Diverse lamprophyres origins corresponding to lithospheric thinning: a case study in the Jiurui district of Middle-Lower Yangtze River Belt, South China Craton

Abstract

Late Mesozoic magmatic rocks that range in composition from mafic to felsic are widespread in the Jiurui district of the Middle-Lower Yangtze River belt in eastern Yangtze Craton of southeastern China. Among which the lamprophyre dikes represent mantle-derived magma and record deep mantle magmatic processes in the region. In this paper, we report a detailed study of geochronology, mineral chemistry, petrochemistry and Sr-Nd-Pb isotopes on the lamprophyres from the Nangang area in the Jiurui district, in an attempt to provide a comprehensive understanding in their petrogenesis, as well as the Mesozoic lithospheric mantle characteristics, and geodynamical control of the extensive Mesozoic magmatism in the Jiurui district. The Nangang lamprophyres belong to alkaline lamprophyre. They are characterized by low contents of SiO2 and TiO2, high Mg# values and high contents of compatible elements, and are enriched in LREE and LILE but depleted in HFSE (with Nb-Ta-Ti negative anomalies). They show moderate enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr values of 0.70598 to 0.70715 and εNd(t) values of −2.2 to −1.6. They also show radiogenic Pb isotopic compositions with (206Pb/204Pb)i values of 17.678 to 18.558, (207Pb/204Pb)i values of 15.544 to 15.591 and (208Pb/204Pb)i values of 38.349 to 38.664. Combining new data from the Nangang lamprophyres with those of previously published data for lamprophyres in the Jiurui (i.e. Wushan I and II lamprophyres) and Edong districts (i.e. Ruanjiawan lamprophyres), we suggest that the Jiurui lamprophyres were likely derived from partial melting of the enriched lithospheric mantle characterized by phlogopite-bearing lherzolite in spinel-garnet transition zone at 75–85km depth. The feature of enriched-mantle source region was formed by metasomatism of subducted slab-derived fluids and melts, which is likely related to the Mesozoic subduction of the paleo-Pacific plate underneath the eastern China continent and the Neoproterozoic subduction of Cathaysia Block towards the Yangtze Craton. In contrast, the Ruanjiawan lamprophyres from the Edong district have the geochemical characteristics (such as the Nb-Ta positive anomalies) which are obviously different from the Jiurui lamprophyres, and we conclude that the Ruanjiawan lamprophyres, which are located more inland than the Jiurui district, were likely derived from the depleted asthenospheric mantle that show no significant influence by the subduction-related metasomatism. Furthermore, interpretation of the Sr-Nd-Pb isotope data for the Jiurui lamprophyres suggests slightly different petrogenesis for the various lamprophyre dikes, namely, the Nangang and Wushan–I lamprophyres were derived mainly from the enriched lithospheric mantle, but with addition of depleted asthenospheric mantle materials, whereas the Wushan–II lamprophyre dikes were product of the enriched lithospheric mantle without any signature for addition of depleted asthenospheric mantle materials. Given that these lamprophyres with distinct different isotopic compositions are sympatric and coeval, we consider that in the Late Jurassic to Early Cretaceous period (ca. 155–125Ma), the lithosphere was thinned to about 75–85km depth. The enriched lithospheric mantle was heated by the underlying convective asthenosphere and underwent partial melting to generate the Wushan–II lamprophyres. Subsequently, the enriched lithospheric mantle delaminated into depleted asthenospheric mantle, followed by partial melting coupled with an intensive interaction with the asthenospheric mantle to generate the Nangang and Wushan–I lamprophyres, respectively. Towards more inland, the Ruanjiawan lamprophyres were derived from the depleted asthenospheric mantle.