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Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

Farkhondeh Khademi, Abbas Asiabanha, John Foden, Zeinab Davoodi


The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a
suitable area for considering the effects of tectonic structures on development of the
volcanic facies in the magmatic provinces. The main structure in the area is a dextral
fault (so called Rahjerd Fault) that has produced the local extensional basins and
also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic
deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive
subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded
in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline
suite that had been changed and evolved in the magma chambers of local extensional
basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling
as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and
An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite
and magnesiohornblende) reveal that the AFC process is a dominant process in the
magma chamber. Also, according to geothermobarometric calculations, the investigated
volcanic rocks of the area could be grouped into two types: one type with higher P-T
(>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing
andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600-
800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA)
and the dioritic stock.


Keywords: parallel dykes; extensional basins; AFC; geothermobarometry; Tafresh; central Iran

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