Metamorphic Evolution of the Bulfat Belt (NE-Iraq): Evidence from 40Ar/39Ar Age Spectrum Measurements
The Bulfat Ophiolite Complex (upper allochthonous thrust sheet) which represents a part of the Iraqi Zagros Suture Zone is situated at the boundary between the Arabian Plate and the Iranian Microplate. We have examined the metamorphic and cooling history of metapelitic rocks of Bulfat Complex using the whole-rocks 40Ar/39Ar age determination. The results of four samples match to the definition of the plateau and revealed to 40Ar/ 39Ar age of 42.4 ±0.5 Ma (sample 1), 32.9 ±0.8 Ma (sample 2), 44.3 ±0.8 Ma (sample 3), and 43 .6 ±1.5 Ma (sample 5). However, samples 4 and 6 yielded an age range between 35- 40 Ma and 40- 50 Ma respectively. Generally, plateau ages (32.9-50 Ma) are identical and considered reliable, but the presence of excess argon (40Ar) remains possible in samples (e.g., sample 6) having initial40 Ar/36Ar ratios higher than the present atmospheric 40 Ar/36Ar = 295.5. The excess argon is derived from the mantle by plumes of mafic magmas into the earth crust and/ or released from minerals and rocks during metamorphic processes. Bulfat Complex is subjected to two metamorphic events: the earlier regional occurred during the period Albian-Cenomanian (97-105 Ma) and associated with the intraoceanic infant supra-subduction zone. The later thermal metamorphism overprinted the previous one, and took place due to the intrusion of igneous body pre – 50 Ma age. This igneous activity is considered one of the multi-events magmatism that happened along the Zagros Fold-Thrust Belt during the period from Upper Cretaceous and Mid – Late Miocene. It seems evident that the thermal metamorphic slice of Bulfat Complex (Gemo-Qandil Sequence) represents a part of the oceanic slab that has been subducted beneath the Iranian microplate. This event probably forming the last evolved episode of island arc system ended with the final closure of Neo-Tethys and the collision between the Arabian Plate and Iranian Microplate occurred at Mid-late Miocene.