Porphyritic metadacite to porphyritic meta-andesite from the Precambrian of Egypt. (photo provided by Karen van Opstal)
“Imperial Porphyry” is a beautiful, important, historically-valuable decorative stone. It was initially quarried during the Egyptian Ptolemaic Dynasty and was also used in the Roman Empire. In later centuries, it was reused in southern and southeastern Europe.
This rock type comes from quarries at Mons Porphyrites in eastern Egypt. The locality name is the basis for the petrologic term “porphyritic”, which refers to a mix of large and small crystals in an igneous rock. Imperial Porphyry rocks are dark reddish or dark purplish with light-colored feldspar phenocrysts. The red and purple colors are the result of alteration of the original rock, which is dark gray-colored. These rocks are part of the Dokhan Volcanics, a greater-than-1 kilometer thick succession of late Precambrian-aged, terrestrial, intermediate to felsic volcanic rocks (= lava flows, volcanic tuffs, and volcanic agglomerates). The nature, age, mineralogy, geochemistry, and paleotectonic setting of the Dokhan Volcanics indicate that Imperial Porphyry rocks are lava flows that accompanied subduction zone volcanism during the Pan-African Orogeny. Subduction was followed by a collision event along the Mozambique Belt in the late Precambrian, during which the ancient small supercontinent Gondwana formed (
www.researchgate.net/figure/Map-of-the-Gondwana-supercont...). Gondwana was part of a larger supercontinent called Pannotia, which rifted apart in the latest Precambrian (
upload.wikimedia.org/wikipedia/commons/d/d7/Pannotia.svg).
Geochemical analysis of Imperial Porphyry rocks has shown that they are 62.2 to 64.4% silica, which makes them porphyritic quartz andesites and porphyritic dacites. A detailed mineral analysis of Imperial Porphyry is given in Makovicky et al. (2016). The mineralogy shows that the rocks have been subjected to fluid alteration and greenschist-facies metamorphism, possibly related to the Pan-African Orogeny and/or burial metamorphism and/or Red Sea rifting orogenesis. The reddish to purplish coloration is from partial hematitization of mafic minerals. Because the rocks are slightly metamorphosed, they are better referred to as "meta-andesite" and "metadacite".
Stratigraphy: upper Dokhan Volcanics, Ediacaran, upper Neoproterozoic, ~593-602 Ma
Locality: old Roman quarry at Mons Porphyrites, above Wadi Abu Maamel, Red Sea Mountains, Eastern Desert, eastern Egypt
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Geologic info. mostly synthesized from:
Makovicky et al. (2016) - Imperial Porphyry from Gebel Abu Dokhan, the Red Sea Mountains, Egypt, part I. mineralogy, petrology and occurrence. Neues Jahrbuch für Mineralogie Abhandlungen [Journal of Mineralogy and Geochemistry] 193: 1-27.
