Layered Intrusion - Intrusive Behaviour and Setting

Intrusive Behaviour and Setting

Mafic-ultramafic layered intrusions occur at all levels within the crust, from depths in excess of 50 km to depths of as little as 1.5 to 5 km. The depth at which an intrusion is formed is dependent on several factors:

• Density of the melt. Magmas with high magnesium and iron contents are denser and are therefore less likely to be able to reach the surface.
• Interfaces within the crust. Typically, a horizontal detachment zone, a dense, impermeable layer or even a lithological interface may provide a horizontal plane of weakness which the ascending magma will exploit, forming a sill or lopolith.
• Temperature and viscosity. As an ascending magma rises and cools, it becomes thicker and more viscous. This then restricts the magma from rising further because more energy is required to push it upwards. Conversely, thicker magma is also more efficient at forcing apart the wall rocks, creating volume which the magma may fill.