西天山艾肯达坂二叠纪钾质火山岩的地球化学特征及岩石成因

西天山艾肯达坂组中粗安岩和粗面岩富碱(Na2O+K2O8%)、高铝(Al2O313.5%)和低钛(TiO21.31%),具有典型橄榄安粗岩系火山岩的岩石化学特征。两类钾质火山岩的微量元素和同位素地球化学特征差异显著:粗安岩明显富集大离子亲石元素和轻稀土元素,具有较高的Sr/Y(29～49)比值和相对较低的(87Sr/86Sr)i(0.7044～0.7045);而粗面岩大离子亲石元素和轻稀土元素的富集程度明显减弱,甚至出现了Sr强烈亏损的现象,它具有较低的Sr/Y(1.74～2.50)比值和相对较高的(87Sr/86Sr)i(0.7060～0.7080)。研究结果显示,艾肯达坂组中粗安岩和粗面岩均是后俯冲岩浆作用的产物,但它们分别形成于俯冲演化的不同阶段:粗安岩是后俯冲伸展阶段形成的典型橄榄安粗质碱性基性火山岩,它是富集地幔低程度(熔融程度低于5%)部分熔融的产物,其熔融相以角闪石为主,在残留相中存在着大量的石榴石,因此它具有较高Sr/Y比值;而粗面岩则是后俯冲挤压阶段形成的长英质的碱性火山岩,它是年青下地壳部分熔融的产物,其形成时间早于粗安岩。     

Origin and geochemistry of Pan-African granitoid rocks in the Gabal Um Shomer area,Southwestern Sinai,Egypt

Geological, petrological and geochemical studies indicated that there are two distinct types of granitoid rocks: older quartz diorites to granodiorite assemblage and younger granitoids, the latter occurring in two phases. The older granitoids have a meta-aluminous chemistry and a calc-alkaline character, with high MgO, Fe2O3, TiO2, CaO, P2O5, Sr and low SiO2, K2O, and Rb. Their major and trace elements data, together with low 87Sr/ 86Sr ratios (0.7029±0.0008) are indicative of I-type affinities. The second-...     

Thallium: A potential guide to mineral deposits

Several recent studies have suggested that rocks associated with mineral deposits of hydrothermal origin are generally enriched in Rb. TI is similar in its geochemical characteristics to Rb, but its use as a potential guide to ore deposits has not been suggested. Although TI has the same size and valency as Rb, it is concentrated more in residual melts than Rb and K because the TI-O bond is more covalent and weaker than either the Rb-O or K-O bonds.A new method which is rapid and precise has been developed by one of the authors for the determination of TI in geological materials at the ppb level. About 350 rocks from 4 mineralized areas in northeastern Washington and central Montana were analyzed for TI by this method. Most of these rocks were also analyzed for SiO₂, A1₂O₃,Fe₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, P₂O₅, MnO, Ag, As, Au, Ba, Co, Cu, F, Li, Mo, Ni, Pb, Sr, Sb, Sn, Sr, Te, U, W, and Zn. The ore deposits of the areas studied include the gold-silver deposits of the Republic district, the uranium deposit of the Midnite mine, the coppermolybdenum deposit of Mount Tolman, and the gold deposits of the North Moccasin mining district.In all the mineralized areas there is significantly more TI in hydrothermally altered rocks than in unaltered rocks. The very high abundance of TI in the altered rocks of the North Moccasin gold district, and the significant positive correlations of TI and K and TI and Rb in the mineralized rocks of the other three areas suggest that TI was concentrated in hydrothermal fluids. In altered rocks the average K/Rb and K/TI ratios are lower, and the Rb/Sr and TI/Sr ratios are higher.Often the abundance of TI and the K/TI and TI/Sr ratios are more useful in delineating mineralized areas than the abundance of Rb and the K/Rb and Rb/Sr ratios. The ternary relationships between TI and several other elements, especially Rb, K, Ba, and Sr, show mineralized rocks to fall near the TI apices or near the TI-Rb or TI-K boundaries.     

Geochemical constraints on the petrogenesis of the Paleozoic granitoids of the Sierra de San Luis,Sierras Pampeanas,Argentina

The Paleozoic granitoids of the Sierra de San Luis comprise the Ordovician tonalite suite (OTS; metaluminous to mildly peraluminous calcic tonalite–granodiorites) and granodiorite–granite suite (OGGS; peraluminous calcic to calc-alkaline granodiorite–monzogranites), as well as the Devonian granite suite (DGS; peraluminous alkali-calcic monzogranites) and monzonite–granite suite (DMGS; metaluminous alkali-calcic quartz monzonite–monzogranite ± granodiorite, mildly peraluminous alkalicalcic monzogranites). The OTS has relatively high K₂O, CaO, and Yb_N and low Cr, Ni, Ba, Sr, Rb/Sr, Sr/Y, and (La/Yb)_N, as well as negative Eu/Eu¹, high ⁸⁷Sr/⁸⁶Sr (0.70850–0.71114), and unradiogenic ε_Nd(470Ma) (−5.3 to −6.0), which preclude an origin of variably fractionated mantle melts and favour a mafic lower crustal source. The OGGS consists of two granitoids: (1) high-temperature characterized by low Al₂O₃/TiO₂, Rb/Sr, and (La/Yb)_N, a smooth negative Eu/Eu¹, and relatively high CaO and (2) low-temperature with high Al₂O₃/TiO₂ and Rb/Sr, low CaO, (La/Yb)_N, and Sr/Y, and negative Eu/Eu¹. Melting of metagreywackes at pressures below 10 kbar with a variable supply of water could account for the chemistry of the high-T OGGS, whereas dehydration melting of biotite-bearing metasedimentary sources at low pressures is proposed for the low temperature OGGS. Melting of crustal sources relates to a contemporaneous mafic magmatism.Devonian magmatism is characterized by high Ba, Sr, K₂O, Na₂O, Sr/Y, and (La/Yb)_N. Sources for the DGS include metasedimentary or metatonalitic protoliths. Biotite dehydration melting triggered by the addition of heat, supplied by mantle-derived magmas, is proposed. High Ba, Sr, LREE, MgO, Cr, Ni, Zr, and V of the monzonites suggest an enriched lithospheric mantle source. Low Yb and Y and high Sr and (La/Yb)_N indicate a garnet-rich residual assemblage (P ⩾ 10 kbar). Melts for the peraluminous rocks may have derived from a metasedimentary or metaigneous source at lower pressures in a process dominated by biotite consumption and plagioclase in the residue.The Ordovician granitoids are synkinematic with compressive deformation related to the early stages of Famatinian convergence. The Devonian magmatism is synkinematic with a system of shear zones that were active during the Achalian cycle.     

Strontium isotope composition and petrogenesis of the Kirkpatrick basalt,Queen Alexandra Range,Antarctica

The initial ⁸⁷Sr/⁸⁶Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO₂ (56.50%) and K₂O (1.29%) and are depleted in Al₂O₃ (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial ⁸⁷Sr/⁸⁶Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial ⁸⁷Sr/⁸⁶Sr ratios of successive flows and their concentrations of SiO₂, FeO (total iron), Na₂O, K₂O, P₂O₅, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO₂, Al₂O₃, MgO, CaO and MnO increase upward. The initial ⁸⁷Sr/⁸⁶Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial ⁸⁷Sr/⁸⁶Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different ⁸⁷Sr/⁸⁶Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial ⁸⁷Sr/⁸⁶Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their ⁸⁷Sr/⁸⁶Sr ratios. The chemical composition of the parent basalt (⁸⁷Sr/⁸⁶Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (⁸⁷Sr/⁸⁶Sr=0.720) is enriched in strontium (173 ppm), SiO₂, FeO (total iron) and the alkalies but is depleted in Al₂O₃, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.Laboratory for Isotope Geology and Geochemistry, Contribution No. 33.     

Lower Palar River Sediments,Southern Peninsular,India: Geochemistry,Source-Area Weathering,Provenance and Tectonic Setting

This present study describes the geochemistry of fluvial sediments of the Palar river (lower reaches), Southern Peninsular India, with an aim to evaluate their provenance, weathering and tectonic setting. The bulk sediment chemistry is influenced by grain size. The river sediments are enriched with SiO₂ and depleted in Al₂O₃, K₂O, CaO, Na₂O, MgO, P₂O₅, MnO, Fe₂O₃ as compared with UCC values. Geochemical classification indicate that the sediments are mainly arkose, wacke and shale in composition. Discriminant diagrams together with immobile element ratio plots reveal that, the Palar river sediments are mostly derived from rocks formed in an active continental margin. Additionally, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate felsic rock sources. The chemical indices of alteration suggest that Palar river sediments are chemically immature and have experienced low chemical weathering effects. This is further supported by the Th/U Rb/Sr ratio and A-CN-K ternary diagram, with most of the sample data points falling close to the plagioclase-smectite line. The bivariate plot of Th/Sc versus Zr/Sc suggest a moderate recycled origin of the sediments.     

Distribution and geochemical features of Hercynian-Indosinian granitoids in southeastern China

The tectonic patterns of the Hercynian-Indosinian cycle in southeastern China can be divided into two types. The first type is the Hercynian-Indosinian eugeosynclinal system, which is situated on the outer side of the Caledonian uplift belts. The second type is the Hercynian-Indosinian fault depression zone or superimposed basin system, which is developed on the basement formed by the Caledonian orogenic movement. There are two genetically different series of granitoids corresponding to two tectonic settings in the Hercynian-Indosinian tectonic regime of southeastern China. Among the transformation-series batholiths one of the largest in the Qinzhou Gulf geosyncline is known as the Darongshan granite (NE-extending) with an outcrop area of about 7000 km². The age of emplacement determined by whole-rock Rb-Sr isochron method ranges from 223 to 263 m.y., corresponding to late Permian, with higher Sr inital ratios (0.7179–0.7274). The granite is chemically rich in K₂O but poor in Na₂O with the atomic Na/K ratio<1.0 and the mole ratio Al₂O₃/(Na₂O + K₂O + CaO) > 1.0. K-feldsparg are usually predominated by medium to maximum microclines with a higher degree of Al-Si ordering characterized by the concentration of Al in T₁ sites of the feldspars (t₁ (o) = 0.57–0.99). Biotites are often rich in Fe but poor in Mg, belonging to Fe-biotite or siderophyllite, with a lower MF value (Mg/(Mg + Fe⁺³ +Fe²⁺ +Mn)), normally <0.4, and a lower Fe₃₊/Fe²⁺ ratio of 0.03-0.02 in their octahedron layers. This fact indicates that the oxidation degree of iron is relatively low in the transformation-series granites as compared with the syntexis-series granitoids. Granitic batholiths of the syntexis series distributed along the fault belts in the Hercynian depressions are usually more mafic, meta-aluminous granodiorites and smaller in size than those of the transformation series, with an outcrop area of 1–200 km₂. The Taiping granodiorite, for instance, is situated on the southern side of the Lower Yangtze River depression. It was emplaced 223 m.y. ago as dated by whole-rock Rb-Sr isochron method, with an initial Sr ratio of 0.706. Petrochemically, it is characterized by Na/K > 1.0 (1.31), mole Al₂O₃/(Na₂O + K₂O + CaO) < 1.0(0.95) and high Fe₂O₃/(Fe₂O₃ + FeO)(0.47), evidently differing from the transformation-series granites. K-feldspars are predominated by orthoclase with a lower degree of Si-Al ordering (t₁ (o) = t₁(m) = 0.30). Biotites are commonly rich in Mg in their octahedron layers (MF = 0.42–0.50) with higher Fe³⁺/Fe²⁺ ratios (0.36-0.51), indicating their formation under higher oxygen fugacity conditions. All this goes to prove that the syntexis-series granitoids were probably derived from mafic igneous or metamorphic source rocks in the lower crust.     

OUTLINE OF THE GEOLOGY OF KOREA

A database on a number of elements in oceanic volcanic rocks is presented, including the principal major-element oxides-SiO₂, TiO₂, Al₂O₃, Fe₂O₃(T), MnO, MgO, CaO, Na₂O, K₂O, and P₂O₅ (where T refers to total iron)–and the trace elements–Ba, Ce, Cr, Cu, Ni, Sc, Sr, V, Pb (mainly by isotope dilution), Yb, Zn, and Zr. Interpretations are given for transition metals, with emphasis on Mn, Sc, and V, in order to determine the concentration of the elements in primitive melts and assess their trends in magmatic differentiation. Transition metals are not enriched in plagioclase, so all are incompatible with pure plagioclase removal–that is, they become enriched in the melt. Both Cr and Ni are known to be highly compatible with olivine separation-i.e., they are depleted in the melt early in differentiation. Also, Sc is compatible with clinopyroxene (Cpx) removal from the melt and is depleted by separation of Cpx. Copper does not fit well in any of the principal silicates, but Cu, like Ni, is greatly enriched in sulfides that may remain in the source or separate from the magma. Decreasing Ni abundances and increasing Cu contents during differentiation are a sign of olivine separation. In the analysis presented herein, V–in the absence of Cpx separation–is found to behave remarkably like the moderately incompatible element Zn, and these two elements add to the list of element pairs of similar incompatibility whose ratios are insensitive to differentiation and to submarine weathering as well. Both are enhanced in titanomagnetite, so both would be compatible during titanomagnetite separation. When Cpx separates, however, V becomes compatible like Sc, but Zn remains incompatible. Thus, decreasing V (and Sc) contents and increasing Zn contents during differentiation are a sign of Cpx separation. Manganese often behaves much like Zn and therefore is moderately incompatible, but Mn is less compatible than Zn and V in titanomagnetite. Thus, decreasing Zn and V with increasing Mn is an indication of titanomagnetite removal. Dual compatible and incompatible trends with differentiation are found chiefly for Cu, Sc, and Sr. Distinguishing mid-ocean ridge basalts (MORB), oceanic-island volcanic rocks (OIV), and island-arc volcanic rocks (IAV) may be accomplished by plots of Ce/Yb versus Ba/Ce, where OIV plot to higher values of Ce/Yb than do MORB, and IAV data plot to higher values of Ba/Ce than do those of MORB. These ratios do not seem to be significantly affected by submarine weathering.     

Petrogenesis and timing of emplacement of porphyritic monzonite,dolerite, and basalt associated with the Kuoerzhenkuola Au deposit,Western Junggar,NW China: implications for early Carboniferous tectonic setting and Cu–Au mineralization prospectivity

ABSTRACT

The Kuoerzhenkuola epithermal Au deposit is located in the northern part of the West Junggar region of NW China and is underlain by a recently discovered porphyritic monzonite intrusion that contains Cu–Au mineralization. Zircon LA-ICP-MS U–Pb dating of this intrusion yielded an age of 350 ± 4.7 Ma. The porphyritic monzonite is calc-alkaline and is characterized by high concentrations of Sr (583–892 ppm), significant depletions in the heavy rare earth elements (HREE; e.g. Yb = 0.96–2.57 ppm) and Y (10.4–23.3 ppm), and primitive mantle-normalized multi-element variation diagram patterns with positive Sr and Ba and negative Nb and Ti anomalies, all of which indicate that this intrusion is compositionally similar to adakites elsewhere. The composition of the porphyritic monzonite is indicative of the derivation from magmas generated by the melting of young subducted slab material. The area also contains Nb-enriched basalts that are enriched in sodium (Na₂O/K₂O = 1.20–3.90) and have higher Nb, Zr, TiO₂, and P₂O₅ concentrations and Nb/La and Nb/U ratios than typical arc basalts. The juxtaposition of adakitic rocks, Nb-enriched basalts, and dolerites in this region suggests that the oceanic crust of the expansive oceans within the West Junggar underwent early Carboniferous subduction. Magnetite is widespread throughout the Kuoerzhenkuola Au deposit, as evidenced by the volcanic breccias cemented by late hydrothermal magnetite and pyrite. In addition, the zoned potassic, quartz-sericite alteration, and propylitic and kaolin alteration in the deeper parts of the porphyritic monzonite are similar to those found in porphyry Cu–Au deposits. These findings, coupled with the mineralogy and geochemistry of the alteration associated with the Kuoerzhenkuola Au deposit, suggest that the mineralization in this area is not purely epithermal, with the geology and geochemistry of the porphyritic monzonite in this area suggesting that a porphyry Cu–Au deposit is probably located beneath the Kuoerzhenkuola Au deposit.     

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan,China

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O, and P₂O₅ and of less variable concentrations of SiO₂, TiO₂, Al₂O₃, and K₂O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO₂, FeO, CaO, K₂O, Na₂O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al₂O₃, MgO, Fe₂O₃, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO₂, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na₂O, K₂O, P₂O₅, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO₂ and Al₂O₃). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.