Hons 2006 – IgPet
Assignment 2: geochemical models
The Viejo Cayambe, in Ecuador, is (the basal part of) a subduction-related volcano, about 1 Ma old. It
is mostly made of andesitic lava flows, with minor pyroclastic deposits, that contain more silicic
(dacitic) elements.
Its petrogenesis was studied in two papers (Samaniego et al. 2002; Samaniego et al. 2005), from
which I took the data you’ll work on; you can, of course, read them but it’s unlikely to be really helpful
to you, as the petrogenetic problem I’d like you to discuss is a minor part of the whole story, and
anyway they propose somehow different models in their two successive papers…
The Viejo Cayambe (VCAY) andesites comprise phenocrysts of plagioclase, orthopyroxene,
clinopyroxene and magnetite. Amphibole is occasionally found, but is rare. The minerals are mostly
zoned, suggesting indeed that fractional crystallization operated, but of course making it more difficult
to define a “typical” mineral composition. Several examples of mineral compositions are given in the
datafile.
The datafile Viejo_Cayambe.xls includes whole rock analysis (major and traces) for the Viejo
Cayambe lava, together with representative mineral analysis.
You will propose a petrogenetic model explaining how the Viejo Cayambe lava can be related to each
other; do that using both major and trace elements.
If you have spare time, you can try to discuss the source of the primitive magma; the questions to ask
are amphibolites or peridotites (i.e., slab melt or mantle wedge), garnet-present or garnet free, and
enriched or normal mantle (this is a subduction zone volcano!). This part is not required but would,
however, be regarded as a bonus…
Samaniego, P., H. Martin, et al. (2002). "Transition from calc-alkalic to adakitic magmatism at
Cayambe volcano, Ecuador: insights into slab melts and mantle wedge interactions." Geology
30(11): 967-970.
Samaniego, P., H. Martin, et al. (2005). "Temporal evolution of magmatism in the Northern Volcanic
Zone of the Andes: the geology and petrology of the Cayambe volcanic complex (Ecuador)."
Journal of Petrology 46(11): 2225-2252.