Inverse reaction rim of biotite in early Cretaceous nosean phonolite of Niangniangshan, Ningwu Basin: mineralogical evidence of magma mixing triggered volcanic eruption

The disequilibrium texture of minerals can significantly indicate magmatic processes such as magma mixing, disequilibrium crystallization, and magma-fluid interaction. In the early Cretaceous nosean phonolite in the Niangniangshan Formation of the Ningwu Basin along the middle-lower reaches of Yangtze River. Biotite phenocrysts developed a rare inverse reaction rim texture, that is, a reaction rim composed of mafic minerals such as amphibole and pyroxene at the edge of biotite, but it's been insufficient to in-depth research the origin of this texture. Based on detailed petrographic study and electron probe analysis, this paper identifies a core-mantle-rim texture of the inverse reaction rimmed biotite in the Niangniangshan Formation nosean phonolite, in which the core is primary biotite, the mantle near the inverse reaction rim is biotite formed by high-temperature melt erosion and re-equilibration crystallization, and the rim is amphibole. The temperature-pressure calculation and geochemical characteristics of biotite and amphibole indicate that the crystallization temperature and pressure of biotite is 807~862 ℃ and the crystallization pressure is 231~369 MPa; in contrast, the crystallization temperature of amphibole is 920~949 ℃ and the crystallization pressure is 271~309 MPa. The chemical composition of the host magma is similar to that of the subalkaline basic magma, and is more mafic than the biotite host magma; The difference in the physicochemical conditions of biotite and amphibole crystallization in the "inverse reaction rim" indicates that this special "inverse reaction rim" texture results from the mixing of early alkaline magma and late basic subalkaline magma. Meanwhile, the crystal size distribution (CSD) theory shows that the formation of the reaction rim may only precede the volcanic eruption by around one month, suggesting that magma mixing may be an important trigger for the volcanic eruption.