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Home , Alkali basalt, Basalt, Trachyte

An alkali- through suite, Mesa'Chivato Mount Taylor , New Mexico

L. S. CRUMPLER Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721

Geological Society of America Bulletin, Part II, v. 91,.p. 1293-1331, 8 figs., 5 tables, May 198.0.Doc. no. MOO501

ite) and trachyte compositions. This ABSTRACT' progression, together with minor-element

A complete through trachyt and isotopic data,. suggests that the .. volcanic suite has been documented from suite evolved by fractional crystalliza-

the central part of the Mount Taylor 301- tion of basaltic .

canic field, New Mexico (the southeast Volcanologic diversity of the field is

margin of the Cdlorado Plateau) showing illustrated by the variety of land forms,

mineralogic, chemical , and field relations including numerous , pit craters,

similar to alkalic of both silicic flow-domes, and distinct north-

continental and oceanic settings through- easterly oriented fissures. Northeast-

out the world. Thus, the Mount Taylor southwest oriented faults with as much as

field is fundamentally distinct from the 30-m displacements were active concurrently -. ._ .'I a predominaqfly calc-alkalic to low-Ti with the volcanism, and,.-speral faults y%, I 'L / k: alkali basaltfc volcanism of the Basin are superposed on fYssure lines, suggesting

and Range Provice as a whole. that the volcanism was a consequence of

Eruptions of the rocks began in deep fracturing of the margin of the f Pliocene-Pleistocene time with alkali by late Cenozoic Basin

basalt (basanatoid) and cont iniied through and Range faulting.

intermediate (hawai'ite, , benmor- 1293

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Volcanism in the Pliocene-Pleistocene makes it the only k'&wn example of a

Mount Taylor volcanic field (Mesa Chivato) classic volcanic suite associated

of New Mexico included :he eruption of with the Rio Crande Rift.

two distinct petrologic suites, the first GEOLOGIC SETTJN~ of which is uncommon in the North American General mainland and includes an alkali basalt-

-mugearite benmorite- trachyte The Mount Taylor field is dominated by I volcanic suite with sodic affinities, akin the Mount Taylor composite (Hunt,

to Hawaiian alkalic volcanism. The 1938; Lipman and others, 1979; Crumpler,

second suite consists of a diverse series 1978) and lies on the southwest edge of

of alkali and domi- the Colorado Plateau, 100 km west of. I

nantly aphyric tes, unconforrnably Albuquerque. and the Rio Grande Val iey

overlying the of the preceding of central New Mexico. The' style of

alkalic suite. Petrographic, chemical, volcanism is similar to that of other

and isotopic data, together with detailed Colorado plateau-marginal volcanic fields

mapping (1:24,000) of the interior of (Hunt, 1956; Best and Brinhall, 1974;

the field (Crumpler, 1976, 1977) indicate Thornbury, 1965), such as the 3an Fran-

that both suites are characterized by . cisco and Springerville-W:~ite Piountains

relatively simple trends in which basaltic Fields, Arizona, in that Khere is complex

lavas preceded more silicic flows. Thus, clustering of smaller basaltic and silPbic

volcanism in the Mount Taylor field is vents associated with a single large

significant by virtue of its compositional composite volcano of dominantly andcsi tic

range and uniqueness in the record of to dacitic composition. Detailed mapping d - late Cenoioic volcanism in the western covered part of Mesa'Chivato, which

United States. Furthermor.e, its location extends 50 km northcast from Piount Taylor

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. 2,700 m, some j00 to 400 m above the A few cones. appear to be super- adjoining Rio Puerco,and its tributaries.- imposed on faults several kilometres long, t The mapped part (Fig. 2)' includes about and some cone5 show resulting minor ., ', * 100 cinder-and-spatter cones, trachyte vertical offsets, as do the underlying.

flow-domes, -s, and pit craters, most basalt flows. Referring to the sequence

of which lie on distinct north-northeast- - of fault scarp degradation of Xallace

oriented fissures. In'many cases, indl- (1977, Fig. 3), most of the scarps have I1 0 vidual craters and flow-domes are elonga- been reduced to "wash-controlled" slopes,

ted in a north-northeast sense. the last stage in the degradation series.

Considering such evidence as the age of . Late Cenozoic the flows ('about 3 m.y.), the resistance

The Mount Taylor field lies on the of the basaltic flows, and the present I Jemez lineament (Jones, 1952; Mayo, 1958) ,. gentle slopes of what were formerly steep

and although the field is elongate in a flow margins, the faulting probably

trend approximately parallel to the trend dates cToser to the time of volcanic -.. .. of tne lineament (N50°E), individual activity in the field than to the present.

fissures trend N30°E. Northeasterly Thus, the fault scarps indicate that there

fissures occur in adjacent fields along has been moderate, seismicity in this area,

the lineament as well, including the at least through Pleistocene time. Th,is

Bandera field in the southwest' (Laughlin may likewise be true of some faulting in * and others, 1972) and the pre-Valles Q the adjoining Rio Puerco fault'belt .

Caldera'basalts of the Cerros del Rio (Slack a-nd , 1976). which forms .- field'to the northeast (Aubele, 1978, 1979) the transition zone between the Colorado

Several of the more prominenf fissures Plateau and the Rio Grande rift. The

in the Mount Taylor-field are cut by ubiquitous northeast trends are also

parallel north-northeast-oriented faults manifest in the numerous dikes and volcanic

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Figure 1. Location of- the Mount Taylor volcanic field with respect to other late Cenozoic extrusive volcanic rokcs and the Rio Grand’e rift,

i) New Mexico. Map ’modified from Woodward and others (1975).

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Fi'gure 2. Simplified geologic map UNCONFORMlTy . I-- of the northern interior of the Mount r Taylor volcanic Meld. Mapping based

on the,El Dado.Mesa and Laguna Seca m

7% minuKe -topogfaphic quadrangles.

Circled sample location numbers refer

to analysis numbers of Table 3. Contact

lines within units represent separable

flow margins, or separate vent centers , 'mBENMORITE c and their slopes. "Porphyritic alkali MAAR RIM MUGEARITE __ - EJECTA .. -.. _.-- basalt" locally includes megacryst-

bearing alkali basalt, and "late hawaiite"

locally includes silicic alkali basalt.

Figure 2 is continued on-the following

f raves.

m] TRACHYTE IF]MESAVEROE GROUP

srRllC'TUR4L 'CRATER ,i. FAULT

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U03O' 109m'

Figure 2. (Continued)

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..I ..I ...... I ..4 01I !.:

Figure 2. (Continued) I

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necks of the Rio Pu,erco VaLley (Johnson, folding is not restricted in its effects

1907), and they suggest a history of to the , butcmost influence (or be

tensile stress along ‘the Colorado Plateau- influenced by) the upper as well.

Rio Grande rift juncture through The fact remains that there is a clear

Quaternary time. relationship between the sag and the dis-

tribution of volcanic centers in the Mount Relation to Colorado Plateau Tectonics Taylor field. The,Mount Taylor volcano,

Hunt ‘(1938) first recognized the fact a complex center of andesitic and alkalic

that the Mount Taylor field lavas uncon- lavas, lies on the deepest point of the

formably overlie the Late Cretaceous to Acoma sag; the field to the north, includ-

early Tertiary (Laramide) structural de- ing the entire assemblage of vents for

pression known as the Acoma sag (Kelley, the alkalic suite reported here, lies

1955) and suggested that the sag and the along itas north-northeast structural

volcanism ?re somehow related (Fig. 3). trough-like extension.

Because the Acoma sag predates the It is probable that the relationship I volcanism by as much as 70 m.y., it may does reflect fundamental chemical or

reflect a fundamental influence of uplifts thermal displacements of unknown ‘origin

and basins#*which typify the tectonics of during Laramide time of the .

the Colorado Plateau, on the underlying Why such an event should manifest itself

mantle, or alternately an influence of as surface volcanism 70 m.y. later is

mantle dynamics on vertical crustal move- enigmatic. A number of scenarios can be

ments. Colorado Plateau uplifts and conjured up explaining the significance

’basins (Woodward, 1973) evoke compressional of such tectonic sags with respect to

tectonism, crustal folding, and lateral an alkalic suite in a continental terrain,

displacements caused by regional stresses but such models arc futile until the

on the Colorado. Plateau. Whatever their geophysical nature of the structure is

origin, the Acwa sag indicates that such better, documented.

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/ I J

Figure 3. Structural contours in thousands of feet referenced to the

base of the Dakota Sandstone (after Kelley, 1955). The Mount Taylor

field lies directly on the Acoma Sag, suggesting a fundamental relationship

between the two features.

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occurs strictly as the earliest in EARLY ALKALIC ROCKS V the series, and trachyte occurs as the

The oldest lavas within the mapped part latest flows in the series. The limited .. of Mesa Chivato, basanitoid and hawaiite, area, over which these erupt occurred

'2 . conformably overlie rhyolitic ash and ('

unconfonnably overlie sediments of the' &uption both strongly suggest that the

Mesa Verde Group. Basaltic rocks of rocks evolved from a single magmatic

similar composition have been reported source, which was successively sampled.

among the, earliest flows in the southern Basaltic and hawaiite cinder cones % end of the field (Lipman and Moench, 1972; and fissure eruptions were distributed

Baker and Ridley, 1970), indicating that throughout the area, whereas

basanitoid lavas may have been the pre- of mugearite, benmorite, and trachyte

cursors of alkali basaltic volcanism domes was largely concentrated in a

throughout 'the field. As there are no complex in ,the center of the field in

known rhyolitic vents in the northern which central concentration increased *--- . 3 part. of the field, tVe"rhyoliti6- .ash with more silicic eruptions. 'Dome- . C..P . . underlying the basanitoid -o€ :tahe no'rthern .- ?.:.I. building ended abruptly with explosive field is probably akrfall. materrla1 from * eruptions:of both dark, aphanitic trachyte %p.;y.-old d%mes.,near Mount flows and fluid, dark trachyte 7 -* Taylor (Bassett and other,s, 1963). lavas from summit craters on domes, as i 3. * 'JS,? Basanitoid flovs were succeeded in strati- well as from near-like vents and craters

graphic order by a complete alkalic suite peripheral to the dome complex. . These

of hawaiite, mugearite, benmorite, and peripheral vents lie on a circle about

voluminous trachyte (terminology from 10-km across, enclosing the area of most

Irvine and Baragar, 1971). Although intense alkalic activity. The annular

there is local interfingering of inter- distribution of explosive craters suggests

mediate compositional types, basanitoid that the'eruptions were controlled by a

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ring fracture, but there is no obvious those of the district (Lorenz, 1973)

subsidence of the interior, nor any other formed during this time (Fig. 4). Raised

evidence indicative of three-dimensiofial rims of the maars consist of comminuted

structure. Alternately, the circle may clasts of basalt, mugearite, trachyte,

outline the extent of a former trachytic sandstone, and chert, in a of ,.

chamber, the late-stage explosions * fine-grained juvenile ash and cinder.

resulting from a volatile-enriched magma Several maars were traversed with a proton

comprising the last liquids of an evolved precession total field magnetometer and

* basaktrachy te system. yielded Evidence of magnetic structure

possible indica,tive of ring dikes at LATE ALKAL~ BASALT AND HAWAIITE depth (Crumpler and Aubele, 1979). Dikes

Subsequent to denudation of early with this configuration could result from

cones and minor lateral scarp retreat the instrusion of basalt along lateral f along the margins of the field, volcanism fractures of the 'pipe created during sub-

.was renewed with' eruptions of porphyritic sidence of the interior of the maar.

alkali basalt and hawaiitc from prominent This hypothesis is supported by the obser- c, linear fissures, maars, and pit craters. vations of Aubele (1976) at Montoso maar,

A basalt flow on,the northern end of Cesros del Rio Field, New Mexico. A the field dated at-. 2.8 .I. .02 m.y. B.P. description of these' and other newly dis- (Bachman and MehnwL, 1978) was probably covered maars in New Mexico is given in r erupted early in this late alkali basalt Aubele and others (1976).

stage. Preliminary Field data suggest Several circular and elongated non-

that there were two phases of late alkali explosive subsidence craters, or pit . L basalt activity, interrupted, by further craters (Fig. 5), were the sources of

erosional scarp retreat and minor faulting. the more voluminous porphyritic late

Overlapping and nested circular maars, alkali basalt flows. These appear to be , with surface expressions comparable to I common in the Mount Taylor Field, because

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-- Figure 4. Two of the six identified maars in the field over1a.p ove? a northcast-

southwest-trending fissure to form the Alejandro maars no. 1 and no. 2 (center

and foreground, respectively). Altdough the floors are filled with alluvial

and lacustrine sediments, many of the maars in the field show surface morphologies

*, similar to the much younger maars of the German Eifel district. The distance,

from rim to rim in both maars is about lekm. The truncated cone of the

Mount Taylor composite volcano (elevation 11,301 ft) lies in the distance, .* and the hill at right center is the southernmost of the cinder cones of the ., Cerros de Alejandro fissure (see Figure 2).

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Figure 5. Laguna Cerro pit crater, a 500-m-wide, 1 km-long non-explosive. crater elongate along a northeast-southwest-trending fissure, is the smallest - of several c'ollapse craters throughout the field. Vegetation in the photo is

dominantly ponderosa pine: In the.background, the edge of Mesa Chivato slopes * 2 500 m (1;500 ft) down'to the surrounding desert (8,300 to 6",800 ft in elevation).

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several more have been found beyond the defining successive surfaccs

mapped area. of craters formed during accumulation of b

Nonporphyritic hawaiite forms a prom- ' the monogenetic cones are now exposed by

inent. row of. cinder-and-spatter cones on down-wasting of loose from the

the western side of the field, the Cerros summits. This is typical of cinder cones

de Alejandro. Together .with the adjacent in the 0.5 to 2 m.y. old age range .

Alejandro maar, they are interpreted as throughout New Mexico. An interesting e the most youthful eruptions in the field. aspect of the erosion is that, with few

Locally, hawaiite flows from these and exceptions, t.he -facing slopes are

other cones have descended escarpments much more eroded and shallow than the .... cut into older basalt on the edge of the north slopes. Given that the initial

field, forming extensive flows on lower slope angles of most cinder cones are

surfaces such as El Dado Mesa. Thus, the between 25" and 35", a study of these

late volcanism probably contined while and other cinder cones could provide a ,

erosion continued to cut back the edges baseline for geomorphologists interested

of the field. Sim>lar relationships are in azimuthal variations in erosion rates

I I seen in- basalt flows near Mount Taylor of hillslopes in loose materials.

(Moench and Schlee, 1967; Lipman and Moench, 1972), suggesting that the field

constituted a regional topographic high Mineralogically, the rocks of the

through the Pleisocene, in which flows alkalic suite are relatively simpie

were eiupted on surfaces graded to an (Table 1); aside from , ,

ancestral Rio Puerco. , and opaque , trace

Some indication 'of the state of ero- amounts of phlogapite occur in the early

sion of the late basaltic cinder cones is basanitoid flows, and as much as 1 modal

given in Figure 6. The general config- percent of pieochroic turbid brown

uration is one in which inward-dipping occurs in mugearite lavas. Several

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I

bomb8 cind8r

Figure 6. Schematic section of a typical kinder cone, showing the degree of erosion seen in many of the cones throughout the field. Note that in

general the loose cinder has been stripped from the upper slopes and has

exposed inward-dipping agglomerate and s,patter. The interior configuration

is based on observations of volcanic necks in the adjacent Puerco River.

valley and on cinder cones n'aturally half-sectioned on the edge of Mesa

Chiva to. I

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1 2 3 4 5 6 7 8 -9 10 11 An r70 55 ------

- 65 Andeslne 62

-.

Ollgoclnre 20 30 68 81 10- SQ 92 0 ;*;

Olivfne . 1.;10 tr tr - 14 9 8 8 1:.I5 ?O 9. ; ;- ; Awlte 10 15 7 tr tr tr -. 13 4 16 19

Opaque nin. 20 10. 12 10 14 8 - 42 23 14 17 Accessoy Kin. tr(phl3g. 1 l(apatite) - .- - - - -

&gOCWIt6/ 8 - - - tr tr 20 20 13 tr

Note: 1, beranitoid (Bampk 1, fig. 2); 2, hvaiite (rwle 4, fig. 2); 3, mugenrite (ample 6, Pig.2);

4, burrnorite (snaple 8, ?lg. 2); 5, broun tmchyte (rmle 10, ?@. 2); 6, rblte trachyte, plane &-pen-,- dlcular to foliation (rqle 15, fig. 2); 7, d.rk fmchyte (rample 16, fig.%!); 8, lvgscystlc maflc alkell

bsealt (sample 21,'fig. 2); 9, porpbyr ltlc rlhll brarlt (aqb24, ?I&2); 10, hswllte (ramplsr25, ?ig. 2);

11, elk811 brralt tnnsltlaml to bswiite (rqb 36, fig. 2). Plngloclrre caaporltloon determined OD the

unlverral rtnge wing the "6 - n0~1"technique. Bample locality rme am raar raqhs lirted for Table 3.

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varieties of trachyte are recognized; of the ultramafic nodules implies that they

their characteristic colors probably are fragmented nodules. Similarly,.. the relate to differences in and appearance of olivine, clinopyroxene, and

glass contents, as whole-rock analyses plagioclase megacrysts in the, late alkali

r suggest. Brown (Stony) trachyte commonly basalts could conceivably have occurred ,- forms extensive flows; white trachyte when magmas encountered and entrained -

forms most of the domes; and dark (micro- cumulates of the preceding alkalic suite.

crystalline) trachyte occurs exclus-ively The detailed observations necessary to

as flows, spatter, and ash from late- discriminate mantle versus cumulate b stage craters. sources for the megacrysts are not

Within the alkalic suite, lherzolitic available. The lack of compositional

inclusions are confined to the early zoning in the megacrysts, typically

basanitoid; and rare sandstone and basalt rounded and occasionally corroded rims,

fragments occur in late trachyte dome implies that they are not low-pressure

lavas. With the exception of a few percent phenocrys ts .

of olivine phenocrysts in the basanitoid, The associated with this

the alkalic suite on the whple is devoid second phase of volcanism are generally

of phenocrys ts . more voluminous than the porphyritic

Late alkali basalts are, however, basalts and are characterized by small

strongly porphyritic and contain amounts of olivine phenocrysts and rare

and megacrysts of clinopyroxene, plagio- dunitic clots, but contain no clinopyro-

clase, and olivine. These include alumi- xene megacrysts.

nous titanaugite with occasional ophitic Petrochemistry plagioclase and resorbed mega-

crysts (Table 2), along with lherzolitic Chemical .analyses for all major rock

and pyroxenitic . Similarity types are given in Table 3. Major elements

of these megacrysts to constituent minerals in the alkalic suite'(ana1yses 1 through

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TABLE 2. HICROPROBE ANALYSES OF MEGACRYSTS

1 2 3 4 5 6 7 ~~~~ sio2 48.4 47.8 47.9 47.06 54.7 57.7 52.54 Ti02 1.5 - 1.5 1.8 1.82 n.d. n.d. n.d.

A1203 8.9 8.8 9.2 7.77 28.3 25.9 3.1.26 cr203 tr _.0.0 0.0 tr n.d. n.d. n.d. FeO 8.4 8.3 8.2 9.32 0.4 0.2 n.d. MO 0.2 0.2 0.2 0.20 n.d. n.d. n.d. w 13.7 13.6 12.7 13 52 n.d. n.d., n.d. CaO 17.8 17.4 18.4 19.33 10.0 7.0 12.34

NaeO 1.3 1.2 1.3 0.33 6.1 7.5 3.55 n.d. 0.4 0.42 %O n.d. n.d. n.d. 0.5 SrO n.d. n.d. n.d. n.d. 0.3 0.5 n.d.

Total 100.2 98.8 99.7 99.35 ' 100.2 99.3 99.11

~~ Note: 1, core of 1 cm augite; 2, rim of augite in # 1, from spatter of cone on the south end ofthe Cerros de Guadalupe; 3, from the San Carlos ultramefic locality, Arizona, A209 micropmbe standard, the Institute of Meteoritics, University of New Mexico; 4, pyroxene from basalt on the East Grants Ridge near bunt Taylor, wet chemical deter- mination, Hunt, 1938; 5, 1 cm andesine megacrysts in megacrystic basalt flow from cone at the north end of the Grande Ridge, Figure 2; 6, 1 cm andesine in early hawaiite flows north of the Ia Jolla Alfalfa crater; r) I, plsgioclese in basalt of the East Grants Ridge near Mmnt Taylor. wet chemical determinatim. Hunt, 193P.

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alkalic rocks, and a tabulated comparison implks relatively small amounts of partial

of selected rocks is given in Table 4. melting of a.deep undepleted mantle source,

K20/Na20 is characteristically less than as suggested for their oceanic counter-

0.5 (Fig. 7), and the sodic (Hawaiian) parts (Sun and Nesbitt, 1977). Conditons

alkalic suite terminology is used accord- for the origin of basanitoid.are thought

ingly (Irvinc and Baragar,, 1971). Only to be represented by relatively small '

a few flows of "silicic alkalic basalt" degress of at depths as

(Lipman and Mehnert, 1975) are present in much as 80 km in the presence of ,

the area, and oversaturated basalts are and consequently the occurrence of

totally absent. Variation in major and in .t.he .early basanitoid may

minor eLements (Fig. 8) illustrates reflect volatiles inherited at the time

typical alkalic lineages (for example, see of mapma generation.

Batiza, 1977; Kuno, 1968). Note on Volume Relationships The alkali basalts .of the alkalic

suite and the late alka-lic bas'.alts and The origin of the gap between hawaiite'

hhwaiites are unusually enriched in Ti02 and mugearite is unknown, but one

and total FeO relative to the average possibility is as follows. Early basalts 1 Basin-Range basalt (Leeman and Rogers, are exposed because they are abundant, * 1970), vhich characteristically contains widespread, and mor,e likely to rkmain

only 1.5% TiO? and 10% FeO(T) . Because uncovered, whereas are more

of the over-all similarity of analyses in frequent in because they were

Tabie 4 and thc anomalously high Ti02 extruded last. of mugearite,.

and FeO content$, the Mount Taylor rocks being neither initially abundant nor

are more akin to oceanic than typical extxuded last, would therefore be less

Basin-Range vblcanic s,uites.' This may likely to be exposed than either early

be typical of cratonic and continental basalt or late trachyte. Some credence

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12 3 4 .5 67 89 ban noitold h.nllte weerite kmrlte u.53 45.73 45.90 u.36 u.43 53.72 9.v %.61 57.89 3.27 3.80 2.85 2.U 3.42 1.38 1.52 0.86 l.ok 14.9 17.4 15.7 17.0 16.8 17-5 16.9 18.1 18.1 U.89 12.45 13.16 11.94 11.72 9-33 9-13’ 7.b0 7.97

4.46 2.51 4 .68 4.31 4.97’ 4.39 3.P4 1..% 3.26 9.88 10.19 8.95 8.06 7.25 5.38 6.21 5.6b 5.ok 6.71 6.77 4.53 3.85 4.60 2.d 2.P2 1.28 1.52 . 8.~7.78 7.50 6.57 7.19 4.02 4.19 2.90 2.96 Na2O 3.79 3.82 4.14 4.k3 3.97 - 5.88 6.37 6.55 6.8?

%O 1.66 1.69 1.93 2.11 2.07”. 2.67 2.77 3.’40 3.18 MI0 0.170 0.170 0.178 0.174 0.190 0.170 0.170 0.210 0.?09

st$+ 0.40 0.84 0.56 1.a 0.87 0.22 0.76 0.55 0.13 h H20 - 0.14 0.11 0.14 0.26 ’. 0.13 0.14 0.lk 0.07 0.08 P2O5 0.74 0.71 0.80 0.95 0.9 1.26 1.40 0.9 0.95 BSO o.ok7 0.09 0.~8o.& 0.110 0.130 0.140 . 0.180 0.150

sm 0.m 0.063 0.691 0.097 0.090 0.169 0.168 0.130 0.l20 mtal 9.e 101.7 98.5 ‘99.8 101.0 99.1 100.3 99.k 101.5

Q 0 0 0 0 0 0 0 0 0

ir 10.4 10.0 11.7 12.2 l2.k 16.1 16.3 20.1 18.3 s ab 21.8 24.6 26.2 35.6 36.2 49.3 58.3 %.9 58.3 an 15.3 25.4 18.1 20.3 22.1 13.8 9.2 8.5 9.7

ne 8.k 6.0 5.1 1.4 0 * 2.4 0.6 1.6 0.6

dl 24.0 7.2 17.2 6.1 6.7 ‘(.5 10.7 0,.0

hy 0 0 0 0 3.9 0 0 0 0

01 10.3 18.1. 4.k 9.0 6.8 8.1 9.*3 7.6 6.6 11 4.9 5.3 5.5 4.7 4.8 2.0 6.0 . 1.2 1.h , 5.4 2.6 6.3 5.8 5.3 2.8 6.6 2.1 2.9 89 0.2 0.6 2.0 2.0 2.0 1.5 1.4 1.B 1-9

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TABLE 3. (Continued)

10 11 12 13 14 15 16 17 18 19 Lte Dbmvn" "vhlte" "dnrk" "Iiii ~1c ,, trachyte tmchyte trmchyte 01knI.i brrrlt mo;, 59.52 59.34 60.39 60.60 61.38 61:lg 61.37 49.14 9.22 50.09 ?lo, 0.51 0.62 0.56 0.34 0.M 0.26 0.34 2.54 2.40 l.5P A1203 17.4 17.6 17.4 18.0 18.8 18.4 18.2 17.1 17.7 15.6

re% * 6.18 6.70 6.15 6.12 5.97 4.70 5.81 11.15 10.51 11.20 re2% 6.59 4.23 3.s 6.75 5.80 5.00 2.72 ,8.W J.37' *3#

re0 0.25 2.89 2.68 0.05 0.75 0.20 3.36 3.50 3.88 7.61

0.87 0.63 0.70 0.36 0.48 0.18 0.29 5.53 4.9 5.72 CeO 2.00 1.76 1.64 1-54 1.73 1.44 1.04 7.53 7.10 7.56 -r. I na20 7.20 6.46 6.46 7.34 6.62 6.46 6.93 3.55 4.15 3:b. I 3.W 3.60 3.48 3.94 3.76 3.85 4.73 1.57 2.00 1.65 %O mo . 0.221 0.200 0.189 0.220 0.210 0.180 0.209 0.165 0.162 0.160 Hp', 0.53 0.51 0.10 0.40 0.20 0.27 0.k8 1.16 0.90 0.79

K20- 0.11 0.15 0.03 Of16 0.10 1.23 0.02 0.04 0.09 0.17 '2'5 0.81 0.85 0.71 0.k: 9.39 0.53 0.32 0.66 0.66 0.69 B80 0.220 0.200 0.160 0.128 0.1~0.130 0.130 0.080 0.084 0.056

Bro 0.m 0.080 0.080 0.055 0.070 0.050 Total 100.1 93.1 *i 100.3 100.8 99.4 100.6 101.3 101.4 99.5

P 0 4.8 6.0 0.2 4.7 k.8 0 0 0 0 c or 22.6 21.5 20.8 23.4 22.0 23.0 27.5 9.4 11.8 10.0

rb 62.8' 9.5 58.5 61.8 58.9 58.6 61.1 32.2 37.3 34.9 an 3.7 3.2 3.5 4.7 6.0 3.7 3.0 26.2 23.8 21.4

ne 1.1 0 0 0 0 0 0 0 0 0

di 1.3 0 . 0 0 0 0 0 6.0 6.0 10.8 hy 0 2.8 2.9 0.6 1.3 4.1 2.1 12.k 3.1 5.5 01 4.2 0 0' 0 0 0 0:4 4.0 9.2 11.7 il 0.7 0.9 0.8 0.2 0.1 0.k 0.5 3..6 3.3 2.1 d 2.7 4.5 4.1 0 2.0 1.9 1.9 4.8 4.1 2.8

8P 1.7 1.8 1.5 1.0 0.8 1.1 0.7 1.4 1.4 1.L

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20 21 22 23 24 25 26 27 28 late late “rg.cmtic” gorpwitic br181t baaalt late hnwailte 45.73 45.57 45.u 45.97 48.09 47.00 49.50 49.05 k8.34

2.84 3.40 ’ 3.23 2.16 2.74 2.54 2.u 2.86 2.76 14.7 15.2 13.7’ 14.3 15.8 15.2 15.2 17.4 16.1

12.71 12.81 13.22 12.53 12.10 13.53 12.52 12.39 12.65 U.41 4.16 5.82 5.69 6.00 5.70 4.84 4.06 4.59 8.74 9.07 7.98 7.41 6.70 8.40 8.16 8.74 8.52 7.69 8.00 8.34 8.52 6.67 5.84 6.77 4.41 5.2L 9.67 9.38 10.37 9.26 8.55 7.63 8.37 6.89 7.67 3.29 2.79 3.04 4.11 3.44 3.56 3.67 4.26 4.10

1.9 1.37 1-32 1.59 1.52 1.58 1.33 2.a 1.58 0.162 0.19 0.166 ‘0.178 0.173 0.174 0.175 0.181 0.165 0.37 1-56 0.55 0.20 0.13 0.53 0.13 0.21 0.52

0.12 0.09 0.05 0.08 0.02 0.10 0.18 0.13 0.07 0.79 0.71 0.64 0.96 0.94 1.55 0.71 0.94 0.8s.

, 0.09 0.060 0.042 O.d@ 0.060 0.062 0.049 0.108 0.052 sro . 0.080 0.085 0.075 0.076 0.079 0.132 0.065 0.100 0.l28 Total 100.2 101.9 99.5 100.6 100.9 100.5 101.6 101.4 100.1

P.0 0 0 0 0 0 0 0 0- or 9.2 8.2 7.8 9.5 9.2 9.5 7.8 12.1 9.3 ab 22.6 25.4 20.4 18.3 31.2 25.7 30.9. 37.5 36.8. an 20.9 25.1 19.7 15.8 23.9. li.9 21.1 22.3 20.8

ne * 4.3 0.1 2.8 8.8 0 4.3 o 0.5 o dl 18.3 14.1 22.1 19.8 10.8 13.7 13.0 4.9 6.8

by, . 0 0 0 0 3.7 0 3.1 , 0 1.7 #. 01 - 14.5 16.5 12.9 l6.2 13.5 15.1 ig.8 l2.6 13.3 11 4.0- 4.8 6.1 4.1 3.9 10.2 4.7 4.0 3.8 d 4:6 4.4 7.0 5.3 1.9 5.0 5.8 4.2 4.4

aP , 1.7 . 1.5 1.7 2.4 2.0 3.7 1.7 2.0 2.9

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29 30 htcrt b8re It- tlWl#itiOllO 1 to hniltc

81% u.40 k7.10 , 2102 2.73 2.90

AlS3 15.4 17.7

Te+ u.74 13.21 "203 6.62 3.35

?to 6.78 10.20

&Q 6.97 5.ko cso 8.ig 8.02

%O 3.66 3.78 %O 1.80 1.35

MI0 0.171 0.165 5- 0.40 0.09 %O- 0.18 0.07

'2'5 0.77 0.62 BOO 0.09 0.049 BrO 0.084 0.068 - total 100.2 100.9

Q 0 0 or 10.6 8.0

nb 29.0 31.0

ID 21.0 27.4 oe 3.0 1.8 dl l2.4 7.0

hy 0 0 01 13.9 15.9 I1 3.9 k.1 Bt 5.5 3.5 8P 1.7 1.3

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Note: 1, 2 basaniroid spatter on south flank of cone. 3, 4, 5 hawaiite: 3, 4, from "window" in Cerros de Guadalupe trach*%e dome complex, 5 from base of flows above contact with sandstone. 6, 7 mugearite: flow east of Grande Ridge, 'S, 9 benmorite: 8 from extensive flows north af Cosme crater, 9 from promitory on edge of mesa. 10, 11, 12 ''brown trachyte: 10, 11 from lows adjacent to La Jolla Crater, 12 from "window" on north end of Cerros de Alejandro. 13, 14, 15 "white"trachyte: from flows/dome in the Cerros de Guadalupe trachyte dome complex. 16, "dark" trachyte: trachyte spatter from south slope of Cerro Montoso trachyte dome. 17, 18, 19 "silicic" alkali basalt: 17, 19 from flows north and northwest of Cerro Montoso, 18 from west side of Cosme crater. 20, 21 megacryst-bearing alkali basalt: 20 from flow terminus north of young cone at north end of Grande Ridge, 2l.from spatter, south flank of same cone. 22, 23, 24, porphyritic alkali basalt: 22 from near head of canyofi'on sobth side of Mesa de la Vereda Piedra Bldnca, 23, 24 from head of canyon on south side of Mesa del Dado. 25,'26, 27, 28- late hawaiite: 25. 28 from flank of flows southeast of Cerro Montoso, 26 from flow edge south of Cerro Pit: crater fissure, 27 from flow at south end of Cerros de Guadalupa. 29, 30 latest alkali basalt transi- tional to hawaiite: 29 from flow west and erupted from Cerros de Alejandro fissure, 30 from spatter at summit of cone on north end of Cerros de Alejandro.

Analytical technique: A1 0 ,Ti02, K 0, Na 0, CaO, FhO, 23 2 MgO, FeOT, SrO and BaO were determined by atomic agsorption spectrophoto- metry, against the UEJN B-1 basalt standafd. SiO? was analyzed gravi- metrically, P 0 by colorimetry,.and H 0 qnd FeT3 by titration bet' 5 2 and after: heaging in a furnace. H30- was found by the weight loss .on heating the sample in a low temperature oven.

Before calculating CIWP norms, the excess Fe 0 has been converted to FeO when Fe 0 was gre2t-er than, or equal 1.3 Ti02% as suggested 3 3 by .Trvine and 6aragar (1971). High Fe203/Fe0 ratios seem to characterize trachytes world-wide.

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I

1 2 3 4 5 '. Kr H Kl! H m H Kr H Kr H

~~ ~ ~ ~~ ~~ Si02 '45.73 45.14 48.43 47.26 53.72 53.22 56.61 57'.97 61.87 62.02

Ti02 3.80 3.04 3.42 3.58 . 1.38 1.81 0.86 1.06 0.34 0.31

Ak03 17.4 13.5 16.8 17.2 17.5 17.7 18.1 18.6 78.2 18.7

Fe203 2.51 3.60 4.97 2.87 4.39 2.58 1.96 1.82 2.72 4.30 PeO 10.19 9.27 7.25 9.36/ 5.38 6.14 5.64 4.81 3.36 0.10 -. 430 6.77 10.02 4.60 5.08 2.04 2.79 1.28 1.95 0.29 0.40

CaO 7.78 10.60 7.19 7.82 4.02 5.39 2.90 3.32 1.04 0.86

Na20 3.82 2.24 3.97 3.50 5.88 6.00 6.55 6.74 6.93 6.?0

K2O 1.69 0.80 2.07 1.40 2.67 2.28 3.40 2.79 4.73 4.93

MI0 0.17 0.18 0.19 0.22 0.17 0.21 0.21 0.24 0.20 -

'2'5 0.71 0.26 0.94 0.77 1.26 1.08 0.90 0.54 0.32 0.24

~ ~~ Note:* Source of Hawaiian analyses, 1. Basalt, Hamaku Series, Mama Kee, Hawaii (Mecdmeld and Katsure, 1964, table 4, no. 4), 2. Hawailte, Heleekale, Maui (Mecdonald and Ketsure,' 1964, table 4, no. 7), 3. Mugearite, Hsleakele, Maui (Mecdonald, 1968, table 2, no.8), 4. Benmorite, (Mecdonald end Ket6WB, 1964, table 6, no. 27), 5. Trachyte, Hualelei, Hawaii (Mecdoneld, 1949, p.78, no.8).

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-7

6

5

4

3

2

Nap0 %

HAWAII i GOUGH rn

MT. TAYLOR FIELD 0 TRISTAN DA ClJNHAA

Figure 7. K O/Na 0 diagram illustrating the sodic affinity 22 of the Mount Taylor field and Hawaiian alkalic suites as

compared with the potassic suites of Gough Island and Tristan

da Cunha.

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0

.Y

Figure 8. (A) Alkalies-silica plo~f& all of the analyzed Mount Taylor

field rocks of Mesa'Chivato. The dotted line separates the alkalic and subalkalic

fields of Irvine and Baragar (1971) and also approximates the dividing line

between th? alkalic and tholeiitic fields of Macdonald and Katsura (1964).

Circles = early alkalic suite; crosses = late porphyritic alkali basalt and

hawaiites. The Mesa Chivato lavas are entirely alkalic. B = basanitoid,

H = hawa'iite, M = mugearite, Bm = benmorite, T = trachyte.

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0 6

4 0. I I

0 o\" 2.Y1 0 . 0

lop 00 -1

30 20 10 0 30 20 10 0 ,s.1. ,. S.I. Figure 8. (B) Major and minor elements plotted against solidification index

[(MgO x 100)/(Mg0 + FeO + Na 0 + K O)] for both the early alkalic suite and late T 2 2 porphyritic alkali basalts and hawaiites. Open circles average Basin-Range basalt

of Leeman and Rogers (1970); the closest Mount Taylor field equivalent is silicic

alkalic basalt. Solidification- index is used here to facilitate comparison with

other recent works using this or similar MpO-based variation plots. Qualitatively,

SiO .increases with decreasing 'i.9. spite of rocks reported here. 2 3.1. Figure 8 (a) is continued on the following frame.

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1 I 1 0 Si02 - 6C 0. 0 0

0 0 0 5a 8. O.0 0 ao0. 00 0.8 0 0 0

0 p205 1

0 SrO ,...

.1

a

a b. 30 20 10 0 S.I.

\ Figure 8. (B) (Continued)

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is given to this by the occurrence of various magma evolution schemes. Since

mugearite and benmorite blocks in the about one part of trachyte can be gener-

A Alejandro maars east of the Cerros de ated by five parts of parental basalt in

Alejandro, an area surrounded by late a fractional crystallization model, one

alkali baslats, thus illustrating that would expect decreasiqg abundance of

some of the early alkalic rocks are buried silicic rocks in the volcani? record if

and unavailable for sampling. Elsewhere such a process were responsible for the

in the field, small residual mesas of observed suite. Therefore, where

early basalt lying a few metres above decreasing abundance with increasing

the local land surface are evidence for silica is not seen,. or where trachyte

at least some erosion of the earliest far exceeds basalt 'in volume, as it the

basalts. case of the East African Rift, it is

About 3 km of trachyte is exposed, often concluded that fractional crystalli-

which from a theoretical.. 1:5 volume ratio zaticn was not responsible for the suite. (Mukherjee, 1967) suggests that about However, volume ratios are non sequitur,

15 km3 of basanitoid magmas were gener- because the amount of given magma type

ated at the onset of alkalic volcanism'. generated and emplaced at depth, and

As trachyte is more voluminous in out- fractionated, says nothing about the

crop than) basalt, either the "missing" amount of the liquid whi:h reaches the 1 'voluke of basalt suggested above -was surface and gets extruded. For example,

-\ never erlpted, or it was buried by the it is conceivable that a basanitoid melt

subsequent volcanism in the field. The . 'could be emplaced in the upper mantle,

significance of volume relations seen in subsequently fractionated to tracliyte,

the petrologic members or rock types of and erupted as trachyte without a single

a volcanic suite at the surface is ques- eruption of basalt ever occurring. The

tionable, although many authors have used . basis for the abundance ratio arguments

it in the past as evidence for or against seems to be probability; the more melt

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that is generated, the more likely it be, we can base our speculations about

should be for some of it to find its way magma genesis only on geochemical models

to the surface. Presumably, if each for the time being. Therefore, the

unit volume of magma, regardless of evidewe skt forth for a

composition, is given the same probability fractional crystallization origin of the

of reaching the surface,.then the amount Mount Taylor Field-Mesa Chivato alkalic

at the surface will reflect the initial suite is geochemical.

volume at depth. Unfortunately, this MACMA EVOILUTION need not be true, and the real question

that must be answered before such an Sr and Ba trends are suggestive of

assumption can be made is as follows: fractional crystall izat in the a1kalic

Are silicic rocks or basaltic rocks more suite, as exemplified by Zielinski and

easily delivered to the surface from a Frey (1978), Bishop and JJooly (1973),

deep magma source region? and Weaver and others (1973), for the

Volcaxiology, magma dynamics, and Gough Island, Marquesas ,- and ._

physical processes in general have been * East African suites, respectively. The

less studied in recent decades than geo- Ba depletion iq the last trachyte is in

chemistry' and petrology, and our ignor- .accord with removal of early crystallized

ance of the -mechanics of volcanism is Ba-rich alkali from late

reflected accordingly in simplistic think- t'rachytic liquids during fractional

ing such as that illustrated in arguments crystallization; the Ba-depleted trachytes

using magma volume ratios. Geochemical were the most explosive and potassic,

and petrologic arguments based on volcano- as well as the last rocks of the suite L logic phenomena must be viewed with that were erupted. . Sr enrichment prob-

caution until our knowledge of volcanology ably occurred when removal of clinopyroxene,

is comparable to our knowledge of geo- plivine, and calcic plagioclase left a chemistry. However dissatisfying it may Sr-bearing, sodic and potassic ..

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liquid species (mugearite) ; decreasing Inclusion of sandstone are particularly

plagioclase content with evolution of common in the late alkali basalts; but

more alkali feldspar content thereafter only three cases of accidental inclusion

depleted the Sr from the trachyte liquids. were found in trachyte lavas; two inclu-

sions were petrographically similar to the Isotopic Data early basalt, and one was of recrystall-

e7Sr/e6Sr data (Table 5) show minor lized arkosic sandstone.

variations between the various rock types CONCLUSIONS of the alkalic suite within the accuracy

of the determinations. The anomalous mug- An alkali basalt through trachyte

earite value is enigmatic, especially volcanic suite is documented from the

considering that the high Sr content of Pliocene-Pleistocene Mount Taylor vol-

mugearite the world over (for example, canic field. Available geochemical evi-

Hlava, 1974) makes it relatively insen- dence is consistent with the interpre-

sitive to contamination. Basalts of the tation that the suite evolved by

later porphyritic suite have characser- fractional crystallization of a basaltic

istically higher 87Sr/86Sr ratios, 'and parent magma, and that samples of this

it is suggested that the higher ratios . magma were erupted during its evolution.

indicdte at least small amounts of con- The occurrence of numerous parcllel

tamina t ion. fissures and faults, all trending north-

Because of the higher visosity of northeast, suggests that deep fracturing

trachytic liquids, complete mixing with of the along the margin

assimilated country rock during ascent of the Colorado Plateau was fundamental

may not take place as effectively in in the empZacemeni; of the volcanism.

trachytic as in basaltic liquids, thus The large volume of basanitoid needed to

increasing the observed contamination of generate the observed quantity of trachyte,

basalts relative to the trachytes. together with the superposition of the

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. 3' hawaiite 0.7053 6 mugearlte 0.7142

8 benmorite 0.7047

11 trachyte 0.7047

15 trachyte 0.7+9 .. 16 trachyte 0.7039

21 ahali basalt 0.7056

27 hnwallte 0.7061*

Note: mrSr separation, values to 2 0.001; values de- temlned in the laboratory of D.C.Bn>okins, Unlv. New Mexico (Brookins, and others, 197).

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~~ ~ P field on a major structural sag, however, types rep)esents a broad cross Secti3n

suggests that therm,al and chemical ano- of ma'ntle mgmagenesis in a spatially

malies beneath the field may have been limited area OF the continental interior,

more important in initiating volcanism a characteristic of Colorado Pl.ateau-

than simple tectonic fracturing of the marginal "3asaltic" volcanic f ields in

lithosphere and accessing mantle melts. general. The alkalic suite shows a

Whether one chooses to include the similarity to volcanism of aseismic

Mount Taylor volcanic field in the Rio oceanic islands, continental rift volcan- , Grandc rift or'not is a matter of defini- ism, and alkalic volcanism in other areas a tion. However, it lies on the topographic with major vertical lithospheric discon-

flank of the rift, as Cordell (1978) has tinuities that is conspicuously rare in

suggested. Tlie petrologic style or the the North American .

Mesa Chivato part of the Mount Taylor ACKNOWLEDGMENTS field is similar to that of other well- r

known , making it the only rift-type This study was -guided by 14. E. Elston

.volcanic field near the Rio Grande rift while the author was at the University of \ province that embodyies fa complete sample New Mexico, and funded by NASA Grant

of the main members of an,alkalic suite. NGR 32-004-062 from the Planetology

The existence of an alkalic suite Program Office. P. I.]. Lipmanhnd H. H.

further substantiates the chemical Mehnert, of the U.S. Geological Survey,

heterogeneity of basaltic Java types in provided preliminary results of field r the Mount Taylor area, which is know to and K-Ar geochronological studies of

include tholeiitic basalts (Lipman and the Mount Taylor field, which were impor-

Floench, 1972; Laughlin and others, 1972), tant in crystallizing some of the ideas

as well as andesitic rocks of Mount Taylor presented in this paper.

(Baker and Ridley, 1970; Hunt, 1938).

Taken together, this range of chemical

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bearing on the origin of the Mt. Taylor REFERENCES CITED volcanic field, New Nexico, U.S.A.:

Aubele, J. C., 1976, PIontoso Naar: A and Planetaiy Science Letters,

detailed view of internal structure V. 10, p. 106-114.

and stratigraphy: Geological Society Bassett, N. A., and others, 1963, Potassiuw

of America Abstracts with Prdgrams, argon ages of volcanic rocks near

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PIANUSCRIPT RECEIVED BY TIIE SOCIETY PMCH 8, 1979.

REVTSET) FIANIISCRIPT RECEIVJD DECEMBER 10, 1979

MANUSCRIPT ACCEPTED DECEMBER 13, 1979

Printed in U.S.A.

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