Detrital zircon U-Pb geochronology of Cryogenian diamictites and Lower Paleozoic sandstone in Ethiopia (Tigrai): Age constraints on Neoproterozoic glaciation and crustal evolution of the southern Arabian-Nubian Shield

Detrital zircon geochronology of Neoproterozoic diamictites and Ordovician siliciclastics in northern Ethiopia reveals that the southern Arabian-Nubian Shield (ANS) formed in two major episodes. The earlier episode at 0.9-0.74 Ga represents island arc volcanism, whereas the later phase culminated at 0.62 Ga and comprised late to post orogenic granitoids related to crustal differentiation associated with thickening and orogeny accompanying Gondwana fusion. These magmatic episodes were separated by about similar to 100 my of reduced igneous activity (a magmatic lull is detected at about 0.69 Ga), during which subsidence and deposition of marine carbonates and mudrocks displaying Snowball-type C-isotope excursions (Tambien Group) occurred. Cryogenian diamictite interpreted as glacigenic (Negash synclinoria, Tigrai) and polymict conglomerates and arkose of possible peri-glacial origin (Shiraro area, west Tigrai), deformed and metamorphosed within the Ne oproterozoic orogenic edifice, occur at the top of the Tambien Group. They were formed well after the shutdown of island arc igneous activity in this region and are pierced by the post-collision granitoids. Negash diamictite and Shiraro sequence contain detrital zircons derived from underlying similar to 0.85-0.74 Ga volcanics, a small number of 1.1 Ga zircons (likely inherited within the underlying arc crust) were also detected. The youngest detrital zircons in these sequences are 0.75 and 0.74 Ga. A broadly Sturtian timing (i.e. similar to 0.70 Ga) is plausible, but we note this is a lower time limit. Our investigation shows that clasts in the diamictite have a proximal provenance and are derived from underlying igneous rocks and metasediments (including Tambien carbonates). Diamictites were formed when subsidence and basin sedimentation ceased and the Tambien and its underlying igneous complex (Tsaliet Group) were uplifted and eroded (incision exceeded 1500 m). Thus, although hearing the hallmark of a Snowball Earth, the properties of Tambien diamictites indicate relief differentiation and vertical motions may have played a significant role in shaping the glacial record of the southern ANS. (c) 2007 Elsevier B.V. All rights reserved.