A-type (the A denoting anorogenic and/or anhydrous) granitoids occurred along rift zones and within stable continental blocks. The identification of A-type granitoids was based on both tectonic setting and chemical characteristics.

A-types are characterized by their relatively high alkali contents and low CaO contents (at SiO2 = 70%: Na2O+K2O = 7-11%, CaO

Very informative thanks Asoori and Leonardo

Hi Saleh,  classification of granites is a very controversial topic as source-related schemes such as the classical A-, I- and S-type may dramatically fail turning to be useless whenever orogenic processes yield to crustal reworking. As an example, in the Corsica-Sardinia Batholith (Italy-France), the place I'm working on - there are many 300 Ma-old anatectic granites that show I-type geochemical characteristics. The reason for this apparent contradiction is that partial melting occurred close to the liquidus involving Ordovician (450-480 Ma) granitoids as source rocks. These examples are pretty common, particularly in cratons. Therefore, my recommendation is use any classification scheme very critically.  

I strongly agree with you Leonardo most of the classification is contradicting by tectonic factors.

Dear Saleh,

You have got plenty of good suggestions. I am attaching Table 18.4, p. 386, from "Principals of Igneous and Metamorphic Petrology (JD Winter, 2010) which is a nice summary of the characteristics of various granitoids. Hope you  find it useful.

Best, Qasim Jan

Dear Saleh, the best starting point is to use the mineralogical classification of Streckheisen et al. A lot of people even today do not utilise the basic mineralogy and hence misname the various granitoids. Once you have the fundamental classification by rock-name, you can then start compiling the geochemical data.  Alan White was a great believer in using the major elements for the I- and S-type scheme, but modern approaches focus more on the trace element and isotopic characteristics. Collisional belts can contain I-, S- and A-type granites, so tectonic setting may be non-unique. However, if you have geochronology on the various granites then this commonly allows you to track changes in tectonism with time. Best wishes, Simon

Dear Saleh,

As many people has already outlined, the alphabetic classification of granites is a bit controversial. The problem with that classification is that we define a rock as granite in the QAP diagram: after removing all non quartzofeldspathic minerals, if the rock plot within the syenogranite-monzogranite field, it is a granite. This classification is mineralogical, but it does not take into account the other minerals: muscovite, biotite, garnet, cordierite, amphibole, pyroxene and the accessories. Consequently, rock with different mineralogies and different histories (primary melts, melts+residual minerals, fractionated melts, etc) will be all called granites if they happen to have the right Qtz-Kfs-Pl proportions.

When you consider these other minerals, then is when the alphabetic classification comes. If the granites have Al-rich minerals like Al-silicate and/or muscovite and/or cordierite and/or garnet and/or Al-rich biotite, they have come from metasedimentary rocks and thus are S-type granites. If the granites have Al-poor minerals like amphibole, orthopyroxene, clinopyroxene or Al-poor biotite, they have come from rocks with bulk andesitic composition (amphibolites, andesites, basalt+sediments, and many other source rocks) and thus they are I-type granites. I'm not an expert with M-type granites and thus I do not say anything about them.

Regarding A-type granites, there is an immportant difference with the S- and I-type granites. The name "S- and I-type" refers to their probable protoliths (metasedimentary/metaigneousm according to their original definition by Chappell and White), but "A-type" refers to its tectonic place (anorogenic or, more properly, extensional regimes), and thus you have A-type granites thar range from peralkaline to peraluminous, with features overlapping with either S- or I-type granites.

So this is more or less the question about the alphabetic classification. May your mineralogy and geochemistry lead you to the classification, and not the opposite.

With kind regards,

Marcos

The mineralogical classification of Streckheisen et al. will indeed be a good start, but you would need A) making thin sections of the samples in all three dimensions, B) staining or scanning of the thin sections, and C) using point counter for correct modal estimates. Point B is absolutely essential, because grains of quartz, potash feldspar and plagioclase are not easy to distinguish mutually.

Frost et al., 2001 and 2008 classification scheme based on major oxide might help classify granite compared to the trace element classification of granite into I, A and S type.

In continuity with books suggested by Dr. Shah Faisal; Whalen et al.,1987 Barbarain (1990), Eby (1992), Frost and Frost (2011), and Whalen and Hidebrand, (2019) are more helpful for differentiating the A1, A2, I, M and S type granitoids on the basis of geochemical/tectonic classification.

As many Scholar has already outlined. But personally i would like to recommend a book for such pertinent questions .....Principles of Igneous and Metamorphic Petrology John D. Winter Second Edition (Page number 404). Table is attached here .......

A. Ghaffari