Tectonic Implications Of The Very Fast Cooling Shown By Concordant 230-228 Ma( 40) Ar/(39) Ar Laser Probe Hornblende And Biotite Single Grain Ages In The Hongseong Area

We obtained identical Ar-40/Ar-39 (pseudo) plateau ages of 230.1 +/- 1.0 and 229.8 +/- 1.0 Ma (1s) on two homblendes from garnet-bearing corona-textured amphibolites in the Hongseong area. These ages are concordant with the 228.1 +/- 1.0 Ma plateau age of biotite in the slightly older amphibolite. The concordant ages of hornblende and biotite, minerals with very different closure temperatures, show that the samples cooled very rapidly, probably in the order of 100-150 degrees C/Ma. The efficiency of cooling is further underlined by the near-coincidence of these Ar-40/Ar-39 ages with 243-229 Ma (error 2-4%, average: 234.5 Ma) zircon U-Pb ages in the Gyeonggi Massif and the Hongseong belt, reported in the literature. Very fast cooling rates require a fundamental tectonic control. Consequently, we discuss our data in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected the crust in Korea in the Late Triassic. This could have been post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off to which the 237-219 Ma mantle-sourced potassic Mg-rich magmatic rocks that are widespread in Korea, also points.