Isotopic dating of Neoproterozoic crustal growth in the Usambara Mountains of Northeastern Tanzania: evidence for coeval crust formation in the Mozambique Belt and the Arabian–Nubian Shield

Precambrian Research 113 (2002) 227– 242 www.elsevier.com/locate/precamres

Isotopic dating of Neoproterozoic crustal growth in the Usambara Mountains of Northeastern Tanzania: evidence for coeval crust formation in the Mozambique Belt and the Arabian–Nubian Shield M.A.H. Maboko a,*, E. Nakamura b a

b

Department of Geology, Uni6ersity of Dar Es Salaam, PO Box 35052, Dar Es Salaam, Tanzania The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry Institute for the Study of the Earth’s Interior, Okayama Uni6ersity, Misasa, Tottori-ken, 682 -01 Japan Received 29 February 2000; accepted 27 July 2001

Abstract Granulite-facies orthogneisses of andesitic to dacitic composition in the Usambara Mountains of north eastern Tanzania yield a Sm–Nd whole rock isochron age of 815 9 58 Ma and an initial m(Nd) value of 4.1. This age is interpreted as dating Sm–Nd fractionation during extraction from the mantle and immediate subsequent crystallisation of the granulite protolith during an event of regional calc-alkaline magmatism in the area. Isotopic and geochemical characteristics of the rocks are consistent with a convergent margin setting for the magmatism with minimal contamination by older continental crust. The isotopic data from the Usambara Mountains demonstrate that Neoproterozoic crust formation in the Arabian–Nubian Shield and parts of the Mozambique Belt was broadly contemporaneous. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Mozambique Belt; Tanzania; Sm–Nd geochronology; Crust formation; Granulites

The Neoproterozoic East African orogeny (Stern, 1994) played the most important role in shaping the geology of a large part of the eastern seaboard of the African continent, stretching from Mozambique in the south northwards into the Arabian Peninsula (Fig. 1). Whereas models for the formation of the northern part of the orogen * Corresponding author. Fax: + 255-51-410258. E-mail address: [email protected] Maboko).

(M.A.H.

(the Arabian –Nubian Shield) in terms of plate tectonic processes, involving the assembly of several arc and back-arc basin systems, are widely accepted (Stoeser and Camp, 1985; Vail, 1985; Kroner et al., 1987; Stern, 1994), the origin of the southern part, the Mozambique Belt of Holmes (1951) is still open to debate. In particular, the question of whether Neoproterozoic tectonism in the Mozambique Belt was preceded by significant juvenile crust formation, as was the case in the Arabian –Nubian Shield or only involved rework-

0301-9268/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0301-9268(01)00213-3

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ing of pre-existing crust remains contentious. Recent isotopic data indicate that many rocks in the Mozambique Belt of Tanzania yield early Proterozoic to late Archaean mean crustal residence ages (Maboko 1995; Moller et al. 1998; Maboko 2000a). This has reinforced earlier arguments based on geological relations (Hepworth

1972) that, unlike in the Arabian– Nubian Shield, the bulk of the Mozambique Belt crust is composed of older basement that was tectonothermally reactivated during Neoproterozoic time. Isotopic data from eastern Tanzania, however, suggest that some of the granulite terranes in the Mozambique Belt may be composed of juvenile

Fig. 1. Sketch geological map of eastern Tanzania showing the major occurrences of Neoproterozoic granulites in the northern part of the Mozambique Belt (Modified from Appel et al., 1998). The heavy dashed line marks the eastern limit of Archaean crust as reflected in TDM ages \2.5 Ga (Maboko 2000a). Also indicated are TDM ages (in Ga) for Uluguru (Maboko 1995; Moller et al. 1998), Wami (Maboko 1995; Moller et al. 1998; Maboko, 2001b), Usambara (Moller et al. 1998, this work), Pare (Moller et al. 1998) and Kitumbi (Moller et al. 1998; Maboko, 2000b) areas. The data of Moller et al. (1998) have been recalculated using the parameters of Michard et al. (1985). The inset in the lower left-hand corner shows the spatial relationship between the Mozambique Belt and the Arabian – Nubian Shield. The location of Fig. 2 is shown as an open square in the south western part of the Usambara Mountains.

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Neoproterozoic crust (Maboko, 1995; Moller et al., 1998; Maboko, 2000a). In this paper, we present new isotopic and geochemical data for granulites sampled from a small part of the Western Usambara Mountains of north eastern Tanzania. The data are used to constrain the age and tectonic setting of crust formation in the area and to argue for coeval Neoproterozoic crust formation in the Mozambique Belt and the Arabian– Nubian Shield.

1. Geological background The Usambara granulite complex, together with the adjoining Pare Mountains granulites, constitutes the largest and most northerly of the Eastern Granulites (Fig. 1), a semi-continuous, north– south trending belt of granulite facies rocks in the eastern part of the Mozambique Belt in Tanzania (Hepworth, 1972). Coolen (1980) provided a summary of the geology of the Eastern Granulites. In the Usambara Mountains, the rocks represent a thick sequence of highly metamorphosed igneous rocks intercalated with pelitic, calc-argillaceous and calcareous sediments, with minor carbonaceous layers (Bagnall et al., 1963). Mafic meta-igneous marker horizons form persistent bands, some as long as 30 km along strike, interlayered with more felsic rocks (Fig. 2). The granulites are cut by minor intrusive bodies of gabbro, serpentinite, pyroxenite and anorthosite which also bear the granulite facies metamorphic overprint (Bagnall et al., 1963). In the investigated area, the rocks display regular moderate north– east dips of layering and foliation and down-dip plunges of pervasive stretching lineations (Shackleton, 1993). Shackleton (1993) interpreted the entire Usambara Mountains rock sequence as, an initially nearly flat, lower-crustal shear zone that developed under granulite facies conditions during Neoproterozoic time. According to Shackleton (1993), continued movement imbricated and refolded the initially flat foliation, giving rise to the regional northeastward dip. In the area around Lushoto (Fig. 2), the rocks consist of alternating layers of quartzo-feldpathic granulites and hornblende/pyroxene granulites

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(Bagnall et al., 1963). The quartzo-feldspathic granulites are intimately interbanded with hornblende-and pyroxene granulites, such that all gradations between hornblende-free and pyroxenefree rocks occur. The quartzo-feldspathic granulites consist of quartz, K-feldpar and plagioclase9 garnet, biotite and magnetite. Typical mineral assemblages in the hornblende/pyroxene granulites are hornblende+ plagioclase+ quartz 9 garnet9 clinopyroxene9 orthopyroxene9 biotite9 K-feldspar and clinopyroxene+ plagioclase + quartz 9 garnet 9 hornblende 9 orthopyroxene9 biotite9 K-feldspar. Apatite, magnetite, ilmenite and zircon are common accessory minerals. Phase equilibria indicate peak metamorphic conditions of 9.5− 11 kb and  800 °C consistent with metamorphic conditions typical of the base of continental crust of normal thickness (Appel et al., 1998). Muhongo et al. (2001) reported zircon U–Pb ages of  640 Ma for Usambara ortho-granulites which they interpret as dating the age of peak metamorphism. Other constraints on the metamorphic age of the Usambara granulites include monazite U–Pb ages of  625 Ma obtained by Moller et al. (2000) and Sm–Nd garnet ages of  605 reported by Maboko (2001a).

2. Analytical methods Eight granulite samples (Table 1) collected within a  15 km across-strike traverse between Mombo and Lushoto (Fig. 2) were analysed for Sr and Nd isotopic compositions as well as Sr, Nd, Rb and Sm concentrations using a Finnigan MAT262 mass spectrometer at the Pheasant Memorial Laboratory (PML) for Geochemistry and Cosmochemistry of the Institute for the Study of the Earth’s Interior at Misasa, Japan. The analytical procedure for chemical separation and mass spectrometry are described in Yoshikawa and Nakamura (1993) for Rb–Sr and Shibata et al. (1989) for Sm–Nd and are essentially similar to those described in Maboko and Nakamura (1996). The isotopic ratios were normalised to 87Sr/86Sr=0.1194 and 146Nd/144Nd = 0.7219. Replicate analyses of the La Jolla Nd

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M.A.H. Maboko, E. Nakamura / Precambrian Research 113 (2002) 227–242

Fig. 2. Geological map of part of the Usambara granulite complex showing the sampling localities (Adapted from Bagnall et al. 1963).

standard gave 143Nd/144Nd = 0.511920 913 (2s, n= 9), whereas the NBS 987 standard gave 87Sr/ 86 Sr =0.71022498 (2s, n =8). Maximum 2| uncertainty in the Sm/Nd and Rb/Sr ratios derived from long-term reproducibility of standard samples is 2%. Typical blank values are 5 and 10 pg for Sm and Nd, respectively, and negligible. Corresponding values for Rb and Sr are 25 and 35 pg respectively and equally negligible. In order to

attempt to constrain the nature of the granulite protolith, the samples were analysed for major and trace elements compositions, also at Misasa. Major elements, together with Ni and Cr contents, were determined on fused disks using a Phillips PM2400 X-ray fluorescence spectrometer. Trace element analyses were performed on a YOKOGAWA PM@2000 ICP-MS using the flow injection method (Makishima and Nakamura,

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1997). The analytical reproducibility is better than 6.5% and typically  2.5% for trace elements and better than 0.2% for the major elements. The accuracy of the ICP measurements as calibrated against the USGS rock standard BCR-1 is better than 2%. Blank values are negligible and the data has, therefore, not been blank-corrected. The petrography of the analysed samples is summarised in Table 1. Sampling localities are shown on Fig. 2.

3. Results

3.1. Major and trace element composition Major and trace element compositions of the samples are shown in Table 2. The samples have bulk compositions comparable to those of modern convergent margin andesites and dacites (Table 3). On the Total Alkali versus SiO2 (TAS) diagram of Le Maitre et al. (1989), sample US6 plots in the basaltic andesite field, US3, US5 and US10 plot in the andesite field whereas US1, US4, 9L and US9B plot in the dacite field (Fig. 3). Using the K2O –SiO2 classification of Gill (1981),

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all the andesitic samples are medium K andesites whereas according to the FeOtot/MgO –SiO2 classification of Miyashiro (1974) all samples are calcalkaline. The dacitic samples US1, US4, 9L and US9B have major element compositions that overlap with the compositional field of graywackes (Table 3). The nature of the protolith (sedimentary versus igneous) of such rocks can be distinguished on the basis of discrimination functions that essentially reflect the effects of progressive feldspar weathering (Shaw, 1972; Nesbitt and Young, 1984; Raith et al., 1999). Shaw (1972) developed a discrimination function (DF= 10.44–0.21SiO2 – 0.32Fe2O3tot –0.98MgO+ 0.55CaO+1.46Na2O+ 0.54K2O, values in weight%), which distinguishes quartzo-feldspathic gneisses of igneous and sedimentary origin on the basis of their DF values. Meta-igneous rocks yield positive DF values whereas meta-sediments yield negative values. Samples US4 and 9L yield positive DF values (2.1 and 1.5, respectively) suggesting an igneous parentage whereas sample US9B yields a value of − 0.4 indicative of a sedimentary protolith. Sample US1 has a DF value of −0.01, close to the sedimentary-igneous boundary, and can, there-

Table 1 Petrography Sample number Mineral assemblagea Texture US1 US3 US4 US5 US6

9L US9B US10

Qtz+Plag+Opx +Bio Plag+Qtz+Hb +Opx+Gt Qtz+Plag+Cpx +Opx9Gt+Hb Qtz+Plag+Cpx +Opx9 Gt+Hb Qtz+Plag+Cpx +Opx+Hb+Bio +Gt Plag+Qtz+Gt +Cpx+Hb Qtz+Plag+Bio +Bio+Gt Qtz+Plag+Cpx + Hb+Bio

Some Opx rimmed by fine-grained second-generation Bio. Qtz elongated sub-parallel to the regional foliation. Some Opx partly altered elongated sub-parallel to Some Opx partly altered elongated sub-parallel to Some Opx partly altered

to fine-grained second-generation Bio and Hb. Qtz and Gt the regional foliation. to fine-grained second-generation Bio and Hb. Qtz and Gt the regional foliation. to fine-grained second-generation Bio and Hb.

Qtz elongated sub-parallel to the regional foliation. Elongated Qtz partially re-crystallised into strain-free subgrains. Some Cpx rimmed by fine-grained grained second-generation Bio and Hb.

a Qtz, Plag, Opx, Cpx, Bio, Hb and Gt refer to quartz, plagioclase, orthopyroxene, clinopyroxene, biotite, hornblende and garnet, respectively.

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Table 2 Major element (in wt.%) and trace element (in ppm) composition US1

US3

US4

US5

US6

US9B

9L

US10

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total

67.7 0.343 13.4 5.05 0.101 2.40 3.40 3.66 0.97 0.062 97.1

57.8 0.560 16.1 7.36 0.116 3.24 7.19 3.51 0.923 0.131 96.9

61.5 0.448 15.8 6.21 0.113 2.18 5.79 3.31 1.21 0.1 96.9

60.2 0.690 16.2 6.39 0.131 2.52 5.30 3.61 1.57 0.273 96.9

53.5 0.532 14.8 10.8 0.187 5.37 8.94 2.84 0.616 0.134 97.3

66.1 0.757 14.8 5.68 0.058 2.26 2.44 3.26 1.72 0.047 97.1

62.2 0.426 15.7 6.55 0.124 2.34 5.90 3.14 1.21 0.106 97.7

56.2 0.762 16.2 6.52 0.091 4.37 7.73 4.39 0.854 0.154 97.3

Ni Cr Rb Sr Y Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Pb Th U Zr Hf Nb Ta La/Ta Ba/La La/Nb Sr/Y

9.80 52.7 12.6 427 16.5 0.130 601 15.4 26.2 2.7 6.50 1.2 0.98 1.5 0.27 2.1 0.51 1.60 0.27 1.9 0.28 8.00 3.6 0.34 84.8 2.6 2.6 0.10 150 39.1 5.92 25.0

20.3 88.9 5.32 413 33.8 0.010 383 17.9 42.7 5.8 25.5 5.8 1.0 5.3 0.88 5.2 1.1 3.00 0.49 3.2 0.48 4.20 0.39 0.09 46.1 1.4 6.4 0.17 107 21.5 2.77 12.7

5.40 21.5 13.8 399 16.8 0.140 727 13.4 26.8 3.1 12.7 2.7 0.72 2.4 0.40 2.4 0.53 1.40 0.24 1.7 0.27 7.70 0.27 0.24 35.8 1.1 4.3 0.17 80 54.1 3.14 21.8

4.10 21.3 18.7 613 13.8 0.010 2095 20.8 37.0 3.9 14.6 2.4 1.3 2.0 0.28 1.8 0.45 1.47 0.26 2.0 0.35 5.80 0.50 0.12 145 3.1 8.2 0.28 74 101 2.53 41.6

37.1 167 1.6 445 19.7 0.003 205 8.9 20.9 2.8 12.6 3.1 0.88 3.0 0.50 3.1 0.66 1.80 0.28 2.0 0.29 3.20 0.10 0.02 37.5 1.1 2.9 0.09 96 23.2 3.08 24.8

41.4 112 150 323 22.3 0.180 568 47.7 93.7 10.6 29.3 5.8 1.5 5.8 0.81 4.1 0.70 1.60 0.24 1.7 0.27 11.5 15.0 0.46 237 8.2 5.5 0.20 239 11.9 8.7 13.2

6.80 19.1 13.5 413 21.3 0.010 972 16.3 31.7 3.7 12.9 2.8 0.90 2.9 0.49 3.1 0.66 1.90 0.30 2.2 0.34 7.00 0.62 0.11 75.8 1.8 3.1 0.12 141 59.7 5.19 21.6

63.1 126 1.86 360 28.2 0.010 356 22.6 47.6 5.8 25.1 5.7 1.4 4.9 0.80 4.8 0.98 2.50 0.37 2.5 0.35 3.90 0.51 0.15 115 2.9 9.2 0.57 40 15.8 2.44 12.2

fore, not be efficiently discriminated using this approach. Any loss of alkalis (Na2O and K2O) as a result of metamorphism would tend to decrease the DF value and would, therefore, not affect the classification of the samples classified as of igneous parentage.

Nesbitt and Young (1982, 1984) proposed a triangular diagram based on the chemical index of alteration (CIA), which can also be used to distinguish between metamorphic rocks of igneous and sedimentary percentage (Fig. 4). On this diagram sample US1, US4 and 9L, together with the

Element

1

1A

2

2A

2B

2C

2D

3

3A

3B

4

4A

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K 2O P2O5

54.8 0.544 15.2 11.1 0.191 5.50 9.15 2.91 0.630 0.137

55.42 0.991 17.95 8.487 0.099 4.561 7.535 3.867 0.892 0.198

59.9 (57.8–62.1) 0.691 (0.578–0.783) 16.7 (16.6–16.7) 6.69 (6.59–7.60) 0.116 (0.093–0.135) 3.48 (2.60–4.49) 6.95 (5.47–7.95) 3.95 (3.62–4.51) 1.150 (0.878–1.62) 0.192 (0.135–0.282)

58.31 0.886 15.30 6.932 0.179 3.204 6.985 3.821 1.453 0.249

59.98 0.894 17.38 7.061 0.099 2.781 6.157 4.270 1.192 0.199

60.30 0.904 16.28 8.451 0.201 2.613 6.934 3.316 0.804 0.201

58.69 0.807 17.42 7.768 – 3.259 6.992 3.238 1.624 0.202

65.7 (63.6–69.7) 0.418 (0.353–0.463) 15.4 (13.8–16.4) 6.11 (5.20–6.70) 0.116 (0.104–0.127) 2.37 (2.26–2.47) 5.18 (3.50–6.04) 3.47 (3.21–3.77) 1.16 (0.999–1.25) 0.092 (0.064–0.108)

65.26 0.696 16.32 5.193 0.100 1.691 4.377 4.576 1.592 0.199

64.88 0.794 13.69 9.127 0.198 1.389 4.663 3.968 1.091 0.198

68.1 0.779 15.2 5.85 0.060 2.327 2.512 3.357 1.77 0.048

67.7 0.61 14.9 5.72 0.10 2.59 2.63 3.42 1.95 0.30

a Sample US6; 1A: Typical calc-alkaline basaltic andesite from the Quartenary High Cascades (McBirney 1985). 2: Average (range in brackets) of samples US3, US5 and US10; 2A, 2B, 2C and 2D are, respectively, typical Quarternary island arc andesite from New Britain (Brownlow 1996), typical calc-alkaline andesite from the Cascades (McBirney 1985), average of 6 andesites from the Scotia arc (Hughes 1982) and average of 1775 Cenozoic andesites (Hughes 1982). 3: Average (range in brackets) of samples US1, US4 and 9L; 3A and 3B are, respectively, typical dacite from the Cascades (McBirney 1985) and average of two dacites from the Scotia arc (Hughes 1982). 4: Sample US9B. 4A: Representative graywacke analysis (Pettijohn et al. 1972).

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Table 3 Comparison of the chemical composition (re-calculated to 100% on a water-free basis) of the Usambara granulites with the compositions of possible protolith rock typesa

233

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Fig. 3. Classification of the Usambara granulites according to the Total Alkali versus SiO2 scheme of Le Maitre et al. (1989).

andesitic samples US6, US3, US5 and US10, plot in the igneous field whereas sample US9B plots in the field of sedimentary rocks. Thus, on the basis of the major element chemistry we conclude that of the eight analysed samples, only sample US9B has a sedimentary (graywacke) protolith, whereas the remaining 7 are ortho-granulites of andesitic to dacitic composition. The trace element data are plotted in chondrite normalised REE plots (normalising values after Boynton, 1984) and Primitive Mantle normalised diagrams (Normalising values after McDonough and Sun, 1995) in Fig. 5. On chondrite normalised diagrams, the meta-igneous samples have light (L)REE-enriched patterns with (LaN /YbN ) values ranging from 3.0 to 7.0 and nearly flat heavy (H)REE patterns (Fig. 5a) as indicated by (TbN /YbN ) values of 0.64 to 2.10 (The subscript N denotes concentrations normalised to the respective chondritic values of Boynton, 1984). The meta-sedimentary sample US9B shows considerably higher LREE enrichment with a La content 154 times the chondritic value and a (LaN/YbN) of 18.8. Like the meta-igneous samples, US9B is

also characterised by a nearly flat HREE pattern (TbN/YbN = 1.43). Similar to the REE patterns, primitive mantle normalised trace element patterns also show an over all enrichment in the most incompatible ele-

Fig. 4. Chemical index of alteration (CIA) diagram, with oxides plotted as molecular proportions (After Nesbitt and Young 1984), for the Usambara granulites. All samples except, US9B, plot in the field of igneous (I) protoliths. Sample US9B plots in the sedimentary (S) field suggesting a graywacke protolith.

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Fig. 5. (a) Chondrite normalised REE patterns for the Usambara granulites (Normalizing values after Boynton 1984). (b) Trace element pattern for the Usambara granulites normalized to Primitive Mantle (Normalizing values after McDonough and Sun, 1995). Note the strong depletion in Nb, Ta and Ti relative to adjacent elements.

ments relative to the less incompatible ones (Fig. 5b). Superimposed on this general pattern, the samples show a strong depletion in Ta and Nb relative to K and La and in Ti relative to Eu and Tb (Fig. 5b). This prominent depletion in the High Field Strength Elements (HFSE) is a feature

235

typical of magmatic rocks formed in convergent margins (Brique et al., 1984). All samples are also strongly depleted in Rb relative to Ba and in Th relative to K and Ta (except samples US1 and US9B, which do not show the Th depletion). In summary, the major and trace element composition of the Usambara rocks suggests a calc-alkaline magmatic protolith (except for the meta-sedimentary sample US9B) with convergent margin geochemical affinities. Even the meta-sedimentary sample appears to have been derived from the weathering and erosion of source rocks with geochemical affinities similar to the protolith of the meta-igneous granulites. The LREE enrichment, the nearly flat HREE patterns and the prominent negative anomalies in the HFSE are all features that characterise magmatic rocks formed in convergent margin tectonic settings (Pearce et al., 1984). A convergent margin setting is also suggested by ratios of the highly incompatible trace elements La/Ta (40–150), La/Nb (2.44– 5.92) and Ba/La (21.5–101) which fall in the respective ranges (\30, 2–5 and 15– 80, respectively) for convergent margin intermediate to silicic rocks (Gill, 1981).

3.2. Isotopic data The Nd and Sr isotopic data are shown in Tables 4 and 5. Also shown are depleted mantle mean crustal residence ages (TDM) calculated assuming a linear evolution model for the mantle together with present-day mantle 143Nd/144Nd and 147 Sm/144Nd values of 0.513114 and 0.222, respec-

Table 4 Sm–Nd data Sample

143

Nd (ppm)

Sm (ppm)

147

TDM

m(0.815)a

US1 US3 US4 US5 US6 9L US9B US10

0.512375 9 10 0.5125229 8 0.5124809 7 0.5123399 8 0.512611 99 0.512470 9 8 0.5124149 9 0.5125299 7

6.500 25.51 12.68 14.57 12.55 12.93 29.27 25.06

1.18 5.83 2.73 2.43 3.13 2.75 5.79 5.73

0.1095 0.1376 0.1295 0.1004 0.1502 0.1280 0.1190 0.1377

1001 1068 1044 971 1068 1044 1036 1057

4.10 4.01 4.04 4.37 4.40 4.01 3.87 4.13

a

Nd/144Nd

Refers to m(Nd) at 815 Ma.

Sm/144Nd

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236 Table 5 Rb–Sr data Sample

87

Sr (ppm)

Rb (ppm)

87

(87Sr/86Sr)Ia

US1 US3 US4 US5 US6 9L US9B US10

0.7043569 10 0.704224 9 8 0.7048029 9 0.7046829 8 0.7034809 8 0.7045509 7 0.7085049 7 0.703742 9 7

427. 413 399 613 445 413 323 360

12.6 5.32 13.8 18.7 1.62 13.5 150 1.86

0.0851 0.0371 0.0996 0.0878 0.0105 0.0940 1.341 0.0149

0.7035 0.7039 0.7038 0.7038 0.7034 0.7036 0.6951 0.7036

a

Sr/86Sr

Refers to the calculated

87

Rb/86Sr

Sr/86Sr ratio at 815 Ma.

tively (Michard et al., 1985). These parameters result in TDM ages which are  0.2 Ga younger than those obtained using the parameters of Goldstein et al. (1984), but are similar to those calculated according to the method of DePaolo (1981). Tables 4 and 5 also list, initial m(Nd) and initial 87Sr/86Sr ratios calculated for an age of 815 Ma which we consider to be the emplacement age for the granulite protolith (see discussion section below). All age calculations are based on decay constants of 6.54× 10 − 12/a for 147Sm and 1.42× 10 − 11/a for 87Rb. All errors quoted on mean values represent calculated 2 standard errors of the mean (2 S.E.). The granulites yield TDM ages which lie in a very narrow range (970– 1070 Ma, mean= 1036 Ma, Table 3) and o(Nd) values of 3.87-4.40 with a mean of 4.129 0.12 (actual maximum analytical uncertainty encompassing the error in emplacement age is 0.7 m(Nd) units). If the meta-sedimentary sample US9B is excluded, the mean TDM age and o(Nd) values remain unchanged at 1036 Ma and 4.1590.12, respectively. There is no correlation between the SiO2 content and the m(Nd) value or between the m(Nd) value and 1/Nd. The measured 87Sr/86Sr ratios of the meta-igneous samples lie in the range 0.703480–0.704802 whereas the meta-sedimentary sample US9B has a ratio of 0.708504. If the meta-sedimentary sample US9B, which gives unrealistically low initial ratio of 0.6951 is excluded, the remaining samples have mantle-like calculated initial 87Sr/86Sr ratios which fall in the range 0.7034– 0.7039 with a mean value of 0.703590.1 (2 S.E.).

4. Discussion

4.1. Emplacement age of the granulite protolith The Nd isotopic data define a positively correlated trend in 147Sm/144Nd – 143Nd/144Nd space, which corresponds to an isochron age of 825964 Ma (2| error, MSWD =1.2) and an initial o(Nd) value of 4.2, similar to the calculated mean value of the individual analyses (Isochron calculated using the Isoplot program of Ludwig, 1994). If the meta-sedimentary sample US9B is excluded from the regression, the age and initial m(Nd) value are 815958 Ma (MSWD = 1.17) and 4.1, respectively, (Fig. 6). This age is significantly older than the  640 Ma zircon U–Pb age which is considered by Muhongo et al. (2001) as dating granulite facies metamorphism in the area. It is also older than the monazite U–Pb ages of  625 Ma which are considered by Moller et al. (2000) as also dating peak metamorphism in the Usambara area. This rules out an interpretation of the isochron age as reflecting homogenisation of the Sm–Nd isotope systematics during granulite facies metamorphism. An alternative interpretation of the isochron as a mixing line between a young, post-815 Ma component and an older component with a low m(Nd) is precluded by the Pb isotope of data of Moller et al. (1998) which show a narrow spread (15.46191–15.5559 2) in the 207Pb/204Pb ratios of K-feldspars from the Usambara granulites. Such a narrow spread in Pb isotope ratios preclude the incorporation of a significant amount of older crustal components in the granulite pro-

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Fig. 6. Sm –Nd whole rock isochron for the Usambara ortho-granulites.

tolith because any such components would result in a much larger spread in the 207Pb/204Pb ratios (Moller et al., 1998). The positive m(Nd) values and the low initial Sr isotope ratios of the samples also indicate mantle-derivation of the protolith with at most a minimal contribution from older continental crust. We, therefore, interpret the positive correlated trend obtained from the meta-igneous samples as a true isochron dating Sm– Nd fractionation during extraction from the mantle and immediate subsequent crystallisation of the granulite protolith at 815958 Ma. Unlike the Nd data, the Sr isotopic data do not define an isochron suggesting mobility of Rb and/or Sr after emplacement.

4.2. Mean residence ages Using the depleted mantle model of Michard et al. (1985), the Usambara granulites yield mean crustal residence ages of between 970 and 1070 Ma. These ages are much younger than the Neoarchaean ages obtained from the amphibolite to granulite facies rocks that outcrop west of the Eastern Granulites (Maboko, 1995; Moller et al., 1998; Maboko, 2000a,b). The Usambara TDM ages are, however, between 155 and 255 Ma older than the inferred protolith emplacement age of  815 Ma. Taking into account their mantle-like Nd and Sr isotopic signatures, three alternative explanations can be advanced to account for the discrepancy in the TDM and emplacement ages of

the samples. (1) The samples represent material that was extracted from the mantle at  970– 1070 Ma and was subsequently re-melted at  815 Ma such that the re-melting event did not result in any significant fractionation of the Sm/ Nd ratio. (2) They represent  815 Ma old juvenile material that was extracted from a mantle source which was less depleted than the Michard et al. (1985) model mantle. (3) They represent juvenile  815 Ma old mantle material which has been contaminated by a relatively small amount of an older crustal component. Of the three alternatives, only (1) would result in TDM ages that date a specific geological event. However, given the fact that at  815 Ma the samples had a homogeneous Nd isotopic composition but fractionated Sm/Nd ratios as attested by their plotting on an isochron of that age, we find alternative (1) unlikely. Alone, the data from the Usambara area do not allow for an unequivocal discrimination between the two remaining alternatives (i.e. 2 and 3). The occurrence over widely separated areas of the East African Orogen of rocks with similar TDM ages and similar Nd, Sr and Pb isotopic characteristics (see the regional correlation section), however, tends to favour a source origin for the difference (alternative 2), rather than the fortuitous contamination of rocks to more or less the same degree on a regional scale (alternative 3). Either way, the Usambara TDM ages show convincingly that the granulite protolith represent juvenile additions to the conti-

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Fig. 7. TDM versus Crystallization age diagram (After Harms et al. 1990; Stern and Abdelsalam 1998) for the Usambara granulites. The Usambara field includes the 7 ortho-granulites from this study and 3 charnockite analyses from Moller et al. (1998). Also shown is the field for the Wami River granulites, which includes data for samples MB2, MB3 and MB4 of Maboko (2001b), WAM1 of Maboko (1995) and T-144-2 of Moller et al. (1998). All samples plot to the left of or close to the line separating the juvenile and reworked fields indicating regional Neoproterozoic crust formation in the Tanzanian sector of the Mozambique Belt. For the Wami River samples a crystallization age of 0.7 Ga (Muhongo et al. 2001) has been assumed whereas an age of 0.82 Ga (this work) has been assumed for the Usambara samples.

nental crust during Neoproterozoic time. The latter conclusion is best illustrated by Fig. 7 which distinguishes the samples on whether the calculated mean crustal residence and emplacement ages are similar (expected of juvenile crust) or different as would be expected in older re-mobilised crust. Following the approach of Harms et al. (1990) and Stern and Abdelsalam (1998) in the Arabian–Nubian Shield, we consider differences of up to 300 Ma between TDM and crystallisation ages as indicative of juvenile crust. Using this criterion, all Usambara ortho-granulites plot in the juvenile field (Fig. 7), buttressing the conclusion that they were extracted from the mantle in Neoproterozoic time. The meta-sedimentary sample US9B also plots in the juvenile field indicating derivation from source lithologies with a mean crustal residence age similar to that of the metaigneous samples. Plotted on Fig. 7 are also data

for three ortho-granulite samples collected over a wider area of the Usambara Mountains by Moller et al. (1998) and data for 5 ortho-granulite samples from the Wami River area previously reported by Maboko (1995) Maboko (2001b) and Moller et al. (1998). For ease of comparison, the data of Moller et al. (1998) have been recalculated using the parameters of Michard et al. (1985). The entire data set plots in the juvenile crust field indicating Neoproterozoic crustal growth in widely separated areas of the Tanzanian sector of the Mozambique Belt.

4.3. Correlation with the Arabian Nubian shield and adjacent areas The  815 Ma age of crust formation in the Usambara area is within range of the 650–850 Ma phase of convergent margin calc-alkaline

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magmatic activity which led to wide spread crust formation in the northern part (Saudi Arabia, Israel, Jordan Egypt and Sudan) of the Arabian– Nubian Shield (Bentor, 1985; Stein and Goldstein, 1996). Although scarce, isotopic and geochemical data from the southern part of the Arabian– Nubian Shield (northern Ethiopia and Eritrea) also indicate Neoperoterozoic crustal growth in a convergent margin setting (Tadesse et al., 2000 and references therein). Tadesse et al., (2000) described 800–750 Ma old syntectonic granitoids from Axum, close to the boundary between the Arabian–Nubian Shield and the Mozambique Belt, with isotopic and geochemical signatures similar to the Usambara granulites. Nine samples of the Axum granitoids yield TDM ages of between 810 and 1085 Ma (mean=946 Ma) and initial m(Nd) values of 3.3– 6.0 (mean= 4.5 90.6), which are similar to values obtained for the Usambara granulites (TDM ages re-calculated using the parameters of Michard et al., 1985). The TDM and m(Nd) values from the Axum granitoids are also similar to the values reported by Maboko, 2001b for the Wami River granulites. This suggests that Neoproterozoic crust formation in the East African Orogen extended from Tanzania in the south, through the southern part of the Arabian– Nubian Shield into the better-documented northern part of the shield. Handke et al. (1999) reported U– Pb zircon and baddeleyite ages of between 804 and 776 Ma for a 450 km long belt of plutonic rocks in west central Madagascar. They interpreted these rocks as constituting the root of a  800 Ma continental magmatic arc at the time of, or slightly preceding, the break up of Rodinia. Convergent margin magmatism in western Madagascar, therefore, was coeval with similar activity in the Arabian– Nubian Shield and in the Mozambique Belt of Tanzania. Together, the Malagasy and Tanzanian rocks provide further evidence of the fact that Rodinia was breaking up, as tectonic and magmatic activity associated with Gondwana assembly was ongoing (Dalziel, 1992). The occurrence of juvenile crust in widely separated areas of the Mozambique Belt demonstrates that Neoproterozoic crust formation in the East African Orogen was not confined to the Arabian –

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Nubian Shield only. The data further suggest a similarity in the isotopic source characteristics of the new crust in the two parts of the orogen, as reflected in similar isotopic signatures of juvenile rocks. The similarity in the Nd isotope systematics of Neoproterozoic juvenile crust in the Mozambique Belt and the Arabian– Nubian Shield is illustrated in an initial m(Nd) versus emplacement age curve (Fig. 8). Initial o(Nd) values for juvenile rocks of the Mozambique Belt (Maboko, 1995; Moller et al., 1998, Maboko, 2001b, this work) lie along the same trend as those of rocks from the Arabian–Nubian Shield (Stein and Goldstein, 1996; Stern and Abdelsalam, 1998; Tadesse et al., 2000). Both suites plot between 1.0 and 4.5 m(Nd) units below the de-

Fig. 8. Initial m(Nd) isotopic compositions relative to that of depleted mantle (Michard et al., 1985) for Neoproterozoic juvenile rocks in the East African Orogen. At the time of their crystallization, the samples plot 1 – 4.5 m(Nd) units below depleted mantle suggesting derivation from a common source that is enriched relative to the MORB source. The dotted lines enclose the inferred Nd isotope growth envelope for such a source (Usambara data from this study and Moller et al. 1998, Wami data from Maboko, 2001b, data for the northern part of the Arabian – Nubian Shield (ANS) from Stein and Goldstein 1996 whereas data for Axum (southern part of the Arabian – Nubian Shield) is from Tadesse et al. 2000). Errors in age are 9 60 Ma or better and do not move the samples from the inferred growth envelope.

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pleted mantle growth curve suggesting their generation from a source that was depleted relative to the Bulk Earth but was enriched relative to the MORB source. The isotopic similarity between juvenile rocks of the Mozambique Belt and those from the Arabian– Nubian Shield is also revealed in the Sr isotope systematics. Calculated initial Sr isotope ratios for the Usambara (Moller et al., 1998, this work) and Wami River rocks (Maboko, 2001b) are indistinguishable from those reported for juvenile crust from the northern part of the Arabian–Nubian Shield (Henjes-Kunst et al., 1990). Isotopic similarity between the two parts of the East African Orogen is further demonstrated by Pb isotope data. Moller et al. (1998) reported feldspar 207Pb/204Pb ratios from the Usambara and Pare granulites that are similar to those obtained by Stacey et al. (1980) and Stacey and Stoeser (1983) for juvenile rocks from the Arabian– Nubian Shield. Overall, the isotopic similarity between Neoproterozoic juvenile rocks in the Mozambique Belt and the Arabian-Nubian Shield suggests that in both areas, new crust was formed from mantle sources with similar Nd, Sr and Pb isotope characteristics.

nile isotopic signature have been reported in the Pare and Usambara Mountains by Moller et al. (1998). These data indicate the existence during Neoproterozoic time of a considerable area that was underlain by juvenile crust from which the meta-sediments were derived. The available data, therefore, confine the distribution of Neoproterozoic juvenile crust to a relatively small part of the Mozambique Belt in the northern part of the discontinuous belt of Eastern Granulites close to the Indian Ocean coastline (Fig. 1). Amphibolite to granulite facies rocks from the more extensive area west of the Eastern Granulites yield late Archaean TDM ages, similar to those found in the Tanzania Craton (Maboko, 1995; Moller et al., 1998; Maboko, 2000a,b). However, there are still large tracts of the Eastern Granulites for which isotopic data is lacking. More isotopic work, particularly in the granulite complexes of south eastern Tanzania, is necessary before the full extent of Neoproterozoic juvenile material in the Mozambique Belt can be ascertained.

4.4. Extent of ju6enile crust in the Mozambique Belt

Granulite facies ortho-gneisses which are exposed in the Usambara Mountains of north eastern Tanzania are a product of 815 Ma old mantle-derived andesitic to dacitic magmatism with convergent-margin geochemical affinities. The rocks were subsequently metamorphosed into the granulite facies at  625–640 Ma (Moller et al., 2000; Muhongo et al., 2001). The isotopic and geochemical data from the Usambara area demonstrate the existence in parts of the Mozambique Belt of juvenile Neoproterozoic crust that is largely coeval with, and has isotopic signatures similar to, the juvenile crust exposed in the Arabian–Nubian Shield.

Despite the existence of a significant amount of juvenile Neoproterozoic rocks in the northeastern part of the Mozambique Belt of Tanzania, the overall proportion of such material remains unclear. In the Usambara Mountains and adjoining areas, a total of 4000 km2 (Shackleton, 1993) is underlain by rocks similar to those described here, whereas in the Wami River area juvenile rocks cover an estimated 600 km2. Maboko (1995) and Moller et al. (1998) reported Nd isotope data which suggest the existence of mixed Archaean and Neoproterozoic crust in the Western Uluguru Mountains, to the south of the Wami River area (see Fig. 1 for locations). Moller et al. (1998) also reported data for a metapelite sample from the Eastern Uluguru Mountains with a juvenile Neoproterozoic Nd isotopic signature (recalculated TDM and m(Nd) values are 1000 Ma and 3.4, respectively). Meta-sediments with a similar juve-

5. Conclusions

Acknowledgements We acknowledge Chie Sakaguchi, Nobuko Takeuchi, Ryoji Tanaka, Hiroyuki Takei and Akio Makishima for technical assistance during different stages of the analytical work. The re-

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search was conducted while MAHM was a Visiting Research Scholar at the Institute for the Study of the Earths Interior, Okayama University at Misasa, Japan. Fieldwork was funded by a grant from the Tanzania Commission for Science and Technology. The paper has benefited from the critical reviews of A. Kroner, N. Harris, A. Moller, B. Mocek and R. Thomas.

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