geochemical characteristics of the volcanic rocks of the lo valdés

TOMO 2 - Volcanismo y Magmatismo
GEOCHEMICAL CHARACTERISTICS OF THE VOLCANIC
ROCKS OF THE LO VALDÉS AND EASTERN ABANICO
FORMATIONS IN THE ANDES OF CENTRAL CHILE (33º50’S).
Lay-Son, F.1, Vergara, M.1, Aguirre, L.1, Nyström, J.O.2
1
Departamento de Geología U. de Chile, Santiago, Chile. Proyecto Fondecyt 1061266
2
Swedish Museum of Natural History, Stockholm, Sweden.
At the upper Volcán valley, 80 km east from Santiago (Fig.1), volcanic and sedimentary
rocks of Upper Jurassic to Oligocene-Miocene ages are exposed. The Upper Jurassic-Lower
Cretaceous succession corresponds to the eastern wing of a Mesozoic synclinorium whose western
wing is exposed 80 km to the west, at the Coastal Range. At the upper Volcán valley the Mesozoic
Lo Valdés Formation (LVF) crops out as a strongly folded belt with strata dipping close to the
vertical. A volcanic belt, the Oligocene-Miocene Eastern Abanico Formation (EAF) with ages between
34.3 and 21.4 Ma (Muñoz et al.2006) is exposed along the same region.
This work aims to contribute with a preliminary geochemical approach through the study of
the lavas of the LVF and their comparison with those belonging to the EAF (Lay-Son, 2006).
Geologic setting and petrography
The LVF (Tithonian-Neocomian) consists in a 1500 m thick succession of littoral to sublittoral
limestones, volcanoclastic rocks, and basic lava flows (pillow lavas, pillow breccias) deposited in a
predominantly marine environment. They are intermediate to basic lava flows deposited in a
predominantly marine environment Lavas are porphyritic (ocoitas) with phenocrysts of Ca-plagioclase,
augite and titanomagnetite.
The AEF is 3300m thick at the upper Volcán valley with lavas making up to 50% of the total
succession; they correspond to flows of basalt, basaltic andesite and andesite together with
volcaniclastic deposits of similar composition. Basalts and basaltic andesites are porphyritic with
phenocrysts of Ca-plagioclase, augite, olivine pseudomorphs and Fe-Ti oxides. The assemblage
olivine-Ca-plagioclase-pyroxene + Fe-Ti oxides would indicate crystallization of basaltic magma
under low-pressure conditions.
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XI CONGRESO GEOLOGICO CHILENO
GEOCHEMISTRY
In spite of the fact that mineral composition and texture of the basic rocks, both in the LV and
EAF formations, are mostly similar, their chemical compositions are markedly different. The oldest
volcanic rocks in the LVF have SiO2 in the range 51.2-56.5% whereas the rocks of the EAF show
a wide compositional variation from 47 to 70% SiO2. The figure 2 indicates that the rocks of the
LVF are essentially K-rich calcoalkalines, partly shoshonitics, whereas those from the EAF are
mainly calcoalkaline with medium to low K (tholeiitic). Basic rocks in both formations are Al2O3rich, over 17% in Lo Valdés and 19% in the EAF, which, according to Middlemost (1975)
corresponds to aluminum-rich basalts (calcoalkaline). The AFM diagram shows the tholeiitic affinity
of the lavas from the EAF contrasting with those, mostly calcoalkalines, of the LVF. MgO contents
of the most basic rocks (basalts and basaltic andesites) in both formations are commonly low, 3.2–
3.5% in the LVF and 6.55 -1.65% in the EAF. Ni, Cr and Co contents are generally higher in the
EAF (Ni 61-5ppm; Cr 204-10 ppm; Co 55-15ppm) as compared with those in the LVF (Ni 2110ppm; Cr 41-9 ppm; Co 20-16 ppm).Th is higher in the LVF rocks (14-9 ppm) than in the EAF
(7-1 ppm). Cr, Ni and Mg contents of the most basic rocks in both formations are too low to
represent primary magmas. Both formations have features typical of magmas associated with
subduction zones, e.g. enrichment in LILE relative to HFSE such as Zr, Hf, Ti, enrichment in light
REE and a pronounced Nb-Ta trough. The Fig. 3 indicates a relative enrichment in LREE in the Lo
Valdés samples as compared with those of the EAF. The Ce/Yb ratio, used as an estimation of
crustal thickness, gives values of 25-28 for the Lo Valdés rocks contrasting with values of 12-23 for
the EAF rocks. This feature suggests that the crust was more attenuated during the Oligo-Miocene.
Initial 87Sr/86Sr ratios vary between 0.70386 and 0.70392 in rocks of LVF whereas they range from
0.70345 to 0.70380 in those of the EAF. åNd values are of (+3.8 to +4.4) and (+4.0 to +5.2) for
rocks of the LVF and the EAF respectively. All the samples analysed plot inside the mantle array
(Fig.4) indicating a relatively primitive magma contaminated with crustal material (and/or fluids derived
from the subducted slab) during the processes of subduction and emplacement.
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TOMO 2 - Volcanismo y Magmatismo
CONCLUSIONS
The geochemical patterns in both units correspond to volcanic arcs related to subduction.
However, contrasted features are observed in some major and trace elements. Thus, the Lo Valdés
rocks are poorer in SiO2, richer in K, Rb, Ba, Th, Ce and Zr and their REE pattern is more enriched
in LREE than the rocks of the EAF. All these characteristics point to a larger extent of crustal
contamination in the Lo Valdés rocks a conclusion also supported by their slightly less primitive
isotopic ratios.
The Lo Valdés volcanics have features akin to a back-arc tectonic setting (Vergara & Nyström,
1996) as suggested by the Ba/La and Ba/Nb for the basic lavas which are lower than the values of
contemporary lavas with the same bulk chemical composition from the Coastal Range at the same
latitude. According to Vergara et al. (1995) lavas in the Coastal Range were emplaced in an islandarc separated from the continent by a marginal sea developed on a moderately thick crust. The EAF
lavas have geochemical and isotopic features akin to volcanic arcs developed in an active continental
margin with an attenuated continental crust, possibly due to extension (Muñoz et al., 2006).
REFERENCES
Lay-Son, F. 2006. Geoquímica de las unidades del Cretácico inferior y del Oligo-Mioceno en las áreas de Portillo
(32º50´S) y Los Valdés (33º50´S) en la Cordillera de los Andes de Chile central. Thesis (unpublished),
Universidad de Chile, Departamento de Geología, 111p.
Middlemost, E.A.K. 1975. The basalt clan. Earth Science Review 11 (4): 337-364.
Muñoz, M.; Fuentes, F.; Vergara, M.; Aguirre, Luis.; Nyström, J.O.; Féraud, G.; Demant, A. 2006. Abanico East
Formation: petrology and geochemistry of volcanic rocks behind the Cenozoic arc front in the Andean
Cordillera, central Chile (33°50’S). Revista Geológica de Chile 33 (1): 109-140.
Nakamura, N. 1974. Determination of REE, Ba, Mg, Na, and K in carbonaceous and ordinary chondrites.
Geochimica et Cosmichimica Acta 38 (5): 757-775.
Peccerillo, A.; Taylor, S. R. 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area,
northern Turkey. Contribution to Mineralogy and Petrology 58 (1): 63-81.
Vergara, M.; Nyström, J.O. 1996. Geochemical features of Lower Cretaceous back-arc lavas in the Andean
Cordillera, central Chile (31-34°S). Revista Geológica de Chile 23 (1): 97-106.
Vergara, M.; Levi, B.; Nyström, J.O.; Cancino, A. 1995. Jurassic and Early Cretaceous island arc volcanism,
extension, and subsidence in the Coast Range of central Chile. GSA Bulletin 107 (12): 1427-1440.
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Figure 1: Location of the studied area
Figure 3: Chondrite normalized (Nakamura, 1974) REE
diagram for basalts and basaltic andesites of the Lo
Valdés and Eastern Abanico formations
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Figure 2: SiO2 vs K2O in rocks of the Lo Valdés and
Eastern Abanico formations. Diagram from Pecerillo
& Taylor (1976).
Figure: 4 εNd vs (87Sr/86Sr)o showing the position of
the rocks from Lo Valdés and Eastern Abanico
formations on the mantle array.