Dating of monazite-apatite-allanite-epidote corona from the Bayan Obo Group in the northern margin of the North China Craton: implications for the time of regional Au and REE mineralization

The Mesoproterozoic Bayan Obo Group located along the northern margin of the North China Craton (NMNCC) hosts a world's largest known rare-earth element (REE) deposit (Bayan Obo Fe-REE-Nb deposit) [1,2] and a number of large gold deposits(eg.,Haoyaoerhugong and Zhulazhaga gold deposits;Fig.1a) [3],and has a long and protracted thermal history spanning from～1.3 Ga to 250 Ma [1].The tectonic history was associated with Proterozoic rifting with carbonatite magmatism at ～1.3 Ga [4]and subsequent subduction-accretion processes of the Paleo-Asian Ocean [5,6].The Paleo-Asian oceanic subduction beneath the North China Craton (NCC) initiated in the Middle Silurian [7,8] and the ocean was closed in the Permian to induce the accretion of arcs and terranes with the NCC[2].This long and protracted tectonic history increases the difficulties both in dating and understanding the genesis of the Bayan Obo REE deposit and black shale-hosted gold deposits (e.g.,Haoyaoerhudong deposit).Thus,it is important to understand the thermal history of the hosting strata in order to constrain the tectonic drive and timing of polymetallic mineraliza-tion.There have been few attempts to date the multistage thermal events which the Bayan Obo Group has witnessed,predominantly because of greenschist facies overprint of the host rocks and lack of reliable geochronometers [9,10].