Contact and Endomorphic Phenomena Associated with a Part of the Idaho Batholith

Abstract

The intrusion of the Idaho batholith in the Paradise Ridge area of north Idaho was accompanied by clearly defined contact and endomorphic phenomena. Hydro thermal solutions emanating from deeper parts of the consolidating magma changed a part of the early, chilled marginal hornblende-biotite diorite of the batholith to a less calcic biotite-quartz diorite and some of the quartzite above to biotite-quartz diorite and to a quartz-biotite-diorite gneiss. In a part of the area the emanations were enriched in potash and these, permeating through the gneissic rock, changed much of it to an augen gneiss with microcline porphyroblasts and the adjacent biotite-quartz diorite to granodiorite. Toward the end of the period of igneous activity the emanations were highly silicic and introduced exceptionally large amounts of quartz into all the rocks, even causing silicification of the invaded quartzite. Minerals added to the rocks by hydrothermal replacements include biotite, andesirie, microcline, quartz, sphene, zircon, apatite, magnetite, garnet, allanite, zoisite, epidote, muscovite, sericite, and calcite. As consolidation progressed downward and hydrothermal solutions were drawn off from deeper levels, the solutions moved upward along fractures in the already endomorphosed rock and by replacement changed the bordering rock to coarse but variable-grained bodies of biotite pegmatite. One of these bodies is extraordinarily rich in garnet. As the solutions became more silicic, fissures were filled with quartz, or the wall rock was replaced by quartz, and bodies of silexite were formed. This study shows the importance of igneous emanations in the formation of quartz-feldspathic gneisses, augen gneisses, and pegmatites, and the extent to which hydrothermal replacements enter into their origin. It is suggested that such phenomena may be widespread in the crystalline schists of extensive metamorphic terranes.