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Geology and petrology of Volc5n Ceboruco, Nayarit, Mexico

STEPHEN A. NELSON Departirieitt of Geology and Geophysics, University of California. Berkelea. Cnlifornia 94720 'Preserit nddress: Department of Geology, Ticllane University, New Orleans' Louisiana 701 18

Geological Society of Arriericd Bullctiii, Part II, v. 91, p. 2290-2431, 19 figs., 11 tables, Nove$t.r, 1980, Doc. no. M01102

- __ __ -

These dacites contain xenoliths 0.f ABSTRACT high-"1, ba,salt that were apparentl'y

Volch C(,boruch is a modc:atcly partially rnoltcm'at the time of their

sized stratovolcanc located in the ir.clusion, sul:pc-is t.ing that the daci te

northwestern ;:art 05 the ?:cxican magma iorred as R result of masma mixing

Volcanic Belt. The bulk of the involving Ja1.a p;mice and basaltic magmas

volcano consists of relatively The dacitc donic? later collapsed *to form

alkali and incompatible element-poor the inner cal.dern, and Ceboruco again

hypcrsthene andcsites erupted prior erupted andesi tcs. These postcaldera

to the format.ion of two concentric andesites cont.nin both augite and

hypersthene and arc enriched .in

were' followed in tlic eruptive sequence alkalis antl incompntiblc clement4

by ;tie crupp'ior! 1,000 vr ago of a relative to tho precaldera andesikes.

white $rliyo?i>cj t c' pumice, tcrmetl the . fin all.^, bc?twc,c!n 1870 and 1.875,

Jala pumice. The eruption apparently rhyodacite 1.ava flows wre' eruptkd

caused the> ' f.orma t: on of Ceborrico ' s during (:cboruco's only Iiistdc

cuter ca1.dcra. !o!,--si lica dacite act-ivity.

anci partially fi lletl ttiis Caldera. ImtIi major and [:race elements' suggest 2290

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that the 1870 dacites and the Jala of the volcanoes in the belt have had

pumice 'could have been derived from reported activity in historic times

magma with postcaldera andesite compo- (Voosfr and otlic,rs. 1958), inclr~tling .- sition. To determine the o&gin of the Volca'n Ceboruccl, which erupted during

Ceboruco andesites, trace-element the years 1870 tb 1875 (Caravantes,

partial-melting models tested three 1870; 1,glesias and others, 1877). The

possible sources: subducted oceanic belt has rccei.Ced little study by earth

crust in the eclogite facies and the scientists, and its pdsition in'the

amphibolite facies and mantle peridotite. global tectonic framework is little

All models were found unreliable be- understood. Chcmical analyses of rocks . cause of the many assumptions involved. from the belt hnyc been reported by

It is concluded that a cornplex process Burri (1930), Williams (1950) ,. Vilcdx

involving partial nelting, fractional (1956), Gunn and Xooser (1971),

crystalization, magma mixing, and, Negendank (1972), Bloomfield (1975),

perhaps, crustal assimilation was and Pichler and Weyl (1976). These

responsible for the generation of published studie:, reveal that the pre-

the hdesites. Such a model cannot be dominant rock types are basaltic andesite , \ test'ed, however, because there are to andesite, with substantial quantities -.. at present no viable constraints that of olivine basn1-t and lesser quantities

\ can be placed on this complex+equence of alkali basalt., dacite, and rhyolite.

of events. The western part of the Mexican

.Volcanic Belt overlaps onto an older INTRODUCTION Tertiary province of rhyolitic ash-

Thc Yerican Volcanic Belt consists flow tuffs (Gastil 3nd others, 1979;

of a chain of Yiocene to Holocene Gunn and >!ooser, 1971). Rhyolitic

volcanoes that span Mexico from its vol.canism&s continued into rece'n-t

Pac'ific coast to its gulf coast. Nine times ill tJie region of Guadalajara,

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where compositions are peralkaline Ahuacatlin (Fig. 1.). It is built on

(Maliood, 1978) (Fig. 1). the northern slopes of the valley of

De Cserna (1961, 1965) suggested Ahuacatlsn and covers an area of about

3 that ci thrust-fault relation existed 80 Itm'. It rises to .an elevation of

along the Middle America Trench along 2,200 m, or about 1,200 m above the

the coast of Mexico, ahd Molnar and present valley floor, and is crowned

Sykes (1969) showed that a Benioff by two concentric calderas, the older of

zone exists lwncath Mexico, where which measures. 3.7. km in diameter and

earthquake.foci are reported to a depth the younger 1.5 km in diameter.

of 150 km. Although the zone is not Caravantes (1870), Iglesias and.

well defined, the active region ap- others (1877), and OrdoGez (1897) re- -. ?arently ends bcneath, or slightly ported ofl the 1870 eruption of Cebo&.

oceanward of, the existing volcanic .l.r'aitz (1920) discussed the possibility

front. , Focal mechanism studies re- that Cebo-ruco produced a nu6e ardente

ported by the above authors are con- during that eriiption: Barrera (1931)

sistent with the postulate that sub- discussed the 1870 eruption and some

duction is takinj; place along the geologic featurcs of Ceboruco in his

Middle America TrLnch off the investigation of the geology along the

southern coast of Mexico. railroad route linking Guadalajara with

Volcsn Ceboruco lies in the the Pacific coast. Segerstrom (1950)

northwestern part of the Mexican reported on the erosion that has taken

Volcanic Belt , where no signjficant place since the 1870 event. Thorpe

earthquake activity is currently and Ffancis (1975) have prese'nted a

observed (Mol nar and Sykes , 1969). photogeologic map and eight chemical,

The volcano IS located at lat 21' 7' analyses of rocks from Ceboruco.

30" N, long 104" 20'61 in a broad basin Volcsn Ceboruco rests in a broad

comprising thc, valleys of Jala and valley whose northern margin is marked

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=.. \G \G \. BAWIA GUADALAJARA Of LAS BAhOfRAS

COLIMA

105’W. . 104’ 103’ I I

Figure 1,. ?lap of western Xcxico showing maj0.r volcanic centers: 1, Volcsn Tequila;

2, Volciin San Pedro; 3, Volcsn Tepetiltic; 4j’Santa Maria del Oro caldera; 5, Vo ca’n

Sanganguey; 6, La Laguna del Tesoro caldera. Inset shqws map area and locations of & historically artive volcanoes of Mexico.

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by an escarpmcnt trending S6Oo15, from several of Ceboruco’ s flank cinder

rising about 300 m above the gently cones.

slopin; valley floor. The walls of The growth of Cebdruco has altered

this escarpment are composed of and blocked the drainage system in the

rhyolitic ash-flow tuffs, probably of valley of Ahuacatlsn. Tributaries to

TerFiary age. The, escarpment fades to the Rio Ahuacatlsn, such as the Arroyo

the east, where similar ash-flow tuffs Jala (Fig. 2) have steep gradients near

slope gently down to the valley floor

southern valley margins and are oyer- water regions of such‘ tributaries are

lain by ash-flow tuffs, which form a present in the cliffs norfh of the volcano. I xountain range termed the Sierra These drainage systems have been cut

Guamuchil. parther south, these ash- off by the growth of Ceboruco and now

ffow tuffs that compose_ the Sierra give the cliffs a scalloped appearance.

Guamuchil Overlie gran.$t ic and The relatively- gentle gradients of ”’ gabbroic rocks that%lhave been dated streams flowing in.the valley of < at 61 and 87 m.y. old, respectively hhuacatlsn have res’uf ted from the growth

(Gastil and others, 1976). of the volcano, causing the Rio

It thus seems likely that the bedrock Ahuaca t lsn to continually grade itself

concealed beneath Ceboruco is also .to the rising level of vglcanic material

composed of rhyolitic ash-flow. tuffs expelled into the valley.

similar. to those found in both vaLley Projecting the steep gradients of

walls. Deeper levels beneath tribu;aries such a$ the Arroyo Jala

Ceboruco must he composed in part of beneath the alluvial fill of the valley

grani tic material, because xenoli ths suggests that the’valley of Ahuacatlsn

of such rock are found in boxbs erupted was at least 500 m dee?er prior to the

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Figure -21: Geologic map of VolcLn Ceboruco and surrounding area.

Figure 2 appears on 'the follow@g frames.

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Figure 2.

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Alluvium

1870 Daclte

Caldera fill

El Ceboruco Andeslte

Andesite

El Mo'lcajete

Coapan [-I Andesite El Centro Andesite Dome

rjCopales Dacite

Dos Equis Dacite Dome

Marquesado . . . Ash Flow

Cerro Pedregoso Rhyodacite .

La Picha,ncha

El Cajon Andesite

Cerro Pochelero Rhyolite Dome rjDestiladero Rhyodaci te

Ceboruquilo Andcsite

Pre Caldera

II-Andesite

Pre Ceboruco ' U Ash flow'tuffs

Contacts -- - - dashed where Inferred

A Cinder Cones

constructed cross section of the, been .observed. Demant (1978) con- :. valley prior to the growth of Ceboruco sidered the region betbeen Tepic and

/ is shown in Figure 3. Thus, it ap- Lake Chapala (Fig. 1) to lie In a

pears that Ceboruco was built on the northwest-trending graben in' which

northern slopes of the valley of Ceboruco and the volcanoes San Juan, I Ahqacatlzn in an area once dissected Sanl:arguey, Tepetiltic, and Tequila

by tributary streams cut in 'rhyblitic' are found. However, no geologic evi-

ash-flow tuffs. dence other than northwest-trending

The steep escarpment 'north"of cinder-cone alignments is offered in

Ceboruco is suggestive of a fault support of this conclusion.

scarp. Although no dfrect evidence ?'he eruptive history af. Volcsn,

is available to substantiate this Ceboruco has been divided into three - > possibility, a lincar zone of cinder. stages of activity separated by episodes

cone$ and volcanic domes trends of caldera formation. The geolqgy of

parallel to the escarpment, suggesting Ceboruco is discussed in the following

that a zone of crustal weakness does sections with regard to these three

exist along the trend marked by both stagcs of activity. Sample localities

features . are listed and d.escribed in Appendix 1. ThortYe and Francis. (1975) stated '. PRECALDERA GEOLOGY OF VCLCAN CEBORUCO that Volc& Ck-boruco lies in a

volcano-tectonic depression bounded The, first stage of Ceboruco's history

n by the steep escarpment to the north produced about 60 km', of material,

and the valley walrs of the Sicrra compared with about 7 km3 in the

Guamachi 1 to the south. Although second and third stages. From this

. this is a possibility, no dircAct evi- it can be reasonably inferred that the

dence indicating that the southern first stage was by far the longest.

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Figure 3. Diagrammatic cross sections of Volciin Ceboruco showing' its

development through time. (a) Near end of first eruptive stage, showing

generalized reconstruction of volcano prior to .its collapse. (b) After

eruption of Jala pumice and Marquesado ash, showing newly. formed outer

caldera. (c) Near end of second stage of activity, showing reconstructed

Dos Equis dome and Copales lava flow. (d) At present, showLng El Centro

dome on floor of inner caldera and Coapan and El Norte lava flow:

cqver-ing northern slopes- and caldera walls. 'F'i'gure .3 appears-0% the following frame.

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8 II P,ecalde'a ondesites

o P,e -e,Ceborucc rccks

...... + : .: :F E : .. i:: ' SW NE b D Ma,quesad<> ash

···:::::0::::;;... ;;;,;,;;;;;;~a ..::: :~ ~ ~ ~ ~ ~ ~: :~ : : ...... ::::::::::::::::::::::::::: .

c D Secand-stage dacites

d • Pastcaldera andesites

......

Figure 3.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/11_Part_II/2290/3433969/i0016-7606-91-11-2290.pdf by guest on 29 September 2021 The end of the first stage was marked by

the formation of a caldera 3.7 km in

diame'er. Prior to the caldera-forming The oldest expoSed lavas or CvhorucD

episode, several deep canyons were cut arc found at the base of the outcr ,' in the eastern and tqestern flanks of caldera walk There is at most 200 m

the volcano (Figs. 2, 8), suggesting of vertical exposure in the cnldcra I that Ceboruco may have remained dormant wah and these lgva flows represent

for a long time before the collapse onfy the later stages in the construflion

event , of the cone. The exposcd lava $lows

A bed of white rhyodacite pumice, are from 2 to 10 m thick an2 are

herein termed the Jata pumice, is interbedded with breccias. The thinness

the uppermost unit expose? in the of the lava flows indicates that in-

caldera wall and is believed to have dividual eruptions that built. thc major

been erupted during the calder&-forming part of the stratovolcanq tended. to be

episode. The pumice has been dated of small volume.

>- at 1020 B.P. by radiocarbon methods. On the sobtheastern flank of

All material found to 6e strati- Ceboruco, near the Cerro Pedregoso

graphically below primary deposits dome (Fig. 2) is' exposed one of the few

of the Jala pumice has been grouped thick flows erupted during the stage.

jnto the units of the first eruptive Only its distal margins can be observed

stage (Fig. 2), of which two sub- protruding from a cover of more rt'cent - groups will be discussed: (1) lavas lava flows. The flow was apparently

forming the caldera walls and main more than 100 m thick and was erupted

volcanic edifice, and (2) /material either from a flank vent or from the.

erupted From vents'on the flanks of surnni.t vent when it was at a much lower

Ceboruco. elevation.

flows consist of angular blocks zoue of cinder cones and volcanic domes,

ranging in size fron a few ccntimetres trending K6OoV through Ceboruco. There

to 5 m, contained within n much are at least 16 such features in this

finer grained matrix. The breccias 45-km-long zone, but only those vents

represent the brecciated tops and within 10 km of either side of Ceboruco

bottoms of block or aa lavas, formed have produced lava flows or domes. The

in the process of flow (Macllonald, rest are cinder cones composed of

1372, p. 86). No pyroclastic breccias andesitic scoria.

have been observed, indicating that The Ceboruquito ('?ab-h 1, analysis

during the first stage explosive 105) and La Pichancha andesites (Fig. 2)

activity was uncommon. are apparently the oldest of these

All of'the lava flows and breccias flank eruptions in the area close to

exposed in the caldera walls and outer Ceboruco. They are similar petro-

flagks of Ceboruco are andesites (Table graphically, having sparse phenocrysts

1, analyses 47 and 66). They are of plagioclase and occasional hypersthene

porphyritic with a glassy to micro- crystals set in a groundmass consisting

crystalline groundmass and contain of plagioclase and titanomagnetite

p henocrys ts of oscillatory- zoned. microlites and dark glass (Table 2).

i plagioclase and lesser amounts of Both flows are covered with a soil

hypersthene and tibanomagnetite (Table layer thick enough for the cultivation -.

2). , Some flows also contain microphenc of crops. The vent of the Ceboruquito

crysts of clinopyrox,ene and an oc- flow appears to have been the small

casional olivine crystal. cinder cone northwest of the Cerro

Pedregoso dome (Fig. 2). The La Flank Lavas Pichancha flow is nearly flat topped

Lavas erupted from vents on the and was apparent,ly erupted from cinder

Precaldera and es ites Flank lavas. * Jala pumice 47 66 136. 105 1o/k 122 353 364 346 S102 57.90 59 :45 56.04 61.87 73.78 69.23 67.50 68.37 67.42 Ti02 1.08 0.87 1.67 1.07 0.12 0.32 . 0.32, 0.30 0.42

AI2O3 17.33 17.36 16.95 16.97 13.80 15.55 15.29 15.40 15.75 1.15 Fe203 1.90 1.61 1.52 0.44 0.93 1.09 0.64 0.73 FeO 4.49 4.33 6.27 3.79 1.01 1.29 1.20 1.51 1.68 MnO 0.11 0.10 0.18 0.17 0.07 0.11 0.10 0.16 0.10 MgO 3.03 3.08 2.87 1.58s. 0.03 0.37 0.53 0.37 0.69 CaO 6.53 6.31 6.21 3.86 0.55 1.48 1 .a2 1.51 1.67 BaO 0.'07 0.08 0.08 0.11 0.02 0.12 0.12 0.12 0.11 Na20 4.14 4;06 4.80 5,56 5.21 5.59 5.07 5.30 5.32

K2° 1.75 1.86 1.75 2.44 4.45 3.30 3.29 3.22 3.08 0.28 0.15 0,. p2°5 0%4 0.28 0.00 0.08 17 0.09 0.11 H20+ 0.33 ' 0.49 0.19 0.07 0.11 0.85 2.52 2.03 1.84 H20- 0.16 0.1.8 0.10 0.08 0.11 0.10 0.45 0.46 0.61 Total 99.10 99.47 99.24 99.37 99.79 99.32 99 :47 99.51 99.53

CIPW norms (mole %)

4 8.12 9.18 3.44 8.43 22.80 18.65 19.79 19.53 18.71 C 0.24 0.46 0.64 0.91 or 10.46 11.08 10.44 14.44 26.90 19.65 20.06 19.50 18.56 -ab 37.64 36.83 43.50 49.98 46.79 50.64 46.95 48.74 48.75 an 23.92 23.90 19.75 14-17 0.84 7.09 8.38 7.33 7.94 di 5.79 5.33 5.70 2.71 1.53 hY 9.93 10.73 11.76 6.60 0.51 2.11 2.34 2.96 3.85 mt '2.02 1.21 1.70 1.59 0.46 0.99 1.18 0.69 0.47. il 1.53 1.22 2.35 1.50 0.17 0.L6 0.46 0.43 0.59 ap 0.59 0.31 1.36 0.58 0.00 0.17 0.37 0.19 0.23

-Note: Analyses by S. A. Nelgon using classical methods. Alkalis by J. Hampel, flame photometer. Ba by H. Bowman and F. Asaro, neutron activation analysis.

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TABLE 1, (Continued)

Jala Second-stage pumice dacites Postcaldera andesites 348 55 106 33 15 30 133 117 1 sio2 67.63 63.62 63.99 60.39 60.57 61.05 60.36 60.86 60.90 Ti02 a. 38 0.89 0.81 1.18 1.18 1.21 1.30 1.26 1221 15.73 17.11 16.67 16.75 16..64 16.58 16.50 16.84 16.69 A1203 0.70 1.83 1.63 1.25 1.21 .'..1.34 1-71 1.27 1.41 Fe203 FeO 1.69 2.52 .2,91 4.30 4.58 4.34 -4.80 4.50 4.31 EInO 0.10 0.09 0.12 0.12 , 0.13 0.14 0.15 0.15 0.12 NgO 0.61 1.62 1.38 2:29 2.38 '2.16 2.38 2.24 2.22 CaO 1.77 3.76 3.66 5.23 5.04 5.10 . 5.21 5.21 5.10 BaO 0.12 0.10 0.10 0.10 0.10 . 0.10 0.10 0.10 0.10 Na 0 5.22 5.24 5.36 4.64 4.65 4.63 4.58 4.68 4.79 2 3.05 2.42 2.44 2.12 2.15 2.18 2.19 2.08 2.19 K2° 0.10 0.32 0.40 0.32 0.40 0.35 0.38 0.50 0.33 zi0S ki20* 1.77 0.02 0.21 0.29 0.17 0.25 0.22 0.13 0.19 I H20- 0.62 0.14 0.05 0.05 0.08 0.07 0.07 0-03 IO-O~ TO~~I99.49 99.57 99.52 99.02 99.15 99.49 99.86 99.48 99.63

CTPI~norms "(mole %)

q 18.77 12.53 12.63 9.66 9.77 10.60 9.55, 10.18 9.85 C 0.87 or 18.52 14.28 14.41 12.67 12.81 12.95 13.02 12.30 ,13.02 ab 48.22 47.07 48.18 42.20 42.17 41.87 41.37 42.14 43.55 an 8.58 15.99 14.24 18.83 .18.35 18;42 18.10 18.90 17.74 di 0.61 1.21 4.41 3.56 3.92 4.97 3.15 4.62 hY 3.54 5.70. 5.66 8.58 9.58 8.45 9.19 9.19 7.54 mt 0.75 1.91 1.71, 1.32 1.27 1.4f 1.80 1.33 1.49 il 0.54 1.23 1.12 1.32 ' 1.27 1.66 1.70 1.76 1.70 aP 0.21 0.67 0.83 0.67 0.84 0.73 0.80 1.04 0.69

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TABLE 1. (Continued)

Post- caldera andesites 1870 dacites 61 3 67 113 181 $i02 59.61 68.17 68.03 67.91 67.61 Ti02 1.35 0.59 0.64 0.61 0.65 16.83- 15.29 A1203 .15.36 15.23 15.31 1.39 0.61 0.96 1.25 1.27 Fe2Q3 FeO 4.98 2.44 2.49 2,40 2.48 MnO 0.11 0.08 - 0.08 0.09 0.08 MgO 2.74 0.69 0.66 0.70 0.71 .CaO 5.76 1.99 2.11 2.09 2.12 BaO 0.10 0.12 0.14 0.15 0.15 Na20 4.42 5.30- 5.21 5.31 5.14 2.02 K2° 3.78 3.36 3.73 3.51 0.30 0.12 '2'5 0.11 0.14 0.13

H20+ 0.11 0.23 0.13 I 0.26 0.21

H20- 0.08 0.09 10.09 0.08 0.08 Total 99.80 99 .,50 99.27 99.95 99.34

cww norms (mole %)

(1 8 58 15.75 17.49 15.78 17.10

01: 12.00 22.40 19.78 22.03 20.82 ab 39.89 47.71 47.15 47.73 46.40 an 20.21 6.82 8.77 6.73 8.40 di 5.37 2.03 1.92 2.41 1.28 hY 9.96 3.58 3.75 2.87 '3.60 mt 1.46 0.65 1.01 1.30 1.23 il 1.90 0.82 0.09 0.85 0.90 aP 0.43 0.25 0.23 0.29 0.27

.Sample Plag San OPX CPX 01 Bi Hb Oxide Groundmass

-~- ~~~ ~~ ~ 47 44.8 -- 5.0 0.3 -- -- _- 0.7 49.2 . 66 42.5 -- 5.7 Tr. Tr., -- -- 1.1 50.7 136 56.4 -- 11.2 10.9 2.5 -7 -- 3.5 15.5 105 5.5 -- 0.4 -- -- _- -- 1.3 92.8 104 10.6 4.8 1.1 0.2. -- 0,2 83.2 122 19.2 ------l'r . 0.9 78.1 55 86.0 -- 6.5 Tr . -- -- 1.7 5.7 106 24.2 -- 4.7 0.2 -- 1.7 69.2 33 37.2 -- 3.7 1.4 -- 1.7 56.0 15 26.68 -- 5.0 2.2 -- 2.7 63.4 30 26.0 -- 5.2 1.5 -- 1.9 65.4 133 37.2 -- 4.3 3.0 -- 3.6 51.9 i 117 36.2 -- 6.2 2.4 -- 2.4 52.8

1 32.4 -- ' 2.9 1.1 -- 1.5 62.0 61 19.3 -- 3.5 1.1 -- 0.9 75.2 3 14.8 -- 1.3 0.5 -- 1.3 82.1 67 12.6 -- 1.9 1.1 -- -_ 0.8 83.5

113 10.2 -- ' 1.3 0.7 -- 1.1 86 .'7

I Note: Values are mlume precent, calculated vesicle-free.

margins of the flow. The combihed To the northwest of the Cerro _.

volume of these two flows is about Pochetero dome lies the rhyodacite 3 0.4 km . dome of Cerro Pedregoso (Fig. 2;

Near the contact of the Ceb6ruquito Tible 1, analysis 122). ..:. . ?his dome is

flow with the northwest side of (he sirnil.ar in composition to ’ the Jala

Cerro Pedreltoso dome is an exposure pumice that covers it. The dome rocks

of limited extent of an older contain pl!enocrysts of plagioclase,

andesite. This andesite (Table 1, biotite, titanomngnetite, and ilmenite

analysis 136) contains micropheno- set in a groundmiss of brown glass and

crysts of plagioclase, clinopyroxene,. feldsp-ar rcicro1it:es (Table 2). Several

hypersthenc, and olivine in a flows of similar character were erupted

microcrystall ine groundmass of feld- from the dome at a later stage, covering

..*< spar, titanomagnetite, ilmeni te, and parts of the Ccboruquito flow.

dark glass (Table 2). It is the mo%t On ‘:the northwestern flanks of ! n basic andesite found at Ceboruco. Ceboruco a flan/k eruption produced a’

Also found on the southeastern 5-km-long lava flow that is similar

flanks oY the volcano is the most mineralogical! y and petrographically

silicic mateijial observed at to the Cerro- Pcdrcgoso dome. The

Ceboruco, t,hct sodic rhyolite dome of distal margins of this flow, termed

Cerro Pochete‘ro (Table 1, analysis the Destil.adero flow in Figure 2,

104). This dome has a volume of are covcred by lavas of the thiPd 3 about 0.03 km and is composed of eruptive stage. Its probab1.e vcnt

contorted bands of obsidian and on the flanks of; CeSoruco is Roi,’

puci.ceous glass, containing sFarse covered by pyroclastic floiss.ar.2 later

phenocrysts of plagioclase, sodic lavas. The combined volume of tlie

sanidine, hupcrsthene, clinopyroxene, Destiladcro flow and Ccrro Pedrcgoso

hypersthene, hornblende, titanomagnetite, JALA PPIICE AND and ilmenite. Also found are crystals PIARQUESADO PYROCLASTIC FLOW , of forsteritic olivine and high-alumina

The Jala pumice is one of the most clinopyroxene which are believed to be

striking eruptive products of Volcsn xenocrysts because they could never

Ceboruco'. Tt is a white rhyodacitic have been in equilibrium with the

pumice that c'ontrasts markedly with si-licic host. These xenocrysts decrease

p-redmthant 1y Wack -and gray lavas in abundance from the top to the bottom

typical of Ceboruco's volume, and it of the deposit.

marks an abrupt change in.composition X type locality has been defined 10

from the previously erupted andesi tes. kin northwest of Jala, where the pumice

Its occurrence as a thin bed at the is seen to consist of five units: three

top Zf Ceboruco's outer cddera indi- layers of'air-fall pumice separated by

tates that it was one of theelast, if two thin layers of dark-colored fine-

not the last., $bases erupted prior to grsined ash. A stratigraphic.section ' 0 collapse of the volcano. This is sub- and a more detailed description of

strnntiated .by the fact that it does the various units at the type locality

not occur in the interior of the are presented in Figure 4.'

caldera .and that it is not found as The five units are distinguisl-lable

a primary dep0si.t on any lava flow as far as 15 Lfi from the center of

known to have a vent withirr=the Ce-boruco. At greater distances, the

confines of the outer caldera. two dark bands disappear and the coarse

The Jala pumice consists prf- air-fall units cannot bc distinguished

doxinantly of clear, high1.y vesicular fron' one aaot!ier. Closer to Ceboruco

glass. . Phenocrysts are not easily more than five units sometimes occur. 'J 'J seen in hand specimen, but mineral picse local. variations probably resul ted

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/Very finr mined reddish. groy osh wTih ondesitic soil overlying thinly bedded lithics up to 3cm 8 3-5% Soil overlying sandy loke deposits. subrounded pumice lopilli up to 4 cm. onderitic orh. Cross-bedded streom White ongulor oirfoll deposits oirfoll pumice with oumice. clasts UD to 5cm. 10% onderitic Iithics

Very fine- roined gray ash with 28% ongulor ondelitic lithicr up to 3cm, 8, 2% white pumice Iopilli pumice Icod uo to 3 cn up to 2 cm. Ash Northof White oir fo I I pumice, c I o s t s eboruco up to dcm, 10-20% ondesitic lithics up to 3cm. Very fine-groined reddash - subrounded white pumice 'Very fine-groined groy arh gray osh with 15% ondesitic lopilli 8 blocks up to 8cm , 'with loo$ondesitic lithics. \

* White' oirfall pumice, clarts up to 8 cm, with 10% ondesitic lithics up to 4cm.

Very fine-groined reddish SO51 very fine-grained groy ash with 305: onderitic reddish-gray osh, 45% lithics 8 5% subrounded subrounded white pumjce blocks up to 18 cm, and white pumice lopili up to6cm. 5% subangulor ondesltic Ilthlcr up to 8cm Bottom not m exposed -4Tertiary? ashflow tuff.

Figure 4. Stratigraphic sections of Jala pumice, Marquesado ash, and small ash unit

north of Ceboruco, showing tentative correlations, hetween units.

eruptive intensity. isopach. This area gepresents only a

.Figure 5 shows the thickness and fraction of the area originally covered,

distributioa of the Jala pumice. The because the thickness of pumice deposits

map is incomplete to the north because generally decreases logarithmically .a the deeply cut tributaries of the Rio with distance from t,he -vent. With some

Santiago, lying about 30 km farther .extrapolation, based on such a logarithmic north, make the area difficult to I decrease with distance, the volume of reach. The thickest sections of pumice material erupted can be estimated.

occur'near the top of Ceboruco on the Such an estimate will be a minimum

north side and withi.n 10 km of the becauscl it is likely that a large amount

present summit. The thickness of material was carried along by the

diminishes to the northwest, where wind as fine particulate matter and

the thinnest deposits found measure distributed over large' distances. The

40 cm. No pumice was found farther minimum volume is estimated to be about 3 3 from Ceboruco, probably because the 5 km . About 10% (0.5 km )'of this

topography was not favorable for the volume is represented by lithic frag-

preservation of thin ,beds of pyroclastic ments. If the bulk density of the

material. deposit without lithic fragments is Although the Jala pumice is rarely I assumed to be 0.7 g/cm3 and the pre- .found in thick section on the flanks ' eruption density of the rhyodacitq magma 3 of Ceboruco, because of poor exposure, was about 2.5 g/cm , then the volume

e'rosion, and burial by later lava of magma erupted was abobt 1.3 ltm3 .

flows, the distribut-ion shown by the Samples of the pumice collected at

isopach map indicates that Ceboruco was varying distances from Ceboruco have been

t?ie source. subjected to mechanical analysis (Fig.

The area covered by the Ja1.a pumice i 6). ~ The grain size and standard

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THICKNESS & DISTRIBUTION

JALA PUMICE

Figure 5. Isopach map of Jala pumice,

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5 10 15 20 25 30 Distance from Ceboruco (km)

1.5

b" 1.a

I I 1 1 0.5 -4 -3 -2 -1 Md @

Figure 6. Cfain size characteristics of

unit of Jala pumice. Top: M versus lower d0 distance from Ceboruco, ,where M is median d@ diameter of pumice Fragments on phi scale of

Krumbien (1936). Isottorn: fl 0 versus i.1 d0 where 00 is a measure of sorting within c1e;iosit.

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deviation from the mean of the grain Two samples of charcoal have been

size both decrease with increasing collected -from beneath the Jala pumice

distance.from the source, typical of on the southeastern flanks of Ceboruco.

air-fall pumice deposits. Meyer Rubin dated these samples, USGS

Walker (1973) has presented a 14-3490 and 14-3493, at 1,010 200 and

classification scheme for pyroclastic 1,030 f 200 yr, ’respectively.

rocks based on the values of two Two deposits of unbedded, unsorted

parameters termed D and F. D is the pyroclastic material occur at Ceboruco.

area covered by a thickness of material These consist of various proportions of

I greater than 0.01 Tmax, where is angular lithic fragments and rounded Tma x the maximum thickness of the deposit. pumice lapilli similar to the .Jala

F is thk fragmentation index, defined pumice, contained in a matrix of very

as the weight percent of materizl fine grained buff-colored ash. Figure

less than 1 mm in diameter at a 4 includes stratigraphic sections and

distaqcc along the dispers’al axis detailed descriptions of the deposits.

where the thickness is 0.1 Tmax. Both appear to be pyroclastic flow

Extrapolating a log-log plot of deposits. The largest of theso ‘is thickness versus distance from the found-south and southwest of Ceboruco,. vent gives of 30 m for the Jala (Fig. 2) where it is cut by the Rio Tmax pumice. The values of D and F are Ahuacatlsn. The deposit is here termed 2 650 km and 5%, resfiectively, al- the Marquesado ash after the village

though I: could not be determined where it was first observed. It !has

along the axis of dispersal as re- a nearly flat top, except wherc erosion- quired by Walker’s classification. has removed - the upper parts, and is Nevertheless, these values indicate apparently banked up against: the

that the eruption which produced ‘the southern wall of the valley of

Jala pumice was of the plinian type. Ahuacatlsn. It also fingers into

of the village of Tetitlsn. The lithic fragments. A similar layer is

thickest parts are found south of found in the smaller ash deposit north

Marquesado, where the Rio Ahuacatlsn of Ceboruco, but it is only 30 cm thick.

has incised a narrow canyon cutting The lithic fragments of both deposits

completely through to the underlying are andesitic?, similar to rocks found

stream gravels deposited by the in the walls of Ceboruco's outer

river prior to the emplacement of caldera. Because &is lithic-rich

the pyroclastic flow. Only thin layer is widespread and shows little

deposits can be found on the flanks variation in thickness within an in-

of VolcAn Ccboruco, probably because dividual deposit, its mode of enplacement

most of the material had enough is puzzling. It seems unlikely that it

momentum to be carried into the valley represents a mudflow deposit, since there

1 bottom. is little matrix. Individual fragments

Four units are present +n the vary from a few centimctres to 40 cm

Plarquesado ash (Fig. 4J. The lower- in maximum dimension. They are found

most unit reaches a maximum thickness in a matrix of fine ash that composes

of about 40 m; it is unsorted and only 10% to 20% of the volume. Its

unbedded. Relative to other ash-flow constant thickness and wide occurrence

units in the sequence, it contains argue against stream deposition, and the

more lithic fragments and fewer lack 0.f distinct angularity of individual

pumice lapilli. Cliarcoal found iii fragments givcs little-evidence of

*the deposit at a height 10 m above explosive disaggregarion,. although

its base has been datcd at 1,500 +- this may be of little importance if

80 yr, 500 yr older than the Jala abrasion occurrkd during transport

pumice. The ash-flow sequence is and deposition. As can be seen from

broken above this lower unit by a Figures 2 and 7, the outer calder,a wall

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Figure 7. Aerial photograph of top of Volczn Ceboruco, showing both caldcra

and postcaldera lava flows. Noflh-is to bottom of photograph to permit easy

viewing of topography. Microwave station can be seen a{ end of road,that runs

along Side of Dos Equis dome (cozpare fig. 2).

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is either missing or extremeljr low The uppermost units of the Marquesado

directly above the ash deposits. 'Thus, ash consist'of a sequence of interbedded

the 1 ithic-rich layers could represent air-fall pumice and fine-grained ash

disaggregated parts of the caldera wall containing pumice lapilli and lithic

eqlosively ejected during the erup- fragments. Because this sequence is

tion of the ash units. similar to the lower four mi= of the

Above the lithic-rich layer the Jala pumice, a tentative correlation

deposit is again largely unsorted, f h%s been made (Fig. 4). The 'fine- -" with the excgprion of several lenses grained ash units in thjs Sequence are,

of pumice in which the matrix corn-. however, different from those found

poses less than 20% of the volume, and in the Jala pumice deposit. In the

the-naximum size of the pumice clasts Marquesado deposits, they have charac-

is 12 cm. These lenses are usually teristics similar to the ash-flow units

1 to 2 m thick within a normal unbroken in the lower parts of the sequence,

sequence 0.f unbedded, unsorted ash-flow whereas in the Jala pumice deposits

deposit. Kuno (1941) and Taneda (1954) they appear to bc of air-fall origin,

have reported similar features in $he fine dust settling out after suc-

ash-flow &Gosits from Japan, and cessive pulses of eruption. They are

Smith (1960) reported that they are also considerably thicker in the

common features in other ash-flow Marquesado deposi t.

deposits. Kuno has suggested that No erosional breaks occur in the

these lenses vere formed as a result entire sequence of the Marquesado

of differential movement of particles deposit. This presents a problem in

during flowage; _r a1terna tively , they correlating its upper units with the

may have formed as a result of rafting Jala pumice, because the Marquesado ash

of pumice on the top of successive has been dated as 500 yr older than

pulses during the eruption. the Jala pumice. The problem cannot

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be resolved'at present because no side, the walls are buried by more

further samples of charcoal from the recent lava flows. On the western and . r Narquesado ash are available. The southwestern sides of (:eboruco, the

charcoal from the Marquesado deposit caldera walls are again covered by more

co,uld have been older material included recent lava flows, but even these flows

in'the ash flow during deposition lie at an elevation 100 m below the

(Blong and Gillespie, 1978) ; however, flak floor of the caldcra on the eastern

until further dating of the Ffarquesado and southern sides, indicating that the

ash is accomplished, the correlation caldera was at least 200 m deep at the

between the air-fall and ash-flow time of its formation.

deposits is plausible. The exposed parts of the caldera

wall are composed of st-ratified lavas OUTER CALDERA AND ITS OKIGIN and breccias that dip avny from the

The end of VolcGn Ccboruco's center of the volcano. ftadial dikes

first eruptive stage was marked by cut through the entire sc'qucnce of

the formation of an almost circular stratified lavasi indicating that

caldera 3.7 km in diameter (Fig. 3b). they were injected aL n late stage

The caldera has since been'partially in the first part of Cc>boruco's de- .. filled by material erupted during velopment. 'helve of these dikes are

two later stages of the volcano's exposed on the south and east sides

activity. The caliera walls are of the caldera. They range in

well exposed on the eastern and thickness from 2 to 8 in and are

southern sides, where they rise similar in composition and mineral

about 100 m above the nearly flat content to the andesitcs 'they cut.

caldera floor. The northern part The intrusion of these dikes represents

of the structure has been completely a radial expansion of thc volcanic

filled, and, except on the northwestern structure of about 2.5%.

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Thorpe and Francis (1975) gave no To test this hypothesis, an accounting

explanation for the first caldera- must be made of the volume relationships

forming event at Ceboruco but sug- involved in the collapse. If the outer

gested that thc white pumice (Jala slopes of Ceboruco are projected upward

pumice) tliat hey found in small to an assumed conical form prior to

volume on the southeastern flanks collapse, then the volume of material 1

of Ceboruco could account for the missing 'due to collapse can be estimated.

second collapse that formed the smaller Such a reconstruction is shown in

inner caldera. Since the Jala pumice Figure 3a. It indicates that prior

is found only on the outer flanks of to the formation of the caldera,

Ceboruco and is nowhere inside either Ceboruco attained an elevation of

caldera or on the surface of an; lava about 2,700 n, or 500 m higher than its

flow known to have a vent within the present caldera wall. Assuming that

confines of the caldera, it could not the caldera was originally 250 m deep,

have been associated with -the second the volume of missing material is 3 caldera. Became the Jala pumice is estimated to be 3.4 km .

found at the top of th? outer caldera The missinl; volume must be accounted

wall, it was one of thk last products for either by ejected fragments of

erupted prior to the caldera formation. precaldera andcsites or as magma Its volume .(5 lCm3 minimum) is not as erupted from an underlying magma small as Thorpe and Francis (1975) chambcr. In the preceding section

suggested. Thvsc facts suggest that of this popcr, the Jala pumice has been

the eruption of the Jala pumice and recalculated to an equivalent volume 3 possibly the Xarquesado ash deposit of magma of 1.3 km , on the basis of

accox?pan5+d~and were responsible €or tbe assurptioris discussed previously.

the collapse of the volcanic edifice The ash flows and lithic fragments in

and formation of .the outer caldera. the Marquesado ash deposits may account

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can be accounted for by erupted history began with the extrusion of

material. a dacite dome onto the floor of the

As the volume estimated for the outer caldera. Only two units in

Jala pumice is considered to be a Figure 2 represent second-stage

minimum, the accounting above seems eruptive products: the 110s Equis- dome

to indicate that the eruption of the and the Copales lava flow (Table 1,

Jala pumice was responsible for the analyses 55 and 106, respectively).

collapse of Ceb:ruco. Willi?ms and Both of these units could have been

Go'lcs (1968) accounted for 40 km3 of erupted during a sing1c event ; thus , 3 the 60-km missing from Mount Mazama the second stage of Cc.horuco's

in the formtion of Crater Lake development is considrred to have been

caldera. They suggested that the of short duration. .

20 km deficiency could be accounted Because of the subsequent formation

for b$ withdrawal of magma from a of the inncr caldera arid the eruption

magma chamber and injrction into the of more recent material, only remnhnts

countiy rock. Although this may have of the Dos"'Equis dome rc,main exposgd.

occurred at Ceboruco, it seems more These form a semicircular body ex-

probable that a consi derable volume tending from the nortlic,nstern side to

of pumice was carried by the wind the southwestern side of the volcano

to great distances from Ccboruco. (Fig. 2). The centcr of thc dome

Further geological investigations collapsed to form the inncr caldera.

in the area northeast of the volcano Parts of the dome to tli~'r.ort:i probably

may reduce t:ie hl% volume discrepancy underiie the inare rccc.nt!y cruntcd

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cinder cones and lava flows that now Joints and flow banding in the

form the higher topographic features caldera wall are difficult to internret

in these areas (Figs. 2, 7). because they often grade into one

Where the outer flanks of the dome another. Generally the flow banding is

are still exposed, they have steep weak, diDping gently away from the

slopes extending lo call!^ to the dome in basal sections of the,caldera

outer caldera walls and are covered wall and nearly vertically in higher

i by talus and rock slides with sparse ' sections of the wall. The joint pattern

vegetation. The best exposure of the is! generally the same, with basal

dome is in the walls of the inner . sections showing outward1y dipping platy

caldera where a continuous outcrop jointing spaced 3 to 5 cm apart and

extends from the northeast to south- higher sections showing vertical

east corners of the caldera. Except for jointing. Id some secti-ons of the

beheaded lava flows capping the top wall the jointing is ncarly vertical

of the caldera wall on lie east and near fhe caldera floor but fans outward

0 soutliwest sides, the rocks forming the to 45 dips in higher parts of the wall.

1. Dos Equis dome are unstratified, con- This type of jointing suy:gests that

trasting markedly witli the s tratif ica- parts of the structure wCre intruded

tion of the outcr cald~rawalls. The separately and cooled ns individual

walls of the inner caldcra generally units, the joints representing con-

form nearly vertical cliffs, frequently traction cracks in each unit. Indeed,

interrupted by more gclritly sloped rock- some of these separate bodies remain

slides. On the eastern side, a 500-m- as spired standing above the other

long blockpf the wall has slumped into parts of the caldera wall.

the caldera af-g an nrFunte fracture, On the basis of these general patterns

offsetting a lava flow c,ipping the or flow banding and jointing, some

top of Lhe wall 5y 20 m. speculations can be Fade concerning the

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mode of growth of the dome. The out- titanomagnetite, and ilmenitc! occurring

wardly dipping fegtures foimd at t.he singly and in clusters (Table 2). +he

base of the caldera wall suggest that groundmass is almost entirely crystal-

during early stages of the dome's forma- line, composed of niicrolites of plagio-

tion material tended to flow away fqm clase and minor hypersthen& with \ a cent~elvent, indrcating exogenous little glass. The rocks appciar. fresh,.. growth. However, the vertical jointing except for thpse at the top of the

and flow banding higher in the .caldera caldera'walls, where they have a

wall suggest that the later stages of pinkish oxidation stain and the oxide

dome growth occurred in an endogenous minerals have oxidized and exsolved.

fashion. The lava fl.&ws capping some parts

The beheaded lava flows, such as the of the caldera wall are similar in

Copales flow' (Fig. 2), at the top of . ni.neral content and composition to the

the caldera wall dip gently until they Dos Equis do!ne rocks, except that the

reach the steep f-lanks of the outer flow rodks havc a glassy groundmass. ,

walls of the dome. This suggests . The flows erupted over the eastern

thak the dome had a relatively flat sidelof the dome are small but extend

top prior to the formaLion of the to the floor of thc.outer caldera.

inner caldcrcl. A diagrammatic cross The Copales flow (Fig. 2), howevclr,

section of the Dos Equis dome before is I.ocally as much as 100 m thick and 3 the caldera was formed is shown in has an estimated volume of 1.4 km .

Figure 3 c. The vol.uxe of the d0rr.e ik After flowing dovn the sides of the 3 estimated to have been 1.3 km . dome, it covered the low ;'art of the

The rocks of the Dos Equis dome are outer caldera wall and spread in two

light gray and have a glomeroporphyritic I.obes over the southwestern flanks of

.% texture with phenocrysts of oscillatory Ceboruco. In aerial. photographs the-

zoned plagioclase, hypersthene, augite, flow is seen to consist of several

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scale. The surface of the floi.; con- are shown in Figure 8.

sists of block lava and is covered The textures found in thc xenoliths

by extensive vegetation and two more are interpreted as cooling textures;

recent lava flows. the larger phenocrysts we?-c presumably

All of the lavas erupted during present in a basic magma that became .

Ceboruco's second stage of activity included in the dacite, resulting in

contain xenoliths, but they are more crystallizat_ion of the basaltic ground-

common in the%flow rocks than in the mass. The ,dacitic host is never

dome rocks. The xenoliths range in size obgerved to have quenched,agninst the

from 1 to 5 cm but are also found to basnltic.xenoliths, arguing against

be broken up and scattered tlirough the the hypothe.sis that, these xenoliths

ciacite matrix. They consist of represent already solidified ba'salt

porphyritic basalt containing large picked up by the dacite. As the

phenocrysts, as large as 1 cm, of xenoliths are found.in many places

o 1 i vine, CJiqopyroxene, ant1 plagio- broken into many smaller aggregates

clnse set in a finer-grained ground- and invaded by the more silicic host,

rn:iL;s of plagioclase, hyperc,thcne, and a reaction relationship with the host d brown glass. The glass corirmonly oc- lava is indicated and the possibility

curs as spherical globulcs surrounding of magma mixing is inferred and will

a hole that apparently contained a bc discussed below.

vapor phase. The glass is siliceous Origin yf the Inner Caldera ('l'nbk 3)and is \iholly contained

wi'thin the xenoliths, where it Nearly half of the walls of Ceboruco's

probablv represents the rc:;idual inner caldera are either missing or

1iq1iid left after crystal1 ization of art? covered by more recently erupted

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Figure 8. Photomicrographs of xenoliths in second-stage dacites.

(a) xenol ith (left) iith groundnass consisting of interg\roim plagioclase

and Frthopyroxene. Olivine crystal from the xenolith is nt right, Floating

in dacitic matrix. (b) Large olivine xeriocryst witiL augite reaction rim

in dacite matrix. (c) Siliceous glass bleb in xenolith. Glass surrounds

hole that probably containdd a vapor phase. (d) Xenolith containing

large phenocryst of olivine (right). Groundmass consists of p1,agioclase

and orthopyroxene with some titanomagnetite and ilmenite. Glass blebs

nre seen in lower left and u?per left.

Figure 8 appears on - the following frames.

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Figure 8,.

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lavas; its diamter is estimated to younger parts of the Copales flow may

have'been 1.5 km. The deepest parts have issued through a now-concealed

are now only 80 m below the highest vent on the flanks of the dome at an

sections of the caldera walls; but elevation lower than the present caldera

it has been filled extensively with floor. Either of these mechanisms could

more recent ash and lava and probably easily account for the 0.5 km3 of

extended at least another 100 m down- missing naterial, as the Copales flow

3 ward at the time of,formation. The has a volume of 1.4 km .

volume 0; material ,missing from the ANDESITES OF THI.: THIRD STAGE OF ACTIVITY 3 structure is estimated to be 0.5 km .

The cause of collapse of the Dos Equis Following the collapse of the Dos

dome is puzzling. No pyroclastic Equis dome, Ceboruco again erupted

deposits are associated with ttic nndesites (Table 1, analyses 33; 15,

caldera; explosive eruptions therefore 30, 133, 117, 1, 61).. The postcaldera

cannot have triggered collapse. It is andesitcs have a glomeroporphyritic

p o s sible t

by evacuation of an underlying magma zoned plagioclase, euhedral hyperdthene,

chamber by eruption of lava, such as augite, titanomagnetite, and ilmenite

the Copales flo:d, through the top of occurring as single crystals and in

the dome. In t:iis case subsidence would clusters. Although variable, the

tiave occurred slowly, as a delayed groundmass is generally glassy and

response to evacuation, and must contains microlites of tabular pl.agio-

have progressed from the center of clase. The postcaldira andesites are

the dome toward the rim, because the distinguishable from the precaldera

beheaded nature of the Copales flow nndesites by their generally 'more

indicates that it once had a vent near glass? groundmass, the presence of

the top of the dome. Alternatively, the ilmenite as a microphenocryst phase,

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and the greater abundance of augite are pinkish gray, similar to the upper

phenocrysts. parts of the Dos Equis dome, except

The earliest exposed andesites of that they are andesites. They differ

the third stage are found on the floor from most of the third-stage andesites

4; of the inner caldera (Figs. 2, 3d, and in that they have a microcrystalline 7); although it cannot be said with ' rather than a glassy grounclniass.- certainty that some of those exposed A dark andesite (sarnp1.e 30) ,was

to the north of Ceboruco are not older. extruded from the central parts of the

In the center of the inner caldera El Centro dome in its last stages of

stands a small andesiite dome, here development (Fig. 7). 'I'hfs flow, moves

termed El Centro dome (sample 33). It over the southeast side of the dome to the

measures 400 m in diameter at its base caldera floor.

2nd has steep-sided margins that rise At some point in the third stage,

to about' 80 m above the caldera floor. activity shifted to the northern margins

Its steeF flanks are covered with a of the calderas, wherr. a considerable

mantle of talus, and its surface is vol.ume of andesitic lava was extruded

rough and irregular. On the ,western to cover both the inner and outer

side of El Centro dome, remnants of caldera walls. The Coapan flow (F'ig. 2)

an -associated tephra cone are still is one of the older flows exposed to

in:act, and on the south and eastern the north. It is a blocky andesite

sides thk dome is surrounded by several (sample 117) that presumably covered

blocky andesite .flows (sample 15) that most bf the northern flanks of Ceboruco,

are uybubtedly relaied to its con- extended to the north into the valley

struction. The northern margins are of El Cajon, and moved along a

buqied by at least two scoria cones relatively, gentle gradient to thc cast

built later in the thi.rd stage. and west, overriding the Ikstiladero

The rocks that compose the dome flow. The vo1ur.e of the Coapan flow

Overlying the Cospan flow and . between the El Centro dome, the thick

covering the northern flanks of the bulbous dacite flow associated with, the

volcano is a petrographically similar 1870 eruption, and the El Norte lava

flow, termed El Norte flow in Figure flow. These cones have flat crater

2. It was apparently extrude4 along floors roughly 50 m in diameter and arcuate fissure vents, most of the overlap one another, the easternmost. cone materials pouring to the north. being lowest and the wcsternmost highest

Some lava extends into the eastern (Fig. 7).

part of the inner caldera, and some Two other cifider cones are present on

flowed into the outer caldera south the floor of the inner caldera. One is

of the mi.crowave station (Fig. 7). the highest structure on the volcano

One small. lava cone, representing a and occurs on the western margin of the

vent during the last stages of the inner caldera. Although this cone eruption, is about 1 km west of the provjdes the vent for the.. bdbous microwave station. The flow is &cite-'flow believed to have been

made up of several flow units, and erupted during tie' 1870 eruption, it

many c.0l.l apsed lava tubes are found was present ,prior to that eruption .- on its surface. (Caqavantes, 1870). Within the crater

The vents for the El Sorte lava of this cinder cone there are three

define the highest parts of Ceboruco's smaller nested cones, one of which

north side. Their arcuate arrangement contains a small dacite plug dome about

indicates that they are associated 10 m in diameter.

with a ring fracture related to the The other cinder cone is located in

margins of the calderas. the southwestern corner of the inner

Three scoria cones, evidently built caldera. Its walls are coxposed in

during the eruptions of lava along the part of the inner caldera-Dos Equis

posit of sandy ash exposed on the bboruco and El. Norte flows suggests

western flanks of 'Ceboruco (see that they wcrc erupted within the past

Segerstrom, 1950, Fig. 69) was- prob-

.ably,. formed during its construction. (1877) stated that there is nothing

On the caldera floor, parts of the in the local legends concerning ._ cone have been eroded an-d it can be eruptions of Ceboruco prior to the

seen to consist of andesitic scoria 1870 event; however, PeGa Navarro

with abundant bombs of dark-colorcd (1946), in tiis history of the State

andesite with inclusions of white of Nayarit, quoted Padre T/ello to the

pumiceous material. A smaller cone effect that Ccboruco erupted in 1567.

that has been partially rexoved by In another history, Pena Kavarro (1956)

erosion is nested within this cone. stated that Viceroy D. Antonio de / Altho-ugh the relative time of Mendoza was in :he regiop,in 1542 and

formation of thi's cinder cone is that Ceboruco was in erupt'jon then. It

uncertain, it may be related to a is possible that one of these reportc>d

lava flow for which the vent is lower on eruptions could have produced the

the flanks of the Dos Equis dome. Ceboruco or El Sorte lava floi;s.

This flow, -termed the Ceboruco flow During the 1,000 yr that have

(Fig. 2) is an almost barren blocky elapsed since the formation;o'f the

andesite (sample 61). It is' one of outer caldcra, Ceboyuc?o, has erupted

the most striking flo6s ii-t Ceboruco, eight times (including the Jala pumice

mainly because I-Iexican. Highway 15 ad 1870 eruptions) and'produced a

crosses it. Its fresh appearance is volure of about 8 km3 of nagrra. This

'so deceiving that Thorpe and Frahcis gives an eruption rate of about 0.008

(1975) mistakenly identified it as Itm3 /yr, or 0.125 times that of the

the 1-870 lava flow. world's most active volcano, Kilauca ._

esLimated worldwide magma production of it fell on the village of Jala.

per year (Nakamura, 1974). * Ceboruco Cara'vantes (1870), who was one of the

has had one eruption every 125 yr on first observers to arrive at Ceboruco, 1 the avcraj;e, a periodicity that would was told of ash flowing down the Arroyo

predict a future eruption in tiit. year de 10s Cuates 1-ikc water. Waitz (1920)

1995. If the average eruption rate is read this description and concluded

projected into the past, then Ccboruco's that the flow of sandy ash was the 3 entire volume of 70 km could have product of a nube ardente (any such

been produced in, about 8,800,yr! deposit is at prFsent covercld .by the

lava flow that followed in the eruption). L870 ERUPTION By the time Carnvantes arrived at

Beginning about Fc.bruary 18, 1870, Ceboruco, lava tiadstarted to flow from

the people in the vil lages surrounding the- crater, first toward the south, but

Volczn Ceobruco felt the ground shake, changing course to follow the stream

heard-_ "subterranean noises," and bed of the Arroyo de 10s Cuates. By

noticed a white cloud of vapor above the Lime Iglesias, Bilrcena, and Platute

the summit of the volcano. Shortly arrived in 1875, the lava had completely

thereafter, on February 23, the erup- filled the arroyo and had reached 7.5

9.. tion began (Cara'hntes, 1870; Iglesias kz fron its source. E-Jen at this stage,

and others, 1877). The main vent the f ].ow would occasionally avalanche,

was &ente&d beneath the highest pt.alc revealing.incandescent matrrial in its

on tile west side of tile volcano, interior. These investigators noted

near thc head of a decp canyon, known that the arroyo had been fjlled in some

then as Arroyo de 10s Cuates. Great places with more than 500 m of new

clouds of asti and vapor rose above lava, and they estimated the volume 3 Celioruco, most of the ash being blown of thc flow to bc 3.3 km . A more

Although most investigators who have beneath the main part of the flow on

studied Ceboruco have claimed that the. the former outer caldera floor. A .. 1870"lava is andesite, chemical . dike in the western wall of the crater

analyses reveal that it is dacitic leads to the final vent of the eruption,

(Table 1, analyses 3, 67, 113, and 181). which produced the small dark tongue

The rock is gray and contains pheno- OY lava near the top of the flow between

crysts of plagioclase, hypersthene, the levees formed during the major part

augite, titanomagnetite, and ilmenite of the eruption. These flow levees

in single grai6s and in clusters.. The occur at an elevation 15 m above the. .. k groundmass is predominantly glass but last vent and indicate that the vent

contains lath-shaped microlites of which erupted the bulk of the lava flow

plagioclase, small amounts of was above the present location of the

hypersthene, and Fe-Ti oxides (Table 2). :plug dome on the elliptical crater

The'vent for the 1870 eruption can floor. This plug dome measures about

be seen on the western flanks of the 30 m in basal diameter and still has

Dos Equispdome (Figs. 2, 7), where several active fumaroles on its"i1anks.

there is a small elliptical crater A small bulbous lava flow on the

containing a small plug dome. The lower flanks of the large cinder cone on the

parts of the walls of the crater are weskern margin of the inner caldera is

composed of beds of sandy ash that dip similar in all respects to the lavas - to the west, indicating that the of the main 1870 eruption. It may have

crater formed by explosive- excavation been et-upted prior to the 1870 eruption

of the flanks of an of-&er-cinder or during it. No mention of this flow

cone. Dikes in the southwestern is made in the reports of the observers

corner of the crater can be followed of the 1870 event.

through to the earliest flow unit of

Five distinct chemical groups are from the precaldera volcanic structure

recognized in the lavas from Ceboruco by X-ray fluorescence methods, .to

on the basis of both major- and trace- determine if indeed they all fall in

element ahundances. -These gr_oups are, the same chemical group. The results,

in order of decreasing stratigraphic along with standard deviations and

age, precaldera andesites, Jala pumice averages for each group, are given in

dacites, postcaldera andesites, and 1870 The Ceboruco lavas define a trend on

dacites. A sixKh group of lavas, all an AFM diagram (Fig. 9) showing no iron

erupted from vents on the southwestern enrichment. The more basic members are

flank of Ceboruco prior to formation slightly more enriched in Fe than the I of'the outer caldera, contains a average of lavas from the Cascades of

variety of chemical typ6s and is the western United States (Smith and

distinct only in the geographic I Carmichnel, 1968), but they are not

locality of the eruptive vents. This as Fe rich as the Island Arc Tholeiitic

group consists of an older low-silica suite (k'ig. lo), and are apparently

andegite (1.36), the Cehoruquito calc-alkalic (Ewart and others, 1973;

andesite (105), and the Cerro Pochetero Jakes and White, 1372),

rhyolite dome (104). No major basaltic Andes i t c s members occur, except as xenoliths

in the second-stage dacites. 6r ttie two major groups of andesites

Chemical analyses of 23 lavas and found at Ceboruco, the older precaldera

pyroclast ics are presented in Tables 1, andesitc group is generally the most

3, and 4. Since only two lavas from basic observed. Con:pared to- the later,

tile preca I dera andes i te group were Tnore siliccous andesites of the

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TABLE 3. TR4CE-ELEPIENT ANALYSES OF CEBORUCO LAVAS

sc 16.10 (Q.05) 15.15 (0.05) 18.69 (0.04) 9.58 (0.04) v 193 (20) 127 (22) 165 (25) 1.15 Cr 23.2 (1.7) 29.3 (1.7) 0.9 (0.7) < 1 (0.1) co 18.114 (0.27) 17.09 (9.25) 25.54 (0.37) 5.4 (0.13) Ni 30 (13) 42 (13) 16 (9) <10 Zn 96.1 (5.6) 75.8 (5.3) 120 (6) 114 (5) cs C.8 (0.2) 0.6 (0.2) 0.19 (0.18) 0.5 (0.2) Ba 645 (17) 733, (18) 722 (39) 952 (22) La 19.17 (0.48) 18.12 (0.46) 30.7 (1.1) 34.04 (0.60) Ce 42.16 (0.63) 38.55 (0.59) 63.38 (3.84) 71.15 (0.79) Nd 20.5 (0.8) 15.5 (0.8) 29.0 (3.6) 34.6 (1.0) Sm 4.27 (0.01) 3.71 (0.01) 7.05 (0.04) 6.77 (0.02) EU 1.36 (0.01) 1.13 (0.01) 2.59 (0.32) 2.35 (0.02) Tb 0.59 (0.04) 0.54 (O.Or+) 0.90 (0.03) 1.30 (0.05j JJY 3.87 (0.13) 3.20'(0.13) 6.35 (0.17') 5.72 .(0.20) Yb 1.97 (0.03) 1.77 (0.03) 2.95 (0.04) 3.31 (0.04j TAU 0.28 (0.02) 0.26 (0.02) 0.31 (0.02) 0.50 (0.02) 11 f 4.22 (0.08) 3.57 (0.08) 5.68 (0.11) 7.27 (0.11) 'r a 0.61 (0.00) 0.40 (0.00) 1.39 (0.01) 1.65 (0.01) 'Ch 2.56 (0.07) 2.87 (0.07) 2.68 (0.05) 3.93 (0.07) U 0.75 (0.03) 0.75.(0.03) 0.89 (0.05) 1.23 (0.03) K/ Rb 518 386 632 722 Ilb / Sr 0.041 O,..062 0.038 0.057 Th/Y 3.41 3.83 3.01 3.20 Eu/ Eu2 1.271 .996 1.24 1.091 I,a/Yb 10.00 10.24 10.41 10.28 -

Note: Values are parts per million. Values in parentheses represent. 1 standard deviation as'rneasure cf precision. Neutron activation analyses by F. Asaro and H. I?orman, Lawrence Herlieley Laboratory.

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sc 13.50 (0.05) 15.55 (0.05) 7.55 (0.03) 8.53 (0.04) v 107 (17) 143 (21) 36 (1.5). 21 (23) cs 4.5 (1.6) 14.6 (1.7) 4:% (1.2) 2.1 (0.5) co 12.18 (0.20) 15.79 (0.2L) 3.2 (0.10) Ni .;5 .: 10 5 <5 Zn 103 (5) 98 (6) 79 (4) 69 (5) cs 0.8 (0.2) 0.8 (0.2) 1.3 (0.1) 1.4 (0.1) B A 921 (21) 855 (20) 1079 (22) 1273 (36) La 28.01 (0.54) 27.5% (0.54) 32.64 (0.53) 39.5 (1..'3) c C' 58.92 (0.72) 56 :.67 (3.12) 75.69 io.78j 84.09 (0.90) Nd 28.1 (1.0) 28.8 (1.0) 31.4 (0.9) 29.1 (1.1.) Sm 5.62 (r3.01) 5.38 (0.01) 6.57.(0.03) ELI 1.49 (0.02) 1.65 (0.02) Tb , 9.86 (0.04) 0.81 (0.03) DY 5.31 (0.13) 6.08 (0.15) Y!, 2.76 (0.03) 3.56 (0.04) 3.70 (0.04) Lu 0.39 (0.02). 0.34.(0.02) 0.42 (0.02) 0.45 (0.04) H f 5.98 (0.10) 5.49 (0.10) 9.52 10.19 (0.14) T ;i 1:lO (0.p) 1.11 (0.01) 1.57 (0.01) 1.97 (0.01) Tli 3.28 (0.07) 3.11 ((3.08) 5.99 (0.07) 6.17 (0.05) U 1.03 (0.03) 0.9/+(0.03) 1.50 (0.03) 1.82 (0.06) K/!?b 700 700 k68 b32 ,Rb/ Sr 0.052 0.050 0.272 0.261 TI?/ U 3.15 3.30 3.99 3.39 l&/Ec" 0.962 0.986 0.863 0.854 La/Yb 10.15 10.63 9.16 10.68

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sc ' 13.25 (0.05) 13.42 (0.05]! 13.37 (0.05) 13.58 (0.05) v 83 (24). 120 (19) 136 (22) Il+4 (25) Cr 7.3 (1.5) 10.3 (1.5) 8.3 (1.6) 7.0 (1.6) co 12.57 (0.21) 12.40 (0.21) 12.43 (0.21) 12.29 (0.20) Ni 33.0 (12.0) ;lo 110 L5 Zn 105 (5) 96 (5) 100 (5) 35 (5) CS 0.5 (0.2) 0.6 (0.2) 0.5 (0..2) . 0.8 (0.2) Ba 923 (31) 928 (21) 897 (21) ' -9%-6,(21) La 27.68 (0'.54) 28.25 (0.54) 28.79 (0.54) %9.$3 (0.56) Ce 56.00 (0.69) 56.43 (0.70) 59.15 (3.781, 59.e7 (0.73) N d 27.4 (0.9) 28.6 (0.9) 30.7 (1.0) 28.& ;1.0) Sm 5.47 (0.01) 5.60 (0.01) 5.64 (0.01) 5.67 ~(0.01) Eu 1.59 (0.02) 1.61 (0.02) 1.63 (0.02) 1.68 (0.02) TS 0.76 (0.04) c.75 (3.C4) 0.77 (0.01) 0.31 \h. C4) DY 4.65 (0.15) 5.06 (0.14) 4.91 (0.15) 4.55 (0.19) Yb 2.52 (0.03) 2.63 (0.03) 2.61 (0.03) 2.70 (0.03) Lu 0.34 (0.02) 0.41 (0.02) 0.36 (0.02) 0.43 (0.02 I-! f 5.63 (0.09) 5.85 (0.10) 5.71 (9.10) 5.95 (0.10) Ta 1.01 (0.01) 1.07 (0.01) 1.07 (3.01) 1.07 (0.01) Th 3.20 (0.07) 3.37 (0.07) 3:20 (3.07) .3.36 (0.07) U 1.00 (0.03) 1.03 (0.03) 1.01 (0.03) 1.00 (0.03) K/ RS 518 508 546 5 74 Rb/Sr 0.066 0.066 0.062 0.054 Th/ U 3.20 3.27 3.17 3.36 Eu/Eu';. 0.950 0.959 0.942 0.966 LaIYS 10.96 10.74 11.03, 10.90

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sc 1.41 (0.04) 1.84 (0.02) V 20 30 25 (20) Cr 4.8 (0.9j 1.8 (0.9) 1.4 (0.9) CO 0.15 (0.04) 0.59 (0.04) 0.55 (0.04) Ni <5 <5 9.0 (6.5) Zn 89 (3) 84 (3) 87 (3) c.s 1.7 (0.1) 0.9 (0.1) , . 0.9 (0.1) Ba 201 (19) 1046 (22) 1076 (21) La lt6.09 (0.64) 34.17 (0.57) 33.52 (0.56) Ce 97.86 (0.88) 67.93 (0.91) 68.0k (0.72) Nd '39. 3 (1.1). 28.3 (0.9) 27..2 (0.8) Sm 7.71 (0.02) 4.64 (0.01) 4.63 (0.01) Eu 0.32 (0.01) 1.03 (0.01) 0.98 (0.01) Tb 1.20 (0.05) 0.64 (0.03) 0.64 (0.03) DY 8.31 (0.15) 3.75 (0.19) 3.66 (0.20) Yb 5. 3?.::(0. @5) 2.90 (0..03) 2.90 (0.03) LC 0.71' (0.C3) 0.45 (0.02) 0.45 (0.02) H f 8.89 (0.11) 7.63 (0.10) 7.57 (0.10) Ta '3.99 (0.01) 1.75 (0.01) 1.73 (0.01) Th 13.55 (0.08) 5.73 (0.06) 5.63 (0.06) U '3.38 (0.04) 1.67 (0.03) 1.70 (0.03) K/ Rb 3 /0 526 558 Rb/Sr 5.37 0.192 0.171 Th/ U -3.71 3. /+3 3.31 Eu/ Eu" 0.131 0.731. 0.697 La/Yb 8.66 11.78 11.56

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___ . ___ - .7;56------348 5 5 - . 1 Oh sc 2.32 (0.02) 2.55 (0.32) 7.67 (0.03) v 23 (.IS) 21 (16) . 54 (18) Cl- 3 ..5 (0 ..*9) 1.5 (1.0) 3.7 (1.3) 2.3 (1.3) co 1.1.3 (0.06) 1.15 (0.06.) 5.01 (0.1.7) 5.29 (1.2)- Ni 14.0 (6.0) 20.0 (?.O) . .7 ’7 Zn 95 (3) 82 (3) 104 (4) 94 (4) cs 0.9 (0.1) 0.9 (0.1) 0.6 (0.2) 0.8 (0.2) Ba 1027 (21) 1038 (22) 899 (21) 938 (21) La 3.3.61 (0.56) 34.74 (0.57) 30.6 (0.55) 30.48 (0.57) CC? 66.G9 (0.71) 67.06 *(0.72) 60.82 (0.70) 64.46 (0.73) Nd 26.0 (0.8) 26.2 (0.08) 2.8.7 (0.3) 28.4 (0.9) SKI 4.73 (0.01) 4.86 (0.01) 5.44. (0.01) 5.78 (0.02) ELI 1.08 (0.01) 1.12 (0.01) 1.62 (0.079 1.66 (0.02) Th 0.63 (0.03) 0.64 (0.03) 0.73 (O.O/+> 0.85 (0.04) DY 3.59 (0.19) 3.97. >0.19> 4.55 (0.15) 4.21 (0.18) Yb 2.88 (0.03) 2.90 (0.03) 2.71 (0.03) 2.90 (0.03) Lu 0.44 (0.02) 0.42 (0.02) 0.39 (0.02) 0.44 (0.02) Hf 7.36 (0.10) 7.19 (0.10) 6.24 (0.10) 6.52 (0.10) Ta 1.69 (0.01) 1.66 (0.01) 1.27 (0.01) 1.36 (0.01) ?’h 5.52 (0.06) 5.48 (0.06) 3.95 (0.06) 4.11 (0.06) U 1.61 (0.03) 1.64 (0.03) 1.24 (0.03) 1.23 (0.03) IZ 1 Rb 556 552 500 470 ‘Rb/ Sr 0.146 0.149 0.083 0.Q89 ‘r 11 lu 3.43 3.34 3.18 3 . 3-4 r / *Ji: Y 0..760 0.718 0.991 1.00 La/Yb li.67 11.98 11.29 iO.51

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TABLE 3. (Continued) ~- 13 3 181 sc 8.42 (0.04) 8.96 (0.04) v 33 (19) 25 (18) Cr 3.7 (1.3) 3.4 (1.4) co 3.87 .(o, ii) ;< z- <8 Zn 80 (4) 94 (5) * ,cs 1.3 (0.1) 11.0 (0.2) Ba 1299 (25) 1308 (26) La 39.35 (0.61) - 39.89 (0.64) Ce 78.13 (0.81) 19.98 (0.83)-, Sd 33.3 (1.0) 35.3 (i.0) Sm -6.63 (0.02) 6.73 (0.02) Eu 1.60 (9.62) 1.62 (0.02) Tb 0.07 (0.05) 0.97 (0.05) DY 5.It1 (0.18) 5.57 (0.22) Yb 3.78 (0.04) 3.79 (0.04) LC 0.53 (0.02) 0.53 (0.02) FI 6 9.35 (0.12) 9.7 (0.13) Ta 1.62 (-0.01) 1.85 (0.01) Th 5.84 (0.07) 6.04 (0.07) ,u 1.70 (0.03) 1.82 (0.031, i? /. I?b 4 70 512 Rb/Sr 0.364 0.219 Th/U 3.111 3.32 Eu/Eu:; 0.771 0.776 LaIYb 10.64 10.53

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TABLE'4. Rb, Sr, Y, AND Zr fN CEBORUCO LAVAS

Precaldera andesites 47 66 *76 127 71 74 51 48 77 49 Av~ -+ 2~ Rb 78 40 $1 37 39 38 35 28 49 31 37 12

Sr 680 644 655 654 620 636 650 676 597 653 647+- 49 22 21 22 23 26 . 22 22 19 21 29 23+ 6 Y 0- Zr 178 153 159 155 155 176 170 171 191 183 169+- 25

Flank lavas 104' 136 105 116 Rb 102 23 28 35 Sr 19 6QO 487 653 >- Y 45 34 40 22 Zr 265 222 325 170

Jala pumice group 3'64 346 348 353 122 178 Ave -+ 2a

Sr 281 316 309 295 271. 280 292+- 36 Y 25 25 29 23 23 24 25+- 5 Zr 298 290 297 278 302 295 293+- 1.7

Second- st'age dacites Postcaldera andesites + 55 106 33 15 30 133 117 1 61 99 Avg - 2a ~ L. Rb 40 , 43 34 35 -33 49 33 26 24 30 33+- 15 Sr 480 483 514 531 530 534 554 500 484 576 528+- 60 Y 35 31 30 27 23 40 29 31 30 25 29+ 10 I - Zr 250 268 223 234 247 244 257 235 210 230 235+- 29

1870 dacites . + 67 113 -181 3 ~vg- 2a Rb 64 66 57 67 64+- 9 Sr 245 250 260 246 250+- 14 Y 34 27 33 33 32+- 6 ZK 390 385 370 382 382+- 17 -Note: Values are parts per million,

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A M

Figure 9. hW diagram for Ceboruco lavas. A = Na 0 + K20, 2 ?I = MgO, and F = FeO + 'Fe 0 all in ratios of weight persent. 2 3, Solid circles = precaldera andesites; oFen circles = flank lavas;

open squares = Jala pumice; triangles = second-stage dacites; I dia-onds = postcaldera andesites; solid squares = 1870 dacites.

Also shown is average trend for Cascades of western United States

(SxitF. cind CarT.ic1iae.l , 1968) .

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5- -

0 0 m 3-

1 3. 5 7 %FeO + Fe203

Figure 10. P1-o; of weight percent Fe0 + Fe 0 versus wegght: percent Flg0 23 e for. Ceborrlco lavas. Symbols are same as in Figure 9. Also shown is average

trend for Cascades of wes.tern United States (solid line) and iron-enrichmefit

trend of an island-arc tholeiite suite from Talasea, New Britian (dashed

line) (Lowder and Carmichael, 1970).

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pos tcaldera group, the prccaldera content is high relative-to most

andesites are”‘enriched in Ca ;Ind Flg, andesites (Taylor, 1969; Jnkes and

siiow a 1ot;er Fe/?:g ratio, and ‘nave k% i t c, 1972), although Pichler and

\: 1:reater abundances of the compatible ZeiL (1972) reported similar Ba con-

trace elements Sc, V, Cr, Co, and Ni. centrations in andesites from northern

‘I‘liey show a fractionated rnre-earth- Chile. Gunn and Mooser (1971) showed

element (KEE) pattern, with La about hif;h Ba concentrations in andesites

50 times chondritic and La/Yb about from this part of Mexico, suggesting

1C (Fig. 11). Con?ared to Taylor’s chat this property pay D,e characteristic

(1969) average andesite, the precaldera of the province.

andesites are slightly enriched in Ti K/Rb ratios in both groups. of

and light REE and show greater andcsites are variable, but they are

abundances of Ba, Sr, Th, U, and Zr. generally high compared with average \ The postcaldera andesitcs are andesites (Taylor, 1969; Jakes and

s1.i gk tly more siliceous than the Vhite, 1970). Although pfagioclase is

prccaldera andesites and are rela- the predominant phenocryst phase in * tively enriched in Na and K., Their both groups, neither shows a signifi-

Ti contents are relatively high for. cant Eu anomaly (Fig. 11).

andesiccy (Taylor, 1969), which Two anddsites erupted from vents on

probably accounts for the appearance the southeastern flank of Ceboruco show I of ilrcenite as a phase in these afiinities with neither of the above

rocks. They show greater abundances groups, althouih both were erup,ted prior

o f incompatible tra’ce e lenien t s except the caldera-forming event. Sample 136

Sr than the precaldera group and (Table 1) is the most basic lava found

are hjghly enriched in REE over at Ccboruco. It is extremely rich in

Taylor’s (1969) average andcsite, with Fe (7.72% total FeO), with a high

La about 78 times chondritic. Ba Fe/Mg ratio, and it,contains R high

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100 - - PRE CALDERA ANDESITES

Zl I II II II Lo Ce Nd Sm Eu Tb Dy Yb Lu La Ce Nd Sm Eu Tb Dy Yb Lu

Figure 11. Chondrite-normalized REE patterns for five chernical1.y distinct

'groups of Ce'boruco lavas and for grrmp of d ivcrse lavns crilpted on soot1ieastc.r-n

flank of Ceboruco. Concentrations are normalized to Leedy chondrite (Masuda

and others, 1973).

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concentrabion of '1150 Sample 136 is they are similar in composition to the 2' relatively enricticd in REE,, with Jala pumice itself. The second-stage 0 La abo~t80 times chondritic.' dacites were erupted next and consist .

, The Ceboruquito flow (sample 105) of two units, the Dos Equis dome (sample

also erupted on the southeastern 55) and the Copales flow (sample 106)

flank of the volcano, is the most these are low-silica dacites. The third

siliceous and potassi,c of Ceboruco's group comprisr;s the products of Ceboruco' s

andesites, and it has an extremely most recent eruption, the 1870 dacitcs.

high Fe/Mg ratio because of its low Five analyses of rocks from,the Jala , MgO content. This lava is strongly pupice group are presented in Tables 1,

enriched in REE, with La about 90 3, and 4. This is the most silice'ous

times chondritic, and it sbws a group of rocks at Ceboruco when con7

slight positive Eu anomaly (Eu/Eu+c = sidered on an anhydrous basis, dis-

\, 1.09), probably carised by accumula- counting the small volume of Cerro

tion of plagioclase (see discussion Pochetero sodic rhyolite. All rocks in

in section on cineralogy). Ba is very this grou? are corundur, normative.

abundant, and the K/Rb ratio is the Heming (1974) reported corundum-normative

highest observed at Ceboruco. dacites from Ihhaul, New Guinea, and

Ceboruco's dacites fall.into three lime from the pumice. This seems

distinct groups, all erupted after t.he likely, because the corun~un-nornative

1 precaldera andesitcs. The .Jala pumice property increases with increasing.

group is the oldest of the. siliceous amounts of H 0 reported in the 2 rocks. The f.larquc,sado ash, Cerro analyses. Luhr (1978) reported

i, Pedregoso dome, and Destiladcro lava compositionally simiiar corundum-

flow are included in this group, because nornative pumice from Volczn San 'Juan

(Fig. -I)-. Further evidence of xenocryst control 0 The samples of the Jala pumice of the major-element trends is suggested

group‘are arranged in Tables 1 and 3 by the increasing amounts of Sc and Co

i-n order of decreasing stratigraphic and a fifteenfold enrichment of Ni over

age. The Cerro Pedregoso dome the dome rock exhibited by the pumice

(sample 122), the oldest of the group, erupted last.

is the most siliceous and the younger, The Jala, pumice group is extremely

upper unit of the air-fall deposit erkiched in REE, with La about 90 times.

(sample 348) is the least siliceous. chondritic (Fig. 11) and La/Yb of’

The proportion of normative quartz, 11.5 to 12. The rockg also show

however, decreases in this sequence. slight negative Eu anomalies (Eu/Eu“

This is possibly due to the same al- = 0.7 to 0.73). Ba is very ab.undant, t teration that cuases corundum to appear with valup as high as 1,067 ppm.

in the norm. Well-defined trends The dacites that, were erupted during

through the sequence are not seen in Ceboruco’s second stage of activity i other elements, but there is a general have low silica contents and thus border

trend of increJsing Mgand Fe with a on being andesites. Sample 55 (Tables

decrease in Fe/Mg upward in the 1, 3, and 4) is from the Dos Equis dome,

stratigraphic sequence. All of the which was followed in the eruptive

rocks, except the Cerro Pedregoso dome sequence by the Copales flow (sample

lava (sample 122) contain xenocrysts 106). In general, the Copales rocks

of forsteritic olivine and ’high-A1 are more differentiated, Leing richer

augite, with greater amounts of these in Si, Fe, Na, and K and lower in Ca,

xenocrysts occurring upward through Mg, and Ti than the Dos Equis dome.

the section. The general trends in The second-stage clacitcs show only

major elements may thus be a reflection slight enrichment in the compatilile

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elements Rb, Zr, Cs, and Ba over the depletcxd in Si relative to the Jala

postcal dera andesites that followed pumice group, they are slightly enriched

them in the eruptive sequence. They in K, Fe, Rb, Th, and U, and are highly

are, however, significantly depleted enrichc'd in Ba, Sc, V, Cr, and Co

in Sc, V, Cr, Co, Ni, and Sr and en- relative to the Jala pumice. The 1870

riched in Th, U, and Ta relative to lavas show considerable REE enrichment

the later andesites. REE pattems relative to the Jala pumice, with La as

(Fig. 11) are similar to the post- high as 105 times chondritic in the

caldera andesites, with La about 80 most basic sample (181) (Fig. ll),

times chondritic and La/Yb values of but they have lower T,a/Yb ratios. These

10.5 to 11.3. Similarly, they exhibit lavas have negative Eu anomalies ranging

no Eu anomaly. from 0.86 to 0.77 (Eii/Eu") but are

The 1870 dacites form the third . less negative than the Jala pumice

group of siliceous rocks. These wcre group.

erupted as different lobes of the Khyolit e sar;,e.flow over a 2-yr period (listed

*in Tables 1, 3, and 4 in the-order The Cerro Pochetero dome on the

in which they were erupted). There southeastern. flank of Ceboruco is the

is some uncertainty about the relative most siLiceous rock associated with

age of sample 3, from a small bulbous the volcano. It is a sodic rhyolite

flow in the inner caldeFa, because it with relatively low normative quartz.

is not in cbntnct with' the other flow It is similar in composition to sodic

mi ts. rhyolitcs reported from Ixtlan del Rio,

Si decreases through the 1870 10 km southeast of Ceboruco (Gunn and

lava series, but, no consistent Mooser, 1971) and Volca'n Tequila and

trends arc exhibited by the other Orizaba (Burri, 1930). Although it, is

c>lemcnt\. Although thcse lavas are not peralkaline (Na20 + K20/A1203 =

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0.978), it may be transitional to the to the pyroxenes and lack of *any sig-

commendites reported by ?lahood (1978) nificant positivc Eu anomlies (except

for the Sierra La Primavera dome in the Ceboruquito flow), suggests that

complex (Fig. 1). plagioclase was a liquidus' or near-

The rock shows a low KfRb ratio, liquidus phase in all of the lavas.

high values of light REE (La 20 times Plagioclase in the andesite occurs

chondritic), and a large negative Eu as both phenocrysts and microphenocrysts

anomaly (Eu/EuS; = 0.13) (Fig. 11). in the glassy groundmass. Generally,

Its high Th/U ratio and strong deple- the phenocrysts are oscillatory zoncd,

tion in Ba relative to other Ceboruco with the most' An-rich compositions

siliceous rocks suggsst that it, like occurring not in the center of the

other lavas erupted on the southeastern crystal, hilt in a zone surrounding the

flank of the volcano, is not related center. Marsh (1976) and Lwart (1976)

to the main groups of Ceboruco. have used this zoning pattern to deduce

that anqes?tic mapas wem undersaturated MINERALOGY i with respect to water at the time of

Fc 1dsp a rs phenocryst precipitation. This deduc-

Plagioclase is the pr*edoT,inant tion is based on the fagt that a near-

phenocryst in all lavas from Ceboruco, isothermal rise of a water-saturated ,

composing as much as 422 by volume system should produce a normally zoned

(calculated on a ve's icle-free basis) plagioclnse, wherc?as a completely dry. .

in the precnldera andesites to as system 'undergoing a similar dcpressuriza-

little as 5.5X in the Ceboruquito flgnk tion would producci a reversely zoned

andesite, where it still predominates plaggoclase. The effcct should be / over orthopyroxene (fable 2). This intermediate for- systems containing

fact, combined with the larger size 11 0 but not at saturation. Eggl.cr 2 of the plagioclase crystals corr,I:ared (1972) arid Eggler 2nd Bur?.li;!y. (1973)

liquidus phase in andesitic magma only mafyetites found as inclusions in

when the 0 content is h~lowI wt H 2 .%. plagioclase do not differ signifi- Thus, the dominanc’e of p1nl:ioclase in cantly from those found in the

Ceboruco’s andesites gives further * groundmass of Ceboruco andpsites (see

suggestion of low water content in. discussion of Fe-Ti oxides below). \ these magmas. On the other hand, n terms of the magnetite inclu- Thus ’. > plagioclase in the Ceboruco andesites sions it appears unlikely that they , is sonetims observed to contain ap- crystallized prior to plagioclase. / parent inclusions of orthopyroxene and A reasonable explanation for these

titanomagnetite, which upon first included phases has been touched upon

consideration would tend to raise by liming (1977). Because most of

doubt th’at plagioclase was a liquidus the Ceboruco andesites contain plagio-

phase. clase with glassy inclusions with

Osborn (1969) suggested that at high coxpositions between the whole rock -7 fugacities of oxygen (so 2 10 atm), and groundmass glass (see discussion 2 magnetite could appear on the liquidus of !;lass compositions below), it seems

of an andesitic magma, but Kggler and possible that some of these glassy

Burnham (1973) have shown that inclusions, whiJe still in the liquid

magnetite cannot be a liquidus or state, could crystallize to an

near-liquidus phase for oxygen assemblage of plagioclase , pyroxene,

/ fqacities expected in andcsitic and oxides in similar proportions to

liquids (lobetween the qi.71 and the modes of the rock. This would 2 Ni-NiO buffers). As discussed by lenvcl the volume once occupied by

Boc?ttcher (1973), there is 1.ittI.e liquid in the plagioclase host now

evidence to support such high r fillcd by mostly plagioclase, with ’ 02

iri natural andes-itic liquid:;. some pyroxene and titanomagxwtite. The

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plagioclase would prcfedontially zrc similar in composition, zoned

nucleate o,h the walls of th&inclu- from An to An with the largcr 70 45' sion and grow inwapd, leaving the An content occcrring only in the

mafic phases totally enclosed by phenocryst group. The' groundmass

plagioclase. This explanation seems crystals are considerably morc Ab t?> i reasonable since pyroxenes and oxides rich and contain as much 3s 9 mol. X Or,

are commonly observed in the glassy but they overlap with the phenocryst

inclusions. themselves. It is there- group at about An 46. Minor elements .

fore conckuded that pla$oclase was . 'such as Sr never reach more than 0.4

the liquidus phase in the Ceboruco wt %, and I:e203 is always between \ \ andesites, inferring that these lavas 0.5% and 1.0%, showing a general, but

I contained relatively little Ii20 at not well-defined, tendency to conc"6n-

1 pressures of phenocryst precipitation. trate in 'the more An-rich compositions.

Compositions of individual points In the Ceboruquito andesi.tc (samp1.e

in plagioclase grains are shown in lOS), plagioclase is the only major

Figurc 12 in terms of An, Ab, and Or phenocryst other than snall amounts of 1 1 components. hypersthene. The rest of the rock, is

The precaldera andesitcs generally composed almost wholly of glass, with

have three sets of plagioclase some plagioclase microlites. The

crysta1.s: il larger phenocrystic larger plagioclase grains arc ri.ddl cd

group, a smaller microphenocryst group with glassy inclusions, 'and the crystals

consisting of skeletal crystals, and all appear to have been partially

an even smaller groundmass group. resorbed by the surrounding liquid.

The phenocrysts and microptienocrysts They thus appea+ to -he xcnocrystic and

would exp'laiy the sm;ill positive Eu

"Complctc microprobc analysis of anomaly observed in this rock (Fig. 11). all ninerals in the Ceboruco rocks are availablc from the author on request. Compositions range .from' An56 to An45 for

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Figure 12. Coinpositions of feldspars in Ceboruco lavas in term:; of'

three feldspar coznonents: abl ite, anorthite, and orthoclase.

Figure 12 appears on the following frame,

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/11_Part_II/2290/3433969/i0016-7606-91-11-2290.pdf by guest on 29 September 2021 )Y .- the larzer crystals and An to An 45 35 compositjons of the lavas, with $re- for the groundmass crystals. caldera andesites richer in Ca, 121, and

Similar conipositi ons are observed in Sr and having more nnorthit ic plagio-

the other flank andesite (sample 13h), clase, and postcaldera andesites having

except for slightly more sotlic ground- morc albitic feldspars containing less

mass plagioclase and a higher Fe203 Sr. This is not surprising in view of

content (0.6 to 0.8 wt X), reflecting the fact that the andesites contain

the high total Fe content of this rock. such high proportions of normative

The feldspars in the postcaldera plagioclase.

andesites cover a wide compositional The dacites from Ceboruco show the

range, bu: ,they are more restricted same general zoning features obscrvcd

for any bne sample. The most basic in the andesites. Corrpositions are

rock of the group, sample 61, has shown in Figure 12.

'the widest range, varying between Plagioclase in the Jala pumltce group - ,.

&In and ~:i~~,almbst no Or varies somewhat with the various units 73 wits (Fig. 1.2). 'The more anorthitic in the sequence of eruption.. The

compositions, however, are similar oldest unit is the Cerro Pedregoso

to tliose- in plagioclase iq xc:riol.iths dome (sample 122), which contains

found in this rock and' they may not feldspars zoned from An to An_lO, 3 -._ 35 be primary. Sample 1 contains the with 3% 0,r in the more'albitic

most albitic feldspars, with compo- compositions. This is contrasted

sitions betwen An44 -2nd An The with the feldspars in the lowest unit: 28' other postcaldera antlesitcs all fall of. the Jala purnice air-fall deposit,

int e rmcd iate be tween t hesc ext rcmc s , which has the same range of An

with values between An57 and An content but has Or a:; high as 8 mol id 37'

'rile feldspar c'ompositions of 9 in tlie more sodic feldspars. Because

the andesitcs re€l.ect tlic bulk . the bu1.k composition:; of the two rocks

ing is required to explain the dif- pyroxene that ate considered to be

ference. The donie rocks also contain xenocrystic, suggesting, that this An-r.icli

biotite and hornblende, whereas the plagioclase may have a simil.nr oriy,jn.

lo.c>Ter unit of the punice contains horn- Kinor elcmnts ii: the plagioclases

blende and orthopyroxene, with only from the Jala plimice are,.similar to

trace amounts of biotite. Thus, a those found in the andesites, with the

reaction of the form below is exception of re203, wh-i.ch is generally

suggested: lower in the Jala pumice, rangtng from’

0.2 to 0.35 lit %. KFe AlSi3010(OFi)2 3 Si02 = 3 + Plagioclasc in the second-stage biotite liquid dacites is more An rich than it is in

I KAlSi 0 3 I:eSi03 1120. the more siliceous Jala pumice gro~ip, 38+ + Or in Opx steam with compositions ranging from An GI Pm. to Anzo. The groundmass feldspars in

Thus, if f. were higher in the these rocks overl.ap with the ?heno- rI 20 dome lavas than in the pumice, the crysts having compositions between

reaction above,is driven to the 21 and 35 mole %, An, with as much a:;

left, increasing the amount of 5.5% Or.

biotite, with sulisequent depletion The 1.870 dacites have plagioclase

of the Or component of the feld- similar in composition to feldspars

spars in the dome lava. of the Jala pumice group; li3wever,

Several grains in sample 346 the forrr,er arc enriched in 1:e 2 0 3’ from the upper unit2 of the Jala probabl\<’a rpflection of the hir,lier

pumice have compositions consider- total Fe contcxnt of the 1870 lavas.

ably more anorthi t.ic than other Occasional anor:thite-rich grains occur

units of the series. These punice . in the 1870 dzcitcs. !3eci?use they

Found in the postcaldera andesites of augite is variable in the Jala

and their occyrrence is not wide- pu-nice group. The Cerro Pedregoso dome

spread, these grains are thought to contains no pyroxene at all. The lowest

be xcnocrystic. unit of the air-fall deposit contains

The only rhyolite at Ceboruco, only an orthopyroxene, and the upper

the Cerro Pochetero dorr.e, contains units of the deposit along the

both a plagioclase and alkali feldspar. Marquesado ash contain several differ-

This plagioclase is the? most albitic ent populations of augite along with

of any found at Ceboruco and contains orthopyrosene. The augite found in

no detectable ,Fe203 or SrO. The these pumiceous rocks tends to have a

alkali feldspar is about Orh3, rp- high but variable A1 content along

flectirig the sodic nature of the rock. with relatively high abrmdance of Cr.

Since they occur with olivine xenocrysts P y roscqes - and ai-c similar in coTposition to Cpx

Compositions of pyroxenes in found in xenoliths in the second-stage

Cehoruco' lavas are depicted in dacites, it is doubtful that they are

Figurcl 13. -Orthopyroxene is the pre- primary to the Jala pumice: Both

do3inn:i: nyroxene' in all of the lavhs, pyroxenes occur in the 1870 dacites,

with aiigite appearing only in trace and pigeonite ,is found in the flank

amounts in the precaldera andesites, andesitc sample 136.

becoming more common in the postcaldera Rims on pyroxene phenocrysts and

rocks, yhere it tends to be found the groundmass pyrhxenes all tend

more commonly in glomeroporphyritic toward pigeonite in both the andesites

clusters with plagioclase and Fe-Tf and dacites erupted from Ceboruco. I oxides. Only the slightly more maf;ic Zoning within indj-vidual phenocrysts

dome-forming rocks of the second-staEe of Opx is usually no more than 5 mol % - - -I

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Flank Lavas Pre - C a ldera 0 0 136

/ Andesites ~- - 104

4' . \\/ A ;>.. r(r. "' ,J\/ \/ *.. \/ \/ \/ \, "j *A A I\ /\ A

Post-Caldera Andesites

\I Y?-- \/ \/ V V v YY Y

A A A A

* 1870 Dacit

En ,/*a ,* \/ \I / Fs

Figure 13. Compositions of pyroxenes in Ceboruco lavas in terms of enstntite,

f errosilite, dio,)side, and hedenbergite components. 01 ivine composit i ons are shown by

triangles along base "of quadri1ateral.s in terms of FeO/(FeO + HgO) ratio's.

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in terms of the En component, and about Olivine the same in Cpx in terms of-the Di

coniponent. k!owever, ill the dacites, Olivine is rare iR &he lavas frorr. - .* particularly those from the 1870 Ceboruco. It occurs as small anhedral

erupt ion and the second-s tage dac i tes, crystals in some of thc precaldera

the compositions of different phcno- andesitcs, as a groundrnass phase in

crysts vary as much as 19%, with thq the flank andesite, sample 136, 9nd

more Pig-rich pyroxenes comparable in as apparent xenocrysts in the pumiceous

' composition to those found in the units of the Jala pumice group. Rare

pos tcal dera andesi'tes. This leads crystals of olivine arc also found in

to the suspicion that these two the second-stage dacites, hut these

groups of dacites may not have been crystals always occur close to olivine-

true liquids, but were contaminated ' bearing basaltic xenoliths -and show a with solid material picked up in reaction rirn'of pyroxene (Figs. 8a and

4. passing through the predominantly 8b). Compositiond of olivines in the

andcsite volcanic 7ile. Ceboruco lavas are shown along the

Xinor- e 1e me ri t ' con t e n t o f t lie base of the pyroxene diagrams in

pyroxenes is generally low. Ti and Figure 13.

A1 are both greater in the cline: In the two rocks where olivine

pyroxcncs than in the orthopyroxenes. appears to be a primary phase (samplhs

Usually, more than 50% OF the A1 is 47'and 136), the CaO and NiO contents

found in the tetraiiedral (Z) sites. of the cry.stals are'0.3 and 0.1, re-

Notable exceptions are the ortho- spectively. Tlils is contrasted with

pyrvxenes in the Ccboruquito flow, the xenocrystjc olivincs found in the

.in yhich virtually all of the A1 is kJala pumice group which have CaO of

ifi octatiedral coordination. 0.15 toCs9.2 and NiO from 0.15 to 0.3.

Only samples from the Jala p-uMe The biotite is peraluminous, as is

group contain biotite and amphibole. the enclosing rock, but a peraluminous

Biotite is the only, hydrous mineral 1iqui.d is not n.ecessarily required to

-founi in the Cerro Pedregoso dome, ptecipitate a pekaluminous hiotite,

and trace quadtities of this mineral as observed by Carmichael (1967).

occur in the lower unit of tlie air-f5ll The amphibole found in the air-fall

deposit. I?mp.hibole is by far the and air-flow units'of the Jaln pumice

prc,dominant mafic mineral in the air- group is a titani ferous hornblende that

fhl.1 and ash-flow units of the group, , occurs as euhedral phenocrysts as much

Average analyses .of hornblende and JS 3 mm long. Within any-individual

the biotite from the Cerro PedrSgoso unit of the group the IiornblendP shows . '1 domci (sample 122) are presented in almost no zoning; crystal cores being at

Table 5. most I.. wt % ric$eY in MgO than their

7'l;e biotite from the dome occurs rims. From unit to unit the hornblendcs

.with plagioclase and Fe-Ti oxides vary slightly in composition, with

in ci mostly glassy groundmass. Th'e sample 346 from the middle of the

crystals are generally 1. to 2 .Lon8 air-f'ill unit b$ing less Fe rich and

and are nearly opaque in thin section. sample 353 from, the Marquesado ash

Compared to th'e biotites in dacites deposit containing more Ti02 than the

from Plount Lassen, California .amphiboles.from other units.

(Carrrdchael, 1967) and Rabaul cal-dera, These amphiboles all contain

,Paput1 New Guinea (Fleming and molecular propbrtions bf) CaO +lNa20

Carmichael, 1.973), this, biotite is. K 0 in e,scess of A1203 and could, if + 2 relatively Fc rich, reflecting the f rnct i ongtqd, generate the pernluminous

II iglier I:e/Mg rat io of the Cerro character of this group of rocks.

Mornb 1end e Biotite 364 346 .348 353 122 Si02 42.75 43.64 4k.01 42.23 39.95 Ti0 2.34 2.93 2.80 3.14 3.16 2 11.49 10.75 10.92 14.23 A1203 10.69 FeO 14.34 11.65 14.41 13.09 16.18 RnO 0.47 0.23 .0.42 0.34 0.25 I PlgO 12.44 12.25 11.91 11.37 11.75 CaO 10.51 11.12 10.75 10.76 0.09 SaiO 2.46 2.74 2.67 2.65 1.18 0.51 0.51 0.50 0.50 8.26 K2O I: 0.21 0.18 0.21 0.20 1.14 H20;'c . 1.86 1.94 1.93 1.87 3.48 Total 98.57 98.68 100.37 97.07 99.17 O=F 0.09 0.08 0.09 0.08 0.48 Total 98.48 98.60 100.28 96.99, 98.69

Formulae on Iiasis of 24 0, OH, F1-

Si 6.44 6.47 . 6.50 6.43 2.98 Ti 0.27 0.33 0.31 0.36 0.18 Al 1.89 2.01 1.87 1.96 1.25 Fe 1.81 1.44 1.79 1.67 1.01 ?ln 0.06 0.03 0.05 0.04 0.02 ?Ig 2.79 2.71 2.62 2.58 1.31 C 3,. 1.70 I .77 1-70 1.76 0.01 Na . 0.72 0.79 0.76 0.78 0.17 K 0.10 0.10 0.10 0.10 0.79 F 0.10 0.08 0.10 0.10 0.27 OH 1.87 1.92 51.90 1.90 1.73' Fe / Fe+Xg 0.393 0.347 0.40G. 0.393 0.435'

-Note: Oxide values are wight %. "H20 calculated assuming- that (OH; -F) site i--,filled. 'i Biotite formula calculated an basis of 12 0, Off, 1;.

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and Lindsley (1 964). Iron-Ti tanium Oxides C In general, oxide phases- are

Titanomagywti te is a ubiquitous homogeneous and have identical Gompo-

phase in ail of Ceboruco's lavas, sitions whether they occur as large

always occurring as euhed?al pheno- phenocrysts or groundmass micropheno-

crysts or microphenocrysts. Ilmenite- crysts. Exceptions are found in the

hematite solid solutions coexist with 1870 lavas, where two populations of

the titanomagnetite only in rocks *with the spinel and rhombohedra1 phas'es

greate'r than 602 Si02 and/or greater occur, and in the youngest postcaldera

than 1.1%Ti02. Thus, the precaldera andesites (samp1.c~1 and 61) \>here a

andesites'contain only titanomagnetite. few grains of titanonfagnetite with

The Fe- and ri-ricli ildnk ldva (bdl,ipIe coKipositions different from the rest

&36) contains both solid solltions; were observed.

the Ceboruquito flow with 1.07% Ti02 In the postcaldera andesites it was

has titanomagnc t i t e only; the assumed that the grains in greater

postcaldera andesites with 1.1%to 1.3% abundance were in equilibrium with the

Ti0 contain both phases, and the Jala single rhombol!edrnl phase 'population. 2 pumice, second-s t.age dacites, 1870 This BSsumption appears reasonable

clacites, and Ccrro Pochetero sodic as it gives temperatures expected for

rhyoJi'te, all with SiO, greater than these postcaldera andesi tes by compari- I, - 63%, contain coexisting oxide phases. son with other rocks of the group.

Average analyses of the Fe-Ti oxides Temperatures obtained using the diTfer-

are presented in I'able 6, where Pe203 ent titanomagnc tit:e compositions are

has. ?ee rccalculatec! using the rethod of lower but are nl:;o reported in Table 6.

Carpichael (l967), and tca7eratures For the 1870 !.nvas two populations

and oxygen fupici ties have been ob- of both oxide phases are found. One

tained from thc curves of Buddington set' is confined t-o large phcnocrysts

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TABLE 6.. ANALYSE:S OF IRON-TITANILJTI OXIDES IN CEBORUCO LAVAS

. .. .. 4 7m 6 6m ~-105m' 105m 104m-. - 10qi-1 '136m 136i.- Si02 0.73 0.33 0.35 0.15 0.15 0 .'03 0.24 0.15 Ti02 13.47' 18.99 20.03 16.79 18.27 51.19 19.70 48.08 "1ZD3 1.06 1.33 1.50 2.23 0.00 0.00 0.29 0.00 '2'3 0.63 0.98 0 :I0 0.07 0.00 0.01 0.51 0.12 Cr203 0.15 0.04 0.02 0.00 0.00 0.. 00 0.02 0.00 FeO 75,.74 72.36 69.78 72'.93 75.13 46.98 72.99 46./+9 MnO 0.54 0.45 0.85 0'.70 1.08 1.65 '0.60 0 . 6.3 %no 0.05 0.15 0.13 0.12 0.00 0.00 0.14 0.03

PlgO 1.67 0.88 1.70 , 2.08 ' 0.40 0.53 . 1.47 1.56, CaO ' 0'. 00 0.03 0.09 0.00 . 9.00 .O.QO 0.02 0.08 'I'otal 94.04 95.54 94.55 94.27 95.05 100.39 96.0'0 97.111

iie c a 1cu 1a t e c! an a 1y s e s Fe203 38.97 28.29 26.64 31.63 32.81 3.92 '29.56 7. '36 1:eO 40.68' 46:91 1t6.26 44.27 44.47 46.18 43.45 46.39 'I'otal 97.95 98.38 97.67 98..24 98.89 10Q 78 98. 96 97. 88 B

?'I5 US? 40.46 55.25 58.27 48.53 52.94 56. 68

15.33

T (OC) 81 '3

NoL(J: Values are weight % "III refers I to O?LtGnomagnctite. i -refers to *,iImEnite 5mole percent. /,Groundmass crystals or inclusions in othci- phases.

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S i02 0.00 ' 0.00 0.11 0.06 0.08 0.04 0.13 0.06

Ti02 13.41 /+7.31 13.06 46.06 8 12.95 47.21 12.'86 48.90 2.42' 19 2.28 0.20 2.45 0.17 2.65 0.175 *l2'3 0'. - 00 0.02 0.00) 0.03 0.00 0.00 ' '2'3 0.02 0'. 0.00 0.00 Cr203 0.00 0.00 0.00 0.00' 0.00 0.00 0.00 FeO 78.33 l-7.11 79,.75 1+6.9C 80.17. 18.02 e.79.47 L7.65 l?nO 0.95 1.65 0.92 1.28 0.87 1.32 0.88 1.25

ZnO 0.00 0.02 0.20 0.08 0.21 0.05 * 0.20 0.07 Kg0 1.22 1.67 1.G8 2.65' 1.45 2.75 1..41 2.68 CaO 0.00 0.00 0.01 . 0.01 0.09 0.03 0.03 0.01 Total 96.35 97.j4 97.83 97.24 98.q5 99.59 97.63 100.79

Fe203 41.21 10.25 43.07 13.26 43.46 12.16 42.79- 10.80 FeO 41.25 3739 41.00 35.87 41.02 36.18 . 40.97 37.93 Total 100.48 98.97. 102.25 .

36 .i3 35.84 35.67 .M (7,) R203 10.03 11 ."90 13.64 10.18 i2.8 12.9 13.0 13.0 -log b2

2' (OC) 855 845 83s 835

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-- I 3 5 3111 353i 5 5m 55i' 106m , 106i- 15m 15i- - -7 S i02 0.07 0.12 0.14 0.07 0.14 0.16 , 0.23 0.20 Ti02 13.17 45.30 12,59 146.80 .13.58 40.87 18.48 46.07 "1293 2.85 0.00 0.79 ,o . 00 1.55 0.00 1.68 0.15 '2'3 0.00 0.00 0.25 0.01 0.85 0.12 0.85 0.12 0.00 Cr203 0.00 0.c1 0.00 0.25 0.01- d.Ql 0.00 FeO 74.54 46.25 79.65 48.52 77.65 47.13 71.89 46.18 Mn 0 0.40 1.35 0.48 0.97 0.66 ' 0.92' 0.53. 0.55 ZnO 0.00 0.00 0.20 0.13 0 .,lS 0.04. 0.04 0.02 rig0 4.02 2.67 0.47 1.44 1.86 2.66 2.39 3.33 CaO 0.00 Q.00 0.00 0. oc 0.00 0.03 0.01 0.01 Total 95.44 95.69 94.49 97.93 96.17 97.80. 96.11- 96.63

Fe203 41.53 12.78 42.62 j,,lt. 12 41 .'17 11. 67 30.63 12.26 FeO 37.18 3-L. 75 41.30 38.51 40.61 36:63 44.33 35.15 Total 99.61 99.67 98.76 99.04 100.30 98.97 99.18 97.87 C

36.30 38.28 52.35 ' 12.37 lp. 62 11.09 12.06 -log i 12.9.: 12.9 1c.4 O7_

I' (OC) 845 847 1005

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3 Om 30i 117~ 1171' 1III lm li Si02 0.17 0.17 0.13 0.15 0.15 0.17 0.10 Ti02 19.29 46.88 18.64 46.91 17.56 19.40 47: 51 A1203 0.84 0.00 1.25 0.00 1.97 1.81, 0.'.3 6 '2'3 0,.52 0.11 0.57 0.11 0.60 0.52 0.09, Cr203 0.06 0.00 0.11 0.11 0.06 0.08 0.00 FeO 72.37 46 .,34 72.13 47.17 73.20 71.65 46.29

Mno - 0.56 0.62 0.54 0.59 0.48 0.57 , 0.59 ZnO 0.09 0.02 0.08 0.02 0.05 0.08 0.03 NgO 2.33 , 3.21 2.14 2.44 2.58 2.45 .3.27 CaO 0.00 0.00 0.02 0.00 0.04 0.06 0.05

Total 96.23 -: 97.35 95.61 97.39 96.69 96.79 $8.29

30.52 11.50 30.78 10.86 Fe203 33.14 29.54 11.07 FeO 44.91 35.99 4/+.44 37.40 43.38 45.07i ,_ 36.33 Total 99.29 98.50 98.70 98.48 100.01 99.75 99.40

52.91 49.. 01 54.30 PI (x) R203 11.03 10.48 11.00 -log f 10.9 11.1 11.4 10.8 O2

2' (OC) 98 970 94 5 990

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/. ' S i02 0.20 0.20. 0.09 0.3.3 +O.ll 0'14 0.07 Ti02 19.82 17.74 46.91 14i79 47.84. 17.14 48.72 Al20j 1.91 2.40 0.17 1.53 0.00 1.59 0.05 '2'3 0.71 0.65 0.18 0.23 0.00 0.30 0.00 0.00 Cr203 0.00 0.00 0.00 0.00 0.01 0.00 FeO 7.0.71 7%. 16 46 .'45 77.46 47.14' 76.05 46.71 FhO 0.45 0.48 0.45 0.69 ' 0.95, 0.60 0.73 ZnO 0.33 0. oc 0.00 0.17 0.14 0.18 0.07. NgO 2.45 2.61 3.24 0.86 1.52 1.20 2.32 CaO 0.01 .o. 07 0.03 0.01 Q.12 0.00 0.00 Total 96.29 96.31 97.54 95.97 97.82 97.18 98.67

Fe203 27.97 31.86 11.61 38.16 8.82 34.38 8.61 FeO 45.54 43.50 36.00 43.1% 39.20 45.03 38.96

Total 99.09 99.51 98.70 99 .s79 g8.70 * 100..63 99.53

49. 78 42.51 48.18 :I (%) R203 11.LO '8.45 8.20 -log f 1O.L 11.2 13.0 12.7 Q12

2' (OC)1015 955 965 890

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Si02 0.14 0.09 0.18 0.10 0.11 0.13 Ti02 17.16 46.71 17.16 47.73 16.78 46.37 .A1203 1.03 0.00 1.15 0.00 1.11 0.00 0.. 31 0.01 0.23 0.00 . 0.28 io "2'3 '0. 0.02 0.00 0.01 0.00 Cr203 0.00 0.00 FeO 76.41 47,. 67 76.13 47.?5 75.79 47.36

lYnO 0.64 , 0.83 0.68 1.02 0.66 0.55 ZnO 3.20' 0-. 05 0.03 . 0, so 0.21 0.03 HgO 1.30 2.03 0.97 1.36 1.28 2.87

CaO 0.00 0 0.00 0.00 0.05 0.00 0.05 Total 97.19 99.39 96.56 97.51 96.23 97.46

Fe203 34.17 , 9.1'8 34.33 8.59 35.24 12.52 FeO 44.76 39.41 45.24 39.52 44.09 36.09 4 ., Total 100.71 100.31 99.99 98.37 99.77 . 98.71

48.86 4 7 :5 1'. 8.76 11.99 -log I 12.5 12.5 i1.3 ' O2 890 9'4 0

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hit lSlm 181i 18 18'ri$ xen m xen i .- S i02 0.15 0.14 0.02 0.21 0.32 0.25 Ti02 16.22 47.99 17.53 46.67 15.41 45.30 *'2'3 3.19 0.10 1.63 0.11 0.00 0.00 0.14 0.42 0.08 '2'3 0.00 '1.33 0.03 Cr203 0.01 0.00 0.01 ' 0:Ol 0.00 0.00 'FeO 76.09 45.68 74.05 47.13 73.43 47.41 .\ .Mno 0.76 1.13 0.71 0.75 0.75 0.77 ZnO 0.14 0.02 0.14 0.03 0.00. 0.00 Plgo 0.87 1.53 1. 3'0 2.77 1.94 3.11 CaO 0.06 0.49 0.00 0.31 0'.00 0.01 Total 95.63 97.07 96.71 98.02 92.27 97.10

Fe203 35.68 7.63 32.47 12.26 36.27 14.13 FeO 43.99 38.81 44.83 36.10 40.79 34.70 Total. 99.21 97'. 83 99.96 99.25 95.90 98.52'

49.0s . 46.29 7.51 11.80 13.57 11.1 11.3

B (OC) 860 957 940

plagioclase grains. Tlie other set is laJ,as. It is generally found as

confined to microphenocrysts. In inclusions in plagioclase.

order to determine which of the two Sulfides are observed as inclusions

populations were equilibrium pairs, it in pla,gioclase ip the Jala pumice but

was first assumed.that the large phcno- ha%e not been analyzed.

crysts And idFlus'fons within grains Glass belonged to the same set, while the

microphenocrysts belonged to the Analyses of the glassy groundmasses

other set. This assumption appears and, in some cases, glassy inclusions

j us tif ied because the phenocry st contained within skeletal plagioclase

and inclusion pairs give higher crystals are presented in Table 7.

temperatures than the microphenocrysts. The glasses were analyzed using the

FIinor 'elements in the oxide phases electron microprobe with a wide beam

show a tendency to v%,ry with temper- diameter an! low sample current: (0.03 ma).

ature as .observed by I-Iil-clreth (1977) They were run against analyzed standards

for the-Bishop Tuff. This is true and checked for accuracy by running also in the Ceboruco lavas, whfre a internal standards of known composi-f ion. posttive correlation is obse'rvecl fort The glassy groundmass of precaldera

Pig0 versus tem?erature and a negative andesite sample 47 is less siliceous

correlat.ion for Fin0 versus temperature. than that of sample 66, but sample 47 - Thege correlations further justify shows a Significant increase in Ti0 2 the assumption made concerning and FeO over that of the whole rock. total equilibrium pairs. The cause of this is believed to be

that sample 47 had just begun to Accessory Mino ra Is crystallize titanomagnetite prior,to

Apatite occurs as a ubiquitous phas'e eruption, whereas sample- 66 had been

66P;~ 66Gsr 47G l05P 105G 364G 346C: - 348G 353C 106P 106C + Si02. 70.75 70.86 63.21 71.54 67.63 68.79 69.15 70.03 .66.91 72.10 71.86 Ti02 0.70 0.97 1.86 1.32 0.97 0.27 0.29 0.29 0.28 0.86 0.52 12.64 12.03 13.99 13.64 13.34 15.64 15.47 25.21 15.37 12.06 12.32 A1203 F6OT 1.73 1.71 6.88 3.17 8.85 1.95 1.95 1.88 1.95, 2.34 2.11 - FlgO 0.23 0.15 2.59 0.19 1.02 0.41 0.41 0,38 0.38 0.34 0.34 CaO 0.84 0.57 5.25 1.54 1.19 1.38 1.35 1.38 1.56 0.57 0.88 Na20 1.69 1.99 4.27 4.83 . 5.06 4.93 4.79 4.93 3.99 4.52 4.33 4.73 4.93 2.76 1.32 5.20 3.19 3.25 3.18 3.30 1.07 3.89 K2° Total 93.32 93.21 100.08 100.54 99.27 .96..56 96.65 97.28 93.8% 96.86 96.25

Xen 15G 3 OP -30C 117G 1G 3G 67G 113G 181P 181G Sj02' 73.03 ,69:,27 65.97 .68,11 69.76 70.83 70y72 68.73 71.70 71.07 72.L3 Ti02 0. G1 1.03 1.35 1.07 1.09 0.35 0.30 0.35 0.28 0.63. 0.26 12.22 13.45 13.38 13.33 14.40 14.43 14.52 15.76 14.96 15.01 15.17 A1203 FeO 2.38 3.62 5.34 3.89 2,j3 1.87 2.05 1.98 1.54 2.71 1.47 T ElgO 0.34 0.55 , 1.09 0.60 0.23 0.28 0.19 0.30 0.16 0.20 0.16 CaO 0.70 1.58 ,3 2.13 1-45 1.06 1:15 1.01 1.14 1.07 0.56 1.43 Na20 6.05 4.26 5.23 4.48 4.95 4.80 k.95 4.87 5.37 4.26 5.24 1z20 3.89 4.40 4.56 4.91 3.65 (1.51 5.43 3.88 4.22 5.16 4.22 Total 100.23 98.16 98.75 98.04 97 86 98.26 '99.27 97.01 99.30 99.60 101.37

aP indicates glassy inclusions in plagioclase; G represents glassy groundmass,

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crystallizing the oxide phase for the feldspar in another environment.

some time prior to eruption. This Thesewdata lend support to the conclu-

o bse rva tion o u I d furt he r subs t ant i a t e sion reached earlier that the po-sitive

the clairr, Tade earlier that oxide I;Y anomaly observed in this rock is'

I. phases precipitate late in. the a result of senocrystic plagioclase.

.I crystallization sequence. Glass of tlie .Jalo pumice is not

Classy inclusions were analyzed in substantially di fferent from the whole-

skeletal microIi1ienocrysts of ?lagio- rock bulk cox?osi ti-on. ?:go, TiO?,

clase in sample 66. They have composi- and CgO are lower, wh'ereas tot a1 t ions simiI.ar to the 'glassy groiindmass, Na 0, and Si02 are slightly higher 2 K20, suggesting that: the glass was in the glass relative to tho, whole rpck,

included and ti!nt these skeletal as eqected from the mineral Dhascs

crystals grew during quenching of the present and the degree of crystal1 iza-

lava. tion. The glasses from each UniWare,

Analyses of glassy groundmass and however, closer in composition to each

inclusions in plagioclase are shown other than the whole-rock compositions

in Table 7 for the Ceboruquito flank are. Thus, 'the increase in MgO and

lava (sample 105). Whereasj the (:a0 as well as compatible trace dements

L groundmass compositions are to be observed in the whole-rock cpmpos i tions

expected from crystallization of of the last two erupted units of the

the phases present in the lava, the air-fall can be attributed to the xeno-

inclusions in plagioclase fall out- crystic 'olivine anti. augite found in these

side the range of compositidns between samples.

the whole rock and groundmass. As Class from sample 106, th'e Copales I

these inclusions are found in I.nrge lava flow of the secondyst-age dacite

co'rroded plagioclase phenocrys t:;, j:roup, show subst:nn t ial decrease:; in

they may represent n liquid trapped by all elements except Si02, K2Q, andsNa 2O-

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Classy inclusions in p1a~;ioclasc the inclusion 'formed. Other oxides,

are again similar to t,Ii(! groundmass except, Na20, ' fall on inte,rmediate points

cotnposi tions hna thu:i probably forced Setween w1:ole-rock and groundmass values.

jubt prior to eruption. Thus, in theaposfcaldera andesites the .- Analyses of glasses in the post- oxides apparently formed late in the

caldera dndcsites arc. :;hewn in Figure crystallization sequence. As each of

14 and Table 7. Classy inclusions the rocks in this group show different

in plagioclase were an,~lyzcd in only degrees of crystallization, the points

mc sample (30). Thcse inclusions plotted in Figure 14, if connected,

were apparently formed at an inter- sfiow an example of crysta! Cractiwation

mediate stage in ttic crystallization in one lava type'and ir&icate that if V history; their coinposi t ions are inter- liquids were separated from crysals at

mediate in silica content between the how degrees of crystall ization they would

whole "rock and groundmass glass. . One show a moderate iron-enrichment trend.

icclcsion plotted ir, Figure 14 contained Data for the 1870 dacites are diffi-

a grain of titanomagnctit'c which cult to interpret, as no consistent

apparently arystallikcd from the liquid trends a're observed. This is probably

\ cnclosrd in the feldspar. Analysis due to J.oca.1 inhomogencit ics in the

oC this iqclusion shows that it is con- glass.

9istently. different fr6rii the other The. totals 'for all' glasses reported r inclusions for all el'trnents. Tlic in Table 7, except sample 66 and the b inclusions that eontain no 'crystals Jala pumice group' rocks, are close to

IA, show higher values of TiO. and FeO 100; even in the glassy inclusions 2 tbtal' 0 which is interprercd to, have resulted in plagioclase. Tf thvsc totals,,

from t.he ,lack of Fc>-ii .oxidc crystal- especially those of the inclusions, .* lizat im from, the' Lava 'up to the point give 2n indicatiqn of thc' pre-eruptive

ill tlic crystal.!ization history w!icn water content of the n%y,ria 3s suggested

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CaO

Figure 14. Variation .diagram-

for postcaldera nndekites showing

compositions of whole-yock samples

0 (circles), glassy inclusions in 2 plagioclase. (triangles), and 1 grour.daass g! ass (squares) .

a I= I

2 16, 15 14 13

Ceboruco’s lnvns apparently, contained lava, indicating .that the xefloliths were

less than 3% water at pressures relatively cool when included in the lava:

corresponding to phcnocryst precipi ta- Small pieces of shale are also common

tion. ‘i‘his conclusion cantlot be and are likewi.sc! surrounded by glassy

made tor karnp1~66 with totals of 92 host lavas. ‘I’tiese were probably in-

to 93 in both groundmass and inclusions, cluded at rclativcly shallow depth

nor in the .lala pumice with similarly low beneath the volcano-again, just prior

totals. The new totals for sample 66 to eruptiop.

indicatk a pre-eruptive water content Xenoliths of granitic material are . .. oi 7 wt’ % for the glass, or perhaps commonly found ih the Cerro Pedregoso

as’-high as wt Z for the bulk rock. dome nenber of the Jala Furnice group

Evidence basc?cl,on water-fugacity and in cinder cones surrounding the

calculations for the Jala pumice La Pinchancha lava flow on the south-

(presented below) show that this eastern flank of Ceboruco (Fig. 2).

-group could tinve been near saturation These are genc~rnllysmall, ranging from

with respect to water ac,prcssures on 0.5 to 2 cm, and are composed‘of .

the order of 1 kb. alkali’ feldspar or Ab32) and

quartz with iptcrstitial dlass of XENOLITHS I varia’ile c0mpo:;it ion. They thus

Four types of xenoliths occur i? appear to have been partially €used

the Ceboruc_o lavas. The most common’ by the host lavas. It is uncertain

typ? is simply pieces of previously, whether they’ rcprcsent material that

Grup ted roczk, usua l.ly andesite , that was accidentally inclhed in the

were presupably picked up’ 5y the lava lava on its pntii to the surface or if

as it was .erupted or €lowed out over they are a residual product of a

,E the ground surface... These are al.ways possible granit- ir. source,rock. It is

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likely that the lavas have reacted in Tables 6. and 7.

with, the granitic rmaterial, but the Because the xenoliths are small and \ extent of the react ion is uncertain. contain large crystals of three dif-

As discussed previously', the second- ferent ..phases, all three of whi.ch are

stage dacites all contain xenoliths rarely found in a sinj;lt.xenolith, no

of porphyritic basal ts. P1:enocrysts anaiysis was per:ormrxd on them. It ;I in these xenoliths consist of plagio- was felt that it wou1.d be difficult

. clase zoned from An to A%6o' high-- to obtain a represent.ntive analysis due 84 L. alumina augite with as much as 0.35 to the Lnconsistent size and mine,ral

wt % triog,,and olivine (FO to ~o ) 81 76 distribution. . l-Iowev(.r, an estihage'd often found. as inclusions in plagioclase mo'de of the xenoliths obtained froin

rims. These are 'set: .in a groundmass ten thin sections.. is presented in Table consisting 0.f plagiocLase (An to 60 - .8A. IJi th considerabl tl iincer tain ty ; .. i An ) orthopyroxene (En71 to En the modes have been used along with tile 42 62) ' -. Fe-Ti oxides, and glass. The glass known ccvositbns of all of the

is o'ften found in Spherical globules phases, includ'ing glass,. to calculate

which surround holes that appaqently an estimated composition of.basal.tic

contained a vapor phase. The glass magma (Table 8B.). l'hc uncertainty is

is siliceous and is wlio.Lly contained particblarly, great for 1*'(:0 \and Ti0 2'

with the xenoliths, presumably as since tlierc is il large, c!rrur iiivul.ved

the residual liquid left after in the determination of the prop.ortion' !

crystallization ,of tlic phenocryst of titanomagnet'i te an(] j lnienite in

and groundmass phascs. Photomicro- th,c xenoliths. Also shown i.n Table 81%

graphs of several of t.hc!sc xenoliths are compositions.'of othcr high-aluniinci

are shown in Figure 8. Analyses basal ts from orogcnic regions. The

c;.f tile individual pliascs are shown cal)culatcd compcsi tion of tlic basal tic

J:i Figures 12 and 13 anti reported xenoliths is cicscs: t:) TI hig::-:ll

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Plag phenocrysts 9.5 9.0 P lag groundma s s 60.3 57.8

CPX 3.4 3.9 OPX 6.9 8.2 01 3.4 3.7 >It 2 .,5 4.2 '0.9 1.4 12.9 11.8

::Obtained by rnrilt-iplying volume percent by avc:ray,e den'sity of the phase, summing and recalculating to 100';. _._-

i -. 2,- 3 4 5 ','SiO, 52.7')' 5:!.2i) 46.90 51.,78 '5l.46 \ ' - ..~- TiO, 3.61 1.69 1.54 2.6j 1:OO \- . A1203 20.33 19.10 17-90 15.33 17.08 Pe0. 6.98 7.07 9;h8 10.91 8.49 I' NnO 0.09 . 0.69 0.14. 0.18 0.15 \ 'lg0 4.!8 5.33 6.98 6.21 7.40 ca 0 7.95 8.57 10.00 9.22 9.96 Sa20 4.43 4.72 2.91 3.76 3.05

K,)d- ' 0.70 0.80 .0.?3 1.20 0.85

reported by Lopez-Escobar and others event could have been rcsponsib1.c for tlie

(1977) , although the calculated chemical composition of the upper units

composition is much richer' i'n Ti0 of the pumice and the second-stage 2 - and lower in MnO than the ,Chilean dacites. This ,hypothesis' is further

exaqle. None of the basalts 5hovn suggested by thr enriclincnts bf Cd,

in the tabJe is as rich in Ti02' as Ni, Sc, MgO, and CaO in the upper units

tlie 'Ceboruco xenoliths, except for of the Jala pumice relative 'to the lower .i a basalt analyzed ky Gunn andMooser units and the Cerro F%dregoso dome unit.

(1971) from 5 km east of Ixtlan del A linear. ].east squares computer

Rio, Sayarit, a locality about 20 routine sircilar to that of Vright and

Icm eaqt of Ceboruco. Doherty (1970) was used to test this

magma-mixing hypothesl s. *Input data MACEL4 MIXING, THE JALA included the major-elcmcnt compositions PlJbIICE AND SECON@STAGE DACITES of the lower and' upper Jala pumice .. The textures of; basaltic xenoliths units, the second-stage dacit;?s, and

foun'd &. Ceboruco s second-s tage the calculated, cornposit io6 of the

daci.tes suggest that material of b-asaltic xenoliths from Table 8B. Tlie

basaltic composition,was' injected resuits arc shown in l'ablc b9 and \ intq a more si~.icwusand cooler confirm that a s/ma11 amount (5.9 wt 7)

aagma in a partially molten state. of basal tic material adgpcf- to the more

\ The basaltic xenol.iths in the siliceous unit of thc Jala pumice could

dacitcs are .found in all' stages of yield -the I~bstsiliceous and Last-

decompositJon, and xenocrysts of erupted unit of tlle pumice; and that a

compositioh similar to phenocrystlc mixtuxe of 68.5 parts lower Jala piimi ce

phascs in ti;e xenoliths are found and 31.5 parts basalt zoulti give rise

in tlie upper units of tl!c Jala,pumice to tlic second-stage daci tcs. ,llthougll

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Calculated Calculated nixture of mixtuie of 5.9 parts 31.5 parts. high-A1 Basalt high-A1 Basalt Observed and 94.1 parts . Obsbrved ,and 68.5 parts average Cerro Pedre’goso ,upper Jala Cerro Pedregoso seconc-stage rhyodaci? - Si02 68.28 (37.63 64.06 63.81 (, 26)f;

T,i0 0.45 0.38 1.04 0.85(. 06)

15.82 1-5.7 3 17.05 16.89(.32) *’2’3 FeOt 2.41 2.32 3.64 4.27 (. 15)

‘MnO 0:11 0.10 0.10 0.11 (. 02)

Ng0 0.59 0.61 1.56 1. SO(. 16)

CaO 1.85 1.77 3.51 3.71 (.07)

N2 0 5.22 5.22 5.30( .h8) 2 5.52 .. ?y f.15 . 3.05 2.48 2.43(.01)

J: A2 = 0.545 E ‘A2 = 0.574

~ “Values in parentheses represent standard deviations from mean.

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the uncertainty in the basalt compo- dacites are not chilled against the

sition is large, the €it for both basaltic inclusions .and that the

mixj ng models is 'good. basaltic inclusions aRe found to be

Sparks a;id others (1977) have disaggregated and strung out through

proposed that magma mixing could be7 the dacitic matrix. The chemical

a mchanism for triggering explosive nixing model offers furthcr support;

eruptions of acid mzgma. they en- however, the fact that the compoSition

visioned a basic malyna injected into of the !lasalt is cstimat-ed and uncer-

t,he acid 'magma chamber, causing super- ta in makes tliis argument less convincing

heating, which induces convection and Unfortunately, thcre arc no tracc-

exsdlution o€ volatiles in the acid e1erner.t data avai'lable on the basaltic \( magma, leading to a pressure increase xenoliths at present that could add

and eruption. further support to ;he hypothesis'.

.A similar process is suggested for MINERAL GEOTHEKMOFIETERS the .Jala pumice-second stzge dacite

series. It is suggested that the Five different methods of estimating

Jala pumice magma chamber was in- temperature have been appIied tcf the

trud(>d by basal tic magmg, triggering Ceboruco lavas. The results arc

the plinian eruption beiore signifi- plotted in Figure 15. Problrms, are

cant mixing could take place. Follow- inherent in interpreting tl:ese est:i:xt'c'd

ing this eruption, fu'rther injection temperatures because .there is little

of basalt into the chpmber would interoal $ration of tlic. vnr:iou,.; methods:

cause rapid convection' and mixing Generally, the Pe-Ti oxid(?

of the tyo magmas, resulting in a thermomc5er oi' Buddington :nu Limdsley

magma OF the composition af the second- (196L)' gives the lowest ~ tcnipcratures ..

stagt dacites. That €his procr.+s did in rocks that contain both tlic ;2 occur is suggested I,yiclie fact that the rhombolicdral. nnd spinel pliascs.

Ceboruco lavas. Solid squares = plagiorlase thermometer of Kudo and Weill (I 970)

and Drake (15)1&) €or equilibration between phenocryst cores and liquids with *

compositions of whole rock. Open squares = plag3oclase tempcratune for

equilibration between phbnocryst rims and glassy groundmass; 0.5, 1, and 5

signify temperatures €or PH 0 of 0.5, 1.0, and 5.0 kb, resuectivcly. Solid 2

inverted triangles = pyroxene thermometer of Idells (1979). Open inverted

triangles = pyroxene thermometer of Wood and Banno .(1973). Solih circles

= iron-titanium oxide thermometer of Buddington and Lindsley (1964).

Temperatures are plotted against sample numbers arranged in order of

strat igra7hic age where possible.

Figure 15 ;ippears on the following frame.

800 850 900 950 1000 1050 1100 I150 1200 1250

Temperature ("C)

Figure 15.

J Ceboruco lnvas are unzoned, temperature than 4 wt % H20 are saturated with

estimates based on th'ese phases are water at pressures less than 2 kb.

assumed to represent the .temperature If this is true, then tzmperatures

of the magma upon' eruption and calculated for equilibrdt ion between

quenching. Buddington and Lindsley plagioclase core compositions and

claimed an accuracy of their method whole-rocks liquid compositions should

of *30 OC, hut studies by Iklz (1973), approximate the liquidus tempetatures

Wright and IJeiblen (1968) and Ulmer of these laws for P below 2 kb, H2° and gthers (1976) indicate soner;hat and should be higher 'than the Fe-Ti

better accuracy. oxide temperatures. Similarly,

Tem?erattires calculated from the' temperatures calculated from plagio-

plagioclase geothermometer of Kudo clase-rim and glassy-groundmass

and 'deill (1970) and a revised ver- .compositions should approxinate quench

sion of Drake (1976) are subject to temperatures for F' between 0 and 1 kb. H2° some uncertainty because of their It can he seen ih Figure 15 that dependence on r"Hp' Fur t he rmo re, 'several samples (346, 61, 113) gfie no account i.s taken of the. effect plagi oclase liquidus temperatures. 100

of pressure rinde r wa t er-un tic? rI- to 200.OC higher than similar rock's. .. saturated 'conditions. However, it from the same group. Because the

has been concluded in an earlier method OF calculation took the most

'section of t-his paper that thp calcic core composition observed in

water :content of the Cebdruco lavas the rock, any xenocrys t ic plagioclase

was gererally low and that plagio- could give an anomal.ous temperature.

clasc \%as a liquidus phase. As the This is the preferred explanation

experimental work of Eggler (1972) for t Ii e se se emi.n i1 y'j spurious temp e ra-

and Egglcr and Burnham (197'3) have turcs. In some cases the plagioclase

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quench temperatures are several tens phenocrysts are rimmed by pigeonitc.

of degrees higher than the Fe-Ti For this reason, average analyses of . oxide quench temperature for reason- pyroxene phenocrysts ,were used to

able water pressures. This. could be corpute teRperatLres. These should,

-, I a result of 'the fact that rea'ctions then, represent intermediate tempera- c- are quenched at different temperatures between liquidus and quenclw

Lures for differen{ phases, or it could values.

be a result of poor sampling 0.f The reliability of these tenipera-

plagioclase rim compositions. tures remains uncertain. Many of those - Trio methods of applying the Lalculated by the method of !Jells are

pyroxene geothermometers of Wood and as much as 100 OC higher than those

Banno (1973) and IJells (1977) were based on the method of Wood'and Banno.

attempted. The first used the Since Wells's (1.977) equation is

compositions of groundmass ortho- essentially a revision of Wood and

pyroxene equilibrated with the rims Banno's (1973) equation and is based

of augite phenocvystk, and the on a larger data set, it is presumed

second used the corb cornpositidns that temperature deterninatiorls based

of coexisting augite and ortho- on the Kells equation are Tnore accurate. ,, pyroxene pairs. In,all cases thz The temperature ranges and sequences , latter method yielded a lower of crystallizati m shown in Figure 15

temperature than the former method. for nndesitcs are in general agrec-

'l'hese unexpected results are probably ment wiLh the experimental work of

due to the fact that no augite was in F,ggler (1972), Eggler nnd Iiurnliam

equilibrium with the groundmass ~ (1973), Green (1.972), and $,tern and

orthopyroxene. This is certainly others (1975) for water cootents of

the case for the postcaldcra the I.io,uitls Icss than abou? 3 wt %,

andesites, in which many of the augite and thus indicate drystallization

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may have' taken place under water- rhyolite, plagioclase liquidus tempera- I

saturated conditions .at pressures tures ran'ge from 975 to 1029 0 C and

less tharr 1 or 2 kb. a temperature calculated frarn the

The second-stage clacites show a two-feldspar geothermometer of Stormcr

[ much larger range of temperatures (1975) is in exa'ct agreement with the.

i \I than the andesite.s; plagioclase dry Fe-Ti oxide quench temperature of

liquidus temperatures are 1168 to 810 'C.

1190 OC, similar to the postcaldera AND PH IN THE JALA PUEtICE andesites, and oxide quench tempera- !'TOTAL 2

tures'are 860 to 865 OC, a range of l'he Kinera1 phases present: in any ,, %ore than 300°.' Thi? could be due individual .unit of the Jala pumice' do

t I' to the fact that the second -stage not allow calculations of pressure.

dncite3arc a nixed .rr.ag;na, as sug- However, if 9 hypothc>tical boundary

gested earlier, and the plagioclase iq the magma'chamber is assum6d between

core covositions were in reality in the first-erupted unit, the Cerro

equilibrium with a basaltic magma. Pedregoso dome, and the later-erupted

In the next secticn it is shown, air-fall pumice, tRen Ptotal by means of a thermodynamic. argu- -can be calcuhted at that boundary.

ment, that the Jala pumice \;as Th.e Ccrro P,edregoso doce contains

Xeclrly saturated with water prior .biotite, Fe-Ti oxides, and plagio- 1 to its eruption. This implies that clase, wJ-lereas the air-fall and ash-

the pl.agioc lase liquidus of this flor~-units ccptain hornblen$c, ortho-

I group is probably rfpresentcd more pyroxene, Fc-Ti oxides, 'and plagio-

realistically by temperatures calcu- clasc,. Thus, the following reaction

lated for water pressures between can I)e used to calcula'te ?J at total Q.5 and 1 kb. the liypothetical boundary:

For the Ccrro Pochetero sodic

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lilt OPX ,Fe+' in thd annite component of the

KFe3A(1Si3Ol0(OH)~2 + 7MgSi03 . ' (1) biotite'fn reaction 2, the phlogopite - biotite OPX component was used as.a check. The

,r ea c t ,ion

Free-energy data and assumptions KMg3A1S+3010(OH)2 + 3Si02

concerning the "modeling of solid-pliase biotite liquid

acfivities are pre'sented in Appendix = 3MgSi03 + KA1Si308+_H20 , (3) . " calculated using reaction 2. Ptotal OPX plag stqan 1 is i88 bar, or a depth of about along with the 'data in Appendix 2, gives 3 km. This is not unreasonable, the compArable result of f .= 769 bar. H,OL because the eruption that* produced ' Fugacity coefficients, for water are

ttie Jaia ,pumice resulted' in collapke given, by Burnham and others (1.969) and f

of Ceboruco and formation of the allow calculation of P of 746 and .- H,O L outer caldera, indicating that 863 bar for, the fugacities. calculated the nagma chamber must have been b from reactions 2 and 3, respectively.\ . gt a relatively shallow depth. .in the Jala pumice prior, pH20 In ordej to calculate the fugacity to the eruption\was slighty lcss thad

the react ion suggesting that the magma ,was of wa'te r , i;,20 , 'totai'

s ligh t I y under sn t urat ed with K~e,3AlSi3010(OH) + 112 O2 respect to water..

biotite gas Garni'chacl and others (1977Fgaue

= KI\;LSi38 0 + Fe 3 @ 4.+ H2C1 , (2) an equatihn (51 in their paper) that

mt st,eam . enables calculation of the mole 'I along iqith the data and assumptions fraction of watrc;r, Xkr20, ,from the

This resu1,~sin '' values of 0.148 to "H,O , dis cuS s ed below .\ L 0.159 for the Jala pumice at the Crystal Fractionation hypothetical boundary between erupted

units. This translates to weight Osborn (1969), Stern 979), Green

percentages of water of 3.7 to 4.0. and Ringwood (1968), and Boettcher

(197-3) have disussed,, the possibility PETROGENESIS OF CEBOKUCO LAVAS of producing cal c-alkalic orogenic

The chemical compositions and rocks by crystal fractionation' of

trends 0-bserved at Ceboruco could basaltic magma. Two of the main

have been produced by several propesses factors; that argye against, this

acting one at a time-or together in proc'ess operatin$ in the prodiirtion

various degrees. These processes' might of Cebmuco's lavas are (1) oniy '3 includk partial melting of some source, about 1.5 km of basaltic magma out 3 #assimilation of country rock along of the tbtal 70 kn. of erupted

the path from source to surface, material is represented in.the i, magga mixing, and crystal fraction% Ceboruco area (the amount required

tion in cl'osed or open systems. Con- to mix with t!!lF Ja1.a pumice to pro-

straints are imposed by the chemical duce the second-stage &cites), and

compositions and mineralogy observed (2) fractionation#of magnetite or

in the lavas, by the temporal, amphibole is required to. suppress

spatial, and structural relations ,the, iron enrichment trend produced

observed in the field, by the by.fractionation of Mg-rich olivines

volumes of material- erupted, and by and pyroxenes. Factor 1 may be .

the temperatutes of the various magmas unimportant if cooler andcs'itic or ,. obtained from mineral geothermoreters. I dacitic magrias prodiice a "shadow effect'' How these constraints apply to.sore i. (Smith: 1979) , shielding basaltic

However, factor 2 is difficult to liquid to produce the derivative liquid and

reconcile with Ceborucojs lavas be- compareh the composition of the derivative

casue titanomagnetike and amphibole - liquid calculated in this manner with the

are never founh as liquidus or near- actu'al composition of the derivative liquid.

liquidus, phases in andesites or The goodness of fit was measured as the sum ' bas'al t i c xeno 1i t h s . of the squires,of the observed coxposition -2 Still, the .more siliceous lavas mints the actual composition (CA ).. Only . 2 at Ceboruco could have been yroduced those models whose C A was lessphan 2

by crystal fractionation of the were deemed acceptable. This limit is

more basic magmas, such as the justified 'because values greater than 2

b nndesites! Three methods of testing allow errors in calcril ated comhtions

possible crystal-fractionation outside the assymed errors in chemrcal

models we,re applied to the Ceboruco analyses and the standard deviations

lavas. Najor elements were tested observed2 for any single group of rocks.

rising a linear least squares com-

puter prpgram 'sirilar to that of Piodels that successfully passed this

Wright and Doherty (197;). -Average major-element test .were then examined

analyses of phenocrysts observed to determine if the proportions of

in assumed parent magmas, along phases required to fractionate were in

with thp analyzed or estimated general agreement with $he modal

(in' the case of ,the basaltic proportions observed in the parent *. xenoliths) major-element concen- lava. Models for which this criterion

trations of assumed parent and deriva- were not met were rejected. for

tive ma!i,mas were used as input. The example, derivjtion of the Jala

program used these data' $ calculate pumice and 1870 dacites from the L

the proi'ortions of various phases xenoli thic basalt would require

by 'height of the fractionating as- elements. The proportion? of phases

semblage, which was deeme2 to. b: and de$ree of crystallization or

outside the limits Qf probability Fractionation obtained from the suc-

based on, modal analysis of the cessful major-element, mode1.s were input

basalt and andesites of the suite. into the Rayleigh distillabion law:

In all, 648 possible fractioqal. d1nC =' (Di - i - 1) dlnF , (4) crystallization models were tested where Ci = concentration of an involvi4g the five major cheniical , ePemcnt i, Di =, bulk/crys

lavas and the basaltic xenoliths, fraction of liquid remaining after re-

1 with from two to five crystalljzing moving the solid phases.

phases for each model. More, than Equation 4 can be integrated and

500 possible models were eliminated solved for the concentration of

n L bccausc the.C A values were much. elements in the resulting liquid

greater than 2, phages h3d.to be relative to the initial liquid only

added, or modakproportions were. if the compositiona1.dependence of D - i 4:. outside of fhe acceptable limits. is known or if it can be assumed

All of the models involving the constant. Certainly Di wou1.d not re-

basaltic xenoliths as a parent maip constant during a process -that

magma were eliminated in this changes- the czmposition of-a melt from

process, a'lthough the uncertainty andesitic to dacitic, as can be seen-

in the cglculated composition of from the, partition coefficieit tabula-

the basalt makes this argument less tion of-Arth (19763 and the discus-

satisfying. sions of Albarede and Bottinga (1972)

Models that successfully passed'the and Allegre and Minster (1978).

test based on major elements were then Eurthermore, D.-.1 is most certainly a

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functioni of both temperature and The 1870 dacites cannot bel'derived

pressure for most elements, from the precaldera andesites for ,? Therefore, in order td apply similar reasons. ,. equation 4 to the Ceboruco lavas, Although fractionation of the pre-

the highevt and lowest values of caldera andesites to the second-'stage

pirtition Coefficients reported . dacites is feasible for major elements

in the literature and estimated and REE, the Rb content is the same in

from other sources were us'ed to both and would be much higher' in the

calculate a high and 1ow.value. of dacites had they resulted from such 'a

Di whiai- coiild%e, used to constrain process. Similarly, fractionation

fracbional-crystallizapion models. models predict too much enrichment .. These values and their'sources' are in Ba, Sc, U, and Ta. A hybrid origin

listed in- Table 10. 'A discussion of the second-stage dacites, as sug-

of the resuits follows. gested earlier,' is thus preferred.

The precaldera andesites could No solutionsewere found that could

n,ot have ?ractionated to the post- relate the three flank lavas 'to each

caldera an,deSites because the other or any other chemical group ob-

major-element models require such served on the vdlcano, consistenryith

small degrees of fractionation .a "conclusion reached in an earlier

-(Gi L@10%) that enrichment in section that these lavas are unrelated

.incompatible' trace elements, to mo?t-of the Ceboruco lavas.

especially REE, would not occur as The second-stage dacites, although

observed. Derivation.of, the Jala believed to be a hybrid magma,-were

pumice from the precaldera andesites found to be suitable parents for the

does not appear possible, because Jqla pumice on the' basis of boa major .- the puinice is so enriched in light and trace elements. The results are

REE, Rb, and Ta and depleted -in Sc. shown in Table 11 along with the'

-, I -. L. Orkhopyroxeve c Iino p y 1: o~ ene . Plagidclase . Magnetite. * Low High Ref. Low High Ref. kow . H5gh Rif. Low . Hig6 kef, ,Rb 0.003 0.35 1,2 QlOl5.- 0.05 1,2 0,016. 0.14 1 Q.001 0.10 3

Sr '* 0.009 0.69 l,i*. 0.093 2.0. '1,2 '1.29 12.9 1,2 0.01 1.69 3,5 Ba 0,003 0.12 ' 1,2 0,004, 0.145 1,2 -0.05 0.59 !'l . 0.001 0.147 5 t

sc- 1.0 ;4.i 3,2 1.0. 12.5 1,2 0,001 ~ 0.025 3,2 1:OO 9.3 3,5

v ,I 1.0 4.08 3,2 1.0 11.5 3,2 0.001 0.08 3,2- 23;O 190' 4 1- co 1;o 10:6 3,2 1.0 6.66 3,2 0.001. 0.08 3,2 41 140 5. Ni 1.0 3.0. 3' 2.. 0 -4.0 3' --' ------La ',o. 002 0.03. 3 0,066 0;]3 3 -- -- -_ -- Ce 0,003 0.038 1 0.0.77 0.21 1 0..023 0.28 1 0.29

Nd . 0,006 0,058 1 0,170 0.43 1 , 0,023 0.20 1 0.42 Sm 0.014 0.100 1 0.26 0.74 1 0.024 0.17 1 0-,5 8 Eu 0,023 0.079 1- 0.27 0.75 1 0.055 0.73 1 0.38.

Dy 0.054 0.293 1- 0.5 1.0 1 , 0.01 0.19 1 0: 32 Yb 0.11 0.67 '-1 0.43 1.0 1 0.006 0.13 1 0.23 I* Lu 0.11 0.84 1 0.27 ' 0.95 1 0.005 0.24 1 0.26 Th 0.01 0.10 3,2 0,001 0.01 22,3 0,0001 0,006 3,2 0.03 u 0.01 0.111 3,2 O..OOl. 1.5 2,3 0.0001 0.006 3 1.0 2.5 3,'5 Ta 0.01 0.111 3,2 0.05 . 0,333 4'2 0.00Oi 0.026 3,2 0.45 0.75 3,5

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Apatite Olivine Garnet Amphibole Lo$ High Ref, Low High Ref. Low High Ref, Low ' High Ref, Rb Q.001 0.12 3 0.009 0.011 1 0.009 0,042 1 0.045 0.43 1

Sr * f.00 6.00 3 0,009 0.02 1 ,0.012 0.015 1 0.19 0.64 1 Ba 0.01 0.6 3 0.009 0.011 1 0,017 0,023 1 0.16 0.73 1

Sc 0:05 0.5 3 Q. 5 1.5 3 4.1 8.3 3 2.18 '10.0 6,3

V 0.05 0.5 3 ------J. - co 0.05 0.5. 3 l.0 - 3.0' 3 2.0 4.0 3 0.98 2.0 - Ni 0,001 -- 3 7.0 12.0 3 0.33 0.66 3 1.0 2.1 3 La 16 50 . 3 0.002 0.01 3 0.2 0.3 3 0.08- 1.6 3 Ce '18' 52.5 1 0.003 0.01 1 0.28 0.35 1 0.094 1.77 1 Nd 27.4 81.1 1 01003 0.01 1 0.068 0.53 1 0.16 4.49 1 Sm 29.3 89.8 '1 0,003 0.011 1 0.29 2.66 1 0.24 8.1 1 \ Eu 20.5 50.2 1 o.po5 0.01 1 0.49 1.5 - 1 0.26. 5.9 1 Dy 25.6 69.2 1 0.006 0.014 1 3.17 28.6 1 0.31 13.5 1 Yb 13.1. 37.0 1 0.009 0.023 1 11.5 39.9; 1 6.23 9..0 1 Lu 11.2 30.2 1 0.009 0.026 1 11.9 29.6 f,7- 0.22 6.3 1 Th 0.3 3.35 2,s ------U 0.300 3.4 2,5* ------

Ta 0.05 0.51 2,s -c - - -- 4------

-Note: References:(l) Arth (1976); (2) I.S.-E. Carmichael, and J. Luhr (unpub. data on andesites from Volcan Colima, Plexico); (3) estimated, by extrapolation or from data on similar elements in sirn?l'lar phases; (4) Ewarth and others (1973); (5) Hildreth (1977); (6) Higuchi and Nagasawa (1969); (7) Gast (1968).

Ob9. Calc. , Obs. Calc. Obs. Calc. &iff -' diff. 'A diff. diff. A diff. diff. A L Si02 ,6.4i 6.31 '0.10 -8.83 8.83 0.00 6.68 6.75 -.07 Ti02 10.57 -0.16 . -0.41 -0.90 -0.68 -0.22 -.061 -0.35 -0.26 -1.40 -1.46 0.06 -0.92 -0.84 -0.08 -1.43 -1.37 -0.06 *I203 FeO- -2.02 -2.18 . 0.16 -3.43 -3.49 0.07 -2.05 -2.08 0.03 >In0 0.02 -0.02 . 0.04 -0.03 -0.02 -0.01 -0.b5 -0.02 ' '-0.03 ?!go -1.26 -1.23 -0.03 -1.81 -1.77' -0.04 -1.48 ,-1.47 -fI.Ol L CaO -2.28 -2.35 0.07 -3.65 -3.67 . 0.02 -3.05. -3.05 0.00- Na20 0.41 0.41 0.00 1.01 0.59 0.42 0.66 0.33 - 0.33

0.92 1.03 -0.11 1.16 >. 1.31 -0.15 %,1.55 1.38 0.17' "20 -0.24 -0.34 0.10 "0.27 -0.26 -0.01 -0i21 -0.12 -0.09 PZ05 C A2 = 0.236 J.A2 = 0.260 C A* = 0.231

$: Cornparism of the observed difference between the proposed parenc or initial magma and the calculated ,difference obtained from the linear least 4qiiare.s * program. A idthe observed difference less the calculat?ed difference.

* wt% Init. Prop. .Obs.-s \?t%Init. Prop. qF. Obs. magma phase m& magma phase mode'Wr mode C1 - / 3px 5.20 0.1792 a 0.1119 3.28 -0,0749 0:1337 9.0953 Cps 0.30 0,0094 0.0041, 7.01 0.1601 0.052.5 0.1843 Plag 23.12 0.7271, '0,8350 29.02 0.6629 0.764;'7 0.6678 y t 1.83 0.0576, 0.0491 3.79 0,0866' 0.0490, 0.0526 m QP ,q,S5 I O.iO268 lr. 0:68 0'.0154 Tr . Tr .

X crysta&lized = 31.80 Z crystallized = 43.77

Postcaldera dodesites to 1870 dacites \dtZ Itiit. Prop. Obs. Obs. magma .phase. mode Wr mode C1-

Opx .3.40 0,0831, 0.1337 ' 0.0953 Cpx 5.79 0.1415 0.0525 0.1843 Plag 28-61 0.6987 0.7647 0.6678 Flt 2.72 0,*0665 O.Od90 0.0526 AP 0.42 0.0101 Tr'. Tr . X crystallized = 40.94

'Average phase compositigns obtained from the electron microprobe .analysts were used for each model. The quantities of each phase are reportsd as weight percent of the ini-tial magma, tk proportions OF. each phase, and t,he weight Fraction mode observed .in the assumed parent magma, LJr = mode obserced in the whole rock C1 =*mode observed in c1;ysta' clusters only. % crystallized = the sum of the weight percei Eages of 'the subtracted phases relative to the' initial magma.

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I Second-stgge dacites Postcaldera andesites to Jala pumice to Jalarpumice r-rrace-element enrichment factorsi I 3 Low High Obs. S;D. Low High bbs. S.D. Rb 1.46 1.37 1.30 (0.28) 1.77 1.64 1.57 .(0.38) Sr 1.01 0.03 0.57 (0.10) 1.07 0.96 0.51 (O.OB)?'c" Ba 1.45 1.21 1.16 (0.05) 1,7L 1.38 1.17 (0.05)""

kC 1.33 0.85 ' 0.25 (O.OO)*~~ 1.48 0.29 0.13 (O,Oopc V 0.82 0.02 0.17 (0.52) 0.49 0.0004 0.06 (0.19) Co 0.55 0.03 0.12 (0.01) 0.20 0.00002 0.05 (0.00)

Ce 1.20 0.78 ' 1.12 (0.02) 1.47 0.96 1.15 (0.03) Nd 1.09 0.59 0.99 (0.06) 1.33 0.75 6.92 (0.06). SP 1.oo 0.54 0.85 (0.00) 1.20 0.67 0.82 (0.00) Eu ..1'. 16 0.47 0.64 (0.01) 1.39 0.45 0.63 (0.01) DY 1.11 0.66 0.82 (0,Oi) 1.32 0.77 0.76 (0.06) Yb 1.26 0.93 . 1. Q7 (0 ,-02) 1.49 1.08 1.11 (0.02)' Lu 1.29 0.93 1.15 (0.11) 1.49 1.08 1-25 (0.13) Th 1.46 1.13 1.45 (0.03) 1.78 1.04 1.79 (0.06) U 1.43, 1.32 1.35 (0.06) 1.69 1.32 1.65 (0.08) Ta 1.45 1.40 1:38 (0.02) 1.73 1.63 1.64 (0.08) I.

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'Postcaldera andesites - to 1870 dad'ites

Law High O'bs. S.D. Rb 1.68 . 1.58 2.00 (0.45) Sr 1.04 0.01 0.47 (0.08) 1.45 (0.06) Ba 1.67 '1.34 sc 1-45 0.40 0.63 (0.01) V 0,68 0.0008 0.24 (0.!8) Ca 0.36 0.005 0.31 (0.01)' Ce 1.50 1.12 1.32 (0.e) Nd 1.41 0.97 1.09 (0.07)

, Sm -- 1.28. 0.89 1.18 (0.01) <. Eu , 1.44 0.54 0.98 (O'.Ol) Dy 1.40 0.96 1.10 (0.09)

Yb 1.51 d1.20 1.'42 (0.02) Lu 1.54 1.86 1.47 (0.15)

Th , 1.69 1 p6 1.83 (0.06)** U 1.63 - .1,15 1.69 (0.08) Ta' 1:66 . 1.58 1.52 (0.02)*~~ ..

~ ~ h'ate: S.D: = standard deviation. 5 trace-element enrichment factors 6 report results of the Ray!.eiih distillation is~lcalculations as. dis- cussed2 in 'text. These are obtained by dividing trace-element concen- tractio6s observed. in derived magma by those in assumed.' ,pargent magma, High = high 't, values and low = low 't, reported in Table 10 for each phase, *;'Values gall outside range calculated for particular element.

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observed modes of the second-stage ari:ounts of hornbleridk w&rc also

dacites. Th:& modes arc not in exact fractionated, Far t$c -187v dac.ites

agreement b,bt 'follow the same order calculated values of Ni -arc too low

of relative abundanci?. Calculated and-those for Ta aro too high. It is

extremes of Sc are Slightly out of also to be noted thaJ the modql'propor-

range f,or this model, but relatively t'ions reqyired for both models .I small degrees of fractionation of a- necessitate removing more cpx than ops

phase such as hornblende, observed from the postcaldera andesites, and

in the Jala pumice, could account this is not observed in'the modes where

for the discrepancy. opx is the predominant pyroxene. In

Differentiation of, the second- these rocks, cpx predominates over'-

stage,.dacites to the 1870 dacites opx in the glommoporphyritic clusters,

cannot enrich the derived liquid which suggests that if fractionation j .;enough in Rb, Ba-3 Coy Sm, Yb, and occurred by crystal settling, it was

c Th and results in Ta values t1;at ark these crystal clustcfs ghat' were prefer- Y.

too high. entially removed from the liq'uid over

-'Models Cnvolving the postcaldera single crystals. Modal prbportions of

andesites as parents for the mqre glomeroporphyritic clusters are

siliceous i-ocks at -Ceboruco were reported in Table 11 and show that

successful in two cases , which -a>e relative proportions of cps and opx

reported in Table 10. They appeqr are in. fair agwement with those

to be possible parents of both tlie required for the fractionation models.

Jala puqice and the 1870 dacites. Derivation of the Jala pumice by

Still, there is slight disagreement crysta: fractionation of ttic polc coal dera u

for the derivation of the Jala andesites, however, seems incompatible

pumice for the elements Sr, Ba, and with a hybrid origin of the secWhd-stage

Sc, which could be resolved if small dacites by mi?* of .Jala punice icit'n 3

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high-A1 basalt. . . Of these two possi- andesites were presepa heneath Ceboruco,

bilities the latter is preferred the first. fractionatiag to produce the

for reasons argued in the section on Jala Rumice and the second erupting to

magma mixing and the second-stage the surface after the mixing event and

dacites. Acceptance of the miiing caldera format ion.

theory for the origin of the second- In order for the postcaldera stage dacites would require an unl.ikely andesites to produce the 1870 dacites,- sequence of events if the Jala pumice 3 a minimum volume of 2.7 km is required,- was derived by fractional 'crystal- with an additional 4 km 3 erupted as

lization, as follows: (1) 'the post- andesite. As' diLdusscd earlier, the

caldera andesitic magma would have last .postcaldera andefi.te may have

to first differentiate to the Ja1.a erhpted about 300 yr prior to the 1870

pumice; (2) basaltic magmg would eruption. Thus, in order for the , ., then have to be injected into the fractional crys,tallization model to

differentiated magma, without intcr- be viable, 41% CrystalJization of a

acting with the parental postcaldera magma body with a minimum volume of 3 andesite; (3) continued mixing of 2.7 km must occur wi-thin about 300 yr,

basalt with the Jala phice would and the temperature must deGrease in

prod uce t he second- s t age dzfci t es ; this body from approximately 1000 to

and (4) the pos'tcaldera andesites 860 'C (Fig. 15). If crystal settling

would then geappear and erupt to the were the mechanism of differentiation,

surface, without shdwing the effects then crystals of average size '0.05

of the basalt passage or the mikng cm must settie about lo5 cm (the

event . approximate radius of an equivalent 3 .*The possibility remains, however, sphere with a volume of 3 km ) in a

that two separate batches of magma magma of density about 2.6 g./cm3 and

with compositions siini lar to postcaddera a viscosity of about lo4 P in 30Q yr.

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Using these quantities the Stokes the presence of 'apparent disequilibrium

settling velocity is roughly 10-5 among mineral assemblages 'fobnd in the .

cm/s. Therefore, in 300 yr crystals rocks. Magma mixing has probably been

I of the prescribed size could tsettl'e. involved at Ceboruco in the production \ 5 about 10 cm. Noting ttpt the corn of the second-stage dacitcs and upper

positifi of the postcaldera andesite units of the Jala pumice. This argu-

glasssapproximates the composition ment is based mainly on.physica1 II of the 1870 dacites (Tables 1 and evidence such as that employed by the

7), all tha.t would be required is above authors. In only one other Case

removal of, all crystals that w'ere is there any physical evidcnce ihat

present in the postcaldera andesites magma mixing could have been involved

'prior to its eruption. in Ceboruco's evolution. This is the

It thus seems reasonable -that the case af the Ceboruquito flank'lava

postcaldera andesites could have .(sample lOS), which contains apparently

been parental toythe 1870 dacites. disequilibrium plagioclase phenocrysts

Strontium isotopic data from that include glass more siliceous than

Moorbath -and others (1978) are in either the groundmass of the rock or

accord with this view. the whole rock. Xenocrysts do occur

in other lavas from Ceboruco; however, Magma Mixing their low abundance does not suggest

Anderson (1976) and Eichelberger magma mixing on any large scale.

(1974, 1975) have evoked rnagma If other magmas at Ceboruco do

mixing as a commonprocess of producing indeed represent mixed magmas, no

andesites and dacites in orogenic physical evidence remai;s to suggest

regions. Their evjdence for such it. To test such possibilities,

conclusions relies on the composifions the least squares computer program

of glassy inclusions in minerals and was used with possible mixing end

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meinbers consisting' of the observed Not preclcded by these tests is the -, chemical groups of lavas found on the possibility. that nixing occurred with.

surfa,ce. Eighty- f our comb inations end-member compositions- not obse.rved

of mixing' the five ch6mical groups, on t-he surface', or that a more complex

three flank lavas, and basaltic process, such as mixing followed by

xenolith compositions were tested for crystal fractionation, was responsible

major-element mixing. Again, the for some of the chemical trends observed 2'. limit of Z';A of 2 was imposed on at Ceboruco. , acceptable solutions, for reasons Assimilafion and Anatexis described above. The mixing propor- of Contine.nta1 CAst tions determined ih the successful

major-element tests were then used Currently , most ,'o f the arguments

*ta calculate the trace-element concerning the involvement of crustal

Mncentrations of the resulting hybGid, material in the genesis of orogenic,

which were then coqared with the magma suite; is based on isotopic

observed values. compositians of the lavas. khurch

pnly one model was found acGeptable and Tilton (1973); 'Rose and others

when these constraints were applied. (1976), Lopez=Escobar and others (1977),

A mixture of 34 parts Jala pumice Thorpe and others (1976), and James.

and 66 phrts.postcaldera ande'site and others (1976) have argued agaknst

reasonably approximates the composi- contamination by crustal material for 9

tion of the second-Gtagq dacites, as ormgenic lavas in the Cascades,

shown in Figure 16. Because there is Central America, and the Andes on the

at least some physical evidence that basis of Sr-isdt.ope evidence. Al-

basaltic, rather than andesitic, mapa ternatively, Church (1976), revising

was mixed with the .Jab pumice, the nis earlher assessment, Tatsumoto

.latter node1 may not Se acceptable. (1969), Francis and others (1977),

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------u- - Q) -I- -lu - 3- 0

- P------

3 -

1 10 100 iooo True.

Figure 16. Calculated conceritr$tions that result fron mixture of 66 parts average

postcaldera andcsite and 34 parts average composition of second-stage dacites. Oxide

values are weight percent; trace elerrent values are parts per million.

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Klerk and others (r977) , "and Ewa$ published Sr-isotope data on four '- I, and Stipp (1968) conc2uded that the rocks from Ceboruco*that include the ". isotopic eyidence does not preclude precaldera and postca'ldera -andesit.es

/ continental crustal involvement in and the 1870 dacites. The r'sported . orogenic. suites from the Cdssades, values fall in a tight group with an 86 Japan, the Andes, and New Zealand. average 87Sr/ Sr ratio of 0.70422

Kay and oth'ers (1978) pointed out 4--0.00006, slightly higher than the values

that_ Sr-isotope data alone cannot reported by the same authors for twelve I Y usua.lly be used as an indicator of rocks from/,the Mexican volcanoes

crustal involvement but must b'e .Colima and San Martin. Without denying

taken in conjunction with Pb-isotope the possibility of crustal contamina-

data. DePaolo and Wasserburg (1977)' tion, Moorbath and others (1978) favored

have shown that Nd isotopes along an isotopically different mantle source 0. with Sr data are useful indicators for the Ceboruco. lavas to give these

of crustal contamination and have? slight enrichments of radiogenici Sr at /' concluded that andesites from Peru Ceboruco relative to the othei Mexican

are indeed contaminated by crustal vblcanoes.

materials, 'a conclusion in sharp No data are currently> available on disagreement with that of James and the isotopic composition 'of crustal

others (19763. It is noteworthy, material in the Ceboruco area, and no

however, that isotopic data' would not Pb or Nd isoCopic data exist for 3 distinguish crustal involvement if -the Ceboruco lavas. However, granitic,

material involved were relatively young xenoliths are found in a partaially

crust re-cently added to a continental resorbed state in some of the Ceboruco

mass by the same process tha6 is mani-' lavas, and this, together with the /' fested as volcanism on the surface. high Th/U values for some of the lavas,

Moorbath and others (1978) hqve the high concentrations of incompatible ,. *

to the average orogenic lavas (Taylor, Volcanic Belt (Elolnar and Sykes,

1969), and the relatively high 87Sr/ 1969) does not eliminate- the possibility

86Sr ratio, suggests that crustil that a subduction process was occurring

material could-have played a role. beneath Ceboruco in times ?ast. Thus, 9 either of the proposed sources of oro- Partial Melting genic magmas remain possibilities for

Because voJcanoes that produce the origin of the Ceboruco andesites.

orogenic andesites generally occur Three possj-ble source materials for

above Beniof f zone%, two sources 0.f the production of the Ceboruco andesites

I origin by direct partial melting are 'are considered here from the standpoint

currently thought plausible. On the of trace-element geochemistry, relevant.

basis of experimental investigations, experimental studies, and mineralogical,

Green and Ringwood (1968), D. H. Green observations on the lavas themselves.

(1973), and'Stern and hthers (1975) These possible sources are (,1)\subducted

concluded that: andesific magmas in oceanic crust in the eclogite facies

orogenic reg'ions could be generaCed (2) subducted oceanic crust in the

by direct partial melting of subducted amphibolite faciFc; and (3) mantle

oceanic crust with an eclogite peridotite. Several authors (Gill,

minerafogy. On th;! other hand, ,1974; Thorpe and others, 1976; Condie

Kushiro (1974), Kushif-o ind others and Hayslip, 1975; Lopez-Escobar and

(1972), and O'llira (1365) argued that others, 1377; among others).hayc

hyarous partial Felting, of the mantle attempted to place constraints on the

is respon&ble for the-genesis of feagibility of deriving anhesitic

. these andesitic magmas. magmas from various source regions by

zone is currentJy present beneath partial melteng. These models suffer

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f;om the assiimptions made in order to element compdsi tions of the Ceboruco

simplify the calculation; among andesites have been used to calculate

these are assumed initial source com- the composition of possible source

/ positions, limited ranges o'f me,lting materials which can be compared to - -_.- fractions, assumed constant values.of postulated trace-element abundances

crystal-liquid partition coefficients * for the particular source considered.

(KD values), and assumotions concerning The models presented here use the

the type of melting process that equations of Shaw (1970), allowing the

could have occurred (fractional or K values to vary within the range D batc!i melting). Thus, these models reported in the literature and listed

are qualitative, at best, and the in Table 10, varying the mClting nodes,

conclusions reached are subject to initial modes, and degrees of melting

the- asgumptions imposed. Ah excellent to give a range of possible source

discussion of tk linitations of these compositions required to produce the

partial-melt ing models is presented Ceboruco andesi tes. Results €or I

in Allegre and Minster (1978). the three possible source materials

Accepting' the fact that trace-. . listed: above are discussed in sthe

element partial-melting models are, following paragraphs.

at present, only qualitative, they Eclogite Source are used in this paper only to

suggest possibilities. Many of the Green and Rinpmod (1968) have

assumptions li sted above have been shown that at pressures of 30 kb

eliniiwScd, and a sl ightly modified under anhydrous .conditions, andes'itic

approach is taken. Instead of compositions lie in a low melting

calculating the trace-elenent trough between more basic and more

concentrat ions of liquids produced acidic compositions and have garnet

by any melting model, the known trace- and clinopyroxenc as near-liquidus

that subducted oceanic' crust in

.eclogite facies could produce andeSites fractional and batch melting are shown

3- if partially melted. Simi-lar conclu- in Figures 17a and 17b.-The-shaded

sions have been reached by Stem and regions and solid bars indicate a

others (1975). range of compositions reported for

In 'order to test the possible oceanic tholeiites 'by Schilling (1971)

role of eclogite melting in the pro- and Kay and nubbard (1978).

duction of the Ceboruco lavas, three As can be seen from the figures,

initial modes were used: an eclogite eclogitic mineral assemblages require

consisting of 55% clinopyroxene, and a V-shaped KEE pattern, wi'th heavy REE

45% garnet, one with 70% clinopyroxene slightly enriched relative to light

and 30% garnet, and one with 65% REE. This contrasts with the flat REE

clinopyroxene, 34% garnet, and 1% pattern observed in ocean-floor

'\ '\ apatite. The apatite model was tholeiites. Although Rb and Sr

included in order to test the role of concentrations in ocean-floor

this mineral in controlling REE, as tholeiites are sufficient to hate

has been suggested by Beswick and produced the Ceboruco andesi tes, .

Carmichael (1978). The proportions reported Ba concentratioGs are sig-

or garnet: aid clinopyroxene entering Iii f icanlly lower and Xi abundances

the melt were allowed to 'vary between slightly higlier. The effect of

-0.4 and 0.6 for each initial node, reducing the amount of garnet in the

and the trace-element abundances in source would lower the heavy REE

the source required to produce the concentrations required but would not

andesites of concern here were affect the Ni and Ba anomalies.

calculated for 10% to 30% melting Apatite, with high I{D values for

using the range of RD values reported l'ight REE-relative to heavy REE,

mineralogy to produce Ceboruco (a) precaldera andesites and. (b) postcaldera

andesites from direc: partial melting (see text). Concentrations are normalized .' .' -* to Leedy chbndrite (Masuda anh others, 1973) for elements Rb through Lu and are

absolute concentrations for elements Co, Sc, and Ni. Observed trace-eleme-nt

concentrations in oceanic tlyole'iites are shod by shaded region €or elements

Kb through Lu and by error'Jbars for elements Co, Sc, and Ni.

Figure 17 appears on the following frame.

100

Rb Sr Ba La Ce Nd SmEu DY Yb Lu Sc Co Ni

I -.55Cpx. .45Gt --- - .7OCpx. .30Gt -.- - .65Cpx. .34Gt..OlAp

100

u Rb Sr Ba La Ce Nd Sm Eu DY Yb Lu Sc Co Ni

Figure 17.

source, would require atsource e

enriched in light REE. The enric'ti- source required, to produce Ceboruco ' s

ments and depletions relative to andsites with 30% partial melting

ocean-floor tholeiites required to becomes somewhat flatter and more in

c produce the precaldera andesites .agreement with the pattern observed in

become even more severe for the pro- oceanic tholeiites. This illustrates duction of the postcaldeta andesites. that it is possible. t? find a se't of At first glance; then, it would K values thapcould produce the D seem unlikely that the Ceboruco Ceboruco andesites, providing that

andesites could be generated ,by direct nature follows-. the sami? assumptions. partial melting, of an eclogite Philpotts and others (1969) have 'I' .. source. However, in .Figures 17a and shown that s'ea-floor basalts are

17b the upper solid 1 ine connecting often altered and light REE, Ba, and , elements Kb tfirough Eu represents Sr? become slightly enriched during this

the values required in the source alteration. Thus, an altered,oceanic

for 30% melting using the highest tholeiitc, if present as the source

values of K-for both a garnet-rich material, could explain the anomalies 1y , -. and a garnet-poor assemblage, while in light KEE, Ba, and S.r. Still, in

the lower dashed line connecting order to produce the Si abundances in

elements Eu through Lu represents the postcaldera-andesites, Ni would

abundances required for a 30% melt have to be about one-third of the

with the lowest vaiues of'KD. This lowest rcported value for 0ceani.c

switch occurs because the K values tholeiites. D. for heavy REE in garnet become greater Because these tracy-element models;

than 1. Thus, if the highest are believed to'be only qualitative, K D values are used for light REE and the it is felt that ,an eclogite'consisting

entirely be ,eliminated as a possib1.e andesites suggesk that hdr near-

source fdr the Ceboruco andesitcs. surface conditions the Ceboruco lavas

contained low amounts of H 0 and had Amphibulite Source 2 liauidus temperatures greater than

Green and Ringwoo'd (1968) suggested lo00 OC (Fig. 15). ,Furthernore, the

that a hydrated oceaiic crust would be postcaldera andesites have eruption

in the amphibolite facies at depths temperatures of 960 to 1115OC, and the

to 100 km, after which an eclogitic precalder? andesites are'expected to I mineGalogy would prevail. The fact have had even. higher eruptive tempera- I that most. island-arc and continental- tures. Thus, if the CGboruco andesites

margin volcanism occurs at heights wereever in equi 1 ibrium with amphibole

above the Benioff zone greater than they would have had to lose substantial

100 km has been used by Wyllie amounts of water and increase in

(i973) to suggest that amphibolite temperature as they ascended from the

could not be the sourc? of orogenic source to the surface. As. this seems

magmas. However, Fyfe and NcBirney highly unlikely, partial melting of

(1975) pointed out that the amphibole amphibolite does' not appear; to be a

breakdown reaction is endothermic, feasible process for generatiing the

causing the stability to be increased andesites.

to deeper levels in the Earth. Despite this conclision, trace-

Eggler (1973) and T. 'Green (1972) element abundances for postulated

have shown that amphibole has a amphibolite sources have been

maximum stability limit in hydrous calculated on 'the dasis of the known

andesitic magmas at 15 to 20 kb and abundances in the Ceboruco andesites.

960 to 1000 OC. The fact that Thi-ee initial source mineral assemblages

plagioclase was apparent 1 y the were chosen: an am:)hibolitc containing

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100% amphibole, one containing 5,0%, source were altered oceanic tholeiite

amphibole and 50% clinopyroxene, and or contained some sedimentary material

one containing 49.5% amphibole, 49.5% . to enrich REE, Ba, and Sr, then the

clinopyroxene, and 1%apatite. As the discrepancies in these elements would

XD values for amphibole and clinopyroxene be further reduced, Thus, with

are approximately the same for most properly chosen KD values and dpatite

elements, only nodal meltLng was con- in the residuum, an altered oceanic

sidered. The results are compared tholeiite'in the amphiboli.te facies,

with a range of compositio2s ok c"ou1d he a possible source.*-for at

oceanic tholeiites in Figures 18a and Yeast the pfecaldera andesites. This

18b. source, however, does not appear likely,

, For all model$.the results indicate on the basis of the mineralogical and

that for extremes of and degrees experimental reasoning discussed at K D of melting ranging from 10% to 40%, light the beginning of.this section.

REE would have to be relat'iyely ei- Mantle Source ' riched in the source, Ni would have

to be in the low range of oceanic The results of experimental work

thol,eiites, and Ba woulJ liave to bc on possible derivation of calc-alkalic

2.5 t'o 10 times enriched over the andesites by partial melting of

observed values in tholeiites. In mantle peridbtite are equivocal (Mysen .. this case, if values for light REE and others, 1969). It docs appear X D were the highest observed.and certain, however, that on.1~under ,DK vahes for heavy REE were the lowest water-saturated conditions would it

observed, then a flatter KEE pattern be possible for a silica-saturated

could approximate the Ceboruco REE magma to be in equilfbrium- with 7, abundmces for models containing olivine in the' mantle -(Nicholls and

apatite as .a residual phase. I If the Ringwood, 1973; Kushiro arid 'others,

mineralogy to produce (a) precaldera andesites, and (b) postcaldera andesites

(see text). Conventions are same asdn Figuie 17.

Figure‘ 18 appears on the following frame.

s A- I00

10 -.-. -.-._

u Rb Sr Ba La Ce Nd SmEu DY Sc Co Ni

- Hb only ---_ 5Cpx. 5Hb T -. 495Cpx. .495Hb. .01Ap

100

\ \ \

IIII.I II I II RbSr Ba La Ce Nd SmEu DY Yb Lu Sc Co Ni #. #. -

Figure 18.

conditions, liquidus temperatures from data on ultramafic nodules

would be depressed below the 1 'atm and high-temperature peridotites.

andesite liquidus to pressures of Only two mogels are shown in the

about 35 kb (T. Green, 1972). Thus, figures, both representing r;odal

in order for the Ceboruco andesites melting. One is for an olivine,

to arrive near the liquidus, they would orthopyroxene-, clinopyroxene-bearing

have to lose from 8 to 20 wt % H20 and peridotite, and the other is for a

rise nearly isothennally. Althougc garnet peridotite. Beth models require

an isothermal ascent is feasible a source enriched in light REE relative

(Lang, -1972), the loss of substantial to heavy REE. The upper curves for

amounts of water in the time envisioned elements Rb t?hrough 1.u represent 20%

for the ascent of andesitic magmas partial melting with the lowest values

does not appear possible (Marsh, of KD from Table i0; the lower curves

1976; Marsh and Kantha, 1978). represent 5% melting with highest KD.

Still, in order to test all possi- For the cornp21tible elements, Sc, Coy ' bilities$ the trace-element composi- and Ni, the upper values are for 20%

tion of possible mantle source melting and high KD and the lower

materials required to produce the values are for 5% welting and low KD.

Ceboruco andesites have been calcu- Thus, for 5% partial melt using the

lated (Figs. 19aeand 19b). As there low values for Rb, Sr, Ra, and K D* is considerable uncertcinty in the .light REE and high ZD values for the -\ trace-element composition of the heavy IIEE, Sc, and Co, the trace-

mantle (see discussion in Ringwood, element concentrations in the precaldera

1975), a range of 1 to 2 times andesites could bc approximated for

chondritic is shown for REE in the all elements except Ni. Despite the

mantle. Other data for possible high uncertainty in mantl'e trace-el ement

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/11_Part_II/2290/3433969/i0016-7606-91-11-2290.pdf by guest on 29 September 2021 Figure 19. Trace-element concentrations required of source with mantle , peridotite, mineralogy required to produce (a) precaldera andesites and

(b) postcaldera andesitcs (see text). Conventions are same as in Figure 17

except that shaded region represents range of chondrite-normalized mantle

concentrationsrfor elements Rb through Lu and error bars for elements

Sc, Co, and Ni represent range of absolute mantle concentrations for

these elements.

Figure 19 appears on the following frace.

I / / / /

I I1 I I1 Rq Sr Ba La Ce Nd SmEu DY Yb Lu

100-- d - - .15Cpx. ,250~~..60 01

* .54 01. ,0661

IIII I II I II

Rb Sr Ba La Ce Nd SmEu ' DY Yb' Lu

Figure 19.

material ever postulated has Ni con- probability of the particular source

centrations low enough to produce decreases. - Mineralogical arguments ,.- Ceboruco's Ni-poor andesitcs. Thus, appear to &le out both an amphiboli'te

only through later fractionation of source and a mantle peridotite source,

a nkyrich phase, such as olivine, leaving on1 y the eclogite source, which

could the Ceboruco andesites be still requires many criteria of the

produced from mantel peridotitc-and source and is thus ndt fully satisfying.

then only in the unlike'ly event that Probably a more complex process than

they lost substantial amounts of simple direct parti.?l melting., was. watcr bJforc arriving at the surface. necessary to produce the Ceboruco

andesites. Summary of Partial Melting Hypotheses Ringwood (1974) ,provided a possible

It is clear from the foregoing model involving partial melting of an , discussion of partial-melting models eclogite source under hydrous condi-

that the source of the Ceboruco tions to produce a magma that reacts

'andesites renains uncertain. AI.- with the overlying mantle', convertSng

though some of the trace-elercent it to garnet pyroxenite. lhe

rnodcls zllow for the Froduction 7jf pyroxenite is envisioned to then

the andesitcs if several. criteria rise diapirically until it again

are net, such as altered sources as partially melts to produce andesitic

I. op?osed to fresh sources, nature's magma. Best (1975) suggested that

choice cf the proper crystal-liquid andcsit.j.c magma may be produced when

Tartition coefficients, the presence hydrous fluids rising from a de-

of minor phases in the source, or a hydrating subducted oceanic crust .. xeans of removing' Si before the magmas react with the overlying 'mantle,

reach the surface, as the number of enrich it i.n incompatible clerne?ts,

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and cause it to inelt and segregate There are, no doubt, thousands of \ andesPtes. Fyfe and >icBirocy. (1975) possible models that will work in

envisioned a similarprocess wherein deriving andesitic magmas, but none of

qydrous fluids from the dehydrating these models will prove viable until

subducted crus; reacts with the a means of placing constraints on the

overlying mantle, product’ng a zone‘ processes involved can be found.

enriched in incompatible elements. SUMMARY AND CONCLUSIONS This enriched mantle is fhen carried

to deeper levels by the subduction The chemical compositional evolution

process until it partially melts to and the structural evolution of Volcsn

produce andesi tes. Ceboruco appear to be intimately related.

All three of these, models -are at Five distinct chemical groups have been

present untestable. They attempt to erupted; the generally moretbasic

explain the anomalous trace-element group of precaldera andesi.tes was

abundances observed in andesitgs and followed, after a period of inactivity,

the general observation that incompatible by silicic pyroclastics, which

elements tend to increase with eepth accompanied caldera fornation. The

to the subduction zone and with higher eruption appears to have marked the

stratigraphic position in the volcanic beginning of a magma-mixing event

pile. All require a hydrous phase involving -the Jala pumice rhyodacit-e

[although Best (1975) mentioned that and a high-Al basaltic nagna. The

the zone-refining process that he resulting hybrid magma was later

suggested could occur in .the magmatic erupted to form the Dos Eqiiis dome

phage], which if present in the case and Copales lava flow. After the

of the Ceboruco andesites would have collapse of the dome, andesites

to ‘be of small amount or lost along distinct from those erupted during

-the ascent path. the precaldera stage were crupted.

which could have resultcd fron frac- in bulk composition or mineral ccntent,

tional crystallization of the post- fron? the lavas erupted in the post-

caldera andesites. caldera periods of a particular

This evolutionary pattern, although volcano's history. Furthermore, caldera

u'nique in its detail?, is not unique formation, in most cases, was ac-

when rompzred with the evolution df companied by the eruption of Parge

other volcanoes in the world. volumes of pumice and ash more siliceous

Williams (1942)' found that the Mount than most of the material expelled

Mazama andesites erupted prior to the before or after caldera formation. - -. formation of the Cratcr Lake caldera Smith (1979) discussed the divergent

rjere of nearly uniform composition. nature and compositional gaps that occur

Just before the caldera-forming in volcanic systems ranging in size

eruption, dacite doms were erupted from small stratovolcanoes to large

around'the flanks of the volcano. systems.such as the Valles and

The caldera-forming event itself Yellowstone systems. He ccnclude?

was accompanied by tile eruption of that many of these systems exhibit

dacitic pumice and ash that changed compositionally zoned magma chambers, 'i abruptly to andesitic ash near the end with silicic tops grading downward

of the eruption (McUirney, 1968). into more basic magma. For smaller

Subsequently, andcsitic lava has systems, such as Ceboruco, he concluded

been erupted on the caldera floor. that it is extremrly likrly for ash- r Kuno (1953) discussed the volranic flow and air-fall eruptions to draw off

evolurion of several. volcanoes 'in magma from the underlying basic parts

Japan which have evolved through of the dagma chamber. The eruptive

episodes of Caldera formation. He history of Ceboruco tends to support

concluded that, in general, the the model of Smith (1979), although,

a system could develop is not readily pendenci cc; -of. trace-element partition

determinable. coefficients and phases in a source

The exact nature of the processes area that control- trace-element behavior.

governing chemical evolution of calc- Although no conclusions that offer

alkaline volcanoes, such as Ceboruco, solutions to the problem of magmatic

are complex. The systems are dynamic source regions are possible from this

and thus dependent on the residence study, it is clear th*at more such

,time of rnagnas in the mantle and crust studies of the chemical &olution

and the chemical compositio’ns of of single volcanoes will, someday

material traversed along the ascent provide a data base large enough to

path. In the case of Ceboruco, it provide solutions.

hks been shown that magma!nixing, ACKNOWLEDGFIEXTS crystal fractionation, and perhaps

assimilation of crustal material- have Financial support for t.his project

all played a role in producing the was provided by National Science

chemical evolutionary trends observed. Foundation Grants EAR 74-12782 and

Detailed mineralogical data demonstrate EAR 78-03642 to I. S. E. Carmichael

that the Ceboruco andesites contained and by Geological Society of America

relatively small amounts of water Penrose research grants to me. I.S.E.

during their crystallization. This Carnichael, C. PI. Gilbert, II. William,

fact is significant in that it seems and J. Luhr read several versions

to preclude any model of magma pro- of the manuscript and provided useful

duction that requiras hydrous melting criticisms. P. Behrman, Tina Cerling,

of the source. This study has also and R. lCorr?.sbec~ler assisted wit!] the

shown the inadequacy, at present, of field work. Special thanks is due

trace-element modeling based on poorly to Elizabeth Duncan for her help.

tt!roughout East of the work. SaPFie 116: La Pincharictia lava flow,

toll of northwesternmost ciridcr cone, APPENDIX .1. SA’E‘LE LOCi\I,ZTIES soutlicast of Ceboruco.

Sample numbers refer to coLlection Sample 136: Beneath Cchoruquito flow,

983 at the Department 01- Geology and near intersection of the former with the

Ceophysics, University of California, Ccrro Pedregoso dome.

Berkeley. Sample 104: Near top of Ccrro

Poctietero ,done. Precaldera Andesites

Samples 47, 48, 49, 51: Outer caldera

wall near sharp bend in Jnl a-Ceboruco ‘road, Samples 364, 346, 348: Jaln pumice

0.25 km after road enters outer caldera. air-fall units 1, 3, and 5, rcspectively.

Savle 66: Outer flanks of ‘Iypc locality, 10 Xm northwest of the

Ceboruco on the Jala-Ccboruco road, vi1,lage of Jala.

riorttwest of Cerro Peclrcgoso dome. Sample 353: Lower unit of Marquesado

Sample 76: Outer caldcra‘wall, ash dcposit, in valley south of thc 2.5 km south of microwave station. i villaf;e of Marquesado. Samples 77, 74: Dike in outer Sample 122: Southwest: flanks of

caldera vall, sam location as Cerrc Pedregoso dome.

sa-lple 76. I SnTple 178: .Eastern margin of Sample 71: Dike in outcr caldera I the DistiLadero lava flow. I I wall, 2.0 km south of microwave I i Sccond-Stage Dacites station. i Sample 127: Dike in outer caldera .Sample 55: Southern wa’ll of inner I wnI 1, northwest corner of caldera. I caldera, interior of the Dos Equis don:c

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Sample 106: Copales lava flow, dacite flow whose vent occurs on the

near Mexican Highway 15, 1.5 km east eastern margin of the inner caldera.

of Copales. Sample 67: South of elliptical

crater on west side of Ceboruco. Postcnldera Andesites Sample 113: Levee near top of the

Sample 33: El Centro dome, near main lobe of the 1870 flow, 20 m west of

top on south side. west side of elliptical crater.

Sample 15: Small flow on inner Sample 181: Dark lava tongue, last-

caldera floor, northeast of El Centro erupted material of 1870 flow, 40 m - dome. below last vent.

Sample 30: Dark andesite flow at APPENDIX 2. THERXODYNAMC DATA AND top of El Centro dome. ACTIVITY RELATIONS Sample 133: Coapan lava flow, near

El Cajon. APPEXDIX TABJ,E 1, TdLRXODYNAPllC DATA __ Sample 117: Coapan lava flow, 1 km Reaction AGOIRT = A/Y + B + C/T(p-1) (see text) vest of the village of Coapan.

Sample 1: El Norte iava flow, 200 m -- A B c. 1 3691.5 .-15.297 -0.3521 south of microwave stat ion. 2 -19973.0 - 5.666 3.00903 Sample 61: Ceboruco lava flow, 3 14766.0 -19.480 '0.1772

at its intersection with Mexican Note: Reaction 1 obtained by adding the reaction of Powell (1975, p. 35), Highway 15. reaction 2 of this paper, and reaction 11 of Nicholls and othc>rs (1971, p. 5). Sample 99: Floor of inner caldera, .Reaction 2, 1Iildreth (1977, p. 12&fd;~Re- action 3, B. .J. Wood (1975, personal bottom of cinder cone in southwest commun. ) . .

corner of the inner caldera.

Activities Rcl at ions 1870 Dac'iites

Sample 3: Toe of small bulbous The activity of cumrningtonitc in tile

method of Powell (1975), with the as- to fill the site over the amount of F,

sumption that H 0 was present in suf- 2 present and the Fe+2/Fef3 ratio was ficicnt arxsunt to fill the (OH,F) site determined to maintain charge bala.nce.

and that sufficient Fe+3 was present Values for fo and T were obtained from 2 to maintain the charge balance. the Fe-Ti oxide geothermometer of

Because plagioclase is the only Buddington and LindsLey (1964), as ex-

feldspar present, cPI% was as- plained in the text. KA 1s i 308 I sumed to equalx 'lag . mu1 tiplied KAlSi308 i REFERESCES CITED by an activity coefficient of 10 [F. J.

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YANUSCRIPT RECELVED BY THE SOCIETY

JULY 27, 1979

REVISED MANUSCRIPT RECEIVED

NARCH 10, 1980

WUSCRIPT ACCEPTED APRIL 8, 1980

Prjnted in U.S.A.

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