Zircon from Orogenic Peridotite: An Ideal Indicator for Mantle-Crust Interaction in Subduction Zones

The orogenic peridotites can be subdivided into crust- and mantle-derived types. They record complex geological processes in subduction and collision zones. The crust-derived peridotites are derived from cumulates crystallized from ultramafic-mafic magmas in deep continental crust, an early mantle-crust interaction, prior to subduction. The mantle-derived orogenic peridotites are originated from subcontinental lithospheric mantle (SCLM) wedge and other mantle domains, and are later involved in the subduction channel and orogenic system. The mantle-derived peridotites usually record complex metasomatism, ultra-high pressure (UHP) metamorphism and mantle-crust interaction during the orogenic processes. Zircons are rarely found in orogenic peridotites. These zircons in orogenic peridotites are generally formed during metasomatism, they can be divided into old zircons (mainly the cores of residual magmatic and recrystallized) and newly grown zircons. Three key factors for zircon formation in orogenic peridotites are that: (1) zircon has strong crystallization ability, and Zr is easier to exchange Si in zircon crystal structure than other elements in the mantle; (2) metamorphic destruction of Zr-bearing minerals and precipitation of intergranular melts during the high-grade metamorphism can nucleate zircon under sub-solidus conditions; (3) the melts/fluids released from the subducted crust can metasomatize the mantle wedge to form zircons. In-situ studies on zircons and zircon inclusions enclosed in mantle minerals indicate that zircon can be an ideal indicator for mantle- crust interaction in subduction zones. The inclusions in zircons and Hf-O isotope of zircons are effective to reflect the composition of the melts/fluids, source properties, and the physical and chemical conditions. Dating of the zircons has been widely used in the studies of lithospheric evolution and crust-mantle interaction. During the complex processes of plate convergence, the orogenic peridotites can be subjected to the melt/fluid metasomatism, modifying the mineral and elemental compositions of peridotites. Thus, zircon is very useful to unravel the history of specific lithospheric mantle and the relationship between continental cratonic cores and their margins.