Scale Crust Formation and Lithosphere Modification Beneath
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,485-13,507,JULY 30, 1991 Large-ScaleCrust Formation and LithosphereModification Beneath Middle to Late Cenozoic Calderas and Volcanic Fields, Western North America CLARK M. JOHNSON Department of Geology and Geophysics, University of Wisconsin, Madison Over 500,000 km3 of intermediate-to silicic-compositionash flow tuffs and lavaswere eruptedfrom large-volume volcanic fields in western North America during the middle to late Cenozoic. Of the commonlyused isotope systems, Nd isotopedata provide the best constrainton the proportionsof crust and mantle componentsin the tuffs; all tuffs that have been studied contain a major, often dominant, mantle component. The proportion of mantle to crustal componentsis best constrainedfor ash flow tuffs that were erupted on Precambrian crust. There is no simple correlation between inception of extensionalfaulting with development of calderas, although all calderas formed in or adjacent to regionsthat ultimately underwentcrustal extension to somedegree. Similarly, there are no first-order correlationsbetween the proportionof mantle-derivedcomponent in the silicic magmasand the tectonic setting. The common thread to all caldera complexes is that they are generatedby large fluxes of mantle-derivedbasaltic magmas. Detailed isotopicand petrologic studies of severalcaldera complexes indicate that the silicic magmas were fundamentally derived by fractional crystallization of mantle- derived magmas, accompanied by assimilation of continental crust. Cenozoic caldera-related magmatismin westernNorth America representsa major episodeof crustalgrowth and hybridization; crustalgrowth rates in the Cenozoicmay rival thosethat occurredin the Proterozoicduring initial crust formation. New results of petrologic and isotopic studiesof multicyclic caldera complexesin the northern Rio Grande rift region, in addition to several other areas of the western United States,confirm previous models that predict large changesin O, Sr, and Nd isotope compositionsof the crust during injection of mantle-derived basaltic magmas. Continued injection of basaltic magmas results in an increasein the mean density of the crust, which may be reflected in upward movement of the locus of silicic magmaevolution in the crust;such changes may be monitoredin areasunderlain by ancientcrust using Pb isotope ratios if the crust is stratified in U/Pb ratios. If accompaniedby assimilation, crystallization of mantle-derivedmagmas that stall near the crust-mantleboundary will return crustal componentsto the mantle in the form of mafic and ultramafic cumulates,in addition to producing a petrologicallycomplex crust-mantle boundary. This model can explain many geophysicalcharacteris- tics of the crust and upper mantle in western North America and supportsthe conclusionsof many recent studiesof lower crustal xenoliths, which proposerecent magmatic underplatingin tectonically active regions. INTRODUCTION caldera complexesalso developedat 40-23 Ma along the easternmargins of what is now the ColoradoPlateau [e.g., Over 500,000 km3 of ash flow tuffs have been erupted Stucklessand Sheridan, 1971; Steven and Lipman, 1976; onto the surface of western North America during the Rattd et al., 1984; Elston, 1984; Pallister and du Bray, middle to late Cenozoic,approximately equally distributed 1989; Lipman et al., 1986]. Caldera-relatedvolcanism in between the western United States and Mexico [Mackin, the Trans-PecosTexas region occurredat 38-28 Ma [Henry 1960; Smith, 1960; McDowell and Clabaugh, 1979; and Price, 1984], and the large volume of ash flow tuffs Swanson and McDowell, 1984; Lipman, 1984]. Taking that belongto the Upper Volcanic Supergroupof the Sierra into accountthe voluminousplutonic rocks that must un- Madre Occidental were erupted at 34-23 Ma [McDowell derlie volcanic centers[Crisp, 1984; Lipman, 1984; Shaw, and Clabaugh, 1979; Swansonand McDowell, 1984]. Vol- 1985], as much as 5,000,000 km 3of intermediate-to silicic- canismin the Mojave and northern SonoranDeserts began compositionmagmas may have been associatedwith this at 30 Ma and migratednorthward over a 20-m.y. period but Cenozoic volcanism. Voluminous, caldera-related Ceno- producedrelatively few calderas [Glazner and Supplee, zoic volcanismin westernNorth America began at 49 Ma 1982]. Well-known young (< 2 Ma) calderasin the western in the Challis volcanic field of central Idaho [Cater et al., United States include those in the Yellowstone and Jemez 1973] and swept southwestalong an arcuate front [e.g., volcanic fields, as well as the Long Valley caldera Armstrong et al., 1969; Best et al., 1989]. Caldera com- [Christiansenand Blank, 1972; Smith et al., 1970; Bailey et plexesin the northeasternGreat Basinformed at 35-30 Ma, al., 1976], and these lie along the outer margins of the whereas calderas in central and western Nevada formed at caldera-related volcanism in the western United States. 30-20 Ma. Calderasat the westernand southernmargins of Cenozoic caldera complexesin westernNorth America the Great Basin formed less than 20 m.y. ago. Large formed in a diverse range of tectonic settings. Although Gans et al. [1989] proposethat major volcanicepisodes are Copyright 1991 by the AmericanGeophysical Union. related to periodsof extensionin the Great Basin, detailed Paper number 91JB00304. studiesof a large number of stratigraphicsections in the 0148-0227/91/91JB-00304505.00 eastern Great Basin indicate that caldera-related volcanism 13,485 13,486 JOHNSON:LARGE-SCALE CRUST FORMATION AND LITHOSPHEREMODIFICATION in the region is not associatedwith extension[Taylor et al., 1989; Best and Christiansen, this issue]. Several large- volume caldera complexes formed significantly prior to extension or in areas that underwent little extension, in- cluding the Sawatch Range calderas in central Colorado, the central Sierra Madre Occidental calderas, and the Marysvale, San Juan, and parts of the Mogollon-Datil vol- canic fields [e.g., Lipman et al., 1970; McDowell and Clabaugh, 1979; Steven et al., 1984; Cather and Johnson, 1986; Shannon, 1988]. Early caldera-relatederuptions in the southwestNevada volcanic field began approximately contemporaneouswith initiation of extension[œkren et al., 1968], and ash flow tuffs were eruptedfor another8 m.y. [Byers et al., 1989]. The McDermitt, Jemez, and Long Valley calderas formed after initiation of extension (see Rytuba and McKee [1984] and referencesabove). Regard- less of the tectonic setting in which calderaswere formed, all developed within or adjacent to areas that underwent mid-Miocene to Quaternaryextensional faulting. Geologistshave long debatedif silicic magmasare gen- eratedprimarily by crystal fractionationof basalticmagmas or partial melting of the crust [e.g., Daly, 1914; Bowen, 1915; Holmes, 1932]. Early Sr isotope analysesof ash- flow tuffs in the Great Basin have been usedto supportboth crystalfractionation and crustalmelting models[Noble and Hedge, 1969; Scott et al., 1971; McKee et al., 1972; Noble et al., 1973; Stuckless and O'Neil, 1973]. In the first Pb isotope study of a large-volume caldera complex, Lipman et al. [ 1978] highlight the stronginfluence of the crust on caldera-relatedmagmatism. This report summarizesNd isotopedata for silicic rocks from middle to late Cenozoiccaldera complexes and volca- Fig. 1. Locationsof Cenozoicvolcanic fields and calderasdiscussed nic fields from westernNorth America (Figure 1). The data in text for which Nd isotopedata are available. Other localitiesnoted highlight the large mantle componentin the ash flow tuffs, in italics. B, Batopilas;BCS, Baja California Sur; GP, Grizzly Peak which, based on detailed study of several caldera com- caldera; J, Jemez volcanic field; K, Kalamazoo volcanic sequence; KSW, Kane SpringsWash volcanic field; L, Latir volcanicfield; LO, plexes,is interpretedto reflect fractionalcrystallization of La Olivina; LH, Los Humeros volcanic field; LV, Long Valley large volumesof basalticmagmas, accompanied by interac- caldera region; MCD, McDermitt volcanic field; M-D, Mogollon~ tion with continentalcrust. Of the three commonlyused Datil volcanic field; MO, Molango; N, Novillo; O, Oaxaca; P-W, radiogenic isotope systems,Rb-Sr, Sm-Nd, U-Th-Pb, de- Source area for Peach SpringsTuff and Woods Mountains volcanic termination of the relative proportionsof crust and mantle center; S, Sonoma volcanic field; SA, San Antonio; SJ, San Juan volcanic field; SLC, Sierra los Cajones; SLP, Sierra La Primavera; componentsis probably least ambiguoususing Nd isotope SV, Sierra Virulento; SWNV, southwest Nevada volcanic field; Z, ratios. Neodymium contentsof mantle-derivedbasalts and Zacatecas. 87Sr/86Sr=0.706line for Mesozoicand Tertiary granitic continental crust are similar, and where the basement con- rocks and is interpretedto reflect the westernmargin of the Precam- sistsof Precambriancrust, the Nd isotopecompositions of brian basement in the western United States [Kistler and Peterman, 1973, 1978]. Farmer and DePaolo [1983] interpret granitic rocks the crust and mantle are often distinct. Present-day that haveend=-7 as indicating the Precambrianbasement margin; this 87Sr/S6Srratios of low-Rb lower crustmay be similar to "line" lies just eastof the 87Sr/a6Sr=0.706line. SP in box refersto those of the modern mantle, which would disguisea crustal Sierran Province (stippled pattern), which containsTertiary basalts component. Conversely, the low-Sr contents of highly that haveexceptionally low endvalues (see references in text). evolved