Mafic units
Mafic units
Middle Jurassic, ~162-163 Ma
Consists of an ultramafic complex composed of concentrically zoned peridotite, gabbro, diorite, granodiorite, and quartz monzonite. Unit forms the southernmost part of the Emigrant Gap composite pluton. Complex exhibits a well-developed magmatic foliation parallel to pluton margins, formed by fractional crystallization and flowage differentiation during forceful emplacement. Complex derived from a single gabbro magma by crystal fractionation, with ultramafic rocks formed by mechanical accumulation of early crystallized mafic minerals, and the slightly younger two-pyroxene granodiorite (part of unit Gegg) crystallized from a felsic differentiate (James, 1971). The age of this unit is based on the ~163 Ma age of the related two-pyroxene granodiorite in unit Gegg (Snoke and others, 1982), and a body of gabbro mapped by Loyd (1995) along the Rubicon River below Hell Hole Reservoir, which has yielded a zircon U-Pb age of ~162 Ma (Langenheim and others, 2025). Wall rocks metamorphosed up to amphibolite facies adjacent to complex. Inclusions of country rock in ultramafic rock metamorphosed to pyroxene--hornfels facies and are partly melted. Complex is similar in structure, rock texture, and mineralogy to zoned ultramafic complexes in southeastern Alaska that form roots to island arc overlap assemblage (James, 1971). Northeast-trending ductile shear zones up to several hundred meters wide occur in the wall rocks along the margins of the southwestern part of the pluton.