MODIFICATION OF ARC LOWER CRUST: HF ISOTOPE EVIDENCE FOR SPATIAL/TEMPORAL CHANGES TO SOURCES OF JURASSIC PLUTONS, KLAMATH MOUNTAINS, CA AND OR

New Hf-in-zircon data combined with existing isotopic data indicate regional spatial and temporal variations in lower crustal sources for Middle to Late Jurassic plutons over several hundred km in the Klamath Mountains. Bulk-rock Sr, Nd, and oxygen data on Jurassic plutons indicate mainly mantle sources prior to 170 Ma, but mantle and metasedimentary sources from 168–154 Ma. One example of lower crustal contributions is the 159–156 Ma Wooley Creek batholith–Slinkard pluton (WCb/Sp) magmatic system. Two-mica and biotite granite in this system have average ε_Hf (zircon) of -3.0 and 2.4, respectively. In contrast, ε_Hf in metaluminous gabbro through granite typically ranges from 6.2 to 11.7. The range of ε_Hf in most samples is larger than analytical uncertainty (±1.5–2 ε units). The data indicate a lower crustal metasedimentary source for the granites and lower crustal contamination of mantle-derived arc basalts prior to solidification of the WCb/Sp in the middle crust.

Further north, average ε_Hf values of intermediate rocks of the 160–156 Ma Ashland, Grayback, and Thompson Ridge plutons are 20.0, 18.0, and 19.3, respectively, and a late-stage granite in the Ashland pluton has ε_Hf of 10.4. Zircon from dioritic and tonalitic rocks of the Chetco complex yield ε_Hf from 7.7 to 17.5. A mafic selvage of the Grants Pass pluton yielded a U-Pb age of 147.9 ± 1.1 Ma and average ε_Hf of 16.8. Three samples from the Bear Mountain complex yielded average ε_Hf of 11.5 (gabbro), 11.2 (monzodiorite), and 10.5 (tonalite). As in the WCb/Sp, the range of ε_Hf typically exceeds analytical uncertainty.

It was expected that all 168–154 Ma plutons would show evidence of mixed mantle and crustal sources. Instead, the northerly suite of plutons have depleted-mantle Hf signatures except for the Ashland pluton granite. Thus, the Ashland granite indicates the presence of deep-seated metasedimentary rocks, but it appears that mafic/intermediate magmas in the northerly plutons were not significantly contaminated by lower crust, or that the lower crust was distinct from crust that contaminated the WCb/Sp. These results indicate the utility of use of Hf (zircon) isotope signatures to trace magma sources but indicate the need for oxygen isotope (zircon) data as a definitive indicator of metasedimentary contributions to arc magmas.