Volcanic and subvolcanic rocks dating in the north of Shahrekord by 40Ar/39Ar method, a new approach regarding the occurrence andduration of the Jurassic volcanism in the Sanandaj-Sirjan zone

The volcano - sedimentary complex in the north of Shahrekord as a belt with NW-SW trendand 396 km2 area is parallel to the Zagros structural zone within the central part of the Sanandaj- Sirjan Zone. The volcanic rocks have mafic and intermediate affinity. The subvolcanic microdiorite rocks were exposed as small stocks. The pyroclastic rocks are composed of tuffs، lapilli tuffs، agglomerates، volcanic breccias and tuffites. To determine the age of the aforementioned volcanic rocks، due to the presence of fresh amphibole crystals in the andesitic and microdioritic rocks، 40Ar/39Ar dating method was applied. On the base of 40Ar/39Ar data، the studied volcanic and subvolcanic rocks have formed between 145 to 169 Ma corresponding to Callovian to Titonian stages (upper Dogger to upper Malm of Jurassic period). The dating results are well consistent with the other geochronological studies performed on some of the intrusive bodies in the Sanandaj-Sirjan Zone (i. e. Aligoodarz and Alvand intrusives) which are related to the subduction of Neothetyan Oceanic Lithosphere below the Central Iran Plate. Nevertheless، the aforesaid volcanism، belonging to middle Kimmerian orogenic phases in this part of the Sanandaj-Sirjan Zone، is reported for the first time.