Piedra Parada, Petrography, Chubut, Argentina
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Piedra Parada photomicrographs: magmatic shearing and quartzite disaggregation
Figure 1a) Barda Colarada Ignimbrite from Canadon de los Buitres. Notice the internal texture of the clast, with dark elongated blebs in an orange matrix. before | 
Figure 1b) Same. |
Figure 2a) Igneous lithic with trachytic texture with matrix demonstrating flow foliation around the clast. Notice devitrified clasts of fiamme. Image is 6mm wide. Sample PP4A, from rock shop at Paso del Sapo, see figures 3k-n in main page. |
Figure 2b) Same, cross-polarized light. Sample PP4A, from rock shop at Paso del Sapo, see figures 3k-n in main page. |
Figure 2c) Ash matrix in this sample showing Y-, platy- and curved-shaped shards in a non-welded matrix. Sample PP4A, from rock shop at Paso del Sapo , see figures 3k-n in main page. |
Figure 2d) Igneous lithic with magmatic shear zones. Lithic has trachytic texture and comprised of a majority of plagioclase grains and a vitreous matrix. Sample PP4A, from rock shop at Paso del Sapo, see figures 3k-n in main page. Image is 2mm wide. |
Figure 2e) Detail of magmatic shear zone in lithic. XPL. Sample PP4A, from rock shop at Paso del Sapo, see figures 3k-n in main page. |
Figure 2e) Detail of magmatic shear zone in lithic. PPL. Sample PP4A, from rock shop at Paso del Sapo, see figures 3k-n in main page. |
Inside splatttered fiamme: disaggregation of quartzite?
This subsection is divided into two parts each from one thin sections of the same area. The first shows a lithic clast of quartzite in the ignimbrite matrix. This clast shows quartz with undulose extinction, incipient subgrain boundaries and serrated grain contacts indicative of bulging due to low-T deformation. The other thin section shows the texture inside the fiamme. This is characterized by dark glass lenses with numerous spheres of quartz, in an orange glassy matrix with well defined flow foliation. The larger quartz spheres are generally formed by multiple quartz grains, with sweeping undulose extinction, serrated contacts indicative of bulging, and are interpreted to have been deformed at low T. Interpretation: this is a result of the assimilation of a weakly deformed quartzite preserved as lithics in the ignimbrite. If this is so we have a great opportunity to study how this assimilation takes place and how it impacts on the composition of the melt preserved in the glass.
Figure 3a) Lithic clast of what seems to be a quartzite. PPL. The question that this raises is whether or not the quartz spheres in the fiamme are disagregated and assimilated country rock. Sample LB3c Canadon de los Buitres, see figures 1a-b above. |
Figure 3b) Same, cross-polarized light. Notice that some of the grains seem to have undulose extinction. Sample LB3C Canadon de los Buitres, see figures 1a-b above. |
Figure 3c) Inside the splattered fiamme, there are darker glass lenses from 1-5mm long with quartz spheres. These are generally surrounded by yellow glass with flow foliation and few quartz spheres. In places, the two have equal proportions and neither can be considered matrix to the other. The spheres comprise multiple quartz grains. PPL. The image is 2.2mm on the vertical. Sample LB3B, Canadon de los Buitres, see figures 1a-b above. |
Figure 3d) Same, cross-polarized light. Sample LB3B Canadon de los Buitres, see figures 1a-b above. |
Figure 3e) Same as above. PPL. Sample LB3B, Canadon de los Buitres, see figures 1a-b above. |
Figure 3f) Same, cross-polarized light. Sample LB3B Canadon de los Buitres, see figures 1a-b above. |
Figure 3g) Same, PPL. Sample LB3B Canadon de los Buitres, see figures 3A-D above. |
Figure 3h) Same, cross-polarized light. Sample LB3B Canadon de los Buitres, see figures 1a-b above. |
Figure 3i) Platy ash shards in matrix indicating that the ignimbrite is non-welded ignimbrite . PPL. Sample LB3B Canadon de los Buitres, see figures 1a-b above. |