Geochemistry of Los Humeros Caldera, Puebla, Mexico

Geochemistry of Los Humeros Caldera, Puebla, Mexico S. P. VERMA * M. LOPEZ M.**

Instituto de Geofisica, Universidad Nacional Aut6noma de Mdxico, Ciudad Universitaria, Delegaci6n Coyoacdn, D.F. 04510, Mexico

ABSTRACT Geochemistry of Pliocene to recent volcanic rocks from Los Humeros caldera (19" 3ff N 19° 50'N and 97 ~'15'W - 97"35'~0 in EastCentral mexico is described. The volcanic rocks from this area seem to represent both alkali and high-alumina basalt series, or both calcalkaline and high-K calc-alkaline sequences. The available bulk-chemical analyses (23 this study and 18 from unpublished literature) show that the entire sequence of rocks from basalts to rhyolites are present in this area. Different degrees of partial melting of the source region followed by extensive shallowlevel crystal differentiation seem to have taken place before most volcanic eruptions. These processes are perhaps the most important mechanisms for magma genesis in Los Humeros caldera. Geophysical studies in this area are not sufficient and more detailed geophysical surveys and a better geological interpretation are needed in order to delimit the underlying magma chamber. FIELD RELATIONS T h e area u n d e r study is in t h e e a s t e r n p a r t of the Mexican Volcanic B e l t (MVB; Fig. 1), an E-W structure whose width varies b e t w e e n 20 a n d 70 k m and which extends from t h e Pacific coast to the coast of Gulf of Mexico. T h e b e l t consists of a

* Earlier publications of the author are also under the name of S. PAL. Present address: Departamento de Geotermia, Divisi6n de Fuentes de Energia, Instituto de Investigaclones Electricas, Apartado Postal 475, Cuernavaca, Mor. 62490 Mexico. ** Present address: Department o[ Physics, University of Toronto, Toronto, Ontario, Canada. Bull. Volcanol, Vol. 45-1, 1982

large n u m b e r of L a t e T e r t i a r y a n d Quaternary cindercones, domes and strato-volcanoes (MOOSER, 1972). S e v e r a l h y p o t h e s e s for t h e origin of t h e MVB have b e e n p u t forth b u t m o s t a u t h o r s (e.g., MOLNAR a n d SYKES, 1969; MOOSER, 1972; PAL a n d URRUTIA-FUCUGAUCHI, 1977; DEMANT, 1978) r e l a t e it to t h e subduction of the Cocos p l a t e b e n e a t h t h e N o r t h American plate at the Middle America Trench. A r e c e n t compilation of all the available b u l k chemical a n a l y s e s of rocks from t h e MVB b y PAL et al. (1978) shows t h a t the MVB is largely characterized b y a calcalkaline sequence of the continental m a r g i n type. However, m a g m a s in t h e n o r t h - e a s t e r n p a r t of the belt, which overlaps with a n o t h e r volcanic province ( E a s t e r n Cordillera) are m o s t l y of alkaline affinity. T h e a r e a of the p r e s e n t study is l o c a t e d near, or p e r h a p s in, this region of ~(overlap , . Los H u m e r o s c a l d e r a is also of i m p o r t a n c e due to its g e o t h e r m a l p o t e n tial (YAI~EZ-GARCIA, 1980) a n d several geophysical studies have b e e n m a d e in this area (FLORES-LUNA et a l , 1978; MENA and GONZALES-MORAN, 1978; ALVAREZ, 1978; PONCE a n d RODRIGUEZ, 1978). T h e a r e a under s t u d y is a b o u t 900 k m 2 a n d is located b e t w e e n t h e l a t i t u d e s of 19°30 ' N a n d 19°5ff N and t h e longitudes 97o15 ' W a n d 97o35 ' W. CANTAGREL a n d ROBIN (1979) who have d a t e d the E a s t e r n Mexican volcanic rocks b y K - A t m e t h o d , state t h a t t h e s e rocks belong to two large magrnatic provinces: t h e T r a n s - M e x i c a n Volcanic B e l t ( l a b e l l e d MVB in this work) is m a i n l y of Miocene a n d Q u a t e r n a r y andesitic rocks w h e r e a s the E a s t e r n Alkaline Province (of o v e r s a t u r e d alkalic and

64

S.P. VERMA - M. LOPEZ M.

I10 °

3ff

I00 °

90 °

50°

,.\

.

".

N O R T l-~'" ..... "',

k-kk~.~ PL TE

J*

~ ~

/,,

It

'V,,,

P~.~L,~.,~

AREA\ "

2-."~

~:~

,

,

~1'-,J

GULF

-

.

,

_ ~ r~

}

20~

ERACRUZ

'

I10 °

"

"

'

',~,,~ I 0'0 °

~

L

,

~

/ --

9'o.

x'

W

FIG. 1 -- Location of the study area (the map has been simplified after LOPEZ-RAMoSand SANCIqEz-MEJORADA, 1976; and ATWATER, 1970). MVB is Mexican Volcanic Belt.

undersaturated nepheline-bearing volcanic rocks) is Oligocene to Quaternary in age. However, no K-At dates from Los Humeros caldera have been reported so far. GEOLOGY AND PETROGRAPHY A preliminary geologic map, constructed from a photogeologic interpretation of the area by PEREZ-R~NOSO (1978), is given as Fig. 2 of which sample locations are presented. A detailed geology of a much larger area of Los Humeros-Derrumbadas has been reported by YAI~Z-GARCIA (1980). The following geologic summary is a synthesis of PEREZ-REYNOSO (1978), DEMANT (1981), H. HARRIS (written comm., 1981) and Fig. 2 (this study). A large andesitic shield volcano was formed in Los Humeros during Pliocene. These andesitic rocks are exposed only outside the caldera but are highly altered and therefore not included in the present study. This volcanic activity was followed by the intrusion of rhyolite (Tpr) and trachyte (Tpt) domes (Cerro Pizarro: samples CH41 and CH42 and Cerro Las Aquilas respectively) and by the eruption of Xaltipan ignimbrite (Tpi; samples

HF15 and 902) that are exposed only outside the caldera. The ignimbrite unit represents at least 80 km 3 of magma and its eruption caused the collapse of the volcanic structure forming Los Humeros caldera. YASEZ-GARCIA (1980) postulates that the rise of andesitic magma caused a partial fusion of the sialic crust and thus gave rise to vast amounts of ignimbrites in this and surrounding areas. The next volcanic activity that took place along the NW border of the caldera, consisted of biotite-, pyroxene- and aphyric-rhyolites (Tprl; Cerro Oyameles dome and lavas. samples 1166, 1181, HF238 and HF239). These rhyolites are perhaps the most voluminous rhyolitic rocks (about 4 km 3) in the area. T h e rhyolites of Cerro Oyameles show a porphyritic texture having plagioclase, quartz and sanidine phenocrysts and minor opaque minerals. Sanidine and biotite microphenocrysts are present in a glass matrix. On the other hand, the rhyolites from slightly northerly positions are almost completely aphyric. Intense mafic volcanic activity took place in this area from Pliocene to Recent times (ROBIN, 1976). Augite basaltic andesite (Tpab; samples CH24, CH25 and 115'8) and olivine basaltic andesite (Tpob;

W

30'

25' 20'

902

Fpl

HF15

___~_

h

m

-2

-- e~. ~-- e , . J

z

7_

\

X

P

L

A

RHYOLITE

TRA CHYTE

OLIVI, E BASAI.T1C ANDESITE

i

A

T

I

O

N

IGNIMBRITE

FLOW DI RECTION

INFERRED CALDERA

STRI K E S L I P FAULT

FAIILT SCA~P

INFERRED GEOLOGICAL CONTACT

G EOLOGIC A L CONTACT

• SAMPLING LOCATIONS

[GNIMBRITE

[ ~

T D ~ II J

BASAEFICio RHYOLITtC PYROCLASTICS

BASALTIC Io RFIYOI.ITIC P Y R O C L A S T | CS

RHYOD&CN~E Io DACITE

~

N

ANDESITE Elow s (3) ~

AU ITE BAY*AI.Tto A~C~.NITE Flowl (1 }

FI aws~}

TERRACE DEPOSI I'S

ALLUVIUM

E

FIG. 2 -- Photogeologic map of Los Humeros caldera, modified after PEREZ-REYNOSO (1978) and D ~ A N T (1981). T h e sampling locations are also presented on this map.

19o

97°3ff

O~

0

0 r~

0

~U

oo

O

66

S.P. VERMA

- M. LOPEZ

sample 1171) formed a large volcanic cone. These basaltic andesites show a porphyritic pilotaxitic texture and have zoned plagioclasephenocrysts. T h e matrix is very free-grained in which feldspar microliths are associated with very smA|l clinopyroxene crystals and magnetite grains. Rather thick sequences (6 to 10 m thick) of basaltic to rhyolitic tuff (Tppc; Faby Tuff; sample I-IF76) were deposited in this and surrounding areas (Plinian deposit) and m a y represent about 10 k m s of total deposit. This volcanic activitywas followed by the eruption of andesitic to dacitic ignimbrite (Tpil; Zaragoza ignimbrite; sample HF2) which represents at least 20 k m 3 of magma. These activities suggest the presence of a stratified m a g m a c h a m b e r similar to t h a t inferred a t C r a t e r Lake, O r e g o n b y RITCHEY (1980). All t h e s e rocks (basaltic a n d e s ites, tufts a n d ignimbrite) a r e e x p o s e d only outside t h e caldera. Thus, T p a b a n d T p o b units are located j u s t outside b u t very close to a large semicircular fault caused b y gravitational collapse of t h e volcanic structure. T h e s e activities ( T p a b , T p o b , T p p c a n d T p i l ) s e e m to h a v e p a r t l y e m p t i e d t h e m a g m a c h a m b e r causing t h e volcanic super-structure to collapse t h e r e b y forming a c a l d e r a of a s o m e w h a t

M.

smaller size than the earlier caldera. T h e present Los Humeros caldera is about 400 m deep and is irregularlycircularwith an approximate d~Ameter of 16.5 km~ T h e first stage of development of Los Humeros caldera is consistent with the scheme of caldera formation proposed by SMrrH and B A I ~ Y (1968). After the subsidence of this area a second magmatic phase developed which consisted of numerous smaller volcanic cones such as el Hilillo(samples C H 1 and CH4), San Antonio, Humeros (samples CH11, CH14 and 1167), Calderita (samples CHT, CH9, C H 1 0 and 1175), Morrito Pequefio (samples C H 1 6 and CH18), etc. that are preferentially concentrated on the western half of the caldera and erupted augite basalts to augite andesitos sometimes containing olivine (Qbl flows). This activity represents a volume of about 5 k m s. S o m e rhyodacitic flows also erupted in this area (Qrd; samples 1164, 1165 and 1176). Contemporaneous with Qbl flows several explosive eruptions occurred that gave rise to thick sheets of pumice and ashes (Qpc; Zebra tuff; samples 1161 and C H 6 8 to CH76) and partiallyfilledthe caldera.This tuff unit represents volume of about i k m 3 and can be distinguished from the earlier ==~

SiO2, H20- and L.O.I.(losson originalignition)were determined by gravimetry;Al2Os, Fe (totaliron),MnO, M g O and CaO by atomic absorption spectrophotometry;Na20 and K20 by flame photometry; TiOz and P205 by colorimetry,and finallyFeO by volumetry.Detailsare given by PAL and HERNANDEZ-CHACON (1973) and LoPEz-M. (1977). Precisions and accuraciesrange between 1 and 5%. CIPW norms have been calculatedon anhydrous basis using a computer program written by one of us (SPV), following the method of KELSEY (1965). (--) H. FERR~ (Writ Comm., 1981); (-{-) DEMANT (1981); (*) NIXON (1981) cited in DEMANT (1981). FeO t = total iron as FeO; Mg-value = 100 Mg2+/(Mg2+ -{- 0.9 (Fe z -t- Fe3+)); atomic. (1) CaO. MgO. 2SIO2, (2) CaO. FeO. 2SIO2, (3) MgO. SiO2, (4) FeO. SiO 2 .

SALIC = sum of salic normative minerals (Q + C + O R + A B + A N + NE). FEMIC ~ sum of m~fic normative minerals (DIlVlg + DIFe + HYMg + HYFe q- FO + FA + + MT + IL + HM). C.I. Crystallization index as defined by POLDERVAARTand PARKER (1964); (AN + 2.1570577 DIug + FO + 0.70007617 HYMg). D.I. Differentiation index of THORNTON and TUTTLE (1960); wt% of normative (Q + OR + + AB + NE + LC), S.I. Solidification index (HUTCHISON, 1974); (100 MgO](MgO + FeO + Fe20 s + NasO + K~O)), all oxides in wt% A.R. AlkaliniW ratio (WRIGHT, 1969); ((A120s + CaO + total ~lk~lis)/(Al~O s + CaO -- total alkalis)), all in wt%, However, when Si02 > 50°£ and 1 < (K20/Na~O) < 2.5, then

2(Na~O) is used in place of total alkalis in the expression.

67

GEOCHEMISTRY OF LOS HUMEROS CALDERA, PUEBLA, MEXICO

tuff unit (Tpp; Faby tuff) by the presence of characteristically thin (less than a meter to a few meters thick) alternating layers of basaltic to rhyodacitic and rhyolitic pumice and ashes in the earlier tuff (Zebra tuff). Such a complex succession of dark and light colored pumice and ash layers has been exposed by a recent drain-

TABLE caldera.

age system in the caldera (e.g., samples CH68 to CH76). The last volcanic activity consisted of a maflc a a flows (Qb2 and Qb3 flows numbered in time sequence). These flows (Qb2; samples 935 and HFll7, and Qb3; samples CH27, CH28, CH30, CH31, CH33, CH34, 903, CH39, CH40, CH46

1 -- Major element chemistry and C I P W CHal

CH42

HFI5(')

902(+)

n o r m of p r e - c a l d e r a volcanism, L o s H u m e r o s

High-K

High-K

Hlgh-K

Hlgh-K

rhyolite (Tpr; Pizarro)

rhyolite (Tpr; Pizarro)

rhyolitic pum~ (Tpi; Xaltlpeal)

dacitic pumice (Tpi; Xaltlpan)

1166(*) 118](*) CH24 Hlgh-K H1gh-K Basaltic Aphyric porphyritic Andesfte rhyolite rhyolite (Tpab) (Tprl; (Tpr!; NW border) Oysmeles)

CH28 Basaltic endeslte (Tpab)

andeslte O]:mb)

]/71(``) (TpOb)

73.13

72.86

74,32

68.63

76.72

75.87

54,02

55.68

54.91

56.77

0.09 18.85 0.74 O. 34 0.05

0.09

0.10

0.38

0.09

0.82

1.02

1.37

1.26

12.84

21.77 2.31 2.83 0.08 3.00 9.46 3.24

20.28 1.96 3.64 0.09 3.15 8.56 3.40 1.44 0.24 0.25 0.68

18.75 0.01 4.77 0.14 4.50 8.91 4.03 1.41 0.29 0.05 0,18

17.26 7.49

1.02 0.00 0.17 0.46 3.10 4.94 0.04 0,15 3.18

13.45 0.96 1.41 0.08 0.15 0.83 4.00 4.42 0.05 0.92 4.08

11.97 0.84

0.05 0.25 2.22

13.25 0.61 0.42 0.0S 0.46 0.44 4.42 4.67 0.05 0.19 2.58

0.11 12.20

Sum

99.69

100.09

100.32

(FeOt /

0.89

0.68

0.86

FeOt/MgO 7.77 Mg-value 20.4

2.11 48.5

1158(+)

Basaltic Andesite

Chemical analysis: SIO2 TiO2 A1203 Fe203 FeO MnO

MgO

o.13

CaO Na20 K20 I~2oO5

0,33 4.20

L.O.I.

4.31

1.02 0.03 0.27 0.45

0.06 0.21 0.39 8.73 4.67

3.85 4.75

1.23

0.12 3.69 7.II 4.43 1.66 0.28

0.01 0.54

0.44

2.12

0.28 0.35 0.91

99.36

99.95

100.38

100.30

100.39

99.12

100.07

0.94

0.77

0.78

0.62

0.63

0.53

0.65

6.00 24.8

15.16 11.6

3.40 36.8

3.60 85.5

1.64 54.8

1.71 53.6

1.11 64.1

1.85 52.0

36.54 1.70 30.06 27.02 2.08 . . . . . . . . . . . . 0.44 1.76 . . . . . . . . . . --0.20 0.10 . . . . . . .

27.47 0.74 27.61 35.78 4.01 . . . . . . 0.39 1.39 . . . . 1.47 0.76 0.13

(FeOt-HVIgO))

CLPW norm: Q C OR

32.33 1.85 26.18 36.53 1.3.5

AB AN

~MgO)

DI|Fo(2 )

-----

o:33

28.44 0.25 28.33 38.40 1.91 . . . . . . . . . 1.18

0.22

35.05 --28.24 32.77 1.99 . . . . . . . . . . . . . . . 0.68 . . . . . . . . . . . . . . --0,06 . . . . I. 03

35.91 0.07 28.19 32.24 1.98 . . .

.

. . 0.53 . . . . . . . . --0.13 . . 0.86

4.01 . . 9.80 37.46 22.29 .

MT

1.03

It, AP HM

0.18 0.12 0.0.5

SALIC FEMIC

98.23 1.71

97.33 2.60

97.40 2.49

95.60 4.15

98.05 1.95

98.39 1.55

83.10 16.90

81.04 18,96

73.36 26.65

73.56 25.85

C.I. D.I. S.I. A .R.

1.58 95.04 1.34 3.91

2.74 95.17 4.35 4.65

2.39 93.62 1.84

4.28 90.86 1.37 3.55

2.47 96.06 2.73 4.11

2.35 96.34 2.22 4.05

51.51 41,48 23.79 1.33

46.96 45.03 23.18 1.40

49.88 44.13 29.61 1.49

38.88 51.27 21.37 1.67

:::

. .

5.54 3.58 6.6I 4.89

. . . . 0.91 0.80 0.12 . .

°

. .

6.46 7.55 1.22 . . . . . . . . . . 7.34 8.55 8.48 27.68 28.92 34.48 41.62 36.01 ~.23 . . . . . . . . 2.51 2.96 7.13 0.61 1.17 3.92 6.38 6.52 7.52 1,78 2.95 4.74 . . . . . . . . . . . . . . . . 3.38 2.86 0.01 1.57 1.95 2.65 0.67 0.57 0.69 . . . . . . . . . .

2.17 2.39 0.66 . .

68

S.P. VERMA . M. L O P E Z M.

TABLE 2 -- Major element chemistry and CIPW norm of post-caldera volcanism, Los Humeros caldera.

1167(+) CHll CH14 I173(*) Basalt Andeslte Andeslte (High-K) AndesRe (Qbl; (Qbl; (Qbl; (Obl; (Qbl) Calderlta) Humeros)HUmeros)Humeros)

CI-R H1gh-K Alxleslte (Obl: Hllfllo) Ct~mlcal analysis:

CH4 High-K AndeaRa (Qm; Hflfllo~

CH7 High-K Andeslte (Qb1: CaIderRa)

CH9 H~gh-K Andeslte (Qbl; Calderlta)

CHI0 HIgh-K Andeslte (Qbl; Calderff~)

1175(*) Basalt

SiO2 TiC2 AI203 FegO.a Feb" iVmO MgO CaO Na20 K20 P205 H20L.O.I.

59.34 1.40 16.72 1.85 4.59 0.12 2.63 5.38 5.00 2.27 O. 35 0.19 0.54

59.23 1.41 16.78 2.13 4.32 0.12 2.65 5.50 4, 84 2.21 O. 35 0.19 0.61

61.74 0,86 16.72 1.98 2.84 0.09 1.84 4.43 5.08 3.16 O. 28 0.32 0.42

61.96 0.89 16.62 2.49 1.97 0.08 2.22 4.39 4.86 2.78 O. 25 0.25 0,56

58.25 0.85 17.92 3.15 1.86 0.09 3.43 5,27 4.24 2.18 O. 24 0.99 1.51

48.61 1.40 17.42 10.40

sam (Fe~/

lo0.33

100.34

99.76

99.52

99.98

0.70 2.35 45.7

0.72 2.51 44.1

0.66 1.90 55.1

0.58 1.37 59.1

(Fe(~-MgO) 0.70 FeOt/MgO 2.38 ]v~;-value 45.4

56.37 1.40 17.27 2.67 4.14 0.10 2.45 6.34 3.82 1.73 0.35 1.45 1.91

56.55 1.34 17.25 1.93 4.61 0.09 2.78 6.99 3.65 1.79 0.37 0.91 2.25

57.70 1.51 16.85 8.21

0.16 7.80 10.50 3.65 0.28 O. 15 0.17

48.50 1.70 16.99 0.83 9.11 0.22 8.57 9.90 3.38 0.48 0.22 0.06

100.54

99.96

100.O0

100.51

1CO.50

O. 55 1.20 62.3

0.53 1.15 63.3

0.73 2.67 42.6

0.70 2.28 46.5

0.71 2.49 44.3

---

11.08

9.93

4.59

0.13 2.97 6.11 4.74 1.95 0.33 0.05

CIPW norm:

Q C OR AB AN NE ~=I~ DI_,.~ , ..~Mg n~lFe o~O MI" 11.

6.65 . . 13.46 42.45 16.53 . . 3.97 2.54 4.73 3.48 . . . . 2.69 2.67

AP

0.83

HM

.

.

7.67 . . . . 13.12 41.14 17.61 . . . . 3.97 2.14 4.79 2.95 . . . . . . . . 3.10 2.69

9.12 11.91 I0.01 --. . . . . . . . . . . . 18.86 16.64 13.21 1.65 43.41 41,66 36.78 26.54 13.62 15.52 24.02 30.21 . . . . . . . . . 2, 82 3.78 4.44 0.92 11.13 1.67 0.17 --5.72 2.88 3.54 8,33 --1.46 0.15 . . . . . . . . . . . . . . . 10.15 . . . . . . . . . 6.56 2.90 3.66 3.92 2.17 1.65 1.71 1.66 2.65

0.83

.

.

.

.

0.67

.

.

.

.

0.60

.

.

0.58

0.52

0.35 - --

2.84 26.12 29,80 1.36 9.39 5.23 ----11.92 8.40 1.20 3.23 0.52

10.57 10.85 11.47 33.41 31.69 39,91 25.70 26.06 18.85 . . . . . . . . . 2.46 3.38 4.11 1.15 2.18 3.69 5,17 5.44 5.65 2.77 3.79 5.80 . . . . . . . . . . . . . . . . . . 4.00 2.87 2.16 2.75 2.61 2.85 0.86 0.90 0.78

SA LIC FEMIC

79.10 20.91

79.54 20.47

85. Ol 15.00

85,74 14.27

84.02 15.93

61.22 38.73

60.11 39.90

80.76 19.15

78,54 21.38

74.81 25.04

C.I. D.I. S.I. A.R.

28.41 62.57 16.10 1.98

29.53 61.93 16.41 1.93

23.79 71.39 12.35 2.28

27.58 70.21 15.50 2.13

31.84 bO.O0 23.08 1.77

64.16 31.01 35.25 1.33

61.97 30.32 38.31 1.34

34.63 55.G6 16.54 1.62

37.59 52.48 18.84 1.58

31.34 55.96 16.62 1,82

See foocn~xes Table I for de[afls on analytical methods and explanation of symbols and parameters.

and CH47) vary in composition from augite- and olivine-basalts to basaltic andesites, showings a pilotaxitic texture in which zoned plagioclase phenocrysts are very well aligned and olivine when present, is finely dispersed in the matrix. The lavas were emitted from scoria and cinder cones located near the border fault (caldera rim) and flowed towards the

southern b o r d e r of the c a l d e r a covering an a r e a of over 100 k m 2. T h e total v o l u m e of m a g m a e r u p t e d during this activity is e s t i m a t e d to b e a b o u t 5 k m 3. F u m a r o l i c activity a n d sulfur deposits are o b s e r v e d in t h e oval-shaped (> crater located in t h e southern p a r t of L o s H u m e r o s caldera. T h i s little c a l d e r a ( L a Calderita) f o r m e d as a direct

GEOCHEMISTRY

OF LOS

HUMEROS

CALDERA,

PUEBLA,

69

MEXICO

Continued: TABLE 2 -- Major element chemistry and CIPW norm of post-ca]dera volcanism, Los Humeros caldera. CH 16 Basaltic Andeslxe (Qbt; Morrito

CH 18 Basaltic Andesite (Qbl; Morrlto

1159(*) Basaltic Andeslte (Qbl)

i173(+) Hlgh-K Aadesite (Qbl)

i176(+) Id]gh-K Daclte (Qrd)

1165(*) High-K Dacite (Qrd)

i164(+) High-K Dacite (Qrd)

1161(*) 935(+) High-K Basalt Dacttlc p u m i c e (Qb2)

52.91 1.54 18.39 1.74 6.12 0,14 4.44 7.54 3.65 1.31 0.39 O, 88 1.60

52.52 1.58 18.38 i . 99 6.13 0.14 4.87 7.82 3.60 1.38 0.36 O. 31 0.71

54.94 1.12 17.41 7.62

62.16 0.91 15.62 0.23

68.39 0.59 15.78 3.61

69.63 0.71 13.95 0.36 2.38

67.69 0.43 14.88 2.63

0.13 4.90 9. Ol 3.75 1.31 0.26 0.19

0.12 3.57 5.04 3.55 3,25 0.22 0.21 0.89

67.25 0.60 15.75 0.64 2.59 0.10 0.77 2.02 5.53 3.85 0.12 0.08 0.14

0.08 0,75 1.98 5.53 3,78 0.09

0.08

0.19

0.59 1,55 5.13 4.29 0, i i 0,01 0.26

0.06 0.64 1.40 4,32 4.39 0.05 0.97 2.78

100,65

99,79

100.64

99.37

99.45

100,83

99.05

(FeOt+MgO)) O. 63 FeGt/Iv~O 1.73 Mg-value 53.4

0.62 1.63 54.9

0.58 1,40 58.6

0.52 1.07 65.0

0.80 4.11 32.5

0.81 4.33 31.4

pequeflo)

pequei~o)

(Qpc)

HFI17(-)

Basalt (Qb2)

Chemical analysis: SIO2 TIC2 AI203 F e 2 03 FeO MnO

MEO CaO Na 2 0 K20

P20S

H20L.O.I. Sum

3.60

0.06

49.23 1.61 16.96 0.10 9.63 0.23 8.59 9.98 3.48 0.58 0.21 0.03

49.20 1.22 16.06

100.24

100.63

99,35

0.82 4.58 30.2

0.79 3.70 34.9

0.53 1.13 63.6

0.44 0.80 71.2

8.88 0,02

U.ll 8.95 3.24 0.49 0.18

~e~

CIPW norm : Q C OR AB AN NE

2.68 --7.88 31.44 30.46 ---

DI

1.53 1o. 6.84

H

4N ]

oq O

MT IL AP HM

:::

2.57

2.98 0.94 ---

1.42

2.42

12.85

14.30

16.38

19.32

22.70

8.26 30,84 30.29

7.71 31.59 26.68

19.54 30,56 17.39

22.93 47.16 6.84

22.21 46.52 7.03

25.66 43,94

26,85

3.63 1.84 10.64 6.37

~.64 4.48 8.14 4.85

3.56 1.80 7.39 4.28

0,82 1.27 1.55 2.77

1.~ 0.7D 1.37 1.04

37.83 2.40 6.83 . . . . . .

1.41 2.55 0.84 1,73

----1.65 0.03

. . . . . .

3.41 25.25

2.91 27.60

28.77

28.01 ---

2.18 9.00 6.01 ---

2.92 3.04 0.86

2,17 2.12 0.61

0,34 1.76 0.53

0.94 1.15 0.29

2.16 1.11 0.21

0.53 1,37 0.26

2.25 0.85 0.12

--11.78 9.33 0.14 3.04 0.49

. . . . . . . . . . . .

9.34 3.12 2.02 0.77 15.07 6.37 2.19 2.33 0.43

SA LIC FEMIC

72.47 27.49

70,80 29.21

68.34 31.01

80.34 19.66

91.22 8.79

92.14 7,55

91.33 8.68

94.85 4.90

59.61 40.41

58.52 41.64

C.I. D.1. 5.I, A .R,

43.07 42.00 25.72 1.47

45.36 40.51 27.10 1.47

51.02 41.71 27.87 1.47

30.25 62.96 25.14 1.98

9.69 84.39 5.76 3.24

10.27 85.11 5.49

6.02 88.93 4.63 4,10

8.01 87.38 5.34 3.26

61.27 30.84 38.38 1.36

64.64 30.51 46.84 1.35

consequence of the collapse of the root of the magma chamber which was in turn related to the emission of Qb3 flows. In this way, the volcanic activity that occurred after the formation of Los Humeros caldera, does not present common features of post-caldera activity, No rhyolitic dome is observed. Instead, a succession of basaltic and rhyodacitic eruptions occurred,

3,20

GEOCHEMISTRY Major oxide abundances and CIPW norms are given in Tables 1 and 2 for preand post-caldera rocks respectively. T h e data are also plotted on variation diagrams (Figs. 3-6). As variable but significant amount of glass is present in most of the samples, chemical rather than mineralogical classification has been used

70

S.P. VERMA - M. L O P E Z M.

Continued: TABLE 2 -- Major element chemistry and CIPW norm of post-caldera volcanism, Los Humeros caldera.

CH27 High-K Basaltic Andeal~e

CH28 Basaltic Andesite (Qb3)

CH30 Basaltic AndesRe (Qb3)

CH31 Basaltic AndesRe (Qb3)

C H 34 High-K Basaltic Andes~e

CH33 (H~h-k) Basalt (Qb3)

(Q~) S~!O~ TIf~ A1208 Fe203 FeO ~O MgO CaO Na20 K20 P205 H20" L.O.I.

(QbS)

90,3(+) Basaltic Andealte (Qb3~ Lim6m)

C H 3 9 CH40 Hlgh-K Hlgh-K Basaltic Basaltic Andesite AB:le~lte (QbS; ( Q b 3 ; L1m6n) Lirn~)

CH46 BaaalUc Andeslte (Qb3; Tepeyehualco.

Baaldtlc AndesRe (Qb3; Tepeye~mlco).

55.09 0.88 20.02 2.10 3.90 0.09 4.25 8.18 2.51 1.92 0.25 0.20 0.84

55.37 0.96 19.80 1.84 4.00 0.09 3.59 8.52 3.51 1.49 0.24 0.19 0.87

55.93 0.95 19.72 1.58 4.04 0.09 3.32 7.82 3.84 1.52 0.23 0.13 0.61

55.83 1.O3 19.40 1.46 4.18 0.09 3.34 7.54 3.99 1.51 0.24 0.13 0.83

51.86 1.40 18.71 3.27 5.39 0.14 5.13 7.03 3.12 1.69 0.47 0.57 0.84

52.54 1.34 19. i 0 2.05 6.34 0.14 5.58 7.08 2.94 1.68 0.39 0.24 0.75

53.65 1.94 17.72 0.05 7.28 0~20 4.97 7.90 4.16 1.37 0.40 0.21

52.69 1.40 18.18 1.24 6.47 0.14 4.90 8.03 3.93 1.87 0.33 0.21 0.90

53.05 1.20 18.82 2.80 5.41 0.14 4.71 7.30 2.96 1.88 0.39 0.48 1.16

54.00 1.46 17.42 2.09 5.76 0.13 4.36 7.71 4.24 1.53 0.35 0.29 0.67

53.93 1.43 17.35 2.47 6.44 0.14 4.60 6.67 3.75 2.02 0.39 0.17 0.58

100.23

100,47

99.78

99.57

99.62

1(]0.17

99.85

100.29

100.30

100. Ol

99.94

(Feor-+MgO) 0.58 FeOt/MgO 1.36 IMg.-valm~ 59.2

0.61 1.58 55.7

0.62 1.64 54.6

0.62 1.64 54.6

0.62 1.62 54.9

0.60 1.47 57.4

0.60 1.47 57.3

0.61 1.55 56.1

0.63 1.68 54.1

0.64 1.75 53.1

0.63 1.88 51.3

3.58 3.00 0,09 0.51 10.17 10.01 26.88 25.08 32.38 32.84 . . . . ......

4.70

1.00

2.15

8.12 35.33 25.72

11.14 33.53 26.66

11.26 25.38 32.95

9.13 36.22 24.21

12.03 31.99 24.74

3.00 2.57 0.93

5.20 3.84 7.13 6.0,5 2.02 1.89 0.07 3.70 0.95

5.69 '3.58 4.25 3.07 3.79 8. Ol 1.81 2.68 0.79

0.66 6.41 0.29 ,3.44 11.58 7.99 5.82 4.92 . . . . . . . . . . . . 4.11 3.06 2.31 2.80 0.94 0.84

3.12 1.82 10.10 6.78 --""" 3.61 2.74 0.93

Sum

(Fec~/

Q C OR AB AN NE cM=z DIIF~ HAMgDFe

o4O MT IL AP H/v~

8.44 5.82 5.76 5.35 . . . . . . . . . . . . 11.44 8.86 9.07 9.05 21.41 29.87 32.81 34.23 37.99 34.07 32.39 30.99 . . . . . . . . . . . . . 0.76 3.89 2.86 2.87 '0.26 '1.67 '1.42 '1.48 10.32 7.19 7.02 7.11 4.04 3.54 4.00 4.22 .

.

3.07 1.68 0.60 . .

.

.

.

.

.

2.68 1.83 0.57 . . .

.

.

.

.

.

2.31 1.82 0.55 . . .

.

.

.

2.15 1.98 0.58 . . .

.

13.01 5.24 .

.

.

.

.

4.83 2.71 1.13 . . .

14.01 8.06 .

. . . . . .

.

SALIC FEMIC

70.28 20.73

78.62 21.38

80.02 19.99

79.62 20.38

73.10 26.91

71.45 28.57

69.18 30.85

71.32 28.68

74.28 25.71

70.56 29.45

70.92 29.10

C.I. D.I. S.I. A .R.

46.86 41.29 28.95 1.37

47.50 44.55 24.88 1.43

43 •48 47.63 23.22 1.48

42.16 48.63 23,07 1.51

41 •50 40.62 27.58 1.46

42.66 38.09 30.02 1.43

43.95 43.45 27.87 1.55

45.70 44.67 26.62 1.57

42.49 41.34 26.52 1.46

43.64 46.35 24.25 1.60

38.53 46.18 23.86 1.63

FIG. 3 -- K20-SiO 2 relationship for Los Humeros volcanic rocks. The boundaries and nomenclature are from PECCERILLOand TAYLOR (1976). The sequences I through IV are: arc tholeiitic series (I), calc-alkaline series (II), high-K co/c-alkaline series (HI) and shoshinite series (IV). Filled symbols are used for pre-caldera rocks and open-symbols are for post-caldera samples. In stratigraphic sequence from oldest to youngest rocks, these sygnbols for this as well as later figures are: Tpr • (CH41 and CH42); T p i , (HF15 and 902); Tprl 9(1166 and 1181); Tpab • (CH24, CH25 .and 1158); Tpob • (1171) and Qbl ~ (CH1, CH4, CH7, CHg, CH10, 1175, 1167, CHll, CH14, 1178, CH16, CH18, 1159 and 1173); Qrd [] (1176, 1165 and 1164); Qpc A (1161, CH68 to CH76); Qb2 Q (935 and HFI17; Q b 3 0 (CH27, CH28, CH30, CH31, CH33, CH34, 903, CH39, CH40, CH46 and CH47). Actually measured SiO2 and K~O (instead of anhydrous 100% adjusted) values are plotted for each sample. The data for samples CH68 to CH76 (Qpc; A) are taken from VERMA(in preparation).

GEOCHEMISTRYOF LOS HUMEROSCALDERA,PUEBLA,IVIEX]CO I

l

,

,

I

I

71 I

I

t

r( Hic.Jh-Krhyo l ire )

A

4

High -KZ ~ [ - ]

docite shoshoniteI

°"

-V-

°," °.."

3

High-K andesffe

©

rhyolite

¢r

obsoro kite

A daci=,e

2

Trr t

c

, S'/

andesffe Low-K rhyolite

L

bose

and~

q

docite

basa It

ondesite

[wx~_Ke.,,." Low, bosc -

T_ "-~ #loleiit,

50

ande

60

% Si02

70

I

I

72

S.P. VERMA - M. LOPEZ M.

PECCERILLO a n d TAYLOR (1976). T h e volcanic rocks from Los H u m e r o s c a l d e r a do n o t r e p r e s e n t one single series on t h e K20-SiO 2 d i a g r a m of PECCERILLO a n d TAYLOR. Instead, t h e s e rocks s e e m to belong to the calc-alkaline a n d high-K calc-alkaline series. T h i s duality is a p p a r e n t in other d i a g r a m s as well. A linear regression of the d a t a p l o t t e d in Fig. 3 gives a good correlation of K20 a n d SiO2 (regression coefficient r = 0.97). T h e total alkalis (Na20 + K20) versus SiO2 d i a g r a m (Fig. 4) shows t h a t t h e rocks under study belong b o t h to the high-alumina b a s a l t a n d alkali rock series of KUNO (1968) a n d confirm the nature of Los H u m e r o s volcanic rocks. Curve C divides the d i a g r a m (Fig. 4) into t h e two fields of alkaline rocks (strongly alkaline and mildly alkaline series) a n d subalkaline rocks (high alumina b a s a l t a n d tholeiitic series) (MACDONALD a n d KATSURA, 1964; SCHWARZER and ROGERS, 1974). Again, Los H u m e r o s volcanic units s e e m to r e p r e s e n t b o t h alkaline and subalkaline rocks.

~DD c

•

7.5 0

~2 5.0 Z

//

2.5 /

. . . .

!

. . . .

5O

! 6O

%

. . . .

!

. . . .

!

70

SiO2

FIG. 4 -- Total alkalis - Si02 relatinship for Los Humeros volcanic rocks. The boundaries dividing the fields of the tholeiitic series, highalumina basalt series and alkali rock series are from KUNO(1966). Curve C is form SCI-IWARZER and ROGERS (1974). Symbols used are same as those in Fig. 3.

FIG. 5 -- (NaeO + K20 ) -- FeO t -- MgO (i.e., AFM) ternary plot of Los Hun]eros volcanics. The fields of pigeonitic and hypersthene rock series form the Izu-Hakone region (KuNo, 1968) are also included. These are bounded by curves P, HP and H, HP respectively. Curves 1 (tholeiitic) and 2 (alkalic) are the average trends for Hawaiian suite, taken from MACDONALD and K A T S U R A (1964). Los H u m e r o s volcanic rocks are p l o t t e d in Fig. 5 on a conventional A F M (N%O + + K20 -- F e O t - MgO) t e r n a r y diagram. T h e rocks from Los H u m e r o s caldera do n o t show m u c h i r o n - e n r i c h m e n t unlike t h e tholeiitic a n d alkalic suites of Hawaii (MACDONALD a n d KATSURA, 1964) or the pigeonitic rock series of J a p a n (KUNO, 1968). T h e fractionation of Los H u m e r o s rocks s e e m s to be s o m e w h a t similar to the H y p e r s t h e n e rock series (KuNo 1968) of the Izu-Hakone region, Japan. CHEN (1978) has o b s e r v e d a calc-alkaline t r e n d with insignificant i r o n - e n r i c h m e n t (similar to Los H u m e r o s rocks) in Pleistocene volcanic rocks from N o r t h e r n Taiwan. His i n t e r p r e t a t i o n is b a s e d on the studies of ALLEN et al. (1972) and CAWTHORN and O'HARA (1976) a n d r e l a t e s t h e production of andesitic t e r m s to a basaltic p a r e n t a l magma. A n a m p h i b o l e fractionation, either by itself or t o g e t h e r with m i n o r olivine a n d clinopyroxene, s e e m s to be capable of producing calc-alkaline t r e n d s without a noticeable iron enrichment. Fig. 6 shows b i n a r y plots of several oxides a n d their combination against D.I. (differentiation index). R a t h e r good linear correlations s e e m to exist in m o s t cases. B e t t e r correlations could in some cases be o b t a i n e d if least-squares linearfit is under-

GEOCHEMISTRY OF LOS HlYMEROS CALDERA, PUEBLA, MEXICO

taken in segments of D.I. or if more complex functions are fit-ted to the data. A correlation matrix of most oxides and D.I. is also included in Fig. 6. Almost all correlations except perhaps those involving Na20 are significant and throw some light on the differentiation history and possible parental magma(s) of Los Humeros volcanic roks. SiO~ and K20 increase steadily with differentiation. Na20 also increases with differentiation but at the latest stages (rhyolitic) shows a slight decrease (Na20 has not plotted in Fig. 6, see Tables 1 and 2). A1203, MgO, total FeO, CaO and TiO~ decrease with differentiation whereas Feo~/MgO ratio shows a general increase with differentiation. Although FeOyMgO ratio has been used as a measure of fractional crystallization (MIYASHmO, 1974), we have used here THORNTON and TUTTLE's (1960) differentiation index (D.I.) for this purpose. This is perhaps justified in the light of generally high linear regression coefficients of most oxides against the D.[. Average bulk chemical analyses are computed from the analyses reported in Tables ! and 2 and compared in Table 3 with recent analyses for the Western and Eastern parts of the Mexican volcanic belt (PAL et al., 1978) and Western Americas (EWART, 1976). Although the comparison of basaltic andesites, andesites and more differentiated rocks m a y be meaningful, the basalt average for Western Americas is significantly higher in SiO2 because mineralogical rather than chemical defmi-

73

tion was used by EWART (1976) for classifying basalts. Table 3 shows t h a t Los Humeros magmas are, in general, similar in their bulk chemical analyses to those from other areas of the Mexican volcanic belt (MVB) and Western Americas. Some small differences are however apparent for some elements: Los Humeros basalts have higher MgO and lower KeO implying that these basaltic magmas m a y have suffered relatively little crystal fractionation before eruption. Their relatively high Mg-value supports this conclusion (Table 3). Basaltic andesites from Los Humeros are richer in A120~ whereas andesites show higher TiO~ and lower MgO. Finally, dacites and rhyolites from Los Humeros have considerably lower FeO t than those from other areas of the MVB implying perhaps a greater involvement of ferromagnesian minerals in the latest stage of differentiation of Los Humeros magmas. T h e variation diagrams and the above mentioned chemical differences are consistent with the production of andesitic to rhyolitic magmas from a basaltic parental magma(s) by fractional crystallization of plagioclase, amphibole, clinopyroxene, olivine and other ferromagneslan minerals. A study involving alkali and alkaline earth elements seems to support this conclusion (VERMA, 1981 and m prep.). I t m a y be of interest to note that the pre-caldera rocks show a bimodal distribution of their SiO2 and D.I. values (Table 1 and Fig. 6). This m a y partly be due to the poorer sampling of pre-c~aldera

TABLE 3 -- A comparison of average bulk chemical analyses of Los Humeros volcanic rocks with compilations* for the Mexican volcanic belt (MVB) and Western Americas (Area code: A Los Humeros; B Western MVB; C Eastern MVB; D Western Americas).

74

S.P. VERMA

- M. LOPEZ

M.

/ - = . .

#-~,

70 "A" ~

0 •-

r= 0.g8

6(~

~0

~ I i I ~ (30 =

I

i

I

""-- 0

,

r =-0.82

©

I

=

I

I

i

I

i

i

,

,

,

,

I

i

I

'

I

@

1}

8

~

,4r ¢r ~ l l

o ~ooo

,

~l~lt

t, "~" T

0 r=

0.65

~r ~r

I

I 40

i

I 60

i

D.

I 80

,

I.

FIG. 6 -- Plots of several oxides FeOt/lVlgO ratio against differentiation index (D.I.). Note the fairly regular variations seen on these plots (r is the regression coefficient for the least-squares linear fit of the data points). A correlation matrix is also included in this Figure.

GEOCHEMISTRY OF LOS HEM~ROS CALDERA, PUEBLA, MEXICO

75

8--

o

o U

~ ' ~ * ~r ~ . . . ~

~_

I

l

0

I

[~ ~--~ ~ ,4~....._...¢~

o

©

~=-0.98

4

'

i

'

~

,,---o.8~

I

i

1

,

I

,

I

i

I

'

I

'

~ i

4 •A[..........

ol

-ik./"

r=

0.98

2

I

I

|

,

,

I

I

#

0

04

..aCt

+ 0 ol Z

~" j ~ r ~ " * / 4

r=

0.96

~o ~:~-%"--'~ I

.

,

i

I 1 SiO2 2 .~ 4 5 6 7 8 9

TiC2 AI2()3 l:cOt MgO Ca() Na20 K2C 13.1.

,

I

60

40

2

3

80

4

5

6

D.I.

7

8

9

1 -0.89 -0.81 -0.95 -(). 87 ~0.96 0.42 (.I.97 0.98

1 0.63 0.92 0.73 O. 82 -0.16 -0.86 -().B5

1 0.64 0.49 O. 83 -0.35 -().80 -0.82

1 0.88 0.87 -0.34 -O.gl -t.1.92

1 0.84 -().48 -0,86 -{t.88

l -0.46 -0,98 -(1.98

l 0.39 0.51

1 (}.0~

1

76

S.P. VERMA - M. LOPEZ M~

rocks, especially from the Tppc unit (basaltic to rhyolitic tephra). However, these rhyolitic rocks show generally slightly higher STSr]S~Sr ratios (0.70420.7048) than the contemporaneous andesitic rocks ( - 0 . 7 0 4 1 ) from this area (VERMA, 1982) and may imply some crustal component in the genesis of these rhyolitic magmas. Post-caldera rocks do not show such a bimodal distribution in their SiO2 and D.I. values (Table 2 and Figs. 3 and 6). Further, their STSr]SeSr ratios do not show significant differences between daciticrhyolitic and basaltic-andesitic magmas (VERMA, 1982). In fact, nine samples (CH68 to CH76; Qpc; Fig. 2) of basaltic to rhyolitic pumice collected from a single locality (a drainage-exposure) within the caldera shows a very uniform STSr]a~Sr ratio ( - 0.7041 + 1; VERMA, 1981, 1982). This implies that the post-caldera magmas are comagmatic, the sialic bulk contribution is rather insignificant and argues in favour of an important role of fractional crystallization (following different degrees of partial melting of the mantle source region to satisfy the duality of Los Humeros rocks) for the magma genesis in this area. The eruptions from smaller volcanic cones (Qbl; Fig. 2) and the alternating succession of basaltic to rhyolitic tephra (Qpc; Fig. 2) seem to ~tap>~ different levels of a stratified magma chamber at fairly different stages of differentiation. This is reflected in the significantly large range of their oxide contents (Figs. 3-6). On the other hand, the youngest flows (Qb3) seem to be more uniform in their composition. According to PEREZ-REYNOSO (1978) these flows are not cut by any kind of drainage, and this observation is taken to support a very young age of eruption ( < 10,000 y) for these flows. The above observations seem to support that a ~ shallow~ magma chamber exists, or at least existed until very recently, beneath Los Humeros area and that the magma chamber or the intrusions are still at a considerably high temperature rendering the area to be a geothermal potential.

GEOPHYSICS Geophysical techniques such as those used on the known shallow magma body at Kilauea Iki have also been used on larger, deeper bodies (SIMKIN, 1979). It is therefore reasonable to expect that such geophysical methods should be able to decipher the deeper hidden structure of Los Humeros caldera and perhaps delineate the subsurface magma chamber. The geophysical techniques applied to Los Humeros caldera, although have been claimed to support the existence of an anomalous structure favorable from the point of view of its geothermal potential, still seem to be somewhat preliminary in nature and lack adequate geological interpretation. Furthermore, somewhat conflicting inferences have been drawn from them.

FLORES-LUNA et al. (1978), in a regional aeromagnetic survey over Los Humeros caldera, have presented a model of a prismatic body near its center. T h e y interpreted that the ~ bipolar ~ anomaly observed was caused by a rectangular prism of 5 km (N-S direction) x 2 km (E-W direction) x 5 km (thickness) at a subsurface depth of 2 km to its top. T h e y interpreted the prismatic body as the magma conduit for the volcanic episodes which occurred prior to the caldera formation. However, the modeling is subject to uncertainty because of the intensity of magnetization of the inferred body (an arbitrary value chosen). Furthermore, their geological interpretation apparently conflicts with the interpretation of a regional gravity study by MENA and GONZALES-MORAN (1978). Gravity results have been interpreted geologically to represent a structure 1.25 km wide at a subsurface depth of about only 500 m measured to its top. However, in their modeling, MENA and GONZALES-MORAN (1978) have assumed a lower density (2.52 g]cm3) for the central body than the surrounding rocks (2.67 g/ cm ~) and pointed out that the subsurface structure can be correlated with other geophysical data. Obviously, this lowdensity mass may not correspond to the intrusive (prismatic) body inferred by aeromagnetic survey by FLORES-LUNA et

GEOCHEMISTRY OF LOS HUMEROS CALDERA, PUEBLA, MEXICO

al. (1978), even if the controversial depth is solved. ALVAREZ (1978) has presented telluric, self-potential, and surface temperature profiles measured along two lines in the caldera. Based on these measurements, ALVAREZ(1978) has delineated an anomalous area of greater geothermal potential on the western portion of the caldera. However, he did not present any interpretation of the telluric and self-potential data in terms of the subsurface geology of the area. His (( surface, temperature measurements (taken at 0.5 to 1 m depth) showed somewhat higher mean tempera~ r e on the western section of the profiles. No near-surface and external effects have, however, been considered for these temperature differences. It is worth pointing out that the ,anomalous>~ area of ALVAREZ(1978) does not coincide with the subsurface ((anomalous, bodies inferred from aeromagnetic and gravity studies discussed above. Finally, PONCE and RODRIGUEZ (1978) have presented a study of microearthquake activity associated with Los Humeros area. This study also does not seem to give much information concerning the subsurface geology related to the magma chamber. It is, thus apparent that we still lack sufficient geophysical information and its geologic interpretation in order to be able to ascertain the presence of and delineate the magma chamber beneath Los Humeros caldera. Teleseismic techniques, seismic reflection profiling, and surface deformation, ground tilt, techniques have been employed with considerable success to the problem of subsurface magma in other areas (SIMKIN, 1979). More detailed geophysical studies are needed in order to gain a better understanding of the tectonically, volcanologically, and geothermally interesting area of Los Humeros caldera.

77

ina series of KUNO (1966) or the calcalkaline and high-K calc-alkaline series of PECCERmLO and TAYLOR (1976). Pure (