Open- and closed-system characteristics of a tilted plutonic system, Klamath Mountains, California

Abstract

The Wooley Creek batholith and Slinkard pluton are adjacent plutons with identical U-Pb zircon ages of 161 +4/−2 Ma. The plutons dip toward each other, which suggests that they are connected in the subsurface. Geologic and thermochemical evidence shows that the Slinkard pluton is the structurally lowest part of the system and the Wooley Creek batholith the structurally highest, with a total structural relief of at least 15 km. The wall rocks adjacent to the highest level of the Wooley Creek batholith are intruded by mafic to felsic dikes derived from the subjacent pluton. The Slinkard pluton is predominantly quartz diorite but grades from two-pyroxene diorite to sparse muscovite-biotite granite and shows little evidence of contamination except near contacts. The Wooley Creek batholith grades upward from two-pyroxene gabbro to biotite-hornblende granite and has a central zone in which injected mafic magma was trapped and mixed. Major and trace element data are consistent with closed-system fractionation of the Slinkard pluton and lower (adjacent) Wooley Creek batholith, but with open-system differentiation of the upper Wooley Creek batholith and roof-zone dikes. Andesitic roof-zone dikes show trace element enrichment similar to the central mixed zone of the Wooley Creek batholith and may have differentiated at that central level of the system.