Petrography

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

Sandstone Composition and Depositional Environment

Abstract: The detrital mineralogy of sandstones is affected significantly by depositional environment. The imprint of the environment is recorded as the relative abundance and size of individual detrital species. Differences between environments within any one basin are accompanied by concomitant changes in detrital-sediment composition. Petrographic analyses of Holocene and ancient sedimentary sequences reveal that the environmentally produced compositional changes are process dependent, and are interrelated closely with textural variations. Fluctuations in rock-fragment content among environments indicate that the relative survivability of unstable detrital grains can influence the overall composition of sandstones. A total of 300 samples of varied grain sizes was selected from contrasting environments in three Holocene sedimentary sequences: (1) the Galveston barrier complex, Texas; (2) the lower alluvial valley of the Mississippi River; and (3) the Mississippi delta. In each sedimentary sequence, bivariate and multivariate analyses of petrographic data demonstrate the environmental sensitivity of both composition and texture. A comparison of data from these three Holocene sequences demonstrates that the most effective environmental segregation based on composition and texture alone is in depositional systems characterized by intermediate or high energy--in systems with a significant proportion of sand-size detritus. Four ancient sedimentary sequences, of different age and geographic location, w re selected to complement the Holocene sequences: (1) Wilcox (Eocene) fluvial rocks, Texas; (2) Wilcox (Eocene) deltaic rocks, Texas; (3) Lower Jurassic (Toarcian) marine rocks, England; and (4) Lower Cretaceous Muddy Sandstone marine, transitional, and nonmarine rocks, northeast Powder River basin, Montana and Wyoming. Petrographic data from each sequence demonstrate that the composition of recent detrital sediments and ancient sandstones is environmentally sensitive. Composition is also sensitive to textural changes. In sediments with a wide range of grain sizes, the relation between the abundance and size of detrital monocrystalline quartz is curvilinear. Compositional and textural data derived exclusively from thin-section analyses can be used jointly to segregate depositional environments within a single sedimentary basin, providing no source changes occur, and providing that certain basic sampling restrictions are adhered to. Thus, small samples such as sidewall cores and cuttings may be used in the delineation of depositional environments. Such small samples are available in abundance in the p troleum industry and, if analyzed petrographically, they can render invaluable assistance in the search for stratigraphic traps.

Field and Geochemical Studies of the Melilite-Bearing Arydzhangsky Suite, and an Overall Perspective on the Siberian Alkaline-Ultramafic Flood-Volcanic Rocks

Abstract: This paper presents the first comprehensive geologic, petrographic, geochemical, and Sr- and Nd-isotopic study of the 350 m thick Arydzhangsky lava suite, the last suite in the entire north Siberian, flood-volcanic sequence to require study by modern methods. Within this sequence, only the Arydzhangsky Suite includes melilite-bearing lavas; it is composed of melanephelinites to limburgites (both melilite-bearing and melilite-free), with rare melilitites and picrites. The lavas contain from 0 to 60 vol% melilite, with MgO contents ranging from 5.7 to 29.5 wt%. Nonetheless, these compositionally diverse lavas are all quite similar in incompatible-element geochemistry. They are distinct from all other Siberian alkaline-ultramafic lavas and, among these lavas, show the most resemblance to the Yakutian kimberlites. With this contribution, all of the north Siberian, alkaline-ultramafic lavas will have received equal geochemical and isotopic characterization. Five rock groups have been identified among them on t...

Aluminium loss during sandstone diagenesis

Abstract: Petrographical observations from a sandstone buried to between 3.2 and 5.7 km in the North Sea Central Graben suggest a significant change in bulk chemistry during burial. The sand contained 35% or more feldspar upon deposition, but due to extensive dissolution, only 5% remains in the deep sandstones. This has released much more Al than is currently contained as authigenic minerals: up to 1460 μ mol cm −3 of Al have been exported. This deep secondary porosity is economically important, but its recognition by petrographic textures is unreliable, as are bulk-chemical provenance studies of deeply buried sedimentary and metamorphic strata.

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

Abstract: Tourmaline in the Martinamor antiform occurs in tourmalinites (rocks with >15–20% tourmaline by volume), clastic metasedimentary rocks of the Upper Proterozoic Monterrubio formation, quartz veins, pre-Variscan orthogneisses and Variscan granitic rocks. Petrographic observations, back-scattered electron (BSE) images, and microprobe data document a multistaged development of tourmaline. Overall, variations in the Mg/(Mg þ Fe) ratios decrease from tourmalinites (0 36–0 75), through veins (0 38–0 66) to granitic rocks (0 23–0 46), whereas Al increases in the same order from 5 84–6 65 to 6 22–6 88 apfu. The incorporation of Al into tourmaline is consistent with combinations of cAl(NaR)–1 and AlO(R(OH))–1 exchange vectors, where c represents X-site vacancy and R is (Mg þ Fe2þ þ Mn). Variations in c/( c þ Na) ratios are similar in all the types of tourmaline occurrences, from 0 10 to 0 53, with low Ca-contents (mostly <0 10 apfu). Based on field and textural criteria, two groups of tourmaline-rich rocks are distinguished: (1) pre-Variscan tourmalinites (probably Cadomian), affected by both deformation and regional metamorphism during the Variscan orogeny; (2) tourmalinites related to the synkinematic granitic complex of Martinamor. Textural and geochemical data are consistent with a psammopelitic parentage for the protolith of the tourmalinites. Boron isotope analyses of tourmaline have a total range of dB values from 15 6 to 6 8%; the lowest corresponding to granitic tourmalines ( 15 6 to 11 7%) and the highest to veins (1 9 to 6 8%). Tourmalines from tourmalinites have intermediate dB values of 8 0 to þ2 0%. The observed variations in dB support an important crustal recycling of boron in the Martinamor area, in which pre-Variscan tourmalinites were remobilized by a combination of mechanical and chemical processes during Variscan deformation, metamorphism and anatexis, leading to the formation of multiple tourmaline-bearing veins and a new stage of boron metasomatism.