SHRIMP U-Pb dating,geochemistry and tectonic implications of the Beiqigetao gabbros in Urad Zhongqi area,Inner Mongolia

Two types of cumulate gabbros are exposed in Wengeng area,Urad Zhongqi,Inner Mongolia,including olive gabbro and amphibole gabbro.The major petrogenetic minerals in the Beiqigetao gabbros are olive,pyroxene,plagioclase and hornblend.The SHRIMP U-Pb dating from olive gabbro give a concordant age of 269±8Ma.Both the gabbros show right-oriented linear REE patterns with LREE enrichment,and are characterized by LILE(K,Sr,Ba) enrichment and negative HFSE(Nb,Ta,U,Th) anomalies.The average 87Sr/86Sr(t),eNd(t),206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of olive gabbro are 0.706776,-8.8,17.2019,15.4252,and 37.3082,and those of amphibole gabbro are 0.706960,-8.9,17.2277,15.4524,and 37.4724,Respectively.Beiqigetao gabbros lie to EMI end member in the 87Sr/86Sr(t)-143Nd/144Nd(t) diagram.All the above evidences suggest that olive gabbro and amphibole gabbro were derived from the same mantle sources,which had undergone subduction-related metasomatism except inheriting the mantle source characteristics of the North China Block.