Fernando VELASCO-TAPIA, Surendra P. VERMA

Magmatic processes at the volcanic front of Central Mexican Volcanic Belt: Sierra de Chichinautzin Volcanic Field (Mexico)

The Sierra de Chichinautzin (SCN) volcanic field is considered one of the key areas to understand the complex petrogenetic processes at the volcanic front of the Mexican Volcanic Belt (MVB). New as well as published major- and trace-element and Sr and Nd isotopic data are used to constrain the magma generation and evolution processes in the SCN. From inverse and direct modelling, combined 87Sr/86Sr and 143Nd/144Nd data, and use of multi-dimensional log-ratio discriminant function based diagrams and other geological and geophysical considerations, we infer that mafic magmas from the SCN were generated by partial melting of continental lithospheric mantle in an extensional setting. Inverse modelling of primary magmas from the SCN further indicates that the source region is not depleted in high-field strength elements (HFSE) compared to large ion lithophile elements (LILE) and rare-earth elements (REE). The petrogenesis of evolved magmas from the SCN is consistent with the partial melting of the continental crust facilitated by influx of mantle-derived magmas. Generally, an extensional setting is indicated for the SCN despite continuing subduction at the Middle America Trench.

Anahtar Kelimeler:

geochemistry, subduction, extension, multi-dimensional discrimination diagrams, isotopes, inverse modelling, direct modeling

Magmatic processes at the volcanic front of Central Mexican Volcanic Belt: Sierra de Chichinautzin Volcanic Field (Mexico)

Keywords:

geochemistry, subduction, extension, multi-dimensional discrimination diagrams, isotopes, inverse modelling, direct modeling,

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Appendix A1. New major-element data for SCN volcanic rocks. Sample CHI20 CHI56 CHI18 CHI59 CHI64 CHI70 CHI69 CHI19 CHI42 CHI16 Sample S Barbara Cima Atlapulco Jumiltepec Tetillas Pelagatos Agua Pehualtepec Jumento Santa Fe Locality 18°88’ 19°033’ 19°98’ 18°50’ 18° 50’ 19° 000’ 19° 00.50’ 19° 07’ 19°16’ 19° 21’ Latitude (°N): 98°42’ 99°50’ 99°75’ 99°83’ 99° 000’ 98° 99’ 98° 50’ 99° 17’ 99°09’ 99° 20’ Longitude (ªW): 1 1 1 1 1 1 1 2 1 1 Analytical method BA BA BTA, sho BA BA BA BA BA BA BA Rock type M M HB1 HMI HMI HMI E1 E1 DISQ E1 Major elements (% m/m; original composition) SiO 2 650 110 320 960 630 100 010 640 250 240 TiO 2 534 459 059 479 0.920 0.770 330 160 0.980 300 Al 2 O 3 730 580 310 690 000 490 710 250 160 070 Fe 2 O 3 t 810 000 800 960 990 580 680 100 970 910 MnO 0.134 0.134 0.129 0.128 0.120 0.120 0.120 0.130 0.110 0.120 MgO 760 850 300 420 510 500 980 390 040 250 CaO 090 650 770 460 900 700 120 600 320 330 Na 2 O 620 910 530 580 500 270 700 910 040 880 K 2 O 230 080 180 0.790 0.980 000 240 170 540 270 P 2 O 0.430 0.280 0.710 0.390 0.190 0.740 0.390 0.250 0.370 0.360 LOI 0.190 0.030 0.140 110 0.160 0.150 0.570 0.290 0.470 0.180 Major elements (% m/m; adjusted composition) (SiO 2 ) adj 018 455 576 414 644 815 296 618 679 738 (Na 2 O) adj 646 936 547 611 567 314 788 908 071 915 (K 2 O) adj 239 087 191 0.797 0.999 013 270 170 552 281 Analytical Method: (1) ICP-OES (‘4LithoRes’ methodology; ActLabs, Inc., Canada); (2) XRF (Guevara et al. 2005; Instituto de Geología, UNAM, Mexico). Rock-types are presented according to total alkalis versus silica diagram (Le Bas et al. 1986; Le Bas 2000) on an anhydrous 100% adjusted basis and Fe 2 O 3 /FeO ratio after Middlemost (1989) using the SINCLAS computer program (Verma et al. 2002). Group subdivision based in phenocryst assemblage and geochemical composition: (a) HMI– high-magnesian intermediate magmas; DISQ– evolved magmas with a ol + opx ± cpx ± plg assemblage; (b) E2– evolved magmas with a opx ± cpx + plg assemblage; and (c) E3 – evolved magmas with textural evidences of mineralogical disequilibrium, such as coexisting olivine and quartz, phenocrysts of plagioclase and pyroxene with oscillatory or more complex zoning and twinning, the presence of biotite and hornblende phenocrysts, and quartz xenocrysts with pyroxene reaction rims.
Figure code Total no. of samples (%) Number of samples (%) discriminated as Diagram type IAB (1) CRB+OIB (2+3) CRB (2) OIB (3) MORB (4) SCN VGA2006 Fig_1234m2 28 (100) 2 (7) 26 (93) Fig_123m2 28 (100) 28 (100) Fig_124m2 28 (100) 28 (100) Fig_134m2 28 (100) 2 5 21 Inapplicable Fig_234m2 28 (100) 28 (100) Before and after DODESSYS
SCN VA2011 Fig_1234t2 4 2 2 Fig_123t2 4 4 Fig_124t2 4 3 1 Fig_134t2 4 4 Inapplicable Fig_234t2 4 3 1
CAVA VGA2006 Fig_1234m2 84 (100) 67 (80) 3 (3) 14 (17) Fig_123m2 84 (100) 64 (76) 12 (14) 8 (10) Fig_124m2 84 (100) 66 (79) 5 (6) 13 (15) Fig_134m2 84 (100) 67 (80) 2 (2) 15 (18) Fig_234m2 84 (100) 61 23 Inapplicable Before DODESSYS
CAVA VA2011 Fig_1234t2 41 (100) 26 (64) 1 (2) 14 (34) Fig_123t2 41 (100) 35 (85) 6 (15) Fig_124t2 41 (100) 26 (64) 1 (2) 14 (34) Fig_134t2 41 (100) 24 (59) 1 (2) 16 (39) Fig_234t2 41 (100) 2 (5) 39 Inapplicable After DODESSYS
CAVA VA2011 Fig_1234t2 38 (100) 24 (63) 1 (3) 13 (34) Fig_123t2 38 (100) 33 (87) 5 (13) Fig_124t2 38 (100) 24 (63) 1 (3) 13 (34) Fig_134t2 38 (100) 22 (58) 1 (3) 15 (39) Fig_234t2 38 (100) 1 37 Inapplicable
In Figure code, numbers 1 to 4 refer to the tectonic settings (1–IAB, 2–CRB, 3–OIB, 4–MORB); m2 and t2 are for, respectively, the diagrams proposed by Verma et al. (2006) and Verma & Agrawal (2011). Computer program DODESSYS is by Verma & Díaz-González (2012). from the SCN, using La as a reference element (the most highly incompatible element). Elem (i) (C La – C i ) E diagram (C La – C La /C i ) E diagram n m i se mi I i se Ii r P c(r,n) m i se mi I i se Ii r P c(r,n) Ce 24 0.78 0.07 2 5 0.920 < 0.0001 0.0044 0.0024 0.96 0.08 0.358 0.0860 Nd 15 0.55 0.05 7 0 0.950 < 0.0001 0.0042 0.0039 49 0.13 0.284 0.3044* Sm 15 0.308 0.044 2 5 0.891 < 0.0001 0.016 0.006 72 0.20 0.625 0.0127 Eu 17 0.196 0.034 7 2 0.827 < 0.0001 0.034 0.007 56 0.22 0.800 < 0.0001 Tb 17 0.158 0.030 5 0 0.805 < 0.0001 0.040 0.013 45 0.44 0.622 0.0080 Yb 15 0.123 0.025 3 0.8 0.809 < 0.0001 0.044 0.022 7 0.7 0.488 0.0647 Lu 15 0.119 0.031 9 1 0.724 0.0023 0.053 0.031 0 0 0.421 0.1178 Ba 26 12 0.26 10 9 0.667 0.0002 0.003 0.009 0.63 0.13 0.155 0.4480* Hf 14 0.23 0.06 2 8 0.762 0.0015 0.030 0.007 09 0.23 0.765 0.0014 Nb 12 0.45 0.16 14 6 0.660 0.0196 0.017 0.007 0.55 0.25 0.631 0.0277 Rb 23 0.46 0.14 20 5 0.568 0.0047 0.0134 0.0044 0.51 0.16 0.549 0.0066 Sr 26 0.06 0.06 7 6 0.200 0.3267* 0.0361 0.0029 0.10 0.09 0.9327 < 0.0001 Ta 14 0.62 0.25 9 8 0.584 0.0283 0.009 0.008 0.86 0.30 0.2542 0.3806* Th 14 0.41 0.16 11 5 0.607 0.0214 0.016 0.007 0.81 0.23 0.550 0.0413 U 13 0.80 0.35 8 11 0.570 0.0417 0.0010 0.0013 0.98 0.43 0.010 0.9689* Y 24 0.047 0.014 5 0.5 0.580 0.0030 0.104 0.010 21 0.35 0.910 < 0.0001 Zr 24 0.39 0.06 3 1 0.806 < 0.0001 0.018 0.005 0.96 0.15 0.632 0.0005 Ti 26 -0.004 0.017 8 0.6 0.049 0.8010* 0.117 0.007 -0.04 0.26 0.955 < 0.0001 P 26 7 0.5 39 17 0.732 < 0.0001 0.0024 0.0011 0.186 0.037 0.410 0.0371 K 26 0.60 0.18 39 6 0.561 0.0029 0.0102 0.0017 0.22 0.06 0.783 < 0.0001 n= number of data pairs considered in the trace-element diagrams; m i = slope of the linear model; I i = intercept value of the linear model; r= correlation coefficient of the linear model; P c(r,n) = probability that the variables are not correlated (i.e. 1 - P c(r,n) is the probability that the two variables are correlated); se mi = standard error for slope in (C La – C i ) E or (C La – C La /C i ) E diagrams; se Ii = standard error for intercept in (C La – C i ) E or (C La – C La /C i ) E diagrams; subscript E refers to normalisation with respect to Silicate Earth values. All concentration data are normalised against Silicate Earth values (in mg.g -1 ) by McDonough & Sun (1995): La= 0.648; Ce= 1.675; Pr= 0.254; Nd= 1.250; Sm= 0.406; Eu= 0.154; Gd= 0.544; Tb= 0.099; Dy= 0.674; Ho= 0.149; Er= 0.438; Tm= 0.068; Yb= 0.441; Lu= 0.0675; Ba= 6.600; Cs= 0.021;
Hf= 0.283; Nb= 0.658; Pb= 0.150; Rb= 0.600; Sr= 19.9; Ta= 0.037; Th= 0.0795; U= 0.0203; Y= 4.30; Zr= 10.5; Ti= 1205; P= 90; K= 240.
Asterisk “*” denotes statistically invalid correlations even at the 95% confidence level (italicized P c(r,n) values). from the CAVA, using La as a reference element (the most highly incompatible element). Elem (i) (C La – C i ) E diagram (C La – C La /C i ) E diagram n m i se mi I i se Ii r P c(r,n) m i se mi I i se Ii r P c(r,n) Ce 20 0.651 0.026 48 0.43 0.986 < 0.0001 0.0133 0.0019 0.900 0.031 0.860 Nd 20 0.440 0.017 69 0.27 0.987 < 0.0001 0.0275 0.0028 0.83 0.05 0.920 Sm 20 0.224 0.019 37 0.32 0.940 < 0.0001 0.0584 0.0038 0.77 0.06 0.964 Eu 20 0.198 0.014 40 0.24 0.955 < 0.0001 0.063 0.005 0.78 0.08 0.948 Gd 20 0.165 0.014 52 0.23 0.942 0.073 0.005 0.75 0.08 0.964 Dy 20 0.135 0.015 70 0.25 0.904 0.091 0.008 0.90 0.14 0.934 Er 20 0.150 0.017 96 0.28 0.904 0.089 0.011 19 0.18 0.890 Yb 20 0.100 0.009 74 0.15 0.934 < 0.0001 0.120 0.009 25 0.15 0.949 Ba 20 0.46 0.28 29 5 0.364 0.1152* 0.0209 0.0021 0.100 0.035 0.918 0.0111 Cs 12 0.007 0.10 8 0 0.020 0.9468* 0.151 0.038 3 0.8 0.766 0.1091 Hf 12 0.36 0.05 2 0.7 0.928 0.0011 0.036 0.012 03 0.19 0.687 0.0135 Nb 20 46 0.14 -2 4 0.922 0.3728 0.008 0.011 18 0.19 0.152 0.5230* Pb 13 0.31 0.08 6 5 0.762 0.0020 0.0394 0.0034 0.33 0.07 0.961 0.0005 Rb 20 0.45 0.10 8 6 0.733 0.0002 0.024 0.005 0.61 0.09 0.698 Sr 20 0.49 0.07 9 1 0.871 0.0260 0.0015 0.220 0.025 0.970 Th 13 0.60 0.05 3 0 0.959 0.2549 0.003 0.007 44 0.14 0.125 0.6838* U 13 0.69 0.18 6 5 0.756 0.0030 0.014 0.005 0.54 0.10 0.06 0.0184 Y 20 0.198 0.014 82 0.24 0.957 0.066 0.009 24 0.14 0.873 Zr 19 0.53 0.05 07 0.8 0.941 0.0160 0.015 0.012 23 0.20 0.301 0.2098* Ti 20 0.140 0.026 84 0.42 0.788 0.019 0.11 0 9 0.373 0.1053* P 20 11 0.07 0.7 2 0.989 0.6057* 0.0001 0.0005 0.484 0.027 0.088 0.7119* K 20 18 0.16 2 7 0.977 0.0013 0.0023 0.0008 0.203 0.013 0.576
For the explanation of variables and symbols see Appendix A14.