Pan-African (intraplate and subduction-related?) metasomatism in the Fawakhir ophiolitic serpentinites, Central Eastern Desert of Egypt: mineralogical and geochemical evidences

Fawakhir serpentinites are the most western ophiolitic ultramafics relative to the Pan-African collision suture at the Qift-Quseir road in the Central Eastern Desert of Egypt. Their location is the basis for their selection in examining the possible contribution of the westerly dipping subducted oceanic slab-related melt/fluid with the intraplate granitic intrusion-related melt/fluid in the metasomatism of the Neoproterozoic ophiolitic serpentinites in the Eastern Desert. Non-residual mineralogy and geochemistry of serpentinites (SF1) far from the post-collision A2-type Fawakhir granitoids and those of serpentinites (SF2) in the vicinity of the granitoid pluton were investigated. The Fawakhir serpentinites are harzburgitic in composition and the Cr# (0.66–0.80) and Mg# (0.32–0.50) of their unaltered spinel cores are indicators for their forearc setting, where they were formed in the oceanic mantle wedge. Based on the spinel Cr# and the whole rock Yb–V bivariate, the melt extraction from the primitive mantle is in excess of 18% up to 24%. The HREE pattern of the SF1 serpentinites refers to the fractional type of melting. The formation of non-residual mineral phases particularly in SF2 samples (amphibole, biotite, apatite thorite, and monazite) and the enrichment of all serpentinites in trace incompatible elements refer to these two serpentinite groups having underwent modal metasomatism. It is suggested that viscous fluid/melt related to the Fawakhir granitoid emplacement metasomatized the SF2 serpentinites, causing a strong enrichment in LREE (display concave LREE; La_N/Sm_N?=?3.32–6.25 and U-type HREE; Gd_N/Yb_N?=?1.14–2.69) and a slight enrichment in Zr (12–16.62 ppm). All serpentinites are enriched in fluid-mobile elements by aqueous fluids, but the SF2 are more enriched in these elements. The spiked B compared to the other fluid-mobile elements (16.97–24.61 and 42.94–60.66?×?PM in SF1 and SF2 samples, respectively) suggests that these elements were added to the obducted ophiolitic Fawakhir serpentinites by the percolation of subduction-related fluids at shallow depths. The contribution of B from shallow continental crust-related fluids is debated. Hosting the Fawakhir serpentinites for the gold deposit at Fawakhir Mine implies a possible genetic relation between gold mineralizations hosted in the ultramafic rocks of the ANS and the processes of recycling of the subducted oceanic slab and the interaction with the mantle. Detailed stable and radiogenic isotopic analyses of the mineralization zones are required to address this question.     

Mantle metasomatism and generation of Early Permian ultramafic rocks in northern North China CratonSCIEI北大核心CSCD

华北克拉通北部高寺台晚古生代环状杂岩体是一套富单斜辉石的超镁铁质岩,是研究华北北部地幔碳酸盐化熔体交代作用的理想对象,同时也是华北克拉通北部晚古生代岩浆活动与深部碳循环联系的纽带。作者在岩体西部边缘识别出一套早二叠世早期富铁角闪单斜辉石岩,形成时代为297.8±3.4Ma。结合岩体东部辉石岩早泥盆世和早石炭世的成岩年龄,表明高寺台超镁铁质杂岩体可能是晚古生代多期岩浆侵位的产物。其次,高寺台环状超镁铁质杂岩体缺少大面积辉长岩和闪长岩、硫化物和Cu-Ni-PGE矿化,单斜辉石成分具有裂谷带堆晶岩矿物特征,角闪石也具有相对低的结晶温度(933~947℃)和压力(287~323MPa)及水含量(4.06%~5.10%),而且区域上尚未发现同期环状杂岩体和蛇绿岩,与岛弧拼贴到陆块后沿俯冲带延伸数万米侵位的阿拉斯加型环状杂岩体明显不同。另外,前人的研究显示,与铬铁矿中富镁单斜辉石包裹体平衡的苦橄质母岩浆显示出OIB的特征,具有强烈的重稀土分馏、富Nb和高Nb/La,异常高的Ca/Al和(La/Yb)N,低的Ti/Eu、Zr/Sm和Hf/Sm,这些特征都表明苦橄质母岩浆起源于石榴石相软流圈地幔,且源区遭受过显著的碳酸盐熔体交代。单斜辉石岩中单斜辉石及角闪石成分及其平衡岩浆成分记录了母岩浆与岩石圈地幔相互作用后的岩浆演化过程,也显示出比母岩浆弱的源区碳酸盐熔体交代印记。此外,岩浆房在地壳浅部经历了多期深部岩浆补给和富铁熔体贯入,同时带来热量和挥发分,触发晶粥体活化,促进了岩浆的上升运移。高寺台环状超镁铁质杂岩体具有板内成因特征,可能是陆内岩石圈伸展的产物。华北克拉通北部晚石炭世-早二叠世三个岩浆事件峰期很好地对应着全球三次升温和CO_(2)回升事件,岩石圈伸展能有效促进强烈的中酸性岩浆活动及起源于碳酸盐化地幔的基性岩浆释放出大量的CO_(2),它们可能是晚石炭末期-早二叠世早期全球大气CO_(2)回升的贡献者之一。     

Sodic metasomatism in a dacite weathering profile in Pinxiang, Guangxi, China

The Pinxiang weathering profile is well developed on Early Triassic dacite lavas of the Baisi Formation. At the top of the profile is developed a red clay zone which is characterized mineralogically by kaolinite, iron oxide minerals, quartz, and a small amount of illite, montmorillonite and vermiculite. In going downwards the red clay zone gives way to a saprolite zone in which plagioclase pseudomorphs have been well preserved although replaced by kaolinite. Beneath the saprolite zone is the saprock zone characterized by less weathering for dacite. At the bottom of the weathering profile is the parent material, dacite, which is composed mainly of plagioclase, quartz, K-feldspar and biotite which have been largely altered into chlorite owing to submarine extrusion of dacite lavas. Some layers in the weathering profile show obvious sodium enrichment and potassium depletion relative to others. In the Al2O3-(CaO* Na2O)-K2O triangular diagram, the weathering trends of these layers in the middle stage are remarkably deviated from normal ones. Both mineralogy and micro-morphology of these layers indicate such deviation resulted from sodic metasomatism of orthoclase.     

Relationship of the thermal conductivity of rocks in the Komsomol’sk ore district (Khabarovsk Territory) with minerageny and metasomatism

Statistical data are given on the thermal conductivity of rocks in the Komsomol’sk ore district (Khabarovsk Territory). The regularities of variations in the thermal-conductivity and thermal-inhomogeneity factors of the studied types of rocks are considered in relation to their mineral composition, texture, structure, and degree of metasomatic alteration. Thermophysical sections of the studied mineralized zones are presented. The conclusion is substantiated that the thermal properties of rocks are highly informative and detailed thermophysical studies of sections might be useful for their lithological subdivision.© 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.     

Combined U/Pb and (U–Th)/He geochronometry of basalt maars in Western Carpathians: implications for age of intraplate volcanism and origin of zircon metasomatism

The age of intraplate volcanism in northern Pannonian Basin of Carpathians is revisited using a combination of zircon U/Pb, zircon (U–Th)/He and apatite (U–Th)/He dating techniques, complemented by electron microprobe (EMP) characterisation of dated minerals. A total of six maar structures and diatremes in the South-Slovakian Volcanic Field (SSVF) were dated and the obtained new ages yielded the following key findings: Two isolated maars in SE part indirectly dated by geomorphologic constraints to Late Pleistocene are actually of Pliocene (2.8 ± 0.2 Ma) and Late Miocene (5.5 ± 0.6 Ma) ages. In contrast, two maars in NW part of the study area are of Late Pliocene age (4.1 ± 0.4 and 5.2–5.4 Ma), younger than the Late Miocene age (~6.5 Ma) inferred previously from K/Ar data on the proximal basaltic lava flows. These maars therefore belong to the second volcanic phase that was previously identified only in SE part of the SSVF. In the light of the new geochronologic data, it seems likely that the Pliocene phreatomagmatic eruptions may have occurred along extension-related, NW- and NE-trending orthogonal faults. EMP analyses and imaging revealed an extensive syn- and post-growth metasomatic replacement by dissolution-reprecipitation in the majority of zircons. Abundant silicate melt inclusions in porous metasomatised parts of the zircons are diagnostic of magmatic rather than hydrothermal metasomatism. Consistent ages of the metasomatised and non-metasomatised zones do not indicate disturbance of the U–Pb system during the metasomatism. Enrichment in U and Th loss in the metasomatised zircons are diagnostic of an increasing oxygen fugacity triggered by degassing of the volatile residual melt during the final stages of alkali basalt fractionation. Rare zircon-to-baddeleyite transformation was probably connected with lowered silica activity in carbonated basaltic magmas in south-eastern part of the study area.     

Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province,Brazil: genesis and constraints on mantle metasomatism

Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO₂; 5.2–6.7 wt.% ZrO₂) occurs in peridotite xenoliths rimming chromite (～50 wt.% Cr₂O₃) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.     

Modal metasomatism in the Kaapvaal craton lithosphere: constraints on timing and genesis from U–Pb zircon dating of metasomatized peridotites and MARID-type xenoliths

Modal metasomatism in the Kaapvaal craton lithosphere is well documented in upper mantle xenoliths sampled by both group I (mainly late Cretaceous) and group II (mainly early Cretaceous to late Jurassic) kimberlites in the Kimberley area. The metasomatic style is characterized by introduction of K, H and large ion lithophile/high field strength (LIL/HFS) elements into the lithospheric mantle leading to the crystallization of hydrous potassic phases such as phlogopite and/or K-amphibole. Textures indicate that the hydrous phases either replace pre-existing assemblages in peridotites, forming the metasomatized peridotite suite (phlogopite–K-richterite–peridotites: PKPs) or crystallize from K-rich melts, forming the mica–amphibole–rutile–ilmenite–diopside (MARID) suite of xenoliths. These K-rich assemblages become potential low melting source components for alkaline incompatible trace element enriched magmas. The timing of metasomatism and its temporal and possible genetic relation to kimberlite magmatism is poorly constrained because of the rarity of phases in the metasomatic assemblages suitable for precise dating. Here we present precise sensitive high resolution ion microprobe (SHRIMP) U–Pb formation ages of 88 ± 2 (1σ=1 standard deviation) and 82 ± 3 Ma data for zircons from a K-richterite–phlogopite-bearing metasomatized peridotite (PKP) and a MARID xenolith respectively, sampled by a group I kimberlite. Both average PKP and MARID zircon ages are indistinguishable from emplacement ages of group I kimberlites in the Kimberley area dated at 83 ± 4 (2σ) and 84 ± 0.9 Ma. One exceptionally old age spot of 102 ± 5 Ma from a PKP zircon provides evidence for modal metasomatism predating group I kimberlite emplacement by several millions of years with minor resetting of the U–Pb isotopic system of most analyzed PKP zircons to a group I emplacement age. Detailed textural and mineral chemical analysis, including high energy X-ray mapping and analysis of fluid inclusion daughter crystals, indicates a complex reaction history for both PKPs and MARIDs. U–Pb zircon ages from this study combined with literature data and experimentally derived models for MARID formation are used to suggest that MARID-formation is concurrent and genetically related to both group I and II kimberlite magmatism in the Kimberley area. MARID and PKP zircon ages are also consistent with the idea first proposed by Dawson and Smith (Geochim Cosmochim Acta 41: 309–323, 1977) that metasomatized peridotites may form from interaction of hydrous fluids expelled by solidifying MARID-type melts with peridotitic wall rocks. Received: 13 December 1999 / Accepted: 13 April 2000     

Mineralogy and Mode of Occurrence of Gold, Silver and Bismuth of the Caijiaying Lead-Zine-Silver Deposit, Hebei Province

The deposit under study is a hydrothermal filling-metasomatic vein type lead-zinc-silver deposit, in whichgold and silver can be recovered as by-products. These metals mainly occur as microgranular native gold,electrum, stephanite, acanthite, pyraragyrite, freibergite, and native silver. Gold minerals tend to be associatedwith galenobismutite, native bismuth and unnamed Bi_2Te. They are either enclosed in pyrite, marmatite,iron-bearing sphalerite and galens or fill the microfissures of these minerals. Silver minerals usually occur incleavages or fissures of galena, marmatite and pyrite, but are not associated with gold and bismuth minerals.Gold and silver mineralizations occurred later than lead and zinc, while the silver mineralization was precededby that of gold.     

Geochemistry of О, Н, C,S, and Sr isotopes in the water and sediments of the Aral basin

The paper presents original authors' data on the O, H, C, S, and Sr isotopic composition of water and sediments from the basins into which the Aral Sea split after its catastrophic shoaling: Chernyshev Bay (CB), the basin of the Great Aral in the north, Lake Tshchebas (LT), and Minor Sea (MS). The data indicate that the δ¹⁸О, δD, δ¹³C, and δ³⁴S of the water correlate with the mineralization (S) of the basins (as of 2014): for CB, S = 135.6‰, δ¹⁸О = 4.8 ± 0.1‰, δD = 5 ± 2‰, δ¹³C (dissolved inorganic carbon, DIC) = 3.5 ± 0.1‰, δ³⁴S = 14.5‰; for LT, S = 83.8‰, δ¹⁸О = 2.0 ± 0.1‰, δD =–13.5 ± 1.5‰, δ¹³C = 2.0 ± 0.1‰, δ³⁴S = 14.2‰; and for MS, S = 9.2‰, δ¹⁸О =–2.0 ± 0.1‰, δD =–29 ± 1‰, δ¹³C =–0.5 ± 0.5‰, δ³⁴S = 13.1‰. The oxygen and hydrogen isotopic composition of the groundwaters are similar to those in MS and principally different from the artesian waters fed by atmospheric precipitation. The mineralization, δ¹³С, and δ³⁴S of the groundwaters broadly vary, reflecting interaction with the host rocks. The average δ¹³С values of the shell and detrital carbonates sampled at the modern dried off zones of the basins are similar: 0.8 ± 0.8‰ for CB, 0.8 ± 1.4‰ for LT, and –0.4 ± 0.3‰ for MS. The oxygen isotopic composition of the carbonates varies much more broadly, and the average values are as follows: 34.2 ± 0.2‰ for CB, 32.0 ± 2.2‰ for LT, and 28.2 ± 0.9‰ for MS. These values correlate with the δ¹⁸O of the water of the corresponding basins. The carbonate cement of the Late Eocene sandstone of the Chengan Formation, which makes up the wave-cut terrace at CB, has anomalously low δ¹³С up to –38.5‰, suggesting origin near a submarine methane seep. The δ³⁴S of the mirabilite and gypsum (11.0 to 16.6‰) from the bottom sediments and young dried off zone also decrease from CB to MS in response to increasing content of sulfates brought by the Syr-Darya River (δ³⁴S = 9.1 to 9.9‰) and weakening sulfate reduction. The ⁸⁷Sr/⁸⁶Sr ratio in the water and carbonates of the Aral basins do not differ, within the analytical error, and is 0.70914 ± 0.00003 on average. This value indicate that the dominant Sr source of the Aral Sea is Mesozoic–Cenozoic carbonate rocks. The Rb–Sr systems of the silicate component of the bottom silt (which is likely dominated by eolian sediments) of MS and LT plot on the Т = 160 ± 5 Ma, I₀ = 0.7091 ± 0.0001, pseudochron. The Rb–Sr systems of CB are less ordered, and the silt is likely a mixture of eolian and alluvial sediments.     

Geochemistry and Sedimentology of Jhill Limestone of Gaj Formation,in Cape Monze and Adjoining Area,Karachi

Among several lithostratigraphic subdivisions of the Gaj Formation of Miocene age, the Jhill limestone is entirely different with respect to its colour, texture and structures. This limestone unit has been evaluated to elaborate its geochemical and sedimentological characteristics. The distribution of various elements in the acid-soluble fraction has been studied in order to determine their mineralogy, sedimentary environment, facies and diagenesis. Mineralogy, recrystallization and other diagenetic changes are the main factors affecting the distribution of trace elements and their mutual relationships in the limestones. Samples of the Jhill limestone show depletion in large-sized ions (Sr, Pb & K) and also in the ions that are not compatible with calcite space group. Elements (Fe, Mn, Zn, Cu & Co) having distribution coefficient (D) above unity for natural calcites, are more enriched. Microscopic and X-ray studies revealed nearly complete conversion of aragonite into stable low-Mg calcite. An attempt has also been made to verify the reefal conditions for these limestones on the basis of geochemical studies. The plots of Sr and other facies-indicator elements show that the majority of the beds belong to forereef flank facies with some algal banks. Low Mg/Ca and Sr/Ca ratios suggest that a phreatic diagenetic environment prevailed after the deposition. High concentrations of Cr, Ni, and Co in the Jhill limestone show a positive correlation with a higher amount of insoluble residue, which reflects a relatively high rate of influx of terrigenous material.     

Interdiffusion of Na,K,and Ca Between Granitic Melts and NaCl Liquid

This study is aimed at determining the diffusion coefficient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl system, in which the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 – 1400°C, pressures of 0.001 × 10⁸ – 2 × 10⁸ Pa, and initial H₂O contents of 0 wt% –6.9 wt% in the granitic melts. The diffusion coefficients of Fe and Mg were difficult to resolve. In all experiments a NaCl melt was present as well. In the absence of H₂O, the diffusion of net-work modifiers follows an Arrhanious equation at 1 × 10⁵ Pa: lgD_ca=−3. 88−5140/T, lgD_k =−3. 79−4040/T, and lgD_Na, =−4.99−3350/T, where D is in cm² /s andT is in K. The diffusion coefficients of Ca, Na, K, and Fe increase non-linearly with increasing H₂O content in the melt. The presence of about 2 wt% H₂O m the melt will lead to a dramatical increase in diffusivity, but higher H₂O content has only a minor effect. This change is probably the result of a change in the melt structure when H₂O is present. The diffusion coefficients measured in this study are significantly different from those in previous works. This may be understood in terms of the “transient two-liquid equilibrium” theory. Element interdiffusion depends not only on its concentration, but also on its activity co-efficient gradient, which is reflected by the distribution coefficient, of the two contacting melts.     

Petrologic and Geochemical Characteristics and Origin of Gusui Cherts,Guangdong Province,China

The characteristic structures of the Precambrian cherts from the Gusui section, Guangdong ,Chi-na, include bedded structure ,laminated structure ,massive structure and pseudobrecciated structure.The chert is characterized by consistently low abundance of TiO2,Al2O3 and most trace elements.Howevver ,it is enriched in Ba,As,Sb,Hg and Se.In Al-Fe-Mn ternary diagrams,it falls into the “hydrothermal field“ .Correspondence analysis and factor analysis show that many elements show up in the factor that represents the leaching of country rocks by hydrothermal solutions,and are the very characteristic element association fo the geochemically anomalous South China basement.Petrologic and geochemical evidence suggests a hydrothermal origin for the chert.The chert may have been formed in a Precambrian fift or an extension zone developed within the Yunkai marginal geosyncline, with a fault system linking it to an unknown heat source at depth.     

Zinc,Copper, and Lead Geochemistry of Oceanic Igneous Rocks—Ridges,Islands, and Arcs

Variations in the abundances of Zn, Cu, and Pb are found to be useful in identifying tectonic regimes and separating oceanisland basalts into enriched- and depleted-source categories. The average Zn, Cu, and Pb contents of normal mid-ocean ridge basalts (N-MORB) are 84, 70, and 0.35 ppm, respectively. Differences in average Zn contents for various ridges reflect more the varying degrees of differentiation than variations of Zn content in the source rocks. At a Mg# of 70, or Mg#₇₀, which is taken to represent primitive MORB, many MORB sequences converge at a Zn content of 58 ± 6 ppm, which is close to the value for primitive mantle (50 ppm) and ordinary chondrites (～55 ppm). Values of 0.1 to 0.15 ppm Pb in MORB at Mg#₇₀, best defined at the superfast-spreading Southern East Pacific Rise, are similar to estimates of Pb in the primitive mantle (0.12 to 0.18 ppm). They also are near the lower end of the range for ordinary chondrites. The very slow spreading Southwest Indian Ocean Ridge has a sequence with higher Pb contents, in addition to a more normal sequence, which has a visual best value of 0.4 ppm Pb at Mg#₇₀. With the exception of the Walvis Ridge, Zn and Cu appear to be little affected by proximity to hotspots (i.e., E-MORB); however, Pb contents are higher and average about 0.6 ppm.

Both Zn and Pb in MORB are incompatible elements (i.e., favor the melt), but Cu is a compatible element. At Mg#₇₀, there is the suggestion of a value of 100 ppm for Cu, with lower values possibly representing partial removal of sulfides and their associated Cu from the source. Nonetheless, Cu contents of primitive MORB tend to be much higher than even high estimates for the primitive mantle (28 ppm), and are closer to ordinary chondrites (～90 ppm). Therefore, Zn, Cu, and Pb all approximate chondritic values in the primitive MORB melt.

Average contents of Zn, Cu, and Pb in oceanic island basalts (OIB) are 115, 62, and 3.2 ppm, respectively. At Mg#₇₀, values of Zn and Cu are similar to the respective averages for OIB, with Zn higher and Cu lower than MORB. At a Mg# of ～40, however, OIB and MORB tend to have similar Zn contents. With further differentiation, OIB trachytes can contain >200 ppm Zn. Unlike MORB, OIB can differentiate to high Cu contents of 200 ppm at Mg#s of 40 to 60. In contrast to Zn and Cu, Pb regresses to a value of 0.83 ppm at Mg#₇₀ for Hawaiian and Reunion volcanics, which is much less than the average value for Pb in OIB volcanics, but higher than for MORB.

Average Zn, Cu, and Pb contents of magmatic-arc basalts are 77, 108, and 1.9 ppm, respectively. In basalts, Zn tends to be incompatible, but a dual incompatible and compatible behavior can occur at high SiO₂ contents. Dacites may average near 55 ppm Zn, but peralkalic rhyolite can contain >300 ppm Zn. A dual compatible and incompatible nature occurs for Cu. Most common, particularly in submarine volcanics, is a compatible trend, with a Cu content of around 80 ppm at a Mg# of 60, which decreases to less than 40 ppm at a Mg# of 30. The incompatible trend of increasing Cu can achieve >200 ppm at a Mg# of 30, leaving a gap approaching 100 ppm at that Mg#. The gap is less obvious on a plot of Cu vs. SiO₂, but is still there. The compatible trend is proposed to result from sulfur-saturated magmas, whereas the incompatible trend is believed to result from sulfur-deficient magmas. Support for this hypothesis is found in sparse sulfur-isotope data. Zn and Cu both can be incompatible over an extended range of Mg#s or silica content. When Zn and Cu are both compatible, Cu decreases more than twice as rapidly as Zn.

Primitive magmas at Mg#₇₀ average about 50 ppm Zn for submarine Mariana arc basalts and 58 ppm for forearc boninites, contents close to MORB values. Mariana arc basalts have a Zn content of ～45 ppm estimated at Mg#₇₀. Cu varies more widely than Zn in primitive magmas, being about 50 ppm Cu for Mariana Islands volcanics and 120 ppm for Kermadec Islands volcanics, a range broadly around MORB values. Average Pb contents are 1.9 ppm for island-arc tholeiites, 5.6 ppm for high-Al basalt, and 3.2 ppm for alkali basalt with average boninite of approximately 1.8 ppm. Back-arc-basin basalts in the deepest parts of the Mariana trough have Pb contents of 0.45 ppm, but more shallow parts may exceed 1.0 ppm Pb. Although the lower contents are similar to MORB values, the ²⁰⁸Pb/²⁰⁴Pb values are greater than Pacific Ocean MORB. At Mg#₇₀ for rocks from the Tonga and Kermadec island arcs, the Pb content is about 0.1 ppm, similar to MORB.     

Platiniferous gold–tourmaline aggregates in the gold–palladium belt of Minas Gerais,Brazil: implications for regional boron metasomatism

Mineralogy and Petrology - The platiniferous gold–palladium belt of Minas Gerais, Brazil, forms an approximately 240-km-long, roughly north–south-trending domain that includes numerous...     

Geochemical and Mineralogical Characteristics of Pleistocene Lignites and Associated Sediments of Marathousa Coal Field,Central Peloponnese,Greece

The mineralogy and geochemistry data are presented for thirty-seven shales,four concretions,two carbonate sediments and seven lignites from the Marathousa coal field of the Megalopolis Basin in Greece.The argillaceous rocks consist of chlorite,illite,kaolinte,albite,quartz.opal-A,calcite and dolomite;the concretions of aragonite,gypsum and pyrite;and the carbonate rocks of calcite,quartz and illite.The mineral matter in the lignites consists of gypsum,quartz,albite,chlorite,illite,opal-A,dolomite,pyrite,and rarely calcite and kaolinite Athree-factor model explains the total variaition of major and trace elements in the argillaceous sediments.The first factor is an aluminosilicate factor and involves the following elements:Al,Si,Mg,Na,K,Ti,Mn,Nb,Y,Rb,Zn,Cu,Ni,Cr,Nband V,associated with chlorite,albite and illite.The second factor involves the elements Ca,Sr,Ba,Znand Sc and is related to carbonate lithology and mainly the carbonate concretions with gypsum.The third factor involves Fe and Ce with a weak association with Mn.The diagenesis of the Marathousa sediments and lignites was not very advanced as indicated by (a) the total thickness of the sequence (500m),(b) the presence of biogenic silica(opal-A) and (c) the age of the deposit(Pleistocene).FOr these reasons the rpresence of chlorite,illite and kaolinite in the sediments and lignite is due not to diagenetic reactions but to weathering of the flysch and metamorphic rocks at the edges of the Megalopolis Basin and transport of the weathering products(illite,chlorite,kaolinite)into the basin of deposition.The diagenetic minerals of the Marathousa sequence include pyrite,gypsum,dolomite and aragonite.     

Nature and timing of magma interactions before,during, and after the caldera-forming eruption of Volcán Ceboruco,Mexico

Volcán Ceboruco, Mexico, erupted ~1,000 years ago, producing the Jala pumice and forming a ~4-km-wide caldera. During that eruption, 2.8 to 3.5 km³ of rhyodacite (~70 wt% SiO₂) magma and 0.2 to 0.5 km³ of mixed dacite (~67 wt% SiO₂) magma were tapped and deposited as the Jala pumice. Subsequently, the caldera was partially filled by extrusion of the Dos Equis dome, a low-silica (~64 wt% SiO₂) dacite dome with a volume of ~1.3 km³. Petrographic evidence indicates that the Jala dacite and Dos Equis dacite originated largely through the mixing of three end-member magmas: (1) rhyodacite magma, (2) dacite magma, and (3) mafic magma. Linear least-squares modeling and detailed modal analysis indicate that the Jala dacite is predominantly a bimodal mixture of rhyodacite and dacite with a small additional mafic component, whereas the Dos Equis dacite is composed of mostly dacite mixed with subordinate amounts of rhyodacite and mafic magma. According to Fe–Ti oxide geothermometry, before the caldera-forming eruption the rhyodacite last equilibrated at ~865 °C, whereas the dacite was originally at ~890 °C but was heated to ~960 °C by intrusion of mafic magma as hot as ~1,030 °C. Zoning profiles in plagioclase and/or magnetite phenocrysts indicate that mixing between mafic and dacite magma occurred ~34–47 days prior to eruption, whereas subsequent mixing between rhyodacite and dacite magmas occurred only 1–4 days prior to eruption. Following the caldera-forming eruption, continued inputs of mafic magma led to effusion of the Dos Equis dome dacite. In this case, timing between mixing and eruption is estimated at ~93–185 days based on the thickness of plagioclase overgrowth rims.Editorial responsibility: T.L. Grove