Petrography

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

Stable isotopic compositions of early Archaean sulphate deposits of probable evaporitic and volcanogenic origins

Abstract: SIGNIFICANT Archaean bedded sulphate deposits have been described only from the Pilbara Block (Western Australia)1,2, the Barberton Mountainland (South Africa)3–6 and southern India7. These are all barite deposits, but there is petrographic evidence suggesting replacement of original evaporitic gypsum in the North Pole (Pilbara) and Barberton deposits. This contrasts with the apparent volcanogenic deposition of barite in the relatively sulphide-rich Big Stubby prospect (Pilbara). This letter summarises the petrographic evidence, concentrating on recent data from North Pole, and reports stable isotopic results for barite and associated sulphides which provide corroborative evidence for the environments of formation of these deposits. Some implications concerning the early hydrosphere, atmosphere and biosphere are outlined.

Petrographic Textures of Detrital Polycrystalline Quartz as an Aid to Interpreting Crystalline Source Rocks

Abstract: The internal textures of detrital polycrystalline quartz can be used to distinguish between polycrystalline quartz populations derived from different crystalline source rocks. This is supported by the petrographic examination of the quartz fraction in 65 metamorphic rocks with known metamorphic histories and the detrital polycrystalline quartz populations in 57 samples of medium sand-size Holocene fluvial sand derived from known crystalline source rocks. Empirical observations by metamorphic petrologists have shown that quartz crystal sizes increase systematically with increase in metamorphic grade of the rocks. Petrographic examination of quartz crystals deformed in controlled laboratory experiments have shown quartz responds to stress via a systematic series of processes: deformation, recovery, primary recrystallization, and secondary recrystallization. Deformation is reflected optically by undulose extinction, deformation lamellae and bands, elongated crystal units, and sutured crystal-crystal boundaries. Recovery is recognized optically be segmented extinction (semi-composite extinction) reflecting polygonization of the crystal. Primary recrystallization occurs when strain-free areas surrounded by dislocation tangles form new crys als, and is recognized optically by the presence of small (50 µm), nonundulose crystals. Secondary recrystallization develops large, nonundulose, strain-free polyhedral crystals with smooth crystal-crystal boundaries. Deformation, recovery, and primary recrystallization features characterize quartz in greenschist-grade metamorphic rocks. Detrital quartz grains with these features are, for the lack of a better term, called "unstable", reflecting the transformation from old crystals to new, recrystallized crystals. Detrital polycrystalline quartz with features of secondary recrystallization are called "stable" grains, and are derived from medium- to high-grade metamorphic rocks. Detrital polycrystalline quartz grains derived from intrusive igneous rocks are also characterized by "stable" features. A plot of a polycrystallinity index (the ratio of detrital polycrystalline quartz with more than 3 crystal units per grain to total detrital polycrystalline quartz) vs. an instability index (the ratio of detrital polycrystalline quartz with "unstable" characteristics to total detrital polycrystalline quartz) discriminates between medium sand-size polycrystalline quartz populations in Holocene sands derived from low-, and medium- to high-grade metamorphic source rocks Detrital polycrystalline quartz populations in Holocene sands derived from granitic plutons partly overlap the medium- to high-grade field.

Metabasalts from the Troodos Massif, Cyprus: Genetic Implication Deduced from Petrography and Trace Element Geochemistry

Abstract: A wide variety of data support the contention that the Troodos massif, Cyprus is a fragment of oceanic lithosphere formed at a constructive margin some 85 m.y. ago. However, Troodos rocks differ significantly in major and trace element content and mineral parageneses from those formed at present day active constructive margins. It is here argued that the geochemical and petrographic characteristics of the massif are compatible with its formation at the axis of a slow spreading ridge within a small, marginal ocean basin.

Deposition and Diagenetic Alteration of Sandstone in Northeast Pacific Arc-Related Basins: Implications for Graywacke Genesis

Abstract: Sedimentary basins characterized by continental to marine shelf depositional regimes are common adjuncts to active lithospheric plate junctions of the northeast Pacific area. Most of these basins are elongate troughs and may form in both fore-arc and back-arc positions. Andesitic composition of volcanic and plutonic rocks that form the major source lithologies in active or recently active arcs produces petrologically distinctive sands that are dominated compositionally by volcanic rock fragments, plagioclase feldspar, and mafic heavy minerals. Quartz content of the sands is low and rarely exceeds 50 percent. Such sands are mineralogically unstable and react upon shallow to moderate burial and consequent increase in temperature and pressure to produce a recurrent sequence of authigenic cements. Three progressive stages of diagenesis are differentiated in sample suites from the Bristol, Queen Charlotte, Grays Harbor, and Chehalis basins: stage 1, early diagenetic calcite pore-filling cement; stage 2, authigenic clay rims and coats around detrital grains; stage 3, authigenic phyllosilicate and (or) laumontite pore-filling cement. Development of silicate overgrowths with chlorite and calcite replacement of rock fragments and plagioclase becomes more pronounced as burial is further increased. As the depth of burial increases, higher temperature, overburden pressure, and fluid pressure result. Of these three factors, geologic evidence favors temperature as the primary control of burial diagenetic reactions. Petrographic data support the concept of a diagenetic origin for graywacke matrix. The second and third diagenetic stages recognized here result in the authigenesis of clay minerals and micas and can reasonably be considered precursors of a graywacke lithology.