Various mechanisms by which pegmatites may have come into place are considered, and the following classification of modes of emplacement, based largely on structural concepts, is suggested:
Displacement: country rocks moved aside from site of emplacement
Forceful emplacement: country rocks moved aside by pegmatite
Permissive emplacement: pre-existing opening in country rock filled by egmatite
Nondisplacement: no movement aside of country rocks occurred
Mobility: material at pegmatitic site was bodily mobile during emplacement
Introduction: pegmatitic material introduced into site of emplacement
Local derivation: pegmatitic material derived locally
Host-rock breakdown: pegmatite derived by chemical breakdown of pre-existing rock
Residual segregation: pegmatite derived by crystallization, in place, of residual magmatic fluids
Nonmobility: material at pegmatitic site not bodily mobile during emplacement
Criteria that may help to distinguish these various modes of emplacement are listed and discussed. Particular emphasis is given to field features. In the literature disagreement exists concerning the worth of some of the criteria, whereas there is substantial agreement on other lines of evidence.
Results of a field study of certain granitic pegmatites of New England are also presented. The purpose of this study was to determine the modes of emplacement of these bodies by application of the various criteria developed. The mode or modes of emplacement of some of these pegmatites could be partly determined, as the bodies exhibit critical features. The modes of emplacement of other bodies were difficult to determine because critical features are lacking.
The various pegmatites developed by several mechanisms. Furthermore, individual pegmatites commonly exhibit evidence for more than one process. Country rock was displaced forcefully to make room for some pegmatites or portions of them and was displaced either forcefully or permissively at other bodies. At some pegmatites the country rock was not bodily displaced, and the material at the sites of most of these bodies was mobile. Pegmatite matter was introduced into emplacement sites where mobile or nonmobile processes were active.