Structural and microstructural analysis of the ophiolite zone, Neyriz area, Fars province, Iran

The Neyriz region, located along the Zagros suture zone, the boundary between Zagros foreland fold-and-thrust belt and Zagros hinterland fold-and thrust belt, is an important area of interest due with presence of obducted ophiolite related to Neo-Tethyan oceanic crust. The aims of this research is to study structural and microstructural data of the Neyriz ophiolite in order to present a new model for the geodynamic evolution of the high temperature solidus lithospheric and asthenosphric diapiric mantle flow of the Neo-Tethyan ocean. Structural and microstructural observation in the southern part of Neyriz ophiolite shows the presence of the shear zones between mantle diapiric flow ultramafic cumulates and overlain gabbroic section. There is evidence of formation of mylonitic foliation in the gabbros/serpentinized harzburgite. Presence of extensional microfaults obliquely cross cutting the mylonitic foliation, the shear sense of S/C fabrics, slickenside and steps on the surface of faults in the mylonitic gabbro suggest the shear zones acted as detachment fault. These features reveal that the Neyriz ophiolite is probably formed as Neo-Thetyan Oceanic Core Complex (OCC) and exhumed by large detachment faults. The characteristics such as, a) serpentinized harzburgite which is overlain directly by local pillow lavas or radiolarites, b) highly deformed and mylonitic gabbro/amphibolite that lies directly on serpentinized harzburgite, and is overlain by pillow lavas, the P-MORB/E-MORB affinity of diabase and basalt and the plagioclase fabrics are consistent with the OCCs model, which provide large exposures of mantle flow and lower crustal rocks on the sea-floor on detachment faults footwalls at slow-spreading ridges. A deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the Neyriz mantle diapir were analyzed using electron backscatter diffraction (EBSD). Both samples have the common amphibole crystallographic preferred orientation (CPO) in which (100) lies perpendicular to foliation and <001> parallel to lineation. Amphibole grains in the layered gabbro sample have little internal deformation, whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries. There is a subtle change in CPO as a function of grain size in the mylonitic gabbro. Coarse grains (porphyroclasts) have a (100) <001> CPO oriented with the main foliation reference frame whilst fine grains have a (100) <001> CPO oriented with the C' shear bands. Detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems, most particularly (110) <1-10> control intracrystalline deformation. Schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to C' shear bands. It is unlikely that the slip systems that control intracrystalline deformation are important in CPO formation. We interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the C' shear bands by grain boundary sliding. This rigid body rotation, possibly in combination with easy slip on (100) <001> are considered the main cause of CPO formation. Amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to CPO. A mylonitic skarn at the margins of a harzburgite of the Neyriz mantle diapir was also analyzed using EBSD. This sample contained garnet porphyroclasts in a wollastonite and pectolite matrix. Detailed analysis of garnet porphyroclast distortions and subgrain boundary trace analysis suggests that (110) <110> dislocation control intracrystalline deformation of garnet grains. Wollastonite and pectolite crystallographic preferred orientations (CPOs) are dominant by (100) parallel to foliation plane and <010> parallel to lineation. However, a dominance of low angle misorientation axes parallel to <010> precludes <010> dislocations from significant involvement in intracrystalline deformation. Schmid factor analysis also shows this slip system does not have high integrated Schmid factor. These observations suggest that oriented grain growth may be responsible for wollastonite and pectolite CPO development. Schmid factor analysis suggests (001) <100> and (100) <001> slip systems in the wollastonite and pectolite were involved in intracrystalline deformation and are linked kinematically to S2 and reactivated S1 planes.