Constraints on timing of peak and retrograde metamorphism in the Dabie Shan Ultrahigh-Pressure Metamorphic Belt, east-central China, using U–Th–Pb dating of zircon and monazite

Abstract

The Dabie Shan Ultrahigh-Pressure Metamorphic (UHPM) Belt occupies the suture between the Yangtze and Sino-Korean blocks in east-central China. The timing of UHPM in the Dabie belt is controversial, and most recent data come from dating of zircons. Monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones, and monazite growth has been documented to occur during prograde, peak, and retrograde metamorphism. Zircons and monazites from UHP mafic rocks from Maowu and a UHP jadeite quartzite from Shuanghe were imaged in thin sections and in mineral separates and dated using a high-resolution ion microprobe. Maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite. Maowu eclogites and garnet pyroxenites contain zircons with mean 206Pb/ 238U age of ∼230±4 Ma, and monazites from a clinopyroxenite have 208Pb/ 232Th ages of ∼209±4 Ma. Multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages (i.e., they do not correspond to the cessation of diffusional Pb loss as rocks cooled below the closure temperature during exhumation). Shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921±22 Ma and lower intercept age of 236±32 Ma, interpreted to represent the ages of the source rocks and of peak metamorphism, respectively. Zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1.5 GPa. Their pooled age of 238±3 Ma agrees well with the lower intercept defined by cores, suggesting that Pb loss from cores and growth of new rims occurred during UHPM at ∼235–240 Ma. Monazite records multiple events during retrograde metamorphism. Maowu clinopyroxenite and Shuanghe jadeite quartzite experienced monazite growth at ∼209 Ma, interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration. Only the jadeite quartzite records growth at 223±1 Ma. Estimated exhumation rates for Maowu are 7–8 km/Ma.