PRECAMBRIAN ALKALINE MAGMATISM

There are very few alkaline rocks in Precambrian terrains. The oldest well-documented examples are 2.7 Ga trachytes and leucite phonolites f rom the Kirkland Lake region of Canada. These rocks are highly potassi c, with major- and trace-element characteristics closely resembling th ose of shoshonitic lavas in modem island arcs, Other examples of Arche an alkaline rocks are limited to rare, volumetrically insignificant la mprophyric dikes and syenitic intrusions, Archean alkaline rocks simil ar to those of modem oceanic islands have not been reported, The oldes t oceanic island suites are found in the 2.0-1.9 Ga Circum-Superior Be lt of Canada which contains several successions of transitional to str ongly alkaline volcanic rocks, Explanations for the paucity of Precamb rian alkaline rocks fall into two main categories, (a) Alkaline magmat ism was not uncommon in the Precambrian, but the rocks that formed did not survive, The alkaline rocks may have been destroyed preferentiall y because they formed late-stage volcanoes composed of friable pyrocla stics and unstable feldspathoids, and were thus particularly vulnerabl e to erosion, Alternatively, the alkaline rock sequences may have erup ted as part of a volcanic series that did not normally become incorpor ated in growing Archean continents, as would have been the case if oce anic plateaus made up the bulk of greenstone belts. (b) Alkaline rocks may indeed have been very rare because conditions in the Archean mant le were not appropriate for the formation of this type of magma, Highe r temperatures may have led to more extensive partial melting, such th at low-degree melts either were not produced or were overwhelmed by hi gh-degree melts, Other possible factors include lower CO2 contents in melting regions, which inhibited the formation of silica-undersaturate d magmas, and the absence of metasomatized lower lithosphere, which pr ecluded the formation of rift-type magmas, The late-Archean shoshonite s apparently formed in a subduction environment, At present our knowle dge of Archean volcanic rocks and Archean tectonic processes is insuff icient to decide between the various possible interpretations.