ANATEXIS AND DYNAMOTHERMAL METAMORPHISM DURING THE EMPLACEMENT OF A SYNTECTONIC OCEANIC ARC-ROOT PLUTON, KLAMATH MOUNTAINS, CALIFORNIA

The Bear Mountain intrusive complex, Klamath Mountains, California, is an oceanic arc-root plutonic suite that intruded rocks of the Rattlesnake Creek terrane during the contractional Late Jurassic Nevadan orogeny. This wehrlite/dunite-gabbro-diorite-tonalite suite imposed an approximately kilometer-wide contact aureole on a sequence of interlayered metabasic and metasedimentary rocks as well as a widespread serpentinite-matrix melange. This aureole has been subdivided into two zones based on textural changes in the metabasic rocks. Inner zone metabasites are characterized by megascopic hornblende and/or pyroxene and by penetrative foliation or mineralogical layering. Hornfelsic rocks are scarce, whereas hornblende schist, streaky, veined, and migmatitic amphibolite, and locally mafic gneiss (pyroxene hornfels-facies rocks) are common. The leucosome in the migmatitic amphibolitic rocks is tonalitic (commonly containing biotite, hornblende, and/or clinopyroxene) and is locally involved in the deformation. Thus anatexis of the aureole rocks accompanied deformation related to the emplacement of the plutonic suite. New petrochemical data indicate that (1) inner zone amphibolites are broadly tholeiitic, whereas outer zone hornfels is variably tholeiitic and alkaline, (2) the leucosomes are enriched in Sr, Ba, Zr, and Al2O3 and deleted in Y and transition metals compared to the amphibolitic rocks. K2O contents in the leucosomes are variable. When combined with the enrichments in Sr, Ba, and Al2O3, this suggests that most leucosomes are at least partial cumulates from which melt was extracted. Kinematic indicators are locally present in the aureole and suggest that during the late-stage emplacement history of this composite intrusion that the plutonic rocks moved downward relative to the rocks of the inner aureole. This chiefly ultramafic-mafic pluton may have foundered due to negative buoyancy (i.e., related to the abundance of dense cumulate rocks). A prominent positive Bouguer gravity anomaly centered on the Bear Mountain intrusive complex indicates the presence of abundant dense rocks at depth.