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The composition and texture characteristics of plagioclase in basalt from the 26°S hydrothermal field of the Southern Mid-Atlantic Ridge provide an important basis for the study of magmatic evolution characteristics. On the basis of observing the petrography of basalt and analyzing the composition of plagioclase, the structure and composition of plagioclase at different stages are compared to explore the process of magma evolution. The results show that the study area is mainly composed of porphyritic basalt, and plagioclase porphyries are commonly characterized by glomerocryst structures, oscillatory-zoned, dissolved and resorbed rim, and growth structures. The plagioclase in the matrix is mainly acicular and swallow-tailed crystals; the basalts in the SMAR-26°S hydrothermal zone are mainly sodium tholeiitic basalts. In the early stage of magma evolution, bytownite was mainly formed, while in the late stage, labradorite was mainly formed. Throughout the entire evolution process, the content of CaO and Al2O3 decreases, while the content of SiO2, Na2O, FeO, and MgO increases; from the core to rim of plagioclase phenocrysts and then to plagioclase microcrystalline in the matrix, the An values decreases overall. The An values in the rim of plagioclase phenocryst partially overlaps with the An values of plagioclase microlites in the matrix, resulting in a gradual decrease in temperature. These represent the changes from the magma origin area to magma eruption. After the formation of mantle magma, it rapidly rises and erupts to form basalt. During the upwelling process, it may undergo the process of decompression accompanied by volatiles escaping, dissolving in the deep, supersaturated crystallizing of the melt, mixing of low-Si same sourced magma, and syn-eruptive fast decompression of degassed magma.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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