link

http://faculty.uml.edu/Nelson_Eby/Research/A-type%20granites/Chappell%20and%20White%20S%20and%20I%20type%20granites.pdf

http://petrology.oxfordjournals.org/content/42/11/2033.full

http://www.geologynet.com/granitetypes.htm

https://www.academia.edu/14065243/The_enigmatic_sources_of_I-type_granites_The_peritectic_connexion

http://wenku.baidu.com/view/e584f38da1c7aa00b42acb3b.html

Dear Malik Othman please go through t this paper hope you will get direction.

Regards

Mastoi

Dear Malik,

ASI gradually grows with the fractionation of phases like amphiboles and pyroxenes, and may exceed 1.1 in some I-type granites. If you have a representative data set for the pluton you are studying (not just one analysis), it would be useful to observe the total span of ASI values.

Thank you all for your helpful reply

Dear Malik,

In literature, I-type granites are commonly represented as equivalents of metaluminous while S-type granites are equals of peraluminous rocks. However, this is not the case in many regions/rocks. I personally work on granitoid bodies (I-type granites) of New Brunswick, Canada, and most of my samples are peraluminous in nature (ASI>1.1). Many papers are explaining these rocks as peraluminous I-type granites. Chappell et al. (2012) examined the effect of assimilation and partial melting of supracrustal rocks in the formation of these peraluminous granites and concluded that the partial melting of more mafic source rocks (in their region), rather than the fractional crystallization of more mafic magmas is the best explanation for the origin/formation of these granitic rocks. This is their explanation "Experimental studies have shown that the melts generated by the partial melting of basaltic to andesitic rocks under crustal conditions are mostly peraluminous. During the dehydrational melting of I-type granite source rocks at pressures below the garnet stability field, biotite and amphibole melt incongruently to yield pyroxenes. The excess Al is incorporated into the felsic liquid, resulting in the generation of peraluminous melts. In this instance, the excess Al in felsic I-type granites is a function of the melting process, and unrelated to the bulk composition of the source (Chappell et al. 2012)"

Please follow the link below for more information: http://www.sciencedirect.com/science/article/pii/S0024493712002824

If your rocks are weathered they have likely lost alkalis, leaving unchanged the amount of Al. This results in an increase of Al/(Na+K+Ca) ratio, possibly reaching values >1.1.

Hi Malik,

We had a similar problem. In the Sierra Nevada, we studied a suite of granites where ASI increased rather dramatically with SiO2 content, to about 1.3 or so for the highest SiO2 samples. But we had very good control on the sedimentary units that comprise the crust and so we could exclude any partial melting of sediments. Take a look at our Fig. 3D and Fig. 13A in Am MIn v. 99, p. 1284-1303. So I think the earlier comments about fractionation are correct - at least in our case in the Sierra Nevada. Best of luck with your work.

Keith

Fractional crystallization with/without assimilation is the most likely process resulting in the increase of ASI in granitic magma system. I think the fractionated product is equal to "highly fractionated I-type granite".

Mostly for the  production of I-type granite source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them

Most of the I-type granites (~ 95%) in the Lachlan Fold Belt formed at lower temperatures and almost half of those rocks for which bulk chemical compositions are available are peraluminous.

The partial melting of more mafic source rocks, rather than the fractional crystallisation of more mafic magmas, is favoured for the origin of these rocks.

Peraluminous granites can form by partial melting of metaluminous crustal sources.  Crustal metaluminous magmas can form by entrainment or dissolution of clinopyroxene.

I really appreciate your explanation, thanks for you all.

This article may have the answer to your question.

Chappell, B.W., Bryant, C.J., Wyborn, D., 2012. Peraluminous I-type granites. Lithos 153, 142–153.

I recommend Roman's answer