Nd-isotopic mapping of the Archaean–Proterozoic boundary in southwestern Tanzania: Implication for the size of the Archaean Tanzania Craton

Gondwana Research 20 (2011) 325–334

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Nd-isotopic mapping of the Archaean–Proterozoic boundary in southwestern Tanzania: Implication for the size of the Archaean Tanzania Craton Shukrani Manya Department of Geology, University of Dar es Salaam, P.O. Box 35052, Dar es Salaam, Tanzania

a r t i c l e

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Article history: Received 16 March 2010 Received in revised form 27 December 2010 Accepted 3 January 2011 Available online 21 January 2011 Handling Editor: A.S. Collins Keywords: Tanzania Craton Nd-isotope Geochronology Archaean granitoids Proterozoic granitoids

a b s t r a c t Nd-isotopic data are presented for the granitoids that straddle the Archaean–Proterozoic (A–P) boundary in southwestern Tanzania along the Itigi–Makongolosi road traverse. On the basis of their Nd depleted mantle (TDM) ages, two groups of granitoids can be identified: those which show TDM ages of 2541–2894 Ma and are therefore belonging to the Archaean and those which have Proterozoic TDM ages of 2031–2430 Ma. Archaean granitoids are deformed, grayish in colour and consist of plagioclase, biotite and quartz and include a few microcline-rich, rare biotite pink alkali granites whereas the Proterozoic ones include mainly undeformed, microcline-rich, pink alkali granites. Although there are several compositional overlaps between the two, Proterozoic granitoids differ from those of the Archaean in showing enrichment in incompatible elements and overall higher abundances of the rare earth elements. Both Archaean and Proterozoic granitoids show negative to slightly positive Eu anomalies. These geochemical characteristics are attributed to the involvement of plagioclase in their magmagenesis indicating their generation at low pressures and shallow depths with the Proterozoic granitoids being derived from a more felsic protolith. Nd-isotopic data, coupled with petrography and lithological field relationships as well as major and trace elements geochemistry of the granitoids, places the A–P boundary approximately 150 km inside from the southern traditionally accepted boundary near Lake Rukwa in southwestern Tanzania. This implies that the size of the Archaean Tanzania Craton is smaller than hitherto understood. Small vestiges/domains showing Archaean ages within the Proterozoic regions could be explained as being slivers of tectonically interleaved Archaean material found within the Proterozoic terrane in southwestern Tanzania. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Mapping and identifying a terrane boundary in ancient continental crustal provinces are complicated by the fact that these provinces frequently experienced poly-phase metamorphism, deep erosion and different episodes of magmatism and deformation. This is frequently observed in Precambrian crustal terranes where most Proterozoic provinces represent juvenile additions to the older crust as well as reworking of Archaean fragments (e.g. Wendt et al., 1972). However, Nd-isotopic studies have shown to be a powerful tool in mapping ancient crustal blocks with different mantle extraction ages (e.g. Kaur et al., 2009). This approach was used by Maboko and Nakamura (1996) and Maboko (2000) to isotopically characterize the boundary between the Tanzania Craton and the Palaeoproterozoic Usagaran belt in southeastern Tanzania and the Pan-African Mozambique belt to the east of the Craton, respectively. In southeastern Tanzania, Maboko and Nakamura (1996) successfully mapped the Archaean–Proterozoic boundary and showed that Archaean granitoids represented juvenile mantle material that was added to continental crust at about 2600 Ma whereas Proterozoic granitoids represented juvenile mantle material E-mail address: [email protected].

that was extracted from the mantle at about 2000 Ma and subsequently assimilated with Archaean crustal component from the adjacent Tanzania Craton. To the east of the Craton (Fig. 1), Maboko (2000) showed that the Pan-African Mozambique belt is composed of reworked Archaean crust with the Neoproterozoic juvenile crust limited to the Eastern Granulites of the Usambara Mountains, some 250 km east of the Craton margin. Other workers in the Neoproterozoic Mozambique belt including Möller et al. (1998), Muhongo et al. (2001) and Sommer et al. (2003) arrived at a similar conclusion. This paper presents Nd-isotopic data from granitoids straddling the Archaean–Proterozoic (A–P) boundary in southwestern Tanzania with the aim of precisely identifying the modal age-boundary between the Tanzania Craton and the Palaeoproterozoic Ubendian belt. These data are also used to put some age constraints on the previously un-dated rocks and to evaluate the extent of granitoid magmatism in the late Archaean of the Tanzania Craton. For clarifications on the need to isotopically map the A–P boundary in southwestern Tanzania, two regional geological maps of Tanzania are used in this study: an older one that shows more generalized geology and is frequently used by many researchers (Fig. 1 after Coolen, 1980) and Fig. 2 being a recently compiled geology and mineral resources map of Tanzania by Pinna et al. (2008) that gives more details of the geological unit subdivisions based on the currently available isotopic age

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.01.002

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Fig. 1. Geological map of Tanzania showing the main lithostratigraphic units (After Coolen, 1980). The traverse followed in this study is shown by samples indicated as dots and numbers. The A–P boundary (this study) is indicated by a thick dashed line between samples IM 18 and IM 19.

data. The results are also compared with a previous study done along the same traverse by Guest (1954) who used petrography, lithological and structural field relationships. 2. Geological background The traverse covered in this study runs along the road from Itigi town in central Tanzania to Makongolosi town in southwestern Tanzania (Fig. 1). The geology under the study area, therefore, covers 6 Quarter Degree Sheets (QDS) namely (from central Tanzania south-westward) QDS 140, 158, 175, 193, 210 and 228. Rocks of the Archaean Tanzania Craton underlie most of the central Tanzania and form the oldest geological nucleus of East Africa (McConnell, 1951). Recent geological mapping on QDS 158 surveyed by Petro et al. (2001) and QDS 175 surveyed by Kajara et al. (2001) reveal that the central and southern parts of the Tanzania Craton are underlain by the rocks of the Archaean Dodoman belt (Fig. 2) consisting of migmatitic quartzo-feldspathic- and banded biotite–gneisses that are surrounded by the alkali- and biotitegranites which are also termed as post-orogenic granites (Quennell et al., 1956). According to these workers, the migmatitic gneisses constitute the older suite whereas the pink alkali- and biotite-granites are younger. The Archean Tanzania Craton is flanked by high grade metamorphic rocks of the Palaeoproterozoic Ubendian Belt (~2.0 Ga, Lennoir et al., 1994) in western and southwestern Tanzania whereas its southeastern margin is bordered by the Palaeoproterozoic Usagaran Belt (~2.0 Ga,

Wendt et al., 1972). Although previous workers considered the Usagaran and Ubendian belts to have formed in two separate orogenies, Wendt et al. (1972), Gabert and Wendt (1974) and Priem et al. (1979) established through detailed mapping and geochronology that the two orogenies are equivalent and are now collectively referred to as the Ubendian–Usagaran belt. In both belts, the geology is described as constituting a basement that preserves a complex record of sedimentation, magmatism and tectonism (Mruma, 1990; Lennoir et al., 1994) which is overlain by metavolcanic rocks. On QDS 210 surveyed by Orridge (1964), the predominant rocks are the granites that have been subdivided (from the oldest to the youngest) into leucocratic biotite granites, coarse-grained porphyritic granites and fine-grained biotite granites. These granites are associated with minor quartzo-feldspathic biotite gneisses. On QDS 228 surveyed by Macfarlane et al. (1963), the geology is comprised of a variety of granites and gneisses with the latter becoming more abundant than in QDS 210. The granites in this area are divided into three types: the porphyritic biotite granites, the coarse-grained hornblende-rich Saza and Chunya granites, and the Ilunga granites which are medium to coarse-grained aplogranites. The last two are named according to their local places of occurrences. These rocks have been cut by numerous dykes and intrusions including syenite, gabbro, and norite of unknown age. It should be noted, however, that the age of granitoids and gneisses covering QDS 210 and 228 is collectively regarded to be Precambrian and it is only on QDS 210 that the author assumed, on the basis of analogy with adjacent QDS, that the gneisses were of Ubendian age and hence Proterozoic. Thus, the results of this study are not only

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Fig. 2. Geological map of southwestern Tanzania (adopted from Pinna et al., 2008). A part of south-westerly traverse from Rungwa to Makongolosi is shown by samples indicated as dots and numbers. The A–P boundary (this study) is indicated by a thick dashed line between samples IM 18 and IM 19.

important in identifying the Archaean–Proterozoic boundary in southwestern Tanzania, but also putting age constraints on the previously undated granitoids in the study area. 3. Samples, petrography and analytical methods Thirty granitoid samples, one syenite sample (IM 29) and one granitic gneiss sample (IM 32), collected along the Itigi–Makongolosi traverse (Figs. 1 and 2), were studied under a polarized microscope. Hand specimens (for sample location and outcrop scale description see Appendix A) and petrographic analysis of the samples indicate that granites in the Makongolosi end of the traverse are mainly composed of alkali feldspar (microcline) with a characteristic pinkish colour, quartz, and rare biotite with few specimens dominated by muscovite (Figs. 3 and 4) whereas granites in the Tanzania Craton end are predominantly made of plagioclase feldspar, quartz and biotite (Figs. 3 and 4). This observation is obvious and notable on the outcrop scale and the change from dominantly grayish coloured granites and gneisses to pinkish and unfoliated granitoids is found a few kilometers south of Rungwa town (Fig. 3). The whole rock samples were analyzed for major elements using a Siemens SRS 3000 X-ray Fluorescence (XRF) Spectrometer at the Southern and Eastern Mineral Centre (SEAMIC) Laboratories, Dar es Salaam. 1 g of the oven dried powdered samples was mixed with 7 g of lithium metaborate and fused in a furnace at 1000 °C for 10 min to make glass beads. The beads were then analyzed for major elements following the procedure described in Messo (2004). The volatile content was

determined by heating the sample powders and measuring the weight loss at 110 °C for the moisture within the samples (H2O−) and later heating at 1100 °C for the water held within the minerals (H2O+; Rollinson, 1993). Trace element concentrations were determined at the Activation Laboratories of Ontario, Canada. Aliquots weighing 0.2 g of sample were fused with a mixture of lithium metaborate/tetraborate in graphite crucibles in an induction furnace. The molten sample was poured into a 5% HNO3 solution and shaken until completely dissolved. The sample was further diluted with HNO3 and an Rh–Ir internal standard added to make a final dilution of 6000. The sample solution was then analyzed on a Perkin Elmer Elan 6000 ICP-MS. The analytical reproducibility deduced from repeated analysis of the USGS standard reference material BHVO1 is better than 6%. The accuracy of the analytical results compared to certified values of the USGS standard BHVO1 is better than 10% for most trace elements. The samples were also analyzed for Nd-isotopic compositions as well as Sm and Nd concentrations using a Triton-MC Thermal Ionization Mass Spectrometer at the Activation Laboratories of Ontario, Canada. Aliquots of the powdered rock samples were spiked with a 149Sm–146Nd mixed solution prior to decomposition using a mixture of HF, HNO3 and HClO4. The REE were separated using conventional cation-exchange techniques. Sm and Nd were separated by extraction chromatography on HDEHP covered Teflon powder. Total blanks are 0.1–0.2 ng for Sm and 0.1–0.5 ng for Nd and are considered negligible. The accuracy of the Sm and Nd analyses is ±0.5% corresponding to errors in the 147Sm/144Nd ratios of ±0.5% (2σ). The 143Nd/144Nd ratios are calculated relative to the value of 0.511860 for the La Jolla standard. During the period of analysis

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Fig. 3. Photographs showing representative samples along the traverse: (a) gray, deformed granite sample representing the majority of Archaean granites and (b) pink, undeformed granite sample representing the majority of the Proterozoic granites (right). For details, see text.

the weighted average of 10 La Jolla Nd-standard runs yielded 0.511874± 10 (2σ) for 143Nd/144Nd, using a 146Nd/144Nd value of 0.7219 for normalization. 4. Results and discussion 4.1. Crustal formation ages Results of the Nd-isotopic analyses are presented in Table 1. All the samples show characteristic crustal 147Sm/144Nd ratios (0.0696– 0.1568) indicating that there was little or no Sm/Nd fractionation during magma genesis. Also shown in the table are depleted mantle crustal formation ages (TDM) calculated assuming a linear evolution model for the mantle together with a present-day 143Nd/144Nd value of 0.513114 and 147Sm/144Nd value of 0.222 (Michard et al., 1985). These parameters give in general TDM ages which are ~0.2 Ga younger than those obtained using the values of Goldstein et al. (1984) but are similar to those calculated according to DePaolo (1981). Using the crustal formation ages, the data set can be divided into two groups. The first group consists of samples IM 01 through IM 18 and sample IM 32 which have TDM ages of 2541–2894 Ma (Table 1) and were mapped as belonging to the Archaean. Sample IM 32 is the most southwesterly sample and the last one collected about 3 km to the south of Makongolosi town and it was expected to belong to the Proterozoic but surprisingly it yielded an Archaean age (Table 1). Going through the data, the samples do not show any spatial trend. The second group consists of

samples IM 19 through IM 31 which have TDM ages of 2031–2430 Ma (Table 1) and previous mapping regarded them to be of Precambrian age without categorizing them into either Archaean or Pretorozoic. Similar to group one samples, the ages in the second group also do not show any spatial trend. These results show that the Archaean–Proterozoic boundary in southwestern Tanzania lie ~35 km south of Rungwa town and that all granitic rocks on QDS 210 (Kipembawe) and 228 (Makongolosi) and part of QDS 193 should be regarded as belonging to the Proterozoic. This places the Archaean–Proterozoic boundary ~150 km inside from the traditionally accepted boundary on the southwestern part which is near Lake Rukwa (Figs. 1 and 2). The results of this study are at variance with the newly compiled geology and mineral resource map of Tanzania (Fig. 2) by Pinna et al. (2008) who placed the Archaean–Proterozoic boundary around Lake Rukwa. In particular, the A–P boundary deduced from this study lies at the southern end of the large Mesoarchaean orthogneisses of Pinna et al. (2008, Fig. 2) which are in contact with what they mapped as Neoarchaean granitoids (traverse covering Rungwa to Makongolosi on part of QDS 193, and whole of QDS 210 and QDS 228), but which are essentially Proterozoic granites. Although Pinna et al. (2008) misinterpreted the Proterozoic granites along the traverse followed by this study; they mapped granitoids to the south of Makongolosi town and considered them to be of Neoarchaean to Palaeoproterozoic age. This is in agreement with a TDM age of 2687 Ma for sample IM 32 of this study. Two explanations can be proposed that fit these observations. Firstly, the fact that the Archaean–Proterozoic boundary lies ~35 km south of Rungwa town (Figs. 1 and 2) which is also ~150 km inside from the traditionally accepted boundary implies that the size of the Archaean Tanzania Craton is smaller than it is always considered. This view can be tested and verified by dating emplacement ages of granitoids in southwest and west Tanzania. Secondly, that rocks with Archaean TDM ages are found south of Makongolosi town which were thought to belong to the Proterozoic (represented by sample IM 32) could either be a product of remelting the Archaean crust which would thus result in Archaean TDM ages or could represent a sliver of tectonically interleaved Archaean material within the Proterozoic terrane. This requires further isotopic mapping of the A–P boundary in western Tanzania to confirm the results and suggestions made from this study. The A–P boundary deduced from the Nd-isotopic studies is in good agreement and coincides with the lithology change explained in Section 3. The traverse from Itigi to Rungwa (Fig. 1) comprises deformed gray granites with occasional pink, microcline-rich alkali granites. From Rungwa to Makongolosi, the granitoids change to microcline rich with a characteristic pink colour and they are relatively undeformed. Similar observations were earlier made by Guest (1954) who, using petrography and lithological and structural field relationships, placed the boundary between the granitoid shield (pink alkali granites) and the deformed gray granites with persistent east–west foliation just south of Rungwa town (Fig. 5). This boundary, surprisingly, coincides with the A–P boundary deduced from this study. 4.2. Major and trace elements geochemistry Major and trace elements composition for the Itigi–Makongolosi traverse granitoids are presented in Table 2 and Figs. 6 and 7. As shown in Fig. 6a, Archaean granitoids are indistinguishable from the Proterozoic granitoids on the basis of their major elements composition although the latter tend to generally higher K2O contents (K2O/Na2O=0.49–2.03, average = 1.26) than the former (K2O/Na2O = 0.29–1.87, average=1.10). Sample IM 32 plots well with other Archaean samples rather than the Proterozoic cluster (Fig. 6a). The main geochemical difference between them (Fig. 6b) is the enrichment in the incompatible elements in Proterozoic granitoids (La=8.90–127 ppm, average 52.5 ppm; Zr=57– 936 ppm, average 268 ppm) compared to the lower concentrations in Archaean granitoids (La=7.10–66.7 ppm, average 32.7 ppm; Zr=14– 467 ppm, average 178 ppm). Interestingly, samples at the boundary

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Fig. 4. Thin section photographs of representative granitoid samples along the traverse. Proterozoic samples are mainly composed of microcline + quartz± plagioclase ± muscovite ± biotite ± sphene whereas Archaean samples consist of plagioclase + biotite + quartz ± microcline.

(sample IMs 19, 20, and 21) within the Proterozoic have the highest contents of the incompatible elements (e.g. Zr and La in Fig. 6c). Moreover, the total rare earth elements concentration is generally lower in Archaean granitoids (REEtotal =38.8–306 ppm, average=149 ppm) than in Proterozoic granitoids (39.7–451 ppm, average=221 ppm). On chondrite-normalized rare earth elements diagram, both Archaean (Fig. 7a), and Proterozoic (Fig. 7b) granitoids show fractionated patterns (La/YbCN =3.00–44.5 and 3.49–70.1, respectively; CN refers to chondritenormalized values). Both of these rock types display negative Eu anomalies (Fig. 7a,b) with few exceptions which show no to slightly positive Eu anomalies (Eu/Eu*=0.19–1.27 for Archaean granitoids and 0.08–1.05 for Proterozoic granitoids). Although there are many compositional overlaps between Archaean and Proterozoic granitoids, the main geochemical differences observed are sufficient to ascertain some constraints regarding their genesis. The negative Eu anomalies observed for both requires that their magma genesis involved plagioclase either as a residual phase during partial melting or plagioclase fractionation during fractional crystallization. This suggests that both were generated at low pressures and shallower depths within the continental crust. The enrichment in incompatible elements such as light rare earth elements (LREE) and high field strength elements (HFSE) as well as

the overall higher abundances of the REE for Proterozoic granitoids requires a more felsic protolith than the Archaean granitoids. As shown in Section 4.1, sample IM 32, although sampled to the southern end of traverse supposed to be of Proterozoic age, it yielded an Archaean age. Field relationship shows that it was sampled from granitic gneisses with a similar foliation trend as the Archaean rocks (Fig. 5). This, coupled with closer compositional similarity to Archaean granitoids tend to favour the explanation that sample IM 32 could represent a sliver of tectonically interleaved Archaean material found within the Proterozoic terrane. 5. Summary and conclusions Combined petrography, field lithological relationships, major and trace elements geochemistry and Nd-isotipic abundances have proved a powerful tool in identifying an A–P boundary in the Precambrian crustal province of southwestern Tanzania. (1) On the basis of the Nd-isotopic data, two groups of granitoids were identified: the Archaean granitoids with TDM ages of 2541– 2894 Ma and Proterozoic granitoids with TDM ages of 2031– 2430 Ma.

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Table 1 Sm–Nd-isotopic data for the Itigi–Makongolosi traverse granitoids. Samples

Sm (ppm)

Nd (ppm)

147

Sm/144Nd

143

Nd/144Nd

Archaean samples IM 01 2.54 10.14 0.1514 0.511849 ± 3 IM 02 5.73 33.47 0.1034 0.511001 ± 5 IM 03 3.35 17.73 0.1142 0.511196 ± 3 IM 04 4.87 32.51 0.0905 0.510773 ± 3 IM 05 3.55 22.59 0.0950 0.510846 ± 2 IM 06 9.68 61.34 0.0954 0.510854 ± 3 IM 07 2.84 16.14 0.1063 0.511065 ± 3 IM 08 8.37 53.68 0.0942 0.510822 ± 3 IM 09 2.11 8.13 0.1568 0.511893 ± 3 IM 10 6.17 32.1 0.1161 0.511227 ± 2 IM 11 2.41 18.29 0.0796 0.510588 ± 3 IM 12 12.32 66.02 0.1128 0.511181 ± 2 IM 13 8.95 54.57 0.0991 0.510939 ± 3 IM 14 6.21 48.14 0.0779 0.510604 ± 3 IM 15 2.53 15.03 0.1017 0.510815 ± 4 IM 16 2.77 18.56 0.0902 0.510808 ± 2 IM 17 1.76 10.64 0.0999 0.510918 ± 3 IM 18 2.71 13.73 0.1193 0.511392 ± 2 Proterozoic samples IM 19 8.4 72.92 0.0696 0.510812 ± 2 IM 20 9.23 59.5 0.0937 0.511059 ± 2 IM 21 9.65 66.66 0.0875 0.511029 ± 2 IM 22 6.19 40.81 0.0916 0.511076 ± 2 IM 23 4.94 34.36 0.0869 0.511080 ± 2 IM 24 9.9 68.34 0.0875 0.511040 ± 2 IM 25 6.44 50.46 0.0771 0.510977 ± 2 IM 26 4.74 32.49 0.0882 0.511012 ± 2 IM 27 1.02 7.15 0.0862 0.511081 ± 5 IM 28 3.18 22.15 0.0867 0.511046 ± 3 IM 29 6.32 39.97 0.0955 0.511380 ± 2 IM 30 2.69 16.84 0.0965 0.511436 ± 2 IM 31 6.41 31.73 0.1221 0.511679 ± 3 Archaean sample at the southwestern end of the traverse IM 32 9.92 62.09 0.0965 0.510889 ± 2

TDM (in Ma) 2714 2701 2696 2698 2705 2704 2685 2718 2838 2701 2689 2682 2682 2641 2894 2652 2727 2541 2292 2430 2351 2372 2284 2340 2239 2383 2272 2320 2082 2031 2180 2688

Calculations are based on a decay constant of 6.54 × 10− 12 per year for 147Sm and DM values for Nd are (143Nd/144Nd) today = 0.513114, (147Sm/144Nd) today = 0.222.

(2) Archaean granitoids are mainly deformed, grey, with plagioclase, biotite, quartz with rare microcline-rich alkali granites whereas Proterozoic granitoids are undeformed, microclinerich, rare biotite and are notable for their pinkish colour. (3) Archaean granitoids compositionally differ from the Proterozoic ones with the latter showing enrichment in incompatible elements including the LREE and HFSE and overall higher concentrations of the REE than the former. (4) The results of this study places the A–P boundary of southwestern Tanzania ~35 km south of Rungwa town and is ~ 150 km inside from the southern traditionally accepted boundary near Lake Rukwa and coincides with the boundary between granitoid shield (here presented as Proterozoic granites) and craton granitoids established by Guest (1954) on the basis of petrography and lithological and structural field relationships. This implies that the size of the craton is smaller than heretofore expected. (5) The presence of small vestiges/domains showing Archaean ages within the Proterozoic regions suggest that they are slivers of tectonically interleaved Archaean material found within the Proterozoic terrane in southwestern Tanzania.

Acknowledgements This research benefited from the Sida/SAREC funding which is highly acknowledged. Elisante Mshiu of Department of Geology, University of Dar es Salaam is acknowledged for GIS work on Figs. 1 and 2. Andy Tindle of Open University, Milton Keynes, UK is acknowledged for assistance with optical microscope photography and petrography. I am indebted to the anonymous reviewers and the Journal Associate Editor, Prof. Alan Collins for their constructive criticism which improved the output of this manuscript.

Appendix A. Itigi–Makongolosi traverse sample locations and brief outcrop description Sample

Latitude (°S)

Longitude (°E)

Field outcrop / hand specimen description

IM 01 IM 02 IM 03 IM 04 IM 05 IM 06 IM 07 IM 08 IM 09 IM 10 IM 11 IM 12 IM 13 IM 14 IM 15 IM 16 IM 17 IM 18 IM 19 IM 20 IM 21 IM 22 IM 23 IM 24 IM 25 IM 26 IM 27 IM 28 IM 29 IM 30 IM 31 IM 32

05°44′21″ 05°54′10″ 05°54′38″ 05°57′22″ 06°00′00″ 06°06′44″ 06°09′17″ 06°11′21″ 06°14′54″ 06°24′12″ 06°27′21″ 06°31′54″ 06°36′43″ 06°39′60″ 06°44′12″ 06°55′14″ 06°57′26″ 07°10′33″ 07°13′55″ 07°15′37″ 07°21′29″ 07°32′05″ 07°33′32″ 07°41′16″ 07°45′42″ 07°51′55″ 08°01′22″ 08°06′03″ 08°10′03″ 08°17′28″ 08°18′15″ 08°24′45″

034°21′15″ 034°11′55″ 034°09′57″ 034°07′47″ 034°03′59″ 033°55′39″ 033°53′48″ 033°53′07″ 033°51′52″ 033°48′06″ 033°46′55″ 033°43′07″ 033°41′47″ 033°40′58″ 033°39′47″ 033°33′03″ 033°30′55″ 033°30′22″ 033°31′00″ 033°29′57″ 033°28′00″ 033°25′54″ 033°24′08″ 033°23′17″ 033°21′27″ 033°20′10″ 033°16′34″ 033°15′29″ 033°15′54″ 033°13′44″ 033°13′09″ 033°10′11″

Doroto village 17 km from Itigi, sample taken from an outcrop 10 km offroad, pegmatite is x-cutting the outcrop Biotite-rich granite that is loaded with amphibolite xenoliths as well as quartz and pegmatite veins Huge fresh quartz–feldspar granitic outcrop 5 km away from Itagata village Quartz–feldspar granite similar to IM 03 Quartz–feldspar, rare biotite granite Quartz–feldspar–biotite granite in a forest, chlorite developing after biotite, few km from Kipili secondary school Quartz–feldspar, rare biotite granite, biotite is slightly altered. The outcrop is enclosing a veined granitic xenolith Quartz–feldspar, rare biotite granite, chlorite forming after biotite and is 4 km from IM 07 Quartz–feldspar–biotite granite Quartz–feldspar–biotite granite Quartz–feldspar–biotite granite A very coarse-grained feldspar–biotite–quartz granite (no more xenoliths seen in granites) A homogenous medium grained granitic sample Coarse-grained granite similar to IM 12 (not more xenoliths seen in granites) Coarse-grained granite with big fields crystals Biotite-rich granite sampled just before arriving at Rungwa game reserve Microcline-rich pink granite collected in the Rungwa river channel at the boundary between Manyoni and Mbeya region Microcline-rich pink granite Microcline-rich, rare biotite granite Microcline-rich, rare biotite granite, the outcrop is associated with tourmaline, big feldspar and x-cut by pegmatites Coarse pink granite exposed along a river channel Slightly pink granite Pink granite enclosing a gray granite xenolith Pink granite rich in mafic minerals beyond Kipembawe Pink granite at Mazimbo primary school Microcline, quartz, hornblende granite Pink microcline, Quartz IIunga granite at Gangwe near Lupa village Pink microcline, Quartz IIunga granite at Gangwe at Mamba village Syenite outcrop at Mtande hill Chlorite–epidote -altered sheared pink granite in a river channel Lodea hills pinkish aplitic granite near Makongolosi Granitic gneiss sampled 3 km south of Makongolosi town towards Chunya

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Fig. 5. Itigi–Chunya road traverse map showing the geology along the traverse (Re-drawn from Guest, 1954). Note that his boundary between the main two types of granitoids along the traverse coincides with the A–P boundary deduced from this study.

References Coolen, J.J.M.M.M., 1980. Chemical Petrology of the Furua granulite complex, southern Tanzania. GUA Papers of Geology, Series 1, No. 13-1980. 258 pp. DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crustal-mantle evolution in the Proterozoic. Nature 291, 193–196. Gabert, G., Wendt, I., 1974. Datierungen von granitischen im Dodoman und Usagaran System und in der Ndembera Serie (Tanzania). Geologisches Jahrbuch 11, 3–55. Goldstein, S.L., O'Nions, R.K., Hamilton, P.J., 1984. A Sm–Nd study of atmospheric dusts and particulates from major rivers systems. Earth and Planetary Science Letters 70, 221–236. Guest, N.J., 1954. Petrographical notes on some rocks outcropping near the road from Chunya to Itigi. Records of the geological survey of Tanganyika, IV, pp. 105–109. Kajara, R.S., Ayubu, S.Y., Makene, C.K., Msechu, M.E., 2001. Geological map of Quarter Degree Sheet 175, Musa River. Geological Survey of Tanzania, Dodoma. Kaur, P., Chaudhri, N., Raczek, I., Kroner, A., Hofmann, A.W., 2009. Record of 1.82 Ga Andeantype continental arc magmatism in NE Rajasthan, India: insights from zircon and Sm–Nd ages, combined with Nd–Sr isotope geochemistry. Gondwana Research 16, 56–71. Lennoir, J.L., Liegeois, J.P., Klerkx, J., 1994. The Palaeoproterozoic Ubendian Shear belt in Tanzania: geochronology and structure. Journal of African Earth Sciences 19, 169–184.

Maboko, M.A.H., 2000. Nd and Sr isotopic investigation of the Archaean–Proterozoic boundary in north eastern Tanzania: constraints on the nature of Neoproterozoic tectonism in the Mozambique belt. Precambrian Research 102, 87–98. Maboko, M.A.H., Nakamura, E., 1996. Nd and Sr isotopic mapping of the Archaean– Proterozoic boundary in south-eastern Tanzania using granites as probes for crustal growth. Precambrian Research 77, 105–115. Macfarlane, A., Mudd, G.C., Orridge, G.P., 1963. Geological map of Quarter Degree Sheet 228, Makongolosi. Geological Survey of Tanganyika, Dodoma. McConnell, R.B., 1951. Rift and shield structures in East Africa. Proceedings of the 18th International Geological Congress, Great Britain, 14, pp. 199–207. Messo, C.W.A., 2004. Geochemistry of Neoarchaean volcanic rocks of the Ikoma area in the Kilimafedha greenstone belt, Northwestern Tanzania. M.Sc (Thesis), University of Dar es Salaam, 101 pp. Michard, A., Gurriet, P., Soudant, M., Albarede, F., 1985. Nd isotopes in French Phanerozoic shales: external vs internal aspects of crustal evolution. Geochimica Cosmochimica Acta 49, 601–610. Möller, A., Mezger, K., Schenk, V., 1998. Crustal age domains and the evolution of the continental crust in the Mozambique belt of Tanzania: combined Sm–Nd, Rb–Sr and Pb–Pb isotopic evidence. Journal of Petrology 39, 749–783.

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Table 2 Major (wt.%) and trace (ppm) elements composition for the Itigi–Makongolosi traverse granitoids. Archaean granitoids

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Cr Zr Hf Nb Ta Rb Sr Y Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pb U La/Yb La/YbCN Eu/Eu* K2O/Na2O

IM 01

IM 02

IM 03

IM 04

IM 05

IM 06

IM 07

IM 08

IM 09

IM 10

IM 11

IM 12

IM 13

IM 14

IM 15

IM 16

78.12 0.06 12.8 0.67 0.02 0.05 0.75 4.39 3.77 0.02 0.07 100.7 190 22 1.4 19 1.9 183 18 14 22 7.1 19.2 2.42 9.8 2.5 0.15 2.3 0.4 2.5 0.5 1.7 0.26 1.7 0.24 12.7 71 17.7 4.2 3.00 0.19 0.86

67.91 0.38 15.5 3.18 0.05 0.99 2.71 4.83 2.67 0.18 0.83 99.26 170 177 5.5 11 1.8 146 332 10 320 36.9 70.1 7.96 26.6 5.3 0.87 4 0.5 2.8 0.5 1.2 0.17 1.1 0.15 13.5 31 8 33.5 24.1 0.58 0.55

75.63 0.13 13.4 1.21 0.02 0.13 1.13 3.75 4.52 0.03 0.26 100.2 180 107 4.7 5 0.4 216 85 7 268 17.2 35.6 4.32 16 3.2 0.41 2.4 0.3 1.8 0.3 0.8 0.12 0.7 0.11 30.8 61 8.9 24.6 17.6 0.45 1.21

74.04 0.15 13.2 1.79 0.03 0.25 1.01 3.41 5.36 0.05 0.43 99.75 180 143 5.2 8 0.5 234 130 9 457 36.9 73.8 8.23 24.5 4.3 0.43 3 0.4 2.1 0.3 0.9 0.12 0.7 0.1 40 78 19.4 52.7 37.8 0.37 1.57

73.83 0.22 13.7 1.98 0.03 0.40 1.54 3.75 4.30 0.08 0.52 100.4 180 141 4.4 6 0.5 178 232 8 852 27.7 53.9 6.1 18.8 3.5 0.65 2.4 0.3 1.8 0.3 0.8 0.12 0.8 0.12 11.2 36 5.4 34.6 24.8 0.69 1.15

72.27 0.34 13.4 2.51 0.05 0.47 1.19 3.52 4.73 0.1 0.54 99.07 120 268 9.1 19 1 266 137 24 611 66.7 142 16.2 52.4 9.2 0.64 6.9 1 5.2 0.9 2.4 0.32 1.9 0.27 28.6 41 11.8 35.1 25.2 0.25 1.34

72.78 0.10 13.4 1.57 0.02 0.23 0.80 3.16 5.92 0.08 0.33 98.35 160 101 3.9 2 0.1 218 188 9 448 18.2 36.5 4.22 14.2 2.8 0.39 2.1 0.2 1.1 0.2 0.5 0.08 0.5 0.08 17 38 4.3 36.4 26.1 0.49 1.87

73.06 0.22 12.9 2.08 0.04 0.25 0.97 3.18 5.27 0.06 0.47 98.48 220 206 7 6 0.4 201 81 11 373 55.8 118 13.6 43.1 7.8 0.48 5.1 0.6 2.6 0.4 1 0.15 0.9 0.14 44 45 7 62.0 44.5 0.23 1.66

73.79 0.09 13.3 1.92 0.04 0.24 0.96 4.23 3.50 0.05 0.42 98.53 160 14 0.8 10 1.9 171 87 8 184 7.7 14.9 1.73 6.7 1.7 0.25 1.7 0.3 1.6 0.3 0.9 0.12 0.8 0.12 8.8 38 6.5 9.6 6.90 0.45 0.83

73.26 0.34 13.1 1.95 0.04 0.31 1.01 3.07 5.40 0.11 0.54 99.1 180 257 9.3 12 1.4 219 148 17 375 31.2 65.9 7.89 28 6 0.57 5.2 0.8 4.4 0.8 2.3 0.34 2 0.28 31.2 38 14 15.6 11.2 0.31 1.76

72.28 0.20 14.5 1.88 0.03 0.38 2.25 4.53 2.67 0.06 0.42 99.24 180 95 3.2 3 0.3 83 375 4 655 21.6 41.6 4.57 13.1 2.1 0.48 1.3 0.2 0.8 0.1 0.4 0.06 0.4 0.06 13.9 22 1.2 54.0 38.7 0.89 0.59

66.67 0.69 14.2 5.49 0.07 0.86 2.75 3.66 4.08 0.21 0.5 99.16 130 467 13.2 16 0.9 114 275 34 1078 42.6 104 14.5 55.2 12 1.92 9.9 1.4 7.2 1.3 3.4 0.47 2.9 0.39 2.7 37 1.3 14.7 10.5 0.54 1.11

72.79 0.23 13.1 2.02 0.05 0.25 1.05 3.22 5.21 0.05 0.64 98.59 150 221 7.5 13 0.7 283 106 17 515 58.5 125 14.7 46.4 8.8 0.65 6.2 0.8 4.2 0.6 1.5 0.18 1 0.13 40.4 29 8 58.5 42.0 0.27 1.62

70.77 0.39 13.7 2.98 0.04 0.72 1.74 3.30 4.70 0.14 0.52 98.99 140 240 7.3 8 0.4 171 382 13 1282 63.8 123 13.3 37.8 6.1 1.13 3.8 0.5 2.7 0.5 1.3 0.18 1.1 0.15 23.3 13 1.6 58.0 41.6 0.72 1.42

69.62 0.38 15.3 3.18 0.04 1.11 3.23 4.31 1.48 0.1 0.63 99.36 240 198 5.7 6 0.6 64 191 7 269 23.8 41.3 4.34 14.2 2.7 0.56 2.3 0.3 1.7 0.3 0.8 0.11 0.8 0.11 11.2 22 1.7 29.8 21.3 0.69 0.34

65.93 0.52 16.7 3.54 0.03 1.22 3.63 5.05 1.47 0.19 0.79 99.09 210 252 7.1 3 0.2 52 886 5 981 21.2 33.6 4.55 15.2 2.8 0.97 2 0.2 1.1 0.2 0.4 0.06 0.4 0.06 3.6 10 0.5 53.0 38.0 1.25 0.29

Mruma, A.H., 1990. Stratigraphy, Metamorphism and tectonic evolution of the Early Proterozoic Usagaran belt, Tanzania. Ph.D. Thesis, University of Dar es Salaam, 252 pp. Muhongo, S., Kroner, A., Nemchin, A.A., 2001. Single zircon evaporation and SHRIMP ages for granulite–facies rocks in the Mozambique Belt of Tanzania. Journal of Geology 109, 171–189. Orridge, G.P., 1964. Geological map of Quarter Degree Sheet 210, Kipembawe. Geological Survey of Tanzania, Dodoma. Petro, F.N., Makyao, F.P., Chiragwire, S.A., 2001. Geological map of Quarter Degree Sheet 158, Kilumbi. Geological Survey of Tanzania, Dodoma. Pinna, P., Muhongo, S., Mcharo, A., Le Goff, E., Deschamps, Y., Ralay, F., Milesi, J.P., 2008. Geology and mineral map of Tanzania. Geological Survey of Tanzania, Dodoma. Priem, H.N.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, E.A.T., Verschure, R.H., Oen, I.S., Westra, L., 1979. Isotopic age determinations on granitic and gneissic rocks from the Ubendian–Usagaran system in Southern Tanzania. Precambrian Research 9, 227–239.

Quennell, A.M., McKinlay, A.C.M., Aitken, W.G., 1956. Summary of the geology of Tanganyika: Part I; Introduction and stratigraphy. Geol. Survey Tanganyika. Memoir 1, 1–264, Dar es Salaam. Rollinson, H., 1993. Using geochemical data: evaluation, presentation, interpretation. Longman Group UK Limited, London. 352 pp. Sommer, H., Kröner, A., Hauzenberger, C., Muhongo, S., Wingate, M.T.D., 2003. Metamorphic petrology and zircon geochronology of high-grade rocks from the central Mozambique Belt of Tanzania: crustal recycling of Archean and Palaeoproterozoic material during the Pan-African orogeny. Journal of Metamorphic Geology 21, 915–934. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematic of oceanic basalts: implication for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.). Magmatic in Ocean Basins. Geological Society of London Special Publication 42, 313–345. Wendt, I., Besang, C., Harre, W., Kreuzer, H., Lenz, H., Muller, P., 1972. Age determinations of granitic intrusions and metamorphic events in the early Precambrian of Tanzania. Proceedings of the 24th International Geological Congress, Montreal, Section 1, pp. 295–314.

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Table 2 Major (wt.%) and trace (ppm) elements composition for the Itigi–Makongolosi traverse granitoids. Proterozoic granitoids IM 17

IM 18

IM 32

IM 19

IM 20

IM 21

IM 22

IM 23

IM 24

IM 25

IM 26

IM 27

IM 28

IM 29

IM 30

IM 31

77.36 0.08 13.1 0.83 0.01 0.10 0.95 4.10 3.89 0.02 0.32 100.7 220 47 2 1 0.1 59 108 2 599 11.3 23.8 2.75 9 1.8 0.61 1.2 0.1 0.5 0.1 0.3 0.05 0.3 0.04 10.4 11 1 37.7 27.0 1.27 0.95

77.15 0.08 13.3 0.88 0.01 0.09 0.85 4.27 4.15 0.03 0.13 101 170 58 2.8 4 0.1 78 98 7 628 11.1 25 3.21 12.3 2.6 0.54 2.1 0.3 1.5 0.3 0.7 0.11 0.7 0.11 11.2 17 3.8 15.9 11.4 0.71 0.97

64.22 0.83 15.5 6.24 0.09 1.17 2.67 4.56 3.91 0.33 0.64 100.2 80 375 8.6 15 0.6 76 440 22 1293 62.6 126 17.8 53.3 9.6 2.04 7.2 0.9 4.7 0.8 2.1 0.27 1.6 0.23 1.2 11 0.5 39.1 28.1 0.75 0.86

63.32 0.415 17.76 3.34 0.052 0.53 1.31 5.85 6.12 0.09 0.57 99.37 100 936 23.6 13 0.7 186 302 11 623 127 223 22.3 57.8 8.5 1.84 4.6 0.6 2.7 0.4 1.2 0.18 1.3 0.21 48.9 39 3.5 97.7 70.1 0.90 1.05

65.67 0.362 14.64 3.98 0.047 1.18 1.27 4.47 6.82 0.34 0.46 99.26 130 557 14.9 10 0.5 158 1208 14 4758 56 124 15.6 51.3 9.5 2.65 6.3 0.7 3.4 0.5 1.3 0.19 1.3 0.19 87 43 7.4 43.1 30.9 1.05 1.53

66.21 0.542 16.32 2.95 0.064 0.66 1.58 4.37 6.28 0.12 0.6 99.69 120 464 12.9 15 0.9 187 252 21 1133 74.4 155 17.8 55.8 9.7 1.44 6.9 0.9 4.4 0.8 2.3 0.34 2.2 0.34 20.9 30 2.7 33.8 24.3 0.54 1.44

73.7 0.202 13.6 1.67 0.043 0.32 1.12 3.49 5.01 0.06 0.55 99.76 140 183 6 16 1.7 260 166 19 851 52.7 98.5 10.9 34.4 5.9 0.81 4.5 0.7 3.8 0.7 2 0.3 1.9 0.28 29.1 23 6.4 27.7 19.9 0.48 1.44

71.2 0.25 13.94 2.24 0.046 0.49 1.32 3.67 4.70 0.09 0.81 98.76 160 189 6.1 11 0.9 229 254 12 1131 49.7 92.1 10 31 5.1 0.86 3.5 0.5 2.6 0.4 1.2 0.18 1.2 0.18 27.3 39 2.9 41.4 29.7 0.62 1.28

70.48 0.428 14.26 2.95 0.053 0.82 2.05 3.46 4.95 0.19 0.7 100.3 130 318 9 21 1.5 220 266 25 1345 86.5 166 18.6 58.3 9.7 1.55 6.7 1 5.1 0.9 2.5 0.37 2.3 0.34 22.6 37 4.1 37.6 27.0 0.59 1.43

74.97 0.145 13.31 1.56 0.026 0.16 0.58 2.78 5.64 0.04 0.98 100.2 210 166 5.7 8 0.6 268 103 18 819 71.8 126 14.6 43.4 6.6 1.09 4.6 0.6 2.9 0.5 1.6 0.24 1.5 0.25 27.8 27 2.6 47.9 34.3 0.61 2.03

70.76 0.309 13.75 2.43 0.048 0.73 1.71 3.63 4.66 0.09 0.38 98.49 170 203 6.4 12 1 179 269 14 831 43.3 82.3 9.08 28.4 4.8 0.95 3.6 0.5 2.8 0.5 1.5 0.23 1.6 0.23 17.9 19 5.1 27.1 19.4 0.70 1.28

76.14 0.077 13.06 0.76 0.048 0.1 0.49 4.05 4.18 0.02 0.42 99.33 170 57 3.5 9 0.8 154 87 7 367 8.9 19.7 1.92 5.5 0.9 0.2 0.6 b 0.1 0.6 0.1 0.4 0.07 0.6 0.1 11.3 39 2.3 14.8 10.6 0.83 1.03

76.08 0.122 12.98 1 0.055 0.11 0.55 3.51 5.16 0.02 0.32 99.9 130 106 3.7 10 1 226 87 7 368 30.5 52.9 5.36 22.2 3.1 0.42 2 0.3 1.6 0.3 0.9 0.13 0.9 0.14 28.9 29 3.6 33.9 24.3 0.57 1.47

62.2 0.456 15.52 4.34 0.107 1.63 5.64 6.46 3.15 0.28 0.24 100 90 58 2 20 1.5 26 1314 16 2131 33.3 73.9 9.49 39.9 6.3 1.7 4.9 0.7 3.5 0.6 1.6 0.24 1.5 0.23 3.9 9 0.8 22.2 15.9 0.94 0.49

70.26 0.2 15.78 2.12 0.046 0.68 2.11 4.95 2.65 0.1 0.94 99.84 110 101 3.1 8 0.3 79 558 7 1607 16.6 32.2 3.73 16.7 2.6 0.68 1.9 0.3 1.4 0.3 0.8 0.12 0.8 0.12 3.5 14 0.5 20.8 14.9 1.00 0.54

76.85 0.091 11.84 1.46 0.017 0.08 0.31 3.47 4.87 0.02 0.45 99.45 230 146 6.7 14 1 128 40 53 184 31.6 83.3 9.75 32.3 6.4 0.15 5.8 1.3 8.9 1.9 6.1 0.99 6.5 0.91 16.8 19 3.7 4.9 3.49 0.08 1.40

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Fig. 7. Chondrite-normalized REE diagrams for (a) Archaean granitoids (b) Proterozoic granitoids along the Itigi–Makongolosi traverse (normalizing values after Sun and McDonough, 1989).

Fig. 6. (a) CaO–K2O–Na2O diagram (b) Zr versus La (c) Incompatible elements traverse across the A–P boundary for the Itigi–Makongolosi traverse granitoids. Archaean granitoids are represented by filled squares whereas open circles represent Proterozoic ones.