40Ar/39Ar data from plutonic rocks in the Mojave Desert near Victorville, California, provide preliminary insight into the complex thermal history of this region and suggest that some plutons here may be significantly older than any plutons in the Sierra Nevada to the north. Near Victorville, multiple deformation and metamorphism of platform-miogeoclinal strata is inferred to postdate deposition of Permian rocks and predate intrusion of monzonite plutons. Monzonite that intrudes multiply deformed Paleozoic marble and that is unconformably overlain by the Mesozoic (Triassic or Jurassic) Fairview Valley Formation has hornblende with an age spectrum typical of severe 40Ar loss. We interpret the age of the highest temperature increment (fusion), 233 ± 14 m.y., as a minimum age for this pluton. The total gas age for the hornblende is only 169.9 m.y. Biotite from the same rock exhibits a plateau age of 76.59 ± 1.24 m.y. The Fairview Valley Formation is overlain by volcanic rocks of the Sidewinder sequence. These rocks were folded and intruded by quartz monzonite that postdates all penetrative deformation in the region. Biotite-hornblende tonalite at Quartzite Mountain is part of one of these younger quartz monzonite plutons, and its biotite yields a plateau age of 73.16 ± 1.11 m.y. This age is the same at the 95% confidence level as the age of biotite from the older monzonite; thus, this younger thermal event was at least the partial cause for the argon loss pattern displayed by the monzonite hornblende. Hornblende gabbro that intrudes Paleozoic marble at Sidewinder Mountain and is in turn contact metamorphosed by later plutonic rocks has hornblende with a disturbed 40Ar/39Ar age spectrum. The 1025, 1050, and 1075 °C steps contain 44.64% of the total argon and yield a common age of 119.2 ± 3.4 m.y., which we interpret as the best estimate for the age of the hornblende.

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