Zircon Geochemical and Geochronological Constraints on Contaminated and Enriched Mantle Sources beneath the Arabian Shield, Saudi Arabia

Abstract

Arabian Shield granitic zircon geochemistry provides insight into the petrogenetic processes involved in generating one of the planet’s largest tracts of juvenile Neoproterozoic crust. New zircon geochemistry supports previous U-Pb and whole-rock data that defined four magmatic groups: (1) ∼870–675 Ma island arc and synorogenic I-type granitoids (IA+Syn), (2) ∼640–585 Ma I- and A-type granitoids from the Nabitah and Halaban Suture (NHSG), (3) ∼610–600 Ma postorogenic perthitic (hypersolvus) A-type granitoids (POPG), and (4) <600 Ma anorogenic aegirine–bearing perthitic (hypersolvus) A-type granitoids (AAPG). The low Nb (∼1–300 ppm) and intrasuite rare earth element variation in IA+Syn and NHSG zircons indicates that these suites are derivatives of contaminated mantle followed by fractionation. AAPG suites, however, have higher Nb content (∼10–400 ppm) and are derived from limited crust-enriched mantle interaction. Each of the IA, Syn, and NHSG suites have discrete granite subsuites distinguished using zircon morphology and geochemistry whose U-Pb ages in each case form three groups. The IA subgroups are ∼867, ∼847, and ∼829 Ma; the Syn subgroups are ∼730, 716, and 696 Ma; and the NHSG subgroups are ∼636, ∼610, and ∼594 Ma. This apparent subevent repetition suggests some form of magmatic pulsing in the Arabian Shield. It is suggested that IA+Syn suites reflect typical volcanic arc granite settings and incremental subduction/accretion of eastward-migrating oceanic fragments of the East African Orogen. The appearance of ∼636 Ma A-type magmatism within suture zones (NHSG) is possibly derived from a long-lived (∼50 m.yr.) melting, assimilation, storage, and homogenization (MASH) zone resulting from an ∼640 Ma slab tear. These A-types are distinguished from more-enriched anorogenic (<600 Ma) A-types, possibly associated with lithospheric delamination.