Cooling cracks form at high temperatures when a basic magma is either viscous or crystalline and hot. It is postulated that fine-grained margins form along the fractures by release of gas through the fracture network. This takes place (a) by raising the liquidus-solidus boundary due to release of volatiles, (b) by expansion of the released gas with consequent cooling. Granitic material introduced along the fractures post-dates and may recrystallize the 'chilled' margins.

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