Magma types of volcanic rocks and continental margin of the Taiwan upper Paleozoic Geosyncline |
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| Authors: | Y Wang T K Liu |
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| Institution: | 1. Department of Geology, National Taiwan Univ., Taipei, Taiwan, Republic of China
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| Abstract: | Greenrocks are very common in the Tananao Schist of eastern Taiwan where the known fossils are of Permian in age. Fourty-four greenrock samples were chemically analysed and their magma types studied. The chemical composition of the greenrocks have marked variation common in volcanic rock series. The greater parts of the greenrocks belong to basalt and a smaller portion to basaltic andesite (SiO2 53 %–58 %). They are probably isochemical with their original igneous rocks except for volatile components. No striking Fe-enrichment exists in a MgO-ΣFeO-(Na2O=K2O) diagram. Based on (Na2O=K2O)-Al2O3-SiO2 diagrams afterKuno (1960), the parent magma of the rocks mostly belong to the high-alumina basalt series and only a few to alkali olivine basalt series. The high-alumina basalt can be looked upon as having an incipient trend for the calc-alkaline or the hypersthene series ofKuno (1959). The average K/Rb ratio of 460, the average TiO2 percentage of 1.5 %, and low K2O of around 0.5 % seem to warrant a conclusion that the basaltic rocks were poured out in the upper Paleozoic eugeosyncline on an embryonic continental crust. Considering the rock association of amphibolite plus serpentine (dismembered ophiolite), meta-graywacke, metachert, crystalline limestone, metaarkose, and metabasites in the Tananao Schist, the most probable site for the eugeosyncline may been an extensional trough near the fragmented paleo-Asiatic margin. |
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