Reconnaissance geology of the Precambrian Shield of Ellesmere, Devon and Coburg Islands, Arctic Archipelago: a preliminary account

The extensively ice- and snow-covered upland areas of southeastern Ellesmere Island, eastern Devon Island and Coburg Island are underlain by granulite facies rocks of the northernmost Canadian Shield. Rocks of supracrustal origin abound and include migmatitic garnet-cordierite-sillimanitebiotite gneisses, marble with diopside, forsterite and wollastonite, sillimanite quartzite, clinopyroxene-orthopyroxene amphibolite, and orthopyroxene-bearing gneisses. Other orthopyroxenebearing quartz of eldspathic gneisses are probably def armed intrusive rocks, which are also represented by massive to crudely gneissic, orthopyroxene-bearing granite and tonalite. The orthopyroxenebearing rocks carry primary biotite invariably and primary hornblende commonly. Various kinds of granite have intruded all these rocks. Granite with biotitic pseudomorphs after orthopyroxene and locally with garnet is com manly intruded into fresh orthopyroxene-bearing granitoid rocks. A similarly retrograded orthopyroxene granite with prominent perthite porphyroblasts farms large bodies at the northern edge of the Shield. Peraluminous granite with garnet, cordierite and/or sillimanite and locally rich in aluminous metasedimentary inclusions is particularly widespread in Ellesmere Island. These granites are thought to have farmed by anatexis of aluminous metasediments under granulite facies conditions. Amphibolite facies rocks are of minor importance in the area and were produced by retrogression from the granulite facies. The granulite facies metamorphism is provisionally envisaged to have taken place at moderate to low pressure and high temperature. The rare presence of the assemblage orthopyroxene + sillimanite +quartz, however, indicates that higher pressures may have prevailed locally. Limited radiometric data suggest a late Archean age for the main granulite f acies metamorphism and possibly a major Hudsonian deformation . The basement rocks are overlain on the east coast of Ellesmere Island by unmetamorphosed elastic sediments and basalt of the Neohelikian Thule Group and are bordered on the west and north by lower Paleozoic strata of the Arctic Platform.