AbstractThe timing of onset of modern plate tectonics is currently in conflict. Some believe that it began in the Archaean whereas others prefer a Neoproterozoic onset. At issue is the lack of reliable recorders of changing styles of subduction. Whilst high-pressure rocks (eclogite and high-P granulites) are present in the rock record from Archaean times, low-temperature, high-pressure and ultrahigh- pressure rocks only appear in the Neoproterozoic. This latter association is the hallmark of steep subduction of cold oceanic crust and is central to the argument. Their disappearance from the rock record older than c.600 Ma may be real or it may be a matter of preservation potential. The scope of this project is to investigate this question by the novel use of detrital rutile, which shows great potential as a provenance indicator for high-pressure metamorphism and tectonic settings.
The best recorders of subduction are blueschists, which are present in the rock record only to ca. 600 Ma ago. Rutiles in blueschist-facies mafic rocks from Syros and pelitic samples from the Sesia Lanzo Zone have been investigated and results show that the Nb vs. Cr diagram is a reliable tool for high-pressure/low temperature conditions, regardless of the lithology of the source rock. Further, rutiles in ultrahigh-pressure/high temperature rocks from the Dora Maira Massif and the Western Gneiss Complex have been analysed. Grains from the first location plot on the correct area of the chart, but do not correlate with the detrital record, whereas grains from the second location show a mixed Nb/Cr signatures, with eclogites plotting along the metamafic – metapelitic borderline, or even on the pelitic region. This indicates that the discrimination diagram requires special care when using it on high grade rutiles.
Provenance studies on Syros and the Sesia Lanzo showed a good host rock – detrital record correlation. Moreover, in the Western Alps, Po River contains a higher percentage of low-temperature rutiles (97%) compared to high temperature grains (3%), that might suggest that the rivers could control this concentration or most likely that the source rocks supply more rutile thus biasing the final population. These results further demonstrate the capability of detrital rutile to provenance highpressure/ low-temperature source rocks, mafic or pelitic, in large riverine systems.
The Zr-in-rutile thermometer gives values consistent with previous estimations for both Syros and the Sesia Lanzo samples, using the calibration with a silica activity of 1. The pressure-dependant calibration has a too big correction for lower pressure and temperature conditions. Moreover, quartz-bearing rocks give almost identical temperatures with quartz-free rocks, suggesting that the silica activity does not have a major effect on the thermometer. This latter thermometer has been used for ultrahigh-pressure/high temperature rutiles from Dora Maira and the Western Gneiss Complex, giving slightly lower results for the first location and considerable higher values for most of the samples from the second location. In the first case, a partial re-setting of the zirconium concentration could be the explanation, whereas in the second case, the study concludes that the Zr-in-rutile thermometer gives more consistent results than any exchange geothermometers. Therefore, this thermometer can be safely applied to rocks from blueschist- to granulite-facies rocks, giving good estimations where diffusion did not took place.
|Date of Award||Sep 2012|
|Supervisor||Craig Storey (Supervisor) & Rob Strachan (Supervisor)|