U–Pb Zircon Geochronology and Geochemistry of Neoproterozoic
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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