Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt,west of Khariar,NW Orissa: Evidence from meso- and microstructures |
| |
| Authors: | T K Biswal Harish Ahuja Himansu Sekhar Sahu |
| |
| Institution: | (1) Indian Institute of Technology Bombay, 400 076 Powai, Mumbai, India |
| |
| Abstract: | Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar
in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds
and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine
show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a
magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites
that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile
Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is
overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures,
along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction
and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites.
Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from
magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt
mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane
Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending
support for thrusting of the mobile belt over the craton around that time. |
| |
| Keywords: | Nepheline syenite Terrane Boundary Shear Zone Eastern Ghats Mobile Belt magmatic fabric synkinematic emplacement |
| 本文献已被 SpringerLink 等数据库收录！ |
|
