|Abstract or Summary
- Columbia River Basalt Group dikes cut the tonalite-granodiorite Wallowa
Batholith in northeastern Oregon, providing a natural setting in which to examine partial
melting. Many dikes have up to 5 m-wide zones of quenched partially melted wallrock at
their margins. This paper examines the progressive partial melting reactions in biotite-and
hornblende-bearing tonalite at the margin of a near-vertical Grande Ronde dike in the
vicinity of Maxwell Lake. Paleodepth at the time of dike emplacement is estimated at 1-
2 km, and dike temperature was about 1140°C. Samples collected from the dike margin
represent five successive stages of melt reaction over a distance of about 5 m from
unmolten wallrock (Stage 1) to about 40 volume percent (vol%) quenched melt (Stage 5).
This melt is now represented by about 31 vol% silicic glass that has undergone little to no
devitrification and about 9 vol% plagioclase, pyroxene, and Fe-Ti oxide quench crystals.
Whole rock major and trace element bulk compositional data are nearly identical to
unmolten rock at each stage, suggesting that the melt did not separate from the restite and
each stage represents essentially a chemically closed system.
With progressive melting, primary hornblende, biotite, and orthoclase are entirely
consumed but residual plagioclase, quartz, Fe-Ti oxides and apatite persist in the restite.
Hornblende dehydration initially produces a dusty intergrowth of augite, pigeonite, lesser
enstatite, and sparse Fe-Ti oxides. Initial biotite dehydration produces titaniferous
magnetite and lesser ilmenite aligned in bands in an intergrowth of enstatite and
plagioclase. Andesine plagioclase develops a spongy texture as the albite component is lost to the melt. Reaction of hornblende, quartz, and feldspar produces sparse tonalitic
(high-Ca) glass, while the reaction of biotite, quartz, and feldspar produces abundant
granitic (high-K) glass. The two glass types are irregularly distributed around mafic
reaction sites and in up to 2 mm seams on quartz-feldspar contacts. With progressive
melting, replacement phases become compositionally more homogeneous, clinopyroxene
is consumed, the proportion of plagioclase decreases, and glasses become slightly more
mafic. Granitic and tonalitic glasses persist in just over 31 vol% glass suggesting that
deformation is required to mix these melt types.