Ashwal, L. D., Armstrong, R. A., Roberts, R. J., Schmitz, M. D., Corfu, F., Hetherington, C. J., Burke, K., Gerber, M. (2007) Geochronology of zircon megacrysts from nepheline-bearing gneisses as constraints on tectonic setting: implications for resetting of the U-Pb and Lu-Hf isotopic systems. Contributions to Mineralogy and Petrology, 153 (4). 389-403 doi:10.1007/s00410-006-0153-9
| Reference Type | Journal (article/letter/editorial) | ||
|---|---|---|---|
| Title | Geochronology of zircon megacrysts from nepheline-bearing gneisses as constraints on tectonic setting: implications for resetting of the U-Pb and Lu-Hf isotopic systems | ||
| Journal | Contributions to Mineralogy and Petrology | ||
| Authors | Ashwal, L. D. | Author | |
| Armstrong, R. A. | Author | ||
| Roberts, R. J. | Author | ||
| Schmitz, M. D. | Author | ||
| Corfu, F. | Author | ||
| Hetherington, C. J. | Author | ||
| Burke, K. | Author | ||
| Gerber, M. | Author | ||
| Year | 2007 (March 1) | Volume | 153 |
| Page(s) | 389-403 | Issue | 4 |
| Publisher | Springer Science and Business Media LLC | ||
| DOI | doi:10.1007/s00410-006-0153-9Search in ResearchGate | ||
| Generate Citation Formats | |||
| Classification | Not set | LoC | Not set |
| Mindat Ref. ID | 78696 | Long-form Identifier | mindat:1:5:78696:4 |
| GUID | 0 | ||
| Full Reference | Ashwal, L. D., Armstrong, R. A., Roberts, R. J., Schmitz, M. D., Corfu, F., Hetherington, C. J., Burke, K., Gerber, M. (2007) Geochronology of zircon megacrysts from nepheline-bearing gneisses as constraints on tectonic setting: implications for resetting of the U-Pb and Lu-Hf isotopic systems. Contributions to Mineralogy and Petrology, 153 (4). 389-403 doi:10.1007/s00410-006-0153-9 | ||
| Plain Text | Ashwal, L. D., Armstrong, R. A., Roberts, R. J., Schmitz, M. D., Corfu, F., Hetherington, C. J., Burke, K., Gerber, M. (2007) Geochronology of zircon megacrysts from nepheline-bearing gneisses as constraints on tectonic setting: implications for resetting of the U-Pb and Lu-Hf isotopic systems. Contributions to Mineralogy and Petrology, 153 (4). 389-403 doi:10.1007/s00410-006-0153-9 | ||
| In | (2007, March) Contributions to Mineralogy and Petrology Vol. 153 (4) Springer Science and Business Media LLC | ||
| Abstract/Notes | Nepheline-bearing gneisses from the 75 km2 Tambani body in the Mozambique Belt of southern Malawi, are miaskitic biotite-nepheline monzodiorites, reflecting an absence of K-feldspar, alkali amphiboles or pyroxenes, and contain euhedral zircon megacrysts up to 5 cm across. The zircons contain U = 1ā1,860 ppm, Th = 0ā2,170 ppm and Y = 400ā1,100 ppm, and very low concentrations of all other measured trace elements except Hf (HfO2 = 0.53ā0.92 wt. %). Cathodoluminescence images reveal oscillatory sector growth zoning and no evidence for xenocrystic cores, indicating that the zircons represent primary magmatic crystallization products that have survived amphibolite grade metamorphism. U-Pb isotopic analyses (by TIMS) yield an upper intercept age of 730 Ā± 4 Ma (MSWD = 1.7), which we interpret as the time of magmatic crystallization of the zircons. This is coincident with 11 SHRIMP spot analyses, which yield a mean age of 729 Ā± 7 Ma (MSWD = 0.37). Metamorphism, at 522 Ā± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization of zircon. Lu-Hf isotopic data for three whole-rocksamples of nepheline-bearing gneiss are collinear with those for zircon megacrysts, and correspond to an age of 584 Ā± 17 Ma (MSWD = 0.37. We interpret the LuHf array to represent a mixing line defined by the Hf isotopic signature of primary zircon and that of the rock-forming minerals reset during metamorphic (re-)crystallization; hence the 584 Ma age is likely geologically meaningless. Given the well-defined association of nepheline syenites (and phonolitic volcanic equivalents) with continental rifting, we suggest that the Tambani body represents a magmatic product formed at 730 Ma during the break-up of the Rodinia supercontinent. The 522 Ma age is akin to other PanAfrican metamorphic ages that record collisional suturing events during the final assembly of Gondwana. Zircon-bearing nepheline gneisses thus preserve a record of intra-continental rifting and of continental collision in southern Malawi. | ||
Locality Pages
| Locality | Citation Details |
|---|---|
| Salambidwe Intrusion, Chikwawa, Southern Region, Malawi |
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