Centimeter-scale metamorphic contact between basaltic lava and limestone: a case study of the Permian Emeishan large igneous province, SW China

Detailed sampling at different intervals was performed to better understand the reaction along the contact between limestones and intruded basaltic lava in the Permian Emeishan large igneous province, SW China. The contact metamorphism is characterized by element migrations, the presence of nontronite with poor crystallinity, and large negative anomalies of stable carbon and oxygen isotopes, which indicates that geochemical reaction between basaltic lava and limestones is only marked at the centimeter scale. The CaCO 3 content decreases from 93.7 to 95.3% in limestone to 58.2% in the metamorphic limestone while the contents of both major elements (Fe 2 O 3 , SiO 2 , Al 2 O 3 , K 2 O, and Na 2 O) and trace elements (Ti, Cr, Cu, Mn, V, P, Ba, Co, Nb, Zn, and Zr) increase in the metamorphic limestone based on chemical gradients and thermal effects due to emplacement of basaltic lava. The presence of nontronite presumably derived from smectite transformation and the absence of wollastonite along the contact reflect the very low temperature of metamorphic reaction and the very low-grade contact metamorphism. The depletions in the δ 13 C and δ 18 O values of the metamorphic limestone achieve 4.066‰–13.792‰ and 7.522‰–9.746‰, respectively. The decreases in stable carbon and oxygen isotopic compositions along the contact relative to the unmetamorphosed limestone indicate that geochemical interaction between basaltic lava and limestone is obvious, suggesting the exchange occurred between the limestone and fluids with low δ 13 C and δ 18 O values. In addition, the narrow metasomatically and thermally affected zone demonstrates the low metamorphic reactivity of the limestone, while the limestone behaved as a barrier against fluid percolation. And the amount of crystallization heat of basaltic lava, as well as the interchange of heat with atmosphere, also play non-negligible roles in the centimeter-scale metamorphic reaction.