Chemical and mineralogical compositions of silicic, mafic, and alkali tonsteins in the late Permian coals from the Songzao Coalfield, Chongqing, Southwest China

摘要

Silicic and alkali intra-seam tonsteins were discovered earlier in southwestern China. This paper reports new data on the geochemical and mineralogical compositions of tonsteins from the Songzao Coalfield, Chongqing, southwestern China, and provides new insights into the origin and distribution of the minerals and elements present. Three types of tonsteins (silicic, mafic, and alkali) were identified based on their lateral correlation over a large coalfield area and the overall similarity of corresponding mineralogical and chemical compositions. The clay minerals in most tonsteins of different sources and the mafic tuffs in the lower portion of the late Permian are dominated by kaolinite or are mainly composed of mixed layers of illite and smectite. The silicic tonsteins contain high quartz while the mafic tonsteins and mafic tuffs have high contents of anatase. All the tonsteins and tuffs contain pyrite, attributed to the seawater influences. The mafic tonsteins and tuffs have higher pyrite contents, probably due to the higher Fe derived from mafic volcanic ashes. The mafic tonsteins and tuffs are enriched in Sc, V, Cr, Co, and Ni, similar to the normal clay sediments in the southwestern China, indicating similar sources. The alkali tonsteins are characterized by high contents of Nb, Ta, Zr, Hf, REEs (rare earth elements), and Ga. The TiO_2/Al_2O_3 values are 0.02-0.08 for alkali, <0.02 for silicic, and >0.10 for mafic tonsteins and mafic tuffs, indicating different magma sources. Mafic tuffs and alkali tonsteins are enriched in rare earth elements, and silicic tonsteins contain a lowest REE but have the highest fractionation between light REE and heavy REE. Tonsteins and tuffs may be at the periphery of the Emeishan Large Igneous Province and probably resulted from a waning activity of the plume. They were probably derived from different mantle sources that were underwent not only low-degree partial melting but also fluid fractionation and contamination by lithospheric mantle. Their source magmas had an alkali-basalt composition and were similar to that of ocean island basalt.