BATHOLITH EVOLUTION IN A 40-KM-THICK ACCRETIONARY COMPLEX, KLAMATH MOUNTAINS, CA

The Wooley Creek batholith and Slinkard pluton (WCb/SP) are part of a vertically extensive magmatic complex intruded into amalgamated terranes of the Klamath Mountains province. At the time of magmatism (162–156 Ma), the crust was ~ 40 km thick. Exposures of the WCb/SP extend from paleo-depths of ~10 km to at least 25 km and consist of lower gabbro–tonalite, upper tonalite–granite, and a narrow transition zone. Porphyritic dikes in the roof zone indicate upward escape of magmas from the system. Two late-stage (156 Ma) granitic bodies, two-mica granite of the SP and biotite granite intruding the upper WCb, have ε_Nd 2.1–4.5; ε_Hf (zircon) -3.0–+2.4; initial ⁸⁷Sr/⁸⁶Sr 0.7039–0.7046; and δ¹⁸O 10.3–12.2. The chemical and isotopic compositions of these (late-stage) granites are consistent with an origin by lower-crustal partial melting of metasedimentary rocks. Basaltic magmas coeval with the WCb/SP were derived from depleted mantle (ε_Nd +10; ε_Hf ~ +19; ⁸⁷Sr/⁸⁶Sr ~ 0.7027). However, these primitive isotopic values are absent in the WCb/SP. Instead, gabbroic through granitic rocks have ε_Nd 2.8–4.5; initial ⁸⁷Sr/⁸⁶Sr 0.70416–0.70474; and δ¹⁸O 8.0–9.9‰. Average ε_Hf (zircon) varies from 2.9–11.7, with intra-sample variation as much as 10 epsilon units. In the upper zone, ε_Hf (zircon) decreases upward, corresponding to upward (1) zoning from tonalite to granite and (2) decrease in abundance of mafic enclaves. The data suggest that magma mixing affected the base of the upper zone, but not the upper part. Refractory, migmatitic xenoliths are locally common and have ε_Nd from +7.6 to -23.7 and ε_Hf (zircon; 159Ma) from +8.9 to -40.9. Evidence for contamination of host magmas is found locally in samples with low ε_Hf (zircon) values.

Taken in total, the data indicate that the WCb/SP magmatic system spanned virtually the entire crustal column. The isotopic signatures of most of the magmas were derived from lower crustal hybridization (~40–30 km paleodepth) of mantle-derived basaltic magmas with melts/rocks that were the source of the late-stage granites. These hybrid magmas were emplaced in the 25–10 km-thick WCb/SP intrusive complex. Within the emplacement zone, local assimilation and magma mixing occurred. Lastly, magmas from the upper parts of the system migrated upward as dikes and probably fed volcanic eruptions.