Bear Mountain Igneous Complex, Klamath Mountains, California: an Ultrabasic to Silicic Cale-Alkaline Suite

Abstract

The Bear Mountain igneous complex, Klamath Mountains, California, can be divided into distinct lithologic suites (order according to apparent relative age): (1) satellitic masses of clinopyroxene-rich ultramafic and gabbroic rocks with subordinate dunite and hornblende-plagioclase pegmatoid; (2) two-pyroxene-biotite diorite and monzodiorite; (3) heterogeneous hornblende-rich rocks varying from gabbro to diorite; (4) leucocratic rocks, chiefly consisting of biotite tonalite and granodiorite; and (5) late dikes (mafic to felsic). Elongate masses of unit (1) flank a composite pluton consisting of units (2–4), while the late dikes (unit 5) intrude the adjacent country rocks. The rocks of the complex invaded an ophiolite allochthon during the Late Jurassic Nevadan orogeny, and well-defined contact aureoles surround the complex. Lower greenschist facies rocks, chiefly metabasalt, impure siliceous metasedimentary rocks, and serpentinized peridotite, have been dynamothermally metamorphosed to mineral assemblages indicative of hornblende-hornfels facies and locally pyroxene-hornfels facies. The emplacement of the igneous complex was chiefly by forcible shouldering aside, although local tectonic features such as faults in the ophiolite allochthon were instrumental in the emplacement history. The ultramafic and gabbroic rocks are interpreted as crystal cumulates of a fractionated basaltic magma. Mineral compositions and whole-rock chemical characteristics of the proposed cumulates suggest that the Mg/Fe ratio of the parental basaltic liquid was high. The activity of silica was low, while water vapor pressure apparently increased through time until it was moderately high during the late magmatic stage. These cumulates were subsequently remobilized during lateral tectonic compression and emplaced higher in the crust as hot, semisolid aggregates. A diverse array of data, including pyroxene compositions, major-, minor-, and rare-earth-element abundances and field relations, suggest that the two-pyroxene-biotite diorite/monzodiorite unit was consanguineous with the clinopyroxene-rich ultramafic and gabbroic rocks. The diorite/monzodiorite unit, therefore, is an intermediate differentiate of an early primitive basalt. Furthermore, major-, trace, and rare-earth-element data characteristic of the diorite/monzodionte unit indicate strong similarities to low-Si andesite and clearly suggest a calc-alkaline affinity. Age relations indicate that the hornblende-rich and leucocratic units are younger and represent the intrusion of other magmas into the same igneous locus. Petrographic and geochemical data from the hornblende-rich unit suggest recrystallization from hydrous magmas similar in composition to high-Al basalt and basaltic andesite. The leucocratic suite, consisting chiefly of calc-alkaline tonalitic rocks, is similar to other quartz-rich felsic rocks widespread throughout the Klamath Mountains-western Sierra Nevada. The available petrographic and geochemical data are consistent with formation of these rocks by either fractional crystallization of a wet basaltic magma or partial melting of amphibolite or eclogite. The Bear Mountain igneous complex is an example of a diverse but distinctive association of ultrabasic to silicic rocks which characterize numerous plutonic complexes in the Klamath Mountains-western Sierra Nevada. These intrusive complexes invade older ensimatic rocks and appear to define the roots of a complex, Middle to Late Jurassic calc-alkaline magmatic arc. The ultramafic and gabbroic rocks characteristic of this plutonic association are similar to Alaskan-type complexes but differ in detail. More significantly, these rocks are important clues to the composition of early magmas as well as the complex processes operative in reservoirs that form the core of calc-alkaline magmatic centers.