Research Repository :: Browsing by Author "Maboko, Makenya A. H."

Browsing by Author "Maboko, Makenya A. H."

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2.67 Ga high-Mg andesites from the Musoma-Mara greenstone belt, northern Tanzania
(2008-07-01) Manya, Shukrani; Maboko, Makenya A. H.
Neoarchaean (2.67 Ga) High-Mg andesites occur in the Musoma-Mara greenstone belt, northern Tanzania. They are associated with dacites and Na-granitoids both of which are adakitic in composition. The high-Mg andesites are characterized by higher contents of MgO (2.42 – 9.47 wt %), Cr (41-797 ppm) and Ni (11-254 ppm) than those of normal island arc andesites. Their La/Yb ratios are 9.87 – 22.5 whereas their Sr/Y ratios are 20 – 131.These characteristics are similar to those shown by Cenozoic Setouchi High Magnesian Andesites (HMA). These rocks are characterized by 143Nd/144Nd ratios that range from 0.511062 ± 7 to 0.511308 ± 12 with corresponding εNd (t) of +0.44 and +1.81. The high contents of Mg, Cr and Ni argue in favour of equilibration of their parental magma with mantle peridotite whereas their relatively low La/Yb ratios argue against the involvement of garnet and amphibole as residual phases during partial melting. Thus, the high-Mg andesites are interpreted to have formed by partial melting of the mantle peridotite that has been fluxed by slab-derived fluids. Overall, the geochemical features of high-Mg andesites, their close association with adakitic rocks in the MMGB together with the short time interval taken for their emplacement are interpreted in terms of a ridge-subduction model. It is considered that such a model was important for generation of late Archaean continental crust.
Aerogeophysical, Geological and Geochemical Investigation of the Late Archaean Granitoids in the Musoma- Mara Greenstone Belt, NE Tanzania
(2008) Mshiu, Elisante E.; Maboko, Makenya A. H.; Marobhe, I.
The Musoma Mara Greenstone Belt (MMGB) is intruded with syn-to-post orogenic suites of granitoids which range in composition from Tonalite Trondhjemite Granodiorites (TTG) to granite. High resolution aerogeophysical data surveyed by GST in 2003 has provided aeromagnetic and radiometric data that were used in this study to classify various granitoids existing in MMGB. The individual radioactive element content of K, Th and U, ternary image and K: U: Th composite classification map have been used in data presentation and interpretation. In addition the radioactive element ratios were used to enhance the radiometric signals. Based on the analysis and interpretations made on airborne radiometric, magnetic data and previous geological maps, a geophysical interpretation map was obtained. This map broadly categorized the MMGB granitoids into two types, the first granitoid type is characterized by high contents of all the three elements (K, U and Th) and low magnetic intensity (< 33997 nT). The second granitoid type is characterized by high K relative to U and Th, and higher magnetic intensity (>33997 nT). The aerogeophysical interpretation map was used as a base map for ground follow-up whereby the granite types were sampled accordingly for geochemical analysis. Geochemical classification of the two granitoid types from geophysical data interpretations further subdivided them into three types i.e. biotite granite, calcic granite and TTG. The overall analysis showed high correlation between aerogeophysical and geochemical data whereby the voluminous biotite and calcic granite are the subdivisions from the first granite type and the less voluminous TTG precisely correlated to the second granite type. Their compositional similarity in geochemistry with the northern MMGB high-K and Na-rich granitoids (Manya et al. 2007a, b) suggested tectonic setting and petrogenetic analogy. Biotite and calcic granites are inferred to have been generated from partial melting of pre-existing materials including TTG, intermediate and felsic volcanic rocks whereby TTGs were generated from partial melting of hydrous basaltic crust that transformed into garnet amphibolites.
Age and geochemistry of coeval felsic volcanism and plutonism in the Palaeoproterozoic Ndembera Group of southwestern Tanzania: Constraints from SHRIMP U–Pb zircon and Sm–Nd data
(Elsevier, 2016-01-31) Bahame, Gagao; Manya, Shukrani; Maboko, Makenya A. H.
The Ndembera metavolcanic rocks represent a continuum of compositions ranging from intermediate to more siliceous calc-alkaline trachyandesites-dacites-trachytes-rhyolites which have been intruded by largely metaluminous, calc-alkaline I-type granites in the Palaeoproterozoic Usagaran Belt of southwestern Tanzania. SHRIMP U–Pb zircon age data show that the Ndembera metavolcanic rocks were extruded at 1871 ± 30 Ma, an event that was largely coeval with the emplacement of cross-cutting microcline-biotite rich granites at 1896 ± 29 Ma. Despite some compositional differences, both the granites and metavolcanic rocks share similar geochemical features including very coherent REE patterns characterized by enrichment of the LREE relative to the HREE (La/YbCN = 13.6–32.8 (average = 23.9) for metavolcanic rocks and La/YbCN = 10.8–38.2 (average = 21.8) for granites), negative Eu anomalies (Eu/Eu* = 0.45–1.01 (average = 0.83) for volcanic rocks and Eu/Eu* = 0.48–0.70 (average = 0.60) for granites), negative Ta, Ti and Nb anomalies (Nb/Lapm = 0.14–0.29 (average = 0.20 for volcanic rocks and Nb/Lapm = 0.01–0.37 (average = 0.16) for granites). The metavolcanic rocks have ɛNd (1871 Ma) values of −0.03 to −4.89 and TDM ages of 2243–2714 which are broadly similar to those of the granites (ɛNd (1896 Ma) values of −3.56 to −4.59 and TDM ages of 2469–2646 Ma). The geochemical similarities between the Ndembera volcanic rocks and granites, coupled with their similar emplacement age, suggest that the two rock suites were derived by the same processes in the same tectonic setting from an isotopically similar source. These geochemical features coupled with the paucity of mafic rocks suggest the derivation of the pluto-volcanic suite by crustal anatexis of basic meta-igneous rocks mixed with a minor metasedimentary component in an intra-continental setting. The more siliceous compositions in the suite may have formed by subsequent fractionation involving hornblende, plagioclase, biotite, titanite and apatite as indicated by the negative correlation between SiO2 and MgO, Fe2O3, Al2O3, CaO, TiO2 and P2O5, low Sr contents and negative Eu and Ti anomalies. The ∼1.87 Ga coeval felsic volcanic and granitic magmatic event documented in the Ndembera Group is also a feature of the nearby Ubendian belt to the west pointing to a Palaeoproterozoic regional thermal event that occurred in southwestern and western Tanzania.
Dampier Ridge, Tasman Sea, as a stranded continental fragment
(Taylor and Francis, 1994) Mcdougall, Ian; Maboko, Makenya A. H.; Symonds, Philip; McCulloch, Malcolm T.; Williams, I. S.; Kudrass, H. R.
Dredging of the Dampier Ridge recovered small fragments of granite, gabbro and sandstone. Dating of the igneous samples by the U-Pb, K-Ar and Rb-Sr methods yielded precise ages mainly in the range 250 to 270 Ma, mid-Permian. An imprecise Sm-Nd mineral age of approximately 310 Ma might reflect slightly earlier emplacement of the gabbro. A granitic fragment has a composition approximating that a manimum melt. Taken together with the Late Palaeozoic emplacement ages and other geochemical and geophysical data, the evidence strongly supports the conclusion that the Dampier Ridge is a continental fragment, formerly part of eastern Australia, with its present location a consequence of continential rifting and opening of the Tasman Basin by sea-floor spreading.
Dating basaltic volcanism in the Neoarchaean Sukumaland Greenstone Belt of the Tanzania Craton using the Sm-Nd method: implications for the geological evolution of the Tanzania Craton
(Elsevier, 2003) Manya, Shukrani; Maboko, Makenya A. H.
Metabasalts from the Sukumaland Greenstone Belt of north western Tanzania yield a whole rock Sm–Nd isochron age of 2823 ± 44 Ma (initial ε(Nd) = 2.7, MSWD = 1.24). This age, which is interpreted as dating the eruption of the oldest mafic volcanics in the belt, is at the 95% confidence level indistinguishable from 2780 ± 3 and 2808 ± 3Ma single zircon U–Pb ages previously reported from stratigraphically higher rhyolitic pyroclastic rocks from the southern margin of the belt. The age equivalency suggests that the entire∼5–7 km thick greenstone sequence that has been traditionally classified into a predominantly mafic lower part overlain by an upper part in which felsic volcanics and BIF predominate, was emplaced within a relatively short time interval not exceeding ∼44 Ma. The Sm–Nd age of the metabasalts is significantly older than a published zircon U–Pb age of 2680 ± 3Ma obtained from a migmatitic gneiss on the southernmost fringe of the Sukumaland Greenstone Belt. This corroborates previous evidence that high-grade metamorphism in the Tanzania Craton postdates emplacement of the greenstones and is most likely associated with the regional emplacement of the large granitic bodies that intrude and flank the greenstones.
Dating Late Pan-African Cooling in the Uluguru Granulite Complex Of Eastern Tanzania Using the 40Ar_39Ar Technique
(Elsevier, 1989) Maboko, Makenya A. H.; Mcdougall, Ian; Zeitler, P. K.
Age spectra measured by the 40Ar39Ar technique on hornblende, muscovite and K-feldspar from the Uluguru granulite complex of Eastern Tanzania indicate that following granulite facies metamorphism at ∼ 715 Ma the terrane cooled slowly, reaching a temperature of ∼475°C about 630 Ma ago. Subsequent cooling was even slower, reaching temperatures of ∼170°C about 420 Ma ago. Assuming a simple relationship between cooling rate and thermal gradient, the cooling history translates into an uplift path characterized by a phase of rapid uplift soon after granulite facies metamorphism followed by a period of slow uplift which began about 630 Ma ago. Such a history is consistent with model thermal histories of crustal segments undergoing thermal relaxation and isostatically-driven uplift following tectonic thickening events.
Discordant ArAr ages from the Musgrave Ranges, Central Australia: Implications for the Significance of Hornblende ArAr Spectra
(1991) Maboko, Makenya A. H.; Mcdougall, Ian; Zeitler, P. K.; Fitzgerald, J. D.
Hornblende and biotite from granulites in the Musgrave Ranges, central Australia, yield variable 40Ar39Ar bulk ages even at the scale of a single outcrop. There is no obvious relationship between age, chemical composition, sample location, or textural characteristics of the minerals. We interpret the age variation as due to the incorporation of variable quantities of excess Ar. However, the age spectra of the minerals do not show the typical gain profiles commonly associated with excess Ar and isotope correlation diagrams do not indicate trapped Ar of non-atmospheric composition. Instead, some of the hornblendes and all the biotites show relatively simple age spectra similar to those commonly interpreted in terms of volume diffusion. These observations lead us to argue that, as is commonly the case with biotites, not all hornblende age spectra with simple patterns, similar to those predicted by diffusion theory, can be interpreted unambiguously in terms of geologically significant Ar concentration profiles. More specifically, we conclude that a relatively simple hornblende age spectrum does not necessarily indicate the absence of excess Ar.Despite their complexity, our results show that post-metamorphic cooling of the granulites was very slow. Although the incorporated excess Ar hinders the reconstruction of a more precise thermal history, the data show that following granulite-facies metamorphism ≈ 1200 Ma ago, the terrane did not cool below temperatures at which hornblende and biotite close to Ar diffusion until after ≈ 930 and ≈ 690 Ma, respectively.
Discussion and Reply: Dampier Ridge, Tasman Sea, as a Stranded Continental Fragment
(Taylor and Francis, 1995) Anfiloff, V.; Mcdougall, Ian; Maboko, Makenya A. H.; Symonds, Philip; McCulloch, M. T.; Williams, I. S.; Kudrass, H. R.
The Emplacement Age of Gabbroic Rocks and Associated Granitoids of the Liganga-Msanyo Complex, South Eastern Tanzania.
(2004) Maboko, Makenya A. H.
The Msanyo Gabbro and the associated Mdando granitoids, which intrude Paleoproterozoic high-grade metamorphic rocks of the Ubendian Belt in southern Tanzania, yield imprecise Sm-Nd whole rock isochron ages of 1608±134 Ma (ε (Nd) = 2.1) and 1642±100 Ma (ε (Nd) = 1.6), respectively. These ages are indistinguishable at the 95% confidence level, indicating that the two rock suites were intruded at the same time. A more precise estimate of the time of emplacement is provided by a 5-point mineral isochron age of 1505±42 obtained from one of the gabbro samples. Calculated initial ε (Nd) values and mean crustal residence ages are similar for both rock suites and range from –0.7 to 1.5 and 1.7 to 1.9 Ga. Respectively. Initial Sr isotope ratios lie between 0.7032 and 0.7039 and these values are similar to those obtained in the mantle and the lower continental crust. The rather evolved Nd isotope systematics, combined with the relatively non-radiogenic Sr isotopic signature, suggest that both suites cystallised from mantle-derived magma that had assimilated a significant amount of lower continental crustal material. Subsequent to their emplacement, the rocks experienced localised shearing and sericitisation during the Ukingan tectonic event. A Rb-Sr plagioclase-whole rock age dates this event at 1137±50 Ma. Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 1-14
Generation of Palaeoproterozoic Tonalites and Associated High-K Granites in Southwestern Tanzania by Partial Melting Of Underplated Mafic Crust in an Intracontinental Setting: Constraints from Geochemical and Isotopic Data
(Elsevier, 2016-06) Manya, Shukrani; Maboko, Makenya A. H.
The southwestern part of the 2.0–1.8 Ga Palaeoproterozoic Usagaran Belt in the Njombe area of SW Tanzania is intruded by two types of synchronous granitic rocks with different chemical and petrological characteristics. The first type consists of hornblende-rich tonalites that have major element compositions similar to those of Archaean TTG but differ significantly in their trace element composition. The tonalites are spatially and closely associated with felsic, high-K, I-type granites, some of which are gneissic and/or aplitic. U–Pb zircon geochronology shows that the emplacement of tonalites at 1887 ± 11 Ma was largely contemporaneous with emplacement of high-K granitic gneisses at 1877 ± 15 Ma and aplitic granites at 1857 ± 19 Ma. The data also reveal the presence of Archaean crust of 2648 ± 25 Ma in the zircon cores of some samples in the otherwise Palaeoproterozoic terrane. The tonalites are characterized by MgO contents of 1.60–4.11 wt.% at a SiO2 range of 58.1–67.9 wt.%, the Mg# of 34–55, lower Sr contents (220–462 ppm) and less fractionated REE patterns (La/YbCN = 3.55–12.9) compared to Archaean TTG (Sr > 500 ppm, La/YbCN > 20). These features, coupled with the εNd (1887 Ma) values of + 0.37 to − 0.66 as well as the associated mafic enclaves are suggestive of derivation of the tonalites by low pressure (below the garnet stability) partial melting of a mantle-derived mafic underplate that was subsequently contaminated with small amounts of pre-existing igneous crustal rocks. The evolved nature of the high-K granites (MgO = 0.20–1.30 wt.%, SiO2 = 65.5–73.9 wt.%, Mg# = 25–42, εNd = − 3.20 to − 4.75) coupled with old TDM ages which are 200–1000 Ma older than their emplacement age requires a higher degree of assimilation of older crustal material by the magma derived from partial melting of the underplated mafic crust which was subsequently followed by crystal fractionation involving plagioclase, pyroxene and amphibole. The close spatial and temporal association of the tonalites, mafic enclaves and the high-K granites and gneisses in the Njombe area provides the first direct evidence of the role of magmatic underplating for the regional thermal anomaly that caused widespread crustal anatexis leading to the generation of the 1.8–1.9 Ga granitic rocks (in Njombe area) and/or associated felsic volcanism in the Usagaran (in Ndembera) and Ubendian (in Ngualla) Belts of SW Tanzania.
Geochemistry and Geochronology of Neoarchaean Volcanic Rocks of the Iramba–Sekenke Greenstone Belt, Central Tanzania
(Elsevier, 2008-06-10) Manya, Shukrani; Maboko, Makenya A. H.
The late Archaean volcanic rocks of the Iramba–Sekenke greenstone belt consist of tholeiitic basalts and basaltic andesites that are associated with a subordinate amount of intermediate to felsic volcanic rocks. Sm–Nd geochronology shows that the entire volcanic suite was emplaced at 2742 ± 27 Ma (MSWD = 1.6, ɛNd = 2.3). REE patterns in the tholeiitic basalts and basaltic andesites are flat or slightly depleted in the LREE and are characterized by La/SmCN and La/YbCN ratios of 0.70–1.17 and 0.74–1.35, respectively. On primitive mantle normalized extended trace element diagrams, two groups of basalts and basaltic andesites can be identified. Rocks of the first group display NMORB-like patterns and are characterized by minor negative Ti anomalies as well as slight depletion of the incompatible elements Th, Nb and Ta relative to the LREE. The second group only differs from the first group in having negative anomalies of Nb and Ta. The intermediate to felsic volcanic rocks are characterized by fractionated REE patterns (La/SmCN = 2.77–6.22 and La/YbCN = 15–103) with only minor to absent Eu anomalies. On primitive mantle- and NMORB-normalized extended trace element diagrams, the samples display large negative anomalies of Nb, Ta and Ti (Nb/Lapm = 0.07–0.21) as well as enrichment of Th relative to the LREE (Th/Lapm = 1.24–3.44). Trace element and isotope geochemistry suggests that crustal contamination did not play a role in the genesis of the Iramba–Sekenke volcanic rocks. Geochemical modelling is consistent with derivation of the tholeiitic basalts and basaltic andesites by 6% non-modal partial melting of a spinel peridotite depleted mantle source. The volumetrically minor intermediate to felsic volcanic rocks are interpreted to have been derived from the parental magmas of the tholeiitic basalts and basaltic andesites by 85–90% Rayleigh fractionation of clinopyroxene, hornblende and minor plagioclase. Tectonic setting discrimination diagrams and trace element ratios in the rocks are consistent with formation of the Iramba–Sekenke greenstone belt in a back-arc setting.
Geochemistry and Geochronology of the Bimodal Volcanic Rocks of the Suguti Area in the Southern Part of the Musoma-Mara Greenstone Belt, Northern Tanzania
(Elsevier, 2009-11-30) Mtoro, Mwache; Maboko, Makenya A. H.; Manya, Shukrani
The Suguti volcanic rocks of the southern Musoma-Mara greenstone belt in northern Tanzania comprise mainly of a bimodal suite of tholeiitic basalts-basaltic andesites and calc-alkaline rhyolites with a subordinate amount of intermediate rocks. Zircon U–Pb and whole rock Sm–Nd geochronology suggests that the two suites are cogenetic and were emplaced at 2755 ± 1 Ma with a common initial ɛNd value of 2.1. The tholeiitic basalts are characterised by relatively flat chondrite-normalised REE patterns with La/YbCN ratios of 0.8–1.6 (mean = 1.0). The basalts also exhibit negative Ti and Nb anomalies in primitive mantle-normalised multi-element diagrams. The flat REE patterns, the presence of prominent negative Nb anomalies and the positive initial ɛNd value of 2.1 suggest that the basalts were formed by low pressure melting of a mantle wedge in an active continental margin setting. Compared to the tholeiitic basalts, the calc-alkaline rhyolites are characterised by low abundances of the transition elements (Cr < 20 ppm, Ni < 20 ppm) and moderately high HFSE (e.g. Zr = 111–250 ppm) abundances. The rhyolites display strongly fractionated, slightly concave upward chondrite normalised REE patterns that are characterised by a slight depletion of the MREE relative to the HREE and minor to large negative Eu anomalies (Eu/Eu* = 0.3–0.9) and their epsilon Nd values range from +2.05 to +2.33. The depletion of the MREE relative to the HREE is an indication of fractionation of clinopyroxene and hornblende during petrogenesis whereas the negative Eu anomalies indicate plagioclase fractionation. The rhyolites are interpreted to have formed from the parental magma of the basalts by fractional crystallization and/or partial melting of a relatively young basaltic crust.
Geochemistry and Nd-isotopic Composition of Potassic Magmatism in the Neoarchaean Musoma-Mara Greenstone Belt, Northern Tanzania
(Elsevier, 2007-11-15) Manya, Shukrani; Maboko, Makenya A. H.; Nakamura, Eizo
The Neoarchaean Musoma-Mara Greenstone Belt (MMGB) of northern Tanzania is underlain in part by the ∼2649 Ma post-orogenic potassic-rich granites, which are the most abundant intrusive rocks in the belt. The rocks are composed of plagioclase + K-feldspar + quartz + biotite ± sphene ± zircon ± hornblende ± chlorite. They are characterized by high contents of SiO2 (68.90–77.76 wt%), K2O (3.71–5.44 wt%) and low Na2O (3.27–5.70 wt%) leading to low ratios of Na2O/K2O (0.63–1.02). The rocks are depleted in CaO (0.22–2.41 wt%) as well as in Sr (15–412 ppm), Cr (≤16 ppm) and Ni (≤5 ppm); and their major element composition are similar to those of experimental melts derived from partial melting of tonalite. On chondrite-normalized REE patterns, these rocks show fractionated patterns (La/YbCN = 1.22–41.32) that are characterised by moderate to strongly negative Eu anomalies (Eu/Eu* = 0.04–0.86). On primitive mantle-normalized spidergrams, these rocks are generally enriched in Th, U, K and Pb and depleted in Ba, Sr, Nb, Ta and Ti relative to adjacent elements. The K-rich granites have ɛNdo (at 2.649 Ga) values of +0.55 to +1.70 that compare well with those of associated volcanic rocks and TTG (ɛNd = +0.44 to +2.66) which predate the emplacement of the K-rich granitoids. Their mean crustal residence ages are 170 to 450 Ma older than their emplacement ages. The overall geochemical features of this suite of rocks, together with evidence from experimental results, are consistent with their generation by partial melting of relatively juvenile igneous rocks within the continental crust at pressures corresponding to depths <15 km where plagioclase was a stable phase. The transition from earlier TTG magmatism to potassic magmatism in the MMGB is interpreted as marking a transition from growth of the Neoarchaean continental crust through the addition of juvenile mantle-derived material to intra-crustal recycling of pre-existing material.
Geochemistry and Petrogenesis of the Late Archaean High-K Granites in the Southern Musoma-Mara Greenstone Belt: Their Influence in Evolution of Archaean Tanzania Craton
(Elsevier, 2012) Mshiu, Elisante E.; Maboko, Makenya A. H.
Musoma-Mara Greenstone Belt (MMGB) is abundantly occupied by the post-orogenic high-K granites which also they mark the end of magmatism in the area. The granites are characterized by high SiO2 and Al2O3 contents that average 74.42% and 13.08% by weight respectively. They have low Na2O content (mean = 3.36 wt.%) and high K2O contents (mean = 4.95 wt.%) which resulted to relatively high K2O/Na2O ratios (mean = 1.50). They also characterized by low Mg# (mean = 33) as well as low contents of transition elements such as Cr and Ni which are below detection limit (<20 ppm). Negative anomalies in Eu (Eu/Eu*, mean = 0.56), Nd, Ta and Ti elements as shown in the chondrite and primitive mantle normalized diagrams indicate MMGB high-K granites originated from a subduction related environments. These high-K granites also characterized by relative enrichment of the LREE compared to HREE as revealed by their high (La/Yb)CN ratio ranging from 8.71 to 50.93 (mean = 26.32). They have relatively flat HREE pattern with (Tb/Yb)CN ratio varying between 0.81 and 2.12 (mean = 1.55). Their linear trend in the variation diagrams of both major and trace elements indicate magmatic differentiation was also an important process during their formation. Conclusively, the geochemical characteristics as well as experimental evidences suggests MMGB high-K granites were formed from partial melting of pre-existing TTG rocks, under low pressure at 15 km depth or less and temperature around 950 °C in which plagioclase minerals were the stable phases in the melt.
Geochemistry and Sm-Nd systematics of the 1.67 Ga Buanji Group of southwestern Tanzania: Paleo-weathering, provenance and paleo-tectonic setting implications
(Elsevier, 2017-09) Kasanzu, Charles H.; Maboko, Makenya A. H.; Manya, Shukrani
We present major and trace and Nd-isotopic data of the ∼1.67 Ga Buanji Group of southwestern Tanzania in order to constrain the nature of their protolith and the intensity of chemical weathering in the source terranes and make inferences on the nature of climatic conditions during the late Paleoproterozoic in the region. Major and trace element contents of the shales from three stratigraphic formations are comparable to those of the post-Archaean Australian Shale (PAAS) and the average Proterozoic Shale (PS). They are characterized by enrichments in LREE relative to HREE ((La/Yb)CN = 9.07–13.2; (Gd/Yb)CN = 1.51–1.85) and negative Eu anomalies (Eu/Eu* = 0.61–0.81); features which are comparable to those of PAAS and PS, indicating upper continental sources. Provenance proxy ratios, together with abundances of Cr, Ni, Co and V that increase up-stratigraphy suggest a decreasing input of felsic detritus up-stratigraphy. Chemical Indices of Alteration (CIA) for the lower, middle and upper Buanji formations are 81, 76 and 79, respectively. These indices largely indicate intermediate (ca. 60–80) to extreme (>80) weathering intensities of the precursor rocks. These observations may suggest the prevalence of warm, humid climates during the late Paleoproterozoic in the region.
The Geochemistry of Banded Iron Formations in the Sukumaland Greenstone Belt of Geita, Northern Tanzania: Evidence for Mixing of Hydrothermal and Clastic Sources of the Chemical Elements
(2004) Maboko, Makenya A. H.
Major and trace element compositions of samples of Banded Iron Formations (BIF) from the Neoarchaean Sukumaland Greenstone Belt of Geita in northern Tanzania reveal that the BIF precipitated from hydrothermal solutions. Fe-Ti-Al-Mn systematics suggest that the hydrothermal deposits have been contaminated, by up to 20% by weight, with detrital material having a composition similar to modern deep-sea pelagic clays. SiO2 and Fe2O3 contents are 48.2 to 88.5% and 8.9 to 49.1% respectively. Al2O3 contents lie between 0.33 and 2.1% and show no correlation with either Fe2O3 or SiO2. Al2O3 is, however, positively correlated with Ti, Ga, Hf, Rb,Th, Zr and Sr but not with CaO, the alkalies and the total Rare Earth Elements (REE). The other major element oxides are generally present in negligible amounts. The samples are characterised by mean Zr/Hf and 144Sm/143Nd ratios of 48± 5 (2 SE) and 0.10±0.01 (2 SE) respectively, similar to mean upper continental crustal values. Shale-normalised REE patterns are nearly flat, except for small positive Eu and very slight negative Ce anomalies and reveal that, compared to average upper crust, the abundances of the REE in the BIF are up to an order of magnitude lower. Chondrite-normalised patterns are characterised by light (L) REE enrichment, flat to slightly depleted heavy (H) REE, slightly positive Eu anomalies and very small negative Ce anomalies. The HREE-depleted patterns are similar to patterns derived from granite-dominated upper continental crust and indicate that the bulk of the REE in the Geita BIF can not have been derived from a mixture of Neoarchaean sea water and bottom hydrothermal solutions. The trace element data, and the REE in particular, indicate that, despite their relatively low proportions, granitic detritus probably derived from contemporaneous felsic flows and pyroclastics are the cause of the dominant trace element geochemical signature of the BIF
Geochemistry of fine-grained clastic sedimentary rocks of the Neoproterozoic Ikorongo Group, NE Tanzania: Implications for provenance and source rock weathering
(Elsevier, 2008-07-30) Kasanzu, Charles; Maboko, Makenya A. H.; Manya, Shukrani
The Neoproterozoic Ikorongo Group, which lies unconformably on the late Archaean Nyanzian Supergroup of the Tanzania Craton, is comprised of conglomerates, quartzites, shales, siltstones, red sandstones with rare flagstones and gritstones and is regionally subdivided into four litho-stratigraphic units namely the Makobo, Kinenge, Sumuji and Masati Formations. We report geochemical data for the mudrocks (i.e., shales and siltstones) from the Ikorongo basin in an attempt to constrain their provenance and source rock weathering. These mudrocks are compositionally similar to PAAS and PS indicating derivation from mixed mafic–felsic sources. However, the siltstones show depletion in the transition elements (Cr, Ni, Cu, Sc and V) and attest to a more felsic protolith than those for PAAS and PS. The Chemical Index of Alteration (CIA: 52–82) reveal a moderately weathered protolith for the mudrocks. The consistent REE patterns with LREE-enriched and HREE-depleted patterns ((La/Yb)CN = 7.3–38.3) coupled with negative Eu anomalies (Eu/Eu* = 0.71 on average), which characteristics are similar to the average PAAS and PS, illustrate cratonic sources that formed by intra-crustal differentiation. Geochemical considerations and palaeocurrent indications suggest that the provenance of the Ikorongo Group include high-Mg basaltic-andesites, dacites, rhyolites and granitoids from the Neoarchaean Musoma-Mara Greenstone Belt to the north of the Ikorongo basin. Mass balance calculations suggest relative contributions of 47%, 42% and 11% from granitoids, high-magnesium basaltic-andesites and dacites, respectively to the detritus that formed the shales. Corresponding contributions to the siltstones detritus are 53%, 43% and 4%.
The Geochemistry of High-Mg Andesite and Associated Adakitic Rocks in the Musoma-Mara Greenstone Belt, Northern Tanzania: Possible Evidence for Neoarchaean Ridge Subduction?
(Elsevier, 2007-11-15) Manya, Shukrani; Maboko, Makenya A. H.; Nakamura, Eizo
Geochemical data are presented for Neoarchaean metavolcanic and plutonic rocks from the Musoma-Mara Greenstone Belt (MMGB) in the northern part of the Tanzania Craton with the aim of inferring their petrogenesis and tectonic settings in which they formed. The MMGB is underlain by two unusual magmatic suites: high magnesium andesites and an adakitic suite. The high-Mg andesites are composed of intermediate volcanic rocks of andesitic composition which, when compared to normal island arc andesites, are characterised by high contents of MgO (2.42–9.47 wt%, Cr (41–797 ppm) and Ni (11–254 ppm). Ratios of Nb/Y are 0.17–0.28, La/Yb are 9.87–22.5 and Sr/Y are 16.1–48.7. These geochemical features are analogous to those shown by modern High Magnesium Andesites (HMA). The adakitic suite is composed of dacites and Na-granitoids and is characterized by Al2O3 contents of 14.75–18.54 wt%, variable contents of MgO (0.31–4.96 wt%), Cr and Ni (5.10–250 and 1.22–115 ppm, respectively). Their ratios of Nb/Y are 0.17–0.66, La/Yb are 11–80 and Sr/Y are 20–131. Compared to the HMA, rocks of the adakitic suite are characterized by lower MgO, Cr and Ni contents, higher contents of Al2O3 and Sr, higher ratios of La/Yb and Sr/Y and are compositionally similar to modern adakites. Rocks from both suites show negative anomalies of Nb, Ta and Ti and have similar ɛNd values (at 2.67 Ga) of +0.44 to +2.66. The geochemical characteristics of the HMA are consistent with the derivation of their parent magma by partial melting of mantle peridotite that has been fluxed by slab-derived aqueous fluids above a continental arc. As the slab further descended into the mantle, partial melting of the subducted oceanic crust occurred in the garnet stability field producing a melt that was depleted in HREE. The slab-derived melts percolated into the mantle wedge and reacted with mantle peridotite resulting in parental magmas of rocks of the adakitic suite. Subsequently, the parental magmas of both rock suites ascended through and were contaminated by older felsic crust forming the continental arc basement. Subsequent fractional crystallization of pyroxene and hornblende led to the range in Mg numbers, CaO, Cr and Ni contents observed in the rocks. The association of some members of the adakitic suite with locally derived clastic sedimentary rocks suggests that the latest volcanic episode in the MMGB occurred in a continental back arc basin. The rapid emplacement of volcanic and plutonic rocks in a relatively short time interval is best explained in terms of the ridge-subduction model of [Iwamori, H., 2000. Thermal effects of ridge subduction and its implication for the origin of granitic batholith and paired metamorphic belts. Earth Planet. Sci. Lett., 181, 131–144.] whereby subduction of the ridge crest results in anomalous high thermal input into the subduction zone leading to rapid arc magmatism within a few tens of kilometers from the slab–crust interface and within a time interval of 30 Ma after ridge.
The geochemistry of high-Mg andesite and associated adakitic rocks in the Musoma-Mara Greenstone Belt, northern Tanzania: Possible evidence for Neoarchaean ridge subduction?
(Elsevier, 2007) Manya, Shukrani; Maboko, Makenya A. H.; Nakamura, Eizo
Geochemical data are presented for Neoarchaean metavolcanic and plutonic rocks from the Musoma-Mara Greenstone Belt (MMGB) in the northern part of the Tanzania Craton with the aim of inferring their petrogenesis and tectonic settings in which they formed. The MMGB is underlain by two unusual magmatic suites: high magnesium andesites and an adakitic suite. The high-Mg andesites are composed of intermediate volcanic rocks of andesitic composition which, when compared to normal island arc andesites, are characterised by high contents of MgO (2.42–9.47 wt%, Cr (41–797 ppm) and Ni (11–254 ppm). Ratios of Nb/Y are 0.17–0.28, La/Yb are 9.87–22.5 and Sr/Y are 16.1–48.7. These geochemical features are analogous to those shown by modern High Magnesium Andesites (HMA). The adakitic suite is composed of dacites and Na-granitoids and is characterized by Al2O3 contents of 14.75–18.54 wt%, variable contents of MgO (0.31–4.96 wt%), Cr and Ni (5.10–250 and 1.22–115 ppm, respectively). Their ratios of Nb/Y are 0.17–0.66, La/Yb are 11–80 and Sr/Y are 20–131. Compared to the HMA, rocks of the adakitic suite are characterized by lower MgO, Cr and Ni contents, higher contents of Al2O3 and Sr, higher ratios of La/Yb and Sr/Y and are compositionally similar to modern adakites. Rocks from both suites show negative anomalies of Nb, Ta and Ti and have similar ɛNd values (at 2.67 Ga) of +0.44 to +2.66. The geochemical characteristics of the HMA are consistent with the derivation of their parent magma by partial melting of mantle peridotite that has been fluxed by slab-derived aqueous fluids above a continental arc. As the slab further descended into the mantle, partial melting of the subducted oceanic crust occurred in the garnet stability field producing a melt that was depleted in HREE. The slab-derived melts percolated into the mantle wedge and reacted with mantle peridotite resulting in parental magmas of rocks of the adakitic suite. Subsequently, the parental magmas of both rock suites ascended through and were contaminated by older felsic crust forming the continental arc basement. Subsequent fractional crystallization of pyroxene and hornblende led to the range in Mg numbers, CaO, Cr and Ni contents observed in the rocks. The association of some members of the adakitic suite with locally derived clastic sedimentary rocks suggests that the latest volcanic episode in the MMGB occurred in a continental back arc basin. The rapid emplacement of volcanic and plutonic rocks in a relatively short time interval is best explained in terms of the ridge-subduction model of [Iwamori, H., 2000. Thermal effects of ridge subduction and its implication for the origin of granitic batholith and paired metamorphic belts. Earth Planet. Sci. Lett., 181, 131–144.] whereby subduction of the ridge crest results in anomalous high thermal input into the subduction zone leading to rapid arc magmatism within a few tens of kilometers from the slab–crust interface and within a time interval of 30 Ma after ridge subduction.
The Geochemistry of Mafic and Ultramafic Rocks in the Wami River Granulite Complex, Central Coastal Tanzania
(1987) Maboko, Makenya A. H.; Basu, N. K.
The geochemistry of mafic and ultramafic rocks in the Wami River granulite complex of the central part of the coastal region, Tanzania, suggests that their protolith consisted of basaltic rocks with a calc-alkaline differentiation trend. Some of the ultramafic rocks show primitive chemical characteristics including high (Mg/Mg + Fetot) ratios and high Ni contents. These primary magmas later evolved mainly by olivine fractionation to yield the parent magmas which cooled to form the protolith of the bulk of the mafic granulites. The effect of the granulite facies metamorphism on the geochemistry of the rocks is restricted to depletion of Nb and Rb and possibly the enrichment of Ba.

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