Browsing by Author "Nakamura, Eizo"
Now showing 1 - 8 of 8
Results Per Page
Sort Options
Item 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, EizoThe 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.Item 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, EizoGeochemical 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.Item 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, EizoGeochemical 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.Item Geochemistry of Volcanic and Plutonic Rocks from the Southern Musoma-Mara Greenstone Belt: Implication for the Evolution of the Tanzania Craton(2000) Messo, Charles W.; Yamashita, K.; Kobayashi, Katsura; Makishima, Akio; Sakaguchi, C.; Nakamura, EizoThe Neoarchean Tanzania craton contains patches of greenstone belts set in granitoid masses that occupy a large percent of the craton. Because of limited geological, geochronological and geochemical information, very little is known about this craton and its constituent terrains. We report new U-Pb chronological, Nd-Hf isotopic and major and trace element data for volcanic and granitoid rocks from the southern Musoma-Mara greenstone belts of the Northeast Tanzania craton. The volcanic rocks are mainly basaltic with minor felsic volcanic rocks. The intrusive rocks are mostly granodiorites and granites with few trondhjemitic samples. Geochemically, the basalts are tholeiitic and MORB-like but with minor depletions of Nb and Ti relative to N-MORB. The felsic volcanic and granitoid rocks are calc-alkaline with normal arc to adakitic signature. The basalts yielded a Sm - Nd isochron age of 2842 ± 65 Ma (εNd = +2.1, MSWD = 1.6) and εHf2840 = +1.7 - +3.2. One granitoid yielded U-Pb zircon age of 2689 ± 12 Ma which is similar to those reported for the oldest rocks in the North Musoma-Mara greenstone belt [1], and is interpreted to represent the age of granitoids and felsic metavolcanic rocks. The felsic volcanic rocks have εNd2689 ~ +1.6 - +2.5 and εHf2689 ~ +0.1 - +1.6 that are within error of those for granitods and meta-basalts. Collectively, these data show that the basaltic rocks may not be related to the felsic volcanic and intrusive rocks by fractionation but were likely formed from equally depleted sources, possibly in arc-back arc environment. The spatial association of MORB-like tholeiites and arc-like volcanic and granitoid rocks is not uncommon in the Tanzania craton [2], as well as other late-Archean cratons worldwide [3, 4], and may imply that the tectono-magmatic processes that produced this association were widespread during this period.Item Ion Microprobe Zircon U–Pb Dating of the Late Archaean Metavolcanics and Associated Granites of the Musoma-Mara Greenstone Belt, Northeast Tanzania: Implications for the Geological Evolution of the Tanzania Craton(Pergamon, 2006-07-31) Manya, Shukrani; Kobayashi, Katsura; Maboko, Makenya A. H.; Nakamura, EizoIon microprobe zircon U–Pb ages from metavolcanic and associated granitic rocks of the late Archaean Musoma-Mara Greenstone Belt (MMGB) of northeast Tanzania reveal that the oldest mafic volcanism in the belt occurred at 2676–2669 Ma followed by felsic volcanism at ∼2668 Ma. The felsic volcanism was coeval with the emplacement of the oldest pulse of massive granitoids that is dated at 2668 Ma. The youngest volcanic episode, represented by a volcanic horizon in the largely sedimentary Kavirondian Supergroup that overlies the greenstone sequence with a marked unconformity, occurred at ∼2667 Ma. A younger phase of post-orogenic granites concluded the magmatic evolution of the MMGB at ∼2649 Ma. Our age data suggests that the entire volcano-sedimentary sequence in MMGB was emplaced in a relatively short time interval between ∼2676 and ∼2667 Ma. It also shows that contrary to arguments based on the degree of deformation, the foliated granites and some amphibolite rafts enclosed in them do not constitute the basement to the greenstone sequence. The data further shows that volcanism in the MMGB was younger than the ∼2820 Ma age of volcanism in the Sukumaland Greenstone Belt (SGB) to the far southwest and the ∼2720 Ma age of volcanism in the nearby Kilimafedha Greenstone Belt (KGB) to the south. The age of granitic magmatism (ca. 2.69–2.55 Ga) in the three belts was, however, largely coeval. Granitic magmatism of this age has also been reported in different parts of the Tanzania Craton suggesting that it was responsible for the late Archaean crustal growth and marks the beginning of a period of stability (or of cratonization).Item 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(2002) Maboko, Makenya A. H.; Nakamura, EizoGranulite-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±58 Ma and an initial ε(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.Item Nd and Sr Isotopic Mapping of the Archaean-Proterozoic Boundary in Southeastern Tanzania Using Granites as Probes for Crustal Growth(Elsevier, 1996) Maboko, Makenya A. H.; Nakamura, EizoGranitoids of two different generations occur across the Archaean-Proterozoic boundary in southeastern Tanzania. The first generation, which is confined to the Archaean Tanzania Craton, yields RbSr whole-rock isochron ages of 2600 Ma and low, mantle-like, initial 87Sr/86Sr ratios (0.702–0.704). These coupled with positive or near-zero ϵNd values (−0.2 to 2.0) suggest that these granitoids represent juvenile mantle material which was added to the continental crust at about 2600 Ma. The second generation of granitoids intrudes the Palaeoproterozoic Usagaran Belt and is characterised by RbSr whole-rock isochron ages of about 1900 Ma. These granitoids show SmNd crustal formation ages which are 200 to 600 Ma older than their emplacement ages and negative ϵNd values (−2.2 to −6.2). Mixing calculations suggest that the Proterozoic granitoids may have formed by partial melting of 2000 Ma (Usagaran) mantle-derived material which had incorporated between 15 and 45% of an Archaean component from the adjacent Tanzania CratonItem SmNd Garnet Ages from the Uluguru Granulite Complex of Eastern Tanzania: Further Evidence for Post-metamorphic Slow Cooling in the Mozambique Belt(Elsevier, 1995) Maboko, Makenya A. H.; Nakamura, EizoTwo samples from the Uluguru granulite complex yield garnet SmNd ages of 633 ± 7 and 618 ± 16 Ma, similar to previously published hornblende 40Ar39Ar and KAr ages. The similarity of the SmNd to the KAr age suggests that the closure temperature of garnet to Nd diffusion is similar to that of hornblende to Ar diffusion. Assuming that published zircon UPb ages of about 700 Ma date peak granulite-facies metamorphism, a mean post-metamorphic cooling rate of 2–3°C/Ma can be calculated for the time interval 700 to 630 Ma. Such slow cooling rates imply thermal relaxation with a thickness length-scale greater than the thickness of average continental crust. This, in turn, implies that the thermal perturbation responsible for metamorphism was preceded by regional crustal thickening probably in a collisional orogen.