Neoproterozoic Arc Construction, NW Midyan terrane, Arabian Shield: 50 m.y. of alkaline and calc-alkaline post-collisional magmatism

The northern Midyan terrane granitoids (NMTG), NW Arabian Shield, comprise two post-collisional suites of volumetrically dominant medium- to high-K calc-alkaline I-types, defined as ones which have 55–70 wt% SiO₂ and alumina saturation index <1.1, and lesser within-plate A-types, defined as ones which have Nb/Y > 6 and Ta + Yb > 60. Granitoids which do not fit these classifications are termed ‘probable’ A- and I-types based on similarities in primitive mantle-normalized plots. The I-types have trace element ratios similar to subduction failure magmas, e.g., mean Sr/Y, Sm/Yb and La/Yb of 37, 3, and 16, respectively. The granitoids exhibit εNd(t) values of +3.80 to +5.77 akin to other Arabian Nubian Shield post-tectonic granitoids and other isotopic compositions similar to western Arabian Shield arc terranes. The A- and probable A-type granites yield U-Pb SHRIMP ages of 640.7 ± 5.3 Ma and 597 ± 2.2 Ma, respectively, and the I- and probable I-types record U-Pb SHRIMP ages of 624 ± 11 Ma to 598.9 ± 6.1 ( n = 4). Based on a recent tectonic model ( Whattam et al., 2024 ) which documents a switch from I-type arc magmatism to within-plate A-type magmatism at 635 Ma, we propose an extension of this model. Subsequent to cessation of a first pulse of A-type magmatism at about 635 Ma, the basaltic carapace of the detached slab began to partially melt and mix with asthenospheric mantle melts by ∼630 Ma, producing calc-alkaline post-collisional I-type granitoids; this continued until 600 Ma. That a sole probable A-type granitoid likewise yields a U-Pb SHRIMP age of 600 Ma makes a simple tectonic model more complex; however, during slab breakoff, small amounts of crustal materials were possibly also partially melted to generate A-types synchronously with I-types.