(A) In collaboration with Prof. Y. Sano, we were able to use nanoSIMS in Tokyo University to date thin zircon rims less than 10-micrometer thick retrieved from clinozoisite rocks associated with the Fengtien nephrite in Taiwan. The results showed that the Fengtien nephrite would have formed at 3.3±1.7Ma and is the youngest jade on Earth’s surface. This is the first study showing that nephrite formation took place during rock exhumation in subduction/collision zones, subjected to Barrovian-type metamorphic-metasomatic conditions.
(B) Based on our series studies on jadeitite from Guatemala, Myanmar and Japan, we were able to set up texture-mineral inclusion-geochemical criteria distinguishing jadeitite zircons into inherited, partially recrystallized, and completely recrystallized/newly-formed ones. The results helped us to conclude that most jadeitites around the world would be of metasomatic replacement origin, demonstrating that Al, Zr and Hf would be immobile elements in subduction environments. However, jadeitite from north of the Motagua Fault, Guatemala might be of precipitation origin. The characteristics of this subduction system responsible for jadeitite formation are not yet clear.
(C) Working on detrital zircons from the Higo metamorphic rocks, Japan, we demonstrated that the eastern extension of the Dabie-Sulu UHP belt may go through Japan as manifested by the coeval (~230-210 Ma) subduction. The proto-Japan should be part of the North China Block, not the South China Block as proposed by many previous studies. On the other hand, the westward subduction of the Paleo-Pacific Plate beneath the South China Block began no later than the very early Jurassic, as evidenced by the subduction-related Talun metagranite (~200 Ma) in Taiwan.
(D) In collaborating with Dr. M-C Liang of RECE, we tried to apply the clumped isotope technique to study marbles in Taiwan basement. Preliminary results showed agreement between clumped isotope composition and occurrences of marbles. However, more quantitative interpretation was hampered due to inconsistency between experimental and theoretical temperature calibration. Further work is now underway to explore the potential problems as well as potential applications.
(E) Orogenic peridotites are conventionally classified into two types: the Mg-Cr type of mantle origin and the Fe-Ti type of crustal cumulate origin. We studied the Svartberget peridotite rock suite in UHP WGC of Norway and found out that this Fe-Ti type peridotite rock assemblage may not necessarily be interpreted to be of crustal cumulate origin. The rock suite is actually similar in many aspects to the Mg-Cr type Kalskaret peridotite suite. The melt-rock interaction observed in the Svartberget rocks may thus an authentic example demonstrating mantle-partial melt interactions in subduction zones.
