The role of the lower continental crust in chemical geodynamics

Recycling of continental crust into the mantle is generally conceptualized in terms of subduction of terrigenous sedimentary rocks produced by weathering and erosion of the upper continental crust. However, another form of crustal recycling is increasingly recognized as adding chemical diversity to the convecting mantle: recycling of lower continental crust. Lower crustal recycling can occur by several processes, including density foundering and subduction erosion. Given the compositional diversity of the continental lower crust, the chemical signature of such recycled material can be highly variable. For example, the radiogenic isotopic signature will depend on the age of the lower crust and its tectono-magmatic history. Newly formed, high density crystal cumulates foundered from colliding arcs may have Nd, Hf, Sr and Pb isotopic signatures similar to mantle-derived arc melts. But if these packages of recycled lower crust remain intact in the convecting mantle, they may evolve to radiogenic Nd and Hf (due to igneous cumulate or restitic garnet), radiogenic Os (due to high Re/Os) and unradiogenic Pb isotope compositions. In contrast, recycling of ancient lower crust should be characterized by evolved Nd, Hf and Os isotopic compositions, but may have variable Sr (depending on whether and when Rb was depleted) and Pb isotopic compositions (depending on the timing of the last orogenic event that homogenized Pb isotopes). Examples of intraplate magmas that derive from foundered ancient lower continental crust are found in eastern China, where thinning and destabilization of the Archean craton occurred coincident with a magmatic flare-up in the Mesozoic. Some of these Mesozoic magmas contain evidence for lower crustal foundering. For example, high Mg# andesites derive from melts of foundered lower crustal eclogite that interacted with mantle peridotite, whereas some high Mg basalts and alkaline picrites derive from a mantle source that was hybridized by silicic melts derived from foundered lower crustal eclogite. This, and other examples of lower crustal recycling point to an additional source of mantle heterogeneity derived from the continental crust.