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 12oø 116ø 112ø ' ' I' ' 36 ø :::::::::::::::::::::::::::::::::::::::::::::::::::• • 111•.•-•WHDF% •) 34 ø :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ß H TS 32 ø 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