Origin of carbonate melts in orogenic belts by anatexis of downthrust carbonate sediments

Carbonate melts produced by partial melting of the carbonated mantle or by liquid immiscibility from mantle-derived silicate melts provide insights into the deep carbon cycle. Sedimentary geochemical signatures in some of the carbonate melts have been attributed to subducted material within their mantle sources. However, subducted carbonates at slab tops may not melt but be transported to the deep mantle, where sedimentary signatures would be diluted by mantle rocks. Here, we report the intruded Cenozoic carbonate melt dykes from the Yanshan orogen – an intracontinental orogenic belt far from the subduction zone – which show strong limestone-like geochemical features. Xenocrysts from the deep crustal basement, together with Sr-Nd-C-O isotope variations, indicate assimilation of the deep continental crust accompanied by CO2 degassing. We suggested they were formed by melting of downthrust limestone strata in the Yanshan orogen, triggered by underplating with basaltic melts during the Cenozoic upwelling of asthenosphere below the area. A compilation of global Phanerozoic carbonate magmas shows that this model can apply to many carbonate melts in orogenic belts which show much more evolved Sr-Nd isotopes and heavier C-O isotopes than those in intraplate settings. Trace elements of these carbonate melts indicate shallow derivation with a negligible contribution of mantle rocks. Spatial-temporal distribution of the orogenic carbonate melts supports them being related to continental crustal collision rather than oceanic subduction. Our findings highlight the carbonate sediments deposited in the continental crust as an important source that may deeply downthrust in the orogenic belt and melt during the post-collision stage. •Anatexis of downthrust limestone produced the Yangyuan carbonate melt dykes.•This model can explain the origin of many carbonate melts in orogenic belts.•Carbonate melts in orogenic and intraplate settings are different in geochemistry.•Many orogenic carbonate melts are not related to subducted material.