Solution Mass-transfer (Veins; Pressure Solution)
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Click image to enlarge |
7. Antitaxial veins – Calcite-filled veins in the Martinsburg Formation of northern Virginia show two
phases of opening recorded by the orientation of growth of calcite fibers. Bedding
is visible at base of image, between the lower, quartz-rich layer and upper, darker,
phyllosilicate-rich layer. In the central vein, the original fracture occurred along
what is now the medial suture, which runs asymmetrically down the vein center. This
contains small fragments of original wall rock, and the first (smallest) calcite
grains to nucleate in the vein as it opened. As opening continued, in a 'top right
to bottom left' orientation, the fibers grew incrementally in that direction, with
the newest material always deposited antitaxially at the vein walls (in a syntaxial
vein, the newest material deposited is always in the vein center). At some point
the opening direction changed in response to external stresses, and extension continued
in a subhorizontal direction. The calcite grains were not deformed, as is evident
from the lack of undulatory extinction, they merely changed orientation of growth.
Phyllosilicates in the wall rock became re-aligned with their basal planes parallel
to the incremental extension direction (i.e., their (001) planes were rotated
to face the incremental shortening direction), which is also parallel to bedding
in this example. Click here to view Flash animation in a new window.
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8. Boudinaged feldspar – Face-controlled quartz fibers fill cracks between boudinaged alkali feldspar fragments
from the Rockfish Valley Fault Zone in northern Virginia. Foliation in the mylonitic
granitoid rock is aligned roughly horizontal. Muscovite rims the feldspar grain at
top right, and very fine-grained recrystallized quartz fills the pressure-shadow
area at lower right. Although evidence for dissolution of quartz is not present in
this image, the deposition of quartz fibers is indicative of solution-transfer processes.
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9. Pressure solution – Large quartz and feldspar grains in metasedimentary rock from the Brooks Range,
Alaska, have planar sides parallel to cleavage, and 'beards' of white mica in the
incremental extension direction: both features are indicative of pressure solution.
Cleavage is defined by dark, iron oxide-rich stripes running lower left to upper
right, by the alignment of white mica, and by the alignment of elongate flattened
quartz grains (very dark grey at lower center). The matrix to the large grains consists
of very fine-grained quartz and phyllosilicates. Click here to view Flash animation in a new window. |
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10. Pressure shadows – In this metasedimentary rock from the central Virginia Blue Ridge province, large
rounded quartz grains behave as rigid clasts in a weaker, very fine-grained matrix
of quartz and phyllosilicates. A dark, iron oxide-rich stripe running from left to
right helps define a poorly developed cleavage. Phyllosilicates in the matrix are
also aligned with their basal planes in this direction. Although the boundaries of
the large, central quartz grain are not planar, as in image # 9,
there are well-developed pressure shadows of newly-precipitated quartz forming 'wings'
parallel to the maximum extension direction on either side. The combination of pressure
shadows and insoluble oxide zones indicates that the rock has undergone significant
pressure solution/re-deposition during deformation. Click here to view Flash animation in a new window. |
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