Geochemistry, petrogenesis and tectonic setting of late Neoproterozoic Dokhan-type volcanic rocks in the Fatira area, eastern Egypt

Abstract

The Neoproterozoic Dokhan volcanics of the Fatira area in eastern Egypt comprise two main rock suites: (a) an intermediate volcanic suite, consisting of basaltic andesite, andesite, dacite, and their associated pyroclastic rocks; and (b) a felsic volcanic suite composed of rhyolite and rhyolitic tuffs. The two suites display well-defined major and trace element trends and a continuum in composition with wide ranges in SiO 2 (54-76%), CaO (8.19-0.14%), MgO (6.96-0.04%), Sr (983-7 ppm), Zr (328-95 ppm), Cr (297-1 ppm), and Ni (72-1 ppm). They are enriched in LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and show strong affinity to calc-alkaline subduction-related rocks. However, their undeformed character, their emplacement temporally and spatially with post-orogenic A-type granite, and their high Zr/Y values suggest that their emplacement follow the cessation of subduction in eastern Egypt in an extensionalrelated within-plate setting. Major and trace element variations in the intermediate volcanics are consistent with their formation via partial melting of an enriched subcontinental lithospheric mantle source followed by a limited low-pressure fractional crystallization of olivine and pyroxene before emplacement. The LILE enrichment relative to HFSE is attributed to the inheritance of a subduction component from mantle material which constituted the mantle wedge during previous subduction events in eastern Egypt. The evolution of the whole volcanic spectrum was governed mainly by crystal/melt fractionation of amphibole, plagioclase, titanomagnetite, and apatite in the intermediate varieties and plagioclase, amphibole, biotite, Fe-Ti oxides, apatite, and zircon in the felsic varieties. At each stage of evolution, crystal fractionation was accompanied by variable degrees of crustal contamination.

Nature and significance of a Cambro-Ordovician high-K, calc-alkaline sub-volcanic suite: the late- to post-orogenic Motru Dyke Swarm (Southern Carpathians, Romania)

“…Hanson 1980;Sisson 1994;Brenan et al 1995) are higher than 1 (REEs are compatible in HT calcic-amphibole in calc-alkaline environment), the REE content decreases during the magmatic evolution from the primitive basaltic andesite to the evolved rhyolite. The same behaviour has been observed and modelled in the Dokhan-type volcanic rocks of Egypt (Mohamed et al 2000). Experimental studies on watersaturated calc-alkaline magma (Grove et al 2003) have shown that calcic amphibole can be the main ferromagnesian phase in the absence of olivine.…”

“…Fig. 11) tectonic settings is hotly debated (e.g., Rock 1984;Zhou 1985;Sloman 1989;Turner et al 1999;Mohamed et al 2000;Moghazi 2003). Moreover, emplacement of high-K magmas with typical arc-type trace element signature can post-date active subduction episode and occur synchronously with uplift, extension or strike-slip movement (Rottura et al 1998;Wang et al 1999;Sloman 1989).…”

Dokhan volcanics of Gabal Monqul area, North Eastern Desert, Egypt: geochemistry and petrogenesis

“…-Transitional setting: transition between compressional and extensional settings (Ressetar and Monard 1983;Mohamed et al 2000;Eliwa et al 2006).…”

Geology, Geochemistry and Radioactivity of Granitic and Volcanic Rocks at Hadarba Area, South Eastern Desert, Egypt

“…Dokhan volcanic forms a successive sequence of acidic lava flows, ash tuffs, crystal lapilli tuffs and agglomerates. Many studies have been carried out on the petrography and geochemistry of the Dokhan volcanics, (Basta et al, (1980); Stern and Gottfried, (1986);El Gaby et al, (1989); Abdel Rahman, (1996); Mohamed et al, (2000); Eliwa, (2000); Moghazi, (2003); El Sayed et al, (2004) ; Eliwa et.al (2006) ; Khalaf (2010) and Alaabed and El Tokhi (2014).These authors show that the Dokhan volcanics have medium-to high-K calc-alkaline affinities. The geotectonic interpretation of the Dokhan volcanics is still controversial, especially whether they have been…”

“…Geologic map for Gabal Hadarba area formed (1) in a subduction environment (Hassan and Hashad, 1990;El Gaby et al, 1990), (2) in association with extension after crustal thickening (Stern et al, 1984(Stern et al, , 1988Stern, 1994;Fritz et al, 1996),(3) during transition between subduction and extension (Ressetar and Monard, 1983;Mohamed et al, 2000).…”