GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 39-13
Presentation Time: 9:00 AM-5:30 PM


BERNARD, Rachel E. and CHIN, Emily J., Scripps Institution of Oceanography, The University of California San Diego, La Jolla, CA 92037

Lower crustal xenoliths from the San Quintin volcanic field, Baja California, Mexico preserve a diversity of igneous textures, ranging from magmatic foliation to reaction coronas with varying morphologies and states of development. We analyzed a suite of 23 lower crustal xenoliths using large-area electron backscatter diffraction (EBSD) mapping. Lithologies include two-pyroxene gabbros containing plagioclase, clinopyroxene and orthopyroxene, and granulites containing plagioclase and clinopyroxene with lesser amounts of olivine, orthopyroxene and oxides. While some samples appear to have foliations resulting from solid state deformation, many of the granulite samples have strong foliations or layering unrelated to deformation. In some samples, this foliation is defined as alternating layers of well-equilibrated plagioclase +- orthopyroxene (triple-junction 120 degree grain boundaries between grains lacking signs of internal deformation) and deformed (e.g., elongated grains, irregular grain boundaries, subgrain formation, kinking, etc.) clinopyroxene + oxides +- orthopyroxene. These relationships are interpreted as reflecting magmatic infiltration of previously deformed lower crustal lithologies. Reaction corona textures include the following mineralogical associations: (1) clinopyroxene cores with coronas of clinopyroxene, olivine and plagioclase symplectites; (2) olivine cores with inner coronas of orthopyroxene and outer coronas of clinopyroxene-oxide symplectites; (3) clinopyroxene cores surrounded by olivine. In addition, at least one sample shows convincing evidence of syn-deformational magmatism, with elongated coronas composed of both internally deformed and undeformed constituent minerals. We will present clinopyroxene major and trace element compositions and calculated melts in equilibrium with clinopyroxene, which will shed insight into the nature of the infiltrating melts. Future investigations will focus on the metamorphic vs. magmatic origin of the reaction coronas and their relationship to the macroscopic foliation.