Concentrations Of Lithophile Elements Research Articles

Magnetite composition in Ni-Cu-PGE deposits worldwide: application to mineral exploration

The concentration of trace elements in magnetite is determined primarily by the geological environment at the time of its formation, which makes magnetite a useful indicator mineral in the exploration of ore deposits. Magnetite is a common accessory mineral in magmatic Ni–Cu–Platinum-Group Element (PGE) sulfide deposits forming primary grains in massive sulfide ore and/or secondary grains during alteration of sulfide ore and host rock. We report magnetite composition from massive and disseminated sulfide samples (n=94), representative of 13 major Ni–Cu–PGE deposits, worldwide, and from a range of geological environments and formation ages (Archean to Permo–Triassic). The samples are divided into 6 different types according to the composition of the parental host magma: (1) komatiite, (2) ferropicrite, (3) picrite–tholeiite, (4) anorthosite–troctolite, (5) flood basalt and (6) impact melt. The minor and trace element composition of magnetite was measured by electron probe micro-analysis and a subset of samples (n=61) by laser ablation-inductively coupled plasma-mass spectrometry.The composition of all primary magnetite, crystallized from an immiscible sulfide liquid, plots on a trend in Ti vs. Cr and V vs. Cr. However, primary magnetite trace element content is controlled by element partitioning during sulfide liquid differentiation, from early-forming Fe-rich monosulfide solid solution (MSS) to Cu-rich intermediate solid solution (ISS). The concentration of most lithophile elements (Cr, Ti, V, Al, Sc, Nb, Ga, Ta, Hf and Zr) is highest in the early-forming magnetite, which crystallized with Fe-rich MSS. The fractionated Cu-rich liquid was depleted in lithophile elements so that late-forming magnetite, which crystallized with the Cu-rich ISS, has a low concentration of these elements. Compatible chalcophile elements partitioning into magnetite depends largely on the co-crystallizing sulfide mineral. Elements such as Ni partition preferentially in Fe-rich sulfide minerals (pyrrhotite or pentlandite) such that coprecipitated magnetite is depleted in chalcophile elements compatible with the sulfides. In contrast, magnetite co-crystallized with Cu-rich ISS is enriched in Ni because Ni is not compatible with the co-crystallizing Cu sulfides. Magnetite composition is also dependent on parental magma composition. Magnetite in massive sulfides from ultramafic parental magmas is most similar in compositions to primitive magnetite from Fe-rich sulfides in fractionated intermediate to mafic hosted orebodies.Two types of secondary magnetite are distinguished from the komatiite-hosted Thompson Ni-deposit (Manitoba, Canada): (1) magnetite formed by replacement of pyrrhotite and (2) magnetite formed during the serpentinization of the ultramafic host rocks. Secondary magnetite is depleted in most trace elements (Ni, Mn, V, Ti, V, Al, Cr), with the exception of Si and Mg, which are enriched.To be useful as a tool for mineral exploration, primary and secondary magnetite should not have the same minor and trace element composition. Primary magnetite from all 13 major Ni–Cu deposits plot in the field for Ni–Cu sulfide deposits in the Ni+Cr vs. Si+Mg discrimination diagram. Secondary magnetite plot outside of the field for Ni–Cu deposits. Therefore using this diagram, we can discriminate between primary and secondary magnetite using the low Ni+Cr content of secondary magnetite. Magnetite is resistant to surficial weathering and destruction during mechanical transport, such that it is a useful indicator mineral in exploration that can be used to detect eroded Ni–Cu–PGE deposits in surficial sediments.

Geochemistry of metamorphic rocks of the Kurai block (Gorny Altai)

We consider the primary nature and sources of the protoliths of metamorphic rocks of the Kurai block located in the large Teletskoe-Kurai system of deep faults separating the Gorny Altai and West Sayan structures. It has been established that the protoliths of the Kurai block metapelites were deposits of transitional crust: They lack typical rocks of mature continental crust (arkoses, litharenites) and have reduced (relative to the upper continental crust) contents of lithophile elements and elevated contents of transition elements. The average Nd model age of the protoliths of the metapelites of the Kurai complex corresponds to the Middle Riphean (1.4-1.6Ga). The metabasites of the Kurai complex are similar in petrologic composition to metamorphic products of oceanic basalts. It is shown that the formation of metamorphic associations of the Kurai block was caused by tectonometamorphic transformations of a compositionally heterogeneous rock unit (basalts, aluminous and volcanomict sediments), which were, most likely, a fragment of Middle Cambrian-Early Ordovician turbidite basin with an oceanic basement.

Use and calibration of portable X-Ray fluorescence analysers: application to lithogeochemical exploration for komatiite-hosted nickel sulphide deposits

Portable X-Ray Fluorescence (pXRF) analysers allow on-site geochemical analysis of rock powders and drill core. The main advantages of pXRF analysis over conventional laboratory analysis are the speed of data collection and the low cost of the analyses, permitting the collection of extensive, spatially representative datasets. However, these factors only become useful if the quality of the data meets the requirements needed for the purposes of the study. Here, we evaluate the possible use of portable XRF to determine element concentrations and ratios used in exploration for komatiite-hosted nickel sulphides. A portable XRF analyser was used to measure a series of chalcophile and lithophile element concentrations (Si, S, K, Ca, Ti, Cr, Fe, Ni, Cu, Zn, As, Sr, and Zr) of 75 samples from three komatiite units associated with nickel sulphide ores in the Yilgarn Craton, Western Australia. Crucial steps in the study were the development of a strict calibration process as well as numerous data quality checks. The 670 analyses collected in this study were compared with conventional laboratory XRF data on discriminant diagrams commonly utilized in exploration for komatiite-hosted nickel sulphides (Cr vs Ni and Ni/Ti vs Ni/Cr). After comparing the results obtained with pXRF during this study with the laboratory values, we can conclude that portable XRF analyses can be used for rapid assessment of the nickel sulphide prospectivity of komatiites provided that strict control protocols are followed. Supplementary Material: is available at http://www.geolsoc.org.uk/SUP18706

Petrology and Trace Element Budgets of High-pressure Peridotites Indicate Subduction Dehydration of Serpentinized Mantle (Cima di Gagnone, Central Alps, Switzerland)

At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.

Episodic magmatism at 105 Ma in the Kinki district, SW Japan: Petrogenesis of Nb-rich lamprophyres and adakites, and geodynamic implications

Cretaceous episodic magmatism produced Nb-rich lamprophyres and adakitic granitoids in the Kinki district of SW Japan. K–Ar dating of minerals from the lamprophyres, adakites, and hornblende peridotite xenoliths yielded ages of 109–99Ma, indicating a short-lived episodic magmatism. The lamprophyres generally display primitive high-Mg basaltic to basaltic andesite compositions with high Mg# and high Cr and Ni contents that preclude substantial differentiation. Some high-Nb basalt (HNB) and Nb-enriched basalt (NEB) compositions also occur. The lamprophyres have high large-ion lithophile element (LILE) and high field-strength element (HFSE) contents and variable (La/Yb)n ratios, and can be divided into high-(La/Yb)n (12.5–22.1) and low-(La/Yb)n (3.6–6.1) groups. The former contains nepheline-normative rocks with positive initial εNd(T) values, whereas the latter contains hypersthene-normative subalkaline rocks with negative initial εNd(T) values. The adakitic granitoids have relatively high TiO2, Nb, and Ta contents compared to more typical high-silica adakites elsewhere, indicating that they were produced by high temperatures (ca. 920 to 970°C) during slab melting.Early Cretaceous slab rollback and the accompanying asthenospheric upwelling at 105Ma could form a transitory thermal anomaly, and hence induce melting of the subducted slab to form adakitic granitoids, and produce metasomatized wedge mantle to form the lamprophyres. The high-(La/Yb)n lamprophyres originated from small degrees of partial melting of an enriched metasomatized mantle wedge within the garnet stability field at depths of ≥70km, whereas the low-(La/Yb)n lamprophyres originated from a different mantle source by a comparatively larger degree of partial melting in a relatively shallow part of the mantle wedge. The magmatic diversity of the Kyoto lamprophyres thus derives primarily from a heterogeneous mantle source that has been variably affected by the results of subduction.

Geochronology of the Hongniu-Hongshan porphyry and skarn Cu deposit, northwestern Yunnan province, China: Implications for mineralization of the Zhongdian arc

The Hongniu-Hongshan porphyry and skarn copper deposit is located in the Triassic Zhongdian island arc, northwestern Yunnan province, China. Single-zircon laser ablation inductively coupled plasma mass spectrometry U–Pb dating suggests that the diorite porphyry and the quartz monzonite porphyry in the deposit area formed at 200Ma and 77Ma, respectively. A Re–Os isotopic date of molybdenite from the ore is 78.9Ma, which indicates that in addition to the known Triassic Cu–(Au) porphyry systems, a Late Cretaceous porphyry Cu–Mo mineralization event also exists in the Zhongdian arc. The quartz monzonite porphyry shows characteristics of a magnetite series intrusion, with a high concentration of Al, K, Rb, Ba, and Pb, low amount of Ta, Ti, Y, and Yb, and a high ratio of Sr/Y (average 26.42). The Cretaceous porphyry also shows a strong fractionation between light and heavy rare earth elements (average (La/Yb)N 37.9), which is similar to those of the Triassic subduction-related diorite porphyry in the Hongniu-Hongshan deposit and the porphyry hosting the Pulang copper deposit. However, in contrast to the older intrusions, the quartz monzonite porphyry contains higher concentrations of large ion lithophile elements and Co, and lesser Sr and Zr. Therefore, whereas the Triassic porphyry Cu–(Au) mineralization is related to slab subduction slab in an arc setting, the quartz monzonite porphyry in the Hongniu-Hongshan deposit formed by the remelting of the residual oceanic slab combined with contributions from subduction-modified arc lithosphere and continental crust, which provided the metals for the Late Cretaceous mineralization.

Multi-stage growth and invisible gold distribution in pyrite from the Kundarkocha sediment-hosted gold deposit, eastern India

Gold mineralization at Kundarkocha, India, is hosted in sheared gray quartz veins that were emplaced in carbonaceous pyritic phyllite. Gold occurs as enclosed grains within sulfides and free grains in quartz. Based on characteristic textural and chemical features, documented by X-ray element imaging, electron probe microanalysis and laser-ablation inductively-coupled plasma mass spectrometry analyses, four pyrite types were identified in carbonaceous phyllites and auriferous veins. Rock-hosted fine-grained syn-sedimentary to early diagenetic pyrite framboids (PyI) have lower contents of Co and As but consistently high gold values. Pyrite of the next generation (PyII) has numerous silicate and rare sulfide inclusions; lower contents of Co and Ni, moderate As values; the highest mean value of invisible gold and maximum concentrations of trace elements such as Li, Ti, Zn, Rb, Sr, Y, Zr, Nb, La, Ce, Ta, Th, U and Cr. Pyrite of the third generation (PyIII) shows evidence of overgrowth over PyII, contains both silicate and sulfide inclusions, and are characterized by moderate contents of Co, high Ni and low Au values and higher concentrations of large ion lithophile elements, but lesser amount of high field strength elements. Pyrites of the latest type (PyIV) occur as polycrystalline aggregates that contain inclusions of gold, sulfides and rare silicates, show oscillatory zoning of Co and As and the lowest concentrations of all other trace elements. Successive decrease in contents of majority of trace elements from PyII to PyIV is attributed to fluid-assisted recrystallization during diagenesis and low grade metamorphism.Later generation pyrites (PyII through PyIV) exhibit higher Au contents regardless of their As values, indicating occurrence of invisible gold mostly as nanoparticles, at times reaching up to 500ppm. Unlike the majority of trace elements that underwent large-scale remobilizations, gold was somehow locked up in pyrite resulting in a rather lean deposit at Kundarkocha.

A two-stage scenario for the formation of the Earth's mantle and core

Various geophysical constraints on the deep Earth point to a chemically heterogeneous mantle. Based on such constraints, Bulk Earth compositions inferred from Enstatite chondrites (E-Earth composition) predict that, whereas the Primitive Upper mantle (PUM) had a pyrolitic composition, the Primitive Lower mantle (PLoM) was enriched in Fe and Si. In E-Earth formalism, this chemical heterogeneity is related to the formation and differentiation of the Early Earth, and mantle Si and Fe variations reflect variations in the efficiency of Si and FeO dissolution in the metal phase during core formation. In the simplest scenario of homogeneous accretion, we calculate by mass balance the composition and the mass fraction of the metallic extract in equilibrium with a pyrolite. The O, Si and Ni contents of this metal extract correspond to a silicate-metal equilibrium at high pressure (50±5GPa) and high temperature (3500±500°C), in line with a giant impact scenario. Mass balance calculations then yield the composition of the proto-core and the proto-mantle prior to the giant impact. We obtain that the core of the proto-Earth was almost devoid of oxygen, hence formed under lower pressure and temperature conditions, in agreement with an early differentiation of planetesimals in the early solar system. In such a two-stage scenario of Earth's core formation, no massive silicate differentiation is required to create a pristine mantle heterogeneity. The concentration of lithophile elements in the Primitive Lower mantle can then be constrained using RLE ratios in E-chondrites and in the upper mantle.

Factors controlling geochemical and mineralogical compositions of coals preserved within marine carbonate successions: A case study from the Heshan Coalfield, southern China

The Late Permian coals in the Heshan Coalfield of southern China are preserved within marine carbonate successions and characterized by super-high organic sulfur (5.13–10.82%). Minerals identified in the coals include quartz, kaolinite, illite, mixed layer illite/smectite, albite, pyrite, marcasite, calcite, and dolomite, along with trace amounts of smectite, fluorite, strontianite, REY-bearing carbonate minerals, jarosite, and water-bearing Fe-oxysulfate. The coals are very rich in trace elements including F (up to 3362μg/g), V (up to 270μg/g), Se (up to 24.4μg/g), Mo (up to 142μg/g), U (up to 111μg/g), and, to a lesser extent, Sr, Y, Zr, Nb, Cd, Cs, heavy rare earth elements, Hf, Ta, W, Hg, and Th.Previous studies attributed the high organic sulfur and elevated trace elements to the seawater influence or the formation of soil horizons before the accumulation of peat in the basin. However, mineralogical and geochemical data presented in this study have shown that the sediment-source region and multi-stage hydrothermal fluids are the dominant influences on the mineralogical composition and elevated trace elements in the coal, although seawater influence also contributed to the composition of the mineral matter.For example, a large proportion of the quartz and clay minerals, as well as almost all the albite, in both the coal benches and the parting mudstones were derived from detrital materials of terrigenous origin in the Yunkai Upland. High concentrations of lithophile trace elements were also derived from the sediment source region. Minerals including fluorite, calcite, dolomite, strontianite, and REY-bearing carbonate minerals were derived from multi-stage hydrothermal activities. High concentrations of V, Mo, and U that occur through the coal seam sections were probably derived from hydrothermal solutions during peat accumulation or at the early diagenetic stages. The hydrothermal fluids also corroded the syngenetically-formed minerals (quartz, albite, and pyrite) and caused re-distribution of lithophile elements from partings to the underlying coal benches, resulting in higher key element ratios (Yb/La, Nb/Ta, and Zr/Hf) and more abundant heavy rare earth elements in the coal benches than in the immediately overlying partings.

Volcanism from fissure zones and the Caldeira central volcano of Faial Island, Azores archipelago: geochemical processes in multiple feeding systems

AbstractMagmas in Faial Island, Azores (Portugal), were mostly erupted from two fissure zones and the Caldeira central volcano during overlapping periods. The fissure zones follow extensional trends oriented WNW and ESE and erupted nepheline- to hypersthene-normative basalts and hawaiites. The Caldeira central volcano builds the central part of the island, which is cut by the fissure zones. Ne-normative basalts show similar high-field-strength element (HFSE) concentrations but higher large ion lithophile element (LILE) concentrations than hy-normative equivalents. Primitive melts were generated by small (3–5%) degrees of partial melting of garnet-bearing peridotite, variably enriched in incompatible elements. Overall, basalts from Faial show relatively higher LILE abundances and LILE/HFSE ratios than those of the other islands of the Azores and of many other volcanoes in the Atlantic area. This feature indicates the existence of chemical heterogeneities in the mantle sources characterized by variable degrees of metasomatism, both at local and regional scales. Hawaiites evolved from basalts through 30–40% fractional crystallization of mafic phases plus some plagioclase, in deep reservoirs, at about 430–425 MPa (~ 15 km). The Caldeira central volcano rocks range from basalts to trachytes. Basalts, produced under similar conditions as fissure basalts, evolved to trachytes through large degrees of polybaric fractional crystallization (100–760 MPa; i.e. ~ 3.6–26 km), involving olivine, clinopyroxene, feldspar and minor quantities of amphibole, biotite, apatite and oxides. In contrast, mafic magmas from the fissure zones were erupted directly onto the surface from magma reservoirs mainly located at the crust–mantle boundary.

Accurate trace element analysis of speleothems and biogenic calcium carbonates by LA-ICP-MS

The high spatial resolution of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) microanalysis of speleothems and biogenic calcium carbonates makes it possible to improve our understanding of past climatic conditions. However, there are analytical problems associated with this method, such as molecular interferences, elemental fractionation, and mass-load-dependent matrix effects, when using NIST silicate reference glasses for calibration.Our study with a sector-field ICP-MS shows that many masses are affected by interferences, such as 24Mg+ by 48Ca++ or 31P+ by 15N16O+. Elemental fractionation and mass-load-dependent matrix effects have been detected for both 213 and 193nm Nd:YAG lasers. They are small for refractory lithophile elements, in particular for the 193nm laser (<5%). For chalcophile/siderophile trace elements with low boiling points, however, these effects are large (up to 40% and 20% for the 213 and 193nm lasers, respectively), and variable for the NIST glasses and carbonates.In this paper, a protocol for precise and accurate LA-ICP-MS analysis of calcium carbonates is established. Isotopes for interference-free measurements at low (M/ΔM ~300) and medium (M/ΔM ~4000) mass resolution have been identified. The NIST glasses are suitable for calibration of refractory lithophile element concentrations. However, matrix-matched calibration with carbonate reference materials, such as USGS MACS-1 or MACS-3, is necessary for accurate analysis of chalcophile/siderophile elements with low boiling points. Analytical reproducibility (RSD) is a factor of ~2 better using the 193nm laser than the 213nm laser. As shown by the analysis of MACS-1 and MACS-3, LA-ICP-MS analyses yield results that agree with the reference values within relative uncertainties of ca. 5–10% at the 95% confidence level.We have applied LA-ICP-MS for the determination of trace element concentrations in calcite and aragonite layers of a stalagmite and found large variations for Mg, Zn, Sr, and U. In ostracod shells, the concentrations of some trace elements (e.g., Sr and Ba) vary significantly, indicating the potential for paleoclimate research.

Tracing the chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of fluorine

AbstractThree Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.

Cenozoic Mg-rich potassic rocks in the Tibetan Plateau: Geochemical variations, heterogeneity of subcontinental lithospheric mantle and tectonic implications

Cenozoic Mg-rich potassic rocks (MPRs; with MgO⩾6wt.%, K2O/Na2O⩾1) that are widespread in the Tibetan Plateau are commonly explained as the products of partial melting of subcontinental lithospheric mantle (SCLM) sources. This is because the MPR have characteristic geochemical features, including high contents of K2O and other large ion lithophile elements, coupled with depletion in the high field strength elements, suggesting an enriched lithospheric mantle source metasomatized by earlier subduction events. In this paper, we report new data together with a synthesis of the Tibetan MPR. Relative to MPR from northern Tibet (i.e., the Songpan–Ganzi and Qiangtang blocks), MPR from southern Tibet (i.e., the Lhasa block) have obviously higher SiO2, K2O, Rb, Th and Zr contents, and higher K2O/Na2O (⩾2) and Rb/Sr (>0.2) ratios, coupled with lower Al2O3, CaO, Na2O, Sr and Ba/Rb (<12). In addition, MPR from southern Tibet show distinctly lower 143Nd/144Nd(i) and higher 87Sr/86Sr(i) isotopic ratios than those of the north. The geochemical variations suggest that MPR from southern Tibet have mantle source compositions and mineral phases different from those from northern Tibet, reflecting the heterogeneity in the SCLM beneath the plateau. The presence of such mantle heterogeneity, combined with the spatial and temporal changes reported in other types of Cenozoic lavas on Tibet, implies that the Tibetan Plateau experienced multiple-stage tectonic events and heterogeneous uplift processes at the surface. Although existing models such as convective removal and oblique subduction can explain some of the geological phenomena related to Tibetan formation, we suggest the integrated effect of multiple tectonic events to explain the spatial and temporal distribution of MPR and other Cenozoic lavas and associated topographic uplift.

Discovery of the Indosinian aluminum A-type granite in Zhejiang Province and its geological significance

Zircon LA-ICP-MS U-Pb dating of the Jingju syenogranites in the southwestern part of Zhejiang Province shows that these rocks were crystallized in the Late Triassic at 215±2 Ma, rather than in the Cretaceous as previously proposed. The Jingju syenogranites are characterized by relatively high K2O and FeO*, and low MgO. They have high concentrations of large ion lithophile elements (LIL) and LREE, such as K, Th, La, and Ce, but are depleted in high field strength elements (HFSE) such as Nb, Ta, and Ti. Their 104Ga/Al ratios and (Zr+Nb+Ce+Y) contents are also high. These characteristics are similar to those of A-type granites. The syenogranites have high ISr (0.7179–0.7203), low ɛNd(t) (from −14.2 to −13.2), and relatively old T2DM ages, similar to those of the ancient metamorphic basement in the Cathaysia Block. It is suggested that the Jingju syenogranites were formed by partial melting of the Cathaysia basement rocks during tectonic extension. This identification of Indosinian A-type granite in Jingju has significant implications for understanding the early Mesozoic tectonic evolution of South China.

The Omolon pallasite: Chemical composition, mineralogy, and genetic implications

The chemical composition of mineral components of the Omolon pallasite was determined by neutron-activation. Six types of olivines were distinguished. Four types differ in the abundance of Co relative to Ni of CI chondrites. The fifth and sixth types were distinguished on the basis of REE distribution in them. Both last types are variably enriched in LREE relative to CI chondrites. In terms of Ca content relative to CI chondrite, these six types are subdivided into two groups: low-calcium and high-calcium. The difference in Ca contents can be caused by different cooling rate of the precursor of these olivines. The distribution pattern of siderophile elements in the pallasite metal indicates that a metallic phase experienced chemical transformations since the time of its formation. The analysis of chemical composition of accessory minerals showed that: (1) HREE are accumulated in tridymite; (2) troilite and daubreelite were formed under different temperature conditions; (3) magnetite is the mineral of the outer zone of melting crust. Four fragments with anomalous contents of lithophile elements were found in the pallasites and studied. The unusual chemical composition of phases and high degree of HREE fractionation in the fragments suggest their formation at high temperatures at the early stage of the Solar system evolution. It is assumed that the Omolon pallasite was formed as impact-brecciated mixture of the asteroid core (with composition close to IIIAB group of iron meteorites) and mantle olivine from incompletely differentiated parent body of chondrite composition.

Geochemistry and zircon geochronology of the Permian A-type Hasanrobat granite, Sanandaj–Sirjan belt: A new record of the Gondwana break-up in Iran

The Sanandaj–Sirjan metamorphic–plutonic Belt (SSB) in west central Iran is a polyphase metamorphic terrain composed of dominantly greenschist-grade metasedimentary and metavolcanic rocks, and felsic to mafic plutons, of Neoproterozoic–Phanerozoic ages. The Hasanrobat granite in central SSB occurs as a single pluton, ~20km2 surface area, with relatively consistent mineralogy and chemistry. Quartz, alkali feldspars (microcline and perthite), sodic plagioclases and biotite are the main constituents, commonly associated with minor amphibole. Accessory phases include zircon, allanite, apatite, and magnetite.The country rocks are Upper Carboniferous–Lower Permian sandstones and dolomitic limestones. Scattered patches of skarn-type assemblages dominated by tremolite and talc occur in the dolomitic limestones, and sandstones are recrystallized to a coarse-grained quartzite at contact with the granite. The granite is metaluminous to slightly peraluminous, and is distinguished by high FeOt/MgO ratios, typical of ferroan (A-type) granites. The A-type affinity is also reflected by high Na2O+K2O, high Ga/Al ratios, high contents of large ion lithophile elements (LILE), high field strength elements (HFSE) and rare earth elements (REE), as well as low contents of Sr, and distinct negative Eu anomalies. The biotites are aluminous, Fe-rich, and plot near the siderophyllite corner in the quadrilateral biotite diagram. They are further distinguished by high fluorine contents (0.61 to 1.33wt.%). Amphibole is ferrohastingsite in composition. Ion microprobe analyses of zircon grains separated from a representative granite sample yielded concordant U–Pb ages with weighted mean 206Pb/238U age of 288.3±3.6Ma.The granite and the country rocks are cut by a set of mafic dykes with asthenosphere-like geochemical signatures. Such association suggests anorogenic intraplate magmatism in Lower Permian in the region. This provides further evidence for, and significantly constrains timing of, a major extension in Upper Paleozoic in Iran, previously inferred from the rock record. The extension led to the Gondwana break-up and the opening of Neotethys Ocean between Sanandaj–Sirjan and Zagros in Permian.

The geochemistry of host arc volcanic rocks to the Co–O epithermal gold deposit, Eastern Mindanao, Philippines

Mindanao is the second largest island of the Philippines and is located in the southern part of the archipelago. It comprises the suture zone between the Eurasian and the Philippine plate, which is displayed in the Philippine Mobile Belt. Eastern Mindanao is part of the Philippine Mobile Belt and outcropping rocks are mainly Eocene to Pliocene in age related to episodes of arc volcanism alternating with sedimentation.New high-precision elemental analysis of the Oligocene magma series, hosting the Co-O epithermal Au deposit, which represents an arc segment in the central part of Eastern Mindanao, revealed dominantly calc-alkaline rocks ranging in composition between basalt and dacites. Major element trends (MgO vs. TiO2 and Fe2O3) are comparable to other magmas in Central and Eastern Mindanao as well as other SW Pacific Islands such as Borneo. Rare earth and trace element distribution patterns display typical island arc signatures highlighted by the conjunction of LILE-enrichment with troughs at Nb, Ta, and Ti. Ratios of Zr/Nb in basalts vary between 17 and 39, signifying a depleted subarc mantle wedge comparable to the range of MORB, and other Indonesian island arc basalts. In basalts, Nb/Ta and Zr/Sm ratios are 12–37 and 14–27 respectively indicative of deep melts of rutile-eclogite subducted slab, as well as fluids, infiltrating the mantle wedge source of basalts.Moderate large ion lithophile element contents and low Th/La and Th/Ce ratios suggest no significant slab-derived components such as sediment or crustal fragments. The comparatively low Ce and Yb values in basalts, but also andesites and dacites, are consistent with a thin arc crust related to an intraoceanic convergent margin setting. This is further supported by Nb contents in basalts that range between 1 and 3ppm and are within the range of modern oceanic convergent margin basalts. The range of HREE fractionation signifies that basaltic melts separated at deeper levels of the subarc wedge, possibly between the forearc and arc axis, followed by a calc-alkaline convergent margin magma suite involving shallower crustal AFC near the central arc sector.The analysed Oligocene arc segment is related to a potentially steep to intermediate dipping subduction zone in an extensional to neutral geotectonic regime. The large subduction accretion complex of the Philippine Mobile Belt provides an ideal setting for significant metal deposits during its entire evolution. This is evidenced in the Eastern Mindanao Ridge, which hosts substantial porphyry Cu and epithermal Au deposits.

Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

PETROGRAPHY, GEOCHEMISTRY AND GEOCHRONOLOGY OF THE METAVOLCANIC ROCKS OF THE MESOPROTEROZOIC LEERKRANS FORMATION, WILGENHOUTSDRIF GROUP, SOUTH AFRICA - BACK-ARC BASIN TO THE AREACHAP VOLCANIC ARC

The Leerkrans Formation of the Wilgenhoutsdrif Group is a succession of highly sheared metasedimentary and metavolcanic rocks separating the western margin of the Archean Kaapvaal Craton from the polydeformed and highly metamorphosed Proterozoic Namaqua Sector of the Namaqua-Natal Province. This study presents the results of a comprehensive petrographic, geochemical and isotopic study of the metavolcanic rocks of the Leerkrans Formation. Rocks of a basaltic member, a quartz porphyry and a second stratigraphically higher basaltic unit were sampled, with those of the lower basaltic member further subdivided based on their petrographic features and geochemical characteristics. Extensive alteration (chloritisation and epidotisation) of the volcanic rocks make identification of the precursor lithology difficult. Highly chloritised and epidotised metabasalts from the Lower Basalt are typically flow-banded, massive, vesicular or amygdaloidal, and have primitive tholeiitic, MORB-like geochemical characteristics and positive epsilon Nd(t) (e Nd(t) ) values (+4.2 to +6.1), with enriched large ion lithophile element (LILE) contents relative to primitive mantle suggesting derivation from a depleted mantle source. Some of the basalts of this unit have alkaline within-plate tholeiite characteristics. The Upper Quartz Porphyry is comprised of less altered felsic volcanic rocks which have pyroclastic or tuffaceous textures. The geochemistry of these rocks, which display a tholeiitic affinity with fractioned REE patterns and LILE enrichment relative to the high field strength elements (HFSE) and primitive mantle, along with negative e Nd(t) values (−4.8) indicate that the rocks are derived from crustal melting. U-Pb single zircon SHRIMP dating of zircons from a felsic pyroclastic rock from this unit yields an age of 1289 ± 9 Ma. The Upper Basalt and Mixed Zone of the Leerkrans Formation are comprised of basaltic lavas showing similar geochemical features to the Lower Basalt. The metavolcanic rocks of the Leerkrans Formation overlap in age with the oldest units of the ~1.3 to ~1.23 Ga Areachap Group, the polydeformed and highly metamorphosed remnants of a volcanic arc that separates the western margin of the Kaapvaal Craton from the Namaqua Sector. The Leerkrans Formation likely represents the remnants of a related back-arc basin to the volcanic arc which was accreted onto the western margin of the craton. The Leerkrans Formation escaped high-grade metamorphism and polyphase deformation that affected the Areachap arc during the Namaquan Orogeny due to being thrust back onto the Kaapvaal Craton during early arc accretion.

Subduction components in Pleistocene to recent Kurile arc magmas in NE Hokkaido, Japan

Samples of Kurile arc lavas erupted between 1.6 Ma and ca. 30,000 years were collected from the Kutcharo, Mashu and Akan caldera area in NE Hokkaido, about 150 km west of the Kurile trench. The samples include rhyolitic pumice, rhyolite, dacite, andesite and, rare, tholeiitic basalt, and show ‘medium’ potassic calc-alkaline affinity. Except for relatively high concentrations of large ionic lithophile elements (LILE), Th and especially Pb, other trace elements, including the rare earths (REE) and high field strength elements (HFSE), show relatively low abundances when compared with those of normal mid-ocean ridge basalts (N-MORB). Their Sr, Nd isotopic compositions are relatively depleted, with 87Sr/ 86Sr ranging from 0.7033 to 0.7034 and 143Nd/ 144Nd from 0.51295 to 0.51230. Pb isotopic compositions are also relatively unradiogenic, with 206Pb/ 204Pb at about 18.4 and 208Pb/ 204Pb ranging from 38.3 to 38.4, significantly more depleted than other Quaternary lavas in NE Japan. The Kurile lavas show typical subduction-type element distributions, with high ratios of fluid-mobile incompatible elements over fluid-immobile HFSE, Ba/Nb, for example, ranging between ca. 200 and 450. The lack of covariance between (e.g.) Ba/Nb and Ba/Th with 87Sr/ 86Sr, and Nd/Pb with Pb isotopic ratios suggests minimal involvement of sediment-derived metasomatism of the magmatic source. Geochemical character of the latter probably reflects contamination by hydrous fluids derived from altered oceanic crust (AOC). This is indicated by the coupling of relatively depleted, MORB-like Sr and Pb isotopic compositions and high Sr and Pb contents. Thus, given their N-MORB-type isotopic compositions, the LILE, and HFSE-like character, evidenced by high ratios of Ba/Nb, and variable Nd/Pb and Th/Nd, suggests NE Hokkaido arc magma genesis is best explained in terms of a binary mixing model involving: a dominantly N-MORB-like (i.e. depleted) convecting mantle ‘wedge’, contaminated by hydrous AOC-derived fluid. Thus magmatic sources for the NE Hokkaido sector appear to have been minimally affected by sedimentary melt-derived metasomatism, consistent with their association with the Kurile arc system.