The origin of laminated and massive anorthosite,Sept Iles layered intrusion,Québec,Canada |
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| Authors: | Michael Denis Higgins |
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| Institution: | (1) Science de la terre, Université du Québec, G7J 1B2 Chicoutimi, Québec, Canada |
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| Abstract: | Well-laminated plagioclase-rich rocks (‘anorthosite’) occur as layers in mineralogically similar but massive rocks in the
Sept Iles intrusion. Several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages
in the production of these rocks. Plagioclase nucleated and initially grew in random orientations in the stationary part of
the boundary layer. There, in the absence of other events, crystallization produced the massive anorthosite. The laminated
anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced
by magmatic density-currents. Such currents would have caused the zone of simple shear of the boundary-layer to migrate outwards
into the previously stationary part. This simple shear would then have rotated the existing plagioclase crystals, as well
as crystals that nucleated at that time, towards the shear-plane. Some rocks show evidence of several periods of enhanced
flow-velocities. The production of the lamination may have weakened the crystal-mush and enabled slumping to occur, producing
folds and contorted layers. Finally, compaction liberated large quantities of intercumulus magma of which some was trapped
as granophyric segregations and some escaped by overturing and churning the crystal-pile. Earlier-formed granophyric segregations
and laminated layers were disrupted during this process to produce a range of more-complex rocks including igneous breccias. |
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