Study of many parts of the Idaho batholith shows considerable endomorphism by post-consolidation solutions. The solutions, enriched particularly in silica but also in variable but generally appreciable quantities of potash and minor amounts of other elements, produced many changes in the solid rock. Biotite, orthoclase, microcline, muscovite, quartz, sphene, apatite, zircon, magnetite, garnet, allanite, epidote, zoisite, chlorite, and sericite replaced earlier minerals. As a result, the final rock is considerably more silicic and generally more alkalic.
Much of the dioritic marginal rock of the batholith has been changed to a quartzrich diorite (tonalite) and granodiorite, and the less calcic rock of the interior from a quartz-bearing diorite and granodiorite to a somewhat calcic quartz monzonite with variations from granodiorite to granite. Dikes of associated aplite show similar end-stage changes, but most of the pegmatites have formed from somewhat later hydrothermal solutions by direct replacement of the aplitic and batholithic rock along fractures.
- Received 31 October 1941.
- © 1942 Geological Society of America