Chemical differentiation of the Cordillera Paine granite (southern Chile) by in situ fractional crystallization |
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Authors: | Peter J Michael |
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Institution: | (1) Department of Geological Sciences, Columbia University, 10027 New York, NY;(2) Lamont-Doherty Geological Observatory of Columbia University, 10964 Palisades, NY, USA;(3) Present address: Department of Geological Sciences, University of British Columbia, V6T 2B4 Vancouver, B.C., Canada |
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Abstract: | Geochemical and field data for the Cordillera Paine (CP) pluton of southern Chile, indicate that differentiation took place
by closed system in situ fractional crystallization. Minor, local and irregular separation of liquids from crystals led to the formation of evolved
granites and aplites which are encountered mostly at the plutons roof and margins.
Chemical trends show strong depletions of Sr, Ba, Mg less intense depletions of Ca, La, Ce, Nd, Fe, Ti, Al and enrichment
of Nb, Y, Th, Rb and Si with differentiation. Pronounced crystal zoning of Ca, Sr and Ba in plagioclase, Ba in orthoclase
and LREE, Y and Th in allanite closely correspond to the whole rock chemical variation. The crystal zoning data suggest that
surface equilibrium only was maintained for the zoned elements during crystallization. Thus, continuous separation of liquids
from crystals was not necessary to generate the kind highly evolved differentiates whose character reflects fractional crystallization.
The schedule of liquid-crystal separation affects mainly the location, degree of dispersion and relative abundance of the
differentiates.
The homogeneity of the CP pluton and the intense crystal zoning suggest that crystal-liquid separation was inefficient, and
that whole rock compositions approach liquid compositions. Assumption of a closed system during crystallization allows estimation
of mineral/melt partition coefficients (K
d
s) using crystal core and whole rock compositions. Crystal zoning and whole rock chemical trends are consistent with models
constructed using the K
d
s thus obtained along with modal abundances from petrographic estimates.
Lamont-Doherty Geological Observatory Contribution Number 3701 |
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Keywords: | |
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