Detrital Zircon U–Pb Geochronology Of

Precambrian Research 154 (2007) 88–106 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 D. Avigad a,∗, R.J. Stern b,M.Beythc, N. Miller b, M.O. McWilliams d a Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel b Geosciences Department, University of Texas at Dallas, Richardson, TX 75083-0688, USA c Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel d Department of Geological and Environmental Sciences, Stanford University, CA 94305-2115, USA Received 1 May 2006; received in revised form 11 December 2006; accepted 14 December 2006 Abstract 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Arabian–Nubian Shield; Cryogenian; Neoproterozoic glaciation; Detrital zircon geochronology; Ethiopia 1. Introduction ∗ Corresponding author. Inasmuch as Rodinia rifted apart during the Neo- E-mail address: [email protected] (D. Avigad). proterozoic (e.g. Wang, 2003; Weil, 2004), great parts 0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2006.12.004 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 89 of that world were the site of plate convergence and were thus decorated by volcanic arcs, mountain belts and plateaus. Thus, Neoproterozoic Snowball Earth (e.g. Hoffman and Schrag, 2000) glaciers covered landscapes that were shaped under different geodynamic regimes. Convergence accommodating the dispersal of Rodinia gave birth to the Neoproterozoic Pan-African-Brasiliano orogeny which culminated in the assembly of Gond- wana (Unrug, 1997; Veevers, 2004). One major branch of the Pan-African-Brasiliano belt is the East African Orogen (EAO; Stern, 1994) which formed a 4000 km elongated mobile belt whose geological history spans almost all of Neoproterozoic time. Recently, glacigenic diamictites interleaved within the folded and weakly metamorphosed Neoproterozoic basement of the EAO in northern Ethiopia (Tigrai) have been reported (Beyth et al., 2003; Miller et al., 2003; Alene et al., 2006) and their potential significance for the Snowball Earth hypothesis (e.g. Hoffman and Schrag, 2000) has been considered. Stern et al. (2006) demonstrated that rocks of this type are found elsewhere in the Arabian–Nubian Shield (the northern segment of the EAO) and reviewed their mode of occurrence and possible timing. Because EAO diamictites are integrated in the Neoproterozoic orogenic edifice in Ethiopia, understanding the origin of the glacigenic sequence and its paleoclimatic signifi- Fig. 1. The Neoproterozoic East African Orogen and the cance requires understanding the orogenic processes that Arabian–Nubian Shield (after Stern, 2002). Approximate location of shaped this region. Here, we present U–Pb SHRIMP Fig. 2 is marked. dating of detrital zircons from Cryogenian diamictites and Lower Paleozoic sandstone to temporally constrain fragments in the east (e.g. the Afif-Abas terranes in Ara- crustal evolution and orogeny in north Ethiopia (south- bia and Azania in East Africa; Collins and Pisarevsky, ern Arabian Nubian Shield). We integrate these results 2005) soon after 630 Ma (Katz et al., 2004). Conver- with dating of igneous rocks and use these data to further gence related to the progressive amalgamation of East clarify the properties of Cryogenian diamictites in this Gondwana and its docking on the SE margins of the region, and attempt to place them within the history of EAO continued until 550 Ma (Meert, 2003; Collins and Neoproterozoic crust formation and orogeny. Pisarevsky, 2005)) leading to strike slip faulting, lateral displacements and northward extrusion (Bonavia and 2. Geological setting Chorowicz, 1993; Jacobs and Thomas, 2004). The ANS (Fig. 1) is dominated by supracrustal The East African Orogen (EAO; Fig. 1, Stern, 1994) metavolcanics including volcaniclastics and immature formed during Neoproterozoic time by closure of the sediments mostly metamorphosed in the greenschist Mozambique Ocean. It comprises two major segments: facies, variously deformed and intruded by granites, gab- the Arabian–Nubian Shield (ANS) in the north, and the bros, and dikes. Geochemical and isotopic signatures Mozambique Belt in the south. ANS is juvenile Neo- indicate that these rocks are dominantly mantle-derived proterozoic crust, its growth involved intra-oceanic arc juvenile crust (Stern, 2002; Stoeser and Frost, 2006). volcanism and perhaps accretion of oceanic plateaux In Ethiopia, the ANS merges with the Mozambique (Bentor, 1985; Stern, 1994; Stein and Goldstein, 1996; Belt which is the southern half of the EAO and which Stern, 2002; Johnson and Woldehaimanot, 2003). Arc accommodated the most intense collision between East terranes were welded together beginning about 780 Ma and West Gondwana fragments (e.g. Stern, 1994). and then tectonically thickened as a result of conver- The Mozambique Belt exposes higher temperature and gence between the East Sahara Metacraton in the west pressure suites with abundant amphibolite and granulite- (Abdelsalam et al., 2002), and a number of continental facies metamorphic rocks and gneiss terranes. 90 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 D. Avigad et al. / Precambrian Research 154 (2007) 88–106 91 The lateral transition between greenschist-facies juvenile volcano-sedimentary sequences of the ANS and high-grade rocks of the Mozambique Belt occurs in Ethiopia. Northern Ethiopia (Tigrai, Fig. 1) and much of Eritrea and Ethiopia plateaus expose ANS-type greenschist facies volcano-sedimentary sequences, whereas high-grade rocks are abundant to the south, east and west. Earlier studies suggested that the high-grade rocks were Archean basement underlying the Neoproterozoic volcano-sedimentary sequence (Kazmin et al., 1978) but later geochronology showed that the high-grade sequence is similarly composed of Neoproteroic pro- tholiths (e.g. Ayalew et al., 1990; Teklay et al., 1998; Yibas et al., 2002; and review in Asrat et al., 2001). The diamictitic sequences studied in the present work occur in north Ethiopia (Tigrai), where they are preserved as greenschist facies metasediments above similarly metamorphosed arc volcanics, carbonates and mudrocks of the southern ANS. 2.1. Ethiopia-Eritrea tectonostratigraphy and basement age relations A simplified geological map of Tigrai is presented in Fig. 2. The ANS basement of northern Ethiopia (Tigrai) is divided (Fig. 3) into a lower Tsaliet Group and an Fig. 3. Schematic geologic columnar section of Tigrai region showing overlying Tambien Group (Beyth, 1972). The Tsaliet main rock units. Two major Neoproterozoic units are distinguished: Group is several kilometers thick and represents an arc Tsaliet Group (metamorphosed arc volcanics and syntectonic grani- volcano-sedimentary sequence (Teklay, 1997; Alene et toid intrusions) and overlying metasediments of the Tambien Group. al., 1998; Tadesse-Alemu, 1998; Tadesse et al., 1999) Diamictites comprise the top of the Tambien Group. The entire Neo- or an arc–back–arc system (Teklay, 2006), whereas the proterozoic section is pierced by post-tectonic Mereb-type granitoids. Ordovician (Enticho) sandstone and associated Endaga Arbi tillites overlying Tambien Group (Miller et al., 2003; Alene et overly the peneplained basement.

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