Ca. 1.7 Ga Magmatism on Southwestern Margin of the Yangtze Block: Response to the Breakup of Columbia

摘要

This paper presents some data of the Jiaopingdu gabbro and Caiyuanzigranite at the southwestern margin of the Yangtze Block,on the geochemical compositions,zircon LA-ICP-MS U-Pb ages and Hf isotopic data.The Jiaopingdu gabbro gives the age of 1721±5 Ma,the Caiyuanzi granite 1732±6 Ma and 1735±4 Ma,and the Wenjiacun porphyry granite 1713±4 Ma,suggesting nearly contemporaneous formation time of the gabbro and granite.The bimodal feature is demonstrated by the gabbro Si O2 content of 44.64-46.87 wt%and granite 73.81-77.03 wt%.In addition,the granite has high content of Si O2 and Na2 O+K2 O,low content of Al2 O3 and Ca O,enriched in REEs(except Eu)and Zr,Nb,Ga and Y,depleted in Sr,implying it belongs to A-type granite geochemistry and origin of within-plate environment.The zirconε(Hf)(t)of the granite and gabbro is at the range of 2-6,which is near the 2.0 Ga evolution line of the crust,implying the parent magma of the gabbro being derived from the depleted mantle and a small amount of crustal material,and the parent magma of the granite from partial melting of the juvenile crust and some ancient crustal material at the same time.Compared with 1.8-1.7 Ga magmatism during breakup of other cratons in the world,we can deduce that the Columbia has initially broken since ca.1.8 Ga,and some continental marginal or intra-continental rifts occurred at ca.1.73 Ga.