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K-Ar and ^Ar/^Ar ages of 170±2 Ma for hornblende approximate the intrusive age of the Humboldt lopolith, Nevada. K-Ar and Rb-Sr ages range from 142Ma to 155 Ma for coexisting biotite and reflect Ar loss due to heating during emplacement of widely scattered Cretaceous and Tertiary granitoid intrusions in the area. The Rb and Sr compositions of the parent magmas for this mafic body lie between tholeiitic and alkalic on a plot of these elements for average oceanic basalts. Variable initial 87Sr/86Sr for gabbros and basalts require at least two magmas from different source materials are represented in the lopolith rocks. Partial chemical analyses, Sr, oxygen and hydrogen isotopic data for rocks of the nearby Permian (?) and Triassic Koipato Group reflect pervasive alteration. Extreme K to Na ratios and SiO2 concentrations are present in these rocks. In addition, their 318O and 3D values reflect alteration by formation and organic waters. Fifteen of sixteen samples of whole-rocks are colinear on a Sr evolution diagram and regression of the data yields an errorchron with T= 168.619.3 Ma, initial "Sr/^Sr = 07141 ± 13, MSWD =48.8. The pervasive alteration of the Koipato Group rocks is the same age as the nearby Jurassic Humboldt lopolith and probably was caused by a circulating hydrothermal system driven by heat supplied during emplacement of the magmas that formed this large mafic igneous rock complex. Paleomagnetic pole positions in the Triassic Koipato group were probably established by remagnetization during the rock alteration in the Jurassic. If so, 1,400 km of southward transport of the Koipato Group on the Golconda thrust before Tertiary time is not required by the paleomagnetic data.

u.s. geological survey open file report/open-file report

40Ar/39Ar, K-Ar, Rb-Sr whole-rock and mineral ages, chemical composition, strontium, oxygen and hydrogen isotopic systematics of Jurassic Humboldt lopolith and Permian(?) and Triassic Koipato Group rocks, Pershing and Churchill Counties, Nevada