1 Introduction

1.1 Regional geological setting

Cambrian sandstones in southern Israel are characteristic of a widespread platform sedimentary cover overlying the Precambrian basement of North Africa and Arabia. Much of the area that formerly constituted the northern margin of Gondwana was shaped by metamorphism, deformation and igneous activity during the Neoproterozoic Pan-African orogeny (Stern, 1994, and references therein). The assembly of the Gondwana supercontinent during the Neoproterozoic to Early Palaeozoic was the consequence of a prolonged history of orogenic amalgamation involving continental fragments from eastern and western Gondwana (e.g., Stern, 1994; Shackleton, 1996; Meert, 2003; Condie, 2003; Collins and Pisarevsky, 2005). The East African Orogen (EAO) extends from the Arabian-Nubian Shield, along the northern sector into East Antarctica (Stern, 1994; Jacobs et al., 2004) (Figure 1). It was recently understood that amalgamation of Gondwana occurred during two major periods of orogeny (Meert, 2003; Collins and Pisarevsky, 2005): the pre-600 Ma East African Orogeny and the subsequent 570-500 Ma Kuunga-Malagasy Orogeny. The latter was mainly documented in eastern Madagascar (Collins and Pisarevsky, 2005) as well as in southeast Kenya (Hauzenberger et al., 2007) and eastern Tanzania (Rossetti et al., 2008). Deposition of the Cambrian cover sandstone heralds erosion of the Pan-African orogen and the cratonisation of Neoproterozoic mobile belts (Avigad et al., 2003). Platform sedimentation in north Gondwana lasted nearly 500 million years (Alsharhan and Nairn, 1997) but was punctuated by long wavelength vertical motion and a number of unconformities, by Triassic-Jurassic foundering of the Eastern-Mediterranean basin (Garfunkel and Derin, 1985), and by Tertiary break-up along the Red Sea - Dead Sea fault system.

During the early Paleozoic, Israel was located near the northern edge of Gondwana at the northern tip of the Arabian-Nubian Shield. The Arabian-Nubian Shield (ANS) of North Africa and Arabia is a collage of accreted Neoproterozoic island arcs squeezed between east and west Gondwana fragments around 630 Ma (Katz et al., 2004; Johnson and Woldehaimanot, 2003). Current exposure of the northern ANS is dominated by widespread late to post-orogenic batholiths intruded at 630-600 Ma (Bentor, 1985 and references therein). Calc-alkaline magmatism was followed by a major phase of erosion, extension and alkaline igneous activity at around 600-580 Ma (Garfunkel, 1999; Beyth et al., 1994). Various Rb/Sr and K-Ar studies have suggested that igneous activity may have lasted until 540 Ma (Bentor, 1985; Ibrahim and McCourt, 1995; Garfunkel, 1999; Beyth and Heimann, 1999) but this has not been verified by U-Pb zircon geochronology. The youngest U-Pb zircon age in the northern ANS is ca. 580Ma (Meert, 2003). By Middle Cambrian time, the northern part of the ANS (including southern Israel) was eroded to sea level and the exhumed basement was subjected to intense weathering. In a world without land plants and under acidic atmospheric conditions, physical and chemical weathering attacked the freshly exhumed basement, and a system of braided rivers deposited vast sheets of Cambro-Ordovician ‘Nubian sandstone’ (Weissbrod, 1980; Weissbrod and Nachmias, 1986; Avigad et al., 2005). The weathered basement was progressively blanketed (from north to south; e.g. Garfunkel, 1999) and the entire ANS as far south as Ethiopia and Yemen was covered by a siliciclastic veneer by Ordovician time (Garfunkel, 2002; Kolodner et al., 2006). Continuous sedimentation in the Levant continued until Late Devonian time, when the sedimentary cover reached a thickness of ~2.5 km (Garfunkel and Derin, 1985; Gvirtzman and Weissbrod, 1985; Segev et al., 1995). Hercynian doming interrupted Paleozoic sedimentation, and a regional unconformity marks the base of the Carboniferous (Gvirtzman and Weissbrod, 1985). The area was later affected by the Triassic to Early Jurassic opening of the Eastern Mediterranean basin (Garfunkel and Derin, 1985). Thick wedges of predominantly marine sediments, including substantial amounts of carbonate, were deposited on the subsiding passive margin until the Late Cretaceous, when tectonism affected the northern and eastern margins of the ANS (Garfunkel and Derin, 1985). Miocene-Oligocene opening of the Red Sea was initially accommodated by rifting in the Gulf of Suez, but at 4-5 Ma, plate motion shifted to the left-lateral Dead Sea transform fault (Bartov et al., 1980). Approximately 105 km of strike-slip motion (Quennell, 1958) formed several pull-apart basins, of which the Dead Sea is the largest (Garfunkel and Ben-Avraham, 1996). We applied low-temperature thermochronology to platform detritus from southern Israel to learn about events that affected the lithosphere before and after their deposition in the Cambrian. Our results illuminate the final stages of Neoproterozoic to Cambrian Pan-African orogeny in this region and help to monitor the long-term post-depositional thermal evolution of the siliciclastic platform deposits.

1.2 The Cambrian of southern Israel

Cambrian strata in southern Israel near Eilat overlie the Neoproterozoic basement of the ANS (Weissbrod 1980; Weissbrod and Nachmias, 1986), which was exposed in the Gulf of Eilat and Sinai in the Tertiary as a result of breakup and rifting along the Red Sea. The Cambrian section at Eilat can be correlated with similar sections across the Dead Sea rift in Jordan, which in turn can be linked to exposures farther east and southeast in Saudi Arabia (e.g. the lower part of the Saq Formation; Powers et al., 1966). At Eilat, the 300m Cambrian section gradually changes upward from subarkose to mature quartz arenite (Figure 2). Fluvial deposits are common at the base, shallow marine sandstones dominate the remainder and shale and carbonate are present locally. From base to top, the Cambrian sequence in Eilat area includes (Weissbrod, 1980; Segev, 1984; Garfunkel, 2002): the Amudei Shelomo Formation, made of relatively immature arkose and subarkose overlying deeply weathered Neoproterozoic basement. A basal polymictic conglomerate is often present; the Timna Formation, made of mainly subarkose with local carbonate facies. Scarce Trilobites in the carbonates (Parnes, 1971) indicate that these rocks and the overlying parts of the section are younger than 520 Ma (e.g., Landing et al., 1998); the Shehoret Formation, consisting of fine- to coarse grained subarkose; the Netafim Formation, comprised of Late Cambrian fine-grained quartz arenite with alternating layers of siltstone and claystone; and the Amir Formation, made of Lower Cretaceous sandstones unconformibly overlying the Cambrian section.

The general trend of increasing sediment maturity is common to the Cambrian section of southern Israel and sections in Jordan, the Sinai, and northern Saudi Arabia (Weissbrod and Nachmias, 1986; Avigad et al., 2005; Kolodner et al., 2006), but the depositional gap represented by the unconformity below the Amir Formation varies. In the subsurface north of Eilat the Cambrian is overlain by Late Carboniferous to Permian sandstones, whereas in southwestern Sinai, the Cambrian is overlain by Carboniferous sediments (Weissbrod 1980; Garfunkel 2002). A more complete lower Paleozoic section is preserved in Jordan and northern Saudi Arabia, where continuous sedimentation occured from Cambrian to Devonian time (Figure 2 of Weissbrod and Nachmias, 1986, and references therein). These geologic constraints are important to the interpretation of the data presented later. Deposition of Late-Middle to Late Cambrian sandstone marks the end of the Pan-African orogeny in the region and the onset of platform sedimentation. Detrital zircon U-Pb geochronology (Avigad et al., 2003; Avigad et al. 2005; Kolodner et al., 2006) revealed that the Cambro-Ordovician sandstones reflect the crustal composition of broad segments of the ANS and beyond it (Kolodner et al., 2006). The Cambro-Ordovician sandstones on the entire North Gondwana margin are mineralogically and often texturally mature (Avigad et al., 2005 and references therein), indicating significant chemical and physical weathering and long-distance transport. The provenance area of the Cambro-Ordovician sandstone was very large, and detritus was transported for hundreds and possibly thousands of kilometers. The Cambro-Ordovician sandstones are thus an excellent monitor of continental-scale geological processes at an important time in Earth history. This study extends the perspectives gained from U-Pb detrital zircon geochronology by applying a suite of thermochronometric techniques to detritus from a fine-grained sub-arkose sample of the Shehoret Formation deposited at ~500 Ma. We present and discuss new detrital K-feldspar ⁴⁰Ar/³⁹Ar ages, zircon and apatite fission track ages and apatite (U-Th)/He ages. Taken together, our new data reveal a lengthy and more detailed picture of the pre- and post-depositional thermal history of the Nubian Sandstone.

2 Methods

Sample K-6 was collected from the Shehoret Formation, close to and from the same horizon as sample K-5 of Kolodner et al. (2006) for which detrital zircon U-Pb ages are available (Figure 3). Because the sample was only weakly consolidated, it was gently crushed by mortar and pestle, thus reducing the potential for damage to apatite grains. After soaking in 10% acetic acid and 3% hydrogen peroxide until reactions ceased, magnetic separation was done by vertical and slope Frantz. The nonmagnetic fraction was placed in lithium metatungstate with a density of 2.96 g/cm³ to separate quartz and feldspar from apatite and zircon. The light and the heavy fractions were next placed in methylene iodide, with density 2.59 g/cm³ and 3.3 g/cm³, respectively, to separate K-feldspar from quartz and plagioclase, and apatite from zircon.

Two aliquots of detrital K-feldspar were prepared for ⁴⁰Ar/³⁹Ar analysis and irradiated at Oregon State University. Neutron flux was monitored by co-irradiating grains of USGS standard Taylor Creek Rhyolite (TCR) sanidine with an assumed age of 28.34 ± 0.14 Ma (Renne et al., 1998). Upon return from the reactor, one aliquot was unwrapped and 50 single grains were placed in a crucible under high vacuum for laser analysis at Stanford University. Apparent ages of single grains were measured by total fusion with an Ar-ion laser. The second aliquot was analyzed by step-wise heating in a double-vacuum resistance furnace. For both aliquots the gettered gas was analyzed in a high sensitivity MAP 216 noble-gas mass spectrometer with Baur-Signer ion source (Baur, 1980).

Zircon fission track ages were measured by Paul O’Sullivan of Apatite to Zircon, Inc. using the external detector method (Hurford and Green, 1983). Fish Canyon Tuff zircon was used as an irradiation standard. Apatites were mounted in epoxy, ground, and polished. The mounts were etched for 15 s in 23% HNO₃ at 25 ^∘C (Jonckheere et al., 2007). They were then covered with muscovite external detectors, and irradiated at the Garching/Munich reactor. The muscovite external detectors were etched in 40% HF for 30 minutes at room temperature. Two mounts of Fish Canyon apatite and three shards of Durango apatite standards were irradiated together with the sample along with mm-sized shards of standard uranium glass. Track counting was performed at ETH Zürich, on c-axis parallel apatite surfaces under 1250× magnification. The muscovite external detectors were repositioned trackside down on the apatite mounts in the same position as during irradiation. Fossil tracks were counted by focusing on the apatite surface through the muscovite detector; induced tracks were counted by focusing on the underside of the external detector without moving the microscope stage (Jonckheere et al., 2003). Chlorine content was used as the kinetic parameter, and measured a Jeol 8100 Superprobe (WDS) at Birkbeck, University of London. Analysis was carried out using an accelerating voltage of 15 kV, current of 2.5 nA and a beam diameter of 1 μm. The analyses were calibrated against standards of natural silicates, oxides and Specpure metals with the data corrected using a ZAF program.

Apatite grain dimensions were measured by binocular microscope at 200× magnification. The alpha-retention factor F_t was calculated from the surface/volume of the grains (Meesters and Dunai, 2002) using an ellipsoidal approximation that was more appropriate to the shape of our detrital apatites (see discussion later). Helium was extracted in a 3 minute laser heating using a 1064 nm wavelength Nd-YAG laser. Re-extraction yielded no detectable helium, indicating complete degassing. The evolved gas was cleaned in a liquid N₂-cooled activated charcoal cold finger and Ti/Zr and Al/Zr getters. ⁴He was measured by peak height calibration to a known amount of ⁴He in a custom-built sector-type mass spectrometer at ETH Zürich. After He extraction, the Pt packets were recovered from the laser pan, partially opened under the binocular microscope, and placed in teflon vials. The samples were spiked with ~50 fmol of ²³³U and ~20 fmol of ²²⁹Th. Approximately 1 ml of concentrated high purity quartz-distilled HNO₃ was added. After digestion on a hot plate (~150^∘C) for 24 h, the HNO₃ was evaporated and ~1 ml of a 6% HNO₃-0.8%HF solution was added. ²²⁹Th, ²³²Th, ²³³U, ²³⁸U and ²³⁵U were measured in low mass resolution by single-collector ICP-SF-MS (Element2).

3 Results

3.1 Previously published zircon U-Pb ages

The detrital zircon age spectrum from sample K-5 resembles that of other samples of Cambrian and Ordovician sandstones in Israel and Jordan (Kolodner et al., 2006), providing an image of the pre-depositional history of the source terrane (Figure 4a). Thirty-four of 55 zircons are of Neoproterozoic Pan-African provenance between 900 and 530 Ma. The U-Pb detrital zircon age spectrum generally overlaps with the history of igneous activity in the ANS that lasted for nearly 300 m.y. spanning much of the Neoproterozoic (Stern, 1994). While the pre-Neoproterozoic zircons have been interpreted as far-traveled, it is possible that some are recycled inherited grains derived from more proximal Neoproterozoic rocks (Avigad et al., 2003; Hargrove et al., 2006). The precise origin of Late Neoproterozoic to Cambrian age (570-530) detrital zircons is also not known as potential source rocks displaying U-Pb zircon ages of this type are uncommon in the northern ANS (Meert 2003). Such rocks are more common in the southern half of the EAO (Kuster and Harms, 1998). All these are plausible sources because the textural and mineralogical maturity of the sandstone evince significant transport. Five grains are actually younger than the inferred 500 Ma depositional age of the Shehoret Formation, and three of these five are concordant. We are confident that these anomalously young ages are not a result of sample contamination because the four samples studied by Kolodner et al. (2006) from the Shehoret area contained a similar proportion of anomalously young grains. Later in this paper we will propose an authigenic origin for these zircons.

3.2 K-feldspar ⁴⁰Ar/³⁹Ar ages

Two sets of ⁴⁰Ar/³⁹Ar data were collected from sample K-6. Fifty single-grain K-feldspar laser total fusion extractions yielded a population of ages that tightly cluster around 535 Ma (Cambrian), indicating a single provenance and thermal history. About half of these grains yield apparent ages that are younger than the zircon U-Pb age spectrum, whereas the other half overlap with the very latest phase of igneous activity as recorded in the zircon U-Pb age spectra. Nearly all K-feldspar grains are older than the depositional age, although some by only a few million years (Figure 4b). A similar picture emerges from the step-heating experiment (Figure 5). Except for the first few temperature steps that amount to less than 10% of the ³⁹Ar released, the spectrum yields late Neoproterozoic to Cambrian apparent ages between 520 and 580 Ma. Because it is possible that even the high temperature part of the age spectrum is affected by hydrothermal alteration, these should be considered minimum ages. The Arrhenius diagram shows multi diffusion-domain behavior typical of K-feldspars (Figure 5). The short time lag between the ⁴⁰Ar/³⁹Ar ages and the depositional age of the sandstone suggests that these grains cooled rapidly, an observation that is corroborated by the step-heating experiment. Because K-feldspars cannot be characterized by a single diffusion domain, they rarely feature easily interpretable age spectra, especially if they experienced a slow and complex thermal history (Lovera et al., 1989). Alternatively, the ‘kink’ in the Arrhenius diagram could also be an artifact of multiple stages of crystal growth (Villa, 2006). In this context, the relatively flat age spectrum of K-6 can be taken as evidence for the simple, rapid, and quasi-simultaneous cooling and/or crystallization of all the K-feldspar grains contained in it. The flat appearance of the age spectrum also lends credibility to the single step total-fusion ages of the detrital K-feldspar grains (Lovera et al., 1999). The ‘partial retention zone’ of the ⁴⁰Ar/³⁹Ar system in K-feldspar is between 200-400^∘C for cooling rates greater than 5^∘C/Ma (Lovera et al., 1993). Because all the detrital K-feldspars and nearly the entire ⁴⁰Ar/³⁹Ar age spectrum are older than the depositional age, the sample was probably not heated to more than 200^∘C after deposition, nor did it undergo complete re-crystallization. Because the sample was a composite collection of unreset detrital grains, we did not attempt to extract a continuous time-temperature model from the data by inverse modeling (Lovera, 1989). Different grains within the sample went through different high-temperature histories, but shared a common low-temperature history. Therefore, the low-temperature part of the step-heating experiment might contain a post-depositional reheating signal, an idea that will be discussed in the following section. The flat section of the age spectrum represents an averaged high temperature age signal of all the component grains. Thus, the average K-feldspar grain in our sample passed through the 300^∘C isotherm before 560 Ma.

3.3 Zircon fission track (ZFT) ages

Fission track ages are roughly proportional to the ratio of the number of spontaneous tracks to the number of induced tracks, both of which have a Poisson distribution. Because the logarithm of the ratio of two Poisson-distributed quantities has an approximately Gaussian distribution, the data of Figure 4 are shown with a logarithmic time-axis (Brandon, 1996). The ZFT ages tightly cluster around 380 Ma (Late Devonian), substantially younger than the depositional age and suggest a significant post-depositional thermal event.

3.4 Apatite fission track (AFT) ages

Sample K-6 contained extremely rounded apatites (Figure 6.a). Morphological and compositional details of these grains are visible in Figure 6. About 20% of the detrital apatites feature distinct overgrowths of euhedral apatite (Figures 6a and 6b). The fission tracks induced in the muscovite external detector reveal that these overgrowths are richer in uranium than their rounded cores (Figure 6c). The spread of the AFT ages is substantially greater than that of the ZFT ages (Figure 4d) because of the lower number of tracks in the apatite. There are fewer induced tracks because the sample was somewhat under-irradiated (we did not expect to find such old ages), and there are fewer spontaneous tracks because apatite generally has a lower uranium concentration than zircon. The AFT ages approximately follow a lognormal distribution (Figure 4d) with a median and central age around 270Ma and an age dispersion (Galbraith, 1990) of 19%. The horizontally confined fission track length distribution is approximately normally distributed with a mean of 9.3μm and standard deviation 2.3μm. Electron microprobe analysis revealed that our sample contains almost pure F-apatite. The Cl content is very low (< 0.1 wt%), significantly lowering the annealing temperature of our sample (Green et al., 1986).

3.5 Apatite (U-Th)/He

Seven apatite grains yielded (U-Th)/He ages between Late Cretaceous and Eocene. As is often the case with (U-Th)/He data, the substantial scatter cannot be explained by the analytical uncertainty of the U, Th and He measurements alone (Fitzgerald et al., 2006). We postulate that in this case the scatter is caused principally by the alpha-ejection correction, which assumes a uniform chemical composition. As noted above, a substantial number of the apatites in sample K-6 are compositionally zoned (Figure 6). If the rim is intact, this will lead to an over-estimation of the alpha-retention factor F_t (Figure 7). Conversely, (partial) removal of the rim during the mineral separation can also lead to an under-estimated alpha-retention factor F_t because of implanted helium.

4 Discussion

4.1 Geodynamic implications

Unreset detrital K-feldspar ⁴⁰Ar/³⁹Ar ages are generally interpreted either as representing cooling or crystallization of the source terrane. Igneous evolution in the ANS is thought to have outlasted the latest Neoproterozoic. Many have reported Rb-Sr, K-Ar and ⁴⁰Ar/³⁹Ar ages in the range of 560-530 Ma and some of these results were interpreted to reflect crystallization (Ibrahim and McCourt, 1995; Beyth et al., 1994, 1999). In particular, it is commonly thought (e.g., Bentor 1985) that post-orogenic and/or extension-related alkaline igneous activity (as well as several diking phases and various volcanics) occurred between 600-540 Ma (at least in the northern ANS). Our literature survey failed to reveal zircon U-Pb ages in support of this; the youngest published zircon age we found is ~580 Ma (Jarrar et al., 1991, 1993; see also summary of ANS geochronological data by Meert, 2003). An extensive summary of available geochronological data provided by Meert (2003) reveals that U-Pb zircon crystallization ages younger than 580Ma are confined to the Kuunga-Malagasy Orogen which flanks the Arabian-Nubian Shield in the SE at a distance of some 2000 km from Eilat. Avigad et al. (2003) and Kolodner et al. (2006) demonstrated the presence of far-traveled pre-Neoproterozoic detrital zircons in Eilat and it is plausible that the ca. 550 Ma detrital zircons in this area are sourced from such a distal provenance. However, it is unlikely that the detrital K-feldspar in the Shehoret Formation has a similarly distal provenance. With respect to Eilat, a mid to short range provenance within the Arabian-Nubian Shield is the most likely source for these feldspars. Moreover, although some alkaline rocks in southern Israel yielded Rb/Sr ages of ca ~530 Ma, U-Pb dating of an alkaline granite from a nearby outcrop yielded ~610 Ma (Beyth et al., 1994). Thus, the interpretation of the 530-560 Ma Rb/Sr and ⁴⁰Ar/³⁹Ar ages in terms of crystallization is questionable. Similar ages in the range of 530-540 Ma were reported from many basement rocks of the ANS and were shown to reflect widespread thermal resetting at the end of the Pan-African orogeny. Resetting of the K-Ar and Rb-Sr systems at around 530 Ma in older basement rocks has been reported from the Sinai peninsula (e.g., Ayalon et al., 1987; Bielski et al., 1979), Jordan, Israel and other parts of the ANS (for example Fleck et al., 1976) and is one of the most fundamental features of the so called ’Pan-African orogeny’ of Kennedy (1964).

It is certainly possible that our K-feldspar ⁴⁰Ar/³⁹Ar have been affected by subsequent (hydro)thermal activity and are, therefore, minimum ages. However, in light of the regionally abundant 560-530 Ma cooling ages discussed in the previous paragraph, we should also consider the alternative possibility that our ⁴⁰Ar/³⁹Ar-ages are ‘real’. We will make this assumption for the remainer of this section. Because none of the basement rocks exposed in Israel, Jordan and Sinai yielded zircon U-Pb ages younger than 580 Ma it is difficult to interpret the K-feldspar ages as crystallization ages. Only the oldest ages in the detrital age spectra could potentially reflect crystallization because they overlap with known crystallization ages defined by U-Pb zircon data. It is more likely that many of the detrital K-feldspars were derived from rocks that were affected by the widespread thermal resetting at the end of Pan-African orogeny. We suggest that insights as to how ANS basement rocks were reset can be obtained from the petrographical observations of sandstone sample K-6. The detritus in this sample is a mixture of sub-rounded and angular grains reflecting mixing between proximal and more distal provenance. Microcline is the most abundant feldspar in sample K-6. Plagioclase and perthite are practically absent, a feature which is suggestive of elimination of the less resistant minerals by weathering. Furthermore, the exclusive presence of microcline in the sample we dated may be a clue to the geodynamic significance of the Pan-African resetting.

The Pan-African orogeny in the ANS culminated in the intrusion of massive calc-alkaline granitoid batholiths (630-600 Ma; Bentor, 1985; Beyth et al., 1994; Garfunkel 1999) which currently make up more than 60% of ANS exposure (Stern and Hedge, 1985). Emplacement of these batholiths was immediately followed by intrusion of shallow alkaline granitoids and dike swarms. Petrological observations indicate that the calc-alkaline phase is dominated by subsolvus granitoids featuring Na-rich plagioclase and microcline as individual phases indicating consolidation at significant depth (Garfunkel, 1999). In contrast, the alkaline rocks are shallow level intrusions and their mineralogy is often (but not exclusively) dominated by perthite. Because perthite is practically absent from sample K-6, and because calc-alkaline granitoids are by far more abundant, many (if not all) of our ⁴⁰Ar/³⁹Ar-dated feldspars were probably derived from the calc-alkaline batholiths. A large body of evidence suggests that calc-alkaline rocks were first exposed by about 600 Ma. Neoproterozoic molasse basins formed in the northern ANS immediately following the calc-alkaline batholitic stage. They are well known in Jordan (Saramuj; Jarrar et al., 1993), the Eastern Desert of Egypt (Hammamat; Willis et al., 1988) and in the subsurface of Israel (Zenifim; Weissbrod and Sneh, 2002). They contain immature detritus and conglomerates derived from exposed calc-alkaline granitoids and older basement rocks, and are frequently interbedded with volcanics. The deposition of the Hammamat and hence exhumation of the underlying basement including the abundant calc-alkaline batholiths is constrained by the intrusion of a ~585 Ma perthitic biotite-granite into the sediments (Willis et al., 1988) and by the presence of 600-580 Ma andesites and rhyolites (Dokhan volcanics) interbedded in this unit (Stern and Hedge, 1985). In southern Jordan, the Saramuj Formation is a thick conglomerate unit derived from the underlying basement (Jarrar et al., 1991) and pierced by a ~590 Ma monzogranite (Jarrar et al., 1993). Thus, field relations and geochronology indicate that the molasse basins were established upon, and fed from, eroded calc-alkaline batholiths whose ages are no younger than ~585 Ma. The observation that the detrital microclines yield 530-560 Ma ⁴⁰Ar/³⁹Ar apparent ages implies that the calc-alkaline source was reheated before exposure and then rapidly cooled. The biotite ⁴⁰Ar/³⁹Ar system in the calc-alkaline basement at Eilat was not reset (620 ± 10 Ma; Cosca et al., 1999), placing tight constraints on the maximum temperature of K-feldspar resetting at 300-350^∘C.

Because the calc-alkaline rocks were exposed by ~590 Ma, the resetting of their K-feldspar at ~300^∘C must have been caused by reburial. Thrust faulting affected segments of the ANS in the Eastern Desert subsequently to the intrusion and exposure of the calc-alkaline granitoids (Greiling et al., 1994; de Wall et al., 2001) and may have caused re-burial but it is questioned whether contraction was regionally widespread. We suggest that late Pan-African thermal resetting was additionally caused by a thick volcanic edifice built upon certain ANS domains some time after ~590 Ma and later removed by erosion beginning ~560 Ma. Considering the elevated geothermal gradient of 50^∘C/km, the thickness of the volcano-sedimentary sequence covering the calc-alkaline basement at ~560 Ma locally reached 6 km. Remnants of such a thick volcanic edifice are preserved within the Late Neoproterozoic extensional basins such as the Hammamat, which is still loaded with a 1 km volcanic sequence (Dokhan) whose base locally bears evidence for low-grade metamorphism (Greiling et al., 1994). Given complete exposure of reset basement by 500 Ma (the estimated age of deposition of Shehoret Formation) the postulated ~6 km thick volcanic section was eroded in >60 m.y.

5 Post-depositional thermal and burial history

The ZFT partial annealing zone is 175 - 250^∘C depending on the thermal history (Brandon et al., 1998). Our ZFT ages cluster at 380 Ma, about 100 m.y. younger than the K-feldspar ages and indicating that the Cambrian sandstone section in southern Israel was heated to temperatures corresponding to low-grade metamorphism subsequent to deposition. Although the ZFT data fail the χ²-test (P(χ²) = 2%), the fact that all zircon grains are younger than the depositional age, and the remarkable coincidence between the ZFT ages and authigenic clays in the same deposits (discussed later) indicates that the ZFT system is almost, if not completely, reset. The fact that the former are thermally reset whereas the latter are not implies that the low-grade metamorphism that affected the Cambrian sandstone during Devonian time did not reach temperatures high enough to completely reset the ⁴⁰Ar/³⁹Ar clock of the detrital K-feldspars. It may seem odd that the ZFT thermochronometer is completely reset by a thermal pulse that has an only minor effect on the K-feldspar ⁴⁰Ar/³⁹Ar system. However, the effective closure temperature of the ZFT system is controversial and has been demonstrated to be sensitive to a range of factors, including cooling rate and radiation damage (Brandon et al., 1998; Rahn et al., 2004). Although the combination of these parameters remains poorly understood, the argon and ZFT data nevertheless put tight constraints (within a few tens of degrees) on the post-depositional thermal history of the Shehoret Formation. While we did not attempt to inverse-model the argon release spectrum for the reasons outlined above, we did forward-model a thermal history in which rapid cooling at 560 Ma is followed by a 1 m.y. heating spike to 200^∘C at 380 Ma (Figure 5). Despite its simplicity (only two diffusion domains were employed) the model adequately replicates the low-temperature part of the ⁴⁰Ar/³⁹Ar age spectrum (Figure 5). Although this model is far from unique (the intensity of the heating peak, for example, could be higher if its duration were shorter), it does illustrate that a single heating pulse could be responsible for the partial loss of radiogenic argon from the least retentive parts of the K-feldspars, while leaving the more retentive zones, which are probed by the high temperature steps of the release spectrum, largely unaffected.

The late Devonian ZFT ages are entirely compatible with the regional geologic context. By itself, the erosional unconformity between the late Cambrian Netafim Formation and the early Cretaceous Amir Formation in southern Israel is not very informative. Tighter constraints are possible if we assume that the northern ANS behaved as a single block and that stratigraphic sections of southern Israel, the Sinai, southern Jordan and northwestern Saudi Arabia can be reliably correlated (Weissbrod and Nachmias, 1986). In the southwestern Sinai, the Netafim is Early Carboniferous, not Cretaceous, and northwestern Saudi Arabia shows a continuous stratigraphic section from the Cambrian until the Devonian (Figure 2 of Weissbrod and Nachmias, 1986). Jointly considered, these observations imply that major erosion occurred at or near the Devonian-Carboniferous boundary, when deepest burial also occurred (Segev et al., 1995).

These conclusions agree with an earlier ZFT study from boreholes in Sinai and southern Israel (Kohn et al., 1992) and basement rocks in the Eastern Desert of Egypt (Bojar et al., 2002). ZFT ages for the Precambrian basement and infra-Cambrian sediments in the subsurface of Israel (Zenifim Formation) are within the range 328-373Ma, consistent with our results for the Shehoret Formation. Although the lower Paleozoic of southern Israel is less than 300 m thick, Kohn et al. (1992) inferred that the basement must have been covered by a thick sedimentary cover that was rapidly removed during an important late Devonian - early Carboniferous tectonic event. Their inference was based on their thermochronometric data and the same regional stratigraphic considerations summarized in the previous paragraph. In addition to counting zircon fission tracks, Kohn et al. (1992) analyzed sphene and found it to be only partially reset. The annealing temperature of sphene is slightly higher than zircon, and thus Kohn et al. (1992) constrained the maximum temperature attained during the Devonian-Carboniferous event to ~225^∘C. This is in agreement with our thermochronological findings based on the paired ⁴⁰Ar/³⁹Ar-ZFT results (see above). Kohn et al. (1992, see also Summer et al., 1995) obtained similar Devonian ZFT ages from Permo-Triassic sandstones and they concluded that these are recycled earlier Paleozoic sandstone, predicting that the Cambrian of southern Israel should also have been thermally reset by a 370 Ma event. Our data provide the first direct confirmation of their prediction.

The impact of a Devonian thermal event is imprinted in several other mineralogical systems too. Rb-Sr ages of < 2μm clays within shales and dolostones of the Early Cambrian Timna Formation are identical (365-381 Ma) to the ZFT data, as are K-Ar and Rb-Sr ages of the insoluble residue of manganese nodules that are found in the same formation (Segev et al., 1995). Identical ages were obtained by Harlavan (1992) on clay minerals from the Cambrian section in Israel using ⁴⁰Ar/³⁹Ar geochronology, and Heimann et al. (1995) obtained ⁴⁰Ar/³⁹Ar ages of ~378 Ma on altered biotites separated from schistose dikes of the Eilat basement and suggested they reflect the influence of a Devonian hydrothermal system. The apatites in the Shehoret Formation give further morphological evidence for a Late Devonian (hydro)thermal event. The cores of the detrital apatites are extremely rounded (Figures 6 - 7), suggestive of long transport. The authigenic overgrowths of some grains are euhedral and are therefore post-depositional. No authigenic apatites have been reported in deposits younger than Devonian (Weissbrod and Nachmias, 1986). While it has been postulated that abundant U-mineralization observed in the Cambrian of southern Israel occurred during the Neogene (Segev, 1992), our observations imply that a pulse of authigenic apatite growth from a uranium-rich fluid phase occurred during the late Devonian in conjunction with the thermal event recorded by the ZFT. The maximum thickness of the sedimentary cover that overlies the Cambrian of Israel during Devonian time can be estimated by correlation with Jordan where it is ~2.5 km thick. It is unlikely that the thermal event recorded by the ZFT was uniquely a consequence of burial, because the overburden was too thin to permit temperatures in the range of 200^∘C at its base. We suggest that the thermal event resulted from hydrothermal fluid circulation related to low-grade anchimetamorphic phenomena that caused the abundant recrystallization of clay minerals and mobilised U.

As discussed above, Kolodner et al. (2006) reported a small but significant fraction of concordant post-depositional zircon U/Pb ages from the Cambrian section of southern Israel. No such grains were found in the Jordanian sections, which were located 50km east of our sample (and 100km south of it, prior to the onset of left-lateral slip on the Dead Sea transform fault). The anomalously young grains are characterized by relatively high U concentrations (Kolodner et al., 2006) and were interpreted as metamict. In view of the U mineralization and the development of U-rich rims around apatites it is possible that some detrital zircons might have experienced an authigenic overgrowth. Similar observations of authigenic zircon growth have been made elsewhere (Saxena, 1966; Dempster et al., 2004) .

Like the zircons, also the apatites were completely reset after deposition. The ~270 Ma AFT ages are systematically younger than the ZFT ages by about ~100 m.y. (Figures 4d & 8). Together with the large spread of the AFT ages, this suggests an extended and/or repeated residence in the AFT Partial Annealing Zone (PAZ: 60 - 110^∘C; Brandon et al., 1998). The unconformity at the base of the Early Cretaceous sandstones that immediately overlie the Cambrian in southern Israel (Figure 3), is not recorded in the AFT data, implying that probably less than 2 km of sedimentary cover had been eroded by then. As explained previously, the Late Cambrian Netafim Formation in the southwestern Sinai is overlain by Carboniferous deposits. Therefore, there have been at least two episodes of surface exposure of lower Paleozoic rocks in this area. The AFT ages and track length distribution therefore probably record a complex history with several stages of burial and erosion in and out of the PAZ. In Elat area (at the surface) the entire Phaneorozoic section (i.e. Cambrian and then Lower Cretaceous to Eocene) is about 1.0-1.2 km thick (Figure 3.c), which is more or less the thickness of the section that was denuded during the Tertiary (Gvirtzman, 2004). Despite the relatively small number (44) of horizontally confined fission tracks in our sample, we inverse modeled the thermal history using the HeFTy program of Ketcham (2005). Chlorine content was used as the kinetic parameter, and default values were used for all settings. The search space was very broadly defined in order to allow non-monotonic thermal histories. A large number of models were found that can explain the data, all of which indicate a prolonged stay in the upper part of the PAZ (Figure 9). The models suggest a late Cretaceous heating pulse, consistent with the local stratigraphy and with our (U-Th)/He data, which are discussed next.

The apatite (U-Th)/He ages range from 33 to 77 Ma. The substantial scatter in these data can be explained by the extreme compositional zonation associated with the authigenic U-rich overgrowths on some of the grains. If the overgrowths survived the mineral separation, then the alpha ejection corrections of these grains are too small and the older (U-Th)/He ages (~70Ma) are more likely to be accurate. With partial retention zone temperatures of about 55-80^∘C (Farley, 2000), our (U-Th)/He ages place an upper limit of 1-2 km (depending on the geothermal gradient) on the exhumation of the Shehoret Formation since the Late Cretaceous. We see no evidence for exposure and denudation related to Tertiary Red Sea rifting. This is compatible with the current moderate relief in the vicinity of the Cambrian outcrops near Eilat, on the western side of the Dead Sea transform. Overall, the base of the sedimentary section in southern Israel and the underlying basement were already close to the surface by 70 Ma. Assuming that our apparent (U-Th)/He ages are correct, there is a striking coincidence between them and the apparent heating pulse that clearly stands out from the AFT inverse model (Figure 10). Because of the imprecision of the latter, it is not clear whether this really reflects a Late Cretaceous tectonic pulse (consistent with Garfunkel and Derin, 1985) or if it coincides with the Tertiary opening of the Red Sea instead. Whatever the underlying cause, the most recent phase in the thermal evolution of the Eilat area is consistent with a decreasing geothermal gradient, as proposed by Kohn et al. (1990).

6 Conclusions

This paper illustrates the utility of applying several geo- and thermochronological techniques to a detrital sample collected from an ancient platform sequence. Together with stratigraphic and petrographic observations, our measurements help create a more detailed picture of the 500 million year history of the Late Cambrian Shehoret Formation of southern Israel. Previous U-Pb detrital zircon geochronology of these deposits revealed a mixture of sources and highlighted the Precambrian crustal evolution in the Pan-African provenance, dominated by erosion of the Arabian-Nubian Shield (ANS) (Kolodner et al., 2006).

Fifty single-grain K-feldspar total fusion argon extractions yielded a population of ages that tightly cluster around 535 Ma (Cambrian). The ⁴⁰Ar/³⁹Ar age spectrum of a multi-grain K-feldspar aliquot displays diffusion behaviour compatible with >560 Ma cooling later affected by a heating event. Because our K-feldspar dates are minimum ages, it is certainly possible that the ANS experienced a simple single phase exhumation history during the Neoproterozoic. It is intriguing, however, that similar 560-530Ma ages have been reported in the literature before. Although the latter were mostly obtained by the K-Ar and Rb-Sr methods and may, therefore, not be very robust, we did consider the consequences of a 560-530 Ma event for our field area. Our literature survey failed to retrieve U-Pb zircon ages younger than 580 Ma in the northern ANS. Much of the northern ANS crustal surface is dominated by ~630 Ma calcalkaline granites and it would therefore be likely that many of the detrital K-feldspar grains in the Shehoret formation issued from the erosion of these rocks. Geological evidence (Garfunkel, 1999) indicates that these granitoids were first exposed by ca. 600 Ma. Therefore a preponderance of the 560-530 Ma ⁴⁰Ar/³⁹Ar ages we measured may represent reburial and reheating of ANS crustal segments below a thick volcano-sedimentary pile. The 535-560 Ma ages correspond to Kennedy’s ‘Pan-African event’ and although further work is required this may be the first time the nature of this event has been revealed.

The Cambian-Ordovician sedimentary cover of northern Gondwana was relatively thin (1-3 km thick; Garfunkel 2002; Kolodner et al., 2006) but covered a very large area from present-day Morocco to Oman (Avigad et al., 2003). The middle Cambrian Shehoret Formation of southern Israel was covered by about 2.5 km of younger sediments by the middle/late Devonian, more than at any other time (Segev et al., 1995). At ~380 Ma, a thermo-tectonic event increased the ambient temperature and heated the Shehoret Formation to ~200^∘C, resetting the ZFT system but leaving the K-feldspar ⁴⁰Ar/³⁹Ar system relatively unaffected. Uranium-rich fluids circulated through the sandstone and formed authigenic clays (Segev et al., 1995) and apatite overgrowths (Weissbrod and Nachmias, 1986), which complicate the alpha-ejection correction applied to our apatite (U-Th)/He data. After this fluid circulation event, the Shehoret Formation was repeatedly buried and exhumed in the upper crust (<200^∘C), as several regionally important stratigraphic unconformities were created.

AFT inverse modeling confirms this theory and hints at a brief Late Cretaceous to Early Tertiary heating pulse. Although this pulse coincides with seven detrital apatite (U-Th)/He ages, those data show substantial scatter between 33 and 77 Ma, likely resulting from extreme compositional zonation associated with the authigenic U-rich overgrowths. The 70 Ma (U-Th)/He ages are more likely to be accurate, setting 1-2 km as an upper limit (depending on the geothermal gradient) on the post-Cretaceous exhumation of the Cambrian sandstone. Our detrital thermochronology shows no evidence for substantial denudation related to Tertiary rifting of the Red Sea, compatible with the idea that this part of the plate boundary was dominated by strike-slip motion and its western side residing at low elevation.

Acknowledgments

This paper greatly benefited from constructive reviews by Barry Kohn, Fred Jourdan, Joseph Meert, Zhengxiang Li (Associate Editor) and Karl Karlstrom (Editor). Pieter Vermeesch would like to thank Jim Metcalf for help with the argon measurements; Paul O’Sullivan for the ZFT dating; Eva Enkelmann and Diane Seward for assistance with the AFT dating; and Andy Beard for the electron microprobe measurements. Dov Avigad was funded by the Israel Science Foundation (ISF) grant number 855/06.

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