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Geology and metamorphism of the Mount Breakenridge area, Harrison Lake, British Columbia

University of British Columbia

The Mount Breakenridge area is underlain by metamorphosed strata of the Upper Paleozoic (?) or older Breakenridge Formation, the Upper Paleozoic (?) or Mesozoic Cairn Needle Formation and the Lower Cretaceous Peninsula Formation. These rocks and enclosed ultramafic pods were subjected to Barrovian-type metamorphism prior to the mid-Cretaceous. Locally, pelitic schists developed andalusite and sillimanite in contact aureoles around Late Cretaceous quartz diorites. Ultramafic and related rocks in the map-area and in the Southern Coast Crystalline Complex and the Northern Cascade Mountains may have formed in a marginal basin which was in existence from Mississippian to Late Triassic. Rock and mineral chemistry of pelites indicates that the composition of white mica is sensitive to metamorphic grade, and composition of plagioclase is controlled by rock bulk chemistry. No obvious relationship exists between biotite and staurolite compositions and rock chemistry or metamorphic grade. Distribution of chemical species between coexisting chlorite, biotite, garnet and staurolite implies a close approach to chemical equilibrium. Linear regression analyses of minerals indicate that assemblages in the sillimanite and kyanite zones are dependent on rock bulk chemistry and that apparent univariant reaction assemblages act as local fH2O buffers. The minerals talc (T), forsterite (F), ehstatite (E), anthophyllite (A) in ultramafites may have formed by the following reaction sequence. The reaction T + F = E produced E at 7kb. Uplift and subsequent reduction in pressure rendered enstatite unstable and anthophyllite formed by the reaction T + F = A. Thermodynamic calculations confirm that the equilibrium temperature of the vapour-absent reaction E + T = A is extremely sensitive to iron solid solution so that the topologies of reactions in the system MgO-SiO2-H20 may be a function of the Mg- content of the system.

Text

2010-02-01

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http://hdl.handle.net/2429/19527

Geological Sciences

University of British Columbia

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