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Arc-rift transition volcanism in the Puertecitos Volcanic Province,northeastern Baja California, Mexico

Arturo Martın-Barajas CICESE, Ensenada, Mexico, P.O. Box 434843, San Diego, California 92143Joann M. Stock Caltech, Pasadena, California 91125Paul Layer University of Alaska, Fairbanks, Alaska 99775Brian Hausback California State University, Sacramento, California 95819Paul Renne Institute for Human Origins, Berkeley, California 94709Margarita Lopez-Martınez CICESE, Ensenada, Mexico, P.O. Box 434843, San Diego, California 92143

ABSTRACT

The Neogene Puertecitos Volcanic Prov-ince of northeastern Baja Californiarecords a transition from arc-related vol-canic activity to rift volcanism associatedwith opening of the Gulf of California. Theeastern Puertecitos Volcanic Province is di-vided into three volcanic sequences basedon mapping, petrology, and 40Ar/39Ar geo-chronology. The lowest sequence comprisesearly to middle Miocene (20–16 Ma) arc-related andesitic lava flows, volcanic necks,and proximal pyroclastic and epiclastic de-posits up to 400 m in thickness, with minorbasaltic lava flows. Following the initiationof crustal extension in the region (11–6Ma), synrift volcanism produced two rhy-olitic sequences that discordantly overliethe arc-related rocks. The older synrift se-quence (6.4–5.8Ma) is composed of rhyolitedomes and a series of pyroclastic flows up to300 m thick. The upper sequence (3.2–2.7Ma) consists of ash-flow tuffs and pumice-lapilli pyroclastic flows, collectively up to200 m thick. Minor andesite eruptions fol-lowed each episode of silicic synrift volcan-ism. Synvolcanic faults produced topo-graphic relief that controlled deposition ofthe pyroclastic flows and caused gentlerdips upsection. Rhyolite domes are alignedparallel to the predominant north-north-west to north-northeast fault pattern.All three volcanic sequences are calc-al-

kaline. However, the synrift andesite ischaracterized by lower K2O, lower incom-patible element concentrations, and lessfractionation of light rare earth elementsthan the arc-related basalt and andesite.

This suggests that the primary melts weremore primitive for synrift andesite than forthe arc-related rocks.

INTRODUCTION

Both the geochemistry and the style of vol-canism in the Baja California peninsulachanged in the last 10–15 Ma. This waspartly due to the cessation of subductionalong the west coast of Baja California andpartly due to the development of the Gulf ofCalifornia rift system (e.g., Hausback, 1984;Saunders et al., 1987; Sawlan and Smith,1984; Sawlan, 1991) (Fig. 1).Postsubduction volcanic rocks have vari-

able chemical compositions. In the centraland southern parts of the peninsula, calc-alkaline lavas with moderate to high K2Oevolved to alkaline lavas rich in MgO duringmiddle and late Miocene time (Sawlan andSmith, 1984; Saunders et al., 1987). Volcan-ism related to continental rifting maintainedits mainly calc-alkaline character along themargins of the Gulf of California (Sawlan,1991). In contrast, compositions are typi-cally tholeiitic within the oceanic spreadingcenters of the gulf itself (Saunders, 1983;Saunders et al., 1982). Synrift calc-alkalinevolcanic rocks in the northern gulf region(Fig. 2a) include ca. 9 Ma andesite and da-cite in the Sierra Pinta (Gastil et al., 1979),a 10–12 Ma subalkaline volcanic field in theSierra San Alberto (Sawlan and Smith,1984), and some 12–14 Ma ignimbrite de-posits in the Bahıa de Los Angeles–SanBorja area (Gastil et al., 1979; Delgado-Argote et al., 1992). In coastal Sonora, re-ported ages for the youngest calc-alkaline

rocks are ca. 12 Ma (Gastil and Krumme-nacher, 1977; Gastil et al., 1979; Mora-Al-varez, 1993). Andesitic to dacitic pyroclasticflows on Isla Tiburon are between 15 and 11Ma (Neuhaus, 1989). In several places calc-alkaline volcanism continued in the gulf de-pression during the mature stage of the riftin Pliocene to Recent(?) time. Plio-Pleisto-cene calc-alkaline volcanic rocks have beenreported in the Tres Vırgenes area, in IslaSan Esteban (Fig. 2a), and possibly in theSanta Ana and La Reforma calderas (Saw-lan, 1981; Desonie, 1992; Demant, 1981).The Miocene-Pliocene Puertecitos Vol-

canic Province is situated along the north-western margin of the Gulf of California(Figs. 2a and 2b) and was a major locus oflate Neogene calc-alkaline volcanism, dom-inantly rhyolite ignimbrite and lava. Normalfaulting north and northwest of the Puerte-citos Volcanic Province began after 11 Mabut before 6 Ma and was accompanied bythe emplacement of voluminous rhyoliticpyroclastic material and lavas (Stock andHodges, 1990; Stock, 1989, 1993). In theeastern and northeastern part of the Puerte-citos Volcanic Province, late Miocene toPliocene rhyolite lava and ignimbrite overlieandesitic rocks from the early Miocene vol-canic arc (Stock et al., 1991). Near the coast,Pliocene volcanic and marine rocks are cutby normal faults, indicating continued ex-tension along the coastal margin of thePuertecitos Volcanic Province (Martın-Barajas and Stock, 1993).Here we (1) describe the volcanic stratig-

raphy and geochronology in the northeast-ern part of the Puertecitos Volcanic Prov-ince, (2) document the relationship between

GSA Bulletin; April 1995; v. 107; no. 4; p. 407–424; 14 figures; 1 table.

407

Data Repository item 9514 contains additional material related to this article.

volcanism and extension, (3) show the geo-chemical and petrologic characteristics ofthe volcanic succession, and (4) examine theevidence for a crustal contribution to thesynrift volcanism.

TECTONIC SETTING

Neogene volcanism in Baja California isrelated to the consumption of the Farallon-Guadalupe plate beneath North America,the cessation of subduction beneath the con-tinent, and the transition to Gulf of Califor-nia rifting and strike-slip motion. The Far-allon-Pacific ridge first contacted the NorthAmerican plate at some location north ofthe Baja California peninsula, not earlierthan 29 Ma (Atwater, 1989; Stock and Mol-nar, 1988). Subsequently, volcanic activitydecreased from north to south as subductionstopped (Fig. 1). In northern Baja Califor-nia, arc-volcanism apparently ended at17–14 Ma, in contrast to southern BajaCalifornia, where it finished at ca. 12 Ma(Sawlan and Smith, 1984; Hausback, 1984;Sawlan, 1991). Calc-alkaline volcanism con-tinued, however, in some areas of the gulfdepression during the development of theGulf of California rift system (Fig. 2a). Agenetic association has been suggested be-tween extensional and transtensional tec-tonics and explosive calc-alkaline volcanism,

due to melting of the continental crust(Gastil et al., 1975; Sawlan, 1991).The Puertecitos Volcanic Province is cut

by north-northwest to north-northeast–strik-ing high-angle normal faults (Fig. 2b) thatare both synthetic and antithetic to themain gulf escarpment (Dokka andMerriam,1982). North of the Puertecitos VolcanicProvince, the escarpment is defined by thenorth-northwest–striking San Pedro Martırfault, whereas to the south its topographicexpression is less dramatic and consists of aseries of smaller normal faults that strikesubparallel to the escarpment. It is clear lo-cally that these faults offset pre–6 Ma rocksmore than post–6 Ma rocks, so that possiblyabout half of the extensional faulting oc-curred prior to 6 Ma (Stock and Hodges,1990). Toward the east, volcanic and sedi-mentary rocks of Plio-Pleistocene age arecommonly tilted, indicating that significantrift-related extensional faulting was still on-going in the northeast part of the Puerteci-tos Volcanic Province into Pliocene andpossibly Quaternary time.

METHODS USED IN THIS STUDY

Our terminology for volcanic and volca-niclastic units follows Fisher (1961), Cas andWright (1987), and Bard (1986). We first es-tablish a descriptive classification (Fisher,

1961), and only in those cases when the pet-rologic characteristics and the field relation-ships define it is a genetic classification pro-posed (Cas and Wright, 1987).

Analytical Techniques

Whole-rock major-oxide analyses wereobtained by X-ray fluorescence (XRF) atSan Diego State University (SDSU) and atthe University of Massachusetts, Amherst(UMass). Standard andesite from GreatValley (AGV) was used for major-oxideanalyses at SDSU. Most trace-element anal-yses were made using XRF at UMass.Selected samples were analyzed for addi-tional trace elements by inductively coupledplasma mass spectrometry (ICP-MS) atWashington State University. The detectionlimits in the ICP-MS analyses were 0.5–1.0times chondrite values, and reported valuesare at 10% confidence level. Standards wereBCR-1 and in-house standards (CharlesKnack, 1992, written commun.). AdditionalXRF analyses for trace elements were per-formed by K. Righter (Berkeley, California)to verify ICP-MS results. Microprobe anal-ysis was done at Harvard University on aCameca probe.40Ar/39Ar dates were done by step heating

at the Geochronology Laboratory, Univer-sity of Alaska, or by laser fusion at the In-

Figure 1. Simplified tectonic map of northwest Mexico showing the age of the youngest magnetic anomalies identified on the PacificPlate along the California and Baja California margins (modified from Atwater, 1989). Diagonal shading shows possible distribution andages of arc-related rocks in southern Baja California (from Sawlan, 1991, and Hausback, 1984). PVP, Puertecitos Volcanic Province; EPR,East Pacific Rise. Dotted lines show the position of a dead rift, representing the cessation of Guadalupe-Pacific spreading, and coevalcessation of subduction, at the time of anomaly 5A (12.5 Ma). Note that, since they formed, these sea-floor features have been displacedrelative to Baja California, because of dextral faulting along the western margin of the Peninsula and in the California ContinentalBorderland region.

MARTIN-BARAJAS ET AL.

408 Geological Society of America Bulletin, April 1995

stitute for Human Origins (IHO) atBerkeley (Tables 1a and 1b). Analyses atIHO generally employed the methods andfacilities of Deino and Potts (1990). Iso-topic methods are described in Appen-dix 1, and data tables are in the GSA DataRepository.1

VOLCANIC STRATIGRAPHY ANDGEOCHRONOLOGY

Based on petrologic characteristics, strat-igraphic position, and isotopic ages, we di-vide the volcanic rocks into three groups(Fig. 3). Group 1 comprises rocks of arc or-igin: early and middle Miocene andesiticand dacitic rocks with minor basalt (Tma).Group 2, Miocene rhyolite (Tmr), consistsof rhyolite domes, flows (Tmrl and Tmru),and ash flows of the Tuff of El Canelo (Tmc)that lie between rhyolite flows. Group 2 also

includes local andesitic flows (Tba). Group3 is a series of pyroclastic flows of variablethickness (Pliocene rhyolite, Tpr) and alsoincludes an andesitic volcano and youngandesitic lava flows (Tpa).

Group 1: Arc-Related Andesite

Miocene arc rocks of Group 1 are wellexposed in Arroyo Los Heme (Fig. 3), whereoutcrops of Tma form a volcanic center ;4km in diameter. The volcanic necks show apronounced paleorelief of up to 400 m. Co-

1GSA Data Repository item 9514, Tables 2, 3,4, and 5 and Appendices 1 and 2, is available onrequest from Documents Secretary, GSA, P.O.Box 9140, Boulder, CO 80301.

Figure 2. (a) Index map of northwest Mexico showing the areas with early-rift calc-alkaline volcanism: SP, Sierra Pinta (Gastil et al.,1979); SRB, Santa Rosa Basin (Bryant, 1984); PVP, Puertecitos Volcanic Province (this study); BA, Bahia de Los Angeles (Delgado-Argoteet al., 1992); SB, San Borja (Sawlan, 1991); IT, Isla Tiburon (Neuhaus, 1989); CS, Central Sonora (Gastil et al., 1979); 3V, Volcan TresVırgenes (Sawlan, 1981, 1991); SA, Sierra San Alberto (Sawlan, 1991); SU, Sierra Santa Ursula (Mora-Alvarez, 1993); SR, Santa Rosalıa;LR, La Reforma and Santa Ana Calderas; SE, Isla San Esteban (Desonie, 1992); CP, Cerro Prieto (tholeiitic; Herzig, 1990). Modified fromSawlan (1991).(b) Simplified geologic map of the Puertecitos Volcanic Province in northeast Baja California, showing location of the studied area.

SPMF, San Pedro Martır Fault; MGS, Main Gulf Escarpment. Geologic background modified from Gastil et al. (1975); geologic map at1:250 000 scale.

PUERTECITOS VOLCANIC PROVINCE, BAJA CALIFORNIA, MEXICO

409Geological Society of America Bulletin, April 1995

eval pyroclastic flow deposits and epiclasticdeposits crop out near the volcanic necksand are interstratified with lava flows. Thepyroclastic rocks consist primarily of blocksand lithic lapilli, and the epiclastic depositsconsist of matrix-supported bedded con-glomeratic breccia and gravel- to sand-sizeddeposits. Suspended particles are up to 20cm in diameter, and the matrix is composedof ash and lithic lapilli. The breccias are in-terpreted as proximal avalanche and debris-flow deposits.In Arroyo Los Heme the arc sequence in-

cludes a cinder cone deposit interstratifiedwith the epiclastic deposits. The cinder conedeposit consists of clast-supported scoriabeds centimeters to decimeters thick. An an-desite dike cuts and a sill overlies the cindercone deposit, which is clearly coeval with theorogenic andesite rocks.Hornblende from two samples of a vol-

canic neck (sample 2) and a breccia (sample1) yielded very similar age spectra (Fig. 4a),but the bulk of the 39Ar was released in oneor two fractions. However, the agreementbetween the ages together with the homo-geneous 37ArCa/39ArK ratios for fractionswith the bulk of the 39Ar released, supportthe 16 Ma age as a good estimate for theclosure age for the hornblendes (Table 1a).

Group 2: Synrift Rhyolite to AndesiteLava and Tuffs

Synrift Rhyolite Lavas. Rhyolite domescrop out over a distance of 10 km in thenorthern region (Fig. 3). Along Arroyo LaCantera, the northernmost rhyolite domecomplex is 3 km in east-west extent. A seriesof glassy flows of unknown age is the lowestunit (Tmrl), which underlies the Tuff of ElCanelo (Tmc), which pinches out against thesouthern flank of the dome. Bedded lithic-lapilli deposits interpreted as airfall depositslocally overlie the lower rhyolite flows. Anupper series of rhyolite flows (Tmru) over-lies the Tuff of El Canelo east and southeastof Arroyo La Cantera.One whole-rock sample of the upper

rhyolite lava (Tmru) in Valle Curbina(sample 3, Table 1a) was dated by the step-heating technique, yielding a two-step pla-teau with an age of 5.80 6 0.03 Ma. Ad-ditionally, we attempted to determine theage of the upper rhyolite lava by dating anandesitic dike that cuts it in Valle Curbina.A whole-rock, step-heating analysis wasrun twice (sample 4, Table 1a). A per-turbed age spectrum was obtained on bothexperiments (see Fig. 4c). Two fractions,representing 40% of the 39Ar released,

yielded a 5.1 6 0.3 Ma age, which may betaken as a lower age estimate for the em-placement of the dike. Due to the largeuncertainty, however, these data do notprovide additional constraints on the ageof the rhyolite sequence. The time of in-ception of the synrift silicic volcanism isnot yet constrained, but the lower rhyoliteflows may be partially coeval or close inage to the Tuff of El Canelo discussed be-low. This is suggested by the air-fall de-posit, likely related to the lower rhyolitedome, that overlies the lowermost unit ofthe Tuff of El Canelo (Table 2 in the DataRepository). This deposit suggests that theemplacement of the rhyolite dome ispartly coeval with the lower units of theTuff of El Canelo.Rhyolite lavas are volumetrically impor-

tant elsewhere in the northern PuertecitosVolcanic Province and, together with out-crops in the study area, may form a latestMiocene rhyolite dome field that nowreaches more than 40 km in a north-southdirection and 15 km in an east-west direc-tion. Other rhyolitic lavas crop out in thesouthern Sierra San Fermin (Fig. 3), northand northwest of Cerro Canelo, and bothnorth and south of ArroyoMatomı (Fig. 2b).Southwest of Puertecitos, the upper rhyoliteflows crop out near the base of some ar-royos, and southward, along the coastalzone, both the rhyolite and the Tuff of ElCanelo pinch out.Small andesite flows and dikes overlie the

rhyolite lava and the Tuff of El Canelo. Inthe Arroyo Los Heme area, a small-volumebasaltic andesite lava flow (Tba) overliesepiclastic deposits derived from the arcrocks along an erosional unconformity(Fig. 3). Additionally, the andesite flow inArroyo Los Heme underlies a heterolitho-logic conglomerate that contains beds a few

TABLE 1. ISOTOPIC AGES OF SELECTED VOLCANIC UNITS IN THE NORTHEASTERN PUERTECITOSVOLCANIC PROVINCE

(a) 40Ar/39Ar ages by step heating

ID number Sample Unit Subunit ti (Ma)* tp (Ma)† %39Ar§

1 II-15 (hbl) Tma LH 16.16 0.3 15.9 6 0.2** 622 21-III-6 (hbl) Tma LH 16.86 0.2 16.3 6 0.1 663 22-III-1 (wr) Tmr Tmru, VC 6.026 0.04 5.80 6 0.03 684 30-IV-4 (wr) Tba VC 4.76 0.6

30-IV-4 (wr) Tba VC 5.26 0.2 5.1 6 0.3 405 I-16 (wr) Tph Tph 28 (?), LH 26 0.1

I-16 (plag) Tph Tph 28 (?), LH 2.56 0.6 2.7 6 0.4 746 II-13 (wr) Tph Tph 28 (?), LH 3.296 0.04 3.36 6 0.03 91

II-13 (plag) Tph Tph 28 (?), LH 5.6 6 0.17 III-6 (wr) Tpr Tpt-b, VC 3.96 0.1 3.5 6 0.1** 92

III-6 (wr) Tpr Tpt-b, VC 3.99 6 0.06 3.39 6 0.06** 568 VP-1 (wr) Tpa VP 2.66 0.1 2.6 6 0.1 81

(b) 40Ar/39Ar ages by laser fusion

ID number Sample Unit Subunit Age (Ma)†† (40Ar/36Ar)i MSWD n§§

9 EST-55 (san) Tmc Tmc1, LC 6.446 0.02 291.6 6 4.9 1.01 2010 12-I-91 (plag) Tpr Tpvc, VC 3.276 0.04 298.7 6 17.4 1.13 1711 MT-91-41 (plag) Tph Tph 28, LH 2.656 0.05 299.7 6 5.0 0.69 2012 EST-65-1 (plag) Tpr Ttg (Tpt-b), SF 3.086 0.04 312.8 6 11.2 1.77 19

Abbreviations: plag, plagioclase; wr, whole rock; san, sanidine; hbl, hornblende.Note: Samples 1–8 were analyzed at the Geochronology Laboratory, University of Alaska at Fairbanks. Samples 9–12 were

analyzed at the Geochronology Center of the Institute of Human Origins. See Appendix 1 for details. Complete analytical resultsare in the Data Repository.*Integrated age.†Plateau age.§Percentage of 39Ar released in the plateau segments.**Preferred ages consisting of only one fraction; see text for details.††Isochron age.§§Number of points fitted.

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Figure 3. Simplified geologic map of thestudied area in the northeastern Puerteci-tos Volcanic Province showing the mainstructural grain. Shaded units are andesiticunits of the three main volcanic groups. SiteA, measured section of Tmec (Table 2 in theData Repository). Site A*, measured sectionof the Tuff of Mesa El Tabano (Group 3, incomposite stratigraphic column Mesa ElTabano–Arroyo La Cantera of Fig. 5). SitesB and B* measured sections of Tph12–Tph29 and Tph1–Tph11, respectively (seeTable 3 in Data Repository).



decimeters thick of well-sorted pumiceous-lapilli. The conglomerate may represent anerosional unconformity with the units ofGroup 3. This andesite flow is interpreted to

be coeval with the ca. 5.0 Ma andesite fromValle Curbina.Tuff of El Canelo (Tmc). This sequence of

multiple ash flows crops out between Arroyo

La Cantera and the Sierra San Fermın(Fig. 3), and for some distance west of CerroCanelo (Fig. 2b). On Mesa El Tabano andMesa La Angostura, the Tuff of El Canelo is

Figure 4. Step heating 40Ar/39Ar versus heat diagrams showing plateau ages for se-lected samples analyzed at the Geochronology Laboratory, University of Alaska. IDsample number as in Table 4 in the Data Repository, and location in Figure 3. Note thatin Figure 4c the progressive increase of apparent age with temperature does not permitidentification of a stable plateau, although a minimum age for the sample would be 4.5Ma (the age of the first plateau).



overlain in angular unconformity by the up-per rhyolite flows and by Pliocene ignim-brites. The Tuff of El Canelo is cut bynorth-northwest–striking high- and low-angle normal faults that produced stronglytilted blocks; in the northern part of thestudy area, the Tuff of El Canelo is tilted508–708WSW, but south and east, the dipdecreases. At site A of Figure 3, the dip de-creases upward, from 258–308 at the base to58–108 at the top. No paleosols or abruptangular unconformities are observed withinthe sequence. In Arroyo La Cantera, theTuff of El Canelo rests upon, and pinchesout against, the older rhyolite flows (Tmrl)and related beds of stratified lithic and pum-ice lapilli, interpreted as airfall deposits.In Arroyo La Cantera (site A of Fig. 3),

the Tuff of El Canelo comprises six coolingunits (Tmc1 to Tmc6) with a cumulativethickness of nearly 300 m (Table 2 in theData Repository). The basal unit (Tmc1), anunwelded crystal-rich pyroclastic flow with amaximum outcrop thickness of 5 m, directlyoverlies and pinches out against the lowerrhyolite flows and bedded airfall depositsand is locally overlain by up to 1m of beddedrhyolite lithic lapilli interpreted as airfall de-posits. Units 2 and 4 are thick compoundcooling units (each;100 m), separated by athin (1.5 m thick) bedded ash and lithic-la-pilli deposit (unit 3). Unit 5 is a 10- to 15-m-thick unwelded lithic tuff, and unit 6 is athick (up to 70 m) densely welded, ash-flowtuff. Unit 6 may correlate northwestwardinto the lower reaches of Arroyo El Canelo,with a 300-m-thick compound cooling unitof densely welded tuffs (unit t12 of the Tuffsof El Canelo of Stock et al., 1991). In thethickest units of Tmc in Arroyo La Cantera,the lithic content is ,25% and glassy juve-nile material predominates. Thickness andwelding variations in the Tuff of El Caneloindicate that the source may be located tothe west and close to our study area.Laser fusion of sanidine from the basal

ash-flow tuff (Tmc1, Table 2 in the Data Re-pository) yielded a well-defined isochronwith an intercept age of 6.446 0.02 Ma (Ta-ble 1b). This result constrains the maximumage for Tmc and is consistent with the 5.860.4 Ma age of the overlying rhyolite flows,discussed above. Thus, most of the rhyoliticvolcanism ofGroup 2 occurred in latestMio-cene time.

Group 3

Rhyolite Ignimbrites (Tpr). This se-quence comprises the Tuff of Los Heme

(Tph) and the Tuff of Mesa El Tabano(Tpt), which are two approximately coevalsequences of pyroclastic flows collectivelylabeled Tpr (Pliocene rhyolites). Tpr formsthe mesas and tilted blocks visible from thecoastal highway from Arroyo Matomı tosouth of Puertecitos. The most complete se-quence of Tuff of Los Heme is observed inthe Arroyo Los Heme area, where it directlyoverlies arc-related andesites, and locallyoverlies the rocks of Group 2. Here, the Tuffof Los Heme contains .25 cooling units,including ash flows, pyroclastic flows withpumice and minor lithic lapilli, and pumi-ceous airfall tuffs (Table 3 in the Data Re-pository). High within the sequence is a lo-cal unit of agglutinated mafic lava spatter.North and west from the mouth of ArroyoLos Heme, the number of individual unitsdecreases, and around Valle Curbina onlythe lower part of the sequence (the Tuff ofValle Curbina and the Tuff of Mesa El Ta-bano) are present (see Martın-Barajas andStock, 1993).The basal unit of the Tuff of Los Heme

(Tph1) correlates with the Tuff of ValleCurbina from the Valle Curbina area(Fig. 3), on the basis of thickness and min-eralogical and textural characteristics. It isconsistent with the isotopic data discussedbelow and with magnetic polarity data(T. Melbourne, unpub. data). In Valle Cur-bina, this unit is .60 m thick in some of thepaleotopographic depressions formed bygraben and half-graben structures, some ofwhich were invaded by marine waters duringlate Miocene and Pliocene time. The Tuff ofValle Curbina is thus interstratified withshallow marine deposits (Stock et al., 1991;Martın-Barajas et al., 1993). The distal partof the Tuff of Valle Curbina reached as farnorth as the southern Sierra San Fermın(Fig. 3), where it is 3–5 m thick. Toward thesouth, where it lies at the base of the Tuff ofLos Heme (Tph1), it is .70 m thick with amiddle welded zone. The unit extends .30km in a north-south direction and .5 km inan east-west direction, with an estimatedminimum volume of 5 km3 for the Tuff ofValle Curbina.The increase in thickness and number of

units toward the south and east within theTpr sequence suggests that their source iseast-southeast of the study area. A collapsestructure west of Volcan Prieto cuts most ofthe Pliocene ignimbrites (Fig. 3) and con-sists of a normal fault dipping 608–458E witha semicircular, concave east pattern in mapview. The uppermost epiclastic and pyro-clastic units of the Tuff of Los Heme pinch

out against the footwall block. Proximal de-posits in the downthrown block include air-fall deposits with ballistic blocks of rhyoliteglass.1 m in diameter, agglutinated spatterdeposits of mafic lava (unit Tph 26), andcoarse epiclastic deposits. The vent or ventsthat produced the ignimbrite deposits donot crop out and may be buried by the py-roclastic deposits or may be under water far-ther east.Isotopic ages for four pyroclastic rocks

from the young synrift sequence (Tpr) rangefrom 3.27 to 2.65Ma. Plagioclase grains sep-arated from three samples of Tpr were laserfused (see Table 1b). The Tuff of Valle Cur-bina (sample 11) yielded a reliable isochronage of 3.27 6 0.04 Ma. For this unit, thefusion step analyses yielded dates rangingfrom 3.06 6 0.13 to 3.44 6 0.04 Ma. Anal-yses yielding lower percentages of radio-genic 40Ar were relatively imprecise, so theisochron age is much better defined than theinitial 40Ar/36Ar age.The mean of 19 fusion analyses of single

plagioclase grains from subunit Tph28 is2.65 6 0.05 Ma (sample 11, Table 1b). Thisash-flow tuff was sampled in site B of Figure3 and is one of the uppermost units of Tphin the coastal zone of Arroyo Los Heme(Table 3 in the Data Repository). The iso-topic age of this subunit constrains well theminimum age of the Pliocene ignimbrite se-quence. Plagioclase from a pumice-lapillituff interstratified in the marine sequence inthe southern Sierra San Fermın (Fig. 3) wasdated at 3.08 6 0.04 Ma (sample 12, Ta-ble 1b). This unit is considered to be thesubmarine equivalent of unit b of the Tuff ofMesa El Tabano, and the age obtained isconsistent with a 3.1 6 0.5 Ma age (K/Ar,plagioclase) for the uppermost unit of theTuff of Mesa El Tabano (Gastil, 1975).The 40Ar/39Ar ages obtained by laser fu-

sion analysis are in good agreement with thestep-heating experiments. Three samplesfrom unit Tpr were analyzed by the step-wise heating technique (Figs. 4d, 4e, and 4f).Samples 5 and 6 in Table 1a are basal vitro-phyres from the uppermost unit of weldedash-flow tuff, 8 km west of the coastlinealong Arroyo Los Heme (Fig. 3). This unitcorrelates with unit Tph28 as discussedabove. On samples 5 and 6, whole-rock andplagioclase separates were analyzed. Sample5 displays a saddle-shaped spectrum for thewhole-rock experiment, and the plagioclaseseparate yielded a segment consisting of twofractions (74% of the 39Ar released) with a2.76 0.4 Ma age. This age is consistent withthe isochron age of Tph28. Sample 6 yielded



a whole-rock plateau age of 3.366 0.03 Ma,and the plagioclase separate yielded a per-turbed, uninterpretable age spectrum.Step-wise heating in duplicate analysis on

whole-rock sample 7 yielded very similar agespectra, with two fractions in agreement at3.5 Ma (see Fig. 4f). This sample was col-lected from unit Tpt-b in the Valle Curbinaarea. However, this age is too old because itis stratigraphically inconsistent with thewell-dated Tuff of Valle Curbina, 3.27 60.04 Ma, which underlies Tpt.Volcan Prieto and Other Young Volcanic

Flows (Tpa).Volcan Prieto (Fig. 3) is a mon-ogenetic, 280-m-high cone that is coevalwith the latest ignimbrite-forming erup-tions. Blocky andesite flows are interstrati-fied with scoriaceous deposits from this vol-cano and cover an elliptical area, 3–4 kmacross, east of the semicircular collapsestructure described above. Most lavas fromVolcan Prieto overlie the Tuff of Los Heme,but some flows to the south underlie one ofthe uppermost ash-flow tuffs of the LosHeme sequence (probably unit Tph29) anda relatively well sorted lithic sandstone. Thissandstone may represent a beach deposit;however, no evidence of lava interactionwith water was observed. Andesitic aggluti-nated spatter (unit Tph26, Table 3 in theData Repository), probably from a nearbyvent, is interstratified in the uppermost partof the Tuff of Los Heme, 5 km north ofVolcan Prieto (site B, Fig. 3). These strati-graphic relationships establish that andesiticvolcanoes and lava flows of Group 3 firsterupted during the waning stages of Plio-cene rhyolitic volcanism and continued afterthe youngest Los Heme ignimbrites.Whole-rock ages from Volcan Prieto

(sample 8, Table 1a) and from an andesiteflow west of Cerro Canelo (sample 20,Fig. 2) are 2.66 0.1 Ma and 0.916 0.67 Ma,respectively. Step-wise heating of a samplefrom Volcan Prieto (Fig. 4g) yielded a clearplateau age with 75% of the 39Ar released,in good agreement with the age of theyoungest Los Heme ignimbrite. The age ofthe andesite flow from west of Cerro Canelois poorly constrained but is consistent withthe youthful morphology of the flow. Addi-tionally, this flow is related to a north-east-striking fault that cuts Quaternary(?)alluvium.Sparse Pliocene andesite in the study area

probably indicates dispersed Plioceneandesitic volcanism in the Puertecitos Vol-canic Province (Stock and Comenetz, 1992).Andesite from the valley west of CerroCanelo (Fig. 2) is similar to the Volcan Prie-

to andesite and has a well-preserved mor-phology. We infer that andesites from bothlocalities (Volcan Prieto and near CerroCanelo) are broadly equivalent in age (latePliocene–Pleistocene [?]). At both localities,the andesites are cut by normal faults (ofstrike north-northwest and north-northeastat Volcan Prieto, and northeast west ofCerro Canelo).

Summary and Regional StratigraphicCorrelations

The eastern Puertecitos Volcanic Prov-ince includes a sequence of arc-relatedandesitic rocks (Group 1) discordantly over-lain by two sequences of dominantly silicicsynrift volcanic rocks (Group 2 andGroup 3,Fig. 5).The ca. 16 Ma ages of the arc-related an-

desite are consistent with the 20–14.5 Maages obtained from similar units to thenorth, in southern Valle Chico (Stock,1989), where the sequence attributed to thevolcanic arc reaches 300 m in thickness andincludes pyroclastic flows, andesitic brec-cias, basaltic lavas, reworked tuffs, and epi-clastic deposits (Fig. 5). There, and north-east of the Puertecitos Volcanic Province inSierra San Fermın (Fig. 3), the andesiticrocks overlie theMesozoic granitic and met-amorphic basement as well as Tertiary base-ment-derived sandstone (Lewis, 1994). Weinfer similar relationships beneath our studyarea. In Arroyo Los Heme, the arc-relatedvolcanic deposits correspond to proximal fa-cies, whereas in Valle Chico and the SierraSan Fermın they vary from proximal to dis-tal. Along the Main Gulf Escarpment at thislatitude, and to the north in the SierraJuarez (north of the Sierra San Pedro Mar-tir), the epiclastic deposits dominate the arc-sequence (Gastil et al., 1975; Dorsey andBurns, 1994).The two synrift sequences (Group 2 and

Group 3) erupted in relatively short periodsof time around 6 and 3Ma, respectively. Thelate Miocene silicic volcanism began nolater than 6.4 Ma, but likely after 7 Ma assuggested by the lack of reported 40Ar/39Arages older than 7 Ma on rhyolite rockswithin the Puertecitos Volcanic Provinceand the Sierra San Fermın (Stock, 1989;Lewis, 1994). The age of the Pliocene ig-nimbrite sequence is well constrained be-tween 3.2 and 2.7 Ma. Both sequences endwith andesite flows and scoria deposits. The6-m.y.-old rocks crop out principally towardthe north and northwest within the studyarea. Many local rhyolite domes and flows

also occur there, and the source of the 6.4Ma Tuff of El Canelo is also probably in thatgeneral vicinity based on the presence of thelava flows and domes along normal faults,the increasing thickness of some unitstoward the west, and the welding variationswithin the compound cooling units. In gen-eral, the Group 2 sequence, which is theolder synrift sequence, pinches out towardthe south in the study area.In southern Valle Chico, a sequence of

rhyolite ignimbrites and lava flows with agesbetween 5.8 6 0.5 Ma (alkali feldspar) and6.1 6 0.16 Ma (alkali feldspar) constitutesthe youngest regionally extensive volcanicsequence near the escarpment (Tmr3 andTmr4, Fig. 5). These rhyolitic rocks overlieintermediate to silicic lavas dated at 6.47 60.2 Ma (whole rock). The rhyolite flows andthe Tuffs of El Canelo in the northernPuertecitos Volcanic Province are possiblycoeval with the 6.4–6.1 Ma rhyolite lavasand tuffs reported by Stock (1989) fromValle Chico. A regionally extensive, denselywelded 11 Ma tuff also crops out in southernValle Chico. The 11 Ma unit and theyounger andesite cones and lavas dated at6.47 Ma by Stock (1989) have no known co-eval units in the area included in this study.The late Miocene rhyolite volcanism ex-

tended as far northward as the Sierra SanFermın (Fig. 2b; Lewis, 1994) and appar-ently ceased around 5.5 Ma in our studyarea. This intense volcanic activity is likelyassociated with the extension of the crust assuggested by the normal faults that pro-duced steplike topography controlling therhyolite flows and that produced shallowdips upsection in the Tuff of El Canelo. Nor-mal faults produced graben structures thatwere invaded by marine waters in late Mio-cene(?) and early Pliocene time. Shallowmarine beds discordantly overlie the lateMiocene volcanic rocks in the northeasternPuertecitos Volcanic Province and some ofthe rhyolite domes may have interacted withsea water as suggested by the rhyolite tephrathat surrounds the lower rhyolite dome inArroyo La Cantera.A second period of explosive volcanism

within the study area occurred during theearly late Pliocene time. Its locus was east ofthe locus of volcanism during late Miocenetime, and the Pliocene ignimbrites pinch outtoward north and west (Fig. 5). The sourceof the Pliocene ignimbrites was east of thepresent coastline, and some units (such asthe Tuff of Valle Curbina) were partly de-posited in a shallow marine environment.Both periods of synrift volcanism ended with



sparse andesite flows, including the eruptionof Volcan Prieto at the end of the late Plio-cene sequence.

PETROGRAPHY AND MINERALCOMPOSITION

Group 1: Arc-Related Andesite

Petrographically, these rocks show por-phyritic to microporphyritic textures, witha glassy to microcrystalline matrix. Horn-blende is the most abundant mafic mineraland occurs in phenocrysts (mostly 1–2mm) randomly distributed or aligned par-allel to the flow direction, defining apseudotrachytic texture. Distinctive, com-positionally zoned plagioclase phenocrystshave labradorite (An50–70) cores and andesine(An30–50) rims (Fig. 6a). The andesitic necks

contain augite in optically zoned, but com-positionally homogeneous crystals (Fig. 7).

Group 2: Rhyolite Flows and Tuffsof El Canelo

In Arroyo La Cantera, the lower rhyolitelavas (Tmrl) are massive, devitrified, rhyo-lite glass flows, with some zones of autoclas-tic flow breccia and some zones of low-tem-perature hydrothermal alteration. Theselavas contain 5%–10% phenocrysts, mainlyoligoclase, augite, and ferrohypersthene(Fig. 7). The hydrothermal alteration is per-vasive and produced pyrophyllite, smectite,and zeolite.The mineralogy of the Tuff of El Canelo

varies from base to top (Table 2 in the DataRepository). The basal unit (Tmc1) containsup to 30% phenocrysts, mainly plagioclase

and quartz, with subordinate opaque min-erals and alkali feldspar. Units 2, 3, and 4probably are a single compound cooling unitwith very consistent phenocryst composi-tions; all three contain ,10% phenocrysts,which consist of plagioclase, orthopyroxene,clinopyroxene, opaques, and hornblende.The latter is generally altered to iron oxides.Unit 6 is a densely welded ash-flow tuff con-taining plagioclase, orthopyroxene (ferrohy-persthene), clinopyroxene (augite), andopaques. In general, hornblende pheno-crysts are not present in Tmc and are rarewithin the Group 2 rocks.The upper rhyolite lavas (Tmru) are

glassy and sparsely porphyritic or aphanitic,with,5% phenocrysts, principally sodic pla-gioclase (oligoclase to andesine), clinopy-roxene (ferroaugite) (Figs. 6b and 7), andopaque minerals in a partially recrystallized

Figure 5. West to east stratigraphic correlation and compilation of isotopic ages of the main volcanic groups in the northern PuertecitosVolcanic Province. Data from this study in Table 1. The Group 3 in the Mesa El Tabano–Arroyo La Cantera is a composite column ofthe Tuff of Mesa El Tabano (site A* in Fig. 3) and Tpvc from Valle Curbina. Plag, plagioclase; wr, whole rock; san, sanidine; ksp,anorthoclase; hbl, hornblende. Note: The age of the uppermost unit in Mesa El Tabano is from Somer and Garcıa (1970).



matrix. Devitrification textures are spheru-litic, composed of intergrowths of alkalifeldspar (Or40–50) and cristobalite. Micro-crystalline calcite occurs in microfracturesand is also disseminated in the matrix.

Group 2: Synrift Andesite Lava (Tba)

Andesite lava from a dike in Valle Cur-bina and from a flow in Arroyo Los Hemeare aphanitic to microporphyritic. Theselava show microlitic flow texture and have10%–50% microphenocrysts—mainly plagio-clase, clinopyroxene (augite), and orthopy-roxene (bronzite) (Fig. 7). Some plagioclasephenocrysts show normal zoning composedof labradorite cores and andesine rims(Fig. 6b). Mafic minerals in some brecciazones within the andesite dike from ValleCurbina are selectively altered to chloriteand limonite, probably by hydrothermal al-teration. Ground mass is mainly plagioclase,pyroxene, opaque minerals and,10% glass.No olivine was observed in these lavas.

Group 3: Synrift Pliocene Ignimbrites(Tpr) and Andesites from Volcan Prietoand West of Cerro Canelo (Tpa)

The pyroclastic units at the base of theTuff of Los Heme (Tph1) and the Tuff ofValle Curbina have similar mineralogiccomposition. Both have plagioclase (An25–50),biotite, clinopyroxene (diopsidic augite-ferro-

augite), and rare hornblende. Quartz is ex-tremely rare. Lithic fragments are princi-pally rhyolite, with minor andesite porphyry,basaltic andesite, and granite. The Tuff ofValle Curbina has two types of glass: onetype is medium brown in thin section andthe other is colorless. Both are rhyolitic incomposition, but the brown variety is1%–2% less silicic and contains 0.5%–1%

more iron. Two compositions of glass shardsare also present in the basal pyroclastic flowdeposit of the Tuff of Los Heme (Tph1).Upsection, thin, densely welded to slightly

welded ash flows predominate. These unitshave phenocrysts of orthopyroxene and cli-nopyroxene (both 1 mm) and lack hydrousphases such as biotite or hornblende. Oli-vine phenocrysts are fairly common in rhyo-lite and dacite rocks. Plagioclase composi-tions vary from andesine to oligoclase(An20–30) (Fig. 6c). Clinopyroxenes are au-gite and ferroaugite; orthopyroxene is hy-persthene (En50–60, Fig. 7); and sparsefayalite is observed with varying degrees ofiddingsite alteration. This phenocryst as-semblage is characteristic of most of the tuffof the Pliocene ignimbrite sequence (Tpr).In andesite lavas from Volcan Prieto

(Tpa) and the andesite flow west of CerroCanelo, textures vary from aphanitic to por-phyritic, and some flows have up to 20%phenocrysts. In thin section, the texture ismicrolitic to felsitic, with orthopyroxene, cli-nopyroxene, and olivine phenocrysts in a mi-crocrystalline groundmass composed princi-pally of acicular plagioclase, pyroxene, andopaque minerals.

GEOCHEMISTRY

Our discussion of chemical compositionsfocuses on the differences between the ba-saltic and andesitic rocks attributed to theold volcanic arc (pre–11 Ma) and the

Figure 6. Plagioclase normative composition in representative rocks. (a) Group 1 pre–11Ma hornblende andesite. (b) Group 2 lateMiocene andesite and rhyolite lavas (Tba-Tmru).(c) Pliocene rhyolite and dacite ignimbrites (Tpr). Data from Martın-Barajas and Stock(1993).

Figure 7. Pyroxene quadrilateral composition in selected samples from the main volcanicsequences in the northeast Puertecitos Volcanic Province. Pre–11 Ma hornblende andesitefrom Arroyo Los Heme (Tma, sample II-1), synrift andesite from Arroyo Los Heme (filledsquare) (Tba, sample 2-V-2), and rhyolite flow from the Valle Curbina area (empty square)(Tmru, sample III-11). Tpt is Tuff of Mesa El Tabano (samples III-6, III-2); Tpvc is the Tuffof Valle Curbina (empty diamonds). The Fo-Fa diagram shows composition of olivine inunit Tpt-c (sample III-2). Data are in Martın-Barajas and Stock (1993) and available onrequest.



andesitic rocks contemporaneous with therift. Because of the scarcity of mafic rocksthat might represent the primary magmas ofsynrift late Miocene and Pliocene rocks, weanalyzed andesite rocks of those sequences.For the trace-element analysis of represen-tative samples, care was taken to analyzeonly the freshest rocks.

Major-Oxide Geochemistry

Arc-Related Rocks. The arc-related se-quence is composed mainly of subalkalineandesite and basalt (Fig. 8). We include twoanalyses of alkalic basalts (20 Ma) and oneof a subalkalic basalt (17 Ma) from southernValle Chico to show that pre–11 Ma rocksalso have geochemical variations apparentlyrelated to the subduction tectonic setting.Subalkalic andesite and basalt plot in thecalc-alkaline field in the ternary diagram ofFigure 9 (Group 1). The alkalic basalt ischaracterized by high K2O (2.3% and 2.7%),MgO, and P2O3 content and very low Al2O3content (Fig. 10). This basalt directly over-lies granitic basement and/or basement-derived sediments (Stock, 1989). For arc-related rocks, a clear decreasing trend isobserved in the FeO*, TiO2, MgO, and CaOcontent with increasing silica, whereasAl2O3 and P2O5 values scatter (Fig. 10).With the exception of the alkalic basalt, K2Oand Na2O in andesitic to dacitic arc-rocksincrease with increasing silica.Synrift Rocks. Synrift volcanic rocks are

predominantly rhyolite to dacite, with someandesite. All synrift andesitic to rhyoliticrocks are subalkaline (Fig. 8) and form abroad calc-alkaline trend in the ternary di-agram (Fig. 9). Na2O and Al2O3 are the ox-ides that show the most scatter on Harkervariation diagrams (Fig. 10); Na2O is par-ticularly scattered in the more silicic sam-ples, whereas Al2O3 tends to scatter in an-desite rocks. The scatter in Na2O is probablydue to sodium mobility during devitrifica-tion that systematically leaches Na2O andincreases K2O content of some ignimbrites(Lipman, 1965; Conrad, 1984). Some of thevitrophyres show incipient devitrification,which could account for the variation inNa2O content. The consistency in the CaO/SiO2 and K2O/SiO2 trends in the same rockssuggests, however, that devitrification didnot modify their chemical composition sig-nificantly. The andesite and basalt analyzedshow no recrystallization or posteruption al-teration effects.Relative to the arc-related andesite and

basalt, synrift rocks are characterized by a

Figure 8. Total alkali versus silica diagram. Dotted lines delimit fields from Le Bas etal. (1986). Dividing lines between alkalic and subalkalic fields from Irvine and Baragar(1971) and McDonald and Katsura (1964). Data from Tables 4 and 5 and Martın-Barajasand Stock (1993). Rock fields: TB, trachybasalt; BTA, basaltic trachyandesite; B, basalt;BA, basaltic andesite; TA, trachyandesite; A, andesite; D, dacite; TD, trachydacite; R,rhyolite.

Figure 9. Ternary diagram for volcanic rocks from the Puertecitos Volcanic Province.Tholeiitic divide from Irvine and Baragar (1971).



Figure 10. Variation of major oxides versus silica in the Puertecitos Volcanic Province. Symbols as in Figure 8. Data from Tables 4 and5 and Martın-Barajas and Stock (1993).


low K2O content (typically ,1.0% K2O).Some synrift andesites and basaltic ande-sites show high TiO2; however, this is notsystematic, and some samples show thesame TiO2 as do the arc rocks. The MgOcontents of the youngest synrift andesiterocks from Volcan Prieto and west of CerroCanelo are high when compared with theorogenic andesite and basalt. In the former,Mg# (Mg#5 atomic Mg/[Mg1 Fe]) variesfrom 55 in dacite and rhyolite rocks to 70–75in andesites with a silica content of 60% (Ta-bles 4 and 5 in the Data Repository). In syn-rift andesites of Group 2 (Tba), Mg# is

lower and ranges from 46 to 56. The Al2O3content in three andesite samples (Tba) isanomalously high (.17 wt%).

Trace-Element Geochemistry

Arc-Related Rocks. The samples analyzedfor trace elements are two alkalic basaltsand two basaltic andesites. Three of thesesamples (VC-86-99, VC-87-10, VC-87-54)are from southern Valle Chico and weredated by K-Ar (Stock, 1989). Sample P38-1is a basaltic andesite from the Arroyo LosHeme (Fig. 3). Orogenic basalt and basaltic

andesite contain large amounts of incom-patible elements and pronounced negativeanomalies in Nb-Ta and Ti (Fig. 11a). Therare-earth-element (REE) patterns arestrongly fractionated between light REEand heavy REE (Lan/Lun ratios of 9.5–20)(Fig. 12a). This fractionation is more pro-nounced in the alkalic basalt of Valle Chicothan in the subalkalic basaltic andesite. Al-kalic basalt from Valle Chico has character-istics similar to synrift alkalic rocks fromsouthern Baja California. Both have sometrace-element characteristics of arc magmas(Sawlan and Smith, 1984; Sawlan, 1991).Synrift Rocks. Trace-element analysis of

synrift rocks includes two basaltic andesites(Tba) from Group 2. FromGroup 3, we alsoanalyzed two andesite flows from VolcanPrieto (VP-1, VP-2) and one andesite fromwest of El Canelo (MT-91-67).The younger synrift andesite is the least-

evolved lava. Most of the incompatible ele-ments are less concentrated in synrift an-desites than in the arc-related basalts(Fig. 11b). This is especially true of the al-kaline-earth and the light rare-earth ele-ments (LREE). Synrift andesite also showsthe same negative Nb-Ta anomaly as do thearc-related rocks, but the negative Ti anom-aly is not as well defined. In synrift andesites,K/Rb ratios vary between 248 and 590 (Ta-ble 4 in the Data Repository). The K/Rbratios in Group 2 andesites (Tba, samples2-V-2 and 30-IV-4) are extremely low (248–282), whereas andesitic rocks ofVolcan Prie-to and west of Cerro Canelo have similarlow K2O contents but have K/Rb ratios from490 to 590. The Ni content of two samplesfrom Volcan Prieto (VP-1 and VP-2) is highfor andesitic rocks (120 ppm); these rocksalso have highMgO and TiO2. This andesitevolcano erupted the least-differentiated la-vas among the Pliocene and late Miocenesynrift volcanic rocks.Most trace elements exhibit similar be-

havior in Group 2 and Group 3 andesite.Variations among synrift andesite withineach group occur in the Ni, Sr, and TiO2contents. Nevertheless, a homogeneous anddistinctive characteristic of synrift andesiteis its REE pattern. Both Group 2 and Group3 andesites have the same REE patternscharacterized by less fractionation of LREEthan arc rocks, with Lan/Lun values between3.8 and 7.2 (Figs. 12a and 12b). The trace-element composition in andesites is alsosimilar in both synrift andesite groups and isclosely clustered in the ratio of some trace-element versus Zr diagrams (Fig. 13). De-spite the scatter in these diagrams, a trend is

Figure 11. Spider diagrams of selected mafic rock samples. Normalization values arechondritic abundances, except those for Rb, K, and P (normalizing values from Thompsonet al., 1984). (a) Pre–11 Ma rocks. (b) Late Miocene to Pliocene rocks. Shaded pattern isarc-related basalt and basaltic andesite of Figure 12a. Data from Table 4 in the DataRepository.



visible in diagrams of Nb/Zr, La/Zr, andY/Zr. The dispersion is larger in the Rb/Zr,Ba/Zr, and Sr/Zr diagrams, and the trend isbest defined when only the andesitic to rhy-olitic lavas are considered. The pyroclasticrocks analyzed (mostly basal vitrophyres)show the most dispersed values.

DISCUSSION

The 16 Ma andesitic rocks from ArroyoLa Cantera represent proximal facies of theearly to middle Miocene volcanic arc, which

is continuously exposed southward along theeastern margin of the peninsula (Hausback,1984). The ages obtained in our study andthose reported for arc-rocks from ValleChico (Stock, 1989) are consistent with pre-vious interpretations of the earlier waningand termination of the subduction-relatedvolcanism in northern Baja California at17–14Ma, earlier than in southern Baja Cal-ifornia (14–12 Ma, Hausback, 1984; Sawlan,1991; Gastil et al., 1979). At the latitude ofPuertecitos, proximal facies of the volcanicarc crop out for 30–40 km from the gulf

coast westward to the southern part of theSierra San Pedro Martir (Stock, 1989;Dorsey and Burns, 1994). Farther west, onlymedial to distal facies crop out. The width ofthe proximal facies in the Puertecitos Vol-canic Province is comparable to the widthreported in southern Baja California byHausback (1984). Although outcrops of theorogenic volcanic belt are discontinuous innorthern Baja California, the PuertecitosVolcanic Province region was a major locusof arc-related volcanism in early to middleMiocene time.The ages of the post–11 Ma rocks (the

older probable age of inception of exten-sion) cluster around two periods of volcan-ism: one at the end of lateMiocene time anda second one at the end of early Pliocenetime. The locus of synrift explosive volcan-ism shifted dramatically eastward betweenthese two periods; we speculate that theshifting may be related to the developmentof the shoulders of the new rift. The youngerignimbrite-forming eruptions are associatedwith a second period of extension that con-tinued along the coast after deposition ofthe youngest ignimbrite.The alignment of rhyolite domes follows

the north-northeast to north-northwest ori-entation of the main fault pattern here(Martın-Barajas and Stock, 1993) and westof the study area, along the escarpment(Stock and Comenetz, 1992; Stock et al.,1993). This suggests that normal faults actedas conduits through which lava flows anddomes were extruded. Additionally, boththe rhyolite domes and the outcrops ofandesitic rocks from Los Heme acted as top-ographic controls on the deposition of thepyroclastic units. There is evidence for syn-extensional tectonism during deposition ofboth the Pliocene and late Miocene ignim-brite sequences, from progressive upwardshallowing of eutaxitic foliation and beddingof the pyroclastic and epiclastic units. Inboth cases the pyroclastic units are wellstratified and have reliable attitudes. In Ar-royo Los Heme, the base of the Tuff of LosHeme is tilted 238E, whereas higher unitsdip ,108E. In Arroyo La Cantera, the unitat the base of the Tuff of El Canelo dips.408ENE, whereas the top of the sequencedips ,108ENE. This suggests syndeposi-tional growth faulting.

Mineralogy and Geochemistry

The mineralogy of the synrift pyroclasticflows, and the rhyolite-andesite lavas, is con-sistent with high temperatures in the magma

Figure 12. Chondrite-normalized REE patterns for selected mafic rock samples. (a)Pre–11 Ma rocks. (b) Late Miocene to Pliocene rocks. Shaded pattern is arc-related basaltand basaltic andesite of Figure 13a. Data from Table 4 in the Data Repository. Normal-ization values from Boynton (1984).



chamber and/or mixing of a hot maficmagma with a more felsic material. Mostrocks in these sequences lack phenocrysts ofhydrous minerals (biotite and hornblende),suggesting that hydrous minerals, if present,broke down under the high temperaturesand (or) decompressional melting underadiabatic conditions during rifting. The as-sociation of augite, hypersthene, and fayal-ite in the rhyolites and dacites is indicativeof a more mafic magma. However, the cli-nopyroxene and orthopyroxene are euhed-ral and homogeneous and appear to have

been in equilibrium with the melt. Only oli-vine phenocrysts in rhyolite to dacite ignim-brites are often altered to iron oxides.Assimilation is suggested by resorption

textures in plagioclase phenocrysts and nor-mal compositional zonation with cores oflabradorite-bytownite and rims of oligo-clase-andesine in synrift rocks. Additionally,modification of the mineralogical character-istics of some pyroclastic flows is inferred tohave occurred by mechanical fragmentationof the granitic wall rocks. The Tuff of ValleCurbina appears to be contaminated with

biotite xenocrysts from the granitic base-ment, because two 40Ar/39Ar ages on biotiteconcentrates yielded Cretaceous ages. Sim-ilar problems have been noted with biotitein younger tuffs interbedded in the marinesection in the Sierra San Fermın (Lewis, 1994).The geochemistry of the synrift rocks sug-

gests that primary melts, probably from themantle, were extensively hybridized bycrustal components. Partial melting from amantle source is suggested by the low K2Oand by relatively high, but variable Ni, TiO2,and Mg# in synrift andesite. The Mg# and

Figure 13. Selected trace ele-ments versus SiO2 for synrift da-cite to rhyolite lavas and ignim-brites in the northern PuertecitosVolcanic Province. Symbols as inFigure 11. Data from Tables 4and 5.



the Ni content are particularly high inVolcan Prieto lavas and constitute the mostprimitive synrift rocks in thePuertecitosVol-canic Province. Evidence of crustal contam-ination is provided by the ratios of some in-compatible elements that do not followtypical differentiation trends from andesiteto rhyolite due to crystal fractionation froma homogeneous source (Fig. 13). Crystalfractionation alone is unlikely to producedifferentiation paths for rhyolite magmas.We would expect, for instance, clinopyrox-ene fractionation to produce a decrease inboth the Al2O3 and CaO trends in theHarker diagrams (Fig. 10). Additionally,REE patterns show no Eu anomaly indica-tive of plagioclase fractionation. Except forthe lowermost units in both the Tuff of ElCanelo and the Tuff of Los Heme, synriftrhyolite-dacite lava and tuff are chiefly crys-tal poor (,10%), so that it is difficult to ap-peal to extensive crystal fractionation as anexplanation for differentiation to producefelsic magma bodies. Magma mixing is aplausible explanation to produce rhyoliteand dacite rocks. Petrographic evidences formagma mixing are few (e.g., the two glasscompositions in the Tuff of Valle Curbina,resorbed hornblende and plagioclase phe-nocrysts, olivine microphenocrysts in rhyo-lite and dacite rocks). Additionally, varia-tions in some trace element ratios alsosuggest magma mixing processes. For in-stance, some 6 Ma rhyolites have the sameCe, La, and Sr values as do coeval andesites,and Zr in synrift rocks varies from 50 ppm to.400 ppm independently of the degree ofdifferentiation.Differences in trace element composi-

tions in andesitic rocks probably indicatedifferences in the degree of partial melting.Andesite from Volcan Prieto and west ofCerro Canelo shows similar low K2O con-tents as Tba andesite but has twice the K/Rbratios. On the other hand, variation in thecontent of some highly incompatible ele-ments in synrift andesite, such as Ni andTiO2, is also consistent with different de-grees of partial melting. Similar composi-tions of most trace elements suggest, how-ever, a similar source.The main differences between synrift an-

desite and arc-related andesite and basaltare the lower K2O, the smaller fractionationof LREE, the smaller negative Nb-Ta anom-aly, and the lower contents of Zr and alka-line-earth elements (Ba, Rb, Sr) in synriftrocks. These suggest that primary melts forsynrift rocks are less evolved than for arc-rocks. The negative Nb-Ta anomaly in arc-

related andesite and basalt rocks is generallyassociated with low values of TiO2 (Arculus,1987) and with the presence of a refractoryresidual phase (rutile, ilmenite, or perov-skite) (Saunders et al., 1980). In synriftrocks, this negative anomaly is least pro-nounced and may also indicate contributionof crustal material. Elsewhere, calc-alkalinevolcanism not related to an active subduc-tion setting is reported as a dominant typeof volcanism in the southwestern UnitedStates. There, crustal contamination previ-ously modified in a paleosubduction settingappears to play a major role in the geochem-istry of late Cenozoic magmas (Glazner,1990).Variations in the ratios of incompatible

elements within the synrift lavas reflect acomplex history of magma mixing and/ordifferent degrees of partial melting. On theother hand, the enrichment of incompatibleelements in the orogenic andesites and thepresence of alkalic basalts with typical oro-genic signatures (negative Nb-Ta and Tianomaly) may be attributed to the tectonichistory of the continental margin, which wasa convergent plate boundary during most ofMesozoic and Cenozoic time (e.g., Atwater,1970, 1989).

Comparison with Other SynriftCalc-Alkaline Volcanic Centersin the Gulf Region

The major recognized center of Plio-Qua-ternary calc-alkaline volcanism in the gulf

depression is the Tres Vırgenes–Santa Anacaldera area, located in the south-centralGulf coast (Sawlan and Smith, 1984; Sawlan,1991). The Tres Vırgenes volcanic centerhas a similar geochemical and mineralogiccomposition to orogenic andesites (Sawlan,1981, 1991). Another Pliocene calc-alkalinevolcanic sequence crops out in Isla San Es-teban (Desonie, 1992; Fig. 2a). There, asuite of rhyolite to andesite domes, lavas,and pyroclastic rocks has similar trace ele-ment compositions to the Tres Vırgenes vol-canoes. The Isla San Esteban sequence iscoevalwith theGroup3 sequence fromPuer-tecitos. Nevertheless, the former has a dis-tinct mineralogic and geochemical compo-sition. Andesite lavas from Isla San Estebanare more fractionated and have higher K2O,lower LREE/HREE ratios, and lower Niand Cr, but similar high-field-strength ele-ments as compared with synrift andesitesfrom Puertecitos (Fig. 14). In Isla San Es-teban, the dominant mafic phase is eitherhornblende or pyroxene, and andesite anddacite rocks have up to 40% phenocrysts.The source for these volcanic rocks remainsunknown, and it has been proposed thatpostsubduction calc-alkaline volcanism maybe related to an asthenospheric window cre-ated by subduction of the spreading ridge(Desonie, 1992).Although speculative, we propose an-

other explanation. Partial melting of a tran-sitional tholeiitic basalt may have producedthe synrift andesite rocks from Puertecitos.Holocene tholeiitic volcanism occurs in the

Figure 14. N-MORB-normalized spider diagrams of synrift andesite, compared withspider diagrams of synrift calc-alkaline andesite and dacite from Isla San Esteban (cross),and Cerro Prieto tholeiitic basalt (Herzig, 1990), andesite, and dacite (shaded pattern).N-MORB normalizing constants are from Pearce (1983).



Cerro Prieto Geothermal Field and in theSalton Trough (Herzig and Elders, 1988;Herzig, 1990). There, differentiation oc-curred mainly by crystal fractionation and,to a lesser extent, by contamination withcrustal rocks (Herzig, 1990). Differentiationof this tholeiitic magma produced a strongfractionation of incompatible elements(Fig. 14). The Cerro Prieto basalt-to-dacitesuite has a higher high-field-strength ele-ment concentration (e.g., Zr, Y, exceptHf) and higher content of HREE than syn-rift andesite from Puertecitos, but Ni inlavas from Volcan Prieto have values sim-ilar to MORB (mid-oceanic-ridge basalt,Fig. 14).In the Puertecitos Volcanic Province, ex-

tension during the rifting may have induceda high geothermal gradient that caused par-tial melting of a lithospheric, MORB-typemantle. Subsequently, andesitic magmabodies where contaminated with crustalmelts to produce rhyolite and dacite rocks.However, isotopic studies are needed toevaluate this hypothesis inferred from ourmineralogical and geochemical data.

CONCLUSIONS

The volcanic succession in the eastern andnorthern Puertecitos Volcanic Provincecomprises three main sequences that recordthe transition from subduction to rifting ofthe Gulf of California. The lower sequence(Group 1) consists of middle Miocene an-desite of the volcanic arc, formed during thefinal subduction of the Farallon-GuadalupePlate beneath northern Baja California. Theolder synrift sequence (Group 2) consists ofa series of rhyolite domes, a sequence of.300 m of pyroclastic flows (the Tuff of ElCanelo), and a terminal phase of andesiticlavas volumetrically small compared to therhyolite. This sequence crops out in thenorthern Puertecitos Volcanic Province,where it is covered discordantly by Plioceneignimbrites. Isotopic ages limit this se-quence to between 6.4 and 5.8 Ma in age.In early Pliocene time (3.24–2.7 Ma), fur-

ther synrift volcanism (Group 3) occurredeast of the locus of the first synrift volcanism(Group 2) and produced a series of pyro-clastic flows up to 200 m thick (Tuff of LosHeme and Tuff of Mesa El Tabano). Theirsource appears to have been located in thesoutheastern part of the Puertecitos Volcan-ic Province. This Pliocene phase of explosivevolcanism culminated with the formation ofa monogenetic andesitic volcano and vari-ous local mafic lava flows (Volcan Prieto

and flows west of Cerro Canelo). The synriftnature of the upper two groups is indicatedby the local alignment of rhyolite domesalong north-south– to north-northwest–south-southeast–striking faults, the upwardshallowing of dips in the volcaniclastic units(indicative of growth faulting), and the fault-related topographic control on deposition ofthe pyroclastic and lava flows.The mineralogic composition of the rift-

related rocks suggests high-temperaturemagmas. The major and the trace elementsignatures of most synrift and arc rocks arecalc-alkaline. However, synrift andesites arelow-K and have a lower content of incom-patible elements than the arc-related ande-site and basalt. The lower content in incom-patible elements in synrift andesite suggestsmantle-derived melts hybridized by crustalcontamination.

ACKNOWLEDGMENTS

Thanks to J. Kimbrough and M. Wala-wender for use of the XRF analytical facil-ities at San Diego State University. Specialthanks to H. Delgado for fruitful discussionsand for a preliminary revision of the draftmanuscript. Thanks to Ch. Herzig andD. Kimbrough for multiple suggestions thatalso improved the manuscript, to V. Friasand G. Rendon for drafting, and to S. Au-gustin and G. Axen for improvement of theEnglish usage. Special thanks to M. Sawlanand P. Weigand for their constructive re-views. A. Martın-Barajas was supported byCONACYT, Mexico, Grant 1224-T9203.J. Stock was partially supported by NationalScience Foundation Grants EAR-89-04022and EAR-92-18381. Additional support forJ. Stock and B. Hausback came from thePetroleum Research Fund of the AmericanChemical Society via Grant No. 21291-G.

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