Geology of the Winokapau Lake area, Grenville Province, central Labrador

Abstract

Precambrian crystalline rocks within the northern part of the Grenville Structural Province near Winokapau Lake southeast of Churchill Falls, Labrador, constitute a mixed paragneiss, orthogneiss, granitoid, and gabbroid terrane. A range of hills in the eastern and southern parts of the area mark an allochthonous crustal block, the Wilson Lake allochthon, underlain primarily by granulite facies quartzofeldspathic paragneiss containing the mineral assemblage hypersthene + sillimanite + biotite + quartz + plagioclase + potassium feldspar ± sapphirine ± cordierite. Lowlands are underlain by parautochthonous rocks comprising amphibolite to granulite facies quartzofeldspathic paragneiss, orthogneiss, and deformed porphyritic granitoid and gabbroid rocks. Paragneiss in the parautochthon is characterized by the mineral assemblage sillimanite + biotite + quartz + plagioclase + potassium feldspar ± hypersthene ± sapphirine ± cordierite ± muscovite ± kyanite ± garnet whereas orthogneiss and deformed granitoid-gabbroid rocks contain the assemblage biotite + plagioclase ± potassium feldspar ± hypersthene ± hornblende ± garnet ± quartz. The protolith of paragneiss in the Winokapau Lake area is at least Aphebian in age, and is correlated with that of the Disappointment Lake gneiss in and around the Red Wine Mountains massif to the north, which is a continuation of the Wilson Lake allochthon that has undergone a similar tectonic history. The orthogneiss is correlated with essentially undeformed Paleohelikian granitoid rocks belonging to the Trans Labrador Batholith, located north of the present study area. Both the parautochthonous and allochthonous blocks have undergone extensive deformation during at least two major orogenic events. U-Pb dates of 1676 and 1603 Ma on zircons obtained from the paragneiss are the result of a newly identified mid-Paleohelikian orogenic event known as the Labradorian orogeny. Similar U-Pb dates ranging from 1653 to 1635 Ma have also been obtained from the orthogneiss. Granulite facies peak metamorphic conditions prevailed during this event with temperatures having exceeded 700 C and pressures 6 x 105 kPa. In contrast, a number of younger U-Pb and 40Ar/39Ar mineral ages have been derived from the orthogneiss, and range from 1013 to 991 Ma. These represent reworking of the rocks during the Grenvillian Orogeny under middle to upper amphibolite facies metamorphic conditions, with indicated temperatures in the range 500 to 600 C and pressures of up to 7 x 105 kPa. Rocks within the Winokapau Lake area form an integral part of the mid-Proterozoic Grenville orogenic terrane, which extends entirely across central Labrador, continuing into both the southwestern United States and the Scandinavian countries.