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

Abstract EN

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 (maficintermediate) and granodiorite-monzogranite (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 / Ybn = 0.96 -3.72).

The granodiorite and monzogranites are late to post orogenic calc-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 / Ybn =2.62-6.97), flat HREE (Gd / Ybn = 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 I-type granites formed in subduction-related, arc environments.

The 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 tNd (+ 6.7 - + 8.5) and initial 87Sr / 86Sr 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 87Sr / 86Sri ratio (0.7025-0.7035) and positive spread tNd (+ 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 follows : 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.