Showing papers on "Petrography published in 1981"

Ore Microscopy and Ore Petrography

Abstract: The Ore Microscope. The Preparation of Samples for Ore Microscopy. Mineral IdentificationQualitative Methods. Reflected Light Optics. Quantitative MethodsReflectance Measurement. Quantitative MethodsMicroindentation Hardness. Ore Mineral Textures. Paragenesis, Formation Conditions, and Fluid Inclusion Geothermometry of Ores. Ore Mineral Assemblages Occurring in Igneous Rocks and Vein Deposits. Ore Mineral Assemblages Occurring in Sedimentary, Volcanic, Metamorphic, and Extraterrestrial Environments. Applications of Ore Microscopy in Mineral Technology. Appendices. Indexes.

Chemistry of rock-forming minerals of the Cretaceous-Paleocene batholith in southwestern Japan and implications for magma genesis

Abstract: Petrographic descriptions and electron microprobe analyses of minerals are presented for 35 specimens from seven suites chosen to examine the transition from magnetite series to ilmenite series granitoids along two transects across the Cretaceous-Paleocene Inner Zone batholith of southwestern Japan. Regularities in chemical compositions of amphiboles, biotites, and feldspars suggest that fundamentally similar processes produced the magmas that formed the two series. Constant or decreasing Fe/(Fe+Mg) for biotites and amphiboles with increasing host rock silica content, coupled with the absence of early formed magnetite and sphene, suggest that magnetite series rocks may have become oxidized during crystallization near the level of intrusion, through the processes of second boiling and differential loss of hydrogen. For the Daito-Yokota, magnetite series suite, Fe/(Fe+Mg) for biotites decreased from 0.48 to 0.37 as SiO2 content of the host rock increased from 55.3 to 75.5 wt %; for an ilmenite series suite from the Takanawa Peninsula, Fe/(Fe+Mg) for biotites increased from 0.51 to 0.77 with an increase in host rock SiO2 from 53.4 to 75.5. Detailed consideration of amphibole chemistry shows predominance of edenitic and tschermakitic substitution schemes as well as coupling between substitutions of Ti in octahedral sites and AlIV. Interrelations between amphibole and biotite chemistry show that Fe/(Fe+Mg) and Mn contents can be interpreted in terms of equilibration, whereas Ti content cannot. The chemistry of chlorites correlates well with that of biotites; primary and secondary muscovites are distinct in composition. Plagioclase in all studied suites shows igneous zoning appropriate to host rock composition; perthitic alkali feldspars in all samples have lost albite component, and temperatures based on the two-feldspar geothermometer are low. The biotite-apatite geothermometer is also inoperative for this group of samples because fully fluorinated apatites typically occur in biotites of modest F content. Whereas magnetites have reequilibrated, analyses of ilmenites for the representative Daito-Yokota and Takanawa suites corroborate biotite compositional data and suggest that fO2 probably differed by 2 to 3 orders of magnitude during crystallization of silica-rich magnetite and ilmenite series granites. Whole-rock chemistry supports mineral chemistry in suggesting that the studied granitoids have crystallized from magmas generated in a lower crustal environment in which mantle-derived magmas partially melted source materials with igneous characteristics.

Agents of low temperature ocean crust alteration

Abstract: δ18O and87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are depositedafter this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.

Burial Cementation in the Devonian, Kaybob Reef Complex, Alberta, Canada

Abstract: Detailed analysis of the petrography and distribution of compositionally zoned ferroan calcite and dolomite cements in the Devonian, Kaybob Reef Complex of Alberta, Canada, has demonstrated that porosity occlusion is predominantly a result of burial diagenesis to depths in excess of 4 km. Different but temporally related mechanisms of formation are indicated for the two cement types: coarsely crystalline dolomite and coarsely crystalline calcite. Calcite cement precipitational history, determined by correlation of compositional zones, demonstrates that pressure solution along stylolites was the essential mechanism of calcite cementation in the reef-interior and to a lesser extent in the reef-slope facies. Zoned dolomite cements are, in contrast, the major cement type in the reef-margi and reef-slope facies; a basinal source for dolomite precipitating fluids is indicated. It is suggested that the composition of basin-derived pore waters was controlled by temperature dependent clay mineral reactions as a function of progressive burial.

Minor and trace element partitioning between immiscible ocelli-matrix pairs from lamprophyre dikes and sills, Monteregian Hills petrographic province, Quebec

Abstract: Many lamprophyre dike and sill rocks in the Monteregian Hills petrographic province of southwestern Quebec contain felsic segregations (ocelli) which have been interpreted as globules of immiscible liquid (Philpotts 1976). Ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements. The major element data, when plotted on a Greig diagram, outline a field of possible silicate-liquid immiscibility at higher alumina+alkali content than that previously mapped in iron-rich experimental systems. The trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high-charge density cations are preferentially concentrated in the matrix (mafic) material, a result which is consistent with theoretical and experimental studies.

Petrography and implications of continental rocks from the Agulhas Plateau, southwest Indian Ocean

Abstract: Seismic-refraction and seismic-reflection profiles reported elsewhere indicate that the Agulhas Plateau is a continental fragment that was rifted from the Falkland Plateau and intruded by basaltic magmas during the early opening of the South Atlantic. Dredge hauls recovering a characteristic suite of low- to high-grade continental metamorphic rocks, indurated sedimentary rocks, and basaltic igneous samples support this origin and evolutionary history. Both the petrography of the gneiss specimens and K-Ar ages of 1,074 ± 36 m.y. and 478 ± 17 m.y. on fresh biotites from two of the rocks are compatible with petrography and ages of the formerly adjacent continental rocks of southern Africa, the Falkland Islands, Falkland Plateau, and Antarctica.

Low grade and very low grade regional metabasic Ordovician rocks of Llŷn and Snowdonia, Gwynedd, North Wales

Abstract: Metabasic assemblages belong to the prehnite-pumpellyite and greenschist facies. The isograds pumpellyite-in, pumpellyite-out/clinozoisite-in and biotite-in have been mapped, and divide the area into four metamorphic zones. Field and petrographic evidence indicate that the metamorphic crystallization was largely syn-and immediately post-tectonic, viz. end-Silurian to Devonian. The use of published Schreinemakers' bundles indicates that most of the prehnite-pumpellyite-bearing assemblages formed at high grades within the facies and that the greenschist facies rocks crystallized in the higher pressure field. As the amount of calcite increases so diagnostic Ca–Al silicates are reduced and eventually eliminated from assemblages. Contrary to recent claims, the evidence indicates that the Gwna Group and the Gwyddel Felsitic Beds of Llŷn were metamorphosed prior to the Arenig.

Diagenesis and Secondary Porosity in Vicksburg Sandstones, McAllen Ranch Field, Hidalgo County, Texas

Abstract: Lower Vicksburg sandstones (Oligocene) at McAllen Ranch field in Hidalgo County, Texas, consist of thin sandstones interbedded with shales. The sands were deposited by turbidity currents as channel and overbank deposits. The sandstones produce gas from depths of 9300 to 15,000 ft (2835 to 4572 m). Depositional patterns were controlled by a diapiric shale uplift and related faults. Petrographic analyses show that primary porosity was reduced during early diagenesis by calcite cementation. However, dissolution of calcite cement, feldspar, and volcanic rock fragments, which took place after deep burial, led to secondary porosity development. Dissolution is evidenced by the formation of intergranular porosity, oversized pores, grain molds, and by microporosity within individual grains. Dissolution was followed by precipitation of quartz overgrowths, formation of authigenic clay minerals (kaolinite, chlorite, illite, smectite, and vermiculite), and by precipitation of iron-rich calcite cement. Scanning electron microscopy confirms that clay minerals are primarily authigenic and uniformly distributed. Chlorite grain coatings and pore lining probably aided in preservation of porosity by inhibiting cementation. Highest porosites and permeabilities are found where the sandstones have the highest secondary porosities, as determined by petrographic study. Porosity increases from about 15 percent to as much as 20 percent in the section from 9642 to 12,586 ft (2939 to 3836 m). This increase is related to the abnormally high pressure gradient of about 0.92 psi/ft (20.8 kPa/m) and to an elevated geothermal gradient of about 2°F/100 ft. Optimum reservoir properties are present where late-stage cementation by clays and iron-rich calcite has not been extensive.

Mesozoic Granitic Rocks of Southern Korea Reviewed from Major Constituents and Petrography

Abstract: Report of 68 new chemical analysis data of granitic rocks collected during a preliminary field trip from Chungju to Busan through non-metamorphic part of the Ogcheon folding belt, Ryongnam land and Gyongsang basin is presented. Special attention is paid to characterize chemical composition of Jurassic and Cretaceous granites of the country. The studies hitherto performed show: (1) Chemical evolution examined from (Na2O+K2O), total Fe as Fe2O3 and MgO variation of Jurassic granite is identical to that of Cretaceous one. (2) For rocks having the same Al2O3/(Al2O3+total Fe as Fe2O3+MgO) index, Jurassic granite is less siliceous than Cretaceous one. (3) Remarkable differences between Cretaceous and Jurassic granites are found in their petrographic texture and the sequence of crystallization of the constituent minerals. Features enumerated above suggest that the two groups of granite separated by their ages differ also each other in their modes of emplacement. Introduction Mesozoic granites in South Korea are divided into two groups according to their ages. They are Jurassic Daebo granites and Cretaceous Bulgugsa granites. The Daebo granites appear on the map as massifs of a hundred to several kilometers in diameter and show a concordant intrusive relation against the surrounding country rocks. The massifs are very often elongated in NNE-SSW direction parallel to the Ogcheon folding belt. The granites intrude in both metamorphic and non-metamorphic terrains of this belt. The contact thermal effect around Daebo granite is not, as far as observed in the present study, so strong in the non-metamorphic terrain. Development of biotite hornfels is observed with a thickness of 200 to 100 meters around the contact. According to the description by D. S. LEE (1971), migmatites are well developed around the granite masses in the metamorphosed part of the Ogcheon belt (his Southeast granitic rock zone). It is said by several authors (O. J. KIM 1971, for example) that the gold mineralization of the country is related to this granite. In contrast with the Jurassic granites, the Cretaceous Bulgugsa granites occur in many cases as intrusive masses of several kilometers to some hundreds of meters in diameter cutting discordantly the surrounding rocks. Few of them can be seen in the Ogcheon belt (LEE'S granites of younger Central zone), but their most typical occurrence is seen in Gyeongsang basin where granite bodies appear as irregular shaped small intrusive masses. Many epigenetic ore deposits of Cu, Zn, Pb and W are said to be related to this Cretaceous Bulgugsa granites (REEDMAN et al., 1973; REEDMAN and UM, 1975; MIYAZAWA, 1977). What is the cause of these contrasts between * Paper presented at the Symposium on Granitic Magmatism and Associated Mineralization in Japan and Korea, held in Tokyo on January 27, 1981. The manuscript received on June 30, accepted on July 10, 1981. ** Geological Institute , Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan. *** Laboratoire de Geologie Structurale, Universite Pierre et Marie Curie, Tour 16, 4 Place Jussieu 75230, Paris Cedex 05, France; Stayed in Geological Institute, Faculty of Science, University of Tokyo during the university year of 1979 as an exchange researcher according CNRS-JSPS Programme.

Method for relating shallow electrical anomalies to the presence of deeper hydrocarbon reservoirs

Abstract: A method is provided for evaluation of shallow electrical anomalies to determine which are likely to be caused by seepage from a hydrocarbon reservoir at depth. The method involves geochemical lithological, and petrographic analyses of anomalous sediments to determine whether these sediments contain certain features characteristic of seepage-induced anomalies. These features include the following: (1) accumulations of seeped (non-indigenous) hydrocarbons; (2) accumulations of carbonate cements that have δ 13 C values between -10 and -58°/oo; (3) pyrite accumulations in a shallow, porous host rock where the pyrite is distributed within fractures or between original grains with a disseminated or cement-like texture, and where it is not associated with kerogen or coal and is not framboidal in morphology; (4) close proximity of said pyrite and carbonate accumulations; and (50 anomalous (high) induced polarization (I.P.) response in conjunction with high apparent resistivity where analyses show the sediments to be in anhydrite-poor formations; and high I.P. response in conjunction with low apparent resistivity where analyses show the sediments to be in anhydrite-rich formations.

Igneous evolution of Hess Rise: Petrologic evidence from DSDP leg 62

Abstract: Igneous rock was recovered at three sites on Hess Rise, north-central Pacific Ocean, during Deep Sea Drilling Project leg 62: tholeiitic basalt at site 464 (northern Hess Rise), trachyte at site 465, and rounded clasts of alkalic basalt at site 466 (both on southern Hess Rise). The depths at which the igneous rocks from sites 464 and 465 were recovered correspond to strong acoustic reflectors, whereas the clasts from site 466 were recovered from within calcareous sediment. Although all the rocks are moderately to highly altered, petrographic and chemical studies, especially rare earth element determinations, provide data on the nature of these rocks. Data from these studies, combined with geophysical models of the evolution of the Pacific plate during the Mesozoic, allow an interpretation of the formation of Hess Rise involving building of a volcanic platform by eruption of tholeiitic mid-oceanic ridge basalt caused by the breakup of a R-R-R-type triple junction during the middle Cretaceous. This period of platform building was accompanied by edifice building along the south edge of Hess Rise involving alkalic basalt and its differentiation products associated with a leaky transform fault. This interpretation agrees with proposed models for the origin of the tholeiitic parts of other oceanic plateaus but requires a different mechanism—namely, volcanism associated with leaky transform faults—to account for the alkalic rocks.

Petrographic Analysis of Late Woodland Ceramics From the Sperry Site, Jackson County, Missouri

Abstract: A sample of 11 sherds from the Late Woodland component of the Sperry Site in Jackson County, Missouri was selected for petrographic analysis. The findings indicate that sherds as well as granite pa...

Primary, Bedded, Structureless Phosphorite of the Khubsugul Basin, Mongolia

Abstract: A phosphatic series of Upper Vendian (Proterozoic) to Lower Cambrian age occurs in the Khubsugul Basin, Mongolia. Unweathered, bedded, aphanitic, structureless phosphorite (microsphorite) is relatively abundant in these phosphorites. Its orthochemical precipitation in shallow water on the inner portion of the Siberian continental shelf of latest Precambrian to Early Cambrian time is established from stratigraphic and petrographic considerations. The microsphorite was the parent material for other petrographic types of phosphorite, which formed both by disintegration of the microsphorite to give intraformational conglomerate and breccia and by granulation of the microsphorite to give granular and pelletal phosphoritc. Although disintegration and granulation were due to a higher energy nvironment, little transportation of grains occurred. Dolomitization of matrix apatite was a significant part of the process of granulation. These various types of phosphorite are intimately interbedded in the lower of two phosphate units.

Petrographic and chemical characteristics of abyssal tholeiites from Deep Sea Drilling Project Leg 63 off Baja California.

Abstract: Tholeiitic basalts were obtained from basaltic basement ranging in age from 6 to 17 m.y. on IPOD/DSDP Leg 63. The main rock types encountered at all sites but 473 are basaltic pillow lavas. Although many of these pillow basalts are highly or moderately altered, fresh glass is usually present. At Site 473, we recovered coarse-grained, massive basalts; no clearly defined pillowed forms were observed. Phenocrysts or microphenocrysts present in the Leg 63 basalts are Plagioclase and clinopyroxene at Site 469; olivine, Plagioclase, and spinel at Site 470; and olivine, Plagioclase, and clinopyroxene at Sites 472 and 473. Olivines of the basalts from Holes 470A and 472 (Fo85_88) are generally more magnesian than those of the Hole 473 basalts (Fo77_81). Also, plagioclases of Holes 470A and 472 basalts (An7O_85) are generally more calcic than those of Holes 469 and 473 basalts (An66_72). Geochemical study of the Leg 63 basalts indicates that in all cases they are large-ion-lithophile (LIL) element depleted tholeiites like typical abyssal tholeiites. In particular, they are very similar in composition to those described from the eastern Pacific, although the degree of iron enrichment found in the Leg 63 basalts is not as extensive as in basalts from the Galapagos spreading center. Hence, the geochemical evidence of the Leg 63 basalts is compatible with their formation at a spreading center. Compositional variations in Leg 63 basalts from any single drill hole is small. Major and trace element data indicate that the samples from Holes 469 and 473 are more fractionated in chemical composition than are the samples from Holes 470A and 472; this compositional variation may be largely ascribed to differences in the extent of shallow-level fractional crystallization of similar parental magma. The Hole 472 samples, however, show a LIL element character distinct from the other Leg 63 samples.

Gabbro-Anorthosite-Granophyre-Leucogranite Relationship Around Gorumahisani Hill, Mayurbhanj District, Orissa

Abstract: Study on the field-relations, petrography and chemical composition of gabbroanorthosite-granophyre-leucogranite suite supports a co-magmatic origin of these rocks.

Cathodoluminescenc E Petrography a Valuable Tool for Teaching and Research

Abstract: Cathodoluminescence is visible light emitted from a specimen when it is bombarded with electrons It can be viewed with a relatively inexpensive instrument such as a luminoscope This device is easy to maintain and can be used by teachers, students and researchers for both instructional and research purposes Although most studies to date have been with carbonate rocks and some clastics, the technique is also applicable to igneous and metamorphic rocks Examples are given to illustrate the kinds of information that cathodoluminescence can provide Many other applications remain to be discovered and the list will grow as other workers apply the technique and their skills to numerous problems in mineralogy and petrology