Journal of African Earth Sciences 86 (2013) 107–118

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Journal of African Earth Sciences

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Geochemistry and U–Pb zircon dating of the high-K calc-alkaline basaltic

andesitic lavas from the Buanji Group, south-western Tanzania
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 i n f o a b s t r a c t

Article history: SHRIMP zircon U–Pb and Sm–Nd isotopic data together with major and trace elements data are presented
Received 15 February 2013 for the basalt andesitic lavas from the Buanji Group, south-western Tanzania in order to establish their
Received in revised form 2 June 2013 emplacement age, ascertain their mantle sources and processes responsible for their generation. Zircon
Accepted 28 June 2013
U–Pb data shows that the Buanji Group lavas were emplaced at 1674 ± 15 Ma and do not therefore belong
Available online 9 July 2013
to the Neoproterozoic Bukoban Supergroup. Having erupted on top of sedimentary rocks, the
1674 ± 15 Ma age of the volcanic rocks provides the younger limit to the age of deposition of underlying
Keywords:
un-dated sediments.
Buanji Group
U-Pbzircon dating
The Bunaji Group lavas are amygdaloidal and contain euhedral phenocrysts of plagioclase + orthopy-
Crustal contamination roxene + clinopyroxene as well as subhedral quartz and opaque minerals mainly magnetite with quartz
Continental arc filling in the interstices. As a suite, they are compositionally uniform and are classified as high-K calc-
Palaeoproterozoic alkaline basaltic andesites. The samples display coherent and fractionated REE patterns with La/SmCN
and La/YbCN ratios of 2.48–2.72 and 4.03–5.92, respectively; and are characterized by negative Eu anom-
alies (Eu/Eu* = 0.76–0.84). They are depleted in Nb, Ta, Ti and Sr but enriched in the most incompatible
elements Rb, Ba, Th, U, K, and Pb leading to high ratios of Ba/Ta (531–1358) and Ba/Nb (35–91, with
one outlier). They exhibit Hf/Yb ratios = 1.31–1.52, sub-chondritic ratios of Nb/Ta (13–18) and Zr/Hf
(32–38), characteristics of shallow melting of mantle derived magmas. The samples display Th/Nb ratios
of 0.48–0.95 and have eNd (1.67 Ga) values of +0.22 to +2.34 which are much lower than the correspond-
ing eNd (1.67 Ga) mantle value of +6.55; characteristics indicative of contamination by the older crust.
The geochemical features of the Buanji Group high-K calc-alkaline basaltic andesites are interpreted in
terms of derivation of these rocks by partial melting of the a spinel lherzolite mantle wedge that has been
metasomatized by the subduction related fluids in a late Paleoproterozoic continental convergent mar-
gin. The resultant magmas were subsequently contaminated by the felsic crust prior to their
emplacement.
Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction convergent margins over those derived from the SCLM and man-
tle plumes akin to the Continental Flood Basalts (CFB) is well doc-
Geochemical studies of continental Proterozoic volcanic rocks umented (e.g. Condie and Kröner, 2013 and references therein). In
provide important insights into the composition of mantle and particular, Condie and Kröner (2013) proposed that the contribu-
the interplay between the mantle and crustal magma sources at tion of accreted oceanic arcs to continental growth during the
that time (e.g. Halama et al., 2003). Three main mantle source post-Archaen times is limited to 10% whereas continental arcs
components have been advocated to explain the geochemical sig- contribute about 40–80%. Supporting this view is the identifica-
natures of the erupted Proterozoic rocks: (1) the Depleted MORB tion of a few Early Proterozoic island arcs (Korsch et al., 2011)
Mantle (DMM), mantle sources of mid-ocean ridge basalts; (2) the but more Proterozoic continental arcs (Cawood and Korsch,
Ocean Island Basalt (OIB)-like mantle, mantle source for ocean is- 2008) in Australia. This is also true for continental growth along
land basalts; and (3) Sub-continental lithospheric mantle (SCLM), the convergent margin which lasted for 300 Ma (1850–1522 Ma)
representing the mantle below the continental crust (e.g. Rashid predominated by continental arcs in the SW Fennoscandia
and Sharma, 2001; Halama et al., 2003; Korsch et al., 2011). (Åhäll and Connelly, 2008).
The predominance of Proterozoic volcanic rocks formed at The Buanji Group of south-western Tanzania (Fig. 1) is largely
a sedimentary dominated Group with igneous activity capping
the sedimentary cycle in form of effusive volcanism represented
E-mail address: shukrani73@yahoo.com by the amygladoidal lavas and numerous gabbro and dolerite

1464-343X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jafrearsci.2013.06.011
108 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118

Fig. 1. Geological map of Tanzania showing the major tectono-lithostratigraphic units (modified after Pinna et al. (2008)). The Ikorongo Group (IG) and Buanji Group (BG, the
focus of this study shown in Fig. 2) which were in past considered to correlate with the Neoproterozoic Bukoban Supergroup are indicated.

dyke intrusions (Harpum and Brown, 1958; Harpum, 1970). The 2. Geological setting
sedimentary rocks associated with the lavas are conglomerates,
quartzites, brown and green shales, dolomites and dolomitic The geology of the area for which the Buanji Group rocks are a
limestones, micaceous siltstones and sandstones, which uncon- part (Fig. 2), was mapped by Harpum and Brown (1958) and de-
formably overlie the 2.0 Ga Ubendian high grade metamorphic scribed on Quarter Degree Sheet 246, which encompasses the Chi-
rocks, anorthosites, gabbros and granitic intrusive (Lenoir et al., mala area and below is the summarized geology of the area. Rocks
1994). The Buanji Group rocks have been lithostratigraphically of the Buanji Group comprise an assemblage of continental sedi-
correlated with the rocks of the Neoproterozoic Bukoban mentary rocks and lavas, which lie unconformably on the Palaeo-
Supergroup of western Tanzania (Harpum, 1970) and are thus, proterozoic (2.0 Ga) Ubendian high grade metamorphic rocks,
traditionally regarded as having formed during the anorthosites, gabbros and granitic intrusives (Lenoir et al., 1994;
Neoproterozoic. Boven et al., 1999). The Ubendian metamorphic rocks include
Previous work on the Buanji Group rocks is limited to geolog- micaceous schists, gneisses and migmatites, garnetiferous quartz-
ical mapping and descriptions (Harpum and Brown, 1958) and ites and marbles. The metamorphic rocks are considered to be of
lithostratigraphical correlations (Harpum, 1970) and modern sedimentary origin that underwent profound regional metamor-
geochemical and geochronological works on the lavas associated phism and migmatization (Lenoir et al., 1994; Boven et al., 1999).
with the sedimentary rocks are lacking. This paper presents The Buanji area was subjected to compressional deformation
SHRIMP U–Pb zircon dating, major and trace element geochem- resulting in several regional and local thrusts (Harpum and Brown,
ical data as well as Sm–Nd isotopic data on the amygladoidal la- 1958).
vas from the Buanji Group of southwest Tanzania. The data are According to Harpum and Brown (1958), the largely sedimen-
used to put constraints on the magmatic emplacement of the tary rocks of the Buanji Group are divided into lower, middle and
volcanic rocks and consequently the deposition age of the under- upper divisions with a maximum thickness of approximately
lying sediments, mantle sources and petrogenesis. The new 27 m, 40 m and 50 m, respectively. The lower division is made of
SHRIMP U–Pb zircon data shows that the Buanji Group lavas conglomerate composed of pebbles of jasper and agate, reddish
were emplaced during the late Palaeoproterozic and correlation shales (Fig. 3c) with occasional quartzitic sandstones. The middle
with Neoproterozoic Bukoban Supergroup is henceforth flawed. division consists of quartzite (locally known as the Gofio quartzite)
Major and trace element data together with Sm–Nd isotopic data and cupriferous shales, above which lies a thin member made up of
shows that the Buanji Group volcanic rocks were formed by par- green shales associated with numerous horizons of dolomitic lime-
tial melting of spinel lhezorlite at a continental convergent mar- stone. In some places, the shales are micaceous and are usually int-
gin and that crustal contamination played a significant role in erbedded with siltstones and quartzitic sandstones. Although the
their genesis. nature of Cu mineralization is uncertain, field observations indicate
 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118 109

Fig. 2. Geological map of the Chimala area showing the rocks of the Buanji Group and sample locations (modified from Harpum and Brown (1958)).

that they are syngenetic with the shales. The base of the upper limestone and amygdaloidal lavas), Bukoba sandstone, Kigonero
division is marked by a dolomitic limestone, above which no red- Flags (shales, sandstones and dolomitic limestones), Busondo and
dish shales are present. Thus, the upper Buanji succession is com- Masontwa (sandstones and shales) (Harpum,1970) with the
posed largely of greenish, grayish or buff-colored shales (Fig. 3b), Bukoban sandstone Group being a type locality for the Bukoban
mudstones, siltstones with intercalated sandstones and horizons Supergroup. The correlation of the Buanji and Ikorongo Groups
of conglomerate and quartzite, above which lies a highly contorted with the Neoproterozoic Bukoban Supergroup is based on litholog-
dolomite with chert banding. The upper Buanji division ends with ical associations. Compounding this lithostratigraphical correlation
effusive volcanism represented by highly vesicular lavas on Chauf- is the evidence from a geochronological study by Deblond et al.
ukwe Mountain (Figs. 2 and 3a). The Buanji Group succession is in- (2001) which showed that the Bukoban sandstones are cross-cut
truded by numerous gabbros and dolerite dykes. by the 1.3 Ga Mesoproterozoic mafic dykes. Deblond et al.
The Buanji Group of SW Tanzania and the Ikorongo Group of NE (2001) showed that the Bukoban sandstone Group belongs to the
Tanzania are two isolated basins that have been lithostratigraphi- Mesoproterozoic rather than the Neoproterozoic and that correla-
cally correlated with Neoproterozoic Bukoban Supergroup of NW tion based on lithological association alone is not sufficient in
Tanzania (Harpum, 1970; Shackleton, 1986; Kasanzu et al., 2008). establishing stratigraphical units.
The Ikorongo Group consists of shales, siltstones, sandstones, con-
glomerates and quartzites that unconformably overlie the
Archaean rocks of the Tanzania Craton (Pickering and Harpum, 3. Petrography and analytical methods
1959; Kasanzu et al., 2008; Kasanzu and Manya, 2010). Groups that
make up the Bukoban Supergroup of NW Tanzania (with major Fifteen volcanic rock samples, collected from the top of Chauf-
lithologies in brackets) include the Uha (red beds, dolomitic ukwe Mountain (Figs. 2 and 3a), were thin-sectioned and studied
110 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118

Fig. 3. Outcrop photographs showing some of the Buanji Group rocks (a) vesicular basaltic andesite, (b) grayish shales of the Upper Buanji, (c) Lower Buanji reddish (brown)
shales with thin horizons of green shales, and (d) thin section photomicrograph of the Buanji Group basaltic andesites (10 magnification).

under a polarizing microscope. Petrographical analysis of the Jarrell-Ash ENVIRO II ICP-OES. Detection limits were 0.01 wt% for
amygdaloidal samples indicates the presence of plagioclase, clino- all major elements and 2 ppm for Sc. Calibration was performed
pyroxene and orthopyroxene phenocrysts, as well as quartz and using seven USGS and Canmet certified reference materials. Loss
opaque minerals (Fig. 3d). Plagioclase is by far the most abundant on Ignition (LOI) was determined from the weight loss after roast-
mineral phase (60% modal abundance) and show interlocking rela- ing the samples at 1050 °C for 2 h. The other aliquot of the sample
tionship with orthopyroxene (20%) and clinopyroxene (15%). Opa- solution was spiked with internal In and Rh standards to cover a
que minerals (most likely magnetite) with a modal abundance of wide mass range, and diluted by a factor of 6000 times prior to
3% and quartz (2%) are the least abundant minerals. Quartz is sub- introduction into a Perkin Elmer SCIEX ELAN 6000 ICP-MS for trace
hedral and occurs in the interstitial spaces between plagioclase and element analysis.
pyroxene phenocrysts (Fig. 3d). The rocks mineral assemblages do The analytical reproducibility as deduced from samples repli-
not show any metamorphic overprint (Fig. 3d). cate analyses is better than 0.5% for major elements but is higher
For geochemical analyses, the samples were crushed in a jig- (0.8–1.5%) for elements with low concentrations like P2O5, K2O,
saw crusher for size reduction and subsequently pulverized in an Na2O, MnO and TiO2. The precision of the ICP-OES measurements
agate mill to fine powder. 5 g Aliquots of each powdered sample as deduced from the replicate analyses of the NIST 1633b standard
were packed and sent to the Activation Laboratories Ltd. of Ontario, is better than 3% for most major elements. For trace elements, the
Canada, for major and trace element determination. For ICP analy- analytical reproducibility of replicate analyses of the samples is
ses, 0.25 g aliquots of each sample were mixed with a flux of lith- better than 8% for most trace elements whereas the precision
ium metaborate and lithium tetraborate and fused in an induction and accuracy of the ICP-MS measurements deduced from replicate
furnace. The melt was immediately poured into a solution of 5% analyses of BIR-1 and W2 standards are 5–10%.
HNO3 containing an internal In standard, and was thoroughly All 15 samples were analyzed for Nd isotopic compositions, as
mixed for 30 min to achieve complete dissolution. An aliquot of well as Sm and Nd concentrations using a Triton-MC Thermal Ion-
the sample solution was analyzed for major oxides and the trace ization Mass Spectrometer at the Activation Laboratories of Ontar-
element Sc, on a combination simultaneous/sequential Thermo io, Canada. Aliquots of the powdered rock samples were spiked

Table 1
SHRIMP U–Th–Pb analytical data for zircons from the Buanji Group volcanic rock.

Analysis ID # Th ppm U ppm Th

U
207 Pb
235 U
±% 206 Pb
238 U
±% Rho 206 Pb
238 U
207 Pb
206 Pb
% Discordant

1.1 central 132 113 1.17 4.2 2.1 0.293 1.3 0.6 1658 ± 20 1697 ± 30 +3
2.1 central, zoned 207 115 1.81 4.4 2.1 0.299 1.4 0.7 1688 ± 21 1736 ± 28 +3
3.1 core 80 45 1.80 4.4 5.7 0.300 1.9 0.3 1693 ± 29 1716 ± 100 +2
4.1 central 79 77 1.03 4.1 3.7 0.299 1.7 0.5 1688 ± 26 1627 ± 62 4
5.1 central 51 100 0.51 4.6 7.2 0.299 1.6 0.2 1685 ± 24 1812 ± 127 +8
6.1 central 175 133 1.31 4.0 3.0 0.290 1.4 0.5 1640 ± 21 1636 ± 50 0
7.1 core, zoned 232 118 1.97 4.1 10.5 0.295 1.3 0.1 1667 ± 20 1644 ± 193 2
8.1 outer 778 344 2.26 2.3 12.7 0.182 2.0 0.2 1076 ± 20 1461 ± 238 +29
9.1 central 68 75 0.91 4.2 5.9 0.293 1.7 0.3 1655 ± 25 1701 ± 105 +3
10.1 central, zoned 190 233 0.82 2.9 7.7 0.205 1.2 0.2 1199 ± 13 1679 ± 141 +31
 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118 111

Fig. 4. Cathodo-Luminiscence (CL) images of zircons from the Buanji Group volcanic sample CH 13 showing the position for analyses as circles and numbers.

Fig. 6. eNd vs t (Ma) for the Buanji Group volcanic samples. The depleted Mantle
curve is from Goldstein et al. (1984).

Fig. 5. U–Pb Concordia diagram for the Buanji Group volcanic sample CH13. The 143Nd/144Nd ratios are calculated relative to the value of
0.511860 for the La Jolla standard. During the period of analysis
the weighted average of 10 La Jolla Nd-standard runs yielded
with a 149Sm–146Nd mixed solution prior to decomposition using a 0.511874 ± 10 (2r) for 143Nd/144Nd, using a 146Nd/144Nd value of
mixture of HF, HNO3 and HClO4. The REE were separated using 0.7219 for normalization.
conventional cation-exchange techniques. Sm and Nd were sepa- Approximately 2 kg of sample CH 13 was crushed and pow-
rated by extraction chromatography on HDEHP covered teflon dered to 250 lm. A heavy mineral concentrate was obtained using
powder. Total blanks are 0.1–0.2 ng for Sm, 0.1–0.5 ng for Nd, heavy liquids and magnetic separation at the Activation Laborato-
and are negligible. The accuracy of Sm and Nd analyses is ±0.5% ries of Ontario, Canada. Hand-picked representative zircons from
corresponding to errors in the 147Sm/144Nd ratios of ±0.5% (2r). the sample were mounted in epoxy resin together with chips of

Table 2
Sm–Nd isotopic data for the Buanji Group volcanic rocks.
147
Sm (ppm) Nd (ppm) Sm/144Nd 143
Nd/144Nd eNd (1.67 Ga) TDM
CH 01 5.08 24.1 0.1274 0.511901 ± 2 0.51 2.21E+09
CH 02 4.98 23.53 0.1279 0.511908 ± 2 0.54 2.21E+09
CH 03 5.36 25.02 0.1294 0.511922 ± 2 0.48 2.23E+09
CH 04 5.04 23.68 0.1286 0.511902 ± 2 0.27 2.24E+09
CH 05 4.94 23.48 0.1271 0.511911 ± 2 0.76 2.19E+09
CH 06 4.95 23.43 0.1277 0.511904 ± 2 0.51 2.22E+09
CH 07 4.88 22.95 0.1285 0.511897 ± 4 0.20 2.25E+09
CH 08 4.98 23.37 0.1288 0.511900 ± 3 0.19 2.25E+09
CH 09 4.83 22.9 0.1274 0.511912 ± 3 0.71 2.20E+09
CH 10 4.95 23.48 0.1274 0.511910 ± 3 0.69 2.20E+09
CH 11 5.27 25.72 0.1238 0.511955 ± 2 2.34 2.04E+09
CH 12 4.09 18.5 0.1336 0.512000 ± 2 1.11 2.20E+09
CH 13 5.00 23.26 0.1299 0.511906 ± 3 0.07 2.27E+09
CH 14 5.01 23.66 0.1279 0.511882 ± 2 0.02 2.26E+09
CH 15 4.75 22.66 0.1267 0.511908 ± 2 0.80 2.18E+09
147
Calculations are based on a decay constant of 6.54  1012 per year for Sm and DM values for Nd are (143Nd/144Nd) today = 0.51316, (147Sm/144Nd) today = 0.2137
(Goldstein et al., 1984).
112 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118

the TEMORA (Middledale Gabbroic Diorite, New South Wales, Aus- ported as two sigma levels. The concordia plots and concordia
tralia) and 91,500 (Geostandard zircon, Wiedenbeck et al., 1995) age calculations have been prepared using ISOPLOT/EX (Ludwig,
reference zircons and polished for imaging. Zircon Back-Scattered 1999).
Electron (BSE) and Cathodo-Luminiscence (CL) images were taken
prior to analysis in order to reveal the internal structure of the zir- 4. Results
con grains. The U–Pb analyses of the zircons were made using a
SHRIMP-II instrument at Activation Laboratories in Canada. The 4.1. SHRIMP U–Pb zircon dating and Sm–Nd systematics
data were reduced in a manner similar to that described by Wil-
liams (1998) and references therein. The SQUID Excel Macro of SHRIMP U–Th–Pb analytical data for zircons from the Buanji
Ludwig (2000) was used for data reduction. Uncertainties given Group volcanic sample CH 13 are reported in Table 1. The sample
for individual analyses (ratios and ages) are at the one sigma level; contain zircons that are either acicular (zircon grains 1 and 4,
however, the uncertainties in calculated concordia ages are re- Fig. 4) or prismatic with euhedral terminals (zircon grains 2, 7

Table 3
Major (wt%) and trace (ppm) element compositions for the Buanji Group volcanic rocks.

CH 01 CH 02 CH 03 CH 04 CH 05 CH 06 CH 07 CH 08 CH 09 CH 10 CH 11 CH 12 CH 13 CH 14 CH 15
SiO2 55.0 54.4 50.3 54.9 54.6 54.5 53.9 53.5 53.9 54.1 54.4 54.2 54.2 54.3 53.9
TiO2 1.18 1.13 1.07 1.14 1.13 1.15 1.10 1.12 1.10 1.09 1.10 0.85 1.14 1.12 1.12
Al2O3 13.7 13.9 14.6 14.0 14.0 13.8 13.6 13.4 13.5 13.5 13.7 13.8 13.4 13.7 13.8
Fe2O3 11.5 11.1 10.4 11.1 11.1 11.2 10.9 11.2 10.8 10.7 11.1 10.4 11.2 10.5 10.8
MnO 0.17 0.16 0.11 0.13 0.17 0.16 0.14 0.14 0.12 0.16 0.14 0.13 0.16 0.12 0.17
MgO 4.79 4.85 4.93 5.24 5.13 5 5.24 5.57 5.68 5.04 5.05 5.78 4.51 4.86 5.5
CaO 6.72 6.63 11.63 6.58 7.49 7.21 6.74 5.76 6.33 6.71 6.71 5.88 7.51 7.76 6.96
Na2O 2.65 2.85 1.03 2.88 3.06 2.84 2.52 2.61 2.49 2.81 2.83 2.58 2.67 2.72 2.68
K2O 2.91 3 0.28 2.62 1.78 2.35 2.41 2.46 2.31 2.62 2.67 2.27 2.87 1.91 2.77
P2O5 0.17 0.16 0.16 0.16 0.17 0.16 0.17 0.16 0.15 0.16 0.17 0.12 0.16 0.17 0.17
LOI 1.9 1.85 4.52 1.99 2.2 1.73 2.24 2.62 3.81 2.2 2.17 2.71 2.01 3.23 2.01
Total 100.7 100 99.06 100.8 100.9 100.2 98.85 98.58 100.2 99.16 100 98.72 99.72 100.4 99.88
Mg # 45 47 48 48 48 47 49 50 51 48 47 52 44 48 50
Ba 601 640 54 541 381 457 529 609 533 540 531 543 544 396 511
Rb 96 104 8 99 77 83 92 87 64 103 101 75 89 63 108
Sr 167 166 419 136 166 152 144 151 173 153 151 144 154 178 166
Ni 20 30 30 30 30 30 30 20 30 30 30 20 30 30 30
Co 41 39 35 38 41 40 39 40 40 40 39 23 40 38 42
Zn 90 80 70 70 90 90 70 70 90 70 80 30 80 80 90
Cr 80 100 110 120 100 100 120 90 110 110 110 80 120 130 100
Sc 33 33 33 34 34 34 33 33 33 33 33 34 32 32 34
V 258 254 257 254 258 255 251 253 249 253 252 244 254 252 256
Th 7.0 6.6 6.7 6.5 6.7 6.5 6.6 6.9 6.5 6.6 6.7 5.7 7.1 6.7 6.7
Pb 14 10 14 10 12 9 28 5 10 15 20 5 8 9 11
U 1.6 1.6 1.5 1.5 1.6 1.6 1.6 1.6 1.5 1.5 2 1.3 1.7 1.6 1.6
Nb 9 8 9 8 11 8 8 9 8 8 14 6 8 8 8
Ta 0.6 0.6 0.5 0.5 0.6 0.5 0.6 0.6 0.6 0.5 1.0 0.4 0.6 0.6 0.6
Zr 149 141 136 148 142 137 137 145 138 139 140 130 139 137 137
Hf 4.3 4 4 4 4.2 4 4.1 4.4 4.1 4.2 4.1 3.4 4.3 4.1 4.1
Y 27 28 28 28 27 27 26 27 26 26 27 19 27 28 26
La 21.7 20.3 23.1 19.7 20.9 20.1 20.1 20.3 19.8 20.6 21.3 14.6 19.6 20.1 19
Ce 45.4 42.9 47 40.9 43.2 42.1 42.2 42.5 41.9 43.6 45 31.4 41.8 41.6 40.6
Pr 6.77 6.34 7 6.14 6.52 6.22 6.28 6.37 6.25 6.52 6.76 4.69 6.35 6.18 6.05
Nd 23.8 22.6 25.7 22 22.9 22 22.7 22.8 22.3 23.5 24.2 17 23.1 22.1 22.1
Sm 5.20 5.00 5.50 4.80 5.00 4.80 4.90 5.00 4.70 5.00 5.10 3.70 5.10 4.90 4.80
Eu 1.43 1.39 2.07 1.33 1.39 1.35 1.35 1.36 1.25 1.37 1.39 1.02 1.32 1.31 1.35
Gd 5.5 5.1 6.1 5 5.4 5.1 5.1 5.2 5 5.3 5.1 4.1 5.5 5.2 5.2
Tb 0.9 0.9 1 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.7 0.9 0.9 0.9
Dy 5.6 5.4 6 5.2 5.6 5.3 5.4 5.4 5.3 5.5 5.4 4.7 5.8 5.4 5.4
Ho 1.1 1.1 1.1 1 1.1 1 1.1 1.1 1.1 1.1 1.1 0.9 1.2 1.1 1.1
Er 3.2 3.1 3.2 2.9 3.1 3 3.1 3.1 3.1 3.1 3 2.8 3.3 3.1 3.1
Tm 0.47 0.45 0.47 0.44 0.47 0.44 0.45 0.46 0.44 0.45 0.44 0.41 0.48 0.45 0.44
Yb 2.9 2.8 2.8 2.7 2.9 2.7 2.8 2.9 2.8 2.9 2.8 2.6 3 2.9 2.8
Lu 0.42 0.40 0.41 0.38 0.41 0.40 0.41 0.41 0.40 0.41 0.39 0.38 0.43 0.41 0.40
La/YbCN 5.37 5.20 5.92 5.23 5.17 5.34 5.15 5.02 5.07 5.10 5.46 4.03 4.69 4.97 4.87
La/SmCN 2.69 2.62 2.71 2.65 2.70 2.70 2.65 2.62 2.72 2.66 2.70 2.55 2.48 2.65 2.56
Eu/Eu* 0.82 0.84 1.09 0.83 0.82 0.84 0.83 0.82 0.79 0.81 0.83 0.80 0.76 0.79 0.83
Nb/La 0.41 0.39 0.39 0.41 0.53 0.40 0.40 0.44 0.40 0.39 0.66 0.41 0.41 0.40 0.42
Nb/Ta 15 13 18 16 18 16 13 15 13 16 14 15 13 13 13
Zr/Hf 35 35 34 37 34 34 33 33 34 33 34 38 32 33 33
Zr/Yb 51 50 49 55 49 51 49 50 49 48 50 50 46 47 49
Th/Yb 2.41 2.36 2.39 2.41 2.31 2.41 2.36 2.38 2.32 2.28 2.39 2.19 2.37 2.31 2.39
Hf/Yb 1.48 1.43 1.43 1.48 1.45 1.48 1.46 1.52 1.46 1.45 1.46 1.31 1.43 1.41 1.46
Ba/Ta 1002 1067 108 1082 635 914 882 1015 888 1080 531 1358 907 660 852

Fe2O3 represents Fe total. Mg# is calculated as 100 Mg/(Mg + Fe2+) with Fe2+ calculated as 0.89*Fe total.
 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118 113

and 8, Fig. 4) to ovoid in shape (zircon grain 10) with lengths vary- time. Also shown in the Table are depleted mantle crustal forma-
ing from 60 to 120 lm. On the basis of the CL images (Fig. 4), some tion ages (TDM) calculated assuming a linear evolution model for
of the zircons from the volcanic rock sample CH 13 show oscilla- the mantle together with a present-day 143Nd/144Nd value of
tory zoning (zircon grains 2 and 7, Fig. 4), a feature indicative of 0.51316 and 147Sm/144Nd value of 0.2127 (Goldstein et al., 1984).
igneous crystallization (e.g. Whitehouse et al., 1999). Some of the The calculated crustal formation ages vary from 2035 to 2270 Ma
zircons, however, show a distinct resorbed core and an overgrowth (Table 3) and are 361–596 Ma older than the emplacement ages.
(zircon grain 8). Ten analyses were performed on ten grains of
sample CH 13 (Table 1). When plotted on a concordia diagram 4.2. Geochemistry
(Fig. 5), eight data points plot on the concordia curve with only
0% to 4% discordancy (Table 1) whereas two data points (Zr 8.1 4.2.1. Alteration and element mobility
and Zr 10.1) plot on the discordia line with +29% to +31% discor- The vesicular nature of the rocks could subject them to the post-
dancy (Table 1). Regression of the data yielded an 8-point concor- crystallization alteration effects. This prompted the assessment of
dant age of 1674 ± 15 Ma (MSWD = 2.4), all regressions employing element mobility that could be caused by post-eruption alteration.
Isoplot/Exe of Ludwig, 1999). The concordant age obtained involve Most workers (e.g. Cann, 1970; Polat and Hofmann, 2003) regard
a regression that includes zircons that crystallized from igneous the low field strength elements (HFSE) and rare earth elements
melt owing to their oscillatory zoning and the 1674 ± 15 Ma age (REE) as being immobile during secondary processes whereas the
is hereby interpreted as the magmatic emplacement of the volca- low field strength elements (LFSE) are considered mobile. The
nic rocks in the Buanji Group of southwestern Tanzania. The Buanji Group rocks were assessed for post-eruption alteration by
1674 ± 15 Ma age of the Buanji Group lavas has henceforth not plotting Zr (a known sensitive indicator of immobility) against
been reported in western Tanzania and cannot in anyway repre- REE (represented by La), HFSE (represented by Nb) and LFSE (rep-
sent the age of an inherited pre-existing crust. The discordancy resented by Ba and Sr) (Fig. 7). The results show very poor correla-
in analysis 8.1 could be attributed to probing on an older core tion for Sr (R2 = 0.04) suggestive of mobility of the LFSE but
and a younger rim, which is reflected in the younger age of ca. moderate positive correlation for Nb (R2 = 0.10), La (R2 = 0.24)
1100 Ma obtained on grain 8 (Table 1). The younger age of ca. and Ba (R2 = 0.11, R2 being the correlation coefficient) suggesting
1200 Ma was also obtained on analysis 10.1 (Table 1) although that the HFSE, REE and some of the LFSE (like Ba) were not signif-
the analysis was performed on the zoned part of the grain. This icantly affected by post-eruption processes and represent original
is suggestive of a younger, post-emplacement event that affected magmatic compositions. The somewhat lower correlation coeffi-
the Buanji rocks in southwestern Tanzania. cient values are a result of the rocks being compositionally uniform
Sm–Nd isotopic data for the 15 volcanic samples from the Buan- (i.e. they show a tight compositional range). This is well reflected
ji Group is presented in Table 2 and Fig. 6. The samples show a in the coherent REE patterns (Fig. 11), which is itself indicative of
tight range of 147Sm/143Nd and 143Nd/144Nd ratios of 0.1238– the REE immobility.
0.1336 and 0.511882–0.512000, respectively. Employing the crys-
tallization age of 1674 ± 15 Ma for the rocks, the calculated eNd(t) 4.2.2. Major and trace element geochemistry
values range from +0.02 to +2.34. These values are lower than the Chemical compositions of the Buanji Group lavas are presented
corresponding mantle value of +6.55 (assuming a linear evolution in Table 3. The lavas are compositionally uniform with a restricted
model of the mantle by Goldstein et al. (1984, Fig. 6) at the same range in SiO2 (53.2–56.4 wt%, average = 55.5 wt%, all major

Fig. 7. Zr vs selected trace elements diagram for the Buanji Group lavas.
114 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118

element compositions expressed on water free basis) and total

alkalis (Na2O + K2O wt%) are in the range of 4.77–5.96 wt% (with
exception of CH 03 showing lower contents of Na2O (1.09 wt%)
and K2O (0.30 wt%), which on this basis behaves like a tholeiite).
When the samples are plotted on TAS diagram of Le Maitre et al.
(1989) the samples straddle the boundary between the field of
basaltic andesites and basaltic trachyandesite (Fig. 8a) with CH
03 plotting as a basaltic andesite. All the samples plot in the lower
side of the sub-alkaline to alkaline divide (Fig. 8a). When plotted
on the Nb/Y vs Zr/Ti diagram of Pearce (1996) which employs
the immobile elements, the samples straddle the boundary be-
tween the sub-alkaline basalt and andesite basalt (Fig. 8b). The
samples were also plotted on the Co–Th classification diagram of
Hastie et al. (2007). Except for sample CH 12 which plotted as a
high-K calc-alkaline basaltic andesite, all other samples including
CH 03 plotted as high-K calc-alkaline basalts (Fig. 9). Taken to-
gether, the Buanji Group lavas are classified as high-K calc-alkaline
basaltic andesites. Because of the restricted range in SiO2, no ob-
served correlations between SiO2 and other oxides as well as some
Fig. 9. Th–Co classification diagram (after Hastie et al. (2007)) for the Buanji Group
selected trace elements (Fig. 10). The MgO and Fe2O3 contents of lavas.
the lavas are 4.61–6.02 wt% (average 5.29 wt%) and 10.8–
11.7 wt% (average 11.2 wt%), respectively. Mg numbers, calculated
as 100Mg2þ =ðMg2þ þ Fe2þ total Þ vary from 44 to 52 (Table 3). The Cr On chondrite normalized rare earth elements diagram
and Ni contents of the lavas are 80–130 ppm and 20–30 ppm, (Fig. 11a), the samples display coherent and fractionated patterns
respectively. with La/SmCN and La/YbCN ratios of 2.48–2.72 (average = 2.64)
and 4.03–5.92 (average = 5.11), respectively; where CN refers to
chondrite normalized values). The samples are characterized by
negative Eu anomalies (Eu/Eu* = 0.76–0.84) except sample CH 03
that has no Eu anomaly (Eu/Eu* = 1.09, Table 1, Fig. 11a). On prim-
itive mantle normalized diagrams (Fig. 11b), the samples are de-
pleted in Nb, Ta, Ti and Sr and are characterized by Nb/Lapm and
Ta/Lapm ratios of 0.37–0.63 (average = 0.41) and 0.36–0.79 (aver-
age = 0.49), respectively; where pm refers to primitive mantle nor-
malized values). The samples are enriched in the most
incompatible elements including Rb, Ba, Th, U, K, and Pb relative
to adjacent elements (Fig. 11b) with characteristic high Th/Ta ra-
tios of 6.70–14.25 (average = 11.66). An exception to this is shown
by sample CH 03 which shows marked depletion in Rb and Ba,
most likely attributed to post-eruption mobility of these elements.

5. Discussion

Primitive mantle derived magmas in equilibrium with mantle

olivine have Mg-numbers of 68–72 and Ni concentrations in the
range of 300–500 ppm (Frey et al., 1978). The lower values of
Mg-numbers (44–52), Cr (80–130 ppm) and Ni (20–30 ppm),
MgO (4.61–6.02 wt%) and higher SiO2 (53.2–56.4 wt%) concentra-
tions of the Buanji Group lavas suggest that the magmas are not
primitive melts and have been modified en-route to the surface.
Incompatible trace elements are useful fingerprints of mantle
sources because they exhibit similar bulk partition coefficients
and their ratios are thus unchanged during partial melting or frac-
tional crystallization (Pearce, 2008). Using this approach, the Buan-
ji group lavas were plotted on the Nb/Yb–Zr/Yb (Fig. 12a) and Nb/
Yb–Th/Yb (Fig. 12b) diagrams of Pearce (2008). In both diagrams,
the Buanji Group lavas are displaced from the MORB–OIB array
and plot between E-MORB and the upper crust (UC). Such a trend
can be explained by (i) the contribution of crustal contamination
and/or (ii) the subduction zone contribution to the genesis of the
Buanji Group lavas.

5.1. Crustal contamination

Fig. 8. (a) TAS classification diagram (Le Maitre et al., 1989) and (b) Ti/Zr–Nb/Y Mafic magmas which undergo crustal contamination are ex-
classification diagram of Pearce (1996) for the Buanji Group lavas. pected to display depletion in Nb and Ti as well as the enrichment
 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118 115

Fig. 10. Selected major and trace element variation diagrams for the Buanji Group lavas.

in Th and the light rare earth elements (LREE) (Taylor and McLen- Group lavas plot on the right end of the ARC-array (high Th/Yb–
nan, 1995). Furthermore, the involvement of older sialic crust high Nb/Yb) of the diagram where continental arc rocks would
(with unradiogenic Nd isotope composition) in the genesis of plot together with the upper crust. This is in contrast to the
basaltic melts should produce lavas that have decreased eNd val- oceanic arc rocks that would plot on the low Nb/Yb end of the
ues. As shown in Figs. 11b and 12b, the Buanji Group lavas exhibit ARC-array (Pearce, 2008). John et al. (2003) employed the
Nb, Ta and Ti depletion in the primitive mantle spidergrams as well Nb/La and La/Sm ratios to distinguish between rocks formed from
as enrichment in Th and LREE. Their eNd values of +0.02 to +2.34 several tectonic settings. These ratios are also effective in dis-
are lower than the corresponding mantle value of +6.55 (assuming criminating between rocks formed in the oceanic arcs from those
a linear mantle evolution model of Goldstein et al. (1984)) and formed in continental arcs and are therefore useful in discerning
show mean crustal residence (TDM) ages which are 361–596 Ma the subduction component. When plotted on the La/Sm vs Nb/La
older than the emplacement ages. Moreover, Th/Nb and Th/Ce diagram (Fig. 13), the Buanji Group lavas plot exclusively in the
are considered to be sensitive indicators of contribution from the field of continental arcs with lower Nb/La ratio (0.39–0.66) than
crustal material (e.g. Pearce, 2008). The Buanji Group lavas show those for NMORB. Other element pairs that are useful in identify-
Th/Nb (0.48–0.95, average = 0.79) and Th/Ce (0.14–0.18, aver- ing the role of subduction component are those which involve
age = 0.16), values that are far higher than those of N-MORB (Th/ the Large Ion Lithophile Elements (LILE) and High Field Strength
Nb = 0.05 and Th/Ce = 0.02, Sun and McDonough, 1989) but are Elements (HFSE). This is due to their opposite behaviors in
indistinguishable from those of the upper crust (Th/Nb = 0.88, Th/ magma generation under hydrous conditions (Gill, 1981). The
Ce = 0.17, Rudnick and Gao, 2003). Thus, the plotting of Buanji use of LILE in inferring magmatic processes and affinity is limited
Group lavas above the mantle array and the elevated Th/Yb in due to the fact these elements can be mobilized during post-
Fig. 12b coupled with low eNd values which are translated into emplacement processes and should be used with a caution. As
older TDM ages than the emplacement ages would suggest the shown in Section 4.2.1, some of the LILE like Sr showed that they
involvement of the continental crust in the genesis of the Buanji are significantly affected by post-eruption processes (R2 = 0.04)
Group lavas. whereas others like Ba have correlation coefficients similar to
those shown by the HFSE and REE indicating that they might rep-
5.2. Subduction zone contribution resent original compositions. According to Gill (1981) and Fitton
et al. (1988), Ba/Ta and Ba/Nb ratios are useful in tracing the role
Another possible explanation for the plotting of the Buanji of subduction derived fluids. High Ba/Ta (>450) and Ba/Nb (>28)
Group lavas above the MORB–OIB array in Fig. 12 is the potential are the most striking features of subduction related magmas (Fit-
involvement of the subduction component in their genesis. The ton et al., 1988). The Buanji Group lavas have Ba/Ta and Ba/Nb
high-K calc-alkaline nature of the rocks as shown in Fig. 9, and ratios of 531–1358 and 35–91, respectively, with one outlier in
the characteristic depletion in Nb and Ta, enrichment in the LREE sample CH 03. These geochemical features and their low Ta/La
and Pb in primitive mantle spidergrams (Fig. 11b) all points to ratios (0.02–0.05, average 0.03) collectively argue in favor of
the convergent margin affinity of the rocks (e.g. Pecerrillo and the Buanji Group lavas to have formed at the Late Palaeoprotero-
Taylor, 1976; Condie and Kröner, 2013). In Fig. 12b, the Buanji zoic continental convergent margin.
116 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118

of garnet in the residue and by implication shallow melting (e.g.

John et al., 2003). Taken together, the low La/Yb and Hf/Yb ratios
for the Buanji Group lavas would indicate that their derivation
was by partial melting of spinel lhezorlite rather than garnet lherz-
olite. The negative Eu anomalies accompanied by Sr and Ti deple-
tion in the spidergrams would indicate fractionation of

Fig. 11. (a) Chondrite- and (b) primitive mantle-normalized diagrams for the Buanji
Group volcanic rocks. The Upper Crust of Rudnick and Gao (2003) is plotted for
comparison. Normalizing values are from Sun and McDonough (1989).

5.3. Source features and melting

The intrinsic geochemical features of the source regions of mafic

magmatism are generally deduced by incompatible elements ra-
tios (e.g. Nb/Ta and Zr/Hf) as they are considered to be unmodified Fig. 12. (a) Nb/Yb–Zr/Yb and (b) Nb/Yb–Th/Yb diagram (Pearce, 2008) for the
during mantle melting (Green, 2006). Mantle-derived magmas are Buanji Group volcanic rocks. In both diagrams, the Buanji Group volcanic rocks plot
characterized by chondritic Nb/Ta and Zr/Hf ratios of 17.6 and 36.3, away from the MORB–OIB array between E-MORB and Upper Crust (UC). N-MORB,
respectively (Sun and McDonough, 1989). The Buanji Group lavas E-MORB and OIB data are from Sun and McDonough (1989) whereas UC data are
from Rudnick and Gao (2003).
show sub-chondritic Nb/Ta ratios of 13–18 and Zr/Hf ratios of
32–38 and thus attesting to the fact they are mantle derived. This
inference is supported by MgO (4.61–6.02 wt%), Ni (20–30 ppm)
and Cr (80–30 ppm) concentrations and Mg# (44–52) which at
SiO2 = 55 wt% are indicative of mantle derived magmas. In the pre-
vious sections, both the crustal and subduction input have been
implicated in the Buanji Group lavas. Having established that they
formed at continental arcs, the Buanji Group lavas are henceforth
interpreted to have been derived from partial melting of mantle
wedge which was metasomatized by the subduction related fluids.
The Buanji group lavas are characterized by uniform and coher-
ent REE patterns that show enrichment of the LREE relative to the
MREE and HREE (La/SmCN = 2.48–2.72 and La/YbCN = 4.03–5.92).
They also exhibit negative Eu anomalies (Eu/Eu* = 0.76–0.84, one
outlier). The La/YbCN ratios of the Buanji Group lavas are lower
than those of the adakites (La/YbCN > 12, Martin, 1986). The higher
ratios in adakites indicate partial melting at greater depths where
garnet is a residual phase (Martin, 1986). The samples also have Hf/ Fig. 13. Chondrite normalized La/Sm vs Nb/La diagram (after John et al. (2003)) for
Yb ratios in the range of 1.31–1.52 which is suggestive of absence the Buanji Group lavas.
 S. Manya / Journal of African Earth Sciences 86 (2013) 107–118 117

plagioclase and magnetite (or ilmenite) during their are highly appreciated. The reviews by Gezahegn Yirgu and an
magmagenesis. anonymous reviewer were helpful in shaping up this manuscript.

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