Precambrian alkaline magmatism

摘要: Abstract There are very few alkaline rocks in Precambrian terrains. The oldest well-documented examples 2.7 Ga trachytes and leucite phonolites from the Kirkland Lake region of Canada. These highly potassic, with major- trace-element characteristics closely resembling those shoshonitic lavas modern island arcs. Other Archean limited to rare, volumetrically insignificant lamprophyric dikes syenitic intrusions. similar oceanic islands have not been reported. suites found 2.0–1.9 Circum-Superior Belt Canada which contains several successions transitional strongly volcanic rocks. Explanations for paucity fall into two main categories. (a) Alkaline magmatism was uncommon Precambrian, but that formed did survive. may destroyed preferentially because they late-stage volcanoes composed friable pyroclastics unstable feldspathoids, were thus particularly vulnerable erosion. Alternatively, rock sequences erupted as part a series normally become incorporated growing continents, would case if plateaus made up bulk greenstone belts. (b) indeed rare conditions mantle appropriate formation this type magma. Higher temperatures led more extensive partial melting, such low-degree melts either produced or overwhelmed by high-degree melts. possible factors include lower CO2 contents melting regions, inhibited silica-undersaturated magmas, absence metasomatized lithosphere, precluded rift-type magmas. late-Archean shoshonites apparently subduction environment. At present our knowledge tectonic processes is insufficient decide between various interpretations.