Showing papers on "Petrography published in 1991"

Quaternary Sediments : Petrographic Methods for the Study of Unlithified Rocks

Abstract: The book considers how the information preserved in sediments, soils and weathering products may be used to reconstruct past environments. It describes and assesses physical and chemical laboratory procedures and shows how to interpret the results of these analyses. The application of each procedure to environmental reconstruction is stressed. Case studies, diagrams and extensive references provide further information.

Oxygen-isotope composition of diagenetic calcite in organic-rich rocks: Evidence for 18O depletion in marine anaerobic pore water

Abstract: Upper Cretaceous organic-rich carbonates in Israel contain benthonic foraminifera and varying amounts of early diagenetic infilling of sparry calcite. The sparry calcite has oxygen-isotope values (down to -9.5‰ PDB) that are significantly lower than those of the coexisting skeletal calcite (averaging -2‰ PDB). Low δ18O values are very common in carbonates that are associated with organic matter. These occurrences are conventionally explained as being due to carbonate formation under conditions of low salinity and/or elevated temperature. On the basis of petrographic data and local geologic evidence, however, none of the conventional explanations can be applied in this case. We advance an alternative model that relates the light oxygen compositions of the diagenetic calcite to depletion of 18O in pore water of normal salinity. The proposed operating mechanism is a direct consequence of organic matter decomposition in the sulfate-reducing zone. Being inherent to reducing marine environments, this model can be applied to many organic-rich rock associations.

Ceramic Petrography As a Technique for Documenting Cultural Interaction: An Example from the Upper Mississippi Valley

Abstract: The petrographic identification of ceramic tempers has long been known to be a fruitful line of inquiry for investigating intersite and interregional cultural interaction. By applying point-counting procedures to the recording of natural as well as humanly added mineral inclusions in ceramic thin sections, considerable power can be added to this traditionally qualitative technique. The effectiveness of this more quantitative approach in discriminating local from nonlocal vessels is demonstrated through a comparative analysis of two Middle Mississippian-contact sites in the upper Mississippi Valley region-Hartley Fort in northeast Iowa and the Fred Edwards site in southwest Wisconsin.

Nd-Sr isotopic and geochemical characteristics of the southern Adamello (northern Italy) intrusives: Implications for crustal versus mantle origin

Abstract: Among the Periadriatic Tertiary calc-alkaline intrusions in northern Italy, the Adamello batholith displays the most convincing evidence for a mafic parentage This is particularly the case for the southern Adamello, where minor ultramafic to gabbroic bodies form the margins of several distinct and dominantly tonalitic plutons Isotopic analyses are presented for the Val Fredda and Blumone ultramafic-gabbroic to tonalitic-leucoquartzdioritic suites, for the more felsic rock types of the spatially predominant Re di Castello group, as well as for a lamprophyre This dike of picrobasaltic composition is thought to represent the parental magma Field relations and a large body of petrographic, geochemical and mineral chemical data point to a cogenetic origin for all of these rocks and to magmatic differentiation with fractional crystallization and accumulation as the main evolutionary mechanism This is compatible with the isotopic results for the Blumone series and the mafic rocks of Val Fredda with eNd = +38 to +15 and eSr = −91 to −46 A common parental magma similar to the lamprophyre (eNd = +31, eSr = −44) also seems plausible The isotopically homogeneous felsic lithologies of Val Fredda (eNd = +01 to −03, eSr = +29 to +07), structurally intimately associated with their mafic counterpart, indicate crustal contamination incurred prior to their differentiation For the Re di Castello group, progressing crustal affinity of isotopic compositions (eNd of −19 to −49 and eSr of +165 to +409) with chemical differentiation implies large-scale interaction with a granitoid crustal component Attempts at quantifying this process reveal individual characteristics and evolution for each pluton The data indicate extraction of this component from a source in the lower crust with time-integrated low Rb/Sr, eSr approximating +50 and eN close to −6 corresponding to a 1000 Ma crustal residence age If a two-stage evolution is assumed, formation of a gabbroic reservoir from the above parental melt would require uptake of some 4–12 wt % of granitoid component by combined assimilation/fractional crystallization, whereas subsequent evolution to tonalitic compositions would necessitate admixing of 40–70 wt % of granitoid partial melt Assimilation thus mainly occurs during evolution from gabbroic to tonalitic compositions, when the mafic magma is ponded near the mantle-crust boundary and where homogenization prior to resuming buoyant ascent is assured Precise locations and more detailed petrography of the samples is available with entire article on microfiche Order from American Geophysical Union, 2000 Florida Avenue, NW, Washington, DC 20009 Document 91B-002; $250 Payment must accompany order

Concrete petrography : a review

Abstract: Concrete strength depends on the amount, composition and quality of the hydrated binder to the fine and coarse aggregate. This paper reviews the way in which concretes can be examined with the petrographic microscope using both thin sections and polished plates. It reports observations made on actual samples, noting the similarity between the densities obtained by petrographic assessment and those established in laboratory tests. The paper highlights the value of carrying out a petrographic investigation to obtain quantitative information in support of and prior to the conventional chemical investigation, including aggregate/cement paste ratio and void content, assessment of the weight fractions of water, cement and aggregate; assessment of the composition of the binder including the slag to Portland cement ratio, assessment of the PFA to Portland cement ratio and possibly the assessment of the type of Portland cement employed in the mixture. Attention is also drawn to the implications of aggregate shape and grading.

Equilibrium and kinetics in contact metamorphism : the Ballachulish Igneous Complex and its aureole

Abstract: I Introduction.- 1 The Setting of the Ballachulish Intrusive Igneous Complex in the Scottish Highlands.- 2 Regional Geology of the Ballachulish Area.- II The Intrusive Complex.- 3 Structure, Petrography and Emplacement of Plutonic Rocks.- 4 Thermal Conditions and Crystallization Sequence, as Deduced from Whole-Rock and Mineral Chemistry.- 5 Nucleation and Growth of Pyroxene in the Hypersthene Diorites.- 6 Microstructures and Thermal Behaviour of Igneous Pyroxenes.- 7 The Shape of the Intrusion Based on Geophysical Data.- III The Contact Aureole and Its Rocks.- 8 Petrography and Mineral Chemistry of Pelites.- 9 Field Relations and Petrography of Partially Melted Pelitic and Semi-Pelitic Rocks.- 10 Decarbonation Reactions in Siliceous Dolomites and Impure Limestones.- 11 Microtextures and Reaction Mechanisms of Carbonate Rocks: A Comparison Between the Thermoaureoles of Ballachulish and Monzoni (N. Italy).- 12 Quartz Grain Coarsening by Collective Crystallization in Contact Quartzites.- 13 Disordering, Re-Ordering and Unmixing in Alkali Feldspars from Contact-Metamorphosed Quartzites.- 14 A Search for Variations in the Structural States of Cordierite in Contact-Metamorphosed Pelites.- 15 Detrital Quartz and K-Feldspar in Quartzites as Indicators of Oxygen Isotope Exchange Kinetics.- IV Interactions Between the Intrusion and the Contact Aureole.- 16 P-T-a(H2O) Conditions in the Thermal Aureole.- 17 Stable Isotope Geochemistry on the Intrusive Complex and Its Metamorphic Aureole.- 18 Thermal Models of Cooling.- V Concluding Discussion.- 19 Evidence of Fluid Phase Behaviour and Controls in the Intrusive Complex and Its Aureole.- 20 Intracrystalline Processes.- 21 Summary and Outlook.- References.

Controls on the petrographic evolution of an active margin sedimentary sequence: the Larsen Basin, Antarctica

Abstract: Abstract The sedimentary fill of the Cretaceous-Tertiary Larsen Basin, located at the northern tip of the Antarctic Peninsula, records the evolution of the Antarctic Peninsula source terrain. Uplift, and the possible renewal of arc volcanism within this area, is documented by the wide petrographic spread in the lower Gustav Group (Barremian-Coniacian). The Hidden Lake Formation (Coniacian-Santonian) records a major pulse in proximal calc-alkaline arc volcanism. The overlying Marambio Group shows a change from lithic-volcanic sandstones to quartz-feldspar-rich sandstones, reflecting a change in source terrain. An abrupt change in sandstone composition in the upper Santa Marta Formation (Campanian) may reflect both a switch in volcanism and plutonism from the east to the west coast of the Antarctic Peninsula (i.e. away from the basin margin), and a change to more mafic volcanism. The Lopez de Bertodano Formation (Campanian-Palaeocene) dominantly reflects a change to a quartzo-feldspathic source terrain (the Trinity Peninsula Group). This is also related to both a decrease in the intensity of arc volcanism and an increase in distance from the location of concurrent volcanism in relation to the site of deposition. Controls on petrography other than simple arc-unroofing (e.g. location and nature of arc volcanism and depositional setting) play an important role in sandstone petrography. These controls on sandstone composition should always be considered in interpreting sequences from active plate margin settings.

Petrography and Mineral Chemistry of Pelites

Abstract: This chapter describes the petrography and mineral chemistry of an exceptionally well-developed sequence of prograde mineral zones in pelitic and semipelitic rocks in the Ballachulish aureole. Two schematic petrogenetic grids are derived: the first is for mineral assemblages below the onset of partial melting, which define the mapped isograds in Maps 1 and 2 and Figure. 8.1; and the second is for high-grade mineral assemblages which occur sporadically within the zone of partial melting (Harte et al., Chap. 9, this Vol.) and within pelitic screens within the igneous complex. The two grids, when linked, provide a continuous petrogenetic grid from the lowest to highest grade in the aureole. In Pattison (Chap. 16, this Vol.) the continuous grid is calibrated in P-T space.

Dolomites, a review of origins, geometry and textures

Abstract: Dolomites form in a range of environments by processes able to drive large volumes of magnesium-rich waters through existing carbonate sediments or rocks. These fluids need not be of unusual composition but there are kinetic barriers to crystallisation which is influenced by factors such as the Mg/Ca ratio, salinity, temperature, the /Ca2+ ratio and time. Dolomite is able to form at near-surface temperatures and pressures within a few thousand years.Textures in dolomitic rocks are controlled by their conditions of formation. A large proportion are replacive but few of these are mimetic, preserving primary structures. Crystals vary from euhedral to anhedral with boundaries ranging from planar to consertal. Solution chemistry and temperatures influence the density and distribution of nuclei together with growth rates and crystal morphology. There is still doubt whether irregular crystal faces are products of high or low temperatures or saturation. Dolomite cements are more important than has previously been realised in massively dolomitised rocks. Differential dissolution of aragonite, calcite, or evaporite minerals leaves space for these cements to occupy. Dolomitisation may also be allied to compaction, generating stylolitic rocks which are progressively enriched in dolomite. Dolomite may be replaced by calcite or it may be dissolved and the resulting pores filled with a calcite cement.There is no general correlation between any set of petrographic features and particular geological models for dolomitisation. Similar physicochemical conditions are reproduced in a range of environments and the most effective guides to origin are in the geometry and regional petrographic variation of dolomite bodies.

Geochemical stratigraphy of two regolith cores from the central highlands of the Moon.

Abstract: High-resolution concentration profiles are presented for 20-22 chemical elements in the under 1-mm grain-size fractions of 60001-7 and 60009/10 Emphasis is placed on the stratigraphic features of the cores, and the fresh results are compared with those of previous petrographic and geochemical studies For elements associated with major mineral phases, the variations in concentration in both cores exceed that observed in some 40 samples of surface and trench soils Most of the variation in lithophile element concentrations at depths of 18 to 21 cm results from the mixing of two components - oil that is relatively mafic and rich in incompatible trace elements (ITEs), and coarse-grained anorthosite The linearity of mixing lines on two-element concentration plots argues that the relative abundances of these various subcomponents are sufficiently uniform from sample to sample and from region to region in the core that the mixture behaves effectively as a single component Soils at depths of 52-55 cm exhibit very low concentrations of ITEs

A genetic link between remagnetization and potassic metasomatism in the Devonian Onondaga Formation, Northern Appalachian Basin

Abstract: Whole-rock chemical analyses indicate that the Onondaga Formation along a Rochester-Syracuse (New York) transect has a marked eastward increasing trend in K/Al ratio, which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization. An eastward increase in the content of some authigenic minerals including K-feldspar, quartz, and Fe-rich chlorite in the rocks is indicated by petrographic and X-ray diffraction studies. These observations suggest a link between remagnetization and potassic alteration of Fe-bearing silicates in the Onondaga. Two hypothetical reactions are proposed in which magnetite authigenesis is related to the breakdown of Fe-bearing smectite in the presence of potassium. Products of these reactions include authigenic K-feldspar, illite, quartz and chlorite, as well as magnetite. Our results suggest that remagnetization was triggered by a basin-wide migration of K-rich fluids during the Alleghenian Orogeny.

Marginal basin—volcanic arc origin of metabasic rocks of the Circum-Rhodope Belt, Thrace, Greece

Abstract: In the upper stratigraphic levels of the Thracian Circum-Rhodope Belt, pillowed or massive metavolcanics and metapyroclastic rocks occur. In the deeper part of the stratigraphic column a composite suite of greenschists, cumulate and noncumulate gabbros, metagabbros, serpentinites, chlorite and talc schists are found. Detailed petrographical study revealed that the metavolcanics consist of four lava types. From the basic to the more evolved types, these lavas are: pyroxeno-phyric lavas, aphyric oligophyric lavas, albite-rich lavas and porphyric felsites. Based on geochemical criteria the metavolcanics are classified as tholeiitic basalts and andesites, to dacites-rhyodacites. The content of MgO, Cr, Ni, TiO2, Zr and REE, and some petrographic features of the pyroxeno-phyric lavas suggest boninitic affinities.

New observations on the quartz monzodiorite-granite suite.

Abstract: Five new fragments of quartz monzodiorite (QMD) were identified in particles from soil 15403, which was collected from the boulder sampled as rock 15405, an impact-melt breccia containing clasts of KREEP basalt, QMD, granite, and a more primitive alkali norite. Petrographic and geochemical studies of the fragments show considerable variation in modal proportions and bulk composition. This heterogeneity is due to unrepresentative sampling in small fragments of coarse-grained rocks. Variations in the proportions of accessory minerals have marked effects on incompatible-trace-element concentrations and ratios. Semiquantitative calculations support the derivation of QMD from 60-percent fractional crystallization of a KREEP basalt magma as suggested by Hess (1989). Apollo 15 KREEP basalt cannot be the actual parent magma because the evolved rocks predate volcanic KREEP basalts. It is suggested that ancient KREEP basalt magmas have crystallized as plutons, with alkali norite clasts offering the only direct evidence of this precursor.

Decarbonation Reactions in Siliceous Dolomites and Impure Limestones

Abstract: In this chapter the petrography and mineral chemistry of carbonates of the Ballachulish aureole are presented. Carbonates are not widespread in the aureole, so the record of progressive metamorphism is incomplete. The outermost thermometamorphic reaction observed in the aureole leads to the appearance of talc + calcite in siliceous dolomites and impure limestones. In the inner aureole from the point where both forsterite and diopside are formed, the outcrop is continuous up to the formation of periclase.

Impact melt rocks from New Quebec Crater, Quebec, Canada

Abstract: — Approximately 1500 g of float samples of impact melt rocks have been recovered from gravel deposits ∼4 km north and northeast of the rim of the 3.4 km diameter New Quebec Crater (61°17′N; 73°40′W) in northern Quebec, Canada. Previously, only two small samples of impact melt rocks were known. The newly recovered samples have cryptocrystalline to microcrystalline matrices with microlites of andesine and pigeonite. Mineral clasts of quartz and feldspar occur and, in some cases, show shock metamorphic features. The melt rocks have a normative mineralogy corresponding to ∼70% quartz, orthoclase and albite and are compositionally similar. Their major element composition can be modeled as a mix of granitic gneisses that make up the target rocks. The melt rocks show enrichments, however, in Cr (21 ppm), Co (9 ppm), Ni (12 ppm) and Ir (1.5 ppb) over the target rocks. Interelement ratios suggest a chondritic impacting body, although they do not define a specific type. Assuming a C-1 chondrite, the impact melt rocks average ∼2% meteoritic contamination. Stepwise 40Ar-39Ar dating using a laser on three chips from three samples give integrated ages of 0.6–2.5 Ma. From the best plateau ages, the age of the New Quebec impact is taken to be 1.4 ± 0.1 Ma, which places it before the first major northern hemisphere continental glaciation of the Pleistocene. A number of considerations suggest that the impact melt rocks were originally deposited in fractures in the crater wall and later transported to their discovery site by glacial ice and melt water.

Mineralogy and petrography of the anomalous carbonaceous chondrites Yamato-86720, Yamato-82162, and Belgica-7904

Abstract: As a part of the consortium study on antarctic meteorites with af­ finities to CI-chondrites we studied the samples of Yamato (Y)-86720, Y-82162, and Belgica (B)-7904. These carbonaceous chondrites are unique samples and do not perfectly fit in the traditional classification schemes. Therefore, they have to be considered as very important samples to carry distinct information about processes in the early solar system. Y-82162 is a very fine-grained carbonaceous chondrite. Based on the occur­ rence of abundant clasts (up to several mm in size) we suggest that this sample is a chondritic breccia. The dominating phases are phyllosilicates; abundant sulfide grains are scattered throughout the entire sample. However, the abundances of sulfides vary from clast to clast. Y-86720 contains about 13 vol% of light objects embedded in a fine-grained, phyllosilicate-rich groundmass. Some of these objects appear to be relict chondrules; however, they essentially consist of phyllosilicates. Most light, round to irregularly-shaped components exhibit well-preserved ac­ cretionary dust mantles (\"dark rims\") similar to those found in CM-chondrites. Y-86720 is mineralogically more closely related to the CI-chondrites than to any other chondrite group; texturally, however, it appears to be an intermediate chondrite between CI and CM as also suggested by bulk chemical criteria (G. W. KALLEMEYN; Papers Presented to the 13th symposium on Antarctic Meteorites, June 7-9, 1988, Tokyo, NIPR, 132, 1988). B-7904 contains 18 vol% of objects larger than about 70 μm in size. 42 vol% of these components are chondrules or chondrule fragments. The most abundant constituents are, however, olivine-bearing, fragment-like objects (45.9 vol%) unknown from other chondrites. The olivines within these components are embedded in a fine-grained brownish-grey matrix. Other constituents include fine-grained CAis, olivine aggregates, and mineral fragments. B-7904 is a new kind of carbonaceous chondrite and we do not like to classify this meteorite as a CM-type chondrite because of the following reasons: a) A great number of chondrules in B-7904 is much larger (0.5-3 mm) than measured for the mean size of chondrules in CM­ chondrites (0.3 mm). b) Many components (olivine-bearing, fragment-like objects, Cr, Al-rich fine-grained particles) are unknown from CM-chondrites. c) The oxygen isotope composition and the low H20-contents are untypical for CM-chondrites.