AGE AND ZIRCON INHERITANCE OF KLAMATH MOUNTAIN PLUTONS: IMPLICATIONS FOR CHANGING SOURCE REGIONS THROUGH TIME

Middle Jurassic to Early Cretaceous magmatic events in the Klamath Mountain (KM) province may be distinguished entirely on a geochronologic basis, but subtle differences exist among nearly coeval units. Moreover, some events are separated by orogenic activity (thrusting) that should modify the composition and “stratigraphy” of the crust. In Middle Jurassic time, magmatism was dominated by the tholeiite to calc-alkaline western Hayfork arc (179 to 171 Ma). Immediately following Wilson Point thrusting, the K-rich Ironside Mountain suite was emplaced (170 to 168 Ma) and it was followed by the calcic to alkalic Wooley Creek suite (166 to 156 Ma). The latter suite contains distinctive 180 to 186 Ma zircon inheritance (but lacks Precambrian inheritance) and has Sr and oxygen isotopic signatures indicative of crustal melting and assimilation. The data suggest that the assimilated crust was metasedimentary (high δ¹⁸O), but contained no significant ancient cratonal component. Following thrusting on the Orleans fault, the Siskiyou suite, a group of calcic to calc-alkalic plutons was emplaced in the western part of the province (151 to 143 Ma), granodioritic plutons (139 to 136 Ma) were intruded locally, and a cluster of tonalite-trondhjemite-granodiorite (TTG) plutons was emplaced in the east-central KM (142 to 136 Ma). The Siskiyou suite plutons and one granodioritic pluton (Grants Pass; Harper et al. 1994; JGR) contain inherited Proterozoic zircons, and laser ablation ICP-MS dating shows additional inheritance at 153-155, 168, 172, ~200, and 416 Ma. These plutons intrude meta-clastic Galice Fm. rocks whose detrital zircon (Miller et al., 2003; GSA Seattle) is virtually identical to the inherited zircon in the plutons. The TTG plutons also contain zircon inheritance, from 146 to 156 Ma, but older inheritance is absent. The distinctive zircon inheritance of three plutonic suites in the KM demonstrates the importance of melting and/or assimilation of crustal rocks during magma genesis. Moreover, the assimilated crust varied as a function of age and possibly of location. In the Wooley Creek suite, the inherited zircons represent a magmatic event unknown in the KM. Thus, zircons from KM plutons provide information about changes in the crustal column and its composition that cannot be deduced from surface exposures.