Discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism. We have applied a combination of 40Ar/39Ar thermochronology, petrology, and thermobarometry to reconstruct the complex Early Proterozoic through early Cenozoic tectonic and metamorphic evolution of continental crust in the Old Woman Mountains area, south-eastern California.
Strong Mesozoic thermal events obscure the earlier history in much of the Old Woman Mountains area. In those areas where Early Proterozoic rocks underwent only lower-greenschist-facies metamorphism during the Mesozoic, thermobarometry of pelitic schists indicates that Proterozoic metamorphism occurred at 9 to 11 kbar and ∼700 °C. 40Ar/39Ar ages of hornblende from samples of interbedded Proterozoic amphibolite indicate that this high-grade metamorphism took place before 1600 Ma. The relatively high-pressure conditions of Early Proterozoic metamorphism in the Old Woman Mountains area contrast with the low-pressure granulite-facies metamorphism that occurred elsewhere in the Mojave Desert at this time. 40Ar/39Ar analyses of hornblende from Proterozoic rocks within Mesozoic shear zones and hornblende barometry from Jurassic intrusive rocks suggest that tectonism and burial of Paleozoic strata to >10 km began between 170 and 150 Ma. This tectonism resulted in regional greenschist-facies metamorphism. Late-stage mineral assemblages in Proterozoic and Paleozoic pelitic rocks in the Old Woman Mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward Late Cretaceous plutons of the 73 Ma Old Woman-Piute batholith. Metamorphic temperatures from garnet-biotite thermometry increase from <450 to >650 °C, and 40Ar/39Ar ages of hornblende from Proterozoic rocks decrease from 1600 to 73 Ma toward these plutons. Barometric calculations from garnet-bearing metamorphic rocks suggest that this Cretaceous metamorphism took place at 3.5 to 5.0 kbar in the Old Woman Mountains. Similar pressures are obtained from hornblende barometry of Late Cretaceous granodiorite. 40Ar/39Ar and apatite fission-track data from the 73 Ma batholith indicate that the rocks exposed in the Old Woman Mountains cooled rapidly to below 350 to 300 °C by 70 ± 1 Ma and that moderately rapid cooling at rates of 10 to 30 C°/m.y. continued until after 60 Ma.
The presence of maximum Mesozoic temperatures on the borders of plutons, hornblende ages of ca. 73 Ma, and similar pressure estimates for plutons and country rocks indicate that the high temperatures needed to cause ductile deformation and amphibolite-facies metamorphism were related to heating from the intrusion of the 73 Ma Old Woman-Piute batholith. Amphibolite-facies metamorphism along pluton contacts was superimposed on the regional greenschist-facies metamorphism related to thrusting and crustal thickening. The extremely rapid cooling in the Old Woman Mountains between 73 and 71 Ma was due largely to conduction of heat into the country rocks, whereas protracted rapid cooling to temperatures below ambient temperatures estimated for the depth of emplacement was due to tectonic denudation at 1 to 2 km/m.y. until about 68 to 65 Ma.