Geochronological and geochemical constraints on Mesozoic suturing in east central Tibet

This paper reports isotopic, major and minor element geochemistry of igneous and metamorphic rocks from the Kokoxili and Yushu regions of central and eastern Tibet. The first region lies along the Kunlun suture, which separates the Bayan Har-Songpan Ganze (Songpan) terrane from the Tarim and Qaidam blocks. Two Kokoxili granitoids yield U-Pb zircon dates of 217+/-10 and 207+/-3 Ma (Late Triassic), which represent the time of emplacement, and Rb-Sr isochron dates of 195+/-3 and 190+/-3 Ma (Early Jurassic), which are interpreted as cooling ages. The geochemical signatures of these granitoids suggest that they are related to subduction continuing into the Late Triassic. In the Yushu area, three samples help constrain the age of the Jinsha suture, which separates the Songpan terranes from the Qiangtang blocks. A leucocratic granite and an orthogneiss in the suture zone yield U-Pb zircon dates of 206+/-7 and 204+/-1 Ma, respectively, and a paragneiss south of it, a U-Pb monazite date of 244+/-4 Ma. The existence of coeval magmatism in both the Jinsha and Kunlun sutures suggests that the two subduction zones were simultaneously active. Combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions, we propose that the Kunlun and Qinling block boundaries, which were distinct in the Permian, subsequently formed a continuous, Late Triassic, northward subducting plate margin. Our data suggest that the Jinsha suture correlates with the Benzilan and Nan-Uttaradit sutures, which together belong to a major Late Triassic subduction zone.