Metamorphic and structural geology of the Mount Cheadle area, northern Monashee Mountains, British Columbia
Creators: name: Digel, Scott G.
Publisher: name: University of Calgary
Year: 1988
Subjects: subject: QE 187 D34 1988a
subject: Geology - British Columbia
subject: Petrography - British Columbia - Monashee Mountains
subject: Mount Cheadle (B.C.)
subject: Monashee Mountains (B.C.)
subject: Geology, Structural - British Columbia - Mount Cheadle area
subject: Geology - Stratigraphic - British Columbia - Mount Cheadle area

description: Bibliography: p. 155-160.
description: Kyanite and fibrolitic sillimanite (fibrolite ) ha v e been found to coexist over 200 km 2 and through a 2 km vertical section in an area of the northern Monashee Mountains, British Columbia, centred on Mount Cheadle. In t he Mica Dam area, 30 km to the southeast, kyanite and sillimanite coexist through about 10 metres locally, and define a steeply-dipping isograd which strikes toward the Mount Cheadle area (MCA). The main purpose of this study is to develop a model to explain the coexistence of these two minerals over such a large area and vertical section in the MCA and account for the changes in nature of the kyanitesillimanite isograd between the Mica Dam area and the MCA. Upside-down stratigraphy of the Upper Precambrian Horsethief Creek Group (HTCG) dominates the study area. The axial surface of the Scrip Nappe , a major Fi structure , projects into the air over the study area. The lower pelite unit sits structurally above the semipelite-amphibolite unit and crops out only at high altitude, occupying the core of the nappe. Pressure and temperature attending the main metamorphism (100 Ma) have been estimated using mineral equilibria in several rock types. Temperatures ranging from 575 to 651 °C and averaging 590 °c ± 10 °C (one standard deviation) have been inferred from garnet-biotite thermometry in metapelites. There is no consistent pattern to the temperatures within the map area, suggesting that they were fairly constant across the MCA. Pressures (± 0.65 kbar ) from 4.85 kbar (at 9500 feet a sl ) to 6. 21 kbar (3500 feet asl ) - were estimated in metapelites using garnet-plagioclase-Al2SiO 5 -quartz (GP AQ) barometry. The increase in GPAQ pressure is approximately linear with increasing depth, suggesting that isobars were essentially flat. Fluid composition attending metamorphism of the pelites was water-rich (XH20 = 0.87 ± 0.11) based on estimates of XH2o from paragonite-albite-kyanite-quartz equilibria. Textural relations and structural data indicate that fibrolite grew in an extensiona l regime after the peak metamorphic event. A subhorizontal lineation defined by oriented fibrolite in the MCA trends ~060-240°. Field and petrographic relations suggest that fluids were a key factor in the formation of fibrolite. The favoured model for the origin of the fibrolite in the MCA proposes that it grew in the presence of fluid during an extensional event at ~63 Ma. This extensional event was accompanied by the intrusion and crystallization of granites similar to those exposed 30 km to the south in the Adams River area. Localization of fluids released from the granites resulted in restriction of fibrolite growth to metre-scale layers within kyanite pelites.