Zircons from the Animas stock and associated rocks, New Mexico

Abstract

Zircons have been studied by measurement of lengths and widths of 200 crystals for each sample. Data are analyzed by the reduced major axis method. Ten samples from the Animas stock, emplaced at shallow depth, have zircons with like reduced major axes. A sample from an apophysis, one from a roof protruberance, and one from near the center of the stock have somewhat different zircon reduced major axes. Contrasting bands in fine- and coarse-grained layered units have zircons with like reduced major axes. The size of zircons varies considerably throughout the stock. Results are compared with those obtained for the Bald Mountain batholith, Oregon, which has been emplaced at moderate depth.Three samples of tuff that originated from the site of the stock, and two samples from a related, nearby vent have zircons with reduced major axes like those of the zircons of the Animas stock. One tuff sample has a somewhat different zircon reduced major axis than that of the Animas stock. Three granitic xenoliths and a younger extrusive have zircon reduced major axes unlike that of the stock. Five different granitic intrusives also have zircons with different reduced major axes. One different granite has the same zircon reduced major axis as that of the Animas stock, but the zircons differ in other respects.It is concluded that zircons may be used to characterize granitic rocks of igneous origin and to distinguish derivatives of one magma from those of older or younger magmas, even if the rocks are of identical chemical and mineralogical composition.