Precambrian Research, 56 (1992) 89-96Elsevier Science Publishers B.V., Amsterdam

89

Reassessment of Proterozoic granitoid ages in Ghana on the basis of U / P b zircon and monazite datingW. Hirdes a, D.W. Davis b and B.N. Eisenlohr~aBundesanstalt 3~r Geowissenschaften und Rohstoffe, Stilleweg 2, D 3000 Hannover, Germany bj. Satterly Geochronology Lab., Royal Ontario Museum, 100 Queen's Park, Toronto, Ont., M5S 2C6 Canada CKeyCentre for Strategic Minerals, University of Western Australia, Nedlands, W.A., Australia(Received March 6, 1991; accepted after revision July 31, 1991 )

ABSTRACT Hirdes, W., Davis, D.W. and Eisenlohr, B.N., 1992. Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating. Precambrian Res., 56: 89-96. The West African Craton in Ghana is of Early Proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the Birimian Supergroup, as well as younger conglomerates and quartzites of the Tarkwaian Group, and extensive granitoid plutons. No consensus exists on absolute and relative ages of plutons within belts and basins. U-Pb geochronology of four different granitoid complexes indicates that, contrary to long-held views, belt granitoid plutons (Ashanti belt, 2172 + 2 Ma; Sefwi belt, 2179 + 2 Ma) are ~ 60 to 90 Ma older than basin plutons (Kumasi basin, 2116 + 2 Ma; Sunyani basin, 2088 + 1 Ma). Belt granitoids are approximately coeval with volcanic rocks of the Birimian Supergroup, while basin plutons are late-kinematic and postdate Tarkwaian sedimentation. The first high-precision geochronological data for rocks from Ghana, i.e., the eastern part of the Birimian domain on the West African craton, presented here show that magmatism within this region occurred over a time-span of at least 90 Ma, and provide further evidence that the period between 2.1 and 2.2 Ga was one of important magmatism in West Africa.

Introduction

Testing of episodic versus continuous models proposed for the growth of the continental crust requires acquisition of precise geochronology for all major areas of exposed basement. Available data suggest that much of Western Africa may be underlain by crust formed ~ 2.1 Ga ago (Taylor et al., 1988; Abouchami et al., 1990), a time that has generally been considered to be one of relatively slow crustal growth, intervening between two well-documented geochronological peaks at approximately 2.6-2.8 Ga and 1.6-1.9 Ga. It is the purpose of this paper to present the first high-precision geochronological data for rocks from Ghana which constitutes the easternmost portion of Proterozoic rocks of the West African craton (Fig. 1 ). Sev-

eral age determinations on zircons from Proterozoic rocks of the western part of the craton (Senegal, Guinea, Mali, part of Ivory Coast) have been carded out by Calvez as well as Boher (summarized by Milesi et al., 1989), giving an age range between 2115 Ma and 2073 Ma for various "gneisses" and granitoids. Rocks of Early Proterozoic age occupy large parts of the West African craton and crop out extensively in Ghana where they are subdivided into the Birimian Supergroup and the volumetrically subordinate Tarkwaian Group. Both were deformed and metamorphosed to greenschist facies in the course of the Eburnean tectono-thermal event. The Birimian comprises an assemblage of sedimentary/volcaniclastic rocks forming sedimentary basins which separate a series of subparallel, roughly

0301-9268/92/$05.00 1992 Elsevier Science Publishers B.V. All fights reserved.

90

W. HIRDES ET AL.

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equally spaced, northeasterly trending volcanic belts of mainly tholeiitic basalts (Fig. 2 ). Rocks of the Tarkwaian Group represent erosional products of the Birimian, are dominated by coarse clastic sediments and are located exclusively within the volcanic belts. Both, belts and basins include extensive outcrops of granitoids. The Birimian supracrustal rocks and, with a minor exception, the granitoids in Ghana represent juvenile crustal additions with Sm/Nd model ages of 2.0 to 2.3 Ga (Taylor et al., 1988 ). Recent field and isotope work suggests that the Birimian volcanic belts and sedimentary basins formed contemporaneously as lateral facies equivalents (Taylor et al., 1988; Leube et al., 1990 ). Traditionally, the Lower Proterozoic in Ghana was interpreted to have undergone two deformational events (Junner, 1940). However, new structural data show that the Birimian Supergroup and the Tarkwaian Group were deformed jointly in a single progressive deformation event (Eisenlohr and Hirdes, in

press). This event involved initially a regional penetrative low-strain phase and subsequently the formation of spatially restricted high-strain zones.Proterozoic granitoids in Ghana

Four types of Proterozoic granitoid suites are present in Ghana; traditionally they have been referred to as Winneba, Cape Coast, Dixcove and Bongo granitoids (Junner, 1940; Kesse, 1985), the latter three having recently been termed "basin", "belt" and "K-rich" granitoids (Mauer, 1990; Leube et al., 1990). Absolute ages of these granitoid types and, in particular, their emplacement age relative to each other and the host rocks are crucial to understanding the evolution of the Proterozoic terrane in Ghana. Basin and belt granitoid rocks are widespread, both other types occur only locally. The Wjnneba granitoid is the only rock suite so far encountered in Ghana which shows evidence for an Archean sialic precursor (Sm/Nd model

PROTEROZOIC GRANITOID AGES IN GHANA3i B U R K I

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