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The timing, composition, and petrogenesis of syn- to post-accretionary magmatism in the northern Cordilleran miogeocline, eastern Yukon and southwestern Northwest Territories Rasmussen, Kirsten Louise

Abstract

Syn- to post-accretionary magmatic episodes of Cretaceous to Eocene age comprises the most voluminous magmatism in the northern Cordillera. Our understanding of the age, composition, and petrogenesis of this magmatism thus far, however, has been severely limited by a lack of robust geochronological, geochemical, and isotopic datasets for these rock units. This study fills the need for better temporal and compositional constraints on mainly Early to ‘mid’-Cretaceous (~115–90 Ma) magmatism in the Cassiar Terrane and eastern Selwyn Basin regions of the northern Cordilleran miogeocline. An extensive set of new U–Pb and ⁴⁰Ar/³⁹Ar age data, together with major and trace element geochemical analyses, radiogenic (Nd, Sr, Pb, and Hf) and stable (S and O) isotopic compositions, and detailed magmatic fluid compositional studies of intrusive rocks in the northern Cordillera are presented here. These results, and the integration of compiled datasets, have led to the identification of several new plutonic suites and sub-suites, as well as modifications of previously defined plutonic suites. Inferences regarding the age and composition of the basement to the northern Cordilleran miogeocline have also been made based on mineralogical, geochemical, and isotopic interpretations of magma sources for crustally derived plutonic suites from across this region. These extensive datasets have also allowed a re-evaluation of previously proposed models for syn- to post-accretionary magmatic episodes, and have resulted in the proposal of modified tectono–magmatic models for the particularly voluminous Early to ‘mid’-Cretaceous magmatic episode. Finally, estimates of magmatic fluid compositions determined from apatite and biotite compositions (with respect to HF, HCl, and H₂O) and extended cooling histories indicated by U–Pb and ⁴⁰Ar/³⁹Ar geochronology, have been used to identify intrusive phases (locally and regionally) that are associated with reduced tungsten skarn mineralization, which comprises some of the most economically important intrusion-related mineralization in the northern Cordillera. Collectively, this work has substantially advanced the current knowledge of syn- to post-accretionary magmatism in the northern Cordillera, and it has broad implications for similar magmatic provinces world-wide. Supplementary data accompanying this dissertation available in cIRcle at: http://hdl.handle.net/2429/44385

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