The building of granitic plutons from pervasive and continuous melting in the lower crust to discontinuous and spaced plutons building in the upper crust
Abstract
The generation of granitic magmas begins when the lower crust starts melting, often with the active help of the underlying mantle. Since melting is driven by heat, it consists in a pervasive and continuous process that develops over wide regions. In contrast, the building of a granitic pluton is highly discontinuous in time. Several inputs of magma, with sometimes a different chemical composition, have to be focused toward a specific region where magmas accumulate. They form a large pluton, often separated from an adjacent pluton by some 50 km. The process of pluton building is therefore discontinuous in space and time. This switch from a continuous to a discontinuous process has rarely been addressed before, though it constitutes a fundamental point of magma generation. Discontinuities result from non-linear processes that develop during the segregation and the ascent of magma. They basically rely on the non-linear rheology of partially molten rocks. Because two phases continuously coexist, namely the matrix and the melt, strain is highly partitioned between them. Thresholds control the change from a solid-like to a liquid-like behaviour of the magma. In between those end-member behaviours, the rheology alternatively changes from one state to the other. Those factors induce highly discontinuous melt segregation, which needs both pure and simple shear to develop. Melt focusing is controlled by the viscosity contrast between the two phases. It gives place to different compaction lengths develop depending on the surrounding, a partially melting source or a nearly brittle crust. Owing to the discontinuous segregation process, intermediate magma chambers could develop with different temperature and magma composition. They could be the place of enhanced magma mixing. Because ascent and emplacement are discontinuous in time, they provide time for the crust to relax, avoiding the room problem for a pluton intruding the upper crust. Finally, the stress conditions, which differ for each tectonic setting, have an influence on the shape of the granitic plutons.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V11F..07R
- Keywords:
-
- 1036 Magma chamber processes (3618);
- 1037 Magma genesis and partial melting (3619);
- 8139 Obduction tectonics