GEOCHEMISTRY AND SHRIMP DATING OF THE KEBU MASSIF FROM WULATEZHONGQI, INNER MONGOLIA: EVIDENCE FOR THE EARLY PERMIAN UNDERPLATING BENEATH THE NORTH CHINA CRATON

Abstract

The Kebu massif is located in Wulatezhongqi, west centre of Inner Mongolia and belongs to the midsection of the northern margin of the North China Craton. The Kebu massif is characterized of adakite. They have SiO2 = 57.66 - 66.94% (>56%), Al2O3 = 16.81 - 18.47% (>15%), MgO = 1.33 - 3.28% (<3%), Y = 7.25 - 13.46 × 10-6 (≤18 × 10-6), Yb = 0.72 - 1.42 × 10-6 (≤1.9 × 10-6) and low in high-fields strength elements (HFSEs, e.g. Nb, Ta, Ti, Zr) relative to continental arc andesite-dacite-rhyolite (ADR), Sr = 540.4 - 655.0 × 10-6 (>400 × 10-6), rich in LRRE (La/Yb = 12.11 - 28.56), slight negative to positive Eu anomaly (δEu* = 0.95 - 1.09). High Sr/Y(48.66 - 74.57), La/Yb and moderate Mg# (38.72 - 46.16), K2O (1.11 - 2.17) is similar to the adakite derived from the thickened lower crust. Low ε Nd (t) values (-15.1) and high (87Sr /86Sr)i (0.7073) are also inconsistent with an origin by slab melting, but resemble the adakitic rock from the East China. However, the Kebu massif is low in Cr, Ni and Mg# relative to the adakite formed by lower crust foundering from the North China craton during the Late Jurassic. This leads us to conclude that the Kebu massif was formed by partial melting of mafic material at the base of ancient continental crust due to basaltic liquid underplating. The SHRIMP U-Pb zircon age of the Kebu massif is 291 ± 4Ma, which means that underplating in the North China craton has been existed since the Early Permian.