Download modern and ancient incompatible element-poor adakite

TOMO 2 - Volcanismo y Magmatismo
MODERN AND ANCIENT INCOMPATIBLE ELEMENT-POOR
ADAKITE-LIKE MAGMAS IN CENTRAL CHILE: NO NEED FOR
CRUSTAL SOURCES.
Daniel Sellés, Michael Dungan, & Carolina Rodríguez.
Department of Mineralogy, Earth Sciences Section, University of Geneva, Switzerland
[email protected]
The Southern Volcanic Zone of the Andes (SVZ) is the volcanic arc developed on the active
western margin of the South American plate between the latitudes of the aseismic Juan Fernandez
Ridge (33°S) and the actively spreading Chile Rise at 46.5°S. The chemical and mineralogical
characteristics of SVZ magmas vary widely along the arc, as do the nature and thickness of the
continental crust. Magmas erupted at volcanic centers to the north of 36°S, where the crust is 45-60
km thick, are generally more evolved and have a more significant crustal component than those to
south, where the crust is <40 km thick. The consequent along-arc variation in the thickness of the
asthenospheric column below the arc, and thus the average extent of melting, might account for some
of the trace element differences in mantle-derived arc magmas (Tormey et al., 1991). However,
most chemical variations have been accounted for an increasing crustal contribution to the north,
either via crustal assimilation (Hildreth and Moorbath, 1988) or as mantle source contamination by
forearc tectonic erosion (Stern, 1991; Kay et al., 2005).
Nevado de Longaví volcano (NLV), located at 36.2°S, has erupted magmas with chemical
and mineralogical characteristics that are unique within the SVZ. Andesitic and more evolved NLV
lavas (SiO2>57) are characterized by incompatible element concentrations that are much lower than
those in lavas to the north where contributions from crustal components are highest (Sellés et al.,
2004). NLV lavas have even lower incompatible element abundances than comparably evolved
magmas at most volcanic centers south of 36°S, where the participation of the crust is minimal. Thus,
magma differentiation mechanisms at NLV appear to be distinct from those at other SVZ centers.
The extent to which incompatible elements are lower in NLV lavas relative to other SVZ volcanoes
is greater for fluid-immobile elements. Elements such as Ba, Pb, and especially B, whose concentrations
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in arc magmas are thought to be mostly provided by slab-derived fluids, are comparatively less
depleted (or actually enriched, as is the case for boron). Thus, mobile-to-immobile element ratios,
such as the Ba/Th are anomalously high at NLV, suggesting a locally very high input of slab-derived
fluids to the mantle source region. The Mocha Fracture Zone of the Nazca plate, which projects
beneath the Quaternary arc at the latitude of NLV, may be responsible for the elevated slab-component
of NLV magmas. Oceanic transform faults commonly host serpentinized peridotite bodies, which
can liberate vast amounts of fluids during subduction, up to an order of magnitude higher than altered
oceanic crust associated sediments.
High-degree partial melts resulting from elevated fluid supply to the mantle wedge are primarily
richer in SiO2 and poorer in incompatible elements than lower-degree melts generated elsewhere
along the SVZ. These hydrous melts evolve following a distinct liquid line of descent, which is
characterized by 1) early fractionation of iron oxides (calcalkaline differentiation trend) 2) enhanced
amphibole stability, and 3) reduced plagioclase stability. Additionally, recent experimental work
indicates that hydrous mafic magmas can fractionate garnet at pressures >~8 kbar (Müntener et al.,
2001; Ulmer et al., 2003), which would explain the pattern of decreasing Y and HREE among NLV
lavas. Trace element modeling indicates that only ~2 wt% garnet needs to be fractionated to reproduce
the observed depletions. Amphibole-dominated fractionation rapidly drives magma evolution towards
elevated SiO2 contents, in keeping with limited enrichment of incompatible elements. Limited plagioclase
fractionation due to the high magmatic water contents results in relatively high abundances of Al2O3
and Sr among evolved magmas.
In the same way NLV represents an “anomaly” in the context of the SVZ, two adakite-like
anomalies occur within the realm of Cenozoic magmatism in Central Chile (33-34°S). The Early
Miocene Manquehue-type stocks immediately postdate a tholeiitic Oligocene sequence at the latitude
of Santiago (Sellés, 1999), and predate the Miocene volcanic sequence constructed to the east. The
El Teniente porphyries intrude the Miocene volcanic sequence and predate the eastward shift of the
arc to its current location (Kay et al., 2005). Both of these intrusives are characterized by La/Yb
and Sr/Y ratios that are considerably higher than those of the preceding and postdating volcanics.
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TOMO 2 - Volcanismo y Magmatismo
The Manquehue and El Teniente intrusives share several key features with the lavas from
NLV (Figure 1).High La/Yb ratios are attained, in all three cases, at relatively low La contents, i.e.
they are due to strong HREE depletions rather than selective LREE enrichment, as is the case in the
northern SVZ. High slab-derived components are indicated by the variable but generally high ratios
of fluid-mobile to immobile elements, such as Ba/Th. The hydrous character of these magmas is also
indicated by the high modal amphibole contents in them. The adakite-like magmas at NLV,
Manquehue and El Teniente are related to more mafic magmas whose La/Yb ratios are low. The
increase of this ratio towards evolved compositions is not accompanied by shifts in the isotopic
composition, unlike what is observed in the northern SVZ. All these characteristics suggest that their
unusual chemical signature is the result of crystal fractionation of hydrous mantle-derived basalts, and
that the garnet signature is due to primary fractionation of this mineral rather than the signature of a
crustal component.
Figure 1: La/Yb ratios of NLV lavas compared to other SVZ lavas (Latitude Variation) as a function of La, Ba/Th,
and Nd isotopic ratio. The bottom diagrams compare the Manquhue-type stocks (black stars) and the El Teniente
porphyries (white stars) to the composition of Oligocene, Miocene, and Quaternary volcanic sequences at the
latitude of Santiago (Time Variations).
Whereas in the case of NLV the cause of a localized high fluid flux can be readily ascribed to
the subduction of the Mocha Fracture Zone, it is less clear what would cause the short-lived “wet”
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pulses during the Miocene. Both the Manquehue-type and El Teniente porphyries seem to coincide
with shifts in the arc position. Consequently, we hypothesize that changes in the stress regime associated
with periods of arc shift favor the serpentinization of the oceanic lithosphere through trench-bend
fractures (Ranero et al., 2003).
REFERENCES
Hildreth, W., and Moorbath, S. (1988) Crustal contributions to arc magmatism in the Andes of Central Chile.
Contributions to Mineralogy and Petrology, 98, 455-89.
Kay, S., Godoy, E., and Kurtz, A. (2005) Episodic arc migration, crustal thickening, subduction erosion, and
magmatism in the south-central Andes. Geological Society of America Bulletin, 117, 67-88.
Müntener, O., Kelemen, P., and Grove, T. (2001) The role of H2O during crystallization of primitive arc magmas
under uppermost mantle conditions and genesis of igneous pyroxenites: An experimental study.
Contributions to Mineralogy and Petrology, 141, 643-58.
Ranero, C.R., Phillips Morgan, J., McIntosh, K., and Reichert, C. (2003) Bending-related faulting and mantle
serpentinization at the Middle America trench. Nature, 425, 367-73.
Sellés, D. (1999) La Formación Abanico en el cuadrángulo Santiago (33°15'-33°30’s; 70°30'-70°45’o), Chile
central: Estratigrafía y geoquímica. MSc. Thesis (unpublished). Departamento de Geología, 154 p.
Universidad de Chile, Santiago.
Sellés, D., Rodríguez, C., Dungan, M.A., Naranjo, J., and Gardeweg, M. (2004) Geochemistry of Nevado de
Longaví volcano (36.2°s): A compositional atypical arc volcano in the Southern Volcanic Zone of the
Andes. Revista Geológica de Chile, 31, 293-315.
Stern, C.R. (1991) Role of subduction erosion in the generation of Andean magmas. Geology, 19, 78-81.
Tormey, D., Hickey-Vargas, R., Frey, F., and López-Escobar, L. (1991) Recent lavas from the Andean volcanic
front (33 to 42°s); interpretations of along-arc compositional variations. In R.S. Harmon, and C.W.
Rapela (eds). Andean magmatism and its tectonic setting, 265, p. 57-77.
Ulmer, P., Müntener, O., and Alonso-Pérez, R. (2003) Potencial role of garnet fractionation in H2Oundersaturated andesite liquids at high pressure: An experimental study and a comparison with the
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