تعيين سن مجموعه نفوذي اشنويه با روش پتاسيم - آرگن

K/Ar age dating of Oshnaviyeh plutonic complex

Oshnaviych plutonic complex (OPC), the western member of Urumiyeh-Golpayehgan intrusive plutons is located in northern part of Sanandaj-Sirjan Zone. OPC, exposing in an area of about 700 kin`, comprises 10 plutons that can be divided into three suites, i.e., diorite, granite and alkali syenite -alkali granite. Dioritic bodies arc the oldest intrusive rocks of the region, which on the basis of the field study , their relative age of emplacement is estimated to be Post- Jurassic and Pre-Miocene. However, with respect to the age of other similar intrusive bodies in Naghadeh area, they are most likely of Post Early Cretaceous - Pre Miocene age. Hybrid intrusive rocks, occurring at the contact of dioritic and granitic rocks may suggest a simultaneous emplacement of both magmas. Syenitic pluton from alkali syenite -alkali granite suite has intruded dioritic and granitic rocks. in contrast, flourine bearing alkali granite pluton from this suite shows no contact with other igneous rocks in the area. K-Ar age determinations obtained on amphibole specimens from diorite suite arc 91.9+2.3. 94.1+2.3 and 100+2.4 Ma. and on biotite specimens from granite suite are 100+1.5 to 989+1.5 Ma. Chronology study using same method on arfvedsonite specimens from syenite pluton shows 78.9±3.1, 79.611.9 and 81.7±2.0 Ma and on K-fledspar samples of flourine bearing alkali granite pluton from the alkali syenite -alkali granite suite presents 76±3.4 and 77.1±1.8. Therefore, based on field evidence and KIAr age dating, OPC presumably formed during two episodes: granite and diorite suites formed simultaneously at about 100 Ma, then plutons of alkali syenite -alkali granite suite emplaced at about 80 Ma.