Abstract

The internal textures of detrital polycrystalline quartz can be used to distinguish between polycrystalline quartz populations derived from different crystalline source rocks. This is supported by the petrographic examination of the quartz fraction in 65 metamorphic rocks with known metamorphic histories and the detrital polycrystalline quartz populations in 57 samples of medium sand-size Holocene fluvial sand derived from known crystalline source rocks. Empirical observations by metamorphic petrologists have shown that quartz crystal sizes increase systematically with increase in metamorphic grade of the rocks. Petrographic examination of quartz crystals deformed in controlled laboratory experiments have shown quartz responds to stress via a systematic series of processes: deformation, recovery, primary recrystallization, and secondary recrystallization. Deformation is reflected optically by undulose extinction, deformation lamellae and bands, elongated crystal units, and sutured crystal-crystal boundaries. Recovery is recognized optically be segmented extinction (semi-composite extinction) reflecting polygonization of the crystal. Primary recrystallization occurs when strain-free areas surrounded by dislocation tangles form new crystals, and is recognized optically by the presence of small (50 mu m), nonundulose crystals. Secondary recrystallization develops large, nonundulose, strain-free polyhedral crystals with smooth crystal-crystal boundaries. Deformation, recovery, and primary recrystallization features characterize quartz in greenschist-grade metamorphic rocks. Detrital quartz grains with these features are, for the lack of a better term, called "unstable", reflecting the transformation from old crystals to new, recrystallized crystals. Detrital polycrystalline quartz with features of secondary recrystallization are called "stable" grains, and are derived from medium- to high-grade metamorphic rocks. Detrital polycrystalline quartz grains derived from intrusive igneous rocks are also characterized by "stable" features. A plot of a polycrystallinity index (the ratio of detrital polycrystalline quartz with more than 3 crystal units per grain to total detrital polycrystalline quartz) vs. an instability index (the ratio of detrital polycrystalline quartz with "unstable" characteristics to total detrital polycrystalline quartz) discriminates between medium sand-size polycrystalline quartz populations in Holocene sands derived from low-, and medium- to high-grade metamorphic source rocks Detrital polycrystalline quartz populations in Holocene sands derived from granitic plutons partly overlap the medium- to high-grade field.

First Page Preview

First page PDF preview
You do not currently have access to this article.