The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite,two-mica granite and garnet-bearing leucogranite.Zircon U-Pb dating yields that(1)tourmaline leucogranite formed at28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma;(2)two-mica granite formed at 19.8±0.5 Ma;(3)both types of leucogranite contain inherited zircon grains with an age peak at^480 Ma.These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions.As compared to the two-mica granites,the tourmaline ones have higher initial Sr and zircon Hf isotope compositions,indicating that they were derived from different source rocks combined with different melting reactions.Combined with available literature data,it is suggested that anatexis at^35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System(STDS),and led to the tectonic transition from compressive shortening to extension.Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt.Crustal anatexis at^28 Ma and^20 Ma represent large-scale melting reactions associated with the movement of the STDS.
GAO LiEZENG LingSenHOU KeJunGUO ChunLiTANG SuoHanXIE KeJiaHU GuYueWANG Li
Geochronological investigation on gneisses of granitic to leucogranitic compositions in Cuona,south Tibet,reveal that their protoliths formed at 808.8 ± 7.9-816.4 ± 3.4 Ma and 855.8 ± 7.0 Ma,respectively.Zircon rims from the granitic gneiss record a metamorphic age of 739.4 ± 4.3 Ma.Lu-Hf isotopic analyses on zircon grains with Neoproterozoic ages yield negative ε_(Hf)(t) values from-9.0 to-4.2,and the corresponding two-stage Hf model ages are 1965-2228 Ma.Whole-rock geochemical data indicate that all granitic gneisses are K-riched calc-alkali series.These new data together with literature data show that(1) the Himalayan terrane experienced an episode of Neoproterozoic magmatism at 850-800 Ma;(2) the Neoproterozoic magma of granitic compositions were derived from partial melting of ancient crusts,possibly due to the thermal perturbation related with the breakup of the Rodinia supercontinent.
Yuhua WangLingsen ZengLi-E GaoChunli GuoKejun HouLifei ZhangWei WangHuiyi Sun
