Neoarchean UHT Metamorphism and Paleoproterozoic UHT Reworking at Uweinat in the East Sahara Ghost Craton, SW Egypt: Evidence from Petrology and Texturally Controlledin situMonazite Dating

Abstract

The Uweinat-Kamil basement complex, in the central part of the East Sahara Ghost Craton (ESGC) in NE Africa, is the only inlier in the craton that contains rocks with Archean formation ages, and is hence a key to understanding the ancient crustal evolution of the otherwise enigmatic and poorly known ESGC. The craton is thought to have been decratonized during the Neoproterozoic after thickening as a result of Pan-African collisional events along its margins. Textural and compositional relationships preserved in the metapelitic granulites from the Uweinat-Kamil inlier suggest a two-stage metamorphic evolution of the rocks. Stage I saw the growth of sapphirine + quartz + garnet 1 at similar to 10 kbar and similar to 1050 degrees C, from an initial assemblage containing kyanite, sillimanite +/- biotite, which are now preserved only as inclusions in porphyroblastic garnet 1. This stage was followed by near-isobaric cooling stabilizing the assemblage garnet 1 + sillimanite 1 +/- orthopyroxene 1 +/- sapphirine coexisting with melt. Stage II saw the breakdown of this assemblage forming a variety of symplectite assemblages (orthopyroxene 2 + cordierite or orthopyroxene 2 + cordierite + sapphirine +/- sillimanite 2 or cordierite + spinel) through near-isothermal decompression from similar to 9 kbar to similar to 6 kbar at ultrahigh temperatures of 900-1000 degrees C. This was followed by near-isobaric cooling of the rocks to temperatures of 700 degrees C at similar to 5.5-6 kbar, as evidenced by the growth of garnet 2 and the formation of late-stage biotite, owing to back-reaction of melt with residual garnet 1 and symplectite minerals. The second stage of the evolution is also observed in the associated metabasic granulites. Complete to partial replacement of garnet 1 porphyroblasts and clinopyroxene by orthopyroxene + plagioclase + hornblende +/- spinel symplectites represents a stage of near-isothermal decompression, whereas the growth of garnet 2 around the symplectitic minerals represents a stage of isobaric cooling. Texturally controlled in situ Th-U-total Pb monazite dating of the metapelitic granulites reveals the polymetamorphic nature of the rocks. Stage I occurred at similar to 2.6 Ga, as indicated by monazite inclusions within garnet 1 porphyroblasts (coexisting with sapphirine + quartz), and represents a previously unknown Neoarchean ultrahigh-temperature metamorphism. Stage II occurred 700 Myr later at similar to 1.9 Ga, as indicated by monazite grains in the symplectites and matrix, and represents a previously uncharacterized Paleoproterozoic ultrahigh-temperature isothermal decompression event. No evidence of any metamorphism during the Neoproterozoic has been found. In this context, it is possible that the proposed decratonization occurred during a Paleoproterozoic decompression event instead of in the Neoproterozoic. The Paleoproterozoic evolution of the Uweinat-Kamil inlier is very similar to that described from other Paleoproterozoic orogenies across the world, which are considered to have resulted from collisions during the formation of the supercontinent Columbia. Future attempts at Columbia reconstructions should take into account the Paleoproterozoic evolution of the Uweinat-Kamil inlier in NE Africa.