Professor Yaoling Niu
Testing the Hypothesis of Continental Collision Zones as Primary Sites for Net Crustal Growth
The significance of the continental crust on which we live is self-evident. However, our knowledge remains limited on the origin, the way and rate of crustal growth, and how it has acquired its "andesitic" composition from mantle derived magmas. The broad similarity in trace element signature between the continental crust and volcanic rocks erupted on islands (like the "Pacific ring of fires") above active subduction zones (where seafloor enters deep mantle) suggests their genetic link, which led to the standard "island arc" model for the origin of the continental crust. However, this standard model has many more difficulties than certainties. The two major problems are: (1) bulk "island arc crust" has too high Mg/Si ratio, which is inappropriate for continental crust with much lower Mg/Si ratio; and (2) it has been noted that crustal production (magmatism) and loss (subduction erosion and sediment recycling) at modern subduction zones are mass-balanced. However, geological observations and models indicate that the continental crust must have been growing continuously over much of the Earth's history.
This requires that geological sites where mantle derived melts similar to continental crust in composition be (1) produced and (2) preserved in the crust, contributing to crustal growth over time. We have shown that some volumetrically significant magmatic rocks formed and preserved during continental collision, such as the India-Asia collision, have the composition very similar to that of the continental crust. Hence, it is essential to test the hypothesis of "continental collision zones as primary sites for net crustal growth."