Precambrian geology of the Aero Lakes area, Beartooth Mountains, Montana
Bahr, Charles H.
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The Aero Lakes area comprises 31.6 square kilometers of the south central Beartooth Mountains located approximately nine kilometers north-northeast of Cooke City, Montana. Precambrian rocks differentiated into four general units: (1) The granitic gneiss complex composed of rocks with varying amounts of potassium feldspar, quartz, plagioclase, and a mafic constituent. Crosscutting relations indicate an intrusive sequence with hornblende diorite the oldest followed in order by gray gneiss, pink granitic gneiss, and tonalite. (2) The migmatite complex composed of schollen, agmatites, and banded migmatites. (3) The amphibolites including both non-lineated and lineated orthoamphibolites which may be remnants of basic igneous intrusions which include the diabase dikes that vary in length from 15 to over 2400 meters and as a large irregular metadiabase pluton which cuts the area north to south. This pluton contains oblique shear zones which have produced layered myIonites from originally homogeneous rock during a post-intrusive deformational event. Folding of the gneissic layering in the nearby granitic rock and migmatitic banding may indicate syntectonic intrusion of the pluton into still-somewhat ductile country rock. (4) The metasediments composed of pelitic schists with inclusions of ironstone and biotite quartzite. Deposition of the original sediments is considered the first detectable geologic event in the area. M₁ was a regional metamorphism of this sequence to the sillimanite grade. Synchronous folding (F₁) resulted in the isoclinal folds found throughout these metasediments. Similarities of rock types in the Aero Lakes area with those in other portions of the southern Beartooth Mountains proven to be part of a composite batholith coupled with establishment of the relative mobility of the leucosome in the migmatites indicates that the granitic rocks were derived from a granitic magma and that the magma was intruded synkinematically into the metamorphosed sedimentary sequence. A second metamorphism (M₂) was confined to the metasedimentary rafts during emplacement of the batholith causing the breakdown of hornblende and the growth of symplectites. During the successive events of emplacement of the granitic stocks the S₁ foliation was developed by flow accompanying deformation. Assimilation of country rock created the folded biotite and hornblende-microcline migmatite. Intrusion of the mafic dikes was the final Precambrian event. Cross-cutting relations indicate at least three episodes of emplacement.