ABSTRACT

In the core of the Ruby Range 20 km east of Dillon, Montana, Precambrian (pre-Belt) metamorphic rocks have undergone metamorphism and recurrent Precambrian, Laramide, and post-Laramide tectonic activity. Exposure of the500 km² crystalline core of therange is a result of: 1) Laramide (Cretaceous to Eocene) arching of the cratonic shelf and attendant upthrusting and erosion of the Rocky Mountain foreland east of the thrust belt, and 2) subsequent mid-late Cenozoic block faulting. The Precambrian history of the range is summarized in this paper.

Protoliths for the metamorphics are (arranged in apparent stratigraphic order, oldest to youngest): impure sandstone and shale (pre-Cherry Creek rocks), a monotonous sequence of illite (?) -quartz mudstones or siltstones (Dillon quartzofeldspathic gneiss), and a shallow marine sequence of sandstone, pelite, carbonates, and iron formation interlayered with basic flows, sills, or tuffs (Cherry Creek Group).

High grade metamorphism at conditions above the "second sillimanite" isograd, contemporaneous with isoclinal folding at 2750 m.y b.p., has produced diopside and forsterite ortremolite in calcareous rocks, sillimanite-K feldspar-almandine in meta-pelite, anthophyllite in magnesian assemblages, hornblende and andesine in basic rocks, and anatectic migmatites This event correlates with the "Beartooth" orogeny farther east

For the Kelly area in the northern Ruby Range, granulite facies (orthopyroxene zone) metamorphism is indicated; various major element geothermometers yield a consensus peak temperature estimate of 745 + 50°C for this regional episode. For the Carter Creek area roughly 20 km to the south, it is somewhat lower temperature (upper-amphibohte facies; 675 + 45°C) for this thermal peak. Anomalously low estimates of temperature provided by oxygen isotopicd ata of coexisting quartz and magnetite in metamorphosed iron formations from the two areas probably result from retrograde or later low-grade metamorphic effects (diffusion or recrystallization).

Minor basic dikes, now represented by discordant amphibolites, were emplaced across migmatitic fold patterns prior to the end of the 2750 m.y.b.p metamorphism Garnet-bearing granitic dikes found locally by C. Hedge in the Ruby Range yield a Rb-Sr whole rock age of 1850 m.y.b.p The somewhat younger, well-known 1600 m y K-Ar date of B. Ciletti (1 966) actually may represent a cooling age for this 1 800 m y. igneous/metamorphic event, wherein argon is retained by muscovite and biotite below 250°C.

Two or possibly three separable mesoscopic and macroscopic Archean fold generations are documented in the central Ruby Range. However, four fold generations are encountered outside the middle third of the range, both north of the McHessor Creek fault and roughly south of theStone Creek fault This discrepancy in fold generations may result from post-2750 m.y, pre-diabase dike (1450 m y.) emplacement of nappes of Cherry Creekrocks nearCarterCreekand east fo the Kephart fault.

F₁ flexural flow and passive flow folds accompanying high-grade metamorphism are isoclinal, similar (or concentric in more competent layers), locally intrafolial and rootless, and display axial plane schistosity. The latter schistosity may be superimposed in fold noses on an earlier schistosity, which parallels compositional layering Transposition of compositional layering associated with F₁ deformation is nearly complete in some intrafolial and highly attenuated mesoscopic folds. Evidence for transposition, which clearly would obscure the regional metamorphic sequence relationships, is generally more abundant north of Hinch Creek (northeastern Ruby Range), suggesting the earliest recognized deformation may be more intense and transposition more complete south of Hinch Creek. Nonetheless, we believe the Cherry Creek rocks structurally overlie the Dillon gneiss.

The dominant northeast trend to foliation in the western foothills of the central and southern Ruby Range is the result of northwest dips to axial planes of macroscopic F₁ folds. At least locally, prior to subsequent Archean folding, F₁ isoclines were approximately recumbent.

F₂ tight to isoclinal, mesoscopic folds deform F₁ axial planes, as seen in a few exposures in the northern and southern Ruby Range. A well-developed macroscopic F₂ fold occurs along Timber Creek, but such examples are not common.

In the northern part of the study area, mesoscopic F₃ tight to gentle, concentric (rarely similar) folds with a spaced cleavage and F₄ open to gentle, concentric folds have geometric counterparts in the southern Ruby Range. Separate F₃ and F₄ folds in the central Ruby Range cannot be easily distinguished geometrically or by orientation, and post-F₁ folds of all scales are flexures and gentle to closed, concentric folds The ma|or warping of metamorphic foliation in the range, the Treasure Chest antiform, is probably an F₄ fold.

Autochthonous F₁ and post-F₁ folds are coaxial (but not coplanar) only in the central Ruby Range. F₃ and F₄ mesoscopic folds are distributed throughout the northern part of the area, and refolding relationships of macroscopic F₃ and F₄ folds are well-developed east of the Kephart fault. There flexural mechanism on a large scale may have been facilitated by the presence of anisotropic interlayering of metasedimentary lithologies.

One model consistent with the geometry and distribution of structures in the northern Ruby Range is the emplacement of at least two nappes, originally as large macroscopic folds Final arching movements upon emplacement caused the youngest macroscopic fold phase. Analogously in the southern Ruby Range, a youngest Archean folding deformation involves large-scale, disharmonic folding or nappe emplacement. Near Carter Creek, the Dillon synform represents refolding along north-plunging axis of a set of east-plunging isoclines, which were translated westward from a source in the central Ruby Range, apparently between the Stone Creek and Kephart faults. Petrographic and chemical analytical evidence of retrograde formation of cordierite in magnesian schist and meta-pelite, at amphibolite-facies temperatures but several kilobars lower pressure, is consistent with regional upwarping and decompression of the central Ruby Range. The prominent northwest-trending fault system transecting the metamorphic foliation of the range may have formed and experienced its first movements at this time (Certainly it was reactivated in late Precambrian and late Cenozoic times). Whether the cordiente formed at the time the nappes were emplaced is unknown.

At 1450 m y.b p., in late Precambrian time, northwest-trending diabases intruded a prominent weakness direction, which we interpret as perpendicular to regional rifting and normal faulting Greenschist metamorphism locally developed probably is of several ages. Hydrothermal alteration of dolomitic marbles formed talc of economic interest. Talc mineralization is generally controlled by stratigraphy, but specifically site-controlled by structure.