Mid Cryogenian NeoProterozoic Lava Rock Found

New insights into the accretion of the Arabian-Nubian Shield: depositional setting, composition and geochronology of a Mid-Cryogenian arc succession (North Eastern Desert, Egypt)

Authors:

Buhler et al

Abstract:

The accretionary phase of the northern Arabian-Nubian Shield (ANS) is poorly constrained. For the first time, this study combines facies analysis, geochemistry, LA-ICP-MS U-Pb zircon geochronology and biotite and hornblende 40Ar/39Ar geochronology on a Mid-Cryogenian volcano-sedimentary succession situated in the North Eastern Desert (NED). The c. 550 m thick Wadi Malaak succession (WMS) nonconformably overlies c. 750 Ma granitoids and is, in turn, overlain by the c. 617 Ma Dokhan Volcanics and Hammamat Group sediments on an angular unconformity. Facies evolution of the WMS switched from an alluvial plain receiving medial to distal felsic ignimbrite sheets to a lacustrine system with mafic phreatomagmatic activity, and then developed back to a subaerial environment with bimodal volcanism. Facies analysis suggests humid paleoclimatic conditions; (peri-)glacial features have not been detected. Reconnaissance geochemistry of WMS samples indicates a compositional range of volcanic and volcaniclastic rocks from basalt to rhyolite with calc-alkaline affinities. Incompatible trace element and REE patterns indicate formation in a subduction setting. Selected tectonic classification diagrams suggest a continental arc setting. U-Pb LA-ICP-MS analyses on zircon extracted from two felsic ignimbrites resulted in ages of 725 ± 7 Ma and 717 ± 8 Ma, constraining the WMS formation to the Mid-Cryogenian. Hornblende and biotite separated from a subvolcanic gabbro that obviously had intruded the WMS yielded 40Ar/39Ar cooling ages of 747 ± 10 Ma and 743 ± 10 Ma, respectively. These 40Ar/39Ar ages are thus in conflict with the U-Pb crystallization ages. This is explained by the presence of excess Ar in biotite and hornblende. The new data presented here indicate that the WMS developed during the waning phase of Mid-Cryogenian ANS accretion, prior to the onset of Sturtian glaciation. It is the first known record of c. 720 Ma arc volcanism in the northernmost ANS.