RbSr geochronologic investigation of precambrian samples from deep geothermal drill holes, Fenton Hill, New Mexico

Analyses of twenty-six gneissic whole rocks from the depth range 732–2588 m in drill hole GT-2 yield a 1.62 ± 0.04 b.y. isochron age with initial 86Sr/86Sr (Ro) = 0.7067 ± 0.0014; this age is in agreement with radiometric ages reported for rocks elsewhere in north-central New Mexico. A preliminary isochron age of 1.92 ± 0.18 b.y. with Ro = 7014 ± 0.0025 from the nearby drill hole GT-1 is defined by eight samples. Because these samples are more representative of mineral systems or granite gneiss -amphibolite mixtures, the age is interpreted as being anomalously old. Samples of non-foliated biotite granodiorite from below 2588 m in GT-2 and from EE-2 yield an 1.50 ± 0.12 b.y. isochron (Ro = 0.7038 ± 0.0016). Hence the biotite granodiorite may either be a sill or an apophysis of a younger pluton.