The Early Ediacaran orogenic history of the northern Arabian-Nubian Shield in a nutshell (Eilat area, Israel)

The Arabian-Nubian Shield (ANS) is a juvenile crust formed over more than 300 my of Neoproterozoic crustal evolution. In the northern ANS two major igneous cycles were most significant in manufacturing the continent: the older comprises Tonian intra-oceanic island arcs whereas the second saw widespread, Early Ediacaran late to post-tectonic granitoids and volcanics. A swarm of schistose dikes of basic to intermediate composition occurs in the Neoproterozoic basement of the Eilat area, in the northern ANS. These dikes were metamorphosed in epidote-amphibolite facies, are vertically-oriented, striking WNW-ENE, with marked vertical schistosity parallel to their walls. They are abundant in a 740 Ma Eilat granite gneiss and crosscut the regional foliation which dips moderately to the south. They are commonly thought to mark a break in the prolonged Pan-African orogenic history but their age is not well defined.

We located a unique field occurrence of a schistose dike crosscut by a granite pegmatite vein which in turn was deformed and folded parallel to the vertical schistosity. The marked foliation in the hinge zone of the folded granite vein formed by crystalline plasticity at elevated temperatures during metamorphism. This key outcrop provides the opportunity to tie high-resolution field observations to accurate, multi-system U-Pb geochronology and to evaluate the relations between dike intrusion, metamorphism, and the invasion of late to post-orogenic granitoids.

Zircon U-Pb geochronology from the schistose dike yielded 645±4 Ma, considered to mark the age of crystallization of the igneous protolith. Zircon from the deformed granite vein yielded an age of 617±17Ma, indicating the vein pertains to the abundant late- to post-orogenic granitoids that invaded the juvenile crust in the aftermath of Pan-African orogeny. Titanite from the schistose dike yielded a lower intercept U-Pb age of 626±4 Ma. With a closure temperature for Pb of ~550-650^◦C, titanite records the age of its crystallization during metamorphism. Apatite yielded a lower intercept U-Pb age of 611±12Ma. With an effective closure temperature for Pb of 450-550°C, apatite serves as an important medium-temperature thermochronometer. Similarly, an apatite U-Pb age of 593±12Ma was determined for the adjacent, garnet-grade Eilat schist. We interpret apatite U-Pb age as representing the timing of cooling of the entire crustal edifice in the Eilat area.

Our study demonstrates that the Pan-African tectonometamorphic history in the Eilat area was punctuated by the intrusion of basic dikes at ~645 Ma. They penetrated an already accreted and metamorphosed island-arc sequence and were subsequently deformed and metamorphosed with their country rocks in the Early Ediacaran. The Early Ediacaran deformation and metamorphism partly overlapped the intrusion of late-orogenic granitoids (see also Elisha et al, 2017) and was immediately followed by rapid exhumation. Our previous work (Katz et al 2004) showed that the protoliths of schistose dikes from the nearby Roded area were high-Mg andesites, resembling boninites which are currently restricted to active subduction zones. We propose, as a working hypothesis, that the schistose dikes signify southward subduction of the proto-Tethys below the Gondwana margin in the late stages of Pan-African orogeny.