Eocene Deformation at Tubbs Hill of Coeur d’Alene, Idaho ... · N By Niki E. Wintzer1 and Reed S....
1
N By Niki E. Wintzer 1 and Reed S. Lewis 2 1 U.S. Geological Survey, 904 W. Riverside Ave., Spokane, WA 99201; 2 Idaho Geological Survey, Morrill Hall, Third Floor, University of Idaho, ID 83844 Acknowledgments Sincere thanks to Jorge Vazquez and Marsha Lidzbarski of the Stanford-USGS SHRIMP laboratory for their indispensable guidance and assistance. The authors thank Da Wang for contribung his rock sample (14DW09). Satellite image of Coeur d’Alene, Idaho is from ESRI World Imagery. Eocene Deformaon at Tubbs Hill of Coeur d’Alene, Idaho, Southeast Priest River Complex Tubbs Hill References Conclusions Armstrong, R.L., Parrish, R.R., van der Heyden, Peter, Reynolds, S.J., and Rehrigh, W.A., 1987, Rb-Sr and U-Pb geochronology of the Priest River metamorphic complex— Precambrian X basement and its Mesozoic-Cenozoic plutonic-metamorphic overprint, northeastern Washington and northern Idaho: Washington Division of Geology and Earth Resources Bullen 77, p. 15–40. Doughty, P.T., and Price, R.A., 1999, Tectonic evoluon of the Priest River complex, northern Idaho and Washington—A reappraisal of the Newport fault with insights on metamorphic core complex formaon: Tectonics, v. 18, p. 375–393. Reid, R.R., Wavra, Craig, Fleck, Robert, Geist, Dennis, and Knowles, Charles, 1993, GeoNote 25 Walking field trip in the Tubbs Hill mylonites, Coeur d’Alene, Idaho: Idaho Geological Survey, 2 p. Rhodes, B.P., and Hyndman, D.W., 1984, Kinemacs of mylonites in the Priest River “metamorphic core complex,” northern Idaho and northeastern Washington: Canadian Journal of Earth Sciences, v. 21, p. 1161–1170. Rhodes, B.P., Hyndman, D.W., and Harms, T.A., 1989, Geology of the southern Priest River Complex and the Newport Fault, in Joseph, N.L., Evans, J.G., Kiilsgaard, T.H., Korosec, M.A., Logan, R.L., Miller, F.K., Phillips, W.M., Powell, R.E., Schasse, H.W., Schuster, J.E., Waggoner, S.Z., Walsh, T.J., and Whipple, J.W., Geologic guidebook for Washington and adjacent areas: Department of Natural Resources, Division of Geology and Earth Resources Informaon Circular 86, 369 p. Stevens, L.M., 2015, Pressure-temperature-me constraints on metamorphism, anatexis, magmasm, and exhumaon in the Priest River complex, northern Idaho, and comparison with geodynamic models: Missoula, University of Montana, Ph.D. dissertaon, 205 p., 33 figs. • Tubbs Hill dike was emplaced circa 50 Ma and emplacement was likely iniated by regional upliſt and exhumaon. • Some zircon cores found in the Tubbs Hill dike were scavenged from underlying rocks of older and variable age—one of which (1579±41) is within the North American magmac gap age range. • The deformed Silver Point pluton (50.13±0.02 Ma) in the northern PRC is the same age as the Tubbs Hill dike (50.1±3.5 and 49.5±1.1 Ma) in the southeastern PRC. • Deformaon connued in the southeastern poron of the PRC unl at least 50 Ma. Figure 9. USGS-Stanford Sensive High-Resoluon Ion Microprobe (SHRIMP) located on the Stanford University campus in Palo Alto, California. Image from hps://shrimprg.stanford.edu/. Figure 8. Two grain mounts. Leſt is an epoxy mount like prepared for this study, and right is an irridium mount. Image from hps://shrimprg.stanford.edu/sample-types -and-sample-preparaon. Figure 7. CL image of a representave zircon grain from 15NW1THD showing a bright core and an oscillatory zoned rim. Figure 6. CL image of a representave zircon grain from 14DW09 showing connuous oscillatory zoning from core to rim. Figure 5. Photograph of zircon grains from sample 15NW1THD taken through the ocular of a microscrope. Methods Rock samples were collected at two separate Tubbs Hill dike localies on the northwest side of Tubbs Hill (Fig. 3 & satellite image above). Mineral grains were separated with a crusher, mill, and water table at the Univerisity of Idaho rock preparaon laboratory. Zircon separaon was completed at Washington State University using a 350 µm sieve, a magnec separator, and methylene iodide (MeI). Near pure zircon fracons (Fig. 5) were dried and placed in folded weigh paper and then in small plasc bags to protect against grain loss. Remaining grain preparaon and analyses were conduced at the Stanford-USGS Sensive High-Resoluon Ion Microprobe (SHRIMP) Laboratory. Zircon grains were handpicked (Fig. 5), mounted in epoxy, allowed to dry overnight, cut, sanded to expose grain interiors, polished, and rinsed. Cathodluminescent (CL) images were taken of the grain mount and used as a guide for analysis spot selecon (Figs. 6 & 7). The grain mount (Fig. 8) was loaded into the SHRIMP (Fig. 9) and exact analysis spots were selected. The two samples analyzed showed similar but disnct zircon zoning. For sample 14DW09, at least 15 rim spots with oscillatory zoning were selected; only a couple grains showed disnct cores (Fig. 6), so no cores were analyzed (a minimum of 5 cores are needed to produce a robust data set). For 15NW1THD, 15 rim spots with oscillatory zoning were selected as well as disnctly zoned cores (Fig. 7). Many of these cores appeared brighter in CL images, which is due to higher lead (Pb) content. Results Discussion 14DW09 15NW1THD Two samples from the same dike, although variably deformed, yielded similar dates well within error of each other providing the dike age of circa 50 Ma (Figs. 10 & 12). This age offers the southeastern most constraint on deformation within the PRC. Stevens (2015) determined that Eocene magmatism within the PRC was a response to uplift and exhumation, and that is likely the case with the Tubbs Hill dike. That also means this dike was emplaced synkinematically like the Silver Point pluton, which shows chlorite microbreccia overprinting mylonitic fabrics (Fig. 1; Rhodes and others, 1989) near the en echelon step over in the Newport Fault (Doughty and Price, 1999). Some zircon cores from 15NW1THD yielded significantly older igneous dates than their rims (Fig. 14; 1295±25 to 2346±176) suggesting the cores were scavenged from older rocks below Tubbs Hill. One zircon core yielded a date of 1579±41 (MSWD=0.58) that is within the North American Magmatic Gap (1610-1440 Ma), which is an interval of geologic time not widely represented in the region. 60 55 50 45 40 65 Date=49.5±1.1 (MSWD=1.59) 40 60 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.006 0.007 0.008 0.009 0.010 207 Pb/ 235 U 206 Pb/ 238 U 50 206 Pb/ 238 U 207 Pb/ 235 U 0.010 0.009 0.008 0.007 0.006 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 40 60 50 Figure 11. Convenonal Wetherill concordia diagram showing U-Pb SHRIMP data distribuon of the spot analyses with 2 sigma error ellipses. Figure 12. 206 Pb/ 238 U dates with weighted mean date for the more deformed sample of the Tubbs Hill dike, 15NW1THD. Figure 13. Convenonal Wetherill concordia diagram showing U-Pb SHRIMP data distribuon of the spot analyses with 2 sigma error ellipses. 40 35 45 50 55 60 65 Date=50.1±3.5 (MSWD=14.6) 206 Pb/ 238 U Date (Ma) Spot Analysis 0.55 0.45 0.25 0.0 0 3 9 11 15 2600 2400 2000 1600 207 Pb/ 235 U 206 Pb/ 238 U 1200 0.35 5 7 13 Dates with error ranges 1295±25 1579±41 1652±87 1693±31 2346±176 Figure 14. Convenonal Wetherill concordia diagram showing various zircon core dates from the U-Pb SHRIMP data distribuon of the spot analyses with 2 sigma error ellipses. 206 Pb/ 238 U Date (Ma) Figure 10. 206 Pb/ 238 U dates with weighted mean date for less deformed sample of the Tubbs Hill dike, 14DW09. Spot Analysis Figure 4. Geologic map of Tubbs Hills in Coeur d’Alene, Idaho showing rock sample locaons (Reid and others, 1993). 15NW1THD 14DW09 14DW09 15NW1THD Geologic Seng Spokane Dome Mylonite Zone Eocene pluton Cretaceous pluton Mesoproterozoic lower Belt-Purcell Supergroup Mesoproterozoic Hauser Lake gneiss Archean amphibolite and Paleoproterozoic granic gneiss Sample localion ID WA MAP LOCATION 20 km N 10 mi Figure 2. Geologic map of the Priest River Complex showing mylonite zone extent and relevant rock ages (modified from Stevens, 2015). EXPLANATION Anformal trace Normal fault CANADA U.S.A. Creston Elmira Lake Pend Oreille Priest Lake Kaniksu Batholith Wrencoe Pluton NEWPORT FAULT Priest River Newport Spirit Lake Rathdrum Coeur d’Alene Lake WASHINGTON IDAHO Silver Point Pluton Spokane Granite and Newman Lake Gneiss Rathdrum Granite Spokane Pend Oreille River Spokane Dome SELKIRK CREST Kaniksu Batholith Coeur d’Alene Rathdrum NEWPORT FAULT Priest River Sandpoint Newport ~49.8 Ma ~49.8 Ma SELKIRK CREST ~50 Ma 50.13±0.02 Ma P u rce l l Tre n ch Fa ult Hope F ault Purcell Trench Fault Figure 4 TX NM AZ UT CO CA NV OR WA MT WY Canada U.S.A. Craton Margin ( 87 Sr/ 86 Sr=0.706) PRIEST RIVER CLEARWATER CLEARWATER BITTERROOT BITTERROOT OKANOGAN GRAND FORKS OKANOGAN GRAND FORKS PRIEST RIVER Sevier Thrust Front Sevier Thrust Front Figure 1. Map of the western United States showing distribuon of metamorphic core complexes (modified from Stevens, 2015). EXPLANATION Metamorphic core complex Stateline or border Thrust fault Geochemical boundary N 150 mi 250 km The Priest River Complex (PRC) is one of many core complexes along western North America (Fig. 1) comprised of mid-crustal metamorphic rocks that were upliſted and exposed at Earth’s surface giving geologists access to otherwise unreachable rocks. Metamorphosed igneous and sedimentary rocks of the Hauser Lake Gneiss, Selkirk igneous complex of the Kaniksu Batholith, the Spokane Granite, Silver Point Pluton, Wrencoe Pluton, and the Tubbs Hill rocks (Fig. 2) make up most of the PRC. Eastward displacement occurred along the Purcell Trench Detachment Fault in conjuncon with regional upliſt. Overlying Belt-Purcell Supergroup supracrustal rocks were eroded including some Hauser Lake Gneiss, which is an amphibolite-facies metamorphosed lower Belt-Purcell sedimentary unit (Fig. 3). The Spokane Dome Mylonite Zone is defined by mylonic fabrics that are collecvely bowed up into an elongate dome that trends NNE and plunges to the north, which projects parally below the Newport Fault (Fig. 2; Rhodes and others, 1989). Ubiquitous lineaons throughout the enre Spokane Dome Mylonite Zone trend generally ENE, plunge moderately, and are defined by elongate mineral grains (Rhodes and Hyndman, 1984; Rhodes and others, 1989). Lineaons are interpreted to represent the eastward movement along the Purcell Trench Fault, which predates the doming of the mylonite zone (Rhodes and Hyndman, 1984). Research about the PRC has been published for over 30 years (e.g.: Rhodes and Hyndman, 1984; Armstrong and others, 1987; Doughty and Price, 1999), and a recent study of the PRC (Stevens, 2015) using advanced geochronologic techniques revealed discrete ages (Cretaceous and Eocene) and magma sources for many of the plutonic bodies within the PRC, but the study did not include the southeastern most poron of the PRC—Tubbs Hill. Comprised chiefly of protomylonic meta-igneous and meta-sedimentary rocks (Fig. 4; Reid and others, 1993), Tubbs Hill also includes a late dike that crosscuts the enre hill and is variably deformed. Abstract Sensive High-Resoluon Ion Microprobe analyses of igneous rims on zircons from deformed (mylonic) granite dike rocks of Tubbs Hill, in downtown Coeur d’Alene, yielded U-Pb dates of 50.1±3.5 Ma and 49.5±1.1 Ma. The dike strikes northwest and is comprised of porphyric biote granodiorite with feldspar phenocrysts 0.5-0.75 cm in length. These robust dates provide a constraint on mylonic deformaon within the southeastern part of the Priest River metamorphic complex (PRC). An Eocene igneous age from this Tubbs Hill dike indicates coeval emplacement with Eocene plutonic suites elsewhere in the PRC, such as the locally deformed Silver Point pluton quartz monzonite dated at 50.13±0.02 Ma (Stevens, 2015). The northern most mylonic deformaon within the PRC, 66 kilometers north of Tubbs Hill is constrained by a recent suite of monazite and zircon U-Pb dates of a leucocrac dike indicang crystallizaon was circa 49.8 Ma (Stevens, 2015). Mylonic deformaon in the PRC appears to be a similar age in both the north and south. Aʹ A A Figure 3. Cross secon through Tubbs Hill region showing eroded fault surface, extensive basement, and offset along the sub-horizontal Purcell Trench Detachment Fault. Tubbs Hill Cougar Bay Highway 95 I-90 Eroded Fault Eroded Fault 0 4,000 feet 2x vertical exaggeration 3000 2000 1000 sea level WEST EAST Elevaon in feet Eocene plutonic rock Cretaceous plutonic rock Mesoproterozoic lower Belt-Purcell Supergroup Mesoproterozoic Hauser Lake gneiss Archean amphibolite and Paleoproterozoic granic gneiss EXPLANATION Normal fault Holocene glacial deposits Miocene Columbia River Basalt Proterozoic quartzite (Foliaon orientaon is 307,28)
Transcript of Eocene Deformation at Tubbs Hill of Coeur d’Alene, Idaho ... · N By Niki E. Wintzer1 and Reed S....
N
By Niki E. Wintzer1 and Reed S. Lewis2
1U.S. Geological Survey, 904 W. Riverside Ave., Spokane, WA 99201; 2Idaho Geological Survey, Morrill Hall, Third Floor, University of Idaho, ID 83844
AcknowledgmentsSincere thanks to Jorge Vazquez and Marsha Lidzbarski of the Stanford-USGS SHRIMP laboratory for their
indispensable guidance and assistance. The authors thank Da Wang for contributing his rock sample (14DW09). Satellite image of Coeur d’Alene, Idaho is from ESRI World Imagery.
Eocene Deformation at Tubbs Hill of Coeur d’Alene, Idaho, Southeast Priest River Complex
Tubbs Hill
References
Conclusions
Armstrong, R.L., Parrish, R.R., van der Heyden, Peter, Reynolds, S.J., and Rehrigh, W.A., 1987, Rb-Sr and U-Pb geochronology of the Priest River metamorphic complex— Precambrian X basement and its Mesozoic-Cenozoic plutonic-metamorphic overprint, northeastern Washington and northern Idaho: Washington Division of Geology and Earth Resources Bulletin 77, p. 15–40. Doughty, P.T., and Price, R.A., 1999, Tectonic evolution of the Priest River complex, northern Idaho and Washington—A reappraisal of the Newport fault with insights on metamorphic core complex formation: Tectonics, v. 18, p. 375–393.Reid, R.R., Wavra, Craig, Fleck, Robert, Geist, Dennis, and Knowles, Charles, 1993, GeoNote 25 Walking field trip in the Tubbs Hill mylonites, Coeur d’Alene, Idaho: Idaho Geological Survey, 2 p. Rhodes, B.P., and Hyndman, D.W., 1984, Kinematics of mylonites in the Priest River “metamorphic core complex,” northern Idaho and northeastern Washington: Canadian Journal of Earth Sciences, v. 21, p. 1161–1170.Rhodes, B.P., Hyndman, D.W., and Harms, T.A., 1989, Geology of the southern Priest River Complex and the Newport Fault, in Joseph, N.L., Evans, J.G., Kiilsgaard, T.H., Korosec, M.A., Logan, R.L., Miller, F.K., Phillips, W.M., Powell, R.E., Schasse, H.W., Schuster, J.E., Waggoner, S.Z., Walsh, T.J., and Whipple, J.W., Geologic guidebook for Washington and adjacent areas: Department of Natural Resources, Division of Geology and Earth Resources Information Circular 86, 369 p. Stevens, L.M., 2015, Pressure-temperature-time constraints on metamorphism, anatexis, magmatism, and exhumation in the Priest River complex, northern Idaho, and comparison with geodynamic models: Missoula, University of Montana, Ph.D. dissertation, 205 p., 33 figs.
• Tubbs Hill dike was emplaced circa 50 Ma and emplacement was likely initiated by regional uplift and exhumation.
• Some zircon cores found in the Tubbs Hill dike were scavenged from underlying rocks of older and variable age—one of which (1579±41) is within the North American magmatic gap age range.
• The deformed Silver Point pluton (50.13±0.02 Ma) in the northern PRC is the same age as the Tubbs Hill dike (50.1±3.5 and 49.5±1.1 Ma) in the southeastern PRC.
• Deformation continued in the southeastern portion of the PRC until at least 50 Ma.
Figure 9. USGS-Stanford Sensitive High-Resolution Ion Microprobe (SHRIMP) located on the Stanford University campus in Palo Alto, California. Image from https://shrimprg.stanford.edu/.
Figure 8. Two grain mounts. Left is an epoxy mount like prepared for this study, and right is an irridium mount. Image from https://shrimprg.stanford.edu/sample-types-and-sample-preparation.
Figure 7. CL image of a representative zircon grain from 15NW1THD showing a bright core and an oscillatory zoned rim.
Figure 6. CL image of a representative zircon grain from 14DW09 showing continuous oscillatory zoning from core to rim.
Figure 5. Photograph of zircon grains from sample 15NW1THD taken through the ocularof a microscrope.
MethodsRock samples were collected at two separate Tubbs Hill dike localities on the northwest side of Tubbs Hill (Fig. 3 & satellite image above). Mineral grains were separated with a crusher, mill, and water table at the Univerisity of Idaho rock preparation laboratory. Zircon separation was completed at Washington State University using a 350 µm sieve, a magnetic separator, and methylene iodide (MeI). Near pure zircon fractions (Fig. 5) were dried and placed in folded weigh paper and then in small plastic bags to protect against grain loss.
Remaining grain preparation and analyses were conduced at the Stanford-USGS Sensitive High-Resolution Ion Microprobe (SHRIMP) Laboratory. Zircon grains were handpicked (Fig. 5), mounted in epoxy, allowed to dry overnight, cut, sanded to expose grain interiors, polished, and rinsed. Cathodluminescent (CL) images were taken of the grain mount and used as a guide for analysis spot selection (Figs. 6 & 7). The grain mount (Fig. 8) was loaded into the SHRIMP (Fig. 9) and exact analysis spots were selected.
The two samples analyzed showed similar but distinct zircon zoning. For sample 14DW09, at least 15 rim spots with oscillatory zoning were selected; only a couple grains showed distinct cores (Fig. 6), so no cores were analyzed (a minimum of 5 cores are needed to produce a robust data set). For 15NW1THD, 15 rim spots with oscillatory zoning were selected as well as distinctly zoned cores (Fig. 7). Many of these cores appeared brighter in CL images, which is due to higher lead (Pb) content.
Results
Discussion
14DW
09
15N
W1T
HD
Two samples from the same dike, although variably deformed, yielded similar dates well within error of each other providing the
dike age of circa 50 Ma (Figs. 10 & 12). This age offers the southeastern most constraint on deformation within the PRC. Stevens
(2015) determined that Eocene magmatism within the PRC was a response to uplift and exhumation, and that is likely the case with
the Tubbs Hill dike. That also means this dike was emplaced synkinematically like the Silver Point pluton, which shows chlorite
microbreccia overprinting mylonitic fabrics (Fig. 1; Rhodes and others, 1989) near the en echelon step over in the Newport Fault
(Doughty and Price, 1999). Some zircon cores from 15NW1THD yielded significantly older igneous dates than their rims (Fig. 14;
1295±25 to 2346±176) suggesting the cores were scavenged from older rocks below Tubbs Hill. One zircon core yielded a date of
1579±41 (MSWD=0.58) that is within the North American Magmatic Gap (1610-1440 Ma), which is an interval of geologic time not
widely represented in the region.
60
55
50
45
40
65
Date=49.5±1.1(MSWD=1.59)
40
60
0.00 0.02 0.04 0.06 0.08 0.10 0.120.006
0.007
0.008
0.009
0.010
207Pb/235U
206 Pb
/238 U
50
206 Pb
/238 U
207Pb/235U
0.010
0.009
0.008
0.007
0.006
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
40
60
50
Figure 11. Conventional Wetherill concordia diagram showing U-Pb SHRIMP data distribution of the spot analyses with 2 sigma error ellipses.
Figure 12.206Pb/238U dates with weighted mean date for the more deformed sample of the Tubbs Hill dike, 15NW1THD.
Figure 13. Conventional Wetherill concordia diagram showing U-Pb SHRIMP data distribution of the spot analyses with 2 sigma error ellipses.
40
35
45
50
55
60
65
Date=50.1±3.5(MSWD=14.6)
206 Pb
/238 U
Dat
e (M
a)
Spot Analysis
0.55
0.45
0.25
0.00 3 9 11 15
2600
2400
2000
1600
207Pb/235U
206 Pb
/238 U
1200
0.35
5 7 13
Dates with error ranges
1295±251579±411652±871693±31
2346±176
Figure 14. Conventional Wetherill concordia diagram showing various zircon core dates from the U-Pb SHRIMP data distribution of the spot analyses with 2 sigma error ellipses.
206 Pb
/238 U
Dat
e (M
a)
Figure 10.206Pb/238U dates with weighted mean date for less deformed sample of the Tubbs Hill dike, 14DW09.
Spot Analysis
Figure 4. Geologic map of Tubbs Hills in Coeur d’Alene, Idaho showing rock sample locations (Reid and others, 1993).
15NW1THD
14DW09
14DW09
15NW1THD
Geologic Setting
Spokane Dome Mylonite Zone
Eocene pluton
Cretaceous pluton
Mesoproterozoic lower Belt-Purcell Supergroup
Mesoproterozoic Hauser Lake gneiss
Archean amphibolite andPaleoproterozoic graniticgneiss
Sample localition
ID
WA
MAPLOCATION
20 km
N
10 mi
Figure 2. Geologic map of the Priest River Complex showing mylonite zone extent and relevant rock ages (modified from Stevens, 2015).
EXPLANATION
Antiformal traceNormal fault
CANADAU.S.A.
Creston
Elmira
LakePend Oreille
PriestLake
KaniksuBatholith
WrencoePluton
NEWPORT FAULT
PriestRiver
Newport
Spirit Lake
Rathdrum
Coeurd’Alene
Lake
WAS
HIN
GTO
NID
AHO
Silver Point Pluton
Spokane Granite and
Newman LakeGneiss
Rathdrum Granite
Spokane
PendO
reilleRiver
SpokaneDome
SELKIRK CREST
KaniksuBatholith
Coeur d’Alene
Rathdrum
NEWPORT FAULT
PriestRiver Sandpoint
Newport~49.8 Ma~49.8 Ma
SELKIRK CREST
~50 Ma
50.13±0.02 Ma
Purcell Tre nch Fault
Hope Fault
Purc
ellTr
ench
Faul
t
Figure 4
TX
NMAZUT CO
CA NV
ORWA
MT
WY
CanadaU.S.A.
Craton Margin(87Sr/86Sr=0.706)
PRIEST RIVER CLEARWATERCLEARWATER
BITTERROOTBITTERROOT
OKANOGAN
GRAND FORKS
OKANOGAN
GRAND FORKS PRIEST RIVER
Sevier Thrust FrontSevier Thrust Front
Figure 1. Map of the western United States showing distribution of metamorphic core complexes (modified from Stevens, 2015).
EXPLANATION
Metamorphic core complex
Stateline or border
Thrust fault
Geochemicalboundary
N
150 mi250 km
The Priest River Complex (PRC) is one of many core complexes along western North America (Fig. 1) comprised of mid-crustal metamorphic rocks that
were uplifted and exposed at Earth’s surface giving geologists access to otherwise unreachable rocks. Metamorphosed igneous and sedimentary rocks
of the Hauser Lake Gneiss, Selkirk igneous complex of the Kaniksu Batholith, the Spokane Granite, Silver Point Pluton, Wrencoe Pluton, and the Tubbs
Hill rocks (Fig. 2) make up most of the PRC. Eastward displacement occurred along the Purcell Trench Detachment
Fault in conjunction with regional uplift. Overlying Belt-Purcell Supergroup supracrustal rocks were eroded including
some Hauser Lake Gneiss, which is an amphibolite-facies metamorphosed lower Belt-Purcell sedimentary unit (Fig. 3).
The Spokane Dome Mylonite Zone is defined by mylonitic fabrics that are collectively bowed up into an elongate
dome that trends NNE and plunges to the north, which projects partially below the Newport Fault (Fig. 2; Rhodes
and others, 1989). Ubiquitous lineations throughout the entire Spokane Dome Mylonite Zone trend generally ENE,
plunge moderately, and are defined by elongate mineral grains (Rhodes and Hyndman, 1984; Rhodes and others,
1989). Lineations are interpreted to represent the eastward movement along the Purcell Trench Fault, which
predates the doming of the mylonite zone (Rhodes and Hyndman, 1984).
Research about the PRC has been published for over 30 years (e.g.: Rhodes and Hyndman, 1984; Armstrong and
others, 1987; Doughty and Price, 1999), and a recent study of the PRC (Stevens, 2015) using advanced
geochronologic techniques revealed discrete ages (Cretaceous and Eocene) and magma sources for many of the
plutonic bodies within the PRC, but the study did not include the southeastern most portion of the PRC—Tubbs
Hill. Comprised chiefly of protomylonitic meta-igneous and meta-sedimentary rocks (Fig. 4; Reid and others, 1993),
Tubbs Hill also includes a late dike that crosscuts the entire hill and is variably deformed.
AbstractSensitive High-Resolution Ion Microprobe analyses of igneous rims on zircons from deformed (mylonitic) granite dike rocks of Tubbs Hill,
in downtown Coeur d’Alene, yielded U-Pb dates of 50.1±3.5 Ma and 49.5±1.1 Ma. The dike strikes northwest and is comprised of porphyritic
biotite granodiorite with feldspar phenocrysts 0.5-0.75 cm in length. These robust dates provide a constraint on mylonitic deformation
within the southeastern part of the Priest River metamorphic complex (PRC). An Eocene igneous age from this Tubbs Hill dike indicates
coeval emplacement with Eocene plutonic suites elsewhere in the PRC, such as the locally deformed Silver Point pluton quartz monzonite
dated at 50.13±0.02 Ma (Stevens, 2015). The northern most mylonitic deformation within the PRC, 66 kilometers north of Tubbs Hill is
constrained by a recent suite of monazite and zircon U-Pb dates of a leucocratic dike indicating crystallization was circa 49.8 Ma
(Stevens, 2015). Mylonitic deformation in the PRC appears to be a similar age in both the north and south.
AʹAA
Figure 3. Cross section through Tubbs Hill region showing eroded fault surface, extensive basement, and offset along the sub-horizontalPurcell Trench Detachment Fault.
Tubbs Hill
Cougar Bay
Highway 95 I-90
Eroded Fault
Eroded Fault
0 4,000 feet2x vertical exaggeration
3000
2000
1000
sea level
WESTEAST
Elev
ation
in fe
et
Eocene plutonic rock
Cretaceous plutonic rock
Mesoproterozoic lower Belt-Purcell Supergroup
Mesoproterozoic Hauser Lake gneiss
Archean amphibolite andPaleoproterozoic graniticgneiss
EXPLANATION
Normal fault
Holocene glacial deposits
Miocene Columbia River Basalt
Proterozoic quartzite
(Foliation orientation is 307,28)
