Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along...Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite （more than 8 wt.% MgO）. This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios （Zr/Hf = 32-35, Ti/Zr - 107-147）, producing flat REE and HFSE profles [（La/Yb）pm = 0.9 -1.3], with abundances of 3-10 times primitive mantle and with minor negative anomalies of Nb [（Nb/ La）pm - 0.6-0.8] and Th [（Th/La）pm = 0.6-0.9]. Initial isotope compositions （εNd） range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping -50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between -2830 and-2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.展开更多
Borehole and surface samples from the Archean Tanzania Craton were analysed for apatite fission track(AFT) and(U-Th)/He data with the aim of deciphering cooling histories of the basement rocks. Fission track dates fro...Borehole and surface samples from the Archean Tanzania Craton were analysed for apatite fission track(AFT) and(U-Th)/He data with the aim of deciphering cooling histories of the basement rocks. Fission track dates from borehole and outcrop samples are Carboniferous-Permian(345± 33.3 Ma to271±31.7 Ma) whereas(U-Th)/He dates are Carboniferous-Triassic(336±45.8 Ma to 213±29 Ma) for outcrop grains and are consistently younger than corresponding AFT dates. Single grain(U-Th)/He dates from the borehole are likely to be flawed by excessive helium implantation due to their very low effective uranium contents, radiation damage and grain sizes. All AFT and(U-Th)/He dates are significantly younger than the stratigraphic ages of their host rocks, implying that the samples have experienced Phanerozoic elevated paleo-temperatures. Considerations of the data indicate removal of up to 9 km overburden since the Palaeozoic.Thermal modelling reveals a protracted rapid cooling event commencing during the early Carboniferous(ca. 350 Ma) at rates of 46 m/Ma ending in the Triassic(ca. 220 Ma). The model also suggests minor cooling during the Cretaceous of the samples to surface temperatures. The suggested later cooling event remains to be tested. The major cooling phase during the Carboniferous is interpreted to be associated with compressional tectonics during the Variscan Orogeny sensu far field induced stresses. Coeval sedimentation in the Karoo basins in the region suggests that most of the cooling of cratonic rocks during the Carboniferous was associated with denudation.展开更多
On 5th November 2003,the central part of Tanzania (Dodoma),located within the eastern branch of the East African Rift System was struck by an M_s 5.5 earthquake that destroyed a school,a dispensary near its epicenter ...On 5th November 2003,the central part of Tanzania (Dodoma),located within the eastern branch of the East African Rift System was struck by an M_s 5.5 earthquake that destroyed a school,a dispensary near its epicenter and caused a huge crack on the Parliamentary building of Tanzania.This was one of the relatively large earthquakes that affected the area after the M_s 6.1 that occurred north of the Dodoma within the Manyara-Dodoma rift segment in May 7,展开更多
文摘Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite （more than 8 wt.% MgO）. This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios （Zr/Hf = 32-35, Ti/Zr - 107-147）, producing flat REE and HFSE profles [（La/Yb）pm = 0.9 -1.3], with abundances of 3-10 times primitive mantle and with minor negative anomalies of Nb [（Nb/ La）pm - 0.6-0.8] and Th [（Th/La）pm = 0.6-0.9]. Initial isotope compositions （εNd） range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping -50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between -2830 and-2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.
基金supported by the John Elleman scholarship at the University of Cape Town through the African Earth Observatory Network(AEON) initiativethe Natural Environment Research Council, UK, grant number NE/H008276/1the Scottish Universities Environmental Research Centre (SUERC)
文摘Borehole and surface samples from the Archean Tanzania Craton were analysed for apatite fission track(AFT) and(U-Th)/He data with the aim of deciphering cooling histories of the basement rocks. Fission track dates from borehole and outcrop samples are Carboniferous-Permian(345± 33.3 Ma to271±31.7 Ma) whereas(U-Th)/He dates are Carboniferous-Triassic(336±45.8 Ma to 213±29 Ma) for outcrop grains and are consistently younger than corresponding AFT dates. Single grain(U-Th)/He dates from the borehole are likely to be flawed by excessive helium implantation due to their very low effective uranium contents, radiation damage and grain sizes. All AFT and(U-Th)/He dates are significantly younger than the stratigraphic ages of their host rocks, implying that the samples have experienced Phanerozoic elevated paleo-temperatures. Considerations of the data indicate removal of up to 9 km overburden since the Palaeozoic.Thermal modelling reveals a protracted rapid cooling event commencing during the early Carboniferous(ca. 350 Ma) at rates of 46 m/Ma ending in the Triassic(ca. 220 Ma). The model also suggests minor cooling during the Cretaceous of the samples to surface temperatures. The suggested later cooling event remains to be tested. The major cooling phase during the Carboniferous is interpreted to be associated with compressional tectonics during the Variscan Orogeny sensu far field induced stresses. Coeval sedimentation in the Karoo basins in the region suggests that most of the cooling of cratonic rocks during the Carboniferous was associated with denudation.
文摘On 5th November 2003,the central part of Tanzania (Dodoma),located within the eastern branch of the East African Rift System was struck by an M_s 5.5 earthquake that destroyed a school,a dispensary near its epicenter and caused a huge crack on the Parliamentary building of Tanzania.This was one of the relatively large earthquakes that affected the area after the M_s 6.1 that occurred north of the Dodoma within the Manyara-Dodoma rift segment in May 7,
