Characteristics of detrital chromian spinels in sandstones from the Nam Duk Formation, Amphoe Lom Sak and Amphoe Nam Nao, Changwat Phetchabun

Abstract

Detailed field investigation along the Lom Sak-Chum Phae Highway along km 16.0 to km 42.0 in Phetchabun reveals Nam Duk Formation is subdivided into 6 associated rock units from older to younger, namely lower sandstone-shale alternation, limestone-shale alternation, upper sandstone-shale alternation, shale-sandstone alternation, sandstone-shale-limestone alternation, and shale units. The first three units are located in the west part of the study area and exhibit stronger deformation features and deeper environment than those of the units situated mostly in the east. Along the highway, numerous detrital chromian spinels are discovered as the accessory mineral in lithic graywacke (less than 1 percent) of mostly deep-water facies. Petrographically detrital chromian spinels are generally deep brown to almost opaque in transmitted light corresponding to their Cr-Al-Fe3+ substitutions. Their grains vary between 30 and 75 mum in size and some rise to 100 mum across. They are mostly sub-angular to angular. Some of them exhibit sub-hedral to euhedral habits suggesting the preservation of the original crystal shape. Thirty-five grains of detrital apinels were chosen for electron microprobe analysis (EMPA). Characteristically these spinels have relatively high Cr content and relatively vary in TiO2 content. The atomic ratios Cr/(Cr+Al) or Cr# are almostly above 0.5. The Fe3+ values are consistently low. The atomic ratios Fe3+/(Cr+Al+Fe3+) in detrital spinels are below 0.2. Composition plots between the Cr/(Cr+Al) and Mg/(Mg+Fe2+) and ternary plots of the major trivalent cations (Cr, Al and Fe3+) for the different chromian spinels, regarded powerful for discriminating mafic to ultramafic rocks of different sources, indicate that the Nam Duk detrital chromian spinels lie within the composition field of the arc-related source rocks. Considering petrographic investigation, the encountered sub-hedral to euhedral-shape spinels with a few inclusions probably suggests volcanic origin. Occurrence and morphology of detrital chromian spinels mostly in the deep-water sediments of the Nam Duk Formation point to the existence of exposed ultramafic-mafic rocks during sedimentation of the Nam Duk Formation. The result reveals that the arc-related tectonic setting may have developed in the region during Middle Permian prior to more violent subduction activity of Permo-Triassic period.