Contrasting response of monazite and zircon to a high- T thermal overprint
Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and sp...
| Published in: | Lithos |
|---|---|
| Main Authors: | , , |
| Format: | Article in Journal/Newspaper |
| Language: | unknown |
| Published: |
Elsevier
|
| Subjects: | |
| Online Access: | http://hdl.handle.net/1885/84370 https://doi.org/10.1016/j.lithos.2005.08.008 https://openresearch-repository.anu.edu.au/bitstream/1885/84370/5/MigratedxPub12652_2006.pdf.jpg https://openresearch-repository.anu.edu.au/bitstream/1885/84370/7/01_McFarlane_Contrasting_response_of_2005.pdf.jpg |
| Summary: | Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage. In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of207Pb / 206Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This ... |
|---|