Geochronological, geochemical, and Nd-Hf isotopic constraints on the origin of Neoproterozoic plagiogranites in the Tasriwine ophiolite, Anti-Atlas orogen, Morocco

Small leucogranite bodies within the Tasriwine ophiolite, Anti-Atlas Mountains, Morocco have chemical characteristics consistent with being plagiogranites sensu stricto. Total REE abundances are low, as are K ₂O (<0.5%) and Rb (<15 ppm) contents. REE patterns of the plagiogranites display an enrichment of LREE compared to HREE, and are characterized by flat to slightly concave-up patterns from Gd to Yb. Such patterns contrast sharply with those of plagiogranites in more complete ophiolite sequences, such as the Semail ophiolite, Oman, or the Troodos ophiolite, Cyprus, where patterns are much flatter and slightly LREE-depleted. Initial Nd isotopic ratios are remarkably uniform; ε _Nd(762 Ma) values range from +6.0 to +6.3, and are consistent with direct derivation from a depleted mantle source. Hafnium isotopic ratios were determined on the same zircons as used for U-Pb geochronological analyses. Zircons from a folded plagiogranite dyke have present-day ¹⁷⁶Hf/ ¹⁷⁷Hf of 0.282699 ± 17, corresponding to a maximum initial ε _Hf of +14.2 ± 0.6. Four batches of zircons from a second pluton have overlapping maximum initial ε _Hf values (ε _Hf(762 Ma) = +14.0 ± 0.3 to +14.9 ± 0.6) and are indistinguishable from the value for the dyke. The zircon hafnium isotope data are consistent with the Nd isotope whole-rock values, implying no decoupling between Hf and REE during or after plagiogranite emplacement. Two of the plagiogranites were dated using high-precision U-Pb zircon techniques. Analyses of four batches of zircon from one pluton lie on a line with an upper intersection with Concordia of 761.1 + 1.9/-1.6 Ma, considered the age of emplacement of the pluton. Five small batches of zircon analyzed from a folded plagiogranite dyke lie on a chord that intersects Concordia at 762 + 1/-2 Ma, which is considered the age of emplacement of the dyke, and the most precise estimate of the age of the plagiogranite as a whole. The chemical and isotopic data for the plagiogranites are consistent with their generation by partial melting of a chemically enriched, but isotopically depleted mantle, followed by extensive fractional crystallization.