Mineralogical and Petrographical Constraints on the Magnetic Susceptibility of Alkaline Igneous Rocks: A Case Study from the Gölcük Volcano (Isparta), Turkey

Magnetic susceptibility (SI) measurements and mineralogical and petrographical studies were carried out on the alkaline potassium-rich Gölcük volcanic rocks, situated in the apex of the Isparta Angle in the southwest of Turkey. Magnetic susceptibility was measured on the pyroclastic rocks and outcrops using hand-held KT10-R Magnetic Susceptibility meter. The classification and SI values of the igneous rocks from the study area are as trachyte (SI between 0.45 × 10^–3 and 36.3 × 10^–3), trachyandesite (SI between 0.5 × 10^–3 and 5.7 × 10^–3), Clinopyroxene (Cpx)-phyric basaltic trachyandesite (SI: 36.3 × 10^–3), Clinopyroxene (Cpx)-phyric trachyandesite (SI: 0.63 × 10^–3), alkali feldspar syenite (SI: 20.9 × 10^–3 and 26.4 × 10^–3), syenite (SI: between 16 × 10^–3 and 35.6 × 10^–3), leucocratic monzosyenite (SI: 23.3 × 10^–3), monzosyenite (SI: 31.2 × 10^–3), mafic monzosyenite (SI: 15.4 × 10^–3 and 20.7 × 10^–3), pyroxene monzonite (SI: 0.56 × 10^–3), micaceous pyroxenite (SI: 0.42 × 10^–3), tephriphonolites (SI: between 11.3 × 10^–3 and 3.32 × 10^–3 but most of them between 17 × 10^–3 and 25 × 10^–3). Based on thin-section investigations, it is clearly seen that igneous rocks contain various amount of opaque minerals. X-ray data show that the opaque minerals are mainly magnetite minerals. Petrographical determinations and magnetic susceptibility SI measurements show that igneous rocks with the same composition and texture may have different magnetic susceptibility values and varying amount of magnetite minerals. On the other hand, some of the volcanic rocks with the same composition but different textures have also different magnetic susceptibility values. The presence of opaque minerals is the dominant factor controlling the magnetic susceptibility value of the Gölcük volcanites and the amount of magnetite mineral is directly proportional to the magnitude of the magnetic susceptibility. It is not plausible to suggest that rocks existing in the same classification should have the same magnetic susceptibility values.