Geochronology of intrusive and metamorphic rocks in the Pilot Range, Utah and Nevada, and comparison with regional patterns iiiCMrw^P I U-S- Geological Survey, 345 MiddlefleldRoad, Menlo Park, California 94025 WENDY C. HlLLHOUofc, f ROBERT E. ZARTMAN U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 MARVIN A. LANPHERE U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025 ABSTRACT tonism; these ambiguities have resulted in diver- clude Archean gneiss, upper Proterozoic to gent views of the regional metamorphic and lower Mesozoic sedimentary rocks that are K-Ar and U-Pb isotopic studies demon- tectonic development. These views range from unmetamorphosed or metamorphosed to green- strate middle Mesozoic metamorphism and assigning the metamorphism entirely to the schist and amphibolite facies, Mesozoic granit- plutonism followed by Eocene plutonism in Mesozoic (Misch, 1960; Armstrong and Hansen, oids, and Cenozoic igneous and sedimentary the Pilot Range, northeastern Great Basin. 1966) to emphasizing the Cenozoic metamor- rocks. Geologic relations in mountain ranges in Combined with field relations, the age con- phism (Compton and others, 1977; Miller and which amphibolite-facies rocks crop out gener- straints indicate that local amphibolite-facies others, 1983) and primarily attributing the at- ally demonstrate convincingly that Cenozoic and widespread greenschist-facies metamor- tendant deformation to either shortening or ex- metamorphism, ductile deformation, and low- phism peaked between 165 and 150 Ma and tension modes. The divergent interpretations angle normal faulting occurred (Compton and that plutons were emplaced early in the de- seem to require models intermediate between others, 1977; Miller and others, 1983; Snoke formational and metamorphic history, be- the extremes (Armstrong, 1982; Miller and oth- and Lush, 1984), which has led to application of tween 165 and 155 Ma. Cenozoic plutons and ers, 1987; Snoke and Miller, 1987), but geologic the term "metamorphic core complex" for these widespread dikes that were emplaced at relations presenting convincing evidence for areas (Davis and Coney, 1979). Despite the about 40 Ma cut folds and faults, including a both Mesozoic and Cenozoic metamorphism are strong overprint of Cenozoic deformation, some major detachment fault that placed unmeta- few (Allmendinger and others, 1984). Studies in of the metamorphic core complexes retain evi- morphosed on metamorphosed rocks. the Pilot Range were begun with the hope that dence for Mesozoic metamorphism (Armstrong, The Mesozoic and early Cenozoic history analyzing the variably developed metamor- 1976; Compton and others, 1977; DeWitt, of metamorphism and plutonism in the Pilot phism, widespread deformation, and many ig- 1980; Snoke and Lush, 1984; Miller and others, Range is consistent with age data and field neous rocks would elucidate Mesozoic and 1987), supporting early proposals by Misch relations throughout the northeastern Great Cenozoic regional tectonics. (1960), Misch and Hazzard (1962), and Arm- Basin that indicate Late Jurassic plutonism This paper presents data from U-Pb and K-Ar strong and Hansen (1966) of widespread Meso- and metamorphism followed by Eocene and geochronologic studies and describes the con- zoic metamorphism (Armstrong, 1982). Oligocene plutonism and local metamor- straints that they place on timing of metamor- Geologic relations in ranges of the eastern phism. In many areas, rocks that were meta- phism, intrusion, and deformation in the Pilot Great Basin in which greenschist-facies rocks morphosed during Late Jurassic time were Range. The new data and interpretations sup- crop out have been less studied recently than cooled through Ar blocking temperatures for port the concepts of Mesozoic and Cenozoic re- those in ranges in which amphibolite-facies micas between 85 and 60 Ma. The wide- gional intrusive and metamorphic episodes in rocks occur. In many places, the greenschist spread occurrenc e of these mica ages suggests the eastern Great Basin. metamorphism is confined to Precambrian and a regional mechanism for cooling. Cambrian strata, and these lower grade meta- REGIONAL GEOLOGIC SETTING morphic rocks bear few of the characteristics of INTRODUCTION metamorphic core complexes. Lee and others Metamorphic rocks crop out widely in the (1970). Smith (1982), and Allmendinger and The Pilot Ranj;e is one of several mountain Pilot Range and several other mountain ranges others (1984) showed that the structural devel- ranges in the northeastern Great Basin that con- in the eastern Great Basin (Fig. 1). These ranges opment and metamorphism of some of these tains a rock record of complexly overlapping comprise a part of the discontinuously exposed low-grade areas are Mesozoic in age. Mesozoic and Cenozoic metamorphism, defor- Cordilleran metamorphic belt (Miller, 1980) Imposed upon the Mesozoic Cordilleran oro- mation, and plutonism. Ambiguities for the that extends the length of the Cordillera and genic belt was Cenozoic upper crustal extension, Mesozoic tectonic history of the region persist forms the western part of the Mesozoic and early resulting in complex low-angle fault slices, despite detailed studies because Mesozoic struc- Cenozoic Cordilleran orogenic belt (Drewes, normal-faulted tilted panels, and the present tures are difficult to directly date and in places 1978). Within the northeastern Great Basin, pattern of fault-block ranges separated by allu- have been modified by pervasive Cenozoic tec- rocks exposed within this metamorphic belt in- vial valleys. Extension in the northeastern Great Geological Society of America Bulletin, v. 99, p. 866-879, 9 figs., 4 tables, December 1987. 866 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/6/866/3998452/i0016-7606-99-6-866.pdf by guest on 02 October 2021 GEOCHRONOLOGY OF ROCKS, PILOT RANGE, UTAH AND NEVADA 867 Basin may have begun as early as Late Creta- sic and during late Eocene into Oligocene time areas, an additional Cretaceous episode oc- ceous but certainly was evident by latest Eocene (Moore and McKee, 1983). Limited data sug- curred (Coats and others, 1965; Armstrong, (Miller and others, 1983). gest that the same two episodes occurred in 1970; Armstrong and Suppe, 1973; Roberts and Previous geochronologic studies of granitoid northeastern Nevada (Coats and others, 1965; others, 1971; Lee and others, 1980). The Ceno- plutons in central and northern Utah indicated Armstrong, 1970), but farther south in the Ely zoic episode in places is divisible into latest two episodes of intrusion during the Late Juras- area and west in the Eureka and Mountain City Eocene and late Oligocene parts. Upper Eocene 114° 112° 40" TO COMPILE e FIGURE 8 O e EXPLANATION <o •"J 5 Granitoid rocks Low-grade metamorphic «e rocks a High-grade * o c <0 O 50 km h L- _J O Figure 1. Map of northeastern Great Basin, showing locations of mountain ranges discussed in text and generalized outcrop pattern of metamorphic rocks and plutons. Outlined area shows region sampled for data analysis shown in Figure 8. Willard-Paris thrust dotted where inferred. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/6/866/3998452/i0016-7606-99-6-866.pdf by guest on 02 October 2021 868 MILLER AND OTHERS Figure 2. Bedrock geologic map of the Pilot Range, showing major tectonostrati- graphic blocks, plutons, and locations of samples for isotopic studies. plutons are sparsely spread through the region, but upper Oligocene plutons appear to be con- fined to metamorphic core complexes. GEOLOGY OF THE PILOT RANGE The Pilot Range is a north-trending fault block in which structures and strata are gently warped about an east-trending axis, dipping south and north away from a central window of metamorphosed rocks. Rocks in this window are structurally ssparated from overlying un- metamorphosed rocks by a low-angle normal (detachment) fault, the Pilot Peak décollement (Fig. 2). Strata above and below the detachment fault are cut by faults nearly parallel to bedding, and those above the detachment are cut by a younger set of normal faults that have rotated tilt blocks into the detachment. The over-all picture is one of Mesozoic metamorphism and ductile deformation, succeeded by detachment faulting and tilting of fault blocks as the upper crust thinned. Structural Blocks and Metamorphism Strata in the Pilot Range lie in three structural blocks. Metamorphic rocks in the central win- dow of the range, underlying the Pilot Peak décollement, comprise upper Proterozoic and Lower and Middle Cambrian metamorphosed clastic and carbon ate rocks. Structurally overly- ing the metamoiphic rocks are unmetamor- phosed miogeoclinal Upper Cambrian to Upper Permian carbonate rocks. Unmetamorphosed Cambrian strata are distinguished from structur- ally underlying metamorphosed Cambrian strata by different ages and depositional environments (McCollumandMcCollum, 1984; Miller, 1984) as well as metamorphic grade. The structurally highest block is composed of Oligocene and Miocene volcanic; and terrigenous sedimentary rocks and occupies a position structurally above unmetamorphosed Paleozoic rocks. Compre- hensive descriptions of strata in the Pilot Range were given by Miller (1984). Rocks beneath the Pilot Peak décollement contain greenschist- and amphibolite-facies metamorphic assemblages. Rocks containing retrograded staurolite and probable garnet occur within an area efist of Pilot Peak (Fig. 2), but elsewhere, rocks are typically