Evolution of a Neoproterozoic island arc in the northern Arabian-Nubian Shield: Volcanic rocks and their plutonic equivalents in the Hamash area, south Eastern Desert, Egypt

• Published in: Precambrian research 2021-06, Vol.358, p.106145, Article 106145
• Main Authors: Gharib, Moustafa E., Maurice, Ayman E., Ahmed, Ahmed H., Asimow, Paul D., Bakhit, Bottros R., Selim, Hussam A.
• Format: Article
• Language: English
• Subjects: Arabian-Nubian Shield; Arc evolution; Eastern Desert; Egypt; Magma generation; Neoproterozoic
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2021.106145

•The Hamash volcanic and plutonic rocks represent a Neoproterozoic arc crustal sections.•Hamash rocks were formed during different stages of arc maturity.•The mantle source depletion changed with arc maturity.•The role of subduction components was more significant in the mature stage. A suite of Neoproterozoic island arc volcanic and plutonic rocks are exposed in the Hamash area of the south Eastern Desert of Egypt. The volcanic assemblage includes metabasalt, a group of andesites with subordinate basalts (the Hamash Volcanics), and felsic volcanic porphyry. The plutonic rocks include a diorite-tonalite series, fine-grained granite, and granite porphyry. The volcanic rocks are generally sub-alkaline and feature a strong depletion in Nb and Ta. The metabasalt belongs to the low-K tholeiitic series and has nearly flat rare-earth element patterns. Its compositional characteristics indicate generation by partial melting of a depleted mantle source in an immature oceanic island arc setting. On the other hand, the basalt, andesite and felsic volcanic porphyry have low- to medium-K calc-alkaline affinity and are enriched in light rare-earth elements and large ion lithophile elements. Their parental magmas likely formed by fluid-assisted partial melting of a less depleted mantle source with major subduction input, in a mature island arc setting. The andesite cannot be derived from the basalt by fractional crystallization; their parental magmas likely represent different degrees of melting underneath crust of different thickness. Estimation of the arc crustal thickness during formation of these rocks yields ~ 5 km for the metabasalt, ~20 km for the Hamash basalt, and ~ 30 km for the Hamash andesite, consistent with a progression in maturity of the arc over time. The geochemical characteristics of the diorite-tonalite series suggest that it represents an intrusive equivalent of the medium-K calc-alkaline Hamash andesite, and the fine-grained granite represents residual liquid after continued fractionation of the parental magma of the andesite and diorite-tonalite. We present thermodynamic fractionation models that constrain the conditions of fractionation for the andesite, diorite-tonalite, and fine-grained granite suite. The compositional similarity of the granite and rhyolite porphyries, especially their SiO2 and Al2O3 contents and Y/Nb ratios, indicate generation by partial melting of similar crustal sources, although greater assimilation of upper crust is eviden