Résumé
The late Proterozoic granitoids from two terranes of different tectonic regimes, namely the Afif and Al Hijaz were studied to constrain the mature of the magmatic processes during the waning stage of crustal evolution in the Arabian Shield. The granitoid rocks from the Afif and Al Hijaz terranes include three major rock types [diorite-quartz diorite, granodiorite and monzogranite] which have wide composition range in term of major oxides, SiO[2][54.74-76.80%], Al[2]O[3] [12.29-17.27]; Fe[2]O[3][0.91- 11.63], CaO [0.57-6.62]. The granitoid rocks in the two terranes are talc-alkaline, metaluminous to mildly peraluminous and belong petrologically and chemically to the I-type granites. Although, some monzogranitic samples chemically resemble A-type granites, they still have high LILE/HFSE ratios and belong to the high Y/Nb A-subtype granites. This can be attributed to the highly fractionated nature of some monzogranites. All the granitoid rocks are depleted in Ba, Sr, Nb, Ti and Eu and enriched in K, Zr, Y, Rb and LREE. Trace element modeling indicates that the dioritequartz diorite was formed by partial melting of the lower crust. Further, two stages of differentiation are necessary to explain the origin of the granodiorite and monzogranite. An early stage, high pressure, crystal-melt fractionation was dominated by separation of plagioclase, hornblende, apatite and magnetite from dioritic magma to yield the granodiorite. Low pressure fractionation [second stage] of the granodioritic liquid yielded the ultimate composition of the monzogranites with separation of plagioclase, hornblende., biotite, apatite and magnetite. In spite of the different tectonic regimes between the Afif [continental] and 41 Hijaz [oceanic] terranes, the granitoid rocks of the two terranes show a complete overlap in term of major and most of trace element abundances. Nevertheless, the Afif granitoids are slightly more differentiated than those of the Al Hijaz terrane. Geochemiral considerations suggest that the calc-alkaline granitoids of the two terranes probably have been derived from chemically similar source regions or have evolved by comparable petrogeaetic processes. Alternatively, the long fractionation history overshadowed the parental chemical fingerprints of these rocks