Overview of Early Paleozoic Magmatism in the Eastern Klamath Mountains, California

WALLIN, E. T., University of Las Vegas, Las Vegas, NV, and N. LINDSLEY-GRIFFIN and J. R. GRIFFIN, University of Nebraska, Lincoln, NE

Igneous rocks of the Yreka and Trinity terranes record a complex history of magmatic events that occurred discontinuously over a 200-m.y. period between the Early Cambrian and the Middle Devonian. Lower Cambrian rocks occur as fault-bounded massifs along the northwestern margin of the Trinity terrane, and as tectonic blocks in melange of the Yreka terrane. The textures and structures of the strongly foliated, ductilely deformed Cambrian rocks indicate a complicated tectonic history prior to their juxtaposition with younger rocks of the Trinity and Yreka terranes. Samples dated as Cambrian (570-565 Ma) consist of tonalite and metagabbro; one tonalite contains inherited Precambrian zircon.

Ordovician ultramific and mafic rocks of the Trinity terrane range in age from 472 to 435 Ma. Two plagiogranites dated at 475 and 469 Ma are presumably also related genetically to the oceanic lithosphere represented by the ultramific rocks. A fault that emplaces Ordovician mantle tectonite of the Trinity terrane against Cambrian metagabbro is intruded by Late Silurian pegmatitic gabbro of the China Mountain pluton, indicating that the Cambrian and Ordovician rocks were juxtaposed before or during the Late Silurian. Isotopic dating and our recent mapping indicate that the Late Silurian magmatism was voluminous and widespread throughout the Trinity terrane and along its margins.

Basaltic to andesitic pillow lavas, flows and dikes occur locally within the Yreka and Trinity terranes. Within melange, these poorly dated occurrences may be Cambrian, Ordovician, or Silurian in age. However, volcanic rocks that overlie the melange are constrained to be Early to Middle Devonian in age and may be a minor northern manifestation of the well developed Devonian magmatic arc in the Redding terrane to the south. These minor Lower to Middle Devonian volcanics and the dikes that presumably fed them are undeformed, indicating that they were erupted after the Yreka and Trinity terranes were amalgamated. Small swarms of sheeted dikes related to this volcanism suggest eruption during post-amalgamation extension.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)