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TitleU-Pb zircon geochronology and phase equilibria modelling of a mafic eclogite from the Sumdo complex of south-east Tibet: insights into prograde zircon growth and the assembly of the Tibetan plateau
AuthorWeller, O M; St-Onge, M R; Rayner, N; Waters, D J; Searle, M P; Palin, R M
SourceLithos vol. 262, 2016 p. 729-741,
Alt SeriesEarth Sciences Sector, Contribution Series 20150104
Mediapaper; on-line; digital
File formatpdf; html
AreaTibet; Sumdo
Lat/Long WENS 92.4667 92.8833 30.0000 29.7833
Subjectsgeochronology; tectonics; igneous and metamorphic petrology; radiometric dating; uranium lead dating; petrography; phase equilibria; tectonic history; terranes; sutures; subduction zones; metamorphism, prograde; pressure-temperature conditions; thermal history; crystallization; recrystallization; morphology, crystal; mafic rocks; eclogites; zircon; bedrock geology; lithology; igneous rocks; intrusive rocks; magmas; metamorphic rocks; chemical analyses; x-ray fluorescence; trace element analyses; major element analyses; Tibetan Plateau; Sumdo Complex; Luobadui-Milashan Suture; North Lhasa Terrane; South Lhasa Terrane; Main Mantle Thrust; Jiale Fault; eclogite exhumation; dissolution-precipitation; Phanerozoic; Mesozoic; Triassic; Paleozoic; Permian
Illustrationsgeological sketch maps; photomicrographs; tables; phase diagrams; plots
ProgramBaffin Bedrock Mapping, GEM2: Geo-mapping for Energy and Minerals
AbstractThe Sumdo complex is a Permian-Triassic eclogitic metamorphic belt in south-east Tibet, which marks the location of a suture zone that separates the northern and southern Lhasa terranes. An integrated geochronological and petrological study of a mafic eclogite from the complex has constrained its tectonometamorphic history and provides a case study of zircon growth in eclogite as a product of prograde dissolution-precipitation. In situ U-Pb geochronology indicates that the eclogite contains a single population of zircon with a crystallisation age of 273.6 ± 2.8 Ma. The morphology and chemistry of the zircon grains are consistent with growth by dissolution-precipitation of protolith magmatic zircon. The presence of zircon grains as inclusions in the cores of peak phases indicates that zircon dissolution-precipitation occurred during prograde metamorphism, and calculated pressure and temperature conditions over which mineral inclusions in zircon are stable suggest that the zircon most likely precipitated at ~15.5-16.5 kbar and 500-560 °C. Subsequent peak metamorphism is calculated to have reached pressure-temperature conditions of 27±1 kbar and 670±50 °C. Previous studies, which have documented a range of peak metamorphic conditions from high- to ultrahigh-pressure at c. 266-230 Ma, indicate that the Sumdo complex is a composite belt that experienced protracted eclogite exhumation. The results of this study are consistent with this interpretation, and extend the age range of high-pressure metamorphism in the complex to over 40 Myr. Analysis of published pressure-temperature-time data indicates two systematic behaviours within this spread. First, peak metamorphic temperatures declined over time. Second, eclogite exhumation occurred in two discrete intervals: soon after formation, and during the demise of the subduction zone. The latter behaviour serves as a reminder that eclogite exhumation is the exception rather than the rule.
Summary(Plain Language Summary, not published)
Zircon is a common accessory mineral that is present in a variety of rocks. It is an extremely important mineral to geologists as it is possible to determine the age of zircon crystallisation, using a technique known as uranium-lead dating. For igneous rocks, a zircon age corresponds to the timing of crystallisation of the magmatic host. However, understanding when zircon crystallised within a metamorphic rock is more complicated. In this paper we show that zircon crystallises in a particular type of metamorphic rock known as an eclogite during its burial (prograde) history, as opposed to previous studies that assumed crystallisation occurred at the metamorphic peak pressure or post-peak (retrograde) history. The results have implications for understanding the rates of rock burial and exhumation.