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Idaho Batholith and Its Southern Extension

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Idaho Batholith and Its Southern Extension WILLIAM H. TAUBENECK Department of Geology, Oregon State University, Corvallis, Oregon 97331 Idaho Batholith and Its Southern Extension ABSTRACT for 28 mi in the most westerly exposures of the batholith are about S. 20° E. Southeast trends Distribution of minerals and rock types, em- also occur near South Mountain in pre-Tertiary placement structures, and postconsolidation country rocks west of the southernmost expo- history are more complex within the Idaho sures of the batholith. The southeast trends batholith than most published descriptions sug- within and outside the batholith indicate that a gest. Hornblende occurs in some areas many significant change in structural direction occurs miles within the batholith, and planar structure in southwest Idaho in the region near South also is present in some of its interior regions. Mountain. In the vicinity of the Cascade Reservoir in The locations of the south and southeast con- west-central Idaho, field relationships and mo- tacts of the Idaho batholith are uncertain, but dal data for granitic rocks are inconsistent with some inferences regarding the position of the the conclusion (Schmidt, 1964) that the bed- batholith are possible from isolated occurrences rock systematically and gradationally changes of Ordovician sedimentary rocks south of Twin from schist and gneiss to directionless granitic Falls and from exposures of pre-Tertiary sedi- rock along 35-mi traverses west to east across mentary and igneous rocks near the Idaho- the border and interior of the batholith. Nevada state line. Major fault blocks within the Idaho batholith Northward continuity of the Sierra Nevada invalidate the concept (Hamilton and Myers, batholith to the Nevada-Oregon boundary is 1966) of a resistant mass that defied internal well established. The trend of the batholith deformation during the Cenozoic evolution of bends toward the northeast before the batholith western North America. disappears under Cenozoic volcanic rocks in Gabbro and norite typically occur west of the southeast Oregon and northern Nevada. The batholith. Granitic intrusions near the batholith distribution and composition of the plutonic in western Idaho are characterized by mega- rocks near the Nevada-Oregon border suggest scopic crystals of epidote. Interstitial zeolites that the quartz diorite boundary line is about also occur in satellite masses rather than in gra- 160 mi east of the inferred location (Moore, nitic rocks of the batholith. 1959) in northern California. Granitic rocks of southwest Idaho are cor- If the Idaho and Sierra Nevada batholiths are related with the Idaho batholith primarily by connected, the Idaho batholith southeast of the location and trend of gneissic border rocks South Mountain must veer sharply west on either side of the Snake River Plain. The beneath Cenozoic volcanic rocks. Any connect- trend of S. 20° W. in the gneissic border zone ing link between the two batholiths must be north of the Snake River Plain also is present in confined to a narrow belt that extends east- rocks lying between 40 and 5 5 mi to the south- northeast for about 75 mi near the Idaho- southwest where gneissic granitic rocks reap- Nevada and Oregon-Nevada boundaries. pear in the westernmost exposures of An appreciable change in the relative posi- pre-Tertiary rocks south of the Snake River. tion of the Idaho and Sierra Nevada batholiths Gross mineralogical characteristics of granitic has occurred since Oligocene time. If the sug- rocks near the Snake River in southwest Idaho gested magnitudes of displacement from nor- closely resemble those in the west part of the mal faulting, dike intrusion, and right-lateral batholith just north of the Snake River Plain. faulting are approximately correct, the east- Southwest structural trends in granitic rocks west change in the alignment of the batholiths in southwest Idaho near the Snake River begin is as much as 50 mi. to deviate to the southeast about 25 mi due Gravity and seismic data considered in terms south of Marsing. Farther to the south, trends of surface geology and the distribution of gra- Geological Society of America Bulletin, v. 82, p. 1899-1928, 4 figs. July 1971 1899 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/7/1899/3417691/i0016-7606-82-7-1899.pdf by guest on 01 October 2021 1900 W. H. TAUBENECK—IDAHO BATHOLITH AND ITS SOUTHERN EXTENSION nitic rocks are consistent with the interpretation specimens from near the plain were collected that models of crustal structure should include for modal analyses. In northern Nevada, on the a granitic layer underlying nearly all of south- other hand, excellent exposures of granitic west Idaho. rocks imposed no restrictions on sampling. Each modal analysis (Tables 1 to 9) represents INTRODUCTION at least 2000 points for each of two to seven thin sections per rock; the number of thin sec- Purpose tions for each analysis is a function of grain size. During 1967 field studies of post-Oligocene Structural trends (Fig. 1) in some areas of dikes and dike swarms of the Basin and Range granitic rocks in Idaho vary as much as 15° structural province (Taubeneck, 1969, 1970), within 100 ft or less. Such variations are most gneissic granitic rocks similar to the distinctive common in exposures in southwest Idaho more border zone rocks of the west part of the Idaho than 17 mi south-southwest of the Snake River. batholith were observed south of the Snake Wherever possible, trends shown in Figure 1 River Plain in southwest Idaho. Moreover, the represent the average of 20 to 40 observations structural trends in these rocks were found to in an area of at least 1 sq mi. be on strike with trends in the gneissic border rocks on the north side of the plain. These dis- THE IDAHO BATHOLITH coveries in southwest Idaho focused my atten- The Idaho batholith is the least known of the tion on the possible continuity of Mesozoic large batholiths of the western United States. granitic rocks between the Idaho batholith and Although hundreds of papers discuss various the Sierra Nevada batholith. aspects of the batholith, almost no detailed This paper (1) presents new data on the petrographic studies are available (Ross, 1963, Idaho batholith, (2) recognizes a southward ex- p. 45). tension of the batholith that includes nearly all General knowledge of the batholith was sum- granitic rocks of southwest Idaho, (3) discusses marized most recently by Ross (1963), near the a possible connection between the Sierra end of a lifetime devoted largely to the geology Nevada batholith and the Idaho batholith, and of Idaho. His conclusions were based partly on (4) concludes that models of crustal structure a reconnaissance of much of the batholith dur- should show a layer of low-velocity ("gra- ing the summer of 1962. Ross (1963, p. 52) nitic") continental crust underlying nearly all correctly reported that "both the border zone of southwest Idaho. and the interior mass are more complex in de- tail than would be supposed from published Methods descriptions." Field studies concentrated on relationships of The batholith traditionally has been de- granitic rocks in Idaho and Nevada included 19 scribed in terms of a gneissic shell of quartz days in northern Nevada, 17 days in southwest diorite that encloses quartz monzonite and Idaho, and 34 days along the west border of the granodiorite (for example, Ross, 1936). Some Idaho batholith in west-central Idaho. In addi- modern generalizations are rather misleading tion, observations of granitic rocks in central in referring to the rocks within the gneissic and south-central Idaho were made during 23 shell as "continuous massive granodiorite and days of reconnaissance studies of Cenozoic quartz monzonite" (Hamilton, 1962, p. 513). dikes and dike swarms within the Idaho bath- Planar structure, in some areas many miles olith. Knowledge of pre-Tertiary rocks in west- within the batholith, will permit detailed struc- ern Idaho, several miles or more west of the tural studies that ultimately will provide consid- batholith and between the Snake River Plain erable information regarding its emplacement and the Clearwater River some 150 mi to the history. north, was acquired mostly in 36 days, during Another misconception regarding the Idaho which Cenozoic dikes were studied. batholith is that hornblende occurs only in bor- Except in glaciated alpine areas, rocks of the der rocks. Actually, hornblende is present in Idaho batholith commonly are deeply weath- some interior parts of the batholith, especially ered (Russell, 1902, p. 40; Larsen and Schmidt, south and southwest of Atlanta (Fig. 1). Near 1958, p. 3), especially near the Snake River Trinity Peak, about 21 mi southwest of Atlanta, Plain where fresh specimens generally are diffi- granitic rocks contain several percent of horn- cult or impossible to obtain. Therefore, few blende. The distribution of hornblende is of Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/7/1899/3417691/i0016-7606-82-7-1899.pdf by guest on 01 October 2021 THE IDAHO BATHOLITH 1901 117 116° 117° 116° Figure 1. Map showing inferred contacts of Idaho bered localities discussed in text. batholith, structural trends, and locations (dots) of num- almost immediate importance, for example, in trate that the petrography and the distribution facilitating the radiometric investigations of of rock types within the batholith are more geochronologists. complex that implied by published statements. Modal data (Tables 1 and 2) for granitic This part of the batholith was studied by rocks in the area between the Cascade Reser- semireconnaissance methods that involved ob- voir (Fig. 1) and the Snake River Plain illus- servations restricted mostly to localities not Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/7/1899/3417691/i0016-7606-82-7-1899.pdf by guest on 01 October 2021 TABLE 1.
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