Magmatic lithology and genesis of Lingshan Island, northwest of the Yellow Sea, northeast of China,Indian Journal of Geo-Marine Sciences

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Magmatic lithology and genesis of Lingshan Island, northwest of the Yellow Sea, northeast of China
Indian Journal of Geo-Marine Sciences ( IF 0.5 ) Pub Date : 2020-07-23
J Yan, J Wang, Q Yu

According to field observation, the magmatic rocks of Lingshan island are grey-white in colour, with massive structure on the whole, and local stomatal, almond and rhyolite structures. Microscopic observation shows that they mainly have porphyritic texture. The main Phenocryst minerals include plagioclase (20 %), quartz (4 %), alkaline feldspar (3 %) and pyroxene (2 %), which are mostly semi-automorphic or heteromorphic, and the matrix is mainly glassy. The plagioclase number ranges from 23 to 35, belonging to the oligoclase-andesine series, indicating that the magmatic rocks in this area are close to intermediate. The SiO₂ content of the magmatic rocks on Lingshan island is 56.54 % - 61.76 % (average: 59.63 %) which gradually increases from north to south. It also can be concluded that the lithology in the study area is a series of intermediate rocks. The analysis of trace element experimental data shows that crustal elements such as Th, Zr, U, Ba and Pb are enriched, whereas incompatible elements such as Nb and Ti are deficient, this indicates that the magmatic rocks originated from partial melting of the lower crust. The rocks have high strontium and low rubidium contents, which indicates that amphibole, garnet and a small amount of potassium feldspar remain in the source area. Finally, from detailed analysis of Nb-Y, Rb-Nb+Y, Al₂O₃-TiO₂ and Harker diagrams, it is concluded that the magmatic rocks are dacite formed by differentiation in the late stage of lower crust partial melting in an island arc extensional tectonic environment, not the rhyolite formed by eruption mentioned in most literature. In addition, we believe that the magmatic rocks of Lingshan island undergo obvious crystallization differentiation and assimilation-contamination during their ascent, which is based on the consolidation index (SI), n (Rb)/n (Sr), Nb/Ta and (La/Yb) N-Sr/Y maps of rock samples.

更新日期：2020-07-28

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