If you have any problems related to the accessibility of any content (or if you want to request that a specific publication be accessible), please contact us at email@example.com.
Formation and emplacement of an ultramafic plutonic complex in continental crust constrained from structural, thermal and rheological properties of contact aureole: a case study from the Emigrant Gap Complex in California
AltmetricsView Usage Statistics
The occurrence of ultramafic plutonic complexes in the upper continental crust is a puzzling geologic phenomenon given the density contrast between the pluton and the felsic-intermediate continental crust. The Emigrant Gap Complex (EGC) located in California is a Middle Jurassic ultramafic intrusive complex. Previous studies on the EGC focus on its igneous compositions. Yet, little study has been done on the contact aureole. To better understand the formation and emplacement processes of the EGC and how the upper crust responds to the intrusion of hot mafic/ultramafic magma, we conducted field mapping, finite strain, microstructural and Electron Backscatter Diffraction (EBSD) analyses in the country rocks. We found that the EGC develops an aureole of increasing temperature and deformation, starting from regional greenschist-facies cleavage >500 meters away from the contact and culminating in a high-temperature (~700℃) mylonitic shear zone at the contact. Field mapping reveals the presence of three zones in the aureole. The inner mylonite zone (0-50 m from the contact) has a moderately plunging (40°) lineation associated with a maximum of 70% contact-perpendicular shortening. Microstructures and quartz c-axis are indicative of deformation at ~700 ℃ or higher. The outer mylonite zone (50-150 m from the contact) has a shallower plunging (20°) lineation associated with 60% shortening. Microstructures and quartz c-axis are indicative of deformation at >500-700 ℃. The outer aureole (150-500 m from the contact) has a horizontal lineation associated with 30-50% shortening. The deformation temperature in the outer aureole is ~400-500 ℃. We use a 1D thermal conduction model to constrain the timing of intrusion and country rock deformation. We estimated the magnitude of the differential stress causing the contact aureole deformation to be ~30-70 MPa based on flow laws. Our differential stress estimate is consistent with the independant stress calculation based on quartz paleo-piezometry. Such magnitude of differential stress cannot be explained by the buoyancy force associated with a mafic or ultramafic magma body. Instead, we attribute the differential stress to the overpressure imposed on the viscous country rocks during the injection of magma into a magma chamber. Our result suggests the EGC was emplaced as a mafic magma that underwent fractional crystallization. Our study highlights the coupling between the structural, thermal and rheological evolutions in a contact aureole that can be used to constrain the formation and emplacement processes of an ultramafic intrusive complex.