Patchy zoning plagioclase rock

The presense of parallel striae on cleavage surfaces distinguishes plagioclase from kfeldspar. Pdf plagioclase zoning as an indicator of magma processes at. Patchy zoning this sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal. Whole rock maiorelement compositions, modal abundances, and rheological properties of turf point formation.

This paper is concerned with the significance of patchy zoning, a feature which, though widespread in igneous plagioclases, has been ignored by many petrologists. Distribution of plagioclase in carbonate rocks from the tertiary. Often a grey color in comparison to other feldspars. This type of zoning may also be produced by physical inclusions of rock or other minerals. Mineral textures and zoning as evidence for open system. Fluids enter the cores of the plagioclase via microcracks andor twin planes. To better understand consider the plagioclase zoning, in which the zoning is particularly common and conspicuous. An example of this would be clay, hematite, or chlorite inclusions coating the surface of an inner layer of crystal growth. Inclusions of this sodic plagioclase within the corroded cores give the feldspar a mottled appearance and suggest the term patchy zoning for these textures. In the geological literature on the lepontine alps, there are several reports on irregularly zoned patchy plagioclase with wide variations in composition.

Linking microcracks and mineral zoning of detachment. Plagioclase zoning in midpleistocene lavas from the seguam volcanic center, central aleutian arc, alaska. Can be confused with quartz if there is no twinning, however the optic axis figure will resolve this. This type of zoning is interpreted as a result of a decrease in confining p on waterdeficient magma during its rise in the crust vance, 1965. Peninsula, i have observed many examples of patchy zoning in plagioclase and, al though this zoning is generally. Pdf backscattered electron bsederived zoning patterns of plagioclase phenocrysts are used to identify magma processes at. Many igneous plagioclases exhibit irregular zoning consisting of corroded cores embayed and surrounded in crystallographic continuity by more sodic plagioclase. Patchy zoning is exceedingly widespread in magmatic plagioclase, especially in the common plagioclaserich volcanic and plu tonic rocks of calcalkaline. Magma storage constrains by compositional zoning of plagioclase. Plagioclase zoning in midpleistocene lavas from the. Wholerock major element compositions and point counts.

Patchy zoning in plagioclase has been observed in gabbroic rocks in southwestern oregon. Plagioclase phenocrysts in the plagioclase phyric basalt from hole u1433b of iodp expedition 349 in the south china sea show complex zoning patterns e. Chemical zoning is common the calcic core has a larger extinction angle than the sodic rim. Concentric zoning may be more subtle as with garnet and feldspar, for example. Origin of patchy zoning in plagioclase from gabbroic rocks of.