Petrological and geochemical characterization of Abu Murrat syn-orogenic I-type and post-orogenic A-type...

The Abu Murrat area, Northern Eastern Desert (NED) of Egypt, exposes two Neoproterozoic granitoid suites recording successive Pan-African evolution. The older syn-orogenic suite comprises quartz diorite, tonalite, and granodiorite, whereas the younger post-orogenic suite consists mainly of monzogranite with minor syenogranite. The syn-orogenic rocks have a moderate SiO₂ (59.7‒69.3 wt%), high CaO (4.0‒7.3 wt%), Na₂O (4.7‒5.7 wt%), and MgO (0.6‒2.2 wt%), and weak Eu anomalies (Eu/Eu*= ~0.8‒1). Field relations, modal QAP classification, and R1-R2, normative Ab-An-Or, A/NK-A/CNK, Ga/Al, and Nb-Y-Rb diagrams indicate calc-alkaline, magnesian, dominantly metaluminous volcanic-arc/I-type affinities. Their evolution involved assimilation and fractional crystallization (AFC), shown by decreasing MgO, Fe₂O₃^T, TiO₂, CaO, P₂O₅, and Sr with increasing SiO₂, variable K/Rb and Ba/Nb, declining CaO/Na₂O, and feldspar-controlled depletion of Eu, Sr, and Ba. The post-orogenic granites are richer in SiO₂ (70.2‒75.8 wt%), K₂O (5.1‒8.0 wt%), Rb (78‒178 ppm), Ga/Al, and HFSE, but poorer in CaO (0.6‒1.5 wt%) and MgO (0.04‒0.62 wt%). They also exhibit strong Eu anomalies (Eu/Eu*= ~0.05‒0.39), consistent with a within-plate A-type affinity. Zircon saturation temperatures are higher in the post-orogenic suite (~ 794-835 °C) than in the syn-orogenic granitoids (~ 635-731 °C), and monazite saturation estimates show a comparable tendency. The post-orogenic melts are interpreted as products of tonalitic lower-crustal melting during post-collisional extension, probably promoted by slab break-off and lithospheric delamination. Abu Murrat thus records the transition from subduction-related I-type arc magmatism to post-collisional A-type magmatism during Pan-African orogen collapse.