Geochemical characteristics of Cenozoic high-K igneous rocks from Liuhe-Xiangduo area, eastern Tibet

Abstract

The major elements, trace elements and Nd-Sr isotopic composition of Cenozoic high-K igneous rocks and mafic deep-derived enclaves from the Liuhe-Xiangduo area, eastern Tibet, indicate the high-K igneous rocks are characterized as being enriched in Ca (CaO = 1.20% – 8.80%), alkali (Na2O + K2O =3.47% – 10.65%), especially K (K2O up to 5.96%) and depleted in Ti (TiO2 =0.27% – 1.50%). Their REE contents are very high (REE =91.29 – 231.11 μg/g). Their REE distribution patterns are of the right-inclined type, characterized by intense LREE enrichment [(La/Yb)N = 7.44 – 15.73]. The rocks are distinctly enriched in Rb, Sr and Ba (46.3 – 316 μg/g, 349–1220 μg/g and 386 – 2394 μg/g, respectively), high in U and Th (1.17 – 8.10 μg/g and 2.38 – 27.0 μg/g, respectively), moderate in Zr and Hf (87.5 – 241 μg/g and 2. 83 – 7.52 μg/g, respectively), and depleted in Nb and Ta (4.81 – 16.8 μg/g and 0.332 – 1.04 μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, U, K, Sr and Hf show positive anomalies, whereas Th, Nb, Ta, P, and Ti show negative anomalies. The rocks are strongly depleted in Cr and Ni (21.4 – 1470 μg/g and 7.79 – 562 μ/g, respectively), and their transition element distribution curves are obviously of type-W. The (87Sr/86Sr)i ratios range from 0.704184 to 0.707539; (143Nd/144Nd)i from 0.512265 to 0.512564; and eNd(t) from -6.3 to -0.4. These geochemical features might suggest that the potential source of the high-K igneous rocks in the Liuhe-Xiangduo area is similar to the EM2, which may be similar to the material enriched K that is located under the crust-mantle mixed layer. The mafic deep-derived enclaves in the high-K igneous rocks belong to chance xenoliths. Their (87Sr/86Sr)i ratios range from 0.706314 to 0.707198; (143Nd/144Nd), from 0.512947 to 0.513046; and eNd(t) from +7.0 to +9.0. These geochemical features might indicate that the enclaves probably came from the depleted mantle. TheP-T conditions of the enclaves also showed that the enclaves are middle-lower crust metamorphic rocks, which were accidentally captured at 20 – 50 km level by rapidly entrained high-K magma, whose source is over 50 km in depth.