HREE Enrichment Research Articles

A COMPARISON OF THE MINERALOGICAL AND GEOCHEMICAL CHARACTER AND GEOLOGICAL SETTING OF PROTEROZOIC REE-RICH GRANITIC PEGMATITES OF THE NORTH-CENTRAL AND SOUTHWESTERN US

Pegmatites enriched in the rare-earth elements ranging in age from Mesoproterozoic through Neoproterozoic occur along a southwestern trend from Wisconsin through Colorado to Arizona. Four widely geographically spaced pegmatite districts that are temporally associated with two distinct post-orogenic tectonic events are compared and contrasted in this study. The pegmatite districts include the circa 1.5 Ga Wausau syenite complex (WSC), in Wisconsin, the 1.0 Ga South Platte (SP), in Colorado, circa 1.6 Ga Trout Creek Pass (TCP), also in Colorado, and circa 1.5 Ga Mojave (MOJ) district in northwestern Arizona. The extreme enrichment of HREE in minerals such as polycrase-(Y), euxenite-(Y), samarskite-(Y) and xenotime-(Y), the LREE enrichment in monazite-(Ce), allanite-(Ce) and bastnasite-(Ce), the REE enrichment in fluorite (where present), enrichments in high field-strength elements (HFSE) illustrated in chondrite-normalized plots and spider diagrams, as well as the intraplate signatures demonstrated in Pearce discrimination diagrams, indicate an anorogenic character for all the granite–syenite–pegmatite systems. All of the pegmatite districts are REE- and Nb-enriched, but TCP and MOJ are less strongly enriched than SP and WSC. The SP and WSC have the typical anorogenic NYF chemical signature, but TCP and MOJ are strongly depleted in F. These pegmatite-forming melts are all inferred to be crustally derived, with differences resulting from variable source-rock composition, differing degrees of partial melting, and perhaps the extent of rifting. The older TCP pegmatite district and the MOJ pegmatite district formed from melts derived by partial melting of lower crustal rocks with a significant metasedimentary component and are associated with smaller, localized extensional events subsequent to the Mazatzal orogeny, possibly related to back-arc spreading. The SP and WSC pegmatites are related to melts formed by the anatexis of lower crust during larger-scale rifting events. The greater enrichment in incompatible elements of WSC and SP pegmatites suggests that they originated from melts derived from smaller degrees of partial melting than pegmatites of TCP and MOJ, which are less strongly enriched and require larger degrees of partial melting. The F-depletion observed in TCP and MOJ pegmatites suggests a paucity of F-bearing minerals in the source region. More subtle geochemical differences between individual pegmatites appear to relate to the degree of evolution, which differs both within and across the pegmatite districts.

THE INTRAGRANITIC POTRERILLOS NYF PEGMATITES AND THEIR A-TYPE HOST GRANITES OF THE LAS CHACRAS - POTRERILLOS BATHOLITH, SIERRA DE SAN LUIS, ARGENTINA

The Potrerillos granitic pegmatites, in the Sierras Pampeanas Orientales of San Luis Province, Argentina, consist of mediumsized bodies with rounded, elliptical, or irregular outcrops varying from 10 to 60 m; vertically, they mostly look like thick lenses. Zoning is ill-defined, with a border zone that grades into a coarse-grained intermediate zone. The core zone also is poorly defined, normally containing milky to pink or gray quartz with large crystals of K-feldspar and, locally, pods of primary fluorite. The pegmatite-forming minerals are quartz, microcline and albite, with accessory biotite, muscovite, tourmaline, beryl, allanite, fluorite, fluorapatite, ilmenite, niobian rutile, rutile and pyrite. Rare primary species are monazite-(Ce), U-rich pyrochlore, scheelite, bismuthinite and molybdenite. Subordinate species are hematite, goethite, Mn oxides, clinobisvanite, bismuthite, and opal. The pegmatites are ascribed to the allanite–monazite type, REL–REE subclass, NYF family. Data on melt+fluid inclusion are consistent with crystallization of an aqueous, CO2-bearing melt at 475° < T < 600°C and ~3.0 to 3.7 kbar, that cooled down to a fluid of low to moderate salinity. The pegmatites are hosted in three dominant facies of two plutons of the Las Chacras – Potrerillos batholith, described as biotite porphyritic granite, biotite-bearing equigranular red granite, and muscovite-bearing equigranular red granite. All of them have high-K compositions, and are metaluminous to mildly peraluminous (ASI in the range 1 to 1.07); the least evolved porphyritic biotite granites of the Las Chacras pluton are depleted in SiO2 and enriched in TiO2, Fe2O3(T), CaO and MgO, poor in Nb, Rb, Th, and HREE, and enriched in P, Zr, Sr and Ba and LREE relative to the red equigranular muscovite bearing facies of the Potrerillos pluton. The asymmetric REE patterns likely reflect the control by accessory monazite-(Ce) and allanite-(Ce) in the porphyritic facies, contrasting with relative HREE enrichment in the muscovite-bearing equigranular granite. Both granite types display Eu/Eu* values <1, indicative of feldspar fractionation in the source; the HREE are strongly depleted, more accentuated in less evolved biotite porphyritic facies, likely reflecting garnet retained in the source. The crystallization-emplacement history of the suite spanned from the Upper Devonian to the Lower Carboniferous in a within-plate regime crossed by deep shear zones. The granite facies that host the pegmatites are redefined as A-type granites. The A-type magmas probably originated by partial melting of a depleted garnet-bearing lower crust, of likely granodiorite or tonalite composition in the minimum melt field of Opx + Cpx + Grt ± Qtz, triggered by mantle-derived heat but previously fertilized by alkali metasomatism produced by mantle degassing.

Episodic fluid action during exhumation of deeply subducted continental crust: Geochemical constraints from zoisite–quartz vein and host metabasite in the Dabie orogen

A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―quartz vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The vein has a mineral paragenesis of quartz+zoisite+epidote+white mica+garnet+rutile+zircon+apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for veining and physical transport of the tiny accessory mineral from the host rock to veins. There are significant differences in major and trace elements between vein-forming minerals, which are attributed to two episodes of fluid action for veining. Metamorphic zircon from the vein yields concordant U–Pb ages of 215±4 to 218±4Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683°C. Thus the veining postdates the UHP metamorphic event at 225–240Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the vein suggests very local saturation of Zr and Ti in the Zo–Qtz vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust.

The Volch’etundrovsky Massif of the autonomous anorthosite complex of the Main Range, the Kola Peninsula: Geological, petrogeochemical, and isotope-geochronological studies

The Volch’etundrovsky Massif occupies the middle part of the autonomous anorthosite complex of the Main Range, has a sheet morphology and marks the tectonic suture between the Kola block and the Belomorian mobile belt. The massif is characterized by homogenous structure and consists of the volumetrically dominant Main Zone including leucogabbro, leucogabbronorites, and anorthosites, and Marginal Zone made up of leuconorites and gabbronorites with subordinate plagioclasites and orthopyroxenites. Chemically, the rocks of the Volch’etundrovsky Massif are ascribed to the normal (tholeiitic and calc-alkaline) petrochemical series with typomorphic high Al2O3 contents (11.71–29.32 wt %). With Al2O3 increase in the leuconorite-anorthosite series, the SiO2 and TiO2 contents show weak variations, CaO and alkalis insignificantly increase, whereas the MgO and FeO contents sharply decrease. The rocks of the Volch’etundrovsky Massif reveal significant REE fractionation and increase in total REE content in the leuconorite-anorthosite series, most approximating the Paleoproterozoic (Sumian) anorthosites of the Kola region. The anorthosites and leucogabbro are characterized by flat HREE, while the leuconorites is strongly depleted in HREE due to garnet fractionation. All rocks of the massif have significant positive Eu anomalies caused by the plagioclase accumulation. Zircons are characterized by LREE depletion and enrichment in HREE. This defines the steep positive slope of the plots complicated by the negative Eu and positive Ce (in zircons from leucogabbro) anomalies, which is typical of the REE distribution patterns in the unaltered zircons from igneous rocks. In zircons from anorthosites, the Ce anomaly is weak to absent. The trace-element distribution in the rocks of the Volch’etundrovsky Massif show positive Ba, Ta, Pb, Sr, Sc, and V anomalies, being controlled by the mineral specifics of the massif and the presence of definite accessory minerals. New U-Pb zircon data on the rocks of the Volch’etundrovsky Massif indicate that the leuconorites from the Marginal Zone were formed 2473 ± 7 Ma and 2463 ± 2.4 Ma ago, and the leucogabbro from the Main Zone, 2467 ± 8 Ma. These rocks have negative ɛNd(T) from -1.54 up to -3.10, which indicates their derivation from enriched mantle reservoir variably contaminated by crustal material. The anorthosites of the Main Zone define an U-Pb age of 2407 ± 3 Ma and ɛNd(T) = −3.78, which presumably reflect the timing of hydrothermal-metasomatic alterations in the upper part of the magmatic chamber accompanied by significant crustal contamination.

Garnet clinopyroxenite layers from the mantle sequences of the Northern Apennine ophiolites (Italy): Evidence for recycling of crustal material

This study aims to define the origin of garnet clinopyroxenite layers from the mantle sequences of the External Ligurian ophiolites. These mantle sequences retain a subcontinental origin and were exposed at a Jurassic ocean–continent transition. The garnet clinopyroxenites are mafic rocks with Mg# values of 66–71. Their chondrite-normalised REE patterns are characterised by severe LREE depletion (CeN/SmN=0.1–0.2) and nearly flat (Type-A pyroxenites) to moderately enriched HREE (Type-B pyroxenites). In addition, Type-A pyroxenites display a small positive Eu anomaly. The whole-rock REE variations are paralleled by the garnet REE compositions. We attribute the major and trace element characteristics of the garnet clinopyroxenites to recycling of gabbroic protoliths that underwent partial melting under eclogite facies conditions. The garnet clinopyroxenites may represent variably evolved garnet+clinopyroxene cumulates formed by eclogite-derived melts. In an alternative hypothesis, Type-A and -B pyroxenites are residual rocks after eclogite melting and cumulates derived from the eclogite melts, respectively. The high pressure fractionation event that gave rise to the garnet clinopyroxenites is considered of Triassic age on the basis of Sm–Nd and Lu–Hf isotope correlations. The Nd–Hf isotopic compositions of the garnet clinopyroxenites in the Triassic (εNd=+4.7 to +7.6, εHf=+4.4 to +12.8) lie below the mantle array, in agreement with recycled ancient MOR-type material. The oxygen isotopic composition of garnet and clinopyroxene from the garnet clinopyroxenites (δ18O=+4.9‰ to +5.2‰) may be reconciled with subduction-related recycling of the lowermost oceanic crust, or delamination and foundering of underplated gabbros from the continental lithosphere. The potential involvement of the garnet clinopyroxenites in the melting processes that gave rise to the MOR-type oceanic crust in the Jurassic would account for the moderate Nd isotope variability and the garnet geochemical signature of the ophiolitic basalts.

Origin and evolution of the Ilmeny–Vishnevogorsky carbonatites (Urals, Russia): insights from trace-element compositions, and Rb-Sr, Sm-Nd, U-Pb, Lu-Hf isotope data

The carbonatites of the Ilmeny-Vishnevogorsky Alkaline Complex (IVAC) are specific in geological and geochemical aspects and differ by some characteristics from classic carbonatites of the zoned alkaline-ultramafic complexes. Geological, geochemical and isotopic data and comparison with relevant experimental systems show that the IVAC carbonatites are genetically related to miaskites, and seem to be formed as a result of separation of carbonatite liquid from a miaskitic magma. Appreciable role of a carbonate fluid is established at the later stages of carbonatite formation. The trace element contents in the IVAC carbonatites are similar to carbonatites of the ultramafic-alkaline complexes. The characteristic signatures of the IVAC carbonatites are a high Sr content, a slight depletion in Ba, Nb, Та, Ti, Zr, and Hf, and enrichment in HREE in comparison with carbonatites of ultramafic-alkaline complexes. This testifies a specific nature of the IVAC carbonatites related to the fractionation of a miaskitic magma and to further Late Paleozoic metamorphism. Isotope data suggest a mantle source for IVAC carbonatites and indicate that moderately depleted mantle and enriched EMI-type components participated in magma generation. The lower crust could have been involved in the generation of the IVAC magma.

Andesite Geochemistry Features of Late Triassic Tumugou Formation, in Zhongdian, Yunnan Province

The Late Triassic Tumugou Formation volcanic rocks which belongs to typical island arc volcanic rocks in southern end of Yidun island arc belt is located at the eastern of the Zhongdian ,NW Yunnan, SW China. The volcanic rocks can be divided into three categories:andesitic basalt, andesite, quartz andesite, etc. Through geochemical analysis the major elements, rare earth ele and trace element in volcanic rocks, SiO255.18-57.59×10-2,TiO21.16-1.45×10-2,Na2O+K2O5.11-8.05×10-2.consider it is calc-alkaline- alkaline Series of high-K andesite, volcanic may be controlled by the crystal fractionation of magma.Rb31.50-101×10-6,Ba1310-12300×10-6,Nb/Ta11.4-15.5,REE166.07-240.78×10-6,δEu0.74-1.00,REE distribution patterns show oblique to the HREE side and enrichment in LREE .Eu anomaly is not obvious. It is can see from the relevant figure about trace element, it is very similar in magmatic distribution patterns between volcanic rock and Volcanic-arc rock, indicating that the volcanic in this area may be formed in volcanic-arc environment. From east to west, Magma source depth have regular change with the really thickness of mainland shell. Explain that Tumugou Formation volcanic rock is subduction by Ganzi- Litang Ocean basin from east to west. Hongshan-Ousaila region of eastern edge of Zhongdian is the volcanic island arc system during the passive continental margin into an active continental margin.

Sm-Nd geochronology, REE geochemistry and C and O isotope characteristics of calcites and stibnites from the Banian antimony deposit, Guizhou Province, China

The Banian sediment-hosted antimony deposit is a medium-sized deposit located in the Dushan County, South Guizhou, China. Calcite is the dominant gangue mineral and can be divided into two types: white and pink. Pink calcite is intimately associated with mineralization. Sm‐Nd dating of five white calcites yield an isochron age of 128.2 ± 3.2 Ma, with initial eNd = ‐4.3 and MSWD = 0.54, while six pink calcites yield an identical age of 126.4 ± 2.7 Ma with initial eNd = ‐6.0 and MSWD = 0.68. These new age constraints suggest that the Banian deposit formed during the late Yanshanian. Negative initial eNd values, C (‐0.5 to ‐1.8), O (12.5 to 14.6) isotopic characteristics of calcites, and field observations of abundance of vuggy open spaces, all suggest significant interaction between fluid and wall rocks. Calcites are characterized by strongly LREE-depleted and MREE- and HREE-enriched (LREE/HREE = 0.11‐0.61). Stibnites are LREEenriched, and MREE- and HREE-depleted (LREE/HREE = 105‐500). Wall rocks are characterized by moderate LREEenriched (LREE/HREE = 6.2‐12). Pink calcites are more enriched in LREE (LREE/HREE = 0.11‐0.61) than white calcites (LREE/HREE = 0.11‐0.23), while the concentration of HREE remains nearly constant in both. The high contents of Fe and Mn in calcites and the negative correlation between Fe+Mn (ppm) and LREE/HREE ratios suggest that the Fe‐Mn impurities in calcites may dominate the enrichment of MREE and HREE in calcites from the Banian deposit.

Solos desenvolvidos sobre diferentes rochas vulcânicas da ilha de Fernando de Noronha: padrão de elementos terras raras e composição isotópica do chumbo

This study examines the influence of pedogenesis on the distribution of rare earth elements in soils derived from different rock types and formed under tropical humid climate, as well as the possible contribution of airborne Pb to these soils. We studied 5 soil profiles developed from different volcanic rocks cropping out in the Fernando de Noronha island. Results show that in the course of weathering, the soils were enriched in REE. The REE patterns of the soils are similar to those of the parent material, except for a slight HREE enrichment. Lead-isotope data indicate the presence of a nonradiogenic anthropogenic component in the upper horizons of the soil profiles.

Low-Al and high-Al trondhjemites in the Huai’an Complex, North China Craton: Geochemistry, zircon U–Pb and Hf isotopes, and implications for Neoarchean crustal growth and remelting

Voluminous tonalite–trondhjemite–granodiorite (TTG) magmas were emplaced in the Huai’an Complex of North China Craton (NCC) during the Neoarchean. Here we present the geochemical characteristics of these rocks and identify low-Al (LAl) and high-Al (HAl) types of trondhjemites within the TTG suite. The LAl group displays high silica (SiO2=77–80wt.%), and low Al2O3 contents (11.06–11.89wt.%), a strongly peraluminous (A/CNK=1.16 and 1.19) composition and enrichment in HREE (LaN/YbN=4 and 5, GdN/YbN=0.35 and 0.9). Their low Rb (13 and 28ppm), Th (0.3 and 7.0ppm), relatively high Na2O/K2O values (3.6 and 4.1) and MgO (0.63 and 0.68wt.%) with moderately high Mg# (36 and 53), suggest crystallization from a primitive source. The HAl group is characterized by relatively low SiO2 (67.22–71.57wt.%) and high Al2O3 contents (16.35–16.41wt.%) with trace element contents and distribution patterns closely comparable to those of Archean high-Al TTGs. Our geochemical data do not favor a direct genetic link between the LAl and HAl trondhjemite groups. The zircon U–Pb data reveal that the LAl and HAl groups represent coeval intrusions at ca. 2.55–2.5Ga. In situ analysis of Hf isotopes in the zircons (εHf=+5.6 to +10, and TDM=2462–2595Ma), together with the geochemical features of the low-Al trondhjemites suggest that they were generated under relatively low pressure (<8kbar), through low degree partial melting of a newly accreted amphibolitic crust. Taking into consideration the contemporaneous (2.55–2.50Ga) high-Al TTGs widely exposed in the Huai’an Complex, we suggest that the low-Al trondhjemites were formed by intraplating of mantle-derived magmas, and the coeval high-Al TTGs were produced by partial melting of a thickened lower crust, triggered by underplating. Our study attests to a major episode of Neoarchean crustal growth in the NCC, and remelting induced by underplating and intraplating of mantle-derived magmas.

Using zircon U–Pb ages and trace element chemistry to constrain the timing of metamorphic events, pegmatite dike emplacement, and shearing in the southern Parry Sound domain, Grenville Province, Canada

The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic relations from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending development of the underlying Twelve Mile Bay shear zone (TMBSZ). Intact granulites preserve ca. 1145 Ma ages, slightly younger than those farther to the north in the interior PSD, but similar to that of the underlying granulite-to-amphibolite facies Parry Sound shear zone. Weaker zircon HREE enrichment in sheared and retrogressed samples with partially resorbed garnet porphyroblasts (containing abundant zircon inclusions) suggests that the 1128–1143 Ma ages common to sheared rocks in the southern PSD record an earlier phase of metamorphism and deformation, and not the timing of shear zone development. Deformed pegmatite dikes bounded by sheared and retrogressed wall rocks from across the southern interior PSD consistently record ca. 1100 Ma ages, indicating synchronous pegmatite emplacement across the transect and constraining the bounding amphibolite-facies shear zones to ≤1100 Ma. TMBSZ samples retain evidence for (1) the older ( ca. 1145 Ma) high-grade metamorphic event, (2) pegmatite emplacement and shear deformation at ca. 1100 Ma, and (3) a later ( ca. 1070 Ma) shearing event. Combined with published structural and petrologic data, these ages indicate that pegmatite emplacement and shearing in the TMBSZ was synchronous with that in the southern interior PSD; further confirming published models for TMBSZ development. Additionally, the data suggest that the Twelve Mile Bay assemblage (within the TMBSZ) experienced high-grade metamorphism synchronously with the interior PSD, and may therefore be correlative with the basal PSD to the north.

Petrology, mineralogy, and geochemistry of the Ge-rich coal from the Wulantuga Ge ore deposit, Inner Mongolia, China: New data and genetic implications

Abstract The Early Cretaceous Wulantuga high-Ge coal deposit in Inner Mongolia is one of the major coal-hosted Ge deposits in China. This paper reports new data on the petrological, mineralogical, and geochemical compositions of 13 bench samples of the high-Ge No. 6 coal from the Wulantuga deposit, and provides new insight into the origin and modes of occurrence of the minerals and elements present. The No. 6 Coal has a low rank (Ro,max = 0.45%) and is a low-ash coal (8.77%). The total content of inertinite (52.5 vol.% on average) in most coal benches is higher than that of huminite (46.8 vol.% on average). The dominant huminite maceral is textinite (averages 43.9%), and the dominant inertinite macerals are mainly fusinite (averages 33%) and semifusinite (12.5%), along with trace portions of intertodetrinite, secretinite, funginite, and macrinite. Fungus, seen as the maceral funginite, played a role in the development of degraded maceral forms in the Wulantuga coals. Funginite is present in samples examined in this study, but is not easily extracted during palynological studies; recovered fungal taxa are saprophytes, indicating woody decomposition prior to incorporation in the mire. Palynology revealed a sparse flora that is consistent with the early Cretaceous age. Minerals in the coal include quartz, kaolinite, illite (and/or illite/smectite), gypsum, pyrite, and traces of rutile and anatase. A varying proportion of bassanite was observed in the low-temperature ashes (LTAs). Bassanite in the LTAs was derived both from the dehydration of gypsum in the raw coals and from the interaction between organically-associated Ca and S during the low-temperature ashing. In addition to a proportion of detrital quartz, fine-grained and cell-filling quartz of authigenic origin is also present. Pyrite is of syngenetic origin and derived from sulfate-rich hydrothermal fluids. Compared to common Chinese and world low-rank coals, the No. 6 Coal is enriched in Be (25.7 μg/g), F (336 μg/g), Ge (274 μg/g), As (499 μg/g), Sb (240 μg/g), Cs (5.29 μg/g), W (115 μg/g), Hg (3.165 μg/g), and Tl (3.15 μg/g). Germanium in the coal is organically associated, and its enrichment is attributed to hydrothermal fluids from the adjacent granitoids. Beryllium is probably associated with Ca- and Mn-bearing carbonate minerals and to a lesser extent with clay minerals, rather than with organic matter. Fluorine largely occurs in clay minerals (kaolinite and illite). The elevated concentrations of Tl, Hg, As, and Sb are mainly distributed in pyrite and were derived from the same hydrothermal source. The high W in the coal occurs in both the organic matter and the authigenic quartz. Illite is the major carrier of Cs in the coal. The accumulation of rare earth elements (REE) in the coals had a polygenetic and multistage nature, including two syngenetic stages (early hydrothermal and terrigenous) and one diagenetic (late hydrothermal) stage. The REE distribution patterns of the early hydrothermal and terrigenous stages were characterized by the enrichment of medium REE (M-type REE) and light REE (L-type REE), respectively. A H-type REE distribution pattern (HREE enrichment) occurred in the late diagenetic hydrothermal stage. The high-Ge Wulantuga coals are also abnormally enriched in precious metals. Gold, Pt, and Pd in the coals, calculated from their concentrations in the LTAs, are 3.5–25.8,

Mineralogical, REE-geochemical and fluid inclusion studies on some uranium occurrences, Gabal Gattar, Northeastern Desert, Egypt

Altered granite occurrences (GII and GV) at Gabal (G.) Gattar are the most important mineralized rocks hosting U-mineralization in Egypt. This mineralized granite is affected by multi-stage hydrothermal alteration processes along brittle structures. This alteration comprises pre-ore alkali-metasomatism and ore-stage hydrothermal hematitization, silicification, kaolinitization, fluoritization, carbonatization and episyenitization. Autoradiographic examination indicates that primary uranium mineralization is present as massive or disseminated ores along tectonic fractures, interstitially in granular minerals, and as cements of breccias. Uraninite, pitchblende and coffinite are the main primary minerals, while uranophane and kasolite are the secondary ones. Galena, chalcopyrite, pyrrohtite and pyrite are the most abundant sulphides in association with uranium mineralizations. Hydrothermal alterations at the GII occurrence exhibit an increase in Ni, Cu, Zn, Pb, Mo, Nb, Rb beside HREEs and a decrease in Ba, Co, Sr, and V, while alterations at the GV occurrence show enrichment in Ni, Cu, Pb, Mo, Nb, Y, HREEs and depletion in Zn, Ba, Co, Rb, Sr, and V. Hydrothermal alteration during the late magmatic stage is also identified through the development of unusual REE patterns and fractionation of ratios like Zr/Hf and Y/Ho out of the CHARAC (Charge Radius Controlled) range. Non-chondritic Y/Ho ratio at GV is of larger value than chondritic one which may result from water-rock interaction or print from the encompassing sedimentary rocks. Fluid inclusion studies on quartz and fluorite revealed that temperatures of the mineralizing fluids range between 126 and 240°C at pressure ⩽0.5×108 Pa and give salinity values ranging between 1 wt% and 7.8 wt% NaCl equivalent.

U–Pb zircon chronology of the Pangidi–Kondapalle layered intrusion, Eastern Ghats belt, India: Constraints on Mesoproterozoic arc magmatism in a convergent margin setting

The Pangidi–Kondapalle layered intrusion is emplaced within high-grade supracrustal rocks in the southern sector of the Eastern Ghats Granulite Belt proximal to the south-eastern periphery of the Indian plate. This layered complex consists mainly of anorthosites and gabbronorites with pyroxenite cumulates containing orthopyroxenites and chromitites in a regressive intrusive sequence. Here we report laser ablation ICP-MS data and REE geochemistry of zircons from three co-spatial anorthositic bodies in this complex. Our results show mean 207Pb/206Pb ages of 1634±18Ma (MSWD=0.55) for the anorthosite from Pangidi. Two anorthosite samples from Kondapalle yield mean207Pb/206Pb ages of 1628±36Ma (MSWD=0.54) and 1693±110Ma (MSWD=5.5). The zircons from all the anorthosite samples show high REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly, suggesting a common melt source. The prominent Mesoproterozoic ages revealed from our U–Pb dating of zircons from these layered complexes suggest subduction-related magmatic underplating in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic supercontinent.

An estimate for the bulk composition of juvenile upper continental crust derived from glacial till in the North American Cordillera

We use a compilation of bulk chemical analyses for 2771 rocks and 3487 till samples (silt + clay fraction) from British Columbia, Canada to construct an estimate of juvenile upper crust composition in the North American Cordillera for comparison with upper crust estimates for older continental crust. Principle component analyses demonstrate that glacial tills are a robust natural average of their surface rock provenance. The chemical weathering during the generation and deposition of tills is small, yet they preserve the chemical weathering history of their source regions. Our results show that the upper continental crust of the Cordillera is broadly similar to other estimates in terms of the abundance of Si, Al, Mg, Ca, Na, Zr, Y, Sr, Ba, Th and U. The till-based upper crustal estimate, however, shows consistent and marked enrichment in Fe, Ti, Mn, P and HREE and depletion in LREE and K. These attributes are not obviously attributed to any analytical or sampling bias; surface rock averages and modern river sediments from British Columbia show identical patterns of enrichment and depletion as the tills. We attribute the unique character of our Cordilleran juvenile upper crust estimate to a significant component of basaltic soils in till generation, as compared to upper crustal estimates from more mature denuded terrains. Nonetheless, our crustal estimate remains overall ‘andesitic’, similar to those from Precambrian shields, supporting the notion that reworking of old crust is volumetrically far more important than production of juvenile crust in the growth of the North American continent.

Geochemistry of Neoproterozoic limestones of the Shahabad Formation, Bhima Basin, Karnataka, southern India

Major, trace and rare earth element (REE) geochemistry of carbonate rocks of the Neoproterozoic Shahabad Formation, southern India were studied in order to investigate the depositional environment and source for the REEs. The PAAS (Post Archaean Australian Shale) normalized REE + Y pattern of Shahabad limestones have consistent seawater-like pattern i.e., i) LREE depletion (average (Nd/Yb)SN = 0.64 ± 0.08), ii) negative Ce anomaly, iii) positive Gd anomaly (average GdSN/Gd* = 1.05 ± 0.16), iv) superchondritic Y/Ho ratio (average Y/Ho = 38.13 ± 21.35). The depletion of LREE and enrichment of HREE are clearly indicated by the (La/Yb)SN, (Dy/Yb)SN and (Nd/Yb)SN ratios, which suggest the retention of seawater characteristics in these limestones. The negative Ce anomaly reflects the incorporation of REE directly from seawater or from the pore water under oxic condition, and also reveals the mixing of two-component systems with terrigenous clay (detrital) in the marine sediments. The terrigenous input in these limestones is confirmed by positive correlation of ΣREE with Al2O3, negative correlation of ΣREE with CaO and differences in Y/Ho ratios. V, Cr, and Sc, are positively correlated with Ti, and strong positive correlation of ΣREE with Fe2O3, Ni, Cr, Sc, and Y also indicate the presence of terrigenous materials in the Shahabad limestones.

LA-ICP-MS U–Pb zircon age constraints on the Paleoproterozoic and Neoarchean history of the Sandmata Complex in Rajasthan within the NW Indian Plate

The Sandmata Complex in Rajasthan within the NW Indian shield represents a metamorphosed volcano-sedimentary sequence incorporated within a late Paleoproterozoic suture zone. Here we report laser ablation ICP-MS age data and REE geochemistry of zircons from four rock types in the Sandmata Complex – a foliated biotite–quartz gneiss, a leucocratic gneiss, a biotite gneiss and a porphyritic granite. The zircons in the biotite gneiss and porphyritic granite show magmatic textures with a single Paleoproterozoic age population, whereas the leucocratic gneiss and biotite–quartz gneiss samples carry zircons with an Archean provenance history and metamorphic characteristics. U–Pb dating of zircon domains in these samples shows two dominant age populations: Paleoproterozoic and Neoarchean, and indicate that (a) granulite facies metamorphism (biotite–quartz gneiss: ca. 1708 Ma) and the granitic magmatism (porphyritic granite: ca. 1733 Ma) were synchronous; and (b) the igneous protoliths of the leucocratic gneiss (ca. 2750 Ma) and biotite gneiss (ca. 2698 Ma) were formed in the Archean. The zircons from both gniesses and granite show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly suggesting a common melt source. We correlate the Paleoproterozoic magmatic ages from Sandmata Complex with a subduction–accretion history prior to the final collisional assembly of the continental fragments in the Neoproterozoic, comparable with a similar history all along the Central Indian Tectonic Zone, as well as from the Eastern Indian Suture, probably indicating the continuity of the suture all along Peninsular India. Our zircon data also reveal the vestiges of Archean rocks within the Paleoproterozoic suture.

Mesoproterozoic ophiolitic mélange from the SE periphery of the Indian plate: U–Pb zircon ages and tectonic implications

The Kanigiri mélange within the Proterozoic Nellore–Khammam schist belt in southern Peninsular India includes ophiolitic fragments that represent the remnants of an oceanic plate. The ophiolitic units were accreted along a NE-trending suture that juxtaposes the Proterozoic Eastern Ghats Granulite Belt (EGGB) against the Archean Nellore Schist Belt of the Dharwar craton. The ophiolite components in the Kanigiri mélange include plagiogranites and gabbros which show mutually intrusive relations indicating their coeval nature. We report laser ablation-ICP-MS age data and REE geochemistry of zircons from the gabbro and granite. The zircons from both gabbro and granite show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. Zircon REE abundances and normalized patterns show little intersample and intrasample variations. U–Pb dating of the zircons reveals prominent Mesoproterozoic ages for the plagiogranite, with the ca.1.33 Ga age of the Kanigiri ophiolitic mélange offering important clues for arc–continent collision during the final stages of amalgamation of the Columbia-derived fragments within the Neoproterozoic supercontinent assembly.

Laterites-bearing REEs, Wadi Natash, Southeastern Desert, Egypt

The present paper recorded the occurrence of laterites-bearing REEs in Natash area. Natash laterite deposits occur as horizons (5–15 m in thickness) at the boundary between the Cretaceous sandstones, and Natash flows. Laterite was recorded in more than twenty six occurrence distributed over an area of about 500 km 2 along and around Wadi Natash. It ranges in color from light-grey, dark-grey to reddish-brown. The distribution of laterite deposits is structural controlled. Mineralogical results of laterites proved the presence of both kaolinite and montmorillonite as clay minerals. The presence of hyperthene, diopside and tremolite indicated that their main source was basic Natash flows. The presence of gypsum, hematite, goethite, calcite and kaolinite indicated that, the Natash laterites were related to tropical weathering conditions. The presence of ilmenorutile and rutile in the laterites declared the reducing and low pH. All the Natash laterite samples were characterized by very highly irregular REE pattern with HREE enrichment. Generally, the ΣREEs+Y, ranged from 0.36%–0.71% with an average of about 0.59%. The LREE was less concentrated than the HREE by a factor of 0.57. Σ(LREE/HREE=0.57) which suggested that, the HREE contents in laterites were derived from the basic and intermediate flows rather than from acidic lavas flows.

Geology of Pulikonda and Dancherla alkaline complexes, Andhra Pradesh

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas — all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it’s simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively. Dancherla syenite body has yielded a six point Rb-Sr whole rock isochron age of 2211±110 Ma with low initial 87Sr/86Sr ratio of 0.7004±0.00046; MSWD = 5.8.The low initial Sr(i) ratio suggests mantle derived juvenile additions to the crust. The Pulikonda syenite body yielded a six point whole rock isochron age of 1500 ±100 Ma with initial 87Sr/86Sr ratio of 0.7085±0.0047 and MSWD = 2%. The higher initial Sr(i) value for this body suggests metasomatised source. Resetting of Rb-Sr systematics has resulted younger age for Pulikonda syenite though it is supposed to be older as per the geological setting and this anomaly is interpreted interms of repeated deformation and reactivation of shear zone, emplacement of alkali granite into Pulikonda syenite and the role of Rudravaram shear zone (basement tectonics) in the formation of the Nallamali fold belt of the Cuddapah Basin. Alkaline magmatism was initially triggered along microplate margin tectonic setting and later confined to the intraplate tectonic setting.