Discovery of deerite from the Aksu Precambrian blueschist terrane and its geological significance |
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| Authors: | Lifei Zhang Wenbo Jiang Chunjing Wei Shenbao Dong |
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| Institution: | 1. Department of Geology, Peking University, 100871, Beijing, China
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| Abstract: | Deerite Fe 12 2+ Fe 6 3+ Si12O40 (OH)10] was first discovered in the metamorphic magnetite-bearing quartzites which are interlayered with blueschists in Aksu Precambrian blueschist terrane. The deerite-bearing mineral association includes (1) deerite + riebeckite + stilpnomelane + ilvaite + magnetite + quartz and (2) deerite + stilpnomelane + magnetite +quartz based on the investigation under microscope. The study of mineral chemistry shows that the deerites from Aksu Precambrian blueschist are Mn-poor deerite similar to that from Alps. But the deerite from Aksu is the nearest to the end member of deerite. According to the equilibrium P-T region of deerite determined by Lattard and Breton (1994), the deerites in metamorphic magnetite-bearing quartzites from Aksu Precambrian blueschists were formed under the conditions of pressure lower than 1.0 Gpa, temperature ranging from 300 to 400°C and about the l0°C/km geothermal gradient. The deerite in metamorphic magnetite-bearing quartzites from Aksu blueschists is the only one Precambrian deerite reported now. This suggests that the earth began to cool, and the modern cooling subduction regime between plates started at Late Proterozoic. The Late Proterozoic may be the important period during the evolution of the earth. |
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