Previous section Volume contents The Petrogenesis of the Colorado Plateau Laccoliths and Their Relationship to Regional Magmatism By Stephen T. Nelson1 and Jon P. Davidson2 CONTENTS Abstract........................................................................................................................... 86 Introduction ....................................................................................................................86 Regional Geologic Setting...................................................................................... 86 Colorado Plateau Structure, Geophysics, and Composition................................... 88 Field Relations........................................................................................................ 88 Geochemical Characteristics of the Laccoliths .............................................................. 89 Magma Chemistry And Petrogenesis in the Henry Mountains...................................... 92 Regional Tectonomagmatic Considerations................................................................... 94 Conclusions .................................................................................................................... 97 Acknowledgments .......................................................................................................... 98 References Cited............................................................................................................. 98 FIGURES 1. Maps showing locations of the Henry, La Sal, and Abajo Mountains, the Colorado Plateau, basement terrane boundaries, and other geographic and tectonomagmatic features of interest.................................................................... 87 2–5. Diagrams showing: 2. Total alkalies versus silica for the Henry, Abajo, and La Sal Mountains ....... 89 3. Relative trace-element abundances in plagioclase-hornblende porphyry and syenite porphyry from the Henry and La Sal Mountains, compared to those of typical calc-alkaline basalts and intraplate basalts of the Basin and Range province......................................................................................... 92 4. εNd versus 87Sr/86Sr in rocks of the Henry, La Sal, and Abajo Mountains... 92 5. Barium versus niobium in rocks of the Henry and La Sal Mountains, the High Plateaus, the San Juan field, and the Indian Peak field .......................... 93 6. Harker variation diagrams comparing rocks of the Henry Mountains to those of the La Sal Mountains .............................................................................. 93 7. Ternary diagram plotting normative compositions of Henry Mountains laccoliths, ash-flow sheets of the Indian Peak and San Juan fields, melts derived from mafic crust, and granite minimum melts......................................... 94 8. εNd versus 87Sr/86Sr isotope diagram for Colorado Plateau laccoliths, rocks of the High Plateaus, and ignimbrites of the San Juan and Latir fields................ 95 9. Plate-tectonic reconstruction of the Western United States during Tertiary time at 50 and 30 Ma ............................................................................................ 96 85 86 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH 10. Cartoon diagram illustrating differences in crustal structure, composition, and magma volumes across the Basin and Range Province, the transition zone, and the Colorado Plateau interior ........................................................................... 97 TABLE 1. Representative geochemical data and sample localities for rocks of the La Sal and Abajo Mountains, Utah....................................................................................... 90 ABSTRACT of the igneous activity of the Colorado Plateau interior during mid-Tertiary time. Understanding the origin of the The petrogenetic processes that formed the Henry laccoliths will help clarify the fundamental differences in Mountains, Utah, may be the same as those responsible for contemporaneous magmatism on and off the plateau, crust- other laccolithic intrusions in the Colorado Plateau, specifi- mantle dynamics involved in the formation of the magmas, cally the La Sal and Abajo Mountains. Each range consists and regional tectonomagmatic processes. of small separate intrusive centers where magma was em- We review existing data and present new data for all placed into Phanerozoic sediments at shallow crustal levels. three of the laccolithic ranges (fig. 1), and contemporaneous Two major rock suites, plagioclase-hornblende porphyry (95 rocks in adjacent regions of the Western United States to volume percent) and syenite porphyry (5 volume percent) address the following topics: exist in both the Henry and La Sal Mountains, whereas pla- 1. We compare the geochemistry of the rocks from all gioclase-hornblende porphyry alone is found in the Abajo three mountain ranges in order to assess their differences, Mountains. Plagioclase-hornblende porphyry evolved from their similarities, and their possible tectonic affiliation. mantle-derived magma, which was ponded in the deep crust 2. We outline the petrogenesis of the Henry Mountains and assimilated amphibolite crust during open-system differ- laccoliths, which we propose as a model system to illustrate entiation before being emplaced at shallow crustal levels. the interactions between mantle-derived melts and continen- Plagioclase-hornblende porphyry also shows isotopic pro- tal crust in the Colorado Plateau. A detailed study is given vinciality at each intrusive center, which may, in part, reflect in Nelson and Davidson (1993). the isotopic diversity of the basement rocks. 3. We consider the regional relationships of the lacco- Geochemically, the laccoliths and contemporaneous liths to roughly contemporaneous and similarly evolved volcanic rocks outside the plateau appear to have strong volcanic rocks in the vicinity of the Colorado Plateau in affinities to arc rocks, although their volume is much less order to evaluate the origins of mid-Tertiary magmatism. than that of a typical volcanic arc. We contend that the lac- coliths are part of an east-west-oriented magmatic belt, itself a portion of a larger mid-Tertiary magmatic system in west- REGIONAL GEOLOGIC SETTING ern North America, and that the minor volume of the lacco- liths reflects the inability of large volumes of magma to Deformational and magmatic events have left the Colo- penetrate the thick, strong, stable crust of the Colorado rado Plateau interior relatively unaffected during the entire Plateau. Phanerozoic Eon (Allmendinger and others, 1987). Howev- er, the Colorado Plateau (fig. 1) is bounded by areas of in- tense Mesozoic and Cenozoic deformation and magmatism. INTRODUCTION Cretaceous to Eocene Laramide shortening may have been the result of shallow or flat subduction (Bird, 1988; Hamil- The laccolithic intrusions of the Henry, La Sal, and ton, 1988) and an accompanying magmatic lull in the region Abajo Mountains of southwest Utah (fig. 1) represent much of the laccoliths and farther west (Armstrong and Ward, ________________________ 1991). Mid-Tertiary andesitic to dacitic volcanism in the 1 Reno-Marysvale, San Juan, and Mogollon-Datil belts pro- Woodward-Clyde Federal Services, 101 Convention Center Drive, ≈ 5 3 Suite P110, MS 423, Las Vegas, NV 89109. duced great volumes ( 5x10 km ; Johnson, 1991) of ignim- 2Department of Earth and Space Sciences, University of California, brite and similar rocks, whereas contemporaneous (middle to Los Angeles, CA 90024–1567. late Oligocene) magmatism in the Henry, La Sal, and Abajo PETROGENESIS OF COLORADO PLATEAU LOCCOLITHS 87 120° 115° 110° 105° 34 38 42 LARAMIDE 32 UTAH Great Basin 40° COLORADO OROGEN 28 Reno-Marysvale Belt Yavapai Reno Province t 1.7-1.8 Ga n o r F 34 ic n Indian Peak e g Area shown Mazatzal field ro San Juan O in 28 Province field 1.6-1.7 Ga NEVADA r map B ie v Rio Grande Rift CALIFORNIA e S Colorado Latir Plateau field 35° Basin and Range Province 25 Mogollon-Datil belt 0 200 KILOMETERS NEW MEXICO ARIZONA A 28 112° 111° 110 109° North LA SAL Mountain Green River MOUNTAINS Moab Brumley Middle Mountain Ridge Laccolith South Mountain HENRY MOUNTAINS Mt. Ellen Shay Mountain East West Mountain Mountain 38° Mt. Pennell ABAJO MOUNTAINS Mt. Hillers Mt. Holmes Blanding Mt. Ellsworth Colorado River 0 30 60 KILOMETERS 37° UTAH COLORADO B ARIZONA NEW MEXICO Figure 1. Locations of the Henry, La Sal, and Abajo Mountains, the Colorado Plateau, basement terrane boundaries, and other geographic and tectonomagmatic features of interest. A, General setting of the Southwestern United States: Numbers labeling hachured contours show the age (in millions of years) of the onset of mid-Tertiary magmatism as it swept through the Cordillera from the north and from the south (adapted from Cross and Pilger, 1978; Burke and McKee, 1979; and Glazner and Bartley, 1984). Proterozoic basement terrane boundaries from Bowring and Karlstrom (1990). B, Detail of the Henry, La Sal, and Abajo Mountains in the central part of the Colorado Plateau. 88 LACCOLITH COMPLEXES OF SOUTHEASTERN UTAH Mountains was volumetrically minor (69, 50, and 20 km3, re- physical properties that distributed strain into surrounding spectively) (Hunt, 1953; 1958; Witkind, 1964). The lacco- terranes. Lower crustal Pn velocities of 6.8 km/s or greater liths may have been part of a large east-west-oriented late in the Colorado Plateau (Smith and others, 1989; Wolf and Oligocene magmatic belt extending from Reno, Nev., to the Cipar, 1993) are