The Cloudy Pass epizonal batholith and associated subvolcanic rocks

Abstract

The Cloudy Pass batholith, one of several small epizonal Tertiary batholiths in the Northern Cascade Mountains, discordantly intrudes metamorphic rocks of pre-Late Cretaceous age. The batholith is remarkable for its chilled borders, associated porphyry plugs, and intrusive breccias. The main body of the batholith consists largely of labradorite granodiorite. Part of the northeast side of the batholith is bordered by a complex more than half a mile thick of chilled rocks of Hart Lake that consists of separately injected, contrasting layers of porphyry. At lower levels these are an early, outer layer of dacite; a younger, inner layer of dacite and labradorite-by-townite andesite; and a middle, still younger layer of autobreccia compositionally similar to the inner layer. Contacts between layers are complex and consist at lower levels of intermixed zones, suggesting that at these levels the early rocks were still molten when injected by the later rocks. At higher levels the younger rocks split into separate dikes, and contacts between younger and older rocks are sharp, suggesting that at these levels the older rocks had solidified before they were intruded by the younger rocks. The border complex is thought to have been cooled largely by expanding gases which were released from the batholith and which escaped to the surface through this zone. The contact between the labradorite granodiorite and the inner layer is gradational locally, but in most places the granodiorite intrudes the inner layer, and the contact is sharp. Where the middle and inner layers of the complex are absent, the contact of granodiorite and dacite of the outer layer is gradational. Porphyry plugs which puncture the adjacent metamorphic rocks, although more siliceous than the rocks of the complex, consist largely of dacite and are petrographically indistinguishable from dacite of the complex. Intrusive breccias are of two types: one consists of rounded fragments of batholithic rocks in a “marble cake” mixture of calcic quartz diorite and white quartz monzonite and is confined to the core of the batholith; the other consists of fragments of batholithic rocks and gneiss in a matrix of varying proportions of igneous and finely comminuted materials and is confined to the porphyry plugs and gneiss. This second type seems to have been injected explosively, probably accompanying “second boiling” of the labradorite granodiorite. Plagioclases range from high- to low-temperature varieties. The transitional and high-temperature plagioclases are confined to the deuterically least-altered parts of the chilled margins. The low-temperature plagioclase occurs in the batholithic core and the complex of Hart Lake, and is thought to have inverted from an original high-temperature form because of long-continued existence at elevated, although subsolidus, temperatures, or because of the action of volatile constituents, or both. The batholith seems to have made room for itself by lifting its roof, and the complex of Hart Lake developed along the upfaulted eastern side.