Abstract
This study presents microscopic results of 50 samples and 5 in situ magnetic susceptibility profiles, field investigations and bedrock characteristics of northern Harrat Rahat Quaternary volcanic rocks, located in the southern part of Al-Madinah city. The petrographic data of volcanic rocks show mainly aphanitic (fine-grained) holocrystalline basaltic units, rich in olivine depicting porphyritic, glomeroporphyritic, ophitic, subophitic, dictytaxitic, seriatic, interstitial, and trachytic textures. Field observations and petrographic studies reveal 9 rock units, from oldest to youngest: olivine basalt, hawaiite, vesicular olivine basalt, alkali olivine basalt, mugearite, benmoreite, scoriaceous basalt, trachytoid basalt and pyroclastic rocks. The magnetic susceptibility data vary from one rock to another based on texture, alteration, grain size, opaque minerals content, and weathering. The Fe-rich and massive basaltic rocks generally reveal higher susceptibility, while basaltic scoria and vesicular, fractured and weathered basaltic rocks generally reveal lower susceptibility. Occurrence of highly mature opaque minerals might suggest an associated sulphide mineralization.