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Geology and geochronometry of the Eagle Plutonic Complex, Coquihalla area, southwestern British Columbia (92H/6,7,10,11) Greig, Charles James


The Eagle plutonic complex forms the southern half of the 200 kilometer long, north-northwest trending Mt. Lytton-Eagle plutonic complex, which parallels the regional structural grain in southwestern British Columbia and extends from south of the forty-ninth parallel to near Lytton (50° 30'N). The Mt. Lytton-Eagle complex lies along the westernmost margin of Quesnellia (Intermontane Belt), separating it from terranes of the Coast-Cascade belt. In the Coquihalla area, the Eagle complex is bounded on the east by a west-dipping, syn-deformational intrusive contact against intensely foliated rocks of the Upper Triassic Nicola Group. On the west, the Eagle complex is in fault contact with Jurassic to Middle Eocene sedimentary rocks of the Methow-Pasayten trough. Four map-units have been distinguished within the Eagle complex in the Coquihalla area. Foliated Late Jurassic Eagle tonalite (155 ± 4 Ma, U-Pb) at the eastern margin of the Eagle complex intrudes mylonitic Upper Triassic Nicola Group rocks and structurally overlies them along the moderately southwest dipping Eagle shear zone, which has a structural thickness of >1 kilometer and a strike length of >100 kilometers. Kinematic indicators suggest a top-to-the-east sense of shear. Locally, randomly oriented, strongly foliated Nicola Group inclusions occur in Eagle tonalite, indicating that deformation in part pre-dated emplacement of Eagle tonalite. More commonly, Nicola Group inclusions in Eagle tonalite are concordant, oblate and may contain foliated, boudined, or folded Eagle tonallte apophyses, suggesting that deformation also post-dated emplacement of Eagle tonalite. Contact relations are interpreted to represent east-directed syn-kinematic emplacement of Eagle tonalite into the Nicola Group in Late Jurassic time. In the central and western Eagle complex, Eagle tonalite grades into well-layered tonalite orthogneiss of Eagle gneiss, which also comprises subordinate amphibolite and rare calc-silicate. Eagle tonalite and gneiss are crosscut by mid-Cretaceous, muscovite-bearing, peraluminous plutons of the Fallslake plutonic suite (110.5 ± 0.4 Ma, U-Pb), which have extensively reset K-Ar and Rb-Sr systematics in their host rocks. Fallslake suite rocks clearly crosscut Late Jurassic fabrics in Eagle tonalite and gneiss, but are themselves variably deformed. Near their western margin, a west-northwest trending foliation is developed that increases in intensity to the southwest, where intensely foliated muscovite-bearing plutons are juxtaposed with Late Jurassic Zoa complex hornblende quartz diorite (153 ± 10 Ma, U-Pb) along the Pasayten fault. Kinematic indicators from ductilely strained rocks in the Pasayten fault give a consistent sinistral sense of shear with an east-side-up component. Rubidium-Strontium and K-Ar muscovite dates from intensely deformed rocks yield mid-Cretaceous dates of 97.2 ± 2.1 and 105 ± 4 Ma. Mid-Cretaceous cooling dates from less deformed rocks of the Fallslake suite are concordant with dates from the deformed rocks. Together with provenance ties between the Eagle complex and the late Albian to Cenomanian Spences Bridge and Pasayten groups, the dates indicate that the Eagle complex was unroofed approximately 10 to 15 Ma after intrusion of the Fallslake suite. Uplift and unroofing was probably accommodated by movement on the Pasayten fault. On the west side of the Eagle complex, Middle Eocene sedimentary rocks of the Pasayten trough overlie the Zoa complex along a moderately west dipping sheared contact that is interpreted as a nonconformity disrupted during east-vergent thrust faulting. In the south, the Middle Eocene clastic sequence is structurally overlain on its west side by middle Cretaceous clastic rocks; in the north, Middle Eocene rocks are intruded on their west side by the Middle Eocene Needle Peak pluton (46.0 ± 1.6, 45.8 ± 1.6 Ma, K-Ar Hb, Bi), which constrains east-vergent thrust faulting involving the Middle Eocene sequence to Middle Eocene time. Up to 10 kilometers of dextral strike-slip movement occurred on the north-northeast trending Zoa fault in post-Middle Eocene time. Movement on the Zoa fault may be coeval with post-Middle Eocene to Early Miocene movement on the northeast trending, southeast-side-down, dextral Coquihalla fault.

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