Geochemical and geochronological characterization of the Tamanrasset W-Sn-(±Ta-Nb-Li) granites: Prolonged rare metal magmatism in the Laouni terrane (Central Hoggar, Algeria)

The origin of topaz-mica granites within the central Hoggar massif, along with their associated rare metal mineralization, remains a compelling subject despite extensive geochemical and geochronological research. This study presents new bulk-rock geochemical data and zircon UPb ages and Hf isotopic composition for the Tamanrasset W-Sn-(±Ta-Nb-Li) granites in the Laouni terrane, supplemented by NdSm isotope analyses. These data enhance the understanding of the formation and tectonic setting of the investigated granites.

Petrographic and geochemical analyses identify four granite varieties: fine-grained monzogranite, fluorite-bearing two-mica granite, porphyritic topaz-alkali feldspar granite, and topaz-mica-albite granite. Generally, these rocks are low-P (P₂O₅ < 0.14 wt%) and range from weakly metaluminous to strongly peraluminous (A/CNK = 0.94–1.83), with high degrees of fractional crystallization (50–90 %) evidenced by enrichment in F, Rb, W, Sn, Cs, Y, Zr, and Ta. The studied rocks feature distinct REE patterns, including strong negative Eu anomalies (Eu/Eu* ≤ 0.1) and M-type tetrad effects in topaz- and fluorite-bearing granites, while monzogranite shows weaker anomalies. Zircon saturation thermometry and P_H2O barometry suggest crystallization under low-temperature, water-rich conditions (11–15 MPa, < 800 °C), with high fluorine activity and volatile-rich phases, suggesting shallow, evolved magmas. Zircon UPb dating defined ages of 605.4 ± 3.7 Ma for Aleméda monzogranite, 603.2 ± 2.6 Ma for topaz-alkali feldspar granite, and 592.9 ± 3.9 Ma for topaz-mica-albite granite, with younger ages (∼575–572 Ma) in two-mica granites from Tin Amzi-El Karoussa. Conversely, the Hanana-Hananère topaz-mica-albite granite has a distinctly older age of 638.0 ± 3.1 Ma.

The studied granites show diverse REE profiles, zircon εHf^(t) (−3.6 to −13.3), bulk-rock εNd^(t) (0.3 to −16.3), and model ages (1.3–2.4 Ga), indicating crustal reworking, mantle delamination, and partial melting. Orogenic collapse and mantle delamination triggered magmatic activity, leading to volatile-rich, evolved ferroan A2-type granites with rare-metal mineralization potential. Their isotopic variability suggests a transcrustal MASH system driven by mantle-crust interactions, with strong spatial association to mega shear zones, highlighting tectonic control on magma ascent and crustal reworking, and incorporation of ancient crustal materials.