MATERIAL Y MÉTODO
4.1 ANÁLISIS DESCRIPTIVO
4.1.2. Características del LES
Throughout the 1990s and 2000s, Jim Luhr, Carlos Navarro-Ochoa and Ivan Savov sampled and described CVC eruption deposits preserved and exposed on the flanks of Nevado de Colima and in quarries and gullies on the rift floor (Figure 1.14; Luhr et al., 2010). Through detailed sampling of charcoal found within ash horizons and tephra fallout deposits, they were able to identify at least 25 deposits, erupted between 30,000 yrs B.P. and the present day (Figure 1.15; Luhr et al., 2010). Further field campaigns carried out as part of this study in January 2010 and February 2011 have built upon the work of Luhr et al. (2010) on the tephrochronology of the CVC. To date, eruption deposits have been described at 89 localities across an area of ~500 km2, including exposures in quarries on the rift floor (Figure 1.14).
Figure 1.14 Sample location map for the Holocene explosive eruption deposits.
Locations are from Luhr et al. (2010) and new sites sampled in 2010 and 2011. The majority of samples are located in access roads on the upper slopes of Nevado de Colima. The lower slopes are densely vegetated therefore access is difficult.
In total, 181 radiocarbon ages have been obtained from the CVC eruptive stratigraphy, yielding uncalibrated ages from 80±50 to 29,930±210 yrs B.P. (Figure 1.15; Luhr et al., 2010). Of these, 143 ages represent the Holocene stratigraphy (0 to 9000 yrs B.P.). Nevado de Colima is densely vegetated; therefore the eruption deposits on the volcano’s flanks are only exposed in road cuts.
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Nevado de Colima is a national park which has one main access road on the northeast flank. Once inside the park entrance at 3480m a.s.l., there are numerous roads; therefore the majority of the deposits are exposed at high elevations, where the older units have been buried, eroded or destroyed by subsequent eruptions. There are a few roads on the lower slopes; however, the area is prone to landslides, and the roads are not well maintained. In the early 2000s, Luhr and Navarro-Ochoa collected samples for geochemical and petrological analysis from the older eruption deposits along roads on the lower slopes (Luhr et al., 2010). They also collected charcoal samples for radiocarbon dating. In an effort to re-sample some of these deposits, and to search for more charcoal samples to build on the tephrochronology of Luhr et al. (2010), these roads were re-visited in February 2011. Unfortunately, only one of the roads was passable, and no charcoal was found.
Figure 1.15 Uncalibrated radiocarbon 14C ages for the explosive plinian CVC eruption deposits (Komorowski et al 1997; Luhr et al. 2010).
Out of a total of 181 dates, 143 of these were sampled between 9000 yrs B.P. and present. The younger eruption deposits are well exposed high on the flanks of Nevado de Colima, while the older eruption deposits are less well exposed, confined to lower elevations where the slopes are densely vegetated and access is difficult. Symbols are larger than analytical error (typically ±60 years).
The stratigraphy of eruption deposits exposed on the flanks of Nevado de Colima comprise tephra fallout deposits varying in thickness from a few centimetres to over 1m (Figure 1.16). The fallout deposits comprise pumice or scoria, with clasts varying from <1cm to 17cm. Lithics are abundant in all the erupted units, typically occurring as angular fragments up to 2cm across (measured along the long-axis). Interbedded with the tephra layers are ash-rich surge or pyroclastic flow horizons (Figure 1.16). These ash beds are typically rich in charcoal fragments; however they commonly contained reworked pumice from the underlying fallout deposits. As a result, care must be taken when using the radiocarbon dating for tephrochronology.
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Figure 1.16 Composite stratigraphic section and field photographs of explosive eruption deposits exposed in road-cuts on the flanks of Nevado de Colima.
The unit names and uncalibrated 14C ages are as reported in Luhr et al (2010). The Group II eruption deposits (units N, F and D) are highlighted in bold.
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Using a combination of radiocarbon dating, together with field mapping, petrology and geochemistry, a detailed stratigraphy of the CVC Holocene explosive eruption deposits has been compiled up to 13,000 yrs B.P. (Figure 1.16, Appendix A). Older eruption deposits are difficult to correlate due to poor exposure and a lack of detailed charcoal sampling for radiocarbon dating. Many of the units also show very similar petrological and geochemical characteristics; therefore correlating units based on these data alone is not possible.
The highly explosive (plinian) eruption deposits show a wide range in composition from basalt to andesite based on the TAS and K2O vs. SiO2 diagrams of Le Maitre et al. (2002; Figure 1.12).
The samples are all pumice and scoria clasts exposed in road-cuts, therefore, the classification of altered volcanic rocks after Hastie et al. (2007) is also used (Figure 1.17). This classification uses the fluid immobile trace elements Co and Th in order to see through possible effects of post-eruption hydrothermal alteration. According to this classification, the plinian eruption deposits range in composition from basaltic-andesite to rhyolite (Figure 1.17). The majority of the deposits are medium-K; however, some samples appear to be trending towards the high-K alkaline cinder cone compositions (Figures 1.12 and 1.17).
Figure 1.17 Classification of the interplinian and plinian CVC eruption deposits based on the classification of altered volcanic rocks of Hastie et al. (2007).
This classification uses immobile trace elements in place of the mobile elements in order to see through the effects of alteration.
The geochemical evolution of the CVC eruption deposits is shown on time series plots (Figure 1.18). The interplinian eruption deposits show little variation in whole-rock major element geochemistry, while the explosive (plinian) eruption deposits show a wide variation in SiO2,
MgO, K2O and P2O5. Overall the SiO2 content appears to decrease from 30,000 yrs B.P. to
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is reflected in the MgO content, which shows the opposite trend. However, there are fewer samples from 10,000 to 30,000 yrs B.P., so it is difficult to draw concrete conclusions from the data.
Figure 1.18 Time plots showing the variations in whole-rock major element geochemistry for the CVC eruption deposits over the past 30,000 years.
The deposits are divided into two groups based on the geochemical distinctions; the deposits with high MgO, K2O and P2O5 (circled) form Group II.
The plots of K2O and P2O5 against age reveal the majority of the eruption deposits show little
compositional variation; however, some of the samples have distinctly high K2O and P2O5
(Figure 1.18). Based on geochemistry and mineralogy, the plinian deposits can be divided into two groups. Group I represents the bulk of the tephra fallout deposits exposed on the flanks of Nevado de Colima, while Group II comprises three units with distinctive mineralogy and geochemistry (Figure 1.18). The main differences between the two groups are summarised in Table 1.1.
The Group I eruption deposits are medium-K, sub-alkaline basaltic-andesites to andesites (50.7 to 60.4 wt.% SiO2) representing the products of differentiation from primitive mantle-derived
melts. This evolutionary trend is shown by increasing whole-rock Al2O3, K2O and Na2O, and
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Unit
Eruption Age
(yrs B.P.) Group Composition
Crystallinity (vol.%) Plag (vol.%) Pyroxene (vol.%) Hbd (vol.%) Olivine (vol.%) Phlog (vol.%) SiO2 (wt.%) MgO (wt.%) K2O (wt.%) Y 4460±40 I Andesite 10 - 15 5 - 10 < 1 2 - 4 - - 53.9 – 60.4 1.8 – 2.3 1.0 – 1.5 W 4480±60 – 4540±60 I Basaltic-Andesite 13 - 19 7 - 10 2 - 5 2 - 5 < 1 - 54.7 – 57.1 3.9 – 5.9 0.8 – 1.1 U 4740±40 – 4760±70 I Basaltic-Andesite 20 - 25 12 - 15 3 - 6 2 - 5 1 - 2 - 50.7 – 56.8 2.3 – 6.1 0.7 – 1.3 S 5430±50 – 5500±60 I Basaltic-Andesite 12 - 17 7 - 10 1 - 2 1 - 3 1 - 2 - 54.6 – 58.6 3.6 – 5.8 0.9 – 1.3 P 5980±50 – 6150±40 I Basaltic-Andesite 14 - 22 10 - 15 1 - 2 1 - 3 < 1 - 55.4 – 59.7 2.6 – 6.2 1.0 – 1.3 N 6950±50 – 7070±60 II Basaltic-Andesite 9 - 13 5 - 10 1 - 2 < 1 1 < 1 52.8 – 56.4 3.3 – 6.2 1.6 – 2.5 L 7520±50 – 7530±80 I Andesite 11 - 19 5 - 10 < 1 1 3 - 5 - 58.0 – 59.9 3.9 – 5.1 1.0 – 1.2 J 7750±60 – 7760±50 I Basaltic-Andesite 11 - 16 7 - 10 1 1 - 2 1 - 53.4 – 59.5 4.6 – 6.0 1.1 – 1.9 H 9770±60 – 10,310±50 I Basaltic-Andesite 13 - 17 2 - 10 1 - 2 2 - 3 < 1 - 5 - 54.5 – 58.7 4.0 – 7.3 0.8 – 1.1 F 11,840±70 – 12,080±150 II Basalt 7 - 12 <1 - 1 2 - 5 < 1 1 - 3 1 - 2 43.9 – 53.3 6.4 – 8.1 0.4 – 2.3 D 12,460±60 – 13,350±130 II Basalt 10 - 15 1 - 3 < 1 - 2 3 - 7 1 1 53.6 – 57.5 2.9 – 6.0 1.0 – 2.2
Table 1.1 Summary table of the characteristics of the CVC eruption deposits.
The Group II units, highlighted in bold, are distinguished by the presence of phlogopite, and their high K2O contents. Mineral abbreviations are: Plag = plagioclase;
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The differentiation trend of the Group I eruptions deposits is illustrated by units U, W and Y, which erupted sequentially between c.4700 to c.4400 yrs B.P. (Table 1.1; Luhr et al., 2010). These units vary in composition from basaltic (50.7 wt.% SiO2) to high-silica andesite (60.3
wt.% SiO2) from oldest (unit U) to youngest (unit Y). Pumice clasts from these units show a
decrease in crystallinity over time, varying from 25 vol.% in unit U to 10 vol.% in unit Y. Similar trends are also observed in plagioclase, pyroxene and olivine abundances (Table 1.1). Accordingly olivine is not present in the more evolved unit Y, but here hornblende is common at up to 4 vol.%.
The Group I eruption deposits show very similar subduction-related trace element abundance patterns to the interplinian eruption deposits with enrichments in LREE and LILE relative to HFSE (Figure 1.19). Detailed petrological and geochemical analyses of the Group I eruption deposits is presented in Chapter 2.
Figure 1.19 Incompatible whole-rock trace element abundances normalised to N-MORB and REE normalised to Chondrite for the CVC eruption deposits.
Normalising values are from Sun and McDonough (1989) and Nakamura (1974), respectively. All the CVC eruption deposits, including the alkaline cinder cones, have subduction-related trace element abundances characterised by depletions in the HFSE (Ta, Nb) relative to the LILE (Rb, Ba, K). The interplinian and Group I plinian samples have overlapping compositions, while the Group II samples show stronger enrichments in all the incompatible trace elements, partially overlapping the alkaline cinder cone compositions.
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The Group II eruption deposits comprise units N, F and D of the Luhr et al. (2010) stratigraphy, which erupted c.7000, c.12,000 and c.13,000 yrs B.P. respectively (Figure 1.16). These units range in composition from basalt to basaltic-andesite (43.9 to 57.5 wt.% SiO2; Table 1.1) but are
geochemically distinct from the CVC sub-alkaline trend, distinguished by higher contents of K2O (up to 2.5 wt.%), MgO (up to 8 wt.%) and P2O5 (up to 0.68 wt.%; Figure 1.18; Table 1.1).
Scoria from these eruption deposits are petrologically distinct, characterised by the presence of phlogopite. Samples typically have low crystallinity of 10-15 vol.% and relatively low abundances of plagioclase and orthopyroxene and higher contents of clinopyroxene and olivine with respect to the Group I deposits (Table 1.1).
The three eruption deposits that form Group II display subduction-related trace element signatures; however, relative to Group I and the interplinian deposits, the Group II magmas display stronger enrichments in all of the incompatible trace elements, particularly in the LREE (La, Ce, Pr, Nd) and fluid mobile elements (Rb, Ba, K, Cs; Figure 1.19). The trace element abundance patterns of the Group II samples partially overlap the alkaline cinder cone field suggesting a petrogenetic link. The Group II eruptions deposits are discussed in detail in Chapter 3.
In the first study of the Holocene tephra deposits, Luhr and Carmichael (1982) discovered a scoria fallout deposit (unit N) exposed on the flanks of Nevado de Colima that was mineralogically and geochemically distinct from the rest of the fallout deposits. The authors proposed that this unit represented mixing between the medium-K, sub-alkaline CVC magma, and the alkaline magmas of the cinder cones. Based on morphology, the ages of the cinder cones were estimated at 1500 to >20,000 years old (Luhr and Carmichael, 1981) coinciding with the explosive eruptions that formed the Holocene CVC stratigraphy. Luhr and Carmichael (1982) therefore proposed that the geochemically distinct unit resulted from direct mixing between these two contemporaneously erupting magmas; however, this hypothesis was never fully explored. Allan and Carmichael (1984) and Carmichael et al. (2006) dated the cinder cones using K-Ar and 40Ar/39Ar techniques, which yielded ages of 450 ka to 62 ka; much older than the proposed mixed units of Luhr and Carmichael (1982). The relationship between the alkaline cinder cone magmas and the CVC magmas is explored throughout the thesis, and is discussed in detail in Chapter 5.