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CENOZOIC SOLDIERS PASS VOLCANIC FIELD, CENTRAL UTAH—IMPLICATIONS FOR THE TRANSITION TO EXTENSION-RELATED MAGMATISM IN THE BASIN AND RANGE PROVINCE by Eric H. Christiansen1, Nichelle Baxter1, Thomas P. Ward1, Elizabeth Zobell1, Matthew R. Chandler1, Michael J. Dorais1, Bart J. Kowallis1, Donald L. Clark2 ABSTRACT The late Cenozoic transition from subduction-related to extension-related volcanism is recorded in the Soldiers Pass volcanic field of the southern Lake Mountains, north-central Utah. The Soldiers Pass Formation (new formal name) is a Paleogene (35–33 Ma) suite of intermediate to silicic volcanic rocks interstratified with and overlain by lake and hot-spring deposits. In ascending order, its volcanic units include the trachydacite tuff member, Chimney Rock Pass Tuff Member, breccia member, and andesite member. Nearly horizontal lake and hot-spring deposits (White Knoll Member) are interlayered with and cap the volcanic strata. The volcanic rocks are very high-K, magnesian rocks with large negative Nb anom- alies on normalized trace-element diagrams. Mineral compositions show they crystallized at high oxygen and water fugacities. Magma mixing is evidenced by high concentrations of compatible elements in the intermediate composition rocks, plagioclase and sanidine compositions and textures, and disequilibrium mineral assemblages. In short, the Paleogene suite has the characteristics of magmas formed at continen- tal subduction zones. There is no structural evidence of extension during the eruption of the Paleogene suite. Following a lull in volcanic activity of about 14 million years, the 19 Ma Mosida Basalt (new formal name) erupted as one of the oldest basaltic magmas in the eastern Great Basin. This mildly alkaline, potas- sic trachybasalt has phenocrysts of olivine (Fo60), plagioclase (An65), and clinopyroxene. Trace-element patterns lack large negative Nb and Ti anomalies, consistent with a nonsubduction origin. Tectonic dis- crimination diagrams also imply a within-plate, alkalic character. We conclude that it is one of the oldest asthenosphere-derived magmas in the Great Basin, but low Mg/(Mg+Fe), Ni and Cr concentrations, and relatively Fe-rich olivine compositions show that it is not primary. This transitional magmatic sequence is probably the result of the progressive foundering of a shallow- ly dipping subducting slab that began during the Eocene below this part of the Great Basin. Foundering produced widespread dehydration of the subducted lithosphere and generated voluminous arc like magma that intruded, hybridized, and differentiated in the crust. Compensating inflow of asthenospheric mantle beneath the Great Basin, along with`1 the development of a transform boundary and lithospheric exten- sion, eventually resulted in decompression melting of the mantle by 19 Ma. The Mosida Basalt has not been tilted but the Lake Mountains horst is bounded on the east and west by normal faults. INTRODUCTION The onset of lithospheric extension has both struc- tural and magmatic aspects. Some aspects of the struc- The Basin and Range Province of the western United tural transition and many aspects of the magmatic transi- States is one of Earth’s most studied regions of continen- tion (from subduction related to extension related vol- tal rifting, and yet the timing and character of the onset canism) are recorded in the southern part of the Lake of Cenozoic extension are still controversial. Some struc- Mountains of western Utah, which lie near the eastern tural studies have concluded that extension in the Great border of the Great Basin (figure 1). In the Soldiers Pass Basin part of the Basin and Range Province is as old as volcanic field, a Paleogene suite of silicic ignimbrites 41–37 Ma (Rahl and others, 2002) or 32–30 Ma (e.g., and intermediate-composition lavas is interlayered with Axen and others, 1993). Others have concluded that sig- lake and hot spring deposits (figure 2). Following a hia- nificant extension on large, high-angle normal faults tus in magmatism, a mafic lava erupted as one of the old- started much later. For example, Parry and Bruhn (1986) est basaltic magmas in the eastern Great Basin. concluded that the Wasatch fault has been active since Our study aims to answer questions about the timing about 17–18 Ma. and nature of the onset of extension in the Great Basin 1 Brigham Young University, Department of Geological Sciences, Provo, Utah 84602 [email protected] 2 Utah Geological Survey, P.O. Box 146100, Salt Lake City, Utah 84114-6100 2 Central Utah - Diverse Geology of a Dynamic Landscape Figure 1. Index map of north-central Utah. Rectangle shows location of Soldiers Pass volcanic field (figure 2) in the south- ern Lake Mountains. The Lake Mountains form a horst between grabens in Utah and Goshen Valleys on the east and Cedar Valley on the west. Figure 2. North map: Generalized geologic map of the Soldiers Pass volcanic field (modified from Proctor, 1985; Biek and others, 2006; Clark and others, 2006). 2007 UGA Publication 36 - Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C. Jr., editors 3 Figure 2. South map: Generalized geologic map of the Soldiers Pass volcanic field (modified from Proctor, 1985; Biek and others, 2006; Clark and others, 2006). using the petrochemistry of volcanic rocks. We focus on magmatism swept across what is now the eastern Great evidence for a petrologic transition from subduction- to Basin, and was later supplanted by a younger suite of extension-related magmatism. Using new geochemical bimodal basalt and rhyolite (Best and Christiansen, data and 40Ar/39Ar ages, we show that extension-related 1991). The volcanic products of these episodes form magmatism began about 19.5 Ma in this region. some of the low hills south of Soldiers Pass and consti- tute the Soldiers Pass volcanic field. We show below that the petrologic transition is contemporaneous with the GEOLOGIC SETTING onset of normal faulting that created the Basin and Range Province. The Lake Mountains form a horst bounded by Soldiers Pass lies in the southern Lake Mountains of the Cedar Valley graben on the west and the composite north-central Utah (figure 1). The pass separates the main Utah-Goshen Valley graben on the east. The Lake mountain range from a series of low hills to the south. Mountains are the easternmost range in this part of the The sedimentary rocks of the southern Lake Mountains Great Basin (figure 1) and lie about 30 km west of the are dominated by shallow-marine limestone and sand- province-bounding Wasatch fault zone. stone of Mississippian to Pennsylvanian age. South of The Soldiers Pass volcanic field lies between two of Soldiers Pass sedimentary rocks exposed in the low hills Utah’s major volcanic fields and their associated mining are as old as Silurian. These sedimentary rocks were districts: the Bingham district to the north and the Tintic folded into a broad syncline in the core of the range and district to the south. The Bingham area was volcanically smaller, tighter folds formed to the south and east of active between 39 and 33 Ma (Babcock and others, 1997; Soldiers Pass (Biek and others, 2006; Clark and others, Deino and Keith, 1997). The Tintic district is 30 km 2006). Folding was presumably related to the Sevier south of Soldiers Pass (Morris and Lovering, 1979) and orogeny of Cretaceous to early Paleogene age (DeCelles, was volcanically active from about 39 to 32 Ma (Clark, 2004). Beginning in the Eocene, intermediate to silicic 2003; Moore and others, this volume). No mineralization 4 Central Utah - Diverse Geology of a Dynamic Landscape is known in the Soldiers Pass area; the only known tions and members is depicted on several detailed geo- hydrothermal deposits are veins and irregular masses of logic maps, except for a small unmapped area in the coarse brown calcite that cut the Paleozoic limestones Goshen Pass quadrangle (figure 2; Proctor, 1985; Biek (Hoffman, 1951). and others, 2006; Clark and others, 2006). Formalizing these mappable rock units will aid in reconstructing the geologic history, making regional correlations, and in CENOZOIC STRATIGRAPHY land use planning. New lithologic and stratigraphic data, 40Ar/39Ar Background and Stratigraphic Nomenclature ages, mineral chemistry, and whole-rock geochemical 40 39 2 data for each unit are given below. Ar/ Ar ages were The Soldiers Pass volcanic field covers about 85 km determined at the New Mexico Geochronology Research extending from near Soldiers Pass on the north, south- Laboratory and are reported in table 1. Chemical anal- ward along the southern Lake Mountains between Cedar yses were performed at Brigham Young University. A Valley (west) and Utah-Goshen Valley (east) to Chimney description of the geochemical techniques used and X-ray Rock Pass in the northernmost East Tintic Mountains fluorescence analyses of international reference materials (figure 2). This volcanic field encompasses parts of the is available at http://www.geology.byu.edu/faculty/ehc/. Soldiers Pass, Goshen Valley North, Goshen Pass, and Representative analyses are given in table 2 and a larger ′ Allens Ranch 7.5 quadrangles. Volcanic rocks from the whole-rock data set is included in Biek and others (2006). Tintic volcanic field overlap in age with those of the Soldiers Pass volcanic field in Chimney Rock Pass. The Soldiers Pass Formation volcanic and sedimentary rocks of this area were origi- nally mapped in detail in the late 1940s to early 1950s, The Paleogene rocks of the southern Lake Mountains mostly by individuals affiliated with Brigham Young are included in the Soldiers Pass Formation (new formal University (Rigby, 1949; Stringham and Sharp, 1950; name), named for exposures in and near the pass. The Bullock, 1951; Hoffman, 1951; Smith, 1951; Williams, formation consists of four volcanic members and an 1951; Ornelas, 1953). Their map units included geo- interstratified lacustrine-fluvial deposit (figure 3). Bul- graphic names that are not on modern maps, including lock (1951) grouped the volcanic rocks in the area with the Fox Hills, Mosida Hills, and Selma Hills.

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