The Mount Gharib A-type granite, Nubian Shield: petrogenesis and role of metasomatism at the source

Abstract

The Mount Gharib peralkaline A-type complex (476±2 Ma), located in the Nubian Shield of Egypt, contains sodic-calcic to sodic amphiboles, accessory astrophyllite, zircon, fluorite, apatite, allanite, aenigmatite, elpidite(?) and ilmenite. This “within plate” hypersolvus suite is enriched in large-ion lithophile (LIL) and high field-strength (HFS) elements, and characterized by a fractionated REE pattern (Ce/Yb=49) and a significant negative Eu anomaly. A fine-grained acicular-amphibole-bearing roof facies shows further enrichment in the LIL and HFS elements. The suite was emplaced in a Pan-African granodiorite-adamellite host, which it locally metasomatized. The affected rocks contain hydrothermal albite, end-member arfvedsonite, astrophyllite, and levels of the LIL and HFS elements intermediate between those in the peralkaline granite and the roof facies. Trace element and isotopic modeling of this A-type granite, with its high initial 87Sr/86Sr value (0.7110), documents an active role of the lithosphere in magma generation. Lithospheric extension, expressed by regional dyke-swarms, was caused by cooling, fracturing and relaxation of the thin, newly formed Pan-African crust. Localized partial melting took place in an open system, possibly as a result of an influx of alkali-rich fluid derived from a sublithospheric source. Metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the A-type melt.