PETROGENESIS OF THE OLIGOCENE EAST TINTIC VOLCbyANIC FIELD, UTAH Daniel K. Moore1, Jeffrey D. Keith2, Eric H. Christiansen2, Choon-Sik Kim3, David G. Tingey2, Stephen T. Nelson2, and Douglas S. Flamm2 ABSTRACT The early Oligocene East Tintic volcanic field of central Utah, located near the eastern margin of the Basin and Range Province, consists of mafic to silicic volcanic (mostly intermediate-composition lava flows) and shallow intrusive rocks associated with the formation of small, nested calderas. Radiometric ages establish a minimum age for initiation (34.94 ± 0.10 Ma) and cessation (32.70 ± 0.28 Ma) of East Tintic magmatism. The igneous rocks of the field are calc-alkalic, potassic, silica-oversaturated, and met- aluminous, and can be categorized into the following three compositional groups: the shoshonite-trachyte series, the trachyandesite series, and the rhyolite series. Based on composition and phenocryst assemblage, the shoshonite-trachyte series is divided into two groups: a clinopyroxene group and a two-pyroxene group. The rhyolite series consists of three field units: the Packard Quartz Latite, the Fernow Quartz Latite, and the rhyolite of Keystone Springs. The trachyandesite series is by far the most voluminous. This series is also subdivided into a clinopyroxene group and a two-pyroxene group. Temperature and oxygen fugacity estimates indicate that shoshonite-trachyte series magmas were the hottest and least oxidizing and that two-pyroxene trachyandesite series magmas were the coolest and most oxidizing. Clinopyroxene shoshonite-trachyte series magma evolved mainly by fractional crystallization. The high K2O, Rb, and Al2O3/CaO ratios and modest SiO2 enrichment of these rocks appear to result from extensive, high-pressure fractional crystallization of clinopyroxene (without plagioclase). Two-pyroxene shoshonite-trachyte series magma was likely produced by mixing between mafic and silicic clinopyroxene shoshonite-trachyte series magmas at low pressure. Assimilation of crustal material appears not to have been important for shoshonite-trachyte series magmas. We believe that parental clinopyroxene shosh- onite-trachyte series magma originated in the mantle wedge above a Cenozoic subduction zone and then interacted with older subduction-metasomatized lithospheric mantle. Rhyolite series magma was likely the differentiate of a lower crustal partial melt. Trachyandesite series magma likely evolved by magma mix- ing and subsequent fractional crystallization. Trace-element compositions indicate that the mixing that produced trachyandesite series magmas was between mafic clinopyroxene shoshonite-trachyte series magma and Fernow Quartz Latite magma, at low pressure for two-pyroxene trachyandesite series magma, and at high pressure for clinopyroxene trachyandesite series magma. INTRODUCTION (figure 1) was mapped at a scale of 1:24,000 using con- ventional methods (Morris, 1975; Hannah and Macbeth, The East Tintic Mountains of central Utah are a 1990; Kim, 1992; Moore, 1993; Keith and others, in Basin and Range-style horst that exposes Paleozoic sed- preparation). Fresh samples were collected from most imentary rocks and the volcanic, sedimentary, and shal- igneous rock units. Standard petrographic techniques low intrusive rocks of the early Oligocene East Tintic were used to determine rock texture and modal mineral- volcanic field (figure 1). In the East Tintic mining dis- ogy. New 40Ar/39Ar age determinations were done on a trict, sulfide-rich alteration and mineralization (Lindgren VG1200S automated mass spectrometer using standard and Laughlin, 1919; Lovering, 1949; Morris and techniques (like those of Harrison and Fitzgerald, 1986) Lovering, 1979) are the products of magmatism (Ames, at the University of California at Los Angeles by S.T. 1962; Morris and Lovering, 1979; Keith and others, Nelson, and are reported in table 1. 1989; Hannah and others, 1991; Moore, 1993). The East The East Tintic Mountains are characterized by vol- Tintic volcanic field represents an ideal locale for study- canic and sedimentary rocks of early Oligocene age (fol- ing the processes responsible for the genesis of subduc- lowing the scale of Hansen, 1991) that lie unconformably tion-related, potassic, silica-saturated magmas related to upon folded and faulted Paleozoic sedimentary rocks, all ore genesis. This report explores the petrogenesis of the of which are cut by shallow intrusions associated with igneous rocks of the East Tintic volcanic field. the volcanic rocks (figure 1). The west side of the range is more deeply eroded and exposes older rocks. Morris GEOLOGIC SETTING and Anderson (1962) studied the Paleozoic-Tertiary unconformity and concluded that there was substan- To establish stratigraphic and temporal relationships, tial topographic relief at the onset of volcanism. An the bedrock geology of the central East Tintic Mountains 40Ar/39Ar sanidine age of 34.94 ± 0.10 Ma from the 1 Department of Geology, Brigham Young University-Idaho, Rexburg, ID 83460 [email protected] 2 Department of Geological Sciences, Brigham Young University, Provo, UT 84602 [email protected] 3 Department of Geology, Pusan National University, Pusan, South Korea 164 Central Utah — Diverse Geology of a Dynamic Landscape 112˚07’ 30’’ W 112˚00’ W 40˚00’ N N 0 2 km Goshen Valley Tintic Valley Figure 1. Index map and generalized geologic map of the East Tintic volcanic field. Geologic contacts are from Keith and others (in preparation). 39˚45’ N Ore-related latite flows Ore-related Salt Lake monzonite intrusions City Tintic Other volcanic and District intrusive rocks undivided Utah Paleozoic sedimentary rocks undivided Table 1. Summary of new 40Ar / 39Ar radiometric ages.1 Table 1. Summary of new 40Ar / 39Ar radiometric ages.1 Sample2 SP292 SP192 TJ77 TJ197 TJ108 ET134 ET134 ET121 Mineral biotite sanidine sanidine biotite biotite biotite hornblende hornblende 3 Latitude (N) 40.174296° 40.204185° 39°48’23” 39°53’35” 39°49’21” 39°59’37” 39°59’37” 39°57’59” Longitude(W) 111.955592° 111.977750° 112°2’54” 112°3’2” 112°4’7” 112°2’38” 112°2’38” 112°3’51” Jx10-6 7865 7862 7856 7860 7858 7870 7869 7876 Fusions 8 10 10 5 5 7 - - 40/36 Ar - - - - - - 294±7.60 301±4.90 40/39 Ar - - - - - - 2.37±0.01 2.33±0.02 MWSD4 - - - - - - 10.75 1.96 Age (Ma)5 34.71 34.18 34.03 33.87 33.72 33.34 33.29 32.70 1 6 0.19 0.24 0.18 0.13 0.08 0.15 0.09 0.28 1 The details of the analytical methods used for age determinations are described in Moore (1993). 2 Samples are in stratigraphic order: the youngest are on right. 3 Location data based on NAD27. 4 MWSD = mean weighted standard deviation. 5 The age reported for biotite and sanidine analyses are mean ages; hornblende analyses are isotope correlation ages. 6 1σ = one-sigma standard deviation. 2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 165 Fernow Quartz Latite, one of the oldest volcanic units, Province. These observations are compatible with pro- establishes a minimum age for initiation of magmatism duction of subduction-related magmas beneath the East in the East Tintic region (Utah Geological Survey and Tintic volcanic field during the early Oligocene, although New Mexico Geochronology Research Laboratory, East Tintic magmatism would have been behind the 2007; Keith and others, in preparation). Clark (2003) active volcanic front, which was several hundred kilome- obtained a similar argon age (34.83 ± 0.15 Ma) for the ters farther south. Fernow to the south in Sage Valley. East Tintic magma- tism continued until at least 32.70 ± 0.28 Ma (table 1). COMPOSITION The two “SP” samples in table 1 are from locations just north of the East Tintic Mountains and are from units that Whole-rock major- and trace-element analyses were overlie possibly the oldest East Tintic volcanic field obtained by wavelength dispersive X-ray fluorescence unit—a welded ash-flow tuff, the Packard Quartz Latite. spectrometry using a Seimens SRS 303 at Brigham Christiansen and others (2007, this volume) describe Young University-Provo. Representative analyses are these “SP” units and associated volcanics. given in table 2. A description of techniques and the Outcrops of Paleozoic sedimentary rocks are most complete geochemical data set (as an Excel file) are abundant in the northern and southern parts of the range available at http://www.geology.byu.edu/faculty/ehc (figure 1). These Paleozoic rocks experienced significant under the heading Resources. Phenocryst compositions folding and faulting during the Mesozoic orogenies of were determined using a JEOL JXA-8600 Superprobe by western North America. Farmer and DePaolo (1983) Choon Sun Kim at the University of Georgia with an suggested that basement rocks beneath this portion of the accelerating potential of 15 keV, a 15 nA beam, and Basin and Range Province are Proterozoic. Nelson and counting times of 40s for standards and 20s for un- others (2002) show that the nearby Santaquin Complex knowns. was accreted to the Archean craton no earlier than ca. Igneous rocks of the East Tintic volcanic field are 1700 Ma and underwent metamorphism prior to ca. 1670 potassic, silica-oversaturated, metaluminous to slightly Ma. peraluminous, and range from shoshonite to rhyolite— Moore (1993) reported on the volcanic stratigraphy 53 to 78 weight percent (wt. %) SiO2; (table 2; figure 2). and age relations of the field (see also the stratigraphic Compositions plot mostly in the alkali-calcic field on the summary of Hintze, 1988, p. 149). This work substan- modified alkali-lime versus silica diagram of Frost and tially updates and revises that of Morris and others others (2001). On the silica versus FeOtotal/(FeOtotal + (Morris and Anderson, 1962; Morris, 1975; Morris and MgO) diagram of Frost and others (2001), using the Lovering, 1979), and Hannah and Macbeth (1990). dividing line of Miyashiro (1974), East Tintic samples Further clarification of the volcanic stratigraphy and age straddle the ferroan/magnesian boundary.