Mantle-crustal source of mafic-felsic magmas in the dubr-igla intrusive complex, Egypt: infference from geochemistry and Sr-Nd isotopic study

The Dubr-Igla intrusive complex [DIC] is a composite batholith intruded into the Pan-African terrain of the Egyptian basement complex. It represents an uplifted tectonic block, comprising a diverse rock association ranging from gabbro to granite. These rocks belong to two distinct suites; the gabbroid-diorite [mafic-intermediate] and granodioritemonzogranite [felsic] suites. The gabbroid-diorite suite has a calc-alkaline affinity and an overall depletion of Nb, Y, K, and Rb with slightly fractionated REE pattern [La/Yb[N]= 0.96 -3.72]. The granodiorite and monzogranites are late-to post orogenic talc-alkaline rocks with metaluminous to mildly peraluminous character and are relatively enriched in LIL. elements but depleted in HFS elements. They have fractionated REE patterns [La/1b[n]=2.62-6.97], flat HREE [Gd/Yb[N]= 0.89-1.10] and well-developed negative Eu anomalies [Eu/Eu[=0.74-3.0]. The granitoid rocks of the DIC have trace element and REE signatures characteristic of 1-type granites formed in Subduction-related arc environments. Whole-rock Rb-Sr isochron ages of the monzogranites and the diorite-quartz diorite from DIC are 644 +/- 7 Ma and 704 +/- 13 Ma, respectively. The diorite-quartz diorite shows a narrow range of ot [Nd] [+6.7-+8.5] and initial [87]Sr/[86]Sr ratio [0.7022-0.7025]. These isotopic characteristics and the overall depletion of the LIL elements and LREE and low Rb/Sr suggest that they had been derived from a depleted mantle source. The granodiorite and monzogranites have low initial [87]Sr/[86]Sr ratio [0.7025 - 0.7035] and positive spread of[Nd][+3.4 - +5.2], which indicate that their protolith was either mantle or most likely juvenile lower crust that had been separated from the mantle. Isotopic, REE data and numerical modeling performed on the three rock types of the DIC [diorite-quartz diorite, granodiorite and monzogranite] indicate that complex petrogenetic processes generated them as follow. 1] the diorite-quartz diorite was formed by 10%-15% partial melting of mantle-derived basaltic source similar in composition to the associated gabbroid rocks; 2] the monzogranites were derived through: a dehydration melting [30%] of mafic metaigneous lower crustal material due to underplating by a mantle-derived magma which supplies heat budget for melting-followed by -about 70%-80% fractional crystallization of a crustal derived- primitive granitic melt to yield the differentiated monzogranites in the DIC; 3] the granodiorite shows abundant field and petrographic evidence for variable extent of mafic-felsic magma interactions [mingling, mixing and heterogeneous hybridization]. Least squares mixing tests, using trace and major elements support the formation of the granodiorite rocks by a simple mixing of two end-members, the diorite and the less differentiated monzogranite. A proportion of the diorite end-member ranging between 15 and 50% can explain the observed chemical variations in the granodiorite

Geochemical aspects and origin of ounger granites from afif and al hijaz terranes in the arabian shield

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