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GEOCHEMISTRY AND PETROGENESIS OF LAVAS FROM THE CASITAS SHIELD,
VOLCAN CERRO AZUL, SOUTHERN VOLCANIC ZONE, CHILE
Mollie K. LAIRD, and WULFF, Andrew H., Dept. of Geography and Geology, Western Kentucky Univ., Bowling Green, KY, 42101; lairdmk@wku.edu
Abstract
The Cerro Azul/Descabezado Grande (DGCA) volcanic complex is located at 35.5o S in the
Southern Volcanic Zone of the Chilean Andes. This region is compositionally transitional
between more primitive compositions in the south and more evolved compositions in the north.
The two main edifices, Cerro Azul and Descabezado Grande are early Holocene and latest
Pleistocene in age and overlie the more mafic Casitas Shield. Pleistocene glaciers deeply incised
the sides of these edifices exposing vertical stacks of lavas. Lavas were sampled in seven vertical
stratigraphic sections during two field seasons in order to construct chemical stratigraphic
columns, and ultimately, to construct a composite chemo-stratigraphy for the Casitas shield stage.
All samples were analyzed for complete major and trace element abundances. At least ten
different eruptive episodes, comprising 3-15 individual flows, have been identified primarily by
various field criteria. Each episode may be modeled separately, revealing the influence of shortterm,
shallow processes superimposed over the longer-term behavior of the volcanic complex with
time.
Whole rock geochemistry, petrography and modal analysis, and mineral compositional data were
analyzed to determine the petrogenetic history of an eruptive episode. Changes in elemental
abundances and ratios were used to facilitate the correlation between lava flows from section to
section. Petrogenetic models constructed using the geochemical data show fractional
crystallization as the dominant process responsible for the lavas, followed by a period of recharge
late in the episode. The whole magma system shows a decrease in Sr but individual eruptive
episodes have short-term increases indicating evidence of other processes (e.g. magma mixing,
assimilation). The abundance of Si decreases up section but fluctuations in Mg within flows are
supportive of other magmatic processes.
Similar comprehensive sampling at the nearby Tatara-San Pedro complex resulted in the
development of a detailed composite volcanic stratigraphy. This study will compare lavas of the
same age at the two neighboring volcanic centers to examine the possibility of identifying
regional/tectonic controls on the magma generation and modification.
Field relationships
between sections
CSS1, CSS2, CSN1
and CSN2. Photo
taken from below
CSN.3 looking east up
to the top of the Casitas
shield.
CSS2 CSN2
CSS1
CSN1
Background
■ The transitional SVZ represents a boundary separating dominantly basaltic andesite volcanoes in the
southern SVZ from volcanoes composed dominantly of andesite to dacite in the northern SVZ.
■ Compositional variations at individual eruptive centers have been attributed to multiple sources for
magmas and complicated interactions between different petrogenetic processes.
■ The Tatara-San Pedro (TSP) volcanic complex in the transitional SVZ revealed a range of
compositions, at one location, that is essentially equivalent to that of the entire SVZ.
■ Composite chemostratigraphy demonstrates changes in petrogenetic processes, sources, and eruptive
activity and styles at a single location, with time, in addition to identifying periods of eruptive activity or
quiescence.
■ The Descabezado Grande-Cerro Azul (DGCA) volcanic complex is characterized by excellent
exposures of stacks of lavas in steep-walled canyons surrounding the present edifices.
■ Every lava in vertical stacks of flows was sampled in stratigraphic sections located in three adjacent
canyons south of the Cerro Azul edifice.
■ Sixteen physically correlative flows were sampled in more than one section to facilitate unambiguous
correlations between sections, and to assess within-flow geochemical variation.
CSS
CSN
CDCS
CDCN
Desc. Grande Cerro Azul
Map of the Southern Volcanic Zone (SVZ) of the Chilean Andes showing the location
of this study and the Tatara-San Pedro complex referred to. In general, volcanoes to
the south are more mafic, almost totally lacking evolved compositions, while
volcanoes to the north of this study are more evolved/contaminated, almost totally
lacking basalts.
Aerial photograph of the Casitas shield portion of the DGCA complex. The
yellow dashed line is a line of traverse along the edge of the shield, with sampled
sections identified in yellow letters. Blue dots are camps and the dotted blue line
is a line of traverse connecting sections CSN and CSS. Flows in this study are
physically correlated to both of these sections.
Topographic map of the DGCA volcanic region. Sampled stratigraphic sections
are located west and north of Laguna Invernada (refer to aerial photo). Map from
Hildreth and Drake (1992)
CAS
CSS.3-9
Data
■ The DGCA lavas are primarily basalts to basaltic andesites in composition, and are tholeiitic in character.
■ The basalts and basaltic andesites are generally aphanitic in texture, with larger (subhedral/rounded)
xenocrysts, phenocrysts and glomerocrysts, and are composed primarily of plagioclase, olivine, and pyroxenes.
■ Olivine phenocrysts are small and euhedral with compositions ranging from Fo65-70. ■ Olivine xenocrysts are generally larger, subhedral to slightly rounded and with higher Fo content (Fo74-80). ■ Feldspars generally fall into two populations – one generally larger and more rounded with higher An
content and the other comprising smaller, more euhedral laths with lower An content.
■ SiO2 content ranges from 51.7 to 54.3 wt%, with MgO ranges of 3.87-4.75 wt% and Mg# ranging from 48-
50.5; demonstrating that none of these lavas represent primary magmas – but all have undergone substantial
fractional crystallization.
■ CaO ranges of 7.91 – 8.79 wt% and Al2O3 forms a narrow range from 18.87 – 19.23wt% - reflecting high
feldspar content in all lavas.
■ Sr content: 801-902 ppm ■ Rb content: 11.4-21.1 ppm
■ Ba content: 297-363 ppm ■ Ni content: 13-19 ppm
■ Cr content: 1-10 ppm ■ Ce content: 23-31 ppm
■ In comparison to the rest of the SVZ and TSVZ, the lavas of the DGCA complex exhibit high abundances of
aluminum, calcium, and strontium.
■ The lavas also exhibit low levels of nickel, chromium, rubidium, and magnesium, as well as low levels of
other large ion lithophile elements.
■ Within-flow compositional variation is sufficiently low that flows from the same eruptive episode can be
chemically correlated between different sampled stratigraphic sections.
CSS.10-17
CSS.18-27
34
31-33
30
29
28
27
26
Eruptive
break
25
24
23
Eruptive
break
19-22
17-18
16
12
15
14
13
11
10
8
9
7
6
5
Photographs of sections CSS1 and CSN1 showing flows in stratigraphic position. Flows at the top of the CSS section cover
the top of the Casitas shield and extend from Volcan Cerro Azul west to abut on outcrops of the Invernada pluton. Samples
were taken from the massive middle of lava flows to avoid weathering and alteration. Field observations (soil development,
mingled flow tops and bottoms, erosion surfaces, etc.) were the basis for identifying eruptive episodes. Several flows were
quite laterally extensive and were used to physically correlate the sampled sections.
Yellow numbers show the location and number of each sample.
Plots showing flows from this study compared with all published compositional data for volcanic centers of the
Southern Volcanic Zone (in gray symbols). Lavas are medium-K, tholeiitic basaltic andesites. Flows CSN.24-34
and CSS.1-9 are filled red circles; lower CSN flows are in green open circles. Plots after LaBas et al. (1986), Gill
(1981), Miyashiro (1974).

Casitas Shield Flows
sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Nb Zr Sr Zn Ni Cr V Ce Ba La Y U Rb Th Pb
CSN-24 53.90 0.94 19.17 8.48 0.14 3.96 7.96 4.18 1.16 0.25 2.8 85 902 72 15 1 180 30 349 16 16.2 2 21.1 3 8
CSN-25 52.56 1.07 19.12 8.86 0.14 4.39 8.45 3.86 1.03 0.23 3.2 80 845 67 14 3 188 29 315 12 17.1 1 17.2 2 5
CSN-26 52.70 1.08 19.22 8.93 0.15 4.37 8.44 4.15 1.02 0.22 3.1 75 842 66 13 4 174 23 332 11 17.3 0 15.8 2 6
CSN-27 52.59 1.09 19.11 8.98 0.14 4.40 8.47 4.09 1.02 0.22 2.7 80 848 64 15 5 167 27 326 14 17.2 1 16.5 3 7
CSN-28 52.31 1.11 19.23 8.98 0.14 4.55 8.65 4.05 0.98 0.21 3.4 80 842 65 16 9 224 26 317 14 17.3 1 15.3 3 5
CSN-29 51.68 1.13 19.08 9.24 0.15 4.75 8.72 4.06 0.93 0.21 3.3 78 828 67 19 9 193 25 322 12 17.6 1 11.4 1 7
CSN-30 51.86 1.11 19.06 9.03 0.15 4.65 8.79 3.79 0.93 0.21 2.5 78 826 66 15 8 187 24 308 9 17.8 1 12.7 2 7
CSN-31 52.10 1.13 19.17 9.09 0.15 4.67 8.79 3.83 0.92 0.21 2.7 75 822 69 15 10 217 23 297 11 18.1 1 11.5 1 8
CSN-32 52.13 1.12 19.13 9.06 0.15 4.59 8.79 4.11 0.94 0.21 2.9 73 817 65 18 7 200 24 309 12 17.8 1 13.1 2 5
CSN-33 52.11 1.14 19.08 9.12 0.15 4.59 8.74 3.86 0.95 0.21 2.8 73 801 65 15 9 200 24 308 12 17.9 1 13.8 1 5
CSN-33 50.66 1.10 18.91 9.00 0.14 4.52 8.51 4.11 0.91 0.21 BD 82 795 94 7 21 298 11 15 17
CSN-34 54.27 0.94 18.87 8.21 0.15 3.87 7.91 4.31 1.19 0.26 3.3 90 861 69 14 8 159 31 363 15 16.1 2 19.5 3 9
CSS-11 3.8 91 828 76 13 4 166 29 331 10 18.5 1 16.2 2 10
CSS-11 52.81 0.96 19.60 8.32 0.13 3.58 8.23 3.95 1.03 0.25 BD 106 808 68 BD 9 355 13 22 21
CSS-10 52.16 1.08 19.37 8.98 0.15 4.27 8.69 3.74 0.95 0.23 3.3 81 777 64 19 8 181 27 311 11 18.0 2 14.6 2 7
CSS-9 3.1 84 761 76 15 8 204 22 300 12 18.7 1 14.5 2 6
CSS-9 51.78 1.06 19.27 8.82 0.13 4.18 8.51 3.92 0.94 0.23 BD 94 749 65 10 19 267 10 19 20
CSS-8 3.2 85 773 62 11 10 172 27 318 12 18.8 2 16.3 2 9
CSS-8 51.93 1.07 19.37 8.89 0.14 4.22 8.46 3.93 0.95 0.23 BD 97 749 65 7 15 307 12 21 23
CSS-7 52.37 1.02 19.22 8.71 0.15 4.30 8.60 3.80 0.97 0.23 2.5 83 783 61 24 10 168 26 308 13 17.7 2 15.5 1 5
CSS-6 3.1 86 861 71 11 8 133 35 388 17 16.6 2 21.6 3 10
CSS-6 54.56 0.86 18.64 7.90 0.13 3.57 7.51 4.12 1.17 0.27 BD 99 844 71 BD 16 361 18 19 29
CSS-5 2.4 70 792 64 21 14 182 24 307 9 17.2 2 13.3 1 8
CSS-5 51.40 1.09 18.81 9.21 0.14 4.78 8.77 3.80 0.88 0.20 BD 84 761 68 10 28 273 9 19 20
CSS-4 2.1 65 801 63 17 13 191 24 309 12 17.2 1 13.7 2 7
CSS-4 50.95 1.11 19.02 9.09 0.14 4.70 8.48 3.89 0.91 0.21 BD 84 785 67 7 18 254 9 19 22
CSS-3 2.5 71 888 67 16 2 165 29 363 15 16.5 2 19.0 3 7
CSS-3 52.55 0.92 19.13 8.42 0.13 4.09 7.73 3.97 1.09 0.24 BD 88 873 73 7 9 320 17 18 24
CSS-2 2.7 74 910 70 14 1 171 28 363 14 16.2 1 13.4 3 7
CSS-2 52.59 0.92 19.15 8.55 0.13 4.03 7.91 4.01 1.03 0.24 BD 89 890 99 5 13 332 13 13 15
CSS-1 2.7 71 885 66 16 3 173 29 362 15 16.4 2 14.4 3 9
CSS-1 52.50 0.96 19.18 8.70 0.13 4.29 8.20 3.89 1.00 0.22 BD 88 864 99 9 11 335 15 15 17
Major and trace element concentrations for lavas from the same eruptive episode from sections CSS and CSN. Data in blue
represent analyses obtained from the X-Ray Facility at Michigan State University. All other data are from the Ronald B.
Gilmore X-Ray Facility at Univ. of Massachusetts. Analyses were obtained from the same powders but using different fused
disk size and techniques. Analyses show some interlab differences, but analyses are consistent for each data set..
Variation diagrams showing behavior of several Large Ion
Lithophile (LIL) and High Field Strength (HFS) Elements,
all plotted versus Rb which is the among the most
incompatible elements for this suite of lavas.
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Nb Zr Sr Zn Ni Cr V Ce Ba La Y U Rb Th Pb
Si Ti Al Fe Mn Mg Ca Na K P Nb Zr Sr Zn Ni Cr V Ce Ba La Y U Rb Th Pb
CSN.24 n24
s18
CSS.8
Plot of major and trace element concentrations compared to flow CSN.18 to show
the degree of compositional variation within the eruptive episode. Heavy black
lines delineate the individual high and low concentrations of flows CSN.25-33.
Flow CSN.25 represents the preceding eruptive episode and is shown to
demonstrate the difference between episodes. Flow CSS.8 is the same as flow
CSN.32 sampled approximately 1 km to the west along the wall of the casitas
shield, and is shown to demonstrate the degree of similarity within the eruptive
package of flows. Variations in U, Th, La and Nb probably represent analytical
uncertainty, while the variation in Cr is a consequence of local accumulations of
pyroxene and olivine.
Chemo-stratigraphic Columns show changes in element concentrations with stratigraphic position. Shortterm
(generally shallow) processes are superimposed on longer-term (generally deeper) processes that are
more indicative of potentially regional controls on petrogenesis. Within-flow compositional variation made
be evaluated between section CSS and CSN lavas. Symbols are as follows:
= lower CSN lavas; = CSN. 24-34; = lower CSS flows (CSS.1-9)
CSN.34 olivines
CSN.29 olivines
Olivine compositions for flows CSN.29 and
CSN.34. Olivine cores (filled symbols) are higher
in Fo content than rims. Small euhedral olivine
compositions are generally the same as the rim
compositions.
CSN.34
CSN.29
Representative plots of plagioclase compositions
(from microprobe) showing a typical
disequilibrium assemblage (CSN.34) with many
different compositions and zoning types. Flow
CSN.29 shows similar plagioclase core and rim
compositions for sampled feldspars in a texture
more indicative of fractional crystallization.
Filled symbols = core compositions
Open symbols = rim compositions
Volcan Cerro Azul as viewed from the southwest. Flows of
the Casitas shield extend to the left (west) and the flat-topped
peak in the back left is Volcan Descabezado Grande.
Proposed Composite Stratigraphy for upper CSN and lower CSS Lavas
CSS2 elevations CSS elevations CSN elevations CSN2 elevations
n34 2470m?
s9 2425m n33
s8 n32 2437m
s7 2419m n31
s6 n30
s5 2390m n29
n28
2395m
s4 n27
n26
2375m
s3
s2
n25 2360m
s1 2333m n24
n23
n22
n21
n20
7
6
2350m n19 2323m
5 2320m n18
4 2308m n17
3
2
Dike 3 n16 2310m
1 2295m n15 2317m
n14
n13
n12
2300m
n11 2252m
n10 2240m
3 2285m
n9 2220m 2 2237m
n8 2202m 1 2222m
n7 2182m
Dike 2 n6
Dike 1 n5 2107m
n4 2100m
n3 2088m
n2 2072m
Acknowledgements
We gratefully acknowledge the Western Kentucky University Department of Geography and Geology for travel and research
funds, as well as continuing support of undergraduate research; Dr. John Andersland, of Western Kentucky University, for his
expert knowledge and assistance with the SEM; and Dr. David Moecher, of the University of Kentucky, for his assistance with the
electron microprobe.
References
n1 2030m
Chart showing the correlation between four sampled
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