which kind of pluton would produce the highest grade metamophism,a granite intrusion 20km deep or gabbro 5km?

Answer 1

The answer is Gabbro at 5 km for two reasons.

1. The gabbro starts hotter, usually a few hundred degrees higher than grantie

2. the higher temperature contrast, being shallower, will produce a more dramatic metamorphism

This, of course, assumes similar size and geothermal gradient.

Answer 2

Assuming the geothermal gradient is the same at both of your intrusions, the deeper seated intrusion will always result in a higher grade of metamorphism. Why? Metamorphism is caused by a combination of heat and pressure. You should be able to figure out the rest!

But QFL247 is also correct - a shallower intrusion will have a higher temperature contrast than a deeper intrusion, and mafic intrusions are hotter than felsic intrusions. Note that the chemistry of the intrusion has nothing to do with metamorphism, but it does with metasomatism.

So the correct answer depends on the question - do you mean metamorphism resulting solely from the intrusion, or the metamorphic grade resulting from the combination of the intrusion and its ambient temperature and pressure?

Answer 3

I think 5km.