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Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP dating D. Avigad* Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel K. Kolodner* M. McWilliams* Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA H. Persing* U.S. Geological Survey, Stanford, California 94305-2115, USA T. Weissbrod* Israel Geological Survey, 30 Malchai-Israel Street, Jerusalem 95501, Israel ABSTRACT Voluminous Paleozoic sandstone sequences were deposited in northern Africa and Arabia following an extended Neoproterozoic orogenic cycle that culminated in the assembly of Gondwana. We measured sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of detrital zircons separated from several Cambrian units in the Elat area of southern Israel in order to unravel their provenance. This sandstone forms the base of the widespread siliciclastic section now exposed on the periphery of the Arabian-Nubian shield in northeastern Africa and Arabia. Most of the detrital zircons we analyzed yielded Neoproterozoic concordant ages with a marked concentration at 0.55–0.65 Ga. The most likely provenance of the Neoproterozoic detritus is the Arabian-Nubian shield; 0.55–0.65 Ga was a time of posttectonic igneous activity, rift-related volcanism, and strike-slip faulting there. Of the zircons, 30% yielded pre-Neoproterozoic ages grouped at 0.9–1.1 Ga (Kibaran), 1.65–1.85 Ga, and 2.45–2.7 Ga. The majority of the pre-Neoproterozoic zircons underwent Pb loss, possibly as a consequence of the Pan-African orogeny resetting their provenance. Rocks of the Saharan metacraton and the southern Afif terrane in Saudi Arabia (;1000 km south of Elat) are plausible sources of these zircons. Kibaran basement rocks are currently exposed more than 3000 km south of Elat (flanking the Mozambique belt), but the shape of the detrital zircons of that age and the presence of feldspar in the host sandstone are not fully consistent with such a long-distance transport. Reworking of Neoproteorozoic glacial detritus may explain the presence of Kibaran detrital zircons in the Cambrian of Elat, but the possibility that the Arabian-Nubian shield contains Kibaran rocks (hitherto not recognized) should also be explored. Keywords: Pan-African orogeny, erosion, Cambrian sandstone, detrital zircon, U-Pb SHRIMP dating, Gondwana. INTRODUCTION The 900–540 Ma Pan-African–Brasiliano orogenic cycle ended with the assembly of Gondwana in late Neoproterozoic time (Unrug, 1997) and was followed by the deposition of thick siliciclastic sequences on the margins of the supercontinent (Wolfart, 1981; Klitzsch, 1981; Courjault-Rade et al., 1999). These Paleozoic strata are the most voluminous siliciclastic sequences ever deposited on continental crust (Burke and Kraus, 2000), and cover large parts of North Africa and Arabia (Fig. 1; Powers et al., 1966; Klitzsch, 1981; Weissbrod, 1980; Wolfart, 1981; Amireh, 1991; Alsharhan and Nairn, 1997). Throughout North Africa and Arabia, the PanAfrican orogeny was followed by continentalscale uplift and erosion, and a vast peneplain that can be traced from Morocco in the west to Oman in the east developed on top of the exhumed Precambrian basement (Powers et *E-mail addresses: Avigad—[email protected]. il; Kolodner—[email protected]; McWilliams— [email protected]; Persing—Harold.persing@ vsea.com; Weissbrod—[email protected]. al., 1966; Bennacef et al., 1971; Bentor, 1985; Garfunkel, 1999). Late– to post–Pan-African unroofing is the most logical source for the thick Phanerozoic siliciclastic sequences of northern Africa and Arabia. However, previous conventional U-Pb ages of detrital zircons (multigrain samples) from Ordovician and younger strata exposed in northern Africa show that the siliciclastic material was derived from Mesoproterozoic sources rather than from Neoproterozoic crust (Gaudette and Hurley, 1979; Abdel-Monem and Hurley, 1979). Detrital zircons of Neoproterozoic age have since been detected in continental fragments detached from northern Gondwana (Kröner and Şengör, 1990; Keay and Lister, 2002; Gebauer et al., 1989; Fernández-Suárez et al., 1999) and in Mesozoic sandstone from Israel (Becker and Becker, 1996). Zircons from Cambrian siliciclastic strata of North Africa and Arabia have not yet been dated. These rocks extend well to the south and cover a significant part of the Neoproterozoic orogen (Fig. 1). Thus, it is plausible that by Late Cambrian time, the crust of northern Africa was no longer a source for detritus, and erosion migrated to more southerly areas dominated by Mesoproterozoic crust (Niger, Chad, and Sudan; e.g., Klitzsch, 1981; Garfunkel, 1999). We therefore reasoned that investigation of Cambrian sandstones might help us to discover if they had a Neoproterozoic provenance. Our samples come from southern Israel, where Cambrian sandstones unconformably overlie Neoproterozoic crystalline basement of the Arabian-Nubian shield (Fig. 2). GEOLOGICAL SETTING Phanerozoic sedimentary basins and platform type sequences along northern Gondwana store a huge amount of siliciclastic rocks (Fig. 1). In North Africa, Paleozoic siliciclastics fill a number of basins, the depths of which may reach 3–4 km (Fig. 1; Petters, 1991). In the Middle East, Paleozoic rocks are exposed in an arcuate belt around the northern and eastern flanks of the Arabian-Nubian shield (Fig. 1); they are 2–3 km thick at the eastern edge of the Arabian plate (Wolfart, 1981; Alsharhan and Nairn, 1997). During the early Paleozoic, the sedimentary source area generally was in the south, in the interior of Gondwana, whereas the open sea was to the north (Alsharhan and Nairn, 1997, and references therein). Precambrian crystalline rocks of the ArabianNubian shield form the basement for the Phanerozoic platform strata of Israel, Jordan, Egypt, and Saudi Arabia (Fig. 1). The ArabianNubian shield evolved from 0.90 to 0.53 Ga during the Neoproterozoic Pan-African orogeny (Shimron, 1980; Bentor, 1985; Vail, 1985; Stern, 1994; Reymer and Schubert, 1984; Stein and Goldstein, 1996), and was deeply denuded at the cessation of tectonic activity (Bentor, 1985; Jarrar et al., 1991; Garfunkel, 1999). Detrital zircons derived from juvenile crust of the Arabian-Nubian shield should yield ages in the range of 0.90–0.53 Ga. The northern Arabian-Nubian shield in southern Israel, underlying the Cambrian sandstone that we sampled, is dominated by igneous and metamorphic rocks ranging in age (U-Pb zircon) from 0.81 to 0.60 Ga (Eyal et al., 1992; Kröner et al., 1990; Beyth et al., 1994); preNeoproterozoic rocks are not known. Base- q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; March 2003; v. 31; no. 3; p. 227–230; 3 figures; Data Repository item 2003020. 227 Figure 1. Simplified geological map of North Africa and Arabia showing distribution of Precambrian basement rocks and Paleozoic sediments. Thickness contours (in km) of subsurface Paleozoic rocks are also shown (post-Paleozoic configuration after Petters, 1991; Alsharhan and Nairn, 1997). We analyzed detrital zircons separated from Cambrian sandstone exposed in southern Israel near Elat (arrow). Figure 2. Columnar section of Paleozoic clastic sequence in southern Israel (after Segev, 1984). We sampled four different levels within Cambrian sandstone section. Sample positions are marked by circles. A. Shelomo is Amudei Shelomo Formation, PA is Pan-African basement. 228 ment rocks exposed in Jordan have a similar age range (Ibrahim and McCourt, 1995). Around the edge of the Arabian-Nubian shield (eastern Saudi Arabia, Yemen, southwestern Egypt, and northern Sudan), older Proterozoic basement segments were remobilized during Neoproterozoic time (Stacey and Hedge, 1984; Harms et al., 1990; Sultan et al., 1990; Kröner et al., 1987; Schandelmeier et al., 1988; Stern et al., 1993; Black and Liégeois, 1993; Windley et al., 1996; Abdelsalam et al., 2002). Farther south, the Arabian-Nubian shield merges with the Mozambique belt (Stern, 1994), which involved Neoproterozoic igneous activity and high-grade metamorphism alongside remobilized and intact older Proterozoic crustal segments (Kröner, 2001, and references therein). that the carbonate and overlying section are younger than 510 Ma (e.g., Landing et al., 1998). Paleocurrent indicators in the Cambrian section generally indicate transport toward the northern Gondwana margin (Karcz and Key, 1966). Heavy minerals in the Cambrian section of southern Israel and Jordan (Weissbrod and Nachmias, 1986; Amireh, 1991, respectively) are characterized by the stable zircontourmaline-rutile assemblage, indicating sediment maturity and a fairly long distance transport (Weissbrod and Nachmias, 1986), possibly 1000 km (Garfunkel, 1999). In contrast, Harlavan (1992) obtained Neoproterozoic 40Ar/39Ar ages on detrital clays in the Cambrian section and inferred derivation from a more proximal source. Cambrian Sedimentary Rocks of Southern Israel Cambrian sandstones are exposed in southern Israel near Elat (Fig. 1; Garfunkel, 1978; Weissbrod, 1980). The section can be correlated with the lower part of the Saq Formation of Saudia Arabia (Powers et al., 1966). In Elat, the Cambrian section is 300 m thick (Fig. 2), changing gradually upward from subarkose and arkose to mature quartz arenite. Fluvial deposits are common at the base of the section, but shallow-marine sandstones dominate the remainder; shale and carbonate are present. Scarce Trilobites (Parnes, 1971) indicate SAMPLING AND PROCEDURES We separated zircons from four sandstone samples collected at different stratigraphic levels in the Cambrian section in the Elat area (Fig. 2). The zircons range in size from 80 to 200 mm, are primarily pale gray, and are euhedral to slightly rounded. The majority of the zircons exhibit oscillatory cathodoluminescence zoning characteristic of a felsic igneous protholith, but metamorphic zircons were also observed. U-Pb measurements were obtained from 191 zircon grains using the Stanford– U.S. Geological Survey sensitive highresolution ion microprobe (SHRIMP)-Reverse GEOLOGY, March 2003 Figure 3. Histogram showing age distribution of detrital zircon from Cambrian siliciclastic section of southern Israel. Total number of zircons 5 200. 157 grains yielded concordant ages. 206Pb/238U ages are used for zircons younger than 0.8 Ga; 207Pb/206Pb ages are quoted for older grains. 43 discordant grains are plotted on basis of their 207Pb/206Pb ages. Geometry. We analyzed 200 zircons: 65 grains from sample K-5, 46 grains from K-15, 48 from K-20, and 41 grains from sample K-2. Analytical data were processed using the Prawn and Lead codes written by T. Ireland. Plotting was performed using Isoplot (Ludwig, 1994). The 204Pb was generally ,0.01% of total Pb. The four Cambrian samples we analyzed revealed similar age distribution patterns, allowing them to be treated collectively. RESULTS OF ION PROBE U-Pb ANALYSES An age histogram of all zircons is presented in Figure 3. It shows that the overall proportions of Neoproterozoic and pre-Neoproterozoic detrital zircons in the Cambrian section are 70% and 30%, respectively1. Of the zircons (157 grains), 75% gave concordant ages (e.g., Wetherill, 1956), i.e., 135 grains yielded Neoproterozoic ages and 22 grains yielded older ages; 9 grains yielded 0.5–0.55 Ga (Cambrian) ages. Of the analyzed zircons, 20% proved to be discordant, yielding pre-Neoproterozoic 207Pb/206Pb ages. Of the Neoproterozoic zircons, 50% con1GSA Data Repository item 2003020, U-Pb geochronological data of detrital zircons from the Cambrian sandstone of southern Israel, is available on request from Document Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA, [email protected], or at www.geosociety. org/pubs/ft2003.htm. GEOLOGY, March 2003 centrate at 0.55–0.65 Ga (Fig. 3). Most of the zircons are igneous in origin, but several metamorphic zircons also yielded late Neoproterozoic ages. The most likely source of the Neoproterozoic zircons is the Arabian-Nubian shield. Of the zircons, ;30% yielded preNeoproterozoic ages: ;60% of the preNeoproterozoic zircons underwent Pb loss and are discordant on a Wetherill concordia diagram. Relying on the 207Pb/206Pb ages of preNeoproterozoic concordant and discordant grains, we distinguish the age groups 0.9–1.1 Ga, 1.65–1.85 Ga, and 2.45–2.7 Ga (Fig. 3). SUMMARY AND CONCLUSIONS Cambrian sandstones exposed in Israel belong to a regionally extensive siliciclastic sequence deposited over the northern margin of Gondwana in the aftermath of the Pan-African orogeny. We obtained U-Pb ages from detrital zircons separated from four samples of Cambrian sandstone of southern Israel: these are the first zircon ages reported from the Cambrian of North Africa and Arabia. Our data show a significant concentration of detrital zircons at 0.55–0.65 Ga; the timing of intense calc-alkaline and alkaline igneous activity, rift-related volcanism, and strike-slip faulting in the Arabian-Nubian shield (e.g., Stern, 1994). The provenance of 0.65–0.55 Ga igneous zircons is probably the voluminous late Neoproterozoic calc-alkaline granitoids (stage III; e.g., Bentor, 1985) and the widespread posttectonic alkaline igneous rocks (stage IV) of the Arabian-Nubian shield. The distribution pattern in Figure 3 is consistent with the latest Neoproterozoic being the most intense phase of igneous activity in the Arabian-Nubian shield. The presence of several 0.5–0.55 Ga detrital zircons indicates that igneous activity in the northern Arabian-Nubian shield outlasted the Precambrian-Cambrian boundary (e.g., Beyth and Heimann, 1999). Pre-Neoproterozoic zircons, grouped at 0.9–1.1 Ga (Kibaran), 1.65–1.85 Ga, and 2.45–2.7 Ga, compose ;30% of the total zircons analyzed. For the 1.65–2.7 Ga zircons, rocks from the Saharan metacraton (Fig. 1; Abdelsalam et al., 2002; Schandelmeir et al., 1988; Stern et al., 1993, and references therein) and the southern Afif terrane in southeastern Saudi Arabia (Stacey and Hedge, 1984) are plausible source terranes ;1000 km south of Elat (Fig. 1). However, the source of 0.9– 1.1 Ga (Kibaran) zircons is enigmatic. The nearest 0.9–1.1 Ga (Kibaran) granitoids are in central Africa (Burundi, Rwanda; Cahen et al., 1984) and flanking the Mozambique belt in southeastern Africa (Malawi; Kröner, 2001), more than 3000 km south of Elat. Likewise, detrital zircons of that age are abundant on the Pacific margins of Gondwana (Ireland, 1992; Williams et al., 2002). However, the Kibaran detrital zircons that we detected are not particularly rounded, and they occur in subarkose that still contains some feldspar, so a distal provenance may not be indicated. Dixon (1981) reported 1.1 Ga zircons from granitic cobbles (as well as other cobbles ranging in age from 1.1 to 2.3 Ga) in Neoproterozoic sections in the Eastern Desert, and suggested derivation from continental areas west of the ArabianNubian shield (such as Uweinat), but 1.1 Ga U-Pb zircon ages have not been recognized there (e.g., Abdelsalam et al., 2002). It is remarkable that the cobbles Dixon dated were as large as 30 cm, so they couldn’t have been transported a long distance. Thus, the possibility that the Arabian-Nubian shield contains 0.9–1.1 Ga rocks (hitherto not recognized) should be explored. Nonetheless, Pan-African orogeny coalesced with repeated Neoproterozoic glaciations (Hoffman et al., 1998; Jacobsen and Kaufman, 1999; for West Africa see Trompette, 1996, and references therein). Neoproterozoic glaciers may have transported toward Gondwana margins a large quantity of detritus that was later reworked and deposited in the great pile of early Paleozoic siliciclastics. This may prove an alternative explanation for the presence of unrounded Kibaran zircons in Elat, thousands of kilometers from any known source. ACKNOWLEDGMENTS Our study was funded by the Israel Ministry of Energy and Infrastructure. We are grateful to S. Feinstein, M. 229 Beyth, A. Segev, Z. Garfunkel, and J. Wooden for advice and discussions. We thank M. Bröcker for support and advice throughout our study, and O. Navon for providing access to his Raman laboratory. Reviews by K. Burke and R.J. Stern and an anonymous reader helped to improve this manuscript and are greatly appreciated. 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Williams, I., Goodge, J., Myrow, P., Burke, K., and Kraus, J., 2002, Large scale sediment dispersal associated with the late Neoproterozoic assembly of Gondwana [abs.]: Australian Geological Convention, 16th, Adelaide, Australia, v. 67, p. 238. Windley, B.F., Whitehouse, M.J., and Ba-Bttat, M.A.O., 1996, Early Precambrian gneiss terranes and PanAfrican island arcs in Yemen: Crustal accretion of the Eastern Arabian shield: Geology, v. 24, p. 131–134. Wolfart, R., 1981, Lower Paleozoic rocks of the Middle East, in Holland, C.H., ed., Lower Paleozoic rocks of the Middle East, eastern and southern Africa and Antarctica: London, Wiley, p. 5–127. Manuscript received 9 July 2002 Revised manuscript received 18 October 2002 Manuscript accepted 21 October 2002 Printed in USA GEOLOGY, March 2003