2004 Fiscal Year Annual Research Report
アフリカ南部クラトンの結合:原生代中期から後期における変動
| Project/Area Number |
15740307
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| Research Institution | Japan Agency for Marine-Earth Science and Technology |
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Principal Investigator |
JOHNSON Simon 独立行政法人海洋研究開発機構, 地球内部変動研究センター, 研究員 (20360753)
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| Keywords | Congo Craton / Kalahri Craton / Zambezi Belt / Continental collision / High-pressure rocks / Metamorphism / U-Pb-Th SHRIMP geochronology / Zambia |
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| Research Abstract |
During 2004 (16年) I completed a final field season in Zambia. The season was conducted over seven weeks in eastern Zambia examining and refining our terrane sub-division of the Southern Irumide Belt, especially that between the Chipata Terrane and the Petauke-Sinda Terrane, The boundary between the two is marked by an extremely high-strain, ophiolite-decorated shear/suture zone, a feature yet to be documented in the literature. Extensive samples were collected and shipped back to Japan for laboratory analyses. From the combined two field seasons (15-16年) I have mapped two majorophiolite/arc bodies (Chakwenga and Chongwe bodies), both of which are critical for elucidating the timing of collision, and pre-collision history between the Congo and Kalahari Cratons.
During this years laboratory analyses I have characterized the whole-rock major and trace element geochemistry from 40 rocks from the two ophiolite/arc bodies. Samples are about to be sent for Rare Earth Element and Sm-Nd isotope
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analyses. Zircons from four samples in the Chakwenga body have been separated, mounted, polished and imaged and I am awaiting time on the LA-Q-ICPMS at the Tokyo Institute of Technology to determine their age, and hence age of crystallization of the Chakwenga ophiolite/arc. In the previous year I had dated, by SHRIMP technique, samples from the Chongwe body and this year I have conducted preliminary Hf isotope analyses on the zircons by LA-MC-ICPMS at the Tokyo Institute of Technology. A further session is planned in order to further constrain the Hf isotopic composition of the zircons and hence the mantle source from which they were derived. An important finding from the Chongwe Body was that metamorphic zircon overgrowths around igneous cores were dated consistently at 575 Ma, some 40-50 m.y. older than the age for metamorphism in the Zambezi Belt. It appears that this metamorphic episode occurred under extremely high pressure (>2.0 GPa), which I interpret asbeing the timing of first contact between the Congo and Kalahari cratons and the attempted subduction of the Congo Craton margin beneath the Kalahri Craton. These results have been written up and submitted to the journal Geology and has just been accepted pending minor revisions. This result is incredibly important with regard to the timing and assembly history of the supercontinent Gondwana and will change the way Gondwanan-geologists view this supercontinent.
During 2004, I attended two international conferences and presented 5 papers (2 oral and 3 posters). I have published 2 papers (directly associated with the kakeni-hi grant), have another accepted and one more submitted. When I receive the Rare earth Element geochemical and Sm-Nd isotope data, another two papers will be submitted to top international journals. Less
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Research Products
(4 results)
