Magmatic evolution characteristics and heterogeneity of the outer zone of the Emeishan Large Igneous Province: a case study of basalts from the Tianquan and Huayingshan areas

The Emeishan Large Igneous Province (ELIP) is extensively exposed in Southwest China, representing the first Mesozoic large igneous province recognised internationally. The mantle plume petrogenetic model has been widely accepted; however, most existing studies have focused on the ‘inner zone’ corresponding to the plume head region, leaving the magmatic evolution patterns of the outer zone lacking systematic constraints. The results show that: (i) Three eruptive cycles and eight rhythms can be identified in the Tianquan basalts, while two eruptive cycles and four rhythms are recognised in the Huayingshan basalts. Both sequences are contemporaneous with the ELIP flood basalts and intercalated in the coeval strata. (ii) The high-Ti basalts from the two areas have similar trace-element distribution patterns and their Sr–Nd–Pb isotopic compositions, which are comparable with those of the high-Ti basalts in the ELIP. (iii) The Tianquan samples exhibit slight crustal contamination, whereas the Huayingshan samples are almost free from crustal contamination. Trace-element ratios indicate that the magma source of Tianquan is located in the garnet stability field, with a partial melting degree (2–4%) lower than that of Huayingshan. The latter is derived from the spinel–garnet transition zone, with a relatively higher partial melting degree (4–6%). In general, the basalts from Tianquan and Huayingshan have similar geochemical characteristics and were both formed in an intracontinental setting. The mantle source of the former was at a greater depth and had a lower degree of partial melting. The compositional differences in magmas between the two areas arise from variations in lithospheric thickness and thermal state, which are controlled by the distance of their outcrop locations from the mantle plume centre, which directly triggered the differences in magma melting source regions and evolutionary processes.