Ediacaran (∼620 Ma) high-grade regional metamorphism in the northern Arabian Nubian Shield: U-Th-Pb monazite ages of the Elat schist

Abstract

Ediacaran times witnessed a hemisphere-scale orogenesis, forming the extensive Pan-African mountain ranges and resulting in the final assembly of the Gondwana supercontinent. The Elat metamorphic association (S Israel), located at the northernmost tip of the Arabian Nubian Shield (ANS), comprises amphibolite-facies pelitic and psammitic schists and orthogneisses and was presumably shaped by Ediacaran East African orogenesis. However, the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are inconclusive and a major emphasis was previously given to pre-Ediacaran island-arc related tectonics. Here U-Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat association.In-situ analysis of metamorphic monazites by LASS-ICPMS (Laser-Ablation Split-Stream Inductively Coupled Plasma Mass Spectrometry) yields two concordant age clusters at ∼710 and 620Ma, recording two regional metamorphic events. The corresponding REE patterns of the dated monazite grains indicate that garnet porphyroblast growth took place only during the younger event (M2). Likewise, the regional south-dipping penetrative foliation (S2), common to the Elat schist and the surrounding orthogneisses, formed during the Ediacaran event (M2). The peak M2 staurolite-garnet-sillimanite assemblage was overprinted by retrograde andalusite and cordierite during decompression, followed by the intrusion of andesitic dykes that were immediately metamorphosed at greenschist-facies conditions. This metamorphic P-T-d-t path corresponds to the collision of the already accreted arc terranes of the ANS with the Saharan Metacraton, as also suggested by the large geochronological database of 610–580Ma post collisional granitoid batholiths throughout NE Africa and Arabia. The Cryogenian M1 metamorphism therefore records arc-arc collision during major convergence forming the proto-ANS to the south.