ND AND SR ISOTOPIC DATA FROM AN ARC-ROOT PLUTONIC SUITE AND HOST ROCKS: BEAR MOUNTAIN INTRUSIVE COMPLEX, KLAMATH MOUNTAINS, CALIFORNIA

The ~148 m.y-old Bear Mountain intrusive complex (BMIC) consists of two principal plutonic units, the Blue Ridge and Bear Mountain intrusions. The Blue Ridge ultramafic/mafic unit chiefly includes dunite, wehrlite, hornblende-olivine clinopyroxenite, and olivine gabbro. Satellite bodies related to a widespread hornblende gabbro/diorite unit, which forms the bulk of the Bear Mountain pluton, intrude the Blue Ridge body. A smaller volume monzodioritic unit is interpreted to intrude the hornblende gabbro/diorite unit. All these units are intruded by small masses and dikes of tonalitic rocks. Initial epsilon Nd values for the BMIC vary from +5.4 to +8.1; initial Sr values range from 0.7030 to 0.7035; and delta 18O values vary from 6.8 to 9.9 per mil. No distinction among units can be made on the basis of Nd and Sr isotopes. However, the tonalitic rocks have delta 18O >8.4 per mil, whereas all other units are <7.6 per mil.

The BMIC intruded metabasalt, meta-argillite, and metaserpentinite of the Rattlesnake Creek terrane (RCT). Metabasaltic rocks of the inner dynamothermal aureole of the BMIC experienced partial melting synchronous with emplacement of the plutonic suite. Argillitic host rocks of the RCT have initial epsilon Nd from –4.9 to –8.9 and Sr from 0.7095 to 0.7141. Slaty argillitic rocks of the structurally lower Late Jurassic Galice Formation have initial epsilon Nd values of –0.9 to –2.5 and initial Sr values of 0.7066 to 0.7071.

The uniformity of isotopic ratios of the ultramafic and mafic parts of the BMIC argues for the lack of significant contamination by host rocks and for isotopically homogeneous mantle sources. However, the higher delta 18 O of the tonalitic rocks combined with their lower initial Sr and higher epsilon Nd values suggests an origin by partial melting of metabasaltic rocks. The source for the tonalites may be the migmatitic rocks of the inner aureole but alternatively could be structurally lower metabasites of the Josephine ophiolite.