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 |
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| Authors: | L D Ashwal R A Armstrong R J Roberts M D Schmitz F Corfu C J Hetherington K Burke M Gerber |
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| Institution: | (1) School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050 Johannesburg, South Africa;(2) Research School of Earth Sciences, The Australian National University, Mills Road, Canberra, 0200, ACT, Australia;(3) Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, Washington, DC 20015, USA;(4) Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway;(5) Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, MA 01003-9297, USA;(6) Department of Geosciences, University of Houston, Houston, TX 77204-5007, USA;(7) Present address: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725-1535, USA |
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| Abstract: | 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 = 1–1,860 ppm, Th = 0–2,170 ppm and Y = 400–1,100 ppm, and very low concentrations of all other measured trace elements
except Hf (HfO2 = 0.53–0.92 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 ± 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 ±
7 Ma (MSWD = 0.37). Metamorphism, at 522 ± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization
of zircon. Lu-Hf isotopic data for three whole-rock samples of nepheline-bearing gneiss are collinear with those for zircon
megacrysts, and correspond to an age of 584 ± 17 Ma (MSWD = 0.37. We interpret the Lu-Hf 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 Pan-African 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. |
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