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The Upper Triassic Kutcho Formation Cassiar Mountains, north-central British-Columbia

University of British Columbia

The stratigraphic, lithologic, structural and chemical characteristics of the Kutcho Formation, an informally named Upper Triassic volcano-sedimentary rock sequence in the Cry Lake map area of north-central British Columbia, are presented and discussed in detail. The Kutcho Formation and overlying Upper Triassic Sinwa Limestone and Lower Jurassic Inklin Formation form the King Salmon assemblage which makes up a west vergent, fault-bounded allochthon whose basement is part of the ancient Pacific ocean floor represented by the Cache Creek Group. Major and trace element chemistry of the Kutcho volcanic rocks demonstrates that they are a calc-alkaline suite, formed within a volcanic island arc which was conterminous with the North American continent. The suite is distinctive in that it is composed mainly of a bimodal assemblage of basalt/basaltic andesite and rhyodacite/rhyolite. Unlike most ancient and present volcanic island arcs, andesitic rocks are rare. Chemically anomalous rocks (basic schist unit) display marked alkaline characteristics. Metasomatism, associated with deformation and remobi1ization of copper-zinc Kuroko-type volcanogenic massive sulphide deposits, may in part account for the alkaline nature of the basic schist. A Rb-Sr whole-rock isochron of 210 ± 10 Ma, with Sr87/Sr86 initial ratio of 0.7042, dates Kutcho volcanism as Upper Triassic. The King Salmon assemblage was isoclinally folded, metamorphosed, and thrust southwestward along the King Salmon fault in Toarcian to Middle Bajocian (Lower to pre-Middle Jurassic) time. Rocks developed a strong penetrative axial planar cleavage which is subparallel to the the sole of the King Salmon thrust. Metamorphic recrystallization at 300 to 450 degrees Centigrade produced greenschist facies mineral assemblages. The Kutcho Formation, and other coeval Triassic island arc sequences, were probably formed within an extensive island arc to the east of the Cache creek subduction complex. Positioning of Kutcho and related Takla, Stuhini, and Sitlika arc systems to the west of the Cache Creek subduction complex was accomplished by dextral, transcurrent motions in post Late Triassic time. The southwest vergence of faults bounding the King Salmon assemblage may have been the result of emplacement and underthrusting of the arc and related subduction sequences by the exotic terrane, known as Stikinia, in late Middle Jurassic time.

Text

2010-05-16

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http://hdl.handle.net/2429/24784

Geological Sciences

University of British Columbia

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