The continental basement in the Eastern Mediterranean area represents the northern edge of Gondwana, which has been the site of repeated crustal accretion and has subsequently been modified by consecutive rifting events. We investigated the geologic and thermal history of the North Gondwana lower crust by examining the U-Pb-Hf isotope systematics in zircons within 6 mafic granulite xenoliths from Pliocene lava cone in North Israel. The lava cone protrudes through the platform cover that seals the late Neoproterozoic junction between the Arabian-Nubian basement to the South and the Cadomian basement exposed in the Taurides to the North. The mafic granulite xenoliths are composed of plagioclase + orthopyroxene + clinopyroxene ± garnet ± spinel ± secondary amphibole. U-Pb zircon ages from the granulites vary among the different samples with distinct zircon age populations at 400–1200 Ma, 170–350 Ma, and 3.6–4.2 Ma, attesting the lower crust preserves a prolonged thermal and igneous history. While 400–550 Ma U-Pb ages are interpreted to be the result of Pb loss, the wide scatter of zircon grains aged between 550 and 1200 Ma, alongside their diverse εHf(t) values (−25–+10), is an extraordinary evidence for the accretion of Neoproterozoic sediments into the North Gondwana arc root lower crust. The U-Pb-Hf signature of these zircons resembles Cadomian sediments of the Tauride block to the north, indicating southward (present coordinates) subduction under North Gondwana and possible accretion of fore-arc sediments to the lower crust through relamination in the latest Neoproterozoic. One xenolith contained metamorphic-shaped zircons aged 170–350 Ma with positive εHf values and Hf-TDM of 0.85 Ga interpreted to reflect Paleozoic recycling of the Neoproterozoic juvenile Arabian basement, which we consider to form a major component of the lower crust in the region. An overwhelming cluster of Carboniferous zircons concentrating at 305 Ma with exclusively negative εHf values around −6, was retrieved from three xenoliths. Some of these zircons portrayed igneous textures and shape. While Carboniferous igneous activity is the hallmark of Western Europe's Variscan orogeny, the latter did not affect the southern rifted edge of Neo-Tethys where our xenoliths were retrieved. The Paleozoic age-Hf composition in our xenoliths is therefore interpreted to result from syn-Variscan recycling of Neoproterozoic sedimentary remains in the lower crust, and some degree of melting in a non-orogenic environment. Rather than with horizontal plate motions and orogeny, the Carboniferous zircon ages in the xenoliths appear to coalesce with significant vertical movements that created continental scale unconformities and a broad basin and swell architecture known to develop over the entire North Gondwana margin at that time. The Carboniferous aged zircons in northern Israel lower crustal xenoliths are therefore a unique gauge of the thermal perturbation that accompanied the large-scale mantle dynamics below the then passive North African margin of Gondwana, while Variscan orogenic accretion occurred on the Eurasian margin. These lower crustal granulite xenoliths therefore contain important information with respect to the nature of the lower crust under Israel, with implications on the geodynamic setting during the Cadomian and Variscan cycles.
Bibliographical noteFunding Information:
This research was supported by the G.I.F., the German-Israeli Foundation for Scientific Research and Development (grant number 1248301.8/2014 ). The Authors wish to thank O. Navon and M. Stein for providing HL xenolith samples and for numerous stimulating discussions. A. Abbo is grateful to the Azrieli Foundation for the award of an Azrieli Fellowship. Y. Geller is thanked for technical assistance in mineral separation, and L. Marko for invaluable analytical support at GUF. We greatly thank Cristiano Lana for allowing us to perform Hf isotopic measurements in Ouro Preto. We would also like to thank N. Morag for assistance in field excursions to Qarnei-Hittin and helpful discussions and advice. This manuscript has benefitted greatly from the reviews given by Vinod O. Samuel and two anonymous reviewers.
© 2018 International Association for Gondwana Research
- Carboniferous thermal event
- Lower continental crust
- Mafic granulite xenoliths
- Sediment relamination
- Zircon U-Pb-Hf