Abstract:

By studying geochemical characteristics of Ordovician volcanic rocks in the Beishan area of Gansu and Inner Mongolia, the authors consider that the volcanic rocks in this area mainly belong to the andesitic basalt predominated by tholeiite series. The samples collected from Huaniushan and Xichangjing are presented nearly flatdepleted of LREE and the slightly enriched HREE in the chondritenormalized REE pattern, and their HFSE characteristic is similar to that of the N-MORB ophiolite. The samples from Yihatuoli and Xirehada are enriched in LREE and depleted in HFSE (Nb, Ta, Ti), which are similar with those of the island-arc volcanic rock. however, they have ratio values of (Th/Nb)_N>>1 and Nb/La＜1, as well as Zr/Nb similar to the the N-MORB, which suggested that the volcanic rocks were contaminated by the crust. The features of Zr>70.00×10^-6 and Zr/Y>3.00 of the samples are accordant with those of intra-plate basalts. Combined with the research of tectonic evolution in the study area, the authors concluded that Ordovician volcanic rocks in Huaniushan and Xichangjing formed at mid ocean ridge settings, and Ordovician volcanic rocks in Yihatuoli and Xirehada formed at a extensional rifting setting within the plate.

Key words: Ordovician, volcanic rocks, geochemical characteristic, tectonic settings, Beishan