| Summary: | Granites from the Midyan terrain, NW Saudi Arabia: Petrology, geochemistry and geochronology Five, Neoproterozoic, poorly-studied felsic intrusives from NW Saudi Arabia have been subjected to a detailed geological study to understand their characteristics, evolutionary history, tectonic setting, ages and economical potential for rare metal mineralization. The work has utilized field observations, mappmg, mineralogy, petrography, geochemistry (ICP-AES and MS, XRF, scanning electron microscopy and microprobe) and geochronology (Rb/Sr whole rock isochrons and U-Pb zircon analysis). The results have indicated that the individual plutons in the region are each quite different. This is mainly displayed by the existence of different types of granites, based on chemistry and petrography (peralkaline, alkaline and calc-alkaline) and mineralogy (particularly the presence of different types of amphibole, both sodicand calcic). The plutons probably formed at low crystallizaton pressures (~ 1.2 to 3.4 Kbar) and at shallow (~4 to 12 km ±O.5 km) depths, but seem to have been derived from similar source regions. The granites were most likely derived by low-pressure, partial melting of crustal rocks with possible crustal contamination, followed by fractional crystallization and later sub-solidus alteration by fluids. The plutons' ages span a range from 630Ma (Cryogenian) to 554Ma (Ediacaran). The geochemical differences between the granites could be related to their formation during different stages of the region's evolution. A gradual change in tectonic setting is indicated, from island arc accretion in the middle stages of the Panafrican orogen, to a post-collisional setting in the early stages of the Panafrican event. Economically, the granites contain relatively high contents of rare earths elements (REEs) and rare-metals (RMs), mostly hosted by phosphates (monazite-Ce and xenotime), Nb-oxide (fergusonite- Y) and possible rare earth carbonates (synchysite). These elevated REEs and Nb contents and their mineralogical forms highlight the presence of suitable exploration targets for rare earth elements late stage deposits. xiii
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