Heat Transport by Fluids During Late Cretaceous Regional Metamorphism in the Big Maria Mountains, Southeastern California

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Heat transport by fluids during Late Cretaceous regional metamorphism in the Big Maria Mountains, southeastern California

THOMAS D. HOISCH U.S. Geological Survey, M.S. 964, Federal Center, Denver, Colorado 80225

ABSTRACT

The Big María Mountains of southeastern California comprise a Late Cretaceous regional metamorphic terrain involving Paleo- zoic cratonal sediments. Siliceous limestone of the upper Paleozoic Supai Formation has reacted to form massive wollastonite, requir- ing an enormous fluid flux. The minimum volume ratio of fluid:rock that is necessary to explain the formation of wollastonite may be calculated from the reaction quartz + calcite =

wollastonite + C02, using the method of Rice and Ferry. Given average conditions of 3 kbar, 500 °C, an infiltrating fluid of composi-

tion XH2O = 1.00, an equilibrium fluid compo-

sition of XH2Q = 0.97, and 90% wollastonite in the final rock, a fluid:rock ratio of 17:1 may be calculated. Infiltrating fluids of composi-

tion 1.00 > XH2O > 0.97 require still higher

ratios. Metamorphic reactions which took A ' Two-feldspar thermometry (°C) place in other units of the sequence (metape- O - Calcite-dolomite thermometry (°C) lites, massive carbonates, metavolcaniclastics) LTZ = Lower Talc Zone did not record the massive fluid flux, but UTZ = Upper Talc Zone DTZ = Diopside-Tremolite Zone fluids must have passed through them to have FZ = Forsterite Zone affected structurally isolated masses of Supai Formation. Passage of fluids must have occurred along fractures. 0 1 2 3 4 5 km 33°45'+ Neither magmatism nor radioactive heat 114°40' sources are adequate to explain the temperatures of metamorphism. If the minimum quantity of fluid that is estimated to have passed through the area was initially at 680 °C, it would result in a 300-degree rise in temperature over that of the stable- craton geotherm. Late metamorphic pegmatite dikes which are most abundant in areas of high metamorphic grade may stem from melts anatectically derived at deeper levels, perhaps Figure 1. Map of the Big Maria Mountains, showing metamorphic temperatures which are as a result of the same fluid flux. Heat that is determined by calcite-dolomite and two-feldspar thermometry and isograds which are deter- introduced by large fluid fluxes may be an mined on the basis of mineral assemblages observed in siliceous dolomites (adapted from important cause of anomalously high temper- Hoisch and others, in press). The calcite-dolomite geothermometer used is that of Rice (1977), atures which are observed in many cases in and the two-feldspar geothermometer used is that of Whitney and Stormer (1977) modified by regional metamorphic terrains. adding 50°. See Hoisch and others (in press) for a detailed discussion of thermobarometry.

Geological Society of America Bulletin, v. 98, p. 549-553, 5 figs., May 1987.

549

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Figure 2. Schematic isobaric T-X^ diagram showing reactions in the siliceous dolomite system MgO-

Ca0-Si02-H20-C02 (for example, Skippen, 1974) and the basis for isograds and metamorphic zones. Volatfles in reactions are emitted for clarity. Mineral abbrevia- tions: Cc = calcife, Di = diopside, Do = dolomite, Fo = forsterite, Qz = quartz, Tc = talc, Tr = tremolite. Meta- morphic zones: 1LTZ = lower talc zone, UTZ = upper talc zone, DTZ = diopside-tremolite zone, FZ = forsterite zone.

INTRODUCTION

The conditions of metamorphism which are preserved in regional metamorphic terrains are in many cases hotter than those predicted by steady-state geotberms. Higher temperatures I may result from the introduction of hot fluids or 0 0.5 1.0 magmas (see, for example, Lachenbruch and X(C02) Sass, 1977), from rapid crustal thinning (Lachenbruch and Sass, 1978; Wickham and REGIONAL METAMORPHISM IN ing. Tectonism in southeastern California was Oxburgh, 1985), or from radioactive decay THE BIG MARIA MOUNTAINS concurrent with Sevier thrusting in southern following crustal thickening (for example, Eng- Nevada. (Hoisch and others, in press) but is land and Thompson, 1984). Briefly elevated iso- The Big Maria Mountains of southeastern much different in character. Differences may re- therms may result from sudden uplift (England California constitute a Late Cretaceous regional flect the response of different crustal levels and Thompson, 1984). In areas which expe- metamorphic terrain -200 km2 in area. Meta- (Haxel and others, 1984) or different deep- rience peak metamorphic temperatures during morphism occurred at -3.0 ± 1 kbar and at crustal structures to the same general compres- crustal thickening, only the introduction of heat temperatures that increased northwestward from sive regime. In southern Nevada, thrusting was by fluids or magmas may be called upon to 430 to 590 °C (Hoisch, 1985; Fig. 1). Isograds initiated along the Paleozoic miogeocline and explain anomalously high temperatures. If such are crudely defined from mineral assemblages does not involve crystalline basement, whereas areas also lack evidence of synmetamorphic within the siliceous dolomitic limestones of the in southeastern California, structures formed plutonism, then the introduction of heat by cratonal sequence by consideration of isobaric within the ancient craton. fluids may remair as the only plausible explana- invariant points and isobaric univariant thermal tion. Such areas are common, but evidence of maxima within the system Mg0-Ca0-Si02- Evidence for Fluid Flux during Regional fluid fluxes of sufficient magnitude to explain H20-C02 (method of Rice, 1977; Fig. 2). Four Metamorphism significant high-temperature anomalies is not re- metamorphic zones are distinguished, the lower corded in many ciises. That fluids may efficiently talc zone (LTZ), upper talc zone (UTZ), Within the Paleozoic to lower Mesozoic cra- transport heat within the crust has been well diopside-tremolite zone (DTZ), and forsterite tonal aissemblage, there is a wide variety of documented. Circulating fluids may distribute zone (FZ), in order of increasing grade (Fig. 1). sedimentary protoliths, including sandstones, pe- heat around intruding plutons (Cathles, 1977; In the direction of increasing temperature, zone lites, siliceous limestones and dolomites, and Ferry, 1980) and cause anomalous surface heat boundaries are recognized by (1) the limit of volcaniclastic sediments. Siliceous limestone of flow (Donaldson, 1962; Lowell, 1975). stability of quartz + talc + calcite, (2) the limit of the upper Paleozoic Supai Formation has In southeastern California, Late Cretaceous stability of talc + calcite, and (3) the occurrence reacted to form massive wollastonite every- regional metamo:rphism is preserved in several of the assemblage calcite + dolomite + tremolite where but in the lowest grade southeastern part mountain ranges, associated with ductile defor- + diopside + forsterite. The isograds corroborate of the area (Fig. 3). The resultant rock is typi- mation of ancient craton and overlying plat- the trend of northwestward increasing tempera- cally 80%-95% wollastonite, containing also form-facies Paleozoic and lower Mesozoic strata ture indicated by geothermometry (Fig. 1). varying amounts of grossular, diopside, vesuvi- correlative with those of the Grand Canyon (for The Paleozoic and lower Mesozoic cratonal anite, and sphene. Quartz, calcite, and It- example, Stone and others, 1983; Miller and strata were attenuated by as much as a factor of feldspar are also present in some cases. others, 1982; Hoisch and others, in press). These 100 concomitant with basement-involved re- The formation of the massive wollastonite re-

areas were metamorphosed at conditions hotter cumbent and isoclinal folding (Hamilton, 1982, quired an enormous flux of nearly pure H20 than those predicted by steady-state geotherms 1984). Mineral lineations in rocks showing fluid. The minimum ratio of fluid:rock that is (Hoisch and others, in press). In the present granoblastic polygonal elongate texture parallel needed may be calculated based on the reaction

study, evidence for a large flux of hot fluids the axes of large and small folds, indicating that calcite + quartz = wollastonite + C02, using the during regional metamorphism in one area, the their formation was synmetamorphic. The cra- method of Rice and Ferry (1982). Mineral reac-

Big Maria Mountains, is presented, and the pos- tonal strata were probably carried to mid-crustal tions which buffer the con: position of a H20-

sibility that the fluids introduced heat sufficient depths during the formation of these structures. C02 fluid will progress to an extent that is to cause a 300-Celsius-degree increase in This implies that the peak conditions of meta- dependent upon the composition and quantity temperature is discussed. morphism were achieved during crustal thicken- of infiltrating fluids. For such reactions, a fluid

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semblages in the siliceous dolomitic limestones

were richer in CO2 (XH2Q = 0.8-0.1) than were fluid compositions in the Supai Formation (Hoisch and others, in press). This is indicated by mineral assemblages which were buffered along isobaric univariant or invariant equilibria (Fig. 2). Fluid:rock ratios which are required by these assemblages are also much less, generally less than 1:1. At most, only a small fraction of the fluid flux is recorded by mineral reactions which took place in quartzites, pelites, and vol- caniclastics. Carbonate-consuming reactions within pelites may have buffered the fluid-phase composition at an early stage of metamorphism (Ferry, 1984), but only so long as carbonate remained. Pure quartzites are nonreactive and so show no mineralogical indications of fluid interaction. Differences in fluid interaction between a spa- tially associated wollastonite-bearing unit and siliceous dolomites have also been recognized in the Notch Peak aureole, Utah (Nabelek and others, 1984; Hover-Granath and others, 1983; Labotka and others, 1985). It was concluded in these studies that fluid was channeled through the wollastonitic unit. Although channeling may have occurred locally within the Supai Forma- tion in the Big Maria Mountains, fluids must have passed through adjacent units to have affected structurally isolated masses. Passage of fluids may have occurred along fractures or- iented at high angles to stratal boundaries. Many such fractures are filled by quartz veins. A sim- Figure 3. Map showing distribution of Supai Formation within the Paleozoic to lower ilar interpretation was made by Walther and Mesozoic cratonal sequence and of metagranites. Axis of main overturned syncline and eastern Orville (1982), who found that quartz veins rep- limit of wollastonite in Supai Formation are also shown. Distribution of units is based on resent channels which accommodate the es- mapping by Hamilton (1984). cape of fluids during the metamorphism of pelites. The wollastonite-forming reaction involves a of unique composition is in equilibrium with Given average conditions of 3 kbar and 500 reduction in the volume of solid phases of both reactants and products at a specific pressure °C (Hoisch, 1985; Hoisch and others, in press), -33%. This results in enhanced permeability

and temperature. In cases in which the composi- an infiltrating fluid of composition XH2Q = 1-00, which is reinforced as long as the reaction con-

tion of fluid introduced differs from the equilib- an equilibrium fluid composition of XH2Q = 0.97 tinues (for example, Nabelek and others, 1984). rium composition, the reaction will proceed if (that is, approximately that of Rice and Ferry, Permeability enhancement during reaction volatiles liberated by it modify the fluid com- 1982, their Fig. 11), and 90% wollastonite in the and fluid infiltration may have precluded the position toward the equilibrium composition. If final rock, a fluid.rock ratio of 17:1 may be need for fractures to accommodate fluid flow infiltrating fluids continuously impose disequil- calculated. The P-T-X relations of the wollas- and may explain why quartz veins are not ibrium fluid compositions, reactions will pro- tonite-forming reaction require infiltrating fluid observed within the wollastonite-bearing Supai

gress until either infiltration stops or at least one compositions of XH2O > 0-97. This is confirmed Formation. of the reactants is spent. In the method of Rice by the presence of vesuvianite in all samples and Ferry (1982), the amount of fluid that is (Valley and others, 1985). The calculated ratio HEAT TRANSPORT BY FLUIDS needed to explain the observed reaction progress increases as the assumed composition of the in- as indicated by the abundances of product filtrating fluid departs from pure water (Fig. 4). In considering whether fluids might have phases is calculated. It is assumed that during Because the wollastonite-forming reaction is a played a role in carrying large quantities of heat infiltration, the fluid composition was buffered pure decarbonation reaction, the calculated to middle crustal levels during crustal thicken- to that of the equilibrium composition and that fluid:rock ratio is insensitive to uncertainties in ing, two factors must be addressed, (1) the all fluid which flowed through the rock inter- the estimated temperature. amount and temperature of fluid which are reacted chemically. Errors in these assumptions The massive fluid flux is recorded by mineral quired to cause the observed temperature anom- will cause the fluid:rock ratio to be under- reactions only in the Supai Formation. Fluid aly and (2) whether heat was contributed by estimated. compositions in equilibrium with mineral as- magmas.

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Amount and Temperature of Fluids overlying rock. The presence of an underlying pluton and a possible relationship to the meta- An approximate estimate of the thermal morphism are suggested by abundant granite anomaly may be made by comparing the condi- pegmatite dikes in the area of highest rnetamor- tions of metamorphism in the Big Maria Moun- phic grade, locally constituting as much as 30% tains with steady-state geotherms (for example, of the total rock volume (Warren Hamilton, Lachenbruch and Sass, 1977). At the estimated 1985, personal commun.). The pegmatites depth of 11 km, rnetamorphic temperatures av- themselves could not have contributed heat di- erage -300 Celsius degrees higher than the rectly to the metamorphism because they cross- stable-craton geo therm, -390 degrees higher cut rnetamorphic fabrics. Model thermal profiles than the Sierra Nevada geotherm, and —220 around intrusions show steep decreases in degrees higher i:han the Basin and Range temperature away from contacts (Ingersoll and geotherm. Southeastern California was part of others, 1948, p. 141; Turner, 1981, p. 20-22). If the stable craton until Early or Middle Jurassic a pluton caused metamorphism by dry heat time (for example, Burchfiel and Davis, 1972). conduction in the Big Maria Mountains, then it Crustal thickening during the 80-m.y. interval would have to immediately underlie the area, preceding Late Cretaceous metamorphism may and significant decreases in grade which corre- have affected crustal temperatures, but probably late with elevation would be observed. Meta- not more than 80 Celsius degrees at depths of 11 morphic grade does not appear to vary as a Percent C02 in Infiltrating Fluid km, with the most likely effect being cooler function of elevation within "he 1,000 m of relief temperatures (based on thermal models of Eng- Figure 4. Calculated variation of fluid:rock in the area. land and Thompson, 1984). volume ratio with composition of infiltrating It is possible that the fluid flux facilitated ana- An expression i:hat relates the fluid:rock ratio, fluid for the reaction quartz + calcite = wollas- texis and that the pegmatite:; were derived from

temperature of the fluid introduced, initial tonite + C02) assuming 90% wollastonite in the melts. It is unlikely that the pegmatites were temperature of th e rock, and final temperature final rock and conditions of 500 °C, 3 kbar. derived from the same source as that of the of the system can be formulated on the basis of Composition of fluid in equilibrium with fluids because this would require that the melt the assumption that fluid and rock reached the quartz, calcite, and wollastonite is assumed to expel most of its water before generating peg- _ same temperature through the exchange of heat. be X|]2o 0-97 (for example, approximately matites. Although no independent constraints that of Rice and Ferry, 1982, their Fig. 11). are yet available as to the origin of the pegma- Two curves are calculated, one relative to the tites, granitic plutons of probable similar age in final rock volume and the other relative to the the nearby Old Woman Mountains may be initial rock volume. analogous. These have been shown to be largely where of crustal origin (Miller anc. others, 1984). The priate for the Big Maria Mountains; it shows the Old Woman Mountains are similar in many re- r Cp = heat capacity of rock (constant variation of the fluid:rock ratio with the initial spects to the Big Maria Mountains; massive wol- pressure) temperature of the infiltrating fluid such that a lastonite is locally developed within the Supai f Cp = heat capacity of fluid (constant terrain initially at 200 °C is heated to 500 °C. In Formation, ductile thrusts and nappes were pressure) the calculations, the heat capacity of common formed within ancient craton at conditions of f/r V = volume ratio fluid:rock rock is considered to be 0.77 cal/cm3 °C (Ferry, greenschist to amphibolite facies, and granitic 3 ATr = change i n temperature of rock 1983) and of water, 0.76 cal/cm °C (calcu- melts were emplaced shortly after the peak of ATf = change in temperature of fluid lated from the data of Burnham and others, metamorphism (Miller and others, 1982; Hoisch and others, in press). In both areas, the timing of 1969). The ATr thus equals 300, and the fluid: Equation 1 doe;; not take into consideration rock ratio and ATf are unknowns where specifi- events allows for the inteipretation that melt heats of reaction, PV work done by reactions, or cation of either determines the other. The generation was induced by the same flux of hot variations in heat capacities with temperature, calculated minimum fluid:rock ratio of 1.7:1 is fluids that caused wollastonite to develop in the all of which are likely to produce only small seen in Figure 5 to require a temperature of 680 Supai Formation. Hot fluids streaming through effects. °C for the fluid introduced. Greater fluid:rock the crust should cause melting close to their source while facilitating textural and chemical A minimum constraint on the fluid:rock ratio ratios require lower initial fluid temperatures to equilibration at shallower, cooler levels. A short for the cratonal sequence may be ascertained by achieve the same effect. A flux of hot fluids of a period of time is needed for magmas to segregate considering that all of the fluid which passed reasonable volume and temperature can thus and intrude the overlying rocks. This could ex- through reacted with the Supai Formation to explain the observed thermal anomaly. plain the observed relationship between the form massive wollastonite. The Supai Forma- abundance of pegmatites and grade of meta- tion comprises ~ 10% of the cratonal sequence Heat Contributed by Magmas? morphism in the Big Maria Mountains. (Fig. 3); thus, tb.e minimum determined fluid: rock ratio of 17:1 for the Supai Formation (see Heat that is transferred conductively from above discussion and Fig. 4) results in a min- magmatic sources is an unlikely explanation of POSSIBLE SOURCES OF HEAT imum fluid:rock ratio for the cratonal sequence the observed grade of metamorphism in the Big AND FLUIDS of 1.7:1. This is probably characteristic of the Maria Mountains. Plutons correlative with the entire Big Maria Mountains. metamorphism are not exposed in the area, but Large-scale fluid fluxes may originate from The curve shown in Figure 5 is calculated this does not preclude the possibility of plutons the compaction of buried sediments or from the using equation 1 and reflects conditions appro- at depth which may have supplied heat to the dehydration of a subducting slab (Fyfe and Ker-

800 1983, Applications of the reaction progress variable in metamorphic petrology: Journal of Petrology, v. 24, p. 343-376. 1984, A biotite isograd in south-central Maine, U.S.A.: Mineral reac- Initial temperature of terrain = 200° C tions, fluid transfer, and heat transfer: Journal of Petrology, v. 25, p. 871-893. Final temperature - 500°C Fyfe, W. S., and Kerrich, R., 1985, Fluids and thrusting: Chemical Geology, v. 49, p. 353-362. 3kb pressure Figure 5. Calculated Hamilton, W., 1982, Structural evolution of the Big Maria Mountains, north- 700 eastern Riverside County, southeastern California, in Frost, E. G., and variation of fluid:rock ra- Martin, D. L., eds., Mesozoic-Cenozoic tectonic evolution of the Colo- è tio with initial tempera- rado River region, California, Arizona, and Nevada, Anderson- 2 Hamilton Volume: San Diego, California, Cordilleran Publishers, ture of infiltrating fluid p. 1-28. 1984, Generalized geologic map of the Big Maria Mountains, such that a terrain initi- northeastern Riverside County, southeastern California: U.S. Geological Survey Open-File Report 84-407. ally at 200 °C is heated to Haxel, G. B„ Tosdal, R. M., May, D. J., and Wright, J. E„ 1984, Latest S 600 500 °C. Cretaceous and early Tertiary orogenesis in south-central Arizona: Thrust faulting, regional metamorphism, and granitic plutonism: Geo- logical Society of America Bulletin, v. 95, p. 631-653. Hoisch, T. D., 1985, Metamorphism in the Big Maria Mountains, southeastern California [Ph.D. thesis]: Los Angeles, California, University of South- em California, 264 p. Hoisch, T. D„ Miller, C. F., Heizler, M. T„ Harrison, T, M„ and Stoddard, 500 E. F., in press, Late Cretaceous regional metamorphism in southeastern California, in Ernst, W. G., ed., Metamorphism and crustal evolution in the western conterminous United States, Rubey Volume 7: Englewood Cliffs, New Jersey, Prentice-Hall. Fluid/Rock Hover-Granath, V. C„ Papike, J. J., and Labotka, T. C„ 1983, The Notch Peak contact metamorphic aureole, Utah: Petrology of the Big Horse Lime- stone Member of the Orr Formation: Geological Society of America rich, 1985). Plate-tectonic models indicate that tity of fluid estimated to have passed through the Bulletin, v. 94, p. 889-906. Ingercoll, L. R., Zobel, O. J., and Ingersoll, A. G, 1948, Heat conduction with the east-directed subduction of oceanic crust was area (1.7 rock volumes) is easily sufficient to engineering and geological applications: New York, McGraw-Hill, taking place beneath the area of southeastern explain the elevated temperatures if the fluid 278 p. Labotka, T. C„ Papike, J. J., and Nabelek, P. I., 1985, Fluid evolution in the California in Late Cretaceous time (for example, was approximately at granite solidus tempera- Notch Peak aureole [abs.]: EOS (American Geophysical Union Trans- actions), v. 66, p. 389. Burchfiel and Davis, 1972). Volatiles which are ture at the start. Hot fluids may have been de- Lachenbruch, A. H., and Sass, J. H., 1977, Heat flow in the United States and released by the breakdown of amphibole and rived from crystallizing plutons at depth. the thermal regime of the crust, in Heacock, J. G., ed., The nature and physical properties of the Earth's crust: American Geophysical Union phlogopite in a subducting slab might flux melt- Deep penetrative structures within the crust Geophysical Monograph Series, v. 20, p. 626-675. 1978, Models of an extending lithosphere and heat flow in the Basin ing in overlying hotter mantle (for example, may serve to channel fluids (Fyfe and Kerrich, and Range: Geological Society of America Memoir 152, p. 209-249. Wyllie and Sekine, 1982). As these melts hy- Lowell, R. P., 1975, Circulation in fractures, hot springs, and convective heat 1985), thereby localizing anatexis and meta- transport on mid-ocean ridge crests: Royal Astronomical Society Geo- bridize and crystallize, aqueous fluids are ex- morphism. Zones weakened by fluids may physical Journal, v. 40, p. 351-365. Miller, C. F., Howard, K. A., and Hoisch, T. D., 1982, Mesozoic thrusting, pelled, ultimately infiltrating the crust. Such become susceptible to deformation in a com- metamorphism, and plutonism, Old Woman-Piute Range, southeastern fluids would initially be at the solidus tempera- California, in Frost, E. G., and Martin, D. L., eds., Mesozoic-Cenozoic pressive regime. This may explain an association tectonic evolution of the Colorado River region, California, Arizona, ture and should transport heat to higher levels as between regional metamorphism and the devel- and Nevada, Anderson-Hamilton Volume: San Diego, California, Cordilleran Publishers, p. 561-581. they stream through overlying rock. The fluids opment of nappes observed in southeastern Cali- Miller, C. F, Bennett, V., Wooden, J. L„ Soloman, G. C., Wright, J. E„ 1984, which affected the Big Maria Mountains need Origin of the composite metaluminous/peraluminous Old Woman- fornia (Hoisch and others, in press). Piute batholith, S.E. California: Isotopic constraints: Geological Society not have been derived from any single body; of America Abstracts with Programs, v. 16, p. 596. Nabelek, P. I., Labotka, T. C, O'Niel, J. R„ and Papike, J. J., 1984, simultaneously crystallizing plutons at different ACKNOWLEDGMENTS Contrasting fluid/rock interaction between the Notch Peak granitic levels within the crust could have contributed to intrusion and argillites and limestones in western Utah: Evidence from stable isotopes and phase assemblages: Contributions to Mineralogy and the over-all fluid flux. The plutons need not have This work benefited greatly from the thought- Petrology, v. 86, p. 25-34. been close to the area as the fluids could have Rice, J. M., 1977, Progressive metamorphism of impure dolomitic limestone in ful comments of John Valley, Brian Smith, the Marysville aureole, Montana: American Journal of Science, v. 277, been transported through conduits. Calculations p. 1-24. Warren Hamilton, Jane Selverstone, Calvin Rice, J. M., and Ferry, J. M., 1982, Buffering, infiltration, and the control of show that if a volume of fluid-saturated granite intensive variables during metamorphism, in Ferry, J. M., ed.. Charac- Miller, Gregg Swayze, Mike Sorey, and an terization of metamorphism through mineral equilibria: Mineralogical melt 5.5 times the volume of the metamor- anonymous reviewer. This project was com- Society of America Reviews in Mineralogy, v. 10, p. 263-326. phosed country rock were to crystallize, it would Skippen, G., 1974, An experimental model for low pressure metamorphism of pleted while the author held a National siliceous dolomitic marble: American Journal of Science, v. 274, result in the minimum estimated fluid:rock ratio Research Council-U.S. Geological Survey p. 487-509. of 1.7:1. Stone, P., Howard, K. A., and Hamilton, W. A., 1983, Correlation of meta- Research Associateship. morphosed Paleozoic strata of the southwestern Mojave Desert region, California and Arizona: Geological Society of America Bulletin, v. 94, p. 1135-1147. Turner, F. J., 1981, Metamorphic petrology (2nd edition): New York, CONCLUSIONS McGraw-Hill, 524 p. REFERENCES CITED Valley, J. V., Peacor, D. 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