Nature and Thermal State of the Lithosphere beneath the Western Margin of the Yangtze Block in South China during the Neoproterozoic

Abstract

The western margin of the Yangtze Block is characterized by numerous felsic intrusions and many mafic-ultramafic plutons that record the lithospheric thermal state of South China during the Neoproterozoic. Numerous ∼800 Ma mafic dikes, intruded into the Neoproterozoic Dengxiangying Group, consist of fine- to medium-grained diabase and have variable SiO2 (42.80–57.20 wt%), MgO (5.16–13.15 wt%), and total alkalis (0.05–4.41 wt%). They show light rare earth element–enriched patterns with negative to slightly positive Eu anomalies (Eu/Eu*=0.67–1.09) and display enrichment of Pb and depletion of Nb, Ta, and Sr in the primitive mantle–normalized trace-element diagram. Rocks from the Dengxiangying mafic dikes have variable initial 87Sr/86Sr ratios (0.652431–0.730039) but relatively constant εNd values (−2.5 to +2.6) and Pb isotope ratios (Pb206/Pb204=17.53–17.88, Pb207/Pb204=15.60–15.63, Pb208/Pb204=37.66–38.38). These geochemical features suggest that the mafic dikes were derived from a lithospheric mantle enriched by slab-derived materials. PRIMELT3 modeling for the Neoproterozoic mafic rocks in the western margin of the Yangtze Block reveals that their primary magmas were in equilibrium with harzburgite and spinel peridotite with potential temperatures of 1377°–1498°C. They have high primary magmatic water contents (1900–72,000 ppm) that are similar to those of other arc basalts. These lines of evidences confirm that the Neoproterozoic mafic and ultramafic rocks were formed in an active continental setting.