TTG series, such as the Hawk Lake Granitic Complex and the Jubilee Lake Stock, are characterised by moderate to low K 2O (<2.5 wt.%), Rb (<60 ppm), and Sr concentrations (<400 ppm), moderate TiO 2 levels (0.1–1.0 wt.%), La Sm (2–12), and Gd Yb (0.8–2.9), moderate to large negative Eu-anomalies, and small negative Ta + Nb troughs on ocean ridge granite (ORG)-normalised plots. The TTG series are compositionally similar to synvolcanic MGB felsic volcanics of cycles 1 to 3. The high La Sm , La Ta and low Zr Th are features of sialic crust; a contribution of ancient sialic crust in the genesis of the felsic volcanics and TTG-series granitoids of all three felsic volcanic cycles and synvolcanic TTG stocks is required in the MGB. The parental magma, like that of the TTG series of the Abitibi Greenstone belt (AGB) may originate in oceanic mantle. The SMG series have elevated K 2O (2.0–6.3 wt.%), Rb (40–280 ppm), Ba (10–2800 ppm), and Sr (250–1350 ppm), steep REE patterns reflected in elevated La Sm (3–12) and Gd Yb (1.8–5.0) without Eu-anomalies, a moderate to large negative Ta + Nb trough on ocean ridge granite-normalised plots, and low TiO 2 (<0.5 wt.%) and CaO (Na 2O + K 2O) . The SMG series is compositionally different to the MGB felsic volcanic rocks and to the TTG-series granitoids. The source is tentatively placed in the mantle wedge overlying a southward-dipping subducted plate of oceanic crust, and the elevated LILE signature is attributed to metasomatism of this source by fluids derived from the subducted slab of oceanic crust. The SMG-series granitoids, like those of the Abitibi, are different when compared to the sanukitoids in that they are compositionally more evolved; however, they may be related to these rocks by low-pressure fractionation, and may be derivatives of sanukitoid-like magmas. The alkali feldspar syenite-alkali feldspar quartz syenite, represented by the Herman Lake Stock of the SS series, has a primitive composition (<60 wt.% SiO 2) with low REE abundances and relatively flat LREE and HREE, but steep MREE profiles. These rocks are strongly LILE-enriched, but do not develop pronounced Ta + Nb troughs on ORG-normalised profiles. These rocks may be derived by low degrees of partial melting of a depleted mantle wedge, previously metasomatised by alkali-rich fluids released from a subducting slab. The elevated Ti Y and Zr Y of these rocks appear to suggest that small amounts of partial melting were responsible for the genesis of these rocks. The granitoid rocks record an overall progression from contributions made from the continental crust in a continental margin environment to magmas dominated by contributions from subducted crust or the overlying variably metasomatised mantle wedge during the terminal phase of arc-continent collision.

Tetrad effects and fine structures of REE abundance patterns of granitic and rhyolitic rocks: ICP-AES determinations of REE and Y in eight GSJ reference rocks.

All fourteen REE and Y have been determined for eight GSJ reference rocks including granitic and rhyolitic samples by ICP-AES with a preconcentration method. The two rhyolites (JR-1 and JR-2) and one granite (JG-2) exhibit convex tetrad effects of the M-type even in their chondrite-normalized REE patterns. Their tetrad effects, however, are modified from ideal ones by some step-like variations in their abundances from Tm to Lu. Similar step-like irregularities are seen commonly in chondrite-normalized patterns of other samples: granodiorites (JG-1, JG-1a, and JG-3), a fresh water lake sediment (JLk-1), and an alkali basalt (JB-1). We normalized the REE analyses for JR-1, JR-2, and JG-2 by those for JB-1, and then found that they clearly indicate quite regular convex tetrad effects of the M-type without apparent irregularities in heavy REE. The other samples, when similarly normalized by JB-1, also display analogous but less marked convex tetrad effects except for JG-3. Our present results are strong evidence for the tetrad effect in magmatic processes relevant to granitic and rhyolitic rocks formations. There are common chemical characteristics among JG-2, JR-1, and JR-2: (1) large negative Eu anomalies, and (2) rather high F contents of about 1, 000 ppm. The chemical characteristics and the theoretical basis of tetrad effects may suggest that the granitic and rhyolitic rocks with the convex tetrad effects of the M-type are residual silicate melts having partitioned REE with fluids in late stages of differentiation processes. The basalt-normalization appears to be so effective for extracting tetrad effects of granitic and rhyolitic rocks. It filters off the REE characteristics common in magmatic products like the step-like irregularities in heavy REE abundances when normalized by chondrite. This makes it easier to identify lanthanide tetrad effects in magmatic rocks.

Clastic metasediments of the Early Proterozoic Broken Hill Group, New South Wales, Australia: Geochemistry, provenance, and metallogenic significance

Whole-rock analyses of samples of pelite, psammite, and psammopelite from the Early Proterozoic Broken Hill Group (Willyama Supergroup) in the Broken Hill Block, New South Wales, Australia, reveal distinctive geochemical signatures. Major-element data show high Al2O3 and K2O, low MgO and Na2O, and relatively high Fe2O3TMgO ratios, compared to average Early Proterozoic clastic metasediments. High field strength elements (HFSE) are especially abundant, including Nb (most 15–27 ppm), Ta (most 1.0–2.2 ppm), Th (17–36 ppm), Hf (4–15 ppm), and Zr (most 170–400 ppm); Y (33–74 ppm) is also high. Concentrations of ferromagnesian elements are generally low (Sc = < 20 ppm, Ni = ≤ 62 ppm, Co = <26 ppm; Cr = most < 100 ppm). Data for rare earth elements (REEs) show high abundances of light REEs (LaCN = 116–250 × chondrite; LaCN = 437 in one sample), high LaCNYbCN ratios (5.6–13.9), and large negative Eu anomalies (EuEu∗ = 0.32–0.57).The geochemical data indicate derivation of the metasedimentary rocks of the Broken Hill Group by the erosion mainly of felsic igneous (or meta-igneous) rocks. High concentrations of HFSE, Y, and REEs in the metasediments suggest a provenance dominanted by anorogenic granites and(or) rhyolites, including those with A-type chemistry. Likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz + feldspar ± biotite ± garnet gneisses (e.g., Potosi-type gneiss) that occur within the lower part of the Willyama Supergroup, or chemically similar basement rocks in the region; alternative sources may have included Early Proterozoic anorogenic granites and(or) rhyolites in the Mount Isa and(or) Pine Creek Blocks of northern Australia, or in the Gawler craton of South Australia.Metallogenic considerations suggest that the metasediments of the Broken Hill Block formed enriched source rocks during the generation of pegmatite-hosted deposits and concentrations of La, Ce, Nb, Ta, Th, and Sn in the region. Li, Be, B, W, and U in pegmatite minerals of the district may have been acquired during granulite-facies metamorphism of the local metasediments.

Geochronology and Petrogenesis of the Archean Silicic Volcanoplutonic Series of the Verkhovtsevo Greenstone Structure, Ukraine

Geological and isotope-geochemical studies of acid volcanics in the Verkhovtsevo greenstone belt and surrounding tonalite-trondhjemite plutons within the central Dnieper gneiss-green- stone terrain were conducted in the search for genetic relationships and increased understanding of the petrogenesis of acid melts. The acid volcanic and plutonic rocks are similar in mineral composition and form a unified calc-alkaline-like trend from dacite/tonalite to rhyolite/ trondhjemite. Dacites and tonalites have the same rare-earth element (REE) patterns with moderately fractionated light and heavy REE as well as small negative Eu anomalies. Rhyolite and trondhjemites have less-fractionated REE patterns with larger negative Eu anomalies. Whole-rock data for the acid volcanic and plutonic rocks yielded a single isochron of 3117 ± 204 Ma, eNd = +1.14 ± 0.80. The data suggest a temporal and genetic relationship between the acid volcanics of the greenstone sequences and the surrounding plutonic rocks; both appear to bel...

Rare Earth element composition of precipitation, precipitation particles, and aerosols

We report the rare earth element (REE) composition of aerosols and precipitation (dissolved and particle phases) from Bermuda, Lewes (Delaware), and Woods Hole (Massachusetts). There is large‐scale fractionation of REEs between the continents, atmosphere, and the oceans. The REE compositions of atmospheric samples are highly fractionated relative to (1) each other (e.g., aerosol/precipitation pairs), (2) the upper crust of the Earth, and (3) seawater. Precipitation samples exhibit markedly convex‐up shale‐normalized patterns, large negative Eu anomalies and large and systematic depletions in the heavy REE (Lu &gt; Yb &gt; Er &gt; Dy). Aerosols are strongly depleted in the heavy REE which dissolutions experiments show are attributable to the presence of HREE‐enriched refractory minerals. We argue that in situ selective dissolution of mineral aerosol particles is responsible for the unusual REE composition of precipitation. This has major implications for the interpretation of the biogeochemical cycles of REEs in the oceans, particularly the partitioning between solution and particle phases, residence times, and the composition of seawater.

Rare earth element selenochemistry of immiscible liquids and zircon at Apollo 14: An ion probe study of evolved rocks on the Moon

Secondary Ion Mass Spectrometry (SIMS) analyses of trace elements in two zircons from 14001,7003 are similar in most respects to one reported recently ( Hinton and Meyer, 1991; Hinton and Upton, 1991). The major difference being the lack of a positive Ce anomaly in the zircon REE pattern reported here which obviates an oxidizing environment for lunar zircon. Using recently published REE partition coefficients, all three lunar zircons indicate separation from a primitive magma with a large negative Eu anomaly which is slightly LREE enriched and contains REEs 10–30 × Cl chondrites. However, this calculated liquid has a REE pattern which is unlike the host material or any lunar granite. It is postulated that these lunar zircons were precipitated in the upper crust of the Moon from late-stage, gabbroic to anorthositic magmas which have undergone extensive fractionation. The exact nature of these evolved magmas remains uncertain. Evidence for suspected immiscible liquids has been found as glasses in fragments from sample 14001,7003. Though the granitic complement to this immiscible pair is a viable immiscible liquid composition, the mafic complement has likely undergone some post-immiscibility modification. However, the original liquid-liquid element partitioning has not been grossly affected by the post-immiscibility modification. Analyses of the glasses by SIMS yields a set of partitioning data for the REE in actual rocks from the Moon. For the most part, the partitioning of the REEs between the glasses is consistent with that determined experimentally and indicates the importance of liquid immiscibility in late-stage lunar igneous processes.

Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India

The Kerala Khondalite Belt (KKB) comprises a suite of supracrustal rocks of probable Proterozoic age that was metamorphosed to the granulite facies. The KKB consists of an interlayered sequence of: (1) garnet-biotite ± orthopyroxene ± graphite gneisses; (2) khondalites (graphite-garnet-biotite-sillimanite ± cordierite gneisses); and (3) cordierite gneisses (garnet-biotite-cordierite ± orthopyroxene), as well as less abundant mafic granulites, calc-silicates and quartzites. Petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments (possibly accompanied by felsic volcanic rocks), along with minor intercalations of basalts, carbonates and quartz-rich sandstones. The REE patterns of the metasediments are typically LREE enriched with large negative Eu anomalies, indicating a continental provenance. The major- and trace-element compositions of some of the arkosic rocks of the KKB resemble those of the massif charnockites that bound the KKB to the north which suggests that the massif charnockites may have been the source of some of the KKB sediments. Modern-day analogues of the pelite-arkose lithologic association observed in the KKB occur in intracratonic rifts and rifted continental margins. This association is clearly distinct from the graywacke-mafic volcanic association found in the southern Indian greenstone belts and many other lower-grade Archean sedimentary sequences. The KKB supracrustals show both lithologic and chemical similarities to many Proterozoic supracrustal successions.

Granites of Edward VII Peninsula, Marie Byrd Land: anorogenic magmatism related to Antarctic-New Zealand rifting

ABSTRACTSyenogranites and monzogranites of Edward VII Peninsula, Marie Byrd Land, represent magmatism associated with continental rifting and the separation of New Zealand from W Antarctica in the mid-Cretaceous. These coarse-grained, leucocratic, subsolvus biotite granites occur as five small plutons cutting Lower Palaeozoic metasediments. Petrographic features include the predominance of microcline perthite over albite, bipyramidal smoky quartz, red-brown biotite and accessory ilmenite, zircon, apatite, monazite and fluorite. Enclaves are absent and miarolitic cavites are rare.The granites are a weakly peraluminous, potassic, and highly fractionated suite with high concentrations of Rb, Nb, Y, HREE and F in the most evolved compositions. REE patterns vary from LREE-enriched (CeN/YbN = 8·4), to flat REE patterns (CeN/YbN = 1·1) with large negative Eu anomalies (Eu/Eu* = 0·02). Initial 87Sr/86Sr ratios are 0·7116-0·7206 and initial εNd values are −5·5 to −7·7. Generalised fractionation trends for the suite are explicable in terms of the modal mineralogy. Monazite crystallisation exerted a predominant control on LREE concentrations.The geochemistry of the Edward VII Peninsula granites suggests an infracrustal I-type source, and regionally available Devonian-Carboniferous I-type granodiorites and tonalites satisfy the isotopic constraints. The granites classify as A-type (preferred term A-subtype) and Within-Plate Granites on standard diagrams, but the least fractionated rocks clearly indicate the I-type, Volcanic Arc Granite geochemical signatures of their inferred crustal sources.

The Northeast Kingdom batholith, Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO 40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.

Chemical and mineralogical comparison of the Pennsylvanian Fountain Formation, Colorado, U.S.A. (an uplifted continental block) to sedimentary rocks from other tectonic environments

The mineralogic and chemical composition of composites of cobble and pebble-sized rock clasts, clay-stones, siltstones, fine sandstones and size fractions of two sandstones of the Fountain Formation near Beulah, Colorado were determined. The sandstones are arkosic and most plot in the basement uplift or transitional continental fields of QFL diagrams. The mineralogic composition of the sandstones is similar to that of the composite of rock clasts at a given location. Al, Mg, Fe, Ti, Mn, Sc, Th, REE and Cs are concentrated in the clay minerals or heavy minerals and are diluted by quartz or feldspar. Consequently they are inversely correlated with Si. Cr, Hf, K, Rb, Ba and Sr are poorly correlated with Si. Cr is concentrated in clay and in heavy minerals in the silts. K, Rb, Ba and Sr are correlated with K-feldspar, mica, or clay mineral abundances. Sedimentary rocks from the Fountain formed from an uplifted continental block. The element concentrations plot in the fields of active or passive continental margins in many tectonic discrimination diagrams probably as a result of derivation from granitoid sources of similar composition. The REE patterns, Eu anomaly size, K and Rb concentrations of the fine sandstones are mostly similar to those of the corresponding composite of rock clasts at a given location, suggesting that these parameters might be the most reliable in distinguishing sources. The most reliable discrimination diagram for uplifted tectonic blocks like the Fountain may be a plot of K 2O vs. Eu anomaly size since both of these parameters were very similar in concentration in both the sedimentary rocks and presumed source of the Fountain. Most sedimentary rocks in this study have larger negative Eu anomalies than those from the NASC (North American Shale composite) or MCPS (midcontinent platform sediment). This suggests a more evolved source for the Fountain than that for average platform sediment.

An Ordovician basalt to peralkaline rhyolite fractionation series from Avoca, Ireland

The Upper Ordovician Avoca volcanic rocks host a Kuroko-type massive sulphide deposit in southeast Co. Wicklow, Ireland. Immobile element geochemistry of lavas shows a bimodal subdivision into acid and basic groups. Basalts have characteristics of magmas from both subduction zone and enriched mantle sources, and are interpreted as products of the rifting of a volcanic arc. Rhyolites are highly enriched in HFS-elements and were originally peralkaline. Constant incompatible element ratios throughout the compositional spectrum indicate a fractional crystallization relationship between the basic and acid end members. The basaltic magmas evolved by crystallization of an assemblage in which plagioclase feldspar was an important component, producing large negative Eu anomalies. The transition to peralkalinity is marked by LREE depletion and no further increase in the Eu anomaly, which may represent the interval of minor phase precipitation and fO 2 variation seen in other peralkaline series. A group of acid pyroclastic Avoca volcanic rocks are subalkaline and are probably unrelated to the main evolutionary series. The Avoca volcanic rocks were erupted in an extensional tectonic setting within a continental margin volcanic arc, similar to the contemporaneous Welsh marginal basin.

Behaviour of rare-earth elements in highly evolved granitic systems: Evidence from Proterozoic molybdenite mineralized aplites and associated granites in northern Sweden

Field relations, mineralogy, major- and trace-element contents (including REE analyses of whole-rock samples and minerals) of three Proterozoic granites and their associated molybdenite mineralized aplites have been studied at Allebuoda, Munka and Kåtaberget in northern Sweden. The granites crystallized from melts that were not saturated with water. The mineralized potassic aplites formed by quenching of residual melts caused by sudden pressure drop, H 2O saturation and vapour escape during tectonic rupturing. Leucogranites with higher Na 2O K 2O ratios from Allebuoda and Munka crystallized during H 2O-saturated equilibrium conditions in which the exsolved vapour could continuously migrate away. The pressure was probably ∼ 3 kbar at Munka, and somewhat lower at Allebuoda. The granites have REE patterns characterized by LREE enrichments and negative Eu anomalies. In comparison, the potassic aplites and the more sodic leucogranites are depleted in LREE, enriched in HREE and have larger negative Eu anomalies. Allanite and monazite are the most important REE carriers in the granites. These minerals are strongly enriched in LREE, whereas fluorite and xenotime, which are more abundant in the aplites, are most enriched in HREE. Due to the strong control of accessory minerals on the REE balance, REE are of limited use in petrogenetic modelling of highly evolved granitic systems.

Rare earth element geochemistry of W-Mo-(Au) skarns and granites from Western Bergslagen, central Sweden

Proterozoic, anorogenic, high silica granites from Western Bergslagen, Sweden are characterized by enhanced W, Mo, Au and F contents, whereas associated W-Mo(Au) skarns have high quartz and fluorite contents. Correlations between major and trace elements in the skarns are weak or absent. The granites responsible for mineralization have flat REE patterns with large negative Eu anomalies. Skarn types vary from pneumatolytic, closest to the granites, through high temperature calc-silicate skarns to low temperature biotite schists, generally furthest from the mineralizing source. REE patterns in pneumatolytic skarns are similar to those found in the granite. A heavy REE enrichment is seen in high temperature skarns followed by an increase in light over heavy REE in the biotite schists, which also have the highest REE contents. The regular variation in REE from granites through the different skarn types suggests that the REE pattern may be used to indicate the position of samples in the mineralizing system.

Rare-earth element mobility around the volcanogenic polymentallic massive sulfide deposit at Que River, Tasmania, Australia

The Que River volcanogenic base- and precious-metal deposit is located in the Cambrian Mt. Read Volcanics of western Tasmania. The host andesites and dacites adjacent to mineralization have been hydrothermally altered during mineralization and the entire sequence has undergone Devonian regional prehnite-pumpellyite facies metamorphism. Rocks petrographically recognizable as andesites exhibit chondrite-normalized REE patterns similar to modern orogenic high-K andesitic rocks with ( Ce Yb ) n ratios ranging from 6.4 to 8.6. Dacites are more fractionated with ( Ce Yb) n ranging from 10.6 to 16.0. Like the andesites, the dacites are characterized by negative Eu anomalies ( Eu Eu ∗ = 0.73–0.85 ). Except for a more extreme Eu anomaly in one sample ( Eu Eu ∗ = 0.57 ), polymict volcaniclastic rocks have patterns very similar to those of the andesites. The chondrite-normalized REE patterns for the hydrothermally altered volcanics fall into three groups that can also be distinguished petrographically. A single chloriterich sample has a substantial Eu anomaly and shows less relative depletion in the middle REE than do either the andesites or the dacites. Highly silicified samples are all characterized by large negative Eu anomalies ( Eu Eu ∗ = 0.37–0.46 ) and many are relatively unfractionated with ( Ce Yb ) n ratios ranging from 3.5 to 7.7. A third group of samples, characterized by the development of phengitic mica and carbonate show REE patterns similar to the andesites, although their Eu anomalies are generally smaller. A sample of massive Zn/1bPb ore has very low REE abundances showing light REE enrichment and a small negative Eu anomaly ( Eu Eu ∗ = 0.82 ). Massive pyrite has a similar pattern except for higher overall abundances and a small positive Eu anomaly ( Eu Eu ∗ = 1.10 ). Except for the Eu anomalies, the andesites and dacites show no evidence for REE mobility. Eu anomalies in the andesites are probably due to alteration of phenocrystic plagioclase and subsequent removal of divalent Eu. Substantial REE mobility has occurred in the hydrothermally altered volcanic rocks, the degree of mobility correlating with alteration style and intensity. Eu has been leached from both former plagioclase phenocrysts and the groundmass. Some of the Eu leached from zones of the most intense alteration may have been redeposited in lower-temperature, more distal alteration zones.