473 Thz Canadian Mfuuralo gist Vol.35, pp. 473-5@(1,997)

EXTENSIONALTECTONICS AND THEDIVERSE PRIMITIVE VOLCANIC ROCKS IN THEWESTERN MEXICAN VOLCANIC BELT

JAMES F. LU}IR1 Deparnnentof Mineral Sciences,NHB-119, Smithsonian In$iafiw Washington,D,C, 20560, U,S-A.,

ABSTRACT In the westernportion of tle MexicanVolcanic Belt, primitive volcanicrocks arerelatively commonin comparisonwith most of the world's subduction-relatedarcs. Theseprimitive rocks are convenientlygrouped in three suites,which have erupted concurrentlyat le,astsince the Ptocene: (1) Dy-normativecalc-alkaline basalts, (2) ne- and lry-normatvelamprophyres such as minettes,leucitites, spssartites, and kersantites,characterized by phlogopite and hornblendephenocrysts in the absenceof plagioclasephenocrysts, and (3) ne- to fty-normativeintraplate-tlpe alkaline basalts.The abundanceand diversiry of primitive volcanic rocks in ftre westemMexican Volcanic Belt appearto be primarily relatedto Plio-Quatemaryrifting within and along the marginsof the JaliscoBlock" which favoredpassage of primitive rnagmasto the surface.The lamprophyresare interpreted to form mainly from melting of phlogopite-bearingpyroxenite veins generatedin the mantle wedgeas melts rising from the subductingslab react with peridotite. Such veins are probably formed by hybridizationreactions in most subductionzones, but the lowdegree lamprophyre-formingmelts rarely erupt to the surface;instead, they stagnateas crustal lamprophyric dikes. Although uncommon,other examples of volcaniclamprophyres are known in activearcs, from Baja Califomia, Japan,and Papua New Guinea.With greater dilution of the vein componentby melts from the peridotitic wallrock, calc-alkalinebasalts are generated.This processappea$ to dominatein manyarcs. Although abundancesofincompatible elenentsare considerably lower thanforthelamprophyres,thetworocktypes sharesimilarpat0erns of relativeemichmentof elementsand Sr, Nd andPbisotopic ratios. The "essence"of subduction-zonegeochemistry, therefore, is most purely representedby the lamprophyres,with the calc-alkalinebasalts being dilutedrelatives. The intraplafe-typealkaline basalts seem to reflect partial melting sf gslyecfing qpper mantlethat wascompositionally unaffected by subductior;but advectedinto the regionbeneafl the rifting continental Keywords: subduction,rifting, primitive, volcanic,lamprophyre, Mexico.

Soilnaarns Dans la portion occidentalede la Ceinture Volcanique Mexicaine, les roches volcaniquesprimitives sont relativement courantes,en comparaisonde la ptupart des arcs li6s aux zonesde suMuction. Ces rochesprimitives sont regroup6esen trois suitesde laves, qui semblentavoir 6t6 en 6ruption en m€metemps au moins depuisle Pliocdne: (l) basaltescalco-alcalins i orthopltoxbne normatif, (2) des variantes lamprophyriquesi orthopyroxdneet n6ph6line normatifs, par exemplg minette, leucititg spessartiteet kersantite,que caract6risentla pr6sencede phdnocristauxde phlogopiteet de homblendeen I'absencede ph6nocristauxde plagioclase,et (3) des basaltesalcalins de type intra-plaque, i orthopyroxBneet n6ph61inenormatifs. L'abondanceet la diversit6des roches volcaniques primitives dansce secteurde la ceinturesemblent d'abord li6es I la pr6sence derifts plio-quatemairesi l'int6rieur du bloc deJalisco et le long desbordures limitrophes, qui ont favoris6le passagede nagnas primitifs vers la surface.Les rocheslamprophyriques auraient cristallisd h partir de magmasrepr6sentant une firsion partielle de veinesde pyrox6nitel phlogopitedqns le coin de manteau.Ces veines i leur tour repr6senteraientdes produits de r6actionde maemasissus de la zone de subductionavec la pdridotite sus-jacente.De telles veines rdsultentprobablement de rdactions d'hybridation dansla plupart deszones de subduction,mais les magmasaptes A donnerdes lamprophyres, formds suitei un taux de fusion partielle assez1imit6, n'atteiglent que rarement la surface. Ils formeraient, par contre, des filons l tendance lamprophyriquedans la crotte. Quoiqu'ils sontrelativement rares, d'autres exemples de rochesvolcaniques lamprophyriques ont 6td signal6sdans des arcs pr6sentementactifs, par exempleen BasseCalifornie, au Japonet en Papouasie- Nouvelle Guin6e. I,es basaltesgalgs-alsalins prennent naissance suite i une plus grandedilution de la composanteissue de veinesde pyroxdnite par m6langeavec un liquide issudes roches encaissantes p6ridotitiques. Ce processusest, semble-t-il, trn6domin:nt dans plusieurs arcs. Quoique les abondancesd'6l6ments incompatibles sont considdrablemenlplus faibles dans les lamprophyres,les deux variantesde laves primitives partagentplusieurs caractdristiques d'emichissements gdochimiques; de plus, leurs rapportsdes isotopesde Sr, Nd et Pb seressemblent. L'essence mOme des magmas form6s dansun contextede subductionserait repr6sent€e par les magmasformateurs de laves lamprophyriques,les basaltesa bndatrce calco-alcalinereprdsentant dss mapas dans lesquelscette composantea 6t6 dilu6e. ks basaltesalcalins d caractire intraplaquesemblent r6sulter d'une fusion partielle du rnanteausut'rieur en convection,non affectdpar le ph6nombnede subduction,mais mis en place dam la r€gion en dessousde la lithosphBrecontinentale en extension. (Iraduit par la R6daction) Mots-cl6s: suMuction, extensio&lave primitive, volcanique,lamprophyre, Mexique.

1 E-mail address:luhr@volcano.si.edu

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INTR,oDUciloN whose volcanoespresumably overlie the aseismic extensionof the subductedCocos plate. himitive volcanic rocks, defined as having MgO > The existenceof the Riveraplate, moving separately 6 wt.Voand Mg# > 62, with Mg# = 100 X Mg(Mg + from both the Pacific and North American plates,has 0.8 Febd), are found in remarkable abundanceand been debatedsince the pioneering work of Atwater diversity in the westernpart ofthe active, subduction- (1970). A review of ideasconcerning the Rivera plate related Mexican Volcanic Belt. Theseinclude calc- can be found in DeMets & Stein (1990). Today, the alkaline varieties, lamprophyreswith phenocrystsof Rivera plate appearsto be slowly subducfingnorth- amphiboleor phlogopiteand an absenceofplagioclase eastwardbeneath westem Mexico. DeMets & St€in phenocrysts, and intraplate-type alkaline basalts. A (1990) determinedthat Rivera - North American large body of petrological and geochemicaldata has convergencevaries from 0.6 crn/yr in the north, near accumulatedon these primitive rocks and their more the Tres Marias Islands,to 2.0 cm/yr in the south,near abundantdifferentiates since the early 1970s.Summa- Manzanillo (Fig. 1). Although seismicityis sparse ries of these studiesfrom regional and plate-tectonic along the Rivera - North American trench segment, perspectivescan be found in Luh et aL (1985,1989), occasionallarge tlrust-type events have been noted N\at et al. (L991), and Wallaceet al. (1992). @issler & McNally 1984, Singh et al. 1981, 1985, The purposeof this report is to provide an updated DeMets& Stein 1990).Pardo & Sudrez(1993) evalu- review focusedon the diverseprimitive volcanicrocks atedseismicity related to subductionof the Riveraplate from the westernMexican Volcanic Belt andtheir com- and found that most epicenterscluster southwestof plex tectonic setling. heviously unpublishedSr- and Colima Volcano, with very few events north of 20o Nd-isotopic data are presentedfor 17 volcanic and latitude. Foci reach 130 km depth and define a basementsamples. Along with other Sr, Nd, and Pb Wadati-Benioff zone that dips -10" down to 20 kn, isotopic darafrom the literature and recentlypublished but then steepensto -50o below 40 km, considerably B andBe resultsfor 22 of the primitive rocks,these data steeperthan the Cocosplate subductionjust to the east. are used to support a vein-and-wallrockmelting rela- Many parallelsexist betweensubduction of the Rivera tionship in the sub-arcmantle for the lamprophyresand plate off westernMexico and subductionof the Juande calc-alkalinebasalts, respectively, and a distinct source Fuca plate off Washinglonand Oregon.In both cases, in the convectingupper manfle for the intraplate-type young and hot lithosphereis subductingsteeply in a alkalinebasalts. mainly aseismic fashion, yet recurrent large earth- quakestake place. TECToNIC SE"TIING Seismologistsand oceanographershave long puz- zled over the nafureof the Rivera{ocos boundary.On The areaofinterest (Fig. 1) extendseastward from the basisof estimatedrates of subductionfor the Rivera the Pacific coastsof Nayarit and Jalisco statesto the (-2 cmlyr) and Cocos plates (-5 cm/yr) in this area eastern boundary of the Michoacdn-Guanajuato their boundarywas hypothesizedto be a zone of left- Volcanic Field (-100.5'W). Volcanic and tectonic lateral strike slip at a rate of -3 cm/yr trending-N45"8 activity in this region is controlledby interactionsofthe (Nixon 1982,Eissler & McNally 1984).This boundary Nortl American,Cocos, Rivera, and Pacific plates. has been dfficult to define becauseseismicity east of 106.5' longitude is diffrrse, and focal mechanisms Subductionofthe Cocosand Riveraplates reveala variety of stylesof faulting @issler& McNally 1984,DeMets & Stein1990). Bandy et al. (1995)have The Cocos plate subductsnortheastward into the associatedthe differential rates of subductionof the Middle America Trench (Fisher 1961) beneath the Rivera and Cocos plates with the developmentof the Pacific coastsof Mexico and CgntralAmerica (Molnar southernColima Rift. & Sykes 1969, Nixon 1982). The convergencerate increasesfrom -5 cm/yr in the west to -8 cm/yr in the Plate tectonicsand rifting of the Jalisco Block east(Minster & Jordan1978, McNally & Minster1981, DeMets et al. 1.990). Large (M, > 7) earthquakes, Three large rift zonesintersect in westernMexico, mostly shallow (15-20 km depth) thrust-type events, about60 km south-southwestofGuadalajara, to form a are commonalong the Cocos- Norttr Americanpart of structural triple junction (Fig. 1): the Tepic-Zacoalco the Middle America TrenchOlcNally & Minster 1981, Rift, the Colima Rift, and the Chapalall.ift.Lubr et aL Singb et aI. 1981, Chael & Stewart 1982, UNAM (1985)and Allanet aL (1991)proposed that rhis sfuc- Seismology Group 1986). The Wadati-Benioff zone tural triple junction reflects initiation of a major conti- for the Cocos plate beneath Mexico is diffrrse and nental rifting event, which will ultimately lead to extendsto a depth of only about 100 km @4olnar& tansfer of the Rivera plate and the Jalisco Block Sykes 1969, Burbach et al. 1,984).As such, the (currently part of the Nortl American plate) to the Wadati-Benioffzone seismicityis virtually confinedto Paciflc plate. The Jalisco Block is the southwestern tle area south of the active Mexican Volcanic Belt cornerof Mexico, boundedby the Tepic-Zacoalcoand

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Colima Rifts @ig. 1). This rifting was argued to be surrounds the timing and nature of faulting in the relatedto jumping of the Pacific-Riveraspreading ridge Tepic-Zacoalco Rift, however. Some investigators to the site of the Colima Rift. Four similar eastward argue that the youngestepisodes of stike-slip motion ridge-jumps have progressednorttrward in sequence arerelated to openingof the Gulf of Califomia andthat along the EastPasific Rise dudng the last 12 Ma (Van such faulting ceasedin the mid-Miocene (Michaud Andel et al. 1975, Klitgord & Mammerickx 1982, et al. 1991,Fenai et al. 1994,Ferrarj1995, Ferrari & Mammerickx & Klitgord 1982), and the proposed Rosas-Elguera1997). Othershave presentedevidence on-going ridge-jump would be a continuationof this that strike-slipmotions have continued into tle Quater- seriesand the fust eventto intersectcontinental crust. nary (Nieto-Obreg6net al. 1985, Allan 1986, Nieto- Several alternatives have been proposed to this Obreg6n 1989, Allan et al. L991, Garduio & Tibaldi model @eMets & Stein 1990, Michaud et al. 1990, r99r). Bourgois& Michaud 1991,Ferrari et al. l994,Ferrai During the Quatemary,fwo main magmaseries have & Rosas-Elguera1997), but no consensushas yet been eruptedin the Tepic-ZacoalcoRift: calc-alkalineand reached. However, a long-term Global Positioning intraplate-type alkaline. Volumetrically dominant Systemsurvey of westernMexico, conductedby a team amongthe Quaternaryeruptive centers are calc-alkaline of Mexican and U.S. scientists,was begunin March of andesites,dacites, and rhyodacites of the composite 1995(DeMets et al. L995).Fifteen stationswere estab- volcanoes Tequila (Wallace & Carmichael 1994), lished: eight within the Jalisco Blockn one near the Ceboruco(Nelson 1980),Tepetiltic (Verma & Nelson on-lald triple junction, onewithin the Colima Rift, one 1989), Sangangiiey(Nelson & Livieres 1986), and north of the Tepic-Z.acoalcoRift, and four east of the San Juan (Luh 1978),which are stung out along the Colima Rift. Results of this study should eventually southeast-northwesttrend of the rift zone @ig. 1). provide unequivocalevidence for the kinematic rates Other calc-alkalinemagmas were eruptedduring Plio- and directionsof motion for the JaliscoBlock and other Quaternary time along the Soutlern Guadalajara crustalterranes in southwesternMexico. VolcanicChain (Luh &Lazaar 1985)and from small Various investigators have emphasizedtlat the cones located west of Volc6n San Juan in coastal on-going continentalrifting event in westernMexico Nayarit (Righter et al. 1995). Moore et al. (1994) provides an excellent analog for the initial rifting of reported calc-alkaline basaltic andesitesof Miocene, Baja California at -14 Ma (Luhr et al. 1985, AJlan Plioceneoand Pleistoceneage near Guadalajar4 and et al. L991.,Lyle & Ness 1991).In the caseof Baja's Gilbert et aL (1,985)described the Pliocene(^4.8 Ma) rifting, the slow continentalstage lasted -5-6 Ma and SanGaspar igrimbrite of andesitic-daciticcomposition. wasprobably followed by a rapid transitionto sea-floor spreading(Lyle & Ness1991). The ColimaRift, whose Alkaline basaltsand derivative hawaiites,mugear- floor is no\il at an averageelevation of -1 km, has ites, and benmoreitesalso are volumetrically important apparentlybeen extending for -5 Ma" and by analogy in the Tepic-Zacoalco Rift. The geatest volumes of with Baja rifting, may soooevolve to marineincursion these magmas apparently erupted in the Miocene and sea-floor spreading. Modeling of gravity data (-10 Ma) at both endsof the rift zone;they areexposed collectedalong the axis of the Colima Rift indicatesa at the easternend in the deepcanyons ofthe Rlo Grande crustalthickness of 30-46 km. similar to resultsfor the de Santiago and its tributaries (Moore et al. 1994, rift fl anks(Jrrutia-Fucugauchi & Molna-Garza 1992). Watkins etal. L97L, Gil&rt etal. 1985),and at the Likewise, heatflow is not anomalouslyhigh within the western end, north of Tepic (Gastl. et aL 1979u b, mag- Colima Rift (Sedlock et al. 1993). Thus, the Colima Righteret al. L995,Ferrai et al. 1994).Similar Rift is at an early stageof continentalrifting that hasnot mas have erupted intermittently since the Mocene, evolved to the point of demonstrablecrustal thinning with the youngest(Quaternary) examples known from and enhancedheat flow. the canyon of the RJo Grande de Santiagonorth of Volcdn Tequila (IVIooreet aL L994) and from cinder The Te p i c-Zaco al co Rijl coneson the flanks ofVolc6n SangangiieyQ.{elson & Carmichael1984, Nelson & Livieres 1986, Verma & Nelson1989). From the structuraltriple junction, the Tepic--Zacoalco Rift extendsnorthwest for more than 250 km as a zone A third major group of magmas erupted in the of normal and strike-slip fault scarps,grabens, half- Tepic-ZacoalcoRift is volcanologicallyand volumetri- grabens,and Quatemaryvolcanic centers(Allan el a/. cally importantbut haslittle direct relevancefor discus- 1991).The Tepic-ZacoalcoRift hasbeen interpreted to sions of primitive magmas;these are the peralkaline be part of a larger zone of major strike-slip faulting trachytes and rhyolites of the Sierra La Primavera (Gastil& Jensky1973, Johnson & Harrison1990) and (I\4ahood1980, 1981, Mahood & Drake1982, Mahood has also been shown to correspondto the northern et al. 1985,Mahood & Halliday 1988),Volcdn Las boundary of the Jalisco Block on the basis of abrupt Navajas (Nelson & Hegre 1990, Verma & Nelson offsets in tle agesof ash-flow tuffs acrossit @errari 1989), and the GuadalajaraIgnimbrite (Gtlbet et al. 1995, Righter et aL 1995). Considerablecontroversy 1985,Mahood et al. L985).

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On the basis of the definition of "primitive" stated Quaternary,as shownby lamprophyriccinder coneson earlier, 15 primitive samplesfrom the Tepic-Zacoalco the flanks of the concurrently active calc-alkaline Rift areused in this study:one is calc-alkaline,two are Colima statovolcanoes(Luhr & Carmichael1981). transitional,and 12 are alkaline.All primitive samples Sincethe mid-Pleistocene,calc-alkaline eruptions have includedin this study axelisted in the Appendix. been mainly confined to the floor of the central seg- ment, where a southward-younging chain of three The ColimaRift major calc-alkaline stratovolcanoeshas grown: Cdntaro,Nevado de Colima, and Colima. Magmacom- The Colima Rift extends soutlward for -160 km positionshave become progressively more primitive as from the structuraltriple junction to the Pacific Coast vent position has migraled toward the trench (Luhr (Fig. 1), and is conveniently divided into three seg- 1993). Similal trends are evident a:nong the lampro- ments: northern, cenfral, and southern (Allan et al. phyres,for which the mostprimitive varietiesare found 1991). The northern segmentis 20 km wide and dis- in the youngestand most southerlyexamples, the cinder plays a well-formed grabenstructure, with steepwalls coneson the flanks of the Colima sffatovolcanoes.This rising up to -1,500 m abovethe grabenfloor and its study includes 18 primitive samplesfrom the Colima playa lakes.Modeling of gravity dataindicates thar (1) Rift: two are calc-alkaline, and 16 are lamprophyric the grabenfloor is underlainby -1,000 m of sediments, rocks from the cental sesment of the Colima Rift (2) downdropping of the graben floor has totaled (Appendix). 2.5 km, and (3) east-west extensionhas reached 1.5-3.3tu (6-l3Eo) (Allan 1986, AJlanet al. l99L). The ChapalaRirt The centralsegment (Fig. 1) beginsat the latitudeof the Colima Volcanic Complex,where graben the Toliman The central axis of the E-W-trending ChapalaRift (Henera 1967) mergesfrom the northwest to broaden containsLake Chapala,Mexico's largest natural lake the Colima Rift to a width of 50-60 (N7an km et al. (Fig. 1).The lake is boundedby a graben100 km long 1991).The southemsegment begins -5 km southof and 10-30 km wide, whose walls expose middle Colima City, andthe extensionalstyle changesdramati- Miocene- to Pleistocene-agecalc-alkaline andesitic cally. This segmentlacks obvious graben an stucture; volcanic rocks and lacustrinedeposits that have been on the basisof field mappingand geophysical surveys, tilted away from the lake by 15-40" (Gardufio & Serpa el aL (1992) have argued that north-south- Tibatdi 1991, Delgado-Granados1993). A family of trending landforms in the area formed by erosion of rotatedfault-blocks on both the northern and southern Cretaceousstructures and are not related to modern sidesofthe grabenserve to widen the ChapalaRift to extension.This interpretation is inconsistentwith evi- 50-60 km. ChapalaRift faulting mergesto the eastwith dencefor extensionboth to the north and the south. the Chapala-Tula Fault Zone, which continues Bandy et aL (1993) modeledgravity datain the south- ^400 km to the east(Johnson & Harrison 1990).In the sm Qslima Rift as indicating two major northeast- westemrift zone,the east-west-ftendingLake Chapala southwest-trendinggrabens witl a total width of (lake elevation 1520 m) is stranded160-170 m above -100 km and up to 8 km of sedimentary fil1. Offshore, the floors of the northwest-southeast-trendingCitala the Colima Rift mergeswith the northeast-southwest- DepressionandZacoalco Graben to its west, which are trending Manzanillo Rift (Bourgois etaI. 1988, clearly younger (Fig. 1).To date,no detaileddescrip- Michaud et al. 1990, Bourgois & Michaud 1991, tion of primitive volcanic rocks from the ChapalaRift Bandy et aL 1995). The Manzanillo Rift is a 60-h- has appearedin the literature. wide depressionwith >1 km of vertical offset, contain- ing a growing quartzofeldspathicsedimentary fan and fault-controlled submarinecanyons. The base of the TheJalisco Block sedimentaryfan may mark the beginningof the Colima Rift extension,and may serveas an analogfor birth of Geologically, the Jalisco Block is the least known the MagdalenaFan (14.5-13 Ma) at the mouth of the part of the western Mexican Volcanic Belt. Grano- nascentGulf of California during the early stagesof dioritic batholiths with emplacementages of continental rifting associatedwith the separationof 85-106 Ma are prominent in the westernpart of the Baja (Lyle & Ness1991). JaliscoBlock (Ktihler et al. 1988,7immermannet al. Two main magma serieshave erupted vrithin the 1988, Righter et al. 1995). Roughly coeval rhyolitic Q6lima Rift, calc-alkaline and lamprophyre-forming. ash-flow tuffs (7G-114 Ma) cover large parts of it Thesetwo seriescan be found interbeddedwithin the (Gastil.et al. 1979a,b, Wallace& Carmichael1989). grabenwalls in the northernand centralsegments of the These are distinctly older than ash-flow tuffs found rift" where calc-alkalinerocks extendback to -10 Ma north of the Tepic-ZacoalcoRift, andthis facthas been and the lamprophyresextend to 4.7 Mu which may used to confirm the northern boundary of the Jalisco represent the time of initial extension (Nlan et al. Block (Ferrari 1995, Righter et aL 1995).The Jalisco 1991). Both series continued to erupt during the Block standshigh relative to the terrane north of the

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Tepic-ZacoalcoRift, apparentlythe result of Neogene lava cones, lava flows, maars, tuff rings, and lava uplift @ettari1995, Righter et aL.7995). domes(Hasenaka & Carmichael1985b). Among these Few late-Cenozoicsfructures have been mappedin small centers.two cinder cones associatedwith lava theJalisco Block. Johnson & Ha:rison(1990) identified flows were active historically: Jorullo during several graben structures in the same area, which 1759-1774 (Luhr & Carmichael 1985) and Paricutin Wallace & Carmichael(1989, 1992) and Carmichael during 1943-1952 (Wilcox 1.954,McBkney et aL et al. (1996) later named the Talpa, Mascota, and 1987,Luhr & Simkin 1993).The estimated78 small Atenguillo Grabens(Frg. 1).Thesegnbens are 2-5 km conesthat have eruptedin the Michoacdn-Cuanajuato wide, with up to 900 m of relief. Topographyis sub- Volcanic Field during the last 40,000 years,including dued, and few young fault scarpshave beenidentified. Jorullo and Parfcutin, are exclusively found in the Thesestructures are well defined,however, by the nortl southern part (Hasenaka& Carmichael 1985a). The to west-northwestcourses of fributariesto the Ameca volcanic field also has about 300 medium-sizedvolca- River @g. 1). noes,mostly shield volcanoes,about 10 lm in diame- Four discretevolcanic fields havebeen defined in the ter; these too show a pronounced southward shift JaliscoBlock (Frg. 1). In eachone, two distinct magma during the last 2 Ma @an et al. 1992). Theseresults serieserupted from neighboringcones during the same conform with many other observationsfrom the main time interval: calc-alkalineand lamprophyre-forming. MexicanVolcanic Belt, which cleady showtrenchward Eruptionsin the MascotaVolcanic Field, including the migration of magmaticactivity during the Quatemary Talpa Graben,began ^.0.5Ma and continuedperhaps (Cantagrel& Robin L979, Ltt}lr & Carmichael1980, into the Holocene.Mascota is dominatedby lampro- Nixon etaL 1987, Delgado-Granadosetal. 1.995). phyrx (minette,absarokite, spe,ssartite) and calc-alkaline Little work has been done on the Miocene-Pliocene basalt, basaltic andesite, and andesite (Lange & volcanicrocks of the Michoacdn-GuanajuatoVolcanic Carmichael1990, Carmichael et al. L996).T\eMascota Field, which are mostly covered by the Quaternary minettes are the youngestknown lamprophyres,with units, but like the youngersequences, they appearto be agesas low as68,000 t 80,000years (Carmichaelet al. dominantlycalc-alkaline (Williams l950,Fenari et al. 1990. About 50 km north-northeast of Mascota is 1990). the San SebastianField, where lamprophyre-forming On the basis of studiesin the northeastemquadrant magmas (representedby minefte, absarokite,and of the Michoac6n-CuanajuatroVolcanic Field, Pasquard kersantite)a1d gals-alkalinemagmas (represented by et al. (7986) and Ferrari et al. (1990) discussedthe basaltic andesiteand hornblendeandesite) also issued evolutionofthe stressfield sincethe Pliocene,as north- from neighboring cinder cones during the interval east-southwestcompression associated with reverse (0.48-0.26Ma: Lange& Carmichael1990, 1991). Los and strike-slip faulting evolved to Late Quatemary Volcanes Field lies at the southernend of the Aten- transtension,as reflectedin the east-westfaults of the guillo Graben, and also includes a wide variety of Chapala-Tula Fault Zone. Other investigators have lamprophyres(minetteo leucitite, vogesite,absarokite, mainly focusedon Quaternaryfault patterns.Johnson trachybasalt,trachyandesite) and calc-alkalinerock & Harrison (1990) recognizedtwo major fault zones rypes (basalt,basaltic andesite,andesite). Dated sam- extendingthrough the volcanic field from the Chapala ples rangefrom 3.4 to 1.7 Ma (Wallace& Carmichael Rift @g. 1). The Chapala-TulaFault Zone continues 1989, L992). At the northem end of the Atenguillo eastwardf.or >420 km, and the Chapala-OaxacaFault Graben, near the Tepic-ZacoaTco Rift, Righter & Zone trendsto the southeast.The latter fault is consid- Carmichael(1992) describedseveral Plio-Pleistocene ered to form a structural suture-zone between the (3.6-{.6 Ma) shield volcanoesand lava plateausbuilt Michoacdn Block to the southwestand the Guerrero either of aJkalinebasalt hawaiite, and mugearite,or of Block to the northeast (Johnson& Hanison 1990). calc-alkalinebasaltic andesite. A large number (65) of Along the Chapala-OaxacaFault Zone lie both histori- analyzedvolcanic rocks from the JaliscoBlock satisf cally active cones of the Michoac6n-Guanajuato tle criteria for primitive characterused in this paper:13 Volcanic Field: Jorullo and Parfcutin. Furthermore. are calc-alkaline, 50 are lamprophyric, and two are Paricutin lies within the densestcluster of Quatemary alkaline (Appendix). cinder cones.Thus, the Chapala-OaxacaFavlt Zote seemsto have focusedmuch of the magmaascending TheMichoacdn-Guanajuato Volcanic Field beneath the Michoac6n-Cuanajuato Volcanic Field during the Quatemary(Johnson & Harrison 1990). As defined by Hasenaka& Carmichael(1985a b, Hasenaka& Carmichael (1987) showed that the 1987),tle Mchoac6n-GuanajuatoVolcanic Field lies petrology of magmas erupted in the Michoacdn- between19.00" and 20.75"N latitude and 101.33"and GuanajuatoVolcanic Field is related to distancefrom 102.66"W longitude, covering >40,000 km2 in the the Middle America Trench.Most Quaternaryvolcanic southernpart of Guanajuatostate and the northernpart rocks are calc-alkaline,but a group of morphologically of Michoac6nstate. It containsmore than 1.000 small old cinder conesin the northem part of the volcanic volcanic centers.includins more than 900 cinder and field is built of evolved alkaline rocks. Primitive

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magrmsexclusively erupted in the southernpart of the Major elementsand classification field mostly alongnortheast-soor5*gs1-6snding aligtr- ments(Hasenaka & Carmichael1987, Connor 1990). On the basisof thesefwo criteri4 122 compositions This study includes24 samplesof primitive rocks from of primitive volcanic rocks from the westernMexican the southern Michoacdn-GuanajuatoVolcanic Field: Volcanic Belt were compiled (Appendix). These are 18 arecalc-alkaline, two arelamprophyric, and four are groupedand discussedby both location androck series: transitional(Appendix). The presenceof lamprophl'res calc-alkaline(34 samples),alkaline (14), lamprophyres in the Michoacdn-GuanajuatoVolcanic Field shows (68), and transitional (6). Compositionaldistinctions that they are not unique to the extensionaltectonic among these rock series are illustrated on Figure 2 regimeof the JaliscoBlock. using the total alkalies versus sihca classificationdia- gram of Le Bas et al. (L986).CIPW normativeparame- ters ne + lc or hy + q arc plotted againstK2OA.{a2O on h"rrnor.ocy oF rrn Pnnrrrrw Volcauc Rocrs Figure 3.

The definition of primitive rocks The calc-allaline rocles Various criteria have been proposed to identify The calc-alkalinerocks have Iry ! q n their CIPW primitive magrnas,those formed by direct partial melt- norms and K2OA{a2Ovalues (fi.7o basis) in the low ing of peridotitic mantle:Mg# x Mg/(Mg + Fd+)] [100 range 0.19-0.40 3). They have phenocrystsof > 63 (Green 1971), FeObd/tr4go < I et al. @ig. Qat$rrni olivine plagioclase, 1983),and Ni > 235 ppm (SatoL977). As emphasized with spinel inclusions, t clinopy- by Myers (1988) and Wallace& Carmichael(1939), roxenet orthopyroxene.On Figure2, primitive calc- however,these criteria derived for applicationto mid- alkaline rocks (circles) are mostly basaltic andesites, ocean-ridgeand ocean-islandbasalts may be inappro- with a few basaltsand andesites. priate to the source regions of subduction-zone magmas. For the calc-alkaline and lamprophyre- The allealineroclcs forming magmas of western Mexico, Ltthr et al. (1989), Wallace & Carmichael(1989, 1992), Allan Jfte alkaline rocks range fromhy- to ze-normative; et aI (L991),and Carmichael et aL (1996)havesuggested two sampleshave hy + q (338,243). T\ei K2OA.{a2O a sourcein the mantlewedge with veinsof phlogopite-, values also are low, from 0.09 to 0.46 Gig. 3). These amphibole-, and apatite-bearing pyroxenife cutting rocks have phenocrystsof olivine with spinel inclu- acrosstlte host peridotite (seealso Foley 1992b).Mag- sions,plagioclase, * slinpptt xene.They eruptedonly mas formed mainly by melting of these veins, with within tle Teprc-Zacoalco Rift and in the nearby, variable contributions from the wallrock peridotite, northernmostJalisco Block. On Figure 2, alkalinecom- need not fulfill the criteria listed above.Furthermore, positions (squares)fall mostly in the basaltand trachy- calculationof Mg# is plaguedby uncertaintyover the basalt fields, except for a single example of basaltic oxidationstate of the sourceregion in the mantle,which andesite.In this study, thesealkaline rocks are distin- is probably quite highly oxidized in the case of the guished from all calc-alkalinerocks by having TiO2 lamprophyre-formingmagmas, in which much of the values(normalized alhydrous) greaterthan I wLVo.For iron is in tle form of Fe3*(Wallace & Carmichael1989. comparisonwith thesealkaline rocls from the westem Lange & Carmichael 1991, Carmichaelet al. 1996). Mexican Volcanic Belt, 16 spinel-lherzolite-bearing Despitethese complications, a consistentbasis for the ne-normatwebasanitic rocks from the Holocene-ageLa definition of primitive sals-alkaline,lamprophyric, and Brefla - El Jagriey Maar Complex (Aranda-G6mez alkalinerocks in the westernMexican Volcanic Belt is et al.1992,Yreret aL.1.992)ofDurango State, ^450 km desireable.In this paper, for purposesof both CIPW NNW of Guadalajara"also are plot0ed(crosses). These norm calculationand classification,primitive rocks are arethe youngestand closest of the major intraplate-type defined as having Mg# 2 62, with Fd+ taken to be lherzolite-bearingmagmas that have eruptedacross the 0.8 X Febd. This Fe3*/Fd+value is equivalent(Kress MexicanBasin-and-Range hovince during the Quater- & Carmichael1991) to an oxygen fugacity along the nary (Luh et al. 1995). The La Brefla - El Jagtiey Ni-NiO buffer (Chou 1978). This oxidation state is basanitesstraddle the basalt,trachybasalt, and basanite probably close to that of the calc-alkalinebasalts and fields on Figure 2, and have K2OA.{a2Ovalues of basaltic andesites,too high for the alkalins magnus, 0.53-{.71. and too low for the lamprophyre-formingmagmas, but a good consistentcompromise. In addition to the The lnmprophyres constraintMg#> 62, rocks with lessthan 6 wt.7oMgO (normalizedanhydrous) also were exclude4 so ns to The lamprophyresare the most diverse rock suite, remove andesitesand dacites with quite low MgO both compositionally and mineralogically. True lam- values,yet Mg#> 62. prophyresare characterizedby phenocrysts(>0.3 mm)

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of phlogopiteor amphibole,with feldsparsrestricted to andesite.In this paper,the designationlamprophyre is the groundmass(<0.03 mm). In the westernMexican usedto includetrue lamprophyresas well asotherrocks Volcanic Belt, a completefansition of rock types is that do not meet the snict petrographiccriteria, yet p, presentbetween lamprophyres and calc-alkalinetypes, show similar geochemicalfeatures, such as high K and no precise distinction is possible. Some of the Sr, and Ba contentsClable 1). Most of the field names transitionalvarieties resulted from mixing betweenthe on Figure 2 are tot used for these lamprophyres,in two end-membermagmas (Luhr & Carmichael1982). favor of pefrographicallydefined rock-narnes based on The lamprophynesform a broad band at high rlkali criteria as [sted in Table 1. Minette is the dominant contentsacross the classificationdiagram Gig. 2), fatl- lamprophyre type in tlre western Mexican Volcanic ing in al1 labeled fields except basaltic andesiteand Belt. As seenon Figure 3, most (56 of 68) lampro-

s10 c.*i o \

o(\l (U z

BasalticAndesite

SiO2 (wt.%)

Calo-Alkaline Lmprophyres Alkaline Trmsitional TZR O tr CR o A o JB o V T MGVF N a LBEJ I

Frc. 2. Systemof classificationbased on silica versustotal alkalies Q-aBaaet al.1986), showing 122primitive volcanic rocks from the westemMexican Volcanic Belt and, for comparison,16 intraplate-typebasanites from the La Breia - El Jagiiey Maar Complex (Appendix). All compositionson this and subsequentdiagrams are normalized on an anhydrousbasis to l,ffiwt.%o, witl f*- = 0.2 X FeM. Namesof volcanic field in the legend:TZR"Tepic-Zacoalco Rift; CR, Colima Rift; JB, JaliscoBlock; 146yp, Michoac6n-GuanajuatoVolcanic Field; LBEI, La Brefla - El JagiieyMaar Complex.

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sl mprophyres

ct +

-F Alkaline

Calc-Alkaline

0.0 0.5 {.0 1.5 2.0 2.5 3.0 K2OlNa20

Ftc. 3. KrOA'{qO versasCIPW normativeparameters ne + lc or lry + 4. This plot excludesthe only three lc-normativesamples, all from Wallace & Carmichael(1989), which plot off-scaletothe upperright minette 100,Krb/Na2O = 5.2, ne + lc = l|.z; olivine leucitite 155,K2O/lrIa2O = 3.9, ne + lc = 1,5.5,and phlogopiteleucitite 166,KroA.{arO = 2.0, ne + lc = 13.0.prior to normcalculation, all compositionswere normalizedon an anhydrousbasis to 100 wt.To,withF** = 0.2 X Fed. Symbols as in Figure 2.

phyres are nz'norarative,and three of these are also The rock nameused by Tats,rmi& Koyaguchi(1989) /c-normative(plotting off-scaleto the upperright). The derives from the absarokite- shoshonite- banakite remarying12 samples,mostly trachybasaltsand trachy- seriesdescribed by Iddings(1895). For thepurposes of basaltic andesitesbut including two samplesof absa- this discussion,key featuresof the shoshoniteassocia- rokite and one of kersantite, are fry-normative; one tion (Morrison 1980)are: (l) typically found in sub- hachyandesiteis also slightly 4-normative.The high duction zones, (2) high total alkalies (Na2O+K2O total alkalies of the lamprophyrcsare a result of ele- >5 wt.Vo),(3) high K2O/1.{a2O(>0.6 at SOVoSiO), (4) vatedK, asseen by the high K2O/t{a2Ovalues (0.4-5.2) low TiO2 Ql.3Vo), and (5) abundantphenocrysts of on Figure 3. olivine, clinopyroxene, and plagioclase.Criteria l-4 Problems of nomenclature plague any literature are sharedby calc-alkalinelamprophyres (nomencla- analysisof volcaniclamprophyres. Most alkalinelqsks ture: Rock 1991).Only by petrography,criterion 5, can from subductionzones are now regardedas belonging shoshonitesand lamprophyresbe unequivocallydistin- to the shoshonite association (Monison 1980). For guished. The unfortunate tendencyin many modern example, Tats,rmi & Koyaguchi (1989) described a petrological studies not to report modal analysesof Quaternary "absarokite" from Katamata volcano in each samplemakes this imfortant distinction difficult solthwestert Honshu (Japan)with olivine, phlogopite, to evaluate.Lamprophyres in some citsesare mistak- clinopyroxene,and hornblende phenocrysts. According enly labeled as shoshonites,but the presenceof phe- to the nomenclatureused in this paper (Table l), how- nocrystic phlogopite or amFhibole (or both) and ever,rhis rock is a kersantite,a variety oflamprophyle. the absenceof plagioclasephenocrysts can clearly

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TABLE I. CHARACTERISTICSOF PRIMITIVE ROCK.ryPES CONSIDEREDAS LAMPROPHYRICIN THIS STUDY

SiO, (wt.%) CIPW Norm PhenocrYstPhases Microphenocrystsand gloundmilssphases

TRUE LAMPROPFIYRES

Minette 48.5 - 55.4 0.6 - 10.1ne Phlogopite+ Olivine + Augite either glassy or rich in Sanidine+ Plagioslase+ Leucite Phlogopiteleucitite 47.7 12.3ne,13.0 lc Phlogopite * Augite Augite, Leuoite, !e-Ti oxides, Apatite Spessartite 53.3 3.9 ne Hornblende * Augite Plagioclas, Sanidine,Augite, Fe-Ti oxides Kersantite 52.7 - 55.9 13.4hy to 1.8ne Homblende+ Augite + Olivine Glass, Augite, Fe-Ti oxides, * Phlogopits + Plagioclrse

RELATED TO LAMPROPHYRES

Basanite 48.5 - 50.5 3.2 - 8.6 ne Olivine + Augite + Plagioclase Augite, Plagioclase,Fe-Ti oxides, Glass Olivine leucitite 46.2 7.5 ne, 15.5Ic Olivine + Augite Phlogopite,Leucite, Fe-Ti oxides, Apatite, Sr-rich Anorthoclase Absarokite 49.2 - 54.3 3.0q,18.6 hl tn 9.1 ne Olivine + Augite Pyroxene,Plagioclase, Sanidine + Phlogopite Trachybasatt 50.4 - 50.5 7.3 - l0.l ne Hornblende+ Olivine + Plagioclase,Olivine, Augite, Augite + Plagioclase Fe-Ti oxides, Glass + Orthopyroxene Trachybasaltic 52.2 - 56,3 3.6- 11.0hy,4.9 ne Homblende+ Olivine + Plagioclase,Olivine, Augite, mdesite Augite+ Plagioclme Fe--Ti oxides, Glass * Orthopyroxene

distiaguishthem. Minettes,kersantites, and other calc- other incompatibleelements increase steadily. For the alkalinelamprophyres should be regardedas hydrous Mascotaregion, Lange & Carmichael(1990) similarly equivalentsof typical shoshonites. notedan apparentfiansition from basalticandesite with olivine and plagioclasephenocrysts to kersantitewith Thz transitional rocks hornblende phenocrysts and no plagioclase phe- nocrysts. This transition also is accompaniedby A final goup of six s:rnplesis designatedas transi- increabesin K and incompatibletrace elements.These tional. Four of theseare hansitional betweenthe calc- observationshighlight &s important role played by alkaline snd alkaline suites;they are otherwisesimilar HzO in the pefiogenesisof lamprophyres,in stabilizing to calc-alkalinerocks, but havel.L-L.z wt.VoTiOz.The phlogopiteand hornblendeand in dsgtabilizingplagio- other two sruaple$are transitional between the calc- clase. The increasesin abundancesof incompatible alkaline suite and the lamprophyres; they have elementsthat accompanythe transition to OH-bearing 1.0-1.1 wt.VoTiOz,just abovethe definedrange for phenocrystsin these suites arc tle oppositeof trends calc-alkalinerocks, elevated Sr (1150-1200ppm), but expectedif increasedHzO in the sourceregion lowered no OH-bearingphenocrysts and abundantphenocrysts the solidus temperatureand eDhancedthe degree of par- of plagioclase.These transitional rocks ran;gefron W- partial melting. Rather,a modelinvolving variable to ze-normative. tial melting of phlogopite-bearingpyroxenite veins and In severalof the rock suitesincluded in this study, surroundingperidotitic mantle is advocatedin a later evidenceexists for transitionsfrom non-lamprophyres section, to true lamprophyreswith increasingcontents of mag- matic H2O and abundancesof incompatibleelements. Trace - element characteristic s For the Quaternarycinder cones of Colim4 Luhr & Carmichael(1981) showedthat a completegxadation is With the exceptionof high-Mg# lamprophyreswith presentfrom basanitewith olivine and minor plaglo- evidencefor accumulationof olivine and inclusionsof clasephenocrysts, to minette with phlogopite,olivine, chromian spinel (Luhr & Carmichael1981), the tbree and augite phenocrysts,microphenocrysts of apatite, main rock-suitesfrom the westernMexican Volcanic and an absenceof plagioclase phenocrysts.In this Belt (calc-atkaline,lamprophyres, and alkaline) show a sequence,the abundancesof P, Sr, Ba, La, Th, and broad overlapin abundancesof compatibleelements on

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plots of Mg# versus Cr and Ni contents.At a given are the alkaline rocks. The lanrprophyreshave consid- Mg#, however,magmas erupted in the JaliscoBlock, of erably steeperREE patternsthat cross the other two both the calc-alkalineand lamprophyre-formingseries, rypes. are depletedin Cr comparedto equivalentsuites from Representativesamples from the tbree suites are the Colima Rift and Mchoacdn-Guanajuato,probably displayed on a multi-elementplot in Figure 7. Three reflecting source-regiondifferences. calc-alkalinesamples (circles), which have the lowest The distinctions among the three rock suites are values of most elements, show the typical patterns easily apparenton plots showingthe concenfation of a associatedwith subduction-zonemagmas: relative high-field-strengthelement, such as Tioversus that of a enrichmentsin the large-radiuslithophile elementsBa" large-radiuslithophile elemen! suchas Ba @ig.4). The K, and Sr, and relative depletionsthe high-field- calc-alkalinerocks (circles) have the lowest values of strength elementsTa, Nb, Hf, and Ti. Four lampro- each parameter.They grade into the broad swarm of phyres (triangles)mimic the calc-alkalinepatterns and lamprophyres(tiangle,s) with increasesin both elemens. their characteristicenrichments @a, K, Sr) and deple- Along a much flatter BlTi trend, the calc-alkaline tions (Ta, Nb, Hf, Ti), but are shifted toward much rocks merge with the alkaline rocks (squares).This higher concenfationsfor nearly all elements.The alka- tend continuestoward the field for basanitesfrom La lins samples,two from the westernMexican Volcanic Brefla- El Jagtiey(crosses). The inlermediatenature of Belt (squares)and one from La Breffa - El Jagiiey the six sampleslabeled transitionat (open and closed (crosses),have entirely different patterns.They extend diamonds)is evident on this plot. through the middle of the plot, with maxima at Ta and Distinctions among the suites in terms of the rare- Nb, and do not show the characteristic peaks and earth elements(REE) areseen on a plot of concentra- troughsof subduction-zonemagm:ts. tions of Yb versus La in Figure 5. The calc-alkaline rocks (circles)have the lowestabundances ofLa and of B and Be systemntics most incompatibleelements. The lamprophyres(trian- gles) attain extremely high values of La and other Hochstaedteret al. (1996) presentedB and Be con- incompatibleelements at relatively low values of yb. centrationsfor a large set of volcanic rocks from the The highest Yb contents are shown by the alkaline westem Mexican Volcanic Belt, including 22 of the rocks (squares)and by the similar basanitesfrom La primitive rocks discussedin this paper:seven are calc- Breia - El Jagiiey(crosses). Representative chondrite- alkaline, 1I arelamprophyric, one is alkaline,and tbree nomralizedREZplotsare shownon Figure 6. The calc- are of transitionalcharacter. Concentrations of Be are alkaline sampleshave the lowest patterns.Subparallel well correlatedwith thoseof I9O (R = 0.94), P2O5(R to them,but shiftedto higher concentrationsof aL|REE, = 0.94),Ba (R - 0.92),and Sr (R = 0.90).Calc-alkaline

5000 4500 4000 3500 E 3ooo ct g 2500 fi 2ooo 1500 1000 500

0.0 0.5 ,|.0 1.5 2.0 2.5 3.0 3.5 4.O TiO2 (wt.%)

Frc.4. TiO, rarsrr Ba contents.Symbols as in Figure 2.

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140

120 Lamprophyres E {00 CL g 80 o J 60 Alkaline 40

2A Calc-Alkaline 0 2 2.5 Yb (ppm)

FIG.5. Yb rlerswI,a contents.Symbols as in Figure2.

rocks have 0.6-1.1 ppm Be, lamprophyreshave M agmnrt c oxyg en fu gac itY 2.84.7 ppm Be, and the alkaline rock is intermediate, with 1.6 ppm Be. Concentrationsof B show no such If a volcanic rock has not undergonepost-eruptive systematics,and are roughly simil2l 1e1 all suites: oxidation,its Fe3VFe2*value and whole-rockcomposi- 2.34.5 ppm in calc-alkalinerocls, 0.7-10.6 ppm in tion can be usedto calculatethe oxygenfugacity of the the lamprophyres,and 1.4 ppm in the alkaline rock. magma(Sack et aL 1980,Kress & Carmichael1991). B/Be values, controlled by the Be concentrations,are The influenceof temperaturein this calculationis only 2.1-8.8 in calc-alkalinerocks and distinctly lower to shift the oxygenfugacity parallel to syntheticoxygen in lamprophyres(0.2-1.9) and alkaline rock (0.9). buffers, and thus it has becomecommon practice to Figure 8 is a plot of Ba concenfrationsversas B[Be expressthese oxygen fugacitiesrelative to the Ni-NiO values,on which the threesuites are clearly distinct and O{NO) solid oxygenbuffer usingthe notationAI'{NO = atransitionalsample bridges the gap.As emphasizedby log/(Oz) Gample)- log/(Od (NNO) butrer (Huebner Hochstaedteret aI. (1,996),the primitive rocks from the & Sato 1970) at the sarnetemperature (Carmichael & westem Mexican Volcanic BeIt have low concentra- Ghiorso 1986,Christie et aL 1986). tions of B and low B/Be values, comparableto other "hot" subductionzones" such as the Cascades(t eeman Of the I22 rocks consideredin this study, 82 were et al. 1990),where young oceaniclithosphere is sub- analyzedfor both loss on ignition (LOI) and FeO by ducting. Theselow ratios contraststrongly with B/Be titration. From this subset,16 samples(20Vo) withLOl values up to 100 for rocks from "cold" subduction greater than L.5 wt.Vo were excluded from oxygen zones,such as the Cental American or Aleutian arcs fugacity evaluation,as theseare the most likely to have (Morris et al. 1990,Ryan & Langmuir 1993,Leeman been oxidized during post-eruptivealteration. The set et al. 1994).In *hot" subductionzones, much of the B of 82 samples,howevero shows no good correlation may be releasedfrom the slab at relativd shallow between LOI and ANNO, and all excluded samples depthsand may not be carriedinto the zone of sub-arc have ANNO > 2; theseare 15 samplesof lamprophyre melting (Beboutet al. 1993). and one alkalinerock.

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!t o .N (6 E L 100 t- zo o 'trrtr !,tr o F o {0

3 tttlttlll rm La Ce Pr Nd Pm Sm Eu cd Tb Dy Ho Er Tm Yb Lu

Calc-Alkaline Lamprophyres Alkaline -+ 22E(cR) -+ 7E (cR) -€F 1 OZR) --F -+ MAS-21(JB) M.4A(JB) -EF 34 CrZR) {- Jor-44(lVlcVD -F 15s(JB) -*- 111(LBEJ) -{- Jor46 (NlcVF)

Ftc. 6. Chondrite-normalizedREE dragran, witl valuesnormalized to 1.274times the concentrationslisted for tle mean Cl chondriteofAnders & Grevesse(1989), which correctsthe latterto a volatile-freeequivalent. Legend identifies sample names (Appendix)followed by volcanic field in parentheses(see Figure 2, caption).

The ANNO valuesof the remaining 66 samplesare Sr,N4 and Pb isotopes plottedin histogramform in Figure 9. The calc-alkaline rocks (z = 27) have a mean ANNO value of 0.9 + Previouslyunpublished results of Sr andNd isotopic 0.8 (1o). Only two sampleseach of the alkatineand analysesby D.J. DePaolo for various rocks from the transitionalgroups are included, and theseplot in the westernMexican Volcanic Belt are listed in Table 2, rangeof the calc-alkalinesamples. The basanitesfrom along with resultsof Pb isotopic analysesfor many of La Brefla - El Jagtiey show a broad range of ANNO tle samesamples from Heatherington(1988). values,exiending down to -1.5. The lamprophyres(n = Sr and Nd isotopic data for all available primitive 33) are much more oxidized than these otler groups, rocks of the westernMexican Volcanic Belt are shown rvith a meanANNO valueof 3.3 t 1.4. in Figure l0; againthe compositionsof the La Breffa-

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!t o .N (E tr ol- 100 z o rlr tr (! E .zo E 10 E o-t-

z lllllllllll CsRbBaThU KTaNb LaCeSrNdP HfZr EuTiTbY

Frc. 7. Multi-elementplol normrlized to primordial-mantlevalues of Wcr;ldet al. (1981).Symbols as in Figure 6.

El Jag0eybasanites are grouped with the alkalinerocks lies about 1100 km west of the East Pacific Rise at from the Tepic-ZacoalcoRifL The calc-alkalinesam- -17'N, however, is more enriched than any of the ples andlamprophyres show considerableoverlap, with primitive Mexican rocks (Grahametal. 1988). calc-alkalinesample 22E defining the depletedend, and ShimadaSeamount is importantin the interpretationof lamprophyreJor46 marking the emiched end. Curi- isotoperatios for the Mexican primitive rocks because ously, the six lamprophyresfrom the Colima Rift form it demonstratesthat the adjacentsub-oceanic mantle a vertical trend on this plot, alongwhich the abundances containsenriched domains of high Sr and low Nd iso- of P, K, La, and other incompatibleelements increase topic ratios. These enricheddomains, perhaps former with decreasingeN6. The two alkaline samplesfrom the basalts,are envisionedto be intimately mixed into the Tepic-Zacoalco Rift and the La Brefla - El Jagtiey dominant depleted peridotite (Zindler et al. L984' basanitesform a trend that is crudely subparallelto the Graham et al. 1988" Prinzhofer et al. 1989). Low overlapping calc-alkalineand lamprophyrefields, but degreesof partial melting can generateliquids that sffied to lower 8?51165rvalues at a given ep6value. mainly tap the endcheddomains, which begin melting This distinction may reflect involvement of seawater at relatively lower temperaturescompared to the during subduction-relatedmetasomatism of the sub-arc surroundingdepleted peridotite. mantle sourcefor both the calc-alkalinesuite and the Only nine samplesof primitive calc-alkaline and lamprophyres,which is consideredto raise 87SrF6Sr lamprophyric character from the western Mexican values with little effect on eN6@ePaolo & Johnson Volcanic Belt have been analyzed for Pb isotopes 1979).Also shownonFigure 10 areresultsofanalyses Qleatherington1988). In Figure 11, these data are of mid-oceanridge basaltsfrom the East Pacific Rise shownon aplotof MPbPMPbversusNPbP@Pb, along (Macdougall & Lugmair 1986, White etal. 1987, with data for seven basanitesfrom La Breia - El Prinzhofer et al. 1989), and of basalts from nearby Jagiiey,which might hdicate the Pb isotopic composi- seamounts(Zindler et al. 1,984,Graham et aI. 1988). tions for alkaline rocks from the westem Mexican Most of these oceanicbasalts are more depletedthan Volcanic Bel! none of which havebeen analyzed. The any of the westem Mexican Volcanic Belt samples, total isotopic variation is relatively small, and broad with most seamountsamples having similar to slightly overlap is apparentamong the different suites. Also enriched Sr-Nd isotopis s6mFositions compared to shown are data for basaltsfrom the East Pacific Rise East Pacific Rise basalts.Shimada Seamount,which and nearby seamounts.The primitive Mexican rocks

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16

14 812 cl- E10 u)ct b8 .86 E 24

-2 -1 234 A NNO

I catc-Rltetinefl tamprophyresI Alkaline

FIG.9. Histogramof ANNO valuesfor 66 rocks ftom the westernMexican Volcanic with valuesof LOI <1.5 wt.Voand 16 La Breffa- El Jaeiievbasanit€s.

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TABLE 2. Sr, Nd, AND Pb ISOTOPICRATIOS POR VOLCANIC ROCKS FROM THE WESTERNMEXICAN VOLCANIC BELT

ttsrAsr t4\d/tnNd epb/rqpb 201pblv pb r*pbfopb Sample Type &a

LAMPROPHYRIC ROCKS

Colina Rirt 500 Bss 0.70390 0.512076 +4.7 18.696 15.604 38.492 507 Bas 0.70386 0.s12094 +5.0 5l I Min 0.70393 0.512091 +5.0 18.664 15.595 38,446 8H Min 0.70391 0.5t2048 +4.1 7E Min 0.70388 0.511990 +3.0 18.636 15.559 38.365 S33 TBA 1E.683 15.600 38.472 B43 Sp 0.70385 0.512112 +5.4

Cerro La Pilita Jor-46 TB 0.70443 0.511948 18.702 15.600 38.518

Mascota FieA M.4A Min 18.677 15.586 38.430 M.l8 Min 18.649 15.580 38.372

CALC-ALKALINE AND ALKALINE ROCKS

Colima Volcano and Adjacent Cinder Cones 228 B 0.70363 0.512134 +5.E 18.575 15.558 38.226 Col-ll BA 0.70374 0.512153 +6.2 18.589 15.584 38.312 Col-Z A 0.70355 0.512125 +5.6 18.578 15.550 38.290 t7B BA 0.7035t 0.512098 +5.1

M ichoacdn-GuanajuatoYolcanic Field Jor-44 B 0.70405 0.512031 +3.8 18.619 15.571 38.344 Jor-lI BA 0.10404 0.512051 +4.2 18.645 15.616 38.380 10s0400A BA 0.70385 0.512039 +4.0 1050-558A AB 0.70375 0.512005 +3.3

Mascota MAS-2r 18.690 15.615 38.452

BASEMENTGRANITIC ROCKS

LHG GT 0.70u1 0.511984 +2.9 18.862 15.599 38.699 r050-540GGd 0.70550 0.512001 +3.2 MAS-4B GX 18.804 15.6t8 38.662

l'he Sr and Nd isotopic analyseswere rnadeby Don DePaoloat UCLA using techniquesdescribed 'n1..Id/rqNd, in DePaolo(l9Sl). Analltical uncertainties(2o) are: in 87SrASr,0.00004, in 0.000022, equivalent to 0.5 in sNd, Results of Pb isotopic analyses are reprinted from Heatherington (1988). Type: rock types abbreviated as Bas, basanite; Min, minette; TB' trachyblsalq TBA, trachybasalticandesite; Sp, spessanite;B, basalt; BA, basaltic andesit€;Cr' granite; Gd, granodiorite; GX, granitic xenolith. Several ofthese samplesare not primitive, and are not discussedin this paper or listed in rhe Appendix: M.l 8 (Carmichael et al. 1996), Col'll and Col-2 (Luhr & Carmichael1980); l7B (Luhr & Carmichaell98l); Jor-l 1 and LHG (Luhr & Carmichael1985);2100A and 558A (Hasenaka& Carmichael1987); 1050-540Gand MAS'4B (unpubl.data). Locations ofbasement granitic rocks: LHG in Luhr & Carmichael(1985), 1050' 540G in Hasenaka(19S6), and MAS-48 from tlre same minetle cone as M.4, described in Carmichaelet al. (1996).

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12

10 seawater contamination z!r8 EPR i c6 Seamounts' : .,\'a o4 CL Alkal tu 2

Shimadal- 0 Seamount\+ -2 0.7020 0.7025 0.7030 0.7035 0.7040 0.7045 0.7050 87sr/86st

Fic. 10. Sr-Nd isotopicplot for primitive volcanicrocks of the wesfernMexican Volcaaic Belt. Samplesplotted are from Table2, andare shown with symbolsfrom Figure2: (228,500,507,511, 8H, TE,B43,Jor-44,Jor46),Verma& Nelson(1989:46, 66), and T.B. Housh (unpublished:109089). Also shom are data for La Breia - El Jagiiey basanites@er et at, 1992), mid-oceanridge basaltsfrom tle East Pacific Rise @PRMORB) [small dots: Macdougall& Lupair (1986), Whrteet al. (1987), hiuhofer et al. (1989)1,and basaltsfrom East Paciflc seamounts[small squares,open plus signs show Shimada Seamountbasalts: Zindler et aI. (1984), Graharnet al. (1988)1.

definea steeperfiend than that formed by most oceanic alkalinerocks and lamprophyresfrom westernMexico, basalts, but a trend nearly identical to that of the may reflect dominanceof the Pb budget by the same ShimadaSeamount basalts. As in the caseof Sr-Nd small-scale,enriched mantle componentas advocated variationsGrg. 10), the Pb isotopic data for the Mexi- by Graham et al. (1988) to explain basalts from can volcanic rocks can be interpretedas a source- ShimadaSeamount (Fig. ll). mixing trend betweenEast Pacific Rise basaltsand a componentrepresented by the extreme sample from DIscussIoN Shimada Seamount.Similar trends toward high 207PbPMW values are characteristicof volcanic rodrs A schematiccross-section depicting subductionof from many subduction-relatedarcs, and are generally tle Rivera Plate beneaththe Jalisco Block is given in interpretedas evidencefor subductionofmarine sedi- Figure 12, which servesas a focus for the following ments, either continent-derivedor pelagic, with high discussion.The mantleis shownto contain small-scale 2MPbPo4Pb values (Churcb 1973,1976, Church & geochemicalheterogeneities, which account for the Tilton 1973,Kay et aL 1978,Davidson 1987). rmpor- enriched isotopic componentsfound in seamount tantly though, the intraplate-typealkaline basaltsfrom basalts(Figs. 10, 11).Five key stepsare numbered on La Breia - El Jagriey,proxies for unanalyzedrelatives Figure 12: from the study areq range to the highest 207Pbln4Pb (l) Fluids begin to migratefrom the young and hot values amongthe primitive volcanic rocks of westem subductedRivera slab at shallow depths, carrying B Mexico (Frg. 11). Theserocks, completely lacking in into the overlying mantleperidotite. the geochemicalsignature of subductionzones @igs. 4, (2) Amphibole breaks don'n in the subductedcrust 7), cannothavegained their elevabd 2o7PbP04Pbvalues by -gO km depth, producing hydrous melts that rise from sedimentsubduction. Instead, these elevated into the mantlewedge and react with peridotiteto form values,and by extensionthose of tle primitive calc- phlogopite-pyroxeniteveins. These veins and the

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enclosingperidotite are draggeddownward by induced Primitiv e lamprophyre-formingand calc-all

, Shimada 15.7 ,+t Seamount

Alkalin It .cl Lamprophyres -'a o- 15.6 -----J $ o , EPR N Seamounts .ct o- N EPRMORB o {5.5 N ;,T.{'t) 15.4 17.8 18.0 18.2 18.4 18.6 {8.8 19.0 19,2 19.4 {9.6 19.8 20.0 ,0. 204 Pb/ Pb Itc. 11. Pb isotopic data for primitive volcanic rocks of the westemMexican Volcanic Belt. Samplesplotted are fiom Table 2 (228, 5@, 5Il,7B, S33,MAS-21, M.4A, Jor-44, Jor-46). Also shownare datafor La Brefla - El Jagiieybasanites @er er aL 1992),mid-ocea:r ridge basaltsfrom the East Pacific Rise (White et al. 1987,hiuhofer et al, 1989),and basaltsfrom EastPacific seamounts(Graham et al. 1988).Symbols as in Figure 10.

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oo

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- 2.4 | ^x t: .erag v -i 9A

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Pra FO >:t -o o .t) td8 lqO 6> :A .9ts N q3 TEEffiT qiOE :>6i .6 bo

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thus the lamprophyreshave low B/Be values Ni contentsof the melts and giving them a "primitive" (Hochstaedteret al. L996). With increasing contribu- character. At a higher temperature,this dissolution tion from the wallrock peridotite, a transition to the grades into partial melting of the peridotite, which primitive calc-alkaline basalts takes place @g. 12, would act to further dilute the vein-derivedmelt. Foley no. 4). The slab-modifiedperidotite apparently contains noted that even where diluted, the geochemicalsigna- significantB @ig. l2,no. 1), andthus the sals-alkaline ture of the vein would be evident in the paflern of basaltshave relatively high B/Be values(Hochstaedter relative enrichmentsof incompatibleelements. et al. 1996). The vein-plus-wallrock melting mechanismadvo- The importance of compositionally distinct veins catedby Foley (1992b) is stongly supportedby data within the mantle was recognizedmore than 25 years for primitive lamprophyres and calc-alkaline rocks ago from studies of composite mantle xenoliths from the westernMexican Volcanic Belt. The lampro- (Wilshire & Trask 1971). Although these veins vary phyres, with their highly elevated KzOA'{azOvalues considerablyin their mineralogy, pyroxenitesare par- (Fig. 3), are likely to have formed primarily by partial ticularly common,as are minerals such as phlogopite, melting of phlogopite pyroxenite veins generatedby amphibole,and apatite, which carry K, P, H, F, and hybridization reactions between hydrous siliceous other elementsthat are not abundantin typical four- melts and peridotite abovethe subductingRivera and phase peridotites. Nicholls & Ringwood (1973) flrst Cocosplates. The lamprophyreshave the highestMg#' suggestedthat hydrous siliceous melts, generatedin Cr, and Ni contentsof all westernMexican Volcanic subductedoceanic lithosphere, could migrate into the Belt rocks. Although theseparameters were augmented overlying mantlewedge and react with the peridotiteto in somerocks tbrough accumulation of olivine with inclu- form hybrid zonesof olivine pyroxenite. These were sionsofchromian spinel [-uhr & Carmichael1981), other envisionedto give rise to calc-alkalinemagmas follow- meltsmay havebeen influenced by olivine and orthopy- ing ascentand partial melting. The processof hybridi- roxene dissolutionin the wallrock peridotite. With zation between hydrous siliceous melt and mantle increasing contribution of the wallrock, the vein- peridotitewas later investigatedexperimentally at high derivedmelt componentis diluted toward compositions pressuresby Wyllie and co-workers for equivalent typicat of calc-alkalinebasalt. As emphasizedby Foley syntheticsystems (Sekine & Wyllie 1982a,b, Wyllie & (L99'2h),however,these still retainthe isotopicand trace- Sekine 1982)and for naturalperidotite-granite couples element of the vein component,although con- (Sekine&Wyllie 1982c,1983, Wyllie et aL 1989).T\ese siderablydiluted in elementabundances. In this sense,the mechanismsfor the lamprophyresof the western Mexican Volcanic Belt studiesdemonstrated a variety of ooessence" productionof olivine-free phlogopitepyroxenites with represent the geochemical of subduction- jadeitic clinopyroxeneas products of the hybridization. zone magmatism.Following the terminology of the Either buoyant diapiric ascentor induced convective BasalticVolcanism Study Ftoject (1981),the western motion was envisioned as carrying these phlogopite Mexican Volcanic Belt lamprophyresare viewed as 'oprobes" p1'roxenitesacross their solidusto causepartial melting. of phlogopitepyroxenite veins in the mantle In a review of experimentaland geochemicalcon- wedgeabove the subductedRivera and Cocos slabs. straints on the origin of ultrapotassicvolcanic rocks, Foley (1992a) showed that liquidus experimentson lnmprophyres in subductionzones theserocks matchpoorly with resultsof partial-melting experiments on phlogopite-rich peridotites, a com- If minettes, kersantites,and other calc-alkaline monly suggestedsource composition. Rather, Foley lamprophyres[nomenclature: Rock (1991)] are the favored phlogopite ha:zburgite or phlogopite pyro- "essence"of subduction-zonemagmatism, why aren't xenite domainsin the mantle. with rare to absentoli- lamprophyres more commonly reported in active vine, as the sourceof potassicmelts, althoughhe noted volcanic -"s'r gimilar veins of phlogopite pyroxenite that few relevantexperiments have yet beenconducted may form abovemost subductedslabs, but the nearly to test this hypothesis.In a companionpaper, Foley pure lamprophyre-forming, vein-derived melts may (L992b) explored the partial-melting mechanismsfor rarely erupt to the surfacein the typical compressive veins containing phlogopite, amphibole, apatite, car- crustalstress regimes of subductionzones. More likely, bonate,and titanates,cutting acrossgamet peridotite. these low-volume hydrous melts lose heat and water Foley emphasizedthe importance of solid-solution during ascen! crystallizing and stagnatingin the upper behavior in phlogopite, amphibole,apatite, and other mantle or crust to form lamprophyric dikes. As vein minerals in extending the temperaturerange of reviewedby Rock (1991), calc-alkalinelamprophyres, melting below the solidustemperature of the surround- of which minetteis the most commonvariety, are usu- ing peridotite. Infiltration of the early vein-derived ally emplacedin convergent-marginssettings as dikes melts into the peridotite along grain boundariescould intruding calc-alkalinegranitoid rocks. Rock (1991) lead to strongdissolution of olivine and orthopyroxene, cited compositeintrusions and other field and geo- mineralsout of equilibrium with the melt. This process chronological evidence indicating that lamprophyre- was envisionedas responsiblefor raising the Mg# and forming and granitic magrnaswere commonly coeval;

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these are equivalent to the coeval lamprophyresand et al. 1990)and rifting eventsin the Volcano arc of the calc-alkalinerocks of the westemMexican Volcanic Belr northernMarianas (Ln et al. 1990) and in Fiji (Gill & The unusualextensional tectonic settingin western Whelan 1989).These cases of oceanicintra-arc rifting Mexico has apparentlyallowed a diversesuite of such provideinsight into mantleprocesses occurring beneath lamprophyre-formingmagmas to rise to the surfaceand the continentalriffs of westemMexico. Interpretations erupt. Other examplesof young lamprophyreserupted of the aboveauthors indicate that lithosphericrifting is in active subduction-relatedarcs also can be cited, accompaniedby advectionof convectingupper mantle, althoughthe role of extensionaltectonics is not gener- unmoffied by subductionand thus low in B andB/Be, ally so clear for these.The best analogis probably the into the sub-arcenvironment @ig. 12, no. 5), to serve 12.44.5 Ma trachybasaltsreported by Hausback asthe sourceof the intraplate-typemagmas. As empha- (1984)from the southernend of Baja Californi4 which sizedby Reagan& Gill (1989),however, the edgeof eruptedat a stagein the rifting history of Baja similar the Rivera plate may be important to intraplate-type to that proposedby Luhr et al. (1985) and Allan magmatismof the Tepic-ZacoalcoRift, perhapschan- et al. (1991) for rifting of the Jalisco Block. Other neling these melts to the surface.In either case,the equivalentsare presentin the Tabar-Feni arc of Papua convecting upper mantle ascendsbeneath the rifting New Guinea,where partial melts from the subductibn- lithosphereand partialy melts during decompression. modified mantle wedge have risen along extensional Becausethis occurs at relatively shallow levels in the faults that fransect the remnant New Ireland forearc mantle, where spinel peridotite is stable rather than (Mclnnes& Cameronl994,Hetzig et aL 1994). garnetperidotite, the intaplate-type alkalinebasalts are relatively rich in the heavyKEE Gigs. 5-7). Primitive intraplate-typeallealine basalts: decompressionmelting of the convectingupper mantle CoNCLUSIoNS

The intraplate-typealkaline basalts of the western In the westernpart of the Mexican Volcanic Belt, Mexican Volcanic Belt must have a distinct sourcein activerifting of the JaliscoBlock is superimposedupon the mantle sem.paredto the calc-alkaline typresand the compressivestress regime relatedto subductionof lamprophyres, because of their differing Ba-Ti the Cocosand Rivera platesbeneath the North Ameri- trends @ig. 4), multi-element patterns @ig. 7), and can plate. This rifting has allowed an unusualdiversity Sr-Nd isotopic compositions(Fig. 10). Worldwide, of primitive magmasto erupt at the surface.The primi- intraplate-typealkaline basaltsare uncommonnear the tive volcanic rocks are convenientlydivided into tlnee volcanicfronts of subduction-relatedarcs. As discussed types: calc-alkaline,lamprophyres, and alkaline. by Gill (1981),Gill & Whelan(1989), and Reagan & The primitive calc-alkalinerocks and lamprophyres Gitl (1989),known post-Mioceneexamples are found are both clearly related to subduction.They have the where the descendingplate is <25 Ma and deep high Ba/Ti values that are typical of subduction-zone Wadati-Benioff zone earthquakesare absen! and in rocks, similuly shapedpatterns on multi-elementplots, many casesthey overlie the edge of the subducting and overlapping Sr, Nd, and Pb isotopic ratios. They plate. Reagan& GiX (1989) discussedthe casefor differ dramatically,however, in major-elementcompo- Turrialba Volcano in Costa Rica" which marks the sitions, proportions of normative minerals, mineral southeasternend of tle Central American volcanic assemblages,and levels of incomFatible-element chain relatedto subductionof the Cocosplate. At the enrichment.The lanrprophyresand calc-alkalinerocks opposite,northwestern end of the Cocos-Riverasub- can be relatedto one anotherthrough a vein-wallrock duction system,the intraplate-typealkaline basaltsof melting mechanismin which the lamprophyresformby the westernMexican Volcanic Belt are virtually con- preferentialpartial melting of low-temperatureveins of fined to the Tqic-Zacoalco Rift, which marks the phlogopite pyroxenite, which carry the "geochemical northernboundary of the JaliscoBlock and also over- essence"of subduction-zonemagmatism. Rifting has lies the nortlem edge of the subductingRivera plate. allowedthese low-degree hydrous melts to rise through Parallelsbetween the westemMexican Volcanic Belt the crust and erupt. Lamprophyre-formingmelts are and Turrialba extendto Sr and Nd isotopic variations. probably generatedbeneath most subduction-related Two samplesanalyzedfrom Turrialba @eagan& Gill arcs, but the compressivestress regimes in most arcs 1989) have similar eN6,but the intraplate-typesample causethem to solidi$ in the upper mantle or crust as has a slightly lower 875r/865rvalue than the coeval classic lamprophyric dikes. Higher degreesof partial enls-alkalinesample, just asseems to bethe case for the melting, involving the wallrock peridotite, dilute this westernMexican Volcanic Belt suites(Fig. l0). vein componentand producethe calc-alkalinebasalts Magmas with either pure or partial intraplate-type that feed the volumetricallydominant andesitic subduc- signatureshave also erupted during and immediately tion-zonevolcanoes. following intra-arc rifting events(Gill 1981,Pearce & The primitive alkalinerocks eruptedonly in proxim- Peate 1995). The best describedexamples are the ity to the Tepic-Zacoalcorift and showno geochemical Sumisu Rift of the Izu-Ogasawaraarc (Hochstaedter evidence of a subduction-zoneorigin. Instead, they

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have the typical characteristicsof intraplate-typemag- is it, and what is its relationship to the Colima rift? mas, with low BalTi values. These alkaline magmas Ge ophy s. Res.Izft . 22, 3M 5-3078. probably formed at relatively low pressuresin the Volcarqsu SrUDYPRoJEc"r (198L): Basaltic Vol' spinel peridotite field, during decompressivepartial BAsALTTc ing canivn on the Terrestrial Plancts. Pergamonhess, Inc., melting of convec upper mantle that rose beneath New York, N.Y. the thinning sub-arclithosphere. BEBou'r,G.8., RYAN,J.G. & LSEMAN,W.P. (1993):B-Be Acrnqowueocnmxrs systematics in subduction-relatedmetamorphic rocks: Characterizationof the subductioncomponent. Geochim" Don DePaolokindly permittedinclusion in this paper Cosmo chim. Acta 57, 22n -2238. of resultsof his Sr and Nd isotopicanalyses for vol- canic rocks from western Mexico. The manuscript BorrRGoIs,I. & Mcrauo, F. (1991):Active frapentation of Plateat the Mexicantriple junction off was improved considerablyin focus and logic the North American Manzanillo. Geo-Marinelztt. 11, 59-65. through reviewsby Ja:nesF. Allan, William P. Leeman, Robert F. Martin, an anonymousindividual, and par- RENARD,V., ALIBounq,J., Barov, W., Bannnn, E., ticularly Brian Cousens.I'm also grateful to Alfred G. CADrus, T., CARFANTAN,J.-C., Gumnsno, J., MAM- Hochstaedterand Ian S.E Ca:michaelfor discussions MERIcKX,J., Mr,ncm. DB LEPDIAY,B., Mcuuo, F. & about the origins of westernMexican magmas,and to SossoN,M. (1988):Fragmentation en coursdubord Ouest GenyongPeng, who tutored me in computergraphics du continentNord Am6ricain: les frontihres sous-rurines during preparationof this manuscript. du Blm Jalisco(Mexique). C. R Acad. Sci.Paris 3Vl ' n', 11.21.-1130. REFERENCES BunsecH, G.V., FRoHlcH, C., Pmtnwcror, W.D. & MAru- Moro, T. (1984):Seismicity andtectonics of the suMucted ArAN, J.F. (1986): Geology of the norftern Colima and CocosPlate. J. Geophys.Res. E9, 7719-7735. Zacoalcograbens, southwest Mexico: Late-Cenozoicrift- ing in the MexicanVolcanic Belt. Geol,Soc, Am-, BulI. 97, J.-M. & Rostr{,C. (1979):K-Ar dafingon east€m 473485. Carr"reons,, Mexicanvolcanic rocls: relationsbetrreen the andesitic and the alkaline . J. VolcunL Geothemt"Res. 5,99-11'4. & CAR!flcHAEL, I.S.E. (1984): Lamprophyric lavas Eovinces in the Colima Graben, SW Mexico. Contrib. Mineral. & Gmonso,M.S. (1986): Oxidation- Petrol.88.203-216. CARrvflcHAEL,I.S.E. reductionrelations in basic magma:a casefor homogene- Earth Plarrct Sci.Ictt.78,2W-210. Ns-soN, S.A., Lun, J.F., Camacueu,, I.S.E., ousequitbria- Woper, M. & WAu,AcE,P.J. (1991): Pliocene-Holocene (1996): minetoes rifting and associatedvolcanism in southwestMexico: an . LAN@ RA & LUID"J.F. Quaternary Mascots,westetn Mexico: a exotic terranein the making. /n The Gulf and Peninsular and associa.tedvolcanic mcls of pkte a zubdrction-modified hovince of the Californias(J.P. Dauphin& B.A. Simoneit, consequerceof extensionabove wedge.Contrb. MineraL PetroL 72,4,342333. eds.).Am. Assoc.Petrol. Geol.,Mem. {1 , 425445. mantle (1982): largeearthquakes ANDERS,E. & GnsvBssn,N. (1989): Abundancesofthe ele- CIIAEI-E.P. & Srswanl G.S. Recent for ments: meteoritic and solm. Geochirn.Cosmochim. Acta alongthe Middle AmericanTrenchand theirimplications 53.197-21,4. the subductionprocess. J. Geoplrys.Rel87,329-338.

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APPENDIX:PzuMITIVE SAMPLESINCLUDED IN THIS STUDY (Mgt> 62, Mgo > 6 wt.%)

Tepic-Zacoalco Rift

Calc-alkaline: Righteret al. (1995):411 Transitional: Moore et al. (1994):A-3, 3-106 Carmichael(1984): l, 16,34,46,66 Intraplate-typealkaline: ' Nelson& Nelson& Livieres(1986): 338 Moore et al. (1994): A-2, A-30,A-73,4-107,48-166 Righteret al. (1995):396

Colima Rift

Calc-alkaline: Luhr & Carmichael(1980): Col-I3 Luhr & Carmichael(1981): 22E Lamprophyric: Luhr & Carmichael(1981): 500, 501, 507, 508, 510, 5l l, 5A, 6A, 6D, 6E, 7E, 8G, 8H Luhr & Carmichael(1982): S33, S34 Allan & Carmichael(1984): 843

Jalisco Block

Calc-alkaline: Lange& Carmichael(1990): MAS-19, MAS-21, MAS- 22,i|lASAI, MAS-42,MAS-45, MAS-108, MAS- 127,MAS-132, MAS-149, MAS-I 50, MAS-309 Wallace& Carmichael(1992):235 Lamprophyric: Wallace& Carmichael(1989): 100, ll3,121,154, 155, 166, 207, 21 1, 227,231, 242 Lange& Carmichael(1991): MAS-I41, MAS-142, MAS' 189,MAS-196, MAS-I98, MAS-I99 Wallace& Carmichael(1992): I 12' ll4' ll7, 120,72+A' 125, 128, 143, 145, 164,208, 2',18,219, 220, 236, 243,245,246 Carmichaelet al. (1996):M.3A, M.4A, M.6, M.9, M.11, M.15,M.30, M.39, M.48, M.52, M.102, M.134, M.l77b,M.177c, M.405 Intraplate-We alkaline: Righter& Carmichael(1992):243,248

Michoacdn-GuanajuatoVolcsnic Field

Calc-alkaline: Luhr& Carmichael(1985): Jor-lA, Jor-2,Jor-l5, Jor-16, Jor-19,Jor-25, J0126,Jor-29, Jor-31, Jor-39B, Jor42, Ior44 T.B. Houshand J.F. Luhr (unpubl.data): 109089 Hasenaka& Carmichael(1987): 416A, 4268, 517A, 534, 536L Transitionali Hasenaka& Carmichael(1987): 4084, 417A,520,542 Lamprophyric: Luhr & Carmichael(1985): Jor-46, Jor-46d

Mexican Basin and Range Province

Intlaplate-typealkaline: Pieret al. (1992):101, 1.02,103,104, 105, 106, 107, 108, 109,r10, lll,114, lls, l16,rt7,120

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