5th Swiss Geoscience Meeting, Geneva 2007
Epidote in calc-alkaline igneous rocks: field examples
Dessimoz Mathias*, Müntener, Othmar*, Jagoutz, Oliver**, Dawood, Hussain***
*Institute of Mineralogy and Geochemistry, Anthropole, CH-1015 Lausanne
(mathias.dessimoz@unil.ch)
**Institut für Geologie, Baltzerstr. 3, CH-3012 Bern
*** Pakistan Museum of Natural History, Islamabad, Pakistan
The role of epidote during igneous crystallization is relatively well understood for
intermediate magmas (granodiorite-tonalite-trondjemite, TTG), and
crystallization temperatures and sequences are experimentally calibrated in
H2O – saturated systems (e.g. [Schmidt and Thompson, 1996), and confirmed
from natural occurrences (e.g. Zen and Hammarstrom 1984). Several
experimental phase equilibrium studies on the role of epidote in calc-alkaline
magmas under variable pressure – temperature – X – fO2 conditions have been
performed over the last decade (see [Schmidt and Poli, 2004] for a review).
These studies show that epidote crystallizes in intermediate magmas above
pressures of ~0.3 t to 0.7 GPa, depending mainly on the bulk composition and
the oxygen fugacity. Much less is known about the phase relations of magmatic
epidote at H2O-undersaturated conditions and in ‘unusual’ bulk compositions
(e.g. dacite, gabbro/diorite), volcanic dikes and high-pressure migmatite
terrains.
We present preliminary results on epidote-bearing assemblages from the
Kohistan island arc complex (Pakistan) the Chelan complex (Western US) and
from epidote-bearing volcanic dikes (Boulder County, Colorado). Volcanic
epidote in dacitic rocks from Colorado is characterized by oscillatory zoning,
which is probably caused by variable allanite component (Fig. 1a) suggesting
that epidote is a near-liquidus phase in dacitic to rhyodacitic compositions.
Crystallization pressures are estimated to be in excess of 0.5 GPa (Evans and
Vance, 1987). Field relationships in the Jijal complex of the Kohistan island arc
show that epidote- garnet gabbros and epidote-pegmatites are intercalated with
epidote-free garnet gabbros and garnet hornblendite cumulates, strongly
supporting the igneous nature of epidote, in contrast to previous studies (e.g.
Garrido et al. 2006)
Back Scatter electron imaging (Fig. 1) displays ‘perthite-like’ internal structures
of epidote, which is mainly caused by Fe3+ - Al variations. Textural and mineral
compositional data indicate that epidote forms in two different ways: (i) by a
discontinuous reaction at the expense of clinopyroxene (cpx+hydrous liquid ->
epidote + amphibole + quartz + H2O) and (ii) by an amphibole-absent
continuous reaction at the expense of plagioclase (plagioclase+hydrous liquid > epidote + quartz). Estimated crystallization conditions are in excess of 1 GPa
and between 700 and 800°C.
5th Swiss Geoscience Meeting, Geneva 2007
Figure 1. Backscatter electron images of volcanic (left) and plutonic (right)
epidote. The internal structure of epidote might be a sensitive monitor of
crystallization conditions. The oscillatory zoning reflects variable allanite
component. The ‘flame-like’ textures of epidote (intergrown with quartz: black) in
Kohistan gabbros are not dramatically different from ‘perthitic’ exsolution of
alkali-feldspar. However, the textures are not unequivocal and could also be
interpreted as cogenetic growth of two epidote minerals of slightly different
composition at near solidus conditions.
REFERENCES
Evans, B.W., and J.A. Vance, Epidote phenocrysts in dacitic dikes, Boulder
county, Colorado, Contributions to Mineralogy and Petrology, 96, 178-185,
1987.
Garrido, C.J., J.L. Bodinier, J.-P. Burg, G. Zeilinger, S.S. Hussain, H. Dawood,
M.N. Chaudry, and F. Gervilla, Petrogenesis of mafic garnet granulite in the the
lower crust of the Kohistan Paleo-arc complex (Northern Pakistan): implications
for intra-crustal differentiation of island arcs and generation of continental crust,
J. Petrol., 47, 1873-1914, 2006.
Schmidt, M.W., and S. Poli, Magmatic epidote, in Reviews in Mineralogy and
Geochemistry: Epidotes, edited by A. Liebscher, and G. Franz, pp. 399-430,
2004.
Schmidt, M.W., and A.B. Thompson, Epidote in calc-alkaline magmas: An
experimental study of stability, phase relationships, and the role of epidote in
magmatic evolution, American Mineralogist, 81 (3-4), 462-474, 1996.
Zen, E.-A. and Hammarstrom, J.M., Magmatic epidote and its petrologic
significance. Geology 12, 515-518