Open Session - SWISS GEOSCIENCE MEETINGs

2.10
The Chelan complex (Washington Cascades): insights into the roots of
continental arcs
Dessimoz Mathias*, Müntener Othmar*, Ulmer Peter**
*Institut de Minéralogie et Géochimie, Université de Lausanne, 1015 Lausanne (mathias.dessimoz@unil.ch)
**Department of Earth Sciences, ETH Zurich, Clausiusstrasse, 25, CH-8092 Zurich, Switzerland
The Chelan complex is a deep plutonic complex exposed in the North Cascades and crops out at the southern end of the
northwest trending Chelan block. This block, bounded by the Entiat fault on the southwest and by the Ross lake fault zone
on the north east, comprises much of the crystalline core of the north cascades and records Cretaceous to Palaeogene arc
magmatism (from 100 Ma to 45 Ma) (Valley et al., 2003)
For a long time, Chelan migmatitic complex was interpreted as metamorphic / migmatitic unit consisting mainly of metatonalite,
metabasite and widespread metaplutonic migmatite (Hopson and Mattinson, 1994) metamorphosed under upper
amphibolite/lower granulite facies. The pressure of emplacement of the complex inferred from the surrounding rocks
(Swakane gneiss and Napeequa complex) is thought to be around 9-12 kbar for a temperature of about 750°C (Valley et al.,
2003)
Field observations in particular comb layers, pegmatitic gabbros and mafic cumulates clearly indicate that the magmatic
origin is well preserved within the entire complex. While subsolidus deformations occur locally, most of the rocks are deformed
in presence of melt, as indicated by a pervasive magmatic (almost no recrystallization of quartz is observed in thin
section) foliation well developed in the tonalite, syn-magmatic shear zones and magmatic folding. At some places, in particular
within melanocratic tonalite, numerous felsic rocks occur that are always associated with a more intense deformation.
The local occurrence and discrete features of these zones are not consistent with a migmatitic origin. Thus, these “migmatite-like”
rocks could rather be interpreted as plutonic feeder zones in which more evolved liquids are concentrated and
migrate to high crustal levels.
Whole rock chemistry performed on hornblendite, hornblende gabbros, diorite, tonalite and mafic dykes display continuous
trends for various oxides that are consistent with an evolution through magmatic processes. Comparison with experimental
data (Alonso Perez, 2006; Kägi, 2000) is in good agreement with an evolution of the suite through crystal fractionation at
high pressure (around 1.0 GPa, Fig. 1). This is well indicated by TiO 2 , Al 2 O 3 , FeO and CaO, which show a good correlation with
the experiments done at 1.0 GPa. The high pressure of crystallization of the Chelan complex is consistent with the widespread
occurrence of primary epidote (P>0.6 GPa) in tonalite and diorite.
According to these observations, the Chelan Complex provide insights into the deep-seated processes occurring in the roots
of continental arcs and could be an example of the Mixing-Assimilation-Storage-Hybridization-Zone (MASH), which form the
base of the plutonic systems (Hildreth and Moorbath, 1988).
REFERENCES
Alonso Perez, R. (2006) The role of garnet in the evolution of hydrous, calc-alkaline magmas an experimental study at 0.8 -
1.5 GPa, p. 1 Band. ETH, Zürich.
Hildreth, W., and Moorbath, S. (1988) Crustal Contributions to Arc Magmatism in the Andes of Central Chile. Contributions
to Mineralogy and Petrology, 98(4), 455-489.
Hopson, C.A., and Mattinson, J.M. (1994) Chelan Migmatite Complex, Washington; Cretaceous mafic magmatism, crustal
anatexis, magma mixing and commingling, and protodiapiric emplacement. Geologic Field Trips in the Pacific
Northwest: 1994 Geological Society of America Annual Meeting Geological Society of America, 1-21.
Kägi, R. (2000) The liquid line of descent of hydrous, primary, calc-alkaline magmas under elevated pressure an
experimental approach, p. 115 S., Zürich.
Valley, P.M., Whitney, D.L., Paterson, S.R., Miller, R.B., and Alsleben, H. (2003) Metamorphism of the deepest exposed arc
rocks in the Cretaceous to Paleogene Cascades belt, Washington: evidence for large-scale vertical motion in a continental
arc. Journal of Metamorphic Geology, 21(2), 203-220.
83
Symposium 2: Mineralogy-Petrology-Geochemistry