Lower Greenschist Facies Oscillations Across the Brittle-Ductile Transition Induced by Alternating Reaction Softening and Hardening Events

Reaction textures associated with multiple fabrics in the Red River shear zone show evidence for both brittle and ductile deformation. Rocks on the eastern flank of the shear zone in the Diancang Shan block near Dali, Yunnan Province, China are dominated by porphyritic granodiorites and form the protoliths of the Red River fault rocks. The earliest fault-rocks are cataclasites in which fracture processes broke matrix feldspar grains. Grain-scale brittle fractures and crushed grains are well preserved in the >cm size relic K-feldspar phenocrysts, where multiple fractures are filled with newly precipitated quartz. Progressive deformation has displaced broken K-feldspar fragments into the evolving matrix establishing a cataclastic texture with abundant feldspar porphyroclasts. Ductile deformation is manifest in multiple generations of fabrics that wrap around these broken phenocrysts. The fabrics are defined primarily by muscovite and chlorite that form folia that may exceed the length of a thin section (several cm). Truncations and embayments of these minerals show that earlier magmatic feldspars and biotites have been dissolved and muscovite + chlorite + quartz have crystallized as reaction products. Reaction softening is clearly manifest by the replacement of K-feldspar and the crystallization of quartz as ribbons and muscovite in well aligned folia that define the mylonitic foliation. In some microstructural sites K-feldspar porphyroclasts themselves are truncated and engulfed by muscovite alone. These relationships suggest the simultaneous operation of the locally metasomatic ionic replacement reactions: K-feldspar + Na+ = Albite + K+ , and ΔV = -10% 3K-feldspar + 2H+ = muscovite + 6 quartz + 2K+ and ΔV = -16% 3Albite + K+ + 2H+ = muscovite + 6 quartz + 3Na+. ΔV = - 8% The latter two reactions produce muscovite and quartz, both much weaker than the reactant K-feldspar; these reactions constitute reaction softening. Moreover, the muscovite tends to align in contiguous bands constituting textural softening (Shea and Kronenberg, 1993). These reactions occur without any demonstrable change in temperature, and so produce a shift from brittle to ductile behavior. The reactions also involve a volume loss, such that they will be driven by a high normal stress; thus they are readily driven by a high normal stress. These ductile fabrics are in turn cut by K-feldspar veins that interrupt the mylonitic fabric produced by the above reactions. The K-feldspar veins add K-feldspar to the assemblage and interrupt the mylonitic fabric. Thus these structures constitute both reaction and textural hardening. Finally these may become boudinaged by continued ductile deformation in the mylonitic matrix, thus establishing a late ductile strain event. Together these overprinting textures and microstructures demonstrate two oscillations of brittle to ductile deformation all at lower greenschist facies conditions where only frictional behavior is predicted by experiments.