JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B8, PAGES 15,249-15,266, AUGUST 10, 1995

The Mount Perkins block, northwestern Arizona: An exposedcross section of an evolving, preextensional to synextensionalmagmatic system

JamesE. Faulds,• Daniel L. Feuerbach,1 Mark K. Reagan,1 Rodney V. Metcalf,2 Phil Gans,3 and J. D. Walker4

Abstract. The steeplytilted Mount Perkinsblock, northwestern Arizona, exposes a cross sectionof a magmaticsystem that evolved through the onsetof regionalextension. New 4øAr/39Arages of variably tilted (0-90 ø) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensionalintrusive activity includedemplacement of a compositeMiocene laccolithand stock,trachydacite dome complex, and east striking rhyolite dikes. Related volcanicactivity producedan -18-16 Ma stratovolcano,cored by trachydacitedomes and flankedby trachydacite-trachyandesiteflows, and -16 Ma rhyoliteflows. Similar compositionsindicate a geneticlink betweenthe stratovolcanoand granodioriticphase of the laccolith. Magmatic activity synchronouswith early regionalextension (15.7-14.5 Ma) generateda thick, felsic volcanicsequence, a swarmof northerlystriking subvertical rhyolite dikes,and rhyolite domes. Field relationsand compositionsindicate that the dike swarmand felsicvolcanic sequence are cogenetic.Modes of magmaemplacement changed during the onsetof extensionfrom subhorizontalsheets, east striking dikes, and stocksto northerly striking,subvertical dike swarms,as the regionalstress field shiftedfrom nearly isotropicto decidedlyanisotropic with an east-westtrending, horizontal least principal stress. Preextensionaltrachydacitic and preextensionalto synextensionalrhyolitic magmas were part of an evolving system,which involvedthe pondingof mantle-derivedbasaltic magmas and ensuingcrustal melting and assimilationat progressivelyshallower levels. Major extension haltedthis systemby generatingabundant pathways to the surface(fractures), which flushed out preexistingcrustal melts and hybrid magmas.Remaining silicic meltswere quenchedby rapid,upper crustal cooling induced by tectonicdenudation. These processes facilitated eruptionof mafic magmas.Accordingly, silicic magmatism at Mount Perkinsended abruptly duringpeak extension-14.5 Ma and gaveway to mafic magmatism,which continueduntil extension ceased.

Introduction extendedregions are essentiallyamagmatic (e.g., southern Nevada in the Basin and Range province; [Eaton, 1982]), The relationsbetween magmatism and the dynamic and whereas some areas of intense volcanism within extensional kinematic evolution of extensional terranes have been the settings(e.g., Mogollon-Datil volcanic field, New Mexico; subject of significant controversy. Many workers have [Elston, 1989]) have experiencedlittle mechanicalextension proposedrelationships between magmatismand extensionin (i.e., faulting and tilting). One approachto elucidatingthe which (1) the emplacementof large volumes of magma at interplaybetween magmatism and extensionis to examinean depth triggers extension at shallow crustal levels [e.g., individualmagmatic system that evolvedduring the onsetor Anderson, 1971; deVoogd et al., 1988; Gans et al., 1989; termination of extension. Meyer and Foland, 1991; Lister and Baldwin, 1993] or (2) Locating volcanic centers in extended terranes can be the geometryof major normal faults, particularlydetachment difficult, however, especiallyin the more highly extended faults, controlsthe distributionand type of magmatism[e.g., regions. For example,sources for much of the widespread Wernicke, 1985; Bosworth, 1987]. Others, however, have and voluminousTertiary volcanismin the Basin and Range noted diachroneities in the timing of magmatism and province have not been located [Lipman, 1984]. Moreover, extensionwithin extendedterranes [Bartley et al., 1988; Best mostof the well-documentedcenters occur in mildly extended and Christiansen, 1991; Taylor et al., 1989; Taylor and regions [Best and Christiansen, 1991], where tilts of fault Bartley, 1992; Axen et al., 1993]. For example, somehighly blocksare generally less than 30 ø . Althoughmuch can be gleanedfrom volcanic centersin the mildly extendedareas, •Departmentof Geology, University of Iowa,Iowa City. 2Departmentof Geoscience, University of Nevada,Las Vegas. the exposedstructural levels in such regionsare generally 3Departmentof GeologicalSciences, University of California, limited to the upperfew kilometersof crust,which precludes Santa Barbara. comprehensiveanalysis of magmaticplumbing systems and 4Departmentof Geology, University of Kansas,Lawrence. emplacementmechanisms. The difficultyin identifyingvolcanic centers, especially in Copyright1995 by the AmericanGeophysical Union. highly extended regions, is a function of structural Paper number 95JB01375. dismembermentby normal and strike-slip faults [e.g., 0148-0227/95/95 JB-01375 $05.00 Anderson,1,973], tilting, and erosion. Large-magnitude

15,249 15,250 FAULDSET AL.: EVOLVINGMAGMATIC SYSTEM, ARIZONA tiltingmay be the most effective agent for obscuring volcanic corridor,several volcanic sequences have been correlated edifices.Tilting induces burial of the downthrownportions withcogenetic plutons on thebasis of structuralrelations and of fault blocks and erosionof the upthrownparts and thus geochemistry[Weber and Smith, 1987]. greatlyreduces the "exposureprobability" of volcanic edifices. In some cases, however, tilting can reveal cross Mount Perkins Block sectionsof magmaticsystems from plutonic roots, through TheMount Perkins block is a steeply(-90 ø) westtilted hypabyssaldike swarms, to surficialvolcanic complexes [e.g., fault block that containsan apparentlycogenetic suite of Proffett,1977; Gans and Miller, 1983;Asmerom et al., 1990; Mioceneplutonic, hypabyssal, and volcanicrock [Faulds, Holm and Wernicke,1990; John, 1995]. Studiesof such 1993b](Figures 1 and 2). The across-strikewidth of the systemscan elucidate the interplay between magmatism and steeplytilted exposures suggests that as much as 9 km of crust extensionby permittinganalysis of the emplacementmay be exposed on end within the Mount Perkins block. The mechanismsof intrusions, structural controls on volcanic blockis borderedon the eastby the gently(5-30 ø) east- centers,and petrogenesis of magmas. northeastdipping Mockingbird Mine fault and on the west by The steeplytilted Mount Perkinsblock, northwesternthe moderatelyeast dipping Mount Davis faultø The 30-km- Arizona,affords a cross-sectionalview of the upperseveral long Mount Perkins block terminatesabruptly northwardat a kilometersof a magmaticsystem that was activebefore, major accommodation zone [Faulds et al., 1990] in during, and after the onset of major extension. We first conjunction with the along-strike termination of the east describe the field relations and geochronologicaland dipping normal fault system. The more diffuse, southern geochemicalevidence that indicate exposureof a tilted boundary of the block is related to a gradual southward magmaticsystem and genetic ties between different structural decreasein displacementon the Mockingbird Mine fault levels of the system. The evolutionof the systemis then (Figure 1). discussedwithin the contextof regionalextension. The Mockingbird Mine fault originated as a steeply dippingnormal fault and was rotatedto a gentledip by fault blocktilting, as shown by steeptilts (70-90 ø) withinboth its Geologic Setting hangingwall and footwall. Nucleationof the fault probably Northern Colorado River Extensional Corridor occurred shortly after the onset of extension and tilting, The Mount Perkins block lies within the central Black however,as 90 ø of rotationwould imply initiation as a steeply dipping reversefault. Middle to late Tertiary reversefaults Mountains in the northern part of the Colorado River are very rare within the region. Asymmetricmicrostructures extensionalcorridor (Figure 1), a 70- to 100-km-wideregion developedin breccia and gouge along the fault, such as R• of moderatelyto severelyextended crust situated between the Riedel shearsand rough facets [Angelieret al., 1985; Petit, relativelyunextended Colorado Plateau on the east and Spring 1987], indicate an east-northeasttrending slip line (i.e., Range-OldWoman Mountains region on the west[Howard essentiallydip-slip). Althoughpiercing points have not been and John, 1987; Faulds et al., 1990]. The extensional corridor terminates northward at the left-lateral Lake Mead observed,offset volcanic strataand the orientationof the slip line suggestthat the MockingbirdMine fault accommodateda fault systemand right-lateralLas VegasValley shearzone. maximumof 5-6 km of slip in the vicinity of Mount Perkins. Thick sections(generally > 3 km) of Miocenevolcanic and In the Mount Perkins region, west tilting increases sedimentarystrata rest directlyon Proterozoicand late eastward across many of the major, east dipping normal Cretaceousmetamorphic and plutonic rock within the faults. This suggeststhat fault block tilting was largely extensionalcorridor [Longwell, 1963; Anderson, 1971; Frost accommodatedby displacementon concave upward, east and Martin, 1982; Sherrodand Nielson, 1993]. dipping normal faults and dominolike tilting of intervening Calc-alkalicmagmatism and extensionswept northward fault blocks[e.g., Proffett, 1977]. The DupontMountain fault acrossthe northernpart of the corridorin early to late on the east flank of the southern Eldorado Mountains Miocenetime [Glaznerand Bartley,1984; Gans et al., 1989; accommodatedthe greatestamount of slip (>10 km) in the Armstrongand Ward,1991; Faulds et al., 1994a]. Mostof Mount Perkinsregion and probablyfacilitated considerable the documentedvolcanic centersand plutonsoccur within tilting of the Mount Perkinsblock. The Dupont Mountain threeeast-west trending magmatic belts [Smithand Faulds, fault may link southward with major detachmentfaults 1994]. Magmatismbegan 1-2 m.y. beforeextension and [Faulds et al., 1992] and eastwardwith a major subhorizontal generallycontinued through most of theextensional episode. discontinuityimaged at 5-6 km depth on seismicreflection Thus the corridoris dominatedby highly fragmented,thick profilesfrom Detrital Valley directlyeast of Mount Perkins volcanicpiles contained within complex arrays of tiltedfault (J. Faulds, unpublished data, 1995). The discontinuity blocks. The averagestrike of tiltedMiocene strata and slip probably accommodatedmuch of the tilting of the Mount data [Anderson,1971, Angelieret al., 1985; J. E. Faulds, Perkins block. However, the amount of slip on the unpublisheddata, 1994] indicate an east-northeast (-075ø) to discontinuityhas not been constrained. east-west extension direction within the Mount Perkins region. Timing of Extension and Magmatism Becausemagmatism preceded and continuedduring extension,the effectsof regionalextension on the evolution As part of a regional investigationof the timing of of individual magmatic systems can be evaluated. magmatismand extensionin the northernColorado River Appreciabletilting in manyareas permits examination of both extensionalcorridor, we haveconducted a detailed 4øAr/39Ar the plumbingsystems and surficialvolcanic edifices of studyof magmaticactivity in the Mount Perkinsblock and individualmagmatic centers. In thenorthernmost part of the adjacentareas. Here we report10 new high-precisionages of FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM,ARIZONA 15,251

LatebasinTertiary-Quaternary fill sediments. East tilted, Miocene volcanic andsedimentary strata. i[• Westand sedimentarytilted, Miocene strata. volcanic

r•• swarms.Miocene plutonsand dike LateCretaceous plutons. Paleozolcsedimentary strata.

..'• Proterozolcmetamorphicplutonlc rock. and

Gonfiydipping normal fault (dashedwhere concealed). Moderatelytosteeply dipping normal fault.

Anticlineaccommodation(Interference zone).

Figure1. Generalizedgeologic map of thenorthern Colorado River extensional corridor [from Faulds, 1993a,also unpublished mapping and reconnaissance, 1994;Anderson, 1971, 1977, 1978]. Blocked out area refersto region shown in Figure2. AZ,accommodation zone(separates east and west tilted domains); AW, AztecWash pluton; BM, Black Mountains; DM, Dupont Mountain fault; DV, Detrital Valley; EM, Eldorado Mountains;HS, Highland Spring Range; LM, Lake Mead; MB, Mockingbird Mine fault; MD, Mount Davis fault;ML, LakeMohave; MP, Mount Perkins; MR, McCullough Range; RM, River Mountains; WH, White Hills; WR, Wilson Ridge pluton. volcanicand plutonicrocks that constrainthe timingand Preextensional Magmatism durationof volcanicactivity, establish links between volcanic andintrusive activity, and tightly bracket the timing and rates Preextensional(-15.7-20 Ma) magmatismwas primarily of tilting associatedwith regionalextension (Table 1 and intermediatein compositionand produced rock units exposed Figure 3). Analyticaltechniques are describedin the in the easternand centralparts of the Mount Perkinsblock appendix. (Table2). The structurallylower, eastern part of the block The 4øAr/39Ardata demonstratethat volcanismwithin the contains the Miocene Mount Perkins pluton that was MountPerkins region began at-20 Ma andcontinued without emplacedin Proterozoicorthogneiss. In the centraland significantinterruption until --11 Ma (Table1 andFigure 3). northernparts of the block,the foliationwithin the gneiss Majorintrusions were emplaced from at least16 to --14.5Ma. generallydips moderately southwest. Normal drag along the Extensionwithin the regionis tightly bracketedbetween MockingbirdMine faulthas warped this foliation into gentle 15.7and 11.3 Ma by4øAr/39Ar ages of variablytilted volcanic eastwarddips alongthe easternmostmargin of the block stratawithin majorgrowth-fault basins (Table 1 and Figure (Figure2). Southof thepluton, east striking, steeply dipping 3). Withinthe MountPerkins block, west tilting of thelower foliationsdominate the gneiss. Largegranitoid dikes with partof theMiocene section locally exceeds 90 ø . Tiltsfan relativelylow aspectratios (length/widthratio <- 10.0) upwardto aslittle as 30 ø within a majorgrowth-fault basin in constitutethe bulk of the Mount Perkinspluton. Most of the westernpart of the block(Figures 2b and3). Directly thesedikes, as well asthe easternand western margins of the west and east of the Mount Perkins block, flits decrease pluton,dip steeply eastward and thus crosscut the foliations in upsectionfrom 90 ø to 0ø. Theyoungest of themost steeply thegneiss. The pluton consists of threedistinct phases: (1) an tiltedunits consistently yields ages of 15.7-15.8Ma, whereas older gabbrothat at least locally includessome diorite, cappingflat-lying units range from 11.3 to 11.5Ma (Table1). (2) quartzdiorite to hornblendegranodiorite of intermediate In the Mount Perkinsregion, peak extension (i.e., highest age,and (3) youngerbiotite granodiorite and granite [Metcalf ratesof tilting)occurred between 15.2 and 14.3 Ma when et al., 1993]. Phases2 and 3 containmafic enclaves. tiltingrates ranged from 50 ø to 80ø/Ma(Figure 3). The Mount Perkinspluton was originallyemplaced as a Volcanicand plutonic rocks at MountPerkins are divided subhorizontaltabular mass but was subsequentlytilted intopreextensional (15.7-20 Ma) andsynextensional (11.3- significantlyto the west. Paleomagneticdata indicatea 15.7Ma) groupson the basisof well-exposedcrosscutting minimum of 50ø of west tilting of the pluton [Fauldset al., relationsand high-precision 4øAff39Ar geechronologic dataon 1992]. Moreover,an 4øAr/39Ar age on biotite of 15.96_+0.06 criticalunits. This subdivisionhelps to clarifythe relations Ma fromthe younger granodiorite phase of thepluton (Table between intrusive and extrusive rocks and the interplay 1) suggeststhat the plutonwas emplaced and cooled below betweenextension and magmatism. theclosure temperature of biotite (--350-373øC) [Berger and 15,252 FAULDSET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA

EXPLANATION

Quaternary-latebasin fill sediments.Miocene 55 Mr. Davis vV•!•MockIn Mr. DavisVolcanlcs and Upper Member 'Mine of PatsyMine Volcanlcs; mainly 14.3-15.3 ß . . Ma basalt flows; Includes15.2 Ma Tuff of BridgeSpring.

II1• RedrhyollteGap flows Mine andvolcanlcs: tuffs. 14.5-15.8Ma

rhyollteMt.Perkins dikedike swarm.swarm: 14.5-15.7 Ma .Mr. Perkins __•15.7-16.0 Marhyollte dikes.

ß

! TraohydaolteQueen Mine voloanlos.domesof the Dixie

Mr. Dc I Dixie Queen Mine voloanlos: 1,5.8-18.,5 / Ma traohydaoiteand traohyandeslte flows.Also Includes 18.5 Ma Peach / SpringsTuff, 19-20 Ma basalt flows, I and basal arkosloconglomerate. Dixie Quee• / I andMr. Perkinsgranite; pluton, Includes16.0 olderMa granodlorltediorite and gabbro phases.

N EarlyProterozolc orthogneiss. Samplelocation for geochronology.

Strikeand dip of layering.

Strikeand dip of follafion.

Genfiydipping normal fault (dashed VVVVV VVVVV where concealed).

vvvv vVVV Moderatelytosteeply dipping normal 'VVV fault (dashedwhere concealed).

b

"• .• .• o c' 2.0t •o• • , '• • , o•-5.•

A A'

Figure2. (a)Generalized geologic map of the Mount Perkins blockß Because ofthe steep tilting, the map essentiallyrepresents a downplunge view of tilted volcanoes. Circled letters represent sample locations for geochronologicdatashown inFigure 3 and Table 1. (b)Generalized crosssection (no vertical exaggeration) ofthe Mount Perkins block. Thicknesses of stratigraphic unitsare slightly exaggerated, asnumerous minor faultshave been omitted forclarity. Tbs, 15.2 Ma Tuff of Bridge Spring; Tps, 18.5 Ma Peach Springs Tuff; Tri, rhyolitedikes. FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA 15,253

Table 1. GeochronologicData

Sample LatitudeN LongitudeW Magnitude RockType MaterialDated ApparentAge, Source of West Tilt Ma

Mount Perkins Block

a (JF92-27) 35039'09" 114035'48" 30ø basaltflow Whole Rock 14.27+0.12 A b (JF92-26) 35ø39'14" 114035'23" 55ø basaltflow Whole Rock 14.5+0.1 A c (JF92-25) 35039'25" 114035'04" 90ø rhyolitetuff Biotite 15.74+0.10 A d* (JF91-41) 35046'36" 114ø36'01" 90ø PeachSprings Tuff Sanidine 18.46+0.05 A e*(JF92-32) 35044'45" 114033'43" 90ø basaltflow Whole Rock 19.9+0.5 A f (JF91-61) 35035'08" 114028'35" 90ø* granodiorite Biotite 15.96+0.06 A f (JF92-242) 35034'55" 114029'28" 90ø* granodiorite Apatite 14.60+3.0 B f (JF92-242) 35034'55" 114029'28" 90ø* granodiorite Zircon 16.10+1.0 B g (JF92-244) 35035'33" 114ø28'16" 90ø* gabbro Apatite 14.10+4.0 B h (JF813) 35035'00" 114ø31'17" 45ø* rhyolitedike K-spar 14.50+0.30 C i (Ti-1) 35029'59" 114030'25" 90ø* rhyolitedike Biotite 15.7+0.4 D Directly East of Mount Perkins Block j (TPV-AND) 35ø24'21" 114ø26'21" 0ø basalticandesite flow WholeRock 11.4+0.4 D k (87-9) 35ø27'11" 114ø29'10" 80ø tuff Biotite 15.7+0.3 D Directly Westof Mount Perkins Block 1 (JF92-44) 35034'45" 114045'40" 0ø basaltflow Whole Rock 11.35+0.10 A m (JF92-45) 35033'05" 114045'00" 15ø basaltflow Whole Rock 12.78+0.02 A n (JF92-34) 35033'08" 114ø38'23" 30ø basaltflow Whole Rock 14.79+0.03 A o (JF92-35) 35032'52" 114038'02" 40ø rhyolitetuff Sanidine 14.97+0.02 A p (JF92-37) 35032'52" 114ø37'54" 60ø tuffof BridgeSpring Sanidine 15.21+0.02 A A, 4øAr/39Ardates from P. Gans, University of California,Santa Barbara; B, fissiontrack ages from D. Foster,LaTrobe University; C, K/Ar datefrom Fauldset al. [1992]; D, K/Ar datefrom Conradet al. [1990]. Errorsquoted at +1o. Analyticaldata for A are reported in TableA1 •. Moredetailed information for B andC canbe obtained from J. Faulds upon request. Sample locations (except 1and m) are shownon Figure 2. *Sampleswere not obtainedfrom Mount Perkinsblock, but correlativeunits crop out near baseof sectionin the Mount Perkinsblock. *Tiltinginferred from paleomagnetic dataand geologic relations [Faulds et al., 1992].

York, 1981] prior to the onsetof extensionat 15.7 Ma. Thus Ma basalticlava flows, (2) a thin (0-30 m) 18.46 Ma (Table the plutonhas probablybeen tilted -90 ø to the west along 1) rhyolitetuff, which is probablycorrelative with the Peach with the bulk of the Mount Perkins block. The steeply SpringsTuff [cf. Nielson et al., 1990, Glazner et al., 1986]; dipping dikes that comprise the pluton were probably (3) a 2-km-thicksequence of trachyandesiteand trachydacite emplacedas subhorizontalsheets. flows, which is here referred to as the Dixie Queen Mine Reconstructionof the Mount Perkins block suggeststhat volcanics;the lowertrachyandesite flows interfingerwith and the centralpart of the Mount Perkinspluton was emplacedat are overlain by trachydaciteflows and domes; and (4) the a depth of approximately5.5+1.0 km. The reconstruction lowermostpart of a 15.8-14.5Ma felsic volcanicsequence, involvesrestoring several minor faults (not shownon Figure which includes quartz-phenocryst-bearingrhyolite flows, 2), untilting, and removal of younger intrusions. The tuffs, surgedeposits, and tuffaceoussedimentary rocks and is "reconstructed"emplacement depth is compatiblewith Al-in- here referred to as the Red Gap Mine volcanicsbased on hornblendegeobarometry [Schmidt, 1992] for granodiorite excellentexposures near the Red Gap Mine (Table 2 and phasesof the pluton, which yield pressuresof 2.1_-t-0.6kbars Figure2). The Dixie Queenand Red GapMine volcanicsare indicatingcrystallization depths of 7.5+2.2 km [Metcalf et al., temporally correlative with the Lower and Middle Members, 1995]. respectively, of the Patsy Mine Volcanics of Anderson A set of widely spaced,east to northeaststriking, rhyolite [1971]. However, the volcanic sequenceswithin the Mt. dikes is also interpreted as part of the preextensional Perkinsblock and PatsyMine Volcanicswere probablynot magmaticsystem. These steeplydipping, quartz-phenocryst- derivedfrom the samevolcanic centers. The lower, pre- beating dikes cut Proterozoicgneisses and the preextensional extensionalpart of the Red Gap Mine volcanicsand the east lower part of the Miocene sectionin the Mount Perkins-Dixie to northeaststriking set of rhyolitedikes have similar spatial Queen Mine area but do not cut the synextensionalparts of distributions,compositions, and agesand may thereforebe the Miocene section. Biotite from one dike gave a K/Ar age cogenetic. Although a slight angular unconformity(10 ø) of 15.7 + 0.4 Ma [Conrad et al., 1990]. The east to northeast locally separatesthe basal arkoseand PeachSprings Tuff, strikesof the dikes and their absencein synextensionalparts dips are generallyconcordant within the lower part of the of the Miocene section suggestemplacement prior to the Miocenesection and average90 ø (Figure3). This suggests onsetof major east-westextension. thatthe lowerpart of theMiocene section was deposited prior In ascendingorder above an early Miocene (~20 Ma) non- to any significantregional extension. conformity, the preextensional,lower part of the Miocene Several features indicate that the Dixie Queen Mine sectionwithin the Mount Perkinsblock consistsof (1) up to volcanicslargely comprise a preextensionalstratovolcano that 30 m of arkosicconglomerate locally cappedby thin -19-20 hasbeen tilted 90ø and is thereforeexposed in crosssection 15,254 FAULDSET AL.: EVOLVINGMAGMATIC SYSTEM, ARIZONA

JF9244 whole rock Pref. Age = 11.35-+--O.1Ma •,12.00+ C •ll.60:•600o 7.0•_.0__..•o850o 1000ø JF9245 whole rock Pref. Age = 12.78_.+0.02Ma /•e II20• 500ø • •.07•4•ri 13.00 ß. ß • ß • ß•! ! • M• • lO.SO •soA(72•)•11.43•.0•

10.•l• ', : : : : : I I • 0.0 0.2 0.4 0.6 0.8 .0

1140•- IsoA(84•)=12.72:k0.04(mswd=0.92)

11.•:: : : : 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative Fraction 39Ar Released • 13.• 550ø JF9234 whole rock Pref. Age -- 14. 79_+0.03Ma 16.00 • ' ' .....• • • IsoA(•)•14.53•.16 (•wd•.70)I

. 14.80- o 1150 ø 13.60- / / / / / / • 0.0•ative 0.2 0.4F•ction 0.639At 0.8Re• 1.0 12.40- WMPA (62%) = 14.792k0.03 •&79 •922S •iotite IsoA = 14.872k0.05 //// • 11.20• TFA = 14.58 Ma

lO.OO ,i , , ti it ti I I I o.o 0.2 0.4 0.6 0.8 1. 14.97Ma CumulativeFraction39Ar Released (O) JF9235 Sanidine 15.50Pref. Age = 14.97:ff1.02Ma • • lsoA(•) :1•.•.16 (•w•.14) 15.102• 950 ø 1040ø 1080 ø 1140ø

14.70'•00ø 1000 •m•ve F•n 39•F Rel• 14.30' ' WMPA (54•) = 14.952k0.02 ' ' IsoA (all) = 14.982k0.03(MSWD=5.5) (if) JF9•2 wholerock 13.90--.. TFA = 14.97Ma 15.21Ma 18.4• Mo Pref.Age = 19.9_+.-0.5 Ma 0.0 0.2 0.4 0.6 0.8 1.0 19.9Mo 22.01 800ø I CumulativeFraction39ArReleased / , JF9237 Sanidine Pref. Age = 15.212•'0.02Ma 16'00.•700ø •,. •T•.•o IsoA (8-1• ø) = 18.0•0.2 15.60'•d II • •'•"• •A =18.88 Ma 12.o/ I I I I • I I I I lO4OO 0.0 02 0.4 0.6 0.8 1.0 15.20..I.• 1080ø .... I Creative F•n 39At Re!• 14.80']-WMPA (90ok)= 15.212L'0.02 I JF9161 biotite ß ,,,T IsoA(all) = 15.182L-0.02(mswd=0.99)] Pref. Age = 15.962•'0.06Ma ' • TFA=I5.22Ma I 1Aft/ I I I I I i i i i I ,_,17'04...... , .-v.• i i i i i i I i i i i 0.0 0.2 0.4 0.6 0.8 1.0 oolO3OO ,1ool Cumulative Fraction 39Ar Released • 13.0+ 820ø I • +•0ø •A •)= 1•9•.• I • 11.0TH IsoA(82•) =16• •.2 I • T• •A= 15.14 Ma I 7.o• ', ', I I I • • I I I (• 0.0 0.2 0.4 0.6 0.8 1.0 1•.96Ma ••e F•n 39ArRe•

Figure40 39 3. Compositestratigraphic column.... of theMount Perkins region showing approximate tilts. The Ar/ Ar agespectra are shown for critical umts. Letters in parenthesesrepresent sample locations shown in Figure2 and Table 1. Weightedmean plateau ages (WMPA) were calculated forshaded parts of the spectra. Temperaturesarein øC. TFA, total fusion age; IsoA, Isochron age. Data quality are assessed inthe appendix. FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA 15,255

Table 2. StratigraphicColumn, Mount PerkinsBlock

Extrusive Intrusive

Synextensional(15.7-11.3 Ma) Basalt flows, isolated (-11.3 Ma) Basin-fillingfanglomerates Mount Davis Volcanics (15.2-12.8 Ma) Basalt and basaltic andesite flows Tuff of BridgeSpring (15.2 Ma) Upper Member, PatsyMine Volcanics Basaltic andesite flows (15.3 Ma) , Bulk of Red Gap Mine volcanics Mount Perkinsrhyolite dike swarm Rhyolite flows and domes NNW striking,east dipping Nonweldedrhyolite tuffs Emplacedat steepdips Quartzphenocrysts Quartz phenocrysts 15.7-14.5 Ma 14.5-15.7 Ma

Preextensional (-20-15.7 Ma) LowermostRed Gap Mine volcanics Rhyolite dikes (- 15.7 Ma) Rhyolite flows and tuffs Quartzphenocrysts Quartz phenocrysts ENE striking,steeply dipping ~16.0-15.7 Ma Dixie Queen Mine yolcanics Mount Perkinspluton: Laccolith and stock Upper trachydaciteflows and domes 15.96 Ma granite-granodiorite Lower trachyandesiteflows Older diorite-gabbro PeachSprings Tuff (18.5 Ma) Basalt flows (19-20 Ma) Arkosic conglomerate -20 Ma nonconformity Early Proterozoicorthogneiss

Dashed lines denote unconformities; solid lines are conformable contacts.

(Figure 2). Just to the north of the Dixie Queen Mine, lava poratesa large growth-faultbasin within which the synexten- flows within the Dixie Queen Mine volcanicsgive way to a sionalvolcanic rocks accumulated. In ascendingorder, the massivebody of trachydacitethat exceeds2 km in thickness. sectionwithin the basinincludes (1) the middleand upper The massivetrachydacite cuts a 5-km-long segmentof the partsof the Red Gap Mine volcanics,which consistof 300- early Miocene nonconformityand is thus interpretedas a 1500+ m of quartz-phenocryst-bearingrhyolite flows, non- tilted trachydacitedome complex. The trachydacitedome welded tuffs, surge deposits,and tuffaceoussedimentary appears to merge eastward in the vicinity of the rocksbracketed between 15.8 and 15.2 Ma in the Red Gap nonconformitywith isolatedexposures of altereddiorite and Mine areaand 15.8 and 14.5 Ma nearthe northernmargin of granite, which are cut by youngerdikes. The diorite and the basin (Figure 2 and Table 1), (2) as much as 100 m of granitein this area are interpretedas the upperpart of a tilted basalticandesite lava flows that correlatewith the Upper stock. Strikes of steeplytilted stratawithin the trachydacite Member of the Patsy Mine Volcanics of Anderson [1971], and trachyandesiteunits swing from northwest to north- (3) the 15.2 Ma Tuff of Bridge Spring [Anderson,1971; northeastfrom south to north aroundthe trachydacitedome Smith et al., 1993], which was probably erupted from a complex (Figure 2). These relations suggest that the caldera in the northernEldorado Mountains [Gans et al., trachydacite dome complex and trachydacite and 1994] and thuscorresponds to an outflow sheetin the Mount trachyandesiteflows correspondto the central core and Perkins area, (4) as much as 1 km of basalt and basaltic flanks, respectively,of a stratovolcanothat has been tilted andesite flows, which correlate with the Mount Davis ~90 ø. The discordance in the strike of strata north and south Volcanicsof Anderson[1971 ] and rangefrom 15.2 to 14.3 of the dacitedome complex probably reflects primary dips off Ma in the Mount Perkinsblock and 15.2 to 12.8 Ma just to the flanks of the stratovolcano(Figure 2). The centralpart of the west and north of Mount Davis, and (5) middle to late the inferred stratovolcano lies west-southwest of the Mount Miocenebasin-filling fanglomerates (Figures 2 and 3). The Perkinspluton and thus originally rested above the pluton. transitionfrom felsic to mafic magmatismwithin the Mount Perkins block occurred rapidly, as demonstratedby conformablerelations and relatively little interfingering SynextensionalMagmatism between the Red Gap Mine and Mount Davis volcanic sequences(Figures 2b and 3). Tilts within the Mount Perkins Within the Mount Perkinsblock, voluminousoutpourings and nearby structuralblocks fan from 90 to 55ø betweenthe of felsic volcanic rock coincided with the onset of extension lowerpart of theRed GapMine volcanicsand Tuff of Bridge -15.7 Ma but gave way to widespread mafic volcanism Spring,55 to 15ø in the Mount Davis Volcanics,and 15 to 0ø essentiallyduring peak extension14.5-15.2 Ma (Figure 3 and in the fanglomeratesand isolatedexposures of flat-lying, Table 2). The westernpart of the Mount Perkinsblock incor- basalticlava flows (Figure2b). 15,256 FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA

Synextensional rhyolitic magmatism also included complex, whereas the predominantnonwelded tuffs to the emplacement of the Mount Perkins dike swarm, a massive20- north and south accumulatedon distal apronsof the rhyolite km-long, felsic dike swarm within the central part of the domes. Rhyolite domeshave not been observedin the Red Mount Perkins block (Figure 2). The east dipping, northerly Gap Mine area but are presentin the hanging wall of the striking dike swarm invades the Proterozoic gneissesand Mockingbird Mine fault (Figures 1 and 2), where massive lower part of the Miocene section. It is essentiallya sheeted bodies of rhyolite with vitrophyricrinds are surroundedby dike complex that rangesup to 1 km in thickness. Directly thick accumulationsof interbeddednonwelded tuffs, surge eastof the Red Gap Mine area,nearly the entire west flank of deposits,and tuffaceous sedimentary rocks. Mount Perkins consists of felsic dikes. Individual dikes are characterizedby widths of less than 10 m and high aspect Geochemistry ratios (length/width > 100). Some dikes extend laterally for several kilometers. The thickest part of the dike swarm lies The major element,trace element,and isotopiccomposi- betweenthe Mount Perkins pluton and dacite dome complex tionsof Miocene igneousrocks from the Mount Perkinsblock (Figure 2). The dikes are generallylow to high silica rhyolites have been determined to investigate their petrogenesis and contain quartz phenocrysts,but a few hornblendedacite [Feuerbach et al., 1994; Metcalf et al., 1995]. Here these dikes are also present. Some dikes contain inclusionsof compositionsare used to evaluate the genetic relationship granodioritethat resemblethe granodioriteand granitephases between volcanic and intrusive rocks within the Mount of the Mount Perkins pluton. Widely spaced northerly Perkins block and the associationbetween the preextensional striking,east dipping rhyolite dikesthat cut the plutonmay be intermediateand preextensionalto synextensionalrhyolitic cogenetic with the larger dike swarm. Similar spatial magmatism.Geochemical data of representativesamples are distributionsand compositionsimply a geneticlink between shownin Table 3. Analytical methodsare discussedin the the synextensionalnortherly striking dike swarm and the appendix. preextensionalset of eastto northeaststriking dikes. Much of the Dixie Queen Mine volcanics and some dike A significant range in dip magnitudes, crosscutting rocks have been altered to varying degreesby hydrothermal relations, and paleomagneticdata indicate a synextensional activity. The alteration most commonly involves sericitiza- origin for the Mount Perkinsdike swarm. The dikesdip 0 to tion of plagioclaseand conversionof mafic constituentsto 45 ø eastward and thus crosscut foliation in the Proterozoic Fe-oxidesand sheetsilicates. Minor sulfidesare also locally gneisses. Many of the more gently dipping, older dikes are developed. Samples analyzed from the Dixie Queen Mine cut by moderatelydipping, younger dikes. Paleomagnetic volcanicscontained no sulfidesand only minor alterationof data suggestat least 45ø of west-tiltingof the dike swarm silicate minerals. The Red Gap Mine volcanicshave experi- [Faulds et al., 1992]. One moderatelydipping dike yieldeda enced little or no alteration. Geochemicalplots of all ele- K/Ar age of 14.5 _+0.30 Ma on K-spar (Table 1). The dike ments show coherenttrends, suggestinglittle metasomatism swarmterminates upsection in the lower part of the 14.5-15.8 and essentiallyprimary compositions.Nonetheless, to avoid Ma Red Gap Mine volcanicsand is thus roughly bracketed potential problems associated with alteration, relatively between 15.8 and 14.5 Ma. These relationssuggest that most immobile rare earth and high field strengthelements, such as of the dikes were emplaced at approximately vertical Hf and Ta, were employedto assessthe proposedcorrelations orientationsduring extensionand were progressivelyrotated wheneverpossible. to easterlydips by west tilting of the Mount Perkinsblock. The lava flows and intrusive rocks within the Mount The rangein dip magnitudesis complementaryto the fanning Perkins block can be divided into intermediate, felsic, and of tilts within the Red Gap Mine volcanics. mafic groupson the basis of SiO2 compositions(Figure 4). Field relations indicate a genetic tie between the Mount The intermediategroup includes the preextensionaltrachy- Perkins dike swarm and Red Gap Mine volcanics. For andesiteflows (56.4% wt % SiO2) and trachydaciteflows and example, the rhyolite dike swarm terminatesupward in the domes(61.8-66.3 wt % SiO2) of the 18.5-16 Ma Dixie Queen lower part of the Red Gap Mine volcanics. In addition, Mine volcanicsand quartz diorite to hornblendegranodiorite along-striketerminations of the dike swarm roughly coincide phase (55.8-67.6 wt % SiO2) of the Mount Perkins pluton. with appreciablethinning or pinchouts of the Red Gap Mine The preextensional15.96 Ma biotite granodioriteto granite volcanics (Figure 2). Abundant and thick rhyolite flows phase (70.1-74.2 wt % SiO2) of the Mount Perkins pluton, furtherimply a nearbysource for the felsic volcanicsequence 14.5-15.8 Ma preextensionalto synextensionalRed Gap Mine in the Red Gap Mine area, which lies directly west of the volcanics (72.0-77.4 wt % SiO2), and synextensionalMount thickestpart of the Mount Perkinsdike swarm. To the north Perkins dike swarm (73.01-77.4 wt % SiO2) constitutethe and south,rhyolite flows give way to nonweldedtuffs and felsic group. The mafic group incorporatesthe gabbroand tuffaceoussedimentary rocks. The commonquartz-bearing mafic enclavesfrom the Mount Perkins pluton [Metcalf et al., compositionof the Mount Perkinsdike swarm and rhyolite 1995], an older (> 16 Ma) dioritic phase(51.6 wt % SiO2) of flows in the Red Gap Mine volcanicsis alsorelatively distinct the Mount Perkinspluton, -15.3 Ma synextensionalbasaltic within the region, as other rhyolite sequencesof similar age andesiteflows of the Upper Member of the Patsy Mine are generallyfree of quartzphenocrysts [e.g., Anderson, 1977, Volcanics, and 14.3-15.2 Ma basalt flows of the Mount Davis 1978]. Volcanics. Sampling efforts for this study focused on the The physicaland temporalrelations strongly suggest that intermediateand felsic groups,because these compositions the Mount Perkinsblock exposes an obliquecross section of a bracketthe onsetof major regionalextension and makeup the synextensionalrhyolite dome complex. Prior to tilting, the extrusive and intrusive parts of volcanic centers at Mount Red Gap Mine volcanicsrested directly above the Mount Perkins. Perkinsdike swarm. The rhyolitelava flows in the Red Gap Striking similaritiesin the abundancesand trendsof rare Mine area were probablydeposited proximal to the dome earthand high field strengthelements and Nd and Sr isotopic FAULDS ET AL.' EVOLVING MAGMATIC SYSTEM, ARIZONA 15,257

z• 15,258 FAULDSET AL.' EVOLVINGMAGMATIC SYSTEM,ARIZONA

12 rhyolite flows [] rhyolite dikes o• 10 trachyandesit•trachydaciterhyolite granite (F)

granite (M)

trachydacite

%6 granodiorite (F) / •. f group group granodiorite (M)

trachyandesite X diorite 40 50 60 70 80 SiO2 (wt. %)

Figure4. Na20+ K20versus SiO2 for volcanic and intrusive rocks associated with the Mount Perkins magmaticsystem. The classification scheme is fromLeBas et al. [ 1986].

compositionssuggest that plutonicand volcanic rocks within 500 the intermediateand felsic groups are cogenetic. For exam- ple, relativelyshallow rare earthelement (REE) slopes, '{:J 200 a. depletedmiddle REE, andnegative Eu anomaliescharacterize O loo theextrusive and intrusive rocks of thefelsic group, including the rhyoliteflows and dikes,as well as the granodiorite- O so granitephase of the MountPerkins pluton (Figure 5b). In addition,the rhyolite dikes and flows and granodiorite-granite LU 20 ...... phaseof the MountPerkins pluton have similar Hf/Ta ratios

(2.0to 4.1) andTa concentrations(1.3 to 2.2ppm) (Figure 6). 5 Nearlyidentical Sr andNd isotopiccompositions (Figure 7) of thegranodiorite-granite phase of thepluton [Metcalf et al., 2 1995],rhyolite dikes, and rhyolite flows further suggest that La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er TmYb Lu theyare cogenetic. Within the intermediate group, both the quartzdiorite to granodioritephase [Metcalf et al., 1995]and trachydaciteshave moderateREE abundances,steep REE 1,000 trends,and no Eu anomaly(Figure 5a). Thoughcompositions 500i. of the plutonic rocks are more scatteredthan those of the bl trachydaciteflows and domes,Hf/Ta ratiosof the trachy- I-- 200 dacitesand quartzdiorite to granodioritephase overlap .•O100 "•,,.x (valuesrange from 4 to 8; Figure6a) andare consistently (.• {513 higherthan that of thefelsic group. The trachydacite flows anddomes and higher silica (> 62% SiO2)granodiorite of the LU 20 MountPerkins pluton also have similar Sr andNd isotopic rv' •o compositions(Figure 7). Limiteddata precludea thoroughanalysis of potential 5 correlations between extrusive and intrusive rocks of the mafic group,but do permitsome speculations. The diorite andtrachyandesite samples have similar REE abundancesand La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu patternsand trace element ratios (Figures5a and 6), suggestingthat the older diorite phase of thepluton and some Figure5. (a)REE/chondrite plot for the felsic magma group. of the-16-18.5 Ma trachyandesiteflows in theDixie Queen Heavy solid lines are rhyolitedikes; medium solid linesare Mine volcanicsmay be geneticallyrelated. However,the rhyoliteflows; thin dashed lines are granites from the Mount traceelement compositions of the trachyandesiteand older Perkinspluton. The two dikes plotted represent the least and dioritephase of thepluton differ significantly from those of mostsilicic of the six thatwere analyzed. (b) REE/chondrite plotfor the moremafic and trachydacitic groups from the themafic enclaves and older gabbro reported by Metcalf et al. Mount Perkinsmagmatic system. Heavy solid lines are [1995] (Figure5a). Diversemafic sources may account for granodioritefrom the MountPerkins pluton; medium solid these differences. linesare trachydaciteflows; thin dottedline is a dioritefrom The geochemicaldata do notafford unequivocal evidence theMount Perkins pluton; thin dashed line is a trachyandesite for correlatingvolcanic and plutonicunits within the Mount flow. FAULDSET AL.: EVOLVING MAGMATIC SYSTEM,ARIZONA 15,259

4O 10

3O • 6 • 2O

I 0:5 '5 2.5 0o

140 'I Intermediate + X 120 't trend -'-•-•' I -t Doz, 100 80

60- <>

40 20 Cm 1C•0' 1•0'160 La Figure6. Volcanicand intrusive rocks within the Mount Perkins and nearby magmatic systems. (a)Hf/Ta versusTa. Symbolsare the same as for Figure 4. Thearea enclosed bythe dashed line shows the intermediateand felsic rocks from the River Mountains-Wilson Ridge system [Smith et al., 1990].The area enclosedbythe dotted line shows the granodiorite andgranite from the Aztec Wash pluton [Falkner etal., 1995].(b) Th/Ta versus Ta. Symbolsand line types are the same as in Figure 6a. (c)Nd versus La for volcanicand intrusive rocks within the Mount Perkins magmatic system showing parallel but not colinear trends for the intermediate and felsic magma groups.

Perkinsmagmatic system. These data are, however, The preextensionalsystem at MountPerkins primarily consistentwith the field relations and rule out potential involvedthe production of intermediate group magmas of the correlationswith mostother magmatic systems in the region. MountPerkins pluton and Dixie Queen Mine volcanics and Forexample, compared to the intermediate to felsic groups of constructionof a stratovolcano,as manifested in the the 14.5-16.0 Ma Mount Perkinssystem, the 15.7 Ma Aztec trachydacitedomes and trachydacite and trachyandesite flows Washpluton in thenearby Eldorado Mountains [Falkner et al., 1995]has similar REE patterns and isotopic compositions but noticeabledifferences in Hf, Th, andTa concentrations (Figures6aand 6b). In contrast,subtle differences in the Sr andNd isotopiccompositions distinguish the 14.5-16.0 Ma MountPerkins system from the younger 13.0-13.5 Ma Wilson Ridge-RiverMountains system [Weber and Smith, 1987; Larsenand Smith, 1990; Feuerbach et al., 1993](Figure 7a), z whichlies well to thenorth of MountPerkins (Figure 1).

Discussion

-1o Tilted Volcanoes Thefield, geochronologic, and geochemical data together demonstratethat the Mount Perkins block exposes a cross 0.702 0.704 0.706 0.708 o.71o o.712 o.714 sectionof a preextensionalto synextensionalmagmatic system,including volcanic edifices, hypabyssal dike swarms, andpart of theplutonic root. Themost compelling evidence Figure7. Nd andSr isotopic compositions forvolcanic and in supportof thishypothesis is the relative proximity of the intrusiverocks of the Mount Perkinsand nearbymagmatic Red Gap Mine volcanics,Mount Perkinsdike swarm, systems.Symbols are the same as for Figure4. Thearea trachydacitedomes and flows of the Dixie QueenMine enclosedby the dashedline representstrachydacitic and volcanics,and Mount Perkins pluton. For example, restoring rhyoliticrocks from the River Mountains and a granodiorite the Mount Perkinsblock to paleohorizontalplaces the Red fromthe Wilson Ridge pluton [Feuerbach et al., 1993].The GapMine volcanics and trachydacite dome complex above areaenclosed by thedotted line shows plutonic rocks from theMount Perkins dike swarmand pluton, respectively. theAztec Wash pluton [Falkner et al., 1995]. 15,260 FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA of the Dixie Queen Mine volcanics. The current level of Gap Mine volcanics. It blossomedduring and immediately erosion intersects the upper and hypabyssallevels of the after the onset of extension (15.7 to 14.5 Ma) with the stratovolcano. Although the geochemical data indicate a emplacementof the massivenortherly striking, Mount Perkins genetic link between the trachydacitedomes and flows and dike swarm and depositionof the bulk of the Red Gap Mine quartz diorite to hornblendegranodiorite phases of the Mount volcanics. The rhyolitic magmatismat Mount Perkins shut Perkins pluton (Figures. 5b, 6, and 7), the pluton and down, however, by -14.5 Ma, essentially during peak trachydacite dome complex are not physically linked at extension. exposed structural levels (Figure 2). Becausethe steeply Emplacementmodes of magmaswithin the Mount Perkins dipping dikes that comprisethe Mount Perkinspluton restore magmatic system changed during the onset of regional to a subhorizontalorientation, we conclude that the pluton extension. Prior to extension,intrusions were emplaced as was emplacedas a laccolithlike body, which emanatedfrom a subhorizontaldikes with relatively low aspectratios, east to poorly exposed stock situated beneath the core of the northeaststriking steeplydipping dikes, and stocks. Someof stratovolcano(Figures 2b and 8). the subhorizontal intrusions were probably laccolithlike The late preextensionalsystem at Mount Perkinsproduced bodiesassociated with stocks. Similar preextensionaleast to the granodiorite-granitephase of the Mount Perkinspluton, northeaststriking dikes, subhorizontalintrusions, and stocks eastto northeaststriking rhyolite dikes, and lowermostpart of have been observedin other parts of the northernColorado the Red Gap Mine volcanics. The granodiorite-granitephases River extensional corridor, including the Oatman mining of the pluton also restore to a subhorizontalorientation. district in the southern Black Mountains [Thorson, 1971; Preextensionaleast-northeast striking basaltic andesitedikes DeWitt et al., 1986], northern Eldorado Mountains feed sills just to the north of the Mount Perkins block in the [Anderson,1971], and central Black Mountains directly north central Black Mountains [Faulds, 1993a]. Similar relations of the Mount Perkins block [Faulds, 1993a]. Best and may characterizethe preextensionalrhyolitic systemat Mount Christiansen [1991] suggestedthat within the Great Basin, Perkinsbut cannotbe verified basedon currentexposures. large volumes of magma were emplaced as subhorizontal Synextensionalmagmatism generated a largerhyolite dome sheetsduring peak Tertiary volcanism,which coincidedwith complex at Mount Perkins, which incorporatesthe bulk of the a period of little regional extension. A large, present-day Red Gap Mine volcanicsand northerlystriking Mount Perkins subhorizontalintrusion may also underlie part of the Rio dike swarm. The rhyolite dome complex developeddirectly Grande rift [Rinehart et al., 1979]. In contrast, on top of the preextensionalstratovolcano. Physical and synextensionalintrusions at Mount Perkins and elsewherein geochemicalrelations strongly indicate a geneticlink between the extensionalcorridor [e.g., Nakata, 1982; Anderson,1993; the Mount Perkins dike swarm and Red Gap Mine volcanics. Falkner et al., 1995] are characterizedby large north- The present erosional surface intersectsthe rhyolite dome northweststriking sheeteddike complexeswith high aspect complex at hypabyssallevels within the rhyolite dike swarm. ratios. Rhyolite domeshave been largely erodedfrom the west flank The changes in emplacement modes at the onset of of Mount Perkinsbut are preservedin the hangingwall of the extensionreflect a major reorganizationof the regionalstress Mockingbird Mine fault. Normal displacement on the field. The preextensional intrusions indicate a nearly Mockingbird Mine fault essentiallydecapitated the rhyolite isotropic regional stress field that may have experienced dome complexand translatedthe upperpart of the complex5- periodic permutations. The subhorizontalintrusions were 6 km to the east of its hypabyssalroots. Such relations probably emplaced during periods when the least principal exemplify how tilting and the subsequenterosion of the stress had a subvertical orientation, whereas east to northeast upthrown parts of fault blocks, as well as structural striking dikes probably reflect a subhorizontal,north to dismembermentby normal faulting, can significantlyobscure northwest trending orientation of the least principal stress. major volcanic centers. The latterorientation may imply small amountsof north-south The Inyo and Mono domes of east central California, extensionin the Mount Perkins region prior to -15.7 Ma. which lie within the actively extendingOwens Valley region Synvolcanicnorth-south extension has also been noted in the [Hill et al., 1985], may represent modem-day, untilted northern Basin and Range province [Best, 1988; Bartley, analogsto the Miocene magmatic systemat Mount Perkins. 1990]. In the Mount Perkins region, however, north-south The dome-upon-domestructure of the Mono domesis similar extensionceased at-15.7 Ma, significantlybefore volcanism to that of the trachydaciticand rhyolitic sequencesat Mount terminated at -11 Ma. Although difficult to quantify based Perkins,although the volumeof the basaldacitic lavas in the on available data, the volume of subhorizontal intrusions in Mono domes [Kelleher and Cameron, 1990] is considerably the northern part of the extensionalcorridor appearsto be less than at Mount Perkins. In addition, both the Mono roughly comparableto that of the east to northeaststriking domes [Bursik and Sieh, 1989] and 11-km-long Inyo volcanic dikes, which suggeststhat vertical inflation of the crustmay chain [Eichelberger et al., 1985, 1988; Fink, 1985; Mastin have been as great as north-south extension. Some and Pollard, 1988; Suemnichtand Varga, 1988] were fed by permutationsin the regional stress field may have been dikessimilar in lengthto thosewithin the Mount Perkinsdike magmaticallyinduced, as emplacementof dikes can locally swarm. overwhelm the regional stressfield leading to shifts in the least principal stressdirection [e.g., Parsonsand Thompson, Interplay Between Magrnatisrn and Extension 1991]. Significantpermutations in the regionalstress field of The rhyoliticmagmatism at Mount Perkinsbegan immedi- probable magmatic origin have been documented for atelyprior to the onsetof regionaleast-west extension, as evi- relatively brief time intervals elsewherein the Basin and dencedby the widely spacedeast to northeaststriking rhyolite Range province [Minor, 1995]. It is noteworthythat even dikes and concordanttilts in the lowermostpart of the Red duringthe inferred period of little differentialstress, magmas FAULDSET AL.' EVOLVINGMAGMATIC SYSTEM, ARIZONA 15,261

PREEXTENSIONAL SYNEXTENSIONAL Red Gap Mine Volcanlcs Volcanlcs

(c)

West East ~ 16.0 Ma ~15.5 Ma

RedGap Mine LateRange-Front Fault (b) nlcs

ENEStrikJn• Proson RhyoliteDike

•. •erl½•---•

ßß .. •

~14.0 Ma ...... ~15.8 Ma Figure8. Schematicmodel of the evolving magmatic system atMount Perkins. (a) Preextensional system at --16.0Ma; regionalstress field is nearlyisotropic. A trachydaciticstratovolcano is situated above a granodioritestock and attendant laccoliths. Magmas are mostly trapped assubhorizontal intrusions, which generatesignificant crustal heating and melting and prevent basaltic melts from reaching the surface. (b)Immediately prior to the onset of extension at•-15.8 Ma; regional stress field is nearly isotropic. Rhyolitic magmatismsupersedes trachydacitic magmatism andproduces east to northeast striking dikes and domes, as the level of crustalmelting shallows and/or the contributionof uppercrustal melt increases. (c)Synextensional system--15.5 Ma; regional stress field is decidedly anisotropic withan east-west trending, subhorizontalleast principal stress. Rhyolitic magmatism continues with the emplacement of a large, northerlystriking dike swarm and associated rhyolite domes, which are largely superimposed onthe older stratovolcano.Abundant dilational fractures and normal faults allow highly viscous rhyolitic melts to reach thesurface. Crustal heating and melting slows as residence time of meltswithin the crust decreases. Silicic magmachambers are eventually evacuated or quenched leading to anabrupt termination in rhyolitic magmatismduring peak extension (•-14.5 Ma). (d) Synextensional maficmagmatism •-14.0 Ma; stress field is anisotropic.Mafic magmas derived largely from the mantle lithosphere haveready access tothe surface. The thermalanomaly beneath Mount Perkins has died out. were not emplacedalong preexistinganisotropies (e.g., bodiesare compatiblewith establishedemplacement modes foliationin Proterozoicgneisses), at leastnot at exposed for extensionalsettings [e.g., Paterson and Fowler, 1993]. structurallevels. Oncemajor east-west extension began at Althoughthe relativetiming of magmatismand extension 15.7 Ma, magmaswere emplaced as subverticalnortherly at Mount Perkinsis compatiblewith extensionbeing driven striking,sheeted-dike complexes oriented perpendicular tothe by uplift, doming,and subsequentgravity gliding above easterlytrending least principal stress. Such large sheetlike shallowly emplaced plutons [e.g., Rowleyet al., 1989], 15,262 FAULDSET AL.:EVOLVING MAGMATIC SYSTEM, ARIZONA

regionalrelations suggest that such a processdid not play a generatedat an increasinglyrapid pace just priorto being majorrole. For example,the MountPerkins block lies within completelyexpended from the magmatic system. the30,000 km 2 ColoradoRiver extensional corridor and is A modelsimilar to thatproposed by Bestand Christiansen partof a 5000km 2 regionthat underwent large magnitude [1991] may be applicableto the MountPerkins magmatic extensionbetween 15.7 and 11.3 Ma. It is notan anomalous, system. Prior to extension,large volumes of maficmagma highlyextended block, as might be expected in a gravityglide derivedfrom the mantle lithosphere may have been trapped as model. Moreover,large magnitudeextension within the subhorizontalsheets within the crust,which promoted corridoris notconfined to individualmagmatic centers nor to crystallizationof the basalt, as well as extensive and rapid the east-westtrending magmatic belts of Smithand Faulds crustalmelting [Huppert and Sparks, 1988]. Themagma [1994]. Someof the morehighly extended parts of the probablyponded at levels within the crust where it was corridorhave actually experienced relatively little magmatism neutrally buoyant. Early in the evolutionof the Mount [e.g., Fryxell et al., 1992], whereassome of the leastextended Perkinssystem, the mafic magma ponded at relativelydeep regions,in termsof mechanicalextension, tend to be loci for levelsbecause of thefelsic nature of thecrust [Glazner and magmatism[Faulds et al., 1994b].These relations suggest Ussler, 1989]. Mixing of the crustal melts with the that mostof the extensionat Mount Perkinswas not induced differentiatingbasaltic magmas then produced low-silica end by localuplift and gravitygliding above shallow intrusions membersof theintermediate group [see Gans et al., 1989]. butwas instead part of a regionalevent influenced primarily Subsequentbatches of maficmagma were blocked by the by theconfiguration of theregional stress field. developingsubhorizontal intrusions of intermediate magma, The changein the stressfield at the onset of extension whichled to furtherheating and melting of thecrust as well occurredafter magma compositions had changed from domi- as crystallizationof the intermediatemagmas. Further nantlytrachydacitic to rhyolitic. The onsetof tilt fanning inmixingof crustalmelt and crystallization of the more mafic withinthe Red GapMine volcanicsdemonstrates that rhy- meltsproduced the more silicic, intermediate group magmas oliteswere erupted before, during, and afterthe onsetof ex- (e.g., trachydacites).The lower densityof the silicic tension.Thus, although the spatial and temporal relationships intermediate magmas promoted their ascent to relatively betweenthe intermediate and felsic groups indicate that both shallow crustal levels. The ascentof the intermediate groupsare manifestationsof the samethermal anomaly and magmasmay have removeddensity barriers to basaltsand possiblyutilized the samemagma chamber, the evolutionof thuspermitted their rise to shallowerlevels. Subsequent the magmaticsystem to progressivelymore silicic composi- pondingof the basaltsat shallowercrustal levels and the tionswas at leastpartly independent of thestyle of deforma- resultantheating and crustal melting may have generated the tion in the uppercrust. felsicgroup of magmas.The preextensionalMount Perkins Manyof the trendsobserved between REE andhigh field systemhad probably evolved to thispoint, as evidenced by strengthelements are parallel and roughly colinear (Figure 6), thecopious amounts of preextensionalintermediate magmas whichindicates that the intermediateand felsic groups andlesser rhyolites. evolvedfrom similar sourcesand by similarprocesses. Althoughthe Mount Perkins system may have eventually However,the significantdepletion in Nd and enrichmentin evolvedinto a largerhyolitic center without major extension, La of the leastevolved rhyolite with respectto the most full-blownregional extension facilitated the ventingof the evolvedtrachydacite (Figure 6c) cannotbe explainedby any highly viscousrhyolitic magmas by creatingabundant reasonableclosed or open systemfractionation model. Thus dilationalfractures and faults, which served as channelways thetwo groups probably are not linked by fractionation along forintrusion. Once major extension began and the rhyolitic oneliquid line of descent.The parentalmagmas for the two magmaswere evacuated,mafic magmaswere no longer groupseither were generatedfrom slightlydifferent sources trapped as subhorizontalsheets within the crust and thus or werefractionated along slightly different paths at depth. cmstalmelting was minimized.This mayexplain why Thehigher Sr isotopiccompositions of the felsic group rela- rhyoliticmagmatism atMount Perkins ended abruptly during tive to theintermediate group (Figure 7) suggestthe former, peakextension and gaveway to maficmagmatism. The which may indicatethat continentalcrust was more involved quantityof rhyoliticmagma available to the systemwas in the genesisof the felsicgroup. Major andtrace element probablyfinite so long as majorextension continued and variationswithin both groups suggest that they differentiated mafic magmas had readyaccess to the surfacethrough largelyby crystalfractionation of theobserved phases includ- abundantchannelways. Major extensionserved to flushout ing suchaccessory minerals as sphene,apatite, zircon, and preexistingcrustal melts and hybrid magmas by generating chevkinite[Feuerbach et al., 1994]. However,the within- abundantpathways to the surface.Any remainingcrustal groupvariations in Sr andNd isotopiccompositions indicate meltswere probably largely quenched through rapid cooling thatassimilation of continentalcrust or inmixingof meltsof inducedby tectonicdenudation. Thick sequences of 15.2- continental crust also contributedto the differentiation. 12.8Ma basaltflows within the synextensional Mount Davis The high-Srand low-Nd isotopiccompositions of the volcanics(Figures 2 and 3), with little if any intercalated intermediateand felsic magmas suggest that their genesis rhyolite,attest both to thepresence of largevolumes of mafic involveda significantcomponent of uppercrust. Further- magmaand easeof accessof thesemagmas to the surface. more,the radiogenic Sr isotopiccompositions are greatest in Maficmagmatism in theMount Perkins region ended -11 Ma thefelsic group, which indicates a correspondingincrease in coincidental with the cessationof extension. thecontribution of uppercrustal melt. Theeruption rate of Extensionand magmatismmay have also interactedin rhyoliticmagmas increased significantly immediately after the moresubtle ways at Mount Perkins. For example, injection of onsetof extensionbefore diminishing abruptly at peakexten- the dike swarmand increased pore pressure associated with sionwhen mafic magmas began erupting. Thus as extension the magmaticsystem may have facilitated slip alongthe accelerated, upper crustal melts were evacuated and/or Mockingbird Mine fault and controlled the location of the FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA 15,263 maximum displacement. Alternatively, greater tectonic Modifications Limited double vacuum-typeresistance furnace denudation and associatedpressure reduction may have for 10 min on each step. Extracted gas was purified with focusedventing of the synextensionalrhyolitic systemwithin SAES-typeST-172 (Zr-V-Fe) gettersand then analyzedon a the footwall of the Mockingbird Mine fault in the area of Mass Analyzer Products Limited 216 mass spectrometer maximum displacement. It is noteworthy that the Mount equipped with a Bauer Signer source and a Johnston• Perkins structuralblock terminatesor loses definition along- Multiplier. At the typical operating gain, the mass strike to the north and south only a short distancefrom the spectrometerhas sensitivity of--2 x 10-•4 mol/V. Typical lateral terminationsof the magmaticsystem. systemblanks are 2 x 10-•6 mol 4øAr and 1.5 x10 '•8 mol 3tAr. Initiation of magmatism clearly preceded the onset of All sampleswere irradiated in the U.S. Geological Survey extension within the northern Colorado River extensional Triga reactor. The neutronflux was monitoredusing sanidine corridor. Magmatism may have indeed promoted extension from Taylor CreekRhyolite (85G003) with an acceptedage of through thermal weakening of the crust, increased pore 27.92 Ma [Dalrymple and Duffield, 1988]. Reactorconstants pressure,and dike emplacementat depth [e.g., Gans et al., for the productionof interferingisotopes were not measured 1989; Meyer and Foland, 1991]. In addition, thickening of directlybut were assignedthe nominalvalues determined by the crust through the inflationary effects of subhorizontal Dalrymple et al. [1981]. Analytical data were reducedto intrusionsmay havebeen significantenough to partiallydrive producespectra and isochronplots using the EyeSoreChron extension[e.g., Hill et al., 1992]. Quantificationof these programdeveloped by B. Hackerat StanfordUniversity. processesis difficult, however,even at Mount Perkins,where Stepheating experiments (10-16 steps)were carriedout on the upper several kilometers of an individual magmatic. all samplesto obtain higher precisionages and to evaluate systemhave been exposed in crosssection. whetherthe isotopicsystematics of the sampleshave been The evolutionof the Mount Perkinsmagmatic system does affected by either alterationor xenocrysticcontamination. illustrate that the onset of major regional extension and Nearly all samplesyielded easily interpretable ages. All ages accompanyingdevelopment of large differential stressescan are in agreementwith their known stratigraphicposition. have a demonstrableimpact on the emplacementmodes of Most samplesyielded simple flat releasespectra with a well- magmaswithin the upper crustby rapidly inducinginjection defined plateau over a significant part of the spectrum. into large, subvertical sheeted dike complexes oriented However, severalof the "whole rock" samplesyielded hump- perpendicularto the least principalstress. This may, in turn, shaped,S-shaped, or more complicatedspectra and poorly facilitate "crustal flushing" and "quenching" of existing defined isochronsthat are more difficult to interpret. It has magmatic systems. It also appears that preextensional been determinedempirically that the ages correspondingto magmaticsystems may partly controlthe segmentationof the the tops of the humpsor the middle, flattishpart of the S- upper crustduring extension,particularly its partitioninginto shapedspectra from whole rock samplesare the bestmeasure large structuralblocks. of the eruptive age [Nauert and Gans, 1994; P. Gans, unpublisheddata, 1994]. Assignmentof uncertaintiesto Appendix: Analytical Techniques these more disturbed spectra is rather subjective, as contiguoussteps are not strictlywithin analytical error of each The4øAr/39Ar Geochronology other. In any case,our estimatedanalytical uncertainties on Mineral separationswere carried out utilizing standard mostsamples is lessthan 0.05 Ma andin the worstcases does techniquesat the University of Iowa under the supervisionof not exceed --0.5 Ma. J. Faulds. Wherever possible,high-K mineral phaseswere extracted. However, basalt and basaltic andesite flows that Geochemical Analyses lack a K-bearing phenocrystphase constitute a large part of the stratigraphic section in the Mount Perkins region. Major and trace element analysesfor 30 sampleswere Groundmass concentrates were therefore extracted from the carried out at the GeoAnalytical Laboratory at Washington interior of severalholocrystalline basalt and basalticandesite State University using automatedX-ray spectrometryand flows. Such "whole rock" sampleshave been shownto yield inductively coupled plasma source mass spectrometer. excellentresults in many cases[Nauert and Gans, 1994]. Proceduresare describedby Hooper et al. [1993] and Knaack All 4øAr/39Aranalyses were performed at theLaboratory et al. [ 1995]. for Argon Geochronologyat the University of California, Sevensamples were analyzedfor isotopiccompositions at Santa Barbara, under the supervision of P. Gans. The theUniversity of Kansas.Samples were dissolved at --180øC 4øAr/39Ardata for individualsamples are availableas an in a sealedbomb using an HF/HNO3 mixture. Sampleswere electronicsupplement through AGU (Table A1 •). Irradiatedtotal spikesfor Rb, Sr, Nd, andSm. Separationof Rb, St, and samples weighing 2 to 15 mg were step heated in a REE group elementswas done using standardcation ex- changetechniques. The HDEHP-on-Teflonmethod of White nanelectronic supplement of this material may be obtained on a and Patcherr[1984] was usedfor separationof Sm and Nd. disketteor AnonymousFTP from KOSMOS.AGU.ORG. (LOGIN to The Hbr and HNO3 methodswere used for separationof Pb AGU's FTP account using ANONYMOUS as the usemameand and U, respectively,on aliquotsfrom the wholerock solution. GUEST as the password. Go to the right directoryby typing CD APPEND. Type LS to seewhat files are available. Type GET and All isotopic analyseswere done on a VG Sector 54 mass the nameof the file to get it. Finally, type EXIT to leavethe system.) spectrometerat the Universityof Kansas.Analysis of Sr and (Paper number95JB01375, The Mount Perkinsblock, northwestern Nd wasdone in dynamicmulticollector mode with 88Sr=4V Arizona: An exposedcross section of an evolving,preextensional to and•44Nd=lV; Rb andSm were analyzed in staticmulticol- synextensionalmagmatic system, by J. E. Faulds et al.). Diskette lector mode with 87Rb=200 mV and •47Sm=500mV. may be orderedfrom the AmericanGeophysical Union, 2000 Florida Avenue, N.W., Washington,DC 20009; $15.00. Paymentmust Analysesfor Sr and Sm were doneon singleTa filaments;Rb accompanyorder. wasrun on singleRe filaments.Nd wasrun both as NdO + 15,264 FAULDS ET AL.: EVOLVING MAGMATIC SYSTEM, ARIZONA andNd +. Analyticalblanks were less than 100 pg for all Berger,G.W., and D. York,Geothermometry from4øAr/39Ar dating elements. Sr isotopic compositionsare normalized for experiments,Geochim. Cosmochim. Acta, 45, 795-811,1981. 86Sr/88Sr=0.1194and referenced to NBS-987 Best, M.G., Early Miocene changein direction of least principal stress,southwestern United States: Conflicting inferencesfrom 87Sr/86Sr=0.710250.Reproducibility of Sr valuesare better dikes and metamorphiccore-detachment fault terranes,Tectonics, than +0.000020 basedon replicateruns of NBS-987. Nd 7, 249-259, 1988. isotopiccompositions are normalized to •46Nd/•44Nd=0.7219Best, M.G., and E.H. Christiansen,Limited extensionduring peak andreferenced for LaJolla143Nd/•44Nd=0.511850. Epsilon Tertiaryvolcanism, Great Basin of Nevadaand Utah, J. Geophys. Res., 96, 13,509-13,528, 1991. values for Nd at crystallization are calculated using Bosworth,W., Off-axis volcanismin the Gregory rift, east Africa: (143Nd/144Nd)(CHUR, 0 Ma)=0.512638and 147Sm/144NdImplications for modelsof continentalrifting, Geology,15, 397- (CHUR, 0 Ma)=0.1967. Reproducibilityon Nd values is 400, 1987. -0.25 epsilonunits based on replicateanalyses of LaJollaand Bursik, M., and K. Sieh, Range-frontfaulting and volcanismin the in-house standards. Mono basin, easternCalifornia, J. Geophys.Res., 94, 15,587- 15,609, 1989. Conrad, J.E., R.H. Hill, and R.C. Jachens, Mineral resourcesof the Acknowledgments. This work was supportedprimarily by Black MountainsNorth and BurnsSpring wilderness study areas, National Science Foundationgrant EAR 91-20383 awarded to J. Mohave County, Arizona, U.S. Geol. Surv. Bull., 1737-C, 22 pp., Fauldsand partly by NSF grant EAR 92-06055. Isotopicanalyses 1990. were funded by the Center for Volcanic and TectonicStudies at the Dalrymple,G.B., and W. Duffield,High-precision 4øAr/•9Ar dating Universityof Nevada,Las Vegas. We thankJames Riley andGlenn of Oligocene rhyolites from the Mogollon-Datil volcanic field Anderson of the National Park Service at Lake Mead National usinga continuouslaser system, Geophys. Res. Lett., 15, 463-466, RecreationArea (LMNRA) for providing boat accessto isolated 1988. areas. We also thank Kent Turner and William Burke of the Dalrymple, G.B E.C., Alexander,M.A. Lanphere,and G.P. Kraker, LMNRA for logisticalsupport in the park. We especiallyappreciate Irradiationo•'samples for4øAr/•9Ar dating using the Geological the careful work of Bridget Tompkins who preparedmost of the SurveyTRIGA reactor,U.S. Geol. Surv.,Prof. Pap., 1176, 1981. mineralseparations for4øAr/39Ar analysis. We arealso grateful to deVoogd, B., L. Serpa, and L. Brown, Crustal extension and JamesConrad, U.S. GeologicalSurvey, for sharingK/Ar data and magmatic processes:COCORP profiles from Death Valley and sample locations, Jack Hamm of Combined Metals Reduction the Rio Grande rift, Geol. Soc. Am. Bull., 100, 1550-1567, 1988. Companyfor enlighteningdiscussions on the local geology,and DeWitt, E., J.P. Thorson,and R.C. Smith, Geologyand gold deposits David Fosterof LaTrobeUniversity for the fission-trackages on the of the Oatman district, northwestern Arizona, U.S. Geol. Surv. Mount Perkinspluton. Discussionswith R. E. Andersonand Eugene Open File Rep., 86-0638, 34 pp., 1986. Smith were also helpful. Myron Best, Allen Glazner,Calvin Miller, Eaton, G.P., The basin and range province: Origin and tectonic JaneNielson, and SteveReynolds provided helpful reviews. significance,Annu. Rev. Earth Planet. Sci., 10, 409-440, 1982. Eichelberger, J.C., P.C. Lysne, C.D. Miller, and L.W. Younker, References Researchdrilling at Inyo Domes, California: 1984 results, Eos Trans. AGU, 66, 186-187, 1985. Anderson, R.E., Thin skin distension in Tertiary rocks oœ Eichelberger, J.C., T.A. Vogel, L.W. Younker, C.D. Miller, G.H. southeasternNevada, Geol. Soc. Am. Bull., 82, 43-58, 1971. Heiken, and K.H. 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