Total REE Abundances Research Articles

A systematic evaluation of titanite reference materials for optimizing trace element and U[sbnd]Pb analysis by LA-ICP-MS

Titanite (CaTiSiO5) is a commonly occurring and versatile accessory mineral with broad applications in petrochronology. In situ U-Pb and trace element analyses via SIMS and LA-ICPMS are routinely performed using a matrix-matched reference material for U-Pb and standard glasses (non-matrix matched reference material) for elemental abundance determination. We report U-Pb isotopic ratios and major and trace element concentrations for three titanite samples (Ecstall, McClure and FCT) which are commonly used as reference materials in petrochronology studies. In addition, we characterize two new samples which can potentially serve as matrix-matched reference materials for titanite trace element geochemistry (BLR-2 and BRA-1). Based on electron microprobe analysis, samples BLR-1 and BLR-2 are homogeneous and suitable for use as a primary reference material for trace element concentrations. Whereas Ecstall, McClure, and FCT titanite reference materials show high intra-grain heterogeneity, yielding relative standard deviations for most trace elements between 5% and 40%, with higher standard deviations for U of 70% for Ecstall (n = 26), 265% for McClure (n = 22), and 202% for FCT (n = 26). Therefore, we suggest that these grains are unsuited to serve as reference materials for trace element quantification. The BRA-1 titanite has low trace element concentrations and is chemically heterogeneous (total REE abundances of 40 ppm for the rim and 95 ppm for the core of the grain), thus is not suitable for standardization of chemical composition using LA-ICPMS. It is commonly asserted that a matrix-matched standardization provides a more robust downhole fractionation correction compared to a non-matrix matched standardization. However, it remains unclear which standardization approach (matrix-matched vs non-matrix matched/glass) is more accurate for titanite trace element quantification. To resolve this, we tested several standardization approaches for trace element quantification, comparing matrix-matched (BLR-1) and non-matrix-matched (NIST612) standardizations with different internal elemental standards (IES; Ca, Si and Ti) and without internal standardization (semi-quantitative). To provide an independent constraint on the accuracy of the various trace element standardization techniques we compared results to trace element concentrations obtained via solution Q-ICPMS on crushed BLR-2 and BRA-1 aliquots. The matrix-matched standardization using Si as the IES yields the best reproducibility of trace element concentrations followed by the matrix-matched reduction using Ti as the IES. Moreover, the matrix-matched semi-quantitative correction yielded the lowest weighted percentage of difference compared to reference trace composition quantified by solution ICPMS. Finally, in this contribution we also benchmark sampling-size for precise U-Pb dating of common-Pb rich phases like titanite.

Association of A‐ and I‐type granitoids in the central Aravalli orogen, Rajasthan: Implications for the Neoproterozoic tectonic evolution of north‐west India

We present new mineralogical and geochemical data for the granitoid plutons of the Neoproterozoic Sendra Granitoid Suite, central Aravalli orogen, north‐west India to constrain their petrogenesis and tectonic setting. The granitoids can be classified into two groups: (a) A‐type Chang, Chitar, Borwar, and Seliberi plutons and (b) I‐type Jaitpura pluton. The available geochronological data (zircon U–Pb ages) suggest that all the granitoids are coeval at 990–970 Ma. The granitoids are mostly granite sensu stricto, with subordinate granodiorite and rare tonalite. The rocks are metaluminous to mildly peraluminous and have high total REE abundances. Nevertheless, there are also certain geochemical differences between A‐type and I‐type granitoids. The I‐type granitoids have relatively high Al2O3 and CaO concentrations and show significant fractionation with increasing SiO2. The least fractionated sample contains low abundances of the HFSE and has low Fe‐number and Ga/Al ratios. In contrast, the A‐type granitoids have lower CaO and higher HFSE concentrations, Fe‐number, and Ga/Al ratios. Both A‐type and I‐type granitoids show negative Ba, Sr, Nb, P, Ti, and Eu anomalies, and are characterized by high zircon‐saturation temperatures (801–977°C). The present data suggest that rocks of the Sendra Granitoid Suite were generated predominantly by partial melting of mafic sources at high temperatures and emplaced in a post‐collisional extensional regime. Moreover, our study provides evidence of the preserved post‐collisional extensional magmatism subsequent to the subduction during the assembly of the Rodinia supercontinent, which is otherwise relatively less prevalent over intra‐plate or within‐plate magmatism during this time period.

Rare Earth Elements Enrichment in the Upper Eocene Tošići-Dujići Bauxite Deposit, Croatia, and Relation to REE Mineralogy, Parent Material and Weathering Pattern

Tošići-Dujići bauxite deposit, situated in Dalmatian inlands, Croatia, contains minor remaining bauxite reserves. The deposit lies on Lower Eocene foraminiferal limestone and is covered by Upper Eocene Promina sediments. Bauxite samples were analyzed for textural, mineralogical, and geochemical features in order to determine absolute REE abundances and their relation to mineralogy, as well as to devise the origin of REE enrichment and to trace weathering and bauxitization paths of the parent material. The samples show total REE abundances up to 3500 mg/kg with significant HREE enrichment in some cases. All samples are gibbsitic with hematite and anatase as major phases. Kaolinite occurs in most of the samples, and goethite, böhmite, and nordstrandite are minor phases. Monazite-(Ce) and xenotime-(Y) were identified as detrital REE minerals as well as authigenic florencite-(Ce). In the REE most abundant sample, REE are most likely bound to Fe- and Ti-oxide phases as suggested by correlation analysis. Chemical weathering proxies show intensive weathering. Geochemical and textural data imply that the REE enrichment is influenced by intensive weathering (CIA 97.87–99.26) of detrital material, and also by possible deposition/redeposition of residual material potentially derived and mobilized from various sedimentary rocks of the area.

Geochemical characteristics, depositional environment, and provenance attitude of the Middle Jurassic Yangye Formation lacustrine mudstones in Kashi Sag, south‐western Tarim Basin

Middle Jurassic mudstones from the Yangye Formation are widely exposed in the Kashi Sag and regarded as significant potential source rocks. Geochemical characteristics of mudstones in the Keziletao Depression have been analysed to decipher their source‐area weathering, provenance, tectonic setting, and depositional environment that have received significantly little attention. Mudstones are characterized by high Ca, U, Th, and REE contents when compared to those of PAAS. Their total REE abundances range from 155.2 to 234.2 ppm, with the enrichment of LREEs and distinct negative Eu anomalies, implying a principal felsic provenance. Various indicators (e.g., CIA, Th/Sc, ICV, and Rb/Sr) and A‐CN‐K ternary diagram suggest parental rocks experienced a moderate to intense chemical weathering degree and simple recycling and sorting. Furthermore, paleoenvironmental indexes (e.g., B, “S,” Sr/Ba, “C,” Sr/Cu, and V/(V + Ni)) indicate that they were likely deposited in a suboxic to anoxic freshwater setting, with semi‐arid to semi‐humid paleoclimatic conditions during the deposition of Yangye Formation. The geochemical data also demonstrate that the provenances were derived predominantly from the unroofing of felsic rocks with a minor intermediate supply. These parental rocks were commonly formed in the continental island arc, active continental margin, and/or collision tectonic settings. The results of this study, combined with published data, suggest the West Kunlun Orogen and thrusting zone to the west and south‐west of the basin were probably arrested to provide substantial detritus for sediments, stimulating a better understanding for the formation mechanism of the Yangye Formation sediments (especially for potential source rocks) in the Kashi Sag.

The Origin and Tectonic Setting of Precambrian Greywacke of Ribandar‐Chimbel, Goa, India: Petrological and Geochemical Evidence

AbstractThe Precambrian greywacke of Ribandar‐Chimbel belonging to the Sanvordem Formation of the Goa Group, India, has been studied for petrography and analyzed for major trace elements. The greywacke is characterized by angular to sub‐round grains of quartz, feldspar, biotite, chlorite and clay minerals. The abundance of clay in the matrix seems to have influenced the Al2O3 content and the K2O/Al2O3 ratio. The variation diagrams indicate a decreasing trend of TiO2, Al2O3, Fe2O3 and MgO; whereas Na2O and CaO exhibit a scatter which could be a result of the variable presence of feldspar within the sediments. The immobile elements, vanadium (25 to 144 ppm), nickel (up to 107 ppm) and chromium (up to 184 ppm), reflect abundance of clay minerals. The greywacke shows strongly fractionated REE patterns with LaN/YbN = 8 to 26 and with higher total REE abundances (up to 245 ppm). The low REE enrichment and depletion in heavier REE with prominent negative Eu anomaly (Eu/Eu*= 0.54 to 0.79) suggest a derivation of the greywacke from an old upper continental crust composed chiefly of felsic components. Petrological evidence and geochemical data suggest that the deposition of the greywacke largely took place in a deep to shallow basin that progressively changed from that of a continental island arc to an active continental setting.

Zircon U–Pb dating, geochemistry and Sr–Nd–Pb–Hf isotopes of the Wajilitag alkali mafic dikes, and associated diorite and syenitic rocks: Implications for magmatic evolution of the Tarim large igneous province

Abstract The Early Permian Tarim large igneous province (Tarim LIP) consists mainly of basaltic lavas, mafic–ultramafic intrusions including dikes and, syenite bodies in the Tarim Basin, NW China. A major unit of the Tarim LIP, the Wajilitag intrusive complex, consists of olivine pyroxenite, clinopyroxenite and gabbro units (from bottom to top), diorite and syenite rocks occurred in the upper part of the complex and alkali mafic dikes intrude the clinopyroxenite phase. Here we report the zircon U–Pb age and Hf isotopes, geochemical characteristics and Sr–Nd–Pb isotopic data of the alkali mafic dikes, and diorite, aegirine–nepheline syenite and syenite porphyry units in the Wajilitag intrusive complex. Zircons from the diorite and alkali mafic rocks yield concordant crystallization ages of 275.2 ± 1.2 Ma and 281.4 ± 1.7 Ma, respectively. The diorite and syenitic rocks in Wajilitag area have a narrow range of SiO 2 contents (51.9–57.3 wt.%), and are enriched in total alkalis (Na 2 O + K 2 O = 8.3–14.3 wt.%), among which the aegirine–nepheline syenite and syenite porphyry have the geochemical affinity of A-type granites. The alkali mafic rocks and syenitic rocks have high Al 2 O 3 (19.4–21.1 wt.%), Zr, Hf, Ba contents, total rare earth element abundances and LREE/HREE ratios and low Mg# value, K, P and Ti contents. Diorites have lower Al 2 O 3 contents, total REE abundances and LREE/HREE ratios and higher Mg# values than the alkali mafic rocks and syenitic rocks. The diorites and syenitic rocks have low initial 87 Sr/ 86 Sr ratios (0.7034–0.7046), and high eNd( t ) values (0.1–4.1) and zircon eHf( t ) values (− 0.9–4.4). All the diorites and syenitic rocks show the 206 Pb/ 204 Pb ratios ranging of 18.0–19.5, 207 Pb/ 204 Pb of 15.4–15.6 and 208 Pb/ 204 Pb of 38.0–39.9. Sr–Nd isotopic ratios indicate a FOZO-like mantle source for the diorite and syenitic rocks, similar to that of the mafic–ultramafic rocks in the Wajilitag complex. In contrast, zircon Hf isotopes of basalt and syenite elsewhere in the Tarim LIP indicate a FOZO-like component may also contribute to Tarim LIP magmatism. Geochemical and Sr–Nd–Pb–Hf isotopic features reflect that diorites and syenitic rocks are probably derived from a FOZO-like mantle source, consistent with a plume mantle origin and then underwent crystal fractionation process.

Formation of a hydrothermal kaolinite deposit from rhyolitic tuff in Jiangxi, China

The Longmen kaolinite deposit is one of the largest hydrothermal clay deposits of Ganxi volcanic basin (northern Wuyi Mountain area, China). The pristine host rocks are rhyolitic crystal-vitric tuff and minor lapilli tuff from the Late Jurassic Ehuling Formation. The ore consists of kaolin-group minerals (kaolinite, dickite), pyrophyllite with minor quartz, sericite, pyrite, etc.. From the host rocks to the transition zones (altered rocks) then to the vein ores, contents of SiO2 and TFe2O3 decrease, whereas Al2O3 and LOI increase, consistent with the contents increase of kaolin minerals and pyrophyllite in the samples. The total REE abundances of the ores are much lower than that of the host and altered rocks, Rb, Nb, Nd, Zr, Ti and Y are significantly depleted. Apparent zoning features of bulk geochemistry and mineral component reflect that the kaolinite deposit occurred at the expense of the host rock by ascending hydrothermal fluids with distinct removal of SiO2, TFe2O3, Na2O, K2O. According to the mineral assemblage, the formation temperature of this deposit falls within the range of 270–350 °C. With regard to the industrial applications, the kaolinized ores are suitable for use in ceramics and gemologic materials crafted for seal stones. Moreover, in mineralogical terms, this deposit is also proved to be an excellent example for studying channeled hydrothermal alterations of rhyolitic tuff.

Zircon U–Pb ages and Lu–Hf isotope systematics from late-tectonic granites, Geita Greenstone Belt: Implications for crustal growth of the Tanzania Craton

Granite plutons that intruded south of lake Victoria and north of the Geita Greenstone Belt have geochemical characteristics similar to high-K granites. When compared to the late high-K granites from the Musoma–Mara region, they have lower SiO2 content and higher TiO2, Al2O3, CaO, Na2O, P2O5, MgO and FeOt. They also show higher values for V, Sr, Zr, Ba and Hf concentrations. All samples display high total REE abundances (∑REE 139–393) and weak to moderate Eu depletion (Eu/Eu*: 0.25–0.77). Their chondrite-normalized pattern indicates that the light REE are moderately fractionated (3.03<Lan/Smn<6.24), whereas the heavy REE are weakly fractionated (1.63<Gdn/Ybn<2.80). The granite plutons are generally undeformed with 207Pb/206Pb ages between 2660 and 2620Ma, and intruded after the main ductile deformation that affected the Geita Greenstone Belt. The 176Hf/177Hf ratios for zircons from all samples are identical within error suggesting evolution from an isotopically uniform reservoir. The ɛHf values plot along the CHUR evolution line indicating that the granite represents juvenile crustal melts derived from greenstone material.Available zircon ages from the Tanzania Craton suggest that crustal growth occurred in three distinct periods, 2850–2800Ma, 2770–2730Ma and 2700–2620Ma, with the dominant period of crustal growth around 2700Ma. Zircon ages from the Sukumaland and Musoma–Mara Greenstone Belts indicate that the greenstone sequences formed during the same three periods of crustal growth.

Modification of the lithospheric mantle by melt derived from recycled continental crust evidenced by wehrlite xenoliths in Early Cretaceous high-Mg diorites from western Shandong, China

This paper reports petrographic, mineral chemical, olivine oxygen isotopic, and whole-rock geochemical data for wehrlite xenoliths from the Early Cretaceous Tietonggou high-Mg diorites in western Shandong Province, in the eastern part of the North China Craton (NCC), and describes the origin of these wehrlites and the processes that affected the deep lithospheric mantle in this area. Wehrlite xenoliths are rounded and vary in size between 3 cm × 4 cm × 5 cm and 3 cm × 2 cm × 1 cm. Olivine within these xenoliths occurs as an isolated residual phase within clinopyroxene, has Fo contents between 89 and 91, and contains between 1414 and 3629 ppm Ni, similar to the values of olivine from peridotite xenoliths in the Cenozoic basalts of eastern China, but lower than the values of olivine from harzburgite xenoliths in the Early Cretaceous high-Mg diorites in western Shandong. In situ oxygen isotope analysis yielded δ 18O values of olivine from (6.03±0.33)‰ to (6.82±0.35)‰, averaging (6.5±0.4)‰; this is higher than typical mantle-derived olivine ((5.2±0.3)‰). Compared with clinopyroxenes from peridotite xenoliths in the Late Cretaceous and Cenozoic basalts, clinopyroxenes in the wehrlites contain relatively low concentrations of Na2O, TiO2, and Al2O3, high concentrations of CaO, and higher Mg# (91.2–94.1) and Ti/Eu ratios (2082–2845), being similar in composition to clinopyroxenes within harzburgite xenoliths in the Early Cretaceous high-Mg diorites. Clinopyroxenes from wehrlite xenoliths are characterized by low total REE abundance, enrichment in light REEs, and depletion in high field strength elements such as Nb, Ta, Zr, and Hf. Moreover, the 87Sr/86Sr, 143Nd/144Nd, and 187Os/188Os (125 Ma) ratios of these wehrlites vary from 0.70596 to 0.70737, 0.512181 to 0.512416, and 0.12661 to 0.57650, respectively. These data suggest that these wehrlite xenoliths were formed by modification of the lithospheric mantle by melts derived from recycled continental crust.

Coupled elemental and isotopic analyses of polygenetic zircons from granitic rocks by ion microprobe, with implications for melt evolution and the sources of granitic magmas

Sequential Pb isotopic and trace element analyses were applied to the geochemical characterization of polygenetic zircons in the San Bernardino and Granite Mountains intrusive suites, two contemporaneous but compositionally distinct suites emplaced in the early evolutionary stage of the western U.S. Cordillera magmatic arc. These coupled analyses allow characterization of both melt evolution and regional diversity in the principal rock types present in the crustal sources of these two suites. Zircon grains in both intrusive suites exhibit consistent compositional zonation — Hf solid solution in zircon generally increases with cooling and crystallization, and with other indices of melt evolution, including total REE abundances and especially the MREE as indicated by increasing Yb/Gd ratios. These temperature-dependent fractionations are superimposed on persistent parental melt compositional differences in Hf/Zr and Yb/Gd ratios. Premagmatic zircons of Paleoproterozoic to Mesoproterozoic age are common across multiple samples from each suite, and their age and compositional variations image the crustal sources of these magmas. The high K San Bernardino suite had a lithologically comparatively simple 1.7 Ga metaigneous source, whereas the Granite Mountains suite had a source distinguished by the presence of hotter, 1.7 Ga metaigneous rocks hybridized by later intrusion of 1.4 Ga rocks. The absence of evidence for Grenville-age or Paleozoic-age zircons severely limits input in these intrusive suites from either upper crustal sedimentary rocks at the level of emplacement or underthrust sediments derived from erosion of the continental margin.

Zircon geochronology and trace element characteristics of eclogites and granulites from the Orlica-Śnieżnik complex, Bohemian Massif

AbstractU–Pb zircon geochronology and trace element analysis was applied to eclogites and (ultra)high-pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the Orlica-Śnieżnik complex, Bohemian Massif. Under favourable circumstances such data may help to unravel protolith ages and yet-undetermined aspects of the metamorphic evolution, for example, the time span over which eclogite-facies conditions were attained. By means of ion-probe and laser ablation techniques, a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences. The 206Pb/238U dates for zircons of all samples show a large variability, and no single age can be calculated. The protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra. The spread in apparent ages is interpreted to be mainly caused by variable and possibly multi-stage Pb-loss. Further complexities are added by metamorphic zircon growth and re-equilibration processes, the unknown relevance of inherited components and possible mixing of different aged domains during analysis. A reliable interpretation of igneous crystallization ages is not yet possible. Previous studies and the new data document the importance of a Carboniferous metamorphic event at c. 340 Ma. The geological significance of this age group is controversial. Such ages have previously either been related to peak (U)HP conditions, the waning stages of eclogite-facies metamorphism or the amphibolite-facies overprint. This study provides new arguments for this discussion because, in both rock types, metamorphic zircon is characterized by very low total REE abundances, flat HREE patterns and the absence of an Eu anomaly. These features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the Carboniferous ages document late-stage eclogite-facies metamorphism, and not amphibolite-facies overprinting.

Petrography and geochemical behaviour of trace element, REE and precious metal signatures of sulphidic banded iron formations from the Chikkasiddavanahalli area, Chitradurga schist belt, India

The ∼1.2 km long and ∼250 m wide Chikkasiddavanahalli (C.S. Halli) hill range consists of mixed sulphidic-oxide banded iron formations (BIFs) and Fe-rich phyllites (±carbonaceous), which overlie carbonated schistose and massive meta volcanics. In stratigraphic succession, the lithologies represent the Ingaldhal Formation and are an integral part of the Chitradurga schist belt in the Western Dharwar Craton. The general strike at C.S. Halli varies from N–S to 340° with vertical to steep dips towards east and west. The sulphides, oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist- and amphibolites- facies metamorphism. The BIFs show mesobands of recrystallised cherts and iron sulphides such as pyrite, arsenopyrite, and silicates such as subordinate grunerite, hornblende, chlorite, muscovite, actinolite and minor carbonates such as ankerite, calcite and magnesian siderite. Chemical data indicate depletion in Ti, Mn, Co, Cu, Cr and Ni in these iron formations. Most chondrite normalized REE patterns of the iron formation show moderate LREE and HREE enrichment coupled with strong positive Eu anomaly; the mineralized portions exhibit characteristic negative Ce and Eu anomalies (Eu/Eu∗ 0.21 to 3.00). The total REE abundance varies, correlates well with the iron contents of the BIFs, and similar to those exhibited by hydrothermal plumes [Chown, E.H., Dah, E.N., Mueller, W.G., 2000. The relation between iron formation and low temperature alteration in a Archean volcanic environment. Precambrian Research 101, 263–275]. Trace and REE data suggest that primary mantle-derived hydrothermal solutions were added to the pore fluids of sediments of the Chitradurga basin and supplied chemical constituents such as FeO, SiO 2 and REE. Oxidation of FeO to Fe 2O 3 was caused by the photosynthesis of primitive stromatolite-building cyanobacteria. Geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation, quartz-calcite vein activity and sulphidation in the mixed sulphide oxide facies BIF and associated iron phyllites in the C.S. Halli area, Western Dharwar Craton, India.

Ion microprobe zircon U–Pb age and geochemistry of the Myanmar jadeitite

Combined geochemistry and geochronology of the Myanmar jadeitite were determined. Bulk-rock trace element compositions display U-shaped REE patterns with pronounced positive Eu anomalies. The total REE abundances are very low, less than half chondritic, and the high field strength elements and some large ion lithophile elements are moderately enriched. These features indicate a metasomatic origin. There are three groups of zircons with different interior characteristics, cathodoluminescence, mineral inclusions, chemical compositions and sensitive high-resolution ion microprobe U–Pb ages. Group-I zircons, with a mean age of 163.2 ± 3.3 Ma, mostly have distinct oscillatory zoning, highest U and Th contents, and Na-free, Mg-rich mineral inclusions, and thus indicate an igneous (formation of oceanic crust) or hydrothermal (serpentinization and/or rodingitization) event in the Middle Jurassic. Group-II zircons, with a mean age of 146.5 ± 3.4 Ma, have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions, suggesting that formation of the Myanmar jadeitites, as well as subduction of the eastern Indian oceanic plate, occurred in the Late Jurassic. Group-III zircons have an age of 122.2 ± 4.8 Ma, which represents a later unknown thermal event. Discovery of the Middle Jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern Indo-Burman Range.

Provenance of the Qingshuijian Formation (Late Carboniferous), NE China: Implications for tectonic processes in the northern margin of the North China block

The Upper Carboniferous Qingshuijian Formation in the Western Hill, NE China, was deposited disconformably on a Lower Paleozoic platformal carbonate sequence. The provenance and the tectonic setting of the source area of the Qingshuijian Formation were studied using sandstone petrography and mudstone geochemistry. Sandstones consist mainly of quartz and (meta) sedimentary lithic fragments and are classified as litharenite. Feldspar is lacking in sandstones, and negative Eu anomalies similar to those displayed by Post-Archean Australian Shale (PAAS), along with depleted Ca, Na, Cs, and Rb, and low K 2O/Al 2O 3 ratios in mudstones all suggest that feldspar was scarce in the source terrane. Abundant sedimentary lithic fragments in sandstones, low values of the index of compositional variability (< 1), values of chemical index of alteration higher than PAAS, depleted transition metals, higher total REE abundances and more fractionated REE patterns than PAAS all indicate that the source area was dominated by sedimentary rocks. Geochemical and sandstone tectonic discrimination diagrams suggest that the provenance of the Qingshuijian Formation was a recycled orogen composed of quartzose to lithic sedimentary rocks that were deposited originally in a passive margin setting. The Yinshan Uplift, located in the northern margin of the North China block, that formed as a result of the collision between the North China block and the Mongolian continent, is interpreted to be the provenance for the Qingshuijian Formation. During collision the intervening sea floor was subducted northward, and an accretionary prism was elevated, forming a barrier to arc-derived sediments.

Geochronological, geochemical, and Nd–Hf isotopic constraints on the origin of Neoproterozoic plagiogranites in the Tasriwine ophiolite, Anti-Atlas orogen, Morocco

Small leucogranite bodies within the Tasriwine ophiolite, Anti-Atlas Mountains, Morocco have chemical characteristics consistent with being plagiogranites sensu stricto. Total REE abundances are low, as are K 2O (<0.5%) and Rb (<15 ppm) contents. REE patterns of the plagiogranites display an enrichment of LREE compared to HREE, and are characterized by flat to slightly concave-up patterns from Gd to Yb. Such patterns contrast sharply with those of plagiogranites in more complete ophiolite sequences, such as the Semail ophiolite, Oman, or the Troodos ophiolite, Cyprus, where patterns are much flatter and slightly LREE-depleted. Initial Nd isotopic ratios are remarkably uniform; ɛ Nd(762 Ma) values range from +6.0 to +6.3, and are consistent with direct derivation from a depleted mantle source. Hafnium isotopic ratios were determined on the same zircons as used for U–Pb geochronological analyses. Zircons from a folded plagiogranite dyke have present-day 176Hf/ 177Hf of 0.282699 ± 17, corresponding to a maximum initial ɛ Hf of +14.2 ± 0.6. Four batches of zircons from a second pluton have overlapping maximum initial ɛ Hf values ( ɛ Hf(762 Ma) = +14.0 ± 0.3 to +14.9 ± 0.6) and are indistinguishable from the value for the dyke. The zircon hafnium isotope data are consistent with the Nd isotope whole-rock values, implying no decoupling between Hf and REE during or after plagiogranite emplacement. Two of the plagiogranites were dated using high-precision U–Pb zircon techniques. Analyses of four batches of zircon from one pluton lie on a line with an upper intersection with Concordia of 761.1 + 1.9/−1.6 Ma, considered the age of emplacement of the pluton. Five small batches of zircon analyzed from a folded plagiogranite dyke lie on a chord that intersects Concordia at 762 + 1/−2 Ma, which is considered the age of emplacement of the dyke, and the most precise estimate of the age of the plagiogranite as a whole. The chemical and isotopic data for the plagiogranites are consistent with their generation by partial melting of a chemically enriched, but isotopically depleted mantle, followed by extensive fractional crystallization.

The alkaline porphyry associated Yao?an gold deposit, Yunnan, China: rare earth element and stable isotope evidence for magmatic-hydrothermal ore formation

The Yao’an vein-type gold deposit is located in the Ailaoshan-Jinshajiang alkaline intrusive belt, Yunnan Province, China, and is associated both in time and space with 33.5±1.0-Ma-old alkaline intrusions. The gold mineralization is associated with potassic wall-rock alteration. The REE distribution patterns of secondary K-feldspar are generally similar to those of the igneous perthite but with about seven times higher total REE abundances. The alteration is ascribed to a high-REE magmatic fluid derived from the Yao’an alkaline intrusive suite. The hydrothermal Yao’an gold deposit formed during two gold-bearing stages, i.e. a sulfide (pyrite) stage and a sulfide-oxide stage (pyrite-specularite). The REE abundance of early stage I pyrite is relatively high with strong enrichment in LREE, (La/Yb)n of 40–290, generally positive Eu anomalies (Eu/Eu*=0.86–1.55), and REE patterns very similar to those of secondary feldspar. In contrast, the later stage II pyrite has much lower REE concentrations and lower (La/Yb)n of 5.5–11.8, Eu/Eu* of 0.49–0.76, and flat chondrite-normalized spidergram patterns. The stage I pyrite has δ34S in the range of –2.2 to +3.2‰, and overlaps with regionally distributed pyrite in least-altered syenite porphyry. In contrast, stage II pyrite has much higher δ34S values between +7.8 and +16.5‰. Carbon isotope data for four samples from stage II revealed δ13C (PDB) values between –6 and –8‰. These stable isotope and REE data suggest that magmatic fluids of the alkaline intrusions caused both potassic alteration and stage I sulfide mineralization. The system evolved from stage I to stage II mineralization by influx of meteoric fluids with relatively heavier sulfur, although calcite carbon isotope data suggest that the CO2 remained dominantly of magmatic origin.

Geochemical Behaviors of REE and Other Trace Elements during the Formation of Strata‐bound Skarns and Related Deposits: A Case Study of the Dongguashan Cu (Au) Deposit, Anhui Province, China

Abstract REE and other trace elements in the altered marbles, massive skarns and ores, as well as garnet and quartz were determined in order to examine the behaviors of trace elements during hydrothermal alteration. It is demonstrated that the high‐field‐strength (HFS) elements Zr, Hf, Th and Nb were immobile while other trace elements were mobile during the formation of skarns and related deposits. REE and ore‐forming elements such as Cu and Ag in hydrothermally‐altered marbles and skarns were provided primarily by hydrothermal fluids. In the direction transverse of the strata, the more deeply the marbles were altered, the higher the total REE abundance and the larger the negative Eu anomalies would be. The chondrite‐normalized REE patterns of skarns are similar to those of the marbles, but the former are distinguished by much higher REE contents and more remarkable negative Eu anomalies. Those patterns were apparently not inherited from the marble protolith, but were controlled by garnets, which were determined by the hydrothermal solutions involved in the infiltration metasomatism. The hydrothermal fluids are characterized by having slightly right‐hand dipping REE patterns, being enriched in LREE and having strongly negative Eu anomalies. The uptake of REE from hydrothermal fluids during the growth of garnet crystals resulted in the garnets having similar REE patterns to the hydrothermal fluids. Based on the spatial variation of REE in skarns and the structures of the hosting strata, we can get a better understanding of the transport path and pattern of the fluids involved in the formation of skarns and their related deposits. REE geochemistry investigation can help elucidate the genesis of skarns and skarn‐related deposits.

Possible komatiitic affinity and potencial PGE mineralization of the archean Ribeirão dos Motas layered sequence, Brazil

The southern portion of the Sao Francisco Craton (SSFC), located in Minas Gerais State, underwent a long crustal development that occurred from Mesoarchean to Neoproterozoic times (Carneiro et al., 1998a; 1998b; Teixeira et al., 1996). The Archean rocks of the SSFC, which formed between 3.5 – 2.5 Ga are essentially gneisses and granitoids (TTG suite), a greenstone belt sequence (Rio das Velhas Greenstone Belt - RVGB), an intrusive ultramafic-mafic layered complex (Ribeirao dos Motas Layered Sequence - SARM), and mafic dikes swarms. Locally, the Archean rocks are covered by Proterozoic intracratonic sedimentary basin sequences containing quartzite-carbonate-shale with minor contribution of the bimodal volcanics-quartzite-arkose and volcanics-graywacke and intruded by swarms of mafic dikes and/or granitoid bodies. The SARM comprises alternate layers of main peridotitic and pyroxenitic rocks with minor gabronorite and amphibolite rocks, outcropping as several disrupted bodies (Carneiro et al., 1997). Olivine, spinel, amphibole and pyroxene (orto- and clino-) constitute the peridotite mineralogy. The pyroxenite is constituted by pyroxenes (orto- and clino-), amphibole and spinel. Besides the typical petrographic features and texture commonly found in layered complexes such as: igneous layering, euhedral crystals and cumulus textures, the studied rocks present some sui generis petrographic aspects such as chadacristal of euhedral amphibole included in euhedral oikocristal of pyroxene. Major and minor elements have been performed on 15 whole rock samples from different SARM outcrops. The SARM ultramafic rocks have MgO contents ranging from 23.8 to 34.9 wt% (MgO = 30.08 average wt%); TiO2 = 0.182 average wt%; CaO/Al2O3 ratio of 0.72 (average); Al2O3/TiO2 ratio of 33 (average). Analyses of REE and PGE have been performed on 6 samples. All samples are low in total REE abundance; Pt and Pd average are 28 and 10.6 ppb respectively. These chemical data suggest that SARM magmas were derived from komatiite magmas similar to siliceous high magnesian basalts (SHMB). Furthermore, the higher Pt and Pd (average ppb) suggest that magmas feeding the SARM could be carrying conspicuous amounts of PGE. Regionally, the geochemistry studies reveal great similarity between the SARM and the komatiite rocks of the RVGB (Padilha et al., 1984; Sichel et al., 1983; Schorscher et al., 1992). However, the SARM ultramafic rocks have higher average contents of MgO, Cr, Ni and lower average contents of SiO2, TiO2, K2O and REE. These geochemical features suggest that the RVGB komatiite may have been generated from more evolved SARM magmas. However, it is also possible that the mantle source region for both ultramafic magmas had been depleted during a melting episode that caused extraction of the RVGB komatiite magma. Thus, in spite of the lack of unequivocal geological relationships between SARM and the komatiite rocks of the RVGB, the two sequences referred to above may have had the same magmatic source and, as a consequence, been tectonically related. Assuming that this is true, the SARM rocks are a fraction of the komatiite magma that was encapsulated in the lower crust forming a layered ultramafic-mafic complex. Another portion of the komatiite magma would have reached the surface forming ultramafic volcanic supracrustal sequences of the SGRV. Finally, the SARM geochemical data when compared with those of the Archean Australian layered complexes (Sun et al., 1991; Hoatson et al., 1992) exhibit higher average Pt, Pd, Al2O3/TiO2 ratio, Cr and Ni, while Ti/Sc ratio, Ti, SiO2 and K2O average (wt%) are lower. The higher Pt and Pd values in comparison with those values of the Archean Australian layered complexes, which were presented by Sun et al. (1991) and Hoatson et al. (1992), are suggestive of a potential PGE mineralization in the SARM.

ORIGIN OF POST-VARISCAN GABBRO-DERIVED GRANULITES (NORTHERN APENNINES, ITALY): CRUSTAL CONTAMINATION PROCESSES AND IMPLICATIONS FOR MANTLE SOURCES

The Late Cretaceous sedimentary melanges from the External Liguride Units of Northern Apennines include large slide-blocks of subcontinental mantle peridotites, MORbasalts and lower and upper continental crust rocks. The slide-block association has been interpreted as representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of mafic and felsic granulites, which locally preserve primary contacts. The mafic granulites commonly display a metamorphic layering, but undeformed rocks preserving a gabbroic fabric are locally found. Undeformed mafic granulites are mostly represented by spinel-bearing gabbronorites, usually containing significant amounts of either olivine or Fe-Ti-oxides. Olivine- and Fe-Ti oxide-bearing rocks locally show spinel-pyroxene symplectites and garnet coronas, respectively. The felsic granulites are mainly quartzo- feldspathic rocks consisting of mesoperthitic to perthitic feldspar, quartz and garnet. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallisation under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900°C). They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times. The gabbro-derived granulites can be recognized as cumulus rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/Al2O3 ratios and overall low contents of incompatible trace elements. The Mg# value ranges from 80 to 52, and point to negative correlations with TiO2 and MnO, thus indicating a tholeiitic differentiation trend. Most gabbro-derived granulites have slightly LREEenriched patterns showing decreasing Eu positive anomaly with increasing total REE abundances. Chondrite normalization of incompatible trace elements reveals spikes at Ba and Sr, and a slight Zr depletion. The quartzo-feldspathic granulites have LREE enriched patterns, with nearly flat HREE and no or slightly positive Eu anomaly; Ba is abruptly enriched relative to REE, whereas Nb and Ti are depleted. The gabbro-derived granulites show a wide range in Sr and Nd isotopic compositions. The Sr isotopic ratio recalculated at 290 Ma varies between 0.7031 and 0.7077, and the initial eNd ranges between +6.8 and -4.5. Two samples of quartzo-feldspathic granulite yield age-corrected Sr isotopic ratios of 0.7107 and 0.7109, and eNd of -8.0 and -5.7. As a whole, the Nd and Sr isotopic data at 290 Ma form a hyperbolic array, in which the olivine-bearing gabbronorites have the highest eNd values and the lowest Sr isotopic ratios. Clinopyroxenes have been analyzed for trace elements by ion microprobe. Clinopyroxene from olivine-bearing gabbronorites shows peculiar compositions that indicate a metamorphic origin through olivine-plagioclase reaction, i.e. the igneous protoliths of the olivine-bearing gabbronorites were most likely troctolite-type cumulates. Clinopyroxenes from Fe-Ti oxide bearing gabbronorites show igneous geochemical trends, thus suggesting that these rocks contained clinopyroxene as original igneous phase. Petrography, bulk-rock and mineral composition indicate that the gabbro-derived granulites can be related to a fractional crystallization process, with early separation of olivine and plagioclase, followed by the replacement of olivine by pyroxene at the liquidus. Trace element modelization of the parental liquid compositions applied to the olivine-bearing rocks yields LREE- and LILE-enriched liquids, with absence of negative Nb anomaly, similar to plume-type MOR-basalts and continental tholeiites. However, a P-MORB origin seems to contrast with the initial Nd and Sr isotopic compositions, which are close to depleted mantle values at the time of emplacement. AFC modelization was successfully applied to obtain the isotopic compositions of the most contaminated samples, starting from the trace element and isotopic compositions of the parental liquids of the olivine gabbronorites and assuming a crustal contaminant with low Sr/Nd and isotopic composition comparable to that of the quartzo-feldspathic granulites. AFC calculations also indicate that the parental liquids of the olivine-bearing gabbronorites cannot be ascribed to N-MORB primary liquids. The primary mantle magma was necessarily characterized by moderate LILE enrichment, although an increase in LILE concentrations could have been enhanced by a small crustal contribution. The LILE enrichment in the parental liquids of the gabbro-derived granulites may be explained with a low degree partial melting of a rather fertile lithospheric mantle source. Alternatively, the primary liquids of the gabbro-derived granulites were related to a mantle source enriched in LILE as a result of the Variscan subduction event.

Manganese microchimneys buried in the Central Pacific pelagic sediments: evidence of intraplate water circulation?

Manganese oxide deposits of an irregular tube-like shape with unique composition and structure were found within Pliocene siliceous sediments from an abyssal hill in the Central Pacific manganese nodule province. Effective separation of Mn from Fe and probably a rapid rate of Mn deposition are suggested from the chemical composition: a very high Mn/Fe ratio (up to 400) and low concentrations of transition metals. The REE pattern is similar to other marine low-temperature hydrothermal and sub-oxic diagenetic Mn deposits, low total REE abundance and strong negative Ce anomaly. The deposit consists of major todorokite with minor buserites, showing a hydrothermal signature. Blade-like crystals of the minerals typically grow within open tubes (several mm thick) and lack detrital minerals. All data indicate a low-temperature hydrothermal or cold seep origin, or less likely a sub-oxic diagenetic origin. This unusual manganese deposit from a pelagic environment is thought to have formed on the sea floor from Mn-bearing waters that passed through sediments or basement rocks. We speculate that intraplate water circulation took place during the Late Pliocene to Pleistocene due to the uplift and/or block faulting after the deposition of pelagic Pliocene sediments.