Early Paleozoic Magmatism And Cenozoic Metamorphism Of The Ya Dong Gneisses, Central Himalayan Orogen

The High Himalayan Crystalline Sequences in the Ya dong area, the central Himalayan orogenic belt, consists mainly of pelitic schists, flesic gneisses, calc-silicate rocks, mafic rocks and a small amount of quartzite and marble. These rocks were intruded by the abundant leucogranites. This paper presents a study of petrology and zircon chronology of the orthogneiss. The gneisses include the dark and light gneisses. The dark gneiss consists of quartz(30~35%), plagioclase(30~35%), K-feldspar(20~25%), biotite(3~5%), and with and without minor muscovite and hornblende. Whole-rock chemical analysis shows that the dard genisses have Si O2 of 68.50~76.55 wt. %, Al2O3 of 13.22~14.97 wt.%, Ca O of 1.21~ 3.45 wt. %, Na2O+K2O of 5.64~7.64 wt. %, A/CNK values of 1.06~1.14. They have high REE contents and low Nb, Sr, Ti and Ba contents, high Rb, K and Ta contents. The two light gneisses contain Si O2 of 77.83 and 775.19 wt.%, Al2O3 of 13.47 and 14.15 wt. %, Ca O of 0.31 to 0.60 wt. %, Na2 O + K2 O of 7.01 and 7.78 wt. %, A/CNK values of 1.84 and 1.48. They have low REE, Th, Ba and Sr contents, high Rb contents.Zircons from the gneisses have core-rim structure, including a magmatic core and a thin metamorphic rim. The zircon magmatic cores have concentric oscillatory zoning, HREE-enriched patterns with negative Eu anomalies. The metamorphic rims and metamorphic zircons have low REE contents with negative Eu anomalies. The zircon U-Pb dating results show that the five samples have the weighted mean ages of 478±14Ma to 507±40Ma, representing their protolith ages. The metamorphic rims and metamorphic zircons give the weighted mean ages of 20.8±1.8 to 22.8±5.5 Ma. Besides, the zircon εHf(t) values of the three samples range from –8.1 to +0.1、–5.9 to –2.1 and –4.8 to +6.7 with TDM2 ranging from 1.44 to 1.96 Ga, 1.59 to 1.83 Ga and 1.03 to 1.76 Ga, which are significantly older than their crystallization ages. This shows that the gneisses were derived from the partial melting of ancient crustal rocks.Based on the above results, we consider that the gneisses recorded the early Paleozoic magmatic event, which was related to the subduction of the proto-Tethys ocean beneath the northern margin of Gondwana continent, and that the gneissse recorded the Cenozoic metamorphic and anatectic event, which was related to the Himalayan orogeny resulted from the India and Eurasian continent collision.