SPECULATIONS ON MAGMA ADDITIONS AND TERRANE ACCRETIONS IN THE KLAMATH MOUNTAINS, CA/OR

We present new data on arc magmatism and terrane area in the Klamath Mountains Province, CA/OR, to evaluate the space/time evolution of Mesozoic magmatism and terrane accretion. A compendium of >300 U-Pb crystallization ages and limited K-Ar and Ar-Ar ages are integrated with measurements of the area and volume of plutonic bodies and areas of accreted terranes within the province. These data were combined with digital elevation models to derive minimum volume estimates of individual plutons and areal extents of host terranes. Because the area of the KMP increased over time due to successive accretion events (mostly in the Jurassic), we derived magma addition rates (MARs) by normalizing pluton area to mapped/projected terrane area at the time of pluton emplacement.

The total area of the KMP is 29,807 km²; 4,981 km² (17%) of the KMP is underlain by exposed plutonic rocks. Magmatic flare ups occurred at 430–400 Ma, 174–154 Ma, and 150–136 Ma, ranging between 16-31 m.y. in duration. The two Mesozoic events could be viewed as a single flare up with a minor pause; however, isotopic and trace element data indicate distinct, tectonically-induced changes in the lower crust between the two events. This difference is also evident in the diversity of rock types and isotopic signatures within mid- to late-Jurassic plutons compared to the relative uniformity of mainly TTG-type early Cretaceous plutons. The Paleozoic event added > 1,893 km³ of plutonic rock (MAR = 0.0086 km³/myr/km²). The Middle to Late Jurassic event was the largest with >25,289 km³ added (MARs = 0.052 km³/myr/km²). Late Jur-Early K additions were >9,952 km3 (MAR = 0.020 km³/my/km²).

Excluding the much older Paleozoic magmatism, we suggest the following: 1) Jurassic to Early Cretaceous MAR’s for the KMP are similar to those for the northern Sierra Nevada; however, mid to Late Cretaceous magmatism that represents the largest additions to the Sierra Nevada batholith is not observed in the KMP. 2) Episodes of Jurassic terrane accretion are broadly correlated with increased MAR, indicating a relationship between deep crustal modification and potential thickening, which in turn controlled the crustal response to heating, and resulting magmatism.