PETROLOGY OF THE BEAR MOUNTAIN INTRUSIVE COMPLEX, KLAMATH MOUNTAINS, CALIFORNIA: EXPLORING THE ROOTS OF AN OCEANIC-ARC PLUTONIC SUITE

The Bear Mountain intrusive complex (BMIC) was emplaced into accreted oceanic terranes of the Klamath Mountains province at ca. 148 Ma. The two principal units of the complex are the Blue Ridge ultramafic-mafic intrusion and Bear Mountain composite pluton. The former consists of dunite, wehrlite, olivine +/- hornblende clinopyroxenite and subordinate gabbro. The latter is subdivided on the basis on mineral assemblage and geochemical compositions into a hornblende-rich gabbro/diorite unit and a hornblende-poor dioritic to monzodioritic unit. Late-stage tonalitic to granitic dikes and small intrusive masses intrude these older intrusive bodies. In the surrounding metamorphic host rocks, at least two sets of synplutonic dikes are recognized. One is distinguished by conspicuous clinopyroxene phenocrysts or altered equivalents, whereas the other is plagioclase- +/- hornblende-phyric.

The Blue Ridge intrusion is chiefly cumulate in origin, with high CaO and MgO, and low Al2O3 contents. The Mg# of gabbroic rocks (0.64 to 0.76) is identical to that of the clinopyroxene-phyric dikes, which are interpreted to be related to the ultramafic-mafic intrusion and may be representative of parental magma. This parent was MgO-rich basalt (10 to 15 wt %), had slightly LREE-enriched patterns and no Eu anomaly, and was H2O-rich, as indicated by primary hornblende in the cumulates.

Rocks of the composite Bear Mountain pluton are distinct from the Blue Ridge in Mg# (0.4 to 0.54). The gabbroic parts of the pluton are chiefly cumulate in origin (plag ± hornblende), as shown by CaO (up to 14 wt. %) and Al2O3 contents (up to 27 wt. %) and REE patterns, typically with positive Eu anomalies. Plagioclase-phyric dikes in the host rocks could be parental to the cumulate gabbro, which is consistent with hornblende as an early phase in both. If so, the parent was evolved, high-Al basalt. In contrast, the monzodioritic part of the pluton shows evidence for in-situ enrichment of K2O, Rb, Ba, and Zr in an H2O-poor mafic (andesitic?) magma.

Parental magmas to the BMIC appear to represent stages of differentiation of arc-related basaltic magmas, from primitive MgO-rich basalt in the Blue Ridge, evolved, H2O-rich, high-Al basalt in the Bear Mountain gabbro/diorite, and evolved, H2O-poor andesite in the monzodiorite.