SIMS ZIRCON U-Pb GEOCHRONOLOGY AND Sr-Nd ISOTOPES OF Ni-Cu-BEARING MAFIC-ULTRAMAFIC INTRUSIONS IN EASTERN TIANSHAN AND BEISHAN IN CORRELATION WITH FLOOD BASALTS IN TARIM BASIN (NW CHINA): CONSTRAINTS ON A CA. 280 MA MANTLE PLUME

摘要

Zircon SIMS U-Pb dating of the Poshi, Hongshishan, Bijiashan, and Huangshan Ni-Cu-bearing and Xiangshan Ni-Cu-Ti-Fe-bearing mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift yields a relatively restricted range of 278.6 Ma to 284.0 Ma. The histogram of compiled age data of basalts in the Tarim Basin and mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift has a peak of 280 Ma, which probably represents the time of mantle plume activity. The basalts have lower ε_(Nd)(t) values in the range of -9.2 ～-1.7 and Mg# of <50, and higher TiO_2 contents (>2 wt.%), indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting. The mafic-ultramafic intrusions have higher ε_(Nd)(t) of-1.3～11.2 and Mg# of 33～90, and lower TiO_2<1.8 weight percent, suggesting that their parental magmas were produced from lithospheric mantle source by high degree of melting due to higher temperature of the mantle plume head. A possible mantle plume model beneath lithospheric mantle of the Tarim Basin, Tianshan and Beishan and its spatial framework is suggested.