168 MA GRANULITE FACIES METAMORPHISM AND 154 MA REFRIGERATION IN THE SEIAD METAOPHIOLITE COMPLEX, RATTLESNAKE CREEK TERRANE, KLAMATH MOUNTAINS

The Rattlesnake Creek terrane (RCT) is an oceanic crust and mantle mélange overlain by a Late Triassic to Early Jurassic arc, which together constitute the basement for the Middle Jurassic western Hayfork arc terrane (WHT). Middle Jurassic Siskiyou metamorphism produced widespread amphibolite facies assemblages in the RCT, but granulite facies metamorphic rocks, previously of uncertain age, occur locally in deep levels of the RCT adjacent to the Condrey Mountain window (CMW).

The Seiad metaophiolite complex, which occurs in the lowest exposed level of the RCT immediately west of the CMW, contains granulite to upper amphibolite facies mineral assemblages in ultramafic, mafic, calcsilicate, and quartzofeldspathic lithologies. Geothermobarometry for mafic granulite yields 810-860 ºC and ≤ 9.5-10.0 kbar for exsolved pyroxene compositions and 965-980 ºC for pre-exsolved pyroxenes. Garnet amphibolite in the core of the complex equilibrated at 715 ºC, 7.3 kbar, in contrast to garnet amphibolite 2 km structurally above the complex at 660 ºC, 4.9 kbar.

U-Pb analysis of two rutile fractions from granulite yields a concordia age of 168.1 ± 1.4 Ma (MSWD of concordance = 0.025). This result demonstrates that the subordinate granulite facies rocks in deep levels of the RCT are a product of Siskiyou metamorphism, rather than relict from an older, cryptic metamorphic event.

⁴⁰Ar/³⁹Ar analysis of pargasitic amphibole in websterite yields a largely concordant age spectrum with a plateau age of 154 ± 2 Ma. This cooling age is consistent with numerous other published ⁴⁰Ar/³⁹Ar ages of 154-147 Ma for amphibole in the RCT.

Siskiyou metamorphism (172-167 Ma) in the RCT is generally ascribed to contraction along regional thrust faults and juxtaposition of the RCT and WHT, with heat being provided by magmatic activity in the formerly overlying WHT island arc (177-169 Ma). Elevated temperatures persisted in much of the RCT due to widespread intrusion of the Wooley Creek plutonic suite (167-154 Ma), being terminated at ~150 Ma by underthrusting of the Western Klamath and Condrey Mountain terranes. Siskiyou granulite metamorphism is found only where deeper levels of the RCT have been exposed by Neogene tilting and uplift around the CMW.