Geology and geochemistry of the granitic rocks and associated dykes, East Gabal Nuqra, South Eastern Desert, Egypt----Institute of Geochemistry Chinese Academy of Sciences

Ibrahim H. Ibrahim* and Mostafa E. Darwish

Nuclear Material Authority, 530 P.O. El Maadi, Cairo, Egypt

Abstract A crescent-shape granitic stock and associated dykes is located to the East Gabal Nuqra at the extreme western part of Wadi Natash, South Eastern Desert, Egypt. The examined granites are classified as alkali-feldspar granites and mainly consist of quartz, potash feldspars, plagioclases, and aegirine-augite. Xenotime, zircon, apatite and allanite are accessories representing the source of Y, U, Th and REEs in these rocks. These granites are characterized by high K2O, Na2O and Zn contents and Rb/Sr ratio. Also, they are highly enriched in high field strength elements (HFSE), especially Zr (1529×10^-6), Nb (100×10-6), Hf (91×10-6) and Y (624×10-6) and light rare-earth elements (LREE, 141×10-6) concentrations and strongly depleted in Ca, Mg, Sr and Eu contents. These features suggest that they are similar to A-type granites (type-2). The rhyolite dykes and granites have similar geochemical characteristics whereas the chondrite-normalized REE patterns show a LREE enriched feature with strong negative Eu-anomaly, whereas the REE pattern of trachydacites show slightly fractionated pattern with no Eu-anomaly. It is suggested that the trachydacites were generated by small degree of partial-melting deep-seated basic source. Such liquid, when subjected to fractional crystallization involving separation of plagioclases as residue, generated the alkali-feldspar granites. And further fractional crystallization gave rise to the alkali rhyolites. The igneous rock suite originated from metaluminous to alkaline trachytic magma, and was developed in a within-plate tectonic environment. The extension caused by NW-SE right-lateral shear in area led to the emplacement of the alkali-feldspar granites. The later extrusion of the alkali rhyolite and trachydacite dykes was due to cauldron subsidence.

Key words Gabal Nuqra; A-type granite; fractional crystallization; cauldron subsidence

* Corresponding author, E-mail: Ibrahim170@yahoo.com

CHINESE JOURNAL OF GEOCHEMISTRY Vol. 31, No. 4, 2012, page 376-389