arizpe sub-basin: a sedimentary and volcanic record of basin and...

González-León et al.292

Arizpesub-basin:AsedimentaryandvolcanicrecordofBasinandRangeextensioninnorth-centralSonora,Mexico

Carlos M. González-León1,*, Víctor A. Valencia2, Margarita López-Martínez3, Hervé Bellon4, Martín Valencia-Moreno1, and Thierry Calmus1

1 Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, ApartadoPostal 1039, 83000 Hermosillo, Sonora, Mexico.

2 Geosciences Department, University of Arizona, Tucson, Arizona 85721, USA. 3 Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE),

Km 107 Carretera Ensenada-Tijuana No. 3918, 22860 Ensenada, Baja California, Mexico. 4 UMR 6538, Domaines Océaniques, IUEM, Université de Bretagne Occidentale,

6, Av. Le Gorgeu, Brest, 29285, France. * [email protected]

ABSTRACT

The Arizpe sub-basin located in the northern part of the Río Sonora basin is a Basin and Range half-graben that initiated during Late Oligocene time in north-central Sonora. Its ~2.1 km-thick, east-dipping volcanic and sedimentary fill assigned to the Báucarit Formation is divided, from base upwards, into the following informal members. The La Cieneguita member composed of interbedded conglomerate, siltstone and gypsum beds which unconformably overlay older Cenozoic volcanic rocks; the El Toro Muerto basalt composed of basalt flows, basalt breccia and subordinate conglomerate beds; the Arzipe conglomerate composed of three fining-upwards conglomerate sequences that interdigitates with flows of the Tierras Prietas basalt in its lower part and the Agua Caliente basalt in its upper part; the Bamori member is a coarsening-upward succession of siltstone, sandstone and conglomerate that unconformably overlies the Arizpe conglomerate and it is unconformably overlain by the sedimentary El Catalán breccia. Basin accommodation started at ~25 Ma when deposition of the La Cieneguita member, followed by alkaline basaltic volcanism of the El Toro Muerto and contemporaneous rhyolitic volcanism, floored the area predating significant clastic deposition. The Agua Caliente basalt (~21 Ma ) in the upper part of the basin fill indicates the basin was rapidly subsiding.

Multiple phases of normal faulting affected the Arizpe sub-basin. The main controlling structure may be the steep (80°), west-dipping, sub-parallel El Fuste and Granaditas normal faults that bound the Arizpe sub-basin at its present-day eastern margin, or there may be a fault or faults that were subsequently buried beneath younger basin fill near the eastern margin of the basin. The basin was disorganized by an even younger NW-SE phase of normal faulting represented by the southwest-dipping Crisanto and Tahuichopa faults. Growth strata within basin fill suggests that syntectonic deposition was active during all phases of normal faulting. However, punctuated tectonic activity on these faults may have controlled deposition of conglomerate sequences of the Arizpe conglomerate.

Geochemical data from the El Toro Muerto, the Tierras Prietas and the Agua Caliente basalt members indicate they are high-K, alkaline to subalkaline basaltic trachyandesites with light REE-enriched patterns, initial Sr ratios between 0.7069 and 0.7076, and εNd values between -3.76 and -4.88. Pb isotopic values from two samples of the El Toro Muerto basalt yielded very similar results, and along

Revista Mexicana de Ciencias Geológicas, v. 27, núm. 2, 2010, p. 292-312

González-León,C.M.,Valencia,V.A.,López-Martínez,M.,Bellon,H.,Valencia-Moreno,M.,Calmus,T.,2010,Arizpesub-basin:AsedimentaryandvolcanicrecordofBasinandRangeextensioninnorth-centralSonora,Mexico:RevistaMexicanadeCienciasGeológicas,v.27,núm.2,p.292-312.

Arizpe sub-basin: A sedimentary and volcanic record of Basin and Range extension 293

with the other geochemical data suggest an important participation of the continental lithosphere as magma source for this volcanism.

The data herein reported are supported by eight new geochronologic ages and they contribute to better document and constrain the timing of magmatism and extension in the Basin and Range tectonic province in Sonora.

Key words: magmatism, geochronology, Oligocene, Miocene, Basin and Range, Sonora, Mexico.

RESUMEN

La subcuenca de Arizpe, localizada en la parte norte de la cuenca del Río Sonora, es un medio graben que empezó a formarse durante el Oligoceno Tardío asociado a la deformación de Sierras y Valles Paralelos (Basin and Range). Su relleno volcánico y sedimentario de ~2.1 km de espesor, el cual buza hacia el oriente, se asigna a la Formación Báucarit y se divide, de la base a la cima, en los siguientes miembros informales. El miembro La Cieneguita, formado por conglomerado con intercalaciones de limolita y yeso, que sobreyace discordantemente a rocas volcánicas cenozoicas más antiguas; el basalto El Toro Muerto, formado por derrames de basalto, brecha basáltica y en menor proporción por conglomerado. El conglomerado Arizpe, formado por tres secuencias conglomeráticas grano decreciente hacia su cima y con interdigitaciones del basalto Tierras Prietas en su parte inferior y del basalto Agua Caliente en su parte superior. El miembro Bamori, formado por limolita, arenisca y conglomerado en secuencia grano-creciente hacia su cima, sobreyace en discordancia al conglomerado Arizpe y está a su vez sobreyacido discordantemente por la brecha El Catalán formada por clastos de basalto. La subcuenca Arizpe empezó a formarse hace ~25 Ma cuando la sedimentación terrígena del miembro La Cieneguita y el volcanismo alcalino del basalto El Toro Muerto precedieron a la sedimentación clástica del conglomerado Arizpe. La edad de ~21 Ma obtenida del basalto Agua Caliente, que ocurre en la parte superior del relleno de la cuenca, indica que ésta fue una cuenca de subsidencia rápida.

El fallamiento normal que inició a la cuenca tuvo lugar cerca de su actual margen oriental y controló el depósito de las secuencias conglomeráticas. En esa posición se ubican las fallas normales El Fuste y Granaditas que buzan al poniente, pero es probable que la falla principal se encuentre actualmente cubierta debajo de rocas más jóvenes del relleno de la cuenca. El fallamiento sinsedimentario fue importante y, después de su formación, la sub-cuenca Arizpe fue desorganizada por fallamiento normal de rumbo NW-SE representado por las fallas Crisanto y Tahuichopa.

Datos geoquímicos de los basaltos El Toro Muerto, Tierras Prietas y Agua Caliente indican que corresponden a traquiandesitas basálticas ricas en K, que varían de alcalinas a subalcalinas con un patrón de enriquecimiento en tierras raras ligeras, con relaciones isotópicas de Sr inicial entre 0.7069 y 0.7076 y valores iniciales de εNd entre -3.76 y -4.88. Análisis isotópicos de Pb en dos muestras del basalto El Toro Muerto arrojan valores muy parecidos y, en conjunto, estos datos sugieren una participación importante de la litósfera continental como fuente de los magmas de este volcanismo.

Los datos reportados se apoyan además en ocho nuevos fechamientos y contribuyen a documentar el tiempo del magmatismo y la extensión de la provincia tectónica de la Basin and Range en Sonora.

Palabras clave: magmatismo, tectónica, geocronología, Oligoceno, Mioceno, Basin and Range, Sonora, México.

INTRODUCTION

AlthoughmostofthestateofSonoraislocatedwithinthelateCenozoicsouthernBasinandRangeextensionalprovince and the sedimentary and volcanic fill of numer-ousNNW-SSE-orientedbasinsthatformedduringthistectoniceventarewellexposed,onlyafewstudieshavebeenconductedtounderstandtheirsedimentary,magmaticand tectonic history. King (1939) first noted that the fills of thesevalleysinsouthernSonoraaremostlycomposedofalowerbasalticmemberandanupperconglomeratememberthathenamedastheBáucaritFormationinoutcropsatthetownofBáucarit(Figure1).

Otherworkerssuggested thatnotallof the lateCenozoicextensionalbasinsinSonorasharesimilarsedi-mentaryandmagmatichistories(Gans,1997;McDowellet al.,1997)andapplieddifferentinformalnamestotheirfill deposits. The first attempts to understand the sedimen-taryhistoryofsomeofthesebasinswereconductedbyMiranda-GascaandDeJong(1992)andDelaO-Villanueva(1993),whostudiedtheMagdalenaandtheRíoYaquibasins(Figure1),respectively.OtherauthorsstudiedtheUresbasinincentralSonora(Calles-Montijo,1999;Vega-GranilloandCalmus,2003).McDowellet al.(1997)andGans(1997),however,studiedinmoredetailthemagma-tismandtectonicdevelopmentofthelateCenozoicbasins


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itsstratigraphicandstructuralrelationshipstobeeasilystudied.Ouraimistocontributetotheunderstandingofthesedimentaryandmagmatichistoryofatypical,com-plexly-deformedgrabenlocatedwithintheBasinandRangeprovinceofSonorabasedongeologicmapping,detailedmeasurementoftheentirebasinstratigraphicsection,andongeochronology,geochemicalandradiogenicisotopicanalyses of some of the interbedded volcanic flows.

GEOLOGIC SETTING

TheArizpesub-basinisaN-S-strikinghalf-grabenfilled by a Late Oligocene-Early Miocene, eastward thick-ening succession of interbedded mafic flows, subordinate ash-falltuffsandignimbrite,andclasticrocksthatGonzález-Leónet al. (2000) first assigned to the Arizpe conglomer-ate.ThebasinwasdevelopedoverabasementcomposedofstrataassignedtotheLowerCretaceousBisbeeGroupandtoUpperCretaceous-CenozoicvolcanicrocksoftheTarahumaraFormation(González-Leónet al.,2000)(Figure2). A rhyolitic unit with minor basalt flows that crops out

intheRíoYaquiregionandnearbyareasofsouthcentralSonora. Their results indicate that the fills of the Basin and Range basins are dominated by mafic flows in their lower partsandbyconglomerateintheirupperparts,andinmostcasesthenameBáucaritFormationwasappliedtodescribethatsuccession.

TheRíoSonorabasinisaninformalnamethatweapplytooneoftheseN-S-elongatedBasinandRangegra-benslocatedinthenorth-centralpartofSonora,betweenthetownsofArizpeandMazocahui(Figure1).Thisbasinis100km-longand15to25km-wide.OurresultsindicatethattheRíoSonorabasinisatectonicallyandsedimen-tologicallycomplexhalf-grabenthatislimitedtotheeastandwestbyelevatedmountainsrangescomposedofsedi-mentary,volcanicandplutonicrocks,whichrangeinagefromProterozoictoMiocene.Atitsnorthernandsouthernboundaries,theRíoSonorabasinisinfaultcontactwithhighmountainousranges.

Inthiswork,wepresentresultsforthenorthernmostpartoftheRíoSonorabasinlocatednearthetownofArizpe,whichwenametheArizpesub-basin.Thewell-exposedsedimentary and volcanic fill of this sub-basin allows for

Figure1.LocationmapoftheRíoSonorabasin,andotherBasinandRangeSonoranbasins,andlocalitiesmentionedinthiswork.Tb,Tubutamabasin;Mb,Magdalenabasin;Bb,Bacanuchibasin;Rsb,RíoSonorabasin;Ub,Uresbasin;Sb,Sahuaripabasin;Ryb,RíoYaquibasin;Rb,ElRodeobasin;Bab,Báucaritbasin.BlackrectangleindicatesthelocationoftheArizpesub-basinandofgeologicmapofFigure2;areaenclosedbydashedlineindicatesthelocationofSierraSantaÚrsulaandSierraLibre.


Figure2.GeologicmapoftheArizpesub-basinintheRíoSonorabasin,andaccompanyingstructuralsectionsA-BandC-D.


CerroEl Picacho

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Sinoquipe conglomerateCerro El Picachotrachydacite

Bamori member

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Tierras Prietas basalt

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intheeasternpartoftheareamightbecorrelativewiththeCerroCebadehuachivolcanicrocks,aunitofOligoceneagethatwasreportedfromitsoutcropsabout10kmnorthofthestudyarea(González-Leónet al.,2000).Basedonourmore detailed stratigraphic study, we divide the basin-fill successionintosevenmembers,restrictthetermArizpeconglomeratetooneofthemandreassignthewholesuc-cessiontotheBáucaritFormation.

Frombaseupwards,theBáucaritFormationiscom-posedofthefollowinginformalmembers:LaCieneguitamember,ElToroMuertobasalt,theArizpeconglomeratethat laterally interfingers with the Tierras Prietas and Agua Calientebasaltmembers,theBamorimemberandtheElCatalánbreccia(Figures2and3).

TheBáucaritFormationisexposedintheArizpeval-leybutinthisstudythecontinuousstratigraphicthickness

oftheLaCieneguitamember,ElToroMuertobasaltandtheArizpeconglomerateweremeasuredwithaJacob’sstaffalongtheElToroMuertocreek,betweenranchoLaCieneguitatothewestandthetownofBamoritotheeast(Figure2).TheuppermostpartoftheArizpeconglomeratewasmeasuredalongInterstateroad89fromtheArroyoAguaCalientetothetownofTahuichopa(Figure2).AlongtheElToroMuertosectiontheArizpeconglomerateinterdigitatesinitslowerpartwiththeTierrasPrietasbasalt,andinitsupperpartwiththeAguaCalientebasaltmembers(Figure3).ThethicknessoftheTierrasPrietasandAguaCalientebasaltswereestimatedfromtheirmorecompleteoutcropsalongthearroyoTetuachicreeklocatedabout6kmsouthoftheElToroMuertoCreekandalongthegorgecutbytheRíoSonorajustsouthofthetownofBamori,respectively(Figure2).ThethicknessoftheBamorimemberwasmea-

Figure2(continued).GeologicmapoftheArizpesub-basinintheRíoSonorabasin,andaccompanyingstructuralsectionsA-BandC-D.


Interdigitationsof the Agua

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suredinoutcropslocatedjusteastofthetownofBamori,andthethicknessoftheCatalánbrecciaswasestimatedfromoutcropsalongtheCañadaCatalánlocatedsoutheastofthetownofBamori.A1:50,000-scalegeologicmappingofthestudy area was refined from previous works by González-Leónet al.(2000)andGonzález-Gallegoset al.(2003).ToconstraintheageoftheBáucaritFormationwereportfournewAr-Arages,threeK-AragesandoneU-Pbzirconageofmafic and rhyolite rocks. These data are complemented with twoAr-AragesreportedbyGonzález-Leónet al.(2000)fromthestudyarea,asdiscussedbelow.Additionally,weincludesevennewgeochemicalanalysesandthreeSr-Nd-Pb

isotopicanalysesofvolcanicrockswhicharecomplementedwithpreviousgeochemicaldatareportedinGonzález-Leónet al.(2000).

STRATIGRAPHY

TheLa Cieneguita member (Figures2and3)consistsinitslowerpartofa60m-thickpackageofpebblecon-glomerateandcoarse-grainedsandstonethatstratigraphi-callygradesupwardsintoa50m-thickintervalofreddishbrownmudstone-siltstone,withinterbeddedgypsum(beds

Figure3.a)StratigraphiccolumnoftheBáucaritFormationintheArizpesub-basinshowingthepositionofdatedsamples.Theplottotherightshowsthe size in cm of the largest measured clasts, and defines three fining-upward sequences in the Arizpe conglomerate. b) Non-decompacted subsidence curveoftheBáucaritFormationintheArizpesub-basin.


< 5 cm thick) and granule to fine-grained sandstone (thin to mediumbeds).ThismemberunconformablyrestsonalteredvolcanicrocksassignedtotheTarahumaraFormationoronrhyolitictuffsofprobableOligoceneage(notdifferenti-atedinFigure2).TheLaCieneguitamemberisnotalwayspresentatthebaseoftheArizpeConglomerate,probablybecauseitwaspartlyerodedafterdepositionorbecauseitwasdepositedonanunevenerosionalsurface,andwherethisoccurs,theElToroMuertobasaltunconformablyrestsontheolderrocks.

TheEl Toro Muerto basalt sharplyoverliestheLaCieneguita member, and consists of basalt flows, basalt brecciaandsubordinateconglomeratebeds(Figures2and3). The basalt flows are fine-grained and petrographically classifyasporphyriticolivinebasaltwithphenocrystsofplagioclaseandscarceaugitepyroxeneinapseudotraquiticmatrixofmicroliticandesine(An34-50).Alocal,100m-thickoutcropofarhyoliticweldedtuffexposedthroughafaultcontactinthecentralpartoftheareaandthatweinformallynametheTetuachiignimbrite,isaprobableinterdigitationoftheTierrasPrietasbasalt,accordingtoitsgeochrono-logicage(Figure3).TheElToroMuertobasaltisinturnstratigraphicallyoverlainbytheArizpeconglomerate,buttowardthesouthwesternpartofthestudyareathesemem-bersareinterbedded(Figure2).Theestimatedthicknessofthismemberis200m.

ThestratigraphiccolumnoftheArizpe conglomerate is1,600m-thickalongthemeasuredsectionoftheElToroMuertocreek.ThelowermostpartoftheElToroMuertoconglomerateconsistsofa20m-thickintervalofmassive,greentolightgraysiltstone,whereastheremainderupperpartofthesuccessionispredominantlyconglomeratethatforms three fining-upward sequences distinguished on the basisoftheirmaximumclastsizes.Thesesequencesaredescribedbelowasthelower,middleandupperconglomer-atesequences(Figure3).

Thelowerconglomeratesequenceis340mthick(Figure3)andinitslowerpartconsistsofpoorly-sorted,pebbletocobbleconglomeratewithclasts(mostly<10cmlong)ofbasalt (~60%),andesite(~20%),rhyolite(~15%)andscarcediorite(Figure4a).Thelargestclastsarescatteredcobblesofbasaltupto80cm-long.Afewbedsofwhite,lithic-andcrystal-richash-falltuffsupto3mthick(Figure4b)areinterbeddedwithinthispartofthesection, along with a 50 m-thick flow of the Tierras Prietas basalt.Themiddleandupperpartsofthislowersequencemostlyconsistsofpoorlysorted,subangulartosubroundedpebble-conglomerateinbedsupto3mthick,buttheymorecommonlyoccurinbedslessthan60cmthick,becauseofabundantreactivationsurfaces.Theconglomeratebedsaremostlyclast-supportedandcommonlyhaveinterbedsofthin,lenticular,coarse-grainedtogranulesandstone,whichoccasionally show fining-upward cycles. Planar cross-strati-fication and large-scale trough cross-stratification are locally present.Intheupperpartofthissequence,thelargestclastsareofbasaltanddecreaseinsizeto20cm.

Themiddleconglomeratesequenceis590mthickandalsohasinterdigitationsoftheTierrasPrietasbasalt.Conglomeratebedsinthiscycleareupto3mthickbutabundantreactivationsurfaces(multistory)obliteratetheoriginal thicknesses of the fining-upward cycles. These bedsaremostlypebbleandrarelycobble,poorlysorted,subangularconglomeratewithlocal,poorly-developedpla-nar and trough cross-stratification. The largest clasts in this sequencearescatteredfragmentsofbasaltthatreachupto40cmlonginthelowerpartandareupto6cmlonginitsupperpart.Nearitsupperpart,thesequencehasintervalsup to 30 m thick of well stratified and well consolidated fine- to coarse-grained sandstone with interbedded yellow toreddish,bioturbatedsiltstoneandwhiteash-falltuffsbeds(Figure4c).Thesequenceendswitha110mthickpebbleconglomerate(clasts<5cmlong)withinterbedsofthin-tomedium-bedded, lenticular, fine- to coarse-grained sand-stone.Clastcompositionsinthissequenceareofandesite(~50%),basalt(~35%)andrhyolite(~15%).

Theupperconglomeratesequenceis650mthickandconsists of medium to thick, poorly defined beds of pebble conglomerate,sandypebbleconglomerate,pebblesandstoneandmorerarelycobbleconglomerateinbedsupto4mthick,withlenticularbedsofcoarse-tomedium-grainedsandstone.Bedscomposingfining-upwardsequencescanbepresent,buttheirupperpartsaregenerallycutbyabundantreactivationsurfaces.Theconglomerateismostlyclast-supported,butmatrix-supportedconglomeratealsooccur.Clastsarepoorlysorted,subroundedtosubangular,andconsistofandesite(~50%),rhyolite(~40%)andbasalt(~10%). Planar and trough cross stratification in bedsets up to 40 cm thick and crude horizontal stratification of clasts oc-casionallyoccurs.Thelargestclastsinthelowerpartofthissequenceareofbasaltandreachupto35cminlength.Theuppermost150mofthissequenceconsistsofpoorlysorted,clast-supported,subroundedpebbleconglomerate,witharichmatrixofpebbletogranulesandstonethatinterbedswith fine- to coarse-grained, lenticular sandstone. These lithologiesgradetothetopofthesequencetowell-strati-fied granule and coarse-grained sandstone. Reactivation surfacesareabundantintheupperpartofthesequence,whereassandstonebedsupto50cmthickaremorelaterallycontinuous.Maximumclastsizesinthispartofthesequencereaches10cminlengthandconsistofandesite(~50%),rhyolite(~30%),dacite(~10%)andbasalt(~10%).AtitsmiddleandupperpartsthissequencehasinterdigitationsoftheAguaCalientebasalt.

TheTierras Prietas basalt hasathicknessofabout700mestimatedfromoutcropsalongtheTetuachicreek,locatedsouthoftheElToroMuertocreek(Figure2).Thismemberconsists of superimposed 50 to 100 m-thick, fine-grained flows of basalt and basaltic breccias, which in their upper partsdevelopreddish,alteredintervalsupto20mthickofprobablypedogenicorigin(Figure4d).Petrographically,thisrockisaporphyriticolivinebasaltwithphenocrystsofplagioclase(An38-50)andsubordinateclinpyroxene(au-


Figure4.OutcropphotographsoftheBáucaritFormationintheArizpesub-basin.a)Poorlysorted,pebbletocobbleconglomerateofthelowerpartoftheArizpeconglomeratemember,alongarroyoElToroMuerto.Outcropheightabout35m.b)Lithic-andcrystal-richash-falltuffsinterbeddedinthelowerpartoftheArizpeconglomeratemember.c)Fine-tocoarse-grainedsandstoneandinterbeddedyellowtoreddish,bioturbatedsiltstoneandwhiteash-falltuffsbedsintheupperpartofthemiddlesequenceoftheArizpeconglomeratemember.d)BasalticandbasalticbrecciaoftheTierrasPrietasbasaltdevelopingreddish,alteredintervalsofprobablypedogenicoriginattheirtops.OutcropatTetuachicreek.Heightofhillsinforegroundfrombaseofcreekis~60m.e)Unconformablecontact(yellowline)betweentheBamoriandtheElCatalánmembersintheRíoSonoraeastofthetownofBamori.ElFustefault(thick,dashedblackline)separatestheElCatalánbrecciafrominterbeddedrhyoliteandbasaltofsupposedOligoceneage.f)Outcropofthe crudely stratified El Catalán breccia dipping to the west in hills located just east of the town of Bamori (photograph viewing toward the southwest).


gite?),sortedinarichpseudotraquiticmatrixofmicroliticplagioclase.AlongtheElToroMuertomeasuredsection,flows of the Tierras Prietas basalt laterally interdigitate with thelowerandmiddleconglomeratesequencesoftheArizpeconglomerate(Figure3).

TheAgua Caliente basalt hasanestimatedthicknessof300minoutcropsalongtheRíoSonoragorge,justsouthofthetownofBamori(Figure2).TheAguaCalientebasaltconsists of poorly stratified, fine-grained mafic flows and breccia. The basalt flows display abundant calcite-filled cav-itiesandpetrographicallyshowanabundantpseudotraquiticmatrixofmicroliticplagioclasewithscarcephenocrystsofplagioclase(An39-53),olivineandclynopyroxene.

TheBamori member isa60m-thickclasticintervalthatunconformablyoverliestheupperconglomeratese-quenceinoutcropexposedeastoftheRíoSonora(Figure4e).Itformsathin,E-NEstripoutcropwithshallowdipstotheeast.Itisacoarsening-upwardsuccessionofmassive,bioturbated siltstone and fine grained sandstone in its lower part, and massive, fine-grained sandstone with intercalations ofmedium-bedded,coarse-grainedsandstoneincosetsup to 5 m thick that show fining-upward successions in itsmiddlepart.Theuppermost20mofthismembercon-

sistsofgranulesandstoneandlenticular,medium-beddedpebbleconglomeratethatarethelagsoflarge-scale,epsiloncross-stratification.

TheEl Catalán breccia hasanestimatedthicknessof150mandcropsoutintheeasternmarginoftheArizpebasin. It is a coarse, crudely stratified breccia with clasts up to2mlong(Figure4f).Theclastsareangularandpoorlysorted,andmostbedsarematrix-supported.Thematrixisamixtureofcoarsesandandgranuleswithoccasionalthintomedium,lenticularbedsofgranuleconglomerate.Theclastsaremostlyderivedfrombasalt(90%),buttherearealsoclastsofandesiteandrhyolite.ThismembershallowlydipstothewestandunconformablyoverliestheBamorimemberwhichdipstotheeast.

GEOCHRONOLOGY

AsamplefromthelowermostpartoftheElToroMuertobasaltyieldedaK-Arageat24.89±0.64Ma(sample6-3-05-1;Tables1and2,Figure2),similar(withinerror)toanAr-Ar(plateauage),sanidineageof24.2±0.4MaobtainedfromtheTetuachiignimbrite(sample10-10-

Unit Sample UTM Location Age

Tetuachiignimbrite 10-10-06-1 12R571932E;3345454N 24.2±0.2Ma(Ar/Ar)ElToroMuertobasalt(lowermostpart) 6-3-05-1 12R566338E;3356954N 24.89±0.64Ma(K/Ar,whole-rock)ElToroMuertobasalt 10-9-06-1 12R578845E;3348784N 23.3±0.3Ma(Ar/Ar)ElToroMuertobasalt 2-15-07-1 12R579239E;3345637N, 23.5±0.2Ma(Ar/Ar).ElToroMuerto(uppermostpart) A-101 12R571640E;3352300N 23.52±0.17Ma(Ar/Ar;González-Leónet al.,2000).CerroElPicacho 11-20-07-2 12R583523E;3334488N 23.3±0.6Ma(U/Pb)TierrasPrietasbasalt A-102D 12R573903E;3352512N 21.70±0.20Ma(Ar/Ar;González-Leónet al.,2000).AguaCalientebasalt 6-3-05-3 12R577704E;3354968N 21.29±0.55Maage(K/Ar,whole-rock).AguaCalientebasalt 6-3-05-2 12R579372E;3357108N 21.51±0.6Ma(K/Ar,whole-rock)ElPajaritobasalt 9-23-08-5 12R605576E;3321056N 18.6±0.2(Ar/Ar)

Table1.Summaryofgeochronologicagesofdatedsamplesfromthestudyarea,includingdatapreviouslyreportedbyGonzález-Leónet al.(2000).

1 2 3 4 5 6 7 8Sample

codeLocation Lab.Ref. Mean age

(Ma)Age(Ma)

± error (1 σ)

K2O(wt. %)

40Ar R

(10-7 cm3 g-1)40Ar R

(%)Molten weight

(g)

6-3-05-2 AguaCaliente B7204-1 21.51 21.31 ± 0.63 2.48 17.41 55.7 0.9871B6916-4 21.73 ± 0.61 2.48 17.49 70.9 0.8090

6-3-05-3 AguaCaliente B6944-4 21.29 ± 0.55 2.86 19.75 73.5 0.8195

6-3-05-1 ElTorroMuerto B6921-9 24.89 ± 0.64 1.99 16.08 79.1 0.8342

Column1:Laboratoryreference;BmeansBrestlaboratory,7204isthechronologicalorderofthisexperimentinthewholerangeofargonextractionsdoneinthelaboratory;-1istheorderofthisextractioninafullseriesofexperiments,i.e.,9to10samplesusingthesamemolybdenumcrucible,includ-ingatleastonestandardsampleofglauconiteGLO,foranoverallcalibrationandcontroltest.Column2:Themeanageresultsfromtwoindependentagedeterminations,i.e.,fortwodifferentargonextractionsfromrockgrainsundervacuumandtheirfurtherisotopicanalysesbymassspectrometry.Columns3and4:AgesarecalculatedwiththeconstantsrecommendedbySteigerandJäger(1977)anderrorsarequotedatonesigmalevelafterthecalculationsbyMahoodandDrake(1982).Column5:Kcontentsweremeasuredbyatomicabsorptionspectrometryafterachemicalattackat80°Cofa representative powder sample rock using hydrofluorhydric acid.

Table2.K-AragedataforthreesamplesoftheBáucaritFormationinthestudyarea.



2

1

01.00.80.60.40.20

30

20

10

0

2-15-07-1 groundmass


1

01.00.80.60.40.20

30

20

10

0


Tetuachi ignimbrite

0.05

01.00.80.60.40.20

30

20

10

0

10-10-06-1 sanidine

El Pajarito basalt

0.5

01.00.80.60.40.20

40

30

20

10

0


Fraction of Ar released39 Fraction of Ar released39 Fraction of Ar released39 Fraction of Ar released39

3739

Ar

/ A

rCa

KA

ge in

Ma

t = 23.8 0.2 Mai +t = 22.7 0.4 Mai +

t = 22.4 0.3 Mai +t = 21.8 0.3 Mai + t = 24.0 0.2 Mai + t = 18.6 0.2 Map +

t = 23.5 0.2 Map + t = 23.3 0.3 Map +t = 24.2 0.2 Map +

t = 20.2 0.5 Mai +

t = 23.7 0.2 Ma( Ar / Ar) = 314 18

SumS / (n-2) = 1.2 , n = 10

c

i

++40 36

0.060.040.02

0.003air

0.002

0.001

00

1

23

45

6

7

1

23

45

6

7

39 40Ar/ Ar

3640

Ar/

Ar

0.060.040.02

0.003

0.002

0.001

00

airt = 23.1 0.4 Ma

( Ar / Ar) = 283 20SumS / (n-2) = 1.2 , n = 7

c

i

++40 36

1

2

345

6

1

2

3

45

39 40Ar/ Ar

3640

Ar/

Ar

0.060.040.02

0.003air

0.002

0.001

00

t = 24.1 0.2 Ma( Ar / Ar) = 313 68

SumS / (n-2) = 2.8 , n = 7

c

i

++40 361

23-9

39 40Ar/ Ar

3640

Ar/

Ar

0.100.060.02

0.003air

0.002

0.001

00

t = 18.8 0.3 Ma( Ar / Ar) = 219 19

SumS / (n-2) = 0.8 , n = 5

c

i

++40 36

2

1

8

3

4 76

5

39 40Ar/ Ar

3640

Ar/

Ar

a)

b)

c)

06-1;Figure5,Tables1and3,Appendix).Wealsodatedtwobasalticsamplesfromoutcropsinthesoutheasternpartofthearea,whichyieldedsimilarAr-Ar(plateau)agesat23.3±0.3and23.5±0.2Ma(samples10-9-06-1and2-15-07-1;Figure 5; Tables 1 and 3). These basalt flows underlie, and areinfaultcontactwiththeElCatalánbrecciaandtheAguaCalientebasalt(Figure2)andweinterpretthemtobelongtotheupperpartoftheElToroMuertobasaltonthebasisofasimilarageof23.52±0.17Ma(Ar-Ar,sampleA-101,Table1,Figure3)previouslyreportedbyGonzález-Leónet al.(2000)fromoutcropsofthatunitinthecentralpartofthestudyarea.González-Leónet al.(2000)alsoreporteda21.70 ± 0.20 Ma Ar-Ar age for a basalt flow from the middle partoftheTierrasPrietasbasalt(sampleA-102D,Table1,Figure3).Inthisstudy,weobtainedtwoK-AragesfromtwosamplesoftheAguaCalientebasalt.Thestratigraphi-callylowermostofthesesamplesyieldedanageof21.29±0.55Maage(samples6-3-05-3,Tables1and2),whilethestratigraphicallyuppermostoneyieldedanageof21.51±0.6(sample6-3-05-2,Tables1and2)(Figure3).

Furthermore,wehereinreportaU-Pbzirconageof23.3±0.6Ma(sample11-20-07-2,Tables1and4,Figure6)obtainedfromasampleofarhyolitedomeexposedatCerroElPicacho,locatedat11kmeastofthetownofSinoquipe(Figure2),whichiscontemporaneouswiththe

ElToroMuertobasaltandcouldrepresentthesourcefortheTetuachiignimbrite.Also,wereporta18.6±0.2MaAr-Ar,plateauage(sample9-23-08-5,Tables1and3,Figure5)for a mafic flow informally named the El Pajarito basalt, whichisinterbeddedintheupperpartoftheconglomeraticbasin fill of the El Rodeo basin located east of the Arizpe sub-basin(Figure1);thesedatahelptoconstrainyoungerageofBasinandRangedevelopmentinthisregion.

GEOCHEMISTRY AND ISOTOPIC RESULTS

Inanefforttocharacterizetheoriginandnatureofthesourceofthemagmasofthestudyarea,threesamplesfromtheElToroMuertobasaltandonesamplefromtheAguaCalientebasaltwereanalyzedformajorandtraceelementsandSr-Nd-Pbisotopiccompositions(Tables5and6;analyticalproceduresaredescribedintheAppendix).WealsoanalyzedmajorandtraceelementsfromonesampleoftheCerroElPicachodome,onesamplefromtheElPajaritobasaltandanotheronefromtheElHuarachebasalt(Table 5); this last mafic flows comes from the basal part of the El Rodeo basin fill and we tentatively assume it to be equivalent in age to the El Toro Muerto basalt. The mafic samplesfromtheArizpesub-basinplotmostlyasalkaline

Figure5.40Ar/39Arresults.Samples2-15-07-1and10-9-06-1oftheElToroMuertobasalt,and10-10-06-1oftheTetuachiignimbritearefromthestudyarea.Sample9-23-08-5isfromtheElPajaritobasaltintheElRodeobasin.Foreachsample:a)agespectrum;b)37ArCa/39Arkdiagram;c)36Ar/40Arversus39Ar/40Arcorrelationdiagram.


El Toro Muerto basalt 2-15-07-1 groundmass 1st experiment

pwr F39Ar 40Ar*/39ArK AgeinMa %40Ar* 40Ar/36Ar 37ArCa/39ArK tiinMa tpinMa tcinMa (40Ar/36Ar)i SumS/(n-2)/n

0.25 0.0229 -1.78±1.64 -9.8±9.0 † -2.82 287.40 1.340.50 0.0824 4.29±0.28 23.4±1.5 § 61.58 769.05 1.510.80 0.1403 4.45±0.13 24.3±0.7 § 94.01 4935.11 1.231.10 0.2194 4.26±0.08 23.3±0.5 § 90.49 3107.49 1.021.50 0.1966 4.32±0.07 23.6±0.4 § 96.18 7737.04 0.682.10 0.1970 4.29±0.11 23.4±0.6 § 94.67 5539.12 0.746.00 0.1413 4.19±0.19 22.9±1.0 § 78.02 1344.67 2.34 22.7±0.4 23.5±0.2

El Toro Muerto basalt 2-15-07-1 groundmass 2nd experiment


0.26 0.0174 2.98±0.69 16.3±3.8 † 4.38 309.04 1.240.50 0.0583 4.28±0.12 23.4±0.7 † 33.56 444.78 1.510.80 0.1430 4.47±0.07 24.4±0.4 § 75.17 1189.89 1.381.10 0.2123 4.46±0.07 24.4±0.4 § 91.86 3628.75 1.081.50 0.2939 4.42±0.07 24.2±0.4 § 93.81 4773.23 0.702.10 0.1839 4.32±0.05 23.6±0.3 § 91.37 3424.58 0.796.00 0.0912 4.08±0.09 22.3±0.5 † 45.11 538.32 2.06 23.8±0.2 24.0±0.2 23.7 ± 0.2 ‡ 314±18 1.2/10

El Toro Muerto basalt 10-9-06-1 groundmass 1st experiment


0.30 0.0331 1.39±0.90 7.6±5.0 † 3.40 305.90 0.570.60 0.1035 3.57±0.16 19.5±0.8 † 54.92 655.52 0.880.95 0.1458 4.10±0.11 22.4±0.6 § 85.57 2047.12 0.901.35 0.2022 4.14±0.08 22.6±0.4 § 88.25 2514.10 0.711.90 0.2807 4.21±0.08 23.0±0.4 § 82.29 1668.51 0.586.00 0.2347 4.08±0.09 22.3±0.5 § 67.02 895.92 1.56 21.8±0.3 22.6 ± 0.2

El Toro Muerto basalt 10-9-06-1 groundmass 2nd experiment

pwr F39Ar 40Ar*/39ArK AgeinMa %40Ar* 40Ar/36Ar37ArCa/39ArK tiinMa tpinMa tcinMa (40Ar/36Ar)i SumS/(n-2)/n

0.30 0.0310 1.83±0.68 10.0±3.7 † 7.32 318.85 0.480.60 0.0634 3.04±0.10 16.6±0.6 † 34.29 449.68 0.740.95 0.0896 4.10±0.08 22.4±0.4 80.25 1496.25 0.851.40 0.2501 4.24±0.07 23.2±0.4 § 89.69 2867.14 0.866.00 0.5659 4.27±0.07 23.4±0.4 § 78.14 1351.84 1.10 22.4±0.3 23.3±0.3 23.1±0.4‡ 283±20 1.2/7

Continues →

Table3.SummaryofAr/Ardataofsamplesfromthestudyarea.

basaltictrachyandesiteinthetotalalkaliesversussilica(TAS)diagram(LeBaset al.,1986),similartopreviousresultsobtainedfromthreesamplesoftheTierrasPrietasbasalt(samplesA-160,A-161ofGonzález-Leónet al.,2000) (Figure 7). The El Huarache and the El Pajarito mafic flows classify as trachybasalt and alkaline trachyandesite, respectively,andtheCerroElPicachodomeisasubalkalinetrachydacite(Figure7).TheREE-chondrite-normalized-patternsareveryconsistentforallthesesamples,showinganenrichmentinthelightREE,similartodatareportedfortheTierrasPrietasbasaltbyGonzález-Leónet al.(2000)(Figure8a),buttheCerroElPicachodisplaysanegativeEuanomaly(Figure8a).The87Sr/86SrisotopicratiosoftheElToroMuertoandAguaCalientebasaltsshowinitialvalues

between 0.7069 and 0.7076, coupled with negative εNd valuesbetween-3.8and-4.9(Figure8b).ThePbisotopicvaluesforthetwosamplesoftheElToroMuertobasaltshowconsistent208Pb/204Pbratiosbetween38.7and39,207Pb/204Pbratiosnear15.7,and206Pb/204Pbratiosbetween18.7and19(Figure8c).

SEDIMENTARY AND STRUCTURAL BASIN DEVELOPMENT

Onthebasisofitsregionallocationandage,andasindicatedbyitssedimentary,magmaticandstructuralrecord(discussedbelow),theArizpesub-basinisatypicalhalfgra-


benthatdevelopedduringLateOligocene–EarlyMiocene(GeologicalSocietyofAmericaTimeScale,2009)regionalextensioninthesouthernBasinandRangeprovince.Amaximumageforbasininitiationisconstrainedto~25Ma(latestOligocene)bycontemporaneousbimodalvolcanismoftheElToroMuertobasaltandtheTetuachiignimbrite.TheLaCieneguitamemberandElToroMuertobasaltbothfloored the area prior to significant clastic deposition of the Arizpeconglomerate.TheyoungerdatedvolcanicrocksintheArizpesub-basinistheAguaCalientebasalt(~21Ma),which is interbedded in the upper part of the basin fill of theBáucaritFormation.Theseagesindicatethatthiswasarapidlysubsidingbasinasmostofitssedimentaryandvolcanicaccumulationoccurredinashortperiodoftime.

TheBáucaritFormationisa2.1km-thick,east-dip-ping,clasticandvolcanicsuccessionwhosetiltingmarkedlydecreases upsection from ~35º to 5-10º. The three fining-upward,multistoriedpebbleconglomeratesequencesoftheArizpeconglomerateareinterpretedtorepresentdepositsofbed-loaddominatedstreamsinproximalbraidedriversand/ormidalluvialfans.Thesandstoneandsiltstoneinter-valsintheupperpartsofthemiddleanduppersequencesindicatedepositionindistalbraidplainswithassociated

developmentoflacustrineenvironments.Thefactorscontrollingdepositionof large-scale

alluvial-fansequenceshavebeenattributedtoallocyclicforcessuchastectonicsandclimate.Weinterpretthatnormalfaultingthatinitiatedsubsidenceofthedownthrownblock was the primary control on deposition of the fining-upwardsequencesintheArizpeconglomerate,aswellasrelativeupliftoftheadjacentblocktotheeastthatfeddetritustothebasin.Thisinferenceisbasedonthethicknessof the succession, coarseness of the detritus, and unroofing ofthesourceareaasrecordedbyupwardenrichmentofvolcanicclastsintheArizpeconglomerate,derivedfromtheTarahumaraFormationbasement.Thenormalfaultingthatinitiatedthebasinformationandsubsequentbasindevelopmentoccurredclosetothepresenteasternmarginofthisbasin,asindicatedbyeastwardtiltingandinternalgrowthstratageometryofthesedimentaryfill.Inthisinterpretation(followingEinsele,1992),activephasesofnormalfaultingwereresponsiblefordepositionofeachof the conglomerate sequences whereas the fine-grained intervalsattheirtopsindicateperiodsoftectonicquiescencewhenretreatofthescarpfrontandloweringofreliefofthesourceareaoccurred.Basedonourgeochronology,it

Tetuachi ignimbrite 10-10-06-1 sanidine


0.88 0.0068 2.27±0.41 12.4±2.3 † 8.72 323.73 0.0392.00 0.0407 4.14±0.05 22.6±0.3 † 98.05 15136.48 0.0243.20 0.0576 4.30±0.04 23.5±0.2 96.13 7625.99 0.0164.50 0.0876 4.41±0.05 24.1±0.3 § 97.06 10052.72 0.0145.50 0.1415 4.43±0.04 24.2±0.2 § 98.69 22547.31 0.0126.50 0.0753 4.39±0.04 24.0±0.2 § 98.70 22721.53 0.0137.30 0.2813 4.45±0.02 24.3±0.1 § 95.67 6820.96 0.0127.50 0.1119 4.43±0.02 24.2±0.1 § 98.98 28989.58 0.0128.30 0.1974 4.46±0.05 24.4±0.3 § 98.01 14865.22 0.012 24.0±0.2 24.2±0.2 24.1 ± 0.2 322±69 2.8/9

El Pajarito basalt 9-23-08-05 groundmass


0.30 0.0064 46.0±2.7 244.8±13.2 † 70.50 1001.68 <0.0010.70 0.0432 4.23±1.07 24.0±6.0 † 24.61 391.97 0.4091.20 0.1337 2.87±0.15 16.3±0.9 59.48 729.31 0.6121.80 0.2196 3.21±0.06 18.2±0.4 § 90.81 3215.28 0.6562.20 0.1687 3.36±0.05 19.1±0.3 § 80.00 1168.58 0.5122.80 0.1486 3.34±0.05 18.9±0.3 § 77.78 1030.68 0.4334.00 0.1552 3.19±0.08 18.1±0.4 § 94.22 5112.95 0.4087.00 0.1246 3.55±0.07 20.1±0.4 † 99.65 84031.11 0.809 20.2±0.5 18.6 ± 0.2 18.8±0.3 219±19 0.8/5

Table3(continued).SummaryofAr/Ardataofsamplesfromthestudyarea.

pwr:laserpowerinwattsapplyiedtoreleaseargon;ageoftheindividualfractiondoesnotincludetheuncertaintyinJ;ti:integratedage;tp:plateauage;tc: isochron age calculated with all the fractions except those identified with the symbol †; (40Ar/36Ar)i:initial(40Ar/36Ar)calculatedfromtheinverseofthey-interceptoftheisochronlineinthecorrelationdiagram(36Ar/40Ar)versus(39Ar/40Ar); SumS /(n-2) goodness of fit of the best straight line calculated and numberofpointsused;§fractionsselectedforplateaucalculation;‡isochronagecalculatedcombiningthefractionsofthetwoexperimentsperformedon the sample; all errors are given at 1σ level; J for El Toro Muerto basalt and Tetuachi ignimbrite samples is 0.003048 ± 0.000017; El Pajarito basalt samplewasirradiatedseparately,J=0.003157±0.000050.ti,tpandtcincludetheuncertaintyinJ.Preferredageishighlightedinboldtypeface.


Isot

ope

ratio

sA

ppar

ent a

ges (

Ma)

Ana

lysi

sU

(ppm

)20

6 Pb/

204 P

bU

/Th

206 P

b*/

207 P

b*± (%

)20

7 Pb*

/23

5 U*

± (%)

206 P

b*23

8 U± (%

)er

ror

corr

.20

6 Pb*

/23

8 U*

±(M

a)20

7 Pb*

/23

5 U±

(Ma)

206 P

b*/

207 P

b*±

(Ma)

Bes

t age

(Ma)

±(M

a)

1120

072-

318

543

80.

715

.879

144

.20.

0327

44.6

0.00

385.

70.

1324

.21.

432

.714

.370

7.4

988.

524

.21.

411

2007

2-5

798

1746

0.6

20.2

824

12.7

0.02

5912

.80.

0038

1.5

0.12

24.5

0.4

26.0

3.3

162.

329

7.9

24.5

0.4

1120

072-

5A23

263

60.

816

.899

636

.50.

0298

36.9

0.00

365.

60.

1523

.51.

329

.810

.857

3.4

819.

923

.51.

311

2007

2-6

1374

3411

0.8

21.2

384

9.7

0.02

539.

80.

0039

1.0

0.10

25.0

0.2

25.3

2.4

53.5

232.

025

.00.

211

2007

2-9

219

612

0.5

15.2

488

37.4

0.03

5338

.00.

0039

7.0

0.18

25.1

1.8

35.2

13.2

792.

981

2.2

25.1

1.8

1120

072-

1020

080

40.

815

.728

937

.10.

0337

37.3

0.00

383.

80.

1024

.80.

933

.712

.472

7.6

813.

724

.80.

911

2007

2-11

180

549

0.6

25.2

560

71.2

0.01

9372

.60.

0035

13.9

0.19

22.8

3.2

19.4

14.0

NA

NA

22.8

3.2

1120

072-

1210

841

70.

834

.598

510

2.0

0.01

4310

2.1

0.00

365.

70.

0623

.11.

314

.414

.6N

AN

A23

.11.

311

2007

2-13

9442

60.

714

.251

992

.50.

0353

93.1

0.00

3611

.00.

1223

.42.

635

.232

.293

3.2

298.

323

.42.

611

2007

2-14

9833

30.

611

.712

915

2.4

0.04

1815

2.4

0.00

354.

00.

0322

.80.

941

.662

.113

24.1

402.

122

.80.

911

2007

2-15

202

714

0.6

16.8

684

57.2

0.03

0757

.30.

0038

3.5

0.06

24.2

0.9

30.7

17.4

577.

513

58.1

24.2

0.9

1120

072-

1636

966

90.

718

.251

431

.30.

0281

31.4

0.00

371.

80.

0623

.90.

428

.18.

740

3.7

717.

823

.90.

411

2007

2-17

327

615

0.9

14.5

870

28.4

0.03

3429

.10.

0035

6.6

0.23

22.7

1.5

33.3

9.6

885.

359

8.3

22.7

1.5

1120

072-

1924

453

10.

617

.418

036

.10.

0281

36.7

0.00

356.

20.

1722

.81.

428

.110

.250

7.4

819.

522

.81.

411

2007

2-20

480

582

0.6

15.6

926

15.4

0.03

3615

.50.

0038

1.7

0.11

24.6

0.4

33.6

5.1

732.

532

7.1

24.6

0.4

1120

072-

2132

960

60.

616

.524

024

.60.

0309

24.9

0.00

373.

50.

1423

.90.

830

.97.

662

2.1

539.

023

.90.

811

2007

2-22

302

435

0.5

15.6

758

37.2

0.03

4837

.40.

0040

4.4

0.12

25.5

1.1

34.8

12.8

734.

781

4.0

25.5

1.1

1120

072-

2349

466

00.

713

.100

529

.10.

0389

29.2

0.00

371.

00.

0323

.80.

238

.711

.111

03.8

595.

023

.80.

211

2007

2-24

306

516

0.5

15.7

717

23.0

0.03

3623

.10.

0038

2.2

0.10

24.7

0.5

33.6

7.6

721.

849

3.7

24.7

0.5

1120

072-

2522

232

70.

69.

8615

36.1

0.04

9936

.20.

0036

3.1

0.09

23.0

0.7

49.4

17.5

1650

.069

2.8

23.0

0.7

1120

072-

2613

565

70.

716

.776

752

.10.

0297

52.6

0.00

367.

40.

1423

.21.

729

.715

.458

9.3

1211

.723

.21.

711

2007

2-27

8343

80.

814

.096

515

4.0

0.03

6315

4.2

0.00

376.

70.

0423

.91.

636

.254

.995

5.7

692.

423

.91.

611

2007

2-28

222

2298

2.0

16.1

271

7.2

0.08

637.

30.

0101

1.1

0.15

64.7

0.7

84.0

5.9

674.

315

4.8

64.7

0.7

1120

072-

3130

376

20.

818

.025

120

.40.

0287

20.4

0.00

371.

20.

0624

.10.

328

.75.

843

1.5

459.

124

.10.

311

2007

2-32

466

948

0.5

17.1

558

16.9

0.02

9117

.10.

0036

2.4

0.14

23.3

0.5

29.1

4.9

540.

637

3.0

23.3

0.5

1120

072-

3362

281

30.

517

.198

110

.40.

0280

10.6

0.00

352.

20.

2022

.40.

528

.02.

953

5.2

227.

822

.40.

511

2007

2-34

118

636

0.6

17.4

082

40.0

0.02

7440

.80.

0035

7.9

0.19

22.2

1.7

27.4

11.0

508.

691

3.9

22.2

1.7

Tabl

e4.

Ana

litic

ald

ata

ofU

-Pb

geoc

hron

olog

ical

ana

lyse

sfor

sam

ple

11-2

0-07

-2fr

omth

eEl

Pic

acho

dom

e.

Min

eral

sepa

ratio

nan

dU

-Pb

geoc

hron

olog

yon

indi

vidu

alzi

rcon

gra

insw

asco

nduc

ted

byla

ser-a

blat

ion-

mul

ticol

lect

orin

duct

ivel

yco

uple

dpl

asm

a-m

asss

pect

rom

etry

(LA

-MC

-IC

P-M

S)at

theA

rizon

aLa

serC

hron

Cen

teri

nth

eD

epar

tmen

tofG

eosc

ienc

es,U

nive

rsity

ofA

rizon

aus

ing

the

proc

edur

esd

escr

ibed

by

Geh

rels

et a

l. (2

006)

. All

age

erro

rs a

re q

uote

d at

1σ

(Ma)

leve

l, an

d er

rors

for i

soto

pic

ratio

s are

quo

ted

at 1

σ(%

). N

A=

not a

vaila

ble.


appearsthatinitiallythebasinsubsidedrelativelyslow,between~25and23.5Mawhendepositionof theLaCieneguitaandElToroMuertobasaltmembersoccurred,andthensubsidenceratebecamerelativelyfasterbythetimewhentheArizpeconglomeratewasbeingdeposited(Figure3b).

ThemostevidentfaultsthatcropoutintheeasternmarginofthebasinaretheElFusteandtheGranaditasfaults(namesaccordingtoGonzález-Gallegoset al.,2003,and traces modified in this work), which are parallel, planar normalfaultsthatdip80ºtothewest.Althoughthesefaultscut the younger stratigraphic member of the basin fill (El Catalánmember),theycouldbeolderfaultsthatwerereac-tivated during the final stages of basin evolution. Otherwise, themaincontrollingfault(s)couldbealesssteeply-dippingfault that was covered beneath younger basin-fill, someplace westofthepresentbasinmargin.TheupliftedblockoftheElToroMuertobasaltthatunderliestheElCatalánbreccia

15

17

19

21

23

25

27

29

31

11-20-07-2Age = 24.2±0.4 Ma

Mean = 24.2±0.3MSWD = 1.4

(2 )σ

206

238

Pb/

P

b

Figure6.U-Pbhistogramofageforsample11-20-07-2ofthetrachydacitefromtheCerroElPicachodome.Theweightaverageageat24.2±0.4MamakesitofthesameageastheElToroMuertobasaltoftheBáucaritFormationinthestudyarea.

Sample 6-3-05-1 2-15-07-1 10-9-06-1 6-3-05-3 11-20-07-2 9-23-08-5 4-8-08-3

Unit ELToroMuerto(lowerpart)

ToroMuerto(upperpart)

ToroMuerto(upperpart)

AguaCalientebasalt

ElPicachodome

ElPajaritobasalt

ElHuarachebasalt

Age(Ma) 24.89±0.64 23.5±0.2 23.3±0.3 21.29±0.55 23.3±0.6 18.6±0.2 Notdated

Major oxides (wt. %)SiO2 51.90 51.37 51.57 53.08 68.71 56.50 49.70Al2O3 16.59 17.33 16.7 15.61 13.56 17.43 16.10Fe2O3 9.78 8.66 9.74 8.52 1.32 6.48 10.60MgO 4.24 3.51 4.06 2.79 0.27 2.39 3.61CaO 6.96 6.63 7.02 7.26 1.11 5.80 9.00Na2O 4.01 3.89 3.95 3.60 3.57 3.98 3.61K2O 2.21 2.39 2.15 2.77 4.81 3.20 1.90TiO2 1.59 1.38 1.72 1.54 0.27 0.90 1.80P2O5 0.649 0.60 0.63 0.71 0.05 0.50 0.60MnO 0.15 0.13 0.19 0.11 0.05 0.08 0.14LOI 1.80 3.70 2.00 3.70 1.35 2.00 1.80TOT/C 0.01 <0.01 0.25TOT/S <0.01 <0.01 <0.01SUM 99.96 99.59 99.75 99.7 99.5 99.7A.I. 4.4 4.18 3.72 1.91

Trace elements (ppm)Ba 1002 1012. 979.2 1123 984.0 1316 819Be 2.0 2.0 2.0Co 27.0 27.6 29.7 25.2 22.0 18 37Cs 2.3 0.7 2.1Ga 20.1 20.4 19.9Hf 7.0 6.7 8.6 9.4Nb 16.0 15.3 16.2 20.1 24.9 22 21Rb 45.0 52.9 42.8 72.2 168.0 63 32Sn 1.0 1.0 9.0Sr 710.0 713.8 769.1 624.5 127.0 698 649Ta 0.7 0.8 0.9Th <3.0 3.8 3.8 6.8 16.8 5 3

Continues →

Table5.Dataresultsofgeochemicalanalysesofmajor,traceandREEforvolcanicrocksofthestudyarea.


Sample 6-3-05-1 2-15-07-1 10-9-06-1 6-3-05-3 11-20-07-2 9-23-08-5 4-8-08-3

U 1.2 1.0 1.8V 175 164 197 168 13.1 97 173W 1.3 0.4 2.7Zr 241 303.1 277.2 378.1 261.0 374 227Y 28 34.3 34.9 40.7 26.39 28 31La 44.12 42.6 40.1 53.5 60.23 65.32 38.71Ce 97.72 97.7 91.1 120.2 121.2 131.19 85.29Pr 11.38 11.24 11.09 13.67 12.51 13.97 10.16Nd 46.51 44.2 43.3 52.4 43.13 51.43 41.86Sm 8.78 7.58 7.69 9.1 6.97 8.57 8.40Eu 2.30 1.96 2.13 2.3 1.19 1.92 2.21Gd 7.68 6.37 6.84 7.82 5.87 7.02 7.53Tb 1.16 1.11 1.14 1.35 0.83 1.01 1.16Dy 5.77 5 5.42 6.31 4.97 5.02 5.83Ho 1.25 1.01 0.98 1.15 0.99 1.03 1.25Er 3.24 2.8 2.77 3.26 2.98 2.82 3.27Tm 0.44 0.43 0.45 0.54 0.44 0.41 0.44Yb 2.91 2.59 2.72 3.04 3.12 2.74 2.89Lu 0.45 0.4 0.41 0.45 0.46 0.41 0.45Mo 0.5 0.4 0.9Cu 19 22.9 17.7 107.1 4 21 35Pb 9 2.6 1.9 11.4 24.9 12 5Zn 107 66 45 71 54 85 99Ni 38 24.1 23.2 25.8 0As 5.2 5.7 13.4Cd <0.1 0.1 0.2Sb 0.5 <0.1 0.3Bi <0.1 <0.1 <0.1Ag <0.1 <0.1 <0.1Au <0.5 <0.5 <0.5Hg 0.21 0.01 0.01Tl <0.1 0.1 <0.1Se <0.5 <0.5 <0.5Cr 53 17 30 72Cr2O3 0.003 0.007 0.007Ni 38.0 25.0 28.0 19.0 0.0 23Sc 14.0 16.0 14.0 0.0

Samples6-3-05-1,9-23-08-5,and4-8-08-3wereanalyzedbyXRFandICP-MSintheLaboratorioUniversitariodeGeoquímicaIsotópica,UniversidadNacionalAutónomadeMéxico.Samples2-15-07-1,10-9-06-1,and6-3-05-3wereanalyzedbyXRFandICP-MSattheGeosciencesDepartmentoftheUniversityofArizona,andsample11-20-07-2wasanalyzedbyHR-ICP-MSattheDepartmentofGeology,UniversityofWisconsin-EauClaire.

Table5(continued).Dataresultsofgeochemicalanalysesofmajor,traceandREEforvolcanicrocksofthestudyarea.

nearthebasinmargin(Figure2sectionA-B)mayindicatethat synsedimentary tectonism was significant during basin evolution.

Otherinterpretedsynsedimentaryfaultsincludethenormal,NW-SELaCieneguitaandtheTetuachifaults(Figure2).TheformeroneaffectedtheLaCieneguitamemberbeforedepositionoftheToroMuertobasalt,whilethelatteronewasactivefollowingdepositionoftheTierrasPrietasbasaltandproducedthehorst thatexposestheTetuachiignimbrite(Figure2).Ontheotherhand,theLaCieneguitaandTetuachifaultsmaybereactivatedstructuresofawidespread,althoughstilllittledocumentedNW-SE-strikingnormalfaultingeventthatoccurredinnorth-central

SonoraduringCenozoictimepriortoBasinandRangetectonism(asreportedbyGonzález-Leónet al.,2000).

AfterdepositionoftheBáucaritFormationceased,theArizpesub-basinwasdisorganizedbyayoungerphaseofnormalfaulting.ThemostimportantfaultsassociatedwiththiseventaretheNW-strikingCrisantoandTahuichopafaults,andprobablytheLasHigueritasfault(Figure2).TheCrisantofault,inthesouthernpartofthebasin,upliftedtheTarahumarabasementformingtheNW-SE-orientedhorstofLomasElQuemado(Figure2)anditsdownthrownblock formed a basin to the southwest that was filled by theSinoquipeconglomerate.TheSinoquipeconglomerate(informalname)isapoorlyconsolidatedconglomerate


A-168A-160A-161

A-161A

7065555045

2

4

6

10

8

Na O

+ K

O (w

eigh

t %)

22

BasaltBasalticandesite

Andesite

Trachydacite

Dacite

Alkaline

Subalkaline

Trachybasalt

Composition of rocks of theBáucarit Formation in the Arizpe

sub-basin and El Rodeo basin

TrachyandesiteBasaltic

trachyandesite

Tierras Prietas basaltMesa Pedregosavolcanic rocksEl Toro Muerto basalt

Nearby localitiesCerro Cebadehuachivolcanic rocksBasins of central SonoraSierra Madre Occidental

El Picacho dome


El Rodeo basin

SiO (weight %)2

60

Central Sonora

Figure7.Totalalkalivs.silicadiagram(afterLeBaset al., 1986) of geochemical classification of volcanic rocks of the El Toro Muerto and Agua Caliente basaltmembers,includingpreviouspublisheddatafromtheTierrasPrietasbasalt(samplesA-160,A-161andA-161AfromGonzález-Leónet al.,2000).AlsoplottedforcomparisonaredatafromtheMesaPedregosavolcanicrocks(A-168ofGonzález-Leónet al.,2000)andourdatafromtheElHuaracheandElPajaritobasaltsfromtheElRodeobasins.Otherdatafromvolcanicrockswithagesbetween28to17Mathatarementionedinthemanuscriptinclude:nearbyareasoftheRíoSonoraregion(bluepentagons)byPazMoreno(1992);theCerroCebadéhuachivolcanicrocks(fromGonzález-Leónet al.,2000),theRíoYaquiregion(blankrectangles)(fromMcDowellet al.,1997),westernSierraMadreOccidental(fromDemantet al.,1989)anddatafromcentralSonora(Tillet al.,2009).

successionthatweinferburiestheArizpeconglomerateintheSinoquipevalley(Figure2,sectionC-D).EvidenceforthisincludetheoutliersoftheElToroMuertobasaltcoveredbythisunitinthewesternmarginofthissag,andtheabsencewithinitoftheotherbasaltmembersoftheBáucaritFormationthatarepresentintheArizpesub-basin.TheTahuichopafaultdeformedtheArizpesub-basininitsnorthernparttoformthePicachodeArizpehorst,anditsverticaldisplacementaccountsforatleast2kmconsideringitexposesstrataoftheBisbeeGroupandtheTarahumaraFormationinthisarea(González-Leónet al.,2000).

ARIZPE SUB-BASIN AND REGIONAL CONTEXT

ThedatahereinreportedfromtheArizpesub-basinhelptoconstrainthetimeofextensionandmagmatismassociatedwiththeBasinandRangeeventinSonoraandinthissectionourresultsarecomparedwithdatareportedfromsimilarbasinsinotherpartsofthestate.

GeochronologyofmagmatismhereinreportedformtheArizpesub-basinmostprobablyindicateanincompleterecordofbasinevolutionconstrainedbetween~25and21Ma.TheyoungerageofdurationoftheBasinandRangeinthisregionofnorth-centralSonora,however,canbeconstrainedbytheageatnear18MathatweobtainedfromtheElPajaritobasaltinterbeddedintheupperpartofthe

sedimentary fill of the El Rodeo basin (Figure 1). These data areconsistentwiththegeochronologyofvolcanicrocksinbasinsofcentralandnorthernSonora,whichconstrainbasininitiation,between27to25Ma,toendatnear15Ma.Forinstance, mafic rock of the Báucarit Formation in the Yécora basinweredatedbetween~24and~17Ma(CocheméandDemant,1991),whileattheRíoYaquibasin(Figure1)theoldervolcanicrockshaveagesbetween~27to22Maandtheyoungerareat~17Ma(McDowellet al.,1997).LateOligocene–EarlyMioceneageswerealsoobtainedbyGans(1997)forbasalticrocksintheSantaRosaarea(Figure1)and,fortheSahuaripabasin(Figure1),BlairandGans(2003) reported ages ~28 Ma for basaltic flows at the base of the Báucarit Formation, ~25 Ma for interbedded mafic flows, and ~15 Ma for andesitic flows in the upper part of the basin fill. In northern Sonora, the lower conglomer-ateoftheBacanuchibasin(González-Leónet al.,2000)(Figure 1) interfingers with ~25 Ma rhyolitic tuffs of the “MesaPedregosavolcanicrocks”(González-Leónet al.,2000)andbothunitsunconformablyoverlythedacitictorhyoliticsuccessionofthe“CerroCebadéhuachivolcanicrocks”datedbetween28and27Ma(González-Leónet al.,2000).BasinsassociatedtometamorphiccorecomplexessuchastheMagdalena,TubutamaandUresbasins(Figure1) have interbedded mafic to rhyolitic volcanic rocks dated between~27and15Ma(Miranda-GascaandDeJong,1992;Miranda-Gascaet al.,1998;Vega-GranilloandCalmus,


c) 15.9

15.7

39

38.5

18 19.518.5 19 20

15.5

206 204Pb/ Pb

C o n t i n e n t a l r i f t

b a s a l t s

Upper Crust

Lower Crust

208

204

Pb/

P

b20

720

4P

b/

Pb



Pre-Pinacate rocks (20–19 Ma)

Volcanic rocks of Sierra Santa Úrsula

0.706 0.7080.704

-5

0

5

Asthenosphere

Lithosphere

b)

87 86Sr/ Sr

εNd

Study areasamples

Sm Eu Gd Tb Dy Ho Er Tm Yb LuLa Ce Pr Nd

1000

100

10

a)Sa

mpl

e/C

hond

rite


El Picacho rhyodacite


Tierras Prietas basalt andMesa Pedregosa basalt

STUDY AREA

El Pajarito basalt

El Huarache basalt

Sierra Santa Ursula

Cerro Cebadehuachi

Central Sonora

2003;WongandGans,2008).MiocenemagmatismalongcoastalSonorawithages

from~23and~14Maisandesitictorhyolitic,althoughitsassociationtoextensionalbasindevelopmentisnotclearlydocumented.ItoccursintheSierraSantaÚrsulaandintheSanCarlos-Guaymasarea(Mora-ÁlvarezandMcDowell,2000;Mora-KlepeisandMcDowell,2004)(Figure1),intheSierraLibre(MacMillanet al.,2003)(Figure1)andbetweenBahíadeKinoandPuertoLibertad,includingtheIslaTiburón(GastilandKrummenacher,1977;Gastilet al.,1999)(Figure1).

MostauthorsconsiderthattheSonoranLateOligocenetoEarlyMiocenemagmatismwasassociatedtoextensionthatoccurredinanintra-arcthatdevelopedduringlatestagesubduction of the Farallon plate along the Pacific margin ofnorthwestMexico(e.g.,Demantet al.,1989;Martin-Barajas,2000;Gans,1997;Umhoeferet al.,2001;Mora-KlepeisandMcDowell,2004;Vidal-Solanoet al.,2008).Gans(1997)andGanset al.(2007)alsonotedthatmajor

extensionaldeformationmigratedfromeasternSonora,whereitoccurredbetween28and24Ma,tocoastalSonorawhereittookplacebetween12and9Ma.

The mafic rocks of the Báucarit Formation in the Arizpesub-basinclassifyinanarrowrangeofalkalineba-saltic trachyandesite, similar to contemporaneous flows of thenearbyMesaPedregosavolcanicrocks(sampleA-168inFigure7takenfromGonzález-Leónet al.,2000).OursamplesfromtheElRodeobasinandnearbyareasreportedbyPaz-Moreno(1992)classifybetweentrachybasaltandtrachyandesite(Figure7).MaficflowsinbasinsofthewesternSierraMadreOccidentalarealkalinetrachybasaltandbasaltictrachyandesite(Báucarit,Demantet al.,1989;CocheméandDemant,1991)(Figure7),andcontemporane-ousvolcanicrocksoftheRíoYaquibasinrangefrombasal-tictrachyandesitetoandesite(McDowellet al.,1997(Figure7).SimilarrocksattheBahíaKino–PuertoLibertad–IslaTiburónregionaremostlybasaltandandesite(Gastilet al.,1979).However,amorecompletegeochemicaldata

Figure8.a)RockRREchondrite-normalizedpatternsforsamplesfromthestudyarea(includingthosefromtheElHuaracheandElPajaritobasalts)comparedwithpreviousresultsfromtheTierrasPrietasbasalt,andMesaPedregosaandCerroCebadéhuachivolcanicrocksfromtheBacanuchibasin(González-Leónet al.,2000).ComparisonisalsomadewithcontemporaneousvolcanicrocksoftheSierraSantaÚrsula(fromMora-KlepeisandMcDowell,2004)andcentralSonora(Tillet al., 2009). b) εNd vs.87Sr/86Srdiagram(afterDePaoloandDaley,2000)comparingourdatawiththepre-PinacatesuiteofVidal-Solanoet al. (2008), and the La Espuela and Mezquite Formations from the Sierra Santa Úrsula (the εNd and the Sr initials valuesofthesetwoformationswerecalculatedfromtheRb,Sr,SmandNdconcentrationsreportedintab.1ofMora-KlepeisandMcDowell,2004).c)208Pb/204Pbvs.206Pb/204Pb(afterZartmanandDoe,1981)and207Pb/204Pbvs.206Pb/204Pb (from Wilson, 2001) isotope correlation diagrams showing fields ofupperandlowercontinentalcrustandcontinentalriftbasalts,respectively.Ourdataplottedalongwithcontemporaneouspre-Pinacaterocks(Vidal-Solanoet al., 2008) and mafic rocks from Sierra Santa Úrsula (from Mora-Klepeis and McDowell, 2004) indicate apparent similar sources for these contemporaneousmagmas.


base of the mafic to intermediate volcanism from central Sonorawithagesbetween~28and~17Ma,reportedbyTillet al.(2009),indicateamostlysubalkalinecharacterwithacompositionalrangefromtrachybasalttoandesite(Figure7).

TheREEchondrite-normalizedpatternsofoursam-plesoftheElToroMuerto,AguaCaliente,ElHuaracheandElPajaritobasalts,andElPichachotrachydacite(Figure8a)plotinanarrow,consistentpatternalongwiththeREEspec-traobtainedpreviouslyfromtheTierrasPrietasbasaltandfromtheMesaPedregosavolcanicrocks(González-Leónet al.,2000)(Figure8a).Thispatternexhibitanenrich-mentinlightREEanddepletioninheavyREE;itisalsosimilarbutmoredifferentiatedthantheREEspectrafromtheCerroCebadéhuachivolcanicrocks(Figure8a),whosepatternresemblesvolcanicrocks(LaEspuelaandMezquiteformations)oftheSierraSantaÚrsula(Mora-KlepeisandMcDowell,2004)(Figure8a).OurREEspectraarealsosimilartothoseofvolcanicrocksfromcentralSonorawithagesolderthan14MareportedbyTillet al.(2009).

Availableregionalisotopicdataofnearlycontempo-raneousrocksarescarceandtheyarerestrictedtotheIslaTiburón,SierraSantaÚrsulaandElPinacateareas.FortheIslaTiburón,Gastilet al.(1999)interpretedthatSrinitialisotopicratiosvaryingbetween0.7031to0.7055in20to15M.y.oldandesitictodaciticrockswerecompatiblewitha contaminated source in the mantle-wedge, without signifi-cantcontinentalcrustinteraction.TheSrratiosreportedbyMora-KlepeisandMcDowell(2004)fortheSierraSantaÚrsulaarelow(87Sr/86Sr=0.7039)fortheolderandesitic

rocks(22to18Ma),andmoreradiogenicfortheyoungerdaciticrocks(18to15Ma;87Sr/86Sr=0.7051to0.7055).ThebasaltsfromthestudyareahavehigherSrinitialratiosthanthecontemporaneousIslaTiburón,SierraSantaÚrsula,andthe pre-Pinacate mafic rocks (20 to 19 Ma, Vidal-Solano et al., 2008), and when coupled with their negative εNd values theyindicatethatmagmaswerederivedfrompartialmeltingoftheattenuatedcontinentallithosphere(Figure8b).ThePbisotopicvaluesoftheElToroMuertobasaltcomparealsowithvaluesofvolcanicrocksofthesierraSantaÚrsulaandthepre-Pinacatevolcanics,andhavesignaturesofcontinen-talriftbasaltsthatforminthecontinentallithosphere(Figure8c).Insummary,thegeochronologyandgeochemicaldatafromtheArizpesub-basincorrelatewellwithinterpreta-tionsoninitiation,durationandmagmacompositionoftheBasinandRangeregionalsetting,althoughnootherbasininSonorahasbeenstudiedindetailregardingitssedimentaryand volcanic fill.

CONCLUSIONS

TheArizpesub-basinisatypicalBasinandRangehalf-grabenwhosesedimentaryandvolcanic,east-dippingbasin fill is the 2.1 km-thick Báucarit Formation that we divideintosevenmembers.Basindurationisconstrainedbetween~25and~21MaaccordingtogeochronologyoftheElToroMuertoandAguaCalientebasaltmembersofitslower and upper parts, respectively. However, mafic flows interbeddedintheupperpartoftheElRodeobasinlocatedjusteastofthestudyareamayconstrainageofdurationoftheBasinandRangeeventbetween~25and~18Mainthisregion.

Normalfaultingwasprobablythemaincontrollingfactorofbasinsubsidenceandsedimentation.Subsidencewasrelativelyslowbetween~25and23.5Maduringde-positionoftheLaCieneguitaandElToroMuertobasaltmembers,andthenbecamerelativelyfasterbythetimetheArizpeconglomeratewasbeingdeposited.FaultsthatinitiatedbasinformationareprobablyreactivatedalongtheElFusteandGranaditasfaults,ortheyareburiedbeneaththe younger fill of the basin. Synsedimentary and post-basin faultsthatdisorganizedthebasinareNW-SE-,andN-S-strikingnormalfaults.

Geochemicaldataindicatethatmagmacompositionofthe older volcanic rocks in the basin is mafic and felsic. The ElToroMuerto,TierrasPriestasandAguaCalientebasaltmembersaremostlyalkalinebasaltictrachyandesite,andthe~23Maold,theElPicachodomeistrachydacite.ThisdomewastheprobablysourceoftheTetuachiignimbrite.REEchondrite-normalizedpatternsoftheserocksplotinanarrow,consistentpatternthatexhibitenrichmentinlightREEanddepletioninheavyREEwhichisconsistentwithcontemporaneousmagmatismofnearbybasins.ThispatternisalsosimilartotheREEspectraofsubalkalinevolcanicrocks,basalttodacite,withagesbetween28and14Ma

Sample 6-3-05-3 Agua Caliente

basalt

10-9-06-1 El Toro Muerto

basalt (upper part)

2-15-07-1 El Toro Muerto

basalt (upper part)

Rb 62.593 41.11 55.738Sr 687.023 690.677 750.082Rb/Sr 0.0911 0.0595 0.074387Rb/86Sr 0.261981 0.171143 0.21367787Sr/86Sr(t=0) 0.707594 0.706881 0.707575stderr% 0.0011 0.0012 0.0014Sm 8.96 8.645 8.312Nd 48.375 42.044 42.84Sm/Nd 0.1852 0.2056 0.1941147Sm/144Nd 0.111973 0.124699 0.117323143Nd/144Nd(t=0) 0.512388 0.512445 0.51239stderr% 0.0014 0.0012 0.0011εNd(t=0) -4.88 -3.76 -4.84Age(Ma) 21.3 24.9 23.5206Pb/204Pb 18.7552 19.0294207Pb/204Pb 15.6171 15.8241208Pb/204Pb 38.6923 39.1189

Table6.IsotopicdatafromtheElToroMuertoandAguaCalientebasaltmembersoftheBáucaritFormationinthestudyarea.

AnalysiswereperformedattheIsotopicLaboratoryoftheUniversityofArizonaandmethodsarepresentedintheAppendix.


reportedbyTillet al.(2009)fromcentralSonora.Ourgeochemicaldatacoupledwithisotopicanalyses

oftheElToroMuertoandAguaCalientebasaltmembersmostprobablyindicatethatmagmatismoftheArizpesub-basincorrespondstocontinentalriftbasaltsderivedfrompartialmeltingatthebaseofthecontinentallithosphere.

ACKNOWLEDGMENTS

WeacknowledgefinancialsupportfromConsejoNacionaldeCienciayTecnología(CONACYT)throughprojectNo.24893,andextendourthankstoMihaiDuceaandBrianMahoneyforpermissiontoperformtheisotopicanalysesherereportedattheIsotopicLaboratoryoftheGeosciencesDepartment,UniversityofArizona.OurthankstoSr.JesúsElíasMolina,ofArizpe,forgrantingpermitforaccesstoranchoLaCieneguitaandotherlandproperties.AZLaserChronispartiallysupportedbyNSF-EAR0443387and0732436.WearegratefultoCathyBusby,ScottBennettandCarlosPallaresforcriticalreviewsthathelpedtoimprovethemanuscript,toJuanPabloBernalUruchurtuand Rufino Lozano Santacruz who provided geochemical analyses and to Pablo Peñaflor Escarcega who made sample preparationforgeochemicalanalyses.

APPENDIX. ANALYTICAL METHODS

Ar-Ar dating

The 40Ar/39Ar analyses were performed at theGeochronologyLaboratoryoftheDepartmentodeGeología,CICESE.Theargonisotopeexperimentswereconductedonrockfragmentsandsanidinecrystals.ThesewereheatedwithaCoherentAr-ionInnova70claser.TheextractionsystemisonlinewithaVG5400massspectrometer.Allthesamplesandirradiationmonitors,wereirradiatedintheU-enrichedresearchreactorofUniversityofMcMasterinHamilton,Canada,atposition5CincapsuleCIC-55Aandreceivedadoseof30MW.Toblockthermalneutrons,thecapsulewascoveredwithacadmiumlinerduringirradia-tion.Asirradiationmonitors,aliquotsofstandardsanidineTCR-2(splitG93)(27.87±0.04Ma)andsanidineFCT-2C(27.84±0.04Ma)wereirradiatedalongsidethesamplesand distributed among them to determine the neutron flux variations.Uponirradiation,themonitorswerefusedinonestepwhilethesampleswerestep-heated.Theargonisotopeswerecorrectedforblank,massdiscrimination,radioactivedecayof37Arand39Arandatmosphericcontamination.FortheCaneutroninterferencereactions,thefactorsgivenbyMasliwec(1984)wereused.Thedecayconstantsrecom-mendedbySteigerandJäger(1977)wereappliedinthedataprocessing.TheequationsreportedbyYorket al.(2004) were used in all the straight line fitting routines of theargondatareduction.Theplateauagewascalculated

fromtheweightedmeanofconsecutivefractionsthatwerein agreement within 1 σ. The error in the plateau, integrated andisochronagesincludesthescatterintheirradiationmonitors[J=0.003048±0.000017].Theanalyticalpreci-sion is reported as one standard deviation (1 σ). For each sample,therelevant40Ar/39Ardataforalltheexperimentsispresented,itincludestheresultsforindividualstepsandtheintegratedages.Intheresultstable,thefractionsselectedtocalculate the plateau age are identified as well as the frac-tionsignoredintheisochronagecalculation.Thepreferredageforeachsampleishighlightedinboldtypeface.

Isotopic analysis

Theisotopicratiosof87Sr/86Sr,143Nd/144Nd,andthetraceelementconcentrationsofRb,Sr,Sm,andNdweremeasuredbythermalionizationmassspectrometryonwholerocksamples.Rocksampleswerecrushedtoaboutonethirdoftheirgrainsize.RockpowderswereputinlargeSavillexvialsanddissolvedinmixturesofhotconcentratedHF-HNO3oralternatively,mixturesofcoldconcentratedHF-HClO4.ThedissolvedsampleswerespikedwiththeCaltechRb,Sr,andmixedSm-Ndspikes(Wasserburget al.,1981;DuceaandSaleeby,1998)afterdissolution.Rb,Sr,andthebulkoftheREEswereseparatedincationcol-umnscontainingAG50W-X4resin,using1Nto4NHCl.SeparationofSmandNdwasachievedinanioncolumncontainingLNSpecresin,using0.1Nto2.5NHCl.Rbwasloaded onto single Re filaments using silica gel and H3PO4.Sr was loaded onto single Ta filaments with Ta2O5powder.Sm and Nd were loaded onto single Re filaments using platinizedcarbon,andresinbeads,respectively.

MassspectrometricanalyseswerecarriedoutattheUniversityofArizonaonanautomatedVGSectormulti-collector instrument fitted with adjustable 1011Ω FaradaycollectorsandaDalyphotomultiplier(DuceaandSaleeby,1998).ConcentrationsofRb,Sr,Sm,Ndweredeterminedbyisotopedilution,withisotopiccompositionsdeterminedonthesamespikedruns.Anoff-linemanipulationprogramwasusedforisotopedilutioncalculations.Typicalrunsconsistedofacquisitionof100isotopicratios.ThemeanresultoftenanalysesofthestandardNRbAAAperformedduringthecourseofthisstudyis:85Rb/87Rb=2.61199±20.FifteenanalysesofstandardSr987yieldedmeanratiosof:87Sr/86Sr=0.710285±7and84Sr/86Sr=0.056316±12.The mean results of five analyses of the standard nSmβperformedduringthecourseofthisstudyare:148Sm/147Sm=0.74880±21,and148Sm/152Sm=0.42110±6.Fifteenmea-surementsoftheLaJollaNdstandardwereperformedduringthecourseofthisstudy.Thestandardrunsyieldedthefol-lowingisotopicratios:142Nd/144Nd=1.14184±2,143Nd/144Nd=511853±2,145Nd/144Nd=0.348390±2,and150Nd/144Nd=0.23638±2.TheSrisotopicratiosofstandardsandsampleswerenormalizedto86Sr/88Sr=0.1194,whereastheNdiso-topicratioswerenormalizedto146Nd/144Nd=0.7219.The


estimatedanalytical±2σ uncertaintiesforsamplesanalyzedinthisstudyare:87Rb/86Sr=0.35%,87Sr/86Sr=0.0014%,147Sm/144Nd=0.4%,and143Nd/144Nd=0.0012%.Proceduralblanks averaged from five determinations were: Rb 10 pg, Sr150pg,Sm2.7pg,andNd5.5pg.

WashesfromthecationcolumnseparationwereusedforseparatingPbinSr-Specresin(Eichrom,Darien,IL)columnsbyusingprotocoldevelopedattheUniversityofArizona.Sampleswereloadedin8MHNO3intheSrspeccolumns.Pbelutionisachievedvia8MHCl.LeadisotopeanalysiswasconductedonaGVInstruments(Hudson,NH)multicollectorinductivelycoupledplasmamassspectrom-eter(MC-ICPMS)attheUniversityofArizona(Thibodeauet al.,2007).Sampleswereintroducedintotheinstrumentby free aspiration with a low-flow concentric nebulizer into awatercooledchamber.Ablank,consistingof2%HNO3,wasrunbeforeeachsample.Beforeanalysis,allsampleswerespikedwithaTlsolutiontoachieveaPb/Tlratioof10.Throughouttheexperiment,thestandardNationalBureauofStandards(NBS)-981wasruntomonitorthestabilityoftheinstrument.

AllresultswereHg-correctedandempiricallynor-malizedtoTlbyusinganexponentiallawcorrection.Tocorrectformachineandinterlaboratorybias,allresultswerenormalizedtovaluesreportedbyGalerandAbouchami(2004)fortheNationalBureauofStandardsNBS-981standard(206Pb/204Pb=16.9405,207Pb/204Pb=15.4963,and208Pb/204Pb = 36.7219). Internal error reflects the reproduc-ibilityofthemeasurementsonindividualsamples,whereasexternalerrorsarederivedfromlongtermreproducibilityofNBS-981Pbstandardandresultinpartfromthemassbiaseffectswithintheinstrument.Inallcases,externalerrorexceedstheinternalerrorsandisreportedbelow.ExternalerrorsassociatedwitheachPbisotopicratioareasfollows:206Pb/204Pb=0.028%,207Pb/204Pb=0.028%,and208Pb/204Pb=0.031%.

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Manuscriptreceived:July31,2009Correctedmanuscriptreceived:May16,2010Manuscriptaccepted:May19,2010

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