Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

The origin of certain chromite deposits of the eastern part of the Bushveld complex

Abstract: Field relations and petrographic characteristics of chromitite layers (Steelport Main and Leader seams) and the enclosing pyroxenites in a section of the so-called critical zone of the Bushveld complex (South Africa) are reviewed in the light of various modes of origin proposed for the chrome-rich rocks. It is concluded that textural and other features can best be explained by postulating differential mechanical settling of chromite and bronzite crystals controlled by variations in the velocity of currents in the magma chamber, followed by crystallization of interstitial liquid. Some of the chromite evidently crystallized in place; this could be accounted for by resolution of settled crystals.

Cordierite-gedrite rocks and associated gneisses of Fishtail Lake, Harcourt Township, Ontario

Abstract: The cordierite–gedrite rocks and associated gneisses on the north side of Fishtail Lake, Harcourt Township, Ontario, occur within the Grenville gneiss complex of the Haliburton Highlands The investigation comprises a petrographic study, based on geologic mapping, and supplemented by new chemical analyses of gedrites, cordierites, garnets, biotites, and typical rocks Comparison with compositional data from other metamorphic environments shows that the compositions of associated cordierite, garnet, and anthophyllite (gedrite) and garnet–cordierite–biotite have lower FeO/(MgO + FeO) ratios in high-grade regional metamorphic environments such as exemplified at Fishtail Lake than in contact metamorphic associations The chemical composition of the rocks is characterized by high MgO and FeO and low lime and alkalies, compared with argillaceous rocks and metamorphic rocks derived from them It is suggested that this unusual composition results from the removal of an anatectic granite fluid from the parent rock

Evolution of gabbroic rocks of the Northern Apennine ophiolites (Italy): Comparison with the lower oceanic crust from modern slow-spreading ridges

Abstract: Field and petrographic relationships together with major and trace element mineral chemistry have allowed us to determine the igneous to high-temperature metamorphic evolution of the gabbroic rocks of the Northern Apennine ophiolites. Gabbroic rocks formed by the intrusion of liquids of normal mid-oceanic-ridge (N-MORB) type in a heterogeneous mantle section under low-pressure conditions. These liquids underwent an igneous-differentiation process controlled by fractional crystallization, most likely associated with late percolation in the gabbroic cumulate pile of a volatile-bearing igneous agent, possibly a trondhjemite-type liquid. Such an igneous-differentiation process produced highly evolved liquids that gave rise to Fe-rich rocks (mostly Fe-Ti oxide-bearing diorites). The gabbroic rocks underwent high-temperature (T 900 °C) recrystallization in ductile shear zones in the absence of seawater-derived fluids. The gabbroic rocks of the Northern Apennine ophiolites bear striking similarities to those recovered from modern slow-spreading ridges, such as the Southwest Indian Ridge, the Mid-Atlantic Ridge (at its intersection with the Kane Fracture Zone, i.e., the MARK area), and the Mid-Cayman Rise. The Northern Apennine ophiolites are interpreted to represent a fossil analogue of a magma-poor slow-spreading center that formed as a result of continental lithospheric extension.

Geochronology of Metasomatic Events

Abstract: In order to date any geological event, suitable mineral geochronometers that record that and only that event must be identified and analyzed. In the case of metasomatism, recrystallisation is a key process that controls both the petrology and the isotopic record of minerals. It can occur both in the form of complete neocrystallisation (e.g. in a vein) and in the form of pseudomorphism, whereby dissolution/reprecipitation at the submicroscopic scale plays a central role. Recrystallisation may be complete or not, raising the possibility that relicts of a pre-metasomatic assemblage may be preserved. Because recrystallisation is energetically less costly at almost any temperature than diffusion, and because radiogenic isotopes (except 4He) never diffuse faster than major elements forming the mineral structure, there is a strong causal link between petrographic relicts and isotopic inheritance (as demonstrated for zircon, monazite, titanite, amphibole, K-feldspar, biotite, and muscovite). Metasomatic assemblages commonly contain such mixtures between relicts and newly formed phases, whose geochronology is slightly more complex than that of simple, ideal systems, but can be managed by techniques that have become routine in the last decade and which are described in this chapter. Because of its crucial role in controlling the isotope systematics, the petrogenesis of a mineral needs to be understood in extreme detail, especially using microchemical analyses and micro-imaging techniques, before mineral ages can be correctly interpreted. As the occurrence of recrystallization is limited by the availability of water, minerals act as “geohygrometers” that allow constraints to be placed on the nature and age of fluid circulation episodes, especially metasomatic events.

The petrography and reservoir properties of some triassic sandstones of the Northern Irish Sea Basin

Abstract: Triassic sandstones of the Northern Irish Sea have been correlated on wire-line logs with the succession at the outcrop in Cheshire (Colter & Barr 1975). Recent work on well sections confirms the lithological similarities of these units with the supposed onshore equivalents. Porosity and permeability of the sandstones are related in the first place to original grain size, particularly argillaceous content. Superimposed on this primary variation are the effects of various diagenetic phenomena: (1) Tangential illite coatings; (2) Platy illite normal to grain surfaces; (3) Calcite cement; (4) Compaction and pressure solution; (5) Quartz overgrowths. Porosities and permeabilities measured in core plugs have been related to primary textures and to the occurrence of these diagenetic changes.