Geologic mapping, petrography and geochemistry of the Middle Jurassic Black Buttes mafic complex and French Lake pluton, northern Sierra Nevada, California

Abstract

The Black Buttes Mafic Complex is an intrusive body composed of at least two compositionally and texturally inhomogeneous plutons: 1) the Black Buttes Pluton and 2) the Beyers Lakes Pluton. The Black Buttes Pluton (52-58 % Si02) is fine to medium-grained and ranges in composition from a two pyroxene gabbro to an amphibole quartz monzonite. The gabbro crops out along the southern margin of the pluton and grades into the more alkalic monzonitic rocks to the north. The medium to coarse-grained Beyers Lake Pluton (52-59 % Si02) crops out along the southern margin of the Black Buttes Pluton. The rocks of the Beyers Lakes Pluton range in composition from a medium to coarse-grained two pyroxene gabbro to a porphyntic- pyroxene quartz monzodiorite. The contact between these two plutons varies from sharp on hand specimen scale to a zone of brecciation with fragments of the Black Buttes Pluton completely surrounded by quartz monzodiorite from the Beyers Lakes Pluton.