ORDOVICIAN ARC ROCKS OF THE YREKA SUBTERRANE, EASTERN KLAMATH MOUNTAINS, CALIFORNIA, AND POSSIBLE LINKS TO THE ALEXANDER TERRANE AND THE ARCTIC CALEDONIDES

The Yreka subterrane of the eastern Klamath Mountains hosts Late Ordovician blueschists and Devonian sandstones that have previously yielded detrital zircon (DZ) suites with “Baltica” age distributions including 1.46-1.55 Ga populations. New SHRIMP-RG U-Pb ages of 563±5 Ma (metagabbro), 560±5 and 565±6 Ma (plagiogranites) confirm the age of Neoproterozoic rocks (565-570 Ma) previously dated by Wallin et al. (1988, 1991).

New mapping, geochemistry and geochronology document that the SE region of the Yreka subterrane (Kangaroo Lake to Lover’s Leap) consists of structurally intact island arc affinity hornblende gabbros through diorites to quartz diorites cut by tonalite and plagiogranite veins. Associated volcanic rocks are basalts, andesites and dacites. Crystallization ages range from 461± 5 to 437±4 (N=8) and overlap blueschist ages. Low Ti, K, Zr, LILE > HFSE and depletion of Ta and Nb document a calc-alkaline island arc affinity.

In support of prior work, we found additional evidence for a profound unconformity above the Ordovician arc rocks: Cobble to boulder conglomerates contain clasts of underlying Ordovician rocks and red beds have Ordovician DZ populations (MDA Early Devonian). Basalt flows, limestone cobble conglomerate (mostly Ordovician fossils in clasts) and map-scale olistolith/avalanche blocks of fossiliferous (mostly Silurian) limestone, thin-bedded chert and shallow water sandstone comprise the chaotic Gregg Ranch complex. Two sandstone blocks yield Ordovician magmatic arc DZ and Baltic shield DZ signatures. Overlying undeformed deep water strata of the Gazelle Fm contain mostly Late Devonian zircon as does the Late Devonian Bragdon Fm that overlies the adjacent Trinity peridotite.

Considered together, existing and new U-Pb crystallization ages and DZ results indicate that the Neoproterozoic plutons and Ordovician arc rocks of the Yreka subterrane correlate with the basement of the Alexander terrane and its Ordovician Descon arc. The Yreka Neoproterozoic intrusions are age equivalent to ~750-550 Ma arc complexes accreted to Baltica during the Timan orogeny (~550 Ma). Displaced parts of this belt in Arctic Alaska and Alexander terrane likely originated within the northernmost prolongation of the Caledonide belt as it crossed through the Timanides of Baltica.