Post-collisional volcanism with adakitic signatures in the Arabian-Nubian Shield: A case study of calc-alkaline Dokhan volcanics in the Eastern Desert of Egypt

Abstract

A volcano-sedimentary succession, including the Guruf volcanic series, is exposed at Gabal Um Guruf in the north Eastern Desert of Egypt. A series of intermediate to silicic volcanic flows and associated pyroclastic deposits unconformably overlie quartz-diorite and granodiorite plutons and are intruded by high-level alkali feldspar granites. The metavolcanic xenoliths in the granodiorite are shown to be unrelated to the Guruf volcanics. Two volcanic cycles can be recognized. The lower unit is a high-K calc-alkaline series with minor basaltic andesite and mostly andesite through dacite. The upper unit consists of rhyolite and rhyodacite that is transitional towards alkaline, A-type magma. Both units are assigned to the Dokhan volcanics and interpreted as post-collisional. REE patterns are characterized by slow progressive enrichment throughout the suite, with heavy REE apparently acting more incompatibly than light REE; concave-upwards REE patterns in basalt andesite suggest residual amphibole in the source. Only the upper rhyolite displays prominent negative Eu anomalies (Eu/Eu* = 0.35–0.73). Apparent crystallization temperatures of primary pyroxene are 800–1000 °C and of primary amphiboles 781–944 °C. The major and trace element chemistry of the suite can be modeled as a single liquid line of descent, with pressure decreasing as the magma cools and evolves, and a prominent role for apatite in sequestering CaO, P₂O₅, and light REE. Although upper crustal contamination and magma mixing cannot be ruled out, the data do not require them. The suite includes pyrophanite, an unusual Mn-rich ilmenite-group solid solution, not previously recorded in volcanic rocks in the Arabian-Nubian Shield (ANS). The Guruf volcanic series is undeformed and unmetamorphosed, unlike the typical subduction-related volcanic units of the ANS. The geochemical characteristics are consistent with those of many post-collisional ANS rocks. The mostly calc-alkaline character and other traits previously interpreted to indicate an active arc setting instead more likely reflect remelting of earlier arc-related material from the pre-collisional stage (850–740 Ma). The lava flows of the lower succession have adakitic characteristics, including high Sr, low Y, low Yb, high Sr/Y, and high (La/Yb)n. This likely reflects the influence of residual or fractionated amphibole in the ANS crust, rather than slab melting during active subduction. A post-collisional episode of delamination and heating of tonalitic lower crustal material, followed by mixing with asthenospheric mantle, created a primary magma with the characteristics of the Guruf basaltic andesite, including residual amphibole. There is no need for either late subduction in the area or long-term persistence of subduction-influenced material in the local convecting mantle.

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