Petrogenesis of the late Mesozoic Lingshang ultramafic intrusion in northern Jiangxi Province:Chronologic and geochemical constraints

Extensive lithospheric extensional thinning events occurred during the late Mesozoic in the interior of South China block. As a record of the early Cretaceous ((120.8±1.4)Ma) mantle-derived magmatic activities in the eastern part of the Qingzhou-Hangzhou juncture belt, the Lingshang ultramafic intrusion in Xinyu of northern Jiangxi is a good object for studying the mantle attribute and geodynamic process of the late Mesozoic in central South China. Based on a systematic analysis of the geochronology from La-MC-ICP-MS zircon U-Pb, Lu -hf isotopes and element geochemistry of the Lingshang ultramafic rocks, the authors investigated the intrusion, the source area characteristics of the ultramafic intrusion and the tectonic background reflected by them. Lingshang ultramafic intrusion suffered no significant crustal contamination. The content of MgO is concentrated with no obvious linear relationship with TiO₂, Al₂O₃, Ni, Th, and some other components, implying no significant crystallization differentiation. However, the range of Mg# and La-Sm differentiation indicates that heap crystallization and partial melting might have exerted an effect on magmatic evolution. The characteristics of rare earth and trace elements (Nb, Ta, Zr, Hf, etc.) are similar to those of intraplate basalt (OIB), and the characteristics of low SiO₂, high Ti, high Fe/Mn ratio and Ni are all closely related to asthenosphere mantle. However, the ε_Hf(t) of zircons (6.83-11.41) are less than the value of the depleted mantle, and Nb/Ta ratios are close to the ratio of lithospheric mantle, suggesting low degree partial melting of spinel peridotite mantle source region in the relevant element diagram. All of these data suggest that the Lingshang ultramafic intrusion may be the result of the interaction between asthenosphere mantle and lithospheric mantle under the background of late Mesozoic intracontinental extension. The upwelling asthenosphere material led to the low-degree partial melting of the preexisting spinel peridotite mantle source area, and the deep ultramafic magma was emplaced rapidly along the tectonic weak zone to form the Lingshang ultramafic intrusion.