The Age of the Metamorphic Complex of Northernmost Ellesmere Island

Аннотация

Among the geological results of the Nares Expedition of 1875-76 to northern Ellesmere Island was the mapping of an area of mica schists and other altered rocks between Stubbs Point and Markham Inlet, and in the same general area Peary noted the presence of igneous rocks. These observations led Schuchert to postulate a borderland of Archaean rocks, the supposed source of the sediments deposited in the Franklinian Geosyncline, to which he gave the name "Pearyea". Recently the Precambrian age of these rocks has been questioned and the view has been expressed that the metamorphic rocks of northern Ellesmere Island are merely highly metamorphosed Palaeozoic strata. Support for this view comes from north Greenland where it appears that Palaeozoic beds can be traced into regions of metamorphic strata. In 1953 the writer mapped a group of migmatites and gneisses between Cape Aldrich and Markham Inlet and named them the Cape Columbia group. Christie continued geological mapping in the area during the 1954 field season and extended the group to include all rocks of advanced metamorphic grade. He mapped outcrops of the Cape Columbia group between Cape Aldrich and Cape Albert Edward, on Ward Hunt Island, at the head of M'Clintock Inlet, and between Ayles Fiord and Phillips Inlet. He also found a conglomerate containing pebbles of Cape Columbia group rocks in what he named the M'Clintock group; on other evidence the M'Clintock group was shown to be older than the Middle Ordovician. Christie also found fragments of metamorphic rocks in a fossiliferous conglomerate in the Challenger group of Upper Ordovician age. Thus from geological evidence it was possible to state with some confidence that the age of the Cape Columbia group was preOrdovician. A specimen of biotite-rich gneiss was recently submitted by the writer to the Isotope and Nuclear Research Section of the Geological Survey of Canada for age determination using the potassium-argon method, which gave an age of 545 million years. The significance of this result in terms of geochronology is discussed in the remaining part of this note. ... If we accept the consensus of current thought, it appears probable that the last metamorphism to which Cape Columbia group rocks were subjected occurred in lowermost Palaeozoic time or uppermost Precambrian time; the rocks themselves may be much older as the method dates only the most recent metamorphism. The existence of metamorphosed strata in northern Ellesmere Island suggests that orogenic forces may have been involved and the resulting landmass may have been the source of the clastic sediments that Thorsteinsson and Tozer note in the Parry Islands and Ellesmere Island. By the close of the Palaeozoic era, the area occupied by the Cape Columbia group rocks had been lowered and limestone of Permian age was being deposited with angular unconformity on the gneissic and other metamorphic rocks of the Cape Columbia group.