Microstructure and Geochemistry of Magmatic Dykes from the Arabian Margin, Red Sea

Abstract

The Red Sea rifting nucleated within the Neoproterozoic Arabian-Nubian shield, which formed during the Pan-African orogeny over a time period of about 300 million years. The Red Sea rifting started about at 30 Ma and was assisted by much diffused magmatism that lasted until recent times. The majority of magmatic rocks consist of basalts that constitute volcanic plateaux (harrats), which represent one of the largest Cenozoic volcanic provinces in the world. Volcanic rocks are distributed all along the Arabian plate margin of the Red Sea, from Yemen to Jordan. Some of the oldest magmatic rocks are acidic in composition, especially along the southern part of the Arabian margin. In this chapter microstructure and geochemistry of acidic, intermediate, and basic dykes sampled along the Arabian margin are described. Acidic dykes consist of granitoids and porphyritic rhyolites. Intermediate and basic dykes consist of andesite and basanite/basalt, respectively. Granitoid dykes show equal-granular coarse-grained texture and mostly consist of euhedral crystals. Other than local displays of crystal-plastic deformation in quartz, these dykes have primary magmatic textures. Dykes consisting of rhyolites contain euhedral K-feldspar phenocrysts with frequent perthitic intergrowth of albite. The rock matrix consists of quartz, K-feldspar, and albite. Basanite/basaltic dykes consist of plagioclase, pyroxene, and amphibole phenocrysts. Plagioclase is also abundant in the groundmass where glass is also preserved. Andesite dykes are characterised by a pervasive alteration that in some instances prevents the identification of original phenocrysts. Where identifiable, phenocrysts consist of plagioclase, amphibole, and pyroxene. Locally, in the groundmass interstitial quartz shows crystal-plastic deformation. Fluidal magmatic structures are recorded locally in basanitic and basaltic dykes and only weakly in rhyolitic dykes. The fine-grained texture of the rock groundmass and vesicular structures indicate that dyke emplacement is quite shallow (hypabyssal conditions). However, the Al-content in amphibole phenocrysts of basanite/basalt dykes is consistent with phenocryst crystallisation depths of 15–20 km. The first U-Pb tests on granitoids reveal that they contain zircon grains from Cryogenian to Ediacaran (middle to late Neoproterozoic) age. Geochemical results indicate that basanite/basaltic dykes are compatible with a divergent environment such as the Red Sea rifting, whereas andesite dykes are compatible with a convergent setting. The rhyolitic dykes are interpreted as related to the Red Sea rifting as they show geochemical signatures compatible with divergent tectonics and are from a region where rhyolitic dykes were dated around 20 Ma.