Work on the Precambrian rocks of the Beartooth Mountains has been mainly in terrain dominated by gneiss. We undertook the current project in the belief that an intensive study of an area where the rocks perhaps were not as thoroughly reconstituted by metamorphic and igneous activity (as is the case farther east) and where metasedimentary rocks are exposed in substantial amounts might reveal a somewhat more detailed picture of Precambrian events than has emerged from earlier studies in the region. The North Snowy block constitutes such an area.
The northwestern part of the Beartooth Mountains is divisible into structural blocks separated by major faults, and the rocks therein are treated as seven formations composing the North Snowy Group. The formations, from oldest to youngest, are the Barney Creek Amphibolite, George Lake Marble, Jewel Quartzite, Davis Creek Schist, Mount Delano Gneiss, Mount Cowen Gneiss, and Falls Creek Gneiss. All are newly named and described herein.
The Barney Creek Amphibolite, George Lake Marble, Jewel Quartzite, and Davis Creek Schist are older than the Mount Delano Gneiss. Pb207/Pb206 ratios from zircons indicate that the age of the Mount Delano Gneiss is about 3,000 m.y., the Mount Cowen Gneiss is about 2,565 m.y., and K-Ar dates indicate that the age of the Falls Creek Gneiss is about 2,200 m.y.
The dominating structural feature in the North Snowy block is a fragmented nappe that extends for 22.5 km before disappearing beneath the Paleozoic sedimentary cover. Its axial surface strikes northeast and dips northwest, bisecting an amphibolite core with flanks made up of mirror-image repetitions of metasedimentary and gneiss units; throughout the nappe structure the main foliation parallels the axial surface.
Five sets of deformation structures were recognized in the field and designated D1 through D5. Folds in D1, the earliest structure for which evidence was recognized, trend northwesterly. These large to small passive-flow folds, characterized by isoclinal form, possess an axial-plane schistosity that is the major structural element deformed in younger events. D2 folds trend north, deform the earlier schistosity, and approach isoclinal form; their axial planes are parallel to the gross compositional layering. Deformation structures in D3 were divided into several sets (D3a to D3f). The rocks were essentially flat lying (that is, recumbent folds and nearly horizontal gross layering) during episodes D1 through D3. D4 folds have northeast trends and steep axial planes; they are of concentric character and overprint D3 and older structures. D5 folds are of diverse trend and are decidedly subordinate. They overprint, kink, and shear all earlier structures.
There is a general progression from more mafic to more silicic gneiss through an 800-m.y. time span: Mount Delano to Mount Cowen to Falls Creek.
Zircon morphology, relict igneous texture, rotated metamorphic xenoliths, and relict intrusive contacts support an origin for the gneisses of the North Snowy block by intrusion and consolidation followed by continuing metamorphism parallel to older lines (that is, synkinematic). The only rock attributable to granitization is the sheared gneiss unit, which apparently was formed through feldspathization of schist by late magmatic fluids emanating from tonalitic melt.
A study of zircon morphology follows. (1) The detrital zircons of the schists retained their rounded sedimentary form following metamorphism in the amphibolite or perhaps hornblende granulite facies. Two colors of detrital zircon, one rimming the other in part, suggest derivation from a complex terrain in which at least two “events” had occurred. (2) In paragneiss formed by feldspathization of schist (granitization), the zircon entirely retained its sedimentary character. No overgrowths or outgrowths were formed. Relict, rounded overgrowths exist only on detrital zircons and had formed in an earlier cycle. (3) The zircon in the orthogneisses retained its igneous character. Minor quantities of detrital zircon assimilated in the igneous gneisses retained their sedimentary character—no outgrowths, overgrowths, or partial refaceting have been observed. Some outgrowths occur on igneous zircons; they grew during the respective igneous crystallization phases. (4) The igneous character of the zircon suites in the several gneisses supports field and fabric data suggesting intrusive igneous origin. Migmatites appear to be subordinate, as are gneisses produced by feldspathization of schists.
Similarity of zircon morphology in gneisses throughout the Beartooth Mountains, as well as in the Little Belt Mountains and perhaps in all areas of basement rocks of southwestern Montana, generally indicate a predominantly igneous mode of origin, although metasomatic processes have operated in varying degrees.
Concepts regarding structural evolution of the Beartooth Mountains have changed. Our view, based on the multiplicity of data enumerated in the text, indicates that there were at least four bona fide orogenies widely separated in time, each differing in its grade of metamorphism and effect on the geometry of structures. The first three were accompanied by widespread granitic intrusion. The four orogenies are, from oldest to youngest, the Pine Creek (circa 3,000 m.y.), the Beartooth (circa 2,600 m.y.), and two unnamed orogenies (circa 2,200 m.y. and 1,700 m.y.). The latter events were followed consecutively by locally intense mylonitization (circa 1,176 m.y.), locally strong development of concentric upright folds (D4 or post-1,176 m.y.), and major wrench movements on west-northwest- and north-northeast-trending faults.
The basement blocks of the northwestern part of the Beartooth Mountains have had a long history of development. Most of them were outlined by shear zones at least as early as 1,700 m.y. ago; the Pine Creek Lake-Mount Cowen block was split by the Marten Peak reverse fault during D4 (post-1,176 m.y.); major wrench movements accentuated the westerly and northerly borders early in D5 (prediabase) and to some extent were further accentuated after these young diabases were intruded (late D5). We conclude that these predominantly wrench movements were all Precambrian.
Fractures of the area are grouped into four sets. Sets 1 (northwest) and 4 (northeast) are interpreted as extensional ac and be fractures, respectively, having formed in D5 but in part expressing stress systems involving northwest-southeast shortening of D3 to D4 time. Sets 2 and 3 formed also in D5 but represent conjugate shears; tension associated with this system was apparently largely taken up along sets 1 and 2.
Most of the major faults that had their origins in Precambrian time have been reactivated during Late Cretaceous and early Cenozoic time (Laramide orogeny). These later movements were apparently largely dip slip, the minimum stratigraphic displacements ranging from perhaps 900 to more than 1,800 m. Dip-slip movement apparently began as early as Late Cretaceous and seems to have ceased on most of the faults prior to volcanism during the middle Eocene. The Deep Creek fault, however, has continued to be active up to the Holocene as shown by scarplets in glacial material and alluvial fans.
