Fractional crystallization is an important process that controls magma evolution, especially for mafic magmas in arc-related settings. However, we know, the granitic magas have high viscosity, which would certainly impede the separation of minerals from the host magmas. In this case, I want to ask wether the fractional crystallization has influences on the granite magma evolution? To what extent?
This is a long-existing and tough question that has pullzed me for along time. Welcome anybody to discuss here.
Regards.
Lu
Dear Mr. Lu:
The fractional crystallization is a common petrological process observed in intermediate to felsic magmas leading at the very end to special rocks like greisen or pegmatites. In a silica-enriched magma minerals form during the initial stage but due to some settling down they are unable to equilibrate with the remaining magma. Often granodioritic early granitoids result from this process or they form a small rim around the granitic body which will subsequently take on a monzo- to syeno-granitic composition. As a result of this, special elements such as Sn, W, Be, Li etc. which cannot be accommodated in the structure of the rock-forming minerals quartz or feldspar get concentrated in the marginal or apical parts of a granitic complex and give rise to greisens or granitic pegmatites.
As far as the pegmatites are concerned this is one way of being emplaced.
With best regards
H.G.Dill
Dear all,
I would like to add that crystal fractionation in granitic magmas has only minor effect on major element composition, especially when primary melt compositionally approaches the temperature minimum in Q-Ab-Or system. But the effect on some trace elements (including volatiles) may be significant, as it has been noted by Prof. Dill. Unfortunately, presence of numerous accessory phases which concentarte REE, LILE, and HFSE seriously impedes any geochemical modelling.
Dear Mr.Lu,
Fractionation from the melt generated due to the melting of the upper crustal rocks may serve as good example. Most of the cases, the fractionation is limited in extent. However, as pointed out by HG Gill, the high viscosity will retard the fractionation to continue. High concentration of LILE, REE observed in leucogranites formed from crustal melting I found in Himalayan as well as TransHimalayan regions.
Sncerely
T.K.Goswami
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