Showing papers on "Petrography published in 1984"

The relationship between acoustic properties and the petrographic character of carbonate rocks

Abstract: Laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine‐ and air‐saturated carbonate rocks with a wide range of facies, porosities, lithologies, and rock fabrics indicate that porosity is the major factor influencing both P- and S-wave impedance and velocity. Primary lithology and secondary mineralogy have only a small influence on impedance and velocity. Combined use of P- and S-wave velocity data discriminates porosity changes from lithologic changes. All other variables, including pore‐fluid type and petrographic fabric, have no significant influence on velocities. Laboratory measurements of P‐wave velocity under simulated in‐situ conditions reproduce well‐log velocity values reliably. Laboratory porosity‐velocity trends agree with the time‐average equation when the correct matrix velocities are used. Rock property results were used to interpret porosity/lithology variations for an inverted seismic section from the Williston basin. Where well...

Alteration of the upper oceanic crust, DSDP site 417: mineralogy and chemistry

Abstract: Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.

The trapped fluid phase in upper mantle xenoliths from Victoria, Australia: implications for mantle metasomatism

Abstract: Mantle-derived xenoliths of spinel lherzolite, spinel pyroxenite, garnet pyroxenite and wehrlite from Bullenmerri and Gnotuk maars, southwestern Victoria, Australia contain up to 3 vol.% of fluids trapped at high pressures. The fluid-filled cavities range in size from fluid inclusions (1–100 μm) up to vugs 11/2 cm across, lined with euhedral high-pressure phases. The larger cavities form an integral part of the mosaic microstructure. Microthermometry and Raman laser microprobe analysis show that the fluids are dominantly CO2. Small isolated inclusions may have densities ≥1.19 g/cm3, but most inclusions show microstructural evidence of partial decrepitation during eruption, and these have lower fluid densities. Mass-spectrometric analysis of gases released by crushing or heating shows the presence of He, N2, Ar, H2S, COs and SO2 in small quantities; these may explain the small freezing-point depressions observed in some inclusions. Petrographic, SEM and microprobe studies show that the trapped fluids have reacted with the cavity walls (in clinopyroxene grains) to produce secondary amphiboles and carbonates. The trapped CO2 thus represents only a small residual proportion of an original volatile phase, which has undergone at least two stages of modification — first by equilibration with spinel lherzolite to form amphibole (±mica±apatite), then by limited reaction with the walls of the fluid inclusions. The inferred original fluid was a CO2-H2O mixture, with significant contents of (at least) Cl and sulfur species. Generation of this fluid phase in the garnet-peridotite stability field, followed by its migration to the spinel peridotite stability field, would provide an efficient mechanism for metasomatic enrichment of the upper mantle in LIL elements. This migration could involve either a volatile flux or transport in small volumes of silicate melt that crystallize in the spinel peridotite field. These observations suggest that some portions of the subcontinental upper mantle contain large reservoirs of free fluid CO2, which may be liberated during episodes of rifting or magmatism, to induce granulite-facies metamorphism of the lower crust.

Zoning in highly alkaline magma bodies

Abstract: We present chemical data on magmatically heterogeneous pyroclastic deposits of late Quaternary age erupted from zoned magma systems underlying Tenerife (Canary Islands), Sao Miguel and Faial (Azores), and Vesuvius. The most fractionated magmas present at each centre are respectively Na-rich phonolite, trachyte, and K-rich phonolite. Within any one deposit, chemical variation is either accompanied by changes in the phenocryst assemblage (petrographic zonation) or is largely manifested in trace element abundances, unaccompanied by any petrographic change (occult zonation). Zoning is analogous to that in calc-alkaline systems where the most fractionated products are high-silica rhyolites. When a range of magma types are considered, a correlation emerges between roofward depletion of trace elements (especially REE) in the zoned system and compatability of those same trace elements in the accessory phenocryst phases present. Thus, allanite- or chevkinite-bearing rhyolitic systems are light-REE depleted roofwards, the sphene-bearing Tenerife system is middle-REE depleted roofwards, the melanite-bearing Vesuvius system is heavy-REE depleted roofwards, while the Azores systems, which lack these phases, display roofward REE enrichment. Therefore, the behaviour of trace elements may in each case be explained by fractionation of observed phenocryst assemblages. The resemblance between features of zoned magma systems and published work on the dynamic consequences of cooling saturated aqueous solutions prompts us to suggest that sidewall crystallization and consequent boundary-layer uprise to form a capping layer at top of the system may be a plausible mechanism for the generation of both petrographic and occult zonation. Reverse zoning occurs among the first-erupted tephra of some deposits, demonstrating that the most highly differentiated magma available is not always the first to be tapped during an eruption from a zoned system.

Chemical differentiation of the Cordillera Paine granite (southern Chile) by in situ fractional crystallization

Abstract: Geochemical and field data for the Cordillera Paine (CP) pluton of southern Chile, indicate that differentiation took place by closed system in situ fractional crystallization. Minor, local and irregular separation of liquids from crystals led to the formation of evolved granites and aplites which are encountered mostly at the plutons roof and margins. Chemical trends show strong depletions of Sr, Ba, Mg less intense depletions of Ca, La, Ce, Nd, Fe, Ti, Al and enrichment of Nb, Y, Th, Rb and Si with differentiation. Pronounced crystal zoning of Ca, Sr and Ba in plagioclase, Ba in orthoclase and LREE, Y and Th in allanite closely correspond to the whole rock chemical variation. The crystal zoning data suggest that surface equilibrium only was maintained for the zoned elements during crystallization. Thus, continuous separation of liquids from crystals was not necessary to generate the kind highly evolved differentiates whose character reflects fractional crystallization. The schedule of liquid-crystal separation affects mainly the location, degree of dispersion and relative abundance of the differentiates. The homogeneity of the CP pluton and the intense crystal zoning suggest that crystal-liquid separation was inefficient, and that whole rock compositions approach liquid compositions. Assumption of a closed system during crystallization allows estimation of mineral/melt partition coefficients (K d s) using crystal core and whole rock compositions. Crystal zoning and whole rock chemical trends are consistent with models constructed using the K d s thus obtained along with modal abundances from petrographic estimates.

Petrographic studies of refractory inclusions from the Murchison meteorite

Abstract: Textural and mineral-chemical data on freeze-thaw disaggregated refractory inclusions from the Murchison meteorite are reported. The data were obtained with neutron activation analysis, SEM, and spectroscopy, the study revealed corundum-bearing inclusions, spinel-hibonite aggregates and spherules, and spinel-pyroxene and elivine-pyroxene inclusions. One of the three spinel-, pyroxene-, forsterite-rich inclusions had an amoeba-shaped spinel-pyroxene core, implying vapor-to-solid condensation and therefore crystallization from a melt. It is concluded that the meteorite formation encompassed diverse nebular materials, and that further studies of the meteorite will enhance the data base on the planetary nebular processes.

Dating diagenesis in a petroleum basin, a new fluid inclusion method

Abstract: The final porosity and permeability of sandstone petroleum reservoirs is greatly affected by the diagenetic growth of minerals after deposition. For example a sand may be deposited with a porosity of 25% and a permeability of 5,000 mdarcy (mD)1; diagenetic growth of quartz around detrital sand grains may leave a rock with only 10% porosity, and later growth of clays may partly fill these remaining pores and block inter-pore connections, reducing permeability to 100 mD (ref. 2). If the depth and timing of such diagenetic alteration can be measured and the extent of diagenesis estimated, then prediction of the diagenetic state of undrilled sandstones may become possible and diagenesis related more closely to the timing of hydrocarbon migration and the formation of hydrocarbon traps. We present an example of a new method for estimating the date of quartz diagenesis using a combination of techniques from thin section petrography, fluid inclusion thermometry, organic geochemical thermometry and sedimentary basin stratigraphic analysis. These results suggest that quartz in the Beatrice oilfield was precipitated from moving and cooling pore fluids, at a temperature between 68 °C and 94 °C in the late Jurassic.

Petrography and classification of Ca, Al‐rich and olivine‐rich inclusions in the Allende CV3 chondrite

Abstract: The results of a detailed, systematic petrographic survey of Ca, Al-rich and olivine-rich inclusions in the Allende CV3 chondrite are reported, and a new classification system based on clearly defined and readily applied petrographic criteria is presented. Most Allende inclusions are aggregates containing one or more of three distinct constituents: (1) rimmed concentric objects enriched in Al- and Ti-rich oxide minerals and various amounts of Ca-rich silicates; (2) porous, 'fine-grained' chaotic material enriched in Ca-rich silicates, especially clinopyroxenes and garnets; and (3) porous, 'fine-grained', mafic inclusion matrix, enriched in olivine, pyroxene, and feldspathoids. Two texturally distinct varieties of inclusions consist primarily of inclusion matrix: unrimmed olivine aggregates and rimmed olivine aggregates. Ca, Al-rich inclusions are classified on the basis of the size and abundance of their constituent concentric objects. Some fundamental relationships among Allende inclusions that previusly have not been emphasized are discussed.

A New Look at Prairie Creek, Arkansas

Abstract: Previous studies of the Prairie Creek occurrence have identified three main rock types namely: “volcanic breccias”, “tuffs and fine-grained breccias” and “hypabyssal kimberlite or peridotite”. Our investigation confirms the presence of three distinct rock groups which include both magmatic and crater-facies types. The so-called “volcanic breccias” and “tuffs” are both considered to be predominantly of pyroclastic origin. Many features of these rocks are atypical of kimberlite and indicate a complex intrusion history. The magmatic rocks contain two generations of relatively abundant olivine in a fine-grained matrix composed of phlogopite, clinopyroxene, amphibole, perovskite, spinel, serpentine and glass. Although some petrographic features of these rocks are similar to those of kimberlites, the form of the euhedral olivine, presence of abundant glass and occurrence of potassic richterite are uncharacteristic of kimberlite but typical of lamproitic rocks. Both the groundmass phlogopite and the bulk rock have compositions intermediate between known lamproites and kimberlites. The data presented here shows that the Prairie Creek intrusion is not a kimberlite. Although in many respects Prairie Creek appears to be transitional between kimberlite and lamproite, it is considered that these rocks form an extension of the lamproite field.

Isotopic and Petrographic Record of Phreatic Diagenesis: Lower Cretaceous Sligo and Cupido Formations

Abstract: Equant calcite results from a complex spectrum of processes in shallow to burial phreatic environments. Variations in temperature, composition of pore fluids, and redox potential of the environment may give rise to successive generations of equant spar which are not easily characterized through conventional petrographic approaches. Because petrographic criteria alone are not conclusive as diagenetic indicators, it is essential to combine petrographic and geochemical techniques to obtain a more complete understanding of the environmental setting of phreatic diagenesis. Integration of petrographic and stable isotopic analyses of equant cements within reefal facies of the Lower Cretaceous Sligo and Cupido formations, south Texas and northeastern Mexico, indicate multiple phases of cementation in phreatic environments and attest to the chemical evolution of the precipitational environment through time. Two isotopically distinct early generations of clear equant calcite, stages I and 2, with variable luminescence, occur immediately below intervals of wholesale dissolution and exhibit oxygen isotopic signatures approximately 1 and 3 lighter than the estimated original isotopic composition of Cretaceous marine carbonate. Moreover, both stages of these c ments yield lighter carbon isotopic signatures, relative to marine carbonate compositions, which reflect the addition of vadose-derived bicarbonate during recharge to the phreatic lens. On the basis of combined petrographic and isotopic evidence, these two early stages of cementation are interpreted to have formed in a shallow, meteoric, phreatic environment. A third generation of brightly luminescent equant calcite, stage 3, exhibits significantly lighter oxygen and invariant carbon isotopic compositions and occurs in fractures which postdate earlier stages of diagenesis. Two-phase fluid inclusions and the lighter oxygen isotopic composition indicate precipitation at elevated temperatures. The fourth cement, stage 4, is a coarsely crystalline calcite of variable luminescence which occur in fractures, moldic pores, and stylolitic seams, postdates the third stage of cementation, and contains abundant two-phase fluid inclusions and inclusions of bitumen. These petrographic criteria indicate precipitation at greater depths and elevated temperatures, and, as such, stage 4 cements should exhibit lighter oxygen isotopic signatures. What is observed, however, is that the isotopic compositions of this calcite are approximately 6 heavier in 18O than those of stage 3. Such isotopic values are in marked contrast to the progressive trend towards lighter carbon and oxygen exhibited by the first three generations of cement and must therefore reflect precipitation from fluids enriched in 18O, which counteracts t e effect of elevated temperatures. By combining isotopic and petrographic data we have succeeded in identifying four distinct episodes of phreatic cementation. The isotopic trends and the corresponding petrographic characteristics indicate that pore fluids evolved from a meteoric to a connate composition throughout the diagenetic history of the Sligo-Cupido sediments. Furthermore, based on the trends exhibited at the four locations analyzed, it appears that the marginal facies of these formations underwent comparable subsidence and diagenetic histories.

Pristine highland clasts in Consortium Breccia 14305: Petrology and geochemistry

Abstract: Data are presented on the petrography and mineral chemistry of six pristine highland clasts chipped from the polymict lunar breccia 14305. Major and trace elements in the clasts were determined by instrumental neutron activation analysis, and mineral analyses were performed by electron microprobe. Mg-suite clasts have 'eastern' geochemical affinities, reaffirming the importance of local variations in geochemistry. These local variations are superimposed on the moon-wide, longitudinal variations noted by Warren and Wasson (1980). Alkali anorthosites and Mg-suite troctolites and anorthosites are not comagmatic, and cannot be related to a single parent magma by either fractional crystallization or variable assimilation of KREEP. Both magma suites may have assimilated varied amounts of KREEP into distinct parent magmas. Alternatively, alkali anorthosites may have crystallized directly from a KREEP-basalt parent magma. A thick crust of ferroan anorthosite probably never existed on the western lunar nearside, or was removed by basin-forming impacts prior to intrusion of later plutonic suites.

1984 SEPM Presidential Address: Diagenetic Albitization of Potassium Feldspar in Arkosic Sandstones

Abstract: Petrographic studies of subsurface cores of the Fountain Formation (Pennsylvanian) in the Denver region, Colorado, show that detrital grains of potassium feldspar have been pervasively and complexly albitized during burial diagenesis. In some strata, all detrital grains of potassium feldspar have been convened to albite. In some instances, the conversion has taken place directly; in others it has involved prior replacement of the potassium feldspar by other minerals, particularly anhydrite, calcite, and dolomite, which in turn are replaced by albite. The albitized grains display fabrics such as perthitic intergrowths and chessboard twinning that are strikingly similar to replacement fabrics produced in feldspars by igneous and metamorphic processes. If successive stages of the convers on to albite were not preserved, the diagenetically albitized grains might easily be interpreted erroneously as detrital grains of plagioclase, perthite, or antiperthite derived from igneous or metamorphic source rocks. The sodium required for the albitization is presumed to have been derived from intraformational sources, particularly from diagenetically altered detrital grains of sodium-rich plagioclase, which have been extensively replaced by sodium-free clay minerals. Ions released in the course of the albitization have provided important intrastratal sources of potassium, aluminum, and silicon, which have been reprecipitated locally in authigenic minerals, particularly albite, illitic clays, and adularia.

The alleged kimberlite-carbonatite relationship: evidence from ilmenite and spinel from Premier and Wesselton mines and the Benfontein sill, South Africa

Abstract: Carbonate-rich, SiO2-poor residua are developed in some kimberlites solidifying as ocelli, layers, or discrete dikes which satisfy petrographic definitions of carbonatite. Arguments that these rocks have mineralogies, antecedents, and comagmatic rocks differing from those of the carbonatites in alkaline rock complexes, including the specific observation that kimberlites and carbonatites contain ilmenites and spinels of different composition, have been used to refute the “alleged kimberlite-carbonatite relationship”. New microprobe analyses of ilmenites and spinels from carbonate-rich rocks associated with kimberlites in three South African localities correspond to spinels and ilmenites of carbonatites from alkalic complexes, or have characteristics intermediate between those of carbonatites and kimberlites. The ilmenites are distinguished from kimberlite ilmenites by higher MnO, FeTiO3, and Nb2O5, and by negligible Cr2O3. The spinels are distinguished from kimberlite spinels by their Al2O3 and Cr2O3 contents. There is clearly a genetic relationship between the kimberlites and the carbonate-rich rocks, despite the observation that their ilmenites and spinels are distinctly different, which indicates that the same observation is not a valid argument against a petrogenetic relationship between kimberlites and carbonatites. These rocks are among the diverse products from mantle processes influenced by CO2, and we believe that the petrogenetic links among them are forged in the upper mantle. We see insufficient justification to deny the name “carbonatite” to carbonate-rich rocks associated with kimberlites if they satisfy the petrographic definition in terms of major mineralogy.

Fluid inclusions in high-grade gneisses of the Kapuskasing structural zone, Ontario: metamorphic fluids and uplift/erosion path

Abstract: Fluid inclusions in quartz grains from five samples of high-grade rocks (two paragneisses, an amphibolite, a mafic gneiss and a tonalite dike) from the 2.7 Ga Kapuskasing structural zone (KSZ), Ontario, were examined with petrographic, microthermometric and laser Raman techniques. Three types of fluid inclusions were observed: CO2-rich, H2O-rich and mixed CO2-H2O. CO2-rich fluid inclusions are pseudosecondary or secondary in nature and are generally pure CO2; a few contain varying amounts of CH4·H2O-rich fluid inclusions are secondary in nature, contain variable amounts of dissolved salts, and generally contain daughter crystals. Mixed CO2-H2O fluid inclusions occur where trails of H2O-rich inclusions intersect trails of CO2-rich inclusions. Isochores for high density (p=1.03 g/cm3) pseudosecondary, pure CO2 inclusions intersect the lower pressure portion of the estimated P-T field for high-grade metamorphism, implying that pure CO2 was the peak metamorphic fluid. The variable CH4 content of CO2 inclusions within graphite-bearing samples suggests that CH4 was introduced locally after the formation of the CO2 inclusions; however the origin of the CH4 remains problematic. An aqueous fluid clearly penetrated the gneisses after the peak metamorphism (during uplift/erosion), forming secondary inclusions and contributing to the minor retrogressive hydration observed in these rocks. The presence of the pseudosecondary, high-density CO2 inclusions in quartz crystals in the KSZ rocks constrains the uplift/ erosion path for the KSZ to one of simultaneous decrease in pressure and temperature.

Petrology and chemistry of hyperferroan anorthosites and other clasts from lunar meteorite ALHA81005

Abstract: The results of petrographic and chemical studies of 11 previously undescribed clasts from the lunar meteorite Allan Hills A81005 are reported. The majority of lithic clasts in this regolith breccia are granular to cataclastic polymict breccias that are mixtures of ferroan anorthosites and troctolitic Mg-suite plutonic rocks with mg' greater than 84, An 97, and REE abundances consistent with those of known Mg-suite rocks. Clasts of appropriate Mg-suite end members have not been found in 81005, although magnesian olivine fragments are present. Impact-melt clasts similar in composition to bulk 81005 also occur. AH81005 is low in KREEP.

The Use of Back-Scattered Electron Imaging for the Petrographic Study of Sandstones and Shales

Abstract: In this paper, the imaging of mineral grains in sedimentary rocks by atomic-number contrast using back-scattered electrons (BSE) in the SEM is discussed. The analyses of polished thin sections of a sandstone and a shale are used to illustrate the technique and particularly to demonstrate its usefulness in petrographic studies of fine-grained sedimentary rocks. The coupling of energy or wavelength-dispersive X-ray elemental analysis to atomic-number contrast allows minerals to be readily identified and paves the way for detailed geochemical and mineral crystallochemical studies of fine-grained sediments.

A Petrographic and Chemical Section through the Northern Idaho Batholith

Abstract: Petrographic and chemical study of a well-exposed section through the northern Idaho batholith documents a high degree of mineralogical and chemical homogeneity and suggests that all of the rocks belong to a single comagmatic series or superunit. Felsic biotite granodiorite to granite with minor muscovite forms a main-phase unit 37 km across, flanked on the southwest by 10 km of near-vertical sheets of granodiorite with intervening high-grade country rocks. Widespread metasedimentary schlieren appear to be "restite" left by voluminous partial melting, the magma not rising far above its level of generation. The lack of mineralogical and chemical variation across the batholith suggests little differentiation, except for separation of late, volatile-rich fluids to form widespread pegmatites. Abundant mafic dikes emplaced into fractures in the granitic rocks just before crystallization of the last pegmatitic fluids may represent the later stages of rising basalts which aided the initial crustal melting to for...

Geochemistry and Petrography of Organic Matter in Sediments from Deep Sea Drilling Project Sites 545 and 547, Mazagan Escarpment

Abstract: A series of sediment samples of probable Triassic to Miocene age from DSDP Sites 545 and 547 seaward of the Mazagan Plateau on the Northwest African Continental Margin off Morocco were investigated by organic geochemical methods including organic carbon determination, Rock-Eval pyrolysis, gas chromatography and combined gas chromatography/mass spectrometry (GC/MS) of extractable hydrocarbons, and kerogen microscopy. The total organic matter content and the organofacies types are significantly variable, but a generally strong influence of terrigenous organic matter was found in all sediments. The organic matter preservation at Sites 545 and 547 was controlled by the intensity of the terrigenous organic matter supply from the nearby continent, which appeared to be particularly strong during the Cretaceous and had diminished at least by early Miocene times. A noticeable amount of marine organic matter has been preserved in the deep-water sediments by a sequence of processes starting with primary deposition in oxygen-depleted shallow waters on the outer shelf followed by downslope transport and rapid burial at the continental rise. These mass movements happened in the Eocene and throughout the Cretaceous as far as this process could be traced by the sedimentary record. Thin black shale layers in the Jurassic interval at Site 547 indicate strongly fluctuating organic matter preservation conditions probably also controlled by the intensity of terrigenous organic matter supply. All of the primary organic matter except that in the Triassic sediment was thermally immature as expressed by low vitrinite reflectance values and an abundance of olefins in the extractable hydrocarbons. INTRODUCTION The geochemistry and petrography of organic matter in Northwest African continental margin sediments have been studied in the past on a large number of samples from various deep holes drilled during DSDP Leg 41 (Site 367, Cape Verde Basin; Site 368, Cape Verde Rise; Site 369, off Cape Bojador; Site 370, Moroccan Basin); Leg 47A (Site 397, off Cape Bojador); and Leg 50 (Site 415, Agadir Canyon; Site 416, Moroccan Basin). A recent synthesis (Rullkötter, Cornford, et al., 1982) of the results concluded that the quantity and provenance of the organic matter as well as the depositional environment varied considerably with geological time and geographic location along the Northwest African continental margin, so that a number of organofacies types could be differentiated. During DSDP Leg 79, additional deep holes were drilled at Sites 545 and 547 seaward of the Mazagan Plateau, about 150 km west of Casablanca (Fig. 1). The sedimentary sequences penetrated extend to the oldest sediments recovered so far from the deep Atlantic Ocean. These sediments offer another excellent opportunity to study the deposition of organic matter on the Northwest African continental margin since the early phase of the opening of the Atlantic Ocean. Figure 2 illustrates the geological setting of Sites 545 and 547 that were projected onto a northwest-southeast seismic line (A-A) through the rectangle shown in Figure 1. Site 545 is located near the foot of the steep Hinz, K., Winterer, E. L., et al., Init. Repts. DSDP, Washington (U.S. Govt. Printing Office). 2 Institute of Petroleum and Organic Geochemistry (ICH-5), KFA Jülich GmbH, P.O. Box 1913, D-5170 Jülich 1, Federal Republic of Germany. Mazagan Escarpment in 3150 m water depth. Drilling was terminated in calcareous Middle to Late Jurassic sediments at 701 m depth after penetrating a Neogene and an incomplete Cretaceous section (Aptian-Cenomanian). Site 547 is located on the landward flank of a gneissic basement high in 3940.5 m water depth. As can be seen in Figure 2, the stratigraphic sequence at Site 547, in contrast to Site 545, includes a Late Cretaceous and Paleogene section that pinches out toward the Mazagan Escarpment and ends in Triassic continental red beds at a terminal depth of 1030 m probably within a few (?)tens of meters from the basement. In this study, the amount, type, and composition of organic matter in the Site 545 and 547 sediments are described, and the results are discussed in terms of its origin, depositional environment, and diagenetic alteration. A comparison will be made between the stratigraphically equivalent sediments from both sites. Furthermore, the results will be incorporated into the present knowledge of organic matter sedimentation along the Northwest African continental margin to improve the organofacies picture gained so far. EXPERIMENTAL METHODS The frozen samples were dried at 50°C for 12 hours and ground. Total organic carbon was determined with a LECO IR-112 carbon analyzer after treatment of the sediments with hydrochloric acid to remove the carbonate carbon. Extraction was performed using a modified flow-blending technique (Radke et al., 1978) and with dichloromethane as the extraction solvent. The total extracts were separated into nonaromatic hydrocarbons, aromatic hydrocarbons, and heterocoraponents by automated medium-pressure liquid chromatography (Radke et al., 1980). A Siemens L350 gas chromatograph equipped with a 23 m × 0.3 mm ID glass capillary column coated with SE54 was used for gas chromatographic (GC) analysis of the nonaromatic hydrocarbon frac-

Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore, Massif Central, France)

Abstract: Three magmatic units (Grande Cascade pyroclastic deposits, Grande Cascade lava flow, Durbise nuee ardente deposits) from the Quaternary volcano Sancy (Mont-Dore area, Auvergne, France) show textural evidences of magma mixing between a silica undersaturated basic magma (alkali basalt and hawaiite) and an acid magma (quartz-bearing trachyte). Three kinds of mixed rock types are described: basic inclusions within an acid host, « emulsified rocks » showing infracentimetric basic globules disseminated within an acid groundmass, and « banded rocks » in which elongated acid and basic zones alternate. The chemical compositions of mixed rocks plot systematically onto linear trends in petrographic diagrams. Microprobe analyses of the groundmass show similar linear variations between basic and acid end-members. The mineralogical associations of these mixed rocks are highly complex and present many disequilibrium features. Olivine is stable in the basic component and becomes rimmed by orthopyroxene in the acid one. Zoning patterns of feldspars are complex. Clinopyroxene, kaersutite and phlogopite phenocrysts have increasing component Mg contents from core to rim both in the basic and the acid. Titanomagnetite and hemoilmenite phenocrysts were equilibrated at 900-800° C under high oxygen fugacities. Mixing results primarily from a mechanical disintegration of partly liquid basic inclusions within their acid host, and also from a mechanical transfer of phenocrysts from one component to the other, in which they often remain surrounded by a coating of their original groundmass. Chemical data on the groundmass indicates that some « true » hybridization between coexisting acid and basic liquids may also have occurred. The extent of mixing is controlled by the type of emplacement. For pyroclastic deposits a chemical gap exists between basic inclusions and their acid hosts; in contrast, mechanical mixing was enhanced during the emplacement of the viscous Grande Cascade lava flow, and complete transitions occur between basic and acid components. The two end-members are genetically associated, the latter deriving from the former by crystal fractionation. Mixing appears as a late-stage phenomenon in the petrogenetic history of the Mont-Dore series; in the case of the Grande Cascade lava flow, its extent is primarily dependent on emplacement modalities.

The Mid-Cretaceous Frontier Formation near the Moxa Arch, southwestern Wyoming

Abstract: Stratigraphic data, paleontologic and petrographic information is presented for the Frontier Formation in the Green River Basin of Wyoming, Utah, and Colorado. In addition, interpretations concerning the areal extent and depositional origin of some strata within the formation are presented. Data indicate that most of the sampled shale units are thermally mature, in terms of oil generation, and some are probably source rocks for oil and gas. 12 figures, 11 tables.

Petrographic analysis and correlation of volcanic rocks in Bostic 1-A well near Mountain Home, Idaho

Abstract: Detailed examination of volcanic rock cuttings from the Bostic 1-A well near Mountain Home, Idaho, provides data that correlate the stratigraphy of the well with the regional stratigraphy of the western Snake River Plain. The Bostic 1-A well penetrates basalt of the Middle Pleistocene Bruneau Formation and underlying sedimentary rocks of the Upper Pliocene Glenns Ferry Formation. Basalt underlying the Glenns Ferry Formation is most likely Banbury Basalt of Middle Pliocene age or Banbury equivalent. A 350-ft interval of felsic volcanics is then intersected above another 600 ft of basalt. The well bottoms in altered felsic volcanics. The lowest 600 ft of basalt flows has not been correlated with any basalt observed on the surface. From the established stratigraphy of the region, and from petrographic evidence, the silicic volcanic rocks occurring both above and below the lowermost basalts in the well are probably lower Pliocene Idavada Volcanics. North of the well, in the Mt. Bennett Hills, Idavada Volcanics overlie crystalline rocks of the Idaho batholith. No estimate of depth to plutonic bedrock can be made from the well data alone. Stratigraphic comparisons suggest as little as 0.2 to 0.3 km more of Idavada lie beneath the Bostic 1-A well. Resultsmore » of geophysical studies suggest additional basalt lies beneath the Bostic 1-A rather than granitic rocks of the batholith.« less

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex, Maine: U-Pb on zircons and Rb-Sr on whole rocks

Abstract: The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende 40Ar/39Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262. Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the 207Pb/206Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data. Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.

Diagenetic Matrix in Proterozoic Graywackes from Sweden

Abstract: Four diagenetic mechanisms of matrix formation were recognized in graywacke sandstones from the Visingso Group (Upper Proterozoic, southern Sweden). These mechanisms are 1) chemical alteration of detrital silicate minerals, 2) authigenesis (crystallization from pore solutions), 3) pseudoplastic flow of soft argillaceous fragments, forming pseudomatrix, and 4) alteration of pseudomatrix either through fixation or leaching of cations. Compared to the previously published studies dealing with graywacke matrix, the new contribution in the present study is 1) alteration of detrital unstable particles is shown to occur through dissolution and replacement by clay minerals, and 2) the microprobe and X-ray diffraction analyses, scanning electron and petrographic microscopes were used together in the mineralogical and textural investigation.