Boulder, Colo., USA: Earth has two types of crust: oceanic crust is relatively young, dense, and made at mid-oceanic spreading centers, whereas continental crust is buoyant, old, and believed to be created in magmatic arcs. However, making continental crust in arcs is problematic because arc magmas are basaltic with 48-50% silica, yet continental crust has ~60% silica and less magnesium than arc basalts. Paradoxically, how scientists think crust is created doesn't match the composition of the continental crust.
In their latest contribution, Robert Hildebrand and Joseph Whalen examine the geochemistry and tectonic setting of large granitic bodies in the Cordillera of North America and use their findings to resolve the continental crust paradox. They conclude that most of the granitic bodies, called plutons, were not formed beneath arcs but instead were emplaced during and after collisions between arcs and continents. The plutons contain 60-70 percent silica and were derived from the mantle.
During collision, arc magmatism shuts down and the leading edge of the continental mass is pulled beneath the arc by the dense oceanic plate. Because continents are buoyant, they resist being pulled completely into the mantle, so the ocean floor, along with the leading edge of the continent, tears off and sinks into the mantle. Fresh mantle upwells through the tear and new magmas, termed "slab failure magmas," containing 60-70 percent silica, are generated. The slab failure magmas rise into the collision zone where they constitute a significant volume of new continental crust.
The authors note that magmas of similar composition can be traced back in time at least 3.6 billion years, which suggests to them that both subduction and slab failure have been long-lived processes that combined to create the continents we live on.
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"The Tectonic Setting and Origin of Cretaceous Batholiths within the North American Cordillera: The Case for Slab Failure Magmatism and Its Significance for Crustal Growth"
by Robert S. Hildebrand and Joseph B. Whalen
Geological Society of America Special Paper 532
SPE532, 113 p. plus insert, $58.00; GSA member price $40.00
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Contact: April Leo