Abstract

A variety of igneous textures displayed by stocks associated with the Henderson molybdenite deposit are interpreted to result from unidirectional growth from the walls of magma chambers inward. These igneous textures are present in layers and multilayered sequences consisting of crenulate quartz and feldspar pegmatites, crenulate quartz, dendritic alkali feldspar and quartz, and dendritic and micrographic alkali feldspar–quartz intergrowths. Minerals or intergrowths defining a layer within any one stock consistently terminate or branch inward, away from the stock contacts. The layers are separated by aplite or aplite porphyry and are most common in the apical and marginal parts of an individual stock. Layering generally parallels intrusive contacts.

Differences in morphologies of phases among the various layers suggest changing conditions of growth from an undercooled melt and/or aqueous solution. Changes in these physical and chemical conditions as the stock crystallized inward produced the observed textural and compositional layering.

Unidirectional solidification textures can be used to determine the relative ages of individual stocks as well as the approximate orientation of stock contacts. In the absence of other independent evidence, unidirectional textures can also be used to indicate proximity to an intrusive contact. These features are an invaluable aid in detailed mapping of multiple-intrusion systems in which the units are compositionally identical or nearly identical.

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