Multi-stage evolution of the Xuhuai rift: Insights from the occurrence and compositional profiles of doleritic sills in the southeastern margin of the North China Craton

摘要

Various types of igneous suites occurred between 930 Ma and 890 Ma in the North China craton are known to represent a large igneous province (UP). However, the magma plumbing system of this LIP and its potential consequence to a breakup event are poorly understood. We report morphology, aystal size distribution of plagioclase, and whole-rock and mineral geochemistry profiles of this LIP represented by the two decoupled mafic sill complexes (Niutishan and Pingshan) in the Xuhuai basin. In terms of petrography and whole-rock geochemistry, unlike the lower-level doleritic Niutishan sill, the Pingshan sill is subdivided into an upper and a lower sequence that meets at a Sandwich Horizon but lacks stratification. The significant En reversals present an almost M-shaped profile that differs from a C-shaped profile as previously revealed in the Niutishan sill. Quantitative textural analyses show relatively strong plagioclase orientation in the lower sequence of the Pingshan sill, which is attributed to magma flow. The Pingshan sill is interpreted to have been constructed as multiple magma injections from crustal magma chambers based on the crystal size distribution and the varied crystallization pressure (5.1-7.5 kbar) of dinopyroxene crystals.