NEW GEOLOGIC MAPPING OF THE PRECAMBRIAN TONALITE ASSOCIATED WITH THE ORDOVICIAN TRINITY OPHIOLITE, KLAMATH MOUNTAINS, CALIFORNIA

The northern part of the Trinity Terrane near Callahan, California consists of a tectonostratigraphic sequence of Ordovician ultramafic rocks and amphibolitic gabbro, Neoproterozoic mylonitic tonalite, and undated basalt of Devonian or earlier age. While the Ordovician rocks appear to make up a parts of an ophiolitic sequence, the Neoproterozoic rocks are clearly not part of the ophiolite. Although previously published mapping indicates the tonalite is a continuous tectonic unit between amphibolitic gabbro and undated basalts, some field observations suggest that the tonalite is discontinuous and may be blocks in the mélange of the overlying Gregg Ranch Complex.

To resolve this question we present detailed geologic maps and geochemical data of the Neoproterozoic tonalite outcrops along Scott Creek, Rail Creek, and the Mountain House area. Although the area was mapped by Lindsley-Griffin and her students, the present study focuses on structural deformation within the tonalite and the nature of contact between the tonalite and overlying basalts, Gregg Ranch Complex mélange, or Gazelle Formation.

The tonalite is retrograded, with hornblende converted entirely into chlorite, and locally crosscut by greenschist-facies mafic dikes that seem to be associated with a brittle brecciation event. The mylonitic foliation is gently folded in each individual mapped area; however, different areas do not appear to share the same fold axis. The upper contact of tonalite is a faulted contact with Gregg Ranch mélange in places; in others, it is in apparent thrust contact with the undated basalt. Geochemical data from the tonalities show a high La/Lu ratio.

We will present observations on several questions, including: What is the relationship of the Neoproterozoic tonalite to the underlying Ordovician ophiolite and overlying mélange? What is the cause for the high La/Lu ratio in the tonalite? Can the source of the tonalite be determined using geochemical modeling?