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Belt Series in the Region Around Snow Peak and Mallard Peak, Idaho

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Belt Series in the Region Around Snow Peak and Mallard Peak, Idaho Belt Series in the Region Around Snow Peak and Mallard Peak, Idaho GEOLOGICAL SURVEY PROFESSIONAL PAPER 344-E Belt Series in the Region Around Snow Peak and Mallard Peak, Idaho By ANNA HIETANEN METAM ORPHIC AND IGNEOUS ROCKS ALONG THE NORTHWEST BORDER ZONE OF THE IDAHO BATHOLITH GEOLOGICAL SURVEY PROFESSIONAL PAPER 344-E Petrologic and structural study of a part of the northwestern contact aureole of the Idaho batholith UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract __________________________________________ _ E1 Metamorphic rocks-Continued Introduction ______________________________________ _ 1 Petrographic description-Continued Metamorphic rocks ________________________________ _ 2 Structure _________________________________ _ E21 Stratigraphy and correlation ____________________ _ 2 FauUs ________________________________ _ 21 Prichard Formation ________________________ _ 3 North west-trending faults ____ - _____ _ 22 Ravalli Group ____________________________ _ 3 North-trending faults ______________ _ 22 Wallace Formation ________________________ _ 6 Folding and lineation __________________ _ 23 Petrographic description ________________________ _ 7 Two sets of folds __________________ _ 23 Prichard Formation ________________________ _ 7 Influence of the grade of metamor- Schist ________________________________ _ 7 phism __________________________ _ 24 QuartzUe _____________________________ _ 8 Influence of the type of material Burke Formation __________________________ _ 8 folded __________________________ _ 25 Quartzite unit _________________________ _ 8 Folds related to faults ______________ _ 25 Schist unit ____________________________ _ 9 Differences in trends between indivi- Revett Formation _________________________ _ 9 dual fault blocks _________________ _ 26 St. Regis Formation _______________________ _ 9 Foliation, cleavage, and fracture cleavage __ 26 Wallace Formation _________________________ _ 10 Igneous rocks _____________________________________ _ 27 Quartzite unit _________________________ _ 10 Plutonic rocks _________________________________ _ 27 Schist units ___________________________ _ 11 Quartz diorite _____________________________ _ 27 Biotite zone _______________________ _ 12 Quartz monzonite and granite _______________ _ 28 Garnet zone ______________________ _ 12 Pegmatite and quartz veins _________________ _ 28 Staurolite zone ____________________ _ 13 Hypabyssal rocks ______________________________ _ 29 Kyanite-staurolite zone _____________ _ 13 Gabbroic sills and dikes ____________________ - 29 Kyanite zone ______________________ _ 15 Dioritic dikes _____________________________ - 30 Sillimanite-kyanite zone ____________ _ 16 Quartz monzonitic dikes and sills ____________ _ 30 Sillimanite zone ___________________ _ 16 Granitic dikes _______________________ - _____ - 31 Metamorphic reactions and metamorphic fa- Conclusions _______________________________________ _ 32 cies ____________________________________ _ 16 References ________________________________________ _ 33 ILLUSTRATIONS Page PLATE 1. Geologic map of the Mallard Peak area, Idaho __________________________________________________ In pocket 2. Geologic map of the Snow Peak area, Idaho ____________________________________________________ In pocket FIGURE 1. Index map of northern Idaho_____________________________________________________________________ E2 2. Generalized stratigraphic section of the Belt Series near Snow Peak ____________________ --------------- 4 3. Photomicrograph of quartzite of the Burke Formation_______________________________________________ 9 4-6. Camera lucida drawings of: 4. Biotite porphyroblasts in biotite zone on West Sister__________________________________________ 12 5. Garnet in garnet zone on Middle Sister_____________________________________________________ 12 6. A postkinematic staurolite in garnet-mica schist of the staurolite zone north of Junction Peak ____ - 13 7. Photographs showing pseudomorphs after staurolite in staurolite-kyanite zone__________________________ 14 8. Photograph of staurolite schist a mile southeast of Bathtub Mountain___________________________ _ _ _ _ _ _ 15 9. A, Photograph of herringbone structure in pseudomorph after a staurolite twin. B, Drawing showing pseudo- morph after staurolite and two episodes of recrystallization of garnet________________________________ 16 10. Diagram showing stability ranges of the index minerals______________________________________________ 18 11. A tentative pressure-temperature diagram showing a possible pressure-temperature gradient of recrystalli- zation_______________________________________________________________________________________ 20 12. Photograph of two sets of folds in garnet-mica schist of the Prichard Formation________________________ 23 13. Drawing of two sets of small folds in th_ schist of the St. Regis Formation_____________________________ 24 14. Drawing of folded quartzite layers in intensely wrinkled schist of the Prichard and St. Regis Formations__ 25 15. Photomicrograph of garnet-mica schist showing bedding and transecting foliation_______________________ 26 16. Photograph of quartz vein in actinolite-diopside granofels southwest of Dismal Lake____________________ 28 17-19. Photomicrographs: 17. Gabbroic rock in Wishards sill______________________________________________________________ 29 18. Quartz monzonitic dike rock________________________________________________________________ 30 19. Spherules of granophyric intergrowth of quartz and feldspars in a granitic dike____________________ 31 m IV CONTENTS TABLES Page TABLES 1, 2. Chemical composition in weight and ionic percentages and molecular norm and mode: 1. Staurolite schist from locality 2096----------------------------------------------------------- E15 2. Granitic dike 2155 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 31 METAMORPHIC AND IGNEOUS ROCKS ALONG THE NORTHWEST BORDER ZONE OF THE IDAHO BATHOLITH BELT SERIES IN THE REGION AROUND SNOW PEAK AND MALLARD PEAK, IDAHO By ANNA HIETANEN ABSTRACT their pressure-temperature fields can be determined. The Snow Peak and Mallard Peak areas provide a far elearer Biotite, garnet, staurolite, kyanite, ·and sillimanite iso­ and more complete account of various geologic events in the grads show a uniform increase in the grade of meta­ northwestern contact aureole of the Idaho batholith than do the morphism southward toward the batholith. Moreover, Headquarters, Boehls Butte, and Elk River-Clarkia areas studied the area offers excellent opportunity for a. study of the previously (Hietanen 1962a, 1963a, b). The normal stratigraphic sequence of the five lowest formations of the Belt Series of Pre­ style of folding in layers of different bulk compositions cambrian age is well exposed in many localities. This sequence under various metamorphic conditions. allows correlation with known equivalents farther north with The area studied comprises 400 square miles in the greater certainty than could be done previously. All metamorphic southeastern part of Shoshone County and in the north­ zones are represented in the mapped area described in this report central part of Clearwater County (fig. 1). It joins the starting with biotite zone in the north and followed succes.sively by zones of garnet, staurolite, staurolite-kyanite, kyanite, kya­ Bungalow area (Hietanen, 1963c) to the south and the nite-sillimanite, and sillimanite-muscovite toward the south. Two Boehls Butte quadrangle and vicinity (Hietanen, episodes of recrystallization are ·evident: a synkinematic and a 1963a) to the west. The southern part of the mapped later postkinematic one during which the temperature was some­ area (pl. 1) , called the Mallard Peak area. in this report, what higher. is bounded by long 115°25' and 115°40' W. and by lat Many of the structural features, ·such as the style of folding and orientation :of foliation and cleavage, change with the grade 46°45' and 47°00' N. This area is covered by the follow­ of metamorphism and With the material folded. Structure is ing 7-l;2-minute topographic quadrangle sheets: the complicated by two to three sets of folds, whose axes intersect eastern part of Buzzard Roost, the Mallard Peak, the at angles of 60°-80°. The folds range in size from minute wrinkles western part of Pole Mountain, the Sheep Mountain, to large folds having wave lengths of several miles. Several high-angle faults--most trending north, ·some trending north­ and the western part of TheNub. west-interrupt the stratigraphic and structural continuity. The northern part of the mapped area (pl. 2), which lies between the St. Joe River and lat 47°00', is called INTRODUCTION the Snow Peak area after a prominent mountain that This report is pa.rt of a series of petrologic ·and struc­ rises 5,000 feet above the canyon of the Little North tural studies of the northwestern contact aureole of the Fork of the Clearwater River in the southwest. The Idaho batholith. The mapped area (pis. 1, 2) is a 12- easternmost part o~ this area is covered by the Simmons mile-wide belt through the metamorphic contact aureole Peak 30-minute quadrangle map. of the batholith and has several unique features tha,t The topography
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