Host control of recrystallized quartz grains

Abstract

SUMMARY. The results of a petrofabric study on recrystallization of quartz in nature are presented and compared with recent experimental work. The orientations of newly recrystallized quartz grains are shown to be dependent on the orientation of host grains from or in which they grew. The results are compatible with the experiments of Hobbs 0968). The orientation of new grains appears to be unrelated to stress.Tr~E experimental work of Hobbs 0968) on the recrystallization of single crystals of quartz showed that the orientation of newly recrystallized grains was controlled fundamentally by the orientation of the host grain from or in which they grew. This observation contradicts previous theory (Kamb, I959, I96I ; Brace, ~96o), experiment (Griggs et aL, I96o), and field studies (Sylvester and Christie, I968), which suggested that the orientation of newly recrystallized grains and any resulting preferred orientation was controlled by stress. These studies largely ignored the long recognized phenomenon of host-controlled recrystallization in metals (Barrett and Massalski, ~966).In order to verify that a host control operates in the orientation of newly recrystallized quartz grains in nature and hence contributes to the development of natural preferred orientations, a study of quartz recrystaHization was undertaken on a suite of retrograde rocks from the Broken Hill area of western New South Wales, Australia. These rocks occur in and near retrograde schist-zones, described in detail by Vernon 0969), Hobbs et aL 0968), and Vernon and Ransom (I97O), which are narrow (from a few millimetres to over Ioom in width), conjugate, planar zones (similar to mylonite zones) transecting the high-grade metamorphic rocks of the region and locally reducing the grade of metamorphism from granulite to amphibolite facies. The specimens studied were collected from a quarry in Eyre Street, Broken Hill, where there outcrops 9

Dynamic Recrystallization in a Naturally Deformed Albite

“…These grains will continue to rotate throughout further deformation, but, since they are rotating largely in response to dislocation motion, their orientations will remain within the same fabric pattern as that oftheir deformed host grain(s). In both naturally and experimentally deformed minerals, fabric patterns of deformed original grains and their recrystallized products are generally indistinguishable (Ransom, 1971;Bell and Etheridge, 1976;Etheridge and Kirby, 1983), providing further support for this recrystallization mechanism. Any form of homogeneous nucleation, whether or not the nucleus orientation is stress-controlled (Av6 Lallement and Carter, 1970), is likely to produce orientations outside the deformed grain fabric.…”

“…With respect to fabric (texture) development, recrystallizcd grains with initial orientations close to that of their deformed host (Hobbs, 1968;Ransom, 1971;Wilson, 1973;Bell and Etheridge, 1976) can be explained by the subgrain rotation model. These grains will continue to rotate throughout further deformation, but, since they are rotating largely in response to dislocation motion, their orientations will remain within the same fabric pattern as that oftheir deformed host grain(s).…”

The role of the fluid phase during regional metamorphism and deformation

“…The weight of published work thus suggests that dislocation flow processes dominate in shear zones, but this is somewhat misleading, because most of the studies have concentrated on zones in essentially monomineralic rocks, or on the outer (lower strain) parts of zones where dislocation flow dominates even in many polyphase rocks. Many polyphase rocks in retrograde shear zones are characterized, at least in their more highly strained portions, by unstrained, relatively equidimensional grains, low-energy grain shapes, uniform grain sizes, predominant contacts between unlike phases and commonly weak quartz c-axis preferred orientations (Ransom, 1969(Ransom, , 1971Vernon & Ransom, 1971;Bell & Etheridge, 1973. In other words, they are similar to progrude rocks of similar grade and composition.…”

The structural geology of the Red Cloud thrust system, southern Eastern Transverse Ranges, California

“…The curve for a random population Is Included on the plot. As found by Ransom (1971) and Wilson (1973), there are fewer new grains at angles of 60^ or greater from the host orientation than would be expected for a random population.…”

“…Figure 13 is a histogram of the c-axis host/subgrain angle for sample BW 34 (cf. Ransom, 1971). The curve for a random population Is Included on the plot.…”