Triassic Plutons

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.

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