Comparison of the Central Metamorphic Belt and Trinity terrane of the Klamath Mountains with the Feather River terrane of the Sierra Nevada

Abstract

The Central Metamorphic Belt and Trinity terrane of the Klamath Mountains and the Feather River terrane of the northern Sierra Nevada share strong similarities in their protolith types, metamorphic parageneses, structures, ages of formation and metamorphism, and relations with surrounding units. These terranes consist of ductilely deformed ultramafic to mafic plutonic and volcanic rocks and minor oceanic sedimentary rocks interpreted as oceanic lithosphere. Ultramafic rocks in the Trinity terrane and the Feather River terrane were hydrated and metasomatized under lower amphibolite-facies conditions. Mafic and sedimentary rocks in the Central Metamorphic Belt and the Feather River terrane contain upper greenschist to amphibolite-facies parageneses formed during Devonian time. Phase assemblages and mineral chemistries indicate peak PT conditions of 5000 to 650 :i: 50°C and 500 :i: 300 MPa for both terranes. The similarities imply that the Klamath Mountains and Sierra Nevada share a common early to middle Paleozoic history. The Trinity terrane/Central Metamorphic Belt represents an arc basement/subduction zone couple; the Feather River terrane may also represent such a couple. INTRODUCTION The Klamath Mountains and the Sierra Nevada of the formational and metamorphic events. Phase relations and mineral North American Cordi\lera contain a variety of rocks that are compositions determined by electron probe microanalysis coninterpreted as magmatic arcs, oceanic crust, and subduction comstrain the PT conditions of metamorphism. Analytical techplexes, and formed during different phases and styles of ocean/ niques used in this study are discussed in Peacock and Norris continent interaction. This chapter summarizes the results of (1989). Our observations, combined with those of other authors, detailed studies of the Central Metamorphic Belt and Trinity permit correlations of the Central Metamorphic Belt and Trinity terrane of the Klamath Mountains, and the Feather River terrane terrane of the Klamath Mountains with the Feather River terrane of the Sierra Nevada, to i\lustrate the similarities and differences of the Sierra Nevada, and provide the petrogenetic framework in between early Paleozoic rocks of the Klamath Mountains and which to discuss the early to middle Paleozoic history of this area. Sierra Nevada. Textural relations, determined by back-scattered To understand the geologic relations among the Central electron and optical microscopy, constrain the sequence of deMetamorphic Belt, the Trinity terrane, the Feather River terrane, and their surrounding units, the fo\lowing section summarizes the .Present address: Department of Geology, Stanford University, Stanford, early to middle Paleozoic rocks in both mountain belts. The California 94305-2115. descriptions of the Central Metamorphic Belt, the Trinity terrane, Hacker, B. R., and Peacock, S. M., 1990, Comparison of the Central Metamorphic Belt and Trinity terrane of the Klamath Mountains with the Feather River terrane of the Sierra Nevada, in Harwood, D. S., and Miller, M. M., eds., Paleozoic and early Mesozoic paleogeographic relations; Sierra Nevada, Klamath Mountains, and related terranes: Boulder, Colorado, Geological Society of America Special Paper 255.