Petrography

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

Drainage and Emplacement of Magmas along an Inclined Transcurrent Shear Zone: Petrophysical Evidence from a Granite-Charnockite Pluton (Rahama, Nigeria)

Abstract: A Pan-African granite-charnockite pluton of northeast Nigeria has been core-sampled at an average grid spacing of 2 km. Detailed petrographic and microstructural observations are combined with an AMS study of 172 stations. The continuum between charnockites and granites is revealed by their constituent minerals and the magnetic susceptibility map. The contemporaneity of emplacement of all the petrographic types is demonstrated by their similar mineral- and magnetic-fabrics. Inclined strike-slip shear zones were active during and slightly after the emplacement of this pluton. Earlier geochemical data indicate that magmas originated from several vertically juxtaposed lower-crust sources. The proposed model of emplacement implies that the shear zones were responsible for the migration, drainage and tapping of magmas from the source region. The inclined attitude of the shear zones would have acted as a roof-guide for the up-rising magmas. Finally, the transcurrent movement most likely enhanced the upward migration of magmas.

Diagenesis of radiaxial fibrous calcites in a subunconformity, shallow-burial setting: Upper Triassic and Liassic, Northern Calcareous Alps, Austria

Abstract: Multiple generations (I-IV) of radiaxial fibrous calcite (RFC) cement crusts are intercalated with meteoric internal sediments and microsparitic calcite cements within karst dissolution cavities in Upper Triassic Steinplatte and basal Liassic Hirlatz carbonates in the Northern Calcareous Alps of Austria. Stratigraphic, petrographic and geochemical evidence suggest that RFC precipitated as high-Mg calcite from marine fluids in the shallow-burial (<180m) environment. Estimated original δ18O and δ13C isotopic compositions of Late Triassic and Early Jurassic marine waters are −1.88 to −2.21%‰ and +2.37 to +2.89%‰ (PDB), respectively. The RFC subsequently was altered to low-Mg calcite by meteoric or mixed meteoric-marine fluids introduced during periods of subaerial exposure. RFC cement generations I-IV in Steinplatte reef-slope beds, and I-II in Steinplatte reef and lagoon and Hirlatz samples retained near-original marine oxygen and carbon isotopic compositions (-1.88 and +2.89%‰ PDB, −2.11 and +2.37%‰ PDB, respectively) and petrographic identity during the conversion to low-Mg calcite. In contrast, RFC generations III and IV in the Hirlatz and upper Steinplatte were intensely leached and partly replaced by coarse, blocky calcites of meteoric origin, with attendant modification of original crystal fabrics and isotopic compositions (−4.65 to −5.22%‰ PDB δ18O, +2.54 to +3.14%‰ PDB δ13C). These differences in diagenetic behaviour are related to contrasting processes of fluid-rock systems and original microfabric variations in the RFC cements. The karst-affected, phreatic-marine burial environment of RFC formation and subsequent diagenetic alteration inferred for these rocks have not been previously documented in other ancient rocks.