Sr, Nd, and Pb isotopic evidence for the origin and evolution of the Cántaro–Colima volcanic chain, Western Mexican Volcanic Belt

Journal of Volcanology and Geothermal Research 197 (2010) 33–51

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

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Sr, Nd, and Pb isotopic evidence for the origin and evolution of the Cántaro–Colima volcanic chain, Western Mexican Volcanic Belt Surendra P. Verma a,⁎, James F. Luhr b,1 a Departamento de Sistemas Energéticos, Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Priv. Xochicalco s/no., Col. Centro, Apartado Postal 34, Temixco, Mor. 62580, Mexico b Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, NHB-119, Washington, D.C. 20013-7012, USA

a r t i c l e

i n f o

Article history: Received 6 August 2008 Accepted 19 August 2010 Available online 15 September 2010 Keywords: Mexico radiogenic isotopes rift subduction geochemistry arc

a b s t r a c t We report new geochemical and Sr, Nd, and Pb radiogenic isotope data on Pleistocene to Recent subalkaline, basanite–minette, and mixed alkaline magmas from the Cántaro–Colima volcanic chain in the western part of the Mexican Volcanic Belt (MVB). All rocks from this area, including the basanites and minettes, show enrichment in LILE (e.g., Cs, Ba, and Sr) and depletion in HFSE (e.g., Ta and Ti), generally considered to be a characteristic of subduction-related magmas. The isotopic ratios of Cántaro–Colima rocks show the following ranges: 87Sr/86Sr 0.70282–0.70395, 143Nd/144Nd 0.51282–0.51305, 206Pb/204Pb 18.54–18.70, 207Pb/204Pb 15.53–15.61, and 208Pb/204Pb 38.09–38.52. The most mafic basaltic andesite from Volcán Cántaro is isotopically the most depleted rock, similar to MORB and oceanic basalts from seamounts in the adjacent oceanic basins. It has the least radiogenic Sr and Pb and the most radiogenic Nd yet measured from any locality in the MVB. The combined geochemical and isotopic evidence is compatible with the generation of both alkaline and subalkaline magmas of the Cántaro–Colima volcanic chain in the mantle wedge by fluidtransport to the mantle from the subducted plate. The isotopic data are also generally consistent with the actual physical mixing of subalkaline and alkaline basanite–minette magmas to generate intermediate alkaline magmas. The geochemical and isotopic data from the western part of the MVB, including the Cántaro– Colima volcanic chain, however, also reflect the tectonic complexity of simultaneously ongoing subduction and rifting, requiring more complex petrogenetic processes and a chemically and isotopically heterogeneous mantle. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In this paper we discuss the evolution of magma compositions accompanying trenchward migration of volcanism along the Cántaro– Colima volcanic chain in the Colima Rift based on abundances of 37 elements and 5 radiogenic isotope ratios in a total of 15 whole-rock samples (less for Pb isotopes). We also discuss the relationship between the two Quaternary magma types of the Colima Rift, subalkaline basalts to andesites and alkaline basanites to minettes, including analyses of several natural mixtures of the two magma types that erupted in the area. Finally, we use an extensive geochemical database from the W-MVB and elsewhere to comment on the origin and evolution of this tectonically complex area. Migration of arc volcanic fronts with respect to the trenches may range from 1000 km in western North America during the Cenozoic (Coney and Reynolds, 1977; Cross and Pilger, 1978), to distances of a

⁎ Corresponding author. Tel.: +52 55 56229745; fax: +52 777 3250018. E-mail address: [email protected] (S.P. Verma). 1 Deceased 1st January, 2007. 0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2010.08.019

few tens of km over 1 Ma. The latter is the case of the Cántaro– Colima volcanic chain (Fig. 1) of the Western Mexican Volcanic Belt (W-MVB) where subalkaline volcanism seems to have migrated progressively southward or trenchward with time. The migration rate is estimated at 13 km/m.y., using the oldest age of 1.7 Ma at Volcán Cántaro by Allan (1986) and the youngest zero age at the presently active Volcán Colima, see, e.g., Saucedo et al. (2005) or Savov et al. (2008) and a distance of about 22 km between these two volcanic centers (Fig. 1). Two major plate tectonic processes are simultaneously taking place in the W-MVB (Fig. 2): the subduction of the Rivera plate in the northern part of western Mexico and of the Cocos plate in its southern part, with a noticeable gap between the two subducted slabs forming at the Colima Rift. Both slabs, with initial shallow dips, bend sharply downward into the mantle (Yang et al., 2009) and the extension or rifting along three well-defined rift zones (Colima Rift, Tepic–Zacoalco Rift, and Chapala Rift; Luhr et al., 1985; Allan et al., 1991) creates the triple junction towards the north of the study area. These multiple processes complicate any simple subduction-related petrogenetic model to explain the origin of the Cántaro–Colima volcanic chain.

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S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

Fig. 1. Generalized location map for the area of this study. Sample locations are shown by open circles, with sample numbers next to them (sample SAY-22E cited in the text is also shown). Triangles (filled) indicate Quaternary volcanic vents (V. = Volcán), including eleven cinder cones (nine of the basanite–minette series), the peak of Volcán Cántaro and the dome C. El Ixcapil to its north, and the cones of Nevado de Colima and Volcán Colima. Four towns along Hwy 54 are also indicated for reference. The map area is shown by a rectangle on the inset map. Also on inset, the three rift areas are (Allan et al., 1991): CR = Colima Rift, TZR = Tepic–Zacoalco Rift, and ChR = Chapala Rift, and other Quaternary volcanoes mentioned in the text are shown by dots and labeled: U = Volcán San Juan, N = Volcán Las Navajas, S = Volcán Sanganguey, T = Volcán Tepetiltic, B = Volcán Ceboruco, P = Sierra La Primavera, J = Jorullo, L = Cerro La Pilita. These rift areas, other grabens and volcanoes together constitute the Western Mexican Volcanic Belt (W-MVB); BC = Baja California. Important tectonic elements of the adjacent Pacific Ocean basin are: RP = Rivera Plate, PP = Pacific Plate, CP = Cocos Plate, MAT = Middle America Trench, and MT = Manzanillo Trough.

2. The Cántaro–Colima volcanic chain The western end of the MVB is dominated by three large intersecting rifts (Colima Rift, Tepic–Zacoalco Rift, and Chapala Rift; Fig. 2) and associated volcanic centers (Luhr et al., 1985; Allan et al., 1991). From the graben intersection area, the Colima Rift runs southward for about 200 km to the Pacific coast; extension continues offshore in the Manzanillo Rift and El Gordo graben (MR and EGG in Fig. 2). These rift zones probably merge with the Cocos–Rivera Plate boundary (Bourgois et al., 1988). The northern Colima Rift has a fairly flat floor at about 1350 m elevation, occupied by several playa lakes. Near the city of Sayula, continued drainage to the south was blocked in the early Quaternary by lava flows, domes, and pyroclastic deposits related to Volcán Cántaro (Fig. 1).

The primary constructional form of Volcán Cántaro (2920 m) has been obscured by erosion. It may have been a small composite volcano, but many prominent, well-preserved andesitic–dacitic domes are also present on its northern flanks. Although outcrops are typically weathered, relatively fresh samples can be collected from blasted roadcuts, boulders in stream beds, and blocks from colluvium. Four samples from Volcán Cántaro were dated by K–Ar at 1.0 to 1.7 Ma (Allan, 1986). The most silica-rich of these samples, a biotite dacite pumice tuff (1004-505), gave an anomalously old age of 4.6 Ma, which is considered questionable (J.F. Allan, pers. comm.). Nevado de Colima, located about 17 km south of Volcán Cántaro, comprises the largest volume of volcanic rocks in the Cántaro– Colima volcanic chain (Fig. 1). Four phases for the development of Nevado de Colima, separated by three episodes of caldera formation,

S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

35

Fig. 2. Simpilifed tectonic features of the western Mexican Volcanic Belt and offshore area (modified after Luhr, 1997). The onland rifts are: CR = Colima Rift, TZR = Tepic–Zacoalco Rift, ChR = Chapala Rift. The cities are: C = Colima; G = Guadalajara; M = Manzanillo; PV = Puerto Vallarta; T = Tepic. Major volcanic centers associated to these rifts are numbered here as: 1 = Volcán San Juan; 2 = Volcán Las Navajas; 3 = Volcán Sanganguey; 4 = Volcán Tepetiltic; 5 = Volcán San Pedro; 6 = Volcán Ceboruco; 7 = Volcán Tequila; 8 = Sierra La Primavera; 9 = Volcán Cántaro; 10 = Nevado de Colima; and 11 = Volcán Colima. FZ = fracture zone. Abbreviated names of some offshore tectonic features are: MR = Manzanillo Rift; EGG = El Gordo Graben; MAT = Middle America Trench.

have been recognized (Robin et al., 1987). The ages of these eruptive phases were constrained by five whole-rock K–Ar dates, which range from about 0.53 to 0.08 Ma, considerably younger than K–Ar ages from Volcán Cántaro but older than the presently active Volcán Colima. The last eruptive phase saw the growth of a cone in the southwestern portion of the third collapsed region, thus continuing the general southward migration of eruptive activity along the Cántaro–Colima chain. This cone (4320 m), which marks the highest point in the chain, has been extensively glaciated, indicating that Nevado de Colima probably became extinct prior to the last glaciation about 8000 years ago (Robin et al., 1984, 1987). In the late Pleistocene or early Holocene the focus of activity shifted southward by another 5 km as the cone of Volcán Colima began to grow on the southern flank of Nevado. The age of the first eruption from Volcán Colima is not known, but Robin et al. (1987) argued that the active periods of the two volcanoes may have overlapped for a considerable time period. As emphasized by Komorowski et al. (1997), edifice collapse of the Mount St. Helens type has been a repeated phenomenon throughout the development of both Colima volcanoes. They estimated that these volcanoes have collapsed as many as 12 times during the last 45 ka, with as many as 9 discrete collapses for Colima volcano alone. Each of these collapses produced major debris avalanches, whose deposits now complexly interfinger and interfill to blanket a huge area to the south, extending all the way to the Pacific coast. The last of these events appears to have occurred about 2500 and 3700 years ago (Robin et al., 1984; Luhr and Carmichael, 1990a,b). Volcán Colima clearly shows a horseshoeshaped scar of just one of these events, 5 km in diameter, open to the SW. An andesitic composite cone has since grown within that region. The cone is now nearing a height of 4000 m, and is one of the most active volcanoes in North America (Luhr and Carmichael, 1980; Siebert and Simkin, 2002; Savov et al., 2008). Volcán Colima has erupted frequently in historical times (at least 25 eruptions since 1560), including major explosive eruptions in 1818 and 1913, and important lava eruptions in 1869, 1961, 1975–76, 1981–82, 1991, 1998–1999, and 2002–2004 (Luhr and Carmichael, 1990a,b; De la Cruz-Reyna, 1993; Bretón González et al., 2002; Luhr, 2002; Saucedo et al., 2005; Savov et al., 2008).

In addition to the volumetrically dominant subalkaline magmas of the Cántaro–Colima volcanoes, a much smaller volume of highly alkaline, silica-undersaturated magmas has erupted in the same area during the Quaternary to the east and west of the andesitic chain (Fig. 1). These eruptions formed a group of nine cinder cones and associated lava flows located about 13–17 km, with one of them about 30 km from the chain of large subalkaline volcanoes. Samples form a continuous series from basanites to minettes with phenocrystic phlogopite (Luhr and Carmichael, 1981). Farthest to the east (about 22 km from the chain) is Volcán Tezontle, constructed of subalkaline basalt, and farthest to the north (about 20 km NNE from Volcán Cántaro) is Volcán Usmajac, whose basaltic andesite is intermediate between the subalkaline and alkalic suites (Luhr and Carmichael, 1981). Hooper (1995) used morphological models of cinder cone degradation to estimate ages in the range 300–20 ka for the Colima cinder cones. Thus, these mostly alkaline cinder cones were erupting on the flanks of the subalkaline Cántaro–Colima volcanic chain throughout the growth of Nevado and Volcán Colima. 3. Sampling details, analytical methods, and database Whole-rock major-element compositions and modes for 17 rock specimens from Volcán Cántaro were determined by Luhr and Carmichael (1990a). Five of these samples (basaltic andesite M81-17, andesite M79-234, and three dacites M79-169, M79-231, and 1004-505), were included in this study (Fig. 1; Table 1). Major-element data for Nevado rocks (Robin et al., 1984, 1987) and Sr isotopic analyses for three Nevado andesites have been reported (Moorbath et al., 1978). Five samples (1004-122, 1004-126, 1004-121, 1004-105, and M81-21; Fig. 1 and Table 1) from Nevado were included in this study. Also included were one alkaline lava block found in a debris-avalanche deposit (M82-4) and an andesitic scoria sample (S8.1) from a fall layer that may correlate with the 3700 year old collapse event (Luhr and Carmichael, 1982; Luhr and Prestegaard, 1988). The latter sample is from a thick sequence of ashflow and scoria-fall deposits on the NE flank of ancestral Volcán Colima, downwind from the active vent; three ash alkaline scoria and ash samples from lower in this same sequence were also analyzed. We also included two historically erupted hornblende andesites: Col-30

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S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

Table 1 Major element, CIPW norm and trace element data of rocks from the Cántaro–Colima volcanic chain, Mexico. Volcano

Volcán Cántaro

Sample

M81-17

Volcán Nevado de Colima M79-234

M79-169

M79-231

1004-505

1004-122

1004-126

1004-121

1004-105

M81-21

Rock type

BA

A

D

D

D

BTA, mug

TA, ben

A

A

D

Long. (°W)

103.62588

103.60985

103.61057

103.61340

103.61033

103.61430

103.61740

103.61590

103.60790

103.58690

Lat. (°N)

19.78695

19.70698

19.82463

19.71318

19.82400

19.55800

19.56080

19.55390

19.56460

19.64830

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Sum Mg # FeOt/MgO (SiO2)adj Q Ne Hy Ol V Ni Cu Zn Y Ba Sc Cr Co Rb Sr Zr Sb Cs La Ce Nd Sm Eu Tb Dy Yb Lu Hf Ta Th U

55.28 1.07 16.59 6.83

58.83 0.63 18.08 6.20

62.51 0.55 16.50 4.37 – 0.08 2.38 4.76 4.34 2.10 0.17 1.28 99.04 57.3 1.80 64.15 16.6 – 8.0 – 82 22 12 50 11 722 8.86 22.1 11.77 25 864 135 – 0.70 16.9 34.2 17.0 3.12 0.91 0.31 – 1.00 0.152 3.14 0.231 2.49 0.92

62.80 0.50 16.68 3.97 – 0.08 1.98 4.36 4.49 1.98 0.16 1.89 98.89 56.7 1.79 64.94 8.4 – 11.9 – 67 21 13.5 49.5 11 668 8.18 22.6 10.44 35 857 129 – 0.77 17.4 35.1 16.5 3.07 0.89 0.30 – 0.96 0.145 3.23 0.238 2.43 0.84

63.58 0.47 16.14 3.68

56.51 0.96 17.46 3.87 3.00 0.11 4.09 7.25 4.31 1.83 0.50 0.03 99.93 59.7 1.58 56.69 3.5

57.49 0.92 17.29 3.49 3.05 0.11 3.94 6.91 4.09 2.04 0.42 0.29 100.04 60.7 1.57 57.73 5.8 – 11.9 – 147 15 42 43 21 612 17.1 20.6 20.0 24 1090 170 0.15 0.28 24.4 54.7 29 6.15 1.74 0.635 – 1.71 0.281 5.06 0.242 3.13 1.35

59.35 0.67 17.58 2.29 3.78 0.11 3.83 6.88 4.04 1.26 0.25 − 0.11 99.93 60.6 1.52 59.37 9.9 – 13.0 – 129 29 30 49 18 469 17.9 61.3 20.6 18.5 560 100 0.146 0.28 10.19 22.7 11.5 3.08 0.954 0.46 – 1.69 0.255 3.17 0.220 1.57 0.64

61.09 0.61 17.21 2.17 3.31 0.10 3.43 5.94 4.11 1.73 0.21 0.22 100.13 60.4 1.53 61.19 11.9 – 12.1 – 97 25 44 41 19 637 14.9 82.8 19.0 24.4 620 170 0.21 0.37 14.5 31.0 16.0 3.46 0.963 0.47 – 1.62 0.254 3.97 0.244 2.44 0.97

62.95 0.44 17.86 1.66 2.39 0.09 1.76 4.94 4.57 1.65 0.23 1.61 100.15 52.3 2.21 63.92 16.0 – 8.2 – 59 9 17 40 14 626 7.45 5.2 10.36 22.4 690 124 0.272 0.77 13.33 27.6 14.6 2.83 0.839 0.354 – 1.32 0.214 3.32 0.242 2.02 0.80

– 0.11 7.21 8.29 3.43 1.18 0.25 0.76 101.00 72.6 0.85 55.45 3.1 – 19.1 – 188 128 47 78 22 313 21.08 242 34.20 14 1371 186 – 0.17 29.1 55.7 32.0 5.78 1.80 0.59 – 1.58 0.227 4.31 0.179 2.78 0.86

– 0.10 3.12 6.16 4.55 1.33 0.18 0.12 99.30 59.5 1.65 59.60 15.4 – 8.5 – 101 17 23 52 12 547 13.60 12.1 19.32 18 772 115 – 0.27 11.6 24.6 13.4 2.84 0.87 0.34 – 1.20 0.182 2.90 0.160 1.31 0.39

– 0.07 1.80 4.34 4.22 2.01 0.16 2.75 99.22 56.8 1.84 66.09 19.5 –

– 7.2

12.6

–

–

59 19 14 46 11 677 7.82 24.2 9.83 31 785 120 – 0.90 17.4 35.2 18.0 3.23 0.88 0.32 – 1.02 0.150 3.26 0.240 2.55 0.94

149 18 44 54 19 605 17.5 28.8 21.6 17 1000 190 0.12 0.32 23.7 52.8 31 5.81 1.62 0.62 – 1.84 0.284 4.1 0.272 2.24 1.09

Rock-types are: B = basalt; BA = basaltic andesite; A = andesite; D = dacite; T-B = trachybasalt; BTA = basaltic trachyandesite; TA = trachyandesite. Mg # = 100Mg 2+/(Mg2+ + 0.9(Fe2+ + Fe3+)), atomic; FeOt = total iron expressed as FeO. The 100% adjusted SiO2 contents are also listed in the (SiO2)adj row. Exact locations of all samples are also included in the rows of longitudes and latitudes.

from the lava of 1982 and 1004-421 from the pyroclastic-flow deposit of 1913. In the tephra section on the NE flank of ancestral Volcán Colima (Fig. 1), below the oldest 14C-dated horizon (8360 years), Luhr and Carmichael (1982) identified a sequence beginning with a basanitic ash (sample A-38) and showing upwards increasing ‘minette-like’ elemental enrichments culminating in two coarse scoria-fall layers, S34 and the phlogopite-bearing layer S33. These scoriae analyzed by Luhr and Carmichael (1982) have diverse and corroded phenocrysts that result from mixing of subalkaline and alkaline magmas. Two basanites (SAY-5A and 1004-501) and one minette (SAY-6E) were included in this study (Luhr and Carmichael, 1981). The basanite ash (A-38) and culminating scoriae (S34 and S33) have also been

analyzed for isotopes in this study. Finally, a high-Mg basalt (SAY22E), analyzed by Luhr and Carmichael (1981), Luhr et al. (1989), and Luhr (1997), was also included in Table 1, because it probably represents a parent composition for some samples of this study. Similarly, basement granite (LHG; Luhr and Carmichael, 1985) – a possible assimilant – was also analyzed in this study for its isotopic composition. Major- and trace- (V, Cr, Ni, Cu, Zn, Rb, Sr, Y, and Zr) elements listed in Table 1 were analyzed by J.F. Luhr using x-ray fluorescence spectrometry (XRF). Instrumental neutron activation analysis (INAA) was used for the determination of other trace elements. Precision estimates were reported earlier (Luhr and Carmichael, 1982). New analyses carried out by J.F. Luhr include: all trace elements for Cántaro

S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

Volcán Colima M82-4

37

Basanite–minette S8.1

Col-30

1004-421

S33

S34

A-38

SAY-5A

SAY-6E

1004-501

V.Tezontle

Basement

SAY-22E

LHG

TA

A

A

A

BTA

BTA

B

T-B

BTA

T-B

B

Granite

103.60057

103.56485

106.61562

103.62723

103.59923

103.59923

103.59923

103.75782

103.75470

103.49272

103.38850

101.81408

19.27687

19.53443

19.50470

19.52565

19.52602

19.52602

19.52602

19.66390

19.62863

19.62948

19.60750

18.99952

56.29 0.89 17.65 3.64 3.46 0.11 3.94 6.79 4.86 1.90 0.46 0.42 100.41 58.6 1.71 56.39 1.0

55.67 0.66 16.89 2.52 3.99 0.11 5.70 6.96 3.97 0.94 0.16 1.89 99.46 68.1 1.10 57.11 4.9

58.54 0.72 17.41 2.32 3.72 0.12 4.42 6.50 4.54 1.18 0.19 0.22 99.88 64.1 1.31 58.79 6.6

56.88 0.79 17.58 3.00 3.42 0.11 4.11 6.92 4.45 1.09 0.19 0.97 99.51 61.2 1.49 57.80 5.5

51.52 1.01 14.37 4.56 2.72 0.12 7.52 8.18 3.28 2.77 0.62 2.71 99.38 72.1 0.91 53.45

53.47 0.96 15.72 3.51 3.64 0.13 5.89 7.94 3.75 2.38 0.48 1.49 99.36 67.0 1.15 54.73

48.30 1.16 11.98 6.29 1.72 0.13 13.27 9.25 3.85 2.55 1.02 0.29 99.81 80.3 0.56 48.76

49.01 1.48 12.41 5.36 2.58 0.11 10.05 8.81 4.16 2.77 1.13 1.76 99.63 76.1 0.74 50.25

49.48 1.60 13.51 2.84 5.36 0.15 9.80 9.07 2.97 3.69 0.99 0.74 100.20 73.3 0.81 49.80

49.42 0.77 16.92 4.40 5.30 0.15 9.27 10.12 2.49 0.65 0.20 0.19 99.88 67.8 1.00 49.72

– 12.1 – 215 23 48 84 22 749 16.35 44.3 20.9 17 1232 174 – 0.22 25.6 56.6 29 5.57 1.55 0.54 – 1.7 0.256 4.52 0.307 2.08 0.67

–

–

–

– –

– –

18.0 – 199 77 35 59 18 382 18.92 193 23.60 12 597 121 – 0.38 10.6 23.0 12.9 3.15 0.93 0.44 – 1.46 0.232 2.79 0.167 1.27 0.52

14.0 – 128 30 24 62 17 421 15.12 74 17.50 20 558 120 – 0.56 9.4 24.4 13.1 2.56 0.88 0.42 2.89 1.61 0.210 2.88 0.192 1.38 0.54

13.2 – 138 26 14 65 16 416 17.78 60 19.60 19 583 125 – 0.47 10.8 24.1 12.9 3.27 1.02 0.44 – 1.64 0.255 3.14 0.204 1.38 0.61

4.2 10.2 190 93 90 71 21 1767 24.04 330 31.2 24 2689 232 – 0.6 54.0 121 67 9.59 2.54 0.69 3.70 1.52 0.19 6.32 0.237 4.69 1.51

10.5 3.2 193 40 62 72 22 792 24.63 189 29.5 26 1434 198 – 0.4 34 82 49 8.34 2.30 0.68 4.00 1.70 0.20 5.12 0.211 4.05 1.34

samples (M81-17, M79-234, M79-169, M79-231, and 1004-505), performed using the Smithsonian's Philips PW 1480 XRF spectrometer using techniques described in Luhr et al. (1995), INAA data for SAY-5A, SAY-6E, and LHG determined at Washington University using procedures described elsewhere (Lindstrom and Korotev, 1982), and all major- and trace-element data for A-38 and M82-4 (performed at UC Berkeley using techniques described in Luhr and Carmichael, 1982). Analyses were recalculated on anhydrous 100% adjusted basis and using Fe2O3/FeO ratio after Middlemost (1989). Rock types and CIPW norms were computed on an anhydrous, 100% adjusted basis and Fe2O3/FeO ratio adjustment using SINCLAS computer program (Le Bas et al., 1986; Verma et al., 2002, 2003). Major-element data used in various plots refer to these 100% adjusted (adj. wt.%) values.

47.38 0.91 10.09 – 8.45 0.16 18.24 10.33 2.15 1.57 0.56 – 99.84 82.0 0.46 47.39 – 4.8 – 30.9 153 608 49 72 20 891 25.13 992 56.6 13 988 156 – 0.37 30.2 70.9 38 7.71 2.06 0.62 – 1.32 0.187 5.15 0.217 2.45 1.04

–

– 10.8

–

– 8.8

– 20.7

5.7 –

14.9

–

–

455 71 72 23 1770 23.52 836 45.90 37 2308 312 – 0.37 54.8 122 61 11.30 2.92 0.78 – 1.30 0.186 9.01 0.320 5.49 2.02

230 49 76 22 2140 24.30 421 36.7 51 2372 390 – 0.5 62.3 137.4 76 12.31 3.1 0.88 – 1.48 0.215 11.62 0.42 6.2 2.32

15.9 271 212 68 74 25 1710 29.30 535 – 52 1566 420 – – 28.4 63 36 7.07 1.55 0.92 4.20 1.57 0.130 13.20 – 1.45 0.90

– – 12.3 12.2 196 221 24 66 18 161 35.5 378 – 7 444 92 – – 9.3 22.0 12.0 2.96 1.13 0.61 3.7 2.08 0.26 2.6 0.15 0.44 0.33

67.20 0.64 14.05 1.40 2.83 0.07 1.74 3.47 3.04 4.10 0.11 0.82 99.47 50.8 2.35 68.13 23.6 – 6.4 – 107 2 43 59 32 537 13.01 26 – 204 135 251 – 8.84 22.2 48.1 23.0 4.83 0.72 0.76 – 2.67 0.396 8.54 – 20.52 6.19

Radiogenic isotopes were analyzed on two fully-automated MAT 261 mass spectrometers at the Max-Planck-Institut für Chemie (MPI), Mainz, Germany, using procedures summarized by Verma (1992): (i) triple-collector for Nd and Pb; and (ii) double- and later transformed to multi-collector for Sr. A few isotopic analyses were also carried out at the Laboratorio Universitario de Geoquímica Isotópica (LUGIS-UNAM). The analytical errors are directly quoted for individual isotopic measurements. The 87Sr/86Sr ratios are normalized to 86Sr/88Sr = 0.11940 and adjusted to SRM987 87Sr/86Sr of 0.710230. The 143Nd/144Nd are normalized to 146Nd/144Nd = 0.72190 and adjusted to La Jolla 143Nd/144Nd of 0.511860. εNd = {((143Nd/ 144 Nd)m/(143Nd/144Nd)CHUR)) − 1}104 (DePaolo and Wasserburg, 1976), using (143Nd/144Nd)CHUR = 0.512638. At the MPI, the measured

38

S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

87 Sr/86Sr for the SRM987 standard was 0.710216 ± 11 (1 s; n = 36) and the measured 143Nd/144Nd for the La Jolla standard was 0.511833 ± 12 (1 s; n = 82) during the period of measurement of about 1 year (September, 1986−August, 1987). At LUGIS (UNAM) the SRM987 gave 87Sr/86Sr = 0.710234 ± 18 (1 s; n = 222) and La Jolla standard 0.511880 ± 22 (1 s; n = 123). Further, the errors reported on individual Sr and Nd isotopic ratios are 2 times the standard error of the mean (2sE) multiplied by 106. Errors on average ratios are 1 s values for multiple determinations. Note the measured isotopic ratios were adjusted following the convention of Mainz (see Verma, 1992, 2002, 2006). At the MPI, the Pb isotopic ratios were corrected for fractionation estimated by running simultaneously the NBS982 standard and are relative to values of 206Pb/204Pb = 36.73845, 207Pb/ 204 Pb = 17.15946, 208Pb/204Pb = 36.74432, and 207Pb/206Pb = 0.46707 for this standard. The analytical uncertainties are 1 s (one standard deviation) values of multiple determinations. At LUGIS (UNAM), the NBS981 standard was used and the values are reported relative

206 Pb/ 204 Pb = 16.937096, 207 Pb/ 204 Pb = 15.491345, 208 Pb/ Pb = 36.721317. The reported errors for the LUGIS data are 1 s values of individual runs. The Sr and Nd isotopic ratios in samples M81-17 and S34 were confirmed by analyzing separate aliquots of these samples and determining duplicate analyses. Their good agreement ensured the high overall reproducibility of the entire data set because these results had also taken into account the sampling variance (sample heterogeneity). The Sr−Nd isotopic data on sample LHG were compared with those from DePaolo's lab at University of California Los Angeles, UCLA after converting εNd values from the UCLA lab (DePaolo and Wasserburg, 1976) to the same international convention as used in Mainz, Germany (see also Verma et al., 2008, for more discussion on the conversion and equivalence of 143Nd/144Nd from CalTech- and Lamont-type laboratories). Data on sample S33 from Washington University, St. Louis agree well with the data obtained in Mainz. Thus, the general agreement between the different sets of Sr and Nd isotopic data (Mainz, Los

to

204

Table 2 Sr and Nd isotopic data from the Cántaro–Colima volcanic chain samples. Sample

Rock name

Rock type (TAS)

Volcán Cántaro (data from MPI-Mainz) M81-17 Basaltic andesite

BA

M79-234

Andesite

A

M79-169 M79-231

Andesite Hornblende-biotite andesite

D D

1004-505

Hornblende-biotite dacitic pumice

D

87

Sr/86Sr

143

Nd/144Nd

εNd

Ref.

0.513050 ± 11

8.0

J

0.512889 ± 13

0.512889

4.9

J

0.703604 0.703624 ± 4

0.512898 ± 15 0.512895 ± 11

0.512898 0.512895

5.1 5.0

J J

0.703638 ± 13

0.512900 ± 19 0.512915 ± 25

0.512908 ± 11

5.3

J

Individual runs

Average

Individual runs

Average

0.702849 ± 24 0.702798 ± 12 0.702816 ± 10 0.702810 ± 9 0.703661 ± 22 0.703664 ± 24 0.703604 ± 9 0.703621 ± 27 0.703626 ± 27 0.703648 ± 21 0.703629 ± 20

0.702818 ± 22

0.513041 ± 12 0.513047 ± 13 0.513062 ± 5

0.703663 ± 2

Volcán Nevado de Colima (data from LUGIS-UNAM-Mexico) 1004-122 BTA, mug 1004-126 TA, ben 1004-121 A 1004-105 A M81-21 D

0.703596 ± 11 0.703383 ± 14 0.703556 ± 10 0.703529 ± 13 0.703596 ± 11

0.703596 0.703383 0.703556 0.703529 0.703596

0.512922 ± 5 0.512957 ± 6 0.512929 ± 5 0.512923 ± 10 0.512908 ± 5

0.512922 0.512957 0.512929 0.512923 0.512908

5.5 6.2 5.7 5.6 5.3

J J J J J

Volcán Colima (data from MPI-Mainz) M82-4 Alkaline block S8.1 Scoria Col-30 Hornblende andesite 1004-421 1913 Hornblende andesite scoria

TA A A A

0.703751 ± 24 0.703577 ± 34 0.703573 ± 27 0.703554 ± 23

0.703751 0.703577 0.703573 0.703554

0.512928 0.512941 0.512984 0.512951 ± 11

5.7 5.9 6.7 6.1

J VL VL VL

S33 S34

Alkaline scoria Alkaline scoria

BTA BTA

0.703950 ± 28 0.703606 ± 23 0.703643 ± 11 0.703631 ± 13

0.703950 0.703627 ± 19

0.512822 0.512943 ± 11

3.6 6.0

J J

A-38

Basanite ash

B

0.703725 ± 21 0.703710 ± 14

0.703718 ± 11

0.512928 ± 23 0.512941 ± 16 0.512984 ± 23 0.512943 ± 23 0.512959 ± 19 0.512822 ± 18 0.512974 ± 15 0.512950 ± 25 0.512927 ± 10 0.512922 ± 12 0.512923 ± 10

0.512923

5.6

J

Basanite–minette (data from MPI-Mainz) SAY-5A Leucite–basanite lava

T-B

0.703796 ± 15

0.512880 ± 13

0.512880

4.7

J

SAY-6E

Basanite–minette

BTA

0.703738 ± 12

4.8

J

Basanite scoria

T-B

0.512886 ± 17 0.512885 ± 20 0.512828 ± 14

0.512886 ± 1

1004-501

0.703793 ± 17 0.703800 ± 13 0.703730 ± 28 0.703747 ± 14 0.703888 ± 25 0.703868 ± 20

0.512828

3.7

J

0.512935a

5.8

L

0.512772 0.51279a

2.6 2.9

J L

Volcán Tezontle SAY-22E High alumina basalt

B

Upper crust LHG

Granodiorite

Basement granite

0.703878 ± 14

0.70358

0.706278 ± 22 0.706300 ± 25

0.706289 ± 16 0.70641

0.512772 ± 23

Rock names are taken from the initial descriptions by Luhr and coworkers. Rock types are the same as in Table 1 (TAS classification). Ref. are: J = this work; VL = Verma and Luhr (1993); L = Luhr (1997). a Ratio estimated from reported εNd.

S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

39

Table 3 Pb isotopic data from the Cántaro–Colima samples. Sample

206

Pb/204Pb

Individual runs Volcán Cántaro (MPI-Mainz) M81-17 18.534 18.536 18.534 M79-234 18.562 18.562 18.565

207

Pb/204Pb

208

Pb/204Pb

Ref.

Average

Individual runs

Average

Individual runs

Average

18.535 ± 0.001

15.525 15.525 15.527 15.563 15.565 15.568

15.526 ± 0.001

38.085 38.090 38.095 38.085 38.090 38.095

38.090 ± 0.005

J

38.271 ± 0.009

J

18.563 ± 0.002

15.565 ± 0.003

Volcán Nevado de Colima (LUGIS-UNAM-Mexico) 1004-126 18.583 ± 0.015 18.583 M81-21 18.572 ± 0.016 18.572

15.541 ± 0.013 18.549 ± 0.012

15.541 18.549

38.205 ± 0.033 38.238 ± 0.031

38.205 38.238

J J

Volcán Colima (MPI-Mainz) M82-4 18.618 18.611 18.616 Col-30 18.568 18.565 S33 18.677 18.675 18.676 S34 18.570 18.564 18.575 1004-501 18.707 18.703

15.568 15.563 15.568 15.564 15.560 15.591 15.589 15.590 15.564 15.559 15.570 15.611 15.606

15.566 ± 0.003

38.311 38.301 38.319 38.261 38.270 38.449 38.437 38.441 38.264 38.248 38.287 38.528 38.519

38.310 ± 0.009

J

38.266 ± 0.006

VL

38.442 ± 0.006

J L

18.615 ± 0.004

18.566 ± 0.002 18.676 ± 0.001 18.683 18.570 ± 0.006

18.705 ± 0.003

15.562 ± 0.003 15.590 ± 0.001 15.600 15.564 ± 0.006

15.608 ± 0.004

38.472 38.266 ± 0.020

J

38.524 ± 0.006

J

Volcán Tezontle SAY-22E

18.575

15.558

38.226

L

Upper crust LHG

18.804

15.618

38.662

L

Ref. same as in Table 2.

Angeles, St. Louis, and UNAM labs) allows us to pull together confidently all Sr and Nd isotopic ratios from Cántaro−Colima area as well as from the Tepic−Zacoalco Rift. The size of the symbols used is generally larger than or similar to the analytical errors in isotopic determinations. This means that the differences on isotope diagrams mentioned below (see Discussion section) are statistically significant, i.e., beyond the experimental errors, and therefore represent real differences among samples and suites. Besides these new analyses, an extensive database for all types of magmas from the W-MVB (Fig. 1) was established from published literature, including these new data (Tables 1−3) and 11 unpublished analyses from Nevado de Colima by J.F. Luhr. The data sources are: Gunn and Mooser (1971), Thorpe and Francis (1975), Mahood (1977, 1981), Gastil et al. (1979), Luhr and Carmichael (1980, 1981, 1982, 1990a,b), Nelson (1980, 1986), Demant (1981), Allan and Carmichael (1984), Robin et al. (1984, 1987, 1990, 1991), Gilbert et al. (1985), Luhr and Lazaar (1985), Nieto et al. (1985), Allan (1986), Nelson and Livieres (1986), Martin del Pozzo et al. (1987), Luhr and Prestegaard (1988), Mahood and Halliday (1988), Luhr et al. (1989), Verma and Nelson (1989), Wallace and Carmichael (1989, 1992, 1994), Lange and Carmichael (1990, 1991), Allan et al. (1991), Righter and Carmichael (1992), Luhr (1993, 1997, 2000, 2002), Robin and Potrel (1993), Verma and Luhr (1993), Ferrari et al. (1994, 2000), Moore et al. (1994), Carmichael et al. (1996, 2006), Righter et al. (1995), Rodríguez-Elizarrarás (1995), Righter and Rosas-Elguera (2001), Mora et al. (2002), Varley et al. (2002), Petrone et al. (2003), Maldonado-Sánchez and Schaaf (2005), Valdez-Moreno et al. (2007), and Frey et al. (2007). Our database included numerous samples from the Tepic−Zacoalco Rift, which are compared with the Colima Rift samples in some diagrams (see Fig. 2 for these rifts). In this context, particularly important is the comparison of the Sr and Nd isotopic data

of Verma and Nelson (1989) from the Tepic−Zacoalco Rift with the present data. Additional database for isotopes for the rest of the MVB was also established from Verma (1983, 2000a, 2002), McBirney et al. (1987), Verma and Hasenaka (2004), Carrasco-Núñez et al. (2005), Gómez-Tuena et al. (2005), and Meriggi et al. (2008). Another extensive database for arcs, continental rifts, Ocean Island basalts, MORB and the rest of the MVB established by Verma (2006) was also used for the interpretation of the data from the W-MVB. 4. Results Major- and trace-element and isotopic data on samples from Cántaro−Colima volcanic chain are given in Tables 1−3. A total alkalis-silica (TAS) classification diagram is used for the Cántaro– Colima samples (Fig. 3a). Although most rocks from Volcán Cántaro, Nevado Colima and Volcán Colima are basalt to dacite, those from the Colima complex are trachybasalt to trachyte. A K2O versus SiO2 plot (Fig. 3b) shows the presence of three series (subalkaline subA, alkaline A, and Mixed) in the vicinity of the Cántaro–Colima volcanic chain. Note also the somewhat higher K2O enrichment of Volcán Cántaro subalkaline products as compared to Nevado and Volcán Colima samples. Subalkaline volcanic rocks from Volcán Cántaro differ from those of the Colima volcanoes (Nevado de Colima and Volcán Colima) in several important ways. Cántaro rocks are primarily andesites and dacites with about 58 to 64 wt.% SiO2 (Luhr and Carmichael, 1990a); a single basaltic andesite was also analyzed (M81-17, 55.3% SiO2; Table 1). Products of Nevado and Volcán Colima are poorer in silica: basaltic andesites to andesites (about 56 to 62% SiO2) dominate and dacites are rare. Andesites from the Colima volcanoes are porphyritic, with 30 to 50 vol.% crystals up to 4 mm across, and about half of the rocks contain hornblende. Andesites and dacites from Volcán Cántaro are even more porphyritic and more-

40

S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

Fig. 3. Total alkalis (Na2O + K2O) versus SiO2 (TAS) diagram for chemical classification (a) and K2O versus SiO2 diagram (b) for volcanic rocks from the Cántaro–Colima volcanic chain, based on recalculated major-element whole-rock concentrations normalized to 100% volatile-free with Fe2O3/FeO after Middlemost (1989) from SINCLAS computer program (Verma et al., 2002) for the samples discussed in this paper (solid symbols) and other samples from the same suites reported in the literature (open symbols). The symbols used are given as inset. Closed and open diamonds are for subalkaline rocks from Volcán Cántaro analyzed in this work and from the literature, respectively. Olivine-bearing hornblende-lamprophyre B43 from Colima graben (Allan and Carmichael, 1984; Luhr, 1997) and SAY-17B from Volcán Usmajac are also included (Luhr and Carmichael, 1981; Luhr, 1997). Colima SubA (subalkaline), A (alkaline), and Mixed trends are also shown. Basanite ash A-38 from the Colima pyroclastic series, the isotopically anomalous basaltic andesite M81-17 from Volcán Cántaro and subalkaline basalt SAY22-E from Volcán Tezontle are labeled. The rock types are: B — basalt, T_B — trachybasalt, BA — basaltic andesite, BTA — basaltic trachyandesite, A — andesite, TA — trachyandesite, D — dacite, T — trachyte, and R — rhyolite.

coarsely crystalline, with phenocrysts up to 10 mm across. Virtually all of the andesites and dacites from Volcán Cántaro contain hornblende, and many also contain biotite, which is unknown from the andesites of the Colima volcanoes.

As an example, the first row of statistical data in Table 4 will be described in detail. This row marked TiO2 presents the parameters of the linear correlation in TiO2–SiO2 plot. For reference, all bivariate data are also plotted in Fig. 4. Note the highly cumbersome nature of this plot when all data from seven stratovolcanoes are plotted. The inferred trends (the first row of Table 4) are also shown schematically in Fig. 4. In Table 4, the number of samples for each volcanic center is identified by the variable n, which, in the case of Volcán Cántaro, is 14. The intercept “a” representing the TiO2 value corresponding to the zero SiO2 concentration (and its error “sa”; note this error is a quantitative measure of the dispersion of the data around the linear fit) of these correlations (for Volcán Cántaro, a and sa are 3.39 and 0.42, respectively; Table 4) would have little geological meaning because SiO2 used as the independent (x-variable) is not likely to be less than about 40% for silicate magmas and it would never reach the value of zero concentration. The slope (b) quantitatively gives the mean tendency of one variable with respect to the other whereas its error (sb) is the estimate of the dispersion of the data around a linear fit. One can similarly observe the results of correlations for other volcanic centers, such as Nevado and Colima, listed in Table 4. Thus, TiO2 shows a statistically meaningful negative correlation with SiO2 for all three stratovolcanoes of the Colima Rift (see negative slopes for Cántaro, Nevado, and Colima) as well as for the four volcanic centers (Tequila, Ceboruco, Snaganguey, and San Juan) of the Tepic– Zacoalco Rift. For Cántaro samples, TiO2 decreases with SiO2 (slopes of − 0.044 ± 0.007) more sharply than for Nevado (−0.035 ± 0.009) and Volcán Colima (−0.0308 ± 0.0035) although significance tests should be applied to ascertain its statistical validity (Verma, 2009a). Similarly, the four volcanoes of the Tepic–Zacoalco Rift show a negative correlation for these two major oxides (TiO2 versus SiO2); TiO2 decreases with SiO2 more sharply for Ceboruco and Sanganguey (slopes of about −0.064 and − 0.068) than for Tequila and San Juan (for both, slopes of about − 0.038). The elements TiO2, MgO and CaO show negative correlations with SiO2 for all and P2O5 for some volcanic centers, whereas K2O indicates positive ones (Table 4). For trace elements and isotopic ratios, the lack of significant correlation for numerous cases is shown by dashed lines in Table 4. Significant positive correlations are shown for large-ion lithophile elements Rb and Ba with SiO2 for most volcanic centers. The alkaline basanite–minette suite from the Colima Rift has very low SiO2 contents accompanied by very high concentrations of K2O (Fig. 3b) and related elements (Table 2). Despite these very high abundances of incompatible elements, relative element abundance patterns for the alkaline samples, in particular, the HFSE (e.g., Ta, and Ti) and LILE (e.g., Cs, Ba, and Sr) signatures, are similar to spatially associated subalkaline samples (Fig. 5), implying a genetic

4.1. Elemental variations among subalkaline and alkaline suites Instead of presenting a series of bivariate plots, elemental variations against SiO2 were explored for statistically significant correlations (at 99% confidence level) using the OYNYL software (Verma et al., 2006a), although no attempt was made to identify discordant outliers (Barnett and Lewis, 1994; Verma and Quiroz-Ruiz, 2006a,b, 2008; Verma et al., 2008) and interpret them separately. The results for the main stratovolcanoes from the W-MVB (three from Colima Rift and four from the Tepic–Zacoalco Rift) are summarized in Table 4. When the correlation is statistically significant for a volcanic suite, most major- and some trace-element abundances define smooth variations when plotted against SiO2 as judged from significant linear correlations at 99% confidence level (Table 4). Furthermore, whenever required, more complex functions can be fitted to the experimental data and geological processes better explained, or a more appropriate statistical treatment of compositional data, such as log-ratio transformation, could be called for (Aitchison, 1986; Agrawal and Verma, 2007).

Fig. 4. TiO2 versus SiO2 (Harker-type) diagram, based on recalculated major-element whole-rock concentrations normalized to 100% volatile-free with Fe2O3/FeO after Middlemost (1989) from SINCLAS computer program (Verma et al., 2002), for the three volcanic centers of the Cántaro–Colima volcanic chain and four centers from the Tepic–Zacoalco Rift. Regression lines for all volcanic centers (Table 4) are also included for reference.

Table 4 Statistically valid linear correlations (at 99% confidence level; Pc(r;n) b 0.01) against SiO2 for different stratovolcanoes in the W-MVB. Chemical parameter against SiO2 (x-variable)

Colima Rift (CR)

Tepic–Zacoalco Rift (TZR)

Cántaro n

a (sa)

Nevado b (sb)

Trace elements Cs 9

–

13 −81 (23) 13 −1390 (350) 13 –

Rb Ba Sr

a (sa)

47 2.8 (0.5) 47 23.1 (2.5) 47 24.1 (1.4) a

47 −1.0 (1.1) 47 −2.4 (0.7) 21 –

–

14 8.5 (2.0)

1.65 (0.37) 32 (6) –

14 – 14 −1750 (450) 14 –

b (sb)

n

− 0.035 (0.009) − 0.326 (0.041) − 0.300 (0.024) 0.112 (0.018) 0.064 (0.011) –

190 2.55 (0.21)

0.145 (0.034) – 38 (7) –

a (sa)

190 35.9 (1.5) 190 25.5 (0.5) 190 − 3.4 (0.7) 190 − 2.96 (0.32) 157 –

71 – 121 − 24a (11) 136 − 1050 (150) 136 − 800 (300)

Tequila b (sb)

n

a (sa)

−0.0308 (0.0035) −0.535 (0.025) −0.320 (0.008) 0.153 (0.011) 0.071 (0.005) –

25

–

Ceboruco b (sb)

n

a (sa)

Sanganguey b (sb)

n

53 5.08 (0.31)

25

− 0.038 (0.006) 16.1 (1.5) − 0.212 (0.024) 26.4 (1.2) − 0.336 (0.020) −2.3a (1.4) 0.135 (0.022)

18 −9.7 (0.6)

−0.064 (0.006) −0.227 (0.009) −0.429 (0.014) 0.263 (0.009)

25

−6.7 (1.5)

0.145 (0.024)

18 −7.0 (0.7)

0.148 (0.010)

53 − 4.6 (0.5)

25

1.41 (0.22)

− 0.0193 (0.0035)

16 1.15 (0.14)

−0.0148 (0.0021)

53 1.07 (0.11)

–

–

9

–

–

−337 (14) −1990 (400) 2950 (350) – –

6.19 (0.22) 44 (6)

9 – 9 –

– –

− 38 (5)

9

2970 (200)

−38.5 (3.0)

49 − 111 (11) 49 − 1200 (180) 49 1500 (190)

– –

9 9

−78 (11) –

1.61 (0.16) –

22 – 0 –

–

–

9

−1.08 (0.30)

17 – 1 –

– –

0 – 9 −9.6a (2.8)

0.0222 (0.0045) – 0.165 (0.041)

1

–

–

9

−15.2 (1.1)

0.323 (0.016)

17

–

–

9

−400 (130)

25 25

1

0.72 (0.18) 25.7 (2.5)

17 17

23 (5)

17 17 1

3.18 (0.39)

18 4.86 (0.36) 18 16.6 (0.6) 18 31.9 (0.9)

−0.068 (0.005) −0.288 (0.014) −0.347 (0.011) 0.172 (0.008) 0.109 (0.007) −0.0144 (0.0018)

27

3.04 (0.13)

27

15.2 (0.5)

27

27.94 (0.41) −2.8 (0.6)

0.092 (0.007)

27

0.65 (0.08)

−0.0075 (0.0011)

13

−1.9 (0.6)

0.038 (0.009)

2.32 (0.18) 23 38 (7) 23

−68 (9) −980 (240)

1.47 (0.14) 29.5 (3.6)

−14.7 (3.0) – –

23

2920 (290)

−33.9 (4.3)

13 13

– 7.3 (0.9)

0 –

–

13

–

– −0.066 (0.014) –

15 – 0 –

– –

0 11

– –

– –

0 –

–

13

−1.4a (1.3)

0.078 (0.020)

9.7a (1.9)

49 –

–

23

−90a (50)

3.5 (0.8)

15 0.70227 (4.2E−4) 15 0.51308 (9E−5)

2.8E−5 (7E−6) −4.0E−6 (1E−6)

46

0.70215 (3.6E−4) –

2.9E−5 (6E−4) –

3 – 14 –

– –

85 − 27 (7) 74 –

0.64 (0.11) –

Lu

9

–

–

14 –

–

71 –

–

– 0.0086 (0.0020) –

0 – 14 –

– –

46 – 66 –

– –

Hf

13 – 9 −0.31a (0.12) 9 –

14 –

–

71 − 1.9a (0.8)

Zr

13 –

–

14 –

–

121 –

0.086 (0.014) –

5

–

–

3

–

–

8 –

–

2

–

–

3

–

–

5

–

–

3

–

–

8 –

–

2

–

–

3

–

–

Nd/144Nd

b (sb)

−3.9 (0.5)

– –

143

a (sa)

27

– –

Isotope ratios 87 Sr/86Sr

n

27

9 9

1

San Juan b (sb)

−0.0378 (0.0019) −0.204 (0.008) −0.358 (0.006) 0.143 (0.008)

La Sm

Nb Ta

a (sa)

53 20.5 (0.9) 53 27.0 (0.7) 53 − 4.4 (0.5)

0 –

–

8

S.P. Verma, J.F. Luhr / Journal of Volcanology and Geothermal Research 197 (2010) 33–51

Major elements 14 3.39 (0.42) − 0.044 TiO2 (0.007) MgO 14 31 (5) −0.45 (0.08) CaO 14 26.2 (2.5) − 0.333 (0.040) Alkalis 14 −3.8a (1.9) 0.162 (0.030) K2O 14 −4.2 (1.2) 0.097 (0.018) P2O5 17 0.69 (0.11) − 0.0080 (0.0018)

n

Colima

Pc(r;n) — probability of no-correlation for n paired data with linear correlation coefficient r; n — number of paired data regressed; a — intercept; (sa) — error of intercept; b — slope; (sb) — error of slope; – linear correlation not valid at 99% confidence level and therefore not reported. For major elements (or oxides), slope and intercept are the same as expressed in this table whereas for trace elements (in mg/kg or ppm with SiO2 in %m/m) both slope and intercept should be divided by 104 (if both axes were to be expressed in the same measurement units). a

Not valid at 99% confidence level although being reported; the statistical parameters are reported as rounded values following the rules put forth by Bevington and Robinson (2003) and Verma (2005).

41

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Fig. 5. Multi-element primitive mantle-normalized plots (normalizing values from Sun and McDonough, 1989) for the Cántaro–Colima rocks. (a) all rock samples from Volcán Cántaro, including the most mafic basaltic andesite M81-17, (note that Nb, instead of Ta, data are plotted for Cántaro rocks and that subalkaline basalt SAY-22E from cinder cone Volcán Tezontle (Luhr and Carmichael, 1981; Luhr, 1997) is included for reference here and in most later Figures); (b) subalkaline (SAY-22E, Col-30, and 1004-421) and alkaline (SAY-5A and, SAY-6E) rocks from Colima Rift Zone (CR); (c) basanitic ash A-38 and mixed rocks (S33, S34, and M82-4) from Volcán Colima.

relationship between the two suites (Luhr et al., 1989). Although most samples from Volcán Colima fall along the tight subalkaline fractionation trend (Fig. 3b), a small number of samples plot between the subalkaline and alkaline fields, and were interpreted as pre-eruptive mixtures of the two suites by Luhr and Carmichael (1982). Among these samples are the avalanche block M82-4 and scoriae S33 and S34; the latter two samples also show petrographic evidence in support of this mixing scenario (Luhr and Carmichael, 1981). In multi-element primitive mantle-normalized diagrams (Fig. 5) for different rock types from the Colima Rift, all Cántaro, Nevado and Colima rocks show similar subduction-related patterns of Ta depletion with respect to Ba and La, or U and K, as well as Sr enrichment with respect to Ce and P, or La and Nd. A MORB-normalized multielement plot for basaltic andesite M81-17 from Volcán Cántaro (the isotopically unusual sample from the Colima Rift having the lowest 87 Sr/86Sr of 0.70282 and the highest 143Nd/144Nd of 0.51305; Table 2) is also shown in Fig. 6 and compared to mantle-derived rift-related basalt SN46 from the Tepic–Zacoalco Rift, having the lowest 87Sr/86Sr ratio (0.70314) and a high 143Nd/144Nd ratio (0.51291) from this graben system (Verma and Nelson, 1989). In a MORB-normalized diagram (Fig. 6), the samples SN46 and M81-17 show similar patterns that are distinct from the MORB rocks, although all have similar isotopic compositions. Mantle-normalized {La/Yb}n values are included here (Fig. 7c) to show the extreme light-REE enrichment in alkaline rocks as compared to subalkaline rocks from this area. The subalkaline rocks from Volcán Cántaro show somewhat higher {La/Yb}n than similar rocks from Volcán Colima.

4.2. Sr, Nd and Pb isotope ratios Fifteen Sr and Nd and seven Pb isotopic data from this study are shown in Figs. 7–9. The isotopic ratios of Cántaro–Colima rocks show the following ranges: 87 Sr/86Sr 0.70282–0.70395, 143Nd/144Nd 0.51282–0.51305, 206Pb/204Pb 18.54–18.70, 207Pb/204Pb 15.53–15.61, and 208Pb/204Pb 38.09–38.52. Fresh MORB and seamount basalt data (Figs. 8 and 9), are also included for comparison purposes and to give an idea of the isotopic

Fig. 6. Multi-element N-MORB-normalized plot (normalizing values from Pearce, 1982) for high-Mg basaltic andesite M81-17 from Volcán Cántaro (the lowest 87Sr/86Sr of 0.70282 yet documented from the Mexican Volcanic Belt) and its comparison to mantle-derived primitive basalt SN46 from the Tepic–Zacoalco Rift (TZR; the lowest 87 Sr/86Sr of 0.70314 from the TZR; Verma and Nelson, 1989). Also included are individual MORB samples having complete datasets (from Verma, 2006) for this figure; samples with incomplete data were not included.

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Fig. 7. Radiogenic isotope ratio and La/Yb versus SiO2 plots for the Cántaro–Colima rocks and their comparison with the Tepic–Zacoalco Rift lavas. (a) 87Sr/86Sr; (b) 143Nd/144Nd; (c) {La/Yb}n; (d) 206Pb/204Pb; (e) 207Pb/204Pb; and (f) 208Pb/204Pb. Symbols used are explained in Fig. 5a, see also Fig. 2 for more explanation. {La/Yb}n signifies that La/Yb ratios are normalized with respect to primitive mantle (normalizing values from Sun and McDonough, 1989). Average error-bars (1 s) are included next to the letters a, b, and d–f in all isotope–isotope diagrams. Average error-bars (1 s) are shown next to the letters a, b, and d–f in all isotope diagrams. Also included on these plots are ten additional published Sr and Nd and seven Pb isotopic analyses for Colima subalkaline and alkaline samples (Luhr, 1997), as well as the isotopic data for subalkaline and alkaline suites from elsewhere in the western MVB (Tepic–Zacoalco Rift from Verma and Nelson, 1989; Wallace and Carmichael, 1994; and Luhr, 1997, 2000; Petrone et al., 2003; Maldonado-Sánchez and Schaaf, 2005, and La Primavera from Mahood and Halliday, 1988).

signature of the source for basalts erupted in the adjacent oceanic basins (Macdougall and Lugmair, 1986; White et al., 1987; Graham et al., 1988; Prinzhofer et al., 1989). Subalkaline rocks from the Cántaro– Colima volcanic chain plot near the prevalent mantle field (PREMA in Fig. 8) whereas alkaline rocks from this chain lie between this mantle field and the bulk silicate earth (i.e., shifted towards BSE or PUM in Fig. 8). Andesite sample M79-234 from Volcán Cántaro is similar to the subalkaline rocks from Colima and Nevado, whereas basaltic andesite M81-17 is much less radiogenic in Pb (Figs. 7d–f, 9). The six subalkaline rocks (basalt SAY-22E from Volcán Tezontle, and basaltic andesite Col-11 and four andesites Col-2, S8.1, Col-30,

1004-421 from Volcán Colima) and the basaltic trachyandesite (SAY-17B) from Volcán Usmajac are isotopically similar in Sr, showing no systematic variation with SiO2 content (Fig. 7a; see also Luhr, 1997). Nd isotopic ratios, on the contrary, display more variable values (Fig. 7b). As for Sr and Nd isotopic ratios, the four subalkaline samples from Volcán Colima cluster tightly at lower isotopic values in 206 Pb/204Pb (Fig. 7d) and 208Pb/204Pb (Fig. 7e), but one of them (Col-11) is more variable in 207Pb/204Pb (Fig. 7f). The three subalkaline rocks (two andesites 1004-121 and 1004-105 and one dacite M81-21) and two alkaline rocks (basaltic trachyandesite 1004-122 and trachyandesite 1004-126) from Nevado

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Fig. 8. Conventional 87Sr/86Sr–143Nd/144Nd diagram for the Cántaro–Colima rocks and their comparison with mantle reservoirs (Zindler and Hart, 1986) and the downgoing slab (Verma, 2000a, 2002). Symbols used are shown as insets. The dashed lines show approximate trace of “mantle array” (White, 1985). Isotopically anomalous basaltic andesite M81-17 and primitive basalt SAY-22E are labeled. Average error-bar (1 s) is included in the lower middle part of the diagram, to the right of the “mantle array” (Faure, 1986). Included for comparison are MORB and seamount data from adjoining oceanic basins (Macdougall and Lugmair, 1986; White et al., 1987; Graham et al., 1988; Prinzhofer et al., 1989) and subalkaline and alkaline rock data from the Tepic–Zacoalco Rift (Verma and Nelson, 1989; Wallace and Carmichael, 1994; Luhr, 1997) and La Primavera caldera (Mahood and Halliday, 1988). Also included are data from continental rifts and island and continental arcs compiled by Verma (2006). Mantle components: BSE bulk silicate earth or PUM primitive uniform mantle reservoir; PREMA prevalent mantle composition; HIMU high U/Pb mantle component. The mixing line (thick blue solid curve) of two-component mixing of altered basalts and sediments from the downgoing Cocos plate is designated as “downgoing slab”. The numbers (1–20%) indicate the wt.% of the sediment component in this mixture. Upper crust represented by granite LHG is also included and its possible mixing trend with M81-17 is shown as a dashed curve.

de Colima are isotopically similar in Sr and Nd, showing no systematic variation with SiO2 content (Fig. 7a, b). The samples from Nevado de Colima also cluster tightly at low Pb isotopic values (Fig. 7d, e, f). Compared to the subalkaline rocks from Colima, the alkaline basanites and minettes from flanking cinder cones (Tables 3 and 4) have slightly higher Sr and Pb but slightly lower Nd isotopic ratios. Similar differences between the two Colima suites were reported for Sr, Nd, and Pb isotopes by Luhr (1997) and for Sr isotopes by Moorbath et al. (1978). The Colima alkaline suite has rather uniform 87 Sr/86Sr but more variable 143Nd/144Nd and Pb isotope ratios (Figs. 7–9). Although the Colima alkaline suite is isotopically distinct from the Colima subalkaline suite, it falls between the latter and the subalkaline suites from volcanoes in the Tepic–Zacoalco Rift in the western MVB (Figs. 1 and 8).

5. Discussion 5.1. Volcán Cántaro The most mafic Cántaro sample (basaltic andesite M81-17) shows the lowest 87Sr/86Sr and the highest 143Nd/144Nd ratios yet recorded for volcanic rocks from the MVB. Its Pb isotopic ratios are also the lowest of all whole-rock data from the western part of the MVB (Luhr, 1997) and among the lowest from elsewhere in the MVB (e.g., Torres-Alvarado et al., 2000; Verma, 2000a, 2002; Carrasco-Núñez et al., 2005; Gómez-Tuena et al., 2005). This rock is isotopically similar to oceanic basalts from seamounts and fresh MORB from the adjoining Rivera, Cocos, and Pacific plates (Figs. 8 and 9; Macdougall and Lugmair, 1986; White et al., 1987; Graham et al., 1988; Prinzhofer et al., 1989). This means that the isotopic signature of M81-17 was derived from a MORB-type mantle, made possible because of the extensional regime of the Colima Rift. Therefore, the basaltic andesite sample analyzed from Volcán Cántaro may have been derived from a

MORB mantle whose existence beneath the Colima Rift having been facilitated by the emplacement and evolution of the landward ridge jump (Luhr et al., 1985; Allan et al., 1991). Andesites and dacites from Volcán Cántaro are homogeneous isotopically, with Sr isotopic ratios similar to subalkaline rocks from Nevado and Volcán Colima, but with significantly lower Nd isotopic values (Figs. 7a, b, and 8). Basaltic andesite M81-17 is clearly not related by simple fractionation to the more-siliceous Cántaro samples, although it has an appropriate trace-element signature for it to be a parent rock for the evolved magmas. The differences in isotopic signatures between the basaltic andesite and andesites and dacites can only be reconciled by both crustal assimilation and fractional crystallization processes. The Cántaro andesites and dacites, showing no systematic isotopic variations with SiO2 (Fig. 7), may be related by fractional crystallization without significant assimilation of continental crust. A simple mixing model (Faure, 1986; Verma, 2000b) of basaltic andesite M81-17 with upper crustal granite LHG (Fig. 8) shows that Cántaro andesites and dacites could possibly be generated by crustal assimilation. The upper crust is likely to be highly heterogeneous and more complex petrogenetic processes of assimilation and fractional crystallization might be involved, both of which would make the mixing curve to pass closer to the Cántaro andesites and dacites than the curve currently drawn (Fig. 8) and, thus, could better explain the relationship of Cántaro basaltic andesite with more evolved magmas. However, because such complex models require more assumptions concerning the relative proportions of assimilation and fractional crystallization, thermal energy constrained chemical models should be practiced (Verma and Andaverde, 2007). 5.2. Nevado de Colima The isotopic data from Nevado are consistent with fractional crystallization as the dominant process, without significant crustal assimilation (Figs. 7 and 8). The basaltic parent SAY-22E (Table 1; V. Tezontle; Fig. 1) proposed for the Volcán Colima andesites and dacites (see below) cannot represent the correct composition for Nevado de Colima rocks, because it is richer in many incompatible elements than the Nevado samples (Fig. 5b). Instead, a different primitive magma poorer in incompatible elements than SAY-22E is required, which would support a chemically heterogeneous source beneath the Colima Rift (see below the interpretation of Volcán Colima). 5.3. Volcán Colima A primitive subalkaline basalt (SAY-22E from the easternmost cinder cone Volcán Tezontle; Luhr, 1997) is a suitable parent to the subalkaline suite (Luhr and Carmichael, 1981; see also Fig. 5c, d) by fractional crystallization. The variability in 143Nd/144Nd and 207Pb/ 204 Pb ratios, however, does suggest some crustal contamination in the andesitic magmas, or else this isotopic variability could be inherited from different batches of isotopically different basaltic parents in the magma chamber, i.e. from an isotopically heterogeneous mantle. The geochemical data for the Colima andesites are largely consistent with fractional crystallization from a mafic parent (Luhr and Carmichael, 1980). 5.4. Basanite–minette cinder cones The Sr and Nd isotopic relationship of alkaline and subalkaline suites from Volcán Colima was interpreted by Luhr et al. (1989) to support a close genetic relationship between the alkaline and subalkaline rocks of the W-MVB, although the significant isotopic differences should be interpreted as reflecting mantle heterogeneity in the source regions of the W-MVB, or the involvement of crustal assimilation, or both processes as done for other parts of the MVB

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Fig. 9. Radiogenic isotope ratio plots for the Cántaro–Colima rocks and their comparison with mantle reservoirs (Zindler and Hart, 1986) and the downgoing slab (Verma, 2000a). (a) 87Sr/86Sr, (b) 143Nd/144Nd, (c) 207Pb/204Pb, and (d) 208Pb/204Pb; all versus 206Pb/204Pb. Isotopically anomalous basaltic andesite M81-17 from Volcán Cántaro and primitive basalt SAY-22E are labeled. The data for comparison are for MORB and seamount from adjacent oceanic basins (Macdougall and Lugmair, 1986; White et al., 1987; Graham et al., 1988; Prinzhofer et al., 1989) including the peculiar Shimada seamount (Graham et al., 1988), subalkaline rocks from the Tepic–Zacoalco Rift (Verma and Nelson, 1989; Wallace and Carmichael, 1994; Luhr, 1997), and rift-related basalt and basaltic trachyandesite rocks from the volcanic front of the central Mexican Volcanic Belt (C-MVB) (Verma, 2000a). The mixing curve (designated “Downgoing slab”) is also shown (the numbers 1, 2, 5, 10, and 20, refer to the wt.% of the sediment component in the basalt–sediment mixture of the upper part of the Cocos plate). The Northern Hemisphere Reference Line (NHRL; Hart, 1984) is included as a reference in Pb–Pb plots. The mantle component PREMA is also shown; the other components plot somewhat outside of the field of these figures. Upper crust represented by granite LHG is also included.

from combined geochemical and isotopic data and their petrogenetic modeling (Verma, 1999, 2000c, 2001a,b,c). The fact that the Sr–Nd isotopic relationship between subalkaline and alkaline magmas from the Tepic–Zacoalco Rift are reverse from that for the Colima Rift (Fig. 8) has some important implications. Basically, we see evidence for a continental lithospheric source in the Tepic– Zacoalco Rift alkaline rocks (with increasing crustal contamination in more evolved compositions), and for enriched, crustally modified isotopic compositions in the Tepic–Zacoalco Rift subalkaline rocks (Luhr et al., 1989; Verma and Nelson, 1989). In the Colima area, the Cántaro– Colima subalkaline rocks show relatively depleted isotopic signatures (weakly contaminated or uncontaminated by crustal rocks), probably implying that their mantle source was probably less enriched with subduction-derived fluids. The Colima alkaline rocks are probably consistent with a more enriched “subduction-fluid”, or “veined mantle” source (see Verma and Hasenaka, 2004). The mafic alkaline magmas in the Tepic–Zacoalco Rift (having HFSE and LILE signatures consistent with derivation from a lithospheric source in the mantle, without the involvement of the subducting Rivera plate; Verma and Nelson, 1989), have lower 87Sr/86Sr and higher 143Nd/144Nd than the mafic subalkaline magmas from the Tepic–Zacoalco Rift, the latter presumably derived with the participation of the subducting slab (Figs. 7–9). In contrast with the Tepic–Zacoalco Rift suites, the alkaline and subalkaline magmas from the Colima Rift have similar HFSE/LILE signatures (Fig. 4). The Colima Rift is also located close to the continuation of the Cocos– Rivera diffuse boundary through the Manzanillo Trough (MT in Fig. 1;

Bourgois et al., 1988). Initially before being affected by subduction, the mantle beneath the Cántaro–Colima area could have been: (a) of continental lithospheric-type sampled by the mafic alkaline magmas from the Tepic–Zacoalco Rift, or (b) MORB-type mantle sampled by a possible parent of the basaltic andesite M81-17 from Volcán Cántaro, or (c) even represented isotopically by some Pacific seamounts and MORB with low 143Nd/144Nd values (see Figs. 8 and 9). As compared to the mantle wedge, the fluids from subduction of altered MORB and overlying sediments from the Rivera plate could be enriched in 87Sr/ 86 Sr and Pb isotopes and relatively depleted in 143Nd/144Nd, with the net effect to shift the mantle wedge towards the isotopic signatures of the Cántaro–Colima volcanic chain (Verma, 1992). However, the difficulty with this scenario is that altered MORB should shift the isotopic compositions to the right of the mantle array (Verma, 2000a, 2002, 2006). No published isotopic or trace-element data are yet available for the sediment column and altered MORB from the subducting Rivera plate, which makes it difficult to better constrain this process, although limited Sr isotope data for relatively fresh MORB from DSDP Site 473 at the interior of the Rivera plate have been reported by Verma (1981). It is likely that the trench basalts might be more altered (Verma, 2000a) and modified in their isotopic compositions than the interior of the oceanic plate (see section 5.6). The alkaline magmas from the Cántaro–Colima volcanic chain have higher 87Sr/86Sr, lower 143Nd/144Nd and higher Pb isotope ratios than the subalkaline magmas (Figs. 8 and 9). The combined Sr, Nd, and Pb isotopic data of alkaline magmas in this chain are probably compatible with their generation in the mantle wedge with a larger participation of the fluids

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Fig. 10. Conventional binary tectonic discrimination diagrams for the W-MVB basic and ultrabasic rocks. Note that none of the diagram indicates an arc setting for the W-MVB; instead, a within-plate (continental rift) setting is more plausible. (a) Ti/Y–Zr/Y (after Pearce and Gale, 1977); (b) Ti/Y–Zr/Y (after Pearce and Norry, 1979); (c) Ti–V (after Shervais, 1982); and (d) Score1–Score2 (after Butler and Woronow, 1986), for details on Score functions see Verma (2010).

released by the subducting oceanic plate than for the subalkaline magmas, although alternative interpretation is possible (see Section 5.6). Thus, both magma types could be generated by subduction-related process as already proposed (Luhr and Carmichael, 1990a; Luhr, 1997; Lange and Carmichael, 1990; Carmichael et al., 1996; Hochstaedter et al., 1996). The highly evolved rhyolitic rocks from La Primavera seem to have acquired a significant crustal component and lie on the right of the fields represented by other less-evolved rocks from the Colima Rift and Tepic–Zacoalco Rift and also away from the “mantle array” (Fig. 8). 5.5. Mixed rocks from Volcán Colima Basanite ash A-38 from the Volcán Colima tephra sequence falls between the fields defined by the subalkaline and other alkaline samples from Colima (Fig. 5). The alkaline avalanche block M82-4 plots in a similar position (see overlapping symbols in Fig. 5). We view these samples as extending the alkaline field to lower Sr and higher Nd isotopic values (Table 2; Fig. 8). The Sr–Nd relationship of mixed rocks from this volcano probably strengthens further the subductionrelated model proposed earlier (Luhr, 1997) and outlined above. Of the two mixed scoria samples, surprisingly S34 falls clearly among the subalkaline andesites, showing no tendencies towards alkaline isotopic characteristics (Table 2; Fig. 8). The overlying, phlogopitebearing S33, on the other hand, falls near the isotopically enriched end of the alkaline spectrum, supporting a genetic relationship with the basanite–minette suite (Table 2; Fig. 8). These observations for the mixed alkaline rocks from the Colima Rift are confirmed on other isotope diagrams (Fig. 9) as well. The alkaline avalanche block M82-4 falls between the minettes and the subalkaline cluster. Pb isotopic data for the scoria samples S33 and

S34 are consistent with the Sr and Nd isotopic results; S34 is indistinguishable from the subalkaline samples, whereas phlogopitebearing S33 falls among the basanite–minette samples (Fig. 9; Table 3). The relatively radiogenic Pb values for the Colima alkaline suite are unlikely to reflect a significant interaction with continental crust (e.g., LHG in Figs. 8 and 9) since these are Mg–Cr–Ni-rich, primitive compositions with high contents of Pb (limited Pb concentration data were reported by Luhr and Carmichael, 1981), Sr, Nd, and other incompatible elements, which make them relatively insensitive to contamination (Luhr, 1997). The combined isotopic data in all suites from the Cántaro–Colima volcanic chain are compatible with their derivation from the mantle wedge and fluid transport in the subduction zone (Brenan et al., 1995; Regelous et al., 1997). According to Pardo and Suárez (1995) the subducted Rivera plate reaches depths on the order of 110 km beneath the Cántaro–Colima area, which apparently makes this subduction-related model appropriate for the Colima Rift. However, in a recent seismic tomography study Yang et al. (2009) have interpreted that both Rivera and Cocos plates steepened into the mantle and, as a consequence, a gap between them is being formed beneath the Colima Rift, whose existence and implications have yet to be explored from geochemical data on these rocks. Additionally, some difficulties still remain (see also below) because the isotope shift to the right of the “mantle array” present in nearly all arcs around the world (Verma, 2002, 2006; see also Fig. 8) is not observed in the Colima Rift. 5.6. Complex tectonic regime of the W-MVB From a compilation of data from a large number of arcs, Verma (2006) showed that on Sr–Nd isotope diagrams numerous samples

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Fig. 11. Four of the five (the fifth diagram does not have the clearly indicated continental rift setting and was therefore not included) discriminate function diagrams based on log-ratio transformation of major-elements in basic and ultrabasic rocks from the W-MVB for tectonic discrimination of four tectonic settings (Verma et al., 2006b): island arc (IAB); continental rift (CRB); Ocean Island (OIB); and Mid-Ocean Ridge (MORB). For details on the discriminant functions and statistical field boundaries see Verma et al. (2006b) and Verma (2010). Note that numerous samples plot in the CRB field. For the use of these discriminant function diagrams the SINCLAS computer program (Verma et al., 2002) is required. (a) four tectonic settings (IAB-CRB-OIB-MORB); (b) three tectonic settings (IAB-CRB-OIB); (c) three tectonic settings (IAB-CRB-MORB); and (d) three tectonic settings (CRB-OIB-MORB).

from all arcs plot at the right hand side of the mantle array, but none of the magmas from the Colima–Cántaro volcanic chain shows this tendency (Fig. 8). In fact, all W-MVB magmas are isotopically similar to those of numerous continental rifts (Fig. 8; see also Verma, 2006). The Sr, Nd and Pb isotopic data from the W-MVB are particularly similar to those from the western U.S.A. (Alibert et al., 1986; Farmer et al., 1995; and the references compiled by Verma, 2006, e.g., Kempton et al., 1991). Because the composition of subducting Rivera plate is not yet known, isotopic data from the subducting Cocos plate are plotted in Figs. 8 and 9. Note in Fig. 8 the shift towards the “Downgoing slab” (shown by numerous arc magmas; see also Verma, 2006) is not noted in subalkaline or alkaline magmas from the Cántaro–Colima volcanic chain. Two lava samples from Volcán Ceboruco (W-MVB) and one from the Cuicuilco area of the central part of the MVB showed consistently low 10Be concentrations (106 atoms/g) of 0.3–0.5 (Tera et al., 1986) as compared to arcs (e.g., Central America Volcanic Arc 0.6–24; Aleutians 0.3–15.3; Chile 1.0–2.9; and Japan 0.1–13.5; Tera et al., 1986; Monaghan et al., 1988; Morris and Tera, 1989; Sigmarsson et al., 1990). Although more 10Be data on historic samples from the MVB are needed, the existing information precludes sediment incorporation (from subducted slab or upper continental crust) in the source or evolution of these western and central MVB magmas. The 10Be data can probably be alternatively interpreted in terms of the tectonic erosion data compiled by Clift and Vannucchi (2004). According to this study, the Mexican case represents an erosive arc. The 10Be data in arc rocks point to subduction of young sediment. The basic samples from the W-MVB (Colima Rift, Chapala Rift; Tepic–Zacoalco Rift, and other grabens) on conventional tectonic

discrimination diagrams (Fig. 10a–d; Pearce and Gale, 1977; Pearce and Norry, 1979; Shervais, 1982; Butler and Woronow, 1986) do not show an arc setting; instead of this, a within-plate (or continental rift) setting is more likely although these older discrimination diagrams might not be working well (Verma, 2010; J.C. Varekamp, personal communication, July 2010). The indication of a rift setting for the W-MVB inferred from these discrimination diagrams (Fig. 10a–d) only means that for the chemical elements involved in these diagrams, viz., Ti, Y, Zr, V, and Sr, in the basic magmas are more consistent with the prevailing rift setting rather than the subduction setting. In this context, it may be worthwhile to mention that this interpretation of a rift setting has been recently reinforced for the central part of the MVB from several lines of evidence including the discrimination diagrams (Verma, 2009b; Verma et al., 2010). Recently, Verma et al. (2006b) and Agrawal et al. (2008) have proposed new discriminant function diagrams for basic and ultrabasic magmas through a statistically correct methodology of log-ratio transformation of major- and trace elements, respectively (for the discussion of the statistically correct handling of compositional data, see Agrawal and Verma, 2007). The samples from the W-MVB were plotted on these diagrams (Figs. 11 and 12), in which the identity of individual rifts or grabens was maintained. Fig. 11 clearly shows a continental rift setting and precludes an arc setting for the W-MVB basic and ultrabasic magmas. Note that this conclusion includes the Colima Rift, but refers to only the less voluminous magma types. Diagrams based on the relatively immobile trace elements La, Sm, Yb, Nb and Th, also support this rift setting. The major-elements and La, Sm, Yb, Nb, and Th, in the basic magmas from the W-MVB are more

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Fig. 12. All five discriminate function diagrams based on log-ratio transformation of five trace elements (La, Sm, Yb, Nb, and Th) in basic and ultrabasic rocks from the W-MVB for tectonic discrimination of four tectonic settings (Agrawal et al., 2008): island arc (IAB); continental rift (CRB); Ocean Island (OIB); and Mid-Ocean Ridge (MORB). The discriminant functions and statistical field boundaries are reported by Agrawal et al. (2008). Note that most samples plot in the CRB and/or OIB fields. For explanation on DF1 and DF2 functions of each diagram see Verma (2010). (a) four tectonic settings (IAB-CRB + OIB-MORB); (b) three tectonic settings (IAB-CRB-OIB); (c) three tectonic settings (IAB-CRB-MORB); (d) three tectonic settings (IAB-OIB-MORB); and (e) three tectonic settings (CRB-OIB-MORB).

akin to continental rifts than to arcs. Approximately linear trends in at least some of these diagrams (Figs. 11 and 12) suggest a transitional nature from an arc to a rift setting for basic volcanism from the Cántaro–Colima volcanic chain.

Finally, also the B and Be data plotted in Fig. 13 (B/Be versus SiO2) clearly show that the W-MVB (and WC-MVB) magmas show more affinity to continental rift rather than to arc setting. Thus, the differences in geochemical and isotopic compositions call for

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The tectonic regime around Colima of rifting within an arc setting can also be traced further in the MVB, where trace element ratios suggest both the influence of rifting and arc processes. Acknowledgements

Fig. 13. B/Be versus SiO2 for the western (W-MVB) and the west-central (WC-MVB) Mexican Volcanic Belt and their comparison with Central American Volcanic Arc (CAVA), other arcs, continental rifts and Ocean Islands (OIB). The database compiled by Verma (2006) was used for this purpose. Note both W-MVB and WC-MVB show riftlike B/Be ratios.

significant chemical and isotopic heterogeneity in the mantle beneath the W-MVB and require more complex petrogenetic processes than the ones envisioned here (see e.g., Verma and Hasenaka, 2004). 6. Conclusions During the last 1.5 Ma the focus of subalkaline magmatism in the Colima Rift has migrated some 35 km toward the active Middle America Trench. Volcán Cántaro, the oldest volcanic center consists primarily of siliceous andesites and dacites that are relatively enriched in K and related incompatible elements, contain biotite and hornblende, and are more strongly porphyritic in comparison to more mafic pyroxene and hornblende andesites from the active Volcán Colima. Sr and Nd isotopic compositions showed no significant variation during this trenchward migration of volcanism. The most mafic sample from Volcán Cántaro, basaltic andesite M81-17, has appropriate elemental abundances to serve as a parent to the dominant siliceous andesites and dacites, but is isotopically similar to the MORB and seamount basalts from the adjacent oceanic basins. The Sr and Pb isotopic data for the subalkaline suites from Volcán Cántaro, Nevado de Colima, and Volcán Colima are consistent with simple crystal fractionation from isotopically similar parent basalts. The siliceous Cántaro samples have lower Nd isotopic ratios than the Nevado and Colima andesites. Volcán Colima samples show variation in their Nd isotopic ratios, in conflict with an evolution through simple crystal fractionation from an isotopically homogeneous basaltic parent, and may require the presence of an isotopically heterogeneous mantle. Compared to the Nevado and Volcán Colima subalkaline suites, the alkaline basanite–minette samples have somewhat higher Sr and Pb isotopic compositions and lower Nd isotopic ratios. These results and the similar HFSE/LILE signatures of these two suites are compatible with the generation of both subalkaline and alkaline magmas from the mantle wedge that has been contaminated by slab-derived fluids, but probably greater fluid input is required for the alkaline magmas from the Colima Rift. The ancestral Volcán Colima and the currently active cone have produced a number of relatively alkaline products in addition to the volumetrically dominant andesites. These alkaline rocks plot between the subalkaline and basanite–minette suites on elemental diagrams and many show petrographic evidence presented by Luhr and Carmichael (1981, 1982) of formation by physical mixture of the two magma series. Isotopic data support the mixing scenario for several of these samples (M82-4 and S33), although mixed scoria S34 displays no isotopic evidence for the alkaline component.

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