Metamorphism and structure of the Esplanade Range, British Columbia
Creators: name: DeVries, Christiaan D. S.
Publisher: name: University of Calgary
Year: 1971
Subjects: subject: QE 475 D47 1971
subject: Rocks, Metamorphic
subject: Metamorphism (Geology)
subject: Geology - British Columbia - Esplanade Range

description: Bibliography: p. 102-105.
description: The Esplanade Range is underlain by a sequence of predominantly metasedimentary rocks correlated with the Horsethief Creek Formation of Hadrynian age. The dominant structural feature of the Esplanade Range is a northwest trending macroscopic syncline, formed prior to Upper Jurassic time. The form of the syncline changes along the trace of its axial plane from upright in the southern part of the map area to easterly overturned in the area around Sentry Mountain. Associated with the macroscopic fold event (F) are NW trending mesoscopic folds and a penetrative axial plane foliation (S1) defined by the preferred orientation of platy minerals. A second deformation produced mesoscopic folds with a crenulation axial plane cleavage ) striking approximately N-S and dipping steeply. No macroscopic folds can be correlated with F deformation. Three metamorphic zones, Biotite, Garnet and Staurolite can be distinguished on the basis of the first appearance of almandine-rich garnet in aluminous schists (garnet 11isograd11 ) and the first appearance of staurolite -- often accompanied by kyanite -- in aluminous schists (staurolite 11isograd11 ). The high grade part of the central Biotite Zone is marked by the appearance of garnet in metasandstones and quartz-hornblende schists, abundant biotite in pelites and metasandÂ­stones, and the appearance of oligoclase instead of albite. A good correlation between metamorphic grade and anorthite content of plagioÂ­clase, grossularite content of garnet and clinozoisite content of epidote in quartz-hornblende schists can be demonstrated, suggesting that these rocks may be useful as a potential grade indicator. Estimates of the physical conditions of metamorphism in the Esplanade Range are as follows: p fluid = 5 kb Biotite Zone temperature = 390° C Lower temperature 1 imit for the Staurol i te Zone = 500° C Upper temperature limit for the Esplanade Range = 625° C Relationships between porphyroblastic minerals and S-surfaces indicate that at least garnet grade temperatures were reached during F1 deformation in what is now the Staurolite Zone. The climax of metamorphism, however, was not reached until the static period of mineral growth foll owing F1, as evidenced by the growth of unoriented porphyroblasts. With the exception of the area around Sentry Mountain, where staurolite grade temperatures outlasted F2 deformation, the metamorphic t??mperatures had fallen well below garnet grade before the onset of F2 deformation. Attitudes of the garnet isogradic surface suggest that the central Biotite Zone has the geometry of a southeast plunging "thermal syn­ cline". The configuration of the isogradic surfaces was presumably established during the climax of metamorphism, i.e. after F1, which precludes a fold origin for the present geometry of the isogradic surfaces. It is proposed that one or more deep-seated heat anomalies, possibly aided by heat convecting fluids, produced the two narrow southeastward extending apophyses of the Selkirk-Cariboo metamorphic complex.