Metamorphism and structure of the Encampment Creek area, British Columbia
Abstract
Late Proterozoic to Early Paleozoic metasediments of the Encampment Creek are tentatively correlated with the Horsethief Creek Formation i n the Selkirk Mountains and part of the Cariboo Group in the Cariboo Mountains. Mesoscopic structures t rend NW-SE and are dominated b y F 3 (third phase) folds which have produced a strong crenulation cleavage. F 1 folds are characterized by penetrative axial plane foliation. The spaced axial plane foliation of F 2 folds is penetrative through hinges, having a style which is intermediate between the axial plane foliation of F 1 and F 2 folds . F 1 a n d F 2 axial planes a re parallel with the regional foliation (Sf) which maintains a fairly constant orientation, moderately dipping to the SW . F 1 , F 2 and F3 folds are nearly coaxial, whereas the hinges of broadly warped F 4 folds are oblique to earlier structures. Macroscopic structures can be interpreted as large, recumbent, isoclinal F 1 f olds which have been refolded by second generation, tight, similar folds. Metamorphism generally outlasted major deformational episodes, however, an early high grade metamorphic event is indicated by a few kyanite porphy roblasts which are wrapped by the foliation and have Se discontinuous with Si. Mappable first appearances of staurolite and of kyanite are accompanied by post-Sf biotite and define the staurolite-kyanite-biotite (SKB) zone. Kyanite may precede the incoming of staurolite by 800 meters. More often, staurolite , kyanite and post-Sf biotite appear simultaneously. The three phase assemblage persists over a minimum distance of 2900 meters. At higher grades of metamorphism, staurolite disappears within the kyanite stability field. A narrow wedge of "almandine zone ", outlined by the SKB zone, defines a thermalsynform which closes to the SE and perhaps to the NW as well. An AFM plot h as insufficient components to represent SKB zone assemblages if phases are compatible. Possible staurolite and kyanite forming reactions mu st account for the absence of chloritoid and stable chlorite at lower grades and the consistency of oxidation state of minerals throughout the area. Tie line intersections in the AFMK tetrahedron show possible univariant reactions which are useful isograds: 1. bt +mu+ gar a btb + st (alterna t ive ly ky) + qtz + H 2 0 2. bt + mu a btb + st ( alternatively ky) + gar + qtz + H2 o Post-Sf biotite has more ( .3 - .9 wt%) K20 than syn-Sf (reactant) biotite, meeting the compositional requirement imposed by the topologies upon the product biotite. Textural relationships implied by each reaction are consistent with petrographic observations. Log Kn (apparent distribution coefficient) values are calculated from electron microprobe analyses of coexisting minerals and suggest that equilibrium is approached. Equilibrium constants are derived for several possible reactions which may occur in staurolite - kyanite bearing pelites. Corrections are made for the nonideality of solid solutions. The temperature, pressure and H20 fugacity at which a given sample could have been in equilibrium can be calculated from the simultaneous solution of these equations. Temperatures ranging from 635 to 641°C are 40°C 18 16 higher when compared to O /0 values from other areas and give H2o fugacities slightly greater than pure water. Pressures average 8 .5 kb. Minimum temperatures of 540°C, at minimum pressures of 4 .6 kb, are calculated from the solid solution of MgC0 3 in calcite coexisting with dolomite.
Description
Bibliography: p. 141-147.
Keywords
Citation
Robbins, D. B. (1976). Metamorphism and structure of the Encampment Creek area, British Columbia (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/12174