广西那坡裂陷盆地晚古生代硅质岩地球化学特征及其地质意义

广西那坡裂陷盆地位于右江盆地南缘,晚古生代该盆地广泛分布包括硅质岩、泥岩和海相玄武岩在内的深水相沉积.对盆地内上泥盆统榴江组和中下二叠统四大寨组硅质岩地球化学特征研究表明,硅质岩SiO₂含量为88.55%~99.03%,PAAS组成含量小于20%,指示其含有较低的陆源碎屑组成.硅质岩的Al/(Al+Fe+Mn)值为0.45~0.94,Eu/Eu*值为0.51~0.95,为非热液成因硅质岩.除去SiO₂稀释作用的影响后,硅质岩具有较高的稀土元素含量(∑REE+Y含量相当于PAAS组成的2~5倍),指示其形成于相对远离陆源供应的环境.岩信和鱼塘上泥盆统榴江组硅质岩具有中等的Ce负异常(Ce/Ce*值分别为0.37~0.72和0.58~0.89)以及较明显的Y正异常(Y/Ho值分别为39.05~83.74和34.33~36.70),形成于远离陆源的开阔裂谷盆地环境.鱼塘中下二叠统四大寨组硅质岩具有明显的Ce负异常(Ce/Ce*值为0.12~0.33),显示成熟洋盆的地球化学特征.结合右江其他地区硅质岩的地球化学特征认为,晚古生代硅质岩的地球化学特征记录了右江盆地从晚泥盆世裂谷盆地到早中二叠世扩张为开阔洋盆的过程.      

陆相硅质岩成因类型及构造—气候指示意义

硅质岩形成于特定的地球化学条件,具有重要的构造—气候—成岩指示意义.我国硅质岩的研究主要集中于海相地层,陆相硅质岩虽分布广泛但研究却很薄弱,成因解释由于借鉴海相燧石经验,以地球化学分析为首要手段,结论存在片面性和单一性,可能会引起古老大陆重要气候—构造—环境信息的遗漏缺失.国内外陆相硅质岩全面调研表明,陆相燧石主要存在...     

Trend Surface Modelling of Karlapat Bauxite Deposit, Eastern Ghat Group, Orissa, India

Abstract: The Karlapat bauxite deposit occurs in the Eastern Ghat Group of rocks in Orissa and has developed in the khondalites. Mineralogical and physical observations on bore hole samples reflect the presence of a maximum of six weathered zones from top to bottom. These zones are termed as topsoil, siliceous laterite, ferruginous laterite, bauxite, lithomarge and altered khondalite. Four-dimensional trend surface models are developed for the data of 45 and 36 bore holes from north and south blocks, respectively, on Al₂O₃ and SiO₂ to delineate the zones of metallurgical grade bauxite (SiO₂5 % and Al₂O₃40 %). The results indicate about 15 m thickness of bauxite in the north block while it could be up to 20 m thick in the south block, leaving about 10 m of lat-erites at the top of each block. High grade bauxite (>47 % Al₂O₃) is also encountered at specific locations.     

西秦岭中志留统含铀岩系中硅质岩的岩石学、地球化学及其成因

根据产状和成因,本文将西秦岭中志留统含铀岩系中的硅质岩分为,(1) 成岩过程中硅化交代碳酸盐岩所形成的硅质岩,产于碳酸盐岩和泥质岩类所组成的透镜层的顶部或碳酸盐岩层内;(2) 早期成岩阶段形成的纹层状硅质岩,主要产于碳酸盐岩和泥质岩透镜层底部或呈薄层夹于粉砂岩、泥质岩层中。根据沉积环境分析及硅质岩的化学成分、REE分布模式和氧同位素组成,提出本区硅质岩形成于受大气淡水影响的局限浅海环境。     

Evidence from Mössbauer spectroscopy for intense localization of metamorphic fluid flow during the formation of spaced cleavage

Arenites in the low-grade metasedimentary rocks of the Stirling-Barren Group, Western Australia, contain discrete spaced cleavages. These in part are pure white mica, but up to 30% of the cleavage zones consists of fine-grained quartz resulting from crystal plastic deformation. A modified Gresens analysis shows that the cleavage domains formed by a 50% volume loss associated primarily with SiO₂ and MgO concentration decreases of 58% and 37%, respectively, and a 12% decrease in Fe₂O₃. Mössbauer spectroscopy shows that the Fe²⁺: Fe³⁺ ratio from lithon to cleavage changed from 50:SO to 15:85. This substantial difference cannot be accounted for by loss of iron. The data are best explained by the focusing of a highly oxidizing fluid through the cleavage domains where volumetric fluid rock ratios around 150 to 1 are indicated. In this example advectic processes appear to be important in low-grade regional metamorphism.     

The evolution of fluorine-rich felsic magmas: source dichotomy, magmatic convergence and the origins of topaz granite

Topaz granite is alkali-feldspar granite that contains essential albite, quartz, K-feldspar, lithium-mica, and topaz. As a group topaz granites are characterized by their extreme enrichment in F (up to 3 wt%) and a wide variety of lithophile elements. They can be subdivided into a 'low-P₂O₅ subtype' (P₂O₅ < 0.1 wt%, Al₂O₃ < 14.5 wt%, SiO₂ > 73 wt%) and a 'high-P₂O₅ subtype' (P₂O₅ > 0.4 wt%, Al₂O₃ > 14.5 wt%, SiO₂ < 73 wt%), the δ¹⁸O values of which indicate a dichotomy of source rock: the low-P₂O₅ subtype (δ¹⁸O < 10‰) having a meta-igneous protolith and the high-P₂O₅ subtype (δ¹⁸O > 10 ‰) a source with a significant component of pelitic material. The unusually high F contents enhance the efficacy of melt segregation and crystal-melt fractionation and so facilitate extreme differentiation in topaz granite magmas. Very low melt volumes restrict the bulk composition of the partial melts regardless of the nature of the source; and extreme fractionation forces them along a path of magmatic convergence, to produce a group of granitic rocks with near-minimum compositions so enriched in a variety of lithophile elements (Li, Nb, Ta, Sn) that economic mineralization often results.     

Magma mixing processes in volcanic contexts, a thermodynamic approach with the examples of St Vincent Soufrière volcano, West lndies and Cerro Chiquito, Guatemala

Different modalities and thermodynamic conditions of magma mixing are compared: heterogeneity and chemistry, temperature and water pressure, volatile (= water) content and water solubility, density and viscosity. Such data are somewhat scarce in the literature.
Two case studies have been studied in detail to highlight this review. (1) St Vincent Soufrière (West Indies ). Magma mixing may have been a major process in triggering the 1979 explosive phase. All products (whole rock analyses) are basaltic andesite (52.3–56.2 wt% SiO₂) but dacitic (60–64 wt% SiO₂) glass occurs as micropumice or inclusions in crystals. (2) Cerro Chiquito (Guatemala). A Quaternary extinct dome consisting of dacite (62.5–67.1 wt% SiO₂) enclosing 20–30% in volume of basaltic andesitic (54.9–55.3 wt% SiO₂) enclaves.
The nearly thorough mixing process of case 1 strongly differs from the commingling process of case 2. The main phases may be mafic (case 1) or acidic (case 2). Thermodynamic parameters are calculated using classical geothermometers and formulae. The range of values and errors are estimated (60–80°C in case of temperature). In addition, other processes, such as crystallization of mineralo-gical phases and fluid vesiculation, may modify thermodynamical conditions. Difference of magma temperature ranges from about 60°C (case 1) up to about 130°C (case 2). Water concentrations of 2.5 wt% in mafic magmas and 4 wt% in acidic magmas, are always lower than the upper limit of water solubility at depths where contrasting magmas were mixed. The difference between the viscosities of the two magmas varies from one order of magnitude up to five.
Various mixing scenarios may occur, each of them being linked to different petrological processes including effusive or explosive volcanism as well as plutonism.     

Ijolite versus carbonatite as sources of fenitization

Ijolite-carbonatite complexes are ubiquitously surrounded of an aureole of metasomatically altered rocks. The process of alteration is termed fenitization and is generally caused by peralkaline fluids emanating from cooling alkaline, i.e. ijolite and carbonatite magmas. Ijolites and carbonatites normally occur together and attempts to determine the source of the fenitizing fluids may therefore lead to controversial, if not erroneous, conclusions.
Mineralogical and chemical data of fenites from Oldoinyo Lengai (Tanzania), Fen (Norway), and Alnö (Sweden) are reviewed in the present paper in order to reveal the main factors controlling the fenitization around ijolite and carbonatite. Despite the overall alkaline nature of the process, variables such as XCO₂ of the fluid, activity gradients of SiO₂, Al₂O₃ and CaO, FeO/MgO ratio, f O₂ and temperature gradients may differ, producing distinctive patterns of fenitization around the two magmatic sources. The ijolitic-type fluid has low XCO₂, high activities of alkalies, SiO₂ and Al₂O₃, and low activity of CaO. The f O₂ evolves along the hm-mt buffer conditions and the temperature falls gradually with distance from the magmatic source. The carbonatitic-type fluid has high XCO₂, high activities of alkalies and CaO, and low activities of SiO₂ and Al₂O₃. Temperatures and f O₂ are initially high, but decrease sharply with distance from the source. Moreover, the CO₂-rich fluid may complex and transport the REE.     

Archean cherts derived from chemical, biogenic and clastic sedimentation in a shallow restricted basin: examples from the Gorge Creek Group in the Pilbara Block

The >3·0 Ga chert sequence of the Gorge Creek Group is exposed at Ord Ranges about 50 km east of Port Hedland in the Pilbara Block. The chert sequence examined in this study is 15 m thick and consists of oxide-rich laminated chert, grey chert (silicified clastic rock), carbonaceous black chert and carbonate-rich laminated chert. Although the cherts have undergone postdepositional silica enrichment, such as cementation and metasomatic silicification, primary precipitation of silica at the site of deposition is indicated by abundant microstructures (mosaic and spherulitic structures). Other primary to early diagenetic components were carbonates, sulphates (gypsum and anhydrite) and organic matter. Although these mineral associations, on the whole, correspond to those of modern marine evaporites, they are different from modern equivalents with respect to abundant precipitation of amorphous silica and presumed primary precipitation of iron-carbonate (siderite). This feature is a possible manifestation of peculiar physicochemical conditions in the water mass from which the chemical sediments were precipitated; compared with modern ocean waters, the concentrations of Fe and Si were significantly higher and the pH value might have been lower. These conditions could be obtained by contributions of Fe- and Si-enriched hydrothermal solutions and continental run-off to the site of deposition. Grey cherts contain detrital quartz and altered Fe–Ti oxides and were formed in a period of input of terrigenous detrital materials. They are characterized by higher concentrations of TiO₂, Al₂O₃, Cr, Ni, Zn, Rb and Zr compared with the other types of chert and by very low (< 4) Al₂O₃/TiO₂ values. These features are attributed to the supply of terrigenous detrital materials that contain abundant Fe–Ti oxides (ilmenite and titanomagnetite) and fine TiO₂ particles. Such detrital materials might have been formed by extensive chemical alteration of source rocks and residual enrichment of Ti relative to Al.     

Sedimentary vs. diagenetic control on rhythmic calcareous successions (Pliensbachian of Asturias, Spain)

The diagenetic overprint in rhythmic hemipelagic successions can either enhance or change the original distribution of CaCO₃, but it is difficult to evaluate its effect because in most field examples, it is not possible to distinguish between sedimentary and diagenetic features. The rhythmic succession of the Pliensbachian of Asturias (Spain) shows alternation of bioclastic and laminated/burrowed intervals. The original content of carbonate brought by storms from shallow areas was larger in the bioclastic horizons. However, there is a widespread mismatch between facies alternations and the observed lithological rhythms. The diagenetic redistribution of CaCO₃ resulted in successive limestone–marl/clay couplets that do not match (either in number or in the location of the boundaries) the sedimentary cycles defined by facies alternations. We conclude that interpreting the limestone–marl rhythms as a direct response to primary changes is highly questionable, unless there is unequivocal proof of a sedimentary origin of the alternation of the two lithologies.     

Chemical criteria to identify the depositional environment of chert: general principles and applications

Many chert sequences are located at key stratigraphic intervals or in structurally allochthonous packages, and because such sequences provide critical information regarding the formation of sedimentary basins and tectonic orogens it is important to determine precisely their original depositional position. Chemical methods of delineating environments are becoming increasingly sophisticated and useful, in part because even slight post-depositional recrystallization may obscure primary rock textures. Most chemical criteria have developed only on a case-by-case and local basis, however, with limited broad applicability. Here I synthesize major, trace, and rare earth element (REE) data in bulk chert from 49 sequences from continental margin, pelagic, and ridge-proximal environments, ranging in age from Early Paleozoic to Neogene, and develop a set of depositional chemical criteria applicable to cherts regardless of age, diagenetic history, or tectonic region. The criteria allow for analytical limitations and diagenetic modifications (precluding use of Si, Ca, Mn, Mg, P, Sr, and Ba, and other elements) intrinsic to chert chemistry. Major element ratios, capitalizing on the affiliations of Al₂O₃ and TiO₂ to terrigenous matter and of Fe₂O₃ to metalliferous input, can distinguish continental margin from ridge-proximal sedimentation, but cannot uniquely identify pelagic deposition. Trace element data are too few to interpret rigorously, with substantial overlap occurring between depositional regimes. REE ratios (i.e., La_n/ Ce_n) allow the best resolution of all three depositional regimes and are independent of diagenetic modification. I present new diagrams illustrating the chemical depositional criteria, and apply them to several sequences described in the literature that have historically yielded problematic genetic interpretations.     

A thermodynamic model for titanium and ferric iron solution in biotite

Recent crystallographic data indicate that in biotite Ti orders preferentially onto the M2 octahedral site rather than onto the M1 site as assumed in previous solution models for K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (KFMASHTO) biotite. In view of these data, we reformulate and reparameterize former biotite solution models. Our reparameterization takes into account Fe–Mg order–disorder and ferric iron contents of natural biotite as well as both natural and experimental observations on biotite Ti-content over a wide range of physicochemical conditions. In comparison with previous biotite models, the new model reproduces the Ti-content and stability field of biotite as constrained by experiments with significantly better accuracy. The predictive power of the model is tested by comparison with petrologically well-characterized natural samples of SiO₂-saturated and SiO₂-undersaturated rocks that were not used in the parameterization. In all these tests, the reformulated model performs well.     

滇东南丘北地区上二叠统吴家坪阶硅质岩地球化学特征及地质意义

运用岩石学和地球化学的方法研究滇东南丘北地区晚二叠世吴家坪期硅质岩,取得以下主要认识:（1）滇东南丘北地区硅质岩SiO₂/Al₂O₃值除样品P29外均大于80,Al/（Al+Fe+Mn）为0.46~0.76,表明硅质岩主要为生物成因,受陆源碎屑影响较小;（2）硅质岩Ce/Ce*为0.517~0.858,La_N/Ce_N为1.12～1.80,经球粒陨石标准化的稀土模式则表现为轻稀土元素富集,且具有明显负Eu异常的右倾型,表明沉积环境为大陆边缘;（3）样品P29硅质岩Eu/Eu*为1.129,反映其受到一定程度热液的影响,可能受峨眉山地幔柱或（和）断裂的共同影响。     

Determination of Traces of Rare-Earth elements, Yttrium and Thorium in Several International Geological Reference Samples and Comparison Of Data with Other Published Values

Experimental results for most rare-earth elements, yttrium and thorium in several international geological reference samples are presented and compared with other published values, where available. The analytical method used to obtain these results involved a preliminary concentration of the rare-earths On milligram quantities of iron as carrier for atomic-absorption, flame-emission and spectrophotometric determinations, or on milligram quantities of Fe₂O₃, Al₂O₃ and SiO₂ as carriers for optical-emission spectrometric determination.     

Silica diagenesis in Eocene shallow-water platform carbonates, southern Pyrenees

Spectacularly developed lower Eocene chert in the Corones platform carbonates of the Spanish Pyrenees is concentrated within a restricted, brackish-water, laminated ostracod-rich facies, which also contains abundant sponge spicules. The chert occurs as nodular, bedded and mottled varieties, and four petrographic types of quartz are developed: microquartz; length-fast (LF) chalcedony; megaquartz; and microspheres. δ¹⁸O values of chert range from 29·6‰ to 30·9‰ (SMOW), which correspond to a broad isotope rank common for biogenic and diagenetic replacement cherts. Calcian dolomite crystals with high Fe and Na are disseminated within the microquartz and LF-chalcedony, but are absent from the megaquartz and host carbonate. The chert is closely associated with desiccation cracks and with interstratal dewatering structures. Load casts are silicified, and laminae rich in sponge spicules are convoluted. Early cracks related to dewatering are filled by microquartz and quartz cements. Ostracod shells within chert are locally fractured; those in the host carbonate are commonly flattened. Late fractures are filled by LF-chalcedony and megaquartz. There is much evidence for the dissolution of sponge spicules and their calcitization in the carbonate host rock. Silica for the Corones cherts was derived from sponges during early diagenesis and shallow burial. Early mechanical compaction and sediment dewatering played a major role in sponge spicule dissolution, migration of silica-rich fluids and the consequent precipitation of chert. Quartz cements continued to be precipitated into the burial environment.     

Heavy Mineral Placer Deposits of Ekakula Beach, Gahiramatha Coast, Orissa, India

Abstract: Ilmenite, hematite, garnet, monazite, zircon, rutile, magnetite, sillimanite, pyroxene and amphibole from the beach sands of Ekakula, Gahiramatha coast, Orissa, India are reported here for the first time. Their total concentration varies from 26. 4 to 100%. Ilmenite, monazite and zircon are between 100 and 300 um in size and are well rounded in shape. Ilmenite-hematite intergrowth is common. Ilmenite has 50. 02–54. 73% TiO₂, 42. 42–46. 90% FeO (total Fe) and small amounts of Al, Mn, Mg, Ca, Ba, Si, V, Cr, and Zn. The bulk samples contain 10. 63–41. 42 % TiO₂, 6. 15–26. 07 % FeO, 5. 86–16. 75 % Fe₂O₃, 7. 41–61. 74 % SiO₂, 1. 39–12. 83% A1₂O₃, 0. 32–4. 97% CaO, 0. 53–4. 24% P₂O₅, 0. 17–3. 27% MgO, 0. 15–2. 97% Na₂O, 0. 07–2. 34% K₂O, and 0. 05–0. 71% V₂O₅ together with appreciable amounts of La, Ce, Pr, Nd, Sm, Eu, Y, U, Th, Zr, and trace amounts of Pb, Zn, Cu, Ni, Co, and Cr. Khondalite, charnockite, calc-silicate granulite, leptynite, migmatite, gneiss, basic granulite and pegmatite of the Eastern Ghats appear to be the major source for the above heavy mineral assemblages. The samples are amenable to gravity and magnetic methods of beneficiation.     

Mo-related Adakitic Granitoids from Non-island-arc Setting: Jecheon Pluton of South Korea

Abstract. The Jecheon granitoids, having an elongated shape of NE-SW 27 km and NW-SE 13 km (190 km²), are composed mostly of magnetite-series hornblende-biotite granodiorite and biotite granite, which intrude into the Neoproterozoic metamor-phic and Paleozoic sedimentary rocks of the Ogcheon Belt. The granitoids have Triassic-Jurassic age of 202.7 ±1.9 Ma with very high ⁸⁷Sr/⁸⁶Sr initial ratio of 0.7140. The granodiorite has 63–69 % SiO₂, 15.1–17.3 % Al₂O₃, <1.6 % MgO, 6–15 ppm Y and Sr/Y ratios of 24–76, and is depleted in HREE. Biotite granite together, the Jecheon pluton has adakitic characteristics, which are unique in a continental tectonic setting. The granitoids may have been generated by partial melting of an older adakitic granitoid of I-type basement, or by separation of early crystallized garnet and hornblende from an anatectic melt.     

Very high 18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

Stable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (> 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar-Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ¹⁸O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ¹⁸O of 0%₀, calculation of temperature based on the maximum δ¹⁸O value of 31.4%₀ yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert-phosphate pairs. Diagenetically recrystallized microcrystalline chert-dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.     

Singularities in NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O)

A new petrogenetic grid is presented for the system NCKFMASH (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O), in which Ca is incorporated in garnet, and CaAlNa₋₁Si₋₁ solid solution is considered for both the plagioclase and white-mica structures. A compatibility diagram for plagioclase-bearing metapelitic assemblages within the NCKFMASH system also has been derived. A dominant feature of the NCKFMASH grid is the singularities and associated singular reactions which occur along plagioclase+margarite- and plagioclase+paragonite-bearing univariant equilibria. The singularities represent compositional coplanarities which occur in response to the CaAlNa₋₁Si₋₁ substitution occurring at different rates in plagioclase and white-mica. This is controlled by a fundamental difference in the mixing within the two mineral structures. The singularities give rise to a number of intriguing phase diagram features, including azeotropes. From the results presented here, it is predicted that the occurrence of margarite and paragonite in pelitic rocks is controlled by equilibria related to the singularities. The presence of these white-micas is strongly dependent upon bulk composition, and plagioclase-bearing, margarite/paragonite-free assemblages, typical of Barrovian-type terranes, are predicted for bulk compositions of Mg/(Mg+Fe)≈0.4 and Ca/(Ca+Na)≈0.4 at for example 5.5  kbar.     

重庆石柱二叠纪栖霞组地球化学特征及其环境意义

在对重庆市石柱县二叠纪栖霞组地层野外露头沉积特征研究的基础上,结合室内薄片观察,根据碳酸盐岩样品的元素含量、比值及相关性,探讨了栖霞组沉积地球化学特征与古环境意义.用于古环境分析的10件栖霞组样品SiO₂、MgO含量低,且两者具有较好的正相关性,Sr含量高,平均为1 751×10-6,Mn/Sr<0.1,Fe/Sr<1,δ18O_PDB>-10‰,表明成岩过程中样品几乎不受硅化及白云岩化作用的影响,未发生化学性质上的改变,可作为古环境分析的可靠对象.栖霞组碳酸盐岩CaO含量高,平均为53.81%,接近纯灰岩的理论化学组成,陆源元素Al₂O₃、TiO₂含量很低,平均值分别为0.17%与0.012%,表明其形成于陆源碎屑影响微弱的稳定古海洋环境.古气候指标MgO/CaO比值极低,平均为0.026,古盐度指标100×(MgO/Al₂O₃)比值高,平均为1 241,古水深判别指标Sr/Ba、1000×(Sr/Ca)比值高,平均值分别为286和4.54,指示了栖霞组碳酸盐岩为潮湿气候下的陆表海(远岸)沉积.古氧相地球化学指标U/Al、V/Al、Mo/Al、Cr/Al、Co/Al、V/(V+Ni)和U/Th特征一致表明栖霞组碳酸盐岩沉积于贫氧—缺氧环境.垂向上,陆源元素∑(Al₂O₃+TiO₂)含量逐渐降低,古水深指标Sr/Ba、1000×(Sr/Ca)比值逐渐增大,表明栖霞组沉积过程中离岸渐远、陆源供给逐步减少、水体持续加深.