A New Geological Framework for South-Central Madagascar, and Its Relevance to the "Out-Of- Africa" Hypothesis

Elsevier Editorial System(tm) for Precambrian Research Manuscript Draft

Manuscript Number: PRECAM2978

Title: A new geological framework for south-central Madagascar, and its relevance to the "out-of- Africa" hypothesis

Article Type: Research Paper

Keywords: Paleoproterozoic; south Madagascar; U-Pb geochronology; detrital zircons

Corresponding Author: Dr. Robert D. Tucker,

Corresponding Author's Institution: USGS

First Author: Robert D. Tucker

Order of Authors: Robert D. Tucker; Jean-Yves Roig, Ph.D.; Paul H Macey, Ph.D.; Claude Delor, Ph.D.; Yuri Amelin, Ph.D.; Richard Armstrong, PhD.; Mamy H Rabarimanana, Ph.D.

Abstract: The Precambrian shield of south-central Madagascar consists of three geologic domains, from north to south: Antananarivo, Ikalamavony-Itremo, and Anosyen-Androyen. The northern Antananarivo domain represents the Neoarchean sector of the Greater Dharwar Craton itself amalgamated at 2.52-2.48 Ga. The Greater Dharwar is overlain by a craton-wide assemblage of earliest Mesoproterozoic supracrustal rocks (Itremo, Sahantaha, and Maha groups), having a diagnostic signature of Paleoproterozoic detrital zircons (1.91-1.79 Ga). The central domain (Ikalamavony- Itremo) is characterized by a suite of Mesoproterozoic igneous rocks (Dabolava Suite) and volcano- clastic sediments (Ikalamavony Group) formed between 1.03 Ga and 0.98 Ga (Irumide time). These, and part of the neighboring Antananarivo domain, were translated eastward as fold-and-thrust nappes in latest Neoproterozoic time. The southern domain (Anosyen) consists of a newly-recognized suite of Paleoproterozoic igneous rocks (2.0-1.8 Ga) and stratified supracrustal gneisses of sedimentary origin, some of which were deposited before the earliest Neoproterozoic. The contact between the Anosyen- Androyen and Ikalamavony-Itremo domains, formerly known as the Ranotsara shear zone, is a tightly folded and highly sheared boundary that was ductilely deformed in latest Neoproterozoic time. It is roughly equivalent to the KKPTSZ in south India, and it defines approximately the boundary between the Archean Greater Dharwar Craton (to the north) and the newly-identified Paleoproterozoic terrane (to the south).

The diagnostic signature of Paleoproterozoic (2.0-1.8 Ga) detritus in the Itremo Group has been cited as evidence that central Madagascar was derived from Africa and sutured to India in Neoproterozoic time. The discovery of Paleoproterozoic rocks in the Anosyen Domain, and their linkage to central Madagascar by Mesoproterozoic time (~ 1000 Ma), forces a reconsideration of the "out-of-Africa" hypothesis. In its place we propose that the enigmatic zircons (2.0-1.8 Ga) were derived from the combined terranes of South Madagascar-India, the Wanni and Highland complexes of Sri Lanka, and the Rayner Complex of Antarctica (SMIWHR) which were sutured with the Greater Dharwar Craton at ca.1.8 Ga. Thus a Neoproterozoic suture in east Madagascar (i.e. Betsimisaraka) is not required and the concept of "terrane-transfer" across the paleo-Mozambique ocean should be abandoned.

Cover letter

United States Department of the Interior

U.S. GEOLOGICAL SURVEY Reston, Virginia 20192 8 June 2010 Editorial Office Precambrian Research

Dear Sir:

I have submitted to Precambrian Research a manuscript which I would like to be considered for future publication: A new geological framework for south-central Madagascar, and its relevance to the “out-of-Africa” hypothesis by R.D. Tucker a,*, J.Y. Roig b, P.H. Macey c, C. Delor b, Y. Amelin d, R.A. Armstrong d, M.H. Rabarimanana e, and A.V. Ralison e

a United States Geological Survey, National Center, MS 926A, 12201 Sunrise Valley Drive, Reston, VA 20192, USA b Bureau des Recherches Géologiques et Minières, 3 avenue C. Guillemin, BP 6009, 45060 Orléans, Cedex 2, France c Council for Geoscience - Western Cape, SOUTH AFRICA d Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AUSTRALIA e Project de Gouvernance des Ressources Minerales, 101 Antananarivo, MADAGASCAR

* Corresponding author: Robert D. Tucker, [email protected], (office) 703-648-6087

This manuscript represents original work that is not being considered for publication elsewhere.

All of our references are from the published literature except for contract reports (BGS et al. 2008, CGS 2009, GAF-BRG 2008e-f) which are in the public domain and may be obtained through the offices of the Project de Gouvernance des Ressources Minerales, 101 Antananarivo, MADAGASCAR. I have included with the submission a letter (pdf file) from the PGRM granting us permission to cite this work. Please note that the PGRM makes reference to submission to J. Geol. Society of London, but because of submission delays, we are submitting our work to Precambrian Research.

All of the authors have contributed substantially to the manuscript and they have approved the final submission. Geologists Bill Burton and Greg Walsh of the US Geological Survey have kindly reviewed the manuscript for its content and presentation, and the manuscript is presented with permission of the Director of the USGS.

Objective scientists who are familiar with the geology of this part of Madagascar and south India, and who would serve as potential reviewers are:

1. Bernard Moine. Laboratoire de Mineralogie, 39 Allées Jules Guesde, 31000 Toulouse, FRANCE; [email protected] 2. Jean-Louis Paquette, Universite Blaise Pascal-CNRS, Clermont-Ferrand FRANCE, [email protected] 3. Dr. Bernard Bingen, Geological Survey of Norway, Trondheim, NORWAY, [email protected] 4. Dr. Nigel Harris, Department of Earth sciences, Open University, Milton Keynes MK7 6AA, UK, [email protected]

Because of the contentious nature of our conclusions, I respectfully request that the manuscript not be reviewed by Maarten deWit, Alan Collins, Ronadh Cox or Bert De Waele whose views on this topic are well known and contrary to ours. We would be delighted to address their concerns in a formal reply should they choose to discuss our published work. Should you have questions, please contact me at the address below. I will be out of the country on a field mission until mid- July, 2010.

Sincerely,

Robert D. Tucker

Robert D. Tucker, Ph.D. U.S. Geological Survey 12201 Sunrise Valley Drive National Center, MS 926A Reston, VA 20192 [email protected] Office No: 3C412 office: 703-648-6087 fax: 703-648-6953 [email protected]

Abstract

A new geological framework for south-central Madagascar, and its relevance to the “out-of-Africa” hypothesis R.D. Tucker a,*, J.Y. Roig b, P.H. Macey c, C. Delor b, Y. Amelin d, R.A.

Armstrong d, M.H. Rabarimanana e, and A.V. Ralison e

ABSTRACT

The Precambrian shield of south-central Madagascar consists of three geologic domains, from

north to south: Antananarivo, Ikalamavony-Itremo, and Anosyen-Androyen. The northern

Antananarivo domain represents the Neoarchean sector of the Greater Dharwar Craton itself

amalgamated at 2.52-2.48 Ga. The Greater Dharwar is overlain by a craton-wide assemblage of

earliest Mesoproterozoic supracrustal rocks (Itremo, Sahantaha, and Maha groups), having a

diagnostic signature of Paleoproterozoic detrital zircons (1.91-1.79 Ga). The central domain

(Ikalamavony-Itremo) is characterized by a suite of Mesoproterozoic igneous rocks (Dabolava

Suite) and volcano-clastic sediments (Ikalamavony Group) formed between 1.03 Ga and 0.98 Ga

(Irumide time). These, and part of the neighboring Antananarivo domain, were translated

eastward as fold-and-thrust nappes in latest Neoproterozoic time. The southern domain

(Anosyen) consists of a newly-recognized suite of Paleoproterozoic igneous rocks (2.0-1.8 Ga)

and stratified supracrustal gneisses of sedimentary origin, some of which were deposited before

the earliest Neoproterozoic. The contact between the Anosyen-Androyen and Ikalamavony-

Itremo domains, formerly known as the Ranotsara shear zone, is a tightly folded and highly

sheared boundary that was ductilely deformed in latest Neoproterozoic time. It is roughly

equivalent to the KKPTSZ in south India, and it defines approximately the boundary between the

Archean Greater Dharwar Craton (to the north) and the newly-identified Paleoproterozoic terrane

(to the south).

The diagnostic signature of Paleoproterozoic (2.0-1.8 Ga) detritus in the Itremo Group has been cited as evidence that central Madagascar was derived from Africa and sutured to India in

Neoproterozoic time. The discovery of Paleoproterozoic rocks in the Anosyen Domain, and their linkage to central Madagascar by Mesoproterozoic time (~ 1000 Ma), forces a reconsideration of the “out-of-Africa” hypothesis. In its place we propose that the enigmatic zircons (2.0-1.8 Ga) were derived from the combined terranes of South Madagascar-India, the

Wanni and Highland complexes of Sri Lanka, and the Rayner Complex of Antarctica

(SMIWHR) which were sutured with the Greater Dharwar Craton at ca.1.8 Ga. Thus a

Neoproterozoic suture in east Madagascar (i.e. Betsimisaraka) is not required and the concept of

“terrane-transfer” across the paleo-Mozambique ocean should be abandoned.

Manuscript Click here to view linked References

A new geological framework for south-central Madagascar, and its relevance to the “out-of-Africa” hypothesis

R.D. Tucker a,*, J.Y. Roig b, P.H. Macey c, C. Delor b, Y. Amelin d, R.A.

Armstrong d, M.H. Rabarimanana e, and A.V. Ralison e

a United States Geological Survey, National Center, MS 926A, 12201 Sunrise Valley Drive, Reston, VA 20192, USA

b Bureau des Recherches Géologiques et Minières, 3 avenue C. Guillemin, BP 6009, 45060 Orléans, Cedex 2, France

c Council for Geoscience - Western Cape, SOUTH AFRICA

d Research School of Earth Sciences, Australian National University, Canberra ACT 0200, AUSTRALIA

e Project de Gouvernance des Ressources Minerales, 101 Antananarivo, MADAGASCAR

* Corresponding author. Tel.: +1 703 648 6087; fax: +1 703 648 6953

E-mail address: [email protected] (R.D. Tucker)

Keywords: Paleoproterozoic terrane, Irumide orogeny, Gondwana, Madagascar, U-Pb geochronology

For Submission to: Precambrian Research

ABSTRACT

The Precambrian shield of south-central Madagascar consists of three geologic domains, from north to south: Antananarivo, Ikalamavony-Itremo, and Anosyen-Androyen. The northern

Antananarivo domain represents the Neoarchean sector of the Greater Dharwar Craton itself amalgamated at 2.52-2.48 Ga. The Greater Dharwar is overlain by a craton-wide assemblage of earliest Mesoproterozoic supracrustal rocks (Itremo, Sahantaha, and Maha groups), having a diagnostic signature of Paleoproterozoic detrital zircons (1.91-1.79 Ga). The central domain

(Ikalamavony-Itremo) is characterized by a suite of Mesoproterozoic igneous rocks (Dabolava

Suite) and volcano-clastic sediments (Ikalamavony Group) formed between 1.03 Ga and 0.98 Ga

(Irumide time). These, and part of the neighboring Antananarivo domain, were translated eastward as fold-and-thrust nappes in latest Neoproterozoic time. The southern domain

(Anosyen) consists of a newly-recognized suite of Paleoproterozoic igneous rocks (2.0-1.8 Ga) and stratified supracrustal gneisses of sedimentary origin, some of which were deposited before the earliest Neoproterozoic. The contact between the Anosyen-Androyen and Ikalamavony-

Itremo domains, formerly known as the Ranotsara shear zone, is a tightly folded and highly sheared boundary that was ductilely deformed in latest Neoproterozoic time. It is roughly equivalent to the KKPTSZ in south India, and it defines approximately the boundary between the

Archean Greater Dharwar Craton (to the north) and the newly-identified Paleoproterozoic terrane

(to the south).

The diagnostic signature of Paleoproterozoic (2.0-1.8 Ga) detritus in the Itremo Group has been cited as evidence that central Madagascar was derived from Africa and sutured to India in

Neoproterozoic time. The discovery of Paleoproterozoic rocks in the Anosyen Domain, and their linkage to central Madagascar by Mesoproterozoic time (~ 1000 Ma), forces a

2 reconsideration of the ―out-of-Africa‖ hypothesis. In its place we propose that the enigmatic zircons (2.0-1.8 Ga) were derived from the combined terranes of South Madagascar-India, the

Wanni and Highland complexes of Sri Lanka, and the Rayner Complex of Antarctica

(SMIWHR) which were sutured with the Greater Dharwar Craton at ca.1.8 Ga. Thus a

Neoproterozoic suture in east Madagascar (i.e. Betsimisaraka) is not required and the concept of

―terrane-transfer‖ across the paleo-Mozambique ocean should be abandoned.

1. Introduction

The Neoproterozoic East African Orogen records the closure of several ocean basins and the final assembly of Gondwanaland. A widely cited, yet contentious, boundary within the orogen is the Betsimisaraka suture of east Madagascar (Collins et al. 2000, Kröner et al. 2000), purportedly a convergent Neoproterozoic margin that sutures the continental fragments of ―Azania‖ (of

African origin) and the Dharwar Craton, India (of East Gondwana, Collins and Pisarevsky 2005;

Fig. 1).

The Betsimisaraka suture is premised on several lines of evidence, all of them recently questioned by Tucker et al. (2010). One of these is the presence of a low-grade medial

Proterozoic sedimentary sequence in central Madagascar (Itremo Group, Moine 1974), containing a subset of detrital zircons (2.2-1.8 Ga), purportedly sourced from the Congo Craton

(Cox et al. 1998, 2004, Fitzsimons and Hulscher 2005). Thus it is postulated that Azania, comprised of the Itremo Group and its Archean basement, was rifted ―out-of-Africa‖ and accreted to the Dharwar Craton before the final amalgamation of Gondwanaland (Cox et al.

2004, Fitzsimons and Hulscher 2005, Collins 2006). A similar line of logic was used by Collins et al. (2007 a,b) to propose an extension of the Betsimisaraka Suture into south India as the

Palghat-Cauvery shear zone. Recently BGS et al. (2008) have identified two other sequences in north and east Madagascar, the Sahantaha and Maha Groups (Fig. 1), also with the same enigmatic signature of Paleoproterozoic detrital zircons (2.2-1.8 Ga). In a modification of the

―out-of-Africa‖ hypothesis, DeWaele et al. (2008) suggest that the Itremo, Sahantaha, and Maha groups are allochthonous and exotic, having been translated many kilometers from their depositional site onto an entirely different basement terrane.

Is there an alternative solution to these extraordinary proposals? Can the provenance of the groups be reconciled in the context of our present knowledge? We examine more closely both versions of the ―terrane-transfer‖ hypothesis, and present an alternative proposal that attempts to reconcile all data into a new geological framework for Madagascar.

2. Regional geologic domains

The present paper benefits from a recent project (PRSM-2), funded by the International

Development Agency, and implemented to improve the knowledge and management of

Madagascar’s mineral resources. A major task of PRSM-2 was to create a comprehensive and uniform coverage of geological maps and geophysical data over large parts of north, central and south Madagascar. This project was completed in 2007 by an international consortium of national surveys and federal institutions from Britain, France, Germany, Madagascar, South

Africa, and the United States (BGS et al. 2008, CGS 2009 a,b, and GAF-BGR 2008 a-e).

The international consortiums defined the tectonic domains of Madagascar illustrated in

Figures 2 and 3. In this paper we draw special attention to the Ikalamavony-Itremo and Anosyen domains of south Madagascar, and focus our discussion on their context within the greater

Gondwana framework. Superimposed within and across the domains are km-wide zones of

4 steeply-dipping, highly-strained rocks that record the effects of latest Neoproterozoic transpressive shortening (Pili et al. 1997, Martelat et al. 2000, Nédélec et al. 2000, de Wit et al.

2001).

2.1 Antananarivo Domain

The Neoarchean rocks of the Antananarivo Domain consists of granitic gneisses (Betsiboka

Suite), with older (but undated) vestiges of paragneisses (Sofia and Vondroza Groups), and four distinctive belts of mafic gneiss and schist (Tsaratanana complex) (BGS et al. 2008, GAF-BGR

2008a, CGS 2009a and references therein). In central Madagascar the Antananarivo Domain is overlain by Proterozoic (mostly) metaclastic rocks (Itremo, Ambatolampy, and Manampotsy

Groups) all with an enigmatic signature of Paleoproterozoic (2.0-1.8 Ga) detritus (Cox et al.

2004, Fitzsimons and Hulscher 2005, Fernandez et al. 2003, De Waele et al. 2008, Tucker et al.

2010). The Maha Group, also of the same age and provenance (DeWaele et al. 2008), crops out in the eastern domain of Masora (the Dharwar Craton). All of the Proterozoic groups, and their

Archean basement, are intruded by Early Neoproterozoic (820-740 Ma) and Late Neoproterozoic

(560-530 Ma) batholiths, stocks, and stratiform massifs. The earliest of these, the Imorona-

Itsindro Suite that ranges in composition from gabbro to peraluminous granite (Handke 2001,

McMillan et al. 2003, CGS 2009 a,b), is commonly ascribed a subduction origin (e.g. Handke et al. 1999, Kröner et al. 2000, Bybee et al. 2010). The youngest suite of intrusive igneous rocks

(Maevarano-Ambalavao Suite, Goodenough et al. 2010) are widely attributed to manifestations of the ―Pan-African‖ orogeny (BGS et al. 2008, CGS 2009a,b, GAF-BGR 2008a and references therein).

In a widely-held view, the Neoarchean basement and the Itremo Group constitute the mini- continent of Azania of purported East African origin (Cox et al. 2004, Fitzsimons and Hulscher

2005, Collins 2006, Kröner et al. 2001, BGS et al. 2008). The eastern boundary of Azania, and the site of its Neoproterozoic collision with India, is the Betsimisaraka suture (Fig. 1). In a different view, the mostly juvenile Neoarchean rocks of central Madagascar are part of the

Greater Dharwar Craton itself coalesced during a Neoarchean accretion event (Tucker et al.

2010). In this latter view, the ―Betsimisaraka suture‖ is a zone of highly-strained,

Neoproterozoic metasedimentary rocks that mostly masks the boundary between Meso- and

Neoarchean elements of the Greater Dharwar Craton. In both views, the younger igneous suites are the product of convergent margin magmatism (Imorona-Itsindro Suite) and orogenic convergence (Maevarano-Ambalavao Suite), respectively, that accompanied the youngest phases of Gondwana’s amalgamation.

2.2 Itremo-Ikalamavony Domain

The Itremo–Ikalamavony Domain represents a Late Neoproterozoic–early Cambrian fold- and-thrust belt comprising Neoarchean and various Proterozoic gneisses thrust eastwards over the southwestern edge of the Antananarivo Domain and intruded by late-tectonic Cambrian granites during the East African Orogeny (CGS 2009a,b; GAF-BGR, 2008a; Tucker et al., 2007).

The <100-km to 200-km-wide NW-SE trending orogenic belt extends from 23°- 17°S where it is observed in the Bekadoka inlier (Fig. 3, CGS 2009 a,b). The Itremo–Ikalamavony Domain has been further subdivided into two sub-domains based on differences in component lithostratigraphy, structural style, and metamorphic grade (CGS 2009 a,b, GAF-BGR 2008a).

The Itremo Sub-domain, in the east, is comprised of an imbricate, near-recumbent nappe stack of greenschist- to low amphibolite grade quartzite, marble and schist of the Proterozoic Itremo

Group (Moine 1963, 1974). These are intruded by and tectonically interleaved with granitic and gabbroic orthogneiss of the Mid-Neoproterozoic Imorona-Itsindro Suite (Handke et al. 1999,

CGS 2009 a,b; GAF-BGR 2008a).

The lithostratigraphy of the overlying and higher-grade Ikalamavony Sub-domain is variable but, in general, it is dominated by volcanic or volcaniclastic units. Lithotectonic sheets of strongly sheared Itremo, Mesoproterozoic Ikalamavony and Neoproterozoic Molo Group supracrustal rocks occur tectonically interleaved with orthogneisses of the Mesoproterozoic

Dabolava and the Itsindro-Imorona Suites (CGS, 2009a, b; GAF-BGR, 2008a; Tucker et al.,

2007; Cox et al., 2004; this study). Although rare, lenticular bodies of Antananarivo Domain migmatites occur as thrust slices within the Ikalamavony Sub-domain (CGS, 2009a, b). The

Ikalamavony Group, of Mesoproterozoic age (Rakotoarimanana 2001, CGS 2009 a,b; this study), is comprised of feldspar-rich psammitic gneiss and amphibolite interlayered with subordinate quartz-feldspar gneiss (metarhyolite). The Dabolava Suite, which intrudes the

Ikalamavony Group and consists of gabbro, diorite, and tonalite gneiss, is dated between 1020 and 982 Ma and has arc-type geochemical compositions (Rakotoarimanana 2001, Tucker et al.

2007, CGS, 2009a, this study). The volcano-sedimentary gneisses of the Neoproterozoic Molo

Group are restricted to the eastern margin of the Ikalamavony Sub-domain just wewst of the

Itremo Group (Figure 3; Cox et al., 1998, 2004). Late- to post-tectonic potassic Ambalavao Suite granites intrude the Itremo-Ikalamavony and Antananarivo in Cambrian times (Tucker et al.

1999, 2007; CGS 2009a; Goodenough et al. 2010). The metamorphic grade of rocks in the

Ikalamavony Sub-domain is upper amphibolite- to granulite grade and thus the overall setting across the Ikalamavony-Itremo Domain is one of warm rocks thrust over cool.

The age of the east- to northeast-vergent recumbent folding and thrust-nappe emplacement, is somewhat controversial. Collins et al (2003a,b), Fernandez et al (2003), and GAF-BGR

(2009b) propose that the Ikalamavony-Itremo supracrustals were translated, recumbently folded, and metamorphosed prior to emplacement of the Imorona-Itsindro Suite (ca. 800 Ma), although they were also deformed and metamorphosed in the Neoproterozoic. Tucker et al. (2007) and

CGS (2009 a,b) believe that all of the regional metamorphism and polyphase deformation, is of latest Proterozoic to earliest Cambrian age (560-540 Ma). All agree that megascale open refolding, about N-S sub-horizontal axes, has produced a complex pattern of superposed folds across all of the Itremo–Ikalamavony Domain.

2.3 Anosyen Domain

Thick units of granulite-facies garnet-cordierite leucogneiss and quartzo-feldspathic gneiss comprise most of the Anosyen Domain. The domain is bounded on the west by the Beraketa high-strain zone, and to the north and east by the Ikalamavony Sub-domain. Its northeast boundary lies within the Ranotsara plain and it follows a zone of highly-strained and mylonitic rocks. Thus the ―Bongolava-Ranotsara shear zone‖ was regarded by early workers as a nearly linear boundary that separates granulite grade Neoproterozoic gneisses (to the south) from amphibolite grade gneisses (to the north). New geophysical imaging indicates this is not the case

(GAF-BGR 2008 b,c). Figure 2 illustrates that the Anosyen-Ikalamavony boundary has a highly irregular shape reflecting, in part, the ductile deformation of the contact. In the southeast, the

8 boundary is marked by a change from generally weakly-magnetic strata of the Anosyen Domain to generally magnetic strata of the Ikalamavony Sub-domain. Based on the distinct differences in aeromagnetic response, it is possible to map rocks of the Anosyen Domain across the trace of the

―Ranotsara-Bongolava shear zone‖ with apparent deflections limited to approximately 60 km of left-lateral offset. Analysis of the gamma ray radiometric image reveals the same conclusion

(GAF-BGR 2008 b,c). Field observations near Fort Dauphin suggest that the Anosyen Domain overlies the Ikalamavony Sub-domain but, along the northwest part of the contact, rocks of the

Ikalamavony Group are tightly folded deep within the Anosyen Domain (Fig. 2). In other places, rocks of the Anosyen Domain overlie the Ikalamavony Group as klippen (GAF-BGR 2008 b-d).

Precambrian stratified rocks of the Anosyen Domain were deformed and metamorphosed in the Neoproterozoic to granulite grade and, as a result, stratified quartzo-feldspathic gneiss

(leptynite) and paragneiss (± hyp-sill) predominate. These were interpreted by early workers as massive outpourings of rhyolite ignimbrite and volcaniclastic sediment (Fort Dauphin and

Tranomaro groups, Besairie 1964), some of them intruded by sheets of stratiform granite hundreds of feet thick (e.g. Anosyen massif). Also common, and structurally overlying the leptynites, are cordierite and/or spinel-garnet schist and gneiss (Ihosy Group, Besairie 1964) some of them intercalated with beds of marble and calc-silicate. Whereas all of the stratified gneisses are of volcanic or sedimentary origin, the age of their protoliths is very much in doubt.

Whole-rock Rb-Sr data (e.g. Vachette and Hottin 1979, Cahen et al. 1984 summarized by

Windley et al. 1994) suggests that the lowest, stratified leptynites of the eastern Anosyen

Domain (Fort Dauphin and Tranomaro groups) may be as old as ca. 2.4-2.1 Ga. Support for this comes from Paquette et al. (1994) and Müller (2000) who reported Nd data (TDM Nd = 2.8-2.1

Ga) and an upper-intercept U-Pb age (zircon, ~1840 Ma) for leptynites of the Ft. Dauphin

Group. Paquette et al. (1994) also reported a U-Pb zircon upper intercept age (zircon, 1.679 Ga) for the Vohimena Granite, shown to intrude (or be melted from) the Tranomaro Group west of

Fort Dauphin. The upper-intercept, however, was interpreted as the mean age of zircon inheritance (Paquette et al 1994) because the concordia lower intercept (572 ± 14 Ma) agreed well with metamorphic monazite ages (570-520 ma) and it seemed likely that granite emplacement accompanied high-grade metamorphism. Indeed, Paquette et al. (994), Kroner et al

(1996, 1999), and de Wit et al. (2003) recovered Neoproterozoic detrital zircons from other localities, and de Wit et al. (2001) directly dated a metavolcanic gneiss, thus confirming a

Neoproterozoic age for at least some of the stratified rocks of south Madagascar. Thus, given the ambiguity of the Nd and U-Pb data, it is widely inferred that the Anosyen Domain consists of high-grade Neoproterozoic metasedimentary rocks, some of them very young (< 600 Ma), and perhaps correlative with the Itremo or Molo groups to the north (de Wit 2003, Collins 2006).

GAF-BGR (2008 c-e) has recently confirmed the young age for the Iakora Group with a date of

736 ± 16 Ma for a volcanic layer (metarhyolite in the Horombe Group) interpreted to date the deposition of the stratified rocks (GAF-BGR 2008 c-e). They also demonstrated the presence of

Paleoproterozoic and Archean detritus in samples of the Iakora Group near Ft. Dauphin but, unlike Kröner et al. (1999, 1996) and de Wit (2001), they do not report detrital zircons of

Neoproterozoic age in their samples (GAF-BGR 2008 c-e).

U-Pb SHRIMP geochronology was undertaken to address several issues relevant to the terrane-transfer hypothesis: (1) the possible Paleoproterozoic ancestry of the Anosyen Domain,

(2) the geographic range of the Dabolava Suite, (3) the age and provenance of the Ikalamavony

Group, and (4) age and provenance of the Iakora Group (Anosyen Domain).

3. Analytical methods and results of U-Pb geochronology

U–Pb ages of zircon were measured on the SHRIMP RG ion microprobe at the Research School of Earth Sciences (RSES), the Australian National University, using the methods described by

Williams (1998) and Hiess et al. (2009). The data were collected during nine analytical sessions between April and May, 2007 and April and October 2009. A ca. 3–5 nA mass filtered O2- primary beam was focused to elliptical spots with the sizes between 17x24 µm and 25x30 µm.

Before data were acquired, the beam was rastered for 120 s to clean the mount surface. The

90 16 204 206 207 208 238 232 16 magnet was stepped through peaks of Zr2 O, Pb, Pb, Pb, Pb, U, Th O and

238U16O; each analysis included 5-8 cycles. The data were reduced using the SQUID+ISOPLOT software package (Ludwig 2001, 2003). Uranium concentrations were calculated relative to U =

238 ppm in the SL13 zircon reference material, and U/Pb ratios were calculated relative to

206Pb/238U age of 417 Ma for the Temora standard (Black et al. 2004). The ages were calculated using the decay constants of uranium of Jaffey et al. (1971), and the 238U/235U ratio of 137.88.

Non-radiogenic Pb was subtracted from the measured Pb isotopic composition using measured

204Pb, and the present-day average terrestrial Pb isotopic composition in the model of Stacey and

Kramers (1975). Concordia regressions, concordia age and weighted average age values are calculated with ISOPLOT (Ludwig 2003). The errors of the ages are reported at 95% confidence level.

U-Pb isotopic data are reported in Table S1 (Supplementary material), and ages are summarized in Table 1 and Figs. 5,7 and 9. Cathodoluminescence images of selected grains are presented in Figs. 4,6, and 8.

3.1 Paleoproterozoic ancestry of south Madagascar

3.1.1 IHY-08-20A: Granite migmatite gneiss (Androyen Domain) (Laborde:

356435/40224). Sample IHY-08-20A was collected from an isolated pavement outcrop of granite migmatite gneiss on the Horombe plateau approximately 20 km WSW of Ihosy. Bedrock outcrop on the plateau is extremely poor and the context of the gneiss to the surrounding stratified rocks of the Androyen Domain cannot be established. In outcrop, three different rocks are visible: (A) a medium-grained, granite gneiss, (B) concordant sheets of fine-grained dark amphibolite 5-15 cm thick, and (C) leucosomes, 5-20 cm thick of coarse- and medium-grained leucocratic granite of anatectic origin. Both the granite gneiss (A) and amphibolite (B) are isoclinally folded, and the granite gneiss is partially melted with its leucosomes (C) coalesced into discordant veins and layers to the gneissic layering. Sample IHY-08-20A is the granite gneiss which is established to be the oldest rock based on cross-cutting relationships with amphibolite and leucosomes.

The recovered zircons in IHY-08-20A are very complex and contain the following sequence of CL domains (Fig. 4a-b): (1) Cores with moderate to low CL brightness and indistinct zoning,

(2) CL-bright cores with oscillatory or sector zoning, (3) Very CL-dark, structureless inner rims, and (4) moderately CL-dark outermost zones (rims, tips and whole grains) with faint zoning or no visible structure. The boundaries between all core and rim domains have complex shapes, suggesting replacement of older generations of zircon with younger material.

The systematics of the isotopic data is as complex as the CL domains (Fig. 5a-d). Two of the domain 1 cores, with moderate CL brightness, have 207Pb/ 206Pb dates of ca. 2.55 Ga, the oldest in this population. Most domain 2 cores, and one domain 1 core (with moderate brightness), yield 207Pb/ 206Pb dates close to 1.8 Ga. Seven of these data points are concordant within 10%,

12 and define an upper concordia intercept at 1793±19 Ma, our best estimate for the emplacement age of the granite protolith (Fig. 5b).

Domain 3 inner rims have higher U, and lower Th/U, than the cores. All five analyses of inner rims are discordant, and form a slightly scattered array (Fig. 5a) that intercepts concordia at ca. 2.55 Ga and 593 ± 58 Ma; the lower intercept is interpreted as the reworking of high-U

Neoarchean or Paleoproterozoic zircon during Neoproterozoic metamorphism and/or partial melting.

All 14 analyses of outer domain 4 rims are concordant (or nearly so), and have a considerable spread of ages well outside analytical error. In figure 5d, these data are arranged in the sequence of descending 206Pb/ 238U dates. The origin of this spread of the dates is not clear, but it is unlikely to be caused by Pb loss, because the dates do not correlate with U content and the range of U concentrations is not exceptionally high. Moreover, Pb loss is unlikely to produce a bimodal age distribution but, rather, many apparent ages that are progressively younger than the age of zircon formation. A more likely interpretation is that the domain 4 outer rims grew in two (or more) events at 595 ± 11 Ma and 522 ± 9 Ma (Fig. 5d).

3.2.2 IHY-08-12:Coarse-grained gabbro gneiss (Anosyen Domain) (Laborde:

362149/467348). Sample IHY-08-12 is a very coarse-grained phase of gabbro gneiss and amphibolite in a small stream exposure ca. 55 km NNE of Ihosy near Amboboky village. Most of the outcrop is a well-foliated and strongly lineated coarse-grained gabbro gneiss consisting of

3-5 mm-sized crystals of clinopyroxene, hornblende, and plagioclase. The outcrop is mapped as stratified gneiss of the Iakora Group by GAF-BGR (2008c), but it is clearly not a supracrustal gneiss, and its relationship to nearby psammitic paragneiss and marble is unknown because of poor exposure.

The recovered zircons from this sample consist of three distinct domain types (Fig. 4c-d):

(1) CL-dark cores, some with faint oscillatory zoning, (2) CL-bright inner rims with concentric or block-shaped zoning and radial cracking, and (3) CL-dark, structureless outer rims. There are also a few whole crystals (4) with CL-dark uniform-reflectance resembling outer rims domains.

Domain 1 cores have high U concentrations between 790 and 1500 ppm, and relatively low

Th/U. Their U-Pb data are slightly discordant (8-16%), and 207Pb/ 206Pb dates are slightly variable between 1915 and 1828 Ma. Discordia regression through all core analyses yields a concordia upper intercept at 2002 ± 68 Ma, which is a rough approximation of the crystallization age. A more precise date is given by the weighted mean 207Pb/ 206 Pb age of the five oldest concordant analyses of 1908 ± 8 Ma (Fig. 5e). This is interpreted as the crystallization age of the gabbro protolith.

Data from domain 2 low-U inner rims spread along a discordia with upper- and lower- intercepts of 1874 ± 130 Ma and 563 ± 63 Ma, respectively. The upper intercept overlaps with the age of domain 1 cores. Corrosive structural relations between inner rims and cores in many grains, and the coincidence of the upper intercept ages, suggest that the inner rims grew

(completely or in part) by re-crystallization of the core material under conditions that facilitated loss of U and radiogenic Pb. The timing of this process is best estimated by the weighted mean

206Pb / 238U dates of the youngest concordant inner cores, yielding an age of 555 ± 17 Ma (Table

S1). Nearly all analyses of domain 3 outer rims and domain 4 crystals are concordant and equivalent. The only exceptions are analyses 1.1 (negatively discordant) and 11.2, the latter from an outermost rim of high CL brightness that surrounds a CL-dark grain. Eight other analyses from domains 3 and (outer rims and uniform grains) yield a weighted average 206Pb / 238U age of

526 ± 8 Ma, which represent the timing of rim growth (Fig. 5f). The latter age is resolved at 2σ

14 level from the age of the youngest inner rims, and it is reasonable to suggest that the zircons record two separate geological processes that occurred at ca. 555 Ma and 526 Ma.

3.2 Dabolava Suite in the central Ikalamavony Sub-domain

3.2.1 IHY-08-23: trondhjemite gneiss (Laborde: 391592/432412). Outcrop IHY-08-23 is located 30 km northeast of Ihosy and 3 km south of Zazafotsy village along National Route 7. It is a homogeneous, but highly deformed, fine- to medium-grained leucocratic trondhjemite gneiss. The essential minerals are quartz and plagioclase feldspar, and accessory mafic minerals include biotite and hornblende, visible as individual crystals or elongated patches, aligned in the foliation plane.

The CL domains in the recovered zircons of this sample consist of four types: (1) Rare dark inner cores, enclosed by, (2) abundant CL-bright cores, many with sector zoning, (3) inner rims of variable brightness, in most grains darker than the cores. Many inner rims are oscillatory zoned, and (4) thin, CL-bright outer rims, almost all are too thin for analysis by SHRIMP (Fig.

6a,b).

All zircons are concordant or slightly normally discordant, and occupy a range of 207Pb/

206Pb ages between ca. 500 Ma and 1100 Ma (Fig. 7a). With exception of one analysis of the outer rim (spot 5.1) and two analyses of an anomalous grain 2, all analyses representing cores and inner rims plot on a single array with upper- and lower-intercepts of 1035 ± 30 Ma and 399

± 170 Ma, respectively. There is no systematic difference between the cores and inner rims in their 206Pb/ 238U and 207Pb/ 206Pb dates, U concentration, or in Th/U ratios. It is likely that the zircons (both cores and inner rims) formed in a single process at ca. 1035 Ma, and experienced

Pb loss and minor additional zircon growth (outer rims) at ca. 399 Ma. The lower concordia

15 intercept gives only an imprecise minimum age estimate for this younger event, because it is probably affected by recent Pb-loss.

3.3 Neoproterozoic orthogneiss in the Anosyen Domain

3.3.1 IHY-08-3A: granite gneiss north of Ihosy (Laborde: 379701/421929). Outcrop IHY-

08-3 is located 10 km west of Ihosy village c. 100 m north of National Route 7. Our sample was collected from a natural hill-top exposure of strongly foliated and lineated biotite granite gneiss

(A), having thick layer-parallel aplite sheets (B) and discordant veins and dikes of granite pegmatite (C). The rock is mapped as the Benato Gneiss of the Horombe Group and interpreted by GAR-BGR (2008 c) to be of volcanic origin. Our sample IHY-08-3A is a pink-gray coarse- to medium-grained biotite granite gneiss, of clear intrusive origin, and thus the date of this rock will establish the minimum age of the host stratified gneisses (Iakora Group).

Three domain types are visible in the CL images (Fig. 6a,b) of zircon from this sample: (1) zircon cores having sharply defined sector- and oscillatory-zoning, (2) CL dark and mostly structureless cores, many showing signs of corrosion between them and more massive outer rims, and (3) visually homogeneous, CL-dark rims and whole-grains with indistinct sector zoning or no zoning at all.

The U-Pb ages of all domain types are concordant or nearly so, and they have a range of

206Pb/ 238U dates between 492 Ma and 837 Ma. There is no systematic difference among cores, rims, and uniform grains in their U concentrations and Th/U ratios, but there is a clear distinction in age. All domain 3 rims and uniform grains (except one), have consistent 206Pb/ 238U dates with a weighted average value of 548.3 ± 7.9 Ma (Fig. 7d). Their relatively high Th/U, similar to the

16 values for the domain 1 cores, suggests that zircon growth at 548 Ma involved partial melting or injection of a melt, rather than crystallization from an aqueous-rich fluid.

The ages of the cores vary. Two analyses (9.1 and 12.1) from grains that are corroded and partially replaced by rim material are younger than the other core ages. This suggests that the U-

Pb ages of at least some of the domain 2 cores are affected by the same processes that formed the domain 3 rims, probably Pb loss or recrystallization or both. The age variation between 837 Ma and 740 Ma in the group of domain 1 cores may be attributed to a lesser influence of the same process. If this explanation is correct, then the weighted average of 206Pb/ 238U dates of 810 ± 11

Ma from eleven ―oldest‖ magmatic cores (Fig. 7c) gives the age of granite emplacement, whereas the dates of other cores are meaningless. However, a roughly bimodal distribution of core ages suggests an alternative explanation – that igneous zircon formed in two processes: one at 810 ± 11 Ma, and another at 745 ± 14 Ma (the weighted average of 206Pb/ 238U dates for the group of six second oldest grains). The apparent age variations among the cores are not due solely to analytical uncertainty, because an attempt to calculate a weighted average 206Pb/ 238U dates for both groups indicates significant dispersion. Distinguishing between the two explanations requires more precise TIMS analysis but it is clear, nonetheless, that the magmatic cores crystallized in early Neoproterozoic time (c. 810 Ma or 745 Ma). As we explain below, our preferred emplacement age for this rock is 810 ± 11 Ma.

3.3.2 IHY-08-10: biotite granite gneiss (Laborde: 392180/412335). Outcrop IHY-08-10 is located 5 km west of Morarano-Ivily village on a poorly-maintained track leading southeast from

Ihosy. The sample was collected from a large pavement outcrop of strongly foliated and lineated biotite granite gneiss, having thick layer-parallel granitic pegmatite sheets ca. 20 cm thick. The

17 rock is mapped as the Benato Gneiss of the Horombe Group and interpreted by GAR-BGR (2008 c) to be of volcanic origin. Our sample is homogeneous coarse- to medium-grained granite gneiss, clearly of intrusive igneous origin, whose essential minerals are alkali feldspar, quartz, and plagioclase. Biotite is present in accessory amounts and defines the gneissic foliation.

The CL domains in zircon from IHY-08-10 consist of two types: (1) cores with moderate CL brightness and well- preserved sector or oscillatory zoning, and (2) CL-dark, structureless rims

(Fig. 6c-e).

The cores have moderate and rather uniform U concentrations of 84-228 ppm, and relatively high and also uniform Th/U ratios of 0.83 to 2.19, consistent with igneous origin. Their U-Pb data are concordant or nearly concordant, but the spread of 206Pb/ 238U dates from 976 Ma to 595

Ma greatly exceeds analytical error. A weighed average of all 21 core analyses of 785 ± 47 Ma

(MSWD=12) has no geological meaning. There is no correlation between the 206Pb/238U and either U concentration or Th/U ratios. An unusual feature of the age distribution of these analyses is a well defined plateau in the middle of the age distribution that includes majority of analyses (12 of 21), which gives the weighted average 206Pb/ 238U age of 822 ± 17 Ma,

MSWD=0.54. The meaning of this pattern is not immediately obvious. It is possible that the five grains with 206Pb/238U below the plateau value experienced loss of radiogenic Pb, but these grains have no distinct features such as high U, cracks, or visible alteration that would suggest that Pb loss was likely. Similarly, there is no independent evidence that the grains with 206Pb/

238U above the plateau would represent, or contain, inherited cores.

A hint to the possible origin of this age spectrum is given by grain 19. Two analyses from the core of this grain yielded the highest and the lowest 206Pb/ 238U of the entire core population, despite being taken in the same uniform-looking core, and close U concentrations and Th/U. This

18 suggests the possibility of re-distribution of radiogenic Pb within the grain. The mechanism of such re-distribution is unclear, but since the zircons are relatively young, low-U and CL-bright, and thus probably crystalline, it is unlikely to be through volume diffusion. Regardless of the cause of this re-distribution: if it did occur, the grains were probably affected to various degrees, grain 19 being the most affected among the analyzed cores. In this case, the plateau would represent the population of the least affected grains, and give the accurate age of crystallization.

We completed additional analyses to test if the apparent age heterogeneity, observed in grain

19, commonly occurs in other grains. Additional analyses were made from previously analyzed cores of the grains 10, 15, 18, 19 and 20, and two analyses were taken at the core of one more grain (#21). All cores have reasonably well preserved igneous zoning (oscillatory or sector) and are visually homogeneous. Five out of six cores are heterogeneous in 206Pb/ 238U dates. New analyses of three grains (nos. 10, 15 and 18), that appear older than the plateau value (now revised at 803 ± 13 Ma, based in 18 data points), are consistent with the plateau value. The initial analysis from the grain 20 was one of the plateau values, but the new analysis from the same grain at 920 Ma is distinctly older than the plateau. Two additional analyses from the grain 19 that appeared the most heterogeneous initially are both inconsistent with the initial analyses. One of the new analyses is consistent with the plateau; the other one is slightly lower. Both analyses from the only homogeneous grain #21 are consistent with the plateau. We thus conclude: (1)

Intra-grain migration of radiogenic Pb is a common phenomenon in this rock, and (2) the 206Pb/

238U plateau value of 803 ± 13 Ma (Fig. 7f) is the best estimate for the date of core crystallization, and thus the emplacement age of the granite protolith.

It is now clear that the age spectrum in this sample, resulting from the entrapment of old and young radiogenic Pb in different part of the same crystalline domain (Fig. 7f), is different from

19 that of IHY-08-3A which shows a monotonic diminution of age from a plateau of ~810 ± 11 Ma to less than 600 Ma (Fig. 7c). Coincidentally, the interpreted age of both samples is approximately 800 Ma, thus proving (1) that intrusive rocks of the Imorona-Itsindro Suite are present throughout this part of the Anosyen Domain, and (2) the supracrustal gneisses they intrude must be older than 800 Ma.

3.4 Age and Provenance of the Ikalamavony Group (Ikalamavony Sub-domain)

3.4.1 PR06090: Quartzo-feldspathic gneiss (metarhyolite) (Laborde: ). Sample PR06080 is a quartzo-feldspathic gneiss, interlayered with fine-grained amphibolite of the Ikalamavony

Group, in the Lower Mahajilo River eight km east-northeast of Miandrivazo. Field relationships suggest that the interlayering is primary, and whole-rock geochemistry is consistent with the interpretation that the protolith is a meta-rhyolite (Rakotoarimanana, 2001).

Sample PR06090B yielded few zircons, but ~ 75% of them are colorless, elongate euhedral crystals with flat prismatic faces. The remaining zircons are a bit larger, opaque caramel in color, with pitted surfaces and elongate prismatic-tabular habit. Roughly half of the zircons are completely or partially metamict, but four domain types could be identified by CL imagery (Fig. 8 a,b). Most of the grains have obvious cores, many of them consisting of low-U inner cores (domain 1) and outer core domains (domain 2) generally oscillatory-zoned (Fig. 8a,b). Domain 1 and 2 cores are considered of igneous origin. Commonly, both core domains are resorbed and overgrown by rims of high-U zircon (domain 3); in rare cases, whole-gains of high-U zircon are also evident (domain 4).

Domains 3 and 4 owe their origin to metamorphic growth, either upon preexisting grains or as wholly-new crystals.

Analyses of the magmatic zircon type define a discordia with an upper intercept age of 1009 ±

12 Ma (Fig. 9b). Some of these grains show a gradational change to high-U rims (―old rims‖ in

Figure 9a) which appear to be part of the same magmatic phase in most cases. There are also some large, discrete grains of this same high-U zircon. Apart from one analysis, these tend to be discordant (shown as ―old‖ rims in Fig. 9a) consistent with their high U concentrations. These analyses, combined with the clearly magmatic cores, define an upper intercept age of 1013 ± 9.7 Ma which we interpret as the eruption age of the Ikalamavony metarhyolite. Some rims have extremely high U abundance, and very low Th/U, and thus appear to be younger than the rim analyses described above. Regression of these very discordant data gives an imprecise upper intercept age of

519 ± 56 Ma on 5 points (MSWD = 1.3; probability = 0.27) which we attribute to a period of

Neoproterozoic zircon growth.

3.4.2 PD07018 and PD07019: Detrital zircons in paragneiss of the Ikalamavony Group

(Laborde: ). Samples PD07018 and PD07019, two garnet-biotite paragneisses, were collected in the river valley ~ 3 kilometers south of Anjoma Ramartina, and separated by about 200m from each other (Fig. 3). They have the same zircon morphological characteristics and age patterns and will, therefore, be described together.

The zircons in both samples are generally round (100-200 µm diameter), or tabular to blocky in shape, and have flat crystal terminations (Fig. 8c,d). In CL images, most have an anhedral cores

(domain 1) with coarse sector- and concentric-zoning (Fig. 8 c,d) that are interpreted as detrital in origin. The domain 1 cores are commonly surrounded by a CL-bright, low-U euhedral rim (domain

2) of metamorphic origin. Rare, spherical metamorphic zircon with no interior cores was also observed (Fig. 8d).

Sixty-four zircon analyses are plotted in Figure 9 c,d. Most of the analyses overlap concordia

(<5% normal and reverse discordant), and the two domain types have very different ages. Most of the detrital cores (domain 1) display a range of 207Pb/ 206Pb ages from about 1070 Ma to 1010 Ma, whereas the metamorphic rims yield a mean 207Pb/ 206Pb age of 998.0 ± 3.7 Ma (Fig. 9d). Two of the domain 1 core analyses are slightly younger, < 980 Ma, but these are slightly discordant and we attribute their young 207Pb/ 206Pb dates to ancient loss of radiogenic Pb. Together the analyses indicate that the protolith paragneiss was deposited between ~ 1010 Ma and 998 Ma. This agrees well with the data from sample PR06090 (above) ) which implies deposition of the Ikalamavony

Group at ca. 1013 Ma.

3.4.3 MHY-08-55: Detrital zircon in metaquartzite, Ankaramena (Ikalamavony Group)

(Laborde: 418512/463270). Sample MJY-08-55 was collected from a small roadside quarry on

National Route 7 a few km south of the village of Ankaramena. The quarry exposes a ca. 7 m thick sequence of coarse-grained metaquartzite of the Ikalamavony Group. The rock at this locality is very pure (> 95%) quartzite with only a trace amount of accessory sillimanite and muscovite. The outcrop is more than 100 km south of known exposures of the Itremo Group and it is mapped as Ikalamavony Group by GAF-BGR (2008 b).

The zircons in this rock are all rounded or sub-rounded, consistent with their detrital origin

(Fig. 8e,f). Under CL, their brightness varies from medium-low to very high, and most are sector zoned or show little or no zoning; a few cores (e.g. 8.1) show sharp oscillatory zoning

(Fig. 8 e,f). Because there are no clearly recognizable types among the cores, all are assigned to domain 1 type. Many grains contain CL-dark, structureless rims and tips (domain 2 type) wide enough to be analyzed by SHRIMP.

Uranium concentrations in the domain 1 cores are variable, but low. Their Th/U values are also variable, but all are in the range typical for igneous zircon. Because the domain1 core analyses are concordant or nearly so, their 207Pb/ 206Pb dates can be used as a reasonable proxy for the age of their source rocks. The distribution of 207Pb/ 206Pb dates shows five principal peaks: the largest at 2.4-2.5 Ga, and smaller peaks at about 1.8 Ga, 2.0-2.1 Ga, 2.7 Ga and 2.9

Ga (Fig. 9e). There is no correlation between U or Th concentrations, Th/U ratios, and 207Pb/

206Pb dates. The detrital zircons were thus derived from terranes with exposed Neoarchean and

Paleoproterozoic source rocks. Their provincial signature is thus identical to that of the Itremo,

Sahantaha, and Maha groups of central and east Madagascar (Cox et al. 1998, 2004, Fitzsimons and Hulscher 2005, De Waele et al. 2008) as well as the Iakora Group of the Anosyen Domain

(GAF-BGR 2008 c,d).

The domain 2 rims have higher U concentrations, lower Th concentrations, and much lower

Th/U than the domain 1 cores. Their 206Pb/ 238U dates are concordant, but variable, spanning a range from 595 Ma to 440 Ma, with pronounced groupings at 550, 500 and 450 Ma (Fig. 9f).

The domain 2 rims are indistinguishable in their appearance, U concentrations, and Th/U ratio.

Thus the origin of the age variations is unclear. It is possible that all rims grew at ca. 550 Ma, but some experienced partial Pb-loss at a slightly younger time (e.g. 520 Ma). It is not clear, however, what made some rims more susceptible to Pb-loss than the others. It cannot be attributed to radiation damage or cracking, as there is no correlation between U concentration, common-Pb abundance, age of rim, or CL appearance. It is also possible that there are many rims, of different ages, that are too thin to be resolved with a 20 µm ion beam.

4. Discussion

Our new U-Pb SHRIMP ages, together with new field observations and the PRSM-2 geophysical data, allow us to construct a new geological framework for south Madagascar.

4.1 Age and Areal Extent of the Dabolava Suite

The age of the Dabolava Suite in the Ikalamavony Sub-domain is now established as 1035 Ma to

982 Ma (Tucker et al. 2007; CGS 2009 a,b; GAF-BGR 2008b, this paper). This is the first clear sign of Irumide activity in Madagascar, but the geodynamical context of the suite, and its extent, is unclear. Recent work by CGS (2009 a,b) and GAF-BGR (2008b), demonstrates that plutons of the suite are widespread in the northwest and west-central part of the domain (Figs. 2,3), some of them are associated with mesothermal gold and pyrite mineralization. Two predominant and associated assemblages are most common (CGS 2009 a): a gabbro-gabbronorite-pyroxenite assemblage (Vongoa type) and granodiorite-tonalite± trondhjemite assemblage (Ambatomeify type). Granitic augen gneiss is also an important but minor constituent of the latter assemblage.

All of these intrude the Ikalamavony Group paragneisses and amphibolites, and the two units were deformed together during the intense late Neoproterozoic to Cambrian East African

Orogeny.

Our data expand the coverage of the suite to include the southern part of the Ikalamavony

Sub-domain. Figure 2 identifies the major plutons of the Dabolava Suite which are now mapped over a strike length of ca. 350 km from Vongoa in the north to Zazafotsy in the south.

Unpublished data imply that it also extends into the Anosyen Domain as Müller (2000) has dated a gabbro gneiss (BM-127-95), north of Ihosy, with a U-Pb zircon age of 982 ± 3 Ma. The age falls squarely into the period of Dabolava magmatism as defined in its type area (Tucker et al.

2007, CGS 2009 a,b). Müller’s work is significant because it implies that the two domains,

Ikalamavony to the north and the Anosyen to the south, were juxtaposed before the time of

Dabolava magmatism (~1.0 Ga).

The chemistry, isotopic signatures, rock associations, and restricted age of the Dabolava suite are consistent with its formation in a magmatic arc, or marginal volcano-sedimentary basin, perhaps in a back-arc continental setting. Unpublished Nd data (Macey et al. in prep) may also imply a near juvenile contribution to the whole-rock analyses, but the data are equivocal in this regard, and do not preclude a small contribution from Paleoproterozoic sources. Whatever their setting, continental or oceanic, the Dabolava Suite and the volcanic strata of the Ikalamavony

Group are different in both age and origin from the passive platformal or continental marginal setting of the Itremo Group (Moine 1974, Cox et al. 1998).

4.2 Age, provenance, and metamorphism of supracrustal rocks in the Ikalamavony Sub-domain

Geological mapping and modern geochronology has established three groups of stratified rocks in the Ikalamavony Sub-domain (CGS, 2009a, 2009b): the Ikalamavony, Itremo, and Molo groups. The Itremo group, the best known of the three, consists of pure metaquartzite, massive dolomitic and calcitic marble, and lesser metapelite (Moine 1963). In contrast, metaclastics of the Ikalmavony and Molo groups are dominated by feldspar-rich rocks, quartzo-feldspathic gneiss, paragneiss, and psammitic schist, suggesting a greater input from proximal felsic igneous rocks (volcanic or plutonic). Based on field appearance and rock associations, early workers considered the Ikalamavony a distal facies of the Itremo, and thus the sequences of pure metaquartzite (and conglomerate) within the Ikalamavony Domain (e.g. the Kinangaly-

Tsinjomay-Bevitsika ridge) were correlated with the Itremo Group to the east (Moine 1963; Joo

1963; Alsac 1963 a,b). This correlation may not be entirely valid.

The Itremo Group has a diagnostic signature of Mesoarchean to Paleoproterozoic detrital zircons (Cox et al. 2004; Fernandez et al., 2003) with major age peaks at ~ 2.50 Ga and ~1.9 Ga, and minor peaks at ~ 2.70 Ga, 2.0 Ga, 2.25 – 2.21 Ga. Its age of deposition is approximately

Statherian (1.75-1.6 Ga) based on its youngest detrital zircons (~1.75 Ga), the oldest igneous rocks that intrude it (800 Ma), and stromatolite morphology (Cox et al. 1998, 2004). The

Ikalamavony Group, however, is different in three significant ways: (1) it was deposited in

Stenian time (~ 1.013 Ga) based on the age of metarhyolite sample PR06090, and cross-cutting relationships with the Dabolava Suite, (2) it is dominated by feldspar-rich metaclastic rocks, some of them interlayered with metavolcanic units (felsic and mafic) and thin calc-silicate rock,

(3) the detrital zircons in some samples (PD07018 and PD08019) are distinctly young and unimodal (1070-1020 Ma) implying their singular derivation from the Dabolava Suite which locally intrudes the group, and (4) at least part of it experienced growth of metamorphic zircon at

~1.0 Ga (c.f. samples PF06090B and PD7018-19). Thus it would appear that the Ikalamavony

Group is not a coeval, facies variant of the Itremo Group. We suggest, rather, it was deposited marginal to an evolving magmatic arc, receiving detritus from both early arc material and

Archean-Paleoproterozoic source rocks, and intruded by later magmas of the arc system over a period of ca. 70 Ma.

Still unresolved is the significance of pure metaquartzite (i.e. MJY-08-55) in the

Ikalamavony Group. Two very different interpretations are possible: (1) The quartzites are structural slices of the Itremo Group intercalated within the domain during Neoproterozoic thrusting, or (2) They are quartz-rich varieties of the Ikalamavony Group derived from Archean

26 and Paleoproterozoic source rocks. The first view is supported by their association with conglomerate and thick marble (a typical Itremo assemblage), the age spectrum of detrital zircon ages (which match exactly those of the Itremo Group; CGS 2009 a,b, this paper); sample MJY-

08-55 also lacks rims of metamorphic zircon (~1.0 Ga) which are present in PD7018 and 7019.

The second view is supported by the abundance of metaquartzite throughout the sub-domain, not all of them bound by obvious faults, and in the case of MJY-08-55, some of them more than 100 km south of the type Itremo area. Until more work is done, we cannot distinguish between these two interpretations.

Two of our samples (PD7018 and 7019) experienced growth of metamorphic zircon at ~998

Ma. Because of the high-degree of younger Neoproterozoic (540-520 Ma) overprinting throughout the sub-domain, it is unclear if the ~1.0 Ga age reflects a period of regional metamorphism, or local hydrothermal growth in a regime of extensive plutonism. Supporting the first view are the data of GAF-BGR (2008) who determined anatectic melting in the Dabolava

Suite at 925 Ma (GAF-BGR 2008b). But evidence for regional folding and metamorphism older than 800 Ma is not abundant, and it is possible that the metamorphic rims and anatectic fabrics

(998-925 Ma) are the result of static emplacement of slightly younger plutons of the Dabolava suite.

The third (and youngest) group of stratified rocks is the Neoproterozoic Molo Group, first identified as a separate unit by Cox et al. (2004) based on their young detrital zircons. The detrital zircons range in age from 700 Ma to 620 Ma, and they have major age modes of ca.

1,050 Ma, 800 Ma and 650 Ma. The depositional age of the Molo Group is constrained between

613 ± 9 Ma, the age of the youngest detrital zircon, and ~560 Ma, the age of metamorphic zircon growth (Cox et al., 2004). The Molo Group is presently restricted to a small northeastern part of

27 the Ikalamavony Sub-domain, although the presence of young detrital zircon in rocks of the

Anosyen domain, has led some to propose its extension to the south (e.g. de Wit 2003, Collins

2006).

4.3 Age and provenance of Anosyen Domain supracrustal rocks

As interpreted by GAF-BGR (2008), the bedrock of the Anosyen Domain is comprised of a single group of stratified gneiss and schist (Iakora Group) intercalated with meta-volcanic rocks of sub-alkaline rhyolitic composition (Horombe Group). An age of 736 ±16 Ma for one of the volcanic layers (metarhyolite) is interpreted to date eruption of the volcanic formations and deposition of the Iakora Group (GAF-BGR 2008 c). This is consistent with a maximum depositional age for the group of ca. 900 -720 Ma implied from detrital zircons near Tôlanaro and Ihosy (Kröner et al. 1996, 1999), as well as their own detrital zircon data near Ft. Dauphin.

Other data, however, suggest a more complex picture. In the course of our work, north and south of Ihosy, we discovered a section of paragneiss with sheets of intrusive granite (gneiss) dated between 810-803 Ma (samples IHY-08-3A and IHY-08-10). In the same general area, but to the north, Müller (2000) reports gabbro gneiss with an emplacement age of 982 ± 3 Ma (Fig.

3). Thus part of the Iakora Group is older than ~980 Ma, and both the Anosyen and

Ikalamavony domains share in common the Dabolava and Imorona-Itsindro suites of intrusive igneous rock.

U-Pb and Sm-Nd isotopic data tell a similar story in the southern part of Anosyen Domain.

GAF-BGR (2009) report an old suite of detrital zircons, with U-Pb ages between 2.30 – 1.66 Ga, in the quartzo-feldspathic gneisses (leptynite) formerly mapped as the Ft. Dauphin Group; not a

28 single detrital zircon of Neoproterozoic age was detected in a population of over 70 analyses.

Perhaps more convincingly, Müller (2000) reports a U-Pb zircon age of ~1.84 Ga for leptynitic gneiss interpreted as the crystallization age of the rhyolite protolith. Similarly, and in the same section of rock, Paquette et al. (1994) and Kröner et al. (1996, 1999) report Nd mean crustal residence ages of 2.8-2.1 Ga (TDM ) and detrital zircons solely in the age range of 1.88-1.70 Ga.

Thus a large part of the southeast Anosyen Domain, in rocks formerly mapped at the Ft. Dauphin and Tranomaro groups, consists of Paleoproterozoic protoliths. In a significant study, Paquette et al. (1994) report that the anatectic Vohimena granite, forming concordant sheets within rocks formerly mapped as the Tranomaro Group, crystallized at 1.68 Ga thus demonstrating the antiquity of this region. Despite the obvious implications, their work has received very little attention.

4.4 Paleoproterozoic Crust in the South

Our SHRIMP data confirm the findings of Paquette et al (1994) and Müller (2000), and prove a Paleoproterozoic age for at least part of the Anosyen Domain. We describe two orthogneisses near Ihosy (IHY-08-20a and IHY-08-12), both of them strongly deformed and partially melted in latest Neoproterozoic time. One, a coarse-gained gabbro gneiss, has a uniform age of zircon cores demonstrating crystallization of the protolith at 1908 ± 8 Ma. The other, a granite migmatite gneiss, has zircon cores demonstrating granite emplacement at 1793 ±

19 Ma. The ages for both of these fall squarely into the period of detrital zircons reported by

GAF-BGR (2008c; 2.3 -1.66 Ga) and Kröner et al. (1996, 1999), and both have rims of metamorphic zircon indicating growth at 522 -526 Ma. One of our samples (IHY-08-20a) has

29 even older zircon cores of Neoarchean age (c. 2.55 Ga), consistent with the findings of Paquette et al. (1994) and de Wit et al (2001), implying the presence of Archean material (rocks or detritus) elsewhere in south Madagascar. These are some of the oldest inherited zircons in south

Madagascar, and their ages comport quite well with the detrital zircon data of GAF-BGR

(2008c-e).

4.5 Relevance to the Terrane-Transfer Model (―Out-of Africa‖)

A current model for the Neoproterozoic assembly of Madagascar involves a two-step process (Cox et al. 2004, Fitzsimons and Hulscher 2005, Collins 2006). First, the transfer of central Madagascar with its Proterozoic strata (Azania) across the paleo-Mozambique ocean from East Africa (West Gondwana) to India (East Gondwana). The inferred period of terrane- transfer is between ~ 1.75 Ga (after deposition of the Itremo Group) and ~650 Ma, the time of its docking with India (Fitzsimons and Hulscher 2005, Collins 2006, Key et al. submitted). The postulated zone that accommodated this convergence is the Betsimisaraka suture in Madagascar

(Fig. 1). Second, the final assembly of Gondwana, including re-amalgamation of Azania with

East Africa, during two or more accretionary phases (de Wit 2003, Collins 2006). These are postulated to have joined the southern sequences of Androyen, Graphite, and Vohibory with

Azania by subduction of back-arc basins or an intervening ocean, and final continental collision between West and East Gondwanaland (de Wit et al. 2001, Jamal and de Wit 2002, de Wit

2003).

The ―out-of-Africa‖ or terrane-transfer model is predicated on the apparent absence of

Paleoproterozoic bedrock (2.2-1.8 Ga) in or near the Itremo Group at the time of its deposition

(Statherian-Calymmian). Similar logic was used by Collins et al. (2007a,b) to explain the position of the Neoproterozoic sedimentary provenance of south India against the Dharwar

Craton. In a variant of the terrane-transfer model, De Waele et al. (2008) propose that the Itremo

Group (as well as other Paleo-Mesoproterozic groups of Madagascar) are allochthonous and exotic, having been transported many kilometers from their original site of deposition to a completely foreign terrane. In contrast to all these, we offer an alternative proposal that is consistent with new and existing data and more compatible with the new domain designations.

4.6 An Alternative Proposal

Identification of an extensive Paleoproterozoic terrane in south Madagascar is a key element of our proposal. The TDM Nd data of Paquette et al. (2004), together with the U-Pb zircon data of

Kröner et al. (1996, 1999), GAF-BGR (2008 c), and Jöns (2006), suggest that a large part of the

Anosyen Domain is underlain by Neoarchean to Paleoproterozoic crust (2.8-1.8 Ga). This was confirmed by Paquette et al. (1994) and Müller (2000) who document Paleoproterozoic granite and rhyolite (1.84-1.68 Ga) in the south. We have dated two other intrusive igneous rocks, both north of Ihosy, with Paleoproterozoic emplacement ages of 1.90-1.79 Ga. It would seem, therefore, that a large part of the Anosyen domain is underlain by Paleoproterozoic and older crust although the true extent of its distribution is yet to be resolved. Other detrital zircon data, mostly from rocks mapped previously as the Ihosy Group, attest to the abundance of

Neoproterozoic detritus indicating that stratified rocks of Neoproterozoic age (~700-600 Ma) are also widespread. All of the Anosyen Domain was metamorphosed to granulite grade and intruded by voluminous granite in latest Neoproterozoic time.

Other terranes with Paleoproterozoic protoliths flank the southern margin of the Greater

Dharwar Craton (reconstruction after Reeves et al., 2004). These include the Southern Granulite

Terrain of India, the Highland and Wanni Complexes of Sri Lanka, and part of the Rayner

Complex of Antarctica (Fig. 10, Table 2). All of these are characterized by having

Paleoproterozoic mean residence ages (TDM Nd), stratified metamorphic rocks with detrital zircons between 3.2-1.8 Ga, upper amphibolite facies metamorphism at ~2.1-1.65 Ga, and widespread orthogneiss emplacement over the period 2.0-1.85 Ga. The clear parallels among their geologic histories leads us to suggest that they comprise part of a large Paleoproterozoic terrane, hereafter SMIWHR (South Madagascar-India-Wanni-Highland-Rayner), that may have been the source of Paleoproterozoic detrital zircons. All of these are strongly reworked in the

Meso- and Neoproterozoic (1.1-0.8 Ga), in common with the Ikalamavony and Anosyen domains of south Madagascar, implying they were joined with the Dharwar Craton before this time.

The question is: how to bring this Paleoproterozoic terrane into proximity with central

Madagascar by Statherian-Calymmian time (1.7-1.5 Ga)? We propose that a mostly

Paleoproterozoic terrane (SMIWHR), but with some Archean material, accreted to the southern margin of the Greater Dharwar Craton (Ghosh et al. 2004, Tucker et al. 2010) by middle

Paleoproterozoic time (Orosirian, 2.0-1.8 Ga). The mechanism by which this terrane was accreted is wholly conjectural but two lines of evidence imply it involved subduction of oceanic crust beneath SMIWHR: (1) igneous rocks of Paleoproterozoic age are unknown in the

Antananarivo Domain and correlative parts of India (i.e. Transition Zone), and (2) the chemistry of the 1.9-1.8 Ga gneisses is broadly calc-alkaline and consistent with a subduction origin.

Because all parts of the terrane experienced crustal reworking and metamorphism in latest

Mesoproterozoic and earliest Neoproterozoic time (~1.1-0.98 Ga) their amalgamation with the

Greater Dharwar Craton was over before the start of the Irumide events that affected all of them.

By early Mesoproterozic time (c. 1.5 Ga) SMIWHR and the Greater Dharwar were assembled and stabilized to form a craton with Paleoproterozoic terranes on its northern and southern flanks (e.g. Rao and Reddy 2002). A sign of its stability are the Mesoproterozoic sedimentary rocks were deposited within it (Fig. 10) including, in India, the Cuddapah, Pakhal, and Chattisgarh supergroups and, in central Madagascar, the Itremo, Sahantaha, Maha, and

Iakora groups. Common to all of these are detrital zircons with ages between 2.1-1.8 Ga which, in our view, were derived from the Paleoproterozoic terranes of SMIWHR and the Aravalli/Delhi belt that fringe the Archean nucleus. In Madagascar, these sequences are clearly intruded by igneous rocks of middle Neoproterozoic age (Imorona-Itsindro Suite) and in the Anosyen

Domain we speculate that the Iakora Group is intruded by early Mesoproterozic plutons (~982

Ma, Müller 2000).

In the most recent compilation before his death, Besairie (1969) subdivided the

Precambrian supracrustal rocks of west Madagascar into two main units: the eastern Itremo

Group and a western Ikalamavony Group, the latter dominated by felsic paragneiss and amphibolite, and the former with ubiquitous metaquartzite, schist, and marble. The U-Pb geochronology performed by Macey (CGS 2009), and presented here, proves that the two groups are not lateral facies equivalents; rather, they are distinct stratified sequences of vastly different age and origin. The Itremo Group is a platformal sedimentary sequence of probable Calymmian age (1.7-1.5 Ga, Moine 1974, Cox et al. 1998, Fernandez et al 2003) and the Ikalamavony Group is a volcaniclastic dominated sequence of Stenian-Tonian age (1.1-0.98 Ga, CGS 2009).

Based on whole-rock geochemistry (CGS 2009a,b, Rakotoarimanana 2001), the Ikalamavony

Group and the coeval igneous rocks of the Dabolava Suite, are considered the product of oceanic subduction. We offer two contrasting views to account for the Ikalamavony Group and the suite of Irumide intrusions: (1) In one view, the Ikalamavony Group and Dabolava suite represents the

Mesoproterozoic volcanic (- sedimentary) and intrusive igneous rocks, respectively, of an ensialic (continental) magmatic arc. Because igneous rocks of the Dabolava Suite intrude both the Anosyen and Ikalamavony domains, we speculate that subduction was west-dipping (present- day) beneath the Anosyen domain and active over the period 1035-980 Ma. (2) In another view, the Ikalamavony Group represents the volcano-sedimentary rocks of a juvenile (ensimatic) arc built on the oceanic crust between the rifted Anosyen and Antananarivo domains. This model predicts an Irumide-age collision between the Anosyen and Antananarivo domains; a prediction that finds support in the fieldwork of Hulscher et al. (2001),Collins et al (2003a), Fernandez et al.

(2003) and GAF-BGR 2008 a,b). In either case, the arc-like rocks of the Ikalamavony Sub- domain must have formed within or near the Paleoproterozoic suture between the Anosyen and

Antananarivo domains. Both hypotheses have key points in their favour, and both imply that, by the end of the Tonian (~850 Ma), the western edge of Madagascar (present-day direction) faced an open ocean. Thus if the still younger suite of intrusive igneous rocks (Imorona-Itsindro Suite) formed above a subduction zone, the convergent margin was west of central Madagascar as has been suggested by several authors (e.g. Handke et al. 1999, Ashwal et al. 2002, Bybee et al.

2010).

5. Conclusions

The Anosyen Domain is redefined as a terrane of Paleoproterozoic igneous and sedimentary rocks, formed between 1.90-1.65 Ga, and intruded by igneous rocks of Stenian-Tonian (1.03-

0.98 Ga) and Cryogenian (0.80 Ga) age. Based on published model Nd ages of 2.8-2.1 Ga

(Paquette et al. 1994), and the abundance of Neoarchean detrital and inherited zircons (GAF-

BGR 2008c), it is likely the Orosirian igneous rocks intrude continental crust but cited examples of Archean rock (e.g. de Wit et al. 2001) are equivocal. The Anosyen Domain also includes a vast amount of stratified paragneiss deposited after ~740 Ma and before 560-520 Ma (the age of regional metamorphism and granite magmatism). The latter package of Neoproterozoic stratified paragneiss is limited to rocks formerly mapped as the Ihosy Group although its exact demarcations are unknown. The Paleoproterozoic and older rocks include those formerly mapped as the Tranomaro and Fort Dauphin groups, as well as an unmapped section of paragneiss north of Ihosy which are intruded by igneous rocks of the Dabolava and Imorona-

Itsindro suites. In a Gondwanan context, the Anosyen Domain is part of a larger terrane of mostly Paleoproterozoic rocks (SMIWHR) that was welded to the southern flank of Greater

Dharwar in Orosirian time (1.9-1.8 Ga)

The southern-most outcrops of the Antananarivo Domain are in the Itremo-Ikalamavony domain where they occur as structural slices within an east-vergent stack of recumbent fold-and- thrust nappes, some of them involving very young rocks of Neoproterozoic age. Thus the boundary between the newly-identified Paleoproterozoic terrane (SMIWHR) and Archean crust

(Greater Dharwar Craton) is largely overprinted within a greatly fore-shortened zone of latest

Neoproterozoic-early Cambrian convergence. The geometry of the Ikalamavony-Anosyen contact is very complex, requiring ductile folding, but it seems to mark approximately the

35 proposed southern edge of the Archean shield (Greater Dharwar). Slices of Archean rock deep within the Ikalamavony Domain imply that the proposed Paleoproterozoic suture may itself be allochthonous.

The presence of gabbro and granite (gneiss) of the Dabolava (982 Ma) and Imorona-Itsindro suites (822-810 Ma) within the Anosyen domain establishes its linkage with the Ikalamavony

Sub-domain before Mesoproterozoic time. In the Ikalamavony Sub-domain, the period of

Dabolava magmatism is lengthened to 1035-982 Ma, and its geographic range is extended southward to the region between Finarantsoa and Ihosy. We demonstrate a Stenian age for the

Ikalamavony Group (~1013 Ma) and establish its independence from the Orosirian Itremo

Group. The Ikalamavony Group is reinterpreted as a volcanic-dominated supracrustal sequence formed within a continental or juvenile arc system, linked both temporally and spatially with the

Dabolava Suite which locally intrudes it.

The discovery of Paleoproterozoic rocks in the Anosyen-Androyen domain, and their linkage to central Madagascar by Mesoproterozoic time (ca. 1000 Ma), forces a reconsideration of the ―out-of-Africa‖ hypothesis. In its place we propose that the enigmatic zircons (2.0-1.8 Ga) were derived from the combined terranes of South Madagascar-India, and the Wanni and

Highland provinces of Sri Lanka (SMIWHR) that was sutured with the Greater Dharwar Craton at ca.1.8 Ga. Thus a Neoproterozoic suture (Betsimisaraka) through east Madagascar is not required, and the concept of ―terrane-transfer‖ across the paleo-Mozambique ocean should be abandoned.

Acknowledgements

Our collaborative research was funded by the World Bank, with a contract from the PRGM

(Madagascar) to the BRGM-USGS consortium, under the title Synthèses Géologique de

Madagascar à l’échelle du millionnième. We acknowledge, in particular, Drs. Ortega, Moine and Rakotomanana, who financed our field work and U-Pb geochronology through the Synthesis

Project, provided intellectual stimulation throughout the project, and who granted permission to cite the Report of Consultants in this paper. Greg Walsh and Bill Burton provided careful and constructive reviews of an earlier version of the manuscript.

Appendix A. Supplementary descriptions

Supplementary descriptions associated with this article may be found in the online version at doi:

Appendix B. Supplementary data

Supplementary data associated with this article may be found in the online version at doi:

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Figure Captions.

Fig. 1. The microcontinent of “Azania” and its position within Gondwana after Collins and Pisarevsky (2005) and Collins (2006). The eastern convergent margin of Azania, and its boundary with East Gondwana, is the “Betsimisaraka suture” (BS) and the Palghat-Cauvery shear zone (P-C), both interpreted as the Neoproterozoic site of the Azania-India collision. According to Cox et al. (2004), Fitzsimons and Hulscher (2005) the terranes derived from East Africa include the Antananarivo domain (Ant) and the Itremo Group (Itr); Collins et al. (2006) has since expanded them to include the Neoproterozoic sediments of the Southern Granulites, India (SG). East Gondwana is the Dharwar Craton of India (DC), and the Antongil/ Masora domains of east Madagascar (A). According to De Waele et al. (2008) only the Paleoproterozoic sequences are allochthonous and exotic: It (Itremo Group), M (= Maha Group), and Sa (Sahantana Group). MB = Mozambique belt (accreted southern terranes of Androyen and Vohibory); P-C = Palghat-Cauvery shear zone; Sey = Seychelles.

Fig. 2. Total magnetic intensity image for part of south-central Madagascar acquired by the PRGM in 2005-2007. Geologic domains are indicated by color in the inset map and labeled on the larger scale map. Also shown are the Dabolava (black fill) and Imorona-Itsindro (diagonal line fill) intrusive suites. Note: (1) the major change in the strike of units from north to south, (2) the continuity of geologic units in the Anosyen Domain across the inferred trace of the ―Ranotsara-Bongolava high strain zone‖, (3) the change in total magnetic intensity from east (high) to west (low) in south Madagascar, (4) the prominent high-magnetic circular form of the late Cretaceous Volcan de l’Androy in the south part of the image. The trace of the Ampanihy and Beraketa high-strain zones, separating the Vohibory, Androyen and Anosyen domain, are also indicated. At = Antananarivo, An = Antsirabe, Fi = Finarantsoa, Ih = Ihosy, FD = Fort Dauphin. Ik = Ikalamavony Sub-domain, Ano = Anosyen Domain.

Fig. 3. Geologic map of south-central Madagascar as defined by the national consortiums (BGS et al. 2008, CGS 2009 a,b, GAF-BGR 2008a-e) and this paper. Inset map shows the geologic domains of interest to this paper: Antananarivo (A), Itremo (It), Ikalamavony (Ik), Anosyen (An), Androyen (Ad), and Vohibory (V). The magmatic suites in these domains include Dabolava (~1.0 Ga), Ankiliabo (~920 Ma), Imorona-Itsinidro (~800 Ma), and Ambalavao (~540 Ma). The metamorphosed stratified rocks in each of the domains is indicated in the legend. Samples discussed in the text are indicated by a red star.

Fig. 4. Cathodoluminescence (CL) images of zircon from samples IHY-08-20A (a.b), IHY-08- 12 (c,d) and IHY-08-23 (e,f). Numbers refer to spot analyses in Appendix A.

Fig. 5. Concordia diagrams of zircon analyses from samples IHY-08-20A (a-d and IHY-08-12 (e-f).

Fig. 6. Cathodoluminescence (CL) images of zircon from samples IHY-08-3A (a,b), and IHY- 08-10 (c-e). Numbers refer to spot analyses in Appendix A.

Fig. 7. Concordia diagrams of zircon analyses from samples IHY-08-23 (a), IHY-08-3A (b-d), and IHY-08-10 (e,f).

Fig. 8. Cathodoluminescence (CL) images of zircon from samples PF06090B (a,b), PD07018 (c) , PD07019 (d) and MJY-08-55 (e,f). Numbers refer to spot analyses in Appendix A.

Fig. 9. Concordia diagrams of zircon analyses from samples PF06090B (a,b), PD7018 and PD7019 (c,d), and MJY-08-55 (e,f).

Fig. 10. A reconstruction of Madagascar and India (after Reeves et al. 2004 and Ghosh et al. 2004) showing the Greater Dharwar Craton and a Paleoproterozoic terrane on its southern margin (SMIWHR, South Madagascar-India, Wanni and Highland complex of Sri Lanka, and the Rayner Complex, Antarctica). The Paleoproterozoic terrane of SMIWHR was the likely source of the enigmatic Paleoproterozoic detritus (2.1-1.8 Ga) in the Mesoproterozoic clastic rocks of Madagascar and India (in red) and thus it was welded to the Greater Dharwar Craton before ~1.8 Ga. The common occurrence of medial Proterozoic igneous rocks (1.0-0.75 Ga) in the Greater Dharwar Craton and SMIWHR supports this view. Also shown is the Ikalamavony (Ik) Sub-domain interpreted as a volcanic arc terrane that formed between the Greater Dharwar and SMIWHR in the period 1.03-0.98 Ga. A = Anchovil high strain zone, AI = Angavo- Ifanadiana high strain zone, Ad = Androyen Domain, As = Anoysen Domain, B = Bemarivo Domain, Bi = Bhima basin, Ch = Chattisgarh basin, Cu = Cuddapah basin, CITZ = Central India tectonic zone, CDML = Central Drönning Maud land, E = Enderby land, EG = Eastern Ghats, H = Highland Complex, Ik = Ikalamavony Sub-domain, It = Itremo Sub-domain, KT = Karur- Kamban-Painavu-Trichur shear zone, LHC = Lutz Hölm Complex, P = Pakhal basin, P-C = Palghat-Cauvery shear zone, RC = Rayner Complex, S = Sahantaha Group, Sey = Seychelles, SG = southern granulites of India, Vo = Vohibory, Vi = Vindhyan basin, W = Wanni Complex.

Table 1

Table 1. Summary of U-Pb Zircon Ages, south-central Madagascar.

SAMPLE LABORDE AGE (Ma) COMMENTS Paleoproterozoic rocks Emplacement age defined by the upper intercept of seven near-concordant domain 1 zircon analyses (MSWD = 0.20). 1793 ± 19 b IHY-08-20A granite migmatite 207 206 402240/356435 Archean inheritance is detected by two near concordant domain 1 zircon cores with Pb/ Pb dates of ~2.55 Ga. gneiss (Anosyen Domain) 595 ± 11 b 522 206 238 ± 9 b Fourteen domain 4 zircon analyses define a bimodal distribution of Pb/ U dates of ~590 Ma and 520 Ma best interpreted as growth during two events.

a Emplacement age defined by weighted average 207Pb/206Pb age of five oldest domain 1 core analyses (MSWD = IHY-08-12 coarse-grained 1908 ± 8 595 1.71). Early metamorphism , and corrosion of domain 1 cores, is definied by the weighted mean 238U/206Pb dates of a gabbro gneiss (Anosyen 467348/362149 ± 11 522 ± 9 the youngest concordant inner cores (555 ± 17 Ma). Eight other domain 3 outer rims date the time of rim growth (526 ± 8 Domain) a Ma) Mesoproterozoic rocks IHY-08-23 trondhjemite gneiss Emplacement age is defined by the upper-intercept age of concordant and discordant domain 1 zircon analyses of 23 1035 ± 30 a analyses (14 grains) (MSWD = 2.4). The lower-intercept age (~399 Ma) is an imprecise minimum age of Pb-loss. (Dabolava Suite, Ikalamavony 432414/391592 a Sub-domain) 399 ± 170 Neoproterozoic rocks IHY-08-3A granite gneiss 810 ± 11 b Twenty-five concordant analyses define a spectrum of 206Pb/238U ages between ~850-650 Ma; Age of emplacement is defined by the oldest plateau of mean 206Pb/238U ages (MSWD = 0.84); 10 concordant rim analyses define age of Pb- (Imorona-Itsindro Suite, 421929/379701 b Anosyen Domain) 548 ± 8 loss and new ziricon growth (MSWD = 0.82) IHY-08-10 granite gneiss Twenty-eight concordant analyses define a weighted mean 206Pb/238U age of 799 ± 27 Ma (MSWD = 8.2) but with (Imorona-Itsindro Suite, 412335/392180 803 ± 13 a range of dates between 976-630 Ma. Emplacement age defined by the weighted mean 206Pb/238U age of 18 concordant Anosyen Domain) core analyses less than 3% discordant (MSWD = 1.19). Ikalamavony Group Upper-intercept age of domain 1 magmatic cores is 1009 ± 12 Ma (MSWD = 0.73); Regression of all domain 1 and 2 PR0609B quartzo-feldspathic 1013 ± 10 a 412335/392180 magmatic cores yields upper intercept age of 1013 ± 10 Ma (MSWD = 0.75). gneiss (metarhyolite) 1009 ± 12 a

PD7018 & PD7019 garnet- Twenty-three domain 1 detrital zircon cores define a narrow range of concordant analyses between 1070-1010 Ma; 1070-1010 a biotite paragneiss (detrital 444010/399170 Thirty-six zircon rim analyses define the mean 206Pb/238U age of metamorphism of 998 Ma (MSWD=1.09). 998 ± 4 zircons) 2995 - 1800 c Fifty-five SHRIMP analyses of detrital zircons, 70% of the population, define three major age modes at ~2.55 - 2.40 Ga, MJY-08-55 metaquartzite of 463270/418512 2.7 Ga, and 2.9 Ga; 30% define age modes at 2.1-2.0 Ga and 1.8 Ga. Fifteen concordant rims yield weighted mean Ikalamavony Group a 550-450 206Pb/238U age of metamorphism between 550-450 Ma.

a Weighted age calculated with ISOPLOT (Ludwig 2003), b Regression age calculated using algorithm of Ludwig (2003), c Range of concordant (± 10 %) detrital zircon ages. Table 2

Table 2. Paleo- Mesoproterozoic crust and metamorphism, SMIWHR terrane

Mean crustal residence age, Detrital and inherited U-Pb zircon (emplacement SAMPLE TDM Nd zircon, monazite ages) COMMENTS

South 1.71-1.68 upper-intercept Evidence for ancient crust between 17 2.8-2.1 ga, intruded by gabbro and Madagascar 2.8-2.1 whole-rock analyses (Fort 2.0-1.0 Ga inherited zircon 10, Vohimena granite ; 1.84 Ft. granite between 1.9-1.8 Ga and again Dauphin-Tranomaro Groups) 15 13, 14 Dauphin leptynite 15; 1.90-1.79 (Anoysen at 0.8 Ga; younger Neoproterozoic 18 Domain) Ga gabbro and granite sediments are also present

Indian ~2.44 Ga granite gneiss 2, 1.6 Southern Nd model ages of 3.0-2.1 Ga 2; 2.0 Ga zircon iheritance in Evidence for ancient crust between Ga granite gneiss 3, 0.95 Ga Granulite Protolith ages between 2.4-2.1 Ga charnockite3; 2.0 Ga 3.0-2.1 Ga intruded by younger granite gneiss enclave 3; 0.8 Ga Terrane 3,9 metamoprhic monazites 3 orthogneisses at 1.6, 0.95, and 0.8 Ga for syntectonic granite 3 (north)

Indian Evidence for ancient continiental Southern > 2.0 Ga detrital grains, and Whole-rock Nd model ages of crust between 2.9-1.3 Ga, intruded by Granulite other samples with detrital 2.0 Ga granite emplacement 3 high-grade gneisses 2.9-1.3 Ga 5-7 granite at 2.0 Ga; other supracrustal zircons between 1.0-0.75 Ga 2 Terrane rocks between 1.0-0.75 Ga (south)

Detrital zircons are not older 1.0-1.1 Ga for orthogneiss of the Evidence for ancient crust as old as Wanni Whole-rock Nd model ages of than ~1.3 Ga 8, 11-13, Upper Vavuniya charnockites province 2.0 Ga; intruded by igneous rocks Complex high-grade gneisses 2.0-1.0 T DM intercept of detrital zircons of 13 ; Neoproterozoic volcanic and between 1.1-1.0 Ga; and again at 0.79- Nd 16 1077 ± 328 Ma, and zircon (Sri Lanka) intrusive igneous rocks 2,8 0.77 Ga evaporation age of 1329 Ma 13

Evidence for ancient crust between Highland Sedimentation at 1.9-2.0 Ga Main period of magmatism, Mean T DM Nd crustal ages ~3.0- 3.0-2.0 Ga, intruded by igneous rocks Complex 1,8, and detrital zircons defined by 5 orthogneisses, 1.9- 2.4 Ga 16 between 1.9-1.85 Ga; and again at 12-14 1.85 Ga 1,8,12 (Sri Lanka) between 3.2-2.0 Ga 0.67 Ga

Rayner Main period of granite and Evidence for ancient crust between 2.18-1.65 T DM Nd model ages, Complex 2.0-1.3 Ga inherited zircon in charnockite emplacement (1.9- 2.5-2.2 Ga, intruded by igneous rocks Hf analyses indicate crustal 4 1.7 Ga); Irumide granites (East 4 various orthogneisses between 1.9-1.7 Ga; and 1.02-0.97 residence of 2.55-2.15 Ga 4 Antarctica) between 1.02-0.91 Ga Ga

References: 1 Baur et al. 1994; 2 Bartlet et al. 1998; 3 Ghosh et al. 2004; 4 Halpin et al. 2005; 5 Harris 1999; 6 Harris et al. 1994; 7 Harris et al. 1996; 8 Hölzl et al. 1994; 9 Janardhan 1999; 10 Jöns 2006; 11 Kröner et al. 1987; 12 Kröner and Williams 1993; 13 Kröner et al. 1994; 14 Kröner et al. 1996; 15 Paquette et al. 1994; 16 Milisendra et al. 1994; 17 Müller 2000; 18 this paper. Figure 1

Terrane Amalgamation

800-600 Ma 600-500 Ma Arabian-Nubian Shield Az

Sa E Sey a s t

A A

f r Ant i c

n BS

O r

Congo o M India g

e n

P-C

Ru Kalahari Antarctica

Tucker et al. Figure 1 Figure 2

45°E 46°E 47°E

Imorona-Itsindro At 19°S Vo Dabolava 19°S mGals 5725.26

-812.564

20°S An

Itremo 20°S B

Ts A/M Ts Ts

21°S A Ano It Ik

Ik a 21°S A/M la

Ad v

An o Fi

n V y Fig. 2 Ano

Ano Ik 22°S 22°S Za mGals Ih 39410.1

R a 29658.5 n o 23°S ts a ra -B 23°S o n g o l a v Z a

S H Vohibory H S

a Z

a 24°S

Z e

24°S H

Anosyen

n i

n a Androyen p

Indian

Ocean 25°S 25°S 45°E 46°E FD

Figure 2 Tucker et al. Figure 3

19°S 45°E 46°E 47°E ! At B 19°S

PR-0609B PD-07018 An Bk A/M PD-07019 ! Ts Ts

Ts 20°S

A 20°S

It Ik A/M

Fi An 21°S ! V Ad 21°S

Magmatic Suites Ambalavao Suite IHY-08-15B IHY-08-18

22°S MJY-08-55 Imorona-Itsindro Suite IHY-08-12

Ankiliabo Suite BM-127-95 22°S IHY-08-23 Dabolava Suite IHY-08-03A WHI-0375 ! Antananarivo Domain Ih IHY-08-10 Ambatolampy Group IHY-08-20A BM-303-96 Manampotsy Group 23°S Neoarchean gneiss 23°S Itremo-Ikalamavony Molo Group Itremo Group Ikamalavony Group

Anoysen Domain 24°S

Iakora Group SBO-0867 24°S CFE-0790 Horombe Group Androyen Domain Mangoky Group A1210

Imalato Group 25°S ! Vohibory Domain FD 25°S undifferentiated 45°E 46°E 47°E

Figure 3 Tucker et al. Figure 4

Figure 4 Tucker et al. Figure 5

2600 A B U IHY-08-20A IHY-08-20A

0.5 U

0.36 8 8 3 3 2 granite migmatite gneiss granite migmatite gneiss 2

/ / 1900 b 2200 b P

0.4 0.34 P

6 1793 19 Ma 6 0 0

2 ~ 2.55 Ga

2 1793 19 Ma Domain 2 1800 Domain 1 MSWD = 0.20 1800 0.3 magmatic cores embayed cores 0.32 (inherited) 1400 Fig. 5B 1700 Domain 2 0.2 0.30 oscillatory-sector zoned 1000 Domain 3 inner rims magmatic cores 1600 600 0.1 593 58 Ma 0.28 Domain 4 outer rims, whole grains 207 Pb/ 235 U 207 Pb/ 235 U 0.26 0 2 4 6 8 10 12 3.4 3.8 4.2 4.6 5.0 5.4 5.8

Outer domain (rims, tips, whole grains: 206 238 D

U C IHY-08-20A 660 two groups of Pb/ U dates 0.11 8 3 2

granite migmatite gneiss / 640 Mean = 595 11 Ma b Wtd by data-pt errs only, 1 of 6 rej. (black) P

620 6 MSWD - 0.83, prob. = 0.50 0

0.10 2 Domain 4 600 outer rims, whole grains 595 11 Ma 580 560 0.09 Neoproterozoic 520 540 zircon growth between Mean = 522 9 Ma ~590-520 Ma 2 of 8 rejected (black) 0.08 MSWD - 0.68 480 500 522 9 Ma 207 Pb/ 235 U Box heights are 2 sigma IHY-08-20A 0.07 0.55 0.65 0.75 0.85 0.95 0.4 0.100

U Oldest U 2002 68 Ma IHY-08-12 8 8 IHY-08-12 E F 3

3 outer rims 2

2 Upper-intercept of all analyses

gabbro gneiss / / gabbro gneiss 0.096

1800 b b Main group of outer rims P P

0.3 6

6 1908 8 Ma Domain 1

0 and uniform grains 0 0.092 2 2 cores 570 1400 526 8 Ma 0.088 MSWD = 0.24 Domain 2 550 inner rims 0.2 207 206 Weighted mean Pb/ Pb 530 1000 Domain 3,4 1925 0.084 1908 8 Ma 510 1915 (Error bars are 2 sigma) 0.080 490 0.1 600 206 238 1905 Weighted mean Pb/ U Outermost rim of a uniform Domains 3,4 CL-dark grain rims and uniform grains 1895 0.076 207 235 MSWD = 1.7 207 235 Box heights are 2 sigma Pb / U 1885 Pb / U 0.0 0.072 0 2 4 6 0.4 0.5 0.6 0.7 0.8

Figure 5 Tucker et al. Figure 6

Figure 6 Tucker et al Figure 7

206 238 0 0 0 0 0 . . . . Pb / U . 0 1 2 0 1 5 6 7 8 9 0 0 0 0 0 0 Tucker etal Figure 7 8 6 0 4 2 0 0 0 0 0 ...... 0 1 2 0 1 0 0 0 0 0 0 0 8 6 0 4 2 0 0 . 1 0 . 0 0 0 206 238 206 238 Pb / U

E W W - 3 Pb / U Domain 2(gray)

x b 0 4 M c t G l d 0 e 0 0 a l u S . . c 0

i d 4 B W r 0 k g I e a

y H . s D M d h 4

R C m Z q n d

e r Y t = u a i o i i e a a m m a

t a t r 5 - 0 a n e r e s d z 0 s 0 . - 6 e g

4 ( p s =

s a 0 h

8 - 0 t

g m s

i 0 - 7 , g f g 0 -

4 n o p e o h r 3 a . 5 e r r a

p 8 e t c o r G

A t s y

e s a o b 0

i i

O n m a c o s y s

. r n 1 8 b n I

q u a m

e 4 I H i l

7 c t t u l y H

l e i 2 o n 0 r , t l M a 0 o i

y r Y n 8 o p 0 0

7 i

Y r a r - 0 = t P

r s e - i P n o m e e

0 - e 0 s 0 f b

206 238 1 b 0

W . 6 ) s a 8 . 8 Pb / U g 1 /

2 8 h n 5 6 7 8 9 5 r t I

0 M - d e n d n 0 0 0 0 0 2 - 0 j 1 . 0 0 0 0 0 j

n 0 e S . 0

2 C a e 1 B 6 e 0 W n P m o . 3 r y i 2 o

s Domain 1(black) r

D r M e d m b i i m s

s a e t = 9 a 1

e t s a -

0 a l 0

0 o

n 1 g - 1 0 -

. u 0 p

g r 8 b . = s a 0 t 0 0 6 4 8 s l

n y a 7 8 g ,

e c 0 3 1 P r s p B e 2 a k r q 0 o r r 0 3

i o x i 5 s 1 u

n s

s 7

b b h

q a

s . o e P

u r

i n - g 1

b a

h l

1 d s /

y i t r b l 1 s

, i a e

M t

2 a y 0 s s 3 3 2 r

e a h .

3 = o 0

e 2 0 1 5

f M

0 d

s 1 U 1 i 2 .

g 0 1 5 e m E 3 M a 0

9 l a 5 l r i e 2 p A U C j s a . . 0 e s 2 .

206 238 4 1 Pb / U 1 0 0 0 0 0 0 0 1 5 6 7 8 9 206 238 ...... 0 0 0 0 0 0 0 0 0 1 1 1 1 5 6 4 5 5 5 5 0 0 0 0 0 0 0 Pb / U 8 6 6 0 4 2 9 1 9 1 3 5 7 0 0 0 0 0 0 0 0 . 1 W Domain 3 G

t 206 238 d M r . I

Pb / U S a B H 0 W W n R y G . Y Domains 1,2 3

D i h i 8 M m d r t -

i a M a e t = e s 0 1 e M y 5 t

a S

0 a a

8 n b g 0 e b . n W - 4 8 r

o p a o - d I = n 2 a

4 1 H x t

n x D 8

, u .

0 5 e 5

e e

p 0 n

e 4 = Y . 0 s =

A s i r

i 5 8 i r f s o

t o 1 l r - 7 a

a 8 l b

e s

Rims

s r 0

9 r r d

a . m 1

r e 2 e

o 9 8 I b 8 a 8 a

5 H ,

n W i g

r l

- M i a

5 p M n r e l n

t c y 3 a y

Y M t r 0 j a 2 c , d i

i e e o 0

n = A t - e 7 0 . c b a . e s

i 0 7 p

0 a

B t s . M o e

. t 8 y = 5 s e g f d a

d - M 2 6

d 0 n a 3

8 5 a a S n e M

0 A t n r a a W U R C i e e a l 0 s - y n i o j p a D m l e . P y s s i r 9 t n f c e

s . e s o l =

a t

s e

e s r e = -

t m

s 1 r

7 e - d

g r

8 . a 5

o s r 1 g 0 a u 0

p 9 r o 3 y

a e 1 , v n

i n

. s n a p l

1 y

s l r s 1 , u o u

q B 3 - 0 e 2 b a

b 0 8 . r M o a

x e 7 l 5 0 a r f

e h Uniform 0 . c P 1 2 e grains s k 1 8 6 i

g .

b s 3 r h q e t s

u j e

/ a a c

r r e t e 2 e s

3 2 d 1 5

s U F . B i 5 g D m a Figure 8

E F

Figure 8 Tucker et al. Figure 9

0.21 0.19 A B PF06090B U PF06090B 8 U 3 8 2 3

0.19 Ikalamavony metarhyolite Ikalamavony metarhyolite 2 1100 1080 /

/ b

b 0.18 P 6

0.17 P

1013 10 Ma 0

6 1040 2 0 2 Magmatic cores only 0.15 900 0.17 Magmatic cores 1000 plus “old” rims 0.13 960 1009 12 Ma 700 Upper intercept age 0.16 0.11 MSWD = 0.75 Upper intercept age 920 MSWD = 0.73 0.09 500 “old” rims 0.15 0.07 207 235 Pb/ U 207Pb/ 235U 0.05 0.14 0.4 0.8 1.2 1.6 2.0 1.4 1.5 1.6 1.7 1.8 1.9 0.22

U PD7018 & PD7019 C D 8 PD7018 & PD7019

3 1100 2

Ikalamavony paragneiss

/ Ikalamavony paragneiss 0.20 1200 b rim domains P metamorphic rims

6 1060

0 1100

2 magmatic detrital 0.18 cores 1020 1000 0.16 1070-1010 Ma 900 980 5 0.14 4 800 3 940 2

1 998 4 Ma 0.12 900 Weighted mean 206Pb/ 238U 990 1010 1030 1050 1070 MSWD = 1.09 207 235 Pb/ U Core 207Pb/ 206Pb ages (Ma) 0.10 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.8 MJY-08-55 E MJY-08-55 F U U

8 0.12 8

3 Ikalamavony metaquartzite 3

2 Ikalamavony metaquartzite 2

/ /

rim domains 3000 650 b 0.6 b P P 6 6 0 0

2 2600 0.10 2

550 207 206 2200 Pb/ Pb ages (Ma) 0.4 14 550-450 Ma zircon rim dates detrital zircon 0.08 5 y

12 t

1800 i l age modes i

b 4

450 y a t i 10 b l i o r r r b p e e

3 a b e b 1400 8 b v i o m t m r a u u l 2 p

e N N e

6 R

0.2 v i 1000 0.06 t 1 a 4 350 l e

0 R 600 2 300 400 500 600 700 800 206 238 207 235 207 235 Pb / U date (Ma) Pb / U 1700 2100 2500 2900 Pb / U 0.0 0.04 0 4 8 12 16 20 24 0.1 0.3 0.5 0.7 0.9 1.1

Figure 9 Tucker et al. Figure 10

Malani

ANS Aravall i - India Tanzanian M Delhi EG Craton R Sey CDM B East Antarctic Kalahari Craton CITZ V

AI Greater Dharwar Singhbhum Ch Bi P Ik Bastar Cu EG K Vo As E PC Ad KT RC SG Legend A Young cover Neoproterozoic igneous rocks SMIWHR H W Neoproterozoic metaclastic rocks (Manampotsy, etc) Exotic Neoproterozoic nappes V Mesoproterozoic belts with Neoproterozoic overprint Mesoproterozoic belts with Neoproterozoic overprint Paleo-Mesoproterozic sediments LHC Paleoproterozoic crust Meso-Neoproterozoic belts with Neoproterozoic oveprint Paleoproterozoic belts with Neoproterozoic overprint Sor Neoarchean craton Mesoarchean crust CDML Rondane

Figure 10 Tucker et al Supplementary material for on-line publication Click here to download Supplementary material for on-line publication: Table S1.xls