Arabian Shield magmatic cycles and their relationship with Gondwana assembly: Insights from zircon U–Pb and Hf isotopes

Earth and Planetary Science Letters 408 (2014) 207–225

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Arabian Shield magmatic cycles and their relationship with Gondwana assembly: Insights from zircon U–Pb and Hf isotopes F.A. Robinson ∗ , J.D. Foden, A.S. Collins, J.L. Payne Department of Earth Sciences, The University of Adelaide, SA, 5005, Australia

a r t i c l e

i n f o

Article history: Received 21 June 2014 Received in revised form 2 October 2014 Accepted 10 October 2014 Available online xxxx Editor: T.M. Harrison Keywords: Arabian Shield U–Pb geochronology Lu–Hf zircon Gondwana magmatism

a b s t r a c t The Arabian Shield preserves a protracted magmatic record of amalgamated juvenile terranes that host a diverse range of early Neoproterozoic to Cambrian granitoids intruding volcanosedimentary basin assemblages that have corollaries in other parts of the East African Orogen. New zircon U–Pb geochronology of 19 granitoids intruding eight Arabian Shield terranes, define four discrete magmatic events: island arc (∼845 Ma), syncollisional (∼710 Ma), post-tectonic (∼620 Ma) and anorogenic (∼525 Ma). Zircon Lu–Hf isotopic analyses indicate that all studied granitoids are juvenile with typical ε Hf values of >+5 to +10 and Stenian-Tonian (∼1100–900 Ma) model ages, regardless of their precise intrusive ages or spatial relationship. Subtle changes in isotopic signatures between ∼850 and 600 Ma, suggest the result from changes in granite source materials brought about by; basaltic underplating, limited crustal interaction with Palaeoproterozoic basement and a change to lithospheric delamination/subduction roll-back processes driving juvenile ANS crustal growth. The cycle of granite intrusion reflects accretionary cycles initiated during Mozambique Ocean closure and during Gondwana amalgamation and final assembly. Post-tectonic magmatism is divided into a ∼636–600 Ma phase and post 600 Ma event that reflects first subduction and then within-plate related processes. The identification of magmatism at ∼525 Ma is now the youngest granitoid identified so far in the Saudi Arabian Shield and may change the identified age of the regional, basal Palaeozoic unconformity. This late magmatism may be generated by the Najd Fault reactivation correlating with the Malagasy/Kunnga Orogeny that marked the final stages of Gondwana assembly. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The Arabian–Nubian Shield (ANS) is a composite terrane generated by the closure and accretion of juvenile volcanic arcs and back-arc basins forming an important part of Gondwana (Johnson et al., 2011). The precise timing and, tectonic significance of these accreted terranes within the East African Orogen (EAO, Stern, 1994), remains a highly debated topic (Cox et al., 2012; Johnson et al., 2011; Stern and Johnson, 2010). Nevertheless, a review by Fritz et al. (2013) recognises five main phases of tectonic evolution in the ANS: (1) rifting of the African craton (∼1200–950 Ma); (2) ensimatic island-arc development (∼950–715 Ma); (3) formation of the Arabian–Nubian neocraton by microplate accretion and continental collision (∼715–640 Ma); (4) collision-related intracratonic

*

Corresponding author at: 14 Johnson Road, Athelstone, SA, 5076 Australia. Tel.: +61 8 83370577. E-mail addresses: [email protected] (F.A. Robinson), [email protected] (J.D. Foden), [email protected] (A.S. Collins), [email protected] (J.L. Payne). http://dx.doi.org/10.1016/j.epsl.2014.10.010 0012-821X/© 2014 Elsevier B.V. All rights reserved.

magmatism and tectonism (∼640–550 Ma); and (5) epicontinental subsidence (<550 Ma), commonly found in fault-bound basins (Nettle et al., 2014). This series of events produced one of the planet’s largest tract of juvenile Neoproterozoic crust (Patchett and Chase, 2002), the eastern half of the ANS in particular, representing extreme juvenile crustal growth (Be’eri-Shlevin et al., 2010). The earliest magmatic phases include emplacement of thick oceanic tholeiite sequences in intra-oceanic settings followed by 950–650 Ma bimodal volcanism with island arc chemistry. These phases are recognised by Bentor (1985), Stein (2003 and references therein), Stein and Goldstein (1996) and Stein and Hofmann (1994) for producing large volumes of mafic crust and lithospheric mantle within the ANS and with the addition of crustal overprinting, these upwelling plumes possibly provide the lithophile element-enriched sources for younger calc-alkaline (640–590 Ma) and alkaline (590–550 Ma) magmatism. Nevertheless, the controversy remains in the explanation of ANS magmatic sources. Models variously include the roles of enriched or depleted mantle and/or incorporation of continental crust (Be’eri-Shlevin et al., 2010; Stoeser and Frost, 2006).

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F.A. Robinson et al. / Earth and Planetary Science Letters 408 (2014) 207–225

Fig. 1. Left) A regional map of the Middle East and Africa highlighting the Arabian–Nubian Shield (Shield boundary obtained from Vail, 1985 and Najd Fault Zone from Stern, 1985). The ANS forms a series of Proterozoic (2500–542 Ma) accreted terranes, which are interrupted by mobile orogenic belts and pull apart basins. The orogenic cycles associated with the ANS emplace it firmly with the formation of Gondwana (∼650–500 Ma). The Red Sea separates the ANS, which rifted apart in the Tertiary (Vail, 1985). Right) A geological map of the Arabian Shield highlighting 20 sampled plutonic suites that correlate to the suite names on the left.

The development of the widespread and voluminous post-tectonic (often A-type granitic) magmatic suites that occur at the final stages of Gondwana assembly are often attributed to lithospheric delamination (Avigad and Gvirtzman, 2009) and/or slab roll-back (Flowerdew et al., 2013). This work, coupled with previous geochronology, focuses on the Saudi Arabian part of the ANS, resulting from a succession of convergence and accretion events during supercontinental formation. Studies from other areas of the ANS such as the Eastern Desert and Sinai (Be’eri-Shlevin et al., 2009, Eyel et al., 2014, 2010) have resulted in extensive published, high quality geochronological, geochemical and petrological data on the timing of calc-alkalinealkaline post-tectonic intrusives. However, this type of data is limited from the Saudi Arabian sequences. New U–Pb zircon data from 19 plutons covering 8 discrete terranes (Fig. 1) are presented here, and will help to define which terranes were arcs, the sequence of convergence and accretion during supercontinental formation, and to provide new evidence for the final amalgamation of Gondwana. Adding to the existing Nd, Sr, Pb, O isotope database (Stoeser and Frost, 2006), this work also presents the first Hf isotope data from the Saudi Arabian part of the ANS. This provides an opportunity

to further emphasise the juvenile character of crustal growth in the ANS and results from prior studies such as Be’eri-Shlevin et al. (2010). 2. Arabian Shield geological setting and previous geochronology The Arabian Shield is widely accepted to have formed by the accretion of juvenile terranes in which eight distinct terranes separated by five ophiolite-bearing suture zones have been isotopically and geochronologically identified with a general younging trend towards the east (Stoeser and Camp, 1985; Stoeser and Frost, 2006; Johnson et al., 2011). In general terms, the Shield structure consists of two parts: the western side comprising the Midyan, Hijaz, Jiddah and Asir island arc terranes; and the eastern side composed of the Tathlith, Ha’il, Afif (including Khida subterrane), Ad Dawadimi, and Ar Ryan Terranes (Fig. 1). These tectonostratigrahphic terranes are composed of ∼850–640 Ma geochemically diverse plutonic and volcanosedimentary assemblages, which according to Flowerdew et al. (2013), Hargrove et al. (2006) and Johnson et al. (2011), have been deformed by at least four periods of arc collision and suturing, are overlain and intruded by post-amalgamation (<640 Ma)

Table 1 A summary of the sampled plutons and previous geochronology. Shield terrane

Geological map unit

Longitude

Rock type

Mineralogy

Published geochronological age (Johnson, 2006)

Asir

Makkah Suite (dm)

3

21◦ 21 36.45 N

40◦ 15 44.86 E

Tonalite

plag+hbl+bi+minor qtz+mag+ti

817–678 Ma (poorly constrained) method not stated. 859 Ma from Kennedy et al. (2011)

Al Hawiyah Suite (hwg)

5

21◦ 26 54.04 N

40◦ 27 16.45 E

Granite

perthitic alk-fld+qtz+bi+minor plag+hast+ti. Accessory mag

Rb–Sr & SHRIMP = 630–590 Ma and as low as 515 Ma and 117 Ma (poorly constrained).

Kawr Suite (kw)

35

21◦ 20 04.76 N

42◦ 44 56.78 E

Gabbro

plag+olv+cpx+minor mag

21◦ 07 05.04 N

42◦ 53 16.43 E

Granite

qtz+alk-fld+plag+bi+minor mag+hast. Mafic enclave = plag+hbl+cpx

U–Pb in zircon = 650–620 Ma and localised intrusions at 605 Ma (poorly constrained)

21◦ 20 10.17 N

42◦ 45 05 29 E

AlkaliGranite

perthitic alk-fld+qtz+aeg+minor bi+ti. Accessory mag+ap

20◦ 18 21.19 N

42◦ 41 39.41 E

AlkaliGranite

perthitic alk-fld+qtz+mic+minor bi. Accessory mag+ap. Granitic enclave +qtz+alk-fld+plag+minor hast+bi

Wadbah Suite (wb)

4

19◦ 27 09.68 N

42◦ 49 58.70 E

AlkaliGranite

perthitic alk-fld+qtz+bi+minor hbl+mic+plag. Accessory mag+zirc

SHRIMP zircon crystallisation age of 606 Ma

Ibn Hashbal Suite (ih)

6

19◦ 29 13.91 N

42◦ 59 44.39 E

AlkaliGranite

mic+qtz+bi+minor plag+hbl+hast

U–Pb in zircon = 640–617 Ma (poorly constrained). Coveal relationship with neighbouring Kawr Suite

Tathlith

Al Hafoor Suite (ao)

8

20◦ 23 40.32 N

44◦ 18 05.19 E

AlkaliGranite

alk-fld+qtz+minor hbl+hast+bi. Accessory mag

Age Unknown. Structural relationships suggest it post dates the 630–620 Ma Bani Ghayy Group volcanics

Afif

Haml Suite (hla)

4

21◦ 18 13.29 N

43◦ 51 21.78 E

QuartzMonzonite

alk-fld+plag+minor qtz+hbl+mic+bi. Accessory mag+zirc+ap

2 units dated by U–Pb in zircon: Samim (640–625 Ma) and Himarah (<610 Ma)

Al Khushaymiyah Suite (ky)

5

23◦ 49 39.69 N

43◦ 11 42.07 E

QuartzMonzonite

perthitic alk-fld+qtz+plag+hbl+minor mag+bi+mic+ti+hast

Intrudes 630 Ma Murdama Group and main body is dated at 611–595 Ma by U–Pb in zircon

Ar Ruwaydah Suite (ku)

4

24◦ 22 43.75 N

44◦ 21 40.56 E

Granite

alk-fld +qtz+plag+bi+minor mag+hast

2 units: Khurs Granite dated with SHRIMP (605–565 Ma) and Arwa Granite with Rb–Sr isochron (587 Ma) and SHRIMP (575 Ma)

Malik Granite (kg)

5

25◦ 07 56.42 N

43◦ 47 10.82 E

Leucogranite

qtz+alk-fld+minor plag+gt

Age Unknown. Structural relationships suggest 620–615 Ma (intrudes Idah Suite)

Najirah Granite (nr)

4

23◦ 43 43.93 N

44◦ 41 21.06 E

Granite/ Alkali-Granite

alk-fld+qtz+plag+bi+minor mag

Main body has been dated at 641 Ma (U–Pb in zircon), but also 576 Ma (SHRIMP)

Abanat Suite (aa)

4

27◦ 18 43.63 N

41◦ 24 33.52 E

AlkaliGranite

perthitic alk-fld+qtz+minor plag+aeg+arfv

U–Pb in zircon = 585–570 Ma

Idah Suite (id)

5

27◦ 03 44.28 N

41◦ 17 58.70 E

AlkaliGranite

perthitic alk-fld+qtz+minor plag+hbl+bi+hast

U–Pb in zircon = 620–615 Ma

Midyan

Al Bad Granite Super Suite (abg)

6

28◦ 44 32.20 N

35◦ 20 12.32 E

Alkali-Granite

perthitic alk-fld+qtz+minor plag+bi. Accessory mag+zirc+ap+fl

Main body has been dated at 586 Ma (Rb–Sr isochron) and 577 Ma (U–Pb in zircon)

Hijaz

Rithmah Complex (rt)

4

25◦ 09 07.90 N

38◦ 11 20.41 E

Diorite/ Gabbro

plag+cpx+hbl+minor olv+opx+mag

Jar-Salajah Complex (js)

4

Granodiorite

qtz+plag+bi+alk-fld+minor mag and hbl

Ad Dawadimi

Ha’il

24◦ 24 37.62 N

38◦ 21 23.72 E

F.A. Robinson et al. / Earth and Planetary Science Letters 408 (2014) 207–225

Latitude

No. samples

Age Unknown. Structural relationships suggest

<600 Ma (intrudes Admar Suite) Contains unreliable age of 745–695 Ma (U–Pb in zircon). Also intrudes the deformed Al Ays (745–700 Ma) and Zaam (760–710 Ma) Groups (continued on next page) 209

Note the coordinates, rock type and mineralogy displayed correlate with samples analysed for U–Pb and Hf isotopes, not the entire suite (all samples are described in Robinson (2014). Mineral abbreviations are as follows: aeg = aegirine, alk-fld = alkali-feldspar, ap = apatite, arfv = arfvedsonite, bi = biotite, cpx = clinopyroxene, fl = fluorite, gt = garnet, hast = hastingsite, hbl = hornblende, mag = magnetite, mic = microcline, opx = orthopyroxene, olv = olivine, plag = plagioclase, qtz = quartz, ti = titanite and zirc = zircon.

Abdel-Monem et al. (1989), Agar (1992), Aleinikoff and Stoeser (1989), Al-Saleh and Boyle (2001), Al-Saleh et al. (1998), Al-Shanti and Gass (1983), Calvez et al. (1983), Calvez and Delfour (1986), Calvez and Kemp (1987), Cole and Hedge (1986), Cooper et al. (1979), Cox et al. (2012), Darbyshire et al. (1983), Doebrich et al. (2004), Duyverman and Harris (1982), Fleck and Hadley (1982), Hargrove (2006), Hargrove et al. (2006), Jackson et al. (1984), Johnson (2003), Johnson et al. (2003), Johnson and Kattan (2007), Johnson et al. (2001), Johnson and Woldehaimanot (2003), Kemp et al. (1980), Kennedy et al. (2004), Kennedy et al. (2005), Kennedy et al. (2011), Kennedy et al. (2010b), Kennedy et al. (2010a), Kröner et al. (1979), Lewis (2009), Matash and Kusky (2001), Nettle et al. (2014), Pallister et al. (1988), Stacey and Agar (1985), Stacey and Hedge (1984), Stacey et al. (1984), Stoeser et al. (1984), Stuckless and Futa (1987), Stuckless et al. (1984) and Whitehouse et al. (2001). Previous geochronology used in this study. See Appendix F for reference details. Arabian Shield

Hargrove (2006) produced an age of 715 Ma by U–Pb in zircon (undocumented) plag+qtz+hbl+bi. Accessory mag 38◦ 46 50.11 E 3 Shufayyah Complex (su)

23◦ 44 42.76 N

Granodiorite/ Tonalite

Aleinikoff and Stoeser (1989) produced an age of 696 Ma from 3 unrealiable zircons. It also contains a whole-rock Rb–Sr isochron age of 659 Ma Rhyolite: alk-fld+qtz+plag (pheoncrysts). Groundmass = qtz+alk-fld 38◦ 45 09.74 E 4 Subh Suite (sf)

23◦ 45 39.13 N

Rhyolite

Contains 3 unrealiable Rb–Sr isochron ages of 640 Ma, 602 Ma and 583 Ma perthitic alk-fld+bi+hbl+minor qtz+plag+mic+ti. Accessory mag+zirc+ap 38◦ 24 43.55 E 5 Admar Suite (ad)

24◦ 17 53.09 N

Syenite

perthitic alk-fld+bi+hbl+minor qtz+plag+olv. Accessory mag+zirc+ap 38◦ 29 35.33 E 6 Mardabah Complex (mr)

25◦ 11 29.42 N

Seynite

Mineralogy Rock type Longitude Latitude No. samples Geological map unit Shield terrane

Table 1 (continued)

Age Unknown. Structural relationships suggest <600 Ma (thought to be one of the youngest Shield plutons)

F.A. Robinson et al. / Earth and Planetary Science Letters 408 (2014) 207–225

Published geochronological age (Johnson, 2006)

210

basins and granitoids, and, in turn, are affected by multiple exhumation and erosion events. A Cambrian regional unconformity at 541 Ma, marked by peneplain development and the absence of intruding magmatic bodies after this age, overlies the deformed and metamorphosed Cambrian–Ordovician rocks of the Arabian Shield (Johnson, 2006). Cenozoic rifting associated with the Red Sea opening (Vail, 1985) has produced the only Phanerozoic magmatism recorded thus far in the ANS. The Proterozoic magmatic history of the Arabian Shield is divided into a series of phases initially defined and described by Bentor (1985) and later Stein and Goldstein (1996). In general, the earliest (∼950–650 Ma) oceanic tholeiite and bimodal volcanism is confined to the western Arabian Shield, has island arc chemistry and is emplaced in intra-oceanic settings. The younger phases cover vast areas of the western and eastern Arabian Shield and are dominated by 640–590 Ma calc-alkaline batholiths terminating with a strong uplift and a switch to 590–550 Ma alkaline granites and volcanics. The N-S trending Nabitah Belt, together with smaller ophiolitic bearing sutures (Yanbu, B’ir Umq, Halaban), form a significant part of the Arabian Shield magmatic history created during the ∼715–640 Ma accretion of juvenile adjacent terranes. The Nabitah Belt is interpreted to result from collision between western island arc and the eastern Afif terrane that is floored with pre-Neoproterozoic continental crust (Stoeser and Camp, 1985; Stoeser and Frost, 2006) and furthermore, has been considered as the northern part of the main suture between Indian and African Gondwana (previously referred to as ‘East and West Gondwana’; Shackleton, 1996). Johnson et al. (2011) indicates the Nabitah collision was not synchronous, but initiated at ∼680 Ma in the north (Midyan, Hijaz and Afif Terranes), while subduction was still underway in the south (Asir, Tathlith and Afif Terranes). The appearance of ∼ 640 Ma post-collision magmatism either side of the Nabitah Belt indicates west and east terrane amalgamation. However, according to Cox et al. (2012) and Doebrich et al. (2007) subduction and accretion continued further east in the Ar Ryan and Ad Dawadimi Terranes, until after ∼600 Ma. Further east still, little is known about the crust beneath the Phanerozoic of Arabia, except that broadly N-S magnetic highs, similar to the island-arc like Ar-Ryan Terrane, can be made out (Johnson and Stewart, 1995) and may represent even younger accreted arc-terranes. This younger age, supported by an Ediacaran– Cambrian Western Deformation Front in subsurface Oman has led to a suggestion that the final Mozambique Ocean suture lies beneath the Arabian Phanerozoic cover, between the Ar Ryan Terrane and the Oman exposures (Collins and Pisarevsky, 2005; Johnson et al., 2011). Despite the differing accretion ages across the Arabian Shield, it is clear that post-orogenic magmatism at <640 Ma signifies the cessation of the microplate accretion with a distinct transition from calc-alkaline to alkaline dominated assemblages (Black and Liegeois, 1993). Taking this into account, this study will examine the tectonic phases associated with accretion and final amalgamation in the Arabian Shield. Twenty plutonic suites from eight Arabian Shield terranes with limited geochronology and geochemical data are selected using 1:100,000 geological maps and technical reports complied by Johnson (2006). A summary of these collected granitoids, and additional Arabian geochronological references used in this study, are presented in Table 1 and Fig. 2. 3. Analytical techniques Detailed analytical procedures for both U–Pb and Hf isotopes are presented in Appendix E and described in Payne et al. (2013). U–Pb zircon analysis was performed by Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) and at Adelaide

F.A. Robinson et al. / Earth and Planetary Science Letters 408 (2014) 207–225

211

Fig. 2. A summary of all reliable geochronology data from the Arabian Shield (references presented in Table 1). The highlighted green sections in the northern Arabian and Nubian Shield represent areas of extensive geochronological data discussed by Stern and Johnson (2010). Within the Arabian Shield, there is a general younging and convergence towards the east (yellow arrows). This is with the exception of the westward dipping Ad Dawadimi Terrane (Cox et al., 2012), which is associated with the Afif-Abas Block (Cox et al., 2012). Data from this study (red circles) aims to reinforce the timing and nature of the accretionary cycles observed across the Arabian Shield. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

Microscopy, University of Adelaide on an Agilent 7500cs ICPMS coupled with a New Wave 213 nm Nd-YAG laser. Ablation and machine U–Pb isotope fractionation were corrected using the GEMOC GJ-1 standard (206 Pb/238 U age of 600.7 [±1.1] Ma; Jackson et al., 2004) and internal accuracy was checked using the Plešovice standard (206 Pb/238 U age of 337.1 [±0.37] Ma; Sláma et al., 2008). GJ-1 and Plešovice analyses yielded 206 Pb/238 U ages of 600.9 [±1.0] Ma (2SD, n = 268) and 342.6 [±1.6] Ma (2SD, n = 92) respectively. The elevated Plešovice age is attributed to sample dm01a (351.3 [±3.9] Ma). Consequently, the uncertainty on U– Pb ages from this sample have an ∼3% error. Data were processed using Glitter software (Griffin et al., 2008) and the age calculations and diagrams constructed using the Isoplot macro (Ludwig, 2000). Zircons with greater than 90% concordance were ablated for Hf isotopic compositions on a Laser Ablation – Multi Collector ICPMS with a New Wave UP-193 Excimer Laser (193 nm) at the University of Adelaide – CSIRO joint facility, Waite Campus, South Australia. Lu/Hf measurements were conducted using a ThermoScientific Neptune Multicollector ICPMS equipped with Faraday detectors and 1012  amplifiers. Specific methods and integration and idle times of Hf–Lu–Yb–Gd–Dy–Ho–Er isotopes are described in Payne et al. (2013). Analytical accuracy was monitored by a combination of MudTank (176 Hf/177 Hf values of 0.282507 [±6] 2SD; Woodhead and Hergt, 2005) and Plešovice (176 Hf/177 Hf val-

ues of 0.282482 [±13] 2SD; Sláma et al., 2008). MudTank and Plešovice analyses yielded 176 Hf/177 Hf values of 0.282501 [±22] (2SD, n = 19) and 0.282474 [±22] (2SD, n = 38) respectively. 176 Hf/177 Hf model values were calculated based on the 176 Lu decay constant 1.87 × 10−11 after Scherer et al. (2001). TDM(crust) ages were obtained following the methods of Griffin et al. (2002), assuming an average crustal composition of 176 Lu/177 Hf = 0.015. 4. Results From the 137 samples collected in the Arabian Shield, 19 were selected for U–Pb and Lu–Hf isotopic analysis (Table 2). A total of 449 zircons were dated and 270 analysed for Hf composition. Supplementary Appendices A, B and C contain all data and the corresponding zircon CL images described in this section. 4.1. U–Pb geochronology Zircon morphology details are presented in Appendix D and the statistics are described below. Many of the dated plutons below form a series of magmatic bodies referred to as a ‘suite’, hence the crystallisation age refers only to the individual pluton, but helps constrain the suite age.

212

Table 2 U–Pb zircon geochronology and Hf isotopes from 19 sampled granitoids across the Arabian Shield. Sample

Terrane

MSWD

U–Pb geochronology

Magmatic timing

No. spots

Lu–Hf isotopes

Asir

38

845.6

4 .9

99

1.6

Island Arc

19

0.282556

0.002036

0.080807

1.467252

0.282523

9.97

1.10

Asir

27

591.9

5 .2

86

2.1

Anorogenic

14

0.282636

0.001680

0.077359

1.467232

0.282617

7.62

1.06

Asir Asir Asir Asir

26 11 26 19

611.7 608.0 615.9 617.6

6 .5 12.0 4 .9 5 .2

95 88 98 91

2.4 4.6 2.0 0.8

Post-tectonic Post-tectonic Post-tectonic Post-tectonic

11 11 16 16

0.282599 0.282695 0.282639 0.282661

0.002112 0.001802 0.000885 0.001473

0.089964 0.083608 0.038315 0.074615

1.467342 1.467247 1.467281 1.467271

0.282574 0.282675 0.282629 0.282644

6.60 10.00 8.54 9.10

1.14 0.92 1.02 0.98

Tathlith

16

636.0

4 .0

88

0.8

Post-tectonic

13

0.282608

0.001893

0.084630

1.467257

0.282585

7.38

1.10

Afif Afif

15 24

608.6 601.2

8 .1 5 .2

88 82

2.3 2.0

Post-tectonic Post-tectonic

15 17

0.282655 0.282562

0.001026 0.001334

0.044365 0.058208

1.467266 1.467271

0.282643 0.282547

8.90 5.31

1.00 1.21

Ad Dawadimi

18

611.0

6 .5

100

0.1

Post-tectonic

13

0.282641

0.001167

0.053102

1.467249

0.282628

8.43

1.02

Ad Dawadimi

20

607.0

7 .9

89

2.9

Post-tectonic

6

0.282674

0.003228

0.159475

1.467228

0.282637

8.46

1.01

Ad Dawadimi

11

599.4

5 .1

91

0.3

Anorogenic

17

0.282633

0.002036

0.107078

1.467247

0.282610

7.62

1.06

Ha’il Midyan

19 31

605.8 597.4

5 .9 4 .8

90 93

1.0 1.2

Post-tectonic Anorogenic

14 15

0.282654 0.282656

0.001856 0.002065

0.093554 0.097518

1.467240 1.467265

0.282633 0.282633

8.54 8.30

1.01 1.02

Hijaz

29

709.5

8 .4

96

3.5

Synorogenic

19

0.282634

0.002907

0.135679

1.467287

0.282596

9.40

1.02

Hijaz

17

525.6

4 .7

105

1.2

Post-tectonic

15

0.282667

0.000975

0.051687

1.467269

0.282657

7.50

1.01

Hijaz Hijaz Hijaz

33 22 47

599.2 698.7 715.4

3 .8 5 .5 3 .6

96 98 101

0.7 0.2 1.2

Anorogenic Synorogenic Synorogenic

15 9 15

0.282634 0.282598 0.282632

0.000579 0.003061 0.001187

0.026077 0.119417 0.049987

1.467272 1.467353 1.467317

0.282628 0.282558 0.282616

8.08 7.86 10.26

1.03 1.12 0.98

206

Pb/238 U

2σ

Conc.

176

Hf/177 Hf

176

Lu/177 Hf

176

Yb/177 Hf

178

Hf/177 Hf

176

Hf/177 Hf

(t) Makkah Suite (dm01a) Al Hawiyah Suite (hwg07) Kawr Suite (kw42) Kawr Suite (kw51p) Wadbah Suite (wb65) Ibn Hashbal Suite (ih68) Al Hafoor Suite (ao85) Haml Suite (hla110) Al Khushaymiyah Suite (ky129) Ar Ruwaydah Suite (ku139) Najirah Granite (nr120) Malik Granite (kg150) Idah Suite (id159) Al Bad Granite Suite (abg179) Jar-Salajah Complex (js202) Mardabah Complex (mr191) Admar Suite (ad194) Subh Suite (sf209) Shufayyah Complex (su216)

ε Hf (t)

TDM (Crustal) age (Ga)

U–Pb ages are the weighted average ages with the exception of samples js202 and ku139, which correspond with upper intercept and concordia ages respectively. Hf isotope data presented are the average values. All raw data is presented in supplementary Appendices A and C.

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No. Zircons

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4.1.1. Makkah Suite (dm): Gabbro-Diorite Sample dm01a contains a range of zircons from yellow-brown, stubby euhedral prisms (∼50–100 μm) with complex core – rim zoning to yellow, elongate prisms (∼100–300 μm) with simple magmatic zoning in CL response (Fig. 3a). Thirty eight analyses from 36 individual zircon grains yielded a 206 Pb/238 U weighted average age of 845.6 [±4.9] Ma (MSWD = 1.6). The mean concordancy is 99% with only 2 out of 38 grains showing >10% discordance (82% and 87%). Overall, it is interpreted that this produces a tight crystallisation age. 4.1.2. Al Hawiyah Suite (hwg): Granite Sample hwg07 zircons are predominantly colourless, brown and pink elongate, euhedral prisms (∼150–300 μm) with mediumfine, simple oscillatory zoning in CL response (Fig. 3b). There are also less common thickly banded, simple magmatic zoned grains. Twenty seven analyses from 23 individual zircon grains yielded a 206 Pb–238 U weighted average age of 591.9 [±5.2] Ma (MSWD = 2.1). The mean concordancy is 86% with 10 out of 27 analyses showing <10% discordance. The remaining 17 have fluctuating 207 Pb/206 Pb ages that result in concordancies of 68%, 70–80% (5 analyses) and the remainder between 80–90%. This creates a poor concordance, but the weighted age is interpreted to yield a robust crystallisation age. 4.1.3. Kawr Suite (kw): Granodiorite Sample kw42 displays a range of zircons from yellow, stubby, euhedral prisms (<100 μm) with medium-fine, simple oscillatory zoned rims to colourless-pink, elongate, euhedral prisms (∼100–200 μm) with very fine, simple oscillatory zoning in CL response (Fig. 3c). Twenty six individual zircon grain analyses yielded a 206 Pb/238 U weighted average age of 611.7 [±6.5] Ma (MSWD = 2.4). The mean concordancy is 95% with 11 out of 26 analyses showing >10% discordance. The lowest and highest values are 80% and 116% respectively, but overall, this is interpreted to provide a tight crystallisation age 4.1.4. Kawr Suite (kw): Alkali-Granite Sample kw51p zircons are predominantly yellow-brown, stubby, euhedral prisms (∼50–100 μm) with thickly banded, simple magmatic zoning in CL response (Fig. 3d). Eleven individual zircon grain analyses yielded a 206 Pb/238 U weighted average age of 608 [±12] Ma (MSWD = 4.6). The mean concordancy is 88%, which is just outside >10% discordance and is mainly attributed to 2 grains yielding a concordance of 65% and 69%. Five analyses have <10% discordance with the remaining 4 at ∼80–90%. Many zircon analyses contain elevated 204 Pb and thus the suite contains a poor concordance, but the weighted average age is still interpreted as a meaningful crystallisation age. 4.1.5. Wadbah Suite (wb): Alkali-Granite Sample wb65 contains zircons ranging from yellow, stubby, euhedral prisms (<100 μm) with fine, simple oscillatory zoned rims to colourless-brown, elongate prisms (∼100–200 μm) with medium-fine, simple oscillatory zoning in CL response (Fig. 3e). Twenty six individual zircon grain analyses yielded a 206 Pb/238 U weighted average age of 615.9 [±4.9] Ma (MSWD = 2.0). The mean concordancy is 98% with only 6 of 26 showing >10% discordance (lowest and highest values are 81 and 111%). Overall, the weighted average age is interpreted yield a tight crystallisation age. 4.1.6. Ibn Hashbal Suite (ih): Alkali-Granite Sample ih68 zircons are yellow-brown, elongate, euhedral prisms (∼150–300 μm) with medium-fine, simple oscillatory zoning in CL response (Fig. 3f). Nineteen analyses from 15 individual zircon grains yielded a 206 Pb–238 U weighted average age

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of 617.6 [±5.2] Ma (MSWD = 0.76) and a concordia age of 618.4 [±5.3] Ma (MSWD = 12). The mean concordancy is 91%, but two grains at 71% and 117% are recorded (elevated 207 Pb/206 Pb ages). Ten of 19 analyses have >10% discordance and range from 71–117%. Overall, both ages are interpreted to yield a tight crystallisation age, but the weighted average is used because of its lower MSWD. 4.1.7. Al Hafoor Suite (ao): Alkali-Granite Sample ao85 contains zircons that are predominantly yellowbrown, stubby, euhedral prisms (∼50–100 μm) with fine, simple oscillatory zoning in CL response (Fig. 3g). Sixteen individual zircon analyses yielded a 206 Pb/238 U weighted average age of 636 [±4] Ma (MSWD = 0.81). The mean concordancy is 88% with only 5 of 16 displaying a <10% discordance. The lowest value recorded is 78% with the remaining 11 analyses between 80–90%. All grains have slightly elevated 207 Pb/206 Pb ages, which create poor concordance, but the weighted average age is interpreted to provide a robust crystallisation age. 4.1.8. Haml Suite (hla): Quartz-Monzonite Sample hla110 zircons are yellow-brown, elongate, euhedral prisms (∼100–200 μm) that display medium-fine, simple oscillatory zoning in CL response (Fig. 3h). Fifteen individual zircon grains yielded a 206 Pb–238 U weighted average age of 608.6 [±8.1] Ma (MSWD = 2.3). The mean concordancy is 88% with 8 of 15 grains containing <10% discordance, whilst the remaining 7 have slightly elevated 207 Pb/206 Pb ages. The weighted age of all 15 analyses is interpreted to provide a robust crystallisation age. 4.1.9. Al Khushaymiyah Suite (ky): Quartz-Monzonite Sample ky129 contains a range in zircons from colourlessyellow, elongate, euhedral prisms (∼100–200 μm) with simple magmatic zoning to colourless-brown, elongate prisms (∼150–250 μm) with medium-fine, simple oscillatory zoning in CL response (Fig. 3i). Twenty four analyses from 22 individual zircon grains yielded a 206 Pb/238 U weighted average age of 601.2 [±5.2] Ma (MSWD = 2). The mean concordancy is 82% with only 5 out of 24 analyses showing <10% discordance. The remaining 19 values have elevated 207 Pb/206 Pb ages with the lowest values in the 60% concordance range, but most have 80–90% concordance. The weighted age of all 24 analyses is interpreted to yield a robust crystallisation age. 4.1.10. Ar Ruwaydah Suite (ku): Granite Sample ku139 zircons are predominantly yellow-brown stubby, euhedral prisms (∼50–100 μm) with medium-fine, simple oscillatory zoning (particularly towards the rims) in CL response (Fig. 3j). Eighteen individual zircon grain analyses yielded a 206 Pb/238 U weighted average age of 611.7 [±6.1] Ma (MSWD = 1.2) and a concordia age of 611 [±6.5] Ma (MSWD = 0.13). The mean concordancy is 100%, but 2 grains at 81% and 118% are recorded (elevated 207 Pb/206 Pb ages). 10 of 18 analyses have >10% discordance and range from 80–120%. Overall, both ages are interpreted to provide a tight crystallisation age, but the concordia age is used because of its lower MSWD. 4.1.11. Najirah Granite (nr): Granite Sample nr120 zircons range from yellow, stubby, euhedral prisms (<100 μm) with medium-fine, simple magmatic zoning to pink, elongate, euhedral prisms (∼150–250 μm) with distinct medium-fine, simple oscillatory zoning in CL response (Fig. 3k). Twenty analyses from 19 individual zircon grains yielded a 206 Pb/238 U weighted average age of 607 [±7.9] Ma (MSWD = 2.9). The mean concordancy is 89% with 12 of 20 analyses showing <10% discordance. Of the remaining 8 analyses, the lowest value

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Fig. 3. (a–d): Concordia plots illustrating the 206 Pb/238 U age of dated plutons from the Arabian Shield. Weighted average plots (1σ error) have been numerically rearranged in descending order to highlight the minimum and maximum values. The accompanying CL images represent typical zircon morphologies found within the dated suites. Calculated initial ε Hf (at U–Pb age) from Hf isotope analysis are also shown.

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Fig. 3. (continued)

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Fig. 3. (continued)

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Fig. 3. (continued)

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Fig. 3. (continued)

recorded is 65% and the rest lie between 70–80%. The weighted average age is interpreted to provide a meaningful crystallisation age. 4.1.12. Malik Granite (kg): Leucogranite Sample kg150 zircons are predominantly colourless, euhedral prisms (∼100–200 μm) with distinct thickly banded, simple magmatic zoning in CL response (Fig. 3l). Eleven analyses from 10 individual zircon grains yielded a 206 Pb/238 U weighted average age of 599.4 [±5.1] Ma (MSWD = 0.27). The mean concordancy is 91%, but 1 grain yielding a concordance of 78% is recorded. Four analyses have <10% discordance with the remaining 5 between ∼80–90%. The fluctuating concordance is consistent with elevated 207 Pb/206 Pb, however, the weighted average age is interpreted to yield a robust crystallisation age. 4.1.13. Idah Suite (id): Alkali-Granite Sample id159 zircons are brown, stubby, euhedral prisms (∼50–100 μm) with fine, simple oscillatory zoning in CL response (Fig. 3m). Nineteen individual zircon grain analyses yielded a

Pb/238 U weighted average age of 605.8 [±5.9] Ma (MSWD = 0.96). The mean concordancy is 90%, but two grains as low as 59% and 62% are recorded (elevated 207 Pb/206 Pb ages). Six of 19 analyses have >10% discordance and the elevated Pb observed in many grains create poor concordancy, but the weighted age produced is interpreted to provide a robust crystallisation age. 206

4.1.14. Al Bad Granite Super Suite (abg): Alkali-Granite Sample abg179 zircons are distinctly pink-brown, stubby, euhedral prisms (∼50–150 μm) with very fine, simple oscillatory zoning in CL response (Fig. 3n). Thirty analyses from 29 individual zircon grains yielded a 206 Pb/238 U weighted average age of 597.4 [±4.8] Ma (MSWD = 1.2) and a concordia age of 596.8 [±5.2] Ma (MSWD = 5.8). The mean concordancy is 93% with 11 of 31 displaying a >10% discordance (2 zircon grains at 74% and 77% and 8 at 80–90%) that is consistent with elevated 207 Pb/206 Pb ages. Overall, both ages are interpreted to yield a tight crystallisation age, but the weighted average age is used because of its lower MSWD.

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4.1.15. Jar-Salajah Complex (js): Granodiorite Sample js202 contains a variety of zircons from brown, stubby, euhedral pyramids (<100 μm) with distinct complex core-rim zoning to yellow, elongate prisms (∼100–200 μm) with fine, simple oscillatory zoning in CL response (Fig. 3o). Twenty nine analyses from 28 individual zircon grains yielded an upper intercept age of 709.5 [±8.4] Ma and a lower intercept age of −636 [±810] Ma with an MSWD = 1.05 indicative of small amounts of Pb loss. The mean concordancy is 96% with only 5 out of 29 analyses showing >10% discordance (80–90% from fluctuating 207 Pb/206 Pb ages). There are also two distinct peaks in the probability curve at ∼710 Ma and ∼675 Ma, possibly indicating metamorphism. Overall, the upper intercept age is interpreted to provide a meaningful crystallisation age. 4.1.16. Mardabah Complex (mr): Syenite mr191 zircons are pink, euhedral prisms Sample (∼100–600 μm) with distinct medium-fine, simple oscillatory zoned rims in CL response (Fig. 3p). Seventeen analyses from 14 individual zircon grains yielded a 206 Pb/238 U weighted average age of 525.6 [±3.8] Ma (MSWD = 1.15). The mean concordancy is 105%, but two grains at 72% and 125% were recorded. Overall, the weighted average age yields a tight crystallisation age. 4.1.17. Admar Suite (ad): Syenite Sample ad194 zircons are predominantly colourless-pink, elongate, euhedral prisms (∼100–200 μm) that display medium-fine, simple oscillatory zoning in CL response (Fig. 3q). Thirty three analyses from 30 individual zircon grains yielded a 206 Pb/238 U weighted average age of 599.2 [±3.8] Ma (MSWD = 0.68) and a concordia age of 599.7 [±3.8] Ma (MSWD = 5.3). The mean concordancy is 96%, but two grains at 74% and 124% were recorded. Overall, both yield a tight crystallisation age, but the weighted average age is used because of its lower MSWD. 4.1.18. Subh Suite (sf): Rhyolite Sample sf209 zircons are predominantly colourless, elongate, euhedral prisms (∼100–200 μm) and display medium-fine, simple oscillatory zoning in CL response (Fig. 3r). Twenty two individual zircon analyses yielded a 206 Pb/238 U weighted average age of 698.7 [±5.5] Ma (MSWD = 0.21) and a concordia age of 699.4 [±5.4] Ma (MSWD = 1.3). The mean concordancy is 98% with 2 grains displaying a discordance of 111%. Overall, both ages are interpreted as a tight crystallisation age, but the weighted average is used because of its lower MSWD. 4.1.19. Shufayyah Complex (su): Tonalite Sample su215 zircons range from yellow, stubby, euhedral pyramids (<100 μm) with thickly banded, simple magmatic zoning to colourless-brown, elongate prisms (∼100–150 μm) with medium, simple oscillatory zoning in CL response (Fig. 3s). Forty seven individual zircon analyses yielded a 206 Pb–238 U weighted average age of 715.4 [±3.6] Ma (MSWD = 1.2) and a concordia age of 715.5 [±3.7] Ma (MSWD = 1.3). The mean concordancy is 101% with 8 out of 47 analyses showing >10% discordance. The lowest and highest values are 88% and 119% respectively. Overall, both ages are interpreted as a tight crystallisation age, but the weighted average age is used because of its lower MSWD. 4.2. Hf isotopes The Hf isotopes reported here from zircons with U–Pb age determinations (Table 2) are the first Hf isotopic data from the Saudi Arabian part of the Arabian Shield. Selected grains that exhibited obvious core and rim compositional zoning were targeted for multiple analyses and exhibited no significant difference in Hf composition e.g. Jar-Salajah (∼700 Ma), Ibn Hashbal (∼600 Ma) and Al

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Hawiyah (∼600 Ma) Suites. Most samples have 10–20 Lu–Hf analyses are thought to adequately represent primary magmatic zircon trends (ε Hf +5 to +10) and within a single pluton, calculated ε Hf values typically range 3–4 units. 4.2.1. Western Arabian Shield magmatism The western Arabian Shield contains four distinct U–Pb age populations at ∼850 Ma, ∼700 Ma, ∼600 Ma and ∼525 Ma that produce similar juvenile compositions and Hf model ages (Fig. 4). The oldest magmatism from the Makkah Suite consists of a tight cluster of ε Hf values ranging from +8 to +11 and TDM (crust) model ages of 1.19–0.99 Ga. Syncollisional (∼700 Ma) suites such as the Shufayyah Complex have tightly constrained ε Hf values from +9 to +11 and TDM (crust) ages of 1.07–0.90 Ga, whilst the Jar-Salajah Complex and Subh Suite exhibit more Hf diversity. These range from ε Hf +4 to +11 and −2 to +10 with TDM (crust) model ages from 1.36–0.86 Ga and 1.73–0.95 Ga respectively. Western Arabian Shield post-tectonic suites convey two age groups at ∼600 Ma and ∼525 Ma that define some of the youngest and most juvenile plutons (Fig. 4). The ∼600 Ma Al Bad Suite (Midyan Terrane) and Al Hawiyah Suite (Asir Terrane) define ε Hf ranges from +6 to +10 and +6 to +9 and TDM (crust) model ages of 1.12–0.89 Ga and 1.13–0.96 Ga respectively. The ∼600 Ma syenitic Admar Suite and the ∼525 Ma syenitic Mardabah Complex both reside in the Hijaz Terrane and display similar ε Hf ranges from +6 to +9 and TDM (crust) model ages of 1.16–0.92 Ga and 1.18–0.87 Ga respectively. 4.2.2. Nabitah and Halaban Suture magmatism The Nabitah Suture separates the western and eastern Arabian Shield and incorporates post-tectonic (<636 Ma) plutons such as Kawr, Ibn Hashbal and Wadbah Suites (Asir Terrane) and the Al Hafoor Suite (Tathlith Terrane) at its eastern margins. The Kawr Suite has two samples at ∼611 Ma (kw42) and ∼608 Ma (kw51p) with the former defining a ε Hf range from +4 to +8 and TDM (crust) model ages of 1.3–0.98 Ga, whilst the younger recording higher ε Hf values from +7 to +14 and TDM (crust) model ages of 1.06–0.66 Ga. By contrast, the Ibn Hashbal Suite has a very tight cluster of ε Hf values from +8 to +10 and TDM (crust) model ages of 1.06–0.88 Ga. The Wadbah Suite shows a larger Hf isotopic with ε Hf values from +5 to +10 and TDM (crust) model ages of 1.21–0.92 Ga. The Al Hafoor Suite records a tight ε Hf cluster from +6 to +8 and TDM (crust) model ages of 1.18–1.04 Ga. Further east, the Najirah Granite (Ad Dawadimi Terrane, Halaban Suture) defines a ε Hf spread from +4 to +9 and TDM (crust) model ages of 1.24–0.94 Ga (Fig. 4). 4.2.3. Eastern Arabian Shield This eastern Arabian Shield is composed of the post-tectonic (∼636–600 Ma) Al Khushaymiyah, Ar Ruwaydah, Haml and Idah Suites and the Malik Granite, which all define tightly constrained, juvenile data (Fig. 4). The Al Khushaymiyah and Haml Suites in the southern Afif Terrane record ε Hf ranges from +3 to +6 and +8 to +10 and TDM (crust) model ages of 1.33–1.11 Ga and 1.04–0.91 Ga respectively. To the east of the Halaban Suture, the Ar Ruwaydah Suite and Malik Granite in the Ad Dawadimi Terrane record ε Hf values from +7 to +9 and +7 to +9 and TDM (crust) model ages of 1.08–0.91 Ga and 1.07–0.93 Ga respectively. The Idah Suite in the Ha’il Terrane (north-eastern Shield) defines a spread of ε Hf values from +6 to +11 and TDM (crust) model ages of 1.12–0.86 Ga. 5. Discussion Zircon U–Pb and Hf isotopic data presented in this study define island arc (∼850 Ma) and accretion (∼710 Ma) events followed by two post-tectonic magmatic phases (∼630 Ma and 525 Ma) that

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Fig. 4. ε Hf vs. suites.

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Pb/238 U age diagrams illustrating the juvenile nature of magmatic events in the Arabian Shield. Note the similar

mark the cessation of continental accretion (Fig. 5). This series of events coincides with the magmatic phases defined by Bentor (1985) and to evaluate the tectonic implications of these ages, the Hf data and possible mechanisms for generating juvenile crustal growth in the ANS are discussed first. This is followed by the tectonic evolution of the Saudi Arabian Shield and finally, the East African Orogen and Gondwana significance. 5.1. Generating juvenile crustal growth in the Arabian Shield A defining characteristic of the Arabian Shield is the continuous series of accretion followed by post-tectonic magmatism migrating from west to east, reflecting supercontinental amalgamation. One would assume that the interaction between mantle and overlying crust in island arc (intra-oceanic), syncollisional (subduction related) and post-tectonic (extension related) settings would produce very distinctive mantle Hf isotopic signatures. However, Arabian

ε Hf and model ages for all Arabian Shield

Shield data presented in this study (Fig. 6), records a series of juvenile crustal growth events (ε Hf +5 to +12) in which the mantle repeatedly provides large volumes of basaltic crust for long periods of time. These new Hf data do not substantiate the suggestion of a distinctive less depleted east to west mantle and possible incorporation of cratonic crust proposed by Stoeser and Frost (2006). Although Stoeser and Frost’s (2006) whole-rock Nd, Sr, Pb and O isotope data are of high quality and cover vast areas of the Arabian Shield, our new Hf data suggest a broadly homogeneous east-west mantle source with limited crust–mantle interaction. A caveat to this is that only one of four samples (ky) was directly from the Afif Terrane (that proposed to be floored with pre-Neoproterozoic crust). This sample is the most evolved of those analysed in the eastern Arabian Shield (Fig. 4). In addition, evidence for more juvenile mantle at ∼600 Ma associated with suture zones is presented (Fig. 6).

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Fig. 5. U–Pb concordia summarising 449 zircon analyses obtained from 19 Arabian Shield suites. There are 4 distinct magmatic phases in the Arabian Shield; Island arc, syncollisional and two post-tectonic events symbolising the cessation of accretion and final Gondwana assembly respectively.

The juvenility of the Arabian suites examined here encompass U–Pb ages no older than 900 Ma. A Paleoproterozoic (∼1800–1670 Ma) terrane, the Khida terrane, has previously been identified as associated with the proposed Neoproterozoic continent Azania (Collins and Windley, 2002). Recently, Late Mesoproterozoic volcanic arc rocks have been reported from the Sinai Peninsula of Egypt (Be’eri-Shlevin et al., 2012) demonstrating that pre-Neoproterozoic crust does exist in the ANS. However, the juvenility of the samples analysed here is emphasized when they are compared to coeval magmatism in the Madurai Block of southern India (Teale et al., 2011), which is also associated with Mozambique Ocean closure, but demonstrates considerable crustal reworking (Fig. 6). Generating large volumes of juvenile crustal growth within the Mozambique Ocean is highly debated, but can possibly incorporate both plume-plateau/subduction and migrating arc/back arc mechanisms. The former is suggested by Stein and Goldstein (1996), who proposed that an enriched upwelling plume associated with ∼1000 Ma intra-oceanic tholeiites can be geochemically traced across the ANS and with the addition of crustal overprinting, possibly provide the enriched sources for younger calc-alkaline (∼640–590 Ma) and alkaline (∼590–550 Ma) magmatism. However, it may be expected any juvenile source trends associated with plumes would be uniform or any crustal overprinting to produce a negative magmatic trend away from the DM. As illustrated in Fig. 6, there is a rough trend between ∼850–600 Ma in which the primitive Hf values become closer to the DM, while the lower values define a trend that parallels typical crustal growth of 0.015 (176 Lu/177 Hf). Although these data cannot rule out the interplay of lithospheric plumes between the juvenile source trends, the increase in juvenility at ∼ 600 Ma must indicate a change in tectonic process. Arabian Shield Hf data could also suggest juvenile arc magmatism in the form of basaltic underplated crust that switched to within plate lithospheric delamination (Avigad and Gvirtzman, 2009) at ∼600 Ma. The geochemical aspects of the subduction related (∼700 Ma) and post-tectonic A-types are not presented in this paper, but evidence from Robinson (2014) indicates that Arabian post-tectonic A-types involve both depleted and enriched mantle sources. This places some doubt over the migrating enriched plume (Stein and Goldstein, 1996) and comple-

ments Be’eri-Shlevin et al. (2010) who propose a more depleted mantle influence beneath the ANS. The Hf data can be used to instead propose a series of western and eastern migrating arcs (oceanic affinity, subduction/suture related), which continued until ∼615 Ma (Doebrich et al., 2007) followed by the closure of back arc basins immediately following (depleted A-type, ∼640–590 Ma, subduction/suture related) and disconnected (enriched A-type, ∼590–550 Ma, within plate delamination) with orogenesis. It may also accommodate subduction slab roll-back (tear) processes suggested by Flowerdew et al. (2013). The change in DM values occur predominantly within the Nabitah and Halaban Suture suites, which are preceded by subduction-related arc magmatism. 5.2. 950–640 Ma tectonic evolution of the Saudi Arabian Shield A tectonic summary of all the cycles discussed in the Saudi Arabian Shield are illustrated in Fig. 7. The generation of a chain of juvenile island arcs, now accreted to form key elements of the western Arabian Shield (Midyan, Hijaz and Asir Terranes), was the preliminary phase in the closure of the Mozambique Ocean. It is suggested that the ∼845 Ma Makkah Suite (Jeddah Terrane) represents the initiation of western island arc magmatism and was emplaced in the upper plate above eastward subducting ocean plates of the EAO. Ophiolite-decorated sutures (Yanbu and B’ir Umq) define accretion between western terranes and eastward impingements of the EAO. This closure is further constrained by syncollisional magmatism in the northern Shield at ∼736–709 Ma (Shufayyah and Jar-Salajah Complexes). This magmatism stitches the Midyan and Hijaz Terranes (Fig. 8). This phase may support the interpretation of Stein (2003) who suggests that accretion of oceanic plateaus and the transformation of enriched plume material into juvenile lithospheric mantle and crust (via subduction), was the main driving force for early ANS growth. 5.3. 640–590 Ma tectonic evolution of the Saudi Arabian Shield Calc-alkaline magmatism in central Arabia is a significant phase involving subduction of eastward migrating plates. The ∼630 Ma A-type (Robinson, 2014) data in this study helps to constrain the

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Fig. 6. ε Hf vs. 206 Pb/238 U age diagram illustrating the juvenile crustal growth of the Arabian–Nubian Shield. The juvenility of the Arabian samples suggests that there has been limited crustal influence and mean model ages (min and max dashed lines) indicate an association with Mozambique Ocean rifting. The bottom trend runs roughly parallel to typical crustal growth 0.015 (176 Lu/177 Hf), while the Hf values become closer to the DM between 850–600 Ma (black dashed arrow), possibly suggesting basaltic underplated crust. The mechanisms for generating large volumes of mafic crust can accommodate initial plume-plateau magmatism, but at ∼600 Ma possibly indicates back arc magmatism in the form of lithospheric/slab roll-back processes. The southern Indian gabbros (Teale et al., 2011) highlight similar-aged magmatism associated with Gondwana amalgamation and Mozambique Ocean closure, but formed with considerable crustal reworking, highlighting the juvenility of the ANS.

final accretion of eastward migrating arc terranes. This is particularly evident from the appearance of the ∼636 Ma Al Hafoor Suite and ∼620–610 Ma Kawr, Wadbah, and Ibn Hashbal Suites intruding the Nabitah Belt (Fig. 8), which supports a southern Shield accretion age of ∼640 Ma proposed by Johnson (2006). As discussed in Section 5.1, generating magmatism in this area of the Arabian Shield may reflect slab roll-back processes, which supports Flowerdew et al. (2013). East of the Afif Terrane, westward subduction magmatism was still occurring until ∼600 Ma in the Ar Ryan Terrane (Collins and Pisarevsky, 2005; Cox et al., 2012; Johnson et al., 2011). The appearance of the ∼611 Ma Ar Ruwaydah Suite and ∼607 Ma Najirah Granitoid along the Halaban Suture (Ad Dawadimi Terrane), lie east of the ∼615 Ma fore-arc sedimentary basin represented by the Abt Schist (Cox et al., 2012). This significant relationship not only helps constrain the final accretion in the eastern Arabian Shield, but the back arc A-type magmatism supports the reversed westward dipping polarity. 5.4. 590–550 Ma tectonic evolution of the Saudi Arabian Shield According to Bentor (1985), the termination of calc-alkaline magmatism was associated with uplift and the appearance of alka-

line dominated magmatic assemblages. This is consistent with the generation of the Al Bad (∼597 Ma), Al Hawiyah (∼591 Ma) and Abanat (∼585 Ma) Suites, which cover vast areas of both the western and eastern Arabian Shield in back arc settings. As described in Section 5.1, this juvenile crustal growth may be associated with lithospheric delamination. However, this is not the case for the small volume ∼525 Ma Mardabah Complex residing in the Hijaz terrane. This A-type syenite (Robinson, 2014) is significant for two reasons: 1) it postdates the supposed age of the Cambrian regional unconformity at 541 Ma, which is marked in the ANS by peneplain development and absence of field evidence for intruding magmatic bodies after this age; and 2) it is now the youngest recorded A-type magmatism in the Saudi Arabian Shield (Fig. 8). Similar age granites and volcanism reside in Sinai (Odin et al., 1983), southern Israel (Beyth and Heimann, 1999) and the Nubian Shield (Harris and Gass, 1981) and together with the Mardabah Complex, demonstrate that ANS magmatism continued into the Cambrian and that the basal Phanerozoic unconformity must post-date the early Cambrian.

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Fig. 7. A tectonic cartoon of the Midyan-Hijaz microplates in the north western Shield (complements Fig. 8). Geochronology data, combined with geochemical information from Robinson (2014), highlight the important Arabian Shield processes that were involved in the formation of Gondwana. Note the Al Ays Group (740–700 Ma) is taken from Johnson (2006).

5.5. Tectonic implications for the East African Orogen and Gondwana amalgamation Following the development of ∼850 Ma island arc magmatism associated with Mozambique Ocean closure, syncollisional magmatism in the ANS at ∼715 Ma represents the first stage of ter-

rane amalgamation within the Mozambique Ocean due to the east Africa and eastern Gondwana subduction related convergence. This process of ocean basin closure between ∼715–640 Ma appears to be consistent with Meert (2003) who discuss ∼750–650 Ma tectonic movements involving the African and South American continental fragments in the northern part of the Mozambique

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Fig. 8. A time space plot summarising all reliable geochronology data from the Arabian Shield. Note the absence of island arc age magmatism in the eastern Shield i.e. a younging trend towards the eastern terranes. The red lines illustrate some significant tectonic processes e.g. closure of back-arc and fore arc basins constrained by A-type magmatism (Nabitah Belt and Abt Schist respectively). The Proterozoic 1850–1670 Ma Khida Terrane in the eastern Arabian Shield is not included in this diagram. The references for the U–Pb, SHRIMP, Rb–Sr and Ar–Ar age data are presented in Table 1. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

belt. Collins and Windley (2002) and Collins and Pisarevsky (2005) suggest that the collision of Azania with the Congo–Tanzania– Bangweulu block caused the termination of accretion/arc collision in the western ANS at ∼630 Ma (west of Afif). The appearance of ∼640–600 Ma post-tectonic magmatism intruding the Nabitah Belt and development of western ANS back-arcs reinforces this interpretation. East of Afif, the appearance of ∼611–607 Ma posttectonic magmatism cross-cutting the Halaban Suture, possibly corresponds with the collision of the Afif-Abas block and eastern Arabia discussed in Collins and Pisarevsky (2005). The final stages of Gondwana assembly are recorded in the northern EAO from ∼570–530 Ma and are split into two orogenic episodes, the Kunnga and Malagasy respectively (Collins et al., 2014 and references therein). It is suggested that the ∼ 525 Ma A-type magmatism (Hijaz Terrane) was generated by the impact of India with Congo–Tanzania–Bangweulu block (Malagasy Orogeny). It is well documented by Stern (1985) and Johnson et al. (2011) that major transform fault systems such as the Najd (Kusky and Matesh, 1999 place the Najd system at 576.6[±5.3] Ma) are scattered across the northern Arabian Shield and form a series of reactivated events that induce magmatism. It is interpreted that the ∼525 Ma Mardabah Complex is a product of localised extension induced by stress transferal along the Najd, which possibly marks the termination of Gondwana assembly in central Arabia at ∼525 Ma. 6. Conclusions U–Pb isotope analysis of 449 zircons obtained from 19 granitoids from 8 of the identified discrete Arabian Shield terranes, both reveal new magmatic suite ages as well as further constraining existing crystallisation ages in the Arabian Shield. These data are sharply separated into 4 magmatic events identified as; island arc (∼845 Ma), syncollisional (∼710 Ma), post-tectonic (∼620 Ma)

and anorogenic (∼525 Ma). Most represented are the post-tectonic suites, which highlight processes directly after the final amalgamation phases of continental accretion. These are stitching plutons providing minimum ages for microplate accretion. Two hundred and seventy Hf isotopic analyses indicate that all ANS melts are isotopically juvenile with typical ε Hf values of >+5 to +10, regardless of their age or spatial relationships. Subtle changes in isotopic signatures between 850 and 600 Ma, suggest changing granite sources through this time interval. These changes may result first from the development of a basaltic crustal underplate and from limited interaction with Paleoproterozoic crustal sources (also with primitive isotopic signatures) and later from the onset of lithospheric delamination and/or/-subduction roll-back. While granitic suite model ages range from ∼1100–900 Ma, and are probably due to the source mixing, they are coincidently the same age as Mozambique Ocean derived magmatism. These granitoids mark accretionary cycles initiated as the Mozambique Ocean began to subduct and close (island arcsyncollisional ages) leading towards final Gondwana amalgamation (post-tectonic ages). Newly defined ∼525 Ma magmatism is the phase of this final post-accretionary Gondwanan accretion stage so far defined in the Saudi Arabian part of the Arabian Shield. This may require redefinition of the age of the basal Phanerozoic regional unconformity. This final stage of magmatism is synchronous with the reactivation of the Najd Faults (Johnson et al., 2011; Stern, 1985), and possibly correlates with the Malagasy–Kunnga Orogen (India–EAO collision) at the final stages of Gondwana assembly. Acknowledgements The Saudi Geological Survey, in particular Khalid Kadi and Mubarak M Nahdi, are thanked for providing research funding and gracious hospitality in Saudi Arabia during February 2010 field

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