Mojave River Valley Museum
Triassic plutonic rocks are widely scattered in regions mostly west and southwest of the
EMNSA (Barth and others, 1991; Miller, 1978). In the Clark Mountain area, several small
dioritic stocks intrude Paleozoic strata, and one of these bodies has yielded K-Ar hornblende
ages of 190 and 200 Ma (Burchfiel and Davis, 1971; Mueller and others, 1979). However,
preliminary U-Pb zircon ages for these bodies are Late Jurassic (J.D. Walker, 1993, oral
commun.). Evidently, the hornblende contained excess argon, resulting in ages spuriously old.
No Triassic U-Pb ages have been reported for plutons in the EMNSA, but few candidates have
been dated by U-Pb. A dioritic orthogneiss unit in the Granite Mountains, assigned a probable
Triassic age by Howard and others (1987), is now known to be Jurassic (Young and others,
1992). Small bodies of hornblende monzonite, commonly present between the Teutonia
batholith and septa of Paleozoic marble in the western New York Mountains, is similar to
Triassic monzonites known in the western Mojave Desert. This hornblende monzonite is
younger than Paleozoic rocks and older than the Early Cretaceous Mid Hills Adamellite.