Comment and Reply on 'S-type granites and their probable absence in southwestern North America'

The problem of muscovite in granites is also raised by Dickson. Muscovite is common in felsic S-type granites, as stated by Chappell and White (1974), but how much is primary and how much is secondary are debatable. We currently think that very little is primary (above the solidus). We are not arguing that muscovite (whether primary or secondary), garnet, and andalusite are not found in S-types; we are arguing that these minerals may also be found in some I-types. Dickson is quite correct in saying that replacement of cordierite by micas results from hydration, which may also affect the alkali/aluminum ratio of the rock. However, he is not necessarily correct in his assertion that water must be introduced into the system; it may be magmatic water released from the magma near the solidus during cooling and crystallization. More systematic goechemical and isotopic work is required on petrographically well-documented granite suites before this question is resolved.