SHRIMP zircon U/Pb dating on ultrahigh pressure rocks from Yangkou: Implications for the timing of partial melting in the Sulu UHP metamorphic belt.
苏鲁仰口超高压岩石SHRIMP锆石U/Pb定年与部分熔融时限

Fertile components in UHP rocks could undergo partial melting at both water-present or -absent conditions during deep subduction as well as early stages of rapid exhumation of continental materials. Petrographic and geochemical observations had shown that the Sulu UHP rocks experienced partial melting at UHP-HP conditions, which led to the formation of quartzo-feldspathic inclusions, granitic veins or plutons, and substantial geochemical effects. To further constrain the timing of these partial melting events, we have carried out SHRIMP zircon U/Pb dating on UHP metapelite as well as concordant K-rich granitic dike from Yangkou, Shandong Province. Zircon separates from both samples have core-mantle-rim structure. Zircon cores from metapelite yield 206Pb/238U ages greater than 282Ma, which represents different crustal components incorporated into the protiolith of metapelite. The mantle and rim yield 206Pb/238U ages of 233±3Ma and 214±4Ma, representing the timing of UHP and amphibolite facies retrograde reactions, respectively. Zircon grains from the synkinematic peraluminous granitic dike (A/CNK=1.2) yield similarly wide range 206Pb/238U ages (483~800Ma) from the core, implying that it might be derived from metasediments like sample SYK08-08. Except for one spot from the mantle yields 206Pb/238U age of 234.6±3.9Ma, all the other spots cluster around a concordant 206Pb/238U age of 220.8±2.9Ma. These mantle domains also contain mineral inclusions of Pl+Kfs+Qtz+Ap, typical of granitic composition. Such an association indicates that the Sulu UHP metasediments underwent partial melting at ~221Ma and prior to the widespread amphibolite facies retrograde metamorphism.