Grant: Discovery Grants Program - Individual

The continental crust can reach temperatures over 800 degrees celsius at depths greater than 30 kilometres. At this temperature most rocks have started to melt, making granite liquid and a solid residue. The granite liquid is less dense and rises. Thus, the upper 15 kilometers of the crust is more granitic in composition, and the lower crust less granitic, than the crustal average. The crust is differentiated by this process. But the composition of the residue, tells us that more granite was made than reached the upper crust; some got stuck between the lower and upper crust. I want to locate this missing granite and find out why it got stuck where it did, how much got stuck and what its composition was. If the missing magma did not move far, it could occur as a rock called diatexite, or as a multitude of centimetre-sized bodies called leucosomes. If it moved farther, it could occur as masses of veins comprising an ""injection complex"", or as thick sheets called sills. Field mapping in northern Quebec, western Ontario and elsewhere will find out which. Granites in the upper crust do not have quite the same composition as the magma that formed deeper down because the magma crystallised as it rose. Analysis of samples collected should show that some were made from the original, or initial melt, but most will be rich in the minerals that crystallised first from the magma, a rock called cumulate, whereas a few might be made from the left over, evolved magma. With this information I can work out the volume of magma stuck in the middle crust and make the budget for crustal differentiation (also called a mass balance). This information will be useful to geologists interested in how the continental crust formed and became differentiated so that light rocks to overlie dense ones. This density stratification is responsible for the longevity of the shield areas, such as most of central Canada, by ensuring that it cannot be returned to the mantle.