Earth Sciences Report 1984-05

Earth Sciences Report 1984-05

ESR 1984-05

Geology of the Bocquene-Turtle Lakes District, Alberta

Author(s) Godfrey, J.D. Date 1987-01-01

The bedrock geology of the study area consists of 8 north-trending belts of Archean granite gneisses in the east intruded by an Aphebian granitoid complex in the west. The migmatitic gneissic belt consists of classic granitic gneisses with minor components of granitoid bodies high-grade metasediments and amphibolite. The granitoid complex is dominated by the Slave Granitoids group and there are minor components of Arch Lake and La Butte Granitoids. Screens and a ghost stratigraphy of high-grade metasediments and granite gneiss are fairly common within the granitoid complex. The internal structure of the granitoids reveals a series of imperfect basins and domes. The granitoids appear to be ultrametamorphic partial melt derivatives from the protolithic granite gneisses. The major contact between the granitoids and the gneissic belt is intrusive with gneissic wall wedges and tongues projecting into the granitoids.

Most of the rocks have undergone a two-cycle polyphase metamorphism. Geochronology and electron microprobe mineral analyses show that an Archean high-pressure granulite facies metamorphism was followed by an Aphebian moderate-pressure facies metamorphism. Mineral assemblages show that the latter retrogressed through amphibolite facies and greenschist facies conditions. From Rb-Sr isochron analyses the moderate-pressure granulite event was dated at 1900 Ma. K-Ar dates on biotite and hornblende of approximately 1800 Ma reveal that the greenschist facies event occurred at the end of a widespread severe thermal event that reset all of the K-Ar isotopic ratios. The metamorphic foliation has a regional northerly trend but a wide range of variations exists locally within both the granitoids and the gneissic belt.

The map area is crossed by regional faults of two principal orientations north to northeasterly and west to west northwesterly. The latter is the more common orientation. The Allan Fault represented by a prominent mylonite zone and localized fault surfaces trends northerly. It principally cuts the granite gneiss belt.

A continental ice sheet of Pleistocene Age has scoured the region leaving abundant erosional and depositional evidence of recent glaciation. The Classical Wisconsin ice advance came from almost due east. Aeolian reworking of the typically sandy glacial deposits by southeasterly storm winds resulted in the formation of sand sheets and dunes. Associated wind polish and abrasion can be commonly found on bedrock surfaces.

Scattered minor mineralization is present in the high-grade metasediments and Slave Granitoids and less commonly in the gneisses and other granitoids. Of particular note are minor uranium stains, copper and dispersed magnetite.

Godfrey, J.D. (1987): Geology of the Bocquene-Turtle Lakes district, Alberta; Alberta Research Council, ARC/AGS Earth Sciences Report 1984-05, 37 p.