Metamorphic And Anatetic Events In The Yardoi Gneiss Dome, Southern Tibet

The Himalayan orogenic belt is the type example of collisional belts. One of the prominent features in this orogen is the development of sub-parallel distributed High Himalayan Crystalline Sequence (HHC) and the Northern Himalayan Gneiss Dome (NHGD), which provides a key field laboratory for studying the tectonic evolution of large-scale collisional belts. The Yardoi Gneiss Dome (YGD) is the easternmost one in the nearly east-west trending NHGD. It consists of different types of high-grade metamorphic rocks and Cenozoic granites. These high-grade metamorphic rocks have experienced multiple stages of metamorphism and partial melting. Garnets from most of these high-grade rocks contain inherited cores formed prior to the Himalayan orogeny. Such garnets can be subdivided into four stages based on their texture and mineral inclusions. The latest is anhedral and elongated crystal that mimics the kyanite and contains rich inclusions of several sets of oriented ilmenite and rutile, which is interpreted to be resulted from biotite dehydration melting. This further indicates that the latest garnet could be linked to the formation of the Yardoi granites. Major and trace element geochemistry (depletion in HREE and covariation between Zr and Hf) in garnet grains of the latest stage also are consistent with such an inference. Due to intensive overprinting by later stages of metamorphism, pressure and temperature estimations from gneisses or schists yield relatively low temperatures (624-648℃) and pressures (0.81～1.01GPa), respectively. However, garnet amphibolites still yield relatively higher P and T at 0.97～1.13GPa and 872～892℃, respectively. Such a condition indicates that amphibolite could undergo partial melting at the early stages of tectonic evolution of the Himalayan orogen. SHRIMP zircon U/Pb data indicate that:(1) granite porphyrys formed at-20.3 Ma, leucogranite at～35.3 Ma and two-mica granite at～43 Ma; (2) amphibolite experienced amphibolite-facies or higher grade metamorphism at～45 Ma; (3) both granite gneiss and garnet-and staurolite-bearing schist also experienced episodes of metamorphism at～47-37 Ma. These zircon U/Pb data also suggest that (1) formation of the Yadoi granites was closely related to metamorphic reactions that resulted in amphibolites; (2) amphibolite partial melting had played a more important role in the production of two-mica granite than that in the leucogranite; (3) gneiss might also have experienced various degrees of melting and contributed subordinately to the generation of the Yardoi granites; and (4) the protoliths for some of the Yardoi high-grade rocks might derived from>520 Ma tectono-thermal events and had experienced pre-Cenozoic metamorphism. Whole-rock element and Sr-Nd isotope geochernical data demonstrate that:(1) the Yardoi granites are of Na-rich and peraluminous; (2) similar to those in gneisses, they are enriched in large ion lithophile elements (LILEs), but highly depleted in high field strength element (HFSEs); (3) most of the granites have an adakite-like geochemistry; and (4) both Sr-Nd isotope systematics and Rb-Sr covariation relationship suggest that partial melting of a source mixed of garnet amphibolite with subordinate pelitic gneiss could account for the formation of the Yardoi granite. Diluted by pelite-derived melts, the amphibolite-derived melts still preserved weakened adakite-like characteristics.The Himalayan orogen has commonly experienced>30 Ma episodes of anatexis, which may be a common feature in large-scale collisional belts. At the early stage of orogen development and overthickened crust, amphibolite partial melting dominated the anatectic event. This event might be a major factor that led to the transition from compressional to extensional deformation at>30 Ma in the southern Tibet.