Young Inclusions in an Old Host: Dating Alteration Events With Electron-Beam Techniques
Abstract
A basic premise in deciphering the history of a rock is that textures such as mineral zoning, fold interference patterns, and unconformities establish a relative time line for an outcrop. A similar argument can be applied to poikiloblastic minerals, which provide information on pre-metamorphic mineral assemblages, a rock's P-T path, and, in cases where the inclusions are good chronometers, a P-T-t path. The focus of this study are zircons from an alkaline complex in the Seiland Igneous Province (Northern Norway) that presented several technical challenges during ID-TIMS analyses owing to the presence of xenotime and thorite mineral inclusions. These zircons also display irregular trace element distribution, localised internal porosity, and crystal face embayment. Careful selection of inclusion-free grains allowed the bracketing of alkaline magmatism to between 570 and 560 Ma. Discordant zircon grains give similar upper intercept ages, whilst the lower intercepts scatter widely (± 150) around 400 Ma. The data has been used to argue that there was only one demonstrable metamorphic event post-dating magmatism in the Seiland Igneous Province, corresponding to the Scandian phase of the Caledonian Orogeny. However, from the ID-TIMS analysis alone, this conclusion is not definitive; older Pb-loss events may be masked by Scandian metamorphism. The textural features of the zircon are diagnostic of dissolution-reprecipitation processes; a complex fluid-rock interaction commonly resulting in volume changes, porosity, replacement textures, and growth of mineral inclusions, and would also result in Pb loss from the primary zircon. To test for a correlation between Pb loss, zircon alteration, regional metamorphism, and dissolution-reprecipitation, electron-probe chemical-dating techniques were applied to suitable mineral inclusions in the zircon. The data yield a clustering of ages around 420 Ma, with no evidence for an older inclusion assemblage. These results support the hypothesis established by ID-TIMS. It is also noted that the age of the inclusions is in conflict with a classical interpretation of the poikilitic texture that requires the inclusions be coeval or older than the host zircon.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2006
- Bibcode:
- 2006AGUSM.U41B..07H
- Keywords:
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- 3625 Petrography;
- microstructures;
- and textures