Crustal evolution of Tonian island arcs in the Arabian–Nubian Shield: A coupled U-Pb-Hf-O isotope study in detrital zircon

The Arabian–Nubian Shield (ANS) is a classic example of Neoproterozoic crustal growth. Its prime and oldest crustal constituents are island-arc terranes, comprising principally Tonian, juvenile intra-oceanic magmatism. Herein we present a coupled U-Pb-Hf-O study of detrital zircons retrieved from Tonian metasedimentary sections storing island-arc detritus in the northern ANS (Eilat area, Israel), intended to explore ANS arcs juvenility and crustal reworking processes. Detrital zircon U-Pb geochronology places island-arc activity at ca. 1040–740 Ma, peaking between ca. 870–740 Ma. Juvenile arc crust formation is demonstrated by predominant positive εHf_(t) values of +6 to +12 in zircons with mantle-like δ¹⁸O (5.0–6.5‰), commencing at ca. 1040 Ma, thus designating one of the oldest uncontaminated magmatic phases in the ANS. The predominance of positive εHf_(t) values, alongside only two Paleoproterozoic grains detected, precludes substantial incorporation of older crustal components in the (mainly) Tonian island-arc system. This confirms that the juvenile island-arc edifice was chiefly intra-oceanic, isolated from pre-Neoproterozoic continental margins during its entire history. Although constantly positive, zircon εHf_(t) values temporally decline, implying continuous reworking of preceding juvenile island-arc crust. Reworking is further highlighted by abundant zircons with high δ¹⁸O values (6.5–9.6‰) that indicate assimilation of ¹⁸O-rich supracrustal components, likely sediments, in arc magmas. Numerous zircons yielded both high εHf_(t) and δ¹⁸O values, suggesting their parent magmas assimilated juvenile, volcanogenic island-arc detritus. Mixing modeling indicates that as much as 10–40% of such sediments were assimilated into arc magmas between ca. 940 and 740 Ma, suggesting continuous cannibalization of juvenile sediments throughout most of the arc system's lifespan. Crustal reworking and sediment assimilation may have promoted crust maturation, resulting in declining εHf_(t) values and culminating with quartzofeldspathic intrusions during the mid-Tonian. The present work therefore highlights the essential role of sedimentary assimilation and crustal reworking in the evolution of juvenile ANS island arcs.