DEVELOPMENT OF JURASSIC ARC COMPLEXES IN THE KLAMATH MOUNTAIN PROVINCE, CA AND OR

Jurassic magmatism in the Klamath Mountain province (KMp) occurred during episodic changes from contractional to transtensional arc settings. The calc-alkalic to adakitic Middle Jurassic western Hayfork arc (179-171 Ma) was deposited on ophiolitic mélange basement. This arc was extinguished by regional thrusting along the Wilson Point fault and was nearly immediately followed by granulite facies metamorphism of the arc basement (~168 Ma). Heat for this high-T, moderate-P metamorphism may have been related to formation of a slab window or to relamination of subducted metasedimentary rocks. Subsequent late Middle Jurassic magmatism consisted of the outboard Rogue-Chetco arc (169–154 Ma), the inboard Josephine ophiolite (JO; 165-162 Ma) and retro-arc magmatism of the Wooley Creek belt (170–156 Ma). Rogue-Chetco rocks show mainly isotopically primitive arc tholeiite affinities, whereas the JO ranged from E-MORB and IAT to boninitic in character. Wooley Creek belt mafic-intermediate rocks have calcic to alkali-calcic characteristics and clear isotopic evidence for lower crustal contamination; granites in this belt were derived from lower crustal metasedimentary rocks. Nevadan orogenesis disrupted late Middle Jurassic magmatism. Post-Nevadan activity consisted of outboard 152–144 Ma, predominantly mafic, calcic/calc-alkalic plutonism and inboard 142–136 Ma tonalitic–granodioritic (TTG) plutonism. The former magmas were isotopically primitive with evidence for local shallow crustal assimilation, whereas the latter TTG rocks were derived by partial melting of underthrust, lower crustal amphibolitic (± garnet) terranes with scant evidence for significant metasedimentary input.

In sum, Jurassic KMp magmatism reflects development of arc systems, at least some of which were transtensional, that were disrupted by regional contraction and terrane amalgamation. The two post-amalgamation arc systems spanned the width of the province. In each, outboard mantle-derived magmas underwent minor or no interaction with deep crustal rocks, whereas inboard mantle-derived magmas interacted substantially with the lower crust, and granitic/TTG magmas were sourced in the lower crust.