Intrusive rocks northeast of Steamboat Springs, Park Range, Colorado

Abstract

Major Precambrian and minor Tertiary intrusive rocks northeast of Steamboat Springs in the Park Range between 40°30' and 40°45' N. lat. are described and compared with related rocks elsewhere in Colorado and Wyoming. The Precambrian intrusives were emplaced in a sequence of high-grade interlayered felsic gneisses, amphibolites, and pelitic schists of sedimentary and volcanic origin. These rocks are cut by a major northeast-trending Precambrian shear zone where mainly left lateral movement of 1/ 2 to 1 mile is certain. Cumulative movement of many miles is possible. The Precambrian intrusives consist of a batholith, the Mount Ethel pluton, a smaller Buffalo Pass pluton, and small dikes or lenses of fine-grained porphyry, pegmatites, and ultramafics. The Mount Ethel pluton is an oval shaped body 7 miles wide by about 40 miles long (shown by geophysical data to extend beneath younger sediments in North Park). Outer batholithic contacts are sharp and dip steeply outward at about 85°. Five mappable internal variants consist, in order of decreasing age, of granodiorite, quartz monzonite porphyry of Rocky Peak, quartz monzonite of Roxy Ann Lake, granite and quartz monzonite, and. leucogranite. Internal contacts between these plutonic variants are sharp, and evidence of liquid-solid relationships abounds; despite this, all rocks except the granodiorite contribute to an Rb-Sr whole-rock isochron indicating emplacement about 1.4 b.y. (billion years) ago. The most important variants volumetrically are: the quartz monzonite porphyry of Rocky Peak, which forms an irregular 2-mile-thick carapace or mapped band around the west edge of the pluton and is lithologically similar to nearby Sherman Granite, and the quartz monzonite of Roxy Ann Lake, which forms most of the rest of the pluton and is lithologically similar to Silver Plume Granite. An apparent Sherman -Silver Plume dichotomy with similar rock types and similar relative ages is noted throughout Colorado plutons of that age. The Buffalo Pass pluton consists of the quartz monzonite and gra- nodiorite augen gneiss of Buffalo Mountain and equigranular quartz monzonite gneiss. Internal contacts are not exposed. These rocks contribute to an Rb-Sr whole-rock isochron indicating syntectonic emplacement 1.7-1.8 b.y. ago, essentially the same as the metamorphism of the felsic gneiss wallrocks in the area of this report, and of rocks of Boulder Creek age elsewhere in Colorado. The fine-grained porphyry dikes cut the Buffalo Pass pluton, the ultramafics, and some pegmatites. The dikes are within the age range of the Mount Ethel pluton and are older than the mylonite and shear zones. They occur in both an older northwest-trending and a somewhat younger northeast-trending set but do not appear to change compositionally from one set to the other. Regional considerations indicate that they were emplaced between about 1.1 and 1.5 b.y. ago, a time when intermediate to mafic dikes were commonly emplaced throughout Colorado, Wyoming, and southwestern Montana. The pegmatite and ultramafic bodies are not dated directly, but clustering of many pegmatites outside the contacts of the Mount Ethel pluton may indicate a genetic relation of the pegmatites to the Mount Ethel rocks. Fluorite is a common accessory mineral in the rocks of the Mount Ethel pluton; it has not been observed in this area in the petrographically similar rocks of the Buffalo Pass pluton. Fluorite was precipitated most abundantly from the Precambrian magma that formed the quartz monzonite of Roxy Ann Lake. In 70 percent of these rocks fluorite is observed in amounts as great as 2 percent and is successively less abundant in both older and younger plutonic phases. Textural evidence indicates that, although most fluorite is intergrown with and contemporaneous with other magmatic minerals, some fluorite is associated with alteration minerals in a manner demonstrating its mobility since its initial deposition.