Neoproterozoic juvenile continental crust formation in the Arabian Shield (Khamal intrusive complex, Western Arabia)

Abstract

The Arabian–Nubian Shield (ANS) is one of the largest Neoproterozoic juvenile crustal provinces on Earth, providing insights into crustal growth, mantle dynamics, and continental assembly. The Wadi Khamal Complex, in the northwestern Arabian Shield, is a mafic intrusive body comprising a central anorthositic core and marginal gabbronorites, with subordinate granites and younger felsic to basaltic dykes. This study integrates petrography, whole-rock and mineral geochemistry, Sr–Nd–Pb isotopes, zircon UPb geochronology, and equilibrium-melt modeling from minerals to reconstruct the origin, age, and conditions of emplacement of this anorthosite complex.

In the complex, the anorthosites are dominated by cumulate plagioclase (An₈₁–₃₆) with interstitial pyroxenes, whereas gabbronorites exhibit modal layering and magmatic foliation defined by plagioclase–pyroxene (± olivine). The granites are in primary contact with anorthosite. The anorthosite–gabbronorite–granite association is crosscut by multiple generations of felsic dykes; additional basaltic dykes are attributed, by field relations and chemistry, to Cenozoic alkaline magmatism coeval with Red Sea rifting. The complex intrudes older arc-related lithologies—including amphibolitized mélanges and a voluminous diorite–tonalite–granodiorite batholith—that are structurally important but not petrogenetically related to Wadi Khamal.

Bulk-rock and mineral systematics define trends consistent with assembly of cumulate anorthosite and gabbronorite from tholeiitic parental melts; because the data are cumulate-dominated, bulk trends need not follow liquid lines of descent. Equilibrium-melt proxies indicate LREE-enriched tholeiitic melts lacking arc-type NbTa troughs, consistent with small to moderate degrees of partial melting of a juvenile (depleted to slightly enriched) mantle. Nd–Sr–Pb isotope signatures (εNd(i) = +4.5 to +6.1; ⁸⁷Sr/⁸⁶Sr(i) = 0.70256–0.70277; ²⁰⁶Pb/²⁰⁴Pb(i) = 17.3–17.9; ²⁰⁷Pb/²⁰⁴Pb(i) = 15.4–15.5; ²⁰⁸Pb/²⁰⁴Pb(i) = 36.2–37.2) indicate juvenile, non-arc mantle with limited crustal input. New zircon UPb dating yields an emplacement age of 614 ± 3.4 Ma.

We conclude that the Wadi Khamal Complex represents a small-scale, late Ediacaran massif-type anorthosite assembled as a crystal-mush from juvenile tholeiitic melts during post-collisional extension. The isotopic contrast between the intrusion and later basaltic dykes underscores a long-term shift from juvenile asthenosphere-dominated sources to enriched, SCLM-influenced sources during Red Sea rifting.