Evolution and stabilization of a juvenile crust: zircon U-Pb and Hf isotopic perspectives from the Northern Arabian-Nubian Shield

The Arabian-Nubian Shield (ANS) is a major example of a juvenile continental crust. Its rock inventory exposed in the Elat massif (southern Israel) spans all ANS evolution stages. U-Pb dating of detrital zircons from the oldest metasediments indicates that island-arc activity occurred at ca. 900-750 Ma. These zircons display a wide range of εHf(t) values, from the expected Neoproterozoic depleted mantle value, ~14 ε units, down to less radiogenic values of ~5 ε units. This indicates that the newly formed arcs recycled a component of an older continental origin. It is thus difficult to envisage the earliest arc igneous phase as entirely intraoceanic. A swarm of (now schistose) dikes penetrated at ~700 Ma, in the course of an otherwise igneous tranquility. Zircons from one of these dikes yielded relatively elevated εHf(t) values of ~12 ε units, which plot within the depleted mantle range. Crustal thickening along the entire East African Orogen was followed by late to post-tectonic calc-alkaline magmatism dated in the Elat massif at ca. 670-600 Ma. Subordinate, shallower alkaline magmatism was dated at ca, 610-590 Ma. Some of these later intrusions carry inherited pre-Neoproterozoic zircons of various ages between 2.5 and 1 Ga, indicating the presence of older crustal sources at the subsurface. Zircon εHf(t) values from the calc-alkaline and alkaline granitoids resembles, albeit slightly lower, those of the island-arc magmas, ca. 10-4 ε units, suggesting that the calc-alkaline/alkaline magmas were produced chiefly by extensive remelting of the older island-arc crust. As a whole, our study indicate that overall ANS juvenile crustal evolution lasted ca. 300 myr, but the most significant episode of mantle extraction took place almost exclusively during the earliest 150 myr of island-arc magmatism. Cratonization of the shield via widespread calc-alkaline batholitic intrusions, resulted from large-scale differentiation of the island-arc crust but lagged some 100-250 myr after initial extraction from the mantle.