The petrology and tectonic setting of Quaternary—Recent volcanic centres of Lombok and Sumbawa,Sunda arc

In the Sunda arc, only the Bali—Lombok—Sumbawa sector is apparently flanked both north and south by oceanic crust. South of Lombok Island the oceanic crust is probably of Early Cretaceous or Late Jurassic age, whereas the oldest rocks known from Lombok and Sumbawa islands are the Lower Miocene to Pliocene sediments and volcanics of the basement beneath the Quaternary—Recent volcanic centres.Three large active volcanoes form the northern parts of Lombok and Sumbawa. The volcanic rocks of Rindjani on Lombok belong to a basalt—andesite—dacite association, rich in plagioclase and hy- and Q-normative. East of Lombok, the volcanic rocks of Tambora and Sangeang Api on Sumbawa belong to a potassic ne—trachybasalt—trachy-andesite association. All three volcanoes occur only 150–190 km above the active north-dipping Benioff zone.Extinct Quaternary centres occur south of the active volcanoes on Sumbawa. Two of these centres, Soromundi and Sangenges, erupted markedly ne- and lc-normative leucitites together with andesites, dacites and trachybasalts.The volcanic composition—space—time relations in the Lombok—Sumbawa sector of the Sunda arc are not in accordance with the generalized island-arc schemata. Conventionally, potassic ne-mnormative island-arc associations are supposed to occur over the deep part of the Benioff zones, far from the trenches of mature island arcs. The SiO₂|K₂O relations of the Rindjani association are reasonably appropriate for a volcano overlying intermediate Benioff-zone depths, but both the Tambora and the Sangeang Api associations are far more potassic than would be predicted by generalized schemata, and also occur in a relatively young arc sector that apparently has developed only since Miocene time.Basalts, trachybasalts and leucitites from the Lombok—Sumbawa sector have been compared: at similar MgO contents and Mg/(Mg+Fe), the progression from hy- and Q-normative to ne- and lc-normative magmas is not marked by significant enrichment in TiO₂, Na₂O, Zr, Nb and P, but is accompanied by a substantial increase in K₂O, Rb, Sr and LREE, by increasing $\text{K}{}_2\text{O}\text{Na}{}_2\text{O}$ and by decreasing K/Rb.${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from Rindjani (0.70386–0.70402) and Tambora (0.70385–0.70389) are very similar and among the lowest for the Sunda arc, but from Sangeang Api (0.70460–0.70500) are significantly higher and more variable in spite of the similar tectonic setting and petrological affinities. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of leucities tend to be higher (0.70488–0.70529).The petrogenesis of the volcanic associations of Lombok and Sumbawa cannot be readily explained. Although even the leucitites display the poverty in TiO₂ that generally characterizes volcanics from simple island-arc tectonic settings, there is very obvious uncoupling within the “incompatible elements”: enrichment in the LIL group (K, Rb, Sr but not Na) is not accompanied by similar behaviour in the group of small highly-charged ions (Ti, Zr, Nb, P). It has proved impossible to model this behaviour without invoking inhomogeneities in the source regions, both in mineralogy and in chemical composition. Similar uncoupling within the incompatible elements has also been reported from basalt groups from the Mid-Atlantic Ridge, may also occur in the Birunga province, and might not arise from processes unique to the island-arc environment.We suggest that a LIL-rich component is being progressively added to the source regions. This component could be incorporated by the crystallization of additional phases such as phlogopite or paragasite. If this component occurs deep within the mantle, it might gain passage to shallower regions either by percolating up the downgoing slab to yield the familiar arc magma zonation, or up substantial cross-arc fractures.     

The carbon isotope biogeochemistry of the individual hydrocarbons in bat guano and the ecology of the insectivorous bats in the region of Carlsbad,New Mexico

The structures and ¹³C contents of individual alkanes extracted from bat guano found in the Carlsbad region of New Mexico can be related to both the photosynthetic pathways of the local plants and the feeding habits of the insects that support the bats.Carbon isotopic analyses show that equivalent numbers of C₃ and C₄ native plant species occupy the Pecos River Valley, a very significant feeding area for the Carlsbad bats. During the seasons when bats frequent the area, the agricultural crops consist principally of alfalfa and cotton, both C₃ plants.The molecular composition of the bat guano hydrocarbons is fully consistent with an insect origin. Two isotopically distinct groups of insect branched alkanes were discerned. These two groups of alkanes derived from two chemotaxonomically distinct populations of insects possessing distinctly different feeding habits. It is possible that one population grazes predominantly on crops whereas the other population prefers native vegetation. This and other isotopic evidence suggests that crop pests constitute a major percentage of the bats' diet.Because the guano sample was less than 40 years old, this material reflects the present day plant community in the Pecos River Valley. Future studies of more ancient guano deposits should reveal a measurable influence of both natural and man-induced vegetative changes with time upon the ¹³C content of the bat guano hydrocarbons.     

Chemistry of micas from kimberlites and xenoliths—II. Primary- and secondary-textured micas from peridotite xenoliths

Micas from coarse granular Iherzolites in S. African kimberlites may be separated into two groups; those showing primary textural relationships with coexisting silicates and those with secondary, alteration relationships with other silicates. Primary-textured micas form a tight cluster with a mean composition from 10 coarse garnet lherzolites of: SiO₂ 41.0, TiO₂ 0.18, Al₂O₃ 13.5, Cr₂O₃ 0.82, total Fe as FeO 2.60, MnO 0.02, MgO 26.0, NiO 0.22, CaO 0.01, BaO 0.29, Na₂O 0.31, K₂O 10.0, Rb₂O 0.028, Cl 0.08, F 0.43 wt%. Primary-textured micas in aggregates with clinopyroxene have higher TiO₂ and four specimens which look similar to the primary group but have textural ambiguities have still higher TiO₂Micas with secondary textures have wide ranges of composition which may be correlated with details of the textural parageneses. Micas from kelyphitic rims around garnets tend to be Cr-rich while those from veinlets are Cr-poor. Both groups tend to have higher FeO and TiO₂ than the primary group. Micas produced by alteration of, or filling veinlets through, orthopyroxene have a wide compositional range which overlaps that of the primary-textured micas, especially for harzburgite specimens.The primary-textured micas show a positive correlation with coexisting pyroxenes for MgO/(MgO + FeO) and TiO₂, but not for Cr₂O₃. Secondary-textured micas do not show correlations with coexisting pyroxenes for any elements.The ‘primary-metasomatic’ micas described by Harte and Gurney (1975) and metasomatic and other micas described by Boettcher et al. (1979) and Boettcher and O'neil (1979) are richer in FeO and TiO₂ than the present primary-textured micas, and are attributed to crystallization from fractionated fluids.     

Sur la présence de nickel dans un gisement de bauxite près de Mégare

Résumé L'exploitation récente d'un gisement de bauxite près de Mégare (Grèce) a fait apparaître sur les calcaires de base une concentration verte d'oxyde de nickel relativement élevée. La présence dans le gisement d'argiles rouges, renfermant de fortes proportions d'oxyde de fer avec des teneurs relativement faibles en alumine et des traces d'oxyde de nickel, induit à penser qu'il s'agit d'un produit latéritique dérivé d'une roche ultrabasique et transporté ultérieurement sur la région calcaire karstifiée avec ses remplissages bauxitiques. L'étude se termine par une hypothèse sur la genèse de ce gîte singulier.

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The recent mining of a bauxite deposit near Megara (Greece) has shown on the footwall limestone a relatively high concentration of a green nickel oxide. The occurrence in the same deposit of red clays with a rather high percentage of iron oxide and a low content of alumina, together with some traces of nickel, induces to think that this material is composed of a lateritic soil derived from an ultrabasic rock and subsequently transported on karstified limestone with its bauxitic fillings. — The study ends with an hypothesis on the genesis of this unusual deposit.

     

Sulfur isotopic evidence for the supergene origin of alunite deposits,Tolfa district,Italy

Sulfur isotope abundance data for minerals of the historically important alunite deposits of the Tolfa district indicate (1) compositional similarity between sulfates and sulfides (mean S³⁴ values of +5.5⁴ and +6.3⁴ permil respectively), (2) negligible fractionation factors (0.995 to 1.004) for sulfate-sulfide mineral pairs, (3) two distinct sulfide isotopic populations characterized by pyrite (+8.3⁴ permil) and marcasite (–1.0⁵ permil) and with associated alunite displaying a covariant isotopic shift, and (4) possible contamination of the hydrothermal system by evaporite sulfate (+25.1 permil) because hypogene pyrite, galena, and cinnabar are enriched in S³⁴ (+6.1 to +10.3 permil). We suggest that alunite, barite, and possibly marcasite are of supergene origin and that the sulfate-sulfur was derived from the surficial oxidation of sulfides. Our conclusion is compatible with the geologic evidence and confirms the supergene hypothesis for the genesis of Tolfa alunites proposed by Fuchs and De Launay (1893) and De Launay (1906; 1913).

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Zusammenfassung Die Schwefel-Isotopen-Häufigkeiten von Mineralen der historisch wichtigen Alunit-Vorkommen des Gebietes um Tolfa zeigen (1) eine große Ähnlichkeit der Isotopen-Zusammensetzungen zwischen Sulfaten und Sulfiden (Mittelwerte für S³⁴ von +5,5₄ bzw. +6,3₄), (2) höchstens sehr geringe Fraktionierungs-Faktoren (0,995 bis 1,004) für Mineralpaare von Sulfat und Sulfid, (3) zwei verschiedene Gruppen von Isotopenverhältnissen bei den Sulfiden, und zwar für Pyrit (+8,3₄) und Markasit (–1,0₅), wobei der jeweils begleitende Alunit ähnliche Unterschiede aufweist, und (4) eine mögliche Beeinflussung des hydrothermalen Systems durch Sulfat aus Salzlagerstätten (+25,1), weil hypogener Pyrit, Galenit und Zinnober eine Anreicherung von S³⁴ zeigen (+6,1 bis +10,3). Wir nehmen an, daß Alunit, Baryt und möglicherweise auch Markasit supergen gebildet wurden, und daß der Sulfat-Schwefel durch die Oxydation von Sulfiden an der Erdoberfläche entstand. Unsere Schlußfolgerung ist mit den geologischen Befunden vereinbar und bestätigt die supergene Hypothese für die Bildung der Alunitvorkommen von Tolfa, wie sie von Fuchs und De Launay (1893) und De Launay (1906; 1913) vertreten wurde.

     

Electron-probe check of fe-distribution in sphalerite grains of the Nagybörzsöny hydrothermal ore deposits,Hungary

Micron-scale iron-distribution patterns in sphalerite grains derived from two contrasting mineral assemblages of the Nagybörzsöny hydrothermal ore deposits (high temperature stockwork and low temperature veins), were determined with electron microprobe, pointcounter and continuous chart-recorder measurements along radial profiles. A significant oscillation of iron-content ranging from 8.1 to 18 per cent for stockwork and 4.3 to 15.8 per cent for veins took place during crystal growth. Homogeneous crystal cores are characteristic for vein-filling, while replacement occurring in deeper levels of the stockwork results in mosaic cores. Frequent oscillation of iron content is relevant to top-sections of the ore bodies, copious supply being recorded in recurring shells, whereas scanty, unilateral ones produce crescents arranged in onion-like structure.

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Zusammenfassung Bestimmungen der Eisenverteilung wurden an Zinkblendekörnern aus zwei verschiedenen Mineralparagenesen (Hochtemperatur-Stockwerk und Gangfüllungen niederiger Bildungstemperatur) der hydrothermalen Erzlagerstätte Nagybörzsöny im Mikronbereich durch Elektronen-Mikrosonde-Messungen (Punktzählung und kontinuierliche Band-Registrierung) entlang radialer Profile vorgenommen. Bedeutende Schwankungen des Eisengehaltes (8,1–18% für das Stockwerk und 4,3–15,8% für die Gänge) wurden registriert. Homogene Kristallkerne kennzeichnen die Gangfüllungen, wogegen in tieferen Horizonten des Stockwerks Verdrängungen zur Bildung von Kristallen mit Mosaik-Kernen führen. Allgemein ist in den Gipfel-Partien der Erzkörper Fluktuation des Eisengehaltes häufig. Gleichmäßiger Nachschub während des Kristallwachstums führte zu ringförmigen Zonen gleicher Eisengehalte, spärlicher oder einseitiger dagegen zu halbmondförmigen Hüllen mit Zwiebelstruktur.

     

Authigenie gypsum in deep-sea manganese nodules

Gypsum crystals of authigenic origin have been found in manganese nodules of the deep-sea sediment surface (4500-5500 m water depth) from the Central Pacific Ocean.     

Holocene displacement along branch and secondary faults in the San Andreas fault zone, southern California

Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.     

Carbonaceous chondrites—I. Characterization and significance of carbonaceous chondrite (CM) xenoliths in the Jodzie howardite

Mineralogical, chemical, textural, and isotopic studies of the abundant carbonaceous inclusions in the Jodzie howardite are consistent with CM characteristics. These CM xenoliths show regolith alteration on a level comparable to the Murray and Murchison meteorites but less than Nogoya, flow-oriented development of phyllosilicates and ‘poorly characterized phases’, and partial oxidation of sulfides. Temperature-programmed pyrolysis mass spectrometry (25°–1400°C) indicates that gas release patterns of volatiles and hydrocarbon components and percent contents of N(0.15), C(2.3) and S(2.4) are typical of CM meteorites. Release of significant amounts of SO₂ is attributed to the thermal breakdown of ‘poorly characterized phases’ (Fe-Ni-C-S-O) that formed during low temperature aqueous alteration in the CM parent body.Noble gas abundances are well within the reported range of CM meteorites. The fact that the Ne composition is typical for ‘solar’ values and the isotopic structure of Xe is ‘planetary’ argues that these gases were entrapped by different mechanisms. Cosmic ray exposure ages for the xenoliths (³He, 5 × 10⁶; ²¹Ne, 6.7 × 10⁶; ³⁸Ar, 6.9 × 10⁶ yr) agree with the reported exposure age for the eucritic host. Volatile abundances, presence of intact organic molecules, and phyllosilicates in the CM xenoliths preclude regolith temperatures in excess of 200°C after CM incorporation. Mixing of the host and xenoliths probably occurred during a low-velocity collision of main belt asteroids.     

Mössbauer spectroscopic studies of iron in organic material from natural sedimentary environments

⁵⁷Fe Mössbauer spectra have been obtained from samples of humic acid, fulvic acid and kerogen and from the organic material extracted from bituminous chalk with benzene-methanol. The spectra indicate that iron occurs in a trivalent form in the silicate residue of the humic acid fraction, as hydrated ferrous ions associated with the fulvic acid fraction, as pyrite in kerogen and in a form not detectable by Mössbauer spectroscopy in the benzene-methanol extract.