Contributions to Contrib Petrol (1983) 83: 16-30 Mineralogy and Petrology

N~BSpringer-Verlag 1983

The Petrogenesis of Topaz Rhyolites from the Western United States

E.H. Christiansen', D.M. Burt, M.F. Sheridan, and R.T. Wilson2 Department of Geology, Arizona State University, Tempe, Arizona 85287, USA ' Department of Geology, University of Iowa, Iowa City, Iowa 52242, USA Applied Geologic Studies, Englewood, Colorado 80110, USA

Abstract. High-silica topaz-bearing rhyolites of Cenozoic emplaced as domes, lava flows or shallow intrusions. They age are widely distributed across the western USA and may also contain Fe-Mn garnet, beryl, bixbyite, pseudo- Mexico. They are characteristically enriched in fluorine , hematite and fluorite in miarolitic cavities or with- (>0.2 wt.%) and incompatible lithophile elements (e.g. Li, in the groundmass of devitrified samples. They are geo- Rb, Cs, U, Th, Be). In addition to topaz, the rhyolites chemically distinct as well, due to their dramatic enrichment contain garnet, bixbyite, pseudobrookite, hematite and in lithophile elements such as U, Be, Li, Rb, Th and Nb. fluorite in cavities or in their devitrified groundmasses. Based on a review of topaz rhyolites from the western Magmatic phases include sanidine, quartz, oligoclase and United States (Christiansen et al. 1982), this paper summar- Fe-rich biotite. Allanite, fluorite, zircon, apatite and magne- izes the petrogenetic implications of their geochemistry and tite occur in most; pyroxene, hornblende, and ti- distribution. We suggest that topaz rhyolites are the fluor- tanite occur in some. The rhyolites crystallized over a wide ine-rich equivalents of anorogeneic granites - as such they temperature interval (850" to 600" C) at,fo2 that ranges from provide some insights regarding the origin of anorogenic QFM to NNO. The REE patterns of most topaz rhyolites granites and about the possible role of fluorine in their are almost flat (La/Yb, = 1 to 3) and have deep Eu anoma- evolution. lies (Eu/Eu* = 0.01 to 0.02). Both parameters decrease with Practical interest in topaz rhyolites stems from their ge- differentiation. -bearing rhyolites have prominent netic association with volcanogenic deposits of Be, Sn, U, middle REE depletions. Topaz rhyolites appear to have and F (Burt and Sheridan 1981). The similarity of these evolved from partial melts of a residual granulitic source rocks to those associated with porphyry molybdenum de- in the Precambrian lower crust. According to the proposed posits suggests that topaz rhyolites may be indicators of model, the passage of hot mafic magmas through the crust subsurface porphyry, greisen or pegmatite mineralization produced partial melts as a result of the decomposition (Burt et al. 1982a). The petrogenetic model outlined here of F-rich biotite or amphibole. An extensional tectonic carries important implications about the ultimate source setting allowed these small batches of magma to rise with- of metals and fluids in these systems. out substantial mixing with contemporaneous mafic magmas. Some of the compositional differences between topaz Distribution and Ages rhyolites and peralkaline rhyolites may be attributed to the accumulation of fluorine and fluorphile elements (Al, Be, Topaz rhyolites are widespread in western North America Li, Rb, U, Th, HREE) in melts which give rise to topaz and their occurrence closely coincides with the limit of late rhyolites and chlorine and chlorophile elements (Ti, Fe, Cenozoic extensional faulting (Fig. 1). In the United States, Mn, Zn, Zr, Nb and LREE) in melts which yield peralkaline the emplacement of these rhyolites appears to have spanned rhyolites. Hence the F/CI ratio of the melt or its source most of the Cenozoic Era; their isotopic ages range from may determine the alumina saturation of the magma series. 50 m.y. (Little Belt Mountains, Montana) to 0.5 m.y. (Min- Topaz rhyolites are distinguishable from calc-alkaline rhyo- eral Mountains, Utah), although all but two are younger lites by lower Sr, Ba, Eu and higher F, Rb, U and Th. than 30 m.y. old (Table 1). Most topaz rhyolites lie within The usually low La/Yb ratios of topaz rhyolites distinguish the eastern and southern Basin and Range province (Idaho, them from both peralkaline and calc-alkaline rhyolite Nevada, Utah and Arizona) and along the Rio Grande suites. rift (New Mexico and Colorado) and thus appear to sur- round the Colorado Plateau. Similar middle to late Tertiary topaz rhyolites appear to be widespread in central Mexico (Sinkankas 1959,1976; Smith et al. 1950; Foshag and Fries Introduction 1942). The Mexican occurrences are associated with small A distinctive suite of fluorine-rich rhyolites is widely distrib- volcanogenic tin deposits (Huspeni et al. 1982) and may uted across the western United States. These rhyolites char- form a continuous belt with the topaz rhyolites in the west- acteristically contain topaz (AI,SiO,F,) and were generally ern United States. However, little geochemical data is avail- able about the Mexican occurrences and they are not con- Offprint requests to: E.H. Christiansen sidered further here. 17

120' lxoO looo go0 soo Table 1. Ages of Cenozoic Topaz Rhyolites in the Western United States

Location Age (m.y.1 Reference

1. Specimen Mountain, 28-27 Corbett Colorado 1968 2. Chalk Mountain, 28-27 Tweto and Case Colorado 1972 3. Nathrop, Colorado 28-29 Van Alstine 1969 4. Tomichi Dome, mid-Tertiary Stark and Behre Colorado 1936 5. Silver Cliff-Rosita, 26 Sharp Colorado 1978 6. Lake City, 18.5 Lipman et al. Colorado 1978a 7. Grants Ridge, 3.3 Bassett et al. New Mexico 1963 8. Black Range, 24 Elston et al. New Mexico 1976 120' llOD 100° 90' 9. Saddle Mountain, - Anthony et al. Fig. 1. Locations of Cenozoic topaz rhyolites in western North Arizona 1977 America (Christiansen et al. 1982). The limit of late Cenozoic ex- tensional faulting is also shown (Stewart 1978) 10. Burro Creek, Arizona mid-Tertiary Burt et al. 1981 11. Wah Wah Mountains 22-20 Lindsey and area, Utah Osmonson 1978, and M.G. Best Mineralogy 12 unpub. data 12. Mineral Mountains, 0.5 Lipman et al. Topaz rhyolites are normally phenocryst-poor, but in some Utah 1978 b lavas and shallow intrusions the phenocryst content may be as high as 40 percent. The major phenocrysts of topaz 13. Smelter Knolls, 3.4 Turley and Nash Utah 1980 rhyolites are sanidine (approximately Or,,), quartz and sodic plagioclase (usually oligoclase). Ferromagnesian min- 14. Thomas Range, 6 Lindsey Utah 1981 erals are rare, but phenocyrsts of Fe-rich biotite are present in crystal-rich varieties; Fe-rich hornblende and Fe- Mn 15. Keg 8 Lindsey et al. garnet are rare phenocrysts. Fe-rich pyroxene occurs in only Utah 1975 a few localities in lavas with high equilibration tempera- I6 Honeycomb Hills, 4.7 Turley and Nash Utah 1980 tures. Accessory phases include Fe - Mn - Ti oxides, apa- tite, fluorite, zircon and allanite. Two-feldspar and Fe-Ti 17. Spor Mountain, Utah 21 Lindsey oxide geothermometry indicate equilibration temperatures 1981 of 600" to 850" C (Christiansen et al. 1980; Turley and Nash 18. Cortez Mountains, 14-15 Wells et al. 1980; Evans and Nash 1978) that are negatively correlated Nevada 1971 with fluorine content at individual centers. The mineralogy 19. Sheep Creek Range, 14 Stewart et al. of the rhyolites from west-central Utah indicates crystalliza- Nevada 1977 tion at relatively low Oxygen fugacities (Fig. 2), near the 20. Jarbidge, Nevada 16 Coats et al. QFM buffer and at high HF/H,O fugacity ratios (lo-' 1977 to Turley and Nash 1980). In these rhyolites, low 21, ~lkh~~~~~~~~~i~~, 36 Chadwick fo2 may have stabilized ilmenite and clinopyroxene over Montana 1978 titanite or high f,, may have stabilized fluorite and ilmenite 22, Little Belt Mountains, 50 Witkind over titanite (Burt 1981). Some topaz rhyolites from other Montana 1973 regions contain titanite (Christiansen et al. 1982). Judging from fo2 estimates reported by Evans and Nash (1978) for Note: Numbers correspond to locations in Fig. 8 titanite-bearing rhyolites from the Mineral Mountains, Utah, these rhyolites may have evolved at higher fo2 than those from west-central Utah (Fig. 2). Fayalite is notable for its absence in both types, apparently as a result of high cooling and devitrification and generally reflect the high f,, which stabilizes biotite over fayalite in aluminous melts. activity of fluorine in the magmas and the higher fo2 that Along with topaz (nearly fluortopaz), fluorite, sanidine, prevailed after eruption (Burt et al. 1980). The presence of quartz, Fe - Mn-Ti oxides, cassiterite, biotite, garnet and these demonstrates that F, K, Na, Al, Si, Fe, Mn, beryl occur along fractures, in cavities and within the Be and Sn are partitioned into a vapor phase after eruption. groundmass of the rhyolites. They are the result of crystalli- The escape of such vapors could significantly alter the com- zation from a vapor phase released from the lavas during position of crystalline rocks. 18

-6 I I ~II!/I 1 I I 0 .. .., : Rock Chemistry and Differentiation ,*? 0 -8 - , ,, , 0;The chemical composition of typical topaz rhyolites from , / -10 - / , / Colorado, New Mexico, Utah, Nevada and Montana are / ,OQv* , ..- ., / presented in Table 2. There is little chemical diversity in . . -I2- >*' ,' ,'/B~~~~~TU~I,. .' >d;'/ the group as a whole. They are generally high in SiO, ".0" (>74%), Na,O (>3.6%), K,O/Na,O ratio (> 1.0), and g -14 -'/ , ...pi.':, fluorine (0.1-1%) and low in TiO, (<0.2%), CaO - ,, / (<0.9%), MgO (<0.2%), and P,O, (-0.01%) relative to -16 - most rhyolites. The composition of an "average" topaz

-18 rhyolite is shown in Table 2 (modal values from composi- , Ihyolites tional histograms of 89 analyses of topaz rhyolites from -20 - / ',/:/ wes+central Utah 1 17 of the numbered localities listed in Table as compiled - / - 1 by Christiansen et al. 1982). Most of these characteristics -22 are typical of "bimodal" rhyolites (those erupted with

I I ,I~,,I I I basaltic lavas in continental environments; Ewart 1979) and boo 700 800 900 1000 1100 topaz rhyolites appear to be a sublcass of this group. Temperature ( OC) Many topaz rhyolites are slightly peraluminous, as dem- Fig. 2. Compilation of T-fo2 data for silicic andesites, dacites and onstrated by the presence of corundum (C) in the whole- rhyolites from western U.S.A. and Alaska (after Ewart 1979, except rock norms (even when calculated on a fluorine-free basis). as noted). The oxygen buffer curves (HM hematite-magnetite, Residual glass, analyzed by microprobe, is also peralumi- NNO nickel-nickel oxide; QFM quartz-fayalite-magnetite) are for nous (~~~l~~and ~~~h 1980; christiansen et 1980). ~h~ 1 atmosphere pressure. Solid circles are Californian bimodal rhyo- presence of garnet and topaz (absent as vapor-phase miner- lites, dots are "orogenic" (calc-alkaline or high-K) magmas; open circles are topaz rhyolites from the Mineral Mountains, Utah als in peralkaline volcanics) also reflects the aluminous (Evans and Nash 1978). The field of the Bishop Tuff (Hildreth character of rh~O1ites.In short, rh~o- 1979) is also shown. Other topaz rhyolite data are from Turley lites have been called "alkali" rhyolites, they are not "~er- and Nash (1 980) and Christiansen et al. (1980), and include samples alkaline. " Ferromagnesian vapor-phase and magmatic min- from the Thomas Range, Spor Mountain, and Smelter Knolls, erals in silicic peralkaline rocks from the western U.S. Utah

Table 2. Chemical Composition and CIPW Norms of Topaz Rhy from the Western United States (* Fe,,,,, as Fe,O,)"

1 2 3 4 5 6 7 8 9 10 11 SM-35 SM-62 WW-41 SK-34 Ave. (9) HC-8 IR-I - - WL-103 Ave.

Si02 73.9 75.9 75.8 76.0 75.63 77.3 77.6 74.9 75.1 75.4 76.0 TiO, 0.06 0.09 0.05 0.03 0.04 0.16 0.12 - 0.06 0.02 0.6 A1@, 13.1 12.9 12.9 12.6 12.7 12.0 12.5 14.8 13.1 13.6 13.0 Fez03 1.43* 1.09* 1.26* 0.03 0.79* 1.14* 1.56* 0.62* 0.15 0.20 1.0 * FeO - - - 1.06 - - - - 0.59 0.23 - MnO 0.06 0.08 0.12 0.04 0.09 0.06 0.04 0.23 0.14 0.10 0.06 MgO 0.08 0.09 0.04 0.03 0.09 0.03 0.09 0.37 0.06 0.25 0.08 CaO 1.27 0.74 0.62 0.53 0.71 0.40 0.52 0.84 0.24 0.50 0.6 Na,O 4.33 4.1 1 3.84 3.81 4.25 3.48 3.00 4.00 3.6 4.5 4.0 K2O 3.65 4.69 4.80 4.86 4.47 4.70 5.20 4.56 4.2 4.2 4.8 P2°5 0.00 0.00 - 0.00 0.01 0.02 0.02 0.01 0.00 0.00 0.00 F 1.06 0.64 0.45 0.66 0.18 0.38 0.28 - 0.09 - 0.3 CIPW Norms or 21.57 27.72 29.37 28.72 26.41 27.77 30.73 26.95 24.82 24.82 ab 36.64 34.78 32.49 32.24 35.92 29.45 25.38 33.85 30.46 38.08 an 5.56 2.79 3.08 2.63 2.36 1.86 2.45 4.10 1.19 2.48 Q 31.72 32.32 32.62 33.69 31.28 37.96 38.65 31.46 37.19 31.51 hy 0.54 0.35 1.07 0.90 0.98 0.68 1.13 1.80 0.81 1.07 di 0.64 0.75 0.00 0.00 0.96 0.00 0.00 0.00 0.00 0.00 aP 0.00 0.00 0.00 0.00 0.02 0.05 0.05 0.02 0.00 0.00 il 0.11 0.11 0.09 0.06 0.15 0.30 0.23 0.00 0.11 0.04 C 0.00 0.00 0.08 0.11 0.00 0.51 1.04 1.78 2.20 0.74 rnt 1.OO 0.76 0.73 0.70 0.58 0.66 0.90 0.40 0.44 0.26 1. Spor Mountain, UT (Christiansen et al. 1982) 7. Sheep Creek Range, NV (Christiansen et al. 1980) 2. Thomas Range, UT (Christiansen et al. 1982) 8. Chalk Mountain, CO (Cross 1886) 3. Wah Wah Mountains, UT (Christiansen et al. 1980) 9. Silver Cliff/Rosita, CO (Phair and Jenkins 1975) 4. Smelter Knolls, UT (Turley and Nash 1980) 10. Little Belt Mountains, MT (Witkind 1973) 5. Burro Creek, AZ (Moyer 1982) 11. Modal values of histograms including 89 analyses from western U.S. 6. Black Range, NM (Correa 1980) (Christiansen et al. 1982) " Norms calculated on F-free basis; all analyses recalculated H,O-free 19

0 2 0.4 0.6 0 8 I0 1.2 rt% ------L------FiCl 4 6 8 10 12 I I I - - I

5 6 ~~~0;-- ...... a - - C-c-+-4 COO O2 04 06 0.8 10 1.2 ------.... - - 8 ...... ------I 1 I 7 "0" i_ !.... -- - Zr -....4-4-4- 250 500 750 J 1000 1250 1500 ppm - -- - Hf lo 20 30 40 5 0 60 ...... --_ I I I - - L__ Zn loo 200 300 400 500 600 ---... -- C,-L-C-,. I I Lo loo 200 300 400 500 6W - ...- I I - - LoiYbN ...., I.. LA2 0 2 5 30 ------I - t ' I"' ;I -LAp 4 Eu O.' 02 1.0 ...... , ...... - - I ------

Fig. 3. Comparison of the geochemistry of topaz rhyolite~(solid bar), with calc-alkaline rhyolite~(upper dotted line), ongonites (upper dashed line) and peralkaline rhyolites (lower dashed line). Data for topaz rhyolites are from Christiansen et al. (1980, 1982); Bikun (1980); Correa (1980); Turley and Nash (1980); Evans and Nash (1978); Keith (1980); Lipman et al. (1978b); Lindsey (1981); Zielinski et al. (1977). Depending on the element, the data represent 50 to 100 analyses from nineteen localities in the western U.S. Data for peralkaline rhyolites are from Villari (1974); Noble et al. (3979); Hargove (1982); Mahood (1981); Ewart et al. (1977); Ewart (1982); Bevier (1981); Bailey (1980); Barberi et al. (1975) and Noble et al. (1974). They represent 60 to 120 samples from more than a dozen localities in addition to the compilation by Macdonald and Bailey (1973). Data for ongonites are from Kovalenko and Kovalenko (1976); Antipin et al. (1980); Kovalenko et al. (1977a, 1978) and represent approximately 100 analyses from 3 localities. The data for calc-alkaline rhyolites are from the compilation of Ewart (1979)

include aegerine, riebeckite, and aenigmetite - none of Perhaps even more significant are the low La/Yb ratios which has been identified in topaz-bearing rhyolites from (La/Yb, = 1 to 3) of most samples (Christiansen et al. 1982). the western United States. The term alkali rhyolite is better The differentiation trends of topaz rhyolites can be dis- reserved for those with alkali pyriboles as suggested by cerned by comparing the chemistry of lavas periodically Streckeisen (1979). Topaz rhyolites, in the IUGS system, erupted from the same center (Mineral Mountains - Evans are classified as alkali feldspar rhyolites. Use of this term and Nash 1978; Spor Mountain - Bikun 1980; Wah Wah avoids confusing them with peralkaline rhyolites which are Mountains - Christiansen 1980; Smelter Knolls, Thomas contemporaneous with topaz rhyolites in the western Unit- Range - Turley and Nash 1980, Thomas Range - Christian- ed States. sen et al. 1982). Although differences in absolute concentra- The trace element compositions of topaz rhyolites are tions occur from center to center, a consistent trend of more distinctive than their major element chemistry decreasing Sc, Ti, Fe, Co, Mg, Ca, K, P, Sr, Ba, Zr, Hf, (Fig. 3). Trace element analyses of variable quality are LREE (light REE) and Eu with increasing Na, F, U, Th, available for samples from 14 of the 22 topaz rhyolite locali- Li, Rb, Cs, Be, Ta, Y, Nb and HREE (heavy REE) emerges. ties included here (summarized in Christiansen et al. 1982). These chemical variations appear to correlate with decreas- As a group, topaz rhyolites are enriched in Cs, Rb, U, ing temperature as estimated from two-feldspar and Fe-Ti Th, Li, Be, Sn, Mo, Nb, Ta and Ga (Christiansen et al. oxide geothermometry. Si generally increases and A1 gener- 1982) and most other incompatible lithophile elements rela- ally declines with evolution, but opposing trends are ob- tive to many other silicic rocks (Fig. 3). They are strongly served in topaz rhyolites with greater than about 0.8% F. depleted in Ba, Sr, Cr and Co (Fig. 3). Rare earth element Most of these chemical trends are qualitatively consistent (REE) concentrations have been determined for over with fractional crystallization processes, but the enrichment 20 samples from 9 separate localities (Keith 1980, Christian- of HREE and the simultaneous depletion of LREE with sen et al. 1982; Turley and Nash 1980; Lipman et al. 1978 b, differentiation is opposite that observed in many other Zielinski et al. 1977). Chondrite-normalized REE patterns silicic suites that are thought to have evolved by crystal for three topaz rhyolites that span the compositional range fractionation (e.g. Frey et al. 1978). are shown in Fig. 4. The most striking features of the pat- The HREE enrichment of evolved low temperature terns are the deep Eu anomalies (Eu/Eu* = 0.45 to 0.01). magmas may be the result of their migration as halide com-

21

Peralkaline rhyolites occur predominantly in continental 0.8 - rift or rift-like settings (Macdonald 197413). Peralkaline rocks also occur in oceanic islands and late orogenic suites but they are almost always associated with lithospheric ex- - tension in both of these environments. 0.6 Peralkaline rhyolites generally contain phenocrysts of anorthoclase, sanidine, quartz, sodic ferrohedenbergite, aenigmatite and fayalite (Sutherland 1974). Arfvedsonite and riebeckite generally crystallize as devitrification prod- ucts. Zircon and Fe-Ti oxides occur as accessories. Their mineralogy and the common enrichment in C1 suggests that most peralkaline rhyolites were not fluid-saturated prior to eruption (Bailey 1980). o The most important chemical features of peralkaline rhyolites (relative to other silicic magmas) are high Fe, Mn, 0.2 0.4 0.6 0.8 1.0 1.2 Ti, F and C1, and low A1 and Ca (Fig. 3; Macdonald FLUORINE 1974a). They are distinct from topaz rhyolites in each of Fig. 5. Fluorine versus chlorine variation diagram comparing glas- these characteristics except their generally high fluorine SY topaz rhyolites (0)and peralkaline rhyolites (b).A F/Cl ratio content. The F-C1 content of peralkaline obsidians is com- of 1 divides oceanic from continental peralkaline rhyolites (Bailey pared to that of glassy topaz rhyolites in Fig. 5. Peralkaline 1980). A F/C1 ratio of 3 separates peralkaline rhyolites from topaz rhyolites have F/Cl ratios of less than 3 and are easily distin- rhyolites. The composition of the Bishop Tuff (B) is shown for guished from topaz rhyolites on this diagram. In many re- comparison (Hildreth 1979). Data are from Christiansen et al. (1982); Moyer (1982); Turley and Nash (1 980); Bailey (1980); spects their trace element chemistry (characterized by ex- Macdonald and Bailey (1973); and Mahood (1981) treme enrichments or depletions of many elements) is simi- lar to that of topaz rhyolites but peralkaline rhyolites from the western United States generally have lower concentra- tions of Rb, U, Th and Ba and higher concentrations of sen et al. 1982). However, they are chemically and minera- Zr than found in aluminous F-rich magmas (Fig. 3 and logically distinct from both the old calc-alkaline and the 6). In addition, they generally have higher concentrations young peralkaline rhyolites. of LREE and steeper chondrite-normalized REE patterns (Fig. 7). Peralkaline rhyolites show differentiation trends Peralkaline Rhj>olite,r (indexed by increasing (Na,O + K20/A1,0, and higher in- compatible trace element contents) that differ markedly Peralkaline rhyolites contain a molecular excess of Na20+ from topaz rhyolites. Peralkaline rhyolites show progressive K,O over A1,0, expressed as normative acmite for F- and depletions of Si, Al, Ca, Ba, Sr, Sc, and sometimes Eu C1-free analyses. They are most easily recognized by the that correlate with increasing Na, C1, Ti, Mn, Fe, Zn, Hf, presence of sodic pyroxenes or amphiboles as phenocrysts Ta, Y, Zr, Nb, REE, U, Th, Rb (Macdonald and Bailey or as vapor-phase minerals. Some peralkaline rhyolites in 1973; Noble et al. 1979; Villari 1974). The enrichment of the Great Basin were erupted during large caldera-forming Ti, Mn, Fe and Zn may seem remarkable in view of their eruptions (e.g. McDermitt, Black Mountain, and Kane modal mineralogy which contains minerals with high parti- Springs Wash calderas, all in Nevada) and almost all erup- tion coefficients for some of these elements (e.g. pyroxene, ted after about 20 m.y. ago (Noble and Parker 1974). olivine and amphibole). The only moderate enrichment of

1200 I I I I I I I I

0

1000 0 F-RICH METALUMINWS RHYOLITES - .a + METALUMINWS RHYOLITES o PERALKALINE RHYOLITES

0 D 1 - 800 - k -a 00

5600- -0 o - D Fig. 6. Rubidium versus zirconium variation m Odb diagram comparing metaluminous (or 2 slightly peraluminous) and peralkaline 400 - 0 @ rhyolites. Included are data from Turley and O OO Nash (1980); Noble et al. (1979); Keith + +o++ .. . (1980); Hildreth (1979); Macdonald and oO++ 200 . .. - Bailey (1973); Ewart et al. (1977); -+ + Z *). ++$+++. .* *.*..- s~~:*-:.. Christiansen et al. (1982); data from ++ +I+ ++ + . 0. +++A Christiansen et al. (1980) are not included L I I I , I I I because of incorrect Zr concentrations 0 200 400 600 800 1000 1200 1400 1600 ZIRCONIUM(pprn) 22

I I I I I I I I I~- - *- . 1000 - -.--. ... - '...... BLACK MTN. ------• ..- .. .. ., . , , . I00 - ..._ - f .- "._ / 6 TOPAZ RHYOLITE THOMAS RNO. 9 - - lo , .. . a ...... _ .... '*-... CALC-ALKALINE RHYOLITE Fig. 7. Chondrite-normalized REE patterns for BAN JUAN MOUNTAINS peralkaline rhyolite (Nevada - Noble et al. 1979), topaz rhyolite (Utah - Christiansen et al. 1982), and calc-alkaline rhyolite (Colorado - Zielinski and I - - I I I1 I I I I Lipman 1976) La CI Nd 9rn Eu Tb Dy ~b Lu

Rb, U and Th (Fig. 3 and 6) appears anomalous in that other calc-alkaline rhyolites, they crystallized under condi- peralkaline rhyolites lack potentially fractionating minerals tions of high T and ,fo2 (approximately 3 log units above with high partition coefficients for these elements (e.g. QFM, Fig. 2). biotite, zircon, or allanite). From this brief review it is suggested that topaz rhyolites are chemically distinct from both calc-alkaline and peralka- Calc-Alkaline Rhyolites line rhyolites, even though they are in part contemporan- eous with both types of magmatism in the western United Calc-alkaline rhyolites are the silicic representatives of the States. Apparently their mode of origin and/or evolutionary orogenic magma series characterized by a lack of -en- paths are different from either magma series. richment during its differentiation. Calc-alkaline rhyolites Topaz rhyolites are most similar to other metaluminous are typically associated with andesitic volcanism on conti- to peraluminous rhyolites that occur as small domes and nental margins overlying subduction zones. They generally lava flows in the Basin and Range province (e.g. Bacon occur as small domes or lava flows associated with compos- and Duffield 1981; Crecraft et al. 1981). They are similar ite volcanoes or calderas but may form voluminous ash- to "ongonites" that have been described from Mongolia flow sheets. Large volumes of calc-alkaline rhyolite were and the Soviet Union (Kovalenko et al. 1971; Kovalenko erupted during the mid-Cenozoic of the western United and Kovalenko 1976; Fig. 3). Ongonites are fluorine-rich States. (0.8 to 3.5 wt.%), topaz-bearing subvolcanic rocks and Calc-alkaline rhyolites generally contain phenocrysts of lavas. Ongonites and topaz rhyolites bear striking resem- plagioclase, Mg-augite, Mg-hypersthene, Ca - Mg horn- blances in their chemistry, mineralogy and associated ore blende, Mg-biotite, Fe - Ti oxides and occasionally olivine deposits. We prefer the term "topaz rhyolite" because it (Ewart 1979). More silicic, high-K varieties contain quartz is more descriptive and because of historical precedence; and sanidine. Titanite and allanite are notable accessories. the first scientific description of topaz in rhyolite from Col- Although generally not fluid-saturated the mineralogy indi- orado dates from nearly 100 years ago (Cross 1886). cates they are relatively hydrous. The T- fo2 relationships for some calc-alkaline rhyolites are shown in Fig. 2. Discussion From Ewart's (1979) compilation there appears to be substantial chemical variation among calc-alkaline rhyolites The nature of volcanic rock associations and contemporan- but they are generally higher in Al, Ti, Fe, Mg, Ca and eous tectonic activity gives some clues about the generation lower in total alkalies and F than topaz rhyolites (Fig. 3). of magmas, for example, active calc-alkaline magmatism Calc-alkaline rhyolites generally have lower concentrations (basalt-andesite-dacite-rhyolite) is consistently associated of Rb, U, and Th (and other incompatible elements) and with lithospheric subduction. However, topaz rhyolites have higher concentrations of Ba and Sr (and other compat- appear to be associated with a variety of igneous rocks. ible elements) than topaz rhyolites (Ewart 1979), attesting Indeed, they show no consistent spatial or temporal rela- to their less "differentiated" nature (Fig. 3). The differenti- tionship to a single magma series from which they could ation trends of calc-alkaline rhyolites appear to be similar be derived by differentiation. to those of topaz rhyolites but the extreme enrichments/ Christiansen et al. (1982) proposed that topaz rhyolites depletions of trace elements are not observed (Fig. 3). Ewart are associated with three different types of magmatism: (1979) has identified a series of biotite rhyolites from the I) fundamentally calc-alkaline, 2) "transitional" basaltic western U.S. with moderate enrichments of Rb, U, and andesites, and 3) fundamentally basaltic. For example, the Th, that appear to be geochemically transitional to topaz 50 m.y. old rhyolites of the Little Belt Mountains, rhyolites (their elemental concentrations are included with Montana, are associated with the emplacement of granite, calc-alkaline rhyolites in Fig. 3). However as with most monzonite, syenite and minor lamprophyre of "calc-alka- 23

(Eaton 1979, Elston and Bornhorst 1979); 2) in the Great Basin where basin and range faulting may have begun as early as 21-20 m.y. ago (Rowley et al. 1978); 3) in Montana where block faulting began about 40 m.y. ago (Chadwick 1978), and 4) along Nevada's Cortez rift that opened 16 m.y. ago (Stewart et al. 1975). The extension occurred in back- or intra-arc and post-arc environments (Eaton 1979; Elston and Bornhorst 1979). The intimate association of extensional tectonics and topaz rhyolite magmatism (Fig. 1) strongly implies a genetic connection. The association in space and time of topaz rhyolites with at least two, and possibly three, different types of more mafic magma suggests that they may be derived by partial melting of the continental crust through which the magmas passed rather than by differentiation of the contrasting magma types. The geophysical character of the Great Basin (Prohdehl 1979), where most young (<15 m.y.) topaz rhyolites occur, is suggestive of crustal anatexis. This region has character- istically high heat flow (90 mW/m2), a thin crust (25-30 km thick) and low upper mantle P-wave velocities (7.4 kmlsec). The coincidence of these features beneath the Thomas Range, Utah, area where topaz rhyolites as young as 3.4 m.y. old occur, is striking. Based on an extrapolation of heat flow observed at the surface, Lachenbruch and Sass (1978) have proposed that partial melting could occur with- Fig. 8. Known topaz rhyolite (.) occurrences in the western United in the crust, and Smith (1978) has suggested that a seismic States compared with the edge of the Precambrian crystalline base- ment. The solid line represents the outcrop limit of Precambrian low-velocity zone detected within the crust of the Great rocks (King 1977) and the dashed line represents the inferred edge Basin may be caused by the presence partially of the Precambrian continent (Kistler et al. 1981 ; Armstrong et al. rock. Figure 9 shows the presumed geothermal gradient for 1977). The numbers refer to the localities listed in Table 1 ; unla- the Battle Mountain High, Nevada (Lachenbruch and Sass belled localities are from Shawe (1976). Note that the volcanic 1978), superimposed on biotite and amphibole decomposi- centers for peralkaline rhyolites (0;Noble and Parker 1974) gener- tion curves and solidi for granite and amphibolite. This ally do not occur in the same areas as topaz rhyolites. An east-west figure demonstrates that magma production is possible even dichotomy is apparent in the northern Great Basin within the thin crust of the Basin and Range province. Pre- sumably similar geothermal gradients existed at other local- ities in the past as the result of underplating of mafic magma line" affinity (Witkind 1973). In Colorado, a variety of or its passage through the crust (Lachenbruch and Sass calc-alkaline lavas and tuffs of the San Juan volcanic field 1978). Lower thermal gradients in thicker portions of the and elsewhere are concentemporaneous with (>25 m.y.) to- crust could also produce crustal anatexis by biotite or am- paz rhyolites along the developing Rio Grande rift (e.g. phibole breakdown at higher pressures. Steven 1975). However, in Utah early Miocene topaz rhyo- The proposal that topaz rhyolites are produced by lites from the Wah Wah Mountains form a bimodal associa- crustal anatexis is also supported by the geochemistry of tion with trachyandesite or K-rich mafic lavas - true basalts the lavas. The metaluminous to slightly peraluminous com- seem to be absent (Best et al. 1980). Likewise the 24 m.y. positions of topaz rhyolites suggest that neither muscovite old topaz rhyolites of the Black Range, New Mexico (Elston nor aluminosilicates (andalusite, sillimanite, kyanite) were and Bornhorst 1979), and the 14-16 m.y. old topaz rhyolites present in the source. Equilibration of igneous melts with from northern Nevada are associated with basaltic andes- these minerals produce strongly peraluminous liquids with ites (Stewart et al. 1977) that are chemically similar to the 3 to 8% normative corundum (Thompson and Tracy 1977; trachyandesites of Utah. Topaz rhyolites younger than Clemens and Wall 1981). In addition, the relatively high about 12 m.y. old are generally associated with fundamen- temperatures of the lavas (up to 850" C, compare with tally basaltic volcanism in the Great Basin (Best et al. 1980), Fig. 9), their apparently low H,O-content (indicated by the in western Arizona (Burt et al. 1981), and at Grants Ridge, late crystallization of biotite and low ,ft,20) and their rise New Mexico (Bassett et al. 1963). The younger group of to shallow crustal levels suggest that muscovite was not topaz rhyolites is also contemporaneous with peralkaline involved in their genesis. As an alternative, biotite (or am- volcanism in the western Great Basin (Noble and Parker phibole) may have provided the volatiles for melting. Ap- 1974). but the two bimodal groups of basalt-rhyolite are propriate temperatures are reached at pressures in excess spatially distinct (Fig. 8). of about 6 kb for the relatively high geothermal gradient By contrast with the rock associations, the tectonic set- illustrated in Fig. 9. (Higher pressures would be required tings for the generation and emplacement of the various by shallower geothermal gradients.) Biotite may not have topaz rhyolites seem to be relatively similar. Topaz rhyolite been residual and could have supplied considerable Rb and magmatism coincide with periods of lithospheric exten- F to the melts. Due to its limited stability at pressures above sion I) along the Rio Grande rift and its northern extension 5 kb, cordierite is unlikely to have been a residual phase into Colorado which initially developed about 30 m.y. ago (Clemens and Wall 1981). 24

experimental evidence of Watson (1979) that suggests zircon would be residual in partial melting events that pro- duce metaluminous liquids. Nonetheless, it is difficult to 40 separate the effect of residual zircon or garnet on the REE from the effect produced by melting a HREE-depleted pro- tolith typical of granulitic rocks (e.g. Collerson and Fryer 1978). In view of our ignorance of the REE pattern of 30 2 the source and of the abundance of REE-rich trace minerals & - quantitative modeling of partial melting is considered to 5 be pointless. 20 EL Phases that probably were present in the source include: 8 alkali feldspar (indicated by its early saturation, negative Eu anomalies, and the high activity of KA13Si08 in the 10 lavas), relatively socic plagioclase (indicated by negative Eu anomalies, and the high activity of NaA13Si08 - residual plagioclase was probably calcic), quartz (indicated by high activity of SiO,), pyroxene (indicated by its early saturation 0 200 400 600 800 1000 1200 in "primitive" topaz rhyolites and fo2 relations), and F-rich Temperature (OC) biotite (indicated by high initial temperatures and the F Fig. 9. Calculated geotherm for average conditions in the crust and Rb enrichment of the magmas). Small amounts of Fe - of the Battle Mountain, Nevada, high heat-flow province (Lachen- Ti oxides, zircon, garnet, apatite, or scapolite may also have bruch and Sass 1978), compared with the P- T curves for the been present in the regi0n. beginning of melting of granite and amphibolite under fluid-present The apparent absence of muscovite or aluminosilicates and fluid-absent conditions following the breakdown of muscovite, from the source mineralogy, the moderate fo2 and the high blotite, or amphibole. The production of magmas within the rela- Na/K ratios of the magmas all suggest that the source was tively thin crust of the Great Basin (GB) seems possible under not pelitic but was instead a meta-igneous or residual meta- conditions of high heat flow. Shallower geotherms could intiate morphic rock, In fact, the suggested source mineralogy is melting in thicker sections of the crust (e.g. Colorado). References: similar to many (charnockitic) granulites that may be pro- granite and amphibolite solidi from Wyllie (1977); muscovite+ duced as the residue after the removal of an earlier melt quartz -+ K-feldspar + andalusite + fluid (Evans 1965), biotite + fraction (e.g. et 1980; Nesbitt 1980). quartz --+ K-feldspar + fayalite + fluid (Wones 1972) over a range of Fe/Fe + Mg, tremolite -, diopside + enstatite + quartz + fluid Although isotopic data are still sparse, they are compati- (Holloway 1977) over a range of F/(F+OH) from 0 to 0.5. FOH-, ble with this type of source. Initial s7Sr/86Srratios for topaz exchange could extend the stability of biotite enough to overlap rhyolites range from 0.705 to over 0.710 (Christiansen et al. with the amphibole stability field 1982) and indicate lower crustal sources with initially low to moderate Rb/Sr ratios. The oxygen-isotope composition of topaz rhyolites from the Mineral Mountains, Utah (6 The moderate fo2 of most topaz rhyolites (QFM k 1 log to 7%,, Bowman et al. 1982) and from Lake City, Colorado unit, Fig. 2) indicates that graphite was not a residual (7 to lo%,, R.A. Zielinski 1982, written communication), phase. Ewart (1979) suggests that the fo2 of bimodal rhyo- are consistent with magma generation from a high-grade lites may be the result of their differentiation from (or par- metamorphic lower crustal source. Pb-isotope ratios for to- tial melting of) basalt. However, Ewart et al. (1977) pro- paz rhyolites from Colorado (Lipman et al. 1978a) indicate posed an alternative oxygen buffer that we feel is more they were derived from a lower crustal source with low appropriate for the crustal origin of topaz rhyolites: U/Pb and moderate Th/Pb ratios. Significant U depletion and little or no Th depletion occurs during granulite-facies 4 0, + FeSiO, + CaAl,SiO, metamorphism (e.g. Rollinson and Windley 1980). Pb- (gas) (orthopyroxene (clinopyroxene s.s.) isotope compositions of topaz rhyolites from Nevada (Rye solid solution) et al. 1974) indicate relatively high U/Pb ratios in the source =+ Fe,O, + CaAl,Si,O,. (unlike residual granulites) but the lavas may have been contaminated by upper crustal Pb during their rise since (ilmenite s.s.) (plagioclase s.s.) older, geochemically dissimilar volcanic rocks have the Assuming ideality and activities representative of those in same Pb-isotope ratios. pyroxene granulites the resultant buffer curve is virtually The proposed source mineralogy, the extensional tec- indistinguishable from the QFM buffer at 10 Kb. However tonic setting and the geochemical features of topaz rhyolites the reaction is pressure-dependent and at 5 kb the buffer imply that they may be the extrusive equivalents of A-type lies near WM and at 1 bar it lies about 3 log units below (anorogenic of Loiselle and Wones 1979, Collins et al. 1982) QFM. or R-type (residual of White 1979) granites. The character- The source of topaz rhyolites may have contained small istics of R-type granites are compared with those of topaz amounts of residual zircon or garnet which have high distri- rhyolites in Table 3. There are two "species7' of anorogenic bution coefficients for HREE. The most primitive rhyolites granites - one metaluminous to slightly peraluminous (anal- from the Thomas Range, Utah, (Christiansen et al. 1982), ogous to topaz rhyolites), the other peralkaline (analogous are moderately depleted in HREE (La/Yb,= 12) possibly to peralkaline rhyolites). In granitic complexes both types as a result of equilibration with residual zircon and/or may coexist one intruding the other or one grading into garnet. This suggestion is consistent with the usually low the other (e.g. Arabian Shield, Stuckless et al. 1982). How- Zr concentration (Fig. 6) of topaz rhyolites and with the ever, in the Basin and Range province topaz rhyolite occur- 25

Table 3. Geochemical Comparison of Anorogenic (R-Type) Gran- of the Proterozoic continent (Stewart 1978; Burchfield ites and Topaz Rhyolites 1979). Topaz rhyolites appear to be restricted to areas which are underlain by this crust (Fig. 8). It should also Feature Anorogenic granitea Topaz rhyolite be noted that uraniferous Precambrian granites with fluor- ite and beryl occur near topaz rhyolite localities in Arizona low ~H~O low (Heinrich 1960; Silver et al. 1980). Utah (Moore and Soren- HF/H20 high high sen 1978) and Colorado (Eckel 1961) suggesting that the fo, low to moderate low to moderate geochemical "anomaly" has persisted since the Precam- (near QFM) brian and is not necessarily the result of recent events. We T high low to moderate know of no topaz rhyolites along the western margin of (60G800" C) the Great Basin or in Oregon, in spite of high heat-flow SiO, high (-76%) high (73-78%) (Blackwell 1978) and young bimodal volcanism, e.g., north- Na20 high moderate-high western Nevada and along the Brothers fault zone in Ore- (34.5%) gon. Presumably the dearth of topaz rhyolites in the north- CaO low low (<0.8%) western Great Basin and Oregon is due to the absence of ancient crystalline Precambrian crust beneath this region. Trace elements However, peralkaline rhyolites are fairly common in the REE high, except Eu moderate LREE, western Great Basin and may result from partial melting high HREE, low Eu of a younger crustal component accreted to the continent Enriched Ga, Y, Nb, Sn, Ga, Y, Nb, Sn, Ta, in post-Belt times or by differentiation of basaltic magmas. Zr, Ta Rb, Th, U, Li The role of fluorine in the origin and evolution of topaz Depleted Co, Sc, Cr, Ni, Ba, Co, Sc, Cr, Ba, Sr, rhyolite magmas may be critically important. For instance, Sr, ELI Eu, Zr large ash-flow eruptions caused by the rapid exsolution of F and C1 high high; F(0.3-1.5%) volatiles are not common in aluminous F-rich magmas - ~1(700-1,700ppm) probably because fluorine has a relatively high solubility in silicate melts (Koster Van Groos and Wyllie 1968; Fuge Fe/Fe Mg high high + 1977) and also because it increases the solubility of water K,0/Na20 high moderate to high in magmas (Koster Van Groos and Wyllie 1968), reducing the likelihood of volatile saturation of large amounts of " From White (19791, Loiselle and Wanes (1979) and Wanes (1979) magma. Also, the addition of fluorine to hydrous assemblages dramatically lowers their solidus tem- peratures (Wyllie and Tuttle 1961, Glyuk and Anfiligov rences appear to be spatially distinct from peralkaline rhyo- 1974; Manning 1981), probably accounting for the low lites (Fig. 8), possibly as a result of differences in the nature equilibration temperatures of evolved topaz rhyolites. In of the crust. addition, fluorine-rich melts may have lower viscosities as Anorogenic granites are thought to result from differen- the result of the depolymerization of alumino-silicate units tiation of variably contaminated alkali basalts (e.g. Loiselle in the melt (Manning et al. 1980; Kogarko 1974), perhaps and Wones 1979) or from small degrees of partial melting enhancing crystal fractionation and liquid state diffusion. of "residual" crustal materials from which earlier water- As noted above, fluorine may stabilize a variety of trace rich magmas had been removed during granulite facies elements within silicate melts either by forming complexes metamorphism (Collins et al. 1982). Fillippov et al. (1974) with them or by otherwise altering the melt structure (Diet- have shown that biotites from granulite facies metamorphic rich 1968; Collins et al. 1982). Hence, elevated concentra- rocks contain greater amounts of fluorine than those in tions of such "fluorophile" elements (Be, Li, U, Sn, Th, amphibolite-facies rocks (0.65% F versus 0.24 to 0.38% F). Mo, Rb, Cs, etc.) may occur at the site of anatexis by These analytical results are consistent with the experimental scavenging from the solid residue and then become further work of Holloway and Ford (1975) that demonstrated high- enriched in differentiates of the magma under conditions er thermal stability for F-rich amphibole relative to hydrous of enhanced diffusion/convection and/or crystal fractiona- varieties. Collins et al. (1982) and White (1979) also suggest tion. In this regard it may be noteworthy that Keith (1980) that the breakdown of fluorine-rich biotites or amphiboles reports molybdenum concentrations that exceed 20 ppm in provides the volatiles for partial melting. These fluorine- a sample of unaltered topaz rhyolite from the Wah Wah rich melts may complex highly charged cations (U, Th, Nb Mountains, Utah. Uranium concentrations exceed 40 ppm etc.) that were rejected by earlier water-rich melts from re- in rhyolites from Spor Mountain, Utah (Bikun 1980). These sidual phases such as zircon. Differentiation may further values are perhaps five times those in other high-silica rhy- enrich portions of the melt in fluorine and these generally lites and suggest an effective Mo and U concentration incompatible elements. process. As Rb-Sr and Pb isotopic studies suggest that the If topaz rhyolites are indeed derived from the crust, source for these rhyolites was not anomalously rich in Rb they should reflect the chemical nature of the source from or U relative to Sr or Pb - we prefer a magmatic concentra- which they were derived. Isotopic (Zartman 1974; Arm- tion process. Thus as fluorine concentrates in the upper strong et al. 1977; Kistler and Peterman 1973) and geophys- portions (or residual liquid) of magma chamber (either by ical (Mabey et al. 1978) investigations of the crust that un- convective diffusion or crystal fractionation) it provides the derlies the western United States demonstrate that it is com- opportunity for extreme enrichments of Mo, U, Be, Li, posed of distinct domains. The most prominent breaks in Sn (and probably Nb, Ta and W, as well). These magmatic characteristics coincide with the interpreted margin of the concentrations may in turn, enhance the probability for crystalline Precambrian basement, which marks the edge the generation of ore in volcanogenic deposits of U, Be, F and Li or for greisen and porphyry deposits of Mo, W should be marked by moderate enrichment or depletion and other elements. of U, Zr, Hf, Fe, Zn, Rb, LREE and Th. For, Hf, Fe The most important characteristics of topaz rhyolites and the LREE this prediction appears to hold true. How- are their substantial enrichment in F and fluorophile ele- ever Rb, Th and U all show enrichments that appear to ments (Be, Li, Sn, U, Th and Rb). In contrast, peralkaline be greater than those in peralkaline rocks. Thus either the rhyolites contain considerable amounts of C1 and substan- source of topaz rhyolites is relatively rich in these elements tially different trace element characteristics (Fig. 3). Ma- (and possibly Li, Be, Ga, Mo and Sn) or some factor that hood (1981) has suggested that the dominant control on does not involve crystal-liquid equilibria (volatile-complex- the trace-element variation in rhyolitic magmas may be ing?) selectively enriches these elements in topaz-rhyolite their roofward migration as volatile complexes in evolving melts. magma chambers. If this is the case, due to varying elemen- It may be that the combined effect of F on alumina tal affinities, different proportions of volatile constituents saturation, phase equilibria and complex formation leads (H,O, HF, HCl, CO, and B) in granitic magmas should to the accumulation of F-associated ("fluorophile") ele- produce distinctive trace-element signatures. When com- ments (Be, Li, U, Th, Rb, HREE) in the apical portions bined with the effect of volatiles on phase equilibria and of magma chambers or in residual liquids which are usually melt structure, the relative proportions of volatile elements metaluminous. (The OH-component of a magma probably could determine the course of magma evolution. We suggest behaves like F because of its similarity in size and charge.) that many of the trace and major element characteristics The evolution of chlorine-enriched peralkaline magmas of peralkaline and topaz rhyolites are the result of their may be dominated by the accumulation of C1-associated C1 or F dominated character. ("chlorophile") elements (Ti, Mn, Fe, Zn, Zr, Nb and pos- For example, Manning et al. (1980) have suggested that sibly LREE) in residual liquids. As peralkaline rhyolites F (and possibly H,O) and A1 have a strong affinity in gran- are rich in both C1 and F their trace element chemistry itic melts - so much so that A1 is removed from tetrahedral may show accumulation of both fluorophile and chloro- coordination in the aluminosilicate framework and placed phile elements (e.g. enrichments in LREE and HREE are in interstitial sites in octahedral coordination. The subse- typical - Villari 1974). quent crystallization of minerals with octahedral A1 (garnet The apparent volatile-affinities just described may ex- and topaz) from topaz-rhyolite melts and glasses supports plain why peralkaline and metaluminous magmas are often this notion. Possibly, as a result of this association, topaz erupted from the same volcanic centers (e.g. Noble and rhyolite melts remain metaluminous or slightly peralumi- Parker 1974; Mahood 1981 ; Rytuba 1979) or occur within nous throughout their differentiation. In contrast, White the same intrusive complexes (e.g. Barker et al. 1975; Harris (1979) has suggested that C1 forms complexes with Na and and Marriner 1980; Lyons and Kreuger 1976; Stuckless reduces its activity in the melt. This may result in the frac- et al. 1982). Presumably, small changes in the F/C1 ratio tionation of a relatively calcic plagioclase - enhancing the of a melt caused by the preferential partitioning of C1 plagioclase effect of Bowen (1928) and producing peralka- (Burnham 1979) into escaping fluids (or conversely enrich- line magmas. ment of Na and C1 in melt by the digestion of fluids or Another im~ortantdistinction between veralkaline and hydrothermally altered rocks) could drive shallow magma topaz rhyolites is the substantial enrichment of Fe in peral- chambers to evolve one way or the other (cf. Lyons and kaline magmas. Consideration of the free energy change Krueger 1976). Likewise the F/Cl ratio imposed by the associated with the exchange reaction source mineralogy may be important and could have impor- tant implications about its nature. Because chlorine is less compatible in hydrous silicates than fluorine (Burnham 1979), as melts or hydrous fluids are removed from a rock during progressive metamorphism the fluorine/chlorine suggests that A1-F and Fe-Cl bonds are favored (at ratio of the residuum rises and subsequent melts of this 1,000" K - data from JANAF Thermochemical Tables). Al- residuum would be F-rich. The apparent ease with which though there are many possible competing ion association, C1 can be mobilized and its high concentration in peralka- the geochemistry of topaz and peralkaline rhyolites suggests line rhyolites suggests that perhaps C1 is introduced into that Al- F and Fe- Cl bonds persits even in chemically the source regions of peralkaline rhyolites by metasomatic complex silicate magmas. solutions prior to melting (Bailey 1980; Boettcher and Some of the unique chemical attributes of peralkaline O'Neil 1981). However, we see no evidence that such meta- and topaz rhyolites can be explained by the nature of their somatizing solutions are important for the genesis of fluor- stable mineral assemblages which reflect the degree of ine-rich topaz rhyolites. alumina saturation. For example zircon, biotite, allanite (and possibly monazite and thorite) are not stable in peral- Conclusions kaline melts (e.g. Dietrich 1968; Watson 1979, for zircon). Apatite is only occasionally reported as a phenocryst in The topaz rhyolites of the western United States are distinc- mildly peralkaline volcanic rocks (e.g. Mahood 1981). Sub- tive fluorine-rich volcanic rocks that were erupted through- stantial enrichments of U, Zr, Hf (controlled by zircon), out most of the Cenozoic. Although they are contemporan- Fe, Zn, Rb (controlled by biotite), LREE and Th (con- eous with rhyolites of peralkaline and calc-alkaline lineages trolled by allanite-etc.) should result during evolution of they are not genetically related to either. Topaz rhyolites such a melt by fractional crystallization. In contrast all of are distinct from the former in their magmatic and vapor- the above mentioned trace minerals are found in topaz phase mineralogy, their mode of emplacement and their rhyolites (and other aluminous melts). Consequently evolu- lower content of C1, Fe, Mg, Ti, Zn, Zr, Nb, and LREE tion by fractional crystallization of an aluminous magma and higher F, Al, Rb, U and Th. Topaz rhyolites are distin- guishable from calc-alkaline rhyolites by their Fe-enriched Anthony JW, Williams SA, Bideauz RA (1977) Mineralogy of mafic mineralogy and by their trace element chemistry Arizona. The University of Arizona Press, Tucson (lower Sr, Ba, Eu, Ti, Mg and higher F, Rb, U, Th and Armstrong RL, Taubeneck W, Hales P (1977) Rb - Sr and K - Ar other incompatible elements). geochronomentry of Mesozoic rocks and their Sr isotopic com- The trace and major element chemistry, mineralogy, T- position. Oregon, Washington, and Idaho. Geol Soc Am Bull 88: 397-41 1 fo2 relationships and the high level of emplacement of topaz Bacon CR, Macdonald R, Smith RL, Baedecker PA (1981) Pleisto- rhyolites suggest that they are derived by relatively small cene high-silica rhyolites of the Coso Volcanic Field, Inyo degrees of partial melting of a residual granulitic source County, California. J Geophys Res 86: 10223-10241 in the lower crust. The apparent limitation of topaz rhyo- Bailey DK (1980) Volcanism, Earth degassing and replenished lith- lites to regions underlain by crystalline Precambrian crust osphere mantle. Philos Trans R Soc London, series A, greater than about 1 b.y. old is in accord with this model 297:309-322 for their formation. Melting was probably the result of bio- Bailey JC (1977) Fluorine in granitic rocks and melts: A review. tite decomposition at temperatures in excess of 850" C and Chem Geol 19 : 1-42 at pressures over about 8 kb. Heat for melting was probably Barberi F, Ferrara G, Santacroce R, Treuil M, Varet J (1975) A transistional basalt-pantellerite sequence of fractional crystal- supplied by the residence of more mafic magmas in (or lization, the Boina Centre (Afar Rift, Ethiopia). J Petrol at the base of) the crust. As a result of comtemporaneous 16:22-56 extensional tectonics the silicic melts were allowed to rise Barker F, Wones DR, Sharp WN, Desborough GA (1975) The separately without substantial mixing with the mafic mag- Pikes Peak batholith, Colorado Front Range, and a model for mas. 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J Volcan Geotherm Res tant in determining the geochemical evolution and mineral- 11:225-251 ogy of these contrasting types. Fluorine appears to be asso- Bikun JV (1980) Fluorine and lithophile element mineralization ciated with metaluminous to slightly peraluminous compo- at Spor Mountain, Utah: Department of Energy Open-File sitions and with substantial enrichments of U, Rb, Th, Li, Report GJBX-225 (80) : 167-377 Be and HREE. Chlorine is associated with veralkaline Blackwell DD (1978) Heat flow and energy loss in the western United States, In: Smith RB, Eaton GP (eds) Cenozoic tecton- magmas and substantial enrichments of the first row of ics and regional geophysics of the western Cordillera. Geol Soc transition elements (Fe, Ti, Mn, Zn, and Cu) as well as Am Mem 152: 145-174 Na, Zr, Hf, Nb and LREE. Although many of these differ- Boettcher AL, O'Neil JR (1980) Stable isotope, chemical, and pe- ences may be explained by fractional crystallization with trographic studies of high-pressure amphiboles and micas: Evi- different stable mineral assemblages, some of these con- dence for metasomatism in the mantle source regions of alkali trasts appear to be opposed to crystal/liquid fractionation. basalts and kimberlites. Am J Sci 280-A: 594-621 We suggest that the differences may be the result of distinct Bowen NL (1928) The evolution of the igneous rocks. Princeton source compositions or of the formation of stable volatile- University Press, Princeton, p 332 Bowman JR, Evans SH, Nash WP (1982) Oxygen isotope geochem- complexes l'n the melt that migrate toward the roofs of istry of Quaternary rhyolite from the Mineral Mountains, magma chambers. If this is the case, the alumina saturation Utah, U.S.A. Dept of Energy, Contract DE-ACO7-8OID 12079, and chemical evolution of a single system could be deter- P 22 mined by the initial F/C1 ratio of the source or could be Burchfield BC (1979) Geologic history of the central western Unit- altered by changes in F/C1 ratio of the melt. ed States. Nevada Bureau of Mines and Geology Report 33: 1-1 1 Acknowledgments. This work was supported by the Department Burnham CW (1979) Magmas and hydrothermal fluids. In: Barnes of Energy (Bendix Field Engineering Corporation Subcontract HL (ed) Geochemistry of hydrothermal ore deposits 2nd ed., # 79-720-E to D.M.B. and M.F.S.) and by the National Science 71-1 36 Foundation (Graduate Fellowship to E.H.C.). We are grateful for Burt DM (1981) Acidity-salinity diagrams -- Application to greisen the assistance of J.V. Bikun, B. Correa, B. Murphy and G. Goles and porphyry deposits. Econ Geol76: 832-843 in performing some of the analytical work presented. Discussions Burt DM, Sheridan MF (1981) A model for the formation of urani- with J.D. Keith, M.G. Best, C. Lesher, K. Hon. R.A. Zielinski, um/lithophile element deposits in fluorine-enriched volcanic J.S. Stuckless, J.R. Holloway and L. Webster were helpful in for- rock. Am Assoc Petrol Geol, Studies in Geology, no 13, pp 99- mulating some of the ideas presented in this paper. Reviews of 109 an earlier manuscript by W.P. Nash and W. Hildreth were also Burt DM, Sheridan MF, Bikun JV, Christiansen EH (1982) Topaz helpful. We are also grateful to S. Selkirk who prepared the illustra- rhyolites - Distribution, origin, and significance for explora- tions and to E. Haman who typed the manuscript. tion. Econ Geol 77, no. 8 Burt DM, Moyer TC, Christiansen EH (1981) Garnet- and topaz- bearing rhyolites from near Burro Creek, Mohave County, western Arizona - Possible exploration significance. 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