Croker Island Complex, Ontario Canada

• Type: Syenite/Pluton
• Age Ma: 1510 ±50^a
• Diameter: ~10 km
• Location: N 46° 05.8’ W 082° 13.5’

^a Meaningful paleomagnetic results were obtained from 23 of 30 oriented samples from the Croker Island Complex, North Channel of Lake Huron, Canada. The age of the complex is estimated to be 1475 ± 50 m.y. from a Rb–Sr isochron. The paleomagnetic pole calculated from the directions of remanent magnetization after partial ac demagnetization is 143 °West, Graphic North. This result, when compared with other paleomagnetic results from North America, indicates little or no polar wandering in the time interval 1.48 b.y. to 1.1 b.y. However, a significant shift in the pole position is indicated during the time interval 1.7 to 1.48 b.y.

The earlier estimate of 1475 + 50m.y. for the age of the Croker Island Complex (Van Schmus 1965), two biotite K-Ar ages have been determined by the Geological Survey of Canada (Wanless et al. 1966). These ages of 1530 + 50 m.y. and 1585 ±50 m.y. are somewhat older than the 1475 m.y. estimate. If these two K-Ar ages are averaged with the Rb-Sr mica ages from the Croker Island Complex (1470, 1475, 1490, and 1520 m.y. Van Schmus 1965), an average mica age of 1511 m.y. is obtained. The last three of the Rb-Sr ages quoted above are from pegmatitic muscovite. Based on these data 1510 ±50 m.y. is a better estimate of the minimum age of the Croker Island Complex. (VAN SCHMUS, 1971).

ABSTRACT: The Croker Island Complex is a pluton about 6 1/2 miles in diameter that is exposed on a group of islands in the North Channel of Lake Huron, south of Massey, Ontario. It intrudes metasedimentary rocks of supposed Huronian age. The pluton consists of an outer granitic (quartz monzonite) ring, an interior syenitic (monzonite) and syenodiorite portion, and a central diorite-gabbro core. Granitic and syenitic dikes intrude the core rocks, and gabbroic and dioritic dikes intrude the granitic rocks. The various rock-types were apparently emplaced in rapid succession, and the general order of emplacement is from gabbroic to granitic. The rocks have a relatively high magnetite-ilmenite content and consequently have pronounced magnetic responses. There is good correlation between the magnetite content of the various rocks and the airborne magnetometer readings over them.

Precambrian rocks of the Croker Island Complex are exposed on a group of islands, including Croker Island, Fox Island, and the Benjamin Islands, just off the north shore of Lake Huron south of Massey, Ontario. Collins (1925) referred to the body as the “Eagle Island batholith”, and Schmus (1964) referred to it as “Eagle granite”. However, because the intrusive body is not of batholithic dimension, does not occur on Eagle Island, and contains little or no granite, the term “Croker Island Complex” more appropriately describes its situation and character.

Deformed Huronian rocks of the Espanola Wedge are transected by the 1.475 Ga Croker Island complex, described by Card et al. (1972) as a an epizonal, post-tectonic intrusion exhibiting a narrow hornfels contact zone.

The island-group has a distinct circular pattern that is discernible on air photographs and on topographic maps. In 1963 a federal provincial aeromagnetic survey disclosed the presence of a large magnetic anomaly over the islands. The magnetic contours follow the topography closely and show that the complex is a circular body about 6 1/2 miles in diameter. The Croker Island Complex anomaly is only one of several in the district.

The rocks of the Croker Island Complex were probably derived from a common magma. This is indicated by the close association in time and space, and the petrographic similarity, of the various rock-types. They were apparently emplaced as a series of magmatic intrusions. There were at least three pulses of magma: an early gabbroic magma; a magma of intermediate composition; and a granitic magma. Possibly there were later additional pulses of late basic and granitic magmas. The gradational contact relationships and contradictory cutting relationships indicate that all pulses were emplaced in rapid succession. The various magma-types may have originated by differentiation of the parent magma at depth. The general order of intrusion from mafic to silicic suggests that fractional crystallization was the differentiation mechanism. The trend of differentiation appears to be along the normal calcalkaline trend toward potassic granitic rocks. The establishing of the mechanism of transition from saturated syenitic rocks to oversaturated granitic rocks is a problem. It is possible that the outer granitic rocks of the complex were formed by contamination from the quartzitic country rocks. There is a general increase of quartz in the rocks from the inside to the outside of the intrusive body. (Card, 1965).

Ground Exploration

References

CARD K.D. The Croker Island Complex North Channel of Lake Huron Geological Circular No. 14 – ONTARIO DEPARTMENT OF MINES

Palmer H.C. The paleomagnetism of the Croker Island Complex, Ontario, Canada Canadian Journal of Earth Sciences, 1969, 6(2): 213-218, 10.1139/e69-02

VAN SCHMUS W.R. Ages of Lamprophyre Dikes and of the Mongowin Pluton, North Shore of Lake Huron, Ontario, Canada Departmemt of Geology, University of Kansas, 1971