Extensional Setting of Hainan Island in Mesoproterozoic: Evidence from Granitic Intrusions in the Baoban Group

Abstract

AbstractThe Mesoproterozoic Baoban Group is the oldest basement in Hainan Island and has played an important role in Columbia (Nuna) supercontinent reconstructions. The Mesoproterozoic granitic intrusions in the Baoban Group are the most widely‐exposed Precambrian magmatic rocks and are the key to understanding the tectonic settings of Hainan Island and its relationship with the South China Block and the Columbia supercontinent. New LA‐ICP‐MS zircon U‐Pb dating on three mylonitic granite samples from the Tuwaishan and Baoban areas yield ages ranging from 1447 Ma to 1437 Ma, representing the absolute timing of the emplacement of the granitic intrusions. Combined with previously published geochronological data for rocks from the Baoban Group and regional mafic intrusions, it is concluded that the Baoban Group formed at 1460–1430 Ma, coeval with the emplacement of the granitic and mafic intrusions. New in‐situ zircon Lu‐Hf isotope analyses for the three mylonitic granite samples yielded positive ∊Hf(t) values, ranging from +0.49 to +8.27, with model ages () ranging from 2181 Ma to 1687 Ma, suggesting that the granitic intrusions originated from a mixed source of Paleoproterozoic crust with juvenile crust. New zircon trace element data show characteristics of high Th/U values of 0.24–1.50, steep slopes from LREE to HREE and negative Pr, Eu anomalies with positive Ce, Sm anomalies, representing typical magmatic zircons formed in continental crust. Compared with available magmatic and detrital zircon ages from Precambrian rocks in the Cathaysia Block, Yangtze Block and western Laurentia, it is inferred that Hainan Island was separated from both the Cathaysia Block and the Yangtze Block, instead being connected with western Laurentia in the Columbia supercontinent. Considering the decreasing tendency of basin deposition time along the western margin of Laurentia, it is proposed that Hainan Island was located to the north or northwest of the Belt‐Purcell Supergroup, along the western margin of Laurentia, during the breakup of the Columbia supercontinent.