Geochemical and U-Th isotopic study of Cenozoic high-K volcanics from Wudalianchi and Keluo, NE China

J.-X. Zhao (Dept. of Earth Sci., Univ. of Queensland, Brisbane, Qld 4072; M. Zhang (School of Earth Sci., Macquarie Univ., NSW 2109); K. D. Collerson (Dept. of Earth Sci., Univ. of Queensland, Brisbane, Qld 4072)

The Wudalianchi and Keluo volcanic fields (WK) (126°20'E/48°50'N) consist of the most potassic Cenozoic basalts in NE China (average SiO2 = 51.2%; K2O = 5.0% and K2O/Na2O = 1.36). Major eruptions occurred during the periods of 9.6 -7.0 Ma, 0.13 - 0.56 Ma and 1719-1721 A.D.. The historic eruption, together with that in Changbaishan Mountains, had a strong impact on global climate.

Preliminary trace element (by ICP-MS) and isotopic study was carried out on 15 samples from Late Pleistocene to Recent lava flows in the two volcanic fields. The ICP-MS data suggest that the volcanics are very uniform, regardless of rock types, ages and locations, characterised by: Sr 1175-2480 ppm, Ba 1497-2239 ppm, La 68-109 ppm, Nb 51-80 ppm, Zr 330-655 ppm, Th 5.2-10.1 ppm, Th/U 4.4-6.2 (ave. 5.0), La/Nb 1.16-1.54, Sr/Nd 18.2 -22.6, Nb/Ta 18.2-23.1, Ce/Pb 10.3-19.2, Nb/U 39.4-55.9, Rb/Sr 0.04-0.07, Sr/Ba 0.72-1.22, Zr/Hf 45.5-48.2, Zr/Nb 5.25-8.18, La/Yb 64.0-99.9 (ave. 81.3). Whilst these data are consistent with a metasomatised mantle source, there is little evidence for metasomatism to be subduction related, except for, perhaps, the slightly higher than chondritic La/Nb ratios. Compared with the Gaussberg Lamproite (GL) of East Antarctica, the WK suite has much lower K2O, Th/U, La/Yb, and La/Nb and much higher Sr/Ba, and Nb/U.

Measured (230Th/232Th) ratios are 0.745-0.804 for the 1719-1721 AD lavas, and 0.577-0.681 for the older ones. The results suggest that: (1) All the samples analysed are in U-Th isotopic disequilibrium, indicating they are probably younger than 375 ka; (2) All the samples plot above the Equiline, suggesting that Th is more incompatible during partial melting, and no residual phlogopite present; (3) If the measured (230Th/232Th) ratios of the 1719-1721 AD samples are taken as initial ratios, Th/U ratios of 3.76 - 4.05 are obtained for the source, which are significantly lower than that of the GL; (4) Assuming the same initial ratio for the 1719-1721 AD and older samples, the older samples yield 238U-230Th model ages of 92-369 ka.