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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dc.contributor.authorAshwal, L. D.
dc.contributor.authorArmstrong, Richard
dc.contributor.authorRoberts, R.J.
dc.contributor.authorSchmitz, Mark
dc.contributor.authorCorfu, Fernando
dc.contributor.authorHetherington, C.J.
dc.contributor.authorBurke, K.
dc.contributor.authorGerber, M.
dc.date.accessioned2015-12-07T22:24:52Z
dc.date.issued2007
dc.date.updated2015-12-07T09:29:34Z
dc.description.abstractNepheline-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.
dc.identifier.issn0010-7999
dc.identifier.urihttp://hdl.handle.net/1885/21020
dc.publisherSpringer
dc.sourceContributions to Mineralogy and Petrology
dc.subjectKeywords: cathodoluminescence; collision zone; geochronology; gneiss; Gondwana; isotopic composition; megacryst; metamorphism; recrystallization; SHRIMP dating; tectonic setting; trace element; uranium-lead dating; zircon; Africa; East Africa; Malawi; Mozambique Be
dc.titleGeochronology of zircon megacrysts from nepheline-bearing gneisses as constraints on tectonic setting: implications for resetting of the U-Pb and Lu-Hf isotopic systems
dc.typeJournal article
local.bibliographicCitation.lastpage403
local.bibliographicCitation.startpage389
local.contributor.affiliationAshwal, L. D., University of the Witwatersrand
local.contributor.affiliationArmstrong, Richard, College of Physical and Mathematical Sciences, ANU
local.contributor.affiliationRoberts, R.J., University of the Witwatersrand
local.contributor.affiliationSchmitz, Mark, Carnegie Institution of Washington
local.contributor.affiliationCorfu, Fernando, University of Oslo
local.contributor.affiliationHetherington, C.J., University of Massachusetts
local.contributor.affiliationBurke, K., University of Houston
local.contributor.affiliationGerber, M., University of the Witwatersrand
local.contributor.authoremailu4029979@anu.edu.au
local.contributor.authoruidArmstrong, Richard, u4029979
local.description.embargo2037-12-31
local.description.notesImported from ARIES
local.identifier.absfor040203 - Isotope Geochemistry
local.identifier.absfor040313 - Tectonics
local.identifier.ariespublicationu4027924xPUB15
local.identifier.citationvolume153
local.identifier.doi10.1007/s00410-006-0153-9
local.identifier.scopusID2-s2.0-33847687615
local.identifier.uidSubmittedByu4027924
local.type.statusPublished Version

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