A newly recorded Cryogenian-Ediacaran Dokhan Volcanic caldera with resurgent uplift in the Arabian-Nubian Shield, southwest Safaga, Egypt

Abstract

The late Neoproterozoic Dokhan Volcanics (DV) of the Egyptian Eastern Desert (northern Arabian-Nubian Shield), were investigated at Gebel Nuqara, near the boundary between the Northern and Central Eastern Deserts. In particular, the nature of the peculiar circular and annular regional structures at Nuqara, were targeted for study. This included exploring the relationships of DV and intrusive units to each other and to regional dikes and faults. The central part of the Nuqara DV exposures consists of two volcanic units: 1) “Older DV” (intermediate varieties, dominantly andesites; 659 ± 16 Ma), with associated arc granitoids (663.2 ± 8.4 Ma), followed by andesite dikes (636.9 ± 7.2 Ma); and 2) “Younger DV” (acidic varieties; 602.3 ± 4.4 Ma for rhyolites, and 589.4 ± 6.1 Ma for rhyolite porphyry). This central area of DV units is enclosed within an annular intrusion of granite. The geochemical data for the Nuqara DV reveal intermediate low-silica adakitic (basaltic-andesite and andesite) and acidic high-silica adakitic (rhyolite and rhyolite porphyry). Both adakite groups share a common geochemical trend, and show evidence for magma diversification via fractional crystallization. The Nuqara DV is identified as a caldera sequence, with resurgent uplift, based on a positive comparison of the volcanic morphology, lithology, structure and mechanism of DV eruptions in the study area with those of other resurgent calderas. The geodynamic setting, volcanic architecture, and geochemistry of the DV, as well as its in-situ zircon LA–ICP–MS estimated ages, allow recognition of a series of eruptive stages, leading to collapse to form the Nuqara caldera, followed by resurgent uplift of the caldera floor during intrusion of rhyolite porphyry domes. The resurgent caldera model assigns a greater significance for the rhyolite porphyry as the exposed top of a sub-volcanic magma chamber. Resurgence is viewed as resulting from increased pressure or volume changes in the magma reservoir. A consequence of resurgence was uplift and erosion of the previous collapse-related topography.