U–Pb zircon geochronology and geochemistry of Neoproterozoic granitoids of the Maevatananaarea, Madagascar: implications for Neoproterozoic crustal extension of the Imorona–Itsindro

Suite and subsequent lithospheric subduction

Xi-An Yanga–c, Yu-Chuan Chenb, Shan-Bao Liub, Ke-Jun Houb, Zhen-Yu Chenb and Jia-Jun Liua*aState Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing, China; bInstitute ofMineral Resources, Chinese Academy of Geological Sciences, Beijing, China; cZijin Mining Group Company Limited, Xiamen, China

(Received 22 July 2014; accepted 14 October 2014)

Voluminous Neoproterozoic granitoid sheets of the Imorona–Itsindro Suite are important components of exposed basementin west-central Madagascar. Here, we report precise new zircon U–Pb ages and whole-rock geochemistry for granitoidswithin the Maevatanana area of Madagascar. The new laser ablation inductively coupled plasma mass spectrometry zirconU–Pb dating undertaken during this study indicates that Antanimbary granitoid and Antasakoamamy granitoid wereemplaced at 747 ± 9 Ma and 729 ± 9–727 ± 8 Ma, respectively. Geochemically, the Antanimbary granitoids show poorNb, Ta anomalies, pronounced positive Zr anomalies, and are K-rich (K2O/Na2O > 1), but the Antasakoamamy granitoidsare relatively depleted in Nb, Ta, show slightly negative Zr anomalies, and are Na-rich (Na2O/K2O > 1). Both suites containzircons with strongly negative εHf(t), indicating participation of much older (Palaeoproterozoic and Archaean) crust. Theirgeochemical characteristics, along with the use of various discrimination diagrams, reveals that crustal delamination andasthenospheric upwelling resulted in crustal extension of the region before ~747 Ma, with subsequent lithosphericsubduction and arc magmatism after 729–727 Ma.

Keywords: U–Pb zircon geochronology; geochemistry; Neoproterozoic granitoids; Maevatanana; Madagascar

1. Introduction

The island of Madagascar consists of a collage ofPrecambrian basement terranes overlain by Phanerozoicsedimentary basins along the west coast of the island.These Precambrian terranes were juxtaposed during theNeoproterozoic–Cambrian (Pan-African) East Africanand Malagasy orogenies (Collins and Pisarevsky 2005;Collins 2006). The East African Orogen (EAO) extendsfrom southern Israel and Jordan in the north to Antarcticain the south (Stern 1994; Meert 2003; Jacobs and Thomas2004) and represents the Neoproterozoic collision zonebetween India, the Congo–Tanzania–Bangweulu Block,and the Saharan Metacraton (Meert 2003; Collins andPisarevsky 2005; Collins 2006). The EAO is the world’slargest Neoproterozoic to Cambrian orogenic complex andconsists of a collage of individual oceanic domains andcontinental fragments. Consolidation of these fragmentsoccurred between ~800 and 500 Ma (Meert and Van DerVoo 1997; Meert et al. 2001; Meert 2003; Fritz et al.2013). Madagascar lies in the heart of the EAO, and theexposed basement in this area has been the subject ofmuch research, including research focused on the archi-tecture of the Archaean basement (Tucker et al. 1999,2011a, 2011b; Collins et al. 2003a) and its history ofNeoproterozoic metamorphism, magmatism, and structuraldevelopment (Buchwaldt et al. 2003; Jöns et al. 2006),

structural geology (Collins et al. 2003a, 2003b; Tuckeret al. 2007; Thomas et al. 2009), and magmatic processesthat operated in this area (Nédélec et al. 1995; Paquetteand Nédélec 1998; Meert et al. 2001; Goodenough et al.2010). Suites of Cryogenian granitoids and gabbros withinthe Seychelles, north-central Madagascar, and northwestRajasthan in India (the Malani igneous suite) are thoughtto represent part of an active continental margin on thewestern edge of Rodinia (Handke et al. 1999; Torsvik2001a, 2001b; Ashwal 2002; Thomas et al. 2009).Zircon U–Pb geochronology and isotopic, geochemical,and petrological evidence from these areas provide evi-dence of the existence of this convergent boundarybetween 800 and 700 Ma. However, other researcherssuggest that gabbroic and granitoid rocks from theSeychelles and Madagascar formed in an intra-plateplume or rift scenario or alternatively may have formedas a result of lithospheric delamination (Stephens et al.1997; Tucker et al. 2011a). Handke et al. (1999) reportedU–Pb ages of 804–776 Ma for a 450 km-long belt ofgabbroic and granitoid plutons stretching from Ambositrato Maevatanana within west-central Madagascar and sug-gested that this belt represents the roots of a continental‘Andean-type’ arc on the western margin of Rodinia; thisarc formed in the middle Neoproterozoic during fragmen-tation of the Rodinian supercontinent. These gabbroic and

*Corresponding author. Email: liujiajun@cugb.edu.cn

International Geology Review, 2015Vol. 57, Nos. 11–12, 1633–1649, http://dx.doi.org/10.1080/00206814.2014.977969

© 2014 Taylor & Francis

granitoid plutons have been named the Imorona–ItsindroSuite (Key et al. 2011; Tucker et al. 2011a; Roig et al.2012) after the type localities in the Itremo area (Moine1968). Igneous rocks of the Imorona type are felsic, mean-ing granitic, syenitic, and monzonitic in composition.Igneous rocks of the Itsindro type are mafic and includegabbro, gabbro-diorite, and granodiorite (Key et al. 2011;Moine et al. 2014; Tucker et al. 2014). Bybee et al. (2010)also suggested that ultramafic complexes within theAndriamena region of north-central Madagascar formedin a similar setting during the middle Neoproterozoic.Tucker et al. (2014) reviewed dating of the Imorona-Itsindro suite and showed that these granitoids formedbetween 851 and 719 Ma. Recent studies suggest thatthe Imorona–Itsindro Suite, emplaced in Cryogenian timefrom 840 to 760 Ma, was linked to continental dilation (Liet al. 2008; Tucker et al. 2011a; Moine et al. 2014). Incontrast, Thomas et al. (2009) reported thatNeoproterozoic granitoids to the north in the BemarivoBelt are somewhat younger at ca. 750–705 Ma and pro-posed that the plutonic rocks have an arc-related nature.

Granitoid intrusions are ubiquitous components of themajor orogenic belts within the Maevatanana area of central

Madagascar, and the geochemical and geochronologicalconstraints provided by these granitoids are useful in recon-structing the geodynamic setting for the magmatic eventsthat formed these intrusions. Here, we present new geo-chemistry and geochronology of granitic rocks from theMaevatanana area. These data constrain the timing ofmajor Imorona–Itsindro Suite magmatism withinMadagascar and constrain the tectonic setting in whichthese rocks formed.

2. Geological setting and petrography

The eastern two thirds of Madagascar is dominated byPrecambrian rocks, whereas the western third is coveredby basins that preserve an extensive sedimentary recordfrom the late Carboniferous to Recent (de Wit 2003). Keyet al. (2011) reviewed previous work and showed thatcentral and north Madagascar consists of five crustaldomains (Figure 1). Five crustal domains consist ofAntongil Cratons, Masora Cratons, AntananarivoCratons, Tsaratanana Complex, and Bemarivo Belt. Fourdomains consist largely of Archaean metamorphic rocks,and the fifth (Bemarivo Belt) consists of Proterozoic

Figure 1. The major Precambrian crustal terranes of Madagascar.Source: Modified after Key et al. (2011) and Moine et al. (2014).

1634 X.-A. Yang et al.

meta-igneous rocks. Each domain has distinctive litholo-gies and histories of sedimentation, magmatism, deforma-tion, and metamorphism and is bounded by tectonicboundaries. Tucker et al. (2011a) proposes that they aredifferent parts of a common craton (the Greater DharwarCraton) amalgamated during Neoarchaean cratonization(ca. 2.5 Ga).

The Maevatanana Belt is the westernmost of threeN–S-trending belts of Neoarchaean amphibolite-faciesmafic gneiss and schist (granulite facies in theAndriamena belt). This belt is dominated by migmatiticgneiss, amphibolite, magnetite-rich quartzite, and metaba-sic to ultrabasic rocks (soapstones). The easternMaevatanana belt and a series of migmatitic gneiss andaugen gneiss are separated by fault.

The Neoproterozoic Antanimbary granitoid andAntasakoamamy granitoid are exposed as plutons withinthe southern part of the Maevatanana area (Figure 2).These granitoid are part of the Imorona–Itsindro suite

(BGS-USGS-GLW 2008; Key et al. 2011; Moine et al.2014). The Antanimbary granitoid contains 35–40%quartz, 25–35% K-feldspar, 15–20% plagioclase, and bio-tite (5%), with accessory zircon, apatite, titanite, and ilme-nite (Figure 3A–C). The Antasakoamamy granitoid isdivided into marginal granite and central quartz monzonitefacies that are separated by a transition zone. The granitoidis medium coarse-grained and varies in composition fromintermediate at the centre of the intrusion to felsic at themargin. The granitoid stocks, including related sheets anddikes, are intruded into the already metamorphosed maficgneisses and schists of the Tsaratanana Complex andBetsiboka Suite (Tucker et al. 2011a). The quartz monzo-nite contains subhedral phenocrysts and is medium coarse-grained. It contains K-feldspar (40–45%), plagioclase(35%), quartz (15–20%), and minor biotite (<5%), zircon,apatite, and ilmenite (Figure 3D–F). In comparison, thegranite is subhedral and fine-grained, is massive, and con-tains quartz (40%), plagioclase (30%), K-feldspar (25%),and biotite (5%), with accessory zircon, apatite, andtitanite (Figure 3G–I). On a modal quartz-alkali feldspar-plagioclase (QAP) classification basis (Figure 4), theAntanimbary granitoid and Antasakoamamy granitoid aremade up of syenogranite (M6), quartz monzonite (M8),and monzogranite (M1).

Samples M6-1, M6-2, M7-1, M7-2, and M7-3 arefrom the Antanimbary granitoid pluton, taken from out-crops along the Ikopa river (S 17°08′1.26″, E 46°48′12.36″). The sampled lithology is coarse-grained, fresh,pinkish granite. Samples M8-1, M8-2, M8-3, M9-1, andM9-2 are from the central facies of the Antasakoamamygranitoid pluton (S 16°59′2.8″, E 46°48′56.16″), takenfrom river outcrops. The sampled lithology is coarse-grained, fresh, light grey, quartz monzonite. Samples M1,M2, M3, M4, and M5 is from southern marginal facies ofthe Antasakoamamy granitoid pluton, taken from a quarry(S 17°1′3.06″, E 46°48′7.22″). The sampled lithology isfine-grained, fresh, grey granite.

3. Analytical methods

3.1. Zircon U–Pb and Lu–Hf isotopic data

High purity zircon separates were obtained by heavyliquid and magnetic separation methods, before handpick-ing under a binocular microscope. Transparent euhedralzircon grains were mounted in epoxy resin and polisheduntil grain interiors were exposed. Zircon grains were thenU–Pb dated by laser ablation inductively coupled plasmamass spectrometry (LA–ICP–MS) using an Agilent 7500aICP–MS equipped with a UP193SS laser ablation systemat the MLR Key Laboratory of Metallogeny and MineralAssessment, Institute of Mineral Resources, ChineseAcademy of Geological Sciences, Beijing, China. Details

Figure 2. Geological map of the Maevatanana region showingsample locations.Source: Modified after Rantoanina et al. (1969).

International Geology Review 1635

of the U–Pb analytical procedures may be found in Yanget al. (2014).

A laser spot of 36 μm was used for analysis, and aHarvard zircon 91500 standard with a recommended206Pb/238U age of 1065.4 ± 0.6 Ma (Wiedenbeck et al.

2004) was used for external standardization; this standardwas analysed after every four unknown zircon analyses.Corrections for common Pb were made following theapproach of Andersen (2002) and data were processedusing the GLITTER and ISOPLOT programs (Ludwing

Figure 3. Photographs showing typical occurrence of Neoproterozoic granitoids studied here. (A) Antanimbary granitoid (M6), (B)photograph of specimen of Antanimbary granitoid (M6), and (C) Antanimbary granitoid (M6) under crossed polarized light; (D)Antasakoamamy granitoid (M8), (E) photograph of specimen of Antasakoamamy granitoid (M8), and (F) Antasakoamamy granitoid(M8) under crossed polarized light; (G) Antasakoamamy granitoid (M1), (H) photograph of specimen of Antasakoamamy granitoid (M1),and (I) Antasakoamamy granitoid (M1) under crossed polarized light. © [Xi-An Yang]. Reproduced by permission of Xi-An Yang.Note: Qtz, quartz; Mc, microcline; Ab, albite; Bt, biotite.

Figure 4. Plots of some studied granitoid samples on the quartz-alkali feldspar-plagioclase diagram of Streckeisen (1976).

1636 X.-A. Yang et al.

2003). The uncertainties on individual LA–ICP–MS ana-lyses are quoted at the 95% (1σ) confidence level; moredetails of the analytical procedure are provided in Blacket al. (2004).

Zircon Lu–Hf isotopic analysis was carried out in situusing a New Wave UP213 laser-ablation microprobe,attached to a Neptune multicollector ICP-MS at theInstitute of Mineral Resources, Chinese Academy ofGeological Sciences, Beijing. Hou et al. (2007) compre-hensively described instrumental conditions and dataacquisition. A stationary spot was used for the presentanalyses, with a beam diameter of either 40 or 55 μmdepending on the size of ablated domains. Helium wasused as carrier gas to transport the ablated sample from thelaser-ablation cell to the ICP-MS torch via a mixingchamber mixed with argon. In order to correct the isobaricinterferences of 176Lu and 176Yb on 176Hf,176Lu/175Lu = 0.02658 and 176Yb/173Yb = 0.796218 ratioswere determined (Chu et al. 2002). For instrumental massbias correction, Yb isotope ratios were normalized to172Yb/173Yb of 1.35274 and Hf isotope ratios werenormalized to 179Hf/177Hf of 0.7325 using an exponentiallaw. The mass bias behaviour of Lu was assumedto follow that of Yb. Zircon GJ1 was used as thereference standard, with a weighted mean 176Hf/177Hfratio of 0.282013 ± 0.00008 (2σ, n = 10) or0.282013 ± 0.000024 (2σ, n = 10) during our routineanalyses. It is not distinguishable from a weighted mean176Hf/177Hf ratio of 0.2820 (2σ) from in situ analysis byElhlou et al. (2006).

3.2. Whole-rock geochemical analysis

Fifteen whole-rock samples from representativeAntanimbary granitoid and Antasakoamamy granitoidhave been analysed for major, trace, and rare earth ele-ments (REEs) during this study. These whole-rock sam-ples were trimmed to remove weathered surfaces beforebeing cleaned with deionized water, crushed, and thenpowdered using an agate mill.

Major element concentrations were determined usingX-ray fluorescence and a PANalytical Axios-Advancedinstrument at the Geological Analysis Laboratory (GAL)of the Ministry of Nuclear Industry, Beijing, China. Thisanalysis used fused glass discs and the analytical preci-sion, as determined using the Chinese National standardGSR-1, was better than 5%. Loss on ignition (LOI) wasobtained using 1 g of powder heated to 1100°C for 1 h.

Trace elements were determined using a plasma opticalemission mass spectrometer (POEMS) ICP–MS system atthe GAL of the Ministry of Nuclear Industry, Beijing,China. The discrepancy between triplicate analyses wasless than 5% for all elements, and analysis of the OU-6and GBPG-1 international standards was in agreementwith their recommended values.

4. Results

4.1. Zircon U–Pb dating

Zircons are abundant in the Antanimbary granitoid, andthey range in size from 50 to 100 μm and have length towidth ratios of 2:1. The crystals are subhedral to euhedral,and the majority are transparent to light brown in colour;CL images reveal concentric zoning patterns consistentwith magmatic crystallization (Figure 5A). Table 1 pro-vides age data for 12 analyses of zircon within sample M6.The measured concentrations for these zircon grains varyfrom 33 to 556 ppm for U; from 39 to 934 ppm for Th;and Th/U ratios range from 0.46 to 1.90, within the rangeexpected for magmatic zircon. The seven most concordantanalyses yield a concordia age of 747 ± 9 Ma (2σ, meansquare weighted deviation (MSWD) = 0.18; Table 1;Figure 6A), indicative of the crystallization age of thegranite. The other five zircons yield much older ages(2434–2074 Ma) and are interpreted to be inherited.Tucker et al. (1999, 2011a) dated the nearby Archaeanrocks of the Maevatanana belt, and the age of these is~2.5 Ga; therefore, the ages of inherited zircon is in keep-ing with the age of palaeoproterozoic granitoid (Tuckeret al. 2014).

Zircon is also abundant within the samples ofAntasakoamamy granitoid analysed during this study.The crystals are generally euhedral, transparent prisms,up to 100 μm in length, that display magmatic oscillatoryzoning (Figure 5B and C). Fifteen zircons from sampleM8, which is quartz-monzonite, were analysed. The mea-sured concentrations for these zircon grains vary from 0.2to 45 ppm for U; from 1.66 to 121 ppm for Th; and Th/Uratios range from 1.52 to 8.30, much higher than for thoseof metamorphic zircon. Eleven of the zircons yield aweighted mean 206Pb/238U age of 729 ± 9 Ma (2σ,MSWD = 0.49; Table 2; Figure 6B), indicative of thecrystallization age of the granite. The other four zirconsyield much older ages (2875–1020 Ma) and are interpretedto be inherited.

Eight zircons from sample M1, which is granite, wereanalysed. The measured concentrations for these zircongrains vary from 15 to 220 ppm for U; from 20 to 213 ppmfor Th; and Th/U ratios range from 0.46 to 2.44, largelywithin the range expected for magmatic zircon. Five ofthese zircons yield a weighted mean 206Pb/238U age of727 ± 8 Ma (2σ, MSWD = 0.028; Table 3; Figure 6C)indicative of the crystallization age of the granite. The otherthree zircons yield much older ages (2445–1943Ma) and areinterpreted to be inherited.

4.2. Zircon Lu–Hf isotopes

Lu–Hf analyses were obtained for zircon from theAntanimbary granitoid. The results are given in Table 4.The 176Lu/177Hf ratios range from 0.001091 to 0.002418,

International Geology Review 1637

Table 1. Results of zircon U–Pb dating of sample M6 from the Antanimbary granitoids.

Isotopic ratios Age (Ma)

SpotTh

(ppm)U

(ppm) Th/U 207Pb/206Pb 1δ 207Pb/235U 1δ 206Pb/238U 1δ 207Pb/206Pb 1δ 207Pb/235U 1δ 206Pb/238U 1δ

1 95 96 0.99 0.066 0.001 1.120 0.021 0.123 0.002 809 27 763 10 747 112 541 363 1.49 0.064 0.002 1.074 0.042 0.122 0.002 748 50 741 21 739 123 300 158 1.90 0.067 0.001 1.138 0.033 0.122 0.003 856 22 771 16 744 174 278 185 1.51 0.064 0.002 1.086 0.044 0.122 0.002 750 81 747 22 745 125 188 132 1.42 0.067 0.001 1.148 0.022 0.124 0.002 839 19 776 10 754 126 934 556 1.68 0.067 0.001 1.126 0.032 0.123 0.002 833 39 766 15 746 127 347 254 1.37 0.064 0.001 1.096 0.021 0.124 0.002 767 34 751 10 752 128 87 78 1.11 0.166 0 10.358 0.100 0.452 0.004 2520 5 2467 9 2405 209 122 178 0.69 0.166 0.001 10.411 0.218 0.456 0.010 2514 7 2472 19 2421 4310 80 78 1.03 0.166 0.001 10.274 0.236 0.450 0.010 2515 13 2460 21 2394 4611 39 33 1.19 0.141 0.004 7.196 0.283 0.380 0.017 2242 46 2136 35 2074 7712 185 400 0.46 0.166 0.001 10.477 0.064 0.459 0.002 2514 8 2478 6 2434 9

Figure 5. Cathodoluminescence images and ages of zircon from the dated samples of Neoproterozoic granitoid. (A) Zircon grains fromsample M6 of Antanimbary granitoid; (B) zircon grains from sample M8 of Antasakoamamy granitoid; and (C) zircon grains fromsample M1 of Antasakoamamy granitoid.

1638 X.-A. Yang et al.

with a mean of 0.001872, indicating that these zircons arevery low in radiogenic Hf. Seven analyses were performedon seven zircon grains from sample M6. The analyses yield

Figure 6. Zircon U–Pb concordia diagram of the dated samplesof Neoproterozoic granitoid. (A) Zircon grains from sample M6of Antanimbary granitoid; (B) zircon grains from sample M8 ofAntasakoamamy granitoid; and (C) zircon grains from sampleM1 of Antasakoamamy granitoid.

Table

2.Resultsof

zircon

U–P

bdatin

gof

sampleM8from

theAntasakoamam

ygranito

ids.

Isotop

icratio

sAge

(Ma)

Spo

tTh(ppm

)U

(ppm

)Th/U

207Pb/

206Pb

1δ207Pb/

235U

1δ206Pb/

238U

1δ207Pb/

206Pb

1δ207Pb/

235U

1δ206Pb/

238U

1δ

136

162.17

0.06

50.00

41.14

40.09

00.119

0.00

278

713

777

443

728

132

2310

2.30

0.06

70.01

41.14

20.26

60.12

20.00

783

343

577

412

774

242

312

127

4.49

0.06

60.00

21.08

60.03

50.12

00.00

179

663

747

1773

18

439

182.16

0.06

50.00

51.04

60.08

40.117

0.00

478

316

472

742

715

235

8145

1.78

0.06

90.00

51.15

60.07

20.12

20.00

289

814

478

034

744

126

2513

1.93

0.06

80.00

51.12

30.09

40.119

0.00

385

915

476

545

724

177

1910

1.90

0.06

50.00

61.12

00.115

0.12

00.00

578

118

376

355

730

298

1912

1.58

0.06

70.00

51.13

80.09

50.12

30.00

485

015

677

245

745

239

3420

1.70

0.06

60.00

51.09

10.08

90.118

0.00

281

315

674

943

716

1010

159

1.59

0.06

40.00

61.05

10.10

90.117

0.00

475

019

772

954

712

2411

148

1.70

0.06

50.00

51.13

50.12

60.12

00.00

478

916

677

060

733

2512

1.66

0.2

8.30

0.62

10.15

430

.358

24.945

0.56

20.44

345

5841

934

981103

2875

1878

131.76

1.16

1.52

0.39

20.06

914

.105

3.29

00.26

70.04

338

7926

827

5722

515

2521

814

116

1.77

0.21

20.01

24.97

80.42

30.17

10.011

2920

9518

1672

1020

5815

168

2.00

0.45

50.011

16.96

0.83

10.26

10.00

841

0336

2933

4714

9342

International Geology Review 1639

variable 176Hf/177Hf ratios between 0.281815 and 0.281908,corresponding εHf(t) values from −18.2 to −15.1 (calculatedat t = 746.9 Ma) (Figure 7), with a mean of −16.67.Corresponding TDM is calculated at 2055–1942 Ma.

Lu–Hf analyses were obtained for zircon from theAntasakoamamy granitoid. The results are given in Table 4.The 176Lu/177Hf ratios range from 0.000439 to 0.003870,with a mean of 0.001367, indicating that these zircons arevery weak in radiogenic Hf. Eleven igneous zircons of M8gave 176Hf/177Hf ratios ranging from 0.281865 to 0.281977.Computations based on crystallization ages (728.9 Ma)of the magmas yielded εHf(t) between −16.7 and −13.2(Figure 7), with a mean of −14.9. Corresponding TDM iscalculated at 2037–1804 Ma. Five igneous zircons of M1gave 176Hf/177Hf ratios ranging from 0.281250 to 0.281452.Computations based on crystallization ages (726.5 Ma) ofthe magmas yielded εHf(t) between −38.5 and −32.6, with amean of −35.6 (Figure 7). Corresponding TDM is calculatedat 2811–2633 Ma.

4.3. Whole-rock geochemistry

4.3.1 Major elements

The whole-rock major and trace element compositions of theNeoproterozoic granitoid analysed during this study are givenin Table 5. The Antanimbary granitoid contains high concen-trations of SiO2 (73.20–73.91 wt.%, average 73.60 wt.%),Al2O3 (14.22–14.71 wt.%; average, 14.35 wt.%), and K2O(5.61–5.68 wt.%, average 5.63 wt.%), low concentrations ofTiO2 (0.12–0.16 wt.%; average 0.14 wt.%) and MgO (0.10–0.11 wt.%; average 0.11 wt.%), and moderate Na2O concen-trations (3.94–4.22 wt.%, average 4.07 wt.%) and Na2O/K2Oratios (Na2O/K2O = 0.70–0.75, average 0.72). All of thesesamples are classified as granites on a total alkali versus silica(TAS) diagram (Figure 8A) and as shoshonitic on a K2Oversus SiO2 diagram (Figure 8B). These samples all plotalong the boundary betweenmetaluminous and peraluminousgranites and within the I-type granite field, using a molar ratioAl/(Na + K) (A/NK) versus Al/(Ca + Na + K) (A/CNK oralumina saturation index) diagram (Figure 8C).

Quartz monzonite samples from the centralAntasakoamamy granitoid pluton (M8-1, M8-2, M8-3,M9-1, and M9-2) range from 62.87 to 65.86 wt.% for SiO2,from 15.95 to 16.52 wt.% for Al2O3, from 0.63 to 0.76 wt.%for TiO2, from 2.97 to 4.00 wt.% for CaO, from 0.22 to0.34 wt.% for P2O5; have high K2O (2.98–3.79 wt.%),Na2O (4.26–4.66 wt.%), and K2O + Na2O (8.34–8.39 wt.%); and have low MgO (1.34–1.59 wt.%). Allthe samples of the granitoid display a Na-rich character-istic (Na2O/K2O > 1). All of these samples are classified asquartz monzonite on a TAS diagram (Figure 8A) and havehigh-K calc-alkaline characteristics, as seen on a K2Oversus SiO2 diagram (Figure 8B). All samples are metalu-minous and within the I-type granite field, using a molarTa

ble3.

Resultsof

zircon

U–P

bdatin

gof

sampleM1from

theAntasakoamam

ygranito

ids.

Isotopic

ratio

sAge

(Ma)

Spo

tTh

UTh/U

207Pb/

206Pb

1δ207Pb/

235U

1δ206Pb/

238U

1δ207Pb/

206Pb

1δ207Pb/

235U

1δ206Pb/

238U

1δ

110

022

00.46

0.06

80.00

11.119

0.01

70.119

0.00

186

522

763

872

75

220

171.12

0.06

70.00

21.09

50.04

30.119

0.00

383

369

751

2172

515

331

251.27

0.06

70.00

41.11

0.07

60.12

0.00

383

513

275

837

731

184

3515

2.44

0.06

60

1.09

10.05

10.119

0.00

681

713

749

2572

732

547

281.72

0.06

40.00

11.04

50.02

30.119

0.00

173

344

726

1172

56

621

314

01.52

0.16

010

.155

0.07

70.46

10.00

324

545

2449

724

4515

7114

911.25

0.14

60.00

17.08

10.08

80.35

20.00

423

026

2122

1119

4321

859

670.88

0.16

30.00

19.90

30.111

0.44

20.00

524

855

2426

1023

5822

1640 X.-A. Yang et al.

ratio Al/(Na + K) (A/NK) versus Al/(Ca + Na + K) (A/CNK or alumina saturation index) diagram (Figure 8C).

Granite samples from southern marginal facies of theAntasakoamamy granitoid pluton (M1, M2, M3, M4, andM5) (Figure 2) range from 69.59 to 71.57 wt.% for SiO2,from 14.85 to 16.33 wt.% for Al2O3, from 0.25 to0.27 wt.% for TiO2, from 1.85 to 2.04 wt.% for CaO,and 0.09 wt.% for P2O5; have high K2O (3.60–4.45 wt.%),Na2O (3.82–4.96 wt.%), and K2O +Na2O (8.24–8.61 wt.%);and have low MgO (0.03 wt.%). All the samples of the

granitoid display a Na-rich characteristic (average Na2O/K2O > 1). All of these samples are classified as graniteson a TAS diagram (Figure 8A) and as high-K calc-alkaline on a K2O versus SiO2 diagram (Figure 8B).All samples are peraluminous and within the I-typegranite field, using a molar ratio Al/(Na + K) (A/NK)versus Al/(Ca + Na + K) (A/CNK or alumina saturationindex) diagram (Figure 8C).

4.3.2 Trace elements

The Antanimbary granitoid contains high Rb (116–123ppm), moderate Nb (12.0–15.2 ppm) and Ta (0.9–1.2ppm), and low Sr (158–168 ppm), Nd (5.9–9.8 ppm),and Ni (4.8-8.3 ppm), leading to the relatively high Rb/Sr (0.70–0.75) and Sr/Nd (17–28) ratios. Samples of thegranitoid have low total REE contents ranging from 46.5ppm to 72.3 ppm, and they show moderate enrichment oflight rare earth elements (LREEs) relative to heavy rareearth elements (HREEs) [(La/Yb)N = 8.40–9.22, average8.89], with negative Eu anomalies (Eu/Eu* = 0.49–0.59)(Figure 9A), and their MREE–HREE patterns are flat andindeed curve up slightly towards Yb and Lu. Primitive-mantle-normalized trace element variation diagrams forthe granitoid (Figure 9B) are enriched in the large ionlithophile elements (Rb, Ba, Th, and K) and significantlydepleted in the high field strength elements (P and Ti). TheLILE enrichments of the Imorona–Itinsdo Suite weremade by Tucker et al. (2014).

Table 4. Zircon Hf isotopic data for the Neoproterozoic granitoids.

Sample Age (Ma) 176Hf/177Hf 2σ 176Lu/177Hf 2σ 176Yb/177Hf 2σ ƐHf(t) TDM fLu/Hf

M6-1 746.9 0.281846 0.000018 0.001568 0.000018 0.046115 0.000363 −17.1 2003 −0.95M6-2 746.9 0.281895 0.000021 0.002418 0.000012 0.073878 0.000283 −15.8 1980 −0.93M6-3 746.9 0.281854 0.000018 0.001091 0.000012 0.037081 0.000317 −16.6 1968 −0.97M6-4 746.9 0.281857 0.000020 0.002118 0.000049 0.065437 0.001073 −16.9 2018 −0.94M6-5 746.9 0.281908 0.000019 0.002033 0.000019 0.065253 0.000663 −15.1 1942 −0.94M6-6 746.9 0.281815 0.000020 0.001715 0.000045 0.060646 0.001414 −18.2 2055 −0.95M6-7 746.9 0.281855 0.000016 0.002159 0.000017 0.061087 0.000410 −17.0 2023 −0.93M8-1 728.9 0.281900 0.000022 0.001404 0.000026 0.057086 0.001529 −15.5 1920 −0.96M8-2 728.9 0.281950 0.000021 0.000439 0.000007 0.014546 0.000288 −13.2 1804 −0.99M8-3 728.9 0.281977 0.000019 0.002552 0.000064 0.107712 0.003015 −13.3 1869 −0.92M8-4 728.9 0.281882 0.000022 0.000445 0.000027 0.015947 0.001079 −15.6 1897 −0.99M8-5 728.9 0.281894 0.000017 0.000712 0.000005 0.022261 0.000332 −15.3 1893 −0.98M8-6 728.9 0.281947 0.000023 0.000941 0.000022 0.034612 0.000759 −13.6 1831 −0.97M8-7 728.9 0.281914 0.000018 0.000948 0.000011 0.032938 0.000472 −14.7 1877 −0.97M8–8 728.9 0.281924 0.000023 0.000686 0.000007 0.023371 0.000282 −14.3 1851 −0.98M8-9 728.9 0.281905 0.000026 0.001467 0.000020 0.052163 0.000659 −15.3 1915 −0.96M8-10 728.9 0.281865 0.000023 0.000670 0.000011 0.023535 0.000505 −16.4 1931 −0.98M8-11 728.9 0.281892 0.000020 0.003372 0.000047 0.133523 0.001567 −16.7 2037 −0.90M1-1 726.5 0.281328 0.000024 0.00126 0.000009 0.125761 0.001357 −35.709 2703 −0.96M1-2 726.5 0.281351 0.000019 0.000719 0.000020 0.025326 0.000429 −34.609 2633 −0.98M1-3 726.5 0.281452 0.000024 0.003870 0.000104 0.214565 0.006181 −32.552 2718 −0.88M1-4 726.5 0.281296 0.000019 0.001111 0.000020 0.051021 0.001425 −36.746 2735 −0.97M1-5 726.5 0.281250 0.000025 0.001281 0.000009 0.058132 0.000511 −38.475 2811 −0.96

Note: λ = 1.867 × 10−11year−1 (Söderlund et al. 2004). © [Söderlund]. Reproduced by permission of Söderlund.

Figure 7. Histogram showing the distribution of ƐHf(t) valuesfor all analysed zircon grains from the Neoproterozoic granitoids.

International Geology Review 1641

Table 5. Major element (wt.%), trace element (ppm), and REE (ppm) composition of the Neoproterozoic granitoids.

From the central part of theAntasakoamamy pluton

From the southern part of theAntasakoamamy pluton

Sample M6-1 M6-2 M7-1 M7-2 M7-3 M8-1 M8-2 M8-3 M9-1 M9-2 M1 M2 M3 M4 M5

SiO2 73.91 73.65 73.66 73.57 73.20 65.54 62.87 65.86 65.34 65.84 69.59 70.88 71.36 71.57 71.40TiO2 0.14 0.14 0.13 0.16 0.12 0.69 0.76 0.66 0.68 0.63 0.27 0.25 0.26 0.26 0.27Al2O3 14.22 14.34 14.22 14.28 14.71 16.22 16.52 15.95 15.95 16.08 16.33 15.53 14.87 14.87 14.85Fe2O3 0.34 0.32 0.38 0.44 0.35 1.63 1.70 1.42 1.39 1.40 0.77 0.73 0.90 0.84 0.76FeO 0.63 0.73 0.67 0.66 0.59 2.59 3.47 2.67 2.73 2.79 1.52 1.51 1.36 1.45 1.55MnO 0.01 0.01 0.02 0.01 0.01 0.04 0.05 0.04 0.04 0.04 0.03 0.03 0.03 0.03 0.03MgO 0.10 0.11 0.11 0.11 0.10 1.47 1.59 1.34 1.34 1.38 0.50 0.50 0.56 0.55 0.57CaO 0.78 0.81 0.77 0.81 0.76 2.97 4.00 3.08 3.31 3.02 2.04 1.91 1.89 1.85 1.86Na2O 3.94 3.97 3.99 4.21 4.22 4.43 4.66 4.29 4.26 4.28 4.96 4.67 4.17 4.04 3.82K2O 5.61 5.61 5.68 5.62 5.64 3.69 2.98 3.66 3.79 3.36 3.65 3.60 4.25 4.20 4.45P2O5 0.05 0.05 0.05 0.05 0.05 0.29 0.34 0.27 0.28 0.22 0.09 0.09 0.09 0.09 0.09LOI 0.17 0.15 0.21 0.10 0.15 0.34 0.56 0.17 0.37 0.37 0.15 0.21 0.13 0.15 0.25Total 99.88 99.90 99.89 99.89 99.89 99.89 99.89 99.89 99.89 99.89 99.89 99.90 99.88 99.90 99.89FeOT 2.17 2.23 2.20 2.19 2.20 2.21 2.20 2.20 2.21 2.20 2.21 2.17 2.17 2.21 2.23Na2O + K2O 9.55 9.58 9.67 9.83 9.86 8.12 7.64 7.95 8.05 7.64 8.61 8.27 8.42 8.24 8.27K2O/Na2O 1.42 1.41 1.42 1.33 1.34 0.83 0.64 0.85 0.89 0.79 0.74 0.77 1.02 1.04 1.16Mg# 0.29 0.32 0.30 0.30 0.31 0.31 0.30 0.30 0.31 0.30 0.29 0.29 0.32 0.31 0.31A/NK 1.13 1.14 1.12 1.10 1.13 1.44 1.52 1.45 1.44 1.51 1.35 1.34 1.30 1.33 1.34A/CNK 1.02 1.02 1.01 0.99 1.02 0.97 0.91 0.96 0.93 0.99 1.03 1.03 1.00 1.02 1.03T(Zr) 743 759 751 750 753 753 752 751 753 752 748 743 752 752 759P 205 214 197 227 197 1271 1498 1179 1232 970 380 384 406 402 397Ti 851 845 773 965 743 4149 4550 3963 4089 3795 1607 1475 1577 1535 1601Au 1.05 1.15 1.01 1.38 1.15 1.25 2.19 1.42 1.69 1.65 1.25 1.55 1.25 1.08 1.42Ag 0.08 0.07 0.07 0.038 0.067 0.22 1.86 0.169 0.2 0.464 2.91 0.38 0.09 0.30 0.41Ba 328 344 346 330 338 3632 2974 4011 4030 3373 2387 1928 1911 1847 1960Rb 116 118 118 120 123 27 24 26 26 23 67 54 74 71 83Sr 159 168 162 167 164 816 919 914 894 826 605 501 375 347 348Ta 1.2 1.2 0.9 1.2 1.2 0.3 0.3 0.2 0.2 0.2 0.7 0.5 0.7 0.7 0.8Nb 14.1 14.1 12.0 15.2 13.8 6.3 7.5 5.2 6.1 4.9 6.9 5.7 9.1 8.8 11.3Hf 3.7 5.0 3.7 4.0 4.2 2.5 2.5 1.9 2.0 1.9 2.4 2.6 4.8 4.7 5.0Zr 79 106 81 88 89 73 71 53 56 54 90 92 143 140 160Y 8.5 8.1 6.8 9.8 5.5 9.0 11.0 8.5 9.8 8.1 6.6 4.8 11.8 11.4 14.0Sc 1.9 2.0 1.8 2.0 1.9 5.4 8.2 6.0 6.0 5.7 2.4 2.0 2.8 2.7 3.2V 6.6 7.0 6.2 7.1 6.4 59.0 70.0 59.8 60.8 56.9 21.3 17.9 19.2 17.5 19.8Co 1.3 1.5 1.3 1.3 1.3 11.2 12.1 10.4 9.6 9.4 4.3 3.6 4.0 3.8 4.6Ni 4.8 8.3 5.1 5.3 5.2 20.3 23.2 21.0 20.7 19.3 7.7 14.2 10.4 8.0 9.6Ga 15.8 16.7 15.4 16.3 16.2 16.3 18.7 17.3 16.9 16.8 19.4 16.2 17.7 16.9 20.0Pb 17.6 18.5 18.2 18.7 18.6 15.3 93.6 17.4 23.6 31.5 27.1 24.2 24.9 25.8 32.8Th 6.0 6.5 4.6 13.0 10.9 1.1 1.1 1.6 1.3 1.4 2.9 2.1 4.9 5.0 5.6U 0.8 0.9 0.7 1.2 1.4 0.2 0.3 0.2 0.2 0.2 0.2 0.3 0.8 0.8 0.9La 14.3 14.4 11.5 16.9 10.8 27.0 37.6 66.6 54.6 56.6 38.7 28.2 48.7 48.1 56.3Ce 29.1 28.3 21.2 34.6 27.7 54.1 73.1 98.1 87.4 84.7 58.7 43.5 76.1 74.2 86.9Pr 2.8 2.7 2.1 3.3 1.9 6.3 8.2 9.2 8.6 7.9 6.0 4.3 7.2 7.0 8.3Nd 9.0 8.3 6.5 9.8 5.9 23.6 32.8 32.5 32.2 28.5 19.4 13.8 24.9 24.8 28.7Sm 1.4 1.3 1.0 1.5 0.9 3.2 4.5 3.7 3.9 3.4 2.7 2.0 3.6 3.6 4.1Eu 0.2 0.2 0.2 0.3 0.2 0.9 1.4 1.1 1.1 0.9 0.6 0.4 0.6 0.6 0.7Gd 1.4 1.3 1.1 1.6 1.0 3.2 4.3 4.3 4.3 3.8 2.7 2.0 3.9 3.7 4.4Tb 0.2 0.2 0.1 0.2 0.1 0.4 0.5 0.4 0.5 0.4 0.3 0.2 0.5 0.5 0.5Dy 1.1 1.0 0.8 1.2 0.7 1.5 2.1 1.5 1.7 1.4 1.2 0.8 2.1 2.1 2.3Ho 0.2 0.2 0.2 0.3 0.2 0.3 0.4 0.3 0.3 0.3 0.2 0.1 0.4 0.4 0.4Er 0.8 0.8 0.6 0.9 0.6 0.9 1.2 1.0 1.1 0.9 0.6 0.5 1.2 1.2 1.3Tm 0.2 0.2 0.1 0.2 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2Yb 1.1 1.2 0.9 1.3 0.9 0.8 1.1 0.8 0.9 0.8 0.5 0.4 1.1 1.1 1.1Lu 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2LREE/HREE 11.07 10.97 10.81 11.35 12.77 15.56 15.75 24.78 20.97 23.32 22.51 22.49 17.00 17.30 17.67(La/Yb)N 9.08 8.68 9.22 9.05 8.40 22.97 25.21 62.37 45.12 53.49 57.83 55.27 31.47 32.86 35.42δEu 0.51 0.49 0.53 0.56 0.59 0.89 0.94 0.81 0.84 0.76 0.68 0.58 0.52 0.51 0.52

1642 X.-A. Yang et al.

Quartz monzonite samples from central facies of theAntasakoamamy granitoid pluton (M8-1, M8-2, M8-3,M9-1, and M9-2) contain high Ba (2974–4030 ppm),

moderate Sr (816‒919 ppm) and Nd (23.6–32.8 ppm),and low Rb (23–27 ppm), Nb (4.9–7.5 ppm), Ta (0.2–0.3ppm), and Ni (19.2–23.2 ppm), leading to the low Rb/Sr(0.03) and relatively high Sr/Nd (28–35) ratios. They havetotal REE contents varying from 122.4 ppm to 219.7 ppm,and the REE are highly fractionated ((La/Yb)N = 22.97–62.37, average 41.83).

Granite samples from southern marginal facies of theAntasakoamamy granitoid pluton (M1, M2, M3, M4, andM5) contain high Ba (1847–2387 ppm), moderate Sr(347–605 ppm) and Nd (13.8–23.7 ppm), and low Rb(54–83 ppm), Nb (5.7–11.3 ppm), Ta (0.5–0.8 ppm), andNi (7.7–14.2 ppm), leading to the low Rb/Sr (0.11–0.24)and relatively high Sr/Nd (12.13–36.30) ratios. They havetotal REE contents varying from 96.4 ppm to 195.4 ppm,and the REEs are highly fractionated ((La/Yb)N = 31.47–57.83, average 42.57).

The Antasakoamamy granitoids are enriched in lightrare earth elements (LREEs), and the samples have a nega-tive Eu anomaly indicating fractionation of plagioclase.Chondrite-normalized REE patterns for these sampleshave steep LREE slopes and flat and low heavy rare earthelement (HREE) patterns (Figure 9C). The HREE deple-tions present within these granitoids may indicate removalof garnet or another HREE-enriched phase. Primitive-man-tle-normalized trace element variation diagrams for thegranitoid (Figure 9D) are enriched in the large ion lithophileelements (Ba, K, and La) and significantly depleted in Rb,Th, U, Ta, Nb, P, and Ti, with the Nb, Ta, and Ti depletions.

5. Discussion

5.1. Mixing or fractional crystallization

The narrow εHf(t) values (from −18.2 to −15.1) and theirHf model ages (2.06–1.94 Ga) of the Antanimbary gran-itoid suggest a mixture of both primary mantle(Cryogenian) and inherited crustal (Neoarchaean) sources.The granitoids have negative Eu anomalies (Figure 9A),which could be because of the result of plagioclaseremoval during fractional crystallization. Additionally,the fractionation of apatite and ilmenite and/or titanite isalso recorded by variable degrees of negative P and Tianomalies (Figure 9B). The granitoid shows positive Zranomalies, possibly the result of lower crustal contamina-tion during its ascent (Sun and McDonough 1989).

The Antasakoamamy granitoids are composed of tworock types, including granite and quartz monzonite, andhave wide SiO2 contents ranging from 62.87 to 71.57 wt.%. The diversity of the granitoid may have resulted from afractional crystallization process. The granitoid samples dis-play wide ranges in Eu/Eu* values (0.51–0.94), suggestingthat the diversity of the granitoids was partially caused byfeldspar fractionation. In addition, the granitoid are stronglydepleted in Ta, Nb, P, and Ti (Figure 9D), indicative of

Figure 8. Classification of the studied granitoids on the basis of (A)the TAS diagram (Middlemost 1994); (B) K2O-SiO2 diagram; and(C) Al2O3/(Na2O+K2O) molar-Al2O3/(CaO+Na2O+K2O) molar.

International Geology Review 1643

differentiation during the formation of the granite. Thesmall P and Ti anomalies present within these granitoidsmay also be indicative of the early formation and separationof apatite and ilmenite, magnetite, and/or titanite, or varia-tions in the source of the magmas that formed the granitoid.Narrow εHf(t) values (from −16.7 to −13.2) of the quartzmonzonite and their calculated crustal model ages indicatethat they sourced recycled crust with model ages of 2.04–1.80 Ga. However, the εHf(t) values (from −38.5 to −32.6)of the granite samples from the southern marginal facies ofthe Antasakoamamy granitoid pluton and their calculatedcrustal model ages suggest that the granite could have beenderived from reworked ancient lower crustal rocks withmodel ages of 2811–2633 Ma. Rocks of this age areknown in the Andriamena belt of central Madagascar(Kabete et al. 2006), implying that magma mixed with orwas derived from older lower crust.

5.2. Petrogenesis of Neoproterozoic Antanimbarygranitoid and Antasakoamamy granitoid

The Antanimbary granitoid and Antasakoamamy granitoidin the study area have lower Mg# (Mg# = Mg2+/(Mg2+ +

TFe3+) × 100) values (29–32) and lower MgO (0.10–0.11 wt.% and 0.50–1.59 wt.%, respectively), Ni (4.8–8.3 and 7.7–23.2 ppm, respectively) and Sr (159–168and 347–919 ppm, respectively) concentrations, suggest-ing that these granitoids probably did not generate atsignificant depths and under high heat flow conditions bymelting of subducting plate and the mantle wedge. Meltsderived from the subducting plate are ubiquitously con-taminated by high Mg# mantle peridotite, causing themelts to be enriched in MgO and Ni (Smithies 2000;Condie and Kröner 2008); studied granitoids might haveformed by a mixing between crustal and mantle-derivedmagmas. Contribution from older crust is also supportedby the predominantly Proterozoic Hf-TDM ages of thesezircons (2.04–1.80 Ga). Therefore, these granitoids couldhave been formed by melting of crust of Palaeoproterozoictonalite–trondhjemite–granodiorite (TTG) composition(McMillan et al. 2003).

The Antanimbary granitoid contains modest amountsof Nb (12.0–15.2 ppm), Ta (0.9–1.2 ppm), and Zr (79–106ppm). The Antasakoamamy granitoid contains low Nb(4.9–11.3 ppm) and Ta (0.2–0.8 ppm) and moderate Zr(53–160 ppm). As stated earlier, both granitoids were the

Figure 9. Chondrite-normalized REE patterns and primitive-mantle-normalized spider diagrams for the Neoproterozoic granitoidsamples. REE abundances for chondrites and trace element abundance for primitive mantle are after Sun and McDonough (1989). (A)Chondrite-normalized REE pattern of Antanimbary granitoid; (B) primitive-mantle-normalized spider diagram of Antanimbary granitoid;(C) chondrite-normalized REE pattern of Antasakoamamy granitoid; and (D) primitive-mantle-normalized spider diagram ofAntasakoamamy granitoid.

1644 X.-A. Yang et al.

result of a mixture of both primary mantle (Cryogenian)and Palaeoproterozoic TTG sources. Nevertheless, thepetrogenesis of the Antasakoamamy granitoid was differ-ent from the Antanimbary granitoid. The 747 ± 9 MaAntanimbary granitoid lacks any Nb-Ta anomaly and haspositive Zr anomaly, whereas the 729 ± 9 MaAntasakoamamy granitoid has a negative Nb-Ta anomaly,suggesting that it was likely related to subduction. TheAntasakoamamy granitoid exhibits strongly negative εHf(t)values (from −16.7 to −13.2; from −38.5 to −32.6), indi-cating a significant contribution of old crustal material andprecluding an origin through partial melting of subductedoceanic lithosphere or slab-melt-modified peridotitic man-tle wedge (Martin et al. 2005). This implies that thegranitoid could have been generated by partial melting ofthickened lower crust.

5.3. Tectonic significance

The geochemistry of igneous rocks bear a close relation-ship to their tectonic setting of formation. TheAntanimbary granitoid plots within the post-collisionalgranite fields within the Yb + Ta versus Rb (Figure 10A)tectonic discrimination diagram. The same conclusion issupported using the multicationic R1‒R2 [R1 = 4Si4+-11(Na++K+)-2(Fe3++Ti4+), molar; R2 = 6Ca2++2Mg2++Al3+,molar] tectonic discrimination diagram, with samplesplotted within the post-orogenic areas of the diagram(Figure 10B). The Antanimbary granitoid compositionsare consistent with formation in an extensional environ-ment. In the Andriamena Belt, between 820 and 785 Ma,the basement rocks formed by extension associated withextensive magmatism (Kabete et al. 2006). Li et al. (2008)proposed that widespread continental rifting occurredbetween ca. 825 and 740 Ma. Tucker et al. (2011a) sug-gested that the Itsindro–Imorona Suite (840‒760 Ma) andthe formation of two long, narrow belts of continentalsediments (Ambatolampy and Manampotsy) are the pro-ducts of continental dilation (and pressure-release meltingof upwelling lithosphere). The Antanimbary granitoidlacks a significant Nb-Ta anomaly, and the granitoids dis-play K-rich (K2O/Na2O > 1) and alkaline characteristics.These features are incompatible with subduction-relatedgranites. The age of the Antanimbary granitoid is747 Ma, suggesting that it formed in this earlyCryogenian extensional setting. We conclude that theAntanimbary granitoid is the product of extension, perhapsas a result of underplating of mantle plume-derived mag-mas that triggered partial melting of lower crust (Tuckeret al. 2014).

On the Yb + Ta versus Rb and R1‒R2 (Figure 10)tectonic discrimination diagrams, the 729–727 MaAntasakoamamy granitoid falls within the fields of volca-nic arc granite and, straddling the post-collision uplift,late-orogenic and syn-collision areas of the diagram,

indicating that the granitoid was emplaced in a convergentmargin setting. This contraction thickened the crust topromote deep melting of the lower crust after 729‒727 Ma. The Antasakoamamy granitoid exhibits stronglynegative ƐHf(t) values (from −16.7 to −13.2; from −38.5 to−32.6), precluding an origin through partial melting ofsubducted oceanic lithosphere or slab-melt-modified peri-dotitic mantle wedge alone. However, the granitoid showsa pronounced negative Nb, Ta anomaly, and small P andTi anomalies (Figure 9D), and most samples display Na-rich (average Na2O/K2O > 1) characteristics. The geo-chemistry of the Antasakoamamy granitoid is thus differ-ent from the Antanimbary granitoid. These features of the

Figure 10. (A) Trace element tectonic discrimination diagramsof the Neoproterozoic granitoids studied here (fields are afterPearce et al. 1984); (B) R1‒R2 multicationic variation diagram[R1= 4Si4+—11(Na++K+)-2(Fe3++Ti4+), molar; R2 = 6Ca2++2Mg2+

+Al3+, molar]. VAG, volcanic arc granite; Syn-COLG, syn-collisiongranite; WPG, within-plate granite; ORG, oceanic ridge granite;Post-COLG, post-collision granite.

International Geology Review 1645

Antasakoamamy granitoid can indicate that the source ofthese magmas contained subduction-related material.

Thomas et al. (2009) proposed that the Bemarivodomain is divided into northern and southern ‘terranes’;U–Pb zircon studies had revealed a bimodal age distribu-tion of the plutonic rocks, predominantly 750 Ma in thesouth and 718‒705 Ma in the north. Moreover, Thomaset al. (2009) suggested that some of these igneous rocksshow the geochemical characteristics of subduction-relatedmagmatism. The ages of the Antasakoamamy granitoidoverlap the ages of other Cryogenian igneous rocks thatare related to subduction. Thus, the bimodality of plutonicrock ages indicates that the Imorona–Itsindro Suite can bebroadly divided into two age domains (Figure 11). Thesetwo igneous pulses reflect crustal delamination, astheno-spheric upwelling, and crustal extension before ~747 Ma,followed by lithospheric subduction and arc magmatismaround 729‒727 Ma.

There are two quasi-parallel, N-striking belts of meta-clastic rocks cover the Antananarivo domain in Madagascar.The eastern belt is outlined by the Manampotsy Group,which is 800 km long and extends from Manakara throughto Bealanana. The western belt is outlined by theAmbatolampy Group, which is 30 km wide and extendsfrom southern to northern Madagascar (Tucker et al. 2014).Based on the depositional age of the Manampotsy Group(840–780 Ma) and igneous rocks of the Imorona-ItsindroSuite that intrude it, Tucker et al. (2014) inferred that deposi-tion of the Manampotsy Group occurred in a long, narrowdepositional basin that formed during early Cryogenian con-tinental extension and magmatism.

The recently acquired maximum deposition age is lessthan ~650 Ma, and the minimum deposition age of theAmbatolampy Group is ~560 Ma (Tucker et al. 2014).Igneous rocks of the Imorona–Itsindro Suite intrude theAmbatolamlpy Group. There are lit-par-lit relationshipsbetween the Ambatolamlpy Group and the 840–760 MaImorona–Itsindro Suite (Tucker et al. 2014), so theAmbatolamlpy Group must be older. Tucker et al. (2014)infer that deposition of the Ambatolampy Group wasdiachronous throughout the late Neoproterozoic, occurringboth before and after the youngest members of theImorona–Itsindro Suite (750–700 Ma) were emplaced.Tucker et al. (2011a) suggest that metaigneous rocks(Itsindro–Imorona Suite) formed during a period of con-tinental extension and intrusive igneous activity between840 and 760 Ma. Thomas et al. (2009) suggest that theCryogenian igneous rocks of the Bemarivo domain (750–708 Ma) were generated above a subduction zone withinthe palaeo-Mozambique Ocean outboard of cratonicMadagascar. Tucker et al. (2014) propose that Gondwanaamalgamated and shortened in Ediacaran–Cambrian time(560–520 Ma), and younger orogenic convergence (560–520 Ma) resulted in east-directed overthrusting throughoutsouth Madagascar, steepening with local inversion of thedomain in central Madagascar (Tucker et al. 2011a). Weinfer that deposition of the Ambatolampy Group occurredin an extensive depositional basin that formed during aperiod of continental extension and magmatism before750 Ma and a period of subsequent subduction and mag-matism in Neoproterozoic time.

Handke et al. (1999) reported U–Pb ages along a450 km-long belt of gabbroic and granitoid plutons fromAmbositra to Maevatanana within west-centralMadagascar; these gabbroic and granitoid plutons formedat 804–776 Ma. They suggested that this belt representedthe root of a continental magmatic arc on the westernmargin of a rifting Rodinia and constrained the criticalperiod of Rodinia’s transformation into Gondwana.However, McMillan et al. (2003) proposed that the gab-broic and granitic components evolved from melting of anenriched subcontinental mantle to cause advective heatingand anatexis at the base of thickened continental crust,implying that the generation of both the Antanimbarygranitoid from the Maevatanana and the gabbroic andgranitoid plutons between Ambositra and Maevatananathrough partial melting of lower crust was most likelytriggered by underplating of mantle-derived magmas.These igneous rocks show LREE-enriched patterns withno negative Eu anomalies; some samples show slightHREE enrichment and weak Nb, Ta anomalies on normal-ized incompatible element patterns (Bybee et al. 2010).The ages of the Antanimbary granitoid presented hererecord the younger phase of this magmatic event, withthe gabbroic and granitoid plutons between Ambositraand Maevatanana in west-central Madagascar recording

Figure 11. Two-stage plate-tectonic model for the developmentof the Neoproterozoic granitoids studied here.Source: Modified from Key et al. (2011) and Tucker et al. (2011a).

1646 X.-A. Yang et al.

older magmatism. Tucker et al. (2011a) suggest that meta-igneous rocks of the Itsindro–Imorona Suite formed dur-ing continental extension and intrusive igneous activitybetween 840 and 760 Ma. Key et al. (2011) proposedthat Rodinia fragmented during the early Neoproterozoicwith intracratonic rifts that sometimes developed intooceanic basins, and middle Neoproterozoic smaller cra-tonic blocks subsequently collided. Our data demonstratea major transition from an extensional tectonic environ-ment before ~747 Ma to a contractional setting after 729‒727 Ma over a protracted period of almost 20 Ma, imply-ing that Rodinia fragmented before ~747 Ma, and subse-quent lithospheric subduction and collision of smallercratonic blocks.

6. Conclusions

(1) Zircon LA–ICP–MS dating of the Antanimbarygranitoid and Antasakoamamy granitoid indicatesthat these intrusions were emplaced at 747 and729–727 Ma, respectively.

(2) The Antanimbary granitoid was generated by par-tial melting of thinned lower crust, which wasmost likely triggered by underplating of mantleplume-derived magmas. The Antasakoamamygranitoids were generated by partial melting ofthickened lower crust, which was related tosubduction.

(3) The geochemistry of these granitoids suggestscrustal extension of the Imorona–Itsindro Suitebefore ~747 Ma followed by lithospheric subduc-tion and arc magmatism after 729‒727 Ma.

AcknowledgementsComments by R.D. Tucker and an anonymous reviewer contrib-uted to improving the manuscript. Edits in English by Dr RobertJ. Stern are much appreciated.

FundingThis research was jointly supported by the China PostdoctoralScience Foundation (project 2013M541000) and the ‘PreliminaryReconnaissance on the Tectonic Setting and Mineral ExplorationPotential of the Global Giant Metallogenic Belts’ project of theChina Geological Survey (CGS; project 12120113102100).

ReferencesAndersen, T., 2002, Correction of common lead in U–Pb ana-

lyses that do not report 204Pb: Chemical Geology, v. 192, p.59–79. doi:10.1016/S0009-2541(02)00195-X

Ashwal, L.D., 2002, Petrogenesis of Neoproterozoic granitoidsand related rocks from the Seychelles: The case for anAndean-type arc origin: Journal of Petrology, v. 43, p. 45–83. doi:10.1093/petrology/43.1.45

BGS-USGS-GLW, 2008. Final Report: Revision de la cartogra-phie géologique et minière des zones Nord et Centre deMadagascar (Zones A, B et D). Republique de Madagascar,Ministère deL’energie et des Mines (MEM/SG/DG/UCP/PGRM), 1049.

Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J.,Williams, I.S., and Foudoulis, C., 2004, Improved 206Pb/238U microprobe geochronology by the monitoring of atrace-element-related matrix effect; SHRIMP, ID–TIMS,ELA–ICP–MS and oxygen isotope documentation for a ser-ies of zircon standards: Chemical Geology, v. 205, p. 115–140. doi:10.1016/j.chemgeo.2004.01.003

Buchwaldt, R., Tucker, R.D., and Dymek, R.F., 2003,Geothermobarometry and U–Pb geochronology of metapeli-tic granulites and pelitic migmatites from the Lokoho region,Northern Madagascar: American Mineralogist, v. 88, p.1753–1768.

Bybee, G.M., Ashwal, L.D., and Wilson, A.H., 2010, Newevidence for a volcanic arc on the western margin of a riftingRodinia from ultramafic intrusions in the Andriamena region,north-central Madagascar: Earth and Planetary ScienceLetters, v. 293, p. 42‒53. doi:10.1016/j.epsl.2010.02.017

Chu, N.-C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella,R.M., Milton, J.A., German, C.R., Bayon, G., and Burton,K., 2002, Hf isotope ratio analysis using multi-collectorinductively coupled plasma mass spectrometry: An evalua-tion of isobaric interference corrections: Journal ofAnalytical Atomic Spectrometry, v. 17, p. 1567‒1574.doi:10.1039/b206707b

Collins, A.S., 2006, Madagascar and the amalgamation ofCentral Gondwana: Gondwana Research, v. 9, p. 3–16.doi:10.1016/j.gr.2005.10.001

Collins, A.S., Fitzsimons, I.C.W., Hulscher, B., andRazakamanana, T., 2003a, Structure of the eastern marginof the East African Orogen in central Madagascar:Precambrian Research, v. 123, p. 111‒133. doi:10.1016/S0301-9268(03)00064-0

Collins, A.S., Johnson, S., Fitzsimons, I.C.W., Powell, C.M.,Hulscher, B., Abello, J., and Razakamanana, T., 2003b,Neoproterozoic deformation in central Madagascar: A struc-tural section through part of the East African Orogen:Geological Society, London, Special Publications, v. 206,p. 363‒379. doi:10.1144/GSL.SP.2003.206.01.17

Collins, A.S., and Pisarevsky, S.A., 2005, Amalgamating easternGondwana: The evolution of the Circum-Indian Orogens:Earth Science Reviews, v. 71, p. 229–270. doi:10.1016/j.earscirev.2005.02.004

Condie, K.C., and Kröner, A., 2008, When did plate tectonicsbegin? Evidence from the geologic record: When Did PlateTectonics Begin on Planet Earth: Geological Society ofAmerica Special Papers, v. 440, p. 281–294. doi:10.1130/2008.2440(14)

de Wit, M.J., 2003, Madagascar: Heads its a continent, tails itsan island: Annual Review of Earth and Planetary Sciences,v. 31, p. 213–248. doi:10.1146/annurev.earth.31.100901.141337

Elhlou, S., Belousova, E., Griffin, W.L., Pearson, N.J., andO’Reilly, S.Y., 2006, Trace element and isotopic compositionof GJ-red zircon standard by laser ablation: Geochimica etCosmochimica Acta, v. 70, p. A158. doi:10.1016/j.gca.2006.06.1383

Fritz, H., Abdelsalam, M., Ali, K.A., Bingen, B., Collins, A.S.,Fowler, A.R., Ghebreab, W., Hauzenberger, C.A., Johnson,P.R., Kusky, T.M., Macey, P., Muhongo, S., Stern, R.J., and

International Geology Review 1647

Viola, G., 2013, Orogen styles in the East African Orogen: Areview of the Neoproterozoic to Cambrian tectonic evolu-tion: Journal of African Earth Sciences, v. 86, p. 65–106.doi:10.1016/j.jafrearsci.2013.06.004

Goodenough, K.M., Thomas, R.J., De Waele, B., Key, R.M.,Schofield, D.I., Bauer, W., Tucker, R.D., Rafahatelo, J.-M.,Rabarimanana, M., Ralison, A.V., and Randriamananjara, T.,2010, Post-collisional magmatism in the central East AfricanOrogen: The Maevarano Suite of north Madagascar: Lithos,v. 116, p. 18–34. doi:10.1016/j.lithos.2009.12.005

Handke, M.J., Tucker, R.D., and Ashwal, L.D., 1999,Neoproterozoic continental arc magmatism in west-centralMadagascar: Geology, v. 27, p. 351‒354. doi:10.1130/0091-7613(1999)027<0351:NCAMIW>2.3.CO;2

Hou, K.J., Li, Y.H., Zou, T.R., Qu, X.M., Shi, Y.R., and Xie, G.Q., 2007, Laser ablation‒MC‒ICPMS technique for Hf iso-tope microanalysis of zircon and its geological applications:Acta Petrologica Sinica, v. 23, p. 2595‒2604. [In Chinesewith English abstract].

Jacobs, J., and Thomas, R.J., 2004, Himalayan-type indenter-escape tectonics model for the southern part of the lateNeoproterozoic–early Paleozoic East African– Antarctic oro-gen: Geology, v. 32, p. 721‒724. doi:10.1130/G20516.1

Jöns, N., Schenk, V., Appel, P., and Razakamanana, T., 2006,Two-stage metamorphic evolution of the Bemarivo Belt ofnorthern Madagascar: Constraints from reaction textures andin situ monazite dating: Journal of Metamorphic Geology, v.24, p. 329–347.

Kabete, J., Groves, D., McNaughton, N., and Dunphy, J., 2006, Thegeology, SHRIMP U-Pb geochronology and metallogenic sig-nificance of the Ankisatra-Besakay District, Andriamena belt,northern Madagascar: Journal of African Earth Sciences, v. 45,p. 87‒122. doi:10.1016/j.jafrearsci.2006.01.008

Key, R.M., Pitfield, P.E.J., Thomas, R.J., Goodenough, K.M., DeWaele, B., Schofield, D.I., Bauer, W., Horstwood, M.S.A.,Styles, M.T., Conrad, J., Encarnacion, J., Lidke, D.J.,O’Connor, E.A., Potter, C., Smith, R.A., Walsh, G.J.,Ralison, A.V., Randriamananjara, T., Rafahatelo, J.-M., andRabarimanana, M., 2011, Polyphase Neoproterozoic orogen-esis within the East Africa–Antarctica Orogenic Belt in cen-tral and northern Madagascar: Geological Society, London,Special Publications, v. 357, p. 49‒68. doi:10.1144/SP357.4

Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., DeWaele, B., Ernst, R.E., Fitzsimons, I.C.W., Fuck, R.A.,Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S.,Natapov, L.M., Pease, V., Pisarevsky, S.A., Thrane, K., andVernikovsky, V., 2008, Assembly, configuration, and break-up history of Rodinia: A synthesis: Precambrian Research, v.160, p. 179‒210. doi:10.1016/j.precamres.2007.04.021

Ludwing, K.R., 2003, ISOPLOT 3.00: A Geochronologicaltoolkit for Microsoft Excel: Berkeley GeochronologyCenter Special Publication, 70 p.

Martin, H., Smithies, R.H., Rapp, R., Moyen, J.-F., andChampion, D., 2005, An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid:Relationships and some implications for crustal evolution:Lithos, v. 79, p. 1–24. doi:10.1016/j.lithos.2004.04.048

McMillan, A., Harris, N.B.W., Holness, M., Ashwal, L., Kelley,S., and Rambeloson, R., 2003, A granite–gabbro complexfrom Madagascar: Constraints on melting of the lower crust:Contributions to Mineralogy and Petrology, v. 145, p. 585–599. doi:10.1007/s00410-003-0470-1

Meert, J.G., 2003, A synopsis of events related to the assemblyof eastern Gondwana: Tectonophysics, v. 362, p. 1–40.doi:10.1016/S0040-1951(02)00629-7

Meert, J.G., Nédélec, A., Hall, C., Wingate, M.T.D., andRakotondrazafy, M., 2001, Paleomagnetism, geochronologyand tectonic implications of the Cambrian-age CarionGranite, central Madagascar: Tectonophysics, v. 340, p. 1‒21. doi:10.1016/S0040-1951(01)00163-9

Meert, J.G., and Van Der Voo, R., 1997, The assembly ofGondwana 800–550 Ma: Journal of Geodynamics, v. 23, p.223–235. doi:10.1016/S0264-3707(96)00046-4

Middlemost, E.A.K., 1994, Naming materials in the magma/igneous rock system: Earth-Science Reviews, v. 74, p.193–227.

Moine, B., 1968, Carte du massif schisto-quartzo-dolomitique,région d’Ambatofinadrahana, centre-ouest du socle cristallinprécambrien de Madagascar: Centre de l’InstitutGéographique National à Tananarive (Imprimeur), Sciencesde la terre, Nancy (Editeur), scale: 1:200,000 (color).

Moine, B., Bosse, V., Paquette, J.-L., and Ortega, E., 2014, Theoccurrence of a Tonian–Cryogenian (~850Ma) regionalmetamorphic event in Central Madagascar and the geody-namic setting of the Imorona–Itsindro (~800Ma) magmaticsuite: Journal of African Earth Sciences, v. 94, p. 58‒73.doi:10.1016/j.jafrearsci.2013.11.016

Nédélec, A., Stephens, W.E., and Fallick, A.E., 1995, ThePanafrican stratoid granites of Madagascar: Alkaline mag-matism in a post-collisional extensional setting: Journal ofPetrology, v. 36, p. 1367–1391. doi:10.1093/petrology/36.5.1367

Paquette, J.-L., and Nédélec, A., 1998, A new insight into Pan-African tectonics in the East–West Gondwana collision zoneby U–Pb zircon dating of granites from central Madagascar:Earth and Planetary Science Letters, v. 155, p. 45‒56.doi:10.1016/S0012-821X(97)00205-7

Pearce, J.A., Harris, N.B.W., and Tindle, A.G., 1984, Traceelement discrimination diagrams for the tectonic interpreta-tion of granitic rocks: Journal of Petrology, v. 25, p. 956–983. doi:10.1093/petrology/25.4.956

Rantoanina, M., Ramarokoto, A., Rabe, M., and Mara, J.B.,1969, Carte geologique Maevatanana (N42): 1/100.000.Service Geologique Madagascara, 1 color sheet.

Roig, J.Y., Tucker, R.D., Delor, C., Peters, S.G., and Théveniaut,H., 2012, Carte géologique de la République de Madagascarà 1/1,000,000: Ministère des Mines, PGRM, Antananarivo,République de Madagascar, 1 color sheet.

Smithies, R.H., 2000, The Archaean tonalite–trondhjemite–gran-odiorite (TTG) series is not an analogue of Cenozoic adakite:Earth and Planetary Science Letters, v. 182, p. 115–125.doi:10.1016/S0012-821X(00)00236-3

Söderlund, U., Patchett, P.J., Vervoort, J.D., and Isachsen, C.E.,2004, The 176Lu decay constant determined by Lu–Hf andU–Pb isotope systematics of Precambrian mafic intrusions:Earth and Planetary Science Letters, v. 219, p. 311–324.doi:10.1016/S0012-821X(04)00012-3

Stephens, W.E., Jemielita, R.A., and Davies, D., 1997, Evidencefor ca. 750 Ma intra-plate extensional tectonics from granitemagmatism on the Seychelles: New geochronological dataand implications for Rodinia reconstructions and fragmenta-tion: Terra Nova, v. 9, p. 166.

Stern, R.J., 1994, ARC Assembly and Continental Collision inthe Neoproterozoic East African Orogen: Implications for theConsolidation of Gondwanaland: Annual Review of Earthand Planetary Sciences, v. 22, p. 319–351. doi:10.1146/annurev.ea.22.050194.001535

Streckeisen, A., 1976, To each plutonic rock its proper name:Earth-Science Reviews, v. 12, p. 1–33. doi:10.1016/0012-8252(76)90052-0

1648 X.-A. Yang et al.

Sun, -S.-S., and McDonough, W.F., 1989, Chemical and isotopicsystematics of oceanic basalts: Implications for mantle com-position and processes: Geological Society, London, SpecialPublications, v. 42, p. 313‒345. doi:10.1144/GSL.SP.1989.042.01.19

Thomas, R.J., De Waele, B., Schofield, D.I., Goodenough, K.M.,Horstwood, M., Tucker, R., Bauer, W., Annells, R., Howard,K., Walsh, G., Rabarimanana, M., Rafahatelo, J.M., Ralison,A.V., and Randriamananjara, T., 2009, Geological evolutionof the Neoproterozoic Bemarivo Belt, northern Madagascar:Precambrian Research, v. 172, p. 279–300. doi:10.1016/j.precamres.2009.04.008

Torsvik, T., 2001a, Neoproterozoic geochronology and palaeo-geography of the Seychelles microcontinent: The India link:Precambrian Research, v. 110, p. 47–59. doi:10.1016/S0301-9268(01)00180-2

Torsvik, T., 2001b, Rodinia refined or obscured: Palaeomagnetismof the Malani igneous suite (NW India): Precambrian Research,v. 108, p. 319–333. doi:10.1016/S0301-9268(01)00139-5

Tucker, R.D., Ashwal, L.D., Handke, M.J., Hamilton, M.A., LeGrange, M., and Rambeloson, R.A., 1999, U-Pb geochronol-ogy and isotope geochemistry of the Archean andProterozoic rocks of north-central Madagascar: The Journalof Geology, v. 107, p. 135–153. doi:10.1086/314337

Tucker, R.D., Kusky, T.M., Buchwaldt, R., and Handke, M.J.,2007, Neoproterozoic nappes and superposed folding of theItremo Group, west-central Madagascar: GondwanaResearch, v. 12, p. 356–379. doi:10.1016/j.gr.2006.12.001

Tucker, R.D., Roig, J.Y., Delor, C., Amelin, Y., Goncalves, P.,Rabarimanana, M.H., Ralison, A.V., and Belcher, R.W.,

2011a, Neoproterozoic extension in the Greater DharwarCraton: A reevaluation of the “Betsimisaraka Suture” inMadagascar: Canadian Journal of Earth Science, v. 48, p.389‒417.

Tucker, R.D., Roig, J.Y., Macey, P.H., Delor, C., Amelin, Y.,Armstrong, R.A., Rabarimanana, M.H., and Ralison, A.V.,2011b, A new geological framework for south-centralMadagascar, and its relevance to the “out-of-Africa” hypoth-esis: Precambrian Research, v. 185, p. 109–130. doi:10.1016/j.precamres.2010.12.008

Tucker, R.D., Roig, J.Y., Moine, B., Delor, C., and Peters, S.G.,2014, A geological synthesis of the Precambrian shield inMadagascar: Journal of African Earth Sciences, v. 94, p. 9‒30. doi:10.1016/j.jafrearsci.2014.02.001

Wiedenbeck, M., Hanchar, J.M., Peck, W.H., Sylvester, P., Valley,J., Whitehouse, M., Kronz, A., Morishita, Y., Nasdala, L.,Fiebig, J., Franchi, I., Girard, J.-P., Greenwood, R.C.,Hinton, R., Kita, N., Mason, P.R.D., Norman, M.,Ogasawara, M., Piccoli, P.M., Rhede, D., Satoh, H., Schulz-Dobrick, B., Skår, O., Spicuzza, M.J., Terada, K., Tindle, A.,Togashi, S., Vennemann, T., Xie, Q., and Zheng, Y.-F., 2004,Further characterisation of the 91500 zircon crystal:Geostandards and Geoanalytical Research, v. 28, p. 9–39.doi:10.1111/j.1751-908X.2004.tb01041.x

Yang, X.-A., Chen, Y.-C., Hou, K.-J., Liu, S.-B., and Liu, -J.-J.,2014, U–Pb zircon geochronology and geochemistry of LateJurassic basalts in Maevatanana, Madagascar: Implicationsfor the timing of separation of Madagascar from Africa:Journal of African Earth Sciences, v. 100, p. 569–578.doi:10.1016/j.jafrearsci.2014.07.022

International Geology Review 1649

Copyright of International Geology Review is the property of Taylor & Francis Ltd and itscontent may not be copied or emailed to multiple sites or posted to a listserv without thecopyright holder's express written permission. However, users may print, download, or emailarticles for individual use.