Abstract
The mafic minerals of the two essential gabbroic rocks from the central Eastern Desert of Egypt (Sid Metagabbros, MG, and Younger Gabbro, YG) have been subjected to EPMA. There are distinct geochemical differences between the amphiboles of these two types. The amphiboles of the YG show relatively high TiO 2, A1 2O 3, Na 2O, K 2O and low SiO 2 and Fe 2O 3 than the MG. The YG have also (Na+K) > 0.5 and A1 IV > 1.3. The amphiboles of the MG are mainly magnesio-hornblende with subordinate actinolitic hornblende, whereas, the YG amphiboles is edenitic hornblende in composition. The A1 VI and Ti contents of the amphibole of the MG indicate that these rocks are subjected to low pressure metamorphism (below 5 k bars) within the greenschist-amphibolite facies. The pyroxenes of YG show diopside and augite compositions developed in subalkaline tectonic environments. The olivine of the YG is mainly iron rich (fa = 77.61 − 76.61).
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