## 6 Discussion
### Genetic Model
Figure 16: A simplified model for the tectonomagmatic evolution of the juvenile crust in the Jabal Sayid and Dayheen areas, the western AS of Saudi Arabia. (**A**) The subduction of the oceanic crust, ophiolite detachment, and thrusting along the Bir Umq suture zone with the arc volcanism and arc-related plutonism of the granodiorite–diorite association. (**B**) The collisional phase, crustal anatexis, and monozgranite intrusions. (**C**) The partial melting of the crustal material leads to the generation of peralkaline granites and related pegmatites in post-collisional extension settings. The crust and mantle thickness are not to scale. The reported ages of the peralkaline granites in Jabal Sayid and Dayheen are in agreement with the broader post-orogenic or anorogenic plutonic activity that occurred in the Arabian-Nubian Shield (AS) between 680 and 550 Ma, as documented by [62]. Ref. [63] reported a whole-rock Rb-Sr isochron age of 584 \(\pm\) 26 Ma for the Dayheen ring complex, indicating the timing of granite emplacement in that area. This age aligns with the estimated age of zircon from the alkali granite at Jabal Sayid, which is reported as 593 \(\pm\) 2.4 Ma, according to [7]. Furthermore, ref. [10] reported zircon U-Pb ages for various granitic rocks in the Dayheen area, including the hornblende-biotic granite, microgranite, and aegirine-riebeckite granites. These ages range from 613.3 \(\pm\) 8.1 Ma to 596 \(\pm\) 5.6 Ma. The ages reported by [7; 10] are in close agreement, indicating that the emplacement of peralkaline granites in the Dayheen and Jabal Sayid areas occurred between 613 and 584 Ma. The \(\epsilon\)Nd values of the Jabal Sayid and Dayheen plutons are positive and relatively uniform, demonstrating similarity to the Jabal Sayid and Dayheen plutons in the Jeddah Terrane. The initial \(\epsilon\)Nd for the alkaline granite in Jabal Sayid is +4.9 to 5.3 [7], while that of the Dayheen monzogranite, hornblende-biotic granite, and aegirine granite is +4.1 to 5.3 [6]. The observed similarity in isotopic composition between the Jabal Sayid and Dayheen peralkaline granites suggests a homogeneous isotopic signature in the source region. This similarity implies that these granites and their associated pegmatities were likely derived from the partial melting of the lower part of the juvenile AS crust. It is possible that the melting process involved the lower crust with or without direct contributions from upper mantle materials. This interpretation aligns with the findings of [10], who concluded that the granitic rocks in the Dayheen ring complex primarily originated from the juvenile crust, although there may have been some involvement of mantle-derived materials. Ref. [64] concluded that the S-type two-mica granitoids in the Bafouassam area of western Cameroon had heterogeneous isotopic signatures, which they attributed to anactic melts of the upper continental crust with variable contributions from the lower continental crust or mantle melts. It is worth mentioning that the present geochemical data do not support a hybridized source for the formation of the Jabal Sayid and Dayheen peralkaline granites. If hybridization had occurred, it would likely result in scattered isotope signatures of Sm-Nd. However, the restricted \(\epsilon\)Nd values observed in Jabal Sayid and Dayheen suggest a more homogeneous source without significant hybridization.