Rb--Sr, K--Ar, and Fission-Track Geochronological Studies of Samples from LASL Drill Holes GT-1, GT-2, and EE-1

摘要

Geochronological investigations using the Rb-Sr, K-Ar, and fission-track methods have been completed on core samples from the three LASL deep drill holes, GT-1, GT-2, and EE-1. This work indicates a complex history for these Precambrian rocks beginning with a metamorphic event at 1.66 b.y. which generated the gneisses and schists from older sedimentary and igneous rocks. The metamorphic complex was intruded by at least two different magmas at 1.3--1.4 b.y. producing thin felsic dikes and a major biotite granodiorite pluton. This igneous activity caused pervasive argon loss to occur, lowering the K-Ar ages to about 1.4 b.y. Plio-Pleistocene igneous activity related to formation of the Valles Caldera increased the local geothermal gradient to 50--60 exp 0 C/km and produced fission track annealing in apatite and again argon loss from the biotite in deeper samples. (ERA citation 02:051326)