Summary of February lecture given by Prof. John Clemens, Kingston University
Despite teasing us with the title of his
presentation, John Clemens
took us on a swift tour of the petrology of granites to demonstrate that they are, in fact, a remarkable group of rocks.After showing us slides of different granite landscapes Professor Clemens
turned to the detail in the rocks with a short discussion of some of the enigmatic structures that can be seen in granite outcrops.Some features such as graded layering and cross layering of crystals can be explained by sedimentation and flow structures operating in the fluid magma.Other features are less well understood.These include orbicular structures, comb layering and ladder dykes, all of which Professor Clemens
believes are created by rhythmic precipitation of mineral layers within volatile-rich patches of magma.
Having shown us some of the variety that granites can exhibit in the landscape and outcrop, Professor Clemens
posed the question: where do granites come from?Much of the thinking on this has been influenced by the work of N L Bowen who, with O F Tuttle, showed that granite could be formed by the melting of crustal rocks under pressure and temperature conditions found within the amphibolite facies of metamorphism.For this to happen the rocks have to be saturated with water but Professor Clemens' own research showed that instead of always being water saturated, granite magmas had water contents that were related to their melting temperatures.The drier the magma, the higher it's melting temperature.Further experimental research has shown that instead of water saturated melting in the amphibolite facies of metamorphism creating granite magmas, fluid-absent dehydration reactions in the much hotter granulite facies of metamorphism seem to be responsible.
described several possible dehydration reactions that could be responsible but in essence it is the OH-bearing minerals such as muscovite, biotite and hornblende which break down with increasing temperature to release a water-undersaturated granitic magma.The heat for such reactions could not be generated by deep burial of crustal rocks alone.Additional heat would be required from the mantle, most probably by the intrusion of mafic magma.Instead of regarding granite magmas as relatively cool bodies (around 6500C according to Bowen), the dehydration reactions suggest that temperatures of granite magma may vary between 700 and 11000C.
Rocks that are undergoing this high temperature melting show both metamorphic and magmatic features.Professors Clemens
showed slides of several of these mixed rocks, or migmatites as they are known, which revealed how the new granite magma is localised in patches throughout the melting rock.
described ascent rates of 20km in hours to just a few months.He
also explained that such ascent rates could build a granite pluton of 1000km3 in a little over 1000 years.
While talking about granite statistics, Professor Clemens
told us that a 3 km thick sheet of granite magma in the upper crust could go from a liquid state to a solid state (though still hot) in about 30,000 years.