The discovery of the oceanic island basalts in Jiageda Formation of northern Da Hinggan Mountains and its geological implications

Abstract: Lying in the Ergun massif, the Jiageda Formation is a part of the Xing’an orogenic belt. It is composed of pillow lava, schist, mica-schist, marble, marbleized limestone and meta-basalt rocks. The meta-basalt rocks with pillow structure in the Jiageda Formation are Na-rich Na alkaline basalts, with high content of TiO2, P2O5 and Na2O; the rare-earth elements are characterized by right-oblique strong LREE-enrichment patterns and high ∑REE values of 107.71×10-6~127×10-6; the rocks are enriched in such HFSH as Nb, Ta, Zr, Hf as well as such LILE as Rb, Sr, Ba and LREE, with LREE/HREE ratios being 5.15-5.56, (La/Yb)N being 4.44-5.18, and δEu being 0.86~1.08.The trace element patterns are of the upward-bulging K-Ti enrichment type, and their multi-component plots fall within the fields of oceanic island basalts and alkali basalts. These basalts are characterized by the tectonic setting of the oceanic intraplate alkaline basalts. The geochemical characteristics indicate that the meta-basalt rocks belong to the OIB. The discovery of the OIB shows that there once existed well-developed Palaeo-oceanic crust. Judging from the geological and geochemical features of the meta-basalt rocks in the area,the authors conclude that they were derived from the asthenosphere mantle or mantle plume.