The Bir Safsaf Precambrian inlier of South West Egypt revisited. A model for ～1.5 Ga T_(DM) late Pan-African granite generation by crustal reworking

摘要

Bir Safsaf is one of the four Precambrian inliers of the southern Egyptian Western Desert; it is located midway between the juvenile crust of the Arabian-Nubian Shield and the Archean terranes of Gebel Kamil, near the Libyan border. Contrary to previous belief, Bir Safsaf is exclusively composed of late Pan-African granitoids, with U/Pb SHRIMP ages between 627 Ma and 595 Ma. The presence of pre-Pan-African materials is limited to scarce inherited zircons with -2.1 Ga and ～2.7 Ga, ages that are well-represented in the pre-Pan-African terranes of the neighboring Gebel Kamil inlier. Early Pan-African inherited zircons with ages of ～640-650 Ma and ～750 Ma are also found. The granitoids of Bir Safsaf show a large geochemical variability, with an overall composition similar to subduction-related granites, but they lack the inter-elemental correlations characteristic of magmatic differentiation, magma mixing or hybridization. This chemically heterogeneous set of granitoids has, nonetheless, nearly uniform initial ~(87)Sr/~(86)Sr and ~(143)Nd/~(144)Nd, and a ～1.5 Ga Nd model age (T_(DM)) despite no new crust being formed at that time in northeast Africa. To reconcile the large chemical variability, the isotopic homogeneity, and the "mixed" Nd model age, we propose that these granitoids were derived from a lithologically heterogeneous pre-Pan African source that, prior to melting, was thoroughly homogenized with respect to Sr and Nd isotopes by convective metasomatism caused by juvenile hydrothermal fluids probably released from a subduction zone.