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 ∼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 Neoproterozoic 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 ∼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. ∼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 bearing 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.
Bibliographical noteFunding Information:
Our study in Ethiopia is funded by USA–Israel Binational Science Foundation (BSF). We thank D. Küster (Mekelle University) and Kiros Mehari (Hisana Mining) for hospitality and assistance during our work in Tigrai and T. Tadesse for providing helpful comments on the manuscript. F. Mazdab and B. Wiegand (USGS) guided us on the ion probe and their help is greatly appreciated. T. Hurgrove is thanked for analysing igneous rock samples T3-11-10 and T3-17-01. We also thank A. Abraham, chief geologist of the Ethiopian Geological Survey for support and M. Alene from the University of Addis Ababa for joining us on the 2004 field trip. D.A. acknowledges hospitality and discussions with P. Henry, O. Bellier and P. Affaton while on a sabbatical at CEREGE (France). Comments by two anonymous reviewers helped to improve this manuscript and are greatly appreciated.
- Arabian-Nubian Shield
- Detrital zircon geochronology
- Neoproterozoic glaciation