Origin of Permian basalts and clastic rocks in Napo, Southwest China: Implications for the erosion and eruption of the Emeishan large igneous province

摘要

The Linghao Formation is composed of interbedded clastic and basaltic rocks in the Napo region of Youjiang Basin, Southwest China. The basalts are geochemically divided into low-Ti and high-Ti groups. Relative to the high-Ti group, the low-Ti group exhibits lower Ti/Y ratios, higher ratios of Th/Nb and Th/La, more significant negative Nb-Ta anomalies and lower ε_(Nd)(t) values (—7.26 to + 0.29).The high-Ti group is characterized by primitive mantle normalized OIB-like incompatible element patterns. These geochemical signatures are also comparable to those of the Emeishan high-Ti and low-Ti basalts in Southwest China, respectively, and support a view that both of the low-Ti and high-Ti basalts at Napo are part of the Emeishan large igneous province (ELIP). The geochemical features imply that the high-Ti basalts could have been generated by low degree of melting of the garnet perido-tite, whereas the low-Ti basalts may be derived from an EMII-like source. All clastic rocks exhibit no Nb-Ta anomalies on primitive mantle normalized elemental diagrams. Detrital zircons from clastic rocks yield U-Pb ages of -260 Ma and have a geochemical affinity to within-plate-type magmas, implying a sedimentary source dominated by the ELIP. Most clastic rocks from the upper part of the Linghao Formation show higher TiO2 contents and lower ratios of Al2O3/TiO2, Th/Sc, and Zr/Sc than those from the lower part. The former were related to the Emeishan high-Ti basaltic rocks, whereas the latter may have mixed felsic source compositions. In terms of chemostatigraphic correlation, we propose that the eruption of the ELIP may be more complex than previously thought and a prolonged and punctuated history of ELIP formation may exist in the Youjiang Basin.