Rock One

Rock, from the geologic map

Silver Plume Granite — intrusive igneous (plutonic). Mesoproterozoic, about 1,600 to 1,000 million years old.

Light- to medium-gray, grayish-orange, orange-pink, or red-purple monzogranite to syenogranite that contains characteristic tabular microcline phenocrysts. Equigranular matrix is locally fine, medium, or coarse grained, and proportion of phenocrysts is variable throughout the region; flow alignment strong in rocks with abundant phenocrysts. Average mineral composition is similar regardless of texture, and contains 30 percent quartz, 25-28 percent oligoclase, 32-35 percent microcline, 7 percent biotite, trace amounts of magmatic garnet and sillimanite, 1 percent subsolidus muscovite, and minor apatite, monazite, and magnetite. Irregular-shaped outcrop masses in east and central parts of map area form northwest part of the composite Longs Peak-St. Vrain batholith, which extends south to Ward and east to Lyons, Colorado, over an area of more than 1,200 km2. Granite bodies forming the north, northeast, and south parts of batholith generally have steep walls, are sharply discordant, and are characterized by gradual transitions between internal textural varieties. In contrast, granite bodies forming the southwest part of batholith (along Continental Divide) were emplaced as anastomosing, flat-lying sheets that are largely concordant with foliation in the surrounding, gently dipping metamorphic rocks. Silver Plume exposed in the Glacier Gorge-Chiefs Head Peak-Longs Peak-Mount Meeker-Estes Cone region consists of sharply bounded granite sheets that vary in color, texture, or composition, and that are locally separated by thin sheets of metamorphic rock. Granite exposed west of Wild Basin is compositionally more homogeneous, but is surrounded by zones of metamorphic rocks that contain 10-75 percent intrusive sheets or irregular bodies of granite (indicated by pattern on older rocks). Age of the Silver Plume Granite in the Longs Peak-St. Vrain batholith is 1,420+/-25 Ma (Peterman and others, 1968, p. 2289). Several rock types that are believed to be related to the Silver Plume Granite have been mapped in local areas. GRI Source Map ID 141 (I-1973).

This is the map unit under this crag's map point, read from the 1:100,000 sheet (National Park Service Geologic Resources Evaluation program, 2006, National Park Service). The point can be a few hundred metres from the rock, a cliff often stands on the line between two units, and at that scale a line on the map is about 100 m wide on the ground, so the wall itself may be the neighbouring unit. The rock named at the top of this page comes from the public geology of this area; this card is the map's answer. Every crag on this unit: Silver Plume Granite. Geology data: Macrostrat (CC BY 4.0), serving USGS and Colorado Geological Survey maps.