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  ARTICLE Received 20 Sep 2016 | Accepted 25 Nov 2016 | Published 31 Jan 2017 Archaean zircons in Miocene oceanic hotspot rocksestablish ancient continental crust beneathMauritius Lewis D. Ashwal 1 , Michael Wiedenbeck 1,2 & Trond H. Torsvik 1,2,3,4, w A fragment of continental crust has been postulated to underlie the young plume-relatedlavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zirconsfrom basaltic beach sands. Here we document the first U–Pb zircon ages recovered directlyfrom 5.7Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrysticzircons ranging in age between 2.5 and 3.0Ga within a trachyte plug that crosscuts OlderSeries plume-related basalts of Mauritius. Our results demonstrate the existence of ancientcontinental crust beneath Mauritius; based on the entire spectrum of U–Pb ages for oldMauritian zircons, we demonstrate that this ancient crust is of central-east Madagascaraffinity, which is presently located  B 700km west of Mauritius. This makes possible adetailed reconstruction of Mauritius and other Mauritian continental fragments, which onceformed part of the ancient nucleus of Madagascar and southern India. DOI: 10.1038/ncomms14086  OPEN 1 School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS, Johannesburg 2050, South Africa.  2 Deutsches GeoForschungsZentrum GFZ,Telegrafenberg, D14473 Potsdam, Germany.  3 Center for Earth Evolution and Dynamics (CEED), University of Oslo, 0316 Oslo, Norway.  4 Geodynamics,Norges Geologiske Undersøkelse (NGU), N-7491 Trondheim, Norway.  w Present address: Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, D14473Potsdam, Germany. Correspondence and requests for materials should be addressed to L.D.A. (email: NATURE COMMUNICATIONS|8:14086|DOI: 10.1038/ncomms14086|  1  E  vidence is accumulating that old continental crust may occur beneath some young ocean-island volcanoes, con-tributing contaminating components to their chemical andisotopic compositions. For example, the Sr, Pb and Nd isotopiccompositions of basalts to rhyolites in the O¨ræfajo¨kull volcano of southeastern Iceland were modelled to have incorporated 2–6% of Precambrian continental crust 1 . This, combined with inversionof gravity anomaly data that indicate unusually thick crust of  4 30km, has been used to infer that the Jan MayenMicrocontinent extends southwestward to underlie southeastIceland 1 . Similarly, a fragment of continental crust was suggestedto underlie the young plume-related lavas of the IndianOcean island of Mauritius, on the basis of gravity inversionmodelling (crustal thickness) and the recovery of Proterozoic(660–1,971Ma) zircons from basaltic beach sands 2 . The island of Mauritius (2,040km 2 ) is the second youngest member of ahotspot track extending from the active plume site of Re´union,through the Mascarene Plateau, the Laccadive-Chagos Ridgeand into the 65.5Ma Deccan Large Igneous Province 3,4 . Threephases of Mauritian basaltic volcanism have been named theOlder (9.0–4.7Ma), Intermediate (3.5–1.66Ma) and Younger(1.0–0.03Ma) Series 5,6 . Minor volumes of trachytic rocks occuras intrusive masses or plugs  o 300m across and are associatedwith Older Series basalts, as confirmed by U–Pb zirconthermal ionization mass spectrometry dating, which yielded atone location an age of 6.849 ± 0.012Ma (ref. 7). Mauritiantrachytes are variably altered from probable primary phonoliticmagmas 7 , forming a prominent Daly Gap when plotted withcoeval basalts; this suggests formation either by extreme fractionalcrystallization from the basalts 8 or by direct partial melting of  CCNC LAC SM Deccan Traps WesternDharwar LRS     M  a  s  o   r  a 40 ° S45 ° E40 ° E35 ° S SMAGMarion plume 57 ° 20 ′ E57 ° 30 ′ E57 ° 40 ′ E20 ° 00 ′ S20 ° 10 ′ S20 ° 20 ′ S20 ° 30 ′ S MAU 8 Younger series (1.0–0 Ma)Intermediate series (3.5–1.66 Ma)Older series (9.0–4.7 Ma)Trachyte localitiesDating of zircons from beach sand MLegend Marion/deccan trapsNeoproterozoicPaleoproterozoic/ mesoproterozoicNeoarcheanPaleoarchean nucleus Figure 1 | Simplified geology of Madagascar and India reconstructed to 90–85Ma.  Mauritius (M) is reconstructed in a likely location nearArchaean–Neoproterozoic rocks in central-east Madagascar just prior to break-up 2 . The exact size and geometries of Mauritius and other potentialMauritian continental fragments (collectively known as Mauritia, including SM Saya de Malha; C, Chagos; CC, Cargados-Carajos Banks; LAC, Laccadives;N, Nazreth; see present location in Fig. 6) are unknown, and are generously drawn in the diagram. We propose that Mauritia is dominantly underlainby Archaean continental crust, and part of the ancient nucleus of Madagascar 25,46 and India 20,21 (stippled black line). A Large Igneous Province event(linked to the Marion plume) occurred from 92 to 84Ma, and most of Madagascar was covered with flood basalts (full extent not shown for simplicity).Blue stippled line indicates the site of Cretaceous pre-breakup strike-slip faulting. AG, Analava gabbro (91.6Ma); LR, Laxmi Ridge; S, Seychelles;SM, St Mary rhyolites (91.2Ma) 41 . The black–white box (geology of Madagascar) is enlarged in the inset to Fig. 5. Inset map shows simplified geology ofMauritius, including trachyte plugs 7 . Star symbol marked MAU-8 is the sampling area for the present study and black bars indicate locations of zirconsrecovered from beach sand samples 2 . ARTICLE  NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14086 2  NATURE COMMUNICATIONS|8:14086|DOI: 10.1038/ncomms14086|  metasomatized mantle 7 . Isotopic compositions of the trachytesshow relatively constant  e Nd  of   þ 4.03 ± 0.15, with highly variableI Sr  of 0.70408–0.71034, interpreted as reflecting small amounts(0.4–3.5%) of contamination by Precambrian continental crust,successfully modelled using as a proxy   B 750Ma granitoidcomposition from the Seychelles. It has been noted that othercompositions, including older Proterozoic or even Archaeancomponents, can also be plausibly modelled 7 . It is from such aMauritian trachyte that a population of zircons was extractedand analyzed for this paper. A subset of these zircons yieldsArchaean U–Pb ages, confirming the existence of a fragment of Precambrian continental crust beneath Mauritius, and wepropose here that Mauritius and other Mauritian continentalfragments are dominantly underlain by Archaean continentalcrust, and that these srcinally formed part of the ancient nucleusof Madagascar and India. 10 µ m10 µ m6 µ m3/9:3022Ma3/8:2950Ma3/1:2930Ma3/7:2874Ma3/10:2849Ma3/11:2967Ma3/3:2934Ma3/4:2897Ma3/5:3003Ma3/2:3030Ma3/6:3006Ma5/3:2810Ma5/2:2830Ma5/1:2834Ma8/3:2836Ma8/4:2552Ma8/1:2887Ma8/2:2807Ma8/6:2766Ma8/5:2447MaQtzMonQtz +KspKspQtzGrain 3Grain 5Grain 8 abcdef Figure 2 | Scanning electron microscope images of three Archaean zircon grains.  These three grains were recovered from the MAU-8 trachytesample. Backscattered electron (BSE) images ( a – c ) of the three grains taken after completing all U–Th–Pb isotopic analyses. Cathodoluminescence (CL)images ( d – f  ) taken prior to acquiring our SIMS data. The indicated analysis numbers correspond to those in Supplementary Data 1. The indicated ages arethe radiogenic  207 Pb/ 206 Pb ages for the corresponding craters.NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14086  ARTICLE NATURE COMMUNICATIONS|8:14086|DOI: 10.1038/ncomms14086|  3  Results U–Pb systematics . A single trachyte sample (MAU-8; locationgiven in inset to Fig. 1) was selected for zircon separation basedon available sample mass ( B 1kg) and Zr concentration(1,165 m gg   1 ); extreme care was taken during sample processing to avoid any risk of contamination. Thirteen zircon grains wererecovered, from which we report 68 individual point analysesacquired using the Cameca 1280-HR SIMS (secondary ion massspectrometer) instrument at GFZ Potsdam; details of the sampleprocessing and analytical methods are provided in the Methodssection. Three zircon grains show uniquely mid- to late-ArchaeanU–Pb systematics, with no evidence for Phanerozoic components.Cathodoluminescence (CL) and backscattered electron (BSE)images (Fig. 2) show that these three crystals contain internalstructures such as metamict cores, partially resorbed idiomorphicbanding and numerous mineral inclusions. We conducted 20individual U–Th–Pb spot analyses on these grains, where theconcordant or near-concordant  207 Pb/ 206 Pb ages range from3,030 ± 5Ma to 2,552 ± 11Ma (Fig. 3 and Supplementary Data 1). We interpret these data as indicating the crystallizationages of a complex Archaean xenocrystic component; no over-growths of young zircon are evident in any of the three Archaeangrains, despite great efforts to identify such rims. The mineralinclusions in these ancient zircons include quartz, K-feldspar andmonazite, as determined by EDS measurements; this assemblagewould be consistent with crystallization from granitic or syeniticmagmas. As the three grains are distinct in terms of theircrystallization ages as well as their Th/U systematics, we concludethat trachytic magma traversed through and incorporated siliciccontinental crustal material that preserves a record of severalhundred Myr of Archaean evolution. This is consistent withSr–Nd isotopic systematics of the Mauritian trachytes 7 , asdiscussed above.Ten of the 13 grains differ from the older zircons in that they are featureless, with no internal structures visible in CL or BSEimages (Fig. 4); these were determined to have late Miocene U–Pbsystematics with no traces of inherited components, yielding anunweighted mean  206 Pb/ 238 U age of 5.7 ± 0.2Ma (1 s.d.,  N  ¼ 48,Fig. 3). We interpret these results as representing the crystal-lization age of the MAU-8 trachyte magma, which is consistentwith their association with the Older Series of Mauritian basaltic volcanism (9.0–4.7Ma, as constrained mainly by   40 Ar/ 39 Ardating  6 ). Th/U ratios . U and Th concentrations of all measured individualspots in the zircon grains are given in Supplementary Data 1. Forthe three Archaean grains, Th/U ratios show distinct values, withGrains 3 and 5 yielding Th/U between 0.05 and 0.47, which istypical of variably metamorphosed and/or recrystallized igneousrocks 9,10 . In contrast, Grain 8 is very depleted in Th, withconsistently low Th/U values around 0.01, more typical of high-grade metamorphic rocks 11,12 ; we note that among theArchaean zircons, Grain 8 contains the youngest  207 Pb/ 206 Pb spotage (2,552 ± 11Ma, excluding analyses that partly overlappedepoxy or mineral inclusions). In Grains 3 and 8, there is atendency for spots with lower Th/U to occur on grain margins.These features suggest that the Archaean zircons were derivedfrom source rocks that experienced a complex history of magmatic and metamorphic events. The young (Late Miocene)zircon grains all show much higher Th/U ratios of 0.7–2.6(average Th/U ¼ 1.4 ± 0.6,  N  ¼ 48) typical of magmatic zircons inalkaline rocks 10,13 . Oxygen isotopes . In order to further characterize the sources of the zircons, we undertook spot analyses of oxygen isotopes by SIMS for some of the grains (Supplementary Data 2). For theArchaean zircons, the results yield  d 18 O SMOW  values between5.5 %  (Grain 3) and 9.7–9.9 %  (Grain 8). The lower value iscomparable to the average  d 18 O SMOW  value of 5.5 ± 0.4 %  forArchaean zircons in tonalite–trondhjemite–granodiorite rocksin the Superior Province of Canada 14 . The higher values areoutside the range of typical  d 18 O SMOW  for magmatic zircons 15 ,although grains with  d 18 O SMOW 4 8.5 %  have been reportedin Precambrian granitoids from Fennoscandia 16 . In contrast,oxygen isotope data for 29 spot analyses of the Miocene agezircons give a mean  d 18 O SMOW  value of 4.59 ± 0.20 %  (1 s.d.)(Supplementary Data 2). These values are lower than the average d 18 O SMOW  of mantle zircons (5.3 ± 0.3 % ; ref. 15), but arewithin the range of zircons recovered from some kimberlites( d 18 O SMOW ¼ 3.4–4.7 % ; ref. 17). This would be consistentwith an origin for Mauritian trachytes as low-degree partialmelts of fertile metasomatized mantle 7 . Comparison of U–Pb ages with adjacent continents . Can thespectrum of U–Pb ages for old Mauritian zircons be correlatedwith exposed Precambrian terranes in nearby continental entities?We considered major continental masses like India, as wellas large (e.g., Madagascar, 587  10 3 km 2 ) and smaller (e.g.,Seychelles, 459km 2 ) continental fragments as potential correla-tives of the sub-Mauritius continental crust. Granitoid rocks of the Seychelles 18 range from  B 700–800Ma, with the vastmajority of ages  B 750Ma (ref. 19); no Archaean componentshave been identified there. The Dharwar Craton of southern India(Fig. 1) consists of a nucleus of Palaeoarchaean to Neoarchaean(3.4–2.5Ga) migmatitic orthogneisses flanked by juvenileNeoarchaean (2.7–2.5Ga), dominantly granitoid gneisses 20 .Palaeo- and Mesoproterozoic rocks are present south of the Dharwar Craton in India 21 . Neoproterozoic igneous ormetaigneous rocks are rare to absent in this region, although the B 750Ma Malani Igneous Suite of Rajasthan, some 1,000kmto the NNW, has been correlated with the granitoids of the Seychelles 22,23 . The best match to the age spectrum of Precambrian zircons recovered from Mauritius occurs in east- Grain 3Grain 5Grain 8Data from Torsvik et al. (2013) 2 3,000 Ma2,6002,2001,8001,4001,000207 Pb/  235 U         2        0        7       P      b       /          2        3         8        U  048121620240. 48 younger analysesnear the srcin Figure 3 | Concordia plot.  Includes all 20 data points from the threeArchean zircons found in the MAU-8 trachyte sample. The ellipses indicatethe 1 s.d. analytical uncertainties for each of the SIMS determinations.Also shown as red symbols are the TIMS wet chemical results reportedby Torsvik  et al. 2 for eight Proterozoic zircons recovered from Mauritianbasaltic beach sands. Not shown are the 48 SIMS determinations on the10 Miocene zircons, here indicated with the arrow. Tick marks on theConcordia curve are in Ma. TIMS, Thermal Ionization Mass Spectrometry. ARTICLE  NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14086 4  NATURE COMMUNICATIONS|8:14086|DOI: 10.1038/ncomms14086|
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