Petrography

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

Chronology of the structural and petrologic development of the southwest Sierra Nevada foothills, California: Summary

Abstract: Metamorphic country rocks of the southwest Sierra Nevada foothills represent the southernmost exposures of the western foothills metamorphic belt. The rocks of the foothills belt in this region are disrupted and metamorphosed by the Cretaceous batholith to a much greater extent than to the north, probably owing to a deeper level of exposure. The oldest country rocks in the region consist of the Carboniferous to Triassic Kings—Kaweah ophiolite belt which served as basement for Jurassic volcanic arc and continent-derived clastic sequences (Saleeby, 1978, 1979). The history of ophiolite generation, sea-floor transport, continental margin accretion, and volcano-plutonic arc growth is recorded by numerous petrogenetic and deformational episodes. Useful time constraints have been derived for a significant number of these episodes by U/Pb and K/Ar geochronometry. These time constraints have facilitated the construction of an absolute chronology for the structural and petrologic evolution of the region. In this paper, the critical geological and geochronological relations are summarized. The isotopic data and their interpretation, and the detailed field relations of the rocks dated along with supporting petrographic and petrochemical data are presented in the complete article in Part II.

Characterization of granitoid profiles in the Sila Massif (Calabria, southern Italy) and reconstruction of weathering processes by mineralogy, chemistry, and reaction path modeling

Abstract: This paper concerns the development of a multidisciplinary research on weathering profiles of granitoid rocks in the Sila Massif (Calabria, southern Italy), based on field investigations, chemical and minero-petrographic data, and geochemical modeling. It aims at evaluating modes and degree of weathering and focuses on the main transformation processes of the parent rock, integrating information obtained by direct observations/analyses on samples with results of simulations of pertinent water–rock interaction processes, carried out through the reaction-path-modeling approach. Physical responses to geological hammer hitting and morphological weathering features were used to characterize increasing weathering grade (I–VI weathering classes) of a representative weathering profile. Also, the main microfabric and minero-petrographic features were observed in thin sections; XRPD and SEM-EDS were used to identify primary minerals and neoformed products. Progressive dissolution of the granitoid rock was simulated using the software package EQ3/6, 8.0 and a modified version of the thermodynamic database data0.ymp.R5. A kinetic mode under closed-system conditions was applied with respect to a defined set of secondary solid phases, which are allowed to precipitate. The partial transformation of biotite and the partial destruction of feldspars, associated with the neoformation of secondary minerals and a substitution of the original rock fabric are the main mineralogical changes observed. Neoformed clay minerals and ferric products replaced feldspars and biotite during the most advanced weathering stage. Microfractures and morphological variations occur on the original rock. The final result of weathering process is a soil-like rock characterized by sand-gravel grain-size fraction. Results of geochemical modeling suggest that the chemical weathering processes occurring in the study area are relatively close to an iso-chemical transformation of the original granitoid rock, based on the small predicted changes in the concentrations of solutes from the initial to the final state of the simulation. The CO2-controlled dissolution of albite-rich plagioclase is the most important reaction, followed by the dissolution of K-feldspar, biotite, chlorite, and muscovite, in order of decreasing importance. The precipitating secondary minerals are ferrihydrite, saponite, vermiculite, illite, and kaolinite in order of decreasing pH values. The order of appearance and the quantities of secondary (product) minerals depend on the setup of geochemical modeling. Based on the standard thermodynamic properties of different illite endmembers, calculated in this work, it was possible to predict the varying compositions of the illite solid solution, which resulted to be similar to those observed in natural systems.

Isotopic signature of burial diagenesis and primary lithological contrasts in periplatform carbonates (Miocene, Great Bahama Bank)

Abstract: The stable isotope geochemistry of Miocene sediments from the leeward margin of the Great Bahama Bank was examined to investigate burial diagenetic processes in periplatform carbonates. Data indicate that, in addition to differences in bulk proportions of neritic and pelagic carbonate along the slope, rhythmic variation in primary carbonate content has controlled patterns of burial diagenesis and associated geochemical signatures throughout much of the succession examined. The present study focuses on Ocean Drilling Program Sites 1006 and 1007, the most distal of five sites drilled from marginal to deep basin environments during Leg 166. These Miocene sections are characterized by their cyclic appearance, manifest as decimeter- to meter-scale alternations between light-colored ooze/chalk/limestone and dark-colored marl/marlstone. The section at Site 1006 contains a high proportion of pelagic carbonate and is unlithified to a subbottom depth of ~675 m. Fluctuations in δ 18 O and δ 13 C values at this site are independent of lithological variation and reflect primary conditions. At Site 1007, located at the toe-of-slope and composed of a mixture of bank-derived and pelagic carbonate, limestones are densely cemented, show little evidence of compaction and have δ 18 O values up to 2‰ higher than coeval sediments at Site 1006. Marlstones at Site 1007 are poorly cemented, exhibit an increase in compaction-related features with depth and have lower and more variable δ 18 O values that are similar to those of coeval sediments at Site 1006. Isotopic and petrographic characteristics of limestone interbeds result from cement precipitation from cold sea water during the first ~100 m of burial. Higher proportions of insoluble materials and pelagic carbonate seem to have inhibited diagenetic alteration in adjacent marlstones; in spite of significant compaction and pressure solution during burial, original isoto pic compositions appear to be best preserved in these intervals at Site 1007. The documented contrasts in petrographic and isotopic patterns illustrate the role of primary sediment composition in controlling lithification processes in periplatform carbonates and stress the importance of consid ering such factors when interpreting geochemical data from ancient shelf and slope limestones.

Petrographic and Geochemical Characterization of Coals from Tiru Valley, Nagaland, NE India

Abstract: In Tiru valley region of Mon district coals occur in Barail Formation and are of Oligocene age. This study is the first investigation on the petrographic and geochemical characterization of these coals. These thinly bedded seams (<2 m) need systematic exploration for proper evaluation of the resource.The coals are bright to moderately bright in appearance and at places look moderately dull. The study reveals that Tiru valley coals are sub-bituminous to bituminous-D in rank. The chemical study reveals that these coals are perhydrous in nature with moderately high volatile matter content. The sulphur content was probably increased with pyrite inclusion as the most abundant mineral. Vitrinite is the most dominant maceral group constituting nearly 98% of all the macerals while liptinite and inertinite occur in very low concentration. Facies-critical models used to decipher the paleodepositional environment suggest that these coals evolved under wet moor with moderately high flooding in the paleomire which all...