The discovery of the Elegen ophiolite in Beishan orogenic belt, Inner Mongolia: Evidence for the east extension of the Hongshishan-Baiheshan ophiolite belt

There is a set of ophiolite tectonic melange belt composed of basalt, plagiogranite, siliceous rock and sand slate which was discovered in the Elegen area of the Beishan orogenic belt in Inner Mongolia, based on the project of 1: 50000 Heihongshan Sheet regional geological survey.The ophiolitic melange belt is in NWW stripe-shaped distribution, with a width of about 2.5 to 8 km and an extension of about 25 km.The basalts in the Elegen ophiolite are rich in Na₂O(3.02%~6.04%), MgO(4.29%~5.46%)and poor in K₂O(0.03%~0.23%)and TiO₂(0.44%~0.59%), and chondrite-normalized LREE patterns are flat.The primitive mantle normalized trace element patterns are characterized by the enrichment of the LILE(Ba, U)and depletion of the HFSE(Nd, Ta, P, Ti), which also have the characteristics of MORB and IAB, showing the lava characteristics of the SSZ-type tectonic background.The trace elements are enriched in the large ion lithophile elements Ba and U, while the high field strength elements Nb, Ta, P and Ti are obviously depleted, and have the characteristics of MORB and IAB, showing the lava characteristics of the SSZ-type tectonic background.The basalts in the the ophiolite and plagiogranite have similar rock geochemical characteristics, suggesting that they were formed in the partial melting of the basaltic magma, and the REE patterns show a flat-like curve with weak negative Eu anomalies.The LA-ICP-MS zircon U-Pb age of 342±4.7 Ma was obtained in the plagiogranite, and it is determined that the Elegen ophiolite was formed in the Early Carboniferous.Through the analysis of the tectonic setting of the ophiolite and the volcanic rocks in different ages around this area, the authors believe that the Elegen ophiolite might have been formed in the expansion stage of the post-arc basin in the Hongshishan-Baiheshan area during the southward reduction of the Paleo-Asian Ocean.In the Late Mesozoic, a series of NE-striking sinistral strike-slip faults caused the Hongshishan-Baiheshan tectonic belt to move northward to the area of Elegen.The discovery of the ophiolite explains the east extension of the Hongshishan-Baiheshan ophiolite belt, thus providing important information for the Paleozoic tectonic evolution of the Beishan area and even the whole Central Asian Orogenic Belt(CAOB).