SEM study of a deformed quartzite from the
Darjeeling-Sikkim Himalayas, India.

 

Abstract :

At shallow crustal levels (~ <10 km) rocks commonly undergo deformation by  cataclasis, i.e.,  brittle fragmentation of mineral grains accompanied by frictional grain boundary sliding. This study looked at a quartzite which deformed by cataclasis in a fault zone. In this rock evidence of fracturing was preserved at both outcrop and microscopic scales. Electron microscopic study showed that the two constituent minerals, quartz and feldspar, fractured differently as a result of their crystal structure. Quartz, having no cleavage plane, deformed by conchoidal fracturing and flaking of edges. Feldspars, having well developed cleavage planes, formed planar fractures parallel to the cleavage planes. X-ray mapping suggests that the rock is dominantly composed of oxides.

 

Introduction :

The collision and subsequent subduction of the Indian plate beneath the Eurasian plate has resulted in the formation of the Himalayas (Powell & Conaghan, 1973). A large component of the resulting deformation is localized along fault zones across which there is relative movement between the bounding blocks. At shallow crustal levels ( ~ < 10 km) , such fault zones are commonly formed by “cataclastic deformation”, i.e., brittle fragmentation of mineral grains accompanied by frictional grain boundary sliding and dilatancy (Sibson, 1977). In general, fractures formed within mineral grains (intragranular fractures) form linkages giving rise to longer transgranular fractures (fractures across grains) which ultimately give rise to angular, internally fractured clasts of various sizes. The resulting fault zone rock is called “cataclasite”.
This study looked at a deformed quartzite from a fault zone in the Darjeeling-Sikkim Himalayas, India. The rock is dominantly composed of quartz (SiO2) with minor amounts of feldspars (CaAlSi2O8) and coal.  Evidence of cataclastic deformation can be observed at the outcrop scale (Fig 1) and under optical microscope (Fig 2). The purpose of this study is to observe the evidence of deformation at the grain scale and how it varies with mineralogy.

 

 kb
a

 

Figure 1. Outcrop of a cataclasite from a fault zone in the Darjeeling-Sikkim Himalayas. Fractures are seen at the scale of this outcrop. An internally fractured clast has been marked. A 10 cm long pen is for scale. The sample for this study was collected from close to this outcrop.

Fig 2. Crossed polarized light view of the sample. Field of view is approximately 2.5 mm. White arrow is showing grain scale fracturing in quartz. Black arrow is showing broken quartz grains surrounding the parent grain.

 

Method :

 

Thin section was prepared from the quartzite. It was mounted on an aluminium stub by using a carbon tape. Carbon paint was used to ground the sample following which it was sputter coated with gold. An accelerating voltage of 25 KV and working distance of 28 mm were used. Both the in-lens Secondary Electron (SE) detector and in-chamber SE detector were mixed to capture the images. Back scattered electron (BSE) detector was also used. Since this rock is dominantly composed of single mineral, quartz, BSE images were not very helpful for this study. X-ray mapping was done to determine a qualitative spatial distribution and association of different elements across the sample.

Results :

The images were taken at a wide range of magnification varying from 500x to 10,000x. This study looked at grain scale fracturing for which the best information was available at the lower range of magnification (950x – 5750x) as shown below.

 

 

 
 

Figure 3. SE image of a grain scale opening mode fracture. The elongate grains in the center are of feldspar, surrounded by quartz.

Figure 4. SE image of conchoidal fractures in quartz. Finer flakes of quartz formed as a result of such fracturing.

 c
 d

 

Figure 5. SE image of a quartz fracture flake with smooth boundary. The smaller scale conchoidal fractures can be seen within the flake.

 

Figure 6. SE image of fractured feldspar grains. Unlike the quartz grains ( of the previous image), the fractures in the feldspar grains are planar and more regular. They are parallel to the cleavage planes and hence are cleavage controlled.

 

 

Thin section study of the rock under optical microscope indicated it to be dominantly composed of quartz with minor amounts of feldspar and coal. The X-ray analysis of the of the rock agreed well with these general observations with peaks corresponding to quartz (Si, O), feldspars ( Ca, Na, Al, Si, O), coal ( C) ( Fig 7). The Au peaks resulted as an artifact of sample preparation which involved sputtering the sample with gold.

a

Figure 7. X-ray spectrometry of the rock sample.

 

To observe the qualitative spatial distribution of the different elements present in the rock X-ray mapping was done along a transect through the sample as shown in Fig 8-14. Si, O, Ca, Al, C and Ti are plotted.

 

Figure.8

Figure 8. BSE image of the sample area for which x-ray mapping was done.

   

 Figure 9. Si K

 Figure 10. O K

 

 d  c

 Figure 11. Ca K

 Figure 12. C K

 

 

 d  c

 Figure 13. Al K

 Figure 14. Ti K

 

The maps indicate a conspicuous spatial correlation between O and all the other elements excepting C. Therefore, excepting C , all the other elements exist as oxide phases. The negative correlation between C and O indicate that the former exists a s an independent phase, namely, coal. Also the strong correlation between Ca,Al, Si and O indicate the presence of feldspars and quartz in the rock. TiO2 is an accessory mineral often associated with quartz.

 

Conclusions :

 Fracturing is the most dominant manifestation of deformation for this rock at three different scales of observation, namely, outcrop scale (cm), optic microscopic scale (mm) and electron microscopic scale ( μm). However, there is a significant difference between the style of fracturing of quartz (SiO2) and feldspar (Ca Al Si2 O8).  Quartz, having no cleavage planes, deforms by forming smooth, conchoidal fractures and flaking of edges. Such quartz fracture flakes (~8-10μm in dimension) are abundant in the sample. On the other hand, feldspars form planar fractures which are more regular and parallel the cleavage planes. Hence, fracturing in feldspars is dominantly cleavage controlled. Transgranular fracturing is common in the feldspar grains. X-ray mapping shows a strong spatial correlation between Si, Al, Ca, Ti and O indicating all of these elements exist as oxide phases. The negative correlation between C and O indicates that the former occurs as an independent phase, namely, coal.

References :

Sibson, R.H., 1977, Fault rocks and fault mechanisms, Journal of Geological Society of London 133, p. 191-213.
Powell, C., McA, Conaghan, P.J., 1973, Plate tectonics and the Himalayas, Earth and Planetary Science Letters, 20, p.1-12.

Please enter any comments, criticisms, questions, etc. below.

Your name:

Email address:

 

 

 

 

 

 

F