Earth Sciences Report 1984-07

Earth Sciences Report 1984-07

ESR 1984-07

Fitzgerald, Tulip-Mercredi-Charles Lakes district, Alberta

Author(s) Godfrey, J.D. Langenberg, C.W. Date 1986-01-01

The bedrock geology consists of a north-trending belt of Archean granite gneisses in the east that is intruded by an Aphebian granitoid batholithic complex in the west portion of the map area. The migmatitic gneissic belt consists of classic granitic gneisses with minor components of small granitoid bodies, high-grade metasediments and amphibolite. The granitoid batholith consists of two principal lithologies, the Slave and Arch Lake Granitoids, both of which exhibit a deformed foliation. A major structural feature in the Slave Granitoid body, the Tulip Dome, contains symmetrical screens of narrow high grade metasedimentary rock bands that define a ghost stratigraphy. Small scale domal and basinal structures within the north section of the Arch Lake Granitoids suggest proximity to a roofal section and may be related to lateral pressure from an active adjacent Tulip Lake Dome. It appears that the granitoids are 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 protruding into the granitoids.

The above rocks indicate that the region has 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 granulite 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 facies event was dated at 1900 Ma. K-Ar dates on biotite and hornblende reveal that the greenschist facies and closure of the K-Ar system occurred at about 1800 Ma. Those events are coincident with the end of a widespread and severe thermal event (the Hudsonian Orogeny). Regionally, the metamorphic foliation has a northerly trend, but a wide range of variations exists locally, within both the granitoids and the gneissic belt.

The map area is crossed by two, north to northeast striking, wide regional shear zones. One lies largely within the main granite gneiss belt, and the other forms a complex boundary between the major bodies of Slave and Arch Lake Granitoids. These shear zones are mylonitic and represent a deep seated environment of ductile shear.

A Pleistocene ice sheet has scoured the region, leaving abundant erosional and depositional evidence of that continental glaciation. The Classical Wisconsin ice sheet flowed almost due west. Its retreat is marked by recessional moraines in the western part of the map area. 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 found on the adjacent bedrock surfaces.

Godfrey, J.D. and Langenberg, C.W. (1986): Geology of the Fitzgerald, Tulip-Mercredi-Charles Lakes district, Alberta; Alberta Research Council, ARC/AGS Earth Sciences Report 1984-07, 43 p.