JURASSIC ARC MAGMATISM IN THE KLAMATH MOUNTAINS, CALIFORNIA-OREGON

Middle to Late Jurassic plutonic and volcanic activity in the Klamath Mountain province records a 25 m.y.-long history of arc-related magmatism traditionally interpreted as straightforward island-arc activity, with individual plutons used to constrain the timing of tectonic imbrication. In fact, Jurassic magmatic rocks can be divided into specific age groups, and these groups correspond to distinctive isotopic compositions and geochemical trends: (1) 179 to 170 Ma western Hayfork calc-alkaline activity. Following or during thrusting, (2) 170 to 169 Ma alkali-calcic Ironside Mountain plutonism. After further thrusting, (3) 168 to 159 Ma Wooley Creek calcic to calc-alkaline plutonism and synchronous outboard formation of the Josephine ophiolite (JO). (4) West of the JO, development of the calc-alkaline Rogue-Chetco arc at ~159 Ma. Following or during 155 to 150 Ma Nevadan thrusting (5) 153 to 149 Ma calcic to alkali-calcic Bear Mountain intrusive complex. (6) Widespread, post-Nevadan calc-alkaline to calcic plutons. Episodes 2 and 5 are characterized by low-H2O magmas, inverted pigeonite, and magmatic trends toward K2O and HFSE enrichment. Along with episode 1, they display low delta 18O, high epsilon Nd, and low initial 87Sr/86Sr. In contrast, episodes 3 and 4 are characterized by high-H2O magmas, elemental abundances typical of calc-alkaline arcs, and (for 3) isotopic values indicative of significant crustal assimilation and melting. We suggest that episodes 2 and 5 are unusual in the Klamath arc because they follow or are synchronous with contractional deformation. Primitive magmas stalled at deep crustal or Moho? levels, where they underwent fractionation toward K2O enrichment but were not significantly contaminated by crust. Episodes 1, 3, and 4 were probably in extensional environments, with higher level magma storage and significant crustal interaction.