METAMORPHIC FOLIATION DEVELOPMENT IN LOW-GRADE SCHIST: QUARTZ AND WHITE MICA GRAIN-SHAPE DATA FROM THE OTAGO SCHIST, NEW ZEALAND

Shape, size and orientation measurements of quartz and white mica grains across increasing metamorphic grade in the Otago Schist, New Zealand, reveal the progressive development of metamorphic foliation that we describe below in three stages:

(i) At sub-greenschist facies conditions, a shape preferred orientation (SPO) of both quartz and white mica developed rapidly and early in the deformation history. The SPO is subparallel to bedding and forms a composite foliation parallel to the axial planes of F₁ folds. The mica SPO developed by oriented growth of white mica following the breakdown of weakly oriented detrital grains, while quartz SPO resulted from pressure solution modification of detrital grains.

(ii) With continued deformation, the foliation intensified by an increase in the aspect ratio and alignment of grains. Both quartz and mica increased in size and both phases contribute to define the foliation. Minerals elongation defines a prominent lineation parallel to fold axes.

(iii) A change in the foliation is marked by the development of segregation layering, enabled by crenulation, pressure solution and the reorientation of veins. In the Otago Schist, this occurs in the chlorite zone, greenschist facies. The foliation is less planar than in Stage ii, and grains record a decrease in aspect ratio and reduction in alignment parallel to the foliation. The occurrence of quartz changes from isolated detrital grains modified by pressure solution, to aggregates and layers of recrystallised quartz of reduced aspect ratio. The foliation is defined increasingly by segregation layering and less by the shape and orientation of individual quartz grains.

The following trends are evident from the SPO data: Grains are longer and have a greater aspect ratio parallel to lineation; grains with a greater aspect ratio are more closely aligned to the foliation plane; the length and thickness of white mica and quartz grains increases with increasing metamorphic grade; trends in grain SPO are influenced by deformation mechanism and segregation of the rock mass following crenulation cleavage development.

The key stages in SPO development are the oriented growth of white mica from detrital grains and the transition from grain-defined to layer-defined foliation coincident with the development of metamorphic segregation.