Petrography

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

White marble from italy and turkey: an archaeometric study based on minor- and trace-element analysis and petrography

Abstract: In 94 marble samples from 4 quarry districts in Italy (Carrara) and Turkey (Proconnesus, Dokimeion, Usak), minor and trace elements were determined by instrumental neutron activation analysis (INAA) and atomic absorption spectrometry (AAS). The maximum size of the calcite grains (MGS) of the rocks was measured in thin section. For 16 elements considered in this work, the concentration ranges show important inter-district overlaps; this also applies to the maximum grain size. However, the application of cluster analysis, using selected attributes, allows one to discriminate every pair of districts; 90 samples are classified correctly in all classification dendrograms.

Source or Sink? An Assessment of the Role of the Old Red Sandstone in the Genesis of the Irish Zn-Pb Deposits

Abstract: The Zn-Pb (-Ba) deposits of central Ireland are hosted by Lower Carboniferous Waulsortian limestone and Navan Group carbonate sediments. The deposits are located near the margins of subbasins in close association with extensional faults. The source of metals and the flow pathways utilized by the hydrothermal fluids responsible for ore genesis have long been a matter of conjecture. The Devonian to Lower Carboniferous Old Red Sandstone has been suggested as a possible regional aquifer and metals source. Therefore, a petrographic and lead isotope study of this unit was conducted to assess its putative role in ore formation. Hydrothermally unaltered Old Red Sandstone has 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.71 to 22.45, 15.52 to 15.86, and 38.57 to 42.71, respectively. The sedimentological characteristics of the red beds, coupled with results of isotopic modeling, support the conclusion that the lead isotope signature of the unit is predominantly controlled by radiogenic Caledonian granite detritus that has an Avalonian-like character. The modeled lead isotope composition of the Old Red Sandstone at the time of the Lower Carboniferous ore-forming event is too radiogenic for it to have acted as the main source of lead for the Irish ore field. Examination of drill core, outcrop, and underground exposures reveals that hydrothermal alteration within the Old Red Sandstone forms a halo around faults, indicating that fluid flow in the unit was predominantly structurally focused. These hydrothermally altered sediments have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.21 to 21.77, 15.54 to 15.91, and 38.02 to 41.57, respectively. Unlike the unaltered sediments, several altered samples have lead isotope compositions similar to galena from the Lower Carboniferous deposits. Petrographic examination reveals that these samples contain trace hydrothermal sulfides that are probably responsible for the orelike signature. Therefore, it appears that, at least locally, the Old Red Sandstone functioned as a sink for lead during the Lower Carboniferous mineralizing event. The conclusions from this study eliminate the Old Red Sandstone as the main source of lead in the ore field and suggest that deposit lead is derived primarily from lower Paleozoic rocks (± Precambrian crystalline basement). In view of these results, regional gravity-driven fluid flow models calling upon an Old Red Sandstone-focused flow regime and metals source can be discounted. Rather, deep circulation of fluids through the fractured basement underlying central Ireland plays an integral role in the evolution of the ore-forming system.

Oxygen Isotopic and Petrographic Evidence Relevant to the Origin of the Arkansas Novaculite

Abstract: O18 values for 73 chert samples from the Arkansas Novaculite (Devonian-Mississippian) range from + 17.8 to 33.7. Much of this variation is related to the effects of local metamorphism and isotopic impurities such as detrital quartz and clay minerals. Pure, unrecrystallized samples from the western outcrops in Oklahoma range from 30.3 to 33.7, whereas samples from Arkansas range from 25.1 to 29.7. As a group, west rn outcrops of the Novaculite are distinctly richer in O18. Petrographic and SEM examination of the O18-rich western samples reveal well preserved early diagenetic textures including abundant relict opal-CT lepispheres (now replaced by microquartz). Radiolaria and sponge spicules show well preserved morphologies. One western sample yielding the highest -value is microbrecciated, contains abundant dolomite rhombs and is laced with quartzine (length-slow chalcedony) suggesting silicification under evaporitic conditions. Samples from eastern localities display no evidence of early diagenetic textures, and radiolaria and sponge spicules are preserved only as petrographic ghosts. The isotopic and petrographic data can be interpreted in terms of a siliceous biogenic ooze deposited in environments ranging from supratidal to open marine. Shallow (platform?) deposition occurred in western areas with early silicification contributing to the preservation of diagenetic textures. Assuming that O18 of the diagenetic waters was within 1 of seawater the higher -values in the west reflect chert crystallization at isotopic temperatures of 20° to 29° C. The lower -values in the east suggest chert crystallization during deeper burial at isoto ic temperatures of 31° to 53° C, similar to burial diagenesis in Tertiary deep sea cherts.

Petrology of later Tertiary and Quaternary rocks of the north-central Cascade Mountains in Oregon, with notes on similar rocks in western Nevada

Abstract: INTRODUCTION AND ACKNOWLEDGMENTS The Cascade Mountains in Oregon have been known since pioneer days as the central part of a volcanic mountain chain extending south-ward into California as far as Lassen Peak, and northward across Washington to Mt. Baker. The rugged nature of the mountains, the heavy timber, and the volcanic rocks combined to render early geological reconnaissance difficult and inconclusive in many respects. It was soon recognized that the Cascade lavas overlie pre-Tertiary rocks in Washington and California, and that the conspicuous peaks are pre-glacial in age, and may have erupted within the last two or three centuries. However, it was by no means known, nor is it yet known, how many geological epochs are represented by the volcanic rocks of the Cascade Mountains, nor how many types of rocks occur, and in what proportions. Hague and Iddings in 1883 gave the first detailed petrographic description of lavas from . . .