INVESTIGATING EMPLACEMENT MECHANISMS OF ULTRAMAFIC PLUTONS IN THE UPPER CRUST: SOLVING A GEOLOGICAL CONUNDRUM

Continental arcs represent major sites of growth of the continental crust. The crust’s bulk intermediate composition makes the emplacement of ultramafic plutons puzzling based on density consideration. The Emigrant Gap Complex (EGC) is a Middle Jurassic intermediate-ultramafic intrusive complex located in the northern Sierra Nevada that consists of ultramafic bodies with coeval intervening mafics and intermediates. EGC intruded the Shoo Fly complex and overlying Paleozoic sediments and Triassic-Jurassic arc rocks. In this study, we will use field mapping, structural analysis, and barometry to investigate the emplacement mechanisms of the EGC. EGC’s ultramafic units consist of pyroxenite, peridotite, and dunite. The dunite pods have varying degrees of serpentinization and deformation, with all sharing a NE striking cleavage, whereas the peridotite is mostly undeformed. The pyroxenite unit features angular pyroxenite breccia intruded by anorthositic and gabbroic dikes. The intermediate-mafics include gabbro, two-pyroxene granodiorite, and porphyritic granodiorite. The host rock pendant dips steeply E and has a N-NW striking cleavage cut by the EGC. A mylonitic shear zone developed in the pyroxenite and granitoid porphyritic units at the contact with the host rocks. Pyroxenite and porphyry form alternative cm-wide shear bands. At some places, the stronger pyroxenite forms a symmetrical phi-type clast suggesting pure shear kinematics. The shear zone parallels the regional cleavage in the host rock and the contact itself. We hypothesize that the margin-parallel shear zone is related to the emplacement of the EGC and its deformational conditions, stress state, and thermal evolution bear information on how the felsic upper crust accommodates the EGC’s mafic-ultramafic units. We are working on the barometry, microstructures and paleo-piezometry to understand the emplacement mechanism of the EGC.