ANATECTIC ORIGIN FOR BAS DRAA APLITE-PEGMATITE DYKES? INSIGHT FROM WHOLE ROCK GEOCHEMISTRY AND TOURMALINE CHEMISTRY

Although many rare metals used today originate from granitic pegmatites, ongoing debate persists regarding the origin of these mysterious rocks. Some pegmatites clearly represent highly fractionated products derived from parent granitic bodies, while others formed through the anatexis of local protoliths. The Bas Draa inlier, situated in the western part of Morocco's Anti-Atlas belt, contains numerous pegmatitic and aplitic dykes. These intrusions primarily cross-cut Paleoproterozoic metasedimentary rocks and some Paleoproterozoic intrusions of basement, varying in width from a few centimeters to over 10 meters and extending several hundred meters in length. Comprising predominantly quartz, muscovite, feldspars, tourmaline, petalite, apatite, zircon, garnet, and gahnite, these rocks exhibit a distinctly peraluminous character with low total rare earth element content (<500 ppm). Their mineral assemblage (petalite, tourmaline, garnet, gahnite), enrichment in Pb, Rb, Cs, and depletion of Ba and Nb compared to the primitive mantle, suggest affiliation with the LCT pegmatite family.

The presence of highly mafic tourmaline compositions (Schorl-Foitite) and epidote, alongside geological features, challenges the notion that these pegmatites derive solely from nearby granites, indicating they likely evolved through crustal anatexis of hosted metasedimentary rocks. Their peraluminous nature strongly supports a metasedimentary origin, notably concentrated in areas where such sources are abundant within the Bas Draa inlier. These findings echo earlier studies questioning the prevailing model that attributes all rare-metal pegmatites to fractionation from granite, thereby prompting reconsideration of the origins of similar mineralized pegmatites globally.