The Geology of the Southern Cariboo Mountains near Blue River, British Columbia

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The study area is underlain by deformed metasediments belonging to the latest Precambrian Semipelite - Amphibolite Unit, the Middle Marble Unit and the Kaza Group. It is suggested that only one thick grit dominated package exists within the Precambrian of southeast B.C. Thus, units below the Kaza Group are tentatively correlated to the Purcell Group. Three main deformational events are recognised: a southwest verging Dl associated with isoclines and a pervasive schistosity; a northeast verging D2 associated with folds and the Thunder River Thrust (new structure), which is a large fault, overturned at high structural levels, that forms the northeast boundary of the Shuswap Metamorphic Complex in this region; D3, associated with upright folds and minor faults. A minor event, late in D2, produced southwest verging structures. Late normal faulting along the North Thompson River Normal Fault down-dropped the rocks of the Cariboo Mountains relative to those of the Monashee Mountains. The metamorphic peak occurred after the second phase of deformation but prior to the late D2 event. Metamorphic grade was of a Barrovian Facies Series type and ranged from sillimanite + kyanite + staurolite grade near the Thunder River, to sillimanite + K feldspar at the headwaters of the Blue River in the south. Migmatite formation occurred in aluminosilicate-free lithologies within the sillimanite zone. At higher metamorphic grade, sillimanite + melt was produced in more aluminous rocks, while at even higher grade, muscovite + quartz gave way to sillimanite + K feldspar+ melt. Extensive retrogression took place at near peak temperatures to produce large late muscovite + ilmenite from Fe+Ti rich biotite + sillimanite. The composition of those muscovites and associated biotites do not now record the conditions at the peak of metamorphism. Retrogression of sillimanite + K feldspar bearing rocks produced large muscovites symplectically intergrown with quartz. The sillimanite + K feldspar+ muscovite and the sillimanite + K feldspar (without muscovite) isograds are lines of preservation, and not prograde isograds. With the above in mind, garnet - biotite temperatures range from 590~ in the north to 832~ in the south. The pressure estimates aiso increase in the same direction: from 550 MPa to 790 MPa. Most of the study area equilibrated at 600 MPa. Upper greenschist conditions were attained during D3 and lower greenschist conditions occurred during normal faulting.
Bibliography: p. 235-248.
Dechesne, R. G. (1986). The Geology of the Southern Cariboo Mountains near Blue River, British Columbia (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/18354