TECTONICS, VOL. 12, NO. 2, PAGES 372-386, APRIL 1993
GEOLOGIC EVOLUTION OF IRON withinthe ranges are regionally extensive and can be accountedfor in tectonicmodels for theMojave Desert. MOUNTAIN, CENTRAL MOJAVE DESERT, New geologicmapping and U-Pb geochronology at Iron CALIFORNIA Mountain,located 20 km southwestof Barstow,California placeconstraints on thelocation of regionalstructures in the StefanS. Boettcher1 centralMojave Desert(Figures 1 and 2). Iron Mountain Departmentof Geology,University of NorthCarolina at providesan excellentlocality to addressMesozoic tectonic ChapelHill modelsthat predict contraction or extensionfor theMojave Desertand eastern Sierra Nevada owing to its proximityto J. DouglasWalker rangeswhere evidencefor currentmodels has been obtained. Departmentof Geology Specifically,ignimbrite and quartz sandstone of the Universityof Kansas Sidewindervolcanic series, exposed in theSidewinder Lawrence, Kansas Mountains~20 km southeastof Iron Mountain,has been cited asevidence for theextensional nature of theearly Mesozoic arcin theMojave Desert [Busby-Spera, 1988]. In contrast, Late Jurassic contractile deformation has been documentedin Abstract.Geologic mapping, structural analysis, petrologic the CroneseHills [Walker et al., 1990a],located ~50 km to the study,and U-Pb geochronologyat Iron Mountain, 20 km northeastof IronMountain and in theWhite-Inyo Mountains southwestof Barstow,California, place important constraints andArgus Range north of the Garlockfault in eastern on thepaleogeographic affinities of metasedimentaryrocks in California[Dunne et al., 1978;Dunne, 1986]. Specific thearea and provide new data to testMesozoic and Cenozoic geologicproblems at Iron Mountainthat bear on thisquestion tectonicmodels for the centralMojave Desert. Rockspresent includethe timingand tectonic significance of an inferred at Iron Mountain include:Precambrian(?) and/or lower kilometer-scale,isoclinal fold andmeter- to centimeter-scale, Paleozoic(?)miogeoclinal rocks, Middle Jurassic tonalite, tight-to isoclinal-typefolds. Middle JurassicHodge volcanic series, Late Jurassic In addition,evaluation of rocktypes present in a 1 km wide hornblendediorite, Cretaceous(?) peruluminous granite, and exposureof metasedimentaryrocks in thenorthem part of Iron Cretaceous(?)granodiorite prophyry. Two phasesof ductile Mountainprovides new dataon the locationof the deformationare present at kon Mountain.The first phase, miogeoclinal/eugeoclinalboundary in thecentral Mojave whichpenetratively deforms the miogeoclinal rocks, tonalite, Desert. The correlationof metasedimentaryrocks at Iron Mountain with Late Precambrian to lower Paleozoic andHodge volcanic series, developed under amphibolite and greenschistfacies metamorphic conditions. Fabric sedimentaryrocks of theCordilleran miogeocline and its developmentin theHodge volcanic series preceded bearingon the locationof proposedregional structures will be emplacementof 151+ 11Ma granite.The second fabric addressedin thispaper. deformsperaluminous granite and Late Jurassic hornblende Last,the postition of Tertiarydetachment faults relative to dioriteas well, andconsists of spacedmylonitic shear zones. numerousranges in the centralMojave Desertremains Theshear zones predate emplacement of LateCretaceous unclear. Iron Mountain, located ~ 15 km southwestof dikes(83 + 1 Ma). Thepresence of probablemiogeoclinal exposuresof knownfootwall rocks in theHinkley Hills has strataindicates that the boundarybetween allochthonous beendescribed as possibly lying in thefootwall of thecentral eugeoclinalrocks and parautochthonous miogeoclinal/cratonal Mojavemetamorphic core complex [Walker et al., 1990b]. rocksmust lie northof Iron Mountain. The olderamphibolite The natureand tectonic significance of structuresat Iron faciesmetamorphism and conlxactile deformation at Iron Mountainare discussedin thispaper in thecontext of recently Mountainare interpreted to be partof a beltof Middleto Late proposedmodels for themagnitude and significance of Jurassicage deformation that runs northeastward through the Tertiaryextension in theMojave Desert. MojaveDesert and forms the southern continuation of theeast Sierrancontractile belt. Newly recognizedsubvertical LITHOLOGIC DESCRIPTIONS myloniticshear zones of Cretaceousage at IronMountain have not beendocumented elsewhere in the centralMojave Previousgeologic investigations at Iron Mountaininclude Desert.No significantTertiary ductile deformation fabrics are workby Miller [ 1944],who discussed lithologies and possible presentat Iron Mountain. regionalcorrelations, and by Bowen[1954], who published a 1:125,000geologic map of the Barstow30' quadrangle. INTRODUCTION Dibblee[1960] published a geologicmap of theBarstow 15' quadrangle,mapped Iron Mountain[Dibblee, 1967] at a scale of 1:62,500,and presented the first description of manyof the Much of the geologichistory of the centralMojave Desertin rock typesat Iron Mountain. southernCalifornia is recordedin mountainranges separated Detailedgeologic mapping at 1:12,000of the entirerange, by largeexpanses of alluvium. Eachrange contains important undertakenby S.S. Boettcherin the Fall of 1989, hasled to the information,but correlationof structural,lithologic, and refinementof earlierdescriptions, documentation of field geochronologicdata from rangeto rangehas proven difficult. relations,and discovery of previouslyunrecognized structural In recentyears, detailed mapping coupled with U-Pb complexities(Figure 3; Boettcher,[ 1990a,b]; Boettcherand geochronologyhas shown that structural elements exposed Walker,[1990]). To betterdefine the chronology of intrusion/extrusion,metamorphism and deformation, U-Pb analysisof zirconand monazite was done on severalof the INowat Department ofGeological Sciences, University of igneousunits. Results are given in Table1 andpresented in Texas at Austin. Figure4. We analyzedsamples of theHodge volcanic series (sampleQBmv) andpostkinematic granite (WCG) fromthe Copyright1993 by theAmerican Geophysical Union. southernarea, and gneissictonalite (IM-101) and Papernumber 92TC02423. postkinematicmuscovite garnet granite dikes (mgg-1) from 0278-7407/93/92TC-02423510.00 thenorthern area. This new U-Pb geochronologyand Boettcherand Walker: Geologic Evolution of IronMountain 373
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Fig.1. Mapof thesouthwestern United States regionalstructures and localities include the San showingregional structures and the location of Andreasfault (SAF), Garlockfault (GF), Old IronMountain (IM). ThePermo-Triassic WomanMountains (OWM) andthe Maria fold and truncationboundary (PTTB), east Sierran thrust thrustbelt (MFTB). Abbreviationsfor geographic system(ESTS), Mojave-Snow Lake fault (MSLF), markersare: LV, Las Vegas;ND, Needles;PH, andWaterman Hills detachmentfault (WHDF) are Phoenix;TS, Tucson;LA, Los Angeles;and SB, ofparticular interest to this study. Other important Santa Barbara. lithologiccomparison with similar rock types elsewhere inthe Biotite-muscoviteschist contains rare large (up to 1 cm), MojaveDesert are the basis of age constraints forunits in this subhedralgarnet crystals and both euhedral-prismatic and study.Table 2 isa summaryofage constraints and lithologies anhedral-fibroliticsillimanite at sixdifferent localities in the for thedifferent rock types at IronMountain. northernarea. The equilibriumassemblage garnet + sillimanite+ quartz + plagioclase+ biotite + muscoviteoccurs inat least two 5 m2 areasof weatheredbiotite-muscovite MetasedimentarySequence schistthat are separated by severalkilometers. Sillimanite and The northernarea of Iron Mountainconsists of light colored, biotiteare locally kinked and folded. low hillsdominated by coarselycrystalline dolomitic marble Calc-silicategneiss is a distinctlybanded rock with thatis massiveto finelylaminated. A distinct,micaceous alternating1 to 20 mmthick, light plagioclase-quartz-rich and quartziteunit, up to 50 m thick,forms a prominentmarker. It darkhornblende-rich layers, interrupted by pelitic layers rich containsabundant, closely spaced, dark laminations of biotite, in biotiteand sillimanite. The equilibriumassemblage magnetiteand other heavy minerals and is commingled with hornblende+ plagioclase + diopside + quartz is commonly isoclinallyfolded quartz-feldspar injection gneiss. The retrogradedtoepidote + albite+ sphene+ diopside + quartz. laminationsin the quartzite and the compositional layering in In threethin sectionsof dolomiticmarble, forsterite and themarble are probably primary bedding structures that have enstatiteoccur in equalproportions asanhedral crystals that beentransposed during deformation and metamorphism. Four areround in thin section.Diopside occurs in greater othervitreous quartzite units, generally less than 5 m thick, proportionsand is subhedral toanhedral. The marble is highly occuras 25 to 500 m longpods along strike and locally recrystallized,with dolomite crystals commonly aslarge as 1 containa biotitelineafion that plunges shallowly to the cm.The equilibrium assemblages in the marble are dolomite northeast.Dark-weathering, banded calc-silicate gneiss and + diopside+ forsterite + enstatite + phlogopite + calcite + muscovite-biotiteschist occur locally and are discontinuous quartz. alongstrike. Sedimentary structures are not recognized and Thepresence ofchlorite, serpentine, epidote and albite in havepresumably been obliterated by deformation. rockscontaining amphibolite facies assemblages indicates the The local occurrenceof fibrolitic sillimanitein micaceous instabilityof thehigher grade assemblage. Secondary quartzite,prismatic sillimanite and garnet in biotite-muscovite alterationof forsteriteto serpentineis common. In addition, schist,hornblende and plagioclase in calc-silicategneiss, and local5 to15 m 2 podsof very coarse-grained gamet-quartz- morewidespread occurrences of diopside, forsterire and epidotehomfels and black-weathering calc-silicate rock occur enstatitein dolomiticmarble indicate amphibolite facies in areaswhere granitic rocks and marble are cornmingled. metamorphismin the metasedimentary sequence [Winkler, Theserocks lack the high-temperature assemblage of the 1979]. In themicaceous quartzite, fibrolite occurs in small metasedimentarysequence and represent skam mineralization matsintergrown with, and possibly forming from, 1 to2 mm or metasomatismassociated with igneous events that postdate longpackets of muscovite in a rockotherwise dominated by theamphibolite facies metamorphism ofthe metasedimentary quartzand 2 to5 mmthick layers of biotite. The equilibrium sequence. assemblagein the micaceous quartzite isquartz + biotite+ Miller[1944] included the metasedimentary sequence with muscovite + sillimanite. theHinkley Valley complex, which he proposed ismiddle 374 Boettcherand Walker: Geologic Evolution of IronMountain
117o15 '
El Paso Mountains
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v..,',:xx .c',?,•_ x • • '- Cronese •?.'."%...•"•'•'••e'•(?."-• Buttes •.'a,,,,,• i• 4• Alvord•] Hills (• •...o c•\ • , Mountain x 30km - W • • X•'arstow% ountinZ , hadow.,.• SM ,, • • .•::•...• .. , ,,;, t.
;,';[•Tertiary toJurassic granitoids I rocksPaleozoic eugeoclinal LateJurassic gabbro '"•,:• Mesozoicstrata • PrecambriantoPaleozoic ,,,, ,, miogeoclinaland cratonal rocks • schistPrecambrianand gneiss(?)to Tertiary Cenozoicright-lateral •• strikeslipfault • detachmentWaterman Hillsfault
Fig.2. Simplifiedgeologic map of thecentral CroneseHills, Shadow Mountains, Sidewinder MojaveDesert showing the distribution of pre- Mountain(SM), and Iron Mountain are the regional Tertiaryrocks and major Cenozoic faults. The focusof this study [after Jennings, 1977].
Paleozoicor olderbased on regional lithologic and cross- dolomite,and calc-silicate rocks overlain by finely laminated cuttingrelationships. Bowen [ 1954]correlated the thickquartzite [Stewart, 1970]. metasedimentarysequence with the Oro Grande series of the Kiser[1981] correlated a metasedimentary sequence inthe Victorvillearea and assigned it a Carboniferousagebased on HinkleyHills, located 15 km northeast of Iron Mountain, with twopoorly preserved brachiopods in the Shadow Mountains. theLower Cambrian Wood Canyon Formation. The Wood Dibblee[1967] stated that the metasedimentary sequence is CanyonFormation is a predominantlyclastic unit with lesser unlikerocks of the Oro Grande series and grouped the amountsof dolomiteand limestone that is grouped in the sequenceat Iron Mountain with the WatermanGneiss and DeathValley facies of theCordilleran miogeocline [Stewart, inferreda Precambrianage. 1970]. Thelarge proportion of carbonatein theIron Mountain Thelarge proportion of marbleand quartzite relative to calc- sequencemakes correlation with the Wood Canyon Formation silicaterocks and pelitic schists atIron Mountain suggests that unlikely.Miller [1981a, b] identified a pre-Wood Canyon themetasedimentary sequence was deposited in a Formationsequence of dolomite with thin layers of calc- miogeoclinalorcraton margin setting [Boettcher, 1990b]. On silicateand quartzite at QuartziteMountain, north of thebasis of comparison with rocks described byHazzard Victorville(Figure 2). Shehad difficulty finding a [1937],Stewart [1970], and Stewart and Poole [1975], we stratigraphicmatch for this sequence but suggested a late correlatethe metasedimentary sequence with the Cordilleran Precambrianage based on gross lithologic comparison with miogeoclinalrocks of the Late Proterozoic Noonday Dolomite unitsin theSan Bernardino Mountains. The Iron Mountain andlower part of the Johnnie Formation. The Noonday sequenceexhibits some similarities tothe upper part of the Dolomite-JohnnieFormation interval contains massive GreenSpot Formation and the Delamar Mountain Formation dolomite,interlayered thin clean quartzites, laminated of theBig BearGroup in theSan Bernardino Mountains Boettcherand Walker: GeologicEvolution of Iron Mountain 375
117o15 ' angularfragments cemented by hematiteor limoniteat the I ends of the boudins. Barstow16krn No mineralsindicative of temperatureshigher than greenschistfacies were observed in theHodge volcanic series. Theassemblage biotite + quartz+ muscovite+ clinozoisite+ plagioclaseis typical of themetavolcanic rocks of theHodge volcanicseries. Plagioclase crystals commonly show alterationto sericiteand, less commonly, calcite. Additionally,the muscovite-sericite schist unit of theHodge volcanicseries appears to haveundergone an episode of IM Rd alterationthat produced fine-grained mica, characteristiclight tan color, and friable nature. The proximityof the Hodgevolcanic series to the Sidewindervolcanic series and similaritiesin lithologies(J. D. Walker andE. Schermer,personal communication, 1990) betweenthese units suggest that these series are correlative. The Sidewindervolcanic series (U-Pb ageon zirconfrom basalunit ~ 171 Ma, Graubardet al. [ 1988]) is a dominantly pyroclasticunit with interbeddedquartz-rich volcanogenic •...a ...... Compositionsare mainlyrhyo!ific *•' 4•;,,. with lessabundant exposures of andesite[Bowen, 1954]. The Sidewindervolcanic series and the Hodgevolcanic series are separatedby graniticplutons that intrude both series. The closestoutcrops of the two seriesare approximately10 km o lkm apart. A zirconage from foliatedquartz-plagioclase-biotite metavolcanicrock (sampleQBmv) stronglysupports Fig.3. Simplifiedgeologic map showing the subdivision of correlationof the Hodgeand Sidewindervolcanic series. Iron Mountaininto a northernarea of metasedimentaryand Threebulk fractionsfrom thissample yield a lower intercept plutonicrocks, acentral area of mainly hornblende diorite, and ageof 160 Ma. Two air-abradedfractions give an olderage of a southernarea consisting of theHodge volcanic series. Cross 164 Ma indicatingthat the zirconshave suffered some Pb loss sectionlines A-A' andB-B' shownfor Figure9 (compiled (Figure4). Resultsfrom othervolcanic rocks in theregion fromDibblee, [1967] and Boettcher, [1990b]). [e.g.,Busby-Spera et al., 1990] andsimilar volcanic rocks from the southernInyo Mountains[Dunne and Walker, 1991; and unpublisheddata, 1991] indicatethat minor Pb lossis [Cameron,1982], but the quartzites at Iron Mountaindo not typicalof zirconsin thesesequences. Given the degree of Pb containabundant feldspar grains that are characteristic of the lossexhibited by theseother volcanic sequences, we interpret 164 Ma to be a minimumage and consider it probablethat the quartziteunits in theGreen Spot-Delamar Mountain interval. There are few similaritiesbetween the metasedimentary true agemay be 170 Ma or very slightlyolder. sequenceat kon Mountainand younger sections of the Cordilleranmiogeocline or eugeocline.In particular,the Granite of Wild Crossing Middle CambrianBonanza King Formationand the Upper CambrianNopah Formation, exposed in DeathValley, At the extremesouthern end of the area,just northof the California,are both predominantly dolomite [Stewart, 1970], Mojave River, an isotropic,pink-stained, equigranular granite but lackthe distinctive, thin layersof orthoquartziteand locallycontains xenoliths of foliatedquartz-plagioclase-biotite thickerlayer of micaceousquartzite found at IronMountain. metavolcanicrock. Zirconsfrom thispost-kinematic granite (sampleWCG) showthe combinedaffects of Pb-lossand HodgeVolcanic Series inheritedxenocrystic zircon. We haveair abradeda fraction of the moremagnetic zircon. The abradedfraction gives older TheHodge volcanic series is exposedapproximately 3 km U-Pb andPb/Pb ages than the correspondingunabraded to thesouth of themetasedimentary sequence in low, variably fraction(Figure 4). This fractionand two nonmagnetic coloredhills thatare dissected by numerousgullies. The fractionslie on a chordgiving a lowerintercept age of 151+_ dominantrock type is a darkweathering, porphyritic quartz- 11 Ma (Figure4). We interpretthis as a minimum plagioclase-biotitemetavolcanic rock with a foliationthat crystallizationage, but considerthis not to be significantly weakensto the southeast.The foliationis generallyparallel to youngerthan the trueage of therock. thecontacts between compositional layers but locally crosses thelayering at lowangles. The compositional layering is most Gneissic Tonalite obviousin the hillsjust north of theMojave River where 3 to 10m thicksubvertical light tan, pink, and dark brown to black The metasedimentarysequence in the northernpart of the layersproduce a stripedpattern on the low hills. Thequartz- areais intrudedby severaligneous bodies. The oldestigneous plagioclase-biotitemetavolcanic rock is interlayeredwith a phasewas pervasively deformed along with thehost sediments fine-grained,white to lighttan, strongly foliated (0.1 to 1 cm to a schistoseto gneissictonalite. The equilibriummineral spacingof foliationplanes) muscovite-sericite schist with both assemblagein the gneissictonalite is plagioclase+ quartz+ quartz-and chlorite-rich layers and, locally abundant elongate biotite+ muscovite.Plagioclase crystals are locally quartzpebbles. Additionally, distinct, resistant, desert- retrogradedto epidoteand calcite. Mineralassemblages varnishedquartzite boudins occur within the muscovite- characteristicof a particularmetamorphic facies were not sericiteschist unit. Thequartzite is thick-bedded(up to 1 m), observedin the gneissictonalite. Analyses of zirconfractions containsgreater than 95% quartz and is commonlybroken into from the gneissictonalite (sample IM-101) did notyield easily 376 Boettcherand Walker: Geologic Evolution of IronMountain
0 0 0 0
,--,o
oc5 • c5
0 •o•o . g •V A •• V A•'• A Boettcherand Walker: GeologicEvolution of Iron Mountain 377
0.033•_• 21•/ _ •_SampleQBmv 2/.•/• _
- I /•• Interceptsat _ •/ '/' 164•2 and 1735 ß338 Ma
207Pb/235U
O.O32 _ SampleWCG • 0.028 - 160..•••'-•" J •" - 0.024
O.O2O 0.16 0.18 0.20 0.22 0.24 207Pb/235U
0.025'
0.02z - SampleIM-101 152 1••,•• -
0.02,'. _
0.022 _
0.021 0.150 0.154 0.158 0.162 0.166 0.170 207Pb/235U
Fig. 4. Concordiadiagrams for U-Pb analyses.(a) fractions.Air abrasionof thenm(1) fraction drove Air-abradedfraction from sampleQBmv yieldsa thepoint towrod the regression line of thenm(0) lowerintercept age of 164+ 2 Ma. Resultsfrom data. Fromthis we interpretthe sample to have othervolcanic rocks in theregion [e.g., Busby- undergonePb lossand to containan inherited Speraet al., 1990] and similarvolcanic rocks from xenocrysticzircon component. The ageof 151 + the southernInyo Mountains[Dunne and Walker, 11 Ma thereforerepresents a minimum age for this 1991;also unpublished data, 1991] typicallyexhibit sample,(c) Bulk fractionsfrom sampleIM- 101 Pb-loss.Given the degree of Pb-lossexhibited by cluster at about 148 to 150 Ma. Air-abraded theseother volcanic sequences, we interpret164 fractionsplot at olderU-Pb ages, thus indicating Pb Ma to be a minimumage and consider it likely that loss.Our preferred interpretation is that the sample the trueage is 165 to 170 Ma, (b) For sample is 170Ma, theage of similargranitic rocks in the WCG, four fractionsare plotted but only threeare region(see discussion) and has an inherited usedin the regression:data for nm(0) and nm(1)AA componentand has sufferedPb loss. 378 Boettcherand Walker: Geologic Evolution of IronMountain
TABLE 2. Summaryof AgeConstraints for RocksPresent at Iron Mountain IronMountain RegionalOccurrence AgeConstraints NorthernArea Peraluminousgranite FremontPeak and The Buttes iLS. Youngerthan Mesozoic 148 Ma(?) Miller et al., 1992], gabbro/dioriteand older than 83+ 1 Ma Old Woman Mountains and Piute uncleformeddikes at Iron Mountain; Mountains[C.F. Miller et al., 1982] LateCretaceous in eastern Mojave Desert; LateCretaceous biotite monzogranite in Fry andOral Mountains[Karish et al., 1987]
Gneissic tonalite Western San Bernardino Mountains ProbablyJurassic, U/Pb 152-190 Ma; [Dibblee, 1967], olderthan 148 Ma(?) gabbro/diorite and Lane Mountain[McCulloh, 1952] youngerthan metsedimentary sequence at Iron Mountain
Metasedimentary Miogeoclinalfacies; Similar facies at Similarin lithologyto LatePrecambrian sequence QuartziteMountain and Sidewinder NoondayDolomite and Johnnie Mountain,[Miller, 1981],Hinkley Formationin DeathValley area and Late Hills [Kiser,1981], Newberry Precambrianto earlyPaleozoic Mountains,[Glazner et al., 1988] and metasedimentaryrocks present in the SanBernardino Mountains [Cameron, Victorvillearea [Miller, 1981a,b] 1982] Central Area Granodioriteporphyry Alvord Mountain 82 Ma at AlvordMountain [Walker et al., 1990c];
Hornblende diorite ShadowMountains, Goldstone, 148Ma U/Pbzircon age from Shadow CoolgardieCamp, and Snow Lake Mountains[Martin et al., 1990], Snow pendant[Sierra Nevada] Lake pendant[Lahren, et al., 1990]; 148Ma Ar/Ar agefrom Goldstone [Miller andSutter, 1982] Southern Area Granite Cronese Hills 151+11 Ma U/Pb zircon at Iron Mountain; 154+1 Ma U/Pb zircon in the Cronese Hills [Walker et al., 1990a]
Hodgevolcanic series Sidewinder volcanic series Jurassic,U/Pb minimumage ~ 164 Ma; U/Pb zirconage of 171Ma for Sidewindervolcanic series [Graubard, 1988]
interpretableresults (Figure 4). Bulk fractionscluster at about occurred,then the sample is probablyabout 190 Ma. Thisis 148to 150Ma. Air-abradedfractions plot at older U-Pb ages, theaverage of thePb/Pb ages of threeolder fractions. Our thusindicating Pb loss. Unfortunately, air abrasiondid not preferredinterpretation is that the sample is about170 Ma, the createany sort of discordiaarray, and we interpret this sample ageof similargranitic rocks in theregion (see discussion to haveundergone extensive Pb loss (possibly combined with below)and hasboth suffered Pb lossand contains an inherited inheritedxenocrystic zircon). This is not unexpected. First, component. thesample collected, although as fresh as could be found, was somewhatweathered and friable (a pervasivefeature of this Hornblende Diorite unit).Second, the extreme deformation and metamorphism sufferedby thisunit probably opened the zircons to Pbloss. A stock-sizedbody of hornblendediorite is present in the We offer two interpretationsfor thesedata. First,if we centralpart of IronMountain and commonly has a colorindex assumethat the sample contains an inherited component, then closeto 65. It is composedmainly of hornblendeand theminimum age of thissample is 152Ma. Thisis the plagioclasewith lesser amounts of sphene,augite, and opaque approximateage of a chordbetween the oldest fraction and an minerals,has a poikilitictexture, and contains four mappable assumed1800 Ma upperintercept (typical inheritance age for (at 1:12,000)0.1 m to 10m wideshear zones. Gabbro, witha granitesin thisregion). Alternatively, if only Pb loss has colorindex of 80 or higher,occurs as small bodies within the Boettcherand Walker: Geologic Evolution of IronMountain 379
diorite.Medium- to verycoarse-grained gabbro dikes and Barstow,are similarin mineralogyand texture to the xenolithsare locallypresent. Greenschist facies granodioriteporphyry body at IronMountain. metamorphismassociated with myloniticshear zones in the hornblendediorite is characterizedby theinstability of Felsite Dikes hornblendeand the presenceof actinolite,clinozoisite/zoisite, chlorite, and albite. Lightgray, 1 to 3 m wide,fine-grained dikes crosscut all Theage of thehornblende diorite complex is inferredby deformationfabrics and previously mentioned rock types. correlationsof intrusiverelations to adjacentareas. Bowen Thesedikes locally contain complex refolded fold patterns that [ 1954] statedthat small intrusionsof hornblendediorite cut the likelyformed during emplacement. Similar felsite dikes are Sidewindervolcanic series across the Mojave River, 30 km presentto thenorth and east at FremontPeak, Hinkley Hills, southeastof Iron Mountain. No intrusionsof gabbroor diorite andMitchel Range. Dikes in theseareas are Early Miocene were observed in the southern metavolcanic rocks at Iron (~23 Ma; Walkeret al. [1990b]and Walker and Martin, Mountain,however. Both U-Pb zirconages of similargabbros unpublisheddata, [1991]), which we interpretto bethe age of in the ShadowMountains [Martin andWalker, 1989] and dikes at Iron Mountain. 40Ar-39Arages of similar gabbros from the Goldstone area STRUCTURE [Miller and Sutter, 1982] 40 km northeastof Iron Mountain are~ 148 Ma. Thesedata currently provide the best correlation Fourmajor structural elements are present at IronMountain: for theage of thegabbro/diorite complex in thecentral part of (1) gneissicfabric in thetonalite and schistose fabric in the Iron Mountain. Hodgevolcanic series; (2) close-to isoclinal-typefolds involvingthe metasedimentary sequence and the gneissic Peraluminous Granite tonalite,and close- to tight-typefolds of theschistosity and myloniticfoliation in theHodge volcanic series; (3) northeast Two phasesof variablydeformed peraluminous granite are striking,subvertical mylonitic shear zones with low-plunging presentin the northernand centralareas of Iron Mountain. stretchinglineation; and (4) andopen folds with subvertical Dikesof shearedbiotite granite with localoccurrences of axesof boththe gneissic and mylonitic fabric. The structures muscoviteand a leucocratic(color index <5) biotitegranite canbe dividedbased on styleand cross-cutting relations into thatcontains pods of abundantgarnet and muscovite intrude an oldergneissic to schistosefoliation and tight to isoclinal thehornblende diorite body, gneissic tonalite, and folds,and younger mylonitic shear zones and open folds. metasedimentarysequence. Metamorphism of thesephases in shearzones is characterizedby thebreakdown of plagioclase Planar Fabrics to form albite,calcite and epidote. The apparentlack of plutonismduring the interval 147-125 Well-defined,northeast trending, subvertical planar fabrics Ma [Chenand Moore, 1982]followed by Early to Late arepresent in all threeareas at IronMountain. The fabric Cretaceousgranitic plutonism in thecentral Mojave Desert andSierra Nevada suggests that sheared biotite granite dikes in thediorite body and sheared peraluminous granite in the n=257 northernarea may be significantlyyounger than 150 Ma. In particular,~98 Ma synkinematicperaluminous granite is ,,....* presentat FremontPeak in theNW Mojave Desert(U-Pb zircon,J.S. Miller et al., [1992]) andLate Cretaceousbiotite ß -I- ß monzograniteis presenteast of Victorville[Karish et al., 1987].
Muscovite-Garnet Granite
Undeformed,2 to 20 m thick,northwest trending dikes of coarse-grainedmuscovite-garnet granite crosscut mylonitic fabricsin the gneissictonalite. U-Pb resultson monazitefrom thesedikes consist of two reverselydiscordant fractions that yieldednearly identical analyses (Table 1). Followingthe reasoningoutlined by Parrish[1990], we interpretthe age of thissample tocorrespond tothe average ofthe 207pb/235U ages. Thus,this sampleis datedat 83 _+1 Ma.
GranodioritePorphyry
An undeformedgranodiorite porphyry separates the Hodge volcanicseries from the hornblendediorite body. This unit is characterizedby large(2 to 4 cm long),randomly oriented K- feldsparmegacrysts in a matrixof medium-grained plagioclase,biotite, and quartz. Granodioriteporphyry exposedto the northof the Hodgevolcanic series is not mylonitic,nor is the southernmargin of thehornblende diorite complexof the centralarea, suggesting that the granodiorite n=345 porphyrywas intrudedalong a now obliteratedcontact betweenthe Hodgevolcanic series and the hornblendediorite body. Granodioriteporphyry dikes at AlvordMountain (82 Fig. 5. Polesto foliationin the northern,central and southern Ma U-Pb zircon;Walker et al., [1990c]), located30 km eastof areas of Iron Mountain. 380 Boettcherand Walker: Geologic Evolution ofIron Mountain
appearsas gneissicbanding in the tonaliteand A. Polesto foliationin all units B. Polesto foliat•onin igneousunits metasedimentaryrocks of the northernarea, occurs within L-S tectonitesin 0.1 to 10 m wide myloniticshear zones in the centralplutonic complex and northern intrusive and metasedimentaryrocks, and is a pervasiveschistosity which becomesmylonitic in the mostintensely deformed S tectonites of the Hodgevolcanic se,'ies. Poles to foliationplanes in the threeareas show a fairly wide scatterabout a pointmaximum (Figure5). Mineral stretchinglineations in the ductileshear zonesalso show a fairly wide scatterand plunge both to the northeastand southwest, typically at low angles(Figure 6). No marked discordance in foliation or lineation orientation occursbetween the areas,although the northern area contains a kilometer-scaleopen fold of the foliationwith a continuous changefrom northeast-to northwest-strikingunits at the northeastern extreme of the area. Gneissicand schistosefabric. The tonalitethat intrudes the metasedimentarysequence contains two distinctdeformation fabricsin outcropand thin sectionscale: a crystalloblastic gneissicbanding/schistosity and a myloniticfoliation and associatedstretching lineation. The gneissicfabric in the tonalite occurs as 1 mm to 5 cm thick bands of biotite- C. Polesto myloniticfoliation D. Polesto compositionallayering dominatedmelanocratic layers and plagioclase- and quartz- unit BG m metased•mentaryrocks dominatedleucocratic layers. Locally, where coarse-grained biotiteis abundant,the fabricis bettertermed a schistosity. Fig. 7. Polesto foliation in selectedunits of thenorthern area. Quartzcrystals in the gneissicrocks do not showevidence of (a) Polesto foliationin all unitsin thenorthern area. (b) Poles grainsize reduction and exhibit only weak undulatory to foliationin igneousrocks in the northernarea; concentration extinction. The compositionallayering in the of poleswith morenortheasterly trend and shallower dip is metasedimentarysequence defines a gneissicbanding that is frommylonitic foliation in shearedperaluminous granite (map likely a productof transposedoriginal bedding, as discussed unitbg) andsheared biotite tonalite (map unit bt). (c) Polesto earlier. myloniticfoliation in unitbg in thenorthern and central areas. The Hodgevolcanic series possesses a penetrative (d) Polesto compositionallayering in themetasedimentary schistositydefined by theplanar orientation of biotite,white sequencein the northernarea.
mica,and elongate quartz ribbons that becomes mylonitic in A. Lineations B. F-1N Hingelines themost intense zones of deformation.The fabric is most myloniticnear the contact with the central plutonic complex andweakens considerably to thesoutheast. The decrease in intensityoffabric is characterized bythe gradual change from highlystrained quartz in elongate ribbons tolarge phenocrysts ofquartz in a fine-grainedmatrix in thequartz-plagioclase- biotitemetavolcanic rock. Also, plagioclase isdeformed in a brittlemanner in themylonitic areas but appears as unreformedphenocrysts toward the southeast. The penetrativenature of thefabric in theHodge volcanic series revealsthat strain was distributed broadly across the southern area. Mylonitic shear zones. The deformationcharacteristics associatedwith the gneissic banding contrast sharply with thoseof themylonitic tonalite, biotite granite and hornblende diorite.The most obvious distinction between the mylonitic fabricand the gneissic fabric is the amount of strainin quartz crystals.In themylonitic rocks, elongate quartz ribbons show bothdynamic recovery and dynamic recrystallization. The quartzgrains are typically small, show uniform extinction, and havesharp polygonal boundaries but can also show well- C. F-1S Hingelines D. Polesto foliationfrom limbs developedsubgrains separated by low-angleboundaries. of F-2 fold in northern area Furthermore,plagioclase feldspar porphyroclasts are Fig.6. (a) Trendand plunge of stretchinglineation in the commonlybroken and boudinaged in theshear zones, but not northern,central, and southern areas of IronMountain. (b) in theareas showing a crystalloblasticfabric. Trendand plunge of hingelinesofF1N folds (folds of the Lowerhemisphere projections of polesto foliationshow compositionallayering in themetasedimentary sequence and thatfoliation in thegneissic rocks is subparallel tothe gneissictonali•e in thenorthern area). (c) Trend and plunge of myloniticfoliation, except for a smallconcentration ofpoles hingelinesof F1Sfolds (folds of theschistosity and mylonitic tothe mylonitic foliation that show a morenortheasterly trend anda shallowerdip (Figure7). fabricin thesouthern area). (d) Polesto foliationfrom un'•ts Thedegree of mylonitizationin theshear zones in the onlimbs of kilometer-scale, open, upright F2 foldin extreme northernand central areas varies considerably. Ultmmylonites northernpart of northernarea. Measurementson northwest andmylonites are common along the contact between the trendinglimb are sparse due to poor exposure. northernand centralareas and in severaldiscrete bands within Boettcherand Walker: Geologic Evolution of IronMountain 381 a largearea of protomylonitenorth of thenorthern/central porphyroclastsare equivocal in theirsense of shear,possibly contact[Boettcher, 1990b]. A generaltrend of decreasing due to grain-to-graininterference. In the southernarea, s-c intensityof myloniticfabric from south to north can be fabric observed at two localities shows sinistral shear. distinguished.Shear zones are less common, do not show as However,fold asymmetryindicates dextral shear sense at two strongof a grainsize reduction, and are not as continuous in other localities. thenorthernmost igneous body as compared to shearzones farther south in the northern area. Folds Shear-senseindicators are scarcein all three areasand FiN folds(here defined as first generation folds in the provideconflicting senses of motion.In total,shear-sense indicatorswere observedat 16 different localities, with dextral northernarea) of the compositionallayering in the indicatorsobserved 10 times and sinistralindicators 6 times. metasedimentarysequence and gneissic tonalite generally Onlyin thecentral area, where s-c fabric is well developed, havesteeply dipping, northeast trending axial surfaces and are the shear-senseindicators consistent: dextral indicators moderatelyplunging hinge lines that trend either northeast or occurat four localities,whereas no unequivocalsinistral southwest(Figure 6). Thesefolds are commonly symmetric, indicatorswere located. In the northemarea, sigma-type close-to isoclinal-type[Fleuty, 1964] andoccur on a meter porphyroclastsshow dextral shear sense at four localities and scale.No FiN foldsof themylonitic foliation were identified. sinistral shear senseat three localities. Most often, Hingelines of close-to isoclinal-typefolds of thegneissic
m
X2548
f'\ j-..F'-" N
400m
Fig. 8. Geologicmap of partof thenorthern area of presenceof numeroussmaller scale folds with Iron Mountain.Note therepetition of themap unit northeasttrending, steeply inclined axial planes mq. Facingindicators have not been identified in suggestthat a synformalfold is present(mapping theseunits, but thedip directionsof theunits and by S.S. Boettcher,[1990b]). 382 Boettcherand Walker: Geologic Evolution ofIron Mountain
GeologicCross Section of Northernand Central Areas of Iron Mountain A A' NW SE 2500' t• bg
1500' inferredfoldaxis
B'
NWB GeologicCross Section ofCentral and Southern Areas of Iron Mountain
qbmv 000, ffi qtz qbmv mss
Explanationforgeologic map (Fig. 8) andcross sections (Fig. 9)
• undifferentiatedQuatemaryalluvium I qbmvIto quartz-biotite myionitic,fabdcmetavolcanic weakens rock,to SE,darkU/Pb gray-weathering,minimum age ~164 highlyMa schistose granodioriteporphyry, tan-to gray-weathering, C1=15-35, contains 3-6cm gl•'l K-feldsparphenocrysts, undeformed ql•'--•Jquartzisoclinal feldsparfolds, local orthogneiss,myionitic distinctlyoverprint banded, injected intoquartzite layers, I-• locallybiotitegranite,present, lightmylonitic tan-weathering, shear zones coarse-grained,intruded by 83ñ1 garnetMa anddikes muscovite • fabricbiotitewithtonalits, local darkmylonitic gray-, overprint,spheroidal-weathering, U/Pb 152-190 MaC1=15-25, containsgneissic [-• hornblende30-90, containsdiorite,thin darkmylonitic brown- shear toblack-weathering, zones coarse-grained,Cl- i;• occurrencesmicaceousquartzite, of sillimanitethinly layered, greaterthan 80% quartz, local • granite,151ñ11 Malight tan- to red/pink-weathering, C1-15-25,undeformed, U/Pb
• localmuscoviteoccurrences biotiteschist,of garnet light and tan- sillimaniteto dark brown weathering, coarse-grained, • containsmuscovite-sedcterounded quartzschist, pebbles,light tan-weathering, shaded line indicateshighly foliated,quartzite locally boudine • ofmarble,diopside, light isoclinaltan-weathering, folds coarse-grained, dolomitic, common occurrences •,\. fault,dashed whereinferred --•j'('- inferred foldaxis • geologiccontact id• subverticalmylonitic shear zone • strikeofvertical foliation ,•r12trend and plunge oflineation inthe planeof foliation /•75 strikeand dip of foliation
Fig. 9. Geologiccross sections of northern,central in theinferred fold; thisunit doescontain mylonitic and southernareas of Iron Mountain; sectionlines shear zones. The contact between the central and shownin Figure3. The contactbetween the southernareas occurs between map units grip and northernarea and centralarea occurs between map qbmvand is inferredto be faulted. The dip of units unitsbt andhd andis highlysheared. Cross-cutting is highly variablebut is typicallysubvertical. relationshipsshow that map unit bg is not involved bandingin thenorthern area (F1N) are subparallel tomineral closefolds with rounded hinges in thequartz-rich layers. F1S stretchinglineations (Figure 6). A large(wavelength of foldshave steeply inclined, north striking axial surfaces and hundredsof meters),tight fold (F1N) parallel to the trend of moderatelyplunging, north trending hinge lines (Figure 6). theoutcrop scale folds has been inferred for thenorthern area F1Sfolds of theschistosity and mylonitic foliation in the basedon therepetition of distinctivequartzite units and broad southernarea of IronMountain are sparse, except for two bandsof marble(Figures 3 and8). Themetasedimentary distinctchlorite-rich and quartz-rich layers within the units,gneissic tonalite, and inferredfold in thenorthern area muscovite-sericiteschist unit. Only20 lineationswere arenow incompletely preserved as a pendantsurrounded by measuredin thepredominantly S tectonites of theHodge intrusionsof crosscuttingperaluminous granite (Figures 8 and volcanicseries, compared with almost100 in thenorthern and 9). centralareas combined. Additionally, hinge lines of F1S folds Thesouthern area contains local folds of theschistosity and arepoorly exposed, making comparison between hinge line myloniticfoliation (here defined as F1S), showing "Z" and lineation orientation difficult. On the basisof the few asymmetryand spaced axial planar cleavage. These folds measurementsof these structural elements (Figure 6), hinge appearas centimeter-scale, close, angular kink foldsin linesand lineations are oblique to oneanother in thesouthern chlorite-richlayers of themuscovite-sericite schist and small, area.Furthermore, axial planes, locally well represented by Boettcherand Walker: Geologic Evolution of IronMountain 383 axialplanar cleavage, are oblique to the schistose/mylonific onquartzite boudins south of thecontact. Steep striations on foliation.This differs from axial planes of FiN folds,which steeplyinclined, north dipping planar surfaces also occur in a aretypically parallel to regional fabric. fewoutcrops of felsitedikes and in muscovite-garnetgranite All threeareas contain gentle to openfolds of thefoliation dikesin the northernarea. Thesefaults involve the youngest (F2) about subvertical axes that change the strike of the igneousphases and may be related to N-S directed Neogene contractiledeformation in the centraland western Mojave foliationup to 90ø. Bothmylonific and gneissic fabrics are affectedby the F 2 folds.This style of folding is most evident Desert[Bartley et al., 1990]. in the easternend of the northernarea where the foliafionin all DEFORMATION HISTORY AND REGIONAL unitschanges strike from about N40øE to N 10øW(Figures 3 .IMPLICATIONS and6). Dibblee[1968] mapped geometrically similar folds withsubvertical axes in twogneissic complexes west of Comparisonof metamorphic facies, structure, and lithology HarperLake, approximately 15km northwest of Iron withother areas in theMojave Desert, combined with U-Pb Mountain. geochronologyoffour igneous units at Iron Mountain, form thebasis for assessmentof the timing of structuresat Iron Brittle Faults Mountain.The relative timing of thesestructures iscrucial to theestablishment of a regionaltectonic setting. Figure 10 is a Northto northwesttrending strike-slip faults, presumably summaryofPermian to Paleocene deformation, plutonism, relatedto Cenozoic right-lateral faults in thecentral Mojave volcanism,and sedimentation across the central Mojave Desert(Figure 2), aresparse in all threeareas of Iron Desert. Mountain.Four map-scale (1:12,000) faults were observed, withseparations limited to tens of meters,commonly in an Early MesozoicExtension apparentright-lateral sense. Additionally, modification ofthe contactbetween the granodiorite porphyry and the Hodge No structuralevidence for earlyMesozoic extension was volcanicseries along a high-anglereverse fault with minor foundat Iron Mountain.However, the Hodge volcanic series offsetis suggested bythe presence ofsteeply raking striations containslayers of orthoquartziteand muscovite-sericite schist
IRON CRONESE SHADOW SIDEWINDER
lg83 DNAG MOUNTAIN HILLS MOUNTAINS MOUNTAIN SCALE 57.8 Ma P•ieo•ne 66.4 post-tectonic biotite monzogranite
0 97.5 zones X{•dikesgranite • E•ly shearin I ? peraluminous northern? and central 144 granite XXX Independence dikes 163 ._q amphibolite-I io I SE-vergentductile W-vergent{•gabbro grade meta. thrusts ? ductile deformation Middleinnorth, areas l. ll••grad•••dege ritz • faults,folds 187 greenschist- Volcanic tonalite • Vmonzonite grademeta. Series II sili½i•to Highanglei? • gran•,•iorite Sidewinder in south, F1N •' intermediate Volcanic 208 folds ? volcanism Series
230
240 I•l Fairview Early 245 sedimentation t•ValleyFm. 258 W-vergent low-angle 'V rnonzonite •. Early ductile j Illsedimentation? faults,folds i W/quartz 266 deformation andrnetamorphisrn
Fig.10. Comparisonof deformation histories trendingdikes of 83 + 1 Ma muscovite-garnet acrossthe central Mojave Desert. Cronese Hills granite(U-Pb on monazite, Boettcher and Walker, datafrom Walker et al. [1990];Shadow Mountains [1990]).Gneissic fabric, F1N folds,and datafrom Martin et al. [1990]; Sidewinder amphibolitefacies metamorphism in the northem Mountain data from Karish et al. [ 1987] and E. areaare considered coeval with highlyschistose Miller [1981];1983 DNAG timescale from Palmer andlocally mylonitic fabric and greenschist facies [1983].Designation of shearzones in thenorthern metamorphismin the southern area on the basis that andcentral areas as Cretaceous is dependenton the this deformation occurred before the intrusion of ageof thesheared peraluminous granite in the 148 Ma? homblendediorite and 151 Ma granite, northernarea. The shearzones are younger than but afterthe intrusion of Jurassicgneissic tonalite thesheared hornblende diorite, which is probably andextrusion of JurassicHodge volcanic series. 148 Ma, andolder than undeformed, northwest 384 Boettcherand Walker: GeologicEvolution of Iron Mountain with quartzpebbles that elsewhere have been interpreted as layerson eitherside of the contact.D•es of biotitegranite accumulatingin extensionalbasins along the early Mesozoic that intrude the hornblende diorite in the central area are also volcano-plutonicarc [Busby-Spera,1988]. Late Triassic(?)to mylonitic,with subparallelorientations to othermylonites in Middle Jurassicvolcanic and sedimentary rocks in thecentral thearea, indicating that tectonic modification of thecontact Mojave Desertoccur in a roughlyE-W trendingband from between the northern and central areas occurred after the Quartzite Mountain to the Rodman Mountains. Volcanic intrusionof the biotite granitedikes into the dioritecomplex. rocksof thisage are alsocommon in thenortheastern Mojave The gabbrodikes, biotite granite dikes, and the leucocratic Desert[Burchfiel and Davis, 1981]. An alternativehypothesis biotitegranite cross-cut the transposedcompositional layering to extensionalong the arc involvescycles of intermittent in the metasedimentarysequence and do not containtight to volcanismfollowed by erosionand redeposition of the isoclinalfolds. The absenceof distributedamphibolite or volcanicsequence with periodic burial by eolian[?]quartz greenschistfacies mineral assemblages outside of the sand. myloniticshear zones in thebiotite granite and hornblende diorite indicates that the intrusion of these units occurred after JurassicContractional Deformation regionalmetamorphism of the northernand southern areas. Thereforewe concludethat they were intruded and deformed Walkeret al. [1990a]have proposed that the east Sierran followingthe initial deformationof thegneissic tonalite, contractilebelt [Dunne,1986, Figure 1] is expressedas injectiongneiss, and metasedimentary units. southeastto westvergent structures that occur in a beltfrom Cretaceousregional deformation is not documentedin the the CroneseHills (50 km northeastof Barstow,California) to centralMojave Desert but is well documentedin theOld the ShadowMountains (30 km southwestof Barstow, Woman and Piute Mountains [C.F. Miller et al., 1982; Fletcher California)(Figure 2). In the CroneseHills, a ductileshear andKarlstrom, 1990] in the easternMojave Desert, in the zonewith severalkilometers of offset,greenschist facies Mafia fold and thrust belt in westernArizona [Laubachet al., metamorphism,and macroscopic isoclinal folds formed 1989], in the Rand Mountains[Jacobson, 1990] andat between169-154 Ma, basedon dateson pretectonic and FremontPeak [J.S. Miller et al., 1992] in the northeastern posttectonicplutons [Walker et al., 1990a]. In the Shadow MojaveDesert, andat CajonPass north of theSan Andreas Mountains,a macroscopic,west vergent, recumbent anticline, Fault [Ehlig, 1988;Silver et al., 1988]. schistosefabric and transposedcompositional layering formed prior to the intrusionof an undeformed148 Ma gabbro Eugeoclinal/MiogeoclinalBoundary complex[Martin andWalker, 1990, 1991]. Penetrative deformation in the southern area of Iron Investigationof metasedimentaryrocks at IronMountain Mountain occurred between ~164 Ma and 151 + 11 Ma. providesnew data on thelocation of the Roughlyequivalent deformation in the northernarea of Iron eugeoclinal/miogeoclinalboundary in thecentral Mojave Mountain occurred before the intrusion of Late Jurassic Desert. Allochthonouseugeoclinal rocks in thenorthwestern hornblendediorite and afterthe intrusionof Jurassicgneissic MojaveDesert are thought to havebeen transported south tonalite(probably between 170 and 148 Ma). Direct evidence fromthe Antler flysch belt along the truncated margin of the for ductilethrust faults at IronMountain is absent,but F1N Cordilleranmiogeocline during the late Paleozoic [Burchfiel foldsare probably similar in ageto deformationin theCronese and Davis, 1975, 1981; Davis et al., 1978;Walker, 1988]. Hills andShadow Mountains. The decreasein grainsize Alternatively,Snow [1992] has proposed that eugeoclinal reductiontoward the southeastin theHodge volcanic series rocksare carded in thehanging wall of thePermian Last andthe decreasein metamorphicgrade from north to south Chancethrust system. Eugeoclinal rocks have been identified leavesopen the possibility that the metasedimentarysequence in the Goldstonearea [Miller and Sutter,1982], in the Paradise wasthrust over the Hodgevolcanic series before the intrusion Range[Wust, 1981] and at LaneMountain [McCulloh, 1952]. andsheafing of the centralplutonic complex. Miogeoclinalrocks have been identified in theVictorville area Kiser [1981] reportedsillimanite + K-feldsparassemblages [Stewartand Poole, 1975;Miller, 1977, 1981a,b], in the andquartz + hornblende+ garnet+ plagioclaseassemblages in HinkleyHills [Kiser,1981], and in theShadow Mountains metasedimentaryrocks of theHinkley Hills. Thissuggests [Martin andWalker, 1990, 1991] (Figure2). The large thatregional metamorphism affected both the HinkleyHills proportionof marbleand quartzite relative to calc-silicate and the northernarea of Iron Mountainprior to the intrusionof rocksand pelitic schists at Iron Mountainsuggests that the the Mesozoicgabbro complex, which is distinctly metasedimentarysequence there was deposited in a metamorphosedonly in the myloniticshear zones. The miogeoclinalor cratonmargin setting [Boettcher, 1990b]. The presenceof K-feldsparin metasedimentaryrocks of the tectonicboundary between eugeoclinal and miogeoclinal rocks Hinkley Hills suggeststhat temperatures were higher there thus lies to the north and west of Iron Mountain. thanat Iron Mountain,where muscovite appears as a stable phasein the amphibolitefacies assemblage. The presenceof CentralMojave Metamorphic Core Complex kinked and folded biotite and sillimanite at Iron Mountain suggeststhat Jurassic deformation may haveoutlasted peak Finally, Walker et al. [ 1990b]have described Iron Mountain metamorphicconditions. aspossibly lying in thefootwall of thecentral Mojave metamorphiccore complex. Mylonitic fabrics at Iron CretaceousDeformation Mountainare no younger than 83 + 1 Ma,northwest trending, muscovite-garnetgranite dikes and, therefore, are unrelatedto Field evidencereveals that the myloniticshear zones in the EarlyMiocene extension in thecentral Mojave Desert. High- northernand central areas originated after the intrusion of Late anglenormal faults and Tertiary strata, common to hanging Cretaceous(?)peraluminous granite, but before the intrusion of wall blockselsewhere in thecentral Mojave Desert, are absent 83 + 1 Ma, northwesttrending, peraluminous granite dikes. at IronMountain. The lackof Tertiaryextensional features at Ultramylonitealong the contactbetween the central and IronMountain, and elsewhere in thewestern Mojave Desert northern areas and the linear nature of this contact are of [Bartleyet al., 1990],suggests that Iron Mountainis located tectonicorigin. Bothhornblende diorite and tonalite show a westof thearea of Tertiaryextension in thecentral Mojave highdegree of grainsize reduction and thinly spaced planar Desert.If so,the breakaway zone for Tertiarydetachment Boettcherand Walker: Geologic Evolution of IronMountain 385 faultsin thecentral Mojave Desert, prior to lateCenozoic southern area to the interval 151-164 Ma. Deformation in the erosion, is located east of Iron Mountain and west of the northern area of Iron Mountain occurred before the intrusion westernmostknown exposures of Tertiarymylonitic rocks in of Late Jurassichornblende diorite (148 Ma?) and after the theHinkley Hills andHarper Dry Lake. intrusionof Middle (?) Jurassicgneissic tonalite (170 Ma?). Resultsfrom reconnaissancethermochronologic studies at This deformationis interpretedto lie in a belt of Middle to Iron Mountainare compatiblewith the interpretationthat Iron Late Jurassic contractile deformation that runs northeastward Mountainlies westof the main zoneof Tertiaryextension in acrossthe MojaveDesert and forms the continuation of the thecentral Mojave Desert. Specifically, biotite 40Ar-39Ar east Sierran contractile belt. andsphene, zircon, and apatite fission track analyses suggest A secondepisode of deformationat Iron Mountainproduced 0.1 to 10 m wide, subverticalmylonitic shear zones and thatIron Mountaincooled slowly from greater than ~300øC at greenschistfacies metamorphism in thenorthern and central ~65 Ma to lessthan ~100øC by ~30 Ma [Dokkaet al., 1991; areas.U-Pb geochronologyon posttectonicdikes and Joneset al., 1990]. Thus,no significanttectonic unroofing assignmentof deformedperaluminous granite to the80-110 occurredat Iron Mountainduring the period of maximum Ma periodof Sierranmagmatism constrains this deformation extension(22-20 Ma) in the centralMojave Desert. to the interval 83-110 Ma.
CONCLUSIONS Finally,evidence for Tertiaryextension is absentat Iron Mountain,possibly indicating that Iron Mountainlies southwestof the breakawayzone for thecentral Mojave Metasedimentaryrocks in thenorthem area of Iron metamorphiccore complex. Mountainare probably miogeoclinal in characterand are relatedto passivemargin sedimentation along the continental marginof westernNorth America during Late Precambrian to Acknowledgments.Financial support during field study earlyPaleozoic time. The metasedimentarysequence in the wasprovided by a MacCarthyFellowship from the University northernarea likely predatesthe Wood Canyon Formation and of NorthCarolina at ChapelHill andgraduate student research is similarto a sequenceof dominantlydolmnitic marble with awardsfrom theGeological Society of America-Southeastern lesseramounts of calc-silicateand quartzite at Quartzite Sectionand Sigma Xi. Additionally,National Science Mountain,near Victorville. The presenceof miogeoclinal Foundationgrants EAR-8817076 and EAR-8917291 awarded rocksat Iron Mountain indicatesthat the eugeoclinal/ to A.F. Glazner and EAR-8816628 and EAR-8916802 miogeoclinalboundary in thecentral Mojave Desert and the awardedto Walker supportedthis project. A.F. Glazneris proposedCretaceous Mojave-Snow Lake fault mustpass north particularlythanked for initiatingthis study, sharing ideas in and west of Iron Mountain. thefield, andfor significantcomments on the manuscript. Penetrative deformation in the northern and southern areas Reviewsof the manuscriptby K.G. Stewart,J.R. Butler,and of Iron Mountainproduced greenschist to amphibolitefacies M. Cloosgreatly improved the final products of thisproject. metamorphism,transposition of compositionallayering, Additionalreviews by P. Stoneand an anonymousreviewer centimeter-to meter-scaleclose- to isoclinal-typefolds, and an helpedto consolidateand focus the manuscript. Discussions inferredkilometer-scale isoclinal fold. U-Pb geochronology withJ.M. Bartley,M.W. Martinand J.M. Fletcheraided in on posttectonicgranite and the Hodge volcanic series along establishinga regionalcontext. Field assistanceand withregional correlation of theHodge volcanic series with the suggestionsfrom J.A. Roskaand J.S. Miller wereinvaluable to Sidewinder volcanic series constrains deformation in the thisproject.
REFERENCES
Bartley,J.M., A.F. Glazner,and E.R. PrenticeHall, EnglewoodCliffs, N.J., batholith,California, J. Geophys.Res., 87, Schermer, North-south contraction of the 1981. 4761-4784, 1982. Mojaveblock andstrike-slip tectonics in Busby-Spera,C.J., Speculative tectonic Davis, G.A., J.W.H. Monger, andB.C. southernCalifornia, Science,248, 1398- modelfor the early Mesozoicarc of the Burchfiel, Mesozoic construction of the 1401, 1990. southwest Cordilleran United States, Cordilleran"collage" central British Boettcher, S.S., Structureof sheared Geology,16, 1121-1125,1988. Colombia to central California, in Mesozoic rocks at Iron Mountain, central Busby-Spera,C.J., J.M. Mattinson,N.R. MesozoicPaleogeography of the Western Mojave Desert,California, Geol. Soc. Am. Riggs,and E.R. Schermer,The Triassic- UnitedStates, edited by D.G. Howell, and Abstr.Programs, 22, A9, 1990a. Jurassicmagmatic arc in theMojave- K.A. McDougall,vol. 2, pp. 1-32, Society Boettcher,S.S., Structureand petrology of Sonoran Deserts and the Sierran-Klamath of EconomicPaleontologists and Iron Mountain,central Mojave Desert, region:Similarities and differences in Mineralogists,Tulsa, Okla., 1978. California,M.S. thesis,University of North paleogeographicevolution, in Paleozoic Dibblee,T.W., Preliminarygeologic map of Carolinaat ChapelHill, 94 pp., 1990b. andEarly MesozoicPaleogeographic the Barstowquadrangle, California, U.S. Boettcher,S.S., and J.D. Walker, Mesozoic Relations: Sierra Nevada, Klamath Geol. Surv. Mineral Invest. Field Stud. deformation at Iron Mountain, central Mountains, and Related Terranes,edited Map MF-229, 1960. MojaveDesert, California, Geol. Soc. Am. by D.S. Harwood,and M.M. Miller, Dibblee,T.W., Areal geologyof thewestern Abstr.Programs, 22, A275, 1990. Boulder,Colorado, Geol. Soc.Am. Spec. Mojave Desert,California, U.S. Geol.Surv. Bowen,O.E., Jr., Geologyand mineral Pap.,225, pp. 325-337,1990. Prof. Pap. 522, 153 pp., 1967. depositsof the BarstowQuadrangle, Cameron,C.S., Stratigraphyand significance Dibblee,T.W., Geologyof the FremontPeak BulletinCalif. Div. of MinesGeol., 165, 7- of theupper Precambrian Big BearGroup, andOpal Mountainquadrangles, 185, 1954. in Geologyof SelectedAreas in theSan California,Bulletin Calif. Div. of Mines Burchfiel, B.C, and G.A. Davis, Nature and BernardinoMountains, Western Mojave Geol., 188, 64 pp., 1968. controlsof Cordilleraorogenesis, western Desert, and SouthernGreat Basin, Dokka, R.K., D.J. Henry, T.M. Ross,A.K. United States: Extensions of an earlier California,edited by J.P.Cooper, B.W. Baksi, J. Lambert, C.J. Travis, S.M. Jones, synthesis:Am. J. Sci.,275A, 363-395, Troxel,and L.W. Wright, pp. 5-20, 78 th C. Jacobsen,M.M. McCurry,M.O. 1975. AnnualMeeting of theCordilleran Section, Woodbume,J.P. Ford, Aspects of the Burchfiel,B.C., andG.A. Davis, Mojave GeologicalSociety of America,Fresno, Mesozoicand Cenozoic geologic evolution Desert and environs, in The Geotectonic Calif., 1982. of the Mojave Desert,Guidebook for the Developmentof California,Rubey Syrup., Chen, J.H., and J.G. Moore, Uranium-lead 1991 AnnualMeeting of the Geological vol. 1, editedby W.G. Ernst,pp. 217-252, isotopicages from the SierraNevada SocietyofAmerica, pp. 143, Geological Societyof America,San Diego, Calif., determinations,Geochim. Cosmochim. Desert,California, EOS, 73,574, 1992. 1991. Acta, 37, 485-494, 1973. Miller, W.J., Geologyof partsof theBarstow Dunne,G.C., Geologicevolution of the Krogh,T.E., Improvedaccuracy of U-?b Quadrangle,San Bernardino County, southernInyo Range,Darwin Plateau,and zirconages by thecreation of more California,Calif. J. Minesand Geol., 73- Argusand Slate Ranges, east-central concordantsystems using an air abrasion 112, 1944. California: An overview,Geological technique,Geochim. Cosmochim. Acta, 46, Palmer,A.R., (compiler),The decade of Societyof AmericaCordilleran Section 637-649, 1982. NorthAmerican geology 1983 geologic Field Trip Guidebook,pp. 3-21, Geological Lahren, M.M., R.A. Schweickert,J.M. time scale,Geology, II, 503-504,1983. Societyof America,Los Angeles,1986. Martinson, and J.D. Walker, Evidenceof Parrish,R.R., An improvedmicro-capsule for Dunne,G.C., andJ.D. Walker,New age uppermostProterozoic to LowerCambrian zircon dissolution and U-Pb constraints on Jurassic volcanism and miogeoclinalrocks and the Mojave-Snow geochronology,Isotope Geosci., 66, 99- tectonism,southern Owens Valley area, Lake fault: Snow-Lakependant, Central 102, 1987. east-centralCalifornia, Geol. Soc. Am. Sierra Nevada, California, Tectonics,9, Parrish,R.R., U-Pb dating of monaziteand its Abstr.Programs, 23, A248-A249, 1991. 1585-1608, 1990. applicationto geologicproblems, Can. J. Dunne, G.C., R.M. Gulliver, and A.G. Laubach,S.E., S.J.Reynolds, J.E. Spencer, Earth Sci., 27, 1431-1450,1990. Sylvester,Mesozoic evolution of rocksof andS. Marshak,Progressive deformation Silver,L.T., E.W.James, and B.W. Chappel, theWhite, Inyo, Argusand Slate Ranges, andsuperposed fabrics related to Petrologicaland geochemical easternCalifornia, in Mesozoic Cretaceouscrustal underthrusting in investigationsat the Cajon Pass deep Paleogeographyof the WesternUnited western Arizona, U.S.A., J. Struct. Geol., drillhole,Geophys. Res. Lett., 15, 961-964, States,edited by D.G. Howell, andK.A. 11,735-749, 1989. 1988. McDougall,vol. 2, pp. 189-208,Society of Ludwig,K.R., Plottingand regression Snow,J.K., Large magnitude Permian EconomicPaleontologists and programsfor isotopegeochemists for use shorteningand continental margin tectonics Mineralogists,Tulsa, Okla.,1978. with HP-86/87 microcomputers,U.S. Geol. in the southernCordilerra, Geol. Soc. Am. Ehlig,P.C., Characterof basementrocks Surv.Open File Rep.83-849, 94 pp., 1983. Bull., 104, 80-105, 1992. exposednear the CajonPass scientific drill Martin, M.W., and J.D. Walker, New Stacey,J.S., andJ.D. Kramers, hole,Geophys. Res. Lett., 15,949-952, stratigraphicand structural relationships Approximationof terrestrial lead isotope 1988. from the Shadow Mountains, western evolutionby a two-stagemodel, Earth Fletcher, J.M., and K.E. Karlstrom, Late MojaveDesert, Geol. Soc. Am. Abstr. Planet.Sci. Lett., 26, 207-221, 1975. Cretaceousductile deformation, Programs,21,268, 1989. Steiger,R.H., andE. Jager,Subcommission metamorphismand plutonism in thePiute Martin, M.W., and J.D. Walker, New on geochronology:Convention on theuse Mountains,eastern Mojave Desert, J. stratigraphicrelationships from the Shadow of decayconstants in geo-and Geophys.Res., 95,487-500, 1990. Mountains,western Mojave Desert: cosmochronology,Earth Planet.Sci. Lett., Fleuty,M.J., The descriptionof folds,Proc. Implicationsfor late Paleozoic 36, 359-362, 1977. Geol. Assoc.London, 75, 461-492, 1964. paleogeography,Geol. Soc. Am. Abstr. Stewart,J.H., UpperPrecambrian and Lower Glazner,A.F., J.D. Walker, J.M. Bartley,and Programs,22, 64, 1990. Cambrian strata in the southernGreat S.B. Dent, Distribution and relationsof Martin, M.W., andJ.D. Walker,Upper Basin,California andNevada, U.S. Geol. Precambrian and Paleozoic rocks in the Precambrianto Paleozoicpaleogeographic Surv.Prof. Pap. 620, 206pp., 1970. centralMojave Desert,California, Geol. reconstructionof the Mojave Desert, Stewart,J.H., andF.G. Poole,Extension of Soc.Am. Abstr. Programs, 20, 163, 1988. California,in PaleozoicPaleogeography of theCordilleran miogeosynclinal belt to the Graubard,C.M., J.M. Mattinson,and C.J. the WesternUnited States-11, edited by J.D. SanAndreas Fault, southern California, Busby-Spera,Age of the lowerSidewinder Cooper,and L.H. Stevens,Society of Geol.Soc. Am. Bull., 86, 205-212,1975. volcanicsand reconstruction of theearly EconomicPaleontologists and Walker, J.D., Permian and Triassicrocks of Mesozoicarc in the MojaveDesert, Geol. Mineralogists,vol. 67, pp. 167-192,Tulsa, theMojave Desert and their implications Soc.Am. Abstr.Programs, 20, A274, 1988. Okla. 1991. for thetiming and mechanism of Hazzard,J.C., Paleozoic section in theNopah McCulloh,T.H., Geologyof the southernhalf continentaltruncation, Tectonics, 7, 685- andResting Springs Mountains, Inyo of theLane Mountainquadrangle, 709, 1988. County,California, Calif. J. Mines Geol., California,Ph.D dissertation,180 pp. Univ. Walker,J.D., M.W. Martin,J.M. Bartley, and 33, 273-339, 1937. of Calif., Los Angeles,Calif., 1952. D.S. Coleman,Timing and kinematics of Jacobson,C.E., The 40Ar/39Ar Miller, C.F., K.A. Howard, and T.D. Hoisch, deformationin theCronese Hills, geochronologyof the PelonaSchist and Mesozoicthrusting, metamorphism, and California,and implications for Mesozoic related rocks, southernCalifornia, J. plutonism,Old Woman-PiuteRange, structureof the southwestCordillera, Geophys.Res., 95, 509-528, 1990. southeasternCalifornia, in Mesozoic- Geology,18, 554-557, 1990a. Jennings,C.W. (compiler),Geologic map of CenozoicTectonic Evolution of the Walker,J.D., J.M. Bartley,and A.F. Glazner, California,Calif. Geol.Data Map Ser., ColoradoRiver Region,California, Large-magnitudeMiocene extension in the map2, scale1:750,000, Calif. Div. Mines Arizona,and Nevada, edited by E.G. Frost, centralMojave Desert: Implications for and Geol., 1977. andD.L. Martin, GeologicalSociety of Paleozoicto Tertiary paleogeography and Jones,S.M., A.K. Baksi, and R.K. Dokka, America,pp. 561-582, 1982. tectonics,J. Geophys.Res., 95, 557-569, 1990b. Coolinghistories of upperand lower plate Miller, E.L., Geologyof the Victorville rocksin metamorphiccore complexes of region,California, Ph.D. dissertation,226 Walker,J.D., M.W. Martin,J.M. Bartley, and the Mojave ExtensionalBelt, California, pp., Rice University,Houston, Texas, A.F. Glazner, Middle to Late Jurassic Geol. Soc.Am. Abstr.Programs, 22, A33, 1977. deformationbelt through the Mojave 1990. Miller, E.L., Geologyof theVictorville Desert,Geol. Soc. Am. Abstr. Programs, Karish, C.R., E.L., Miller, and J.F. Sutter, region,California, Part I, Geol.Soc. Am. 22, 91, 1990c. Mesozoictectonic and magmatic history of Bull., 92, 160-163, 1981a. Winkler,H.G.F., Petrogenesisof the centralMojave Desert, in Mesozoic Miller, E.L., Geologyof theVictorville metamorphicrocks, 348 pp., Springer- rocksof southernArizona and adjacent region,California, Part II, Geol.Soc. Am. Verlag,New York, 1979. regions,edited by W. Dickinson,and M. Bull., 92, 554-608, 1981b. Wust,S.L., Geologyand structure of the Klute,Ariz. Geol.Soc. Dig., 18, 15-32, Miller, E.L., and J.F. Sutter, Structural ParadiseRange, San Bernardino County, 1987. geologyand 40Ar/39Ar geochronology of CaliforniaM.S. thesis,69 pp.,Stanford University,Stanford, Calif., 1981. Kiser,N.L., Stratigraphy,structure and the Goldstone-Lane Mountain area, metamorphismin theHinkley Hills, MojaveDesert, California, Geol. Soc. Am. S.S. Boettcher,Department of Geological Barstow,California, M.S. thesis,70 pp., Bull., 93, 1191-1207, 1982. Sdences,University of Texasat Austin,Austin, TX 78712 StanfordUniversity, Palo Alto, Calif., Miller, J.S.,J.M. Fletcher,S.S. Boettchcr, J.D.Walker, Department of Geology,University 1981. M.W. Martin, A.F. Glazner, andUNC of Kansas,Lawrence, KS 66045-2124. Krogh,T.E., A low-contaminationmethod for AdvancedField Camp,Late Cretaceous (ReceivedFebruary 10, 1992; hydrothermald•omposition of zirconand deformation,plutonism, and cooling revisedSeptember 16, 1992; extractionof U andPb for isotopicage aroundFremont Peak, central Mojave acceptedOctober 9, 1992.)