Middle Cretaceous Andean‐Type Arc Evolution in the Sanandaj–Sirjan Zone (Shalair Valley), Iraq: New Geochronological and Geochemical Constraints

The middle Cretaceous granitoid plutons of the Shalair Valley, situated in northeastern Iraq, constitute a principal magmatic component of the Sanandaj–Sirjan Zone within the northwestern segment of the Zagros Orogenic Belt. Among these plutons, the equigranular Aulan body (AG) and the porphyritic Laladar body (LG) were crystallized at 111.0 ± 1.5 Ma and 112.4 ± 2.1 Ma, respectively, as determined by zircon U‐Pb dating. These bodies are metaluminous, calc‐alkaline I‐type granitoids. The AG rocks show positive εNd (t) values (+2.1 to +2.6) with identical Nd model ages of 0.78–1.27 Ga, whereas the εNd (t) values of the LG rocks range between +0.3 and +4.1 with Nd model ages of 0.87–1.5 Ga. These isotopic signatures collectively suggest derivation from a younger, juvenile crustal source for both granitoid bodies. Integration of the present results with previous studies on the middle Cretaceous granitoids of the Shalair Valley indicates that these intrusions were broadly synchronous and syngenetic. Moreover, Sr–Nd isotopic data reveal a distinct compositional contrast between the northern and southern limbs of the Shalair Valley volcanic arc‐related anticline, with the northern intrusions incorporating a higher proportion of crustal material during magma evolution. Taken together, the geochemical and isotopic evidence substantiates the interpretation that the Shalair Valley granitoids were emplaced through fractional crystallization (FC) processes coupled with crustal assimilation, within a subduction‐governed, mature Andean‐type continental arc that developed along the northwestern Zagros Orogenic belt during the middle Cretaceous.