Preferred orientation development of phyllosilicates in diagenetic to greenschist facies environments, and related geological processes.

Abstract

Phyllosilicates are the principal components of phyllosilicate-rich diagenetic to low-grade metamorphic rocks such as mudstones, shales, and slates, and are among the most common rock types on the Earth's surface. Understanding how the chemical composition, structure and textures of phyllosilicates in those rocks evolved under different geological conditions provides critical information about many geological processes in the upper crust. Advances in electron microbeam technology have made it possible to study fine-grained phyllosilicates in detail. However, a piece of critical information, the dimensional and crystallographic preferred orientation of phyllosilicates, is not easily obtained. This dissertation was designed to develop a method and systematically examine preferred orientation development of phyllosilicates in environments from diagenetic to low metamorphic grades, using a combination of transmission X-ray texture goniometry and electron microscopy. Samples from various environments and geologic settings were studied. The results of these studies show that (1) in a burial environment, the preferred orientation development of phyllosilicates is largely affected by the mineralogical transformation from smectite to illite (S-I); this supports the proposal that coalescence of illite packets during the S-I transition provides the hydraulic seal for the generation of basin overpressure. (2) In the presence of tectonic stress, cleavages form as the result of reorientation of phyllosilicates in a new orientation. In the reorientation process, larger detrital grains (tens of $\rm\mu m)$ change orientation by mechanical processes, whereas fine-grained authigenic grains (tens of nm) change orientation primarily by chemical processes (dissolution and neocrystallization). (3) Mechanical processes are favored at low metamorphic grade and low strain, whereas chemical processes are favored in relatively high-grade and high-strain environments. This suggests that the dominant cleavage-forming mechanism is a function of the combined effects of thermal and strain energy in rocks. The contribution of either source to the total energy of the system is complementary and fully interchangeable. (4) After deformation stopped, preferred orientation of phyllosilicates continues to improve with metamorphic grade by extending cleavage-parallel phyllosilicate grains along the (001) planes at the expense of phyllosilicate grains in less favorable orientations, by dissolution and neocrystallization.