NEW LOOK AT NEOPROTEROZOIC ROCKS OF THE YREKA TERRANE, KLAMATH MOUNTAINS, N. CALIFORNIA

Interest in the “Baltic” elements of Cordillera terranes and their possible origin in the current Arctic led to a closer look at parts of the eastern Klamath Mountains. Here, Ordovician blueschists and Devonian sandstones of the Yreka terrane are enriched in non-western Laurentian detrital zircon (DZ) populations including 1.46- 1.55 Ga U-Pb ages specifically linked to Baltica. Wallin et al. (1988; 1995) dated adjacent Neoproterozoic (565-570Ma) intrusive rocks, ages common to the ~ 750-550 Ma arc complexes accreted to the margin of Baltica during the Timan orogeny (~ 550 Ma) and in possibly displaced parts of this belt in the Arctic Alaska and Alexander terranes.

We investigated the contact relationships between map units in the region near Kangaroo Lake and Lover’s Leap to sample additional rocks and strata for U-Pb geochronology, geochemistry and detrital zircon studies. We confirmed that undated ultramafic rocks of the Trinity terrane and the coarse-grained Silurian gabbros that intrude them (Wallin et al., 1995) are in fault contact with the Neoproterozoic igneous rocks. The Neoproterozoic consists of structurally intact intrusive, extrusive and hypabyssal rocks. Intrusive rocks vary from hornblende gabbros through diorites to quartz diorites. Tonalite and plagiogranite veins cut this succession. Geochemistry indicates a basalt-andesite-dacite calc-alkaline assemblage with island arc affinities– low Ti, low K, low Zr, and LILE concentrations relatively higher than HFSE. Mafic rocks are depleted in Ta and Nb. Following Rohr et al (1975), we observed that the Neoproterozoic complex is unconformably overlain by conglomerates and sandstones, basalt and limestone of Ordovician and Silurian age. Conglomerates, perhaps coeval with carbonate buildups, contain mainly limestone and volcanic rock clasts (ranging from basalt to dacite). The limestone clasts have yielded mostly Ordovician fauna (Rohr et al., 1975). All of the rocks appear to be overlain (or overthrust) by the adjacent Devonian Gazelle Fm.

Detrital zircon geochronology of Paleozoic sediments overlying the Neoproterozoic arc sequence are compared to those to the Yreka terrane and to established DZ signatures of coeval strata of the Arctic region.