The whole-rock major and trace element geochemical datasets of Luhr and Carmichael (1981), Carmichael et al. (2006), Maria and Luhr (2008), Vigouroux et al. (2008) and Cai (2009) are considered here. Luhr and Carmichael (1981) analysed 15 samples from the alkaline and sub- alkaline cinder cones by ICP-MS and XRF. The incompatible and rare-earth element abundances in ten of the samples were measured by INAA. Carmichael et al. (2006) re-sampled each of the cinder cones, and analysed them for whole-rock major and trace elements by XRF. No REE data were reported as part of their study. The focus of the work by Maria and Luhr (2008) was on melt inclusions from cinder cone magmas from the Colima and Mascota volcanic fields in the western TMVB. As part of their study, they analysed six samples from the Colima cinder cones for the full suite of major and trace element abundances by XRF and ICP-MS (Maria and Luhr, 2008). Similarly, Vigouroux et al. (2008) studied melt inclusions in the minettes and basanites of the CVC, but also analysed a suite of six whole-rock samples for their major and trace element abundances by XRF and ICP-MS. Cai (2009) re-analysed the trace element compositions of five of the cinder cones by ICP-MS as part of her PhD thesis at the University of Columbia.
- 140
-
Sample: 500 507 508 510 511 5A 6A 6D 7E 8G
Composition: Basanite Basanite Basanite Basanite Minette Leucite-basanite Minette Minette Minette Minette Location: Apaxtepec Telcampana Telcampana Cuauhtemoc La Erita La Erita San Isidro San Isidro El Carpintero Norte Comal Chico
SiO2 50.27 47.78 48.45 47.96 47.60 48.30 47.96 48.32 48.20 48.49 TiO2 1.75 1.10 1.01 1.14 1.61 1.16 1.53 1.61 1.64 1.32 Al2O3 14.76 12.26 11.85 14.50 11.14 11.98 11.44 11.39 11.62 12.46 FeOt 8.60 7.35 7.47 8.23 7.21 7.37 7.49 7.47 7.06 7.67 MnO 0.15 0.14 0.14 0.16 0.13 0.13 0.13 0.13 0.11 0.15 MgO 7.41 13.40 15.25 10.44 12.35 13.27 12.21 11.65 11.81 11.07 CaO 9.39 9.11 8.86 10.29 8.78 9.25 8.57 8.64 8.32 8.84 Na2O 3.20 2.60 2.12 2.63 3.83 3.85 3.45 3.51 3.28 3.23 K2O 3.02 3.69 3.05 2.64 2.48 2.55 3.01 3.11 3.58 2.98 P2O5 0.76 0.91 0.71 0.74 1.31 1.02 1.23 1.27 1.32 1.05 L.O.I. 0.39 0.87 0.55 0.75 2.23 0.29 1.99 1.84 1.96 1.46 SUM 99.70 99.21 99.46 99.48 98.67 99.17 99.01 98.94 98.90 98.72 XRF (ppm) Cr 313 844 1280 580 805 755 655 658 265 700 Ni 82 449 506 192 506 392 344 406 436 272 Cu 50 62 39 74 50 71 82 100 90 41 Zn 84 81 74 69 83 72 84 85 80 83 Rb 38 32 21 16 62 37 51 49 73 31 Sr 1220 2142 1715 1464 1670 2308 2342 2456 3079 2273 Y 27 18 16 18 19 23 22 23 29 22 Zr 346 251 215 188 411 312 404 413 554 347 Nb 11 6 6 10 14 11 14 15 16 9 Ba - - - 1117 - 2009 2425 - - 2162
Table 4.2 A subset of whole-rock ICP-MS and XRF analyses for the alkaline cinder cones from Luhr and Carmichael (1981). The full datasets for the cinder cones are given in Appendix I, and details of the analytical techniques are given in Appendix D.
- 141 -
Figure 4.4 Whole-rock major element variation diagrams for the cinder cone magmatic rocks.
Compiled using data from Luhr and Carmichael (1981), Carmichael et al. (2006), Vigouroux et al. (2008), Maria and Luhr (2008) and Cai (2009). The grey shaded area represents the Group I deposits, and the field outlined by the dashed line represents the Group II eruption deposits. The sub-alkaline cinder cone samples lie within the compositional fields of the Group I and/or Group II eruption deposits. The alkaline magmas form a distinct group which do not lie on the same trend as the Group I and II deposits, indicating the alkaline and sub-alkaline magmas cannot be linked by fractional crystallisation.
- 142 -
The full geochemical datasets are given in Appendix I, and the analytical techniques are described in detail in Appendix D. Table 4.2 reports data for a subset of ten samples from Luhr and Carmichael (1981), spanning the series from basanite to minette.
4.2.1 Major Elements
The alkaline cinder cone deposits are characterised by low SiO2 and Al2O3, and high TiO2,
MnO, MgO, CaO, K2O and P2O5 (Figure 4.4). These magmatic rocks show relatively little
variation in SiO2 content (47.6 – 50.3 wt.%), but show large variations in MgO (7.4 – 15.3
wt.%), K2O (2.5 – 4.4 wt.%) and P2O5 (0.7 – 1.3 wt.%; Figure 4.4). The high MgO, K2O and
P2O5 contents are reflected in the mineralogy, with mineral assemblages dominated by mafic
minerals, and the presence of phlogopite, sanidine and apatite phenocrysts and microphenocrysts (Figure 4.3).
The major element geochemistry for the Group I and Group II eruption deposits shows overall differentiation trends from a mafic parent magma (see Sections 2.4 and 3.4). Samples from the two sub-alkaline cinder cones form two clusters, generally lying within the compositional fields of the Group I and II eruption deposits, suggesting a link between these magmas. The alkaline cinder cones, however, form a distinct group that lies off this trend indicating a separate source or fractionation trend (Figure 4.4). The major elements show the CVC Group I and Group II magmas and the alkaline cinder cone magmas cannot be linked by fractional crystallisation. However, the composition of the basaltic cinder cone, Volcán Tezontal, lies along the trend displayed by the alkaline cinder cones (Figure 4.4). This indicates there is a petrogenetic relationship between high-K alkaline and medium-K, sub-alkaline magmas at the CVC.
4.2.2 Trace Elements
The basanites to minettes are primitive magmas with high Cr and Ni contents of 195 – 1300 ppm and 200 – 500 ppm respectively (Figure 4.5). These abundances of Cr and Ni are significantly higher than in the basalts to andesites of the Group I and Group II eruption deposits. Again this is reflected in the mineralogy, with mafic minerals dominating the assemblage of the alkaline cinder cone scoria (see Section 4.1).
The N-MORB normalised trace element abundance patterns of the alkaline cinder cones reveal that these magmas have a typical subduction-related signature with elevated LILE (Rb, Ba, K, Sr ± Th) relative to HFSE (Nb, Ta, Ti, Hf, Zr, Y ± P; Figure 4.6a). The cinder cone deposits display strong enrichments in the majority of the incompatible elements relative to the Group I eruption deposits, and less so relative to the Group II units. The fluid mobile elements (Rb, Ba and K) show stronger enrichments than the less mobile elements (Figure 4.6a).
- 143 -
Figure 4.5 Compatible whole-rock trace elements, Cr and Ni versus SiO2 for the cinder cone magmas.
The Group I and II eruption deposits are shown by the shaded area and dashed line, respectively. The alkaline cinder cones are primitive magmas indicated by their high Cr and Ni contents.
A Chondrite-normalised REE abundance plot reveals that the LREE (La, Ce, Pr and Nd) are strongly enriched relative to the MREE and HREE (Sm to Lu), and the Group I eruption deposits (Figure 4.6b). The abundances of the REE in the Group II eruption deposits overlap the abundances in the alkaline cinder cone magmas, and have similar patterns (Figure 4.6b).
Figure 4.6 N-MORB normalised trace element and Chondrite-normalised REE abundance diagrams for the alkaline cinder cone magmas.
Normalising values are from Sun and McDonough (1989) and Nakamura (1974) for N-MORB and Chondrite, respectively. The fields for the Group I and Group II eruption deposits are indicated. The cinder cone magmas display characteristic subduction-related trace element abundances with enriched LILE relative to HFSE. Samples from Group II overlap the cinder cone magma field suggesting a link between the suites of samples.
- 144 -