Publications

High-angle misorientations can significantly influence material properties. In this study, optical microscopy, Scanning Electron Microscope-Electron Backscatter Diffraction (SEM-EBSD), and Atomic Force Microscopy (AFM) have been used to investigate high-angle misorientations in quartz-bearing crustal rocks. Thin sections of high-grade quartzofeldspathic rocks were subjected to chemical mechanical polishing (CMP) with colloidal silica. In quartz, high-angle misorientations like random high angle grain boundaries (RHAGBs) and Dauphiné twin boundaries (DTBs) could be discriminated using EBSD techniques but not optical microscopy. In nanoscale AFM images, indented channels are observed along RHAGBs but not DTBs; these result from material removal during CMP, indicating lower compactness of RHAGBs compared to DTBs. Along any RHAGB, EBSD reveals different misorientations across segments between consecutive RHAGB-DTB intersections. Grains adjacent to these RHAGB segments have angles between their c-axes varying from 61-66° with parallel {101‾2} planes, and 81–84° with parallel {112‾2} planes, respectively. These symmetries represent the Japan and Sardinian twin laws of quartz, indicating that the RHAGB segments become low-energy twin boundaries, thereby reducing the overall surface energy of the aggregate. Finally, these results suggest that apart from surface topography quantification and high-resolution nanoscale imaging, AFM in conjunction with SEM-EBSD can be used for precisely locating sites for TEM study.

The supercontinents Rodinia and Gondwana are believed to have assembled through orogenic events at ∼1000 Ma and ∼550 Ma, respectively. The Larsemann Hills, part of the extended Rayner Complex of East Antarctica, is a Neoproterozoic granulite terrane that was a part of both supercontinents. The tectonic evolution of the terrane in the Rodinia to Gondwana interval remains uncertain. This study integrates new field, microstructural, metamorphic and geochronological information to unveil a previously undocumented mid-Neoproterozoic thermo-tectonic event in the Larsemann Hills. Granulite facies metamorphism (M1), synchronous with the deformation, D1, is interpreted to have occurred in the Larsemann Hills at ∼1000 Ma, followed by a crustal shortening (D2) event that continued from ∼990–900 Ma, signifying the incorporation of the Rayner crustal unit into Rodinia. Subsequent mid-Neoproterozoic extensional deformation (D3), characterized by NW-SE trending S3 shear zones, reoriented pre-existing structural fabrics. The extensional deformation operated under amphibolite facies metamorphic conditions (M2) and resulted in a post-peak decompressive P-T trajectory. UPb LA-ICPMS dates on zircons separated from metapelites within high D3 strain zone yield clusters at ∼700 Ma; zircons immediately adjacent to the shear zone yield ages around ∼990–900 Ma. A thermal overprint at ∼550 Ma is evident but weakly manifested in the western Larsemann Hills, with no unambiguously correlatable structural or metamorphic evidence, suggesting distance from the Gondwana orogenic front. The extensional deformation documented here is contemporaneous with extension in the northern Eastern Ghats Province, India that was contiguous with the Rayner Complex in the Neoproterozoic. This suggests that a major terrane-wide extensional event was associated with the disintegration of Rodinia. In the Neoproterozoic, the EGP-Rayner unit was therefore a tectonic entity distinct from cratonic India, with only the former being an integral component of Rodinia.