Original Depth of Paleoproterozoic Carbonatites in North China Craton and Onset of Modern-Style Plate Tectonics

Subduction is one of the key factors that change the evolution of the interior of the Earth, and there have been controversies over the starting time of modern-style plate tectonics. The discovery of majorite garnet inclusions and eclogite xenoliths hosted by the Paleoproterozoic carbonatites in the Fengzhen area, Inner Mongolia, provides a rare window into the origin depth of magma and plate tectonics. Mineralogy and high temperature and pressure experiments constrain the ferric iron-rich (Fe³⁺/∑Fe~0.8) majoritic garnets (Si ~3.18 pfu) originated from about ~400 km in depth, indicating that the carbonatitic magma originated from the mantle transition zone. The mineral-pair thermobarometer and P-T phase diagram determine the peak metamorphic pressure and temperature at ~660 ℃ and ~2.65 GPa, which gives a peak thermal gradient of ~250 ℃·GPa^-1, similar to the product of modern plate deep subduction, indicating that the modern plate tectonics has started since the Paleoproterozoic. Statistics show that the global Paleoproterozoic carbonatites are closely associated with high-pressure metamorphic rocks in Paleoproterozoic orogens. Global Paleoproterozoic slab subduction might be linked to the Columbia supercontinent amalgamation. Large-scale slab subduction inputted crustal sediments into the deep mantle, forming carbonatitic magmas and oxidized ferric iron-rich majoritic garnets. Crustal materials recycled for about 2 billion years in the mantle source region, leading to the compositional heterogeneities and carbon cycle in the deep mantle.