THE INTERPLAY BETWEEN OROGENIC MODIFICATION OF THE LOWER CRUST AND PLUTONISM FROM ZIRCON AGE, HF, AND O DATA, KLAMATH MOUNTAIN PROVINCE, CALIFORNIA AND OREGON

Among Doug Yule’s many contributions to Klamath Mountain geology is the geochronology of Jurassic magmatism in the Western Klamath terrane, which highlighted the importance of distributed Middle Jurassic intrusions and the Middle–Late Jurassic Chetco complex (aka Illinois River batholith). The latter results laid the groundwork for recent petrologic and geochronological comparisons of Jurassic plutonism presented here.

Middle Jurassic magmatism began with ca. 178–172 Ma deposition of arc apron sediments of the western Hayfork terrane (WHT) upon ophiolitic mélange basement of the Rattlesnake Creek terrane (RCT). Latest Middle Jurassic contraction placed the WHT/RCT assemblage beneath older terranes to the east: the “Siskiyou orogeny”.

Between 172 and ca. 156 Ma (U-Pb Zrn), plutonism spanned the province. Isotopically, two groups of plutons stand out based on ε_Hf and δ¹⁸O(Zrn). The outboard 167–159 Ma Chetco complex and four small, widely dispersed 172–168 Ma inboard plutons display near mantle-like ε_Hf (>+12) and δ¹⁸O (< +6.3‰). In contrast, inboard Ironside Mtn and Wooley Creek suite (WCS) plutons display ε_Hf < +12 and δ¹⁸O > +6.3‰. In particular, the WCS displays ‘draw-down’ toward crustal isotopic values (ε_Hf to -3; δ¹⁸O to +9.7‰) similar to those seen in arc flare-ups. Within this group, inherited zircon is mainly Mesozoic. This plutonic activity ceased with onset of the Nevadan orogeny (beginning ca. 157–155 Ma).

Renewed arc magmatism was 152–136 Ma. All of these plutons, from gabbro to granite, display limited ranges of ε_Hf (8.6–12.8) and δ¹⁸O (5.8–6.8‰). Within this group, zircon inheritance includes Tr, Pz, and Proterozoic grains.

The data suggest that pre- and perhaps syn-Siskiyou magmas intruded lower crust dominated by Middle Jurassic igneous rocks, whereas post-Siskiyou, pre-Nevadan magmas interacted with metasedimentary rocks in the lower crust, leading to isotopic draw down. Post-Nevadan magmas encountered isotopically primitive lower crust, but also acquired a range of old inherited grains (assimilation of Galice Fm host rocks?). The data attest to rapid changes in location, composition, and intensity of Jurassic–Early Cretaceous magmatism due to orogenic modification of the lower crust.