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Geology Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian −Ordovician sandstones Navot Morag, Dov Avigad, Axel Gerdes, Elena Belousova and Yehudit Harlavan Geology 2011;39;955-958 doi: 10.1130/G32184.1
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Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian–Ordovician sandstones Navot Morag1, Dov Avigad1, Axel Gerdes2, Elena Belousova3, and Yehudit Harlavan4 1
Institute of Earth Sciences, Hebrew University of Jerusalem, Giv’at Ram, Jerusalem 91904, Israel Institut für Geowissenschaften, Johann Wolfgang Goethe Universität, Altenhoeferallee 1, Frankfurt am Main D-60438, Germany 3 GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW 2109, Australia 4 Geological Survey of Israel, 30 Malchai-Israel Street, Jerusalem 95501, Israel 2
the sample’s 176Hf/177Hf ratio from the concurrent chondritic uniform reservoir ratio in parts per 10,000) in zircon are indicative of juvenile crustal provinces, whereas negative values are indicative of crustal reworking (e.g., Condie et al., 2005; Kinny and Maas, 2003). The ANS is considered to be a predominantly juvenile Neoproterozoic province, whereas other Neoproterozoic–Cambrian orogenic belts within Gondwana are usually dominated by reworking of older crust (e.g., Abdelsalam et al., 2002; Stern, 2002, and references therein). Thus, in principal, the isotopic composition of Hf in detrital zircons can be used to differentiate between sediments derived from the adjacent ANS and those derived from other, more distant orogenic belts. Herein we present new Lu-Hf isotopic data acquired for detrital zircons from the Cambrian– Ordovician sandstones of southern Israel and Jordan, the U-Pb ages of which were reported by Avigad et al. (2003) and Kolodner et al. (2006). The data obtained allow a new appraisal regarding the provenance of these sandstones along with additional perspectives on the late Precambrian– Cambrian landscape of northern Gondwana.
ABSTRACT A voluminous Cambrian–Ordovician sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly by a series of Neoproterozoic–Cambrian orogenic events. Paleocurrent markers indicate that the sediments were carried from Gondwana hinterland toward the supercontinent margins in the north (present coordinates). Derivation from Neoproterozoic terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambrian–Ordovician sandstone that tops the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. It is remarkable that the detrital zircon Hf isotopic signal is in marked contrast to the Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambrian–Ordovician sandstones yielded negative εHf(t) values incompatible with a juvenile source. Therefore, most of the detrital zircons were derived from distant terranes comprising pre-Neoproterozoic crust reworked during the assembly of Gondwana, rather than from the adjacent Arabian-Nubian Shield. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Neoproterozoic orogens of northeast Africa were completely worn down by the onset of Cambrian deposition and that vast areas in the northern part of Gondwana were low lying at that time. Applying Hf isotopic data to single detrital zircons of known age may provide new insight on the provenance of the Cambrian–Ordovician sandstones. Positive εHf(t) values (the deviation of 35 oN 10 oW
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INTRODUCTION The early Paleozoic sequence at the northern periphery (in present coordinates) of Gondwana is probably the most voluminous siliciclastic sediment body ever deposited on the continental crust (Burke et al., 2003). This sequence, which covers an ~2000-km-wide belt along northern Africa and Arabia (Fig. 1A; Alsharhan and Nairn, 1997; Beuf et al., 1971; Klitzsch, 1981; Wolfart, 1981), was deposited following the amalgamation of Gondwana by a series of orogenic events spanning the interval from ca. 750 to ca. 530 Ma (Neoproterozoic–Cambrian) (Fig. 1B; e.g., Collins and Pisarevsky, 2005; Meert, 2003; Meert and Lieberman; 2008; Unrug, 1997). At the base of the sequence is a continental-scale peneplain extending from the Atlantic coast in the west to the Persian Gulf in the east (Avigad et al., 2003, 2005). U-Pb dating of detrital zircons from the Cambrian–Ordovician sandstones of southern Israel and Jordan yielded predominantly Neoproterozoic ages, indicating that Neoproterozoic (‘Pan-African’) terranes were the main source for these sediments (Avigad et al., 2003; Kolodner et al., 2006). In particular, the underlying Arabian-Nubian Shield (ANS) was designated a likely provenance. Nonetheless, Neoproterozoic-aged detrital zircons are consistent with derivation from any of the Gondwanan Neoproterozoic terranes.
o 15 N o 55 E
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Neoproterozoic Neoproterozoic juvenile crust remobilized crust
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S limit of Sand Cambrian– Ordovician Cambrian– transport sandstones Ordovician direction sandstones Cratonic basement
Figure 1. A: Simplified geological map of North Africa and Arabia showing distribution of Precambrian basement rocks and Paleozoic sediments (after Avigad et al., 2003, 2005). B: General reconstruction of Gondwana (after Meert and Lieberman, 2008). Dashed rectangle marks extent of area in A.
© 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. GEOLOGY, October 2011 Geology, October 2011; v. 39; no. 10; p. 955–958; doi:10.1130/G32184.1; 3 figures; Data Repository item 2011280.
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SAMPLING AND ANALYTICAL PROCEDURES Zircons from six Cambrian–Ordovician sandstone samples were studied: three from southern Israel and three from southern Jordan (Fig. 2). Zircons separates were U-Pb dated by Avigad et al. (2003) and Kolodner et al. (2006).
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Clay Quartz pebbles Quartz arenite Sub-arkose and arkose
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GEOLOGICAL BACKGROUND In the Middle East, Paleozoic sediments are exposed on top and around the northern and eastern peripheries of the ANS (Fig. 1A; Alsharhan and Nairn, 1997; Wolfart, 1981). The ANS, exposed over more than 2000 km along the Red Sea margins, is a crustal province of Neoproterozoic age composing the northern sector of the East African orogen (Stern, 1994). Ample Nd, Sr, and Pb isotopic data indicate that the ANS is generally a juvenile Neoproterozoic crustal addition (e.g., Duyverman et al., 1982; Stacey and Stoeser, 1983; Stein and Goldstein, 1996; Stern, 2002; Stoeser and Frost, 2006). To the south, the ANS merges with the Mozambique belt (Meert, 2003; Stern, 1994), which is dominated by ancient crust reworked during Neoproterozoic time (Kröner, 2001; Stern, 2002). By the early to middle Cambrian, the ANS basement had subsided and a widespread deposition of platform-type sediments had begun (Garfunkel, 2002). Throughout northern Gondwana, the early Paleozoic sedimentation was rather uniform, beginning on the northern peripheries in the early-middle Cambrian and gradually spreading more than 1000 km southward, reaching hinterland terranes such as Ethiopia and Yemen in the Ordovician (Alsharhan and Nairn, 1997; Garfunkel, 2002; Klitzsch, 1981; Kumpulainen et al., 2006). The Cambrian section exposed in southern Israel is ~300 m thick (Weissbrod, 1980), whereas in southern Jordan, the Cambrian–Ordovician section reaches nearly 1300 m (Powell, 1989). Nevertheless, both sections display a remarkably similar depositional history. In both localities the section changes gradually upward from subarkose and arkose to mature quartz arenite. Fluvial sandstones and conglomerates are common at the base of the section, whereas shallow-marine sandstones dominate the remainder; shale and carbonate are also present (Fig. 2; Amireh et al., 1994; Selley, 1972; Weissbrod, 1980). Scarce trilobites (Parnes, 1971) indicate that the carbonate and overlying sandstones are younger than ca. 510 Ma (early Cambrian; Landing et al., 1998). Paleocurrent indicators throughout the section generally indicate transport toward the northern Gondwana margin (Amireh et al., 1994; Selley, 1972). Similar indications for northward transport were obtained from other Cambrian–Ordovician sequences throughout northern Gondwana (Beuf et al., 1971; Dabbagh and Rogers, 1983; Kumpulainen et al., 2006).
m
K-20 22
Shehoret K-5 148 Timna 49 Amudei Shelomo K-2 90 Precambrian basement
P-2 Salib
300
Precambrian basement
Figure 2. Schematic lithostratigraphic columns of Cambrian–Ordovician sequences in southern Israel and Jordan (see text) (after Kolodner et al., 2006).
Analyses of Lu-Hf isotopic composition were carried out in situ using a laser-ablation microprobe attached to a multicollector−inductively coupled plasma−mass spectrometer (LA-MCICP-MS) in two laboratories. Samples from the Israeli section were analyzed in the Australian National Key Center for Geochemical Evolution and Metallogeny of Continents (GEMOC) at Macquarie University, Sydney. Samples from the Jordanian section were analyzed in Goethe University, Frankfurt (GUF). In both laboratories, the typical laser spot size was 40–60 μm, and the data were acquired as time-resolved profiles of isotopic ratios, which were processed offline to check the homogeneity of the ablated zircon. Corrections made for background signal, instrumental mass bias, and isobaric interferences of Lu and Yb isotopes on mass 176 were described by Griffin et al. (2000; GEMOC) and Gerdes and Zeh (2009; GUF). Note that Lu-Hf spots were located in the same zircon sector as the U-Pb spot, usually adjacent or overlapping it. Overall, a total of 282 zircon grains were analyzed. RESULTS OF LA-MC-ICP-MS Hf ISOTOPIC ANALYSIS The complete Lu-Hf isotopic data are given in Table DR1 in the GSA Data Repository.1 1 GSA Data Repository item 2011280, Table DR1 (Lu-Hf isotopic data of detrital zircons from the Cambrian-Ordovician sandstones of southern Israel and Jordan), is available online at www.geosociety .org/pubs/ft2011.htm, or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Detrital zircons εHf(t) values versus U-Pb ages (concordant only) are presented in Figure 3A. The total range of εHf(t) values for all zircons is between −44 and +15; zircons with negative εHf(t) compose 68% of all analyzed zircons. Similarly, of the Neoproterozoic-aged zircons (concordant ages only, 140 grains), 61% yielded negative εHf(t) values. The proportion of zircons with negative εHf(t) values is relatively constant throughout the Cambrian–Ordovician section (between 67% and 84%); one exception is the lowermost unit of the Israeli section (K-2, Amudei-Shelomo Formation), in which negative εHf(t) values represent only 30% of the analyzed zircons. Relative probability diagrams of Hf model ages (TDM; depleted mantle) calculated for the detrital zircons are presented in Figure 3B. For comparison, zircon Hf TDM from the nearby Elat basement (Morag et al., 2011) and whole-rock Nd TDM from the entire ANS (Stern, 2002, and references therein) are also presented. The Hf TDM of detrital zircons in the Cambrian–Ordovician sandstones range between 0.7 and 3.8 Ga with a broad peak at ca. 2 Ga, significantly different than Hf and Nd TDM obtained for the ANS basement, which are generally restricted to between 0.6 and 1.5 Ga. DISCUSSION AND CONCLUSIONS Our study reveals a remarkable contrast between the Hf isotopic composition of the detrital zircons from the Cambrian–Ordovician sandstone and that of the underlying ANS basement: while εNd(t) and εHf(t) values generally indicate that the ANS is a juvenile Neoproterozoic terrane (Morag et al., 2011; Stern, 2002, and references therein), the majority of Neoproterozoic detrital zircons (61%) from the sandstone have negative εHf(t) values, inconsistent with a juvenile provenance. This negates the ANS as the dominant source for the Cambrian–Ordovician sandstones. Hence, their main provenance must be beyond the ANS boundaries, in Neoproterozoic terrains dominated by recycling of an older crust. Some constraint on the sediment transport distance may be set by comparing Hf TDM of the detrital zircons with the available Nd TDM data for the East African orogen and neighboring areas. Recycled pre-Neoproterozoic crust with Nd TDM older than 1.5 Ga is exposed in the Afif terrane and in the eastern margins of the Saharan metacraton (Abdelsalam et al., 2002; Stern, 2002, and references therein) that are ~700–1000 km to the southwest and southeast, respectively, of the studied Cambrian–Ordovician section (Fig. 1A). However, for these terranes there are no reports of Nd TDM older than ca. 3 Ga, dates that are nevertheless found in detrital zircons from the Cambrian–Ordovician section (Fig. 3B). The nearest Neoproterozoic terrane containing such Nd TDM ages is in
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εHf(t)
Figure 3. A: εHf(t) (i.e., de20 viation of 176Hf/177Hf ratio Neoproterozoic grains (n = 140) DM from concurrent chonPositive εHf(t) 54 86 10 ANS field dritic uniform reservoir Negative εHf(t) ratio in parts per 10,000) values of detrital zircons 0 CHUR from Cambrian–Ordovician of southern Israel and Jordan (see Fig. 2) -10 plotted versus their U-Pb Israel Jordan K-2 P-2 ages (concordant ages K-5 UI-4 -20 only). Abbreviations: K-20 US-2 ANS—Arabian-Nubian (total n = 177) Shield; CHUR—chon-30 dritic uniform reservoir; 0 500 1000 1500 2000 2500 3000 3500 DM—depleted mantle. B: U-Pb age (Ma) Relative probability diagrams of concordant zirNeoMesoPaleoproterozoic Archean proterozo proterozo con Hf model ages (TDM) Cambrian-Ordovician sandstones from Cambrian–OrdoviNeoproterozoic TDM 31 17 45 Mesoproterozoic TDM cian sequence and from 84 Elat basement Paleoproterozoic TDM Elat Precambrian basezircon Hf TDM (n = 24) Archean TDM ment (Morag et al., 2011), and whole-rock (WR) ANS WR Nd TDM Nd TDM from entire ANS (n = 324) (Stern, 2002, and references therein). Zircon Hf Cambrian-Ordovician detrital zircons Hf TDM TDM are two-stage model (n = 177) ages calculated using 176 177 measured Lu/ Hf of each zircon (first stage = age of zircon) and 0 1 2 3 4 value of 0.0113 (Rudnick TDM (Ga) and Gao, 2003) for average continental crust (second stage). Nd TDM are single-stage model ages calculated using WR 147Sm/144Nd data. The DM was assigned linear evolution model beginning at 4.5 Ga and ending with present-day mean mid-oceanic ridge basalt Hf and Nd isotopic composition (Chauvel and Blichert-Toft, 2001). Diagrams were plotted using Isoplot (Ludwig, 2003).
Relative probabilty
A
B
Yemen (Windley et al., 1996), ~2000 km to the southeast of the studied section. Nevertheless, since northward-directed detrital transport was also documented in the Ordovician strata of Yemen (Dabbagh and Rogers, 1983), sediment sources could have been even farther to the south. Therefore, derivation of the Israeli and Jordanian Cambrian–Ordovician sandstones from a distance of >2000 km is a conservative estimate. A likely southern limit of the northward-directed Cambrian–Ordovician fluviatile systems may have been located along the early Cambrian Damara-Kuunga orogenic belt, which extended from Namibia in the west to Western Australia in the east (Collins and Pisarevsky, 2005; Meert, 2003), but we note that Cambrianaged zircons are rare in the Cambrian–Orodovician section. Trans-Gondwanan transport of Cambrian–Ordovician sands from the central portions of the East African orogen and their dispersal throughout Gondwana’s peripheries was suggested in a number of previous studies (e.g., Myrow et al., 2010; Squire et al., 2006; Veevers et al., 2006; Williams et al., 2002). For the Middle Eastern Cambrian–Ordovician section, a distal provenance was suggested by Weissbrod and Bogoch (2010), Kolodner et al. (2006), and Grafunkel (2000). Our new detrital zircon Hf data provide independent indication
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for the long-distance transport and dispersal of these sands. Distant sourcing of the sandstones from areas south of the ANS implies that by the middle Cambrian, the entire ANS must have been leveled such as to allow sand transfer and dispersal across it. This is consistent with previous indications for intense erosion and unroofing of the ANS as early as 600 Ma (Avigad and Gvirtzman, 2009; Garfunkel, 1999), and suggests that the final shaping of the subCambrian peneplain involved only minute denudation (Garfunkel, 2002). Despite the flat landscape of the Cambrian peneplain, monadnocks are known in several places in the northern ANS, both in Israel (Karcz and Key, 1966; Segev, 1984) and in Jordan (Selley, 1972). The somewhat higher proportion of zircons with positive εHf(t) at the lowermost unit of the Cambrian in Israel, as well as other zircons with positive εHf(t) values throughout the section, were probably sourced from the erosion of these monadnocks and their pediplains. The dominance of late Neoproterozoic zircons in the detrital U-Pb age spectra indicates that the Cambrian–Ordovician sandstones are generally first cycle in origin (Avigad et al., 2003). However, these sandstones are quartz rich and mineralogically mature (Amireh,
1991; Weissbrod and Nachmias, 1986); it has therefore been suggested that chemical weathering played a major role in their mass production (Avigad et al., 2005). The results of our study suggest that long-distance transport may have also contributed to the great mineralogical maturity of these unique siliciclastic sequences. ACKNOWLEDGMENTS We thank K. Kolodner, who allowed us to use her zircon samples and U-Pb data. We also thank R.J. Stern for providing his Sm-Nd data collection and Z. Garfunkel for advice and discussions. Remarks by two anonymous reviewers helped to improve this manuscript. This is contribution 731 from the Australian Research Council National Key Centre for the Geochemical Evolution and Metallogeny of Continents (http://www.gemoc.mq.edu.au). This research was supported by the G.I.F., the German-Israeli Foundation for Scientific Research and Development (grant no. 977-168.8/2007) and by the Israel Science Foundation (grant no. 855/06). REFERENCES CITED Abdelsalam, M.G., Liegeois, J.P., and Stern, R.J., 2002, The Saharan metacraton: Journal of African Earth Sciences, v. 34, p. 119–136, doi:10.1016/ S0899-5362(02)00013-1. Alsharhan, A.S., and Nairn, A.E.M., 1997, Sedimentary basins and petroleum geology of the Middle East: Amsterdam, Elsevier, 811 p. Amireh, B.S., 1991, Mineral composition of the Cambrian-Cretaceous Nubian Series of Jordan— Provenance, tectonic setting and climatological implications: Sedimentary Geology, v. 71, p. 99–119, doi:10.1016/0037-0738(91)90009-3. Amireh, B.S., Schneider, W., and Abed, A.M., 1994, Evolving fluvial-transitional-marine deposition through the Cambrian sequence of Jordan: Sedimentary Geology, v. 89, p. 65–90, doi:10.1016/0037-0738(94)90084-1. Avigad, D., and Gvirtzman, Z., 2009, Late Neoproterozoic rise and fall of the northern Arabian-Nubian shield: The role of lithospheric mantle delamination and subsequent thermal subsidence: Tectonophysics, v. 477, p. 217– 228, doi:10.1016/j.tecto.2009.04.018. Avigad, D., Kolodner, K., McWilliams, M., Persing, H., and Weissbrod, T., 2003, Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP dating: Geology, v. 31, p. 227–230, doi:10.1130/0091 -7613(2003)0312.0.CO;2. Avigad, D., Sandler, A., Kolodner, K., Stern, R.J., McWilliams, M.O., Miller, N., and Beyth, M., 2005, Mass-production of Cambro–Ordovician quartz-rich sandstone as a consequence of chemical weathering of Pan-African terranes: Environmental implications: Earth and Planetary Science Letters, v. 240, p. 818–826, doi:10.1016/j.epsl.2005.09.021. Beuf, S., Biju-Duval, B., de Charpal, O., Rognon, P., Gariel, O., and Bennacef, A., 1971, Les Grès du Paléozoique Inférieur au Sahara: Sédimentation et Discontinités évolution structurale d’un Craton: Paris, Institut Francais du Pétrol, 464 p. Burke, K., McGregor, D.S., and Cameron, N.R., 2003, African petroleum systems: Four tectonic aces in the past 600 million years, in Arthur, T., et al., eds., Petroleum geology of Africa: New themes and developing technologies: Geological Society of London Special
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