formation3

Deep below the mountains a root of continental crust was pushed down into the upper mantle perhaps to a depth of 40 or more kilometres. Here the sediments it carried would melt, mixing with each other and the surrounding rocks of the upper mantle and lower crust to form a magma.

After Bristow 1996

As it cooled this magma would separate by crystal fractionation to produce dense magma, which settled down under gravitational forces. The less dense component of the magma rose up to form granite. These granite intrusions rose up into the mountain chain where they met resistance from over laying rocks, a process known as stopping. As they rose they assimilated the surrounding rock, changing the chemistry of the melt.

Compressive forces built the mountain range as continental plates met, however once formed the mountains would soon feel the forces of gravity and begin to subside. Unstable at high elevations rocks act like a very stiff liquid, flowing back to a level. Thrust faults, which built the mountains, were now reactivated as normal faults due to gravitational collapse.

Granite contains radioactive minerals which keep the granite hot over vast periods of time. Very slow cooling occurs, producing coarse massive crystals. These processes are envisaged by geologists as taking many millions of years, spanning the Permian, Triassic and even Jurassic periods of earth history

The variation in the granites is a product of this assimilation of surrounding rock and alteration of the granite by late stage processes. With a lid of crystallised granite the internal heat and pressure resulted in volatile fluids and gases being punched up through cracks and fissures in the capping.

These attacked the minerals in the granite changing them to secondary constituents, like tourmaline and white mica. They also carried the rarer minerals like tin and copper, these injections would produce the ore bodies, exploited for their mineral wealth in the last millennium by man.

Quartz Tourmaline Schorl, vain rock x1/2

These fissures were capitalised by the hot volatile fluids and gases as they circulated around the margins of the granite. Radioactive heat keeping the convection moving, drawing in groundwater as well as the fluid from the magma. The result was several processes, which altered the granite, softening areas, rotting the granite from without, and within. This created schorl rocks and greisien, and may have led ultimately to kaolinisation of the granite.
Greisen, highly altered granite, showing tormaline stringers x1/2

Where rotting did not take place granite remained hard and resistant to subsequent weathering and erosion. Throughout the Jurassic and Cretaceous the granite uplands formed an island covered by tropical forests, deep-seated weathering occurred rotting feldspars to clay minerals, producing kaolin. This process continues to this day, as our more temperate climates attacks the granite.