Precambrian alkaline magmatism

Abstract

There are very few alkaline rocks in Precambrian terrains. The oldest well-documented examples are 2.7 Ga trachytes and leucite phonolites from the Kirkland Lake region of Canada. These rocks are highly potassic, with major- and trace-element characteristics closely resembling those of shoshonitic lavas in modern island arcs. Other examples of Archean alkaline rocks are limited to rare, volumetrically insignificant lamprophyric dikes and syenitic intrusions. Archean alkaline rocks similar to those of modern oceanic islands have not been reported. The oldest oceanic island suites are found in the 2.0–1.9 Ga Circum-Superior Belt of Canada which contains several successions of transitional to strongly alkaline volcanic rocks. Explanations for the paucity of Precambrian alkaline rocks fall into two main categories. (a) Alkaline magmatism was not uncommon in the Precambrian, but the rocks that formed did not survive. The alkaline rocks may have been destroyed preferentially because they formed late-stage volcanoes composed of friable pyroclastics and unstable feldspathoids, and were thus particularly vulnerable to erosion. Alternatively, the alkaline rock sequences may have erupted as part of a volcanic series that did not normally become incorporated in growing Archean continents, as would have been the case if oceanic plateaus made up the bulk of greenstone belts. (b) Alkaline rocks may indeed have been very rare because conditions in the Archean mantle were not appropriate for the formation of this type of magma. Higher temperatures may have led to more extensive partial melting, such that low-degree melts either were not produced or were overwhelmed by high-degree melts. Other possible factors include lower CO 2 contents in melting regions, which inhibited the formation of silica-undersaturated magmas, and the absence of metasomatized lower lithosphere, which precluded the formation of rift-type magmas. The late-Archean shoshonites apparently formed in a subduction environment. At present our knowledge of Archean volcanic rocks and Archean tectonic processes is insufficient to decide between the various possible interpretations.