OPEN-SYSTEM BEHAVIOR IN A CRUSTAL-SCALE MAGMA COLUMN, KLAMATH MOUNTAINS, CALIFORNIA

The Wooley Creek batholith (WCB) and Slinkard pluton (SP) are mid- to upper-crustal components of a late-Jurassic, trans-crustal magma system. Tilting and erosion have exposed > 11 km of structural relief in the system. The main stage may be divided into an older lower zone (SP & lower WCB; ca. 159 Ma), a younger upper zone (upper WCB; ca. 158–156 Ma) and a complex central zone in the WCB, which represents the transition between the lower and upper zones. Xenoliths in the lower and central zones display bulk-rock ε_Nd and ε_Hf (zircon) values that correlate with rocks in the three host terranes; therefore, these xenoliths preserve a ghost stratigraphy of the host terranes. In many individual pluton samples, zircon Hf and oxygen isotope data display ranges too large to represent uniform magma compositions, and data consistent with an uncontaminated mantle-derived magma are scant. Zoning of Zr and Hf in augite and hornblende show that zircon crystallization was at temperatures (T) near or below 800°C, which are lower than T of emplacement. Thus, diversity in zircon isotope compositions in individual samples indicates in-situ crystallization from melts that were heterogeneous in terms of Hf, and locally O isotopes. These and published data reveal that the magma compositions were modified first in a deep-seated MASH-zone at T > zircon stability, yielding elevated δ¹⁸O and lowered ε_Hf in the magmas. At the level of emplacement, further differentiation, particularly assimilation-fractional crystallization driven by episodic influx of mafic magmas, occurred locally as individual magma batches interacted with partial melts from host-rock xenoliths. This piecemeal assimilation was accompanied by zircon crystallization, resulting in the heterogeneous isotopic signatures. Magmatism ended at ca. 156 Ma with emplacement of late-stage, isotopically evolved, two-mica granite in the SP and biotite granite in the upper WCB. These magmas probably represent partial melts from the deep-crustal MASH environment.