U–Pb Zircon Geochronology and Geochemistry of Neoproterozoic

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

U–Pb zircon geochronology and geochemistry of Neoproterozoic granitoids of the Maevatanana area, 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 of Mineral 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 basement in west-central Madagascar. Here, we report precise new zircon U–Pb ages and whole-rock geochemistry for granitoids within the Maevatanana area of Madagascar. The new laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating undertaken during this study indicates that Antanimbary granitoid and Antasakoamamy granitoid were emplaced at 747 ± 9 Ma and 729 ± 9–727 ± 8 Ma, respectively. Geochemically, the Antanimbary granitoids show poor Nb, Ta anomalies, pronounced positive Zr anomalies, and are K-rich (K2O/Na2O > 1), but the Antasakoamamy granitoids are relatively depleted in Nb, Ta, show slightly negative Zr anomalies, and are Na-rich (Na2O/K2O > 1). Both suites contain zircons with strongly negative εHf(t), indicating participation of much older (Palaeoproterozoic and Archaean) crust. Their geochemical characteristics, along with the use of various discrimination diagrams, reveals that crustal delamination and asthenospheric upwelling resulted in crustal extension of the region before ~747 Ma, with subsequent lithospheric subduction and arc magmatism after 729–727 Ma. Keywords: U–Pb zircon geochronology; geochemistry; Neoproterozoic granitoids; Maevatanana; Madagascar

1. Introduction structural geology (Collins et al. 2003a, 2003b; Tucker The island of Madagascar consists of a collage of et al. 2007; Thomas et al. 2009), and magmatic processes Precambrian basement terranes overlain by Phanerozoic that operated in this area (Nédélec et al. 1995; Paquette sedimentary basins along the west coast of the island. and Nédélec 1998; Meert et al. 2001; Goodenough et al. These Precambrian terranes were juxtaposed during the 2010). Suites of Cryogenian granitoids and gabbros within Neoproterozoic–Cambrian (Pan-African) East African the Seychelles, north-central Madagascar, and northwest and Malagasy orogenies (Collins and Pisarevsky 2005; Rajasthan in India (the Malani igneous suite) are thought Collins 2006). The East African Orogen (EAO) extends to represent part of an active continental margin on the from southern Israel and Jordan in the north to Antarctica western edge of Rodinia (Handke et al. 1999; Torsvik in the south (Stern 1994; Meert 2003; Jacobs and Thomas 2001a, 2001b; Ashwal 2002; Thomas et al. 2009). – 2004) and represents the Neoproterozoic collision zone Zircon U Pb geochronology and isotopic, geochemical, between India, the Congo–Tanzania–Bangweulu Block, and petrological evidence from these areas provide evi- and the Saharan Metacraton (Meert 2003; Collins and dence of the existence of this convergent boundary Pisarevsky 2005; Collins 2006). The EAO is the world’s between 800 and 700 Ma. However, other researchers largest Neoproterozoic to Cambrian orogenic complex and suggest that gabbroic and granitoid rocks from the consists of a collage of individual oceanic domains and Seychelles and Madagascar formed in an intra-plate continental fragments. Consolidation of these fragments plume or rift scenario or alternatively may have formed occurred between ~800 and 500 Ma (Meert and Van Der as a result of lithospheric delamination (Stephens et al. Voo 1997; Meert et al. 2001; Meert 2003; Fritz et al. 1997; Tucker et al. 2011a). Handke et al. (1999) reported – – 2013). Madagascar lies in the heart of the EAO, and the U Pb ages of 804 776 Ma for a 450 km-long belt of exposed basement in this area has been the subject of gabbroic and granitoid plutons stretching from Ambositra much research, including research focused on the archi- to Maevatanana within west-central Madagascar and sug- tecture of the Archaean basement (Tucker et al. 1999, gested that this belt represents the roots of a continental ‘ ’ 2011a, 2011b; Collins et al. 2003a) and its history of Andean-type arc on the western margin of Rodinia; this Neoproterozoic metamorphism, magmatism, and structural arc formed in the middle Neoproterozoic during fragmen- development (Buchwaldt et al. 2003; Jöns et al. 2006), tation of the Rodinian supercontinent. These gabbroic and

*Corresponding author. Email: [email protected]

© 2014 Taylor & Francis 1634 X.-A. Yang et al. granitoid plutons have been named the Imorona–Itsindro Madagascar, and the geochemical and geochronological Suite (Key et al. 2011; Tucker et al. 2011a; Roig et al. constraints provided by these granitoids are useful in recon- 2012) after the type localities in the Itremo area (Moine structing the geodynamic setting for the magmatic events 1968). Igneous rocks of the Imorona type are felsic, mean- that formed these intrusions. Here, we present new geo- ing granitic, syenitic, and monzonitic in composition. chemistry and geochronology of granitic rocks from the Igneous rocks of the Itsindro type are mafic and include Maevatanana area. These data constrain the timing of gabbro, gabbro-diorite, and granodiorite (Key et al. 2011; major Imorona–Itsindro Suite magmatism within Moine et al. 2014; Tucker et al. 2014). Bybee et al. (2010) Madagascar and constrain the tectonic setting in which also suggested that ultramafic complexes within the these rocks formed. Andriamena region of north-central Madagascar formed in a similar setting during the middle Neoproterozoic. Tucker et al. (2014) reviewed dating of the Imorona- 2. Geological setting and petrography Itsindro suite and showed that these granitoids formed The eastern two thirds of Madagascar is dominated by between 851 and 719 Ma. Recent studies suggest that Precambrian rocks, whereas the western third is covered – the Imorona Itsindro Suite, emplaced in Cryogenian time by basins that preserve an extensive sedimentary record from 840 to 760 Ma, was linked to continental dilation (Li from the late Carboniferous to Recent (de Wit 2003). Key et al. 2008; Tucker et al. 2011a; Moine et al. 2014). In et al. (2011) reviewed previous work and showed that contrast, Thomas et al. (2009) reported that central and north Madagascar consists of five crustal Neoproterozoic granitoids to the north in the Bemarivo domains (Figure 1). Five crustal domains consist of – Belt are somewhat younger at ca. 750 705 Ma and pro- Antongil Cratons, Masora Cratons, Antananarivo posed that the plutonic rocks have an arc-related nature. Cratons, Tsaratanana Complex, and Bemarivo Belt. Four Granitoid intrusions are ubiquitous components of the domains consist largely of Archaean metamorphic rocks, major orogenic belts within the Maevatanana area of central 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). International Geology Review 1635 meta-igneous rocks. Each domain has distinctive litholo- (BGS-USGS-GLW 2008; Key et al. 2011; Moine et al. gies and histories of sedimentation, magmatism, deforma- 2014). The Antanimbary granitoid contains 35–40% tion, and metamorphism and is bounded by tectonic quartz, 25–35% K-feldspar, 15–20% plagioclase, and bio- boundaries. Tucker et al. (2011a) proposes that they are tite (5%), with accessory zircon, apatite, titanite, and ilme- different parts of a common craton (the Greater Dharwar nite (Figure 3A–C). The Antasakoamamy granitoid is Craton) amalgamated during Neoarchaean cratonization divided into marginal granite and central quartz monzonite (ca. 2.5 Ga). facies that are separated by a transition zone. The granitoid The Maevatanana Belt is the westernmost of three is medium coarse-grained and varies in composition from N–S-trending belts of Neoarchaean amphibolite-facies intermediate at the centre of the intrusion to felsic at the mafic gneiss and schist (granulite facies in the margin. The granitoid stocks, including related sheets and Andriamena belt). This belt is dominated by migmatitic dikes, are intruded into the already metamorphosed mafic gneiss, amphibolite, magnetite-rich quartzite, and metaba- gneisses and schists of the Tsaratanana Complex and sic to ultrabasic rocks (soapstones). The eastern Betsiboka Suite (Tucker et al. 2011a). The quartz monzo- Maevatanana belt and a series of migmatitic gneiss and nite contains subhedral phenocrysts and is medium coarse- augen gneiss are separated by fault. grained. It contains K-feldspar (40–45%), plagioclase The Neoproterozoic Antanimbary granitoid and (35%), quartz (15–20%), and minor biotite (<5%), zircon, Antasakoamamy granitoid are exposed as plutons within apatite, and ilmenite (Figure 3D–F). In comparison, the the southern part of the Maevatanana area (Figure 2). granite is subhedral and fine-grained, is massive, and con- These granitoid are part of the Imorona–Itsindro suite tains quartz (40%), plagioclase (30%), K-feldspar (25%), and biotite (5%), with accessory zircon, apatite, and titanite (Figure 3G–I). On a modal quartz-alkali feldspar- plagioclase (QAP) classification basis (Figure 4), the Antanimbary granitoid and Antasakoamamy granitoid are made up of syenogranite (M6), quartz monzonite (M8), and monzogranite (M1). Samples M6-1, M6-2, M7-1, M7-2, and M7-3 are from the Antanimbary granitoid pluton, taken from outcrops 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, and M9-2 are from the central facies of the Antasakoamamy granitoid pluton (S 16°59′2.8″, E 46°48′56.16″), taken from 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 of the Antasakoamamy granitoid pluton, taken from a quarry (S 17°1′3.06″, E 46°48′7.22″). The sampled lithology is fine-grained, fresh, grey granite.

3. Analytical methods 3.1. Zircon U–Pb and Lu–Hf isotopic data High purity zircon separates were obtained by heavy liquid and magnetic separation methods, before handpick- ing under a binocular microscope. Transparent euhedral zircon grains were mounted in epoxy resin and polished until grain interiors were exposed. Zircon grains were then U–Pb dated by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) using an Agilent 7500a ICP–MS equipped with a UP193SS laser ablation system Figure 2. Geological map of the Maevatanana region showing at the MLR Key Laboratory of Metallogeny and Mineral sample locations. Assessment, Institute of Mineral Resources, Chinese Source: Modiﬁed after Rantoanina et al. (1969). Academy of Geological Sciences, Beijing, China. Details 1636 X.-A. Yang et al.

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).

of the U–Pb analytical procedures may be found in Yang 2004) was used for external standardization; this standard et al. (2014). was analysed after every four unknown zircon analyses. A laser spot of 36 μm was used for analysis, and a Corrections for common Pb were made following the Harvard zircon 91500 standard with a recommended approach of Andersen (2002) and data were processed 206Pb/238U age of 1065.4 ± 0.6 Ma (Wiedenbeck et al. using the GLITTER and ISOPLOT programs (Ludwing International Geology Review 1637

2003). The uncertainties on individual LA–ICP–MS ana- 4. Results σ fi lyses are quoted at the 95% (1 ) con dence level; more 4.1. Zircon U–Pb dating details of the analytical procedure are provided in Black Zircons are abundant in the Antanimbary granitoid, and et al. (2004). they range in size from 50 to 100 μm and have length to Zircon Lu–Hf isotopic analysis was carried out in situ width ratios of 2:1. The crystals are subhedral to euhedral, using a New Wave UP213 laser-ablation microprobe, and the majority are transparent to light brown in colour; attached to a Neptune multicollector ICP-MS at the CL images reveal concentric zoning patterns consistent Institute of Mineral Resources, Chinese Academy of with magmatic crystallization (Figure 5A). Table 1 pro- Geological Sciences, Beijing. Hou et al. (2007) compre- vides age data for 12 analyses of zircon within sample M6. hensively described instrumental conditions and data The measured concentrations for these zircon grains vary acquisition. A stationary spot was used for the present from 33 to 556 ppm for U; from 39 to 934 ppm for Th; analyses, with a beam diameter of either 40 or 55 μm and Th/U ratios range from 0.46 to 1.90, within the range depending on the size of ablated domains. Helium was expected for magmatic zircon. The seven most concordant used as carrier gas to transport the ablated sample from the analyses yield a concordia age of 747 ± 9 Ma (2σ, mean laser-ablation cell to the ICP-MS torch via a mixing square weighted deviation (MSWD) = 0.18; Table 1; chamber mixed with argon. In order to correct the isobaric Figure 6A), indicative of the crystallization age of the interferences of 176Lu and 176Yb on 176Hf, granite. The other five zircons yield much older ages 176Lu/175Lu = 0.02658 and 176Yb/173Yb = 0.796218 ratios (2434–2074 Ma) and are interpreted to be inherited. were determined (Chu et al. 2002). For instrumental mass Tucker et al. (1999, 2011a) dated the nearby Archaean bias correction, Yb isotope ratios were normalized to rocks of the Maevatanana belt, and the age of these is 172Yb/173Yb of 1.35274 and Hf isotope ratios were ~2.5 Ga; therefore, the ages of inherited zircon is in keep- normalized to 179Hf/177Hf of 0.7325 using an exponential ing with the age of palaeoproterozoic granitoid (Tucker law. The mass bias behaviour of Lu was assumed et al. 2014). to follow that of Yb. Zircon GJ1 was used as the Zircon is also abundant within the samples of reference standard, with a weighted mean 176Hf/177Hf Antasakoamamy granitoid analysed during this study. ratio of 0.282013 ± 0.00008 (2σ, n = 10) or The crystals are generally euhedral, transparent prisms, 0.282013 ± 0.000024 (2σ, n = 10) during our routine up to 100 μm in length, that display magmatic oscillatory analyses. It is not distinguishable from a weighted mean zoning (Figure 5B and C). Fifteen zircons from sample 176Hf/177Hf ratio of 0.2820 (2σ) from in situ analysis by M8, which is quartz-monzonite, were analysed. The mea- Elhlou et al. (2006). sured concentrations for these zircon grains vary from 0.2 to 45 ppm for U; from 1.66 to 121 ppm for Th; and Th/U 3.2. Whole-rock geochemical analysis ratios range from 1.52 to 8.30, much higher than for those of metamorphic zircon. Eleven of the zircons yield a Fifteen whole-rock samples from representative weighted mean 206Pb/238U age of 729 ± 9 Ma (2σ, Antanimbary granitoid and Antasakoamamy granitoid MSWD = 0.49; Table 2; Figure 6B), indicative of the have been analysed for major, trace, and rare earth ele- crystallization age of the granite. The other four zircons ments (REEs) during this study. These whole-rock sam- yield much older ages (2875–1020 Ma) and are interpreted ples were trimmed to remove weathered surfaces before to be inherited. being cleaned with deionized water, crushed, and then Eight zircons from sample M1, which is granite, were powdered using an agate mill. analysed. The measured concentrations for these zircon Major element concentrations were determined using grains vary from 15 to 220 ppm for U; from 20 to 213 ppm X-ray fluorescence and a PANalytical Axios-Advanced for Th; and Th/U ratios range from 0.46 to 2.44, largely instrument at the Geological Analysis Laboratory (GAL) within the range expected for magmatic zircon. Five of of the Ministry of Nuclear Industry, Beijing, China. This these zircons yield a weighted mean 206Pb/238Uageof analysis used fused glass discs and the analytical preci- 727±8Ma(2σ, MSWD = 0.028; Table 3; Figure 6C) sion, as determined using the Chinese National standard indicative of the crystallization age of the granite. The other GSR-1, was better than 5%. Loss on ignition (LOI) was three zircons yield much older ages (2445–1943 Ma) and are obtained using 1 g of powder heated to 1100°C for 1 h. interpreted to be inherited. Trace elements were determined using a plasma optical emission mass spectrometer (POEMS) ICP–MS system at the GAL of the Ministry of Nuclear Industry, Beijing, – China. The discrepancy between triplicate analyses was 4.2. Zircon Lu Hf isotopes less than 5% for all elements, and analysis of the OU-6 Lu–Hf analyses were obtained for zircon from the and GBPG-1 international standards was in agreement Antanimbary granitoid. The results are given in Table 4. with their recommended values. The 176Lu/177Hf ratios range from 0.001091 to 0.002418, 1638 X.-A. Yang et al.

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

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

Isotopic ratios Age (Ma)

Th U Spot (ppm) (ppm) Th/U 207Pb/206Pb 1δ 207Pb/235U1δ 206Pb/238U1δ 207Pb/206Pb 1δ 207Pb/235U1δ 206Pb/238U1δ

1 95 96 0.99 0.066 0.001 1.120 0.021 0.123 0.002 809 27 763 10 747 11 2 541 363 1.49 0.064 0.002 1.074 0.042 0.122 0.002 748 50 741 21 739 12 3 300 158 1.90 0.067 0.001 1.138 0.033 0.122 0.003 856 22 771 16 744 17 4 278 185 1.51 0.064 0.002 1.086 0.044 0.122 0.002 750 81 747 22 745 12 5 188 132 1.42 0.067 0.001 1.148 0.022 0.124 0.002 839 19 776 10 754 12 6 934 556 1.68 0.067 0.001 1.126 0.032 0.123 0.002 833 39 766 15 746 12 7 347 254 1.37 0.064 0.001 1.096 0.021 0.124 0.002 767 34 751 10 752 12 8 87 78 1.11 0.166 0 10.358 0.100 0.452 0.004 2520 5 2467 9 2405 20 9 122 178 0.69 0.166 0.001 10.411 0.218 0.456 0.010 2514 7 2472 19 2421 43 10 80 78 1.03 0.166 0.001 10.274 0.236 0.450 0.010 2515 13 2460 21 2394 46 11 39 33 1.19 0.141 0.004 7.196 0.283 0.380 0.017 2242 46 2136 35 2074 77 12 185 400 0.46 0.166 0.001 10.477 0.064 0.459 0.002 2514 8 2478 6 2434 9 International Geology Review 1639 δ U1 238 Pb/ 206 δ U1 235 Age (Ma) Pb/ 207 δ Pb 1 206 Pb/ 207 δ U1 238 Pb/ 206 δ U1 235 Pb/ 207 δ Pb 1 206 Pb/ 207 Pb dating of sample M8 from the Antasakoamamy granitoids. –

Figure 6. Zircon U–Pb concordia diagram of the dated samples of Neoproterozoic granitoid. (A) Zircon grains from sample M6 of Antanimbary granitoid; (B) zircon grains from sample M8 of Antasakoamamy granitoid; and (C) zircon grains from sample M1 of Antasakoamamy granitoid. with a mean of 0.001872, indicating that these zircons are very low in radiogenic Hf. Seven analyses were performed Spot Th (ppm) U (ppm) Th/U Table 2. Results of zircon U Isotopic ratios on seven zircon grains from sample M6. The analyses yield 1234 365 236 1217 39 168 81 109 25 2710 19 1811 2.17 19 4512 2.30 34 4.49 1313 15 0.065 1014 14 2.16 0.067 1215 1.78 0.066 1.66 20 0.004 1.93 1.76 0.065 9 11 0.014 1.90 0.002 0.069 8 16 1.58 0.2 1.144 0.068 0.005 1.70 1.16 1.142 0.065 0.005 1.59 1.086 0.067 8.30 6 0.090 0.005 1.70 1.52 1.046 0.066 8 0.266 0.006 1.156 0.035 0.064 0.005 0.119 0.621 1.123 0.065 0.392 0.084 0.005 1.77 0.122 1.120 0.120 0.072 0.006 2.00 1.138 0.002 0.154 0.094 0.005 0.069 0.117 1.091 0.007 0.212 0.115 0.001 0.122 1.051 0.455 30.358 0.095 0.119 14.105 787 1.135 0.004 0.089 0.012 0.120 833 0.002 24.945 0.109 0.011 796 0.123 0.003 3.290 0.126 137 0.118 783 4.978 0.005 0.562 435 16.96 0.117 898 0.004 0.267 63 0.120 859 0.002 774 0.423 164 0.443 781 0.004 774 0.831 144 0.043 850 0.004 747 154 0.171 813 4558 727 183 43 0.261 750 127 3879 780 156 789 0.011 765 156 17 419 0.008 763 728 197 42 268 742 772 166 2920 34 3498 749 731 4103 45 2757 729 715 55 13 42 770 744 95 1103 45 724 36 43 225 8 730 54 23 1816 2875 745 60 12 2933 1525 716 17 1878 712 29 72 733 23 47 218 10 1020 24 1493 25 58 42 1640 X.-A. Yang et al.

176 177 δ variable Hf/ Hf ratios between 0.281815 and 0.281908, corresponding εHf(t)valuesfrom−18.2 to −15.1 (calculated −

U1 at t =746.9Ma)(Figure 7), with a mean of 16.67. – 238 Corresponding TDM is calculated at 2055 1942 Ma. – Pb/ Lu Hf analyses were obtained for zircon from the

206 Antasakoamamy granitoid. The results are given in Table 4. The 176Lu/177Hf ratios range from 0.000439 to 0.003870,

δ with a mean of 0.001367, indicating that these zircons are very weak in radiogenic Hf. Eleven igneous zircons of M8 176 177

U1 gave Hf/ Hf ratios ranging from 0.281865 to 0.281977.

235 Computations based on crystallization ages (728.9 Ma) Age (Ma) Pb/ of the magmas yielded εHf(t)between−16.7 and −13.2 207 (Figure 7), with a mean of −14.9. Corresponding TDM is calculated at 2037–1804 Ma. Five igneous zircons of M1 176 177 δ gave Hf/ Hf ratios ranging from 0.281250 to 0.281452. Computations based on crystallization ages (726.5 Ma) of the magmas yielded εHf(t)between−38.5 and −32.6, with a Pb 1 mean of −35.6 (Figure 7). Corresponding TDM is calculated 206 at 2811–2633 Ma. Pb/ 207

δ 4.3. Whole-rock geochemistry 4.3.1 Major elements The whole-rock major and trace element compositions of the U1 Neoproterozoic granitoid analysed during this study are given 238 in Table 5. The Antanimbary granitoid contains high concen- Pb/ – 206 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 of δ TiO2 (0.12–0.16 wt.%; average 0.14 wt.%) and MgO (0.10– 0.11 wt.%; average 0.11 wt.%), and moderate Na2O concentrations (3.94–4.22 wt.%, average 4.07 wt.%) and Na O/K O

U1 2 2 – 235 ratios (Na2O/K2O=0.700.75, average 0.72). All of these ﬁ Pb/ samples are classi ed as granites on a total alkali versus silica

207 (TAS) diagram (Figure 8A) and as shoshonitic on a K2O versus SiO2 diagram (Figure 8B). These samples all plot along the boundary between metaluminous and peraluminous δ granites and within the I-type granite ﬁeld, using a molar ratio Al/(Na + K) (A/NK) versus Al/(Ca + Na + K) (A/CNK or alumina saturation index) diagram (Figure 8C). Pb 1

206 Quartz monzonite samples from the central

Pb/ Antasakoamamy granitoid pluton (M8-1, M8-2, M8-3,

207 M9-1, and M9-2) range from 62.87 to 65.86 wt.% for SiO2, from 15.95 to 16.52 wt.% for Al2O3,from0.63to0.76wt.% Pb dating of sample M1 from the Antasakoamamy granitoids. – for TiO2,from2.97to4.00wt.%forCaO,from0.22to 0.34 wt.% for P2O5;havehighK2O(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.%). All the samples of the granitoid display a Na-rich characteristic (Na2O/K2O > 1). All of these samples are classiﬁed as quartz monzonite on a TAS diagram (Figure 8A) and have high-K calc-alkaline characteristics, as seen on a K2O versus SiO2 diagram (Figure 8B). All samples are metalu-

Isotopic ratios Spot Th U Th/U ﬁ Table 3. Results of zircon U 123 10045 20 2206 317 35 17 0.468 47 25 213 1.12 15 114 1.27 0.068 28 140 59 2.44 91 0.067 1.72 1.52 0.001 0.067 67 1.25 0.066 0.002 0.064 0.88 0.16minous 1.119 0.004 0.146 0 1.095 0.001 0.163 0.017 0and 1.11 0.001 0.043 within 1.045 0.001 0.119 1.091 0.076 7.081 10.155 0.119 0.023 the 0.001 9.903 0.051 0.12 0.088 0.077I-type 0.003 0.119 0.111 0.119 865 0.352 0.003 0.461granite 0.001 0.442 833 0.006 0.004 0.003 22 835eld, 0.005 733 69 817 2302 2454using 763 132 2485 44 751 a 13 molar 6 8 5 758 5 21 726 749 2122 2449 37 727 2426 11 725 25 11 7 731 5 10 15 725 727 1943 2445 18 2358 6 32 21 15 22 International Geology Review 1641

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

176 177 176 177 176 177 Sample Age (Ma) Hf/ Hf 2σ Lu/ Hf 2σ Yb/ Hf 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 −0.95 M6-2 746.9 0.281895 0.000021 0.002418 0.000012 0.073878 0.000283 −15.8 1980 −0.93 M6-3 746.9 0.281854 0.000018 0.001091 0.000012 0.037081 0.000317 −16.6 1968 −0.97 M6-4 746.9 0.281857 0.000020 0.002118 0.000049 0.065437 0.001073 −16.9 2018 −0.94 M6-5 746.9 0.281908 0.000019 0.002033 0.000019 0.065253 0.000663 −15.1 1942 −0.94 M6-6 746.9 0.281815 0.000020 0.001715 0.000045 0.060646 0.001414 −18.2 2055 −0.95 M6-7 746.9 0.281855 0.000016 0.002159 0.000017 0.061087 0.000410 −17.0 2023 −0.93 M8-1 728.9 0.281900 0.000022 0.001404 0.000026 0.057086 0.001529 −15.5 1920 −0.96 M8-2 728.9 0.281950 0.000021 0.000439 0.000007 0.014546 0.000288 −13.2 1804 −0.99 M8-3 728.9 0.281977 0.000019 0.002552 0.000064 0.107712 0.003015 −13.3 1869 −0.92 M8-4 728.9 0.281882 0.000022 0.000445 0.000027 0.015947 0.001079 −15.6 1897 −0.99 M8-5 728.9 0.281894 0.000017 0.000712 0.000005 0.022261 0.000332 −15.3 1893 −0.98 M8-6 728.9 0.281947 0.000023 0.000941 0.000022 0.034612 0.000759 −13.6 1831 −0.97 M8-7 728.9 0.281914 0.000018 0.000948 0.000011 0.032938 0.000472 −14.7 1877 −0.97 M8–8 728.9 0.281924 0.000023 0.000686 0.000007 0.023371 0.000282 −14.3 1851 −0.98 M8-9 728.9 0.281905 0.000026 0.001467 0.000020 0.052163 0.000659 −15.3 1915 −0.96 M8-10 728.9 0.281865 0.000023 0.000670 0.000011 0.023535 0.000505 −16.4 1931 −0.98 M8-11 728.9 0.281892 0.000020 0.003372 0.000047 0.133523 0.001567 −16.7 2037 −0.90 M1-1 726.5 0.281328 0.000024 0.00126 0.000009 0.125761 0.001357 −35.709 2703 −0.96 M1-2 726.5 0.281351 0.000019 0.000719 0.000020 0.025326 0.000429 −34.609 2633 −0.98 M1-3 726.5 0.281452 0.000024 0.003870 0.000104 0.214565 0.006181 −32.552 2718 −0.88 M1-4 726.5 0.281296 0.000019 0.001111 0.000020 0.051021 0.001425 −36.746 2735 −0.97 M1-5 726.5 0.281250 0.000025 0.001281 0.000009 0.058132 0.000511 −38.475 2811 −0.96

granitoid display a Na-rich characteristic (average Na2O/ K2O > 1). All of these samples are classiﬁed as granites on 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-type granite ﬁeld, using a molar ratio Al/(Na + K) (A/NK) versus Al/(Ca + Na + K) (A/CNK or alumina saturation index) diagram (Figure 8C).

4.3.2 Trace elements The Antanimbary granitoid contains high Rb (116–123 ppm), moderate Nb (12.0–15.2 ppm) and Ta (0.9–1.2 ppm), 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 the Figure 7. Histogram showing the distribution of ƐHf(t) values for all analysed zircon grains from the Neoproterozoic granitoids. granitoid have low total REE contents ranging from 46.5 ppm to 72.3 ppm, and they show moderate enrichment of light rare earth elements (LREEs) relative to heavy rare ratio Al/(Na + K) (A/NK) versus Al/(Ca + Na + K) (A/ earth elements (HREEs) [(La/Yb)N = 8.40–9.22, average CNK or alumina saturation index) diagram (Figure 8C). 8.89], with negative Eu anomalies (Eu/Eu* = 0.49–0.59) Granite samples from southern marginal facies of the (Figure 9A), and their MREE–HREE patterns are flat and Antasakoamamy granitoid pluton (M1, M2, M3, M4, and indeed curve up slightly towards Yb and Lu. Primitive- M5) (Figure 2) range from 69.59 to 71.57 wt.% for SiO2, mantle-normalized trace element variation diagrams for from 14.85 to 16.33 wt.% for Al2O3, from 0.25 to the granitoid (Figure 9B) are enriched in the large ion 0.27 wt.% for TiO2,from1.85to2.04wt.%forCaO, lithophile elements (Rb, Ba, Th, and K) and significantly and 0.09 wt.% for P2O5;havehighK2O(3.60–4.45 wt.%), depleted in the high field strength elements (P and Ti). The Na2O(3.82–4.96 wt.%), and K2O+Na2O(8.24–8.61 wt.%); LILE enrichments of the Imorona–Itinsdo Suite were and have low MgO (0.03 wt.%). All the samples of the made by Tucker et al. (2014). 1642 X.-A. Yang et al.

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

From the central part of the From the southern part of the Antasakoamamy pluton Antasakoamamy 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 71.40 TiO2 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 0.27 Al2O3 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 14.85 Fe2O3 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 0.76 FeO 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 1.55 MnO 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 0.03 MgO 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 0.57 CaO 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 1.86 Na2O 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 3.82 K2O 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 4.45 P2O5 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 0.09 LOI 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 0.25 Total 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 99.89 FeOT 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 2.23 Na2O+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 8.27 K2O/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 1.16 Mg# 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 0.31 A/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 1.34 A/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 1.03 T(Zr) 743 759 751 750 753 753 752 751 753 752 748 743 752 752 759 P 205 214 197 227 197 1271 1498 1179 1232 970 380 384 406 402 397 Ti 851 845 773 965 743 4149 4550 3963 4089 3795 1607 1475 1577 1535 1601 Au 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 1.42 Ag 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 0.41 Ba 328 344 346 330 338 3632 2974 4011 4030 3373 2387 1928 1911 1847 1960 Rb 116 118 118 120 123 27 24 26 26 23 67 54 74 71 83 Sr 159 168 162 167 164 816 919 914 894 826 605 501 375 347 348 Ta 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 0.8 Nb 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 11.3 Hf 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 5.0 Zr 79 106 81 88 89 73 71 53 56 54 90 92 143 140 160 Y 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 14.0 Sc 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 3.2 V 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 19.8 Co 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 4.6 Ni 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 9.6 Ga 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 20.0 Pb 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 32.8 Th 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 5.6 U 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 0.9 La 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 56.3 Ce 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 86.9 Pr 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 8.3 Nd 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 28.7 Sm 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 4.1 Eu 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 0.7 Gd 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 4.4 Tb 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 0.5 Dy 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 2.3 Ho 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 0.4 Er 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 1.3 Tm 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 0.2 Yb 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 1.1 Lu 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 0.2 LREE/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 35.42 δEu 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 International Geology Review 1643

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 (0.2–0.3 ppm), and Ni (19.2–23.2 ppm), leading to the low Rb/Sr (0.03) and relatively high Sr/Nd (28–35) ratios. They have total 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 the Antasakoamamy granitoid pluton (M1, M2, M3, M4, and M5) 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), and Ni (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 have total 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 light rare earth elements (LREEs), and the samples have a negative Eu anomaly indicating fractionation of plagioclase. Chondrite-normalized REE patterns for these samples have steep LREE slopes and ﬂat and low heavy rare earth element (HREE) patterns (Figure 9C). The HREE depletions present within these granitoids may indicate removal of garnet or another HREE-enriched phase. Primitive-mantle-normalized trace element variation diagrams for the granitoid (Figure 9D) are enriched in the large ion lithophile elements (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 their Hf model ages (2.06–1.94 Ga) of the Antanimbary granitoid 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 plagioclase removal during fractional crystallization. Additionally, the fractionation of apatite and ilmenite and/or titanite is also recorded by variable degrees of negative P and Ti anomalies (Figure 9B). The granitoid shows positive Zr anomalies, possibly the result of lower crustal contamina- tion during its ascent (Sun and McDonough 1989). The Antasakoamamy granitoids are composed of two rock types, including granite and quartz monzonite, and Figure 8. Classiﬁcation of the studied granitoids on the basis of (A) have wide SiO2 contents ranging from 62.87 to 71.57 wt. the TAS diagram (Middlemost 1994); (B) K2O-SiO2 diagram; and (C) Al O /(Na O+K O) molar-Al O /(CaO+Na O+K O) molar. %. The diversity of the granitoid may have resulted from a 2 3 2 2 2 3 2 2 fractional crystallization process. The granitoid samples display wide ranges in Eu/Eu* values (0.51–0.94), suggesting Quartz monzonite samples from central facies of the that the diversity of the granitoids was partially caused by Antasakoamamy granitoid pluton (M8-1, M8-2, M8-3, feldspar fractionation. In addition, the granitoid are strongly M9-1, and M9-2) contain high Ba (2974–4030 ppm), depleted in Ta, Nb, P, and Ti (Figure 9D), indicative of 1644 X.-A. Yang et al.

Figure 9. Chondrite-normalized REE patterns and primitive-mantle-normalized spider diagrams for the Neoproterozoic granitoid samples. 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 of Antasakoamamy granitoid. differentiation during the formation of the granite. The TFe3+) × 100) values (29–32) and lower MgO (0.10– small P and Ti anomalies present within these granitoids 0.11 wt.% and 0.50–1.59 wt.%, respectively), Ni (4.8– may also be indicative of the early formation and separation 8.3 and 7.7–23.2 ppm, respectively) and Sr (159–168 of apatite and ilmenite, magnetite, and/or titanite, or varia- and 347–919 ppm, respectively) concentrations, suggest- tions in the source of the magmas that formed the granitoid. ing that these granitoids probably did not generate at Narrow εHf(t)values(from−16.7 to −13.2) of the quartz significant depths and under high heat flow conditions by monzonite and their calculated crustal model ages indicate melting of subducting plate and the mantle wedge. Melts that they sourced recycled crust with model ages of 2.04– derived from the subducting plate are ubiquitously con- # 1.80 Ga. However, the εHf(t)values(from−38.5 to −32.6) taminated by high Mg mantle peridotite, causing the of the granite samples from the southern marginal facies of melts to be enriched in MgO and Ni (Smithies 2000; the Antasakoamamy granitoid pluton and their calculated Condie and Kröner 2008); studied granitoids might have crustal model ages suggest that the granite could have been formed by a mixing between crustal and mantle-derived derived from reworked ancient lower crustal rocks with magmas. Contribution from older crust is also supported model ages of 2811–2633 Ma. Rocks of this age are by the predominantly Proterozoic Hf-TDM ages of these known in the Andriamena belt of central Madagascar zircons (2.04–1.80 Ga). Therefore, these granitoids could (Kabete et al. 2006), implying that magma mixed with or have been formed by melting of crust of Palaeoproterozoic was derived from older lower crust. tonalite–trondhjemite–granodiorite (TTG) composition (McMillan et al. 2003). The Antanimbary granitoid contains modest amounts 5.2. Petrogenesis of Neoproterozoic Antanimbary of Nb (12.0–15.2 ppm), Ta (0.9–1.2 ppm), and Zr (79–106 granitoid and Antasakoamamy granitoid ppm). The Antasakoamamy granitoid contains low Nb The Antanimbary granitoid and Antasakoamamy granitoid (4.9–11.3 ppm) and Ta (0.2–0.8 ppm) and moderate Zr in the study area have lower Mg# (Mg# =Mg2+/(Mg2+ + (53–160 ppm). As stated earlier, both granitoids were the International Geology Review 1645 result of a mixture of both primary mantle (Cryogenian) and Palaeoproterozoic TTG sources. Nevertheless, the petrogenesis of the Antasakoamamy granitoid was different from the Antanimbary granitoid. The 747 ± 9 Ma Antanimbary granitoid lacks any Nb-Ta anomaly and has positive Zr anomaly, whereas the 729 ± 9 Ma Antasakoamamy granitoid has a negative Nb-Ta anomaly, suggesting that it was likely related to subduction. The Antasakoamamy granitoid exhibits strongly negative εHf(t) values (from −16.7 to −13.2; from −38.5 to −32.6), indicating a significant contribution of old crustal material and precluding an origin through partial melting of subducted oceanic lithosphere or slab-melt-modified peridotitic mantle wedge (Martin et al. 2005). This implies that the granitoid could have been generated by partial melting of thickened lower crust.

5.3. Tectonic significance The geochemistry of igneous rocks bear a close relation- ship to their tectonic setting of formation. The Antanimbary granitoid plots within the post-collisional granite fields within the Yb + Ta versus Rb (Figure 10A) tectonic discrimination diagram. The same conclusion is supported using the multicationic R1‒R2 [R1 = 4Si4+-11 (Na++K+)-2(Fe3++Ti4+), molar; R2 = 6Ca2++2Mg2++Al3+, molar] tectonic discrimination diagram, with samples plotted within the post-orogenic areas of the diagram (Figure 10B). The Antanimbary granitoid compositions are consistent with formation in an extensional environ- ment. In the Andriamena Belt, between 820 and 785 Ma, the basement rocks formed by extension associated with extensive magmatism (Kabete et al. 2006). Li et al. (2008) proposed that widespread continental rifting occurred between ca. 825 and 740 Ma. Tucker et al. (2011a) sug- Figure 10. (A) Trace element tectonic discrimination diagrams gested that the Itsindro–Imorona Suite (840‒760 Ma) and of the Neoproterozoic granitoids studied here (fields are after the formation of two long, narrow belts of continental Pearce et al. 1984); (B) R1‒R2 multicationic variation diagram [R1 = 4Si4+—11(Na++K+)-2(Fe3++Ti4+), molar; R2 = 6Ca2++2Mg2+ sediments (Ambatolampy and Manampotsy) are the pro- 3+ ducts of continental dilation (and pressure-release melting +Al , molar]. VAG, volcanic arc granite; Syn-COLG, syn-collision granite; WPG, within-plate granite; ORG, oceanic ridge granite; of upwelling lithosphere). The Antanimbary granitoid Post-COLG, post-collision granite. lacks a significant Nb-Ta anomaly, and the granitoids display K-rich (K2O/Na2O > 1) and alkaline characteristics. These features are incompatible with subduction-related indicating that the granitoid was emplaced in a convergent granites. The age of the Antanimbary granitoid is margin setting. This contraction thickened the crust to 747 Ma, suggesting that it formed in this early promote deep melting of the lower crust after 729‒ Cryogenian extensional setting. We conclude that the 727 Ma. The Antasakoamamy granitoid exhibits strongly Antanimbary granitoid is the product of extension, perhaps negative ƐHf(t) values (from −16.7 to −13.2; from −38.5 to as a result of underplating of mantle plume-derived mag- −32.6), precluding an origin through partial melting of mas that triggered partial melting of lower crust (Tucker subducted oceanic lithosphere or slab-melt-modified peri- et al. 2014). dotitic mantle wedge alone. However, the granitoid shows On the Yb + Ta versus Rb and R1‒R2 (Figure 10) a pronounced negative Nb, Ta anomaly, and small P and tectonic discrimination diagrams, the 729–727 Ma Ti anomalies (Figure 9D), and most samples display Na- Antasakoamamy granitoid falls within the fields of volca- rich (average Na2O/K2O > 1) characteristics. The geo- nic arc granite and, straddling the post-collision uplift, chemistry of the Antasakoamamy granitoid is thus differ- late-orogenic and syn-collision areas of the diagram, ent from the Antanimbary granitoid. These features of the 1646 X.-A. Yang et al.

The recently acquired maximum deposition age is less than ~650 Ma, and the minimum deposition age of the Ambatolampy Group is ~560 Ma (Tucker et al. 2014). Igneous rocks of the Imorona–Itsindro Suite intrude the Ambatolamlpy Group. There are lit-par-lit relationships between the Ambatolamlpy Group and the 840–760 Ma Imorona–Itsindro Suite (Tucker et al. 2014), so the Ambatolamlpy Group must be older. Tucker et al. (2014) infer that deposition of the Ambatolampy Group was diachronous throughout the late Neoproterozoic, occurring both before and after the youngest members of the Imorona–Itsindro Suite (750–700 Ma) were emplaced. Tucker et al. (2011a) suggest that metaigneous rocks (Itsindro–Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Thomas et al. (2009) suggest that the Cryogenian igneous rocks of the Bemarivo domain (750– 708 Ma) were generated above a subduction zone within the palaeo-Mozambique Ocean outboard of cratonic Madagascar. Tucker et al. (2014) propose that Gondwana Figure 11. Two-stage plate-tectonic model for the development amalgamated and shortened in Ediacaran– Cambrian time of the Neoproterozoic granitoids studied here. – – Source: Modified from Key et al. (2011) and Tucker et al. (2011a). (560 520 Ma), and younger orogenic convergence (560 520 Ma) resulted in east-directed overthrusting throughout south Madagascar, steepening with local inversion of the Antasakoamamy granitoid can indicate that the source of domain in central Madagascar (Tucker et al. 2011a). We these magmas contained subduction-related material. infer that deposition of the Ambatolampy Group occurred Thomas et al. (2009) proposed that the Bemarivo in an extensive depositional basin that formed during a domain is divided into northern and southern ‘terranes’; period of continental extension and magmatism before U–Pb zircon studies had revealed a bimodal age distribu- 750 Ma and a period of subsequent subduction and mag- tion of the plutonic rocks, predominantly 750 Ma in the matism in Neoproterozoic time. south and 718‒705 Ma in the north. Moreover, Thomas Handke et al. (1999) reported U–Pb ages along a et al. (2009) suggested that some of these igneous rocks 450 km-long belt of gabbroic and granitoid plutons from show the geochemical characteristics of subduction-related Ambositra to Maevatanana within west-central magmatism. The ages of the Antasakoamamy granitoid Madagascar; these gabbroic and granitoid plutons formed overlap the ages of other Cryogenian igneous rocks that at 804–776 Ma. They suggested that this belt represented are related to subduction. Thus, the bimodality of plutonic the root of a continental magmatic arc on the western rock ages indicates that the Imorona–Itsindro Suite can be margin of a rifting Rodinia and constrained the critical broadly divided into two age domains (Figure 11). These period of Rodinia’s transformation into Gondwana. two igneous pulses reflect crustal delamination, astheno- However, McMillan et al. (2003) proposed that the gab- spheric upwelling, and crustal extension before ~747 Ma, broic and granitic components evolved from melting of an followed by lithospheric subduction and arc magmatism enriched subcontinental mantle to cause advective heating around 729‒727 Ma. and anatexis at the base of thickened continental crust, There are two quasi-parallel, N-striking belts of meta- implying that the generation of both the Antanimbary clastic rocks cover the Antananarivo domain in Madagascar. granitoid from the Maevatanana and the gabbroic and The eastern belt is outlined by the Manampotsy Group, granitoid plutons between Ambositra and Maevatanana which is 800 km long and extends from Manakara through through partial melting of lower crust was most likely to Bealanana. The western belt is outlined by the triggered by underplating of mantle-derived magmas. Ambatolampy Group, which is 30 km wide and extends These igneous rocks show LREE-enriched patterns with from southern to northern Madagascar (Tucker et al. 2014). no negative Eu anomalies; some samples show slight Based on the depositional age of the Manampotsy Group HREE enrichment and weak Nb, Ta anomalies on normal- (840–780 Ma) and igneous rocks of the Imorona-Itsindro ized incompatible element patterns (Bybee et al. 2010). Suite that intrude it, Tucker et al. (2014) inferred that deposi- The ages of the Antanimbary granitoid presented here tion of the Manampotsy Group occurred in a long, narrow record the younger phase of this magmatic event, with depositional basin that formed during early Cryogenian con- the gabbroic and granitoid plutons between Ambositra tinental extension and magmatism. and Maevatanana in west-central Madagascar recording International Geology Review 1647 older magmatism. Tucker et al. (2011a) suggest that meta- BGS-USGS-GLW, 2008. Final Report: Revision de la cartogra- igneous rocks of the Itsindro–Imorona Suite formed dur- phie géologique et minière des zones Nord et Centre de ing continental extension and intrusive igneous activity Madagascar (Zones A, B et D). Republique de Madagascar, Ministère deL’energie et des Mines (MEM/SG/DG/UCP/ between 840 and 760 Ma. Key et al. (2011) proposed PGRM), 1049. that Rodinia fragmented during the early Neoproterozoic Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J. with intracratonic rifts that sometimes developed into N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., oceanic basins, and middle Neoproterozoic smaller cra- Williams, I.S., and Foudoulis, C., 2004, Improved 206Pb/ tonic blocks subsequently collided. Our data demonstrate 238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID–TIMS, a major transition from an extensional tectonic environ- ELA–ICP–MS and oxygen isotope documentation for a ser- ment before ~747 Ma to a contractional setting after 729‒ ies of zircon standards: Chemical Geology, v. 205, p. 115– 727 Ma over a protracted period of almost 20 Ma, imply- 140. doi:10.1016/j.chemgeo.2004.01.003 ing that Rodinia fragmented before ~747 Ma, and subse- Buchwaldt, R., Tucker, R.D., and Dymek, R.F., 2003, – quent lithospheric subduction and collision of smaller Geothermobarometry and U Pb geochronology of metapeli- tic granulites and pelitic migmatites from the Lokoho region, cratonic blocks. Northern Madagascar: American Mineralogist, v. 88, p. 1753–1768. Bybee, G.M., Ashwal, L.D., and Wilson, A.H., 2010, New 6. Conclusions evidence for a volcanic arc on the western margin of a rifting Rodinia from ultramafic intrusions in the Andriamena region, – – north-central Madagascar: Earth and Planetary Science (1) Zircon LA ICP MS dating of the Antanimbary Letters, v. 293, p. 42‒53. doi:10.1016/j.epsl.2010.02.017 granitoid and Antasakoamamy granitoid indicates Chu, N.-C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella, that these intrusions were emplaced at 747 and R.M., Milton, J.A., German, C.R., Bayon, G., and Burton, 729–727 Ma, respectively. K., 2002, Hf isotope ratio analysis using multi-collector (2) The Antanimbary granitoid was generated by par- inductively coupled plasma mass spectrometry: An evalua- tion of isobaric interference corrections: Journal of tial melting of thinned lower crust, which was Analytical Atomic Spectrometry, v. 17, p. 1567‒1574. most likely triggered by underplating of mantle doi:10.1039/b206707b plume-derived magmas. The Antasakoamamy Collins, A.S., 2006, Madagascar and the amalgamation of granitoids were generated by partial melting of Central Gondwana: Gondwana Research, v. 9, p. 3–16. thickened lower crust, which was related to doi:10.1016/j.gr.2005.10.001 Collins, A.S., Fitzsimons, I.C.W., Hulscher, B., and subduction. Razakamanana, T., 2003a, Structure of the eastern margin (3) The geochemistry of these granitoids suggests of the East African Orogen in central Madagascar: crustal extension of the Imorona–Itsindro Suite Precambrian Research, v. 123, p. 111‒133. doi:10.1016/ before ~747 Ma followed by lithospheric subduc- S0301-9268(03)00064-0 tion and arc magmatism after 729‒727 Ma. 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 structural section through part of the East African Orogen: Acknowledgements Geological Society, London, Special Publications, v. 206, Comments by R.D. Tucker and an anonymous reviewer contrib- p. 363‒379. doi:10.1144/GSL.SP.2003.206.01.17 uted to improving the manuscript. Edits in English by Dr Robert Collins, A.S., and Pisarevsky, S.A., 2005, Amalgamating eastern J. Stern are much appreciated. Gondwana: The evolution of the Circum-Indian Orogens: Earth Science Reviews, v. 71, p. 229–270. doi:10.1016/j. earscirev.2005.02.004 Funding Condie, K.C., and Kröner, A., 2008, When did plate tectonics This research was jointly supported by the China Postdoctoral begin? Evidence from the geologic record: When Did Plate ‘ Tectonics Begin on Planet Earth: Geological Society of Science Foundation (project 2013M541000) and the Preliminary – Reconnaissance on the Tectonic Setting and Mineral Exploration America Special Papers, v. 440, p. 281 294. doi:10.1130/ Potential of the Global Giant Metallogenic Belts’ project of the 2008.2440(14) China Geological Survey (CGS; project 12120113102100). de Wit, M.J., 2003, Madagascar: Heads its a continent, tails its an island: Annual Review of Earth and Planetary Sciences, v. 31, p. 213–248. doi:10.1146/annurev.earth.31.100901. 141337 References Elhlou, S., Belousova, E., Griffin, W.L., Pearson, N.J., and Andersen, T., 2002, Correction of common lead in U–Pb ana- O’Reilly, S.Y., 2006, Trace element and isotopic composition lyses that do not report 204Pb: Chemical Geology, v. 192, p. of GJ-red zircon standard by laser ablation: Geochimica et 59–79. doi:10.1016/S0009-2541(02)00195-X Cosmochimica Acta, v. 70, p. 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