MECHANISMS OF STRAIN LOCALIZATION: A CASE STUDY OF MAFIC METAVOLCANIC ROCKS OF THE WESTERN HAYFORK SUBTERRANE, KLAMATH MOUNTAINS, NW CALIFORNIA

Variables controlling ductile strain localization include matrix grain size, mineralogy, fluid composition, and temperature. To investigate the effects of these variables, we present a case study of 4 deformed and 3 undeformed samples of mafic metavolcanic rocks from the Western Hayfork subterrane, Klamath Mountains, NW California. Matrix grain sizes were measured using a petrographic microscope, phases were identified using SEM-EDS, and whole-rock mineralogy was quantified using powder XRD. Average matrix grain size in undeformed samples ranges from 9.5 μm to 15.8 μm. Two deformed samples exhibit average matrix grain sizes of 9.8 and 12.3 μm. However, the other two deformed samples yield average matrix grain sizes of 57.7 and 111.5 μm.

Undeformed samples contain 18.1-20.9% albite, 21.2-23.6% oligoclase, 17.2-20.2% andesine, 13.1-17.7% hornblende, 8.6-9.5% chlorite, 6.8-7.2% clinozoisite., 3.3-9.7% biotite, and 1.0-2.4% quartz. The two fine-grained deformed samples yield average mineral abundances of 16.6% labradorite, 15.4% andesine, 14.8% albite, 13.6% oligoclase, 11.6% muscovite, 10.4% hornblende, 7.7% clinozoisite, 7.3% chlorite, and 2.6% calcite. Whereas the coarse-grained deformed samples contain an average of 17.7% oligoclase, 15.7% albite, 15.7% labradorite, 15.2% andesine, 11.6% hornblende, 9.4% muscovite, 6% chlorite, 5.5% clinozoisite, and 3.5% calcite.

To summarize, two deformed samples have similar grain sizes to the undeformed samples. However, the other two contain noticeably coarser grain sizes, which may indicate a period of higher-temperature metamorphism. In addition, there are detectable differences in mineral assemblage and abundance between undeformed, fine-grained deformed, and coarse-grained deformed samples. Deformed samples do not contain biotite and quartz, but do contain labradorite, muscovite, and calcite. Free calcite vs. free quartz may record different fluid compositions between deformed and undeformed samples. Also, undeformed samples show higher abundances of plagioclase and hornblende compared to the deformed samples. The coarse-grained deformed samples display lower abundances of muscovite by ~2.2% and clinozoisite by ~2.3% compared to the fine-grained deformed samples; otherwise, they are mineralogically similar.