Geochemical characteristics of Cenozoic basaltic high-K volcanic rocks from Maguan area, eastern Tibet

Abstract

The major element, trace element and Nd-Sr isotopic composition of Cenozoic basaltic volcanic rocks from the Maguan area, eastern Tibet, indicates that the volcanic rocks are enriched in alkalis, especially K (K2O up to 3.81%) and depleted in Ti (TiO2 = 1.27%–2.00%). These rocks may be classified as two groups, based on their Mg # numbers: one may represent primary magma (Mg numbers from 68 to 69), and the other, the evolved magma (Mg* numbers from 49 to 57). Their REE contents are very high (Σ REE = 155.06-239.04 μ/g). Their REE distribution patterns are of the right-inclined type, characterized by LREE enrichment [(La/Yb)N = 12.0–19.2], no Ce anomaly (Ce/Ce* =1.0), and weak negative Eu anomaly (Eu/Eu* = 0.9). The rocks are highly enriched in Rb, Sr and Ba (59.5-93.8 μ/g, 732–999 μ/g, and 450–632 ν/g, respectively), high in U and Th (1.59–2.31 μ/g and 4.73–8.16 μ/g, respectively), and high in Nb, Ta, Zr and Hf (70-118 μ/g, 3.72–5.93 μ/g, 215–381 μ/g, and 5.47–9.03 μ/g, respectively). In the primitive mantle-normalized incompatible element spidergram, Nb, Ta, Zr, Hf and P show positive anomalies, whereas Ba, Ti and Y show negative anomalies. The87Sr/86Sr ratios range from 0.704029 to 0.704761;143Nd/144Nd from 0.512769 to 0.512949; and eNd from +2.6 to + 6.1. These geochemical features might suggest that the potential source of the basaltic high-K volcanic rocks in the Maguan area is similar to the OIB-source mande of Hawaii and Kergeulen volcanic rocks.