O18/O16 ratios of the Johnny Lyon granodiorite and Texas Canyon quartz monzonite plutons, Arizona, and their contact aureoles

Abstract

O18/O16 ratios have been measured for 29 quartz samples, 6 whole-rocks, 3 muscovites, and 1 K-feldspar from two adjacent granitic plutons of vastly different age (about 1660 m.y, and 70 m.y.) intruded into the same type of country rock, the Precambrian Pinal schist. Sample traverses were made across 3 different contact zones of these intrusive bodies. Except for 2 quartz veins with δO18=+11.0 and + 12.3, all quartz samples collected more than 15 cm from the margin of the Early Tertiary Texas Canyon pluton are isotopically exceedingly uniform with δO18=9.47±0.11. Four quartz samples collected more than 10 m from the margin of the Precambrian Johnny Lyon pluton have δO18=10.43±0.08. Compared with previous studies of this type, only relatively minor O18-enrichments have occurred in the border zones of the plutons. This is in part because the original δO18 differences between the metasedimentary rocks and the intrusives are relatively small (only 3 to 6‰), but is mainly due to the lack of H2O in the contact zones during intrusion as a result of the general impermeability and prior dehydration of the schist. There is no isotopic evidence for significant influx of “external” H2O into either of the plutons during their crystallization and cooling. However, in roof-zones where metasedimentary rocks overlie the plutons there is a strong δO18 lowering in the contact metamorphic aureoles, indicating upward expulsion of low-O18 “magmatic” H2O into these rocks.