Insights Into Middle Crustal Processes In Magmatic Arcs, Coast Mountains, British Columbia
This thesis explores processes occurring in the middle and lower crust of magmatic arc-settings. This study is based on observations on the Coast Mountains of British Columbia, Canada, at ~53 degree N latitude. The rocks exposed at the surface in this area represent the roots of a Mesozoic to early Tertiary magmatic arc. Transpressional deformation occurred synchronously with magmatism and metamorphism from the Late Cretaceous to the Early Eocene. Between 61 and 50 Ma, the entire Canadian Cordillera tracks a period of extension and exhumation of high-grade metamorphic rocks, synchronous with intensified magmatism. The thermal state of the crust during this time period was explored using numerical models. The models attempt to reproduce a geotherm that explains observations found in the field, such as abundance of amphibolitic migmatites, widespread magmatism, granulite facies metamorphism, and high-temperature low-pressure metamorphism. The resulting geotherm is isothermal from the pluton emplacement depth to the Moho. The temperature of the profile is buffered by the solidus temperature of the country-rock. The upper crust is characterized by a steep temperature gradient of ~50 degree C/km. The structure and petrology of three plutons was analyzed to better understand pluton emplacement processes. Two of the plutons, the Kitlope and Quottoon plutons; are characterized by steep foliations and intruded during transpression. The third pluton, the Chief Matthew's pluton; intruded as a sub-horizontal sill during extension. The transposition of the foliation from vertical to horizontal is explained by the development of melt-filled tension gashes oriented perpendicular to the sub-vertical foliation. Melt migration results in melt accumulation in the gashes. This process coupled with a change from regional transtension caused development of flat foliation localized around the Chief Mathew's pluton. The exhumation path and cooling history of the three plutons was also explored. Petrology, geothermobarometry, geochemistry, and cooling ages were analyzed to determine that: 1) exhumation of ~6 km of rock occurred between the emplacement of the Quottoon and Chief Matthew's plutons at a rate of ~2-3 mm/yr; 2) the rocks cooled at a rate of ~50 degree C/Ma; 3) the area cooled below the biotite closure temperature by 51 Ma. An exhumation model integrating the studied area with the rest of the Canadian Cordillera is also proposed.
dissertation or thesis