Petrography, Mineral Chemistry and P–T Conditions of the Tertiary Hornblende-bearing Andesites in the Altınpınar (Torul-Gümüşhane)
Abstract
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.
Keywords
Altınpınar andesites,Eastern Pontides,Tertiary volcanism,geothermobarometry
Kaynakça
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