Characterization and Engineering Evaluation of Tholeiitic to Calc-Alkaline Rock Assemblages in Achin District, Nangarhar Province, Eastern Afghanistan

Abstract

Magmatic lithofacies assemblages serve as crucial archives of lithospheric evolution, while simultaneously acting as high-durability geomaterials for heavy industrial infrastructure. This study presents an integrated petrological, whole-rock geochemical, and petrophysical characterization of the sub-alkaline intrusive suites within the Gaodara area (Achin District, Nangarhar Province, Eastern Afghanistan) to resolve their petrogenesis and engineering utility. Systematically sampled outcropping suites and adjacent country formations were evaluated utilizing polarized-light thin-section microscopy, X-ray fluorescence (XRF) spectrometry, and hydrostatic displacement densitometry. Petrographic analysis identifies a distinct plutonic-hypabyssal bimodal system. The intrusive core comprises phaneritic, unevolved olivine gabbros exhibiting relict hypidiomorphic-granular to ophitic fabrics dominated by calcic plagioclase, clinopyroxene, and fractured olivine. Conversely, the enclosing country rock framework consists of intermediate, fine-to-medium-grained calc-alkaline tonalites containing quartz, plagioclase, biotite, and hornblende. Whole-rock major-oxide geochemistry confirms a distinct sub-alkaline bimodal distribution. The primitive, mantle-derived gabbroic core displays a highly refined tholeiitic differentiation path characterized by low silica (SiO₂ down to 36.69 wt.%), prominent iron enrichment (Fe₂O₃ up to 18.31 wt.%), and elevated magnesium numbers (MgO up to 20.70 wt.%). Conversely, the surrounding tonalitic country rocks follow a more evolved calc-alkaline trend, reaching high silica parameters (SiO₂ up to 82.36 wt.%) and elevated potassium fractions (K₂O up to 4.09 wt.%). Geotechnical testing reveals that these crystalline microstructural arrays directly dictate the rock mass durability. The dense, interlocking tholeiitic fabric yields a highly compact rock matrix with a low mean effective water absorption of 0.13%, an elevated mean bulk density of 3.06 g/cm³, and a mean specific gravity of 3.07. Ultimately, these exceptional petrophysical and geochemical attributes confirm that the Gaodara tholeiitic-to-calc-alkaline bimodal suite provides high-resistance, low-porosity geological formations ideally suited for heavy civil engineering, industrial dimension stone production, and high-durability construction applications.