Flow And Deformation Mechanisms Of Hornblende Rocks From The Paleoproterozoic Fold Belt In Liaodong Peninsula

The composition, structure and evolution of the middle-lower crust has been an subject of extensive study in Earth sciences. Since amphibolites rocks constitute the major parts of the middle-lower crust, their rheological and mechanical behavior constrains the nature and state of latter, and therefore influences the evolution of continental plate. Detailed study on the deformation characteristics and deformation mechanisms of hornblende and hornblende rocks would promote understanding the creep laws and rheology of middle-lower continental crustal rocks, and establishing a comprehensive mechanical and rheological model of crustal lithosphere.This paper discusses about, from microstructural studies of three types of tectonites with different deformation characteristics (gneissic, banded and mylonitic structures) from high grade metamorphic amphibolites from the Paleoproterozoic Fold Belt in Liaodong Peninsula, the flow mechanisms of hornblende tectonites, on the basis of field geological investigation and analysis of geological settings, and by means of optical microstructural analysis, transmission electron microscope (TEM) analysis, electron microprobe analysis and digital shape fabric analysis. In combination with experiments preformed by Hacker, this thesis discusses the dynamic and rheological structure of the lithosphere.It is shown from microstructural analysis that amphibolites with different structural characteristics differ from each other distinctly in many aspects. Hornblende grains in gneissic amphibolites have a large grain size, short prism shape and their aspect ratios (short axis to long axis) between 0.2~0.4, with a maximum at about 0.3-0.4. Plagioclase grains are not strongly elongated, but hornblende grains have obvious crystallographic preferred orientation. It may suggest that the rocks have been transformed rather weakly during structural deformation.Cleavage domains mainly of hornblende grains and microlithons of plagioclase grains constitute the banded amphibolites. They result in differentiation and orientation of the hornblende and plagioclase grains in the amphibolites. Hornblende grains show undulose extinction, crenulation and strip subgrains with their long axes paralleling to foliation. The orientation of hornblende grains in banded amphibolites is much stronger than that in gneissic amphibolites. The orientations of long axes of the hornblende grains are centered between 0 and 20°. The grains have aspect ratios between 0.1 and 0.4. The hornblende-plagioclase contacts are amoeboid shaped, bulging into hornblende grains that indicate that direction of grain boundary migration toward hornblende grains during recrystallization. Plagioclase grains have weak orientation that intersect with foliation in rocks in a small angle. Their aspect ratios vary between 0.4~0.7. The mylonitic amphibolites experienced intensive shearing deformation and show obvious deformation characteristics. Hornblendes grains that make up the cleavage domains are strongly oriented. Both hornblende and plagioclase grains constitute mylonitic foliation. Hornblende grains have sizes varying from 0.3mm to 0.6mm and have aspect ratios highly concentrated between 0.1 and 0.2, indicating a strong unaxial extension. Plagioclase grains occur as single grains and are obviously oriented. There are lots evidences of deformation, as is undulose extinction, irregular grain boundaries, subgrains and boundary granulation. The grain sizes the plagioclase grains vary between 0.1mm and 0.4mm and have aspect ratios from 0.2 to 0.5. They are strongly oriented at 10°～35°in reference to the orientation of the direction of the thin sections, which imply that they may have contributed to the development of foliation in the rocks.Submicrostructural analysis revealed the dislocation substructures and dislocation associations. Minor amount of free dislocations, dislocation loops, dislocation arrays, dislocation chains and tangled dislocations are observed in hornblende grains from gneissic amphibolites. Dislocations are rare...