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The evolution and significance of microfracturing within feldspars in low-grade granitic mylonites: A case study from the Eastern Ghats Mobile Belt, India

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Title The evolution and significance of microfracturing within feldspars in low-grade granitic mylonites: A case study from the Eastern Ghats Mobile Belt, India
 
Creator SINHA, S
ALSOP, GI
BISWAL, TK
 
Subject history
rocks
crust
antarctica
kinematics
movement
criteria
zones
sense
shear
microfractures
feldspar clasts
shear zone
rigid body rotation
 
Description Patterns of microfracturing are investigated in plagioclase and K-feldspar porphyroclasts formed within granitic mylonites along the boundary of the Eastern Ghats Mobile Belt, India. The mineral assemblage comprising quartz, feldspar, biotite and hornblende suggests lower greenschist facies conditions during mylonitisation, with the contrasting ductile behaviour of quartz and brittle fracturing of feldspars restricting the temperature range during deformation to 300-350 degrees C. Microfracturing of feldspars takes place by concentration of pure shear within the feldspar-rich layers. This may reflect strain partitioning into dominantly pure and simple shear due to the competency contrasts between the two major constituent minerals (quartz and feldspar). The microfractures occur in conjugates (here designated T(1) and T(2)) with T(1) inclined in the same direction as the S-fabric and showing an antithetic sense to the NW verging shear, while T(2) is inclined in an opposite sense to the S-fabric and displays synthetic shear. The direction of maximum compression occurs at high angles to the C-fabric, and the T(1) and T(2) fractures are the result of pure shear localized into brittle layers within the mylonite. With progressive shear, the fractures along with their host feldspar grains are rotated. Theoretical graphs are plotted between bulk shear (gamma) and the angle of initiation (alpha) of T(1) and T(2) with respect to C-planes, for fractures hosted in a circular or elliptical objects. The kinematics of these fractures are also analyzed with regard to variations in shear strain and sense of shear along them. The sense of shear may vary or remain stable within fractures, depending on their initial angle of inclination with respect to the C-fabric. As T(1) is inclined at low angles to the XY plane and in the same direction as the S-fabric, it undergoes maximum shear strain compared to T(2) and may even exceed the bulk shear. This facilitates breakdown of feldspar porphyroclasts during progressive deformation. The T(1) set maintains an antithetic sense of shear even at low angles with the C-fabric, while T(2) displays a synthetic sense in spite of being at high angles to C. The T(1) and T(2) fractures therefore differ significantly from the classic Riedel, P and Y shears where low angle fractures display synthetic shear and high angle ones (R') show antithetic displacement with respect to the bulk shear sense.
 
Publisher PERGAMON-ELSEVIER SCIENCE LTD
 
Date 2011-08-27T00:39:00Z
2011-12-26T12:57:38Z
2011-12-27T05:44:24Z
2011-08-27T00:39:00Z
2011-12-26T12:57:38Z
2011-12-27T05:44:24Z
2010
 
Type Article
 
Identifier JOURNAL OF STRUCTURAL GEOLOGY, 32(10), 1417-1429
0191-8141
http://dx.doi.org/10.1016/j.jsg.2010.07.006
http://dspace.library.iitb.ac.in/xmlui/handle/10054/11479
http://hdl.handle.net/10054/11479
 
Language en