The Kanasi potassic magnesianrich dacites|Altai|northwestern Chinese： Metasomatism by the melts of subducted sediments
Wang Yujing1 Yuan Chao1 Long Xiaoping1 Sun Min2 Xiao Wenjiao3
1.Guangzhou Institute of Geochemistry|Chinese Academy of Sciences|Guangzhou 510640；2.Department of Earth Sciences|The University of Hong Kong|Pokfulam Road|Hong Kong 3.Institute of Geology and Geophysics|Chinese Academy of Sciences|Beijing 100029
Potassic magnesianrich dacites occur to the south of the Kanasi，Altai，northwestern Chinese. UPb dating of zircons from these rocks yields an early Devonian age of 411±4 Ma. The dacites are enriched in SiO2(6471～6777 wt％)，MgO(328～353 wt％)and K2O(212～275 wt％)，and depleted in Na2O(170～185 wt％). The rocks have low CaO content(112～128 wt％)，high Al2O3 content(1412～1533 wt％)and K2O/Na2O ratios(12～16)，and ASI indexes larger than 18，indicating features of peraluminous potassic calcalkali igneous rocks，which are clearly different to primitive HighMg andesites. The dacites possess moderate Cr(134×10-6～153×10-6)，Co(175×10-6～187×10-6)，and Ni(72×10-6～81×10-6)contents. These rocks are enriched in LREEs，large ion lithophile elements(eg. Rb，K)and high field strength element(eg. U、Th、Pb)with moderate Sr，Ba，and “TNT”(Ta，Nb，Ti)negative anomalies，showing geochemical characteristics of igneous rocks in subduction zone. Their high Mg#(517～525)and low εNd(t)ratios(-46～-42)imply partial melting of metasomatic peridotites in the mantle wedge. The relative high Ba/La，Pb/La and low Th/Yb，Nb/Y ratios suggest that the metasomatism of mantle peridotites was predominated by the melts of subducted sediments. Therefore，the above geochemistry indicates that the potassic magnesianrich dacites were originated from the partial melting of metasomatic mantle peridotites mainly altered by the melts of subducted sediments，showing an active continental margin in the Devonian. In addition，the timing of the potassic magnesianrich dacites provides a good constrain on the minimum depositional age of the underlying Habahe group and suggests the sediments of the group deposited before the early Devonian.