Geochemistry of evolved magmas and their relationship to subduction-unrelated mafic volcanism at the volcanic front of the central Mexican Volcanic Belt

Journal of Volcanology and Geothermal Research 93 Ž1999. 151–171 www.elsevier.comrlocaterjvolgeores

Geochemistry of evolved magmas and their relationship to subduction-unrelated mafic volcanism at the volcanic front of the central Mexican Volcanic Belt Surendra P. Verma

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Max-Planck-Institut fur ¨ Chemie, Abteilung Geochemie, Mainz, Germany Centro de InÕestigacion ´ en Energıa, ´ UNAM, PriÕ. Xochicalco S r No., Col. Centro, Apartado Postal 34, Temixco, Mor. 62580, Mexico Received 13 May 1998; accepted 3 May 1999

Abstract This study reports new geochemical and radiogenic isotope data on hy-normative Pleistocene to recent Ž- 40,000 years. andesitic to dacitic volcanic rocks from Sierra de Chichinautzin ŽSCN., located south of Mexico City, in the central part of the Mexican Volcanic Belt ŽMVB.. Their rare-earth element, Nb, Zr, and Y concentrations are generally lower than in mafic rocks from this area, which have been shown to be subduction-unrelated. The isotopic ratios of these evolved magmas show the following ranges: 87Srr86 Sr 0.70370–0.70469, 143 Ndr144 Nd 0.51278–0.51289, 206 Pbr204 Pb 18.64–18.72, 207 Pbr204 Pb 15.59–15.62, and 208 Pbr204 Pb 38.39–38.52. These isotopic ratios are generally similar to those for the associated mafic rocks, although some differences do exist between them, particularly for the 87Srr86 Sr ratios that are somewhat higher and 143 Ndr144 Nd that are slightly lower for the evolved rocks. The available geochemical evidence does not support the generation of the SCN evolved magmas by simple fractional crystallization ŽFC. or even by assimilation coupled with fractional crystallization ŽAFC. of such mafic magmas, nor by direct Žslab melting. or indirect Žfluid transport to the mantle. participation of the subducted Cocos plate. The viable mechanism seems to be that the most evolved andesitic and dacitic magmas in the SCN were produced by partial melting of a heterogeneous mafic granulite source in the lower crust. The magmas of intermediate compositions may reflect magma mixing processes between the most evolved andesitic and dacitic magmas generated in the lower crust and the mantle-derived mafic magmas Žbasalt and basaltic trachy-andesite. at the volcanic front of the central part of the MVB. These results have direct bearing to other volcanic areas with complex tectonic setting. q 1999 Elsevier Science B.V. All rights reserved. Keywords: geochemistry; subduction; rifting; magma mixing; Mexico; Mexican Volcanic Belt

1. Introduction The Mexican Volcanic Belt ŽMVB., a presumed continental arc, extends approximately East–West,

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from near Veracruz to Puerto Vallarta ŽFig. 1.. Contrasting hypotheses for its origin range from a direct link between volcanism and subduction, to no direct association between them but instead volcanism related to past and present tectonics Žrifting andror faulting. within the MVB Že.g., Verma and Nelson, 1989; Urrutia-Fucugauchi and Bohnel, 1988; Verma, ¨ 1987, 1999; Wallace and Carmichael, 1992; Righter

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 8 6 - 4

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S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

Fig. 1. Location and present tectonic setting of the Mexican Volcanic Belt ŽMVB., modified after Verma Ž1999.. The cities for reference are: MC s Mexico City; PV s Puerto Vallarta; V s Veracruz. MAT s Middle America Trench; EPR s East Pacific Rise; OFZs Orozco Fracture Zone; RFZ s Rivera Fracture Zone; CO s Cocos Plate; RI s Rivera Plate; PA s Pacific Plate; NA s North American Plate; symbols of crossed circless IPOD-DSDP Leg 66 Sites 487 and 488. Thick arrows representing maximum horizontal tensional stress vectors and lines showing local tectonics Žfractures and faults shown by continuous, dashed and dotted lines. are from Singh and Pardo Ž1993.. The box SCN ŽSierra Chichinautzin., South of Mexico City, is amplified in Fig. 2.

et al., 1995; Hochstaedter et al., 1996; Luhr, 1997; Luhr et al., 1989a; Marquez et al., 1999a,b.. Primi´ tive volcanic rocks, defined by Luhr Ž1997. as having MgO ) 6% and Mga ) 62, are found rather commonly in the western part of the MVB. Many occurrences of such primitive rocks are also present in the central MVB ŽVerma, 1999.. Their study can provide useful constraints on the possible magmatic source regions. In spite of the fact that the Cocos plate subducts at a shallow angle and that there is no direct seismic evidence of the actual position of the subducted slab at depths below about 50 km Že.g., Nixon, 1982; Suarez ´ et al., 1990; Barbosa et al., 1993; Singh and Pardo, 1993; Pardo and Suarez, 1995., most authors, ´ Ž including Pardo and Suarez 1995. believe that there ´ is a link between volcanism and subduction. Verma Ž1999. constrained the origin of mafic volcanism in the central MVB from major and trace

elements and radiogenic isotopes in basalt and sediment samples from IPOD-DSDP Sites 487 and 488, located respectively on the Cocos plate and the landward-dipping packages of hanging-wall sediments Žaccretionary wedge. on the edge of the North American plate, close to the Middle America Trench ŽMAT. off Acapulco ŽFig. 1.. The same chemical and isotopic parameters were also reported by Verma Ž1999. for six samples of mafic rocks from Sierra de Chichinautzin ŽSCN., a monogenetic volcanic field of Pleistocene to recent age, situated south of Mexico City, close to the volcanic front in this part of the MVB Že.g., Martin del Pozzo, 1982; Verma and Armienta-H., 1985.. Published geochemical results on four additional mafic rock samples from the SCN ŽRobin, 1982; Nixon, 1988. were also included in the discussion. These results show that the mafic magmas have no direct genetic link with the subducted Cocos plate ŽVerma, 1999..

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

With this discovery of subduction-unrelated mafic magmas at the volcanic front of the central MVB, the origin of the evolved magmas becomes a very important issue, which also has bearing on the final fate of the subducting Cocos plate, its contribution to the volcanism, and the importance of ongoing rifting processes throughout the MVB Že.g., Suter et al., 1992, 1995; Campos-Enrıquez et al., 1999; Marquez ´ ´ et al., 1999a,b.. If the evolved magmas can be shown to originate from partial melting of the lower crust, subduction plays no direct role in the volcanism of the SCN, because it was already shown by Verma Ž1999. that the mafic magmas from the SCN have no direct relationship to the subducting Cocos plate. If, on the contrary, the evolved magmas can be generated from a direct Žpartial melting. or indirect Žfluid transport to the mantle wedge. contribution of subducting plate or fractionation of subduction-related mafic magmas, the role of subduction can be affirmed at least in the genesis of the evolved magmas. Answers to these issues are also important to obtain a unified model for the origin of the MVB. In order to throw further light on this complex problem and provide more constraints, I present here new geochemical and radiogenic isotope data on andesitic to dacitic rocks from the volcanic front of the central part of the MVB and discuss their petrogenetic implications.

2. Sample description The geology and geochemistry of volcanic rocks from the SCN have been described by several authors Že.g., Gunn and Mooser, 1971; Negendank, 1972; Bloomfield, 1973, 1975; Demant, 1981; Martin del Pozzo, 1982; Verma and Armienta-H., 1985; Delgado Granados and Martin del Pozzo, 1993; Marquez et al., 1999b.. However, this is the first ´ paper that addresses, in a quantitative way, the origin of andesitic and dacitic magmas and explores their possible relationship to the closely associated Žboth in space and time. mafic magmas that are not directly related to subduction. Fig. 2 gives a simplified geologic map of the SCN area with locations of ten new Pleistocene to recent andesitic and dacitic rock samples analyzed in this study Žwith their coordinates in Table 1., two other

153

samples from the literature, and the earlier mafic samples from Verma Ž1999., as well as rocks analyzed by Swinamer Ž1989.. Precise localities for other literature analyses Že.g., Gunn and Mooser, 1971; Negendank, 1972; Martin del Pozzo, 1989. were not reported by the authors and therefore could not be included in Fig. 2. According to Marquez et ´ al. Ž1999b., volcanic activity in the SCN during the last 40,000 years has built more than 220 monogenetic volcanoes Žsome prominent ones are shown in Fig. 2., with an estimated total volume of 470 km3 in an area larger than 600 km2 and a high eruption rate of about 11.7 km3r1000 years. All analyzed rocks from the SCN are generally very young Ž40,000 years.. Andesitic and dacitic rocks are closely related to mafic rocks, both in space and time ŽFig. 2.. The evolved rocks from the SCN have an aphanitic texture, with generally - 10% phenocrysts. Basaltic andesites have clinopyroxene ŽCpx., olivine ŽOl., plagioclase ŽPlg., and orthopyroxene ŽOpx., with some accessory minerals, such as magnetite and ilmenite, in both phenocryst and groundmass phases ŽNegendank, 1972; Milan et ´ et al., 1988; Marquez ´ al., 1999b.. Andesites and dacites from the SCN have Cpx, Opx, Plg, and some accessory minerals. Therefore, participation of these major modal minerals ŽOl, Opx, Plg, and Cpx. and minor opaque minerals will be considered in fractional crystallization ŽFC. and combined assimilation-fractional crystallization ŽAFC. models. Mass-balance estimates based on microprobe data on selected SCN rocks show the relative importance of modal minerals as Plg, Ol, pyroxenes ŽCpx and Opx., and opaque minerals. However, because of very large residuals in such mass-balance computations, simple FC is ruled out as a viable petrogenetic process for the SCN rocks ŽMarquez Gonzalez, 1998; Marquez et ´ ´ ´ al., unpublished data..

3. Analytical methods Very fresh interior parts of large sample blocks were crushed at the Max-Planck-Institut fur ¨ Chemie, Mainz, Germany, using agate or hardened iron container. This latter container is particularly useful because it is much faster to grind rocks than using an

154

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

Fig. 2. A simplified geologic map of the SCN Žmodified after Bloomfield, 1975; Martin del Pozzo, 1989; Swinamer, 1989; Marquez et al., ´ this issue.. Main volcanic edifices are shown as stars. Sampling locations are given using symbols explained as inset ŽV99 s Verma, 1999; S89 s Swinamer, 1989.. Samples reported in this work are labeled; however, sample name is simplified ŽC stands for CHI and M for MXC.. Sample locations not provided by authors ŽGunn and Mooser, 1971; Negendank, 1972; Martin del Pozzo, 1989. could not be included here. Coordinates of the present sampling are given in Table 1 and locations are as follows: CHI02 from about 1 km South of Tres Marıas, ´ CHI03 from about 5 km S of Parres, CHI05 from about 1 km N of Parres Žall three from the Cuernavaca–Mexico highway.; CHI06 from near Monte Casino on the Cuernavaca–Mexico federal road; CHI07 from NE of Palo Grande village; CHI08 from La Lagunilla village; CHI09 from San Pedro Techuchulco; CHI10 from Santa Marıa ´ Jajalpa; CHI11 from about 2 km SE on Tenango de Arista; CHI12 from about 0.5 km E of the location of CHI07. Two additional samples from the SCN ŽTable 1. are included because their major and trace element data have important bearing on the petrogenesis of the SCN, which was not adequately discussed in the original paper ŽNixon, 1988..

agate container. The only possible contamination is for a major element ŽFe., instead of SiO 2 in an agate container. The iron container is made of highly pure Fe and therefore contamination of trace elements is almost negligible ŽA.W. Hofmann, personal communication.. Major and trace elements were analyzed by X-ray fluorescence spectrometry ŽXRF. at the Universitat ¨ Mainz, Germany ŽVerma et al., 1992.. Other analytical work was carried out at the Max-Planck-Institut fur ¨ Chemie. Rare-earth elements ŽREE. were determined by high performance liquid chromatography ŽHPLC; Cassidy, 1988; Verma, 1991.. Radiogenic

isotopes were determined on two fully-automated triple- Žfor Nd and Pb. and double- as well as multi-collector Žfor Sr. MAT 261 mass spectrometers ŽVerma, 1992.. The mass spectrometer initially equipped with a double-collector was upgraded as a multi-collector machine during the course of this study. The analytical errors in the concentration range measured in the present samples are as follows: generally 1–5% for major and 5–10% for trace elements by XRF; 1–5% for the REE by HPLC, except for Tm and Lu Ž5–15%.. The analytical uncertainties for isotopes are directly quoted for each sample.

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

CIPW norms were calculated for all samples, including the literature data from the SCN, on an anhydrous 100% adjusted basis, with Fe 2 O 3rFeO ratio depending on the rock type such as basalt, trachybasalt, basaltic trachy-andesite, andesite, dacite, or rhyolite ŽMiddlemost, 1989.. Rock types inferred for the SCN magmas were based on total alkali–silica ŽTAS. classification ŽLe Bas et al., 1986., using the recalculated adjusted data according to Middlemost Ž1989..

4. Results The compositions of the SCN rocks range from basaltic andesites to acites ŽTable 1.. Their trace-element concentrations and radiogenic isotope ratios are presented in Tables 2–4. Average compositions of the SCN rocks and other pertinent data used in modeling computations are reported in Tables 5–7. All chemical data were adjusted for Fe 2 O 3rFeO ratio recommended by Middlemost Ž1989. and to 100% anhydrous basis before the norm calculation and major element plots. All samples analyzed in this study are Hy- and Q-normative, except two basaltic andesite samples ŽNixon, 1988. which have Ol, instead of Q in the norm ŽTable 1.. The more mafic rocks from the SCN are also hy-normative ŽVerma, 1999.. There is a wide dispersion in composition with respect to total alkalis at given SiO 2 ŽFig. 3.. A part of this scatter may in fact be analytical, e.g., Bloomfield’s data seem to be consistently higher in total alkalies. The rocks include basalt, trachybasalt, basaltic trachy-andesite, basaltic andesite, trachyandesite, andesite, and dacite. Major elements versus MgO also show considerable scatter ŽFig. 4., implying that fractional crystallization cannot be the only process for the evolution of the SCN rocks. Mga is plotted against SiO 2 in Fig. 5 for all rocks from the SCN, except Bloomfield’s results which may be biased. Although a wide scatter is seen for all available data, the limited data from a single laboratory reported in this work on evolved rocks ŽTable 1. and by Verma Ž1999. for mafic rocks from the SCN Žlarge filled and open squares respectively in Fig. 5. show relatively constant Mga with SiO 2 throughout the sequence from basalt to dacite.

155

This general constancy is also seen in the FeO trMgO ratio ŽTable 1; FeO trMgO versus SiO 2 plot not shown.. Chondrite-normalized REE patterns Židentified with their SiO 2 concentrations. are light-REE enriched and show very small or no negative Eu anomaly ŽFig. 6.. In general, the mafic rocks from the SCN have higher REE concentrations than the evolved andesitic and dacitic rocks. Even the basaltic sample ŽCHI13. with the lowest REE has higher REE concentrations than an andesite ŽCHI12. or a dacite ŽCHI10.. Nb, Zr, and Y concentrations are generally lower in the evolved rocks from the SCN than in mafic rocks ŽTables 2 and 5.. Average dacite from the SCN ŽTable 5. shows light REE-enriched pattern ŽFig. 7a, b. and significant Nb depletion with respect to Ba and Ce in MORB-normalized multi-element diagram ŽFig. 7c, d.. These characteristics are generally comparable to high-degree partial melts ŽFig. 7. of an average mafic lower crust ŽMLC. from San Luis Potosı´ area, approximately 400 km NW of the SCN and will be discussed in the following section. Fig. 8 shows a false-correlation-free ŽChayes, 1971; Rollinson, 1993. ratio–ratio plot ŽLarYb– BarNb. for the SCN rocks. As compared to the basaltic samples, the andesitic and dacitic rocks have higher BarNb, but similar or higher LarYb ratio. The Nb concentrations of the evolved rocks ŽTable 2. are lower than those of the closely related mafic rocks from the SCN analyzed by Verma Ž1999.. Results of some simple model computations included in Figs. 8 and 9 are also discussed in the following section. Isotopic ratios of the SCN evolved rocks range as follows: 87 Srr86 Sr 0.70370–0.70469, 143 Ndr144 Nd 0.51278 – 0.51289, 206 Pbr 204 Pb 18.64 – 18.72, 207 Pbr204 Pb 15.59–15.62, and 208 Pbr204 Pb 38.39– 38.52 ŽTables 3 and 4; Fig. 9.. For comparison, the mafic rocks from the SCN show the following ranges ŽVerma, 1999 .: 87 Srr 86 Sr 0.70348 – 0.70433, 143 Ndr144 Nd 0.51275–0.51293, 206 Pbr204 Pb 18.65– 18.76, 207 Pbr204 Pb 15.57–15.61, and 208 Pbr204 Pb 38.34–38.53. Thus, minor but significant differences between these isotopic ratios are observed for 87 Srr86 Sr and to a lesser extent for 143 Ndr144 Nd, which are respectively slightly higher and lower for the evolved rocks from the SCN. However, all

156

Rock-type Basaltic andesite

Andesite

Dacite

Sample Long. Ž8W. Lat. Ž8N. Ref.

CHI06 X 99815.58 X 18859.83 j

MXC7 X 98856.67 X 1988.25 n

MXC5 X 98854.50 X 1987.00 n

CHI11 X 99836.37 X 1985.62 j

CHI09 X 99831.51 X 1986.42 j

CHI08 X 99825.51 X 1985.82 j

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3t MnO MgO CaO Na 2 O K 2O P2 O5 LOI

53.98 1.58 16.59 8.80 0.14 5.86 6.81 3.97 1.27 0.48 0.44

53.63 0.79 15.69 7.68 0.13 9.71 7.65 4.05 0.86 0.18 –

54.15 0.78 15.46 7.59 0.12 9.55 7.73 3.67 0.88 0.19 –

60.70 0.72 16.33 5.66 0.09 4.39 5.81 4.01 1.73 0.17 0.51

61.00 0.78 16.01 5.79 0.01 4.22 5.53 4.13 1.79 0.24 0.39

60.83 0.80 16.11 5.48 0.09 4.12 5.62 4.17 1.66 0.24 y0.07

CHI07 X 99824.89 18850.69y j 59.67 1.00 15.92 6.23 0.10 4.77 5.41 3.81 2.30 0.30 0.69

CHI12 X 99824.72 X 18850.68 j

CHI03 X 99811.37 X 1986.30 j

CHI05 X 99810.14 y 1988.51 j

CHI10 X 99832.34 X 1986.90 j

57.97 0.97 16.11 6.79 0.11 5.92 6.24 3.87 1.60 0.24 0.53

61.72 0.75 15.42 5.65 0.09 5.23 5.05 4.27 1.65 0.22 0.42

60.54 1.11 16.16 6.34 0.11 3.87 5.28 4.35 1.83 0.31 0.53

63.30 0.71 16.51 4.92 0.09 3.33 4.94 4.32 1.80 0.17 0.20

CHI02 99814.13y X 1983.26 j 60.91 0.78 15.21 5.03 0.09 3.16 4.54 3.99 2.01 0.22 2.70

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

Table 1 Major-element composition and CIPW norm of evolved volcanic rocks from the Sierra de Chichinautzin, Mexico

– – 99.92

0.00 0.24 100.61

0.00 0.15 100.27

– – 100.12

– – 99.89

– – 99.05

– – 100.10

– – 100.35

– – 100.47

– – 100.43

– – 100.29

– – 98.64

Mga FeO t rMgO Fe 2 O 3 rFeO Q Or Ab An Di Hy Ol Mt Il Ap Salic Femic C.I. D.I. S.I. A.R.

62.6 1.35 0.30 2.14 7.60 34.01 23.99 5.70 19.66 – 2.75 3.04 1.15 67.73 32.30 41.54 43.74 29.45 1.58

76.1 0.71 0.30 – 5.09 34.35 22.14 11.78 12.50 9.84 2.37 1.50 0.43 61.59 38.42 56.08 39.44 43.54 1.53

76.0 0.72 0.30 – 5.23 31.20 23.22 11.25 21.73 3.10 2.35 1.49 0.45 59.65 40.36 56.15 36.43 44.03 1.49

66.9 1.16 0.35 11.42 10.31 34.21 21.63 5.10 13.57 – 1.98 1.38 0.41 77.57 22.44 36.04 55.94 27.80 1.70

65.5 1.23 0.35 11.86 10.68 35.28 20.05 4.90 13.15 – 2.03 1.50 0.57 77.86 22.15 33.88 57.82 26.49 1.76

66.2 1.20 0.35 12.05 9.94 35.75 20.60 4.88 12.74 – 1.93 1.54 0.58 78.34 21.68 34.31 57.73 26.70 1.73

66.6 1.18 0.35 9.67 13.74 32.59 19.75 4.35 15.09 – 2.19 1.92 0.72 75.75 24.27 33.96 55.99 27.88 1.80

69.4 1.03 0.35 6.85 9.52 32.98 22.01 6.10 17.74 – 2.37 1.86 0.57 71.37 28.65 40.74 49.35 32.56 1.65

70.7 0.97 0.35 11.59 9.79 36.27 18.10 4.50 15.84 – 1.97 1.43 0.52 75.75 24.27 33.47 57.64 31.13 1.81

61.4 1.47 0.35 10.86 10.88 37.02 19.28 4.09 12.83 – 2.21 2.12 0.74 78.03 21.98 31.19 58.76 23.61 1.81

64.6 1.33 0.40 15.14 10.67 36.65 20.40 2.48 11.03 – 1.89 1.35 0.40 82.86 17.15 29.44 62.46 23.17 1.80

62.9 1.43 0.40 15.86 12.43 35.33 18.47 2.91 10.90 – 2.02 1.55 0.55 82.09 17.92 27.96 63.62 22.27 1.87

Ref.: js this work; n s Nixon Ž1988.. Mgas100 Mg 2qrŽMg 2q q0.9ŽFe 2q qFe 3q .., atomic; Fe 2q and Fe 3q are calculated from an iron oxidation ratio ŽMiddlemost, 1989., using a computer program for CIPW norm calculation, written by Verma and Navarro-L. Fe 2 O 3 rFeO values are those recommended by Middlemost Ž1989. for each rock type. Salic ssum of salic normative minerals ŽQqOrqAbqAn.; Femic ssum of femic normative minerals ŽDiqHyqOlqMtqIlqHm.; C.I.s crystallization index ŽAnq 2.1570577DimgqFoq0.7007616Hymg; Poldervaart and Parker, 1964.; D.I.sdifferentiation index ŽQqOrqAbqNeqLc; Thornton and Tuttle, 1960.; S.I.ssolidification index Ž100 MgOrŽMgOqFeOqFe2 O 3 qNa 2 OqK 2 O.; Huchinson, 1974.; A.R.salkalinity ratio ŽAl 2 O 3 qCaOqtotal alkalies.rŽAl 2 O 3 qCaOytotal alkalies.; however, when SiO 2 ) 50% and 1- K 2 OrNa 2 O- 2.5, then 2ŽNa 2 O. is used in place of total alkalies; ŽWright, 1969.. Subdivision of Di, Hy, and Ol in Mg- and Fe-varieties is automatically done in the computer program for the estimation of the indices that involve such a subdivision.

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

H 2 Oq H 2 Oy Sum

157

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

158

Table 2 Rare-earth and other trace-element data for evolved volcanic rocks from Sierra de Chichinautzin, Mexico Sample Rock type Ref.

CHI06 BA j

MXC7 BA n

MXC5 BA n

CHI11 A j

CHI09 A j

CHI08 A j

CHI07 A j

CHI12 A j

CHI03 A j

CHI05 A j

CHI10 D j

CHI02 D j

La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu Ba Nb Zr Y Sr Rb Zn Cu Ni Co Cr V

17.7 44 4.9 22.1 4.53 1.41 4.53 0.70 0.80 2.30 0.33 2.14 0.40 388 19.4 255 28.1 520 24.6 91 27 85 30 154 128

– – – – – – – – – – – – – 246 5 123 21 413 20 – – 247 – 393 155

– – – – – – – – – – – – – 240 5 121 21 386 21 – – 250 – 447 161

16.4 35 3.5 14.1 3.01 1.01 3.47 0.70 0.80 1.92 0.28 2.16 0.39 402 3.8 134 16.8 640 29.3 71 13 48 19 104 102

16.4 38 4.0 17.4 3.60 1.05 2.91 0.50 0.50 1.40 0.16 0.98 0.24 499 5.5 170 18.8 555 34.0 80 22 71 16 138 95

18.1 47 5.3 23.3 4.71 1.33 4.10 0.60 0.63 1.70 0.27 1.58 0.35 489 6.1 179 18.1 614 36.0 78 20 67 15 113 76

23.1 50 6.0 26.7 5.25 1.48 5.01 0.70 0.78 2.18 0.31 1.98 0.37 553 9.9 219 26.1 678 47.9 79 22 98 20 146 95

10.4 24.4 3.2 12.6 2.80 0.88 2.55 0.45 0.47 1.28 0.19 1.22 0.24 417 8.3 164 21.3 466 34.0 74 22 110 24 173 109

19.3 42 4.8 20.7 4.10 1.16 3.80 0.60 0.64 1.77 0.24 1.66 0.26 459 6.2 174 17.8 454 38.0 82 21 126 18 194 86

21.0 46 5.4 23.1 7.10 143 4.70 0.80 0.86 2.19 0.34 2.04 0.45 486 13.9 213 22.7 494 40.2 77 21 59 18 118 85

10.8 23 2.8 12.1 2.65 0.85 2.62 0.40 0.43 1.24 0.19 1.30 0.15 412 4.5 130 17.0 529 32.6 62 10 36 14 70 50

17.2 48 4.5 19.3 4.10 1.12 3.90 0.58 0.72 2.11 0.28 1.99 0.38 612 9.8 223 21.5 404 52.6 75 16 46 14 88 72

Ref.: Same as in Table 1. Rock type: BA s Basaltic andesite, A s Andesite, D s Dacite.

SCN rocks fall within the ‘‘mantle array’’ on the 87 Srr86 Sr y143 Ndr144 Nd plot of Fig. 9.

5. Discussion: origin of the SCN evolved magmas The most important geochemical characteristics of the SCN magmas that must be explained by any model are: Ž1. generally lower REE concentrations in the andesitic and dacitic rocks as compared to the mafic rocks; Ž2. similar Pb isotopic compositions but slightly higher 87 Srr86 Sr and somewhat lower 143 Ndr144 Nd of evolved rocks compared to mafic rocks; and Ž3. lower Nb concentrations and higher BarNb ratios in the evolved rocks compared to the mafic ones. Several different models are considered here and evaluated using the available geochemical and iso-

topic data from the SCN ŽTables 1–5. as well as for different source components ŽTables 6 and 7.. Although there is no direct evidence for the compositions of the mantle or the crust beneath the SCN, geochemical and isotopic data are available for mantle and crustal xenoliths and crustal rocks from nearby San Luis Potosı´ and Molango areas ŽRuiz et al., 1988a,b; Luhr et al., 1989b; Pier et al., 1989; Heinrich and Besch, 1992; Schaaf et al., 1994.. Similarly, compositions of altered Mid-Ocean Ridge Basalt ŽMORB. and sediments from IPOD-DSDP Site 487 located at the subducting Cocos plate corresponding to the central part of the MVB ŽFig. 1; Verma, 1999. are also used for this purpose. 5.1. Partial melting of the subducted slab Some authors have argued that the slab can melt to generate arc magmas, particularly in subduction of

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

159

Table 3 Sr and Nd isotopic data for evolved volcanic rocks from Sierra de Chichinautzin, Mexico Sample

87

Srr86 Sr

Individual runs Basaltic andesite CHI06 0.703996 " 14 0.703985 " 9 0.703970 " 20 MXC5 0.703900 " 70 Andesite CHI11

CHI09 CHI08

CHI07 CHI12 CHI03

CHI05

Dacite CHI10

CHI02

143

Ndr144 Nd

e Nd

Ref.

2.9

j

–

n

0.512862

4.4

j

0.512892

5.0

j

0.512889

4.9

j

0.512817

3.5

j

4.1

j

–

j

Average

Individual runs

Average

0.703984

0.512786 " 7 0.512783 " 9

0.512785

–

0.703704 " 9 0.703690 " 20 0.703706 " 5 0.704182 " 18 0.704160 " 14 0.703951 " 14 0.703966 " 10

0.703700

0.704333

0.512857 " 9 0.512856 " 9 0.512874 " 11 0.512881 " 7 0.512902 " 11 0.512890 " 10 0.512890 " 14 0.512886 " 14 0.512823 " 5 0.512810 " 6 0.512847 " 13

0.704212 " 11 0.704230 " 12 0.704321 " 11 0.704344 " 11 0.704377 " 10 0.704385 " 20 0.704371 " 12 0.704377 " 13 0.703990 " 18 0.703992 " 11 0.703967 " 16

0.704221

0.704378

–

0.703983

0.512851 " 14 0.512827 " 9

0.512839

3.9

j

0.703842 " 17 0.703848" 20 0.703867 " 9 0.703862 " 23 0.703857 " 18 0.704674 " 14 0.704696 " 15 0.704695 " 9

0.703855

0.512871 " 13 0.512883 " 14 0.512871 " 13

0.512875

4.6

j

0.704688

0.512719 " 13

1.6

j

0.704171 0.703959

The 87 Srr86 Sr ratios are normalized to 86 Srr88 Sr s 0.11940 and adjusted to SRM987 87 Srr86 Sr ratio of 0.710230. The measured 87 Srr86 Sr ratio for the SRM987 standard during the period of measurements of this study was 0.710216 " 11 Ž1 s ; n s 36.. The 143 Ndr144 Nd ratios are normalized to 146 Ndr144 Nd s 0.72190 and adjusted to La Jolla 143 Ndr144 Nd ratio of 0.511860. The measured 143 Ndr144 Nd ratio for the La Jolla standard was 0.511833 " 12 Ž1 s ; n s 82. during the period of measurement of about 1 year ŽSeptember, 1986–August, 1987.. e N d s wŽ143 Ndr144 Nd. m rŽ143 Ndr144 Nd. C H U R 4 y 1x10 4 ŽDePaolo and Wasserburg, 1976., using Ž143 Ndr144 Nd. C H U R s 0.512638. Further, the errors reported on individual Sr and Nd isotope ratios are 2 times the standard error of the mean Ž2 s E . multiplied by 10 6 . Ref. same as in Table 1.

young slabs Že.g., Defant et al., 1991; Peacock et al., 1994.. Although the actual position of the subducted Cocos plate beneath the SCN is largely uncertain, no deep earthquakes Žbelow about 50 km. are observed. The plate becomes largely aseismic before reaching

the volcanic front in this part of the MVB Že.g., Pardo and Suarez, 1995.. The subducted plate is ´ relatively young and can therefore melt, in principle, to generate SCN evolved magmas Že.g., proposal of Defant et al., 1991.. Its possible melting products

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160

Table 4 Pb isotopic data for evolved volcanic rocks from Sierra de Chichinautzin, Mexico Sample

206

Pbr204 Pb

Individual runs Basaltic andesite CHI06 18.719 18.716 18.720 Andesite CHI09 CHI12 CHI08

Dacite CHI02

18.638 18.634 18.712 18.717 18.666 18.672

18.710 18.708

207

Pbr204 Pb

208

Pbr204 Pb

Ref.

Average

Individual runs

Average

Individual runs

Average

18.718 " 2

15.590 15.584 15.594

15.589 " 4

38.469 38.458 38.481

38.469 " 9

j

18.636 " 2

15.588 15.589 15.615 15.615 15.600 15.597

15.588 " 1

38.398 38.390 38.522 38.526 38.455 38.460

38.394 " 4

j

38.524 " 2

j

38.457 " 3

j

38.483 " 3

j

18.714 " 3 18.669 " 3

18.709 " 1

15.596 15.595

15.615 " 1 15.598 " 2

15.595 " 1

38.480 38.486

The Pb isotope ratios are corrected for fractionation estimated by running simultaneously the NBS982 standard and are relative to values of 206 Pbr204 Pb s 36.73845, 207 Pbr204 Pb s 17.15946, 208 Pbr204 Pb s 36.74432, and 207 Pbr206 Pb s 0.46707 for this standard. All Pb data are corrected for mass fractionation Ža factor of 1.48 " 0.04; 1 s ; n s 9.. The analytical uncertainties quoted for Pb isotopes are the combined uncertainties in within-run statistics and in the estimation of fractionation correction, and are multiplied by 10 3. Ref. same as in Table 1.

Žbased on chemical and mineralogical data for MORB and sediments from DSDP Sites 487 and 488; Verma, 1999; see Fig. 1 and Table 7. are compared to andesitic and dacitic magmas from the SCN, using key parameters, such as Pb-isotopic ratios, LarYb– BarNb ŽFig. 8., and 87 Srr86 Sr y143 Ndr144 Nd ŽFig. 9.. The maximum limit of about 2% sediment was modeled by Verma Ž1999. from Pb-isotopic data for the subducting Cocos plate and the SCN mafic rocks. Because the evolved andesitic and dacitic rocks from the SCN have very similar Pb-isotopic compositions to the mafic rocks, such a conclusion will also be valid for these evolved rocks. Therefore, the altered MORB and the mixture of 98:2 altered MORBq sediment represent the limits of source composition for the partial melting of the subducting Cocos plate. Their equilibrium partial melting represents partial melts up to a small Ž1%. degree of partial melting Žheavy dotted lines M and MS2 in Fig. 8; MORB mineralogy in Table 7.. The LarYb ratios of magmas from the SCN are significantly greater than even the low-degree partial melts of the subducted slab.

Furthermore, the hy-normative nature of nearly all SCN magmas is not consistent for them to be such low-degree melts Ž1%.. If garnet is assumed in the altered MORB source Žeclogite mineralogy., the LarYb ratios of equilibrium partial melts can be easily matched with the andesitic and dacitic rocks from the SCN. Similarly, partial melting of mafic granulite ŽMLC. type composition ŽTables 6 and 7., having higher LarYb than the subducting Cocos plate ŽFig. 8., should generate magmas with trace element characteristics similar to those of evolved andesitic and dacitic rocks. However, the segment of a mixing curve between MORB and sediments from Site 487 Židentified by 2, 5, 10, and 20%; Fig. 9., representing 2% to 20% sediment component in the MORB-sediment mixture, lies far to the right of all the SCN rocks and therefore, partial melting of the subducted Cocos plate cannot generate the SCN magmas. 5.2. Simple differentiation of mafic magmas Verma Ž1999. has shown that the mafic magmas in the SCN could be generated by partial melting of

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

161

Table 5 Average composition and other statistical parameters for the SCN rocks ElementrIsotope ratio B n SiO 2 TiO 2 Al 2 O 3 FeO t MnO MgO CaO Na 2 O K 2O P2 O5 La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu Ba Nb Zr Y Sr Rb Ni Cr V Co Cs Hf Ta Th U 87 Srr86 Sr 143 Ndr144 Nd 206 Pbr204 Pb 207 Pbr204 Pb 208 Pbr204 Pb

BTA x " 1s

n

BA x " 1s

9 51.1 " 0.5 11 53.5 " 1.0 8 1.73 " 0.09 11 1.78 " 0.40 8 15.71 " 0.41 11 15.4 " 1.7 8 9.11 " 0.33 11 9.1 " 1.5 8 0.15 " 0.01 11 0.13 " 0.04 8 8.66 " 0.31 11 6.6 " 1.3 8 8.4 " 0.7 11 7.22 " 0.38 8 3.57 " 0.27 11 4.15 " 0.34 8 1.17 " 0.16 11 1.52 " 0.26 8 0.41 " 0.14 11 0.56 " 0.22 4 19 " 5 4 25 " 7 3 47 " 16 2 66 " 22 3 5.4 " 1.8 2 7.7 " 2.8 3 25 " 9 2 34 " 11 3 5.7 " 2.3 2 7.0 " 1.9 4 1.8 " 0.5 4 2.05 " 0.27 3 5.5 " 2.1 2 6.8 " 1.9 4 0.81 " 0.25 4 0.92 " 0.25 3 0.90 " 0.34 2 1.17 " 0.35 3 2.5 " 0.9 2 3.2 " 0.8 3 0.36 " 0.14 2 0.44 " 0.13 3 2.3 " 0.9 2 3.3 " 1.6 3 0.39 " 0.16 2 0.53 " 0.18 7 292 " 33 7 430 " 80 3 18.2 " 1.5 5 25 " 6 4 214 " 13 7 290 " 80 3 29.5 " 0.7 5 36 " 6 8 520 " 50 7 530 " 43 8 18.4 " 2.7 7 27 " 5 6 147 " 15 7 132 " 33 4 295 " 39 7 209 " 35 3 151 " 7 3 136 " 14 4 39 " 8 6 35 " 9 3 0.47 " 0.06 2 0.74 " 0.44 2 5.0 " 0.5 2 5.0 " 0.7 2 1.38 " 0.30 2 1.28 " 0.13 3 1.4 " 0.8 4 4.0 " 1.2 2 0.75 " 0.21 2 0.84 " 0.11 5 0.70366 " 19 5 0.70409 " 41 4 0.51288 " 4 2 0.51284 " 12 2 18.752 " 0.012 1 18.650 2 15.609 " 0.003 1 15.570 2 38.518 " 0.018 1 38.343

n 20 17 17 17 17 17 17 17 17 17 1 1 1 1 1 1 1 1 1 1 1 1 1 10 7 7 7 14 14 11 11 3 5 3

8 7 1 1 1 1

A x " 1s 54.6 " 1.0 1.3 " 0.5 16.7 " 0.8 7.8 " 0.6 0.14 " 0.04 6.2 " 1.5 7.7 " 0.6 3.80 " 0.27 1.25 " 0.23 0.34 " 0.22 17.7 44 4.9 22.1 4.53 1.41 4.53 0.70 0.80 2.30 0.36 2.14 0.50 430 " 150 12 " 7 220 " 90 27 " 6 500 " 80 26 " 5 110 " 80 210 " 110 148 " 18 27 " 5 0.91 " 0.17 – – 3.8 " 0.9 – 0.70408 " 20 0.51278 18.718 15.589 38.469

n 33 32 32 32 32 32 32 32 32 32 7 7 7 7 7 7 7 7 7 7 7 7 7 19 18 18 18 19 19 18 18 7 18 1

11 8 6 3 3 3

TA x " 1s

n

60.6 " 1.5 3 0.85 " 0.17 3 16.2 " 0.7 3 5.8 " 0.7 3 0.10 " 0.04 3 4.6 " 1.0 3 5.7 " 0.7 3 4.01 " 0.39 3 1.70 " 0.33 3 0.22 " 0.08 3 17.8 " 4.1 40 " 9 4.6 " 1.1 20 " 5 4.4 " 1.5 1.19 " 0.23 3.8 " 0.9 0.62 " 0.12 0.67 " 0.15 1.78 " 0.35 0.27 " 0.05 1.66 " 0.44 0.35 " 0.08 500 " 120 1 6.6 " 2.9 1 175 " 41 1 20.8 " 3.3 1 550 " 110 1 35 " 8 1 74 " 39 1 130 " 60 1 93 " 11 18.4 " 3.8 1 1.61 – – 4.9 " 0.9 1 – 0.70412 " 22 0.51286 " 3 18.673 " 0.039 15.600 " 0.014 38.46 " 0.07

D x " 1s

n

58.1 " 0.9 0.80 " 0.16 15.6 " 2.0 7.5 " 0.8 0.11 " 0.01 4.9 " 1.1 6.49 " 0.16 4.31 " 0.23 1.98 " 0.14 0.32 " 0.23 – – – – – – – – – – – – – 1005 4 122 28 1025 34 30 40 – 19 – – – 4 – – – – – –

17 17 17 17 17 17 17 17 17 17 2 2 2 2 2 2 2 2 2 2 2 2 2 11 11 11 11 12 12 12 12 2 11

10 4 2 1 1 1

x " 1s 64.2 " 1.1 0.69 " 0.13 16.8 " 1.1 4.41 " 0.46 0.09 " 0.02 2.5 " 0.8 4.92 " 0.43 4.30 " 0.33 1.83 " 0.31 0.19 " 0.05 14 " 5 36 " 18 3.6 " 1.2 16 " 5 3.4 " 1.0 0.98 " 0.19 3.3 " 0.9 0.49 " 0.13 0.56 " 0.21 1.7 " 0.6 0.24 " 0.06 1.6 " 0.5 0.25 " 0.18 520 " 90 5.3 " 1.7 160 " 30 17.2 " 2.0 560 " 90 37 " 9 26 " 21 46 " 40 61 " 16 11.0 " 2.3 – – – 4.8 " 1.0 – 0.70384 " 62 0.51280 " 11 18.709 15.595 38.483

Uncertainties on major and trace elements are one standard deviation Ž1s. values of the mean Žx. of n determinations for each rock-type. Average 87 Srr86 Sr and 143 Ndr144 Nd are reported to five decimal places with uncertainties expressed as one standard deviation multiplied by 10 5. For literature sources see Fig. 3. Note that Bloomfield’s data may be biased and therefore were not included in these computations.

the upper mantle. However, the generation of the andesitic and dacitic rocks from simple fractional crystallization ŽFC. of mafic magmas does not seem

to be supported by the analytical data presented here, particularly the REE data. All andesitic and dacitic rocks have REE concentrations lower than most

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

162

Table 6 Average composition of mafic lower crust and partition coefficients used in modeling Elementr Isotope ration

MLC n

La Ce Nd Sm Eu Tb Yb Lu Sr K 2O Rb Ba Nb P2 O5 Zr TiO 2 Y Cr SiO 2 87 Srr86 Sr 143 Ndr144 Nd

8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

x"1s 6.1"2.1 16"6 10.4"4.2 3.4"1.2 1.30"0.28 0.66"0.22 2.1"0.8 0.34"0.14 390"300 0.44"0.25 4.4"2.1 130"60 8"7 0.13"0.06 71"31 1.3"0.8 22"7 300"180 49.0"2.9 0.70450"60 0.51276"13

Partition coefficients D ML C1 D MLC2 0.097 0.103 0.181 0.398 0.550 0.460 2.278 3.498 0.560 0.154 0.068 0.300 0.121 – 0.295 0.578 1.920 5.840 – – –

0.090 0.096 0.164 0.403 0.504 0.482 2.312 3.533 0.419 0.186 0.076 0.238 0.164 – 0.304 0.621 1.927 6.855 – – –

MLC s mafic lower crust from San Luis Potosı´ area ŽSchaaf et al., 1994.; the uncertainties in all major and trace components as well as isotopic ratios are same as in Table 5. The number of determinations, on which mean values are based, is in the column n. Mineralrmelt partition coefficients were calculated as mean values of individual data complied from several sources: Nicholls and Harris Ž1980.; Green and Pearson Ž1987.; Green et al. Ž1989.; Nielson et al. Ž1992.; Rollinson Ž1993.; Dunn and Sen Ž1994.; Forsythe et al. Ž1994.; Hack et al. Ž1994.; Hauri et al. Ž1994.; Jenner et al. Ž1994.; Skulski et al. Ž1994.. Then bulk partition coefficient data were estimated for two different MLC mineralogies as follows: Ž1. D ML C1 from 30% plagioclase, 30% clinopyroxene, 20% garnet, 5% orthopyroxene, 10% amphibole, and 5% opaque minerals; Ž2. D ML C2 for 20% plagioclase, 20% clinopyroxene, 20% garnet, 20% orthopyroxene, 15% amphibole, and 5% opaque minerals.

mafic rocks, except CHI13 ŽFig. 6.. Even this basalt has higher REE than a few evolved rocks from the SCN. The results of calculations of FC removal of 50% Plg, Ol, or Opx, from average SCN basalt ŽFig. 8. and basaltic trachy-andesite Žresults not included in Fig. 8. show that although the range of LarYb ratio in the SCN rocks could in theory be explained by an extreme participation of Opx, variation in BarNb ratio is larger than expected for simple FC.

Predicted Nb concentrations for evolved magmas are greater than the initial mafic magma Že.g., average SCN basalt or basaltic trachy-andesite, Table 5.. However, Nb, Zr, and Y concentrations measured in andesitic and dacitic magmas from the SCN are lower than the mafic magmas ŽTable 5., which precludes simple FC as a viable process for generating andesitic and dacitic magmas in the SCN. It is of interest to mention that such a decrease with increasing SiO 2 has also been observed for rift-related basalt, trachybasalt, and basaltic andesite rocks from continental extension in Eastern China ŽFan and Hooper, 1991.. Finally, small but significant differ-

Table 7 Concentration and partition coefficient data for different source components used in modeling computations Source La

Yb

Ba

MORB Sed. MS2 FMS2 MF2 PM MLC LC Av. B CHI13

1.98 3.80 2.02 0.58 0.54 0.493 2.1 2.9 2.3 1.29

7.00 1.50 1640 8.60 39.6 1.64 26.8 0.21 16.9 0.46 6.987 0.713 130 8 280 13 292 18.2 354 17.1

D MO RB D TM DAF C D PlgŽB. DOlŽB. DOp xŽB.

2.23 29.0 2.77 0.95 0.82 0.687 6.1 13.5 19.0 11.4 0.081 0.045 0.071 0.1275 0.0053 0.0021

0.370 0.259 0.355 0.0091 0.029 0.655

Nb

0.141 0.001 0.175 0.915 0.01 0.015

LarYb BarNb 1.13 7.63 1.37 1.63 1.52 1.39 4.7 4.7 8.3 8.84

4.67 190.7 24.1 127.4 36.6 9.80 21.5 21.5 16.0 20.7

0.058 0.052 0.008 0.0265 0.004 0.027

MORBsaltered MORB average data from Site 487 ŽVerma, 1999.; Sed.s overlying sediment average from Site 487 ŽVerma, 1999.; FMS2 s fluid released from 98:2 mixture of altered MORB and sediment from Site 487; PMs Primitive mantle ŽSun and McDonough, 1989.; MLC saverage mafic lower crust, LC s average lower crust ŽSchaaf et al., 1994.; Av. Bsaverage basalt data from Table 5; D MO RB was estimated for a mineralogy of 40% Ol, 35% Cpx, 20% Plg, and 5% opaque minerals; D PM was similarly estimated for mineralogy of 50% Ol, 30% Opx, 15% Cpx, and 5% Spinel ŽSpinel D’s were not available and were replaced by D’s for opaque minerals.. This particular set of DAF C is for a mineralogy of 25% Ol, 25% Opx, 25% Cpx, and 25% Plg. Individual mineralrmelt partition coefficient data Ž D PlgŽB. , etc.. are from Henderson Ž1982., Wilson Ž1991., Lopez Ruiz and ´ Ž1990., Hart and Dunn Ž1993., Rollinson Ž1993., Cabria´ Gomez ´ Dunn and Sen Ž1994., and Green Ž1994..

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

163

Fig. 3. A total alkali–silica ŽTAS. diagram for the SCN rocks. The abbreviations for the fields are: B s basalt; T_Bs trachybasalt; BTA s basaltic trachy-andesite; TA s trachy-andesite; BA s basaltic andesite; A s andesite; D s dacite. The literature references are: V99 s Verma Ž1999.; M89 s Martin del Pozzo Ž1989.; S89 s Swinamer Ž1989.; NI88 s Nixon Ž1988.; R82 s Robin Ž1982.; B75 s Bloomfield Ž1975.; B73 s Bloomfield Ž1973.; NE72 s Negendank Ž1972.; GM71s Gunn and Mooser Ž1971..

ences in 87 Srr86 Sr and to a lesser extent in 143 Ndr144 Nd ŽFig. 9. rule out a simple FC of mafic magmas to generate evolved andesitic and dacitic magmas in the SCN.

5.3. Mantle melting in presence of fluids released from subducted slab If the mantle wedge melts in the presence of fluids from the subducted Cocos plate and the resulting mafic magmas undergo differentiation before eruption, this combined process could generate evolved magmas of the SCN. Partial melting of the upper mantle is shown schematically in Fig. 8, using the data for a primitive mantle inferred by Sun and McDonough Ž1989.. Although chemical and isotopic data were reported for spinel-lherzolite xenoliths in alkali basalt from San Luis Potosı´ area in Mexico ŽHeinrich and Besch, 1992; Schaaf et al., 1994., only La and Yb but no Ba or Nb data were available and therefore could not be used for modeling estimates in Fig. 8. The extreme composition on the mantle partial melting curve ŽPM. is for 1% melting and lies somewhat away from the andesites of the SCN.

In order to test models of mantle melting in the presence of fluids released from the subducting Cocos plate, as a first approximation the possible composition of such fluids is estimated from relative element mobility as a function of ionic radiusrcharge ratio ŽTatsumi et al., 1986; Hawkesworth et al., 1991.. If such fluids Že.g., FMS2 in Fig. 8. released from the subducted Cocos plate mix with the overlying mantle Žrepresented by solid circle. and the resulting 50:50 mixture Žshown by solid triangle. melts partially ŽMF2 dashed curve in Fig. 8., this process will not generate the LarYb ratios of any of the SCN rocks. This would be worse if a MORB-type mantle were assumed to underlie the SCN area. In fact, a fluid, with still higher LarYb ratio than these fluids released from the slab, is required to mix with the primitive mantle. Further, the arguments based on radiogenic isotopes ŽVerma, 1999. also apply to rule out this model Žfluids released from the slab and their mixing with the overlying mantle wedge prior to melting. as a possible mechanism for the generation of the SCN magmas. Finally, although it is not possible at present to fully constrain this model and obtain mass-balance estimates, other major and trace element data are also generally inconsistent with this hypothesis.

164

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

Fig. 4. Major element plots against MgO for all SCN rocks. tw s this work; V99 s Verma Ž1999.; lit.s all literature references given in Fig. 3. The final box shows the symbols used in these plots. Rock-types ŽB, BTA, BA, A, and D. are same as in Fig. 3.

5.4. Crustal contamination and AFC of mafic magmas The general failure of the above simple alternative models to explain the geochemical characteristics of the SCN magmas requires consideration of a more complicated model. Therefore, a combined assimilation of crust and fractional crystallization of common minerals ŽAFC process; DePaolo, 1981. from a parental basaltic magma is evaluated in this section. If significantly large proportions of the crust were to be assimilated to generate the SCN evolved magmas through AFC, the geochemical characteristics of the SCN rocks require that the crustal end-member should have very low REE concentrations Žless than

the lowest REE values in the SCN evolved rocks., low Nb Žless than a few ppm., and rather similar radiogenic isotope compositions to the evolved SCN rocks. Two different assimilants were tested: average composition of eight mafic metaigneous xenoliths Žshown by MLC in Fig. 8. and of all fourteen mafic, intermediate and metasedimentary xenoliths ŽLC. from the SLP area ŽSchaaf et al., 1994.. Several different possibilities in terms of bulk partition coefficients and assimilantrfractionated mass ratios were evaluated. For most mass ratios of assimilantrfractionating phases and for fractions of liquid remaining equivalent to less than about 0.75, the predicted La, Yb, and Nb concentrations became too large to be

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

165

Fig. 5. SiO 2 versus Mga for the SCN rocks. The symbols used are shown as an inset. The rock-types are same as in Fig. 3. Biased data from Bloomfield Ž1973; 1975. are not included in this diagram.

consistent with the andesitic and dacitic melts from the SCN Žresults not shown.. Representative results of AFC modeling for the evaluation of the REE, Ba, and Nb data are presented in Fig. 8 Žtwo overlapping dotted curves

identified by AFC., using assimilation to fractional crystallization ratio of 0.2 and fraction of liquid remaining up to 0.1. Average basaltic parental magma ŽTable 5. was used as initial composition and two different assimilants ŽMLC and LC. were evaluated.

Fig. 6. Chondrite-normalized REE plots for selected SCN rocks ŽBTA, B; BA, A, and D are same as in Fig. 3.. The BTA and B data are from Verma Ž1999.. The chondrite REE data used for normalization are Žin ppm.: La s 0.329, Ce s 0.865, Pr s 0.112, Nd s 0.63, Sm s 0.203, Eu s 0.077, Gd s 0.276, Tb s 0.047, Dy s 0.343, Ho s 0.07, Er s 0.225, Tm s 0.03, Yb s 0.22, and Lu s 0.0339 Žafter Haskin et al., 1968; Nakamura, 1974.. All rocks are also identified by their SiO 2 concentrations Žadjusted 100% anhydrous basis..

166

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

Fig. 7. Multi-element normalized diagrams for average dacite from the SCN ŽTable 5., average mafic lower crust ŽMLC. from San Luis Potosı´ ŽSchaaf et al., 1994., and 50% partial melt of the MLC. The set of continuous curves represent 50% partial melts of x " 1s Žmean " one standard deviation value. of the MLC ŽTable 6.; the other set of dotted curves are for x " 1s values of SCN dacites ŽTable 5.. Ža. chondrite-normalized REE diagram, model using D ML C1 ŽTable 6.; Žb. REE diagram, model using D MLC2 ŽTable 6.; Žc. MORB-normalized diagram, model using D ML C1; and Žd. MORB-normalized diagram, model using D MLC2 . Chondrite normalization data are given in Fig. 6. MORB values for normalization are Žin ppm unless specified.: Sr s 120, K 2 O s 0.15%, Rb s 2.0, Ba s 20, Nb s 3.5, Ce s 10, P2 O5 s 0.12%, Zr s 90, Sm s 3.3, TiO 2 s 1.5%, Y s 30, Yb s 3.4, and Cr s 250 Žafter Pearce, 1982..

The results clearly show that although the LarYb ratios of evolved andesitic to dacitic magmas from the SCN can be matched, the BarNb ratios predicted by AFC are too low ŽFig. 8. and the La, Yb, and Nb concentrations too high for the process to be viable for the SCN rocks. Another diagram ŽBarNb versus Nb; not included here for space limitations. for the SCN rocks also suggests a similar conclusion. Even when the assimilantrfractionating mass ratio was assumed to be rather high Ž0.9., none of the two parameters Žneither Nb concentration nor BarNb ratio. predicted by the AFC model could be matched

with the decreasing Nb and increasing BarNb ratios in the SCN evolved rocks. However, Nb data could be reconciled with an AFC model if amphiboles or micas rich in Nb were dominantly involved in the fractionation of mafic magmas ŽIonov and Hofmann, 1995.. Nevertheless, these minerals have not been observed as major modal phases in the SCN rocks. 5.5. Partial melting of lower crust and magma mixing None of the above models satisfactorily explains the combined geochemical and isotopic character-

S.P. Verma r Journal of Volcanology and Geothermal Research 93 (1999) 151–171

Fig. 8. LarYb–BarNb plot for the SCN rocks. The abbreviations for rock-types are same as in Fig. 3. The models included give the predicted compositions for: Ž1. equilibrium partial melting of subducting altered MORB from Site 487 on Cocos plate Žsolid square. represented by heavy dotted curve ŽM.; Ž2. equilibrium partial melting of a mixture of 98% altered MORB and 2% sediment from Site 487 on Cocos plate Žsolid square. represented by heavy dotted curve ŽMS2.; Ž3. equilibrium partial melting of primitive mantle Žsolid circle. using average data from Sun and McDonough Ž1989., represented by heavy dashed curve ŽPM.; partial melting of a 50:50 mixture of primitive mantle with fluids released from 98:2 altered MORB and sediment Žfluid composition FMS2 is shown by q symbol., represented by a solid triangle and a heavy dashed curve ŽMF2.. The extreme compositions on all curves correspond to 1% equilibrium partial melting of the source. Estimates of average mafic lower crust ŽMLC., intermediate lower crust ŽILC., and lower crust ŽLC. are included using filled diamonds from a nearby area of San Luis Potosı´ ŽSchaaf et al., 1994.. Also included are results of AFC modeling Žtwo overlapping dotted curves marked AFC. using SCN average basalt ŽTable 5. and two different assimilants represented by average of mafic granulites ŽMLC. and of all granulites ŽLC. from San Luis Potosı. ´ Similarly, simple fractional crystallization ŽFC. of 50% plagioclase ŽPlg., olivine ŽOl., or orthopyroxene ŽOpx. from SCN average basalt ŽTable 5. are shown by continuous curves. Ol fractionation leaves both LarYb and BarNb ratios practically unchanged.

istics of the SCN magmas. Partial melting of lower crust is evaluated as a viable process to generate at least some of the evolved rocks. Smith and Leeman Ž1987. proposed a similar model for the petrogenesis of Mount St. Helens dacitic magmas. The lower crust represented by eight analyses of mafic granulite xenoliths from the SLP area Žmafic lower crust-MLC. is heterogeneous ŽTable 6. and its melting is likely to produce a wide compositional range of andesitic to dacitic melts, unless the melting is controlled by an ‘‘invariant point’’ assemblage. The mineralogy of these granulitic xenoliths ŽTable 6. is inferred from

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the data presented by Schaaf et al. Ž1994.. High-degree Ž50%. partial melts of average MLC are generally similar in their light- and heavy-REE to the average dacite from the SCN ŽFig. 7a, b., except for the middle-REE ŽSm to Tb.. Predicted compositions of such partial melts of heterogeneous MLC Žrepresented by x " 1s; set of two continuous curves in Fig. 7a–d. show overlapping patterns to the dacites Žrepresented by x " 1s; set of dotted curves. from the SCN. The predicted LarYb ratios ŽFig. 7a, b. and negative Nb anomalies ŽFig. 7c, d. of these partial melts are very similar to the SCN dacites. However, discrepancies between predicted and measured compositions are observed for middle-REE ŽSm to Tb. and some LILE ŽK, Rb, and Ba.. Nevertheless, these two sets of compositions could be fully matched if the lower crust beneath the SCN had a slightly lower Sm to Tb and a somewhat higher K, Rb, and Ba concentrations than the MLC from San Luis Potosı, ´ or if minor mineral phases possibly

Fig. 9. 87 Srr86 Sr– 143 Ndr144 Nd plot for the SCN rocks. The size of the symbols used for these rocks is similar to or even larger than the analytical errors involved. Approximate trace of ‘‘Mantle-array’’ is shown for easy reference using dashed lines. The mixing curve identified by 2, 5, 10, and 20% represents a physical mixture of Site 487 altered MORBqsediment, where the numbers refer to the percent of the sediment component in this mixture ŽVerma, 1999.. All individual analyses of mafic granulites from San Luis Potosı´ area are shown by small solid circles. The field marked by dotted curve encloses these mafic granulites. The average mafic lower crust Žsolid circle represented by MLC. based on eight analyses and lower crust Žsolid diamond represented by LC. based on fourteen analyses of mafic and intermediate metaigneous and metasedimentary granulites from San Luis Potosı´ area are also included for easy reference ŽSchaaf et al., 1994..

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present in the MLC were taken into account in the partial melting models. The available Sr and Nd isotopic data are consistent with this possible mechanism to generate at least some of the SCN evolved magmas, if mafic lower crust similar to the mafic granulitic xenoliths ŽSchaaf et al., 1994., was involved in melting Žsolid circles in Fig. 9., simply because the field of these mafic xenoliths Ždotted area. encloses all isotopic data for evolved SCN rocks Žtriangles and squares in Fig. 9.. The upper mantle is also likely to be heterogeneous Žalthough probably less so than the lower crust. and could produce wide chemical compositions of mafic magmas. These could be subjected to crystal fractionation effects of some of the common minerals, such as olivine, prior to eruption ŽVerma, 1999.. Thus, as explained below, two different source regions, both heterogeneous, seem to be involved in the genesis of the SCN magmas: a spinel-lherzolite source in the upper mantle for the mafic magmas Žbasalt and basaltic trachy-andesite. and a maficgranulite source in the lower crust for the evolved Žandesitic and dacitic. magmas. Both of these groups of magmas are likely to be chemically and isotopically heterogeneous. Their mixing prior to eruption could generate magmas of intermediate compositions. The parallel REE patterns observed for the SCN rocks as well as their apparent inversion with respect to their SiO 2 concentrations Žrocks with higher SiO 2 have generally lower REE concentrations; see Fig. 6. can be easily explained by mixing of two heterogeneous end-members: Ž1. basalt and basaltic trachyandesite from the upper mantle Žfor example, CHI04, CHI01, and CUI1 with higher REE concentrations; Fig. 6. and Ž2. evolved andesite and dacite from the lower crust ŽCHI12 and CHI10 with lower REE concentrations; Fig. 6.. The parallel REE patterns are a strong argument in favor of a melting rather than a fractionation origin for the SCN evolved magmas. There is ample petrographic evidence consistent with this mixing scenario ŽMarquez Gonzalez, 1998; ´ ´ Marquez et al., 1999b.. Similarly, mixing of two ´ heterogeneous end-members in the BarNb versus LarYb diagram ŽFig. 8; mixing curve not shown. can explain the compositions of other intermediate magmas Že.g., basaltic andesite. erupted in the SCN. Further FC or AFC of these magmas in the middle

and upper crust could contribute additional complexities to the SCN magmas. However, the similarities in isotopic ratios between the mafic and evolved rocks rule out a major role for upper crust in the petrogenesis of SCN magmas, because compared to the SCN rocks there is a large contrast in the isotopic compositions of upper crustal intrusive and sedimentary rocks beneath the MVB ŽS.P. Verma, unpublished data.. This also requires that the lower crust should have very similar Pb-isotopic ratios to the SCN volcanic rocks. Another possibility for an AFC model to be successful might be to presume a different type of mafic basaltic magma with higher BarNb and 87 Srr86 Sr, but slightly lower 143 Ndr144 Nd than the available SCN mafic magmas, which has not actually erupted in the SCN. The main difficulty is assuming such a hypothesis is that the reasons for its non-eruption are not clear. A more comprehensive treatment of this mixing model will only be warranted when detailed microprobe major and trace element mineral data for the SCN rocks and Pb-isotopic and key trace elements such as Ba and Nb in Mexican lower crust become available, leading to well-constrained mass-balance estimates and mineralrmagma partition coefficients.

6. Final remarks In spite of the evidence for ongoing subduction beneath central Mexico, the location of the subducted Cocos plate below about 50 km is not well constrained ŽSingh and Pardo, 1993; Pardo and Suarez, 1995.. The mafic magmas in the SCN have ´ been shown to be generated from the upper mantle, without a material contribution from the subducted slab ŽVerma, 1999.. There is abundant evidence for the presence of major faulting and rifting in several areas of the MVB, including close to the SCN ŽSuter et al., 1992, 1995; Campos-Enrıquez et al., 1999.. In ´ fact, a N–S extension has recently been inferred for the SCN from the main E–W trend of the lineaments traced by remote sensing and the volcanic center alignments ŽMarquez et al., 1997, 1999b.. This in´ ferred extension in the SCN seems to have facilitated passage of relatively low-volume batches of partial melts both from the upper mantle and the lower

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crust, as well as the mixing of these two types of melts at sub-crustal depths, possibly followed by FC or AFC processes in the middle to upper crust, prior to eruption. These results have direct bearing to magma genesis in other volcanic areas with complex tectonic setting. The actual thermal regime, crustal thickness and compositions, as well as major and trace element compositions of constituent minerals must be evaluated in detail to further constrain the complex magmatic processes involved in this large monogenetic field at the volcanic front of the central MVB.

7. Conclusions On the basis of new geochemical and isotopic data for andesitic and dacitic rocks from the SCN and published data on the SCN mafic magmas and the upper part of the subducting Cocos plate, as well as estimates for the upper mantle and lower crust from a nearby area of San Luis Potosı, ´ the origin of the SCN can be visualized in terms of two distinct sources. The mafic magmas were produced from partial melting of a heterogeneous upper mantle, whereas the evolved magmas were generated in a more heterogeneous lower crust. Finally, mixing of mantle-derived basaltic and lower crustal andesitic and dacitic magmas, prior to eruption, seems to be a viable process for the genesis of the intermediate compositions in the SCN.

Acknowledgements I am grateful to: Al Hofmann and B. Schultz Dobrich for use of experimental facilities at MPI and Univ. Mainz, respectively; AvH Foundation and IIE for support to carry out the analytical work in Germany; Marcos Milan ´ and Gerardo Carrasco for help in the field; Fernando Velasco, Alvaro Marquez, ´ Ignacio Torres, and Manuel Vasconcelos for help with preparation of the first two figures. This work was partly supported by CONACyT grant 400359-50196P-T and DGAPA grant ES-100596. I am also grateful to Jim Luhr, Joaquın ´ Ruiz, Anita Grunder, an anonymous reviewer, and the editor Brush Marsh;

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all of them provided many helpful comments on earlier versions of this paper. The anonymous reviewer was very critical of this work and therefore his comments were particularly helpful to refine my presentation.

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