Petrography

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

A re-evaluation of aragonite versus calcite seas

Abstract: Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous, and Jurassic-Cretaceous to Early/Middle Cenozoic (e.g. Sandberg 1983; Wilkinson and Algeo 1989). Variation in carbonate mineralogy has been related to the position of global sea level (emergent or submergent modes, Wilkinson et al. 1985), change in rates of seafloor spreading (e.g. Mackenzie and Pigott 1981; Hardie 1996),PCO 2 level (e.g. Sandberg 1985; Mackenzie and Morse 1992; Hallock 1997) and Mg/Ca ratios related to spreading rate (e.g. Stanley and Hardie 1998). However, other researchers suggest that the assumption of change of original carbonate mineralogy through time needs to be re-evaluated in the light of mineralogical change that is related to water temperature or latitudes (e.g. Nelson 1988). Evaluation of Ordovician (Arenig to Ashgill) Gordon Group carbonates of Tasmania (Australia), based on petrographic (e.g. abundantChlorozoan biota, and oomold texture) and geochemical criteria (such as high Sr/Na ratios) indicated that aragonite (not calcite) was the predominant mineral in these warm water, subtropical carbonates (Rao 1990). Petrographic (e.g. presence or absence of aragonite relicts, abundant acicular to fibrous isopachous marine cement, presence of diffuse laminae and a number of spalled ooids) and geochemical evidences (such as elevated Sr) in the Upper Jurassic Mozduran limestone, in Kopet-Dagh Basin in northeast Iran, showed variation in carbonate mineralogy, in spite of similar atmosphericPCO 2 level, global sea-level and tectonic setting. This is evidenced by aragonite occurring in the shallowest part of the basin (Adabi and Rao 1991) and mainly calcite with some aragonite forming in the relatively deeper water (below wave base) (Adabi 1997). Carbonate mineralogy in Recent shallow marine carbonates, and in experimental studies, varies with seawater temperature. In the Recent, aragonite is the predominant mineral in warm, shallow marine carbonates and calcite the dominant mineral in marine cool water carbonates (James and Clarke 1997). Therefore, variations in carbonate mineralogy in the Mozduran limestone are attributed to seawater temperature assuming invariant seawater chemistry prevailed in the Upper Jurassic. Several Jurassic examples show variations in ooid carbonate mineralogy, such as the Upper Jurassic Smackover oolite of the Gulf Coast region (southern Arkansas and northern Louisiana) and Upper Jurassic ooids in the Purbeck limestones of Swiss and French Jura. These results are not in agreement with the concept of a “calcite sea” during the Ordovician and the Upper Jurassic periods. Very recently, Westphal and Munnecke (2003) showed that in spite of the tendency of abiotic precipitates (Sandberg 1983) and skeletal mineralogy (Stanley and Hardie 1999) to follow the general trend of calcite seas and aragonite seas, organisms with calcite and aragonite mineralogy coexisted throughout the Phanerozoic. They have examined the temporal and spatial distribution of limestone-marl alternations in Ordovician, Jurassic and Cretaceous (times of calcite seas). Limestone-marl alternations are most abundant in settings that favored aragonite production and accumulation analogous to the modern environment (Westphal and Munnecke 2003). If the above observations confirmed, the proposed secular variation in Phanerozoic carbonate mineralogy needs to be re-evaluated.

Petrographic variations within thick turbidite sequences: an example from the late Palaeozoic of eastern Australia

Abstract: The subdivision of thick sequences of turbidite sediments has been problematical because of the monotonous nature of the units. One method, of using detailed detrital petrography for a large number of specimens, has delineated variations with a sequence of Late Palaeozoic age in eastern Australia. The rocks occur within a single structural block and are all members of one sedimentary petrographic province. They have been subdivided into three stratigraphic units (Moombil Beds, Brooklana Beds and Coramba Beds) and greywackes from these units are quartz-poor to quartz-intermediate, feldspathic or volcanolithic types. Dacitic volcanism has provided most of the detritus and the contribution from non-volcanic sources is small. The Coramba Beds are further subdivided into four petrographic units which are parallel to the stratigraphic boundaries. These lithostratigraphic units are based on the presence or absence of detrital hornblende, and the relative ratio of volcanic lithic fragments to feldspar. Vertical petrographic variations within the entire sequence indicate that although the acid volcanic source was predominant throughout the time of deposition, there is a noticeable increase in the contribution from intermediate-volcanic, acid-plutonic, low-grade metamorphic and sedimentary sources towards the top of the sequence. Detrital hornblende is also present in the upper parts of the sequence.

Petrogenesis of bimodal volcanic rocks from Maoershan Formation in Zhangguangcai Range:evidence from geochronology and geochemistry

Abstract: LA-ICP-MS zircon U-Pb dating and geochemical data of the volcanic rocks from Mesozoic Maoershan Formation in Zhangguangcai Range were obtained for to constraining their formation time and the regional tectonic background.Zircons from two representative volcanic rocks are euhedral-subhedral in shape and display striped absorption or oscillatory zoning in CL images with high Th/U ratios(0.40～2.08),implying their magmatic origin.The dating results indicate that the volcanic rocks from Maoershan Formation were formed in the Early Jurassic(179～184 Ma) which is older than the previously believed Late Jurassic.The volcanic rocks from the Maoershan Formation display a bimodal volcanic rock association based on their petrographic and geochemical data.The trachyte and trachyandesite are characterized by high alkali(especially K2O),enrichment in large ion lithophile elements(LILE) and light rare earth elements(LREE)in contents,whereas the rhyolites are chemically similar to the A-type rhyolite.The bimodal volcanic rock association in Lesser Xing'an Range and Zhangguangcai Range in the studied areas implies an intense extensional environment.Combined with the spacial compositional variation of the Early Jurassic igneous rocks,it is suggested that the volcanic rocks from Maoershan Formation were formed under an extensional environment similar to back-arc basin which could be related to subduction of the Paleo-Pacific plate(Izanagi) beneath the Eurasian continent.