NEW AGES AND TECTONIC HISTORY FOR THE TRINITY TERRANE, A POLYGENETIC OPHIOLITIC COMPLEX, KLAMATH MOUNTAINS, CALIFORNIA

New age data from the ophiolitic Trinity terrane shows that most map units previously reported to be Vendian (Ediacaran) are actually Ordovician. The discredited age dates were from gabbro of the Trinity ophiolite. New work yielding multiple ages suggests that the older ophiolite sequence of gabbro-diabase-basalt is Ordovician. Our revised interpretation of Trinity terrane stratigraphy is:

1) Scott River block – Ediacaran oceanic plagiogranite formed 571-565 Ma;

2) China Mountain block – Early to Late Ordovician oceanic layered gabbro, dikes + plagiogranite, pillow basalt;

3) Trinity River block – pre-Early Ordovician mantle intruded by Early Ordovician and Siluro-Devonian plutons;

4) Siluro-Devonian SSZ (Supra-Subduction Zone) ophiolite sequence of pegmatitic gabbro-sheeted dikes-pillow basalt built about 435-412 Ma after 1-3 were juxtaposed.

5) Trinity Alps block, sutured to 1-4 after the Siluro-Devonian SSZ component formed.

Ordovician tectonic history is intraoceanic but these rocks exhibit a wide range of tectonic styles and deformation intensity. Because the ages do not correlate with type and degree of deformation we suspect formation within a complex oceanic rifting and transform fault environment involving multiple microplates and triple junctions.

The Forest Mountain terrane west of Yreka represents a separate slab of oceanic lithosphere that underthrust the Yreka-Trinity composite terrane in Early Devonian (~407-398 Ma).