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
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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. Compositional relations is ongoing by Keith and others (in preparation). trends for major and trace elements are shown on SiO2 At least 100 km3 of magma was erupted during the variation diagrams (figures 3 and 4). Representative roughly 2-million-year life span of the volcanic field mineral modes are shown in table 3. We group the com- (Moore, 1993). The magmatic history of the field con- positions of field units of the East Tintic volcanic field sists of the formation of nested calderas that produced into the following three compositional series: the small deposits of tuff and numerous lava flows. By vol- shoshonite-trachyte series, the trachyandesite series, and ume, roughly twice as much lava as ash was erupted. the rhyolite series. These series are based on composi- The volcanic field is one of the easternmost members of tional groupings and trends that we determined were pet- the Tintic–Deep Creek magmatic belt, an east-west elon- rogenetically important. We describe these series in this gate zone of Cenozoic magmatism that extends from the section and interpret them in the next. Colorado Plateau westward into the Deep Creek Range Shoshonite-trachyte series rocks are petrogenetically of west-central Utah (Stewart and others, 1977). Activity important (see below), but volumetrically small. This in nearby magmatic centers, including those in the West series includes the most mafic samples of the field and Tintic Mountains (Stein and others, 1990), Salt Creek ranges from 53 to 57 and 63 to 68 wt. % SiO2. Compared area (Keith and others, 1991; New Mexico Geochron- with trachyandesite series rocks at equal SiO2, shoshon- ology Research Laboratory and Utah Geological Survey, ite-trachyte series rocks have higher K2O, Rb, Zr, Ba, 2005), and Bingham mining district (Moore, 1973), was and Al2O3/CaO and lower Ni, Cr, Fe2O3, and MgO (table roughly contemporaneous with that in the early Oligo- 3; figures 3 and 4). Based on composition and pheno- cene East Tintic volcanic field. Severinghaus and cryst assemblage, the shoshonite-trachyte series is divid- Atwater (1990) reconstructed the time-integrated geome- ed into two groups: a clinopyroxene (cpx) group, and a try and thermal history of the Farallon and Vancouver two-pyroxene (2-px) group. Compositionally, the cpx plates, which were subducted beneath western North group follows a tight mineral-control line, whereas the 2- America during the Cenozoic. Best and others (1989), px group is more scattered (e.g., figures 3e and 4f). The Best and Christiansen (1991), and Christiansen and oth- cpx group consists of the shoshonite of Buckhorn ers (2007, this volume) suggest that, beginning at around Mountain and the Latite Ridge Latite units, and the 2-px 45 Ma in northern Utah and Nevada, volcanism spread group consists of the Dry Herd Canyon Latite and the southward across what is now the Basin and Range Big Canyon Latite units (Moore, 1993). Patterns on 166 Central Utah — Diverse Geology of a Dynamic Landscape
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Figure 2. International Union of Geological Sciences (IUGS) classification diagram (Le Bas and others, 1986) for East Tintic rocks.
T able 3. Representative phenocryst modes. Series Shoshonite-Trachyte Rhyolite Trachyandesite Group Clinopyroxene Two-pyroxene Fernow Two-pyroxene Clinopyroxene Sample1 TD18 TD39 TD55 ET153 TJ198B TD47 TJ181 TD45 TD46C TD46E TD56 TD63 TD68 TD6 TJ126 TJ51
Latitude (39°N) 50’10” 51’13” 48’20” 55’30” 55’4” 46’15” 55’42” 45’37” 46’25” 47’26” 47’19” 49’13” 49’20” 47’44” 51’42” 51’48” Longitude (112°W) 4’43” 5’38” 4’45” 2’3” 1’33” 5’56” 3’23” 6’49” 6’33” 6’28” 5’19” 4’8” 4’29” 3’17” 6’0” 3’30”
Phenocryst % 19.8 34.2 8.8 4.5 11.1 15.0 31 47.9 32.9 14.1 23.9 32.1 35.0 22.3 37.0 31.0 Lithic % - - - 5.0 6.1 - - 1.1 - - 3.8 - - - - - Points Counted 2000 2000 2000 1959 2264 2000 2261 2000 2000 2000 2000 2000 2000 1000 2000 2000
Plagioclase 60.1 57.3 - 63.0 - 58.5 62.4 33.5 64.4 68.7 58.8 52.0 63.1 65 63.2 55.3 Sanidine - - - trace - - - 26.7 ------1.5 Quartz ------31.7 - - 9.2 - - - - - Clinopyroxene 30.3 31.9 31.8 - 3.6 17.7 25.4 - 12.5 14.2 12.0 24.5 10.9 5.8 15.4 13.4 Orthopyroxene - - - - - 9.4 5.1 - 9.4 8.5 9.7 11.4 4.4 10.3 - - Biotite - - - 26.1 15.2 - - 6.7 4.0 - - trace 9.2 - 14.3 19.8 Hornblende ------6.3 - - Olivine - - 68.2 trace ------Fe-Ti Oxides 9.6 10.8 trace 10.9 8.0 14.4 7.1 2.9 9.7 8.5 8.8 11.4 12.4 12.6 7.0 9.5 Apatite trace trace trace trace trace trace trace trace trace trace trace trace trace trace trace trace
1 Samples were chosen to represent the variations in unit mineral mode. Sample – field unit correlations, and a complete list of all measured modal analyses are reported in Moore (1993). 168 Central Utah — Diverse Geology of a Dynamic Landscape
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Figure 3. Major-element variation diagrams for East Tintic rocks. The classification lines in figure 3A are after Ewart (1982). Symbols as for figure 2. 2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 169
140 3000 E A 2700 120 2400 100
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100 100
52 54 56 58 60 62 64 66 68 70 72 74 76 52 54 56 58 60 62 64 66 68 70 72 74 76 SiO2 (wt. %) SiO2 (wt. %)
Figure 4. Trace-element variation diagrams for East Tintic rocks. Symbols as for figure 2. 170 Central Utah — Diverse Geology of a Dynamic Landscape chondrite-normalized trace-element diagrams of mafic North Standard Latite, both non-mineralizing units shoshonite-trachyte series samples (especially TD55) are (Moore, 1993). Trachyandesite sample patterns on chon- smooth curves—suggesting they are relatively undiffer- drite-normalized trace-element diagrams show negative entiated rocks—that peak at Th, and have negative Nb Sr, P, and Ti anomalies that become larger as SiO2 and Ti anomalies (figure 5). Negative Sr, P, and Ti anom- increases (figure 5). We attribute this to fractional crys- alies become larger as SiO2 increases. We attribute this tallization of the observed phenocryst assemblages. to fractionation of the observed minerals (table 2). In The temperatures and oxygen fugacities of shoshon- evolved shoshonite-trachyte series samples positive light ite-trachyte series and trachyandesite series rocks were rare earth element (LREE) and Zr anomalies increase estimated by Kim (1992) using the compositions of with evolution, suggesting the absence of LREE- and Zr- pyroxenes (Davidson and Lindsley, 1985) and Fe-Ti sequestering phases like monazite, xenotime, or allanite oxides (Andersen and Lindsley, 1988). For cpx shoshon- and zircon, respectively. Zircon saturation calculations ite-trachyte series magma, three determinations from a show that shoshonite-trachyte series magma was, for its silicic unit (Latite Ridge Latite) yielded Fe-Ti oxide tem- composition, too hot and too Na-, K-, and Ca-rich to sta- peratures between 950 and 960°C and oxygen fugacity bilize zircon (Watson and Harrison, 1983). values between 0.88 and 0.94 log units above the fay- The rhyolite series is distinguished by high silica alite-magnetite-quartz oxygen buffer (FMQ) (Kim, content (70 to 78 wt. %), and consists of three field units: 1992). Additional temperature information for cpx the Fernow Quartz Latite, the rhyolite of Keystone shoshonite-trachyte series rocks can be gleaned from a Springs, and the Packard Quartz Latite (Moore, 1993). model for apatite saturation derived from experiments Because rhyolite series compositions do not overlap in (Watson, 1980). Assuming apatite saturation occurred in silica content with the other series, comparisons with the mafic samples of this series as predicted by peak con- those series are not meaningful. The units which com- centrations of P2O5 at 55 wt % SiO2 (figure 3e), the sat- prise this series have relatively uniform mineral modes uration model predicts a magmatic temperature of and chemical compositions, except that the Packard ~1000°C. These independent estimates of cpx shoshon- Quartz Latite has extremely high concentrations of Ba, ite-trachyte series magmatic temperatures are consis- ~2700 parts per million (ppm), and relatively high con- tent—as expected, the less evolved magma is hotter. For centrations of Sr, ~425 ppm. Contamination by crustal 2-px shoshonite-trachyte series magma, determinations rocks could explain the anomalous composition of from two lava flows (Latite of Dry Herd Canyon and Packard rhyolite series magma. The Jurassic Arapien Latite of Rock Canyon) yielded 2-px temperatures Shale, which contains evaporite deposits, is exposed near between 1063 and 1162°C, an Fe-Ti oxide temperature of the volcanic field (Hintze, 1980). It is possible that this 1028°C, and oxygen fugacity values between 0.6 and 1.7 unit, or some other unit containing evaporites, underlies above FMQ. For 2-px trachyandesite series magma, four the volcanic field at depths where rhyolite series magma determinations from an extrusive unit (Latite of Rock could have been contaminated by them. If this did occur, Canyon) and four from an intrusive unit (Silver City the magma preferentially assimilated Sr and Ba, because Monzonite) yielded 2-px temperatures between 904 and the concentrations of other elements that would be abun- 990°C, Fe-Ti oxide temperatures between 888 and dant in evaporite deposits (e.g., Rb) appear not to have 958°C, and oxygen fugacity values between 1.72 and been affected. Some of the silicic rocks in the Soldiers 2.45 log units above FMQ. No temperature or oxygen Pass area to the north of the volcanic field also display fugacity estimates were made for cpx trachyandesite elevated Ba concentrations (Christiansen and others, 2007, series or rhyolite series samples. The mineral assem- this volume). blages of rhyolite series units indicate that rhyolite series The trachyandesite series is by far the most volum- magma was significantly cooler than shoshonite-trachyte inous. Rocks of this series range from 56 to 68 wt. % series or trachyandesite series magma. These tempera- SiO2 (i.e., between the mafic and silicic rocks of the vol- ture and oxygen fugacity estimates support the series canic field) and are characterized by the lack of strong groupings we created based on compositional character- mineral control (e.g., figures 3g, 4b, 4d, and 4g). Based istics. on differences in mineral mode, this series is subdivided into two groups: a cpx group, and a 2-px group. There are no significant compositional differences (for the ele- PETROGENESIS ments we measured) between these two groups; how- ever, intrusions of 2-px group magma are associated with Major- and trace-element trends on SiO2 variation mineralization while those of the cpx-group are not diagrams (figures 3 and 4) indicate strong mineral con- (Keith and others, 1993, 1997; Stavast and others, 2006). trol in the evolution of magmas of the East Tintic vol- The 2-px group, shown in figure 1 as the ore-related latite canic field, but are too incoherent to be explained by evo- flows and monzonite instrusions, consists of the Latite of lution along a single fractional crystallization line. These Rock Canyon, the Silver City Monzonite, the Copper- trends and major- and trace-element models suggest that opolis Latite Tuff, and the Andesite of Rock Canyon magma mixing was also important in generating the units (Moore, 1993; Stavast and others, 2006). The cpx- observed compositions (explained below). Major-ele- group consists of the Latite of Sunrise Peak and the ment modeling of fractional crystallization was done
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 171
1000 Shoshonite-Trachyte Series
e l t 100 n a M
e v i t i m
i 10 r P / k c o R 1
0.1 Rb Ba U K Nb La Ce Sr Nd P Zr Ti Y
1000 Rhyolite Series e l 100 t n a M
e v i t i 10 m i r P / k c o
R 1
0.1 Rb Ba U K Nb La Ce Sr Nd P Zr Ti Y
1000 Trachyandesite Series
e l t 100 n a M
e v i t i m
i 10 r P / k c o R 1
0.1 Rb Ba U K Nb La Ce Sr Nd P Zr Ti Y
Figure 5. Trace-element patterns for each of the compositional series using the normalizing values of McDonough and Sun (1995). Symbols as for figure 2. 172 Central Utah — Diverse Geology of a Dynamic Landscape using mass balance calculations (Stormer and Nicholls, The first segment of the major-element fractional 1978) with the phenocryst compositions of Kim (1992) crystallization model (table 4) predicts ~13% fractionat- and the whole-rock chemical analyses collected for this ed material and has a very large sum of the squared resid- study (table 2). Trace-element modeling of fractional uals, 3.66. Adding plagioclase and/or magnetite does not crystallization was done using multi-sequence fractional improve the model. The second segment of the major- crystallization calculations (Allégre and Minster, 1978). element model predicts subtraction of ~37% additional The parameters for the major- and trace-element frac- material, and has a small sum of the squared residuals, tional crystallization models are found in tables 4 and 5. 0.46. A major-element model that combines segments The trace-element fractional crystallization and mixing one and two has a low sum of the squared residuals, 0.07, models were applied to Rb, Sr, Ba, Ni, Cr, Sc, V, Zr, U, and predicts ~73% subtracted material. The third seg- Th, La, and Ce compositions with equal success. The fits ment of the major-element model predicts subtraction of of the models to Rb, Sr, and Ba (and Zr and Cr) data are ~14% additional material, and has a sum of the squared shown in figures 6, 7, and 8. residuals equal to 0.79. Table 5 shows the mineral modes and partition coef- Shoshonite-Trachyte Series ficients that make up the trace-element fractional crystal- lization models. These models are illustrated in figure 6. Major- and trace-element models suggest that frac- The first segment models evolution from the parent tional crystallization dominated the evolution of cpx magma (TD55) to TD39 by removing 30% of the magma shoshonite-trachyte series magmas, which range in com- as olivine and clinopyroxene. The second segment mod- position from ~53 to 68 wt. % SiO2. The composition of els evolution from TD39 to TJ198B by removing an the most mafic sample, TD55, was used as the parent additional 33% of the magma as plagioclase, clinopyrox- magma in modeling the petrogenesis of this series. The ene, magnetite, and apatite. The third segment models whole-rock and olivine compositions of this sample have evolution from TJ198B to ET153 by removing an addi- too little MgO and Ni to be a simple partial melt of man- tional 17% of the magma as plagioclase, pyroxene, bio- tle peridotite, indicating that some amount of evolution tite, hornblende, and apatite. A likely cause for the scat- took place in the primitive magma generated by partial ter of the data about the trace-element models is that melting to produce the composition of this sample. sample compositions evolved from a parent magma more Based on changes in the phenocryst (= fractionating) mafic than TD55, but followed slightly different frac- assemblage, the evolution of this series was divided into tional crystallization paths. As noted above, while only three segments: (1) olivine + clinopyroxene, (2) plagio- the fit of the model to Rb, Sr, and Ba data are shown in clase + clinopyroxene + magnetite + apatite, and (3) pla- figure 6, the model was applied with equal success to Ni, gioclase + pyroxene + biotite + hornblende + magnetite Cr, Sc, V, Zr, U, Th, La, and Ce compositional data. + apatite. The first segment models changes from sam- Segments one and three of the major-element frac- ples TD55 to TD39, the second from samples TD39 to tional crystallization models are impaired because SiO2 TJ198B, and the third from samples TJ198B to ET153. changes so little over these segments. Small SiO2 The inception of plagioclase and biotite fractionation for changes do not allow the necessary natural averaging trace-element models was estimated from inflection effects needed in mass balance calculations. The major- points on, respectively, SiO2 vs. Sr (and Ba) and SiO2 vs. element models are also impaired by the mineral compo- Cl (and F) variation diagrams. sitions upon which they are based. The models would be Table 4. Major-element fractional crystallization models1 1 Tfableor cl inopyroxene4. Major-eleme shoshonite-trachytent fractional crystallization series models magma.for clino- pyroxene shoshonite-trachyte series magma. Segment 1 2 1+2 3
Parent TD 55 TD39 TD55 TJ198B Daughter TD39 TJ198B TJ198B ET153
Plagioclase - 43.0 51.3 68.0 Clinopyroxene 83.0 33.7 33.3 23.3 Biotite - 11.7 7.1 3.2 Olivine 17.0 - 2.0 - Magnetite - 11.6 6.4 5.5
Sum % Subtracted 13.3 37.6 73.2 14.4
Sum R2 3.66 0.46 0.07 0.79
1 Phenocryst modes represent weight percent crystals subtracted (as % of all mineral phases) from the parent magma. 2 Sum of the squares of the residuals of the nine elements used. 2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 173
Table 5. Trace-element fractional crystallization model parameters. Table 5. Trace-element fractional crystallization model parameters. Mineral Mode1 & Partition Coefficient2 (D)
Series Pl San Cpx Opx Bt Hb Ol Ap Dbulk
Clinopyroxene mode - - 0.31 - - - 0.68 -- Shoshonite- DRb - - 0.01 - - - 0.01 0.11 0.010
Trachyte DSr - - 0.14 - - - 0.00 0.39 0.044 F=1.0–0.70 3 DBa - - 0.03 - - - 0.01 0.05 0.016
4 Clinopyroxene mode 0.60 - 0.30 - - - - trace - Shoshonite- DRb 0.02 - 0.01 - - - - 0.11 0.016
Trachyte DSr 3.00 - 0.08 - - - - 0.39 1.826 F= 0.70–0.37 3 DBa 0.48 - 0.03 - - - - 0.05 0.297
4 Clinopyroxene mode 0.67 - 0.04 - 0.20 - - trace - Shoshonite- DRb 0.02 - 0.01 - 4.00 - - 0.11 0.814
Trachyte DSr 3.00 - 0.08 - 0.27 - - 0.39 2.069 F= 0.37–0.20 3 DBa 0.48 - 0.03 - 10.0 - - 0.05 2.323
mode 0.60 - 0.22 0.07 - - - trace4 - Two-pyroxene DRb 0.02 - 0.01 0.02 - - - 0.11 0.382 Shoshonite- Trachyte 5 DSr 3.00 - 0.08 0.02 - - - 0.39 0.707 DBa 0.48 - 0.03 0.02 - - - 0.05 1.830
mode 0.60 - 0.14 0.09 0.03 0.02 - trace4 -
Two-pyroxene DRb 0.02 - 0.01 0.02 4.00 0.05 - 0.11 0.140 5 Trachyandesite DSr 3.00 - 0.08 0.02 0.27 0.23 - 0.39 1.835
DBa 0.48 - 0.03 0.02 10.0 0.35 - 0.05 0.606
mode 0.59 0.01 0.15 - 0.17 - - trace4 -
Clinopyroxene DRb 0.02 0.38 0.01 - 4.00 - - 0.11 0.140 5 Trachyandesite DSr 3.00 9.40 0.08 - 0.27 - - 0.39 1.835
DBa 0.48 6.60 0.03 - 10.0 - - 0.05 0.606
1 Pl = plagioclase; San = sanidine; Cpx = clinopyroxene; Opx = orthopyroxene; Bt = biotite; Hb = hornblende; Ol = olivine; Ap = apatite. 2 Partition Coefficients are from Arth (1976) and Henderson (1990). Because the partition coefficients for Rb, Sr, and Ba in Fe-Ti oxides are all 0, their modes were not included in the table, though magnetite was included in the models. 3 F = the fraction of liquid remaining. 4 The apatite modes used were: 0.001% for segment 1 (F = 1-0.7) and 0.005% for all other segments and models. 5 For this series, bulk D was calculated using an average of the sample mineral modes (table 3). 174 Central Utah — Diverse Geology of a Dynamic Landscape
Figure 6. The cpx shoshonite-trachyte series trace-element fractional crystallization model, which uses the data of table 5. The heavy line is the model. The model was applied to Rb, Sr, Ba, Ni, Cr, Sc, V, Zr, U, Th, La, and Ce compositional data with equal success. The fit of the model to Rb, Sr, and Ba data is shown here—Rb vs. Sr in figure A, Rb vs. Ba in figure B, and Sr vs. Ba in figure C. F- values indicate fraction of melt remaining (F = 1.0 indicates all melt; F = 0 indicates all solid). The cpx shoshonite-trachyte series frac- tional crystallization model (heavy line) is also the 2-px shoshonite-trachyte series mixing envelope. The arrow indicates the direction of compositional change caused by fractional crystallization (after mixing) for 2-px shoshonite-trachyte series magma. Symbols as for figure 2. 2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 175 more accurate if we had mineral composition data for The high Al2O3/CaO ratio of shoshonite-trachyte series samples from each segment. Unfortunately, the data of magma is likely due to clinopyroxene fractionation Kim (1992) are from a single cpx shoshonite-trachyte (without plagioclase). Meen (1987) proposed that high- series sample. In spite of these problems, the major-ele- pressure fractional crystallization of orthopyroxene from ment models do predict roughly the same amount of frac- basaltic magma at or near the base of continental crust tionated material as the trace-element models (tables 4 could cause the K2O/SiO2 ratio of the magma to increase and 5). We attribute the discrepancies between the dramatically. There is no orthopyroxene in cpx shoshon- major- and trace-element models to the problems with ite-trachyte series samples, but the SiO2 content and the major-element model just described. K2O/SiO2 ratio in clinopyroxene is essentially the same Taken as a whole, the major- and trace-element mod- as in orthopyroxene. Further, Draper and Johnston els predict that ~80% of an initial magma with the com- (1992) show that in Mg-rich arc basalts, clinopyroxene is position of TD55 was subtracted as phenocrysts during stable at high pressure whereas both clinopyroxene and the evolution from shoshonite to trachyte (from TD55 to orthopyroxene are stable at low pressure. We propose ET153). The percent of magma subtracted as minerals to that the elevated K2O, Rb, and Al2O3/CaO ratio and mod- produce trachyte from the actual parent magma would be est SiO2 enrichment of cpx shoshonite-trachyte series greater than 80% since TD55 is too Ni- and MgO-poor to magma result from extensive high-pressure fractionation be a direct partial melt of peridotite. Our calculations of clinopyroxene (with plagioclase absent) from a primi- suggest that fractional crystallization dominated the evo- tive magma. As fractionation progressed, K2O and H2O lution of cpx shoshonite-trachyte series magma. concentrations would have increased to levels sufficient Two-pyroxene shoshonite-trachyte series samples to stabilize biotite at the expense of clinopyroxene. have compositions that lie between the mafic and the sili- cic cpx shoshonite-trachyte series samples (figures 4 and Rhyolite Series 6). These compositional variations, supported by mag- matic temperatures and oxygen fugacity values, are con- There is little direct evidence for the source of rhyo- sistent with production of 2-px shoshonite-trachyte series lite series magma. The fundamentally basaltic models magmas by mixing between mafic and silicic cpx for arc magmatism mentioned above predict that conti- shoshonite-trachyte series magmas. Low Zr concentra- nental crustal melts would be produced by heat released tions of silicic 2-px shoshonite-trachyte series samples from ponded, crystallizing basaltic magma. As these (figure 4d) are, however, not consistent with this genetic continental partial melts would be intimately associated model. Since silicic 2-px shoshonite-trachyte series with the basaltic magmas, extensive mixing between the magma at ~1000°C would be zircon-undersaturated by two would be expected. We believe that Fernow, roughly 400 ppm Zr, the low Zr concentrations could not Keystone, and Packard rhyolite series magmas are likely have been produced by zircon removal (Watson and the differentiates of lower crustal partial melts, and that Harrison, 1983). Mixing in a small amount of Fernow these partial melts were produced as a result of the heat rhyolite series magma would explain the Zr-concentra- produced by the crystallization (including clinopyrox- tion discrepancy. We propose that 2-px shoshonite-tra- ene) of ponded primitive cpx shoshonite-trachyte series chyte series magma was generated by mixing between magma in the lower crust. mafic and silicic cpx shoshonite-trachyte series magma, with perhaps some involvement of Fernow rhyolite series Trachyandesite Series magma in generating silicic 2-px shoshonite-trachyte series magma. Trachyandesite series samples have compositions The compositional characteristics of shoshonite-tra- that lie between the mafic and silicic magmas of the vol- chyte series magma (e.g., negative Nb and Ti anomalies), canic field (figures 3 and 4). The compositional varia- the temporal and spatial association with a subducting tions of this series indicate mineral control, but are too oceanic plate (Severinghaus and Atwater, 1990), and the varied to have been produced by fractional crystallization association with subduction-related mineralization (e.g., alone along a single trend (e.g., figures 3a, 3d, 4d, 4f, and porphyry Cu-Mo and Ag-Au vein deposits; Lindgren and 4g). These compositional characteristics, supported by Laughlin, 1919; Morris and Lovering, 1979; Keith and magmatic temperature and oxygen fugacity estimates, others, 1989) suggest that cpx shoshonite-trachyte series suggest that magma mixing and subsequent fractional magma originated above a Cenozoic subduction zone. crystallization governed the generation and evolution of Many workers have proposed a fundamentally basaltic trachyandesite series magma. Compositional variations view for arc-related magmatism (e.g., Hildreth, 1981; suggest that the mafic end member for mixing was mafic Fyfe, 1982; Grove and Kinzler, 1986). These models cpx shoshonite-trachyte series magma. Zirconium con- predict injection and ponding of basaltic magma at or centrations (figure 4d) as well as estimated magmatic near the base of the crust. As noted before, shoshonite- temperatures and oxygen fugacity values indicate that trachyte series samples are characterized by high silicic shoshonite-trachyte series magma can be ruled out Al2O3/CaO, Zr, Rb, and K and modest SiO2 enrichment. as the silicic end member for the trachyandesite series. Clinopyroxene and/or plagioclase fractionation affects Compositional variations are, however, consistent with the Al2O3/CaO ratios in cogenetic fractionating magmas. rhyolite series magma as the silicic end member of mix- 176 Central Utah — Diverse Geology of a Dynamic Landscape ing. The compositional trends illustrated in figure 7 indi- include silicic cpx shoshonite-trachyte series magmas is cate that neither Packard nor Keystone rhyolite series a valid alternative, but nearly eliminates the need for magmas are the silicic end members of mixing—since fractional crystallization. Our model allows an important Packard rhyolite series magma has too much Ba and too role for both magma mixing and subsequent fractional little Zr, and Keystone rhyolite series magma has too lit- crystallization and is meant to be illustrative, not defini- tle Cr and Rb/Zr ratios that are too low. We propose that tive. We suggest that primitive trachyandesite series trachyandesite series magmas were generated by mixing magmas were produced by magma mixing (within a mix- between mafic cpx shoshonite-trachyte series and ing envelope similar to the one we have proposed) and Fernow rhyolite series magmas. Figure 8 illustrates the then evolved (often to compositions outside the mixing compositions that could be produced by the combined envelope) by fractional crystallization. effects of magma mixing and fractional crystallization. If the magmas of both trachyandesite series groups The fractional crystallization arrows in figure 8 were cal- were produced by this mechanism, why does one group culated using the parameters in table 5 and indicate the contain only clinopyroxene while the other contains both direction in which primitive (recently mixed) trachyan- clinopyroxene and orthopyroxene? In addition, if 2-px desite series magmas would evolve by fractionation (of shoshonite-trachyte magma was produced by mixing the observed minerals). We are not able to determine the between two orthopyroxene-absent magmas, why does it relative importance of magma mixing and fractional contain orthopyroxene? As mentioned above, Draper crystallization in generating trachyandesite series mag- and Johnston (1992) show that in high-Mg arc basalts, mas. For example, widening the mixing envelope to only clinopyroxene is stable at high pressure whereas
Figure 7. Cr vs. Ba (A) and Zr vs. Rb/Zr (B) variation diagrams, showing (1) the rhyolite series magma responsible for mixing with mafic cpx shoshonite-trachyte series magma to produce trachyandesite series magmas was Fernow rhyolite series magma rather than Keystone or Packard rhyolite series magmas, and (2) assimilation of crustal material was not important in generating the com- positions of shoshonite-trachyte series magma. Symbols as for figure 2. 2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 177
Figure 8. The trace-element magma mixing and fractional crystallization models for trachyandesite series magmas. The models were applied to Rb, Sr, Ba, Ni, Cr, Sc, V, Zr, U, Th, La, and Ce compositional data with equal success. The fit of the model to Rb, Sr, and Ba data is shown here—Rb vs. Sr in figure A, Rb vs. Ba in figure B, and Sr vs. Ba in figure C. The mafic end member of mixing is mafic cpx shoshonite-trachyte series magma (represented by the mafic portion of the fractional crystallization model of figure 6). The silicic end member of mixing is Fernow rhyolite series magma. Arrows show the directions that magmatic compositions would be dis- placed, after mixing, as a result of fractional crystallization. The directions were calculated using the parameters of table 5. Symbols as for figure 2. 178 Central Utah — Diverse Geology of a Dynamic Landscape both clinopyroxene and orthopyroxene are stable at low Tintic volcanic field. The cpx shoshonite-trachyte series pressure. Assuming that their interpretations regarding magma evolved mainly by fractional crystallization. The pyroxene stability are applicable to the mafic and inter- high K2O, Rb, and Al2O3/CaO ratios and modest SiO2 mediate magmas of the East Tintic volcanic field, then enrichment of this series appear to result from extensive, the observed differences in mineral assemblages (specif- high-pressure fractional crystallization of clinopyroxene ically pyroxene) could indicate that phenocryst assem- (without plagioclase). The 2-px shoshonite-trachyte blages in 2-px trachyandesite series and 2-px shoshonite- series magma was likely produced by magma mixing trachyte series magmas last equilibrated at low pressure, between mafic and silicic shoshonite-trachyte series and that the phenocryst assemblages of cpx trachyan- magma at low pressure. Assimilation appears not to have desite series and cpx shoshonite-trachyte series magmas been important for the shoshonite-trachyte series. last equilibrated at high pressure. This may indicate that Fernow rhyolite series magma was likely the differenti- mixing occurred in these groups at those pressures. ate of a lower crustal partial melt. Trachyandesite series magma evolved by magma mixing—between mafic Assimilation shoshonite-trachyte series and Fernow rhyolite series magmas—and subsequent fractional crystallization, at The role of assimilation of crustal material can be low pressure for 2-px trachyandesite series magma and at estimated most effectively with isotopic data; however, high pressure for cpx trachyandesite series magma. We the variability of incompatible trace-element ratios is believe parental cpx shoshonite-trachyte series magma also a reasonably good indicator of open-system process- originated in the mantle wedge above a Cenozoic sub- es. Figure 7 illustrates the variability of two incompati- duction zone and then interacted with older subduction- ble trace elements, Rb and Zr. The Rb/Zr ratio is rela- metasomatized lithospheric mantle. Figure 9 is a schem- tively constant for shoshonite-trachyte series samples, atic diagram that illustrates our petrogenetic interpreta- which is consistent with our interpretations that magmat- tions—namely, how cpx shoshonite-trachyte series and ic differentiation for cpx shoshonite-trachyte series Fernow rhyolite series magmas evolved and interacted to produce the magmas of the East Tintic volcanic field. magma was dominated by fractional crystallization, that Our petrogenetic model has important implications for 2-px shoshonite-trachyte series magma may have been the genesis of ore bodies in the Tintic mining district and generated by mixing of mafic and silicic cpx shoshonite- related areas (see Keith and others, 1993, 1997; Stavast trachyte series magma (because mixing would not and others, 2006). change the ratio), and that assimilation was not signifi- cant for shoshonite-trachyte series magma. If we have interpreted the origin of Fernow rhyolite series magma ACKNOWLEDGMENTS correctly, then the mixing of Fernow rhyolite series and shoshonite-trachyte series magmas could be considered This study was funded by grants from the Brigham crustal assimilation. Young University Department of Geology and National Science Foundation (# EAR-9114980). We thank Dave CONCLUSIONS Wark (Rensselaer Polytechnic Institute), Tobi Kosanke (Shell Oil Company), and Don Clark, Tom Chidsey, and We propose that fractional crystallization and magma Grant Willis (Utah Geological Survey) for helpful re- mixing controlled the evolution of magmas in the East views of this manuscript.
Figure 9. Schematic diagram showing the proposed petrogenesis of East Tintic volcanic field magmas. Subduction-related cpx shoshonite-trachyte series magma ponded at or near the base of the crust. Heat from the crystallizing magma induced melting in continental material producing rhyolite series melts. Two-pyroxene shoshonite-trachyte magma was produced by mixing between primitive and evolved shoshonite-trachyte series magma at low pressure. Trachyandesite series magma was generated by mixing between Fernow rhyolite series and mafic cpx shoshonite-trachyte series magmas, at high pressure for cpx trachyandesite magma and at low pressure for 2-px trachyandesite magma. 2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors 179
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