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| Titel |
Zircon formation during hornblende-breakdown at 600-650 |
| VerfasserIn |
A. Zeh, A. Gerdes |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250020313
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| Zusammenfassung |
| Detailed petrological observation of rock forming and accessory minerals provide evidence that zircon in tholeiitic metabasites from the central part of the Limpopo Belt were formed during two distinct processes: during magma crystallisation at ca. 3.3 Ga, and a metamorphic overprint at about 2.0 Ga. This is well supported by zircon textures (CL images), as well as by U-Th-Pb and Lu-Hf isotope analyses of zircon obtained by laser-ablation-ICP-MS in thin sections and zircon mounts. Textural observation, thermodynamic calculations and trace element analyses furthermore constrain that the metamorphic zircon overgrowths formed under amphibolite-facies conditions at ca. 600-650°C / 5-7 kbar, and result from the prograde mineral reaction: hornblende (Zr-rich = 30ppm) + chlorite + garnet = cummingtonit (Zr poor = 3 ppm) + plagioclase + zircon. The results also show that the primary magmatic zircon cores – even such affected by multiple Pb-loss - still preserve their original 176Hf/177Hf of ca. 0.28065, whereas the metamorphic overgrowths have higher 176Hf/177Hf of 0.28090-0.28115. This indicates that the metamorphic zircon overgrowths incorporated additional radiogenic hafnium, perhaps formed by 176Lu decay in the matrix between 3.3 and 2.0 Ga. Furthermore, our results show that zirconium and hafnium are mobile in the environment of hornblende-breakdown. |
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