Zircons from the Copper Flat Intrusion, Hillsboro, New Mexico
Wilbanks, John Randall
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Zircon crystal growth trends of the Copper Flat quartz monzonite intrusion, Hillsboro, New Mexico, are more variable near the intrusive contacts than toward the central portion of the intrusion. Length and width measurements of zircon crystals recovered from 36 samples were statistically treated using the method of the reduced major axis. Eight of the 36 samples have reduced major axes significantly different from the mean growth trend calculated for 34 of the 36 samples. Six of these eight samples were obtained from locations and elevations near the roof of the intrusion. The other two samples represent material from the bottom of holes drilled near an inward-sloping contact of the intrusion. Because zircon is an early-forming mineral in granitic magmas, different physico-chemical conditions, particularly water concentration near the intrusive margins, are thought to control growth patterns. Fewer differences in zircon growth trends are found near the central portion of the intrusion, reflecting the more uniform environment during zircon crystallization. The two anomalous samples from greater depths may represent the marginal fades of the intrusion, thus supporting the hypothesis that the intrusion is a funnel-shaped pluton.