Özet:

In the eastern Pontides orogenic belt (NE Turkey) there are different volcanic rocks exposed that formed under different types of magma. Tertiary volcanism consists of acidic, notr and basic volcanic and pyroclastic rocks in Gümüşhane and surrounding area. The Eocene volcanism played an important role during the evolution of the eastern Pontides. In this study, petrography, mineral chemistry and P–T conditions of the Tertiary hornblende-bearing andesites in the Altınpınar (Torul-Gümüşhane) are investigated. The andesites are mainly consisting of hornblende (amphibole), plagioclase (An40-61Ab38-57Or1-5), clinopyroxene (Wo44-45En43-44Fs12), opaque (generally magnetite and pyrite) minerals, with secondary clay, sericite, calcite, chlorite minerals. . The samples commonly exhibit microlitic porphyry and vitrophyric porphyry textures. Some of the hornblende aggregates, occurring in large plates, contain inclusions of plagioclase and opaque minerals. All hornblende minerals are exclusively Ca-type with CaB ≥ 1.5, (Na+K)A <0.5 and CaA <0.5 (apfu) parameters, and magnesiohornblende in composition with Mg# (Mg/Mg+Fe2+) = 0.72-0.91. Estimation of geothermometry (T), geobarometry (P) and oxygen fugacity (ƒO2 / DNNO) calculations in andesites using calcic amphibole geothermobarometry equations indicates that the crystallization is estimated to have taken place at 765 – 867 °C (mean= 790±20 °C); 2.5 – 4 kbar and, from –14.4 to –11.1, respectively. These calculated values indicate that hornblende minerals are crystallized at depths of 9 - 15 km in continental crust. According to the chemical composition of minerals the andesites are crystallized from a calc-alkaline parent magma.