Rb-Sr Geochronologic Investigation Of Precambrian Samples From Deep Geothermal Drill Holes, Fenton Hill, New Mexico

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Journal Article: Rb-Sr Geochronologic Investigation Of Precambrian Samples From Deep Geothermal Drill Holes, Fenton Hill, New Mexico

Abstract
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. Dikes of fine-grained leucocratic and biotite monzogranites, which intrude the more coarse-grained 1.62-b.y. old gneisses are dated at 1.44 ± 0.03 b.y. (Ro = 0.7040 ± 0.0009). We thus recognize metamorphic activity at 1.62 b.y.; major magmatism at about 1.50 b.y.; and more local dike emplacement at 1.44 b.y. Present elevated in-situ temperatures have not disturbed the Rb-Sr systematics.

Authors 
D. G. Brookins and A. W. Laughlin








Published Journal 
Journal of Volcanology and Geothermal Research, 1983





DOI 
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Online 
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Citation

D. G. Brookins,A. W. Laughlin. 1983. Rb-Sr Geochronologic Investigation Of Precambrian Samples From Deep Geothermal Drill Holes, Fenton Hill, New Mexico. Journal of Volcanology and Geothermal Research. 15(1-3):43-58.