Gabbro-diorite-tonalite¡granodiorite complexes, derived from subduction-related, calc alkaline
magmas represent a distinct unit in the Precambrian basement of Egypt. These intrusive
complexes are commonly intruded into ophiolitic me´ lange, schistose or gneissic rock
assemblages. Well-constrained geological mapping of the Um Eleiga complex, combined with
geochemical studies, should enable a better understanding of its genesis, especially its potential
as a host for gold mineralisation. This complex represents a composite pluton intruded into the
allochthonous, ophiolitic Abu Dahr massif and its associated undifferentiated metavolcanosedimentary
me´lange matrix in the southern Eastern Desert of Egypt. Field and petrographic
investigations have revealed a compositional continuum from gabbro to granodiorite through
diorite and subordinate tonalite. Petrography and bulk rock geochemical data have been used to
determine the magma type and tectonic setting of the Um Eleiga complex, whereas the electron
microprobe data were valuable for the purpose of determining the pressure–temperature and
oxygen fugacity conditions. The compositional continuum, pattern of zonation, shallow magma
origin and marked common negative Nb anomalies suggest that a close petrogenetic relationship
between the gabbro, diorite, tonalite and granodiorite units of the Um Eleiga complex. Subducted
upper mantle wedge is proposed as being the source region for the gabbroic magma, whereas
zoning of the complex is attributed to interplay of fractional crystallisation and assimilation.
Pressure estimates based on the Al(iv)-in-hornblende geobarometry indicate that the gabbroic
and dioritic rocks crystallised under 4–5 kbar, whereas tonalite and granodiorite were formed at
3–4 kbar. The hornblende-plagioclase thermometer indicates a formation temperature range of
about 720 to 880uC for the complex. Varied oxidation states are inferred from the mineral
assemblages, including magnetite, titanite and ilmenite and checked by Fe/(FezMg) ratios of
biotite in the granitoid rocks. The tectonic setting, mineralogical, geochemical characteristics and
varied redox state of the Um Eleiga intrusive complex are typical for arc-related intrusives
potential for hosting intrusion-related gold in a collisional environment.